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Ore-Forming Processes in Irish-Type Carbonate-Hosted Zn-Pb Deposits: Evidence from Mineralogy, Chemistry, and Isotopic Composition of Sulfides at the Lisheen Mine

J. J. Wilkinson and S. L. Eyre^*

Department of Earth Science and Engineering, Royal School of Mines, Imperial College London, Exhibition Road, London SW7 2AZ, United Kingdom

A. J. Boyce

Scottish Universities Environmental Research Centre, East Kilbride, Glasgow G75 0QF, Scotland

Corresponding author: e-mail, j.wilkinson{at}imperial.ac.uk

Lisheen is a strata-bound zinc-lead deposit formed during the Mississippian by replacement of hydrothermally dolomitized, grossly stratiform breccia bodies located near the base of the Carboniferous Waulsortian Limestone. It represents one of a number of carbonate-hosted massive sulfide ore deposits in the Irish ore field that, due to several unique features, have been classified as Irish type.

Disseminated pyrite occurs in preore dolomite and around the margins of preore dolomite clasts within dolomite breccias. Early fine-grained sphalerite-pyrite mineralization occurs as infill of intergranular dolomite porosity. Locally, massive to semimassive iron sulfide is observed, mainly comprising pyrite with lesser marcasite. A complex polymetallic sulfide assemblage typifies the main ore stage, dominated by fine-grained disseminated, massive or colloform sphalerite and galena, with minor pyrite, chalcopyrite, arsenopyrite, tennantite, nickel- and cobalt-bearing minerals. Silver occurs in solid solution in tennantite, galena, and sphalerite. Dolomite and barite dominate the gangue, with lesser calcite. Main-stage mineralization involved the progressive replacement of preexisting iron sulfides and the dolomite breccias, initially by replacement of the breccia matrix and ultimately by replacement of clasts. Coarse crystalline sphalerite and euhedral galena crystals are generally restricted to fracture-fill mineralization or vugs within main ore-stage assemblages where they occur with euhedral dolomite and calcite.

Barite intergrown with main ore-stage sulfides has ³⁴S values of 14.3 to 18.1 per mil, consistent with the derivation of sulfate from coeval Carboniferous seawater. The ³⁴S values for sulfides range from –44.1 to +11.8 per mil, with a mean value of –13.7 per mil, typical of the Irish ore deposits. The dominant low ³⁴S signature is considered to be the result of bacterial reduction of coeval seawater sulfate. Extremely low ³⁴S values, in the range of –38 to –44 per mil, are only observed in preore disseminated pyrite; such extreme fractionations are thought to be due to low bacterial sulfate reduction rates coupled with oxidative cycles in near-sea floor pore waters. Main ore-stage sulfides have ³⁴S values in the range of –4 to –18 per mil, with a mode of –10 per mil, consistent with a typical bacterial fractionation from coeval seawater sulfate. Isotopic equilibrium between cogenetic sulfides is not observed. The bacteriogenic sulfur component was probably transported from bacterial colonies fringing the ore system by low-temperature brines.

The ³⁴S values of late ore-stage sulfides mainly range from –20.2 to +12.0 per mil, with the majority having relatively high values (mean = –3.0 ± 8.5, 1) interpreted as being due to the presence of a hydrothermal sulfur component, leached from the lower Paleozoic basement. For galena and sphalerite there is a general increase in ³⁴S values with depth in the system, with time, and with proximity to east-west– and northwest-trending faults. These relationships suggest that input of hydrothermal sulfur from depth via fractures became increasingly important. Hydrothermal sulfur appears to be more important at Lisheen than the other major Irish deposits.

Galena lead isotope analyses gave average ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb values of 18.183, 15.594, and 38.080, respectively. These data do not correlate with ore-stage, galena texture or ³⁴S. The results are comparable to previous data from Lisheen and from Silvermines, 35 km to the west, implying a common lead source in the lower Paleozoic basement.

The textural, mineral, chemical, and isotopic evidence suggests that main-stage ore was precipitated as a consequence of rapid supersaturation, caused by fluid mixing within the permeable dolomite breccias. This process involved relatively high temperature (ca. 240°C), metal-bearing solutions derived from a basement-equilibrated fluid reservoir (carrying Zn, Pb, Fe, Cd) and shallow, saline (ca. 25 wt % NaCl equiv) formation waters rich in bacteriogenic H₂S. Minor metals (Cu, As, Ni, Co) are thought to have been stripped from the footwall Old Red Sandstone during hydrothermal alteration around fault conduits. The availability of abundant seawater sulfate, operation of open-system bacterial sulfate reduction, and episodic availability of free oxygen imply that ore formation cannot have occurred at significant depth below the paleosea floor. Cessation of mineralization was due to a cut-off of the sulfur-rich brine supply, possibly by deposition of impermeable hanging-wall sediments. This process of ore formation is consistent with evidence from the other economic Irish-type deposits in the ore field.

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