Peraluminous granite related to tin mineralization formed by magmatic differentiation and fluid exsolution of metaluminous melt: A case study from the Bozhushan batholith, South China Block

Abstract

Most primary tin deposits are closely temporally and spatially related to peraluminous granites. These granites usually exhibit highly evolved geochemical characteristics that disguise their specific origin. This paper reports a study of whole-rock geochemistry and Sm–Nd isotopes, zircon U–Pb geochronology and Lu–Hf isotopes, and mineral chemistry of the Bozhushan granitic batholith in the Youjiang basin, South China. The Bozhushan batholith, producing the Bainiuchang large tin–polymetallic deposit, consists of metaluminous tin-barren monzogranites and peraluminous tin-bearing monzogranites. LA-ICP-MS zircon U-Pb dating suggests that the peraluminous rocks were emplaced during the Late Cretaceous (88.5 ± 0.7 Ma), which is largely consistent with the emplacement age of the metaluminous rocks. These granitic rocks have similar Nd-Hf isotopic compositions, supporting a common crustal source. Relative to the metaluminous monzogranites, the peraluminous tin-bearing rocks exhibit more evolved geochemical compositions with higher Si and lower Ca, P, Mg, Fe, and Ti and stronger depletion in Eu, Ba, Sr, Zr, Hf, and rare earth elements. These differing geochemical characteristics can be explained by the differentiation of biotite, plagioclase, zircon and apatite. The extremely low Nb/Ta (<5) and K/Rb (<150) ratios of the peraluminous rocks indicate a fluid exsolution process. We suggest that the peraluminous tin-bearing rocks were derived by magmatic differentiation and fluid exsolution of the metaluminous melts. Combining our new results with previous studies, we conclude that on a worldwide scale some peraluminous granites related to tin mineralization are derived by magmatic differentiation and fluid exsolution of metaluminous melts rather than from metasedimentary rocks.