Genesis and hydrothermal evolution of the Tiantangshan tin-polymetallic deposit, south-eastern Nanling Range, South China

The Tiantangshan tin-polymetallic deposit, located in the Nanling Range of South China, is a medium-sized polymetallic deposit found in the region in recent years. The deposit is hosted within volcanic rocks, and the orebodies occur in the cupola and outer contact zone of the concealed quartz porphyry, as well as the altered fracture zone of the volcanic rocks close to the intrusive contact. In light of field evidence and petrographic observations, the mineralization can be divided into four stages: greisenization stage (stage I), quartz-cassiterite-wolframite stage (stage II), quartz-fluorite-cassiterite-sulfides stage (stage III), and post-ore stage (stage IV). Three types of fluid inclusions are present in the hydrothermal topaz, quartz, and fluorite, including H2O-rich (W-type, WL- and WV- subtypes), CO2-bearing (C-type), and solid-bearing (S-type). Four stages of fluid evolution are observed by detailed fluid inclusion studies: (a) Stage I fluids are trapped under two-phase immiscible condition, as evidenced by the coexistence of primary aqueous (W-type) and aqueous-carbonic (C-type) fluid inclusions preserved in topaz and quartz; the fluid inclusions display homogenization temperatures of 378-448 degrees C and salinities of 6.0-17.5 wt.% NaCl equiv. (b) Similarly, fluid inclusions in stage II quartz also record immiscible condition, as identified by the coexistence of W-type and C-type fluid inclusions with lower homogenization temperatures (308-400 degrees C) and salinities (1.8-14.5 wt.% NaCl equiv.). (c) Stage III fluids are characterized by the coexistence of widespread WL-subtype, minor S-type, and rare WV-subtype fluid inclusions, with similar homogenization temperatures (230-369 degrees C) but contrasting salinities (0.5-39.5 wt.% NaCl equiv.), which indicates an episode of fluid boiling occurred in this stage. (d) Stage IV marks the end of the hydrothermal system characterized by the lower temperatures (175-298 degrees C) and salinities (0.2-3.9 wt.% NaCl equiv.) W-type fluid inclusions trapped. Microthermometry and H-O isotopes indicate that the early ore-forming fluids (in stage I) exsolved from the granitic magma and underwent progressive mixing with meteoric water during subsequent ore-forming process (in stages III and IV). The water-CO2 fluid immiscibility is the main mechanism of cassiterite and wolframite precipitation, while fluid boiling and mixing are probably thought to be the dominant mechanisms for the deposition of sulfide at Tiantangshan. Ar-40/Ar-39 dating of hydrothermal biotite intergrown with cassiterite shows that tin-polymetallic mineralization occurred at similar to 133 Ma which is coeval with the hidden intrusions. Taken together, these lines of evidence confirm that the Tiantangshan deposit is a magmatic hydrothermal greisen-quartz-vein type tin-polymetallic deposit that formed in the lithospheric extension and thinning setting associated with the postsubduction Paleo-Pacific Plate tectonic regime that influenced South China.