Taking downhole EM underground, at Hill 50 Decline, Mount Magnet, Western Australia

Abstract

Hill 50 Decline (part of the Hill 50 Gold Mine group of deposits) is located at Mount Magnet, 560 km north-northeast of Perth, in the Murchison Province of the Yilgarn Craton. The Hill 50 orebody is hosted by banded iron formation (BIF) "bars" of the Sirdar Formation, generally having a width greater than 20 m. Gold mineralisation is localised along the north-northeast to northeast striking faults, known as Boogardie Breaks, and accompanying sub-horizontal fractures. Sulphide replacement of the adjacent oxide facies BIF has resulted in steeply plunging stratabound shoots ("pencils") of massive gold-bearing pyrite? pyrrhotite ore, with pyrrhotite being the dominant sulphide at depth and at the centre of the ore zones. Previously, pyrrhotite-rich BIFs have been shown to be characterised by significantly higher conductivities than their unmineralised counterparts. Downhole electromagnetic (DHEM) surveying carried out in the Hill 50 underground was aimed at detecting the massive, pyrrhotite conductors. A 65 m ´ 80 m transmitter loop was laid out underground and drillholes were surveyed using the three-component, Crone pulse EM system. Logistical problems experienced during data collection were largely related to firing times and data quality was greatly influenced by the numerous noise sources encountered in an operating underground mine (eg, drill rigs, trucks, etc.). Modelling and interpretation of the resulting DHEM data has identified several conductors. All the interpreted conductors are positioned within the Hill 50 BIF Bar, except for one conductor, lying in the Outer BIF Bar 1. Most conductors are modelled as steeply dipping, while those modelled as shallow-dipping are considered to be related to mineralisation within the sub-horizontal fractures. Of the models tested so far, all have been shown to be directly related to massive sulphide mineralisation, with the majority being associated with economic gold grades, demonstrating that the technique has been effective in delineating the pyrrhotite-rich BIF target underground.