Exploring through cover – the integrated interpretation of high resolution aeromagnetic, airborne electromagnetic and ground gravity data from the Grant’s Patch area, Eastern Goldfields Province, Archaean Yilgarn Craton. Part C: Combining geophysical meth

Abstract

Archaean lode gold deposits usually follow narrow structures, such as lithological contacts, ductile shears and brittle cross-faults. These features cannot be identified directly in covered areas and are often impossible to correlate between exploration drillholes. High-resolution geophysical methods provide dense, regularly spaced coverage for mapping continuity of structure and the geometry of the regolith at depth. Such geophysical ?pattern mapping? contrasts to direct anomaly targeting in that it requires image processing and integration of geological and geochemical data to produce ?holistic? regolith and bedrock geology maps used for targeting gold bearing structures. Grant?s Patch is a semi-mature lode gold corridor located 45 km NW of Kalgoorlie in Western Australia. The southern part of this corridor is covered by high-resolution aeromagnetic, airborne electromagnetic and gravity data, has uniform exploration drillhole coverage to non-weathered bedrock, and has very little surface disturbance from mining infrastructure. These geophysical methods are shown to produce effective results, yet no method is superior as they each provide complementary information. At Grant?s Patch, aeromagnetics identifies magnetic greenstone units, such as mafic to ultramafic sills and dolerite horizons, faults as disruptions crossing magnetic units, and paleochannels containing ferruginous gravel. Gravity maps density differences between greenstone rock units, identifies buried granitic intrusions, and identifies large paleochannels. High-pass filtered grids of gravity data reveal differential weathering between rock units and along faults zones, even where no pattern is revealed in the aeromagnetic data. The airborne electromagnetic (AEM) response is dominated by conductive clays, predominately in situ as saprolite, and saline groundwaters in the regolith that usually follow litho-dependant weathering of bedrock units. AEM data can also identify graphitic shale units as bedrock conductors where conductive regolith overburden is thin and allows for penetration. Cross-faults are mapped as disruptions of the conductive units and may themselves be either conductive or resistive.