The Waihi Epithermal Gold-Silver-Base Metal Sulfide-Quartz Vein System, New Zealand: Temperature and Salinity Controls on Electrum and Sulfide Deposition

Abstract

The large, andesite-hosted quartz vein system at Waihi contains a major epithermal Au-Ag deposit (total production of 6.4 Moz Au and 39.9 Moz Ag, Ag:Au = 6.2). It is notable for a large vertical extent (up to 575 m) of economic Au-Ag ore. A pervasive adularia-quartz-illite-chlorite-pyrite alteration zone bounds the larger quartz veins and is associated with veinlets of quartz + calcite + pyrite ± adularia ± illite. This zone is enveloped by an illite-quartz-adularia-chlorite-calcite-pyrite alteration zone, which grades out through a transition zone characterized by interlayered illite-smectite into a smectite-chlorite-calcite alteration zone in the least altered andesites. The formation of the adularia, quartz, chlorite, calcite, pyrite, and some illite in these alteration assemblages was associated with an initial stage of vein filling by platy calcite and quartz. This was followed by a main stage of quartz and sulfide vein filling, with crustiform quartz at deeper levels and colloform quartz plus minor adularia at shallow levels. Deposition of main-stage quartz and sulfides was accompanied by illite overprinting adularia in the wall rock and filling fractures in the quartz-calcite-pyrite ± adularia veinlets. Sulfide-bearing bands associated with the main-stage quartz show a zonation with decreasing depth from a coarse-grained sphalerite-galena-pyrite-chlorite assemblage at deep levels to a finer grained sphalerite-galena-pyrite-chalcopyrite-acanthite-electrum assemblage at medium to shallow levels. Fluid inclusion studies indicate that the main-stage quartz was deposited from fluids which ranged in temperature and composition from up to 295°C and 1.7 wt percent NaCl equiv at depth, to 190°C and 0.2 wt percent NaCl equiv at shallow levels. These are true salinities, because the fluids extracted from the fluid inclusions have low CO 2 contents (0.05–0.24 wt %). Quartz within electrum-poor Zn-Pb-Fe sulfide bands was deposited at higher temperatures (295°–242°C) and salinities than quartz in electrum-bearing Zn-Pb-Fe-Cu-Ag sulfide bands (260°–190°C). Temperature versus depth profiles for fluid inclusions with evidence of boiling indicate that the paleowater table was located about 160 m above the present erosion level during deposition of the main-stage quartz. Minor amethyst as the final stage of vein filling was locally preceded by inesite and was not associated with sulfides and electrum. Amethyst was deposited at 253° to 170°C from fluids containing up to 4.5 wt percent NaCl equiv. These higher salinities are attributed to concentration of salts in the residual liquid by localized, open-system boiling. Chloride-depth, chloride-enthalpy, and δ 18 O water -enthalpy plots indicate mixing of deep fluids (up to 1.7 wt % NaCl for the main-stage quartz) with very low salinity, steam-heated ground water. Oxygen isotope δ 18 O values for vein quartz and amethyst range from 6.1 to 11.6 per mil and for vein calcite from 2.29 to 9.4 per mil. Calculated δ 18 O water values define a trend from –2.9 per mil at shallow levels to +0.6 per mil at depth. Extrapolation of the dilution trend on a δ 18 O water -enthalpy plot indicates that the deep fluid ( δ 18 O water +0.6‰) was mixed with heated (160°–180°C) ground water of local meteoric water composition (–5.5‰). Dilution of a deep chloride fluid (1–2 wt % NaCl equiv), carrying zinc and lead as chloride complexes, with steam-heated ground water is inferred to have been responsible for deposition of the electrum-poor Zn-Pb-Fe sulfide assemblage from 300° to 240°C at a paleodepth below about 500 m. Deposition of this assemblage may also have been promoted by decrease of sulfur in solution through loss of H 2 S gas by boiling during episodic fracturing. The decrease in salinity following dilution favored the transport of gold, silver, and copper as hydrosulfide complexes and led to the deposition of the electrum-bearing Zn-Pb-Fe-Cu-Ag sulfide assemblage over a temperature range of 260° to less than 190°C at paleodepths of 500 to less than 250 m. In this assemblage, the decrease of sulfur in solution as a result of deposition of the associated chalcopyrite, pyrite, and galena is a likely mechanism for electrum deposition.