Origin and fluid geochemistry of unconformity-related gold at the Black Ridge gold deposit, Clermont, Queensland

Abstract

Gold in the Black Ridge deposit is mainly concentrated along the unconformity between the Proterozoic(?) Anakie Metamorphics and the conglomerates of the Permian Blair Athol coal measures. Much lower gold concentrations occur through all upper rock units including the Tertiary basalts. The deposit is conventionally regarded as a fossil placer. Systematic studies of the mineralogy, fluid inclusion, stable isotope geochemistry, and thermodynamics of the Black Ridge deposit based on detailed surface and underground mapping and on logging of 57 drill holes have provided new geologic, mineralogical, and geochemical data for the deposit which are inconsistent with the placer model and favor a hydrothermal fluid mixing model.Siderite alteration has been observed in all rock units including the Tertiary basalt. A close spatial association of gold with siderite is revealed by core logging and field observations. This spatial relationship is further confirmed by the correlation between gold grade and intensity of siderite alteration according to chemical analysis and microscope observation. The direct paragenetic coexistence of gold with siderite and marcasite is also commonly observed 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 structurally controlled.Stable isotope studies of carbon and oxygen in siderite suggest that there were two different fluid sources involved in the alteration. One (fluid I) was a descending fluid in the conglomerates above the unconformity, and the other (fluid II) rose from the metamorphic rocks below the unconformity. Both the carbon and oxygen isotope data indicate fluid-mixing processes along the unconformity. The deposition of Black Ridge gold, which is directly associated with siderite, may have been caused by fluid mixing. Mineralogic and fluid inclusion studies and thermodynamic analyses suggest that fluid II was relatively reduced and roughly 250 degrees C, with a pH > or = 6.6, whereas the mixed fluid (III) was slightly oxidized (at the sulfide-oxide boundary), and about 120 degrees to 150 degrees C, with a pH < or = 5. The original fluid I was probably cool, relatively oxidized, and slightly acidic, although the quantitative parameters remain unknown.It is proposed that carbon may have been largely introduced by fluid II, and siderite precipitation controlled mainly by the content and oxidation state of iron in the host environments. Thermodynamic modeling indicates that fluid mixing just above the unconformity should have led to the gold solubility dropping more than four orders of magnitude in the fluids at Black Ridge. The calculation further suggests that in a system which is in equilibrium with siderite, gold may precipitate at very low fluid concentrations.Hydrothermal alteration and mineralization may have been generated by either igneous activity or tectonic movement-related metamorphism after Permian sedimentation. In either case, the potential for post-Permian hydrothermal gold mineralization may be significant in central Queensland and eastern Australia generally.