REE and stable isotope (C, O, S) geochemistry of fluorite from the Kamar-Mehdi deposit, southwest of Tabas, Iran

Abstract

Fluorite mineralization occurs in Middle Triassic dolomite of the Shotori formation in the Kamar-Mehdi region, southwest of Tabas, Iran. Ore deposits consist of fluorite, barite, galena, calcite, and quartz. The accompanying minerals are iron and manganese oxides, pyrite and malachite. Alteration of wall-rocks include silicification and dolomitization. The character of the mineralization and the relationship between ore-host rock indicates that fluorite mineralization is epigenetic. Analysis of rare earth elements (REE) in the fluorite samples indicates low values in ranges between 3.01 and 6.79 ppm for early fluorites and 25.86 and 17.46 ppm for late fluorites. These concentrations indicate that fluorites have originated from a sedimentary environment. Also, Tb/Ca versus Tb/La suggests a hydrothermal-sedimentary origin for the Kamar-Mehdi fluorites. La/Yb and chondrite-normalized REE patterns reveal that the early fluorites are enriched in LREE (light rare earth elements) and the late fluorites are enriched in HREE (heavy rare earth elements). The late fluorites have resulted from the mobilization and recrystallization of the early fluorites. Ce/Ce* ratios show negative Ce anomalies for all fluorites, indicating a high oxygen fugacity in hydrothermal fluids. Eu/Eu* ratios show negative Eu anomalies for the early fluorites and positive Eu anomalies for the late fluorites, indicating that the early fluorites have formed above 200 °C and the late ones below 200 °C in temperature. La/Yb ratios and chondrite-normalized REE patterns are similar for fluorites and the host rocks, indicating that these elements have originated from the host rocks. There is an overlap between the δ13C composition of calcite associated with the fluorite (−0.9 ‰, −0.10 ‰) and that of carbonate sediments of the contemporaneous Triassic sea. This indicates the participation of Triassic seawater in the ore-forming fluid. The δ18O composition of the calcites (−10.94 ‰, −10.03 ‰) is similar to or a little lower than Triassic seawater. This difference is most likely due to the increase in the temperature of the hydrothermal fluids. The average δ34S values are −5.9 ‰ for galena and +20.8 ‰ for barite samples. These values show that seawater sulfate is the most probable source of sulfur. Reduced sulfur is most likely to be provided via the thermochemical reduction of sulfate. The existing data (such as tectonic setting, host rock composition, REE geochemistry, and oxygen, carbon, and sulfur isotopic compositions) indicate a sedimentary-hydrothermal origin for Kamar-Mehdi fluorite deposit.