Textures, mineralogy and mineral chemistry of the Cerro de Maimón deposit (Dominican Republic): Weathering and metallurgical implications

Abstract

The aim of this thesis is to study the textures mineralogy, and mineral chemistry of the sulfide and oxide ores from the Cerro de Maimón deposit (a Cu-(Zn)-Au-Ag VMS deposit) in the Dominican Republic. The samples studied were collected by the author in June 2009 as part of an internship done at the Cerro de Maimón mine. The deposit is hosted in Maimón Formation, a metamorphosed and deformed volcanogenic bimodal suite formed in the first stages of the development of the primitive Caribbean island arc in the Early Cretaceous.

The deposit crops out as a gossan (or oxide zone) developed above the water table as result of weathering and leaching of the sulfides. Below the water table, percolating Cu-rich waters replaces the primary sulfides to form a supergene Cu-enriched zone. GlobeStar Mining Company-Cormidom produces a Cu-concentrate from the sulfide ore, and recover Au-Ag doré bars from the oxide ore.

The primary sulfide ore is composed of mainly pyrite with chalcopyrite, sphalerite and pennantite as interstitial matrix. Minor galena and traces of Pb, Au-Ag tellurides are also present. The samples studied show annealing textures (triple junctions) as a result of metamorphism. Supergene Cu-sulfides include chalcocite and covellite finely intergrown with each other. These minerals replace first sphalerite, then chalcopyrite and finally corrode pyrite. The oxide paragenesis is composed of goethite, hematite, quartz and barite (i.e. limonite s.l.). The gossan shows in-situ textures (p.e. cellular boxwork) and “transported” textures (p.e. botryoidal-colloform aggregates and microbreccias). Yellowish fine goethite layers with high gold content are also present. Gold is found as extremely pure grains (99.1 at% Au) located in pores or fractures suggesting a late and in situ precipitation. Silver is found as iodargyrite (AgI) in pores of botryoidal aggregates with evidences of in situ precipitation. Iodargyrite has been cited as an arid climate indicator. However, we disagree as all the evidence indicates that the gossan developed in tropical climate and we suggest a marine influence as a source of the iodine.

Metallurgical problems in the sulfide and oxide treatment plants have been studied. Since sphalerite is naturally activated by Cu-rich mine waters, an effective differential flotation may be difficult to achieve. We suggest a hydrothermal treatment to purify the Cu-concentrate. Regarding the lower than expected Agrecuperation in the oxide ore, more studies are required to identify how the non-recovered silver is hosted and located. A dilute solid solution in the goethite assemblage could be an explanation.