Anatomy Of A Giant – The Navan Orebody

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 These localized basin-margin processes were superseded by the widespread deposition of calc-turbidites in the Dublin Basin in the Arundian to Asbian.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)