西藏申扎侧波积异地区深源喷出型铁钴镍矿石的发现及意义

Discovery and Significance of Iron-cobalt-nickel Ore with Deep Source Emission Type in Cebojiyi District of Xainza,Tibet

侧波积异地区位于西藏申扎境内,区内分布着巨大的超基性杂岩体,在超基性杂岩体中出露有铁矿体。在野外地质特征研究的基础上,选择该铁矿体的代表性样品,运用显微镜下鉴定、扫描电镜与电镜能谱分析、X射线衍射分析、粉末样品化学分析等手段,系统研究了其岩(矿)相学、矿物学与地球化学特征。结果表明:矿石呈棕褐色,致密坚硬,相对密度大,显微镜下具有典型的熔结凝灰结构、假流动构造;主要晶屑矿物为铝铁镁铬铁矿、方铁矿,次要晶屑矿物有镁橄榄石,并可见爆碎的铝铁镁铬铁矿与高铁铬铁尖晶石胶结而成的混合晶屑,以及由铝铁镁铬铁矿与方铁矿交生的混合晶屑;玻屑、塑性玻屑为富Ni的硅铝铁质玻屑,已完全脱玻化而转变成含镍铁铝蛇纹石,并析出大量的微—细粒方铁矿,但仍保留完好的玻屑、塑性玻屑形态特征;矿石SiO2质量分数为13.59%～20.62%,平均为16.15%,在微量元素蛛网图上表现为K、P、Ti等3个元素明显亏损型,在稀土元素配分模式图上表现为轻稀土富集型,属于火山成因喷出型铁钴镍矿浆,物源区是源自地核但受到地幔或地壳的混染;Pb-Sr-Nd同位素特征显示矿石物质应该来自富集型地幔Ⅱ;矿石的TFe2O3质量分数平均为53.40%,Ni平均为6 991×10-6,Co平均为555×10-6,为一种含钴镍的铁矿石,在区域上需要注意这种类型矿石的找矿评价工作。

Cebojiyi district is located in Xainza of Tibet with widely distributed ultrabasic complex, which contains iron ores. Based on field research of geologic feature, the representative samples were selected to analysis the litho-mineragraphic, mineralogic and geochemical characteristics, using microscopy, scanning electron microscope, electron microscope energy spectrum, X-ray diffraction analysis and chemical analysis of powder sample. The results show that ores are dark brown, hard and dense with typical weld-tuff texture, pseudo-flowagestructure and higher relative density; A1-Fe-Mg chromite and wustite are the main crystal minerals, secondary with forsterite, and the mixed crystal minerals fracturing A1-Fe-Mg chromite and Fe-rich chromohercynite are found occasionally with some crystal minerals intergrown by A1- Fe-Mg chromite and wustite; the vitric materials are Ni-rich siallite-ferrite, devitrificated to Ni- Fe-A1 serpentine entirely producing a lot of fine-grain wustite with the primary form; mass fractions of ore SiOz are 13. 59%-20.62% with an average of 16.15%, K, P and Ti are obviously depleted according to the trace element spider diagram, and light rare earth elements are enrich according to the REE pattern, so that the ores are derived from the volcanic Fe-Co-Ni melt from the core but contaminated by mantle and crust ; ore material should come from EM II according to the characteristic of Pb-Sr-Nd isotope; the average mass fractions of TFe2O3, Ni and Co are 53.4%, 6 991×10^-6 and 555×10^- 6 respectively, indicating the ore is Co-Ni-bearing iron ore, which should be focused in prospective evaluation in this region.