河南窑场和辽宁思山岭铁矿磁铁矿矿物学和氧同位素特征对比——对BIF型铁矿成因与形成环境的启示

A comparative study of mineralogical and oxygen isotope characteristics between the Yaochang iron deposit in Henan Province and the Sishanling iron deposit in Liaoning Province: Insights into ore genesis and forming environment of BIF

河南窑场铁矿和辽宁思山岭分别位于华北陆块的南缘和东北缘。通过对其主要矿石矿物磁铁矿单矿物的化学成分、热电性及氧同位素的分析发现,磁铁矿均具高Ni低Co的特征,暗示成矿来源与深部物质有关。它们均相对富集轻稀土元素,显示Eu正异常和无明显Ce负异常,其中辽宁思山岭铁矿磁铁矿Eu异常较为强烈。此外,窑场和思山岭磁铁矿的δ18O、热电系数平均值分别为7.1‰、-59.05μV/℃和2.3‰、-57.18μV/℃。化学成分显示两者均应属火山沉积变质成因,其中思山岭磁铁矿类质同像程度较低,结构较为均匀。对华北陆块产出的主要铁建造岩群的磁铁矿δ18O值和稀土元素组成的对比分析结果显示,贫富矿石中磁铁矿氧同位素组成具有规律性,与富化机制有密切联系;不同岩群中贫矿石磁铁矿δ18O值高低与变质程度具有一定联系。根据中太古代-古元古代BIFs的Ce、Eu异常程度的综合分析结果,推测华北陆块BIFs形成时的古海洋整体上处于缺氧或少氧环境,但伴随华北陆块演化,还原环境影响趋弱,不利于条带状铁建造的形成。

The Yaochang and Sishanling iron deposits are located respectively in southern and northwestern margin of North China block. A systematic study of chemical composition, pyroelectricity and oxygen isotope components of magnetite samples revealed some important information. The magnetite samples are all characterized by high values of Ni and low values of Co, indicating that the mineralization was related to deep materials. They have similar PAAS-normalized REY distribution patterns and are enriched in light REE, with moderately positive Eu anomalies and lightly negative Ce anomalies; nevertheless, the magnetite in the Sishanling iron deposit has a more obvious Eu positive anomaly. The average δ18O values and thermoelectric coefficients of magnetites from these two deposits are 7.1‰ and 2.3‰, -59.05 μV/℃ and -57.18 μV/℃ respectively. Based on chemical analyses and other typomorphic peculiarities of magnetite, the authors hold that the magnetite is of the volcanic-sedimentary metamorphic origin, but magnetite samples from the Sishanling iron deposit have relatively low isomorphic extent and a more homogeneous structure. A systematic comparison with the main terrains in North China block in such aspects as oxygen isotope and REE compositions indicates that the oxygen isotope components of rich and poor ores show regularity related to different mechanisms of mineralization and enrichment. The values of δ18O in poor ores are related to the metamorphic grade of different terrains, consistent with oxygen isotope fractionation. Systematic investigation of the Ce/Ce＊ and Eu/Eu＊ ratios of BIFs from Archean-Peleoproterozoic strata in North China block indicates relatively low redox levels in the seawater, but with the growth of North China block, the oxygen-free environment exerted less effect on the formation of BIFs, which was unfavorable for the generation of banded iron formation.