太原西山七里沟剖面本溪组铁质鲕粒成因探讨

著名的“山西式”铁矿几乎分布于整个华北地区，其成因曾是一个古老的地质问题。本文在前人研究的基础上，通过野外露头观察，并结合镜下鉴定与能谱分析对太原西山七里沟剖面本溪组铁质层中铁质鲕粒结构及其特征进行了详细研究。研究表明，大部分铁质鲕粒具有放射结构，并能隐约看到同心圈层，同时，在鲕粒内部及其周围发现了大量的似蓝细菌丝状体，以及铁质凝块和球粒，具有明显的微生物成因特征。因此，认为铁质鲕粒的形成与微生物密切相关，微生物参与了铁矿的形成。

Origin of the Ferriferous Ooids in the Benxi Formation at the Qiligou Section, Taiyuan Xishan

The “Shangxi type” iron deposits are well known for their wide distribution on  North China platform.  The origins of the deposits including their mineralization processes and  the mineral sources, together with their ages, were once concerned by many geologists. Although the geological problems seem clear, the microbial activity is  documented to have influence on the formation of  the  iron deposits just as the Xuanlong iron deposits. 
  The iron deposits outcropped well in Taiyuan Xishan favors our study, and the ferriferous oolites developped in the iron layer of the Benxi Formation at the Qiligou section became our main study object. Through field observation,  thin section examination and analysis of Energy spectrum, the composition  and  texture  of the ferriferous oolites are studied in detail. The oolites are matrix supported, with  45%  of  ooids   and a few other grains (3%) such as bauxite debris and biotic fragments(shell,etc.) floating in clay minerals. The ooids are well sorted  with  the sizes  mainly ranging  from 0.5 mm to 1 mm in diameter. In the thin section, most of  the ooids are  round,  and   some rombic and oval shaped. The cortical layers of the ooids are mainly composed by micritic hematites, whereas the nuclei consist of micritic hematites or clay minerals, which indicate certain common origin
between  some nuclei and the matrix. Most of the ooids are characterized by radial structure with 2 or 3 obscure concentric layers, which indicate that the ooids probably formed in a relative stable condition, coinciding with the previous conclusion that the iron deposits formed in the restricted lagoon environment. Additionally, abundant intertwined cyanobacteria filaments are found to aggregate around and envelop most of the ooids and the distribution of ferriferous material (micritic hematites) in the matrix is closely related to these filaments. Spherical cyanobacteria which are interlaid in the intertwined filaments are also recognized under SEM. At  the  same time, ferriferous clots and pellets are identified in the cortical layers and even the nuclei of the ooids, and the notched margins of some ooids are probably related to microbial activity. All of these characteristics suggest that the microbial activity plays a significant role in the formation of the ferriferous ooids. The microbes are thus proposed to be involved in the formation of the iron deposits which formed in the relative stable lagoon environment.