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Formation and preservation of Paleoproterozoic boron-rich evaporites
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DOI：10.1007/s00410-026-02329-6.png)
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Abstract
En 中文
Ancient evaporitic borate deposits are rare in the geological record, and the mechanisms governing their formation and long-term preservation remain poorly constrained. The Paleoproterozoic Liao–Ji orogenic belt of the North China Craton hosts several metamorphosed stratiform borate deposits that provide a unique opportunity to address these issues. Heavy δ11B (+ 7.5 to + 9.8‰) of borate minerals (suanite, ludwigite and szaibelyite) and δ34SV−CDT (+ 8.7 to + 16.1‰) of pyrrhotite demonstrate marine contributions, whereas whole-rock geochemistry, mineral chemistry, and negative εNd values (mostly between − 2 and − 7) reflect substantial crustal input. MORB-like iron isotopes (-0.049 to + 0.292‰) indicate a dominantly hydrothermal source of iron. These features point to boron concentration within an evaporitic basin at the continent margin that received mixed marine and terrigenous contributions. Phase modelling and Zr-in-titanite temperatures constrain peak P–T conditions of regional granulite-facies metamorphism of the Wengquangou deposit to 750–800 °C at 0.6–1.0 GPa. In situ LA–ICP–MS U–Pb dating of zircon defines two age populations at ca. 2.15 Ga and 1.87–1.84 Ga, corresponding to the ages of deposition and granulite-facies metamorphism, respectively. Apatite U–Pb ages of ca. 1.71–1.64 Ga record a later post-peak thermal or fluid-related overprint. Prograde dehydration reactions converted soluble hydrous borates into stable anhydrous assemblages, fundamentally modifying mineralogy while largely retaining primary isotopic characteristics. This high-grade metamorphic overprint played a key role in stabilizing the deposit against post-depositional dissolution. Our data show that ancient evaporitic borate deposits formed by similar processes as modern ones, involving the leaching and mobilization of B from volcanic and sedimentary rocks, transport of B into a continent-margin basin, and concentration of B by evaporation. The rare preservation of ancient borate deposits does not indicate that they rarely formed, but that they are generally not preserved as regions hosting major evaporitic borate deposits seldom undergo subsequent high-grade metamorphism.
Keywords:
Boron isotopes
High-grade metamorphism
Paleoproterozoic
Evaporation
Borate deposits
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