滇中腊梅铁矿床磁铁矿地球化学特征及其指示意义

Geochemical characteristics of magnetite from Lamei iron deposit in Central Yunnan and its indicative significance

滇中腊梅铁矿床位于西南“三江”地区金沙江-哀牢山-红河富碱斑岩带中段,其矿化类型不明,成矿机制和资源潜力不清,制约了该区找矿勘查的深入研究。本次研究工作在详细的地质调查基础上,根据矿(化)体产出特征,将磁铁矿划分为正长斑岩内的豆状-星点状磁铁矿(Ⅰ-Mag)、透辉石角岩内浸染状-团块状磁铁矿(Ⅱ-Mag)和层间破碎带内脉状及囊状充填型磁铁矿(Ⅲ-Mag)3种类型。通过LA-ICP-MS原位微区成分对比,发现3类磁铁矿总体富集Ti、Mn、V、Mg、Zn及Ni,贫Sn、Ga及Sc等元素,并且从Ⅰ-Mag→Ⅱ-Mag→Ⅲ-Mag, Mg、V、Zn含量及Ni/Cr值逐渐增加,Cr、Sn及REE含量逐渐降低,反映3类磁铁矿具有从岩浆成因向热液成因演化的特征。结合Ni/(Cr+Mn)-(Ti+V)、(Al+Mn)-(Ti+V)图解及磁铁矿矿相学特征,认为该矿床属于接触交代热液型铁矿床,其成矿物质来源与富碱斑岩密切相关。3类磁铁矿形成温度约300~500℃,Ⅰ-Mag→Ⅱ-Mag→Ⅲ-Mag的氧逸度有逐渐降低的趋势。地球化学特征及成矿温度、氧逸度等信息指示,腊梅铁矿床深部具斑岩型铜多金属矿床的成矿潜力。

Lamei iron deposit in Central Yunnan is located in the middle section of the Jinshajiang-Ailaoshan-Honghe alkali-rich porphyry belt in the “Sanjiang” area of Southwest China. Its mineralization type is unknown and its metallogenic mechanism and resource potential are unclear, which restricts the in-depth study of prospecting and exploration in this area. On the basis of detailed geological survey, and according to the output characteristics of the ore bodies, magnetite is divided into three types: bean-star point magnetite in syenite porphyry(Ⅰ-Mag), disseminated-agglomerate magnetite in diopside hornstone(Ⅱ-Mag) and vein-shaped and sac-like filling magnetite in interlayer fracture zone(Ⅲ-Mag). Through LA-ICP-MS in-situ micro-area composition comparison, it is found that the three types of magnetites are generally enriched in Ti, Mn, V, Mg, Zn and Ni, poor in Sn, Ga and Sc and other elements. Ⅰ-Mag→Ⅱ-Mag→Ⅲ-Mag, the content of Mg, V, Zn and the ratio of Ni/Cr gradually increase, and the content of Cr, Sn and REE gradually decrease, reflecting the evolution characteristics of the three types of magmatite from magmatic origin to hydrothermal origin. Combined with Ni/(Cr+Mn)-(Ti+V),(Al+Mn)-(Ti+V) diagrams and the ore facies characteristics of magnetite, the deposit is considered to be a contact metasomatic hydrothermal iron deposit, and its mineralization material source is closely related to alkali-rich porphyry. The formation temperature of three types of magnetite is about 300 ~ 500℃, and the oxygen fugacity ofⅠ-Mag→Ⅱ-Mag→Ⅲ-Mag tends to decrease gradually. Geochemical characteristics, mineralization temperature, oxygen fugacity and other information indicate that the deep part of Lamei iron deposit has the metallogenic potential of a porphyry copper polymetallic deposit.