S H Rasul
Articles written in Proceedings – Section A
Volume 57 Issue 3 March 1963 pp 190-198
Talwara (23° 34′: 74° 22′) is a small Manganese-producing locality in the district of Banswara, Rajasthan. From the Banswara Town, Talwara can be reached by a motorable road. In the past, the Manganese ores of the locality have drawn very little attention of the geologists.
The ores of Manganese, being associated exclusively with the Aravalli limestone of Rajasthan, occur in the form of scattered masses occupying solution cavities, fissures, etc., in limestone. Aravalli quartzites, conglomerates, phyllites and slates also occur in the vicinity of Talwara.
The white limestone with a mosaic structure is of common occurrence and is composed of calcite and dolomite with some magnetite. The impurer varieties of limestone are characterised by the presence of tremolite, hornblende and quartz with a little of biotite. Schistosity is developed only in a calc-amphibole rock having alternating bands of hornblende and limestone. Crush-breccia is noticed occurring in association with manganiferous limestone at a distance of about 2 miles west of Talwara. This breccia might have originated due to severe local crushing of the country-rock. The limestones seemed to be a product of low-grade metamorphism of a calcareous sediment.
Fresh ores are generally hard and cavernous but on weathering they become softer and soily. Fragments of unreplaced limestone are occasionally present in the ore-bodies. This limestone has also been replaced by the Manganese ores to a greater or lesser extent, depending on the structural characters of the host rock.
The ores are composed largely of massive and colloform cryptomelane. Pyrolusite forms an insignificant part of the ores. There are two generations of cryptomelane, of which, the earlier colloform one is veined by the later generation of cryptomelane.
There are also examples of pseudomorphous replacement of limestone by cryptomelane. Most of the pyrolusite was derived from the supergene alteration of cryptomelane. From their nature and mode of occurrence the Manganese ores are classified as outcrop secondary ores formed in cavities in crystalline limestones belonging to the less metamorphosed type of Dharwar.
Volume 58 Issue 6 December 1963 pp 343-352
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.