Abstract:

Lodes in the Central Wales Orefield are mineralized oblique-slip normal faults that postdate the regional joints. They developed shortly after the (Acadian) deformation of the Welsh Basin in the late Early Devonian and many were reactivated with further mineralization in the early Carboniferous. Although orefield extent is clearly centred on the structural culmination around the Plynlimon Dome the major areas of lead–zinc production are strongly off-centered. These areas lie on the depositional pathways of three horizons of late Ordovician–early Silurian massive turbidite sandstones. It is argued that these basin-floor sand bodies are preferential sites of lode nucleation and that their distribution controls the size and abundance of the lodes and hence their chances to link laterally and at depth. This in turn controls their ability to capture and transmit mineralizing fluids and thus their chances of hosting large ore deposits. The prime cause of high fluid pressure gradients during the first phase of mineralization was rapid uplift of ‘tight' rock with dehydration fluids from Acadian metamorphism. Second phase fluids probably originated as seawater and were driven by convective overturn at essentially hydrostatic pressures. The stratigraphic control of lode productivity has implications for mineral exploration in such terranes.

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