Evolution of magmatic sulfide liquids: how and when base metal sulfides crystallize?

Experimental studies on binary, ternary and quaternary Cu–Fe–Ni–S systems are fundamental for the investigation of magmatic sulfide deposits, the main source of Ni, Co and platinum group elements (PGE). Previous experimental studies successfully formulated our general understanding of the evolution of magmatic sulfide systems but, yet, in many cases could not explain some of the key geological, mineralogical and geochemical features of sulfide ore deposits. The challenges are imposed by not well-defined solidus of the Cu-rich sulfide melts, yet poorly constrained phase stability at subliquidus conditions, and poorly resolved subsolidus evolution of magmatic sulfide phases. In this study we aim at better understanding how base metal sulfides crystallize from the evolving sulfide liquid during cooling from superliquidus to room temperatures. We report on controlled cooling (15 °C/day) experiments from 1100 to 25 °C in evacuated silica tubes of a single composition of the quaternary Cu–Ni–Fe–S system similar to the Merensky Reef sulfide ore. Run products were sampled at various temperatures along the cooling path and examined on the microscopic scale by back scattered electron imaging and on the nanometer scale by transmission electron microscopy. The compositions of coexisting phases were analysed using electron microprobe. We show that the sulfide melt (SM) coexists with monosulfide solid solution (MSS) above 950 °C and persists to 700 ± 25 °C before it crystallizes to intermediate solid solution (ISS). The transition to subsolidus state is clearly traced by abrupt change in the Cu/(Fe + Ni) distribution between MSS and Cu-rich phase (either SM or ISS) and in the composition of SM or ISS. The compositional jump at ca. 700 °C is also accompanied by the inverse change in the proportions of coexisting phases indicating significant subsolidus reactions and evolution with decreasing temperature. Pentlandite commences crystallization around MSS grains between 550 and 450 °C and coarsens to a granular-type pentlandite at 450 °C by diffusion of nano pentlandite exsolutions from MSS. Pentlandite exsolves and grows as “flames” and “brushes” in both MSS and ISS at lower temperatures (