Rare-earth elements in Precambrian iron formations in Väyrylänkylä, South Puolanka area, Finland

Abstract

Partial rare-earth element (REE) and ore analyses of 19 samples of oxide-, carbonate-, silicate-, and sulphide-facies rocks from the Superior-type Precambrian (Karelian) iron formations and associated rocks (pelitic metasediments, dolomite, basic tuffite and metadiabase) are given. For comparison one analysis of an Archean (Prekarelidic) iron formation of the Algoma type is also included. The Karelian iron formations are relatively rich in REE and their REE distribution patterns show depletion of Ce. This is due to the regularly occurring apatite, in which these features are multiplied as is indicated by the analysis of the apatite-rich band. This stratum is interpreted as a marine phosphorite interband and, consequently, the apatite in the iron formations as of marine origin. With the exception of Ce, the REE distribution patterns in iron formations and associated pelitic metasediments and dolomite are relatively similar. All these rocks show a clear trend towards relative depletion of the lighter REE and Yb and Lu in comparison with the North American shale (NAS). Metadiabase and basic tuffite are poor in REE and depleted in the lighter REE in the same way as are tholeitic basalts. Archean iron formations seem to have a greater Eu:Sm ratio than the younger iron formations. More REE data from geologically wellknown iron formations are needed, however, before any definitive conclusions can be drawn.