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Yauliyacu |
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Peru |
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Ag Zn Pb Cu
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Super Porphyry Cu and Au
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The Yauliyacu zinc-lead-silver mine in the Casapalca District is an underground operation owned and operated by Empresa Minera Los Quenuales S.A., a subsidiary of Glencore International. It is located at a height of 4200 m asl in the district of Chicla, province of Huarochirí, Department of Lima, central Peru, ~100 km ENE of Lima (#Location: 11° 38' 59"S, 76° 14' 5"W).
The Yauliyacu mine has been in continuous production for over 100 years during which time >6100 t of Ag has been recovered from it and the surrounding Casapalca District. It is ~12 km WSW of the major Toromocho porphyry copper-molybdenum mine and the broad halo of epithermal polymetallic veins that surround that deposit. It is ~100 km south of the Cerro de Pasco Zn Pb Cu Ag deposits.
Mining in the Casapalca District dates back to the early Spanish colonial period when it was restricted to outcropping, or near surface, veins, primarily recovering native silver from rich hydrothermal veins or from the oxidised zones. Modern style mining commenced in 1887 with Cia. de Minas Los Andes on the Rayo vein, followed by exploration, development and exploitation of several of the mineralised structures in the Casapalca district, including Carlos Francisco, Carmen, Bella Union and Aguas Calientes. In 1921, Cerro de Pasco Corp. acquired the Casapalca mine and most of the mining permits and licenses that now make up the Yauliyacu mine. Cerro de Pasco Corp. also built the 11.5 km Graton drainage tunnel at 3240 m asl that extends under the Yauliyacu mine. In January 1974, the state owned Centromin Peru, compulsorily acquired the mines of the Casapalca mining district. In 1997, Empresa Mineral Yauliyacu SA, whose largest shareholder is Quenuales International, purchased the mine on behalf of Glencore International. As part of the purchase agreement, the Casapalca mining district was split into two mining areas, the Yauliyacu mine to the west, and on the eastern side, the Casapalca mine. The Casapalca mine is owned by Cia. Minera Casapalca S.A., a privately owned company, and although both mines are connected underground in some of the workings, Casapalca is operated separately (as of 2022).
Regional Setting
The stratigraphic sequence in the Casapalca District includes rock units from the Palaeozoic to the present. The oldest rocks exposed in the area around the Yauliyacu mine belong to the Excelsior Group, a pelitic sequence that was regionally metamorphosed by an Upper Devonian deformation event. This unit is discordantly overlain by a volcanoclastic series belonging to the regional Late Permian to Middle Triassic Mitu Group that followed the intensive erosion that followed the Upper Devonian event. This orogenic event produced a zone of uplift that separated a basin on the east and on the west. These basins persisted until the Albian stage of the Lower Cretaceous. Mesozoic deposition commenced with a marine transgression, represented in the east by the limestones of the Late Triassic to Early Jurassic Pucará Group. Two principal facies were accumulated during the Lower Cretaceous: i). a sequence of mainly sandstones and limestones in the western basin, represented by the Chimú, Santa, Carhuaz and Farrat Formations; and ii). sandstones, quartzites and interbedded shales in the eastern basin, represented by the Gollarisquizga Group. Subsequently, during the Albian stage of the Lower Cretaceous, a general marine transgression occurred due to subsidence within the basin, giving rise to the deposition in both the eastern and western basins of a calcareous sequence that constituted the Pariahuanca, Chúlec, Pariatambo, Jumasha and Celendin formations. At the same time as these limestones were being deposited, interbedded volcanic and sedimentary rocks of the Quilmaná Group dominated the western-most part of the combined basin. At the end of the Cretaceous and beginning of the Tertiary, together with the uplift of the Andean cordillera, large plutonic intrusions of the Coastal Batholith were being emplaced. To the east, a red-bed molasse sequence, the Casapalca Formation was deposited, fed by the uplift of the Andean mountains.
The principal deformation took place in the Eocene, represented by the Incaic phases, which folded the Mesozoic sequence, including the Casapalca Formation red beds. In its final stage, this tectonic event produced magmatic extrusives that covered the area. Volcanic ash and lava flows were interbedded with continental sediments, represented by the Rímac and Colqui groups in the western part of the basin, and by the volcanics of the Carlos Francisco, Bellavista and Rio Blanco Formations further to the east.
Late Oligocene tectonic activity folded these units and generated new faults that followed the pre-existing structural framework. Subsequently, the region was overlain by the volcanic-sedimentary-volcanic sequence of the Millotingo Formation, which was later deformed by the episodic Miocene Quechua tectonic phases that gave rise to the explosive volcanism of the Huarochirí Formation.
Across the Miocene-Pliocene boundary, towards the end of the Quechua tectonic event, a centre of explosive eruptions and lava flows occurred, marking the end of the Andean deformation cycle and the beginning of the period of orogeny that produced the Puna surface. Later, during the Pliocene-Pleistocene, the Puna peneplain was gradually uplifted to 4000 m asl across a set of gravitational faults.
Cenozoic faulting, folding and emplacement of plutonic and hypabasal intrusions was accompanied by mineralising fluids, particularly those related to the magmatism that followed the Miocene deformation, and probably before the deformation of the Lower Pliocene (Spring and MacFarlane, 2006). A significant number of mineral deposits in the district are interpreted to have been emplaced into Tertiary volcanic rocks as fracture infillings from such hydrothermal solutions.
Fluvial and glacial erosion intensified with the Pliocene-Pleistocene uplift, producing deeply incised valleys.
For more detail of the regional setting and a geological location map, see the separate Peruvian Segment of the Andean Cu-Au Province record particularly the Central Peru Mineral Belt paragraphs of the Distribution of Mineralisation section.
Deposit Geology
The Yauliyacu deposits are underlain and hosted by a series of Tertiary age bedded rocks that principally comprise sandstones, calcareous shales, limestone, breccias, tuffs and lavas, with a total cumulative thickness of ~5400 m. The sequence is exposed in a series of anticlines and synclines within the 340° trending Casapalca Anticlinorium that is the dominant structure in the deposit area. The oldest exposed rocks belong to the Casapalca Formation which forms the core of the broad Casapalca anticline that is bisected by the Rimac River. At depth, this formation unconformably overlies the Cretaceous Jumasha Formation and Gollarisquizga Group.
The Casapalca Formation comprises a series of clastic continental sedimentary rocks interpreted to have been deposited in a distal fluvial system, and has been divided into two distinct members:
i). The older, 1300 to 1400 m thick Capas Rojas Member (Red Beds) that is composed of intercalated shales and calcareous sandstones whose characteristic red colour is due to finely disseminated hematite. The sandstones range from fine to coarse grained and commonly have laminar and cross stratification, and contains no economic mineralisation; and
ii). the overlying Carmen Conglomerate Member, composed of a series of conglomerate and limestone units interbedded with sandstones, shales, tuffs and volcanic conglomerates with a variable thickness from 80 to 200 m. The conglomerates also occur as lenses containing cobbles and rounded quartzites and limestones gravels in a sandy clay matrix with a calcareous cement. Where mineralised veins cross coarse sandstone and conglomerate layers, replacement of limestone clasts and calcareous matrix is evident.
The overlying Carlos Francisco Formation, comprises a thick series of volcanic rocks overlying the sedimentary units, and has been sub-divided into three members, from the base:
i). The Tablachaca Member, which overlies the Carmen Member, and is composed of a 150 to 200 m thick succession of volcanic rocks made up of tuffs, breccias, agglomerates and extrusive porphyritic rocks.
ii). The volcanic lithofacies of the Carlos Franciso Member are up to 450 m thick and overlie the Tablachaca Member. They comprise massive andesitic flows and fragmentals/breccias, with intercalated layers of breccia and porphyritic andesites indicating the top, bottom and centre of the flow. The breccia beds are composed of angular, porphyritic fragments, that are generally green and are enclosed in a matrix of red, porphyritic rock.
iii). The Yauliyacu Member is composed of ~50 m of fine-grained red tuffs that conformably overlie the Carlos Francisco volcanic lithofacies.
The Carlos Francisco Formation is overlain by the Bellavista Formation in the southern part of the deposit, where it comprises a varied suite of sedimentary and volcanic rocks. The principal sedimentary facies are limestone and siliciclastic (sandstones to siltstones) rocks, whilst the volcanic facies range from tuffs to andesites. Thin beds of grey limestones with occasional dark grey limestone interbeds are a prominent characteristic of the formation. The dark grey interbeds contain nodules of quartz or fragments of fine grain tuffs and red shales.
The Bellavista Formation is overlain by the Rio Blanco Formation, which is predominately composed of finely bedded volcanic rocks, mainly red lapilli tuff with interbedded breccia units, with a basal interval containing some interbedded limestone.
The Quaternary sequence in the Casapalca area occurs as a series of glacial deposits and Recent superficial cover.
Intrusive rocks at Yauliyacu include various Tertiary Age rocks of intermediate composition that are commonly observed in the northern part of the deposit area. They are chemically similar, with a high content of soda, but vary in texture, and degree and type of alteration. The Taruca Porphyry occurs as dykes and sills that intrude the volcanic lithofacies of the Carlos Franciso Member in the SE part of the deposit area. A stock of these rocks, elongated in a north-south direction, outcrops on the Taruca Mountain.
Three major, generally parallel, inverse faults cut the deposit area: i). the Infiernillo Fault which strikes at 332° and dips 70°SW; ii). the Rosaura Fault that strikes at 317° and dips 80°SW and contains mineralisation; and, iii). the Americana Fault, striking at 322° and dipping at 70°NE. Other faults include the Rio Blanco Fault in the SW of the district, which has a strike close to 35°, and parallels the M and C system of veins. The Grand Fault, which strikes at 305° displaces all of the veins.
Mineralisation
Mineralisation in the Casapalca district occurs as i). hydrothermal polymetallic veins and as ii). disseminated orebodies. Both styles are hosted in late Cretaceous to Tertiary volcaniclastic and fluvial sedimentary rocks. Whilst the mineralisation crosses the stratigraphic sequence, it is concentrated within the Casapalca and Carlos Francisco Formations. The veins are known to persist for up to 5 km along strike at surface, of which 4 km have been exposed underground. Veins are typically 0.3 to 1.2 m thick with a known vertical range of >2 km. The major vein structures dip at 60 to 80°NW. These vein structures have undergone strike-slip faulting, prior to the mineralising event, with the formation of duplexes. Hydrothermal brecciation of the host rock is often seen to link faults.
The ore forming minerals within the veins are principally sphalerite, galena, tetrahedrite, tennantite and chalcopyrite with typical gangue minerals of pyrite, quartz, calcite, rhodocrosite, dolomite, sericite and manganiferous calcite.
A four stage paragenesis has been observed by Rye and Sawkin, namely:
• Stage I - NE-SW trending veins containing Zn, Pb, Ag and Cu polymetallic mineralisation;
• Stage II - north-south veins containing Cu mineralisation;
• Stage III - east-west veins with Ag and Pb;
• Stage IV - deposition of quartz and carbonate gangue minerals.
Mineralisation was initiated at temperatures of ~370°C and terminated at ~200°C, from mineralising fluids with salinities estimated to have varied between 4 and 40% NaCl weight equivalent (Independent Mining Consultants Inc., (1996).
The main (or principal) mineralised veins of the Casapalca District are located in the central sections and continue to depth, designated as the "L", "M", "N" and "N3" veins. The L and M veins both strike at 20° and dip moderately west, whilst the N and N3 veins strike east-west and dip steeply north. Offshoots and splays from the main vein structures are a common and been given various names.
Strong hydrothermal alteration associated with the veining is typical, occurring as proximal silica, pyrite and sericite, with distal propylitic alteration extending outwards.
The disseminated mineralisation was discovered in the late 1980s and is referred to as 'cuerpos' orebodies that have more recently been shown to be an important resource in the Yauliyacu mining plan. The following types of cuerpo orebodies have been distinguished:
i). Stockwork and disseminated mineralisation in the hanging and footwalls of the large veins, including remnants fringing the veins previously exploited by Centromin. Economic bodies of this type surrounding veins, some of which are <1 m thick, can reach widths of up to 8 m.
ii). Stockwork and disseminated mineralisation in sigmoidal shaped structures occuring at strong bends in veins.
iii). Stratabound replacement of limestone clasts and matrix in conglomerates and coarse grained sandstones of the Carmen Member, proximal to cross cutting veins. Large orebodies of this type often occur between two main vein systems. This implies mineralising solutions encountered susceptible units, e.g., sandstones and conglomerates, with suitable porosity and permeability to permit sulphide deposition. These orebodies can be up to 120 m in length, 15 to 20 m wide and 80 m in vertical extent. A single orebody of this size represents a mineral resource of >0.5 Mt (million tonnes). This type of mineralisation has dominant propylitic alteration with abundant epidote.
The mineralisation at the Yauliyacu mine is zoned both vertically and laterally. Vertically, high grade silver occurs near the surface while high grade zinc is found in the lowest levels of the mine. Laterally there is an overlapping zonal trend centered on the Casapalca Red Beds Zone 1, grading away on both sides into Zone 2 and further away from Zone 2 into Zone 3, from sphalerite to galena to chalcopyrite and tetrahedrite/tennantite and trace bornite.
Resources and Reserves
At 31 December, 2005 (Spring and MacFarlane, 2006)
Measured + Indicated Mineral Resource - 5.119 Mt @ 4.46% Zn, 2.39% Pb, 0.51% Cu, 317 g/t Ag,
Inferred Mineral Resource - 6.886 Mt @ 3.83% Zn, 2.07% Pb, 0.47% Cu, 293 g/t Ag,
Proved + Probable Ore Reserve - 2.476 Mt @ 2.74% Zn, 1.4% Pb, 0.28% Cu, 162 g/t Ag.
NOTE: Mineral Resources are exclusive of Ore Reserves
At 31 July, 2010 (Burns, 2011)
Measured + Indicated Mineral Resource - 4.459 Mt @ 3.28% Zn, 0.90% Pb, 0.37% Cu, 152.3 g/t Ag,
Inferred Mineral Resource - 16.754 Mt @ 3.46% Zn, 1.34% Pb, 0.37% Cu, 176.6 g/t Ag,
Proved + Probable Ore Reserve - 3.311 Mt @ 2.12% Zn, 1.05% Pb, 0.21% Cu, 118 g/t Ag.
NOTE: Mineral Resources are exclusive of Ore Reserves
At 31 December, 2020 (Glencore Resources and Reserves Report, 2021)
Measured + Indicated Mineral Resource - 18.9 Mt @ 2.9% Zn, 1.1% Pb, 0.1% Cu, 135 g/t Ag,
Inferred Mineral Resource - 13.4 Mt @ 2.8% Zn, 1.5% Pb, 0.3% Cu, 247 g/t Ag,
Proved + Probable Ore Reserve - 8.2 Mt @ 1.6% Zn, 0.7% Pb, 0.2% Cu, 97 g/t Ag.
NOTE: Mineral Resources are inclusive of Ore Reserves
The information in this summary is drawn from "Spring, V. and MacFarlane, R., 2006 - A technical review on the Yauliyacu lead/zinc mine, Junin Province, Peru; a technical report prepared for Silver Wheaton Corp., by Watts, Griffis and McOuat Limited, 90p. and
"Burns, N., 2011 - 2010 Resource and Reserve Update Yauliyacu Mine, Peru; an NI 43-101 Technical Report prepared by Silver Wheaton Corp., Canada, 149p.
NOTE: Wheaton Precious Metals (Silver Wheaton Corp.) is precious metals streaming company that has entered into agreements to purchase all or a portion of the silver production from Yauliyacu for an upfront payment and an additional payment upon delivery of the metal.
The most recent source geological information used to prepare this decription was dated: 2010.
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.
Yauliyacu
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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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