H091-0006
Stable isotope study of the Drake Goldfield, north-east New South Wales, Australia

Thursday, 10 December 2020
Poster
Hongyan Quan, University of New South Wales, PANGEA Research Centre, School of Biological, Earth and Environmental Sciences, Sydney, NSW, Australia, Ian Graham, University of New South Wales, PANGEA Research Centre, School of Biological, Earth and Environmental Sciences, Sydney, Australia, Rohan Worland, WHITE ROCK MINERALS LTD, West Ballarat, Australia, Lewis Adler, University of New South Wales, Bioanalytical Mass Spectrometry Facility, Kensington, Australia and Christian Dietz, University of Tasmania, Central Science Laboratory, Hobart, Australia
Abstract:
The Drake Goldfield is located in north-east New South Wales, Australia. It contains numerous low sulfidation epithermal precious metal deposits, which occur exclusively within the Drake Volcanics, a NW-SE trending sequence of Late Permian shallow volcanics and related epiclastics. Although being known and mined for ~100 years, there have been few detailed studies on the deposits associated with the Drake Volcanics, with the only comprehensive study being that on the Red Rock Field, one of the smallest within this area. The Drake Volcanics are centred upon a geophysical anomaly called “the Drake Quite Zone”, interpreted to be a collapsed volcanic caldera structure.

A total of 92 fresh carbonate and 36 sulfide samples (primarily sphalerite and pyrite) were micro-drilled from diamond drillcores from across the field and at various depths. pXRD analysis of these carbonates identified five species as follows: ankerite (5), calcite (62), dolomite (14), magnesite (8) and siderite (2). Overall, the δ13C varies little in comparison to the δ 18O which is highly variable(δ13CPDB from -21.32‰ to +1.42‰ and δ 18Osmow from -0.92‰ to +17.94 ‰ ). For S isotope analysis, overall, these have a wide range in δ34SCDT values from -16.54‰ to +2.10‰. Although there is much variation, they show a trend of δ34S approaching values of 0 with increasing depth.

The carbon and oxygen isotope results indicate that the carbon is mainly of magmatic origin and little affected by the widespread and pervasive low-temperature alteration and atmospheric precipitation. Calcite shows a widespread near horizontal distribution in oxygen isotope values suggesting derivation of oxygen from a large range of sources including meteoric, low-temperature hydrothermal and possibly magmatic. For magnesite, the spread in oxygen isotope values once again forms a linear array towards more positive δ18O values, suggesting derivation from low-temperature alteration fluids. Dolomite and ankerite have similar trends, both opposite to that for magnesite trending towards more negative oxygen isotope values and with much less variation. Sulphur isotope ratios of sulfide minerals indicate that the sulfur was most likely derived from at least two different sources; sedimentary sulphate and magmatic SO2, the relative importance of each varying from one deposit to another.

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