MASS BALANCE CALCULATIONS USING TRACE ELEMENT ZIRCON TO INDICATE SILICA ENRICHMENT IN A VOLCANIC SETTING

Constructing the original composition of weathered tephra can be a daunting task. A previous study by Wadia (1998 and 2007) demonstrated the enrichment in silica using the hypothesis that the percentage of aluminum oxide had remained constant between the unweathered parent material and its weathered counterpart (Faure, 1991), which indicated the average enrichment in silica to be 6.5%. The study attempts to demonstrate the enrichment of micro-cryptocrystalline quartz in the Reid’s Mistake formation of the Newcastle Coal Measures of Sydney Basin, Australia, using zircon as the immobile element. Zircon is more favorable for mass balance calculations (MBC) regarding silica enrichment as it is a trace element that tends to be immobile in the chemical weathering process. Composition of the unweathered reference samples was gathered from the Tertiary volcanic complexes of Northeastern New South Wales and Southeastern Queensland. The chemical composition of the weathered samples was obtained using the inductively coupled plasma-atomic emission spectroscopic (ICP-AES) analysis. The study was supplemented with petrographic and x-ray diffraction (XRD) analyses. The petrographic analyses indicated the presence of micro-crytocrystalline quartz, mica-biotite, glass shards, feldspar, and volcanic quartz. XRD analyses for size fractions 44-20, 20-5, and 5-2 micron size fractions indicated the presence of quartz, feldspar, and clay minerals (illite-montorillonite and kaolinite). MBC using zircon as the immobile trace element provided the quantitative enrichment in silica. Results from both the petrographic and XRD analyses indicated that the tephra was subjected to chemical weathering in a meteoric regime, as indicated by the minerals observed in the weathered tephra. The MBC involving zircon resulted in the silica enrichment of 40.11%, indicative of chemical weathering, and additionally supported by the presence of chert lenses in the volcanic horizon, therefore indicating chemical weathering to be responsible for the enrichment of silica.