Petrology of Mn carbonate–silicate rocks from the Gangpur Group, India B.K. Mohapatra a, * , B. Nayak b a Regional Research Laboratory (CSIR), Bhubaneswar 751 013, India b National Metallurgical Laboratory (CSIR), Jamshedpur 831 007, India Received 11 April 2003; revised 6 February 2004; accepted 21 July 2004 Abstract Metamorphosed Mn carbonate–silicate rocks with or without oxides (assemblage I) and Mn silicate–oxide rocks with minor Mn carbonate (assemblage II) occur as conformable lenses within metapelites and metacherts of the Precambrian Gangpur Group, India. The petrology of the carbonate minerals: rhodochrosite, kutnahorite, and calcite that occur in these two assemblages is reported. Early stabilisation of spessartine, aegirine, quartz, and carbonates (in a wide solid solution range) was followed by pyroxmangite, tephroite, rhodonite through decarbonation reactions. Subsequently, jacobsite, hematite, braunite, hollandite and hausmannite have formed by decarbonation–oxidation processes during prograde metamorphism. Textural characteristics and chemical composition of constituent phases suggest that the mineral assemblages reflect a complex relationship between protolithic composition, variation of X CO 2 (!0.2 to w0.3) and oxygen fugacity. A variation of X CO 2 and f O 2 would imply internal buffering of pore fluids through mineral reactions that produced diverse assemblages in the carbonate bearing manganiferous rocks. A minor change in temperature (from around 400 to 450 8C) does not appear to have had any major influence on the formation of different mineral associations. q 2004 Elsevier Ltd. All rights reserved. Keywords: Mn carbonates; Mn carbonate–silicate rocks; Gondites; Gangpur Group; India 1. Introduction The Precambrian Gangpur Group, India comprises a volcanic free sedimentary sequence metamorphosed up to amphibolite facies. This sequence is well known for the occurrence of manganese oxide ores that are interstratified with Mn silicate rocks (Roy, 1966, 1981). Supergene alteration process enriched the manganese concentration and transformed these units into workable deposits that were exploited intermittently between 1907 and 1948. The manganese ores in this area are conformably enclosed within the pelitic Goriajhar Formation of the Gangpur Group. These manganiferous members interstratified with phyllite, mica schist, and meta chert, were deposited at about 1700–2000 Ma and metamorphosed at around 850 Ma (Sarkar et al., 1969). These manganese-rich assemblages are considered as analogs of the Precambrian Sausar Group rocks in Madhya Pradesh and Maharastra region of India (Roy, 1981). The manganiferous rocks that have been studied during the present investigation from Goriajhar and Manomunda area, Sundargarh district, Orissa, India are shown in Fig. 1. The manganiferous rocks of the Goriajhar Formation have been often, if not always, assigned to be of gonditic nature (e.g. Fermor, 1909; 1911; Krishnan, 1937; Prasad Rao and Murty, 1956; Roy, 1966, 1981; Nicholson et al., 1997). Gondites are defined as a metamorphosed equivalent of non-calcareous, manganiferous, arenaceous and argillac- eous sediment which is composed of spessartine and quartz with minor Mn silicates and oxides (Roy, 1965). These rocks do occur in the Goriajhar Formation of the Gangpur Group, but are of very minor abundance. However, spessartine, in general, is widely distributed in the rocks of the study area which do not have any affiliation with true gondite. Recent studies by the authors established for the first time that in addition to gondite (i.e. Mn silicate rocks with minor Mn oxides, type IIA of Dasgupta et al., 1990), 1367-9120/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2004.07.006 Journal of Asian Earth Sciences 25 (2005) 773–780 www.elsevier.com/locate/jaes * Corresponding author. E-mail address: bkmohapatra@rrlbhu.res.in (B.K. Mohapatra).