-
oni
2, W
Article history:Received 24 September 2012Received in revised
form30 July 2013Accepted 22 August 2013Available online 31 August
2013
Keywords:Veins
exceeds the stresses acting on the fracture/fault plane, thus
causingfailure and mesh development. Subsequently, the uid ows
intothe mesh of fractures, a phenomenon analogous to burping(Sibson
and Scott, 1998), and the uid pressure suddenly drops.
veins (hydrother-., 1991, 2001; Cox,ns seal the faults,ext cycle
of uidnd vein formationcrosscutting veins1996; Miller et al.,and
uid pressure
conditions necessary to open pre-existing fractures during
veinformation is critical in the evaluation of the structural
control onmineralization (including gold).
In the past, 3-D Mohr circle analysis has been carried out
usingvein orientation data in order to evaluate the
relativemagnitudes ofstress and uid pressure at the time of
fracture opening (e.g.,Indarama lode-gold deposit of Zimbabwe;
McKeagney et al., 2004;also see Yamaji et al., 2010). Themethodwas
developed by Jolly andSanderson (1997) to examine the conditions of
stress and uid
* Corresponding author. Tel.: 91 3222 283388; fax: 91 3222
255303/282700.E-mail addresses: [email protected],
[email protected]
Contents lists availab
Journal of Struc
w.e
Journal of Structural Geology 56 (2013) 45e56(T.K. Mondal),
[email protected] (M.A. Mamtani).1. Introduction
One of the important mechanisms for the formation of veinsthat
host mesothermal lode-gold deposits is fault-valve action(Sibson et
al., 1988; Sibson, 1992, 1996, 2000; Boullier and Robert,1992;
Sibson and Scott, 1998; Cox et al., 2001 among others).
Thisrequires presence of fractures that are misoriented for
reactivationand a uid pressure build up (supralithostatic levels)
so that it
This drop in uid pressure leads to formation ofmal
precipitation) and ore deposition (Cox et al1995; Wilkinson and
Johnston, 1996). The veiand the system is once again ready for the
npressure build-up, failure, uid ow (burping) a(in that order);
repeated cycles (above) lead toand fractures (mesh-like structure;
Sibson, 1992,1994). Thus, understanding the relative stressMohr
circleFluid pressureGold mineralizationAnisotropy of magnetic
susceptibilityGadagIndia0191-8141/$ e see front matter 2013
Elsevier Ltd.http://dx.doi.org/10.1016/j.jsg.2013.08.005In this
paper orientations of quartz veins from the Archaean age lode-gold
bearing region of Gadag(southern India) are used to determine the
relative stress and uid pressure (Pf) conditions by con-structing
3-D Mohr circle. Anisotropy of magnetic susceptibility (AMS)
analysis of the host massivemetabasalt reveals that the magnetic
foliation is NWeSE striking, which is related to early
NEeSWcompression (D1/D2 deformation) that affected the region. The
quartz veins have a wide range of ori-entations, with NWeSE
striking veins (steep northeasterly dips) being the most prominent.
Veinemplacement is inferred to have taken place under NWeSE
compression that is known to have causedlate deformation (D3) in
the region. It is argued that the NWeSE fabric dened the
pre-existinganisotropy and channelized uid ow during D3. The
permeability was initially low, which resulted inhigh Pf (>s2).
3-D Mohr circle analysis indicates that the driving pressure ratio
(R0) was 0.94, a conditionthat favoured fracturing and reactivation
of fabric elements (foliations and fractures) having a wide rangeof
orientations. This led to an increase in permeability and uid owed
(burped) into the fractures.Resulting vein emplacement and sealing
of fractures led to a reduction of Pf (s2). Thus, it is concluded
that the quartz veins inthe Gadag region are a consequence of an
interplay between conditions that uctuated from Pf > s2 toPf
< s2.
2013 Elsevier Ltd. All rights reserved.a r t i c l e i n f o a b
s t r a c t3-D Mohr circle construction using vein(southern India)
e Implications to recog
Tridib Kumar Mondal*, Manish A. MamtaniDepartment of Geology
& Geophysics, Indian Institute of Technology, Kharagpur
72130
journal homepage: wwAll rights reserved.rientation data from
Gadagze uid pressure uctuation
est Bengal, India
le at ScienceDirect
tural Geology
lsevier .com/locate/ jsg
-
1400
b
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78 00
0 N
- - - - - - - -
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-
Supracrustals(WDC)Peninsular Gneiss(Basement)
Chitradurga
Shimoga
Gadag
- - -- - - -
- - -
100 km
EDC
Arabian Sea
GadagWDC
EDC
CD
DT
SGT
14
10
8076
Delhi
Gadag
a
Hyderabad
ChennaiBangalore
N100 km
Arabian Sea
GADAG
MUNDARGI
MULGUND
1500
1530
75 30 76 00
HOSUR
Metabasalt
Metamorphosed Greywacke-Argillite
Polymict Conglomerate
Ferruginous Chert
c
SHIRHATTI
0 12 kmN
ATTIKATTI
T.K. Mondal, M.A. Mamtani / Journal of Structural Geology 56
(2013) 45e5646
-
pressure responsible for the opening of pre-existing
fracturesduring dyke emplacement. It was based on the approach
adoptedby Delaney et al. (1986) according towhich older joints can
be lledup by magma if magmatic pressure exceeds the horizontal
stress
coaxial with NW striking axial plane. According to Chadwick et
al.(2003), NEeSW compression was responsible for NWeSE
orientedstructural elements in the Gadag schist belt. D3 is
characterized byregional warps with NE striking vertical axial
plane; this super-
r Sars. Daical m
T.K. Mondal, M.A. Mamtani / Journal of Structural Geology 56
(2013) 45e56 47acting across the joint plane. McKeagney et al.
(2004) argued thatsince the formation of veins, like dykes, is also
dependent on therelative stress and uid pressure, these conditions
could beanalyzed using the 3-D Mohr circle construction suggested
by Jollyand Sanderson (1997). In the present paper, the authors
presentvein orientation data from the Gadag region (southern
India),which is a province of lode-gold deposit (Curtis and
Radhakrishna,1993; Radhakrishna and Curtis, 1999). In accordance
with the workof Jolly and Sanderson (1997) and McKeagney et al.
(2004), 3-DMohr circle is constructed using these data to analyze
the relativestress/uid pressure conditions that led to vein
formation. It isargued that vein emplacement took place by
fault-valve action,which (as mentioned above) involved cyclic rise
of uid pressureand fracturing/faulting as well as intermittent fall
of uid pressureduring uid ow and vein formation. According to the
authors, thisuctuation of uid pressure can be recognized from the
distribu-tion of vein orientations from Gadag and their 3-D Mohr
circleanalysis.
2. Geology of the study area
In this study, the quartz veins from the vicinity of Gadag
locatedin southern India have been investigated (Fig. 1a). The
southernIndian shield comprises two major units e Dharwar Craton
andSouthern Granulite Terrain (SGT) (e.g., Naqvi and Rogers,
1987;Chakrabarti et al., 2006). The Peninsular Gneiss (3.4e3.0
Ga)comprises the oldest rocks of the Dharwar Craton (Beckinsale et
al.,1980; Taylor et al., 1984; Sarma et al., 2011). This craton is
consid-ered to have formed by the accretion of East Dharwar Craton
(EDC)and West Dharwar Craton (WDC) at 2.75e2.51 Ga, and the zone
ofaccretion is marked by a shear zone; this is variously referred
to asthe Chitradurga Boundary Fault by some workers, and an
easterlydipping thrust by others (e.g., Naqvi and Rogers, 1987;
Chadwicket al., 2003; Meert et al., 2010). The eastern part of WDC
com-prises Chitradurga schist belt and Gadag lies in the
northernmostpart of this belt (Fig. 1b). The 120 km2 area to the
south of Gadag iscommonly referred to as the Gadag schist belt
(Fig. 1c), whichcomprises metabasalts (greenschist facies/lower
amphibolite faciesmetamorphism), metamorphosed greywacke-argillite
(inter-banded with ferruginous chert and banded iron formation),
poly-mict conglomerate, and ferruginous chert. Limited
metamorphicand geochemical studies have been carried out on the
area in thepast. These indicate actinolite, albite, chlorite,
epidote, quartz andmagnetite are present in the basaltic rocks, and
that geochemicallythey maybe considered of island-arc afnity
(Chakrabarti et al.,2006). Regionally, this belt trends NWeSE and
is surrounded byolder Peninsular Gneisses and younger granitoids
(around Mul-gund; Fig. 1c). The Moyar Bhavani Shear Zone (dashed
line inFig.1a) separates the Dharwar craton from the SGT (w2.5 Ga;
Plavsaet al., 2012) lying to its south.
Previous structural geological studies on the Gadag schist
belthave revealed three phases of deformation viz. D1, D2 and
D3(referred to as FD1, FD2 and FD3 by Beeriah et al., 2001;
Chakrabartiet al., 2006). D1 folds are very tight, isoclinal and
asymmetric, whileD2 folds are reported to be open to tight upright.
Both these are
Fig. 1. (a) Map showing location of Gadag region in the southern
Indian shield (afteWDC Western Dharwar Craton; CD Cuddapah Basin
rocks; DT Deccan Trap basaltof (b) is demarcated by the rectangular
box (dashed) around Gadag. (b) Regional geolog
Gadag schist belt that is shown in (c). (c) Simplied geological
map of the Gadag schist belt (box. White lines in the vicinity of
Hosur and Attikati represent the gold bearing lodes. Blacimposition
resulted in the development of dome-basin structure inthe region
(Chakrabarti et al., 2006). Geophysical (gravity) in-vestigations
have also revealed presence of NEeSW trending steepdeep-seated
faults that dissect NWeSE trending structures of theGadag schist
belt (Ramadass et al., 2003, 2006). Thus, the abovestudies indicate
that the structural evolution of the Gadag schistbelt is controlled
by early NEeSW compression followed by lateNWeSE compression.
The Gadag region is a province of gold mineralization, which
hastaken place in quartz veins that form lodes. These lodes
aredistributed in different parts of the area, especially east of
Hosurand around Attikatti (Fig. 1c). The general trend of most
lodes isNWeSE (Fig. 1c). Recently Sarma et al. (2011) performed
geochro-nological studies (UePb dating of monazite and xenotime) on
rocksof the region and concluded that the gold mineralization
occurredat w2.52 Ga. They suggested that the hydrothermal activity
andresultant gold deposition could be related to the nal stages
ofcratonization of the Dharwar craton. The present study is
concen-trated on vein orientation and fabric data from the vicinity
of Hosurthat lies 20 km to the SWof Gadag (Fig. 1c), and as stated
in Section1, the objective is to analyze the relative stress/uid
pressure con-ditions that led to vein formation.
3. Methods of analysis and results
Lithologically the region around Hosur comprises metabasaltsthat
are massive and devoid of any prominent mesoscopic foliation(Fig.
2a and b). Quartz veins have emplaced in the metabasalts. Atplaces,
veins of one orientation are dissected and displaced (offset)by
veins of another orientation that implies different phases of
veinemplacement (Fig. 2b). At several places, fractures and quartz
veinsform a mesh (Fig. 2c). It has been argued by Sibson (1992)
that sucha mesh forms when a rock contains fractures that are
severelymisoriented for reactivation under a prevailing stress eld,
but getreactivated due to rise in uid pressure (supralithostatic
levels). Inthe Gadag region veins of several orientations
showmutually cross-cutting relationships (Fig. 2c), which implies
repeated cycles of veinemplacement (Sibson,1992). Although veins of
various orientationsare recorded from the area, a majority of them
are NWeSE striking(maximum range N310eN315) with sub-maxima in
NEeSW di-rection (Fig. 2d). Field studies reveal that quartz grains
growperpendicular to the vein wall indicating that dilation was
impor-tant and that these are extensional veins (Fig. 3). It is
observed that,there are large numbers of fractures in the rocks
that are not lledby quartz veins (Fig. 2c). Although these have
various orientations,NWeSE and NEeSW striking fractures are the
most common(Fig. 2e).
It maybe noted that NWeSE direction, which denes themaximum
orientation of quartz veins, is also the orientation of theboundary
between WDC and EDC, the overall trend of the schistbelt, the
orientation of the structural elements developed duringthe D1 and
D2 folds, the direction of compression that led to D3, aswell as
the orientation of most mineralized lodes of the region.
Thisimplies that there is a strong structural control on the
formation ofthese veins (and gold deposition). These veins, as
mentioned above,
ma et al., 2012). SGT Southern Granulite Terrain; EDC Eastern
Dharwar Craton;shed line represents the Moyar Bhavani Shear Zone.
Inset shows the map of India. Areaap of Dharwar craton (after
Chadwick et al., 2003). Dashed box near Gadag marks themodied after
Curtis and Radhakrishna, 1993). The study area is enclosed in the
dashedk arrow points to the area around Hosur that is shown in Fig.
4a.
-
of ST.K. Mondal, M.A. Mamtani / Journal48occur in massive
metabasalt, which does not show a eld foliation.However, such
visibly massive rocks may preserve an internalfabric, which in many
cases can be recognized on the basis ofanisotropy of magnetic
susceptibility (AMS) studies (e.g., Tarlingand Hrouda, 1993;
Mamtani and Greiling, 2005; Raposo et al.,2007; Loock et al., 2008;
Mamtani and Sengupta, 2010). From theworks of Sanderson and Zhang
(1999) and Cox et al. (2001) it isknown that pre-existing
anisotropy can play a critical role inchannelizing uids in nature.
It is also known from rock mechanicsinvestigations that
well-foliated rocks (such as schists) are weakerin directions
parallel to the foliation than perpendicular to it (e.g.,Tsidzi,
1990). In a recent study on massive quartzites that did notcontain
visible foliations, Vishnu et al. (2010) identied the folia-tion
from AMS studies and demonstrated that the strength of therock is
higher perpendicular to the magnetic foliation than parallelto it.
This implies that in massive rocks such as the metabasalts ofthe
study area (which host quartz veins), it is important to
analyzetheir internal fabric vis--vis vein orientation to evaluate
the con-trol that the fabric may have had on vein emplacement. The
authorshave performed an AMS analysis of metabasalts to identify
thefabric in them, and 3-D Mohr circle construction using quartz
veinorientations to analyze the relative stress/uid pressure
conditionsunder which they were emplaced. A combination of data
from boththese methods provides a comprehensive evaluation of the
condi-tions of vein formation in the Gadag region.
Fig. 2. (a) and (b) are representative eld photographs
documenting quartz veins in massiveand quartz veins in metabasalt
near Hosur. Camera facing due north and due west in (a) andstudy
area. (e) Rose diagram of 111 fracture orientations around quartz
veins highlightingtructural Geology 56 (2013) 45e563.1. Fabric
analysis using AMS in metabasalt
AMS analysis involves inducing magnetism in a sample indifferent
directions and measurement of the induced magnetiza-tion in each
direction. The magnetic susceptibility (K in SI units) ofa mineral
relates the induced magnetization (M) to the externalmagnetic eld
(H) into which it is immersed by the relationM KH. The induction
takes place along the long crystallographicaxis of the mineral
(long shape axis in case the mineral belongs toisometric system) of
the mineral. Therefore, its magnetic suscep-tibility is not the
same in every direction, a property that isreferred to as
anisotropy of magnetic susceptibility (AMS). In thecase of
naturally deformed rocks, minerals tend to develop apreferred
orientation, and the above property of AMS is useful inpetrofabric
analysis. AMS analysis gives the orientations andmagnitudes of the
three principal axis of the magnetic suscepti-bility ellipsoid viz.
K1, K2 and K3, where K1 > K2 > K3. The mag-nitudes are then
used to determine the mean susceptibility (Km),strength of magnetic
foliation (F), strength of magnetic lineation(L), shape parameter
(T) and the degree of magnetic anisotropy (Pj).T denes the shape of
the AMS ellipsoid and its value variesfrom 1 to 1; negative and
positive T values indicate prolate andoblate shapes, respectively.
Pj is a measure of the eccentricity of theAMS ellipsoid (see
Tarling and Hrouda, 1993 for detailed formulae).Further, it maybe
noted that the orientation of K1 denes the
metabasalt of the Gadag region. (c) Field photograph documenting
a mesh of fractures(c) respectively. (d) Rose diagram of
orientations of 296 quartz veins recorded from thethat most
fractures have NWeSE or NEeSW strike.
-
l of ST.K. Mondal, M.A. Mamtani / Journamagnetic lineation, and
K3 is the pole to the magnetic foliation(K1K2 plane).
In the present study, AMS was measured using the
KLY-4SKappabridge (AGICO, Czech Republic) in the Department of
Geol-ogy & Geophysics, Indian Institute of Technology,
Kharagpur (In-dia). Oriented samples of metabasalts from a total of
31 sites fromthe vicinity of Hosur were collected (Fig. 4a).
Multiple cores(2.54 cm diameter, 2.2 cm length) were investigated
from eachsample; a total of 189 cores from the 31 sites were
analyzed. Usingthe program Anisoft (version 4.2; AGICO, Czech
Republic), meanvalues of the various AMS parameters (Jelinek
statistics, Jelinek,1981) were calculated for each site. It is
noted that the Km variesbetween 470 and 69,700 106 SI units, with
most of the sampleshaving Km below 1000 106 SI units. This
indicates that para-magnetic and ferromagnetic minerals contribute
signicantly tothe AMS. Petrographic studies reveal the presence of
actinolite,hornblende, chlorite, albite, epidote, pyrite and
magnetite, whichare inferred to contribute to the high positive
susceptibilitiesrecorded in the samples. The Pj lies between 1.025
and 1.35, and theshape of the AMS ellipsoid is dominantly oblate
(positive T values).Since the present study is focused on
evaluating the orientation offabric in metabasalts vis--vis vein
orientation, only the magneticfoliation and lineation orientation
data are presented here (Fig. 4b).The reader is referred to
Supplementary Data-1 for complete AMSdata of the 31 samples
analyzed. It is noted that the magneticfoliation (K1K2 plane) is
consistently NWeSE striking (mean
Fig. 3. Field photographs documenting quartz grains growing
perpendicular to the vein walthe area shown in (b). Camera faces
due SE in (a), (b) and (c). In (d), (e) and (f) camera factructural
Geology 56 (2013) 45e56 49orientation: N336E/60 towards NE; Fig.
4b). The magnetic linea-tions plunge to the NW and SE (crosses in
Fig. 4b).
3.2. 3-D Mohr circle construction from vein orientation
Here, the present authors have used the method proposed byJolly
and Sanderson (1997) to analyze the relative stress/Pf condi-tions
that led to vein emplacement in the metabasalts of Gadagregion
(southern India). The methodology involves the use of
lowerhemisphere equal area projection of poles to veins.
Dependingupon the type of distribution (clustered or girdle) of the
poles, theorientations of the principal stress axes (s1, s2 and s3)
are obtained.Also, the angle (q) between fractures susceptible to
reactivation andprincipal stresses are determined. Here q1 is the
angle between s2and poles to range of fractures susceptible to
reactivation lying onthe s2s3 plane. q2 is the angle between s1 and
poles to range offractures susceptible to reactivation lying on the
s1s3 plane. q3 isthe angle between s1 and range of poles to
fractures susceptible toreactivation lying on the s1s2 plane.
According to Jolly andSanderson (1997), a clustered distribution is
indicative of Pf < s2and fractures with limited range of
orientations get reactivated.Conversely, a girdle distribution is a
result of Pf > s2, where frac-tures of a wide range of
orientations get reactivated. The 3-D workof Jolly and Sanderson
(1997) was developed theoretically from the2-D study done by
Delaney et al. (1986). Since McKeagney et al.(2004) have given a
detailed description of the theoretical basis
ls that indicate that the veins formed due to extension
(dilation). Arrow in (a) points toes due NW, S and W
respectively.
-
N0 200 mts
SHIRUNJ
HOSUR
1519
75
35
1518
1517
G63
G64
G66
G69
G71
G86
G80
G81
G128
G15
G14
G37
G62G16
G74
G72
G70
G65
G61G131
G12
G73
G76
G75
G9
G174G52G36
G54
G55
G49G59
G46
G58
G43
G44
G57
G13G60
G56
G87
75
36
a
INDEX
AMS Sampling SiteVein LocationLode
N
N336E/60
b
n=31: K , contour lines: K31
Fig. 4. Map showing the distribution of gold bearing lodes
within the massive metabasalt occurring in the vicinity of Hosur
(modied after Curtis and Radhakrishna, 1993). AMSsampling sites and
locations of veins are also shown. (b) Lower hemisphere equal area
projection of pole to magnetic foliation (K3) and magnetic
lineation (K1) recorded in themassive metabasalts around Hosur.
Dashed great circle (N336E/60NE) represents the mean orientation of
the magnetic foliation (K1K2) plane.
T.K. Mondal, M.A. Mamtani / Journal of Structural Geology 56
(2013) 45e5650
-
l of Sof this method, the present authors have preferred to
present onlythe salient aspects of the principles involved in the
3-D Mohr circleconstruction.
Delaney et al. (1986) gave two Equations (1) and (2) below
toexplain opening of a fracture under the condition Pf > sn
(normalstress)
Pf smax smin
2 smax smin
2cos2q: (1)
R Pf smaxsmin
2smaxsmin
2 Pf sm
smax cos2q: (2)
Here, R is referred to as the driving stress ratio, smax and
smin arethe maximum and minimum principal stress respectively, smax
isthe maximum shear stress and sm is the mean stress. q
representsthe angle between the normal to the fracture plane and
maximumprincipal stress, which illustrates the range of
orientations offracture that can open under a given Pf
condition.
Subsequently, Baer et al. (1994) extended the work to 3D
andintroduced equations (3) and (4) to describe the state of stress
andPf.
R0 Pf s3s1 s3
: (3)
F s2 s3s1 s3
: (4)
Here, R0 is the driving pressure ratio and F is the stress
ratio.Fractures can open only if R 0 > 0. If Pf s3, then R 0 0,
whichimplies that no fracture will open. If Pf s1, then R 0 1,
whichindicates that fractures of all possible orientations are
susceptiblefor dilation/reactivation.
The above theory was further integrated with
stereographicprojections of dike orientations by Jolly and
Sanderson (1997)based on the principle that e (a) clustered
distribution of poles todike orientations is obtained if dikes have
similar orientations,implying that only a limited range of
fractures can dilate to allowdike emplacement; here Pf < s2 (b)
girdle distribution of poles todikes is obtained if they have
varied orientations, implying that awide range of fractures dilated
to allow dike emplacement; herePf> s2. According to them, in
case of a girdle distribution of poles todikes, an empty elliptical
space can be identied, the centre ofwhich denes s1 (i.e., normal to
the girdle distribution). This helpsrecognize s1s2 and s1s3 planes
and the angles q2 and q3 on therespective two planes. In case of a
clustered distribution of poles todikes, an ellipse enclosing the
cluster maxima enables identify s3,and helps dene the s1s3 and s2s3
planes along which the angles q1and q2 can be respectively
measured. Following Baer et al. (1994)and Jolly and Sanderson
(1997), the driving pressure ratio (R0 )can be calculated using
Equation (5), and stress ratio (F) can bedetermined using Equations
(6) and (7) in case of Pf < s2 (clustereddistribution) and
Pf> s2 (girdle distribution), respectively. Based onthese data,
3-D Mohr circle can be constructed.
R0 Pf s3s1 s3
1 cos2q22
: (5)
F s2 s3s1 s3
1 cos2q21 cos2q1
: (6)
s2 s3 1 cos2q2
T.K. Mondal, M.A. Mamtani / JournaF s1 s3
11 cos2q3
: (7)It maybe noted that in the present study the terms girdle
andcluster distribution are used in a descriptive (qualitative) way
aswas done by Jolly and Sanderson (1997), and subsequently
appliedby several other researchers (e.g., McKeagney et al.,
2004;Mazzarini et al., 2010); these terms must not be confused
withthose used by Woodcock (1977) for dening shape of
distributionby calculating eigen values. As shown in the rose
diagram of ori-entations of 296 quartz veins recorded in the study
area, themaxima ranges from N310eN315 (Fig. 2d). Of these, 120
quartzveins were exposed in 3D for which complete orientation
data(strike/dip amount/dip direction) were recorded. Following
Jollyand Sanderson (1997), the poles to these veins are plotted
inFig. 5a. The distribution of the poles is dominantly of girdle
typeindicating a large range of orientations. This girdle pattern
is alsorevealed in the contoured diagram (Fig. 5b). For this girdle
distri-bution, an elliptical open space is recognized that helps
determineorientation of s1, s2 and s3, and the angles q2 and q3
(Fig. 5c). Hereq2 14 and q3 44. Equation (5) was used to calculate
drivingpressure ratio (R0), which was found to be 0.94. Girdle
distributionindicates Pf > s2, and therefore Equation (7) was
used to calculatethe stress ratio (F), which was found to be 0.87.
Fig. 5d is the 3-DMohr circle construction using these data, which
indicates that Pfwas relatively high (close to s1), thus implying
that a wide range offabric elements (foliations and fractures) were
susceptible to dila-tion/reactivation to allow vein emplacement. It
may however benoted that although the overall distribution is of
girdle type, thereis a signicant cluster of poles in the southwest
(maximum at 42
towards N225E) implying that large number of veins strike
dueNWeSE with northeasterly dips (mean orientation:
N315E/48NE;dashed great circle in Fig. 5b). The implications of
this are discussedin Section 4.3.
4. Discussion
4.1. Relative timing of magnetic fabric development
vis--visregional deformation
As documented in Fig. 2, the metabasalts of Gadag region
thatcontain quartz veins (and lodes) are dominantly massive, and
donot have a well developed mesoscopic foliation. However,
AMSanalysis of these rocks reveals that the magnetic foliation
(K1K2) isconsistently NWeSE striking (mean strike N336E and 60 dip
to-wards the northeasterly direction; Fig. 4b). Regionally, the
orien-tation of themagnetic foliation is parallel to the general
trend of theGadag schist belt as well as to the zone of accretion
between theEDC and WDC (Fig. 1b and c). According to Chadwick et
al. (2003),the accretion between EDC and WDC is inferred to have
occurredduring 2.75e2.51 Ga. According to Sarma et al. (2011), the
volcanicsin the vicinity of Gadag arew2.58 Ga in age (UePb zircon)
and goldmineralization is w2.52 Ga (UePb monazite and xenotime).
Thepresent authors infer that the NWeSE oriented fabric in the
met-abasalts developed due to ductile deformation controlled by
NEeSW compression that resulted in D1 and D2 folds (Chakrabarti et
al.,2006), and NWeSE striking thrusts (Chadwick et al., 2003) in
thearea. Since the maximum extension direction during this
defor-mation was NWeSE oriented, some weak planes (mode-I
cracks)tended to develop in NEeSW direction (i.e., perpendicular to
theextension). This is manifested in the presence of NEeSW
orientedfractures recorded in the area (Fig. 2e). It is known that
late stageNWeSE compression led to warping of the Gadag schist belt
thatresulted in formation of dome-basin geometry in the argillites
(D3of Chakrabarti et al., 2006) as well as the development of
NEeSWoriented deep seated faults that dissect NWeSE oriented
structures(Ramadass et al., 2003, 2006). Magnetic lineations in the
meta-
tructural Geology 56 (2013) 45e56 51basalt are doubly plunging
(NW and SE; Fig. 4b). In the past,
-
Nn=120 n=120N
a b
d
f
2
2
n
n
2
2
n
n
Pf
P >f 2
P f 2
P s2. The empty elliptical space enables identify s1, and helps
dene the s1s2 and s1s3 planes.The angles q2 and q3 are determined,
which are then used to prepare the 3-D Mohr circle shown in (d). It
maybe noted that all the fractures (lled by veins) whose poles lie
outsidethe vacant ellipse in (c), and all the fracture orientations
that lie to the left of the grey line dening Pf in (d), are
susceptible to reactivation (see Section 4.2 for details). (e)
T.K. Mondal, M.A. Mamtani / Journal of Structural Geology 56
(2013) 45e5652
-
superposed deformation in rocks that lack mesoscopic
foliationsand lineations has been recognized from magnetic fabric
(e.g.,Mamtani and Sengupta, 2010). Accordingly, the variation in
plungeof magnetic lineation on the NWeSE striking magnetic
foliationplane is inferred to be due to superposition of D3 over
D1/D2deformation. Thus, based on AMS analysis it is clear that the
met-
high levels (supralithostatic). This led to dilation
perpendicular tothe magnetic foliation, and reactivation of
fractures of various ori-entations (Fig. 5d), thus resulting in
formation of a mesh of frac-tures, into which the uids burped and
quartz veins precipitated.As a consequence, the veins in the
metabasalt have a wide-range oforientations and a girdle
distribution on the equal area projection
thatto reduce, andy lin
T.K. Mondal, M.A. Mamtani / Journal of Structural Geology 56
(2013) 45e56 53abasalt already had a NWeSE oriented fabric before
the last (D3)deformation took place. According to the authors, this
played animportant role in vein emplacement (see Section 4.2
below).
4.2. Fabric in the metabasalt and its inuence on vein
emplacementat high Pf
It was mentioned earlier (Section 3) that (a) pre-existing
me-chanical anisotropies tend to localize uid ow (Sanderson
andZhang, 1999; Cox et al., 2001), and (b) rocks are weaker (i.e.,
frac-ture more easily) in a direction parallel to an existing
foliation(including magnetic foliation in massive rocks) than
perpendicularto it (Tsidzi,1990; Vishnu et al., 2010). Accordingly,
the authors inferthat NWeSE oriented fabric played an important
role in providingpathways for emplacement of quartz veins during
the third defor-mation (NWeSE compression). It is envisaged that
since the pre-existing foliation (magnetic fabric) was NWeSE
oriented, themetabasalt would have a lower strength in this
direction. As aconsequence, under NWeSE compression, the rocks
would bemostsusceptible to fracturing and would tend to dilate
perpendicular tothe magnetic foliation. This is one of the reasons
for the occurrenceof large number of veins and fractures in this
direction (Fig. 2d ande). 3-D Mohr circle analysis of vein
orientation data (Section 3.2)indicates that during vein
emplacement Pf was high (>s2). More-over, there is a sub-maxima
of vein orientations in NEeSW direc-tion (Fig. 2d) and the
metabasalt also has large number of NE-SWstriking fractures (Fig.
2e), which are inferred to have developedduring D1/D2 (as discussed
in Section 4.1). Based on all this infor-mation, it is concluded
that under NWeSE compression and high Pf,fractures and foliations
of all orientations got reactivated anddilated thus resulting in
emplacement of veins. Since NWeSE ori-ented fabric and NEeSW cracks
were most common, these werethe directions inwhichmaximumveins
emplaced, thus resulting inNWeSE oriented maxima and sub-maxima in
NEeSW direction(Fig. 2d). The overall pattern of the vein
orientations (poles) istherefore a girdle distribution described by
Jolly and Sanderson(1997), which implies that fabric elements of a
wide range of ori-entations get reactivated/dilated (Fig. 5a). This
indicates Pf> s2 (i.e.,s2 < Pf< s1), and the girdle is
normal to s1 (Fig. 5c and d). Based onthese plots it is therefore
logical to infer that in the present studyarea, Pf must have
reached supralithostatic levels in order to reac-tivate the
structural elements that dene the pre-existing anisot-ropy such as
foliation (direction dened bymagnetic foliation) and/or
faults/fractures. 3-D Mohr circle construction reveals that thes1s2
plane has N315E orientation, which is parallel to the
meanorientation of the veins in the region and also sub-parallel to
themagnetic foliation that denes the anisotropy in the basalts.
Thus,based on these data it is envisaged that uid ow tended to
localizetowards the pre-existing anisotropy in the metabasalt.
However,the permeability in the system must have been initially
weak as aconsequence of which uid ow was restricted and Pf rose to
very
Determination of state of stress when Pf < s2 considering
that at low Pf, fabric elements(1997), the poles to veins forming a
maxima in the SW (see a and b), can be consideredthose orientations
whose poles (black squares) were susceptible to reactivation at the
reand were inactive at low Pf. The ellipse enclosing the cluster
maxima enables identify s3then used to prepare the 3-D Mohr circle
shown in (f). Fractures to the left of the gre
emplacement (see Section 4.3 for details). sn and sn in (d) and
(f) represent the normal and sprincipal stresses,
respectively.(Fig. 5a and b). The authors envisage that when the
uid burped,fracture reactivation and vein emplacement continued
even as Pfdropped, which is discussed in the following section.
4.3. Vein emplacement at low Pf
Although the overall distribution of poles to veins is of
girdletype, there is a signicant cluster to the SW (Fig. 5b). Also,
there arelarge number of fractures and veins that cross-cut each
other toform a mesh, which indicates repeated cycles of uid ow
(veinemplacement). Thus the authors envisage that after the
initialburping of uid at high Pf into fractures of various
orientations,there was a reduction in Pf, and sealing of several
fractures tookplace. Since the NWeSE compression continued, only
the NWeSEoriented fabric elements (fractures and foliations) were
susceptiblefor reactivation at reduced Pf, while the other
orientationsremained inactive (sealed). Thus the dominance of NWeSE
ori-ented veins is inferred to be not only due to dilation that
took placeat high Pf (when veins emplaced in all orientations), but
also due tocontinued reactivation and vein emplacement at reduced
Pf whenonly the NWeSE fabric elements were suitably oriented for
reac-tivation. Therefore, the authors believe that the cluster of
poles toveins in Fig. 5 can be used to comment on the relative
stress/uidpressure conditions that prevailed at low Pf. Following
Jolly andSanderson (1997), an ellipse is drawn around the region of
clusterof poles to veins. It maybe noted that under reduced Pf, the
fracturesrepresented by poles lying outside this ellipse (open
squares inFig. 5e) would be inactive/sealed, and only the fractures
repre-sented by poles lying within the ellipse (dark squares in
Fig. 5e)would be susceptible to reactivation/vein emplacement.
Therefore,using the ellipse around the cluster in Fig. 5e,
orientation of s3(centre of the ellipse), s1 and s2, and the angles
q1 and q2 weredetermined. Here q1 58 and q2 74. Using Equations 5
and 6(which is for clustered distribution) R0 0.07, and F 0.27.Fig.
5f is the 3-D Mohr circle construction using these data,
whichindicates that when Pf was very low (close to s3), a very
limitedrange of fabric elements (NWeSE oriented) were susceptible
todilation/reactivation to allow vein emplacement.
Based on the above discussion, the authors conclude that
thenatural situation that led to vein formation in the Gadag
regionduring D3 deformation actually had interplay of conditions
thatuctuated between Pf > s2 and Pf < s2 (Fig. 6b). During
the initialstages of D3, network permeability was poor, uid owwas
low, asa consequence of which uid pressure would have increased
tohigh levels. This resulted in the condition Pf > s2, as a
consequenceof which fabric elements (foliations and fractures) of
all orienta-tions underwent dilation and uids burped into them
(Fig. 6b-i).This resulted in emplacement of veins in varied
directions that arerecorded in the study area (Figs. 2d and 5a).
The state of stress andPf at this stage is represented in Fig. 5c
and d. Once the uid hadburped, Pf became low thus resulting in Pf
< s2 (Fig. 6b-ii). Under
had NWeSE orientation were susceptible to reactivation.
Following Jolly and Sandersonpresent a clustered distribution. An
ellipse is drawn around this cluster, which enclosesd Pf. All the
orientations whose poles lie outside this ellipse (open squares)
were sealedhelps dene the s1s3 and s1s2 planes. The angles q1 and
q2 are determined, which aree dening Pf in (f) were the only ones
that were susceptible to reactivation and vein
hear stress respectively. s1, s2 and s3 represent maximum,
intermediate and minimum
- Fig. 6. Schematic diagram (not to scale) documenting the
sequence of events that led to vein emplacement in the metabasalt
of Gadag region. Magnetic fabric (represented by NWeSE oriented
dashed lines) developed during D1/D2 deformation under NEeSW
compression (as shown in a). Extension in NWeSE direction (small
arrow in a) led to development ofsome NEeSW oriented cracks. D3
deformation (b) took place under NWeSE compression. At high Pf
(>s2) reactivation of fabric elements (foliations and fractures)
having variedorientations took place. This resulted in burping of
uid and vein emplacement in different orientations (b-i). Once the
uid had burped, Pf reduced to levels
- l of Sthis reduced Pf condition only the NWeSE striking fabric
elementswere suitably oriented for reactivation/vein emplacement.
All otherorientations were sealed and inactive (dotted grey veins
in Fig. 6b-ii). The state of stress and Pf at this stage is
represented in Fig. 5eand f. At some point of time during uid ow
and vein emplace-ment, NWeSE oriented fractures also got sealed,
thus reducing thepermeability of the system and leading to the next
cycle of rise in Pfto very high levels (Pf > s2), followed by
reactivation of fractures ofa wide range of orientations and
consequent uid ow (burping).Thus, the crosscutting network of veins
and fractures (mesh-structure) and the dominance of NWeSE oriented
veins in thestudy area is because of the repeated cycles and
interplay of theevents that uctuated between high Pf (>s2) and
low Pf (s2 (s2 < Pf < s1) and
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3-D Mohr circle construction using vein orientation data from
Gadag (southern India) Implications to recognize fluid pres ...1
Introduction2 Geology of the study area3 Methods of analysis and
results3.1 Fabric analysis using AMS in metabasalt3.2 3-D Mohr
circle construction from vein orientation
4 Discussion4.1 Relative timing of magnetic fabric development
vis--vis regional deformation4.2 Fabric in the metabasalt and its
influence on vein emplacement at high Pf4.3 Vein emplacement at low
Pf
5 ConclusionsAcknowledgementsAppendix A Supplementary
dataReferences