Mineralogy, micromorphology and distribution pattern of major and trace elements in the Mn-nodules of Nasirabad manganese occurrence, Neyriz, Fars Province

Articles in Press

Document Type : Research Article

Authors

• Mohsen Rezaei ¹
• Inana Sadrinejad ²
• Alireza Zarasvandi ³

¹ Assistant Professor, Department of Geology, Faculty of Earth Sciences, Shahid Chamran University of Ahvaz, Ahvaz, Iran

² M.Sc. Student, Department of Geology, Faculty of Earth Sciences, Shahid Chamran University of Ahvaz, Ahvaz, Iran

³ Professor, Department of Geology, Faculty of Earth Sciences, Shahid Chamran University of Ahvaz, Ahvaz, Iran

10.22067/econg.2025.1133

Abstract

This study deals with the results of Raman laser spectroscopy, BSE electron images, EPMA and WDS elemental mappings to characterize the micromorphology, mineral chemistry, and the distribution pattern of Mn, Fe, Mg, Al, Cu, Ni, Co, Sr, Ti, P, Si, V, and Pb in the Mn-nodules of Nasirabad manganese occurrence located to SE of Zagros thrust belt near the Neyriz ophiolite. The results of micromorphology indicated that the studied nodules are of multi-core structure dominated by the accumulation of siliceous microfossils and spheroidal colloform textures. In the core areas, Mn-minerals are fine-grained containing mostly todorokite, pyrochroite, pyrolusite, and ramsdellite. On the contrary, to the marginal parts, the Mn-minerals are coarser and consist mainly of pyrolusite, and ramsdellite. Results imply for the depletion of Fe, Pb with Cu, Ni, and Co, which may indicate the contribution of distal hydrothermal fluids in the formation of nodular structures. Additionally, evidences such as the diagenetic replacement of siliceous microfossil shells with manganese oxide minerals, especially todorokite, shows the role of diagenetic fluids in the evolution of nodular structures. No sighting of rhodochrosite along with the absence of enrichment patterns in the bio-essential elements (e.g., iron, arsenic, barium, strontium, cerium and cobalt) imply for the absence or insignificant role of biological-microbial processes in the formation of studied nodules.
 
Introduction
The main processes involved in the formation of marine Mn deposits include hydrothermal, hydrogenous, diagenetic, and biogenetic - bacterial (e.g., Zarasvandi et al., 2013a). It is important to note these processes could act solely, however, in many cases a combination of these processes is involved in the formation of marine manganese mineralizations (Polgári et al., 2012; Kim et al., 2023). The main structural zones hosting the Mn-deposits in Iran include: (1) Urumieh-Dokhtar volcano-plutonic belt, (2) Central Iran, (3) Sabzevar zone, (4) Alborz magmatic belt, and (5) Cretaceous ophiolites (Maghfouri et al., 2019). Along with some of the Iran's Mn-deposits, especially those associated with ophiolitic belts, i.e., the Abadeh Tashk manganese deposit and the Nasirabad manganese occurrence, nodular Mn-mineralizations have also been reported (e.g., Zarasvandi et al., 2013a). However, it should be noted that this is not a general feature, as no nodular Mn-mineralization has been reported in the Sorkhvand manganese deposit, which is related to the radiolaritic mudstones of the Kermanshah ophiolite (Zarasvandi et al., 2016b). In general, manganese nodules may have various mineralogical phases (Kim et al., 2023). Generally, iron and manganese oxide/hydroxides have various mineralogical phases, depending on the geochemical conditions of the sedimentary environment (Kim et al., 2023). This is due to their sensitivity to the redox conditions of the sedimentary environment (Ling et al., 2018). The occurrence of Mn-bearing nodular structures in the Nasirabad manganese occurrence was previously reported in Zarasvandi et al. (2013a). The present study tried to characterize the mineralogy, micromorphology and the geochemistry of these nodules using Raman laser spectroscopy, BSE electron images, EPMA analyses, and WDS elemental maps.
 
Geological setting
The Nasirabad manganese occurrence is located to the southwest of Neyriz city in the Fars Province. Structurally and lithologically the studied area is located in the southeastern part of Zagros thrust belt close to the exposure of Neyriz ophiolite. In this area, Mn-mineralization occurred as nodular and layered forms in interlayers with the chert successions of the Pichakan radiolarite zone. This zone represents the abyssal facies of Neotethys from the Late Triassic to Cretaceous (Tangestani et al., 2011). The Neyriz ophiolite is thrust over the Pichakan radiolarite zone (Babaie et al., 2001). This zone in the lower parts contains the Upper Triassic limestone turbidites, dark marl and serpentinite diapirs (Zarasvandi et al., 2013a). This changes upward to <5 cm bedded cherty radiolarites, alternating with up to 5m bedded green siliceous shale and detrital limestone. To the upper parts this overlies by a thick (∼ 500 m) sequence of radiolarites. The radiolarites of the upper parts hosting the Mn-mineralization are younger than Middle Jurassic (Zarasvandi et al., 2013a).
 
Materials and methods
In this study, the preparation of thin-polished sections and all analysis steps were carried out in the Montanuniversitat Leoben, Austria. In order to correctly determine the type and paragenesis of manganese ore minerals, BSE electron images and EDX spectra of samples were obtained. Also, all Raman spectra were taken by a Jobin Yvon LabRAM instrument. An electron probe micro-analyzer (EPMA) instrument, Jeol JXA 8200 superprobe model, was used to prepare the elemental maps using the X-ray wavelength dispersion (WDS) method via overnight running of instrument.
 
Results and discussion
The studied nodules represent multi-core structure dominated by the accumulation of siliceous microfossils and spheroidal colloform textures. BSE electron images accompanied by Raman laser spectroscopy analysis confirmed that in the core areas, Mn-minerals are fine-grained containing mostly todorokite [(Na, Ca, K, Ba, Sr)_1-x(Mn, Mg, Al)₆O₁₂]،, pyrochroite [Mn(OH)₂], pyrolusite [Mn⁴⁺O₂], and ramsdellite [Mn⁴⁺O₂]. On the contrary, to the marginal parts, the Mn-minerals are coarser and consist mainly of pyrolusite, and ramsdellite. The patterns of major and trace elements (i.e., Mn, Fe, Mg, Al, Cu, Ni, Co, Sr, Ti, P, Si, V, and Pb) imply for the depletion of Fe, Pb with Cu, Ni, and Co, providing insight into the contribution of distal hydrothermal fluids in the formation of nodular structures. Additionally, diagenetic replacement of siliceous microfossil shells with manganese oxide minerals, especially todorokite, shows the role of diagenetic pore-fluids in the evolution of nodular structures. No sighting of Mn-carbonate ore minerals (i.e., rhodochrosite) along with the absence of enrichment patterns in the bio-essential elements (e.g., iron, arsenic, barium, strontium, cerium and cobalt) imply for the absence or insignificant role of biological-microbial processes in the formation of studied nodules.
 
Conclusion
This study deals with the micromorphology, in situ mineralogy, and the distribution pattern of major and trace elements in the Mn-nodules of Nasirabad manganese occurrence, Neyriz, Fars Province. The analysis of the presented data implies that distal hydrothermal fluids accompanied with diagenetic pore-fluids were involved in the formation of multi-core nodular structures. Compared with modern Mn-bearing nodules, the studied nodules have no Fe-Mn-bearing hydroxide minerals (i.e., vernadite) highlighting the role of diagenetic fluids. Additionally, the ubiquitous occurrence of stable and unstable polymorphs of Mn oxides (i.e., pyrolusite and ramsdellite, respectively) is attributed to the local changes in the crystallization conditions during the slow growth rate of Mn-nodules.
 
Acknowledgements
We hereby acknowledge the support of the office of vicechancellor for research and technology, Shahid Chamran University of Ahvaz for providing the research grant No: SCU.EG1403.38600.

Keywords

• Manganese nodule
• Mineralogy
• Geochemistry
• Nasirabad
• Neyriz

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