ORIGIN AND FLUID GEOCHEMISTRY OF UNCONFORMITY-RELATED GOLD AT THE BLACK-RIDGE GOLD DEPOSIT, CLERMONT, QUEENSLAND

Gold in the Black Ridge deposit is mainly concentrated along the uncon formity between the Proterozoic(?) Anakie Metamorphics and the conglom erates of the Permian Blair Athol coal measures. Much lower gold conce ntrations occur through all upper rock units including the Tertiary ba salts. The deposit is conventionally regarded as a fossil placer. Syst ematic studies of the mineralogy, fluid inclusion, stable isotope geoc hemistry, and thermodynamics of the Black Ridge deposit based on detai led surface and underground mapping and on logging of 57 drill holes h ave provided new geologic, mineralogical, and geochemical data for the deposit which are inconsistent with the placer model and favor a hydr othermal fluid mixing model. Siderite alteration has been observed in all rock units including the Tertiary basalt. A close spatial associat ion of gold with siderite is revealed by core logging and field observ ations. This spatial relationship is further confirmed by the correlat ion between gold grade and intensity of siderite alteration according to chemical analysis and microscope observation. The direct parageneti c coexistence of gold with siderite and marcasite is also commonly obs erved under the microscope, SEM, and electron microprobe. The grade of gold is correlated with compositional variation of zoning in siderite . Moreover, the main ore zones and the individual orebodies are struct urally controlled. Stable isotope studies of carbon and oxygen in side rite suggest that there were two different fluid sources involved in t he alteration. One (fluid I) was a descending fluid in the conglomerat es above the unconformity, and the other (fluid II) rose from the meta morphic rocks below the unconformity. Both the carbon and oxygen isoto pe data indicate fluid-mixing processes along the unconformity. The de position of Black Ridge gold, which is directly assocaited with sideri te, may have been caused by fluid mixing. Mineralogic and fluid inclus ion studies and thermodynamic analyses suggest that fluid II was relat ively reduced and roughly 250-degrees-C, with a pH greater-than-or-equ al-to 6.6, whereas the mixed fluid (III) was slightly oxidized (at the sulfide-oxide boundary), and about 120-degrees-C to 150-degrees-C, wi th a pH less-than-or-equal-to 5. The original fluid I was probably coo l, relatively oxidized, and slightly acidic, although the quantitative parameters remain unknown. It is proposed that carbon may have been l argely introduced by fluid II, and siderite precipitation controlled m ainly by the content and oxidation state of iron in the host environme nts. Thermodynamic modeling indicates that fluid mixing just above the unconformity should have led to the gold solubility dropping more tha n four orders of magnitude in the fluids at Black Ridge. The calculati on further suggests that in a system which is in equilibrium with side rite, gold may precipitate at very low fluid concentrations. Hydrother mal alteration and mineralization may have been generated by either ig neous activity or tectonic movement-related metamorphism after Permian sedimentation. In either case, the potential for post-Permian hydroth ermal gold mineralization may be significant in central Queensland and eastern Australia generally.