Relationship between regolith materials, petrophysical properties, hydrologeology and mineralisation at the Cawse Ni laterite deposits, Western Australia: Implications for exploring with airborne EM

Abstract

In the Eastern Goldfields of Western Australia, airborne electromagnetic (AEM) data have been acquired over areas prospective for nickeliferous laterite deposits, although their application in exploring for this style of mineralisation is not well understood. In part, this can be attributed to a lack of detail concerning relationships between supergene Ni enrichment and the petrophysical, particularly electrical, properties associated with prospective regolith materials and settings. This issue was addressed using a combination of multi-parameter borehole geophysical techniques and laboratory analyses of mineralogy, geochemistry and soluble salt content for 24 drill holes in a zone of known mineralisation associated with the Cawse lateritic Ni deposits, located in the Eastern Goldfields, W.A. The regolith associated with supergene Ni enrichment exhibited a complex vertical conductivity structure. In places, elevated conductivities were coincident with Ni-Mn-Co mineralisation. These correlations were noted to occur at hydromorphic barriers associated with structural discontinuities, or in places where regolith textural changes were marked. A better understanding of the complex spatial patterns associated with the electrical character of the regolith at Cawse, was achieved by examining the weathered profile as a set of hydrostratigraphic units, namely the transported cover, zone of illuviation and in situ regolith. This framework allows the observed regolith electrical structure to be explained in terms of contemporary groundwater processes and the accumulation of soluble salts. The observed petrophysical (electrical) response of the regolith at Cawse suggests that AEM has a role in locating impermeable barriers and faults which may, respectively, represent sites for shallow, high grade Mn-Co-Ni and garnierite mineralisation, particularly where they are coincident with local accumulations of soluble salt and moisture.