Xin-Fu Zhao

南秦岭地区是我国重要的铌-稀土成矿带,特别是发育了碳酸岩-碱性岩型铌-稀土矿床和碱性火山岩型铌(钽)矿床两类矿床类型,是研究铌和稀土成矿过程以及差异富集机制的良好对象。本文在系统总结区域地质背景和矿床地质特征的基础上,对近年来发表的岩石地球化学和年代学数据进行了系统整理,结合岩浆演化特征和矿物结构-地球化学特征,探讨了成岩成矿过程与元素富集机制。南秦岭这两类矿床均形成于早志留世(约430~445Ma),与扬子板块北缘的陆内裂谷作用密切相关。两类矿床相关的岩浆岩具有十分一致Sr-Nd-Pb同位素特征,均主要落入DMM-HIMU-EMⅠ三端元混合区域,结合基性岩具有高TiO₂含量、高Na₂O/K₂O和Ba/Rb比值以及低Rb/Sr比值的特征,指示成矿岩浆具有一致的地幔源区,均起源于含角闪石和碳酸盐组分的交代岩石圈地幔。两类矿床的成矿差异是由于富CO₂的碱性玄武质母岩浆沿不同路径演化导致:在火山岩型铌(钽)成矿系统中,母岩浆经强烈分离结晶作用形成粗面质岩浆,铌元素随岩浆演化富集于高演化的粗面岩中;而在侵入岩系统中,母岩浆经分离结晶形成含CO₂的正长质岩浆,随后经液态不混溶作用形成碳酸质岩浆,在此过程中稀土元素优先分配进入碳酸盐熔体。铌在正长岩阶段以及碳酸盐岩浆演化早期饱和成矿,而稀土元素则在晚期富挥发分的碳酸岩"盐熔体"阶段富集成矿。此外,两类矿床均遭受了三叠纪区域造山变质事件的叠加改造,导致了局部稀土活化再富集及铌矿物相的转变,但此阶段没有新生成矿物质的加入。本研究构建了南秦岭地区铌-稀土矿床的成矿模型,为区域找矿勘查提供了理论依据和借鉴。

Kiruna-type iron oxide–apatite (IOA) deposits, an important source of iron, show close associations with andesitic subvolcanic intrusions. However, the processes of ore formation and the mechanism controlling iron concentration remain uncertain. Here, we report the widespread presence of high-temperature (>800°C) water-poor multisolid hydrosaline liquid inclusions in pre- and syn-ore minerals from IOA deposits of eastern China. These inclusions consistently homogenize to a liquid phase by vapor disappearance and mostly contain 3 to 10 wt % Fe, signifying a substantial capacity for iron transportation by such hydrosaline liquids. We propose that the hydrosaline liquids were likely immiscible from the dioritic magmas with high Cl/H ₂ O in subvolcanic settings. Subsequent reaction with host rocks and/or decompression and cooling of the hydrosaline liquids is deemed responsible for the simultaneous formation of high-temperature alteration and magnetite ores, thereby providing important insights into the distinctive characteristics of IOA deposits in shallow magmatic-hydrothermal systems.

Apatite can incorporate significant amounts of H₂O and CO₂, potentially recording volatile abundance and behavior during magma evolution. Here we conducted in situ measurements of CO₂ and H₂O concentrations in apatite, along with elemental compositions, from two contemporaneous alkaline volcanic suites (Tianbao and Tudiling) in the South Qinling belt in central China to better understand the CO₂ and H₂O contents and behavior in evolving alkaline melts. Apatite from alkali basalts in Tianbao contains variable CO₂ contents ranging from 2O contents ranging from 0.1 to 0.6 wt.%. Apatite from REE-enriched trachytes, which evolved from alkali basalt, shows significantly lower CO₂ contents and a continuous decrease in H₂O during magma fractionation. The observations suggest that CO₂ loss commenced at the early stages of magma evolution, whereas significant H₂O loss occurred during subsequent magma fractionation in tandem with REE-enrichment. In comparison, apatite grains from the Tudiling trachyte, which is genetically linked with carbonatite, contain higher CO₂ contents (0.6 to 1.5 wt.%) but lower REE concentrations than the Tianbao trachytes. Apatite in the Tudiling trachyte is inferred to have crystallized from a carbonated alkaline magma prior to significant CO₂ loss and the separation of Tudiling carbonatitic melts, where subsequent liquid immiscibility led to REE enrichment into the carbonatitic melts. The volatile characteristics of apatite from the two volcanic suites provide valuable insights into two different evolutionary processes of alkaline/carbonatitic magmas, the behavior of CO₂ and H₂O, and the enrichment of REE in alkaline systems.