Constraints on the volume of sulfide melt generated through metamorphism of ore deposits

Abstract

Phase relations between mineral assemblages in common sulfide ore deposit types are examined to place constraints on the amount of polymetallic melt produced within the limits of crustal metamorphism. Most sulfide deposit types have the potential to undergo minor melting at moderate temperatures, provided that sulfosalts or tellurides are present. Deposits that contain these phases may start to melt at conditions ranging from lowest greenschist facies to amphibolite facies, whereas those lacking sulfosalts and/or tellurides may only melt once P-T conditions are well into the granulite facies, particularly if galena is also absent. Epithermal gold deposits would melt at the lowest temperatures of all sulfide deposit types. Massive Pb-Zn(-Cu) deposits may start to melt in the lower to middle amphibolite facies, but only if pyrite and arsenopyrite coexist at these conditions. Whereas massive Ni-Cu-PGE deposits can typically generate only trace quantities of melt. Similarly, disseminated Cu deposits, excepting sulfosalt-bearing occurrences, are incapable of generating melt until the granulite facies when melting commences in bornite-rich regions, in the system Cu-Fe-S. Even at granulite facies conditions, the amount of polymetallic melt that can be generated in most deposit types is <1 vol.%. The exception is massive Pb-Zn(-Cu) deposits, where melt volumes significantly exceeding 1 vol.% may be segregated into sulfide magma dykes, allowing mobilization over large distances.