Petrography and geochemistry of tourmaline breccia in the Longtoushan Au deposit, South China: Genesis and its exploration significance

The Longtoushan Au deposit, located in the Qin-Hang Metallogenic Belt, South China, is a breccia-hosted epithermal Au deposit. It is characterized by distinctive occurrence of magmatic-hydrothermal tourmaline-cemented breccias. Tourmaline-group minerals have an extensive range of chemical composition that could reflect their host environments, including some of economic interest. To investigate the potential of tourmaline as a geochemical indicator for ore exploration, B-isotopic and major element compositions of tourmaline from the Longtoushan Au deposit have been determined. Tourmaline mainly occurs as orbicular aggregates, clusters and veinlets in rhyolite porphyries, granite porphyries and in spatially associated hydrothermal breccias (Au-mineralized and Au-barren). Three types of tourmaline (I, II and III) are recognized at Longtoushan, most of which being the alkali-deficient schorl-dravites. The pre-ore Tourmaline I in the rhyolite porphyry is Fe-rich schorl, contrasting with the dravitic ore-stage Tourmaline II in the mineralized breccia and Tourmaline III in the granite porphyry. Locally, Tourmaline II shows concentric zonings which correlate with core-rim chemical variations (higher Fe, Al in the rim than in the core). Tourmalines from Longtoushan are characterized by a dominant MgFe-1 exchange vector and have relatively low Na contents (avg. 0.66 apfu; formula normalized on 15 total cations), indicating they possibly crystallized in reduced fluids of low-salinity. The δ11B values of tourmalines from this study fall in a narrow range of –14.5 to –10.2‰, indicating dominant magmatic-hydrothermal fluids of continent crust origin. The results suggest that tourmaline from Longtoushan may originate from boron-rich vapors fluids unmixed from the felsic magma, which was favorable