The evolution of authigenic Zn-Pb-Fe-bearing phases in the Grieves Siding peat, western Tasmania

A thick peat profile overlying mineralized metasediments possesses exceptionally high base metal contents (up to 28.6 wt% Zn and up to 3.8 wt% Pb) in the form of abundant detrital and authigenic minerals. This metal-rich peat was examined using X-ray diffraction, scanning electron microscopy and Raman spectroscopy to determine the characteristics, mineral associations, phase evolution and conditions of formation of Zn-Pb-Fe-bearing minerals within the peat. Mineral assemblages consisting of sulfides, silicates, sulfates, oxides, carbonates and phosphates could be classified as follows: (1) detrital minerals supplied by the surrounding rocks (i.e., Cambrian volcanics and sediments, Ordovician carbonates) and (2) authigenic phases that are precipitated in situ, including the predominant Zn-Pb-Fe-bearing phases. Detrital minerals are characterized by weathering-related morphologies (e.g., round, smooth surfaces and angular edges and dissolution pits), whereas authigenic phases are recognized by their delicate microparticle textures (e.g., bladed, framboidal and botryoidal textures). Zinc-bearing phases are represented by non-stoichiometric phases, also containing S, C, O and Al; sphalerite, baileychlore and Fe-Zn-Pb carbonate. Authigenic Pb- and Fe-bearing phases are also present in the peat such as galena, anglesite, plumbojarosite, magnetite and pyrite. A "line of descent" of authigenic sulfides has been established, suggesting that a non-stoichiometric, possibly amorphous Zn-rich phase is a precursor to the sphalerite. Stages of pyrite formation, where massive polycrystalline pyrite is produced via disseminated and framboidal pyrite, have also been hypothesized in this study. The assemblages of authigenic minerals in the peat reflect dynamic physical and chemical conditions, including biological processes, and are not necessarily in equilibrium with each other.