Geoquímica dos pórfiros da cordilheira da costa no norte do Chile: um estudo de caso

Abstract

The present work presents two distinct studies carried out in the region between Antofagasta and Iquique, in the coastal mountain range. The regional scale has analyzes of total rock geochemistry which were analyzed with emphasis on rare earths. At the local scale the company Sociedade Química e Mineira de Chile (SQM) provided surface samples and 2 drill holes, in which mineral chemistry analysis was carried out in the Scanning Electron Microscope (SEM) of the prospective area of Búfalo, Antofagasta region . The objective of the regional study was to perform a geochemical comparison, with emphasis on rare earth elements (REE) with verification and organization of the geochemical patterns of the different lithologies, in order to suggest geological processes and controls that may influence the mineralization. In the area of Buffalo, with the study of microscopy and mineral chemistry, it was possible to determine ore mineralogy, microstructures and micro-textures. The isotopic dates previously known in the area point to ages between 51 to 53 MA, confirming that these are Paleogene porphyries that lie along the central depression inserted in the Domeyko fault system. When considering the abundance of REE in porphyry, the later sequences tend to concentrate the greatest abundances of total REE. Due to the fact that REE are more incompatible than the metal elements of porphyry mineralization, REE are concentrated in the residual magma and can be used to mark the magma stages and this allowed dividing the samples into groups and also highlighting which groups would have larger possibilities of mineralization. The groups with the lowest ETR levels present higher potential of mineralization, being able to highlight prospective areas such as Galenosa, Quillagua and Puntillas. The SEM besides producing the images of the mineralogy of the sampled slides also characterized the geochemistry of the copper ore allowing to affirm that these are minerals of enargite, associated with pyrite. Due to the metallic content of the enargite the rock may have undergone a leaching process that mobilized copper contents reducing its concentration in the mineralogical composition and producing changes of hematite enriched in copper.