Geochemical behavior and mineral control in distribution and mobilization of trace and rare earth elements during the formation of argillic alteration zone in Mahiroud gold prospect, southeast of Birjand

The Mahiroud mineral prospect area in the flyschic basin of the eastern Iran is located in northeastern part of the Sistan suture zone in southeast of the Sarbisheh, Southern Khorasan province. The igneous suite (intrusive-extrusive) in the Mahiroud area constitutes the major geologic rock units which include andesite-basalt, andesite, microgabbro (diabase), pillow lavas, spilite, and dacitic dikes. The intrusion of tonalitic igneous bodies of upper Cretaceous age into the Mahiroud suite caused the occurrence of widespread argillic alteration in the area. Mineralogical considerations revealed that the argillic alteration zone in the area contains minerals such as quartz, albite, kaolinite, montmorillonite, illite, chlorite, hematite, goethite, and pyrite. The distribution pattern of rare earth elements normalized to chondrite is indicative of differentiation and weak enrichment of LREE relative to HREE. Consideration of REE distribution pattern depicts negative Eu and Ce anomalies. It appears that the alteration of plagioclase in andesitic unit by hydrothermal fluids and oxidation of hypogene pyrite are two effective and principal agents for occurrence of negative Eu anomaly in the alteration zone of the study area. The occurrence of negative Ce anomaly indicates that the hypogene solutions had significant role in creation of the argillic alteration zone. Consideration of the correlation coefficients among elements showed that both adsorption and scavenging had no role in distribution and fixation of REE in the area, and also kaolinite, smectite, and Mn-oxides were not the host minerals for REE. The positive correlations between REE and certain elements like Be, Hf, and Nb exhibit that the REE-bearing minerals played prominent and essential role in concentration and distribution of these elements in the studied argillic alteration zone.