The Textures and Mineralogy of the Kohuamuri Siliceous Sinter, Coromandel Peninsula, and its Economic Potential

Abstract

The Kohuamuri sinter is an extensive (0.2km2) and well-preserved, fossilised, siliceous sinter in the eastern province of the Hauraki Goldfield, within the Coromandel Volcanic Zone. Sinters are surface expressions of geothermal systems. The Kohuamuri sinter consists of two siliceous sinter deposits, the Kohuamuri and Kaitoke deposits, which are surrounded by silicified Miocene and Pliocene volcanics of the Whitianga volcanic centre. The Kohuamuri deposit consists of a large in situ sinter outcrop, with previous mining attempts visible at the base, as well as a sinter boulder field lining the stream. The Kaitoke deposit constitutes large sinter boulders within a landslide situated on a normal fault. In situ and float blocks of sinter, exhibit patchy lithofacies within re-­‐ crystallised quartz, which has obliterated some of the textures due to silica diagenesis and overprinting by late stage acidic burial fluids. The well-­‐preserved textures resemble similar fabrics, at macroscopic and microscopic scale, to those observed in actively forming and fossilised hot spring deposits, derived from the surface discharge of near-­‐neutral alkali chloride fluids. Geyserite was found at both deposits suggesting spring-­‐fluid temperatures >90°C proximal to the vent. Other facies of the Kohuamuri sinter contain textures synonymous with the mid-­‐apron areas, including palisade and conical tufted, and distal apron areas of plant-­‐rich facies and clotted fabric of geothermally influenced marshes. For these samples collected in place, lithofacies were used as vectors to interpret the location of geothermal upflow and infer the paleo-­‐environment of the sinter deposits. Other textures preserved within the Kohuamuri sinter suggest intermittent flow of geothermal fluids, such as network fabrics and fragmented sinter, as well as a hydrothermal breccia formed due to over-­‐pressurisation. Mineralogy of the deposits showed changes in fluid composition over time, from alkali chloride fluids to late stage acidic fluids, which caused corrosion of diagenetically mature silica. Mineralogy of the hydrothermal breccia revealed igneous minerals with hydrothermal alteration to illite clay, a process that occurs at temperatures >210°C. Trace elemental analysis showed a relative elevation in precious and base metals, including gold, silver and pathfinder elements associated with epithermal deposits. This provides evidence of possible ore mineralisation at depth. Elemental association also suggested that some occur as sulphides, as in other active geothermal systems and epithermal deposits. This method of assessing the paleo-­‐environment, paleo-­‐fluids and elemental composition has been used in other goldfields throughout the world. One such is the Jurassic Deseado Massif Ag-­‐Au province of Patagonia, Argentina, where Jurassic hot spring deposits, epithermal mineralisation and structures are aligned regionally. This method could be used at other sites throughout the Hauraki Goldfield.