oa Borehole MMR at Marvel Loch gold mine – solving for the conductive overburden

Multi-parameter logging at Marvel Loch gold mine in the Yilgarn Craton of Western Australia indicated that the mineralisation was more conductive than the host rocks and thus potentially targeted using downhole electrical methods. Borehole MMR was the preferred to borehole EM due to the lack of large zones of consistently conductive mineralisation and the lack of conductive features within the surrounding host rock.

The drilling program consisted of a parent hole with a number of wedges. Two of these wedge holes were surveyed with borehole MMR however the upper portion of the hole through the overburden/regolith (ie.the parent hole) could not be surveyed due to steel casing and thus the nature of the conductive overburden could not be readily determined. Hence an empirical approach was initially taken whereby stripping the raw total MMR field with a layered-earth field was computed for a range of different overburden conductivities until a credible background to the axial component of the anomalous MMR response is obtained. TEM soundings acquired post survey in the vicinity of the drillhole collar to assist in determining the overburden characteristics and improved the stripping process.

The results of the two wedge holes were similar with subtle negative axial anomalies related to the upper and lower mineralised zones and thus attributable to conductors below the drillholes. The modelling indicates a southerly plunge to the lower conductive zone which is consistent with the southerly plunge of the mineralisation within the pit. The adjacent pit has been modelled as conductor carrying current in the reverse direction to simulate a resistor producing a minimum at around 150m and influences the shape and amplitude of the anomaly due to the upper alteration zone.