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 t o simulate the geologic processes of fault-controlled fluid migration during hydrothermal ore genesis. The history of hydrothermal ore genes is is strongly affected by the interaction between fault deformation a nd fluid flow. An injected fluid induces fault slip by lowering the ef fective stress across a fault. The subsequent movement on the fault en hances the fault aperture which provides a channelway that both facili tates and focuses fluid flow. The variation of fluid injection uIlder certain stress conditions causes cyclic movement on faults. This cycli c failure gives rise to pulsating fluid flow along the faults which ma y be responsible in part for the precipitation zoning of mineral depos its. As an important structural condition for ore formation in any fau lt-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 deposit ion at particular structural sites. This study indicates that differen t fluid flow patterns (i.e., changes in fluid flow rate or velocity wi th 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 tile northwest-southeast-tren ding fault and the western half of the east-west-trending Yuanlao faul t in the center of the Malage ore field. These are the main pathways f or the ore-forming fluids. In tile smaller secondary faults, the flow velocity fluctuates more rapidly with time and space, in some instance s causing conditions that are favorable to mineral deposition. Tile ma in ore deposition sites occur on the smaller secondary faults. The det ailed variation of fluid and aperture conditions modeled in this compl ex fault array can be related to the formation and location of mineral deposits, both predictively and conceptually. The modeling also demon strates that the fault deformation and fluid flow are sensitive to the principal stress orientation. If the orientation of sigma(1) is chang ed by more than 15 degrees, there are very different results for the c alculated fault motion pattern, hydraulic aperture, and fluid flow.