白云鄂博富稀土碳酸岩的地球化学特征

Geochemical Characteristics of a Carbonatite Dyke Rich in Rare Earths from Bayan Obo, China

重点解剖了一条距白云鄂博ＲＥＥＮｂＦｅ 矿床东矿， ＮＥ 方向３ ｋｍ 并切割白云鄂博群Ｈ１ 及Ｈ２岩性段的细粒方解石碳酸岩岩墙的岩石地球化学特征。结果表明， 碳酸岩的稀土元素含量变化大， 最高可达２０ ％ （ 质量分数）， 已构成稀土富矿石。碳酸岩的轻稀土元素高度富集， 轻、重稀土元素之间发生了极度分馏， 但无铕异常显示。形成这种岩石地球化学特征的可能机制为： 碳酸岩浆直接来源于岩石圈富集地幔的低程度部分熔融作用（ Ｆ＜１ ％ ） ， 残留地幔矿物组合以富含柘榴石为特征； 碳酸岩浆在地壳深部岩浆房中的分离结晶作用。碳酸岩的稀土元素和微量元素分布型式均与白云鄂博ＲＥＥＮｂＦｅ矿床的赋矿细粒白云石大理岩十分相似。然而， 碳酸岩的主要元素以钙质为特征， 不同于赋矿细粒白云石大理岩， 造成这种差异的原因在于碳酸岩岩墙没有遭受大规模的白云岩化作用， 而赋矿细粒白云石大理岩则可能是碳酸岩经白云岩化作用的产物。

This contribution analyses in detail the whole rock geochemistry of a rare earth element(REE) rich carbonatite dyke that is located at Dulahala and lies 3 km NE of the East Ore body of the giant Bayan Obo REE Nb Fe deposit. The dyke cuts cross H1 coarse quartz sandstone and H2 fine quartzite of the Proterozolic Obo group. REE content in the dyke varies greatly over 20%(mass fraction), which comprises rich REE ores. Light REE in carbonatites are extremely enriched and strongly fractionated relative to heavy REE, but without Eu anomaly. The carbonatite may be produced by mechanisms as follows: the carbonatite magma is directly formed by very low degree( F <1%) partial melting of enriched lithospheric mantle, leaving residual minerals characterized by abundant garnet; then the magma arises into a chamber within the crust where they will undergo fractional crystallization, which makes REE further concentrate in carbonatite. The REE patterns and spider diagrams of the carbonatite are identical to those of fine grained dolomite marble that is the ore host rock for the Bayan Obo deposit. However, the carbonatite is calcic, which is different from the fine grained dolomite marble in major element geochemistry. The difference is suggested to be resulted from that the carbonatite dyke has not affected by a large scale dolomitization, while the fine grained dolomite marble might be the product of dolomitized carbonatite intrusive body that might set up a hydrothermal system in the region, which transported Mg from the Bayan Obo sediments, especially from the shales, to the carbonatite intrusion.