Behaviour of rare earth and high field strength elements during hydrothermal alteration of meta-turbidites associated with mesothermal gold mineralization in central Victoria, Australia

Abstract

A number of soil- and bio-geochemical surveys have shown that it is not uncommon for mesothermal gold deposits to display distinct enrichment in rare earth and high field strength elements in the supergene environment by up to two orders of magnitude. Potential sources for these anomalies and correlations with hydrothermal alteration in the hypogene zone, however, have remained undetermined. This investigation was undertaken in order to assess the behaviour, and potential as a geochemical exploration tool, of rare earth and high field strength elements in meta-turbidites during hydrothermal alteration associated with mesothermal gold mineralization. Sediment-hosted mesothermal gold deposits from central Victoria show evidence of intense wallrock alteration involving enrichment of CO 2, K 2O, S, As and Au, and loss of Na 2O. However, prolonged and extensive hydrothermal alteration of the Cambro-Ordovician meta-sedimentary host rocks did not result in significant mobilization or enrichment of rare earth elements in the wallrock assemblages investigated. With the exception of gold deposits within contact aureoles of Devonian granites, where fluid temperatures of 500° to 600°C resulted in minor preferential depletion of heavy rare earth elements in the alteration assemblages, the physico-chemical characteristics of the ore-bearing fluids (near-neutral pH, low salinity, 200°–300°C) meant that little or no mobilization or fractionation of rare earth elements took place during hydrothermal alteration and ore formation. This implies that enrichment in rare earth and high field strength elements associated with vegetation over mesothermal gold deposits reflect elevated values of these elements in the ore-bearing vein systems themselves, or, alternatively, resulted from dissolution of REE-enriched phases immediately adjacent to the mineralized structures during weathering and oxidation of sulphides. Based on generally extremely low concentrations of rare earths in the mineralized quartz veins themselves, the latter of the two scenarios is favoured to have led to the observed redistribution and concentration of rare earth and high field strength elements in the regolith zone above ore-bearing vein systems. These REE-enriched haloes generally are of laterally limited extent and therefore represent specific targets for mineral exploration.