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Cerro Verde, Santa Rosa, Cerro Negro |
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Peru |
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Cu Mo
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Super Porphyry Cu and Au
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The contiguous Cerro Verde and Santa Rosa deposits and the neighbouring Cerro Negro prospect are located in southern Peru, ~15 to 20 km SSW of the city of Arequipa (#Location: 16° 32' 4"S, 71° 35' 38"W).
Cerro Verde and Santa Rosa outcrop at altitudes of 2680 to 2750 m on the subplanar 'Santa Rosa' erosional pediment which was eroded into the older 'La Caldera' surface that is ~200 m higher.
Historic high grade artisanal mining was undertaken by the pre-Colombian Incas and subsequently by Spanish colonists. The earliest recorded production in the district comprised several thousand tonnes of high-grade direct shipping copper oxide ore from Cerro Verde between 1868 and 1879 by Spanish miners. This ore was shipped to Wales for smelting. A concession covering the deposit was acquired by Anaconda in 1916 and held until 1970 with only limited work undertaken in 1917-18, and again between 1964 and 1967. In 1970, the Peruvian government passed a law requiring resources within mining concessions to be put into production or relinquished. As Anaconda was unwilling to commit to production, the mining rights were resumed in 1970. The title was passed to the government owned Minero Perú which commenced exploiting brochantite-dominated oxide ores at Cerro Verde, based on a reserve of 29 Mt @ 1.07% Cu in the upper parts of the deposit. The operation involved one of the world's first Solvent Extraction/Electro-winning (SX/EW) facilities, which was constructed from 1972 and brought into production in 1977, by which time oxide reserves had grown to 61 Mt @ 1.01% Cu. At the same time, 1.2 Gt of underlying sulphide ore @ 0.6% Cu had been indicated. In 1994, as part of a privatisation program, the operation was purchased by the Sociedad Minera Cerro Verde SAA, initially 82.5% owned by Cyprus Amax (subsequently acquired by Phelps Dodge Copper Corporation in 1999), with a 9.2% equity held by Compañía de Minas Buenaventura. Substantial capital was invested in the property to upgrade and improve productivity. In the eight years after privatisation, copper production increased by ~350% whilst costs were reduced by >40%. In 2005 Sumitomo Metal Mining Company Ltd. acquired a share in the operation and in 2007 Freeport-McMoRan Inc., merged with Phelps Dodge. By 2015, the ownership had evolved to 53.56% held by Freeport-McMoRan Inc., while SMM Cerro Verde Netherlands N.V. (a subsidiary of Sumitomo Metal Mining Company Ltd.) owns 21% of the operation and the balance is held by Compañía de Minas Buenaventura S.A.A. (19.58%) and other shareholders (5.86%). A large-scale expansion project was completed between 2012 and 2015 to a capacity of ~275 000 tpa of Cu in cathodes and concentrate and 6800 tpa of Mo.
Geological Setting and Structure
Cerro Verde, Santa Rosa and Cerro Negro represent the northernmost economic hydrothermal system in the 800 km long, upper Paleocene to early Eocene porphyry Cu-Mo belt of the central Andes of southern Peru and northern Chile. They were emplaced into and above the Arequipa Terrane basement block in the Bolivian Orocline section of the northern Central Andes. See the Central Andes and Bolivian Orocline record for an overview of the regional setting.
In the Cerro Verde-Arequipa district, the Arequipa Terrane basement is principally represented by the Palaeoproterozoic granulite facies Charcani Gneiss, which underwent three principal metamorphic events, namely i). 1820 to 1800 Ma, followed by ii). a Grenvillean episode between 1200 and 970 Ma; and iii). a late ~440 Ma metamorphism (Torres et al., 2008). This lithofacies comprises regularly banded, pink, granular quartzo-feldspathic psammitic gneisses that has undergone partial granitic melting, with concordant amphibolite layers after basic sills (and dykes) and injections of foliated granite and pegmatite. It represents both sedimentary and igneous protoliths.
The Charcani Gneiss is unconformably overlain by the:
• Upper Triassic to Lower Jurassic Tinajones Conglomerate which contains rounded clasts of gneissic diorite and quartzite with intercalated layers of coarse-grained reddish-brown sandstone.
• Lower Jurassic Chocolate Formation composed of grey porphyritic andesites and andesitic volcanic rocks with some intercalations of medium-grained sandstones, shales and occasionally light grey limestone lenses.
• Lower to Middle Jurassic Socosani Formation comprising a series of limestones, partly metamorphosed and recrystallised to fine-grained marble.
• Upper Jurassic to Lower Cretaceous Yura Group made up of intercalations of quartz sandstones, quartzites and finely stratified dark siltstones and shales, but also including Puente Formation black shales and carbonatic sandstones (Martinez et al., 2017; Horning, 1988).
All of these rocks are intruded by the extensive late Cretaceous to Tertiary Tiabaya (~78 Ma) and Yarabamba (67 to 62±1 Ma by U-Pb zircon; Mukasa and Tilton, 1985; Mukasa 1986; 68±3 Ma, Rb-Sr; Le Bel, 1985) granodiorites of the Coastal Batholith. Both are composed of monzonitic-tonalitic porphyry of similar composition, although the latter has a generally finer texture. These intrusions enclose associated explosive breccias of varying composition. A north-south structural grain accompanied intrusion of the granodiorites.
The hypogene Cu-Mo mineralisation in the three deposits is associated with a cluster of hypabyssal intrusions that were emplaced near the contact of the Charcani Gneiss and the Yarabamba Granodiorite, and were controlled by the intersections of NW-SE structures and local NE-SW tensional faults. Extensive post-mineral NE-SW and east-west fractures and tensional faults which cut the NW-SE regional trend were the result reverse reactivation.
All of these rocks are unconformably overlain by the Lower Pliocene Sencca Volcanics, which consisting of rhyolitic and dacitic tufts, white and pink, with abundant small pumice fragments and castings.
Each of the deposits is spatially related to a steep walled stock of quartz- and feldspar-phyric rock with surface areas of ~0.15 km2. These intrusions are described as a 'dacite-monzonite porphyry' or 'quartz-bearing monzonite porphyry' with an overall dacitic composition. They are the youngest significant intrusives in the district, although a dyke of post-mineral dacitic, quartz-feldspar porphyry is exposed in the southern part of the Cerro Verde pit, and a weakly altered quartz porphyry in the Santa Rosa pit may represent late phases of the same intrusive event. At the Cerro Verde deposit, the mineralisation related intrusions have been dated at 61±1 Ma by U-Pb zircon; Mukasa (1986); 62±2 Ma by Rb-Sr whole rock; Beckinsale et al., (1985), and 59 to 56±2 Ma by K-Ar biotite; Estrada (1978), although the much younger latter are considered to reflect the loss of radiogenic 40Ar (Quang et al., 2003).
The Santa Rosa and Cerro Negro deposits are hosted entirely by the Paleogene granitoids, i.e., both the Yarabamba Granodiorite and the younger porphyry stocks, while Cerro Verde straddles the contact between these granitoids and the enclosing Charcani Gneiss. Tourmaline-cemented breccias are developed in association with both the Cerro Verde and Cerro Negro deposits, while small volumes of tourmaline-free 'silica breccia' occur at all three mineralised centres. NW-SE striking bodies of tourmaline-rich granitic pegmatite and aplite are locally widespread, cutting all granitoid and basement rocks, although their relationship to the tourmaline breccias is uncertain (Quang et al., 2003).
The strong NW-SE aligned fabric in the district is also reflected in the elongation of the Santa Rosa and Cerro Verde hydrothermal systems, the overall alignment of the Cerro Verde, Santa Rosa and Cerro Negro deposits, breccia zones and vein copper mineralisation over an interval of >7 km. This trend parallels a system of NW trending and steep NE dipping regional faults in the region, possibly a continuation or extension of the major Incapuquio fault system in the Cuajone-Quellaveco-Toquepala district some 115 km to the southeast.
Hypogene Mineralisation and Alteration
Hydrothermal alteration is correspondingly distributed over a NW elongated zone covering an area of 5 x 1.5 km in the Cerro Verde-Santa Rosa district. Potassic and phyllic zones are enveloped by a lower temperature propylitic halo. The higher temperature potassic zone occurs in the core of the deposit. It has a 'blotchy' appearance and is best developed at depth and is lithology dependent, with two varieties:
• mainly orthoclase with lesser biotite (around 30%) and magnetite (<5%) in the Yarabamba Granodiorite and the dacitic porphyry stocks; and
• biotite-magnetite in the Charcani Gneiss, whilst a deeper low grade (0.1 to 0.15% Cu) sub-facies of the potassic zone comprises a magnetite-biotite-albite assemblage. Magnetite cemented hydrothermal breccias at Santa Rosa are probably contemporaneous with this variety.
The average sulphide content in the potassic zones is estimated at ~3%, with a chalcopyrite:pyrite ratio of ~3 (Perea et al.,, 1983).
The potassic zones are surrounded in the upper parts of the deposits by quartz-sericite-pyrite phyllic alteration which is the principal host to economic grade mineralisation, with, on average, 5 to 7% sulphide and a chalcopyrite:pyrite ratio of 0.3 to 0.7. The phyllic assemblage is best developed in Yarabamba Granodiorite and the dacitic porphyry stocks, but is also observed in Charcani Gneiss. The temporal relationships between phyllic alteration and tourmaline breccias in the Cerro Verde deposits is uncertain, with breccia clasts having undergone silicification and intense quartz>sericite, but pyrite-free alteration. The Bonanza Breccia, the principal body of silica breccia at Santa Rosa is composed of angular fragments with intense sericite>quartz alteration and disseminated chalcopyrite in a matrix dominated by massive chalcopyrite and minor pyrite, magnetite and ferberite.
The principal hypogene sulphide/sulphates within the Cerro Verde deposits are pyrite, chalcopyrite, molybdenite, magnetite, galena, sphalerite, pyrrhotite, tetrahedrite, native copper, free gold, silver and anhydrite. Chalcopyrite is the dominant primary copper sulphide. Hypogene copper mineralisation is distributed within the Yarabamba Granodiorite (40%), Charcani Gneiss (29%), the 10 monzontic-tonalitic porphyry stocks ( 21%) and 10% in the quartz and quartz-tourmaline breccias (INGEMMET 2007).
Hydrothermal activity of phyllic alteration is constrained by laser-induced incremental-heating 40Ar-39Ar of sericite (muscovite-2M1) dates of 61.8±0.7 and 62.0±1.1 Ma at Cerro Verde, and 62.2±Ma at Santa Rosa (Quang et al., 2003). Cerro Negro, is the youngest of the three deposits and was emplaced in the shallowest environment (Núñez et al., 2000).
The mineralised body of Cerra Verde plunges at 70 to 85°SW (towards Santa Rosa), whilst the Santa Rosa mineralisation plunges to the NE (towards Cerro Verde). Both occur within a wide common halo of alteration, and are interpreted to coalesce at depth.
Supergene Mineralisation
Supergene oxide and sulphide mineralisation is variably developed in all three deposits and is zoned as follows:
• an upper zone of leaching composed of carbonates, hematite, goethite and lesser jarosite which averages 70 m in thickness and is best developed in tourmaline breccias where it locally persists to depths of >250 m, as at the Cerro Verde deposit. The widespread presence of hematite indicates weathering and oxidation of an earlier chalcocite blanket (Anderson, 1982), whilst relict zones of chalcocite-kaolinite occur throughout the supergene profile. Jarosite is locally abundant over the originally more pyritic margins of the Cerro Verde and Santa Rosa deposits.
• an underlying oxide and mixed oxide-sulphide layer characterised by a brochantite stockwork cut by minor chrysocolla veins, with associated neotocite, malachite, tenorite, antlerite, chrysocolla, chalcanthite, cuprite and chalcedony. The neotocite or copper pitch, which is a Cu-Fe-Mn oxide mineral, occurs above the main development of brochantite. The distribution of the oxide ores is erratic and commonly spatially associated with chalcocite-kaolinite or hematitic zones.
• an irregular and discontinuous blanket of secondary sulphide enrichment comprising chalcocite, covelite, digenite and bornite.
At Cerro Verde there are at least two supergene sulphide enrichment blankets: i). an older, ~15 m thick remnant horizon, discontinuously preserved, hanging within hematitic leached cap; and ii). a deeper, younger, more localised, but thicker zone located within a large body of tourmaline breccia, juxtaposed upon the underlying hypogene mineralisation, and overlain by the main zone of copper oxides. This lower blanket averages 60 to 80 m thick, but increases to 100 m, and locally reaches 150 m in the main tourmaline breccia body. In general, the supergene sulphides thin to the north and NE. The single preserved blanket at Santa Rosa ranges from 20 to 45 m in thickness, with an underlying transition where hypogene chalcopyrite is partially replaced by chalcocite, covellite and bornite.
Multiple dating of supergene related alunite partially replaced by natroalunite revealed two populations, of 38.8 to 36.1 Ma and 28.0 to 24.4 Ma, demonstrating that supergene activity had commenced by the latest Eocene, during the Incaic orogeny, and continued into the Oligocene. Deep (~300 to 350 m) leaching in the Santa Rosa deposit is indicated by ~26 Ma natroalunite during the late Oligocene, inferred to have formed beneath the older La Caldera surface. Veins of ~23 Ma alunite and ~21 Ma natroalunite in a hematitic leached capping at the top of the Cerro Verde deposit are truncated by the Santa Rosa surface, which is inferred to have developed after 21 Ma. At Cerro Verde, the ages of alunite group minerals decrease with increasing depth to 6.7-4.9 Ma at >120 m depth, suggesting deepening of the supergene profile into the Miocene beneath the Santa Rosa surface. Jarosite dates of 1.3 to 0.7 Ma indicate the persistence of minor supergene activity into the Pleistocene (Quang et al., 2003).
The preservation of remnants of Late Eocene alunite and supergene sulphides within Late Oligocene to Late Miocene supergene leached capping at Cerro Verde suggests the overall physiographic configuration of the district was very consistent from the Late Eocene to the Middle Miocene, and that semi-arid climatic conditions favouring supergene enrichment prevailed from the Mid-Eocene until the Late Miocene or Early Pliocene onset of the current hyper-aridity (Quang et al., 2005).
Ore Reserves and Mineral Resources
The reserves at Cerro Verde and Santa Rosa in 2003 comprised (Quang et al., 2003):
Supergene ore - 331 Mt @ 0.52% Cu, and
Mostly hypogene ore - 464 Mt @ 0.61% Cu.
Remaining proved + probable reserves at Cerro Verde, as at December 31, 2011 (Freeport-McMoRan, 2012) were:
Mill ore - 3.752 Gt @ 0.39% Cu, 0.015% Mo, 1.67 g/t Ag (Cu - 86.0%; Mo - 54.4% recovery);
Crushed leach ore - 146.0 Mt @ 0.50% Cu (79.8% recovery);
ROM leach ore - 79 Mt @ 0.21% Cu (41.0% recovery).
Remaining Reserves and Resources at the Cerro Verde operation, as at December 31, 2017 (Freeport-McMoRan, 2018; US Securities and Exchange Commission, Form 10-K) were:
Proved + Probable Ore Reserves
Mill ore - 3.471 Gt @ 0.37% Cu, 0.01% Mo, 1.94 g/t Ag (0.885 Gt Proved and 2.586 Gt Probable, both at the same quoted grades)
Recovery, Cu - 86.4%; Mo - 54.4%; Ag - 44.8%;
Crushed leach ore - 75.0 Mt @ 0.33% Cu;
ROM leach ore - 31 Mt @ 0.21% Cu.
Mineralised material, in addition to Ore Reserves, equivalent to Mineral Resources
Milling material - 969 Mt @ 0.36% Cu, 0.02% Mo, 1.9 g/t Ag;
Leaching material - 6 Mt @ 0.24% Cu.
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2007.
Record last updated: 18/3/2018
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Cerro Verde and Santa Rosa
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Selected References:
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Demouy, S., Paquette, J.L., Saint Blanquat, de M., Benoit, M., Belousova, E. A. OReilly, S.W., Garcia, F, Tejada, L.C., Gallegos, R. and Sempere, T., 2012 - Spatial and temporal evolution of Liassic to Paleocene arc activity in southern Peru unraveled by zircon U-Pb and Hf in-situ data on plutonic rocks: in Lithos v.155, pp.183-200.
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Quang, C.X., Clark, A.H., Lee, J.K.W. and Guillen, B.J., 2003 - 40Ar-39Ar Ages of Hypogene and Supergene Mineralization in the Cerro Verde-Santa Rosa Porphyry Cu-Mo Cluster, Arequipa, Peru: in Econ. Geol. v.98, pp. 1683-1696.
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Quang, C.X., Clark, A.H., Lee, J.K.W. and Hawkes, N., 2005 - Response of Supergene Processes to Episodic Cenozoic Uplift, Pediment Erosion, and Ignimbrite Eruption in the Porphyry Copper Province of Southern Peru: in Econ. Geol. v.100, pp. 87-114.
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Tumialan, P.H., (Ed.) 2003 - Cerro Verde, Santa Rosa y Cerro Negro: in Tumialan, P.H., (Ed.) 2003 Compendio de Yacimientos Minerales del Peru, Instituto Geologico Minero y Metalurgico, Republic del Peru, Capitulo VII, Porfidos de Cobre, Boletin no. B-10, pp. 137-139, 141 (in Spanish with English translation).
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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