ABSTRACT
In the southern part of the Irish Zn-Pb(-Ag) orefield the main hostrocks for mineralization are black dolomite matrix breccias. The origin of these breccias is poorly understood at present and their genetic relationship to economic mineralization is uncertain. The Cooleen zone is a weakly mineralized satellite of the Silvermines deposit, Co. Tipperary, where early cementation and alteration stages have not been completely overprinted by extensive hydrothermal alteration. Detailed studies demonstrate that the breccias formed initially as sedimentary slump breccias in response to fault movement. Hydrothermal fluids travelled up through faults and preferentially dolomitized the relatively permeable limestone breccias, enhancing the breccia textures, thereby forming the primary hostrock for later replacive sulphide mineralization. Crystalline white dolomite and calcite vein/stockworks cut the breccias and sulphides and are thought to represent fluid activity during the waning stages of the mineralizing system.
