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The source of the mineralizing fluid in Archean greenstone-hosted orogenic gold deposits is widely debated, with the available geochemical and isotope data interpreted as reflecting the involvement of metamorphogenic fluid produced by devolatilization of greenstone belt metaigneous/metasedimentary rocks or granite-derived magmatic-hydrothermal fluids or both. Orogenic gold deposits form in complex geologic environment involving multiple fluid sources, reflected in the large variation in the B-isotope composition of tourmaline (δ11B = −24.8‰ to +19.8‰). The δ11B distribution of tourmaline from world-wide orogenic gold deposits define two peaks, one at ca. −15‰, and the other between −5 and 0‰. In this study, we modelled the release of boron and associated B-isotope fractionation during prograde metamorphic dehydration of representative mafic and pelitic greenstone rocks using a mass balance approach and compared the results with the B-isotope composition of tourmalines measured globally from orogenic gold deposits. The results indicate that the boron content of greenstone belt metabasalts decreases from ∼26 ppm to ∼2.7 ppm by 530 °C while those in metapelites decrease from ∼60 ppm at 300 °C to ∼40 ppm by the terminal chlorite breakdown temperature of about 570 °C. In both mafic and pelitic assemblages, terminal chlorite breakdown reactions occurring during the greenschist to amphibolite facies transition (530–570 °C) release large amounts of 10B-rich fluids, which rapidly lower the δ11B of the metamorphic fluid. The δ11B of the fluids released from metabasalts and metapelites at these conditions are estimated to be about −14.2 to −10.8‰ and −5.7 to 0.0‰, respectively. Tourmalines precipitating from such metabasalt-derived fluid are expected to have δ11B between −18.0‰ and −12.4‰ while those crystallizing from metapelite-derived fluid will have δ11B between −9.5‰ and −1.6‰ for the temperature range of ore formation (300–550 °C) in orogenic gold deposits. Our calculated range of δ11B for the metabasalt- and metapelite-derived metamorphic fluids matches closely with the two peaks in the δ11B distribution of tourmaline from orogenic gold deposits. The δ11B of metabasalt-derived fluid is highly variable (+2‰ to −15‰ between 450 °C and 610 °C) and strongly dependent on the metamorphic grade attained by the greenstone belt lithologies. The majority of measured tourmaline δ11B values from orogenic gold deposits of all ages can potentially be explained by different peak metamorphic temperatures with a single fluid source from metabasalts, or by mixing between basalt-derived and pelite-derived metamorphic fluids, or both. Prior characterization of the metamorphic grade of the rocks and the temperature of mineralization are desirable before using the B-isotope composition of tourmaline for evaluating fluid source. The δ11B (−14.0‰ to −3.3‰) of tourmalines measured by us from the Hutti and Kolar gold deposits as a case study can be explained by our modelled results with the isotopic variations attributed to mixing between metapelite- and metabasalt-derived fluids. Low Li concentrations in the tourmalines of Hutti and Kolar (avg. 21.7 and 28.0 ppm in Hutti and Kolar, respectively) further support a metamorphic origin for the hydrothermal fluid. The relatively higher V contents of the Hutti tourmalines (avg. 962 ppm) vis-a-vis the Kolar tourmalines (avg. 615 ppm) is suggestive of greater pelitic input in Hutti.

In the Proterozoic tungsten belts of Balda and Motiya in western India, tungsten mineralization is hosted in tourmaline-bearing quartz veins intrusive into pelitic schists and granites. Tourmaline is a ubiquitous phase in all rock types, and in this study, we use its major, trace element and B-isotope composition to constrain the nature of the tungsten (W)-bearing hydrothermal fluid and the processes involved in the precipitation of wolframite. We have also reconstructed the compositions of the W-precipitating fluids from mineral-fluid trace element partition coefficients that were extrapolated using the Lattice Strain Model. The tourmalines from both belts are of schorl composition and have high alkali, low Ca content, and moderate X-site vacancies. High Li, Mn, Zn, and Sn in the fluid in Motiya is suggestive of relatively saline fluid possibly derived from a granitic source. The high V/Sc ratios of tourmalines in the mineralized veins and wall-rock tourmalinites indicate an important role of biotite dissolution and fluid-rock interaction that contributed Fe–Mn for the precipitation of tourmaline and wolframite. The tourmalines in the mineralized veins at Balda (δ11Btur = −10.9 ± 0.7‰, 2σ; n = 10) and those in the associated granites (δ11Btur = −11.5 ± 0.7‰, 2σ; n = 6) have similar B-isotope composition, while those of the associated topaz granites and pegmatites (−13.9 ± 0.7‰, 2σ; n = 19) are isotopically lighter than the granites. The B-isotopic variation in the granite-pegmatite-vein system can be explained by fluid exsolution with the mineralized vein forming from exsolved fluid and the topaz-bearing granites and the pegmatites crystallizing from the residual melts. The δ11B of the mineralizing fluid is estimated to be ca. −7.4‰ at Balda and ca. −7.6‰ at Motiya, and were possibly derived from granites, consistent with extensive tourmalinization and muscovitization of the adjacent wall rocks, and high concentration of elements such as F, Li, B, Sn, Mn in the tourmalines and the reconstructed fluid. The chemistry and B-isotope composition of tourmalines in both the belts support the hypothesis that W-bearing hydrothermal fluid was primarily derived from a fractionated granitic source, and that precipitation of wolframite and tourmaline involved interaction of the granitic fluid with surrounding pelitic rocks.

Linear chains of Deformed Alkaline Rocks and Carbonatites (DARCs) mark sutures where continents had rifted apart and later amalgamated. Since DARCs are products of two well-defined components of Wilson cycle, i.e., continental rifting and subsequent collision, geochronological constraints from DARCs along the Singhbhum/Bastar Craton-Eastern Ghats Belt contact in eastern India is used to unravel the history of continental breakup and amalgamation in the Indo-Antarctic Enderbia continent. Proto India and East Antarctica were involved in several episodes of collision and breakup during the assembly of past supercontinents. The Napier Complex of East Antarctica collided with the Dharwar Craton and Ongole Domain at 1.60 Ga forming the central-eastern Indian shield. Zircon U-Pb ages from DARCs at the craton-Eastern Ghats Belt margin show that the alkaline complexes (Kamakhyanagar: 1350 ± 14 Ma; Rairakhol: 1379 ± 6 Ma; Koraput: 1387 ± 34 Ma; Kunavaram: 1360 ± 5 Ma; Jojuru: 1352 ± 6 Ma) were emplaced between 1320 Ma and 1370 Ma. The alkaline magmatism marks an episode of rifting in the Indo-Antarctic continental fragment, correlatable with the breakup of the Columbia supercontinent. The Khariar alkaline complex was emplaced at 1478 ± 5 Ma during an earlier phase of crustal extension coeval with the formation of the Mesoproterozoic Chattisgarh rift basin. Metamorphic zircons from the alkaline rocks furnish age populations at 953–930 Ma, 808–795 Ma, and 661–563 Ma. The 953–930 Ma ages are correlated with oceanic closure between Ruker Terrane of East Antarctica and India during Rodinia assembly. The resultant collision of the Ruker Terrane with India-Napier Complex composite produced the Grenville-age Eastern Ghats Province-Rayner Complex orogen. The 808–795 Ma ages record disintegration of Rodinia when India broke away from East Antarctica. In the early Paleozoic, India redocked with East Antarctica and Australia during Gondwanaland assembly. The 661–563 Ma zircon ages date the resulting collisions during Pan-African orogenesis. The assembly of the Eastern Ghats Province and the Rayner Complex with the Bastar-Singhbhum Craton occurred in the early Neoproterozoic during the final stages of Rodinia assembly, supporting the existence of Enderbia continent comprising crustal units from India and East Antarctica.

In this study, the chemical and B-isotope composition of tourmalines that coprecipitated with uraninite in schistose rocks of the Mohuldih and Bagjata uranium deposits along the Singhbhum Shear Zone are used to constrain the sources and the nature of the mineralizing fluids, along with the mechanism of primary uraninite precipitation. Three generations of tourmaline are identified, which can be correlated with three generations of uraninite in the host rocks. These correspond to three major episodes of hydrothermal alteration of the shear zone rocks at 1.8-1.9 Ga, 1.66-1.56 Ga, and ca. 1.0 Ga, synchronous with metamorphism/granitoid activity. The texturally earliest tourmaline with δ11B of 8.8 ± 0.3 ‰ precipitated together with primary uraninite from externally-sourced high saline fluids derived from subducting slab during the 1.8–1.9 Ga hydrothermal alteration event. The precipitation of uraninite can be linked to the decreasing salinity of the hydrothermal fluid as a consequence of fluid-rock interaction. Dissolution of chlorite/biotite and the precipitation of more Mg-rich tourmalines resulted in decrease of fluid salinity, which destabilized Fe-chloride and U-chloride complexes leading to the precipitation of Fe-rich tourmaline together with primary uraninite. The texturally later second and third generation tourmalines (δ11B = 2.5 to 6.3 ‰) and uraninites formed from mixed fluids derived from the slab and dehydration of pelitic assemblages during renewed episodes of hydrothermal alteration synchronous with metamorphism at 1.66-1.56 Ga and ca. 1.0 Ga. The δ11B of the tourmalines from the schists of the Singhbhum Shear Zone correspond to the range of values (–10.4 ‰ to +6.0 ‰) reported for IOCG-type deposits associated with metamorphic slab-derived fluid sources.

The Western Dharwar Craton in southern India is underlain by Paleoarchean to Neoarchean granitoids. Here, we use major-trace element chemistry and zircon U-Pb-Hf isotopic composition to identify major components of the crust, constrain the timing of juvenile crust extraction, and discuss the implications for Archean tectonic processes. The granitoids are metaluminous to weakly peraluminous, magnesian, and calcic. They were derived from basaltic protoliths with minor components sourced from pre-existing felsic crust. Low La/Yb and Sr/Y indicate shallow garnet-free plagioclase-bearing amphibolitic sources. The granitoids display large variations in concentration of trace elements, attributed to plagioclase accumulation or fluid-assisted mobilization of REEs during metamorphism. Zircon ages help to constrain four major episodes of granitoid crust formation at 3.43–3.41 Ga, 3.36–3.34 Ga, 3.29–3.25 Ga, and 2.66–2.65 Ga. The 3.43–3.41 Ga, 3.36–3.34 Ga, and 3.29–3.25 Ga granitoid suites have positive eHfi (2.7–4.5) and plot on a common eHfi vs. time trend consistent with repeated granitoid extraction at 3.43–3.41 Ga, 3.36–3.34 Ga, and 3.29–3.25 Ga from mafic sources that separated from model depleted mantle between 3.55 Ga and 3.35 Ga. The eHfi (0.4–0.69) of the 2.66–2.65 Ga Neoarchean granitoids can be explained by melting of similar 3.35 Ga mafic crust or by mixing between juvenile magmas and preexisting granitoids. Uranium-Pb ages from metamorphic zircons indicate polyphase metamorphism of the granitoids at 3353–3329 Ma, 3264–3256 Ma, 3187–3141 Ma, 3083–3062 Ma, and 2574–2526 Ma. Hf-isotopic data from zircons in granitoids from several cratons indicate that prior to c. 3.5 Ga most granitoids have chondritic or crust-like eHfi explained by repeated granitoid extraction from long-lived mafic crusts with limited interaction with juvenile magmas. Juvenile eHfi and short protolith residence times of the Western Dharwar Craton Paleoarchean granitoids is suggestive of a tectonic setting with rapid recycling of basalts as in subduction zones. In contrast, greater protolith residence times and crust-like signature of granitoids older than 3.5 Ga in the crustal record indicate a tectonic setting where basalts persisted for prolonged periods of times such as in an oceanic plateau.