西藏措勤尼雄矿田滚纠铁矿磁铁矿元素地球化学特征及对成矿作用的制约

Geochemistry and controls on the mineralization of the magnetite from the Gunjiu iron deposit in the Nyixung ore field, Coqen, Xizang

关键词:  地球化学  磁铁矿  滚纠铁矿  尼雄矿田

Key words:  geochemistry  magnetite  Gunjiu iron deposit  Nyixung ore field

摘要:

尼雄矿田滚纠铁矿地处拉萨地块隆格尔-工布江达岩浆弧，是冈底斯成矿带中生代铁铜多金属成矿作用的典型代表。在详细的野外地质调查和室内研究基础上，分析了滚纠铁矿床磁铁矿的成因矿物学特征。电子探针测试和ICP-MS分析表明，磁铁矿主量元素具有富SiO₂，贫TiO₂、V₂O₅的特征；微量元素Ba、Ti相对亏损，Cs、U相对富集，Eu、Lu、Tb、Ho、Tm强烈亏损。磁铁矿的Ti-（V+Cr）和（Ti+V）-（Al+Mn）协变图显示氧逸度、温度对矿物元素含量有明显制约作用，同时w（TiO₂）与w（CaO+MgO）、w（Na₂O+K₂O）表现出明显正相关关系，指示矽卡岩系统中流体-岩石相互作用是磁铁矿地球化学元素变化的主要控制因素。通过研究矿床中矿物生成顺序和磁铁矿中Ti、V元素特征并结合前人流体包裹体测温资料，认为矿区铁矿化阶段为高氧逸度的中高温环境，初步限定磁铁矿成矿温度为300~450℃。矿物的w（Ni）均值为8.98×10^-6，Ni/Co比值<1（变化范围0.15~0.59），Ti/V比值为6.71~25.52，从矿物化学角度进一步印证滚纠铁矿的成矿物质来源于矿区中酸性岩浆流体系统。TiO₂-Al₂O₃-（MgO+MnO）和（Ca+Al+Mn）-（Ti+V）等成因判别图解在矿区具有良好适用性，说明磁铁矿是矽卡岩矿床成矿过程的重要指示矿物。

Abstract:

The Gunjiu iron deposit in the Nyixung ore field, Coqen, Xizang is a representative iron deposit within the Mesozoic iron-copper polymetallic deposits in the Gangdise metallogenic zone. The genetic mineralogy of magnetite is dealt with on the basis of field geological investigation and mineralogical and petrological studies. The microprobe and ICP-MS analyses show that magnetite is characterized by the enrichment of the major elements SiO₂ and depletion of TiO₂ and V₂O₅, the depletion of the trace elements Ba and Ti, the enrichment of Cs and U, and highly depletion of Eu, Lu, Tb, Ho and Tm. The covariation diagrams of Ti vs. (V+Cr) and (Ti+V) vs. (Al+Mn) suggest the controlls of the oxygen fugacity (fO₂) and temperatures on the element contents in magnetite. The positively correlation of w(TiO₂) vs. w(CaO+MgO) and w(TiO₂) vs. w(Na₂O+K₂O) also indicate that the fluid-rock interaction in the skarn systems may exert a major control on geochemical signatures of magnetite. The combination of mineral sequence, Ti and V elementary variations, and previous data for fluid inclusion temperature determinations have reflected that the mineralization was initiated in the intermediate- to high-temperature environments with high oxygen fugacity and mineralization temperatures of 300℃ to 450℃. The average w(Ni) value of 8.98×10^-6, Ni/Co ratios of 0.15 to 0.59 (<1), and Ti/V ratios of 6.71 to 25.52 have disclosed that the ore-forming matter was originated from the intermediate to acidic magmatic fluid systems. The feasibility of the discriminant diagrams of TiO₂-Al₂O₃-(MgO+MnO) and (Ca+Al+Mn) vs. (Ti+V) for the genetic interpretation of magnetite have provide the evidence that magnetite from the Gunjiu iron deposit may be an important indicator mineral in the mineralization processes of the skarn deposits.