Now showing 1 - 3 of 3
  • Publication
    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.
  • Publication
    Controls on Metal Distributions at the Lisheen and Silvermines Deposits: Insights into Fluid Flow Pathways in Irish-Type Zn-Pb Deposits
    The world-class Irish Zn-Pb(-Ag) deposits occur within one of the world’s major metallogenic provinces. While it has been well documented that these orebodies are structurally controlled, exactly how fluids migrated from source to trap is still poorly understood. Using 3-D modeling techniques, the current study investigates metal distribution patterns at the Silvermines and Lisheen deposits to gain insights into fluid pathways and structural controls on mineralization. Distinct points along segmented normal faults are identified as the feeders to individual orebodies, allowing hot, hydrothermal, metal-bearing fluids to enter host rocks and form orebodies. These points are characterized by highly localized and elevated Ag, Cu, Co, Ni, and As concentrations as well as low Zn/Pb ratios, which increase away from the feeders. Metal distributions are initially controlled by major and minor normal faults and subsequently affected by later oblique-slip dextral and strike-slip faults. High-tonnage areas without typical feeder signals are interpreted to be structural trap sites, which are distal to fault-controlled feeder points. This study highlights both the importance of a well-connected plumbing system for metal-bearing fluids to reach their basinal traps and the control that an evolving structural framework has on spatial distribution of metals.
      512Scopus© Citations 22
  • Publication
    3-D Modeling of the Lisheen and Silvermines Deposits, County Tipperary, Ireland: Insights into Structural Controls on the Formation of Irish Zn-Pb Deposits
    (Society of Economic Geologists, 2019-02-01) ; ; ; ;
    Faults are important structures in the formation of many mineral deposits, often acting as conduits for ore-forming fluids and sometimes providing, or generating, the bounding structures to associated mineralizing sites. Using 3-D analysis and modeling of the Lisheen and Silvermines deposits within the Irish ore field, we investigate the geometry of normal fault systems and their implications on the origin and nature of associated deposits. These Irish-type deposits are carbonate hosted and developed within the hanging walls of normal faults arising from an Early Carboniferous episode of north-south rifting, with relatively limited amounts of later deformation. Structural analysis of high-quality mine datasets indicates that fault segmentation is ubiquitous with left-stepping segments arising from north-south stretching developed above generally ENE-NE-trending fault arrays, which are subparallel to older Caledonian penetrative fabrics and structure within underlying Silurian and Ordovician rocks. Fault segments occur on different scales and have a profound impact on structural evolution, with larger scale segments and intervening relay ramps defining distinct orebodies within deposits and smaller scale segments and relays potentially providing paths for upfault fluid flow. The difference in behavior is attributed to the integrity of associated relay ramps where intact ramps represent orebody-bounding structures, and smaller breached ramps provide enhanced associated hydraulic properties and act as vertical conduits. Hanging-wall deformation along the rheological boundary between host-rock limestones and underlying shales has an important control on the localization of earlier dolomitization and/or brecciation and later mineralization adjacent to this contact, and on the migration pathways for basinal brines and mineralizing fluids.
      688Scopus© Citations 27