JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.82, December 2013, pp.595-600
Tokapal Tuff-facies Kimberlite, Bastar Craton, Central India: A Nickel Prospect? N.V. CHALAPATHI RAO, BERND LEHMANN1, B.K. PANWAR, ALOK KUMAR and DATTA MAINKAR2 Department of Geology, Centre of Advanced Study, Banaras Hindu University, Varanasi, 221005, India 1 Mineral Resources, Technical University of Clausthal, Clausthal-Zellerfeld, 38678 Germany 2 Directorate of Geology and Mining, Raipur, Chhattisgarh 492007, India Email:
[email protected] Abstract: We report the occurrence of garnierite (a general term referring to Ni-Mg bearing hydrous silicates in laterites) from the crater-facies Tokapal kimberlite of the Bastar craton, Central India. Garnierite occurs as discrete ovoid or amoeboid segregations (up to 200 mm) or as veinlets with up to 18.1 wt% NiO and high iron contents (up to 36.2 wt% FeOT). Chemical composition of the garnierite implies its derivation from a magnesium-rich protolith. Extensive lateritisation of the large crater-facies (~2.5 km diameter) saucer-shaped kimberlite under tropical weathering conditions, aided by suitable topography, drainage and favourable structural set-up, are the factors inferred to be responsible for the formation of garnierite in the Tokapal system. As lateritic nickel ores constitute significant resources for nickel exploration, the perspective of the Tokapal kimberlite as a nickel prospect needs to be investigated. Keywords: Nickel, laterite, garnierite, kimberlite, Tokapal, Bastar craton, India. INTRODUCTION
Nickeliferous laterites provide about half of the world nickel production. They are derived from weathering of ultramafic rocks such as ophiolitic complexes, komatiites, dunites and peridotites. Nickel is liberated from magnesian olivine, which has up to 0.40 wt% Ni, and fixed in “garnierite”, a general field term for Ni-Mg-bearing hydrous silicate phases with a general formula of (Ni,Mg)3Si2O5(OH)4 (e.g. Faust, 1966; Brindley and Maksimovic, 1974; Springer, 1974; Talovina et al., 2008; Wells et al., 2009). It is important to mention here that garnierite has no specific mineralogical connotation and in fact is not recognised as a mineral name by the International Commission on New Minerals and Mineral Names (CNMMN) (see Proenza et al., 2008). Magnesian olivine with up to 0.43 wt% NiO (Kamenetsky et al., 2008) is a dominant mineral in kimberlites. However, no nickeliferous silicate phase is yet known, to the best of our knowledge, from kimberlites around the world. In this communication, we report the occurrence of garnierite from the crater-facies Tokapal kimberlite of Bastar craton, Central India. Characterisation of the garnierite aggregates by various microscopic and electron microprobe techniques (BSE imaging, X-ray elemental mapping, qualitative and quantitative analyses)
is presented, the possible factors responsible for the formation of garnierite in the Tokapal kimberlite system are evaluated, and the possible perspective of the Tokapal kimberlite representing a nickel prospect is expressed. BACKGROUND INFORMATION
The Tokapal kimberlite is a saucer-shaped volcanic sheet, about 2500 m wide, occurring in the western part of the Mesoproterozoic platformal sequence of the Indravati basin of the Bastar craton, central India (Fig. 1). Together with its NW exposure at Bejripadar (Fig. 1), the Tokapal kimberlite is one of the world’s largest (>550 ha) crater-facies kimberlite system (Ramakrishnan, 1987; Mainkar et al., 2004; Lehmann et al., 2006). Gravity-magnetic and resistivity surveys over the kimberlite system in the Tokapal-Duganpal area revealed that the body is >50 m in thickness at many places and well over 90 m at a few sites (Das et al., 2005). In situ U-Pb dating on autometasomatic titanite from Bejripadar tuff facies kimberlite gave a Neoproterozoic age of 620±30 Ma (Lehmann et al., 2007) which, however, may represents a later overprint given the recently obtained 1000-Ma age (UPb on zircon) of rhyolitic tuff in the top sequence of the Indravati basin (Mukherjee et al., 2012).
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spectrometry (EDS) using a Princeton Gamma-Tech (PGT) prism digital spectrometer (model OPE002-1038) attached to a CAMECA-SX100 Electron Probe Micro Analyzer (EPMA) at Mineral Resources, Technical University of Clausthal, Germany. Subsequently, the quantitative chemical composition was determined by wave-length dispersive spectrometry (WDS). TAP, LLIF and PET crystals are used and a number of natural as well as synthetic standards served for calibration. For quantitative analysis, an acceleration voltage of 15 kV, beam current of 20 nA, beam diameter of 1 µm and a counting time of 30 secs was used. The generated data was processed by using the PAP-correction program and PEAKSIGHT software program developed by CAMECA. After repeated analyses of respective standards it was observed that the error on the elements was less than 1%. RESULTS AND DISCUSSION
Fig.1. Geological setting of the Tokapal kimberlite system (adapted from Mainkar et al. 2004). Asterisk refers to the area where garnierite was found.
Petrographically, the Tokapal kimberlite is characterised by two olivine populations, i.e., sub-hedral to rounded macrocrysts (up to 15 mm) and euhedral fine grained microphenocrysts (100) from the various Indian cratons, or (ii) any of the kimberlitic rocks outside India. This raises an important question: What are the factors responsible for the formation of the garnierite in the Tokapal kimberlite? Nickeliferous laterites are residual products derived from
93.30
Garnierite from Brazil
42.16 0.27 14.90 15.95 0.075 1.74 0.25 5.44 19.28 100.067
olivine-rich rocks by either alteration or metamorphism. Their grade, tonnage and mineralogy are a function of climatic and geological factors, such as geomorphological and tectonic history, drainage, structure and lithology, in a dynamic system with each of them exercising a major influence that ultimately determines the characteristics of a deposit (e.g. Brand et al., 1998; Thorne et al,. 2009). Garnierite-type nickeliferous laterites are known to be mainly confined to humid and tropical regions where aggressive leaching of soluble elements such as Mg and Si results in residual concentration of Fe and Ni. Tectonic activity also has a profound influence on the relief, erosion and drainage, and garnierite silicate deposits are mostly associated with regions of moderate to low tectonic activity and relief and
Fig.3. A qualitative EDX elemental scan of a garnierite grain of this study. Abscissa is energy in kiloelectronvolts (keV) and ordinate is the elemental count. Note the lack of any peaks at the position of sulphur. See text for details. JOUR.GEOL.SOC.INDIA, VOL.82, DEC. 2013
TOKAPAL TUFF-FACIES KIMBERLITE, BASTAR CRATON, CENTRAL INDIA: A NICKEL PROSPECT?
Fig.4. Si-Mg-(Fe+Ni) triangular plot for Ni-bearing hydrous silicates showing the composition of garnierite (black circles) from the Tokapal kimberlite. The various other fields are after Brand et al. (1998).
high water table with a freely drained environment (Brand et al., 1998). The present-day geomorphology of peninsular India and the drainage pattern of its rivers are inferred to have been imposed during the Cretaceous due to an upwelling mantle plume that was responsible for the regional uplift and eruption of the Deccan flood basalts (e.g. Cox, 1989; Widdowson and Cox, 1996). During post-Cretaceous times, peninsular India was essentially marked by tectonic quiescence and rapid erosion of its uplifted mass. Hence, all the necessary requisites for garnierite formation viz., suitable climate, erosion and low tectonic activity are fully met in the domain of the Tokapal kimberlite system. However, the most important factors that appear to have decided the formation of garnierite here are the (i) large areal extent of the kimberlite protolith with a Ni content of 0.10-0.12 wt% (Mainkar et al. 2004), (ii) long-term exposure of the kimberlite on the peneplained domain (gently undulating flat surface of around 500m above mean sea level; Dutta et al., 1985) of the Bastar craton, (iii) the saucer-shape
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of the permeable tuff sheet which promoted aggressive leaching of Mg and Si resulting in residual enrichment of Ni and Fe, (iv) the high iron content of the kimberlite protolith of about 12 wt% Fe2O3 which led to a ferricrete cap resistant to erosion, and (v) a structurally controlled very well-developed drainage system in the area that promoted lithological permeability for fluid circulation inside the olivine-rich rocks. Further studies are clearly required to fully evaluate the role of the above, and other factors, in controlling the garnierite formation as well as a detailed characterisation of the garnierite by FTIR, Raman spectroscopy and X-ray diffraction. It is also important to asses the nickel potential of the Tokapal kimberlite as lateritic nickel ores are universally known to constitute significant resources for nickel mining. CONCLUSIONS
Garnierite is reported from the crater-facies Tokapal kimberlite of the Bastar craton, central India. This study also constitutes the first documentation of garnierite from Indian kimberlites. Garnierite essentially occurs as ovoid or amoeboid segregations and has up to 18.1 wt% NiO and high iron contents (up to 36.2 wt% FeOT). Extensive lateritisation of the large saucer-shaped crater-facies (~2.5 km diameter) kimberlite under tropical weathering conditions, aided by suitable topography, drainage and favourable structural set-up, are the factors inferred to be responsible for the formation of garnierite in the Tokapal system. The possibility of the Tokapal kimberlite representing a nickel prospect needs to be further investigated. Acknowledgements: NVCR thanks the Head, Department of Geology, BHU and Humboldt Foundation, Germany, for support. BKP and AK thank UGC and CSIR, respectively, for research fellowships. We thank Dr. Martin Wells (Kensington, Australia) for comments on an earlier version of this MS and an anonymous JGSI reviewer for useful suggestions.
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(Received: 29 April 2013; Revised form accepted: 25 September 2013)
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