Formation of Phosphatic Coated Grains and Peloids by Grain Diminution of Precambrian Apatite Crystals

Due to tropical physical and chemical processes active on a Precambrian apatite marble formation, primary apatite crystals were removed from the parent rock and deposited in different types of depressions within a thick weathering profile. The primary apatite crystal margins show a microcrystalline envelope characteristic of coated grains. The envelope may extend towards the centre of the grain eventually forming a wholly microcrystalline apatite body out of the initial coated grain, thus producing a peloid by the process of grain diminution.

Microbial mats grow within the depressions engulfing the primary apatite crystals. coated grains and peloids of variable size. Continuous surficial processes together with subsurface percolation of sediment and nutrient-rich waters help to maintain the growth of the laminated stromatolitic groundrnass. Grain diminution seems to play an important role during diagenetic process in the phosphate micro-sedimentary environment of the depressions within the apatite marble formation. The grain diminution of Precambrian single apatite crystals to form a phoscrete-type phosphorite is perhaps a unique phenomenon.