MINERALIZING FLUIDS IN THE EPITHERMAL ADULARIA-SERICITE AU-AG DEPOSITS OF THE SOUTHERN HAURAKI GOLDFIELD, NEW ZEALAND

The Hauraki Goldfield on the Coromandel Peninsula of the North Island, New Zealand contains approximately 50 epithermal adularia-sericite Au-Ag deposits hosted by hydrothermally altered Miocene to Pliocene andesite, dacite, and rhyolite. Past production exceeds 11 Moz Au and 51 Moz Ag. In the southern Hauraki Goldfield, epithermal mineralization formed at paleodepths ≤1,400 m in steeply dipping, NE- to E-NE-striking quartz veins. Ore minerals are commonly zoned with depth: electrum is most abundant at shallow to intermediate depths, whereas base metal sulfides are more abundant at greater depths. Microthermometry and LA-ICP-MS analyses of individual fluid inclusions (FIs) from the Waihi and Tui deposits show that chemical composition of mineralizing fluids vary with paleodepth.

Fluids associated with the electrum-rich portion of the Waihi deposit, which formed ~200 to 400 m below the water table, have low salinities (<4.2 wt % NaCl equiv.) and moderate temperatures (FI homogenization temperature, T_h = 189° to 242°C). Coexisting liquid- and vapor-rich FIs reflect deposition from periodically boiling fluids. Fluids trapped in the deeper, base metal sulfide-enriched, part of the Waihi deposit have higher salinities (<8.4 wt % NaCl equiv.) and temperatures (T_h= 229° to 285°C). The FIs are predominantly two-phase liquid-rich, with the exception of some pseudosecondary and secondary FI assemblages comprised of coexisting liquid-rich and vapor-rich FIs that reveal an overprint by boiling fluids. The paleodepth of base metal sulfide bearing veins is estimated at ~500 m below the water table.

The Tui Zn-Pb-Cu deposit represents a deep epithermal environment, with an estimated paleodepth of ~1,400 m below the water table. The mineralizing fluid was a single phase liquid, with a salinity <11.9 wt % NaCl equiv., and T_hof 241° to 291°C.

In the southern Hauraki Goldfield, with increasing paleodepth: salinity, T_h values, Na, K, Ca, Li, and base metal contents increase, whereas Sb and As decrease. Base metals were transported by chloride complexes and deposited as sulfide minerals due to destabilization of the complexes by cooling and dilution of the ore-bearing fluids. In contrast, Au and Ag were transported by dilute fluids that deposited Au and Ag because of cooling and chemical changes associated with periodic boiling.