Radio Image Mapping at the Polymetallic Rosebery Mine, NW Tasmania
- 1. MMG, Melbourne, Australia
- 2. MMG, Rosebery, Australia
Description
In 2019, MMG conducted a suite of thirteen deep crosshole Radio Image Mapping (RIM) tomographic surveys along the northern extension of the polymetallic Rosebery Mine in North-Western Tasmania. The RIM method is based on the attenuation of an electromagnetic wave in a material. A transmitter is located in one drill hole and a receiver in a nearby drill hole. The system measures the amplitude and phase difference between the transmitted and received signal. This determines the attenuation rate, which can be used to estimate the conductivity of the rock between the transmitter and the receiver. The positions of the transmitter and the receiver are varied to produce a tomographic data set. The survey was conducted with the Russian-built FARA borehole system containing several electrical dipoles transmitting or receiving radio signals in the kHz range. Thirteen cross-hole panels were designed to test the range of the RIM system and to search for additional mineralisation along a prospective zone between the Rosebery Fault and Mt. Black Fault in the Cambrian-age Mount Reid volcanic belt. The mineralisation, dominantly consisting of sphalerite, chalcopyrite and galena, is significantly more conductive than the surrounding volcanoclastic host rock. The high electrical resistivity of the volcanoclastic host rock assured an effective range of the transmitter-receiver system in excess of 400m. Tomographic imaging of the survey data has identified several tabular, down-dipping zones of low resistivity. There was relatively little variation in the tomographic imaging with frequency. Follow-up drilling intersected sulphide mineralisation in the vicinity of the RIM target zones. Tomographic imaging of survey data identified several tabular dipping zones of low resistivity that conform to observed lithological and mineralisation layering. Little variation was observed within the imaging across frequency, providing greater confidence in data quality. A series of follow up exploration drill holes located significant mineralisation coincident to RIM target zones. The RIM system provided a cost-effective mechanism to either upgrade or sterilise large volumes of exploration search space between existing drill holes in a brownfields setting. The imaging capabilities of the RIM system could be improved by monitoring time-variations in transmitter strength and taking cable stretch into consideration for transmitter and receiver locations.
Notes
Files
ID055.pdf
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