Structure and mineralization in the Cligga granite stock, Cornwall

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Abstract
In the Cligga stock there are two groups of fractures, produced by separate episodes of brittle deformation which affected the granite and adjacent slates. Generalized models constructed by interpretation of fault and fracture traces on geological cross-sections provide a coherent explanation of the formation mechanisms for the two groups of structure: A—curved primary flat-lying joints, granite-veins and porphyrydyke fissures, early normal faults in slates, marginal thrusts and reverse faults, and B—conjugate normal faults and extensional fissures. The fractures were produced by stresses exerted from within the underlying St Agnes–Cligga cupola which forms part of the buried batholith ridge. The stress system was modified during the first deformation by local conditions within the stock. Structures of one or both generations controlled the widespread greisen (phyllic) wallrock alteration, hydrothermal mineralization and kaolinization (argillic alteration). Time/space models illustrate relationships during the major episodes of ore mineral deposition which produced telescoped metal zonation in the belts of W/Sn/Cu/Fe veins which traverse the intrusion. The relationships between structure and mineralization at Cligga and in other small granites, indicate that undiscovered stocks in SW England may contain potentially economic W/Sn-sheeted vein or stockwork ore bodies.

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