西天山智博铁矿床磁铁矿成分特征及其矿床成因意义

智博大型磁铁矿床位于新疆西天山阿吾拉勒成矿带东段,赋存于石炭系大哈拉军山组玄武质安山岩、安山岩及火山碎屑岩中。智博铁矿床包括东、中、西以及13号矿体4个矿段。矿体主要呈层状、似层状、透镜状。金属矿物以磁铁矿为主,含少量浸染状黄铁矿,局部可见细脉赤铁矿及零星状黄铜矿。矿石构造以块状和浸染状构造为主,角砾状次之,局部为条带状构造、脉状-网脉状构造;矿石结构包括半自形-他形粒状结构、交代残余结构、板条状结构。智博矿区的蚀变矿物组合以透辉石、钠长石、钾长石、绿帘石、阳起石为主,含有少量方解石、石英、绿泥石及榍石。根据矿物共生组合、矿石结构的观察以及矿物化学分析,识别出岩浆期和热液期2个成矿期,进一步细分为3个成矿阶段:磁铁矿-透辉石-绿帘石阶段(a1),磁铁矿-钾长石-绿帘石阶段(b1),石英-硫化物阶段(b2)。磁铁矿的电子探针成分分析显示,岩浆期矿石中FeOT含量较高,而Al2O3、CaO、MgO、SiO2等氧化物含量较低,热液期矿石则相反。角砾状和部分浸染状磁铁矿中V2O5含量相对较高,与火山岩中含量类似,暗示该矿化阶段的铁质部分来源于围岩;块状以及浸染状磁铁矿FeOT含量大部分在90%以上;角砾状、网脉状、树枝状矿石中磁铁矿的w(FeOT)分布相对比较集中,多数在90%~92%之间;纹层状矿石的w(FeOT)则变化于88%~92%之间,其CaO、SiO2等氧化物平均含量相对增加。TiO2-Al2O3-MgO图解和Ca+Al+Mn vs Ti+V图解均表明智博铁矿床的形成与火山活动和岩浆热液的交代作用有关。

Magnetite composition of Zhibo iron deposit in Western Tianshan Mountains and its genetic significance

Located in the eastern Awulale metallogenic belt of Western Tianshan Mountains, the large-size Zhibo iron deposit is hosted in basaltic andesite, andesite and pyroclastic rocks of the Lower Carboniferous Dahalajunshan Formation. The Zhibo iron deposit consists of four main areas: the eastern zone, the middle zone, the western zone, and No.13 ore body, in which the ore bodies occur as layers, stratoid bodies or lenses. Mineralization consists of massive magnetite associated with minor disseminations of pyrite and veinlets of hematite and trace chalcopyrite. The ore structures include mainly massive and disseminated, subordinately brecciated, and occasionally banded or vein-stockwork forms, whereas the ore textures consist of ubhedral - anhedral grainular, metasomatic relict, and platy forms. The principal alteration mineral assemblages in Zhibo include diopside, albite, K-feldspar, epidote, and actinolite, together with calcite, quartz, chlorite and titanite. On the basis of observed mineral assemblages and ore textures as well as mineral analyses, two periods of ore deposition have been recognized: magmatic period and hydrothermal ore-forming period, which could be further subdivided into three metallogenic stages, i.e., magnetite-diopside-epidote stage (a1), magnetite-K-feldspar-epidote stage (b1) and quartz-sulfide stage (b2). Electron microprobe analyses show that the magnetite with magmatic features has higher content of FeO_T and lower content of Al₂O₃, CaO, MgO, SiO₂ in comparison with the magnetite with hydrothermal features. The content of V₂O₅ in brecciated and disseminated ore is relatively high, similar to the content of volcanic rocks, suggesting that part of iron of the ore-forming stage came from the wall rock. Massive and disseminated magnetite has varied FeO_T values, mostly above 90%; FeO_T content of magnetite from brecciated, veined and dendritic ores varies between 90% and 92%; the FeO_T content of magnetite from laminated ore varies between 88% and 92%, with higher average content of other oxides such as CaO and SiO₂. Both ternary plot of TiO₂-Al₂O₃-MgO and Ca+Al+Mn versus Ti+V discriminant diagram suggest that the formation of the Zhibo iron deposit was associated with volcanic activity and hydrothermal metasomatism of magmatic-hydrothermal fluid.