Textural, cathodoluminescence and geochemical evidence for the timing and nature of hydrothermal dolomitization in the Irish base-metal orefield

摘要

The most important host rocks to Zn-Pb (-Ag) mineralization in the southern sector of the Irish ore-field are carbonate breccias located at the base of Waulsortian mudbanks (as found at the Silvermines, Lisheen and Galmoy deposits). The Cooleen zone is a sub-economic satellite of the Silvermines deposit, where the early cementation history of the breccias has not been completely overprinted by hydrothermal alteration. Important constraints on the relative timing of cementation, hydrothermal dolomitization and Zn-Pb mineralization can thus be identified.The Cooleen prospect is in Co. Tipperary, located in the southwest Irish Midlands. Dolomite, limestone and partially dolomitized breccias have been identified and textural relationships have revealed a complex paragenesis, which can be described as six broad 'events'. (1) Fault-controlled, wedge-shaped sedimentary breccias formed one on top of another during formation of the Waulsortian Limestone, accompanied by minor early sulphide mineralization. (2) The limestone breccias acted as favourable horizons for selective dolomitization near the base of the breccia stack. (3) Main-stage mineralization was dominantly replacive, preferentially located within the dolomite breccia host rocks. Minor mineralization is also located in limestone and undolomitized breccias. (4) Late stockwork veins of white dolomite and/or calcite associated with sphalerite mineralization cut the limestone and dolomite breccias. (5) Late crystalline calcite infills remaining void space in the- breccias; stylolites postdate every phase.