Stromatolite Park Jhamarkotra in Aravalli supergroup and A Geo-heritage site

Stromatolite Park Jhamarkotra has located about 25 km. southeast on Udaipur-Salumbar road. it is the largest & richest deposit of phosphorite associated with stromatolite. It is another site preserving pieces of evidence of early life on the earth. The stromatolites occur over a strike length of 15 km in rock phosphate within the Precambrian Aravalli Supergroup of rocks. The rock phosphate occurs in dolomitic limestone associated with stromatolites appearing in grey to bluish-grey color shades and invariable forms and shapes.

Jhamarkotra rock phosphate mine is the largest open cast mine in India and practically the only commercially exploitable deposit of rock phosphate in India, it has the greatest potential of rock phosphate than any other mine in India. The mines have a total estimated reserve of 80 million tons of rock phosphate however the deposit is highly disturbed by the folding and faulting of the beds during early proterozoic orogen known Aravalli-Delhi Orogen. Geological Survey of India declared this Stromatolite park as a National geological monument for their protection and maintenance.

Rock phosphate occurs in cavernous dolomite and it is present as a thin bed dipping at an angle of 45 degrees to 55 degrees to the horizontal. The top surface level is about 600 m. above Msl and mines working has gone to a depth of 190m. up to 410m. above Msl.

Geology

Around Jhamarkptra mines, metasedimentary rocks belonging to the Aravalli group lie unconformably over a basement of gneisses and granites. The banded gneisses are the oldest rock unit in the area.  The Stromatolitic phosphate in the area is hosted by the Dolomitic limestone which in places has become cherty and silicified towards the top. The phosphate are extending for about 15 km. with an average width of 15 meters. The stromatolitic Phosphate occurs as columnar, Brecciated, Stratiform, lenticular, and in minor quantities as a secondary colloidal deposit. The high-grade ore Jhamarkotra phosphate contains P2O5 content between 30 to 35%.

The blue-green algae flourishing in the shallow marine basins are said to be primarily responsible for stromatolitic structures and phosphate deposition under suitable temperature and pH conditions (7.1-7.8).  The study of stromatolites is significant in determining the trend and behavior of phosphate and its grade variations laterally as well as in-depth.

Stromatolite of Jhamarkotra

The presence of unique algal stromatolite fossils in the Proterozoic Aravalli dolomitic limestone 1800 Ma at Stromatolite Park Jhamarkotra attracted the attention of geologists all around the world. Stromatolite Park Jhamarkotra has various varieties, shapes, & sizes of stromatolites. it is characteristic by “crocodile-skin” weathering due to differential weathering of stromatolitic & surrounding material is of special interest.

Stromatolites are biogenic sedimentary algal structures classified on the basis of shape, structure, mode and kind of branching, convexity, etc. Three forms of stromatolites i.e. columnar, brecciated, and stratiform, have been recognized in this area of which brecciated stromatolites are mostly confined to the high-grade phosphorites while columnar stromatolites occur with low-grade deposits. Stratiform stromatolites are comprising interbedded laminae.

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In high-grade phosphorite, the brecciated fragments of stromatolites are welded together with calcareous and cherty matrix and were probably formed by tectonic activity causing brecciation and subsequent leaching and recrystallization of detrital stromatolites which have upgraded the ore. In the low-grade phosphorites, stromatolitic structures are better seen with an intervening carbonate matrix.

The Association of phosphorites with stromatolites showing typical surface manifestation has been of great value in stratigraphic correlation and as a guide for phosphate hunting in areas with similar litho-associations. Some of the features of stromatolites, like growth at right angles to beds and convexity towards the top, have offered valuable field guidance for correct structural interpretation.