Petrophysical characteristics of BIF ? hosted gold deposits and the application of downhole EM to their exploration, with examples from Hill 50 Gold Mine, Mt Magnet, Western Australia

Abstract

Hill 50 Gold Mine is located at Mt Magnet, 560 km NNE of Perth, in the Murchison Province of the Yilgarn Craton. The Mt Magnet greenstone belt consists of a sequence of mafic, ultramafic and felsic volcanic rocks, with interbedded volcanogenic sediments, predominantly banded iron formations (BIFs). The belt has been deformed into a principal structure, known as the Boogardie Synform. Major NNE ? NE striking faults, superimposed on the synform, are referred to as Boogardie Breaks and exhibit important control over many of the ore deposits. Historically, most of the gold production in the Mt Magnet district has come from BIF ? hosted deposits (e.g. Hill 50 and Boomer ? styles). Hill 50 ? style deposits are hosted by BIF "bars" of the Sirdar Formation, having a width generally greater than 20 m, and being structurally controlled by the Boogardie Breaks. Gold mineralisation is closely associated with pyrrhotite and to a lesser degree, pyrite. Hillcrest South and Brown Hill West prospects are currently being explored using a Hill 50 model. Petrophysical measurements carried out on samples from these areas demonstrate that pyrrhotite ? rich BlFs are characterised by lower magnetic susceptibilities and higher Koenigsberger Ratios, conductivities and densities. Use of downhole EM in locating the pyrrhotite ? rich BIF has been highly successful.Boomer ? style deposits are hosted by multiple NW ? trending BIF units, and breccias, and are distinguished by the close association of gold mineralisation with pyrite. Petrophysical testing on samples from the Boomer pit showed that the pyrite ? rich rocks are characterised by higher densities and magnetic susceptibilities than their pyrite ? poor counterparts. Although the conductivities are also slightly higher, Boomer ? style deposits are not detectable using downhole EM. The differing conductivities observed in unmineralised BlFs and BlFs with significant pyrite andlor pyrrhotite can be explained by varying textural and mineralogical characteristics of the metallic lustre minerals involved.