Influence of mineralogical composition and texture on induced polarization effects in gold-bearing rocks from the Hebert-Brent Showing, Yellowknife Greenstone Belt, Northwest Territories

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University of New Brunswick


In June 2015, geological mapping discovered significant concentrations of gold in the Hebert-Brent (HB) Showing situated within an 11 m-wide highly sulphidized sericite-ankerite schist shear zone, hosted in a 10-15 m-wide, quartz-feldspar porphyry. The HB gold showing is located within the Barney Deformation Corridor of the Yellowknife Greenstone Belt (YGB), Northwest Territories. On July 29th, 2016, two 400 m long IP/resistivity surveys, with 5/10 m electrode spacing were acquired using a multi-gradient array. The survey identified zones of high IP response (chargeability) where expected, in areas of known disseminated sulphide mineralization. In addition, the IP survey exposed a previously undiscovered anomalous IP source. In this study, a prototype two electrode laboratory apparatus was used to investigate factors influencing electrical resistivity and IP effects in selected mineralized and non-mineralized rocks from the survey area. The study revealed information that may be utilized in future geophysical exploration in this area. It was determined that samples from Hebert-Brent are generally less resistive than non-mineralized rocks within this area. However, as a consequence of preferential alignment of silicate layers with foliation, anisotropic effects should be anticipated in this area. The IP response of the mineralized samples was in general greater in magnitude than the non-mineralized samples. It was also determined that there are some IP effects related to highly resistive, non-mineralized rocks in this area. These anomalous IP effects are likely a result of some uncertainty in the measurement apparatus. Normalizing the PFE IP effect by resistivity served to make the mineralized samples standout from highly resistive ones that also exhibited elevated IP responses. Comparison of mineralized and non-mineralized spectral curves were shown to be characteristically different. In addition, the peak phase angle between 0.1 - 10 Hz was found to correlate with increased percent frequency effect. It is recommended that surveys which sense the conductivity of an ore body should be used to aid in delineating massive and electrically conductive parts of a sulphide body. In addition, it is recommended that future IP and resistivity surveys in the area use a measurement of IP effect normalized by resistivity (such as the Metal Factor; MF) to highlight anomalous bodies that are prospective for elevated sulphide (and associated gold) content. Based on the findings in this study, spectral induced polarization (SIP) surveying may aid in identifying mineralized rocks in HB, as the phase maximum (between 0.1 and 10 Hz) is in general greater than that of the non-mineralized country rocks. Testing of the two electrode apparatus was shown to provide accurate and repeatable measurements on both the mineralized and non-mineralized rocks used in this study. However, more testing of the measurement apparatus is recommended to assess its reliability for very highly resistive samples.