PGE Mineralization in Metamorphosed Mafic-ultramafic Rocks of Madaura area, Bundelkhand craton, Central India- A Preliminary Study Jayanta K. Pati1, Thierry Augé2, S. Raju3 and Kuldeep Prakash1 1
Department of Earth and Planetary Sciences, Nehru Science Centre, University of Allahabad, Allahabad-211 002, India 2 BRGM, B.P. 6009, F-45060 Orléans cedex 2, France 3
Geological Survey of India (NR), Lucknow-247 667, India e-mail:
[email protected]
INTRODUCTION
GEOLOGY OF MADAURA AREA
Deep-seated, mafic and utramafic itrusives are ideal locales for nobel metal mineralization observed in different parts of the world. PGE mineralization in India is reported from Dharwar craton (Alapieti et al., 1994) and Singhbhum craton (Mondal and Baidya, 1997, Augé et al., 2002) associated with mafic-ultramafic rocks of Archean age. Penensular India comprises Singhbhum, Dharwar and Bundelkhand cratons. The present study is a preliminary assessment of PGE mineralization mainly with respect to Madaura area, Bundelkhand craton, Central India (Fig.1). Metamorphosed maficultramafic rocks are exposed in eight locations (Madaura, Rajola, Dangli, Ikonakhurd, Girar, Hanumatgarh, Siron, and Purani Pindar) over a regional spread of about 200sq km with cumulative outcrop size of about 50sq km area in the southern fringe of Bundelkhand craton. These are sulphidepoor and magnetite-rich rocks similar to the Stella layered intrusion (Maier et al, 2003).
The mafic-ultramafic rocks are exposed in the form of narrow lenticular outcrops within Bundelkhand Granitoids (2.5-2.2 Ga). The outcrops are mostly low lying, partly covered under alluvium and their contact relationship with the host granitoids is still unclear. The field relationship, petrography and chemistry suggest that the metamorphosed maficultramafic rocks possibly represent an
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episodic magmatism in this area. The mafic dykes are the youngest magmatic rock in parts of Bundelkhand craton (2.0-0.9 Ga). Recently Malviya et al. (2005) have reported TDM ages of mafic-ultramafic rocks in the central part of Bundelkhand craton to lie between 4.90 and 3.32 Ga based on Sm-Nd systematics. However, the age of these maficultramafic rocks is unknown and conflicting opinions with respect to their spatial and temporal relationship with Bundelkhand granitoids exist. Prakash et al. (1975) consider them to be younger than Bundelkhand granitoids (2.5-2.2 Ga), and on the other hand Basu (1986) has classified them to be the oldest rock in the Bundelkhand craton. Mylonitised pink granite is exposed closed to this maficultramafic outcrop but their contact relationship is unclear. Petrographically these are altered to various extents. However, on the basis of relict minerals and textures these rocks can be classified as meta-dolerite, meta-gabbro, meta-pyroxinite, meta-dunite. The dolerites show ophitic to sub-ophitic texture and are composed of plagioclase (partly sassuritised) + clinopyroxene (altered to mostly hornblende ± chlorite) + magnetite + epidote ± calcite. The gabbros are coarse grained, inequigranular and composed of tabular plasioclase (partly altered) + clinopyroxene (altered to various extents) + magnetite + epidote ± calcite. Meta-pyroxenite dominantly contains amphiboles (hornblende ± actinolite ± tremolite) in addition to other phases like talc ± serpentine, magnetite and ± calcite. Meta-dunites are extensively altered, retain cumulus texture and contain lots of olivine pseudomorphs (with rare relict core) + serpentine + chlorite + actinolite + magnetite ± calcite.
The mafic dykes are observed to cut-across the metamorphosed ultramafic rocks and granitoids and show a NW-SE trend. The mafic-ultramafic rocks (meta-pyroxenite and meta-dunite) are foliated (E-W) and show development of puckers on an earlier foliation. The gabbroic rocks are exposed as lenticular, E-W trending outcrops parallel to northerly dipping, sub-vertical mylonitic foliation observed in the host pink granitoids. The mafic dykes do not show any imprints of deformation possibly suggesting their postkinematics emplacement with respect to regional E-W ductile deformation. GEOCHEMISTRY The present study includes ten randomly chosen meta-dunite, meta-pyroxenite, metagabbro and dolerite samples. The MgO contents of meta-dunites lie between 25.7 and 36.9 wt.%, whereas meta-gabbro and dolerite samples have MgO values ranging from 8.6 to 13.9 wt.%. Similarly Cr content of metadunite is between 3500 and 7545 ppm and Ni values range from 1266 to 2090 ppm. Metagabbro and dolerite show Ni and Cr values up to 492and 55 ppm, respectively. These rocks are chemically distinct. Table 1. Analyses of select noble ppb) Sample no. Au Pt Mubk66 2 20.2 Mubk92
