The late Precambrian phosphorite deposits of the Aravalli Mountain belt occur as discontinuous outcrops within dolomitic limestone and silicified dolomite of the Aravalli Supergroup. They extend from Udaipur in the north to Jhabua in the south. Phosphorites concentrated in algal stromatolitic columns constitute the bulk of the deposit. Petrological, geochemical, and mineralogical studies of the phosphorites around Matoon (including Kharwaria), Kanpur, and Jhamarkotra of the Udaipur district and Khatama, Kelkua, and Amlamal in the Jhabua district have been carried out and the results have been incorporated in this paper.Matoon and Kharwaria phosphorites are stromatolitic, fragmental rocks, composed essentially of microsphorite (authigenic microcrystalline phosphorite mud), embedded in carbonate-chert matrix. Phosphates occur as nodules, intraclasts, and irregular fragments. Kanpur phosphorite is confined to algal stromatolitic columns and is composed of microsphorite grains embedded in a dolomicrite groundmass. Jhamarkotra phosphorite displays a larger spectrum of petrographic types, including algal microsphorite in a micritic groundmass, fragmental phosphatic intraclasts with recrystallized calcareous cement, and microsphorite clasts embedded in silt-sized quartz and pelitic intercalations. Recrystallized apatite crystallites in the fractures of the carbonate host rocks are also common. Phosphorite of Khatama is also microsphorite, rich in silt-sized detritus and pelitic disseminations. In Kelkua, microsphorite layers alternate with chalcedony layers and comprise micrite matrix with some detrital quartz.Bulk chemical analyses reveal the oxide distribution pattern in various deposits. The F/P 2 O 5 ratio is higher in Jhabua compared to the Udaipur deposits. Similarly, Na is more common in Jhabua phosphorite except for a few samples from Udaipur. The CO 2 content of the bulk rock is more or less uniform except where carbonates constitute a significant fraction of the microsphorite groundmass. CO 2 determinations in apatite concentrates by the X-ray peak-air method show a low CO 2 content in most concentrates indicating a very minor substitution in the fluorapatite structure. The most common phosphate mineral identified by X-ray diffraction studies is carbonate-fluorapatite with partial substitution by Mg for Ca in most samples and Na and Mg in samples from Jhabua. Partial substitution for (PO 4 ) by (CO (sub ) 3) is more pronounced in the Kanpur and Jhabua phosphorites. Electron microprobe analysis of minute apatites and discrete crystallites in the microsphorite was utilized for deducing the structural formula of average Precambrian phosphorites of India. The unit cell dimensions indicate a o = 9.362 and c o = 6.868 for Matoon apatites, a o = 9.366 and c o = 6.88 for Kanpur (stromatolitic) apatites, a o = 9.354 to 9.336 and c o -- 6.87 to 6.868 for Jhamarkotra apatites, and a o = 9.35 to 9.36 and c o = 6.88 for average Jhabua phosphorites. Heating of samples to 900 degrees C in an open platinum crucible for one hour resulted in a loss of water of crystallization and a consequent reduction and sharpening of peak heights with some minor shifts of diffraction lines. Crystallite size measurements by line-broadening techniques indicated a size range of a o = 1,500 to 2,000 Aa and c o = 1,700 to 2,000 Aa, whereas SEM photographs suggested a size range between 0.1 to 0.3mu m for the a o and c o axes, respectively. Infrared spectroscopy indicated the presence of one or more discrete carbonate phases mixed with apatite. These data conform with X-ray data presented here, but the "CO 2 index" does not show any apparent correlation with a o cell dimensions. On the basis of nature of occurrence, and geochemical and petrological characterizations, three important classes of phosphorites have been distinguished. These are (1) stromatolitic-carbonate biostromal phosphorite, (2) massively bedded phosphorite, and (3) fragmental-brecciated phosphorite. Each class is characterized by distinct ranges of P 2 O 5 , CaO, MgO, R 2 O 3 (Fe 2 O 3 and Al 2 O 3 ), and SiO 2 contents. Based on available data it has been postulated that these Aravallian phosphorites formed in protected shallow tidal to intertidal waters. While some stromatolitic assemblages have been recognized as ubiquitous, some deposits display specific and restricted algal forms. Geochemical studies and paleogeographic postulations indicate that the water chemistry in various paleodepressions, where phosphorite accumulated, varied marginally from place to place and supported a primary algal-induced biochemical origin for the microsphorites in these paleobasins. The possibility of phosphorite formation through the transformation of hypophosphites has been suggested as an alternate mechanism to explain the origin of at least some phosphorites of this area.

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