The chemical composition and original nature of the Dharwar phyllites of the Shivrajpur-Bamankua area, district Panch Mahals, Gujarat state

Abstract

The chemical composition of the two most common varieties of non-manganiferous Dharwar phyllites of the Shivrajpur-Bamankua area was examined in an attempt to ascertain their original nature. The lithological and petrographical characteristics of these phyllites, studied in some detail by the author in connection with the main problem of genesis of the workable manganese ore deposits occurring in these phyllites near Shivrajpur (22° 26′ : 73° 37′) and Bamankua (22° 27′ : 73° 37′), indicate that their grade of metamorphism in this part of the country is low and comparable to the green-schist facies. Occasionally, the phyllites are intermixed with soft slate, shale and lithomarge.

The phyllites have a very high silica content, excess of alumina over the amount necessary for feldspar, dominance of magnesia over lime, potash over soda and ferric oxide over ferrous oxide and possess a composition fairly compatible with average pre-Cambrian phyllites and slates of sedimentary origin. The comparative study also suggests that nearly all the important chemical constituents of the phyllites are in normal order except silica, which is exceptionally high and the ferric/ferrous oxide ratio in this case, is in reverse order. However, the unusual order of these two constituents has not affected the conclusions seriously, as may be seen in the justifications given in support of it. The percentage by weight of combined water is unusually low and it might probably be due to progressive dehydration during metamorphism and intense folding of the pre-existing metasediments.

From the present investigation, the phyllites of the Shivrajpur-Bamankua area have been assigned to a sedimentary source consisting of somewhat highly siliceous pelites, which were partially metamorphosed into phyllites.