The fate of sulfur in sub-arc ores

Date & time

1–2pm 8 February 2018


Jaeger 1 Seminar Room, RSES


John Mavrogenes (RSES)

Event series


 Adele Morrison

Volcanic arcs, where continental crust is built, are also the breeding grounds for the biggest ore deposits. Despite their immense metal budgets, arcs stand out mostly as sulfur anomalies.  More sulfur is erupted from arcs than is thought possible, yet even more remains in the crust. How arcs become so sulfur rich and how sulfur travels though the crust is unknown. Arc magmas carry sulfur as dissolved sulfide and/or sulfate in melts with excess as sulfide melt or anhydrite crystals. Once exsolved, S travels as gas, vapor, liquid (brine) or melt, all of which are found in sub-volcanic ores. Veins of quenched sulfosalt melt are present in structural feeder zones to “high-sulfidation” epithermal deposits while chalcopyrite-anhydrite veins are ubiquitous in the deeper porphyry environment. Thus, at least two sulfur valences (S2- and S6+) are found in these environments. Furthermore, local reduction of sulfur at or near the depositional site provides anhydrite plus sulfides in porphyry vein systems. Thus, both copper and sulfur can be transported until wall-rock reactions trigger sulfide/sulfate deposition in veins. These veins often contain the bulk of the copper extracted from porphyry deposits. Sulfur isotopes can be used to track temperatures between coexisting sulfide sulfate pairs. However, temperatures provided by sulfur isotopes are largely ignored due to “re-equilibration”. The term re-equilibration loosely covers diffusion, dissolution/re-precipitation and other more nebulous processes. Diffusive re-equilibration will be discussed in light of recent SHRIMP-SI analyses of such pairs.

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