Secondary phosphate mineralization in a karstic environment in Central Sri Lanka

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<ul><li><p>Mineral. Deposita 24, 169-175 (1989) MINERALIUM DEPOSITA </p><p> Springer-Verlag 1989 </p><p>Secondary phosphate mineralization in a karstic in central Sri Lanka Kapila Dahanayake 1,2 and S. M. N.D. Subasinghe 1 </p><p>1 Institute of Fundamental Studies, Hantana Road, Kandy, Sri Lanka 2 Department of Geology, University of Peradeniya, Peradeniya, Sri Lanka </p><p>environment </p><p>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. </p><p>Sedimentary phosphorite deposits are reported from sedi- ments dating back to the Precambrian (Banerjee et al. 1980, Howard &amp; 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). </p><p>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 </p><p>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). </p><p>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. </p><p>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. </p><p>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. </p><p>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? </p><p>In this paper the author attempts to answer this crucial question at least partially by considering the texture, struc- </p></li><li><p>170 </p><p>ture, and mineralogy of a modern phoscrete-type phos- phorite, forming presently in a tropical terrestrial setting. </p><p>Development of phosphate-rich weathering profiles </p><p>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). </p><p>The study area at Eppawala has daily temperatures ranging 28-36C 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. </p><p>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 </p><p>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). </p><p>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). </p><p>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). </p><p>80 *24 "E 80 "25"E </p><p>F Tat </p><p>From Kurune </p><p>8' </p><p>80 "26"E </p><p>l LANKA ~N </p><p>ura </p><p>$ ~ 1 Migma~itic gneiss I I J ] </p><p>f ~ Charnockite </p><p>"''[ Apat i te Marble </p><p>Ouartzite </p><p>@ Fig. 1. Location and general geology of the Eppawala phosphate occurrence (modified after Tayawardena, 1976) </p></li><li><p>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 </p><p>Phosphatic enrichment and formation of biolaminated neosedimentary structures </p><p>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. </p><p>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%. </p><p>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 </p><p>0 m </p><p>2 m </p><p>LATERITIC ZONE </p><p>Primary chlor-fluor-hydroxyi apatIte grains and sketetal apatite grains In a laterttle reddish brown sell characterized_ by the presence of crandalltte, hydroxyl eiIastadlte and jahnsltlte </p><p>LEACHED ZONE </p><p>Coated chlor-fluor-hydroxyl apatlte grains and pelolds In a laminated grounOmass consisting of secondary fluorapatlte, carbonate hydroxyl apatlte and hydroxyl apatite </p><p>PARENt ROCK (Apatlte marble) </p><p>171 </p><p>LEGEND </p><p>humlc s011 </p><p>laminated groundmass </p><p>[~ primary apattte grains </p><p>~ la ter l t l c ~-----]skeletal apatlte grain soil </p><p>~---~c0ated grains ~pe lo lds </p><p>--~mlcro-uncenformlty </p><p>Fig. 3. Schematic weathering profile (Eppawala Phosphate deposit) showing the generalized mineralogy and microstructures in the lat- eritic and leached zones </p><p>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). </p><p>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 </p></li><li><p>172 </p><p>Fig. 4. Photomicrograph showing large primary apatite grains in a calcite-dolomite matrix of the parent apatite marble (plane- polarized light, bar = 500 gm) </p><p>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) </p><p>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) </p><p>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) </p><p>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). </p><p>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 </p><p>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 pre...</p></li></ul>


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