Alteração hidrotermal e paragênese mineral da mina Cerro Rico – Lavras do Sul, RS – novos dados e interpretações

Abstract

The Cerro Rico Mine is an area which is on east of the city of Lavras do Sul, in the central region of Rio Grande do Sul state and is it under mineral exploration license by Amarillo Gold Ltd. Volcanic rocks of Hilario Formation predominantly pyroclastic (with calcium-alkaline affinity, high potassic to shoshonitic) contains gold and copper mineralization related to hydrothermal processes. Petrographically corresponds to tuffs, andesite and traquite. Structured in veins, venules and fractures fill, the mineralization occurred in the area linked with tardi to post-magmatic events who have developed the regional Geologic context. The central objective of this research consists at the characterization mainly of alteration minerals and ore, characterizing a mineral paragenesis and identifying the processes of hydrothermal alteration who have affected the region and the presence or not of Rare Earth Elements. It was realized 14 days of field work where 4 days was used to collect surface samples and 10 days used to describe 681,25 meters of three boreholes. Petrographic analysis was realized in 16 samples with 11 at the microscope and 4 at the Scanning Electron Microscope (SEM). At SEM it was identified sulfites like bornite, pyrite, arsenopirite; sulfates baryte, jarosite, alunite; the oxides ilmenite, hematite, chromite plus silicates very altered like pyroxenes/amphiboles and clay minerals of chlorite group, ilite, sericite e kaolinite. Also it can be possible to identify phosphates of the crandalita series as well as adulare and pyrophyllite. The main paragenesis found was: - Pyrite, chlorite, quartz, calcite e actinolite e baryte, corresponding to a propylitic zone. -Arsenopirite, chalcopyrite, pyrite with calcite, baryte, adulare; low sulphidation zone of an epithermal deposit. - Esfalerite, pyrite, chalcopyrite, pyrophyllite; high sulphidation zone of an epithermal deposit. The paragenesis covelite, pyrite, pyrophyllite and quartz indicate an advanced argillic association. A supergene action at Cerro Rico Mine allowed the formation of minerals like jarosite, a tracker mineral important in environments where there are auriferous mineralizations. The chemical data has allowed statistical approaches of some elements (Bi, K, Ni, Sb, Zn) which are important for the interpretation of what type of mineral deposit are we working with. The La has presents mean values up to six times greater than the background at the drill cores. Noteworthy is the relation between Bi with Sb which allows the interpretation of the occurrence of a hydrothermal zone of high sulphidation front of negative values prevailing in the correlation between Sb and Zn. Thus, it interprets the location of the Cerro Rico Mine as a border area with rocks that are in a hydrothermal deposit scheme porphyry type and hydrothermal systems of epithermal type (either high sulphidation or low sulphidation).