Down-hole Magnetometric Resistivity inversion for zinc and lead lenses localization at Tobermalug, County Limerick, Irland.

Bouchedda, Abderrezak (INRS-ETE) | Giroux, Bernard (INRS-ETE) | Allard, Michel (Glencore)


Summary Down-hole magnetometric resistivity (MMR) measurements have been conducted in Tobermalug prospect in County Limerick, Ireland. The survey was used as an alternative to down-hole electromagnetic to delineate subhorizontal zinc/lead mineralization lenses that are poorly conductive. Two survey areas were investigated, DHMMR1 and DHMMR2. Interpretation is based on the regularized least-squares inversion of MMR data, in which MMR modeling is performed by resolving electrostatic and magnetostatic equations using finite volume method. Inversion of synthetic data of two conductive horizontal discs model shows that conductive structures are well positioned but their extensions is biased to current electrodes orientation. Inversion allowed localizing a few conductive elongated targets. At DHMMR1, the conductivity is weaker and seems to be associated to disseminated mineralization. DHMMR2 contains a higher conductivity and more elongated target. It seems to be associated to semi-massive sulphides. Introduction Ireland was one of leading European producer of zinc and lead. During the last fifty years, zinc and lead concentrates have been extracted from the Irish Midlands Orefield that covers a surface area greater than 35000km2 (Blaney, 2011). It is one of the world’s major zinc provinces with five major producing mines with large size and high grade ore. In 1999, Minco Ireland signed a joint venture agreement with Noranda (now Glencore) for the exploration of Pallas Green block located between Limerick city and Tipperary town in the centre south of Ireland. The zinc/lead mineralisation at Pallas Green ranges from disseminated and stringer-fracture fill textures to massive (Blaney, 2011). It consists of multiple, subhorizontal, stratiform lenses between 0.5 m to >18 m thick and comprises sphalerite, galena, pyrite and minor marcasite within a Carboniferous limestone. Disseminated and sphalerite rich ores are generally poor targets for electrical resistivity and electromagnetic methods (Denith and Mudge, 2014). In contrast, the MMR technique is more appropriate because it can respond to highly conductive and to weakly conductive targets in a conductive host. For example, it was successfully used in Australian environments for poorly conducting metal sulphide targets, such as sphalerite rich bodies (Asten, 1988; Bishop et al., 1997) and nickel sulphide mineralisation (Bishop et al., 2000).

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