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Recent advances in W-Sn and Rare Metal deposit metallogenesis.
Oral Communication Abstract
Controls on distribution of Sn-W and rare metals (Li, Be, In, Nb, Ta) in the
peraluminous, post-collisional granites of the Cornubian Batholith, SW England.
B. Simons1*, J.C.Ø. Andersen1, R.K. Shail1 and F.E. Jenner2
1Camborne School of Mines, University of Exeter, Penryn Campus, Treliver Road, Penryn, Cornwall, TR10 9FE.
2Open University, Department of Environment, Earth & Ecosystems, Milton Keynes, MK7 6AA.
*corresponding author: B.Simons@exeter.ac.uk
The Early Permian Variscan Cornubian Batholith was generated during an extensional regime
following Variscan convergence, within the Rhenohercynian Zone (RHZ) of SW England [1]. It is a
peraluminous (A/CNK> 1.1) composite pluton intruded into Devonian and Carboniferous
metamorphosed sedimentary and volcanic rocks. The component granites can be classified, using
mineralogical, textural and geochemical criteria, into five main types: G1 (two mica), G2 (muscovite),
G3 (biotite), G4 (tourmaline) and G5 (topaz). The granites were sourced by dehydration melting of a
metagreywacke source. Muscovite and minor biotite breakdown during ~25% partial melting at
moderate temperatures and pressures formed the G1 granites. Higher degrees (~30%) of biotite-
dominated partial melting at higher temperatures and lower pressures are required to form the G3
granites. Partial melting was strongly influenced by the progressive lower-mid crustal emplacement of
mafic igneous rocks during post-Variscan extension and a minor (5%-10%) mantle-derived
component in the G3 granites is possible. G1-G2 and G3-G4 are linked through fractionation of an
assemblage dominated by feldspars and biotite, although G4 show some deflections of trace element
plots likely due to fluid-rock interactions. G5 formed through flux-induced melting of biotite-rich restite
in the lower crust [2,3].
Overall, Li, Be, Nb, Ta, In, Sn, W and Bi are enriched in the Cornubian Batholith relative to average
crustal abundances. Up to 20% fractionation from G1 granites produces G2 granites which are
enriched in Li (average 315 ppm), Be (12 ppm), Ta (4.4 ppm), In (74 ppb), Sn (18 ppm) and W (12
ppm). Gallium (24 ppm), Nb (16 ppm) and Bi (0.46 ppm) increase with fractionation but are not overall
significantly enriched, while Sb (0.16 ppm) is depleted relative to the average upper and lower crust.
Muscovite, a late-stage magmatic/subsolidus mineral, is the major host of Li, Nb, In, Sn and W in G2
granites. G2 granites are spatially associated with W-Sn greisen mineralisation across the region.
Fractionation within the younger G3-G4 granite system resulted in the enriched Li (average 364 ppm),
Ga (28 ppm), In (80 ppb), Sn (14 ppm), Nb (27 ppm), Ta (4.6 ppm), W (6.3 ppm) and Bi (0.61 ppm) in
the majority of the G4 granites. Depletion of Be in G4 granite is attributed to partitioning of Be into
cordierite during fractionation. The distribution of Nb and Ta is controlled by accessory phases (e.g.,
rutile) within the G4 granites, facilitated by high F and leading to disseminated Nb and Ta
mineralisation. Lithium, In, Sn and W are hosted in biotite group micas, which may prove favourable
for breakdown on ingress of hydrothermal fluids with metals then available for partitioning into
mineralising fluids. Cínovec
Topaz (G5) granites are enriched in Li (average 1363 ppm), Ga (38 ppm), Sn (21 ppm), W (24 ppm),
Nb (52 ppm) and Ta (15 ppm). Within G5 granites, the metals partition into accessory minerals such
as rutile, columbite-tantalite and cassiterite, forming disseminated magmatic mineralisation. High
observed concentrations of Li, In, Sn, W, Nb and Ta in G4-G5 granites are likely facilitated by high F,
which are preferentially carried in the melt until fluid exsolution or crystallisation of F-bearing minerals
(e.g. [5]).
[1] Shail & Leveridge, 2009. Comptes Rendus Geoscience.
[2] Darbyshire & Shepherd, 1994, Journal of the Geological Society
[3] Simons et al., 2016, Lithos
[4] Simons et al., submitted.
[5] Linnen, 1998, Economic Geology.