Abstract
The Coed-Y-Brenin porphyry copper deposit represents the largest deposit (200 mt) of its kind within the UK. This study aims to further understand the mineralogy and textures of ore minerals within the Coed-Y-Brenin deposit.The area consists of middle to upper Cambrian sediments, namely the Ffestiniog Flags Formation, Dollgellau Formation, and the Dol-Cyn Afon Formation. These geological units were later intruded by the Rhobell Fawr volcanics during the Ordovician. The deposit is located within the Welsh basin, which was a back-arc setting on the margin of the Avalonian microcontinent. The region underwent subduction-related magmatism due to an oceanic-continental convergent margin. The rocks of Coed-Y-Brenin have undergone many events of deformation, with the majority associated with the Caledonian Orogeny.
The deposit is hosted within the Afon Wen Intrusive body, which intrudes the Ffestiniog Flags Formation and is co-genetic with the Rhobell Fawr Volcanic Complex. Borehole cores were studied at the BGS in Keysworth, Nottingham, and 37 samples were taken from the diorite (Afon Wen) and Ffestiniog Flags Formation for further study.
Host rock samples were studied using digital optical microscopy of thin sections (using a Leica DVM6 microscope) and EDS spot analysis via SEM. Ore samples were analysed using ICP-MS geochemistry and automated SEM-EDS using AMICS and the Leica DVM 6.
ICP-MS geochemistry of the samples detected Ag, Al, As, Au, Ba, Ca, Co, Cr, Cu, Fe, K, La, Mg, Mn, Mo, Na, Ni, P, Pb, S, Sb, Sc, Se, Th, Zn and W, although the emphasis of this study was on Au, Cu, Co and Mo.
Gold was detected as disseminated and sporadic grains, ~5 µm in size, primarily hosted within quartz veins associated with the intrusive complex, although also present within pyrite as inclusions. The upper limit of gold detection (1000 ppb) within the geochemical assay was reached in two samples. Copper occurs at up to ~6620 ppm and is detected in every sample, most commonly occurring as chalcopyrite. Chalcopyrite was noted as associated with pyrite and dolomite. Cobalt was detected exclusively within pyrite (up to 111ppm) in solid solution. Molybdenum was detected within molybdenite (up to 890ppm), included within calcite, and associated with pyrite and chalcopyrite.
Seven stages of ore mineral paragenesis were observed within the samples. Although it is anticipated that more stages are likely present, without further study or dating, they cannot be identified.
In comparison to previous work it was observed several stages of pyrite mineralization as well as the brecciation of pyrite were observed. Similar results were seen throughout as gold was contained in pyrite and quartz veins, although a difference was that Miller found electrum and tellurides, whereas this study did not; instead, native gold was found.
| Date of Award | 2025 |
|---|---|
| Original language | English |
| Supervisor | Ian Skilling (Supervisor), Duncan Pirrie (Supervisor) & Richard E. Bevins (Supervisor) |