国外前寒武纪铁建造的研究进展与有待深入探讨的问题

Research progress of Precambrian iron formations abroad and some problems deserving further discussion

形成于早前寒武纪的铁建造，是一种富铁[W（TFe）〉15％]的硅质化学沉积岩，其主要矿物组成是铁氧化物（磁铁矿和赤铁矿）及石英。根据铁建造的岩相学特征，将其划分为条带状铁建造和粒状铁建造；根据铁建造的沉积环境，将条带状铁建造划分为与火山岩有关的Algorna型和与细碎屑一碳酸盐岩有关的Superior型2种类型。铁建造的出现，起始于38亿年前，主要集中于28～18亿年，在18亿年之后有一个连续的缺失，但在8亿年左右因雪球事件而重新少量出现。Algoma型铁建造主要发育于中一新太古代，而Superior型则集中出现于古元古代；前者多形成于前寒武纪克拉通化之前，与海相火山活动和陆壳巨量增生密切相关，而后者多形成于克拉通化之后，与稳定发育的克拉通盆地和大气氧含量增加有关。Algoma型铁矿具有单个矿体规模较小、品位较低和多层发育等特征，而Superior型铁矿则具有单个矿体规模较大、品位较高、层位稳定等特征。由于铁建造在地质历史上大规模发育且不重复出现，所以，开展铁建造的研究不仅具有经济价值，而且具有重要的科学意义。铁建造的研究趋势是，在世界范围内进一步深化地球早期构造（地幔柱与早期板块构造）演化、水圈及大气圈组成与演化、地球早期生物活动，以及铁建造成因和时空分布规律等方面的研究。

Iron formation （IF）, formed during the early Precambrian period, is an iron-rich （TFe 〉 15%） and siliceous chemical sedimentary rock. It consists chiefly of iron oxides （magnetite and hematite） and quartz. According to its petrographic features, IF can be classified into two types： Banded Iron Formation （BIF） and Granular Iron Formation （GIF） ; BIF can also be further divided into Algoma type related to volcanic rocks and Superior type related to fine clastie-carbonate rocks on the basis of sedimentary environment. IF began its production at 3.8 Ga, mainly occurred from 2.8 Ga to 1.8 Ga, continuously disappeared after 1.8 Ga, and reappeared in small amounts at about 0.8 Ga because of the Snowball Event. Algoma type BIF was dominantly produced in Meso-Neoarchean, whereas Superior-type BIF was much more common in Paleoproterozoic. The former, mainly formed before cratonization, was closely related to marine volcanic activity and continental accretion while the latter, mainly formed after cratonization, was related to stable craton basins and increasing atmospheric oxygen content. Algoma-type iron deposits are often characterized by smaller-size in terms of single ore body, lower grade and multilayer development, whereas the Superior-type ones have features of lairger-size single ore bodies, higher grade and fairly stable layers. Due to extensive production and unrepeatable nature of IF, the research on IF has not only economic value but also important scientific significance. The research trend of IF seems to further focus on such problems as the relationship between IF and early tectonic （mantle plumes and early plate tectonics） evolution, the composition and evolution of hydrosphere and atmosphere, the activity of early organisms in the earth, the genesis of IF and the regularity of its temporal and spatial distribution.