北京十三陵地区青白口系长龙山组含铁建造的发育特征及成因*

含铁建造在整个前寒武纪的地质记录中分布广泛,但针对华北地区青白口系长龙山组出露的含铁建造目前还没有报道。文中通过野外观察及室内XRD、SEM、ICP-MS等手段,对北京十三陵地区出露的长龙山组含铁建造的分布规律、矿物组成、成矿物质来源等开展研究,并探讨含铁建造的成因机制。结果表明,含铁建造的铁质主要为赤铁矿,与石英、海绿石等矿物伴生。含铁建造存在2种分布模式: Ⅰ型含铁建造,与海绿石伴生,产出厚度变化大(1~50 cm),偏光显微镜下海绿石砂岩和铁质条带呈互层状产出,赤铁矿具近球形结构;Ⅱ型含铁建造,无海绿石伴生,产出厚度相对较大(一般大于10 cm),镜下见黑色的富铁层夹碎屑石英透镜体,赤铁矿常发育针状或雪花状结构。依据构造演化和微量元素分析,认为含铁建造的铁质来源主要为下马岭组顶部的铁质风化壳。Ⅰ型含铁建造为化学胶体沉淀与海绿石风化的混合成因;Ⅱ型含铁建造主要属于胶体化学沉积成因,后期可能经受了糜棱化改造作用。这一结论为华北地区前寒武纪含铁建造的找矿提供了沉积地质依据,对于完善含铁建造成矿机理及华北地区元古代构造演化具有重要的意义。

Characteristics and genesis of iron formation in the Changlongshan Formation of Qingbaikou System in the Ming Tombs area,Beijing

Iron formation is widely distributed through the entire Precambrian geological record,however,there is no report on the outcropped Precambrian iron formation in the Changlongshan Formation of the Qingbaikou System in North China. Based on field investigation,XRD,SEM,ICP-MS and other methods,the distribution and mineral composition of iron formation and the source of ore-forming materials outcropped in the Ming Tombs area were studied in this paper to investigate the genesis of iron formation. The study showed that the main component of iron formation was hematite,which was associated with quartz,glauconite and other minerals. The iron formation of the Changlongshan Formation was divided into two types. Type I is associated with glauconite with relatively smaller thickness,and it is characterized by bands of glauconite sandstone interbeded with iron sandstone and near-spherical hematite microscopically. Type II is not accompanied by glauconite with relatively larger thickness,and it is represented by black iron-rich layers intercalated with quartz lens and acicular or snowflake hematite under the microscope. Tectonic evolution and trace element analysis showed that the iron source was mainly derived from the irony weathering crust at the top of the Xiamaling Formation. The type I iron formation is a mixture of chemical colloid and glauconite weathering,and the type II iron formation is mainly formed by colloidal chemical deposition and could undergo mylonitization in the later stage. The research results provide a sedimentary geological basis for distribution and prospecting of the Precambrian iron formation in North China,and it is also of great significance for enriching the metallogenic mechanism of the iron formation and revealing the tectonic evolution of Proterozoic of North China.