新疆西天山阿克萨依铁矿床地质及地球化学特征

Geological and geochemical characteristics of Akesayi iron deposit in Western Tianshan Mountains, Xinjiang

近年新疆阿吾拉勒铁铜成矿带内铁矿找矿勘查取得了突破性进展，阿克萨依铁矿是其中一个新发现并评价的中型规模铁矿床。该矿床的地质特征与带内其他铁矿明显不同，成因不明。矿体主要赋存于下石炭统阿克沙克组的粗安岩、玄武岩、安山岩、安山质凝灰岩，少量赋存于下石炭统大哈拉军山组上段的凝灰岩中。阿克沙克组玄武岩和安山岩富集LREE和LILE（如Rb、Th、K），亏损HREE和HSFE（Nb、Ta、Ti），稀土元素呈右倾的配分模式，结合Th-Ta—Hf／3、Zr／4-Y-Nb＊2和Th—Nb构造环境判别图解，认为这些火山岩可能形成于岛弧环境。阿克萨依铁矿的形成与岛弧内发育的火山机构有关，矿体受火山机构的次级断裂控制，呈脉状、透镜状，围岩蚀变发育，以绿泥石化、碳酸盐化为主，硅化、绿帘石化为次。矿石主要为浸染状和脉状构造，局部发育块状和角砾状构造。成矿过程可划分为3个阶段：磁铁矿阶段、石英-硫化物阶段和绿泥石-碳酸盐阶段。阿克萨依铁矿区磁铁矿低训（TiO））（变化范围0～1．7％、平均0．2％）、w（Cr2O3）（0～0．2％、平均0．06％）和叫（NiO）（平均值接近0），明显不同于岩浆分异型钒钛磁铁矿床磁铁矿的硼（TiO2）（≥5．4％），而与热液型和接触交代型铁矿磁铁矿的训（TiO2）相似（平均0．18％～0．33％）；黄铁矿Co／Ni比值都较高（〉1），具火山成因、热液成因黄铁矿的特征；硫同位素δ34S值介于-0．6‰～0．4‰，成矿流体具岩浆硫的特征。初步认为阿克萨依铁矿床可能是与火山活动紧密相关的火山-次火山热液交代型铁矿床。

In recent years, iron ore prospecting breakthrough was made in the Awulale iron-copper metallogenic belt, Xinjiang. The Akesayi iron deposit is a medium-sized iron deposit which was recently discovered and evaluated. This deposit is obviously different from other iron deposits in the Awulale iron-copper metallogenic belt in geological features, and its genesis is unknown. The orebodies are mainly hosted in basalt, trachyandesite, andesite and andesitic tuff of Lower Carboniferous Akeshake Formation, and subodinately in tuff of the Lower Carboniferous Dahalajunshan Formation. Basalt and andesite of the Akeshake Formation are enriched in LREE and LILE （such as Rb, Th, K） and the depletion of HREE and HSFE （Nb, Ta, Ti）, with the distribution patterns of REE being right-oblique. Combined with Th-Ta-Hf/3, Zr/4-Y-Nb ＊ 2 and Th-Nb diagrams for discrimination of tectonic setting, it is shown that these volcanic rocks might have been formed in an island arc environment. The formation of the Akesayi iron deposit was related to volcanic mechanism developed in the island arc and the orebodies are controlled by secondary faults of volcanic structure, and their shapes are mainly veined and lenticu- lar with well-developed wall rock alterations, mainly chloritization and carbonatization and subordinately silicification and epidotizationss. The ores are mainly of disseminated and veined structure, and partially of massive and brecciated structure. Mineral formation can be divided into three stages： Magnetite stage, quartz-sulfide stage and chlorite-carbonate stage. Magnetites of the Akesayi iron desposit have low w（TiO2） （ranging from 0 to 1.7%, 0.2% on average）, w（Cr2O3） （ranging from 0 to 0.2%, 0.06% on average） and w（NiO） （0 on average）, significantly different from w（TiO2） content of magnetites in the magmatic crystal fractionation deposit （≥5.4 % ） but similar to w （TiO2） of magnetite of the hydrothermal and contact metasomatic type iron deposits （ w （TiO2） ranging from 0.18 % to 0.33 % ） . Pyrite Co/Ni ratios are higher （ 〉 1 ） , with characteristics of volcanic genesis pyrite and hydrothermal genesis pyrite. Sulfur isotope δ34S values range from - 0.6% to 0.4%o, implying that ore-forming fluid had magmatic sulfur characteristics. Through a comprehensive analysis, the authors preliminarily hold that the Akesayi iron deposit might be a volcanic-subvolcanic hydrothermal metasomatic iron deposit closely related to volcanic activities.