mohr - 3-d mohr circle construction using vein ori entation data from gadag

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

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78 00

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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.


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)


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

mohr - 3-d mohr circle construction using vein ori entation data from gadag

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