Textures, mineralogy and geochemistry of the Union, Trio and Amaranth epithermal veins, Waihi, New Zealand

Abstract

The Union, Trio and Amaranth deposits are located under Union Hill, Waihi, near the world class Martha deposit. The Union Hill deposits have similar textures and mineralogy. The deposits display a range of textures, but coarse grained crustiform quartz and quartz after platy calcite pseudomorphs are dominant. Electrum occurs predominantly as inclusions in pyrite and galena with an average grain size of 10 μm and median composition of Au0.45Ag0.54, similar to electrum in the Martha deposit, which occurs in pyrite and chalcopyrite. Base metal sulphides and green corrensite and chlorite clay minerals are very common at the deposit, and the mineral assemblage and textures are similar to those that occur at deeper levels in the Martha and Favona deposits. Clay minerals identified by X-ray diffraction and TerraSpec include corrensite, chlorite, smectite, illite, kaolinite, chlorite/smectite and illite/smectite, and individual clay minerals cannot be identified at the macro scale. There may be some correlation between chlorite/smectite and electrum deposition, but further investigation is necessary. Fluid inclusions have homogenisation temperatures that range between 156 and 300°C with a median of 239°C, marginally hotter than some of the shallower-formed deposits in Waihi. They indicate a paleowater table of approximately 1400 m RL similar to the Martha deposit, and variation in paleowater table during deposit formation. Salinities for the inclusions are mostly low, but range up to 5.1 wt % NaCl equiv. in quartz and between 5.6 and 6.3 wt % NaCl equiv. in sphalerite. Higher salinity inclusions are associated with higher assay samples. The Union Hill deposit is the expression of the lower part of the wider Waihi hydrothermal system, exposed due to post-mineral tilting to the east. As tilting and faulting only accounts for the centre and east Waihi deposits, further investigation is necessary to explain the occurrence of the Martha deposit.