Other
Scientific paper
Oct 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991gecoa..55.2753h&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 55, Issue 10, pp.2753-2765
Other
6
Scientific paper
The Waiotapu geothermal system is the largest in terms of surface extent and heat flow (17 km 2 and 600 MW, respectively) of the 20 major geothermal systems in the Taupo Volcanic Zone, New Zealand. Two extinct dacite cones and a rhyolite dome rise about 500 m above the Waiotapu valley. The majority of thermal features occurs in the valley over a relief of only 100 m and lies within coincident apparent resistivity and 30 m depth thermal anomalies. Thermal features comprise near-neutral pH chloride springs (often boiling), acid sulfate springs and mud pools, and mixed waters, the latter including both bicarbonate-chloride and acid sulfate-chloride springs. The majority of the sulfate discharges to the surface in the northern, higher altitude area, while most chloride that reaches the surface does so in the southern portion of the system. Comparison of measured and calculated surface chloride flux (the latter from measured heat flow and reservoir chloride concentrations) indicates that at least half of the deep chloride does not discharge at the surface, but flows laterally to the south at shallow depths, mixing with groundwater. The composition of well discharges indicates a shallow (400 m) reservoir of chloride fluid (about 1350 mg Cl/kg at 230°C) is diluted by a CO 2 -rich steam-heated water (nil chloride at about 160°C), the latter located on the margins of the system. This same dilution trend is apparent from the compositional variation and K-Na and K-Mg geothermometry of hot spring waters. However, there is chemical and stable isotope evidence from hot springs that cold groundwater also acts as a diluent at shallow levels, and very near the surface steam-heated acid sulfate waters mix with the upflow. Boiling of the deep chloride fluids prior to dilution results in the stabilization of K-feldspar. In contrast, dilution at shallower depths by the steam-heated waters (both CO 2 -rich and acid sulfate) leads to the water compositions initially shifting to K-mica (and interstratified clay) stability, and to kaolinite stability with further dilution and temperature decrease. The composition and distribution of these fluids are consistent with alteration mineralogy and zonation observed in drill core. Although boiling occurs at deeper levels and up to the surface in the upflow zone at Waiotapu, mixing is also common and is the dominant process on the margin and in the near-surface portion of the system. Boiling of deep chloride fluid is very common in the upper 1-2 km of the upflow zone of most geothermal systems, though it may be quenched by mixing with marginal waters, either cold groundwater in high relief terrane, or steam-heated waters in areas of lower relief. CO 2 -rich steam-heated waters are common on the margins of systems and will generally underlie the near-surface, acid sulfate steam-heated waters. Interaction with CO 2 -rich waters will form K-mica and interstratified clays, while acid sulfate waters cause kaolinite and even alunite to form. The former assemblage is commonly associated with typical low sulfidation epithermal gold mineralization as wallrock alteration, where gold-bearing veins comprise adularia and illite. In contrast, kaolinite is rare, and alunite absent, from a direct spatial and genetic association with gold mineralization in this environment. This implies that mixing of the deep, neutral pH chloride fluid with surficial acid sulfate waters does not commonly lead to ore-grade gold deposition. Mixing with (and cooling by) CO 2 -rich, steam-heated waters leads to clay stability, accounting for the large halos of clay alteration and low-grade metal anomalies which often surround veins of high gold grade. In contrast, mixing with surficial acid sulfate waters is unlikely to cause precious metal mineralization because the chloride fluids have already lost the majority of their gold through deeper boiling and dilution.
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