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Mineralogical Profile of Supergene Sulfide Ore in the Western Copper Area, Morenci Mine, Arizona

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Mineralogical Profile of Supergene Sulfide Ore in the Western Copper Area, Morenci Mine, Arizona 391 The Canadian Mineralogist Vol. 57, pp. 391-401 (2019) DOI: 10.3749/canmin.1800020 MINERALOGICAL PROFILE OF SUPERGENE SULFIDE ORE IN THE WESTERN COPPER AREA, MORENCI MINE, ARIZONA § BENJAMIN N. SCHUMER * University of Arizona, Department of Geosciences, 1040 E. 4th Street Tucson, Arizona 85721 USA RALPH J. STEGEN Freeport McMoRan, Inc., 10861 N. Mavinee Dr. Ste. 141, Oro Valley, Arizona 85737 USA MARK D. BARTON University of Arizona, Department of Geosciences 1040 E. 4th Street Tucson, Arizona 85721 USA J. BRENT HISKEY University of Arizona, Department of Mining and Geological Engineering 1235 James E. Rogers Way, Tucson, Arizona 85721 USA ROBERT T. DOWNS University of Arizona, Department of Geosciences 1040 E. 4th Street Tucson, Arizona 85721 USA ABSTRACT The intergrowths and compositions of supergene copper sulfide minerals from drill hole MOR-4511 in the Western Copper area of the Morenci mine, Greenlee County, Arizona, have been examined by reflected light microscopy and electron probe microanalysis (EPMA) to better understand the formation of supergene sulfides with implications for hydrometallurgical processing. The supergene copper sulfides occur in three main textures: partial to complete replacement of chalcopyrite, partial replacement of pyrite, and partial to complete replacement of one another. Compositions of copper sulfides vary widely, but (CuþFe):Sratiosof1.806 0.05, 1.92 6 0.03, and 1.10 6 0.10 are dominant. No stoichiometric Cu2S was found. At shallower depths in the supergene blanket and near/within faults, high (CuþFe):S phases (with ratios of 1.80 6 0.05 and 1.92 6 0.03) replacing primary chalcopyrite and pyrite or lower (CuþFe):S supergene sulfides are dominant, and near the base of the blanket low (CuþFe):S phases (with ratios of 1.10 6 0.10) replacing primary chalcopyrite or higher (CuþFe):S supergene sulfides gradually become more dominant. This indicates high concentration of Fe3þ,Fe2þ, and Cu2þ, necessary to form high (CuþFe):S phases, at shallower depths and near sources of unreacted fluid, such as faults. Formation of low (CuþFe):S phases directly from chalcopyrite or from high (CuþFe):S phases could be controlled by decreased concentrations of iron species and Cu2þ due to reaction with primary chalcopyrite and pyrite as fluids descend or migrate away from faults, reduced access to supergene fluids, and/or lower pyrite-chalcopyrite ratios. The compositional patterns of supergene copper sulfide minerals observed at Morenci are similar to those observed in other supergene enrichment blankets of porphyry copper systems worldwide and are even more similar to compositions seen in leaching experiments of synthetic copper and copper-iron sulfides. Keywords: chalcocite, copper sulfides, supergene enrichment, porphyry copper, electron microprobe analysis, Morenci, Arizona. § Corresponding author e-mail address: [email protected] * Present affiliation: Kappes, Cassiday and Associates, 7950 Security Cir. Reno, Nevada 89506 USA Downloaded from https://pubs.geoscienceworld.org/canmin/article-pdf/57/3/391/4699700/i1499-1276-57-3-391.pdf by University of Arizona user on 29 May 2019 392 THE CANADIAN MINERALOGIST INTRODUCTION TABLE 1. MINERALS IN THE SYSTEM Cu–S Porphyry copper deposits contain some of the Name Formula Cu:S largest endowments of copper on Earth. They are Chalcocite Cu2S2 characterized by large tonnages of relatively low- Djurleite Cu31S16 1.9375 grade Cu-bearing minerals in vein stockworks and Digenite* Cu9S5 1.8 disseminations within hydrothermally altered rock Roxbyite Cu9S5 1.8 (e.g., Seedorff et al. 2005). Supergene processes Anilite Cu7S4 1.75 acting initially on primary low-grade ore in exhumed Geerite Cu8S5 1.6 porphyry systems produce diverse sulfide, oxide, and Spionkopite Cu39S28 1.39 oxysalt copper phases to form supergene enrich- Yarrowite Cu9S8 1.125 ments. It is common for subsequent supergene cycles Covellite CuS 1 to leach and enrich the prior-formed supergene Villamanı´nite CuS2 0.5 deposits to produce masses of rock with chalcocite Note: * Digenite is stable only in the system Cu–Fe–S. and oxide copper having significantly higher grade Bold entries have known crystal structures. Data for than the primary ore (Anderson 1982, Titley & villamanı´nite are from Bayliss (1989), all other data are Marozas 1995, Chavez 2000, Sillitoe 2005). Pro- from Anthony et al. (1990). cessing of supergene sulfide ‘‘chalcocite’’ ores is typically carried out via concentration or leaching whereby the copper oxides, such as chrysocolla, are recovered by solvent-extraction-electrowinning (SX- EW) methods. descriptions of supergene copper sulfides in the Mineralogical characterization of supergene pro- southwestern United States generally consist of cesses in copper deposits is exceedingly important for varying proportions of chalcocite and covellite that mineral processing methods and recovery determina- replaced chalcopyrite and pyrite. One example is the tions. Supergene sulfide deposits often contain a porphyry copper-molybdenum deposit at Morenci, mixture of minerals in the Cu–S system (Table 1). Arizona, where a general overview of supergene The Cu-sulfide minerals typically occur in varying copper sulfide mineralogy was provided by Enders amounts with hypogene sulfides consisting principally (2000), but a detailed mineralogical study of super- of pyrite, chalcopyrite, bornite, and molybdenite, gene enrichment is lacking. which each react differently in processing methods The Morenci mine in Greenlee County, Arizona using sulfuric acid-ferric sulfate leach solutions. (Fig. 1), produces copper from supergene and Leaching studies of chalcopyrite (e.g., Cordoba´ et al. hypogene deposits. Historically, the highest-grade ores 2008) and other copper bearing minerals using acidic at Morenci were oxide copper deposits within skarn Fe3þ solutions determined relative recovery amounts and massive chalcocite from enriched veins and specific to individual minerals that have broad stockworks in monzonitic stocks. With the depletion application to processes used in modern heap-leach of these high-grade ores, mining progressed to open- operations (Sullivan 1933, Goble 1981, Whiteside & pit methods to recover copper from the immense Goble 1986). tonnages of relatively lower-grade chalcocite using However, there are comparatively few detailed concentrators. The implementation of SX-EW tech- mineralogical studies of natural supergene copper nology in the late 1980s at Morenci provided for the sulfides. Sillitoe & Clark (1969) studied the supergene recovery of copper from oxide copper and low-grade copper and copper-iron sulfides in the Copiapo´ mining chalcocite that is not of sufficient grade for concen- district, Chile, and Goble & Smith (1973) conducted trating. Morenci currently recovers copper from an electron microprobe study on copper sulfides in red concentrating, heap leaching of chalcocite, and run- bed copper deposits of Alberta. Reich et al. (2010) of-mine leaching of oxide copper and chalcocite. The used electron microprobe, secondary ion mass spec- reserves at Morenci total 12.1 billion pounds of trometry, and transmission electron microscopy to recoverable copper from 664 metric tonnes of mill investigate trace element chemistry of supergene ore grading 0.42% Cu, 356 million metric tonnes of digenite in Chilean copper deposits. Reich & heap leach averaging 0.51% Cu, and 2243 million Vasconcelos (2015) provided a summary of supergene metric tonnes of run-of-mine leach with an average processes and mineralogy in copper deposits, and grade of 0.18% Cu (Freeport McMoRan 2016 Form Zammit et al. (2015) described the roles of microbial 10-K Report). action in supergene processes that lead to the This study was undertaken to aid ore processing at formation of enrichment blankets. Mineralogical Morenci and to better understand the mineralogy of Downloaded from https://pubs.geoscienceworld.org/canmin/article-pdf/57/3/391/4699700/i1499-1276-57-3-391.pdf by University of Arizona user on 29 May 2019 MINERALOGY OF SUPERGENE SULFIDES, MORENCI MINE 393 2þ þ 2À FeS2 þ 7=2O2 þ H2O ! Fe þ 2H þ 2SO4 ð1Þ 2þ þ 3þ Fe þ 1=4O2 þ H ! Fe þ 1=2H2O ð2Þ 3þ 2þ 2þ CuFeS2 þ 16Fe þ 8H2O ! Cu þ 17Fe þ 2À þ 16H þ 2SO4 ð3Þ Copper-rich solutions migrate downward and precip- itate copper sulfide minerals by reaction with hypo- gene and/or other supergene sulfide minerals lower in the deposit. The simplified supergene profile (Fig. 2) consists of two zones: (1) a barren, leached zone in which hypogene sulfides were oxidized and much of the copper mobilized, leading to very low copper grades occurring with varying amounts and types of iron oxides; the first zone caps (2) a zone of enrichment in which supergene sulfides replace hypogene sulfides (Titley & Marozas 1995). Oxide copper zones that include chrysocolla, brochantite, tenorite, and native copper with hematite and/or goethite occur within or proximal to the leached zone and are the product of in situ oxidation of hypogene or supergene sulfide minerals that likely formed as a result of low pyrite–chalcopyrite ratios, reactive host rocks, and other factors which inhibit the leaching and FIG. 1. Location map, from Enders et al. (2006). Western transport of copper (Chavez 2000). Copper (WC) is at the middle right in the lower right hand Additionally, bacterial action is thought to play an inset. essential role in supergene processes. Acidithiobacillus bacteria, found by Enders et al. (2006) in the 5200 bench of the Metcalf area at Morenci, catalyze the normally very slow process of abiotic pyrite oxidation, supergene sulfide minerals and the natural processes resulting in a rate five orders of magnitude faster than governing the formation of supergene copper sulfides. abiotic oxidation alone [reactions (4) and (5)]: The results from this study also provide mineralogical 2þ þacidithiobacillus 3þ characteristics that could be useful in leach processing Fe þ 1=4O2 þ H ! Fe þ 1=2H2O and recovery determinations. ð4Þ SUPERGENE PROCESSES IN PORPHYRY COPPER DEPOSITS 3þ 2þ FeS2 þ 14Fe þ 8H2O þ Fe 2þ 2À þ The formation of enriched sulfide ore in porphyry ! 15Fe þ 2SO4 þ 16H ð5Þ systems is broadly dependent upon hypogene ore mineralogy, lithology of host rocks, and character of According to Enders et al.
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