Geological Controls of Mineralization of Corundum at Gobbagurthi, Khammam Telangana India

Industrial grade Corundum and gem variety, (ruby and abrasive variety) occurs in and around Lakshmipuram and Gobbagurti areas of Khammam District of Andhra Pradesh. The area is within the Khammam Schist Belt., which is the northern extension of the Nellore Schist Belt of the Indian Peninsular Shield. Local entrepreneurs extract corundum based on certain faiths and beliefs since a long time. The nature of occurrence of corundum in these areas represents in-situ, float, and placer concentrations. Corundum occurs along with biotite gneisses, amphibolites and sillimanite-cordierite-diopsidebearing rocks, and magnesium-aluminium rich pelitic granulites. Tonalitic gneisses, cordieritecorundum rock and sillimanite schists form small lenticular bodies. The host rocks of corundum are intruded younger pegmatite veins that also have cut through the Precambrian suite of rocks. Distinct controls of corundum mineralisation manifested in geomorphology, structure and lithology are observed in an area that have proved to be very useful guides for understanding the various controls of corundum mineralization at Gobbagurthi as well as in surrounding areas of corundum occurrences in Khammam district.


Controls of Mineralization of Corundum
 Corundum mineralization is essentially manifested by lithology and structure. The structurally controlled drainage in prime mineralized areas carry considerable quantities of transported corundum.  The association of corundum both with pelitic schists and with tonalities exhibits lithological control. The chemically depleted rocks with SiO 2 and the well-defined planes of structural weakness is the loci of mineralization.  The gullied land with moderate slopes in continuation of the known corundum lithological signatures forms an important area for prospecting.  The N-S structures are important and significant which facilitated the emplacement of felsic intrusions. They also form the loci of mineralization.
 Distinct controls of corundum mineralization manifested in geomorphology, structure and lithology is observed in this area.

Geomorphologic Controls
The geomorphologic controls are manifested in structural hills, denudation hills, residual hills and pediment areas. The elevated reaches are occupied by thick thorny and bushy vegetation and sometimes inaccessible. The available map on land use/land cover studies indicated built-up land, forest, gullied land and water bodies. The gullied land with moderate slopes in continuation of the known corundum lithological signatures forms an important area for prospecting. The study area possesses dendritic to sub dendritic drainage. 1. Majority of the streams drain along NW-SE and N-S trending major stream and enter into the pediment area and finally discharge into the Wyra Lake. They lose their velocity as they approach the lower reaches and thus help in continuous deposition of their content in the meanders and confluence points of different nallas. They carry along with them colluvial corundum and all the culverts are the focal points for active exploitation of placer accumulations in this area. Felsic intrusive are observed along the Nagarjunasagar Right Canal.

Structural Controls
The Structural controls are envisaged in three major prominent trends along the directions NW-SE to N 35 o W-S35 o E, N-S with few degrees variations towards E-W and NE-SW to N 30 o E-S30 o W. The NNW-SSE or NW-SE trend generally coincides with the plane of schistosity of the amphibolites, which seldom possess mineralization of corundum. The N-S structural feature is the important and significant structure in the amphibolites that facilitated the emplacement of granite / pegmatite / aplitic intrusions. They form the loci of mineralization of corundum. The NE-SW (joints) structures indicate the postmineralized effects in the formation of en-echelon lensoidal occurrences in this area. In addition to the above, WNW-ESE and ENE-WSW trends are also observed which also localize corundum.

Lithological Controls
Lithology plays a protracted and dominant role in the mineralization of corundum. The contact zones between the felsic intrusions with schistose amphibolites localized the rich concentrations of corundum. Generally, the quartzo-felsic rocks do not enclose corundum. Perhaps, the tectonic elements, which facilitated the emplacement of quartzo-felsic and related rocks such as aplites, pegmatites and tonalities must have also acted as carriers of corundum and localized them with desilicified rocks along the contact zones as insitu occurrences in the study area. The garnetiferous amphibolites along their contacts with the felsic and pelitic schists enclose rich occurrences of corundum. However, the amphibolites as such do not exhibit mineralization of corundum. Occurrences of sillimanite-corundum and kyanite-corundum rocks in the study area only substantiate the contact zones where sillimanite schists and kyanite schists as a contact rock with the quartzofelsic intrusive (Pegmatite, tonalities, Cordieritecorundum gneiss) hosting mineralization of corundum. Kasipathi, and Bhaskara Rao, 1998 studied variety of corundum from different areas within the vicinity of Wyra, at Pallabhavi, Kodaratimetta, Lakshmipuram, Mekalkunta and Pallipadu areas and based on XRD studies, they were identified as potential resource for abrasive industry.
The XRD data confirmed their potentiality.