青海东昆仑洪水河铁矿床新元古代含铁建造铁同位素特征及其成因意义

青海省东昆仑造山带洪水河铁矿床为一中型铁矿床，其含铁建造产于狼牙山组千枚岩中，矿石类型主要为块状磁铁石英岩型，少量为条带状磁铁石英岩型，前人一般认为其属于沉积变质型铁矿床。本文在前人研究基础上，对洪水河铁矿区含铁建造中块状铁矿石进行了铁同位素、主量元素、稀土元素和微量元素分析。结果显示：除1件样品外，其余含铁建造样品的铁同位素δ56FeIRMM014均介于0.97‰~1.97‰之间，和全球典型新元古代含铁建造的Fe同位素特征基本一致；铁矿石的SiO₂+Fe₂O₃质量分数高达78.56%~98.06%，具有极低的Al/(Al+Fe+Mn)值（0.00~0.06），为典型的化学沉积岩；总稀土元素（w (∑REE)）变化范围为(16.49~80.89)×10^-6，没有明显的Ce异常(Ce/Ce*为0.93~1.05)，轻稀土元素轻微亏损，显示出类似新元古代含铁建造型的特点。综合对比洪水河铁矿区含铁建造的Fe同位素组成、沉积时代和地球化学特征，推断洪水河铁矿区含铁建造的沉积环境为新元古代柴达木—东昆北陆块的被动大陆边缘构造环境，铁等成矿物质主要来源于海相热液流体；富含Fe²+的海相热液流体上涌并逐渐演变为低温热液后在亚氧化水体环境中与含氧海水混合，最后导致Fe²+被部分氧化并形成氢氧化铁，氢氧化铁逐渐沉积在大陆斜坡上最终形成含铁建造。洪水河铁矿的成因类型可划归为拉皮坦型新元古代含铁建造。

Fe Isotope Characteristics and Their GeneticSignificance of the Neoproterozoic Iron Formations in Hongshuihe Iron Deposit,Eastern Kunlun Orogenic Belt,Qinghai Province

Hongshuiheiron deposit in the Eastern Kunlun orogenic belt, Qinghai Province, is amiddle-sized one in which iron formations are localized within the phyllite ofthe Langyashan Formation. The ore types are mainly massive magnetite quartzitetype, and minor banded type, which was generally considered as sedimentarymetamorphic iron deposit by predecessors. Based on previous researches, thispaper presents new data from major, rare earth and trace elements as well as Feisotope analyses for massive iron ores in the iron formations of the Hongshuiheiron deposit. New Fe isotope results yield variableδ56FeIRMM-014values mostly rangingfrom 0.97‰to1.97‰, with one exception, showing similarFe isotope compositions with typical Neoproterozoic iron formations of theworld. The results of major, rare earth and trace elements show that these oreshave high SiO₂+Fe₂O₃ contents (78.56%-98.06%)and very low Al/(Al+Fe+Mn) ratiosof 0.00-0.06, indicating that they are typical chemical sedimentary rocks. Thetotal rare-earth-element contents (∑REE)vary from 16.49×10^-6 to80.89×10^-6, withno obvious Ce abnormally, and light-rare-earth-element are slightly depleted,which shows a similar characteristic to the Neoproterozoic iron-bearingstructures. Based on the comprehensive comparison of Fe isotope compositions,depositional ages and geochemical characteristics of iron formations in theHongshuihe iron deposit, we can conclude that there was a passive continentalmargin tectonic environment of the Qaidam-East Kunbei block in theNeoproterozoic during iron formation, and iron and other ore-forming materialswere predominantly derived from marine hydrothermal fluids. The marinehydrothermal fluids rich in Fe²+ upwelled and gradually evolved intolow-temperature hydrothermal fluids which then mixed with oxygen-bearingsurface seawater in a sub-oxidized condition. Finally, Fe²+ waspartially oxidized and formed ferric hydroxides, which subsequently depositedon the continental slope and eventually formed iron formations. The genetictype of the Hongshuihe iron deposit can be classified as the NeoproterozoicRapitan-type iron formations.