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.
Critical Elements in Alkaline Igneous Rock-Related Epithermal Gold Deposits
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Published:January 01, 2016
Abstract
Alkaline igneous rock-related gold deposits, primarily of Mesozoic to Neogene age, are among the largest epithermal gold deposits in the world. These deposits are a subset of low-sulfidation epithermal deposits and are spatially and genetically linked to small stocks or clusters of intrusions possessing high alkali-element contents. Critical-, near-critical, or energy-critical elements associated with these deposits are F, platinum-group elements (PGEs), rare earth elements (REEs), Te, V, and W. Fluorine and tungsten have been locally recovered in the past, and some other elements could be considered as future by-products depending on trends in demand and supply.
The Jamestown district in Boulder County, Colorado, historically produced F from large lenticular fluoritebearing breccia bodies and Au-Te veins in and adjacent to the Jamestown monzonite stock. Several hundred thousand metric tons (t) of fluorspar were produced. Some alkalic epithermal gold deposits contain tungstenbearing minerals, such as scheelite, ferberite, or wolframite. Small tungsten orebodies adjacent to and/or overlapping the belt of Au telluride epithermal deposits in Boulder County were mined historically, but it is unclear in all cases how the tungsten mineralization is related genetically to the Au-Te stage. Micron-sized gold within deposits in the Ortiz Mountains in New Mexico contain scheelite but no record of tungsten production from these deposits exists.
The most common critical element in alkaline igneous-rock related gold deposits is tellurium, which is enriched (>0.5%) in many deposits and could be considered a future commodity as global demand increases and if developments are made in the processing of Au-Te ores. It occurs as precious metal telluride minerals, although native Te and tetradymite (Bi2Te2S) have been reported in a few localities. Assuming that the Dashigou and adjacent Majiagou deposits in Sichuan province, China, are correctly classified as alkalic-related epithermal gold deposits (exact origin remains unclear), they represent the only primary producers of Te (as tetradymite) from this deposit type.
It is worth noting that some epithermal veins (and spatially or genetically related porphyry deposits) contain high contents of Pt or Pd, or both. The Mount Milligan deposit typically contains >100 ppb Pd, and some values exceed 1,000 ppb. However, owing to the presence of other large known PGE resources in deposits in which PGEs are the primary commodities, it is unlikely that alkaline-related epithermal gold deposits will become a major source of PGEs. Similarly, many epithermal gold deposits related to alkaline rocks have high vanadium contents, but are unlikely to be considered vanadium resources in the future. Roscoelite (V-rich mica) is a characteristic mineral of alkalic-related epithermal deposits and is particularly abundant in deposits in Fiji where it occurs with other V-rich minerals, such as karelianite, Ti-free nolanite, vanadium rutile, schreyerite, and an unnamed vanadium silicate. A few alkaline intrusive complexes that contain anomalous concentrations of gold or were prospected for gold in the past are also host to REE occurrences.The best examples are the Bear Lodge Mountains in Wyoming and Cu-REE-F (±Ag, Au) vein deposits in the Gallinas Mountains in New Mexico, which have REE contents ranging up to 5.6% in addition to anomalous Au.
- alkalic composition
- Asia
- Boulder County Colorado
- British Columbia
- Canada
- Cenozoic
- China
- Colorado
- epithermal processes
- Far East
- gold
- gold ores
- igneous rocks
- mafic composition
- Mesozoic
- metal ores
- metals
- mica group
- mineral deposits, genesis
- minor elements
- Neogene
- New Mexico
- palladium
- platinum
- platinum group
- rare earths
- sheet silicates
- Sichuan China
- silicates
- sulfides
- tellurides
- tellurium
- Tertiary
- tetradymite
- United States
- Western Canada
- Wyoming
- yttrium
- roscoelite
- Ortiz Mountains
- Bear Lodge Mountains
- Gallinas Mountains
- nolanite
- schreyerite
- Jamestown mining district
- Dashigou Deposit
- Mount Milligan Deposit
- Majiagou Deposit