Geochemistry and Stable Isotope Composition of Altered Rocks at the Golden Cross Epithermal Au-Ag Deposit, New Zealand

Abstract

Golden Cross is a late Miocene volcanic-hosted epithermal Au-Ag deposit associated with quartz ± calcite ± adularia ± illite assemblages located in the Coromandel peninsula, New Zealand. Gold- and Ag-bearing quartz veins occur in intensely altered flows and lesser pyroclastic volcanic rocks of andesitic and dacitic composition. The primary control of whole-rock geochemistry at Golden Cross, as at other volcanic-hosted epithermal deposits associated with quartz ± calcite ± adularia ± illite assemblages, is the destruction of igneous plagioclase by hydrothermal alteration and K metasomatism from hydrothermal fluids. Quantitative mass change calculations indicate that, irrespective of the rock type (andesite versus dacite), hydrothermally altered rocks are enriched in K, Rb, As, and Sb around the Empire vein system and stockwork. In contrast, Na and Sr are strongly depleted around the veins, with Fe locally depleted. Silicon, Ca, Mg, and Ba are variably enriched and depleted, with Si enriched immediately adjacent to the veins and Ca only locally enriched. The enrichment and depletion trends for these elements are directly related to the degree of alteration and the alteration mineralogy. Intensely altered rocks show addition of K and Rb, coupled with loss of Na and Sr, which reflect the replacement of plagioclase by adularia and/or illite. Calcium depletion also reflects the destruction of plagioclase, although Ca loss is locally offset by the formation of hydrothermal calcite. Progressively higher Na and Sr away from the veins reflect greater preservation of plagioclase and the transition into less altered rocks on the margins. Potassium, Rb, Na, and Sr all define anomalies that extend 200 to >350 m east of the veins and are more widespread than As and Sb, which are anomalous within 150 to 200 m of the veins. The geochemical zonation patterns identified at Golden Cross are very similar to those at other epithermal deposits associated with quartz ± calcite ± adularia ± illite assemblages. Potassium, Na, Rb, Sr, and Ca enrichment and depletion patterns provide insight into the intensity and type of alteration, thereby complementing traditional analyses of Au, Ag, and other pathfinder elements, such as As and Sb, that are used to explore for epithermal Au-Ag deposits. Potassium, Na, Rb, Sr, and Ca enrichment and depletion patterns at Golden Cross are larger than halos defined by precious metals and other trace element pathfinders, thereby providing a larger target area for exploration. Potassium metasomatism is particularly important because the Empire vein system occurs in altered rocks that are significantly enriched in K. Potassium metasomatism can be evaluated by raw K 2 O data or, more effectively, by K mass changes, molar element ratios of (2Ca + Na + K)/Al versus K/Al (wt % K 2 O * 100)/ppm Sr, Rb/Sr ratios, and K numbers [K/(2Ca + K + Na) molar]. Whole-rock oxygen isotope patterns at Golden Cross are similar to those described at other epithermal deposits associated with quartz ± calcite ± adularia ± illite assemblages. However, the area defined by low δ 18 O values is smaller than the area that can be defined by field mapping of alteration minerals or whole-rock major and trace element geochemistry.