Geophysical and geochemical investigations over the Long Rake, Haddon Fields, Derbyshire

Abstract

Geophysical and geochemical investigations were undertaken over the Long Rake at Haddon Fields, Derbyshire in order to establish methods, or combinations of methods, showing the best response to the mineralisation. The mineralised structure carries high concentrations of fluorite with associated lead and zinc minerals and the gangue minerals baryte and calcite. The ground examined was relatively undisturbed with good geological control from drill hole data. Gravity and magnetic anomalies such as those obtained over the Long Rake could have limited applications for the indirect location of veins the approximate position of which is known. Induced polarisation, resistivity and electro-magnetic measurements failed to produce anomalies which could be directly attributed to the mineralisation or its host structure. However, reconnaissance mapping with very low frequency electro-magnetic (VLF-EM) and Radiohm methods showed that, over a large section of the survey area, the fluorspar vein could be mapped by its association with the subdrift shale/limestone contact. The determination of a wide range of elements in soils and tills showed that the more mobile elements such as F and Zn are particularly useful in detecting mineralisation over broad areas. Less mobile elements tend to exhibit localised disperson patterns which have applications in precisely locating an orebody. Elements enriched in soil above the Long Rake, in areas of thin overburden, include Pb, Ba, Sr, Ca, Zn, Rb and Th. Thickening cover towards the west tends to mask anomalies of many elements above the Rake, only Ba, Sr and Pb maintaining significant contrast. The collection of basal till samples was made difficult by the occurrence of large limestone boulders. However, results indicated that the method has no advantage over soil geochemistry in this environment, as geochemical contrast is not improved. Levels of Ba and Ca were highly variable and the concentration of Ba appeared to be directly related to the sampling depth