THE RHODOCHROSITE SMITHSONITE SOLID-SOLUTION SERIES FROM BROKEN-HILL (NSW), AUSTRALIA - GEOCHEMICAL AND INFRARED SPECTROSCOPIC INVESTIGATIONS

A nearly complete rhodochrosite-smithsonite solid-solution series with minor amounts of Pb2+, Cu2+, Ca2+ and Mg2+ is found in the oxidized z one of the Broken Hill ore body, Australia. DeltaC-13-values between - 10.0 and - 12.3 parts per thousand (versus PDB), and delta O-18-value s between 26.5 and 28.1 parts per thousand (versus SMOW) indicate the influence of organic carbon and confirm formation under low temperatur e conditions, respectively. Based on thermodynamic data and assuming e quilibrium precipitation, element activity ratios of the fluids deposi ting the rhodochrosite-smithsonite solid-solutions are estimated for 2 5-degrees-C. Since Pb2+ and Cu2+ are incorporated into the lattice of rhombohedral carbonates, the stabilities of hypothetical calcite-struc tured PbCO3 and CuCO3 are used in these calculations. Pb2+ and Cu'' we re always depleted with respect to Mn2+ in the modelled fluids. Incorp oration of Pb2+ seems to be related to an increase of the solid-soluti ons mole fraction Mn2+/(Mn2+ + Zn2+). The aqueous environment is calcu lated to have been highly variable with respect to Zn2+/Mn2+, Ca2+/Mn2 + and Mg2+/Mn2+ ratios, but probably rather homogeneous with respect t o Pb2+/Mn2+ ratios. Assuming an ideal solid-solution a Lippmann phase diagram for the system MnCO3-ZnCO3-H2O at 25-degrees-C is constructed representing thermodynamic equilibrium conditions. Infrared spectrosco py is applied here to investigate the effects of cationic substitution on band positions in the infrared absorption spectra, and to test it as an analytical tool for the investigated rhombohedral carbonate soli d-solutions. Significant frequency shifts in the nu2 and nu4 vibration al bands of the carbonate ion group are observed in the rhodochrosite - smithsonite mineral series as a function of cationic substitution. T aking into account analysis of endmember carbonates, a nu2-nu4-plot is constructed, reflecting mineral compositions in the ternary systems M nCO3-ZnCO3-CaCO3 and MnCO3-ZnCO3-MgCO3, which will help to identify rh odochrosite-smithsonite solid-solutions in the investigation of natura l samples.