Geology of Govitzdgarh-Sewariya Area

Transcription

1 Geology of Govitzdgarh-Sewariya Area 3.1 Geological review of study area The present study area lies along the western margin of South Delhi Fold Belt, about 30 km west of Ajmer and 50 km south of the well known tungsten deposit at Degana. In the study area, metamorphosed volcano-sedimentary formations of Barotiya Group (the westernmost basin of SDFB) are flanked to the west by pre-delhi marble and gneisses, and intruded by a large body of foliated biotite granite, known as Sewariya granite (northern equivalent of Erinpura granite, Heron, , and by smaller intrusive bodies and a dyke swarm of tourmaline leucogranite. Wolframite bearing quartz veins are localized along the sheared intrusive contact between Sewariya granite and mica schist of Barotiya Group (Jain and Bhattacharjee, 1992). The Sewariya granite is exposed for a total length of about 36 km along NNE-SSW (the regional trend of Delhi rocks) with a constant width of 4-6 km. Bhattacharjee et al., (1993) consider the biotite-bearing variety of Sewariya granite, which hosts the wolframite deposit, as the source rock for tungsten mineralisation in this area. Banerji and Pandit. (1995) reported the occurrence of a leucogranite in the form of small pods and irregular bodies along an 18 km long NNE-SSW trending belt to the east of tungsten prospects, occurring as concordant bodies within meta-argillites of Barotiya Group. Pegmatites associated with these leucogranite bodies were reported to contain Li mineralisation in the form of lithium phosphate minerals ferrisicklerite and triphyllite (Bhattacharjee et al., 1993). On the basis of spatial association of tungsten deposit with biotite granite, and Li mineralisation with leucogranite, and certain geochemical characteristics of these host rocks, Banerji and Pandit, (1995) have concluded that Sewariya granite and leucogranite are the source rocks respectively for W and Li mineralisation in this area. Pandian and Dutta, (2000) recognized and mapped larger exposures of a tourmaline leucogranite along Luni-Sagarmati river section near Govindgarh, about 15 km northeast of Sewariya. In view of its distinct geological, petrographic and geochemical characteristics and relevance in tungsten mineralisation in Sewariya area, this granite body is named as

2 Gokindgarh granite (Pandian and Dutta, 2000). The leucogranite reported by Banerji and Pandit, ( 1995) represents the westward continuation of the Govindgarh granite. During present investigation, the geological map of Sewariya-Govindgarh area has been compiled from various publications of Geological survey of India (Jain and Bhattacharjee, 1992; Chattapadhyay et al., 1995) and taking into consideration the new observations made by us on the distribution of Govindgarh granite. The modified map is presented in Fig The rocks occurring in Govindgruh-Sewariya area consist of the following four litho-units in decreasing order of age. 1. Pre-Delhi rocks 2. Meta-sediments and vo!canics of Barotiya Group of the Delhi Supergroup 3. Sewariya granite and related rocks 4. Govindgarh granite and related rocks Greater emphasis has been given in the present study to decipher the geological characteristics of Govindgarh granite and related rocks whose widespread occurrence has been recorded for the first time during the present investigation. 3.2 Pre-Delhi rocks The western most of the study area is occupied by pre-delhi Ras marble, and gneiss and schist of the Archaean Banded Gneissic Complex (BGC). The BGC comprise high grade schist and gneiss (quartz-biotite-k-feldspar-garnet-silimanite) and amphibolite (Plagioclase- hornblende + garnet + clinopyroxene), which have recorded folds and fabrics of older penetrative deformation which are not observed in the younger Barotiya rocks of Delhi Supergroup (Bhattacharjee et al., 1993). The Ras marble which has been equated with the Raialo series by Heron (1953) and with carbonates of Delhi Supergroup by Gupta et al., (1991) rests unconformably over the BGC. Apart from few outcrops of Sewariya granite, Ras marble is the only other litho-unit which is exposed along small ridges in the western part of the study area, while rest of the area is a peneplain with sporadic soil cover.

3 Fig. 3.1: Geological Map of Govindgarh-Sewariya Area

4 3.3 Barotiya sequence of Delhi Supergroup The Barotiya Group of rocks of Mesoproterozoic age include an assemblage of meta- sediments (dominantly mica schist, along with conglomerate, meta-arkose) and meta- volcanics (dominantly amphibolite with only one instance of felsic rock in the study area). Mica schist is the dominant rock in the Barotiya sequence exposed in the study area. Large outcrops of mica schist along with meta-volcanic rock occur within and adjacent to Sagarmati river close to Go~indgarh and Pisangan villages. Mica schist consists of biotite, muscovite, quartz, K-feldspar and sodic plagioclase. In the vicinity of wolframite-bearing quartz veins, tourmaline is introduced into the mica schist replacing biotite. Meta-basic volcanic rock rich in diopside, hornblende and calcic plagioclase with variable amount of epidote and chlorite, is another dominant Barotiya rock in the study area. One outcrop of few m thick band of meta-felsic volcanic rock exists near the junction of Luni and Sagarmati rivers (Fig.3.2). This rock consists of K-feldspar, plagioclase, quartz, sericite and secondary calcite. A few m thick conglomerate bed with deformed pebbles aligned parallel to the pervasive foliation trending N25'E and dipping steeply towards SSE, occurs within mica schist and it is best exposed in Luni river to the north of Kalni village (Fig.3.3). Barotiya rocks are well foliated with a prominent foliation trending NNE with steep dips towards SSE or NNW. Few fold patterns are observed in the Barotiya rocks exposed in Sagarmati river (1) broad antiform with N20 E trending horizontal fold axis (Fig.3.4) and (2) broad open folds with axis plunging 10' towards N20 E and axial plane dipping 60' towards S70 E. Meta volcanic rock of Barotiya Group exhibits: (I) asymmetric fold with axial plane dipping 60' towards S70 E and fold axis plunging 10' towards N2OoE (Fig.3.5); (2) broad open fold with vertical axial plane and fold axis plunging 10' towards N45OE; (3) tight symmetric folds with fold axis plunging 16' towards N45OE, in which dykes of Govindgarh granite intrude parallel (Fig.3.6) or oblique to axial plane (Fig.3.7). These folds registered in Barotiya rocks are comparable with F2 folds (Gupta et al., 1995) of prevalent in rocks of Delhi Supergroup. 3.4 Sewariya granite and related rocks The Sewariya granite pluton is one of the largest granite batholith occupying the western margin of South Delhi Fold Belt. This pluton is exposed over a strike length of 6km

5 Fig. 3.2: Meta felsic volcanlc rock exposed in the Sagamati river Fig. 3.3 : Deformed conglomerate outcrop in Luni river to the north of Kalni

6 Fig. -;.?:Coarse grained GG folded nith mica schist to prod~~ce broad antiform with N20 E trending horizontal fold axis in the 'mica schist hill' of the Sagamlati ri-i.er near Go\-indgarh \-illage. Note the buckle folds in GG. Fig. 3.5:Meta-basic rock showing broad antiform whose axial plane dips 60" towards S70 E and fold axis plunges 10" towards N20 E. in Sagam~at~ river south of Govindgarh.

7 Fig. i.6:thin dyke of medium grained Go\-indgarh granite occ-iipying axial plane frachlre trending towards li450e in folded meta-basic rock of Barotiya group (Sagaxmati river section). Fig. 3.7:Dyke of coarse grained Govindgarh granite cuts oblique to axial plane of meta-basic rock of Barotiya group (Sagarmati river section).

8 along NNE direction with an average width of about 6 km at the western flank of SDFB (Jain and Bhattacharjee, 1992). It is flanked by Ras marble and, in very few instances, mica schist and meta-volcanics of Barotiya Group on the west and by various rocks of Barotiya Group on the east. A large inlier of Barotiya mica schist occurs within Sewariya granite in the study area (Fig.3.1). The Sewariya granite is grey coloured, medium to very coarse grained and generally shows a gneissic foliation trending N20-30 E with steep to near-vertical dip (Fig.3.8). It contains quartz, microcline, plagioclase, biotite and muscovite. SG is generally devoid of tourmaline. However, in some areas small pods of tourmaline are observed in SG adjacent to dykes of GG. The metasomatic origin of such tourmaline grains in SG (Fig.3.9) is evident from tourmaline replacing biotite or total absence of biotite in such instances. Sewariya granite is intruded by quartz veins, few 10s of cm thick and extending for few m, consisting only of grey coloured quartz. These quartz veins follow different strike direction (for example N50 E in Kalni; NIOoE in Kotariya) and often show evidence of brittle deformation along with the intruded Sewariya granite (Fig.3.10). Repeated displacement of these quartz veins for few cm to few 10s of cm is commonly observed along fault planes and cataclasite bands trending N-S near Kalni and N20 E near Kotariya villages Along the several km long contact zone between Barotiya rocks and SG: unambiguous intrusive relationship between SG and Barotiya rocks is recognized only at few places (eg: Chhoralo Bala river section to the southeast of Bijathal village). However, there are several instances where xenoliths of mica schist are found within Sewariya granite as well as in Govindgarh granite. Bhattacharjee et al., (1993) reported minor folds in Sewariya granite which are tight to open and plunge moderately towards northeast and southwest. From the style and orientation these folds are comparable with the F2 folds of the Delhi rocks. They have also suggested an antiformal structure based on the attitudes of gneissic foliation, the axial trace of which coincides with the median shear that passes through Sewariya- Ganeshpura area. A late N-S ductile shear zone that has affected the Delhi rocks of the region has also traversed the Sewariya pluton, particularly along its central parts producing a band of mylonite and ultramylonite (Bhattacharjee et al., 1993; Heron, 1953).

9 Fig. 3.5: Coarse gained Se~variya granite showing gneissic foliation trending N20-3WE, which is intruded by younger medium grained Govindgarh granite. Fig. 3.9: Tounnaline rich pods of metasomatic origin in Sewariya granite near to a dyke of Govindgarh granite (north of Kalni).

10 In the study area, we have observed few discrete shear zones in Sewariya granite showing ductile and brittle deformation. Along its contact with mica schist, there are several instances where SG shows mylonite (Fig.3.11) fabric with grain size reduction and evidence of dynamic recrystallisation (eg., development of quartz ribbons) in a few m wide zone. Apart from these ductile shear zones (DSZ) localized along the intrusive margin of SG, there are few DSZ occumng well within SG. Immediately to the north of the hill at Sewariya village, SG shows development of protomylonite with megacrysts of feldspars in a N1O0E trending shear zone (Fig.3.12) which has a maximum width of 35 m at the northern edge of the hill, narrows down to 20 m width at a distance of 100 m further north, and this shear zone could be traced over a total length of about 300 m along NNE-SSW direction. Towards NE of Bijathal (east of Alniyawas- Bijathal road), there is a small ridge where SG shows development of ultramylonite and quartz veins within a 100 m long and few m wide shear zone trending along N30 E direction. A similar type of shear zone in SG containing ultramylonite and quartz veins is also observed to the south of Richmaliyan village where it trends along N-S direction (Fig.3.13). These shear zones are marked in the geological map (Fig. 3.1). In addition to the ductile shearing localized generally along or near to the intrusive contact of SG with Barotiya rocks, effects of brittle shearing are also observed in SG in three locations: (1) In northern part of study area, large outcrops of coarse grained SG occur near Kalni with widespread development of thin bands of cataclasite in various orientations (trending N20W, N50 W, N-S). Among these cataclasite bands, early formed ones are displaced (upto few cm) by later ones indicating their progressive development during the brittle shearing event (Fig.3.14). Grey colored quartz veins occurring within SG are also displaced by these cataclasite bands by few cm to more than a meter (2). In the small ridge to the NE of Bijathal, ultramylonite band of SG is brecciated (Fig.3.15) indicating a later brittle deformation within the DSZ. (3) Veins of pseudotachylite, upto few cm thick and few m long, traverse through mylonitised SG in Chhorola Bala river bank to the east of Kotariya village (Fig.3.16), where these pseudotachylite veins are parallel to sub-parallel to the mylonite foliation of SG which trends N20 E. This is another instance of later brittle deformation within the DSZ.

11 Fig. 3.10: Quartz veil1 in SG showing displacelnent along fault planes near Kalni. Fig. 3.11: Searariya granite showmg mylonite fabrlc (north of Sea ariya v~llage)

12 Fig. 3.12: Sen~ariya granite shoxving mylonite fabric xvit11 megacrysts of feldspars (near Richrnaliyan \-illage). Fig. 3.13: Outcrop of ultramylonite ~ 5th quartz veins developed in Sewariya granite near Richmaliyan village.

13 Fig. 3.14: Cataclasite bands in Selvariya granite near Kalni. The early fonned ones are displaced by later ones. Fig. 3.15: Brecciated ultramylonite in Sewariya granite (NE of Bijathal).

14 In few instances, effects of ductile and brittle shearing are observed in Barotiya rocks along the intrusive contact with SG. 3.5 Govindgarh granite and related rocks From previous studies in this area (Bhattacharjee et al., 1993; Banerjee and Pandit, 1995; Pandian and Dutta, 2000) few isolated and small intrusive bodies of leucogranite were reported. A medium to coarse grained tourmaline leucogranite called as Govindgarh granite (GG), occurs in close association with an assemblage of metasediments and metavolcanics of Mesoproterozoic Barotiya Group and a porphyritic to equigranular biotite bearing granitic gneiss (Sewariya granite). The present study has revealed that Govindgarh granite is a major litho unit occuning in the form of dyke swarm and stock-like bodies intruding into the Barotiya rocks and Sewariya granite in a NNE-SSW trending zone along the western margin of South Delhi Fold Belt (SDFB). Presence of Govindgarh granite as stocks and dykes is recorded within a zone extending for about 20 km from Kalni in the north to Sewariya in the south, and about 8 km wide limited to Govindgarh-Pisangan in the east and Sewariya-Kurki-Bijathal in the west. Within this linear zone, there are some large areas where GG is either the dominant or exclusive rock, and this includes the north-south stretch of 5 km along Luni-Sagarmati river and the 4 knl wide zone to the west of Luni-Sagarmati river up to Pipaliya. Govindgarh granite intrudes into the folded Barotiya sequence in the form of dykes parallel to fold axis (Fig.3.6). Parallel to axial plane of fold whose axis trends N45"E and oblique to fold axis trending N32OE (Fig.3.7). Angular xenoliths of Barotiya rocks are common in GG and at few places Barotiya rocks have undergone brittle defom~ation due to intrusion of GG. Near the "mica schist hill" on western part of Sagarmati river close to Govindgarh village, Barotiya mica schist is found as roof rock to the Govindgarh granite (Fig.3.17). There is a clear intrusive relationship between the younger Govindgarh granite and older Sewariya granite. Dykes of GG intruding across the N30 E trending gneissic foliation of SG are seen near Richmaliyan, Kotariya, Bijathal and Pipaliya (Fig.3.8). While GG intrusion is younger to some folds registered in Barotiya rocks, at few places both these rocks are deformed to develop asymmetric folds (Fig.3.18). The 'mica schist hill' adjoining

15 Fig. 3.16: Veins of pseudotachylite in mylonitised SG in Chhorola Bala river bank.(east of Kotariya tillage). Fig. 3.17: Mica schist of Barotiya rock occurring as roof rock to Govindgarh granite in the western part of Sagannati river close to Govindgarh village.

16 Sagarmati river shows a broad antiform with fold axis trending N20 E, where few dykes of GG intruding into mica schist have developed satellite folds (buckle folding) of different arc length (Fig.?. 19). Two different varieties of Govindgarh granite are well documented in the study area (Sivasubramaniam and Pandian, 2003) both of which are leucocratic, tourmaline bearing, biotite free and intruding into Barotiya rocks and Sewariya granite: (1) an older medium grained granite, which is either massive or layered with alternating tourmaline-rich and tourmaline-poor bands, and (2) a younger coarse grained leucogranite which appears to be the product of pegmatitic stage. The two varieties of GG share a common mineral assemblage of quartz, sodic plagioclase, K-feldspar, tourmaline and muscovite; garnet and apatite are often found as accessory minerals in medium grained GG. Both the types of GG are occasionally intruded by quartz-tourmaline veins (Fig.3.20), many of which are observed in Sagarmati river. The tourmaline present in GG and the GG-hosted quartz veins is black coloured schorl variety. Govindgarh granite is totally devoid of biotite, whereas the Sewariya granite invariably contains biotite but free from magmatic tourmaline. In only one location to the south of Pisangan, few coarse flakes of biotite were observed in GG adjoining its intrusive contact with mica schist, where partially assimilated xenoliths of mica schist occur within GG. We infer that assimilation of mica schist by GG is responsible for the development of biotite in this instance. The medium grained Govindgarh granite is well exposed in the Sagarmati river section. This granite occurs commonly as stock like intrusive body (diameter ranging from 100 to 300m) and in some instances as dyke (upto 4 m wide and exposed length upto about 25m) or small vein (few cm width). In some instances medium grained leucogranite is layered, with alternate dark bands rich in tourmaline and light coloured bands containing less tourmaline (each few cm thick). In one of the GG outcrops within Sagarmati river, a thin layer (< 1 cm) of brown coloured garnet extending for about 5 m length was found in the medium grained GG. These are inferred to be magmatic layering developed during early stages of crystallization of medium grained GG. In some instances these layers follow definite trend between NNW to NNE (Fig.3.21). In a canal section parallel to the road between Govindgarh and Alniyawas (close to Tanka) layering is observed in medium grained GG within a 1 m wide zone for a length of about 400 m, in which the general trend of layers is N30 W with

17 PIS. 3.18: D>.ices of coarse srained GG folded n-ith enclosing mica 5cIiist to de~ clop asymmell-ic folds (Saganna~i ri\-er). Fig. 3.19: Dykes of GG intruding into mica schist have developed satellite folds (buckle folding) of different arc length in the mica schist hill adjoining Sagarmaii river.

18 Fis Inri~l,i\ e conracr bem eel: mcdi~rn~ grained and coai-\ct grair.c,i iiii. li?i::~~i.:. quart/_ \ ei11 cur\ throiigll hot11 (Sagar:nali?I\-i.r!. Fig : -Medium grained GG shows magmatic layering defined by toru~-nlaline rich layers (near dam at Sagamati ri~rr).

19 moderate dip towards SW. At this location, layering is characterized by presence of fewer, cm size tourmaline crystals in light coloured layer and multitude of very fine (mm size) tourmaline crystals in dark coloured layers. In few locations outcrop scale deformation in medium grained GG is evident from folding of the magmatic layering. For example, near the dam across Sagarmati river layering in GG following a trend of N50"E shows broad open folds whose axis is oriented between N40W and N10W. In another outcrop of GG, the trend of layering swerves from N30 E to N70 E (Fig.3.22). The coarse grained Govindgarh granite occurs in the form of dyke swam spread over the entire study area, more abundant in some areas like Sagarmati river, and relatively less frequent in the western part of study area. In many places coarse grained GG cuts across the layered and medium grained GG (Fig.3.23). Many instances of layered GG intruded by coarse grained GG are observed in the canal section near Tanka. Geological Survey of India has reported the presence of Li-phosphate minerals femsicklerite and triphyllite in some of the dykes of coarse grained GG occurring near Pipaliya and Koliyan ki Dhani (Bhattacharya et al., 1993; Baneji and Pandit, 1995). However, we did not come across these minerals in coarse grained GG during our investigation. Similar to the effect of deformation registered in layered medium grained GG, some dykes of coarse grained GG (trending N50 E) exposed in Sagarmati river and intruding into medium grained GG, show wavy walls due to folding, with fold axis oriented along N40W (Fig.3.24). Alignment of tourmaline crystals perpendicular to the dyke wall is commonly observed in coarse grained GG. In some areas (tungsten prospects marked in Fig.3.1), quartz veins containing tourmaline, muscovite and wolframite intrude into mica schist and adjoining Sewariya granite, mostly following the schistosity and gneissic foliation in these rocks which trend along NNE direction with steep dip towards WNW or ESE. These quartz veins contain milky white quartz and these are different from the grey quartz veins which are confined to SG in their mineral assemblage (presence of tourmaline, muscovite and wolframite) and absence of deformation effects like faulting. In all these tungsten prospects, along with wolframitebearing quartz veins dykes of Govindgarh granite also occur in close proximity and intruding into mica schist and Sewariya granite. Quartz veins similar to those of tungsten prospects also occur within the two types of GG, but presence of wolframite is not yet known in these quartz veins.

20 Fig. 3.22: Layered medium grained Govindgarh granite showing deformation of layers (near dam in Sagarmati river). Fig. 3.23: Coarse grained GG cuts across medium grained GG.

21 Fig. 3.24: Dyke of coarse grained GG intruding into nledium grained GG sho~vs Avavy xvalls due to folding (Sagarmati river).

22 3.7 Significant geological events in study area From field relations it is demonstrated that Sewariya granite (biotite-bearing granitic gneiss) and Govindgarh granite (,biotite-free tourmaline leucogranite) formed during two successive magrnatic events, of which GG is younger phase. The deformed grey coloured quartz veins which are confined to Sewariya pluton are inferred to have formed during!ate stages of Sewariya granite magmatism. Contrary to previous observation, we have found that although wolframite bearing quartz veins are hosted by mica schist of Barotiya Group and adjoining marginal portions of SG pluton, dykes or larger intrusives of GG are invariably present in close proxinlity of these mineralized zones. Moreover, there is a striking similarity in the mineral assemblage of medium grained layered GG (representing magmatic phase), coarse grained GG (representing pegmatitic phase), wolframite bearing quartz veins in the prospects and quartz veins occurring within GG (both representing hydrothermal phase), characterized by the ubiquitous presence of tourmaline + muscovite + apatite. From these geological characteristics it is inferred that the younger leucogranite magmatism produced successively the medium grained GG, coarse grained GG and milky white quartz veins some of which emplaced outside GG contain wolframite. The shear zones developed in SG in the study area are part of a major NNE trending shear zone, called Phulad Shear Zone or Phulad Dislocation Zone, occurring all along the western margin of SDFB (Sinha-Roy et al., 1998). The structural features such as Fz antiformal closure, mylonitisation and development of pseudotachylitic mass along the ductile shear zone that passes through the F2 axial zone and along the margins of the Sewariya pluton indicate that this granite was emplaced syn-kinematic to Fz folding. The emplacement of GG, in the form of dykes intruding into Barotiya rocks and SG, could be syn-to post-kinematic to this shearing event. It is inferred that these deformation observed locally in GG are related to the shearing event whose signatures are abundantly present in SG.

23 FoSiow,ing is the succession of major geological events recorded in the study area. Post-Delhi magmatism Milky white quartz veins with tungsten mineralisation Coarse grained Govindgarh granite Medium grained Govindgarh granite... Intrusive contacts... Brittle and Ductile deformation of Barotiya rocks, SG and grey quartz veins Grey quartz veins in Sewariya granite Sewari ya granite... Sheared intrusive contact Delhi Supergroup... Barotiya Group of rocks (Mica schist, meta- volcanics)... Structural hiatus..... Pre-Delhi rocks Ras marble, gneisses and amphibolite