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Chaucha |
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Ecuador |
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Cu Mo
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Super Porphyry Cu and Au
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The Chaucha porphyry Cu-Mo system is located ~75 km SE of Guayaquil, and 40 km west of Cuenca in southwestern Ecuador, at an altitude of ~1500 m asl. (#Location: 2° 56'S, 79° 25'W).
It is one of a number of Miocene porphyry and epithermal copper-gold deposits that occur in two main clusters, which define two mineral districts in western Ecuador, including Chaucha, Gaby-Papa Grande, Quimsacocha and Portovelo-Zaruma (Azuay-El Oro District), and Junín and Alpala (Imbaoeste District). These districts are ~440 km apart and located within 100 km of the Pacific coast, 200 km SSE of Guayaquil and 75 km north of Quito, in southwestern and northwestern Ecuador respectively. Other deposits are distributed along the Miocene porphyry belt between these two districts, and to the north and south into Colombia and Peru.
Tectonic and Regional Setting
For 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.
Chaucha
The late Miocene Chaucha porphyry system is ~10 km to the east of the major north- to NNE-trending Bulubulu fault system, and is mainly hosted by a granodiorite porphyry complex in the SE portion of the mid-Miocene multiphase Chaucha batholith, near the intersection between the NNE-striking Cordillera, and east-west trending Chaucha faults. The Chaucha batholith is largely composed of biotite granodiorite dated at 12.5±1 Ma (K:Ar, Snelling, 1970) and 15.3 to 14.8 Ma (zircon U-Pb; Schütte et al., 2010). A south-tapering wedge of undifferentiated basement(?) metamorphic rocks (biotite schist and granodiorite gneiss), occurs on the eastern margin of the batholith, bounded to the east by the NNE trending Cordillera Fault. To the east of that fault, the country rock is predominantly volcanic and volcaniclastic rocks of the late Oligocene to early Miocene Saraguro Group. The host granodiorite porphyry occurs as a multiple biotite- and hornblende-bearing granodiorite (±quartz-diorite) porphyry intruded by seriate quartz-diorite stocks and dykes. The largest is an ~3 x 1 km, NW-SE elongated stock within the batholith, extending to its eastern margin with the metamorphic rocks. Other, smaller stocks and associated dykes intrude the metamorphic rocks and Saraguro Group, the largest of which is ~2 km to the ENE, covering an area of ~1.5 km in diameter along the Cordilleran Fault. These porphyries have been dated at 9.77±0.29 Ma (K:Ar; Muller-Kahle and Damon, 1970), 9.79±0.03 Ma (zircon U-Pb; Schütte et al., 2010).
The mineralisation is mostly composed of chalcopyrite-, pyrite- and molybdenite-bearing quartz veinlets and disseminations within hydrothermally altered intrusions. Intrusive and hydrothermal brecciation partly occurs at intrusive contacts and close to some fault zones, but is not associated with appreciable mineralisation (MICON 2005).
The outermost alteration zone is propylitic, characterised by chlorite, epidote and some clay minerals, with neither sericite or secondary biotite. Potash metasomatism occurring as disseminated and replacement biotite, quartz, epidote and magnetite, appears inwards, and increases towards the centre of the system. The potassic metasomatism has been overprinted by a phyllic assemblage of sericite, quartz and pyrite, with late gypsum as fracture fillings, with a transitional potassic-phyllic assemblage of disseminated, replacement and veinlet quartz, sericite (partly green), muscovite, K feldspar, biotite and chlorite.
The phyllic zone, which Goossens and Hollister (1973) show as covering an east-west oriented area of ~2 x 1 km, is centred on the intersection of the Chaucha and Cordillera faults but is elongated along the Chaucha fault. Within the phyllic zone, pyrite appears to comprise as much as 8% of the altered rock by volume. The phyllic zone is shown as the core to a broader pyritic halo encompassing a similarly east-west oriented area of ~6 x 3 km. Within this pyrite halo, outside of the phyllic zone, the pyrite content reduces, with ~40% being disseminated and the remainder occurring in veinlets of the ore stockwork. In the outer fringes of the pyrite halo, mainly in the propylitic zone, the pyrite tends to occur in small fractures alone often with, but usually without, other minerals, and disseminated pyrite may be absent (Goossens and Hollister, 1973).
No separate potassic zone has been recognised, and all of the significant Cu-Mo mineralisation occurs within the phyllic zone. Chaucha is a zoned Pb-Zn-Cu-Mo district with the central core of Cu-Mo and rare zinc mineralisation sporadically distributed in veins and fractures in the propyllitic fringe (Goossens and Hollister, 1973).
By far the most important host for the metals is the quartz-sulphide stockwork which has been leached and converted to quartz-limonite at surface. An ~60 to 100 m thick supergene enrichment blanket covers an area of ~900 x 500 m in the NW or Naranjos sector of the deposit, while a smaller blanket is found in the Gur-Gur sector (eastern stock of the porphyry complex). The grade of ore in the supergene zone is 0.6 to 1.5% Cu, compared to the hypogene mineralisation grades of ~0.4% Cu (Goossens and Hollister, 1973).
Within the more potassic zones, mineralisation occurs as disseminated chalcopyrite±bornite and as quartz-chalcopyrite, quartz-molybdenite and quartz-chalcopyrite-pyrite-molybdenite veinlets. In the potassic-phyllic transition, the mineralisation is similar, but lacking in bornite, while the phyllic zone is characterised by veinlets of quartz-chalcopyrite-pyrite±molybdenite. The supergene blanket contains chalcocite, covellite and copper oxide minerals (Schütte et al., 2010).
Published inferred resource estimates (largely supergene ore) for the Chaucha deposit (Schütte et al., 2010) include:
Naranjos sector - 120 Mt @ 0.4 to 0.5% Cu (0.3% Cu cutoff grade); and
Gur-Gur sector - 43.4 Mt @ 0.54% Cu (0.4% Cu cutoff grade).
The most recent source geological information used to prepare this decription 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.
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Goossens P J and Hollister V F, 1973 - Structural Control and Hydrothermal Alteration Pattern of Chaucha Porphyry Copper, Ecuador: in Mineralium Deposita v.8 pp. 321-331
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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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