| The Bangongco–Nujiang copper metallogenic belt, from the Bangong lake of Aliarea to the Nujiangyuan of Naqu area, has experienced formation, subduction, collisionand final closure of the Bangongco–Nujiang ocean. These tectonic magmatic eventsprovide favourable metallogenic geological conditions for endogenetic metallicmineralizations in this area, and lead to large metallogenic and ore-prospecting potential inthis area. Due to its very short discover histry, however, this metallogenic belt has beenpaid attention to in recent several years, and little mineral exploration and theoreticalresearch have been conducted.This thesis focus eight representative deposits, including Duobuza, Bolong, Rongnaand Naruo etc, on their geological and geochemical characteristics, ore-forming fluidcharacteristics, geochronology, tectonic settings and origin of associated magma, in orderto define their ore genesis, constrain the ore-forming material source, and theirgeodynamic settings, and establish metallogenic series, summarize metallogenicregularities, suggest further prospecting direction. Such major conclusions have beendrawn as followings:1. The intermediate-acid intrusions in the Duolong mineralization area formed at115~128Ma and have the characteristics of subduction zone at an active continentalmargin, with the positive Hf (t) values and younger TDM2. Combined with the Mesozoic geodynamic evolution of the Bangongcuo–Nujiang suture zone, it has been indicated thatthe ore deposits in Duolong were formed under early Cretaceous active continental marginsetting and related to the subduction of Bangongco–Nujiang oceanic basin. In addition,their diagenetic and metallogenic materials may be derived from mixture of crust andmantle materials.2. The chronology, geochemistry and Lu–Hf isotope analysis of the ore-bearingporphyry and barren porphyry in the this area indicate that these porphyry intrusions werealmost formed simultaneously under the same tectonic setting and derived from onemagma source. However, LaN/YbNvalues and potassium of the ore-bearing porphyry areevidently larger than those of the barren porphyry, indicating a high-grade magmaevolution, during which K, Cu and Au elements were fully precipitated and enriched. Thisis favorable to forming copper-gold ore bodies with high contents of potassium.3. From the research on geological characteristics, ore-forming physicochemicalconditions, ore-forming fluid and ore-forming material sources of typical ore deposits inthe Duolong mineralization area, it has been indicated that the Rongna and Sena belong tothe high sulfidation epithermal deposit, while other deposits are of porphyry type.Consequently, these copper-gold deposits discovered in this area can be classified into twogenetic types,porphyry (Duobuza, Bolong, Dibaonamugang, etc.) and porphyry-highsulfidation epithermal deposit (Rongna and Sena).4. The results, covering LA ICP-MS zircon U-Pb dating on the intrusions associatedcopper-gold mineralization, S and Pb isotope analysis of metallic sulfide, and C-H-Oisotope analysis of gangue minerals, indicate the formation age of intrusions and ores fromthe deposits in Duolong area is Early Cretaceous and their ore-forming fluid have thecharacteristics of those of the porphyry deposits, which is mainly from magma water,minor meteoric water, as well as little participation of metamorphic water in Gaerqin. Inaddition, although their metallogenic materials were all derived from mixture of crust andmantle, differences in crust depth also occurred among these deposits, for example, theore-forming material of Duobuza and Bolong mainly came from deep mantle and lowercrust, while that of Naruo and Sena mainly came from deep mantle and upper crust.5. At Early Cretaceous, Bangongco–Nujiang oceanic crust subducted northward, and the subduction plate fluid upwelling caused local stretching and thinning on the thickenlithosphere, which triggered the partial melting of depleted mantle material and theforming of basaltic magma with large amounts of metallogenic materials including Cu andAu. The basaltic magma were partially melted in the lower crust, together with somelower crust components, which formed the primary ore-bearing intermediate-acid magma.The magma intruded through EW-, NE-and NW-trending faults, and formed hypabyssaland super hypabyssal intrusions. In the later period of magmatic evolution, the ore-bearingfluid from magmatic differentiation ascended through the secondary fractures and thelocal decompression leaded to boiling. Finally, the metallogenic materials were depositedand enriched in the shallow parts of crust, thereby forming the Duolong mineralizationarea.6. Considering the ore-controlling geological conditions, diagenetic and metallogenicepoch and metallogenic series of the Duolong mineralization area, we summarize themetallogenic regularity. Combined with all available data, we further put forward threepredicted prospective areas, including the south of Sena deposit, the north ofDibaonamugang deposit, the deep of Rongna deposit and Sena deposit. |
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