Mineral exploration in the Harlech Dome, North Wales

This report presents the results of an airborne geophysical (electromagnetic, magnetic and radiometric) survey of the eastern part of the Harlech Dome, North Wales, and the consequent geological, geochemical and geophysical investigations carried out on the ground and in the laboratory. e The report is divided into two parts. In Part 1 are the regional and general investigations including an account of the airborne geophysical surveys, an investigation into the causes of electromagnetic (EM) anomalies, Curie temperature determinations, the geochemistry and relationships to mineralisation of some igneous rocks in the Harlech Dome, a description of intrusion-breccias in the Harlech Dome, and fluid inclusion studies. In Part 2 are the details of 26 ground investigations carried out to explain the airborne geophysical anomalies. These investigations were of a reconnaissance nature aimed at detecting metal enrichments and explaining the causes of the anomalies rather than at delineating ore bodies. The eastern side of the Harlech Dome contains a folded succession of Cambrian and Ordovician 8 sedimentary, volcanic and intrusive rocks metamorphosed to a low grade. The area has long been a centre of mining activity containing the Dolgellau Gold-belt, in which quartz veins were mined for gold and silver with copper, lead and zinc sulphides, and the porphyry-style copper deposit at Coed-y- Brenin. As reported in Part 1 the airborne magnetic and EM results showed a complex pattern of anomalies, whereas the aeroradiometric data showed little variation. Ground studies of aeromagnetic anomalies demonstrated that they were related either to magnetite present in some sandstones and igneous rocks, I or, more importantly, to pyrrhotite enrichments in several lithologies. Mineralogical studies showed that whilst some enrichments in siltstones and mudstones were probably syngenetic in origin, others were possibly related to epigenetic concentrations produced during vein mineralisation. A study of the I EM anomalies showed that many of them were related to relatively low concentrations ( c 3%) of carbonaceous material or non-commercial quantities of sulphides in the dark mudstones of the Dolgellau Member and Clogau Formation. An interpretation of the radiometric data confirmed that the only indications of uraniferous enrichment occur in the black mudstones of the upper Cambrian Dolgellau I Member. The rock geochemistry study indicated that statistical treatment of rock analyses could be used to distinguish intrusions associated with porphyry-style mineralisation. It also showed that the porphyry- I style mineralisation at Coed-y-Brenin was probably co-genetic with the end-Tremadoc magmatism which gave rise to both the volcanic pile on Rhobell Fawr and intermediate intrusions in the Cambrian, and therefore was quite distinct from the end-Silurian quartz-sulphide vein mineralisation. Intrusion breccias which are also linked to the Rhobell Fawr volcanism were recognised in the area for the first I time. One of them contains the worked-out Glasdir copper deposit. The fluid inclusion study showed differences between quartz veins associated with the vein and disseminated styles of mineralisation, which may be useful in exploration. The ground follow-up work reported in detail in Part 2 showed indications of mineralisation in many of the areas examined. Among these, the possibility of vein and stratabound lead and zinc mineralisation was found at Hengwrt Uchaf and Benglog. At Bryn Coch, Tyddyn Gwladys and Hafod Fraith possible I associated but separate bodies or extensions to the proved porphyry-type copper deposit at Coed-y- Brenin were identified in addition to modest vein mineralisation. In three areas, Mynydd Foe1 Uchaf, Hafod-y-fedw and Y-Gors, dispersed epigenetic sulphide 0 mineralisation in bedrock, mainly pyrrhotite with sub-economic base-metal sulphides, were found. Similar metalliferous concentrations were tentatively identified in a number of other areas including Garth Gel1 where pilot studies were carried out on coincident EM and magnetic anomalies. At Mynydd 1 Bach, Craiglaseithin and Do1 Haidd there are indications of either new veins or extensions to known veins containing copper, lead and zinc as well as feeble disseminated copper mineralisation at the last two localities; again the metal concentrations are considered sub-economic. Ffridd Dol-y-moth and Waun Hir are both drift covered areas which may conceal mineralisation but exploration is hampered by 1 contamination problems. At Nannau, slight copper enrichment was found in volcanic rocks which are believed to be cogenetic with the Coed-y-Brenin porphyry copper deposit. Recommendations are made for further work at Glasdir, in the Coed-y-Brenin area, at Hengwrt 1 Uchaf and Benglog, at Ffridd Dol-y-moth and Nannau and, pending the results of the drainage survey, on the nature and extent of the sulphide concentrations in the Clogau Formation