Deformational Structures And Metallogenetic Model Of Sediment-hosted Pb-Zn Deposits In Northern Segment Of The Sanjiang Orogenic Belt, Southwest China

As a new deposit type, the sediment-hosted Pb-Zn deposit in collisional orogenic belt remains poorly understanded. The Northern segment of the Sanjiang orogenic belt (NSOB), which is located in the hinterland of Tibetan continent-continent collisional orogen, has little been influenced by late large-scale strick-slip fault systems, and thus provides an ideal place for study of the sediment-hosted Pb-Zn deposits under collision environments. Here I try to describe the deformational structures, Ore-controlling structures and Ore micro-texture of the Chaqupacha, Dongmozhazhua, and Mohailaheng deposits in the NSOB by geological field mapping, thin section observations, and synthesizing all available informations of multi-desiphilines. Accordingly we got below main progresses:1) made three geological maps with scale of 1:10000 of the Chaqupacha, Dongmozhazhua, and Mohailaheng Pb-Zn deposits, respectively; 2) two phases of Indosinian structures and their interference pattern are identified; 3) new interpretation of ore-control structures is suggested; 4) a three-phases metallogenetic model is proposed to explain the three kinds of sediment-hosted Pb-Zn deposits in the NSOB,5) establishing structure-control model for sediment-hosted Pb-Zn deposits in continent-continent collision orogen; and 6) discussed the correlation between deformation and metallogenesis of basin fluid under upper crustal environments.Detailed geological mapping reveals two stages of folding in the Late Palaeozoic to Early Mesozoic strata. The earlier one is reversal linear fold with numerous asymmetric parasitism folds. The later fold overprinted the earlier one resulting in hook- or crescent- like interference patterns. The angular disconformities suggest that the two stages folding developed in the Late Permian and Late Triassic, respectively.Previously studies have identified two Cenozoic thrust nappe systems in NSOB:the Tanggula Thrust System (TTS), and the Fenghuoshan-Nangqian Thrust System respectively. The Chaqupacha deposit is located at the tip portion of the two systems, while the Dongmozhazhua and Mohailaheng are located within the Fenghuoshan-Nangqian thrust system. The thrusts in the three deposit regions have similar geometry and kinematics; all are characterized by a northward, northwestward, or northeastward dipped plane depended on its location. Fault breccia developed very well. On the other hand, the fault-associeted small structures are quite variable among the three deposit regions. For example, imbricate slices, strike-slip faults and associated tectonic breccia developed very well in the Dongmozhazhua deposit region, which have not been identified in the Chaqupacha area, while en-echelon extensional veins occur in the lower plate of the thrusts in Mohailaheng. Microtectural observations reveal episodic brecciation suggesting episodic faulting for the thrusts, just like the fault activity of the Ganzhi-Yushu fault that caused the 4.14 Yushu earthquake. In the Chaqupacha deposit region, normal fault cut across the thrusts. These normal faults commonly have zig-zag geometry in map-view, and its fracture is locally filled by marl of the Wudaoliang Formation. Taking regional geological data into account, we suggest that the thrust in Mohailaheng developed at the Eocene; that in Dongmozhazhua at the end of Oligocene; and normal fault in Chaqupacha occurred at the Miocene.Although all the Pb-Zn deposits in NSOB are hosted in carbonatic rocks, ore-controlling structure differs from one to other. The surface mineralization of Chaqupacha Pb-Zn deposit is identified mainly along the normal fault; but drilling data reveal that main ore bodies are located at ca.20 m below the surface. Further more, the main ore-body is strictly limited in breccia belts; there is evidence that the breccias formed by dissolution and collapse of paleo-caves. In contrast, the ore-control structures in Dongmozhazhua and Mohailaheng deposits are mainly subsidiary fractures of the thrusts, including locally developed extensional breccias and minor trans-extensional fault.Micro-textural observations reveal that all Pb-Zn minerals deposited in open-space; but the filled voids have different geometrical and/then kinematic feature. The vein-like ore bodies in Chaqupacha interconnect each other to form network, whereas drustic texture is widespread in these ore veins. Among the drusy calcite and sulfides, marl is common displaying stratiform structure. Such phenomenon suggests a syn-sedimentation mineralization. On the other hand, the ore body of the Dongmozhazhua and Mohailaheng deposits presents as minor veins or porphyry ore. The tightly correlationship between the thrust and the ore veins suggests that the precipitation of sulfides in both Dongmozhazhua and Mohailaheng occurred during or slightly later than the N-S direction compression. As evidence, the ores in Mohailaheng are commonly broken up. The ore-forming processes may include cataclasis, strain-solution, dissolution to form cavies and/then filling the cavies. Because of the tight relationship between the evolution of brittle faults and the ore-bodies, we suggest here that the Dongmozhazhua, Mohailaheng and Chaqupacha deposits formed at the Late Eocene, Late Oligocene and Early Miocene, respectively.The suggested three satges of ore-forming process are tightly correlated with two compressional at eaerly and middle periods and an extensional strain at late time respectively. During the ore-forming process, the temperature of fluid gradually decreased while the inner pressure of the fluid increased very likely due to a decrease in the confining pressure. Such a variation is in consistence with the Cenozoic tectonic evolution of the North-central Tibet:A change in tectonic regime from compression to extension completed during the interval of the Oligocene to Miocene in the Tibet Plateau; in corresponding, the the ore-filling structures changed from thrust-related ones to those formed by extensional strain, which have filled by ore-bodies with different texture.Conclusively, the Pb-Zn deposits in NSOB are tightly related with the tectonic evolution of Himalayan-Tibetan Plateau. Intensive and multi-stages compressional and extensional deformations, which are likely alternative, greatly increased permeability of the carbonitic rocks in the study areas. A high permeability in host rock is very significant for fluid moving and ore-mineral precipitation.