Arsenocolusite in Pyrite Ores of the Saum Copper–Zinc Massive Sulfide Deposit, North Urals

Pyrite ores on the flanks of the Saum copper–zinc massive sulfide deposit are clastic sediments intensely transformed under conditions of acid diagenesis. Colusite in ores is confined to fine-grained pyrite clasts, which are rimmed by later small-crystalline pyrite aggregates. Colusite forms fine dissemination (1–3 µm) and large isometric grains (up to 50–100 µm), contains inclusions of sulvanite, and belongs to arsenocolusite in chemical compositions (wt %): 12.62–14.87 As, 0.19–0.64 Sb, 0.29–1.46 Sn, 3.05–3.23 V, and 1.51–3.20 Fe. The LA ICP MS analysis of fine-grained pyrite revealed minor Ga, Ge, In, and Se contents in composition of arsenocolusite, which are correlated with chemical elements of colusite according to isomorphous substitutions in its formula. Galena, tellurobismuthite, native gold, molybdenite, rutile, monazite, uraninite, and Ba-bearing muscovite are found in fine-grained pyrite in assemblage with colusite. Interaction of sulfide sediments with seawater led to their enrichment in seawater elements (V, Mo, and U) and the deposition of colusite in assemblage with molybdenite and uraninite from trace element-rich diagenetic fluids under their dehydration.