Economic sulfide deposits have been identified in only a handful of Proterozoic massif-type anorthosite complexes. Yet exploration interest has expanded greatly in the last decade since discovery of the Voisey’s Bay deposits. The paucity of new discoveries may reflect the unique confluence of geologic conditions required to concentrate sulfides in this environment. Sulfide minerals in anorthosite complexes are generated when the silicate magma reaches sulfide saturation and exsolves an immiscible sulfide melt. If this occurs early in magma emplacement, the dense sulfide melt will tend to stall in feeders or near the base of the magma body. Thus, sulfide concentrations will be spatially confined in large anorthosite complexes, with sulfide exposure dependent entirely on erosional level. Generation of economic sulfide concentrations may require assimilation of crustal sulfide, either from sulfidic metasedimentary rocks or from volcanogenic or sediment-hosted massive sulfides. These sulfide sources are prevalent in rift settings and hence in the suture zones that commonly characterize the country rocks of major anorthosite-associated intrusive complexes. Nevertheless, development of sulfide concentrations may depend on the fortuitous intersection of magma with sulfidic country rocks.
Rhenium-osmium (Re-Os) isotope data are available for parts of three Proterozoic anorthosite complexes; in each case, high initial 187Os/188Os requires a predominantly crustal source for the Os in the sulfide minerals. Modeling of Re-Os isotope data from the 1559 Ma Suwałki anorthosite massif leads to the hypothesis that sulfide saturation results from direct assimilation of crustal sulfide by a mantle-derived magma. Subsequent interaction of the sulfide phase with additional, uncontaminated silicate melt increases the concentrations of Re, Os, and other chalcophile elements in the sulfides and shifts the 187Os/188Os of the crustally derived sulfide toward that of the mantle-derived silicate melt.
Whereas we cannot rule out a crustal source for the parental magma at Suwałki, Re-Os data also permit a mantle-derived melt modified by crustal contamination. We model two end-member processes that can form sulfide concentrations in mantle-derived melts by assimilation of crustal material. The first process assumes that upwelling magmas are contaminated by bulk silicate crust, bringing the magma to sulfide saturation and producing an immiscible sulfide melt. The magma continuously assimilates crustal material so that the silicate melt maintains high 187Os/188Os. With increasing R (the mass ratio of silicate to sulfide liquid), concentrations of chalcophile elements, including Re and Os, increase in the sulfide melt but 187Os/188Os remains roughly constant. The second process assumes that upwelling magmas are contaminated by Re-Os-rich crustal sulfide, generating copious immiscible sulfide melt with Re and Os concentrations and 187Os/188Os not far removed from those of the contaminant. Assimilation of crustal material declines as the magma establishes armored conduits and/or chemical and thermal gradients between the magma and pristine country rock. With increasing R, continued flux of relatively uncontaminated magma increases Re and Os concentrations in the sulfide but draws 187Os/188Os downward toward lower mantle values. If this second process dominates at Suwałki, the high γOs limits R to very low values, probably <50, affirming substantial input of externally derived sulfur without subsequent interaction with silicate melt. Whereas reality no doubt lies between these end-member processes, a model based on the second process, using geologically reasonable model parameters and physical processes, generates Re-Os data that agree well with measured 187Os/188Os values at Suwałki.