Geology, geochemistry and fluid inclusions of the Bianjiadayuan Pb-Zn-Ag deposit, Inner Mongolia, NE China: Implications for tectonic setting and metallogeny

摘要

The Bianjiadayuan Pb-Zn-Ag deposit in the Southern Great Xing'an Range consists of quartz-sulfide vein-type and breccia-type mineralization related to granite. Vein orebodies are localized in NW-trending extensional faults. Hydrothermal alteration is well developed and includes silicification, potassic alteration, chloritization and sericitization. Three stages of mineralization are recognized based on field evidence and petrographic observation and are marked by assemblages of quartz-arsenopyrite-pyrite (stage I), quartz-pyrrhotite-chalcopyrite-sphalerite (stage II) and quartz-galena-silver minerals (stage III). The granite, with a zircon age of 143.2 +/- 1.5 Ma (n = 14, MSWD = 0.93), is subalkaline, peraluminous and is classified as A2-type granite originating in a post-orogenic extensional setting during the opening of suture zone between the North China Craton and the Siberia Craton from the Late Jurassic to the Early Cretaceous. The delta(34)Sayr values of sulfides, ranging from 3.19 to 10.65%., are not consistent with the majority of magmatic hydrothermal deposits in the SGXR, possibly implying accessory source in addition to magmatic source. Microthermometric measurements show that ore minerals were deposited at intermediate temperatures (347.8-136.4 degrees C) with moderate salinities (2.9-14.4 wt.% NaCl). Ore-forming fluids were derived largely from magmatic hydrothermal processes, with the addition of meteoric water in late stage. Successive precipitation of Pb, Zn and Ag occurred with changes of physicochemical conditions. Overall considering mineralization features, ore-forming fluids and materials and tectonic setting and comparing with adjacent deposits, the Bianjiadayuan deposit is a mesothermal magmatic hydrothermal vein-type Pb-Zn-Ag deposit controlled by fractures and related to A2-type granite in response to the tectonic/magmatic/hydrothermal activity in late Jurassic. Besides, the explosive breccias in the west area require more attention in future exploration. (C) 2015 Elsevier B.V. All rights reserved.