Abstract Highly evolved alkaline/peralkaline igneous rocks host deposits of rare earth elements (REE), including Y as well as Zr, Hf, Nb, Ta, U, and Th. The host rocks spanning from silica-undersaturated (nepheline syenites) to silica-oversaturated (granites) occur in intraplate tectonic environments, mainly in continental settings and are typically associated with rifting, faulting, and/or crustal extension. They range in age from Neoarchean/Paleoproterozoic to Mesozoic, but several significant deposits are of Mesoproterozoic age. The deposits/prospects can be subdivided into three types. The first is hosted by nepheline syenitic rocks of large, layered alkaline intrusions where the mineralization commonly occurs in layers rich in REE-bearing minerals, which mostly show cumulate textures (e.g., Thor Lake/Nechalacho, Canada; Ilimaussaq, Greenland; Lovozero, Russia; Kipawa, Canada; Norra Kärr, Sweden; Pilanesberg, South Africa). The second type includes mineralization in peralkaline granitic rocks where REE-bearing minerals are usually disseminated. The mineralization is typically hosted by pegmatites (including the Nb-Y-F type), felsic dikes, and minor granitic intrusions (e.g., Strange Lake, Canada; Khaldzan-Buregtey, Mongolia; Ghurayyah, Saudi Arabia; Bokan, Alaska, United States). The third type is disseminated, very fine grained, and hosted by peralkaline felsic volcanic/volcaniclastic rocks, mostly of trachytic composition (e.g., Dubbo Zirconia and Brockman/Hastings, Australia). The bulk of the REE is present in ore/accessory minerals which in some mineralized zones, particularly in cumulate rocks from alkaline complexes, can reach >10 vol %. Mineralization is composed of a variety of REE-bearing minerals, which frequently show complex replacement textures. They include fluorocarbonates, phosphates, silicates, and oxides. Economically most important are bastnäsite, monazite, xenotime, loparite, eudialyte, synchysite, and parasite. Many other minerals are either sparse or it is difficult with present technology to profitably extract REE from them on a commercial scale. Compared to carbonatite-hosted REE deposits, the REE mineralization in alkaline/peralkaline complexes has lower light REE concentrations but has commonly higher contents of heavy REE and Y and shows a relative depletion of Eu. Elevated concentrations of U and Th in the ore assemblages make gamma-ray (radiometric) surveys an important exploration tool. The host peralkaline (granitic, trachytic, and nepheline syenitic) magmas undergo extensive fractional crystallization, which is protracted in part due to high contents of halogens and alkalis. The REE mineralization in these rocks is related to late stages of magma evolution and typically records two mineralization periods. The first mineralization period produces the primary magmatic ore assemblages, which are associated with the crystallization of fractionated peralkaline magma rich in rare metals. This assemblage is commonly overprinted during the second mineralization period by the late magmatic to hydrothermal fluids, which remobilize and enrich the original ore. The parent magmas are derived from a metasomatically enriched mantle-related lithospheric source by very low degrees of partial melting triggered probably by uplift (adiabatic) or mantle plume activity. The rare metal deposits/mineralization related to peralkaline igneous rocks represent one of the most economically important resources of heavy REE including Y. In addition to REE, some of these deposits contain economically valuable concentrations of other rare metals including Zr, Nb, Ta, Hf, Be, U, and Th, as well as phosphates.

Abstract Ion adsorption-type rare earth element (REE) deposits are the predominant source of heavy REE (HREEs) and yttrium in the world. Economic examples of the deposits are confined almost exclusively to areas underlain by granitic rocks in southern China. These deposits are termed “ion adsorption-type” because the weathered granites contain more than ~50% ion-exchangeable REY (REE + Y), relative to whole-rock REY. The ore grades range from 140 to 6,500 ppm (typically ~800 ppm) REY, and some of the deposits are remarkably enriched in HREEs. The Yanshanian (Jurassic-Cretaceous) granites that weather to form the deposits are products of subduction-related or extensional intraplate magmatism. These parent granites for the REE deposits are biotite- and/or muscovite-bearing granites and are characterized by >70% SiO 2 , <0.08% P 2 O 5 , and metaluminous to weakly peraluminous (ASI < 1.1) compositions. The highly differentiated (SiO2 >~75%) muscovite granites are HREE enriched relative to the biotite granites and are notably characterized by occurrences of fluorite and hydrothermal REE-bearing minerals, particularly REE fluorocarbonates that formed in a deuteric alteration event. Magmatic allanite and titanite are either altered to form hydrothermal REE-bearing minerals or almost completely broken down during weathering. The weatherable REE-bearing minerals, including fluorocarbonates, allanite, and titanite, are the source minerals for the ion adsorption ores. The HREE grades of the ion adsorption ores are strongly influenced by the relative abundances and weathering susceptibilities of these REE-bearing minerals in the parent granites. The presence of easily weathered HREE minerals in the underlying granites appears to be the primary control of the HREE-rich deposits, although solution and solid phase chemistry during development of the weathering profile may influence REE fractionation. Monazite, zircon, and xenotime are also present in the granites, but because they are more resistant to chemical weathering, they are typically not a source of REEs in the weathered materials. The REE-bearing minerals are decomposed by acidic soil water at shallow levels in the weathering profile, and the REE 3 + ions move downward in the profile. The REEs are complexed with humic substances, with carbonate and bicarbonate ions, or carried as REE 3+ ions in soil and ground water at a near-neutral pH of 5 to 9. The REE 3 + ions are removed from solution by adsorption onto or incorporated into secondary minerals. The removal from the aqueous phase is due to a pH increase, which results from either water-rock interaction or mixing with a higher pH ground water. The REEs commonly adsorb on the surfaces of kaolinite and halloysite, to form the ion adsorption ores, due to their abundances and points of zero charge. In addition, some REEs are immobilized in secondary minerals consisting mainly of REE-bearing phosphates (e.g., rhabdophane and florencite). In contrast to the other REEs that move downward in the weathering profile, Ce is less mobile and is incorporated into the Mn oxides and cerianite (CeO 2 ) as Ce 4 + under near-surface, oxidizing conditions. As a result, the weathering profile of the deposits can be divided into a REE-leached zone in the upper part of the profile, with a positive Ce anomaly, and a REE accumulation zone with the ion adsorption ores in the lower part of the profile that is characterized by a negative Ce anomaly. The thickness of the weathering profiles generally ranges from 6 to 10 m but can be as much as 30 m and rarely up to 60 m. The negative Ce anomaly in weathered granite terrane is thus a good exploration indicator for ion adsorption ores. A temperate or tropical climate, with moderate to high temperatures and precipitation rates, is essential for chemical weathering and ion adsorption REE ore formation. Low to moderate denudation, characteristic of such a climate in areas of low relief, are favorable for the preservation of thick weathering profiles with the REE orebodies.