Hydrothermal Alteration And Whole Rock Geochemistry Of The Tauhara Geothermal Field: Traditional Versus Rapid Techniques

Abstract

Established practices that characterise the geology of geothermal fields involve primary (host rock) and secondary (alteration) mineral identification using visual description of cuttings and core, optical microscopy and x-ray diffraction (XRD). More recently, new technologies such as short wave infrared (SWIR) and portable x-ray fluorescence (pXRF) have emerged (primarily driven by demand from the mineral exploration industry) that can obtain mineralogical and geochemical information relevant to geothermal industry and research. Whole rock geochemistry is not routinely carried out during geothermal well logging due to cost and time constraints. For this reason, pXRF data, which can be obtained rapidly and inexpensively, is evaluated against traditional lab-based x-ray fluorescence methods (XRF). Three geothermal wells (TH9, TH10 and TH12) located across the Tauhara Geothermal Field were examined and the hydrothermal alteration mineralogy and whole rock geochemistry of each assessed. The results presented here demonstrate that pXRF data can be used to distinguish lithologies based on immobile element concentrations (e.g. Y, Zr, and Nb). In addition, Si and Sr show significant differences between lithological units. Therefore, immobile element concentrations and ratios may be useful for distinguishing boundaries between lithological units that are otherwise difficult to determine by mineralogy and petrography alone. Higher permeability zones accompanied by more intense hydrothermal alteration, is reflected in elevated concentrations of fluid mobile elements such as Ba, K, Ca, As, Rb and Pb. Comparison between traditional lab-based XRF (fused and press powder discs) and portable XRF on cuttings shows generally excellent correlation (rs more than 0.8) between the two methods for As, Ba, Ca, K, Nb, Rb, Sr, Y, Zr. However Al, Cu, Mn, Mo, Pb, S, Si, Th, Cr, Sb, Sn, Ta, Zn are not as well correlated between the two methods (rs less than 0.7). Thus pXRF is likely an efficient and cost effective technique for distinguishing lithology changes and hydrothermal alteration of host rocks particularly where a preliminary and rapid assessment is required. This study also compares traditional x-ray diffraction data with Terraspec