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The Cotabambas porphyry copper-gold-silver deposit is located within the Andahuaylas-Yauri belt in southern Perú 545 km SE of Lima, 50 km
SW of Cusco, 60 km east of Abancay and 500 m to the NW of the town of Cotabambas (#Location: 13° 44' 14"S, 72° 21' 23"W).
The Middle Eocene to Early Oligocene Andahuaylas-Yauri belt porphyry copper province is situated in southeastern Peru, extending for ~300 km between Andahuaylas in the NW and Yauri to the SE. The belt includes both porphyry related iron-copper skarn mineralisation, such as Tintaya, Atalaya and Las Bambas, and typical porphyry style alteration and copper±Mo±Au) mineralisation, such as Antapaccay, Los Chancas and Cotabambas. Other mineralisation includes replacement, and sediment-hosted oxide zinc deposits, and minor epithermal vein-style ores.
This belt incorporates sections of the intermontane depression between the Eastern and Western Cordillera and the northern Altiplano (Carlier et al., 1996; Chávez et al., 1996), and marks the transition between the normal subduction conditions of southern Peru and northern Chile, and the northern, flat subduction zone of central and northern Peru (Cahill and Isaks, 1992). It is situated 250 to 300 km inboard of the Peru-Chile trench.
The Andahuaylas-Yauri belt is largely truncated to the north by the Abancay Deflection, a continental trench-normal structure that marks the northern limit of the central volcanic zone in Peru, the northern limit of the Precambrian Arequipa basement block to the SW along the coast, and is an extension of the trend of the subducting oceanic Nazca Ridge to the west. It also marks a change in structural trend, from NNW to the north, to NW-SE in southern Peru as well as coinciding with the southern limit of current flat slab subduction.
The Andahuaylas-Yauri batholith dominates the geology of the Andahuaylas-Yauri belt, exposed over the full ~300 km of its extent. It varies from 25 km in width at the SE end, to 130 km near Abancay in the north. It comprises early mafic to intermediate intrusions with cumulate textures, grading to intermediate intrusive rocks with equigranular to porphyritic textures. Batholith emplacement took place in 3 stages (Perelló et al., 2003): i). Early stage, characterised by calc-alkaline cummulates, mainly gabbro, olivine gabbro and diorite that crystallised at the bottoms of shallow magma chambers at temperatures of 1000°C and a pressure of 2 to 3 bar; ii). A second stage of intermediate rocks, mainly monzo-diorite, quartz-diorite, quartz-monzodiorite and granodiorite, which dominated the emplacement, and are lighter grey in color with a medium to coarse grain size and textures that appear to be equigranular to porphyritic. This rocks constitutes the bulk of the batholith; iii). Terminal stage, subvolcanic intrusions with a granodioritic to dacitic composition. K-Ar ages for the batholith, published by Carlier et al. (2003; 1996), Carlotto (1998) and Perelló et al., 2002; 2003), give a Middle Eocene to Early Oligocene age of ~48 to 32 Ma), with the cummulate rocks 48 and 43 Ma, and for the intermediate stage rocks from 40 to 32 Ma.
The batholith intrudes Precambrian to Palaeozoic basement rocks which are exposed to the NE. The basement sequence culminates in the Permian to Early Triassic age Mitu Group volcaniclastic and clastic rocks. The basement is overlain by Mesozoic and Cenozoic sedimentary rocks deposited in the Eastern and Western Peruvian basins. The eastern basin is made up of marine clastic and carbonate rocks. The northeastern edge of the western basin includes the Lagunilla and Yura Groups, made up of middle to late Jurassic quartz-arenite, quartzite and shale, that grades upward into massive micritic limestone, shale and chert of the Mara and Ferrobamba Formations. The upper Yura Group comprises the Soraya Formation, composed of arenite, quartz arenite and quartzite. The Yura Group hosts the Cotabambas deposit.
The sedimentary San Jerónimo Group and the largely volcanic Anta Formation dominated the Eocene and Oligocene stratigraphy, which un-conformably overlies the Mesozoic and Cenozoic sedimentary sequences. Miocene and Pliocene volcanic and sedimentary rocks overlie the Oligocene sedimentary sequence. These are all overlain by a regional discontinuous veneer of Pleistocene fluvio-glacial rocks and re-worked gravels.
The geology of the Cotabambas Project is predominantly composed of the following Eocene to early Oligocene rocks:
• Anta Formation andesite, which overlies widespread Mesozoic carbonate country rocks, and is relatively restricted in the deposit area, occurring on the northern and southern fringes of the diorite;
• Diorite, related to the Andahuaylas-Yauri Batholith, occurring as a ~6 km diameter intrusion into the Mesozoic limestone country rocks and Anta Formation andesites;
• Later, altered, mineralised monzonite porphyry, also related to the Andahuaylas-Yauri Batholith. This occurs as swarms of the 'Principal quartz monzonite' dykes and stocks that are each 100 to 300 m long and 20 to 100 m wide. These are centred on the two main porphyry systems, the larger Ccalla, and the smaller Azulccacca, immediately to its SSW. The swarm at the core of the Ccalla deposit, however, continues beyond the western margin of the deposit. Other 'monzonite' dykes of similar dimensions are interspersed with the 'Principal quartz monzonite', whilst less frequent stocks and dykes of 'Secondary quartz monzonite' are largely peripheral to the mineralised systems. A larger 3 km diameter mass of 'quartz-monzonite latite' is found intruding the diorite mass ~2 km to the north of the Ccalla porphyry system. Larger plugs, up to 1 km in diameter, of the 'Principal quartz monzonite' and associated dykes intrude the diorite away from the main mineralised systems, associated with the Guaclle, Cayrayoc and other mineralised systems;
• A late dacite volcanic dome and associated latite dykes covers an area of ~1 x 0.6 km on the northern margin of the Ccalla porphyry system, while the dykes persist through both deposits, but do not occupy a large volume.
The geometry of the quartz monzonite porphyry and latite dykes are influenced by a strong system of sub-vertical faults and shear zones that trend at ~30°. A second set of structures, normal to this system, and parallel to the regional thrust faulting trend of 120°, runs between the Ccalla and Guaclle areas of the deposit, to the east and west respectively.
Two main porphyry centres have been tested at Cotabambas, the larger, ~1 km diameter, Ccalla, and the smaller, 300 x 400 m Azulccacca, ~800 m to its SSW.
These deposits both comprise an oxide zone which occurs within a well developed leached cap, and hypogene and supergene enriched sulphides below the base of oxidation. The preserved leached cap at Ccalla is up to 200 m thick, while that remaining at Azulccacca is up to 100 m thick.
Hypogene mineralisation has been intersected from ~20 m below surface, to depths of >500 m, and is best developed with pyrite in quartz-sericite-altered quartz monzonite porphyry dykes running parallel to the NNE trending structural corridors at Ccalla and Azulccacca. Mineralisation is present as disseminated chalcopyrite and pyrite, pyrite-chalcopyrite stringers or veinlets and quartz chalcopyrite pyrite veinlets. Locally, patches of hypogene mineralisation are developed in diorite, peripheral to the quartz monzonite porphyry, where the NNE trending structural system passes within 10 to 20 m of the diorite-porphyry contact. The intensity of chalcopyrite mineralisation decreases and disseminated pyrite increases away from the higher grade parts of the hypogene zone.
The pattern of distribution of grade is characterised by a background of 0.1 to 0.3% Cu, with steeply dipping (60 to 80°SE) tongues of > 0.3% Cu, enclosing cores of 0.5 to 1% Cu. The gold distribution is similar, but more diffuse, with broader coincident tongues of 0.1 to 0.3 g/t Au, enclosing cores of 0.3 to 1 g/t Au.
Gold grades are strongly correlated to copper levels in the chalcopyrite and pyrite dominated hypogene zone. Some occurrences of bornite have been noted in deeper portions of the hypogene zones. Silver grades are not as strongly correlated to copper as they are to gold levels, but are generally elevated where copper-gold mineralisation is present.
The mineralisation is cut by steeply dipping, near vertical, <10 to >90 m thick, barren dykes of latite that expand downwards.
Hypogene mineralisation occurs as disseminated stringers and four veinlet styles and assemblages:
• A1, with quartz, anhydrite, magnetite, chalcopyrite and pyrite;
• A2, with quartz, magnetite, chalcopyrite and pyrite;
• B, with quartz, chalcopyrite, molybdenite;
• D, with quartz, pyrite, galena and sphalerite.
Alteration comprises an early potassic phase coincident with the A1 and A2 type veinlets, whilst the type B veinlets are part of a transitional phyllic alteration phase. Type D mineralisation is interpreted to be part of that phyllic alteration stage.
Supergene Sulphide Enrichment Zone
High-grade chalcocite zones with lesser covellite and chalcopyrite overprint the upper sections of the hypogene sulphide mineralisation, at the base of the leached cap. Supergene zones are found at both Ccalla and Azulccacca, and are characterised by high chalcocite content, correspondingly high cyanide-soluble copper levels and generally >1% total copper.
Oxide Copper-Gold Mineralisation
Oxide mineralisation occurs in the leached caps of both the Ccalla and Azulccacca deposits, which are composed of abundant limonite, goethite and manganese wad, and a characteristic mottled orange brown colour. Iron oxides and oxy-hydroxides replace pyrite, and oxide copper-gold mineralisation occurs as patches of green copper oxides, typically chrysoscolla, malachite and broncanthite. Copper oxides occur as coatings on disseminated chalcopyrite grains and as fill in fractures and veinlets. Oxide copper-gold lenses with lateral extents of 100 to 200 m and thicknesses of 10 to 50 m have been encountered in outcrop and intersected in drill holes, typically occurring after hypogene and secondary sulphide mineralisation. However, isolated drill intersections indicate that oxide copper-gold mineralisation may also overlie low-grade hypogene mineralisation, possibly indicating remobilisation of copper in the leached cap.
Oxide Gold Mineralisation
This style of mineralisation has been defined in a lens in the Azulccacca deposit, but has also been intersected in short, isolated 1 to 5 m intervals in other parts of the leached cap of the deposit. It is associated with limonite and occurs near major structures cutting the hypogene sulphide zone that are associated with the quartz monzonite porphyry.
Published NI 43-101 compliant mineral resources as at 30 June 2013 (Twigg, et al., 2015) were:
Hypogene sulphides - 84.2 Mt @ 0.37% Cu, 0.21 g/t Au, 2.73 g/t Ag, 18 ppm Mo (0.2% Cu cut-off);
Supergene sulphides - 8.9 Mt @ 0.73% Cu, 0.31 g/t Au, 3.07 g/t Ag (0.2% Cu cut-off);
Oxide copper-gold - 23.8 Mt @ 0.49% Cu, 0.24 g/t Au, 2.63 g/t Ag (0.2% Cu cut-off);
Oxide gold - 0.2 Mt @ 0.66 g/t Au, 3.74 g/t Ag;
TOTAL - 117.1 Mt @ 0.42% Cu, 0.23 g/t Au, 2.74 g/t Au, 13 ppm Mo (0.2% Cu cut-off)
Hypogene sulphides - 521 Mt @ 0.29% Cu, 0.18 g/t Au, 2.41 g/t Ag, 21 ppm Mo (0.2% Cu cut-off);
Supergene sulphides - 7.4 Mt @ 0.73% Cu, 0.18 g/t Au, 1.93 g/t Ag, 7 ppm Mo (0.2% Cu cut-off);
Oxide copper-gold - 75.8 Mt @ 0.41% Cu, 0.15 g/t Au, 1.82 g/t Ag, 3 ppm Mo (0.2% Cu cut-off);
Oxide gold - 1.2 Mt @ 0.61 g/t Au, 3.27 g/t Ag;
TOTAL - 605.3 Mt @ 0.31% Cu, 0.17 g/t Au, 2.33 g/t Au, 19 ppm Mo (0.2% Cu cut-off).
This summary is largely drawn and paraphrased from "Twigg, S., Colquhoun, W., Khera,V., Aarsen, J. and Vela, L., 2015 - Cotabambas Project, Apurimac, Perú, Updated Preliminary Economic Assessment; An NI 43-101 Technical Report prepared by Amec Foster Wheeler (Perú) S.A. and Moose Mountain Technical Services for Panoro Minerals Ltd., 322p."
The most recent source geological information used to prepare this summary was dated: 2015.
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.
Perello, J., Carlotto, V., Zarate, A., Ramos, P., Posso, H., Neyra, C., Caballero, A., Fuster, N. and Muhr, R., 2003 - Porphyry-Style Alteration and Mineralization of the Middle Eocene to Early Oligocene Andahuaylas-Yauri Belt, Cuzco Region, Peru: in Econ. Geol. v.98., pp. 1575-1605.|
Perello, J., Neyra, C., Posso, H., Zarate, A., Ramos, P., Caballero, A., Martino, R., Fuster, N. and Muhr, R., 2004 - Cotabambas: Late Eocene porphyry copper-gold mineralization southwest of Cuzco, Peru: in Sillitoe, R.H., Perello, J. and Vidal, C.E., (Eds.) 2004 Andean Metallogeny: New Discoveries, Concepts and Updates, Society of Economic Geologists, Denver, SEG Special Publication 11 pp. 213-230.|
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