A geochemical appraisal of potential economic mineral deposits in the advanced argillic alteration zone, Rotokawa geothermal field

Abstract

Volatile loss following boiling of the deep fluid in the Rotokawa geothermal field has resulted in the development of extensive near-surface advanced argillic alteration. The zone is characterised by the mineral assemblage amorphous silica + kaolinite + sulphur + alunite. This thesis considers the potential economic value of these minerals from a geological and chemical perspective. Oruanui Ignimbrite is locally altered at the land surface to a glaring white siliceous residue characterised as opal-A (amorphous) and opal-CT (disordered cristobalite) type precipitated silica. Reported free silica grades are between 84 and 92%, the principal impurities being kaolinite, alunite and detrital quartz. Estimated quantities of silicious material present exceed two million tonnes. No hypogene thermal water discharge occurs near these silica deposits. The active altering agent is supergene sulphuric acid, oxidised from hydrogen sulphide via bacterial action, and dissolved in precipitation and condensed steam, which percolates downward through the rock in response to gravity and concentration gradients. Over much of the thermal area, the Ignimbrite is obliterated by deep-seated hydrothermal eruptions which have blanketed the topography with thick breccia deposits. Where the breccias mantle altered ignimbrite they are frequently silicified to a comparable extent. Breccias which have been reworked in water-filled craters are parental to low grade (=45% kaolin) clay deposits. The kaolinite is difficult to upgrade because of an intimate association with amorphous silica particles. The critical difference with respect to physicochemical conditions that determine whethersilicification or kaolinisation occurs has yet to be fully elucidated. It is tentatively suggested that the flow rate of the descending supergene fluid is a controlling factor. Characterisation of geothermal soils demonstrates that their intense colour derives from the oxidation of iron leached out of the rhyolitic substratum. Hematite, goethite, hisingerite and protoferrihydrite were identified as minor constituents of the soils. Cinnabar and arsenic sulphides are not present. Vapour distillation at atmospheric pressure proves to be an effective procedure for obtaining ash-free sulphur from the Rotokawa sub-surface ore. At 500 °C, the arsenic concentration of the extracted sulphur is reduced by 80 to 98% relative to the parent ore. The results suggest that commercial quantities of sulphur with less than 50 ppm As and 25ppm Hg can be produced from the Rotokawa ore.