オーストラリア, クイーンズランド州, Mount Leyshon金鉱床の地球化学的特微

Gold mineralization at the Mount Leyshon mine, located about 24km south of Charters Towers, eastern Queensland, Australia, is hosted mainly by breccia and quartz porphyry. The quartz porphyly has a Rb-Sr isotope age of 283±19Ma. The Rb-Sr data, combined with lead and K-Ar isotope systematics, reveal that the intrusive phases and related gold mineralization were essentially continuous from latest Carboniferous to earliest Permian, and occurred over a short time period at high levels in the crust. Using the isochron age, the following initial Sr and Nd ratios are obtained: εSr=9.9 to 12.0, εNd=-1.3 to -1.5 for a trachyandesite dike (post-ore and latest magmatic event) ; εSr=86.0 to 100.0, εNd=-7.1 to -13.0 for hydrothermal calcite associated with ores in the breccia; εSr =40.0 to 110.0, εNd=-2.2 to -3.3 for quartz porphyry (related to the mineralization); εSr=138.0 to 162.0, εNd =-14.0 to -15.0 for metasedimentary basement; and εSr=264.0, εNd=-11 for granitic basement. These isotope characteristics are consistent with a two-component mixing model where the bulk earth component is mixed with a limited amount of recycled crustal component.<BR>Based on field relationships and textural evidence, the mineralization and related alteration are divided into two stages, early and late. Ores in the early stage occur as veins and veinlets up to 3 cm wide mainly in quartz porphyry, metasediment and breccia (Main Pipe breccia), while those in the late stage occur as matrix replacement, cavity fillings, disseminations and veins up to 1.5m wide mainly within the Mount Leyshon breccia and tuffisite. The early stage is characterized by Mo and ore fluids of high salinity (40 to 52 eq.wt% NaCl) and higher pH (potash feldspar and biotite), while the late stage fluid is characterized by Cu-Pb-Zn-Bi-Au-Ag, low salinity (3 to 17 eq.wt% NaCl) and lower pH (muscovite, chlorite and kaolinite).<BR>Using available solubility data, gold at the Mount Leyshon deposit can best be interpreted as having precipitated from the breakdown of a thio complex (Au(HS)₂-) at about 300°C possibly due to a decrease of pH in the pyrites table region.