Characteristics and Genesis of Ion Adsorption-Type Rare Earth Element Deposits
Published:January 01, 2016
Kenzo Sanematsu, Yasushi Watanabe, 2016. "Characteristics and Genesis of Ion Adsorption-Type Rare Earth Element Deposits", Rare Earth and Critical Elements in Ore Deposits, Philip L. Verplanck, Murray W. Hitzman
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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% SiO2, <0.08% P2O5, 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 REE3+ ions move downward in the profile. The REEs are complexed with humic substances, with carbonate and bicarbonate ions, or carried as REE3+ ions in soil and ground water at a near-neutral pH of 5 to 9. The REE3+ 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 (CeO2) as Ce4+ 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.
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Rare Earth and Critical Elements in Ore Deposits
This special volume provides a comprehensive review of the current state of knowledge for rare earth and critical elements in ore deposits. The first six chapters are devoted to rare earth elements (REEs) because of the unprecedented interest in these elements during the past several years. The following eight chapters describe critical elements in a number of important ore deposit types. These chapters include a description of the deposit type, major deposits, critical element mineralogy and geochemistry, processes controlling ore-grade enrichment, and exploration guides. This volume represents an important contribution to our understanding of where, how, and why individual critical elements occur and should be of use to both geoscientists and public policy analysts.
The term “critical minerals” was coined in a 2008 National Research Council report (National Research Council, 2008). Although the NRC report used the term “critical minerals,” its focus was primarily on individual chemical elements. The two factors used in the NRC report to rank criticality were (1) the degree to which a commodity is essential, and (2) the risk of supply disruption for the commodity. Technological advancements and changes in lifestyles have changed the criticality of elements; many that had few historic uses are now essential for our current lifestyles, green technologies, and military applications. The concept of element criticality is useful for evaluation of the fragility of commodity markets. This fragility is commonly due to a potential risk of supply disruption, which may be difficult to quantify because it can be affected by political, economic, geologic, geographic, and environmental variables.
Identifying potential sources for some of the elements deemed critical can be challenging. Because many of these elements have had minor historic usage, exploration for them has been limited. Thus, as this volume highlights, the understanding of the occurrence and genesis of critical elements in various ore deposit models is much less well defined than for base and precious metals. A better understanding of the geologic and geochemical processes that lead to ore-grade enrichment of critical elements will aid in determining supply risk and was a driving factor for preparation of this volume. Understanding the gaps in our knowledge of the geology and geochemistry of critical elements should help focus future research priorities.
Critical elements may be recovered either as primary commodities or as by-products from mining of other commodities. For example, nearly 90% of world production of niobium (Nb) is from the Araxá niobium mine (Brazil), whereas gallium (Ga) is recovered primarily as a by-product commodity of bauxite mining or as a by-product of zinc processing from a number of sources worldwide.