Numerical Modeling Of Coupled Nonlinear Dynamic Processes Of Mineralization In Dachang, Guangxi, China

In recent years, new scientific methods for ore deposits exploration are developed due to the following imperative reasons:(1) mining industry is one of major financial resources for many countries in the whole world. The decreasing of mineral output will definitely result in severe economical effects to the society.(2) Some of the famous large-scale mineral resources have been exhausted, and some will be in the foreseeable future. Finding new large-scale ore deposits at low cost is becoming a challenging task.3) Usually field observations and physical instruments detection are main traditional methods for ore exploration. Now it is very difficult to explore and find new ore deposits using them after so many years of exploration. To improve the situation, geologists have committed themselves to modeling the ore-forming processes, mainly related to fully coupled problems between deformation, fluid flow, heat transfer and chemical reactions in the hydrothermal systems.The Dachang ore district in the Guangxi Province was chosen as a field example because of its pervasive Sn-polymetalization enrichment and also since it provides a unique example where hydrothermal fluid flow, tectonic driven-force and heat transfer might have affected Sn-polymetalic ore formation and other diagenesis process.Numerical simulations, which are based on both the finite element method and the finite difference method, were carried out for the first time to construct conceptual models in the Dachang ore district, Southern China. Sensitivity analyses were conducted to test the mechanical behavior through modeling procedures. Fully coupled tectonic deformation, fluid flow and heat transfer were considered to investigate and quantify the effects of topography, structure, driving force, temperature and salinity on fluid flow and the related ore-forming processes in the porous medium.In the simplified models, fault zone width, fault dip-direction, the possibility of re-injecting meteoric water and topography are investigated. Modeling results show that, mechanical and geological conditions having significant effect on fluid flow migration within models. The structural characters demonstrate strong influence of hydraulic head and structure-driven flow in the rocks, the effect of compression drives fluid flow into the more permeable parts within the model. With the increase of the fault zone width, more fluids flow upwards. Because the permeable fault zones can provide a primary fluid pathway, fluid flow is focused into and along the fault zone. High permeability and dilation within the fault zone allows the downward flow of surficial fluids, which might be driven by topography and hydraulic gradient. Pore pressure gradients and evolution of pore pressure due to deformation drives upward flow from the base of the fault, as local area of contraction and high shear strain expel fluids. Downward flow on the upper part of the model becomes more prominent due to a decrease in hydraulic head. Fault dip angle is one of the factors influencing the mineral location through fluid convergence and circulation spots along the stratigraphic layers. The presence of topography in a basic model demonstrates the great effect of hydraulic head and topographic driven flow in homogenous rocks.A geologically more complicated model in Tongkeng-changpo ore field was constructed to investigate the pore-fluid patterns. Simulation results from this finite difference modeling indicate that the shortening deformation expels basal fluid to ascend from the basement along the Dachang fault; while the surface topography and pore pressure reduction resulting from the tectonic deformation allows the surficial fluid to descend along the upper part of the fault. These two fluids mix at depth and then travel laterally into the Upper Devonian silicalite limestone (D31) due to the elevated permeability caused by the shortening rock deformation stress. A large amount of fluids circulating in the hanging wall D31unit implies that it is a favourable site to host ore deposits.Simulation results from hydrothermal modeling indicate that faults serve as important conduits for upwelling fluid flow, which may bring reactants and heat from depth into the shallower aquifer for Sn-polymetalization. The upwelling pore-fluids along the permeable faults bring salinity and heat from underlying rock units at depth into the shallow aquifer, whereas the downward pore-fluids via the permeable faults cool the basement. The width of the permeable faults can influence the pore-fluid flow pattern, salinity, and temperature distributions, because the wider faults allow more pore-fluids to be channeled through them.During the hydrothermal computational simulation, the salinity distribution strongly controls the pore-fluid flow patterns. The higher salinity distribution (32wt.%NaCl) at depth impedes the upwelling hydrothermal pore-fluid flow and enhances the cold seawater to move downwards, resulting in a relatively low temperature in the Upper-Devonian aquifer. However, the low saline pore-fluids at depth (12wt.%NaCl) could be brought upwards into the Upper-Devonian aquifer by a thermally-induced buoyancy force, resulting in a relatively high temperature in the Upper-Devonian aquifer, which is in favor of hydrothermal Sn-polymetallic mineralization. The evaporative conditions at surface allow high saline pore-fluids to move downwards into the aquifer but prevent hot pore-fluids flowing into it from depth.According to the prerequisite of hydrothermal Sn-polymetallic mineralization, high salinity and high temperature are required. However, the present simulation results from the finite element modeling indicate that the Upper-Devonian aquifer cannot reach both high salinity and high temperature simultaneously if only a buoyancy force is involved as the driving mechanism.Coupled deformation-thermal-fluid flow modeling is conducted to investigate the effect of tectonic force and temperature on the pore-fluid flow during the Indium-bearing mineralization processes. In this study, the porosity-permeability relation and the fluid density-temperature relation are replaced by new relations for the fractured rock. The modeling results indicate that dilation and compressive deformation and thermal convection are major contributing factors to control fluid focusing within the upper Devonian sequences, and therefore they play an important role in mineralization within these horizons. High temperature is facilitating the pore-fluid flow in the basement to transport the Indium-bearing minerals. Downward cooling meteoric water enhances the In-bearing minerals to deposit at the dilation zones. Deposition of the indium-bearing sulfide assemblage occurred in response to cooling and dilation of the magmatic ore-bearing fluid as a result of mixing with meteoric water.The following two different type mechanisms of mineralization may contribute to the ore-forming process in the Dachang orefield:(1) during the diagenesis and deformation period, ladder veins were first formed in the silicite then irregular veins were formed in the ore-bearing strata. Ores were also precipitated through the open space-filling and the ore-forming matters may come from the stratabound metal sulfide.(2) The late Yanshanian orogeny resulted in the regional faulting and folding. The original orebodies experienced significant modification in their morphology and some big veins with mineralization were formed crosscutting the earlier formed orebodies. These late ore-bearing veins are closely connected with the granitic instrusion and the metal elements are supposed to be from the granite.