Mathur2012
- 1. LETTER Use of Cu isotopes to distinguish primary and secondary Cu mineralization in the Cañariaco Norte porphyry copper deposit, Northern Peru Ryan Mathur & Joaquin Ruiz & Michael J. Casselman & Peter Megaw & Robert van Egmond Received: 11 December 2011 /Accepted: 15 August 2012 /Published online: 4 September 2012 Cañariaco Norte porphyry copper deposit in northern Peru occurs in chalcocite and covellite-rich veins and dissemina- tions that exist from the surface to depths greater than 1 km. The overall range of Cu isotopic ratios of 42 mineral sepa- rates from Cañariaco varies from −8.42 to 0.61‰, with near-surface chalcocite and Fe oxides having isotopically depleted values compared to chalcocite, covellite, and chal- copyrite from deeper levels. The majority (34 of 36) of measured Cu sulfides have a typical hypogene copper iso- tope composition of δ65 Cu00.18±0.38‰, with no enriched isotopic signature existing in the Cañariaco Norte sulfide data. Thus, the copper isotope data indicate that most of the chalcocite and covellite formed from high-temperature hypo- gene mineralization processes and that only a minor portion of the deposit is enriched by supergene processes. The nonexis- tence of an enriched δ65 Cu reservoir suggest the presence of an undiscovered lateral/exotic Cu occurrence that enriched 65 Cu that remained in solution during weathering. Regardless of the cause, the comparative analysis of the Cu isotope data- set reveals that little exploration potential for an extensive supergene enrichment blanket exists because the weathering history at Cañariaco Norte was not conducive to preservation of enriched Cu at depth beneath the leach cap. Keywords Cu isotope . Supergene . Hypogene . Porphyry copper . Cañariaco . Peru Introduction The application of Cu isotopes to understanding both high- and low-temperature aqueous processes is gaining momentum (Kimball et al. 2009; Mathur et al. 2005, 2009, 2010; Mathur and Schlitt 2010; Asael et al. 2007, 2009; Haest et al. 2009; Larson et al. 2003; Maher and Larson 2007; Rouxel et al. 2004; Li et al. 2010; Palacios et al. 2011). Low-temperature aqueous reactions clearly lead to high fractionation of Cu isotopes in residual minerals, aqueous solutions, and the min- erals precipitated from them. Thus, the deposition of Cu from low-temperature aqueous solutions can be identified by highly fractionated Cu isotope ratios in minerals or solutions as demonstrated by several studies of actively weathering and preserved weathered rocks (Mathur et al. 2005, 2009, 2010; 2012; Haest et al. 2009; Mirnejad et al. 2010; Palacios et al. 2011). All of these studies have demonstrated that the redox reactions occurring during weathering lead to patterns in the copper isotope signatures of minerals found in vertical profiles in supergene ore bodies. In contrast, copper minerals precip- itated from higher-temperature fluids show much lower frac- tionation (Seo et al. 2007; Maher and Larson 2007; Graham et al. 2004; Ikehata et al. 2011). The Cañariaco system possesses abundant chalcocite- dominant veins that occur from near surface to great depths. However, despite having mineralogy throughout that appeared favorable for copper recovery by solvent extrac- tion, it was found that only the most near-surface chalcocite ores responded well to this treatment. This suggested not only that both hypogene and supergene chalcocite were present, but also that chalcocite genesis also affects metal- lurgical behavior. In order to resolve this question and Editorial handling: F. Barra R. Mathur (*) Juniata College, Huntingdon, PA 16652, USA e-mail: mathur@juniata.edu J. Ruiz University of Arizona, Tucson, AZ 85721, USA M. J. Casselman :P. Megaw :R. van Egmond Candente Copper Corp, Vancouver, Canada Miner Deposita (2012) 47:755–762 DOI 10.1007/s00126-012-0439-y # Springer-Verlag 2012 Abstract A significant proportion of the copper in the
- 2. understand the potential extent of supergene effects in the Cañariaco deposit, the copper isotopic composition was measured on 42 mineral separates from drill core intercepts ranging from the near surface leached, through the inter- preted supergene enrichment zone to deep hypogene zones. Geology The Cañariaco project contains a major porphyry–copper– gold deposit, Cañariaco Norte, and two porphyry–copper– gold-mineralized centers that are at a prospect stage of evaluation, Cañariaco Sur and Quebrada Verde. The Cañariaco property is located at latitude 06°05′ South and longitude 79°17′ West, in the Western Cordillera of the Peruvian Andes, in the Cañaris District of the Province of Ferreñafe, Department of Lambayeque of northwestern Peru. The property is situated approximately 700 km northwest of Lima, the capital of Peru, and approximately 110 km to the northeast of the city of Chiclayo. The property is located at elevations between 2,700 and 3,600 m above sea level and encompasses both tropical forest in the Cañariaco River Val- ley and open grassland on ridges and plains at higher eleva- tions. The deposit is located on the west side of the Cañariaco River valley. The topography of the deposit area is very rugged and is made up of a series of steep valleys and narrow ridges. There is significant rainfall during the November to March rainy season. The water table is at or near surface. Copper was first discovered at Cañariaco Norte in the early 1970s during a regional exploration program carried out by the Peruvian Servicio National de Geología y Min- ería (“INGEMMET”), in cooperation with a British Geolog- ical Survey Team. Exploration was sporadic with phases of exploration and drilling in the 1970s and 1990s. Candente Resource Corp acquired the Cañariaco property in 2002 and did extensive exploration and drilling from 2004 to 2010. The Cañariaco Norte deposit has a resource of 750 Mtonne at 0.46 % Cu, 0.07 g/tonne Au, and 1.80 g/tonne Ag. Cañariaco Norte is an example of a primary porphyry– copper–gold system. The Cañariaco Norte Early to Late Tertiary polyphase intrusive-breccia complex is approxi- mately 1.7 km in strike extent, 1.1 km wide, and has been drilled to a depth of about 770 m. The complex has intruded Early Tertiary volcanic units of the Llama and Porculla Formations of the Calipuy Group. Three major intrusive generations have been identified, a crowded quartz–feldspar porphyry, a biotite–feldspar porphyry, and a coarse quartz porphyry. These intrusive units are cross-cut by feldspar porphyry dykes. Three breccia bod- ies that post-date the intrusive rocks are also recognized: hydrothermal breccias, tourmaline breccias, and polymic- tic breccias. Copper mineralization was introduced during series of events closely following the emplacement of each of the three major intrusive units and the polymictic breccia unit. In general, the highest copper grades (>0.5 % Cu) occur in the biotite–feldspar porphyry and polymictic breccia units. Intermediate copper grades (0.35 to 0.5 % Cu) are most common in the hydrothermal breccia and the coarse quartz porphyry, and the lowest grades (0.2 to 0.35 % Cu) domi- nate in the crowded quartz feldspar porphyry, tourmaline breccia, and volcanic host rocks. Alteration forms distinct concentric zones with central potassic alteration; central to intermediate, overlapping, and partly overprinting phyllic, argillic, and advanced argillic alter- ation; and fringing propylitic and minor silicic alteration. Alter- ation development is directly related to the density of fracturing in the hosting lithologies and brecciation in the polymictic breccia unit. Locally, alteration distribution and intensity are controlled by northeast- and northwest-trending faults. Mineralization occurs primarily as disseminations and in fractures, sulfide and quartz veins, faults, and breccias. Fracture density is the single most important factor influ- encing copper grades, although breccias and faults can lo- cally be important. Three main zones have been recognized in the deposit: a variable-thickness leached cap, minor supergene copper min- eralization under the leached cap, and hypogene copper miner- alization; the main component of the deposit. Hypogene copper mineralization consists mainly of chalcopyrite, chalcocite, and covellite, with lesser enargite and minor bornite, tennantite, and digenite. Supergene copper consists of chalcocite and covellite. Sampling and methods Figure 1 shows the location of the sampled drill holes and shows that samples were taken from Cu-rich ores vertically from the leach cap through the supergene and hypogene zones within the major igneous intrusion phases. Mineralization commonly occurred as veins and fine-grained mineral dissem- inations. Minerals from veins were handpicked, whereas rocks containing disseminated sulfide mineralization were either powdered and handpicked, or the whole rock powder (<250- μm fraction) was used. X-ray diffraction (XRD) of the pure mineral powders was done to verify the minerals present where approximately 0.2 g of powder was used for analysis on a Scintag Pad V X-ray powder diffractometer. XRD scans were completed in slow, step scan mode. Table 1 reports the major sulfide or Fe oxide minerals present. Potentially, different copper minerals could possess dif- ferent Cu isotopic compositions caused by either high- or low-temperature aqueous processes. Due to the disseminat- ed nature of the ores, multiple copper-bearing phases exist in the samples as shown in Table 1. To test for isotopic 756 Miner Deposita (2012) 47:755–762
- 3. heterogeneity in the samples, different mineral separates were analyzed from two samples (samples with an asterisk in Table 1). Regardless of the aliquot, the copper isotopic composition remained consistent, and copper isotope values of the minerals overlap within reported error. For instance, the mineral powder for sample 07–137-depth 620 m has the same δ65 Cu0−0.07±0.14‰ as the pure sulfide separate obtained through handpicking δ65 Cu00.15±0.14‰. The minerals were dissolved in heated aqua regia (approx- imately 100 °C) in Teflon beakers for approximately 18 h. All samples possessed a small amount of nondissolved silicate residue because the handpicking did not completely eliminate all silicate and other trace minerals. The XRD did not detect the silicate minerals because they were <3 % of the powder. The presence of silicates or other phases that could provide ions into solution could impact measurements (Pribil et al. 2009; Marechal and Albarede 2002) on the multicollector inductively coupled mass spectrometer (MC-ICP-MS); there- fore, ion exchange chromatography, with the use of the MP-1 BioRad 200–400 mesh size resin, was used to separate Cu from the samples following the procedure described by Mathur et al. (2005), Marechal et al. (1999), and Chapman Fig. 1 Geologic map and drill hole location for samples Miner Deposita (2012) 47:755–762 757
- 4. et al. (2006). All reported results have yields of Cu from the columns at greater than 90 %. Concentrations of Fe, Cu, Mg, and Na were measured on an inductively coupled plasma atomic emission spectrometer at the Pennsylvania State Uni- versity Materials Characterization Lab to determine the amount of ions needed for ion exchange chromatography. The volume of ion exchange resin used here (1.6–1.8 ml) can accommodate approximately 2,000 meq. Fe and Cu were the dominant ions in solution by at least one order of magni- tude in comparison to the other ions. In order to ensure chromatographic separation of Cu, no more than 900 meq of all measured ions was loaded onto the ion exchange resins. The samples were measured on the MC-ICP-MS at Washington State University. All samples were diluted to approximately 100 ppb and produced a 1- to 1.8-V 65 Cu signal on the instrument. Background peaks were measured on peak and subtracted out of each analysis. The combined background of the 2 % nitric acid the solutions were run with and procedural blanks (running acid through the col- umn without samples and measuring the Cu) never reached a value greater than 0.001 V for 65 Cu. Thirty ratios were collected for each block and two blocks were measured for each sample. Mass bias was corrected using the standard– sample–standard bracketing technique. The NIST 976 stan- dard was used and isotope ratios are reported as: d65 Cu ¼ 65Cu 63Cu sample 65Cu 63Cu standard 1 # 1; 000 Isobaric interferences from Mg and Na were checked because elevated concentrations of these and other elements can cause matrix effects and result in spurious data (Marechal and Albarede 2002; Marechal et al. 1999; Pribil et al. 2009; Mathur et al. 2005; Zhu et al. 2000). None of the samples contained significant concentrations of the possible isobars. Measurement errors for these values are in the sixth decimal place for all reported values. This does not represent the true instrument error. In order to assess the error, we analyzed the variation of the standard over 300 times where the 2 σ error is 0.14‰. Results Mineralogy and isotope data are presented in Table 1. The mineralogy of the samples is typical of those found in leached caps, hypogene and supergene zones in porphyry copper deposits (Sillitoe 2010; Titley 1981; Titley and Beane 1981; Seedorff et al. 2008). Reported mineral phases were identified from XRD peaks which have ap- proximately 5 % resolution. Therefore, the reported min- eral phases represent the dominant Cu sulfide species in the diffraction pattern. At the micron scale, all of the Cu species most likely exist throughout the deposit (and in all of the samples analyzed). Table 1 Sample location, mineralogy (hem hematite, goe goethite, cpy chalcopyrite, py pyrite, cc chalcocite, dj djurleite, cv covellite) and Cu isotope measurements Drill core Elevation (m) Mineralogy δ65 Cu (per mil) 07–135 2,933 hem/goe −3.74 07–135 2,906 hem/goe −2.29 07–146 2,865 hem/goe −4.31 07–183 2,856 hem/goe −5.81 07–183 2,842 hem/goe −4.69 07–200 2,952 hem/goe −8.42 07–200a 2,952 hem/goe −8.21 004–008 2,702 cc+cpy 0.10 004–008 2,657 cc+cpy 0.12 004–008 2,833 cc+cpy 0.06 004–008 2,754 cc+cpy 0.08 005–033 2,624 cc+cpy 0.18 07–103 2,861 cc −0.43 07–135 2,903 cc 0.33 07–135 2,852 cpy+cc −3.31 07–146 2,841 cc 0.50 07–183 2,822 cc 0.58 004–001 2,897 dj −2.98 004–001 2,876 dj 0.92 004–001 2,867 dj 0.32 005–033 2,573 cc+cv 0.14 006–74 2,769 cpy+py 0.31 006–74 2,724 cpy 0.37 006–74 2,619 cpy+py 0.16 07–135 2,788 cpy 0.06 07–135 2,737 cpy 0.29 07–135 2,633 cpy −0.03 07–135 2,617 cpy −1.20 07–135 2,398 cpy 0.10 07–135 2,684 cpy 0.45 07–135 2,627 cpy+cv 0.30 07–135 2,589 cpy+cv 0.61 07–135 2,536 cpy 0.55 07–137 2,596 cpy+py 0.07 07–137 2,310 cpy 0.15 07–137a 2,310 cpy −0.07 07–183 2,802 cpy 0.41 07–183 2,668 cpy 0.28 07–183 2,659 cpy+cv 0.20 07–183 2,573 cpy+cv 0.37 07–183 2,468 cpy+cv 0.32 07–183 2,366 cpy 0.42 07–183 2182 cpy 0.32 a indicates samples of rock powder replicates 758 Miner Deposita (2012) 47:755–762
- 5. Values of δ65 Cu ranged from −8.42 to 0.92‰ for all samples analyzed (Table 1). No relationships with the Cu isotope composition and type of host rock exist. Two signif- icant trends exist in the data. First, both Fe oxides and Cu- bearing minerals in the leach cap possess more depleted copper isotope signatures than the hypogene minerals (Table 2) (Mathur and Schlitt 2010). A plot of copper isotope composition versus the elevation (Fig. 2) shows that all sam- ples (Fe oxide or Cu-bearing) within 90 m of the surface have depleted copper isotopic compositions in comparison to Cu sulfide samples from below 90 m depth. The second important trend is illustrated in a histogram of copper-bearing minerals and Fe oxide leach cap minerals (Fig. 3) where all of the Cu isotope ratios of the sample plot within the leach cap and hypogene ranges. Porphyry copper deposits that have signif- icant supergene enrichment generally display three distinct copper isotope reservoirs (Mathur et al. 2005, 2009). The basic pattern observed is that the δ65 Cu values of leach cap mineralshypogene mineralsenrichment minerals. Figure 3 has data from several sources (Kimball et al. 2009; Mathur et al. 2005; Maher and Larson 2007; Larson et al. 2003; Zhu et al. 2000; Borrok et al. 2008; Graham et al. 2004). Importantly, no ores from Cañariaco Norte have enriched copper iso- tope ratios in comparison to hypogene mineralization. The copper isotopic compositions from the leach cap are sim- ilar to the range of copper isotope ratios measured from mineral powders tested in batch leach experiments of Mathur and Schlitt (2010). Discussion The weathering of Cu-rich rocks results in an unequal isotopic redistribution of Cu from hypogene into oxidized species during supergene enrichment. Low-temperature aqueous reac- tions involving oxidation of Cu cause significant fractionation of Cu isotopes and result in the development of distinct isotopic reservoirs. Mathur et al. (2009, 2010) measured sev- eral samples from all three isotopic reservoirs in the Silver Bell, Morenci, and Ray porphyry Cu deposits in southern Arizona and showed a Cu isotopic pattern in the leach cap where the lighter isotopic Fe oxide and Cu oxides occur in the shallowest portions of the deposit whereas deeper portions of the leach cap possess isotopically heavier Fe oxides and Cu oxides. The pattern most likely reflects the continual weath- ering history caused by regional uplift, which exposed the shallower portions of the leach cap to greater degrees of oxidative weathering while the deeper portions of the system did not experience as much oxidative weathering. For in- stance, dissolution of Cu-rich minerals leaves isotopically depleted residues; thus, as Cu-rich minerals weather for longer time intervals in the shallower part of the system, the Cu isotope composition of the residue will become progressively depleted. The reprecipitated copper in the supergene enrich- ment zone beneath the leach cap should possess the enriched copper leached from above. The data presented here clearly display a similar Cu isotopic composition of minerals in the leach cap. Both Fe oxides and Cu sulfide minerals in the leach cap have mea- sured Cu isotope ratios significantly depleted in comparison to typical hypogene mineralization value of approximately 0 ‰. The results indicate that oxidative weathering has re- moved heavy 65 Cu during leaching. Interestingly, the leached cap minerals at the highest elevations possess rela- tively more depleted copper isotope signatures than minerals at lower elevations. More specifically, examination of each drill core (07–135 and 07–183) reveals that the most deplet- ed δ65 Cu values exist at the highest elevation and the iso- tope composition becomes progressively enriched with lower elevations. This general observation could be inter- preted that the higher elevations experienced greater degrees of oxidative weathering and removal of Cu in contrast to areas at lower elevations. Table 2 Average Cu isotope values for the leach cap and Cu mineralized zone Average depth from top of drill core Mineral δ65 Cu (per mil) 1σ Number 0–90 m Fe oxide −4.88 1.85 7 0–90 m Cu sulfides −0.44 1.58 8 90–687 m Cu sulfides 0.18 0.32 28 Fig. 2 Plots from specific cores of δ65 Cu versus elevation Miner Deposita (2012) 47:755–762 759
- 6. The Cañariaco dataset significantly differs from other PCDs with regard to supergene enrichment minerals that possess enriched Cu isotopic signatures. The Cañariaco dataset lacks any isotopic reservoir that possesses the enriched isotopic signature typical of supergene chalcocites found in the other PCD analyzed to date (Fig. 3). Mass balance must exist in the system, and theoretically, there are two possible places the heavier could reside: the copper moved laterally to form an “exotic” deposit or the copper remained in solution. Only 2 of 36 total Cu sulfide-rich samples had measured Cu isotope ratios different from typical hypogene copper isotopic signatures (0±1‰). The two samples clearly exist in the leach cap (because Fe oxides coexist in the sample drill core sample). Therefore, the data indicate that no significant supergene enrichment exists beneath the leached cap. The data do not rule out the possibility that a super- gene enrichment blanket existed at some point in the geologic past. Notice that the two chalcocite samples in the leached cap possess isotopically depleted signatures. A possible interpretation of these depleted copper isoto- pic compositions could be that supergene chalcocite formed below the water table and was subsequently uplifted above the water table. The chalcocite then experienced oxidative weathering which consequently stripped 65 Cu copper leaving the isotopically depleted residual copper mineralization. The bulk of the copper isotope data possesses a hypogene copper isotope composition (0.18±0.32‰, Table 2) suggest- ing that the bulk of the mineralization is of high-temperature hypogene origin. In other words, hypogene mineralization conditions were such that primary covellite, chalcocite, and chalcopyrite were deposited. Petrographic analysis shows cross cutting and overgrowth mineral textures that indicate that multiple overprinting high-temperature mineralization events occurred, so samples must be considered composites of several hypogene events. Cu isotopes can be used to understand high-temperature mineralization (Maher and Larson 2007); however, this requires individual grain analysis which could not be achieved with the mineral separation technique employed here. The Cu isotope data indicate that supergene enrichment did not generate the chalcocite or covellite present at depth in the deposit, but does not provide insight for why little to no supergene enrichment occurred. Mass balance must exist in the copper isotope reservoirs measured. Why does en- richment occur only in weathered remnants within the leach cap and not as a typical enrichment blanket as found in other PCDs? The leach cap is depleted in terms of copper, and the whereabouts of “lost” 65 Cu copper remains unknown. The migration and sequestration of Cu can be controlled by many factors such as climate, pyrite/chalcopyrite ratio (abil- ity to generate sulfuric acid), fracture abundance (secondary permeability), metallic surfaces for copper to precipitate on, uplift, erosion, preservation, solubility/precipitation of min- erals, and preservation of enrichment blankets (Reich et al. 2008; Blanchard 1968; Chavez 2000; Sillitoe and McKee 1996; Hartley and Rice 2005; Anderson 1982). Some or all of these factors probably explain the lack of supergene enrichment; however, the current humid climate and lack of units to “preserve” the enrichment blankets most likely inhibited supergene enrichment. For instance, the isotopi- cally depleted Cu enrichment minerals found in the leach cap indicate current weathering actively dissolves enriched layers in the subsurface without preserving en- richment. Further copper isotopic measurements of ground- and surface waters from the area should show an enriched isotopic signature and confirm that copper is “lost” from the system. Alternatively, the 65 Cu copper isotopic reservoir could exist as lateral “exotic” copper deposits. Instead of envision- ing only vertical flow commonplace in traditional supergene models, lateral flow regimes could be conduits for the copper in solution. So, the enriched copper isotopic reser- voir could exist on the periphery of the deposit but it has not been discovered. Conclusions and future directions Copper isotope analyses can be used to distinguish between high- and low-temperature mineralization processes. In this instance, the copper isotope data from Cañariaco Norte indicate that high-temperature mineralization events provid- ed the majority of copper mineralization and supergene enrichment is negligible. Fig. 3 Histogram plot of copper isotope composition of minerals from porphyry copper deposits; data taken from Mathur et al. (2005, 2010) and Zhu et al. (2000) 760 Miner Deposita (2012) 47:755–762
- 7. The implications possess a twofold significance for ex- ploration geology and hydrometallurgy: (1) From an explo- ration geologic standpoint, the lack of a supergene blanket throughout the deposit eliminates the need to explore for horizontal copper mineralization. However, the data could be congruent with the existence of lateral flow of copper and the existence of “exotic” copper deposits trapped down- stream by a suitable reductant. (2) From a hydrometallugical standpoint, process of the ores should focus on treating hypogene mineral assemblages. In order to further augment the dataset collected here, copper isotope measurements of fluids actively weathering the deposit would be ideal. The solutions should possess a relatively enriched copper isotopic signature. Equally im- portant in understanding the overall mass balance of copper in the system would be copper isotopic measurements from soils derived from the weathering of the igneous rocks at Cañariaco Norte. 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