Tube fossils from gossanites of the Urals VHMS deposits, Russia: Authigenic mineral assemblages and trace element distributions

摘要

The occurrence, types, morphology, and mineralogical characteristics of tube microfossils were studied in gossanites from twelve VHMS deposits of the Urals. Several types of tube microfossils were recognized, including siboglinids, polychaetes and calcerous serpulids, replaced by a variety of minerals (e.g. hematite-quartz, hematite-chlorite, carbonate-hematite) depending on the nature of the substrate prior to the formation of the gossanites. Colonial hematite tube microfossils (similar to 150 mu m across,1-2 mm long) are composed of hematitic outer and inner walls, and may exhibit a cellular structure within their cavities. Spherical forms are saturated with Fe-oxidizing bacteria inside the tubes - probably analogues of trophosomes. Colloform stromatolitic outer wall surfaces are characterized by the presence of numerous interlaced filaments of hematite (2-3 mu m diameter, up to 1-2 mm long). Between tube microfossils, the hematitized cement contains bundles of hematitized filaments with structures similar to the hyphae of fungi. Hematite-chlorite tube microfossils are scattered in gossanites, mostly as biological debris. They are typically 30 to 300 pm in diameter and 1 to 5 mm long. The layered structure of their tube walls is characterized by hematite-quartz and chlorite layers. Abundant filamentous bacteria coated by glycocalix and chlorite stromatolite are associated with hematite-chlorite tubes. The carbonate-hematite tube microfossils (up to 300 pm across, 2-3 mm long) occur in carbonate-rich gossanites. The tubes are characterized by fine (similar to 10 pm thick) walls of hematite and cavities dominated by relatively dark carbonate or hematite. Carbonates may be present both in walls and cavities. Stromatolite-like leucoxene or hematite-carbonate aggregates were also found in association with tubes. Randomly oriented filaments are composed of ankerite. Single filaments are composed of individual cells, typically smaller than 100 nm across, similar to that of magnetotactic bacteria.