Mineralogy, geochemistry, and genesis of Mn mineralization associated with the Noorabad Delfan radiolarites, Northwestern Lorestan

The Borujerd-Kermanshah ophiolite is a part of the ophiolite complex belonging to the Zagros Mountains and is a part of the Alpes-Himalayan belt (Alavi, 1994). The Boroujerd-Kermanshah Ophiolite complex consists of serpentinized peridotite, layered metagabbro, isotropic gabbro, diabase dyke, plagiogranite, pillow lavas and sedimentary rocks (Miocene radiolarite and limestone). In the upper part, the radiolarite layers are covered by jasperoid rocks and pelagic limestone (Mohajjel et al., 2003; Saccani et al., 2013). The Noorabad Delfan manganese deposit is subjected to mineralogical and geochemical studies in order to elucidate its petrogenesis. The Noorabad Delfan manganese deposit is located along the ophiolite belt of the upper Jurassic-lower Cretaceous period. Manganese mineralization occurs as syngenetic to epigenetic with jasperoid and silicified veins in the carbonate and radiolarian chert units. Geochemical studies show that some elements such as Ce, Cu, Ni are enriched in the mineralized zone. Mobile and trace elements (Sr, As, Zn, Ba, Fe, Mn, Si) and the ratio of Mn/Fe, Al/Ti, show similar characteristics with submarine hydrothermal-hydrogenous manganese deposits. In geochemical studies, SEM, XRD and EPMA results show that pyrolusite is the majar mineral in the ore deposit, and nsutite and rhodochrosite are formed as the accessory phase.

During the field work, 53 samples were collected from the mineralization zone and host rocks. A total of 30 polished and thin sections were studied by Zeiss polarized microscope (Axioplan-2), in the Kharazmi University and the Iran Mineral Processing Research Center (IMPRC). Suitable sections were selected for more study by Zeiss 1450vp SEM at the Iranian Mineral Processing Research Center (IMPRC). The SEM-EDS analyses and secondary electron (SEM-SE) images at the IMPRC were acquired on beam currents between 0.05 and 5 nA, and electron acceleration potentials of 5 to 20 kV. The Electron microprobe analysis of selected points by SEM was carried out using a Cameca SX100 at IMPRC in the 20 kV and 20 nA current and 1 to 5 mm beam long. The Cameca PAP correction software was used for data reduction. Backscattered electron images were used in order to select more analytical points. A total of 15 samples were selected for whole rock chemical analysis. Samples were prepared with regular methods and finally they were analyzed for major and rare elements by the XRF and ICP-MS methods in Zarazma and Iran Minerals Processing Research Center Laboratory.

Manganese deposits with hydrothermal origin are usually related to silica gels. These deposits are associated with submarine volcanic eruptions and hydrothermal-hydrogenous activity and they are rich in metal elements. This kind of deposits, is basically emplaced with interlayer marine sediments (Roy, 1992). Titanium in the hydrothermal fluids is an immobile element and can be used as an indicator for measuring the amount of continental crest sediment. The relatively high TiO2 levels in the manganese deposits indicate the composition of the material during sedimentation (Sugisaki, 1984). Therefore, in the hydrothermal deposits, the TiO2 ratio is lower than other kinds of manganese ore mineralization types. Nicholson (Nicholson, 1992a) believes that hydrothermal manganese deposits are known by enrichment of As, Ba, Cu, Pb, Sb, Sr, Li, Cd, Mo, V, Zn, Co, Cu, Ni and sedimentary deposits are distinguished by K, Na, Ca, Mg, Sr (Nicholson, 1992b). According to this statement, the manganese mineralization of the Noorabad Delfan deposit may be classified as hydrothermal to hydrogenous ore deposits related to oceanic crust ophiolitics.

The Noorabad Delfan deposit with 5 km long is formed in radiolarite sequences in the west of Iran. Based on field observation, mineralogy and geochemistry and the major/trace and rare elements ratio, Noorabad Delfan mineralization is classified as a hydrothermal-hydrogenous deposit. In the study area, the manganese mineralization is formed with interlayer of radiolarite as syngenetic to epigenetic mineralized zones.The hydrothermal systems are generated by submarine volcanic activity and seawater. Due to the rotation of submarine volcanic activity related fluids, the hot oceanic water carries metalliferous phases. The metalliferous phase has been deposited during cooling, pressure reductions and Eh-pH changes. The hydrothermal to hydrogenous activity is the main factor in manganese mineralization in the Noorabad Delphan deposit. Mineralogical and geochemical evidences support a primary hydrothermal source for Mn-mineralization.