A Deposit Model for Carbonatite and Peralkaline Intrusion-Related Rare Earth Element Deposits

  • Verplanck P
  • Van Gosen B
  • Seal R
  • et al.
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Abstract

Carbonatite and alkaline intrusive complexes, as well as their weathering products, are the primary sources of rare earth elements. A wide variety of other commodities have been exploited from carbonatites and alkaline igneous rocks includ- ing niobium, phosphate, titanium, vermiculite, barite, fluo- rite, copper, calcite, and zirconium. Other elements enriched in these deposits include manganese, strontium, tantalum, thorium, vanadium, and uranium. Carbonatite and peralkaline intrusion-related rare earth element deposits are presented together in this report because of the spatial, and potentially genetic, association between carbonatite and alkaline rocks. Although these rock types occur together at many locations, carbonatite and peralkaline intrusion-related rare earth element deposits are not generally found together. Carbonatite hosted rare earth element deposits are found throughout the world, but currently only five are being mined for rare earth elements: Bayan Obo, Daluxiang, Maoniuping, and Weishan deposits in China and the Mountain Pass deposit in California, United States. These deposits are enriched in light rare earth elements, including lanthanum, cerium, praseodynium, and neodynium. The principal rare earth element-minerals associated with carbonatites are fluocarbon- ates (bastnäsite, parisite, and synchysite), hydrated carbonates (ancylite), and phosphates (monazite) with bastnäsite being the primary ore mineral. Calcite and dolomite are the pri- mary gangue minerals. At present, the only rare earth element production from a peralkaline intrusion-related deposit is as a byproduct commodity at the Lovozero deposit in Rus- sia. Important rare earth element minerals found in various deposits include apatite, eudialyte, loparite, gittinsite, xeno- time, gadolinite, monazite, bastnäsite, kainosite, mosandrite, britholite, allanite, fergusonite, and zircon, and these minerals tend to be enriched in heavy rare earth elements. Carbonatite and alkaline intrusive complexes are derived from partial melts of mantle material, and neodymium isotopic data are consistent with the rare earth elements being derived from the parental magma. Deposits and these associated rock types tend to occur within stable continental tectonic units, in areas defined as shields, cratons, and crystalline blocks; they are generally associated with intracontinental rift and fault systems. Protracted fractional crystallization of the magma leads to enrichment in rare earth elements and other incompat- ible elements. Rare earth element mineralization associated with carbonatites can occur as either primary mineral phases or as mineralization associated with late stage orthomagmatic fluids. Rare earth element mineralization associated with alkaline intrusive complexes may occur as primary phases in magmatic layered complexes or as late-stage dikes and veins. The greatest environmental challenges associated with carbonatite and peralkaline intrusion-related rare earth element deposits center on the associated uranium and thorium. Con- siderable uncertainty exists around the toxicity of rare earth elements and warrants further investigation. The acid-generat- ing potential of carbonatites and peralkaline intrusion-related deposits is low due to the dominance of carbonate minerals in carbonatite deposits, the presence of feldspars and minor calcite within the alkaline intrusion deposits, and only minor quantities of potentially acid-generating sulfides. Therefore, acid-drainage issues are not likely to be a major concern associated with these deposits. Uranium has the potential to be recovered as a byproduct, which would mitigate some of its environmental effects. However, thorium will likely remain a waste-stream product that will require management since progress is not being made towards the development of thorium-based nuclear reactors in the United States or other large scale commercial uses. Because some deposits are rich in fluorine and beryllium, these elements may be of environmen- tal concern in certain locations.

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APA

Verplanck, P. L., Van Gosen, B. S., Seal, R. R., & McCafferty, A. E. (2014). A Deposit Model for Carbonatite and Peralkaline Intrusion-Related Rare Earth Element Deposits. Mineral Deposit Models for Resource Assessment, 58 p.

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