华北东部淄博中二叠统铝—粘土矿成矿过程与构造驱动机制

The metallogenic materials of karstic bauxite (with carbonate as the substrate) were generally considered to be transported elsewhere (allochthonous), while those in the latertic bauxite with aluminosilicate as substrate were derived in-situ. The applicant has proposed that allochthonous bauxite is the product of orogeny and uplift along continent margin, and it can serve as important“deposit probe”for regional geodynamic evolution study. The genetic type of middle-Permian bauxite-clay deposit at Zibo in eastern North China Craton is unique, because orebody is in conformable contact with roof and floor sandstone and ancient weathering surfaces are absent. This means this kind of bauxite-clay deposit has a genesis of allochthonous. The preliminary study on dating of detrital zircon separated from the ores carried out by the applicant confirmed this viewpoint obtained from geological contact relationship. This cognition is different from the traditional idea about the source of red-clay type bauxite. This research aims to distinguish the sources for bauxite, clay, and the roof, floor, and interlayer sandstone via exploring detrital zircon ages and isotope compositions from these layers and testing Sr-Nd isotopes for the ores, and establish the link between the tectonic uplift, palaeogeography, and bauxite-clay deposit formation. It also plans to identify the mineral generation, to reveal bauxite mineralization process, and to explore the genetic relations between bauxite and its upper clay, finally to elucidate the controlling factors of Permian bauxite-clay deposit at Zibo. This research could provide theoretical support for prospect of the same layer in western Shandong Province and enrich the understanding on bauxite genesis.

喀斯特型铝土矿（底板为碳酸盐岩）的成矿物质一般认为是异地搬运而来，红土型（底板非碳酸盐岩）为就地风化成因。申请者前期对喀斯特型铝土矿的研究认为异地搬运铝土矿为古陆边缘造山和隆起的产物，是区域构造研究的重要“矿床探针”。华北东部淄博二叠纪铝（粘）土矿成因类型独特，成矿过程和构造背景尚需查明。其矿层与顶底板砂岩整合接触，未见古风化面，显示成矿物质是异地搬运，前期碎屑锆石年龄研究也初步证实此观点；这与对红土型铝土矿物源的传统认识存在差异。本研究通过探索铝（粘）土矿和顶、底板及夹层砂岩碎屑锆石年龄和同位素组成的差异，分析矿石Sr-Nd同位素组成，辨别成矿物质来源，确定成矿期构造抬升事件和古地理格局。鉴别矿物生成顺序，揭示铝土矿和上部粘土矿的成因联系和古环境制约。最后，综合探讨淄博二叠纪铝（粘）土矿的控制因素。该研究为鲁西地区同层位矿床找矿提供理论支撑，并可进一步丰富铝土矿成矿理论。

根据基岩类型，铝土矿可分为喀斯特型和红土型。其中喀斯特型铝土矿基岩为喀斯特古风化面，物质来源于异地搬运；红土型铝土矿基岩为铝硅酸盐岩，物质来源于底板岩石风化。华北东部淄博二叠纪铝（粘）土矿成因类型独特，其底板为砂岩且与矿层之间为整合接触，应归为红土型铝土矿；但因为其铝（粘）土矿顶板未见古风化面，显示为异地物质沉积成因。本次研究选取淄博典型的A层铝（粘）土矿为研究对象，通过显微观察、XRD分析、碎屑锆石U-Pb、Lu-Hf年代学等多种手段，探索A层铝（粘）土矿的物质组成及其赋存状态，并且分析其矿物成因，探讨A层铝（粘）土矿物质来源。研究显示，淄博二叠纪铝（粘）土矿的主要矿物成分包括高岭石和锐钛矿，以及少量的勃母石和菱铁矿。矿层底部的高岭石结晶度指数为0.57，属于无序；中上部的高岭石结晶度指数在1.02-1.14之间，属于有序。无序的高岭石由陆源碎屑风化物形成；有序的高岭石属于胶体化学结晶作用成因，由母岩风化所形成胶体，在湖沼的酸性环境下沉淀形成，也可能与沉积后期煤层有机质氧化发热有关。矿层和顶底板中的碎屑锆石大部分具有明显的震荡环带，表明这些锆石为岩浆成因；铝土矿层中的碎屑锆石U-Pb年龄为单一峰值290Ma，而顶底板峰值年龄为~320Ma、~1850Ma、~2500Ma，表明成矿物质为异地搬运。铝土矿层中年龄为290Ma的碎屑锆石的ξHf(t)值范围与华北克拉通北缘的ξHf(t)值范围几乎全部重叠，而顶底板层碎屑锆石的ξHf(t)值同时投在在华北北缘和北秦岭造山带范围中。说明铝土矿物质来源于华北北缘岩浆活动，而顶底板物质来源于华北北缘和北秦岭造山带。进一步通过与华北晚石炭世G层铝土矿的矿物学和年代学特征作对比，揭示了沉积古地理环境对铝土矿中主要矿物的形成起了控制作用。