西天山智博火山岩型铁矿床矿化蚀变机制研究

A number of rich Fe deposits are formed in the Awulale Fe-Cu metallogenic belt, Western Tianshan Mountains, Xinjiang. Almost all of them are hosted in the volcanic or volcaniclastic rocks of Carboniferous Dahalajunshan Formation which have a close relationship with the genesis of these deposits. Various types of mineralization occur in these deposits, as well as all kinds of altered minerals similar to typical skarn, such as epidote , K-feldspar, diopside, actinolite and garnet. Thus, the genesis and metallogenic mechanism of these deposits have been remaining controversial, among which the Zhibo large scale Fe deposit is a typical one. For its relatively simple associated minerals and superimposed alteration, the Zhibo deposit is an extremely proper object for studying the mineralization mechanism from alteration. This project intends to study the deposit from the aspects of mineralogy, geochronology and geochemistry on the basis of predecessors' work. Firstly, the composition of ores of different stages and types and alteration minerals such as epidote and K-feldspar will be analyzed through EPMA, micro-situ major, trace and rare earth elements, in order to explore the relationship between alteration and mineralization. Secondly, 40Ar/39Ar dating method will be used on K-feldspar to get the age of alteration minerals. Thirdly, H-O isotopes, fluid inclusions and other experiments will be conducted on alteration minerals such as epidote and K-feldspar, aiming at being aware of the origin and composition of the fluid. Finally, the alteration and mineralization mechanism of the Zhibo deposit can be revealed through the analysis of altered minerals and the mineral arisen sequence.

西天山阿吾拉勒铁铜成矿带发育大量富铁矿床，均产于石炭纪大哈拉军山组火山岩中，其形成与该套火山岩具有密切的成因联系。这些矿床矿化类型较多，多发育有绿帘石、钾长石、透辉石、阳起石、石榴石等类矽卡岩等蚀变矿物组合，因而在矿床成因和成矿机制方面存在争议。智博大型铁矿是该带上一个典型铁矿床，其共生矿物简单，蚀变叠加较少，适合从蚀变矿物角度对成矿机制方面进行深入研究。本项目拟从矿物学、年代学和矿床地球化学等方面着手，在吸收前人工作成果的基础上，通过对不同期次、类型的矿石以及绿帘石、钾长石等蚀变矿物进行电子探针、微区原位主量、微量和稀土元素成分分析，厘定蚀变与矿化之间的关系；对钾长石进行40Ar/39Ar法年龄测定，获得蚀变矿物年龄；对蚀变矿物绿帘石、钾长石进行H-O同位素、流体包裹体等研究，确定流体来源及成分特征；最终通过分析蚀变矿物和矿化的生成顺序以及蚀变原因，揭示智博铁矿床矿化蚀变机制。

智博大型磁铁矿床位于西天山阿吾拉勒铁成矿带东段，产于石炭纪大哈拉军山组火山岩或火山碎屑岩中，其形成与该套火山岩具有密切的成因联系。矿体主要呈似层状、透镜状，矿石结构主要有半自形-他形粒状结构、板条状结构以及交代残余结构；矿石构造类型有致密块状、浸染状、条带纹层状、角砾状、网脉状等类型，主要发育透辉石化、阳起石化、绿帘石化、钾长石化等蚀变。本次研究选择磁铁矿、黄铁矿、绿帘石、钾长石等作为研究对象，通过电子探针、等离子质谱及微区原位分析获得其地球化学元素特征，并获得他们的H、O、S等稳定同位素数据，测得了绿帘石、钾长石、石英及方解石中包裹体的温度。磁铁矿的稀土、微量元素特征显示，磁铁矿以岩浆作用为主。岩浆期的磁铁矿δ18O值平均为3.8‰，与基鲁纳型和拉科铁矿的磁铁矿δ118O非常接近；而热液期的磁铁矿δ18O值平均为4.0‰，比岩浆期的磁铁矿的δ18O值范围更大些，可能与热液流体参入、矿物重结晶等因素有关。智博铁矿中黄铁矿的Co、Ni、Se含量比较高，Co/Ni比值分布范围较大，多数介于0.1~10之间，S/Se比值多在2500~5000之间，各项数据均与火山成因和热液成因的黄铁矿类似。黄铁矿中硫同位素具有变化范围窄(-1.2‰~0.3‰)、平均值为-0.6‰的特点，表明硫主要来源于深源硫。绿帘石、钾长石、石英、方解石均一温度分布在146℃~409℃，盐度分布于2.4 wt.% ~19.7wt.% NaCleq。之间，表明晚期阶段成矿流体以低温中-低盐度为特征。.智博铁矿的形成与区域内石炭纪陆缘弧岩浆-热液活动有关。成矿母岩浆由俯冲流体交代的亏损地幔部分熔融形成，经液态不混溶作用或者低氧逸度下演化分异形成的富铁流体，沿火山通道与断裂向上运移，在火山岩裂隙或层间薄弱带聚集成矿。主成矿阶段以贯入充填成矿为主，形成大量块状富矿石，蚀变不发育，与围岩截然接触，磁铁矿呈板条状、树枝状；中晚期成矿伴有强烈的围岩蚀变，形成浸染状、条带状矿石。角砾状矿石的形成与隐爆作用或者贯入充填成矿作用有关。