GUWAHATI, June 5 - Based on the lithological composition and age-dating, the rocks of Shillong Shield, parts of Karbi Anglong and Goalpara districts are found to be similar to the Archaean rocks of Rajmahal area in mainland India. This proves that this area was a part of a single geological body. In geological time, it drifted apart due to tectonic movements.
Archaean rocks are the oldest rocks�formed in the pre-Cambrian era, that is � around 4 billion years back. It needs mention here that Guwahati, including north of both Nilachal Hillock and Umananda River Island, forms the northern boundary of the Shillong Shield.
The above was the observation made by noted geologist Dr B C Barooah, a former Director of the State�s Geology and Mining Directorate.� However, under the influence of tectonic movements in some areas of the Brahmaputra Valley, the Archaean basement complex went down and later it was covered by thick alluvial deposits, he maintained.
He also made a significant observation, saying that the Brahmaputra was originally flowing towards south to the Myanmar areas and met the Irawati. Later on, when the Patkai Hills was formed, it shifted towards west through its present course.
Archaean complex is later intruded by younger granite rocks of various colours and texture, which have high economic potential for decorative stone industry. At present, the granite rocks in the State are mostly used as ballast, which is of low economic value.
Moreover, blasting in our hills where decorative stones are available has also been causing irreparable damage to the decorative rocks. Blasting activities in these areas should hence be prohibited once for all. Long back, the Geology and Mining Directorate identified most of the State�s hills with such rocks. The Government has not exploited the potential of the granite deposits in the right manner.
The State should learn from the examples of the states of Gujarat, Rajasthan and South India. In these states, where granitic bodies are there, mining is done in a proper way for extracting decorative stones. Indian Bureau of Mines (IBM) has developed the ways and means for mining the decorative granites.
The State should implement the Minor Mineral Concession Rules, 1995 (as revised in 2013), properly and the Geology and Mining Directorate should be authorised in implementing the Rules.
The Archaean basement complex in Karbi Anglong District also has sillimanites, besides, in its Deopani area, kaolin is found, said Dr Barooah.�
Natural sillimanite rocks are used mainly in furnace and glass industries by cutting them into required shape and size. Sillimanite is the best raw material also for manufacturing high alumina refractories, alumina bricks etc.�On the other hand, kaolin, also known as china clay, is an essential ingredient to manufacture china and porcelain and many other things like paints, talcum powder, plaster of Paris, rubber, paper, etc.
Sillimanite deposits of Karbi Anglong are still to be explored in its entirety, said Dr Barooah.