Study Of The Nature Of Ore-forming Fluids And Metallogenic Model Of The Carlin-type Gold Deposits In The Youjiang Basin, China

The Youjiang Basin, which is also named as the Nanpanjiang Basin, is an important enrichment area of Au-As-Hg-Sb deposits in South China. It is also one of the largest enrichment areas of Carlin-type gold deposits in China and is well studied by many geologists.After years of studies, many important research achievements highly improve the studies of ore-forming process. However, disputes still exist in some problems, such as the source of ore-forming fluids, the role of organic matter in ore-forming process, the relationship between the basin evolution and the migration of ore-forming fluids and so on. In this thesis, the author chose six typical Carlin-type gold deposits and three paleo-oil reservoir in the Youjiang Basin to discuss these questions by using EPMA, fluid inclusions, REE and trace elements, sulfur and H-O isotope, organic geochemistry and tectonic geology.The gold-bearing minerals in the Carlin-type gold deposit is mainly arsenian pyrite and arsenopyrite. The arsenian pyrite can be further divided into four subtypes, including the euhedral and subhedral pyrite, framboidal pyrite, pyrite aggregates and pyrite veins. Most of the Au exists in the rim of euhedral and subhedral pyrite. An antithetic correlation exists between As and S in arsenian pyrite and arsenopyrite, indicating the As may get into the structure of sulfides by substituting the position of S.The relationship between Au and As reveals that the Au may host as a solid solution (Au+) and nanoparticles of native gold (Au0) in the sulfides. The relationship between Co and Ni suggests that most of the pyrites were formed by hydrothermal fluids, but the magmatic and volcanic activities cannot be neglected. The sedimentary forming pyrites also exist in the samples, suggesting that the sedimentary origin pyrites were highly altered by the hydrothermal fluids.The results of fluid inclusions reflect that the ore-forming fluids, which are CO2-riched H2O+NaCl+CO2+CH4+N2 systems, have the nature of low-medium temperature, low salinity and medium-high density. The elemental map of As reflected that the As distribute apparently along the boundary faults, implying that the As may be sourced from the deep earth and migrated by the ore-forming fluids in the form of H3ASO3. Au has been detected in the sulfides of both ores and wall rocks, suggesting that the ore-forming fluids are generally affected the strata and are related to the basinal fluids. The comparison ofREE between the ores and wall rocks suggests that the distribution patterns of REE in different deposits are similar and the Eu and Ce has apparent to weak negative anomalies, reflecting the ore-forming process were mainly related to the basinal reductive fluids.The W and T1 have obvious enrichment, suggesting the existence of Cl-rich magmatic hydrothermal. The Y/Ho ratios of ore samples fluctuate around 28, implying the bicarbonate complexation and fluorine were both involved in the ore-forming process. The sulfur isotopic data suggest that the source of sulfur is complicated and may be the combination of the dissolution of sulfides in the strata, the magmatic and volcanic source and bacterial sulfate reduction, and the dissolution of sulfides in strata is the main source. The H-O isotopic data reflect that the ore-forming fluids are mainly related to the meteoric waters of the basin, but the magmatic water and metamorphic water may be also involved in the fluids. Overall, it can be inferred that the ore-forming fluids of the Carlin-type gold deposit in the Youjiang Basin are the combination of basinal and deep earth fluids, and the basinal fluids are in the dominant position.The ore-forming fluids were rich in Au, As,H2S (HS-),halogen (F, Cl).The organic matter in the deposit can be divided into two major types,including the disseminated and enriched organic matter, they are mainly sourced from the bacteria and algae of marine, and deposited in the reductive environment. The content of organic matter in the deposits has no relationship with the samples are ores or wall rocks, but relates to the lithology. Most of the samples have Ro values larger than 2%, indicating the organic matter is in the over mature stage. The content of chloroform bitumen "A",the conversion rate of organic matter, Ts/(Ts+Tm), C29Ts/(C29H+C29Ts),hopane index, sterane isomerized index all reflect that the maturity of organic matter in the ores is higher than in the wall rocks, implying that the ore-forming hydrothermal fluids accelerated the maturity of organic matter. The results indicate that the migration of hydrocarbon may not the reason for the formation of gold deposit, and the roles of organic matter in the ore-forming process can be reflected at the preconcentration of Au, the reduction of Au ion, the supply of sulfur and providing the accommodation space for Au sedimentation.There were mainly two sets of structures developed in the Youjiang Basin, including the NW- and NE-trending;The NW-trending boundary faults played an important role in the evolution of the basin and the distribution of volcanic rocks.The lithology and structure are the two main factors that control the formation of gold deposits.Evolution of the Youjiang Basin can be divided into six stages, which are the shore-shallow sea shelf developed stage in the middle of Early Devonian (Di2), the rifted ocean basin developed stage during the Middle Devonian to the Middle Permian (D2-P2), the ocean basin disappeared and foreland basin developed stage during the end of Middle Permian to the Middle Triassic (P23-T2), the stage of foreland basin disappeared and post-collision extension during the Late Triassic to the Early Jurassic (T3-J1), the NE-trending compression structure developed stage during the Middle Jurassic to the middle of Early Cretaceous (J2-K12) and the partly extension stage during the late of Early Cretaceous to the Paleogene. The Carlin-type gold deposits were possibly developed after the foreland basin disappeared, and the ore-forming stage are related to the extension tectonic environment. Influenced by the evolution of the basin, there mainly exist two migration systems of ore-forming fluids, which are the horizontal migration system and the vertical migration system, and the ore-forming fluids were mainly migrated horizontally.