21 Genesis of Sedimentary- And Vein-type Magnesite Deposits at Kop 22

8/10/2019 21 Genesis of Sedimentary- And Vein-type Magnesite Deposits at Kop 22

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KADR et al. / Turkish J Earth Sci

Figure 1. Geological map o the Kop Mountain area (a), sedimentary magnesite (b) and vein magnesite (c)in ultramafic units.


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In the study area, the basement consists o PalaeozoicPulur metamorphic units including gneiss, amphibolite andschist (Figure 2). Tese metamorphics are unconormablyoverlain by Liassic volcano-sedimentary rocks o theHamurkesen Formation which comprises intercalations o

sandstone, claystone, marl and basalt. Te Malm - LowerCretaceous Hozbirikyayla Formation conormably oveliesthe Hamurkesen Formation, and comprises predominantlylimestone and sandstone. Te ultramafic rocks were thrustover the Mesozoic and Palaeozoic rocks during UpperCretaceous to Middle Eocene time. Te ultramafic rocksare unconormably overlain by the Kaplankaya Formation(Upper Cretaceous), consisting o limestone interbeddedwith sandstone. Te ertiary units are represented byconglomerate o the Srck Formation (Eocene) andlimestone, sandstone, claystone and marl o the GllerFormation (Miocene).

3. MethodsTin-sections were prepared rom the samples andthen examined using an optical microscope (Nikon Pol400). Mineralogical characteristics o the samples weredetermined by X-ray powder diffractometry (XRD)(Rigaku Geigerflex) and scanning-electron microscopy(SEM-EDX) (JEOL JSM 84A-EDX), differential thermalanalysis-thermal gravimetry (PerkinElmer - DiamondG/DA thermal analyzer) and inrared spectrometry(F-IR)(PerkinElmer 100 F-IR spectrometer).

Representative magnesite, dolomite andhydromagnesite-dominated bulk samples were preparedor SEM-EDX analysis by adhering the resh, brokensurace o each sample onto an aluminium sample holderwith double-sided tape and coating with a thin (350 )gold coating using a Giko ion coater. DA-G curveswere obtained using a 10 mg sample o powdered clay-sized raction (


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the Upper Cretaceous; coralline algae (Figure 4e) probablyrom the ertiary; and also radiolaria and sponge spicules(Figure 4), silicified volcanic rocks, ultramafic rocks, andmetamorphic quartzites. Te basal conglomerate includessandy matrix fill between grains which was later cementedby magnesite. Te host rocks (ultramafics) o magnesites

are highly to partially serpentinized dunite and harzburgite(Figure 4g,h).

5.2. XRD determinations

Te mineralogical compositions o bulk magnesite andassociated rock samples were examined by XRD, andsemi-quantitative mineral contents are given in able 1.Te results indicate that magnesite predominates in thesedimentary rocks and magnesite beds include dolomite,

magnesitic dolomite, smectite and silica (quartz)intercalations. Vein infills in highly altered ultramafics

Figure 3.Field photographs o magnesites. (a)sedimentary magnesite (mg) associated with a shear zone within the ultramafic

rocks (u); (b)general view o the sedimentary magnesites (mg) including claystone intercalations (arrow); (c) close view osedimentary magnesite (mg) including claystone (sm) intercalations; (d) a magnesite block showing conchoidal racturing(hammer); (e) basal breccia consisting o different rock ragments including ultramafic blocks (u); (fk)magnesite veins (mg)in ultramafic units (u); (l) typical magnesite block (mg) in ultramafic rocks (u).


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Figure 4.Photomicrographs o magnesite and associated rocks. (a)micritic magnesite, plane-polarised light (sample: VM-40); (b)microsparitic patches (arrow) in micritic magnesite, plane-polarised light (sample: SY); (c)mostly microsparitic magnesite, plane-polarised light (SZ); (d) limestone and ossil ragments in the breccia underlying sedimentary magnesite, plane-polarised light (SZ-4);(e)ossil ragment (red algae) and microossils (r: oraminiera) in limestone ragment, plane-polarized light (SZ-4); (f) microossils inlimestone ragment. rad: radiolarite, sp: sponge spicule, plane-polarised light (SZ-4); (g) development o vein magnesite in completelyserpentinized dunite showing mesh texture, crossed polars (F-9); (h) development o magnesite vein in dense serpentinized harzburgite,crossed polars (F-9).


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able 1.Mineralogical compositions o magnesite and associated rocks.

Sample No Rock type mg hmg dol srp qtz smc

Sedimentary

SM-1 Magnesite +++++







SM-12 Magnesite ++++ +

SM/D-9 Magnesitic dolomite ++ +++

SM/D-13 Magnesitic dolomite ++ +++ acc

SM-11 Silicate + ++++

Claystone

SM-2 Claystone +++++

SM-3 Claystone +++++

Vein

VZ-1 Magnesite +++++

VZ-2 Magnesite +++++

VB-2 Magnesite +++++ accVB-3 Magnesite +++++

VM-30 Magnesite +++++




VD-31 Dolomite acc +++++

HM-1 Hydromagnesite +++ ++

HM-4 Hydromagnesite ++++ +

Vein

VS-40 Silicate +++++

VS-42 Silicate +++++

Host rock

VD-31Y Altered ultramafic rock +++ ++

L-20 Altered ultramafic rock +++++ acc

DUM-M Altered ultramaficrock +++++

HM-9 Altered ultramafic rock acc +++

J-9 Altered ultramafic rock +++++

mg: magneste, hmg: hydromagneste, dol: dolomte, srp: serpentne, qtz: quartz, smc: smectte, acc: accessory, +: relatve abundance omneral.


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consist mainly o pure magnesite; hydromagesite, silica(quartz) and dolomite are also present. Magnesite hassharp diagnostic basal reflections at 2.74, 2.50, 2.11, 1.94,1.70 (Figure 5). Hydromagnesite was determined by9.18, 5.79, 3.31, 2.90 peaks, dolomite was recognized

by its 3.69, 2.88, 2.19, 1.78 reflections. Serpentine wasdetermined by 7.30, 4.58, 3.66, 2.46, 2.54 reflections.Brucite was determined by 3.14, 4.77 peaks. Smectite isrecognized by its 15.5 reflection which expands to 17, and then collapsed to 10 afer heating at 350C and550C.

5.3. SEM-EDX determinations

Scanning electron microscope analyses were carriedout on vein and sedimentary magnesite, dolomite andhydromagnesite samples. SEM images indicate thatmicritic magnesite rhombs are dominant in vein samples.

Te magnesite samples consist o euhedral or subhedralcrystals 26 m across (Figure 6a). Sedimentary magnesite

samples consist o micriticmagnesite subhedral crystals 300C, volcanogenicsources, deep-seated sources, or combinations o all othese. In this study, the vein-type magnesites have d13C

values predominantly ranging rom 9.00 to 11.23PDB with two exceptions. Tese d13C values differ rom


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able 2.Chemical compositions o magnesite and associated rocks.

Sedmentary

Magneste Magnestc dolomte Claystone Slcate

Oxde (%) SM-1 SM-4 SM-5 SM-6 SM-7 SM-10 SM-12 SM/D-9 SM/D-13 SM-3 SM-11SO

22.06 0.97 0.45 0.78 0.58 3.84 0.94 4.35 1.66 45.28 74.31

Al2O

30.16 0.08 0.06 0.06 0.06 0.62 0.11 1.09 0.30 5.74 0.18

Fe2O

30.54 0.34 0.12


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able 2.Continued.

Ven

Magneste Dolomte Hydromagneste Slcate

Oxde (%) VM-30 VZ-1 VZ-2 VZ-3 VB-3 VD-31 HM-1 HM-4 VS-40 VS-42 VS-43ASO

20.79 0.36 0.14 0.14 0.54 1.31 15.98 8.60 95.45 98.18 97.67

Al2O

3


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deep-seated or mantle sources o CO2 (ranging rom 4

to 8 PDB rom Deines 1980) and indicate a derivation

predominantly rom the decarboxylation o organic-richLiassic and Palaeozoic sediments beneath the obductedultramafic rocks with some contribution rom the thermaldecarbonation o limestones (Fallick et al. 1991; Zedeet al.2000; Ece et al.2005). In contrast, the sedimentarymagnesites exhibit considerably less negative d13C valuesaveraging -3.0 PDB which are due mainly to outgassingo the mineralizing fluid with some involvement romatmospheric carbon. Tese d13C values are coincidentwith values o sedimentary magnesite rom the WestCarpathians o Slovakia (Ilavsky et al. 1991), but differrom evaporatic lacustrine and marine sedimentarymagnesites (d13C > 0 PDB; Melezhik et al.2001; Zedeet al.2000).

As temperature is the main actor controlling theoxygen isotope ractionation, water temperature can beestimated using the ollowing Aharon (1988) equation:

103In = d18Om

d18Ow= A (1062) + B

where 103 In is the per mil ractionation betweenmagnesite (m) and water (w), is temperature in Kelvin,A and B are constants (A= 3.53 and B= 3.58 or magnesiterom Aharon 1988). Te estimated temperatures or waterrom which magnesite has been precipitated are ~24.5C

or the sedimentary magnesite and ~37.2o

C or the veinmagnesite based on the average d18O values (31.3 SMOW or sedimentary and 28.1 SMOW or vein)and assumption o d18O

w(5 SMOW rom Zede et al.

2000). Tese temperatures indicate low and moderate-temperature magnesites ormed under surace or near-surace conditions, respectively.

In the eastern Pontides, the emplacement o ultramaficrocks took place during Upper Cretaceous - MiddleEocene time under a compressional tectonic regime.Later, afer the Middle Eocene the area was subjected toextensional tectonic movements associated with volcanic

activity. Te extensional tectonic movements producedvertical aulting and racturing. Te isotopic data suggest

Host rock

Dunite Harzburgite

Oxide (%) F-9 C-37 VD-31Y

SiO2

43.23 36.14 38.59

Al2O

32.65 0.12 0.60

Fe2O

38.55 5.93 7.72

MgO 33.02 40.89 31.89

CaO 2.74


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low and moderate temperature magnesites, respectively.Te d13C values o vein magnesites indicate derivationpredominantly by decarboxylation in organic-richshales. Te d13C values o sedimentary magnesite showdepletion in light isotopes mainly due to outgassing o

mineralizing water. Co, Ni and i contents o magnesiteand associated rocks indicate that Mg was derived romserpentinized ultramafic rocks. Te suggested mechanisminvolves downward migration o surace water throughthe ultramafic rocks and organic-rich sediments, ollowed

by ascent o the percolating water, rom which magnesiteprecipitated as infill o vein-type ractures and sediment atthe surace.

AcknowledgementTe authors are grateul to Pro. Dr. Nurdan nan andPro. Dr. Kemal asl (Mersin University) or ossilidentifications. Appreciation is extended to Dr. GneKrkolu and Dr. Okan Z. Yeilel (Eskisehir OsmangaziUniversity) or conducting the IR and DA-G analyses,respectively. Te authors are grateul to anonymous

reviewers or their extremely careul and constructivereviews, which significantly improved the quality o thepaper. We are also grateul to Pro. Dr. lkay Kuu andPro. Dr. Erdin Bozkurt or their insightul editorialcomments and suggestions.

Figure 10.A crossplot o stable isotope values o two types omagnesite.

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21 Genesis of Sedimentary- And Vein-type Magnesite Deposits at Kop 22

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