ALBERT

Description: Sperrylite (PtAs 2 ) is one of most common Pt minerals, but the processes whereby it forms are not clearly established. Most commonly it is associated with the major-component base metal sulfide minerals (pyrrhotite, pentlandite, and chalcopyrite), which are believed to have crystallized from magmatic sulfide melts. Hence, sperrylite is thought to have formed by crystallization from a sulfide melt or by exsolution from sulfide minerals. However, sperrylite is also found associated with silicate and oxide minerals where it is thought to have formed by crystallization from the silicate magma. To investigate the conditions under which sperrylite could crystallize from a magmatic sulfide melt we investigated sperrylite saturation in Fe-Ni-Cu-S sulfide melts under controlled $${f}_{{\mathrm{O}}_{2}}$$ and $${f}_{{\mathrm{S}}_{2}}$$ at 910–1060 °C and 1 bar. The As and Pt concentrations in the sulfide melt at sperrylite saturation increase from 0.23–0.41 to 2.2–4.4 wt% and from 0.36–0.65 to 1.9–2.8 wt%, respectively, as the iron concentration in the sulfide melt decreases from 50 to 36 wt% at 910–1060 °C. We show that transitional metal concentrations, particular iron and nickel, as well as sulfur and oxygen fugacities influence As and Pt concentrations in the sulfide melt at sperrylite saturation. These intensive variables appear to effect sperrylite solubility by influencing the oxidation state of As in the sulfide melt. The measured concentrations of As and Pt in sperrylite-saturated sulfide melts produced in our experiments are much higher than that in most natural sulfides, implying that arsenides and sulfarsenides will not reach saturation in natural magmatic sulfide melts at high temperatures unless the magma has been contaminated with an exceptionally As-rich rock. This suggests that the observed arsenides and sulfarsenides in natural sulfide ores were not formed by crystallization from unfractionated sulfide melts at high temperatures above 900 °C, but might form at low temperatures below 900 °C.

Description: Magmatic sulfide ore deposits are products of natural smelting: concentration of immiscible sulfide liquid (‘matte’), enriched in chalcophile elements, derived from silicate magmas (‘slags’). Sulfide ore deposits occupy a spectrum from accumulated pools of matte within small igneous intrusions or lava flows, mined primarily for Ni and Cu, to stratiform layers of weakly disseminated sulfides within large mafic–ultramafic intrusions, mined for platinum-group elements. One of the world's most valuable deposits, the Platreef in the Bushveld Complex (South Africa) has aspects of both of these end members. Natural matte compositions vary widely between and within deposits, and these compositions are controlled largely by the relative volumes of matte and slag that interact with one another.

Description: A large proportion of ores in magmatic sulfide deposits consist of mixtures of cumulus silicate minerals, sulfide liquid, and silicate melt, with characteristic textural relationships that provide essential clues to their origin. Within silicate-sulfide cumulates, there is a range of sulfide abundance in magmatic-textured silicate-sulfide ores between ores with up to about five modal percent sulfides, called "disseminated ores," and "net-textured" (or "matrix") ores containing about 30 to 70 modal percent sulfide forming continuous networks enclosing cumulus silicates. Disseminated ores in cumulates have various textural types relating to the presence or absence of trapped interstitial silicate melt and (rarely) vapor bubbles. Spherical or oblate spherical globules with smooth menisci, as in the Black Swan disseminated ores, are associated with silicate-filled cavities interpreted as amygdales or segregation vesicles. More irregular globules lacking internal differentiation and having partially facetted margins are interpreted as entrainment of previously segregated, partially solidified sulfide. There is a textural continuum between various types of disseminated and net-textured ores, intermediate types commonly taking the form of "patchy net-textured ores" containing sulfide-rich and sulfide-poor domains at centimeter to decimeter scale. These textures are ascribed primarily to the process of sulfide percolation, itself triggered by the process of competitive wetting whereby the silicate melt preferentially wets silicate crystal surfaces. The process is self-reinforcing as sulfide migration causes sulfide networks to grow by coalescence, with a larger rise height and hence a greater gravitational driving force for percolation and silicate melt displacement. Many of the textural variants catalogued here, including poikilitic or leopard-textured ores, can be explained in these terms. Additional complexity is added by factors such as the presence of oikocrysts and segregation of sulfide liquid during strain-rate dependent thixotropic behavior of partially consolidated cumulates. Integrated textural and geochemical studies are critical to full understanding of ore-forming systems.