Abstract
A large number of Paleoproterozoic borate deposits are hosted by the lower units of a volcanic-sedimentary sequence in Liaoning Province, northeastern China, and are a major source of boron in China. The ore-bearing wall rocks in the deposits are serpentinized ultrabasic rocks and carbonates, with layered leptynites, leptites, amphibolites, and migmatites adjacent to the ore. Both the borate ores and country rocks contain tourmaline, although the country rocks have much lower abundances of the mineral. Based on in situ boron isotope measurements using laser ablation–multi-collector–inductively coupled plasma–mass spectrometry (LA–MC–ICP–MS), boron isotope data show that: (1) δ11B values of borate ores range from +6.8‰ to +13.9‰ (mean +10.8‰); (2) tourmalines from the borate ores have δ11B values from +9.5‰ to +12.7‰; and (3) the wall rocks within the borate ores yield slightly lower δ11B values ranging from +5.7‰ to +7.6‰, and those outside the deposits from −9.9‰ to −5.9‰. Positive δ11B values in borates as well as in tourmalines inside the mining area indicate that boron in these Paleoproterozoic borate deposits was derived from marine evaporites. δ34SV-CDT (where V-CDT is Vienna Canyon Diablo Troilite) values of borate ores, serpentinized marbles, and anhydrites range from +16.1‰ to +24.7‰, whereas δ13CV-PDB (where V-PDB is Vienna Pee Dee Belemnite) values of marbles range from +3.2‰ to +5.9‰. These isotopic characteristics are interpreted to reflect formation in a marine evaporative environment. LA–MC–ICP–MS zircon weighted207Pb/206Pb ages of leptite and serpentinized olivine basalt from the hanging wall of the borate deposits are 2139±13Ma and 2130±19Ma, respectively. Therefore, the (~2.2Ga) borate deposits may have originated from marine evaporative boron-bearing sediments, which were interbedded within bimodal volcanic rocks during the early stages of development of the Liaoji rift.
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