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Fluid focusing and its link to vertical morphological zonation at the Dajishanvein-type tungsten deposit, South China
Xiangchong Liu, Huilin Xing, Dehui Zhang
PII: S0169-1368(14)00083-3DOI: doi: 10.1016/j.oregeorev.2014.04.005Reference: OREGEO 1206
To appear in: Ore Geology Reviews
Received date: 5 November 2013Revised date: 30 March 2014Accepted date: 3 April 2014
Please cite this article as: Liu, Xiangchong, Xing, Huilin, Zhang, Dehui, Fluid focusingand its link to vertical morphological zonation at the Dajishan vein-type tungsten deposit,South China, Ore Geology Reviews (2014), doi: 10.1016/j.oregeorev.2014.04.005
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*Corresponding author at: School of Earth Sciences and Resources, China University of Geosciences (Beijing),
Beijing 100083, China. E-mail address:[email protected], [email protected]
Fluid focusing and its link to vertical morphological zonation at
the Dajishan vein-type tungsten deposit, South China
Xiangchong Liua,b, Huilin Xingb*, Dehui Zhanga* a. State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences and Resources,
China University of Geosciences (Beijing), Beijing 100083, China
b. Centre for Geoscience Computing, School of Earth Sciences, The University of Queensland, Brisbane, QLD
4072, Australia
Abstract: Five-floor vertical zonation in vein morphology is a common phenomenon
in exo-contact vein-type tungsten deposits of South China and has been widely
applied for practical exploration, but the causative mechanisms are poorly understood.
We combine fractal methods and finite element based numerical analysis to
investigate the link between the fluid focusing and the vertical morphological
characteristics at the Dajishan tungsten deposit and to determine vein growth
mechanisms. Fractal analysis of vein morphology indicates that the growth
mechanisms of veins in the lower part of ore bodies were mainly controlled by growth
rate. Anisotropic permeability tensor method is applied to evaluate the anisotropic
flow behaviours in joints. Numerical experiments on permeability ratio and aspect
ratio controlled by stress state indicate that the NWW-striking joints in the host rock
significantly affect the fluid focusing in these joints and the other potential conduits.
The higher permeability and aspect ratios in the lower levels of the joint systems lead
to the more fluid focusing in the lower part of ore bodies, which delivers more fluids
required for precipitating silica. The five-floor zonation at this deposit is related to the
regional tectonic evolution from the Indosinian (257~205 Ma) to Yanshanian
(205~135 Ma) in South China. This paper may also help to better understand the
vertical morphological zonation in this type of tungsten deposits in China.
Key words: Dajishan; Tungsten; Morphological zonation; Fluid focusing;
Permeability ratio; Aspect ratio.
1. Introduction
Vertical morphological zonation is a common phenomenon at exo-contact
vein-type tungsten deposits in South China (Gu, 1984). Field geologists have
identified five zones based on the vein morphology since 1960s, and proposed a
five-floor vertical morphological zonation. In the five-floor zonation, the top zone is
called thread vein zone where thin veins not over 1 cm in thickness occur. These
barren thin veins gradually become thicker and richer in tungsten downwards,
forming the veinlet zone then mixing zone of veinlet-thick veins. The veins in the
mixing zone converge downwards into a few wolframite-quartz veins often exceeding
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1 m in thickness, which is called thick vein zone. At the root of the five-floor zonation,
these thick veins shrink downwards and become barren (more details see Clarke
(1983) and Liu and Ma (1993)). The five-floor zonation has played an important role
in prospecting this type of tungsten deposits in the last few decades (Gu, 1984; Mao et
al., 2013; Wang et al., 2010; Xu et al., 2008). However, the reason for such a zonation
is still unclear.
Although vertical morphological zonation is one of the most apparent
characteristics in the vein-type tungsten deposits, South China, it is not well
developed elsewhere. Vein-style tungsten-tin deposits in Cornwall, England and in
Panasqueira, Portugal also show variable vein thicknesses (Foxford et al., 2000; Hall,
1971; Sanderson et al., 2008), but these sheeted vein systems do not exhibit as regular
morphological zonation as those in South China. The distinctive morphological
characteristics of veins in China may indicate complicated coupling of magmatic
hydrothermal processes and regional tectonic evolution (Liu and Ma, 1993; Yu, 2004).
Therefore, it is necessary to better clarify the mechanisms of the five-floor zonation to
understand its links to mineralization and to better target blind deposits.
With regard to hydrogeology, formation of these deposits involved complicated
problems that coupled thermal, hydraulic, mechanical, and chemical processes
(Ingebritsen and Appold, 2012; Zhao et al., 2012). Massive quartz veins at these
deposits require extremely focused flow to deposit silica given its low solubility in
aqueous fluids (Akinfiev and Diamond, 2009; Zhu et al., 1981). Percolation models
suggest that fracture evolution is essential to the locations and styles of tungsten-tin
mineralization, and that the veins tend to develop when localized opening and
channelized flow happens at the backbones of fracture systems (Roberts et al., 1998,
1999; Sanderson et al., 2008). This means that focused flow in fractures is one of the
necessary conditions to account for the morphological zonation. Analytical solutions
(Zhao et al., 2006, 2008) demonstrate that fluid focusing in joints or faults depends on
their hydraulic characteristics (permeability ratio) and geometric characteristics
(aspect ratio).
In this paper, we firstly present the geological background of the Dajishan
tungsten deposit. Then, we illustrate vein thickness distribution at this deposit using
fractal techniques, which characterize its morphological characteristics improving that
described by Liu and Ma (1993). These fractal methods can trace the history of vein
growth and help understand the use of morphological zonation in exploration. These
data and observations are used as constraints on the numerical modeling. Next, we
numerically investigate fluid flow and heat transfer using an anisotropic permeability
tensor. Numerical experiments indicate that permeability and aspect ratios
significantly influence the degree and location of fluid focusing in oriented joints.
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Finally, we build a link between the focused flow in the existing joints and the vertical
morphological zonation constrained by fractal methods.
2. Background
The Dajishan tungsten deposit, located in Quannan County, Jiangxi Province, is a
large-scale vein-type and granite-type tungsten polymetallic deposit. The vein-type
deposit discovered in 1917 has a WO3 reserve of 160,000 tonnes with an average
grade of 2.04% (Mao et al., 2013). The ore deposit lies in the Nanling Range of the
Cathaysian Block that is situated inboard from the subduction zone of the Western
Pacific plate (Fig.1a). Recent data suggest that tungsten concentration of stream
sediments in the Cathaysian Block is 5~250 times the Clark value (0.6 ppm) of crust
in East China (Chi et al., 2012). The Yanshanian (205~135Ma) was the most
important epoch for magmatic activity in South China (Zhou et al., 2006). Mesozoic
granites are present throughout this region, consisting of Jurassic granites mainly
distributed in the Nanling Range and in the interior of South China tectonic block and
Cretaceous granites along the present-day continental margin (Zhou et al., 2006).
Almost all of the polymetallic tungsten deposits in Jiangxi were related to Yanshanian
granites and formed at the endo- or exo-contact of granite intrusions (Guo et al., 2011;
Mao et al., 2013).
The exposed strata include Cambrian, Devonian, and Paleogene rocks (Fig.1b).
The Cambrian strata consist of low-grade metamorphic sandstone and slate that hosts
the Dajishan granite and ore body. These sedimentary rocks strike 300o~320
o and dip
20o~40
o. The Devonian strata consist of coarse sandstone, phyllite, and quartz
conglomerate, strike 30o~50
o and dip 40
o~80
o. The contact relationship between the
Devonian and Cambrian units is an angular unconformity (Zhou, 2009).
The main structure at the Dajishan deposit is a synclinorium with a NE-striking
axis and four groups of structural elements, divided by their orientations: EW, NE,
NNE, and NWW structures (Fig.1c). The EW structures include an EW silification
belt in the contact between the Wuliting granite and the host rock, quartz porphyry
filled in an EW fault and No.14 EW vein in the ore body. The NE structures include
the Dajishanfeng Fault (F5) and the Chuandiwo Fault (F1-F4). The NNE-trending
structures consist of small-scale compressive faults in mineralized area. A few faults
in this group are filled with tungsten-bearing quartz veins, although most are small
faults that cut the ore body. The NWW-striking structures include the ore-bearing
fractures and a fault filled with diorite (Zhou, 2009).
Magmatic rocks in the ore district include biotite granite, two-mica granite,
muscovite granite, diorite, and quartz porphyry (Fig.1). The Wuliting biotite granite is
dated at 238.4±1Ma by the zircon U-Pb method (Zhang et al., 2004), whereas its
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whole-rock Rb-Sr age is 167±1Ma (Sun, 1989) and its biotite K-Ar age is
160.3±3.0Ma and 164.2±3.5Ma (Jiang et al., 2004). The two-mica granite occurring
below the ore body is dated at 161Ma by the whole-rock Rb-Sr isochron (Sun, 1989)
and at 160.6±2.8Ma by the muscovite K-Ar method (Jiang et al., 2004). The
muscovite granite occurs from the level of 517m to 0m. It consists of five small
intrusions, of which the No.69 granite is the largest and Ta-Nb-W-Be mineralized
(Fig.1d). The whole-rock Rb-Sr age of the No.69 granite is 159±5Ma (Sun, 1989),
while its zircon U-Pb age is 151.7±1.6Ma (Zhang et al., 2006). Drill data indicate that
the muscovite granite was intruded later than the two-mica granite. The diorite is the
host rock of the vein-type ore body at the middle set. The quartz porphyry swarm
occurs in the Cambrian strata, but its age is unknown. Geophysical data show that the
Dajishan ore district is situated above EW-trending blind granite with an area of
several thousand km2 (Zhou, 2009).
The tungsten-bearing quartz veins occur in the Cambrian metamorphic sandstone,
slate, and diorite. Alteration of wall rock include silicification, tourmalinization, and
biotitization in sandstone, greisenization in the granite, and biotitization in the diorite
(Sun, 1989). Ore minerals include wolframite, scheelite, tungstite, bismuthinite, native
Bi, and molybdenite, and gangue minerals are quartz, feldspar, muscovite, calcite, and
tourmaline. The typical minerals in the middle vein set are orthoclase, microcline,
perthite, and albite (Zhou, 2009).
The age of mineralization at the Dajishan vein-type deposit is 160±1.3 Ma dated
by the molybdenite Re-Os isochron (Zhang et al., 2011) and 144 Ma and 147 Ma by
the muscovite 40
Ar-39
Ar method (Zhang et al., 2006). Hydrogen and Oxygen isotopic
measurements (δ18
O=9.9~11.5‰, D=-75.1~-60.7‰) in the three types of granite
suggest that they were formed by crustal anatexis, and the fluids were derived from
magmatic water (Zhuang et al., 1991). The ore-forming fluids are interpreted to be
magmatic in the main mineralization stage, and then were mixed with meteoric waters
in a late stage (Zhang et al., 1997). Based on oxygen isotope mineral pairs (quartz and
wolframite, quartz and scheelite, and quartz and mica), mineralization temperatures
were 300~400oC (Zhang et al., 1997). Infrared measurements suggest that the fluid
inclusions in wolframite experienced relatively simple evolution history and had
higher temperature range than those in quartz (Ni et al., 2006). The mineralized fluids
were in the H2O-NaCl system or in the H2O-CO2-CH4-NaCl system (Wang et al.,
2013; Xi et al., 2008). Homogenization temperatures of inclusions in quartz are
200~330oC, and salinities are typically 4~10wt% (Wang et al., 2013; Xi et al., 2008).
The fluid pressure at the time of mineralization is estimated to be 114~132MPa,
indicating that the ore-forming depth was 4.6~5.3km (Xi et al., 2008).
Fig.2 shows the spatial zonation of the granites and veins along with the
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interpreted relative timing of granite and vein emplacement. The
magmatic-hydrothermal system is interpreted to be multi-phrase from a magmatic to
post-magmatic period (Kong, 1982; Teng, 1990), with both the spatial and temporal
distributions the consequence to tectonic activity. At the Dajishan vein-type deposit,
silicates and oxides are present in the upper part of vein systems, whereas oxides,
sulfides, and carbonates are dominant in the lower part (Teng, 1990).
3. Geological structures
Previous studies suggest that the mineralization at the deposit was controlled by
fractures (Liu and Yu, 2009; Zhou, 2009), and therefore the structural characteristics
of joints and faults are summarized below. Joint numbers and orientations were
measured on a line transect normal to vein orientation. A geochemical profile through
the Dajishanfeng Fault was measured at 467m level to constrain fluid flow and heat
transfer therein.
3.1. Joints
Previous studies indicate that two sets of preferred orientated joints occur at this
deposit, and their azimuths are 9o 74
o and 181
o 66o, respectively (Zhou, 2009; see
Fig.3a). The definition of the azimuth is dip direction dip angle. The corresponding
principal stresses are 252o 79
o (σ1), 95
o 11o (σ2), and 5
o 4o
(σ3) (Zhou, 2009). The
joints dipping north are more abundant than those dipping south. The mineral and
associated alteration assemblages in one set show no difference from the other. The
preferred orientations of joints we measured show a similar result to the previous
studies (Fig.3b). Our measurements also indicate that vein density increases with
increasing joint density (Fig.4).
3.2. Faults
The Dajishanfeng Fault and Chuandiwo Fault are situated in the east and in the
west of ore-bearing veins, respectively. The Dajishanfeng Fault outcrops near the
unconformity between the Cambrian and Devonian strata, trending 50o and dipping
40o~60
o NW. It consists of compressive fracture zones and reverse faults without
obvious strike-slip movement. Previous studies indicate that the barren Dajishanfeng
Fault predates mineralization as it is cut by veins both at 467m and at 985m levels
(Zhou, 2009). The geochemical profile through this fault at 467m level indicates that
rock type changes from hornfels to meta-sandstone, and that quartz veins become
fewer and thinner from DP1-1 to DP1-12 (Fig.5). Along the profile, grades of WO3
fall from 1156 ppm to 10 ppm with a significant rise at the eastern side of the fault.
These data indicate this fault played a minor role in mineralization. Previous studies
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suggest that the Chuandiwo Fault also shows little control on formation of
mineralized veins (Zhou, 2009).
3.3. Veins
The ore-bearing veins are divided into three vein sets by their locations: the north
set, the middle set, and the south set. The south and north sets occur in the Cambrian
strata, whereas the middle set mainly occurs in the diorite. The vein sets extend
horizontally 300~650 m with the eastern end close to the Dajishanfeng Fault. Most
veins trend 280o~300
o and dip 65
o~80
o NNE (Zhou, 2009). The ore bodies are
wedge-shaped, and numerous thin veins converge to a few thick veins downwards
(Fig.6).
In section 4, we characterize the vertical morphological zonation using the data
of vein morphology. In section 6 and 7, we conduct numerical simulations using the
data of structures.
4. Morphological characteristics of veins
The middle and north sets were chosen as the objects in this study. The middle
set becomes barren at 517m and wedges out at 267m, whereas the north set extends to
the level of 100m (Fig.1d). The two-mica granite present below the middle and north
sets occurs at the level of 240m and 100m, respectively (Zhou, 2009). Using the
classification of the five-floor zonation (Fig.4 in Liu and Ma, 1993), the middle set
between the 467m and 317m levels represents the fifth zone of the vein system, i.e.
the thin out zone, and the north set between the 467m and 317m levels represents the
fourth zone, i.e. the thick vein zone.
Fifteen horizontal traverses normal to veins were chosen to measure vein
thickness, length, and density in the middle and north sets (Fig.1d; Table 1). True vein
thickness has been transformed from the value measured along scan lines by vein’s
dip angle. The minimum vein thickness recorded was 1.0 cm. Data of vein length in
the middle set were not successfully obtained due to inadequate exposure.
4.1. Methods
The distribution of vein thickness is a useful tool for characterizing fracture
evolution and fluid channelization in vein-style deposits (André-Mayer and Sausse,
2007; Gillespie et al., 1999; Monecke et al., 2001; Sanderson et al., 2008). Given that
fractal-geometry techniques have been widely applied to quantify rock structures
(Kruhl, 2013), these fractal methods were applied to describe the morphological
characteristics of the vein systems at the Dajishan deposit.
Similar to other non strata-bound vein systems, both power law and
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negative-exponential distributions were compared to identify vein thickness
distribution. Power law distribution follows the function (Gillespie et al., 1999):
, (1)
Negative exponential distribution follows the function (Sanderson et al., 2008):
, (2)
where N(t) is the number of objects of size t or larger, t represents vein thickness, and
Dt is the fractal dimension of thickness. Low values of Dt mean that there is a large
proportion of thick veins compared to thin ones (Gillespie et al., 1999).
The relationship between vein thickness and length infers the segment linkage of
vein system. Their relation is generally expressed by (Gillespie et al., 1999):
, (3)
where t is the vein thickness, L is the vein length, and c is the power-law exponent.
Vein systems evolve from isolated veins to interacting veins through segment linkage
(Kim and Sanderson, 2005), increasing the value of c.
4.2. Results
Fig.7 and Table 2 show the fitted distributions of vein thickness and the related
parameters. For the north set, the distribution of vein thickness at 317m is ambiguous
due to large fit goodness in both power law (R2=0.92) and negative exponential
(R2=0.97) distributions, whereas vein thickness at 467m level conforms to a power
law distribution. Veins at 317m level have smaller fractal dimension (the slope of the
fitted line) than those at 467m level either for negative exponential distribution or for
power law distribution. For the middle set, vein thickness at both levels conforms to
power law distributions. Veins at 317m have larger fractal dimension than the ones at
467m. For power law distribution, the middle set has a smaller average fractal
dimension than the north set.
Veins at 317m level in the north set have smaller power-law exponent n than the
ones at 467m level (Table 2).This result was fitted by 10 pairs of thickness-length data
at both levels.
4.3. Vein growth mechanisms
The channelized fluid flow through fractures is often explained by a percolation
model (Sanderson et al., 2008) or by a stochastic model (Monecke et al., 2001). In the
percolation model, a percolation threshold is the critical point, above which
well-linked fractures characterized by decreasing Dt and increasing c allow focused
flow through part of the fracture system (Sanderson et al., 2008). In the stochastic
model, Dt follows the function (Monecke et al., 2001):
(4)
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rn is the nucleation rate defined as the number of newly created veins in a unit time
and a unit volume, and rg is the growth rate defined as the increment of vein thickness
in a unit time. A high nucleation rate rn produces a large fractal dimension Dt, whereas
a high growth rate rg yields a small Dt. Combination of these two models might help
to better understand vein growth mechanisms (André-Mayer and Sausse, 2007).
Data of veins in Table 1 and fractal dimensions of power law distribution in
Table 2 were chosen to make the following comparisons. Dt<1 in the two sets
indicates that the systems are above the threshold and well connected. The north set at
317m level has smaller Dt than the same set at 467m level, therefore characterized by
a relative high amount of thick veins. This is consistent with higher growth rate
represented by larger vein thickness and lower nucleation rate reflected by smaller
vein density at 317m level; therefore, vein morphology in the north set between 467m
and 317m levels was controlled by increasing growth rate and decreasing nucleation
rate downwards. This combined contribution reduced Dt in the north set from 467m to
317m level.
Two lines of evidence were used to explain the growth mechanisms of veins in
the north set. One line is the average strain (Table 1). High strain in the vein system at
317m level is interpreted to be linked to large growth rate, compared to that at 467m
level. The other is the power-law relationship between vein thickness and length
(Table 2). Comparison of power-law exponent c implies that the vein system at 317m
level evolved to a higher level of linkage and allowed more focused flow, therefore
increasing the vein growth rate and decreasing the fractal dimension of vein thickness.
The middle set at 317m level has a larger fractal dimension than at 467m level
(Table 2), meaning a smaller proportion of thick veins. This trend of fractal dimension
is ascribed to growth rate rather than nucleation rate. The vein system at 317m level
has a smaller average thickness but a slightly lower average vein density than at 467m
level. This means that the decrease of growth rate from 467m to 317m level
contributes to the increase of fractal dimension, to a large degree. This downward
trend of growth rate is consistent with the downward decrease of average strain (Table
1).
4.4. Vertical morphological zonation
The fractal methods above illustrate a number of morphological characteristics of
the vein system at the Dajishan deposit.
1. It is characterized by decreasing Dt downwards in the fourth zone, represented by
the north set between 317m and 467m levels.
2. It is characterized by increasing Dt downwards in the fifth zone, represented by the
middle set between 317m and 467m levels.
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3. The middle set at 467m level has the smallest Dt among the four groups of data.
Based on limited data, we concluded a rough trend of Dt that first decreases from
the top zone to the fourth zone, then increases in the fifth zone. Veins between the
fourth and the fifth zone have the lowest Dt, implying the most focused fluid flow.
Growth rate controlled the vein growth in the fifth zone, and both growth rate and
nucleation rate determined the vein thickness distribution in the fourth zone. Vertical
morphological zonation derived from the trend of Dt is that numerous thread veins
become thick downward, increasing the proportion of thick veins, then converge to a
few large veins that eventually shrink at the contact zone between the two-mica
granite and the host rock. Therefore, the five-floor zonation can be well characterized
by fractal dimension of vein thickness.
These characteristics of veins are closely related to magmatic hydrothermal
transition that controlled development of quartz veins in time (Liu and Ma, 1993; Zhu
et al., 1981). Reverse mineral zonation implies that multiple stages of mineralization
were focused at different levels, the main mineralization stage in the middle and the
upper parts and the late stage in the lower part. Despite the complexity,
well-crystallized wolframite that is common in thick veins (Fig.3a) suggests that this
morphological zonation, to a large degree, was formed during the main mineralization
stage. This highlights the close relationship between vein’s morphology and tungsten
mineralization.
Similar morphological zonation has also been discovered in epithermal deposits
(Brathwaite et al., 2001) and porphyry deposits (André-Mayer and Sausse, 2007).
These studies indicate that understanding morphological characteristics of veins can
aid in exploration. In vein-type tungsten deposits, the reason why the five-floor
zonation has become a useful prospecting method can be explained in part by the
fractal dimension of vein thickness. The decrease of fractal dimension at the Dajishan
deposit’s north set between 467m and 317m level means the increase of the proportion
of thick veins. This trend predicts the occurrence of channelized flow and much
potential of mineralization at depth. The increase of fractal dimension in the middle
set between 467m and 317m level implies that the degree of fluid focusing falls and
veins might become barren. These predictions in the north and middle sets are
consistent with the resource potential. It can be seen that fractal methods combined
with the five-floor zonation can play a helpful role in exploration.
Despite its important role in exploration, the five-floor zonation has many
uncertainties and limits because it is an empirical prospecting method rather than a
typical metallogenic model involving process understanding (Wang et al., 2010; Xu et
al., 2008). In the next part, we investigate the factors affecting the link between fluid
focusing and the vertical morphological zonation using the finite element method.
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5. Computational models
5.1. Computational model and governing equations
PANDAS, a novel finite element and/or Lattice Boltzmann method (FEM/LBM)
based software, has been developed to simulate highly coupled systems involving
heterogeneously fractured geomaterials to address the key scientific and technological
challenges in geo-science and –engineering at different spatial and temporal scales by
using supercomputers (Xing, 2013; Xing et al 2010). PANDAS has been applied to
simulate crustal dynamics, fault systems and enhanced reservoir systems across
different scales for underground geoenergies (Xing 2013, 2014; Xing and makinouchi
2002; Xing and Mora 2006; Xing et al 2007; Xing et al 2010). It is applied here to
simulate the transient thermal-fluid flow process, and the governing equations are
briefly listed below (Xing, 2014).
For the conservation of non-deformable rock mass with constant porosity , the
continuity equation is expressed as
(5)
where is the fluid velocity; P is the fluid pressure; is the effective
compressibility; is the density; is the porosity. Bold symbols are used for vector
and tensor/matrix variables throughout this paper.
The relationship of fluid velocity and pressure P for fluid flow though porous
media is normally expressed by Darcy’s law:
(6)
where k is the intrinsic permeability tensor of the porous media; g is the gravitational
acceleration; is the dynamic viscosity; D is the depth.
From the energy conservation, the conductive-convective heat transfer in porous
media can be described as:
(7)
, where , is density, C is specific heat capacity,
and subscript r, f, and m denote rock matrix, fluid, and mixture, respectively; heat
conductivity ; T is the temperature; is the energy source
term.
5.2 Anisotropic permeability tensor method
Permeability is often anisotropic in geologic systems (Rosenberg et al., 1993),
and is strongly controlled by rock fabric (Ingebritsen and Appold, 2012). In
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intrusion-related deposits, permeability is however strongly controlled by fracture
networks (Titley, 1990). Anisotropic permeability of these fractures is often described
by a permeability tensor (Oda, 1985), an equivalent continuous method to model fluid
flow in petroleum and hydrological studies (Snow, 1969). The permeability tensor in
a set of parallel fractures is defined as (Gupta et al, 2001):
(8)
It assumes that all of the fractures have smooth parallel plane walls of infinite extent.
The permeability scalar k accounts for the anisotropic intensity that fractures bring to
the porous media. The unit permeability tensor accounts for the directional effect
in the fluid flow due to fractures in the medium. The unit permeability tensor is
defined as:
(9)
where stand for the orthogonal axes , and ; is the Kronecker delta,
vanishing when and unity when ; is the component of the vector
normal to the fracture plane. The function of is defined by:
(10)
where is the dip direction of fractures, the dip angle. The permeability scalar
is defined as:
(11)
where is the fracture aperture, and a dimensionless constant. The value of
approaches the upper limit , when the fracture system behaves more like parallel
plate conduits. The fracture density f is defined by f=N/H, where N represents the
number of fractures in a sample volume, H the length of a representative line that
crosses the sample volume. The permeability tensor becomes a diagonal matrix, and
the values in the diagonal line are the principal permeability.
6. Numerical modeling of fractures at the deposit
A geometric model has been built along the section line A-A’ (Fig.8). The
Dajishangfeng Fault and Chuandiwo Fault were omitted in the model given their
minor roles in minerlization. Most ore-bearing veins have similar orientations to the
preferred oriented joints, particularly the set dipping north. This implies that these
oriented joints may become potential conduits for mineralized fluids, therefore were
incorporated in the model. The joint system consists of five parts (Є-3 to Є-7) with the
same size, corresponding to the five zones (Є-3 the fifth zone and Є-7 the first zone).
The contact zone (Є-1) refers to the Cambrian strata and early crystallized granite
given multiple magmatic activities. The boundary and initial conditions are shown in
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Fig.8.
6.1. Hydraulic parameters
The permeability tensor of oriented joints was calculated using equation (8)~(10)
(Appendix A). The diagonalized unit permeability tensor is:
(12)
The major axis (k11=1.0) is on the intersection of the two sets and trends NWW, the
minor (k33=0.3) bisects the obtuse angle of the two sets and strikes NNE, and the
intermediate (k22=0.7) bisects the acute angle and is along the vertical dimension
(Fig.9). The coordinate system in simulations is NNE as X-axis, NWW as Y-axis, and
the vertical dimension as Z-axis. X-axis is perpendicular to the oriented joints, Y-axis
along oriented joints. The 2D model in Fig.8 corresponds to X-Z plane and invokes
k22 and k33.
Both the joint aperture and density are required to compute the permeability
scalar k. Aperture that controls permeability scalar of joints varies during the
evolution of hydrothermal systems (Ingebritsen and Manning, 2010). It is also
difficult to know if joints are connected or unconnected. For simplicity, the
permeability’s order of magnitude is constrained by the depth-dependent permeability
(Ingebritsen and Manning, 2010):
(13)
where k is in m2 and Z in km. Mineralization depth is estimated to be ~5 km (Xi et al.,
2008), so the permeability’s order of magnitude is approximately 10-16.7
m2. Existing
quartz veins imply that this value might be a lower limit for the jointed host rock.
Diorite is the host rock for the middle set, and field measurements show that joint
density here is slightly higher than the one in the north set but equal to the value in the
south set (Fig.6). Given the slight difference in joint density between the diorite and
the Cambrian strata, we did not differentiate between the diorite and the Cambrian
strata in the models.
6.2. Key assumptions
Several assumptions have been made to focus the models on the short period of
time when magmatic fluids were released. First, several lines of evidence indicate that
high permeability is localized in the upper crust and has a limited lifetime, ranging
100~10
3 years (Ingebritsen and Manning, 2010). This short-lived high permeability is
the key to form ore deposits (Cathles and Shannon, 2007; Ingebritsen and Appold,
2012; Liu, 2011; Lupi et al., 2011; Weatherley and Henley, 2013), and therefore
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numerical experiments were run for no more than 1000 years. Permeability of all units
was assumed to be fixed and was not changed as a result of thermo-mechanical stress
and chemical precipitation (Germanovich and Lowell, 1992; Lowell et al, 1993).
Second, fluid inclusions in wolframite or in quartz are mainly two-phase
aqueous-rich (Cao et al., 2009; Song et al, 2011; Wang et al., 2007; Wang et al., 2009,
2013; Wei et al., 2012; Xi et al., 2008), implying that boiling was not the dominant
mechanism controlling mineralization in this type of deposits (Wang et al., 2011).
Given this constraint, we assumed that the release of magmatic fluids was driven by
constant overpressure during a short period, neglecting thermal expansion of fluids
and fluid production in wall rocks (Hanson, 1995). Fluid inclusion analysis at the
Dajishan deposit also suggests that lithostatic pressures were reached (Xi et al., 2008),
and that a pressure gradient approximating 105Pa/m existed over 1 kilometer. In this
case, the effect of the gravity term (104Pa/m) in equation (6) was ignored.
Third, the contact zone is considered to be permeable at about 600oC. In
magmatic hydrothermal systems, brittle-ductile transition (BDT) exists around the
contact zone (Fournier, 1999). Recent numerical simulations employing
temperature-dependent permeability indicate BDT is defined at 650~750oC (Coumou
et al., 2009), higher than 350~400oC constrained by Fournier (1999). Generally, the
BDT depends on temperature as well as mineralogy and strain rate (Fournier, 1999;
Ingebritsen, 2012; Simpson, 2001). For simplicity, we consider the contact zone Є-1
as permeable rock when magmatic volatiles were expelled.
6.4. Models
Fluid focusing in joints depends on both the permeability ratio of joints to host
rock and the aspect ratio of long axis to short axis (Zhao et al., 2006, 2008). We
conducted four numerical experiments to illustrate the link between fluid focusing and
the five-floor zonation. In experiment 1, permeability in each unit is isotropic and
each part of the joint system has the same permeability and aspect ratios. This ideal
case acts as a reference experiment. In experiment 2, we investigated how
permeability ratio influences fluid focusing in the joint system. In this experiment, we
did not follow the assumption of an infinitely extended fracture plane in permeability
tensor, because joints in geological environments are not always connected in all
directions. Previous numerical simulations indicate that the maximum connectivity
often occurs in the direction of maximum principle stress (Sanderson and Zhang, 1999;
Zhang and Sanderson, 2001). The maximum principle stress during mineralization
was almost vertical (Zhou, 2009), so we elevated the vertical permeability and
increased the permeability ratio and the degree of anisotropy. In experiment 3, we
studied how geometric characteristics of the joint system affect fluid flow therein. The
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width of the vein set decreases downwards (Liu and Ma, 1993; Zhou, 2009), implying
the lower part of the joint system has larger aspect ratio than the other parts.
Experiment 4 was conducted to examine the combined influence of permeability ratio
and aspect ratio. We displayed the output in 1 year to represent a transient result and
the one in 20 years to represent the steady state.
7. Simulation results
7.1 Experiment1
Hydraulic parameters are shown in Table 3. After 1 year, a large pressure
gradient occurred in the lower part of joint system, driving fluids flow into the joint
system (Fig.10a, b). In the upper part, fluids moved outwards due to high fluid
pressure relative to the host rock. Fluid moved fastest (9.4╳10-8
m/s) in the lower part
of joint system. After 20 years, the pressure gradient and fluid velocity has decreased
relative to 1 year (Fig.10c, d), the maximum velocity decreasing to 8.2╳10-8
m/s, and
flow was mainly focused in the third zone (Є-5). Table 4 shows that pressure
distributions in all four experiments have reached steady state after 20 years. For
convenience, velocity ratio is defined as the ratio of maximum velocity in any
experiment to the maximum in experiment 1 after 20 years. This parameter was used
to highlight the difference between this experiment and the following experiments
(Table 4).
7.2. Experiment 2
Previous studies using convection-type models have shown the significant
impact of anisotropy on flow patterns and heat transfer (Dutrow et al., 2001; Gerdes et
al., 1998; Hurwitz et al., 2002, 2003; López and Smith, 1996; Rosenberg et al., 1993).
In this experiment, porosity and vertical permeability of the fourth and fifth zones was
also increased to 0.14 and 7.0╳10-14
m2, respectively. After 1 year, the pressure
gradient around these two parts was much higher than experiment 1, causing faster
flow in fourth and fifth zones (Fig.11a, b). After 20 years, fluid flow became slightly
slower than before but was still most focused in fourth zone with a velocity ratio of
1.3 (Fig.11c, d, Table 4).
7.3. Experiment 3
We reduced the width of the fourth and fifth zones from 150m to 60m, raising
their aspect ratios from 2 to 5. All units have the same hydraulic parameters as
experiment 1. After 1 year, one large pressure gradient was generated around these
two zones, moving fluids towards them (Fig.12a, b). After 20 years, pressure gradient
in these areas became lower, decreasing fluid velocity therein, and fluids were most
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focused in the fourth zone, not in the fifth zone anymore (Fig.12c, d). The velocity
ratio decreased from 1.9 after 1 year to 1.6 after 20 years.
7.4. Experiment 4
We adopted the same geometric size of joint system as that of experiment 3 and
the same hydraulic parameters as that of experiment 2. Thus, the fourth and fifth
zones have higher aspect ratio and higher permeability than the other zones. This
experiment has the hydraulic and geometric characteristics closest to those of the
Dajishan tungsten deposit, compared to the other experiments. After 1 year, flow
velocity in the fifth zone increased rapidly upwards, reached its peak in the fourth
zone, and then decreased gradually upwards (Fig.13a, b). In the steady state after 20
years, the fourth zone still had larger pressure gradient and higher degree of fluid
focusing than other zones (Fig.13c, d). The velocity ratios in this experiment are
higher than the one in any other experiments.
8. Discussions
8.1. Comparisons with analytical solutions
Fluid focusing in joints has been positively correlated with permeability ratio and
aspect ratio (equation 23 in Zhao et al., 2006). The interaction of these two parameters
implies that fluid focusing is related to an increment of one parameter as well as to the
current value of the other (Zhao et al., 2006). These conclusions, together with
comparisons between experiment 1 and others (Table 4), were used to test the
numerical results.
In experiment 1, fluids were focused in the joint system due to a permeability
ratio greater than 1. This is consistent with one conclusion from the analytical
solutions that a permeability ratio larger than 1 is a prerequisite for fluid focusing in
joints. In experiment 2, elevating vertical permeability of the fourth and fifth zones is
equivalent to increasing the permeability ratio. An increase of flow velocity is
consistent with a positive correlation between fluid focusing and permeability ratio. In
experiment 3, the high aspect ratio in the lower part of the joint system led to
accelerated flow, which is in agreement with the analytical solutions. In experiment 4,
high permeability ratio and aspect ratio in the fourth and fifth zones exerted the most
significant influence on fluid focusing in joints. This is consistent with the nonlinear
relationship of the analytical solutions. The comparisons above suggest that the
numerical experiments match well with the analytical solutions.
8.2. Insights from simulations
The nature of the initial fracture system is of vital importance to the subsequent
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development of vein-style mineralization (Sanderson et al., 2008). Numerical
experiments suggest that the NWW-striking joints acted the main pathways
channelizing fluids. In experiment 1, fluid flow was most focused in the third zone
after a short geological time. However, this ideal case does not agree with the
constraint from fractal methods that fluid flow was most concentrated between the
fourth and the fifth zone. Increasing the permeability ratio (experiment 2) or aspect
ratio (experiment 3) in the fourth and fifth zones improved the degree of fluid
focusing and lowered the focusing location. Experiment 4 indicates that increasing the
permeability ratio and aspect ratio in the fourth and fifth zones together had the most
significant impact on fluid flow. In this case, fluid flow was most focused in the fourth
zone, therefore allowing for the greatest potential to transfer heat and mass. This
implies that high permeability and aspect ratios in the lower part of the joint system
are a necessary but an insufficient condition for precipitating quartz in the fourth
zone.
Permeability and aspect ratios in the joint system are closely related to stress
state during mineralization. Previous numerical models (Sanderson and Zhang, 1999;
Zhang and Sanderson, 2001) indicate that loading direction exerts an important
influence on deformation, and that only those fractures parallel to maximum principle
stress remain open. Hence, the maximum principle stress that was vertical during
mineralization improved the connectivity of subvertical NWW-striking joints,
therefore increasing their permeability ratio. Those models also suggest that enhanced
fluid flow is extremely localized in small areas of the fracture network when a critical
stress state is reached. Fluid pressure in the lower part of the joint system was much
higher than the upper part, allowing it to reach the critical stress state easily. If so,
fluid flow would be highly localized in a few large fractures. This localization of
deformation in the lower part of the deposit corresponds to a high aspect ratio. For the
upper part of the joint system, it is relatively difficult to reach the critical state due to
low fluid pressure. This leads to uniformly distributed apertures and low flow velocity.
In this case, the aspect ratio in the upper part was lower than in the lower part.
Therefore, fluid focusing influenced by permeability ratio and aspect ratio is
essentially controlled by the coupling of mechanical deformation and fluid flow.
8.3. Control of regional structure
The orientations of vein-style tungsten-tin deposits are controlled by granite
emplacement and crystallization or by external stress regimes (Černý et al., 2005). In
this part we focus on the relationship between regional tectonic evolution and the
approximate EW-striking joints.
Regional tectonic stress, as the intermediate principle stress, is interpreted to
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have controlled the joint orientation at the Dajishan tungsten deposit (Zhou, 2009).
There is also a general agreement that this preferred orientation of ore-bearing
structures in the same type of deposits in South China was controlled by regional
tectonic evolution from the Indosinian (257~205 Ma) to Yanshanian (205~135 Ma)
(Liu and Ma, 1993; Mao et al, 2013; Zhou, 2009). During the Indosinian epoch,
widespread EW-striking folds and thrusts in the Nanling Range were formed in a N-S
compressive stress regime dominated by the Tethyanorogenic domain, and resulted in
nucleation of approximate EW-trending joints; In the Yanshanin epoch, regional stress
regime was changed to a WNW-ESE compressive stress regime controlled by
paleo-Pacific tectonic domain (Wan, 2012), which may have played a role in further
propagation of EW-trending mechanical weakness (Wan, 2012). This tectonic
evolution is supported by the time-space distribution of Mesozoic granitoids (Zhou,
2006) and tungsten-tin ore deposits in South China (Mao et al, 2013). Without the
influence of regional structure, it is difficult to explain the vein-type tungsten deposits
that were formed within the short time of 160-150Ma in South China (Mao et al,
2013), over 90% of which trend approximate EW (Zhu et al, 1981). Note that the
vein-type tungsten deposits having morphological zonation can occur in the strata
from the Sinian to Jurassic and in many types of sedimentary and magmatic rocks (Gu,
1984). Additionally, granite related to these deposits is F-rich and B-poor (Chen et al.,
1989; Zhang et al., 2004). Without these oriented joints, F-bearing magma often
creates disseminated rather than vein-style mineralization due to low amount of
mechanical energy released during crystallization (Pollard et al, 1987). These
constraints imply that regional structure played an important role in controlling
large-scale distribution of vein-type tungsten deposits in South China.
8.4. Limitations
Vein thickness data in the other three zones are absent because the upper part of
the vein sets is inaccessible. This constrains the trend of fractal dimension to a narrow
range. Also, the morphological characteristics do not incorporate vein spacing, an
important parameter to infer the link between vein clustering and fluid focusing
(Gillespie et al., 1999). Due to the inadequate data, the vertical morphological
zonation we illustrated is not a full map of the five-floor zonation, but it is an
important part of it.
One deficiency of using the permeability tensor is the assumption of infinite
fractures (Gupta et al., 2001), which neglects the connectivity of the joint networks.
Actually, only a small part of connected joints dominate fluid transport (Taylor et al.,
1999). This implies that a better tool incorporating connectivity structure (Renard and
Allard, 2013) is important to model hydrodynamic processes of mineralization.
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Numerical simulations only dealt with fluid flow in the joint system.
Temperature played a minor role due to its front much slower than pressure front and
limited time. To improve the model, it is necessary to further investigate fluid flow
coupled with other physical and chemical processes in the vein-type tungsten deposits.
In this case, permeability is not constant any more, but varies as physical properties of
rock are changed by deformation, temperature, and chemical reactions.
9. Conclusions
Using the five-floor zonation terminology, the chosen data in the middle set
represents the fifth zone, and the data in the north set correspond to the fourth floor.
Fractal methods suggest that the growth mechanisms of veins in the fourth and fifth
zones were mainly controlled by growth rate. The fractal dimension of vein thickness
indicates that the proportion of thick veins increases downwards in the fourth zone,
but decreases in the fifth zone. Therefore, the degree of fluid focusing would have
greatest between the fourth and the fifth zone. These insights improve on the use of
the five-floor zonation for exploration.
A Permeability tensor was used to model anisotropic permeability of oriented
joints at the Dajishan tungsten deposit. Numerical experiments based on the finite
element method suggest that NWW-striking joints in the host rock acted potential
conduits for mineralized fluids. Consistent with the analytical solutions, two
parameters have an impact on fluid focusing in these joints, permeability ratio and
aspect ratio. High permeability and aspect ratio in the lower part of the joint system
favors fluids focusing in the fourth zone, allowing the necessary fluids to precipitate
silica. Thus, the favorable condition for forming the five-floor type deposits is high
permeability and aspect ratio in the lower part of the joint system relative to higher
areas, a condition controlled by stress state during mineralization.
The origin of these orientated joints is related to regional tectonic evolution from
the Indosinian to Yanshanian in South China. These conclusions reached by the fractal
methods and numerical experiments present a clue to understand the vertical
morphological zonation in this type of deposits in this region.
Acknowledgements
The work is financially supported by the NSFC grant (41373048) and the MLR
special research fund (201411024). Xiangchong Liu also acknowledges the
scholarship from the CSC and the financial support from the GPMR Open Project
Program (NGPMR2011). We would like to thank Liqiang Yang, Shide Mao,
Yongquan Li, Zhiwei Tian, Huaizhong Yu, Jinfang Gao, Yan Liu, Nianchao Zhang,
Quanshu Li, and Paul D. Bons for helpful advices and discussions, as well as the
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staffs of the Dajishan Tungsten Company in field work. David Polya is appreciated
for presenting his paper about wolframite solubility. Xiangchong Liu thanks Chunxi
Lu for her continuing encouragement and appreciates the help of Bo Zhao, Rongzhen
Zhang. The related calculation was carried out by using PANDAS software on the
savanna supercomputer at UQ. We also thank Dr Peter Schaubs of CSIRO Earth
Science and Resource Engineering and an anonymous reviewer for their constructive
comments and advice to improve the manuscript significantly.
Appendix A
The unit permeability tensor of two sets of joints is as follows:
and are the unit permeability tensors of the first set of joints (9o 74
o) and
the second one (181o 66
o), respectively, and and are the corresponding
weighting coefficients. Their values calculated by equation (9)~(10) are:
,
,
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Fig.1. (a) A simplified map showing the tectonic units in the South China Block (modified from Guo et
al, 2011); (b) Regional geological map of the Dajishan tungsten deposit (Sun et al., 1985); (c), (d)
Plane map and cross section map of the Dajishan tungsten deposit, respectively (modified from Sun et
al., 1989)
1-fault; 2-reverse fault; 3-indeterminated fault; 4-geological boundary; 5-unconformity;
6-tungten-bearing quartz veins; 7-location of the sample lines of measuring vein morphology;
-granite; - two-mica granite;
-muscovite granite; -granite-porphyry; -quartz porphyry;
-diorite; Q-Quaternary; E-Paleogene; K-Cretaceous; J-Jurassic; C-Carboniferous; D-Devonian;
Є-Cambrian; A-A’ in Fig.1c is the cross-section line for Fig.1d and is also used in the numerical model.
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Fig.2 Schematic pattern of diagenesis and mineralization at theDajishan deposit (Kong, 1982)
I-Biotite granite; II-Two-mica granite; III1-Muscovite granite; III2-Stockscheiders; III3-Quartz phrase;
IV1- Microcline quartz veins; IV2- Beryl quartz veins; IV3-Wolframite quartz veins
Fig.3 Rose diagrams of dip direction of joints at theDajishan tungsten deposit
(a). 446 joint orientations were measured from the level of 317m to 985m by Zhou (2009). (b). To
supplement the data in Zhou (2009), 32 joint orientations were measured from the level of 267m to
467m. Both two diagrams show that the set of joints that dips NNE is more abundant than the one
dipping SSW.
Fig.4 Vein density increases with joint density. The data were measured at three levels, 467m, 317m,
and 267m. The average joint density is 0.83/m, and the average vein density is 0.42/m. The average
joint density in the north set is 0.67/m, 0.98/m both in the middle and in south sets.
a b
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Fig.5 (a) A simplified map of the deposit at level 467m. It shows the relative location of DP1 to the
deposit. DP1 lies at 467m level. (b) The geochemical profile DP1 and its related geological cross
section. It starts with east of the middle set and strikes 110o, covering 220m. DP1-1 is the starting point.
Blocky and fragmented hornfels occurs in 0-55m, where 2-6cm thick quartz veins trend 5-12o and dip
75o, occurring wolframite and sulfide. In 55-74m, hornfels is well fragmented and has 1cm thick quartz
veins that strike approximate E-W. The fracture zone is the Dajishanfeng Fault. In 74-220m,
metamorphic sandstone is silicified and few barren quartz veins occur. Grades of WO3 drop from 1156
ppm at DP1-1 to 10 ppm at DP1-12.
Fig.6 (a). Euhedral wolframite often occurs in thick veins, the photo taken at 417m level in the north
set with a hammer as the scale. The morphology of wolframite implies that fracture aperture was large
and was held open for enough time for crystal growth; (b) Parallel thread veins (<1 cm in thickness)
outcrop at 985m level (Zhou, 2009). Veins in (a) and (b) trend NWW.
Wolframite Thread veins a b
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Fig.7 Vein thickness distributions
(a)-(b) are fitted power law distribution and negative exponential distribution for vein thickness in the
north set, respectively. (c)-(d) are fitted power law distribution and negative exponential distribution for
vein thickness in middle set, respectively. Fractal dimensions of thickness are the fitted slopes.
Fig.8 The geometry model used in the numerical simulations
The location of cross section A-A’ is shown in Fig.1c. It trends NNE, normal to the vein sets. The
Z-axis represents the paleodepth. Є-1 stands for the contact zone, and joint system consists of Є-3, Є-4,
Є-5, Є-6, and Є-7, corresponding to the fifth floor to the first floor. Each part of the joint system is
0.3km high and 0.015 wide, the same aspect ratio of 2. Magmatic fluids are released from the top of
magma. The boundary conditions include 100oC and 40Mpa at top and 600
oC and 170MPa at the top of
magma. The right and left sides have free boundary conditions.
a b
c d Power law distribution Negative exponent distribution
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Fig.9 The principle permeability in the coordinate system used in the simulations
Fig.10 Numerical results in Experiment 1. (a), (b) Distribution of fluid pressure and flow velocity after
1 year; (c), (d) Distribution of fluid pressure and flow velocity after 20 years. The arrows show the
direction of fluid flow. The unit of pressure is Pa, and the unit of velocity is m/s.
b
a
After 1 year
Maximum: 9.4╳10-8
m/s
After 20 years
c
d
Maximum: 8.2╳10-8
m/s
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Fig.11 Numerical results in Experiment 2. (a), (b) Distribution of fluid pressure and flow velocity after
1 year; (c), (d) Distribution of fluid pressure and flow velocity after 20 years.
Fig.12 Numerical results in Experiment 3. (a), (b) Distribution of fluid pressure and flow velocity after
1 year; (c), (d) Distribution of fluid pressure and flow velocity after 20 years.
b
a
After 1 year
Maximum: 1.4╳10-7
m/s
After 20 years
c
d
Maximum: 1.1╳10-7
m/s
b
a
After 1 year
Maximum: 1.5╳10-7
m/s
After 20 years
c
d
Maximum: 1.3╳10-7
m/s
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Fig.13 Numerical results in Experiment 4. (a), (b) Distribution of fluid pressure and flow velocity after
1 year; (c), (d) Distribution of fluid pressure and flow velocity after 20 years.
Table 1 Database of veins in the middle and north sets
Vein
set Level(m) Section Length(m)
Vein
number
Vein
density(/m) Strain (%)
Average
strain (%)
Average vein
density (/m )
Average vein
thickness (m)
Middle
set
467
1 47 23 0.49 8.37
9.25 0.47 0.19 2 20 11 0.55 3.83
3 22 6 0.27 4.91
4 15 9 0.58 19.91
317
5 29 14 0.48 6.20
3.92 0.39 0.10 6 17 6 0.35 4.63
7 23 8 0.35 0.93
North
set
467
8 30 12 0.40 4.64
4.00 0.43 0.08 9 74 34 0.46 2.82
10 50 22 0.44 4.54
317
11 14 6 0.43 5.16
4.59 0.26 0.21
12 27 7 0.26 4.28
13 37 7 0.19 3.13
14 37 8 0.22 5.42
15 32 6 0.19 4.95
b
a
After 1 year
Maximum: 2.0╳10-7
m/s
After 20 years
c
d
Maximum: 1.6╳10-7
m/s
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Table 2 Summary of the results from fractal methods
Vein set Level (m) Dt-Power Dt-Negative c R2
North
set
467 0.92 7.82 0.91 0.51
317 0.85 4.20 1.14 0.59
Middle
set
467 0.78 3.81
317 0.86 6.19
Dt-Power represents the fractal dimension of power law distribution, Dt-Negative represents
the fractal dimension of negative exponential distribution; R2 is the fit goodness of power-law
relationship between vein thickness and length.
Table 3 Hydraulic parameters in experiment 1
Unit Magma Є-1 Є-2 Є-3,Є-4,Є-5,Є-6,Є-7
kx (m2) 1.0╳10
-18 1.0╳10
-16 2.0╳10
-16 3.0╳10
-15
kz (m2) 1.0╳10
-18 1.0╳10
-16 2.0╳10
-16 3.0╳10
-15
Φ 0.0015 0.015 0.02 0.05
The porosity and permeability are cited from Sheldon and Ord (2005). The density and dynamic
viscosity of fluid are 1╳103 Kg
.m
-3 and 1.3╳10
-4 Pa
.s, respectively. The specific heat capacity of fluid
and rock are 1000 J.kg
-1.K
-1 and 4200 J
.kg
-1.K
-1, respectively. The thermal conductivity of fluid and rock
are 0.6 W.m
-1.K
-1 and 2.0 W
.m
-1.K
-1, respectively.
Table 4 Results of the four numerical experiments
Numerical Experiment 1 2 3 4
Pressure increment during 50
days after 20 years <7Pa <16Pa <6Pa <16Pa
Velocity ratio after 1 year 1.1 1.4 1.9 2.4
Velocity ratio after 20 years 1.0 1.3 1.6 2.0
Pressure change during 50 days indicates that the four experiments reached steady state, given that the
pressure gradient after 20 years is approximately 105Pa/m.
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Highlights
1. Many vein-type tungsten deposits in China have vertical morphological zonations.
2. We build a link between fluid focusing and the morphological zonation.
3. Permeability ratio and aspect ratio influence the fluid focusing in joints.
4. Fractal analysis on vein morphology helps the use of the zonation in exploration.
Graphical Abstract
Fractal analysis of vein morphology at the Dajishan tungsten deposit, South China
indicates that the lower part has a relative high amount of thick veins, and that the
growth mechanisms of veins in the lower part of ore bodies were mainly controlled by
growth rate. The existing oriented joints above granite became the main pathways for
mineralized fluids. The higher permeability and aspect ratios in the lower levels of the
joint systems lead to the more fluid focusing in the lower part of ore bodies, which
delivers more fluids required for precipitating silica.