Chapter 7 By-Products of Porphyry Copper and Molybdenum Deposits
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©2016 Society of Economic Geologists, Inc. Reviews in Economic Geology, v. 18, pp. 137–164 Chapter 7 By-Products of Porphyry Copper and Molybdenum Deposits David A. John1,† and Ryan D. Taylor2 1 U.S. Geological Survey, MS 901, 345 Middlefield Rd., Menlo Park, California 94025-3591 2 U.S. Geological Survey, Denver Federal Center, MS 973, Box 25046, Denver, Colorado 80225 Abstract Porphyry Cu and porphyry Mo deposits are large to giant deposits ranging up to >20 and 1.6 Gt of ore, respec- tively, 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 depos- its. 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. Introduction of porphyry Mo deposits, and these deposits are generally Porphyry copper and porphyry molybdenum deposits are subdivided into two end-member types, arc-related (also the world’s largest sources of copper (~60%) and molybde- called quartz monzonite or low fluorine) and alkali-feldspar num (~95%) and commonly contain 100s of million metric rhyolite-granite (AFRG; also called Climax) types (Sillitoe, tons (Mt) to >20 billion metric tons (Gt) of ore (Seedorff et 1980; White et al., 1981; Westra and Keith, 1981; Ludington al., 2005; Sinclair, 2007; Singer et al., 2008; John et al., 2010; et al., 2009; Taylor et al., 2012). Sillitoe, 2010; Taylor et al., 2012). These deposits formed In addition to Cu, Mo, and Au, significant amounts of from large magmatic-hydrothermal systems that affected other elements, including Ag, As, Re, platinum group metals 10s to >100 km3 of upper crustal rocks, thereby resulting (PGMs, especially Pd), Se, and Te, are recovered from some in enormous mass redistribution and local concentration of porphyry Cu deposits (Table 1). Small amounts of W, Sn, Th many elements (Barton, 2010). There is a broad spectrum of and light rare earth elements (REEs) have been recovered types of porphyry Cu deposits ranging from those in which from alkali-feldspar rhyolite-granite porphyry Mo deposits. Cu is the only metal recovered to Au- and/or Mo-rich depos- Due to the large volume of rocks affected by the ore-form- its in which Au and Mo are co- or important by-products ing hydrothermal systems and the large tonnages of ore pro- to porphyry Au deposits in which Au is the major product cessed from these deposits, other elements concentrated in and only minor Cu is recovered (Sillitoe, 2000; Singer et trace quantities may become economic in future years. In this al., 2008). Similarly, there is a spectrum of characteristics chapter, we review the characteristics of porphyry Cu and Mo deposits and discuss by-products and potential by-products † Corresponding author, e-mail, [email protected] from them. We also briefly mention other types of porphyry 137 138 Table 1. By-Products of Porphyry Copper and Porphyry Molybdenum Deposits Commodity Deposit type Location/paragenesis Mineralogy Grade Examples Notes References Silver Porphyry Cu Mostly in central Cu- Mostly in solid solution <0.1 to 21 g/t Batu Haiju, Ballantyne et al. (1998); (Mo-Au) ores in in Cu-Fe sulfides; less Bingham, Butte, Arif and Baker (2004); potassic alteration commonly in electrum, Chuquicamata, Singer et al. (2008) argentite, tetrahedrite- Escondida tennantite, sphalerite, galena, and Ag tellurides Rhenium Porphyry Cu, In molybdenite; Solid solution in 0.01 to 0.6 g/t Bingham, Recovered from flue Giles and Schilling especially conti- higher Re contents of molybdenite Chuquicamata, dust produced by (1972); Berzina et al. nental arc deposits molybdenite at shallower El Teniente, Pebble roasting molybdenite (2005); Sinclair et al. depths in some deposits concentrates (2009); John et al. (in press); Millensifer et al. (2013) Selenium Porphyry Cu Cu-(Mo-Au) sulfide ores Solid solution in Cu 1 to 600 ppm Bingham, Elatsite, Recovered from anode Tomakchieva (2002); and Fe sulfides; (typically <10 ppm) Pebble, Skouriés slimes that typically Gregory et al. (2013) uncommon selenides contain about 7% Se Tellurium Porphyry Cu Cu-(Mo-Au) sulfide ores Au, Ag, and Pd tellurides <0.1 to >100 ppm Bingham, Elatsite, Recovered from anode Economou-Eliopoulos (petzite, hessite, (typically 1-10 ppm) Pebble, Skouriés slimes that typically and Eliopoulos (2000); merenskyite) contain 1-4 % Te Tomakchieva (2002); Gregory et al. (2013) JOHN ANDTAYLOR Platinum Porphyry Cu, Mostly in central Cu-Au Tellurides (merenskyite); <0.1 to 60 ppb Allard, Elatsite, Pd/Pt ranges from Tarkian and Stribrny Group Metals especially island zone in potassic alteration; solid solution in pyrite Pt + Pd Mt. Polley, Mount 0.6 to >20; commonly (1999;) Economou- (Pd and Pt) arc deposits in in late stage pyrophyllite (Pebble) Milligan, Pebble, Santo only Pd reported Eliopoulos (2005;) alkaline rocks alteration at Pebble Tomas II, Skouriés Gregory et al. (2013) Arsenic Porphyry Cu Commonly in advanced Enargite-luzonite, Highly variable; Bingham, Butte, Recovered from Meyer et al. (1968); argillic alteration in tennantite-tetrahedrite, Schwartz (1995) Chuquicamata smelter flue dust; Schwartz (1995); upper and outer parts of arsenopyrite; solid reports 300 to 2000 considered an Ossandón et al. (2001); deposits and in late stage solution in pyrite, ppm in Cu ore environmental hazard Singer et al. (2008) veins chalcopyrite, bornite Zinc Porphyry Cu Advanced argillic Sphalerite; solid >8,000 ppm as Butte, Mostly with enargite in Meyer et al. (1968); alteration in upper and solution in tennantite sphalerite overgrowths Chuquicamata “Main stage” veins at Ossandón et al. (2001) outer parts of deposits on Cu sulfides at Butte and late veins and in late stage veins Chuquicamata at Chuquicamata Tungsten Alk. granite and arc- Late-stage veins Scheelite, wolframite, 0.1 to 0.3% W in Climax, Endako, Porphyry W deposits Sinclair (2007); related porphyry (porphyry Mo deposits); huebnerite porphyry W-Mo; Pine Nut, Sunrise Singer et al. (2008) Mo, porphyry Cu breccia pipes inferred 0.02 to 0.06% WO3 related to porphyry (porphyry Cu breccias) Cu deposits Bismuth Arc-related Late-stage, upper ore Bismuthinite, aikinite, <150 ppm Davidson, Endako, Treated as an impurity Noble et al. (1995); porphyry Mo zones associated with W native bismuth Koktenkol, Pidgeon in some ores such as Mazurov (1996) at Endako Uranium Porphyry Cu Cu-(Mo-Au) ores and Unknown Unknown Bingham, At Bingham recovered Dahlkamp (2009) in exotic Cu ore Chuquicamata from Cu-leach liquor (Mina Sur exotic that averaged 8 to Cu), Twin Buttes 12 ppm U Tin Alk. Granite Sericitic alteration, Cassiterite 0.2 to 0.5% Sn in Climax, porphyry Primary commodity in Ohta (1995) porphyry Mo; paragenetically related porphyry Sn deposits Sn deposits of porphyry Sn deposits porphyry Sn to W at Climax Bolivia and Japan Table 1. (Cont.) Commodity Deposit type Location/paragenesis Mineralogy Grade Examples Notes References Cerium, Alk. Granite Unknown, possibly Monazite 0.005% monazite in Climax Monazite is a relatively Overstreet (1967); LREE porphyry Mo and derived from the ore at Climax abundant accessory Wallace et al. (1968) porphyry Sn alkaline host intrusion mineral in alkaline plutons Indium Porphyry Mo-W Late-stage in veins Solid solution in Trace to 280 ppm; Mount Pleasant, Mostly in sphalerite; Briskey (2005); and breccia sphalerite, chalcopyrite up to ~7 wt % in Bingham lesser amounts in Sinclair et al.