Leached Cap Research Articles

Cu isotope patterns of whole rocks in the Kerman porphyry copper belt, southeastern Urumieh Dokhtar magmatic arc, Iran

This study presents copper isotope compositions of mineralized whole-rock samples from the Kerman porphyry copper belt (KPCB) southeast of the Urumieh Dokhtar magmatic arc. Samples for this study were specifically collected from the Sar Cheshmeh, Meiduk, Iju, SarKuh, Darreh Zar, Bagh Khoshk, and Jebal Barez deposits and investigated to study the application of Cu isotopes to mineral exploration. While in the leached cap zone of the deposits, δ65Cu values range between −3.41 to 5.82 ‰ (avg. 0.42 ‰), in the supergene enriched zone of these deposits the relatively higher δ65Cu content ranges from 5.18 to 8.71 ‰ (avg. 7.17 ‰), and in the hypogene zone, the intermediate δ65Cu values range from 1.49 to 7.31 ‰ (avg: 4.36 ‰). Most measured δ65Cu values in these investigated porphyry copper deposit are positive, which indicates the presence of a Cu − enriched zone. In the enriched leached cap zones, the δ65Cu of the Darreh Zar and Sar Cheshmeh deposits are overall higher relative to the Iju, Meiduk, SarKuh, and Bagh Khoshk deposits, and these higher δ65Cu values are associated with the highest Cu grade and tonnage in the Sar Cheshmeh and Darreh Zar deposits. It is therefore plausible to conclude that the higher δ65Cu value in the leached areas are diagnostic of the high concentration of copper.These findings are complemented by the presence of Cu(II) carbonates, silicates, and Fe − oxides in each sample that has resulted in the enrichment of 65Cu, relative to the remnant sulfides with a wider copper isotope range. In addition, there is a general shift toward higher δ65Cu isotope values relative to the inferred precursor minerals. The overall apparent weathering and oxidative dissolution is likely to have generated isotopically heavier fluids and lighter residual minerals. The elevated δ65Cu of the bulk samples from porphyry Cu deposits in KPCB is consistent with a preferential loss of 63Cu into fluids during the segregation of aqueous fluid–melt. This is best explained by several pulses of hypogene magmatic fluid which led to repeated enrichment in 65Cu in the magmatic system. It is plausible to conclude that copper isotope values in these mineralized samples can be used as an effective exploration tool to identify buried porphyry Cu systems.

Supergene Turquoise and Associated Phosphate Minerals of the Porphyry-Lode System at Butte, Montana, USA

ABSTRACTWeathering processes superimposed onto exhumed hydrothermal ore deposits in western North America have developed secondary mineral assemblages that inform the near-surface evolution of these systems within the context of Basin and Range extension. The occurrence of the secondary phosphate mineral turquoise [CuAl6(PO4)4(OH)8·4H2O] in the weathering profile of Laramide porphyry copper deposits is widely documented, although previous studies on the composition and distribution of turquoise are largely restricted to the archaeological literature. In this study, we use the world-class Butte porphyry-epithermal system to study the occurrence, paragenesis, and mineral chemistry of turquoise and related phosphates in the weathering profile of the deposit. Field observations, mineral textures by optical microscopy, electron microprobe analyses, and geochemical modeling show that blue, Al-endmember turquoise formed exclusively at or immediately below the pre-mining water table, within the chalcocite enrichment blanket and in the absence of significant FeIII. At higher structural levels above the pre-mining water table, green FeIII-bearing turquoise [Cu(Al,FeIII)6(PO4)4(OH)8·4H2O] is commonly intergrown with jarosite and/or tinticite at the micron scale. We show that jarosite, tinticite, and FeIII-bearing turquoise formed at the expense of pre-existing, Al-endmember turquoise after extensional faulting caused a relative lowering of the water table in the eastern fault block of the deposit. The low solubility of the FeIII-phosphates tinticite and strengite in the leach cap environment suggests that it is unlikely that these minerals formed directly from oxidized meteoric water. Electron probe microanalysis documented the presence of As and F in blue Al-endmember turquoise and Cl in green FeIII-bearing turquoise. The presence of Cl in FeIII-bearing turquoise may represent an evaporitic signal associated with basin development as the Butte district was exhumed. The paragenesis of Cu- and Fe-phosphate minerals provides another tool with which to interrogate spatial and temporal relationships in the near-surface evolution of porphyry copper systems.

Hydrogeochemistry in the Yukon-Tanana upland region of east-central Alaska: Possible exploration tool for porphyry-style deposits

A hydrogeochemical study using high resolution ICP-MS was undertaken at the Taurus and other porphyry Cu–Mo(-Au) occurrences and Ag–Au–Cu (+/− Pb, Zn) occurrences with epithermal-style characteristics in the Yukon-Tanana upland region of eastern Alaska. Surface water samples were collected from 30 sites on creeks that drain known deposits and occurrences and surrounding presumably unmineralized areas. Water samples for the entire ~9 km length of McCord Creek, which drains the Taurus deposit, and those from streams draining the areas at and near the Bluff and Dennison porphyry occurrences have high conductivity values (492–1250 μS/cm) and consistently high concentrations of B (3–250 μg/L), Co (2.3–42 μg/L), Mn (339–4750 μg/L), Re (0.012–0.1 μg/L), and SO42− (>200 mg/L), all of which are well above the median value for this data set and significantly greater than concentrations in water samples from the unmineralized areas. These are the best pathfinder elements specifically for porphyry style deposits because most of them are not anomalous in waters near epithermal occurrences. Copper concentrations are high (up to 115 μg/L) in some low-pH water samples from McCord Creek and drainages around Bluff, and a few near neutral pH waters have high molybdenum (>1 μg/L), but neither element is consistently anomalous in close vicinity to the porphyry occurrences, possibly due to a metal-poor, sulfide-poor leached cap (average of ~50 m) that overlies supergene and hypogene mineralized zones and is the dominant rock at surface. High concentrations of Bi and/or As occur in many waters associated with mineralized areas, particularly the Bluff and Dennison occurrences. In general, the element associations related to porphyry deposits reflect the deposit mineralogy, as well as size of the footprint related to alteration and mineralization.

Copper Isotopic Perspectives on Supergene Processes: Implications for the Global Cu Cycle

Research Article| October 01, 2015 Copper Isotopic Perspectives on Supergene Processes: Implications for the Global Cu Cycle Ryan Mathur; Ryan Mathur 1Department of Geology, Juniata CollegeHuntingdon, PA 16652, USAE-mail: mathurr@juniata.edu Search for other works by this author on: GSW Google Scholar Matthew S. Fantle Matthew S. Fantle 2Department of Geosciences, Penn State UniversityUniversity Park, PA 16802, USAE-mail: mfantle@psu.edu Search for other works by this author on: GSW Google Scholar Author and Article Information Ryan Mathur 1Department of Geology, Juniata CollegeHuntingdon, PA 16652, USAE-mail: mathurr@juniata.edu Matthew S. Fantle 2Department of Geosciences, Penn State UniversityUniversity Park, PA 16802, USAE-mail: mfantle@psu.edu Publisher: Mineralogical Society of America First Online: 09 Mar 2017 Online Issn: 1811-5217 Print Issn: 1811-5209 © 2015 by the Mineralogical Society of America Elements (2015) 11 (5): 323–329. https://doi.org/10.2113/gselements.11.5.323 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Ryan Mathur, Matthew S. Fantle; Copper Isotopic Perspectives on Supergene Processes: Implications for the Global Cu Cycle. Elements 2015;; 11 (5): 323–329. doi: https://doi.org/10.2113/gselements.11.5.323 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyElements Search Advanced Search Abstract A compilation of copper isotopic compositions (δ65Cu) from supergene systems suggests distinct differences in the mean δ65Cu of Cu in leach cap (δ65Cu = −1.2 ± 3.5‰), enrichment zone (mean δ65Cu = +1.2 ± 4.2‰), and fluids (mean δ65Cu = +0.9 ± 1.3‰) relative to the high-temperature sulfides that comprise the primary ore (δ65Cu = +0.1 ± 0.6‰). These isotopic differences can be explained by the oxidative dissolution of primary ore minerals, such as chalcopyrite, and the subsequent precipitation of oxides in the near-surface system and of sulfides at depth. A dynamic mass balance model predicts the observed Cu isotopic compositions of the Cu reservoirs in nature and constrains the temporal isotopic evolution of supergene systems. From the model, these systems isotopically evolve to substantial extents over 500 ka to 5 Ma time scales. In relatively closed systems, percent-level loss of Cu from the solid (with δ65Cu values >>0‰) is possible, suggesting that supergene systems are important components of the global Cu cycle. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Gossans and Leached Capping; Field Assessment

Gossans and Leached Capping; Field Assessment. Roger Taylor. Pp. 146. 2011. Springer-Verlag. ISBN 978-3-642-22050-0. Price US$129.00. This small hardcover volume, devoted to weathered sulfide orebodies manifested in gossans and leached capping, contains concise and brief scientific descriptions of the weathering processes, accompanied by an extensive set of photographs. It is one more in the long series of pamphlets and books published by the author and his colleagues, first through the Economic Geology Research Unit at James Cook University (Townsville, Queensland Australia) and now through major publishing houses. The current volume focuses on the field recognition and interpretation of weathered ore deposits without the utilization of any of the multitude of currently available high-tech analytical tools. As the author states, it is an attempt to improve upon the seminal early books on leach capping interpretations led by Blanchard (1968), who produced the bible on leached cap interpretation. The current book focuses on observations in outcrop and through the hand lens, at scales from outcrops to hand specimen. The book contains eight chapters, including in each a concise discussion of the topic in the chapter, followed by photographic plates with adequate description such that the …

Use of Cu isotopes to distinguish primary and secondary Cu mineralization in the Cañariaco Norte porphyry copper deposit, Northern Peru

A significant proportion of the copper in the Canariaco Norte porphyry copper deposit in northern Peru occurs in chalcocite and covellite-rich veins and disseminations that exist from the surface to depths greater than 1 km. The overall range of Cu isotopic ratios of 42 mineral separates from Canariaco varies from −8.42 to 0.61 ‰, with near-surface chalcocite and Fe oxides having isotopically depleted values compared to chalcocite, covellite, and chalcopyrite from deeper levels. The majority (34 of 36) of measured Cu sulfides have a typical hypogene copper isotope composition of δ65Cu = 0.18 ± 0.38 ‰, with no enriched isotopic signature existing in the Canariaco Norte sulfide data. Thus, the copper isotope data indicate that most of the chalcocite and covellite formed from high-temperature hypogene mineralization processes and that only a minor portion of the deposit is enriched by supergene processes. The nonexistence of an enriched δ65Cu reservoir suggest the presence of an undiscovered lateral/exotic Cu occurrence that enriched 65Cu that remained in solution during weathering. Regardless of the cause, the comparative analysis of the Cu isotope dataset reveals that little exploration potential for an extensive supergene enrichment blanket exists because the weathering history at Canariaco Norte was not conducive to preservation of enriched Cu at depth beneath the leach cap.

Exploration Applications of Copper Isotopes in the Supergene Environment: A Case Study of the Bayugo Porphyry Copper-Gold Deposit, Southern Philippines

Copper isotope compositions of secondary phases from the supergene profile of the Bayugo porphyry copper-gold deposit (Philippines) show significant isotopic fractionation relative to hypogene sulfide phases and reveal systematic patterns of isotopic enrichment in the leached cap iron oxides and supergene copper sulfides. Thirty-nine iron oxide, copper oxide, and copper sulfide minerals yielded δ 65 Cu values between −3.4 and +6.9 per mil. Samples of hypogene sulfides have a narrow range of values near zero per mil (−0.6 to +1.0‰). In contrast, samples of supergene sulfides (chalcocite and djurleite) collected from the exotic and enriched copper zones define a trend of decreasing δ 65 Cu from the enriched source area and proximal exotic zone (>3‰) downgradient to the distal portions ( These findings are significant, because within exotic copper zones the copper isotope values of supergene sulfides can provide an indication both of proximity and direction to the source area. The technique could be applied in the search for unrecognized porphyry centers upgradient from headless exotic copper occurrences, by exploring for the supergene copper phases with the highest δ 65 Cu values. Independently, copper isotope ratios of iron oxides in surface samples or fossil leached caps might be used to rank prospects and to focus drilling in areas with the greatest potential for mature enrichment profiles, by mapping the distribution of isotopically enriched leached cap iron oxides. The leached cap screening and targeting tool could enhance the iron oxide mapping techniques traditionally applied in leached cap interpretation and exploration.

Patterns in the Copper Isotope Composition of Minerals in Porphyry Copper Deposits in Southwestern United States

Cu isotope measurements of minerals from American southwest porphyry Cu deposits reveal a positive relationship with the grade of Cu and display spatial patterns. We measured the Cu isotope composition of 87 minerals contained within veins from drill core and hand specimen samples from three deposits (the Silver Bell, Ray, and Morenci mines). The overall range of Cu isotope ratios varies from −13.5 to +8.3 per mil. The deposits possess three different Cu reservoirs in which each has unique Cu isotope signatures, where δ65Cu for leach cap minerals < hypogene minerals < supergene enrichment minerals. The pattern exists in all three deposits and resulted from oxidative weathering of Cu at surface and reprecipitation of Cu at depth. A Rayleigh distillation model mimics the Cu isotope patterns found in the leach cap and quantifies the degree of leaching. A comparison of the isotopic composition of the three reservoirs among the three deposits reveals distinct differences—the Silver Bell mine possesses leach cap minerals that are more depleted in 65Cu and enrichment minerals that are more enriched in 65Cu than both the Morenci and Ray mines. A possible explanation for the difference among the deposits could indicate that the Silver Bell deposit experienced a greater degree of leaching than the Ray and Morenci deposits. The comparative analysis of the Cu isotope data reveals the exploration potential of using Cu isotope fractionation in the supergene environments.

A comparative copper isotope study of porphyry copper deposits in Iran

ABSTRACT Copper isotope ratios and Cu concentrations have been measured in oxide and sulphide minerals from leached cap (hematite-goethite-jarosite-malachite), supergene (chalcocite-pyrite-hematite) and hypogene (pyrite-chalcopyrite) zones in the Chahfiroozeh, Darrehzar, Iju and Parkam porphyry Cu deposits from the Kerman porphyry Cu belt of Iran. In general, δ 65 Cu values show distinctive values of 0.23‰ to 0.79‰ in hypogene, 2.52‰ to 4.82‰ in supergene and -1.00‰ to -6.16‰ in leached cap minerals. Leach cap minerals contain less Cu than supergene and hypogene minerals. This study supports previous distinct Cu isotopic patterns for leached cap, supergene and hypogene zones reported from other porphyry Cu deposits. Weathering and oxidative dissolution of Cu sulphide minerals normally generate isotopically heavier fluids and lighter residual minerals. Upon encountering reducing conditions at greater depths, the oxidative solutions precipitate secondary Cu-sulphides in the enriched zone and the new minerals possess heavier Cu isotopic compositions. The degree of isotopic separation and difference in concentrations among the three reservoirs is most likely related to the weathering processes and the isotopic signatures present in the leach cap may have exploration potential.

Exploration potential of Cu isotope fractionation in porphyry copper deposits

We examined the copper isotope ratio of primary high temperature Cu-sulfides, secondary low temperature Cu-sulfides (and Cu-oxides) as well as Fe-oxides in the leach cap, which represent the weathered remains of a spectrum of Cu mineralization, from nine porphyry copper deposits. Copper isotope ratios are reported as δ 65Cu‰ = (( 65Cu/ 63Cu sample/ 65Cu/ 63Cu NIST 976 standard) − 1) ⁎ 10 3. Errors for all the analyses are ± 0.14‰ (determined by multiple analyses of the samples) and mass bias was corrected through standard-sample-standard bracketing. The overall isotopic variability measured in these samples range from − 16.96‰ to 9.98‰. Distinct Cu isotopic reservoirs exist for high temperature hypogene, enrichment, and leach cap minerals. Chalcopyrite from high temperature primary mineralization forms a relatively tight cluster of δ 65Cu values of 1‰ to − 1‰ whereas secondary minerals formed by low temperature reveal a range of δ 65Cu values from − 16.96‰ to 9.98‰. Secondary chalcocite is relatively heavy with δ 65Cu varying from − 0.3‰ to 6.5‰. Leach cap minerals dominated by Fe-oxides (jarosite, hematite and goethite) are relatively light ranging from − 9.9‰ to 0.14‰. Although the dataset is relatively small ( n = 50 total minerals analyzed), a combination of these data with values from previously published reports [Zhu, X.K., O'Nions, R.K., Guo, Y., Belshaw, N.S. and Rickard, D., 2000. Determination of natural Cu-isotope variation by plasma-source mass spectrometry; implications for use as geochemical tracers. Chemical Geology, 163(1–4): 139–149.; Larson et al., 2003; Mathur, R. et al., 2005. Cu isotopic fractionation in the supergene environment with and without bacteria. Geochimica et Cosmochimica Acta, 69(22): 5233–5246.; Markl et al., 2006, and Maher et al., 2007], show a distinct pattern of heavier isotopic signature in supergene samples and a lighter isotopic signature exists in the leach cap and oxidation zone minerals. The pattern could be used as a tool for exploration geology by providing the following information: 1) Identification of highly fractionated copper isotope ratios in copper sulfide and Fe-oxide samples that indicate supergene processes and the extent of leaching and enrichment copper 2) Identification of highly fractionated copper isotope ratios in surface and/or groundwaters that indicate the active weathering copper sulfides that experienced significant enrichment .

The El Galeno and Michiquillay porphyry Cu–Au–Mo deposits: geological descriptions and comparison of Miocene porphyry systems in the Cajamarca district, northern Peru

El Galeno and Michiquillay are early to middle Miocene Cu–Au–Mo porphyry-related deposits located in the auriferous Cajamarca district of northern Peru. The El Galeno deposit (486 Mt at 0.57% Cu, 0.14 g/t Au and 150 ppm Mo) is associated with multiple dioritic intrusions hosted within Lower Cretaceous quartzites and shales. Emplacement of the porphyry stocks (17.5–16.5 Ma) in a hanging wall anticline was structurally controlled by oblique faults superimposed on early WNW-trending fold-thrust structures. Early K-feldspar–biotite–magnetite (potassic) alteration was associated with pyrite and chalcopyrite mineralisation. A quartz–magnetite assemblage that occurs at depth has completely replaced potassically altered rocks. Late- and post-mineralisation stocks are spatially and temporally related to weak quartz–muscovite (phyllic) alteration. High Au grades are associated with early intrusive phases located near the centre of the deposit. Highest Cu grades (~0.9% Cu) are mostly associated with a supergene enrichment blanket, whilst high Mo grades are restricted to contacts with the metasedimentary rocks. The Michiquillay Cu–Au–Mo deposit (631 Mt at 0.69% Cu, 0.15 g/t Au, 100–200 ppm Mo) is associated with a Miocene (20.0–19.8 Ma) dioritic complex that was emplaced within the hanging wall of a back thrust fault. The intrusive complex is hosted in quartzites and limestones. The NE-trending deposit is crosscut by NNW-trending prospect-scale faults that influenced both alteration and metal distribution. In the SW and NE of the deposit, potassic alteration zones contain moderate hypogene grades (0.14 g/t Au and 0.8% Cu) and are characterised by chalcopyrite and pyrite mineralisation. The core of the deposit is defined by a lower grade (0.08 g/t Au and 0.57% Cu) phyllic alteration that overprinted early potassic alteration. Michiquillay contains a supergene enrichment blanket of 45–80 m thickness with an average Cu grade of 1.15%, which is overlain by a deep leached cap (up to 150 m). Cu–Au–Mo (El Galeno-Michiquillay) and Au-rich (Minas Conga) deposits in the Cajamarca region are of similar age (early–middle Miocene) and intrusive rock type (dioritic) associations. Despite these geochronological and geochemical similarities, findings from this study suggest variation in metal grade between the hybrid-type and Au-rich deposits result from a combination of physio-chemical factors. These include variations in temperature and oxygen fugacity conditions during hypogene mineralisation resulting in varied sulphide assemblages, host rock type, precipitation of ubiquitous hydrothermal magnetite, and late hydrothermal fluid flow resulting in a well-developed phyllic alteration zone.

Hydrothermal alteration and physical volcanology of Archean rocks in the vicinity of the Headway-Coulee massive sulfide occurrence, Onaman area, northwestern Ontario

The Headway-Coulee Zn-Pb-Cu-Ag prospect occurs as a series of 13 lenses of vein and disseminated-type sulfides which are associated with hydrothermally altered Archcan mafic lavas and felsic hydroclastic rocks. Mafic lavas form a 4- to 6-km-wide sequence that is locally interrupted by a narrow, lens-shaped horizon (30-200 m X 2 km) of thin to thickly bedded ash-rich tuffs. The felsic tuffs are subdivided into two units which represent two periods of explosive activity.The felsic tuffs and many of the mafic lavas have undergone hydrothermal alteration during the waning stages of explosive activity. Alteration zones and their metamorphosed equivalents both crosscut and are conformable to stratigraphy; they have been divided into six mineralogically and chemically distinct types: least altered, calcite, sericite, iron chlorite, chloritoid, and kyanite.Least altered rocks underlie approximately 50 percent of the area, are all mafic in composition, and have been affected primarily by greenschist facies metamorphism. Calcite-type alteration is the earliest hydrothermal event recognized in the volcanic rocks and forms a 600- by 1,600-m zone which crosscuts stratigraphy. Calcite-rich rocks grade laterally into least altered or sericite-rich rocks; they were formed by the addition of CO 2 and the minor loss of Na, Fe, and Mg whereas Ca remained relatively immobile. Sericite-type alteration postdates calcite alteration and occurs primarily as elongate lenses (up to 1,000 m) within felsic tuffs; it also occurs in anomalous amounts within iron chlorite and chloritoid-rich mafic lavas. Prior to formation of chlorite and chloritoid the sericite alteration zone was 3,000 by 100 to 1,200 m in size and crosscut the stratigraphy. Sericite is thought to have formed from a low-temperature, evolved seawater fluid which circulated only in the very upper portions of the hydrothermal system. At low water/rock ratios this fluid became enriched in K and Si and was heated enough to rise buoyantly along fractures and more permeable lithological units. Water-rock interactions formed sericite and quartz at the expense of plagioclase feldspar.Iron chlorite-type alteration and its metamorphosed equivalent (chloritoid-type alteration) crosscut both calcite and sericite alteration and chlorite replaces sericite within both mafic and felsic rocks. Within felsic tuffs this alteration is characterized by the addition of Fe and Mn and the loss of Na, K, Ca, Si, and CO 2 ; volume may decrease by as much as 30 percent. Within mafic lavas, however, the alteration is characterized by addition of Fe with minor Ca and loss of K and CO 2 ; there was negligible volume change. Iron chlorite is believed to have formed from an evolved, high-temperature seawater fluid which circulated at depth. Interaction with mafic rocks at low water/rock ratios enriched the fluid in Fe, Mn, H (super +) while depleting it in Ca and Na. This acidic and base metal-rich fluid, as it neared the surface, interacted with sericite and calcite-rich rocks to form iron chlorite, iron carbonate, and a hydrous aluminum silicate phase (pyrophyllite?). Within the discharge channel, close to the water-rock interface, the solution extensively leached the ash tuffs to produce a rock composed dominantly of SiO 2 and hydrous aluminum silicate phases. This rock represents a leached cap to the hydrothermal system and a local discharge site. Later greenschist facies metamorphism converted the quartz-hydrous aluminum silicate assemblage to kyanite-quartz; and the iron chlorite plus hydrous aluminum silicate assemblage to chloritoid. Base metals were precipitated below and/or immediately above the discharge site.

A thermodynamic approach to the understanding of the supergene alteration at the Afton copper mine, south-central British Columbia

A thermodynamic reconstruction of the supergene alteration at the Afton copper mine, south-central British Columbia, demonstrates that the dominance of native copper and the lack of copper enrichment in the supergene zone are related to the relatively mafic composition of the wall rocks and the absence of abundant hypogene sulfides. Supergene alteration of porphyry copper deposits in general can be interpreted in terms of a [Formula: see text] plot. Two arbitary "limiting" curves characterized by extreme values of acidic and basic pH can be drawn. A reacting fluid evolving near the acidic limiting curve will produce a well developed leached cap with an enriched supergene ore blanket underneath. In contrast, a reacting fluid evolving near the basic limiting curve will lead to a spectrum of copper oxides and native copper with little enrichment, as typified by Afton.