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Campana Mahuida |
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Neuquen, Argentina |
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Cu Au
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Super Porphyry Cu and Au
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The Campana Mahuida porphyry copper-gold deposit is located ~80 km NNW of Zapala in Neuquén, Argentina, and <40 km east of the Chilean border (#Location: 38° 12' 57"S, 70° 32' 38"W).
The deposit has been the subject of exploration since 1968 by a range of mining companies and institutions, including Plan Cordillerano, Dirección General de Fabricaciones Militares in 1968, Falconbridge in 1972, Dirección General de Fabricaciones Militares in 1974-75, Corporación Minera Neuquina in 1990-91, Recursos Americanos Argentinos in 1993, and Grupo Minero Aconcagua in 1996-1997). In 2007, the Chinese Metallurgical Construction Corporation (MCC) and Corporación Minera del Neuquén (CORMINE) to continue exploration and mine the deposit. Further work has been blocked by community opposition.
For more detail on the setting, see the Southern Andes and Patagonia Terrane record
The Campana Mahuida district is situated in the Cordillera Principal of northwestern Neuquén, on the western margin of the thrust belt segment of the Agrio Fold Belt (Braccacini, 1970; Ramos,1978). It is characterised by a series of related east-vergent folds and thrusts forming a complex thrust front that is bounded to the east by an area of gentle folds. This fold belt is bounded on the west by the extensional Loncopué graben that is filled with Pliocene to Pleistocene basalts (Ramos 1999).
The deposit was formed during a major period of hydrothermal alteration associated with the emplacement and crystallisation of a Paleocene calc-alkaline andesite porphyry stock, with late superimposed supergene processes.
The stratigraphic in the district includes the sedimentary rocks of the Lotena and La Manga Formations which belong to the Jurassic Lotena Group, the Tordillo Formation, and the Vaca Muerta Formation.
The Tordillo Formation hosts the porphyry Cu mineralisation and comprises fine- to medium-grained, laminated siltstone and intercalated sandstone deposited in a lacustrine environment, which was interrupted by brief intervals of fluvial deposition. It is composed of detrital sand-sized grains of poorly sorted, angular quartz and feldspar (potassium feldspar > plagioclase) with minor mafic minerals (amphibole) in a feldspar rich, silt sized matrix. These sedimentary rocks underwent compaction and silicification with the development of carbonate, argillic minerals (Illite and chlorite) and local hematite during diagenesis.
At surface, intrusive rocks comprise two dyke-like stocks (the Pedregoso Formation) and several hypabyssal bodies (El Sillero Formation) that outcrop on the western and southernmost margin of the Agrio fold belt. The Pedregoso Formation stocks are variable in size and intrude the Tordillo and Vaca Muerta Formations with sharp contacts to form the Tres Puntas (1816 m) and Pedregoso (1794 m) hills. They comprise a grey, fine-grained diorite core surrounded by a white greyish, medium-grained granodiorite to tonalite margin.
The El Sillero andesite and quartz-andesite dykes also exhibit sharp contacts with the Jurassic sedimentary rocks, and in the sub-surface have been mapped as a stock-like body, with dimensions of some 1400 x 400 m, elongated in a NNE direction, and is associated with both porphyry Cu and skarn mineralisation.
Field relationships and radiometric dating suggest that the andesitic dykes and stock of the El Sillero Formation (61.0 ± 1.4 Ma) are contemporaneous with the dioritic stocks of the Pedregoso Formation (60.7 ± 1.9 Ma). Both are calc-alkaline, metaluminous, and enriched in incompatible trace elements (K, Rb, Sr, Ba, Nb, La, Ce, Yb, Th, and U).
The area is characterised by intense fracturing with two main fault orientations, namely: i). an older NNE striking and 80° NW dipping set that controlled the intrusion of the El Sillero Formation, and ii). younger, near-vertical NW and E-W striking faults that cut the first set of structures.
Hydrothermal alteration associated with the deposit comprises of a potassic core controlled by a north-northeast fracture zone, surrounded by a large propylitic zone, and a phyllic halo that has overprinted both the outer potassic core and inner propylitic zone, while a small intermediate argillic zone occurs within the phyllic halo.
Potassic alteration affects both the andesite of the El Sillero stock and sediments of the Tordillo Formation and occurs as NNE elongated 800 x 400 m zone. The potassic zone is characterised by an early, high-temperature biotite + quartz ± magnetite assemblage, with rare orthoclase and tourmaline locally preserved. In the deeper sections there are minor to trace amounts of albite with quartz and calcite that occur as veinlets. Brown and dark-brown, fine-grained ragged biotite occurs in clusters with euhedral magnetite replacing amphibole, rimming early biotite phenocrysts, and filling veinlets. K-feldspar is rare, filling veinlets that cut fresh plagioclase. Quartz occurs in early barren veins with parallel walls, as disseminated fine-grained aggregates with biotite clusters, and less commonly, as massive silicification. Chlorite, white mica and fine-grained clay mixtures overprint this alteration.
Cu mineralisation is spatially associated with the potassic alteration and postdates the biotite + quartz ± magnetite assemblage. Metallic minerals include pyrite ± chalcopyrite with minor to trace amounts of bornite ± molybdenite ± gold ± pyrrhotite. Sulphide minerals ranges from 1 to 4 vol%, with pyrite:chalcopyrite ratio which varies from 1:1 in the core of the potassic zone (where the grade averages 0.52% Cu, 48 ppm Mo, and 0.2 g/t Au) to 2:1 in the periphery (where the grade averages 0.11% Cu, 7 ppm Mo, and 0.1 g/t Au). Sulphide minerals occur as disseminations, seams and stringers, and form the centre of early quartz veins.
Propylitic alteration occurs as a zone peripheral to the potassic alteration, affecting other lithologic units in addition to the Tordillo Formation. It is characterised by an assemblage of chlorite ± epidote ± calcite ± quartz ± pyrite and weakly developed as disseminations and thin veinlets of fine-grained clay minerals in siltstone with abundant relic primary K-feldspar and quartz.
An elliptical zone of phyllic alteration surrounds the potassic core, hosted by the Tordillo Formation, El Sillero andesite, Pedregoso Formation, and to the SW, the sandstone and limestone of the Lotena and La Manga Formations, respectively. Phyllic alteration occurs as an assemblage of sericite ± fine-grained clay ± quartz ± pyrite ± chalcopyrite next to and overprinting the fringe of the potassic core, and as massive aggregates, disseminated, and veinlet fillings of sericite ± fine-grained clay ± quartz ± pyrite toward the periphery. The pyrite:chalcopyrite ratio varies from 2:1 with average grades of 0.11 to 0.20% Cu in the internal parts. Grades on the outer margins are 0.005 to 0.01 % Cu; Molybdenum (2 to 27 ppm) and gold (0.02 g/t) contents exhibit similar decreases across the alteration zone.
A second generation of fine-grained clay accompanied by alunite and jarosite veinlets overprint this alteration.
Crosscutting relationships observed during this study show that phyllic alteration overprinted the potassic and propylitic alteration assemblages. To the south, intense kaolinisation accompanies sericite ± quartz ± pyrite alteration of sedimentary rock and porphyry and has been mapped as hypogene intermediate argillic.
Supergene alteration produced a vertical zonation from a leached cap, to an oxide copper zone and a supergene sulphide copper blanket overlying the hypogene sulphides. The leached zone increases in thickness toward the edges of the deposit. The oxide zone overlies the core of the deposit and pinches out toward the periphery and to the SW of the core and is characterised by limonite (hematite, goethite, jarosite), delafossite, malachite and chysocolla. The supergene sulphide zone forms a halo of chalcocite and rare covellite surrounding a core free of secondary sulphides and extends beyond the oxide zone, to the south and SW.
According to the USGS MRDS database, reserves in 1994 comprised:
Oxidised ore - 31 Mt @ 0.4% Cu, 0.14 g/t Au;
Supergene enrichment ore - 11 Mt @ 0.6% Cu, 0.03 g/t Au;
Sulphide ore - 71 Mt @ 0.38% Cu, 0.09 g/t Au.
According to Franchini, et al. (2007), after Chabert (1997); Chabert and Zanettini (1999), at a 0.15% Cu cut-off grade, reserves were:
40 Mt of 0.49% Cu.
The USGS Giant porphyry-related camps of the world: a database of Mutschler et al., 2005 quotes a resource of 20 Mt @ 0.8% Cu.
Current resources (USGS MRDS database, visited September, 2016) - 194 Mt @ 0.49% Cu, 0.098 g/t Au
The most recent source geological information used to prepare this decription was dated: 2007.
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.
Campana Mahuida
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Selected References:
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Franchini M, Impiccini A, Meinert L, Grathoff G and Schalamuk IBA, 2007 - Clay Mineralogy and Zonation in the Campana Mahuida Porphyry Cu Deposit, Neuquen, Argentina: Implications for Porphyry Cu Exploration: in Econ. Geol. v102 pp 27-54
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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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