Geochemistry and stable isotopic composition of wall rock alteration 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.