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Indium fractionation in the granites of SW England


Abstract and Figures

Biotite granites in SW England are enriched in Ge, In, and Sn whereas the topaz granites are enriched in Li, Ga, Nb, Ta, Sb and W. Within biotite granites the micas (biotite and muscovite) are the primary carriers of critical metals. Analysis of metasediment samples from the Porthscatho formation, an analogue for the source of the granites, and subsequent trace element modelling indicates that a mantle contribution to the biotite granites is not required for the critical metals. A model with 30% partial melting of metasedimentary rocks and 10 to 50% subsequent fractional crystallisation is sufficient for the biotite granites to reach the observed indium concentrations. Topaz granites require a more complex model.
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Indium fractionation in the granites of SW England
B. Simons*, J.C.Ø. Andersen & R.K. Shail
Camborne School of Mines, University of Exeter, Tremough Campus, Penryn, TR10 9EZ.
*corresponding author:
Abstract. Biotite granites in SW England are enriched in
Ge, In, and Sn whereas the topaz granites are enriched
in Li, Ga, Nb, Ta, Sb and W. Within biotite granites the
micas (biotite and muscovite) are the primary carriers of
critical metals.
Analysis of metasediment samples from the Porthscatho
formation, an analogue for the source of the granites, and
subsequent trace element modelling indicates that a
mantle contribution to the biotite granites is not required
for the critical metals. A model with 30% partial melting of
metasedimentary rocks and 10 to 50% subsequent
fractional crystallisation is sufficient for the biotite granites
to reach the observed indium concentrations. Topaz
granites require a more complex model.
Keywords: Granite. Trace element modelling. Indium.
1 Introduction
1.1 Project rationale
The polymetallic mining district of SW England has
been exploited for a variety of metals including tin (Sn),
copper (Cu), zinc (Zn) and lead (Pb) since the Bronze
Age with the last mine closing in 1998. Now, with the
increasing need for speciality metals for new
technological developments (in the following called
critical metals) and increases in the price of traditional
commodities such as Sn and Cu, the area is once again
prospective. A combination of the traditional
commodities (Sn, Cu) alongside critical metals (In, W)
are key for current development at Hemerdon (Wolf
Minerals) and exploration at South Crofty (Western
United Mines), St. Columb (Treliver Minerals Ltd.) and
Redmoor (New Age Exploration Ltd.). Despite the rich
history of geological research in the region and current
activities, there has been little research into the
distribution of critical metals in the region.
There have been limited studies on the partitioning of
the critical metals into the common silicate minerals.
Sb, In and W in particular are elements that are poorly
represented in partitioning studies, and there is very
little information in the literature on how they become
concentrated in the crust. This paper presents data for
the abundance of critical metals in biotite and topaz
granites of SW England and the major minerals in
biotite granite and discusses a possible source of indium
for the biotite granites.
1.2 Methods
A total of 70 samples have been collected to encompass
the various granite types and other significant rock types
present in SW England (e.g. lamprophyres,
metasedimentary rocks). These samples have had the
weathered edges removed, been crushed and ground
using an agate barrel. For whole rock analysis, samples
were analysed by an Agilent 7700 ICP-MS using the 4-
acid digestion technique of Garbe-Schönberg (1993).
Sample duplicates and standards were analysed with
each sample batch. For metal distribution studies,
individual minerals were handpicked from crushed
material, washed and powdered using a mortar and
pestle before 4-acid digestion and analysis by ICP-MS.
2 Study Area
2.1 Location and regional geology
The polymetallic mining district of SW England extends
through the county of Cornwall into West Devon (Figure
1). The Early Permian Cornubian Batholith is exposed
as a series of major plutons including, from west to east,
the Isles of Scilly, Land’s End, Tregonning-Godolphin,
Carnmenellis, St. Austell, Bodmin Moor and Dartmoor
granites. The batholith was emplaced into Devonian and
Lower Carboniferous sedimentary and volcanic rocks
that underwent low-grade regional metamorphism and
deformation during the Variscan Orogeny. Magmatic
activity spanned approximately 25 Ma with the earliest
granite pluton dated at ~293 Ma and the youngest at 268
Ma (e.g., Chen et al. 1993; Chesley et al. 1993). Coeval
basaltic magmatism is represented by basaltic lavas and
lamprophyre dykes.
The region had several stages of mineralisation but
the most widespread synbatholith polymetallic “lode”
systems formed during variable mixing of S-bearing
magmatic, meteoric and metamorphic fluids (Jackson et
al. 1989). Establishing the distribution of critical metals
in the granites themselves will aid understanding of
where these metals are likely to be concentrated within
the mineralisation systems.
Figure 1. Map showing the study area summarising different granite types. Major plutons are marked: DT Dartmoor, BD
Bodmin, SA St. Austell, CN Carnmenellis, TG Tregonning-Godolphin, LE Lands End and IOS – Isles of Scilly. Map
modified after Manning et al. (1996) and Dangerfield & Hawkes (1981).
2.2 Granite characteristics
The granites are peraluminous with A/CNK values of
>1.1, relatively low Na2O (<3.2%) and restricted range
of SiO2 compositions. The batholith is enriched in As,
B, F, Li, P, Sn and Zn amongst other elements, and
contains ilmenite as the main ferrous phase (Chappell &
Hine, 2006). Major element data from this study are in
agreement with previous studies.
The granites can be subdivided by mineralogy into
biotite, topaz and tourmaline granites with biotite
granites accounting for over 90% of the upper parts of
the batholith. These mineralogical classifications can be
further refined texturally by variations in groundmass
grain-size and varying abundance / size of alkali feldspar
phenocrysts. The main granite types are accompanied by
later stage aplites, quartz porphyry dykes and quartz-
tourmaline rocks.
Figure 2. Chondrite normalised REE diagram showing the
differences between the biotite and topaz granites. Chondrite
values are from McDonough & Sun (1995).
The source of the granites has long been a subject of
debate with various authors arguing for and against a
mantle contribution. REE plots for biotite granite are
consistent with a crustal source, showing strongly
enriched LREE whereas the plots for topaz granites are
largely flat with lower total REE. A negative Eu
anomaly occurs in both granite types (Figure 2).
3. Critical metal distribution and source
3.1 Whole rock geochemistry
Whole rock samples were analysed by ICP-MS for Li,
Be, Ga, Ge, Nb, In, Sn, Sb, Ta, W and Bi. The results
(Table 1) define two distinct mineral groups that are
significant for different granite types. Overall Be, Ge,
In and Sn are more strongly enriched within the biotite
granites, while Li, Ga, Nb, Sb, Ta, W and Bi are more
enriched in the topaz granites.
Table 1. Table summarising the average abundances (ppm) of
critical metals in SW England biotite and topaz granites
compared to the average continental crustal abundances (Taylor
& McLennan, 1985).
45 km SW
La Ce Pr Nd Sm Eu Gd Dy Ho Er Tm Yb Lu
Sample / Chondrite
Biotite Granite Topaz Granite
3.2 Indium fractionation and source
Trace element modelling can aid in the determination of
the possible source for critical metals within the biotite
granites. Four samples of the Porthscatho Formation
(Darbyshire & Shepherd, 1994), a metasedimentary rock
into which the granites intruded, have also been analysed
for a selection of metals using ICP-MS. These samples
have been used as a bulk compositional analogue for a
lower crustal granite source. Although derived from the
upper plate during the final stages of Variscan
convergence, their peri-Gondwanan Mid-Proterozoic
basement source is likely to be similar to SW England
lower crust and both these samples and granites have
similar TDM model ages (Shail & Leveridge, 2009).
Trace element modelling followed the method by
Williamson et al. (2010) with equations for batch
melting and Rayleigh fractionation. For the purpose of
this exercise, all metals have been assumed to be
completely incompatible with partition coefficients of 0.
Consistent with Williamson et al. (2010), 30% and 50%
partial melting with 10% and 50% fractionation have
been modelled for each metal.
For In, combining 30% partial melting of Portscatho
Formation sandstones with 10% and 50% crystal
fractionation is sufficient to produce a range of In
concentrations that account for the majority of
concentrations of In in the granites (Figure 3).
Figure 3. Modelled In concentrations expected in granites for
different degrees of partial melting (M) and crystal
fractionation (F). In contents of all biotite granites in this
study (n=43) are also shown along with the average Portscatho
Formation (PSF) In content (n=4).
Using a source value of 0.41 ppm In, the average
value obtained in the analysis of the Portscatho
Formation, partial melting of 30% and fractionation of
<10% up to 50% is sufficient to obtain the range of In
values observed in the granites. This implies that there
is no need for a mantle contribution of material, at least
in the biotite granites, as the source can provide enough
material to account for the granite values. The same
model also accounts for the range of Be, Ga, Ge, Nb, Sn,
Sb, Ta and Bi within the biotite granites.
The topaz granites cannot be modelled using the
above method and it is believed that there are different
controls on the enrichment of these metals in the
granites. With the flat REE profile for these granites, as
shown above, it may be possible there is a mantle
component process in these granites or there could be
another fractionation process that is not yet understood.
3.3 Metal concentrations in silicate minerals
The major minerals found in biotite granite have been
analysed for their critical metal content. Abundances are
summarised in Table 2.
Table 2. Table summarising metal abundances (ppm) in major
minerals of biotite granite. Qtz = Quartz; Kfs = Alkali
Feldspar; Pl = Plagioclase; Bt = Biotite; Ms = Muscovite.
The micas appear to be the major carriers of critical
metals. This can be explained by their similar ionic
charge and radius to other elements that are major
constituents in micas. For example, the ionic radius of
Mg2+ (0.86Å) is closely matched by Ge2+ (0.87Å), while
In3+ (0.94Å) is only moderately larger. Substitution of
the critical metals into the mica lattice appears to be a
likely mineralogical control on the metal fractionation
within the biotite granites.
4. Conclusions
Several critical metals are enriched relative to average
continental crust, in the biotite and topaz granites of SW
England. The metals can be broadly subdivided into two
groups that help to outline the potentially fertile granite
stocks that would be prospective for particular metals.
Indium deposits would be expected (along with Ge and
Sn) to be preferentially associated with the biotite
granites, while Li, Ga, Nb, Sb, Ta and W would be
expected to be associated with the topaz granites.
The source of the indium enrichment in biotite
granites can be explained by partial melting of a
metasedimentary host rock (similar to the Portscatho
Formation) without any mantle involvement. The limited
mantle contribution to the biotite granites is supported by
isotope data (Darbyshire & Shepherd, 1985) as well as
REE patterns. There are other controls on concentration
of metals in the topaz granites.
The micas (biotite and muscovite) are the likely
major hosts for many of the critical metals. This is in
agreement with their ionic radii and charges, which make
them compatible with other metals usually contained
within these minerals.
0 0.02 0.04 0.06 0.08
Modelled In in granite (ppm)
In in source (ppm)
Biotite granite
M=0.3, F=0.1
M=0.3, F=0.5
Average PSF
Analytical support from Sharon Uren and Steve Pendray
of Camborne School of Mines, Cornwall is gratefully
acknowledged. This study is part of a PhD project
funded by the European Social Fund.
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Geological mapping of existing and redundant kaolin workings within the St Austell Granite has identified a suite of granitic rocks which show evidence of complex late-stage magmatic and hydrothermal processes. Coarse porphyritic biotite granites, like those which predominate in southwest England, occur much more widely than previously acknowledged, and are intruded by an apparently cogenetic suite of lithium-mica granites and tourmaline granites. The tourmaline granites characteristically exhibit very variable textures, with coarse quartz grains set within a fine grained, tourmaline-rich matrix. A highly evolved fine-grained tourmaline granite represents the most evolved of this suite. Topaz granites intrude the earlier granite varieties, and all are intruded by rhyolite porphyry dykes (elvans). Major and trace element chemical data suggest that the biotite granite-lithium-mica granite-tourmaline granite suite represents the product of crystallization of a granitic magma within which B (but not F) became progressively enriched until water saturation was achieved. Water exsolution effectively quenched any remaining granitic melt, resulting in the very variable textures shown by the tourmaline granites. The topaz granites are chemically distinct from their predecessors, showing marked enrichment in F, Li and P2O5 (but not B). Instead of being products of differentiation of biotite granite magma, the topaz granite melts may have been derived separately in a later episode of partial melting of the same source. Kaolinization is widespread throughout the western part of the St Austell Granite, and deposits worked at present tend to be located in granite varieties other than biotite granite. The geochemical parameters used to distinguish the primary granite types (particularly Nb v. Zr and Ga-Nb-Zr plots) are sufficiently robust to permit the parent granitic rock type to be identified for heavily kaolinized material.
Concordant U-Pb dates of magmatic monazites in the dominant coarse-grained biotite (± muscovite) granites of the late Variscan Cornubian batholith demonstrate that the emplacement of the six plutons was diachronous, ranging from 293.1 ± 1.3 Ma (Carnmenellis) to 274.5 ± 1.4 Ma (Land's End, southern lobe). 40Ar/ 39Ar age spectra for magmatic muscovites confirm that cooling to about 320°C at the present level of exposure was accomplished within 4 to 5 Ma in each intrusion. Moreover, age spectra for hydrothermal muscovites show that development of the major Sn (-Cu) lodes of the South Crofty mine commenced before 286 Ma, only 2 to 3 Ma after cooling of the hosting Carn Brea granite, a satellite of the Carnmenellis pluton. The older plutons had therefore approached ambient temperatures, and had even experienced intense hydrothermal activity, before emplacement of the youngest. -Authors
Combined Nd and Sr isotope data for the Hercynian granites of SW England provide important new evidence for the origin of the Cornubian batholith. The Nd isotope systematics are shown to be robust even in areas of extensive hydrothermal alteration. epsilon(Nd) values for the main plutons range from -4.7 to -7.1 whereas values for the Palaeozoic country rocks are significantly lowe;, typically -8 to -11. Depleted mantle Nd model ages (1.3-1.8 Ga) indicate a heterogeneous source region for the granite magmas and imply that the age of the basement beneath southern England is significantly older than previously estimated. Calculated epsilon(Nd) values for the Palaeozoic country rocks at the time of granite intrusion suggest that they were unlikely to have been the main source material for the magmas. epsilon(Nd)-epsilon(Sr) data for the granites do not define a simple binary mixing array between a mafic, depleted mantle melt and Proterozoic crust. The best fit model is consistent with partial melting of a composite lower crustal source comprising immature metasedimentary and mafic meta-volcanic rocks, and mixing with a relatively minor contribution of basaltic magma extracted from a slightly enriched mantle source. Such hybrid magmas are thought to have been generated during an Ivrea-type underplating of the lower crust during continental collision; their isotope signatures being determined by the relative proportions of crust and mantle material.
The Cornubian ore field of southwest England is spatially associated with the roof of an S-type, ilmenite series, high heat production monzogranite batholith hosted by a deformed and lightly metamorphosed, upper Paleozoic marine mudstone-sandstone sequence. Three metallogenic stages can be recognized within the ore field: a prebatholith stage (300-400 Ma), when minor strata-bound Fe-Mn oxy-hydroxide and Fe-Cu sulfide synsedimentary deposits formed; a synbatholith stage (270-300 Ma), or main-stage event, when hydrothermal deposits of Sn Cu-As-Fe-Zn-Pb were formed; and a postbatholith stage (Mesozoic-Cainozoic), when epithermal vein deposits of Pb, Ag, Sb, Ba, Zn, Fe, U, Co, Ni, and Au, and hydrothermal-supergene kaolinite deposits, formed. These stages are discussed. Contemporary hot spring activity atests to the continuing dynamism of the environment and highlights the superimposed complexity of hydrothermal systems associated with high heat production granite batholiths. -from Authors
The Cornubian Batholith comprises six major and several smaller bodies of S-type granite in southwestern England. These late-Variscan granites comprise two-mica granites, and much less abundant Li-mica granites that are restricted to one of the major bodies (St Austell) and smaller bodies. Some of these intrusive rocks are associated with major Sn mineralization. This paper is concerned with the geochemistry of the two-mica granites, which are felsic, strongly peraluminous, and have a high total alkali content and low Na:K. Rocks with very similar compositions to these granites occur elsewhere, including the Variscan granites of continental Europe, and in southeastern Australia. In detail all of the major plutons of this batholith have distinctive compositions, except for Bodmin Moor and Carnmenellis which cannot be discriminated from each other compositionally. A comparison with experimental data shows that the granites attained their major element composition under conditions of crystal-liquid equilibrium, with the final melt being saturated in H2O, at temperatures close to 770d̀C and pressures about 50 MPa. That temperature estimate is in good agreement with values obtained from zircon saturation thermometry. The specific minimum-temperature composition excludes the possibility of widespread transfer of elements during hydrothermal alteration. Minor elements that are relatively very abundant are Li, B, Cs and U, while F, Ga, Ge, Rb, Sn, Ta, W and Tl are quite abundant and P is high for felsic rocks. Sr, Ba, and the trace transition metals Sc to Zn, are low, but not as low as they commonly are in very felsic granites. These trace element abundances, and the EL2O-saturation, resulted from the fractional crystallization of a melt derived by the partial melting of feldspathic greywackes in the crust. The Cornubian granites have compositions very similar to the more felsic rocks of the Koetong Suite of southeastern Australia, where a full range of granites formed at the various stages of magmatic fractionation postulated for the Cornubian granites, can be observed. The operation of fractional crystallization in the Cornubian granites is confirmed by the high P abundances in the feldspars, with P contents of the plagioclase crystals correlating with Ab-con-tent Most of the granites represent solidified melt compositions but within the Dartmoor pluton there is a significant component of granites that are cumulative, shown by their higher Ca contents. The Cornubian plutons define areas of high heat flow, of a magnitude which requires that fractionated magmas were transported laterally from their sources and concentrated in the exposed plutons. The generation of these granite plutons therefore involved magmatic fractionation during the stages of partial melting, removal of unmelted material from that melt, and fractional crystallization. During the later stages of those processes, movement of those magmas occurred on a crustal scale.