Numerical Simulation Of Magmatic Hydrothermal System

As an important means and trend for ore-forming process study, numerical modeling simulates ore-forming process by computer program and discuss the effects of various geological factors in different time stages during mineralization process,and demonstrates theories and assumptions and provide recommendations for mineral exploration.Numerical simulation tecnique started from 50 years of 20th century and was initially used in reservoir flow and hydro-geological research ,then it entered structural geology.Simulation studeies in these fields led to numercial modeling of solid mineral ore-forming process and was gradually develped .At present, SEDEX-type deposit (Sedimentary Exhalative Deposit) and MVT-type deposit (Mississippi Valley-type) are the main objects which are associated with basin and occur in a series of carbonate rocks and siliciclastic rocks and have no significant volcanism.Another object is volcanic deposit in which VHMS-type(Volcanic Hosted Massive Sulfide) deposit is important. This paper summarizes development process and the trend of numerical modeling in solid mineral hydrothermal ore-forming system,in addition, implementation steps and main geologic factors that be always included in models are introdcued and major softwares such as HYDROTHERM,ANSYS,FLAC are presented,then,domestic researchs are generalized.In this paper ,a distinct element finite difference model by software HYDROTHERM of the tin polymetallic hydrothermal deposits in Laochang-Kafang ore field in Gejiu Mining district is constructed to simulate the geologic process of magmatic hydrothermal system.The simulation results well reflect the evolution of magmatic hydrothermal system, and a series of sensitivity analysis are conducted to determine the role and impact of various factors in the evolution of hydrothermal system.In additon,a finite element model by software ANSYS is established to demonstrate thermal stress of this magmatic system, which is a type of geo-stress but is different from tectonic stress,and two models are compared with each other. The followings are the results:(1)The evolution of magmatic hydrothermal system could last more than 10,000 years after the invasion of Lao-Ka granite ,in which the temperature field and flow field can be maintained until the late stage of the system,while the hydropressure filed is only kept for about 5,000years as it fades rapidly to hydrostatic pressure much more.(2)The evolution of temperature field indicate that the silicate phase of mineralization with a high temperature may be sustained for less than 2000 years and the contact area between granite and carbonate rocks is the zone of action;the oxide phase and sulfide phase with a medium temperature have a duration of nearly 20,000 years and could extend to 700~1000 meters from the location of granite;the carbonate mineralization stage could last until the end of hydrothermal system and could reach nearly 3,000 meters away from the granite.(3)The evolution of hydro-pressure field shows that the existence of zones with high permeability and high pressure by water and steam gathering is favour of mineralization,for it is favorable for metallogenic pressure to be formed in a wider area and to be sustained for a longer period.(4)The evolution of the flow field is most complex and can be divided into four stages in accordance with driving mechanisms and flow patterns:1,(less than 100 years)During the initial stage of hydrothermal system,vertical flow is the main flow pattern and hydro-pressure is the main driver for fulid flow.The fluid mass flow rate is about 0.1~0.5gs-1cm-2 in strata and ten times the number of it in faluts.2,(100~5,000 years)Fluid begin to flow downward in some faults and the convection forms nearby faults gradually,during this stage, fluid flow is to some extent controlled through density difference casued by temperature change and flow rate decreased significantly during the transition process from the first stage to this stage.3,(5,000~50,000 years) The convection in the area between two faults appears and the density difference become the main driver for fluid flow as the hydro-pressure has been recovered to hydrostatic pressure at this time.From the medium term of the second stage to this one, fluid mass flow rate is 0.1~0.3 gs-1cm-2 in the strata and about 1~2 gs-1cm-2 in the faults. 4,(50,000 years~the end)The flow field declines gradually during which the convection disappear first and following which vertical flow in faults and lateral flow between two fauts fade both.(5)Faults play an important role on the evolution of the temperature field and fluid field.Some faults provide the necessary paths for the downward movement of precipitation water ,in addition, the distribution and composition of faults control the fluid flow rate,flow direction and flow pattern(convection or advection) and affect the temperature distribution indirectly.Convection is likely to occur around the faults which break through granite rock ,and also appear in the zone between two faults which may be sustained for about several ten thousand years and probably is the reason why mineral deposits distribute in the zone between two faults as well.The lateral flow distance can be increased by the fracture on the granite's side and fluid mixing seems more likely to occur in the zone between the fracture and granite rock.The attitude of fault also affect the flow field and temperature for that convection is more appropriate to appear nearby vertical faults.(6)Permeability restricts the range and velocity of fluid flow ,as the flow rate in high permeability zone is significantly higher than that in relatively low permeability area and con- vection and fluid mixing can easily occur in the interface of upper high permeability stratum and lower relatively low permeability stratum that may be a favorable zone for mineralization.(7)A simple change of granite invasion depth has no significant effect on the trend of hydrothermal system evolution, while the increase of granite initial pressure would like to enhance the hydro-pressure in stratum during the early stage of hydrothermal system.Both sides of the upper area of granite salient are favorable for convection and mineralization.(8)The boundary of low permeability zone has a higher seepage rate that indicate the transformation effect on basalt-related deposit by granite-related hydrothermal fluid should be considered.(9)The comparative results of HYDROTHERM hydrothermal model and ANSYS thermal stress model show that fluid flow directly control the evolution of temperature field ,therefore sole quantitative discussion on temerpature field or flow field is not reasonable.(10)Thermal stress can be achieved to hundreds of MPa due to magmatism in this area and therefore the geo-stress effect on hydrothermal system including both tectonic stress and thermal stress should be considered in the future work to implement a coupling modeling of deformation and fluid and heat.Numerical simulation of hydrothermal ore-forming system has a bright prospect and will be fully applied to mineral exploration in the future, domestic geologists brought numerical simulation into the research of ore-forming process as early as 90's of 20th century ,but most are simple applications on specific instances and no comprehensive research and professional simulation software are developed and therefore urgent attension is needed.