沉积喷流型矿化的岩石学证据——以大兴安岭南段黄岗和大井矿床为例

Petrological evidence for exhalative min eralization: Case studies of Huanggang and Dajing deposits in the the southern s egment of the Da Hinggan Mountains, China

沉积喷流型矿床往往有各种喷流岩 (热水沉积岩 )的共生 ,正确识别这些喷流岩经常能为认识矿床的成因提供关键证据。大兴安岭南段是我国北方著名的锡 -多金属成矿区 ,燕山期火山 -侵入岩广泛出露 ,因此认为区内矿床大都是与燕山期岩浆活动有关的热液矿床。但是 ,区内近 90 %的矿床却产在二叠纪地层中 ,显示了与二叠纪沉积地层的密切联系。本文以其中的黄岗铁 -锡矿床和大井锡 -多金属矿床为例 ,对与矿石密切共生的热水沉积岩开展了系统的地质学、岩相学、矿物学和组构学研究 ,并结合必要的地球化学数据 ,证明大兴安岭南段在二叠纪沉积盆地演化过程中可能曾经有重要的水下热液沉积喷流成矿作用发生 ,尽管历来被人们所忽视 ,但其重要性可能并不亚于燕山期的岩浆热液成矿作用。黄岗矿床的层状矽卡岩与二叠纪海底火山活动关系密切 ,是一种很具特色的喷流岩 ;而被误认为是燕山期流纹斑岩的大井矿床菱铁绢云硅质岩可能是一种产在陆相断陷盆地、与锡铜铅锌银多金属矿化有关的新类型热水沉积岩。显然 。

Exhalative mineralizations are frequently associated with various types of exhal ites that often provide important evidence for ore genesis. The southern segment of the Da Hinggan Mountains is a well-known tin-polymetallic metallogenic bel t of North China where Jurassic-Cretaceous volcanic-plutonic rocks are widespr ead. Based on this fact, most of the ore deposits were regarded as epigenetic hy drothermal deposits in genetic connection with the Mesozoic magmatism. However, nearly 90% of the deposits occur in Permian strata implying a close relation bet ween mineralization and Permian strata. Case studies were made on the Huanggang Fe-Sn deposit and the Dajing Sn-polymetallic deposit. In combination with geoc hemical data, detailed geological, fabric, petrographical and mineralogical stud ies on the exhalites associated with ores demonstrated that subaqueous exhalativ e mineralization did occur during the basin evolution at the Permian time in the southern segment of the Da Hinggan Mountains, which is ignored and poorly under stood, but might be as important as the hydrothermal mineralization connected wi th the Mesozoic magmatism. The stratiform skarns in the Huanggang deposit presen ts a peculiar example of exhalites. The siderite-sericite chert in the Dajing d eposit, regarded as Mesozoic rhyolite porphyry before this study, is a new type exhalite formed in a lacustrine basin and closely associated with sulfide ore ch aracteristic of complex metal assemblage of Sn-Ag-Cu-Pb-Zn. Exhalite is appa rently one of the most important petrological evidences for exhalative mineraliz ation.