Metallogenic Chronology of Boron Deposits in the Eastern Liaoning Paleoproterozoic Rift Zone

Lead isotopic analytic data of 30 ores gathered from the Zhuanmiao boron deposit, Wengquangou boron (iron) deposit and its Dongtaizi Ore Member constitute three isochrons, the corresponding ages of which are 1902 ± 12 Ma, 1852 ± 9 Ma and 1917 ± 48 Ma. Lead isotopic analyses of marble from the Xiquegou Member of the Qingchenzi orefield yield a Pb-Pb isochron age of 1844 ± 27 Ma. 40Ar-39Ar quick neutron activation dating of phlogopites and microclines coexisting with ore minerals in the Wengquangou boron (iron) and Zhuanmiao boron deposits shows that: (1) the phlogopite from the Wengquangou has a plateau age of 1923 ± 1.5 Ma and an isochron age of 1924 ± 2.5 Ma; (2) the microcline from the Wengquangou has the plateau age of 1407 ± 5.4 Ma and 220 ± 12 Ma and an isochron age of 1403 ± 19 Ma; (3) the phlogopites from the Zhuanmiao yield a plateau age 1918 ± 1.3 Ma and an isochron age of 1918 ± 2.9 Ma; (4) the microclines from the Zhuanmiao yield the plateau age of 1420 ± 16 Ma and 250 ± 8 Ma and an isochron age of 1425 ± 19 Ma and 269 ± 16 Ma. These ages indicate that the eastern Liaoning area happened around 1900 Ma, an important tectonomagmatic event, which is consistent with the worldwide Mid-Proterozoic tectonomagmatic event. During this period, the Proterozoic Liaohe Group was folded and underwent strong normal metamorphism, and the (hydrothermal) sedimentary boron deposits (or source beds) formed earlier were strongly superimposed by mineralization, resulting in enrichment of boron; later regional geological processes made little contribution to the formation of the boron deposits. Lead isotopic components show that the U-Pb and Th-Pb isotopic system reached homogenization in the ores whereas only the U-Pb isotopic system reached homogenization in the marble from the Xiquegou district, which indicates that the boron deposits superimposed in the studied area endured a relatively strong process of hydrothermal migmatization during the end phase of early Proterozoic metamorphism.