The origin of mercury in the area affected by artisanal mining in the Ponce Enriquez Gold District (Southern Ecuador): a preliminary investigation

The Ponce Enriquez mining district (south of Ecuador) is one of the most important gold mining areas in Ecuador, active since 1980’s. The gold mineralization is hosted within Cretaceous mafic rocks (mainly basaltic lavas, with porphyritic or aphanitic texture) of the Pallatanga Unit. The mining activity is a recognized economic activity that can be presented as large-scale mining, mediumscale to small-scale mining, artisanal mining and illegal mining. Gold is recovered by leaching with cyanide and flotation in artisanal activity and by a combination of amalgamation with mercury in the illegal mining activity. In all cases, the lack of efficiency is a common point along with the environmental damage. As a matter of fact, the high level of potentially toxic elements (PTEs) concentration (As, Cu, Cd, Zn, Hg, and Pb) in stream sediments and other environmental matrixes (surface water, soil, and biological) has been discussed in several studies (e.g., Jiménez-Oyola et al., 2021). However, in same case, the presence of PTEs in the mining district has been considered as due to the superposition of geogenic sources and mining activity. To determine whether the environmental matrixes are actually contaminated by anthropic activity or not with mercury, it is necessary to know the natural pre-anthropogenic content or the natural background of these elements. In this framework, we conducted a preliminary investigation to determine the content of Hg into the bedrock, to create a preliminary database to assess the geochemical baselines of the area. We collected twenty bedrock samples, considering the different textures of the rocks and abundance of mineralization, to have a good representative number of this variability. Total Hg concentrations in bedrocks were quantified with a Milestone tri-cell Direct Mercury Analyzer (DMA-80) in the Earth Sciences Department of university of Pisa. Stream sediment geochemical data were obtained from the work of PRODEMINCA (1998) and Jiménez-Oyola et al. (2021). Mercury concentrations in bedrock nearly the mined area is low and are taken to represent background concentrations, ranging from 0.002 to 0.148 ppm. Bulk mercury values in stream sediments are elevated to several times these background concentrations. Indeed, stream sediments have Hg content ranging between 0.15 and 33.84 ppm in the investigated area. These results may suggest an anthropic contribution of mercury concentration in specific areas. These conclusions are considered preliminary because the number of sample sites is relatively small. We plan to expand the number of samples and process this data with statistical methods to obtain the geochemical baseline values. Jiménez-Oyola S., Chavez E., García-Martínez M.J., Ortega M.F., Bolonio D., Guzmán-Martínez F., García-Garizabal I. & Romero P. (2021) - Probabilistic multi-pathway human health risk assessment due to heavy metal(loid)s in a traditional gold mining area in Ecuador. Ecotox. Environ. Safe., 224. https://doi.org/10.1016/j.ecoenv.2021.112629. PRODEMINCA (1998) - Monitoreo Ambiental de las areas mineras en el sur del Ecuador. Ministerio de Energía y Minas.