Mineral formation conditions in acid magmatic systems related to the formation of massive sulfide deposits of the Urals and Altai-Sayany area

Abstract

Studies of melt inclusions in quartz indicate the similarity of acid magmatic systems of massive sulfde deposits in the Urals and Altai-Sayany region. The melts of normal alkalinity corresponding to rhyodacite and rhyolite compositions and related to the tholeiitic series are dominant in all the deposits considered. The magmas are characterized by the same type evolution with a decreasing content of main oxides (TiO2, Al2O3, FeO, MgO, CaO, Na2O, K2O) and an increasing SiO2 content. Our results show the accumulation of Cu in relatively low-H2O acidic melts of ancient (Cambrian) deposits of the Altai-Sayany region and low metal contents in the intermediate (Silurian–Devonian) and H2O-saturated magmas of the Urals. The youngest (Devonian) magmas of Siberia evolve simultaneously along these two directions. The analysis of melt inclusions in quartz suggests that the minimum contents of trace and rare earth elements are characteristic of the Silurian-Devonian acid melts of the Urals, with their maximum contents in the youngest (Devonian) magmas and the intermediate contents of ancient (Cambrian) magmatic systems of the Altai-Sayany region. The features of rare and rare earth element patterns in melt inclusions in quartz indicate the similarity of acid magmatic systems of massive sulfde deposits in the Urals and Altai-Sayany region with present-day suprasubduction melts in the ocean-continent transition zones. Computational modeling using data on melt inclusions in quartz confrms our previous conclusions (Simonov, Maslennikov, 2020) that the occurrence of contrasting (basic and felsic) volcanic complexes with massive sulfde deposits in the Urals and Altai-Sayany region is a result of evolution of basaltoid magmas. Keywords: conditions of mineral crystallization, basaltic-rhyolitic complexes, massive sulfde deposits, melt inclusions, quartz, acidic melts.