Geophysical exploration at the Comet Gold Mine, Western Australia

Abstract

The Comet Gold Mine is in the Murchison mineral field which lies within the Yilgarn Craton of Western Australia. Several different geophysical methods were used in this study to define the geophysical signatures of sedimentary iron formations (SIF) and altered basalt associated with gold mineralisation. The geophysical surveys carried out at the Comet Gold Mine were gravity, sub-audio magnetics (SAM), transient electromagnetics (TEM) and downhole geophysical logging. Data from previous geophysical surveying were also used, and these included highresolution aeromagnetics and TEMPEST airborne electromagnetics. Other exploration information, such as geology and drillhole data, were integrated with geophysical results to study the geophysical responses and generate a geophysical interpretation map.The main aim of this study was to generate an understanding of the various geophysical responses of geology and gold mineralisation in the Comet Mine area for future gold exploration in this region. Particularly, the study focused on the ability of the SAM method to map out geology and geophysical response for gold mineralisation. The response from SAM surveying has been investigated over an area of 13 sq kms. The SAM surveying was completed using a transmitter current of 5-8 Amp with a 50% duty cycle at 4 Hz frequency, which was considered the best setting for the Comet area. The SAM anomalies were compared to results from other geophysical methods. The results of all geophysical surveying suggested that the TEM method was also effective for identifying altered sulphide and magnetic altered rock associated with gold mineralisation.Experiments were carried out using SAM surveying with electrodes in standard surface pits and pits placed directly into the gold mineralised structure. Both surveys showed very similar results, so in this area, surface electrode pits work well for current injection during SAM surveys. The similarities are probably due to the lack of conductive regolith cover in the Comet Mine area.The SAM response was studied for survey grids using different electrode positions and directions. Experiments in changing SAM electrode position over the same area were carried out along and across geological strike to detect the different geological structure directions. The EQMMR response was different for electrodes oriented at 90º when surveys were repeated over the same area. SAM mainly measures conductive features running sub-parallel to the electrode direction, but the EQMMIP response was mostly the same, despite the difference in electrode direction. In addition, the EQMMIP result was very similar for rotated grids, with some distortion occurring around the main EQMMR anomaly near the Venus open cut pit. Therefore, SAM chargeability was not strongly polarised along the electrode direction like the EQMMR response. This is consistent with the theory of MIP that the method detects the effect of induced polarisation in the earth by virtue of the magnetic fields associated with current flow in polarisable bodies within the earth.Gravity data were collected along in 4 transects 500 m apart and at 50 m station spacing. 3D gravity modelling using polygonal shapes was completed to a good fit with felsic and mafic rocks by having rock units dip to the SE. Euler depth solution calculations were applied to locate contacts and deep gravity sources. Gravity surveying has also proved to be a useful survey method for geological mapping and locating regional structures.Ground TEM survey data were used at the Venus prospect to map out conductive zones at depths ranging from 30 to 90 m. All anomaly bodies were interpreted to have a SE dip. The modelled ground TEM results were compared to TEMPEST airborne electromagnetic conductivity depth slices. Both EM survey results showed reasonably similar patterns, but the ground TEM method provided more reliable conductor locations and depth estimations that correlated well with the drilling information and downhole conductivity logging.Geophysical logs of natural gamma and inductive conductivity were surveyed in 5 drillholes that intersected gold alteration zones. The alteration zones associated with gold mineralised sediments, sulphide and magnetic minerals were identified in the downhole logs as increased conductivity, with a sight increase in the natural gamma response. Natural gamma was usually high above a background of host rock in the gold mineralised shear zones. This was likely due to K associated with the clay rich SIF units, and sericite and biotite from gold related alteration. During this study, drillhole CTRC028 was drilled into a modelled TEM anomaly, and gold mineralised SIF was intersected at the predicted location from the model.Geophysical survey information (magnetics, gravity, SAM and TEM) and anomalies in the Comet area were found to be primarily controlled by the local structures and mineralisation along these structures. Modelled ground TEM results were compared to TEMPEST airborne, EM data and showed reasonably similar patterns. The geophysical survey data also highlights black shale units, which can produce a false target commonly running parallel to the sulphide altered fault zones and SIF units, because of graphite and sulphide in the black shale.The TEMPEST data were a valuable guide to bedrock conductivity over the outline project area at Comet, and the follow–up ground TEM and SAM survey data was very useful for accurately pin-pointing anomalies for drill testing.Geophysical and geological data analysed in this study was used to generate a geophysical interpretation map at 1:5,000 scale. The new interpretation of geological units and structures at Comet will provide geologists with a better understand about the geological and structural setting for mineralisation in the Comet area. For example, the Comet Fault represents a faulted limb of the Comet fold structure that has both limbs dipping to the SE, and plunges to the NE. Magnetic anomalies associated with SIF are considered to correlate with some gold bearing horizons and the location of the Comet Fault, that has become more siliceous and altered by sulphide minerals and magnetite minerals.It is recommended that other prospect areas in the region should be surveyed using the SAM method in order to identify shallow gold bearing structures and improve geological interpretations ahead of drilling.