Petrological studies on two East Coast bauxite deposits of India, and implications on their genesis

Sedimentary Geology, 39 (1984) 121-139

121

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

PETROLOGICAL STUDIES ON TWO EAST COAST BAUXITE DEPOSITS OF INDIA, AND IMPLICATIONS ON THEIR GENESIS

M. DEB and A. JOSHI *

Department of Geology, Delhi University, Delhi-l lO007 (India) (Received July 19, 1982; revised and accepted September 8, 1983)

ABSTRACT Deb, M. and Joshi, A., 1984. Petrological studies on two East Coast bauxite deposits of India, and implications on their genesis. Sediment. Geol., 39: 121-139. Lateritic bauxite caps on flat-topped plateaux underlain by Precambrian ldaondalites along a 200-250 km stretch of the Eastern Ghats define the "East Coast bauxite province". Studies conducted in two East Coast deposits--at Galikonda, in Andhra Pradesh, Pottangi and in Orissa--reveal a good profile differentiation, in terms of four horizontal zones of variable thicknesses, which are from top to bottom: a thin, iron-rich indurated soil cover; a lateritic bauxite zone; a lithomarge zone of partially lateritised and kaolinitised ldaondalite and finally, the lowermost, fresh bed-rock khondalite. Detailed and systematic mineralogical and petrographic studies have been carried out along three vertical profiles of these two deposits, using techniques of transmitted and reflected light microscopy, X-ray diffractometry, infra-red spectrophotometry, differential thermal analysis and scanning electron microscopy. These studies along with computations of quantitative mineralogical variations along the profiles confirm that the basement khondalites are the parent rocks of these East Coast bauxite deposits and that the authigenic bauxitisation was brought about by kaolinite-gibbsite transformation through desilication of the upper part of the kaolinitised lithomarge zone which formed as an intermediate stage in the weathering of the basement khondalites.

INTRODUCTION

Large resources of low-grade bauxite deposits have been delineated by Government agencies on known aluminous laterite occurrences capping flat-topped plateaux in a stretch of 200-250 km of the Eastern Ghat belt, between long.82°-83°E and lat.17°51'-18°75'N. These are underlain by Precambrian granulite facies metamorphites (Fig. 1) comprising an interlayered sequence of khondalites and charnoc-

* Present address: Geological Survey of India, Manipur-Nagaland Circle, Dimapur, Nagaland-797112, India.

0037-0738/84/$03.00

© 1984 Elsevier Science Publishers B.V.

122

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kites, the former being the bed rock of the subhorizontal blankets of bauxite deposits with large areal extent ( - 0.5 km), appreciable thickness (max. - 50 m) and, usually, good profile differentiation. This contribution concerns the petrological aspects of two deposits from the "East Coast bauxite province"--one at Galikonda (1ong.82°59 ', lat.18°15 ') in Andhra Pradesh and the other at Pottangi (long.82°58 ', lat.18°35 ') in Orissa (Fig. 1). It attempts to trace the different stages of the bauxitisation process through a study of samples representing the entire lengths of one scarp profile from Galikonda and two complete bore hole profiles from Pottangi, in addition to selected surface samples.

123 THE BAUXITEPROFILE Field studies and reconstruction of borehole logs show that both bauxite deposits have a good profile differentiation (cf. Ramam, 1976). From top to bottom, typical vertical profiles of these deposits can be broadly divided into four horizontal parts of variable thickness: (a) The topmost horizon is made up of a thin (20-40 cm), dark brown to black, soil cover. At places an indurated iron-rich crust, commonly with rounded pisolites, is associated with this recent to subrecent soil cover. This accumulation of iron could possibly be attributed to biological activity. (b) The lateritic bauxite zone commences below this hard ferruginous duricrust; however, no sharp boundary is perceptible between the two. Its thickness is about 20 m along the Galikonda scarp, though the maximum thickness of lateritic bauxite recorded in this deposit is 50.9 m (Ramam, 1976). Similar blanket nature of lateritic bauxite at Pottangi could only be ascertained from the profile sections drawn on the basis of borehole logs. At both deposits erosion of the subhorizontal bauxite cap by breaking along the edges has resulted in the development of conspicuous vertical scarps. (c) The lateritic bauxite zone gradually merges with a lithomarge or saprolite zone, comprising partially lateritised and kaolinitised khondalite. This horizon has attained a considerable thickness at Galikonda (about 100 m), but is relatively thinner at Pottangi. Although intensely weathered, vestigial structures like schistosity and gneissosity of the parent rock are still retained in this zone. The mottled and the pallid zones of McFarlane (1976) are probably present here, though not clearly demarcable in these two deposits. (d) The lowermost horizon contains the fresh bed-rock khondalite. The transition between the lithomarge and basement rocks is observed to be relatively sharp. PETROLOGICALSTUDY Methods used

The mineralogy and petrography of about 50 bauxite samples and their subjacent bed rocks (about 30 samples) were studied by a combination of methods which included the optical identification of minerals and textures, X-ray diffraction analysis using CuKa radiation through a nickel filter, differential thermal analysis to ll00°C together with DTG and TG analyses for certain samples, and infrared spectrophotometric study in the range of 200/600-4000 cm-I using the KBr disc technique. Representative patterns of these analytical techniques for the saprolite and bauxite zones, are presented in Figs. 5 and 9. The opaque minerals present in bauxite were studied under reflected light. Micromorphology and elemental composition of the cryptocrystalline phases were studied under the scanning-electron microscope fitted with X-ray dispersive analyser.

124

Khondalites in the bed rocks and in the lithomarge zone

Totally unaltered khondalites are absent in the vicinity of the bauxite deposits. Thus, the bed rocks of bauxite ores invariably show some amount of alteration which is reflected in the varying proportions of secondary minerals in them. The constituent minerals in khondalites, in order of abundance, are: quartz, almandine garnet, sillimanite, feldspars (both albite-oligoclase and orthoclase), kaolinite, gibbsite and corundum, together with minor amounts of ilmenite, titaniferous magnetite, graphite, goethite, lepidocrocite, haematite, zircon, rutile, anatase and sphene. The major mineral constituents display considerable variation in their relative proportions, depending largely on the degree of alteration of the khondalites and partly on

T A B L E IA M o d a l p e r c e n t a g e of m i n e r a l s in relatively u n a l t e r e d k h o n d a l i t e s S a m p l e No.

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G a l i k o n d a (surface)

P1

P.2

P.3

G.1

G.2

G.3

Quartz

13.47

48.35

29.63

38.10

57.57

87.71

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26.83

24.09

20.54

15.15

10.59

4.98

2.34

6.56

6.42

7.75

13.48

3.63

11.07

11.63

24.01

18.16

3.46

Corundum

0.12

2.26

2.19

8.33

Limonite

25.54

1.54

10.00

12.51

Sillimanite Feldspar

Other opaques

-

Altered mass

-

20.14

5.62

3.68

12.71

-

2.19

7.22

T A B L E IB M o d a l p e r c e n t a g e of m i n e r a l s in a l t e r e d k h o n d a l i t e s Sample No.

Location P o t t a n g i (central) 11

Quartz Lim. Garnet

Galikonda (scarp)

16

17

G.9

G.8

G.7

6.45

29.06

52.60

21.65

25.41

21.07

34.92

28,39

38.23

16.25

7.85

0.73

1.92

10.67

Sillimanite

0.38

Feldspar

.

2.51 .

.

G,6

G.5

G.4

11.43

27.60

22.37

12.39

10.20

1.43

1.36

4.93

.

Corundum

-

-

0.30

-

Other opaques

0.57

-

0.90

6.50

3.82

2.77

2.74

3.50

1.62

53.66

52.22

54.04

48.73

63.42

56.16

18.44

9.33

24.92

Altered mass

46.36

16.56

4.65

Gibbsite

11.77

23.48

2.59

-

-

-

125 the variations in the original composition of the parent sediments (see Table I). The common compatible mineral assemblages in khondalites: almandine + sillimanite + orthoclase + quartz; and almandine + sillimanite + corundum + quartz, indicate that these rocks were regionally metamorphosed under lower granulite facies condition (cf. Turner, 1968; Winkler, 1974). The pronounced schistosity in these rocks is described by elongated quartz grains, stretched garnets and oriented sillimanite laths as well as flakes of graphite, some grains of ilmenite and titaniferous magnetite. The gneissosity of the rocks is produced by alternate lighter and darker bands, the former being made up of quartzo-feldspathic aggregates and latter by a concentration of garnets (Fig. 2).

Mineralogy Quartz, the predominant mineral, is invariably fractured in altered khondalite--the fractures filled up by gibbsite at places. Garnet which is next in abundance, has a high content of almandine molecule indicated by its cell edge

Fig. 2. Polisheddrill-coresample of unaltered khondalite from central block, Pottangi. Gneissosityin the sample is described by the dark garnetiferous, opaque mineral-rich bands and the lighter quartz-feldspar-sillimanite layers.

126 around 11.5024 A. From minor alterations along fractures and rims, through thoroughly limonitised grains with relict patches (Fig, 3) to complete pseudomorphs with skeletal box-work--all stages of garnet alteration are common in altered khondalites. Sillimanite laths, defining the schistosity are, at places, very coarse (4-5 mm in length). The fibrolite variety of sillimanite, though present, is quantitatively insignificant. Alkali feldspars in khondalites, mostly orthoclase--as suggested by their staining with sodium cobaltinitrate and potassium rhodizonate--are also invariably altered with brownish colouration along their accentuated cleavage (Fig. 4). Corundum is an infrequent but important constituent of khondalites (Fig. 3). Its presence was confirmed by one set of parting, very high relief, uniaxial negative figure and two prominent X-ray diffraction peaks at 2.083 and 1.624/k. Amongst the secondary aluminous minerals--kaolinite and gibbsite--the former is the predominant phase, and is present mainly in bands, irregular patches or cloudy amorphous to cryptocrystalline mass, dispersed throughout the altered rock. It also forms along rims and fractures of primary minerals like sillimanite and corundum. X-ray diffraction studies on all altered khondalites showed the two most prominent reflections of kaolinite around 7.18 and 3.58 ,~ from the 001 and 002

Fig. 3. Coarse garnet grains showing advanced stage of alteration into goethite (black) along fractures. Co-existing minerals are sillimanite (with fine cleavage), corundum (high relief, one set of parting, squarish grain) and quartz (white). Parallel polarised light.

127

Fig. 4. Altered feldspar grains in khondalite from Galikonda showing accentuated cleavage. Quartz (white) and garnet (high relief,lowerpart of photograph)are also present in association.Parallelpolarised light. planes, respectively (Fig. 5; cf. Brown, 1961), and the presence of both poorly crystalline and well crystalline varieties with the crystallinity index in the range of 0.60-1.17 (cf. Caroll, 1970). In the DTA studies a strong endothermic deflection in the range of 560°-580°C and another exothermic peak between 975°-1000°C, along with the temperature ranges shown by these two peaks, pointed to the presence of "well ordered to little disordered" kaolinite (cf. Smykatz-Kloss, 1974) in the saprolite (Fig. 5). Besides, most of the important vibrations in the O-H stretching region (3700, 3650, 3630 c m - t ) , Si-O stretching and O-H bending regions (1110, 1035, 1030, 935, 910 cm - t ) and in the far infrared region (785, 750, 535, 475 cm -~) for kaolinite (Gadsden, 1975) could be noticed in the infrared spectra of all saprolite samples studied (cf. Fig. 5). Under the SEM initial stages of development of crystallinity in kaolinite are decipherable in most samples. In one, prismatic crystals of kaolinite with a well-defined hexagonal outline (Fig. 6) were observed to form clusters in a predominantly amorphous groundmass. Gibbsite occurs in clusters of medium to fine grains in the kaolinitic mass or forms rims of in-situ alteration around primary aluminous minerals. Its presence was easily confirmed from X-ray, D.T.A. and IR patterns of the bulk powders of altered khondalite.

128

In certain white bands and patches with a waxy feel, the association of kaolinite and gibbsite is so intimate as cryptocrystalline grains, that the phases could not be identified individually even under a SEM, although their presence was confirmed through X-ray and infra-red studies (Fig. 7). Under the SEM two different forms were noticed--one is micro-crystalline, either lath-like or tabular, 10-60 ~t in length;

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129

Fig. 6. Scanning electron micrograph showing aggregate of well defined crystals of kaolinite. Note the perfectly developed crystals of kaolinite showing hexagonal, prismatic habit.

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130

the other occurs in tiny shapeless aggregates in the matrix. The prominence of A1 and Si in energy dispersive analysis of X-rays showed that both forms were kaolinite (cf. Joshi, 1979). The predominance of kaolinite in such mixed patches, antipathetic variation of the amount of the two phases and the textural relationships wherever the two phases can be approximately distinguished under the microscope, indicate the paragenetic sequence of gibbsite development from kaolinite. The content of the hydrated iron oxides, goethite and lepidocrocite, increases with the degree of alteration of the rocks. Cryptocrystalline irregular patches of these minerals appear to be the result of later seggregation along the vestigial schistosity. The crystalline variety is often radially disposed, indicating open space filling or display comb-structure along walls of box work formed by total leaching of garnet grains or occurs along a ramifying network of veinlets. Texture

In the East Coast Saprotite primary textures have been extensively superimposed by secondary fabrics and no primary mineral has been left unaffected. In higher levels, quartz and alkali feldspars have been extensively corroded or altered, garnets have been thoroughly leached and pseudomorphs of hydrated iron oxides after garnet, with perfect box work, formed. Some of the voids produced by leaching remain vacant to give rise to the micropores, others are partly or fully filled up by aggregate needles or collomorphous bands of secondary kaolinite, gibbsite, or lepidocrocite. Resistant minerals like sillimanite, corundum, rutile and sphene, while occurring as relicts, show evidence of in-situ alteration. The bauxite cap

In the overlying lateritic bauxite zone secondary minerals predominate and their relative proportions change drastically. Here the constituent minerals in order of abundance are: gibbsite, goethite, haematite, kaolinite, anatase, limonite, cliachite and boehmite. The lithorelict minerals: sillimanite, graphite, magnetite, ilmenite, corundum, garnet and quartz, are present sporadically in minor amounts. Mineralogy Gibbsite (Gi) is the predominant constituent of the bauxites. Three varieties of gibbsite are distinguished, mainly on the basis of size and mode of occurrence. The most common is the medium-sized (0.1-0.3 mm) variety which occurs in clusters forming rossettes or lining fractures and voids (Fig. 8). Next in abundance is the fine-grained ( - 0.02 mm) type, present at random as disseminated grains. Least common is the coarse-grained ( - 0.3 ram) variety, forming euhedral to subhedral crystals, showing prismatic or tabular habit and conspicuous lamellar twinning. In X-ray diffraction patterns of bauxite powders (Fig. 9), the prominent reflection of gibbsite is always present at 4.8 ,~. Other reflections are observed at 4.4, 4.3, 2.0 ,~

131

Fig. 8. Medium grained, twinned gibbsite crystals lining the walls of fracture in bauxite. Fine-grained gibbsite mass is also associated (upper right). Crossed Nicols.

and also between 1.80 and 1.75 .~. Sharp and intense reflections of gibbsite in X-ray patterns of these "East Coast" bauxites are indicative of its ordered internal structure and good crystallinity. DTA studies of bauxite samples show a large endothermic peak between 330°-360°C (Fig. 9). The typical dehydration reaction of gibbsite takes place in this range of temperature, though in the lower part. The higher values are attributable to the presence of aluminous goethite in some samples. About 60-65% of the total weight loss in some samples has been found to be associated with the dehydration reaction of gibbsite (cf. TG curve in Fig. 9) of the sort: gibbsite~x-A1203~x-Al203 ~a-A1203 (reaction series I of Brindley and Choe, 1961). Another small and broad endothermic peak has been observed between 520°-540°C. Even in the absence of the more prominent peak of boehmite at 250°C, as in the East Coast samples, such peaks have been attributed by Brindley and Choe (1961) and MacKenzie and Berggren (1970) to a small amount of boehmite in the samples. Almost all the important bands reported by Frederickson (1954), Dorsey (1968), Jonas and Solymar (1970a) and Gadsden (1975) for gibbsite in the near as well as far

132

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Fig. 12) f r o m Pottangi, Central.

infrared spectra have been observed in the samples studied. The most consistent vibrations in the O-H stretching region are around 3630, 3520 and 3380 c m - 1 (Fig. 9). Two strong vibrations, around 1020 and 970 c m - ] have been noticed in the O-H bending region. In the far infrared spectra a prominent band at 370 c m - 1 has been noted for gibbsite. A very low intensity ratio of the 3440 and 3520 c m - 1 bands from

133

Fig. 10. Scanning electron micrograph showing cluster of well crystalline gibbsite grains. Note the prismatic form of individual crystals.

these bauxite samples indicate an ordered crystalline structure of gibbsite (Bardossy et al., 1977). Under the SEM it is revealed that gibbsite in all good grade bauxite samples is not necessarily well crystalline, and may be represented by clay-sized shapeless aggregates. Crystalline gibbsite, with pseudohexagonal, prismatic form is however quite common and occurs in irregular patches or oriented clusters (Fig. 10). Goethite (Go) is an ubiquitous constituent and either rims the inner walls of cavities or occurs as bands and patches within the gibbsite matrix. Under reflected light, its reflectivity (at 546 nm) has been found to vary from 14.3 to 12.5% (in air) in adjacent bands, which could be due to a variation in the content of water or the presence of other impurities in the crystal lattice of goethite. In X-ray diffraction studies most of the d-values indicate a shift from those of pure goethite, possibly/because of Al-substitution, in the lattice. The strongest reflection from (110) plane appears between 4.18 and 4.25 ,~; from (140) between

134

2.18 and 2.23 ,~; from (111) between 2.36 and 2.48 ,~; from (021) between 2.50 and 2.60 .~. As suggested by Jonas and Solymar (1970b), Orban (1971) and Franz (1978), the shift in the (111) reflection is most suitable for determination of Al-substitution in goethite lattice. Using the dlH (,~) vs. mole% A10(OH) in goethite curve of Jonas and Solymar (1970b) it was found that in certain samples in which d~H is between 2.458 and 2.459 ,~,, Al-substitution in goethite is almost negligible whereas in certain other samples, dH~ of goethite around 2.419 A (Fig. 9) indicates a substitution of almost 24 mole% AIO(OH). The broad and low intensity peaks of goethite suggest a low degree of crystallinity of the mineral. Due to the high percentage of gibbsite in the bauxite samples goethite could not be confidently identified in either D.T.A. studies or infrared spectrophotometry. Under the SEM it is characterised by small nodules with corrugated surfaces. Haematite was noted in reflected light, occurring in irregular massive patches, and was confirmed by X-ray diffraction with the strongest peak around 2.72 ,~. Most peaks were broad and of low intensity, suggesting a less perfect crystal structure a n d / o r substitution of AI/Ti in the lattice. In one sample of bauxite, a reflection around 6.24 ,~, indicated the presence of lepidocrocite. Kaolinite was detected only in a few bauxite samples, significantly, in close association with patches of gibbsite aggregates. Clusters of fine-grained, subhedral anatase, characterised by greenish-blue colour, are found to occur within recrystallised gibbsite flakes in many bauxite samples. In certain bauxite samples, a broad X-ray peak between 9 and 10 A (Fig. 9) could be attributed to the presence of cliachite (Schoen and Roberson, 1970) which forms a dull grey amorphous matrix. In addition, a negligibly small amount of boehmite is also detectable by X-ray reflections in the range of 5.980 and 6.137 ,~ and also between 3.170 and 3.204 .~. Limonite identified petrographically, though not as any specific iron oxide, is present in fractures and along cavity walls and rarely, as veinlets. The lithorelict minerals (listed earlier) are all characterised by various advanced stages of alteration. Texture

Bauxite samples display fabrics which are quite different from those noted in lithomarge rocks. This marked change in fabric is primarily brought about by a change in the relative proportions of the primary and secondary mineral constituents, the dominance of gibbsite over kaolinite, the high degree of crystallinity of different varieties of gibbsite as well as by the abundance of ferruginous phases. Most common are the medium- to coarse-grained domains of polysynthetically twinned gibbsite flakes in a ground mass of fine silt-sized gibbsite grains. Gibbsite, as well as goethite, is also commonly restricted to the walls of voids at places as alternating bands. The crystals are either arranged radially towards the centre of the voids, or lie with their long dimensions parallel to the cavity wall. The lithorelict minerals such as sillimanite, garnet, feldspars and corundum, frequently display

135

Fig. ll. Aggregateof gibbsite crystals forming pseudomorphs after sillimanite in lateritic bauxite. Parent sillimanite (grey) can still be seen in some patches. Crossed nicols.

skeletal fabrics due to pseudomorphous replacement by gibbsite or limonite (Fig. 11). Pseudomorphs of goethite after garnet, with conspicuous box work, are frequent in the bauxites, which, with increasing intensity of chemical leaching at upper levels become transformed into irregular patches of limonite. The occasional presence of gibbsite in the cavities of some limonitised garnet may suggest their derivation from the aluminium contained in the host. But gibbsite is also found to occur in the fractures of quartz where it cannot be related to its host genetically. Such fracture and cavity filling by gibbsite rather appears to be due to absolute accumulation which is also supported by the gel-like features occasionally seen. Juxtaposition of these various textures has produced the porous and spongy look of the East Coast bauxites. MINERALOGICAL TRANSFORMATIONS ALONG VERTICAL PROFILES The gradual variation in mineralogy from the bed rock to the bauxite capping, in terms of changing volumetric percentages of major minerals was studied along a scarp profile at Galikonda and two vertical bore holes at Pottangi. The mineralogical

136

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computations, which are at best semiquantitative, are based on X-ray diffraction and differential thermal analysis and have been substantiated by chemical analysis and petrographic study of the samples. Along the subvertical Galikonda scarp section twelve samples were collected at nearly equal intervals. They were all subjected to petrographic studies and seven

137

were selected for detailed mineralogical analysis. In the bore hole from the south block of Pottangi, only the soft powdery cores for the entire length were available while in the other, from the central zone, powdery material was available upto a depth of 19.8 m, and some solid cores below this depth, on which the petrographic studies were conducted. The quantitative mineralogy along these three profiles are depicted in Fig. 12. From the altered lithomarge to the bauxite cap above, the character of the samples changes primarily due to a change in the relative proportion of the secondary minerals and a steady diminution in the quantity of the primary phases of khondalites. In this respect the important minerals considered are: gibbsite, kaolinite, goethite + haematite, anatase, quartz and the litho-relict minerals. These studies indicate that quartz, silimanite, garnet and feldspar comprise the major part of the basement rocks, along with some minor kaolinite. In the lithomarge, kaolinite predominates over others but diminishes suddenly once the bauxite cap is reached and is totally non-existent in the good grade ores in the middle of the bauxite cap. The lower surface of the bauxite cap is defined by a marked drop in the quartz content, sudden increase in the gibbsite content and concommittant decrease in kaolinite. The iron oxides (mostly hydrated) are consistant in proportion throughout the entire length of the profiles, showing a slight increase right at the top. Anatase similarly shows a minor enrichment from the bed rock to the bauxite cap. IMPLICATIONS OF BAUXITE GENESIS

The preceeding description of the mineralogy and petrography of the bed rocks, the intervening lithomarge zone, and the overlying bauxite cap brings out vital clues regarding the nature of the bauxitisation process in these two East Coast deposits. The picture of gradual facies change in the vertical direction, from the basement khondalites to the lateritic bauxite zone at the top through a wide zone of partially altered saprolite, in terms of their mineralogy, leaves no doubt as to the in-situ nature of the bauxitisation process. Field observations, such as vestigial gneissosity and partially altered bands of khondalite in the lateritic bauxite zone, supported by a variety of textural evidences of in-situ alteration of primary (metamorphic) minerals in khondalites into secondary aluminous and ferruginous phases along the weathering profiles--the feature becoming more widespread and complete at upper levels with increasing intensity of alteration--suggest clearly that the basement khondalites are the parent rocks of East Coast bauxite ores (cf. Deb et al., 1978). Though the authigenic nature of the bauxite horizon is amply established, along with the broadly invarient mineralogic nature and its uniform thickness, it is still difficult to trace confidently the different stages of the bauxitisation process, particularly because the role of kaolinisation in bauxite formation still remains controversial. In some deposits the parent rock is directly overlain by the bauxite horizon while in many others as in the present case, between the bauxite and the

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parent rocks there is an intermediate zone of saprolite, rich in kaolinite. In these latter deposits the kaolinite zone is believed, by some workers, to have formed by resilication of the lower portion of the overlying bauxite, while others consider the bauxite cap to represent the desilicated upper portion of the kaolinite horizon. While some workers have sought a clearer understanding of this transformation process through laboratory studies of experimentally aged sesquioxide gels (e.g. MacKenzie and Meldau, 1959; MacKenzie and Gard, 1962), it is apparent that field and petrographic criteria would often provide the decisive clue. In the two East Coast bauxite deposits under study the important petrographic observations in this context are: (1) The antipathetic variation in the content of kaolinite and gibbsite along the weathering profiles with the latter increasing overwhelmingly in the bauxite zones, as brought out in the quantitative mineralogical studies. (2) The presence of mixed cryptocrystalline patches in lithomarge samples, in which initial stages of nucleation of gibbsite is noticed in a predominantly kaolinitic mass. This more or less establishes the paragenetic sequence of gibbsite development from kaolinite. (3) The predominance of kaolinite at lower levels, with incipient development of cryptocrystalline gibbsite in the kaolinitic mass and gradual fall in its volumetric proportion at upper levels which shows a sharp drop at the base of the bauxite cap. Such observations collectively point towards the possibility of bauxitisation taking place by desilication of the upper part of the saprolite zone in the weathering profile. Moreover, evidence of silication of the bauxites into a kaolinitic mass, such as, abundant veinlets and veins of kaolinite across massive bauxite or fragments rich in gibbsite embedded in kaolinite or crystalline silica, is absent in the two deposits under consideration. Thus, we may conclude that the major portion of the gibbsite contained in these bauxite deposits formed by the decompositions of kaolinite which formed as an intermediate stage in the course of weathering of khondalites. Direct and immediate precipitation of Al-hydroxides by the decomposition of primary minerals like feldspar, sillimanite and corundum probably made a subsidiary contribution to the process of bauxitisation. Epigenetic migration of aluminous solution appears to be responsible for the formation of gel textures and coarser gibbsite aggregates and infilling of cavities and fractures in relict garnet and quartz by gibbsite. The enhanced mobility of aluminium in such cases could be attributed to the presence of certain organic acids in the meteoric waters. ACKNOWLEDGEMENTS

We are grateful to the authorities of Bharat Aluminium Company Ltd. for sponsoring the study and for facilities in the field, including study and collection of samples from the bore hole cores. We would like to record our thanks to Dr. M.G. Deshmukh for his interest and co-operation throughout the course of the work.

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