Fluid inclusion study of the Nannihu giant porphyry Mo-W deposit, Henan Province, China: Implications for the nature of porphyry ore-fluid systems formed in a continental collision setting

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• 作者：Yong-Fei Yang^a ; Nuo Li^a ; Yan-Jing Chen^a ; ^b ; ^{yjchen@pku.edu.cn}
• 关键词：Fluid inclusion ; Nannihu porphyry Mo&ndash ; W deposit ; Qinling Orogen ; Continent&ndash ; continent collision ; High-salinity carbonic fluid
• 刊名：Ore Geology Reviews
• 出版年：2012
• 期刊代码：66_01691368
• 类别：ear
• 出版时间：August, 2012
• 卷：46
• 期：Complete
• 页码：83-94
• 文件大小：2878 K

摘要

The Nannihu Mo-W deposit, located in the Henan Province of China, is a giant porphyry deposit formed in the Qinling Orogen during the Jurassic continental collision between the North China and Yangtze Cratons. Mo-W mineralization is associated with the Nannihu porphyritic monzogranite, occurring as numerous stockwork veinlets in the altered wallrocks and the causative porphyry. Both the porphyry stock and wallrocks underwent intense hydrothermal alteration, ranging outwardly from potassic to phyllic alteration, and to carbonation zones with increasing distance from the intrusion. The hydrothermal ore-forming process can be divided into four stages characterized by veinlets, from early to late: (1) quartz + potassic feldspar 卤 molybdenite 卤 pyrite; (2) quartz + molybdenite veins containing minor pyrite and chalcopyrite; (3) quartz + polymetal sulfide; and (4) quartz + carbonate + fluorite. Most of the ores are formed during stages 2 and 3.

Four types of fluid inclusions (FIs) are distinguished in this study based on petrographic and microthermometric criteria, i.e. pure CO₂, NaCl-H₂O, CO₂-H₂O and daughter mineral-bearing fluid inclusions. All of the four types of FIs can be observed in the hydrothermal quartz formed in stages 1, 2 and 3; while the stage 4 minerals contain only NaCl-H₂O fluid inclusions. Fluid inclusions of stage 1 are mainly homogenized between 350 and 460 掳C, with salinities ranging from 5.7 to 17.9 wt.%NaCl equiv. The stage 2 FIs yield homogeneous temperatures of 300-380 掳C and salinities of 3.5-16.9 wt.%NaCl equiv. FIs of stage 3 are homogenized between 250 and 370 掳C, with salinities of 1.8-14.3 and 28.6-39.1 wt.%NaCl equiv. FIs of stage 4 are homogenized at temperatures of 115 to 265 掳C, yielding salinities of 0.5 to 1.2 wt.%NaCl equiv. Daughter minerals, such as halite, chalcopyrite and some unknown minerals, are frequently present in stage 3 FIs, which probably represent a reducing and oversaturated fluid-system resulting from fluid-boiling. The daughter mineral-bearing FIs coexist with vapor- and liquid-rich NaCl-H₂O FIs that have contrasting salinities. These FIs are homogenized in divergent ways at similar temperatures, suggesting that fluid boiling took place in stages 2 and 3. The estimated pressures range from 70 to 270 MPa in stage 1, through 30-150 MPa in stage 2, to 30-85 MPa in stage 3, corresponding to a depth of no less than 3 km. In other words, the ore-forming fluids are characterized by high temperature, high salinity, high fO₂ and high CO₂ content; and fluid-boiling resulted in CO₂ release, fO₂ decrease and ore mineral precipitation. We suggest that the CO₂-rich fluid is a distinctive feature of porphyry systems developed in continental collision setting, in transitional compressional to extensional regime, contrasting to the CO₂-poor NaCl-H₂O fluids observed in volcanic arcs.