The carbonate-hosted Gortdrum Cu-Ag(?Sb-Hg) deposit, SW Ireland: C-O-Sr-Nd isotopes and whole-rock geochemical signatures

Abstract

The Gortdrum Cu-Ag(+/- Sb-Hg) deposit, consisted of a fault-controlled orebody (3.8 Mt @ 1.19 % Cu and 25.1 g/t Ag) formed at the base of the Irish Midlands basin, in Lower Carboniferous rocks laterally time equivalent to Navan Group rocks hosting the giant Navan Zn-Pb deposit, and form the base of the Irish Midlands basin. The ore body is hosted on the hanging-wall of the Gortdrum Fault either along strata or within a wedge of brecciated carbonate rocks. Vertical zonation based on predominant host rock, ore textures, sulfide assemblages, and whole -rock geochemistry allowed the detailing of three ore types: a) the lower ore, representing Cu sulfides hosted within basal carbonated siliciclastic rocks; b) the upper ore, representing Cu, Cu-Sb, and Hg sulfides hosted within upper calcareous rocks and; c) the vein-associated ore, dominantly hosted in the more competent upper carbonate rocks. The origin of the deposit is unambiguously related to the development of the Gortdrum Fault and its associated permeability, which allowed basement/basin-derived fluids to react with carbonates and induce copper-silver mineralization. The mineralogy, ore shoot geometry, and geochemical association of Gortdrum are shared with classic Zn-Pb Irish-type deposits such as Navan, Lisheen, Silvermines, and Tynagh (sub-seafloor replacement). In these Irish-type deposits, copper-silver mineralization is associated with the late stage of Zn-Pb mineralization and shares a common geochemical footprint with Gortdrum of anomalous Ag, As, Sb and Hg. Additionally, the C-O and Sr-Nd isotope range of Gortdrum and Navan samples overlap, indicating that mineralization processes in both deposits were ineffective in modifying the original host-rock signature. These similarities suggest that Gortdrum could represent a variation of Irish-type mineralization where late-stage Cu-Ag-Sb-bearing fluids succeeded in forming a deposit. Hypothetical early-stage Zn-Pb fluids a) never existed; b) deposited disseminated sulfides in country-rocks, c) formed an undiscovered resource or, d) deposited ore concentrations that were eroded off.