The Nuggihalli schist belt (3.0–3.5 Ga; e.g., [14]) occurs as a band of low-grade metamorphosed ultramafic bodies stretching roughly NW–SE to north–south in the Dharwar craton, belongs to the greenstone belt in South India (see map, Fig. 2). Ultramafic rocks of the area consist of unaltered dunite, harzburgite, chromite bearing serpentinite, and talc–chlorite–tremolite schist. Other major rock types are amphibolite, metasediments (fuchsite quartzite, quartz–mica–chlorite schist and staurolite–quartz–mica schist), tonalite trondhjamitic gneisses and meta-anorthosite. Titanomagnetite bodies rich in vanadium are intercalated with the ultramafic rocks. The area under study has been subjected to greenschist-to-amphibolite facies metamorphism and shows increase in grade from north to south [19].

The chromite bodies occur commonly as layers, high angle bands and in irregular tabular or lensoid forms within serpentinite. The size of the chromite deposit varies from lenticular bands ∼300 m in length, with a width of 6 m at Tagadur mines and pods of a few metres to more than 60 m size, extending to a depth of ∼100 m in Byrapur underground mine. The spectacular rosy moss-like aggregates of pink chlorite (Cr-bearing clinochlore) occur as veins and fracture fillings and as pockets in grey chromite ore bodies [4]. Based on the presence of pillow structure and spinifex texture in peridotite, Naqvi and Hussain [15] suggested submarine extrusion to have occurred. From the Cr/Al ratio in serpentinite, komatiitic affiliation was deciphered by Sudhakar [22]. The identification of a komatiitic affinity for the ultramafic rocks of the belt led to the suggestion by these investigators that this belt is an example of Barberton type greenstone belt, in which the komatiitic rocks are considered to be pristine and primordial. The Cr/Fe ratio of chromite indicates a layered intrusion complex [16].

Probable stratigraphic succession of the belt modified from the one reported by Jafri et al. [10] is presented below:

• – dolerite dyke,
• – meta-anorthosite,
• – gneisses,
• – dunite,
• – titanomagnetite, gabbro, metasediments,
• – amphibolite,
• – perodotite/serpentinite, with bands of chromite, tremolite–talc schist.

The chromite samples were collected from chromitite bodies which run as bands and lenses within serpentinised masses. The chromite samples were chemically analysed by a JEOL-733 electron microprobe with wavelength dispersive method at 15 kV with a beam current of 10 nA and a beam diameter of 10 μm. Average spectrum counts (10 s×5 times) were compared with natural standards and data were corrected by the Bence and Albee method [3]. The minerals that were used as the standards are; albite for Si and Na, almandine for Al and Fe, olivine for Mg, rhodonite for Mn, rutile for Ti, diopside for Ca and sanidine for K. Table 1 shows the chemistry of studied chromites. Detail chemistry of relict and metamorphosed mineral phases (viz., olivine, pyroxene, amphibole, chlorite, etc.) will be presented in a subsequent paper.

We have plotted the chromite chemistries in 100×Cr/(Cr+Al) (Cr^∗) vs 100×Mg/(Mg+Fe²⁺) (Mg^∗), which revealed that the studied samples fall in the cluster of the well-known stratiform deposits of the world. The modified 100×Fe³⁺/(Cr+Al+Fe³⁺) (Fe^3+•) vs 100×Mg/(Mg+Fe²⁺) (Fig. 3) and Cr₂O₃ vs Al₂O₃ wt% (Fig. 4a) plots show the komatiitic affinity of the chromitite bodies. However, FeO (total) vs Cr₂O₃ (Fig. 4b) simultaneously brings out its continental layered characteristics. From these plots, it can be emphasised that the chromite from the komatiites and stratiform deposits fall in the same compositional field. We have used Rollinson's Cr/(Cr+Al) vs Fe²⁺/(Fe²⁺+Mg) plot [20] (Fig. 5) to compare the studied chromitites with reported komatiitic chromitite of Zimbabwe (Belingwe, Inyala and Shurugwi). They all fall in the same range (Cr/(Cr+Al) varying from 0.6 to 0.9 whereas Fe²⁺/(Fe²⁺+Al) from 0.26 to 0.87) as reported. This comparison is instructive, because the mentioned komatiites are the most studied and offer the best opportunity to study primary magmatic mineral deposits. We have also plotted our chromite data in the Barnes and Roeder [2] longitudinal spinel projections of Cr/(Cr+Al) and Fe³⁺/(Cr+Al+Fe³⁺) vs Fe²⁺/(Mg+Fe²⁺) and TiO₂ vs Fe³⁺/(Cr+Al+Fe³⁺) diagrams. All the studied samples show their komatiitic affinity.

The chromite–olivine assemblage has been used to determine the temperature of formation of the chromite ores. The calibration proposed by Fabries [8] has been used to determine the temperature in the present study. This thermometer is successfully applied to the olivine having Fo content ∼90% and it has reasonably been used in the present case, since the Fo content of the olivine (83–92%) is close to that value. Detail chemistry will be presenting elsewhere. The thermometric relation is:

In the above equation,

The Fe²⁺ and Fe³⁺ contents in chromites have been based on stoichiometric calculations. Using the above equation, the temperature of formation of Nuggihalli chromite has been determined as 1178 °C.

The oxygen fugacity of the chromite from the Nuggihalli schist belt has been determined by the method discussed in [13]. According to this method the f_O2 is determined from Fe³⁺/(Cr+Al+Fe³⁺) ratio of chromite owing to its strong dependence on f_O2 and very weak dependence on the melt composition. To determine the f_O2 of Stillwater and Bushveld complex, some [13] used this method, the results of which are consistent to the estimation by other methods. The f_O2–T paths of natural basalt are sub-parallel to QFM (quartz–fayalite–magnetite) buffer curves [6]; therefore, f_O2 of natural magma decreases with decreasing temperature. For this reason, to eliminate the normal temperature effect f_O2 values are expressed relative to one of buffer curves. Error in 200 °C temperature of crystallisation makes an error in f_O2 values only of 0.5 log unit. The EPMA and Mössbauer study lead to the determination of 100×Fe⁺³/(Al+Cr+Fe³⁺) as 15.6. The temperature of formation of Nuggihalli chromite has been determined from olivine–spinel geothermometer as 1178 °C. Since most of the chromitite layers form within 1250±100 °C, curves at that temperature of Fig. 6 are reasonable [13]. Therefore, the 1250 °C curve of Fig. 6 has been used to determine log f_O2, which comes as −6.67.

The Nuggihalli chromite which crystallises at ∼1180 °C with an oxygen fugacity of −6.67, appearing from their chemical plots (Figs. 3–5) indicates their komatiitic affinity. Working on the Cr-spinels from the Staré Ransko gabbro-peridotite, Czech Republic, van der Veen and Maaskant [23] reported that all the spinels from the investigated samples plotted below the discriminant curve A (Fig. 1), which indicates a strong possibility for sulphide mineral association. This indeed was true. While the spinels from the Horni Bory xenoliths [12], showing no evident presence of sulphides etc. plotted distinctly above the discriminant curve. Much to the surprise of the present authors the spinels of Nuggihalli plot remarkably along the curve itself. This observation reinforces the logic of van der Veen and Maaskant [23] and redefines the validity of the profile of the boundary curve of Johan [11].

Fig. 1 shows that all the spinels from Nuggihalli schist belt plot on the discriminant curve A but a few plot within the barren zone close to the A curve. This evidently points the possibility of chromitite band (along with its silicate host rocks) being barren rather than fertile. This observation eliminates the strong possibility for chromitite to host any mineralisation of sulphides or their kind.

The reported occurrence of sulphides [17] (chalcopyrite, pyrite, pyrrhotite, cubanite and pentlandite) in the close proximity of the ultramafic intrusions in this belt is inevitably seen to be confined within the marginal part of gabbroic masses, intrusive into the ultramafic (later metamorphosed) bodies hosting the chromite and titanomagnetite bands.

The validity of the discriminant curve A of Johan [11] thus receives a further endorsement from the chromite characters of the greenstone belt of south India.