中国内生稀土矿床类型、成矿规律与资源展望

The types, ore genesis and resource perspective of endogenic REE deposits in China

稀土是我国为数不多的优势战略矿产资源.我国内生稀土矿床与碳酸岩-碱性岩浆演化及氧化性热液活动密切相关,以碳酸岩型、碱性岩-碱性花岗岩型和热液型最为典型,主要包括内蒙古白云鄂博、四川牦牛坪、山东微山、湖北庙垭、内蒙古巴尔哲和云南迤纳厂等矿床.碳酸岩型稀土矿床成矿碳酸岩经历了强烈的分异演化和岩浆热液的交代/叠加作用,常富集轻稀土,伴生有Nb、Th、Sc等资源.碱性岩-碱性花岗岩型稀土矿床多与高分异碱性岩或碱性花岗岩密切相关,且普遍经历强烈的岩浆期后热液交代作用,富集中-重稀土,常伴生有Zr、Nb、Ta等高场强元素矿化.热液型稀土矿床矿体在空间上没有密切共生的岩浆岩体,但其形成往往和隐伏的岩浆岩关系密切,多期次的热液叠加往往导致稀土不断活化-再富集,也可伴随Au、U、Co等元素的矿化.我国稀土成矿主要发生在中元古代和中、新生代,通常受控于克拉通边缘大陆裂谷或陆内伸展构造环境.今后需摸清我国稀土资源家底,评价与寻找富集中-重稀土资源的内生稀土矿床,加强稀土矿床伴生资源的综合利用.

China hosts a large proportion of global REE resources. This paper presents a preliminary review of the types and ore genesis of endogenic REE deposits in China. Three major types of REE deposits are identified, namely carbonatite-type,alkaline and alkali granite-type and hydrothermal-type deposits.The carbonatite-type deposits mainly include Bayan Obo in northern China, Maoniuping in southwestern China,Weishan in eastern China, and Miaoya in Central China, all of which are genetically related to carbonatite/carbonatitealkaline complex. This type of deposit mainly contains economic light REE(LREE), which are mainly hosted in fluorocarbonate and/or phosphate. Apart from LREE, some deposits may also show obvious enrichments in heavy REE(HREE),Nb, Th, and Sc. The above-mentioned carbonatite-type REE deposits were formed in different periods, including MesoProterozoic(Bayan Obo), Paleozoic(Miaoya), Mesozoic(Weishan) and Cenozoic(Maoniuping). They were generated in intra-continental extension(Bayan Obo, Miaoya, and Weishan) or post-collisional(Maoniuping) setting. The ore-related intrusions were mainly derived from very low degrees partial melting of the sub-continental lithosphere mantle(SCLM).Before the formation of carbonatite, the SCLM had been metasomatized by recycled crustal components, which elevated the contents of REE and other components(e.g., Nb, Ba, F, and P). The parental carbonatitic or CO_(2)-rich silicate magmas have experienced intensive fractional crystallization or immiscibility of carbonate-and silicate-rich melts, which further improve the contents of REE in the later-stage magmas. A subset of REE minerals was crystallized in the magmatic evolution process, while a large proportion of REE were retained in the residual melts. Furthermore, the REE were transported out of the magma chamber by volatile-rich fluids(e.g., F^(-), SO_(4)^(2-) and Cl^(-)), and were deposited adjacent to the carbonatitic intrusions.The alkaline and alkali granite-type deposits mainly include Baerzhe and Saima in northeastern China, which have genetic associations with SiO_(2)-saturated or-unsaturated alkaline intrusions, respectively. Compared with the carbonatitetype deposit, this type of deposits contains relatively high contents of HREE, which are mainly hosted in phosphate or silicate. Notably, this type of deposits may also contain economic Zr, Nb, and Ta. The above-mentioned deposits were formed in the Mesozoic under an intra-continental extensional setting. Similar to the carbonatite, the ore-related alkaline intrusions were derived from the sub-continental lithosphere mantle(SCLM), which had been metasomatized by REE-and volatile-bearing components. The parental silicate magmas have experienced intensive fractional crystallization during the evolution process. Many REE-, Nb-, and Zr-bearing minerals were crystallized in the later magmatic process. These minerals were later modified by magmatic-hydrothermal fluids, which lead to remobilization and further enrichment of REE, Zr and Nb.A few IOCG deposits also show obvious enrichment in LREE, which are represented by the Paleoproterozoic Yinachang and Lala deposits in southwestern China. Diverse minerals occur as REE hosts in this type of deposits, mainly including fluoro-carbonate, phosphate, and silicate. No spatially associated magmatic intrusions can be identified in the IOCG-type deposits, but available data indicate that REE enrichment was genetically related to some hidden magmatic intrusions. The intrusions were generated in a rift-setting and vary from mafic to felsic in composition. In this type of deposit, REEs were initially deposited in apatite and allanite. In the later stages, apatite and allanite were modified by multiples phases of fluids.During the fluid/mineral interaction processes, REEs were mobilized and reprecipitated in other REE minerals, including monazite, bastnaesite, xenotime, synchysite. Multiple stages of fluid/mineral interaction are important for the final enrichment of REE in this type of deposit.In the future, more studies are needed to accurately evaluate the resources of REE in China, to target more deposits that are relatively enriched in medium-heavy REE, and to provide guidance for the effective utilization of accompanied metals in the REE deposits.