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Alto Chicama, Lagunas Norte |
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Peru |
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Au
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Super Porphyry Cu and Au
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The Alto Chicama Project and the Lagunas Norte high-sulphidation epithermal gold deposit, are located in the Quiruvilca district, Santiago de Chuco Province, in Northern Peru, roughly half way between the Yanacocha and Pierina deposits. It is approximately 90 kilometres east of the coastal town of Trujillo (#Location: 7° 57' 9"S, 78° 14' 48"W).
The Alto Chicama District is part of a 290 km long gold-rich belt containing world class deposits such as the Yanacocha mine in the north, and the Pierina mine to the south in central to northern Peru. This gold rich belt is located in the western Cordillera of the Peruvian Andes, between the Coastal Batholith to the west, and the Marañon fold and thrust belt to the east.
Regional Setting
For details of the regional setting, see the separate Peruvian Andes Cu-Au Province record.
The Mesozoic units of northern Peru largely comprise sedimentary rocks of the Andean cycle (Mégard, 1987). From the Tithonian, the Western platform and trough of Peru was dominated by the subsiding Chicama basin (Jaillard and Jacay 1989), where up to 2500 m of deep marine shale and intercalated subordinate thin sandstone beds of the Chicama Formation were deposited. In the following Berriasian to Valanginian, a gradual regression from deep marine to shallower siliciclastic sedimentation took place, producing a sequence of quartz sandstones derived from the Guyana and Brasilia shields (Moulin 1989). These fluvio-deltaic sandstones are represented by the Lower Cretaceous Chimú Formation (Benavides-Cáceres 1956; Jaillard and Jacay 1989).
Marine transgressions and regressions dominated the Valanginian, during the deposition of the Santa-Carhuaz Formation (Benavides-Cáceres 1956) which is composed of alternating sandy and shaly beds. Carbonates and black shales of the Chulec and Pariatambo Formations respectively, represent a progressively deepening depositional environment, and overlie the Santa-Carhuaz Formation. The western border of the Chicama basin was the site of intense volcanic activity during the Aptian, producing the Casma Group (Atherton et al., 1985; Soler 1991), before the cessation of marine sedimentation and volcanism in the Albian, during a dextral transpressive event in the arc (Soler and Bonhomme 1990). This volcanic activity was followed by voluminous intrusive activity and the emplacement of the 100 to 55 Ma Coastal Batholith (Cobbing et al., 1981; Soler 1991).
During the late Cretaceous, the inferred offshore Mariana-type subduction to the west was replaced by the current Andean-type subduction (Benavides-Cáceres 1999), resulting in a tectonic inversion and intense compressive deformation which gave rise to the Marañon Fold and Thrust Belt (Benavides-Cáceres 1999). The deformed Mesozoic rocks were unconformably overlain by the Eocene to Miocene Calipuy Group volcanic and volcaniclastic rocks (Cossío and Jaén, 1967; Wilson 1975; Rivera et al., 2005; Montgomery 2012). Volcanism was terminated during the late Miocene along much of northern and central Peru, attributed to the onset of flat slab tectonics along the Peruvian margin due entry of the aseismic Nazca ridge and oceanic Inca plateau into the subduction zone (Gutscher et al., 1999; Hampel 2002).
Deposit Geology
The basement at Lagunas Norte is predominantly Mesozoic pelitic and siliciclastic rocks of the Chicama and Chimú Formations respectively (Reyes 1980), which have been thrusted and folded into NW striking east-verging folds of the Marañon Fold and Thrust Belt (Benavides-Cáceres 1999). These rocks are weakly metamorphosed to slate and quartzite. The Lower Miocene volcanic rocks of the Calipuy Group were unconformably deposited over the folded Mesozoic basement. Gold mineralization is hosted by the siliciclastic rocks of the Chimú Formation and the overlying volcanic strata. The stratigraphy of these rocks on the deposita area is as follows (after Cerpa et al., 2013):
Jurassic Chicama Formation - which is exposed to the west and north of the mining operations. The thickness is unknown locally, but has been estimated to be up to 1500 m thick 40 km to the south (Cossío and Jaén 1967; Jaillard and Jacay 1989). It comprises a succession of dark carbonaceous shale and siltstone with occasional thin beds of fine-grained sandstone, and has been weakly metamorphosed to slate, with an intense cleavage subparallel to bedding. The contact with the overlying Chimú Formation is a gradual transition, characterised by an increasing abundance of quartzite intercalations. The Chicama Formation is not exposed in the deposit area, but clasts of slate in the Dafne breccia (see below) are possibly derived from this unit.
Upper Jurassic to Lower Cretaceous Chimú Formation - which is the main host to mineralisation at Lagunas Norte, comprises a compositionally mature quartz sandstone, typically containing ∼95% silica, with occasional coal beds, and scarce siltstone and shale intercalations. It has undergone weak metamorphism, with some recrystallisation and cementation of quartz grains to form quartzite. The thickness of the formation is estimated to be between 450 and 600 m in the Lagunas Norte area (Benavides-Cáceres 1956).
Miocene Calipuy Group - which is composed of a sequence of volcanic and volcaniclastic rocks (Cossío and Jaén 1967; Rivera et al., 2005; Montgomery 2012) separated from the Mesozoic basement over an angular unconformity. At Lagunas Norte is is subdivided into the following, from the base:
• Quesquenda unit, which is related to an eruptive centre 4 km to the north of the deposit (Rivera et al., 2005) and is exposed in the easternmost sections of the deposit, where it comprises >150 m of andesitic pyroclastic and volcaniclastic rocks, with interstratified lithic-rich tuffaceous deposits containing carbonized wood. It is interpreted to be the product of pyroclastic eruptions interbedded with lahar deposits.
• Dafne unit is composed of a series of breccias in the western part of the deposit area. Overall, the breccia body takes the form of a subvertical, downward tapering cone, and in plan view, a NW elongated ellipsoid that has an up to 1 km long axis. It cuts the Mesozoic basement and has been interpreted to represent a diatreme. Four main lithofacies associations are recognised, namely, the
i). Diatreme margin - generally comprising clast-supported monomict and polymict breccias with in a rock flour or juvenile volcanic matrix, and ubiquitous hydrothermal cement. No evidence of multiple brecciation breccia has been recorded. These breccias have a coarse stratification parallel to the diatreme margin. Three monomictic breccia domains hav been recognised containing quartzite, siltstone or tan-colored hydrothermal quartz clasts (silice parda), respectively. The angular to subangular clasts indicate jigsaw to slightly clast rotated breccias. The polymictic breccias include subangular-rotated clasts of quartzite and siltstone with a rock flour matrix and cement of hydrothermal quartz and, at depth, pyrite. This lithofacies has a gradual transition to the wall rock through jigsaw textures, angular clasts and locally monomictic breccia chimneys. Hydrothermal quartz and pyrite cement dominates the matrix.
ii). Main body, which is volumetrically the most important lithofacies, occupying the central section of the diatreme. It largely consists of polymictic, matrix-supported breccias with quartzite, siltstone, slate and juvenile volcanic (fiamme-like whispy shapes) clasts, as well as occasional silice parda hydrothermal quartz fragments. The clasts are subangular to subrounded, with no apparent internal organisation or sorting of clasts, set in a matrix of rock flour largely derived from siltstone and shale and fine-grained reworked volcanic material.
iii). Crater lithofacies association facies occur in the upper central part of the diatreme, and include polymictic, unstratified and massive breccias, distinguished by large (up to 1.7 m diameter) rounded to subrounded quartzite and andesite blocks with striated clast surfaces, accompanied by smaller clasts rock flour and juvenile volcanic material.
iv). Apron lithofacies, located in upper peripheral sections of the diatreme, characterised by an intercalation of polymictic and monomictic clast-supported breccias with rock flour and volcanic matrix. This facies has coarse bedding, or 'tephra stratification' (Lorenz 2003). Clasts of quartzite and siltstone are subrounded to rounded and locally tabular, with common clast imbrication and widely distributed fiamme in the most peripheral polymictic parts of the breccias. These polymictic breccias are obvious up to 1 km north of the diatreme. The clast size in this unit decreases with increasing distance from the diatreme.
• Josefa unit, which is is sub-divided into two subunits, namely, the
i). Josefa diatreme, which is 45 by 30 m in plan view, and was emplaced in the eastern part of the deposit in the Josefa area. It has a downward tapering cone shape, but in contrast to the larger Dafne diatreme, no siltstone, shale or carbonaceous material is present. Quartz crystals up to 5 mm in diameter occur in the largely juvenile volcanic matrix. The breccia of the diatreme are classified using the same scheme as for the Dafne diatreme, with diatreme margin lithofacies of monomictic clast-supported breccias containing angular quartzite clasts and quartz crystals in a volcanic matrix, also containing quartz-alunite cement. The main body of the breccia is polymictic and matrix supported, with subangular to subrounded and juvenile clasts and quartz crystals in a tuffaceous matrix, which as in the marginal facies, includes quartz-alunite cement. The crater lithofacies in the upper part of the diatreme is contains large, up to 80 cm in diameter quartzite blocks set in a tuffaceous matrix with abundant quartz crystals and juvenile volcanic clasts. The apron lithofacies is only partly preserved on the southern margin of the diatreme, where it occurs is a series of crudely stratified beds.
ii). Volcanic and volcano-sedimentary, succession which overlies the diatreme and is inferred to be largely related to the eruptive activity from the same diatreme. It is composed of two principal units, which crop out at Josefa and Alexa as well as at Dafne, where they overlie the apron lithofacies breccias, and are generally affected by advanced argillic alteration. The lower of the two is the quartz feldspar phyric (QFP) unit characterised by monomictic breccias containing quartzite clasts, which increase in size towards the Josefa diatreme. This breccia is overlain by a pyroclastic flow deposit with small (<2 cm) altered pumice fragments and up to 5 mm quartz crystals. This pyroclastic deposit is overlain by lithic lapilli tuff containing small quartz crystals and rare accretionary lapilli. The QFP is overlain by a dacitic unit, which comprises a series of pyroclastic and volcaniclastic deposits with pumice and lithic fragments but no quartz crystals. These are, in turn, overlain by an ash tuff with only scarce lithic fragments. These rocks have been subjected to advanced argillic alteration.
• Shulcahuanga unit, which comprises porphyritic andesite lavas and andesitic to dacitic domes that are exposed around Cerro Shulcahuanga to the west and south of the deposit. The andesitic lavas overlie the Dafne unit and have been weakly chlorite-smectite/illite altered. Two main lithological associations are mapped, namely,
i). Andesitas Azules, an andesite with a pale blue-green hue imposed by clay alteration, characterised by hornblende phenocrysts in a fine grained aphanitic groundmass, which occurs as dykes that crosscut the southern margin of the Dafne diatreme.
ii). Shulcahuanga dome, and adjacent lava flows to the east of the deposit, which has a porphyritic texture with plagioclase, biotite, and hornblende phenocrysts. These rocks have characteristic prominent flow banding (Macassi, 2005), and are dated at 16.8 to 17.3 Ma (40Ar/39Ar biotite and hornblende; Montgomery 2012).
Alteration and Mineralisation
Hydrothermal alteration at Lagunas Norte varies according to the host rock compositions and textures. The upper volcanic-hosted section of the deposit, is characterised by zonation pattern typical of high-sulphidation systems (e.g., Simmons et al., 2005), where a nucleus of vuggy quartz is surrounded by quartz-alunite and dickite-kaolinite±alunite zones, indicating acidic fluids that were progressively neutralised during reaction with the host rock. In contrast alteration of the quartzite is subtle and difficult to recognise, although kaolinite and, in more silty units, pyrophyllite have been recognised by spectral instruments. Four hydrothermal stages have been defined. Gold was introduced during stages 1 and 3, the latter being the principal pulse of mineralisation. Minor additional gold was introduced during stage 4. Supergene oxidation to depths of up to 80 m below the current surface made the ore amenable to heap leach extraction (Cerpa et al., 2013).
• Stage I - Early hydrothermal - characterised by fine-grained yellowish to tan-coloured quartz-pyrite aggregates with minor rutile, locally referred to as silice parda. This assemblage is only found in the Chimú Formation, where it generally occurs along a network of pre-existing fractures, best developed in silty layers, but also forms the cement of small fault controlled monomictic breccia bodies (Cerpa et al., 2013).
Between Josefa and Dafne diatremes, and in the southern part of the Josefa zone, silice parda is accompanied by chalcopyrite and digenite. Although gold is not visible to SEM or optical microscopy, mineralogical studies show it to be associated with pyrite, probably as solid solution or as nanoparticles in the pyrite. The presence of silice parda clasts in the Dafne diatreme, suggest the first mineralisation stage preceded diatreme emplacement. Montgomery (2012) reports an age of paragenetically early alunite hosted in the Chimú Formation of 17.36±0.14 Ma, providing a minimum age for this stage.
Δ34S on a digenite-pyrite pair from this stage gave a maximum fluid temperature of 360°C (Hubberten, 1980).
• Stage II - phreatic and phreatomagmatic - The Dafne and Josefa diatreme breccia lithofacies suggest they were formed by phreatic and phreatomagmatic activity which Cerpa et al. (2013) define as the second hydrothermal stage. This stage generated ground preparation for subsequent mineralisation by fracturing the adjacent rock and also acted as host to portion of the ore. Mineralisation within the diatremes is controlled by the permeability, which in turn is controlled by matrix type and abundance, type, shape and size of clasts. A minimum age for the brecciation events is indicate by the oldest alunite within the overlying volcanic sequence of 17.05±0.12 Ma (Montgomery 2012).
• Stage III - Main mineralisation - The bulk of the gold deposited during this stage is not optically visible, and is contained within pyrite. This mineralisation and the associated alteration is difficult to detect in the host quartzite, although coarse alunite fracture filling with associated pyrite and enargite is observable at depths of >80 m below the present surface. In addition, spectral sensors are able to detect disseminated kaolinite in quartzite hosts. In the core of Lagunas Norte, pyrophyllite is found within the more silty beds of the Chimú Formation, whereas kaolinite occurs on the periphery of the deposit. Where coal is present, sulphide assemblages that include pyrite, stibnite and arsenopyrite are locally observed (Cerpa et al., 2013).
Alteration patterns are lithologically controlled in both diatremes. Within the internal sections of the Dafne breccia, dickite-kaolinite alteration affected juvenile fragments, and fracture controlled silicification is locally present, while the margins are intensely silicified with minor alunite. The matrix composition determines the alteration intensity and assemblages in the apron lithofacies. Where the matrix is predominantly volcanic, quartz-alunite is the dominant alteration assemblage, whilst in beds with a carbonaceous matrix, the juvenile fragments are preferentially altered to alunite-dickite-kaolinite (Cerpa et al., 2013).
The Josefa breccias are pervasively altered to quartz-alunite and juvenile fragments have commonly been replaced by pyrite and alunite. In the volcano-sedimentary lithofacies at Dafne, Josefa and Alexa, and the Josefa marginal facies and the Dafne breccia, the alteration pattern is typical of high-sulphidation epithermal mineralisation. Vuggy quartz distribution is controlled by small east-west oriented faults and by the permeability of volcanic or breccia facies. Within the overlying volcanic package, vuggy quartz is best developed in pumice- and crystal-rich pyroclastic flow deposits where pumice fragments and feldspar phenocrysts were leached and the volcanic matrix was completely replaced by residual quartz. The vuggy quartz zone is surrounded by an assemblage of quartz-alunite-pyrite alteration, where alunite replaced feldspars and pumice clasts, and the groundmass has been replaced by fine-grained quartz and pyrite. Although pyrite has generally been oxidised, it is preserved with alunite in some silicified strata at Alexa, whereas at Josefa, it occurs together with rutile (Cerpa et al., 2013).
The matrix of the basal breccia of the volcanic pile has been affected by pervasive quartz-alunite±kaolinite-dickite alteration. Disseminated alunite from the volcanic Josefa unit altered by this main mineralisation stage has been dated at 17.0±0.22 Ma (40Ar/39Ar), consistent with the age range of 16.7 to 17.1 Ma inferred for the main hydrothermal activity (Montgomery 2012). The andesitic volcanic rocks surrounding the deposit have been affected by weak to moderate argillic alteration where illite partly replaces hornblende and quartz-chlorite veinlets have been observed (Cerpa et al., 2013).
Cerpa et al. (2013) have shown that coarse alunite from this stage, which is in textural equilibrium with pyrite and enargite, has δ34S values of 24.8 to 29.4‰ and δ18OSO4 values of 6.8 to 13.9‰, consistent with H2S as the dominant sulphur species in the mostly magmatic fluid and constraining the fluid composition to low pH (0 to 2) and logfO2 of -28 to -30. Alunite-pyrite sulphur isotope thermometry records temperatures of 190 to 260°C; the highest temperatures corresponding to samples from near the diatremes.
• Stage IV: late-stage alteration - is only accompanied by modest gold mineralisation. It is characterised by white to yellowish alunite occurring as fine-grained and massive aggregates which form the cement to local fault breccias and fills thin fractures in Chimú Formation quartzite. Pyrite occurs is in textural equilibrium with this alunite, whilst traces of kaolinite and, at Alexa, diaspore are generally present in addition. The massive alunite crosscuts pyrophyllite-altered siltstone beds and has overgrown coarse-grained alunite. Barite, rutile, drusy quartz and late native sulphur filling open spaces in the volcanic rocks represent the final occurrence of hydrothermal activity. Fractures filled with barite are found in the quartzite at Alexa and Josefa. The youngest alunite reported by Montgomery (2012) is 16.45±0.28 Ma interpreted by Cerpa et al. (2013) to constrain hydrothermal stage IV.
Cerpa et al. (2013) obtained δ18OSO4 values from stage IV alunite that vary between 11.5 and 11.7‰ which they interpret to indicate that the fluid was magmatic, supported by the isotopic composition of barite (δ34S = 27.1 to 33.8‰ and δ18OSO4 = 8.1 to 12.7‰). The Δ34Spy-alu isotope thermometry records temperatures of 210 to 280°C, with the highest values concentrated around the Josefa diatreme.
• Stage V: Supergene oxidation - Lagunas Norte has undergone extensive supergene oxidation to depths of 80 m below the current surface, to produce hematite, goethite and locally jarosite and scorodite. Iron oxides are mainly found as cement to tectonic and hydrothermal breccias, as well as on fracture surfaces. Oxidation liberated the gold, making treatment cyanide leaching economically viable (Cerpa et al., 2013).
At the end of 2002, on the basis of 120 000 metres of diamond drilling, in 445 diamond drill holes, on a 50 metre centres grid, the declared resource at Lagunas Norte was:
123.5 Mt @ 1.83 g/t Au, for 226 tonnes (7.3 Moz) of gold, including 192 tonnes (6.2 Moz) in oxides and 34 tonnes of gold in sulphides.
At December 31, 2010, ore reserve and mineral resource figures were (Barrick Gold Annual Report 2010):
Proven + probable reserves - 218 Mt @ 0.95 g/t Au, for 205 tonnes (6.62 Moz) of gold;
Measured resource - 1.23 Mt @ 0.61 g/t Au, for 0.75 tonnes (0.024 Moz) of gold;
Indicateded resource - 40.92 Mt @ 0.58 g/t Au, for 22.8 tonnes (0.732 Moz) of gold;
Inferred resource - 8.27 Mt @ 0.46 g/t Au, for 3.64 tonnes (0.117 Moz) of gold.
At December 31, 2015, remaining ore reserve and mineral resource figures were (Barrick Gold Annual Report 2015):
Proven + probable reserves - 63.641 Mt @ 1.82 g/t Au, 5.17 g/t Ag, for 116 tonnes of gold;
Measured resource - 2.092 Mt @ 1.37 g/t Au, 3.82 g/t Ag, for 2.86 tonnes of gold;
Indicateded resource - 35.461 Mt @ 1.36 g/t Au, 3.74 g/t Ag, for 48.27 tonnes of gold;
Inferred resource - 1.692 Mt @ 0.88 g/t Au, 0.93 g/t Ag, for 1.5 tonnes of gold.
Note: reserves are additional to resources.
This summary is largely drawn/paraphrased from Cerpa et al. (2013), and to a lesser degree from Araneda, R., et al. 2003 - Alto Chicama Project, Quiruvilca District La Libertad Department, Peru; ProEXPLO 2003 Conference
The most recent source geological information used to prepare this decription was dated: 2013.
Record last updated: 30/9/2016
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.
Lagunas Norte
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Selected References:
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Bissig, T., Clark, A.H., Rainbow, A. and Montgomery, A., 2015 - Physiographic and tectonic settings of high-sulfidation epithermal gold-silver deposits of the Andes and their controls on mineralizing processes: in Ore Geology Reviews v.65, pp. 327-364.
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Cerpa, L.M., Bissig, T., Kyser, K., McEwan, C., Macassi, A. and Rios, H.W., 2013 - Lithologic controls on mineralization at the Lagunas Norte high-sulfidation epithermal gold deposit, northern Peru: in Mineralium Deposita v.48, pp. 653-673.
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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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