Hydrothermal Alteration of the Clay-sized Fraction in the Westley Park and Kiama Sandstones (Permian), NSW, Australia

Elizabeth Rampe ('05)

    Two late Permian shallow marine sandstones (Westley Park and Kiama Sandstones) overlain by extrusive shoshonitic basalts (Blow Hole Latite and Bumbo Latite, respectively) were analyzed to determine the extent of hydrothermal alteration. Based on bulk and clay XRD analyses, the Westley Park Sandstone contains mixed-layer chlorite-vermiculite in the top 80 cm, mixed-layer chlorite-smectite 1-12 m below the contact with the overlying Blow Hole Latite, and laumontite. Based on bulk and clay XRD analyses, the Kiama Sandstone contains mixed-layer chlorite-smectite throughout and interstratified chlorite-smectite up to 9 m below the contact with the overlying Bumbo Latite and up to 9 m above the contact with the underlying Blow Hole Latite. The Kiama Samdstone also contains laumontite throughout. XRF data do not show any trends to explain the differences in mineralogy between and within each sandstone, except that there generally is higher weight percent K₂O and Fe₂O₃ in the top 80 cm of the Westley Park Sandstone, due to the presence of chlorite-vermiculite. The presence of chlorite-vermiculite in the top 80 cm and chlorite-smectite below 80 cm in the Westley Park Sandstone is likely due to contact metamorphism, but may be due to different phases of the mica to smectite reaction. The ordering of the chlorite-smectite in the Kiama Sandstone may be due to proximity to basalts and possibly elevated temperatures, amount of volcanic lithics/glass in the sediment, or differences in permeability in the sandstone. The differences in the grain size of the laumontite in the Kiama Sandstone may result from different episodes of hydrothermal fluids, lack of small pore space due to compaction, or less silica-rich fluids near the base of the sandstone. Zoned flow tubes in the Kiama Sandstone represent channelized hydrothermal fluid flow, likely due to the emplacement of the overlying basalt. Sediment pore waters were "squeezed" away from the basalt flow front. Mineral assemblages in both sandstones indicate that the temperature of the hydrothermal fluids did not exceed ~ 300° C.