QUÍMIO-ESTRATIGRAFIA E EVOLUÇÃO DOS JASPILITOS DA FORMAÇÃO CARAJÁS (PA)

Abstract

Banded iron-formations (BIFs) are relatively frequent rocks in the Earth’s crust and in the geological time but, after decades of research, their genesis remains controversial. Although ancient, with ca . 2,75 Ga, the BIFs of the Carajas formation are adequate to litho-geochemical studies since unlike the majority of other worldwide deposits, they were not affected by metamorphic transformations, and their chemical composition remain unchanged. These BIFs are jaspilites of the oxide facies and occur interlayered in volcanic rocks of the Grao Para group. Whole rocks chemical analyses for major, trace, rare earth elements and loss-on-ignition (PF) as related to the original stratigraphic positions are presented together with analyses from separated iron and silica bands. The study involved a qualitative survey using optical-emission-spectrography and chemical analyses by classical methods, ICP-MS, XRF, AAS and INAA. The BIFs of the Carajas Formation have average contents as follow (62 samples): Fe 2 O 3 T= 53,65%; SiO 2 = 44,60 %; Al 2 O 3 = 0,63%; FeO= 1,31%; MnO= 0,06%; MgO= 0,1%; CaO= 0,04%; Na 2 O= 0,04%; K 2 O= 0,02%; P 2 O 5 = 0,01% and PF= 0,73%. The samplig with stratigraphic control was very important to establish that from base to top of the BIFs there is Si enrichment with a proportional decrease in Fe (correlation coefficient=-0,99). The REE presents very low abundances ( S REEmean = 5,27 ppm), when compared with others BIFs and these abundances are higher in the iron-rich bands and at the base of the BIFs. From base to top of the package there is a relative enrichment in HREE and a less pronounced Eu positive anomaly, indicating a gradual reduction on the contribution of the hydrothermal sources to the jaspilite genesis. These results indicate that at circa 2.750 Ma the jaspilites of the Carajas formation were deposited in a wide and tectonically stable shallow marine platform. The foremost contribution of hydrothermal sources in the jaspilite genesis at the base of the sedimentary package and the absence of associated clastics points out to a depositional site away from the coastline. However, while the BIFs were deposited, the contribution of these hydrothermal sources decreased while the Si contents were increased.