Provenance of heavy minerals in beach sands, southeastern Brazil: from Rio Grande to Chui (Rio Grande do Sul State)

Abstract

The sands in the fore- and backshore zones between Rio Grande and Chui (Rio Grande do Sul State, Brazil) are predominantly composed of quartz, but in some places heavy-mineral concentrations appear as dark patches on the surface of the backshore. Opaques (ilmenite, magnetite, leucoxene, limonite) are the predominant minerals. The predominant transparent species are epidote, augite, hypersthene, and hornblende. Minor quantities of tourmaline, staurolite, garnet, zircon, rutile, kyanite, and andalusite are also present. There are two heavy-mineral associations in the study area: (1) a northern assemblage in which epidote, augite, hypersthene and hornblende slightly predominate; and (2) a southern assemblage in which the opaques predominate. The primary source of the heavy minerals was the igneous—metamorphic rocks of the Sul-Riograndense Shield and the basalts from Serra Geral Formation. Minerals from these sources were incorporated in the coastal sediments during the Quaternary eustatic changes in sea level and, as the coastal plain evolved, were spread over the beaches. The inner continental shelf, underlain predominantly by Pleistocene sands, is the main present-day source of the shoreline sediments. Modern rivers supply negligible heavy minerals to the shore; most fluvial sediment is trapped in the lagoons which parallel the shoreline. The northern heavy-mineral association is probably related to the small fraction of fluvial material reaching the ocean through the Rio Grande inlet and bringing mafic minerals to the beach. The La Coronilla headland and mostly the Chui cliffs contribute minerals to the south part of the area causing the development of heavy-mineral concentrations. Based on direct observations of modern beach dynamics and on sedimentary structures observed in trenches, three processes are inferred to have concentrated the heavy minerals: (1) the swash and backwash in stormy weather or during the strongest tides; (2) flow of sheets of water into or out of berm-top spillover and/or channels; and (3) eolian winnowing action. The concentrates occur principally between Albardão lighthouse and Chui village, where the black sands extend for some tens of kilometers. The concentration of heavy minerals in the deposit near Chui reaches 47% by weight. In this concentrate opaques make up to 72% of which ilmenite forms 58.4%; rutile and zircon show maximum local values between 5 and 6%.