Estudio mineralógico y cuantificación por análisis digital de imagen de las menas auríferas de Nariño (Colombia). Aplicación a la mejora del proceso mineralúrgico

Applying Digital Image Analysis (DIA) to produce quantitative mineralogical information for gold ore processing is usually a difficult task. Typical problems are the low grades of the common ores, their small grain size, a not always straightforward mineralogy, or the danger of mistakes in grade definition by nugget effect, among others. The Nueva España (Narin̄o, Colombia) ores provide a good test for the methodology, overcoming these problems, since they are very rich (up to ∼600 ppm), and most of the gold is visible, as native gold or electrum, with reflected light microscope; to avoid nugget effect, the whole surfaces of six polished sections have been measured. The study by DIA was carried out to support the research understaken on the problems related to ore processing by informal miners in the Nueva Esparta Deposit (Pantoja, 1999). The aims of the DIA-study were: 1) to provide the necessary quantitative information on the geometry and mineralogy of the ores to improve the gold recovery, using alternative methods to amalgamation, which is being used in such a way, that it represents a serious threat to health and environment; 2) To test the DIA methodology, by comparing DIA- results with other independent results (chemical analysis, gravimetric tests, etc.) coming out from the research. Grainsize distribution shows that gold grains are frequently very small (<1-7 μm), but the modal analyses carried out on all the grains demonstrate that most of the gold is contained in coarser grains (over 50% > 105 μm in the Bruja, > 77 μm in the Gruesa veins). A high recovery of gold should thus be possible by gravimetric methods, which would allow to prevent the very serious environmental problems of present-day informal amalgamation. The tests carried out attain recoveries of up to ∼95% of the total gold content of the ores. The results of ore processing test fit well with those obtained by DIA on these ores.