Mineral exploration in the Cockermouth area, Cumbria. Part 1 : regional surveys

The results of geochemical, geological and geophysical surveys over Carboniferous rocks along the northern margin of the English Lake District are given in two reports. This report (Part 1) describes broadscale surveys across an area bounded by Caldbeck in the east and the coast at Maryport in the west, and confined roughly by the boundary of Carboniferous rocks to the north and south. Part 2 contains details of orientation and follow-up surveys in the Ruthwaite, Tallentire and Whitrigg areas. Revision geological mapping of the area discovered many new occurrences of baryte mineralisation, which is particularly common in the Dinantian and Namurian rocks of the Tallentire - Bothel area. The mineralisation usually comprises epigenetic fracture fillings of baryte, often accompanied by brown carbonate and minor chalcopyrite or malachite. Locally in the Tallentire area baryte also occurs in disseminated and veinlet form within the Hensingham Grit. Lead-zinc mineralisation is less common; it occurs as epigenetic fracture fillings and locally as syngenetic or diagenetic concentrations in mudstones and shales of the Coal Measures. A geochemical drainage survey involving the analysis of water, stream sediment and panned concentrate samples revealed the presence of numerous metal anomalies. These required careful interpretation due to the presence of extensive contamination and glacial deposits derived from metalliferous source rocks in the Lake District and southern Scotland. The mineralogical examination of panned concentrates was used successfully to discriminate between anomalies caused by natural and artificial sources. In many cases evidence for both sources was found in a single sample. The data confumed the widespread occurrence of baryte and suggested that hitherto undetected mineralisation may be present around Ruthwaite (Ba), Tallentire Hill (Ba, Cu), Broughton Moor (Ba, base-metals), near Binsey (polymetallic) and south of Stockdale (Zn, Pb). Gold and cinnabar were reported for the first time from this area. Gold was observed in 21 panned concentrates, most collected over Upper Carboniferous rocks in the west of the area. It may have a local bedrock source, but comes probably from glacial deposits derived from mineralised Lower Palaeozoic rocks in southern Scotland and the north-east Lake District. Cinnabar was identified in 22 concentrates and is believed to be locally derived. Samples of mineral veins and altered wallrocks contain appreciable (up to 40 ppm) Hg, indicating that it is associated with the epigenetic mineralisation. A re-appraisal of the existing regional magnetic and gravity data for the area suggested that a north-west-trending fracture, named the Bothel Fault, could be a more significant structure than was apparent from existing maps, and may have been active during the Carboniferous. The magnitude of the gravity anomaly over the Solway Basin suggests the presence of either a thickened Carboniferous sequence, perhaps across a concealed growth fault, or more low-density (acid volcanic) rocks in the Lower Palaeozoic basement than is indicated on existing maps. Satellite imagery proved useful for indicating the direction of ice movement from glacial features, as well as suggesting the possible location of major fractures along the southern edge of the Solway Basin. It was concluded that the baryte mineralisation may be present locally in sufficient quantities to be of economic interest, but that base-metal mineralisation was weak. The epigenetic mineralisation is considered to be Upper Carboniferous to Lower Permian in age and to be the product of fluid flow through open fractures at the margin of the Solway Basin. The deposits have several features in common with Irish and Pennine-style ore deposits but there are also some clear differences, notably the paucity of Pb-Zn mineralisation.