The role of granites in volcanic-hosted massive sulphide ore-forming systems: an assessment of magmatic–hydrothermal contributions

Abstract

Assessment of geological, geochemical and isotopic data indicates that a significant subgroup of volcanic-hosted massive sulphide (VHMS) deposits has a major or dominant magmatic–hydrothermal source of ore fluids and metals. This group, which is typically characterised by high Cu and Au grades, includes deposits such as those in the Neoarchean Doyon-Bousquet-LaRonde and Cambrian Mount Lyell districts. These deposits are distinguished by aluminous advanced argillic alteration assemblages or metamorphosed equivalents intimately associated with ore zones. In many of these deposits, δ34Ssulphide is low, with a major population below −3‰; δ34Ssulphate differs from coexisting seawater and Δ34Ssulphate–sulphide ∼ 20–30‰. These characteristics are interpreted as the consequence of disproportionation of magmatic SO2 as magmatic–hydrothermal fluids ascended and cooled and as a definitive evidence for a significant magmatic–hydrothermal contribution. Other characteristics that we consider diagnostic of significant magmatic–hydrothermal input into VHMS ore fluids include uniformly high (>3 times modern seawater values) salinities or very 18O-enriched (δ18O > 5‰) ore fluids. We do not consider other criteria [e.g. variable salinity, moderately high δ18Ofluid (2–5‰), δ34Ssulphide near 0‰, metal assemblages or a spatial association with porphyry Cu or other clearly magmatic-hydrothermal deposits] that have been used previously to advocate significant magmatic–hydrothermal contributions to be diagnostic as they can be produced by non-magmatic processes known to occur in VHMS mineral systems. However, in general, a small magmatic–hydrothermal contribution cannot be excluded in most VHMS systems considered. Conclusive data that imply minimal magmatic–hydrothermal contributions are only available in the Paleoarchean Panorama district where coeval seawater-dominated and magmatic–hydrothermal systems appear to have been physically separated. This district, which is characterised by chloritic and sericitic alteration assemblages and lacks aluminous advanced argillic alteration assemblages, is typical of many VHMS deposits around the world, suggesting that for “garden variety” VHMS deposits, a significant magmatic–hydrothermal contribution is not required. Other than deposits associated with advanced argillic alteration assemblages, the only deposit for which we ascribe a major magmatic–hydrothermal contribution is the Devonian Neves Corvo deposit. This deposit differs from other deposits in the Iberian Pyrite Belt and around the world in being extremely Sn-rich, with the Sn closely associated with Cu and in having formed from high 18O-rich fluids (δ18Ofluid ∼8.5‰). We consider these characteristics, particularly the last, as diagnostic of a significant magmatic hydrothermal contribution. Our analysis indicates that two subgroups of VHMS deposits have a major magmatic–hydrothermal contribution: Cu/Au-rich deposits with aluminous alteration assemblages and reduced, very Sn-rich deposits in which Sn was introduced in a high-temperature ore assemblage. Comparison with “normal” VHMS deposits suggests that these subgroups of VHMS deposits may form in specialised tectonic environments. The Cu/Au-rich deposits appear to form adjacent to magmatic arcs, an environment conducive to the generation of hydrous, oxidised melts by melting metasomatised mantle in the wedge above the subducting slab. This contrasts with the back-arc setting of “normal” VHMS deposits in which relatively dry granites (In this contribution, we use the term granite sensu latto) formed by decompression melting drive seawater-dominated hydrothermal circulation. The tectonic setting of highly Sn-rich VHMS deposits such as Neves Corvo is less clear; however, thick continental crust below the ore-hosting basin may be critical, as it is in other Sn deposits.