Mineralogy , Geochemistry , and Origin of Buyukmahal Manganese Mineralization in the Artova Ophiolitic Complex , Yozgat , Turkey

The Artova ophiolite complex (AOC) is exposed along the northwestern and eastern margins of Yozgat area in Turkey. The Mndeposit in the Buyukmahal area is part of this ophiolite complex. The deposit is in banded and lenticular forms and hosted by a radiolarite unit overlying the volcanics. Pyrolusite and magnetite are the main minerals of the manganese ore in the Buyukmahal (Yozgat) area.The gangminerals in the deposit are composed only of quartz and calcite. In this study,mineralogy, major oxide, trace element andREE contents of Buyukmahalmanganesemineralization are evaluated.Thegeochemical data indicate that Buyukmahal mineralization does not originate from a pure hydrothermal or pure hydrogenous source but from a system consisting of both sources. It is also asserted that the mineralization was first developed on a sea floor spreading center within the Alpine Ophiolite System and then obducted as part of the AOC.


Introduction
The Alpine Ophiolite System (AOC) is exposed along the northwestern and eastern margins of the Yozgat region in Turkey.The Mn mineralization in the Buyukmahal is a part of this ophiolite complex.The mineralization in this area is highly firm and generally fractured and folded, developed in banded and lenticular shape, and syngenetic with radiolarite cherts.Mineralizations are chiefly NW-SW trending and small anticline structures are observed in some parts.Although mineralization in the Buyukmahal area has not been studied, Derbent and Eymir manganese deposits within the AOC were investigated recently by Öksüz [1,2].These deposits were operated from time to time by local miners, but lately none of the deposits is mined out due to low reserve potential.The Eymir manganese deposit which is genetically linked to Buyukmahal mineralization occurs within radiolarite cherts of the lower Cretaceous ophiolite complex [1].Major and trace element contents of the Eymir ore indicate that the deposit is of a hydrothermalhydrogenous type volcano sedimentary mineralization and both oxic and anoxic sedimentation conditions prevailed.
The Derbent manganese mineralization, another manganese deposit in the Yozgat region, occurs as two separate ore bodies [2].Chemical data yield that hydrothermal and hydrogenousdiagenetic processes played important role in formation of Derbent mineralization.The geochemical characteristics of these deposits are consistent with those of several other manganese mineralizations such as Waziristan, Hazara [3], Baby Bare [4], Baft Ophiolitic melange Kerman (Iran) [5], Wakasa [6], C ¸ayırlı [7], and Kasımaga [8] deposits.Particularly Waziristan (Pakistan) and C ¸ayırlı (Turkey) deposits are regarded as hydrothermal exhalative manganese deposits occurring on seafloor spreading centers associated with ophiolite units [9,10].The Buyukmahal deposit under investigation is also thought to be a hydrothermal exhalative manganese mineralization.The aim of this study is to discuss the mineralogical and geochemical mechanisms responsible for development of manganese ore in the Buyukmahal area.

Geological Setting
Turkey comprising the border between Eurasia at the north and Gondwana at the south is an E-W elongating component of the Alpine-Himalayan Orogenic zone.The Alpine Orogenic system is formed by the closure of a different branch of the Tethys Ocean.During the closure of Tethys Ocean, continental parts of the Gondwana and Laurasia continents collided.Turkey as an orogenic mosaic (orogenic collage) is a part of these continental parts including remnant materials between these continentals [11].The AOC is included to the Alpine Orogenic system.The AOC of Upper Cretaceous age shows a wide distribution and hosts several ore mineralizations.
Ore bodies in the study area occur as laminated, banded and lenticular forms (Figures 2(a), 2(b), 2(c) and 2(d)).The mineralization is entirely associated with radiolarite cherts and thickness of lamina and bands is in the range of 1 to 90 cm.Manganese ores are quite fractured and fissured and show an irregular structure (Figure 2(a)).Polished section determinations indicate that ore assemblage is composed of hematite and pyrolusite, whilst quartz and calcite are the gangue minerals.Pyrolusite and magnetite are the main minerals in the Buyukmahal deposit.Hematite peaks were recorded in XRD analysis but it could not be observed in ore microscopy and Raman spectroscopy determinations.

Material and Methods
Twenty ore samples (500 g each) were collected from the Buyukmahal manganese deposit.The whole section of the ore from top to bottom was sampled systematically.Samples were taken at 30 cm intervals.
Powders of 12 samples under 200 mesh were analyzed at ACME Laboratories.Major oxide and trace element contents were determined with ICP-ES and REEs were analyzed with the ICP-MS method.30 g sample was powdered into 100 m for geochemical analysis.0.5 g sample was processed in HCl-HNO 3 -H 2 O solution at ∼95 ∘ C for 1 hour and then the amount of sample was increased 10 mL for the final filtering.Results of analysis are given in Tables 1, 2, and 3.In addition, in order to determine paragenesis and textural characteristics of mineralization, 10 polished sections were studied with ore microscopy.XRD analysis for six samples was done at TPAO (Turkish Petroleum Corporation) laboratories.A Rigaku DMAX IIIC model X-Ray diffractometer with a Cu target (2-70 ∘ 2) was used in the analyses.Ore minerals were also studied with Thermo Scientific DXR Raman Microscope at the Geological Department of the Ankara University.The Raman spectrums obtained were evaluated with Crystal Sleuth program to determine the mineral paragenesis.Chemical composition of pyrolusite was determined with microprobe analysis conducted at Montan Universität in Leoben (Austria).The results are shown in Table 4.

Mineralogy
Mineral paragenesis in the study area was investigated with ore microscopy studies as well as XRD, Raman spectroscopy, and microprobe analysis for pyrolusite.Results show that pyrolusite and magnetite are the main ore minerals in the Buyukmahal area accompanied by little amount of hematite.Gangue minerals are quartz and calcite.Results of microprobe analysis performed on four points in a pyrolusite crystal are shown in Table 4.

Pyrolusite (MnO 2 ).
It is mostly precipitated from lowtemperature hydrothermal fluids.Pyrolusite is a common alternation mineral in oxidized marine environments.Pyrolusite and magnetite, forming the main components of the Buyukmahal area, with a whitish yellow color, are distinct with their strong anisotropic character.Pyrolusite minerals develop in small veins and characteristic with anhedral and subhedral cutaways.Ore microscopy and Raman spectroscopy images of pyrolusite are shown in Figure 3. Using the results of microprobe analysis, the structural formula of pyrolusite (on the basis of two oxygen) is calculated as Mn 1.69 -Fe 0.07 -Si 0.09 -Al 0.02 -Ca 0.01 O 2 (Table 4).

Magnetite (Fe 3 O 4
). Magnetite occurs as small scattered crystals or veins.Vein magnetite is observed cutting the pyrolusite (Figure 4).In single nicol magnetite is seen in brown and gray colors and in the crossed nicols it is in anisotropic character.Samples are slightly magnetic.Ore microscopy and Raman spectroscopy images of magnetite are shown in Figure 4.

Geochemistry
Geochemical data are used to determine the origin of mineralization (e.g., hydrothermal, hydrogenous, and diagenetic).The chemical composition of Buyukmahal deposit is SiO 2 : 85.40 to 10.32 wt%, MnO 2 : 68.54 to 6.79 wt%, and Fe 2 O 3 : 16.73 to 2.31 wt%.Fe and Mn are characteristically fractionated on precipitation from a hydrothermal solution, producing high or low Mn/Fe rations in exhalative sediments [13].Mn/Fe rations of the deposit range from 25.89 to 0.90 wt% (Table 1).These values are conformable with those of hydrothermal exhalative manganese deposits in ophiolitic regions and recent submarine spreading centers [1,14,15].
The Si-Al discrimination diagram, proposed by Peters [16], is used to distinguish hydrothermal from hydrogenous Mn-oxide deposits.Buyukmahal ore samples are almost within the field of hydrothermal field, with only one sample within the field of hydrogenous deposits (Figure 5).
Ba contents of Waziristan [6], Hazara (Pakistan) [3], Binkılıc ¸ [17], C ¸ayırlı [7], and Kasımaga (Turkey) [8] regions are very high (415, 6304, 6892, 1229, and 2719, resp.)indicating a sedimentary contribution.High Ba content of the Buyukmahal deposit (ave.3659) is also indicative of sedimentary origin.Modern submarine hydrothermal Mn-oxide deposits are more enriched in Cu, Zn, Ni, and Co contents in comparison to pelagic sediments.However, they are lower than hydrogenous deposits [13,18].Choi and Hariya [6] discriminated hydrogeneous deposits and submarine hydrothermal Mn-deposits on a Ni-Zn-Co ternary diagram.On this diagram, five samples are plotted near hydrogenous fields and seven samples are close to hydrogenous field (Figure 6).In Fe-(Ni + Co + Cu) * 10-Mn triangular diagram [9,10,19], all samples are plotted in hydrothermal and diagenetic fields (Figure 7).Correlation data on major oxide and some trace elements contents of ore samples are given in Table 5.Major oxide, trace, and REE geochemistry are very useful for understanding the formation conditions of ore deposits.REE contents of 12 samples collected from the Buyukmahal manganese mineralization are shown in Table 6.
REE contents of the hydrothermal and hydrogenous ferromanganese and manganese deposits differ considerably and thus can provide great information on the genetic processes involved in the formation of submarine manganese and ferromanganese ores [20][21][22][23].REE patterns of the studied deposit (Figure 8     deposits show positive Ce anomaly but hydrothermal ferromanganese deposits are characteristic with negative Ce anomaly [22][23][24].All samples of the Buyukmahal manganese mineralization show strong negative Ce anomalies which resemble the pattern of typical submarine hydrothermal deposits (Figure 8(a)).However, the Ce anomaly depends on the temperature of the fluid, the proximity to the hydrothermal source, and redox conditions [23,25,26].Eu also shows negative anomaly in all samples, indicating contamination from the continental crust and/or sediment contribution via dehydration [27].
Y/Ho ratios in the area range from 13.06 to 31.54 (average 25.05).High Y/Ho ratios are indicative of multienvironments for the mineral deposition.In this respect, both deep marine environments and terrigenous materials may be effective for precipitation [30].
Data computed with the formula of Ce anom = log [3 × Ce N /(2 × La N + Nd N )] also yield information on the origin of mineralization.For example, in the case of Ce anom > −0.1, Ce is said to be enriched, which reflects an anoxic character for the water body of sedimentation.If Ce anom < −0.1, there is a negative Ce anomaly which indicates an oxic nature for the water body of sedimentation [31].Ce anomalies in all samples at Buyukmahal are found to be Ce anom < −0.1, indicating an oxic character for the sedimentation environment.

Discussions and Conclusions
The AOC of Upper Cretaceous age is located along the northwestern and eastern margins in Yozgat (Turkey) and  is included to the Alpine Orogenic system.Mineralization in the Buyukmahal area, observed in banded and lenticular forms, occurs in a close association with radiolarite cherts and is intensely affected by the tectonism.
Based on the results of major and trace element data, mineralization in the study area was probably formed from hydrothermal solutions associated with a sea floor spreading center.However, ore minerals at Buyukmahal were not  precipitated entirely from a purely hydrothermal or purely hydrogenous fluid, but certainly from a mixture of these two.For instance, Ti is generally immobile in hydrothermal solutions and could be a measure of clastic input [32].The good correlation observed between Al 2 O 3 and TiO 2 ( = 0.99) can be attributed to the mixing of detrital materials during precipitation [6].showing hydrogenous [29] and hydrothermal [29] fields.
Fe compounds (less stable than Mn) precipitate proximal parts, whilst Mn compounds precipitate distal parts of hydrothermal vents along the sea floor spreading centers [33,34].Eh and/or pH of the hydrothermal solution also exert controls on the precipitation of Mn and Fe and their compounds [34][35][36][37].Mn is more mobile relative to Fe during low Eh and/or pH conditions.The fractionation of Mn compounds from Fe compounds suggests a spatial variation in Eh and/or pH [34].Considering Fe and Mn concentrations of the mineralization in the study area, it can be asserted that Buyukmahal deposit was formed from a hydrothermal source; in addition, considering the high Fe content, mineralization might be formed in a proximal site of the hydrothermal vent.
Although mineralization at Buyukmahal is of a hydrothermal type, it does not originate from a pure hydrothermal or pure hydrogenous source.Geochemical data support a system contributed from both sources.The mineralization was developed on a sea floor spreading center within the Alpin Ophiolite system and then obducted as part of the AOC.

Figure 1 :
Figure 1: Location and geology map of study area (modified from [12]).
The correlation coefficients indicate the presence of strong positive relations between major oxides and various trace elements (Al 2 O 3 -Fe 2 O 3 :  = 0.95; Al 2 O 3 -TiO 2 :  = 0.99; TiO 2 -Fe 2 O 3 :  = 0.94) and the contribution of mafic terrigenous material to the deposition environment.

Table 5 :
Correlation relations for major oxides..70 and higher values and −0.70 and lower values specify the presence of possitive or negative corelation coefficients. 0

Table 6 :
Correlation relations for trace elements..70 and higher values and −0.70 and lower values specify the presence of possitive or negative corelation coefficients. 0