Petrology of the Komata gold - silver epithermal ore deposit, Coromandel Peninsula, New Zealand

Abstract

The Komata epithermal Au-Ag deposit occurs at the southern end of the Coromandel Peninsula, New Zealand, and was produced by hydrothermal activity related to rhyolitic volcanism in the late Pliocene and/or early Pleistocene. Hosting the mineralised veins at Komata are Whitianga Group rhyolites. These rhyolites intrude and overlie a sequence of andesitic to dacitic lavas and pyroclastics belonging to the Coromandel Group. The veinstone is essentially a breccia of country rock (with fragments up to 1m), cemented by quartz, adularia and calcite. Electrum and acanthite are the predominant precious metals, and are locally associated with sphalerite, chalcopyrite and pyrite. Fluid inclusions suggest Au-Ag minerals were deposited by low salinity (i.e. predominantly 0.2-2.0 wt% NaCl equivalent), relatively gas rich fluids at around 240°C and at depths of 150-300m. Base metals are noted to increase with depth and towards the peripheries of the system. Deposition of Pleistocene Omahine Andesite lavas postdates hydrothermal activity. The Komata vein system and the rhyolite intrusives (both trending 030) are postulated to have intruded along tensional gashes within a major N-S trending dextral fault system. Synthetic and antithetic shears (trending 060 and 300 respectively) produced in this system are displayed as minor faults and as prominent photo-lineaments. Quartz-adularia alteration occurs close to the major veins with an assemblage of quartz-illite becoming increasingly important further out. Illite grades out into interlayered illite-smectite clays, which form the stratigraphically highest vestige of the extinct hydrothermal system. Late stage downward percolation of fluids from the near surface condensate zone produced local overprinting of kaolinite, hematite and/or limonite. The vertical pattern of alteration zonation in conjunction with fluid inclusion evidence suggests a level of erosion 100-150m lower than that seen at the neighbouring deposit, Golden Cross, where smectite alteration is preserved at near surface levels.