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Visit of the Campanillas mine, Nambija District, Ecuador
(Field trip UNESCO-SEG Metallogeny Course, June 2003)

J. Vallance 1, M. Chiaradia 2, L. Fontboté1, A. Markowski 1

1 Section des Sciences de la Terre, University of Geneva, Rue des Maraîchers 13, 1205 Geneva, Switzerland
2 School of Earth Sciences, University of Leeds, Leeds LS2 9JT, U.K.

Campanillas is an oxidized gold skarn located in the central pat of the Nambija District, 1 km north of the Nambija village, between 1500 and 1700 m above sea level (Fig. 1). An overview of the geological, mineralogical, and geochemical characteristics of the Nambija District is contained in Vallance et al. (2003) and references therein.

At Campanillas a small-size, high grade gold mine is operated by Comcumay. S.A. There is a main open pit where old galleries are still visible and another pit in the in the northern part of the concession (Katy sector).

Fig. 1: Partial view of the Campanillas main open pit. Old mining workings are visible.

In the main pit gold occurs in three garnet skarn units with a total thickness of around 30 m. Skarn has developed on volcanic and volcaniclastic rocks with limestone intercalation pertaining to the Triassic Piuntza Unit. The bedding of the volcano-sedimentary sequence is still visible (in general N90°E, 30°SW). The skarn bodies are roughly concordant with bedding.

In the pit in the Katy sector, gold-bearing garnet skarn bodies occur too, as well as porphyritic dykes and stocks, which crosscut the volcanosedimentary sequence. Weathering is severe.

The lithological column recognized in the main pit consists of alternating fine-grained and coarse-grained volcaniclastic rocks. Skarn has developed preferentially on fine-grained volcaniclastic rocks as three main units roughly concordant with bedding (Fig. 2). The thickest skarn unit (around 6 m) is located at the base of the mine lithological column and is separated from the second one (2 m thick) by 7 to 10 meters of coarse-grained volcaniclastic rocks. The third unit has a thickness of around 2 m and is separated from the underlying one by andesitic beds containing a 1 meter thick horizon made up of alternating 10 to 20 cm thick fine-grained andesitic volcaniclastic rocks and black chert levels (Fig. 2).

The skarn units consist mainly of massive brown garnet (garnet-1, Ad58-99) with a few cm thick rims of pyroxene, epidote, chlorite, and K-feldspar forming the transition to volcaniclastic rocks. In places, not skarnified levels are observable giving a banded aspect to the rock (Fig. 3, 4).

Fig. 2: Sketch of the lithological column at the Campanillas main pit.

Fig. 3: Preserved volcaniclastic and black-chert layers within the brown garnet skarn
giving a banded aspect to the rock and revealing the bedding.

Fig. 4: Discontinuous lenses of brown garnet (grt) skarn surrounded by green pyroxene-epidote skarn (px-ep).
At the margins occur chlorite (chl), hematite (hm) ± calcite (cal).
The great axis of the lenses is roughly parallel to bedding of the host volcaniclastic rocks.

In several places massive brown garnet skarn grades to a blue-green garnet skarn assemblage of garnet (Ad64-78), pyroxene and quartz (blue-green garnet skarn). Clusters or veins of red-brown garnet (Ad58-100) are observed in this assemblage. The blue-green garnet green skarn grades progressively to irregular open spaces filled with green and/or yellow garnet (garnet-2, Ad53-97) quartz, pink euhedral K-feldspar, chlorite, hematite, calcite, and pyrite. The open spaces often show mineralogical zoning with garnet on the walls, followed by K-feldspar, and quartz at the center.

The open spaces form gradually irregularly shaped 2 cm to 30 cm thick N30°E to N70°E steep dipping veins (“Type I veins”, Vallance et al., 2003). Type I veins form zones up to 6 m thick within the skarn and less than 1 m thick anastomozed networks in the volcaniclastic rocks. The mineralogy and the zoning are the same as the open space but quartz is predominant (>90 vol.%), and the pyrite content is very low (< 1 vol.%). Four networks of Type I veins occur in the Campanillas mine. Interspace between type I veins is 10-15 m. (Fig. 5). Extensions along bedding of the steep dipping Type 1 veins are observed in places.

Gold occurs in open spaces and Type I veins, building grains up to several mm in size and is frequently more abundant than pyrite. It occurs preferentially at the contacts between garnet and quartz (Fig. 6). Gold was not observed in massive brown garnet skarn without open spaces or Type I veins, or in volcaniclastic rocks.

Fig. 5: Map of the Campanillas mine showing the geometry of the Type I veins (Modified from PRODEMINCA, 2000).

Fig. 6: Gold grains (Au) in a Type I veins crosscutting volcaniclastic rocks.
Epidote (ep) and pyroxene (px) are present at the vein walls. (Photo A. Markowski, February 2002)

Acknowledgements

We thank Comcumay SA., Calle Malecon 1707 y 10 de Agosto, Guayaquil, for granting access to the Campanillas mine.

References

Litherland M., Aspden J.A. & Jemielita R.A. 1994. The metamorphic belts of Ecuador. Overseas Memoir 11. BGS, Keyworth, U.K. 147 p.

PRODEMINCA. (2000). Depositos porfidicos y epi-mesotermales relacionados con intrusiones de la cordillera del condor. Evaluacion de distritos mineros del ecuador. Vol 5. UCP Prodeminca Proyecto MEM BIRF 36-55 EC. 223 p.

Vallance, J., Markowski, A., Fontboté, L., & Chiaradia, M. (2003). Mineralogical and fluid inclusion constraints on the genesis of gold-skarn deposits in the Nambija district (Ecuador). in D. Eliopoulos et al. eds., Mineral Exploration and Sustainable Development, Millipress, in press.

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