Barium isotope variations in highly-fractionated granites linked to tin mineralization: New insights into tin recycling and enrichment in the continental crust

Abstract

Most primary tin deposits are intimately associated with highly-fractionated granites (HFGs), but the factors controlling the formation of tin mineralization; including source enrichment, partial melting, fractional crystallization, and fluid processes, are still subject to debate. The potassium-rich Yanbei volcanic-intrusive complex is part of the world-class Nanling WSn metallogenic belt in South China and contains both barren and Sn-rich igneous rocks. We report here Ba isotope compositions of these rocks in an effort to elucidate the relationship between HFGs and related Sn mineralization. Overall, the results show δ138/134Ba values ranging widely from −0.83 to +0.21‰. The δ138/134Ba of the barren volcanic rocks (−0.11 to +0.21‰) are correlated with SiO2, Sr, and Ba contents, and Eu/Eu* values in a manner indicating that fractional crystallization is the main control over their Ba isotope variation. The most fractionated volcanic rocks have the highest δ138/134Ba due to crystallization of alkali-feldspar rich in light Ba isotopes. In contrast, the highly-fractionated tin granites have low Ba contents and δ138/134Ba values of −0.83 to −0.23‰. These low values cannot be explained by either addition of mantle-sourced melts and magmatic fluids, or fractional crystallization and fluid circulation at the shallow emplacement level. Rather, these geochemical features are likely inherited from an isotopically lighter crustal source with enhanced tin contents related to the recycling of intensely chemically weathered sediments. When combined with bulk-rock chemical data, the Ba isotope data suggest that assimilation of this tin-enriched source played a key role in forming the tin granites and related tin deposits.