Mineralogy, geochemistry and genesis of the Zaghdarreh Mn deposit, SW Kerman province, the southern Sanandaj-Sirjan zone

Introduction

Manganese deposits occur in various tectonic settings from mid-ocean ridges to plagic environments and continental margins (e.g. Evensen et al., 1978). Depending on the difference in the source of manganese supply, these deposits are divided into hydrothermal, hydrogenous and diagenetic (Oksuz, 2011; Polgari et al., 2012; Schmidt et al., 2014). The change in origin causes distinct geochemical differences in these deposits.

The studied area in the southwest of Kerman province (Fig. 1) hosts manganese deposits. This area is a part of the South Sanandaj-Sirjan zone, where different rock sequences formed in association with the subduction of the Neotethys oceanic crust beneath the central Iranian micro-plate occur. The aim of this study is to investigate the mineralogical and geochemical properties of the Zaghadareh manganese deposit in order to identify the origin and genesis of this deposit, which has not been studied by researchers so far. Also, the formation of this deposit is compared with some other deposits from the Sanandaj-Sirjan zone to gain a better understanding of the conditions governing the mineralization environment of the Neotethys Ocean during the Mesozoic Era.

Geology of the study area

The study area belongs to the southern part of the Sanandaj-Sirjan zone (Fig 1a). As the other parts of the this zone, this area is composed of metamorphic, igneous and sedimentary rocks. The main lithology of the area includes a color mélange with Cretaceous age (Fig. 1b). These rocks cover the southern and southeastern parts of the region and include some parts of the ophiolitic sequence, volcanic rocks, radiolarian cherts and pelagic limestones. In the northern parts, basaltic and andesitic rocks of Mesozoic-Cenozoic age occur.

The Zaghdarreh deposit occurs at the boundary between the ophiolitic color mélange and pelagic limestones (Fig. 1b). In this area, radiolarian cherts host the Mn-ore mineralization (Fig. 2). The cherts occur as interlayers with pelagic limestones in the region (Fig. 2a, b and c). The thickness of the Mn-ore layers in different parts of the deposit varies from 1 cm to more than 1 m. The occurrence of iron oxides and hydroxides (hematite and limonite) is visible in some parts of the deposit (Fig. 2d), which is evidence of the presence of Fe along with Mn in this deposit. Faulting has caused crushing and uplift in limestones and radiolarite cherts. Based on field surveys, these faults have a NW-SE trend and a 60º dip to the SW.

Mineralogy

XRD Analysis

The results of X-ray diffraction analysis of two samples from the Zaghdarreh deposit are presented in Fig. 4. The analyses show that quartz, calcite, braunite and hematite are the main minerals and fluorapatite, dolomite, clay minerals and amphibole are the minor ones. Pyrolusite is the main mineral in one sample and is considered as a minor mineral in the other sample, indicating that the conditions of formation of the deposit were not uniform in all parts of it.

Petrography

The samples shows a uniform paragenesis in the polished and thin-polished. Pyrolusite is the main manganese-bearing mineral, which is accompanied by braunite. Pyrolusite is generally occur as fine-grained and associated with fine-grained quartz (Fig. 4a). Psilomelane and todorokite were identified in some samples, occurring as thin veins or filling the empty spaces between the other grains. Quartz, hematite and calcite are other minerals in these samples.

The main textures in the studied samples include colloform, micronodular, synchronous fine-grained and stockwork veins (Fig. 4). The colloform texture (Fig. 4b), which is abundant in the samples, indicates formation in a deep marine environment where manganese oxides and hydroxides precipitated slowly. The micronodular texture (Fig. 4c, d, and e) consists of spherical or elliptical aggregates of manganese minerals in a matrix of fine-grained silica and is indicative of slow depositional environments. The cores of the nodules are made of braunite, which is covered by a coating of pyrolusite (Fig. 4d, e). The synchronous fine-grained texture (Fig. 4f) indicates the coeval deposition of silica and manganese minerals, which is often associated with early diagenetic conditions. The presence of thin, interconnected stockwork veins filled with manganese and quartz minerals (Fig. 4g) indicates the intrusion of hydrothermal fluids into the host rock. Another characteristic of these samples is the alternation of manganese-bearing and silica layers, which demonstrate variations in depositional conditions and the influence of hydrothermal fluids on the mineralization process (Fig. 4h and i).

Discussion

Geochemistry

The results of the whole rock geochemical analyses of the Zaghdarreh deposit samples are represented in Table 1. SiO2 is the most abundant oxide and its amount range from 46 to 63 wt% in the samples. MnO range from 11 to 19 wt%. The other important oxides include CaO (8-14 wt%), FeOt (8 to 11 wt%) and Al2O3 (0.8 to 1.4 wt%). The correlation diagrams between Mn and Fe, Co, Cu, Ni, Zn and V are presented in Fig. 6. The results show that manganese has the highest correlation with Fe (R = 0.62) and the lowest correlation with Ni (R = -0.63). The SiO2 vs. Al2O3 variation diagram (Toth, 1980) (Fig. 6a) shows that the Zaghdarreh deposit formed from hydrothermal fluids. Also correlations between Al2O3, TiO2 and MgO can be attributed to detrital materials from the adjacent island arcs (Zarrasvandi et al., 2023) and submarine volcanic activities.

Genesis

Manganese oxides are formed by a variety of processes and in a variety of geological environments. In general, manganese deposits are divided into three main categories: sedimentary (sedimentary-diagenetic or stratiform), hydrothermal, hydrogenic, and supergene (e.g. Roy, 1992; Kuleshov 2011). Maynard (2010) also has divided manganese deposits into two types: primary (resulting from hydrothermal, diagenetic, and hydrogenic activities) and secondary (resulting from supergene processes). Hydrothermal deposits are formed by the deposition of metal-rich hydrothermal fluids near oceanic ridges, seamounts, arc islands, and around submarine hot springs, and hydrogenic (deposition from seawater) and primary diagenetic (deposition from pore water) processes produce Fe-Mn sedimentary nodules in the seas (e.g. Roy, 1992).

Petrographic studies can be useful in determining the origin and genesis of manganese deposits. Deposits close to the source often contain hematite and quartz formed by hydrothermal fluids exhumed from the oceanic crust. Deposits far from the source are associated with jasperite and occur at a distance from mid-ocean ridges (Oksuz, 2011; Brusntinyn and Zhukov, 2012). Hydrothermal deposits usually have coarse-grained crystalline textures with hematite and quartz veins, but hydrogenous deposits have thin-layered textures and ferromanganese crusts (Oksuz, 2011). Also, the presence of stockwork veins and manganese-rich nodules indicates the influence of hydrothermal processes (Maynard, 2010; Oksuz, 2011). The textural and mineralogical characteristics of the studied samples are consistent with hydrothermal deposits.

The Fe2O3-SiO2-MnO ternary diagram (Karakuş et al., 2010) also confirms this (Fig. 6b). In addition, the Mn/Fe ratio is also an important factor in investigating the origin of Mn deposits. The Mn/Fe ratio is less than 1 in lacustrine deposits, 1 in hydrogenous deposits, and higher than 10 in hydrothermal deposits (Nicholson et al., 1997). This ratio range 1.17 to 1.9 (Table 1) for the studied samples which is between those of hydrothermal and hydrogenous deposits. Also, TiO2 contents are higher than 1 for hydrogeous Fe-Mn deposits and lower than 1 for the hydrothermal ones (Ahmadi et al., 2019). The TiO2 values of the Zaghdarreh deposit samples (0.03 to 0.06 wt%, Table 1) are consistent with a hydrothermal origin. The values of some minor elements such as Ni, Cu, V, Co, and Zn are also useful in determining the origin of Mn deposits. Hydrothermal manganese deposits have relatively high concentrations of Co, Ni, and Cu compared to hydrothermal deposits located along mid-ocean ridges (Toth, 1980; Usui and Someya, 1997). Co is closely associated with Mn oxides and its abundance decreases on average from hydrogeous to diagenetic and hydrothermal deposits (Sabatino et al., 2011). It should be noted that manganese and cobalt are oxidized together in the same catalytic pathway, and as a result, microbial processes can cause cobalt enrichment in manganese deposits (Moffett and Ho, 1996; Polgári et al., 2012). The Co/Zn ratio is important for separating hydrothermal and hydrogeous deposits. The average ratios are 0.15 and 2.5 in hydrothermal and hydrogeous deposits, respectively (Toth, 1983). The Co/Zn ratio for the Zaghdarreh deposit samples range from 0.02 to 0.5, which is consistent with hydrothermal deposits.

Trace element diagrams presented for determining the origin of manganese deposits are shown in Fig. 7. The ternary diagram Fe-Mn-(Ni+Co+Cu)*10 (Fig. 7a) indicates a hydrothermal origin for studied the samples. In the Zn-Co-Ni diagram, the samples are mainly located in the hydrothermal deposits, however some samples lie near the hydrogenous deposits (Fig. 7b). In the Zn+Cu+Pb+V vs. Ni+Co diagram (Fig. 7c), the samples fall between the hydrothermal and hydrothermal zones.

Environment of occurrence

Manganese deposits derived from hydrothermal fluids can form in or near mid-ocean ridges and to a lesser extent in arc islands (Roy, 1992). Sedimentary deposits are formed by the slow deposition of Fe-Mn crusts in deep marine waters or by bacterial processes (Toth, 1980; Usui and Someya, 1997; Jach and Dudek, 2005). Deposits close to the mid-ocean ridge have higher iron contents than others and, in turn, lower TiO2 contents (e.g., Murray, 1994; Nicholson et al., 1997; Ahmadi et al., 2019). In the plot of Al2O3/Al2O3+Fe2O3 vs. Fe2O3/TiO2 (Fig. 8), samples from the Zaghdarreh deposit are located near the mid-ocean ridge, where MnO was deposited by the ridge associated hydrothermal fluids. The high Fe contents and presence of hematite in the samples studied (Fig. 4) are consistent with this.

Comparison with the other Mn deposits from

The Zagros and Sanandaj-sirja zones hosts many Mn-deposits, mainly formed as a result of hydrothermal processes (Zarasvandi et al, 2016b). Comparing the geochemistry of samples from the Zaghdarreh Mn deposit with other deposits can confirm the results and interpretations related to its genesis. In most of the graphs presented in Figs. 7 and 8, geochemistry of the Zaghdarreh deposit are in agreement with the other ones, although there are also some differences. The Fig. 8a shows that, unlike other deposits, diagenetic and bacterial processes did not affect the Zaghdarreh deposit. Also, this deposit, like the Nasirabad deposit (Neyriz area), also exhibits some geochemical characteristics of water-borne deposits, indicating their genesis. Also, the Nasirabad deposit was formed in a pelagic environment and at a distance from the oceanic ridge.

Conclusion

The Zaghdarreh Mn deposit occurs in the southern part of the Sanandaj-Sirjan zone and in the color mélange radiolarite cherts of the Neotethys ophiolites. Petrographic studies and X-ray diffraction analyses show that the deposit mineral include pyrolusite, braunite, todorokite, hematite, quartz, and calcite. The main textures are colloform, micronodular, disseminate, and stockwork veins formed by hydrothermal processes. The geochemical properties of samples, such as high of Si, Fe, Zn and low of Ni, Co, and Cu contents is consistent with hydrothermal fluid originated Mn-deposits. Also, the ratios between Al2O3, Fe2O3, and TiO2 indicates that they were formed near a mid-ocean ridge by associated hydrothermal fluids. Submarine volcanic activities and detrital materials from the adjacent island arcs changed the geochemistry of the deposit and elevated Al, Ti and Mg contents. The results obtained for the Zaghdarreh deposit are consistent with other hydrothermal Mn deposits of the Zagros orogeny. Also, differences in position of the adjacent Nasirabad deposit (Neyriz region) and Zaghdarreh deposit indicates that hydrothermal fluids caused both the distal and proximal Mn mineralization in this part of the Neo-Tethys Ocean.