Uncovering the Cobar Basin – new results from the NSW MinEx NDI areas

Folkes, Chris1; Deyssing, Liann2; Trigg, Steven2; Carlton, Astrid1; Schifano, Joe3

1Mineral Exploration Cooperative Research Centre, Geological Survey of New South Wales, Department of Regional NSW, Maitland, NSW, Australia, 2Mineral Exploration Cooperative Research Centre, Geological Survey of New South Wales, Department of Regional NSW, Orange, NSW, Australia, 3Mineral Exploration Cooperative Research Centre, University of New South Wales, Sydney, NSW, Australia

The Cobar region of central-western New South Wales hosts numerous precious and base metal mineral systems, many exploited by active and historical mine workings. These are mostly located around rock outcrops and under shallow cover sequences of the Cobar Basin. The North Cobar and South Cobar MinEx CRC National Drilling Initiative (NDI) areas were selected by the Geological Survey of New South Wales (GSNSW) to improve the understanding of the geology, mineral systems and groundwater in basement and regolith profiles that extend away from the margins of these known mineralised areas.

Understanding the prospective basement geology in the Cobar region is complicated by variations in the depth and nature of the weathering profiles through the region. A mix of transported and in situ weathering reflects a complex palaeo-landscape.

Recent activities by the GSNSW are designed to map the thickness and character of the weathering profile and increase our knowledge of basement geology. This work will inform and complement proposed drilling in the NDI areas to further investigate the geochemical and petrophysical signatures of basement geology and mineral systems. The activities also have implications in understanding groundwater resources.

Comprehensive audit and gaps reports were published in 2020 for the North Cobar and South Cobar NDI areas. These reports reviewed existing data, provided recommendations for new sampling and data capture, and prioritised the scientific questions that should be addressed. Recent mineral potential mapping and mineral-systems studies identified new prospective areas and provided a better understanding of the main controls on, and timing of mineralisation for the Cobar region.

An airborne electromagnetic (AEM) survey was flown in the region in 2019, comprising 116 east‒west lines spaced at 2.5 km and 5 km, totalling approximately 11,000 line km. Ongoing modelling and interpretation of the data have enhanced knowledge of depth and nature of the cover, groundwater systems, structural geology (e.g. fault locations and geometry), mineral systems and stratigraphy. The AEM data, with other datasets such as fault attribution, NSW Seamless Geology, and crustal-scale fault models, are being integrated into a new 3D model of the region that will benefit mineral explorers and hydrogeologists.

The AEM data have also been integrated with drilling, hydrogeochemical, biogeochemical and spectral surveys. Downhole lithology and assay information from legacy drilling has been imported into 3D viewable datasets. Hydrogeochemical analyses from waterbores in the Cobar region were completed by CSIRO and GSNSW and highlighted new areas for potential exploration. Researchers from UNSW analysed Cypress Pine needles to provide a regional biogeochemical dataset with focus areas over known mineralisation. Spectral scanning of legacy drillcore using the HyLoggerTM has provided useful constraints on the depth and nature of weathering, and alteration of basement rocks.

These activities build on recent mineral system studies to advance the understanding of the geology, and mineral and groundwater resources in the North Cobar and South Cobar NDI areas. This has greatly helped to inform the site selection and scientific questions to be answered with future MinEx CRC drilling in these NDI areas.


Chris Folkes is a senior geoscientist in the Regional Mapping team at the Geological Survey of NSW. He completed his undergraduate studies at the University of Bristol in the UK, and a PhD at Monash University in volcanology, igneous geochemistry and geochronology.

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