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

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 (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.