山西原平王家庄条带状磁铁石英岩矿床元素地球化学特征及其地质意义

Element Geochemical Characteristics of the Wangjiazhuang Banded Magnetite-quartzite Deposit in Yuanping City,Shanxi Province,and Its Geological Significance

山西原平王家庄铁矿床为五台山地区产出的条带状铁建造(BIF)的典型代表。含矿岩石为磁铁石英岩,围岩主要为斜长角闪岩、角闪变粒岩、石英片岩等。通过对变质岩的原岩恢复,得出其原岩主要为火山岩,具有从拉斑玄武岩向钙碱性火山岩演化的特点,局部有陆源碎屑的加入。微量元素中相对亏损Ta、Nb、Zr、Ti等高场强元素,相对富集K、Rb等大离子亲石元素,具有较低的w(Y)/w(Nb)比值,表明本区处于岛弧和活动大陆边缘带的构造环境。磁铁石英岩的稀土元素配分型式为向右缓倾斜的轻稀土富集型,具轻微铕正异常和轻微铈负异常,稀土元素特征表明BIF形成于典型的海洋环境,早太古代洋壳俯冲产出的高温气水热液对于BIF的沉积发挥了重要的作用。Eu轻微正异常说明本区火山活动较弱,海底高温热液对BIF的沉积贡献相对较少。BIF与其围岩在原始地幔标准化不相容元素蛛网图中的相关性说明二者有相同或相似的物质来源和成岩环境,二者同属一个沉积盆地,但却有不同的沉积过程和形成条件。

The Wangjiazhuang iron deposit in Shanxi Province is a typical case of banded iron formation in the Wutaishan area.The ore-bearing rock is magnetite-quartzite,with wall rocks mainly being amphibolite,amphibole-metadolerite and quartz schist.Through the restoration of the metamorphic rocks,we conclude that the protolith are principally volcanic rocks.The volcanic rocks in the area evolved from the tholeiite series to the calc-alkaline series,along with some terrestrial clastics.The negative anomalies of HFSE,high contents of LILE and a low ratio of Y/Nb show that this area was in an island-arc and active continental margin tectonic setting.REE patterns of the magnetite-quartzite demonstrate a right inclined curve with slightly LREE enrichment.Eu shows a slightly positive anomaly and Ce shows a slightly negative anomaly.This REE characteristics show that BIF was formed in a typical marine environment.It is predicted that high-temperature hydrothermal fluids created during oceanic crust subduction in the Neoarchean played a considerable role in the formation of BIF.The slightly Eu positive anomaly suggests that the volcanic activity of the area was weak,so high-temperature hydrothermal fluids in the bottom of the ocean played a minor role.The correlatability between BIF and its wall rocks in the primitive mantle-normalized incompatible element spider diagrams suggests that they have the same or similar material source and diagenetic environment,as well as belonging to a same depositional basin,but with different deposition processes and conditions,respectively.