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Abstract of :

Tripodi, D. (2002) The Early Tertiary gold-rich VHMS of La Plata, Western Cordillera, Ecuador. M.Sc Thesis, Département de Minéralogie, Université de Genève, 186 p. (Supervisors: Massimo Chiaradia and Lluís Fontboté)

The VHMS deposit of La Plata is situated in the province of Pichincha (central Ecuador), about 55 km WSW of Quito (longitude: 78°56’E – latitude: 00°23’S), on the western slope of the Cordillera Occidental. It is hosted by the Main Macuchi Unit, part of an early Tertiary (Paleocene to Eocene) ensimatic island arc. 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 mine zone consists of an easterly inclined anticline of north-south direction, plunging to the south (45°). Most of the sulfide mineralization is found as sulfide lenses on the eastern flank of the anticline, at the transition between felsic and mafic rocks. The footwall is composed by highly altered dacite whereas the hanging wall consists of basaltic to andesitic volcanic rocks, partly massive, partly brecciated and hematite-rich. The brecciated layers may correspond to flow tops (suggesting the existence of several lava flows) and/or be the result of tectonic brecciation. Whole rock geochemistry is consistent with previous works pointing to an ensimatic island arc setting and tholeiitic affinity, while perlitic textures found in a footwall rock is consistent with the submarine volcanic environment. No pyroclastic rocks or other trace of explosive activity have been found. The rocks have been affected by low-grade metamorphism as indicated by amygdules visible in the hanging wall rocks filled with chlorite, quartz, albite, zeolites (laumontite ± heulandite?), prehnite, pumpellyite, epidote, and calcite. In the hanging wall rocks, Na2O values up to 5.5 wt.% and very low K contents may be due to regional albitization of the basalts.

The footwall rocks are strongly affected by hydrothermal alteration and display pervasive sericitic alteration, with quartz, pyrite, ± ankerite. Maximum K2O values are around 4.0 % and correlate with low Na2O contents (below 0.5%). A few rocks show intense silicification. Chloritic alteration, typical in other VHMS, has not been recognized nor in the stockwork neither in other areas. Iron enrichment reaches a peak of 29.5 wt% for a highly pyritized sample.

The massive sulfide ores occur in some drill-cores as a single 2 to 12 m thick massive sulfide lens whereas in others they form up to three superposed thinner lenses. It has not been elucidated if the lens superposition is the result of tectonic repetition. In almost all studied drill holes the massive sulfide ores are overlaid by a ca. 2-10 cm massive jasper lever found directly above or at less than 5 meters above the mineralization. In the open pit area a stockwork is recognized.

The main hypogene 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 subordinate quartz. Native gold is present in relatively large sized grains (10-20 µ, up to 300 µ) and has Au:Ag ratios about 85:15. Gold appears to be paragenetically related to chalcopyrite, bornite and (Fe-poor) sphalerite. ICP-MS analyses reveal a good correlation between gold and zinc. Some gold grains are also observed within early pyrite and barite. A few ore samples show weak development of supergene covellite. The La Plata massive sulfide ores are characterized by relatively high values of Sb (up to 3660 ppm), As (up to 1220 ppm), and Co (up to 487 ppm).

High sulfidation conditions, which have been suggested to occur in other Au-rich VHMS deposits, could be indicated at La Plata, among other paragenetic criteria, by the presence of a pyrite-bornite assemblage and of primary covellite and by the very low Fe-contents in sphalerite. However, the absence of alteration assemblages more acid than quartz-sericite does not support high sulfidation conditions during ore formation.

The sulfur isotopic composition of the sulfides (d34S = -2.2 to +1.6 ‰) is typical for volcanogenic massive sulfides. As in other deposits, the results do not allow to discriminate between (i) leaching of basaltic sulfur from underlying volcanic rocks, (ii) non-bacteriogenic reduction of marine sulfate at temperatures of 250° to 300 °C, and (iii) direct magmatic contribution of volcanic SO2 to the hydrothermal system. The S isotopic composition of barites (d34S = 20 to 21.4 ‰) exhibits the signature of marine sulfate at mid to upper Eocene times. Even if the use of sulfur isotope pairs as a geothermometer is problematic in VHMS because isotope equilibrium is rare, the obtained temperatures (250-365 °C for sulfate-sulfide, 205-310 °C for sulfide-sulfide) are in general agreement with accepted temperature ranges for Cu-bearing VHMS. The effects of a possible re-equilibration during low-grade metamorphism should be considered.

The combination of sulfur isotopes and 87Sr/86Sr ratios (87Sr/86Sr = 0.704601±9 to 0.705549±8) of barite favors a mixing model for the precipitation of this mineral. Seawater probably provided the major part of S contained in barite, while Sr (as well as Ba) was mainly supplied by the hydrothermal fluid and derived by the interaction of the fluids with footwall volcanic rocks.

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, the absence of breccias related to mounds, the isotope evidences pointing to mixing between an hydrothermal fluid and seawater, are all features favoring a brine pool model for the VHMS La Plata deposit.

Acknowledgment. This work has benefited from support by Society of Economic Geologists, Cambior, Inc., and the Swiss National Science Foundation.

See also Chiaradia, M., Tripodi, D. & Fontboté, L., 2003, Visit of the La Plata gold rich VHMS deposit, Ecuador (Field trip UNESCO-SEG Metallogeny Course, June 2003)

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