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Junin

Ecuador

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The Junín porphyry Cu-Mo deposit is located in the Intag River Region, west of Cotacachi City in northwestern Ecuador, ~75 km north of Quito (#Location: 0° 19' 1"N, 78° 39' 6"W).

Tectonic and Regional Setting

  The bulk of Ecuador's porphyry Cu±Mo±Au±Ag and porphyry-related epithermal Au±Ag±Cu deposits are of Jurassic and Tertiary (mostly Miocene) age, that define two distinct and parallel, temporal metallogenic belts to the east and west respectively (PRODEMINCA 2000; Sillitoe and Perelló 2005; Chiaradia et al., 2009).
  Junin is one of a number of porphyry and epithermal deposits that form two main geographically separated mineral districts within the broader Tertiary metallogenic belt that follows the western Andean range or Cordillera Occidental.
  The southern of these, the Azuay-El Oro District, is defined by a series of Miocene porphyry Cu-Mo, Cu-Au and high, low and intermediate sulphidation epithermal copper-gold deposits and prospects ~200 km SSE of Guayaquil in southern Ecuador.
  The Imbaoeste District, ~440 km to the north, includes the Eocene Alpala porphyry Cu-Au and Miocene Junín porphyry Cu-Mo deposits. These deposits are respectively associated with the Oligocene to early Miocene Saraguro Group and overlying similar Late Miocene to Recent arc sequences which are both developed and overlap throughout the belt, with the older succession predominating to the west.
  Other deposits and occurrences are distributed along the Tertiary porphyry belt between these two districts, and to the north into Colombia and SE into Peru.

  For more detail of the regional setting and geology, see the separate records for North Andes copper-gold province in Ecuador   and the broader   North Andes and Panama copper-gold province.

Geology and Mineralisation

  The late Miocene Junín porphyry Cu-Mo deposit is hosted by the mid-Miocene Apuela batholith and is the central of three porphyry Cu systems that define the NE-striking belt (subparallel to the Chimbo-Toachi shear zone) known as the Imbaoeste district (MMAJ/JICA 1998; PRODEMINCA 2000; MICON 2005). The system is related to multiple 9 to 6 Ma, NNE to ENE striking and 45 to 70°SE dipping hornblende granodiorite porphyry dykes of variable thickness (generally 50 to 100 m) that are pre-, syn-, and post-mineral (MICON 2005; Salazar 2007). These porphyry dykes are similar in overall mineralogical composition, but display local textural variations where groundmass proportions vary from 30 to 80 vol.% and phenocryst sizes vary from relatively fine-grained (several mm) to coarse-grained (up to ~2 cm; Salazar 2007).
  The porphyry dykes are hosted by seriate plutons forming part of the 19 to 12 Ma Apuela batholith, mostly composed of biotite- and hornblende-bearing granodiorite, tonalite, quartz-diorite and quartz-monzodiorite intrusions, intruding a sequence of mainly basalt and turbidite units of the late Cretaceous Río Cala island arc sequence.
  Porphyry Cu-Mo mineralisation extends to depths of 600 m in the central part of the deposit, and is associated with zones of potassic and, to a lesser extent, phyllic alteration, which partly overprints potassic assemblages. The potassic alteration occurs as disseminated and replacement biotite and quartz±K feldspar±magnetite veinlets, while the phyllic phase is characterised by disseminated and replacement green and white sericite. Propylitic alteration is found in the peripheral parts of the mineralised system, and is also seen to overprint potassic and phyllic alteration in the central parts of the deposit (Salazar 2007).
  Mineralisation accompanying the potassic phase of alteration occurs mainly as chalcopyrite-, bornite- and molybdenite-bearing quartz and quartz-pyrite stockwork veinlets, as well as disseminations of chalcopyrite and bornite, while a suite of quartz-pyrite±chalcopyrite ±molybdenite veinlets with sericite halos is associated with the phyllic phase.
  Alteration and mineralisation affects both the porphyry dykes and the Apuela batholith, with fluids preferentially exploiting pre-existing structures and lithologic contact zones (Salazar 2007). Post-mineral porphyry intrusions, which are mainly distributed in peripheral parts of the deposit, show similar mineralogical and textural features as mineralised porphyry intrusions (Salazar 2007). Molybdenite from "B-type" quartz-molybdenite veinlets cutting through porphyry dykes with potassic alteration, subsequently overprinted by phyllic and, locally, propylitic alteration were dated at 6.13±0.02 and 6.63±0.03 Ma (Schütte et al., 2011), while a host coarse-grained, quartz-rich potassic altered hornblende granodiorite porphyry dyke gave a zircon U-Pb age of 9.01±0.06 Ma (Schütte et al., 2010). The Apuela batholith at nearby Cuellaje has a zircon U-Pb age of 12.87±0.05 Ma (Schütte et al., 2010).

Published inferred resource estimates for the Junín deposit include:
    982 Mt @ 0.89% Cu, 1.9 g/t Ag, 0.04% Mo, 0.01 g/t Au (0.4% Cu cutoff grade; Schütte et al., 2010); or
    1.3 Gt @ 0.73% Cu, 1.6 g/t Ag, 0.03% Mo, 0.01 g/t Au (0.2% Cu cutoff grade; Ascendant Copper, 2006).

The most recent source geological information used to prepare this summary was dated: 2010.    
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
 References to this deposit in the PGC Literature Collection:
Schutte P, Chiaradia M, Barra F, Villagomez D and Beate B,  2012 - Metallogenic features of Miocene porphyry Cu and porphyry-related mineral deposits in Ecuador revealed by Re-Os, 40Ar/39Ar, and U-Pb geochronology: in    Mineralium Deposita   v.47 pp. 383-410


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