The geology and mineralization of geevor tin mine, cornwall

Abstract

Geevor Mine is situated on the northern flank of the Land's End Granite in the St. Just mining district of West Cornwall. Steeply-dipping, NW-SE lodes, with an average width of 2 ft., are confined to the granite. The latter is overlain by metamorphosed 'killas' and a conformable 'greenstone' series which possesses an overall North-West dip. Their previous folding controlled the emplacement of the granite, and their structure was further distorted during the intrusion stage. Tin, as cassiterite, with copper as a by-product, is produced from nearly twenty lodes, many of which have now been completely exploited. A greater degree of fracturing, comprising primary and secondary granite joints initiated by a continuance of the horizontal maximum stress operating during intrusion, allowed the increased vertical stresses associated with mineralization to be accommodated by movement along pre-existing joints. Elsewhere in Cornwall new fissures, now equivalent to the lodes, were created. Preliminary movement comprised block faulting, the limits now being defined by 'post-mineralization' faults and fault zones, and occasionally by a marginally-mineralized lode. Individual blocks were eventually subjected to wedge faulting, each wedge being defined by the lodes. The corresponding movements, both normal and reverse faulting, may be evaluated from displacements exerted upon numerous small dykes. Complete lode mineralization was attained by a series of phases, commencing with wall-rock alteration, and separated in time from late quartz-rich phases by the cassiterite- and sulphide-bearing associations. The extent of distribution of each phase product was influenced by strong structural and temperature controls, demonstrated by constructing contour diagrams. Post-mineralization movements along the earlier limits of block faulting created an additional form of 'cross-course' or 'guide', which was infilled with quartz associations, as opposed to the other which appears only as planes of complex movement.