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Trace-Element Fingerprints of Chromite, Magnetite and Sulfides

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Trace-Element Fingerprints of Chromite, Magnetite and Sulfides Contrib Mineral Petrol (2015) 169:59 DOI 10.1007/s00410-015-1148-1 ORIGINAL PAPER Trace‑element fingerprints of chromite, magnetite and sulfides from the 3.1 Ga ultramafic–mafic rocks of the Nuggihalli greenstone belt, Western Dharwar craton (India) Ria Mukherjee1,2 · Sisir K. Mondal1,4 · José M. González‑Jiménez3,5 · William L. Griffin3 · Norman J. Pearson3 · Suzanne Y. O’Reilly3 Received: 17 December 2014 / Accepted: 25 May 2015 / Published online: 24 June 2015 © Springer-Verlag Berlin Heidelberg 2015 Abstract The 3.1 Ga Nuggihalli greenstone belt in the (LA-ICPMS) of unaltered chromites indicates that the Western Dharwar craton is comprised of chromitite-bear- parental magma of the chromitite ore-bodies was a komati- 3 ing sill-like ultramafic–mafic rocks that are surrounded by ite lacking nickel-sulfide mineralization. In the Ga/Fe +# 3 metavolcanic schists (compositionally komatiitic to komati- versus Ti/Fe +# diagram, the Byrapur chromites plot in itic basalts) and a suite of tonalite–trondhjemite–granodior- the field of suprasubduction zone (SSZ) chromites while ite gneissic rocks. The sill-like plutonic unit consists of a those from Bhaktarhalli lie in the MOR field. The above succession of serpentinite (after dunite)–peridotite–pyrox- results corroborate our previous results based on major- enite and gabbro with bands of titaniferous magnetite ore. element characteristics of the chromites, where the calcu- The chromitite ore-bodies (length 30–500 m; width lated parental melt of the Byrapur chromites was komati- ≈ 2–15 m) are hosted by the serpentinite–peridotite unit. itic to komatiitic basalt, and the Bhaktarhalli chromite was ≈ Unaltered chromites from massive chromitites (>80 % derived from Archean high-Mg basalt. The major-element modal chromite) of the Byrapur and Bhaktarhalli chromite chromite data hinted at the possibility of a SSZ environ- mines in the greenstone belt are characterized by high Cr# ment existing in the Archean. Altered and compositionally (100Cr/(Cr Al)) of 78–86 and moderate Mg# (100 Mg/ zoned chromite grains in our study show a decrease in Ga, 2+ (Mg Fe +)) of 45–55. In situ trace-element analysis V, Co, Zn, Mn and enrichments of Ni and Ti in the fer- + ritchromit rims. Trace-element heterogeneity in the altered Communicated by Timothy L. Grove. chromites is attributed to serpentinization. The trace-ele- ment patterns of magnetite from the massive magnetite Electronic supplementary material The online version of this bands in the greenstone belt are similar to those from mag- article (doi:10.1007/s00410-015-1148-1) contains supplementary matic Fe–Ti–V-rich magnetite bands in layered intrusions, material, which is available to authorized users. and magnetites from andesitic melts, suggesting that mag- * Ria Mukherjee netite crystallized from an evolved gabbroic melt. Enrich- [email protected] ments of Ni, Co, Te, As and Bi in disseminated millerite and niccolite occurring within chromitites, and in dissemi- 1 PGE Research Laboratory, Department of Geological nated bravoite within magnetites, reflect element mobility Sciences, Jadavpur University, Kolkata, India during serpentinization. Monosulfide solid solution inclu- 2 School of Geosciences, University of the Witwatersrand, sions within pyroxenes (altered to actinolite) in pyroxen- Johannesburg 2001, South Africa ite, and interstitial pyrites and chalcopyrites in magnetite, 3 Australian Research Council (ARC) Centre of retain primary characteristics except for Fe-enrichment in Excellence for Core to Crust Fluid Systems (CCFS) and GEMOC, Macquarie University, Sydney, Australia chalcopyrite, probably due to sub-solidus re-equilibration with magnetite. Disseminated sulfides are depleted in plat- 4 Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY, USA inum-group elements (PGE) due to late sulfide saturation and the PGE-depleted nature of the mantle source of the 5 Department of Geology and Andean Geothermal Center of Excellence, Universidad de Chile, Santiago de Chile, sill-like ultramafic–mafic plutonic rocks in the Nuggihalli Chile greenstone belt. 1 3 59 Page 2 of 23 Contrib Mineral Petrol (2015) 169:59 Keywords Archean chromite · In situ trace-element · Chromites from massive chromitites (>80 % modal) of LA-ICPMS · Komatiite · Suprasubduction zone (SSZ) the Archean Nuggihalli greenstone belt in southern India setting · Nuggihalli (Figs. 1, 2) have been analyzed for trace elements using LA-ICPMS. The in situ trace-element study was conducted on both unaltered chromites and in chromites with core-rim Introduction zoning, in order to compare the distribution of trace ele- ments during primary magmatic conditions and second- Chromite is an efficient petrogenetic tool (Irvine 1965; Dick ary alteration. In addition to chromite, the trace elements and Bullen 1984; Stowe 1994; Kamenetsky et al. 2001; of magnetite and sulfides (interstitial to chromite in mas- Ahmed and Arai 2002; Mondal et al. 2006; Rollinson 2008; sive chromitites and to magnetite in magnetite bands, and Mukherjee et al. 2010), because its chemical composition is as inclusions in pyroxene within pyroxenite) from the Nug- controlled by mantle melting processes that are typical of gihalli greenstone belt have been studied. This has been particular tectonic settings. Studying the secular changes undertaken to understand their genesis and to identify the of chromite composition in different chromitite deposits is signatures of sub-solidus processes that modify their pri- therefore useful in understanding the evolution of Earth’s mary composition. Modification of primary magmatic com- upper mantle through geological time (Stowe 1994; Arai positions is expected in these minerals due to the extensive and Ahmed 2013). Chromites in massive chromitites com- hydrothermal alteration and greenschist facies metamor- monly retain primary compositions in terms of major ele- phism experienced by the rocks within the greenstone belt. ments and can thus be used to deduce the parental melt composition and the probable tectonic setting of the chro- mitite deposits (e.g., Mondal et al. 2006, references therein). Geological setting However, metamorphosed chromites from massive chromi- tites within ophiolites may be altered in terms of their minor The Nuggihalli greenstone belt is situated in the Western and trace elements (e.g., González-Jiménez et al. 2013, Dharwar craton in southern India (Fig. 1). This craton com- 2014; Colás et al. 2014). The study of chromitite deposits prises several early to Mesoarchean greenstone belts (3.4– in Archean greenstone belts helps to reveal the nature of the 3.0 Ga; Mukherjee et al. 2012 and references therein), also mantle and the tectonic processes prevalent in the Archean. known as the Sargur Group supracrustals (Swami Nath and The host ultramafic–mafic rocks to the chromitites in Ramakrishnan 1981), which occur as discontinuous lin- Archean greenstone belts typically are metamorphosed and ear belts (30–60 km 2–6 km) over the entire length of × hydrothermally altered and are not reliable indicators, thus the craton. The Nuggihalli greenstone belt belongs to the making the chromites indispensable in petrogenetic studies. Sargur Group, which is represented by conformable meta- In situ trace-element analysis using laser ablation ICP- morphosed (low-grade greenschist to lower amphibolite MS (LA-ICPMS) has revolutionized petrogenetic studies facies) volcanic (e.g., komatiite to komatiitic basalts and as one can investigate the minerals of interest without inter- tholeiites) and sedimentary assemblages (e.g., fuchsite- ference from the surrounding phases (e.g., Dare et al. 2009, quartzites, banded-iron formations, bedded barites and 2011, 2012; Pagé and Barnes 2009; González-Jiménez kyanite-garnet-bearing quartzites and mica schists) that are et al. 2013, 2014; Pagé et al. 2012; Colás et al. 2014). Com- surrounded by the tonalite–trondhjemite–granodiorite suite pared to major elements in chromites, the trace elements of gneisses (TTG; Fig. 1; Mukherjee et al. 2012). Sill-like are more sensitive to parameters like temperature and chromitite-bearing layered peridotitic rocks and magnetite- oxygen fugacity, and they therefore show larger variations bearing gabbroic rocks commonly occur within the early during partial melting and fractional crystallization (Dare to Mesoarchean volcano-sedimentary supracrustals, e.g., et al. 2009; González-Jiménez et al. 2013, 2014). The trace the Nuggihalli–Holenarsipur–Krishnarajpet–Nagamangala 3 elements that enter the trivalent (Cr, Al, Fe +) octahedral greenstone belts (Fig. 1). The chromitite-bearing plutonic site of the spinel structure, e.g., Ti, Ga, V and Co, are par- rocks in the Nuggihalli greenstone belt have been dated at ticularly sensitive to these processes. Thus, trace elements 3125 120 Ma by whole-rock Sm–Nd methods (Mukher- ± in chromites are useful for fingerprinting the parental melt jee et al. 2012). compositions and tectonic settings of the chromite depos- The Nuggihalli greenstones (60 2 km) form a linear × its. No work has been conducted so far to determine the in NNW–SSE trending belt in the Western Dharwar craton situ trace-element patterns of chromites in chromitites from (Fig. 2). The belt is comprised of a plutonic chromitite- Archean greenstone belts. In this respect, the in situ trace- bearing sill-like layered ultramafic–mafic unit that is sur- element data that have been generated in this study are sig- rounded by metavolcanic schists (komatiitic to komati-
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