Rare earth element evidence of origin and depositional environment of Late Proterozoic ironstone beds and manganese-oxide deposits, SW Brazil and SE Bolivia

Abstract

Ironstone beds and high grade Mn-oxide deposits (layers or concretions) of Late Precambrian age occur in SW Brazil and SE Bolivia. These deposits, referred to here as Jacadigo-Boqui Fe-Mn deposits after the stratigraphic units hosting most of them, share many characteristics with other siliceous, Precambrian iron formations. Rare earth element (REE) distributions in the Jacadigo-Boqui Fe-Mn deposits were determined by instrumental neutron activation analysis (INAA) and compared with published data for other iron formations and for various water types. REE patterns of both ironstone beds and Mn-oxide deposits are light REE (LREE) enriched on chondrite-normalized plots. Average REE ratios of all samples are: La/Sm = 5.66, La/Ce = 0.63, Eu/Sm = 0.19, Sm/Lu = 8.09. Sm/Lu values for all samples range from 2 to 16, and Sm/Lu values also vary among samples of ironstone beds and Mn-oxide deposits. REE ratios are generally not related to other chemical parameters. Comparison to REE distributions in river, estuarine, and oceanic waters and to particulate matter forming in the oceans and anoxic basins suggests that the Jacadigo-Boqui Fe-Mn deposits formed in waters most similar to those of estuaries or coastal oceans. Formation in river or lake water appears unlikely. The lower Sm/Lu values of oxide layers suggest that these layers formed in a more marine environment than the jasper layers. The balance of continental and marine inputs may have been changed by glacial cycles or sea level changes, either eustatic or tidal. REE patterns of the Jacadigo-Boqui Fe-Mn deposits are roughly similar to those for most other iron formations. However, La/Ce and Eu/Sm values are much lower. A glaciogenic model suggests that the ironstone beds formed in a basin under an ice shelf as a result of freezing. Nodular, detritus-rich Mn-oxide deposits probably formed off the edge of the ice shelf. Layered Mn-oxide deposits most probably formed rapidly during glacial melting and retreat of the shelf as a result of quantitative oxidation of basin water. Glacial melting or lower sea level should result in higher Sm/Lu and lower La/Ce and La/Sm values of chemical sediments (i.e., values more like those of rivers). However, the similarity of REE ratios of layered Mn-oxides and ironstone beds suggest that overturn and oxidation dominated over mixing of marine and river waters during deposition of layered Mn-oxides. The negative Eu anomalies of the Jacadigo-Boqui Fe-Mn deposits distinguish them from most natural waters and other iron formations and are surprising in light of the hypothesis that banded iron formations derive their Fe and Mn from sea-floor, hydrothermal vent waters. It is unlikely that these anomalies were produced during precipitation of Fe- and Mn-oxides from waters with oceanic or mid-ocean and ridge hydrothermal REE patterns. Mixing of REE from a continental water with a negative Eu anomaly and REE from oceanic and ridge hydrothermal waters is believed unlikely because it can not explain all the REE ratios and presents problems regarding the source of Fe and Mn. Alternatives that must be evaluated are derivation of substantial Fe, Mn, and REE from either a continental source or a hydrothermal fluid that had a REE pattern different from that of mid-ocean ridge hydrothermal fluids.