The influence of magnesium ions during the formation of

Abstract: The stromatolites of the Precambrian Aravalli Supergroup outcropping around Udaipur City are composed of carbonate (dominantly dolomite) and/or phosphate (carbonate-fluorapatite). The phosphate present in th stromatolites occurs in varying proportions, from traces levels to a m jor constituent. Compositional variation is tributed to the differences in the original magnesium contents stromatolites which reflects their different of the depositional environments, Intertidal stromatolites had primary magnesium calcite mineralogies, while those deposited in subtidal areas were composed of aragonite and/or lower magnesium calcite. The Mg/Ca ratio in the stromatolites appears to have been controlled by microorganisms which, it is argued, also played an important role in the process of phosphate fixation. It is inferred that the high Mg content of intertidal stromatolites prevented extensive phosphatisation in this facies. Phosphorites around Udaipur, Rajasthan, India (Fig. 1) are unusual in being strictly confined to stromatolites. They were deposited in marine epicontinental subtidal to supratidal environ-ments and now occur as a series of isolated deposits around the city (Chauhan 1979; Banerjee et al. 1980; Roy & Paliwal 1981; Sisodia 1986). The stromatolites are composed of carbonate and phosphate with both com-ponents in quite variable proportions, such that they can be classified as stromatolites with dominantly phosphate or dominantly carbonate compositions, or as stromatolites having alter-nate phosphate and carbonate laminae. The present investigation involved separation and geochemical analysis of these different cat-egories of stromatolite. In this paper we present the results of these analyses and discuss the distribution of major elements in the deposits. Geological setting The Udaipur phosphorites (see Choudhur