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Numerical modeling of coupled fluid flow, heat transport and mechanical deformation: An example from the Chanziping ore district, South China
GEOSCIENCE FRONTIERS 2(4) (2011) 577e582
available at www.sciencedirect.com
China University of Geosciences (Beijing)
GEOSCIENCE FRONTIERS journal homepage: www.elsevier.com/locate/gsf
ORIGINAL ARTICLE
Numerical modeling of coupled fluid flow, heat transport and mechanical deformation: An example from the Chanziping ore district, South China Minghui Ju a, Jianwen Yang b,c,* a
Computational Geosciences Research Centre, Central South University, Changsha, Hunan 410083, China College of Earth Sciences, Guilin University of Technology, Guilin, Guangxi 541004, China c Department of Earth and Environmental Sciences, University of Windsor, Ontario N9C 3P4, Canada b
Received 11 April 2011; accepted 4 July 2011 Available online 25 August 2011
KEYWORDS Chanziping uranium deposit; Numerical modeling; Tectonic deformation; Fluid flow; Thermal convection
Abstract This paper presents numerical investigation on the ore-forming fluid migration driven by tectonic deformation and thermally-induced buoyancy force in the Chanziping ore district in South China. A series of numerical scenarios are considered to examine the effect of meteoric water precipitation, the dip angle of the faults, unconformity surface, and thermal input on the ore genesis. Our computations reveal that the downward basinal fluid flow driven by extensional stress mixes with the upward basal fluid driven by the thermal input from depth at the junction of two faults at a temperature of about 200 C, triggering the precipitation of the Chanziping uranium deposit. ª 2011, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.
1. Introduction * Corresponding author. Department of Earth and Environmental Sciences, University of Windsor, Ontario N9C 3P4, Canada. E-mail address: [email protected] (J. Yang). 1674-9871 ª 2011, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved. Peer-review under responsibility of China University of Geosciences (Beijing). doi:10.1016/j.gsf.2011.07.005
Production and hosting by Elsevier
Unconformity-related uranium deposits constitute approximately 33% of the Earth’s proven uranium reserves (Singh et al., 2002; Jefferson et al., 2007). Today, all of Canada’s uranium production is from deposits of this type, with active mining concentrated mainly in the Athabasca Basin of Saskatchewan (Jefferson et al., 2007; Le Carlier de Veslud et al., 2009). The largest and highest grade unconformity-type uranium deposits in the world occur in Paleoproterozoic sedimentary basins, particularly in Saskatchewan, Canada, and Northern Territory, Australia; whereas many smaller and lower grade unconformity-type uranium deposits are found in much younger Mesozoic basins in southern China (e.g., Wang et al., 2002). Geological, structural, geochemical and mineral paragenetic studies suggest that uranium deposits of this type may form by the mixing of an oxidized fluid circulating in basinal sandstones with
578
M. Ju, J. Yang / Geoscience Frontiers 2(4) (2011) 577e582
Figure 1
Geological sketch of the Chanziping ore deposits.
a reduced fluid emanating from basement rocks, at or near the unconformity (Cuney and Kyser, 2008; Boiron et al., 2010). Despite this general understanding, the mechanisms of fluid flow, and the heat and mass transport required for ore formation are still not fully understood. In particular, it is unclear what actually controlled the focusing of fluids into specific areas. Computational efforts have been made to simulate hydrothermal fluid flow associated with the formation of unconformity-related uranium deposits. For instance, Schaubs et al. (2005) modeled 2-D fluid flow driven by tectonic deformation for the McArthur River and Cigar Lake deposits in the Athabasca Basin, and Barrie (2006, personal communication) conducted 2-D finite element modeling of fluid flow driven by buoyancy due to heat for the same basin with particular reference to the McArthur River deposit area. However, no attempt has so far been made to simulate ore-forming fluid flow for much younger Mesozoic unconformity-related uranium deposits. This paper aims at a numerical modeling of ore-forming fluid flow related to the formation of the Chanziping deposit, southern China. A series of numerical experiments of fully coupling tectonic deformation, fluid flow and heat transport are conducted by using a finite difference code FLAC (Itasca, 2002) to quantify the role of structural control in governing fluid flow and the resultant ore-forming processes in the targeted area.
the unconformity between late Mesozoic Cretaceous red sandstone and Paleozoic Cambrian granitic basement. The main ore controlling structure is uranium-rich shear zones hosted and surrounded by Cambrian black shale. The uranium deposit occurs in the limbs of the Chanziping syncline located at the southeastern margin of the Jiangnan paleouplift (Huang et al., 1985; Xu et al., 1988). The folding of the SinianeCambrian carbonaceous, siliceous and argillaceous sedimentary rocks in the Caledonian formed an anticlinorium, which was accompanied by the formation of massive Caledonian granitic batholiths. The Cambrian strata and the overlying cretaceous red clastic sedimentary rocks are separated by an unconformity. The ore-bearing lower Cambrian Qingxi Formation, mainly composed of carbonaceous and siliceous shale, experienced weak slate facies metamorphism (Huang et al., 1985; Kang and Liu, 1991). This formation can be divided into six layers from bottom to top, and the ore body is concentrated mainly in the third and forth layers being composed of black carbonaceous pyrite-bearing siliceous slate and carbonaceous slate (Huang et al., 1985). The uranium mineralization was controlled by a fracture
2. Geological background and conceptualized model The Chanziping unconformity-related uranium deposit, located in the Zongfeng Basin straddling the Guangxi and Hunan provinces of southern China (Fig. 1), was discovered in the middleelower Cambrian sedimentary successions and structurally-hosted below
Figure 2 2-D conceptualized model showing major rock units and unconformity surface in the Chanziping ore district.
M. Ju, J. Yang / Geoscience Frontiers 2(4) (2011) 577e582
579
Rock material parameters assigned for the model.
Table 1 Rock types
Density (kg/m3)
Bulk modulus (Pa)
Granite Shale Sandstone Fault
2650 2500 2450 2400
4.95 2.80 1.67 2.30
1010 1010 1010 108
Shear modulus (Pa) 2.90 6.50 1.25 3.00
1010 1010 1010 107
Cohesion (Pa) 4.00 3.00 1.50 1.00
Table 2
Thermal parameters assigned for the model.
Rocks
Specific Linear Conductivity Fluid Grain heat thermal (W/m K) volumetric volumetric capacity expansion thermal thermal (J/kg K) coefficient expansion expansion (10 6/K) (10 6/K) (10 6/K)
Granite 790 Sandstone 839 Fault 2000 Shale 600
7.90 11.60 13.90 10.40
3.0 2.4 2.0 2.0
474 686 824 620
23.7 34.8 41.7 31.4
zone between the third and forth layers (Huang et al., 1985; Kang and Liu, 1991). In addition, a regional fault was developed at the contact between the Qingxi Formation and the granitic intrusion (i.e., the Xuechengling granite), extending several tens kilometers in NNE-direction (Kang and Liu, 1991). Uranium mineralization occurs in its hanging wall. The ore-forming temperature ranges from 150 to 350 C (Huang et al., 1985). Uranium-lead dating of uranium-rich rocks (including carbonaceous siliceous slate and uranium-rich quartz veins) and pitchblende showed that the main ore-forming stage was around 75 4 Ma and the deposit exhibited multi-stage mineralization (43 7 Ma and 22 2 Ma)
Tensile (Pa)
106 106 107 106
4.00 3.00 1.50 5.00
Permeability (m2) 105 105 106 105
2.00 1.00 1.00 1.00
10 10 10 10
16 19 15 14
Friction angle( )
Dilation angle ( )
Porosity
30 30 20 15
3 4 3 4
0.20 0.15 0.18 0.30
(Huang et al., 1985). Moreover, Huang et al. (1985) proposed that the formation age of the Qingxi Formation was ca. 523 16 Ma. Structural studies indicate that tectonic activities were dominated by folding in the Caledonian and extension in late Yanshanian, and the extensional stress was a major tectonic force when the deposit was formed during the Yanshanian in the targeted district (Huang et al., 1985; Chen et al., 2002). Based on the existing geological, geophysical, and structural data as well as the preserved remnants of the Chanziping ore district and the constraints from petrographic and fluid inclusion studies, a 2-D conceptualized hydrological model is constructed to describe the fluid plumbing system when the deposit was formed. As illustrated in Fig. 2, the deposit-scale model is 1500 m wide and 600 m deep, involving four main rock units: Cretaceous red sandstone, Cambrian black shale, Caledonian granite and Yanshanian granite basement. The black shale layer is partitioned by a fault (Fault B) simulating the structural fracture zone between the third and forth layers of the Qingxi Formation and bounded on the right by a thrust fault (Fault A) simulating the NNE-direction regional fault. The uranium ore bodies are hosted below the unconformity separating the Cretaceous red sandstone and the Cambrian black shale and close to the joint of the two faults. The sides and bottom of the model are assumed to be impermeable to fluid flow, while the upper boundary is assumed permeable. Initial pore pressure is equal to the hydrostatic value. Structural studies in
Figure 3 Pore pressure distribution and fluid flow velocity vectors. a: with the precipitation included (the max flow rate is 1.624 10 m/s); b: without the precipitation included (the max flow rate is 1.568 10 10 m/s).
10
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M. Ju, J. Yang / Geoscience Frontiers 2(4) (2011) 577e582
Figure 4 Effect of the dipping angle of Fault B. a: 73 (the max flow rate is 1.568 10 c: 89 (the max flow rate is 2.323 10 10 m/s).
this area indicate that extensional stress was the main tectonic force when the deposit was formed during the Yanshanian. Thus, a horizontal velocity boundary condition reflecting the extension is applied to the side boundaries. Fault A and B are simulated as weak narrow zones to channel fluid flow. The mechanical properties and thermal parameters assigned to these units are tabulated respectively in Table 1 and Table 2, based on field data and literatures (Luo and Ye, 1998; Hu et al., 2008; Ju and Yang, 2010).
10
m/s); b: 69 (the max flow rate is 1.262 10
10
m/s);
3. Numerical modeling results Numerical calculations are undertaken by using a finite difference code FLAC (Fast Lagrangian Analysis of Continua) (Cundall and Board, 1988), in which rock masses are represented as continua by mean mechanical and fluid flow properties. According to a prescribed non-linear stress/strain law, elements will deform in response to the applied boundary conditions. The rheology of the
Figure 5 Pore pressure and fluid flow vectors corresponding to the case that the unconformity functions as a flow pathway. The max flow rate is 1.8919 10 10 m/s.
M. Ju, J. Yang / Geoscience Frontiers 2(4) (2011) 577e582
Figure 6
Volumetric strain distribution in the model (min Z
central to upper crust is characterized by the classical MohreCoulomb material with non-associated plasticity (Vermeer and de Borst, 1984; Hobbs et al., 1990; Ord and Oliver, 1997), and fluid flow is assumed to obey Darcy’s law. Thermal volumetric strains are introduced into the incremental mechanical and fluid constitutive laws to account for thermal-stress and thermal-pore pressure couplings. In this paper, numerical modeling is employed to investigate in particular the effect of meteoric water precipitation, the dip angle of faults, unconformity surface, and thermal input on uranium mineralization since these factors are thought by geologists to have played an important role in the ore-forming processes in the study area. An 8% extension is applied to the side boundaries to simulate the extensional tectonic stress during uranium mineralization. In the first scenario, a precipitation rate of 1.0 10 10 m/s (in the same order of natural light rain) is applied on the upper boundary. Fig. 3 illustrates pore pressure distribution and fluid flow velocity vectors with the precipitation included (Fig. 3a) and without it (Fig. 3b). Comparing Fig. 3a with Fig. 3b indicates that meteoric water enhances fluid infiltration through the overlying permeable red sandstone and leads to a stronger fluid flow into the faults at a higher flow rate. The oxidized meteoric water leaches uranium minerals from the red basin fill and carries them with the flowing water. The oxidized meteoric water channeling into the faults mixes with the reduced basal fluids from the basement, which allows uranium to precipitate in the fault zone. This modeling result is supported by the evidence from fluid inclusion results (e.g., Huang et al., 1985). Obviously, the effect of meteoric water on uranium mineralization will become greater when the precipitation rate increases, and vice versa. In the second scenario, various dip angles of Fault B are considered. Fig. 4 shows the pore pressure contours and fluid velocity
Figure 7
581
1.0 10 3, max Z 1.0 10 2).
vectors corresponding to a dipping angle of 69 (Fig. 4b), 73 (Fig. 4a) and 89 (Fig. 4c), respectively. It can be seen that the dipping angle has an important effect on pore pressure and fluid velocity field, in particular along the fault zone. The larger the dipping angle is, the greater the fluid velocity becomes. Nevertheless, all cases exhibit that fluid flow tends to converge in the joint of the two faults A and B, indicating that it is the favorable locale for ore genesis. So far, the unconformity has been considered only as an interface separating the Cretaceous red sandstone and the underlying Cambrian black shale. Noticing that it could be a preferential flow pathway due to the presence of highly permeable weathered regolith around the unconformity, thus in the third scenario, a 15 m width permeable layer is used to simulate the unconformity surface. The other conditions are kept the same as before. Numerical results are shown in Fig. 5. Clearly, the unconformity surface as a flow conduit facilitates the fluid flow over the solution domain. As a result, the maximum flow rate in the joint of the two faults is now 1.819 10 10 m/s, higher than that without considering the unconformity as a flow pathway. Fig. 6 illustrates the distribution of volumetric strain, from which it is clear that there is a volume increase or dilation around the unconformity surface and along the left hand side of the model, indicating an intensive shearing during deformation, which can lead to a permeability variation. The volumetric increase or dilation around the unconformity allows more fluid flow to travel therein, which in turn facilitates fluid flow to focus into the faults. In the above numerical experiments, no thermal regime has been taken into account. Previous geochemical and fluid inclusion studies have shown that the temperature of ore-forming fluid flow ranges from 150 C to 350 C in the study area (Huang et al., 1985). In the last scenario, therefore, a heat flux of 70 mw/m2 is applied to the bottom of the model. Other thermal parameters are
Temperature contours and fluid flow vectors. The max flow rate is 2.190 10
10
m/s.
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M. Ju, J. Yang / Geoscience Frontiers 2(4) (2011) 577e582
tabulated in Table 2. Fig. 7 shows the temperature distribution and fluid flow field. It can be seen that the flow velocity in this scenario is higher than that without taking into account the heat effect. In addition to enhancing the fluid flow rate, the thermal input also drives the basal fluid to flow upwards to mix with downwelling basinal fluid due to the extensional stress in the joint of the two faults and surrounding area, over which uranium minerals are deposited at a simulated temperature of about 200 C, consistent with the geochemical and fluid inclusion results.
4. Conclusions Our modeling results have indicated that the extensional stress and thermally-induced buoyancy force are the two major mechanisms responsible for the ore-forming fluid flow and the resultant ore genesis in the Chanziping ore district, south China. Aided by these two forces, the oxidized basinal fluid migrates downwards through the red basin fill and then channels into the faults, and finally mixes with upwelling reduced basal fluid from deep basement at the junction of the two faults leading to the formation of the Chanziping uranium deposit.
Acknowledgments This work was supported by “The Program to Sponsor Teams for Innovation in the Construction of Talent Highlands in Guangxi Institutions of Higher Learning”. Research was also partially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) through a Discovery Grant to J. Yang (Grant No. RGPIN 261283). Prof. Guoxiang Chi and one anonymous reviewer are thanked for their constructive comments.
References Boiron, M.C., Cathelineau, M., Richard, A., 2010. Fluid flow and metal deposition near basement/ cover unconformity: lessons and analogies from Pb/ZneFeBa systems for the understanding of Proterozoic U deposits. Geofluids 10, 270e292. Chen, X., Huang, R., Ying, X., 2002. Geologic-hydrogeochemical prospecting criteria and prospecting model for uranium deposits in southern Hunan and northern Guangxi. Uranium Geology 18, 229e234 (in Chinese with English abstract). Cundall, P., Board, M., 1988. A microcomputer program for modeling large-strain plasticity problems. In: Swoboda, C. (Ed.), Numerical Methods in Geomechanics. Proceedings of the 16th International Conference on Numerical Methods in Geomechanics, pp. 2010e2108. Cuney, M., Kyser, K., 2008. Recent and Not-so-recent Developments in Uranium Deposits and Implications for Exploration. Mineralogical Association of Canada (MAC) Short Course Series 39. Canada, Quebec, p. 215.
Hobbs, B.E., Muhlhaus, H.B., Ord, A., 1990. Instability, softening and localization of deformation. In: Knipe, R.J., Rutter, E.H. (Eds.), Deformation Mechanisms, Rheology and Tectonics. Geological Society of London, 54. Special Publications, pp. 143e165. Hu, J.H., Su, J.H., Zhou, K.P., Zhang, S.C., Gu, D.S., 2008. Time-varying numerical analysis of mode in induction caving roof. Rock and Soil Mechanics 29, 931e936 (in Chinese with English abstract). Huang, S., Xia, Y., Xu, W., 1985. The study of isotope geology for the genetic mechanism of Chanziping uranium deposit. Uranium Geology 1, 10e18 (in Chinese with English abstract). Itasca, 2002. FLAC: Fast Lagrangian Analysis of Continua, Version 4.0, User’s Guide. Itasca Consulting Group Inc., Minneapolis. Jefferson, C.W., Thomas, D.J., Gandhi, S.S., Ramaekers, P., Delaney, G., Brisbin, D., Cutts, C., Portella, P., Olson, R.A., 2007. Unconformity-associated uranium deposits of the Athabasca Basin, Saskatchewan and Alberta. In: Jefferson, C.W., Delaney, G. (Eds.), EXTECH IV: Geology and Uranium Exploration and Technology of the Proterozoic Athabasca Basin, Saskatchewan and Alberta. Bulletin 588, Geological Survey of Canada, pp. 23e67. Ju, M., Yang, J., 2010. Preliminary numerical simulation of tectonic deformation-driven fluid flow: implications for ore genesis in the Dachang district, South China. Journal of Geochemical Exploration 106, 133e136. Kang, Z., Liu, H., 1991. On the characteristics of metallotect features and origin of Chanziping uranium deposit. Journal of East China College of Geology 14 (1), 1e10 (in Chinese with English abstract). Le Carlier de Veslud, C., Cuney, M., Lorilleux, G., Royer, J.J., Jebrak, M., 2009. 3D modeling of uranium-bearing solution-collapse breccias in Proterozoic sandstones (Athabasca Basin, Canada) e Metallogenic interpretations. Computer and Geosciences 35, 92e107. Luo, Y.Z., Ye, Y.W., 1998. Processing rock mechanical properties within No.91 orebody, Dachang. Jiangxi Nonferrous Metal 12, 9e12 (in Chinese with English abstract). Ord, A., Oliver, N.H.S., 1997. Mechanical controls on fluid flow during regional metamorphism: some numerical models. Journal of Metamorphic Geology 15, 345e359. Schaubs, P., Ord, A., Annesley, I., Madore, C., Quirt, D., Thomas, D., Portella, P., 2005. A comparison of McArthur River and Cigar Lake unconformity-related uranium deposits, Saskatchewan, Canada: insight from deformation-fluid flow models. Contributions of the Economic Geology Research Unit 64, 121. Singh, R.V., Sinha, R.M., Bisht, B.S., Banerjee, D.C., 2002. Hydrogeochemical exploration for unconformity-related uranium mineralization: example from Palnadu sub-basin, Cuddapah Basin, Andhra Pradesh, India. Journal of Geochemical Exploration 76, 71e92. Vermeer, P.A., de Borst, R., 1984. Non-associated plasticity for soil, concrete and rock. Heron 29, 1e64. Wang, Z.H., Zhou, X.M., Zheng, D.Y., Wang, G., Teng, R.Q., 2002. A metallogenic model for Mesozoic unconformity-related uranium deposits in South China. Geological Review 48, 365e371 (in Chinese with English abstract). Xu, W., Huang, S., Xia, S., 1988. The study of UePb isotopic evolutionary system in Chanziping uranium deposit. Journal of East China College of Geology 11, 11e21 (in Chinese with English abstract).
available at www.sciencedirect.com
China University of Geosciences (Beijing)
GEOSCIENCE FRONTIERS journal homepage: www.elsevier.com/locate/gsf
ORIGINAL ARTICLE
Numerical modeling of coupled fluid flow, heat transport and mechanical deformation: An example from the Chanziping ore district, South China Minghui Ju a, Jianwen Yang b,c,* a
Computational Geosciences Research Centre, Central South University, Changsha, Hunan 410083, China College of Earth Sciences, Guilin University of Technology, Guilin, Guangxi 541004, China c Department of Earth and Environmental Sciences, University of Windsor, Ontario N9C 3P4, Canada b
Received 11 April 2011; accepted 4 July 2011 Available online 25 August 2011
KEYWORDS Chanziping uranium deposit; Numerical modeling; Tectonic deformation; Fluid flow; Thermal convection
Abstract This paper presents numerical investigation on the ore-forming fluid migration driven by tectonic deformation and thermally-induced buoyancy force in the Chanziping ore district in South China. A series of numerical scenarios are considered to examine the effect of meteoric water precipitation, the dip angle of the faults, unconformity surface, and thermal input on the ore genesis. Our computations reveal that the downward basinal fluid flow driven by extensional stress mixes with the upward basal fluid driven by the thermal input from depth at the junction of two faults at a temperature of about 200 C, triggering the precipitation of the Chanziping uranium deposit. ª 2011, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.
1. Introduction * Corresponding author. Department of Earth and Environmental Sciences, University of Windsor, Ontario N9C 3P4, Canada. E-mail address: [email protected] (J. Yang). 1674-9871 ª 2011, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved. Peer-review under responsibility of China University of Geosciences (Beijing). doi:10.1016/j.gsf.2011.07.005
Production and hosting by Elsevier
Unconformity-related uranium deposits constitute approximately 33% of the Earth’s proven uranium reserves (Singh et al., 2002; Jefferson et al., 2007). Today, all of Canada’s uranium production is from deposits of this type, with active mining concentrated mainly in the Athabasca Basin of Saskatchewan (Jefferson et al., 2007; Le Carlier de Veslud et al., 2009). The largest and highest grade unconformity-type uranium deposits in the world occur in Paleoproterozoic sedimentary basins, particularly in Saskatchewan, Canada, and Northern Territory, Australia; whereas many smaller and lower grade unconformity-type uranium deposits are found in much younger Mesozoic basins in southern China (e.g., Wang et al., 2002). Geological, structural, geochemical and mineral paragenetic studies suggest that uranium deposits of this type may form by the mixing of an oxidized fluid circulating in basinal sandstones with
578
M. Ju, J. Yang / Geoscience Frontiers 2(4) (2011) 577e582
Figure 1
Geological sketch of the Chanziping ore deposits.
a reduced fluid emanating from basement rocks, at or near the unconformity (Cuney and Kyser, 2008; Boiron et al., 2010). Despite this general understanding, the mechanisms of fluid flow, and the heat and mass transport required for ore formation are still not fully understood. In particular, it is unclear what actually controlled the focusing of fluids into specific areas. Computational efforts have been made to simulate hydrothermal fluid flow associated with the formation of unconformity-related uranium deposits. For instance, Schaubs et al. (2005) modeled 2-D fluid flow driven by tectonic deformation for the McArthur River and Cigar Lake deposits in the Athabasca Basin, and Barrie (2006, personal communication) conducted 2-D finite element modeling of fluid flow driven by buoyancy due to heat for the same basin with particular reference to the McArthur River deposit area. However, no attempt has so far been made to simulate ore-forming fluid flow for much younger Mesozoic unconformity-related uranium deposits. This paper aims at a numerical modeling of ore-forming fluid flow related to the formation of the Chanziping deposit, southern China. A series of numerical experiments of fully coupling tectonic deformation, fluid flow and heat transport are conducted by using a finite difference code FLAC (Itasca, 2002) to quantify the role of structural control in governing fluid flow and the resultant ore-forming processes in the targeted area.
the unconformity between late Mesozoic Cretaceous red sandstone and Paleozoic Cambrian granitic basement. The main ore controlling structure is uranium-rich shear zones hosted and surrounded by Cambrian black shale. The uranium deposit occurs in the limbs of the Chanziping syncline located at the southeastern margin of the Jiangnan paleouplift (Huang et al., 1985; Xu et al., 1988). The folding of the SinianeCambrian carbonaceous, siliceous and argillaceous sedimentary rocks in the Caledonian formed an anticlinorium, which was accompanied by the formation of massive Caledonian granitic batholiths. The Cambrian strata and the overlying cretaceous red clastic sedimentary rocks are separated by an unconformity. The ore-bearing lower Cambrian Qingxi Formation, mainly composed of carbonaceous and siliceous shale, experienced weak slate facies metamorphism (Huang et al., 1985; Kang and Liu, 1991). This formation can be divided into six layers from bottom to top, and the ore body is concentrated mainly in the third and forth layers being composed of black carbonaceous pyrite-bearing siliceous slate and carbonaceous slate (Huang et al., 1985). The uranium mineralization was controlled by a fracture
2. Geological background and conceptualized model The Chanziping unconformity-related uranium deposit, located in the Zongfeng Basin straddling the Guangxi and Hunan provinces of southern China (Fig. 1), was discovered in the middleelower Cambrian sedimentary successions and structurally-hosted below
Figure 2 2-D conceptualized model showing major rock units and unconformity surface in the Chanziping ore district.
M. Ju, J. Yang / Geoscience Frontiers 2(4) (2011) 577e582
579
Rock material parameters assigned for the model.
Table 1 Rock types
Density (kg/m3)
Bulk modulus (Pa)
Granite Shale Sandstone Fault
2650 2500 2450 2400
4.95 2.80 1.67 2.30
1010 1010 1010 108
Shear modulus (Pa) 2.90 6.50 1.25 3.00
1010 1010 1010 107
Cohesion (Pa) 4.00 3.00 1.50 1.00
Table 2
Thermal parameters assigned for the model.
Rocks
Specific Linear Conductivity Fluid Grain heat thermal (W/m K) volumetric volumetric capacity expansion thermal thermal (J/kg K) coefficient expansion expansion (10 6/K) (10 6/K) (10 6/K)
Granite 790 Sandstone 839 Fault 2000 Shale 600
7.90 11.60 13.90 10.40
3.0 2.4 2.0 2.0
474 686 824 620
23.7 34.8 41.7 31.4
zone between the third and forth layers (Huang et al., 1985; Kang and Liu, 1991). In addition, a regional fault was developed at the contact between the Qingxi Formation and the granitic intrusion (i.e., the Xuechengling granite), extending several tens kilometers in NNE-direction (Kang and Liu, 1991). Uranium mineralization occurs in its hanging wall. The ore-forming temperature ranges from 150 to 350 C (Huang et al., 1985). Uranium-lead dating of uranium-rich rocks (including carbonaceous siliceous slate and uranium-rich quartz veins) and pitchblende showed that the main ore-forming stage was around 75 4 Ma and the deposit exhibited multi-stage mineralization (43 7 Ma and 22 2 Ma)
Tensile (Pa)
106 106 107 106
4.00 3.00 1.50 5.00
Permeability (m2) 105 105 106 105
2.00 1.00 1.00 1.00
10 10 10 10
16 19 15 14
Friction angle( )
Dilation angle ( )
Porosity
30 30 20 15
3 4 3 4
0.20 0.15 0.18 0.30
(Huang et al., 1985). Moreover, Huang et al. (1985) proposed that the formation age of the Qingxi Formation was ca. 523 16 Ma. Structural studies indicate that tectonic activities were dominated by folding in the Caledonian and extension in late Yanshanian, and the extensional stress was a major tectonic force when the deposit was formed during the Yanshanian in the targeted district (Huang et al., 1985; Chen et al., 2002). Based on the existing geological, geophysical, and structural data as well as the preserved remnants of the Chanziping ore district and the constraints from petrographic and fluid inclusion studies, a 2-D conceptualized hydrological model is constructed to describe the fluid plumbing system when the deposit was formed. As illustrated in Fig. 2, the deposit-scale model is 1500 m wide and 600 m deep, involving four main rock units: Cretaceous red sandstone, Cambrian black shale, Caledonian granite and Yanshanian granite basement. The black shale layer is partitioned by a fault (Fault B) simulating the structural fracture zone between the third and forth layers of the Qingxi Formation and bounded on the right by a thrust fault (Fault A) simulating the NNE-direction regional fault. The uranium ore bodies are hosted below the unconformity separating the Cretaceous red sandstone and the Cambrian black shale and close to the joint of the two faults. The sides and bottom of the model are assumed to be impermeable to fluid flow, while the upper boundary is assumed permeable. Initial pore pressure is equal to the hydrostatic value. Structural studies in
Figure 3 Pore pressure distribution and fluid flow velocity vectors. a: with the precipitation included (the max flow rate is 1.624 10 m/s); b: without the precipitation included (the max flow rate is 1.568 10 10 m/s).
10
580
M. Ju, J. Yang / Geoscience Frontiers 2(4) (2011) 577e582
Figure 4 Effect of the dipping angle of Fault B. a: 73 (the max flow rate is 1.568 10 c: 89 (the max flow rate is 2.323 10 10 m/s).
this area indicate that extensional stress was the main tectonic force when the deposit was formed during the Yanshanian. Thus, a horizontal velocity boundary condition reflecting the extension is applied to the side boundaries. Fault A and B are simulated as weak narrow zones to channel fluid flow. The mechanical properties and thermal parameters assigned to these units are tabulated respectively in Table 1 and Table 2, based on field data and literatures (Luo and Ye, 1998; Hu et al., 2008; Ju and Yang, 2010).
10
m/s); b: 69 (the max flow rate is 1.262 10
10
m/s);
3. Numerical modeling results Numerical calculations are undertaken by using a finite difference code FLAC (Fast Lagrangian Analysis of Continua) (Cundall and Board, 1988), in which rock masses are represented as continua by mean mechanical and fluid flow properties. According to a prescribed non-linear stress/strain law, elements will deform in response to the applied boundary conditions. The rheology of the
Figure 5 Pore pressure and fluid flow vectors corresponding to the case that the unconformity functions as a flow pathway. The max flow rate is 1.8919 10 10 m/s.
M. Ju, J. Yang / Geoscience Frontiers 2(4) (2011) 577e582
Figure 6
Volumetric strain distribution in the model (min Z
central to upper crust is characterized by the classical MohreCoulomb material with non-associated plasticity (Vermeer and de Borst, 1984; Hobbs et al., 1990; Ord and Oliver, 1997), and fluid flow is assumed to obey Darcy’s law. Thermal volumetric strains are introduced into the incremental mechanical and fluid constitutive laws to account for thermal-stress and thermal-pore pressure couplings. In this paper, numerical modeling is employed to investigate in particular the effect of meteoric water precipitation, the dip angle of faults, unconformity surface, and thermal input on uranium mineralization since these factors are thought by geologists to have played an important role in the ore-forming processes in the study area. An 8% extension is applied to the side boundaries to simulate the extensional tectonic stress during uranium mineralization. In the first scenario, a precipitation rate of 1.0 10 10 m/s (in the same order of natural light rain) is applied on the upper boundary. Fig. 3 illustrates pore pressure distribution and fluid flow velocity vectors with the precipitation included (Fig. 3a) and without it (Fig. 3b). Comparing Fig. 3a with Fig. 3b indicates that meteoric water enhances fluid infiltration through the overlying permeable red sandstone and leads to a stronger fluid flow into the faults at a higher flow rate. The oxidized meteoric water leaches uranium minerals from the red basin fill and carries them with the flowing water. The oxidized meteoric water channeling into the faults mixes with the reduced basal fluids from the basement, which allows uranium to precipitate in the fault zone. This modeling result is supported by the evidence from fluid inclusion results (e.g., Huang et al., 1985). Obviously, the effect of meteoric water on uranium mineralization will become greater when the precipitation rate increases, and vice versa. In the second scenario, various dip angles of Fault B are considered. Fig. 4 shows the pore pressure contours and fluid velocity
Figure 7
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1.0 10 3, max Z 1.0 10 2).
vectors corresponding to a dipping angle of 69 (Fig. 4b), 73 (Fig. 4a) and 89 (Fig. 4c), respectively. It can be seen that the dipping angle has an important effect on pore pressure and fluid velocity field, in particular along the fault zone. The larger the dipping angle is, the greater the fluid velocity becomes. Nevertheless, all cases exhibit that fluid flow tends to converge in the joint of the two faults A and B, indicating that it is the favorable locale for ore genesis. So far, the unconformity has been considered only as an interface separating the Cretaceous red sandstone and the underlying Cambrian black shale. Noticing that it could be a preferential flow pathway due to the presence of highly permeable weathered regolith around the unconformity, thus in the third scenario, a 15 m width permeable layer is used to simulate the unconformity surface. The other conditions are kept the same as before. Numerical results are shown in Fig. 5. Clearly, the unconformity surface as a flow conduit facilitates the fluid flow over the solution domain. As a result, the maximum flow rate in the joint of the two faults is now 1.819 10 10 m/s, higher than that without considering the unconformity as a flow pathway. Fig. 6 illustrates the distribution of volumetric strain, from which it is clear that there is a volume increase or dilation around the unconformity surface and along the left hand side of the model, indicating an intensive shearing during deformation, which can lead to a permeability variation. The volumetric increase or dilation around the unconformity allows more fluid flow to travel therein, which in turn facilitates fluid flow to focus into the faults. In the above numerical experiments, no thermal regime has been taken into account. Previous geochemical and fluid inclusion studies have shown that the temperature of ore-forming fluid flow ranges from 150 C to 350 C in the study area (Huang et al., 1985). In the last scenario, therefore, a heat flux of 70 mw/m2 is applied to the bottom of the model. Other thermal parameters are
Temperature contours and fluid flow vectors. The max flow rate is 2.190 10
10
m/s.
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tabulated in Table 2. Fig. 7 shows the temperature distribution and fluid flow field. It can be seen that the flow velocity in this scenario is higher than that without taking into account the heat effect. In addition to enhancing the fluid flow rate, the thermal input also drives the basal fluid to flow upwards to mix with downwelling basinal fluid due to the extensional stress in the joint of the two faults and surrounding area, over which uranium minerals are deposited at a simulated temperature of about 200 C, consistent with the geochemical and fluid inclusion results.
4. Conclusions Our modeling results have indicated that the extensional stress and thermally-induced buoyancy force are the two major mechanisms responsible for the ore-forming fluid flow and the resultant ore genesis in the Chanziping ore district, south China. Aided by these two forces, the oxidized basinal fluid migrates downwards through the red basin fill and then channels into the faults, and finally mixes with upwelling reduced basal fluid from deep basement at the junction of the two faults leading to the formation of the Chanziping uranium deposit.
Acknowledgments This work was supported by “The Program to Sponsor Teams for Innovation in the Construction of Talent Highlands in Guangxi Institutions of Higher Learning”. Research was also partially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) through a Discovery Grant to J. Yang (Grant No. RGPIN 261283). Prof. Guoxiang Chi and one anonymous reviewer are thanked for their constructive comments.
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