Multi-Stage Magmatic and Hydrothermal Events Formed Giant Caosiyao Porphyry Mo Deposit in the Northern Margin of North China Craton

Abstract

The formation of Caosiyao giant porphyry Mo deposit is related to three granitic porphyries: coarse-grained granite porphyry (CG), fine-grained granite porphyry (FG), and giant plagioclase phenocryst bearing granite porphyry (PG). To investigate the mineralization significance of three porphyries, Microthermometry, Laser micro-Raman Spectra, and H-O-He-Ar isotope analyses of fluid inclusions were conducted. Intermediate density with high temperatures (> 550 °C) and moderate-low salinities (~10 wt.%) characterizes CG-related initial exsolved fluids. Vapor-rich and brine phases separated from the initial fluid following a continuous decrease in pressure and temperature, inducing molybdenite precipitation. FG-related initial fluids are characterized by high temperatures (> 550 °C) and salinities (> 65 wt.%). The mixing of low-salinity fluids led to a rapid decrease in the salinity of FG-related fluid, promoting the deposit of the Mo element. The lead-zinc mineralization is closely related to the FG-related fluid, and the addition of meteoric fluid induced the formation of galena and sphalerite. The ore-forming fluid related to the PG is CO₂-rich and accompanied by the addition of mantle-derived He-Ar. The presence of CO₂ did not contribute to the solubility of Mo, resulting in the absence of a considerable amount of molybdenite.

Keywords

fluid inclusion / microthermometry / H-O-He-Ar / Caosiyao / northern margin of NCC

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