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Nambija District - Fortuna, Cambana, Campanillas, Nambija, Guaysimi Alto, Sultana del Condor |
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Ecuador |
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Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Nambija skarn gold district is located in the Corriente Copper-Gold Belt, in the Cordillera del Cóndor and Cordillera Real of southeastern Ecuador. The district includes, from north to south, the Fortuna, Cambana, Campanillas, Nambija, Guaysimi (also known as Guaysimi Alto), and Sultana del Cóndor mines (#Location: centred on 4° 4' 20"S, 78° 47' 15"W).
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 metallogenic belts (PRODEMINCA 2000; Sillitoe and Perelló 2005; Chiaradia et al., 2009).
The Jurassic deposits form the 150 km long, NNE-SSW trending Corriente Copper Belt, a narrow eastern, sub-Andean metallogenic belt in the Cordillera Real and Sub-Andean Cordillera del Condor of southeastern Ecuador. These deposits are all associated with Upper Jurassic late porphyry intrusive phases of the Zamora Batholith, and include the Mirador, Mirador Norte, Panantza and San Carlos porphyry Cu, the Fruta del Norte epithermal Au-Ag and the Au-mineralised Nambija skarn field.
A broader Miocene metallogenic belt follows the entire western Andean range or Cordillera Occidental, and has a continuity with the Miocene metallogenic belt of southern Colombia and northern Peru (Sillitoe 1988; PRODEMINCA 2000; Sillitoe and Perelló 2005).
For details 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, Structure and Alteration
The Nambija district is located at the western margin of the Guiana Shield, over the suture with the Lower Palaeozoic arc that was accreted to the shield in the mid Palaeozoic. The skarn deposits are hosted by the Triassic Piuntza unit, which occurs as a 20 x 1 to 2 km, north-south trending contact-metamorphosed lens, within the Jurassic Zamora batholith.
The Zamora batholith is an I-type tonalite and granodiorite, dated at 190 to 170 Ma. The Piuntza unit, which is overlain by the Jurassic Misahuallí unit, rests unconformably on the Carboniferous Isimanchi unit, which in turn lies on Amazon craton basement rocks. The Piuntza unit consists of at least 300 m of shallowly dipping continental and/or marine volcano-sedimentary rocks and occurs as a flat roof over the Zamora batholith, preserved as skarn and metamorphosed rocks along the ridges. The unit includes metamorphosed sandstone, quartzite, black silty shale, fine- and coarse-grained tuff, volcanic flows and volcaniclastic breccia of basaltic andesite to andesite composition, as well as limestone and calcareous shale. Limestone is only a minor component.
Three main sets of structures have been recognised in the Nambija district, namely:
i). a set of north-south dextral reverse fractures and by coeval NE-striking, steeply dipping fractures with local sinistral displacement. The main gold mineralisation is controlled by the NE-striking fractures, in part as tensional veins.
ii). NW-striking reverse faults and thrust planes dipping at 10 to 40° SW, which cut the previously mineralised structures. Both structures resulted from a NE-SW stress field compatible with the oblique NE-directed subduction that prevailed from the Late Jurassic to Early Tertiary.
iii). a set of east-striking, steeply dipping dextral faults which cut all of the structures listed above.
The Piuntza unit and Zamora batholith are cut by several felsic porphyritic intrusions with mainly granodioritic compositions and volcanic arc signatures. These felsic porphyritic intrusions, are frequently altered to quartz-pyrite ±sericite ±K feldspar ±biotite. Pyrite amounts to 3 to 5% by volume as disseminated grains and in network veinlets of 1 to 5 mm thick which extend several tens of metres into the enclosing rocks. Porphyry Cu-Mo mineralisation at Nambija-El Tierrero and Cumay is associated with this and accompanying potassic and shreddy biotite alteration.
In the Nambija district, skarn occurs as massive, coarse-grained, lenticular bodies which mainly replaced fine-grained volcaniclastic rocks. The skarn is dominantly composed of massive brown garnet (mean Ad38). Subordinate pyroxene-epidote skarn is mainly developed on the margins of the brown garnet skarn bodies. A later phase of more andraditic garnet (mean Ad45), blue-green skarn occurs at the transition between the garnet and pyroxene-epidote skarn. The final phase are largely honey-yellow to red-brown clusters and cross-cutting bands of mainly andraditic (mean Ad84) skarn.
A weak retrograde overprint is recognised as infillings of structurally controlled (10 to 60° aligned) vugs and type-1 veins, with a mineralogy of milky quartz, K-feldspar, calcite, chlorite, and hematite ±plagioclase ±muscovite + minor amounts of pyrite, chalcopyrite, hematite, sphalerite and gold. The vugs and type-I veins are are cut by thin (1 to 2 mm) through going type-II veins that have similar orientations and mineralogy.
Mineralisation
Gold deposition, together with that of small amounts of hematite, chalcopyrite and pyrite, took place during fluid cooling in the retrograde skarn stages. This is reflected by the occurrence of native gold as grains up to several mm in size, together with retrograde quartz, K-feldspar, garnet and calcite, associated with retrograde alteration assemblages, mainly in the irregular vugs and type-I veins, and to a lesser degree in interstitial spaces and through-going type II veins.
Gold is not observed during the last retrograde alteration phase, as indicated by the absence of gold in sulphide-rich type-III veins, which cut the previous vein generations.
Some gold occupies interstitial positions between garnet and pyroxene grains in skarn affected by retrograde alteration but lacking vugs and veins. Type-I veins are gold bearing where crosscutting volcaniclastic rocks, but, like the type-I veins themselves, only within a few tens of metres from the skarn front. The total amount of sulphide minerals (pyrite > chalcopyrite > sphalerite) is very low in the Nambija deposits, typically <1%. Gold does not correlate with Cu, Zn, S or As.
Resources
In 1990, the total resources were estimated at 23 Mt @ 15 g/t Au (355 t Au) (Mining Magazine, 1990).
In 2000, total resources were re-evaluated at 125 to 155 t Au and the production since 1980 estimated at 60 to 90 t Au.
The most recent source geological information used to prepare this decription was dated: 2006.
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.
Nambija
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Chiaradia, M., Vallance, J., Fontbote, L., Stein, H., Schaltegger, U., Coder, J., Richards, J., Villeneuve, M. and Gendall, I., 2009 - U-Pb, Re-Os, and 40Ar/39Ar geochronology of the Nambija Au-skarn and Pangui porphyry Cu deposits, Ecuador: implications for the Jurassic metallogenic belt of the Northern Andes: in Mineralium Deposita v.44, pp. 371-387.
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Fontbote, L., Vallance, J., Markowski, A. and Chiaradia, M., 2004 - Oxidized Gold Skarns in the Nambija District, Ecuador: in Sillitoe R H, Perello J and Vidal C E (Eds.), 2004 Andean Metallogeny: New Discoveries, Concepts and Updates Society of Economic Geologists, Special Publication 11, pp. 341-357.
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Markowski, A., Vallance, J., Chiaradia, M. and Fontbote, L., 2006 - Mineral zoning and gold occurrence in the Fortuna skarn mine, Nambija district, Ecuador : in Mineralium Deposita v.41, pp. 301-321.
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Vallance, J., Fontbote, L., Chiaradia, M., Markowski, A., Schmidt, S. and Vennemann, T., 2009 - Magmatic-dominated fluid evolution in the Jurassic Nambija gold skarn deposits (southeastern Ecuador): in Mineralium Deposita v.44. pp. 389-413,
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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, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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