Carbonatites in Western North America—Occurrences and Metallogeny
Published:January 01, 2013
Leo J. Millonig, Lee A. Groat, 2013. "Carbonatites in Western North America—Occurrences and Metallogeny", Tectonics, Metallogeny, and Discovery: The North American Cordillera and Similar Accretionary Settings, M. Colpron, T. Bissig, B. G. Rusk, J. F. H. Thompson
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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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Tectonics, Metallogeny, and Discovery: The North American Cordillera and Similar Accretionary Settings
The northern Pacific Rim—for the purposes of this contribution—comprises the Mesozoic and Cenozoic magmatic-arc and associated terranes of eastern China, Korea, Japan, the Russian Far East, Alaska, Yukon, British Columbia, the western United States, and Mexico. This ~1,800-km-long segment of the Pacific Rim is marked by a broad spectrum of metallogenic environments and mining jurisdictions, which combine to dictate where and how exploration is conducted and the overriding character of any resulting discoveries.
This summary report commences with a brief metallogenic overview of the northern Pacific Rim, with particular attention paid to the world-class Mesozoic and Cenozoic ore deposits that define the region’s premier metallogenic provinces. This is followed by a summary of the relative attractiveness of the region’s various mining jurisdictions, as recorded by recent exploration activity. The major discoveries made along the northern Pacific Rim, particularly during the past half century, are then placed in this metallogenic and regulatory context as a basis for determining the successful exploration methodologies employed. This discovery track record is then used to predict what the future of exploration in this vast and varied region may hold.
Much of the northern Pacific Rim, from eastern China and the Russian Far East in the northwest through Alaska to western parts of Canada, the United States, and Mexico in the southeast (Fig. 1), is characterized by a complex array of oceanic, accretionary prism, magmatic arc, and back-arc basin terranes and associated microcontinental blocks accreted to the North China, Siberian, Hyperborean, and North American cratons, mainly during Mesozoic times (Coney et al., 1980; Campa and Coney, 1983; Kojima, 1989; Nokleberg et al., 2005; Yakubchuk, 2009). The metallogeny of these tectonic collages is dictated by various combinations of pre-, syn-, and postaccretion ore-forming events, the last of which are generally preeminent, except in British Columbia (Nokleberg et al., 2005; Nelson and Colpron, 2007).
Although the Meso-Cenozoic metallogeny of the northwestern and northeastern Pacific quadrants displays some similarities, it is the contrasts that are most marked. The main contrasts stem from the preeminence of tin, tungsten, and antimony in eastern China, Korea, Japan, and the Russian Far East and of copper and silver in Western Canada, the conterminous United States, and Mexico. Nonetheless, both the northwestern and northeastern Pacific quadrants are exceptionally well endowed with gold and molybdenum deposits. The northeasternmost Russian Far East, Alaska, and Yukon Territory display elements of both northwestern and northeastern Pacific metallogeny (Fig. 1). These metallogenic contrasts between the northwestern and northeastern quadrants result in China being the world’s leading producer of tungsten, tin, bismuth, and antimony, mostly from its eastern Mesozoic metallogenic province.