Summary:
The mineralization at El Domo shares most of the features of a VMS deposit (Franklin et al., 2005; Franklin et al., 1981; Large, 1992; Large et al., 2001; Lydon, 1996; Lydon, 1988a; Lydon, 1988b). VMS deposits are major sources of Zn, Cu, Pb, Ag, and Au, and can contain trace metals such as Co, Sn, Se, In, Bi, Te, Tl, Ga, and Ge.
Pratt (2008) was the first to document and describe a Kuroko-type VMS environment on the Project concessions. He established a lithostratigraphy for the Las Naves/El Domo area in which massive sulphide mineralization rests on a footwall sequence of rhyolite and dacitic autobreccias. He divided the sulphide mineralization into five types:
- Massive sulphides with indistinct texture. In some places, a fragmental texture can be seen within the sulphides, suggesting that they may be formed by the replacement of lapilli tuff.
- Sulphide-altered lapilli tuffs and peperites.
- Transported sulphide fragments within polymictic lapilli tuffs.
- Sulphide “pseudo”-fragments within polymictic lapilli tuffs. Rare, thinly laminated siliceous chert with banded sulphides.
The mineralised zone at El Domo is an intact, upright and only mildly disturbed Kuroko-type VMS deposit. As such, it displays the characteristic zoning of the model type from the underlying feeder pipe area through vertical and lateral variations upward to the abrupt termination of the massive sulphides against the characteristic hanging wall grainstone marker defined by Franklin et al. (2005). Over time, the evolution of the hydrothermal mineralizing system and the growth of the mineralised deposit account for the spectrum of mineralization types distinguished by Pratt.
Sphalerite, chalcopyrite, and pyrite are the principal sulphides in the mineralised rocks from the Curipamba prospect. Galena is less common, and tennantite/tetrahedrite and covellite are minor phases. Gold was identified within sphalerite + galena + barite mineralization, where it occurs as minute (5 µm to 50 µm) inclusions in sphalerite. The colloform banded sphalerite also contains an abundance of large, partly dissolved inclusions of skeletal galena. Careful microscopic examination revealed that gold was introduced to sphalerite via fractures with late chalcopyrite. Minute gold also occurs on the rim of some galena and is intergrown with chalcopyrite. The galena is partly replaced by tennantite, and it is rimmed and crosscut by chalcopyrite veinlets. Two small grains of gold were also identified in a late carbonate veinlet that crosscuts the sphalerite. The sphalerite is a pure zinc end-member with little or no iron content. In a number of samples, sphalerite is colloform banded, and just as some pyrite, often has framboidal texture. Textural evidence suggests that galena was largely contemporaneous with sphalerite, and both post-dated the pyrite. Tennantite and tetrahedrite represent a relatively minor phase and both crystallized at the expense of galena and less commonly, pyrite. Chalcopyrite was the last sulphide to crystallize in the polymetallic assemblage. In some samples, fragmented pyrite and sphalerite are “flooded” and partly replaced by massive chalcopyrite. Locally, chalcopyrite is stained to an unusual purple/blue colour. Microprobe analysis showed that in these domains the chalcopyrite has an unusual chemistry, and contains 2.2% to 3.7% bromine (by weight). Galena occurs as a skeletal inclusion in chalcopyrite and as replacement after pyrite. It contains inclusions of, and can be partly replaced by, tennantite and tetrahedrite. Covellite and chalcocyanite occur within sediments. Covellite forms a rim on detrital sphalerite and some pyrite, and chalcocyanite (anhydrous Cu-sulphate) is disseminated through the matrix. Barite is the principal gangue mineral (Schandl, 2009).
Mineralization at El Domo is broadly zoned with an upper “cap” of barite, enriched variably in silica sphalerite, galena, and gold. This cap is underlain by a massive sulphide zone with local zoning of zinc-rich mineralization along the hanging wall contact and a copper-rich base. This zonation, however, is not apparent throughout the massive sulphide zone. Zinc-rich mineralization consists of low iron sphalerite, some sulphosalts, barite, and pyrite. Copper mineralization is characterized by chalcopyrite and abundant pyrite. The base of the sulphide section is typically strongly silicified, with semi-massive pyrite and chalcopyrite as disseminations and stringer veins.
Mineralization in the grainstone shows evidence of at least two mineralization events. The sulphides include breccias which appear to have been caused by some form of collapse (possible anhydrite dissolution), while interstitial spaces were infilled by sphalerite and, in some cases, chalcopyrite. This brecciation replacement texture is common in many massive sulphide mounds as they grow by “displacement” or expansion. During this process, the core of a mound is constantly impregnated by high temperature hydrothermal fluid, displacing the lower temperature minerals outwards and leading to a constant zone refinement of the mineralization.
The sulphide and precious metal compositions have numerous unusual features, usually associated with high temperature systems that have achieved boiling just prior to their expulsion on or near the seafloor. The exceptionally high gold recorded in many of the upper zones in all of the occurrences, together with the anomalous antimony, arsenic, mercury, and bromine contents of some of the minerals, can only be achieved by this process, which enables exceptionally efficient gold precipitation. Gold is conserved in the vapour phase of a hydrothermal fluid, and thus may be deposited over a much wider area than the base metals. At the Project, gold is generally associated with baritic exhalite.