Composition and formation conditions of sulfide globules in pyroxenites of the Shigir Hills, Middle Urals

The Ni-Cu sulfide droplets (globules) firstly has been established in the Precambrian ultramafites of the Shigir Hills in the Western Slope of the Middle Urals. The ultramafites are represented by wehrlites and pyroxenites related with the crystallization of ankaramite melt, which was produced during the rifting in the margin of the East European Platform terms of petrogeochemical signatures, the Shigir pyroxenites are similar to high-Ca ultramafites from Ural-Alaskan-type complexes, which form the Ural Platinum Belt. Sulfide globules consist of a polymineral aggregate of troilite, pyrrhotite, pentlandite, chalcopyrite grains; bornite, cubanite, pyrite, galena, and molybdenite are less common. The compositions of sulfides reflect low-temperature equilibrium below 300 °C. The whole composition of globules corresponds to monosulfide solid solution. It contains about 48–60% of iron, 1–12% nickel, 1–8% copper and less than 1% of cobalt. The formation of sulfide globules reflects the sulfur saturation of melt took place after crystallization of the most volume of olivine and clinopyroxene. Close association of sulfide globules with enstatite, kaersutite, plagioclase and Ti-rich Cr-spinel indicates their formation at temperatures above 900–1000 °C and a pressure of 5 kbar. The values -0.4 – +0.3‰ of δ34S in sulfides reflects the mantle source of sulfur. The presence of sulfide droplets in pyroxenites can be considered as one of the search signs for magmatic sulfide-platinum metal mineralization in the ultramafites of the Shigir complex and expands the potential productivity of Ural-Alaskan-type intrusions to magmatic sulfide mineralization.