以多种证据建立的白云鄂博稀土矿床成矿矿物的生成顺序

MUITIPLE LINES OF EVIDENCE FOR ESTABLISHING THE MINERAL PARAGENETIC SEQUENCE OF THE BAYAN OBO RARE EARTH ORE DEPOSIT OF INNER MONGOLIA,CHINA

通过野外和室内的研究认为白云鄂博稀土矿床是后生热液交代成因的矿床。热液活动始于1200Ma,终于400Ma。473～400Ma为成矿高峰期。导致矿床形成的断裂和热液活动是经历了三次区域变质和两次板块俯冲作用的产物。

The Bayan Obo deposit of Inner Mongolia, China is the world's largest known rare earh element(REE) ore deposit; the deposit also contains Fe and Nb ores. The REE and Fe ores are stratabound in adolomite marble unit (H8) in the upper part of the Middle Proterozoic Bayan Obo Group, and havebeen metamorphosed. The H8 host rock was not a magmatic carbonatite, but rather was probably de-posited as a sediment. Becanse of the complex mineralogy, the complicated regional metamorphic histo-ry, and the numerous episodes of mineralization of this giant ore depesit, we found it necessary to ex-amine multiple lines of evidence in order to describe the deposit and interpret its origin. Field and mi-croscopic textural analysis, geochronological mineral ages, and chemical compositions of different gen-erations of dated minerals were combined to deduce the mineral peragenetic sequence. The general mineral paragenetic sequence established for the Bayan Obo REE ore deposit is: (1)deposition of the H8 carbonate (in Middle Proterozoic time), followed by dolomitization; (2) earlyMgO>FeO magnesio-arfvedsonites of metamorphic origin (minimum K-Ar apes of 802±19and724±17Ma); (3) early magnesio-arfvedsonite in hydrothermal veins crosscutting the lower part ofthd Bayan Obo Group (1200Ma); (4) early monazite in dolomite prior to 628 Ma; (5) Mn-rich mag-nesio-arfvedsonite (minimum K-Ar age of 628±15Ma); (6) disseminated high-Nd monazite(^(232)Th/^(208)Pbmodel op of 596--592Ma); (7) monazite in banded ore (^(232)Th/^(208)Pb internal isochron age473±2Ma); (8) FeO>MgO alkali amphiboles (minimum K-Ar age of 442±11a); (9) early mag-netite; (10) low-Nd monazite, bastnaesite, and apatite associated with aegirine and fluorite in bandedores; and alkali amphiboles in sheared dolomite (^(232)Th/^(208)Pb mineral isochron ages and ^(40)Ar/~39(Ar)plateau ages of about 425Ma); (11) granular hematite replacing fine-grained monazite (post--423Ma); (12) late magnetite; (13) aegirine-rich veins containing huanghoite and aeschynite (^(232)Th/^(208)Pbmineral isochron age of 438±25Ma) crosscuturg banded ores; and (14) Hercynian ape late stage min-eralization. The Bayan Obo ore deposit is concluded to be of epigenetic, hydrothermal, metasomatic orign. In- termittent hydrothermal activity began about 1,200 Ma and diminished after 400Ma. Intermittentprincipal mineralizatiuon started before 628Ma and reached peak activity between 473 and 400Ma.Three periods of regional metamorphism and two periods of subduction are thought to have producedthe fracures and hydrothermal solutions responsible for fomation of the deposit.