Mineralogy , Geochemistry and Stable Isotope Studies of the Dopolan Bauxite Deposit , Zagros Mountain , Iran

The Dopolan deposit is a Mediterranean-type bauxite located in the Zagros Fold-Thrust Zone, Iran. This deposit consists of five lithological members including iron-rich, clay-rich, oolitic, pisolitic and organic matter-containing bauxites. The mineralogy of the deposit includes diaspore, boehmite, and kaolinite, nacrite, with minor pyrite, anatase and rutile. Geochemical studies show that light rare earth elements (LREEs) are enriched relative to heavy rare earth elements (HREEs) in all members, supporting an authigenic origin. Mass changes based on Ti as an immobile element indicate that conventionally-immobile elements (Al, Nb, Ta, Zr, Hf) are enriched in situ in the residual units, whereas mobile elements (K, Ca, Si) were depleted during bauxitization. This study shows that the Khaneh–Kat argillitic dolomite is the likely parent rock. The δ18O (7.63‰to 9.35‰) and δD values ( ́49.91‰to  ́66.49‰) for kaolinite in the bauxite samples suggest equilibration with meteoric waters which supports a supergene origin. Bauxitization occurred in a warm climate with relatively constant isotopic composition suggesting climate stability during the development of bauxite horizons and remobilization of Al (with formation of secondary boehmite). The δ13C values of calcite ( ́7.3‰) in the bauxite support the idea that the Khaneh–Kat Formation has experienced post-depositional isotope exchange with meteoric waters during the karstification process.

Stable isotopes (H, C and O) have been used to study bauxite genesis and indirectly determine paleo-environments associated with its formation [27].The δ 18 O and δD values of minerals such as kaolinite, goethite, gibbsite and boehmite can be used to deduce information about the conditions that prevailed during bauxitization [28].Hydrogen isotope ratios may be affected by post-formation isotope exchange with meteoric water at low surficial temperature or higher temperatures as a result of diagenetic conditions [29].Carbon isotope composition of organic matter can indicate the role of microbial activity during deposition and mineralization of carbonate which may reflect environmental conditions and biogeochemical processes that controlled the formation of bauxite [30].
The most important known bauxite deposits in Iran are distributed in four regions, namely: (1) Sanandaj-Sirjan belt, Permo-Triassic and late Triassic in Bukan and Kanshiteh; (2) The Zagros Fold belt, Upper Cretaceous, Triassic in Mandan, Deh-now, Sarfaryab and Dopolan [9]; (3) The Alborz mountains, Upper Triassic in Tilabad and Jajarm [31]; and (4) The central plateau of Iran, Upper Triassic, Permo-Triassic in Bazargan and Balbaloyeye [31] (Figure 1).Formation of bauxite horizons in the Phanerozoic was a climate dependent process; it has been established that the Phanerozoic bauxites were generally formed in hot or warm humid climates [26].These conditions allowed intensive weathering of carbonate rocks, karstification and subsequent bauxite development [26].
Stable isotopes (H, C and O) have been used to study bauxite genesis and indirectly determine paleo-environments associated with its formation [27].The δ 18 O and δD values of minerals such as kaolinite, goethite, gibbsite and boehmite can be used to deduce information about the conditions that prevailed during bauxitization [28].Hydrogen isotope ratios may be affected by post-formation isotope exchange with meteoric water at low surficial temperature or higher temperatures as a result of diagenetic conditions [29].Carbon isotope composition of organic matter can indicate the role of microbial activity during deposition and mineralization of carbonate which may reflect environmental conditions and biogeochemical processes that controlled the formation of bauxite [30].
The most important known bauxite deposits in Iran are distributed in four regions, namely: (1) Sanandaj-Sirjan belt, Permo-Triassic and late Triassic in Bukan and Kanshiteh; (2) The Zagros Fold belt, Upper Cretaceous, Triassic in Mandan, Deh-now, Sarfaryab and Dopolan [9]; (3) The Alborz mountains, Upper Triassic in Tilabad and Jajarm [31]; and (4) The central plateau of Iran, Upper Triassic, Permo-Triassic in Bazargan and Balbaloyeye [31] (Figure 1).The Triassic Dopolan bauxite deposit is located in the Chaharmahal and Bakhtiary province in a high Zagros structural zone.The Dopolan bauxite deposit has an estimated reserve of 8-15 million tons with an average grade of 45% Al2O3 [32].Previous studies of the Dopolan bauxite deposit show that the bauxite formed as a continental deposit filling karstic cavities at the boundary of the Khaneh-Kat and Neyriz Formations [33].The aim of this study is to determine the precursor rock and understand the bauxitization process during formation of this deposit.To attain these objectives, detailed geological, mineralogical, petrographical and geochemical investigations (including 18 O and D isotopes) were carried out and the results are discussed here.The Triassic Dopolan bauxite deposit is located in the Chaharmahal and Bakhtiary province in a high Zagros structural zone.The Dopolan bauxite deposit has an estimated reserve of 8-15 million tons with an average grade of 45% Al 2 O 3 [32].Previous studies of the Dopolan bauxite deposit show that the bauxite formed as a continental deposit filling karstic cavities at the boundary of the Khaneh-Kat and Neyriz Formations [33].The aim of this study is to determine the precursor rock and understand the bauxitization process during formation of this deposit.To attain these objectives, Minerals 2016, 6, 11 3 of 21 detailed geological, mineralogical, petrographical and geochemical investigations (including 18 O and D isotopes) were carried out and the results are discussed here.

Geology
The Dopolan deposit is located in the High Zagros thrust zone near the border of Zagros and Central Iran zones.The structural framework of the thrust zone is complex, with significant folding and faulting.The Zagros fold belt evolved during the collision event between the Arabian and Eurasian plates [34] during late Cretaceous [35] and Pliocene [36].
The Zagros orogenic/metallogenic belt extends from the Turkish-Iranian border in NW to the Makran zone in SE Iran [37].This belt consists of five tectonically related parallel zones: Zagros simple folded belt, the "Crushed zone" or High Zagros, the Main Zagros thrust, the Sanandaj-Sirjan zone, and the Urumieh-Dokhtar magmatic arc [38].The High Zagros is characterized by earlier deformation, large offsets on basement faults, steep contacts with surrounding zones and more ductile deformation [39] (Figure 1).The geomorphology of the Dopolan area is characterized by many mountain peaks, up to 2200 m high separated by deep valleys.The Dopolan bauxite is located in a large structure called the Sabzkuh-Kelar Synclinorium, which is bounded by two thrust faults.In the studied area, the Zagros stratigraphy consists of Cambrian to Quaternary sequences.The oldest strata are located on limbs and youngest rocks in the core of sycnclinorium, uplifted by faults [32].

Geology
The Dopolan deposit is located in the High Zagros thrust zone near the border of Zagros and Central Iran zones.The structural framework of the thrust zone is complex, with significant folding and faulting.The Zagros fold belt evolved during the collision event between the Arabian and Eurasian plates [34] during late Cretaceous [35] and Pliocene [36].
The Zagros orogenic/metallogenic belt extends from the Turkish-Iranian border in NW to the Makran zone in SE Iran [37].This belt consists of five tectonically related parallel zones: Zagros simple folded belt, the "Crushed zone" or High Zagros, the Main Zagros thrust, the Sanandaj-Sirjan zone, and the Urumieh-Dokhtar magmatic arc [38].The High Zagros is characterized by earlier deformation, large offsets on basement faults, steep contacts with surrounding zones and more ductile deformation [39] (Figure 1).The geomorphology of the Dopolan area is characterized by many mountain peaks, up to 2200 m high separated by deep valleys.The Dopolan bauxite is located in a large structure called the Sabzkuh-Kelar Synclinorium, which is bounded by two thrust faults.In the studied area, the Zagros stratigraphy consists of Cambrian to Quaternary sequences.The oldest strata are located on limbs and youngest rocks in the core of sycnclinorium, uplifted by faults [32].
In the Dopolan deposit, the lithostratigraphic column includes pinkish Permian dolomite (Dalan Formation) at the base, overlain by the Upper Triassic argillitic dolomite (Khaneh-Kat Formation), and Jurassic dolomite (Neyriz Formation) (Figure 2).Argillite and bauxite ore layers lie above the Triassic brecciated argillitic dolomite (Figure 3A).The Khaneh-Kat Formation is predominantly composed of dolostone and marly limestone with a matrix of calcite (Figure 3B) [41].This formation consists of a massive crystalline and porous dolomite at the top with more clay-rich carbonates and shale in the lower parts [42].The Argillite and bauxite ore layers lie above the Triassic brecciated argillitic dolomite (Figure 3A).The Khaneh-Kat Formation is predominantly composed of dolostone and marly limestone with a matrix of calcite (Figure 3B) [41].This formation consists of a massive crystalline and porous dolomite at the top with more clay-rich carbonates and shale in the lower parts [42].The Khaneh-Kat Formation is intensely karstified with bauxite filled karst and caves (sinkholes) at the top of the Khaneh-Kat Formation (Figure 3C,D).Karstification in the Khaneh-Kat Formation is the result of specific paleo-climatic regimes as climate and time are important to the development of extensive karst landforms [43].The studied deposits are situated along a disconformity between the Khaneh-Kat and Neyriz Formations, implying extended periods of subaerial exposure (karstification, weathering, bauxitization).Khaneh-Kat Formation is intensely karstified with bauxite filled karst and caves (sinkholes) at the top of the Khaneh-Kat Formation (Figure 3C,D).Karstification in the Khaneh-Kat Formation is the result of specific paleo-climatic regimes as climate and time are important to the development of extensive karst landforms [43].The studied deposits are situated along a disconformity between the Khaneh-Kat and Neyriz Formations, implying extended periods of subaerial exposure (karstification, weathering, bauxitization).The main bauxite horizon occurs at the contact of the Khaneh-Kat and Neyriz Formations where bauxite-filled paleo-sinkholes in the Khaneh-Kat Formation.The main bauxite horizon is composed of five distinct layers including (from bottom to top): iron-rich, clay-rich, oolitic (powdery white bauxite), pisolitic and organic-rich (black) bauxite layers (Figure 4).The lower part of the bauxite horizon is comprised of iron-rich bauxite, ~1.5 m thick, which directly overlies the Khaneh-Kat Formation.The mineral assemblage of this unit is pyrite, kaolinite and boehmite [44].Clay-rich, oolitic and pisolitic bauxite layers are intercalated between the The main bauxite horizon occurs at the contact of the Khaneh-Kat and Neyriz Formations where bauxite-filled paleo-sinkholes in the Khaneh-Kat Formation.The main bauxite horizon is composed of five distinct layers including (from bottom to top): iron-rich, clay-rich, oolitic (powdery white bauxite), pisolitic and organic-rich (black) bauxite layers (Figure 4).Khaneh-Kat Formation is intensely karstified with bauxite filled karst and caves (sinkholes) at the top of the Khaneh-Kat Formation (Figure 3C,D).Karstification in the Khaneh-Kat Formation is the result of specific paleo-climatic regimes as climate and time are important to the development of extensive karst landforms [43].The studied deposits are situated along a disconformity between the Khaneh-Kat and Neyriz Formations, implying extended periods of subaerial exposure (karstification, weathering, bauxitization).The main bauxite horizon occurs at the contact of the Khaneh-Kat and Neyriz Formations where bauxite-filled paleo-sinkholes in the Khaneh-Kat Formation.The main bauxite horizon is composed of five distinct layers including (from bottom to top): iron-rich, clay-rich, oolitic (powdery white bauxite), pisolitic and organic-rich (black) bauxite layers (Figure 4).The lower part of the bauxite horizon is comprised of iron-rich bauxite, ~1.5 m thick, which directly overlies the Khaneh-Kat Formation.The mineral assemblage of this unit is pyrite, kaolinite and boehmite [44].Clay-rich, oolitic and pisolitic bauxite layers are intercalated between the The lower part of the bauxite horizon is comprised of iron-rich bauxite, ~1.5 m thick, which directly overlies the Khaneh-Kat Formation.The mineral assemblage of this unit is pyrite, kaolinite and boehmite [44].Clay-rich, oolitic and pisolitic bauxite layers are intercalated between the iron-rich bauxite at the base and organic-rich bauxite at the top.The clay-rich bauxite, ~5 m thick, has grey to light grey color and shows fine-grained oolitic texture.It is the main ore zone with 53% Al 2 O 3 , 2% TiO 2 .Kaolinite, diaspore, boehmite and quartz as main minerals [32].Field evidence such as presence of plant root casts show that the deposit formed in a marsh sedimentary basin.The clay-rich bauxite is overlain by the oolitic bauxite which is characterized by a vuggy structure, fine-grained (0.4-5 mm) oolitic texture, low density, with mineralogy dominated by diaspore and boehmite.Pisolitic bauxite, 0.5-3 m thick, overlies the oolitic bauxite.This layer contains 1-5 cm dark grey hard concretions with abundant diaspore and an average of 70% Al 2 O 3 , 3% Fe 2 O 3 and 2% TiO 2 [32].The transition between the upper pisolitic bauxite and the lower part of the black organic-rich bauxite is gradual and indistinct.The average thickness of the organic-rich bauxite is 1.20 m with kaolinite, a small amount of nacrite and rutile, and abundant plant remnants, which are locally converted to small lenses or thin coal layers.

Methodology
Field investigations were carried out over two periods (2013-2014) during which 60 specimens were sampled from the bauxite quarry and argillite (Khaneh-Kat Formation).Thirty-four samples of the five identified bauxite horizons were selected for petrographic studies.A detailed mineralogical analysis of the bauxite samples was performed by X-Ray diffraction (XRD) at the Geological Survey of Iran (Tabriz center) using a Philiphs X pert Step-Tro Model D-5000 diffractometer (Philiphs, Amsterdam, The Netherlands) with Co Kα (1.789 Å) radiation, fixed graphite monochromators, voltage 40 kV current 30 mA, 0.02 step size, scan range 4 ˝-80 ˝, drive axis 2θ.Major and trace element content in the representative samples were obtained using inductively coupled plasma mass spectrometry (ICP-MS) in the Acme Analytical Laboratories, Vancouver, BC, Canada.Sample preparation was based on digestion of 0.2 g sample in lithium metaborate flux and fusion in a furnace at 1000 ˝C.The melt is then dissolved in 100 mL diluted HCl acid.
Before isotopic measurement, samples were crushed with mortar and pure minerals were separated using the method of Bird et al. (1992) [45].Stable isotope analysis was carried out at Cornell Isotope Laboratory in the United States.Isotope ratios were determined using a high precision Thermo Delta V isotope ratio mass spectrometer (Bremen, Germany) interfaced to a Temperature Conversion Elemental Analyzer (TC/EA).Analyses were performed utilizing several in-house and commercial standards for internal checks on instrument accuracy and precision.Results for δ 18 O were compared against the Vienna Standard Mean Ocean Water (V-SMOW) [46].Carbon isotope values reported in standard δ notation in units of per mil (‰) relative to the [46] Vienna Peedee Belemnite standard.The δ 13 C values were reproducible to ˘0.2‰.

Mineralogy
Mineralogical and textural investigations based on optical microscopy and XRD analysis show that diaspore, kaolinite and nacrite are the main mineral components in the bauxite horizons with boehmite, anatase, rutile and muscovite as minor phases (Table 1).Diaspore is the dominant Al mineral found in the Dopolan bauxite.The matrix, authigenic in origin, shows predominantly pelitomorphic, microgranular and panidiomorphic textures; however, there are also some terrigenous grains such as intraclasts, ooids and erosional ooid nucleus which suggest an allochthonous origin for some components of the deposit.In addition, pelitomorphic and fluidal textures in this deposit suggest allochthonous bauxitization of the parent rock.This may be due to insufficient speed to remove dissolved silica that led to the kaolinite formation.
Two forms of pyrite occur in bauxite horizons: framboidal pyrite and euhedral pyrite (Figure 5A,B).Framboidal pyrite commonly forms during early diagenesis under organic-rich, anoxic conditions [7,11,47,48].Euhedral pyrite occurs as cubic grain filling fractures and is interpreted to have formed during the last stages of diagenesis.Petrographic observations show the fragments in the central part of the pisolites are filled with the secondary porous matrix composed of kaolinite (Figure 5C,D).The pisolitic bauxite is characterized by the presence of pisolitic textures.Microscopic investigation suggests two pisolite forms: (1) simple pisolites with a kaolinite core surrounded by a preserved cortex of boehmite (Figure 5E); (2) complex residual rounded fragments typical of pisolitic bauxite, so called allogenic pisolites (Figure 5D) [49].Micromorphological evidence such as pisolites with simple, complex and oolitic cores indicate that deposit can be divided into two types-autochthonous and allochthonous (Figure 5C-E).surrounded by a preserved cortex of boehmite (Figure 5E); (2) complex residual rounded fragments typical of pisolitic bauxite, so called allogenic pisolites (Figure 5D) [49].Micromorphological evidence such as pisolites with simple, complex and oolitic cores indicate that deposit can be divided into two types-autochthonous and allochthonous (Figure 5C-E).

Major and Trace Elements
Whole rock major, trace and rare earth element (REE) analyses of bauxite horizons are provided in Table 2.The average of Al2O3 content of the Dopolan bauxites varies between 26 wt % and

Major and Trace Elements
Whole rock major, trace and rare earth element (REE) analyses of bauxite horizons are provided in Table 2.The average of Al 2 O 3 content of the Dopolan bauxites varies between 26 wt % and 72 wt %, while the average content of Fe 2 O 3 and SiO 2 commonly ranges between 1 wt % and 17 wt % and 1 wt % and 35 wt %, respectively (Table 2).The high SiO 2 content of the bauxite facies is a result of weak lateritization [8] and the presence of kaolinite [11].Based on the Al 2 O 3 -Fe 2 O 3 -SiO 2 ternary diagram [50], organic-rich and clay-rich bauxite horizons show weak and moderate lateritization, respectively, whereas oolitic, pisolitic and iron-rich bauxites plot in the strong lateritization field (Figure 6).Furthermore, the Dopolan bauxite samples plot in the bauxite and kaolinite bauxite fields on the Al 2 O 3 -Fe 2 O 3 -SiO 2 ternary diagram (Figure 7) suggesting tropical paleo-geographic conditions which favored bauxitization and the formation of bauxite horizons [9,51].
The high SiO2 content of the bauxite facies is a result of weak lateritization [8] and the presence of kaolinite [11].Based on the Al2O3-Fe2O3-SiO2 ternary diagram [50], organic-rich and clay-rich bauxite horizons show weak and moderate lateritization, respectively, whereas oolitic, pisolitic and iron-rich bauxites plot in the strong lateritization field (Figure 6).Furthermore, the Dopolan bauxite samples plot in the bauxite and kaolinite bauxite fields on the Al2O3-Fe2O3-SiO2 ternary diagram (Figure 7) suggesting tropical paleo-geographic conditions which favored bauxitization and the formation of bauxite horizons [9,51].High content of trace elements such as Cr (132-1088 ppm), Zr (132-557 ppm) and Nb (28-77 ppm) is a significant and common feature of the Dopolan bauxite (Table 2).Trace elements such as Cr, Ni, V, Co, Zr are considered bauxitophilic [53] and they are used in geochemical calculations such as mass change and identification of parent rocks.Trace elements such as Zr, V, and Ga are enriched in all parts of the bauxite; these trace elements are low in the Khaneh-Kat Formation.Zr, Hf, Nb and Ta are immobile elements and show positive correlations (R 2 = 0.64 to 0.87) with TiO2 (Figure 8).The high SiO2 content of the bauxite facies is a result of weak lateritization [8] and the presence of kaolinite [11].Based on the Al2O3-Fe2O3-SiO2 ternary diagram [50], organic-rich and clay-rich bauxite horizons show weak and moderate lateritization, respectively, whereas oolitic, pisolitic and iron-rich bauxites plot in the strong lateritization field (Figure 6).Furthermore, the Dopolan bauxite samples plot in the bauxite and kaolinite bauxite fields on the Al2O3-Fe2O3-SiO2 ternary diagram (Figure 7) suggesting tropical paleo-geographic conditions which favored bauxitization and the formation of bauxite horizons [9,51].High content of trace elements such as Cr (132-1088 ppm), Zr (132-557 ppm) and Nb (28-77 ppm) is a significant and common feature of the Dopolan bauxite (Table 2).Trace elements such as Cr, Ni, V, Co, Zr are considered bauxitophilic [53] and they are used in geochemical calculations such as mass change and identification of parent rocks.Trace elements such as Zr, V, and Ga are enriched in all parts of the bauxite; these trace elements are low in the Khaneh-Kat Formation.Zr, Hf, Nb and Ta are immobile elements and show positive correlations (R 2 = 0.64 to 0.87) with TiO2 (Figure 8).High content of trace elements such as Cr (132-1088 ppm), Zr (132-557 ppm) and Nb (28-77 ppm) is a significant and common feature of the Dopolan bauxite (Table 2).Trace elements such as Cr, Ni, V, Co, Zr are considered bauxitophilic [53] and they are used in geochemical calculations such as mass change and identification of parent rocks.Trace elements such as Zr, V, and Ga are enriched in all parts of the bauxite; these trace elements are low in the Khaneh-Kat Formation.Zr, Hf, Nb and Ta are immobile elements and show positive correlations (R 2 = 0.64 to 0.87) with TiO 2 (Figure 8).

Rare Earth Elements
The chondrite-normalized REE diagram of the bauxite horizons and argillitic dolomite from the Khaneh-Kat Formation shows a similar pattern with light rare earth element (LREE) enrichment and heavy rare earth element (HREE) depletion and a negative Eu anomaly (Figure 9).The ∑REE of bauxite samples vary from 480 to 1140 ppm (Table 3).There is also a decreasing trend in ∑REE in the bauxite profile from the pisolitic bauxite (1037 ppm) to the iron-rich bauxite (890 ppm).The organic-rich bauxite contains low ∑REE values (522 ppm) (Table 3).

Rare Earth Elements
The chondrite-normalized REE diagram of the bauxite horizons and argillitic dolomite from the Khaneh-Kat Formation shows a similar pattern with light rare earth element (LREE) enrichment and heavy rare earth element (HREE) depletion and a negative Eu anomaly (Figure 9).

Rare Earth Elements
The chondrite-normalized REE diagram of the bauxite horizons and argillitic dolomite from the Khaneh-Kat Formation shows a similar pattern with light rare earth element (LREE) enrichment and heavy rare earth element (HREE) depletion and a negative Eu anomaly (Figure 9).The ∑REE of bauxite samples vary from 480 to 1140 ppm (Table 3).There is also a decreasing trend in ∑REE in the bauxite profile from the pisolitic bauxite (1037 ppm) to the iron-rich bauxite (890 ppm).The organic-rich bauxite contains low ∑REE values (522 ppm) (Table 3).3).There is also a decreasing trend in ř REE in the bauxite profile from the pisolitic bauxite (1037 ppm) to the iron-rich bauxite (890 ppm).The organic-rich bauxite contains low ř REE values (522 ppm) (Table 3).Boynton, 1984 [54].Ny = Nyriz Formation, Ch. = Chondrite.
The ř REE content of the Kaneh-Kat Formation (27 ppm) is within the average range of carbonate rocks (23-27 ppm; [46].Iron-rich and clay-rich bauxites with the lowest content of ř REE the carbonate bed rock.Enrichment of LREEs in the bauxitic zone is a result of authigenic processes [13].
The chondrite-normalized REE patterns of different bauxite horizons and Khaneh-Kat Formation show similar patterns with negative Eu anomalies (0.04-0.08;Table 3).All sequences of the bauxites and bedrock (Khaneh-Kat dolomite) display negative Eu anomalies.Eu/Eu * anomalies in all bauxite horizons show that this ratio is conservative.Variations of La/Y ratio in the bauxites have been used for determining change in pH during weathering [11,13,55]; values of La/Y < 1 imply prevalence of acidic conditions whereas values of La/Y > 1 indicate basic conditions (Figure 10).Calculated La/Y variations in bauxites can be related to LREE variations in the bauxite profile suggesting that the leaching of LREE in the pisolitic bauxite may be the result of low pH conditions.Ce/Ce * ratio in the 0.86-1.73range (Table 3) suggests that Ce +3 was leached out from the oolitic and pisolitc bauxite and was available to be transported to deeper layers and precipitated near the bed rock [15].The ∑REE content of the Kaneh-Kat Formation (27 ppm) is within the average range of carbonate rocks (23-27 ppm; [46].Iron-rich and clay-rich bauxites with the lowest content of ∑REE the carbonate bed rock.Enrichment of LREEs in the bauxitic zone is a result of authigenic processes [13]. The chondrite-normalized REE patterns of different bauxite horizons and Khaneh-Kat Formation show similar patterns with negative Eu anomalies (0.04-0.08;Table 3).All sequences of the bauxites and bedrock (Khaneh-Kat dolomite) display negative Eu anomalies.Eu/Eu * anomalies in all bauxite horizons show that this ratio is conservative.Variations of La/Y ratio in the bauxites have been used for determining change in pH during weathering [11,13,55]; values of La/Y < 1 imply prevalence of acidic conditions whereas values of La/Y > 1 indicate basic conditions (Figure 10).Calculated La/Y variations in bauxites can be related to LREE variations in the bauxite profile suggesting that the leaching of LREE in the pisolitic bauxite may be the result of low pH conditions.Ce/Ce * ratio in the 0.86-1.73range (Table 3) suggests that Ce +3 was leached out from the oolitic and pisolitc bauxite and was available to be transported to deeper layers and precipitated near the bed rock [15].

Protolith
The Cr-Ni binary diagram can be used to predict the protolith of the bauxite deposit [2,11,22,56]; Dopolan samples plot in the karst bauxite field (Figure 11).Karst bauxites form on a variety of parent rocks such as carbonates, rock debris, volcanic ash, and wind-born material [22].To identify the protolith of the Dopolan bauxite, calculated correlation coefficients of major elements are used.The strong positive correlation between Ti and Al in the Dopolan deposit suggests a relationship between the bauxite horizons and the argillitic dolomite of the Khaneh-Kat Formation (Figure 12).

Protolith
The Cr-Ni binary diagram can be used to predict the protolith of the bauxite deposit [2,11,22,56]; Dopolan samples plot in the karst bauxite field (Figure 11).Karst bauxites form on a variety of parent rocks such as carbonates, rock debris, volcanic ash, and wind-born material [22].To identify the protolith of the Dopolan bauxite, calculated correlation coefficients of major elements are used.The strong positive correlation between Ti and Al in the Dopolan deposit suggests a relationship between the bauxite horizons and the argillitic dolomite of the Khaneh-Kat Formation (Figure 12).The ∑REE content of the Kaneh-Kat Formation (27 ppm) is within the average range of carbonate rocks (23-27 ppm; [46].Iron-rich and clay-rich bauxites with the lowest content of ∑REE the carbonate bed rock.Enrichment of LREEs in the bauxitic zone is a result of authigenic processes [13]. The chondrite-normalized REE patterns of different bauxite horizons and Khaneh-Kat Formation show similar patterns with negative Eu anomalies (0.04-0.08;Table 3).All sequences of the bauxites and bedrock (Khaneh-Kat dolomite) display negative Eu anomalies.Eu/Eu * anomalies in all bauxite horizons show that this ratio is conservative.Variations of La/Y ratio in the bauxites have been used for determining change in pH during weathering [11,13,55]; values of La/Y < 1 imply prevalence of acidic conditions whereas values of La/Y > 1 indicate basic conditions (Figure 10).Calculated La/Y variations in bauxites can be related to LREE variations in the bauxite profile suggesting that the leaching of LREE in the pisolitic bauxite may be the result of low pH conditions.Ce/Ce * ratio in the 0.86-1.73range (Table 3) suggests that Ce +3 was leached out from the oolitic and pisolitc bauxite and was available to be transported to deeper layers and precipitated near the bed rock [15].

Protolith
The Cr-Ni binary diagram can be used to predict the protolith of the bauxite deposit [2,11,22,56]; Dopolan samples plot in the karst bauxite field (Figure 11).Karst bauxites form on a variety of parent rocks such as carbonates, rock debris, volcanic ash, and wind-born material [22].To identify the protolith of the Dopolan bauxite, calculated correlation coefficients of major elements are used.The strong positive correlation between Ti and Al in the Dopolan deposit suggests a relationship between the bauxite horizons and the argillitic dolomite of the Khaneh-Kat Formation (Figure 12).

Mass Change
Mass change is associated with enrichment or depletion of elements during bauxitization [2,10,14].The content of immobile elements in relatively fresh (parent) rock and weathered rocks can be used to calculate mass changes during weathering.In the residual accumulated bauxite ore, elements such as Ti, Zr, Hf, Nb, Ta, Cr and Ni are considered to be relatively immobile.Loss and/or gain in mass of the weathered Dopolan profiles were calculated based on Ti, which is strongly immobile during weathering of bauxite [14].These calculations are based on Maclean [57] expression: where EF= TiO2protolith/TiO2bauxite (Table 4), RC = % component in altered rock × EF (Table 4); and MC = RC − precursor composition (Table 5).

Mass Change
Mass change is associated with enrichment or depletion of elements during bauxitization [2,10,14].The content of immobile elements in relatively fresh (parent) rock and weathered rocks can be used to calculate mass changes during weathering.In the residual accumulated bauxite ore, elements such as Ti, Zr, Hf, Nb, Ta, Cr and Ni are considered to be relatively immobile.Loss and/or gain in mass of the weathered Dopolan profiles were calculated based on Ti, which is strongly immobile during weathering of bauxite [14].These calculations are based on Maclean [57] expression: where EF= TiO 2protolith /TiO 2bauxite (Table 4), RC = % component in altered rock ˆEF (Table 4); and MC = RC ´precursor composition (Table 5).Al 2 O 3 is enriched in the bauxite horizons especially in the pisolitic and oolitic bauxite (Figure 13).Iron enrichment in the iron-rich bauxite is due to the presence of pyrite, whereas all other bauxite horizons show Fe depletion.The highest Si enrichment is seen in the organic-rich bauxite which is related to the Eh-pH conditions and the presence of clay minerals.Ca, K, Na and P 2 O 5 were leached out of weathering protolith whereas Al was enriched in the residual debris.

Isotopic Signatures
The geochemical characteristics of O and D isotopes have been widely used to study bauxitization processes and paleo-climate changes (e.g., [58]).Hydroxyl (OH) bearing minerals such as kaolinite and boehmite are considered to be one of the main controls on the isotopic composition of bauxite deposits [28].
Oxygen and hydrogen isotope compositions of the footwall dolomite (Khaneh-Kat Formation) are +28.8‰and ´59.7‰, respectively (Table 6).δ 18 O and δD values of various bauxite horizons range from +7.63‰ to +9.35‰ and ´66.49‰ to ´49.91‰, respectively (Table 6 and Figure 14).One sample from the calcite cement of the footwall dolomite shows δ 13 C value of ´7.34‰ (Table 6).Such depleted δ 13 C values are considered to reflect subaerial exposure and influence of the karstification associated with the unconformity surface [48].as kaolinite and boehmite are considered to be one of the main controls on the isotopic composition of bauxite deposits [28].

Discussion
In karstic terranes, soluble bedrocks such as limestone and dolomite are dissolved by CO2-enriched water.Karstification creates space for bauxite deposition.Bauxite accumulated in the karst cavities is more protected from erosion compared to regolith on silicate rocks [11,62].Such a process leads to the formation of a specific type of karst called ore karst which occurs only on the exokarst floor [63].The term was used in carbonate rocks wherein the ore was developed due to dissolution of carbonates by acidic fluids related to oxidation of sulfide ores [63].Mineralogy, karst characteristics, wall-rock relationship, and geological setting indicate that the Triassic Dopolan bauxite deposit formed in a karstic environment.Boehmite, diaspore, kaolinite, nacrite, pyrite,  ), Yaou (after [58]) and Dehdasht after [27].Kaolinite line (after [59]).SMOW = Standard Mean Ocean Water; S/H = supergene/hypogene line after [61].

Discussion
In karstic terranes, soluble bedrocks such as limestone and dolomite are dissolved by CO2-enriched water.Karstification creates space for bauxite deposition.Bauxite accumulated in the karst cavities is more protected from erosion compared to regolith on silicate rocks [11,62].Such a process leads to the formation of a specific type of karst called ore karst which occurs only on the exokarst floor [63].The term was used in carbonate rocks wherein the ore was developed due to dissolution of carbonates by acidic fluids related to oxidation of sulfide ores [63].Mineralogy, karst characteristics, wall-rock relationship, and geological setting indicate that the Triassic Dopolan bauxite deposit formed in a karstic environment.Boehmite, diaspore, kaolinite, nacrite, pyrite,  [60]), Yaou (after [58]) and Dehdasht after [27].Kaolinite line (after [59]).SMOW = Standard Mean Ocean Water; S/H = supergene/hypogene line after [61].

Discussion
In karstic terranes, soluble bedrocks such as limestone and dolomite are dissolved by CO 2 -enriched water.Karstification creates space for bauxite deposition.Bauxite accumulated in the karst cavities is more protected from erosion compared to regolith on silicate rocks [11,62].Such a process leads to the formation of a specific type of karst called ore karst which occurs only on the exokarst floor [63].The term was used in carbonate rocks wherein the ore was developed due to dissolution of carbonates by acidic fluids related to oxidation of sulfide ores [63].Mineralogy, karst characteristics, wall-rock relationship, and geological setting indicate that the Triassic Dopolan bauxite deposit formed in a karstic environment.Boehmite, diaspore, kaolinite, nacrite, pyrite, anatase and rutile are common minerals.The matrix type suggests an authigenic origin for bauxite; however, terrigenous grains such as intraclasts, ooids and erosional ooid nucleus imply a semi-authigenic origin, at least for part of the deposit.Also, the presence of angular bauxite and detrital phases indicates some displacement into or within the karstic area.Gibbsite can transform to boehmite in surface environments and boehmite can transform to diaspore in a near-surface environment [18,[64][65][66][67]. Kaolinite and other clays such as nacrite can also be present in bauxite deposits [53].The pisolitic texture results from aggregation of discolored pisolites showing evidence of deferrification process.Deferrification in pisolitic bauxites may be controlled by organic ligands which affect the solubility and dissolution of iron oxides [7].Micro-organisms can also reduce Fe 3+ to Fe 2+ in solutions to provide energy for metabolism potentially leading to deferrification [68].The organic ligands could have also contributed to the formation of acidic (pH between 3 and 6) and reducing conditions which prevailed during bauxitization in the Dopolan area.
Trace element composition (e.g., Cr, Ni; Figure 11) show that the Dopolan deposit is a karst bauxite type which located in the Zagros Fold-Thrust Zone.The Zagros fold-thrust belt contains the most important sedimentary basins in Iran, wherein sedimentary rocks reach depths of more than 12 km and are comprised mainly of carbonate and detrital sediments.The lack of magmatic units suggests that the Khaneh-Kat argillic dolomite can be considered as the likely parent rock for the Dopolan bauxite deposit.The similarity of REE pattern of bauxite horizons and the Khaneh-Kat argillic dolomite support that the Khaneh-Kat could be parent rock of the Dopolan bauxite [69].Furthermore, the REEs show affinity to mobilization during bauxitization and subsequent redistribution [15].The REE patterns demonstrate that bauxite samples are enriched in REEs, possibly due to prolonged tropical weathering which led to the breakdown of rock-forming minerals [70,71].
Minor REE variations across the bauxite horizons probably reflect Eh and pH fluctuations during formation of each horizon [72].Mobility of LREEs depends on pH variation in the bauxite profile in the weathering system.Variations of La/Y ratio in bauxite have been used to study pH change during weathering [11,13]; values of La/Y < 1 imply prevalence of acidic conditions whereas values of La/Y > 1 indicate basic conditions (Figure 10).Iron-rich, clay-rich, oolitic, and organic-rich bauxite horizons of Dopolan are enriched in LREEs suggesting high pH conditions during formation of these bauxites (Figure 10).However, the pisolitic bauxite is depleted in LREEs possibly reflecting low pH conditions during formation (Figure 10).The acidic solutions probably resulted from weathering of pyrite which explains HREEs' enrichment in the pisolitic bauxite [73].
Positive correlation between REEs and some major elements implies that the REEs are hosted in the specific minerals (Table 7) or both REEs, and major elements have been enriched by the same processes.In the Dopolan deposit, Ti has positive correlation with all REEs (Table 7) which probably suggests that distribution of REE is controlled by the formation of authigenic heavy minerals such as rutile and anatase [74].The strong positive correlations (R = 0.71-0.80)between HREEs and Al 2 O 3 suggest (Table 7) that they may be hosted by Al oxides (e.g., diaspore and boehmite minerals) or their relative content increased during bauxite formation processes.
In addition, variation in the amount of pyrite, organic matter and other minerals such as oxyhydroxides, phosphates and Fe oxides in the weathering profile can affect Eh-pH conditions under wet-tropical environments [7,13,75,76].The mass change calculations of the bauxite horizons show that Ca, K and P 2 O 5 were largely leached out of the weathering system whereas Al was enriched in the residual system.The bauxitization at Dopolan occurred along a karstification surface between the Khaneh-Kat and Neyriz Formations.Due to high CO 2 and temperature [77], continental weathering was intense during this time period.The depleted δ 13 C value (´7.34‰) supports this hypothesis that the Khaneh-Kat Formation has experienced post-depositional isotope exchange with meteoric waters during the karstification process.Similar to the Early Permian Australian bauxite deposits, the oxygen and hydrogen isotopic compositions of the Dopolan bauxite fall close to the boehmite line (Figures 15  and 17).The Dopolan kaolinite plots on the left of the kaolinite line and the supergene/hypogene line (S/H; Figure 17) indicating kaolinite equilibrium with meteoric water and supports the supergene origin for kaolinite.Furthermore, the isotopic composition of kaolinite (Figure 16) shows that the

Conclusions
The following conclusions can be made based on the data obtained from the Dopolan deposit.1. Field observation and geochemical data suggest that the Dopolan bauxite deposit (in the Zagros sedimentary basin) is a karst bauxite type and originated from the argillitic dolomite of the Khaneh-Kat Formation.
The main mineral assemblage of this deposit consists of diaspore, boehmite, and kaolinite.Oolitic, pisolitic, pelitomorphic, microcrystalline and microgranular are the most important textures.These textures suggest an authigenic origin for the deposit.However, the presence of angular bauxite, detrital phases, pelitomorphic and fluidal textures indicates some small displacement into or within the karstic area.
2. Mass change calculation shows silica dissolution and deferrification in the bauxite horizons, causing Al-enrichment and bauxite formation.
3. The chondrite-normalized REE patterns of the different bauxite horizons and the Khaneh-Kat dolomite are similar, which may suggest that the Khaneh-Kat dolomite is the parent rock.The REEs are most enriched in the pisolitic and oolitic bauxite which is located in the upper part of the bauxite horizon and lie within the strong laterization.Minor REE changes across the bauxite profile probably reflect Eh and pH variations in the bauxitic profile.
4. The Dopolan deposit shows a relatively constant isotopic condition during the development of bauxite horizons and remobilization of Al.The δ 18 O and δD values of kaolinite samples suggest kaolinite equilibrium with meteoric water which supports a supergene origin.
5. The δ 13 C value (´7.34‰) of Khaneh-Kat formation is lower than the values of normal Triassic carbonates (´1‰-+5‰): thus supporting that the Khaneh-Kat Formation experienced post-depositional isotope exchange with organic carbon by meteoric water during karstification processes.

Figure 1 .
Figure 1.Schematic map of Iran with location of the Zagros Fold Belt, Alborz Mountain, Central Iran, and distribution of bauxite deposits.

Figure 1 .
Figure 1.Schematic map of Iran with location of the Zagros Fold Belt, Alborz Mountain, Central Iran, and distribution of bauxite deposits.

Figure 4 .
Figure 4. Schematic profile of the Dopolan bauxite deposit; on the right diverse bauxite facies.

Figure 4 .
Figure 4. Schematic profile of the Dopolan bauxite deposit; on the right diverse bauxite facies.

Figure 4 .
Figure 4. Schematic profile of the Dopolan bauxite deposit; on the right diverse bauxite facies.

Figure 8 .
Figure 8. Variation diagrams showing correlations between TiO2 and trace elements in the Dopolan bauxite deposit.Red circles are carbonate samples, blue circles are bauxite samples.

Figure 9 .
Figure 9. Chondrite-normalized rare earth element (REE) patterns of the bauxite samples as well as argillitic dolomite from the Khaneh-Kat Formation.

Figure 8 .
Figure 8. Variation diagrams showing correlations between TiO 2 and trace elements in the Dopolan bauxite deposit.Red circles are carbonate samples, blue circles are bauxite samples.

Minerals 2016, 6 , 11 9 of 21 Figure 8 .
Figure 8. Variation diagrams showing correlations between TiO2 and trace elements in the Dopolan bauxite deposit.Red circles are carbonate samples, blue circles are bauxite samples.

Figure 9 .
Figure 9. Chondrite-normalized rare earth element (REE) patterns of the bauxite samples as well as argillitic dolomite from the Khaneh-Kat Formation.

Figure 9 .
Figure 9. Chondrite-normalized rare earth element (REE) patterns of the bauxite samples as well as argillitic dolomite from the Khaneh-Kat Formation.

Figure 12 .
Figure 12.Scatter plot of TiO2 versus Al2O3 in the Dopolan deposit.Red circles are carbonate samples, blue circles are bauxite samples.

Figure 12 .
Figure 12.Scatter plot of TiO 2 versus Al 2 O 3 in the Dopolan deposit.Red circles are carbonate samples, blue circles are bauxite samples.

Figure 14 .
Figure 14.Vertical variation of the δ 18 O and δD values of the Dopolan and the Khaneh-Kat Formation.

Figure 14 .
Figure 14.Vertical variation of the δ 18 O and δD values of the Dopolan and the Khaneh-Kat Formation.

Table 1 .
X-Ray diffraction (XRD) mineralogical results and textural characteristics of selected samples of the Dopolan deposit.

Table 1 .
X-Ray diffraction (XRD) mineralogical results and textural characteristics of selected samples of the Dopolan deposit.

Table 2 .
Major and trace element compositions of samples from the Dopolan deposit, Khaneh-Kat and Neyriz Formations.Detection limits for major and trace elements are 0.002% and 0.05 ppm, respectively.

Table 3 .
Rare earth element composition of samples from the Dopolan bauxite deposit.

Table 4 .
Calculated enrichment factor (EF) and reconstructed compositions (RC, in wt %) of samples from bauxite profiles of the Dopolan deposit.

Table 5 .
Mass changes (in %) of the Dopolan samples based on the reconstructed compositions (RC, in wt %).

Table 6 .
Stable isotope composition of selected samples from the Dopolan bauxite deposit.

Table 6 .
Stable isotope composition of selected samples from the Dopolan bauxite deposit.