Hydrothermal gold mineralization at the Hira Buddini gold mine, India: constraints on fluid evolution and fluid sources from boron isotopic compositions of tourmaline

摘要

We determined the boron isotope and chemical compositions of tourmaline from the Hira Buddini gold deposit within the Archean Hutti-Maski greenstone belt in southern India to investigate the evolution of the hydro-thermal system and to constrain its fluid sources. Tourmaline is a minor but widespread constituent in the inner and distal alteration zones of metabasaltic and metadacite host rocks associated with the hydrothermal gold mineralization. The Hira Buddini tourmaline belongs to the dravite-schorl series with variations in Al, Fe/(Fe+Mg), Ca, Ti, and Cr contents that can be related to their host lithology. The total range of delta~(11)B values determined is extreme, from -13.3 per thousand to +9.0 per thousand, but 95 percent of the values are between -4 and +9 per thousand. The boron isotope compositions of metabasalt-hosted tourmaline show a bimodal distribution with peak delta~(11)B values at about -2 per thousand and +6 per thousand. The wide range and bimodal distribution of boron isotope ratios in tourmaline require an origin from at least two isotopically distinct fluid sources, which entered the hydrothermal system separately and were subsequently mixed. The estimated delta~(11)B values of the hydrothermal fluids, based on the peak tourmaline compositions and a mineralization temperature of 550 deg C, are around +1 and +10 per thousand.. The isotopically lighter of the two fluids is consistent with boron released by metamorphic devolatilization reactions from the greenstone lithologies, whereas the ~(11)B-rich fluid is attributed to degassing of I-type granitic magmas that intruded the greenstone sequence, providing heat and fluids to the hydrothermal system.