Trace element geochemistry of magnetite from the Mahura iron placer deposit, Markazi province, Iran: implications for magnetite provenance rocks

Abstract

The Mahura iron placer deposit is located 55 km northeast of Arak city in the Markazi province, Iran. From a geological point of view, it is located in the Central Iran Zone. In this study, the trace element geochemistry of magnetite is used to determine the origin of placer magnetites in the Mahura deposit. Magnetite is the most important placer mineral that occurs as micrometer-sized grains in the Quaternary alluvium. The alluvium consists of sandy, silty and clayey sediments containing a considerable amount of volcanic rock fragments of various sizes. The results of the petrographic studies and the whole-rock geochemistry indicate that the volcanic rock fragments are mostly andesite and basaltic andesite. These volcanics contain 4 to 10% magnetite as disseminated grains in the groundmass and as inclusions in plagioclase and amphibole phenocrysts. The mineral chemistry of the placer magnetites indicates 2.59–3.33% Ti, so that they can be considered as titanomagnetite or as a solid solution between magnetite and ilmenite. The chemical composition of the magnetites in the volcanic rock fragments also falls within the range of titanomagnetite with a tendency towards magnetite in the TiO₂-Fe₂O₃-FeO system. The composition of the placer magnetites and magnetites in the volcanic rock fragments are plotted in the magmatic field in Ti-Al and Ti versus Ni/Cr diagrams. The diagrams Ti + V versus Ni/(Cr + Mn) and Ti + V versus Ca + Al + Mn indicate that placer magnetites and magnetites in the volcanic rock fragments belong to the Fe-Ti, V deposits. In the V-Ti diagram, all magnetites fall into the range of titaniferous iron ores. The comparison of the trace element composition of placer magnetites and magnetites in the volcanic rock fragments in the elemental diagrams above showed that all magnetites have the same origin, so that the placer magnetites were most likely released from the volcanic rock fragments by erosion over time. On the other hand, the comparison of the chemical composition of the volcanic rock fragments in the alluvium with that of volcanic rocks from the Ashtian area north of the Mahura deposit in the petrogenetic diagrams reveals similar nature for these rocks and that the magnetite-bearing volcanic rock fragments originate from the Ashtian Basin.