Mineral reconnaissance in the Northumberland trough

Abstract

A geochemical and geophysical reconnaissance programme was commenced in 1978 to investigate the favourability of the large Carboniferous depositional trough underlying Northumberland for the discovery of new metalliferous mineral deposits and/or new styles of mineralisation. The long historical association of the southern part of the area with successful mining ventures indicated that a more comprehensive evaluation of potential was justified. The importance of the Whin Sill as a host lithology had long been recognised and the disposition of the Sill played a significant part in determining the direction of effort in the area. The association of mineral deposits with faulting in the Whin Sill was considered to justify an airborne magnetic survey for mapping such structures in that part of the basin underlain by the sill at shallow depth. The main geochemical effort involved a drainage reconnaissance of the entire Lower Carboniferous trough area. The geochemical data obtained from stream sediments and panned concentrates were processed bY computer, employing simple statistical techniques from which a number of anomalous areas were defined which are not attributable to either the known ore bodies or artificial contamination. The data derived from the regional geochemical survey identified not only the known (and now largely worked out) mining areas but also a number of other areas with anomalously high metal values. Barium, in stream sediments and panned concentrates, was a reliable indicator of mineralisation and identified the Settlingstones- Whinnetley-Fallowfield area. High barium values in concentrates were also obtained from an area to the south of Rothbury (Ewesley Farm) which is geographically remote from any known mineralisation. Soil samples subsequently collected in this arca also contained elevated barium values, and it is considered that unexposed barium mineralisation exists in the area, probably associated with a fracture cutting the Whin Sill. From the airborne geophysical data a number of linear magnetic anomalies were identified, several of which can be equated with known fault structures or their probable extensions. Of the faults thus indicated, some have carried significant mineralisation, and apparently related magnetic anomalies in their vicinity were thus identified as of possible mineral exploration significance. 1 Several of these linear magnetic anomalies were further examined by geochemical (soil sampling) and ground-geophysical techniques. The geochemical data obtained from the soil traverses in the areas examined did not provide unequivocal information, values for the ore elements being generally low. One magnetic anomaly indicating an eastward extension of the Sun Vein near to Newbrough was identified as a drilling target, and four inclined boreholes were drilled from two sites to test the fault structure affecting the Whin Sill as interpreted from the magnetic data. Three of the holes were continued to sufficient depth to pass through the Whin Sill and into the sediments beneath. Sufficient information was obtained from the holes to permit stratigraphic correlation between them and also to establish structural relationships, while considerable variation in the texture and degree of alteration of the quartzdolerite was apparent in the cores. Base metal mineralisation associated with the alteration of the Sill, and also in some of the carbonate sediments, was identified. Chemical analyses of samples from the Whin Sill quantify the changes in composition effected by the process of hydrothermal alteration. Magnetic susceptibility values determined on the Whin Sill core show great variability, consistent with the variation in the degree of alteration to White Whin