Evidence of Ediacaran glaciation in southernmost Brazil through magmatic to meteoric fluid circulation in the porphyry-epithermal Au-Cu deposits of Lavras do Sul
Résumé
The Lavras do Sul Au-Cu mining district, southern Brazil, hosts an Ediacaran plutonic-volcanic association with hydrothermal alteration. This association is interpreted as a ca. 600-580 Ma tilted porphyry-epithermal system, where the main ore occurs in phyllic and intermediate argillic halos around quartz veins. In this paper we used detailed field mapping, fluid inclusion microthermometry, oxygen and hydrogen isotope measurements of whole-rocks, feldspars, clay minerals and quartz from propylitic, phyllic and intermediate argillic alteration to reconstruct the hydrothermal fluid temperatures and compositions. The results showed that the deeper zones of the hydrothermal system were predominantly influenced by magmatic fluid and its shallower zones by meteoric water with very negative δ18O compositions. Our data support a model of mixing between magmatic-derived fluids with meteoric waters of glacial origin. Based on stable isotope data we propose indirect evidence of high latitude position for the study area during Brasiliano/Pan-African convergence of the Kalahari and Rio de La Plata cratons.