Transport network and flow mechanism of shallow ore-bearing magma in Tongling ore cluster area

Abstract

Abundant studies revealed that shallow intrusions of the Yanshanian epoch resulted in the mass mineralization of the Tongling region. Various evidences showed there existed a concealed magma chamber at −10 km depth in the middle part of this region during Yanshanian epoch, from which the ore-forming magma was generated and then transported to the superficial layer. Yet the transport network and flow mechanism of the shallow ore-bearing magma, the key problem associated with ore-forming process, was relatively little focused on. Integrate analysis of structural mechanics, statistical fractal and geological facts suggested that NE trending high-angle fold-related thrust faults and the tessellated basement ones served as the main pathways for the shallow magma's transporting, moreover, the saddle void spaces among adjacent strata in the folds upon this fault system provided the place for magma's emplacement. So the folds in the upper part and faults in the lower part of the upper crust constituted the fluid's transport and emplacement network. During the deformation of geologic body with multi-layer structure, the layers in the upper part tended to fold when received the jacking stress from the lower part, while the lower one inclined to fault undergoing loads of the upper part. And the producing probability of this structure assemblage was highly increased in the condition, such as in the Tongling area, that the mechanic rigidity of the lower layers was stronger than that of the upper ones. For the pre-existence of fluid-conducting network, the top magma with high volatile in the magma chamber transported rapidly to the superficial layer in dyking pattern, located in the void spaces of folds, filled and reconstructed them. The sudden drop of pressure caused the fluid unmixing from the magma and mass ore-forming elements concentration. Pulse activity of the dyking may be the principal reason why magmatic bodies in the Tongling area were spatially-temporally concomitant and limited flux in chemical compositions.The Tongling area is one of the most important ore cluster areas in the middle to lower Yangtze River metallogenic belt. The ore-forming process in Tongling region was mainly resulted from the medium-acidic magma intrusion activity during Yanshanian epoch [1][2][3][4] . Lots of research of the structure system and intrusion series were carried out in recent decades and the following aspects were mainly focused on: (1) Accurate determination of the petrologic structure, chemical composition and intrusion age of magmatic rocks and the various inclusions in them [5][6][7][8][9][10] .(2) Detailed geological and geochemical research dealing with the significant physical-chemical processes following magma's shallow emplacement [11,12] , for instance, cryptoexplosion and the differentiation between magma and skarn magma. (3) Detecting of the regional deep-seated structure via deep reflection seismic profile technology and remolding on the caprock's 3D strain field by analogue experiment [13][14][15][16][17][18] . (4)...

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