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Cerro Corona |
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Peru |
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Au Cu
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Super Porphyry Cu and Au
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The Cerro Corona porphyry gold - copper deposit is located in the Hualgayoc mining district within the Western Cordillera of the Peruvian Andes in Northwestern Peru, 80 km south-east of the La Granja deposit, 25 km NNW of the Yanacocha mine, 45 km NNW of Cajamarca, approximately 650 km north-west of the capital, Lima, and around 150 km to the east of the Pacific coast port of Chiclayo (#Location: 6° 45' 47"S, 78° 37' 20"W).
The Hualgayoc District has seen significant production of Ag, Pb, Zn, Au and Cu since the Spanish discovered the area in the 18th century. Early mining concentrated on placer deposits and oxidised surface veins. The veins at Cerro Jesus in the district reportedly produced more than 1500 t of silver. The curent open pit operation commenced mining in 2007, operated by Gold Fields Limited.
Regional Setting
The Cerro Corona deposit occurs towards the southern end of the Cajamarca Mineral Belt in the Western Cordillera of the northern Peruvian Andes, a generally north-south to NW-SE trending belt of Oligocene to Miocene porphyry copper and/or gold deposits that extends for 350 km from Cajamarca in the south to the Ecuadorian border and includes two geochemically distinct groups of deposits along this trend namely: i). porphyry Cu-Mo deposits which include Rio Blanco, La Granja, Michiquillay, El Galeno and Cañariaco; and ii). porphyry Cu-Au deposits which include Cerro Corona, Minas Conga and La Carpa deposits, and the concealed Kupfertal porphyry Cu-Mo mineralisation which passes up into the associated high sulphidation lithocap Cu-Au and epithermal Au ores of Yanacocha. These systems are mostly associated with dacite to monzonite to diorite intrusions, which intrude basement rocks of Upper Jurassic to Lower Cretaceous quartzites, limestones and mudstones of the Goyllarisquizga Formation and Early Tertiary sequences of andesitic to dacitic lavas and tuffs of the Llama and Porculla Formations which together comprise the Lower Calipuy Group.
For detail of the broad regional setting and geology of the Peruvian Andes, see the separate Peruvian Andes record.
Basement in the region comprises Precambrian to Early Palaeozoic pelitic schists of the Olmos Complex which includes Precambrian metamorphic rocks, overlain by Ordovician shales and sandstones. These are unconformably overlain by Permian conglomerates, sandstones and volcanic flows and tuffs, which are in turn overlain by Late Triassic-Early Jurassic marine sediments intercalated with minor volcanic units of the La Leche Formation. These are succeeded by the Early to Late Jurassic volcano-sedimentary sequence of the Oyotún Formation. The Mesozoic rocks were deposited in ensialic, extensional, marginal basins related to eastward subduction, which extend the length of the Andes. During the Latest Jurassic to Early Cretaceous the region was uplifted and eroded by the mid-Cretaceous Mochica tectonic phase. By the late Early Cretaceous, subsidence resulted in an eastern sub-basin bounded to the east by the basement Marañon High, and the deposition of 2 to 3 km of Cretaceous strata. The oldest of these sediments are thick regionally extensive deltaic sandstones with shales and coal, and a thin marine limestone which collectively form the Early Cretaceous Goyllarisquisga Group which unconformably overlies the older rocks. From the close of the Early Cretaceous to the middle of the Late Cretaceous, a marine transgressive sequence of up to 1500 m of marls, shales and limestone were deposited across the region. Sedimentation ceased abruptly at the beginning of the Early Tertiary when the basin was deformed by the Late Paleocene Incaic I (59 to 55 Ma) and Middle Eocene Incaic II phases (43 to 42 Ma), which resulted in the formation of a foreland thrust and fold belt with SW-dipping, NE-verging thrust sheets, and the development of open, upright folds. Some thrusts were reactivated and folded during the Quechua 1 orogenic pulse (17 Ma). These periods of activity were accompanied by the eruption and deposition of volcanic units of the Eocene and Miocene Llama and Porculla Formations, which together comprise the Calipuy Group. This episode was followed by uplift and erosion and then by renewed magmatism and volcanic activity resulting in the deposition of 1000 to 3000 m of sub-aerial andesitic to rhyodacitic volcanics and associated coeval intrusions to form the 12 to 10 Ma Yanacocha volcanic complex and the Middle to Late Miocene Huambos Formation which capped the stratigraphic sequence in the region. Subsequently, the area has been variably eroded by glaciation.
The structural fabric of the region is reflected in generally north-west trending thrust faults and fold axes, although, Cerro Corona is located where the structural framework is affected by the Cajamarca flexure, which has produced east-west trending mega-lineaments that transect the dominant northwest fabric. The epithermal gold deposits of Yanacocha and Maqui-Maqui also in this flexure zone. The district structure is characterised by large open folds in the sediments, with axial planes trending approximately 315° and dipping steeply southwest. No large scale thrusts are evident in the Hualgayoc district, with faulting mainly as normal and oblique slip fractures with offsets of a few metres. Three fracture sets have been recognised: 315° with vertical to steep NE dips; 080° with 65°NW dips; and 045° with 65°NW dips. All three host vein mineralisation in the district cutting both sediments and intrusions, indicating that they have been active since emplacement of the intrusions.
Geology
The geology of the Hualgayoc district surrounding Cerro Corona, comprises sandstones, limestone, limy siltstones with minor shale interbeds of the Inca, Chulec, Pariatambo and Yumagual Formations which overlie the more siliciclastic rich Early Cretaceous Goyllarisquisga Group. These are unconformably overlain by andesitic to rhyodacitic volcanic flows and tuffs, and intruded by coeval plugs and stocks of similar composition which belong to the Cerro San Miguel, Cerro Jesus, Cerro San Jose, Cerro Hualgayoc, Cerro Tantahuatay and Cerro Corona intrusions. Based on age dates, these stocks represent a number of intrusive centres, including: i). the 10.5 to 14.3 Ma dioritic Cerro San Miguel, Jesus and Jose intrusions, and ii). the slightly younger than 7.9 Ma rhyodacitic Cerro Hualgayoc intrusive. Alteration of the andesitic Cerro Tantahuatay intrusion is dated at 12.4 Ma, while Cerro Corona, although it is quartz-dioritic, is believed to be coeval with the San Miguel intrusive group. The manto and vein-type Pb-Zn-Ag-Cu mineralisation of the district occur as stratabound metasomatic replacements in limestones and fracture-fill in sediments and intrusives, respectively, and with the porphyry Au-Cu, are genetically associated with the same group of Tertiary intrusions.
The Cerro Corona intrusive that hosts the Cerro Corona copper-gold deposit has a quartz-diorite composition and is a subvertical, cylindrical body, exposed at surface over a NE-SW elongated area of ~1000 x 450 to 700 m. Geological modelling strongly suggests that the Cerro Corona porphyry probably comprises four or five separate nested stocks of similar composition, the last two of which are barren (Gold Fields, 2012). Outcrops are typically strongly silicified and cut by abundant quartz veins forming dense stockworks, particularly in the core of the intrusive. Fresh intrusive in drill core is medium to dark grey and porphyritic, with larger (3 to 10 mm) feldspar phenocrysts, large (3 to 6 mm) biotite booklets, and smaller (1 to 3 mm) hornblende phenocrysts in a matrix of generally very fine-grained (20 to 30µm) quartz and feldspar. The rock comprises around 20 to 40% predominantly alkali feldspar; 20 to 25% secondary orthoclase in altered intrusive; 20 to 50% quartz in matrix and veinlets; 10 to 20% sericite in altered intrusive; 5 to 7% biotite; up to 10% hornblende; and 2% opaques ranging up to 10% with vein material. The intrusive is relatively homogenous, without distinguishable phases and intrusive breccias.
The immediate wall rock succession strikes NW-SE, and dips to the SW. It mostly comprises the carbonate rocks of the Yumagual Formation, underlain by the Pariatambo Formation which is intruded by the northeastern third of the Cerro Corona intrusive, while at surface, the Chulec Formation is found immediately to the NE of the intrusion. The enclosing limestone host rocks of these formations are relatively undisturbed, indicating passively emplaced, although the contacts are locally sheared or faulted. The limestone wall rocks are extensively 'silled' by the intrusive, forming large, typically 10 to 30 m thick 'rafts' of limestone within the quartz-diorite body. Skarn is not well developed, nor significantly mineralised, and is generally limited to a 30 m wide skin outwards from the intrusive contact.
Structure
On a regional scale, the emplacement of the Cerro Corona intrusive appears to have been controlled by the intersection of north-west, Andean parallel and north-east, Andean-normal (transandean) regional structures. A dominant NE-SW trending fault system, running through the intrusion, and densest in the SE half, is referred to as the Mariela Fault trend, which has an important relationship in the distribution of the mineralisation. Other prominent sets trend NW-SE, east-west, NNW-SSE and SWS-ENE. The intrusive and surrounding limestones are in turn cut by numerous sub-vertical faults and fracture zones paralleling these same directions with off-sets of 30 to 70 m. While the bulk of the Au and Cu mineralisation is disseminated, it is also partly structurally controlled, occurring as isolated high grade veins within the intrusive in strong sets of fractures, veins and faults trending at 025°, 075° and 120°, reflecting a radial pattern of faulting and fracturing observed near the centre of the intrusive. The majority of these radial faults dip outward from the centre of the intrusive, showing normal and/or dextral movement.
Alteration
The Cerro Corona quartz-diorite intrusive stock, in general, displays an alteration zonation comprising an early potassic core, surrounded by weak distal propylitic alteration, the product of magmatic dominated potassic metasomatism, overprinted by weak fracture controlled phyllic (quartz-sericite-pyrite), the result of hydrothermal hydrolysis, and strong semi-pervasive sericite-clay argillic alteration. The entire pre-mining surface exposure of the stock appears to have been affected by weak to moderate potassic metasomatism, but has been overprinted by the strong sericite-clay in all but two remnant potassic centres, broadly coinciding with the two low grade/barren cores and the other in the north-eastern sector of the intrusive.
Propylitic alteration is weakly developed on the outer margins and in the 'barren cores' of the intrusive, and along early fractures. It is characterised by chlorite, epidote and calcite, replacing hornblende. Pyrite, with minor chalcopyrite and hematite occur in association with a siderite-ankerite carbonate assemblage.
The generally weakly developed phyllic alteration has a marked fracture control as indicated by the lack of textural destruction of most affected rocks. It is found in narrow (1 to 3 cm) quartz-sericite-pyrite selvages on quartz stockwork veins, but coalesces to form broader 1 to 2 m wide zones where vein density is sufficiently high, and appears to form more strongly developed siliceous patches surrounding the barren cores. The associated up to 6 vol.% pyrite is generally fine-grained and dark grey, occurring as disseminations and in fine veinlets, and is best preserved in the south-western and western margins of the stock.
Sericite-clay or clay-sericite (argillic) alteration is ubiquitous from the surface to a depth of about 360 m below the highest point of the exposed stock. This variable sericite-clay alteration assemblage overprints all other alteration styles while preserving original rock textures, and appears to be partially structurally controlled, being frequently associated with fracture zones and inferred faults.
The youngest alteration is a swelling montmorillonite clay produced by meteoric water dominated leaching along fractures and faults and affected around 22% of the porphyry, to form a distinctive, crumbly 'popcorn rock' texture in drill core, produced when the clays expand in response to interaction with drilling fluids and/or water.
Limestone contacts are typically decalcified and partially silicified, with minimal skarn or marble development, restricted to irregular zones which are generally limited to a 30 to 40 m (locally 50 to 70 m) wide skin to the intrusive. They typically contain disseminated diopside, biotite and minor garnet. However, marble forms a halo up to 100 m wide in the southwest portion of the intrusive contact.
Mineralisation
The Cerro Corona quartz-diorite intrusive stock and porphyry Au-Cu deposit was passively emplaced into sedimentary country rocks at a depth interpretted to be of ≥1 km. The main body of the intrusion seems to comprises four or five nested stocks, the younger two of which are barren, producing a NE and a SW 'barren core'. These 'barren cores', with <0.14 g/t Au and <0.05% Cu, are located in the southwestern and northeastern part of the hypogene zone, forming irregular zones that has a diameters of around 150 m at the surface, expanding downward to about a 100 x 200 m area at 260 m below the general surface. These barren zones appears to coincide with a change in rock type, distinguished by fewer quartz veins, finer-grain size, somewhat darker colour, slightly larger feldspar phenocrysts and potassic alteration in which mineralisation is weak to non-existent.
The main hypogene Au and Cu ore occurs as a circular shell enclosing the 'barren cores', but appears to diminish in diameter at depth, splitting into a series of tapering roots. The main hypogene ore zone passes into an barren diorite outer shell. Deposition of the primary, disseminated Au and Cu mineralisation appears to have accompanied emplacement of the intrusion and was associated with early potassium metasomatism, reflected by K feldspar-biotite alteration.
The primary hypogene mineralisation comprises 3 to 8% pyrite, 1 to 3% chalcopyrite, <1% bornite, trace molybdenite, galena and sphalerite and microscopic gold, with accessory 1 to 4% magnetite, up to 10% earthy and specular hematite and minor carbonates. The gold and the primary sulphides are found within quartz stockwork veinlets and as disseminations within the wall rocks. Quartz veins commonly contain magnetite, in some cases associated with high gold and chalcopyrite contents, although there does not appear to be an exclusive minerals association. Primary magnetite is also disseminated within wall rocks. Secondary copper sulphides, namely chalcocite and covellite (1 to 2% total), are restricted to the supergene enrichment blanket, where they are found as rims or coatings on pyrite and chalcopyrite grains.
Magnetite appears to have been the earliest deposited metallic mineral, while disseminated pyrite typically predates chalcopyrite. Most of the gold is coeval with the chalcopyrite. The later chalcopyrite hematite-quartz mineralisation is observed to have corroded earlier pyrite. Gold typically occurs as inclusions within chalcopyrite, and on chalcopyrite-pyrite contact boundaries, as well as fracture fillings within pyrite. The Au composition generally includes 5 to 15 atomic% Ag. Preliminary studies indicate a paragenetic sequence of magnetite, hematite, pyrite, chalcopyrite, bornite and molybdenite, all most likely related to early potassic alteration. The initial mineralisation was mainly disseminated, although the younger stages were increasingly vein hosted, while the late stage was predominantly barren pyrite veins.
Following the deposition of the primary sulphides and potassic alteration phase, abundant fracturing resulted from the progressive inward cooling of the intrusive, allowed the formation of multiple, crosscutting quartz stockwork veins, oriented in radial and concentric patterns. Between 10 and 25% quartz stockwork veins, locally ranging up to 50 to 70%, are found throughout the intrusive, and are generally sub-vertical. Individual veins vary from 1 mm to 1 m in width, but average between 3 and 10 mm. Local, intensely silicified, zones up to 10 m wide are formed by coalescing veins. One of these zones occurs at the topographic peak of Cerro Corona, where intense silicification and quartz veining form a resistive 'cap' of almost pure saccharoidal silica which is sulphide poor, low grade, and is generally coincident with the 'barren core'. Early veins were generally low grade, but, as the meteoric water dominated hydrothermal system developed, later veins became enriched in Au and Cu remobilised from the intrusive host rocks. This second stage of veining was associated with phyllic alteration, although later stages were accompanied by argillic developments, which strongly overprinted the deposit during the waning stages of the hydrothermal system.
The Cerro Corona stock and hypogene porphyry system was subsequently deeply eroded, exposing the roots of the hydrothermal system and the base of economic mineralisation. The upper portion of the deposit is oxidised and has low Cu grades. Below the 'oxide zone', which varies from 10 to 50 m in thickness, a 10 to 60 m thick blanket of supergene Cu mineralisation is present, separated by a transition or 'mixed zone' of variable thickness, ranging from 10 to 70 m. The 'supergene zone' is underlain in turn by hypogene Au and Cu sulphides, below around 100 m from the surface. Pyrite is present in both the supergene and hypogene ores.
The 'oxide zone' is characterised by ubiquitous iron oxides (3 to 5 vol% goethite, hematite and minor jarosite) and almost total removal of Cu mineralisation by supergene acid leaching. Cu grades vary from 0.02 to 0.15%, but are typically less than 0.05%, while gold grades are slightly upgraded. The 'mixed zone' consists of mixed oxide and sulphide minerals and is characterised by erratic Cu grades, with an assemblage that includes various iron oxides; pyrite, chalcopyrite and minor bornite; sparse chalcocite and covellite; trace molybdenite, galena, sphalerite, malachite, azurite, chalcanthite, brochantite and chrysocolla.
The 'supergene zone' is defined by the general absence of oxide minerals and the occurrence of up to 2 vol.% of the supergene Cu minerals chalcocite and covellite, and Cu grades characteristically elevated to 1 to 2% Cu, although Au grades are generally unaffected.
Reserves and Resources
Reserves and resource figures in 2004, using a 0.5 g/t Au cut-off (Brewer and Davis, 2004) were:
Oxide Mineralisation
Measured resource - 4.0 Mt @ 1.31 g/t Au, 0.06% Cu,
Indicated resource - 4.5 Mt @ 1.27 g/t Au, 0.05% Cu,
Sub-Total Measured + Indicated Oxide resources - 8.5 Mt @ 1.29 g/t Au, 0.05% Cu,
Inferred resource - 0.1 Mt @ 0.90 g/t Au, 0.04% Cu
Sulphide Mineralisation
Measured resource - 71.7 Mt @ 1.02 g/t Au, 0.53% Cu,
Indicated resource - 39.5 Mt @ 0.94 g/t Au, 0.46% Cu,
Sub-Total Measured + Indicated Sulphide resources - 111.2 Mt @ 0.99 g/t Au, 0.51% Cu,
Inferred resource - 7.6 Mt @ 0.72 g/t Au, 0.36% Cu
Total Measured + indicated Resources - 119.7 Mt @ 1.01 g/t Au, 0.47% Cu, for 3.8913 Moz Au, 0.5668 Mt Cu.
Total Inferred Resources - 7.7 Mt @ 0.73 g/t Au, 0.36% Cu for 1.1785 Moz Au, 0.0274 Mt Cu.
At 31 December, 2006, ore reserves and mineral resources, prior to the commencement of mining in 2007 (Gold Fields, 2007), were:
Measured + indicated + inferred resources - 235.3 Mt @ 0.7 g/t Au, for 156 t Au, and 221.1 Mt @ 0.4 % Cu, including
Proved + probable reserves - 99.9 Mt @ 1.0 g/t Au, for 99 t Au, and 94.0 Mt @ 0.5 % Cu.
At 31 December, 2012, reserves and resource statistics were (Gold Fields, 2012):
Mineral Resources
Open pit mine
Measured resource - 90.3 Mt @ 0.84 g/t Au, 0.45% Cu,
Indicated resource - 45.9 Mt @ 0.61 g/t Au, 0.39% Cu,
Inferred resource - 1.1 Mt @ 0.36 g/t Au, 0.35% Cu
Sub-total resources - 137.3 Mt @ 0.76 g/t Au, 0.43% Cu,
Stockpiles
Measured resource - 8.9 Mt @ 1.26 g/t Au,
Measured resource - 2.1 Mt @ 0.42% Cu,
TOTAL resources - 146.2 Mt @ 0.79 g/t Au, 0.43% Cu.
Ore Reserves
Open pit mine
Proved reserve - 77.4 Mt @ 0.89 g/t Au, 0.47% Cu,
Probable reserve - 26.3 Mt @ 0.67 g/t Au, 0.42% Cu,
TOTAL reserve - 103.6 Mt @ 0.83 g/t Au, 0.45% Cu.
This summary is largely based on "Brewer, N.H. and Davis, B., 2004 - Technical Report, Cerro Corona Project, Department of Cajamarca, Peru; Prepared for Gold Fields Limited and IAMGold Corporation, available via https://www.sec.gov"; and "Gold Fields Limited, 2012 - Cerro Corona Mine, Technical Short Form Report; 21p.
The most recent source geological information used to prepare this decription was dated: 2008.
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 Corona
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Selected References:
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Baumgartner, R., Gomez, P. and Escobar, G., 2016 - Comprehensive Mineralogical Characterisation at the Cerro Corona Cu-Au Porphyry Mine – the Fundamental Key for Geometallurgical Applications: in Proceedings The Third AusIMM International Geometallurgy Conference (GeoMet) 2016, The Australasian Institute of Mining and Metallurgy: Melbourne, pp. 221-230.
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Gustafson, L.B., Vidal, C.E., Pinto, R. and Noble, D.C., 2004 - Porphyry-epithermal transition, Cajamarca region, northern Peru: in Sillitoe, R.H., Perello, J. and Vidal, C.E., 2004 Andean metallogeny: new discoveries, concepts, and updates Society of Economic Geologists, Denver, Special Publication 11, Ch. 15, pp. 279-299.
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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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