Modelling B-release and isotopic fractionation during metamorphic dehydration of basalt and pelite: Implications for the source of mineralizing fluid in greenstone-hosted orogenic gold deposits. (1st July 2021)

• Record Type: Journal Article
• Authors: Hazarika, Pranjit; Upadhyay, Dewashish; Mishra, Biswajit; Borah, Pritom; Abhinay, Kumar
• Is Part Of: Geochimica et cosmochimica acta. Volume 304(2021)
• Page Start: 83
• Page End: 100
• Publication Date: 2021-07-01
• Subjects: Tourmaline -- B-isotope -- Orogenic gold -- Greenstone-belt -- Eastern Dharwar Craton -- Hydrothermal fluid; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2021.04.019
• Languages: English
• ISSNs: 0016-7037

Abstract

Highlights: Devolatilized fluids from metabasalts at ∼550 °C have δ 11 B between −14.2 to −10.8‰. Metapelites at similar temperatures release fluids with δ 11 B between −5.7 to 0.0‰. Tourmaline δ 11 B from worldwide orogenic gold deposits match modelled δ 11 B for metamorphic fluids. Tourmaline δ 11 B in Hutti and Kolar (−14.0 to −3.3‰) suggest precipitation from metamorphic fluids. Li and V in tourmaline may be used as reliable source rock indicators. Abstract: 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 (δ 11 B = −24.8‰ to +19.8‰). The δ 11 B 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 10 B-rich fluids, which rapidly lower the δ 11 B of the metamorphic fluid. The δ 11 B 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 δ 11 B between −18.0‰ and −12.4‰ while those crystallizing from metapelite-derived fluid will have δ 11 B between −9.5‰ and −1.6‰ for the temperature range of ore formation (300–550 °C) in orogenic gold deposits. Our calculated range of δ 11 B for the metabasalt- and metapelite-derived metamorphic fluids matches closely with the two peaks in the δ 11 B distribution of tourmaline from orogenic gold deposits. The δ 11 B 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 δ 11 B 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 δ 11 B (−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-à-vis the Kolar tourmalines (avg. 615 ppm) is suggestive of greater pelitic input in Hutti.