Petrogenesis of the Mesoarchaean Stella layered intrusion, South Africa: implications for the origin of PGE reefs in the upper portion of layered intrusions

Abstract

The ~3033 Ma Stella layered intrusion is hosted by supracrustal rocks of the Kraaipan–Madibe greenstone terrane, South Africa. The studied portion of the intrusion consists mainly of magnetite leucogabbro and magnetite anorthosite, as well as several massive magnetite layers. The intrusion hosts a laterally continuous, ~60-m-thick, PGE mineralized interval, with total resources amounting to 108t Pt + Pd + Au, constituting one of the oldest known PGE reef-style mineralizations on Earth. The richest reef, with a grade of 4.4 ppm Pt+Pd over a width of 5–8 m, occurs in semi-massive magnetitite. It is suggested that the mineralized oxide and silicate layers formed through a combination of primary magmatic, late magmatic, and hydrothermal processes, including granular flow and phase sorting of a magnetite- and sulfide-bearing gabbroic crystal mush that crystallized from a tholeiitic basalt, as well as remobilization of S and metals by late magmatic and hydrothermal fluids that led to crystallization of platinum-group minerals.

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Mineralium Deposita (2023) 58:1477–1497
https://doi.org/10.1007/s00126-023-01189-5
ARTICLE
Petrogenesis oftheMesoarchaean Stella layered intrusion, South
Africa: implications fortheorigin ofPGE reefs intheupper portion
oflayered intrusions
WolfgangDMaier1 · Sarah‑JaneBarnes2· WilliamDSmith3
Received: 5 May 2022 / Accepted: 13 June 2023 / Published online: 12 July 2023
© The Author(s) 2023
Abstract
The ~3033 Ma Stella layered intrusion is hosted by supracrustal rocks of the Kraaipan–Madibe greenstone terrane, South
Africa. The studied portion of the intrusion consists mainly of magnetite leucogabbro and magnetite anorthosite, as well
as several massive magnetite layers. The intrusion hosts a laterally continuous, ~60-m-thick, PGE mineralized interval,
with total resources amounting to 108t Pt + Pd + Au, constituting one of the oldest known PGE reef-style mineralizations
on Earth. The richest reef, with a grade of 4.4 ppm Pt+Pd over a width of 5–8 m, occurs in semi-massive magnetitite. It is
suggested that the mineralized oxide and silicate layers formed through a combination of primary magmatic, late magmatic,
and hydrothermal processes, including granular flow and phase sorting of a magnetite- and sulfide-bearing gabbroic crystal
mush that crystallized from a tholeiitic basalt, as well as remobilization of S and metals by late magmatic and hydrothermal
fluids that led to crystallization of platinum-group minerals.
Keywords Layered intrusion· South Africa· Platinum-group elements· Platinum-group minerals· Archaean· Ore deposit
Introduction
Platinum-group elements (PGE) are critical metals required
in the production of catalytic converters and fuel cells. How-
ever, apart from extensions and development of existing ore
bodies, largely in the Bushveld Complex of South Africa
and at Noril’sk, Russia, few new economically viable PGE
deposits have been found in the last decades. Thus, the
world’s primary PGE production remains centered on the
Bushveld Complex and Noril’sk, with smaller contributions
from the Great Dyke of Zimbabwe, the Stillwater Complex,
USA, and the Lac des Iles intrusion of Canada. The Stella
layered intrusion of South Africa (Fig.1) is interesting in
this regard because its mineralized reefs are among the old-
est on Earth and occur in an unusual stratigraphic position,
i.e., within the upper, relatively evolved portion of the intru-
sion. This has opened new search spaces for PGE explora-
tion (Maier etal. 2003). In this paper, we present a detailed
petrological study of the mineralized interval and propose
a refined ore model.
History ofexploration, regional geology,
andtectonic setting
The Stella intrusion is located in the Northwest Province
of South Africa, ~400 km to the west of Pretoria (Fig.1).
The name “Stella” derives from a comet that was visible
in the skies in the 1880s (Theal 1919). The intrusion has
been dated at 3033.5 ± 0.3 Ma (Schmitz etal. 2004). It is
hosted by the western (Stella) limb of the poorly exposed
Kraaipan–Madibe greenstone terrane, which also includes
the Goldridge, Madibe, and Amalia belts (Ramotoroko etal.
2016). The Kraaipan-Madibe terrane comprises a diverse
range of lithologies. In addition to mafic volcanics in the
greenstone belts, there are quartzites and banded iron for-
mations (3410 + 61–64 Ma; Anhaeusser and Walraven
Editorial handling: M. Fiorentini
* Wolfgang D Maier
maierw@cardiff.ac.uk
1 School ofEarth andEnvironmental Sciences, Cardiff
University, Cardiff, UK
2 Sciences de la Terre, Université du Québec à Chicoutimi,
Chicoutimi G7H 2B1, Saguenay, Canada
3 Department ofEarth Sciences, Carleton University, Herzberg
Laboratories, Ottawa, OntarioK1S5B6, Canada
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1478 Mineralium Deposita (2023) 58:1477–1497
1 3
1999), felsic intrusive rocks including tonalite and trond-
hjemite forming the regional basement (3162 to 3070 Ma,
Anhaeusser and Walraven 1999), as well as post-tectonic
rhyolite (2914 Ma, M. de Wit pers. com. to H. Jelsma)
and K-rich granodiorite and adamellite (2880 to 2846 Ma)
(Fig.1).
Geology oftheStella intrusion
The Stella intrusion was originally delineated in the early
1970s by means of aeromagnetic surveys and drilling which
established a strike extent of ~13 km and an estimated strati-
graphic thickness of ~1.5 km (ESM 1a). The intrusion is
thought to represent the upper portion of an originally larger
and thicker, but tectonically dismembered, layered intrusion
(Andrews 2002). Two sections have been delineated, Mor-
ester in the North and Kroomdraai in the South, both being
~6 km long. Several PGE-rich deposits have been deline-
ated by drilling, including Crater and Sirius in the Morester
section, and Orion and Crux in the Kromdraai section.
Deformation resulted in west over east, bedding parallel,
ductile, in places mylonitic, shearing and thrusting, particu-
larly at the contact between competent and less competent
lithologies, e.g., magnetitite and diabase dykes. The widths
of the shear zones can be up to 2 m. Late brittle reverse
faulting caused conjugate fault sets, trending 010–040°
and 100–115°, and compartmentalization of the intrusion,
with up to ~50 m horizonal displacement (ESM 1b), and
unknown vertical displacement. This led to local duplication
of the reef, resulting in thickening to as much as 70 m (at the
Crux deposit, ESM 1a).
The basal contact of the intrusion is not exposed at the
surface. In drill core, the contact zone consists of interca-
lated magnetite–quartz schist, chert, and highly altered intru-
sive rocks, displaying abundant quartz veining. The contact
zone is overlain by medium- to coarse-grained leucogabbro,
magnetite leucogabbro and anorthosite, hosting more than
10 magnetitite layers, 0.4–4 m thick. Contacts between lay-
ers are mostly gradational (Fig.2). The layers dip broadly
sub-vertically or are locally overturned with a westerly dip,
but there are strong variations on a local scale due to fault-
ing and folding.
In the present study, borehole intersection W955D was
examined in detail (ESM 2a). The borehole plunges at ~45°
to the east. The foliation of plagioclase grains in the drill
core suggests that the bore hole intersected the reef interval
at an angle of ~45° (Fig.2). This could suggest that the true
thickness of the ~250-m mineralized interval, including the
1.8-m-thick main mineralized magnetitite, is reduced by a
factor of 0.71.
The uppermost 40m of the drill core, representing the
stratigraphic base of the examined sequence, consist largely
of magnetite-poor leucogabbro that is locally pegmatitic
(Fig.3, ESM 1c). This is underlain by ~50 m of leucogabbro
containing 1–15 modal % magnetite. In most rocks from ~90
to 220 m, the magnetite content of leucogabbro is > 15%, and
numerous massive (> 50 wt.% oxide) to semi-massive (20–50
Fig. 1 Geology of the
Kraaipan–Madibe greenstone
terrane hosting the Stella intru-
sion (modified after Ramo-
toroko etal. 2016)
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1479Mineralium Deposita (2023) 58:1477–1497
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wt.% oxide) magnetite layers occur. The rocks are mostly
medium- to coarse-grained, or locally pegmatitic, consist-
ing of euhedral and subhedral plagioclase now replaced by
amphibole and chlorite, and interstitial magnetite.
There are a number of different felsic rock types present
in the drill cores, including oligoclase–quartz rocks contain-
ing xenoliths of the host leucogabbro and showing strongly
sheared contacts with the host rocks, implying they are pre-
to syn-tectonic. Abundant fine-grained quartz–albite veins
oriented sub-parallel to the igneous layering have thick-
nesses of centimeters to 10s of meters. Many display chilled
margins and are unaltered and undeformed, suggesting they
formed after the deformation of the leucogabbro. Coarse-
to medium-grained granite veins are less common than the
quartz–albite veins and measure up to a few m in thickness.
They consist of plagioclase, quartz, alkali feldspar, locally
biotite and myrmekite and contain blocks of sheared gabbro
indicating that the veins are post-tectonic in age.
There are two distinct sets of mafic dykes: diabase of
Fe-rich tholeiitic composition is oriented predominantly
sub-parallel to the igneous layering and has been altered to
amphibole schist consisting of actinolite, chlorite, quartz,
Fig. 2 Drill core W955D inter-
secting upper main reef magnet-
itite and magnetite leucogabbro
as well as a diabase dyke. Note
sample S19 (~15ppm PGE) col-
lected from 201.1-201.3m.
Fig. 3 Concentrations of
Pt+Pd+Au in drill cores at
Stella (drill line 11150N). Num-
bers next to log of bore hole
W955D denote depth of drill
core in meters. Figure modified
after Andrews (2002)
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1480 Mineralium Deposita (2023) 58:1477–1497
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plagioclase, and epidote. These dykes are less altered and
deformed than the Stella cumulates but have mostly sheared
contacts with the cumulates and thus likely intruded during
the late stages of tectonism (Fig.2). MELTS simulations
(rhyolite MELTS v 1.2.0) yielded a crystallization sequence
and mineral compositions matching the Stella cumulates
suggesting the dykes could represent the parent magmas to
the Stella intrusion. A second suite of mafic dykes trends
ENE and is clearly visible on aeromagnetic maps (ESM 1a).
These magnetite gabbros dykes have chilled margins and
brecciated contact zones.
Some boreholes intersected up to 200 m of diamictite
likely belonging to the Kameeldoorn Formation of the Ven-
tersdorp Supergroup (2.754–2.709 Ga, Gumsley etal. 2020).
Their contacts to the Stella intrusion are steeply dipping,
suggesting the rocks are confined to fault troughs.
The thickness of overburden is typically ~4 m, ranging
from 1 to 11 m, the latter in paleochannels. The depth of
weathering varies from a few meters to 45 m (average 25 m).
PGE andV‑Ti mineralization
The first observation of sulfide mineralization in the
Stella intrusion was made by Kafrarian Metal Holdings
in 1975–1978 who detected a Cu-Ni-Zn soil anomaly and
drilled ferrogabbro. Platinum-group element mineralization
was discovered by Anglo American Prospecting Services
during the 1990s, following a soil sampling program. Drill-
ing subsequently delineated several mineralized domains.
The project is currently estimated to hold measured and
indicated resources totaling 69.91 Mt grading at 1.48 g/t
Pt+Pd+Au and inferred mineral resources of 56.68 Mt grad-
ing 1.62g/t Pt+Pd+Au (unpublished report by Coffey Min-
ing Consultants 2014).
Initial descriptions of the Stella PGE mineralization
have been provided by Andrews (2002) and Maier etal.
(2003). Close-spaced assaying by Harmony Gold estab-
lished that the mineralized interval can be sub-divided into
several sub-reefs, based largely on PGE, Cu, and Au con-
tent as well as Pt/Pd ratio. The stratigraphically lowermost
PGE-enriched horizon contains 0.5–3 ppm PGE. It occurs
some 15–45 m below the main mineralized interval and
has thus been termed “Pre reef kick.” The main mineral-
ized interval has an average thickness of 40–45m (Fig.3).
At its base is the low-grade (LG) reef, hosted by magnetite
leucogabbro and minor magnetitite. This reef has slightly
higher Pt than Pd values, with an overall grade of about 1
g/t Pt+Pd+Au over widths of 10–20 m. The 10–15-m wide
mid-reef is Pd-enriched by a factor of 2–4 times relative
to Pt. It usually contains two higher-grade reefs, termed
MR1 and MR2 (mid-reef 1 and 2, respectively), character-
ized by elevated magnetite content and grades reaching 4
ppm (average 2.5 ppm) over 1–3 m. Next comes a ~5–10
m, predominantly non-magnetic anorthositic interval with
grades of < 0.5 g/t Pt+Pd+Au. The main reef has a width
of up to 20 m, with average grade of 1.9 ppm Pt+Pd+Au
over 14 m. There are two high-grade zones, the 2.3-m
lower main (LM) reef at the base (avg 3.5 g/t Pt+Pd+Au),
and the 5–8 m wide, magnetite-rich, upper main (UM)
reef at the top (4.4 g/t Pt+Pd+Au). Gold and Cu sharply
increase in the top few meters of UM and extend into the
hanging wall rocks which consist of magnetite gabbro,
magnetite anorthosite, and magnetitite. Relatively low
levels of Pt (< 0.3 ppm) and traces of Au persist for ~10
m, whereas elevated Cu continues for ~30 m, to a horizon
termed “Pseudoreef,” consisting of magnetitite with chal-
copyrite but no PGE or Au.
Weathering has resulted in leaching of Pd from the
rocks close to surface. Estimated losses within 10 m of
surface are 25% Pd and within 30 m 15% Pd. Similar
weathering leaching has been reported from the Stillwa-
ter Complex (Fuchs and Rose 1974), the Bushveld Com-
plex (Hey 1999; Junge etal. 2019), and the Great Dyke
(Oberthür etal. 2013).
Andrews (2002) provided an overview of the platinum
group mineral (PGM) variation across the reef interval. He
listed kotulskite, moncheite, sperrylite, arsenopalladinite,
ferroplatinum, and rare gold and electrum, yet PGE-sulfides
are absent. No information on the total number of PGM
grains was provided. Most PGM are hosted by silicates,
whereas sulfides and magnetite contain few PGM. In some
samples, the PGM (particularly sperrylite) may replace sili-
cates and form encrustations (or “atoll structures”) around
Cu sulfides, up to 35 μm in diameter. There is some varia-
tion in the composition of PGM between reefs: LG contains
mainly merenskyite and sperrylite, the mid-reef has mainly
merenskyite and stibiopalladinite, whereas the main reef has
mainly stibiopalladinite and sperrylite. The weathered main
reef has abundant PGE alloys and gold, analogous to weath-
ered reefs elsewhere.
The oxides in the oxide layers consist predominantly of
titanomagnetite (~85%), with much of the remainder being
ilmenite. In addition, some rutile, leucoxene, and hematite
are present close to faults. Grades of magnetic fractions of
the main reef are 1.14% V2O5, 4.83% TiO2, 81.1% Fe2O3
(crater deposit) and 0.93% V2O5, 5.59% TiO2, and 80.7%
Fe2O3 (Orion deposit) (Andrews 2002). Mid-reef grades (at
Orion) are 1.06% V2O5, 4.07% TiO2, 74.2% Fe2O3, whereas
LR grades at Orion are 0.41 % V2O5, 2.36 % TiO2, 51.3 %
Fe2O3. Andrews (2002) reported the results of microprobe
studies across the drilled interval of the intrusion indicating
that magnetite contains an average of 1.3 wt.% V2O5, 0.43
wt.% TiO2, and 0.1 wt.% Cr2O3. Vanadium grade drops to
0.9–0.95% V2O5 near carbonate alteration but is otherwise
relatively constant.
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1481Mineralium Deposita (2023) 58:1477–1497
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Analytical methods
All whole-rock analyses were performed at LabMaTer,
Universitée du Québec à Chicoutimi (UQAC). Tellurium,
As, Bi, Sb, and Se were determined on 0.4 g of sample
using hydride generation-atomic fluorescence spectrome-
try (HG-AFS) following Mansur etal. (2020). This method
has the advantage of low detection limits in the 3 to 10 ppb
range allowing greater precision than standard analytical
methods. More details on the method and results for certi-
fied reference materials determined in the same batch as
the Stella samples are reported in ESM 2b. In addition, a
wide range of major and trace elements were determined
using laser ablation-inductively coupled plasma-mass
spectrometry (LA-ICP-MS) on fused Li-tetraborate disks
containing 0.5 g of sample. Details of the method and
results for reference materials GeoPt-19, GeoPt-36 (both
gabbros), LK-NIP (diabase), and BC28 (massive magnet-
ite) are reported in ESM 2c,d. Analysis of Na, Cs, and Sm
by LA-ICP-MS has a markedly higher detection limit than
INAA and thus the INAA results from Maier etal. (2003)
are used (EMS 2c).
We characterized samples using transmitted and
reflected microscopy as well as scanning electron micros-
copy (SEM). High-resolution element maps were gener-
ated using a Carl Zeiss Sigma HD Analytical Field Emis-
sion SEM equipped with two Oxford Instruments 150
mm2 energy dispersive spectrometers (EDS) at Cardiff
University. The maps were produced using an acceler-
ating voltage of 20 kV, a 120-μm final aperture in high
current mode, a nominal beam current of 8.5 nA, and a
working distance of 8.9 mm. For entire sections, a step
size of 15–20 μm and a pixel dwell time of 10 ms was
used. The acquired maps were processed and exported in
AZtec 5.0 software. For the detection of discrete precious
metal grains, feature maps were produced, which identi-
fies grains of a user-defined contrast during the automated
acquisition of back-scattered electron images. Each identi-
fied platinum-group mineral (PGM) was imaged, analyzed
by EDS, and morphologically characterized in ImageJTM
software to better constrain their physical and chemical
properties.
Results
Drill core observations
Sixty samples from drill core W955D were collected
for study. Of these, 26 were analyzed for whole-rock
geochemistry. Images of these drill core samples are pre-
sented in ESM 1c. The key observations are as follows:
(i) due to the greenschist metamorphism identification
of the original silicate mineralogy is difficult. However,
pseudomorphs of plagioclase can be recognized by their
distinctive tabular crystal shapes. Based on geochemistry,
the proportion of pyroxene rarely exceeds ~10 modal %.
(ii) The rocks are mostly coarse grained, with plagioclase
in, for example, samples S11 and S22–25 typically having
grain sizes of ~1 cm. Sample S1 is relatively unaltered,
showing both mafic minerals (likely hornblende) and pla-
gioclase. (iii) Plagioclase is typically euhedral or subhe-
dral, and, in many cases, defines a well-defined foliation
sub-parallel to banding (ESM 1c, d). (iv) Some samples
contain pyroxene oikocrysts (S15) and resemble mottled
anorthosite. (v) Oxides typically have interstitial habit,
possibly resulting from grain boundary adjustment at the
sub-solidus stage (Duchesne 1999).
Transmitted andreflected microscopy
The Stella rocks are pervasively altered to a greenschist
metamorphic assemblage of clinozoisite, epidote, carbon-
ate, actinolite, and chlorite. This caused near complete
replacement of all silicates and partial replacement of
magnetite by chlorite, carbonate, and leucoxene and of
ilmenite by rutile and sphene. Alteration is particularly
pronounced along faults where secondary hematite, rutile,
and sphene may occur. As a result of the alteration, no
compositional data are available for the primary magmatic
minerals. CIPW norms (assuming Fe3+/Fe2+ = 0.3) of
magnetite-poor samples indicate that original plagioclase
had anorthite contents of ~55 to 62 (with the exception of
sample S1 which appears to have lost some Na), and mafic
silicates have Mg# ~0.16 to 0.33.
Small amounts of sulfides occur throughout, but apart
from uppermost 25m of the studied sequence where up to
~1% sulfides may occur, their concentration is mostly so
low (< 500 ppm S in whole rocks) that they are not readily
visible megascopically. The sulfides consist largely of chal-
copyrite and pyrite (Fig.4). Rare phases include pentland-
ite, millerite, galena, sphalerite, cassiterite, and linnaeite.
Most grains are less than 25 μm in size, but a few grains
measure up to 500 μm. Chalcopyrite forms anhedral grains
that are unaltered, i.e., have no rims of magnetite or bor-
nite. Pyrite mostly forms highly irregular, often vermicular
grains that have a porous appearance. The sulfides mostly
occur along grain boundaries or cleavage planes of meta-
morphic silicate minerals that, in most cases, have com-
pletely replaced subhedral or euhedral plagioclase crystals.
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1482 Mineralium Deposita (2023) 58:1477–1497
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Scanning electron microscopy
In order to study the economically important main reef,
compositional maps were produced for sample S19 (201.3
m) and its platinum-group mineral (PGM) assemblage
was determined. The Fe-Ti-S map (Fig.5a) highlights the
predominance of magnetite over ilmenite, consistent with
whole-rock compositional data. Assuming all TiO2 is in
ilmenite, the most oxide-rich samples consistently give
proportions of 14 wt.% ilmenite to 86 wt.% magnetite. The
Mg-Ca-K-S map (Fig.5b) shows that the silicate minerals
are Ca-rich, with Mg- and K-rich cores, reflecting replace-
ment of plagioclase by sericite and chlorite. The main vein
crossing the center of the slide is relatively Mg rich, likely
consisting of chlorite and serpentine. The Mg content of
the silicates decreases with distance from the vein, pos-
sibly reflecting diffusion of Mg from the vein (ESM 1e).
The rock contains only few, very small (~10 μm) grains of
likely apatite and zircon. The grain areas are close to the step
size used (~10 micron); thus, they were unresolvable in our
maps. The maps also highlight the relatively small grain size
of the sulfides and their predominant association with sili-
cates, whereas oxides are largely sulfide free (Fig.5c and d).
The BSE images illustrate the distribution of the sulfides
and PGM predominantly within silicates (e.g., Fig.6a), but
in rare cases, PGM are associated with sulfide (Fig.6b)
and oxides (Fig.6c–e). The images also illustrate that the
magnetite grains contain trellis-type ilmenite exsolution
lamellae which are typically associated with small grains
of spinel (Fig.6e). Granular ilmenite occurs predominantly
along grain boundaries of magnetite and near fractures. The
ilmenite grains may be surrounded by halos of pure magnet-
ite lacking ilmenite lamellae. Granular ilmenite occasionally
is replaced by rutile and titanomagnetite (Fig.6f).
In total, 164 PGM were characterized in sample S19 (201.1
m), some of which being composite grains of several minerals.
Fig. 4 Petrography of Stella
rocks. a Pervasive replacement
of plagioclase by secondary
silicates, including clinozoisite
and epidote (center of grain in
upper left), as well as dis-
seminated grains of chlorite
and hornblende in remaining
plagioclase grains. Sample S 2,
crossed polars. b Metamorphic
alteration assemblage affecting
plagioclase, sample S2, crossed
polars. Czs: clinozoisite; Cl:
chlorite; Hbl: hornblende. c
Anhedral grains of chalcopyrite,
lacking alteration rims. Plane
polarized reflected light, sample
S19. d Chalcopyrite filling
cleavage planes in actinolite,
sample S 19. e Chalcopyrite
occurring only within silicate
minerals, sample S 23. f Finely
disseminated chalcopyrite in
metamorphic silicates, sample
S23
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1483Mineralium Deposita (2023) 58:1477–1497
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The most abundant mineral is stibiopalladinite (95 grains), fol-
lowed by sperrylite (68 grains) and kotulskite (22 grains) (ESM
1f, 2f). One grain of Pt-sulfide was found included in magnetite
(Fig.6a). The compositions of the PGMs are relatively homog-
enous, but some grains of kotulskite have up to 4.6 wt.% Au
and 1.3 wt.% Ir, sperrylite has up to 6.5 wt.% Pd and 6.7 wt.%
Au, and stibiopalladinite frequently has several wt.% Te and
Bi (ESM 2f). The PGM are mostly very small (< 5 μm). On
average, sperrylite forms the largest grains, followed by sti-
biopalladinite and merenskyite. Most of the PGM occur along
grain boundaries of metamorphic silicates with a few grains
occurring in fractures (7–8% for sperrylite and stibiopalladinite,
1% for merenskyite; Fig.6). Our work showed a less diverse
PGM assemblage than Andrews (2002), likely because we stud-
ied just one sample of the main reef, whereas Andrews (2002)
studied 7 samples from the main, mid and low-grade reefs.
Lithophile geochemistry
Major oxides
Based on petrography, most Stella samples appear to be
mixtures of Fe-Ti oxides and silicates with negligible
sulfides or carbonates. Therefore, plots of SiO2 vs. the
other elements give an idea of the composition of the oxide
component (Fig.7). Based on the y-axis intercept on a plot
of SiO2 vs. Fe2O3 (Fig.7a), when no silicates are present
the Fe oxide component expressed as Fe2O3 is 87 wt.%.
Plots of SiO2 versus TiO2, MgO, and Al2O3 give y-axis
intercepts of 6.6% TiO2, 2.8% MgO, and 2.7% Al2O3 as
the concentrations for the oxide component (Fig.7b–d).
We can compare these compositions to those calculated by
MELTS (see ESM 2g-k) using the composition of a typical
Fig. 5 SEM-EDS element
maps of sample S19 (201.1
m). a, b Note relative scarcity
of primary Mg-Ca silicates
largely confined to secondary
veins and pervasive chloritiza-
tion of plagioclase. Sulfides are
almost exclusively hosted by
silicates. Ti is controlled mostly
by exsolution phases, plus
minor granular ilmenite (and
rutile), as well as Ti-rich veins
(likely consisting of ilmenite
and rutile). c, d Distribution of
PGM, hosted largely in silicates
(black), whereas oxides (dark
grey) and magnesian veins are
largely PGM free
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1484 Mineralium Deposita (2023) 58:1477–1497
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Archean tholeiite as the magma composition. Fractional
crystallization simulations of tholeiite from the Obatoga-
mau Formation, Québec (Boucher etal. 2021), consid-
ered to be parental to the magnetite-rich Lac Doré layered
intrusion, indicate similar spinel compositions to those
estimated at Stella after 75% crystallization. Note that
the TiO2 values are comparable to that of magnetite from
the central portion (sub-zone C) of the upper zone of the
Bushveld Complex (Klemm etal. 1985) and to magnetite
of the Bell River and Lac Doré intrusions, Québec (Poliv-
chuk 2018; Mokchah and Mathieu 2022).
The binary variation plots and the textures suggest that
the main silicate component of the Stella rocks was plagio-
clase. Using the compositions of plagioclase calculated by
MELTS for the Archean tholeiite (after 75% fractionation),
one can see on plots of MgO vs. SiO2 (Fig.7c), CaO vs.
SiO2 (Fig.7e), and CaO vs. Al2O3 (Fig.7f) that the rocks
plot slightly above the plagioclase Fe oxide tie line. Thus,
the rocks contain an additional mineral (or minerals) hosting
the extra CaO and MgO. Based on the MELTS simulation of
Archean tholeiite, this mineral is likely clinopyroxene. Using
the plagioclase, magnetite, and clinopyroxene compositions
predicted by MELTS, most of the Stella rocks plot near the
plagioclase–clinopyroxene tie line with a ratio of plagio-
clase to clinopyroxene ~1 : 9. In summary, the major element
composition of the whole rocks can be modeled as mixtures
of magnetite, plagioclase, and clinopyroxene. The propor-
tion of plagioclase to clinopyroxene is fairly constant at 9 : 1
whereas the magnetite content varies widely from 1 to 90%.
Transition metals
Vanadium, Ni, Co, and Zn all show negative correlations
with SiO2 suggesting that these elements too are mainly
controlled by mixtures of Fe-Ti oxides with silicates (see
Fig.7g for V, ESM 2l for Ni, Co, and Zn). The intercepts for
Fig. 6 PGM in sample S19
(201.1 m) of the Stella intru-
sion. a Stibiopalladinite (stp)
and chalcopyrite (cp) in silicates
interstitial to oxides consist-
ing of magnetite and granular
ilmenite. Also note needle-
shaped PGM at left, likely rep-
resenting a Pt-sulfide. b Large
grain of sperrylite included
in anhedral chalcopyrite. c
Stibiopalladinite within gangue
and sperrylite within ilmenite.
d Kotulskite and sperrylite
included within magnetite. e
Sperrylite (spy) within gangue
and stibiopalladinite at margin
of magnetite. Note trellis-type
ilmenite lamellae as well as
abundant small grains of pleo-
naste. f Magnetite and granular
ilmenite, the latter breaking
down to rutile + titanomagnetite
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1485Mineralium Deposita (2023) 58:1477–1497
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Fig. 7 Binary variation dia-
grams of Stella rocks. a Fe2O3
vs. SiO2, b TiO2 vs. SiO2, c
MgO vs. SiO2, d Al2O3 vs.
SiO2, e CaO vs. SiO2, f Al2O3
vs. CaO, g V vs. SiO2, h Cr vs.
SiO2, i Th vs. SiO2, and j Th/Y
vs. SiO2. Mineral compositions
in c, e, and f are from MELTS
simulation of Obatogamau
Archean tholeiite after 75%
fractionation. Trend lines in a,
b, d, and g are calculated for
data excluding gabbronorite
sample. Stippled line in c, e
and f indicates proportion of
clinopyroxene is 10%. Composi-
tion of UC and PM in j are from
Rudnick and Fountain (1995)
and Sun and McDonough
(1989), respectively
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1486 Mineralium Deposita (2023) 58:1477–1497
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these elements at 100% oxide suggest that magnetite con-
tains ~1.2% V2O5, ~650 ppm Ni, ~250 ppm Co, and ~250
ppm Zn. The V content is slightly lower than that of the
main magnetite layer of the Bushveld Complex (~1.5–2%
V2O5) which is mined for V, but is similar to magnetitite
in the center of the Bushveld upper zone (sub-zone B) and
to magnetite at Bell River and Lac Doré (Polivchuk 2018;
Mokchah and Mathieu 2022).
Manganese (intercept ~0.24 wt%) and Sc (intercept 20
ppm) show slightly weaker correlations with SiO2 (ESM 2l)
suggesting that these elements are slightly less compatible
with magnetite than V, Ni, Co, and Zn. For both elements the
gabbronorite at the base of the mineralized interval (sample
S1) plots at high levels of Sc and Mn, possibly because these
elements are more compatible with clinopyroxene than Fe
oxide (Ewart and Griffin 1994; Otamendi etal. 2016).
Chromium does not correlate with SiO2 or anything else
(Fig.7h). Our data indicate < 100 ppm Cr in most samples,
except for the two lowermost magnetitites (S5 and S7) that
have ~700 ppm Cr. Interestingly, semi-massive magnetite
sample S4, located just 5 m below Cr-rich magnetitite sam-
ple S5, has only 11 ppm Cr although V tenors are broadly
similar as in S5. This indicates a surprising degree of decou-
pling of V and Cr contents of magnetite, considering that
Cr and V are both strongly compatible into magnetite (DCr
mean 67, range 19–340; DV mean 26, range 7–130; Dare
etal. 2012). The data are not an analytic artefact as INAA
analysis (Maier etal. 2003) yielded similar results. Possi-
bly, a more primitive magma intruded at the level of sample
S5, or in the Cr-poor samples clinopyroxene crystallized
together with magnetite, competing for Cr.
Rare earth elements
All REE are incompatible with magnetite and show negative
correlations with Fe2O3+TiO2 and SiO2 (not shown). Euro-
pium is the least incompatible with regard to plagioclase,
with a partition coefficient approaching unity (Bédard 2006).
On REE multi-element diagrams (ESM 1g), the silicate-rich
rocks all have strong positive Eu anomalies reflecting the
presence of cumulus plagioclase in these rocks. The patterns
are relatively flat, resulting in La/YbN ~1.44. Gabbronorite
sample S1 has a relatively flat REE pattern with mostly
higher REE contents than the other cumulates, suggesting
it has a higher melt component than the other samples. The
magnetite-rich samples all have relatively low REE contents,
with less pronounced positive Eu anomalies, consistent with
their low plagioclase content.
HFS andLIL elements
Hafnium, Zr, Nb, and Ta are incompatible with regard
to plagioclase and clinopyroxene. They have small but
significant D values into Fe oxides (0.13 to 0.25, Dare etal.
2012). Hafnium and Ta are both present at very low levels
in all rock types and the concentration of both elements
is similar (0.3–0.6 and < 0.03–0.09 ppm, respectively).
The slight compatibility of Zr, Hf, Nb, and Ta with regard
to magnetite precludes using these immobile elements to
estimate trapped liquid component. Samarium is not com-
patible with any of the crystallizing minerals. Assuming
that the magma contained Sm concentrations similar to the
Archean tholeiite, and allowing for 75% fractional crystalli-
zation, the trapped liquid should contain ~4.5 ppm Sm. The
Stella cumulates contain 0.1 to 0.6 ppm Sm which suggests
2 to 13% trapped liquid.
Thorium is incompatible with plagioclase, clinopy-
roxene, and magnetite and thus should broadly reflect the
trapped liquid component of the rocks. All analyzed sam-
ples have relatively low Th contents (< 0.25 ppm, Fig.7i),
reflecting their cumulate nature. The plagioclase-rich
rocks have the highest Th contents, suggesting either that
they contain a higher liquid component than the magneti-
tites, perhaps due to less efficient compaction, or that they
assimilated some crustal material. To assess the possibil-
ity of crustal contamination, we have elected to use Th/Y
ratios since these elements are both strongly incompatible
with respect to plagioclase and Fe-Ti oxides. In addition,
Th/Y in the upper crust and the mantle is very different
(UC: 0.49, PM: 0.019). Yttrium is slightly compatible into
clinopyroxene, but clinopyroxene is a minor component
of the Stella rocks. The Stella rocks have Th/Y between
0.01 and 0.05, broadly similar to PM (Fig.7j). Based on
these data, contamination does not appear to be a critical
factor in the crystallization of the Stella rocks. This inter-
pretation is consistent with the generally unfractionated
nature of multi-element variation diagrams of lithophile
incompatible trace elements (ESM 1h), with the exception
of positive Sr, Eu, and Ti anomalies representing cumulus
plagioclase and magnetite, and elevated concentrations of
large ion lithophile elements K, Ba, Cs, and Rb. The latter
show erratic correlations with Na, possibly due to vari-
able alteration. Given that the samples have experienced
greenschist facies metamorphism these elements will not
be considered further.
Chalcophile elements
Copper, Au, Se, Sb, As, Te, Bi, and the PGE are all chal-
cophile elements under crustal conditions and are thus
expected to be controlled by sulfides. Therefore, the cor-
relations between these elements and S will be considered
first (Fig.8).
Copper shows a good positive correlation with S, but
there are three distinct populations (Fig.8a). Most samples
containing > 0.5% S have S/Cu ratios of approximately 1.5,
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1487Mineralium Deposita (2023) 58:1477–1497
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Fig. 8 Binary variation dia-
grams of chalcophile metals vs
sulfur (x = gabbro, orange dot is
anorthosite). a Cu, b Au, c Bi, d
Te, e Se, f As, g Sb, h Pd, i Pt,
and j Ir
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1488 Mineralium Deposita (2023) 58:1477–1497
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indicating that the main sulfide phase is probably chalcopy-
rite (S/Cu~1). As mentioned earlier, Ni appears to have been
largely controlled by the oxides. In rocks with > 0.3% S, the
ratio of Ni/S is ~0.05 implying the Ni content of the sulfides
is ~1.7%; therefore, < 5% of the sulfides are pentlandite,
with the remaining sulfide phase probably being Fe-sulfide
(pyrite), consistent with petrographic observations. The
samples with relatively low S concentrations (< 0.5 %) have
S/Cu ratios of ~3; these would then contain ~1/3 chalcopy-
rite and 2/3 pyrite. Seven samples plot below these trends,
possibly indicating S addition or Cu loss. Taken together, the
three populations suggest a Cu-rich sulfide, a medium-Cu
sulfide, and some low-Cu sulfide.
Gold, Te, Bi, and Se also show positive correlations
with S, consistent with their high D values into sulfide melt
(Fig.8b–e) (Barnes 2016), albeit with more scatter than Cu.
Se/S ratios are commonly used to evaluate addition of
external S to a magma (Eckstrand etal. 1989; Queffurus
and Barnes 2015 and references therein) and are thus of
particular interest. For samples with Se contents higher
than the detection limit there is a positive correlation
between S and Se, but with highly variable S/Se. Most
samples have S/Se > 5000, i.e., non-magmatic values
normally explained by addition of external S to a magma
(Queffurus and Barnes 2015). Five samples have S/Se >
10000, yet most of the samples with high S/Se are rela-
tively S-poor. However, when the bulk S and Se contents
of the entire interval are considered the S/Se is 3500, i.e.,
overlapping with the magmatic range of S/Se (2850-4350;
Eckstrand and Hulbert 1987).
Arsenic and Sb do not correlate with S (Fig.8f and g).
There are two main populations, namely, the magnetitites
and the magnetite-bearing anorthosites. Both have moderate
correlations between As and Sb (not shown), with the former
having markedly lower As/Sb ratios.
Correlations between Pt and Pd with S are generally
poor, particularly in the samples with > 0.15% S (Fig.8h
and i). Platinum, Pd and Ir show positive correlations with
each other (ESM 1i), suggesting that none of the PGE was
significantly mobile. The PGE show strong positive correla-
tions with Sb (ESM 1i), but not with As (ESM 1i), suggest-
ing they are mainly controlled by antimonides. For all PGE-
enriched samples the ratio of Pd to Sb is 1 to 2 in terms
of weight. As the atomic weight of Pd and Sb is broadly
similar, the antimonides thus should be predominantly PdSb
and Pd5Sb3, i.e., approaching the composition of stibiopalla-
dinite, in agreement with the observed PGM assemblage.
In order to further constrain the behaviour of the chalco-
phile elements during partial melting, crystallization, and
crustal contamination, we plotted the compositions of high-
S and low-S samples into mantle normalized multi-element
plots after Barnes and Mansur (2022) (Fig.9). Contamina-
tion with continental crust results in enrichment in Th, As,
Sb, and Bi, as well as negative Nb anomalies, as has been
previously shown for the Bushveld Complex (Mansur and
Barnes 2020). The Stella rocks, including the magnetite
poor samples, lack negative Nb anomalies, suggesting that
crustal contamination was insignificant or involved incom-
patible-trace-element-poor material. The S-rich rocks have
broadly similar levels of Te, As, Bi, Sb, Se and PGE, but
many of the S-poor rocks are enriched only in As, Sb and
Bi. This confirms that there is a decoupling between the
PGE and most other chalcophile elements (Fig.8, ESM 1i).
Of particular interest are the patterns of sample S1
(Fig.9). The sample is barren in terms of PGE as well as Se
and Te (ESM 2e) but is enriched in crust-derived As-Sb-Bi,
i.e., PGE and crust-derived chalcophile elements are decou-
pled. This suggests that As, Sb, and Bi were introduced to
the rocks by crust-derived metamorphic fluids or assimila-
tion of crustal rocks.
Fig. 9 Multi-element variation diagram of TABS as well as Th and selected PGE, normalized to primitive mantle (normalization factors from
Lyubetskaya and Korenaga 2007)
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1489Mineralium Deposita (2023) 58:1477–1497
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Fig. 10 a–c Diagrams showing concentrations of lithophile and chal-
cophile elements and element ratios plotted vs. stratigraphic height in
drill core W955D. a Sulfur and noble metals, b copper and TABS,
and c chalcophile element ratios. Numbers next to logs denote depth
of drill core (in meters) which broadly represents stratigraphic height
of intrusion
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1490 Mineralium Deposita (2023) 58:1477–1497
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Vertical compositional variation
The distributions of selected lithophile elements and ele-
ment ratios, as well as S, Cu, TABS and the noble metals
in the analyzed drill core are plotted vs height in Fig.10
and ESM 1j. Several observations may be highlighted:
(i) Most major element oxides, including TiO2 (and Fe2O3,
not shown) as well as trace elements V (and Zr, not
shown) display no clear trend with height (ESM 1j).
Al2O3, reflecting plagioclase mode, increases with
height, whereas Cr decreases with height (ESM 1j).
Th/Y, potentially a proxy for contamination, is slightly
enriched in the reef interval relative to the footwall rocks.
(ii) Sulfur shows relatively low concentrations (200–700
ppm) up to a stratigraphic height of ~195 m, above
which there is a sharp increase to 1100–5000 ppm
(Fig.10a). A similar pattern is seen for Cu, Te, Se, Bi,
and Au (Fig.10b), elements that show positive correla-
tions with S in binary variation plots (Fig.8).
(iii) The concentrations of the PGE (Fig. 10a) and Sb
(Fig.10b) also increase with height, albeit showing
considerable scatter and several peaks.
(iv) Arsenic behaves differently from the other chalcophile
metals, showing the highest concentrations in mag-
netite anorthosite in the central portion of the interval
(Fig.10b).
(v) The thickness of the main mineralized interval, con-
taining > 1000 ppb PGE + Au, amounts to some 70
m (Fig.10a). Particularly elevated concentrations are
found at a stratigraphic height of ~100 m (the so-called
“pre-reef kick”), 145–160m (the “low-grade reef”),
160–185 m (the “mid-reef”), and 190–202 m (the
“main reef”).
(vi) Whereas the highest PGE grades are found in three
magnetite layers, some magnetite layers are virtually
barren and some silicate-rich samples are PGE enriched
such that magnetite and PGE concentrations are decou-
pled (Fig.10a).
(vii) Peak values of different chalcophile metals occur at
different stratigraphic levels (Fig.10a and b); Rh peaks
at 93 m and 145 m above the base of the intrusion, Ir at
145 m and 201 m, Pt+Pd at 201–202 m, Au at 202–205
m and Cu at 205–219 m. Te and Se peak at the same
stratigraphic level as Cu, but As peaks at 140 m, Bi at
195 m, and Sb at 200 m. This pattern of stratigraphic
separation of peak concentrations has been termed
“offset” mineralization in the Munni Munni intrusion
(Barnes etal. 1992) and the Great Dyke (Wilson etal.
2000; Oberthür and Cabri 2002).
(viii) Chalcophile element ratios Cu/Pd and Pt/Pd show
sharp increases above the main reef, analogous to other
PGE reefs, such as the Bushveld Complex (Fig.10c).
Pd/Ir of the mineralised rocks plots mostly between 100
and 800, consistent with the relatively evolved nature
of the Stella rocks.
(ix) Cu/Zr in the studied interval is generally above the level
of the primitive mantle (Fig.10c) reflecting the pres-
ence of sulfides in all rocks. Cu/Zr sharply increases
above the main reef. This is surprising as one would
normally expect that the formation of a PGE reef
depletes Cu relative to Zr.
(x) S/Se is mostly above the range of the primitive man-
tle (Fig.10c), except for the S- and Se-rich uppermost
samples with overlap with the S/Se range of the primi-
tive mantle.
Discussion
Comparison ofStella toother PGE mineralized
layered intrusions
The major PGE reef-type deposits are located in the lower to
central portions of layered intrusions such as the Bushveld
Complex of South Africa, the Stillwater intrusion of Mon-
tana/USA and the Great Dyke of Zimbabwe. Most subeco-
nomic PGE reefs occur in similar settings (Smith and Maier
2021). However, in a number of layered intrusions, elevated
PGE grades have been found in the relatively evolved, mag-
netite-rich upper portions of the intrusions. Possibly the first
such occurrence was reported by von Gruenewaldt (1976),
describing PGE rich sulfides below the main magnetite layer
(MML) of the Bushveld Complex. Subsequently reported
examples include the 2435 Ma Koitelainen intrusion of
northern Finland (Mutanen 1989), the 55 Ma Skaergaard
intrusion of Greenland (Bird etal. 1991), the Birch Lake
deposit of the 1107 Ma Duluth Complex (Hauck etal. 1997),
the 992 Ma Rincon del Tigre intrusion of Bolivia (Prender-
gast 1998), the 1096 Ma Sonju Lake intrusion within the
Duluth Complex of Minnesota (Miller Jr. 1999), the 1108
Ma Coldwell Complex of Ontario (Barrie etal. 2002), the
~2640 Ga Rio Jacare Complex in Brazil (Sa etal. 2005), the
3033 Ma Stella layered intrusion (Maier etal. 2003), the
Jameson intrusion of Western Australia (Maier etal. 2015,
Karykowski etal. 2017), and the Nuasahi intrusion of India
(Prichard etal. 2018). These PGE-enriched horizons share a
number of common features. They have widths of several 10
s of meters (~60 m at Stella, 42–95 m at Rincon del Tigre,
60 m at Skaergaard, 75 m at Koitelainen, at least 25 m at
Sonju Lake, up to 80 m in the Coldwell Complex, ~110 m
in the Rio Jacare Complex, and ~20 m at Birch Lake). Most
show a distinct “offset” pattern, with PGE peaking below
Cu and Au. However, apart from the Stella reefs, only the
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1491Mineralium Deposita (2023) 58:1477–1497
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Skaergaard reefs (~5.5 ppm Pt+Pd in a 1-m-thick layer) and
Birch Lake (7–9 ppm Pt+Pd over ~5 m) approach economic
grade. In contrast, at Rincon del Tigre, peak values are 1.8
ppm Pd over 1 m. At Koitelainen, peak values reach ~1–1.5
ppm over ~4 m. At Rio Jacare, peak values are 1.5 ppm over
1 m. At Skipper Lake in the Coldwell Complex, peak values
are 1.6 ppm PGE over 3.4 m.At Jameson, 1-2 ppm PGE
occur over 1-2m.
Metamorphism
The Stella rocks contain a typical middle greenschist facies
metamorphic assemblage. Based on the strong correla-
tions between SiO2, Al2O3, Fe2O3, and most lithophile ele-
ments, we conclude that the effect of metamorphism has
not changed the concentrations of these elements. Excep-
tions are Cs, Rb, and K which appear to be enriched rela-
tive to less mobile elements, probably when plagioclase was
replaced by sericite, and Na and Ba which appear to have
been partly lost, especially from the magnetite rich rocks.
There is no correlation between K2O or SiO2 and any of
the chalcophile elements, so the metamorphic alteration is
likely not the reason for the decoupling of PGE, Sb and As
with S and Cu.
State ofdifferentiation andmagmatic lineage
oftheStella layered intrusion
Iron–titanium oxide layers are typically found in the upper
portions of layered intrusions which implies that Fe-Ti
oxides crystallize only after the magma has undergone con-
siderable fractionation. In some layered intrusions, oxides
may occur near the base (e.g., Panzihua, Hongge, and
Baima), and in these cases, it has been proposed that the
intruding magma may be relatively evolved and Fe-enriched
(Song etal. 2013).
The crystallization of Fe oxides has been experimentally
investigated by Toplis and Carol (1995) using a fine-grained
dyke from the base of the Skaergaard intrusion as a start-
ing composition (~6.5 wt.% MgO). The exact timing of the
appearance of Fe oxides depends on the oxygen fugacity.
Assuming fO2 at FMQ, some 60% crystal fractionation of
olivine, plagioclase, and clinopyroxene is required before
Fe oxide crystallizes.
The crystallization history of the Stella intrusion has been
modeled using rhyolite MELTS simulations (Smith and Asi-
mow 2005) of fractional crystallization of Archean tholeiite
(6.34 wt.% MgO) from the Obatogamau Formation (Boucher
etal. 2021). The simulations indicate that ~50% fractiona-
tion is needed to bring Fe oxide on the liquidus of the tholei-
ite (Fig.11), but to crystallize minerals of the composition
suggested by the Stella CIPW norms such as the An content
of plagioclase of ~55, the tholeiite likely underwent ~75%
fractionation.
The relatively unfractionated incompatible trace element
patterns (ESM 1h) and essentially flat chondrite-normal-
ized REE (ESM 1g), as well as the mantle-like Th/Y values
(Fig.7j) suggest that the Stella rocks did not undergo sig-
nificant crustal contamination.
Origin oflayering
The major element plots of the Stella rocks suggest two
end-member compositions. The first end-member contains
up to 90% magnetite as well as minor clinopyroxene and
plagioclase (in the ratio of 1 : 9). The second end-member
comprises predominantly silicate minerals, with ~85% pla-
gioclase, ~9% clinopyroxene, and ~6% Fe oxide. This sug-
gests that the studied interval consists essentially of interlay-
ered magnetite-bearing anorthosite and leucogabbros. Such
rocks represent non-cotectic assemblages which could be
explained by a range of petrogenetic models.
Fig. 11 MELTS simulation
of crystallization of Archean
tholeiite (sample 152A1, Oba-
togamau Formation, Boucher
etal. 2021). Pink field denotes
likely crystallization interval
of analyzed Stella rocks. See
text for further explanation, and
ESM 2g-k for the outputs of the
simulation
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1492 Mineralium Deposita (2023) 58:1477–1497
1 3
(i) Accumulation of a broadly cotectic plagio-
clase–clinopyroxene–magnetite crystal mush (in propor-
tions of ~70 : 20 : 10, Fig. 11), followed by granular flow
causing phase sorting and sieving. Analog experiments of
Forien etal. (2015) have simulated granular flow of plagio-
clase–pyroxene–oxide slurries resulting in distinct layers of
oxide, pyroxene, and plagioclase. This model is schematically
shown in Fig.12a. Granular flow would potentially provide
an explanation for the pronounced lateral thickness varia-
tion of layers reported by Andrews (2002) and the preferred
orientation of plagioclase grains (ESM 1c,d). However, none
of the analog experiments generated interlayered oxide-only
and plagioclase-only mushes without interleaved pyroxenite,
as observed at Stella.
(ii) Plagioclase flotation. This model was initially sug-
gested by Bowen (1917) for massif-type anorthosites, by
Vermaak (1976) for Bushveld anorthosites and by Higgins
(2005) and Namur etal. (2011) for the thick anorthosite
under the roof of the Sept-Iles intrusion, Québec. However,
plagioclase flotation cannot readily explain the interlayering
of anorthosite with relatively dense magnetite layers.
(iii) Intrusion of plagioclase supersaturated magma (Laty-
pov etal. 2020) into magnetitite. However, the model liquids
proposed by Latypov etal. (2020) have no match in nature.
(iv) Reactive porous flow. Meurer etal. (1997) have
argued that relatively cool, silica and alkali undersaturated
volatiles ascending through a cumulate pile can trigger
dissolution of pyroxene and sodic plagioclase while sta-
bilizing calcic plagioclase. Maier etal. (2021) used this
model to explain Bushveld anorthosite layers. In the case
of Stella, one could argue that volatiles were trapped in
sulfide-bearing magnetite gabbro beneath a relatively com-
pacted layer of magnetitite (Fig.12b). This could have
triggered transformation of the gabbro to leucogabbro and
anorthosite, while facilitating sinking of dense magnetite
crystals into the volatile-rich silicate mush. If the latter
underwent granular flow, layers of magnetite-anorthosite
as well as magnetitite of laterally highly variable thickness
could have formed, both being locally enriched in PGE.
The model would also offer an explanation for the widely
observed association of oxide layers with anorthosites in
layered intrusions (e.g., Cawthorn and Ashwal 2009).
Origin oftheStella PGE mineralization
The formation of PGE reefs remains debated, likely compris-
ing both syn-magmatic and late-magmatic processes (Naldrett
Fig. 12 Sketch model of pro-
posed formation of Stella layer-
ing. a Granular flow of gabbroic
crystal mushes resulting in
distinct pyroxene-, oxide-, and
plagioclase-rich layers. Arrows
indicate flow direction. b Reac-
tive flow of volatiles trapped
in sulfide-bearing magnetite
gabbro beneath magnetite layers
causes dissolution of pyrox-
ene, resulting in percolation
of magnetite into volatilerich,
sulfide-bearing anorthositic
mush, forming lenses and layers
of oxide-rich rocks
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1493Mineralium Deposita (2023) 58:1477–1497
1 3
2004; Boudreau and McCallum 1992; Smith etal. 2021; Man-
sur etal. 2021). In the following section, we aim to place
constraints on the relative importance of these processes.
PGE reef formation bysyn‑magmatic processes
Sulfur is incompatible into silicate minerals. As a result, crys-
tallization leads to an increase in S content of the magma,
ultimately triggering sulfide melt saturation. For example,
Barnes etal. (2009) have shown that Bushveld B1 magma
reaches sulfide melt saturation after ~15% fractionation,
broadly consistent with estimates of Ripley and Li (2013)
for siliceous high-Mg basalt and picrite (15–25%), explain-
ing the occurrence of most high-grade PGE reefs in the lower
portions of layered intrusions. However, at Stella, high-grade
PGE reefs occur in the upper portion of the intrusion. This
could suggest that initial sulfide melt saturation occurred
relatively late, possibly due to Fe enrichment of the crystal-
lizing magma (Li and Naldrett 1993; Scoates and Mitchell
2000; Ripley and Li 2013). An example is Kiglapait where
sulfide melt saturation occurred at 91% crystallized (Morse
1981). Von Gruenewaldt (1976) proposed that sulfide satu-
ration in evolved magmas may be triggered by magnetite
crystallization resulting in a sharp drop in Fe contents of
the magma, consistent with the observation that many of the
PGE reefs in the upper portions of LI occur in magnetite-rich
intervals. Furthermore, fractional crystallization simulations
of Archean tholeiite indicate that sulfide melt saturation is
attained at ~65% fractional crystallization, just after spinel
has appeared on the liquidus (Fig.11, ESM 2g-k).
Mobilization ofPGE andTABS byhigh‑T fluids
The mineralized interval at Stella shows a number of fea-
tures that could suggest an important role for fluids in petro-
genesis. For example, the silicate minerals are metamor-
phosed. The S/Se ratios are highly erratic and, in much of
the interval, significantly higher than mantle. Pyrite is abun-
dant, locally forming the predominant sulfide. There is a
relatively weak correlation between the PGE and S whereas
the PGE correlate with Sb, suggesting control of PGE by
antimonides. Arsenic and Sb are locally enriched, despite
relatively low D values with respect to sulfide melt.
We consider two main models by which fluids could have
controlled or modified the PGE mineralization: firstly, the
PGE reefs could be of largely secondary origin caused by
introduction of PGE via late magmatic or hydrothermal
fluids originating in weakly mineralized floor cumulates
(Schiffries 1982; Ballhaus and Stumpfl 1986; Boudreau and
McCallum 1992). However, large-scale mobility of Pt and
Pd in fluids at Stella is not consistent with the observed posi-
tive correlation between Pt and Pd with IPGE (ESM 1i), the
latter normally being considered immobile (Holwell etal.
2017), although experimental data are lacking. Secondly,
the Stella reefs could be of largely primary magmatic origin
but were upgraded due to local mobility of S and chalcophile
elements in deuteric fluids, as proposed by Anderson etal.
(1998), Sluzhenikin etal. (2020), Sa etal. (2005), Dijon
andBarnes (2012) and Li and Boudreau (2017) for Skaer-
gaard, Noril’sk, Rio Jacare, and Lac des Iles, respectively.
Mansur etal. (2021) argued that during deuteric or metamor-
phic alteration of magmatic sulfides in the Luanga intrusion,
Brazil, Se, Te, Bi, and Pd were lost together with S, whereas
As and Sb combined with PGE to form PGM. Thus, As and
Sb would have acted as fixing agents for PGE (especially Pt),
along the models of Mountain and Wood (1988) who showed
that the effect of As, Se, and Te is to decrease the mobility
of Pt and Pd, and of Wood andCabri (2002) who suggested
that free arsenide or telluride should trigger the formation
of insoluble PGE phases. Some aspects of this model can be
applied to Stella. The first stage in reef formation involved
sulfide segregation in response to advanced fractionation,
resulting in a ~100-m-thick PGE-enriched gabbro with rela-
tively high PPGE/IPGE ratios and enrichment in Au, Cu, and
TABS (Fig.12b). Late magmatic fluids then redistributed S,
Cu, Te, Bi, and Se from the lower to the upper portion of the
reef interval. The occurrence of S/Se values above primitive
mantle levels in many of the most S and Cu depleted rocks in
the lower part of the PGE mineralized interval suggests that
there were several pulses of fluid flux, with early removal
of S followed by late-stage reprecipitation of S. In contrast,
the PGE and As were relatively immobile, delineating the
original position of the sulfides. Mountain and Wood (1988)
do not provide data for Sb, but if we argue that Pt and As
are not moving significantly then by analogy PdSb has to be
immobile at Stella since it is concentrated in the same rocks.
We acknowledge that there are several observations
which cannot be explained solely by local-scale mobili-
zation of magmatic Pd, Te, Se, and Cu in late magmatic
fluids. For example, Andrews (2002) reported encrusta-
tions of sperrylite around Stella BMS suggesting that Pt
and As were locally mobile (Sullivan etal. 2022), possibly
in domains where the sulfides were completely dissolved
(Holwell etal. 2017). Most significantly, As and Sb contents
are typically decoupled from PGE and the other TABS, as
for example in gabbro sample S1 at the base of the analyzed
interval that is barren in PGE and Te-Se, but enriched in
As, Sb, and Bi (Fig.9). This suggests addition of the latter
metals from the crust, such as sulfide and As-Bi-rich banded
iron formation that was intersected by drill cores at in the
Kalahari Goldridge gold deposit of the Kraaipan greenstone
belt (Hammond and Moore2006). Because Stella shows
little lithophile element evidence for crustal contamination
(ESM 1g,h), As, Sb, and Bi were likely introduced by fluids.
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1494 Mineralium Deposita (2023) 58:1477–1497
1 3
Implications forexploration
Economic PGE reefs are only known from the lower portions
of large layered mafic–ultramafic intrusions (e.g., Bushveld,
Great Dyke, and Stillwater). Although PGE reefs have also
been found in the upper, more differentiated portions of lay-
ered intrusions (e.g., Skaergaard, Rincon del Tigre, Koite-
lainen, Rio Jacare, Coldwell Complex, Nuasahi, Sonju Lake
and Jameson), these are currently all uneconomic. However,
the Stella reef is richer in PGE than these examples.
The formation of Stella-type reefs may be favored by
tholeiitic intrusions that crystallised along a Fenner trend of
differentiation and remained uncontaminated during ascent
and crystallization. We suggest that it may be worthwhile to
reinvestigate many of these intrusions, particularly where
previous analytical programs were widely spaced and/or did
not include PGE.
Stella-type reefs are relatively sulfide-poor, and the peak
mineralization is commonly located below the peak sulfide
occurrence (as also seen at Sonju Lake and Koitelainen).
As a result, the mineralization may be difficult to detect
macroscopically, and detailed sampling and analysis may
be necessary. Barnes (1990) and Maier and Barnes (2005)
have shown that Cu/Pd ratios are a useful tool to pinpoint the
stratigraphic level of PGE reefs in layered intrusions. The
method is based on the relatively more chalcophile behavior,
and thus efficient scavenging by sulfide melt, of Pd than Cu,
resulting in sharply elevated Cu/Pd ratios above PGE reefs.
The lessons from Stella are that high grade PGE reefs
may occur in the upper portions of intrusions if initial sulfide
saturation is delayed. This may possibly be facilitated in
magmas that evolve along a trend of strong Fe enrichment,
such as high-Al tholeiites (Scoates and Mitchell 2000; Rip-
ley and Li 2013) and Archean Fe-rich tholeiites. However,
whether any PGE reef in the upper portions of intrusions
will be economic remains to be seen. A challenge for many
of the known examples could be that they may be relatively
strongly affected by S and metal mobility, due to the rela-
tively high volatile content of evolved magma.
Conclusions
The Stella intrusion hosts one of the richest PGE miner-
alizations known on Earth. The mineralization is unusual
in that it is located in the upper portion of the intrusion,
consisting of interlayered magnetitite and anorthosite–leu-
cogabbro. Much of the interval is relatively sulfide poor
(< 500 ppm S) but contains abundant PGM, mainly anti-
monides and arsenides, which are typically hosted by
silicates rather than sulfides or oxides. The sulfides com-
prise approximately equal proportions of chalcopyrite and
pyrite.
We propose that the reefs formed through a complex
sequence of events. The proto cumulate assemblage con-
sisted of magnetite leucogabbro hosting semi-massive
magnetite layers and magmatic sulfides enriched in PGE,
Cu, and TABS. The sulfides likely formed in response to
Fe depletion of the magma following the appearance of
magnetite on the liquidus. Ascending deuteric and met-
amorphic country rock fluids, the latter enriched in As,
Sb, and Bi were trapped beneath relatively impermeable
magnetite cumulate layers. The fluids mobilized S, Cu,
Au, Se, Bi, and Te from the lower portion of the mineral-
ized interval, whereas PGE were stabilized by As and Sb
to form PGM.
The relative paucity of S may render Stella-type reefs
difficult to identify megascopically. Furthermore, the upper
magnetite-rich portions of layered intrusions have, in the
past, been poorly studied with regard to possible PGE miner-
alization. Therefore, important reef-type PGE mineralization
may have been overlooked.
Supplementary Information The online version contains supplemen-
tary material available at https:// doi. org/ 10. 1007/ s00126- 023- 01189-5.
Acknowledgments Harmony Gold Mining Company Limited permit-
ted access to their properties and provided the samples and invaluable
logistic support. The authors thank Tony Oldroyd for making the thin
sections and Duncan Muir for assisting in the processing of the ele-
ment maps.
Author contributions WDM conceived the project and collected the
samples. SJB conducted the analyses and provided input to discussions
and data interpretations. WDS provided input to discussions and data
interpretation.
Funding University of Pretoria provided a generous funding. Addi-
tional funding was received from NSERC (to SJB).The samples were
collected in 2002 while WDM was working at the University of Preto-
ria who provided generous funding.
Declarations
Competing interest There are no competing interests.
Open Access This article is licensed under a Creative Commons Attri-
bution 4.0 International License, which permits use, sharing, adapta-
tion, distribution and reproduction in any medium or format, as long
as you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons licence, and indicate if changes
were made. The images or other third party material in this article are
included in the article's Creative Commons licence, unless indicated
otherwise in a credit line to the material. If material is not included in
the article's Creative Commons licence and your intended use is not
permitted by statutory regulation or exceeds the permitted use, you will
need to obtain permission directly from the copyright holder. To view a
copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.
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1495Mineralium Deposita (2023) 58:1477–1497
1 3
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