Graphite-bearing dolomite carbonatites in Newania, Western India
Doroshkevich A.G.*, Ripp G.S.*, Viladkar S.G.**.
* Geological Institute SB RAS, Ulan-Ude, Russia
**Carbonatite Research Centre, Amba Dongar Kadipani, India
The features of Newania carbonatites as primary mantle-derived melt have been described in (Viladkar, 1998). There are high Mg#s of carbonatites (up to 19 wt. % of MgO), low initial Sr87/Sr86 rations (0.7021), mantle-like C and O isotope rations (-5.1 and 8.1, repectively), absence of associated silicate rocks, and dolomite composition. Nd isotope studies (Gruau et al., 1995), noble gases and N (Basu & Murty, 2006) in Newania carbonatites also indicate mantle source. Newania certainly fits most of the criteria for a direct derived carbonatite magma; the key mineralogical features of the Newania carbonatites as mantle-like carbonatites are presence of magnesite, graphite and Cr-rich magnetite. Magnesite is an early crystallizing phase. Cr-rich magnetite and graphite are coexistent with carbonatite minerals and precipitated from carbonate magma.
The Newania dolomitic carbonatites are fine- to medium-grained, massive and banded rocks containing crystals and grains of amphibole and apatite. In the fine grained types the groundmass consists of dolomite. Dolomite is ferroan dolomite with 5–12 wt. % FeO. Mineral has up to 1.4 wt. % SrO. Phlogopite, Fe-magnesite, columbite are minor minerals. Monazite, ilmenite, aegerine, pyrrhotine, molibdenite, zircon, bismite, carbocernaite, pyrochlore, barite, rutile are accessory. Amphibole and phlogopite form thin bands. In addition, amphibole contains rare relic of early pyroxene (aegerine). Composition of amphibole varies from magnesio-arfvedsonite to magnesioriebekite. It is TiO2 and Al2O3 poor, but enriched in F (1.3-2 wt. %). Fe3+/Fe2+ varies from 0.5 to 1.0. Phlogopite is characterized by higher Fe/Fe+Mg (2.7-3) and TiO2 (up to 1.1 wt. %). Fe3+/Fe2+ varies from 1.5 to 2.9. The F content ranges from 2.6 to 2.9 wt. %. Magnesite forms rare, rounded to euhedral or subhedral grains (fig. 1, a,b), 0.5 -1.5 mm in size, in dolomite matrix.
Fig. 1. Mode of occurrence of magnesite (mgnst), apatite (ap) and magnetite (mt) in carbonatites from Newania (BSE images). a, b – replacement of magnesite (mgnst) by siderite (sid) with Fe oxides formation; c – apatite (ap) bands in dolomite (dol) matrix; d - thin shell of monazite (mon) around apatite grains; e, f - crystals and anhedral grains of magnetite; g - external composite ilmenite (ilm) in magnetite; h – lamellae of ilmenite in magnetite. dol – dolomite, c – graphite.
Mineral was replaced by siderite along cracks, cleavage cracks and rim of magnesite grains. Siderite is associated with secondary iron oxide (limonitic oxide), pyrite, Ba and Sr sulphates. Magnesite is ferroan magnesite, and siderite is magnesium siderite. Both, magnesite and siderite contain a small amount of MnO and CaO. Apatite is coexisting mineral with phlogopite, amphibole and columbite (fig. 1, c); it forms single grains as well as the bands and lenses. Mineral is fluorapatite with F content up to 4.9 wt. %. Mineral is highly strontium (SrO is up to 2.9 wt. %). Mineral contains solid inclusions of columbite and graphite. Often secondary monazite forms thin shell around apatite grains (fig. 1, d). Graphite forms flakes and their assemblages that dispersed in carbonatite. It also included as solid inclusions in apatite, amphibole, magnetite, columbite and sometimes mineral forms thin shell around magnetite grains (fig. 2).
Fig. 2. Distribution of graphite (c) in Newania carbonatites (BSE images). ap – apatite, dol – dolomite, amf – amphibole, clmb – columbite, mt – magnetite, ilm – ilmenite.
Magnetite occurs as crystals and rounded or anhedral grains with size up to 2-3 mm. Mineral contains irregular and poikilitic inclusions of dolomite and apatite (fig. 1, e, f). The bulk of magnetite contains a larger proportion of magnetite end-member with minor proportions of ulvospinel and magnesioferrite series. Mineral is enriched in chromium (from 0.4 to 1.4 wt. %) and V2O3 ( up to 0.3 wt. %) contents. Magnetite grains usually contain external composite ilmenite with trellis lamellae along {111} planes (fig. 1, g, h). Ilmenite is characteristically depleted in chromium (0.16 wt. %) and enriched in vanadium (0.8 wt. %) relative to magnetite. Mineral is characterized by higher Nb2O5 (1.9 wt. %) and MnO (1.2 wt. %).
Estimates of crystallization temperature for Newania dolomitic carbonatites were obtained from coexisting apatite-phlogopite and ilmenite-magnetite pairs, using apatite-biotite thermometer of Ludington (1978) and Fe-Ti oxide geothermometer of Andersen & Lindsley (1985). The apatite-biotite geothermometer yields temperature estimates in the range 693-9780C. Estimates of the relative HF fugacity, log (fHF/fH2O), based on Andersen & Austrheim (1991), using the apatite-phlogopite equilibrium temperature yield values between (-3.6) and (-4.9). Ilmenite-magnetite pairs gave temperature for the Newania groundmass crystallization from 649 to 675 0C. The oxygen fugacity shows the deviation from FMQ (3 to 3.5 log units relative to FMQ). The oxidation state of Newania source is embodied in presence of Fe-columbite, and composition of phlogopite, amphibole and pyroxene that have higher amount of Fe3+.
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