Precious metal mineralization associated with the Coed-y-Brenin porphyry copper system, North Wales
The Coed-y-Brenin system is a diorite-hosted porphyry copper deposit of early Ordovician age (Tremadoc) situated on the south eastern side of the Harlech Dome: a thick sequence of Cambrian and Ordovician sediments and volcaniclastics that were subject to greenschist facies metamorphism during the Caledonian Orogeny. The copper mineralization occurs as disseminations, coatings on hairline fractures and in late-stage quartz-sulphide-calcite veinlets. Pyrite and chalcopyrite are the main opaque phases but sulphosalts, tellurides and electrum are also present. The copper mineralization is largely confined to the diorite and micro-tonalite dykes and sills associated with the Rhobell Fawr Volcanic centre. This study has shown that epithermal style precious-metal mineralization is also developed in the Coed-y-Brenin area in the shales and siltstones of the Maentwrog and Ffestiniog Flags Formations. It is characterised by pyritization, sericitization and silicification. The gold and silver occur as electrum in fractures and also as tellurides in late stage quartz-pyrite veins. Both styles of mineralization share a close spatial association and are characterised by propylitic and phyllic alteration. Argillic and potassic assemblages are not developed. Geochemical analysis of mineralized and unmineralized material has shown that the diorite-hosted mineralization is enriched in Cu, Au and Ag and depleted in Zn while the epithermal mineralization displays enrichments in Au, Ag, As, Sb and the base metals. Rb:Sr ratios increase with increasing alteration in both styles of mineralization. Sulphur isotope studies have shown that both styles of mineralization have a common, probably magmatic, sulphur source. Fluid inclusion data and O and H isotope studies indicate that following magma emplacement and development of the porphyry copper mineralization a low temperature-low salinity meteoric hydrothermal sytem was established. This was responsible for the late-stage quartz-sulphide-calcite veins in the diorites and for all the sediment-hosted mineralization. The association between epithermal and porphyry deposits has become apparent in recent years but the Coed-y-Brenin system is unusual due to its Lower Palaeozoic age and because both styles of mineralization are developed at the same structural level. This study of the Coed-y-Brenin porphyry system demonstrates the potential for the development and preservation of epithermal mineralisation in porphyry systems and also in older orogenic belts.