Spatial Distribution of the Hydrothermal Black Matrix Breccia and its Impact on the Irish-type Zn-Pb Mineralisation

The Mississippian Waulsortian Formation of the Irish Midlands hosts a number of significant Zn-Pb mines including Lisheen, Galmoy and Silvermines. Consisting predominantly of sphalerite, galena and pyrite, the deposits are intimately associated with hydrothermal dolomite and dolomitic breccias, commonly referred to as “Black Matrix Breccia” (BMB). The hydrothermal dolomite and associated breccias form a predominantly tabular body that largely envelops the zone of sulphide mineralisation. A wide variety of mineralisation styles and textures are recognized, however the majority of the mineralisation resulted through replacement of this hydrothermal dolomite. Recent geochemical evidence indicates that the hydrothermal dolomite contains distinct geochemical signatures that may be useful in sulphide exploration within the Irish Midlands. To date, little work has been conducted on the spatial distribution and variability of this significant hydrothermal dolomite and the role it plays in ore genesis within the Irish Midlands. Through detailed petrographic characterisation, this study documents the distribution of the hydrothermal dolomite. This distribution helps constrain the origin of the massive Zn-Pb deposits and forms an important tool for future mineral exploration in the Irish Orefield.