Abstract
The Early Permian Variscan Cornubian Batholith is a peraluminous, composite pluton intruded into Devonian and Carboniferous metamorphosed sedimentary and volcanic rocks. Within the batholith there are: G1 (two-mica), G2 (muscovite), G3 (biotite), G4 (tourmaline) and G5 (topaz) granites. G1–G2 and G3–G4 are derived from greywacke sources and linked through fractionation of assemblages dominated by feldspars and biotite, with minor mantle involvement in G3. G5 formed though flux-induced biotite-dominate melting in the lower crust during granulite facies metamorphism. Fractionation enriched G2 granites in Li (average 315ppm), Be (12ppm), Ta (4.4ppm), In (74ppb), Sn (18ppm) and W (12ppm) relative to crustal abundances and G1 granites. Gallium (24ppm), Nb (16ppm) and Bi (0.46ppm) are not significantly enriched during fractionation, implying they are more compatible in the fractionating assemblage. Sb (0.16ppm) is depleted in G1-G2 relative to the average upper and lower continental 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.Fractionation within the younger G3–G4 granite system enriched Li (average 364ppm), Ga (28ppm), In (80ppb), Sn (14ppm), Nb (27ppm), Ta (4.6ppm), W (6.3ppm) and Bi (0.61ppm) in the G4 granites with retention of Be in G3 granites due to partitioning of Be into cordierite during fractionation. The distribution of Nb and Ta is controlled by accessory phases such as rutile within the G4 granites, facilitated by high F and lowering the melt temperature, leading to disseminated Nb and Ta mineralisation. Lithium, In, Sn and W are hosted in biotite micas which may prove favourable for breakdown on ingress of hydrothermal fluids. Higher degrees of scattering on trace element plots may be attributable to fluid–rock interactions or variability within the magma chamber. The G3-G4 system is more boron-rich, evidenced by a higher modal abundance of tourmaline. In this system, there is a stronger increase of Sn compared to G1-G2 granites, implying Sn in tourmaline-dominated mineral lodes may represent exsolution from G4 granites.G1–G4 granite abundances can be accounted for by 20–30% partial melting and 10–40% fractionation of a greywacke source. G5 granites are analogues of Rare Metal Granites described in France and Germany. These granites are enriched in Li (average 1363ppm), Ga (38ppm), Sn (21ppm), W (24ppm), Nb (52ppm) and Ta (15ppm). 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 and G5 granites are likely facilitated by high F, Li and P, which lower melt temperature and promote retention of these elements in the melt, prior to crystallisation of disseminated magmatic mineralisation.
Highlights
Growth in the low carbon and consumer electronics sectors has led to a rise in demand for metals that are not typical historical exploration and mining targets
It is well established that the granites of the Cornubian Batholith are peraluminous, with A/CNK [(Al2O3/(CaO + N2O + K2O)] values of N 1.2 and average SiO2 contents N 70 wt.% (e.g. Darbyshire and Shepherd, 1985; Charoy, 1986; Manning et al, 1996; Chappell and Hine, 2006; Müller et al, 2006)
The Cornubian granites are enriched in elements such as As, B, Cl, F, Pb, Rb, U and Zn relative to peraluminous granites elsewhere in the world (Willis-Richards and Jackson, 1989)
Summary
Growth in the low carbon and consumer electronics sectors has led to a rise in demand for metals that are not typical historical exploration and mining targets. Peraluminous granites are a major global source of magmatic and magmatic-hydrothermal ore deposits, for elements often described as lithophile during the petrogenesis of granitic melts (Li, Nb, Ta, Be, Sn and W) Černŷ et al, 2005; Sial et al, 2011) Factors such as source composition (Romer and Kroner, 2014), degree of fractionation (Lehmann, 1987; Taylor and Wall, 1992) and hydrothermal overprinting (Štemprok, 1987; Haapala, 1997) control enrichment of these lithophile metals within granites. A prerequisite for understanding the distribution of rare metals in magmatic-hydrothermal systems is an understanding of their behaviour in precursor magmas prior to, and during, the exsolution of magmatic-hydrothermal fluids
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