Geoelectric properties of the Elura prospect, Cobar, NSW

Abstract

The Bureau of Mineral Resources investigated the geoelectric properties of fresh and weathered rocks at Elura through a program of down-hole logging, Schlumberger soundings, and laboratory measurements on drill core. Resistivity logging showed that resistivities of intensely weathered country rocks, comprising shales, sandstones and siltstones, just below the water-table (at about 80 m) are less than 30 ohm-m, in some instances being as low as 1 ohm-m. Resistivities increase with depth as rocks become less weathered: at 200 m resistivities of approximately 2000 ohm-m were recorded. Induced polarisation logging indicates that the chargeability of country rock is approximately 10 mV/V (1?2% FE) just below the water-table, and increases to approximately 20 mV/V (2?4% FE) at 200 m depth. Laboratory measurements and results of logging indicate that the mineralisation has a resistivity of about 0.2 ohm-m, and a very high chargeability. Interpretation of Schlumberger depth soundings indicates that the top 5 m of section consists of a thin, resistive layer which is sometimes overlain by a thinner, more conductive layer. Below a depth of approximately 5 m, the section has a low resistivity of approximately 10 ohm-m and a very low chargeability which probably represents strongly weathered rocks. This strongly weathered zone appears to be approximately 110 m thick, below which resistivity of the rocks is shown by logging to increase with depth, from approximately 100 ohm-m at a depth of 110 m to approximately 2000 ohm-m at a depth of 200 m. The latter section has a higher chargeability than the stongly weathered zone, as a result of less weathered rocks lying below 110 m. An observed decrease in the resistivity of the country rock in the vicinity of the deposit may indicate a fracture halo or some other effect related to the mineral deposit. Analyses of the sounding data show that over the deposit, the top 50 m of section has a higher conductivity?thickness product than the section on either side of the deposit. A zone with higher resistivity and chargeability is interpreted as lying above the deposit and extending to within about 40 m from surface. This zone may reflect the presence of a siliceous gossan. Southeast of the deposit a sounding at the site of coincident transient electromagnetic and induced polarisation anomalies has a geoelectric section similar to that observed over the Elura deposit.