Fingerprints of S-Cl-bearing fluids deduced from metal and mineral microinclusions in Quaternary Kamchatka lavas

Quaternary ankaramite, basalt, andesite and dacite from the Avacha, Bakening, Mutnovsky and Gorely volcanoes in Kamchatka contain numerous mineral microinclusions in rock-forming silicate and accessory oxide minerals as well as in groundmass volcanic glass. We have studied the composition and textural setting of these microinclusions along with the petrology and geochemictsry of their host lavas using optical petrography, major and trace element geochemistry and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS). The microinclusions are represented by Au and Au-bearing alloys, Ag and Ag-bearing alloys, partially oxidized siderophile (W) and chalcophile (Pb, Zn, Cu, Bi, Sb, Pb) metals. S-Cl-bearing microinclusions as Ag (±Cu) chlorides, composite S-O-C-bearing metal chlorides, Ag (±Cu) sulfides, sulfides of Fe, Ni, Cu, Pb, Zn and Sb as well as barite, cassiterite and apatite are very characteristic for these rocks. Primary-textured, magmatic microinclusions in olivine, pyroxenes, amphibole, plagioclase and fresh volcanic glass show no evidence for the later-stage transformations. Secondary-textured, post-magmatic microinclusions are localized in pores, voids and fractures or along the boundaries of silicate minerals and in association with secondary minerals. Magmatic and post-magmatic S-Cl-bearing microinclusion assemblages almost completely overlap compositionally suggesting that S-Cl-bearing fluids accompanied formation of Kamchatka lavas from magmatic differentiation in sub-volcanic conduits to their emplacement, cooling and degassing above the surface as well as possible post-eruptive transformations near active fumarolic fields. Lavas from the Avacha, Bakening, Mutnovsky and Gorely volcanoes display elevated Pb, Zn, Sb, Cu and Ag concentrations in comparison with the average MORB and upper continental crust. Some volcanic samples with both magmatic and post-magmatic S-Cl-bearing mineral microinclusions have higher Mo, Sn, Sb and Ag contents than previously reported for Kamchatka volcanoes suggesting that sulfur and chlorine potentially contribute to metal enrichment in magmas and associated fluids and facilitate transport of certain ore metals in the Kamchatka subduction zone.