摘要： 鲕粒灰岩是寒武纪重要的碳酸盐岩沉积类型之一,其以方解石质和白云石质鲕粒最为常见,成因已有诸多研究。然而,含铁鲕粒灰岩或铁鲕岩在寒武纪却很少见有报道,该类鲕粒的形成机理仍不清楚。基于此,文中综合运用沉积学与沉积古地理、矿物学和地球化学方法,对豫北卫辉地区馒头组的含铁鲕粒进行了详细的分析。研究区含铁鲕粒包括微亮晶含铁鲕粒和亮晶铁质鲕粒2类,形成于晋豫局限海鲕粒浅滩与滩后微生物丘礁的过渡地带。地球化学测试和显微观察表明含铁鲕粒中的铁矿物为形态不规则且发育锯齿状边缘的赤铁矿自形晶体,未见其他含铁矿物; 样品的主量元素Ca、Fe、Al和Si含量较高,且Fe与Al元素之间有显著的正相关; 微量元素呈现与地壳类似的丰度和变化规律; 稀土元素总量较高,总体偏向于轻稀土相对亏损、重稀土相对富集且无明显Eu异常; 这些特征说明含铁鲕粒的含铁矿物来自陆源。矿物学和沉积古地理分析表明,含铁矿物很可能以Fe(OH)₃胶体溶液的形式自鄂尔多斯陆随径流南下进入研究区。研究区馒头组含铁鲕粒的形成需要具备3个条件: 陆源不断供给的Fe(OH)₃胶体、近岸半封闭海湾环境对铁的富集及规律性搅动的水动力条件对铁矿物的持续氧化。在鲕粒形成过程中,来自陆地风化的铁源的周期性供给会形成含有赤铁矿(包括少量伊利石)的方解石纹层,并与不发育赤铁矿的方解石纹层交替发育组成微亮晶含铁鲕粒; 如果铁源供给充分,则形成以赤铁矿纹层为主、偶夹方解石纹层的亮晶铁质鲕粒。上述结果有助于揭示其形成时期的古海洋环境与大气环境,对铁矿床的开发和利用具有重要的理论意义与实际价值。

关键词: 成因机制, 含铁鲕粒, 铁矿物来源, 寒武系, 馒头组

Abstract: Oolitic limestone is an important type of carbonate rocks in the Cambrian period,with calcite and dolomitic ooids being the most common. While there have been numerous studies on the formation of these ooids,the occurrence of iron-bearing ooids in the Cambrian is rare and the formation mechanism remains unclear. The iron-bearing ooids of the Cambrian Mantou Formation in the Weihui area of Henan Province were systematically analyzed using sedimentology,sedimentary paleogeography,mineralogy and geochemistry. They can be divided into microsparry iron-bearing ooids and sparry iron ooids,which were formed in the transition zone between the oolitic shoal and back-shoal microbial bioherm of a limited sea in Shanxi and Henan Provinces. The geochemical tests and microscopic observations of iron-bearing ooids indicate that the iron minerals are idiomorphic crystals of hematites with irregular morphology and serrated edges,with no other iron minerals present. The contents of major elements such as Ca,Fe,Al and Si in the samples are high,showing a significant positive correlation between Fe and Al. The abundance and variation trend of trace elements resemble those found in the Earth's crust. The total amount of rare earth elements is high,with a deficiency in light rare earth elements,and an enrichment in heavy rare earth elements. There is no obvious Eu anomaly. These characteristics indicate that the iron minerals in iron-bearing ooids originate from terrestrial detrital input. The analysis of mineralogy and sedimentary palaeogeography of iron-bearing ooids reveals that the iron-bearing minerals likely formed in the study area as a result of the precipitation of Fe(OH)₃ colloidal solution from the Ordos land with runoff. Three conditions are necessary for the formation of iron-bearing ooids in the Mantou Formation of the study area: continuous supply of Fe(OH)₃ colloidal solution from terrestrial source,iron enrichment in a semi-enclosed coastal bay environment,and continuous oxidation of iron minerals under the hydrodynamic conditions of regular agitation. During oolitic formation,the intermittent input of iron elements from terrestrial weathering leads to the development of alternating laminae of hematite and illite-bearing calcite,which interbed with calcite lamina lacking hematite crystals to form microsparry iron-bearing ooids. If the iron supply is sufficient,sparry iron ooids are formed,characterized by hematite lamina interbedded with calcite lamina. These findings are crucial for understanding the paleo-marine environment and atmospheric environment during the geological period,with significant theoretical and practical implications for the development and utilization of iron deposits.

Key words: genetic mechanism, iron-bearing ooids, source of iron minerals, Cambrian, Mantou Formation

中图分类号: 

• P588.24+5