Many Paleoproterozoic borate deposits are hosted within the lower units of the volcanic–sedimentary sequence (ca. 2.2 Ga) of the Liaohe Group, Liaodong Peninsula, northeast China. It is debated as to whether these deposits were affected by marine evaporation. The borate ore bodies of the Zhuanmiao deposit in the Liaohe Group have stratiform or lenticular shapes and occur in serpentinized carbonates, with the ores being bordered by layered leptynites, leptites, amphibolites, and migmatites. We carried out a C–O–Mg stable isotope study of the ores and carbonate rocks in the Zhuanmiao deposit. The δ13C values of unserpentinized magnesite marbles in the ores of the Errengou block in the Zhuanmiao deposit range from +3.2 ‰ to +6.0 ‰ and preserve a positive C isotopic anomaly typical of the Lomagundi–Jatuli Event (2.306–2.057 Ga). The δ26Mg values of these unserpentinized magnesite marbles in the Zhuanmiao borate deposit range from –1.01 ‰ to –0.84 ‰, which are typical of marine sediments. The positive C isotopic anomaly of the magnesite marbles in the Zhuanmiao deposit indicates that the C isotopic anomaly of magnesite that formed during the Lomagundi–Jatuli Event is not only a result of global oxygenation of the ocean–atmosphere, but also marine evaporation in individual basins. This evaporation even led to rare examples of borate mineralization.
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