Fluid inclusion and stable isotope study of the Nantai molybdenum polymetallic deposit in North Qinling Mountains

The Nantai Mo polymetallic deposit is hosted in the Kuanping Group in the North Qinling belt, and its stratoid and lenticular orebodies occur in the porphyry and the contact zone between the marble of the Kuanping Group and the porphyry. Mineralization occurs as disseminations, lumps and veinlet-disseminations. Wall rock alteration is well developed and mainly comprises potassium-silicate alteration, skarnization and carbonatization. The ore- forming process can be divided into two metallogenic periods: skarn and quartz- sulfide, and the quartz-sulfide period represents the main molybdenum mineralization episode. Fluid inclusions in quartz phenocrysts and quartz-molybdenite veins can be divided into four types, i. e., L-type aqueous fluid inclusions, V-type aqueous fluid inclusions, C-type three-phase CO2-bearing inclusions and S-type multi-phase inclusions containing daughter minerals. Homogenization temperatures of fluid inclusions at the early stage of quartz phenocrysts can be divided into three intervals, i. e., 340 ~ 400℃, 220 ~ 280℃ and 140 ~ 200℃, with salinity being 57. 90% ~ 73. 96%, 30. 06% ~ 38. 01%, 3. 39% ~ 18. 55%, respectively. The temperatures of the main mineralization stage in quartz-molybdenite veins were 280 ~ 380℃, 200 ~ 240℃ and 120 ~ 180℃, with salinities ranging from0. 43% to 12. 85% w( NaCleq). The late stage in non-mineralization quartz veins has only homogenization temperatures of 120 ~ 240℃, with salinities ranging from 1. 91% to 10. 73% w( NaCleq). Coexistence of two or more types of fluid inclusions in the three homogenization temperature intervals of the principal mineralization stage with corresponding salinities homogenized to divergent phases at similar temperatures suggests that boiling had already occurred several times during the ore-forming evolution and was probably the dominated factor for metal precipitation.This phenomenon is comparable to things of the Qiushuwan Mo( Cu) deposit in North Qinling. δ34S values of sulfides associated with mineralization range from- 0. 3‰ to 7. 2‰, 3. 1‰ on average, indicating that sulfur was derived from deep-seated magma. δDV-SMOWvalues of sulfide-bearing quartz vary between- 103‰ and- 76‰, with corresponding δ18OH2Ovalues between 4. 01‰ and 5. 55‰, impling that the ore-forming fluids of the main mineralization stage in the Nantai Mo polymetallic deposit were mainly derived from magmatic fluids, with some addition of the meteoric water.