The present study deals with the ironstones that occur at the base of the Devonian
Pimenteiras Formation in the northwestern border of the Parnaíba basin. Samples were collected along the Xambioá-Vanderlâdia and Colinas do Tocantins-Couto Magalhães sections about 180 km away from each other. In both sections the sedimentary package lies unconformably over the Proterozoic basement represented by rocks of the Araguaia belt. The ironstones consist of discontinous decimeter-thick layers intercalated in sandstones and shales. The ironstones of the
Xambioá-Vanderlândia show abundant oölites immersed in an iron-oxide/hydroxide-rich matrix. The size of the oölites ranges from 0.2 to 0.5 mm, reaching exceptionally 1.0 mm, while the shape is generally spherical to ellipsoidal. Most oolites present concentric internal structure and only a few are broken. Their nucleus is commonly occupied by detrital grains of quartz, zircon or monazite around which iron-oxide/hydroxide concentric lamellae were formed. Locally some
laminae are composed of chemically precipitated monazite. Goethite/hematite (75-80%), quartz (5-15%) and smectite /kaolinite (0-10%) are the main mineral constituents. Chemically the oolithic ironstones are made up of SiO2 (3-15%), Al2O3 (4.4-7.7%), Fe2O3 (62.5-79%), P2O5 (0.5-3,2%), TiO2 (0.2-0.8%); other components total less than 0,2% and LOI values vary from 9.4 to 13.5%. P2O5 contents are high enough to be accounted for the amounts of the phosphate minerals (monazite and goyasite), so that some P was most likely adsorbed on the iron oxides-hydroxides. Regarding the trace elements, the higher average concentrations were determined for V (780-1990 ppm), Zr (125-600 ppm), Sr (15-296 ppm), Ba (25-266 ppm), Ni (22-225 ppm),Y (39-181ppm) and REE (144-1630 ppm, with Ce>NdLa), which reflect the variable amounts of zircon, monazite and goyasite. The vanadium values in particular may be due to the presence of interbedded clay minerals and /or adsorption on the iron oxides-hydroxides. When normalized to the North American Shale Composite (NASC), the oölithic ironstone samples are enriched in REE by factors greater than 2 few of them exhibiting discrete positive Ce anomalies. In general, the distribution pattern for REE is convex between La and Dy, but towards Lu it tends to become sub-horizontal resembling a “capsized spoon”. Unlike the ironstones described above, those of Colinas do Tocantins-Couto Magalhães area do not present oolithic texture, although they are also basically composed of iron oxides-hydroxides, which not only imparts them a dark red coloration but also acts as a cement for the randomly distributed detrital grains of several minerals. These non-oölithic ironstones consist of hematite/goethite, quartz, clay minerals, muscovite and accessory amounts of monazite and zircon. Their chemical composition reveals Fe2O3 (41-60%), SiO2 (16-39%), Al2O3 (6-11%), K2O (0.5-1,2%), P2O5 (0.32%) and H2O (6-11%) recorded as LOI. MgO, CaO, Na2O, MnO and Cr2O3 are minor components and represent less than 0.4% of the rock. V (83-3.488 ppm), Zr (62-372ppm), Ba (166-347 ppm), Rb (26-62 pm), Zn (19-868 ppm), Ni (3-106 ppm) e Sr (31-51 ppm) are the trace elements with more expressive contents. REE values range from 100 to 300 ppm. LREE (Ce> La> Nd) contents are usually >15 ppm whereas the other REE are in general below of 5 ppm. Although the chemical composition of these ironstones is largely controlled by the amounts of minerals present, the high concentrations of some trace elements may indicate incorporation in the phyllosilicate lattice (Ni and Ba) and adsorption on the iron oxides-hydroxides (V). Comparatively to NASC, the nonoölithic ironstone samples are poorer in REE, whose distribution pattern is dominantly concave and marked by the fractionation of the IREE in relation to both LREE and HREE. The Xambioá-Vanderlândia ironstones are texturally and chemically quite distinct from those of the Colinas do Tocantins-Couto Magalhães area. Besides the presence of oolites, the former show smaller amounts of terrigenous material (especially quartz) and higher iron oxide-hydroxides proportions. They also are more enriched in V, Sr, Zr and REE, and poorer in Al2O3 e Rb. The NASCnormalized REE distribution pattern is also different, especially concerning the IREEN values which, being higher in the oölithic ironstones, display convex curves and, being lower in the nonoolithic ironstones, display concave curves. In the field, however, spatial relationships could not be defined between the two ironstones types. It is then suggested that they represent different lithological facies of the same iron formation. Probably, the deposition of the non-oölith facies occurred in deeper and quieter waters away from the continental border, where larger amounts of detrital sediments were discharged, whereas the deposition of the oölith facies took place in shallower and more agitated waters, with less supply of terrigenous material. Iron was largely derived from the erosion of continental areas where reduced environments favored its mobilization and transported by rivers along with suspended particles, colloids, and organic complexes.
|
