Origin of tourmaline-rich rocks in a Paleoproterozoic terrene(N.E.China):Evidence for evaporite-derived boron

Abstract: We have analysed the major element, trace element and boron isotope composition of borates, tourmaline-rich rocks and their non-tourmalinised equivalents from the lower portion of the Paleoproterozoic South Liaohe Group (N.E. China). These rocks host economically important borate deposits that formed in an evaporitic environment. The sequence contains abundant tourmaline in leptynites, stratiform tourmalinites, quartz-tourmaline veins and pegmatites that have spatial and temporal relationships to the borates.Tourmalines distal to the borates have lower δ1 lB values (-5.2 ‰ to +3.6 ‰) than those proximal to the borates (+1.8 ‰ to +9.4 ‰)that are slightly lower than those of the borates (av. +10.5 ‰). Mg/Fe ratios of the distal tourmalines are low (<1.0) relative to proximal tourmaline-rich rocks (>1 to 2.6). These differences reflect the fact that the evaporites are a major source of B and Mg. The geochemistry of immobile elements in the stratiform tourmalinites is similar to that of their unmineralised equivalents (interpreted as meta-tuffites), suggesting that the tourmalinites formed via alteration of Al-rich tuff layers by boron-rich fluids derived from leaching of the underlying borates. Field evidence and geochemical data support a three-stage model of tourmaline formation. The first stage was coeval with deposition of the borates, during which boron from hot springs was adsorbed by clay minerals in the tuffs that were later metamorphosed to tourmaline-bearing leptynites. In the second stage, hot fluids leached boron from the evaporites and passed through overlying tuffs to form the stratiform tourmalinites. During later emplacement of granites at the base of the sequence, felsic veins and pegmatites intruded the borates and tourmalinites. Boron from the borates diffused into the felsic veins, forming coarse-grained tourmaline at their margins. This association of tourmaline with the borate deposits emphasizes the significance of tourmaline-rich rocks as a prospecting guide for borates in this area and has implications for the origin of tourmalinites in other metamorphic terrenes.