Emmanuel Madayag

The Drake Goldfield, also known as Mount Carrington, is located in north-eastern New South Wales, Australia. It contains a number of low–intermediate-sulfidation epithermal precious metal deposits with a current total resource of 724.51 metric tons of Ag and 10.95 metric tons of Au. These deposits occur exclusively within the Drake Volcanics, a 60 × 20 km NW-SE trending sequence of Late Permian volcanics and related epiclastics. Drilling of the Copper Deeps geochemical anomaly suggests that the volcanics are over 600 m thick. The Drake Volcanics are centered upon a geophysical anomaly called “the Drake Quiet Zone” (DQZ), interpreted to be a collapsed volcanic caldera structure. A total of 105 fresh carbonate samples were micro-drilled from diamond drillcores from across the field and at various depths. A pXRD analysis of these carbonates identified five types as follows: ankerite, calcite, dolomite, magnesite, and siderite. Except for three outlier values (i.e., −21.32, −19.48, and 1.42‰), the δ¹³C_VPDB generally ranges from−15.06 to −5.00‰, which is less variable compared to the δ¹⁸O_VSMOW, which varies from −0.92 to 17.94‰. μ-XRF was used to analyze the elemental distribution, which indicated both syngenetic/epigenetic relationships between calcite and magnesite. In addition, a total of 53 sulfide samples (primarily sphalerite and pyrite) from diamond drillcores from across the Drake Goldfield were micro-drilled for S isotope analysis. Overall, these have a wide range in δ³⁴S_CDT values from −16.54 to 2.10‰. The carbon and oxygen isotope results indicate that the fluids responsible for the precipitation of carbonates from across the Drake Goldfield had complex origins, involving extensive mixing of hydrothermal fluids from several sources including those of magmatic origin, meteoric fluids and fluids associated with low-temperature alteration processes. Sulfur isotope ratios of sulfide minerals indicate that although the sulfur was most likely derived from at least two different sources; magmatic sulfur was the dominant source while sedimentary-derived sulfur was more significant for the deposits distal from the DQZ, with the relative importance of each varying from one deposit to another. Our findings contribute to a greater understanding of Au-Ag formation in epithermal environments, particularly in collapsed calderas, enhancing exploration strategies and models for ore deposition.

The intermediate-sulfidation epithermal Strauss deposit is part of the Drake Goldfield of north-eastern NSW, Australia. The mineralization is gold dominant with minor silver and significant zinc, copper and lead. Strauss has a combined total resource of 2.66 Mt @ 1.5 g/T Au and 1.9 g/T Ag. Petrographic analysis has shown the deposit to be hosted by varying layers of dacitic ash flow tuffs and a cross-cutting lamprophyre dyke though previous authors classified some of the tuffs as intrusive andesites. Mineralization mainly occurs in the form of vein stockworks of three main orientations. Structural analysis indicates that these resulted from two fault system events and one conjugate fault system. Petrographic and SEM analysis shows that there are two different mineralization styles, a pyrite dominant and chalcopyrite dominant assemblage, with three main mineralization events. Correlation analysis for the assay data indicates that Au has a strong relationship with Ag and a moderate correlation with Pb. XRD analysis indicates widespread argillic-phyllic and propylitic alteration. Silver tellurides have been found as late-stage grains associated with the chalcopyrite mineralization assemblage. Sr shows a significant depletion due to the intense and pervasive alteration at Strauss. The carbon and oxygen isotopes of the vein carbonates suggests that the carbon from the late-stage carbonates was derived from a magmatic source with a significant input of low temperature meteoric water, while the sulfur isotopes indicate a magmatic sulfur source.