"Geological and Geochemical Analyses for Emerald Exploration in the Muzo Formation along the Western Emerald belt, Colombia"

Abstract

The Colombian emeralds are distributed in two parallel belts where the host rocks are mainly Cretaceous black shales and limestones. A better understanding of the mineralizing processes which form the emerald deposits in Colombia will improve the exploration activities within both belts. This study focuses on the Western Emerald Belt (WEB) where seven mines were visited. The WEB is located in the central part of Colombia, on the western flank of the Eastern Cordillera. The most important emerald mines are located in Muzo, Maripi, Yacopi, Otanche and Coscuez municipalities. Thirty six samples were collected for analyses of carbon and oxygen isotopes with the purposes of: first, to know the main source of the fluid(s) which interacted with the shales in order to form the emeralds and associated minerals (pyrite, calcite, chalcopyrite, quartz, albite, etc.), and second, to identify if the analyses of stable isotopes combined with field and laboratory information are determinant and useful for the exploration of emeralds in the WEB. Geological information such as structural data, identification of debris flows, hydrothermal and hydraulic breccia as well as mineral assemblage were collected during the field trip. For microscopic analyses, 17 samples were characterized by petrographic observations, such as microstructures, mineral habits, paragenesis and deformation parameters. The identification of different carbonates and supergene minerals was conducted with a RAMAN spectrometer and Scanning Electron Microscope (SEM). The main conclusions of this research suggest that: 1. the mineralizing fluids came from a metamorphic / sedimentary origin. 2. A range for 18O (SMOW) between 18 to 21.8 ‰ and -4 to -10.3‰ for 13C (PDB) represents the isotopic signatures of the hydrothermal fluids which produced the emerald mineralization for the visited mines. 3. Calcites from the Muzo Formation in the WEB would have the possibility of being associated with emeralds, if apart from being in the previously mentioned ranges of isotopic values; the carbonate is in paragenesis with sulphides, albite and/or quartz originating from either hydrothermal or hydraulic breccia associated with thrust planes, folds and overthrust faults. For exploration purposes, it is important to define debris flows to avoid mining development in such areas.