MINERALOGICAL AND TEXTURAL ZONATION IN THE WORLD-CLASS EPITHERMAL WAIHI VEIN SYSTEM, NEW ZEALAND

The Waihi vein system in the Hauraki goldfield is a series of adularia-sericite epithermal Au-Ag veins that includes the world-class Martha mine, which has produced more than 6.8 Moz Au. Critical comparison of opaque minerals and quartz textures within eight major veins of the Waihi district shows significant mineralogical and textural variations among veins. The peripheral veins of the district (Martha, Favona, Moonlight, Cowshed and Silverton) contain abundant colloform, cherty, and black quartz fill textures, with minor crustiform and massive quartz. The central veins (Amaranth, Trio and Union) contain predominantly massive and crustiform textures with minimal colloform and cherty quartz and black quartz fill, and these veins are also commonly coarser grained than peripheral veins. Electrum, acanthite, pyrite, sphalerite and galena are widely distributed throughout the Waihi Vein System; electrum is the only significant Au-bearing mineral, although rare Au-bearing telluride and selenide minerals also occur locally. Acanthite and Ag-bearing selenide minerals such as aguilarite and naumannite are most abundant at relatively shallow levels within individual veins, and also in peripheral veins of the district. In contrast, deeper portions of individual veins, and the central veins contain more abundant concentrations of base metal sulfide minerals. Observed textures and mineralogical changes are consistent with formation of the central veins at a deeper structural level than peripheral veins. Mineralogical variations produce changes in the concentrations of Se, As, and Sb in the ores that have important metallurgical and environmental implications. Gold- and Ag-bearing ore minerals occur only in certain bands of veins, requiring episodic precipitation of those minerals, and suggesting episodic influx of Au- and Ag-rich fluids. Available fluid inclusion data do not suggest large temperature variations among the different veins, and therefore the variability in the Se, As, and Sb contents of the ore assemblages presumably reflects episodic variations in the chemistry of hydrothermal fluids from place to place, and perhaps through time.