Genesis and evolution of the Leimengou porphyry Mo deposit in West Henan Province, East Qinling–Dabie belt, China: Constraints from hydrothermal alteration, fluid inclusions and stable isotope data

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The Leimengou Mo deposit is located in the East Qinling–Dabie metallogenic Mo belt, located at the southern margin of the North China Craton. The deposit is hosted within the Mesozoic Leimengou granite porphyry pluton and at the contact between the granite and Late Archean gneisses. In light of mineral assemblage and crosscutting relationships among veinlets, the mineralization and alteration can be classified in four distinct stages: (1) a Mo–barren pre-mineralization stage of alteration characterized by massive K-feldspar alteration of the granite porphyry; (2) a K-feldspar–quartz vein stage that formed veinlets with minor molybdenite; (3) a quartz–sulfide veinlet stage, which is the main stage of mineralization; and (4) post-mineralization quartz–calcite vein stage.Stage 1 alteration is associated with fluid inclusions of low–medium salinity (2.4–13.1wt.% NaClequiv) that homogenized at medium to high temperatures (238–476°C). Stage 2 mineralization is associated with fluid inclusions that homogenized at 201–389°C and that have salinities of 2.0–13.6wt.% NaClequiv. Fluid inclusions that formed during stage 3 have homogenization temperatures of 205–370°C and corresponding salinities of 1.0–17.6wt.% NaClequiv to 31.4–40.6wt.% NaClequiv, indicative of fluid immiscibility and mixing of hydrothermal fluids with meteoric water. Stage 4 H2O–NaCl fluids were trapped at lower temperatures (148–246°C) and have salinities of 1.7–7.9wt.% NaClequiv.Stages 1 to 4 have calculated δ18Ofluid values of 3.6–4.3‰, 2.7–2.8‰, 0.6–2.3‰, and −3.2‰ to −3.8‰, respectively, with δDfluid values of –72‰ to −80‰, −83‰ to −90‰, −68‰ to −82‰, and −81‰ to −90‰, respectively. These results are consistent with the fluid inclusion data outlined above and suggest that early fluids were exsolved from the granitic magma and underwent progressive mixing with meteoric water. Molybdenum was initially transported from the crystallizing magma in a hot, alkaline, and oxidized fluid, while decreasing temperatures and pH, water–rock interaction, and fluid immiscibility during fluid ascent resulted in its precipitation and deposition.