CONDITIONS OF CHLORITE GROWTH IN THE HILL END GOLDFIELD, NEW SOUTH WALES, AUSTRALIA: SOME PREDICTIONS AND EVALUATIONS

JIANCHUN LU
School of Earth Sciences, The University of Melbourne, Parkville, VIC 3052, Australia

PHILIP K. SECCOMBE
Department of Geology, The University of Newcastle, Callaghan, NSW 2308, Australia

JOHN L. WALSHE
Department of Geology, Australian National University, Canberra, ACT 2601, Australia

Abstract

A solid-solution model for the composition of chlorite has been used to calculate the physicochemical conditions of gold deposition for the slate-belt-hosted Hill End goldfield in the Paleozoic Lachlan Fold Belt of New South Wales, Australia. A P-T range of 295-340 C and 1.4-3 kbar, and the redox conditions for three major stages of gold mineralization are estimated by combining data for T, f(O2) and f(S2), calculated from chlorite compositions, with the results obtained from fluid- inclusion analyses and a knowledge of the mineral paragenesis. Gold deposition occurred in a redox environment where f(O2) ranged from 10-32 to 10-36.4, tracked by fluid and mineral species from below the CO2-CH4 stability boundary to above the pyrite-pyrrhotite stability boundary, respectively. The temperature corrections to fluid-inclusion Th data provided by the chlorite model could be as high as 190 C. With careful paragenetic control, the chlorite geothermometer seems to be a powerful tool in deciphering multistage fluid systems, and offers a solution to some of the ambiguities attending fluid-inclusion studies in metamorphic environments.