Authigenic Ti mineralization as an indicator of halmyrolysis of carbonatesulfide-hyaloclastite sediments in Urals massive sulfide deposits

Research subject. The horizons of ferruginous sedimentary rocks in the massive sulfide deposits of the Urals.Aim. To estimate the Ti behavior upon halmyrolytic transformation and lithogenesis of calcareous hyaloclastites and carbonatesulfide-hyaloclastite sediments.Materials and methods. Microtextures of authigenic aggregates of Ti minerals (anatase, rutile, titanite) in jasperites and gossanites of various Urals massive sulfide deposits were studied. The minerals were identified using microscopic and electron microscopic methods, as well as electron back-scattered diffraction.Results. Upon partial halmyrolysis of calcareous hyaloclastites, Ti was removed with the formation of authigenic anatase rims around hematitized hyaloclasts. The full transformation of hyaloclastites to hematite-quartz jasperites resulted in decomposition of authigenic Ti minerals. Authigenic rutile and titanite formed in gossanites (hematite-quartz and hematite-chlorite products of submarine oxidation of calcareous sulfide-hyaloclastite sediments). The occurrence of abundant bacteriomorphic structures in corroded hyaloclasts indicates a significant role of bacteria in halmyrolysis.Conclusions. Titanium for the formation of Ti minerals in ferruginous sediments was sourced from hyaloclastites. The halmyrolysis of calcareous hyaloclastite sediments and related formation of jasperites occurred under alkaline conditions favorable for the transportation of Ti in the form of hydroxycarbonate complexes. The formation of rutile instead of anatase was associated with lower pH values (<5) due to oxidation of pyrite in sulfide-bearing hyaloclastite sediments. Titanite formed as a result of further alteration of Ti-bearing phases. Our results solve the fundamental problem of Ti mobility during halmyrolysis of hyaloclastites, which contradicts its commonly accepted immobility in hydrothermal processes.