Tourmaline compositions trace the sources of metals in the Tangziwa Sn-Cu deposit, Gejiu ore district, China

• Low Cu contents in magmatic tourmaline from Cu-Sn deposits. • Gejiu granitic magmas and exsolved magmatic fluids have low Cu contents. • Addition of Cu during hydrothermal alteration of granite. • External fluids add Cu and redistribute Sn to form Sn-Cu mineralization. Major Sn-Cu deposits in the Gejiu ore district are genetically related to granitic intrusions. As Sn and Cu are lost during magma evolution in oxidized and reduced systems, respectively, they do not enrich together in the same magma. Therefore, the formation of Sn-Cu deposits in magmatic systems is debated. In particular, it is unclear whether Cu was added during late-stage magma evolution or after the crystallization of the magma. We address this question by analyzing tourmaline from the Tangziwa Sn-Cu deposit. The uniform B isotopic compositions of tourmaline (δ 11 B = −16.61 to −14.45 ‰) indicate that it crystallized from granitic melts or magmatic hydrothermal fluids. The chemical compositions of tourmaline track the evolution of fluids and melts from which tourmaline crystallized and record the availability of compatible elements. Tourmaline has high Sn contents, which shows that Sn was abundant in the melt. In contrast, the Cu contents in toumaline (less than 15 ppm) are low as for typical Sn-rich granites and much lower than in tourmaline from Cu-rich magmas that form Cu-Mo-Au porphyry deposits (Cu contents in such tourmaline may reach ∼1000 ppm). This implies that the granitic melts from Tangziwa had low Cu contents and these melts could not be the source of Cu in the deposit. Our data demonstrate that Cu was introduced after crystallization of tourmaline, possibly by the same fluid that resulted in extensive alteration of the granitic rocks and the mobilization of Sn from magmatic minerals to form alteration, skarn, vein, and sulfide type Sn deposits.