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Arabian Journal of Geosciences ISSN 1866-7511 Arab J GeosciDOI 10.1007/s12517-013-0953-y

Petrology of ultramafic rocks and Mg-richcarbonate minerals in southeast of Dehshir,Central Iran

Azat Eslamizadeh & Shahram Samanirad

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ORIGINAL PAPER

Petrology of ultramafic rocks and Mg-rich carbonateminerals in southeast of Dehshir, Central Iran

Azat Eslamizadeh & Shahram Samanirad

Received: 14 January 2013 /Accepted: 15 April 2013# Saudi Society for Geosciences 2013

Abstract The study area is located southeast of Dehshir be-tween the Urumieh-Dokhtar Magmatic Arc and Nain-BaftOphiolite Belt comprising the Nain, Dehshir, Shahr Babak,and Baft ophiolite complexes. The Dehshir OphioliticComplex which obducted in the Late Cretaceous, consistsmainly of ultramafic rocks. These remnants of oceanic crustare extensively faulted and fractured. The severe faulting andbrecciating of the ophiolite sequence have undergone high-grade alteration and changed it to the tectonic mélange. TheDehshir colored mélange is bounded to the west by Dehshirfault which is a right-lateral offset of the Nain-Baft suture. Inthis research, the petrographic studies of the area showed thatthe ultramafic rocks consist mainly of dunite and harzburgiteintruded by diabasic dikes. Syntectonic hydrothermal fluidscirculated throughout these rocks. Migration of Mg-rich fluidsand hydrothermal brecciating occurred within highly alteredand brecciated zones. Magnesite precipitated from hydrother-mal solutions and formed the massive, lenticular, and vein-typeore deposits in serpentinized-hosted rocks. Later on, magnesiteturned into hydromagnesite due to hydration at the lowerdepths near the surface. According to the X-ray diffractionand X-ray fluorescence analysis, hydromagnesite is the mostdominant and widely occurring Mg-rich carbonate mineral inthis area. The main alteration is serpentinization butbirbiritization also occurs as a result of interaction betweenfluids and ultramafic rocks.

Keywords Central Iran . Ophiolite . Dehshir . Magnesite .

Hydromagnesite

Introduction

Late Cretaceous ophiolites in Iran have been studied bymany geologists in the last decades (Adib and Pamic1982; Lanphere and Pamić 1983; Sabzehei 2002;Rohgoshay et al. 2005; Shafaii Moghadam and Stern2011). The Iran ophiolites lie along the northeastern flankof the Zagros folded belt which is bounded on the northeastby the Main Zagros thrust (Berberian and King 1981). TheArabian plate has been moving northeast since LateCretaceous time which Neotethys was consumed. TheZagros orogenic belt in the position of an accretionary prismmanifests this ongoing plate convergence which is in tran-sition now from subduction to continental collision (ShafaiiMoghadam and Stern 2011).

Magnesite is mainly used in the refractory industry. TheIranian refractory industry is the main consumer of magne-site. As the magnesite production in Iran is not sufficient forthe refractory industry, it is widely imported every year. Theexclusive magnesia producer in Iran is the IranianRefractories Procurement and Production Company whichuses the magnesite of Nehbandan area in South KhorasanProvince. There are also a few minable deposits in Sistan-Blouchestan Province. The current magnesite production istotally 30,000 tons per annum with 86–96 % MgO contentand a density between 3.30 and 3.38 g/cm3. Total magnesitereserves in Iran are estimated at 3.54 Mt of mainly low-grade ore (Weber 2000). Magnesite is a strategic protectorfor the cement, steel, glass, oil, gas, electricity, and energyindustries. Study of the hydromagnesite–magnesite miner-alization and genesis, could lead to find new resources in theresearch area. Although the occurrences of hydromagnesiteare too small in comparison with the large magnesite de-posits of the world, they are important as potential resources

A. Eslamizadeh (*) : S. SamaniradDepartment of Mining Engineering, Islamic Azad University,Bafgh Branch, Bafgh, Irane-mail: meslamizadeh@bafgh-iau.ac.ir

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of the raw materials for production of industrial grade caustic,dead-burned, and fused magnesia or magnesium metal.

Geological outline

The Urumieh-Dokhtar Magmatic Arc (UDMA) with 40-kmwidth and distinct linear intrusive–extrusive complexes, ex-tended parallel to the Zagros Folded Belt (Ghasemi and Talbot

2006). The UDMA volcanism started in Early Eocene androse up in the Middle Eocene (Berberian and King 1981). It isgenerally thought that the UDMA was a long magmatic arcoverlaid the margin of Neotethyan oceanic lithosphere whichsubducted beneath the Central Iran plate. The post-suturingmagmatic activities in UDMA is related with intrusive activityand uplifting the Sanandaj-Sirjan Zone (Fig. 1) forced themargin to break off. This cutoff occurred at the peak ofmagmatic activities across the UDMA in the Middle

Fig. 1 a Distribution of LateCretaceous ophiolites of Iran. bSchematic cross sectionshowing the relationshipsbetween the Outer and the InnerZagros Ophiolitic Belts, theZagros Thrust-Folded Belt, theSanandaj-Sirjan Zone, and theUrumieh-Dokhtar MagmaticArc. (Shafaii Moghadam andStern 2011)

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Eocene. The study area is located at the western margin ofCentral Iran micro-continent, between the UDMA and TheNain-Baft Ophiolite Belt (Fig. 2). The NABA consists of theNain, Dehshir, Shahr Babak, and Baft ophiolite complexes(Shafaii Moghadam and Stern 2011). These massifs are com-posed of associated slices of harzburgites, small bodies ofgabbro and dike swarm complexes, accompanied by variousextrusive rocks from basaltic–andesitic lava flows and brecciato dacite and rhyolite (Shafaii Moghadam et al. 2009). Thestudy area is located approximately 125 km southwest of Yazdin Central Iran. It is a part of the Dehshir ophiolite complexwhich is exposed discontinuously over 150 km along theZagros folded belt (Fig. 1). The Dehshir ophiolite complexconsists of oceanic mantle and crustal sequences that includeharzburgite, gabbro, plagiogranite, sheeted dikes, and pillowlavas capped by Turonian-Maastrichtian Globotrunca bearingpelagic limestone (93.5–65.5 Ma) that rests conformably onthe ophiolite (Shafaii Moghadam and Stern 2011). Currentstudy indicates that the Dehshir ophiolitic complex whichabducted in the Late Cretaceous, consists mainly of ultramaficrocks with subordinate dikes, gabbro, plagiogranite, basalticflows, and pillow lavas. These remnants of the oceanic crustare extensively faulted and fractured. The severe faulting andbrecciating of the ultramafic rocks have undergone high-gradealteration and changed the ophiolite to the colored mélange.According to the geological map of Dehshir (Sabzehei andNabavi 1998) and field works, the Dehshir ophiolite is Pre-Late Cretaceous in age. The Dehshir colored mélange isbounded to the west by a major NW–SE trending fault zone(Fig. 3). The Dehshir fault is a right-lateral offset of the Nain-Baft suture (Meyer et al. 2006). The colored mélange outcropsin contrast with the Eocene volcanic rocks across the Dehshirfault (Fig. 3). Study of NABA indicates that most of ophiolitecomplexes developed as supra-subduction ophiolites withinthe intra-oceanic island arc environments. Intra-oceanic island

arcs and ophiolites now forming the Nain-Baft zone wereemplaced southwestward onto the northeastern margin of thesouth Sanandaj–Sirjan zone (Ghasemi and Talbot 2006).

Petrography

The Dehshir Ophiolitic Complex consists mainly ofultramafic rocks with subordinate dikes, gabbros,plagiogranites, basaltic flows, and pillow lavas. In thisresearch, mineral, and chemical composition of theultramafic rocks and related Mg-rich carbonate mineralsare studied by multistage fieldworks, rock sampling,chemical analysis, and microscopic study. In the studyarea, the ultramafic rocks mostly altered to serpentinite butrelict olivine and pyroxene still exist. Serpentine is the mostcommon alteration product of olivine and orthopyroxene of theultramafic rocks (Figs. 4, 5, and 6). The predominanthydromagnesite–magnesite mineralization occurs in ultramaficrocks which highly serpentinized; however, harzburgite rem-nants can be seen clearly in the mesh texture of serpentine(Fig. 6). Harzburgite with a xenoblastic texture consists mainlyof olivine and orthopyroxene. Serpentinized harzburgite andserpentine are replaced by olivine and bastite. Orthopyroxenereplacement by serpentine is widespread in most ophiolitecomplexes of Iran (Sabzehei and Nabavi 1998), Pakistan andCentral Alps (Bashir et al. 2009). According to these reports,olivine usually altered to mesh-textured serpentine while bastiteis more common after orthopyroxene. Bronzite is the mostdominant orthopyroxene in harzburgite, and minor enstatite ex-hibits variable enrichment in Fe. Due to a partial alteration,bronzite crystals show a bronze-like submetallic luster on theircleavage surfaces. Green to brown bronzite crystals have ametallic luster. The altered bronzite is known as bastite or schillerspar. Bastite is brown or green in color and has a metallic luster-like bronzite (Fig. 6).

Fig. 2 Satellite image of the Nain-Baft Ophiolite Belt at the south-western margin of the Central Iran Micro-continent. The NW–SEOphiolite Belt occurs in contrast with Eocene volcanic rocks alongsidethe major Nain-Baft Fault Zone

Fig. 3 Modified geological map of Dehshir scale 1:100,000 (compiledafter Sabzehei and Nabavi 1998). The study area indicated as a redrectangular. The Dehshir fault appeared in westernmost of the map

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Bronzite occurs in olivine–orthopyroxene-bearing rocks(Fig. 7). A harzburgite sample contains crystals of olivine(Fig. 8). Olivine and orthopyroxene commonly altered to talc(Figs. 9 and 10). Modal-mineral composition of these ultra-mafic rocks indicates a pyroxene-hornblende peridotite.Traces of microcrystalline quartz occur within the mineralizedserpentinite (Fig. 11). Current study proved that the silicifica-tion might happen after the carbonization.

Another accessory mineral detected in thin sections ischrome spinel, an alternate name for the mineral“picotite” (Mg, Fe) (Al, Cr)2O4 which is usually crys-tallized in the cubic system. Picotite and bronzite arefound together as irregular dark brown grains (Figs. 12and 13). Iron oxides and hydroxides as well as chloritewidely occur in serpentinite, which is the best rock forhosting hydromagnesite–magnesite mineralization (Figs.

14 and 15). Figure 16 presents some typical ultramaficrocks and their names based on the modal composition(Le Maitre et al. 1989).

Petrography study of the ultramafic rocks indicated thatolivine and orthpyroxene have been changed to talc (Figs. 9and 10). Magnesite can be formed via talc carbonate meta-somatism as below:

2Mg3Si2O5 OHð Þ4 þ 3CO2 ! Mg3Si4O10 OHð Þ2 þ 3MgCO3 þ 3H2O

Talc and magnesite are generated due to the carbonation ofserpentine. Simandl et al. (2000) and Bashir et al. (2009)described a triangular variation diagram (MgO–CO2–H2O)to illustrate the mineralogical range of the Mg-based min-erals array. According to the samples analysis (Table 1) onthe MgO–CO2–H2O (mole percent) and ternary plots (Fig.17), there are three Mg-rich carbonate minerals in thestudyhydromagnesite Mg3(CO3)4(OH)2, 4H2O, magnesite(MgCO3), and huntite CaMg3(CO3)4.

The pectolite–prehnite veins also occur along diabasedikes (Figs. 18). Diabase dikes with intersertal texture whichintruded the serpentinites consist of plagioclase microlithsthat surrounded microcrystalline and microporphyriticanhedral crystals such as sericite, chlorite, epidote, diopside,and hornblende. The magnesite–bearing rocks consist ofserpentine, hematite, and chlorite in a matrix of magnesitebut the magnesite minerals appear as cauliflower grains(Figs. 19, 20, 21, and 22).

�Fig. 4–17 4 An outcrop of serpentinized ultramafic rocks. 5 Themesh texture of serpentine after olivine. 6 Serpentinized harzburgite withbastite replacing orthopyroxene. 7 Bronzite in olivine orthopyroxenite.8An automorph crystal of olivine in harzburgite. 9 Talc substituted olivinein a pyroxene-hornblende peridotite. 10 Talc substituted orthopyroxene ina pyroxene-hornblende peridotite. 11 Microcrystaline quartz in min-eralized serpentinite. 12 Picotite and bronzite in harzburgite (XPL).13 Picotite and bronzite in harzburgite (PPL). 14, 15 Serpentinizedrocks impregnated by Iron oxides, hydroxides, and chlorite. 16 Theultramafic rocks in the area based on Le Maitre et al. (1989). 17 Aternary MgO–CO2–H2O plot (mole percent) showing mineralogicalcomposition of magnesite

Fig. 18–22 18 A diabase dike. 19 The intersertal texture in a diabase dike including sericite, chlorite, and epidote between plagiclase laths. 20 Themacroscopic samples of serpentinite with iron–oxide minerals and magnesite veins. 21 The microscopic picture of the sample 20 includingserpentine and chlorite in a matrix of magnesite. 22 Cauliflower magnesite samples. Mineral abbreviations used: Srp serpentine, Br bronzite, Ololivine, Tlc talc, Hbl hornblende, Opx orthopyroxene, Op opaque, Ser sericite, Qtz quartz, Mgs magnesite, Pic picotite, Hem hematite, Chl chlorite,and Ep epidote

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Mineralization

Both mafic and ultramafic rocks crop out in this area.Although dunite and harzburgite are the most common rocksbut harzburgite is more dominant than dunite. These ultra-mafic rocks highly altered to serpentine. Serpentinizedharzburgite and dunite with minor lherzolite as well asgabbro-diabase dikes occur as disrupted rodingitized mas-sifs. Serpentinite generally shows a mesh texture in thehighly brecciated zones. Hydromagnesite–magnesite veinsformed within serpentinites. These ore deposits have simi-larities to the Alpine-type magnesite deposits. In the studyarea, magnesite veins and veinlets occur within serpentinizedultramafic rocks. Weathering and alteration of olivine resultedin the formation of hydromagnesite–magnesite in fractures ofthe ultramafic rocks. Also in the tectonic sheared zones, thecrystalline magnesite exists together with dolomite (based onX-ray diffraction (XRD) analysis). After alteration of olivine,the magnesite mobilization formed stock-work veinlets ofhydromagnesite–magnesite. In comparison to serpentinization,birbiritization, and magnesite forming (the hydration develop-mental processes) which disintegrated the mafic minerals likeolivine, more complicated metasomatic processes took place.Hydrothermal metasomatism and mobilization of ions mightbe the reasons for the late forming of talc [Mg (OHSiO4)]. Talcis primarily formed by Mg-metasomatism under hydrationsolutions (Augustithis 1979). The mechanism of serpentineproducing after olivine might happen as below:

3Mg2SiO4 þ SiO2 þ 2H2O ! 2Mg3Si2O5 OHð Þ4Olivineð Þ Serpentineð ÞThe weathering of serpentine by CO2-laden waters could

result in deposition of cryptocrystalline magnesite in cracksand fissures. Alteration of olivine due to hydrating (weatheringof dunite) with participation of CO2 formed the magnesite as apseudomorph after olivine (Augustithis 1979). The followingchemical reactions represent the general concept of the min-erals carbonation:

Mg2SiO4 þ 2CO2 ! 2MgCO3 þ SiO2

Forsteriteð Þ Magnesiteð Þ ð1Þ

Mg3Si2O5 OHð Þ4 þ 3CO2 ! 3MgCO3 þ 2SiO2 þ 2H2O Serpentineð Þð2Þ

or:

2 Mg; Feð Þ2SiO4 þ 2H2Oþ CO2 ! H4 Mg; Feð Þ2Si2O9 þMgCO3:

(Shand 2006)

According to the reactions represented above, magne-sium carbonate precipitated in cracks and fissures whilethe silica carried away by hydrothermal solution. Theexperimental works (Augustithis 1979) showed that byusing the various concentrations of MgSO4-bearing solu-tions at a temperature range 100–300 °C (i.e., the thermalrange of hydrothermal solution), the olivine solubilitycould be achieve without adding acids at the lower con-centrations of Mg+ and SO4+ ions. The role of MgSO4

was considered to be catalytic in this experiment.Based on the XRD and X-ray fluorescence (XRF) analysis

results, hydromagnesite is the most dominant and widelyoccurring Mg-rich carbonate mineral in this area (Table 1).

Huntite is a very soft, fine-grained mineral, and highlydisintegrates in water. In the study area, huntite has beenmined through thousands of low depth holes since theancient times. So the huntite outcrops are very rare. Unlikehuntite, hydromagnesite–magnesite mineralization occurs asmassive to lenticular bodies and various-sized veins however,hydromagnesite also shows a fracture-filling texture withinthe altered ultramafic rocks and serpentinites (Figs. 23a–i).The listvenitic alteration occurred in the faulted-brecciatedzones where the CO2-bearing hydrothermal solutionscirculated.

These outcrops are detected easily by an orange-lightbrown to black color in a dark green background ofserpentinite. The listvenitic zone with magnesite stockworksshown in Fig. 24. The mineral paragenesis are quartz, mag-nesite, calcite, dolomite, and siderite. Magnesite pseudo-morph after olivine occurs as veins and veinlets shown inFigs. 25 and 26. The magnesite coexisting with dolomiteindicates that dolomite has been replaced by magnesite. The

Table 1 The XRF analysis result of hydromagnesite, magnesite, and huntite samples of the study area

Sample WT % SiO2 Al2O3 Fe2O3 CaO Na2O K2O MgO TiO2 MnO2 P2O5 LOI

Hydromagnesite 1 6.49 0.11 0.94 2.95 0.07 0.01 43.21 0.01 0.001 0.001 45.7

Hydromagnesite 2 8.51 0.11 3.28 8.95 0.01 0.01 38.20 0.012 0.006 0.003 40.59

Hydromagnesite 3 1.3 nd 0.19 1.82 0.76 0.02 45.94 nd nd – 50

Hontite 1 0.053 – 0.005 4.73 0.03 – 45.54 0.01 0.002 – 49.63

Hontite 2 0.041 0.01 0.003 39.28 0.02 – 10.21 0.006 0.01 – 50.42

Magnesite 1 0.02 nd 0.21 0.06 – – 45.93 – 0.89 0.004 52.46

Magnesite 2 0.04 0.11 0.8 0.04 – – 46.40 – 0.67 0.005 51.98

nd no detection limit

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magmatic Mg-rich solutions circulated throughout the wholerocks and derived the magnesium ion from the underlyingdolomite beds.

Conclusion

The study area is located approximately 125Km southwest ofYazd in Central Iran. It is a part of the Dehshir ophiolitecomplex which is exposed discontinuously over 150 km along

the Zagros Folded Belt. The Dehshir ophiolite obducted in theLate Cretaceous and consists mainly of ultramafic rocks withsubordinate dikes, gabbros, plagiogranites, basaltic flows, andpillow lavas. These remnants of oceanic crust is extensivelyfaulted and fractured. The severe faulting and brecciating ofthe ultramafic rocks have undergone high-grade alteration andchanged the ophiolite to the colored melange. This coloredmelange is bounded to the west by Dehshir fault which is aright-lateral offset of the Nain-Baft suture. The ultramaficrocks consist mainly of dunite and harzburgite intruded by

Fig. 24–26 Hydrothermal alteration of listvenite together with magnesite stockworks in serpentinite. 25Magnesite as a pseudomorph after olivine.26 Crossed veins and veinlets of hydromagnesite–magnesite

Fig. 23 a–i Massive to lenticular and vein-type mineralization of hydromagnesite–magnesite

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diabase dikes. The highly serpentinized ultramafic rocks aredunite and harburgite but harzburgite is dominant. Althoughserpentinization is the main alteration in this area, birbiritizationalso occurs as a result of interaction between fluids and ultra-mafic rocks. The serpentinized dunite and harzburgite withminor lherzolite, diabase dikes, and gabbro exhibit as disruptedrodingitized bodies. Hydrothermal fluids circulated throughoutthese rocks during the Late Cretaceous tectonic movements.Migration of Mg-rich fluids and hydrothermal brecciating oc-curred within highly altered and brecciated zones. Magnesiteprecipitated from hydrothermal solutions and formed massive,lenticular, and vein-type ore deposits in serpentinized-hostedrocks. Later on, magnesite turned into hydromagnesite due tothe hydration at the lower depths near the surface. Theweathering of serpentine by CO2-rich solutions could resultin deposition of cryptocrystalline magnesite in cracks andfissures. The magnesite occurs here as massive bodies, lentic-ular (lens-shaped) masses, and veins of varying thickness andhydromagnesite occurs generally as fracture fillings in alteredultramafic rocks and serpentinites. According to the XRD andXRF analysis, hydromagnesite is the most dominant and wide-ly occurringMg-rich carbonate mineral in the study area. Eventhough the occurrences of hydromagnesite–magnesite in thestudy area are not as large as the hugemagnesite deposits of theworld, those are important as potential resources of the rawmaterials for production of industrial grade caustic, dead-burned and fused magnesia or magnesium metal.

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