Mineralogical and geochemical studies on some banded iron formations from the Eastern Desert of Egypt, and their industrial uses

Citation

Taman ZI (2005) Mineralogical and geochemical studies on some banded iron formations from the Eastern Desert of Egypt, and their industrial uses. PhD Degree in Geology.

Abstract
Banded Iron Formations (BIFs), associated with Late Proterozoic metavolcanics, occur at Abu Marawat and Semna. As part of the BIFs occurrences in the Eastern Desert of Egypt, their nature arises much debate about their origin. The Precambrian rocks covering the study areas are, starting with the older units: (1) Serpentinite and related rocks, (2) Metavolcanics and BIF, (3) Arc granitoids, and (4) Molasse-type Hammamat sediments. The metavolcanics cover a large part of the area. They form a succession of intercalated metamorphosed volcaniclastics and volcanics. The volcaniclastics are lapilli tuffites, fine and coarse tuffites and agglomerates of basaltic to dacitic compositions. Lamination, cross bedding and graded bedding are reported. Smallscale synsedimentary faulting is observed. Clasts of the volcaniclastics are quartz, feldspar, volcanic rock fragments, jasper and iron oxides (derived from the associated BIF). The volcanic intercalations are represented by porphyritic amygdaloidal basalt and andesite. During this study a set of representative samples of the BIFs and the metavolcanics were collected for mineralogical and geochemical analyses. This involved semi-quantitative mineralogy using the X-ray diffraction technique (XRD) coupled with the matrix flushing technique, mineral chemistry analyses using Scanning Electron microscopy (SEM) and Energy Dispersive Spectrometry (EDS), bulk rock chemistry using X-ray Fluorescence Spectrometry (XRF), Inductively Coupled Plasma - Mass Spectrometry (ICP-MS), and ICP-Ms-4 acid digestion techniques. In addition, Small-scale experiments on upgrading the BIFs were carried out. These are mainly concerning with the ability to use the magnetic properties of the ore to separate the different minerals and upgrade the ore. The main minerals of the BIFs are magnetite, hematite, and quartz. Minor minerals are calcite, chlorite, siderite, dolomite, apatite, pyrite, chalcopyrite and pyrrhotite. Magnetite and quartz are more common in Semna BIF relative to those of Abu Marawat. Hematite and carbonates are relatively common in Abu Marawat. The abundance of chert and magnetite at Semna supports more distal and deeper environment compared to the shallower, more oxidant proximal deposition of the Abu Marawat BIF. Six types of BIF mesobands are described according to the dominant mineral assemblage. The mesobands are: (1) Jasper - magnetite mesobands, (2) Jasper - hematite - magnetite mesobands, (3) Jasper - magnetite - hematite mesobands, (4) Jasper - hematite mesobands, (5) Iron oxide - jasper mesobands, (6) Banded - oolitic mesobands. Primary textures and structures are observed, these includes lamination, banding, spherulitic, oolitic, microfaulting and slumping. SiO2 and Fe2O3 are the dominant constituents in the BIFs. The BIFs show significant enrichment in P and depletion in Ti. The BIFs show negative Ce anomalies, which suggest the existence of oxidizing conditions. The REE patterns of the BIF samples show a considerable depletion in LREE and a positive Eu anomaly. Such positive Eu anomalies are common in most Archean and Proterozoic BIFs and are considered to reflect the result of ocean hydrothermal input into an otherwise highly seawater-dominated system. The geochemistry of the metavolcanics of Abu Marawat and Semna indicates a strong depletion in Ni, Co, and Cr, which might exclude the mantlederived origin. Semna metavolcanics show higher LREE/HREE ratio relative to those of Abu Marawat. The trace and REE distribution of Semna metavolcanics show many typical features basalts formed during plate convergence (arc basalt); including negative Ta and Nb anomalies, positive Th anomaly, enrichment of LILE relative to HFSE, enrichment of LREE relative to the HREE and slight negative Eu anomalies. Abu Marawat metavolcanics show relatively flat REE patterns which are similar to those reported for plume-enriched mid-ocean ridge basalts or backarc basin basalts. Abu Marawat and Semna metavolcanics are believed to be deposited in island arc environs. Back-arc environment can be assumed for Abu Marawat metavolcanics. The Semna BIFs are relatively fine grained (usually < 0.1 mm in grain size) and hard cherty nature, the characteristics which constitute difficulty in the grindability and utilization of the ore. Abu Marawat is slightly coarser and less cherty and is more recommendable for crushing and grinding processes. The wet magnetic separation process was found suitable for enriching the Abu Marawat and Semna BIFs. Using the drum magnetic separator and Davis tube the ores of Abu Marawat and Semna can be upgraded from 50.27 Fe % up to 70.45 Fe % and from 52.33 Fe % to 71.01 respectively. Medium drum speed (60 revolutions per minute) give more enhanced separation compared to higher and lower drum speed. Roasting of the total ore at 750 - 800 °C followed by magnetic separation is found to be adequate. The optimal roasting time is 20 - 30 minutes.