Subsurface andesite geology and hydrothermal alteration at an exploration prospect north of Waihi

The Coromandel Volcanic Zone hosts the Hauraki Goldfield in the North Island of New Zealand. The goldfield consists of approximately 50 low sulphidation Au-Ag deposits. Situated in Coromandel Group rocks, which host 95% of the gold production in the region, the area of focus of this study is a prospect located 4 km north of the world class Martha Hill Mine. Such epithermal deposits are characterised by zones of altered rock called halos. Seven angled exploratory drill holes from the area were examined in this study. These cores were logged using high resolution photographs from a geological perspective for the first time. Samples were taken for petrographic and mineralogical analysis including X-ray diffraction analysis and for X-ray fluorescence spectroscopy. Leapfrog was used to display the spatial relationship of these results. The aim of this study was to determine the volcanic geology and alteration origins of this project. This was achieved by identifying facies as well as their spatial distribution, determining the mineralogical properties of the facies and their variations down-hole, as well as identifying the style and spatial extent of alteration. Eight new facies were identified which are hosted in andesitic rock. Three of the facies were based on visible crystal concentration which ranged from, 3% – 65% and occurred throughout the core. Fresh samples were dominated by plagioclase, with relatively minor amounts of quartz and pyroxenes and altered samples comprised of calcite+quartz assemblages with various suits of sulfides and oxides, predominantly pyrite and magnetite. The mineral assemblages and their abundances are comparable to the Waipupu Formation andesite with vast similarities to active andesite volcanoes and Late Archean examples. The rocks exhibit seriate porphyritic textures and glomeroporphyritic clots that are consistent with polybaric fractionation. The textures resulted in the host rocks innate high porosity and permeability which increases with each propagation of hydrothermal fluid. While the common resorption of quartz indicates magma mingling occurring in the magma chamber, these same resorption features as well as observed swallowtail plagioclase microlites also reflect degassing processes within the magma during ascent. Four breccia facies were described based on the concentration and orientation of breccia clasts. These facies occurred dominantly around the edge of alteration zones, which can be attributed to their relatively higher porosity. While some breccia display properties of auto breccia, hydrothermal breccia’s are dominant with all breccia likely altered to some degree. Finally, an irregular facies was described and relates to hydrothermal alteration processes exclusive to moderate and high alteration zones. Using the mineralogy data, in conjunction with stratigraphic logs, zones of alteration could be identified and designated based on intensity, describing two zones of high intensity alteration separated by a zone of moderate alteration and low alteration occurring closer to the surface. These zones give an insight into the flow of paleo fluid during alteration and can hint at the location of Au bearing veins