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Knowledge Driven GIS Modelling Techniques for Copper Prospectivity Mapping in Singhbhum Copper Belt – A Retrospection
Basab Mukhopadhyay, Asit Saha, Niladri Hazra
Geodata and Database Division, Geological Survey of India
27, J. Nehru Road, Kolkata - 700016
E-Mail: gsi_chq@vsnl.com
(I) Introduction:
Data generated from the modern day exploration campaign are not only diverse but voluminous also. Sophisticated geological, geochemical, remote sensing and geophysical techniques combined with high-resolution ground and air-borne geophysical surveys are not only make mineral exploration more laboratory oriented but also makes the task difficult for interpretation. Many sophisticated techniques like stable and radiometric isotope analysis, fluid inclusion study and litho-geochemistry are introduced in exploration campaign for proper understanding the process of mineralisation and in turn also used for generation of genetic/exploration model for that commodity. Hence, the positive result of modern day exploration lies in the effective analysis of the datasets, the extraction of only the exploration relevant factors and integration of these factors to a single prospectivity map (Knox-Robinson, 2000). Visualisation and integration of these high volume of data require an analytical system viz. GIS, which has been designed for effectively store, interrogate and integrate diverse spatial and non-spatial data to generate prospectivity map depending on a hypothesis. Over the past decade, a number of techniques have been evolved to make use of exploration dataset and construct maps that illustrate how mineralisation potential or prospectivity changes over an area (Knox-Robinson and Wyborn, 1997). The GIS modeling methodologies in prospectivity mapping of a commodity can be categorized either as knowledge driven or data driven. There are several knowledge driven modeling approaches are available and which can be effectively transformed into GIS analytical environment: some of which can be summarized as Boolean logic, Index Overlay, Fuzzy Inference analysis and Vector Fuzzy modeling. The main objective of this paper is threefold: i) application of above mentioned techniques on the exploration datasets of Singhbhum Copper Belt on the basis of proposed exploration model ii) generation of prospectivity maps by different methodologies iii) comparison of results of different methodologies for generation of equivocal conclusion.
(II) Singhbhum Copper Belt - Geology and Mineralisation:
GSI has carried out extensive survey work in the form of systematic geological mapping on 1:63,360; 1:50,000 & 1:25,000 scales followed by detailed geological mapping of prospective locales on 1:10,000, 1:5,000 and 1:2,000 scales. Mapping work is intimately followed by airborne magnetic, electromagnetic and scintillometric sensor surveys accompanied by geochemical and ground geophysical surveys in selected areas to access the mineral potential mainly for copper. Nearly 600 boreholes are drilled to access the nature of copper ore-body disposition and estimation of reserve (Anon, GSI, 1991).
The Singhbhum Copper Belt, located in Jharkhand, Eastern India forms an arcuate highly deformed linear zone in the Singhbhum Crustal Province and considered one of the most potential sulphide bearing stretch of the country. The Singhbhum Shear Zone marks the boundary between a southern platform and a northern mobile belt. The Singhbhum Shear Zone is developed along the southern fringe of the Proterozoic Fold Belt of North Singhbhum. This fold belt is sandwiched between the Early Archean Cratonic Nucleous represented by Singhbhum and Bonai Granite in the south and Proterozoic Chottanagpur Granite Complex to the north. A curvilinear belt of metasedimentaries belonging to Dhanjori and Singhbhum Group of Proteropzoic age occupies the intervening gap area between the Singhbhum and Chottanagpur crustal province. The Singhbhum shear zone, which has developed in this Proterozoic belt, is a northery dipping arcuate ductile shear zone (Ghosh and Sengupta, 1987) marked by lenticular mylonite zone. The width and trend of the shear zone is 10Km & SW-NE in the western part, gradually narrows down to 1 km & E-W in the central part and again widens to more than 5 Km & NW-SE in southeastern part. In the southeastern part the shear zone splits into a number of N-S trending narrow shear zones (Banerji, 1981). In the western part the shear zone branches out and follow the northern and southern boundary of Chakradharpur Granite Gneiss. The rocks within the Singhbhum shear zone form a tectonic mélange comprising of granite mylonite, quartz-mica phyllonite, quart-tourmaline rock and deformed volcanic & volcanoclastic rocks (Mukhopadhyay and Deb, 1995). The shear sense indicators suggest a thrust type of deformation (Mukhopadhyay and Deb, 1995). The copper mineralized zone runs parallel to Singhbhum Shear Zone.
The copper mineralisation along Singhbhum Copper Belt is located along the Dhanjori Group of rocks south of shear zone and Singhbhum Group of rocks north of shear zone. The copper sulphide mineralisation is considered to be associated mainly with the meta-volcanics and meta-tuff sequences of the above mentioned Groups (Anon, GSI, 1991). The predominant chalcopyrite – pyrite – pyrrhotite ore mineral assemblage is concentrated along massive to braided veins, stringers, dissemination, discordant to sheet like bodies.
Stratigraphic control of mineralisation is completely absent in the area: no stratigraphic horizon has been found to exclusively contain the ore bodies. Lithological control in chemical sense is not also obvious (Sarkar, 1966a). In Badia and Mosabani, the orebodies are concentrated in soda-granite rocks whereas in Surda, Turamdih, Bayanbil and Tamapahar, this is concentrated in chlorite schist, chlorite-biotite-quartz and quartz chlorite schist. In many other places, the ore body is concentrated along sheared basic volcanics of Dhanjori Group. It is found that in soda granite rocks the ore bodies are richer and thicker compared to ore bodies in other lithounits (Sarkar, 1966a). Thus, it can be summarized that all ‘shear zone rocks’ are mineralized to a varied extent, out of which the sheared granitoids and metabasics are more mineralized compared to metasediments and metaultrabasic rocks. The orebodies in the area are represented by both sulphide and oxide facies. Chalcopyrite is the predominant sulphide mineral and oxide is represented by magnetite, ilmenite and rutile. Ore bodies are primarily concentrated along the major dislocation surface – the ‘Singhbhum Shear Zone’ and lineaments adjacent and parallel to it. The ore shoots are emplaced along the dislocation planes parallel to the shear surfaces. Mobilisation of the ore body is mainly taken place along shear bands, thus parallel to subparallel discontinous ore bodies (i.e. parallel to prominent structural grains of the area) is controlled by local trend of slip planes (Anon, GSI, 1991). The ore shoots are emplaced along the synformal part of B/ folds (reclined folds with down dip axis) and that too in their steeper southern limb. Axes of these B/ folds are parallel to the transportation direction. The down dip orientation of the lineation / fold axis of these folds indicate the plunge of the ore shoot (Sarkar, 1966a). Sengupta (1972) indicated that pervasive foliation parallel to regional axial planar schistosity developed in the early history of shear movement, has remained a potential plane of weakness along which there have been successive movement of different nature resulting in shear cleavage/ slip planes dipping at a gentler angle along schistose shear zone rocks. Sulphide mineralization which are later to this fractures localized them as ore zone which are lensoid or tabular in shape, linear in strike running parallel to the schistosity.
He also suggested that locally developed down dip cross warps have favoured the opening of foliation planes, also serves the locales of ore deposition. The entire shear zone is characterished by intense hydrothermal activity resulting in silicification, tourmalinisation, biotitisation, chloritisation and sericitisation in the rock. It is found that chloritisation is more characteristics for sulphide and uranium mineralisation (Dunn, 1937; Sengupta, 1972). Dunn (1937), opined that hydrotherms derived from the granitic magma at the later stage, gave rise to apatite-magnetite and sulphide mineralisation in two separate and sequential phases. As ore body with uniform ore mineralogical assemblage occurs along contrasted rock type, the contribution of shear zone rocks in ore formation may be considered as poor because of chemical incompatibility between the rock types. However, the shear zone rocks have generated favourable locales for ore bearing hydrotherms coming from deeper sources. Such ore bearing hydrothermal process can only generate uniform ore mineralogy in contrasting petrochemical assemblage (Sarkar, 1966b). P.R. Sengupta (1972) suggested that sulphide mineralisation is epigenetic and deposited from high temperature hydrothermal solution when deformation and metamorphism is well advanced.
The favourable locales are generated by defomation and metamorphism; preceded and in part accompanied by iron-magnesis metasomatism giving rise to biotitisation in apatite-magnetite mineralization and chloritisation for sulphides. Banerjee(1962, 1981) opined that major concentration of ore deposit is in the central sector where metasomatism predominates over granitic activity, indicating that metasomatism was the dominant process of ore localization. Talapatra (1968), worked around Mosabani supported the view of Banerjee (1962) that copper sulphide and associated minerals are generated from a dual source – partly extraneous and partly indigenous, and metal bearing migmatic solution invading the shear planes dumped its load at suitable sites. It is to be mentioned here that soda-metasomatism and sulphide mineralisation are not spatially closely related except at Badia-Mosabani (Sarkar, 1984). Sarkar (1984) proposed a volcano-hydrothermal replacement +/ volcano – sedimentary precipitation taking place before regional folding and progressive dynamo-thermal matamorphism. He opined that the ore mineralisation is of Cuprous type of volcanic sulphide deposit which are proximal in nature and deposited by near surface replacement of earlier rock/sediment. The d34S data supports the view (Sarkar,1984). From the fluid inclusion and isotope study from the samples of Mosabani mines, Changkakoti et. al. (1987) suggested that the temperature of sulphide mineralization is in between 275 and 4500 C, oxygen and hydrogen isotope studies indicate that ore fluids were evolved either from formation water with meteoric precursors or were deeply circulating meteoric water which equibrated with 18O-rich rock at elevated temperature. From the above discussion, it appears that main process of localization of sulphide minerals is by hydrothermal process. Most of the workers from GSI supported this view.
It is also found that zones of high aeromagnetic and ground geophysical anomalies proved effective in hidden target zones. The high copper anomaly in bed rock is taken for targeting hidden copper mineral deposit (Anon, GSI, 1991).
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