Waiotapu, New Zealand: the geochemical evolution and mineralization of an active hydrothermal system

Abstract

The Waiotapu geothermal system is located within the Taupo Volcanic Zone of New Zealand, an extensional, subduction-related rift comprising dominantly silicic volcanics related to caldera-forming events. Characteristics of the Waiotapu volcanic setting, fluid chemistry, and alteration and mineralization patterns bear close similarity to those of precious and base metal epithermal ore deposits. The physical and chemical evolution of the geothermal system to its present state has been defined through study of the geology, fluid and mineral chemistry and reactions, stable isotope patterns and fluid inclusions. The important factors relating to precious and base metal mineralization have been identified, and the results are applied to understanding fossil epithermal systems. The area of the surface thermal anomaly covers 17 km2 and the natural heat flow is 600 MW; boiling, near-neutral chloride springs plus acid mud pools are common. Seven exploration wells were drilled in the 1950s to depths of 500 to 1100m to determine the field's energy potential; temperatures of up to 295°C were recorded but production was not encouraging. The wells penetrate dominantly rhyolitic ignimbrite flows and air fall tuffs; two dacite volcanoes (last ˜active 159,000 years ago) and an older rhyolite extrusive dome lie on the northern and western margins of the field respectively. Disseminated and vein hydrothermal alteration comprises quartz, albite, adularia, late white mica, chlorite, epidote, sphene, mordenite, laumontite, wairakite, calcite, pyrite, and pyrrhotite in varying assemblages. Locally there is a near-surface zone (usually ˜ 50 m deep) of acid alteration comprising kaolinite, alunite, fine pyrite and cristobalite. Fluid inclusion homogenization temperatures closely approximate a boiling point with depth profile, which is also the case for present measured well temperatures in the southern portion of the system. However, in wells to the north, present measured temperatures have an inversion 20° to 40°C lower than the inclusion trapping temperatures, indicating that some cooling has occurred since inclusion formation. Freezing temperature results are generally consistent with present fluid chemistry, except in the south, where at least a ten fold higher gas (mainly CO2) content was present during inclusion formation than now (0.03 molal C02).This higher gas content contributed to hydraulic fracturing at depths of 100 to 250 m, resulting in hydrothermal eruption activity ˜ 900 years ago. Stable isotope values (δ18O, δD and δ13C) of minerals, coupled with whole rock δ18O values and present fluid isotope concentrations, define a pattern of meteoric fluid flow; the fluid has evolved isotopically (and chemically) through interaction with the rock to produce variable fluid/rock ratios of ˜ 1.0 (in the north) to ˜ 0.1 (in the south). Fluid chemistry of well discharges indicates that a deep up flow of neutral chloride fluid (˜240°C but having boiled from at least 300°C) in the north is diverted southwards by a southerly flowing, cooler (˜170°C), more dilute chloride fluid; this pattern, and near surface boiling, is consistent with the chemistry of the hot springs. Isotope concentrations in the fluid also support mixing and boiling at depth. The δ13C values of some calcite (˜-7 %) coupled with an inverse correlation to CO2 δ18O values, suggests that there was an intermittent, magmatic CO2 input; D values from some fluid inclusions are also anomalously light with respect to the dominantly meteoric fluid, though this does not appear to have been caused by a magmatic input. The hydrothermal mineral assemblage and chemistry, in terms of thermodynamic component equilibria, are consistent with present fluid chemistry; where differences occur, reaction progress may be identified, with textural evidence indicating the path of chemical evolution. At 225°C (a common well fluid discharge temperature), the pH of the present fluid varies from 5.7 to 6.3 (weakly alkaline). The PH2 and PH2S of well discharges allow fO2 and fS2 of the deep fluid to be calculated. These values are consistent with the observed pyrite and pyrrhotite (+ chlorite?) assemblage. The composition of sphalerite and epidote (plus the presence of minor hematite) suggests that fO2 and fS2 have fluctuated at times, probably due to boiling episodes; however, the fluids have (almost) always remained sulfide dominant. Earlier in the system's history (prior to the high CO2 level), CO2 was an order of magnitude lower than now, and an albite-adularia assemblage was stable; the increase in CO2 lowered the pH and resulted in a more recent formation of white mica alteration. Hydrothermal fluids are presently depositing ore grade concentrations of precious metals within and beneath surface siliceous sinters (up to 80 mg/kg gold, 175 mg/kg silver, 2 wt % arsenic and antimony each, and anomalously high mercury and thallium). Base metal sulfides are disseminated in a zone beneath the precious metals between 200 and 800 meters depth. Calculations of lead chloride, silver chloride and gold thio complex stability, compared with metal contents in the fluids, agree with the observed metal zonation and indicate that gold is extremely undersaturated in the deep fluid. Boiling (with a subsequent pH increase) and cooling dominate base metal sulfide precipitation, whereas silver may be precipitating due to dilution as well. The only way to precipitate gold is to mix the deep fluid with surficial acid, oxidized fluids. This probably occurs during boiling episodes and after hydrothermal eruptions, when depressurization serves to draw surficial fluids deeper in to the system. As2S3 in Champagne Pool may be adsorbing gold prior to deposition. The evolution of the gas and therefore fluid chemistry may be closely related to nearby rhyolite-basalt volcanism. This evolution also has an effect on the processes controlling mineralization in the system, and along with the lateral fluid flow, is responsible for the lateral zonation of mineralization. An understanding of the important processes operating in the Waiotapu system, particularly related to fluid flow and mineralization, assist in reconstructing the hydrology and chemistry of a fossil system. This provides exploration criteria for and helps in unraveling aspects of epithemal mineralization.