PROVENANCE OF SAND DEPOSITS OF JACUI RIVER, RS USING HEAVY MINERAL SIGNATURES

Abstract

This paper discuss the provenance of unconsolidated sand deposits of Jacui River, southern Brazil. It is the major river of Rio Grande do Sul and contains the main unconsolidated sand deposits used in the concrete industry of state. The river basin drains distinct geologic terrains with different ages and lithologic associations including Rio Grande do Sul Shield (Precambrian), Parana basin including Serra Geral Formation (Cretaceous) and Tupanciretã Formation (Tertiary). Heavy mineral analysis on selected samples collected along spots from the main course river and his afluents have shown a diverse and large heavy mineral assemblage including: garnet, tourmaline, hornblende, zircon, apatite, staurolite, opaques (magnetite, ilmenite, hematite and pyrite) and minor rutile, pyroxene (mainly hypersthene), kyanite, sillimanite, andaluzite, titanite, actinolite and tremolite. This heavy mineral assemblage indicates that the main source-area for unconsolidated sands is the Rio Grande do Sul Shield through the afluents from south Jacui River margin. Sedimentary rocks from several Parana basin formations are also important sourcearea for unconsolidated river sand. Volcanic rocks of Serra Geral and sedimentary fluvial rocks of Tupanciretã Formation source-areas show minor contribution. ATi and GZi have shown a consistent value decreasing along the main river course, from montante to jusante. Both index patterns indicate that alluvial sands have suffered weathering along the alluvial plain during the parking period. The ZTR (Zircon + Tourmaline + Rutile index) values are low, relatively constant and consistent along the main course river. Also the ARi (Apatite Rounded index) is relatively low and consistent along virtually all river course. Both ZTR and ARi indicate an important contribution from afluents in order to mantain these consistent index values along river. Further, the heavy minerals signature of Jacui River, established in this work, would help to better understand the provenance and filling of young section of Pelotas basin.