Lead Isotope Sign of the Mantle Material Contribution to the Formation of Tin Ore Deposits (by the Example of the Khingan-Badzhal-Komsomolsk Metallogenic Zone, the Far East)
摘要
Using the high-precision (±0.02%) MC-ICP-MS method, the lead isotopic composition of galena from four Late Cretaceous (95–85 Ma) tin ore deposits of the Khingan-Badzhal-Komsomolsk metallogenic zone (KhBKZ), which is part of the Khingan-Sikhote-Alin ore province (KhSAP), is studied. In comparison with other previously studied KhSAP tin deposits, the KhBKZ ore lead has lower isotopic ratios ranging as 206Pb/204Pb = 18.359–18.497, 207Pb/204Pb = 15.565–15.588, and 208Pb/204Pb = 38.369–38.404. In the 206Pb/204Pb–207Pb/204Pb diagram, the points of the KhBKZ lead isotopic compositions are located significantly below the orogenic evolution curve (according to the Zartman-Doe model) and the average crustal curve (µ2 = 9.74) according to the Stacey-Kramers model. According to these evolutionary characteristics, as well as by the values of the Th/U parameter of the source and the Tm model age, the KhBKZ ore lead differs from KhSAP lead, indicating a significant difference in the geochemical and geodynamic conditions of the KhBKZ deposits formation. In particular, it indicates a limited contribution of the continental crust, a significant proportion of lead originated from the mantle and the participation of a regional mantle source in the formation of tin-bearing granitoid magmas. Model calculations of the balance of lead of mantle and crustal sources in the ore lead of KhSAP and KhBKZ show that for KhSAP, the contribution of the mantle source was less than 50%, and, in the case of KhBKZ, it could reach 70–90%. Perhaps the main geological factor that caused the noticeable participation of mantle lead in the KhBKZ deposits was the environment of their formation: unlike the KhSAP, these deposits were formed in the environment of a transform margin, when due to the interaction of moving plates, a larger-scale admission of mantle material into the zone of granite formation and ore deposition occurred. Under these conditions, the oceanic plate could break up with the admission of the hot asthenosphere into the accretionary prism.