Petrologia do Carbonatito Passo Feio, ênfase no comportamento dos ETR, Caçapava do Sul – RS

Abstract

Carbonatites are uncommon igneous rocks, consisting mainly of primary carbonates and which arouse economic interest due to the potentiality of high phosphate content and light rare earth elements. The focus of this study is on the carbonatite Passo Feio, which constitutes a body that dives 30º for NW, which is interspersed with the metamorphic rocks of the Passo Feio Formation. The study area is 4.5km² and is located 17km southwest of the city of Caçapava do Sul, and can be accessed through the vicinal roads from the BR-153. This work had as main objectives the accomplishment of a geological mapping of the study area, rock and soil sampling, elucidation of carbonatite mineral paragenesis assisted by mineral chemistry studies in SEM-EDS. In addition, it was intended to characterize geochemically the carbonatite and selected sampled soils, with emphasis on Rare Earth Elements (REE), through ICP-MS analysis. Stable isotope analyzes of δ 13 C and δ 18 O were also performed in three rock samples, in order to clarify the processes during and after the installation of this body. The Passo Feio carbonatite varies texturally from massive to foliate, being mainly composed by carbonate and subordinately by apatites, phlogopites and tremolites. The opaque found corresponded to iron oxides, iron sulphides and barium sulphates, while the identified accessory minerals were zircons, monazites-(Ce) and aeschynites-(Ce). The presence of REE was not observed in the apatites, only in the monazites-(Ce) and aeschynites-(Ce), and it is suggested that the latter correspond to later phases of crystallization, corroborated by geochemistry, mineral chemistry and microtextures found. Considering the mineral chemistry results and taking into account the textural criteria it was possible to classify this carbonatite as an alvikite, with geochemical patterns that do not indicate economic potential for REE. However, soil geochemistry showed an important enrichment in REE, reflecting a probable concentration of monazite-(Ce) and aeschynite-(Ce), and from this, it was possible to establish a zone in which probably the carbonatite Passo Feio would extend. Although stable isotope results do not fit into the assumed field for the primary igneous carbonatites, the observed variation in the samples can be attributed to the cooling of the magma itself, without suggesting metamorphic processes or subsequent changes. Therefore, the deformation found in this carbonatite probably developed in late-magmatic conditions, guided by a tectonics associated with horizontal movements in shear zones. Thus, due to its proximity to large structures, this work suggests that this carbonatite was the product of the reactivation of mantle sources, within a post-collision magmatic context, corroborated mainly by the petrographic and isotopic evidence.