Ore mineralogy of the mesothermal gold lodes of the Dolgellau gold belt, North Wales

Abstract

The auriferous quartz-sulphide veins of the Dolgellau gold belt represent the finest example of a pre-Acadian, mesothermal gold-lode metallogenic province in the United Kingdom. The gold lodes are composite, multi-stage, ribbon-quartz bodies occupying shear zones in clastic marine sedimentary rocks, including black mudstones, of Middle to Upper Cambrian age and in associated altered, basic-intermediate intrusives. Petrological studies have revealed a complex history of multiphase mineralization and anomalously pervasive hydrothermal alteration. The mesothermal gold-lode mineralization has a four-stage paragenetic sequence comprising early Fe-Co-As followed by localized, bonanza-style Au-Ag-Bi-Te-Pb. Stage three is Cu-Fe-dominated, whereas the final stage is dominated by Pb-Zn. Quartz and carbonates accompany all stages, their relative abundances varying considerably from locality to locality. The textural relationships of the ore minerals have been partially modified by post-depositional processes related to regional, strain-related metamorphism and Acadian deformation, but the veins still reveal a consistent regional paragenesis, a detailed account of which is published here for the first time. A discussion of the redefined paragenetic sequence of the gold lodes is followed by a reinterpretation of the extensive existing pool of previously published analytical data.