GEOCHEMICAL & PETROGRAPHIC ANALYSES OF VOLCANIC ASH IN THE FORMATION OF CHERT

Enrichment of silica in the form of chert is generally thought to be formed in various geologic environments. Some of the environments include bedded marine chert, chert in carbonates, hypersaline environments, pedogenic chert, and chert in volcanic sequences. The research addresses the hypothesis that chert is formed as a result of chemical weathering. Chert lenses and nodules in the volcanic ash horizon of Sydney Basin, Australia, appear to have formed differently. Their provenance and association with paper thin clay layers within the same volcanic sequence suggest that they formed as a result of the chemical weathering process of volcanic ash. The petrographic studies indicate precipitation of microcrystalline-cryptocrystalline quartz, mica, feldspar, and the presence of glass shards. Geochemical analysis conducted using inductively coupled plasma-atomic emission spectroscopy (ICP-AES) for the major, minor, and trace elements of the pyroclastic samples when compared with the reference rock also indicate silica enrichment. Mass balance calculations (MBC) (Wadia, 2013) using trace element zircon also indicated silica enrichment by 40.11%. X-ray diffraction (XRD) analysis indicated the strong presence of quartz in all size fractions (44-20, 20-5, and 5-2 microns), as well as kaolinite. Geochemical and petrographic analyses were instrumental in supporting the initial hypothesis regarding the formation of chert as a result of chemical weathering in a volcanic setting.