碳酸岩流体及其稀土成矿作用

CARBONATITIC FLUIDS AND REE MINERALIZATION

火成碳酸岩熔浆可侵入到大陆和洋壳构造环境 ,多数大陆构造环境下的碳酸岩在时间和空间上与地壳减薄事件有关 ,高温下它具有极强的搬运碱金属、大离子亲石元素和高场强元素的能力。碳酸岩中可能出现的原生包裹体有两相水溶液 (气 +液 )包裹体、含子矿物多相水溶液包裹体、含CO2 包裹体和CO2 +盐水溶液混合多相包裹体以及硅酸盐玻璃质熔融包裹体等。碳酸岩型稀土矿床中赋存的流体包裹体类型也可分为气液相、含碳水溶液相和气液固多相包裹体 ,包裹体中稀土子矿物的出现是该类矿床最显著的特征。富碱碳酸岩流体中REE ,Sr ,Th和Fe等都具有极高的溶解度 ,水岩反应和流体不混溶是造成成矿热液中REE及Fe等沉淀成矿的关键因素 ,REE的迁移沉淀可延续到低温。

Igneous carbonatites can intrude into both continental and oceanic tectonic settings. Most carbonatites in continental settings are associated in space and time with events of crustal thinning. The carbonated alkaline melt shows the high capability of transporting large ion lithophile and high field strength elements at high temperatures. Primary fluid inclusions in carbonatite complex generally comprise liquid vapour, daughter mineral bearing aqueous, CO 2 bearing, CO 2 + daughter mineral bearing solid, and silicate melting inclusions. Fluid inclusions in REE deposits can also be distinguished into liquid vapour, aqueous carbonic and aqueous liquid vapour solid inclusions. REE minerals identified as daughter inclusions are the special feature in this type of deposit. Alkali carbonatite fluids are capable of transporting huge amounts of REE, Sr, Th and Fe. The interaction of the fluids with carbonate host rocks and the fluid immiscibility are the key factors in the precipitation of both REE minerals and Fe oxides. The transport and precipitation of REE can last to the late stage of fluid influx despite of low temperatures and salinities.