Nb-REE ore deposits related to weathered carbonatite within the Tomtor massif, northern Siberia, contain the world's largest carbonatite-related concentrations of Nb and REE. Despite its remote location, economic exploitation of the deposit was made possible by the discovery of rich Sc-REE-Y-Nb ore, The 300-km 2 Tomtor massif is a ring complex in which the Nb-REE deposits are situated within and above a central carbonatite stock. The deposits include: (1) pyrochlore-bearing carbonatite, (2) a lower ore horizon, and (3) an upper ore horizon. The average concentration of niobium increases from a few tenths of a percent in the carbonatite, to a few percent in the lower ore horizon, and up to 12 percent in the upper ore horizon. BEE contents range from 0.5 to 0.7 percent in carbonatite, 4 to 6 percent in the lower ore horizon, and 11 to 30 percent in the upper ore horizon. In a plot of 19) O vs. delta 3 C, the Tomtor carbonates plot along a trend from the Oka box for magmatic carbonates to the field of sedimentary carbonates. The Tomtor carbonatites have initial 87 Sr/ 86 Sr ratios ranging from 0.70346 to 0.70374, a range consistent with mantle derivation for the magmas. The lower ore horizon, which is up to 300 m thick, consists of niobium (Nb 2 O 5 content 1-2%) and phosphorus ores that occur in a variety of original rock types (carbonatites, volcanics, picritic dikes, and country rocks) which have undergone several types of alteration including weathering, hydrothermal alteration, and low-temperature sideritization. 87 Sr/ 86 Sr ratios of the lower ore horizon range from 0.70421 to 0.70374 and suggest a dominantly mantle source for Sr with a very minor addition of crustal Sr. The upper ore horizon is a placer deposit 3 to 25 m thick which has been partly affected by hydrothermal alteration. Rich ores of the upper ore horizon are composed mainly of pyrochlore, phosphates and alumophosphates of REE, Sr and Ba (florencite, monazite, gorceixite, goyazite), rutile, and pyrite. Nb in the Tomtor deposits occurs principally in pyrochlore, columbite, rutile, and ilmenorutile. REE and Y occur in florencite, monazite, rhabdophane, xenotime, and bastnaesite, and Sc occurs in monazite, rhabdophane (0.55-0.95%), minerals of the crandallite group (0.017-.0030%), rutile, and as a thin film of Sc(OH) 3 on phosphates.

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