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The La Arena epithermal and Au-Cu porphyry deposit is located in a relatively smooth terrane with undulating hills at elevations vary between 3000 and 3600 m asl, ~120 km east to ENE of the coastal city Trujillo, and 21 km west of Huamachuco in northern Peru (#Location: 7° 53' 42"S, 78° 8' 8"W).
The deposit was first discovered by geologists working for Sociedad Minera Cambior Perú S.A. in December 1994, and passed to IAMGOLD following its acquisition of Cambior. Rio Alto Mining Limited entered into an option and earn-in agreement with IAMGOLD Quebec Management Inc. in June 2009, and in February 2011, acquired 100% of the La Arena gold-copper project. The first gold was produced from the mine in May 2011. Mining proceeded as a two-staged development, comprising an oxide gold project in the first phase and a copper-gold sulphide project in the second.
The La Arena Deposit is located on the eastern flank of the Andean Cordillera Occidental in northern Peru, which is underlain by sedimentary rocks of the Mesozoic Western Basin that was folded and faulted during Cenozoic deformations. The dominant units in the regional stratigraphy are the folded Upper Jurassic Chicama Formation and the Lower Cretaceous Goyllarisquizga Group, that are mainly composed of siliciclastic sedimentary rocks, with lesser younger Lower to Upper Cretaceous carbonate rocks found in the cores of synclines. West of La Arena, the Cretaceous rocks are unconformably overlain by the Cenozoic volcanic suite of the Calipuy Group.
For detailed background of the regional setting see the Peruvian Andes Cu-Au Province record.
The regional stratigraphy in the district comprises, from the base:
• Chicama Formation, which represents the oldest outcropping rocks in the district, comprising soft, laminated marine black shales with thin sandstone intercalations.
Early to Mid Lower Cretaceous
Goyllarisquizga Group, composed of shallow marine siliciclastic rocks, and subdivided into the
• Oyon Formation, comprising fine to medium grained sandstone and thinly-bedded shale, with some coal seams.
• Chimú Formation, thickly-bedded, medium grained quartzitic sandstones, which constitutes the principal host rock for gold mineralisation at Lagunas Norte, El Toro, La Arena, La Virgin and Santa Rosa elsewhere in the region.
• Santa Formation, dolomitic limestone, marly limestones and shale;
• Carhuaz Formation, sandstones; and
• Farrat Formation, medium to coarse grained, crossbedded quartzite and sandstone with intercalated shale and marl.
Late Lower to Upper Cretaceous
Overlying the Goyllarisquisga Group sediments are Lower-Cretaceous shallow marine carbonates of the Machay Group Inca, Chulec and Pariatambo Formations, mainly carbonate rich formations, and the Upper Cretaceous Yumagual Formation. The Mesozoic sequence was folded and faulted during the Peruvian orogeny towards the end of the Cretaceous, the early stage of the developing Andean Orogeny.
Calipuy Group, which comprise the cordilleran arc volcanics that unconformably overlie the folded and faulted Mesozoic strata south and west of La Arena. These sub-aerial volcanic rocks are associated with Upper Miocene sub-volcanic intrusive bodies of andesitic to dacitic composition. The Calipuy volcanic rocks are mainly tuffs with basal agglomerate horizons, and are inter-bedded with andesitic lavas. They constitute the host rock for high sulfidation, low sulphidation and polymetallic mineralization at Lagunas Norte, Tres Cruces and Quiruvilca respectively.
To the west of La Artena, the Coastal Batholith is emplaced within volcano-sedimentary country rock of the Mesozoic Western Trough, temporal equivalents of the sequence described above. Cainozoic intrusive rocks, including granodiorites, diorites and quartz–feldspar porphyries, occur as isolated stocks cutting both the Mesozoic sedimentary sequence and the overlying Calipuy volcanics, vary in age from ~25 to 23 Ma. One of these intrusions hosts the porphyry style mineralization at La Arena.
The regional structure of the Jurassic-Cretaceous sedimentary comprises a series of linear folds, reverse faults and over-thrusts, all of which trend generally NW-SE. Individual folds range up to 80 km in length with separations of adjacent anticlinal and synclinal axes of ~5 km, varying in character depending on the relative competency of the stratigraphic units involved, e.g., the highly competent Chimu Formation forms structurally complex cores to the main anticlines, and have resisted erosion better than the enclosing strata.
The La Arena deposit is hosted within Mesozoic sedimentary rocks where they have been intruded by intermediate to felsic porphyritic stocks that tend to occupy the cores of anticlinal structures.
The oldest rocks in the deposit area, which are exposed in the cores of anticlines, are thinly bedded and laminated mudstones, minor siltstones and fine grained sandstones with occasional coal seams of the Lower Cretaceous Oyon Formation, the basal formation of the Goyllarisquizga Group. These are overlain by the Chimu Formation, the principal host rock to epithermal gold at La Arena (and elsewhere in the region), which has been sub-divided into the three members from oldest to youngest:
• Transition Member - 130 m of laminated fine to medium grained sandstones, intercalated with siltstones and mudstones. This unit represents a transitional facies from the more shaly Oyon Formation to the more sandy Lower Member of the Chimu Formation.
• Lower Member, 125 m of thickly bedded and compact medium to coarse grained sandstones which, due to their brittle nature, are fractured and often brecciated, and comprise the dominant sedimentary host at La Arena. This unit also hosts similar mineralisation at Lagunas Norte, El Toro, La Virgin and Santa Rosa.
• Upper Member, 150 m of a mixed succession of coarse-grained sandstones, laminated siltstones and carbonaceous mudstones.
Multiple dacitic and andesitic feldspar porphyries intrusions cut the Cretaceous sedimentary sequence at La Arena. These vary from dacitic to andesitic in composition, and are differentiated by texture and composition. Three stages of intrusion are recognised, as described below. U-Pb age dating on zircons within individual intrusives, indicates overlapping dates at ~24.9±0.4 to 0.7 Ma, suggest that all three were emplaced in a short period, within a maximum 1 to 2 Ma interval.
• Dacitic Feldspar Porphyry 1 (FPD1), the earliest intrusion, which is generally barren. Textures are commonly porphyritic and sometimes phyric, with inequigranular, ≤1 to 4 mm subhedral plagioclase phenocrysts, set in a microcrystalline matrix with relicts of ferromagnesian minerals (amphiboles), and abundant pyrite in the matrix and in veinlets. The pyrite veinlets are D veins of different stages, crosscutting each other, and forming a pyrite stockwork. This intrusive lacks a significant quartz stockwork with Cu-Au mineralisation, although it locally carries low grades near the contact with FPD2.
• Dacitic Feldspar Porphyry 2 (FPD2), considered to be the second intrusive stage, characterised by a porphyritic texture which has largely been obliterated by clay and sericite alteration, although 1to 3 mm remnant phenocrysts of plagioclases are recognised. This porphyry is host to most of the Cu-Au porphyry mineralisation. It contains a strong stockwork of 20 to 50% of A, AB and B type veins, at a density of 15 to 30 veins per metre, and vein widths varying from <1 to 7 cm. In the potassic alteration zone, phenocrysts of plagioclases are better preserved, along with primary biotite in small subhedral crystals, and magnetite and calcite veinlets. Under the microscope, the main vein components are: 15 to 43% quartz I; 15 to 50% quartz II; 22 to 39% plagioclases; 1 to 20% biotite; 10 to 15% biotite II 1 to 10%; K feldspar; 10 to 54% sericite; 1 to 3% accessory rutile; 1 to 3% hematitised rutile; 1% epidote, 1% chlorites, 1% hematite, 1 to 5% carbonates.
• Dacitic Feldspar Porphyry 3 (FPD3), the third stage of intrusion, which, in turn is composed of least three intrusion phases, differentiated on their textural features and contact relationships. Textures vary from porphyritic to fine grained phyric. The mineral assemblage includes 30 to 40% subhedral plagioclase phenocrysts, varying in size from ≤1 to 4 mm, and 1 to 10% subhedral biotite, from ≤1 to 3 mm across. This intrusion contains low grade of Cu-Au mineralisation, occurring in a weak, 1 to 8% stockwork of A and B quartz veins at a density of ~8 to 10 veins. The intensity of veining may increase slightly towards the contact with the FPD2 intrusion. Vein widths vary from <1 to 2cm. The intrusions are characterised by potassic alteration, with moderate to strong magnetite in matrix and veins, along with chlorite and K feldspar. Phyllic alteration is characterised by quartz-sericite alteration, typically containing quartz fragments and some ductile A type veins along with late D type veins. Pyrite is common as very fine veins and in the matrix.
Under the microscope, the main components are 10 to 15% quartz II; 1 to 37% quartz I; 5 to 51% clays; 1 to 15% hematitised rutile; 7 to 50% sericite; with accessory 3 to 15% opaques; 1 to 20% K feldspar; 1 to 3% carbonates I and II; 1% hematite; 7% chlorites and 5% limonites.
• Andesitic Dykes and Plugs (FPA), the final intrusive phase, which is barren. These dykes crosscut earlier intrusions, and in hand specimen, have a texture that is porphyritic, with coarse <1 to 6 mm, subhedral and inequigranular phenocrysts of plagioclases crystals, a few pyroxene/hornblende crystals and moderate chlorite in the matrix. On the western margin of the deposit, a laccolith-like development of FPD3 intrusion overlies an argillically-altered heterolithic breccia. The breccia is altered up to advanced argillic (quartz/alunite) facies, with an oxidized, porous matrix dominated by hematite, limonite and quartz, with some remnant sulphides.
The composite intrusion is developed over an east-west width of ~1100 m, and comprises a 200 to 350 m wide core of mineralised FPD2, progressively flanked by FPD1 and an outer shell of FPD3. The FPD1 is best developed in the SE over an irregular, almost rhomboid 1100 x 1100 m area controlled by NW-SE and NE-SW faults, surrounding the porphyry deposit, with the mineralised FDP2 as two main 300 x 500 m clusters of smaller plugs largely within its confines, while FDP3 is best developed to the NW of the main FDP1 body, with dykes injected into both FDP2 and FDP1 and along the outer contact of FDP1.
Major faults within the deposit area have strikes varying from NW-SE to N-S, paralleling the orientation of the fold axes and probably following the same controls. They are mainly reverse faults, and are regarded as probably syn-folding. Other mapped faults, generally lesser structures, strike NE-SW to east-wesr, parallel to the main fold-related stresses, displaying dilationary and tear movements.
Within the mine, mineralization appears to be controlled by the interaction of three fault trends: i). the NW-SE Andean Trend, with dips varying from 50 to 70°NE; ii). 10°, with sub-vertical dips and mainly dextral tear relative displacement; and ii). 40°, dipping 70 to 80° to both the SW and NE, with a sinistral component of movement. The latter fault trend cuts all the others, and appears to have acted as the principal feeder channels for mineralising fluids.
The mineralisation-related porphyry intrusions are hosted in the core of the La Arena anticline, where the strike of the anticlinal axis undergoes a deflection from the region NW-SE trend, to swing north-south for ~1000 m before reverting to the regional strike. This structural abnormality, which also corresponds to a zone of north-south faulting transecting the main NW-SE Andean trend, represents a dilational jog, and is part of a regional flexure. The high sulphidation epithermal Au mineralisation is located to the west of the porphyry, hosted in sandstones of the Chimu formation, at the intersection of NW-SE and NE-SW faults.
Four principal fault systems have been identified in the deposit area. The first two had NW-SE compressive and north-south dextral strike slip components, whilst the third represent NE-SW extension associated with normal and strike slip faults, and the fourth has been reactivated by compressional trust fault displacement.
Both high sulphidation epithermal Au and porphyry Cu-Au mineralisation occurs at La Arena, the former hosted by Chimu Formation sandstones, and the latter by multiple intrusions with an age of ~25 to 24 Ma (Hedenquist; 2012).
Two major clay alteration assemblages are recognised at the surface that are respectively dominated by, i). pyrophyllite-illite-muscovite and kaolinite (probably supergene) in the porphyry zone; and ii). silica-alunite-illite-dickite and supergene kaolinite in the epithermal high sulphidation zone.
Two NW oriented corridors of pyrophyllite alteration occur, one on the NE edge of Calaorco pit epithermal deposit, which is immediately to the SW of the La Arena porphyry deposit, and the second extending to the NW from the porphyry into the Ethel pit epithermal deposit. These two corridors are parallel to the Andean trend which likely reflects the control of mineralisation by major structures. It is suggested that conduits of hot muscovite-stable fluids overprinted the porphyry and cooled as they flowed to the NW along these structures (Hedenquist, 2012). In addition, there is a NE oriented zone of pyrophyllite on the NW end of the Calaorco pit, parallel to a major cross structure.
With depth, the argillic assemblage gives way to strong phyllic alteration (quartz-sericite), which, in turn, overprints the prograde potassic alteration (secondary biotite-magnetite-K feldspar-chlorite) and destroys the magnetite. In addition, there is a later argillic overprint of illite-chlorite along structures that extend deep into the porphyry. The transition from the margins of the porphyry mineralisation to the epithermal deposits, is marked by pyrophyllite, particularly along NW structures, caused by cooling during the phyllic stage, progressing from muscovite to pyrophyllite, and with further cooling, to dickite (Hedenquist, 2012).
Both high-sulphidation epithermal Au and porphyry Cu-Au (Mo) mineralisation are represented at La Arena, developed over a north-south oriented area of ~2.2 x 1.1 km, and a vertical range of ~1 km. Continuity of the mineralisation is generally good, and improves with lower-grade cut-offs, which is a characteristic of this type of deposit.
High-Sulphidation Epithermal Au
Four separate breccia zones containing anomalous gold have been outlined around the western and northern margins of the La Arena Porphyry, the Calaorco, Ethel, Astrid and San Andrés zones.
Epithermal gold mineralisation mined in the Calaorco Open Pit occurs: i). partly within the Calaorco Breccia, located at the contact between well-fractured Chimu quartz sandstones and the overlying intrusive, ii). partly within the un-brecciated but still well fractured sandstones, and iii). partly within the intrusive along the contact.
The Ethel Breccia, which is located to the north of the Calaorco Breccia and NW of the La Arena porphyry, is a similar but smaller oxidised epithermal gold deposit with surface dimensions of the >0.2 ppm Au zone of ~500 x 150 m.
Gold mineralisation is both lithologically and structural controlled, and principally occurs in silicified fractured sandstones and locally in hydrothermal breccias. The Calaorco breccia lies parallel to the contact between the Chimu sandstones and the porphyry, with Chimu sandstone dipping gently towards the east, below the laccolithic porphyry. Gold mineralisation (>0.2 ppm Au) within the Calaorco breccia extends over a NW-SE length of ~700 m length, which tends to swing towards the north at depth. The width of this zone varies from 100 to 300 m outward from the contact between sandstone and porphyry. Gold mineralisation is most pronounced within the oxide zone, which can extend to >250 m below the surface.
Structural control is mainly related to the principle NW-SE Andean orientation, although a secondary tensional fracturing, and the bedding planes also influence the distribution of mineralisation. The tensional fracturing has acted as the principal channel for the ingress of oxidised high sulphidation epithermal fluids and gold mineralisation. Fine grained free native gold occurs with small proportions as electrum.
High grade shoots of gold are directly controlled by the intersection of SW-NE faults and the main mineralised trend, and are oriented towards the NW-SE, e.g. the Tilsa structure, which has a strike length of ~300 m and a grade of 80-100 g/t Au over a variable true thickness of a few centimetres to 1 m. This zone corresponds to the intersection of the NE-SW Calaroco and NW-SE Esperanza faults, and is part of a more extensive high grade gold zone of ≥1 g/t Au which extends towards the north.
The epithermal mineralisation is associated with a steep, NW dipping, downward tapering mass of advanced argillic pyrophyllite-illite-muscovite-kaolinite alteration, enclosing zones of strong silica alteration.
Porphyry Cu-Au (Mo)
Cu-Au mineralisation is accompanied by phyllic (quartz-sericite) and potassic (secondary biotite-magnetite-K feldspar) alteration assemblages, and is dominated principally by pyrite, chalcopyrite, smaller amounts of bornite, covellite and chalcocite, and some molybdenite. It comprises a deep core of remaining potassic alteration, with a width of ~250 m, surrounded vertically and laterally by a shell of strong phyllic and then weak phyllic assemblages, mainly quartz-sericite-muscovite-illite, and an outer thin propylitic band. At the surface and to the east, these zones pass into the argillic silica-alunite-illite-dickite assemblage.
The deposit comprises an upper 40 to 50 m of secondary enriched chalcocite-covellite±copper oxides in the argillic zone, overlying a 10 to 40 m thick mixed zone of chalcocite-chalcopyrite±covellite. The primary zone chalcopyrite±bornite, which predominates at La Arena, commences at depths >100 m below the surface.
The Cu-Au-(Mo) porphyry deposit comprises an elongated NW-SE oriented, 1400 x 200 to 400 m ore body within porphyritic andesite intrusive. Mineralisation persists to a depth of 500 m, with the first 350 m containing the better Cu, Au and Mo grades. At the 3200 m RL (~200 m below the surface), the >0.1% Cu shell of the porphyry body has dimensions of ~1100 x 300 m, enveloping a more irregular >0.2 g/t Au core of ~900 x 250 m zone. Both zones have a common barren core ~200 m in diameter. The main enveloping sulphide zone defines a NW-SE elongated area of ~1400 x 800 m, with the two main zones of oxidised epithermal mineralisation on and overlapping its NW and SW margins.
Mineralisation occurs as disseminations along hairline fractures as well as within larger stockwork veins. Sulpide mineralisation comprises pyrite, chalcopyrite and molybdenite, with accessory pyrrhotite, sphalerite, galena, arsenopyrite, marcasite and rutile. In addition, very fine microscopic native gold has been observed (25µm).
The best Cu-Au mineralisation occurs within the FPD2 intrusion, associated to phyllic and potassic alteration and with ranges from ≥0.5 to >1% Cu and ~0.5 to 1g/t Au respectively. Low grade Cu-Au mineralisation is related to the intra-mineral FPD3 intrusion, which contains from 0.1 to <0.5% Cu and <0.2 to >0.5 g/t Au.
Production, Reserves and Resources
Production 2011 to 2014 (Rio Alto Mining, 2014)
41.01652 Mt @ 0.64 g/t Au for 26.3 t of contained Au, of which 85.2% was recovered from dump leaching.
Mineral resources as at 31 December 2014 (Rio Alto Mining, 2014)
Oxide ore - measured + indicated resources
in sedimentary hosts - 102.0 Mt @ 0.38 g/t Au, 0.01% Cu, 0.5 g/t Ag, 4.5 ppm Mo;
in intrusive hosts - 29.1 Mt @ 0.24 g/t Au, 0.09% Cu, 0.6 g/t Ag, 26.5 ppm Mo;
in colluvium - 2.6 Mt @ 0.34 g/t Au, 0.01% Cu, 0.2 g/t Ag, 2.5 ppm Mo;
Total measured + indicated resources - 133.6 Mt @ 0.35 g/t Au, 0.03% Cu, 0.5 g/t Ag, 9.2 ppm Mo.
Oxide ore - Inferred resource
in sedimentary hosts - 2.2 Mt @ 0.34 g/t Au, 0.01% Cu, 0.4 g/t Ag, 2.9 ppm Mo;
in intrusive hosts - 0.3 Mt @ 0.14 g/t Au, 0.01% Cu, 0.1 g/t Ag, 2.1 ppm Mo;
Total inferred resources - 2.5 Mt @ 0.31 g/t Au, 0.01% Cu, 0.4 g/t Ag, 2.8 ppm Mo.
Sulphide ore as at 31 December, 2014 (Rio Alto Mining, 2014)
Measured + indicated resources - 274.0 Mt @ 0.24 g/t Au, 0.33% Cu, 0.4 g/t Ag, 38.5 ppm Mo;
Inferred resource - 5.4 Mt @ 0.10 g/t Au, 0.19% Cu, 0.4 g/t Ag, 40.7 ppm Mo.
Ore reserves as at 31 December 2013 (Rio Alto Mining, 2014)
in sedimentary hosts - 1.4 Mt @ 0.45 g/t Au, 0.01% Cu, 0.44 g/t Ag;
in intrusive hosts - 0.2 Mt @ 0.38 g/t Au, 0.26% Cu, 0.34 g/t Ag;
in low grade stockpiles - 1.2 Mt @ 0.23 g/t Au, 0.004% Cu, 0.81 g/t Ag;
Total proved reserves - 2.8 Mt @ 0.35 g/t Au, 0.03% Cu, 0.59 g/t Ag;
in sedimentary hosts - 56.9 Mt @ 0.47 g/t Au, 0.01% Cu, 0.46 g/t Ag;
in intrusive hosts - 16.5 Mt @ 0.32 g/t Au, 0.14% Cu, 0.37 g/t Ag;
Total probable reserves - 73.4 Mt @ 0.43 g/t Au, 0.04% Cu, 0.43 g/t Ag;
TOTAL proved + probable reserves - 76.2 Mt @ 0.43 g/t Au, 0.04% Cu, 0.47 g/t Ag.
This summary is largely drawn from "Mining Plus Peru S.A.C., 2014 - La Arena Project, Peru; an NI 43-101 Technical Report prepared by Mining Plus Peru S.A.C. for Rio Alto Mining Limited, 224p."
The most recent source geological information used to prepare this summary was dated: 2014.
This description is a summary from published sources, the chief of which are listed below.
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