Abstract:
The Late Miocene Whitianga Volcanic Centre of the Coromandel Volcanic Zone, North Island, New Zealand, is host to 18 known, siliceous and intensely silicified deposits including: silicified volcanics; sediments of inferred lacustrine, fluvial and mud pool origin; siliceous hot spring deposits, or sinters; and silicic veins. Collectively these siliceous and silicified deposits indicate past geothermal activity that occurred at the Earth's surface and in the shallow subsurface. Sinters, in particular, are of interest because they are derived from deep geothermal reservoirs and precipitate from discharged thermal fluids at the intersection of the water table with the land surface. However, some siliceous and silicified features in hydrothermal paleoenvironments have been misidentified as sinters. By studying textural types, distributions, structural setting, mineral paragenesis and trace elemental composition of the siliceous and silicified deposits in the Whitianga Volcanic Centre, it is possible to differentiate sinters and identify their spring-vent source areas (100–70 °C) from more distal thermal spring discharge locations (<70 °C to ambient), extent of hydrothermal alteration, and, in places, delineate zones of epithermal upflow where Au–Ag mineralisation may have occurred at depth. This has important implications for the exploration of mineralised epithermal ore deposits at local to regional scales, in particular for the Hauraki Goldfield of the Coromandel Volcanic Zone.
