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Abstract

Carbonatites are relatively rare igneous rocks that are of considerable economic interest due to their common enrichment in certain elements, such as the rare earth elements (REEs), Nb, Ta, Th, and P. There are more than 37 known carbonatite occurrences in western North America, six of which have published resource estimates: these are the Aley and Upper Fir in British Columbia (for Nb, Ta, P), the Bear Lodge Mountains (for REEs), the Wet Mountains area in Colorado (for Th, Nb, U, and REEs), Iron Hill in Colorado (for REEs, Nb, and Th), and Mountain Pass in California (for REEs).

Based on their distribution in geologic space and time, the carbonatites in western North America can be divided into four distinct groups: group 1 (~1450-1375 Ma) is Mountain Pass; group 2 (~813-449 Ma) comprises several carbonatites in Canada and the United States; group 3 (~359-328 Ma) comprises most of the carbonatites in western Canada; and group 4 (~52-37 Ma) comprises carbonatites in the northwestern United States and northern Mexico. It is not possible to distinguish between economically more or less favorable carbonatites based on their intrusion ages and geologic settings alone because all of the above groups contain both potentially mineralized and barren carbonatites. Therefore, other factors, such as the source region of the parental magma, the depth and degree of melting of that region, and the evolution of the parental magma during ascent and emplacement, control whether or not a carbonatite reaches ore grade.

In terms of regional exploration, the vast majority of the carbonatites in western North America are situated parallel and in close proximity to the Cordilleran deformation front. These carbonatites are of Neoproterozoic to Paleozoic age, are related to rifting and/or extensional tectonic events, and are commonly hosted by miogeoclinal strata. Carbonatites that are distal to the Cordilleran deformation front are (1) the Eocene carbonatites in Montana and Wyoming, which follow a confined N40°W trend, presumably governed by the edge of the subducted Kula plate, and (2) the Mesoproterozoic Mountain Pass carbonatite, which is unique in terms of age and geologic setting.

The repetitive and localized nature of carbonatite and alkaline magmatism makes areas in proximity to known carbonatite occurrences prime exploration targets; furthermore, Th-REE-rich quartz dikes are possibly indicative of buried carbonatitic and alkaline intrusions, and the lack of REE carbonates in amphibolite facies metacarbonatites suggests that these phases are not stable at elevated metamorphic pressure and temperature conditions.

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