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50th Anniversary Invited Review
Layered intrusions in the Precambrian: Observations and perspectives
William D. Smith
a,b,*
, M. Christopher Jenkins
c
, Claudia T. Augustin
b
, Ville J. Virtanen
d,e
,
Zoja Vukmanovic
f
, Brian O’Driscoll
g
a
CSIRO Mineral Resources, 26 Dick Perry Avenue, Kensington, Perth, WA 6151, Australia
b
Mineral Deposits Group, Herzberg Laboratories, Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada
c
U.S. Geological Survey, Geology, Minerals, Energy, and Geophysics Science Center, 904 W. Riverside Ave Rm. 202, Spokane, WA 99201, USA
d
Institut des Sciences de la Terre d’Orleans, UMR 7327, Universite d’Orleans, CNRS, BRGM, Orleans, F-45071, OSUC, France
e
Department of Geosciences and Geography, University of Helsinki, Gustaf H¨
allstr¨
ominkatu 2, 00014 Helsinki, Finland
f
School of Environmental Sciences, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, United Kingdom
g
Department of Earth and Environmental Sciences, University of Ottawa, Ottawa K1N 6N5, Canada
ARTICLE INFO
Keywords:
Layered intrusion
Precambrian
Mineral system
Critical metals
Igneous petrology
microXRF
Electron backscatter diffraction
ABSTRACT
Layered intrusions are plutonic bodies of cumulates that form by the crystallization of mantle-derived melts.
These intrusions are characterized by igneous layering distinguishable by shifts in mineralogy, texture, or
composition. Layered intrusions have been fundamental to our understanding of igneous petrology; however, it is
their status as important repositories of critical metals – such as platinum-group elements, chromium, and va-
nadium – that has predominantly driven associated research in recent decades. Many layered intrusions were
emplaced during the Precambrian, predominantly at the margins of ancient cratons during intervals of super-
continent accretion and destruction. It appears that large, layered intrusions require rigid crust to ensure their
preservation, and their geometry and layering is primarily controlled by the nature of melt emplacement.
Layered intrusions are best investigated by integrating observations from various length-scales. At the
macroscale, intrusion geometries can be discerned, and their presence understood in the context of the regional
geology. At the mesoscale, the layering of an intrusion may be characterized, intrusion-host rock contact re-
lationships studied, and the nature of stratiform mineral occurrences described. At the microscale, the miner-
alogy and texture of cumulate rocks and any mineralization are elucidated, particularly when novel
microtextural and mineral chemical datasets are integrated. For example, here we demonstrate how mesoscale
observations and microscale datasets can be combined to understand the petrogenesis of the perplexing snowball
oiks outcrop located in the Upper Banded Series of the Stillwater Complex. Our data suggest that the ortho-
pyroxene oikocrysts did not form in their present location, but rather formed in a dynamic magma chamber
where crystals were transported either by convective currents or within crystal-rich slurries.
Critical metals may be transported to the level of a nascent intrusion as dissolved components in the melt.
Alternatively, ore minerals are entrained from elsewhere in a plumbing system, potentially facilitated by volatile-
rich phases. There are many ore-forming processes propounded by researchers to occur at the level of
emplacement; however, each must address the arrival of the ore mineral, its concentration of metals, and its
accumulation into orebodies. In this contribution, several of these processes are described as well as our per-
spectives on the future of layered intrusion research.
1. Layered intrusions and igneous layering
Layered intrusions are important geological features that record
Earth’s subsurface processes, particularly with respect to differentiation
and solidication of mac and ultramac magmas. Over the past half
century, many books and special volumes have been published that
summarize the body of scientic work dedicated to layered intrusions (e.
g., Latypov et al. 2024a and references therein). This article builds on
recent reviews by elaborating on construction mechanisms, parent
melts, and exploration tools, while also showcasing multiscale and
multidisciplinary research through a novel study of the enigmatic
snowball oiks outcrop of the Stillwater Complex in Montana, USA.
* Corresponding author at: CSIRO Mineral Resources, 26 Dick Perry Avenue, Kensington, Perth, WA 6151, Australia.
E-mail address: will.smith@csiro.au (W.D. Smith).
Contents lists available at ScienceDirect
Precambrian Research
journal homepage: www.elsevier.com/locate/precamres
https://doi.org/10.1016/j.precamres.2024.107615
Received 2 August 2024; Received in revised form 1 November 2024; Accepted 4 November 2024
Precambrian Research 415 (2024) 107615
Available online 16 November 2024
0301-9268/© 2024 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (
http://creativecommons.org/licenses/by/4.0/ ).
Layered intrusions are found in a variety of tectonic settings and man-
ifest in many different shapes and sizes throughout geologic time.
Layered intrusions are particularly well represented in the Precambrian;
more than 70 % of known and dated intrusions that exhibit some form of
igneous layering were emplaced in the Precambrian (Smith and Maier
2021). In this contribution, we focus on Precambrian layered intrusions,
but refer in places to igneous processes best preserved in younger in-
trusions. In general, igneous layering in layered intrusions can be
broadly categorized into mineralogically controlled layering and cryptic
layering. Mineralogically controlled layering is dened by conspicuous
variations in the modal proportions (uniform or gradational), grain size,
and (or) texture of the rock-forming minerals. Some form of
mineralogically-controlled layering is characteristic of all layered in-
trusions. For example, Fig. 1 shows mineralogically controlled layering
in outcrop at a variety of length scales from the decimeter scale down to
the millimeter scale, exhibited by rocks from the Archean Stillwater
Complex (USA), Proterozoic Etoile Suite (Canada), and Proterozoic Lake
Owen Complex (USA). Mineralogically controlled layering is often used
to subdivide the stratigraphy of layered intrusions into series, zones, and
subzones (e.g., Cameron 1978; McCallum 1987). This informal strati-
graphic nomenclature is important to correlate the geology of layered
intrusions over tens to hundreds of kilometers along strike. Mineralog-
ically controlled layering appears as sharp or gradational changes in
rock modal mineralogy or as well-developed rhythmic patterns or se-
quences (e.g., Wager and Deer 1939; Hess, 1960; Wager and Brown
1968; Irvine 1974). A special kind of rhythmic layering that presents as
10 s to 100 s of meters thick repeated successions of lithologies, termed
cyclic layering, is observed in so-called open system layered intrusions
and is generally believed to represent the repeated inux of magmas into
the nascent intrusion (e.g., Jackson 1961; 1970; Irvine 1976). Mineral-
ogically controlled layering may be 100 s of meters in stratigraphic
thickness down to the thickness of a single crystal, and can be laterally
continuous for 100 s of km along strike or wispy and discontinuous over
10 s of cm.
In contrast, cryptic layering refers to variations in geochemical
compositions that are unidentiable in the eld. This type of layering is
seen in places like the Archean ultramac rocks in Fiskenæsset,
Greenland (Polat et al. 2012) and Windimurra, Australia (Parks and Hill
1986). It can manifest as mineralogically homogeneous layers with
varying bulk rock major, minor, or trace element geochemistry. Cryptic
layering can also be present in more subtle forms, such as variations in
mineral major or trace element chemistry (e.g., Boudreau and McCallum
1989; Wilson et al. 1996) or variations in mineral or bulk rock isotopic
composition (e.g., Kruger et al. 1987; Kruger 1994).
Various models have been proposed over the years to explain the
different types of igneous layering. At the turn of the twentieth century,
it was commonly thought that the accumulation of minerals near
intrusive body contacts was caused by convection (Becker 1897) or
diffusion (Harker 1909). Following the 1831–1836 voyage of the HMS
Beagle, Darwin (1844) was among the rst to attribute the diversity of
igneous rocks to crystal settling, an idea that was later popularized by
Bowen (1915) who performed experiments showing olivine sinking in a
melt. In an inuential paper by Wager and Deer (1939), the sedimen-
tation of crystals precipitating from convective currents of basaltic melt
was invoked as the layering forming mechanism for the Skaergaard
intrusion in Greenland. This settling mechanism fundamentally changed
how researchers considered the formation of layered mac and ultra-
mac intrusive rocks and led to several transformative publications (e.g.,
Hess 1960; Jackson 1961; Wager and Brown 1968). Since then, a variety
of dynamic and non-dynamic processes (Boudreau and McBirney 1997;
Namur et al. 2015) have been developed and proposed, alone or in
combination. These processes include crystal settling, in situ crystalli-
zation, inltration metasomatism and competitive particle growth
(Fig. 2). Although the relative importance of these and other processes
remains heavily debated, it is likely that many of them operate during
the formation of layered intrusions and, as such, the interpretation of
cumulus rocks should be approached with an open mind and some de-
gree of caution.
Crystal settling (i.e., cumulus theory) refers to the process where
crystals homogeneously nucleate within a silicate melt and are depos-
ited at the base of a magma chamber either by gravity-driven settling or
convective currents. This process gave rise to the rock name cumulate
because it was believed the primocrysts crystallized from the melt and
accumulated on the intrusion oor (Wager and Deer 1939; Wager and
Brown 1968; Irvine 1982). Crystal settling rst garnered attention
because of the noticeable textural similarities between some cumulates
Fig. 1. Photographs of layering at various length scales. A. Decimeter-scale igneous layering exposed in the Contact Mountain area of the Neoarchean Stillwater
Complex, Montana, USA. Photograph by Chris Jenkins, U.S. Geological Survey, 2023. B. A sharp mineralogically-controlled layer contact from the Proterozoic Etoile
Suite in Canada (R. Maier et al. 2024). C. Fine-scale mineralogically controlled layering in gabbronorites from the Proterozoic Lake Owen Complex, Wyoming, USA.
Photograph by Chris Jenkins, U.S. Geological Survey, 2023. Scale bar shows 1 cm tick marks. D. Microscopic-scale igneous layering in peridotites from the B
chromitite zone of the Neoarchean Stillwater Complex, Montana, USA. Photograph by Chris Jenkins, U.S. Geological Survey, 2024.
W.D. Smith et al.
Precambrian Research 415 (2024) 107615
2
and clastic sedimentary rocks. Indeed, sedimentary structures including
cross bedding, graded bedding and slump structures have been docu-
mented in layered intrusions including the Precambrian Kiglapait
intrusion (e.g., Morse 1969). In addition to the evidence of sedimentary
structures preserved in some cumulate rocks, non-cotectic mixtures of
cumulus minerals (e.g., Stillwater Complex, Jenkins and Mungall 2018)
and monomineralic layering (e.g., Bushveld Complex, Maier et al. 2013;
Forien et al. 2015) have been cited as evidence of syn- or post-
depositional mechanical sorting of crystals before the crystal mush
cools and becomes rigid.
In contrast, layer formation by in situ crystallization shares similar-
ities with chemical sedimentary rocks. In the late 1970 s, two seminal
papers by Campbell (1978) and McBirney and Noyes (1979) identied
several features of layered intrusions that were not compatible with
cumulus theory. For example, cumulus theory could not predict how
near vertical primary layered sequences containing steeply dipping se-
quences with apparent sedimentary structures (e.g., cross bedding),
observed in the Cretaceous Skaergaard intrusion (Greenland) or the
Proterozoic Jimberlana intrusion (Australia), could form. They also
pointed out that cumulus theory could not explain how plagioclase
crystals—with densities less than the basaltic magmas they are proposed
to crystallize from—could settle rather than oat (Bottinga and Weill
1970). Further, Campbell (1978) argued that classic nucleation theory
posits that the activation energy required for the heterogeneous nucle-
ation of crystals is far less than for homogeneous nucleation. Because of
this, heterogeneous nucleation is the much more likely nucleation
mechanism operating in layered intrusions. In situ crystallization theory
suggests that primocrysts heterogeneously nucleate and grow on the
outer edges of the intrusion or on pre-existing cumulates (or on a
chamber oor) and grow from the margins of the intrusion inward. In
this scenario, nucleation and growth of crystals occur at the interface
between the cumulates and the remaining melt (Campbell 1978;
McBirney and Noyes 1979). Since that time, overhanging layered se-
quences in the Merensky Reef (Latypov et al. 2017) and UG1 chromitites
(Mukherjee et al. 2017), unidirectional crystal growth patterns (i.e.,
crescumulate or comb layering; e.g., the Proterozoic Rognsund gabbros
from the Seiland Igneous Province in Norway, Robins 1972) and the
unidirectional growth of chains of crystals (e.g., UG-2 chromitite of the
Bushveld Complex, Latypov et al. 2022) have been cited as evidence in
support of this kind of layer-forming mechanism. While crystal settling
or in situ crystallization may form primary igneous layering during
crystal fractionation, metasomatism or competitive particle growth may
modify or overprint the original cumulate textures and generate entirely
new or different layered successions.
Metasomatism or reactive porous ow refers to the movement of
magmatic vapors or uids through the porous crystal mush layer (e.g.
McBirney 1987; McBirney and Sonnenthal 1990). Metasomatism occurs
when melts or magmatic uids interact with crystals in the permeable
mush layer. This process may change primary mineral chemistry or even
the rock type entirely. As crystals accumulate and compact under their
own weight or as melts convect in a semi-solid mush zone, the interstitial
liquid is forced upwards through the crystal mush (Boudreau and
McCallum 1992; Irvine 1980; Latypov et al. 2008; Maier et al. 2021).
Tegner et al. (2009) suggest that differential compaction results in
variations in trapped liquid content contributing to the modal layering.
Others have suggested that the density contrast between overlying dense
melts and lower-density melts in the mush may drive inltration and
metasomatic processes in layered intrusions (Kerr and Tait 1985;
Fig. 2. Cartoon showing various layer forming mechanisms that operate in mac and ultramac intrusions as discussed in the text. Hypothetical phases are sym-
bolized as plagioclase (white), orthopyroxene (brown), clinopyroxene (green), olivine (purple), and sulde liquid (red).
W.D. Smith et al.
Precambrian Research 415 (2024) 107615
3
McBirney 1995; Jenkins et al. 2021). Yao and Mungall (2022) modelled
the formation of the Cr concentration diffusion patterns in the Bush-
veld’s Main Magnetite Layer as the result of metasomatism of a mushy
layer of magnetite by an upward percolating melt. These results are of
particular interest because they demonstrated how diffusion patterns in
an outcrop could be modelled by two distinctly different proc-
esses—metasomatism and in situ crystal growth. Holness (2024) sug-
gested that ne scale igneous layering in the Stillwater Complex was
produced by the percolation and crystallization of silicate melt in a
slowly cooled cumulate pile aided by competitive particle growth. These
examples demonstrate that different layer forming mechanisms may
operate in conjunction.
Competitive particle growth (i.e., Ostwald ripening, textural coars-
ening, crystal aging) theory suggests that small crystals will dissolve at
the expense of large crystals to minimize interfacial energies and achieve
textural equilibrium. It has been suggested that such a process can lead
to ne-scale layering over long cooling times in layered intrusions
because crystal aging occurs at different rates for different phases
(Boudreau 1995). Such postcumulus textural equilibration mechanisms
have been evoked to explain highly regular ne-scale layering features
in layered intrusions including the so-called inch-scale doublet layering
in the Stillwater Complex (e.g., Boudreau 1995; Holness 2024). To some
extent, this process likely occurs in very slowly cooled layered igneous
rocks as demonstrated by their propensity to have equilibrated textures
(e.g., 120-degree dihedral angles at monomineralic triple junctions). The
effects of the textural maturation process may make it difcult to discern
between layer-forming processes because primary igneous textures can
be overprinted or even obliterated.
Still, other layer forming processes may operate to create primary or
secondary layering. Physical processes operating in magma chambers
like magma density currents (e.g., Higgins 1991) or viscous particle
segregation in response to slumping of the cumulate mush (Forein et al.
2015) have been invoked to modify primary igneous fabrics or generate
sequences of modally layered cumulates, respectively.
2. Debates and controversies
Debates, controversies, and advancements in layered intrusion
research have recently been detailed by Latypov et al. (2024a) and as
such, we only briey outline a few issues of contention here. Con-
struction mechanisms remain a controversial topic, with researchers
debating whether layered intrusions are the product of large molten
magma chambers or mushy systems constructed episodically through
repeated sill injection. Key observations driving this debate include the
lack of geophysical evidence for magma chambers in the upper crust
(Cashman et al. 2017) and the fact that high-precision U-Pb zircon
geochronology has revealed that some layers of the Bushveld, West-
Pana, and Stillwater complexes are younger than overlying layers
(Mungall et al. 2016; Groshev and Karykowski 2019; Scoates et al.
2021). Although there are several studies that infer the non-sequential
construction of sill complexes (Marsh et al. 2003; Wilson et al. 2016;
Smith et al. 2021) and layered intrusions (Harker 1909; Mitchell and
Scoon 2012; Hepworth et al. 2018; 2020; Yao et al. 2021) on other
grounds, it remains uncertain as to whether ages derived from
geochronology of zircon and baddeleyite hosted in long-lived and
(possibly) once mushy layers are truly representative of their host rock
crystallization ages (Latypov and Chistyakova 2022). Researchers are
continuing to address these debates by thoroughly examining eld re-
lationships (Latypov et al. 2022), applying geophysical mapping ap-
proaches to the plumbing systems beneath layered intrusions (Cole et al.
2024) and by examining (post-)cumulus processes from the perspective
of expansive geochemical datasets (Barnes and Wiliams 2024).
Since the seminal study of Wager et al. (1960), textures of igneous
cumulates have been used to infer a variety of magmatic processes in
layered intrusions that are expounded in Latypov et al. (2024a; 2024b).
One longstanding debate concerns the mechanisms by which planar
alignments of cumulus crystals, that were initially interpreted as strong
evidence for crystal settling, develop. This phenomenon has since been
attributed to in situ crystallization (McBirney and Noyes 1979), post-
cumulus processes such as compaction (e.g., Meurer and Boudreau
1998), and remobilization or deformation of poorly consolidated crystal
mush (Higgins 1991; Irvine et al. 1998; O’Driscoll et al. 2008). Another
ongoing debate regards the nucleation and growth of chromite in
chromitites, where Holness et al. (2023) showed that the long-accepted
mechanism of ‘self-nucleation’ of chromite crystals (Campbell 1978;
Godel et al. 2013; Prichard et al. 2015) was energetically unreasonable.
It seems likely instead that the common observation of chromite crystals
forming interconnected networks in such rocks is a result of physical
aggregation (i.e., synneusis) or an alternative mechanisms of nucleation
(i.e., secondary nucleation; Latypov et al. 2024b).
Several debates surround the origin of chromitite-anorthosite re-
lationships and reef-style platinum-group element (PGE) mineralization
in layered intrusions. Some researchers argue that coupled chromitite-
anorthosite units result from the ux melting of partially molten
gabbroic cumulates, whereby chromite becomes supersaturated as
Cr
2
O
3
and Al
2
O
3
are liberated (Nicholson and Mathez 1991; Marsh et al.
2021). Alternatively, these units may form through partial melting of
gabbroic cumulates by replenishing melts (O’Driscoll et al. 2009; Scoon
and Costin 2018; Jenkins et al. 2021), consistent with the lack of
accessory olivine and late-stage volatile-bearing phases in these hori-
zons. So-called boulders in the Boulder Bed of the Bushveld Complex
could be an exception because several exhibit features consistent with
volatile involvement, such as amphibole rinds (Smith et al. 2023). Reef-
style PGE mineralization derived from upwelling magmatic volatiles
requires that the PGE and base-metals are liberated from an underlying
cumulate pile and reconcentrated in laterally extensive stratiform ho-
rizons (Boudreau 2019 and references therein). Proponents of this model
report a relative increase in the abundance of Cl-rich halogen-bearing
phases (Boudreau et al. 1986), melt inclusions hosting late-stage sili-
cates (Ballhaus and Stump 1986), and bulk Cl/Br values (Parker et al.
2022) at the level of mineralization. On the other hand, relative Cl en-
richments could reect devolatilization of fractionating sulde melts
(Mungall and Brenan 2003; Liu et al. 2021) and melt inclusions may
sample compositional boundary layers that are not representative of the
original melt (Faure and Schiano 2005). Recent experimental work has
shown that Cl-rich magmatic brines and (or) carbonic uids are capable
of transporting ore-forming concentrations of Pt, Pd and Au (Simakin
et al., 2021; Sullivan et al., 2022a, 2022b). However, there is presently
no compelling evidence that Ni and the iridium-group PGE can be
effectively transported in these media, which is at odds with Ni/Cu and
Pd/Ir values recorded in reef-style occurrences. In the Merensky Reef,
chromitite-hosted orthopyroxene crystals have relatively low H
2
O con-
centrations compared to orthopyroxene from the bracketing lithologies
(Tang et al. 2023). More experimental studies are required to better
constrain the solubility of noble metals in magmatic phases, which
should be coupled with bulk and in situ (e.g., primocrysts, accessory
phases) measurements of volatiles species.
3. Drivers and biases inferred from scientometrics
In layered intrusion research, the number of scientic articles
available online has been increasing since the 1950 s and rapidly
increased over the 2010 s, possibly inuenced by invention of Google
Scholar (Fig. 3A). Recent articles and their supplementary materials are
generally more accessible, which caters to a recency bias in which one
overemphasizes the relevance of recent information over older infor-
mation (Mayo and Crockett 1964). This can ultimately lead to misin-
formation by transmission chaining, and as such, one should always
revisit the earliest works. Other inherent biases in scientic research
include apophenia (i.e., the tendency of identifying meaningful patterns
where none truly exist; Jones and Martin 2021) and conrmation bias (i.
e., the tendency to interpret data in a way that is supportive of existing
W.D. Smith et al.
Precambrian Research 415 (2024) 107615
4
(caption on next page)
W.D. Smith et al.
Precambrian Research 415 (2024) 107615
5
beliefs; Klayman 1995). In general, these biases stem from the human
tendency to nd immediate reward in conrming one’s own hypothesis
– consistency is often conated with correctness. The inuence of such
biases may be muted through interdisciplinary collaborations and
thorough peer-review processes as well as greater data transparency and
reproducibility (Jones and Martin 2021).
The popularity of important petrogenetic processes has changed over
time, likely inuenced by seminal publications, technological ad-
vancements and new discoveries (Fig. 3B). Oftentimes, popularized
concepts become amplied in subsequent literature, which is partly a
consequence of anchoring and availability biases (Yasseri and Reher
2022). For example: (i) mentions of crystal settling decrease at the turn of
the century, coinciding with a rise in the mentions of crystal mush; (ii)
mentions of crystal settling and in situ crystallization have somewhat
antithetical trends over time; (iii) double diffusive convection was only
mentioned to a large extent in the 1980 s; (iv) mentions of trapped liquid
shift and in situ crystallization spike shortly after the publication of Barnes
(1986) and Campbell (1978), respectively. These biases could generally
be easily mitigated through collaborative research, peer-review and
robust statistical examination of interdisciplinary datasets.
Unlike concepts, research methodologies employed to better un-
derstand layered intrusions have remained broadly unchanged (Fig. 3C).
Fundamental methods such as petrography, geochemistry, and isotope
geochemistry have been a staple of layered intrusion research, bolstered
by geochronology and mineral chemistry. Studies utilizing mineral
chemistry increased rapidly during the 2000 s, perhaps due to accessi-
bility and/or technical advancements. Geophysics, microstructural data
and multivariate analysis are becoming increasingly utilized in layered
intrusion research, suggesting that these analytical methods will play an
important role in the future of layered intrusion research. Technological
advancement and software development have meant that data are
becoming increasingly accessible and, as such, interdisciplinary studies
are becoming increasingly common in layered intrusion research. Many
studies on layered intrusion research are published in Journal of
Petrology and Contributions to Mineralogy and Petrology consistent with
the fact that most studies focus heavily on petrology. Economic Geology
and Mineralium Deposita are also commonly utilized journals, high-
lighting that many studies focus on ore-forming processes that operate
during layered intrusion formation (ESM 1).
Large and/or strongly mineralized layered intrusions receive greater
attention than smaller and/or apparently barren intrusions (ESM 1).
This is unsurprising given that the mineral potential of layered in-
trusions is an important reason many institutions are able to study them.
Commercial interest generates extensive geophysical and geochemical
data on a subject intrusion, whilst driving the drilling programs neces-
sary for sampling campaigns. This is exemplied in what we refer to as
post-discovery peaks (Fig. 3D), including: (i) studies on the J-M Reef of
the Stillwater Complex increase shortly after discovery, broadly coin-
ciding with the invention of catalytic converters and Ni-sulde re
assay; (ii) studies on Voisey’s Bay peak shortly after its discovery in the
1990 s; (iii) studies on the Skaergaard intrusion increase after the dis-
covery of the Platinova Reef; (iv) research on the Duluth Complex spikes
shortly after PGE-rich suldes were discovered in the South Kawishiwi
intrusion; (v) research on Nova-Bollinger spikes a few years after its
discovery in 2014. Notwithstanding their size and economic signi-
cance, other reasons why certain layered intrusions are more widely
reported than others include accessibility, as well as their degree of
alteration, deformation and exposure. The tendency to study specic
intrusions unavoidably leads to reporting and sampling biases; obser-
vations from a few intrusions dominate the literature and are somewhat
indiscriminately extrapolated to intrusions from other eras and
geological settings (i.e., generalizability; Simundic 2013; Andringa and
Godfroid 2020). Such biases can be partially mitigated by considering
multiple analogous subjects (e.g., intrusions, mineral occurrences) in
scientic contributions as opposed to centering on a single subject.
4. Layered intrusions in the framework of the Precambrian
The emplacement of layered intrusions in the Precambrian broadly
correlates with supercontinent assembly and disassembly, which, in
turn, correlates with enhanced periods of juvenile crust production and
crustal reworking (Fig. 4A-E; Maier and Groves 2011; Dhuime et al.
2012; Smith and Maier 2021). Moreover, their occurrence generally
correlates with episodes of large igneous province (LIP) magmatism
(Ernst et al. 2021), and as such, periods of crustal thickening and
enhanced magmatic activity appear most favorable for layered intrusion
formation. The earliest period of enhanced layered intrusion emplace-
ment occurs from 3.0 to 2.7 Ga, and primarily corresponds to the West
Pilbara (Australia; e.g., Munni Munni, Andover, Radio Hill) and
Murchison Domain (Australia; e.g., Windimurra, Narndee) intrusions, as
well as other notable intrusions such as Stella (South Africa), Monts de
Cristal (Gabon), and Fiskenæsset (Greenland). This interval is signicant
in Earth history as it follows the broadly accepted interval of global-scale
subduction initiation (Palin et al. 2020), precedes the assembly of the
Superia-Sclavia (or Kenorland) supercontinents (Nance et al. 2014), and
correlates with a ramp-up in large igneous province magmatism (Ernst
et al. 2021) as well as crustal production and differentiation
(Hawkesworth et al. 2010; Dhuime et al. 2015). This newly evolving
crust would have interacted with upwelling komatiitic melts generated
during relatively voluminous melting (>25 %) in a hotter Archean
mantle, leading to the production of siliceous high-Mg basalts that are
believed to be parental to many Archean layered intrusions (West Pil-
bara intrusions, Hoatson and Sun 2002; Stillwater Complex, Jenkins
et al. 2021).
The second period of enhanced layered intrusion emplacement oc-
curs from approximately 2.5 to 2.4 Ga, and includes intrusions associ-
ated with the Baltic (e.g., Monchegorsk, Kemi, Penikat) and Matachewan
(e.g., East Bull Lake, Agnew, River Valley) LIPs. This interval spans the
earliest stages of the Great Oxidation Event (GOE), where it correlates
with increasing crustal production and differentiation. It has been pro-
posed that this period also coincides with a sharp increase in mineral
species resulting from oxidation and biological mediation (Hazen et al.
2008), as well as the cooling of ambient mantle (i.e., Great Thermal
Divergence; Condie et al. 2016). Contaminated komatiitic melts are still
Fig. 3. Layered intrusion research over the last century. A. Number of scientic articles (n =3,404) published for the selected intrusions (ESM 1). B. Number of
citations for year ‘x’ by years since year ‘x’ for articles pertaining to seven important processes. Note the rise in reference to ‘crystal mush’ and the coeval fall in
reference to ‘gravitational settling’ and ‘magma mixing’. Vertical lines correspond to the publication of inuential articles, including: Bowen (1922), Jackson (1961),
Wager & Brown (1968), Campbell (1978), McBirney and Noyes (1979), Campbell and Naldrett (1979), Barnes (1986), Boudreau and McCallum (1992), and Mitchell
and Scoon (2012). C. Number of citations for year ‘x’ by years since year ‘x’ for articles that utilize important methods in layered intrusion research. Note the co-
occurrence of petrography and whole-rock geochemistry, as well as the recent rise in microstructural and multivariate analyses. Vertical lines correspond to the
advent of important technologies and online resources. D. Number of citations for year ‘x’ by years since year ‘x’ for articles pertaining to ten notable intrusions. Note
the dominance of the Bushveld Complex as well as post-discovery research peaks for the Stillwater, Duluth, Voisey’s Bay, and Nova-Bollinger. Vertical lines represent
important discoveries in the eld. E. Global production of metals known to occur in layered intrusions. Note that for scaling reasons, PGE tonnages have been divided
by 10 and Ni tonnages have been multiplied by 20. Vertical lines correspond to events that may have inuenced metal price and production [modied from Zientek
et al. (2014) and Jowitt et al. (2020)]. The data and a detailed explanation for constructing the diagram also provided in ESM 1. Abbreviations/acronyms, WWW =
world wide web, GS =Google Scholar, AI =articial intelligence, VW =Vietnam War, CC =catalytic converters, LiB =Li-ion battery, VFB =Vanadium ow battery,
SU =Soviet Union, SA =South Africa, GR =Great Recession.
W.D. Smith et al.
Precambrian Research 415 (2024) 107615
6
interpreted as being parental to many of these intrusions (Agnew, Vogel
et al. 1999; Karelian intrusions, Guo et al. 2023), and relatively high
degrees of crustal assimilation may have been facilitated by thicker and
more fusible siliceous crust (Dhuime et al. 2015). With the exception of
the 2.1–2.2 Ga Sandikounda Layered Complex (Senegal; Dia et al. 1997),
there are no known layered intrusions emplaced shortly after the GOE
(2.4 to 2.2 Ga), particularly now that the emplacement age of the Bacuri
Layered Complex (Brazil) was recently revised from ~ 2.2 to ~ 3.3 Ga
(Ferreira Filho and Araujo 2009; Spier et al. 2024). The global-scale
episode of magmatic quiescence that followed the GOE is known as
the ‘magmatic shutdown’ and has recently been reinterpreted as the
result of a ~ 2.7 Ga mantle overturn event (Condie et al. 2022a). The lull
in magmatic activity is strongly evidenced in the known layered
intrusion record, yet less apparent in the LIP record which only stalls
between 2340 to 2260 Ma (Condie et al. 2022a).
Magmatic activity apparently resumed sometime between 2.1 and
2.0 Ga, with the emplacement of the world’s largest known igneous
intrusion, the Bushveld Complex, and its associated intrusions (e.g.,
Uitkomst, Molopo Farms Complex, Losberg) at ~ 2.05 Ga. This is fol-
lowed by a peak in layered intrusion emplacement at ~ 1.8 Ga during
the assembly of the supercontinent Nuna (or Columbia), primarily cor-
responding to intrusions in the Scandinavian Kotalahti and Vammala
Nickel Belts (e.g., Rytky, Ylivieska, Kaipola) as well as those in east
Kimberley (e.g., Panton, Savannah, Springvale). There are relative in-
creases in the rate of juvenile crust production at ~ 2.05 Ga and ~ 1.8
Ga (Hawkesworth et al. 2010; Spencer et al. 2018), and crustal
Fig. 4. Summary of magmatism, crustal evolution, and tectonic regimes throughout geological history. A. Emplacement ages of layered intrusions categorized by size
class (Smith and Maier 2021). B. Emplacement ages of large igneous provinces (LIPs) categorized by size class (Ernst et al. 2021). C Juvenile crust production
(Hawkesworth et al. 2010) overlain by several crustal production models. D. Zircon ages (Puetz and Condie 2019) together with rates of crustal reworking and
evolving crustal thickness (Dhuime et al. 2012; 2015). E. Global geodynamics with the continental freeboard (Bada and Korenaga 2018) and ages of local subduction
initiation according to Ba contents of TTG suites (Huang et al. 2022). Plots are underlain by supercontinent ages (Bradley 2008; 2011; Nance et al. 2014), global
subduction initiation (Palin et al. 2020), and the Great Oxygenation Event (GOE).
W.D. Smith et al.
Precambrian Research 415 (2024) 107615
7
reworking rates also relatively increase at ~ 1.8 Ga (Dhuime et al.
2012). Global spikes in the production of mac–ultramac intrusions
(layered or not) that host some degree of magmatic Ni-Cu-PGE sulde
mineralization also occur at ~ 2.05 and ~ 1.8 Ga, which seemingly
manifest in areas of localized extension within overall contractional
tectonic systems (Begg et al. 2010; Pehrsson et al. 2016; Smith and Maier
2021). This metalliferous period is followed by a relative paucity in
magmatic Ni-Cu-PGE sulde occurrences and layered intrusion
emplacement despite no apparent lull in large igneous province mag-
matism (Pehrsson et al. 2016; Ernst et al. 2021). There are, however,
relative decreases in crustal production and reworking rates indicative
of cratonic stability, which may result in the absence of localized
extensional environments favorable for layered intrusion emplacement.
The period of 1800 to 800 Ma is colloquially referred to as the
“boring billion” due to prolonged tectonic stability, climatic stasis and a
slowing of biological evolution (Santosh and Groves 2023). Also, during
this period there is a relative reduction in the number of passive margins
(Bradley 2008) and arc magmatism (Liu et al. 2019) which, in turn, led
to an apparent reduction in ore deposits related to subduction-related
convergent margins (Santosh and Groves 2023). There is a peak in
layered intrusion emplacement during the assembly of Rodinia (1.2–1.1
Ga), which includes intrusions of the Giles Complex (e.g., Jameson
Range, Gosse Pile, Kalka), the Midcontinent Rift System (e.g., Duluth,
Sonju Lake, Coldwell), and the alkaline Greenlandic intrusions (e.g.,
Ilímaussaq, Klokken). Interestingly, each of these magmatic provinces
are associated with failed rift systems (e.g., Ngaanyatjarra Rift, Mid-
continent Rift, Gardar Rift), which manifest at the time the crust was
particularly thick and buoyant (Dhuime et al. 2015; Santosh and Groves
2023). The 1.2–1.1 Ga emplacement peak is followed by a sharp
reduction in LIP magmatism and layered intrusion emplacement, as well
as a shallow reduction in crust thickness and production (i.e., erosion
rates exceed crust production rates for the rst time; Dhuime et al.
2015). There is a relative increase in layered intrusion emplacement
towards the end of the Proterozoic, correlating with the break-up of
Rodinia and assembly of Gondwana (Smith and Maier 2021). Several of
these intrusions were emplaced during the course of the Brasiliano (e.g.,
Mangabal, Americano do Brasil, Canind´
e) and Pan African (e.g., Laouni,
Motaghairat, Korab Kansi) orogenies, coinciding with a relative peak in
crustal reworking rates characteristic of supercontinent construction
(Dhuime et al. 2012).
5. Mantles, melts and magmas
Magmas parental to layered intrusions originate in the mantle and
evolve en route through the crust; primary melts refer to melts extracted
from the mantle source and parental melts refer to the melt from which
the layered intrusion forms. Henceforth, the nature of the mantle (e.g.,
composition) and the conditions under which it melts (e.g., pressure,
temperature, volatile content) are rst order controls on the nature of
primary melt that may eventually form a layered intrusion. The diversity
of parent melt compositions proposed for layered intrusions suggests
that these conditions vary; putative parent melt compositions include
contaminated komatiitic melts (Maier et al. 2016; Solovova et al. 2021;
Jenkins et al. 2021), picritic melts (Emeleus et al. 1996; Duchesne et al.
2004), basaltic melts (Namur et al. 2010; Bai et al. 2019), and alkali
basaltic melts (Upton et al. 1996). It is widely accepted that high-degree
partial melts are required to produce a layered intrusion with magmatic
sulde mineralization, as these conditions are required to ensure total
liberation of chalcophile metals as well as a signicant amount of Ni
from the mantle source (Naldrett 2004). However, relatively low-degree
fertile melts may be produced in the subarc mantle where Ni may be
concentrated in relatively fusible phases (phlogopite, amphibole,
apatite) as opposed to refractory olivine (Straub et al. 2011; Ezad et al.
2024). That said, the convecting mantle and the subcontinental litho-
spheric mantle (SCLM) are the two more commonly invoked mantle
sources for the melts parental to layered intrusions (Maier and Groves
2011) yet distinguishing between these sources has remained a matter of
contention.
The convecting (asthenospheric) mantle must be anomalously hot
relative to ambient mantle to undergo extensive near-adiabatic melting
beneath thick continental lithosphere (i.e., a position with limited
decompression). The anomalous heat required is oftentimes explained
by the involvement of a mantle plume originating at the core-mantle
boundary (Koppers et al. 2021 and references therein). The core-
mantle boundary is thought to be a mixture of primordial mantle ma-
terial, depleted mantle material, subducted lithospheric plates and
perhaps even material entrained from the core, with the relative pro-
portions of these materials at this interface not necessarily having
remained constant through geological time (Koppers et al. 2021; Condie
et al. 2022b). This lithological heterogeneity is interpreted as being
represented in the diverse trace element and isotopic signatures of up-
welling plumes (Hastie et al. 2016, Koppers et al. 2021). Partial melting
of plumes initiates in the upper mantle, where the composition of the
melt is largely controlled by stabilities of simple mineral assemblages.
The bulk of the convecting mantle has a major element composition
that, under upper mantle conditions, is characterized by a peridotitic
mineral assemblage of olivine +orthopyroxene ±clinopyroxene and an
aluminous phase (plagioclase <0.9 GPa, spinel at 0.9–3 GPa, and garnet
at >3 GPa; Arndt et al. 2005). A relatively minor proportion of the
convecting mantle is pyroxenitic, and is relatively fusible compared with
peridotite, as it is mainly composed of clinopyroxene +garnet ±minor
olivine and spinel (Hirschmann et al. 2003, Lambart et al. 2013). Plumes
derived from the convecting mantle have sourced several LIPs (Brown
et al. 2022; Pierru et al. 2022) and layered intrusions (Hoatson and Sun
2002; Arndt 2013; Guo et al. 2023). Identifying a plume source is not a
trivial task as the heterogeneities in the core-mantle boundary and
possible subsequent assimilation of crustal materials can affect primary
melt isotopic and trace element compositions so that they become
largely indistinguishable from other mantle-derived melts (Campbell
2001; Arndt 2013). For this reason, multiple chemical proxies are
required to conclusively constrain the mantle source of a layered
intrusion. As an example, calculated parental melts together with Sr, Nd,
and Os isotopic as well as trace element and PGE systematics were used
to conclude a mantle plume source for most of the 2.5–2.4 Ga Baltic
layered intrusions in the Finnish Lapland (Yang et al. 2016; Guo et al.
2023). In a similar manner, various isotope systems were used in tandem
by Day et al. (2008) to infer a mantle source unaffected by long term
depletion or recycled crustal components for the ~ 1.27 Ga Muskox
layered intrusion, Canada.
The SCLM represents subcratonic buoyant residue that formed as a
result of ancient melting events in the convecting mantle, generally
agreed to have occurred in the Archean (3.5 to 3.0 Ga; Arndt et al. 2009;
Grifn et al. 2013). Following melt extraction, the convecting mantle
protolith became depleted (i.e., removal of incompatible elements)
lherzolite, harzburgite and possibly dunite. The residual SCLM is re-
fractory and unlikely to experience high degrees of partial melting itself.
However, it is theorized that the composition of the SCLM has evolved as
the ambient mantle has cooled through time, becoming decreasingly
refractory from the Archean to the present day (Grifn et al. 2009). In
addition, SCLM xenoliths have shown evidence for inltration by melts
or supercritical uids from various sources, which introduce relatively
fusible components (e.g., alkalis, volatiles; Grifn et al. 2009). The SCLM
may be inltrated by kimberlite or carbonatite melts from the under-
lying mantle, a cocktail of devolatilized supercritical uids, and/or
partial melts from subducting lithospheric slabs, low-degree partial
melts from less refractory mantle, or a combination of all of these
(Grifn et al. 2009; Howarth et al. 2014). The aforementioned processes
act to form SCLM variants that contain higher abundances of relatively
fusible phases; e.g., containing assemblages such as “MARID” (mica,
amphibole, rutile, ilmenite and diopside) and “PIC” (phlogopite,
ilmenite, and clinopyroxene) have been recorded in mantle xenoliths
(Fitzpayne et al. 2018). The melting of metasomatized SCLM would
W.D. Smith et al.
Precambrian Research 415 (2024) 107615
8
produce compositionally diverse primary melts that may ultimately
form layered intrusions; however, the fertility of such melts is a matter
of debate (refer to Arndt 2013). A SCLM mantle source for the Bushveld
Complex has been invoked by several researchers to explain the rela-
tively Pt-rich nature of associated ne-grained rocks (Sharpe 1981;
Maier and Barnes 2004) as well as the radiogenic Re-Os isotopic
signature of Merensky platinum-group minerals (PGM) (Hart and Kin-
loch 1989; Schoenberg et al. 1999). However, it is broadly accepted that
these observations can be explained by primary melts interacting with
SCLM and/or continental crust (Arndt, 2013; Barnes et al., 2010; Gün-
ther et al., 2018; Yudovskaya et al., 2017).
To identify the mantle source and melting conditions that formed a
layered intrusion, it is important to constrain its parental melt compo-
sition. Igneous petrologists utilize the cumulates comprising layered
intrusions as well as associated ne-grained units (i.e., chilled margins,
hypabyssal intrusions, associated volcanic rocks) to constrain the nature
of the parent melt and, thus, infer the physicochemical state of the
mantle from which it derived. It should be noted, however, that primary
melts may experience variable degrees of differentiation, magma mixing
and/or contamination as they traverse the crust, which should be
considered in detail, and possibly amended, before making the parental
to primary melt connection. Parental melt compositions have been
approximated using several methods, including: (1) weighted average
summation of the layers of the intrusion (i.e., bulk composition; Morse
1981; Naslund 1989); (2) considering the compositions of associated
ne-grained rocks as parent melt analogues (Miller and Ripley 1996;
Barnes et al. 2010; Virtanen et al. 2022); (3) using the compositions of
cumulus minerals, with or without corresponding bulk-rock chemistry,
together with parameterized Nernst partition coefcients (B´
edard 1994;
Godel et al. 2011a); (4) measuring the compositions of cumulus mineral-
hosted melt inclusions (Spandler et al. 2000; 2005).
Method (1) utilizes the rocks of the layered intrusion itself; however,
it requires an extensive whole-rock and mineral chemistry dataset to
sufciently constrain the relative proportions and compositions of the
different rock types. This method further requires that the intrusion has
a well constrained geometry, hosts ‘wholesale’ the crystallized products
of the parental melt (i.e., residual melt was not expelled), and that
different magma pulses are properly identied and accounted for. For
these reasons, this method is most effective for intrusions that formed in
a closed system, such as the Kiglapait intrusion (Morse 1981). Morse
(1981) reports the summed bulk composition of the Kiglapait intrusion;
a S-undersaturated, high-Al and −Fe basaltic composition that was
similar to the composition of the chilled margins from the associated
Hettasch (Berg 1980), Barth Island (de Waard 1976), and Jonathon
(Berg et al. 1994) intrusions. Fine-grained rocks later discovered at the
southern margin of Kiglapait were also found to be comparable to the
summed bulk composition and led to a renement of the putative
Kiglapait parent melt composition (Morse and Nolan 1985; Nolan and
Morse 1986). Based on subsequent trace element and isotopic work,
Fourny et al. (2019) proposed that the Kiglapait parent melt represented
25–30 % partial melting of mantle harzburgite that assimilated small
proportions (~5%) of lower crustal material. Utilizing Method (4),
Fourny et al. (2019) showed that the parent melt had a slight enrichment
in incompatible elements, with negative Th-U anomalies and positive
Ba-Pb-Sr anomalies. The approximated trace element composition is
similar to that of marginal rocks from the Kiglapait (Morse and Nolan
1985; Nolan and Morse 1986) yet is less evolved than marginal rocks of
the Hettasch intrusion (Berg et al. 1994). This is in line with the con-
clusions of Morse (1981), who argued that the Hettasch and Kiglapait
primary melts were distinct, as opposed to sharing a genetic lineage, on
the basis of incompatible element concentrations inconsistent with
fractionation or contamination.
Method (2) assumes that some ne-grained mac–ultramac rocks
associated with a layered intrusion are relatively rapidly cooled frac-
tions of cogenetic melt that have experienced negligible differentiation
upon cooling. As such the rock composition is representative of the
parental melt or of a more evolved composition on the liquid line of
descent, which can potentially be computationally back-fractionated to
infer its original composition. It is important that researchers account
for the presence of entrained minerals (primocrysts or xenocrysts; e.g.,
Marsh et al. 2003) and/or the inuence of in situ crustal contamination
(i.e., by adjacent country rocks; e.g., Mungall et al. 2010), that may lead
to erroneous approximations of parental melt composition. It should
also be noted that especially adjacent to marginal reversals (i.e., where
the rocks towards the margin are more evolved) of layered intrusions,
original chilled margins are often disturbed or even replaced by more
evolved secondary pseudo chilled margins (e.g. Latypov et al. 2007).
Chills within, and ne-grained sills associated with, the Bushveld
Complex range from komatiitic to high-Mg basaltic-andesitic to basaltic
(Sharpe 1981; Irvine and Sharpe 1982; Barnes et al. 2010; Wilson 2012;
Maier et al. 2016). Hamlyn and Keays (1986) highlighted similarities
between these ne-grained rocks and boninites; however, it is presently
broadly accepted that these rocks derived from contaminated komatiitic
melts (i.e., siliceous high-Mg basalts or SHMB), based on independent
lines of evidence (Barnes 1989; Eales and Costin 2012; Maier et al. 2016;
Mansur and Barnes 2020; Solovova et al. 2021). Comparable parent
magmas are proposed for many layered intrusions worldwide, but the
uniquely low Pd/Pt values of the Bushveld marginal rocks require that
the primary melt assimilated a fusible SCLM component (Barnes et al.
2010). Several studies have suggested that the Midcontinent Rift ood
basalts are cogenetic with the Duluth Complex layered intrusions based
on their spatiotemporal connection and geochemical evidence (e.g.,
Miller and Weiblen 1990; Miller and Ripley 1996; Swanson-Hysell et al.
2021). These rapidly cooled volcanic rocks suggest a common lineage
from high-Al olivine tholeiitic melts with the lavas and intrusive rocks
experiencing variable amounts of fractionation en route to emplacement
level (e.g., Miller and Weiblen 1990; Miller and Ripley 1996; Swanson-
Hysell et al. 2021; Virtanen et al. 2022). Based on major and trace
element modelling, it has been suggested that the earliest ood basalts
formed from a high-T mantle plume (mantle potential temperature ca.
1480–1630 ◦C) but that the temperature had cooled nearly to the
ambient mantle conditions (ca. 1400–1450 ◦C) during the formation of
the Duluth Complex (Brown et al. 2022). The same models reveal
continuous thinning of the lithosphere from about 60–100 km to 45–65
km during the magmatic event and rapid movement of the lithosphere,
leading to the spatial disconnection between the rift and the plume
(Brown et al. 2022).
Method (3) postulates that the Mg# value of a parent melt can be
approximated from olivine-only (or orthopyroxene-only) cumulates
using the method of Chai and Naldrett (1992), which may be extrapo-
lated to other cumulates so long as the other cumulus phase can be
subtracted from the corresponding whole-rock composition (Li et al.
2000; Smith et al. 2024). Since this approach determines the most
evolved melt capable of crystallizing the most forsteritic olivine
measured, it is recommended to use the approach in conjunction with
Method 2. Trace element concentrations of the parental melt may be
approximated by assuming chemical equilibrium between a cumulus
mineral and the melt from which it crystallized together with parame-
terized Nernst partition coefcients (B´
edard 1994; Godel et al. 2011a).
For such a method to be effective, one must determine the relative phase
proportions and appropriate partition coefcients as well as assume
negligible migration of interstitial liquid (i.e., closed system; ibid.). It
should be noted that in long-lived magmatic systems such as layered
intrusions, reactive ow of interstitial liquid may disturb the primary
mineral compositions (Sparks et al. 2019). This approach has been
applied to the Bushveld Complex (Yang et al. 2019), Mirabela intrusion
(Barnes and Williams 2024), and the Savannah intrusion (Le Vaillant
et al. 2020), amongst many others.
Method (4) uses melt inclusions hosted by early cumulus phases to
constrain the nature of the parent melt from which the host mineral
crystallized (Veksler 2006; Solovova et al. 2021). This method is
analytically challenging as well as prone to sampling compositional
W.D. Smith et al.
Precambrian Research 415 (2024) 107615
9
boundary layers and/or affected by subsolidus diffusion (Faure and
Schiano 2005; Baker 2008); thus, it is seldom utilized in layered intru-
sion research. Despite these difculties, melt inclusions have proven
effective in constraining parent melt compositions as well as for
detecting liquid immiscibility for several Phanerozoic intrusions
(Spandler et al. 2000; Jakobsen et al. 2011; Dong et al. 2013). Spandler
et al. (2005) documented two types of chromite-hosted melt inclusions
in the G Chromitite of the Stillwater Complex; Type I inclusions repre-
sent a relatively volatile-rich high-Mg basaltic magma that were inter-
preted as an analogue for the magma parental to the ultramac series
and Type II inclusions formed from Si-rich silicate melts interpreted as
country rock xenomelts. However, subsequent work on melt inclusions
from the same horizon argued that these are not reliable proxies for the
parent melt composition, instead representing trapped interstitial melts
(Bai et al. 2024), which should be reverse-fractionated to the more
primitive parental melt composition. A similar bimodal occurrence of
melt inclusion compositions, this time hosted in olivine, were docu-
mented in feldspathic peridotite in the lower portion of the Dovyren
intrusion (Konnikov et al. 2005). These were interpreted as lower crustal
microxenoliths or xenomelt droplets, indicating that the Dovyren parent
melt(s) experienced pre-emplacement crustal contamination. A siliceous
komatiitic parent melt was proposed for the Bushveld Complex based on
olivine-hosted melt inclusions from the comagmatic Uitkomst Complex
(Solovova et al. 2021), consistent with the komatiitic chilled margins
documented in the Lower Zone of the Bushveld Complex (Maier et al.
2016). The most primitive melt inclusion (~ 22.5 wt% MgO) was
thought to represent the primary melt composition (i.e., devoid of sig-
nicant contamination), and that its relatively high SiO
2
content (~
51.9 wt%) is a characteristic unique to Bushveld-associated melts (ibid.).
The above examples highlight that constraining mantle melting
conditions, source compositions, and parental melts of layered intrusion
is a challenging task. Several innovative methods, however, have
already made it possible to make reasonably reliable estimates of the
parental melt compositions and their connections to the primary mantle
melts. As comprehensive geochemical and isotopic databases are
becoming increasingly available for many layered intrusions and as
computational models become physiochemically more realistic, we
postulate that future research will ne tune our understanding of the
evolution of the mantle – currently largely based on studies of lavas –
through the lens of layered intrusions.
6. Emplacement conditions and growth mechanisms
Layered intrusions are important components of the Precambrian
crust, with giants like the Bushveld Complex, Windimurra and Sept ˆ
Iles
among the largest known igneous bodies in the world. Although there is
considerable uncertainty in determining and comparing the physical
extents of layered intrusions, their dimensions overlap with those of
granite plutons and, rarely, batholiths, which are constructed sequen-
tially over many millions of years (Fig. 5). These plutonic igneous bodies
are laterally extensive and able to achieve thicknesses in excess of 10 km
(Ivanic et al. 2018), equal to approximately a quarter of the thickness of
typical continental crust (Cruden et al. 2018). Although doming and
assimilation create space, plutonic intrusions are mainly accommodated
by the structural removal of oor rock, driven by mass exchange be-
tween host chambers and underlying melt reservoirs (Cruden 2006;
Fig. 5. Horizontal length versus vertical thickness of igneous bodies superimposed on the regime diagram for tabular intrusion scaling and growth mechanisms
(Cruden et al. 2018). Note that the considered layered intrusions plot together with felsic plutons and batholiths that appear to grow primarily via oor rock
depression over millions of years. 1 =Scoates et al. (2021), 2 =Ivanic et al. (2018), 3 =Namur et al. (2010), 4 =Campbell and Murck (1993), 5 =Miller and
Severson (2009), 6 =Hoatson and Keays (1989), 7 =Alapieti and Lahtinen (1986), 8 =Marks and Markl (2015), 9 =Morse (1969), 10 =Maier et al. (2003), 11 =
Cruden et al. (2018) and references therein.
W.D. Smith et al.
Precambrian Research 415 (2024) 107615
10
Cruden et al. 2018). Depression of the oor rocks is further aided by the
accumulation of dense ultramac cumulates that can induce chamber
subsidence as they accumulate and attempt to delaminate (Roman and
Jaupart 2016). In the case of the Bushveld, chamber subsidence has been
posited as a layer-forming mechanism via slumping of the cumulate pile
(Maier et al. 2013) and delamination of cumulates from the intrusion
base may be responsible for a thick mac layer that occurs at a diffuse
Moho discontinuity beneath the complex (Kgaswane et al. 2012; Roman
and Jaupart 2016). Moreover, several intrusions have cumulate layers
that thicken towards their centers as a result of subsidence (e.g., Kemi,
Alapieti et al. 1989; Bjerkreim-Sokndal, Bolle et al. 2002).
The bifurcation of growth mechanisms between sheet-like intrusions
and laccoliths (Fig. 5) appears to be largely unrelated to emplacement
depth and intensive parameters of the parent magma, but rather arises as
a result of magma supply rate and tectonic setting (Cruden et al. 2018).
However, processes facilitating the thickening of plutonic igneous
bodies vary with depth, primarily as a function of crustal rheology
(Condie 2021). The relative importance of roof uplift and oor depres-
sion as growth mechanisms has an antithetical relationship with crustal
depth, whereas the relative importance of brittle (e.g., fracturing, stop-
ing) and ductile (e.g., viscous ow) processes shifts at the brittle-ductile
transition zone (average heat ow of 50 mW/m
2
; Condie 2021; Acocella
2021). Crustal aging and thickening throughout the Precambrian is
likely to have led to secular changes in the relative importance of the
construction mechanism of plutons. Many notable layered intrusions
were emplaced in relatively brittle upper crust (<12 km; Fig. 6); into
metamorphosed country rocks that collectively dene a relatively high
geothermal gradient (i.e., similar to the average thermal gradient of
LIPs; Jennings et al. 2021). Their apparent preservation in the upper
crust may reect discovery bias (i.e., exposure) and/or that relatively
rigid crust is required to prevent the foundering of dense ultramac
cumulates (Menand 2011; Roman and Jaupart 2016). Rare examples of
Fig. 6. Temperature versus depth (km) diagram showing approximated emplacement conditions of several intrusions underlain by geotherms (Hasterok and
Chapman 2011), the brittle-ductile transition (Condie 2021), and divisions in the crust (Gao et al. 1998). Ranges for the Bushveld Complex, Stillwater Complex, and
Kiglapait intrusion were derived from their contact aureole and as such, temperature ranges represent the maximum temperature of the country rocks upon
emplacement. Temperature ranges for the Sept ˆ
Iles, Ilimaussaq, and Americano do Brasil intrusions are estimated by applying the average Proterozoic geotherm
(Jennings et al. 2021) to their emplacement depths reported in the literature. Temperature range for the Windimurra intrusion is approximated by apply the median
LIP geotherm (Jennings et al. 2021) to the approximated emplacement depth. Emplacement conditions of the Hasvik intrusion, Duluth Complex and Nova-Bollinger
intrusion are as reported in the literature. Note that most considered layered intrusions were emplaced into relatively cool brittle upper crust. 1 =Jennings et al.
(2021), 2 =Augustin et al. (2023), 3 =Waters and Lovegrove (2002), 4 =G´
al et al. (2013), 5 =Sawyer (2014), 6 =Virtanen et al. (2024), 7 =Larsen and Sørensen
(1987), 8 =Markl et al. (2001), 9 =Berg and Docka (1983), 10 =Morse (2015), 11 =Chong et al. (2024), 12 =Namur et al. (2011), 13 =Labotka and Kath (2001),
14 =Ahmat (1986), 15 =Tegner et al. (1999).
W.D. Smith et al.
Precambrian Research 415 (2024) 107615
11
deep-seated intrusions (≥6 kbar) include Nova-Bollinger (Australia;
Taranovic et al. 2022) and Hasvik (Norway; Tegner et al. 1999), which
although relatively volumetrically small, show evidence for enhanced
interaction with their host country rocks in the form of extensive ther-
mal aureoles and incorporation of abundant crustal xenoliths.
With rare exceptions, layered intrusions are products of open-system
processes. This means that their physical dimensions and the nature of
their layering are primarily controlled by inux and efux of magma,
which may be episodic or progressive (Cruden et al. 2018; Latypov et al.
2024a). Many intrusions develop initially from tabular intrusions (i.e.,
sills), which may then thicken by: (i) ination (or ballooning) if magma
supply rate exceeds the rate of crystallization and horizontal length-
ening; or (ii) under- and over-accretion of distinct sheet-like intrusions if
the rate of crystallization far exceeds the magma supply rate (ibid.). The
Kiglapait intrusion is widely accepted to have formed in a closed system
following the progressive emplacement of a single pulse of magma
(Morse 2015 and references therein). Cumulate rocks displaying pre-
dominantly normally graded layering comprise the lower half of the
intrusion and are consistent with having formed in response to gravi-
tational settling of cumulus phases in a convecting melt body (Higgins
2002; Morse 2015). The Muskox intrusion is widely accepted to have
formed in an open system, whereby replenishing melts were successively
emplaced into a resident melt that had already fractionated cumulates
(Irvine 1976). The cumulates display modally graded, reversely modally
graded, and macrorhythmic layering, as well as cross-bedding (Irvine
1976; Scoates and Scoates 2024), indicative of dynamic, open-system
processes. Researchers have proposed that the Bushveld and Stillwater
complexes were, at least in part, constructed by the non-sequential
emplacement of distinct sheet-like intrusions (Mungall et al. 2016;
Wall et al. 2018; Scoon and Mitchell 2023). Although it is broadly
accepted that silicic plutonic bodies grow by the under-accretion of
successive magma pulses (Glazner et al., 2004; Menand et al. 2011), this
remains a matter of serious debate for layered intrusions (Latypov et al.
2024a). Out-of-sequence U-Pb zircon ages in layered intrusions that
appear contradictory to eld observations pose a challenge to the un-
derstanding of mush zones and melt percolation in layered intrusions.
However, Holness et al. (2017a, 2017b) and Latypov et al. (2024c)
suggest that most mush zones are relatively thin (m-scale), and Barnes
and Williams (2024) argue that trapped melt (from which zircon crystals
crystallize) does not percolate signicantly within the mush. Both lines
of evidence suggest that zircon crystallizes from trapped liquid com-
plementary to cumulates representing the emplacement sequence of
layering.
7. Multiscale and multidisciplinary observations from layered
intrusions
Research on layered intrusions intensied following the discovery
that these geological formations may contain economically valuable
metals (Fig. 3). Their critical metal repositories have meant that, over
time, research efforts have employed an impressive array of techniques,
including eld-based studies, petrography, geophysics, geochemistry,
geochronology, uid dynamics, and thermodynamic modelling (Fig. 3;
O’Driscoll and VanTongeren 2017). The study of layered intrusions
exemplies the necessity of multidisciplinary and multiscale approaches
in deciphering the petrogenesis of geological formations. By combining
insights from various scientic elds and examining structures at
different scales—from the microscopic details of minerals to the
macroscopic outline of entire intrusions and their associated magmatic
systems—we can begin to disentangle the complex array of processes
responsible for their formation.
Layered intrusions are components of trans-crustal plumbing systems
that may only be contextualized at the macroscale (km to 10 s of km). At
this scale, one can infer the extent of the magmatic event, the geometry
of the subject intrusion(s), and the relationships between magma(s),
crustal rocks, and tectonic structures. Macroscale investigations require
comprehensive geological mapping, which enabled the discoveries of
the Muskox Intrusion in Canada (Smith and Kapp 1963) and the Giles
Complex intrusions in Australia (Maier et al. 2015). Such observations
also elucidate regional structural context, illustrating how intrusions
interact with surrounding rock formations (Hutton 2009; Schoeld et al.
2014). Analyses at the macroscale are important in highlighting re-
lationships between cogenetic intrusions on a regional scale, such as the
Americano do Brasil Suite (Augustin et al. 2022) and the Giles Intrusions
(Maier et al. 2015), as well as emplacement mechanisms (refer to Section
6). Importantly, geophysical methods have signicantly contributed to
the identication of poorly or unexposed parts of intrusions, sometimes
linking seemingly spatially disconnected intrusions. For example, in
northern Finland, magnetic and gravimetric measurements have
revealed an unexposed ca. 100 km long elongated magma body linking
the N¨
ar¨
ank¨
avaara dike to the Koillismaa intrusion indicating the exis-
tence of a much larger magma system (Alapieti 1982; J¨
arvinen et al.
2022). Geophysical measurements can also reveal staging magma res-
ervoirs or underplating below the exposed levels of intrusions, as for the
Bushveld using gravimetric and seismic data (Cole et al. 2024). Airborne
and ground-based geophysical surveys are essential for a comprehensive
understanding of these complex structures and are frequently used to
constrain how the intrusions extend and behave at depth (e.g., Koivisto
et al. 2012). Entities with coherent physical properties detected in
geophysical data often correlate with specic rock types within the
intrusion, underscoring variations in composition and density (Sun
et al., 2020; Ferr´
e et al., 2009).
On the mesoscale, which we consider ranging from hand-sample to
outcrop size (cm to 10 s of m), studies focus on features visible with
minimal magnication. This level of examination connects microscopic
mineral analyses with broader geological surveys, offering detailed in-
sights into the internal structure of magmatic intrusions. To ensure un-
biased sampling, detailed descriptions of mesoscopic features are
crucial, as they signicantly inuence the representativeness of sample
sets for layered intrusions. Some key mesoscopic observations include
magma-wall rock contact relationships that provide evidence of chilled
margins, for example, are important in the study of parental melts for
layered intrusions (Huppert and Sparks 1989; refer to Section 5). Styles
of in situ magma-wall rock interaction processes (i.e., bulk vs. selective
assimilation) are often visible on the mesoscale (Barnes et al. 2001;
Queffurus and Barnes 2015; Barnes et al. 2023). Identifying how these
processes may have affected the composition of the magma is useful for
the interpretation of the whole-rock and mineral chemical data. In the
case of Pechenga and the Duluth Complex, mesoscopic observations of
magma-black shale interaction were important in the interpretation of
how sulfur was transported from the shale to the magma to instigate the
formation of Cu-Ni(−PGE) deposits (e.g., Barnes et al. 2001; Queffurus
and Barnes 2015). Apart from contact relationships, this length-scale
often uncovers the mechanical and chemical stratication processes
within intrusions, whether through gravitational crystal settling (Irvine,
1980a; Wager and Brown, 1968) or in situ crystallization (Campbell,
1978; Latypov et al., 2017). Textures and structures found on this scale
offer insights into magma chamber dynamics, including convection
currents, crystal settling and magma mingling or mixing. Additionally,
variations in magnetic susceptibility observed in mesoscopic scale
geophysical studies not only reect changes in mineral abundance but
also, particularly with ferrimagnetic minerals, differences in grain sizes.
This helps in identifying stratication, cryptic layering, and post-
magmatic alteration (Ferr´
e et al., 2009). The traditional whole-rock
geochemical methods are nonetheless still of greater importance to
understanding the origins of the rocks.
The mineralogy and texture of cumulates comprising layered in-
trusions are revealed at the microscale. Although recent years have seen
the development of sophisticated micro- to nano-scale techniques for
characterizing cumulates, traditional optical microscopy remains the
principal method and the rst tool to identify mineralogy and textures.
Jackson (1961) described the rocks and textures of the ultramac
W.D. Smith et al.
Precambrian Research 415 (2024) 107615
12
cumulates in the Peridotite zone of the Stillwater Complex in remarkable
detail and several studies has been based on these initial descriptions.
Despite advances in micro- to nano-scale techniques, optical petrog-
raphy remains essential for identifying microtextures that cannot be
discerned through chemical analysis or automated mineralogy. For
example, recognizing the distinction between poikilitic and inter-
cumulus textures is crucial for understanding cumulate formation
mechanisms, and traditional optical petrography is thus indispensable.
The recognition of poikilitic textures has challenged the origins of their
formation, with the classical hypothesis suggesting they form as inter-
stitial postcumulus phases crystallizing from intercumulus liquid within
a segregated crystal mush (Wager et al., 1960). However, this hypothesis
has been contested, with alternative suggestions that oikocrysts may
form in situ as cumulus phases from supersaturated liquids (Campbell,
1968; Mathison, 1987). Petrography can also be used to detail micro-
textures and infer processes, as shown by a recent study on the Sept ˆ
Iles
intrusion, Canada, identifying symplectites to infer redistribution and
concentration of hydrous uids in incompletely solidied rock or an
increase in water activity of the interstitial melt (Keevil et al. 2020).
Recently developed methods related to microstructure character-
ization such as high-resolution X-ray computed tomography (HRXCT),
electron backscattered diffraction (EBSD) and X-ray uorescence
element mapping analysis enhance our understanding of magmatic
processes (Godel et al. 2013; Barnes et al. 2021; Chen et al. 2024). Non-
destructive imaging of microscale or sometimes even nanoscale 3-D
structures (e.g., crystal shapes, sizes, distribution, and aggregation) has
become possible with the development of the HRXCT, revolutionizing
our observational capabilities on microstructures (Cnudde and Boone
2013). Recent research used HRXCT to reveal a continuous 3-D frame-
work of interconnected chromite grains in the UG1 chromitite in the
Bushveld Complex, with the grains predominantly in face-to-face con-
tact and randomly oriented; supporting the hypothesis that the chro-
mitites form through in situ crystallization directly at the magma
chamber oor rather than gravity settling (Latypov et al., 2022). In-
vestigations into dihedral angles by Holness et al. (2022) and Fowler and
Holness (2022) provide critical insights into mineral formation and
growth rates. Crystal nucleation, growth and settling processes can also
be studied using EBSD, which provides information on the crystallo-
graphic orientation of individual grains within a sample (Wieser et al.,
2019). As an example, EBSD analysis revealed that chromite grains in
the UG2 chromitite of the Bushveld Complex show no systematic rela-
tive orientation. These data indicate the random juxtaposition of indi-
vidual grains. Chromite-chromite-plagioclase dihedral angle
measurements from the UG2 chromitite suggest high degrees of textural
equilibration in these rocks. Together these data support the hypothesis
that massive chromitite seams formed by chromite grains nucleating
heterogeneously on the silicate grains, with subsequent accumulation
and sintering of individual grains or clusters (Holness et al., 2023).
Similarly, EBSD analyses showed that the presence of plagioclase
magmatic foliation in anorthosites and massive magnetitites in the
Bushveld Upper Zone is a result of crystal settling from moving, crys-
tal–laden magmas (Vukmanovic et al. 2019).
Microscopic analysis of major and trace elements in minerals and
melt inclusions also provides important hints into the process occurring
in magma chambers. The study by Xing et al. (2022) investigates the
crystallization and cooling history of the Sept ˆ
Iles layered intrusion by
analyzing complex phosphorus zoning in olivine grains using high-
resolution EPMA imaging, EPMA for elemental analysis, EBSD for
crystallographic orientation, and thermal and diffusion modelling. This
study revealed that rapid growth of dendritic and hopper olivine pat-
terns resulted from signicant undercooling in the magma chamber.
Barnes et al. (2016) used microbeam XRF mapping, LA-ICP-MS, and
EPMA to investigate the crystallization mechanisms and zoning patterns
in orthopyroxene oikocrysts from the Neoproterozoic Ntaka Ultramac
Complex, revealing that Cr-rich cores, oscillatory zoning and reverse
zoning support dynamic magma conditions with primary cumulus
growth and secondary inltration metasomatism. Schoneveld et al.
(2024) investigated Cr zoning patterns in pyroxenes at Nova-Bollinger
and Kevitsa, nding that these zoning patterns indicate favorable con-
ditions for upgrading metal content in active and open magmatic sys-
tems. Further, they demonstrated that relict pyroxene zoning remains
detectable under moderate degrees of alteration (amphibolization),
making the indicator robust to moderate and hydrous alteration.
Furthermore, melt inclusions within cumulate minerals reveal a wealth
of information, including the initial melt composition (Solovova et al.,
2021), evidence of rock assimilation (Spandler et al., 2005), and the
onset of immiscibility, when the melt separates into distinct liquid
phases (e.g., Jakobsen et al. 2011; Fischer et al., 2016; Wang et al.,
2018).
8. Investigating the origin of rolled oikocrysts in the Stillwater
Complex
8.1. Introduction
Multidisciplinary studies that integrate quantitative rock textural
analysis, bulk rock geochemistry, mineral chemistry, and/or numerical
modelling form an increasingly large part of the layered intrusion
published literature. Such integrated approaches can elucidate rock
forming processes that are proposed based on eld observation and
conjecture. To demonstrate the utility of such a multidisciplinary
approach, a well-known, but poorly documented outcrop from the
Stillwater Complex was sampled and imaged using several microbeam
techniques, including Maia Mapper microXRF and scanning electron
microscope-electron backscatter diffraction (SEM-EBSD). The purpose
of this exercise is to show the utility of a modern multidisciplinary
analytical approach to constrain petrogenetic processes in layered
intrusions.
8.2. Geological summary of the Stillwater “snowball oiks”
The sampled outcrop is located on the northern side of Picket Pin
Mountain in a part of Gabbronorite III zone within the Upper Banded
series of the Stillwater Complex (McCallum, 1996). Few studies have
focused on rocks from the Upper Banded series, partly due to their
inaccessibility and partly because they lack known economic mineral
potential. The sampled outcrop is a popular stop on geologic eld trips
because it is exposed near the Picket Pin sulde PGE prospect located on
the south side of Picket Pin Mountain—another popular eld trip stop.
The outcrop has been referred to in Stillwater Complex eld guides as
the “snowball oiks outcrop” and is comprised of complexly interlayered
gabbronorite, leucogabbronorite, norite and anorthosite (e.g., Carlson
and Zientek 1985; McCallum and Meurer 2002). A distinct feature in this
outcrop is the presence of spheroidal orthopyroxene oikocrysts ranging
from about 2 to 12 cm in diameter (Fig. 7A-C). These oikocrysts have
been informally referred to as “snowball oiks” given their size and su-
percial resemblance to snowballs. Other features shown in the outcrop
include structures interpreted as scour and ll structures, rip up clasts
and lenticular autoliths of anorthosite (Fig. 7B; Carlson and Zientek,
1985). The snowball oikocrysts contain chadacrysts of euhedral
plagioclase and intercumulus augite (Fig. 7D). The oikocrysts are
wrapped by a planar lamination carried by preferentially oriented
plagioclase, orthopyroxene and clinopyroxene primocrysts in gab-
bronorite with subordinate, wispy layers of anorthosite and norite. By
contrast, plagioclase chadacrysts hosted within the orthopyroxene
oikocrysts appear to have a random orientation. Some of the snowball
oikocrysts are characterized by having anorthosite rinds (Fig. 7D).
Plagioclase shows very little crystal bending and/or undulose extinction.
No deformation twins are present. Field descriptions of this unit have
relied on the observations of sedimentary-like structures and the folia-
tion wrapping the poikilitic orthopyroxene to invoke crystallization in a
dynamic environment.
W.D. Smith et al.
Precambrian Research 415 (2024) 107615
13
8.3. New insights into the petrogenesis of the “snowball oiks” outcrop
We describe the detail of the analytical methods used to quantify
rock texture and the distribution of elements in the sample studied in
ESM 2. This includes short descriptions of the types of maps produced by
EBSD analysis that we present here.
Maia Mapper microXRF: The microXRF data are shown as a false-color
three element (Ca-Cr-Fe) composite image in Fig. 8 (Ryan et al., 2018).
EBSD: The area mapped by EBSD covers two textural domains in the
sample, the area of the orthopyroxene oikocryst (Fig. 9A, shaded area)
and the outside of the oikocryst (Fig. 9A). The orthopyroxene oikocryst
encloses mostly euhedral plagioclase laths with minor anhedral inter-
stitial clinopyroxene (Fig. 9A-C). Within the oikocryst, plagioclase
apparent aspect ratio varies from 1 to 8 (refer to ESM 2). Plagioclase pole
gure data, within the oikocryst, show very weak fabric with point
maxima at the (010) and (001) poles (Fig. 9D). The [100] axes do not
form a girdle but show disperse point maxima instead. Although the
number of plagioclase crystals is lower than outside the oikocryst, this
still implies a weak lineated fabric. The clinopyroxene crystals within
the oikocryst are mostly randomly orientated, however, a small pro-
portion of the clinopyroxene show an almost identical crystallographic
orientation (black arrow; Fig. 9B, D). Orientation data of the large
orthopyroxene oikocryst conrms that the oikocryst is mainly a single
crystal, but the spread of the data points suggests minor crystal plasticity
(black arrow; Fig. 9D), i.e., intra-crystal strain.
Plagioclase outside the oikocryst has a similar apparent aspect ratio
to plagioclase within the oikocryst (refer to ESM 2). Plagioclase orien-
tation data outside of the oikocryst show strong preferred orientation of
Fig. 7. Photographs taken of the snowball oikocryst outcrop near Picket Pin Mountain in the Upper Banded series of the Stillwater Complex. A. Poikilitic ortho-
pyroxenes (opx oiks) and anorthosite lenses (pC lens) distributed throughout modally layered gabbronorite exposed on a glacially polished outcrop. B. Lens-shaped
anorthosite autolith in gabbronorite. C. A dense cluster of poikilitic orthopyroxenes rimmed by anorthosite-leuconorite. D. Snowball orthopyroxene oikocryst
rimmed by anorthosite. Photographs by Chris Jenkins, U.S. Geological Survey, 2023. Mineral abbreviations are plagioclase (pl) clinopyroxene (cpx), and ortho-
pyroxene (opx).
W.D. Smith et al.
Precambrian Research 415 (2024) 107615
14
both [100] axes and the poles to (010) and (001) planes (Fig. 9E). The
[100] axes develop a girdle distribution, parallel to the rock’s fabric
(Fig. 9E). Within the center of the girdle, a population of plagioclase
crystals develops strong point maxima of [100] (Fig. 9D). Poles to cli-
nopyroxene (100) planes develop point maxima parallel to the rock’s
foliation, whereas [001] axes reveal a weak girdle (Fig. 9E). Poles to the
(010) plane show weak multiple point maxima (Fig. 9E). The small
number of orthopyroxene grains prevents proper textural analysis, but
as seen from the rose diagram (Fig. 9), the major axes of the cumulus
orthopyroxene imply the presence of a fabric parallel to the foliation
seen in plagioclase (Fig. 9E). The (100) pole data form point maxima
orthogonal to rock’s foliation, whereas poles to the plane (001) develop
a girdle parallel to the foliation (Fig. 9E). Poles to the (010) plane show
the least developed texture (Fig. 9E).
8.4. Interpretation of results
The microXRF data show that poikilitic and cumulus crystals in the
snowball oikocryst outcrop are not chemically zoned. This contrasts
with the markedly zoned poikilitic pyroxene found near the J-M Reef
PGE deposit in the Stillwater Complex (Jenkins et al. 2022) and in
mineralized mac magmatic conduit-type Ni-Cu-PGE deposits world-
wide (Schoneveld et al. 2020). The absence of chemical zoning indicates
that the orthopyroxene oikocryst grew and/or equilibrated with melts
with a near constant composition for the duration of its crystallization
history. Alternatively, the system may have cooled slowly enough that
any primary zoning pattern was overprinted during chemical equili-
bration; however, this seems unlikely given the large size of the poiki-
litic crystals and the slow rate of chemical diffusion in orthopyroxene
(Cherniak and Dimanov 2010). While the texture of the sample studied
points to the formation of the rock in a dynamic environment, there is no
indication that the composition of the parental melt was heterogeneous
(i.e., by recharge of a melt with a signicantly different composition) as
the poikilitic crystals grew.
Similar aspect ratios of plagioclase crystals within and outside the
oikocryst suggest that all plagioclase crystallized early (refer to ESM 2).
The mild evidence of lineated plagioclase crystals within the oikocryst
represents early evidence of magma ow before the growth of the
oikocryst (Fig. 9D). Outside of the oikocryst, the strong fabric in both
plagioclase (i.e., foliation and lineation) and orthopyroxene (i.e., folia-
tion) suggest that the environment of formation was relatively dynamic
(Fig. 9E). Due to the absence of signicant deformation microstructure
(i.e., crystal plasticity) the role of strain as fabric forming process can be
disregarded (e.g., viscous compaction). Both plagioclase and orthopyr-
oxene outside the oikocryst are cumulus crystals, their fabric is a result
of crystal rearrangement during magma ow of crystal-rich slurry
(Vukmanovic et al. 2018). However, the intercumulus clinopyroxene
also records a mild fabric. We suggest that intercumulus melt pockets in
the pore spaces between plagioclase (and orthopyroxene) acted as lu-
bricants during the ow. They later crystallized parallel to the ow di-
rection. All phases record a mild degree of intracrystalline plasticity
(ESM 2), suggesting that this small amount of strain was recorded in the
subsolidus. A working hypothesis would be that this strain was a result
either of a late viscous compaction or post-emplacement sagging of the
intrusion. To come to a more certain scenario, further microstructural
characterization of samples above and below the sample shown here is
needed.
Chemical mapping and EBSD fabric analysis indicate that the
snowball oikocrysts formed early when the melt was co-saturated in
orthopyroxene and plagioclase and near clinopyroxene saturation. The
oikocrysts interacted with and trapped plagioclase chadacrysts and
continued to grow as the entire assemblage was being transported in a
owing slurry. The snowball oikocrysts have been transported within
the aligned plagioclase (i.e. within the place of foliation). This is notable
because it runs contrary to conventional cumulus theory that posits
poikilitic crystals grow within intercumulus pore spaces due to compo-
sitional convection in the cumulate pile (Kerr and Tait 1985). Further,
these textures could not have been developed by or formed by peritectic
Fig. 8. False color microXRF map of sample 84PP3. The map is a composite made by stacking Ca (blue), Cr (red), and Fe (green) element maps. The approximate area
of polished thin section 84PP3-5 imaged by EBSD is shown by the white box. The rose diagram shows the orientation of orthopyroxene crystals. Mineral abbre-
viations are plagioclase (pl), clinopyroxene (cpx), and orthopyroxene (opx).
W.D. Smith et al.
Precambrian Research 415 (2024) 107615
15
reaction between primocrysts and percolating melts as has been sug-
gested elsewhere for orthopyroxene oikocrysts in olivine crystal mushes
(Jackson 1961; Kaufmann et al. 2018). After deposition, as the melt
proceeded to crystallize the snowball oikocrysts were deformed by the
mass of the overlying mush. An aspect that requires further investigation
is how these large oikocrysts were transported during the magma ow.
8.5. Conclusions
Layered intrusions host many labyrinthine features that can be
constrained using a multidisciplinary approach. By combining micro-
structural (EBSD) data with geochemical data we can correlate how the
same process affects a rock from different perspectives, without dis-
regarding evidence. Our combined data show that these poikilitic
crystals are formed in a dynamic magma chamber where crystals have
been transported either by convective currents (Barnes et al., 2016c;
Fig. 9. EBSD analysis of the snowball oikocrysts. A. Phase map of the sample 84PP3-5. B. Clinopyroxene orientation map. C. Orthopyroxene orientation map. The
orientation maps are colored using the inverse pole gure (IPF) key referenced to X direction of the sample. The dotted area on A-B-C indicates the area of an
orthopyroxene oikocryst. D. Pole gure data of plagioclase, clinopyroxene and orthopyroxene within the oikocryst. Due to a small number of grains, clinopyroxene
and orthopyroxene data are not contoured. E. Pole gure data of plagioclase, clinopyroxene and orthopyroxene outside of the oikocryst. Number of data points for D
and E are reported in supplementary material. m.r.d: multiples of a random distribution. Squared brackets [] indicate crystallographic axis, and parentheses ()
indicate pole to the crystallographic planes.
W.D. Smith et al.
Precambrian Research 415 (2024) 107615
16
Holness et al., 2017b) or in crystal-rich slurries (Maier et al. 2013;
Vukmanovic et al. 2019). The data do not indicate that the poikilitic
orthopyroxene crystals grew at their present location either by compo-
sitional convection or peritectic reaction.
9. Layered intrusions as critical metal repositories
Many Precambrian layered intrusions host repositories of critical
metals, including PGE, chromium (Cr), nickel (Ni), copper (Cu), cobalt
(Co), vanadium (V), and others (Cawthorn 1996). Mineralization is
often stratiform and, between intrusions, varies in stratigraphic occur-
rence, nature of the host rocks, and tenor (Smith and Maier 2021). For
example, reef-style PGE occurrences can occur at almost any strati-
graphic level and are sometimes associated with chromitites (Prevec
2018). Nonetheless, most manifest near to the transition from ultramac
to mac cumulates, derived from a primary melt that itself was gener-
ated from a partial melting degree sufcient to liberate all the PGEs
(Barnes et al. 1985; Keays 1995; Maier 2005). Moreover, Ti-V rich
magnetitites typically occur at higher levels in stratigraphy associated
with anorthosites and leuconorites, such as in the Bushveld and Wind-
imurra complexes (Cawthorn 2015; Ivanic et al, 2018); however, the
Sept ˆ
Iles Ti-V rich magnetite layers and pods occur in the middle of the
stratigraphy intercalated with olivine gabbro and troctolites (Namur
et al. 2010). While the metal content of the primary melt inuences the
potential for an intrusion to host an ore deposit, it is not the only
requirement (Arndt et al. 2005); a combination of melt fertility, fertile
melt migration, ore mineral segregation, and accumulation is vital
(Mungall 2005).
9.1. Mantle source and crustal transport controls on the metallogeny of
layered intrusions
Layered intrusions inherit most of their metallogenic potential from
their mantle source(s). Here we consider the Cr, Ti, V, PGE, Cu and Ni
contents of primary mantle magmas, which depend on the composition
of the mantle source, the degree of melting, and the P-T conditions they
evolve under as well as on certain buoyancy-related processes that may
aid in the transport of the main metal-bearing phases from the mantle
source.
Chromium is compatible in most of the main phases in the con-
vecting mantle (i.e., clino- and orthopyroxene, garnet, and spinel;
McDonough and Sun 1995) and in the metasomatized SCLM (e.g., cli-
nopyroxene, phlogopite, ilmenite, and amphiboles; Foley et al. 2022,
Ezad et al. 2024). As such, the Cr content of primary mantle melts in-
creases with higher degrees of partial melting. Mantle melting experi-
ments conducted with pyrolite compositions show that Cr content
increases approximately linearly up to the maximum of ca. 0.4–0.6 wt%,
when the degree of melting reaches 30–50 % (Hirose and Kushiro 1993;
Baker and Stolper 1994; Walter 1998), which is a typical range for
komatiitic primary melts. Due to the highly heterogeneous nature of the
metasomatized SCLM (refer to Section 5), estimating the Cr contents of
its partial melting products is more complicated, although Cr is
compatible in most of the residual solid phases of PIC and MARID as-
semblages (Foley et al. 2022). Consequently, those layered intrusions
that form by the highest degree of mantle melting should have the
highest Cr concentrations and hence are more likely to form chromitite
deposits (e.g., Bushveld, Stillwater, Great Dyke). Campbell and Murck
(1993) demonstrated through mass balance the difculty in explaining
the occurrence of large stratiform chromite in layered intrusions by
showing that Cr
2
O
3
is only soluble to about 600 ppm in a basaltic melt.
This limitation then requires between 2 and 12 km of complementary
melt for every 1 m of massive chromitite produced depending on how
efcient Cr extraction is from the melt (Fig. 10). Jenkins and Mungall
(2018) suggested that contaminated komatiites were more likely the
parental melts to the chromitites in the Peridotite zone of the Stillwater
Complex on the basis that the Cr solubility in high temperature, high-Mg
melts is considerably higher than a basalt and therefore a still large (700
m to 2 km), but more reasonable complementary melt column was
Fig. 10. A. Mass balance calculation showing the thickness of melt needed to form a 1-m-thick chromitite versus Cr
2
O
3
dissolved in the silicate melt. Curves
represent variable efciency of Cr extraction from the melt from 10 to 50 %. Diamonds indicate the 600 ppm Cr
2
O
3
solubility estimated for a basaltic melt by
Campbell and Murck (1993). Crosses show melt-solid assemblages and temperatures for the model contaminated komatiite parental melt used to model the Peridotite
chromitites from the Stillwater Complex by Jenkins and Mungall (2018). B. Bivariate plot showing the thickness of melt needed versus Pt solubility in silicate melt to
form a 1-m thick Pt reef deposit at variable amounts of sulde (from 0.5 to 10 wt%). The resulting suldes in the 1-m thick reef have Pt tenors of 300 ppm. The
assumption is made that Pt extraction from the silicate melt is 100 % efcient (i.e., Pt is perfectly compatible in sulde liquid).
W.D. Smith et al.
Precambrian Research 415 (2024) 107615
17
required to satisfy the mass balance (Fig. 10).
Contrary to Cr, Ti is incompatible to most of the minerals in the
convecting mantle, hence it is enriched in low degree partial melts and
becomes progressively diluted as the degree of melting increases (Hirose
and Kushiro 1993; Baker and Stolper 1994; Walter 1998). The effect of
pressure during melting is of minor importance for the Ti content of the
primary melt forming from the convecting mantle. Experiments con-
ducted with the typical pyrolite composition of the convecting mantle,
show that the Ti content of the primary melt decreases from ca. 1.0–2.5
wt% at <10 % melting to ca. 0.5–1.0 wt% at 10–20 % melting, before
reaching a fairly steady level of 0.5 wt% at >20 % melting (Hirose and
Kushiro 1993; Baker and Stolper 1994; Walter 1998). The PIC and
MARID assemblages of the metasomatized SCLM contain several min-
erals such as ilmenite, rutile, and phlogopite, which can contain high Ti
contents. These minerals, especially phlogopite, are present in the re-
sidual solid with intermediate degrees of melting (roughly 30 %,
although highly dependent on the composition: Foley et al. 2022; Ezad
et al. 2024) but once they melt completely, the partial melt should attain
high Ti content. However, as mentioned above, due to the heterogeneity
of the metasomatized SCLM composition, quantitative assessment is
difcult. For a layered intrusion to inherit a high Ti budget from a
convecting mantle source, the degree of melting should be relatively
low, whereas high Ti melts form from a metasomatized SCLM when the
Ti-bearing phases are consumed.
Vanadium is a complex trace component in the mantle as it can be
present in multiple valence states, which affect its partitioning between
phases (Canil 1997; Mallmann and O’Neill 2009). Depending on the fO
2
in the mantle, the bulk partitioning of V ranges gradually from mildly
compatible in reducing conditions (<fayalite-magnetite-quartz (FMQ)
to −2 log units) to highly incompatible (>FMQ to +2 log units)
(Mallmann and O’Neill 2009). Considering the common minerals in the
convecting mantle, the relative partition coefcients of V increase
roughly in the following order: olivine <orthopyroxene <clinopyrox-
ene and garnet <Al-spinel <Cr-spinel (Mallmann and O’Neill 2009).
Given that the partitioning is weakest in the more refractory olivine and
orthopyroxene, V becomes increasingly incompatible (enriched in the
melt) as complete melting of the less refractory phases is approached.
Generally, ultramac magmas generated from the convecting mantle
tend to have fO
2
at the FMQ buffer or up to about –3 log units below
based on a global comparison of V-in-olivine oxybarometry (Nicklas
et al. 2024). Based on the same global dataset, there has been gradual
increase in fO
2
from the Archean to modern magmas, which in theory
could have caused secular change from mildly compatible to incom-
patible V partitioning during mantle melting. This, however, is not re-
ected in the temporal occurrence of Fe-Ti-V deposits in layered
intrusions (Fig. 4). As V is sometimes found concentrated together with
Cr and PGE and sometimes with Fe and Ti, it seems that its partitioning is
either controlled by local changes in the fO
2
or that, instead of the de-
gree of mantle melting, the composition of the mantle source composi-
tion is the main factor in forming V-rich melts.
The PGE budget inherited from the accretion of the Earth is thought
to have been largely stripped from the silicate mantle and crust during
the formation of the metallic core (Holzheid et al. 2000). Hence, most of
the PGE now concentrated in layered intrusions were originally intro-
duced to their mantle sources by a late veneer following the core for-
mation (Maier et al. 2009). This extraterrestrial input was spatially
erratic and imposed a rst-order control on the PGE contents of magmas
prior to the mantle-wide homogenization of this newly inherited mate-
rial, which was possibly complete by 2.9 Ga (Maier et al. 2009).
Therefore, it is possible that the oldest layered intrusions in the
geological record could have originated from mantle sources that were
either strongly depleted or enriched in the PGE compared to the mantle
that existed from the Neoarchean onward. As with any other elements,
the PGE contents of primary melts are not only dependent on the
depletion or enrichment of the mantle source.
The iridium-group PGE (Ir, Os, Ru) and palladium-group PGE (Pd, Pt,
Rh) are partly hosted by different phases in the mantle, hence their
behavior during mantle melting is decoupled (Mungall and Brenan
2014; Waterton et al. 2021). The PPGE, together with Cu, are mostly
hosted in suldes in the mantle as they are incompatible in silicates and
oxides (Fellows and Canil 2012; Mungall and Brenan 2014), although it
should be noted that Rh might be more compatible in silicates and ox-
ides than Pt and Pd under some conditions (refer to Barnes et al. 2015).
As a result, primary magmas reach highest PPGE (e.g., for Pt, on the
order of 10
2
ppb) and Cu (on the order of 10
2
ppm) contents at the point
when the mantle suldes become fully dissolved in the partial melt,
whereas further melting causes dilution of these metals (e.g., Mungall
and Brenan 2014; Yao et al. 2018). The point at which sulde is
exhausted from the mantle source is highly dependent on the P-T con-
ditions and melt composition (major elements and Ni +Cu), which
control the S dissolution capability of the magma (refer to Smythe et al.
2017; O’Neill 2021) as well as the S content of the mantle source.
Typically, it is thought that roughly 10–20 % melting is required to fully
dissolve sulde in the mantle (Mungall and Brenan 2014; Yao et al.
2018; Waterton et al. 2021; Virtanen et al. 2024).
Compared to the palladium-group PGE, the iridium-group PGE are
signicantly more refractory, hence their concentrations are not as
strongly controlled by the presence of residual sulde in the mantle
source (Pearson et al. 2004, Barnes et al. 2015). Additionally, iridium-
group PGE-bearing metal alloys with very low solubility to silicate
melt are generally stable in the mantle (Fonseca et al. 2012; Barnes et al.
2015). Due to the many potential IPGE-hosting phases in the residual
mantle assemblage, IPGE concentrations rarely reach as high a primary
enrichment as palladium-group PGE in mantle-derived melts. Like the
stratiform Cr deposits, reef-type PGE deposits hosted in layered in-
trusions require very large volumes of mantle-derived melt to satisfy
mass balance considerations. To demonstrate this, we show that 370 to
13,000 m of a mantle-derived partial melt is required to form a 1-m thick
reef with 1 mass % sulde with tenors of 300 ppm Pt (Fig. 10) This
calculation requires the unrealistic assumption that Pt extraction from
the silicate melt is 100 % efcient.
Due to its abundance, olivine dominates the Ni inventory of con-
vecting mantle, while Ni-rich but modally minor sulde phases are of
lesser importance (Yao et al. 2018). Nickel is compatible in olivine with
the compatibility showing an inverse correlation with forsterite content
and temperature (e.g., Matzen et al. 2017). Typically, generic mantle
melting models show that Ni content of the primary melt increases with
the degree of melting (Naldrett 2004, Mungall 2013); however, more
complex models that account for the P-T path of the upwelling mantle
and phase equilibrium, and their effects on Ni partitioning suggest
contrarily that Ni content decreases with the degree of melting (Yao
et al. 2018). The reason for the contradictory behavior is that when the
melting initiates deep in the mantle, temperatures are hotter, and the
partial melt has relatively high MgO content as controlled by the melting
reactions. However, as upwelling continues, the MgO content of the melt
decreases (refer to e.g., experiments of Walter 1998). Consequently, Ni is
less compatible in residual highly-forsteritic olivine early in the melting
process, compared to less forsteritic residual olivine that forms at a later
stage of decompression melting and adiabatic temperature reduction
(Yao et al. 2018).
The common denominator of the above considerations of metal
contents in primary magmas is that the metals are transported in the
magma mainly as dissolved components. However, this might not al-
ways be the case. Some researchers consider the mobility of precious
metals transported as nanonuggets attached to or dissolved in sulde
liquids. These droplets of sulde liquid may buoyantly rise through
silicate melts when they are attached to supercritical uid bubbles (i.e.,
compound droplets: Mungall et al. 2015) or perhaps when they are
coated by low-density carbonate melt (Cherdantseva et al. 2024). In
general, the transport of sulde liquids by compound droplets has been
invoked for short distance transportation of sulde liquids in low-
pressure settings, where volatile solubility in the silicate melts is
W.D. Smith et al.
Precambrian Research 415 (2024) 107615
18
relatively low. As an example, this process has been suggested to have
facilitated the upgrading of sulde tenors in the deposits of the Norilsk-
Talnakh intrusion (Iacono-Marziano et al. 2022). Similar buoyant two-
phase transport of dense metal-rich phases facilitated either by super-
critical CO
2
uids, with low solubility in silicate melts, or by carbonate
melt jackets, has been suggested to be possible from as deep as SCLM (e.
g., Blanks et al. 2020, Cherdantseva et al. 2024). However, direct evi-
dence for such mantle-to-crust scale two-phase transportation processes
is difcult to nd due to the poor preservation of the transport agent,
and it remains unclear how important this type of metal transport pro-
cess is.
To summarize, the metallogenic potential of layered intrusions is
strongly affected by the mantle source and the degree of partial melting.
In general, high degree partial melts (roughly ≥25 %) inherit high
concentrations of Cr and Ni as well as the PGE, as long as the latter are
not diluted by too high a degree of partial melting. In addition, inher-
iting high V contents may require a high degree of melting if fO
2
in the
mantle is low. Medium degrees of melting, high enough to completely
dissolve mantle sulde (roughly 10–20 %), produces primary melts with
high concentrations of Ni, Cu, and the PPGE. When the degree of mantle
melting is low (roughly ≤10 %), the primary melts formed have the best
metallogenic potential for Ti and for V if the mantle source has suf-
ciently high fO
2
to cause incompatibility of the latter. This general
pattern may, however, be violated if the mantle source has exotic
metasomatic phases or if the metal transport is facilitated by two-phase
transport.
9.2. Deposit-scale ore-forming processes
Deposit-scale ore-forming processes refer to coincident magmatic,
magmatic-hydrothermal, and hydrodynamic processes that ensure the
saturation of a given phase (e.g., sulde melt, chromite, conjugates) and
its concentration into economically viable orebodies (Naldrett 2004;
Namur et al. 2015; Latypov et al. 2024a). Mineral deposits in layered
intrusions often occur as stratiform intervals within which ore minerals
are concentrated. Without sufcient concentration, potential orebodies
are diluted with gangue minerals and can become high tenor, but low
grade disseminated mineral occurrences, such as that reported at the
Duluth Complex (Minnesota; Naldrett 2004) and the Fazenda-Mirabela
intrusion (Brazil; Barnes et al. 2011). With the exception of the Nor-
il’sk-Talnakh intrusions (Russia), virtually all PGEs are sourced from
Precambrian intrusions (refer to Table ST6 of Mudd et al. 2018; Fig. 4).
Stratiform massive chromitites in layered intrusions account for most of
the world’s Cr reserves, and most of these reserves are present within the
Precambrian Bushveld Complex and Great Dyke of Zimbabwe (Liu et al.
2024). Layered intrusions are critical V resources (Simandl and Paradis
2022), where many important Fe-Ti-V deposits occur in Precambrian
layered intrusions, including the ~ 2.8 Ga Murchison intrusions of
Western Australia, the ~ 2.4 Ga Koillismaa Complex in Finland, and the
~ 2.6 Ga Lac Dor´
e intrusion in Qu´
ebec (Smith and Maier 2021).
Moreover, most layered intrusions with reported nelsonites (i.e., Fe-Ti
oxide- and apatite-rich rocks) were emplaced in the Precambrian (e.g.,
Sept ˆ
Iles intrusion, Tollari et al. 2008; Grader intrusion, Charlier et al.
2008).
Early models for the formation of stratiform mineral occurrences
were founded on cumulus theory (Wager and Brown 1968), and
although many ore-forming processes have since been invoked (refer to
review by Smith and Maier 2021), the differentiation of parental melts
and gravitational accumulation of their saturated phases remain rele-
vant for stratiform mineral occurrences (Naldrett et al. 2012; Song et al.
2013; Holness et al. 2023). Gravitational settling and/or density-driven
stratication are key processes in the formation of Fe-Ti-V oxide-rich
occurrences, whether formed by cumulus oxides (Charlier et al. 2008;
Zhang et al. 2012; She et al. 2015) or conjugate (immiscible) liquids
(Holness et al. 2011; Wang et al. 2018). Prolonged fractionation of sil-
icate minerals may produce an Fe- and Ti-rich residual melt from which
oxides crystallize and segregate from coprecipitating silicates according
to density contrast (Charlier et al. 2008; Nebel et al. 2013; Mokchah and
Mathieu 2022). It is worth noting that the Dmgt/melt
V(bulk)value increases with
decreasing fO
2
, which favors the formation of relatively V-rich magne-
tite should negligible V partition into prior-forming clinopyroxene and
ilmenite (Toplis and Carroll 1995; Toplis and Corgne 2002). The Main
Magnetite Layer of the Bushveld Complex is thought to have formed via
in situ crystallization from such an Fe- and Ti-rich residual melt
(Reynolds 1985), whereby the cryptic dome-shaped Cr enrichments in
the constituent magnetite are interpreted as nucleation sites (i.e., growth
nodes; Cawthorn and McCarthy 1980; Cawthorn 1994; Kruger and
Latypov 2020). In situ growth in a chemically stratied magma was also
invoked to explain the formation of the Mustavaara oxide-rich gabbros
in the Koillismaa intrusion (Finland; Karinen et al. 2022). Alternatively,
Yao and Mungall (2022) argued that the Main Magnetite Layer formed
by homogeneous nucleation and settling of dense magnetite crystals,
where the dome-shaped Cr proles were explained as representing sites
of upwelling Cr-bearing melts.
Silicate liquid immiscibility may also be responsible for Fe-Ti-V
oxide-rich occurrences and this process has been recently reviewed by
Veksler and Charlier (2015). Although magma mixing may inuence
silicate liquid immiscibility (Von Gruenewaldt 1993; Charlier and Grove
2012), it is broadly accepted to be a product of extensive fractionation,
where efcient unmixing of Si-rich and Fe-Ti-rich conjugates is facili-
tated by the slow cooling rates of plutonic intrusions (Namur et al. 2012;
Charlier et al. 2013). Experimental studies continue to explore the
triggers (Honour et al. 2019) and relative timing (Hou and Veksler 2015)
of silicate liquid immiscibility in basaltic systems, which has been evi-
denced by the preservation of both conjugate liquids as apatite-hosted
melt inclusions (Fischer et al. 2016; Wang et al. 2018; Lino et al.
2023) and inferred from apatite chemistry (Kieffer et al. 2023). The Sept
ˆ
Iles (Charlier et al. 2011; Namur et al. 2012), Bushveld (VanTongeren
and Mathez 2012), and Duluth (Ripley et al. 1998) intrusions are all
proposed as having experienced large-scale silicate liquid immiscibility
that led to the formation of magnetitites, ferrogabbros, and leucocratic
gabbros. Immiscibility was also invoked to explain the formation of
oxide-rich lenses in ultramac cumulates of the ~ 3.5 Ga S˜
ao Tom´
e
intrusion (Brazil; Ruiz et al. 2019), which would argue that liquid–liquid
immiscibility can occur relatively early in the differentiation of ferro-
basaltic melts. Iron- and Ti-rich conjugate melt is likely to be under-
saturated with respect to sulde melt and will likely partially or
completely dissolve any magmatic sulde it encounters, leading to the
upgrading or redistribution of chalcophile metals (Godel et al. 2014;
Nielsen et al. 2015; 2019).
Chromitites may occur in the lower ultramac (e.g., Stillwater
Complex), central mac (e.g., Bushveld Complex), or upper evolved (e.
g., Akanvaara) portions of layered intrusions (Smith and Maier 2021 and
references therein). It is important to note that many chromitites have
radiogenic isotope signatures consistent with crustal contamination (e.
g., Schoenberg et al. 1999), a process long considered to be involved in
chromitite formation (cf. Irvine 1976). Thin chromitite stringers that are
associated with anorthosite likely form during the reaction between
resident cumulates and replenishing melts (O’Driscoll et al. 2009; Scoon
and Costin 2018) or inltrating supercritical uids (Mathez and Kinzler
2017; Marsh et al. 2021); however, the origin of massive chromitites is
heavily debated. Massive chromitites, even more so than chromitite
stringers, necessitate the existence of melts that volumetrically exceed
that inferred from the preserved cumulates and their lateral persistence
implies chamber-wide phenomena (Campbell and Murck 1993; Caw-
thorn and Walraven 1998; Latypov et al. 2022; Fig. 10). The dilemma of
mass balance for chromitite petrogenesis is mitigated for by many
workers by invoking the existence of an underlying staging chamber
from which chromite crystals are entrained and deposited, either as
monomineralic slurries (Voordouw et al. 2009) or within silicate-
chromite slurries that unmix through kinetic sieving (Eales 2000;
W.D. Smith et al.
Precambrian Research 415 (2024) 107615
19
Mondal and Mathez 2007; Eales and Costin 2012). Indeed, a dike-like
massive chromitite unit associated with the Kemi intrusion, inter-
preted as a feeder conduit, has been used to implicate either the exis-
tence of chromite-charged melts or the backow of chromite-rich
slurries (Alapieti and Lahtinen 2002; Yang et al. 2016). Chamber-wide
phenomena, such as magma mixing (Irvine 1977; Campbell and
Murck 1993; Spandler 2005), shifts in fO
2
(Ulmer 1969), melt strati-
cation (Naldrett et al. 2012), and pressure shifts [decrease (Cameron
1977; Lipin 1993; Cawthorn 2005); increase (Latypov et al. 2018; Drage
and Brenan 2023)] have also been proposed as triggers for chromite
saturation; however, any model that implicates gravitational settling
and sorting has difculty addressing the issues of chromitite topography
and, in some cases, mass balance and uniformity of their bracketing
units (Latypov et al. 2017).
Although PGMs may crystallize directly from mantle-derived melts
(Peck et al. 1992; Maier et al. 2014), it is generally accepted that
immiscible sulde melts play a key role in scavenging chalcophile
metals from silicate melts (Naldrett 2004; Mungall and Brenan 2014).
Recent models show that the S solubility of basaltic melt varies only
slightly under crustal pressures (O’Neill 2021). Relatively high S solu-
bilities at lower pressures (<1 GPa; Mavrogenes and O’Neill 1999;
Smythe et al. 2017) indicates that melts pooling in shallow reservoirs
must either undergo extensive degrees of differentiation and/or assim-
ilate crustal materials to reach sulde saturation (Ripley and Li 2013).
Crustal contamination may trigger sulde melt saturation in basaltic
magmas by altering the composition of the melt (Irvine 1976; Virtanen
et al. 2021), modifying the redox conditions (Jugo and Lesher 2005;
Iacono-Marziano et al. 2017), introducing volatile species (Naldrett
2004; Liu et al. 2007), and/or introducing sulde xenomelts (Lesher
2017). Section 9.1 outlines the importance of pre-emplacement
contamination in PGE reef formation; however, the in situ assimilation
of sulde xenomelts is relevant to some contact-style magmatic sulde
occurrences. One example is the Partridge River Intrusion of the Duluth
Complex, whereby Samalens et al. (2017) showed that semi-metal
concentrations in suldes decrease and PGE concentrations in suldes
increase with increasing distance from country rock xenoliths. Also at
the Duluth Complex, Virtanen et al. (2021) have shown that S and
chalcophile metals could be liberated from the contact aureole during in
situ devolatilization, providing a mechanism for increasing melt fertility
without wholesale assimilation of wall-rocks.
As detailed in Latypov et al. (2015; 2024a), many models have been
proposed for the formation of stratiform PGE reef-style mineralization,
which, like chromitites, necessitate chamber-wide phenomena. They
may occur at any stratigraphic level within an intrusion and can mani-
fest in several different host rocks (Latypov et al. 2024a); no single
petrogenetic model has been able to account for their diversity. Crustal
contamination, likely in an underlying staging chamber, is believed to
have played a role in the formation of relatively uncommon ultramac-
hosted PGE reefs, such as the Sopcha Reef in the Monchegorsk intrusion
(Russia; Karykowski et al. 2018) and Volspruit Zone of the Bushveld
Complex (Tanner et al. 2019). Conversely, PGE reefs hosted in upper,
more evolved portions of layered intrusions, such as in the Jameson
(Musgraves Region, Australia; Polito et al. 2017) and Nuasahi (India;
Prichard et al. 2018) intrusions, are thought to have formed from
strongly fractionated melts in response to magnetite saturation. Each
“reef”, however, requires high R-factors (mass ratio of silicate:sulde
melt; Campbell and Naldrett 1979) that are broadly believed to only be
attained via open-system processes, such as entrainment of sulde melts
from depth (McDonald and Holwell 2011; Kaavera et al. 2020) or
dissolution-upgrading (Kerr and Leitch 2005; Cao et al. 2021). Recog-
nition that the D
sul/sil
values of the PGE are on the order of 10
6
(Mungall
and Brenan 2014) has led to the conclusion that many PGE reefs can be
explained by closed-system processes (Mungall et al. 2020), but it has
been proposed that these D
sul/sil
values are overestimated due to the
mechanical inclusion of PGE nanonuggets in sulde liquid (Anenburg
and Mavrogenes 2020).
Alternatives to orthomagmatic models are hydromagmatic models
that propose PGE concentration in stratiform horizons during the (re-)
dissolution of upwelling Cl- and PGE-bearing uids exsolved from un-
derlying cumulates (Boudreau and McCallum 1992). These models are
supported by, amongst other things, the relative increase in the abun-
dance of Cl-bearing accessory phases (Boudreau et al. 1986; Boudreau
and McCallum 1989), presence of saline uid inclusions (Ballhaus and
Stump 1986; Ripley 2005; Hanley et al. 2008), and the existence of
pegmatitic rocks, discordant pipes, and potholes (Boudreau 2019 and
references therein). Reef-style PGE mineralization in the Stillwater and
Bushveld complexes are taken as key case studies for hydromagmatic
models (Boudreau et al. 1986), yet similar Cl enrichments are not
observed in the Main Sulde Zone of the Great Dike (Boudreau et al.
1995), the Munni Munni intrusion (Boudreau et al. 1993), or Fedorova-
Pana Complex (Sushchenko et al. 2023). Although the role of halogens
in ore formation is debated, it is broadly accepted that PGEs (Pt and Pd)
can be transported as chloride complexes in aqueous brines (Sullivan
et al. 2020a; 2020b).
Although not mutually inclusive, many stratiform chromitites and
magnetitites host elevated PGE concentrations (Maier 2005; Smith and
Maier 2021). Contrary to experimental results (Rose and Brenan 2001),
Godel et al. (2006) argued that chromitites trap sulde liquid as it
cannot percolate as effectively through them as they can through silicate
cumulates. Any “trapped” sulde liquid may become further enriched in
PGEs if it loses Fe to neighboring chromite crystals and S
2
to the sur-
rounding rocks (Naldrett and Lehmann 1988). Alternatively, the asso-
ciation between chromite and suldes could reect sulde, PGM and/or
PGE-nanonuggets preferentially [heterogeneously] nucleating in
reduced boundary layers that encompass crystallizing chromite crystals
(Finnigan et al. 2008; Anenburg and Mavrogenes 2016; Barnes et al.
2021). Magnetitites with elevated PGE concentrations are almost uni-
versally explained as the product of S-saturation triggered by saturation
of magnetite (i.e., removal of Fe
2+
from the melt; Polito et al. 2017), and
further stratication of oxides (±suldes) may be achieved by granular
ow (i.e., Stella intrusion; Maier et al. 2023). The suldes may be PGE-
poor if the silicate melt had previously experienced sulde melt satu-
ration (´
Etoile Suite in Quebec; R. Maier et al. in-press).
Several reef-style PGE occurrences exhibit ‘offsets’ in peak concen-
trations of chalcophile metals and are, thus, aptly referred to as offset-
style reefs. Although the term has been founded on offset-style miner-
alization observed at the mac–ultramac transition in Munni Munni
(Barnes et al. 1992) and the Great Dike (Wilson and Tredoux 1990), this
pattern has been documented in several other intrusions, where it dis-
plays remarkable variability in its relative stratigraphic position [e.g.,
basal ultramac cumulates in Kapalagulu (Prendergast 2021) to upper
magnetitites in Jameson (Karykowski et al. 2017)] and the nature of the
offsets. Although the sequential offsets observed for peak Pd +Pt, Au,
and Cu concentrations are consistent with generally accepted partition
coefcients (i.e., upwards-decreasing D
sul/sil
values), some workers have
suggested that fractional sulde segregation and instantaneous equili-
bration is unable to sufciently account for the offsets (Barnes 1993;
Mungall 2002). This has led to offsets being described as a result of
uctuations in the efciency of PGE scavenging by sulde melt (i.e.,
‘virtual’ partition coefcients; Barnes 1993) or differences in the diffu-
sivities of chalcophile and highly siderophile elements (Mungall 2002);
the latter process has been applied to a number of offset-type reefs
(Holwell et al. 2016; Guice et al. 2017; Prendergast 2021). The latter
models describe the offsets as primary features, but in an alternative
model for the Stella (Maier et al. 2003) and Skaergaard (Andersen et al.
1998) reefs, researchers proposed that offsets form as accumulated
sulde liquid differentiates whilst being displaced upward during
cumulate compaction.
9.3. Detecting mineral occurrences in layered intrusions
The nal key components of the layered intrusion mineral systems
W.D. Smith et al.
Precambrian Research 415 (2024) 107615
20
are detectability and delineation – how can one determine whether an ore-
forming process was active in a given system and can the preserved rock
record be exploited to vector toward mineral deposit occurrences (Maier
and Barnes 2005; Barnes et al. 2016). Layered intrusions typically host
thick mac–ultramac cumulates that may be spatially associated with
trans-crustal structural networks through which fertile parent melts
ascended. Such cumulates often display elevated concentrations of Mg,
Fe, Cr, and Ni, low incompatible trace element contents (e.g., Ti, Zr,
REE), gravitational anomalies (>2.6 g/cm
3
), and anomalous electro-
magnetic signatures that are enhanced by serpentinization as well as the
presence of contact-style suldes and oxide-rich occurrences (Finn et al.
2015; Blanchard et al. 2017; Kharbish et al. 2022).
Cumulus suldes and gossanous outcrops are favorable mineraliza-
tion indicators and translate to anomalous concentrations of S and
chalcophile metals in the rocks, overlying soils and ora, and emanating
stream sediments. In contrast, mac–ultramac rocks that are depleted
in chalcophile elements likely experienced upstream sulde melt
segregation, indicating that cumulus suldes may exist elsewhere in the
accessible plumbing system (Maier 2005). The presence of S-bearing
country rocks and evidence of their interaction with mantle-derived
melts (e.g., thermal aureoles, xenoliths) are also positive indicators,
because the assimilation of such rocks can trigger sulde melt saturation
in fertile mantle-derived melts (Ripley and Li 2013) and chalcophile
metals may even be liberated during country rock devolatilization
(Virtanen et al. 2021). Many of these attributes translate to mappable
criteria that can be used for mineral prospectivity exploration (Porwal
et al. 2010). This exercise leverages increasingly popular machine
learning algorithms to delineate areas of heightened prospectivity,
which has proven to be effective in delineating chromitites in the Gawler
Craton (Farahbakhsh et al. 2023) and Fe-Ti-V occurrences in southwest
China (Cong et al. 2017).
Siliceous high-Mg basalts (SHMBs) are parental to many of the
world’s notable layered intrusions, such as the Bushveld (Solovova et al.
2021), Stillwater (Jenkins et al. 2021) and Penikat (Maier et al. 2018)
complexes. These magmas represent komatiitic melts that assimilated
crustal rocks during their ascent (Arndt and Jenner 1986), perhaps in a
staging chamber within which sulde melts may also segregate and
become entrained (Eales and Costin 2012; Kaavera et al. 2020). Cu-
mulates derived from contaminated melts often contain cumulus
orthopyroxene (Jenkins and Mungall 2018), and these grains may show
erratic zoning proles when associated with dynamic systems conduc-
tive to magmatic sulde ore formation (Schoneveld et al. 2020). How-
ever, subsolidus diffusion of cations under prolonged cooling rates will
dilute zoning proles (Barnes et al. 2016), hence further research could
determine its suitability for vectoring mineral occurrences in layered
intrusions. The degree of interaction between mantle-derived melts and
crustal rocks is often evaluated by determining whether trace element
ratios and isotopic signatures of host rocks have deviated from that
expected of mantle-derived melts (Maier and Barnes 2005; Queffurus
and Barnes 2015; Tang et al. 2021).
In studies of magmatic suldes, δ
34
S [(
34
S/
32
S) sample/(
34
S/
32
S)
standard – 1) ×1000] values have been widely used to evaluate the S
sources, since the assimilation of exogeneous S by mantle-derived melts
typically results in δ
34
S values that deviate toward relatively heavy or
relatively light end members (Lesher and Groves 1986; Ripley and Li
2003). However, because the δ
34
S values of Archean mantle and crust
are similar, many studies in Archean terranes now also consider Δ
33
S
values {δ
33
S – 1000 ×(1 +δ
34
S/1000)
0.515
–1}, which only deviate from
0 prior to 2.4 Ga as a result of UV-induced photochemical processes in
the oxygen-decient Archean atmosphere (Farquhar and Wing 2003). It
must be noted that immiscible sulde melts (or xenomelts) will continue
to equilibrate with coexisting silicate melt, diluting and possibly eradi-
cating non-zero S isotopic signatures (Lesher and Burnham 2001). This
was demonstrated at the Duluth Complex, where diluted S isotopic
signatures coincided with low semi-metal (As and Sb) and high PGE (Pd
and Ir) concentrations of suldes, consistent with increasing R-factors
(Samalens et al. 2017). Nonetheless, the concentrations of semi-metals
(or ‘TABS’; Te, As, Bi, and Sb; Mansur and Barnes 2020) and the pres-
ence of sulfarsenides (Knight et al. 2017) may be used to evaluate the
nature of crustal contamination in each system.
Whole-rock Cu/Pd and Cu/Zr values remain among the most widely
used indicators of sulde melt segregation, having been effective in
inferring pre-emplacement sulde melt segregation and in vectoring
many of the world’s reef-style PGE occurrences regardless of their host
rock and stratigraphic position (Fig. 11; Barnes 1990; Li and Naldrett
1999; Maier 2005). The Ni concentration of olivine is another widely
used indicator of S-saturation and vector of sulde mineralization (Bulle
and Layne 2016; Taranovic et al. 2022; Smith et al. 2025) and refer to
the review of Barnes et al. (2023a). The trace element concentrations of
Fe-Ti oxides are sensitive to the wide variety of conditions under which
they can crystallize (e.g., silicate melts, sulde melts, hydrothermal
uids), meaning they can be used to determine provenance and vector
towards magmatic sulde and Fe-Ti-V occurrences (Dupuis and Beau-
doin 2011; Dare et al. 2014). Oxides exsolved from immiscible sulde
melts may have characteristically high Cr, V, and Ni concentrations
depending on the degree of sulde melt fractionation (Dare et al. 2012),
whereas those associated with Fe-Ti-V occurrences can have elevated Ti
and V concentrations (Dupuis and Beaudoin 2011).
Accessory apatite that is spatially associated with reef-style PGE
occurrences has been known to be relatively Cl-rich (Boudreau et al.
1986) and its trace element concentrations may be used to discover Fe-
Ti-P mineral deposits (Kieffer et al. 2023) or reveal insights into the
petrogenesis of layered intrusions (Kieffer et al. 2024). For example,
cumulus apatite associated with Fe-Ti-P(−V) oxide mineralization has
relatively high Sr concentrations and Eu/Eu* values as well as low ΣREE
+Y concentrations (Kieffer et al. 2023). Lastly, the use of sulde
chemistry for exploration targeting has been discussed by Mansur et al.
(2020). Pentlandite compositions may be most applicable to layered
intrusion mineral occurrences as their Pd, Rh, and TABs concentrations
are sensitive to deposit origin (Ni-Cu versus PGE), sulde melt frac-
tionation, PGM exsolution and metal tenors (Mansur et al. 2020; 2024).
10. Future avenues of layered intrusion research
Layered intrusions occur in a variety of different geodynamic settings
throughout Earth’s history and are well represented by geologically
diverse examples in the Precambrian (Namur et al. 2015). Although
much progress has been made in understanding layered intrusion
petrology, many aspects of their origin remain heavily debated in the
literature. Here we reiterate the next steps of Latypov et al. (2024a)
regarding the future of layered intrusion research in the specic context
of Precambrian geodynamics.
1. Regional mapping campaigns of surface outcrops and underground
mine exposures, as well as logging drill cores, will likely always
underpin layered intrusion research. Primary eld observations and
their correlation on the macroscale facilitate appropriate sampling
campaigns, making them essential for contextualizing subsequent
data and constraining interpretations. Field observations are always
where the maximum uncertainty potentially occurs in a dataset.
Field methods may be bolstered by spectral imagery (Barnes 2020;
Hill et al. 2021) and/or combined with geophysical data (Finn et al.
2015; 2023) to better understand the morphology and architecture
of layered intrusions, particularly in areas with little-to-no exposure
(Lima et al. 2008; Balch et al. 2010). For example, Cole et al. (2024)
demonstrated that geophysical data can elucidate deep high-density
features, which could represent staging chambers or magmatic
underplating beneath layered intrusions.
2. Element mapping and microtextural analyses are powerful in testing
hypotheses and forming new ones. Combinations of these methods
can elucidate the physicochemical conditions under which rocks and
their ores were formed and modied. Although element mapping has
W.D. Smith et al.
Precambrian Research 415 (2024) 107615
21
gained traction for mineralogical characterization (Barnes et al.
2016; 2021; Smith et al. 2021), microtextural analyses remain
underutilized despite the valuable petrogenetic insights they offer
(Vukmanovic et al. 2019; Jenkins et al. 2022; Holness et al. 2024). As
demonstrated in Section 8, element mapping and microtextural
analysis were used to constrain the petrogenesis of the snowball
oikocryst unit of the Stillwater Complex. Our analysis demonstrated
deposition of poikilitic crystals in a dynamic environment prior to
the formation of a mush.
3. High-resolution X-ray computed tomography and novel laboratory
diffraction contrast tomography are non-destructive methods that
allow for visualization and quantication of three-dimensional fea-
tures in polycrystalline rocks (Godel 2013; Chen et al. 2023). These
methods can be utilized in conjunction with two-dimensional
petrographic observations to document magmatic fabrics, melt
percolation, diffusion kinetics, crystal nucleation mechanisms, and
likely other as yet unidentied applications.
4. Greater constraints on the parameters under which a given intrusion
formed can be achieved through the use and discovery of mineral
thermobarometers, oxybarometers, and hygrometers (Lindsley and
Andersen 1983; Ballhaus et al. 1991; Godel et al. 2011; Molina et al.
2021). Empirically determined parameters may then be used to
inform thermodynamically constrained forward modelling (Ghiorso
and Sack 1995; Bohrson et al. 2020; Ariskin et al. 2023) that might,
for example, lead to a better understanding of sulfur content at sul-
de saturation (O’Neill 2021; Wieser and Gleeson 2022), alloy sol-
ubility (Mungall and Brenan 2014), and the effects of degassing
(Burgisser and Degruyter 2015; Ding et al. 2023). These mineral
indicators may help to identify secular changes in the nature of
layered intrusion petrogenesis throughout the Precambrian.
5. The successful implementation of Point (4) could be supported by
experimental studies that explore the various conditions under
which layered intrusions typically form. Petrological experiments
that consider a range of intensive parameters can help to constrain
Fig. 11. Identication of reef-style PGE occurrences in layered intrusions using whole-rock Cu/Pd values. Intrusions and references include the Rustenburg Layered
Suite of the Bushveld Complex (Barnes et al. 2004), Stillwater Complex (Keays et al. 2012), Penikat intrusion (Maier et al. 2018), Great Dyke (Maier et al. 2015),
Munni Munni intrusion (Hoatson and Keays 1989), Sonju Lake intrusion (Miller Jr 1999), Luanga intrusion (Mansur et al. 2020), Stella intrusion (Maier et al. 2023),
N¨
ar¨
ank¨
avaara intrusion (J¨
arvinen et al. 2020), and the Vuruchuaivench occurrence of the Monchegorsk intrusion (Karykowski et al. 2018).
W.D. Smith et al.
Precambrian Research 415 (2024) 107615
22
liquid lines of descent (Leuthold et al. 2015; Drage and Brenan
2023), understand element partitioning (Blundy and Wood 2003;
Shephard et al. 2022), and simulate crustal assimilation (Iacono-
Marziano et al. 2017; Virtanen et al. 2022; Deegan et al. 2022).
Furthermore, analogue experiments may also be used to model
processes such as magma mixing (Sato and Sato 2009) or layer for-
mation (Forien et al. 2015), providing insights into emplacement
mechanisms and layer-forming processes.
6. Future studies could aim to better understand the role of volatile
species in layer- and ore-forming processes. These studies may utilize
experimental or computer-based simulations of the interaction be-
tween immiscible phases (Mungall 2015; Yao and Mungall 2020;
Cherdantseva et al. 2024), as well as directly analyze the volatile
budgets of volatile-bearing and nominally volatile-free minerals
(Boudreau et al. 1995; Pedersen et al. 2021; Bai et al. 2024). These
studies could help to elucidate the volatile budgets of magmas, melts,
and uids associated with layered intrusions, mineralized or not, and
provide insight into the cycling of volatile species throughout the
Precambrian.
7. Development of workows for the handling of expansive and inter-
disciplinary datasets, particularly commercial datasets that may far
exceed academic datasets in size (Jenkins et al. 2020; Barnes and
Williams 2024). Systematic handling of such datasets will allow for
meaningful comparisons between intrusions and their critical metal
occurrences, potentially illuminating spatiotemporal changes in
their nature. Such workows will likely consider machine learning
algorithms that can be leveraged for chemical domaining (Horrocks
et al. 2019) or intensive parameter prediction (Higgins et al. 2022).
8. As layered intrusions are hosts of important critical metal occur-
rences, many studies will likely center on identifying traits that are
characteristic of mineralized examples as well as on developing
practical tools to assist in their delineation (Dare et al. 2014; Scho-
neveld et al. 2020; Barnes et al. 2023). Effective tools are founded on
a deep understanding of the processes that govern critical metal
deposition, yet commercially viable exploration tools are lacking for
mineral occurrences with no systematic haloes. In the absence of
deposit-scale footprints, research could utilize the mineral system
approach (e.g., McCuaig et al. 2010; Barnes et al. 2016) to infer
prospectivity through identifying processes that are favorable for ore
deposition.
9. As multifaceted datasets become commonplace for layered in-
trusions and their genetically associated rocks (i.e., dikes, volcanics),
global spatiotemporal comparisons of their petrogeneses become
possible. These data can potentially shed light on fundamental
planetary scale processes such as the physicochemical evolution of
the mantle through time, mantle heterogeneity on local and regional
scales, material ux between the mantle and the lithosphere,
magmatic fractionation, and stabilization of continents.
The future of layered intrusion research can be enhanced by multi-
disciplinary collaborations between geoscientists, material scientists,
data scientists, mathematicians, and (or) geophysicists, amongst others,
that generate and interpret holistic datasets. Integrated studies that are
transparent and reproducible could lead to new discoveries in layered
intrusion research and support the next generation of researchers.
CRediT authorship contribution statement
William D. Smith: Writing – review & editing, Writing – original
draft, Methodology, Investigation, Formal analysis, Data curation,
Conceptualization. M. Christopher Jenkins: Writing – review & edit-
ing, Writing – original draft, Methodology, Investigation, Formal anal-
ysis, Data curation, Conceptualization. Claudia T. Augustin: Writing –
review & editing, Writing – original draft, Methodology, Investigation,
Conceptualization. Ville J. Virtanen: Writing – review & editing,
Writing – original draft, Methodology, Investigation, Conceptualization.
Zoja Vukmanovic: Writing – review & editing, Writing – original draft,
Methodology, Investigation, Formal analysis, Data curation, Conceptu-
alization. Brian O’Driscoll: Writing – review & editing, Validation,
Supervision, Conceptualization.
Funding
V.J.V is supported by the SEMACRET project (101057741) co-
funded by Horizon Europe program and UK Research Innovation. M.C.
J. is supported by the U.S. Geological Survey’s Mineral Resource Pro-
gram. W.D.S. is presently supported by a CSIRO Early Research Career
Postdoctoral Fellowship. BO’D acknowledges current research support
from the Natural Sciences and Engineering Research Council of Canada
(NSERC Discovery Grant) and from the Newmont Chair in Economic
Geology (University of Ottawa).
Declaration of competing interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Acknowledgements
We thank Precambrian Research for inviting us to contribute to their
50
th
year anniversary special issue. Randolph Maier is thanked for
providing photographs of layering in the Etoile Suite. Prof. Tom Blen-
kinsop is thanked for bringing the importance of geometric scaling to the
attention of the authors. We thank Dr. Iris Buisman and Dr. Charlie
Gordon from the Earth Science Department at the University of Cam-
bridge for providing analytical support for the study of the Stillwater
snowball oikocrysts and Dr. Mike Zientek (U.S. Geological Survey) for
kindly providing sample 84PP3. Jacob Walmsley (CSIRO) is acknowl-
edged for acquiring Maia Mapper images. Dr. Victoria Pease is thanked
for their editorial handling and review of this contribution. Dr. Lisa
Zieman (USGS), Rais Latypov (University of Witwatersrand), and one
anonymous reviewer are thanked for providing constructive reviews of
this manuscript. Lastly, each of us acknowledge the broader research
community for their contributions, discussions, and mentorship. Any use
of trade, rm, or product names is for descriptive purposes only and does
not imply endorsement by the U.S. Government.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.precamres.2024.107615.
Data availability
Data are available in the supplementary materials. Requests for the
raw EBSD and element map data can be made to the authors.
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