Weathering profile of the Cerro de Maimón VMS deposit (Dominican Republic): textures, mineralogy, gossan evolution and mobility of gold and silver.

Lecturer: Lisard Torró

Date: Wednesday the 8th, 10.00h

Place: Room 2, Fac. Geology UB

Abstract:

Cerro de Maimón, in the central Dominican Republic, is currently the only VMS deposit under production in the Caribbean region. It is hosted in the Maimón Formation, of early Cretaceous age, which is part of the oldest and chemically most primitive island-arc in the Caribbean. From bottom to top, this deposit can be divided into (i) a primary sulfide zone, (ii) a supergene enrichment zone and (iii) an oxidized zone. This study reports new data on the textural and mineralogical characteristics of the oxidized zone (gossan/leached capping zone) with emphasis in the Au-Ag-bearing phases. The mineral paragenesis of the oxidized zone is essentially composed of goethite, hematite, quartz and barite. Botryoidal, cellular and brecciated textures can be distinguished. Botryoidal and brecciated textures dominate in the upper parts of the oxidized zone, whereas cellular textures are more common in the intermediate and lower parts. However, the weathering profile is very heterogeneous. The leached capping profile shows evidence of both transported and indigenous gossans. Gold in the oxide paragenesis is extremely pure (99% Au) suggesting that chemical refining took place. Silver occurs mainly as iodargyrite (AgI), and minor AgBr, AgCl, in botryoidal aggregates.

Particles of Au-Ag alloy in the primary mineralization exposed to a weathering environment can be leached and transported by various agents (chemical and biochemical) that may exist simultaneously. In the presence of halides, gold and silver can be leached and transported in a wide range of pH-Eh conditions, especially if iodine is present. Silver is leached more rapidly and over a broader range of pH-Eh conditions, and is preferentially transported as iodine-complexes than other halides. The presence of iodargyrite in the Cerro de Maimón gossan, fully developed under tropical conditions, suggests that this mineral cannot be considered an indicator of extremely arid environments as typically claimed by many authors; iodargyrite occurrence would rather point to the presence of saline-halide rich groundwater in oxide deposits.