Exploration for gold-bearing iron formation in the Burbidge area of the Southern Cross Greenstone Belt, W.A.

Abstract

In the Southern Cross Greenstone Belt, gold mineralisation is often associated with iron-rich sedimentary rocks. In the Burbidge area, a massive iron formation crops out on the eastern limb of the north?south trending Caudan Antiform. On the western limb, the iron formation is rarely exposed, although it has been proved at depth by drilling and has been mined in the Bronco South Pit. Geophysical surveys were undertaken to the south of Bronco South to locate the continuation of the iron formation and to optimise a drilling program. Results of these surveys are described herein. Three phases of ground magnetic data were acquired.I1093 Despite high-amplitude noise originating within surface laterites, a magnetic anomaly of approximately 500 nT amplitude and 300 m wavelength was identified, coinciding with the postulated position of the iron formation. Following wavenumber filtering to suppress the lateritic noise, the data from each line were modelled in terms of a two-dimensional, sub-vertical, sheet-like body at a depth of about 100 m. The iron formation in Bronco South is highly silicified and weathered, and has a very low magnetic susceptibility (< 0.00028 SI units). As this is too low to cause the observed anomalies, a magnetic susceptibility of 0.12 SI units was used, based on measurements of fresher iron formations exposed elsewhere in the area. We interpret the modelled body as representing unweathered iron formation below the weathering profile. Based on the magnetic modelling, a series of structural features affecting the iron formation is recognised. Since gold mineralisation associated with the iron formation is known to be concentrated in such zones, these areas are considered to be the most prospective within the horizon. Subsequent drilling confirmed the inferred position of the iron formation and intersected gold mineralisation at moderate grades.