Drill-Hole Induced Polarisation, Self-potential and Magnetic Surveys: Case Studies from Mineral Belts of Rajasthan

Downhole wide spacing IP (DHIP), Self-potential (SP), magnetic susceptibility and applied potential observations were made along the boreholes drilled for Pb-Zn-Cu sulphide in mineral belts of Rajasthan The aim was to identify mineralised zones, physical property contrast of the mineral and country rock and presence of mineralisation in the vicmity of the borehole but missed in drilling

Number of boreholes were surveyed in different parts of the mineral belts of Rajasthan and the relationship between apparent resistivity and chargeability, SP and magnetic susceptibility and lithology of the cores has been studied and analyzed Chargeability 'high', resistivity 'low', SP 'low/high' and magnetic susceptibility 'high' characterized the sulphide mineralisation The surveys were successful in detecting the presence of mineralisation in the drill hole, studying the m situ physical properties of barren & mineralised rocks and finding the correct depth of the conductor It helped in conducting raise-a-la-masse surveys and interpretation of surface geophysical data m mineral belts of Rajasthan Applied potential (Mise-a-la-masse), surveys were able to give information on the lateral extension of the mineralised zones intersected in the borehole

Five examples of detection of sulphide mineralisation in boreholes of Kayar, Pindwara-Watera and Danba-Bethumbi mineral belts of Rajasthan are presented The results indicated average appaient resistivity low value of the order of 1 to 10 ohm-m and magnetic susceptibility of order of 600-5000 x 10⁶ c g s units in the sulphide zones, while in the country rocks values recorded were m the range of 3000-24,000 ohm-m and 50-500 x 10⁶c g s units respectively The diagnostic resistivity response is simply related to the percentage, size and disposition of the sulphide mineralisation The association of high magnetic susceptibility is indicative of pyrrhotite mineralisation in these zones The SP 'high' and 'low' could be due to the electrochemical potential developed at the interface of country rock and ore body Detection IP/ resistivity measurements have also improved the prospective search radius of drill-holes and are successful in assessing qualitatively the lateral extent ol intersected mineralisation.