Petrology of the Jurassic Shah-Kuh granite (eastern Iran), with reference to tin mineralization

The Shah-Kuh granitic pluton of eastern Central Iran was emplaced ∼165 Ma ago, in an active continental margin setting. It is made of two main units: a granodioritic unit (SiO2=63-71 wt%) to the north-west and a syenogranitic unit (SiO2=73-77 wt%) to the south-east. The former unit displays seriate medium-grained textures and contains locally abundant mafic enclaves. The latter unit is medium- to coarse-grained and porphyritic, with 0.5-3 cm long K-feldspar megacrysts. Fine-grained granitic bodies are present in both units. The rocks are metaluminous to slightly peraluminous (I-type) and peraluminous (S-type) and belong to the ilmenite-series granites. Fractional crystallization appears to have been a very effective differentiation process in both units, and the fractionated mineral assemblages are determined by mass balance calculations. Isotopic data (Sri=0.7065 and εNdt=−2.5) are consistent with a young upper crustal protolith. Tin mineralization in sheeted quartz-tourmaline (-cassiterite) veins is spatially associated with the granodioritic unit. The veins formed by hydraulic fracturing when the granodioritic to monzogranitic magma became water-saturated and exsolved a fluid phase during crystallization. The reduced nature of this magma is responsible for the incompatible behaviour of Sn, likely to favour Sn concentration in the residual melt and then in the exsolved fluid. Another fluid phase was exsolved by the syenogranitic magma and was responsible for local greisenized granites, characterized by high Y and HREE-contents and non-fractionated REE distribution patterns. Field and mineralogical data show that the (B, Sn) vein-forming fluid was different from the (F, Li) greisen-forming fluid.