Epithermal Au-Ag and Related Deposits of the Hauraki Goldfield, Coromandel Volcanic Zone, New Zealand

Abstract

Epithermal Au-Ag vein deposits of the Hauraki goldfield in the Coromandel volcanic zone, New Zealand, produced 320,000 kg Au and 1.5 million kg Ag between 1862 and 2006 from a total land area of approximately 2,900 km2. Approximately 69 percent of the production was from the world-class Martha deposit at Waihi. Some 97 percent of total past gold production was from deposits hosted in Miocene andesite and dacite of the Coromandel Group, although this unit represents only 61 percent of the rocks exposed in the goldfield. The deposits consist of quartz ± calcite vein systems, 0.5 to 4.5 km long, within alteration zones up to 20 km2 in size. They were mined over vertical intervals of 170 to 330 m but up to 575 m at Martha and 700 m at Karangahake. Electrum and acanthite are the main ore minerals, although pyrite, sphalerite, galena, chalcopyrite, sulfosalt minerals, and Te- and Se-bearing minerals are present in some deposits. Stibnite and cinnabar occur locally as late minerals, and some deposits are overprinted by late massive calcite. Fluid inclusion ice-melting temperature (Tm) measurements indicate that fluids associated with base metal and precious metal deposition had apparent salinities of between 0.5 and 2 wt percent NaCl equiv with a maximum of 6.1 wt percent NaCl equiv. When fitted to hydrostatic boiling point with depth curves, the homogenization temperature (Th) data define paleodepths spanning a vertical interval from near the water table to a depth of approximately 1,300 m. The presence of vapor-rich fluid inclusions, and occurrences of lattice textures and vein adularia, indicate that periods of boiling coincided with deposition of some epithermal ores. Sulfur and carbon isotope values of vein minerals are consistent with a magmatic origin for sulfur and carbon. Oxygen isotope values of vein quartz and calcite are consistent with derivation from evolved meteoric water, with possibly a small magmatic water component in some samples. The epithermal deposits can be broadly subdivided into northern, southern, and eastern groups based on their host rocks, age, variability of vein strike, Au/Ag ratios, occurrence or relative abundance of key minerals such as adularia, calcite, and arsenopyrite, the grain size of vein quartz, and differences in vein textures. Early (ca. 14–10 Ma), steeply dipping planar veins occur in the oldest andesite and dacite in the northern portion of the goldfield, but some of these are bonanza-grade veins that contained significant gold resources. Larger veins, typically with well-developed crustiform textures, formed in andesite and dacite in the southern gold-field at 7 to 6 Ma, and these contributed most of the Au and Ag that has been recovered from the goldfield. In contrast, rhyolite in the eastern goldfield hosts smaller, less well developed veins plus stockworks that have produced the least amount of Au and Ag. Epithermal mineral deposits in the Hauraki goldfield evolved through time, reflecting changes in volcanic activity, and more speculatively, tectonic and structural history and setting.