The role of granites in volcanic-hosted massive sulphide ore-forming systems: an assessment of magmatic–hydrothermal contributions * David L. Huston & Jorge M. R. S. Relvas & J. Bruce Gemmell & Susan Drieberg

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

David L. Huston & Jorge M. R. S. Relvas &

J. Bruce Gemmell & Susan Drieberg

Fig. 5 Geology of the Iberian Pyrite Belt showing the location of deposits discussed in the text

Assessment of geological, geochemical and isotopic 

data indicates that a significant subgroup of volcanichosted  

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 CambrianMount  

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 magmatichydrothermal  

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 lacksm  

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‰).

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