Mineralogy and Geochemistry of Late Permian Basalt Breccias, Sydney Basin, Australia

Linda Chernak ('05)

    Volcanic breccias from the Late Permian Shoalhaven Group that formed directly by interaction with seawater contain hydrothermal minerals similar to those found within their associated basalts, the Blow Hole Latite Member (BHLM) and Bumbo Latite Member (BLM), near Kiama, NSW, Australia. X-ray diffraction, SEM-EDS, and petrographic microscope analyses show that the BHLM breccia contains albite, orthoclase, muscovite, celadonite, clinochlore, hematite, and calcite and the BLM breccia contains orthoclase, plagioclase, laumontite, and hematite. Amygdales within the BHLM breccia contain concentric rims indicating at least three episodes of precipitation. EDS and XRD analysis indicate vug-filled materials to be celadonite (an iron-rich illite), clinochlore (a magnesium-iron-rich chlorite), smectite, calcite, and quartz. Black calcite with curved cleavage was found in the BHLM breccia and analyses were undertaken to attempt to determine the origin of the unusual color and crystal characteristics. The only differences found between black calcite and white calcite were slightly higher concentrations of SiO₂, Fe₂O₃, MnO and Al₂O₃ in the black calcite. Fluid inclusion analysis suggests calcite precipitated over a range of temperatures, from 88 - 245°C. Fluid inclusions with high homogenization temperatures have high salinities while fluid inclusions that formed at lower temperatures contain lower salinity fluids. Carbon isotope ratios of calcite indicate the presence of organic material. Oxygen isotope ratios of calcite within the breccias suggest that calcite either precipitated from hot (>200°C) seawater or from cooler fluids that were a mixture between seawater and meteoric waters.