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The genesis of tin–uranium granites in the Scottish Caledonides: Implications for metallogenesis
Author(s) -
Plant J. A.,
Henney P. J.,
Simpson P. R.
Publication year - 1990
Publication title -
geological journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.721
H-Index - 54
eISSN - 1099-1034
pISSN - 0072-1050
DOI - 10.1002/gj.3350250325
Subject(s) - batholith , geology , geochemistry , fractional crystallization (geology) , crust , mantle (geology) , partial melting , metallogeny , uranium , tectonics , seismology , pyrite , materials science , sphalerite , metallurgy
The genesis of granites enriched in Sn and U and their role in the formation of Sn–U deposits is reviewed with particular reference to studies of geochemical, geophysical, and geological data sets from the Scottish Caledonides based on image analysis studies. The chemistry and tectonic setting of radioelement‐enriched granites some of which are also enriched in Sn (e.g. Cairngorm intrusion) is distinct from that of the more commonly occurring calc‐alkaline granites. On mantle‐normalized diagrams they are enriched in U, Th, Rb, Nb, Ta, Li, F, Be, B, and Sn with marked depletion of Sr, Ba, Ti, and P; and on chondrite normalized REE diagrams they have high REE with La N /Lu N values of 1·8–6 and marked negative Eu anomalies (Eu/Eu* 0·3). These features are characteristic of Sn–U granites from S.W. England (Dartmoor) and elsewhere. Geophysical data indicate that the Caledonian Sn–U granites are the high points of a buried batholith (the East Grampian batholith) with a length of approximately 100km, breadth of about 40km, and depth of approximately 7–8km. The batholith is discordant to the Caledonian structural grain and was emplaced post‐orogenically, probably in tensional pull‐apart structures. Petrogenetic models and isotopic data for the Cairngorm granite preclude ‘S’‐type crustal melting models for its genesis; instead the preferred model involves combined assimilation and fractional crystallization (AFC) as the result of the addition of dioritic magma to the lower crust followed by up to 50 per cent fractional crystallization. The concentration of Sn, F, and radioelements in the granite magmas, the release of heat as the result of protracted fractionation, the large volumes of the intrusions, and their emplacement into fault systems are factors favouring the formation of Sn and U deposits. The high calculated heat production of such intrusions and their emplacement into faults along which reactivation can occur may explain the association of such granites with hydrothermal ore deposits formed long after magmatic cooling. Overall the role of granites in ore formation is complex and reflects such factors as the chemistry, heat production (contents of U, Th, and K), density, permeability, and response to deformation of intrusions as well as the availability of water in the rocks into which they are emplaced.