The Proterozoic Chimalpahad Anorthosite Complex (CAC) is a deformed and dismembered complex in South India comprising of anorthosite-leucogabbro-gabbro-ultramafic rocks. It is located in the Nellore-Khammam Schist Belt (NKSB) within the contact zone between the Eastern Dharwar Craton (EDC) and the Eastern Ghats Belt (EGB). The chromitites occur as small dismembered lensoidal bodies within the ultramafic rocks. Presence of micrometric inclusions of iridium-like platinum-group minerals (IPGM), such as laurite (RuS2), erlichmanite (OsS2), irarsite (IrAsS), and Ir-Os-Ru alloys, is reported for the first time in the CAC chromites. The chromite cumulates preserve magmatic character (Cr#: 0.61 to 0.77; Mg#: 0.29 to 0.55), while disseminated chromites and rims of some grains (Cr#: 0.59 to 0.89; Mg#: 0.10 to 0.25) show postmagmatic modifications. The chromites are characterised by higher Cr and Al contents and lower Fe and Ti contents. The mineral chemistry of chromites such as Cr# and Mg# contents of Al2O3 (10.05–20.07 wt.%) and TiO2 (0.1 to 0.25 wt.%) typically suggests that the CAC chromitites and their ultramafic hosts were generated from the S-undersaturated boninite melt in a magmatic arc in suprasubduction tectonic setting. A general negative slope of bulk-rock PGE distribution with upward convex pattern is observed for the CAC chromitites in the chondrite-normalized plot. These geochemical and mineral chemical characters in addition to location of the chromitites and the CAC in the tectonic zone between Eastern Dharwar Craton and Eastern Ghats Mobile Belt point towards ophiolitic affinity of the CAC chromitites. Laurite (RuS2) grains are trapped close to the cores of chromites, whereas erlichmanites (OsS2) are trapped away from the cores. Textural relations indicate primary orthomagmatic nature of laurite and Or-Ir-Ru alloys and their precipitation prior to or concomitant with chromite from an S-undersaturated melt at ~1200–1300°C and logfS2 varying between −2 and −1.3. Ru-Os disulfide and erlichmanite crystallized subsequently with a gradual decrease in temperature in the differentiating ultramafic magma. Precipitation of irarsite and base-metal sulfides can be attributed to local and rapid fluctuations of temperature, fS2, and fAs during the formation of the chromitites due to influx of fresh batch of melts or crystallization in an open magmatic system. The compositional variations of laurite-erlichmanite record heterogeneous physicochemical environment during their crystallization. In addition to zoning, preservation of IPGMs in the CAC chromites points towards fast cooling of the magma possibly associated with rapid exhumation of the chromitites and their ultramafic hosts from the roots of the subcontinental lithospheric mantle (SCLM) towards their final emplacement into the crust. Occurrence of the CAC along a linear extension with the Kondapalli Ultramafic Complex and deformed remnants of ophiolites (Kandra and Kanigiri complexes) along the eastern margin of the Eastern Dharwar Craton (EDC) bordering the Eastern Ghats Belt (EGB) indicates this zone to be a Proterozoic convergent margin.

The chromites and associated platinum-group minerals (PGMs) crystallize from mafic-ultramafic magmas in a wide range of tectonic settings, e.g., stratiform/layered magmatic complexes, podiform complexes in ophiolites, Alaskan zoned complexes, and chromite-rich Archean complexes, also known as “conduit-type” [111]. As the chromite composition is mainly controlled by mantle-melting processes and associated tectonic environment, these provide important constraints on evolution of the mantle-derived ultramafic-mafic magma [12] and associated postmagmatic processes [13, 14]. This further helps in understanding the secular changes in tectonic systems occurred during the early Earth’s evolution (e.g., [14, 15]). In addition, the association of the iridium-like platinum-group elements (IPGE: Os, Ir, and Ru), which behave as compatible elements during partial melting and crystal fractionation of sulfide-undersaturated ultramafic-mafic magmas, is commonly attributed to local perturbation of the redox state of silicate melt surrounding growing chromite crystals [1618]. Besides, the IPGE-bearing minerals such as laurite-erlichmanite commonly exhibit zoning of Os [19]. The heterogeneity in laurite-erlichmanite composition reflects the crystallization condition of the PGMs and their host chromites [1925]. Therefore, these chromite-PGM assemblages have been used as petrogenetic indicators to understand the physicochemical condition of the parental melts and mantle characteristics during their crystallization [4, 8, 12, 19, 2630].

The Dharwar Craton in South India is known for occurrences of PGM-bearing chromiferous mafic-ultramafic complexes. Notable among these include Sittampundi in Tamil Nadu, Channagiri in Karnataka, and Chimalpahad and Kondapalli in Andhra Pradesh. The Chimalpahad Anorthosite Complex (CAC) in the Eastern Dharwar Craton (EDC) is the largest mafic-ultramafic complex in the Precambrian shield of South India. The complex contains a well-exposed assemblage of anorthosite-leucogabbro-gabbro-ultramafite±chromitite [3134]. Based on geochemical characteristics, subchondritic signature of Os in chromitite, and mantle extraction ages (TMA~3.5Ga for chromitites and 2.77–1.41 Ga for websterites and anorthosites), it is interpreted that the Chimalpahad Complex evolved through sequential extraction of ultramafic-mafic magmas over a protracted period from a deep-seated chamber of a magmatic arc [3234]. Dharma Rao et al. [34] also suggested that the range of Os concentrations in the Chimalpahad chromitites is comparable to those from layered intrusions as well as ophiolites.

Krishna Province in the EGB, in which the CAC is located, is divided into the Ongole, Udayagiri, and Vinjamuru domains [35]. The last two of these domains were earlier considered to make up the Nellore-Khammam Schist Belt (NKSB). Krishna Province contains two ophiolite suites (Kandra and Kanigiri), mafic-ultramafic complexes, and extensive felsic and alkaline rocks. These rock suites have imprints of tectono-magmatic events, which provide important evidences for continental rifting and oceanic subduction during Proterozoic time along the eastern margin of the Eastern Dharwar Craton.

In this study, we report the presence of PGM (laurite, erlichmanite, and irarsite) inclusions in chromite grains in the CAC chromitites for the first time. We present the results from petrology, mineral chemistry of orthopyroxene, chromite and PGM, and bulk-rock PGE geochemistry of chromitites of the CAC to constrain the nature of the parental magma and geodynamic setting. In combination with the inferences from chromite chemistry, here we highlight the significance of zoning in the PGMs to understand the physicochemical condition of parental magma during PGM formation.

The Chimalpahad Anorthosite Complex (CAC) is a deformed and metamorphosed mafic-ultramafic complex occurring within the contact region between the Proterozoic Eastern Ghats Belt (EGB) and the Eastern Dharwar Craton (EDC) in South India [31]. This complex is about 30 km long and 5 km wide, arcuate in shape, trending NE-SW. The main body along with smaller dismembered bodies of anorthosite, pyroxenite, gabbro, and peridotite occurring within amphibolites of the adjoining Nellore Schist Belt (NSB) covers an area of nearly 200 km2 [31, 36] (Figure 1). The CAC mainly consists of three major lithological units, viz., anorthositic rocks, gabbroic rocks, and ultramafic rocks with subordinate chromitite and magnetite [36]. The CAC is interpreted to have syntectonically emplaced as a “sill-like” layered intrusion. The anorthositic rocks include predominant gabbroic anorthosite and anorthositic gabbro with subordinate high-Ca anorthosite. Gabbroic rocks occur as concordant bands and lenses in the anorthositic rocks and as intrusive dykes in the Nellore Schist Belt.

Ultramafic rocks are relatively scanty and occur as pods, lenses, and minor bands within the main complex, as well as fragmented and dismembered bodies adjacent to the main complex. Chromitites occur as small elongated to lensoidal bodies and bands within ultramafites. Chromites also occur as dissemination within the ultramafites. Float ores of chromite can be found within deeply weathered country rocks in the area. The prominent primary magmatic features such as massive cumulates with spectacular igneous layering, rhythmic layering, cross-stratification, trough structures, and zebra banding are very common in the complex ([36] and reference therein). This shows similarity with layered structure observed within Kondapalli Igneous Complex ([3739]. The CAC is strongly deformed with development of plunging synformal structure with NW-SE axial plane (Figure 1) [36].

The ultramafic rocks comprising of pyroxenite (websterite and orthopyroxenite) and dunite constitute a minor component in the CAC. The chromitites are dominantly associated with pyroxenite and rarely with anorthositic rocks. However, hydrous metamorphism and alteration have affected both the chromitites and host ultramafites, but the primary magmatic characteristics are still preserved. The serpentinized ultramafites, now occurring as talc-tremolite-chlorite-carbonate rocks, contain chromite as dissemination, pods, and dismembered bands. The major chromite-rich ultramafic bodies are well exposed at Lingannapet (17°20 : 80°26), Jannavaram (17°18 : 80°24), Shriramagiri (17°20 : 80°23), Konayyapalem (17°19 : 80°25), and Yenkur (17°19 : 80°27) (Figure 2). Few detached minor bodies of altered ultramafics associated with chromite occur at Jannaram, Papakollu, Himmatnagar, and Vinobanagar areas (Figure 1).

The contact region of the EGB, which represents a deeply eroded granulite facies crustal segment between the Singhbhum and Dharwar Cratons along the eastern and southeastern parts of the Peninsular India, preserves important imprints of continental growth and recycling in relation to various supercontinent assemblies through Earth’s evolution history [40]. This contact region also hosts Kandra and Kanigiri ophiolite suites (Figures 1(b) and 1(c)) [32, 41] signifying that the region was an active convergent margin [33, 34, 42], which developed due to westward subduction and accretion before final collision with the Dharwar Craton [43]. These features also provide important evidence of the mechanism and timing of Mesoproterozoic ocean closure [44, 45].

During the present study, sampling has been done from unaltered domains of chromitites from different locations. Polished thin sections of the collected chromitite samples were prepared, and detailed petrographic studies of silicates, oxides, and sulfides were carried out using LEICA DMRX at Geological Survey of India, Hyderabad, India. Compositions of chromite and PGM inclusions are determined by CAMECA SX100 Electron Probe Micro Analyser (EPMA) equipped with five numbers of wavelength dispersion spectroscopes (WDS) at Geological Survey of India (GSI), Southern Region, Hyderabad. The equipment was operated with an accelerating voltage of 15 kV and 12 nA beam current for analysis of silicates and oxides. The PGM and sulfides were analyzed with a voltage of 20 kV and a beam current of 20 nA. The beam width was kept 1 μm in diameter. The duration of the analyses varied from 10 to 30 seconds. Natural standards were used for all elements except Mn and Ti for which synthetic standards were used. The raw analytical data were corrected with a PAP matrix correction program. Whole-rock PGE concentrations of chromitites were determined from twenty-five representative samples at Chemical Laboratory, GSI, Hyderabad, by inductively coupled plasma mass spectrometer (ICP-MS) using a modified digestion method [46, 47]. The detection limits of the methods adopted were as follows (values in ppb): Os-2.0, Ir-3.0, Ru-5.0, Rh-4.0, Pt-5.0, and Pd-5.0. The precision and accuracy of analyses were checked by repeated analyses using international standard references.

4.1. Petrography of Chromitites

The chromitites predominantly are massive nature (Figure 2) and show an adcumulate texture (Figure 3). Chromite grains are also heterogeneously distributed as disseminations within serpentinized orthopyroxenite, websterite, and dunite ([see [34]) (Figures 2(d), 3(a) and 3(b)). Massive chromitite is monomineralic (>90% chromite), comprising of coarse-grained anhedral, interlocking, and fractured chromite grains associated with insignificant silicates (Figures 3(c)–3(f)). Interstitial silicates especially pyroxene and olivine are altered to chlorite, tremolite, serpentine, and talc with occasional carbonate, with few relics of orthopyroxene (Figures 3(a)–3(f)). The fresh chromitites generally exhibit overall equilibrium texture suggestive of their early magmatic crystallization at high temperature (Figures 3(c)–3(e)). Some samples also show triple point junctions and 120° between chromite grain boundaries, typical of high-temperature annealing (Figures 3(d), 3(h), and 3(j)) [48]. At places, they show postcumulus to subsolidus reequilibration with development of chromite grain breccia (Figure 3(l)). Fresh chromite grains are translucent and reddish-brown to orange in polarized light, whereas altered grains are opaque. The alteration of chromite to Fe-rich chromite is irregular and inhomogeneous (Figures 3(b), 3(e), 3(g), and 3(j)). Few chromite grains, mostly disseminated type and in silicate-rich domains, are fractured and affected by postmagmatic modifications (Figures 3(g), 3(h), and 3(j)). Tiny grains of arsenopyrite and pentlandite generally occur along the interstitial spaces as well as within the fractures in chromite and silicates (Figures 3(h), 3(i), and 3(k)). The textural association suggests these sulfides and sulfarsenides are related to postmagmatic later event.

4.2. Mineral Chemistry

The mineral chemical compositions of representative chromite, platinum-group minerals, and base metal sulfides from the CAC were determined by EPMA. Analyses of fresh chromite grains from five samples (CHM-21, CHM-22, CHM-28, CHM-29, and CHM-30) were acquired to understand compositional variation and zoning. Zoning present in six PGM grains could be reliably analyzed by the electron microprobe. In most cases, several of these analyses show totals close to 100 wt.% and a satisfactory structural formula could be determined.

4.2.1. Chromite

The chromite cumulates in the chromitite samples of the CAC are mostly fresh. The chromite grains in silicate-rich domains of chromitites and disseminated chromites in ultramafites show irregular altered rims. Compositions of fresh chromite cores (N-33) and rim (N-15) were obtained by EPMA. These analyses are presented in Table 1. The fresh chromites contain Cr2O3 varying from 46.88 to 57.62 wt.%, Al2O3 from 10.05 to 20.07 wt.%, FeOt from 19.37 to 26.34 wt.%, and MgO from 5.80 to 10.18 wt.%, with low TiO2 (0.06 to 0.25 wt.%). Consequently, their Cr# [Cr/(Cr+Al)] and Mg# [Mg/(Mg+Fe2+)] vary in narrow ranges from 0.61 to 0.77 and 0.29 to 0.55, respectively. Some analyses were also obtained from the grains with altered rims to compare the postmagmatic compositional change [13, 4951]. These chromites have lower Cr2O3 (41.84 to 50.39 wt.%), Al2O3 (4.12 to 8.76 wt.%), MgO (1.95 to 6.12 wt.%), but higher FeOt (32.64 to 39.30 wt.%) contents. The Cr# of chromite rims varies from 0.59 to 0.89 and Mg# from 0.10 to 0.25. The content of MnO does not show any significant variation. The compositions of chromite cores from the CAC chromitites are consistent with those of primary magmatic crystallization. In Cr-Al-Fe3+ diagram (Figure 4(a)), compositions of chromites plot in the Al-chromite compositional field, and disseminated grains show Fe-chromite affinity. These chromite compositions also indicate postmagmatic equilibrium under greenschist to lower amphibolite facies metamorphic condition in the temperature range of 500–600°C (Figures 4(b) and 4(c)). In Cr# [Cr/(Cr+Al)] vs. Fe# [Fe2+/(Fe2++Mg)] plot, the CAC chromites show compositional overlap between podiform/ophiolitic and layered complex (Figure 5(a)). Moreover, in Cr# [Cr/(Cr+Al)] vs. Mg# [Mg/(Mg+Fe2+)] diagram, they are plotted typically in the field of fore arc peridotites (Figure 5(b)) [13]. Their TiO2 content vs. Cr# [Cr/(Cr+Al)] further suggests boninite-peridotite (mantle-melt) interaction nature (Figure 5(c)).

4.2.2. Platinum-Group Minerals

The PGMs occur as micrometric inclusions of <10 μm size within chromite grains of the CAC chromitites. The mineralogy and textural characteristics of these PGMs are shown in Figures 3(h)–3(k) and 6, and their compositional variations are presented in Table 2. Among the PGMs, laurite (RuS2) and erlichmanite (OsS2) constitute the most abundant PGM phases followed by irarsite (IrAsS) and Ir-Os-Ru alloys along with sulfarsenides in the studied samples (Figures 6(a)–6(c)). Inclusions of laurite are more common in the chromite cores (Figures 6(a) and 6(b)) than in the rims (Figures 6(c) and 6(d)). Irarsite inclusions occur in core (Figures 6(e) and 6(f)) as well as rim (Figures 6(g)–6(i)) regions of chromites. Extensive solid solution between laurite and erlichmanite is also indicated from their mineral chemistry and BSE images (Figure 7 and Table 2). As compared to erlichmanite-laurite, alloys of Ir-Os-Ru and sulfarsenides are less common in these samples. The laurite (Figures 7(a) and 7(b)) and irarsite (Figures 7(c)–7(f)) grains show compositional variation due to solid solution among Ru, Os, and Ir, particularly in those grains located in the chromite rims and associated with sulfarsenides. Two paragenetically distinct populations of PGMs could be recognized based on their texture and composition. Some of the PGMs are subhedral- to anhedral-shaped independent grains, laurite in composition, and occur within chromite cores without any sulfarsenide association (Figures 6(a) and 6(b)). These are likely to represent relics of primary magmatic phases. Second category includes arsenic-bearing laurite and erlichmanite (Figures 6(c) and 6(d)), and irarsite (Ir-Ru-Os±S±As) (Figures 6(e)–6(i)) grains located mostly in chromite rims and in the matrix. These are likely to be the products of late magmatic to postmagmatic modifications of primary phases. Few laurite grains located inside chromite cores display patchy zoning with lower Ru content and corresponding increase in Os-Ir-As contents (Figures 7(a) and 7(b)). Pentlandite and arsenopyrite grains mostly occur in the interstitial spaces between chromites and silicates or along fractures (Figures 3(h)–3(k)).

Compositionally, the PGMs of laurite-erlichmanite series in chromite cores are Ru-rich (Ru: 44.31–45.47%, S: 35.72–35.82%, Os: 05.56–06.28%, and Ir: 03.30–03.88%) (CHM-28, Pt-1), whereas those in chromite rims are Os-rich (Os: 38.11%, S: 31.08%, Ru: 17.37%, and Ir: 03.24%) (CHM-21, Pt-41). These contain very small amounts of Rh and Fe (Table 2). The erlichmanite grains occurring in chromite rims have Os: 29.38–36.03%, S: 26.92–28.83%, Ru: 13.62–15.45%, and Ir: 2.70–11.14% with low As (0.6–02.67%) and Fe (1.99–5.45%) contents. These do not show significant compositional variation. The compositions of irarsite grains present in the chromite cores vary significantly from those located in the rims. These inclusions in chromite cores are Ir-rich (Ir: 49.40–45.41%, As: 24.73–25.15%, S: 13.19–13.91%, Os: 0–6.17%, and Ru: 0.85–5.10%), whereas those in chromite rims are Os-S-rich (Ir: 19.86–34.83%, S: 9.61–25.41%, As: 9.19–16.75%, Os: 6.09–21.55%, and Ru: 1.40–15.49%). The compositions of laurite-erlichmanite and irarsite grains are plotted in Os-Ru-Ir, OsAsS-RuAsS-IrAsS, and OsS2-(Ru+Rh)S2-(Ir+Pd+Pt)S2 ternary diagrams to show the relation between these minerals (Figure 8).

4.2.3. Orthopyroxene

Pyroxene is present as intercumulus phases in chromitite and is affected by postmagmatic processes. The relic pyroxene grains are Mg-rich confirming to enstatite variety and having restricted composition variation (En89-88). Their MgO content ranges from 32.68 to 34.67 wt.% and Mg# from 0.89 to 0.91. They exhibit moderate contents of Al2O3 (0.94–1.08 wt.%), lower CaO (0.04–0.52 wt.%), and FeO (6.5–7.3 wt.%), and meager MnO, TiO2, and K2O (Table 1(c)). Their Al2O3 content and Mg# ratio are also comparable with those found in suprasubduction zone (SSZ) peridotites.

4.3. Geochemistry of Platinum-Group Elements

The whole-rock PGE concentrations of the chromitite samples of the CAC are presented in Table 3. The total PGE concentration of the CAC chromitites shows wide variation from 30 to 548 ppb. Platinum content varies from 4 to 30 ppb, palladium from 3 to 86 ppb, and rhodium from 3 to 25 ppb, whereas osmium, iridium, and ruthenium concentrations show wider ranges from 3 to 108 ppb, 3 to 131 ppb, and 3 to 259 ppb, respectively. This suggests that the distribution of PGEs in chromitites of the CAC is inhomogeneous. The PGE concentration of the CAC chromitites shows a general negative slope and upward convex pattern in the chondrite-normalized PGE plot, being enriched in Rh and Ru (IPGE) and depleted in Pt-Pd (Figure 9).

5.1. Nature of Chromite

The chromitites of the CAC are fragmented and dismembered. These comprise of coarse anhedral and interlocking grains of chromites, many of which are fractured. The compositions of chromite core and rim parts of cumulate chromite grains and disseminated chromite grains of the CAC are shown in the Cr-Al-Fe3+ diagram (Figure 4). This diagram demonstrates Al-chromite composition for cumulate grains and Fe enrichment for disseminated ones indicating their primary magmatic characteristic and postmagmatic changes. The Cr2O3 content of chromites exhibits perfect negative correlation with Al2O3 and FeOt (figure not shown). Moreover, their TiO2 (up to 0.25 wt.%) content is indicative of postcrystallization processes [12, 13, 39, 4953].

The compositions CAC chromites are compared with the Cr-Al-Fe3+ compositions of spinels from boninite, Al-depleted komatiite, ophiolite, altered komatiite, and different metamorphic grades as well as with the spinel stability limits [12, 49, 5457]. Their physical characteristics and geochemical characteristics such as low contents of TiO2, FeOt, and Al2O3 contents are typical of podiform/ophiolitic chromites. Although the disseminated chromites show noticeable compositional variations, the cumulate grains are relatively fresh and display slight compositional variation from core to rim. Such compositional variations are plotted in the Mg# vs. Cr#, Al2O3 vs. Cr2O3, and FeOt vs. Cr2O3 diagrams to understand the nature of magmatic processes involved during evolution. The chromite of CAC shows compositional variation in core and rim with reverse zoning attributed to metasomatic process (Figures 10(a)–10(c), after Gamal El Dien et al. [13]). These plots show limited composition range for Al, Ti, and Cr, and therefore, these chromite compositions can be used to infer geodynamic setting. In the Fe2+# [Fe2+/(Mg+Fe2+)] vs. Cr# [Cr/(Cr+Al)] plot, the CAC chromite cores fall in the overlapping fields of ophiolitic and layered intrusion chromites (Figure 5(a)) [12]. As the Fe is easily exchanged between chromite and surrounding silicate matrix during subsolidus reaction, therefore, it cannot be emphatically said that the CAC chromitites are related to layered intrusion.

5.2. Nature of Parental Magma

The chromite composition is a reflection of its parental melt composition and gives indication of degree of partial melting and fractional crystallization [12, 58, 59]. The main constituents of chromites such as FeO, MgO, Al2O3, and TiO2 are the functions of their parental melt composition [23, 53, 5964]. The CAC chromites indicate their affinity for suprasubduction fore arc peridotites in Cr/(Cr+Al) vs. Mg/(Mg+Fe2+) discrimination diagram (Figure 5(b)) [13, 65, 66]. The TiO2 (0.1 to 0.25 wt.%) content of chromite cores attests to their boninitic to arc tholeiitic affinity (<0.35 wt.%) [53] and limited effect of assimilation and crustal contamination [50]. This is further supported by boninitic-peridotitic interaction trend in the Cr/Cr+Al vs. TiO2 discrimination plot [67] (Figure 5(c)). The Al2O3 and TiO2 contents of chromite are also commonly used to determine the nature of parental melt and ambient tectono-magmatic environment [53, 63, 68, 69]. We have calculated the contents of the Al2O3 and FeO/MgO ratios of the parental melts, which were in equilibrium with the chromitites, based on the following equations [62].
(1)Al2O3ofspinel=0.035×Al2O3ofmelt2.42,lnFeOMgOspinel=0.471.07Yspinel,Al+0.64Yspinel,Fe3++lnFeOMgOliquid,
where FeO and MgO are in wt.%, Yspinel,Al=Al/Al+Cr+Fe3+, and Yspinel,Fe3+=Fe3+/Al+Cr+Fe3+.

Moreover, the calculated Al2O3 content of the parental melt of the CAC chromites ranges between 10.77 and 13.02 wt.% and the calculated FeO/MgO ratio of the parental melt is 0.8–1.84 (Table 1). Such high Al2O3 contents and FeO/MgO ratios of the parental magma are comparable to the chemical characteristics of boninitic magma formed by partial melting in an arc setting [53, 70]. The calculated data on parental melt, along with the actual Al2O3 content of chromites, also plot very close to the evolutionary trend of an arc system in the diagram of Al2O3 in melt vs. Al2O3 in spinel (Figure 5(d)) [53, 63]. Based on above discussion, we suggest that the parental magma of the CAC chromitites was similar to that of boninitic/tholeiitic basalt derived by high degree of partial melting of mantle peridotite in arc setting. These characters indicate that the chromites and their ultramafic hosts may be part of oceanic crust attributed to oceanic arc/ophiolite rather than layered intrusions related to continental crust.

5.3. Geodynamic Implication

The chromites have been widely used as petrogenetic indicators [12, 5759]. Their Cr2O3, Al2O3, and TiO2 contents are widely used to interpret magma types, source, and tectonic settings [12, 53, 58, 59]. As chromites of low metamorphic grade can also be used for the purpose [49], the compositions of chromite cores of this study are plotted in different tectonic discrimination diagrams to decipher the geodynamic setting of CAC chromitites. In the Fe2+/(Fe2++Mg) vs. Cr/(Cr+Al) diagram, these samples fall close to the overlapping fields of ophiolite and layered intrusion (Figure 5(a)) in fore arc peridotites (Figure 5(b)) indicating boninite peridotite interaction, whereas they fall in the overlapping fields of Alaskan, ophiolitic, and stratiform chromite fields in the Mg/Mg+Fe2+ vs. Fe3+/(Fe3++Cr+Al) diagram (Figure not shown). Similarly, the calculated Al2O3 content of parental melt and TiO2 content of the chromites indicate subduction-related magmatic arc setting [53, 62, 71] (Figure 5(d)). Such tectonic signatures are attributed to closure of the ocean leading to formation of the suture zones. The proximity of the CAC to the Kondapalli Igneous Complex along the contact zone between the Eastern Dharwar Craton and Eastern Ghats Mobile Belt corroborates that this contact zone represents a part of relict magmatic arc [34, 72]. Moreover, its spatial and temporal associations with a basalt-dominated greenstone belt (i.e., Nellore Schist Belt) [73] and syn- to post-tectonic granitoids suggest that these igneous complexes were formed in a suprasubduction zone setting and most likely represent fragments of oceanic crust similar to dismembered subduction-related ophiolites (see [74]). This suture zone has also witnessed multiple phases of arc-related magmatism at ~1.1 Ga and ~1.7 Ga [32, 33].

5.4. Genesis of Laurite and Irarsite

The Os-Ru-Ir-bearing sulfides, sulfarsenides, and alloys such as laurite (RuS2), erlichmanite (OsS2), and irarsite occur as minute inclusions (1–10 μm) enclosed within chromite grains of the CAC chromitites (Figures 6 and 7). The IPGE-bearing minerals are characteristics of chromites associated with ultramafic rocks from subcontinental lithospheric mantle (SCLM) and ophiolitic chromitites, whereas the PPGE-bearing minerals dominate in chromitites of layered complexes [4, 8, 75]. Most of these PGM grains show compositional zoning (Figure 7) and record subtle changes from magmatic to postmagmatic processes during evolution. The temperature and sulfur fugacity (fS2) are the most important parameters, which play significant roles during precipitation of primary magmatic IPG minerals [7678]. Textural relation of these IPGMs with the chromites (Figure 6) suggests that laurite was the first PGM to crystallize from the primary melt prior to or coeval with chromite crystallization, and it was followed by crystallization of erlichmanite. The formation and entrapment of irarsite occurred at a subsequent magmatic stage. Their zoning and distribution of Os-Ir-Ru with sulfur and arsenic follow the strong positive/negative correlation trends (Figures 11 and 12). This can be correlated with decrease in temperature and increase in sulfur fugacity (fS2) (e.g., [8, 30]).

Experimental works suggest that the laurite can form during crystallization of the chromite from the basaltic melts at relatively high temperatures (>1000°C) [19, 7982]. Most of the stoichiometrically Os-poor laurites can crystallize from an S-undersaturated basaltic melt at 1200–1300°C and log fS2 from −2 to −1.3 ([26, 7678, 83, 84]). The temperature of stability of pure laurite is in the range of 1300–1150°C [8, 30, 79]. During the progressive crystallization, temperature gradually decreases with increasing fS2, which facilitates laurite to gradually accommodate more Os and Ir in its lattice during evolution of the basaltic magma [7680]. Similar process of increasing Os solubility in laurite can be inferred in the present study during crystallization from a basaltic melt with decreasing temperature and/or increasing fS2. Therefore, laurite grains are trapped close to the cores of the chromites, whereas erlichmanites are trapped away from the cores. Experiments carried out by Finnigan et al. [82] indicate that laurite can crystalize directly from sulfide-undersaturated silicate melt by reaction of nano-to-micron-sized metallic Ru-(Os-Ir) alloys with S dissolved in the silicate melt. Recently, Fonseca et al. [19] and Zaccarini et al. [30] have demonstrated that laurite and Pt-Ir-Os-Fe alloys crystallize in equilibrium from Ni-Cu-Fe sulfide melts at >1200°C.

The origin of laurite-erlichmanite in the chromites of the present study, therefore, can be related to crystallization from basaltic melt in deep mantle ([7880, 8486]). The upward convex pattern of the whole-rock chondrite normalization plot of PGEs of the CAC chromitites, due to enriched contents of Rh and Ru and depletion in Pt-Pd, favours an S-undersaturated magmatic source (Figure 9). In addition, their high whole-rock (Os+Ir+Ru)/(Rh+Pt+Pd) ratios in chromitites are typical of ophiolitic chromitites ([see [76]]). In the present study, irarsite (IrAsS) is present in the chromite rims and in the neighborhood of pentlandite and arsenopyrite (Figures 3(h) and 3(k)). Some laurite grains display patchy zoning. The spot analyses of laurite present in the investigated CAC chromitites show a wide range of Os content up to the end-member erlichmanite (OsS2) (Table 2). These laurites with varying contents of Os must have formed over a temperatures range (and sulfur fugacity) below the stability of pure laurite, i.e., 1300–1150°C. Similarly, the irarsite grains also show compositional zoning due to variation of Os content (Figures 7, 11, and 12). The precipitation of irarsite may be attributed to local fluctuations of the fugacity of arsenic (fAs) related to influx of melts or fluids in an open system. This mineral chemical evidence of present study is also supported by Os isotopic studies [34].

The presence of Os-Ir (and Ru-poor, Ni-free) inclusions in chromite indicates an initial (and perhaps short-lived) percolation of an S-undersaturated melt, consistent with the suprasubduction zone (SSZ) origin of the CAC chromitites. Influx of fresh batches of melt or fluid in an open system leading to a rapid increase of fS2 might have destabilized the earlier formed alloys and promoted precipitation of the IPGE sulfides and sulfarsenides. During the coarsening coalescence of chromite nuclei, with micro nuggets of laurite, other IPGM grains were gradually trapped in crystallizing chromite grains [8789]. Thus, it is inferred that laurite-erlichmanite series crystallized along with crystallization of the CAC chromites during early magmatic stage at high temperature in an open system, whereas irarsite and base metal sulfide inclusions in chromite of the CAC may be attributed to local and rapid fluctuations of temperature and fS2fAs subsequently due to addition of fresh batches of melt.

5.5. Implications of Zoning in Laurite and Irarsite

The IPGE-bearing minerals in the CAC chromites preserve micro- to nanoscale compositional zoning due to variation in their Os and Ru contents. The observed zoning pattern is mainly due to the substitution of Ru by Os in laurite and vice versa in erlichmanite [30, 52, 75, 9093]. The zoning in laurite-erlichmanite inclusions in chromite grains is widely accepted to be magmatic ([8, 10, 19, 30, 60]. However, some studies suggest that such zoning can also be produced by postmagmatic modifications [94, 95]. The zoning observed in laurite-erlichmanite can be classified into three main types: (i) grains with Os-poor (laurite) core and Os-rich rim (normal zoning), (ii) grains with Os-rich core and Os-poor rim (reverse zoning), and (iii) grains made up of complex intergrowth of Os-rich and Os-poor laurite and/or erlichmanite (oscillatory zoning) [52, 90].

There is a consensus that a progressive increase in the Os content of laurite leading to normal zoning is a reflection of fractional crystallization during cooling of the parental melt due to decrease in temperature and increasing fS2 [76, 77, 96101]. In this case, it is expected that Os, which entered into the structure of laurite at high temperature, gets exsolved upon cooling via occupation of vacancies in the position of Ru. In case of reverse zoning, subsolidus equilibration of a chemically heterogeneous laurite grain by intracrystalline diffusion of Os during cooling of host chromite from very high temperatures (~1300–1150°C) to the conditions of closure of the chromitite system (~600°C) could be the reason (Figure 13). In contrast, oscillatory zoning highlights the existence of fluctuating condition of fS2 (and activity of arsenic) of the magma with or without changes in temperature [10, 52, 63].

Besides these hypotheses, there is possibility of complete mutual substitution of RuOs between laurite and erlichmanite as these two disulfides are isostructural [98101]. Experimental study by Baurier-Aymat et al. [90] revealed that the core-to-rim microscale zoning in laurite reflects condition of nonequilibrium during laurite crystal growth. They further suggested that faster cooling of the chromite-laurite magmatic system associated with exhumation of the host chromitites from the mantle to emplacement in the crust could be responsible for preventing intracrystalline diffusion of Os in the laurite even at high temperatures and thus preservation of microscale zoning. Thus, zoning in laurite-erlichmanite is a record of heterogeneous physicochemical environment prevailed during crystallization of host chromites.

The nanometer-sized domains of erlichmanite present fringing laurite could be due to heterogeneous dissolution of Os in the structure of laurite at high temperature. The zoning in laurite and irarsite in present study is normal to patchy type and is supported by the compositional gap between laurite and erlichmanite. The presence of outer Os-rich zone in laurite can be explained due to solid-solution relation between laurite and erlichmanite in an open magmatic system. The origin of irarsites with noticeable content of As and Os-rich zones is due to local fluctuations of the fugacity of arsenic (fAs) as discussed above, which is also supported by presence of pentlandite and arsenopyrite in the interstices of chromitite in the neighborhood of irarsites. The presence of significant As content in the irarsite may be related to subsequent influx of melt or fluid in an open system, thus indicating protracted evolution of magmas parental to the CAC chromitite in an arc setting.

5.6. Implication for Supercontinent Assembly

Assemblies of ancient continental fragments leading to supercontinent formation are believed to be driven by subduction-related processes as evident from presence of high-pressure rocks and/or magmatic arc rocks along the contact regions. The growth of these Precambrian terrains through subduction-accretion-collision can be understood from the constitution and structural architecture of ophiolites [44]. The Nellore-Khammam Schist Belt (NKSB), in which the Chimalpahad Anorthosite Complex (CAC) is situated, occurs sandwiched between the Eastern Ghats Belt (EGB) and the Eastern Dharwar Craton (EDC). This region is characterised by the presence of deformed remnants of ophiolites of ~1847 Ma Kandra Complex and ~1334 Ma Kanigiri Complex associated with ~1.7 Ga continental arc-related felsic magmas [43, 72, 102, 103]. The ~2.7 Ga Nellore greenstone belt was also intruded by enriched MORB-type gabbros at ~1911 Ma [104]. The ~1.69–1.63 Ga Kondapalli Mafic-Ultramafic Complex [33] and the Chimalpahad Complex are also located in this contact zone along a linear extension, thereby indicating this zone to be a paleoconvergent margin [40, 105]. The Sm-Nd isotopic results for the CAC anorthosites indicate an age of ~1.3 Ga for the complex [73], and their Re-Os isotopic data suggest mantle extraction ages (TMA) between 1.41 and 1.48 Ga [34]. The geochemical characteristics of the CAC rocks indicate that the mafic-ultramafic magmatism in the complex is a related subduction process, which was operative in this segment. Such anorthosite-dominant layered intrusions spatially associated with greenstone belts are interpreted to be fragments of oceanic crust, representing dismembered subduction-related ophiolites (see [73]). Occurrence of two ophiolite suites along with other mafic-ultramafic bodies and arc-related granitic magmatism in the NKSB along the marginal zone between the EDC and the EGB suggests multiple cycles of ocean opening and closure involving repeated accretionary activity during Proterozoic in this crustal segment [35]. The eastern margin of the EDC underwent long-lived (1.8–1.3 Ga) subduction-related growth via accretion during 1.8–1.3 Ga [106]. The emplacement of the CAC, thus, has link with the assembly of Columbia supercontinent. The Palaeoproterozoic mafic magmatism is related to a major extensional event along the EDC margin due to breakup from the southeastern margin of the North China Craton during 1.9-1.8 Ga [104]. According to him, this exposed craton margin later evolved as the site for collisional events during the Late Palaeoproterozoic to Early Mesoproterozoic. Further, the occurrence of alkaline rocks in the Prakasam Alkaline Province in the northern NKSB associated with younger granites has been interpreted to represent a subsequent Mesoproterozoic rifting episode in this domain (e.g., [40, 107110]).

The anorthosite-leucogabbro-gabbro-ultramafic rock suite of the Proterozoic Chimalpahad Anorthosite Complex (CAC) contains chromitites as small dismembered lensoidal bodies. The mineral chemistry of chromites suggests that the CAC chromitite was formed from an S-undersaturated boninitic melt in a magmatic arc related to subduction setting. The present study also reports presence of micrometric inclusions of laurite, erlichmanite, irarsite, and Ir-Os-Ru alloys with micro- to nanoscale zoning within chromite grains in the CAC. Laurite grains with erlichmanite domains represent solid solution series between the pure laurite (RuS2) and erlichmanite (OsS2), i.e., Os-poor and Os-rich laurite. The laurite-erlichmanite series minerals are formed from an S-undersaturated basaltic melt at ~1200–1300°C and log fS2 from −2 to −1.3 during early magmatic stage along with chromite crystallization. The Os-poor irarsite is formed during the late magmatic stage with decreasing temperature and fluctuating in fS2. It can be concluded that the observed zoning in laurite and irarsite developed as a result of very fast exhumation of the CAC chromitites and associated ultramafites from root of the SCLM to their final emplacement in the crust. The mineral chemistry and physical characteristics of chromites, bulk-rock PGE geochemistry of chromitites, and location of the CAC also point towards its ophiolitic affinity. The emplacement of the ca. 1.3 Ga CAC in the proximity of ca. 1.85 Ga Kandra ophiolite complex and ca. 1.33 Ga Kanigiri ophiolitic mélange along the contact zone between the EDC and EGB indicates its close link with the Columbia supercontinent assembly.

The work is the outcome of the field season program of the Geological Survey of India (GSI). All data and reports are available in Geological Survey of India portal (https://www.gsi.gov.in/webcenter/portal/OCBIS/pageReports/pageGsiReports?). The data can also be made available on request to the author. The title of the report is Investigation for Chromite and PGE Mineralisation in the Chimalpahad Ultramafic Complex, Khammam District, Telangana (G4 Stage).

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The work is the outcome of the Field Season Program of the Geological Survey of India (GSI), Southern Region, Hyderabad, carried during the year 2013-15. The authors would like to thank Shri G. Vidyasagar, Additional Director General and HOD, and the Director, Publication Division, GSI, Central Region, for the accord approval for publication of the manuscript. TM and SN acknowledge Mrs. Shubhangi Bhilkar, Senior Geologist, for her help during field work.

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