IGA NEW YEAR LECTURE
Wednesday 18th January, 2012
Dr. John Ashton, (Boliden Tara Mines Ltd) will present a lecture:
ANATOMY OF A GIANT – THE NAVAN OREBODY
VENUE: Synge Lecture Theatre, Art’s Block, (Nassau St. entrance) Trinity College, Dublin 2.
TIME : 8.00pm
The Giant Navan Carbonate-Hosted Zn-Pb Deposit – A Review *
John H. Ashton, Robert Blakeman, Jim Geraghty
Boliden Tara Mines Limited, Navan, Ireland
Alastair Beach, David Coller, Michael Philcox
Scottish Universities Environmental Research Centre, Glasgow, Scotland
Jamie J. Wilkinson
Department of Earth Science and Engineering, Imperial College London, United Kingdom
The Navan Orebody is a world-class carbonate-hosted Zn-Pb deposit (~105Mt @ 8.1% Zn, 2.0% Pb) where Boliden Tara Mines Limited currently extract some 2.7 Mtpa from Europe’s largest Zn mine. The deposit comprises stratabound sulphide lenses hosted by Lower Carboniferous shallow-water carbonates of Courceyan to Chadian age. Sulphides precipitated when hot, metal-carrying fluids, invaded fractured and variably permeable limestones and mixed with cooler, seafloor-derived brines enriched in H2S derived from bacteriogenic reduction of seawater sulphate. The deposit is focused along an early ENE-trending, NW-dipping, extensional relay fault system within the footwall crest of a major tilt block, controlled by a large SE-dipping fault that developed during the early stages of rifting of the Dublin Basin (late Courceyan to Chadian). Footwall uplift on this structure resulted in gravitational instability, further SE-dipping faulting and low angle tectonic slides that eroded the crest of the tilted footwall block, including the pre-rift host rocks, mineralization, and earlier faults. The culmination of this slide event was marked by the deposition of submarine debris-flows and fault talus deposits. These localized basin-margin processes were superseded by the widespread deposition of calc-turbidites in the Dublin Basin in the Arundian to Asbian.
Many structures were subsequently inverted by later (Variscan?) reverse and wrench tectonism. The switch from early faulting with NW throws to faulting with sustained SE throws created an asymmetric horst and generated fracture permeability that allowed mineralizing fluids access from the basement to the host limestones over a prolonged time-span. The development of localized basinal areas in half-grabens allowed sulphur-reducing bacteria to thrive, generating fluids rich in H2S that mixed with basement-sourced metal-rich hydrothermal fluids to precipitate sulphides.
(*based on a presentation to the Zinc 2010 Conference held in Cork by the IAEG)
John Ashton graduated from the Royal School of Mines, London in 1975 in Mining Geology. He undertook PhD research at the University of Aberystwyth until 1978 into ore geology and wallrock geochemistry of Pb/Zn and Au veins in Wales. After working for Mogul of Ireland Ltd at Silvermines, as mine/senior mine geologist he joined Tara Mines Ltd in 1980, where he is currently Chief Mine and Exploration Geologist. Experience and interests include computer graphics and 3D modelling, ore reserve estimation, the genesis of carbonate-hosted Zn/Pb deposits and base metal exploration.