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Porphyry Copper Discovery Beneath the Valeriano Lithocap, Chile Richard H. Sillitoe, 27 West Hill Park, Highgate Village, London N6 6ND, England, Claudio Burgoa, Victor Rae 4777, Las Condes, Santiago, Chile, and David R. Hopper, First Quantum Minerals Ltd., Callao 3461, Las Condes, Santiago, Chile ABSTRACT
Exploration for porphyry copper deposits beneath barren or poorly mineralized, advanced argillic lithocaps is becoming commonplace; however, there have been few discoveries except in cases where the copper ± gold ± molybdenum mineralization has been partly exposed, typically as a result of partial lithocap erosion. At Valeriano, in the high Andes of northern Chile, completely concealed Miocene porphyry copper-gold mineralization was recently discovered beneath a lithocap. Here, the results of the staged drilling program that led to the discovery are summarized, with emphasis on the key geologic, alteration, and mineralization features that provided guidance. The final deep drill holes of the 16-hole program cut well-defined advanced argillic and sericitic alteration zones before entering chalcopyrite ± bornite–bearing, potassic-altered porphyry, with grades of 0.7 to 1.2% Cu equiv, at depths of ~1,000 to >1,800 m.
Lithocaps are large rock volumes— originally 1 to 2 km thick and up to tens of square kilometers in areal extent—that normally constitute the upper parts of one or more porphyry copper systems (Sillitoe, 1995). Typically, they are wholly or partially composed of advanced argillic alteration and can host high-sulfidation epithermal gold-silver ± copper mineralization. The advanced argillic alteration, commonly both structurally and stratigraphically controlled, is typified by vuggy residual quartz and quartz-alunite at shallower to page 20 . . . levels, with FIGURE 1. Location of the Valeriano copper-gold prospect with respect to other latest Oligocene-Miocene porphyry and high-sulfidation (HS) epithermal deposits and principal prospects in the El Indio and Maricunga metallogenic belts, northern Chile.
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Porphyry Copper Discovery Beneath the Valeriano Lithocap, Chile (continued)
increased amounts of pyrophyllite ± diaspore ± topaz at depth as a consequence of higher fluid temperatures. Pyrite is ubiquitous and abundant, and enargite, bornite, chalcocite, and covellite are the typical copper-bearing species in any associated high-sulfidation mineralization. Lithocap remnants can overlie porphyry copper deposits and/or occur alongside them, depending on the vagaries of postmineral tectonism and erosion. In most known examples of porphyry copper deposits associated with lithocap remnants, the porphyrytype mineralization has been at least partially exhumed as a result of either erosion—as at Red Mountain, Arizona (Corn, 1975); Yanacocha, Peru (Bell et al., 2004); El Salvador, Cerro Casale, and Escondida, Chile (Gustafson and Hunt, 1975; Vila and Sillitoe, 1991; Hervé et al., 2012); Batu Hijau, Indonesia (Meldrum et al., 1990); Frieda River, Papua New Guinea (Asami and Britten, 1980); and Kounrad, Kazakhstan (Nakovnik, 1964)—or, uncommonly, structural dismemberment and tilting (Yerington, Nevada; Lipske and Dilles, 2000). In the past decade, deep exploration for porphyry copper deposits completely concealed beneath extensive lithocaps has become increasingly common as near-surface mineralization becomes scarcer but, with rare exceptions (e.g., Recsk, Hungary; Baksa et al., 1980), there have been few successes. Therefore, it is considered worthwhile to present this brief case history of a deep, geologically blind porphyry copper discovery at Valeriano in the Atacama Region of northern Chile (Fig. 1), where the sublithocap exploration drilling was guided by geologic criteria and concepts.
metallogenic belt, which is defined by a series of advanced argillic lithocaps and contained high-sulfidation epithermal gold-silver ± copper deposits and prospects (Fig. 1). The Valeriano prospect is located in the southern part of the eponymous lithocap whereas the El Encierro prospect, controlled by Barrick Gold Corporation, is near its northern extremity (Fig. 2). Lithocap-hosted, high-sulfidation gold-silver-copper mineralization also occurs 9 km farther south in Argentina at Taguas (Fig. 2), a Minera Piuquenes property. The giant Pascua-Lama high-sulfidation gold-silver-copper deposit (e.g., Chouinard et al., 2005) is located ~30 km south of Valeriano (Figs. 1, 2). The Valeriano lithocap is confined to an N-trending, orogen-parallel graben bounded by high-angle reverse faults of Miocene age (Nasi et al., 1990; Ortiz and Merino, 2015; Fig. 2), which are probably tectonically inverted, earlier Tertiary normal faults (Winocur et al., 2015). The graben is underlain mainly by volcanosedimentary rocks assigned to the Doña Ana Formation of
late Oligocene-early Miocene age and bounded east and west by basement units: andesitic and dacitic volcanic and subordinate epiclastic sedimentary rocks of the Late Permian-Early Triassic strata of Paso Guanaco Sonso and bimodal basaltic and rhyolitic volcanic rocks of the Late Triassic Pastos Blancos Formation (Nasi et al., 1990; Ortiz and Merino, 2015; Fig. 2). The Paso Guanaco Sonso unit is intruded by the calc-alkaline, I-type Chollay granitoid batholith of Late Triassic age (Hervé et al., 2014; Fig. 2). Small dioritic intrusions of the Infiernillo unit (~17–14 Ma; Bissig et al., 2001) occur locally (Fig. 2). At depths of >1,000 m, drilling at Valeriano showed that the Pastos Blancos volcanic rocks, dominated by rhyolitic ignimbrites but also including flowfoliated lavas, domes, and possibly sills of similar composition, as well as minor epiclastic horizons, are intruded by biotite-hornblende granodiorite porphyry. Early, intermineral, and late-mineral porphyry phases are recognized and span porphyry copper formation. Molybdenite accompanying the copper mineralization was dated by the Re-Os method at 9.95 ± 0.04 Ma, confirming that the porphyry system is part of the Miocene to early Pliocene metallogenic belt of northern Chile and contiguous Argentina (Sillitoe and Perelló, 2005) and intermediate in age between the Veladero (11.9–10.3 Ma) and Pascua-Lama (9.4–8.1 Ma) high-sulfidation epithermal deposits (Bissig et al., 2001; Holley et al., 2016; Figs. 1, 2).
GEOLOGIC SETTING The Valeriano lithocap, ~13 km long and ~4.5 km wide at surface as defined by the outer limits of kaolinite development and limonite after pyrite (Fig. 2), underlies a prominent glaciated ridge at ~4,200 m above sea level in the Frontal Cordillera of the high Andes. The lithocap lies at the northern end of the El Indio
FIGURE 2. Geologic setting of the Valeriano lithocap and porphyry copper-gold prospect in a Miocene graben bounded by reverse faults (blue lines, triangles on upper plates). Simplified from Ortiz and Merino (2015) and Ragona et al. (1995). Deposit symbols as in Figure 1. Abbreviations: Mio = Miocene, Olig = Oligocene, P = Permian, Tr = Triassic.
PROSPECT HISTORY The Valeriano alteration zone was recognized in 1986 by Jozsef Ambrus, who appreciated that exposed silicic ledges (ribs) and breccias are reminiscent of those in the high-sulfidation epithermal deposits farther south in the El Indio belt, particularly El Indio and Tambo (e.g., Araneda, 1982; Fig. 1). The mineral rights over Valeriano were duly acquired and have been maintained in good standing to the present day by Sociedad Contractual Minera Valleno. The property was optioned from 1988 to 1991 to Rayrock and
then Phelps Dodge, which constructed a 57-km access road and targeted the concealed porphyry copper potential with 2,900 m of core and 3,500 m of reverse-circulation (RC) drilling. The deepest hole, to 550 m, ended in advanced argillic alteration. From 1995 to 1997, Barrick Gold Corporation tested the high-sulfidation epithermal gold potential of the lithocap with 20 shallow RC holes, totalling 6,173 m. The best hole, averaging 1.4 g/t Au over 100 m, cut vuggy residual quartz and underlying hydrothermal breccia containing quartz, alunite, pyrite, and enargite; however, the estimated potential of ~0.4 to 0.6 million ounces of gold was insufficient for Barrick, leading to termination of its option. Hochschild Mining, a midtier, London-listed company best known for its precious metal operations in Peru, optioned the property from 2010 to 2013. The initial intention was to further test the FIGURE 3. The Valeriano lithocap, showing surface alteration epithermal precious metal potentypes, arsenic, gold, copper, and molybdenum soil geochemtial at Valeriano, bearing in mind ical anomalies, and locations of the 16 core holes drilled by Hochschild Mining from 2010 to 2013. Section line (A-A’) for that Hochschild would be satisfied Figures 4 to 7 is also shown. The lithocap is clearly defined with a much smaller deposit than by clay and iron oxide signatures on processed multispectral that required by Barrick. However, Landsat-8 imagery. as work commenced, the likely existence of a concealed porphyry of ~200 m, the alteration copper deposit was also clearly recogcomprises quartz-alunite nized. This discovery case history is in VAL-2 and 6, but prebased on the results of three progresdominantly quartz-kaolinsively deeper drilling campaigns, for a ite elsewhere. Both these total of 14,270 m in 16 holes (Fig. 3), assemblages are underlain by conducted by Hochschild Mining. a quartz-pyrophyllite–rich
FIRST DRILLING CAMPAIGN: LITHOCAP TESTING The first round of drilling, consisting of eight holes (VAL-1–8; Fig. 3) to depths of 400 to 800 m and totalling 4,884 m, was designed to test the high-sulfidation epithermal gold potential at Valeriano; they were sited within the confines of the previously defined high-sulfidation gold-copper mineralization, which lies within a prominent geochemical anomaly defined by arsenic and, to lesser degrees, gold, copper, molybdenum (Fig. 3), and antimony. Logging of the core from these holes provided the first confirmation of likely porphyry copper mineralization at depth. The holes intersected mainly rhyolitic volcanic rocks that display pervasive advanced argillic alteration (Fig. 4a). To depths
zone from 200 to 400 m thick (Fig. 4a). On approach to the end of hole 6, pale green, soapy illite accompanies the pyrophyllite, and specularite appears (Fig. 4a, b). The alunite, kaolinite, and pyrophyllite as well as any specularite and sulfide minerals replace plagioclase phenocrysts and pumice clasts, including fiamme, whereas the accompanying fine-grained quartz occurs principally as a groundmass replacement. Trace amounts of fine-grained dumortierite, the borosilicate characteristic of advanced argillic zones (Sillitoe, 2010), are also present, particularly in the quartz-pyrophyllite zone.
Two dikes were cut in a faulted interval near the end of VAL-4: 20 m of granodiorite porphyry, containing minor sericite (white mica) as well as specularite, and 30 m of andesite porphyry with relict biotite-magnetite alteration. The granodiorite porphyry was suspected to be part of the late Miocene porphyry copper event whereas the andesite porphyry was tentatively considered as part of the Late Triassic wall-rock package. Quartz-bearing veinlets are absent from the surface at Valeriano and first become evident at or near the top of the pyrophyllite zone, at approximately 500-m depth. Most of them are nondescript quartz-pyrite veinlets without halos and of uncertain significance. A few are banded and weakly auriferous, similar to those described from porphyry gold deposits in the Maricunga belt (Vila and Sillitoe, 1991; Muntean and Einaudi, 2000; Fig. 1). In VAL-2, 3, 4, and 6, there are also a few quartz-molybdenite-pyrite veinlets, closely comparable to B-type veinlets in to page 20 . . . porphyry copper
FIGURE 4. North-northwest Valeriano section showing (a) geology and alteration types, and (b) mineralization features and copper and gold grades in core from the first drilling campaign. Drill-hole numbers as in Figure 3. Abbreviations: cv = covellite, en = enargite, py = pyrite.
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Porphyry Copper Discovery Beneath the Valeriano Lithocap, Chile (continued)
deposits (Fig. 4b). Furthermore, in VAL4, scarce D-type porphyry copper veinlets, marked by pyrite center lines and centimetric quartz-sericite halos, were also observed. Although now embedded in advanced argillic alteration, the B-type veinlets must have formed in association with potassic alteration, which was then overprinted by the D-type veinlets (cf. Gustafson and Hunt, 1975). In the first-campaign drill holes, the quartz-alunite and quartz-kaolinite zones lack appreciable amounts of copper and gold, even where unoxidized (Fig. 4b), although enargite is observed locally in scattered alunite veinlets and, at surface, in several alunite- and pyrophyllite-bearing, silicic breccias, ledges, and mantos (Fig. 3). Copper-gold mineralization, averaging 0.1 to 0.15% Cu and 0.1 to 0.15 g/t Au, only becomes widely developed within the underlying quartz-pyrophyllite zone containing quartz-bearing veinlets (Fig. 4b). Most of the copper and gold occur as a high sulfidation-state assemblage characterized by veinlet and disseminated pyrite and trace chalcopyrite coated by thin black films of intergrown chalcocite, covellite, digenite, bornite, enargite, and/or tennantite. Although these copper-bearing sulfide coatings are reminiscent of those resulting from supergene enrichment, they were considered hypogene because, unlike the weak chalcocite enrichment that is also present, they are unrelated to the base of sulfide oxidation (Fig. 4b) and become more strongly developed downward to depths of up to 800 m. It is clear from the widely observed intergrowth of the sulfide minerals and pyrophyllite that the high-sulfidation mineralizing event and advanced argillic alteration were contemporaneous, whereas the scarce chalcopyrite and possibly some of the pyrite were inherited from a former potassic assemblage. Results of a pole-dipole induced polarization survey carried out on behalf of Hochschild Mining revealed the presence of a prominent chargeability feature with its top ranging from 100 to 400 m beneath the surface. The anomaly was first considered as a possible indicator of the position of concealed porphyry copper-gold mineralization. However, as core logging progressed, it was realized that the top of the anomaly, defined by a value of ~10 ohm-m, approximates the base of
supergene sulfide oxidation (Fig. 4b). Therefore, the anomaly has no particular significance other than showing the existence of widespread sulfide minerals, chiefly pyrite, as would be expected in a lithocap. Several lines of evidence, summarized above, were taken to confirm that a porphyry copper center likely underlies the lithocap. The most direct evidence was provided by the scarce B- and D-type veinlets at depth in the holes and the sericite-altered granodiorite porphyry dike cut at the bottom of VAL-4, which could be an offshoot from a larger intrusive body at depth. Furthermore, the presence of specularite in two widely separated holes (VAL-4 and 6) was taken as a possible vector to the underlying porphyry copper center. Consequently, it was suggested that the porphyry copper mineralization underlies the central and southern parts of the surface alunite-rich zone, in the general vicinity of VAL-2, 4, and 6 (Fig. 3). It was clear that the top of the main porphyry intrusion must lie deeper than 800 m, below the maximum tested by drilling at that time, and could easily be as deep as 1,200 m. Although it was accepted that the chance of the porphyry copper mineralization being sufficiently high grade to be an underground bulk-mining proposition was low, it was concluded that two drill holes to a depth of at least 1,200 m in the general area of VAL-4 and 6 could test the potential.
breccia (Fig. 5a) not fully appreciated previously. The tightly packed, centimeter-sized, ignimbrite clasts contain truncated A- and B-type quartz veinlets, but similar veinlets also crosscut the breccia, thereby confirming its intermineral timing. None of the holes intersected appreciable volumes of porphyry, although several biotite-hornblende granodiorite porphyry dikes were cut at depths of >800 m. One of the dikes, in VAL-9, contains quartz veinlets and is early, whereas the others cut quartz veinlets and are clearly of late-mineral timing. The breccia from 850 to 890 m in VAL-12 was observed to contain isolated clasts of this early quartz-veined porphyry. Holes VAL-9, 11, and 12 were drilled from quartz-pyrophyllite alteration into a well-defined quartz-illite zone, which transitions downward to sericitic (quartz-sericite) alteration, thereby confirming the first indications of these hydrolytic alteration assemblages previously seen deep in VAL-4 and 6 (Figs.
SECOND DRILLING CAMPAIGN: SUBLITHOCAP TESTING Management took the decision to probe the Valeriano prospect to greater depths and five holes (VAL-9–13; Fig. 3), totaling 5,920 m, were drilled in 2012 to depths ranging from 800 to 1,200 m. The main lithology intersected to a depth of ~1,200 m remained rhyolitic ignimbrite, which in VAL-11, 12, and 13 has been converted to a vertically extensive, magmatic-hydrothermal
FIGURE 5. North-northwest Valeriano section with addition of (a) geology and alteration types, and (b) mineralization features and copper and gold grades in core from the second drilling campaign. Drill-hole numbers as in Figure 3. Abbreviations: cpy = chalcopyrite, HS = high sulfidation. See Figure 4 for rest of legend and abbreviations.
600 m or so (Fig. 6a). The anhydrite had been leached out by cool ground water at shallower levels (Fig. 6b). The quartz-illite zone is poorly represented (Fig. 6a). The high-sulfidation copper-gold mineralization is more strongly developed in the sericitic zone cut by VAL-14 and 16 than in previous holes, is higher in grade (0.3–0.35% Cu, ~0.2 g/t Au), and extends over a greater vertical interval of ~600 m (Fig. 6b). Beneath the high-sulfidation zone, pyrite/chalcopyrite ratios decrease progressively downward until chalcopyrite is the only sulfide species present and, at depths of 1,500 to 1,600 m, trace amounts of bornite appear (Fig. 6b). The highest tenors, attaining 0.92% Cu and 0.42 g/t Au from 1,596 to 1,644 m in VAL-14 (Fig. 6b), are hosted by the early porphyry, with lower grades reported from the intermineral phases. The third drilling campaign proved conclusively that a well-developed potassic alteration zone exists in association with a mineralized, multiphase porphyry intrusion at depth below the Valeriano lithocap. Although long mineralized intercepts and local high grades are present, including 724 m at 0.60% Cu and 0.27 g/t Au from 1,030 to 1,754 m in VAL-14 (Fig. 6b), the limited drilling failed to intersect the copper-equivalent grades considered necessary for bulk underground mining. Hence, Hochschild Mining declined to conduct further exploration and withdrew with a retained 10% interest. Nevertheless, it remains distinctly possible that copper tenors could increase still further at greater depths in concert with any additional increase of bornite/chalcopyrite ratio, a situation that could also result in higher gold values (cf. Kesler et al., 2002). Indeed, increased Au/Cu ratios do accompany the appearance of bornite (Fig. 7). Drilling to a depth of at least 2,200 m would be required to test the possibility of further downward increases in copper and gold values. Furthermore, well-mineralized magmatic-hydrothermal breccia or mafic FIGURE 6. North-northwest Valeriano section with addition of host rocks at depth (e.g., basalt in (a) geology and alteration types, and (b) mineralization feathe lower part of the Pastos Blantures and copper and gold grades in core from the third drilling cos Formation; Ortiz and Merino, campaign. Drill-hole numbers as in Figure 3. Abbreviations: bn 2015) could local= bornite, cc = chalcocite, dg = digenite. See Figures 4 and 5 to page 20 . . . for rest of legend and abbreviations. ize even higher
4a, 5a). At these greater depths, even the nonbrecciated ignimbrite is intensely and pervasively altered and cut by plentiful multidirectional A- and B-type quartz veinlets inherited from a former potassic zone. Sizable remnants of this potassic alteration, defined by K-feldspar, biotite, and magnetite, were first recognized near the bottom of VAL-11, at a vertical depth of ~1,100 m (Fig. 5a). The high sulfidation-state mineralization that accompanies the quartzpyrophyllite alteration at the base of the lithocap changes to pyrite and chalcopyrite within the quartz-illite zone. This alteration boundary is marked by the appearance of several volume % of specularite (Fig. 5b), derived from magnetite in the former potassic assemblage. The pyrite/chalcopyrite ratio decreases progressively downward and at ~1,200 m approximates 5:1. Underlying the quartz-illite zone, the deeper parts of the sericitic zone average ~0.4% Cu and ~0.15 g/t Au, with individual 2-m samples attaining 0.8% Cu and 0.6 g/t Au (Fig. 5b)—values that were considered particularly high for felsic volcanic rocks distal to a porphyry copper center. The base of the high sulfidation-state mineralization coincides with an appreciable and consistent decrease in Au/Cu ratios (see below). On completion of the second drilling campaign, it was concluded that the prognosis remained good, with an excellent chance of encountering potassic alteration and higher-grade copper-gold mineralization within the next 500 or so vertical meters. This implied drilling to a minimum depth of 1,500 m, but it was again cautioned that ore-grade copper-gold mineralization would be unlikely given the extreme depths involved.
THIRD DRILLING CAMPAIGN: PORPHYRY COPPER DISCOVERY Management decided that the results of the second drilling campaign were sufficiently encouraging to justify deeper testing of the Valeriano prospect. During the 2013 season, two deeper holes, VAL-14 to 1,845 m and VAL-16 to 1,620 m, were drilled and VAL-9, collared in 2012, was reentered and deepened to 1,878 m (Figs. 3, 6).
The most striking finding of the deeper drilling was the greater abundance of granodiorite porphyry, either in the form of a stock, as depicted in Figure 6a, or closely spaced dikes. VAL-14 cuts an early, crowded phase, containing abundant A-type quartz veinlets, and VAL-14 and 16 have two texturally different, intermineral phases. The drilling also showed that at least three relatively mafic rich, intermineral porphyry dikes attain much shallower levels in the system, two of them subcropping (Fig. 6a). The three deep holes passed through the now well-defined advanced argillic and quartz-sericite zones before penetrating intense potassic alteration, defined by K-feldspar, biotite, magnetite, and anhydrite, over the final
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Porphyry Copper Discovery Beneath the Valeriano Lithocap, Chile (continued)
of the system. Shallower alteration zones are variably telescoped over deeper ones, as most clearly evidenced by the presence of A- and B-type quartz veinlets, generated during potassic alteration, in the lower parts of the advanced argillic lithocap. The relative proximity of the quartz-pyrophyllite zone to the present surface, where pyrophyllite commonly occurs with alunite (Figs. 3, 4a), implies that the deeper, higher-temperature root zone of a formerly much thicker lithocap is probably now exposed (cf. Sillitoe, 1995, 2010). Hence, the FIGURE 7. North-northwest Valeriano section showing selected mineralization boundaries (from Fig. 6b) and down-hole Au best high-sulfidation gold (g/t)/Cu (%) ratios. Note the pronounced decrease in Au/Cu mineralization, in either values at the base of the high-sulfidation (HS) overprint and vuggy residual quartz ledges the more subdued increase on entering the deep chalcopyriteor silicic hydrothermal bornite zone. breccias, may have been eroded. Chlorite-sericite metal values as they do, for example, alteration, which commonly intervenes at Resolution, Arizona (Hehnke et al., between sericitic and potassic zones 2012). (Sillitoe, 2000, 2010), is suppressed at Valeriano by the mafic-poor nature of the rhyolitic wall rocks. GEOLOGIC MODEL In common with the alteration, the The Valeriano porphyry copper-gold shallow-level, vertical sulfide zoning at prospect, as currently defined, disValeriano also follows a classic sequence plays a classic vertical zoning pattern from pyrite-enargite at and near the from quartz-alunite/kaolinite through present surface through pyrite-enargquartz-pyrophyllite, quartz-illite, and ite-chalcocite-covellite to pyrite-diquartz-sericite in the volcanic wall rocks genite-bornite (Fig. 6b). This high to potassic where porphyry intrusions sulfidation-state mineralization changes become volumetrically important (Fig. abruptly downward to a pyrite-chalco6a). The quartz-illite zone is absent pyrite assemblage, which transitions at centrally and, hence, appears to chargreater depth to chalcopyrite only and, acterize the lower-temperature fringe eventually, in the presently tested core
of the system, to chalcopyrite-bornite representative of low sulfidation-state conditions (Fig. 6b). The deeper parts of the high-sulfidation mineralization overprinted the pyrite and chalcopyrite and clearly removed some of the latter mineral as well as adding gold. As a consequence, Au/Cu (and Ag/Au) ratios increased (Fig. 7). Specularite accompanies pyrite and subordinate chalcopyrite in the quartz-illite and quartz-sericite zones, but magnetite becomes the dominant iron-bearing species in the underlying potassic zone. Molybdenite constitutes a well-defined shell coincident with the partly sericitized top of the potassic zone (Fig. 6b), in keeping with its marginal position in most porphyry copper-gold deposits (Sillitoe, 2000). Laterally, at least in the lithocap, pyrite is the only abundant sulfide, although sphalerite and galena are also present locally. Based on the drilling to date, there is no suggestion of a lowgrade core resulting from either sulfide nondeposition or emplacement of a late-mineral intrusion. Indeed, volumetrically significant late-mineral porphyry and hydrothermal breccia are present only on the southern edge of the system (Figs. 4–6).
CONCLUSIONS Porphyry copper discovery at Valeriano resulted from a pragmatic observational approach based on systematic use of the hand lens, geologic reasoning, and familiarity with the porphyry copper model and its constituent mineral assemblages. Conventional microscopy and short-wave infrared (SWIR) spectroscopy were also utilized during the third drilling campaign to confirm
and refine the previously defined mineralogic zoning. In particular, the downward increase in the 2,200-nm sericite absorption feature provides a valid vector to the core of the system (cf. Thompson et al., 1999; Halley et al., 2015). No geophysical method assisted the discovery. In retrospect, the most instructive shallow-level geologic features indicative of the concealed porphyry copper mineralization at Valeriano are well-developed, copper sulfide- (as well as enargite-) bearing, high-sulfidation mineralization; sparse banded veinlets at shallow levels; sparse B-type quartz-molybdenite veinlets overprinted by the quartz-pyrophyllite alteration; the shallow remnant of weak potassic alteration in more mafic rock (the andesite porphyry dike of inferred Late Triassic age) cut by VAL-04; and minor intermineral porphyry dikes >1,000 m above the main porphyry intrusions. The enargite- and gold-bearing silicic manto, highest-grade high sulfidation-state mineralization, and highest molybdenum values all appear to be centered above the apex of the currently defined copper-gold core. The key downward alteration and mineralization changes that maintained exploration momentum and underpinned the recommendations to drill deeper are the visually obvious pyrophyllite to illite and illite to sericite transitions; the abrupt transition from a high sulfidation-state sulfide assemblage to pyrite plus chalcopyrite; the gradual downward increase in quartz veinlet intensity and decrease in pyrite/chalcopyrite ratios; and the consistent downward increase of copper and gold grades (Figs. 4–6). The appearance of bornite near the ends of the deepest holes supports the notion of even higher copper and gold tenors at still greater depths.
ACKNOWLEDGMENTS Thanks are due to former Hochschild Mining colleagues for their participation in the Valeriano exploration program, including George Schroer, Isac Burstein, and César Aguirre for management support and Luciano Bocanegra, Juan Burlando, Horacio Gasquéz, Carlos Gordo, Juan Molina, Alvaro Muñoz, Jhonny Peñaranda, and María Eugenia Rodríguez for contributions to geologic mapping and core logging. Carmen Holmgren carried out the mineralogic studies, Paola Gress
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