Numerical modeling of fault-controlled fluid flow in the genesis of tin deposits of the Malage ore field, Gejiu mining district, China

      A distinct element finite difference model based on geologic data from the tin polymetallic hydrothermal deposits of Malage is constructed to simulate the geologic processes of fault-controlled fluid migration during hydrothermal ore genesis. The history of hydrothermal ore genesis is strongly affected by the interaction between fault deformation and fluid flow. An injected fluid induces fault slip by lowering the effective stress across a fault. The subsequent movement on the fault enhances the fault aperture which provides a channelway that both facilitates and focuses fluid flow. The variation of fluid injection under certain stress conditions causes cyclic movement on faults. This cyclic failure gives rise to pulsating fluid flow along the faults which may be responsible in part for the precipitation zoning of mineral deposits. As an important structural condition for ore formation in any fault-related mineralized region like Malage, the linking up of fracture networks not only allows appropriate long-distance channels for fluid migration but also provides the appropriate conditions for ore deposition at particular structural sites. This study indicates that different fluid flow patterns (i.e., changes in fluid flow rate or velocity with time and space) can reflect the variation of structural environment. For instance, the flow velocity is increased gradually over time on the relatively connected faults, such as the northwest-southeast-trending fault and the western half of the east-west-trending Yuanlao fault in the center of the Malage ore field. These are the main pathways for the ore-forming fluids. In the smaller secondary faults, the flow velocity fluctuates more rapidly with time and space, in some instances causing conditions that are favorable to mineral deposition. The main ore deposition sites occur on the smaller secondary faults. The detailed variation of fluid and aperture conditions modeled in this complex fault array can be related to the formation and location of mineral deposits, both predictively and conceptually. The modeling also demonstrates that the fault deformation and fluid flow are sensitive to the principal stress orientation. If the orientation of sigma 1 is changed by more than 15 degrees , there are very different results for the calculated fault motion pattern, hydraulic aperture, and fluid flow.

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