大别造山带沙坪沟特大型斑岩钼矿床年代学及成矿机理研究

摘要

沙坪沟钼矿是我国新发现的最大的斑岩钼矿,位于大别造山带的西缘,是秦岭-大别钼矿带东缘的一部分.主体为隐伏矿床,地表钼矿化较弱,钼矿主要以脉状产于花岗斑岩和石英正长岩中.花岗斑岩是主要赋矿岩体,普遍见有硅化、钾化、黄铁绢英岩化,发育爆发角砾岩,野外观察及钻孔揭示钼矿成矿与斑岩体中的爆发角砾岩有密切关系.地球化学研究显示,花岗斑岩具有偏酸性,钙碱性富钾低钙低钠的特点.斑岩中辉钼矿的Re-Os年龄测定结果表明等时线年龄为111.1±1.2 Ma,含矿斑岩的锆石U-Pb年龄为111.5±1.5 Ma,石英正长岩锆石U-Pb年龄为111.7±1.9 Ma.同时花岗岩中锆石CL图像显示钼矿成矿过程中受到明显的热液活动作用,热液成因的锆石Y/Ho和Ce4+/Ce3+明显高于其他岩浆成因锆石,不在“CHARAC”场范围.锆石的Ce4+/Ce3+和(Eu/Eu*)N值反应出沙坪沟钼矿样品形成过程中岩浆氧逸度偏低,呈现出不同于斑岩铜(钼)矿的演化趋势.全岩锆石钛温度计计算出花岗斑岩和石英正长岩的温度分别为830℃和860℃.由相邻钼矿床成矿年代对比可知,沙坪沟钼矿床与秦岭-大别钼矿带的第三期成矿环境相同,位于三叠纪大别山主碰撞带的北部,属于弧后位置.到燕山中晚期,由于太平洋板块的后撤,中国东部岩石圈大规模拆沉,软流圈被扰动,深埋于地壳的含矿沉积物与酸性岩浆熔融,之后岩浆逐步分异演化,进入成岩和岩浆期后热液阶段,与钼有关的硫化物快速萃取,岩浆沿大小断裂及塌陷盆地边缘上侵形成钼矿床.%The Shapinggou porphyry molybdenum deposit located in the western margin of the Dabie Mountains is the newly-discovered largest Mo deposit in China. It is part of the east extension of the Qinling-Dabie Mo mineralization belt. The main orebody is concealed with weak Mo mineralization. Mo deposits mainly occur in high-K granitic porphyry and quartz syenite. The former is the major host rocks, commonly accompanied with silicification, potassic alteration, and pyritized sericitization, as well as brecciation. Field observation and drilling data reveal that Mo mineralization is closely related to collaspe of breccia. The geochemical data indicate granitic porphyry is characterized by acid, sub-alkaline, high K2O, low Na2O and Ca2O. Re-Os dating of molybdenite for the porphyry deposit yields an isochron age of 111. 1± 1. 2 Ma. Zircon U-Pb ages are 111. 5±1. 5 Ma and 111. 7± 1. 9 Ma for the granitic porphyry and quartz syenite, respectively. Meanwhile, zicron CL images of zircons from the granites show Mo mineralization underwent distinct hydrothermal activities. Y/Ho, Ce4+/Ce3+and (Eu/Eu" )N of hydrothermal zircons are distinctly higher than those of magmatic zircons, distinctly out of the range of the CHARAC field. Zircon Ce4+/Ce3+ and (Eu/Eu* )N reflect a low oxygen fugacity in magma during the Shapingou Mo mineralization, presenting a different evolution trend from that of porphyry Cu or Mo deposit. Temperatures calculation of whole rock zircon Ti indicates that granitic porphyry and quartz syenite has temperatures of 830℃ and 860℃, respectively. Comparison to metallogenic ages of the neighboring Mo deposits shows that the Shapinggou deposit has the similar ore-forming background to the third phase of the Qinling-Dabie Mo metallogenic belt. The fact that it is located at the northern section of the Triassic suture between the North and South China Blocks suggests a back-arc setting. At the middle and late stages of Yanshanian period, retreat of the Pacific plate, large-scale detachement of asthenosphere in eastern China, and mixing between ore-bearing sediments at deep crust and acidic magma asthenosphere resulted in further fractionation of magma to enter the diagenesis and post-magma hydrothermal stage related to quick extraction of Mo. Emplacement of magma upward along various factures and the margins of collapsed basin are responsible for the formation of Mo mineralization.