By-Products of Porphyry Copper and Molybdenum Deposits
Published:January 01, 2016
Porphyry Cu and porphyry Mo deposits are large to giant deposits ranging up to >20 and 1.6 Gt of ore, respectively, that supply about 60 and 95% of the world’s copper and molybdenum, as well as significant amounts of gold and silver. These deposits form from hydrothermal systems that affect 10s to >100 km3 of the upper crust and result in enormous mass redistribution and potential concentration of many elements.
Several critical elements, including Re, Se, and Te, which lack primary ores, are concentrated locally in some porphyry Cu deposits, and despite their low average concentrations in Cu-Mo-Au ores (100s of ppb to a few ppm), about 80% of the Re and nearly all of the Se and Te produced by mining is from porphyry Cu deposits.
Rhenium is concentrated in molybdenite, whose Re content varies from about 100 to 3,000 ppm in porphyry Cu deposits, ≤150 ppm in arc-related porphyry Mo deposits, and ≤35 ppm in alkali-feldspar rhyolite-granite (Climax-type) porphyry Mo deposits. Because of the relatively small size of porphyry Mo deposits compared to porphyry Cu deposits and the generally low Re contents of molybdenites in them, rhenium is not recovered from porphyry Mo deposits. The potential causes of the variation in Re content of molybdenites in porphyry deposits are numerous and complex, and this variation is likely the result of a combination of processes that may change between and within deposits. These processes range from variations in source and composition of parental magmas to physiochemical changes in the shallow hydrothermal environment. Because of the immense size of known and potential porphyry Cu resources, especially continental margin arc deposits, these deposits likely will provide most of the global supply of Re, Te, and Se for the foreseeable future.
Although Pd and lesser Pt are recovered from some deposits, platinum group metals are not strongly enriched in porphyry Cu deposits and PGM resources contained in known porphyry deposits are small. Because there are much larger known PGM resources in deposits in which PGMs are the primary commodities, it is unlikely that porphyry deposits will become a major source of PGMs.
Other critical commodities, such as In and Nb, may eventually be recovered from porphyry Cu and Mo deposits, but available data do not clearly define significant resources of these commodities in porphyry deposits. Although alkali-feldspar rhyolite-granite porphyry Mo deposits and their cogenetic intrusions are locally enriched in many rare metals (such as Li, Nb, Rb, Sn, Ta, and REEs) and minor amounts of REEs and Sn have been recovered from the Climax mine, these elements are generally found in uneconomic concentrations.
As global demand increases for critical elements that are essential for the modern world, porphyry deposits will play an increasingly important role as suppliers of some of these metals. The affinity of these metals and the larger size and greater number of porphyry Cu deposits suggest that they will remain more significant than porphyry Mo deposits in supplying many of these critical metals.
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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.