Base Metal Sulfide Mineralization in Lower Carboniferous Strata, Northwest Ireland

Abstract

Zn-Pb sulfide mineralization at Abbeytown mine and Twigspark quarry comprise the only known carbonate-hosted base metal sulfide deposits in the Sligo syncline, northwest Ireland. Limestone sedimentation occurred uniformly throughout the region during the Early Carboniferous (Mississippian) as observed by field relationships and lithofacies; however, petrographic and stable isotope evidence indicate that host-rock dolomitization occurred under different conditions at localities to the west and east of the Ox Mountains inlier, suggesting significant uplift and geologic isolation of these areas prior to dolomitization. Localized fluid flow systems are thought to be responsible for sulfide mineralization and associated epigenetic carbonate cements. West of the Ox Mountains inlier at Abbeytown, evidence of three geochemically distinct fluids are observed: (1) a lower-temperature, lower-salinity fluid (70°–130-°C, 4–9 wt.% equiv. NaCl); (2) a lower-temperature, higher-salinity fluid (70°–140°C, 15–24 wt.% equiv. NaCl); and (3) a higher-temperature, moderate-salinity fluid (165°–220°C, 8–14 wt.% equiv. NaCl). Similar fluid types were observed at the Twigspark deposit. The source of the higher-salinity fluid is likely seawater evaporated to near the point of halite precipitation. The higher-temperature fluid is thought to have been derived from deep circulation of basinal brines. It is speculated that mixing of the higher-salinity fluid with the high-temperature fluid was vital for ore formation at Abbeytown because fluid inclusions in sphalerite have homogenization temperatures and salinity values that fall along a mixing trajectory of these end member fluids. Also, areas where the high-salinity end member fluid is absent are barren of sulfides. Less complex fluid systems are indicated for sites east of the Ox Mountains inlier where no sulfide mineralization was observed. Data from base metal sulfide prospects in northwest Ireland indicate no connection with the regionally extensive flow system thought to be responsible for Zn-Pb deposits throughout the Irish Midlands. © 2010 Canadian Institute of Mining, Metallurgy and Petroleum. All rights reserved. Sommaire Les mineralisations en sulfures de Zn-Pb de la mine Abbeytown et de la carriere Twigspark sont les seules gites de metaux usuels a encaissant carbonate connus dans le synclinal de Sligo, au nord-ouest de l’Irlande. Ces carbonates se sont deposes d’une maniere uniforme dans la region au Carbonifere inferieur (Mississippien) tel que l’indiquent les relations de terrain et les lithofacies; les donnees petrographiques et d’isotopes stables indiquent toutefois que la dolomitisation de l’encaissant s’est faite sous des conditions differentes aux localites situees a l’ouest ou a l’est de la boutonniere des Ox Mountains, ce qui suggere un soulevement et une isolation geologique significative de ces secteurs avant la dolomitisation. L’action de systemes de circulation locaux sont consideres etre a l’origine de la mineralisation sulfuree et des ciments carbonates epigenetiques qui leur sont associes. A Abbeytown, a l’ouest de la boutonniere des Ox Mountains, on note les effets de trois fluides geochimiquement distincts: (1) un fluide a basse temperature peu salin (70°–130°C, 4–9% poids equivalent NaCl); (2) un fluide a basse temperature fortement salin (70°–140°C, 15–24% poids equivalent NaCl); and (3) un fluide forme a plus haute temperature et salinite moderee (165°–220°C, 8–14% poids equivalent NaCl). Des fluides similaires ont ete observes au gite Twigspark. La source presumee du fluide le plus salin est de l’eau de mer amenee par evaporation pres du point de precipitation de la halite. Le fluide a plus haute temperature est probablement derive de la circulation profonde de saumures derivees d’un bassin sedimentaire. Il est possible que le melange du fluide a haute salinite avec celui a haute temperature ait joue un role central dans la genese de la mineralisation d’Abbeytown parce que les temperatures d’homogeneisation et les valeurs de salinite des inclusions fluides dans la sphalerite se distribuent sur une ligne de melange reliant ces deux poles. De plus, les secteurs ou l’on ne trouve pas trace du terme le plus salin sont steriles en sulfures. Les sites sans mineralisation sulfuree situes a l’est de la boutonniere des Ox Mountains presentent des systemes fluides plus simples. Les donnees provenant des autres occurrences de sulfures de metaux usuels du nord-ouest de l’Irlande ne montrent pas de rattachement avec le systeme de circulation regional presume etre a l’origine des gites de Zn-Pb des Irish Midlands. © 2010 Canadian Institute of Mining, Metallurgy and Petroleum. All rights reserved.