Mineral. Deposita 24, 169-175 (1989) MINERALIUM DEPOSITA

© Springer-Verlag 1989

Secondary phosphate mineralization in a karstic in central Sri Lanka Kapila Dahanayake 1,2 and S. M. N.D. Subasinghe 1

1 Institute of Fundamental Studies, Hantana Road, Kandy, Sri Lanka 2 Department of Geology, University of Peradeniya, Peradeniya, Sri Lanka

environment

Abstract. At Eppawala in central Sri Lanka secondary phos- phate mineralization is intimately associated with laminated fabrics within depressions (sinkholes and smaller cavities) formed in the thick weathering profiles of a hilly terrain underlain by a Precambrian apatite-bearing formation. The lowermost levels of the profile show extensive zones of leaching where derived apatite crystals occur within fine- grained, laminated stromatolite sequences. The stromatoli- tic groundmass, which diagenetically formed by percolating oxygenated phosphate and carbonate-rich groundwaters, is impregnated by the phosphate minerals francolite and collo- phane. Scanning electron microscopy (SEM) reveals that fine filaments, characteristic of microorganisms, are associ- ated with the secondary phosphate mineralization. Continu- ous degradation and fragmentation of the stromatolitic mat has produced pellets, peloids, and intraclasts all enriched in secondary apatite. Degrading recrystallization around the edges of the primary apatite crystals has developed coated grains. The widespread occurrence of phosphate-enriched allochems in stromatolitic groundmasses is a unique devel- opment of a modern phosphorite in a karstic environment.

Sedimentary phosphorite deposits are reported from sedi- ments dating back to the Precambrian (Banerjee et al. 1980, Howard & Hough 1979; Champetier et al. 1980; Horton et al. 1980). Riggs (1979 a, b) recognized phosphatic alloche- mical particles and orthochemical groundmasses in phos- phorites. Coated grains and other allochems occur in mi- crobial mats in phosphate deposits of different ages (Christie 1978; Soudry and Champetier 1983; Dahanayake and Krumbein 1985; Soudry 1987).

In modern sediment systems reported from the conti- nental shelves and slopes of Africa and South America (Baturin 1971, 1982; Burnett 1980; Veeh et al. 1974, Cook 1976) pellets, concretions, and nodules are common in fine- grained groundmasses enriched in organic matter. A widely accepted explanation for the formation of marine phos- phorite on continental shelves and slopes is based by Kaza- kov (1937) on the upwelling of cold phosphate-rich bottom waters. Although oceanic upwellings sometimes coincide with sites of modern phosphorite production, it is doubtful whether such localized phenomena could be responsible for some large phosphorite deposits, e.g. those of North Africa and the Middle East. Furthermore, phosphorites related to upwelling are impoverished in phosphate, e.g., 1.10% PzO5

on the Peru-Chile shelves (Manheim et al. 1975) and less than 0.2% on the continental margins of Brazil (Riggs 1979 a; Milliman et al. 1975).

Kolodny (1969) and Kolodny and Kaplan (1970) suggest that phosphate deposits could form by the reworking of ancient phosphorites. Phoscrete-type phosphorites that oc- cur in ancient and modern settings are derived from such reworking resulting from weathering and subsequent diage- netic phenomena in nonmarine environments (Southgate 1986). The main transformations observed during weather- ing of phosphate deposits are carbonate leaching and neogenic formation of clay and apatite phases. The trans- formation intensity is dependent on climatic conditions (Flicoteaux and Lucas 1984). The resultant weathering pro- files occur within a mixed-mineralogical suite of rocks and are characterized by fabrics and textures identical to those developed in calcretes (Read 1976). Coated grains, peloids, and other allochems dominate such profiles.

One widely accepted factor in the genesis of phosphorite is the relation between biological activity and the phosphate mineralization of all epochs (Christie 1978; Gulbrandsen 1969; Riggs 1979 a, b; Fauconnier and Slansky 1980; Daha- nayake and Krumbein 1985; Soudry 1987). A biogenic ori, gin has been suggested for some phosphorites because of associated organic remains. The possibility of activity by bacteria, fungi, or communities of microorganisms has been suggested. Early workers (Kazakov 1937) theorized on the inorganic precipitation of apatite in seawater undisturbed by strong currents. However, (Cook 1976) emphasized the role of biological decay and postdepositional processes in the production of phosphorite.

The literature on sedimentary phosphate deposits is voluminous. It has been the practice to ascribe marine ori- gins to phosphorites even when available evidence (e.g., as- sociation with ferruginous sediments) may indicate the con- trary. The mechanism of phosphorite formation is much more complicated than simple chemical precipitation of cal- cium phosphate from a nutrient-rich source.

The massive phosphorite deposits of the world and their associated assemblages of abnormal authigenic sediments are a consequence of complex processes which thus far have not been adequately explained. The absence of a simple comprehensive explanation for the origin of phosphorites caused Riggs (1979 b) to ask the question - are phosphorites being generated anywhere in the world today?

In this paper the author attempts to answer this crucial question at least partially by considering the texture, struc-

170

ture, and mineralogy of a modern phoscrete-type phos- phorite, forming presently in a tropical terrestrial setting.

Development of phosphate-rich weathering profiles

A phosphate deposit commonly referred to as the Eppawala phosphate deposit occurs in the township of Eppawala in the Anuradhapura district of Sri Lanka, located about 200 km NNE of Colombo (Fig. I). The deposit occurs in the form of a series of ridges underlain by apatite marble, dikes, pegmatites, and migmatitic gneisses at elevations of about 175 m above mean sea level. The basement rocks display complex folded structures such as kink folds and are tra- versed by several cross-cutting scapolite-diopside-apatite and apatite pegmatites. The rocks, which generally trending NS, are interbanded with charnockites, quartzites, gneisses, and marbles characteristic of the Sri Lankan Precambrian metasedimentary formations dated to be between 2 and 3.5 Ga (German-Sri Lanka Consortium 1987).

The study area at Eppawala has daily temperatures ranging 28°-36°C and is characterized by monsoon rains during the period from October to January with inter- monsoon showers spread throughout the rest of the year. The annual average precipitation is 1,200 mm. The area has been subjected to tropical weathering for a considerable length of time as exemplified by the thick weathering pro- files (on the order of 50-100 m) characteristic of this area.

A typical profile at Eppawala has an upper zone mostly lateritic and reddish brown in color with primary apatite crystals disseminated in the loose soil. Isolated skeletal

apatite grains, which formed due to leaching of their phos- phate contents, are observed in finer, loose or hardened lateritic matrices. The lower ,zone also referred to as the leached zone has larger concentrations of coated apatite grains and peloids within depressions such as sinkholes or smaller cavities developed on the parent rock, which is pre- dominantly an apatite marble. The leached zone is charac- terized by a multitude of micro-unconformities (Figs. 2, 3).

X-ray diffractometry of samples from both zones of the weathering profile shows the occurrence of primary and secondary phosphate minerals. The lateritic zone is cha- racterized by aluminous, siliceous, and ferruginous apa- tites, e.g., crandallite [CaA13(PO4)s(OH) 5 • H20], hydroxyl ellastadite [Cal0(SiO¢)s(SO4)s(OH,F)], and jahnsitite [(Ca-Mga)MnF%(PO~)(OH2) 1. However, the zone of leaching shows a concentration of fluorapatite and hydroxyl apatite, i.e., Caf(PO4)3F, Cas(PO¢)sOH and Cal0(PO¢)fCO3(OU,F) (Fig. 3).

The phosphate deposit at Eppawala has PsO s contents varying from 25% to 43%, and its citric acid solubility is low (1% 3%) (Amerasekera etal. 1981; Jayawardena 1976). Eppawala phosphate is currently used as a fertilizer for long-term crops, such as coconut, rubber, and tea, after crushing to a fine size and mixing with other ingredients. Recently, a method has been reported whereby a more solu- ble fertilizer could be produced from Eppawala phosphate (Tennakone et al. 1988). Considering the present rate of exploitation, the Eppawala deposit with a known reserve of 25 000 000 tonnes of phosphate can be used for more than a century (Tennakone 1988).

80 *24 "E 80 "25"E

F Tat

From Kurune

8'

80 "26"E

l LANKA ~N

ura

$ ~ 1 Migma~it ic gneiss I I J ]

f ~ Charnockite

"°'°'°[ Apa t i te Marble

Ouartzi te

@ Fig. 1. Location and general geology of the Eppawala phosphate occurrence (modified after Tayawardena, 1976)

Fig. 2. Weathering profile at Eppawala showing (a) the upper zone of lateritic soil with loose, irregularly distributed apatite crystals and (b) the lower leached zone with apatite crystals in a hardened groundmass. The modern phosphorite is forming principally in this zone

Phosphatic enrichment and formation of biolaminated neosedimentary structures

The deposit was enriched in phosphate by (1) weathering and dissolution of carbonate and phosphate from the parent apatite marble (Fig. 4), the more resistant apatite crystals becoming separated and accumulating in depressions in the parent rock (Fig. 4), and (2) secondary apatite growth around the apatite grains and within their interstices.

The original apatite crystals, recognized as chlorapatite and fluorapatite (Gunawardane and Glasser 1979) contain- ing between 39%-42% P205 (Jayawardena /976; Tazaki et al. 1987), are found within the weathering profile of the Eppawala phosphate deposit in various matrices and ce- ments. At certain points in the upper levels of the weathering profile single apatite crystals derived from the parent marble can be seen deposited in dark-brown lateritic soils. At other points in the lower leached levels of the profile the apatite crystals occur within a harder, cementlike, laminated groundmass (Figs. 2, 3). The crystals vary in length from a few millimeters to several centimeters, but rarely exceed one meter. The groundmass, which may be clayey, sandy, or conglomeratic, has P20 5 contents varying between 25% -30%.

In the leached zone the apatite crystals (which are mostly coated) are found in conglomeratic groundmasses with a laminated structure of alternating clear and dark layers, characteristic of stromatolites (Fig. 5), visible under the po- larizing microscope. Some primary apatite grains have fine- grained "envelopes" reminiscent of the carbonate-coated grains with outer crusts modified by reerystallization (Folk

0 m

2 m

LATERITIC ZONE

Primary chlor-f luor-hydroxyi apatIte grains and sketetal apatite grains In a laterttle reddish brown sell characterized_ by the presence of crandalltte, hydroxyl eiIastadlte and jahnslt l te

LEACHED ZONE

Coated chlor-f luor-hydroxyl apatlte grains and pelolds In a laminated grounOmass consisting of secondary f luorapatlte, carbonate hydroxyl apatlte and hydroxyl apatite

PARENt ROCK (Apatlte marble)

171

LEGEND

humlc s011

laminated groundmass

[ ~ primary apattte grains

~ l a t e r l t l c ~-----]skeletal apatlte grain soil

~---~c0ated grains ~ p e l o l d s

--~mlcro-uncenformlty

Fig. 3. Schematic weathering profile (Eppawala Phosphate deposit) showing the generalized mineralogy and microstructures in the lat- eritic and leached zones

1965; Voll 1960) or grain diminution (Orme and Brown 1963; Wolf 1965; Bathurst 1971). On closer examination of the envelope around the primary apatite crystals, a mosaic of fine collophane grains is observed. Scanning electron mi- croscopy (SEM) confirms the occurrence of an irregular perforated envelope characterized by finely crystalline, euhedral-subhedral hexagonal microcrystals of apatite on the order of 5 microns. Filaments suggestive of bacterial rods were observed locally in the envelope, which generally shows a gradational boundary with the groundmass. Some coated grains have several outer envelopes resulting from periodic growth, resembling superficial oncoids described by Dahanayake (1977; Fig. 7).

The groundmass supporting the apatite crystals reveals a generally laminated character and a network of micro- crystalline apatite in the form of francolite and collophane. The secondary mineralization is invariably accompanied by dark cryptocrystalline peloids, which appear to have formed by degradation of the microbial mat. When the degradation is minimal, the original stromatolitic laminations are clearly

172

Fig. 4. Photomicrograph showing large primary apatite grains in a calcite-dolomite matrix of the parent apatite marble (plane- polarized light, bar = 500 gm)

Fig. 5. Coated grains associated with thin dark envelopes of micro- crystalline apatite (francolite-collophane) around derived apatite grains. Note the faintly laminated stromatolitic groundmass. The mat has developed dark peloids (arrows in middle center and left center) at points due to diagenetic degradation processes (plane- polarized light, bar = 250 ~tm)

Fig. 6. Stromatolitic laminations (microbial mat) within a cavity formed in the parent apatite marble (left and middle center). Note the apatite of the cavity wall (right) which shows mineralization. The microbial mat shows little or no signs of degradation. The upper part of the depression shows pore spaces characterized by oncolitic growths (see details in Fig. 8; plane-polarized light, bar = 500 gm)

Fig. 7. A new microbial mat formed in the interstices between the intraclasts (arrows) and coated grains (C). The intraclasts have pleoids presumably derived from the older microbial mat (plane- polarized light, bar = 250 gm)

observed. Locally, two generations of microbial mat forma- tion are noted, i.e. the younger generation showing stroma- tolitic laminations with minimal recrystallization and the older episode characterized by pellets and peloids within a profuse francolite-collophane groundmass (Fig. 5). The younger mats also enclose remnants of the older mats, such as intraclasts and peloids (Fig. 7).

Phosphatic peloids (~100 ~tm) are dark-brown, well- rounded to subrounded particles of cryptocrystalline apatite. They formed, perhaps, as end products of degrading crystallization or grain diminution of the primary apatite

grains or fragments of the microbial mat. These pellets and peloids are comparable to those reported from Negev (Israel) which originated from bone fragments or forami- nifers (Soudry and Nathan 1980). Some of the larger peloids resemble pseudooncoids (Dahanayake et al. 1976; Daha- nayake 1977). Generally, these grains are smaller than the coated grains, and only the preliminary stages of grain dimi- nution are encountered within their envelopes. The pellets and peloids are opaque in thin section, though examination with SEM reveals the presence of apatite. Sometimes, these allochems are rare or absent in the phosphate deposit, and in such instances the stromatolitic mat shows more of its laminated character. Elsewhere, the degradation of the mat has preferentially taken place, and the pellets and peloids then appear in large numbers and in a variety of sizes. It was noted that these coated grains may be found within intra- clasts that show a faintly laminated character. The outer margins of the intraclasts may also show the microcrystal- line apatite envelope characteristic of the coated grains de- scribed earlier. It is interesting to note that the mineralized biogenic network of the groundmass merges with the outer margins of both coated apatite grains and intraclasts, which are comparably recrystallized through grain diminution. These observations imply a common genesis for the enve- lopes and the mineralized groundmass.

Within the upper parts of laminated depressions in the phosphate deposit dark, irregular shaped patches are ob- served filling or partly filling small cavities. On closer exami- nation, cauliflower-shaped interconnected or independent oncoid fabrics are noted, particularly in the open pore spaces.

These oncoid structures, comparable with carbonate on- coids in their internal structure and sequential disposition (Dahanayake 1978, 1983), exhibit two types of laminations,

173

Fig. 8. Oncolitic growth in the upper pore spaces of a stromatolitic sequence. The central dark nucleus is a porous microbial mat frag- ment around which concentric laminations have grown (arrows; plane-polarized light, bar - 500 gin)

Fig. 9. SEM micrograph of a cavity within an oncoid. Note the growth of hexagonal secondary apatite grains (bar : 20 gm)

Fig. 10. SEM micrograph showing the details of a laminated groundmass. Note the partially grown multiple apatite crystals with foliate structures (fine filaments branching out of their margins). The filaments of microorganisms (fungi and/or bacteria?) suggest the role of biological activity in the mineralization (bar = 10 gm)

i.e., clear francolite and dark collophane. They alternate concentrically around intraclasts or open pore spaces serv- ing as nucleii. The clear francolite laminae show a radial structure. The overall fabric seems to be the result of precip- itation within open spaces and is analogous to geodic crystallization (Figs. 8, 9).

These oncoid structures often posses central cavities, within which SEM observations reveal microcrystals of sec- ondary apatite fully or partly developed in association with branching (?) fungal or bacterial filaments of about one micron in length (Figs. 9, 10). The microcrystals when fully formed and compacted are about 10-20 microns long, and when partially formed or deformed about 5 microns long. The larger crystals are francolite, whereas the smaller ones are mostly collophane.

Discussion and conclusions

In the Eppawala Precambrian apatite-marble formation, the leached zone shows both primary and secondary phosphate enrichments. The primary enrichment is due to the weather- ing, dissolution, and removal of primary apatite crystals from the parent marble rock and their accumulation in de- pressions such as sinkholes and cavities. Stromatolitic mats, which have acted as sources for the secondary enrichment, have formed within these depressions. In these depressions,

sedimentary microenvironments have persisted, supported by the continual sedimentation from percolating waters en- riched in carbonate and phosphate. Such sedimentation has been facilitated by prolonged weathering and erosion in the tropical monsoon climate of the study area. The formation of coated grains, pellets, peloids, intraclasts, and oncoids within stromatolitic sequences of the Eppawala sedimentary environment reflects diagenesis within these sedimentary microenvironments. Of particular interest is the evidence of microbial activity in the form of clear-dark stromatolitic laminations and the fine filaments associated with the phos- phate mineralization. The dominant presence of phosphatic coated grains, pellets, and peloids, believed to have formed through recrystallization involving microorganisms, indi- cates such biological activity. Recent studies (Gulbrandsen 1969; Lucas and Prevot 1984; Dahanayake and Krumbein 1985; Soudry 1987) have demonstrated the important role played by microorganisms in the precipitation of apatite in some phosphorite deposits.

Concurrent with the formation of allochems, the stroma- tolitic (microbial) mat may also have undergone degrada- tion and mineralization producing a microcrystalline, francolite-collophane groundmass. The weathering, ero- sional, depositional, diagenetic, and mineralization phe- nomena seem to occur episodically as shown by the different generations of new and old microbial mats characterizing the Eppawala neosedimentary phosphate deposit.

174

PRECAMBRIAN RECENT

APATITE MARBLE

Y---..,\o APATITE-SCAPOLITE DIOPSIDE ROCKS

APATITE PEGMATITE

-- I Primary apattte crystals

~7 i2

(A) ~atherlng ~ ~ • (A) Microbial diss01utl0;, and erosion ,A, cdl ;otatloo . . . . . . . . . . . . . . . .

of the rocks peloids 0rowth of microbial ~:. '~:~: ? ' . : ~ ; ! | , o tlooo andlntreclasts tinne depressloos

depressions (slnkholes Grain diminution K8) 0eveloPmant bf oncolds ~i!-!~.',i~i:i!~g:~,~:~Zl

(o Concentration of ==4> coated grains, I ~ N . . . : : ~ . : : t primary apatlte crystals in depressions Secondary phosphate [ ~ ' ~ . t . . ~ & ~ . ~ l

mtnerallzatlon ~:,-v~o~-¢:..,~ .,..., (d) Percolation of (for~tlon of .......... ~... ~-.~ ~ P hp~phorl.te carbonate phosphate francolite ~2:~~ .-.-.:,.~ wK petmos waters and deposition ~ collophanei ~:; : :87:; i :~]] coated grains depresslons°f sediments in ~ / ,.,~omal~" " ~t ~ : ' : ' : ~ i l lntraclasts and o~oids

engulfing the i:.W':{~{~__~.~ _Z./ll (E) Formation of the prlmary apatlte : ' : ' ~ ! ~ : 1

microbial mat In crystals %25, ;:} ~;i~;;~i

~ ~ ~7. " '~ ' , : : . " - . . : [ !

Coated• gralns~,peloids and lntraclas[g in francollte - cellophane groun@nass

~ ~ Coated Grain ~ P e l o l d s ~Secondary ~ Oncold ~ Microbial I ~ I around an Fat , ~mlnerallzatlon apattte grain

Fig. 11. A schematic diagram summarizing the stages of formation of the terrestrial phosphorite at Eppawala

At Eppawala weathering and dissolution of the apatite- marble formation produce carbonate- and phosphate- bearing solutions, which percolate in the leached zone even- tually precipitating apatite when appropriate geological and microbiological conditions are met. Recent studies have shown that apatite precipitates alone (as is the case in the Eppawala sedimentary environment) when excessive phos- phate is supplied to the system by the degradation of organic matter (Gulbrandsen 1969; Lucas and Prevot 1984). Oxida- tion of organic matter to shift the equilibrium toward exclu- sive apatite precipitation requires an abundant supply of oxygen. In the subsurface weathering profile at Eppawala this supply is apparently assured through circulating groundwaters. This allows the phosphate present to precipi- tate as secondary apatite in the sinkholes and minor cavities. One might except that a considerable period of time would be needed to precipitate apatite, but recent studies by Sou- dry (1987) suggest that such mineralization could indeed be achieved within a matter of days. The derived apatite crystals in the continental, freshwater, sedimentary environ- ment at Eppawala are considered to be analogous to the shark teeth and fish bones that contribute to phosphorite formation in some marine environments.

The present study at Eppawala reports a unique environ- ment for the genesis of phosphorite in a continental karstic setting. Coated grains formed around primary apatite crystals, and peloids formed through the extreme phosphati- zation of coated grains and intraclasts are comparable to the foraminifers or bone fragments that produce similar parti- cles in ancient phosphorites (Soudry and Nathan 1980). In its allochemical and orthochemical composition the Eppa- wala deposit is comparable to many phosphorites reported from different stratigraphic levels (Fig. 11).

Thus, this example of modern phosphorite formation at Eppawala could perhaps contribute to solving the complex problem of phosphorite genesis.

Acknowledgements. The author wishes to thank Professor Cyril Ponnamperuma, Director of the Institute of Fundamental Studies, Sri Lanka, for the assistance rendered during the studies of the Eppawala phosphate deposit. Dr. J.W. Herath, Head of the Ceram- ic Research & Development Centre, Piliyandala, Sri Lanka, kindly assisted in the SEM studies. Mr. S.M.B. Amunugama and Ms. S. J. Wijesekera are thanked for drafting work, and Ms. M. Wickrama- suriya for typing the manuscript. An IDRC research grant (3-P-87-0333) is acknowledged.

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Received: July 12, 1988 Accepted: January 31, 1989