Abstract
Marmato, Colombia, has been an important centre of gold mining since before the first Spanish colonizers arrived in 1536. The Marmato deposit is hosted in a dacite and andesite porphyry stock as sheeted sulphide-rich veinlet systems. The district is currently experiencing a surge in both major mining projects and artisanal mining, driven by sustained high gold prices. Ore from small-scale and artisanal gold mining is processed in numerous small mills (entables) around Marmato, which impact surface water quality through the discharge of milled waste rock slurry, highly alkaline cyanide-treated effluent, and high dissolved metal loads. To investigate the impact of artisanal mining and ore processing, water samples were collected in January 2012 from streams around Marmato. The average dissolved metal concentrations in impacted streams were Zn, 78 mg L−1; Pb, 0.43 mg L−1; Cu, 403 µg L−1 Cd, 255 µg L−1; As, 235 µg L−1; Ni, 67 µg L−1; Co, 55 µg L−1; Sb, 7 µg L−1; and Hg, 42 ng L−1, exceeding World Health Organization drinking water guidelines. In addition, arsenic speciation was conducted in-situ and indicated that 91–95% of inorganic arsenic species is in the form of As(V). Spatial analysis of the data suggests that entables processing ore for artisanal miners are the main contributor to water pollution, with high sediment loads, alkalinity and elevated concentrations of dissolved arsenic, cadmium, mercury and lead, caused by the processing of gold-bearing sulphides in the entables. Geochemical data from surface water were compared to a comprehensive data set of whole rock analyses from drill core and channel samples from the deposit, indicating that the deposit is significantly enriched in gold, silver, lead, zinc, arsenic, antimony, and cadmium compared to crustal averages, which is reflected in the surface water geochemistry. However, elevated mercury levels in surface water cannot be explained by enrichment of mercury in the deposit and strongly suggest that mercury is being added to concentrates during ore processing to amalgamate fine gold.
Highlights
Marmato, Colombia, has been an important centre of gold mining since before the first Spanish colonizers arrived in 1536
The purpose of this study was to document the geochemistry of surface water around Marmato, to determine the types of metal pollutants present, and to define their spatial relationship to artisanal mining and ore processing
Marmato has been a centre of hard rock gold mining since its founding by the Spanish conquistadors in 1532, gold in the area was exploited in pre-Colonial times by the Quimbaya people (Redwood, 2011)
Summary
Colombia, has been an important centre of gold mining since before the first Spanish colonizers arrived in 1536. Gold mining is an important economic driver in the impoverished rural regions in Colombia, with around 300,000 miners working in the artisanal and small-scale gold mining (ASGM) sector on a mainly subsistence basis, producing an estimated 54 metric tonnes (Mt) of gold annually (Cordy et al, 2011). Many of these mines operate illegally under Colombian law, without valid mining claims, property title or environmental permits.
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