The Galinge skarn deposit, the largest iron polymetallic skarn deposit in the Qiman Tagh metallogenic belt (western China), was formed via multi-stage fluid-rock interactions. It is divided into six ore domains from east to the west. Skarn-related tourmaline is ubiquitous in the V ore domain of the Galinge deposit, occurring both in the altered basaltic andesite (Tour-I) and in the sandstone (Tour-II). The tourmaline composition in both rock types is within the dravite–uvite solid solution. Some Tour-I crystals show compositional growth zoning in which the early stage uvite cores (Gen-1) are overgrown by second-stage dravite rims (Gen-2). Some Tour-I crystals also show overgrowth rims and fracture-infilled textures (Gen-3). Some other Tour-1 tourmalines without clear growth zoning (Others) show an intermediate composition between Gen-2 and Gen-3.
The varying composition of the zoned tourmalines records important information about the evolving hydrothermal fluids and host rocks. Gen-1 and Gen-2, displaying a narrow and high range of Fe2+/(Fe2+ + Mg) ratios, are much more equilibrated with mafic host rocks. The alkaline (K + Na) content of tourmalines is associated with the salinities of the ore-forming fluids. The lowest Na + K content of Gen-3 indicates that it may have been equilibrated with a low-salinity fluid environment in which the concentration of metal-chlorite complexes decreased. The Gen-3 stage is considered to be the main ore-forming event. Tour-II have similar Ca/(K + Na + Ca) ratios with Gen-1 and Gen-2 ratios, which indicates that they are contemporarily formed by the same fluid as Tour-I.
Through compositional comparison of the tourmalines with those from other hydrothermal deposit types, the Galinge skarn-related tourmalines are overwhelmingly controlled by the MgFe–1 substitution mechanism. This is different from the compositions of tourmalines in porphyry, VMS, and vein-greisen type deposits, which are, respectively, controlled by the Fe3+Al–1, (CaMg)(NaAl)–1 and (NaMg)(□Al)–1, and (Fe2+Fe3+)(MgAl)–1 substitution mechanisms. Different tourmaline compositions and substitution mechanisms could be used as guides for mineral exploration.