Geophysical responses of the Prominent Hill Fe-Cu-Au-U Deposit

Abstract

In November 2001, Minotaur Resources announced the results of discovery drillhole URN001. The drillhole intersected 107m averaging 1.94% copper and 0.66 g/t Au from 200m depth and a further 152m averaging 1.20% Copper and 0.61 g/t Au from 429m, hosted in a hematite breccia. URN001 was targeted at a gravity anomaly, located to the south of an intense magnetic anomaly. Targeting of the gravity anomaly utilised 3D smooth model inversion. Following the announcement of the assay results from URN001, potential field and electrical surveys were completed over the prospect area. Downhole induced polarisation logging of URN001, showed that the high grade copper mineralisation was associated with chargeability and resistivity anomalies. In an attempt to highlight zones of high grade copper mineralisation, a dipole-dipole induced polarisation (IP) survey was performed. A strong phase anomaly, coincident with a gravity anomaly was highlighted by this survey. Drill testing of this anomalous source resulted in the intersection of massive hematite, containing little to no copper mineralisation. Downhole IP logging, and petrophysical work has confirmed that the hematite is chargeable and has a low resistivity. The electrical properties of the ``barren hematite' are comparable to the high grade copper mineralisation. The barren hematite is also a significant contributor to the gravity anomaly seen at Prominent Hill. As a consequence it is difficult to distinguish between mineralised and unmineralised breccias by geophysical methods.