Layered intrusion outcrop areas are limited compared to many mafic intrusive bodies globally. Their origin is reported throughout geological time, from Archaean to Cenozoic, and they vary in size from less than a square kilometer to tens of thousands of square kilometers. Many layered intrusions occur within Archaean craton [1] , and global examples include the Bushweld Complex (South Africa), Great Dyke (Zimbabwe), Stillwater Complex (Montana), Norlilsk Talnakh (Russia), Pilbara (Australia), and Labrador Tough (Canada). These layered intrusions represent ultramafic to mafic litho-sequences with textural and compositional variations and are commonly associated with extrusive komatiite [2] - [8] . The layering is formed due to fractional crystallization by the differential settling process. The layered Intrusions are of significant economic importance as they are known to host base metals and associated precious metals.

Very few layered intrusions have been reported in Indian cratonic blocks. In the Dharwar craton, the layered sequence rocks are seen in the Nuggihalli schist belt and the Hanumalapura complex of West Dharwar Craton; the Kondapalli layered complex and the Sittampudi anorthosite complex of East Dharwar Craton. JC Pura schist belt lacks economic significance compared with other Dharwar schist belts even though they all evolved during Archean. In most of the earlier works, the petrological, textural, and structural characterization has been carried out in the JC Pura schist belt [6] [9] . Also, the layered sill of the ultramafic sequence has been reported from the JC Pura schist belt [10] [11] . In the recent findings, the scope for the JC Pura schist belt has opened up with the preliminary reporting of PGEs and trace elements like Cr, Ni, Fe, and Ti [12] [13] and traces of gold in magnetite grains [14] . The present study focuses on the layered ultramafic sill of the JC Pura schist belt. During our fieldwork, magnetite mineralization was observed as layered bands within the layered sill body and vein-type in adjacent serpentinized komatiites. Petrography and ore mineral phases were characterized for the samples collected during fieldwork to identify the mineralization type and understand compositional variations.

JC Pura schist belt is an oval-shaped Archaean greenstone belt in the western Dharwar craton ( Figure 1 ), dating back to 3.23 - 3.35 Ga [5] [15] . The rocks of the JC Pura schist belt are considered to be Sargur equivalent [16] , and the Dharwar group overlies these Sargur group rocks with an angular unconformity along the Kibbanahalli arm of the Chitradurga schist belt [15] . The JC Pura schist belt appears as a dome in its central portion due to the granitoid intrusion [9] [17] . The predominant rock types of the JC Pura belt are extrusive ultramafic rocks (dunite, peridotite, pyroxenite), basalts (amphibolite), minor cherty meta quartzites, and rare banded iron formation (BIF). The ultramafic rocks exhibit komatiitic characteristics and are intensely serpentinized, chloritized, and carbonatized. The talc-chlorite-tremolite schist indicates greenschist to lower amphibolite

facies metamorphism [6] . These Sargur equivalent rocks are surrounded by Peninsula Gneiss emplaced as synkinematic intrusions followed by granitic activity. The intrusive layered sill body is found within the extrusive komatiitic milieu of the JC Pura schist belt [10] . The layered sill is about 1 kilometer in length and 200 meters in width. Its lithology includes dunite, peridotite, and pyroxenite. All these rocks are partially to completely altered to serpentinite. The ultramafic sequences are of cyclic type with sharp contacts, which could be the result of episodic injection of undifferentiated to slightly differentiated magma pulses which solidified without much fractional crystallization [10] [18] [19] . Magnetite is noticed as band within a layered sill ( Figure 2(a) & Figure 2(b) ), and sometimes it cuts across serpentinite ( Figure 3(a) & Figure 3(b) ) showing feeble to moderate magnetism. Magnetite crystals are also seen as rolled grains near the layered sill and are oxidized. Magnetite vein width ranges from 2 - 4 cm. The serpentinite outcrops exhibit skeletal features due to weathering.

During fieldwork, the rock samples bearing mineralization were collected from the layered sill randomly. Five samples were chosen, and doubly polished thin section slides were prepared for petrographic and electron probe microanalyzer (EPMA) studies to understand the mineral phases and their characteristics. The mineral phases present in the studied rock were analyzed by deploying CAMECA-SX Five Electron Probe Micro Analyzer (EPMA) at the Department of Geology, Centre of Advanced Study, Institute of Science, Banaras Hindu University. Before analysis, the thin section was coated with a 20 nm carbon layer LEICA-EM ACE200 instrument for better conductivity. The CAMECA SXFive instrument was operated using a LaB6 filament source, at 15 kV accelerating voltage and 10 nA current. Wavelength dispersive X-ray spectrometry in combination with LIF (lithium fluoride), PET (penta erythritol), LPET (long penta erythritol), TAP (thallium acid phthalate), LTAP (long thallium acid phthalate) and PC1 crystals were deployed for the quantitative analyses. The beam’s diameter and peak time throughout the analysis was ~1 μm and 10 ns, respectively. X-ray intensities were calculated by using the X-PHI correction. The instrument was calibrated using the CAMECA natural and synthetic standards. The detailed procedure is documented in Pandey et al. [20] .

Metallic deposits in layered sequences such as primary fractionated layers and late hydrothermal emplacements, are ubiquitous worldwide. The lithotypes present in the study area are layered sequences of ultramafic rocks (dunite-peridotite-pyroxenite) with magnetite mineralization as bands and veins. These rocks are affected by significant hydrothermal activity due to the metamorphic process and are entirely serpentinized, forming serpentinite with few residues of olivine and pyroxene. The vein form of magnetite is formed during serpentinization and occurs on a centimetric scale, which is rarely observed in serpentinites [23] .

A petrographic study of samples from layered ultramafic sequence rocks suggests that the ore and silicate minerals developed rhythmically and contemporaneously, primarily due to episodic injection of primitive magma, which cooled with moderate fractional crystallization and could have some genetic relationship with the main komatiitic suites. The layered sill could be a basal adcumulate part of komatiitic magma [8] . The mineral assemblage of serpentine + talc + chlorite + magnesite + tremolite indicates transition facies of greenschist to amphibolite facies metamorphism [8] [24] . The Reflected light microscope study of the samples reveals that the source rocks host oxide and sulfide ore mineral phases. EPMA data reveals that the opaques in layered sill are Cr-magnetite and magnetite ( Table 2 & Table 3 ).

Chrome spinel is a ubiquitous mineral in ultramafic rocks. Cr-magnetite and magnetite are the bimodal alteration phases developed in the spinel transformation system (Cr-Fe³⁺) during the metamorphism or hydrothermal alteration exhibiting wide geochemical variation due to the varied substitutions of Fe and Cr. These spinel alteration phases are metamorphic grade indicators [23] - [25] . The formula for Cr-Fe³⁺ spinel series is

${\text{Fe}}^{2+}{\left({\text{Fe}}_{1-n}^{3+}{\text{Cr}}_n\right)}_2{\text{O}}_4$

where 0 < n < 1. The two end members of the system are pure chromite (FeCr₂O₄) for n = 1 and magnetite (Fe²⁺Fe³⁺₂O₄) for n = 0 [26] [27] .

The alteration of chromite to Cr-magnetite to magnetite is marked by a decrease in Cr³⁺ and an increase in Fe³⁺. Varying Cr³⁺ proportions are noticed in Cr-magnetite grains of the layered band (Cr3+: 0.023 - 0.2108 apfu) and vein forms (Cr³⁺: 0.4266 - 0.4506 apfu), indicating multistage alteration. The Cr-magnetite grains are compositionally homogenous in veins with high Cr concentration values and have slight substitutions of Ti, Mn, Mg, and Al, which are two to three times higher than those in the layered bands. The atomic ratios in Cr-magnetite exhibit high Cr#[Cr/(Cr+Al)]: 0.95 - 1 atomic ratios; low Mg#[Mg/(Mg + Fe²⁺)]: 0.0013 - 0.0470 atomic ratios, and depleted Al of 0.0015 apfu, which shows substantial alteration of the primary chromite [28] - [30] . Cr shows a positive correlation with Mg, Mn, and Ti, no correlation with Al and Ni, and a negative correlation with Fe³⁺, Fe²⁺, and Fe^3+/R³⁺ where R³⁺ is Cr+Al+Fe³⁺ ( Figure 8 ). The high Fe³⁺: 1.376 - 1.933 apfu; high Fe²⁺#[Fe²⁺/(Fe²⁺+Mg)]: 0.97 - 1.00 atomic ratios; and depleted Cr³⁺: 0 - 0.0094 apfu; Mg: 0 - 0.034 apfu; Al: 0 - 0.0011 apfu, indicates the complete transformation of spinel to Magnetite. The Cr-spinels are prone to solid alterations during prograde to retrograde metamorphism of host rocks and hydrothermal alteration [24] [31] . The chromite grains are wholly transformed into Cr-magnetite at amphibolite facies metamorphism with enriched Fe³⁺ and later into magnetite at greenschist facies metamorphism due to the reducing conditions [8] [24] . The altered opaque grains are depleted with Cr, Mg, Al, and Ni due to their redistribution under extreme stages of metamorphism. The EPMA data of studied oxide phases plotting midway of the Cr-Fe³⁺ line in trivalent ions (Cr-Al-Fe³⁺) ternary plot and falls within the fields of metamorphogenic magnetite of greenschist to amphibolite facies [31] ( Figure 9 ). The plot of Mg# vs. Fe³⁺/R³⁺ (R³⁺ = Cr + Al + Fe³⁺) and Fe# vs. Fe³⁺/R³⁺ reveals highly altered compositions characterized by strong enrichment of Fe³⁺ in metamorphogenic magnetite ( Figure 10 ). All Cr-magnetite samples except a few plots in between Cr-magnetite and magnetite represent additional miscibility gaps in bimodal phases of the Cr-Fe³⁺ transformation series of the Cr-Al-Fe³⁺ ternary plot [31] , and these varieties are termed as ishkulite [32] [33] , as the Cr³⁺ proportions range between 0.10 to 0.4506 apfu of studied samples matches with the 0.10 - 0.50 apfu of ishkulite in open database of minerals in mindat.org [34] ( Figure 9 ). Compositional variation is expected in chromites in ultramafic rock due to subsolidus re-equilibration of the chromites with surrounding silicates and interstitial melt [5] [8] [35] - [39] . The chromite grains alter due to hydrothermal effects (serpentinization) and metamorphism by reducing and then oxidizing by highly oxidizing fluid. During this process, chromite grains completely alter to homogenous Cr-magnetite [8] . During the hydrothermal alteration the interaction of thermal fluids with iron bearing minerals (olivine, pyroxenes and accessory chromites) in ultramafic rocks could cause the release of the reducing agents and lead to the formation of Cr-magnetite with an increase in Fe³⁺ by diffusion through oxidizing fluids. Metamorphic process facilitates the oxidation transformation of chromite grains as the temperature and pressure increase, causing breakdown by releasing chromium and iron and then recombine to form bimodal phases of ferrit-chromite and Cr-magnetite in spinel transformation system. This process involves primarily the oxidation of chromium and reduction of iron. Chromite (Fe²⁺Cr₂O₄) containing chromium in +3 oxidation state is oxidized to +6 state forming soluble chromate ions. Simultaneously, iron in chromite in +2 is reduced to a +3 state, to form as magnetite (Fe²⁺Fe₂³⁺O₄).

In the outcrops, mineralization is observed as bands in adcumulate ultramafic rocks of the layered sill and as veins in adjacent serpentinites. Petrographic and EPMA studies of opaques reveal Cr-magnetite and magnetite. The terrain has suffered an episodic prograde to retrograde metamorphism of greenschist to amphibolite facies with significant alteration of precursor rocks and minerals. Precursor chromite grains are completely transformed into homogenous Cr-magnetite and magnetite due to hydrothermal alteration and metamorphic process. During the transformation process, the elements Cr, Mg, Al, Mn, and Ni are depleted, and Fe3+ is enriched significantly. Cr-magnetite, representing an additional miscibility gap, is of the Ishkulite variety. Cr-magnetite appearing as centimeter-scale veins is mainly due to serpentinization, and such occurrences are rare in serpentinite. The cation concentrations of Cr-magnetite in vein form with significant Cr concentration and slight substitutions of Ti, Mg, Mn, and Al show the opposite trend with respect to the other metamorphogenic magnetites, indicating later hydrothermal alteration in episodic metamorphism. The layered sill appears to be the lower adcumulate part of the komatiite suite. The presence of Cr-magnetite as the vein in the study area would indicate possible chromite deposits hidden beneath, and the magnetite grains subjected to hydrothermal alterations could host noble metals like gold and PGEs. The present findings are new and are the result of a preliminary investigation. Hence, there is a scope for detailed studies to understand the full potential of mineralization.

We thank Prof. N. V. Chalapathi Rao, presently Director, National Centre for Earth Science Studies (MoES), Thiruvananthapuram, and earlier Professor, Department of Geology, Centre of Advanced Study, Institute of Science, Banaras Hindu University, India, for granting permission and extending support to conduct EPMA studies at their Laboratory. We also thank Dr. Rohit Pandey, Assistant Professor, Mantle Petrology Lab, Department of Geology, Centre of Advanced Study, Institute of Science, Banaras Hindu University, India, for extending kind support in EPMA analysis. Prof. N. Malarkodi, the Chairperson, Department of Geology, Bangalore University, India, and Prof. P. C. Nagesh, Former Chairperson, Department of Geology, Bangalore University, India, are thankful for their support during the course of our work.