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Volume 31 Issue 5
Oct.  2020
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Khan Muhammad Ayoub, Kakar Muhammad Ishaq, Thomas Ulrich, Liaqat Ali, Andrew C. Kerr, Khalid Mahmood, Siddiqui Rehanul Haq. Genesis of Manganese Deposits in the Ali Khanzai Block of the Zhob Ophiolite, Pakistan: Inferences from Geochemistry and Mineralogy. Journal of Earth Science, 2020, 31(5): 884-895. doi: 10.1007/s12583-020-1337-3
Citation: Khan Muhammad Ayoub, Kakar Muhammad Ishaq, Thomas Ulrich, Liaqat Ali, Andrew C. Kerr, Khalid Mahmood, Siddiqui Rehanul Haq. Genesis of Manganese Deposits in the Ali Khanzai Block of the Zhob Ophiolite, Pakistan: Inferences from Geochemistry and Mineralogy. Journal of Earth Science, 2020, 31(5): 884-895. doi: 10.1007/s12583-020-1337-3

Genesis of Manganese Deposits in the Ali Khanzai Block of the Zhob Ophiolite, Pakistan: Inferences from Geochemistry and Mineralogy

doi: 10.1007/s12583-020-1337-3
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  • The Zhob ophiolite comprises the Naweoba,Omzha and Ali Khanzai blocks,which are surrounded by the sediments of the Alozai Group and Loralai Formation. The Ali Khanzai Block contains metamorphic,ultramafic,gabbroic,volcanic and volcaniclastic rocks with associated chert. The Zhob manganese deposits found in the Ali Khanzai Block,occur in banded,lenticular and massive forms within red to brown coloured metachert. Braunite and pyrolusite are the main constituent manganese-bearing minerals with minor hausmannite,hematite and barite while quartz is the major gangue mineral with some carbonate minerals. Geochemical evidence from the major oxides indicates that the manganese mineralization and associated metachert at Zhob were formed by hydrothermal activity with little contribution from contemporaneous volcanic materials and this is confirmed by high Fe/Mn and low Co/Zn ratios and trace element patterns. These deposits formed along with seafloor spreading centres and were later obducted as part of Ali Khanzai Block of Zhob ophiolite.
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Genesis of Manganese Deposits in the Ali Khanzai Block of the Zhob Ophiolite, Pakistan: Inferences from Geochemistry and Mineralogy

doi: 10.1007/s12583-020-1337-3

Abstract: The Zhob ophiolite comprises the Naweoba,Omzha and Ali Khanzai blocks,which are surrounded by the sediments of the Alozai Group and Loralai Formation. The Ali Khanzai Block contains metamorphic,ultramafic,gabbroic,volcanic and volcaniclastic rocks with associated chert. The Zhob manganese deposits found in the Ali Khanzai Block,occur in banded,lenticular and massive forms within red to brown coloured metachert. Braunite and pyrolusite are the main constituent manganese-bearing minerals with minor hausmannite,hematite and barite while quartz is the major gangue mineral with some carbonate minerals. Geochemical evidence from the major oxides indicates that the manganese mineralization and associated metachert at Zhob were formed by hydrothermal activity with little contribution from contemporaneous volcanic materials and this is confirmed by high Fe/Mn and low Co/Zn ratios and trace element patterns. These deposits formed along with seafloor spreading centres and were later obducted as part of Ali Khanzai Block of Zhob ophiolite.

Khan Muhammad Ayoub, Kakar Muhammad Ishaq, Thomas Ulrich, Liaqat Ali, Andrew C. Kerr, Khalid Mahmood, Siddiqui Rehanul Haq. Genesis of Manganese Deposits in the Ali Khanzai Block of the Zhob Ophiolite, Pakistan: Inferences from Geochemistry and Mineralogy. Journal of Earth Science, 2020, 31(5): 884-895. doi: 10.1007/s12583-020-1337-3
Citation: Khan Muhammad Ayoub, Kakar Muhammad Ishaq, Thomas Ulrich, Liaqat Ali, Andrew C. Kerr, Khalid Mahmood, Siddiqui Rehanul Haq. Genesis of Manganese Deposits in the Ali Khanzai Block of the Zhob Ophiolite, Pakistan: Inferences from Geochemistry and Mineralogy. Journal of Earth Science, 2020, 31(5): 884-895. doi: 10.1007/s12583-020-1337-3
  • Manganese deposits occur in a range of tectonic settings, with ore deposits being associated with ophiolites as well as continental sedimentary sequences. These deposits can be divided into three types based on mineralogy, geochemical composition and tectonic setting: (1) hydrogenous, (2) hydrothermal, and (3) diagenetic/biogenetic-bacterial deposits (Polgári et al., 2012; Öksüz, 2011; Hein et al., 1997; Bolton et al., 1988). Hydrogenous deposits occur as ferromanganese crusts which are slowly precipitated (2-10 mm/Ma) from seawater on the seafloor usually via microbial mediation and are usually depleted in Mn minerals and rich in amorphous iron compounds (Öksüz, 2011; Jach and Dudek, 2005; Usui and Someya, 1997; Toth, 1980). Hydrogenous deposits are characterized by a low Mn/Fe ratio (~1) and high trace element concentrations, particularly Ni and Cu (Hein et al., 1997, 1996; Usui and Someya, 1997; Usui and Nishimura, 1992; Ingram et al., 1990; Toth, 1980).

    In contrast, hydrothermal deposits precipitate directly from low-temperature hydrothermal solutions (Hein et al., 1997; Ingram et al., 1990) and are commonly stratabound and laminated or occur as irregular bodies and epithermal veins. They are found in modern and ancient environments in marine settings adjacent to either spreading centers or intraplate seamounts, or subduction-related island arc settings (Roy, 1997, 1992). Diagenetic or bacterial deposits are mainly found as nodules and precipitate from hydrothermal solutions or pore waters. These manganese deposits are generally related to oxidation of organic matter and formation of Mn-carbonate mineralization (Polgári et al., 2012; Öksüz, 2011; Marta Polgári, 1991), however, they can also form by a combination of these processes. Unlike hydrogenous deposits, hydrothermal and diagenetic deposits have high Mn/Fe ratios and low trace metal contents (Hein et al., 1996, 1994). Although diagenetic and hydrothermal deposits are similar in some ways, these two types can be distinguished on the basis of their morphology, tectonic setting and growth rates (Kuhn et al., 1998).

    In Pakistan, manganese deposits are reported mainly from North Waziristan, Lasbela, Kurram, Bajaur, Mohmand, and Mansehra. All the manganese deposits are associated with ophiolitic rocks except that of Mansehra, which is hosted by a continental sedimentary sequence (Asif, 2014; Shah and Moon, 2004; Shah and Khan, 1999; Naseem et al., 1997; Ahmed, 1992). Among these deposits, the North Waziristan (45 wt.% MnO), Lasbela (40 wt.% MnO) and Bajaur manganese (55.39 wt.% MnO) deposits are economically significant due to their grade and accessibility (Asif, 2014; Shah and Khan, 1999; Naseem et al., 1997).

    The Zhob ophiolite, in the Zhob Valley, is located north-northeast of the Muslim Bagh ophiolite (Fig. 1b) and hosts several types of economic deposits including manganese ores (Ahmed, 1975, 1969; Bilgrami, 1964a, b ; Jones, 1961; Heron and Crookshank, 1954). The manganese deposits in this study are located in the Ali Khanzai Block of the ophiolite which is situated 16 km south of Zhob City (Fig. 1c). This Zhob manganese occurrence was first reported by Ahmed (1975), who described the manganese occurring in the form of veins and layers, associated with basalt and chert. The Zhob manganese is being commercially mined since the deposits contain an average 42.5 wt.% MnO. In the current study, we present the first petrographic descriptions, mineralogy, and geochemistry of the Zhob manganese ores found associated with the Ali Khanzai Block of the Zhob ophiolite and propose a model for its paragenesis.

    Figure 1.  Geological map showing (a) the study area, (b) the Western Sulaiman belt showing occurrences of the Zhob Valley ophiolites and its surrounding rock units and (c) map of Ali Khanzai Block of the Zhob ophiolite with the occurrences of manganese deposits (modified after Rehman, 2019; Siddiqi et al., 2016; Kakar et al., 2013; Bannert et al., 1992; HSC, 1960). BO. Bela ophiolite; KO. Khanozai ophiolite; MO. Muslim Bagh ophiolite; WO. Waziristan ophiolite; DO. Dargai ophiolite; CO. Chilas ophiolite; CF. Chaman fault; PS. Penjwai suture; HF. Herat ophiolite; KB. Karakorum Block; MMT. main mantle thrust; MBT. main boundary thrust.

  • The Zhob ophiolite is exposed in northwestern Pakistan and is part of the suture zone marked by the Bela, Muslim Bagh and Waziristan ophiolite complexes (Şengor, 1987; Fig. 1a). In the south, the rocks of the suture zone are thrusted over the calcareous zone, which is comprised of a marine succession (Triassic-Jurassic) of the Sulaiman fold-thrust belt. In contrast, in the north, the rocks of the suture zone are overlain by the Pishin-Katawaz flysch succession, comprising the Nisai (Eocene) and Khojak formations (Oligocene) (Fig. 1b). The rock units in immediate contact with the Zhob ophiolite include the Jurassic Alozai Group (Durrani, 2012) at the base, and the Nisai Formation (Eocene-Oligocene; Bukhari et al., 2016) at the top.

    The Zhob ophiolite consists of three separate blocks; the Naweoba, Omzha and Ali Khanzai blocks. These blocks contain metamorphic rock, serpentinized peridotite, mafic oceanic crustal rocks, basaltic volcanic rocks and sedimentary rocks such as chert and mudstone. These blocks contain a few narrow elongated bodies of diorite parallel to peridotite and volcanic rocks.

  • The Ali Khanzai Block is approximately 12 km long and up to 3 km wide, and is located about 16 km to the south of Zhob City (Fig. 1c). The Ali Khanzai Block consists of metamorphic rocks, a mantle section, mafic oceanic crustal rocks and a basalt-chert unit (Fig. 1c; Rehman, 2019). The contacts between various rock units of the Ali Khanzai Block are sharp and sometimes irregular because it is tectonically disturbed (Rehman, 2019). Sub-ophiolitic metamorphic rocks found beneath the mantle section are mostly amphibolites, greenschists and mica schist. The mantle section is fragmented and distributed across the base of this block and is composed of dunite and harzburgite which are highly serpentinized. The crustal section contains mainly layered and massive gabbros and some doleritic dykes which are quite deformed and tectonically disturbed. The volcanic-chert-unit is the largest unit of Ali Khanzai Block and comprises volcanic rocks, volcaniclastic rocks and metachert. Basalt is a significant volcanic rock and occurs commonly in pillow and sheet forms. These units cannot be stratigraphically correlated across the Ali Khanzai Block due to intense folding, faulting and the exotic nature of the rocks in the block.

    Chert in the volcanic-chert-unit is highly folded, fractured and metamorphosed, and hosts manganese mineralization. These metacherts are fine-grained and crystalline and are thin to medium-bedded. They have gradational contacts with underlying volcanic rocks and are interbedded with volcanic rocks in some localities. The metacherts of the Ali Khanzai Block are cryptocrystalline and comprise fine-grained quartz along with minor clay minerals and mainly consist of non-detrital free silica. Occasionally the metachert contains radiolarian tests that are filled with detrital quartz and clay minerals.

    Manganese mineralization in the Ali Khanzai Block is identified by its black colour in the field (Fig. 2). Manganese deposits are scattered (50 to 100 m apart) and occur as small deposits associated with reddish-to-brown metachert of the basalt-chert-unit of the Ali Khanzai Block (Fig. 1c). These deposits are collectively described here as the Zhob manganese deposits and are currently being explored and mined, however the actual size of these manganese deposits remains unknown.

    Figure 2.  Field photographs showing (a) lenticular shape of manganese deposit surrounded by metachert, (b) layers of manganese surrounded by metacherts, (c) Mn-rich phases forming alternate bands with host metachert, and (d) massive manganese ore. Mn. Manganese; Ch. metachert.

    The ore deposits in the investigated area are scattered and irregular but conformable with their host rocks. They lie above the volcanic rock-chert unit and are associated with reddish-to-brown metachert. The ore deposits occur at a variable distance from the boundaries between metachert and volcanic rocks. The thickness of the ore bodies varies from 1 to 200 cm, they are separated by layers of reddish-to-brown metachert. The general trend of the mineralization is SW-NE and is consistent with the bedding of the host rock. Manganese ores and host rocks are folded, faulted and fragmented, indicating that considerable deformation occurred after the formation of the deposits. These ore deposits occur in layered, banded, lenticular and massive forms (Fig. 2). The lenticular and layered ore bodies are surrounded by metacherts (Figs. 2a, 2b). The banded form of manganese, containing Mn-rich phases, are thin to medium in size and are clearly visible and alternate with bands of the host the metachert (Fig. 2c). Massive manganese is very thick and irregular (Fig. 2d) and has sharp contacts with the host metacherts. The pockets of manganese ore are enveloped by the surrounding host rock and are disconnected and rounded to subrounded.

  • Around 60 samples of massive, layered and banded ores and metachert (host rocks) were collected in the field. These samples of ores and metachert were studied petrographically. The least-altered 13 samples of ores and 7 samples of metachert were selected for major and trace element analysis. Fe2O3, MnO, MgO, CaO, Na2O, TiO2 and K2O were determined by atomic absorption spectrometry (Perkin Elmer 3300, equipped with graphite furnace), while SiO2, P2O5 and Al2O3 were analysed by a Pye Unicam UV/visible spectrophotometer (detailed methodology and standards are in supplementary material Appendix A). The trace elements such as Cu, Pb, Zn, Ni, Cr, and Co were also determined by the atomic absorption spectrometry (detailed methodology and standards are in supplementary material Appendix A). The loss on ignition (L.O.I.) was determined by heating the samples at 1 000 ℃ in a muffle furnace.

    Petrographical and mineralogical studies were carried out using both polarized and reflected light microscopy of polished thin sections. All of the above studies were carried out at the Geochemistry Laboratory of the National Centre of Excellence in Geology, University of Peshawar, Pakistan.

    The mineralogical studies of ore and gangue minerals are based on a combination of X-ray powder diffraction analysis (XRD Bruker D-8 Advance with Cu anode operating at a generator voltage of 40 kV) and ore microscopy at Center of Excellence in Mineralogy, University of Balochistan, Quetta, Pakistan.

    The qualitative phase composition of the ore and gangue minerals were analysed by Micro X-ray fluorescence spectroscopy (μ-XRF) using polished blocks of the representative samples. The X-ray fluorescence analysis was conducted on a Bruker M4 Tornado X-ray machine at the Institute of Geoscience at Aarhus University, Denmark. The source used was a single rhodium target X-ray tube. All samples were scanned using the

    same parameters. The spot size was 20 μm and the distance between measured points was 25 μm. Two spectrometers were operated at a range of 40 keV/130 kcps. The X-ray tube was run at 50 kV electron voltage and the current was set to 600 μA. All analyses were carried out in a vacuum.

    Quantitative mineral chemistry of manganese minerals was analyzed at the Department of Geoscience, Aarhus University, Denmark, using a JEOL JXA-8600 electron microprobe equipped with EDS (energy dispersive x-ray spectroscopy) with an acceleration voltage set to 15 KeV and a current beam of 10 nA. The beam diameter was 5 μm with a counting time of 20 seconds. For the calibration of the microprobe natural mineral standards were used. The analysis quality was monitored with two in-house standards; clinopyroxene BB1 and labradorite (Oslo plagioclase). These standards were measured intermittently and compared with the known composition to check the quality of the analysis. Carbon-coated polished thin sections were used for this analysis.

  • X-Ray diffraction has determined that braunite and pyrolusite are the most dominant Mn-minerals with associated minor hausmannite, hematite and barite while quartz is most the common gangue mineral. Braunite is the principle manganese mineral and it ranges in shape from microcrystalline to coarse-grained and anhedral to subhedral. Microcrystalline braunite is intermixed with microcrystalline to cryptocrystalline quartz while the coarse-grained braunite grains are interlocked with each other and are occasionally cross-cut by veinlets of quartz (Fig. 3a). Coarse-grained braunite usually occurs as a homogeneous mass but is also found as a rim around pyrolusite (Fig. 3b). Pyrolusite occurs as massive anhedral to subhedral grains as well as disseminated grains in Mn minerals and gangue minerals and it can also fill cracks and spaces between minerals. It is also found in micro-veins formed by filling of fractures and cracks in braunite and gangue minerals particularly cryptocrystalline quartz (Fig. 3c). Massive pyrolusite grains show needle, flake and fibrous shapes (Fig. 3d). Breccia-like textures between pyrolusite and quartz are also common.

    Figure 3.  Photomicrographs and micro-XRF scans showing (a) braunite grains cross-cut by veinlets of quartz, (b) braunite enclosing pyrolusite forming a relic texture, (c) pyrolusite micro-veins formed by filling of fractures and cracks in braunite and gangue minerals, (d) massive pyrolusite grains showing fibrous sections, (e) quartz in veinlets and well-developed crystals, filling vugs or interstitial spaces of manganese and iron minerals and (f) barite filling cavities in manganese-rich phases. Note Figs (b) and (f) are micro-XRF images. Bra. Braunite; Pyr. pyrolusite; Hem. hematite; Qz. quartz.

    Hematite is associated with Mn minerals and occurs as a fine-grained matrix (Fig. 3b) enclosing Mn minerals and forms veins along with gangue minerals. Hematite also occurs as finely disseminated grains together with microcrystalline Mn minerals and cryptocrystalline quartz. Gangue minerals are predominantly quartz and carbonate with minor barite. Quartz is usually crushed due to deformation and is cryptocrystalline to microcrystalline. It occurs as veinlets and well-developed crystals, filling vugs or interstitial spaces of manganese and iron minerals (Fig. 3e). Carbonate is mainly calcite that commonly forms irregular or sometimes oval patches and fills the veins and cracks within manganese and iron oxide matrix. Barite is very rare and fills cavities in manganese-rich phases (Fig. 3f).

    Hence, three stages of mineralization are recognized in Zhob manganese mineralization including; early, middle, and late stages. The early stage is represented by massive pyrolusite minerals, while the middle stage is characterized by braunite with replacement textures and enclosing pyrolusite. Late-stage supergene mineralization comprising pyrolusite, hematite and barite minerals are found infilling fractures and fissures (Fig. 4).

    Figure 4.  Ore paragenetic succession of the Zhob manganese deposits.

  • Compositions of the main minerals identified in the Zhob manganese ore are given in Table 1. Braunite is one of the most significant manganese ore minerals and it normally contains up to 10 wt.% SiO2 (Roy, 1981). However, in silicate-rich braunite SiO2 contents can be up to 40%, depending on temperature and partial oxygen pressure during formation (Naseem, 1996; Maun, 1959). The studied ore from the Ali Khanzai Block contains both normal braunite and silicate-rich braunite. The average composition of normal braunite is SiO2 (13.0 wt.%), Al2O3 (1.5 wt.%), Fe2O3 (1.1 wt.%), Mn2O3 (82.9 wt.%), MgO (0.3 wt.%) and CaO (1.45 wt.%) while silicate-rich braunite contains SiO2 (28.4 wt.%), Al2O3 (1.2 wt.%) and MnO (67.5 wt.%).

    Composition Normal braunite Si-rich braunite Pyrolusite Hematite Barite Detection Limit
    SiO2 13.02 28.41 bdl bdl CaO 2.98 SiO3 0.06
    Al2O3 1.46 1.18 dbl bdl SrO 0.85 Al2O3 0.05
    Fe2O3 1.09 0.85 0.90 89.27 BaO 69.98 Fe2O3 0.10
    Mn2O3 82.19 67.52 - 9.11 MgO bdl Mn2O3 0.08
    MgO 0.29 0.43 0.00 bdl FeO 0.87 MgO 0.07
    CaO 1.45 0.24 bdl bdl MnO 0.12 CaO 0.05
    Na2O 0.20 0.27 bdl bdl SO3 25.05 NaO2 0.04
    K2O bdl bdl bdl bdl Total 99.85 K2O 0.05
    Cr2O3 bdl bdl bdl bdl Cr2O3 0.09
    MnO2 - - 98.31 - TiO2 0.04
    TiO2 - - - 1.62 SrO 0.03
    Total 99.75 98.93 99.24 100 SO 0.08
    Composition Ions calculation based on 12O Based on 2O Based on 3O Based on 4O
    Si 1.54 2.88 0.00 0.00 Ca 0.00
    Al 0.20 0.14 0.00 0.00 Sr 0.01
    Fe3+ 0.10 0.06 0.02 1.89 Ba 1.03
    Mn 8.21 5.80 - - Mg 0.00
    Mg 0.05 0.07 0.00 0.00 Fe 0.00
    Ca 0.18 0.03 0.00 0.00 Mn 0.00
    Na 0.05 0.05 0.00 0.00 S 0.99
    K 0.00 0.00 0.00 0.00
    Cr 0.00 0.00 0.00 -
    Mn4+ - - 1.97 -
    Mn3+ - - - 0.10
    Ti 0.04

    Table 1.  Mineral chemistry of manganese minerals of Zhob manganese deposits. Bdl. Below detection limit; -. no test results

    Pyrolusite is also a dominant mineral in the Zhob deposits with a typical composition of 98.3 wt.% MnO2 and 0.90 wt.% Fe2O3. The low totals indicate the presence of some absorbed water in the pyrolusite (Gutzmer and Beukes, 1997; Frenzel, 1980).

    Hematite is an accessory mineral and its composition averages 89.7 wt.% Fe2O3, 9.1 wt.% Mn2O3 and 1.6 wt.% TiO2. However, the high contents of Mn2O3 suggest that hematite may have incorporated Mn into its structure from the coexisting Mn-phases during metamorphism (Shah and Moon, 2004).

    The few grains of barite that have been observed have an average composition of 70.0 wt.% BaO, 27.1 wt.% SO3 and 0.9 wt.% SrO. Small amounts of FeO, CaO and MnO in the barite composition may have been substituted for BaO.

  • The whole-rock composition of manganese ores and host rocks of the studied deposits are given in Tables 2 and 3, respectively. Manganese ore samples have been divided into Mn-rich and Fe-rich manganese ores. Average major elements contents (wt.%) of Mn-rich and Fe-rich manganese ore samples and their host rocks are compared in Fig. 5a. Mn-rich ores are higher in MnO and lower in Al2O3, Fe2O3 and SiO2 than Fe-rich ores. Host rocks are much higher in SiO2 than manganese ores while almost similar in the other major elements to the manganese ores.

    Mn-rich Fe-rich Detection limit
    Sample Z97b ZA101 ZA102 Z103a ZA104a ZA104b Z105b Z106a Z97a Z105a Z105M Z108 ZA108b
    MnO 49.21 48.18 33.07 54.80 51.36 51.02 51.46 46.08 25.47 36.71 33.23 32.01 39.38 0.005
    Fe2O3 0.31 4.36 3.19 1.16 1.76 0.13 0.83 0.09 16.83 8.95 13.98 10.7 12.5 0.005
    MgO 0.3 0.44 0.09 0.11 0.26 0.12 0.08 0.16 0.15 0.05 0.14 0.24 0.19 0.005
    P2O5 0.01 0.05 0.6 0.1 0.06 0.01 0.02 0.04 0.14 0.16 0.36 0.07 0.05 0.005
    Na2O 0.07 0.18 0.1 2.72 0.09 0.06 0.09 0.08 0.13 0.06 0.09 0.1 0.14 0.005
    Al2O3 0.63 0.16 0.16 0.56 0.33 0.8 1.08 0.71 0.37 0.73 3.21 1.01 0.63 0.005
    TiO2 0.53 0.07 0.1 0.08 0.12 0.07 0.91 0.11 0.07 0.21 0.97 0.84 0.34 0.005
    CaO 0.06 1.44 0.05 0.52 1.81 0.26 bdl 1.44 0.02 0.01 1.39 0.35 1.59 0.005
    K2O 1.38 1.72 1.23 1.83 0.85 1.08 1.23 1.31 1.08 1.08 1.23 1.38 1.08 0.005
    SiO2 43.21 36.5 55.11 32.37 39.19 41.84 41.01 47.44 49.6 51.2 43.95 49.68 42.05 0.005
    LOI 4.35 6.20 5.30 5.50 3.30 4.40 3.50 2.55 5.50 0.90 3.15 3.80 1.20
    Total 100.06 99.30 99.00 99.75 99.13 99.79 100.21 100.01 99.36 100.06 101.70 100.18 99.15
    Mn 38.11 37.31 25.61 42.43 39.77 39.51 39.85 35.68 19.72 28.43 25.73 24.79 30.50
    Fe 0.22 3.05 2.23 0.81 1.23 0.09 0.58 0.06 11.77 6.26 9.78 7.48 8.74
    Cu 43 17.2 5.2 39 8 45 3.6 25 36.4 10 23.2 3.2 7 0.05
    Cr bdl 29.8 bdl 8.4 3.2 9.6 8 3 41.2 16 4 18.8 26 0.05
    Pb 305 706 558.2 29 410 212.4 687 484.4 378 687.4 647 546.2 390.2 0.05
    Co 85.2 21 105 179 168.2 185 124 90 80.2 140 231 131 90.4 0.05
    Ni 313.4 132.2 314.6 198.6 379.4 344.4 147 219.6 333.6 403 437.4 287 239.8 0.05
    Zn 175 163.2 250 305 184.2 240 320 145 161 394.2 408.2 141 115.4 0.05
    Mn/Fe 176 12.24 11.48 52.30 32.31 435 69 567 1.68 4.54 2.63 3.31 3.49
    Co/Zn 0.49 0.13 0.42 0.59 0.91 0.77 0.39 0.62 0.50 0.36 0.56 0.93 0.78
    Co/Ni 0.27 0.16 0.33 0.90 0.44 0.54 0.51 1.59 0.24 0.35 0.53 0.46 0.38
    LOI. Loss of ignition, bdl. below detection limit.

    Table 2.  Whole rock major oxides (wt.%) and trace elements (ppm) geochemical composition of manganese ore of Zhob manganese deposits

    Sample Z97H Z97aH Z104H ZA106 Z111H ZA112 Z113H Detection limit
    MnO 0.01 0.51 0.01 0.01 0.01 0.01 0.01 0.005
    Fe2O3 1.33 1.30 2.34 1.13 2.02 2.13 1.11 0.005
    MgO 0.11 0.08 0.12 0.14 1.42 0.22 0.23 0.005
    P2O5 0.03 0.01 0.34 0.01 0.02 0.01 0.02 0.005
    Na2O 0.08 0.14 0.04 0.05 0.15 0.15 0.14 0.005
    Al2O3 1.58 1.75 1.23 1.54 1.39 3.29 2.91 0.005
    TiO2 0.09 0.08 0.08 0.50 0.07 0.27 0.24 0.005
    CaO 0.62 0.46 4.11 0.06 0.20 1.65 1.52 0.005
    K2O 0.64 0.51 1.23 1.31 0.77 0.93 0.94 0.005
    SiO2 88.43 93.11 84.18 93.83 93.40 91.03 87.63 0.005
    LOI 7.25 1.90 5.90 1.95 0.60 0.95 5.90
    Total 100.17 99.85 99.58 100.53 100.05 100.64 100.65
    Mn 0.01 0.39 0.01 0.01 0.01 0.01 0.01
    Fe 0.93 0.91 1.64 0.79 1.41 1.49 0.78
    Cu 46 21.20 26.20 6.40 5 7 13.00 0.05
    Cr 34.00 16 5.20 12.00 41.40 33.40 15 0.05
    Pb 193 397 302.40 390.40 322.20 316 435.20 0.05
    Co 19.20 26.40 45 24.40 37.20 6 23.40 0.05
    Ni 18.40 88.20 74 83.40 39 6 66.40 0.05
    Zn 30 27 60 38 47 27 40 0.05
    Mn/Fe 0.01 0.43 0.01 0.01 0.00 0.01 0.01
    Co/Zn 0.64 0.98 0.75 0.64 0.79 0.21 0.59
    Co/Ni 1.04 0.30 0.61 0.29 0.95 0.97 0.35

    Table 3.  Whole rock major oxides (wt.%) and trace elements (ppm) geochemical composition of host rocks of Zhob manganese deposits

    Figure 5.  Diagrams showing comparison of (a) major oxides and (b) trace elements in Mn-rich, Fe-rich ores and host rocks.

    Trace element concentrations (ppm) of Mn-rich and Fe-rich manganese ores and their host rocks are compared in Fig. 5b. Fe-rich manganese ores are higher in Pb, Co, Ni, Zn and Cu and more depleted in Cr than the Mn rich ores. Trace element contents in the host rocks are much lower than in the manganese ores except for Cu and Cr which are broadly the same. The samples of Fe and Mn-rich ores will be discussed together in the following section because they have been sampled from the same deposits.

  • The common veining, brecciation, mesh textures and crushed grains of ore and gangue minerals of Zhob manganese deposits reveal that after their formation, the rocks of the studied area may have undergone regional metamorphism (see Gutzmer and Beukes, 1997; Ostwald and Nayak, 1993; Roy et al., 1990; Abs-Wurmbach et al., 1983; Dasgupta and Manickavasagam, 1981; Roy, 1981). The mesh texture, in which pyrolusite is surrounded by braunite, indicates the replacement of pyrolusite by braunite and suggests that braunite was formed due to the reaction between pyrolusite (early formed phase) and iron-oxide (hematite) and subsequent recrystallization and remobilization caused incorporation and readjustment of Mn, Fe, Si, P, Al and Pb from pyrolusite, hematite, quartz, and barite (Shah and Moon, 2004). The occurrence of pyrolusite as relics within braunite confirms this inference. However, the presence of pyrolusite as fracture-filled micro-veins could also be related to supergene enrichment (Shah and Moon, 2004). Braunite is a characteristic mineral of greenschist to granulite facies conditions (Gutzmer and Beukes, 1997; Abs-Wurmbach et al., 1983; Roy, 1981). Hence, the mineralogy and texture of the minerals found in the Zhob manganese ores support the idea that rocks of the area have been locally subjected to greenschist facies metamorphism.

    The major oxides SiO2, Fe2O3, MnO, Al2O3 and TiO2 and trace element contents as well as ratios of major and trace elements are useful in assessing the paragenesis of manganese ore deposits (Zarasvandi et al., 2013; Polgári et al., 2012; Karakuş et al., 2010; Shah and Khan, 1999; Crerar et al., 1982).

    The Si vs. Al binary discrimination diagram proposed by Toth (1980) and Peters (1988), is widely used to assess the hydrothermal, sedimentary or hydrogenous (i.e., from seawater deposition) origins of manganese mineralization (Öksüz, 2011; Karakuş et al., 2010; Mücke et al., 1999; Nicholson, 1992). SiO2 concentration is typically high in hydrothermally-formed sediments as compared with hydrogenous deposits (Karakuş et al., 2010). Almost all the samples from the Zhob deposits plot within the field of hydrothermal deposits similar to the Waziristan (Pakistan) and Dehoo (Iran) manganese deposits (Fig. 6a). In addition to the hydrothermal processes, other processes and components such as diagenesis, clastic materials, and biosilica can increase the Si concentrations (e.g., Polgári et al., 2012; Toth, 1980).

    Figure 6.  (a) Bivariate plot of Si vs. Al (Choi and Hariya, 1992) in which data points of the ore samples fall within the hydrothermal field. (b) Ternary plot of Fe-Mn-(Ni+Co+Cu)×10 (modified after Toth, 1980; Bonatti et al., 1972) with the plotted data points within the hydrothermal field.

    The triangular plot of Mn-Fe-(Co+Ni+Cu)×10 (Hein et al., 1992; Crerar et al., 1982; Toth, 1980; Bonatti et al., 1972) has also been successfully used to help distinguish manganese mineralization of various origins. The Zhob ores again plot in the hydrothermal field (Fig. 6b) almost identical to Waziristan (Pakistan) and Dehoo (Iran) manganese deposits. The Zn-Ni-Co triangular diagram (Choi and Hariya, 1992) also shows the hydrothermal origin of Zhob deposits (Fig. 7a) and illustrates that they are compositionally similar to other hydrothermal manganese deposits like those in Waziristan (Pakistan) and Dehoo (Iran). The Co/Zn vs. Co+Ni+Cu diagram (Toth, 1980), once again reveals the hydrothermal genesis of the Zhob deposits (Fig. 7b).

    Figure 7.  (a) Zn-Co-Ni diagram showing data for the manganese ores of Zhob, with fields for hydrothermal and hydrogenous deposits (after Choi and Hariya, 1992). (b) A plot of Co/Zn vs. Co+Ni+Cu for the manganese ores of Zhob (after Toth, 1980) and (c) TiO2 vs. Al2O3 diagrams of manganese ore samples.

    Mn/Fe ratio is used to delineate the origin of manganese deposits; for example, for lacustrine deposits (Mn/Fe < 1), hydrogenous deposits (Mn/Fe=1), SEDEX deposits (0.1 < Mn/Fe < 10) (Nicholson et al., 1997) while a high Mn/Fe Ratio is characteristic of hydrothermal deposits (Jach and Dudek, 2005). The Mn/Fe ratios of the Zhob manganese deposits range from 1.7 to 561 (mean 106). The available average Mn/Fe ratios of world manganese deposits are as follows: 199 (Waziristan, Shah and Khan, 1999), 97 (Cayirli, Karakuş et al., 2010), 80 (Dehoo, Lotfi, 2017), 2 (Hazara, Shah and Moon, 2004), 880 (Eymir, Öksüz, 2011), 19 (Nasirabad, Zarasvandi et al., 2013), 48 (Buyukmahal, Oskuz and Okuyucu, 2014), 15 (Binkilic, Gultekin, 1998), 19 (Ulukent, Ozturk, 1993) and 133 (Hinode, Choi and Hariya, 1992). The comparison of the world manganese deposits is shown in Table 4. Among these deposits Waziristan, Cayirli and Dahoo are hydrothermal exhalative manganese deposits associated with ophiolites. The Hazara, Nasirabad, Buyukmahal, and Eymir deposits are of hydrothermal- hydrogenous origin while Binkilic, Ulukent, and Hinode deposits are formed in hydrogenous environments. Thus, the range of Mn/Fe ratio (1.7 to 561; avg. 106) of the Zhob manganese ore deposit is consistent with those of hydrothermal exhalative manganese deposits.

    Country Iran Turkey Pakistan Turkey Turkey Japan Pakistan Turkey Iran Turkey Pakistan
    Ore deposit Dehoo (1) Cayirli (2) Wazirstan (3) Ulukent (4) Binkilic (5) Honde (6) Hazara (7) Eymir (8) Nasirabad (9) Buyukmahal (10) Zhob (This study)
    Origins Hydrothermal Sedimentary Hydrogenous Hydrothermal-hydrogenous Hydrothermal
    SiO2 25.87 63.02 43.69 13.6 10.65 12.67 9.41 16.04 84.39 49.62 44.09
    TiO2 0.04 0.03 0.32 0.1 0.02 0.04 0.84 0.02 0.06 0.16 0.34
    Al2O3 1.07 0.65 0.73 2.49 2.85 1.27 12.53 0.73 1.29 3.66 0.80
    Fe2O3 2.44 0.68 2.96 3.72 2.46 0.59 20.33 0.26 0.76 6.63 5.75
    MnO 45.59 29.22 45.88 63.78 33.39 67.21 33.78 69.91 8.82 26.79 42.46
    MgO 0.42 0.2 0.6 1.99 1.27 0.08 0.59 0.59 0.3 1.31 0.18
    CaO 5.17 0.24 1.28 4.05 18.96 1.67 6.43 0.24 0.5 2.94 0.69
    Na2O 0.12 0.05 0.29 0.24 0.39 0.07 0.07 0.01 0.17 0.18 0.30
    K2O 0.21 0.11 0.22 0.05 0.56 0.46 0.88 0.05 0.2 0.67 1.27
    P2O5 0.08 0.04 0.25 0.18 0.31 0.12 3.73 0.07 0.02 0.11 0.13
    Cr (-) 13.7 45 (-) 26 16 247 (-) (-) (-) 29.85
    Co 28.75 25.21 11 13 59 222 404 103.5 85.36 367.4 125.24
    Ni 89.13 69.4 36 10 167 341 305 67.35 86.13 280.4 288.46
    Cu 62.13 154.9 72 56 26 691 375 80.5 117.74 450.8 28.05
    Zn 77.74 66.6 64 70 49 147 580 62.45 35.62 81.8 230.67
    Pb 19.68 6.5 49 65 (-) 18 2357 9.33 2.02 25.8 541.6
    Mn/Fe 56.23 97.17 199 18.98 15.03 133.43 2.16 880.33 18.85 6.06 105.37
    Co/Zn 0.38 0.38 0.17 0.19 1.2 1.51 0.7 2.24 2.2 4.49 0.57
    Co/Ni 0.33 0.36 0.31 1.3 0.35 0.65 1.32 1.54 0.99 1.31 0.51

    Table 4.  Comparison between some major and trace element composition of the Zhob manganese occurrences and different type of manganese ores in the world

    The behavior of Co and Ni are closely related to each other and are also closely associated with Mn oxides. On average, cobalt abundances decrease from hydrogenous to diagenetic and to hydrothermal manganese deposits (Zarasvandi et al., 2013; Sabatino et al., 2011). Deposits with Co/Ni < 1 are characteristic of sediments deposited in hot water (Fernández and Moro, 1998) whereas Co/Ni > 1 indicates a deep marine environment of formation (Toth, 1980). Apart from one sample, Co/Ni ratios in the Zhob manganese ore deposit are lower than 1 suggesting a sedimentary contribution to the mineralization (Table 2). Co/Zn ratios of ~0.15 are characteristic of hydrothermal manganese ore deposits while a ratio of ~2.5 indicates a hydrogenous origin for a manganese ore deposit (Toth, 1980). Co/Zn ratio for the Zhob manganese deposits ranges from 0.13 to 0.9 and indicates a hydrothermal origin for the manganese mineralization. These values are similar to the Waziristan manganese deposit and Dehoo ore deposit (Table 4). Although Co/Zn ratios suggest a hydrothermal origin for the Zhob ore deposits, Co/Ni ratios indicate a small contribution from a sedimentary component during the formation of the manganese deposits.

    Analysis after (1) Lofti et al. (2017); (2) Karakuş et al. (2010); (3) Shah and Khan (1999); (4) Ozturk (1993); (5) Gultekin (1998); (6) Choi and Hariya (1992); (7) Shah and Moon (2004); (8) Öksüz (2011); (9) Zarasvandi et al. (2013); (10) Öksüz and Okuyucu (2014).

    Like Fe and Mn values, concentrations of Ti and Al are also useful for the assessment of the genesis of Mn ore deposits. Al and Ti contents are high in the sedimentary environment and they have the same behavior during the formation of an ore deposit (Sugisaki, 1984; Crerar et al., 1982). Detrital materials are an important source of Al in the chemical precipitates of manganese ores (Zarasvandi et al., 2013; Polgári et al., 2012), while titanium is essentially constant in hydrothermal solutions and is thus a measure of clastic (detrital) input (Şaşmaz et al., 2014; Shah and Khan, 1999; Sugisaki, 1984). The contents of Al and Ti in the Zhob deposits show a positive correlation (Fig. 7c) except for one sample which has markedly high Al2O3, probably due to alteration. This broad positive correlation, therefore, supports the contribution of detrital material in the formation of the deposits.

  • The Zhob ophiolite is part of the Bela-Zhob-Waziristan ophiolitic belt (Şengor, 1987). Ophiolites of this belt were obducted on the western margin of the Indian continental plate as the oceanic crust of Neo-Tethys Ocean and formed in a supra-subduction zone environment in Late Cretaceous (Sawada et al., 1995). The Ali Khanzai Block of Zhob ophiolite was the part of the crust Neo-Tethys Ocean that contains manganese mineralization associated with metachert. By considering the models of Bonatti et al. (1976), Canon and Force (1983) and Buehn et al. (1992), a simple genetic model can be proposed for the origin of the Zhob manganese deposits (Fig. 8). The downward circulation of seawater into the fractured oceanic crust and interaction of the seawater with hot oceanic crust caused progressive warming and reduction and increase in the acidity that generated hydrothermal solutions (Roy, 1992). These solutions were able to leach Mn, Fe and other metals from the basaltic oceanic crust. Finally, the metal-enriched hydrothermal solutions convected upward to the seafloor lead the deposition of manganese and ferromanganese deposits.

    Figure 8.  Hypothetical model showing the geological environment of formation of the Zhob manganese deposits.

    The precipitation of different metals from the hydrothermal solutions are the result of decreasing pressure and temperature and increasing Eh and/or pH in the solution. Cooling of the hydrothermal solution may occur during circulation through cooler rocks and/ or by subsurface mixing with seawater. The origin of the Zhob manganese deposits similar to those of present-day hydrothermal systems within mid-oceanic spreading centers (Shah and Khan, 1999; Roy, 1992). The formation of these deposits can be related to the Neo-Tethys Ocean where these deposits originated along with sea-floor-spreading centres and were later obducted as part of Zhob ophiolite. During obduction, the Zhob ophiolite has undergone metamorphism which may have caused alteration of the manganese deposits.

    It is now understood that Fe compounds are less stable compared to Mn and so precipitate close to hydrothermal vents. Conversely, Mn is more stable in aqueous solution and so precipitates further away from hydrothermal vents at sea floor spreading centers (Roy, 1992; Panagos and Varnavas, 1984). The precipitation of Mn and Fe and their compounds are controlled by Eh and/or pH of the hydrothermal solution (Frakes and Bolton, 1992; Panagos, 1984). In low Eh and/or pH conditions Mn is more mobile relative to Fe. The fractionation of Mn compounds from Fe compounds suggests a spatial variation in Eh and/or pH (Panagos and Varnavas, 1984). Considering Fe and Mn concentrations of mineralization in the study area, it is likely that the Zhob deposits were formed from a hydrothermal source and the high Fe contents, indicate that mineralization was probably proximal to a hydrothermal vent at a sea floor spreading center.

    It is clear from the geochemical data in Figs. 6, 7, and low Mn/Fe ratios (Table 3) of the host rocks (metachert) that metachert was formed by hydrothermal processes with a limited contribution from sedimentary (contemporaneous volcanic) components. The conformable nature of the Zhob manganese deposits and host metachert indicate that Mn and Si were probably both derived from the same hydrothermal source. Metachert contains radiolarian tests and this suggests that the metachert is formed from a mixture of biogenic and hydrothermal sediments.

  • The Zhob manganese ore deposits are hosted by red to brown metachert in the Ali Khanzai Block of the Zhob ophiolite. Braunite and pyrolusite are the main manganese minerals that occur together with minor hematite and barite. Gangue minerals are mainly quartz and carbonate minerals. Major and trace element signatures show that hydrothermal processes were responsible for the formation of mineralization at Zhob with a minor contribution from deterital (contemporaneous volcanic) materials. Therefore, it is concluded that the Zhob manganese deposits are hydrothermal in origin.

  • This research was financially supported by the six months, HRD Foreign scholarships of the Centre of Excellence in Mineralogy, University of Balochistan, Quetta, which was approved by the Higher Education Commission, Pakistan under its PSDP development project "Capacity Building and Strengthening of the Centre of Excellence in Mineralogy". The research was also partly supported by the Higher Education Commission, Pakistan "National Research Program for Universities (NRPU) Project #3593" to Muhammad Ishaq Kakar. The authors are grateful to the reviewers for their constructive comments which improved the manuscript. The final publication is available at Springer via https://doi.org/10.1007/s12583-020-1337-3.

    Electronic Supplementary Material: Supplementary material (Appendix A) is available in the online version of this article at https://doi.org/10.1007/s12583-020-1337-3.

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