Rare native Ni, Fe, Cu, and Sn metals in Hemrin Basalt, northern Iraq: implications for mineral chemistry and ore genesis

The Hemrin Basalt (HB) is an abnormal occurrence because of its genesis, mineralogy, textures, rare native metals content, and odd geographic-geologic-tectonic setting. It occurs as a caprock for few NW/SE trending hills in a nearly flat-lying area. The HB is a high-K calc-alkaline, collision-related, continental-arc basalt and consists of diopside, labradorite and volcanic glass as the predominant constituents displaying rare firework and feathery textures. The accessory minerals in HB comprise small, discrete grains of native Ni, Fe, Cu, and Sn metals, and scattered grains of sulfides (pyrrhotite, pentlandite, molybdenite), oxides (magnetite, ilmenite, Cr-spinel), and apatite. These minerals occur as disseminated individual or composite grains of variable shapes within the groundmass of HB. Vesicle-filling secondary minerals are zeolite, calcite, anhydrite, and gypsum. The native metals were formed under reducing conditions. The reducing conditions were created as a result of assimilation of C- and S-rich sedimentary rocks by the ascending magma from the continental lithospheric mantle through the thick pile of sedimentary formations. The possible C- and S-rich rocks include the carbonates and evaporites of Fatha Formation and the Hemrin Coal Seam within the upper part of Injana Formation hosting the HB and overlying the Fatha Formation, and possibly the petroleum source, reservoir and caprocks forming the sedimentary column underlying the area. Assimilation of S from these rocks created sulfides, while assimilated C resulted in the formation of native metals.