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Constancia, Pampacancha, Katanga, San Jose |
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Peru |
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Cu Mo Ag Au
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Super Porphyry Cu and Au
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The Constancia porphyry Cu-Mo and Pampacancha porphyry-skarn deposits are located in the south-eastern Andes of Peru, in the province of Chumbivilcas, department of Cusco, ~600 km SE of Lima and 100 km south of Cusco at elevations of 4000 to 4500 masl (#Location: 14° 27' 35"S; 71° 45' 57"W).
Copper and gold were exploited at the Katanga deposit, located ~5 km NW of Constancia, from the early twentieth century to the early 1990s. The Katanga deposit comprises narrow skarn bodies developed along the contact between marbles and monzonite stocks, with Cu, Ag and Au mineralisation occurring as hypogene sulphides.
The San José prospect, now part of Constancia, was explored by Mitsui during the 1980s, seeking more high-grade ore for the nearby Katanga mine operation. By 1995, evidence of porphyry style mineralisation had been identified over an area 1.4 x 0.7 km, with some Cu enrichment below a widespread leached cap developed in both porphyry and skarn.
Subsequent exploration by Mitsui and Rio Tinto culminated in an estimated resource of 189 Mt @ 0.68% Cu, 0.02% Mo, using a 0.5% Cu cutoff. This resource was considered too small and a farm-out partner was sought in 2004. By 2005 Norsemont Mining entered the joint venture, and by 2009 had acquired 100% ownership of the project. In March 2011, Hudbay Minerals Inc. took 100% control of Constancia by purchasing Norsemont Mining after a formal take-over bid. Operations commenced at the Constancia mine during the fourth quarter of 2014 and achieved commercial production on April 30, 2015.
Geology
The oldest rocks exposed in the district belong to the Lower Cretaceous Chilloroya (or Murco) Formation, a sequence of white, red, violet or grey medium-grained sandstones with intercalations of reddish mudstones and possibly underlying sandstones, siltstones and occasional calcareous and quartzite horizons of the upper Hualhuani (or Soraya) Formation. These are discordantly overlain by a suite of massive, grey micritic limestones with minor intercalations of shales, calc-arenites and lenses of conglomerates of the Arcurquina Formation, which has been correlated with the Upper Cretaceous Ferrobamba Formation. These rocks are exposed in a 15 x 5 km, north-south elongate corridor, embraced by plutonic intrusions of the Oligocene Andahuaylas-Yauri Batholith. This batholith varies from dioritic to granodioritic in composition, composed of plagioclase and orthoclase, quartz, hornblende, biotite, apatite, zircon and sphene.
Small mantos, veins and lenses of massive magnetite skarn are common in the area, apparently related to emplacement of this batholith. Several stocks, dykes and laccoliths of monzonitic intrude and cross-cut all the lithologies described above. Where these rocks have intruded limestones, it is common to find mineralised skarns, some of which contain Cu-Au-Ag mineralisation e.g., at the Katanga mine. Some of the stocks have characteristics typical of porphyry copper mineralisation as at Constancia, which is located on the eastern margin of the Andahuaylas-Yauri Batholith.
Where the Arcurquina Formation is in contact with intrusive rocks within the Constancia area, they are altered to marble or pyroxene-diopside-garnet-magnetite-epidote skarn, with or without sulphides. The limestones and skarns dip gently to the SE, away from the principal monzonite found in the southern part of the Constancia mineralised zone. The overall thickness of the sedimentary package is unknown. The base of this limestone unit has been interpreted to correlate with the favourable skarn horizon at the Tintaya Mine to the south, and elsewhere in the region.
Glacial moraines cover the northern and eastern margins of the Constancia deposit.
Multiple phases of monzonite and monzonite porphyry characterise much of the deposit area at Constancia. At least four main intrusive phases have been
recognised, which are, from oldest to youngest:
• Diorite, which while not related to the Andahuaylas-Yauri Batholith forms the ‘Intrusive Basement’ to the Constancia deposit.
• Monzonite Porphyry 1, that outcrops as a large stock on Constancia Hill, and extends west to San José, and hosts most of the porphyry-related mineralisation. It comprises 40 to 50% plagioclase as up to 3 mm phenocrysts, with 5 to 7% hornblende as up to 6 mm long elongated crystals, set in a finer matrix pinkish orthoclase crystals and magnetite. The upper levels of this stock are mostly leached.
• Micro Monzonite Porphyry, characterised by a fine-grained texture with 60 to 70% plagioclase crystals up to 2 mm long, 1 to 3% biotite and <1% magnetite in a the light grey matrix. Within this body, 1 to 7 mm, rounded, xenolith-like alteration patches are found. This porphyry outcrops as a stock in the south, but is more widespread west of the deposit, including the San José zone. Some dykes of this lithology have been mapped cutting the Monzonite Porphyry 1.
• Quartz Monzonite Porphyry, which is mostly found in the Constancia zone, occurring as wide, north-south to NNW trending dykes with dark, fine-grained chilled margins. They contain 40 to 60% plagioclase with ~7% tabular, well formed hornblende crystals occurring as phenocrysts in a greenish matrix. The most distinctive feature is 1 to 2% quartz 'eyes' that are three to four mm wide. No mineralisation is associated with the dykes, which may be up to 60 m wide, and are mostly vertical.
• Monzonite Porphyry 2, occurring as a dyke-like monzonite porphyry, mostly outcropping to the west of Constancia, between it and San José zone. It is characterised by 60 to 70% plagioclase phenocrysts up to 4 mm long and normally sub-rounded in a whitish matrix with little magnetite and 5 to 8% mafics where hornblende is > biotite. It occurs as up to 150 m wide, north-south striking dykes that appear to have a steep easterly dip.
• Andesite, occurring as a dark-grey, aphanitic rock which is greenish where subjected to chloritisation, composed of plagioclase and
hornblende phenocrysts, and ~1% magnetite. This andesite occurs mainly as narrow dyke-like bodies, some of which close to the contacts with quartz monzonite porphyries.
Several brecciated units and minor felsic dykes are also found in the area.
Structure
Structural activity has played a significant role in preparing and localising the hydrothermal alteration and accompanying Cu-Mo-Ag-Au mineralisation, including skarn formation. Major inter- and post-mineral fracture systems in the deposit area strike NE. The main structures include the:
• Barite Fault, a late, NE-SW striking structure between the Constancia and San José zones that is generally 5 to 10 m wide, and is characterised by broken and brecciated monzonite-barite-quartz-copper oxides and galena. In detail, it has an en echelon structure, with narrow NE sections separated by ENE to east-west tension gashes containing higher grade mineralisation cut by a series of pits and tunnels over a length of ~1000 m.
Other veins with similar strike and mineralisation are also known. The Barite Fault has been subjected to late post-hypogene copper displacement that post-dates and limits the Constancia mineralisation on its north-western side. This fault system may also control the San José zone on its south side, and it has been suggested that the San José and Constancia mineralised zones may belong to the same body, offset across the sinistral Barite Fault.
• San José Pit Wall Fault, a north-south striking structure that bounds the eastern side of the San José pit.
• Yanak Fault, that represents to the youngest structural event in the area. It strikes NNW-SSE and is also located between the Constancia and San José deposits, and has been recognised over an ~3 km length and width of up to 50 m, represented by gouge and milled rock.
Alteration and Mineralisation
Porphyry style alteration comprises the following:
• Potassic Alteration, comprising an assemblage characterised by secondary K feldspar, and variable quantities of hydrothermal biotite replacing both earlier ferromagnesian minerals and rock matrix. Quartz veining is commonly associated, particularly 'A' and 'B' veinlets. Anhydrite veinlets are also common. The intensity of alteration is variable, varying from weak to strong.Hydrothermal magnetite is found as disseminations and with 'A' veinlets in deeper sections of the deposit. Associated mineralisation includes chalcopyrite ±bornite-molybdenite-pyrite, mainly in 'A' and 'B' veinlets, but also replacing ferromagnesian minerals or filling fractures.
Copper grades generally range from 0.2% to 4%, and are highest where fracture-fill Cu mineralisation is superimposed on earlier disseminated copper sulphides. The high-grade hypogene Cu ore is hosted by a dense 'A'-veinlet stockworks, with low pyrite:chalcopyrite ratios typically of the order of 1:1 to 2:1. Molybdenite commonly increases with depth, related to 'B' veinlets. Bornite is found sporadically, particularly at deeper levels, sometimes associated with anomalous gold.
• Propylitic Alteration is transitional outwards the potassic alteration, extending for >1 km from the porphyry intrusive contacts. It comprises an assemblage that includes epidote-chlorite-calcite-pyrite-rhodochrosite, with subordinate accompanying chalcopyrite filling fractures or replacing mafic minerals. Sphalerite-galena veinlets and veins form a halo to the copper-molybdenum mineralisation within the propylitic alteration zone, occurring as far as 3 km away from the porphyry copper system.
• Phyllic Alteration forms a pervasive carapace surrounding and sometimes overprinting potassic alteration, and results in almost complete destruction of primary rock textures. The phyllic assemblage includes sericite-quartz-pyrite, limited amounts of chalcopyrite and associated occasional 'D' veins and veinlets.
The Constancia deposit is a porphyry Cu-Mo-Ag system that also includes copper-bearing skarn mineralisation, common in the Yauri-Andahuaylas district. Five mineral associations are defined within the Constancia deposit area, as follows:
• Hypogene, porphyry-style, occurring a disseminations, quartz-vein stockworks and fracture-controlled chalcopyrite-molybdenite mineralisation within the intrusive, predominantly associated with the Monzonite Porphyry 1, which, when mineralised, shows either extensive quartz-sericite or potassic alteration. This style constitutes the bulk of the deposit, extending to well below the 3900 m level (compared to the 4000 to 4500 m surface). Two zones of porphyry-style mineralisation are known within the project area, Constancia and San José to the NW. At San José mineralisation occurs at surface, while at Constancia it is deeper and extends over a north-south elongated area of 1200 x 800 m.
• Hypogene, skarn-style, containing chalcopyrite, rare bornite, galena and sphalerite mineralisation. This style occurs at the contact between the intrusives and limestones, most commonly calcareous sandstones and arkoses of the Chilloroya Formation, to form magnetite-garnet skarn, but more commonly pyroxene-diopside±garnet-epidote associations.
• Supergene, including digenite-covellite-chalcocite with rare native copper, mainly hosted by intrusives, occurring immediately beneath, and occasionally as remnants within, the leached cap. The highest Cu grades in the Constancia porphyry are typically associated with this style as well as the skarn zone.
• Transitional (Mixed) mineralisation, including secondary copper sulphides/chalcopyrite in the monzonite, and corresponds to the zone where both supergene and hypogene sulphides co-exist.
• Oxide copper mineralisation, which occurs locally, and while shallow,volumetrically small and not considered relevant to exploitation.
Oxidation and leaching are intense at the surface, with almost no fresh sulphides remaining. A leached cap persists to variable depths, up to a maximum of 100 m where fracturing is more intense, or in rocks with intense stockwork development. Oxidation decreases towards the margins of the deposit and in the magnetite skarn, occurring to only a few tens of metres below the surface. The principal iron-oxides are jarosite and goethite with lesser hematite after supergene chalcocite. Copper grades in the leached cap are typically of the order of 100 to 200 ppm, considered to be strongly geochemically anomalous. Molybdenum and gold values in the leached cap are generally similar to hypogene grades.
Pampacancha
The Main Pampacancha body is located ~3 km SE of the Constancia porphyry, and is associated with massive, grey micritic limestone of Upper Cretaceous
Arcurquina or Ferrobamba Formation in contact with the dioritic porphyry where it is altered to a magnetite skarn, that hosts economic Cu-Au-Mo mineralisation. Intrusive rocks include the unmineralised Oligocene basement diorite, intruded by a diorite porphyry interpreted as the source for skarn alteration, which is, in turn,
cut by intra-mineral monzonite intrusions which contributed Cu-Au and also locally stope out skarn Cu-Au mineralisation that is best developed at the upper and lower margins of the limestone body.
Prograde magnetite-chalcopyrite-pyrite skarn grades to marginal less well mineralised garnet and pyroxene skarn which are locally overprinted by epidote-bearing retrograde skarn. Epithermal low sulphidation quartz-sulphide Au-Cu style mineralisation produces common supergene enriched Au anomalies, and along with other features such as hydrothermal alteration and veins typical of near porphyry settings.
The Pampacancha Main Body has been divided into three sectors:
• Sector 1, that extends over an area of ~300 x 250, characterised by magnetite>>calc-silicates skarns developed from surface
to a depth of ~200 m, but occasional deeper. This sector is intruded by potassically-altered diorite porphyry and fine-to-medium grained diorite, the latter
also showing evidences of potassic alteration. Skarn bodies thin to the south due to erosion.
• Sector 2, a 200 x 250 m zone immediately to the north of Sector 1, characterised by a limestone body sandwiched by two main skarn layers, that extend from about 70 m depth to about 250m. The upper skarn extends from near the surface, alternating with thin limestone layers, all disrupted by diorite porphyry dyke emplacement and development of endoskarn. The lower skarn layer extends to a maximum depth of ~300m.
• Sector 3, that is immediately NW of sector 2 over an area of ~500 x 350m, and is basically the same as the latter, with a limestone body in between two skarn bands, thinning out to the north, east and west.
Reserves and Resources
Published ore reserve and mineral resources (Hudbay Minerals, 2015 Annual Report) were:
Ore reserves at 1 January 2015
Constancia
Proved reserve - 477 Mt @ 0.30% Cu, 0.0094% Mo. 0.038 g/t Au, 2.91 g/t Ag;
Probable reserve - 94 Mt @ 0.22% Cu, 0.0061% Mo. 0.036 g/t Au, 2.77 g/t Ag:
Pampacancha
Proved reserve - 23 Mt @ 0.52% Cu, 0.0142% Mo. 0.298 g/t Au, 4.28 g/t Ag;
Probable reserve - 20 Mt @ 0.44% Cu, 0.0159% Mo. 0.252 g/t Au, 3.74 g/t Ag:
TOTAL reserves - 614 Mt @ 0.30% Cu, 0.0093% Mo. 0.054 g/t Au, 2.97 g/t Ag:
Mineral resources at 30 September 2015 [exclusive of ore reserves]
Constancia
Measured resource - 68 Mt @ 0.22% Cu, 0.0059% Mo. 0.036 g/t Au, 2.17 g/t Ag;
Indicated resource - 293 Mt @ 0.20% Cu, 0.0056% Mo. 0.033 g/t Au, 1.96 g/t Ag:
Inferred resource - 200 Mt @ 0.19% Cu, 0.0051% Mo. 0.031 g/t Au, 1.86 g/t Ag:
Pampacancha
Measured resource - 5 Mt @ 0.41% Cu, 0.0069% Mo. 0.243 g/t Au, 5.46 g/t Ag;
Indicated resource - 6 Mt @ 0.34% Cu, 0.0098% Mo. 0.211 g/t Au, 4.68 g/t Ag:
TOTAL measured + indicated resources - 372 Mt @ 0.20% Cu, 0.0059% Mo. 0.039 g/t Au, 2.09 g/t Ag:
TOTAL inferred resources - 200 Mt @ 0.19% Cu, 0.0051% Mo. 0.031 g/t Au, 1.86 g/t Ag.
The processing plant at Constancia is designed to process a nominal throughput of ~80 000 tpd of ore and average annual throughput of 29 Mt from the Constancia open pit and Pampacancha satellite deposit (Hudbay Minerals, 2016).
The information in this summary is taken from "Meagher, C. and Humphries, M., 2012 - Constancia Project, Province of Chumbivilcas, Department of Cusco, Peru; an NI 43-101 Technical Report prepared for HudBay Minerals Inc., 333p."
The most recent source geological information used to prepare this decription was dated: 2012.
Record last updated: 21/3/2017
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.
Constancia
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