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

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

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δ^(56)Fe_(IRMM014)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SiO_(2)+Fe_(2)O_(3)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(ΣREE))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Fe^(2+)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Fe^(2+)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。

Hongshuihe iron deposit in the Eastern Kunlun orogenic belt,Qinghai Province,is a middle-sized one in which iron formations are localized within the phyllite of the Langyashan Formation.The ore types are mainly massive magnetite quartzite type,and minor banded type,which was generally considered as sedimentary metamorphic iron deposit by predecessors.Based on previous researches,this paper presents new data from major,rare earth and trace elements as well as Fe isotope analyses for massive iron ores in the iron formations of the Hongshuihe iron deposit.New Fe isotope results yield variableδ^(56)FeIRMM-014values mostly ranging from 0.97‰to1.97‰,with one exception,showing similar Fe isotope compositions with typical Neoproterozoic iron formations of the world.The results of major,rare earth and trace elements show that these ores have high SiO_(2)+Fe_(2)O_(3)contents(78.56%-98.06%)and very low Al/(Al+Fe+Mn)ratios of 0.00-0.06,indicating that they are typical chemical sedimentary rocks.The total rare-earth-element contents(∑REE)vary from 16.49×10^(-6)to 80.89×10^(-6),with no obvious Ce abnormally,and light-rare-earth-element are slightly depleted,which shows a similar characteristic to the Neoproterozoic iron-bearing structures.Based on the comprehensive comparison of Fe isotope compositions,depositional ages and geochemical characteristics of iron formations in the Hongshuihe iron deposit,we can conclude that there was a passive continental margin tectonic environment of the Qaidam-East Kunbei block in the Neoproterozoic during iron formation,and iron and other ore-forming materials were predominantly derived from marine hydrothermal fluids.The marine hydrothermal fluids rich in Fe^(2+)upwelled and gradually evolved into low-temperature hydrothermal fluids which then mixed with oxygen-bearing surface seawater in a sub-oxidized condition.Finally,Fe^(2+)was partially oxidized and formed ferric hydroxides,which subsequently deposited on the continental slope and eventually formed iron formations.The genetic type of the Hongshuihe iron deposit can be classified as the Neoproterozoic Rapitan-type iron formations.