A K-feldspar breccia, spatially associated with the Mo-Cu mineralization of a stockwork in the Late Caledonian Galway Granite at Mace Head, is described for the first time. Detailed mapping reveals a network of breccia pods and veins over an area of approximately 6000 m2. The breccia is clast-supported and is composed of sub-angular fragments of perthitic K-feldspar megacrysts (<10 cm), granite and microgranodiorite clasts (<25 m) set in a matrix of quartz (<lcm), biotite (<5 cm) and apatite (<3 mm). Field and textural studies indicated that the feldspar megacrysts and granite clasts were brecciated and silicified as they were carried (to the present structural level) by hydrous K- and SiO2-rich fluids. The residue of these fluids crystallized to form the breccia matrix. The formation of the breccia predates the mineralized quartz veins of the Mo-Cu stockwork. It is concluded that the breccia formation is genetically related to ore-forming processes in the Galway Granite.

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