ORIGIN OF IRON-BEARING NON-SKELETAL ONCOLITE AND ITS ROLE IN CONCENTRATING IRON IN ORDOVICIAN LIMESTONE, ANHUI PROVINCE
摘要: 安徽下扬子区下奥陶统顶部到中奥陶统的灰色生物屑微晶灰岩中,产出一种含铁的非骨架核形石。它形成于宁镇—皖南碳酸盐台地两侧边缘的浅水到潮坪环境中。这种核形石是由蓝绿藻以生物碎屑为核心,围绕核心以底部滚动和悬浮方式生长成同心圆状多层包壳的椭球体,有柱核形石和层柱核形石两种类型。蓝绿藻呈核形石生长过程中,吸附铁质形成含铁核形石,并能吸附铜、铅等金属元素。Abstract: A kind of iron-bearing non-skeletal oncolites has been found in the Middle and Lower Ordovician grey biomicrite of Kuniutan and Datianba Formations at Shitai, Chao and He counties, Auhui. Beds containing this Kind of oncolites are intercalated between the brownish red or grey-green biomicrite, nodular biomicrite and argillaceous biomicrite. The oncolite content is: 1- 5% or more. The fact that the birdseye structure and desiccation cracks occur on some bed surface indicates that the beds are formed in a shallow subtidal or supratidal oxidizing environment. The sediments bearing the oncolite are distributed in a mud-bank-facies belt which is restricted to the southwestern margin of Ninzhen-Wannan carbonate platforms where are near Huaiyang ancient continent and Jiangnan ancient island which are considered as a source of iron for the oncolite, however, the sediments in the same facies belt but far away from the anceient continent and island develop little ironbearing oncolite. The iron-bearing non-skeletal oncolites are oval grains, 15-20mm in lenghth and the longest up to 32.8mm, consisting of a nucleus of bioclast and concentric laminated crust which is formed by the grown blue-green algae. Two types of oncolite can be recognized depending on the distinction of their microtexture. Type one is columoids consisting of columnar microstromatolites in a radiating arrangement around a nucleus. The second type is colu-strigatoids consisting of laminar-columnar micro stromatolites around a nucleus. Their crust can be distinguished into three parts; inner crust, middle crust ( major curst ) and outer crust. Among them the middle crust is the thickest one. The formation of oncolites underwent several processes: the prepartion of nuclei; the growth of original crust rolling on the bottom) the suspention growth of major crust) the calcification of crust) the growth and abrasion of outer crust) and last the burial and consolidation of oncolites. Based on the thickness of the major crust and the number of algae lamina-minae per unit length, it can be determined that columoids grow faster and suspend longer than the colu-strigatoids. The constituents of the oncolite include calcite, limonite, hematite and cham-osite etc. and calcite is the major composition. The total amuont of ironstone in the oncolite comes to 5.92%. It is four times higher than that of country rocks. Most of iron materials concentrate in the algae laminae and the mucilage of algae with colloidal texture. The iron content ia the laminae has a cyclic change. Based on the distribution of iron it can be suggested that the blue-green algae absorb and precipitate iron materials while they are growing. This process is called concentration of iron in growing period.The iron can also be concentrated around the oncolite in cloud pattern during diagenetic process but it is not important while compared with the former. The oncolite not only contains iron but also contains other metal elements. For example, in the oncolite, Pb = 200ppm, Cu=100ppm, their contents are seven and three times as high as that of country rocks respectively. The evidence mentioned above shows that the growth of algae is of significance in concentrating iron and other metal elements, such as lead and copper, which may result in the formation of sedimentary iron deposits or other metal deposits,
 1 贺自爱，1982，石油与天然气地质，1期，41-97页。
4 Dahayake，K.，1978，Sequential position and enviromental significance of dif- ferent types of oncoids. in: Sedimentary Geology, 20, p.301-316.
3 Milliman, D. J., 1974，海洋碳酸盐，中国科学院地质研究所译，地质出版社，1978, 35-94 页。
5 Monty，C. L.and Mas, J. R.，1989, Lower Cietaceous(Wealdian)blue-green algal deposits of the province of valencia. E astern Spain.in; Coated Grain. p. 85-120.
- 文章访问数: 533
- HTML全文浏览量: 7
- PDF下载量: 531
- 被引次数: 0