Preliminary structural constraints from the Tyrone Igneous Complex and its metamorphic basement

The Grampian orogeny of the British and Irish Caledonides records the progressive accretion of a series of arcs, ophiolites and microcontinental blocks to the Laurentian margin between the Late Cambrian and Middle Ordovician. Accreted oceanic tracts contain elevated, but sub-economic concentrations of the Platinum Group Elements in the Shetland and Ballantrae ophiolites, Scotland, and potentially economic Cu-Zn-Pb-Ag-Au deposits in volcanic arc sequences of counties Tyrone and Mayo. Orogenic collapse was associated with significant Au mineralization in the deformed passive margin sequences (=Dalradian Supergroup) of Northern Ireland, with the UKs largest gold deposit at Curraghinalt and only active gold mine at Cavanacaw. The Curraghinalt deposit contains at least 3.5 Moz of Au, with precious metals likely to have been sourced from the underlying Tyrone Igneous Complex (TIC) – a structurally dismembered Ordovician arc-ophiolite sequence that also locally contains high-grade Au mineralization. Despite its importance and extensive research on the TIC in the last ten years, no detailed study has been undertaken on its structural history.

We present preliminary results of structural measurements made across the TIC and its underlying metamorphic basement (=Tyrone Central Inlier). These were made by mapping of the Corvanaghan and Fir Mountain quarries in the Tyrone Central Inlier, as well as mapping of outcrops in the Tyrone Volcanic Group (e.g. Carnanransy Burn, Tullybrick, Cashel Rock, Beaghbeg). The Tyrone Central Inlier exposed in the quarries is composed of amphibolite facies rocks – predominantly metabasite and psammite, and minor metapelites. Leucosome-rich zones occur in the metabasites. Dips of these leucosome fabrics, bedding dips and first foliations are either similarly dipping, or show rotations around an identical SW-NE-oriented axis. Two generations of faulting are apparent in the Tyrone Central Inlier, with high angle oblique-slip to strike-slip faults with dextral indicators, crosscut by a family of dip-slip planar fault zones, with a normal sense where measurable. Zones of gold-bearing pyrite and chalcopyrite (with malachite staining) were observed near veins and faults that are N-S oriented in Corvanaghan quarry. Pegmatite and muscovite granite intrusions crosscut deformation fabrics.