Abstract - It has long been known that the combined effect of structure and preferential host horizons controls the distribution of ore within the carbonate-hosted Zn-Pb deposits of the Irish Midlands. We explore the extent to which structures, and faults in particular, have acted as conduits or traps for mineralising fluids with particular reference to high quality 3-D data drill-hole and underground mapping. Our datasets include underground maps together with large drill-hole databases (sometimes including over 1300 drill-holes), incorporated in 3-D geological models within the Vulcan modelling software system. We demonstrate that an improved definition of the 3-D structure and structural history is an essential step in understanding the hydrothermal flow system in Irish Zn-Pb deposits.
Mineralisation occurs dominantly as replacements of Courceyan-aged carbonates, forming stratabound lenses at the base of a regionally dolomitised limestone and minor stratabound and fault-hosted mineralisation in an underlying sequence of argillaceous limestones. The deposits occur between major ENE-E trending, north-dipping normal fault arrays with displacements in the order of 200m. Stratigraphic constraints indicate that these fault systems initiated in the Late Courceyan. Although they appear to have been originally isolated, these fault systems are now linked by large-scale intervening relay ramps (ca. 500-2500 m wide) containing related minor normal faults with variable trends. These minor faults often bound individual ore lenses or high-grade pods within the deposit area.
The main ore bodies of the Lisheen and Galmoy deposits are bounded to the south-east by the Rathdowney Fault System. This system consists of a kinematically coherent, en-échelon array of E-W trending, north-dipping fault segments with maximum throws of ca. 220 metres. In general, segments are linked by left-stepping relay zones (450 to 500 m wide) with generally intact relay ramps. However, significant smaller-scale lateral and vertical segmentation along segments is accompanied by the formation of relay zones (up to 150 m wide), which breached at an early stage and are now characterised by fault-bounded lenses of densely faulted and brecciated rocks. The thicker and higher-grade ore-lenses are generally found within these breached relays.
Ore textures and cross-cutting relationships indicate that main-stage mineralisation is essentially post-faulting. Ore distributions demonstrate the important role of faulting, and related segmentation, in both the migration and trapping of metal-bearing fluids. Grade distribution patterns and thickness contour maps suggest that upward flow of the metal-bearing fluids was largely restricted to the strongly deformed and brecciated, within-segment, breached relays along the Rathdowney Fault System, rather than the intact relays between segments, which remained essentially un-mineralised. By contrast, minor faults within the deposit often bound individual ore lenses or high-grade pods, a feature which suggests that not only were these faults important controls on lateral flow but they could also act as traps to mineralising fluids. This localised trapping is attributed to the presence of clay material (usually solution seams) along slip surfaces, which either prevented or retarded across-fault fluid flow.
Abstract of talk given to:
North Atlantic Minerals Symposium , Halifax, Nova Scotia, May 2005.