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

A solid-solution model for the composition of chlorite has been used t o calculate the physicochemical conditions of gold deposition for the slate-belt-hosted Hill End goldfield in the Paleozoic Lachlan Fold Bel t of New South Wales, Australia. A P-T range of 295-340 degrees C and 1.4-3 kbar, and the redox conditions for three major stages of gold mi neralization, are estimated by combining data for T, f(O-2) and f(S-2) , calculated from chlorite compositions, with the results obtained fro m fluid-inclusion analyses and a knowledge of the mineral paragenesis. Gold deposition occurred in a redox environment where f(O-2) ranged f rom 10(-32) to 10(-36.4), tracked by fluid and mineral species from be low the CO2-CH4 stability boundary to above the pyrite-pyrrhotite stab ility boundary, respectively. The temperature corrections to fluid-inc lusion T-h data provided by the chlorite model could be as high as 190 degrees C. With careful paragenetic control, the chlorite geothermome ter seems to be a powerful tool in deciphering multistage fluid system s, and offers a solution to some of the ambiguities attending fluid-in clusion studies in metamorphic environments.