Cu Mo Au Ag
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The El Galeno porphyry style Cu-Au-Mo deposit is related to early to middle Miocene porphyry intrusive, and located in the auriferous Cajamarca district of northern Peru, and is ~7 km north of the similar Michiquillay porphyry deposit, and 20 km east of the Yanacocha gold deposit.
(#Location: 6° 58' 20"S, 78° 19' 35"W).
The El Galeno porphyry copper system is situated 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.
For detail of the broad regional setting and geology of the Peruvian Andes, see the separate Peruvian Andes and Michiquillay records.
The El Galeno deposit is associated with 17.5 to 16.5 Ma multiple dioritic intrusions hosted within Lower Cretaceous quartzites and shales. Mineralisation comprises a hypogene zone overlain by a supergene enrichment blanket up to 120 m thick.
A regional thrust fault verging east-northeast has brought the Early Cretaceous sedimentary rocks that host the El Galeno mineralisation, over the Late Cretaceous stratigraphy. The porphyry-style mineralisation is hosted both by sedimentary rocks of the Goyllarisquizga Group and porphyritic dioritic intrusive stocks emplaced in a hanging wall anticline structurally controlled by oblique faults superimposed on early WNW-trending fold-thrust structures. The thrust faulting predates the intrusion of the porphyry and formation of the porphyry-style mineralisation. The principal Early Cretaceous stratigraphic and the intrusive units are as follows, from oldest to younger:
Chimu Formation (Early Cretaceous, Lower Valanginian) - 600 m of alternating sandstones, quartzites and shales. The Chimu Formation quartzite forms the core of the El Galeno Anticline and outcrops on Cerro Quinua Cucho on the SE side of the porphyry deposit. The rock is a massive white quartzite which is highly siliceous with intergranular voids. It generally lacks quartz veins and has disseminated and fracture fill sulphides, with some sandstone beds exhibiting potassic (K-feldspar, biotite) alteration and local quartz veinlet stockwork.
Santa Formation (Early Cretaceous, Middle Valanginian) - grey shales with intercalations of marly limestones and dark grey sandstones.
Carhuaz Formation (Early Cretaceous, Upper Valanginian, Hauterivian and Lower Barremian) - more than 550 m of well bedded in thin to medium beds of maroon and grey shales, sandstones and quartzites.
At El Galeno, the Santa and Carhuaz Formations cannot be distinguished and have been mapped and logged as a single 500 to 600 m thick unit which is the principal host to the El Galeno porphyry and the ore. The lithology is mostly a very fine-grained sandstone which is generally massive and featureless with no evident bedding. The cement of the sandstone is susceptible to hydrothermal alteration, which includes biotite and K-feldspar and argillic alteration that are hard brown, pink and white rocks repectively. It has a well developed quartz veinlet stockwork and the best supergene and hypogene copper grades.
Farrat Formation (Early Cretaceous, Upper Barremian to Aptian) - medium to coarse grained quartzites and sandstones with cross bedding and ripple marks. It forms the curved headwall around the head of the glacial valley in which the El Galeno porphyry deposit is located and lies to the NW of, and structurally above the porphyry deposit.
Inca Formation (Early Cretaceous, Lower Albian Age) - 4 to 100 m of ferruginous sandstone and shale with calcareous intercalations, which discordantly overlies the Farrat Formation.
Chulec Formation (Early Cretaceous, Middle Albian Age) - from 250 m to 474 m of well bedded grey shales, yellow marls and creamy-brown nodular limestones, which concordantly overlies with the Inca Formation.
The El Galeno deposit is located in the hanging wall of a major fault, the El Canche Thrust (or Puntre Fault), which is an important regional control on gold and copper mineralisation with several other intrusions and mineral deposits located in the hanging wall over a distance of 50 km to the NW, including the Michiquillay porphyry Cu to the Cerro Corona porphyry Cu-Au deposit. At El Galeno area, the El Canche thrust changes strike from NW to almost E-W. The axis of the hanging wall anticline forms a dome with a WNW strike, plunging gently (~25°) to the east and west at its ends, and which has a dextral deflection to NW strike in the middle, in the vicinity of the El Galeno porphyry. The deposit is localised where a NE structure (the Kerosene valley Fault) intersects the anticline axis. The El Galeno porphyry deposit was formed after these structural events and is not deformed by these structures, but was probably focused by them.
The El Galeno porphyry has been dated at 16.53±0.18 Ma (Early Miocene) on magmatic biotite in the late mineral PD4 porphyry, while hydrothermal biotite in the PD2 porphyry gives an age of 17.5±0.3 Ma. Six types of igneous intrusive rocks have been distinguished:
Microdiorite - narrow 1 to 2 m wide, premineral dykes which occur in the Cretaceous sediments, are medium grained with plagioclase, biotite, hornblende and magnetite and exhibit biotite alteration and quartz
veinlets. They are probably older and unrelated to the porphyries.
PD1 Porphyry - the most abundant porphyry, with a quartz diorite to granodiorite composition and abundant 3 to 5 mm phenocrysts of plagioclase, biotite and some bi-pyramidal quartz.
PD2 Porphyry - which has a quartz diorite to diorite composition, is more mafic than PD1 and has smaller and less abundant phenocrysts of plagioclase and biotite. PD2 contains xenoliths of PD1 and occurs as dykes or small stocks within PD1. PD2 is also pre-mineral, does not crosscut any veins in PD1, and has the same intensity of veining as PD1.
PD3 Porphyry is a coarse grained granodioritic porphyry with abundant 4 to 5 mm glomerophyric phenocrysts of plagioclase which comprise up to 50% of the rock, and coarse 5 to 8 mm bipyramidal quartz, biotite, and hornblende. PD3, which is also premineral, has the same amount of quartz-magnetite veining as PD1 and 2.
PD3.5 Porphyry is a quartz monzonite porphyry which has a texture similar to PD 4 with phenocrysts of plagioclase, quartz, biotite and rounded orthoclase mega-phenocrysts. It has some quartz-magnetite veinlets but they have a much lower density than PD1.
PD4 Porphyry is a quartz monzonite to monzonite porphyry with a low percentage of ~4 mm phenocrysts of plagioclase, biotite, hornblende, and sparse (1%) coarse (~20 mm) orthoclase megacrysts, in a pale grey to dark grey matrix. PD4 cross-cuts quartz-magnetite and quartz veinlets in PD3 and contains xenoliths of PD 3 and veined PD1. It is affected by weak potassic alteration and has a low percentage of B-type quartz-sulphide veinlets which are cut by pyrite and pyrite-quartz D veinlets with a halo of silica-sericite-pyrite or epidote-chlorite. PD4 forms dykes, typically between a few to 10 m in width, cutting PD3 with steep, irregular contacts and ~45 to 80° dips.
The porphyries are mostly of quartz diorite, granodiorite and quartz monzonite composition, while more mafic varieties have more dioritic and monzonitic compositions, although hole rock analyses reveal a generally dacitic composition and calc-alkaline suite.
The overall porphyry intrusion has the form of an upright cylindrical stock ~600 m in diameter, with sill-like appendages on the margins which have a low to moderate dip conformable to bedding. The total diameter is ~1400 m including these apophyses. It has dimensions of 1200 x 500 m with a NW elongation and an apophysis on the east side. The porphyries are notable for the lack of hydrothermal, phreatomagmatic or phreatic breccias which indicates that the intrusion and mineralization occurred in a passive fashion, with the only breccias present being igneous breccias or pseudobreccias formed by hydrothermal alteration and veining.
The porphyry intrusions has a barren core due to a combination of post-mineral intrusions and mineralogical zonation.
Mineralisation and Alteration
The principal hydrothermal alteration at El Galeno is potassic which affects both the porphyry and sandstones with the development of pervasive secondary biotite and/or potassium feldspar. This alteration is accompanied by the development of a stockwork of multiple phases of cross-cutting A and B-type magnetite, quartz and quartz-magnetite veinlets with sulphides. The quartz-magnetite veinlets are very abundant in the core of the PD1 stock which
has up to 50 to 100% veining, usually barren of Cu-Au mineralisation.
Quartz-sericite-pyrite, phyllic alteration is poorly developed, and is restricted to selvages of narrow D-type pyrite and pyrite-quartz veinlets which extend to the deepest levels in the core of the system.
Argillic alteration occurs as a pervasive supergene overprint of potassic alteration accompanying the
zone of chalcocite enrichment, and comprises white clays which extend to a depth of over 100 m where there is a gradational change to hypogene potassic alteration.
No well defined zone of propylitic alteration is obvious, partly because potassic alteration extends into the sandstones beyond the porphyry stock, which are largely non-reactive. The quartzites only exhibit alteration minerals in intergranular spaces with disseminated pyrite, chalcopyrite and, molybdenite. They largely lack quartz veins and have a low density of fractures with sulphides.
Hypogene mineralization comprises pyrite, chalcopyrite and molybdenite within the A and B quartz veinlets, and as disseminations, accompanied by potassic alteration. Sulphides are generally absent within the central core of intense, 50 to 100% quartz-magnetite veining, and is consequently low grade. The D veinlets have pyrite with molybdenite, chalcopyrite, bornite and minor sphalerite. In addition to pyrite, chalcopyrite and molybdenite, minor bornite and less commonly arsenopyrite, sphalerite and pyrrhotite also occur in veinlets and disseminations, while tennanite and tetrahedrite have been rarely observed along the western margin of the porphyry system.
High grade Au mineralisation is associated with early intrusive phases located near the centre of the deposit, while the highest Cu grades (~0.9% Cu) are largely associated with a supergene enrichment blanket. The high Mo grades are restricted to contacts with the metasedimentary rocks.
The high grade chalcocite zone occurs as a ~100 to 150 m, locally over 300 m thick, blanket in the upper part of the deposit beneath a thin leached capping and oxide zone. The strongest chalcocite mineralisation occurs along the southern and eastern flanks of the porphyry intrusive in sandstones of the Santa and Carhuaz Formations, and is thinner in the central part. Quartzites, and to a lesser degree sandstones, contain supergene copper minerals to a greater depth than the intrusive lithologies, as they neutralise descending acid copper bearing solutions less efficiently. Chalcocite coats and replaces pyrite and chalcopyrite and is associated with argillic alteration. Covellite forms rims and occurs in fractures in chalcocite.
The supergene sulphide blanket is overlain by a thin, 0 to 40 m, averaging 10 m thick, remnant jarositic leached capping with strong argillic alteration. The leached cap is underlain by a 10 to 45 m thick zone of partial leaching and oxidation with variable copper grades. The leached capping and supergene blanket is absent over the ridge on the SW side of the deposit where hypogene sulphides outcrop with surficial oxidation to jarosite, ferrimolybdite and chalcanthite, and oxidation of magnetite to hematite. As the leached capping is too thin to account for the volume of supergene chalcocite enrichment present, it is assumed a much thicker leached capping was originally developed, as at the nearby Michiquillay deposit, and has been eroded by glaciation.
The El Galeno deposit contains an estimated geological resource of (Davies and Williams, 2005):
486 Mt @ 0.57% Cu, 0.14 g/t Au and 0.015% Mo.
NI 43-101 compliant mineral resources in September, 2006 were (Northern Peru Copper Technical Report, 2007):
0.4% Cu cut-off
Indicated resource - 765 Mt @ 0.49% Cu, 0.11 g/t Au, 2.6 g/t Ag, 140 ppm Mo;
Inferred resource - 98 Mt @ 0.35% Cu, 0.11 g/t Au, 2.1 g/t Ag, 100 ppm Mo;
0.5% Cu cut-off
Indicated resource - 526 Mt @ 0.56% Cu, 0.13 g/t Au, 2.7 g/t Ag, 150 ppm Mo;
Inferred resource - 25 Mt @ 0.43% Cu, 0.14 g/t Au, 2.3 g/t Ag, 120 ppm Mo.
The information in this summary has been in part drawn from "Sim, R., Davies, B.M., Rose, W.L., Elfen, S.C. and Hyyppa, R.R., 2007 - El GalenoProject, Prefeasibility Study; An NI 43-101 Technical Report prepared for Northern Peru Copper Corp., by Samuel Engineering, Inc., 200p."
The most recent source geological information used to prepare this summary was dated: 2012.
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 to this deposit in the PGC Literature Collection:
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.|
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