H and O isotopic compositions of layered silicates in metasomatites from the Gai Zn-Cu massive sulfide deposit, southern urals

Abstract

Substantial differences in isotopic compositions of micas and pyrophyllites from metasomatites related to various stages of the process that formed the giant Gai massive sulfide deposit have been established. The illite from the earliest and predominant chlorite-illite-quartz metasomatite is characterized by the least δD values of −(50–85)‰ and δ18O=7–11‰. The pyrophyllite-quartz metasomatite as well as illite and pyrophyllite schists developed locally in the southern part of the deposit that likely correspond to the site of discharge of late geothermal paleosystem, contain pyrophyllite and illite with much higher values of δD=−(25–45)‰ and δ18O=4–9‰. Local zones of illite-paragonite schist complete the mineral formation and are characterized by the transitional δD values of −(30–55)‰ and elevated δ18O of 10–11‰. The most plausible model of isotopic evolution in the hydrothermal system, with an initial temperature of mica formation at 250°C, assumes the mixing of transformed sea water with a magmatic (metamorphic) water at the initial stage when the background metasomatites and massive sulfide orebodies of the northern lode have been formed. Subsequently, after the burial of the northern lode beneath basaltic andesite flows, the repeated sea water invasions took place in the southern discharge site of the system. As a result, the pyrophyllite-quartz metasomatite was formed; the pyrophyllite and illite schists originated in tectonic compression zones. The interaction of this water with silicate rocks was completed by a formation of illite-paragonite schist. In general, the substantial contribution of sea water to the formation of metasomatic halo of the deposit casts no doubt.