Abstract:
The Golden Cross fossil hydrothermal system (probable age Pliocene to Pleistocene) is located 8km northwest of Waihi, N.Z., within calc-alkaline volcanics of the Coromandel Peninsula.
Intense country rock alteration within a volume of 3km x 1.5km x 0.5 - 0.8km is characterised by the assemblage; quartz-adularia-smectite-interstratified illite/smectite-illite-chlorite-calcite-pyrite. Gravity models (2-D and 3-D) and magnetics are consistent with the alteration zone observed in drill core.
Fluid inclusion studies suggest that initially, early, hot brine (Th>350ºC, 12-14 wt% NaCl equiv.) permeated country rock along joints under predominantly lithostatic conditions. Unusual inclusions of high apparent salinity (26-29 wt% NaCl equiv, Th 270°C) containing at least two immiscible liquids of unknown composition are restricted to the upflow portion of the system.
Alkali-chloride fluid of probable meteoric origin (pH~4.7) precipitated main-stage quartz (Th 200-250°C, predominantly 200°C, 0.8-2.7 wt% NaCl equiv.) under predominantly hydrostatic conditions. Permeability was enhanced by faulting, and subsequent hydraulic fracturing. Precious metals are confined to the main-stage quartz zone, and were precipitated as a result of changes in fluid temperature.
Temperature variations at the time of main-stage quartz deposition are reflected in the distribution of clay types. The sequence smectite-interstratified illite/smectite-illite-chlorite is observed with increasing depth and temperature.
Lateral fluid flow is evidenced by δ18O contours and the distribution of quartz and adularia.
Late calcite was deposited from near-neutral Ph alkali-chloride fluid (Th 150-200°C, 0.8-1.0 wt% NaCl equiv.) and is located beneath the main-stage quartz zone.
As the system waned, near-surface acid-sulphate water was drawn down permeable channels within the system to a minimum depth of 500m below surface level, overprinting earlier mineral assemblages with kaolinite ± hematite ± limonite ± jarosite.
Water/rock ratios indicate the system was hydrologically mature.