Whole rock lithogeochemical data from the andesite rocks which host lowsulfidation epithermal gold veins in the Waihi district, New Zealand have been used to interpret hydrothermal alteration processes that variably altered the andesite rocks. The andesite host rocks are interpreted to have similar primary chemical compositions throughout the Waihi area, as reflected in similar ratios of immobile elements (e.g. Zr, Ti and other high field strength elements). The geochemical data reveal that elevated K/Al ratios occur in rocks proximal to low-Au-Ag deposits, reflecting adularia alteration in the host rocks associated with the upflowing hydrothermal fluids. Silicification of host rocks is achieved both by the addition of SiO2, as well as the destruction of plagioclase (reflected in the loss of CaO and Na2O). The rocks have been variably altered, with calcite and pyrite abundance reflecting the addition of carbon and sulfur. Pathfinder element (e.g. As, Sb) concentrations are variable across the district, with high pathfinder element concentrations interpreted to reflect proximity to hydrothermal fluid flow. Together, the results provide a district scale overview of hydrothermal alteration at Waihi, and emphasize the utility of lithogeochemistry for classifying and quantifying hydrothermal alteration that can then be used to interpret hydrothermal processes and assist mineral exploration.
Waihi Au-Ag mineralization is vein hosted within a relatively homogeneous andesite body. Geochemical exploration targets adularia and pathfinder (As, Sb, Hg etc.) anomalies. Adularia is formed as an alteration product of feldspars during hydrothermal alteration and occurs in anomalous concentrations near Au-Ag mineralization. This paper aims to test the potential of portable X-ray fluorescence (pXRF) to infer and map adularia and As anomalies. The pXRF was first tested on standards (CRM and ILD), it yielded accurate and precise data for most of the elements of interest (r 2 > 0.9 and APD < 10%). The second stage applied the pXRF directly on core, most elements were negatively affected, but still to a usable degree of accuracy. As was particularly effected due to its heterogeneous distribution (APD 46%). Lastly, we applied the pXRF to a suite of 300 Favona samples, their K/Al value were used as a proxy for adularia presence. XRD analysis carried out on 60 samples was used to validate adularias presence or absence. When combined with pXRF data we found a K/Al value above 0.4 generally indicated a sample contained adularia. Therefore, with further work, the pXRF could be independently used to infer adularia alteration in the Waihi setting.
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