Geochemical Characteristics of Aluminum-Bearing Iron Ores: A Case Study from the Kolijan Karst-Type Bauxite Deposit, Northwestern Iran

Abstract

The Kolijan bauxite deposit (southeast Mahabad, northwestern Iran) mainly contains aluminum-bearing iron ores and was deposited in karstic depressions and sinkholes of the middle Permian carbonate rocks of the Ruteh Formation. Based on microscopic observations, the aluminum-bearing iron ores were allogenic in origin. According to XRD and SEM-EDS analyses, hematite and goethite are their main constituents, accompanied by lesser amounts of kaolinite, illite, amesite, boehmite, rutile, anatase, calcite, pyrolusite, crandallite, and parisite-(Ce). Chondrite-normalized REE patterns are indicative of fractionation and enrichment of LREE (La–Eu) compared to HREE (Gd–Lu), along with positive Eu and Ce anomalies (Eu/Eu* = 2.29–5.65; Ce/Ce* = 3.63–5.22). Positive Ce anomalies can be attributed to the role of carbonate bedrock as a geochemical barrier and the precipitation of parisite-(Ce). A strong positive correlation between Eu/Eu* and Ce/Ce* (r = 0.84) indicates that Eu anomalies, similar to Ce anomalies, are closely dependent on an alkaline pH. The distribution and fractionation of elements in the iron ores were controlled by a number of factors, including the pH of the environment in which they formed, wet climatic conditions, adsorption, isomorphic substitution, scavenging, co-precipitation, fluctuations of the groundwater table level, and the role of carbonate bedrock as a geochemical barrier. This research indicates that the aluminum-bearing iron ores were probably generated from the weathering of basaltic protolith.