OXYGEN, CARBON AND STRONTIUM ISOTOPE STUDY OF THE CARBONATITIC DOLOMITE HOST OF THE BAYAN OBO FE-NB-REE DEPOSIT, INNER-MONGOLIA, N CHINA

The large Fe-Nb-REE deposit at Bayan Obo is hosted by a dolomite marbl e within the thrust complex of marbles, quartzites and slates that bel ongs to the Bayan Obo Formation of mid-Proterozoic age. The dolomite i s either a dolomitized sedimentary limestone subsequently mineralized and tectonically thrust and folded, or a dolomite (or dolomitized) car bonatite intrusion with late-stage recrystallization and mineralizatio n that has been subsequently tectonically deformed. O and C isotope da ta indicate that the sedimentary limestones and dolomites of the Bayan Obo Formation, which occur in the thrust stack together with quartzit es and slates, have values of 60 c. +20 per mil (SMOW) and delta C c. zero. In contrast, the coarser grained facies of the large (0.5 x 10 k m) dolomite marble which hosts the REE ore body has 60 per mil values between +8 and +12 and delta C values between -5 and -3, whereas the f iner-grained recrystallized and REE-mineralized dolomite marble which occurs close to the ore bodies has 60 between +12 to +16 and delta C b etween -4 and zero. Sr-87/Sr-86 data confirm this distinction: >0.710 for the sedimentary rocks and <0.704 for the coarse-and fine-grained d olomite marbles. These data are taken to indicate that the large and c oarse-grained dolomite was an igneous carbonatite (as borne out by its ferritic contact rocks and trace element geochemistry), and that the finer grained dolomite recrystallized under the influence of mineraliz ing solutions which entrained groundwater. The stratiform features in the coarse-grained dolomite that are evident in the field are interpre ted as tectonic layering.