东天山白山泉铁矿床矿物学和Nd同位素研究

Study on mineralogy and Nd isotope of Baishanquan iron deposit in East Tianshan,NW China

中国东天山石炭纪火山沉积岩系中产出有众多大-中型铁矿床,白山泉铁矿为其中重要代表,前人对其成因研究薄弱。文章主要对白山泉铁矿开展了详细的岩相学、矿物学和Nd同位素研究,结果表明铁矿体主要呈层状产出,矿石显示浸染状和条带状构造,主要由磁铁矿、石英和铁硅酸盐矿物组成,可见少量斜长石和磷灰石。磁铁矿较纯净,主要由TFeO组成,其他氧化物含量大多不足0.1%,显示化学沉积成因的特征,但受到了海底火山作用的显著影响。电子探针分析表明,角闪石为阳起石、镁角闪石以及透闪石,黑云母为镁黑云母,斜长石主要为钠长石,这些矿物均为变质成因,暗示白山泉铁矿经历了绿片岩相变质作用,根据Hbl-Pl地质温度计以及黑云母Ti地质温度计(根据平均值计算)获得变质温度为412~488°C。铁矿石ε_(Nd)(t)范围-2.44~-6.11,暗示铁质来源于海底热液,但受到了碎屑物质的混染。通过与前寒武纪铁建造和海相火山岩型铁矿开展详细对比,笔者提出白山泉铁矿与其他类型海相火山岩型铁矿具有相同的火山机构,可能为前寒武纪铁建造类似物,进一步建立了前寒武纪铁建造和海相火山岩型铁矿之间联系。

Abundant large and medium size iron deposits were hosted in the Carboniferous volcanic sedimentary rocks in the East Tianshan of NW China,among which the Baishanquan iron deposit is one of the important deposits.However,its genesis was poorly understood.In this paper,detailed petrographic,mineralogical and Nd isotope studies have been carried out on samples from the Baishanquan iron deposit.The results show that the Baishanquan iron orebody is stratiform,and the iron ores show disseminated and banded structure,mainly composing of magnetite,quartz and iron silicate minerals,with minor plagioclase and apatite.Magnetite is relatively pure,mainly composed of TFeO,and the contents of other major oxides are less than 0.1%,This suggests that it is of chemical sedimentary origin,but significantly affected by submarine volcanism.EPMA analysis shows that hornblende belongs to actinolite,magnesium hornblende and tremolite,biotite belongs to magnesium biotite and plagioclase is mainly albite.These minerals are of metamorphic origin,suggesting that the Baishanquan iron ore has experienced greenschist facies metamorphism.According to the Hbl-Pl geothermometer and biotite Ti geothermometer(calculated according to the average value),the metamorphic temperature is 412~488°C.ε_(Nd)(t)values of iron ores range from-2.44 to-6.11,which suggest that iron is derived from submarine hydrothermal fluid,but mixed with clastic materials.Through detailed comparison with Precambrian iron formations(IFs)and submarine volcanic-hosted type iron ores,we proposed that the Baishanquan iron ores share the same volcanic system with other types of submarine volcanic-hosted iron ores,which may be the Phanerozoic analogue of Precambrian IFs.This study further establishes the relationship between the Precambrian IFs and submarine volcanic-hosted type iron ores.