LATE TRIASSIC PORPHYRY MOLYBDENUM MINERALIZATION IN THE WESTERN QINLING, CHINA: AN EXAMPLE FROM THE WENQUAN DEPOSIT

The Western Qinling witnessed superimposed orogeny including the north subduction of Paleotethys ocean and collision between North China and South China blocks, however, the precise timing constraints on tectonic regime switchover and their associated mineralization are remaining enigmatic. The Wenquan porphyry Mo deposit, with a resource of 247 million tonnes at 0.048% Mo, is hosted in the Wenquan composite batholith comprising five phases and mafic enclaves yielding an episodic growth lasting nearly 30 myr ranging from ca. 238 Ma to 208 Ma, consistent to Triassic igneous activity recording a transition regime from a subduction setting to a syn-collision setting and a post-collision setting in the Western Qinling, and is therefore an ideal “mineral deposit probe” to unlock this puzzle.

Petrology, whole-rock petrogeochemistry and Pb and Lu-Hf isotopes suggest that the heat and the hot mafic melt initiated by the break-off of the northward subducting South China block lithosphere triggered partial melting of the Mesoproterozoic subcontinental lithospheric mantle to produce mafic magmas, and the underplated mafic magmas caused partial melting of the shallow subducted Mesoproterozoic lower crust generating granitic magmas at Wenquan.

Petrography and microthermometry on fluid inclusions derived from quartz and H-O-S-Fe isotopes on gangue (K-feldspar, sericite, quartz), ore minerals (pyrite, molybdenite) and whole-rock suggest that formation of the Wenquan magmatic-hydrothermal systems span the ductile-brittle transition as constrained by the temperature range of 550~300 °C and a paleodepth of ~5 km. The early, high-temperature potassic alteration developed under a ductile regime, whereas the later, low-temperature phyllic alteration correlates to a brittle environment, where hydraulic rock fracturing enhances permeability and fluid circulation. Most sulfur and metals were precipitated from vapor-rich fluids resulting from cooling and expansion of a single-phase fluid exsolved from a deeply-sourced hybrid magma under lithostatic load. External Late Triassic meteoric or Devonian formational fluids became a part of the hydrothermal system during the post-fracturing final stage of ore formation under a hydrostatic regime.