Manganese

Geological Setting

Manganese mineralization in Oman is primarily associated with the Wahrah Formation, a Permian–Triassic sedimentary succession exposed in the northern Oman Mountains. The Wahrah Formation forms part of the autochthonous to para-autochthonous sedimentary cover beneath and adjacent to the obducted Oman Ophiolite. It consists predominantly of shallow marine carbonates, chert horizons, marl, and subordinate siliciclastic units deposited along a passive continental margin setting prior to ophiolite emplacement.

During the Late Permian to Early Triassic, the region was characterized by shallow epicontinental seas with fluctuating redox conditions. These conditions were favorable for the chemical precipitation of manganese and iron oxides from seawater. Manganese was likely derived from hydrothermal input at distal spreading centers, continental weathering, and basin-scale seawater circulation. Periodic changes in oxygenation levels within the marine basin promoted manganese enrichment in specific stratigraphic horizons, leading to the development of stratiform manganese beds interlayered with carbonates and siliceous sediments.

The manganese mineralization is strongly stratigraphically controlled, occurring within discrete horizons of the Wahrah Formation. These horizons are commonly associated with chert-rich or marl-bearing intervals that provided suitable geochemical traps for manganese precipitation. Fine lamination, nodular textures, and bedded oxide layers suggest primary sedimentary deposition followed by early diagenetic modification.

Subsequent tectonic events significantly influenced the present distribution of manganese deposits. During the Late Cretaceous obduction of the Oman Ophiolite, the sedimentary sequences of the Wahrah Formation were subjected to folding, faulting, and uplift. These tectonic processes fractured the host rocks and locally enhanced fluid circulation, leading to minor remobilization and secondary enrichment of manganese along structural zones. In some areas, surface weathering and supergene processes further concentrated manganese oxides, forming lateritic caps and high-grade surface accumulations.

Lithologically, the host rocks display interbedded carbonate and siliceous units with varying permeability. The presence of chert layers and microfractured carbonate beds facilitated localized fluid movement and secondary enrichment. Iron oxides such as hematite and goethite are commonly associated with manganese mineralization, reflecting similar geochemical precipitation mechanisms under oxidizing conditions.

Regionally, manganese occurrences are distributed along structurally deformed belts where the Wahrah Formation is well exposed. The combination of favorable sedimentary facies, stratigraphic trapping mechanisms, and post-depositional tectonic modification created the geological framework responsible for manganese concentration in Oman.

Overall, the geological setting of Omani manganese deposits reflects a sedimentary-chemical origin within shallow marine environments, later modified by tectonic deformation and surface weathering processes. This integrated stratigraphic and structural control provides a predictable exploration model focused on specific Wahrah Formation horizons and structurally enhanced zones.

Ore Mineralogy

The manganese deposits of Oman, principally hosted within the Wahrah Formation, are characterized by a predominance of manganese oxides and hydroxides formed through primary marine chemical precipitation and subsequently modified by diagenetic and supergene processes. The mineral assemblage reflects deposition in shallow marine environments under fluctuating redox conditions, followed by tectonic uplift and prolonged surface weathering. As a result, most economically significant manganese occurrences in Oman are oxide-rich rather than sulphide-bearing, with mineralogical variations closely tied to stratigraphic position and degree of post-depositional alteration.

The dominant ore mineral is pyrolusite (MnO₂), which commonly represents the highest manganese-grade phase within the deposits. Pyrolusite occurs as massive fine-grained aggregates, compact stratiform layers, botryoidal crusts, and fracture-filling material. In many cases, it forms through the oxidation of earlier manganese-bearing minerals during supergene enrichment, particularly near surface exposures. Its presence is typically associated with well-developed oxidation zones, where meteoric waters have enhanced manganese concentration by leaching and redeposition. The abundance of pyrolusite directly influences the overall grade of the ore, especially in enriched horizons.

Psilomelane, a field term commonly applied to hard, massive barium-bearing manganese oxides such as cryptomelane-group minerals, is also widespread and contributes substantially to bulk ore composition. It typically forms dense, steel-grey to black compact masses, nodules, and layered accumulations within stratiform beds. Cryptomelane (KMn₈O₁₆) frequently occurs as microcrystalline aggregates intergrown with pyrolusite, particularly in oxidized and potassium-enriched zones. These minerals are indicative of supergene processes and secondary mineral stabilization under oxidizing surface conditions.

Manganite (MnO(OH)) occurs locally in less intensely oxidized sections and may represent either an early diagenetic precipitate or an intermediate phase in the transformation from primary manganese carbonate or silicate phases to stable oxides. It is typically observed as prismatic crystals or granular masses within bedded manganese horizons. In certain siliceous intervals, braunite (Mn²⁺Mn³⁺₆SiO₁₂) may be present, especially where silica activity was elevated during diagenesis. Braunite-bearing zones are generally more compact and may indicate primary sedimentary chemical precipitation under specific basin conditions.

Iron oxides such as hematite (Fe₂O₃) and goethite (FeO(OH)) are common accessory minerals and frequently occur interbedded with manganese oxides or as replacement phases within carbonate host rocks. Their presence reflects similar redox-controlled precipitation mechanisms and indicates periodic variations in basin oxygenation. Gangue minerals are predominantly calcite, dolomite, microcrystalline silica (chert), and clay minerals, with carbonate units often showing partial replacement by manganese oxides. Chert-rich intervals within the Wahrah Formation are particularly significant, as they commonly coincide with manganese-rich horizons, suggesting a strong geochemical link between silica precipitation and manganese concentration.

Texturally, the deposits display well-developed stratiform and laminated bedding, nodular and concretionary structures, botryoidal and colloform oxide growth patterns, and localized fracture-filling mineralization. Replacement textures within carbonate beds are also common, indicating post-depositional fluid interaction and secondary enrichment. Overall, the ore mineralogy supports a model of primary chemical sedimentation in a shallow marine setting, subsequently enhanced by diagenetic redistribution and supergene oxidation, which together produced economically viable manganese concentrations in Oman.

Exploration

Exploration for manganese in Oman is primarily focused on identifying favorable stratigraphic horizons within the Wahrah Formation and understanding the structural and geomorphological factors that enhance manganese concentration. Because the deposits are largely stratiform and sediment-hosted, exploration strategies emphasize detailed geological mapping, facies analysis, and recognition of specific lithological markers such as chert-rich intervals, marl layers, and oxidized carbonate beds. Stratigraphic continuity plays a critical role, as manganese mineralization is typically confined to discrete sedimentary levels that can be traced laterally across structurally deformed terrains.

Surface reconnaissance remains an effective first-stage exploration method due to the distinctive appearance of manganese oxides, which commonly form dark, massive, or botryoidal outcrops resistant to weathering. Mapping of these surface expressions is complemented by systematic rock-chip and channel sampling to determine grade distribution and lateral continuity. Geochemical surveys, including soil and stream-sediment sampling, are particularly useful in detecting concealed or partially covered mineralized horizons, especially in areas where structural deformation or colluvial cover obscures direct exposure.

Remote sensing and spectral analysis have become increasingly valuable tools in manganese exploration, particularly for detecting oxide-rich zones. Manganese oxides exhibit diagnostic spectral characteristics that can be identified through satellite imagery and hyperspectral data, enabling regional-scale targeting of prospective Wahrah Formation exposures. These techniques are especially effective in arid environments such as Oman, where limited vegetation cover enhances spectral clarity.

Subsurface evaluation typically involves trenching and shallow drilling to assess thickness, grade variability, and internal layering of mineralized beds. Because manganese deposits in Oman are often lensoidal and laterally variable, drilling programs are designed to test both stratigraphic continuity and structural influences such as faults and fold hinges that may localize enrichment. Downhole sampling provides data for resource estimation, grade modeling, and metallurgical testing, particularly to determine suitability for metallurgical versus chemical-grade applications.

Structural analysis is also an important exploration component. Although manganese mineralization is primarily stratigraphically controlled, post-depositional folding and faulting have influenced present-day distribution. Exploration therefore targets structurally thickened zones, fold closures, and fracture-controlled enrichment areas where secondary concentration may have upgraded manganese content.

Modern exploration in Oman integrates traditional geological methods with geochemical analysis, geospatial modeling, and remote sensing to refine targeting within the Wahrah Formation. The predictable stratigraphic control, combined with surface visibility and favorable oxidation characteristics, makes manganese exploration comparatively straightforward, though economic viability depends on thickness, grade consistency, and accessibility. Continued reassessment of known occurrences and underexplored stratigraphic belts suggests potential for incremental resource expansion within Oman’s manganese-bearing formations.

Sur Deposits

Manganese deposits in the Sur area of eastern Oman constitute a small but geologically significant group of stratiform manganese occurrences developed within deep-marine sedimentary sequences associated with the Hawasina Complex and the eastern margin of the Semail Ophiolite. The Sur region occupies a structurally complex zone where oceanic sediments, ophiolitic units, and post-obduction deformation intersect, creating favorable conditions for chemical sedimentation and metal concentration during the evolution of the Neotethyan ocean basin. The manganese mineralization in the Sur area is primarily hosted by siliceous sedimentary rocks, including radiolarian chert, siliceous shale, and locally marly or calcareous interbeds. These lithologies are typical of distal deep-marine depositional environments and are comparable to manganese-bearing horizons documented within the Wahrah Formation in northern Oman. Mineralization generally occurs as thin stratiform layers, discontinuous lenses, or disseminated impregnations concordant with bedding, rather than as vein-type or structurally controlled bodies.

Available field observations, historical sampling, and limited geochemical analyses indicate that manganese grades in the Sur area are moderate to locally high, but highly variable over short distances. Reported MnO contents from surface and near-surface samples typically range between 10 and 35 wt.% MnO, with localized enrichments exceeding 40 wt.% MnO in small lenses or pockets where manganese oxides are concentrated. Average grades over continuous horizons are generally lower, commonly in the range of 12–25 wt.% MnO, reflecting dilution by silica and clay-rich host rocks. Iron contents are typically low to moderate, resulting in high Mn/Fe ratios, a characteristic feature of sedimentary and hydrogenous manganese deposits. This geochemical signature supports selective manganese precipitation under oxic to suboxic conditions at or near the sediment–water interface. Silica (SiO2) contents are commonly high, often exceeding 50–70 wt.%, due to the dominance of radiolarian chert as the host lithology, which has a direct impact on bulk ore grade and beneficiation potential. Trace element concentrations, where analyzed, show generally low levels of base metals such as Cu, Zn, Ni, and Co, distinguishing the Sur manganese occurrences from hydrothermal or volcanogenic massive sulfide–related systems. Barium may be locally enriched, particularly in association with siliceous horizons, suggesting episodic changes in seawater chemistry or minor low-temperature hydrothermal input.

Individual manganese horizons in the Sur area are typically thin, ranging from a few centimeters up to 0.3–1.0 m in thickness, and laterally persistent over tens to a few hundreds of meters. Based on surface mapping and limited continuity, individual occurrences are interpreted to contain small tonnages, commonly on the order of several thousand to tens of thousands of tonnes per horizon. No large, laterally extensive manganese bodies comparable to major global sedimentary manganese deposits have been documented in the Sur area to date. As a result, these occurrences are best classified as small stratiform manganese deposits or prospects, rather than defined economic deposits. However, their significance lies in their consistency with regional manganese-bearing stratigraphy and their potential to occur as multiple stacked horizons within favorable sedimentary packages.

The manganese deposits of the Sur area are interpreted to have formed predominantly through sedimentary to hydrogenous processes, driven by redox-controlled precipitation of manganese from seawater in a deep-marine basin. Fluctuations in oxygen availability, biological productivity, and sedimentation rates likely governed the episodic formation of manganese-rich layers. Minor hydrothermal contribution cannot be ruled out, particularly given the proximity to ophiolitic volcanic units, but available evidence suggests it played a secondary role. Although the Sur manganese deposits are currently of limited economic importance, they provide valuable insights into the behavior of manganese in ophiolite-related basins and serve as useful analogues to Wahrah Formation deposits elsewhere in Oman. From an exploration perspective, they highlight the importance of targeting siliceous deep-marine stratigraphy, high Mn/Fe ratios, and laterally persistent chert horizons when assessing manganese potential in eastern Oman.

References

Manganese deposits in Oman, although limited in scale compared to major global occurrences, represent a geologically significant component of the country’s mineral wealth and provide important insights into deep-marine sedimentary processes associated with ophiolite-related basins. These deposits are predominantly stratiform, hosted within siliceous sedimentary sequences of the Wahrah Formation, part of the Hawasina Complex, and are typically controlled by lithology rather than structural features. The principal focus in studying these deposits is the recognition of red chert-dominated horizons, high Mn/Fe ratios, and lateral continuity of manganese-bearing layers, which serve as key indicators for prospective exploration. Geochemically, the deposits exhibit moderate to locally high MnO content, low to moderate iron, and minimal base metal contamination, reflecting their sedimentary to hydrogenous origin and oxic to suboxic depositional conditions. From an exploration and academic perspective, understanding the depositional environment, stratigraphic controls, lithological variations, and geochemical signatures is critical for identifying prospective zones, evaluating ore quality, and drawing analogues for similar occurrences elsewhere in Oman. Furthermore, although these deposits are currently of limited economic significance, their consistency with regional manganese-bearing stratigraphy and the presence of multiple stacked horizons emphasize their potential for targeted exploration in the future.

– Geology and Mineralization of the Semail Ophiolite, Oman – Lippard, S.J., Shelton, A.W., & Gass, I.G. (1986), Geological Society of London, Memoir 11.
– The Hawasina Nappes: Deep-Marine Sedimentation and Tectonic Evolution of the Oman Mountains – Glennie, K.W., Boeuf, M.G.A., – Hughes Clarke, M.W., Moody-Stuart, M., Pilaar, W.F.H., & Reinhardt, B.M. (1974), Verhandelingen Koninklijk Nederlands Geologisch Mijnbouwkundig Genootschap.
– Tectonic Evolution of the Oman Mountains and the Hawasina Complex – Searle, M.P. (1988), Geological Society Special Publication
– Sedimentary and Tectonic Evolution of the Hawasina Basin, Oman Mountains – Béchennec, F., Le Métour, J., Rabu, D., & Villey, M. (1992), Bulletin of the Geological Society of France.
– Permian to Triassic Stratigraphy of the Wahrah Formation, Oman Mountains – Béchennec, F., Roger, J., Le Métour, J., & Wyns, R. (1993), Ministry of Petroleum and Minerals, Geological Map Explanatory Notes.
– Geochemistry and Origin of Marine Manganese Deposits – Roy, S. (1981), Economic Geology.
– Hydrogenous and Diagenetic Manganese Deposits in Deep-Marine Environments – Nicholson, K. (1992), Ore Geology Reviews.
– Manganese Mineralization in Ophiolite-Related Sedimentary Sequences, Oman – Al-Rawas, A.D. & Vale, J.F. (1998), Journal of African Earth Sciences.