Re-evaluation of quartz vein history in the Dolgellau Gold-Belt,North Wales,United Kingdom

Conspicuous, gently inclined, lenticular quartz–carbonate veins with fibre-textured infills dissect the steep, gold-bearing quartz veins in the Dolgellau Gold-Belt. Similar quartz veins fill the necks of boudin-like structures affecting the main gold-bearing veins at Clogau mine. They also dissect the post-mineralization ENE–WSW ‘Clogau Stone’ dykes. The uniformly oriented regional cleavage shows marked deflection around the gently inclined quartz veins. These late quartz veins were emplaced whilst the mudstones of the Clogau Formation, which host the steep gold-bearing quartz veins, were undergoing ductile deformation. The stretching involved in the late vein and boudin development requires these structures to be contemporaneous with some stage in the development of the regional sub-vertical cleavage and upright fold system. The earlier gold-bearing veins show evidence of brittle fracture and still retain some drusy space-filling textures. Cleavage is deflected adjacent to these early veins and in breccia clasts within the veins. The rare minor folds seen in the gold-bearing quartz veins have axial planar cleavages. It is concluded that the gold-bearing veins pre-date cleavage development. Thus they were forming when large amounts of water were still available in the sedimentary sequence, removing the severe source and transport problems that are implicit in the previous view that the veins were post-tectonic. Copyright © 1999 John Wiley & Sons, Ltd.