Understanding the 3-dimensional fault geometry of a vein-hosted fluorite deposit: the basis for realistic numerical fluid flow simulations

摘要

Hydrothermal fluid flow dynamics within faults and associated ore deposition is a complex and coupled process incorporating the advection/convection of a (saline) fluid, heat transfer and fluid-rock interactions within a 3-dimensional heterogeneous geological medium. Consequently, numerical simulations can help us to understand these processes qualitatively and quantitatively. To date, most simulations tend to be of generic and over-simplistic 'base-cases' as opposed to deposit-specific, making it difficult to extrapolate results from the simulations to the deposit. In order to simulate a particular mineralising system, a good understanding of its structural and mineralogical evolution is required so that realistic boundary conditions can be ascribed and so that the model is a physically accurate representation of the natural system.Fluid-flow simulations are to be based on the operational fault-hosted Silius fluorite (Pb-Zn-Ba) deposit in south-eastern Sardinia. They will incorporate a highly realistic 3-dimensional geometrical representation of the fault which hosts the mineralisation, as determined from the digitisation of the mine level plans. From underground structural observations, the fault is interpreted to be a dextral strike slip fault, with this interpretation being confirmed by the larger-scale features of the 3-dimensional fault geometry. Faulting and mineralisation, which cross-cut bi-modal Permian dykes are interpreted to be contemporaneous and cyclical. Two main veins are found in the deposit distinguished by their differing parageneses. The (older) San Giorgio vein contains fluorite, quartz, chalcedony, (minor) calcite, sphalerite and marcasite whilst the (younger) San Guiseppe vein contains fluorite, (abundant) calcite and galena with some barite (mostly found in the higher levels of the mine).