On the Initiation of Metamorphic Sulfide Anatexis

Mineral assemblages in common sulfide ore deposits are examined together with phase relations to (1) investigate the pressure-temperature conditions required for the onset of metamorphically induced partial melting involving economic minerals, and (2) place constraints on the amount of melt produced. Deposits that contain sulfosalt or telluride minerals may start to melt at conditions ranging from lowest greenschist facies to amphibolite facies. Deposits lacking sulfosalt and/or telluride minerals may begin to melt once P-T conditions reach the upper amphibolite facies, if galena is present, or well into the granulite facies if galena is absent. The result is two broad melting domains: a low- to medium-temperature, low melt volume domain involving melting of volumetrically minor sulfosalt and/or telluride minerals; and a high-temperature, potentially higher melt volume domain involving partial melting of the major sulfide minerals. Epithermal gold deposits, which are especially rich in sulfosalt minerals, are predicted to commence melting 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 if pyrite and arsenopyrite coexist at these conditions, and in the upper amphibolite facies if they do not. Excepting sulfosalt-bearing occurrences, massive Ni-Cu-PGE (platinum group element) deposits will show little to no melting under common crustal metamorphic conditions, whereas disseminated Cu deposits are typically incapable of generating melt until the granulite facies is reached, when partial melting commences in bornite-bearing rocks. The volume of polymetallic melt that can be generated in most deposit types is therefore largely a function of the abundance of sulfosalt minerals. Even at granulite-facies conditions, this volume is usually less than 0·5%. The exception is massive Pb-Zn deposits, where melt volumes significantly exceeding 0·5 vol. % may be segregated into sulfide magma dykes, allowing mobilization over large distances.