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Abstract: The Western Dharwar Craton in peninsular India comprises a typical Meso- to Neo-Archean granite-greenstone terrain. Detrital zircons from two metagreywackes in a late basin from the Gadag Greenstone Belt preserve at least eight age populations ranging in age from ca 3.34 to 2.55Ga, and grains as old as ca 3.54Ga. The zircon provenances for the two samples appear to be the same up to ca 3.25Ga, with relatively juvenile εHf values largely between zero and depleted mantle values. After 3.25Ga, one sample has similar εHf values whereas the other has only negative values indicative of Hf-evolution in a crustal environment. After ca 3.25Ga the source regions for the two samples were distinctly different. The detrital zircons reflect the age and evolution of the upper crust of the Western Dharwar Craton. Modeling of Hf isotopic evolution of the detrital zircons suggests two major crust-forming events at ca. 3.6 and 3.36Ga, and some indication of juvenile addition to the crust at ca 2.6Ga. The maximum sedimentation age of the greywackes is constrained by the youngest detrital zircon population at 2547±5Ma. Gold mineralization in the belt is dated at 2522±6Ma and constrains greywacke sedimentation, deformation and metamorphism to a ca 25my interval. [Copyright &y& Elsevier]

New zircon U-Pb ages for a felsic volcanic rock (2,588 ± 10 Ma) and an intrusive granite (≥2,555 ± 6 Ma) in the Gadag greenstone belt in the Western Dharwar Craton, southern India, are similar to dates for equivalent rocks in the Eastern Dharwar Craton and indicates docking of the two cratons prior to this time. The zircons in the intrusive granite are strongly overprinted, and coexisting titanites yielded two different age populations: the dominant group gives an age of 2,566 ± 7 Ma, interpreted as the emplacement age, whereas the minor group gives an age of 2,516 ± 10 Ma, reflecting a hydrothermal overprint. In situ U-Pb dating of monazite and xenotime in gold reefs of the Gadag (2,522 ± 6 Ma) and Ajjanahalli (2,520 ± 9 Ma) gold deposits reveal a previously undated episode of gold mineralization at 2.52 Ga, substantially younger than the 2.55 Ga Hutti deposit in the eastern Dharwar Craton. The new dates confirm that both the younger greenstone belts and lode gold mineralization in the Dharwar Craton are about 100-120 My, younger than in other well-dated Archaean cratons. Although gold mineralization across the craton postdates most of the magmatic activity and metamorphism at upper crustal levels, widespread thermal reworking of the lower-middle crust, involving partial melting, metamorphism, and lower crustal granitoid intrusion, occurred concurrently with gold mineralization. It is likely that the large-scale hydrothermal fluid flow that produced widespread gold deposition was also part of this tectono-thermal event during the final stages of cratonization of the Dharwar Craton in southern India. [ABSTRACT FROM AUTHOR]

The Hutti orogenic gold deposit, in the eastern Dharwar craton of South India, is located in a cuspate greenstone belt which is tectonically juxtaposed against tonalite-trondhjemite-granodiorite (TFG) basement on its western margin. The host rocks to gold mineralization are mafic and felsic volcanic rocks metamorphosed to the amphibolite facies. Mineralization is confined to nine, broadly parallel, quartz reefs within north-south--to north-northwest-south-southeast-trending shear zones that dip approximately 70°W. Six of these 2- to 10-m-thick, boudinaged veins have been mined to a depth of over 810 m. Free gold occurs in the veins and alteration haloes and is spatially and temporally associated with quartz and disseminated pyrite and arsenopyrite which, in turn, are closely associated with hydrothermal monazite and xenotime in the biotite-plagioclase wall-rock alteration around veins. Gold is considered to have been deposited during D2 to D3 deformation, broadly coincident with the emplacement of syntectonic granitoids. The history of volcanism, granitic magmatism, and gold mineralization is defined by U-Pb geochronology of magmatic zircons and hydrothermal monazite and xenotime. The felsic volcanic host rocks have a U-Pb zircon age of 2587 ± 7 Ma, about 40 m.y. older than the age of gold mineralization at 2547 ± 10 Ma determined from hydrothermal monazite. The syntectonic Kavital granitoid has a U-Pb zircon age of 2545 ± 7 Ma, which overlaps with the timing of gold deposition and is consistent with structural interpretations. The Ramagiri and Kolar gold deposits elsewhere in the Dharwar craton are considered to have a similar structural timing, and the Hutti geochronology suggests that gold mineralization occurred throughout the Dharwar craton some 80 to 120 m.y. later than the major peak of Late Archean world-class orogenic gold mineralization in most other Archean cratons. [ABSTRACT FROM AUTHOR]

We report in situ and bulk S-isotope analyses of the 3.2 Ga old barite deposit of the Sargur Group, Dharwar Craton, India. Compared with the two other coeval barite deposits in Western Australia (Pilbara Craton, Warrawoona and Sulfur Springs Group) and South Africa (Kaapvaal Craton, Onverwacht and Fig Tree Group), the rocks of the Sargur Group have experienced pervasive ductile deformation and equilibrated under high-grade metamorphic conditions, which could have caused partial S-isotope re-equilibration. However, in situ techniques revealed high spatial heterogeneities in δ 34 S of up to ∼18‰ in barite-bearing pyrites, as well as similar Δ 33 S anomalies compared to the associated sulfate indicating bacterial sulfate reduction isotope effects and a minor impact of the pervasive ductile deformation and metamorphism on the primary/diagenetic signatures. Our data in associated silicate rocks define consistent trends in Δ 33 S/δ 34 S and Δ 36 S/Δ 33 S reflecting metamorphic reworking of primary photolytic signatures with sulfide deriving from thermal reduction of the barite sulfate at >400 °C. Our results match the previous data obtained on other Early Archean barite deposits. This strongly suggests that similar sulfur sources were at play over the three continents during the production and preservation of atmospheric sulfate and elemental sulfur aerosols. This also confirms that the different Early Archean barite deposits known on Earth formed during periods of intense subaerial volcanism and record volcanic photolytic signatures different from those common in the Archean. [ABSTRACT FROM AUTHOR]