Metal Transport and Deposition in the Broadlands Geothermal System

Abstract

Theoretical and analytical studies of the Broadlands geothermal system have lead to development of a thermodynamic model for metal transport and deposition applicable to typical two phase hydrothermal fluids. This model explains how such fluids rising through the earth's crust producing propylitic, argillic and advanced argillic alteration assemblages can give rise to metallically zoned ore deposits in the upper few kilometers as a result of the chemistry of the fluids and the metal complexes they contain. Using fluid phase equilibria data and thermodynamic fluid-mineral equilibria data together with thousands of measured temperature data and 320 bulk rock x-ray diffraction analyses, Broadlands demonstrates that two phase hydrothermal fluids typically have CO2, H2 and H2S buffered by fluid phase equilibria producing “steady states”. These “steady states” consist of a near neutral chloride fluid forming a propylitic zone at temperatures above ≈ 240°C and an acidic mixed fluid forming argillic and advanced argillic zones from ≈ 240 to ≈ 150°C. Ore mineralogical studies of over 200 samples, fluid inclusion homogenisation and freezing data and analyses of 239 heavy mineral separate samples for nine metals by atomic absorption spectrophotometry indicate the “steady state” fluids in Broadlands are depositing discrete heavy minerals with trace solid solutions and/or un-ionised metal atoms forming a zonation of "base" metals below higher and “precious” metals below moderate temperatures. The thermodynamic model developed for metal transport and deposition is based on the slope [dlogm•/dpH]t (where m•= molal concentration of M(L)^c vn) which is derived from general dissolution equilibria of the metal or its homonuclear sulphide. The model can predict the stoichiometry of the dominant metal transporting complexes and the approximate temperatures of their deposition as ore minerals in typical two phase hydrothermal fluids.