Abstract

The Golden Cross epithermal gold-silver deposit is located 120 km north of presently active geothermal systems of the Taupo volcanic zone in the Waihi district, Coromandel Peninsula, New Zealand. Gold and silver of economic grades (1-20 g/metric ton equiv gold over 7 million metric tons) occur in zones dominated by quartz reefs, veins, and silicified hydrothermal breccias with adularia common in these zones. The highest precious metal concentrations occur in an inferred fluid up-flow zone. Gold and silver are thought to have precipitated from cooling hydrothermal fluid largely of meteoric origin, in direct response to hydraulic fracturing and subsequent boiling.Alteration minerals are dominated by clay, quartz, adularia, calcite, and pyrite. Hydrothermal clays show a distinct zonation with the sequence smectite, interstratified illitesmectite, illite, and chlorite corresponding to a regional increase in temperature with depth. Kaolinite overprints the above mineral assemblage to at least 400 m below the present ground surface. Quartz and adularia appear to be good indicators of high permeability in the past, whereas bladed calcite indicates local boiling. Fluid permeability was enhanced by repeated hydraulic fracturing. Alkali-chloride fluids with near-neutral pH were responsible for the dominant alteration described above, whereas the downward percolation of a near-surface acid-sulfate fluid, as hydrothermal activity waned, precipitated kaolinite + or - hematite + or - jarosite.Fluid inclusion studies reveal that in main-stage quartz, type I liquid-rich inclusions with an average T h of 200 degrees C and T (sub m ice ) of -0.5 degrees to -1.6 degrees C are dominant. Type I inclusions within late precipitated calcite show a T h decrease from 210 degrees to 150 degrees C from the center to the periphery of a large calcite body. Early (?) type Ia liquid-rich inclusions within quartz have a T h > 350 degrees C and T (sub m ice ) values of -7 degrees to -10.2 degrees C. Type II (vapor rich) and type III (two-liquid, vapor rich) inclusions within quartz with a T h 270 degrees C have T (sub m ice ) values typically from -25.4 degrees to -29.7 degrees C. Type III inclusions have not previously been described from epithermal deposits in New Zealand and possibly contain organic liquid(s).Oxygen isotopes of hydrothermal quartz exhibit distinct horizontal and vertical gradients in an east-west cross section through the deposit. The delta 18 O SMOW values from 7.0 to 11.7 per mil are inferred to represent a gradient in permeability (variable water/rock ratios) with higher fluid flowthrough in the west of the cross section, where precious metals are concentrated. Calcite delta 18 O SMOW values of 5.2 to 7.8 per mil combined with fluid inclusion homgenization temperatures are consistent with a model whereby calcite was precipitated late and over a relatively short period of time. Calculated values of delta 18 O SMOW of water for main-stage quartz and late-phase calcite indicate a meteoric source of -5 to -6 per mil for the hydrothermal water. Values for delta 34 S CDT from hydrothermal pyrite of -1.4 to +4.8 per mil reflect, in part, late acid-sulfate fluid. Alteration assemblages, inferred fluid compositions, temperature, apparent salinity, and water isotopic compositions of the Golden Cross epithermal gold-silver deposit are analogous to those for some active geothermal systems of the Taupo volcanic zone. Golden Cross may thus have been a part of a much larger geothermal system, not unlike those of the zone today.

First Page Preview

First page PDF preview
You do not currently have access to this article.