| 其他摘要 | The Dahongshan iron-copper deposit in Yunnan province is located in the southwestern margin of the Kangdian Axis, belonging to the central Yunnan synclise between the Honghe and Luzhijiang deep fault. This deposit is hosted in the Mesoproterozoic Dahongshan Group comprising volcano exhalative-sedimentary rocks, and is one of the typical VHMS deposits in China. There are a series of deposits in the ore district，so called as “Dahongshan-type” deposits. All these deposits characterized by different ore-forming mechanism are related to the volcano exhalative and magmatic emplacement events. The economic value of these deposits is significant due to the giant reserves of iron and copper. Previous studies focused on the geological characteristics of the ore deposit, while the geochemical study was relatively poor. Thus, a detailed study on the stratiform copper ore body has been conducted in this work including elaborate petrography study on various types of rocks；qualitative and quantitative study of typical minerals by electron probe；U-Pb dating of zircon from volcanic rock; major elements and trace elements analysis of various rocks; S and Pb isotope of sulfide; fluid inclusion study of quartz-vein and C、O isotope of calcite related to sulfide-rich sample. Finally, the above mentioned studies lead to the following conclusions:
1. The emplacement age of the volcano exhalative-sedimentary rocks of the Mangganghe Group is about 1687±8Ma. It is therefore impossible that the Dahongshan Group had been altered by Lvliang Movement as previously suggested. During 700 ~ 800Ma, the Jinning Movement strongly affected the Dahongshan Group by metamorphism, deformation and fluid alteration.
2. The rocks in the stratiform copper deposit are mainly biotite schist, dolomite-albite rock and dolomite-marble. The main minerals of biotite schist are biotite, albite, ankerite and garnet; main minerals in dolomite-albite rock are albite, ankerite, garnet and minor biotite; those of dolomite-marble are ankerite, with minor albite and biotit. Their protolith are composed of volcanic material, pelitic sediment and exhalative sediment with various proportions.
3. There are at least three stages of metamorphism happened, namely high temperature regional metamorphism, fluid alteration and deformation. The highest temperature of regional metamorphism is 660℃, mainly between 600℃ and 630℃, with the pressure range of 4.9 ~ 5.0kbar. These metamorphism made the protolith altered to epidote-amphibolite facies. REE, especially LREE，and part of HFSE were mobilized during metamorphism. Alteration caused by metamorphic fluid enriched the earliest metal-poor ore body and made it economicly utilized.
4. The study on the sulfur isotope of sulfide indicates that the δ34S range is -0.6 ~ +10.9‰, mainly in +5.0 ~ +10.9‰. It is suggested that the sulfide-rich ores inherited the sulfur characteristics of the earliest sulfide-poor ores and absorbed heavy S during alteration. The sulfur of early-stage sulfide may derived from magma and seawater. The Pb isotopic ratios of sulfides are: 206Pb/204Pb＝ 18.985 ~ 23.318, with an average of 21.222; 207Pb/204Pb＝15.581 ~ 15.904, with an average of 15.747；208Pb/204Pb＝39.803 ~ 45.652, with an average of 42.540. It is obvious that the sulfides are enriched in radiogenic Pb. Some sulfides with relatively low content of radiogenic Pb are consistent with those of the sulfides in reworked ore body reported before. The Pb isotopic characteristics of sulfides represent the mixture of two types of Pb, namely common Pb of the earliest sulfide and radiogenic Pb from the wallrock. The characteristics of S and Pb isotopes of sulfides suggest that the copper of the earliest chalcopyrite came from submarine volcano exhalative-sedimentation, while that of late chalcopyrite-rich ores is inherited the copper from the earliest chalcopyrite and the cu-bearing fluid.
5. The study of fluid inclusion indicate that there were three types of fluid system: ①low-medium salinity H2O-NaCl±KCl±FeCl3±CaCl2 fluid which salinity of 0.53 ~ 24.59% NaClequiv. and density of 0.80 ~ 1.16 g/cm3; ②high salinity and high density H2O-NaCl±CaCl2 fluid, with salinity of 31.2 ~ >59.76% NaCl equiv. and density of 1.14 ~ 1.45 g/cm3; ③ pure liquid CO2 fluid, with density of 0.77 ~ 1.09 g/cm3. The homogenization temperature is 100 ~ 456℃, clustering in two ranges: 150 ~ 260℃ and 260 ~ 456℃. The δ13C and δ18O of calcite are -5.6 ~ -3.1‰ and 12.4 ~ 15.5‰, respectively. Fluid inclusion from quartz and C、O isotope characteristics of calcite suggest that the ore-forming fluid may mainly originate from magma chamber beneath seafloor, and partly mixed with seawater. The ore-forming fluid dissolved the earliest sulfide during alteration, and deposited sulfide-rich ores after short-distance transport.
6. The mechanism of mineralization is summarized as below: During the Mesoproterozoic, the continental rifting induced the submarine volcano exhalative-sedimentation, thereafter gave rise to the metal-poor ore body during this period. In late Neoproterozoic, regional metamorphism, deformation and fluid alteration rebuilt the Dahongshan Group and concentrated the copper from the earliest copper-poor ore body, resulting in the formation of economically utilized ore bodies. |
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