THE BAYAN OBO FE-REE-NB DEPOSIT, INNER MONGOLIA, CHINA: COMPARISONS WITH CARBONATITE-RELATED AND FE-OXIDE-TYPE DEPOSITS

Along with other factors the Fe-oxide-Cu-Au deposit class is commonly characterised by alkali silicate alteration and enrichments in the REE and other high field strength elements. For this reason a link has been proposed with Fe-oxide-rich deposits associated with carbonatites (most notably Phalaborwa, S.A.). The Bayan Obo Fe-REE-Nb deposit, Inner Mongolia, China has been compared with both carbonatite-related deposits and with the hydrothermal Fe-oxide deposit class (with inferences of an A-type granite source rock for metals). The carbonatite-related model is supported by the geochemistry of the deposit, stable isotope data, and the fenite-like silicate alteration assemblage. Sm-Nd and Pb isotope systematics however, indicate an enriched and possibly crustal source for metals. Both textural and fluid inclusion studies indicate that the deposit was formed and then modified by multiple stages of fluid circulation, possibly linked to deformation. An overview of available geochronological data suggest that the most obvious ways to reconcile these differences are either the remobilisation of an old (Proterozoic) alkaline igneous- or carbonatite-related deposit by fluids produced during Caledonian orogenic activity, or through the derivation of carbonatites from enriched upper mantle. The inferred carbonatite source for metals at Bayan Obo is indicative of distinct differences between Bayan Obo and the deposits of the Fe-oxide class. Most notably the differences in alteration are probably a result of the fact that hypersaline brines were not involved in the genesis of the Bayan Obo deposits. The evolution of fluids from carbonatite magmas is just one mechanism that can generate low sulphur, chloride-rich brines, capable of transporting large masses of Fe. Distinctions must begin to be drawn between Fe-oxide rich deposits that may form a common class (e.g. Fe-oxide-Cu-Au deposits associated with regional Na-Cl alteration) and those which, despite having common features, are linked to distinct fluid and metal sources. This distinction may have important implications for exploration and prospectivity.