The Yengejeh diasporic karst bauxite deposit, NW Iran: Constraints on distribution and fractionation of major and rare earth elements

Abstract

The Yengejeh diasporic karst bauxite deposit (NW Iran) was developed as discontinuous layers and lenses along the contact of dolomitic limestone of the Ruteh Formation (middle-upper Permian) and dolomite of the Elika Formation (Triassic). This deposit is constituted by two conspicuous subset layers, (1) green bauxite (GB) and (2) red bauxite (RB). The XRD analytic data revealed that diaspore is accompanied by chlorite and anatase in the GB subset and by hematite in the RB subset. Minerals such as pyrophyllite, kaolinite, illite, and rutile are present within the both subsets. The antithetical distributions of Al with Fe and Al with Ti in the deposit are indicative of the controlling role of climatic changes of the depositional environment and the function of post-formation diagenetic processes. The increment in certain geochemical ratios like La/Y, (La/Yb)N, and (LREE/HREE)N from the top toward the bottom of the horizon is designative of pH increase in depositional milieu by buffering of the percolating solutions, and of preferential scavenging of LREE by hematite. The positive Ce anomaly in the GB subset took place as the result of change in the oxidation state of this element from Ce3+ to Ce4+. However, the positive Ce anomalies in the RB subset implicates preferential scavenging of Ce onto hematite. The marked variation range in anomaly values of Eu/Eu* (0.38-1.01) at Yengejeh reveals that this index acted as non-conservative during the evolution of this deposit, and this behavior is related most likely to influences of post-formation diagenetic processes.