Mineral magnetism identifies the presence of Pyrrhotite in the Navan Zn-Pb deposit, Ireland: Implictions for low temperature Pyrite to Pyrrhotite reduction, timing of mineralization and future exploration strategies S.C. Johnson 1 , T.D. Raub 2 & J.H. Ashton 3 1 CODES, ARC Centre of Excellence in Ore Deposits, University of Tasmania, Sandy Bay, Hobart, Tasmania, Australia 2 Department of Earth and Environmental Sciences, University of St Andrews, North St., St Andrews, Fife, Scotland. 3 Boliden Tara Mines Limited, Knockumber Rd., Navan, County Meath, Ireland Abstract Presented here are the preliminary results of a pilot study from the Navan Zn-Pb deposit utilising the first example of Superconductive Quantum Interference (SQUID) magnetic microscopy in relation to ore deposits. Using this method of microscale, high- resolution magnetic mapping we have identified the pervasive presence of microscopic ferromagnetic pyrrhotite. A previous palaeomagnetic age based on a limited reference frame from N. America by Symons et al. (2002) suggested an epigenetic origin for Navan. We have compiled a new Apparent Polar Wander (APW) path for Scotland that is better constrained for comparison to Navan, to which the Symons et al. (2002) study does not hold true. The identification of pyrrhotite in the ore assemblage holds promise to aid future investigations into the geochemistry of the Irish ore systematics. In addition, our employment of hand- held magnetic susceptibility tests on drillcore has defined orders of magnitude differences between samples of economic grade and those of little to no mineralization, this can be developed in the near-mine environment as a novel, cost-effective and low impact method of vectoring towards ore. Keywords Irish-type Zn-Pb, Navan, Pyrrhotite, magnetics 1 Introduction and rationale There is a plethora of evidence that metallogenesis in the Irish Midlands took place during the early to mid Lower Carboniferous forming the world class Navan deposit and other Irish-type Zn-Pb deposits. Work also supports that the process involved syndiagenetic replacement and, in places, exhalative deposition (Boyce et al., 1983). Complementary to these results, is the fact that the vast majority of base metal occurrences in the Irish ore-field occur within early to mid Lower Carboniferous strata, a significant framework upon which mineral exploration strategy is built. Despite this there has been much debate with regard to the age of mineralization in Ireland (i.e. Hitzman, 1999; Wilkinson, et al., 2005 and Blakeman et al., 2002). Textural and isotopic evidence favours early- to mid-Carboniferous age for ore precipitation (Ashton et al., 2003 and references therein); work by Peace & Wallace (2000) suggests an epigenetic, post-Arundian, origin however their focus was on the carbonate phases and host rock, and palaeomagnetism appears to favour late Carboniferous to early Permian age for the various Irish deposits (Symons et al, 2002 & Pannalal et al., 2008). Extended (tens of millions of years) crustal fluid flow at high geothermal gradient during varying stress regimes of the developing basin, however, is difficult to conceive. Hence workers have favoured one or more discrete, short-lived episodes of mineralization (Ashton et al., 2003 and Blakeman et al., 2002). The main objections to the late, epigenetic age for Navan derived from palaeomagnetism are both external and internal to the method. Proposed ages vary dramatically for the different Iaepetan-suture-proximal Irish Zn-Pb occurrences (Symons et al., 2002 & Pannalal, et al., 2008). In some cases, this age uncertainty reaches ~30 million years between deposits separated only by ~10 km along strike and hosted by units of similar age (Symons et al., 2002), seeming to contradict intuition of reasonable thermal gradients and a common regional history. The late epigenetic age proposed is also in direct conflict with the geology present, namely mineralized clasts present in a submarine flow horizon at Navan, shown to be Chadian in age (345Ma), ~ 10Ma older than the proposed epigenetic age ascertained by palaeomagnetics. Assessing a confident age for mineralization at Navan and the other Irish deposits may aid in defining intelligent exploration strategy and exploitation practice. This can be achieved by assigning enhanced value to, for example, one of the fault generations, or to a particular stratigraphic horizon.