The Cuajone porphyry copper-molybdenum deposit lies within one of a trio of major porphyry centres spread over a north-west trending interval of 30 km, located some 100 km south-east of Arequipa, and 42 km north-east of Moquegua in southern Peru. The other two centres embrace the major Toquepala and Quellaveco deposits respectively. The three deposits together contain more than 25 Mt of copper metal.
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Cuajone, Toquepala and Quellaveco are exposed on the ocean-ward slopes of the central Andean Cordillera Occidental at between 3000 and 3700 m asl and are immediately to the southwest of an inactive string of Pliocene-Quaternary andesitic-dacitic strato-volcanoes forming the crest of the cordillera. They are members of a belt of Paleocene to early Eocene porphyry copper deposits in the Main Arc Domain of Andean magmatism and are believed to have been emplaced at between 52 and 56 Ma. This mineralisation is the culmination in the evolution of a major continental volcano-plutonic arc, the eruptive elements of which are represented by the calc-alkaline to shoshonitic subaerial volcanics of the Cretaceous to Palaeogene Toquepala Group.
Regionally the Toquepala Group rocks are underlain by Upper Triassic to Jurassic marine volcanic and sedimentary suites and then by Precambrian metamorphics (of the Arequipa Massif accreted block) which are cut by Palaeozoic granites.
The Toquepala Group is not fully exposed but comprises a sequence that is several kilometres thick of andesites, dacites and rhyolite flows, breccia flows, agglomerates and ash flows with minor volcaniclastics and conglomerate lenses, as well as prominent quartz porphyritic rhyolite flows found near Cuajone.
Fringing to the south-west and overlapping the volcanic belt are the un-fossiliferous continental clastics and minor rhyolitic ignimbrites of the Oligocene Moquegua Formation developed during a period of protracted mid-Tertiary erosion. These are followed by early Miocene rhyodacitic to rhyolitic ignimbrite sheets of the Huaylillas Formation which overlie both the Moquegua Formation and Toquepala Group above an erosional unconformity. Near Cuajone these are followed by other, younger mid-Miocene ignimbrite units of the Chuntacala Formation. Further to the north-east Miocene continental clastics and volcanics cover extensive areas of the Cordillera Occidental.
All of the units described above, to the top of the Toquepala Group, have been intruded by the composite, polyphase Cretaceous to Palaeogene Andean Batholith ranging in age over a 150 m.y. period and in composition from gabbro to alkali feldspar granite, although quartz-diorite, quartz-monzodiorite and granodiorite predominate. The intrusive phases of this batholith are however, intruded by the porphyry stocks of the Cuajone and Toquepala mine areas.
The mineralisation and alteration at the Cuajone deposit is directly related to a multistage latite porphyry which intrudes basaltic andesites and the overlying 370 m of rhyolite porphyries of the locally 80 to 65 Ma Toquepala Group. The first, pre-mineral intrusive in the mine area was a north-south elongated, 0.7x0.35 km, 66.7 ±1.7 Ma grey to grey-green holocrystalline to equigranular, medium grained, porphyritic diorite stock, approximately 1 to 2 km to the west of the orebody.
The three pulse emplacement between 56 and 52 Ma of the multistage, 2.5x0.7 km, NW-SE elongated intrusive body of latite porphyry was as follows:
i). The first magmatic pulse of the latite porphyry was concentrated in the south-eastern part of the multiple intrusive mass and was responsible for the introduction of the bulk of the hypogene copper and molybdenite mineralisation in the Cuajone orebody and the associated intense alteration of both the latite and surrounding Toquepala Group andesites and rhyolites. This pulse, where not heavily altered, is a porphyry with phenocrysts of quartz to 4 mm in diameter and laths of feldspar in a cryptocrystalline matrix. The alteration takes the form of a potassic core, characterised by biotite-magnetite-K feldspar-silica, grading upwards and outwards to biotite-magnetite-silica, which passes laterally into an extensive outer envelope of chlorite-epidote-calcite-pyrite propylitic alteration which has a radial extent of up to 4 km from the centre of the deposit. The intensity of this alteration has masked the boundary between the latite porphyry and the surrounding Toquepala Group lithologies.
ii). The second magmatic pulse produced the phase known as the Barren Latite Porphyry, represented by two separate exposures, a larger, ovoid 850x550 m mass immediately to the north-west of the first pulse, while a smaller 300x200 m plug occurs within the outcrop of the first pulse. As the name suggests both exposures have only weak associated alteration and no copper or molybdenum mineralisation. However, this pulse did develop breccia bodies on its contacts with the other pulses and country rocks. These breccias comprise heterolithic clasts that range from well rounded to angular within a matrix of latite porphyry.
iii). The third magmatic pulse covers a surface area of around 800 m in diameter immediately to the northwest of the main primary latite porphyry outcrop and has only weak associated alteration and no copper or molybdenum mineralisation. It is porphyritic with quartz grains up to 2 cm across in a microcrystalline to cryptocrystalline matrix.
At a late stage in the emplacement of the Latite Porphyry complex, and during the of the first erosive period, the interaction of downward percolating meteoric waters with the rising hypogene hydrothermal fluids produced an intense phyllic silica-sericite-pyrite zone which was superimposed on the upper parts of the mineralised system associated with the first Latite Porphyry pulse to develop a higher grade zone of copper-molybdenum ore with grades of greater than 0.4% Cu as chalcopyrite and molybdenite. This alteration and mineralisation style is principally developed within the Latite Porphyry and the Toquepala Group rhyolites, and only to a minor degree in the underlying andesites.
The copper of the hypogene deposit is distributed as follows: Basaltic andesite - 51%, Latite Porphyry - 47%, Toquepala Group rhyolite - 1%, Mineralised breccias - 1%.
The 900 m wide hypogene ore zone at Cuajone was overlain by a gently west dipping, average 20 m thick, blanket of sulphide enrichment carrying more than 1.5% Cu. This blanket thickened in its centre, but thinned substantially to the south towards its limit. The main chalcocite layer is overlain by a 15 to 40 m thick partially oxidised upper zone averaging 1.3% Cu, where remnant chalcocite is still apparent, but malachite and chrysocolla dominate. These are in turn overlain by a partially preserved (maximum of 120 m thick) hematite bearing leached capping with 0.1 to 0.2% Cu. Argillic alteration associated with the supergene ores include kaolinite, montmorillonite, illite and dickite.
The deposit has been quoted as comprising a geological resource of: 1600 Mt @ 0.74% Cu.
The original pre-mining reserve was: 470 Mt @ 1% Cu, including,
Enriched ore: 75 Mt @ >1.5% Cu.
To 1999, after 23 years of production, 1.443 Gt of ore and waste had been removed from the pit and
425 Mt @ 0.97% Cu treated by the concentrator.
The reserve in 1999 was estimated to be 1400 Mt @ 0.64% Cu, 0.033% MoS2 at a 0.4% Cu cutoff.
The Cuajone and Toquepala mines and the un-developed Quellaveco deposit are owned and operated by the Southern Peru Copper Corporation, which is in turn owned by Grupo Mexico 54.6%, Phelps Dodge 14%, with the remainder held by others.
For more detail consult the reference(s) listed below which were the principal source of the information on which this summary was based.
The most recent source geological information used to prepare this decription was dated: 2003.
This description is a summary from published sources, the chief of which are listed below.
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Clark A H, Farrar E, Kontak D J, Langridge R J, Arenas M J, France L J, McBride S L, Woodman P L, Wasteneys H A, Sandeman H A, Archibald D A 1990 - Geologic and geochronologic constraints on the metallogenic evolution of the Andes of southeastern Peru: in Econ. Geol. v85 pp 1520-1583
Clark A H, Tosdal R M, Farrar E, Plazolles A 1990 - Geomorphologic environment and age of supergene enrichment of the Cuajone, Quellaveco, Toquepala Porphyry Copper deposits, southeastern Peru: in Econ. Geol. v85 pp 1604-1628
Concha O, Valle J 1999 - Prospeccion, exploracion y desarrolla del yacimiento de Cuajone (Language: Spanish): in ProExplo 1999 Proceedings pp 117-143
Quang, C.X., Clark, A.H., Lee, J.K.W. and Hawkes, N., 2005 - Response of Supergene Processes to Episodic Cenozoic Uplift, Pediment Erosion, and Ignimbrite Eruption in the Porphyry Copper Province of Southern Peru: in Econ. Geol. v.100, pp. 87-114.
Simmons A T, Tosdal R M, Wooden J L, Mattos R, Concha O, McCracken S and Beale T, 2013 - Punctuated Magmatism Associated with Porphyry Cu-Mo Formation in the Paleocene to Eocene of Southern Peru: in Econ. Geol. v.108 pp. 625-639
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