Continental evaporites as brine sources for formation of SEDEX and MVT deposits: Implications from boron isotope compositions of tourmaline

Abstract

In this study, the boron isotopic composition of Paleoproterozoic seawater is reconstructed using δ11B of tourmaline from the Paleoproterozoic Pb-Zn deposits of Rajpura-Dariba, Zawarmala, and Rampura-Agucha in the Aravalli-Delhi Fold Belt (ADFB) as well as non-mineralized tourmaline bearing metapelites from the Mangalwar Complex (MC) and Gangpur Schist Belt (GSB). This is then used to constrain the source of the mineralizing fluids in Sedimentary Exhalative (SEDEX)- and Mississippi Valley Type (MVT)-deposits. Based on textural relations and chemical variations, tourmaline in the Rajpura-Dariba and Zawarmala deposits can be grouped into two generations: syn-mineralization Tur-I and metamorphic Tur-II. In contrast, a single generation of metamorphic tourmaline is identified in the Rampura-Agucha deposit and in the MC. Tur-I from Rajpura-Dariba has XMg of 0.14–0.56, while Tur-II is more magnesian (XMg = 0.58–0.77). Similarly, Tur-I from Zawarmala has a wide compositional range (XMg = 0.22–0.67), whereas Tur-II is Mg-rich (XMg = 0.39–0.67). Tourmalines from Rampura-Agucha and MC are also magnesian (XMg = 0.66–0.70 and 0.59–0.69, respectively). The δ11B values of syn-ore tourmaline from Rajpura-Dariba and Zawarmala vary between −18.8‰ and −16.1‰, and between −6.6‰ and −4.0‰, respectively. Metamorphic tourmalines from Rajpura-Dariba, Zawarmala, Rampura-Agucha and MC have δ11B values between −19.1‰ and −7.7‰.The δ11B of the pelitic sediments in Rajpura-Dariba, Rampura-Agucha, the MC and the GSB were estimated from the δ11B of metamorphic tourmaline hosted in them using mass balance constraints and P-T pseudosection modelling. Applying the B-isotope fractionation factor between Miocene shale and modern clay (Δ11Bmiocene shale − modern clay = −3.8‰ to −2.2‰) to the modelled δ11B of post-diagenetic protoliths of the metapelites, the δ11B of marine sediments for these basins were constrained between −15.1‰ and −10.2‰. Subsequently, using the B-isotope fractionation factor between modern clay and seawater (Δ11Bmodern clay − modern seawater = −45.2‰ to −40.8‰), the δ11B of Paleoproterozoic seawater is estimated to have been between +27.3‰ and +33.4‰.The isotopically light δ11B of syn-mineralization tourmaline from Rajpura-Dariba and Zawarmala (−18.8‰ to −16.1‰ and −6.6‰ to −4.0‰, respectively), and worldwide SEDEX/MVT-type deposits in general (a median value of −9.6‰; compilation of Trumbull et al., 2020), implies that the B was most likely sourced from continental evaporites and not evaporated seawater and/or connate brine. Dissolution of early rift-related continental evaporites most likely act as a major source of basinal brines for the formation of SEDEX/MVT deposits during the sag-phase of basin development.