The Navan Orebody, Ireland - structural control of a word-class carbonate-hosted Zn-Pb deposit
|Location||International Geological Congress,oslo 2008|
|Author||Ashton, John۱; Coller, David۲; Beach, Alastair۳; Blakeman, Robert۱|
|Holding Date||15 September 2008|
Central Ireland contains a world-class carbonate-hosted Zn-Pb province, currently with 3 mines contributing >60% of western European Zinc production. There is wide agreement that a principal control on ore localization was the presence of Caledonide (NE-ENE) trending normal faults. Recent structural studies at Navan reveal how these structures developed and controlled ore genesis.
At Navan, Europe’s largest Zn mine, Bolden Tara Mines Limited extract some 2.7 Mtpa from an orebody with a pre-mining resource of ~100Mt at 8.4% Zn, 2.0% Pb. The orebody comprises stratabound sulphide lenses hosted in Lower Carboniferous shallow-water carbonates of Courceyan to Chadian age. Previous studies indicate that sulphides precipitated when hot metal-carrying hydrothermal fluids (generated by basement convection of evaporite-derived brines), replaced fractured limestones and mixed with cooler seafloor-derived fluids carrying sulphur formed by bacteriogenc reduction of seawater sulphate.
The host-rocks are truncated by a complex of Chadian slides and submarine debris-flows that involved movement of displaced material southwards towards the developing Dublin Basin, where some ~2km of limestone turbidites subsequently accumulated in Arundian to Brigantian times. The differential subsidence and degradation that took place on the developing basin margin is interpreted as the result of footwall uplift along the long-lived NE trending Navan fault system. This fault initialised as a terrane-bounding feature during the Caledonian Orogeny. Multiple reactivation of this structure occurred during the Carboniferous and is reflected by a variety of fault and slide geometries induced in the Lower Carboniferous rocks.
Extensional NW dipping ENE trending normal faulting was initiated during the late Courceyan to Chadian and included a strongly fractured relay ramp system. Subsequent differential subsidence linked to the initiation of the Dublin Basin gave rise to several ENE trending but now SE dipping normal faults. As extension increased during the Chadian, footwall uplift on the larger of these faults resulted in complex southerly directed, low-angle gravity slides. This sliding was succeeded by formation of submarine debris-flows and fault-talus deposits as localized basin-margin processes were superseded by widespread deposition of calc-turbidites in the Dublin Basin. Many of the structures were inverted by Hercynian reverse and wrench movements in late Carboniferous times.
A switch from early faulting with NW directed throws to sustained SE throws generated an asymmetric horst at Navan and generated fracture permeability that allowed basement-escaping mineralizing fluids access to host limestones over a prolonged time-span. The development of localized SE facing half-grabens created basinal areas that allowed sulphur-reducing bacteria to thrive, generating fluids rich in sulphur that mixed with the basement-sourced metal-rich hydrothermal fluids to precipitate sulphides.