新疆西天山智博铁矿床蚀变矿物学、矿物化学特征及矿床成因探讨

Alteration mineralogy,mineral chemistry and genesis of Zhibo iron deposit in western Tianshan Mountains,Xinjiang

智博铁矿床是西天山东部阿吾拉勒铁成矿带新发现的大型磁铁矿矿床之一。赋矿围岩为下石炭统大哈拉军山组火山岩及火山碎屑岩。围岩蚀变广泛发育,识别出3个阶段：第一阶段以辉石＋钠长石＋磁铁矿为主;第二阶段以角闪石＋钾长石＋绿帘石＋磁铁矿＋黄铁矿为主;第三阶段以绿帘石＋绿泥石＋方解石＋石英＋黄铁矿＋赤铁矿±黄铜矿为主。电子探针分析表明,智博铁矿与其他岩浆-热液成因铁矿床具有类似的蚀变矿物化学成分。辉石以透辉石为主（Di=62.97%~83.56%）,含少量钙铁辉石（Hd=16.44%~36.45%）;火山岩中斜长石（Ab47.57-57.82An41.5-51.87Or0.56-0.68）蚀变形成钠长石（Ab77.89-99.33An0.46-2.48Or0.21-20.3）;与热液作用有关的钾长石叠加改造早期蚀变矿物;角闪石主要为阳起石;晚期发育富铁绿帘石〔Fe/（Fe＋Al）=0.2~0.36〕以及绿泥石蚀变矿物。与火山岩中的磁铁矿〔w（TiO2）3.08%〕相比,矿体中磁铁矿具有低w（TiO2）（0.23%）的特点,部分早期浸染状磁铁矿与火山岩中的磁铁矿w（V2O5）相当,暗示该矿化阶段的铁质部分来源于围岩。矿物学及矿物化学表明,热液交代作用对成矿具有重要的贡献。同时,智博铁矿具有一些暗示铁矿浆成因的结构特征,如块状磁铁矿与围岩呈截然接触,磁铁矿胶结围岩角砾,磁铁矿条带呈流动状分布以及板条状磁铁矿等。结合铁矿带区域地质特征,认为智博铁矿可能主要由富铁岩浆流体形成,在形成大量块状富铁矿体的同时,伴随有广泛的围岩蚀变。矿区内大量的磁铁矿矿化与晚石炭世大陆岛弧岩浆活动有密切的成因联系。

The Zhibo iron deposit is one of the recently-discovered large magnetite deposits in the Awulale iron metal- logenic belt, which is located at the eastern corner of western Tianshan Mountains. The ore bodies are hostedmainly by volcanic and volcaniclastic rocks of the Lower Carboniferous Dahalajunshan Formation. The wall rocks of the Zhibo iron deposit have been strongly altered. Three stages of alteration assemblages and associated nfineralization have been recognized： Stage Ⅲ , dominantly composed of pyroxene ＋ albite ＋ magnetite; Stage Ⅱ, characterized by amphibole ＋ K-feldspars ＋ epidote ＋ magnetite ＋ pyrite; and Stage Ⅲ, dominated by epidote ＋ chlorite ＋ cal- cite ＋ quartz ＋ pyrite ＋ hematite _＋ chalcopyrite. Electron microprobe analyses indicate that the Zhibo deposit have compositional features of mineral assemblages similar to many other magmatie-hydrothermal iron oxide deposits. Pyroxene consists mainly of diopside （Di = 62.97 % -- 83.56 % ）, with minor hedenbergite （Hd = 16.44 % 36.45% ）. Igneous plagioclase （Ab47.57_57.s2 An41.5-51.87 Or0.56_0.68） has been altered into albite （Ab77.89-99.33 An0.46_2.48 Or0.21_20.3）. K-feldspar related to late hydrothermal alteration is superimposed upon the pre-existing associations. Amphiboles are entirely of actinolite composition. Abundant Fe-epidote [ Fe/（Fe ＋ A1） = 0.22 0.36] and minor chlorite have pervasively replaced early mineral assemblages. In contrast with high titanium （3.08% TiO2） magnetite in volcanic rocks, most magnetite from the ore bodies has lower w（TiO2） （0.23%）. A small amount of disseminated magnetite has a w （V2O5） similar to the magnetite from volcanic rocks, sug- gesting minor iron movement from the wall rock during the early metasomatism. Mineralogy and mineral chemistry of the altered minerals indicate that metasomatic replacement played an important role in mineralization. In addi- tion, the Zhibo iron deposit also has some textural features suggestive of an Fe-rich melt origin for the deposit, such as the abrupt contact between the wall-rock and massive magnetite ore, the existence of significant amounts of breccia, the arrangement of magnetite that defines a flow texture, and platy magnetite. Combined with re- gional geology of the Awuiale iron metallogenic belt, it might be suggested that Fe-rich magmatic liquids consti- tuted the most plausible origin for the mineralization, producing massive magnetite ore bodies associated with ex- tensive wall rock alteration. There existed a genetic relationship between late Carboniferous continental arc mag- matism and massive magnetite mineralization.