Hydrothermal minerals and precious metals in the Broadlands-Ohaaki geothermal system; implications for understanding low-sulfidation epithermal environments

The Broadlands-Ohaaki geothermal system is a boiling hydrothermal system hosted by a sequence of Quaternary felsic volcanic rocks and Mesozoic metasediments. More than 50 wells have been drilled (400 to 2,600 m deep) to assess the geothermal potential for the production of electricity. Fluids and precipitates sampled from wells, along with descriptions of the alteration minerals in more than 500 drill cores, provide a three-dimensional picture of the distribution of fluid types and secondary minerals. Interpretation of these features and the distribution of gold and silver highlight the relationship between alteration and mineralization in an active, low-sulfidation epithermal environment. Quartz, illite, K feldspar (adularia), albite, chlorite, and pyrite are the main hydrothermal minerals that occur in the deep central upflow zone at approx. 250 degrees C and 600 m depth. These minerals form through recrystallization of the volcanic host rocks and incorporation of H20, CO2, and H2S in the presence of a deeply derived chloride water containing nearly equal 1,000 mg/kg Cl and nearly equal 26,400 mg/kg CO2. At the same time, and on the periphery of the upflow zone, illite, smectite, calcite, and siderite form through hydrolitic alteration in the presence of CO2-rich steam-heated waters that contain 30 mg/kg CI and nearly equal 13,000 mg/kg CO2. Upward and outward from the deep central upflow zone, mineral patterns reflect the shift from rock-dominated to fluid-dominated alteration and the prevailing influence of boiling, mixing and cooling on fluid-mineral equilibria. Accordingly, the abundance of quartz and K feldspar increase toward the upflow zone, whereas clay abundance increases toward the margin of the upflow (with smectite dominating at 150 degrees C and illite dominating at 200 degrees C); the abundance of chlorite, pyrite and calcite varies here, but albite is absent. Geothermal production wells with high fluid fluxes are the main sites of precious-metal mineralisation. The deep chloride water (with or without minor amounts of vapor) enters the well at depths 500 m and undergoes a pressure drop that causes boiling. As a result, precious metals precipitate and accumulate as scales on back-pressure plates or as detritis in surface weir boxes; these deposits contain 10 to 1,000 mg/kg Au, 100 to 10,000 mg/kg Ag and nearly equal 10 to nearly equal 1,000 mg/kg As and Sb, each. Within production wells, platy calcite deposits as a scale at the site of first boiling near the fluid feed point, while crustification-colloform-banded amorphous silica deposits in surface pipe work. By contrast, the hydrothermally altered host rocks contain low concentrations of gold ranging from 0.01 to 1.0 mg/kg Au (68 analyses), and these correlate positively with arsenic (100 to nearly equal 5,000 mg/kg) and antimony (10 to nearly equal 500 mg/kg). Reaction path modeling using SOLVEQ and CHILLER shows that calcite, K feldspar, gold and amorphous silica deposit in sequence from chloride water that cools along an adiabatic boiling path (300 degrees to 100 degrees C) analogous to fluid flow in a production well. By contrast, calcite, quartz, K mica, and pyrite deposit from chloride water cools due to mixing with CO2-rich steam-heated waters; dilution prevents precipitation of precious metals. Thus field observations and reaction path modeling demonstrate that boiling is the main process influencing the deposition of precious metals. The results of this study show how peripheral hydrolytic alteration by CO2-rich steam-heated waters relate to propylitic and potassic alteration by chloride waters in the epithermal environment of a hydrothermal system. Both the distribution of alteration mineral assemblages associated with the different water types and the broad-scale distribution of temperature-sensitive smectite and illite reflect the location of the upflow zone. On a local scale, the occurrence of platy calcite, crustiform-colloform silica, and K feldspar in veins indicates the existence of boiling conditions conducive to precious-metal deposition.