Ore mineralogy and mineral chemistry of the Glen Eden Mo-W-Sn greisen-breccia system, eastern Australia

Abstract

The Glen Eden Mo-W-Sn deposit, New South Wales, is located at a site of igneous activity, which is characterized by both intrusive and extrusive felsic igneous rocks. Mo, W and Sn mineralization is related to the intrusive rocks as evidenced by temporal, spatial and geochemical relationships. The main part of the deposit is hosted by a pipe-like greisen breccia body, ~ 500 m in diameter. The mineralization at Glen Eden occurred in two stages: pre-breccia mineralization in the volcanic wall rocks and post-breccia mineralization in the pipe. Minerals such as molybdenite, wolframite, cassiterite, chalcopyrite, sphalerite, arsenopyrite, pyrite, pyrrhotite, and galena were deposited as disseminated grains in the wall rocks and as open-space fillings in the breccia pipe. Quartz, fluorite, muscovite, K-feldspar, and Bi-bearing minerals are also found in the pipe. In addition to common bibearing ore minerals (native bismuth, bismuthinite, joseite A, bursaite, and galenobismutite), there is an unknown opaque Bi-bearing mineral with the composition of Bi4TeS5. This mineral is white in color with a slight yellowish tint and shows strong reflectance, weak pleochroism, moderate anisotropy in greenish colors, no internal reflections, possible twinning, and low hardness. In the paragenetic sequence, cassiterite and wolframite mostly precipitated before sulfides; however, locally, there was overlap with sulfide mineralization. Hydrothermal minerals were deposited mainly in the temperature range of 350-250°C from hydrothermal fluids with low salinity (<8 wt% NaCl equivalent) and high salinity (>30 wt% NaCl equivalent) at a pressure of approximately 400 bar. At 250-350°C, hydrothermal fluids had pH = 3.1-5.2, fH2S = 10-3.7-101.6 bars, fO2 = 10-47.8-10-30.2 bars in the pre-breccia stage and pH = 4.9-5.9, fH2S = 10-2.7-100 bars, fO2>10-36.7 bars in the post-breccia stage. In the pre-breccia stage, the precipitation of ore minerals occurred mainly due to the increase in pH of hydrothermal fluids as result of the reaction of hydrothermal fluids with wall rock. The deposition in the post-breccia stage mainly occurred by an increase in the pH and fO2 (due to boiling) of the hydrothermal fluid and a decrease in temperature due to the boiling and mixing of magmatic and meteoric waters.