滇西北烂泥塘斑岩铜金矿床铁氧化物LA-ICP-MS微量元素特征及其地质意义

烂泥塘斑岩铜金矿床位于云南省西北部的中甸地区，矿体主要以细脉—浸染状、网脉状产于石英二长斑岩和石英闪长玢岩之中。矿区热液蚀变作用发育，围绕矿体由深部至浅部依次发育钾化带、绿泥石—绢云母化带、绢云母化带和泥化带。钾化带中发育3种不同产状的磁铁矿，根据磁铁矿产出状态与脉体之间的相互穿插关系，将其划分为浸染状分布的磁铁矿（Ⅰ类）、单一脉状磁铁矿（Ⅱ类）和石英—硫化物脉中的磁铁矿（Ⅲ类）。此外，矿区常见产于成矿期后白云石—石英大脉中的镜铁矿。采用激光剥蚀电感耦合等离子体质谱（LA-ICP-MS）对上述铁氧化物进行了原位微区成分测试。结果表明：3类磁铁矿均富集Ti、V、Cr、Ni、Co、Al、Mg、Mn、Ga和Zn等微量元素。早期Ⅰ类磁铁矿含有钛铁矿出溶体，与Ⅱ、Ⅲ类磁铁矿相比，相对富集Mg、Ni和V等元素，属于岩浆磁铁矿；Ⅱ类磁铁矿相对富集Mn、Zn、Sn和Sc等元素，属于热液磁铁矿。岩浆磁铁矿（Ⅰ类磁铁矿）与后期脉状磁铁矿（Ⅱ类和Ⅲ类）相比，Ti、Al和Cr等元素含量相差不大。这可能是由于后期热液蚀变对Ⅰ类磁铁矿的强烈改造，导致其中Ti、Al和Cr等元素含量降低（通常岩浆磁铁矿比热液磁铁矿更富集Ti、Al和Cr）。Ⅱ、Ⅲ类脉状磁铁矿属于热液磁铁矿且二者微量元素含量差别不大，说明它们属于同一期流体中沉淀的产物。与磁铁矿相比，镜铁矿中的Ti、Al和V元素含量相差不大，而Cr、Ga、Ni和Co等元素含量比磁铁矿低一个数量级。结合前人资料，认为Al、Mn、Mg和Sc元素在磁铁矿中主要以类质同象形式存在，而Ca、S、Cu、Ba、Sr和Zr等元素主要以显微包裹体形式存在。钾化带中广泛发育的磁铁矿—赤铁矿共生组合、镜铁矿以及磁铁矿中异常低的Mn含量表明，烂泥塘矿区成矿流体的氧逸度高达赤铁矿—磁铁矿缓冲线。

Trace Elemental Compositions of Iron Oxides from the Lannitang Porphyry Cu-Au Deposit in the Zhongdian Region (Northwest) and the Geological Significances：A LA-ICP-MS Study

The Zhongdian area, located in northwestern Yunnan, is an important porphyry belt in China. It hosts a large number of Triassic intermediate-felsic porphyritic intrusions and porphyry deposits such as Pulang porphyry Cu-Au, Xuejiping porphyry Cu, Chundu porphyry Cu, Langdu Cu skarn and Lannitang porphyry Cu-Au deposit. The Lannitang porphyry Cu-Au deposit is located in west belt of the Zhongdian area. The magnetite in Lannitang porphyry Cu-Au deposit is widespread and it occurred as disseminated and vein types in potassic and chlorite-sericite alteration zone.Specularite is also observed frequently in the post-mineralization dolomite-quartz coarse veins.We conducted the petrography and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to determine the texture and composition of iron oxides (magnetite and specularite). In this study, we identified three types of magnetite. Type-Ⅰ magnetite is disseminated in potassic alteration of deposit. It is generally contains ilmenite lamellas. Type-Ⅱ and Type-Ⅲ magnetite are occurred in magnetite single vein and magnetite-bearing quartz stockwork vein separately. Type-Ⅱ and Type-Ⅲ are distributed in potassic and chlorite-sericite alteration zone. The LA-ICP-MS analyses show that Type-Ⅰ magnetite is relatively rich in V, Ni and Mg than other two types of magnetite. Type-Ⅱ and Type-Ⅲ magnetite are more enriched in Mn, Zn, Sn, Sc and high-Ni/Cr ratio than Type-Ⅰ magnetite.Type-Ⅱ and Type-Ⅲ magnetite has similar content of many trace elements. The concentration of Cr,Ga,Ni and Co in specularite is obviously lower than those of magnetite. The ilmenite lamellae and low-Ni/Cr(Ni/Cr<1) ratio revealed that Type-Ⅰ magnetite belongs to igneous magnetite. Type-Ⅱ and Type-Ⅲ are distributed in veinlets and displayed high-Ni/Cr ratio (Ni/Cr>1). We suggested that they are hydrothermal magnetite. Type-Ⅰ magnetite (igneous) is intergrown with hydrothermal minerals including chlorite and sericite and it has quiet similar contents of Ti, Al and Cr with the other two hydrothermal magnetite.We suggest that Type-Ⅰ magnetite (igneous) experienced late-stage fluid alteration, which induced the loss of Ti, Al and Cr.The similar content of trace element between Type-Ⅱ and Type-Ⅲ magnetite indicated that they may precipitate from same period of fluid.In combination with previous studies, we propose that the presence of elements such as Al, Mn, Mg and Sc are in solid solution within magnetite (and/or specularite),but the Ca, S, Cu, Ba, Sr and Zr may be present in micro-/nano-scale mineral inclusions.The widespread presence of magnetite-hematite and specularite in the potassic alteration zone and low Mn concentration of magnetite indicates a high oxygen fugacity of the Lannitang porphyry Cu-Au deposit (magnetite-hematite buffer).