Mineral. Deposita 17~ 349-362 (1982) MINERALIUM DEPOSITA © Springer-Verlag 1982 Precambrian Phosphorites in the Bijawar Rocks of Hirapur-Bassia Areas, Sagar District, Madhya Pradesh, India,,2 D. M. Banerjee*, M. W. Y. Khan, Neeta Srivastava, and G. C. Saigal Department of Geology, University of Delhi, Delhi, India T h e P r e e a m b r i a n phosphorites of Bijawar G r o u p of rocks show characteristics of a epicontinental sea with restricted and very shallow marine environment of formation along s o m e shoals, which existed during the iron-rich P r e c a m b r i a n times. These phosphorite deposits located in the Hirapur-Bassia areas show extensive leaching of carbonate and phosphate minerals during episodes of weathering. X - r a y diffraction studies indicated that carbonate-flourapatite is the m a j o r apatitic phase in these phosphorites while crandallite developed on the surface outcrops. T h e r e is a general tendency for the depletion of C O 2 in these apatites leading to formation of flourapatite. This C O 2 is an indicator of hidden weathering in the rocks. Major and trace element determinations of phosphorite have been used to indicate various correlation factors responsible for the concentration of elements in these P r e c a m b r i a n leached phosphorites. 1 T h e paper is a contribution to the a i m s and objectives of I G C P Project 156 2 T h e paper is dedicated to Prof. Dr. R.C. Misra, who as a teacher and guide had been a source of inspiration to the senior author for the last two decades ~ Present address: Palaeoatmosphere R e s e a r c h G r o u p Air C h e m i s t r y D e p a r t m e n t Max-Planck-lnstitut for C h e m i e Postfach 3060 D - 6 5 0 0 Mainz Federal Republic of G e r m a n y INTRODUCTION T h e Bijawar Group of rocks constitute a well defined, though poorly studied rock entity, and is n a m e d after the township of Bijawar, w h e r e they define the eastern Bijawar Basin. T h e n a m e Bijawar w a s given to the early P r e c a m b r i a n sediments resting over the primordial Bundelkhand granite m a s sif and underlying the U p p e r Proterozoic Vindhyan G r o u p of sandstones (Medlicott, 1859). Mathur (1960) and Mathur and Mani (1978) carried out geological mapping of the type Bijawar basin and the stratigraphic succession of the area after these authors is given below: 0026-4598/82/0017/0349/ $02.80 350 D.M. VINDHYAN GROUP (Upper P r o t e r o z o i c ) SEMRI FORMATION Gangau Ferruginous Formation Banerjee et al. Unconformity BIJAWAR GROUP Bajna Dolomite A m r o n i a Quartzite Dargawan Traps Malhera Breccia BUNDELKHAND Our present interest lies in the Gangau Ferruginous Formation of the above scheme which contains reddishbrown phosphorite in its lower parts, bordering the granitic rocks of Bundelkhand massif. In the Hirapur-Bassia areas, Bundelkhand granites are either directly overlain by Bajna Dolomite or c o m e in direct contact of Gangau Ferruginous Formation containing conglomerates, breccia and shales (Fig. 2). There is no 'tillite' in the vicinity of the phosphorite horizon, but has been recorded in the adjacent regions (Mathur and iV[ant, 1978). ()pen synclinaI and anticlinal structures are prominent in the shales and the Gangau Formarion is locally bounded by a synformal structure in the Bassia area (Fig. I). Some prelh-ninary information about this deposit is contained in the review by Pant (1980). Similar phosphorites also occur at Sonrai in the Lalitpur district, west of Hirapur, where copper, lead-zinc and uranium minerals are intimately associated with the secondary phosphate horizons. Authors shared responsibilities for different tasks, viz. Banerjee for geological mapping, sampling, geological and geochemical interpretations, mineralogy, overall synthesis and writing, Khan for the major and trace element analyses by AAS and other techniques, and Srivastava and Saigal for X-ray diffraction studies. GRANITE PETROLOGICAL Tillite and Shale Ferruginous conglomerate and breccia and GNEISSES CHARACTERS Since, only the underlying carbonates and the top phosphorites are of direct relevence to the present paper, the litho-characteristics of only these two rock types have been described. The fine grained Bajna Dolomites are of two types, namely: (a) brown ferruginous dolomite and (b) grey dolomitic limestone. The first type grades into overlying Gangau Formation. Their composition vary from cherty, shaly dolomite to pure dolomite rock. The Gangau Formation on the other hand are composed of ferruginous breccia, conglomerate and reddish brown slates and shales. The ferruginous sandstone and ferruginous shale occur as intercalations within the ferruginous conglornerate and breccia with abundant apatite grabls in the groundmass. Based on the nature of occurrence in the area, four categories of phosphorites have been recognized: Type I: Ferruginous, shaly~ laminated pho sphorite Type If: Massively bedded, ferruginous phosphorite Type III: Brecciated/Conglomeratic, ferruginous phosphorite Type IV: Veins/Pore filling type, remobilized, secondary, crystalline apatitic, phosphorite. In Type-I phosphorite, rnicrogranular apatite grains occur within the ferru- "Jr- o C~ B~ c~ I r~ n," c~ x _oo .o n °:5 0 T O 0 o g_ 0 o.~ r~ _.1 o O _..1 O f~ ~9 g_ 0 ~ ~.~ i5 --, -o_ .o J o ~ .o 0 0 x 123 ~ ~t~ • 0 \ -t- • ,~ 0 352 D. IV[. Banerjee SemriSandstone (VindhyanGroup)2 ~Unconformity ~ ~ . ~ /Gangau Ferruginous ~ (Phosphate)1:ormation Bajna Dolornite~ Basement Bundelkhandgranite-gneiss et al. Dargawan Formation ~ ' ~ " ~ + ~ +--------.~__,_~/-'-+ + + + Fig. 2, Schematic/diagrammatic cross section across the phosphorite deposits in Hirapur-Bassia-IV[ardeora area. Scale approximate ginous shaly matrix either as dispersed matter or as well developed microlaminations. Visual estimates of m i c a clay mineral abundance in thin sections gives an inverse relationship with the quantity of apatite grains. Recrystallized coarse (0. 02-0.15 a m ) apatitie veins traverse through the shales (Fig. 3), mostly in a r a n d o m m a n n e r or along secondary surfaces. wall have ferruginous inclusions along successive layerings. S o m e of them show curved twin planes (Fig. 5). Possibly s o m e of the ferruginous layers w e r e pre-existing which were r e m o v e d during dissolution process and precipitation of the apatites; T h e Type-If phosphorites are massive, hard and compact, very fine grained, with thin laminations of coarse-grained quartz. Apatite is randomly distributed in the micaeeous g r o u n d m a s s and occur as small crystallites (0.02-0. 05 a m ) . Interstitial subhedral to anhedral elongated polyerystalline quartz (0. 010. 02 am) show dissolution features on their margins. Subrounded to subangufar fragments of ferruginous shaly phosphorite s o m e t i m e s occur e m b e d d e d in this type of ore, suggesting a later origin of this type of phosphorite. The void filling type of secondary apatite riddled with disseminated ferruginous matter occur with well defined rims of magnetite (Fig. 4). Such apatite are usually large (0. 05-0.25 a m ) and show successive growth layers. P o r e filling episode shows the following stages of evo lution : (c) Precipitation of quartz in the remaining voids of the pore spaces; (a) Apatite crystals growing outward with long axes n o r m a l to the confining (b) Overgrowths of clear apatite on the pre-exisitng clouded apatite grains indicate authigenic precipitation; (d) Iron oxide (magnetlte/goethite) forming in the late stages of diagentie alterations. T h e Type-III phosphorites contain ferruginous cement and occur in association of conglomeratic and brecciated quartzites. ]More or less equant apatite crystallites (0. 01 a m ) in a shaly g r o u n d m a s s contain profuse ironoxide disseminations. The Type-IV phosphorite occur as lateritie cover with well developed reniform surfaces. Large, perfectly crystalline prismatic apatite grains (0. 05-0.25 a m ) radiate into the voids and pores (Fig. 4). T h e microcrystalline g r o u n d m a s s is stained reddish b r o w n by the secondary iron-oxides. A m o r e detailed petrological description is given by Banerjee (1982). Pr ecambrian Phosphorites Fig. 3. P h o t o m i c r o g r a p h of T y p e - I p h o s p h o r i t e showing r e c r y s t a l l i z e d c o a r s e grained apatite crystals occurring within randomly in a predominantly areno-argillaceous groundmass. oriented secondary (X-nicols) veinlets Fig. 4. Photomicrograph of void-filling type phosphorite with well developed apatite crystallites growing in size towards the open void spaces. Dark magnetite layers occur normal to the growth direction of the apatite crystallites. (X-nieols) Fig. 5. Photomicrograph showing curved twin planes of apatite crystallites and their growth along the cavity margins. Secondary magnetite fills the remaining void spaces and also permeates into the twin planes of the apatite. (X-nicols) MINERALOGY prevalence of carbonate-poor (CO2: 0. 10-2.85%) flourapatites. The diffraction pattern of the bulk samples, espeCharacteristic x-ray diffraction patterns cially those f r o m the top, weathered shown in Fig. 6A and B reveal t h a t profiles, show positional shifts in the carbonate-flourapatite is the major con- diffraction peaks f r o m those known f r o m stituent, while quartz is the major an ideal earbonate-flourapatite and a gangue. There is, however, a general n u m b e r of secondary peaks remain sup- 354 D.M. 4888-- 1 2 3 8.144 6 ;Z4 5.¢'54 3 ~5 4.257 526 3.8774"=355 28 Z7 6 3 . 4 4 6 10@ 29 7 3.342 38 38 El 3.171 IS ~ll 9 3.~ 14 32 10 R . 8 ~ 3 8 3 3 3 ! ! 2.776 4I ~4 ~2 2.71~B E~ 35 A 3008-- 14 15 t6 17 |O t9 2~ 21 22 23 2.519 2. 458 2.29@ 2.251 2.22t 2.14~ 2.127 2.~3 2.~ I .~B 2 6 23 3 4 4 6 1 4 et al. I.gSI I t .~8 ,.886 5 t .8~Z 1.859 g 1,7~ 12 1.772. 9 J.74~ 8 1,7~ 19 ! .66| ! .~'38 S I,~8 2 .54~ 39 48 41 42 43 44 45 Banerjee 1.54@ I.$35 I ,b~4 1.5~2 |,4~ I .~9 1.454 1.447 2 :3 3 3 3 e 8 4 2888 1888@9 t 4~4 Z6 3 4 5 2123 B 34 8 18 4888-- 28 38 68 78 B 4 E; 0 7 8 I0 I1 12 13 14 |5 10 17 18 3888 2~88- 58 48 4.54~ 4.254 3.782 3.65~ 3.$88 3.4~ 3.35@ 3 . I58 3.1~7 2.~g 2.506 2.841 2,~@ 2.775 2.708 2 22 l e 23 5 24 77 ~5 I ~8 | 27 IO 2 8 I 29 730 ~ 3t 9 32: @ 33 9 34 4 35 8 36 ~.4J7 2.387 2.356 ;~.325 2.264 2.212 2 . 128 2.~2 2.076 1.9~8 I,STg | .g52 I .934 1.802 1,8~1 :3 4~f 34l 34 42 | 43 | 9 44 4 45 4 46 4 47 1248 249 H 50 ~@ 51 9 52 5 ~ 3 54 J,7Q7 | .793 t.773 t .747 t.74G L723 1 .64~ 1,5~ | .g83 I .~7 I,E~S! { *$48 | ,543 I,~;38 1.465 5 S 1 2 4 2 ] R I ! ! | 3 1 1 13 24 Fig. 6. A A typical X-ray diffractogram of bulk phosphorite showing prominent apatite peaks. B A X-ray diffractogram of phosphorite concentrate after apatire peaks are ~emoved by computer stripping technique. The remnant peaks show the presence of crandallite Precambrian Phosphorites 355 major elements and recalculated molar percentages of average Hirapur apatites. T h e observed charge imbalancing is due to the unaccounted flourine content which w a s not determined during the present work. Higher concentration of P2 05 is noticed in the weathered phosphorites (Type IV). Silica show gradual decrease with increasing P2 05, while Ca increases simultaneously. However, the total Si values have not been considered for any interpretations, whose distribution in the rock is controlled by both diagenetic and secondary quartz. Similarly, the primary association of iron with phosphorite is not established with certainity. Na content is fairly high compared o ~ d 2 k 2 hk) a = 3333 (h 2 + + to any known phosphorite types from f o r (210), ( 3 t 0 ) , (320) and (410) p l a n e s India. All trace elements show low conand e ° : 1. d f o r (004) and (002) p l a n e s . centrations with few exceptions. The L a t t i c e d i m e n s i o n s of the a p a l i t e ( c a r intercorrelation of elements define three b o n a t e - f l o u r a p a t i t e ) c r y s t a l l i t e vary be- major associations such as: (i) apaHtic phosphorite, (2) ferruginous-clayey tween 9 . 2 9 ~ - 9. 379 ~ f o r a o and 6. 880 ~ f o r c o, w h e r e a s f o r c r a n d a l l i t e phosphorite and (3) weathered (leached) a o is r e c o r d e d a s 7 . 8 3 ~ and Co a s aluminous phosphorite. E a c h element 9, 67 ~ . CO 2 w a s d e t e r m i n e d by t h e association reflects a grouping of elep e a k - p a i r m e t h o d of G u l b r a n d s e n (1970), ments: (a) structurally substituted in w h i c h v a r i e s f r o m 0. 10 to 2.85%. major mineral apatite, (b) adsorbed onto the mineral surface or (c) existThe summary of axes in the unit cell ing as discrete minerals derived from apatite by weathering. Most elements of apatite shows no significant difference between the mean length in the actually- exist in more than one associunleached and weathered phosphorites, ation. The element distribution pattern which varies from 9.29 ~ in weathered in the apatite structure indicate a posiphosphorites, to 9. 37 ~ in massive untive correlation of CaO and P2 05 (Fig. 7A). To establish the inter-element leached phosphorites. C O 2 - a o plots linear or otherwise, all show a wide scatter and r a n d o m corre- relationships, lation suggesting extensive leaching dur- the samples with variable P2 05 coning weathering. tents have been plotted. The major element regression plots of CaO - P2 05, P2 05 - CO2 and CaONa20 are given in Fig. 7A-C. Since, the range of valCHEMICAL COMPOSITION ues of minor elements are of less significance, the comparison of major elements has been deemed to be possible Spectrophotometric determination of between different petrological and chemmajor elements were carried out using ical types. Comparison of less-weatherthe U.S. Bureau of Analytical Standards ed phosphorites with the weathered ones B C R 1 and G S P 1 and B.A.S. Standards and variations in the composition of (U.K.) the T h o m a s Phosphate 31-A. their contained carbonate-apadte as reT r a c e elements w e r e determined by flected by the normalised structural atomic absorption and x-ray flouresconcentration values can be taken as eence techniques. Table 1 depicts the the measure of the chemical ingradients pressed due to overlapping by the predominant apatite peaks in most phosphorites. Using a computer p r o g r a m m e , all apatite peaks w e r e stripped-off f r o m the diffraction patterns, which resulted in the enhancement of the suppressed peaks. The residual pattern (Pig. 6B) is comparable to crandallite ( A S T M file no. 16-162) - an aluminous phosphate, which s e e m to have f o r m e d by variable substitutions in the apatite structure along the apatite -~ crandalite -~ millisite weathering series. T h e lattice parameters were calculated f r o m the observed diffraction patterns and by referring to hexagonal axes with the equation: 356 D.M. Banerjee eL al. of the depositional waters at the time of the phosphorite formation. Those ele40 ments which indicate negative correlation e.g. most of the plots of Na20 with respect of CaO suggest that only o~ 30 very small part of Na20 has substitut3 ed in the apatite structure, while the rest of it occurs in association of oth(:D er minerals. This random distribution cU 2O pattern also reflects varying degree of weathering at different locations. There is consistent decrease in the overall I IC trace-element concentrations in the 30 4'0 50 6O more weathered phosphorites except for Mn, which tend to be fairly high in CaO wt % weathered profiles (Table 2). Such a • distribution pattern of trace elements m ./ / / indirectly suggest a positive correla/ tion of various elements with CaO and / / P205 the essential constituents of o~ 2 / the carbonate-flourapatite. Trace elez / ee ments like U and Th show positive / cNJ / interelement correlation along the eno / tire profile (Fig. 8) from weathered to / / least weathered phosphorites. The CaO/ / / • Th, U correlation is represented by a curve which shows that the increase of 010 U and Th with gradual decrease of CaO 20 3'0 40' ' is a product of apatite-calcite leaching/ P205 wt % weathering near to the surface. Since the depth of weathering is not uniform, the distribution pattern of U and Th C with respect to CaO and P2 05 also varies in the same proportion. Therefore, it can be assumed that both U 3 and Th have no strong correlation • • • either with the apatites nor with the crandallites, but have been concentrated in the rocks as a result of leaching. oO,3 2 Figure 8 depicts the absolute abundance C; z of U with progressive leaching within the deposit. The ratio of Th/U varies from of 0. 32 and 0.23 to 0.42 with a mean significantly higher this is considered average of 0. I0 that the worldwide pos(~edepohl, 1969). It is however ] J I I I0 20 30 4O 50 6O sible that Th may have some crude affinity for the crandallite in the shaly CaO wt% phosphatic horizons. This type of relationship can be interpreted to be due to Fig. 7A-C. Regression plots showing: variability in the depths of leaching, A Positive correlation of CaO and and thus, suggest that most of U and P205 . B Crude linear relationship of Th in these rocks have formed by the P205 and CO 2. C Negative correlaprocess of secondary enrichment. CO 2 tion of CaO and Na20 A Precambrian 357 Phosphorites Top BI- -= AI- B2O C5=E AB- E D3- O0 8B- CaO- P205% B~tt0m- _ o ,'o 25 + + ~ 2'0 ~, Uronium (ppml 1o ~o Jo ,60 + ,~0 ,~o + ,~ Fig. 8. Graphic presentation of absolute abundance of uranium with progressive leaching within the phosphorite deposit of Hirapur-Bassia area + Fig. 9. Schematic diagram of the distribution pattern of phosphorite in a weathering/leaching profile at Hirapur-Bassia area (scale arbitrary) shows a weak and crude linear relationship (Fig. 7B) with P205 in most apatites, but in more weathered rocks the CO 2 content is higher, possibly due to secondary vein-filling calcites, and in this, the plots deviate significantly from llnearity. This near random correlation also indicates variable leaching. DISCUSSION The dolomitic limestone bottom at ]3assia is abruptly followed by sandy apatitie phosphorite and then by sandy clayey ferruginous phosphorite layers, each one of these show transition into other phosphorite types depending upon 358 D.M. Banerjee degree and intensity of weathering (Fig. 9) leaching, collapse and recementation. The bottom-most part of the phosphorite unit appears to have formed by the phosphatization of carbonates~ although the sharp boundaries between the two does not convincingly prove such an interpretation. The chemistry shows that these carbonate-flourapatites are poor in CO 2 content. In more weathered phosphorites, mineral phases of crandallite are more prominent, inspite of the presence of a dominant apatite phase. In carbonate-flourapatites, 4% CO 2 is considered normal concentration, consequently the low CO 2 values (Table I) of the Hirapur-Bassia apatites is due to the processes of decarbonation, caused by the dissolution and reprecipitation. The early illite-montmorillonite assemblage seem to have transformed into muscovite, which accounts for m o s t of the K and A1 present in these phosphorites. A part of K s e e m to have been derived f r o m the potassic minerals of the cratonic m a s s of B u n delkhand granitic c o m p l e x which also provided land derived phosphorus through weathering of the terrestrial cover. A s mentioned earlier, trace element content is low in the shaly and lateritic phosphorites c o m p a r e d to m a s sive bedded phosphorites. Most trace elements s e e m to have leached out of the system during prolonged episodes of weathering and leaching. T h e irregular distribution of crandallite is also due to differential weathering of shaly apatitic phosphorites. T h e carbonate flourapatite -* crandallite (Ca) transformation in these phosphorites appear to be in their early stages, c o m p a r e d to phosphorites of the Sonrai area of the western Bijawar basin, w h e r e minerals of m o r e advanced weathering stages -~ millisite -~ wavellite are also recorded. It is possible that the paragenetic sequence would have been the s a m e as those identified in the Sonrai area, ~Vest Senegal, parts of Florida, R u m j u n g l e area in Australia and parts of USSIq, provided the Hirapur-]Bassia et al.: Precambrian Phosphorites basin w a s not protected f r o m the onslaught of weathering for a considerable time after they w e r e lithified and uplifted. It is worth recalling that Mathur and i%,~ani (1978) recorded glacial tillites in the adjacent non-phosphatic area, apparently overriding the G a n g a u Formation and possibly m a r k i n g the unconformity over which the L o w e r Vindhyan sandstones w e r e deposited. It can therefore be visualized that towards the closing phases of G a n g a u sedimentation, warmer climates changed to cooler ones which helped in protecting the basic carbonate-flourapatitic character of the Hirapur-Bassia phosphorite. It is therefore possible that the weath~ ering was not prominent during the Precambrian times but was initiated vigourously during the subsequent periods. During the last phases of evolution of Bijawar rocks, land conditions prevailed with local fluvial influences and groundwater circulation through the exposed rock masses. The ferruginous crust, so common in such environments, were formed locally, but the sandstoneshales developed more extensive ferruginous coatings which also filled the intergranular spaces. This phase is associated with recrystallised aluminocalcic phosphatic cement which occur as rugs and cavity fillings within the alumino-phosphate rich phosphorites. There is gradual decrease in the detrital quartz content from the bottom to the top, and cement is more prominent in the surficial shales and phosphorites. In such places where the leaching was more intense, it induced numerous dissolution features leading to the formation of pseudo-breccia by the removal of cement, collapse and recernenration by circulating groundwater containing dissolved iron. It is significant to note that various mineral transformations noted above have not disrupted any of the p r i m a r y stratification or the microtextures, which suggest that these transformation reactions w e r e apparently isovolumetric due to leaching by p e r m a n e n t groundwater which received its supply through oO o r,q ~ d d ~ d d d d d ~ ddd,~d d ddddd'd I ddd~dddd~ ~ CO d ¢0 d + r~ ~ i ;q ~ 0 ~ i dddddddddg~ ddd~d 0 co C~ 0 ~ ? ~I S d ~ g t"- 0 d ~s 0 dgd O~ I ddddd 0 d'~ ddddl L~ ~ f~ 0 ~l ~' Fgd ddd d~'~ ~ d ~ I + ~> i ~ v ~dd~t d o oo o.1 ~ d g ~ d d d ~ d ddd~ ddddd~d~d~d ddddd O~ 0 g 0 ~.~ ~, d ~ 0 | ~dd~dMddd~'~ ~ddd I d c~ ~o 0 i~ oq i © c~ d~gdddddddg ddgdd CO co co ,-.4 d 0 @ 0 0 0 u~ r~ ;:q O~ o'~ ~ CO d d d d ~ dCOd ~ d dd~dd' d oO ¢0 O~ i d 0 o~ 0 ~ ~ ~ . oO68 % u! SelOU~ I~ [ % "~ ~. uo~n~.sod~uoo •o I ~ o a H @ O~ f~ I::q 0 > 0 o 0 0 0 ~ > ~ 0 ~ 0 0 ~ 0 0 c~ ;D 0 ~ o~ 0 0 I I I 1 "a o ~ • ® c~ 0 (Y 0~ 0 O~ 8~ 0 0 0 0 m % o~ -~ ~3 2 0 ~ "'2; d z ~ ~ cd . D . M . Banerjee et al.: P r e e a m b r i a n Phosphorites the wet tropical rainfall, a condition that could very well be imagined to have existed in the central part of India w h e r e the U p p e r Bijawars are n o w exposed, at least f r o m the Tertiary times, if not earlier. T h e intensity of weathering is dependent on the mineral association and the m o s t c o m m o n effect is the loss of C O 2 in the carbonate flourapatite strue ture. A s long as the carbonate is present in the rock it protects the associated apatite, halting the mineral transformation beyond the flourapatite stage (Lueas et al., 1980). Therefore, the apatites like those of Hirapur-Bassia area with low C O 2 contents could be safely interpreted to have evolved through the processes of weathering. With prolonged period of weathering, w h e n carbonate w a s almost completely leached out, other associated minerals w e r e attacked which released additional cations like Al and Fe. In the argillaceous host rocks, these released cations combined with available phosphates and f o r m e d crandallite, while in the alumina-poor detrital rocks, flourapatite r e m a i n e d the dominant phase and developed magnetite-goethite coatings. That iron w a s not readily available at the time of early phosphate sedimentation, is evident f r o m the fact that no iron-phosphate has f o r m e d in the sequence. However, in the voidfilling type remobilized and recrystallized phosphorites, it s e e m s that s o m e of these voids and fissures w e r e occupied by ferruginous laminations which have been systematically leached and isovolumetrieally replaeed by crystalline apatite grains, growing f r o m the m a r g i n s towards the centre of the void spaces (Fig. 4). Such features suggest that small quantity of iron w a s possibly introduced into the system subsequent to p r i m a r y phosphate sedimentation, possibly at late diagenetic stages, on the sediment-water interface and w a s subsequently leached out during early stages of weathering. W i d e s p r e a d ferruginous coatings of all the rocks in the area is due to region- 861 al ferruginization process, which s e e m to have a c c o m p a n i e d the regional silicification as the last stages of the geological activity in the region. It is also possible that s o m e F e w a s introduced in the sediments through epigenetic processes. To s u m m a r i z e , the phosphorites of Hirapur-Bassia originated along restricted shoals in littoral basins of the intracratonic sea which fringed the margins of the IBundelkhand granitic craton. T h e shallow nature of the basin has been interpreted on the basis of occassional rippled sandy surfaces, current cross-laminations, horizontal and w a v y laminations in the silty layers and p r i m a r y parallel laminations and raindrop-imprints on the ferruginous shales. T h e environment w a s highly oxidizing, as evident f r o m the abundance of hydrated and non-hydrated iron oxides, inspire of the fact that the basin had a restricted circulation. This also explains an observed fact that the organic matter contribution to the phosphate precipitation w a s negligible and the water depth of the sedimentation basin w a s extremely shallow, with occassional subaerial exposures. It is possible, such a shallow oxidizing basin w a s also responsible for higher concentration of lqa in the phosphate rocks. Acknowledgement. Authors are grateful to Vinod K u m a r of the Geological Survey and M. Sc. students V.N. Pathak and I.J. Banerjee for the help in the field, to P.C. P a d m a k s h a n and R a m e s h K u m a r for cooperation in the laboratory investigation, to G u e r r y H. McClellan for s o m e X - r a y diffraction data, to Jim Cathcart for arranging U - T h analyses by neutron activation technique at the U S Q S laboratories and to INarayan D a s and K . P . Cheria for checking u r a n i u m analysis at A t o m i c Mineral Division Laboratory at N e w Delhi. T h e investigation w a s partly funded by Council of Scientific and Industrial Research, N e w Delhi. 362 D.M. Banerjee et al° : P r e c a m b r i a n P h o s p h o r i t e s Banerjee, D.M. : Lithotectonic, phosphate mineralization and regional correlation of Bi.jawar Group of Rocks in the Central India: In Geology of the Vindhyachal, Valdiya, K.S. (ed.) p. 26-39, Hindustan Pub. Corp. Delhi (1982) Gulbrandsen, R.A.: Relation of carbon dioxide content of apatite of phosphorite formation to the regional facies: The Mountain Geologist. 8, 81-84 Medlicott, H.B. : On the Vindhyan rocks and their associates in the Bundelkhand. Mere. Geol. Surv. India 2, 1-95 (1959) Pant, A.: Resource status of rocks phosphate deposits in India and areas of future potential: Proc. Fertilizer Raw Material Resources Workshop, East-West Center, Honolulu. Sheldon, R.P., Burnett, W.C. (eds.) 331-357 (1980) Wedepohl, K.H. : Handbook of Geochemistry. New York: Springer, Berlin, Heidelberg, New York (1969) (1970) L u c a s , J . , F l i c o t e a u x , R . , Nathan, Y . , P r e v o t , L . , Shahar, Y . : Received: Different aspects of phosphorite weathering. S.E.P.M. Spl. Pub. No. 29, 41-51 (1980) Mathur, S.M. : A note on the Bijawar Series in the eastern part of the type area, Chattarpur area, M.P. Rec. Geol. Surv. 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