Geochemical evidence for basement control of the west Cumberland haematite mineralization

Abstract

A geochemical investigation of the West Cumberland meta somatic haematite deposits and related vein deposits in the central Lake District was carried out to establish the origin of the mineralizing fluids. Ore samples we re analysed for a variety of major and trace elements and were found to contain a simple geochemical assemblage characterised by the abundance of arsenic and locally barium. The lack of regional variation in ore geochemistry and the equally uniform mineralogy suggests that the replacement and vein deposits are cogenetic and belong to the same metallogenic province. Minor differences which do exist can be explained, by wallrock - ore fluid interactions. In the Eskdale area the veins are associated with a particular phase of the host granite containing an abundance of free haematite. Relative trace element enrichments in the ores are matched by sympathetic depletions in the adjacent haematized granite. The implied geochemical relationship is verified by a spatial correlation between areas of mineralization and. the distribution of concealed granite in the basement; as indicated by the gravity pattern. A more detailed gravity interpretation in the West Cumberland area shows that the zone of maximum mineralization in the limestones occurs directly above the faulted margin of a concealed, granite shelf extending outwards from the Ennerdale granophyre. Based on the combined/geological, geochemical and fluid, inclusion evidence a new model of haematite ore genesis is proposed which envisages the convective circulation of hot saline brines in the granite basement with the concomitant leaching of iron and its re deposition at higher levels as epigenetic haematite mineralization. The hypothesis is consistent with the known distribution of ore bodies and the observed spatial variation for arsenic and copper in the West Cumberland orefield. Baritic ores are related to the mixing of the ore fluids with formational waters from the Permo-Triassics.