Physico-chemical conditions of crystallization of minerals-phenocrysts in basaltic complexes hosting massive sulfide deposits of the Urals fnd Siberia

Abstract

he paper discusses the physico-chemical formation conditions of minerals-pheno-crysts in basaltic complexes, which host massive sulfde deposits of the Urals and Siberia. It is found as a result of study of melt inclusions that clinopyroxene from basalts of the Valentorka (North Urals) and Kyzyl-Tashtyg (South Siberia) deposits crystallized from melts with similar temperature (1165–1130 and 1210–1085 °С, respectively) and chemical parameters. In both cases, the composi¬tions of basalt-andesite magmas (with features of igneous island arc and back-arc basin systems) evolved with a decrease in FeO, MgO, and CaO contents and increase in K2O and SiO2 contents. Modeling, which is based on the compositions of inclusions and clinopyroxene, showed that miner-als-phenocrysts crystallized from intermediate magma chambers of diferent depth, the parameters of which are consistent with each other and with data on present-day suprasubduction magmatism: Valentorka deposit – 33–27, 23–13, and 10–3 km, 1185–1090 °C; Kyzyl-Tashtyg deposit – 27–20, 15.0–6.7, and 5.0–1.7 km, 1215–1105 °С. Calculations using compositions of melt inclusions show a consistent change of melts from basalts to rhyolites, indicating that the combination of contrasting volcanic complexes of the deposits studied are a result of the evolution of initial basaltic magmas during their uplift to the upper crustal horizons. Our studies of rocks, clinopyroxene and melt inclu¬sions indicate that the basaltic complexes of the Valentorka and Kyzyl-Tashtyg deposits formed in an ancient suprasubduction island arc – back-arc basin system.