Aspects of Hydrothermal Alteration at the Waihi Epithermal Au-Ag Deposit, New Zealand

Abstract

Hydrothrmally altered andesites host mineralization in the epithermal Au-Ag deposit at Waihi. quartz veins form a complcx braided system within an extensionalfracture zone. Several vein-filling events occurred, the main ore stage is seen ascombtocryptocrystalline grey with a minor sulphide assemblage of sphalerite, galena, and chalcopyrite. Hydrothermal alteration is dominated by quartz + adularia + illite +chlorite+pyrite enveloping the veins and grades outwards into a chlorite+calcite+ +pyrite assemblage. A distinct zoning of clay minerals is observed around quartz veins which correlates with increasing temperaturestowards fluid conduits (veins). Steep thermal gradients are indicated by the observed clay mineralogy; such gradients infer alteration at shallow levels. Adularia present in the silicified zone and as a indicates high permeability around veining intervals. The abundance of adularia declines sharply into the host andesite, suggesting the paleohydrology was controlled and enhanced by structures such as faults and joints. The Occurrenceof lattice-texturedcalcite and vein adularia indicates that boiling conditions existed. Geochemical results show that with increasing intensity of alteration, silica and potassium were added, while sodium, calcium, and magnesium were lost. Arsenic and antimony increases towards veins suggests these elements may be used as pathfinders to further mineralization at Waihi. The alterationassemblageand geochemical apparent at Waihi are similar to those in active geothermal systems. This is characteristicof low-sulphidation,adularia-sencitetype deposits.