Geochemistry of gold-bearing carbonates, Beaconsfield gold mine, Tasmania, Australia

Abstract

The major carbonates minerals in the Beaconsfield gold mine are calcite, dolomite and magnesian ankerite. Gold occurs in magnesian ankerite. Ankerite has high Ca, moderate Fe, Mg and Mn and low Na and Sr. The values of Ca and Mg and total amounts of Mg, Fe and Mn, and values of Fe, Mn and Sr are strongly correlated. Thisis due to the two layered structure ofmagnesian ankerite comprised of Mg, Fe and Mn carbonites and Ca carbonites. The elemental and isotopic compositions of magnesian ankerite are related to ordering, substitution of elements, salinity, redox potential, dissolution and reprecipitation, temperature, composition of fluids, environmental setting and gold mineralization. Ca, Sr and Na are derived from dissolution of Ordovician limestone. Mg is derived from leaching of ultramafics. Fe and Mn are leached from both ultramafics and clay minerals. Meteoric water was high in Ca, Fe, Mg and Mn concentrations and formed magnesian ankerite in a reducing burial environment. The heat source was possibly deep burial depth. tectonic hot spots and a few granite intrusions east of Beaconsfield.Gold and ankerite were precipitated in alkaline conditions. Gold and magnesium in ankerite are derived from the leaching of Cambrian ultramafic rocks during the Devonian by the passage of meteoric fluids through tectonically affected Ordovician carbonates.