Evidence of biogenic activity in quartz-hematite rocks of the urals VMS deposits

Abstract

The textural, mineralogical, and geochemical features of quartz-hematite rocks associated with Urals VMS deposits indicate that the tube microfossils are responsible for immobilization and accumulation of chemical elements during precipitation of authigenic minerals. The crystallization of authigenic minerals is a result of submarine transformation of mixed hyaloclastitic, sulfide, and carbonate sediments and diagenetic processes, which modify the mineralogy and geochemistry of sediments. The tube microfossils about 100 μm across and up to 1 mm long consist of the external rim made up of fine-disperse hematite or hematite-quartz aggregates and of the internal channel filled with hematite and/or transparent quartz, fine-disperse hematite-quartz aggregates, leucoxene, rare sulfides, apatite, Fe-chlorite, and Mn-calcite. The carbon isotopic composition of calcite from quartz-hematite rocks with tube microfossils (up to −26.2 ‰) indicates its biogenic origin. The habitat conditions of the tube microfossils favored the mineral precipitation. The newly formed apatite, rutile, illite, monazite, dolomite, ankerite, siderite, monheimite, REE carbonates, anatase, leucoxene, Mn-oxides, titanomagnetite, and hematitized framboidal pyrite are observed in quartz-hematite matrix with abundant tube organisms in contrast to quartz-hematite rocks free of tube microfossils. Biomorphic hematite contains high contents of Mn (up to 9393 ppm), As (up to 1872 ppm), V (up to 779 ppm), W (up to 1091 ppm), Mo (up to 40 ppm), and U (up to 8.68 ppm), which are indicative of biological mechanisms of accumulation and conservation of these metals in the system.