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The Michiquillay porphyry style Cu-Au-Mo deposit is related to early to middle Miocene porphyry intrusive, and is located in the auriferous Cajamarca district of northern Peru, ~7 km south of the similar El Galeno deposit, and 20 km east of the Yanacocha porphyry-epithermal gold deposit cluster (#Location: 7° 2' 5"S, 78° 19' 16"W).

The original claim to the Michiquillay deposit was made in by Mr. Reynaldo De la Puente and others in 1957. Subsequently, at the beginning of the 1960s, American Smelting and Refining Company (ASARCO) acquired the mining titles over the Michiquillay area, and commenced diamond drilling, from which a preliminary reserve of 570 Mt of copper ore was estimated. In 1970, the concessions were acquired by the State and assigned to Minero Perú S.A. In the 1970s, the Japanese government financed a cooperation committee to investigate the deposit, later to become the Michiquillay Copper Corporation, which included a consortium of Japanese companies that undertook geological, mining and metallurgical testing, and the establishment of a pilot plant. In March 2001, Minero Perú S.A. was merged with Centromín Perú S.A. By 2007, the privatisation of Michiquillay was completed, and the Michiquillay concessions were transferred to Anglo American, who continued testing and evaluation. Anglo American withdrew from the project in December 2014 and returned the property to the State.

Regional Setting

The Michiquillay porphyry copper system is located towards the southern end of the Cajamarca mineral belt in the Western Cordillera of the northern Peruvian Andes, a generally north-south trending belt of Oligocene to Miocene porphyry copper deposits that extends for 350 km from Cajamarca in the south to the Ecuadorian border and includes two geochemically distinct groups of deposits along this trend namely: i). porphyry Cu-Mo deposits which include La Granja, Michiquillay, El Galeno, Cañariaco and Rio Blanco; and ii). porphyry Cu-Au deposits which include Cerro Corona, Minas Conga and La Carpa. These systems are mostly associated with dacite to monzonite to diorite intrusions, which intrude basement rocks of Upper Jurassic to Lower Cretaceous quartzites, limestones and mudstones of the Goyllarisquizga Formation deformed in the Marañon (or Incaic) thrust and fold belt, and Early Tertiary sequences of andesitic to dacitic lavas and tuffs of the Llama and Porculla Formations which together comprise the Calipuy Group.

For detail of the broad regional setting and geology of the Peruvian Andes, see the separate Peruvian Andes record.

Basement in the region comprises Precambrian to Early Palaeozoic pelitic schists of the Olmos Complex which includes Precambrian metamorphic rocks, overlain by Ordovician shales and sandstones. These are unconformably overlain by Permian conglomerates, sandstones and volcanic flows and tuffs, which are in turn overlain by Late Triassic-Early Jurassic marine sediments intercalated with minor volcanic units of the La Leche Formation. These are succeeded by the Early to Late Jurassic volcano-sedimentary sequence of the Oyotún Formation. The Mesozoic rocks were deposited in ensialic, extensional, marginal basins related to eastward subduction, which extend the length of the Andes. During the Latest Jurassic to Early Cretaceous the region was uplifted and eroded by the mid-Cretaceous Mochica tectonic phase. By the late Early Cretaceous, subsidence resulted in an eastern sub-basin bounded to the east by the basement Marañon High, and the deposition of 2 to 3 km of Cretaceous strata. The oldest of these sediments are thick regionally extensive deltaic sandstones with shales and coal, and a thin marine limestone which collectively form the Early Cretaceous Goyllarisquisga Group which unconformably overlies the older rocks. From the close of the Early Cretaceous to the middle of the Late Cretaceous, a marine transgressive sequence of up to 1500 m of marls, shales and limestone were deposited across the region. Sedimentation ceased abruptly at the beginning of the Early Tertiary when the basin was deformed by the Late Paleocene Incaic I (59 to 55 Ma) and Middle Eocene Incaic II phases (43 to 42 Ma), which resulted in the formation of a foreland thrust and fold belt with SW-dipping, NE-verging thrust sheets, and the development of open, upright folds. Some thrusts were reactivated and folded during the Quechua 1 orogenic pulse (17 Ma). These periods of activity were accompanied by the eruption and deposition of volcanic units of the Eocene and Miocene Llama and Porculla Formations, which together comprise the Lower Calipuy Group. This episode was followed by uplift and erosion and then by renewed magmatism and volcanic activity with the eruption of the 12 - 10 Ma Yanacocha volcanic complex and the Middle to Late Miocene Huambos Formation which capped the stratigraphic sequence in the region.

The WNW trend of the thrust and fold belt is rotated to WNW to the south of Cajamarca area and is cut by a series of NE-trending structures with sinistral offset. This is interpreted as a major trans-arc structure with greater differential movement towards the foreland on the south side, which focused mineralisation in a NE-trending, 30 to 40 km wide by 200 km long mineral belt in the Cajamarca area known as the Chicama-Yanacocha structural corridor. The Michiquillay and nearby El Galeno porphyry deposits are located within this mineral belt.

Geology and Mineralisation

The immediate country rocks in the deposit area comprise quartzites and white sandstones of the Early Cretaceous Goyllarisquisga Group, sandstones and calcareous shales of the Lower Cretaceous Farrat Formation, and ferruginous Inca, sandy limestone, marl and calcareous shales of middle Cretaceous Chulec Formation. The stratified rocks have a general strike of ~300° and dip of 30 to 40°SW.

Michiquillay is a Cu-Au-Mo deposit associated with a Miocene (20.0 to 19.8 Ma) dioritic complex emplaced within the hanging wall of a back thrust fault, intruding the quartzites and limestones of the country rock sequence. The deposit trends in a north-east direction and is crosscut by NNW-trending prospect-scale faults that influenced both alteration and metal distribution.

The intrusive complex extends over an area of approximately 5 x 1.5 km, parallel to the NW structural trend, and includes at least two major intrusive phases. The most common is a medium-grained crowded porphyritic diorite (D1) with 35 to 45%, 0.3 to 0.5 mm euhedral plagioclase, ~3%, 0.5 to 8 mm biotite books and 1 to 3%, 0.3 to 5 mm hornblende phenocrysts in a fine grained feldspathic matrix, with a local variety characterised by 0.5 to 4% rounded quartz phenocrysts. The composition of this phase is biotite-hornblende diorite.

The second phase of the complex is a weakly porphyritic, medium to fine grained diorite and contains a low vein content and intense biotite alteration. It comprises ~15% euhedral plagioclase, biotised hornblende phenocryts and euhedral biotite books and forms steep 1 to 6 m thick dykes that crosscut and truncate the potassic altered main phase and contain xenoliths of biotite-hornblende diorite and lack well developed stockworks. This phase is interpretted to be syn- to late-mineralisation.

The intusive complex is located in the hangingwall of the NW-trending Michiquillay back-thrust fault which dips at ~60°NE. The thrust is underlain by Cretaceous limestone, while the intrusive contacts of the complex in the hangingwall are with Cretaceous quartzites which underlie the limestones. The Michiquillay fault is characterised by a matrix-supported breccia containing fragments of both limestone and quartzite. The porphyry is cut by a series of sub-vertical, oblique faults towards the centre of the deposit, trending either NNW or ENE. The NNW striking faults are the dominant set within the deposit. They separate potassic alteration zones in the NE and SW parts of the complex and define the outer limit of a strong phyllic zone, while an increase in stockwork veining occurs along these faults, and some contain localised zones of vuggy quartz.

Both intrusive phases contain a well-developed K feldspar-biotite-quartz-magnetite alteration assemblage, forming a NE-SW oriented, approximately 500 m wide zone that lies towards the south-eastern margin of the complex, and is stongest in the NE and SW part of the deposit. This alteration was formed by the replacement of plagioclase by K feldspar and the precipitation of fine-grained hydrothermal biotite. The development of biotite is strongest to the SW, while to the NE there is only moderate biotisation but more strongly developed K feldspar-quartz alteration. Both segments contain thin, 1 to 2 mm wavy, magnetite-biotite veins that are cross-cut by quartz veining. Hypogene sulphides associated with the potassic alteration include chalcopyrite, molybdenite, pyrite, minor bornite and trace pyrrhotite and sphalerite, which occur as vein and fracture fillings, and as disseminations in the groundmass. Grades within these altered potassic zones are of the order of 0.8% Cu and 0.14 g/t Au.

The core of the altered zone and deposit, between the NE and SW potassic alterations zones described previously, is occuppied by intense phyllic quartz-sericite-pyrite alteration, occurring as veins of quartz-pyrite veining with muscovite/illite selvages which are distributed throughout the complex and crosscut all other veins. The phyllic alteration has obliterated previous textures, although towards the margin of the phyllic core it has been shown to replace potasic alteration. The distribution of the alteration zones is influenced by the NNW-striking faults. Sulphides associated with the phyllic alteration include large amounts of pyrite, and minor molybdenite and chalcopyrite, with enargite, luzonite and tetrahedrite-tennantite in the upper-central parts. Within the phyllic zone grades average 0.57% Cu and 0.08 g/t Au, lower than in the potassic zone.

The hypogene mineralised zone is overlain by a 45 to 80 m thick supergene sulphide enrichment zone with grades varying from 0.5 to 2.2% Cu, averaging 1.152% Cu, and a leached cap preserved over a thickness of from a few to 150 m in thickness. The supergene enrichment zone is characterised by covellite and chalcocite replacing hypogene chalcopyrite. Locally, argillic alteration, in the form of kaolinite, has partially to to moderately overprint the phyllic assemblage. This alteration is in part controlled by fault zones, extending to depths of 100 m below the surface.

Michiquillay contains a 45 to 80 m thick supergene enrichment blanket which has an average grade of 1.15% Cu, beneath a deep leached cap which is up to 150 m thick.

Michiquillay comprises an indicated resource (Davies and Williams, 2005) of:
      631 Mt @ 0.69% Cu, 0.15 g/t Au, 0.01 to 0.02% Mo, including
        supergene enriched ore of: 46.2 Mt @ 1.15% Cu.

Prior to 2014, Anglo American estimated the deposit had resources of:
      544 Mt @ 0.69% Cu, 0.1 to 0.5 g/t Au, 2 to 4g/t Ag.

The most recent source geological information used to prepare this summary was dated: 2005.    
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.


  References & Additional Information
   Selected References:
Davies, R.C. and Williams, P.J.,  2005 - The El Galeno and Michiquillay porphyry Cu-Au-Mo deposits: geological descriptions and comparison of Miocene porphyry systems in the Cajamarca district, northern Peru: in    Mineralium Deposita   v.40, pp. 598-616.
Hollister V F and Sirvas E B,  1974 - The Michiquillay porphyry copper deposit : in    Mineralium Deposita   v9 pp 261-269

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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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