Algoma型和Superior型硅铁建造地球化学对比研究

A compare geochemistry study for Algoma-and Superior-type banded iron formations

前寒武纪条带状硅铁建造(BIFs)是世界上最重要的铁矿资源类型和地球早期特有的化学沉积建造类型,广泛分布于太古代-古元古代(3.2～1.8Ga),记录了地球早期岩石圈、水圈、大气圈和生物圈的状态及演化。前人根据BIFs的岩石组合和构造地质环境将其划分为Algoma型和Superior型。本文对比研究了Algoma型和Superior型BIFs的硅、氧、铁和多硫同位素特征。不同时代和不同类型BIFs的硅氧同位素组成非常相似,强烈亏损30Si,δ30SiNBS-28为较大的负值。二者的铁同位素和硫同位素非质量分馏效应明显不同。Algoma型BIF的Δ33S多为负值,而Superior型BIF的Δ33S多为正值;Algoma型BIF富集重铁同位素,δ56FeIRMM-144多为高正值,而Superior型BIF相对富集轻铁同位素,δ56FeIRMM-144多为负值或小正值。研究提出无论是Algoma型,还是Superior型BIFs都是由地球早期的海底火山热液喷气作用形成的,二者属于同一成矿系统,相对而言,Algoma型BIF与火山活动关系更密切,距离同期火山活动中心更近,多形成于深水盆地,环境更加还原。

The Precambrian banded iron formations （BIFs） are the most important type for iron resources in the world and extraordinary chemical marine sediments formation only occurred in the early Earth, which were concentrated during Archean to Early Proterozoic eras （3.2～1.8Ga） and recorded lithosphere, hydrosphere, atmosphere and biosphere status and evolvement then and there. The BIFs were classified into Algoma-and Superior-type on the basis of mineralogical composition and proposed tectonic setting. But contrastive studies about their geochemical characteristic, depositional environment and origin are little. So the silicon, oxygen, iron and multiple sulfur isotopic compositions of Algoma-and Superior-type BIFs were compared. The silicon isotopic compositions of BIFs of different types and ages are similar and strongly depleted in 30Si, the δ30Si values are lower. Their iron and multiple sulfur isotopic compositions are obvious different. Most Δ33S values of Algoma-type BIF are negative, whereas positive Δ33S values appear at Superior-type BIF. The heavy iron isotope are usually enriched in Algoma-type BIF, the δ56Fe are large positive values;whereas the light iron isotope are enriched in Superior-type BIF, the δ56Fe are negative or small positive values. The data imply that BIFs were formed by submarine exhalation, whatever the Algoma-type or the Superior-type, they belong to the same mineralize system. In contrast, Algoma-type BIF were closely related to volcanic activities and formed in depth marine basin and more reductive depositional environment.