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Visit of the La Plata gold rich VHMS deposit, Ecuador

(Field trip UNESCO-SEG Metallogeny Course, June 2003)

M. Chiaradia 1, D. Tripodi 2, L. Fontboté 2
1 School of Earth Sciences, University of Leeds, United Kingdom
2 Earth Sciences, University of Geneva, Switzerland

 

1. Introduction

The volcanic-hosted massive sulfide (VHMS) deposit of La Plata (currently property of the SULTANA DEL CONDOR MINERA S.A.) is situated in the province of Pichincha (central Ecuador), about 57 km SW of Quito and 32 km SE of Santo Domingo de los Colorados (longitude: 78°56'E - latitude: 00°23'S), on the western slope of the Cordillera Occidental, at an altitude ranging from 1350 to 1700 meters (Fig. 1).

Fig. 1: Geological map of the northern Western Cordillera. Inset: geoteconic map of Ecuador
(grey=oceanic terrane; white=continental terrane) (from Chiaradia and Fontoboté, 2001).

A preliminary resource of 840,000 tons @ 4.8 g/t Au, 54 g/t Ag, 4.1% Cu, 0.7% Pb and 4.2% Zn has been measured (Cambior 1998). The high gold grade of the La Plata deposit allows its classification as Au-rich VHMS deposit (Hannington et al. 1999).
La Plata belongs, together with the deposits of Macuchi and El Patiño more to the south (Fig. 1), to an Early Tertiary Au-bearing VHMS metallogenic event within the island arc terrane of Macuchi (Chiaradia and Fontboté, 2001, Tripodi et al. 2003).

Fig. 2: Geological map of the La Plata concession (Tripodi 2002, modified from 0'Dowd, 1999).

2. Geological setting

The oceanic island arc terrane terrane of Macuchi, host to the La Plata deposit, consists of a Paleocene-Eocene submarine volcanosedimentary sequence with pillow lavas and intrusions of basaltic to andesitic composition. Within the Macuchi Unit, Chiaradia & Fontboté (2001) distinguish the Paleocene Basal Macuchi (lower part of the arc pile, MgO-, Ni- and Cr-rich primitive volcanics, SiO₂ = 47.5-55.0 wt%, MgO = 12.5-4.0 wt%) characterized by abundant basaltic pillow lavas intercalated with mudstones, and the Eocene Main Macuchi which is the higher part of the arc pile (Fig. 1). The Main Macuchi consists mostly of basaltic andesites and andesitic pillow lavas, breccias and volcanic sandstones, but also comprises more evolved and SiO₂-rich rocks (SiO₂ as high as 69.0 wt%, MgO as low as 1.0 wt%). The La Plata deposit is hosted by the Main Macuchi sequence.

Fig. 3: Tajo Abierto, the main outcrop at La Plata. A: General view. B: Contact between host rock and mineralization. C: Mineralization (pyrite, chalcopyrite, sphalerite). D: Massive jasper level on top of the mineralization. E and F: stockwork (from Tripodi, 2002).

3. Mine geology and ore mineralogy

The mine zone consists of an easterly inclined anticline of north-south direction, plunging 45° to the south (Figs. 2, 3). Most of the sulfide mineralization is found on the eastern flank of the anticline, at the transition between felsic and mafic rocks (Fig. 2). The footwall is composed by highly altered dacite crosscut by a mineralized stockwork, while the hanging wall encompasses basaltic to andesitic volcanic rocks, partly massive, partly brecciated (Fig. 4). A ubiquitous level of massive jasper (<10 cm thick) is found directly atop of or less than 5 meters above the mineralization (Figs. 2, 3, 4). The Al-Fe-Mn concentrations suggest a hydrothermal exhalative origin for this level. The mineralization and host sequence have undergone only weak zeolite facies burial metamorphism and the geometric relationships between orebody and host rocks are close to the original ones except in areas affected by fracturing.
The geochemistry of the volcanic rocks reveals a tholeiitic affinity and an ensimatic island arc setting. The hydrothermal alteration phases are essentially quartz, sericite, pyrite, ± chlorite, ± illite in the footwall, and hematite, ± pyrite, ± sericite, ± ankerite in the hanging wall.

Fig. 4: Lithostratigraphy of the La Plata area (from Tripodi, 2002).

The orebodies are massive to semi-massive lenses forming a well defined stratigraphic horizon. Their thickness varies from several decimeters to a few meters, rarely exceeding 10 meters, and their lateral extension can reach 100 meters. The main ore minerals are chalcopyrite, pyrite, sphalerite, bornite, and galena, with subordinate tennantite and covellite, accessory digenite, chalcocite, and native gold. Gangue minerals are barite and quartz to a lesser extent. Ore minerals often display fine laminated textures. Resedimented ore breccias (e.g., from mound erosion) are absent at La Plata. The time relationships of the ore phases are presented in the paragenetic sequence (Fig. 5). Gold occurs in small grains ranging in size from 5 to 50 microns, but a large grain reaching 350 microns was also observed (Fig. 6). The smaller grains are often rounded and occur within bornite, chalcopyrite, galena, and digenite. The bigger gold grains fill open spaces within the boundaries of surrounding minerals. Electron microprobe analysis reveals Au-contents ranging from 79.6 to 97.0 wt.%, the remainder being represented essentially by Ag with minor amounts (<1%) of Hg and Cu.

Fig. 5: Paragenetic mineral sequence at La Plata (from Tripodi et al., 2003).

Fig. 6: Microphotographs in reflected light plane polarized. A and B: gold (Au) in early pyrite (py) (TRI-93). C: Au associated with bornite (bo) (E94050). D: Au associated with galena (gn) (E94050). E: Au grain adjacent to bornite, galena, and gangue (gg) (TRI-103). F: Au associated with chalcopyrite (cp) (TRI-77). G: Au associated with digenite (di) (TRI-75). H: Au within gangue (barite). Cc = chalcocite; cv = covellite; fh = fahlore; sl = sphalerite (from Tripodi, 2002 and Chiaradia and Fontboté, 2001).

4. Ore geochemistry

The ³⁴S values of the sulfides range between -2.2 and +1.6 ‰ (N=16: Fig. 4). As in other VHMS deposits, the results do not allow to discriminate between (i) leaching of basaltic sulfur from underlying volcanic rocks, and (ii) non-bacteriogenic reduction of marine sulfate at temperatures of 250° to 300 °C.
The ⁸⁷Sr/⁸⁶Sr compositions of barite show that, whereas seawater probably provided the major part of sulfur contained in barite, Sr (as well as Ba) was mainly supplied by the hydrothermal fluid and derived by the interaction of this fluid with footwall volcanic rocks.
Therefore, the combination of sulfur isotopes and ⁸⁷Sr/⁸⁶Sr ratios (⁸⁷Sr/⁸⁶Sr = 0.704601±9 to 0.705549±8) of the barite favors a mixing model for the precipitation of this mineral.

The lead isotopic compositions of ore minerals (Chiaradia & Fontboté 2001) are very homogenous and overlap the compositional field of the Main Macuchi rocks, suggesting derivation of the majority of ore lead from leaching of the Main Macuchi.

5. Conclusions

Paragenetic, geotectonic, petrographic, and geochemical considerations suggest that La Plata belongs to the Kuroko-type VHMS of Sawkins (1976), or the to the bimodal mafic VHMS-type of Barrie & Hannington (1999). The fact that the massive sulfides form a stratigraphically well defined horizon, the presence of textures showing fine lamination, the proximity of the exhalite jasper horizon, and the absence of breccias related to mounds are features that could be used to support a brine-pool model for the La Plata VHMS deposit, although more data (including fluid inclusions) are necessary and other interpretations are possible (Tripodi et al., 2003).

References

Barrie, C.T. & Hannington, M.D. 1999. Classification of volcanic-associated massive sulfide deposits based on host-rock composition. Reviews in Economic Geology Vol. 8: 325-356.

Cambior, 1998, Annual Report, p. 28.

Chiaradia, M. & Fontboté L. 2001. Radiogenic lead signatures in Au-rich volcanic-hosted massive sulfide ores and associated volcanic rocks of the early Tertiary Macuchi island arc (Western Cordillera of Ecuador). Economic Geology Vol. 96: 1361-1378.

Hannington, M.D., Poulsen, K.H., Thompson, J.F.H. & Sillitoe, R.H. 1999. Volcanogenic gold in the massive sulfide environment. Reviews in Economic Geology Vol. 8: 325-356.

0'Dowd, P., 1999, La Plata polymetallic property systhesis report, Ecuador. Cambior Ltd., unpublished synthesis.

Sawkins, F.J. 1976. Massive sulfide deposits in relation to geotectonics. Geological Association of Canada, Special Publication 14: 221-240.

Tripodi, D., 2002, The Au-rich VHMS deposit of La Plata (Western Cordillera, Ecuador): Unpublished M.Sc. Thesis, University of Geneva (Switzerland), 184 p.

Tripodi, D., Chiaradia, M., & Fontboté, L., 2003, Geological setting, mineralogy and geochemistry of the Early Tertiary Au-rich volcanic-associated massive sulphide deposit of La Plata (Western Cordillera, Ecuador), in D. Eliopoulos et al. eds., Mineral Exploration and Sustainable Development, Millipress, in press (643 Kb pdf document to download).

Acknowledments

This work has benefited from supports by the Society of Economic Geologists (Grant to DT for M.Sc. Thesis research), Cambior Inc., and the Swiss National Science Foundation. We thank acces to the mine by Sultana del Condor Minera, S.A.

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