A study of the petrology and petrophysics of the Black Swan Komatiite, Eastern Goldfields, Western Australia

The Black Swan Project, situated some 43 km south of northeast of Kalgoorlie in Western Australia, formed a case study for an assessment of the petrophysical properties of the Black Swan Succession. The Succession hosts a number of discrete magmatic nickel sulphide bodies and associated disseminated mineralization. The study was undertaken with the aim of establishing possible vectors to ore within the project area and potentially elsewhere in the Yilgam Craton. Komatiite flows that host mineralization were examined using a proprietary downhole logging system, OMS~LOGG, which recorded magnetic susceptibility, inductive conductivity, resistivity, natural gamma and gamma-gamma readings on selected diamond drill core samples sourced from both trough and flanking channel environments. Hand-held magnetic susceptibility, specific gravity and conductivity readings were taken from both ore-grade material and host rock lithologies, and a number of representative drillcore samples described in terms of mineralogy, geology and geomechanical data. This information was then used to relate the geophysical properties to individual lithological units. However, establishing such relationships proved difficult, as the variable metamorphic influences within the Black Swan Succession have resulted in wide ranging and inhomogeneous mineralogical assemblages, with complex distribution and composition of magnetic oxide minerals. In general, potentially mineralised trough environments were characterized by high conductivity and a higher density. Carbonate was the most abundant mineral in this environment. Conversely, the channel flanks had lower conductivities, and lower density readings, while chlorite was the primary mineral constituent. Susceptibility readings returned similar ranges in both environments, indicating the technique could not be used as a discriminator. No correlation was noted between manual estimations of magnetite content and magnetic susceptibility or between manual estimates of opaque mineral percentages and magnetic susceptibility. A weak inverse linear relationship was noted between the percentage of opaque material and the gamma-gamma readings in downhole logs.