De Souza Kovacs, Nadir1 and Lu, Yongjun1
1Geological Survey of Western Australia, Perth, Australia
The South West Terrane of the Yilgarn Craton is prospective for a number of important commodities such as gold, nickel and lithium; however, the region remains poorly geologically understood. The South West Terrane is one of the focus areas of the Geological Survey of Western Australia’s Accelerated Geoscience Program, 2020–21. This terrane is often associated with farming land, viticulture and grain growing; nevertheless it also hosts several word-class mineral deposits such as those at Boddington (Au–Cu), Greenbushes (Li–Ta) and Ravensthorpe (Ni). The geology of these deposits known from open-file statutory exploration reporting (stored in the Geological Surveys MINEDEX and WAMEX databases) provide a wealth of knowledge, that when combined with other publicly available geoscientific data compiled by the survey, data are providing new insights into the geological and mineralisation histories of the terrane.
The gold assay results from 84157 soil samples covering the South West Terrane were extracted from WAMEX and were spatially visualized and evaluated. The distribution of high-grade gold samples (0.53 to 1.37 ppm) cluster along several regional faults, including the Newdegate, Darkan, Manjimup, Tenterden, Koolanooka, Hyden, Pingarning, Dumbleyung Faults and other prominent but unnamed regional faults. By contrast, low-grade gold (<0.001 ppm) samples occur in areas without major faults. The regolith-landform map shows that the southern part of the Lockhart Paleovalley follows the strike of the Newdegate Fault and coincident structures on regional-scale magnetic images. The spatial distribution indicates that gold in soils is genetically linked to the regional faults, which are likely to have acted as fluid pathways during gold mineralization.
One particularly interesting feature is that high-grade gold in soils clustering along the north-trending Newdegate Fault also coincide with the isotopic boundary imaged by whole-rock neodymium isotopes of felsic igneous rocks. This isotopic boundary also correlates well with gravity and magnetic anomaly zones, and has been suggested to be the terrane boundary between the South West Terrane and the Youanmi Terrane (Lu et al. 2021 AESC). This correlation is supported by the spatial distribution of gold deposits including Griffins Find, Tampia and Edna May which are located on or close to this isotopic boundary. It indicates that gold anomalism in soils might be potentially useful for ranking deep crustal structures.
The spatial distribution of gold in soils in the South West Terrane is closely associated with regional faults. Through integration with other data, such as isotopic mapping or regional geophysical compilations, the regional scale gold in soil datasets can assist in prioritising the most prospective areas for exploration.
Nadir de Souza Kovacs is a Senior Regolith Geologist at GSWA, she has compiled several regolith maps including the State Regolith Map of Western Australia. Dr. Yongjun Lu is a renowned Senior Geochronologist Isotope Geologist at GSWA. He received the Waldemar Lindgren Award of Society of Economic Geologists in 2018.