Paper Number: 2081
Gold deposits in the Colombian Amazonia and some of their social-environmental implications
Amaya Perea, Z.1,2 , Cramer, T.1,2 , Bonilla Pérez, A.1,2, Franco Victoria, J.A.1,2, Roberto Reyes, O.S. 1,2and Cano Ramírez, H.1,2
1Department of Geosciences, Universidad Nacional de Colombia, Ciudad Universitaria, Bogotá, zamayap@unal.edu.co
2Grupo de Estudios en Geología Económica y Mineralogía Aplicada (GEGEMA), Universidad Nacional de Colombia, Departamento de Geociencias.
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Metasedimentary rocks of the Mesoproterozoic Maimachi Formation in the NE Amazonas Craton experienced several enrichment processes of valuable elements from gold to tungsten in hypo- to hydrothermal systems. The latter occur as primary and secondary deposits in the Naquén-, Caranacoa- and Taraira serranias within Colombia, which are probably similar to those in the Serranía de la Esperanza in Brazil or Cerro Yacapana in Venezuela. Several of these deposits are being exploited informally and illegally as well known from the Brazilian garimpeiros. In the deposits studied by us until now primary gold mineralization occurs in two main styles: first as gold in milky quartz veins together with pyrite, sphalerite, muscovite, apatite and in some cases together with wolframite and bismuth minerals like russellite, filling structural planes of the strata in greenschist-facies quartzite, metasilts and metaconglomerates. The second style corresponds more to stockwork-like milky-quartz veins cutting both the metasedimentary sequences and the latter veins.
Since the Mesoproterozoic, the strong chemical weathering in the mainly humid tropical climate enhanced the formation of placer and alluvial deposits, which were reworked several times in the context of tectonic events like the Nickerian Orogeny. Pristine gold in the veins may occur as massive aggregates in the quartz veins or as small plates; furthermore weathering and probably interaction with organic diluents in the river waters often produced dendritic to worm-like nuggets.
The research about all these fascinating features is just starting in the complex geopolitical context of the Colombian Amazonia in difficult-to-access study areas, partly controlled by armed groups like the FARC-guerrilla, where illegal and uncontrolled mining activities over placer- and alluvial gold deposits - often using mercury - produced and is producing negative environmental effects in soils, hydrosphere and the whole sensible Amazonian ecosystems. To overcome the missing scientific and basic knowledge as well as the weak implementation of environmental protecting rules is a strong challenge for the next time and will get special importance after signing of the peace agreements which will bring together the different actors in the frame of a new scenario, where academics may play a fundamental role for the sustainable use of the mineral and biotic wealth in the region.
Figure 1: Left: Placer gold mining in the Serranía de Caranacoa – Cerro Wawa. Right: gold in milky quartz from the Serranía del Naquén – Caño Maimachi.