Composition and Conditions of Formation of Sulfide Globules in Pyroxenites of the Shigir Hills, Middle Urals

Ni–Cu sulfide droplets (globules) have been for the first time established in the Precambrian ultramafic rocks of the Shigir Hills on the western slope of the Middle Urals. The ultramafic rocks are represented by wehrlites and pyroxenites related with the crystallization of ankaramite melt, which was derived during rifting at the margin of the East European Platform. In terms of petrogeochemical signatures, the Shigir pyroxenites are similar to high-Ca ultramafic rocks from Ural–Alaskan-type complexes that form the Ural Platinum Belt. The sulfide globules consist of a polymineral aggregate of troilite, pyrrhotite, pentlandite, chalcopyrite; 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 consists about 48–60% of iron, 1–12% of nickel, 1–8% of copper, and less than 1% of cobalt. The formation of sulfide globules reflects the sulfur saturation of melt, which occurred 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 δ34S in sulfides (–0.4…+0.3‰) suggests the mantle source of sulfur. The presence of sulfide droplets in pyroxenites can be considered as one of the signs for searching magmatic sulfide-platinum-group metal mineralization in the ultramafic rocks of the Shigir complex and expands the potential productivity of Ural–Alaskan-type intrusions to magmatic sulfide mineralization.