Abstract:

The Dahu-Qinnan molybdenum deposit is located in northern belt of the Xiaoqinling gold province and metallogenically belongs to the Xiaoqinling-Waifangshan sub-zone of the eastern Qinling Mo-W metallogenic belt, Central China. Spatially associated with Au-bearing quartz veins, the molybdenite mineralization in the area is characterized by Mo-bearing quartz veins. The occurrences, textures and alterations of Mo-bearing quartz veins in Dahu-Qinnan area have been studied based on the field observation and PIMA instrument analysis. These Mo-bearing quartz veins are friable with cream color, and are of vuggy, cloddy, breccias structure. The breccias in these Mo-bearing quartz veins are mostly gneiss of the Taihua Group and porphyritic granite dyke of Mesozoic with stockwork and disseminated molybdenite mineralization. These breccias are replaced more or less by argillic minerals. Alteration associated with molybdenite-bearing quartz veins is mainly potassium alteration(only in the breccias), alteration to sericite, sillicification (residual silica), alteration to kaolinite, smectite and anhydrite. Based on field evidences, alteration in the area can be divided into 1) Potassium+sericite+sillicification+pyrite+molybdenite in early stage and 2) sillicification ( residual silica)+kaolinization+anhydrite in later stage. Molybdenite and other sulfide minerals occur as the infillings in fissures and pore space in molybdenite-bearing quartz veins. The oreforming temperatures of Mobearing quartz veins are from 134℃ to 463℃. The δD_H2O  is from －96‰ to－110‰, and δ¹⁸O_quartz  is from 10.7‰ to 11.5‰ respectively, revealing a mixing processes of magmatic, metamorphic fluids and meteoritic water. δ¹⁸O value of kaolinite is from 0.1‰ to 2.3‰, also suggesting a hydrothermal origin. δ³⁴S_pyrite is from －5.778‰ to －7.841‰,  indicating the involvement of organism in metallogeny. Based on above studies and combined with the geological setting of the deposit, it is suggested that the molybdenite-bearing quartz veins in the area are likely to be porphyry molybdenum orebodies associated with porphyry granite dyke formed probably at ca. 245-211 Ma during the continental-continental collision of the Qinling orogeny. They were leached and enriched by high sulfidation hydrothermal fluids during later gold-forming epoch.

Key words: molybdenum deposit, Mo-bearing quartz vein, porphyry molybdenum body, acid leaching, Mesozoic, Dahu-Qinnan