Mineralogical and geochemical characteristics of the iron-duricrust deposit in Adi-Daero area, northwestern Tigray, Ethiopia: implication for the origin and controlling factors

摘要

Mineralogical and geochemical variations of elements from the Adi-Daero iron-duricrust were studied using petrographic, X-ray diffraction and geochemical analyses. Petrographic studies of the samples confirmed the presence of nodular and colloform textural features. The XRD analysis showed the presence of haematite, goethite, quartz, kaolinite and ilmenite are the mineral constituents. Major-element geochemistry was determined by ICP-AES, and trace and REEs were analysed using ICP-MS. Of the major oxides, Fe_2O_3 (6.8-72.93 wt-%) and SiO_2 (11.07-77.75 wt-%) showed significant variation with generally increasing and decreasing trends from the iron-rich duricrust to the bedrock, respectively. In the profile studied, the alkali and alkaline earth elements (Ca, Mg, Na and K) were highly depleted, suggesting that iron-duricrust near-complete leaching. Elevated values of Sr and Ba in the lower part of the profile are probably due to their abundance in the protolith material (sandstone). A considerable enrichment of V and Cr, and Zr and REEs in the iron-rich duricrust and clay-rich horizon compared to the bedrock confirms their substitution for the Feoxyhydroxides, occurring in heavy mineral phases and adsorbed onto clay minerals, respectively. The La/Y <1 indicates an acidic environment in the upper part of the profile (AD-1 to AD-6;;0.64-0.99) and La/Y >1 a basic environment in the basal part of the profile (AD-7 to AD-12;;1.13-1.58). Similarly, the (La/Yb)N ratio of samples AD-1 to AD-6 (6.81-8.05) is significantly lower than that of AD-7 to AD-12 (10.09-24.54), reflecting an acidic environment. Positive Ce anomalies in the samples of AD-1 to AD-2 (1.17-1.2) are linked to the existence of cerianite due to change in oxidation state of Ce while in AD-8 and AD-9 (0.92-0.93), it is related to adsorption of Ce ions onto clay particles protolith. Therefore, the absolute enrichment of iron in the Adi-Daero iron-duricrust deposit was most likely an in situ process that involved vertical transfer from the iron-poor sandstone source rock.