Newly discovered gossanite-like and sulfide ore bodies associated with microbial activity in the Zagros ophiolites from the Rayat area of NE Iraq

Abstract

• A new gossanite-like and sulfides ore body discovered in Rayat area NE Iraq. • Hydrothermal and sedimentation processes formed Fe-Ni-Co-ore along Cu-Ag-sulfides. • Abundant fossil microbial assemblages are associated with the mineralizations. • A coaly layer provides clues for the evolution of the Walash Ophiolitic mélange. In this article, we report the discovery of a gossanite-type hematite and magnetite ore associated with sulfide mineralization in the Rayat area of NE Iraq. Detailed petrographic, mineralogical, and geochemical data define the features of the gossanite and the associated sulfide-rich rocks, providing an assessment of the mineralization and its relation to hydrothermal, microbial and alteration processes. The identified gossanite-like rocks are interpreted as part of a specific submarine gossan formed on the sulfide-rich serpentinite. In the uppermost part of the studied sequence, jasperite lithofacies occur, within which millimeter-thick layered iron oxide-silica alternations are also visible and comparable to the banded iron formation (BIF). Below the jasperite, an oxidized mineralized zone (OMZ) developed, representing gossanite that is exceptionally rich in iron oxides (up to 70%), chlorite, and quartz with some minor spinel content. The lowermost part of the studied sequence is the sulfide-rich reduced mineralized zone (RMZ) composed of silica, pyrite, chalcopyrite, pentlandite, Ni-chlorite, and siderite. The transition zone (TZ), approximately 1 m thick, between the OMZ and RMZ, contains chlorite, Fe-oxides, quartz, and partially altered sulfide minerals. The potential economic significance of the ore body in addition to Fe is the silver-nickel (Ag-Ni) and cobalt (Co) mineralization, with an average Ag concentration of 5.09 wt% per sulfide grain, which occurs as an exsolution phase within the sulfidic-cherts lithofacies. The Ni concentration ranged from 363 to 4,000 mg/kg, while that of Co ranged from 90 to 441 mg/kg, depending on the lithofacies. A thin lensoid coaly bed associated with the ore body, attributable to the Ophiolitic Mélange, provides some initial clues for the epigenetic thermotectonic evolution of the studied strata. The overall data suggest that the Rayat gossanite, jasperite, and the underlying sulfide-rich zone were formed near the seafloor through a combination of initial hydrothermal alteration of serpentinites, which provided the sulfides, accompanied by the sedimentary deposition of Fe-Si-exhalatives along with the olistostromatic brecciated facies in the upper section, and the subsequent seafloor oxidation/weathering effects along microbial iron uptake. The discovery of gossanite and sulfides in the Rayat area, along with the already known chromitite deposits in the contiguous areas, highlights NE Iraq as a potential mineral-rich domain.