GENERATION OF COMPLEX POLYMETALLIC MELT ASSEMBLAGES IN METAMORPHOSED GOLD DEPOSITS

We have investigated three different types of gold deposits, metamorphosed at amphibolite and granulite facies conditions, to assess development of polymetallic melts and the role that this process plays in ore body evolution. The gold-bearing melts are referred to as polymetallic melts because a large range of compositions is possible. Four different melt types have been observed: (1) sulfide melts dominated by Pb, Fe, Cu, Zn and S; (2) sulfosalt melts involving Au, Ag, Pb, Tl, Hg, Cu and Fe complexed with Sb-S, As-S and/or Bi-S; (3) intermetallic melts developed between alloys of Au, Ag, Hg, As, Sb and Bi; and (4) telluride melts involving Au, Ag and Pb complexed with Te. Of these, the sulfide melts are comparatively unimportant for mobilizing gold, as they appear to be incapable of dissolving >1000 ppm Au and crystallize at high temperatures. In contrast, when As, Sb and/or Bi are present, as either sulfosalt or intermetallic melts, the proportion of gold incorporated can greatly exceed 1% (can be >50%), and melts can persist through fractionation to temperatures below 300ºC. In some deposits, Au-rich telluride melts are intimately associated with sulfosalt and intermetallic melts, and certain compositions are molten below 400ºC. Interaction between early-formed Sb-, As-, Bi- and Te-rich melts and unmelted minerals is facilitated by deformation-driven melt segregation, which allows further melting and incorporation of a wide range of elements into the melt. These polymetallic melts are mobilized from compressional high strain regions into dilational domains such as boudin necks and extensional fractures developed in competent lithologies. Ore minerals that do not participate significantly in melting are not extensively mobilized. Fractional crystallization of polymetallic melts, driven by ongoing deformation during cooling, leads to diverse suites of ore minerals. Although some gold typically remains at moderate concentrations within compressional high strain domains, it tends to become concentrated in dilational domains, creating a nugget effect.