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Geoarchaeology and Archaeomineralogy (Eds. R. I. Kostov, B. Gaydarska, M. Gurova). 2008. Proceedings of the International Conference, 29-30 October 2008 Sofia, Publishing House “St. Ivan Rilski”, Sofia, 147-152.

GEOLOGY AND PETROGRAPHY OF OCHRES AND WHITE CLAY DEPOSITS IN RAJASTHAN STATE, INDIA Giovanni Cavallo1, Manoj Pandit2

1University of Applied Sciences, South Switzerland, DACD, LTS PO Box 12 6952 Canobbio, Switzerland; giovanni.cavallo@supsi.ch 2University of Rajasthan, Dept. of Geology, 302004 Jaipur, India; mpandit_jp1@sancharnet.in

ABSTRACT. Rajasthan State in NW India is the leading producer of ochres used in paint, cement, rubber, glass, linoleum, plastic industries and foundries, lacquers and also for imparting colour to paper and cement. The main mines are located in Bhilwara, Chittaurgarh and Udaipur districts, in the Precambrian Aravalli Delhi Fold Belt (ADBF, also known as Aravalli Mountain region). The NE-trending Aravalli Mountains that run for more than 750 km in NW India represent the most prominent tectonomorphic feature in this region. It marks the Western boundary of the Bundelkhand craton that occurs to the East while in the South the continuity of ADFB is lost under the vast Deccan volcanics of much younger age. Geological evolution of this terrain includes an Archean basement over which green schist- to amphibolite-facies metasedimentary sequences of Palaeo- to Mesoproterozoic (Aravalli Supergroup) and Meso- to Neoproterozoic (Delhi Supergroup) volcano-sedimentary sequences were deposited. The petrographic approach on the materials collected in these areas, inherited by the micromorphology of the soils, is correlated to the geology. Texture and co-exixting mineral characteristics have been used to infer original minerals and host rocks. Minerals non-usable as pigments (quartz, feldspar, etc.) have also been discriminated.

Introduction Clay pigments and, more in general, earthy pigments (Hradil et al., 2003) have been widely used from ancient times to date for decorating bodies, caves, religious temples and architectural surfaces, both walls and stones (Delamare et al., 2000). The use of these materials is primarily related to their easy availability in all the countries and in varied geological contexts being products of weathering of a variety of host rocks. As reported in the ancient treatises, these raw materials were used mixed with inorganic and/or organic binding medium depending upon the tradition of the pictorial techniques and the specific purpose. Despite such a wide application spectrum, not much is known about their origin, chemical and mineralogical composition and physical properties. This impedes the work of specialists in the field of conservation and restoration of cultural heritage especially in terms of choosing the best proxies for original pigments. A number of substandard and synthetic materials available in the market are being used, however, they contain impurities especially sulphates which are detrimental to the historical artworks. The process and technique of preparing the final pigment from the raw material is also misunderstood sometimes.

India has a long tradition, historical and cultural background in the use of ochres and clay pigments in the art work. This is well documented in the ancient and medieval literature (Bhattacharya, 1976; Seth, 2006) and also continues even in the present times as part of the culture. The pigments have

also been described as mineral resources in all the reference literature, such as Krishnaswamy (1988). Rajasthan State in the NW of India is one of the most important localities in the production and export of ochre and clay pigments. Here ochre deposits and mines are located at various places, however, the notable occurrences are in Bikaner, Chtittaurgarh, Jaisalmer, Jhunijhunun, Jodhpur, Nagaur and Udaipur districts. We have recently initiated a detailed geological, mineralogical and geochemical investigation of the ochre deposits of Rajasthan in order to understand the composition of the raw material and the process of transformation of some selective minerals as earthy pigments. This paper presents some preliminary results on geological and petrographic characteristics of some ochre and white clay occurring in the Precambrian terrain (Fig. 1) of central and SE Rajasthan (Bhilwara, Chittaurgarh and Udaipur districts).

Geology of the ochre and clay deposits Bhilwara district Precambrian rocks in the Bhilwara district include a significant Archean (Sandmata Complex, Hidoli Group, Berach Granite, etc.) and a subordinate Proterozoic component (Aravalli Supergroup, Delhi Supergroup and Vimdhyan Supergroup). The oldest rocks in this region have been described as the Bhilwara Supergroup which is predominantly Archean in age with an early Proterozoic component (Gupta et al., 1997). This group includes the Sand Mata Complex (medium to high grade migmatites, gneisses, granulites etc.), Hindoli Group (low-grade metagraywacke, phyllite and tuff) and

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Mangalwar Complex (migmatites, calc-silicate rocks, schist, amphibolite, quartzite etc.). The Berach Granite and presumably time equivalent Jahazpur Granites represent the end Archean magmatism in this area. The basement rocks in the Bhilwara sector define tectonic contacts on either side, with the Aravalli and Delhi metasediments in the West while the Great Boundary Fault juxtaposes these rocks and the sediments of Vindhyan Supergroup in the East. The ochre deposits and operating mining sites are located to the ENE of Bhilwara town and show a geological cum tectonic control. The deposits are located in a NE-trending linear belt

which corresponds to the predominant tectonic direction in the ADFB. On the basis of geological considerations the ochre deposits in the Bhilwara district seem to be genetically associated with the Late Archean Hindoli Group and can be further discriminated into two settings within the Hindoli Group metasediments and along its contact with the Proterozoic Jahazpur Group. A solitary deposit (BI 02) is located within dolomite and biotite schist terrain of Mangalwar Complex close to the contact with Berach Granite. Such a lithological control is clearly seen in the predominant calcareous mineralogy of this deposit.

Fig. 1. Precambrian geological map of Rajasthan (modified after Heron, 1953; Gupta et al., 1997)

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Chittaurgarh district District Chittaurgarh, which is located to the south of Bhilwara, also shares a common geological history with the latter. The rocks of Bhilwara Supergroup (migmatites, gneisses, dolomites, schists) form a peneplained basement in this area. The Berach Granite, exposed in patches, represents a late Archean intrusive phase into the Migmatitic gneisses of the Mangalwar Complex. The rocks of Vindhyan Supergroup (alternating bands of sandstone, shale and limestone) exposed in the eastern part of the region occurs stratigraphically above the basement with an angular unconformity between the two. In the southeastern part of the district all the Proterozoic rocks have been covered by Late Cretaceous – Early Eocene (~65 Ma) basaltic flows known as Deccan Traps. This is one of the major ochre and white clay producing areas in SE Rajasthan where extensive deposits are located within the Proterozoic Lower Vindhyan rocks (Suket Shale). The deposits are clustered in a small area south of Chittaurgarh town and occur close to the boundary between Nimbahera Limestone and Suket Shale representing the top-most units of the Lower Vindhyan sequence. The two other occurrences are located to the north and southeast of Chittaurgarh town. Both these sites are within the Lower Vindhyan terrain, the former along the boundary between Binota Shale and Bari Shale sandstone (no exposures of host rock are seen, however, and the deposits show a complete alteration). The latter deposit is located along the contact between Suket Shale (Lr. Vindhyan) and Kaimur sandstone (Up. Vindhyan). Udaipur district Udaipur district has been hailed as the type area for the Aravalli Supergroup which shows best development in the Udaipur valley. In addition, there are a number of outcrops of the BGC gneisses and granites to the east and southeast while a tectonized contact with the younger Delhi Supergroup is seen towards the western part of the district. The westernmost part of the district shows NE-trending linear ridges of 967 Ma old Sendra – Ambaji granites and host calc-silicate rocks of Kumbhalgarh Group (Delhi Supergroup). The oldest dated rocks (3.3 Ga old TTG gneisses) from NW Indian terrain are located to the southeast of Udaipur city. The now abandoned red ochre quarry at Iswal (to the northwest of Udaipur city) has been one of the major producers of red ochre in the past. The quarry, which exposes a 15 m thick section of red ochre, has presently been filled up as a part of a new road development project in the region. However, it was possible to take samples from the southern face of the quarry. The ochre deposits are located within the chlorite/phyllite/tuff (Bari Lake Group), the uppermost formation of Lower Aravali, and quartzites of the Jharol Group (lower part of the Upper Aravalli). The deposits are located along the transition from shallow (Lower Aravalli) to deep sea facies (Upper Aravalli) depositional conditions.

Materials and methods The samples collected in the Bhilwara (BI), Chittaurgarh (CH) and Udaipur (UD) districts are listed in Table 1. All the samples have been analyzed by means of an optical microscope both in transmitted and incident light.

Table 1 Description and location of ochre and white clay deposits

N Description and location

BI04 Yellow ochres and white clay (Itawa Tahsil, Kotri village)

BI06 Rock (c) yellow ochre (b), reddish horizon (a) (Mine BS Mandora)

BI07 Pebbles mixed with red ferruginous material (near Joralia village)

BI15 Red material (near Manoharpura and Hansed Ka Kheda villages)

CH03 Red (a) and yellow (b) ochre (mine Banasti II, Sawa)

CH04 Red (a), yellow (c) ochre and white clay (b) (mine Banasti IV, near Sawa)

CH05 White sandy clay (a,b) (mine Banasti I, Sawa) CH07 Lens of yellow Ochre (as CH05) CH08 Red ochre (as CH05) CH09 Red ochre (a) and yellow ochre plus white clay

(near Barada village) CH10 White sandy clay (Kantharia village) CH13 Red material (a), contact red material - bauxite

(b), Bauxite and clay (c) (Senwar) CH14 Yellow ochre (a) and white clay (b) (Senwar) CH15 Red ochre (Senwar) UD01 Red ochre (a – bottom; b – middle part; c – top)

(Iswal)

Petrography Bhilwara district Sample BI04 (a: bottom, yellow material; b: middle (white clay, yellow and traces of red material; c: top, white clay). The yellow Ochre sample (BI04a-b) contains metamorphic quartz both as individual crystals and in polycrystalline aggregates while yellow Fe-oxihydroxides are seen disposed along parallel

planes. Quartz grains are about 40 µm in size. Sericite and biotite are also present. Samples BI04a and Bi04b show the preservation of the original microstructure and texture which can be correlated to a biotite schist (Fig. 2) and to a quartz-feldspar rock (Fig. 3). Biotite appears to be the main contributor for Fe-oxihydroxides; opaque minerals (magnetite) are also related to the process of biotite decomposition. The clay minerals (sericite) can be linked to the feldspar alteration.

Fig. 2. Microstructure of the sample BI04a showing the characters of the original biotite schist rock (PPL)

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Fig. 3. Microstructure of the sample BI04b showing the characters of the original quartz-feldspar rock (XPOL)

The fabric of the sample BI04c does not exhibit any layering, excepted for metamorphic quartz (individual crystals and polycrystalline aggregates). The high interference colour of the very fine grain minerals is in accordance with clay minerals (kaolinite group). In some cases polycrystalline quartz aggregates exhibit effects of mechanical stresses which have completely obliterated the original structure of the quartz. Petrographic observations allow the identification of quartz as relict phase while the entire section can be described as an in-situ profile. Sample BI06 (from the bottom to the top. c: host rock 10 cm thick; b: lens of yellow ochre; a: reddish horizon). Sample BI06c seems to be a product of complete alteration of granite and shows a characteristic mineralogy comprising destroyed and secondary calcite (calc-silicate rock). Sample BI06b shows

a close association of quartz (∅med=250 µm), Fe-oxihydroxides and clay minerals; accessory phases include muscovite and amphibole. No layering has been observed. The sample does not show any features of layering. Mineralogical and petrographic characteristics suggest quartzite to be the original rock which has altered to form an ochre deposit. The sample BI06a shows development of red oxides (non in-situ alteration) and secondary calcite in pores. Quartz occurs in accessory quantity while clay minerals occur as interstitial filling.

Sample BI07 includes metamorphic quartz (∅med=1 mm) and presence of fractures which have also acted as channel ways for passage of solutions. Besides, calcite also occurs both as fracture filling around oxides as well as clusters of few mm size grains. The most likely original rock seems to be granite gneiss. Some pebbles of quartzite are present which show re-crystallized mosaic of quartz grains with typical dihedral angles close to 120°, triple point junction. Ochraceous material is both spread around and as round particles, the former ranging in

size between 10 and 100 µm. The particles are sometimes

elliptical in shape with major axis 800 µm and minor axis 400

µm. Muscovite, marble fragments, alteration product of feldspars as secondary epidote, metamorphic quartz filled with secondary calcite are also present (BI07a). Pebbles of quartzite mixed with red oxides, sometimes with radial structures; traces of muscovite are also seen. The sample BI07b seems to be a part of lateritic horizon.

Sample Bl15 includes secondary calcite and clay minerals in a red matrix and can be traced to a felsic rock (biotite gneiss) as the source (Fig. 4).

Fig. 4. Microstructure of the sample BI15 showing the characters of a biotite gneiss (PPL)

Chittaurgarh district Sample CH03 (from the bottom to the top. a: red material; b: yellow Ochres). The sample CH03a shows the presence of red oxides and abundance of probable gibbsite Al(OH)3 crystals (Fig. 5) causing the matrix to be isotropic. Traces of yellow Ochre are observed.

Fig. 5. Gibbsite occurring as randomly orientated crystals (PPL)

Sample CH03b: yellow Fe-oxides (limonitic material?) and

associated clay minerals are seen. Quartz (∅=10-20 µm),

altered feldspars (∅=150 µm) are also present while clay minerals form prominent fractures fills. Sample CH04 (from the bottom to the top. c: yellow Ochre; b: white clay; a: red Ochre). Sample CH04c is a yellow ochre which shows a high abundance of clay minerals and

subordinate quantity of fine grained quartz (∅=20 µm). Muscovite is in traces. In the sample CH04b white clay and

quartz fragments ∅=40 µm are present; large fragments of altered feldspars are also present. The sample CH04a comprises mainly altered plagioclases which can be recognized by the grain boundaries, grain shapes and textural relationship. The sample contains significant amount of opaque

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minerals (Fe-Ti oxides). These features indicate a mafic source which can most likely be the Deccan basalt as confirmed by a relict basaltic/doleritic texture. Sample CH05 comprises abundant clay minerals mixed with

coarse-size quartz and pebbles of granitic rocks (∅=1.0-2.0 mm); the latter show altered feldspars and quartz. At places the relicts of the host rock are seen as individual quartz fragments within fine clay matrix. Some fine crystals of sphene and zircon are also seen which further confirm a granitic source (CH05a). The sample CH 05b (from the top of the profile) is also similar to CH05a. Sample CH07 is a yellow ochre which includes fine bands of clay minerals (imparting a layered appearance) and randomly

oriented individual fine grained quartz (∅=10 µm). Sample CH08 also includes altered plagioclase and Fe-oxides. Although the plagioclase grains have been completely altered the original grain boundaries are still preserved. The still preserved igneous texture of porphyritic basalt and absence of quartz can be traced to a basaltic source (Fig. 6). The sources of released iron are magnetite and pyroxenes.

Fig. 6. Altered plagioclases showing still preserved porphyritic basalt texture (PPL)

Sample CH09 (a: red material; b: yellow and white). Samples from the site CH09 show sub-rounded altered feldspars of variable size set in a matrix of iron oxide (CH09a). Yellow limonitic material is seen mixed with secondary calcite (microsparitic) and sparitic calcite forms veins. Micritic calcite is

also present. Quartz (∅=50-100 µm) is present as accessory mineral. Traces of clay minerals; some fragments of original rock (marble) are still present (CH09b) and allow the identification of impure marble as the source rock. Sample CH10 includes clay minerals and coarse grained quartz along with secondary calcite in veins. The feldspars are completely altered and form the matrix. Secondary calcite veins also have inclusions of quartz fragments. The mineralogical and textural attributes indicate a quartz-feldspatic source rock. Sample CH13 (from the bottom to the top. a: red ochre; b: contact between red ochre/bauxite, c: bauxite). Sample CH13a behaves almost as an opaque rock due to abundant oxides

and very fine grained clay minerals. Some traces of quartz and muscovite are also seen. Closely associated red oxide, fine clay minerals and large grains of altered calcite (fragments of marble) suggest a calc-silicate rock as possible source. Sample CH13c shows predominant clay minerals and red oxide along with some relict feldspar grains and accessory zircon. Sample CH14 (from the bottom to the top. a: white clay; b: yellow Ochre). Sample CH14a is a mix of clay minerals and

yellow Ochres with quartz grains of variable size (∅=20-200

µm); traces of opaque minerals are also present. A large grain showing growth of secondary quartz around Fe-oxihydroxides nuclei can also be seen. Sample CH14b consists of clay

minerals and angular to sub-angular quartz fragments (∅=20-

600 µm) as well as quartz in veins and traces of Fe-oxides. Some relict plagioclases and secondary quartz vein can be seen. Sample CH15 includes predominant iron oxides, completely altered plagioclases (sericite), which still retains the shape and textural relationships seen as a relict basaltic texture (Fig. 7). The iron oxides were derived from Fe-Ti oxides and augite present in the basalt.

Fig. 7. Altered plagioclases in a well preserved texture of basaltic rock (PPL)

Udaipur district Sample UD01 (a, bottom; b, middle part; c top). Sample UD01a includes a red coloured matrix where the Fe-oxides are generally in the form of rounded globules; the nuclei of these globules are generally represented by opaque minerals. Alteration of plagioclase into clay is quite evident which suggests that the original rock must have been volcanic in origin as abundant altered euhedral plagioclases are relict phases associated with red oxides (UD01b); secondary calcite fills cracks and micro-cracks. Red matrix (Fe-oxides) is the most prevalent constituent along with feldspars and traces of quartz (UD01c).

Discussion The samples collected at Bhilwara show a close association between yellow ochres (FeOOH) and clay minerals (BI04a-b-c). The yellow ochres seem to be the alteration products of biotite and the clay minerals (white clay, probably kaolinite) from the alteration of the feldspars. The alteration of biotite into

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FeOOH is clearly seen (Fig. 8). The relict microstructures are typical of schists (biotite schists) as well as metamorphic quartz-feldspar rock (granite gneiss). For these samples the original characters of the host rocks are well preserved. The upper part of the succession is dominated by white clay minerals where the original character of the host rock has been completely obliterated. The lithological and geological setting of this area also supports the petrographic observations. The samples BI06 is mainly dominated by white clay minerals; the geological profile (from the bottom to the top) includes a probably calc-silicate rock and quartzite. The samples BI07 show the character of a lateritic horizon where fragments of marble and quartzite are seen mixed with red iron oxides. The sample BI15 shows the character of an altered schist; the matrix is dominated by opaque red minerals.

Fig. 8. Incipient alteration of biotite into FeOOH (XPOL)

The main characteristic in the samples from Chittaurgarh include complete or almost complete alteration of plagioclases reflecting an original basaltic rock as the source. The source of iron in these samples can be traced to breakdown of iron bearing minerals such as magnetite and pyroxenes. Although the samples from Udaipur are limited to a single face of abandoned mine, the same situation seems likely for the samples collected at Iswal, Udaipur district where the original character of volcanic rock (tuffs) is recognizable.

Conclusions Ochres and clays are not generally studied throughout optical microscopy; the use of petrographic examination allows to reveal many distinctive characters of the deposits otherwise non detectable by means of ancillary techniques such as XRD, XRF, ICP-MS and SEM/EDS. The samples studied show a strong connection between the host rocks and the deposits; the microstructure and the texture

of the original rock are sometimes very well preserved. In many cases the reconstruction of the entire profile could be possible and the deposits can be described as an in-situ alteration profile. The common character in the Bhilwara and Chittaurgarh districts is the association between white clays and yellow ochres; this is controlled by the geology where feldspar alteration is responsible for clay formation (very likely kaolinite) and biotite alteration for yellow ochres (very likely goethite). Another common characteristic is the association of altered plagioclases to red oxides. In these cases the original rocks are volcanic (basalts in Chittaurgarh district and tuffs in Udaipur). Further studies would include mineralogical and chemical analyses that will be carried out to identify the phases non detectable under microscope and for a better understanding of the amorphous phases occurring in the samples. Acknowledgements. We thank Rajiv Rahi of the University of Rajasthan, Dept. of Geology, for his cooperation and assistance during field work. We are thankful to the Rajasthan State Mines and Geology Department for providing help and guidance during field work and in particular to Mr. Labana for his active help during field work in Bhilwara district. The smiles, bright eyes, and colorful dresses of the people working in the mines will eternally remain engraved in our memories. We acknowledge the financial assistance received from Rector’s Conference of the Swiss Universities of Applied Sciences represented by the General Secretary Office (KFH) – grant number P-0710-04.

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