Hot-water deposition of pyritic stromatolite and its relation to biomineralization

Pyritic stromatolite, a rich pyrite ore, is scattered as reef masses in sed ex deposits of the Proterozoic Yanshan rift trough. The pyritic stromatolit e consists of a core and alternating concentric rims of light colloidal pyr ite and dark organic materials. The concentric rims are cemented together b y trichomes highly similar to the trichomic microorganisms inhabiting subst antively around the black chimneys on the current sea beds while the core i s composed chiefly of groups of thermophilous sulphur bacteria. Biomarkers for the molecules of pyritic stromatolite include pristane, phytane, regula r isoprenoids paraffin, methyl-heptadecyl, and so on. This study reveals th e existence of methane-yielding bacteria in the pyritic stromatolite and re flects the evolution of thermophilous thallophyta. Long pulsation of minera lizing thermal solutions venting up along contemporaneous faults in rift tr oughs contributed greatly not just to the reproduction of thermophilous org anisms living around the vents, but to their adsorption of Fe2+ from the so lutions in a reducing environment. Pyritic stromatolite constantly took sha pe through metabolism and reduction of these organisms. Owing to the uneven development of the organic communities close to the vents or the hydrother mal plumes, pyritic stromatolite occurred eventually as scattered reef mass es. This mineralizing mechanism may be summarized as the following procedur e: flowing of hydrothermal fluids associated with submarine exhalation-->ad sorption and metabolism of thermophilous micro-organisms-->reduction of org anic materials-->formation of deposits of pyritic stromatolite.