Andesite volcanic facies and hydrothermal alteration in the subsurface peripheral to existing Waihi mine workings

Abstract

The Waihi vein system is an andesite-hosted epithermal Au-Ag deposit comprising a series of veins located in the southern Hauraki Goldfield, New Zealand. It includes the Martha Mine which is the largest gold producer in the Hauraki Goldfield. Despite a history of over 127 years of prospecting and mining at Waihi, the Favona Au-Ag deposit was recently discovered in blind quartz veins 2 km to the east of Martha Mine. This highlights that there are still new Au-Ag veins to be found and extensions of existing veins to be explored. Little work has been published on the volcanism of the Waipupu Formation andesites, which host the mineralisation. Secondly, the formation of epithermal mineral deposits is accompanied by various chemical reactions which occur between mineralising fluids and host rock, and commonly produce distinct mineralogical and geochemical halos that overprint the volcanic textures and are generally much larger than the ore bodies themselves. The overall aim of this project is, therefore, to reconstruct the volcanic and hydrothermal setting peripheral to the Martha Hill mine. Two northwest-oriented exploration drill holes that are peripheral to the Martha Hill vein system were logged from a volcanological perspective, sampled and analysed by optical mineralogy, TerraSpec, x-ray diffraction on clay separates and x-ray-fluorescence.

Eight new andesite lava facies have been recognised. Six of the facies were identified based on mineral abundances and the presence of quartz phenocrysts. The crystal abundances within these lava facies ranges from 10 to 60% and are dominated by feldspar and mafic minerals. Quartz phenocrysts are minor but prominent features in the rock. These mineral abundances are near synonymous with other active andesite volcanoes and Late Archean examples. The texture of each of the lava facies is dominantly porphyritic, and phenocrysts are mainly plagioclase + ferromagnesian silicates ± quartz. Quartz phenocrysts are commonly resorbed which reflects different levels of magma mixing within the magma chamber before erupting. The other two facies were andesite lava breccias described as either monolithic (containing one clast type) or heterolithic (containing multiple clast types). The monolithic breccia resembles that of an autobreccia, typically associated with the outer parts of a lava flow.

Using alteration mineralogy data (from shortwave infrared spectroscopy and x-ray diffraction on clay separates) and geochemical data (from x-ray fluorescence), a transition from illite to interstratified illite-smectite to smectite was identified heading northwest from the Martha Mine and major mineralised Au-Ag quartz veins. Based on known thermal stabilities for these minerals, a paleoisotherm model was established peripheral to the Martha Hill mine suggesting a “cooling” of the hydrothermal fluid northwest of Martha Mine. Multiple trace elements such as Tl, Bi and Se strongly correlated to the mineralogy, vectoring towards higher temperature mineralisation.