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

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 a dolomite marble unit(H8)in the upper part of the Middle Proterozoic Bayan Obo Group, and have been metamorphosed. The H8 host rock was not a magmatic carbonatite, but rather was probably deposited as a sediment. Becanse of the complex mineralogy, the complicated regional metamorphic history, and the numerous episodes of mineralization of this giant ore depesit, we found it necessary to examine multiple lines of evidence in order to describe the deposit and interpret its origin. Field and microscopic textural analysis, geochronological mineral ages, and chemical compositions of different generations 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)early MgO>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 of thd Bayan Obo Group(1200Ma);(4)early monazite in dolomite prior to 628 Ma;(5)Mn-rich magnesio-arfvedsonite(minimum K-Ar age of 628±15Ma);(6)disseminated high-Nd monazite(²³²Th/²⁰⁸Pbmodel op of 596--592Ma);(7)monazite in banded ore(²³²Th/²⁰⁸Pb internal isochron age 473±2Ma);(8)FeO>MgO alkali amphiboles(minimum K-Ar age of 442±11a);(9)early magnetite;(10)low-Nd monazite, bastnaesite, and apatite associated with aegirine and fluorite in banded ores; and alkali amphiboles in sheared dolomite(²³²Th/²⁰⁸Pb mineral isochron ages and ⁴⁰Ar/³⁹Ar plateau ages of about 425Ma);(11)granular hematite replacing fine-grained monazite(post-423 Ma);(12)late magnetite;(13)aegirine-rich veins containing huanghoite and aeschynite(²³²Th/²⁰⁸Pb mineral isochron age of 438±25Ma)crosscuturg banded ores; and(14)Hercynian ape late stage mineralization. The Bayan Obo ore deposit is concluded to be of epigenetic, hydrothermal, metasomatic orign. Intermittent hydrothermal activity began about 1,200 Ma and diminished after 400Ma. Intermittent principal 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 produced the fracures and hydrothermal solutions responsible for fomation of the deposit.