Geophysics of the Prominent Hill prospect, South Australia

Abstract

In November 2001, Minotaur Resources announced the results of discovery drill hole URN001, at the Prominent Hill prospect. The drill hole intersected 107 m averaging 1.94% Cu and 0.66 g/t Au from 200 m depth, and a further 152 m averaging 1.20% Cu and 0.61 g/t Au from 429 m, hosted in a haematite breccia. URN001 was drilled to test a gravity anomaly, located to the south of an intense magnetic anomaly. 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 IP/resistivity survey was performed. A strong phase anomaly, coincident with the gravity anomaly, was highlighted by this survey. Drill testing of this anomalous source resulted in the intersection of massive haematite, containing little to no copper mineralisation. Downhole IP logging and petrophysical work have confirmed that the haematite is chargeable and has a low resistivity. The electrical properties of the "barren haematite" are comparable to the high-grade copper mineralisation. The barren haematite is also a significant contributor to the gravity anomaly. As a consequence, it is difficult to distinguish between mineralised and unmineralised breccias using geophysical methods. Additional geophysical survey methods, such as TEM and CSAMT have been trialled at Prominent Hill, but with limited success. Interpretation of electrical datasets is hampered by the presence of conductive black shale in the cover sequence.