فصلنامه زمین شناسی اقتصادی
سال چهاردهم شماره 1 (پیاپی 32، بهار 1401)

Varmazyar Pb–Zn (Ag) occurrence, 65 km north of Zanjan, is located in the Tarom–Hashtjin metallogenic belt (THMB). The THMB has been recognized as one of the most important epithermal metallogenic belts in Iran (Kouhestani et al., 2018b) that host numerous small- to medium-sized epithermal deposits (i.e., Gulojeh, Aqkand, Aliabad–Khanchy, Chodarchay, Khalyfehlou, Chargar, Zajkan, Marshoun, Abbasabad, Zehabad, and Shah Ali Beiglou). These epithermal deposits are temporally and spatially related to late Eocene granitoids (Mehrabi et al., 2016; Kouhestani et al., 2018b).Although the general geological characteristics of the region, where the Varmazyar occurrence is located, were determined (Faridi and Anvari, 2000), no detailed studies have been conducted on the mineralogy, geochemistry, and the ore-forming fluids characteristics of the Varmazyar occurrence. In this paper, we investigate the geology, mineralogy, geochemistry, fluid inclusions, and alteration styles of the Varmazyar occurrence to constrain its ore genesis. These results may have implication for the regional exploration of epithermal deposits in the THMB.

Detailed field work has been carried out at different scales in the Varmazyar area. A total of 70 samples were collected from various parts of ore veins and breccias, host tuff units and granitoid intrusion. The samples prepared for thin (n=15) and polished-thin (n=27) sections in the laboratory of University of Zanjan, Zanjan, Iran. Representative 7 samples from the mineralized veins and breccias, 1 sample from host intermediate tuff unit and 1 sample from barren and fresh granite intrusion, were analyzed for rare and rare earth elements using ICP–MS in the Zarazma Analytical Laboratories, Tehran, Iran.Fluid inclusion measurements have been conducted on 4 doubly polished thick (~150 μm) sections including crystalline quartz, and sphalerite from the second, and third stages of ore formation. Microthermometric measurements were performed using a Linkam THMSG-600 heating–freezing stage attached to a ZEISS microscope in the fluid inclusion laboratory of Iranian Mineral Processing Research Center, Tehran, Iran.

The geological units hosting the Varmazyar occurrence are mainly Eocene volcanic and volcaniclastic rocks that were intruded by late Eocene granitoids. The volcaniclastic rocks can be divided into two units as acidic (lithic tuff, lithic crystal tuff and crystal tuff) and intermediate (lithic crystal tuff, crystal tuff, and lithic tuff) units. They are metamorphosed to clinopyroxene hornfels facies near contact intrusions. The granodiorite intrusion is the main rock units in the Varmazyar area. It crops out mainly in the south, southwest and northeast of the Varmazyar occurrence. It ranges in composition from monzogranite to syenogranite and shows porphyritic and granular textures.Mineralization at Varmazyar occurs as epithermal base metal quartz-sulfide brecciated vein that occupy NS-trending faults in the Eocene acidic and intermediate tuff units.  The ore vein extends up to 300 m along, from several cm to 2–3 m wide, and generally dip steeply (65–80°) to the west. Wall-rock alterations developed at the Varmazyar occurrence include silicification, intermediate argillic, carbonate, and propylitic alteration; the first three are closely related to the Pb–Zn (Ag) mineralization. The alteration styles show a systematic zonation pattern, from the silica, via intermediate argillic, to propylitic alteration. Four stages of mineralization can be distinguished at Varmazyar. Stage 1 is represented by silicification of host rocks along with minor disseminated pyrite. This stage is a pre-ore stage and usually crosscut by later stages. Stage 2 is the main ore-stage at the Varmazyar occurrence. It is characterized by up to 5 cm wide quartz veins and breccias that contain variable amounts of disseminated galena, sphalerite, and minor pyrite. Clasts of this stage and associated wall-rock alteration have been recognized in the hydrothermal cements of stage 3 breccias. Stage 3 is marked by quartz-calcite-manganese oxides (psilomelane, pyrolusite, braunite) veins and breccia cements. It is usually crosscut previous mineralization stages and, in turn, is cut by stage 4 calcite veinlets. Stage 4 is a barren post-ore stage represented by < 1 mm wide calcite veinlets. This stage usually crosscuts previous ore stages. No sulfide minerals are recognized with stage 4. The ore minerals at Varmazyar formed as vein-veinlet and hydrothermal breccia cements, and show disseminated, vein-veinlet, brecciated, comb, crustiform, colloform, cockade, bladed, plumose, and vug infill textures. Galena, sphalerite, pyrite, psilomelane, and pyrolusite are the main ore minerals; smithsonite, cerussite, goethite, secondary pyrolusite, and braunite are supergene minerals. Quartz, calcite, and sericite are present in the gangue minerals. Comparison of Chondrite–normalized rare elements and REE patterns of host intermediate tuffs, barren and fresh granite intrusion, and the mineralized samples at Varmazyar indicate that mineralization is probably genetically related with granite intrusions. In this case, leaching of some elements from the host tuff units may have involved in mineralization. Ore-forming fluids associated with the quartz-sulfide veins are represented by two-phase aqueous inclusions and by H2O–NaCl fluids with moderate-temperature (135–249 °C) and low-salinity (0.2–6.4 wt.% NaCl equiv.). Fluid inclusion data indicates that fluid boiling and mixing were important processes in the evolution of the ore-forming fluids at Varmazyar. Our data suggest that Varmazyar is an example of intermediate-sulfidation type of epithermal base metal mineralization.

Iran hosts numerous porphyry and epithermal ore deposits which have mostly been formed at discrete time periods within different tectonic assemblages. Porphyry and epithermal ore deposits are considered to be the important sources of base metals in Iran. Well-known porphyry deposits include the Sarcheshmeh, Meiduk, Sungun, (Shahabpour and Kramers, 1987; Hezarkhani and Williams, 1998; Taghipour et al., 2008), and well-known epithermal deposits include the Sari Gunay, Chah Zard, Touzlar, and Narbaghi (Richards et al., 2006, Kouhestani et al., 2012, Heidari et al., 2018). The Choran deposit exists in the Urumieh-Dokhtar Magmatic Belt (UDMB). This deposit is located in the southern part of the Cenozoic Urumieh-Dokhtar Magmatic Belt, 70 km SW of Bardsir city, SE Iran. In this area, mineralization is associated with Oligocene - Miocene quartz diorite and granodiorite intrusions emplaced within Eocene volcanic–pyroclastic sequences. This study has focus on the spatial and temporal relationships between the porphyry and epithermal styles of mineralization in this area. 

A camp was set up in the field and sampling was performed during the 2017-2018. During the field observations, 286 rock samples were collected from the outcrops and drill core, and 67 thin sections were prepared and studied using a polarizing microscope in the Shahid Chamran University of Ahvaz. In order to correctly characterize the chemical composition of silicates (plagioclase and biotite), samples with least traces of alteration have been selected. The chemical composition of plagioclase and biotite were determined on the carbon coated thin section samples using an Electron Probe Micro Analyzer (EPMA). All the analyses were conducted at the Montanuniversitat Leoben, Austria using a superprobe Jeol JXA 8200 instrument.

Based on drill core logging and petrographic studies, mineralization in the Choran deposit is mainly accompanied with granodiorite intrusions. Overall, both hypogene and supergene mineralizations have been identified in the study area. The hypogene mineralization mainly occurs as disseminated blebs and veins which consist of pyrite, arsenopyrite and chalcopyrite with minor amounts of sphalerite. The supergene mineralizations that involve chalcocite and covellite. The first generation of hydrothermal veins (A-type) are characterized by assemblages of quartz + K-feldspar ± magnetite occurring roughly in the potassic alteration. This is followed by B-type veins characterized by assemblages of quartz + pyrite + chalcopyrite + feldspar ± biotite ± magnetite ± calcite. Type C veinlets (1 mm to 5 cm width) contain quartz + pyrite ± chalcopyrite and exhibit an intense stockwork texture in the potassic and phyllic alteration zones. The supergene sulfide zone is dominated by chalcopyrites and it is completely or partly replaced by chalcocite, digenite, and covellite. The hydrothermal alteration consisting of sodic-potassic, potassic, phyllic alunite and kaolinite are associated with granodiorite and quartz diorite intrusions. The result of EPMA analyses showed that all of the plagioclases in granodiorite and quartz diorite are consistently of andesine type. Based on the diagram of Al / (Ca + Na + K) (a.p.f.u) vs. An%, (Williamson et al., 2016) plagioclase samples of granodiorite intrusions plot collectively in the field of fertile calc-alkaline rocks associated with porphyry mineralization, while the quartz diorite samples are mostly plotted in the barren field. The results of biotite analyses indicate that all biotites of granodiorite and quartz diorite intrusions are of Mg-biotite type. The amounts of IV (F), IV (Cl), and IV (F/Cl) in the biotites of quartz diorite and granodiorite are between (2.28 to 4.08), (-5.62 to -5.52), (7.87 to 9.64) and (2.03 to 2.45), (- 5.81 to - 5.66), (7.74 to 8.18), respectively.

Most of the characteristics of the Choran Cu-Au deposit, i.e. geological setting, textural and structural, mineralogical with alteration features, are analogous to that of porphyry systems having high-sulphidation epithermal lithocap (Hedenquist et al., 1998; Muntean, 2001; Sillitoe, 2010).

The Saveh-Kashan-Qom copper belt, in the northern part of the Urumieh-Dokhtar Magmatic Arc (UDMA) consists of two of the oldest (gold and copper) zones in Iran (Samani, 1998; Rajabpour et al., 2017) where Upper Eocene-Oligocene Mard Abad-Bouin Zahra volcanic suite is situated. This volcanic suite hosts several copper deposits including Jarou, Gomosh Dasht, Ghezel Cheshme, Bidestan and Afshar Abad that are known as the "Kuh-e-Jarou Mining District". The Kuh-e-Jarou Mining District has a total potential ore reserve of 2 Mt Cu with an average grade of 3 wt.% (Zar-Azin Gostar Consultant Engineering Co., 2009). Upper Eocene volcanic and pyroclastic rocks of rhyodacite, trachyandesite, andesite, and trachytic tuff with high-K calc-alkaline to shoshonitic affinity consist of the main host rocks for Cu mineralization. These units are primarily intruded by post Eocene intrusive bodies.  The geochemistry and genesis of ore bodies have not been fully understood since most previous studies in this area have been focused on petrology of volcanic and intrusive rocks. Moreover, the main purpose of this study is to investigate mineralization style, geometry, and textural-structural features of orebodies, alterations, and fluid inclusions with implication for genesis of Jarou, Gomosh Dasht, Ghezel Cheshme, Bidestan and Afshar Abad copper deposits. In addition, this research provides more insight into understanding geology and mineralization conditions in the study area with an implication for future exploration.

Seventeen thin polished sections from the ores and the host rocks were prepared and they were studied by a transmitted/reflected polarizing microscope in the Iran Mineral Processing Research Center (Karaj, Iran). Five rock powdered samples were also analyzed using X-ray diffraction (XRD) spectrometry (X′ pert Philips) in order to identify the mineralogy of clay minerals in the mineralogy laboratory of Salamanca University (Spain). Fluid inclusion microthermometry was performed using a Linkam THMS600 heating-freezing stage (-190 to +600 °C) mounted on a ZEISS Axioplan2 microscope in the fluid inclusion laboratory of the Iranian Mineral Processing Research Center (Karaj, Iran). Salinities (wt.% NaCl eq.), density (g/cm3) and pressure (bars) were calculated using the FLINCOR v.1.4 (Brown, 1989) and FLUIDS (Bakker, 2012).

The orebody is controlled by a series of feather-like ruptures and faults and its dominant mineral compositions are chalcopyrite and chalcocite with minor amounts of pyrite, galena, bornite and sphalerite. The gangue minerals are quartz, barite, calcite and chlorite. Four types of hydrothermal alterations including chloritization, sulfidization, silicification and epidotization were recognized. Based on field and petrographic studies, three mineralization stages were distinguished including (1) the pre-ore mineralization stage characterized by fine-grained disseminated pyrites, (2) the main hydrothermal stage consisting of three substages: I) an early quartz-chalcopyrite ± bornite vein, II) middle bornite-chalcocite ± covellite breccia ore, III) late galena and sphalerite inclusions, and (3) late-stage barite and calcite veins.Based on petrographic studies, five types of aqueous fluid inclusions have been distinguished in the quartz-chalcopyrite ± bornite and barite veins including two-phase liquid-rich (LV), two-phase vapor-rich (VL), liquid monophase (L), vapor monophase (V) and minor halite-bearing liquid-rich fluid inclusions (LVS). The results show that parental fluids with a density of >1 g/cm3 and an approximate depth of 400 meters were formed and they were followed by fluid inclusions with a density of <1 g/cm3 and a depth of <300 meters due to fluid depressurization, faults. Moreover, introducing low temperature meteoric waters have caused fluid mixing and subsequently copper ore deposition (Henley et al., 2015; Cheng et al., 2019). Considering all geological mineralization styles, textures and structures of the orebody, types of alteration and fluid inclusions in copper deposits of the Kuh-e-Jarou Mining District, it can be suggested that these deposits have similarities with the Manto-type copper deposits in Chile or volcanic red beds in northern America.

Subduction-related magmas are characterized by enrichment of large ion lithophile elements (LILEs), light rare earth elements (LREEs) and depletion in high field strength elements (HFSEs) (Harangi et al., 2007). These geochemical signatures of magmatic rocks are commonly explained by the addition of hydrous fluids from subducting oceanic lithosphere combined with the flux of melts from subducted sediments to the mantle wedge, lowering the mantle solidus and leading to magma generation (Aydınçakır, 2016). Asthenospheric mantle, subcontinental lithospheric mantle and/or lower crust may be the principal source of these rocks (Eyuboglu et al., 2018). In addition, magma differentiation processes, such as fractional crystallization, crustal contamination, and magma mixing may also play an important role in the genesis of these rocks.This research study presents new petrological and geochemical data from the volcanic rocks with NW–SE trending, which are situated in the northwestern margin of the Central –East Iranian Microcontinent (CEIM) (south-east of Khur, Isfahan Province) which have been formed during the peak activity of Eocene. Study of this typical small volume subduction- related magmatism will be useful in understanding the origin and geological evolution of the Central Iran in Cenozoic.

The petrographic investigations on Eocene volcanic rocks from the SE of Khur area were carried out with an optical microscope (Olympus-BH2) in the petrology Laboratory of the University of Isfahan, Iran. Major and trace element concentrations of samples from whole- rocks were obtained by a combination of inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) at the Als Chemex Laboratory of Ireland. The chemical compositions of 4 samples (B865, B866, B867, and B868) were determined by Neutron Activation Analysis (NAA) in the Isfahan Activation Center The detection limit was 0.01% for all major element oxides and 0.01 ppm for rare earth elements. Mineral abbreviations were adopted from Whitney and Evans (2010).

Eocene volcanic rocks with trachy-basalt and trachy-basaltic andesite composition are exposed in the northwestern part of the Central-East Iranian Microcontinent (CEIM) (SE of Khur, Isfahan Province, Central Iran). These rocks which have a dominant northwest-southeast trend crosscut the Cretaceous sedimentary rocks.Petrography and mineral chemistry analyses indicate that the predominant rock-forming minerals of volcanic rocks are olivine, plagioclase, clinopyroxene and orthopyroxene. Phenocrysts set in a fine to medium grained matrix of the same minerals plus sanidine with minor amounts of opaque minerals. Secondary minerals are chlorite and calcite. The most common textures of these rocks are porphyritic, microlitic porphyritic, poikiolitic and glomeroporphyritic. Geochemical analyses of whole rock samples show that these rocks have been enriched in alkalies and large ion lithophile elements (Cs, K, Rb, Sr, Ba,), and have been depleted in high field strength elements (HFSE) (Ta, Nb, Ti). All samples indicate moderate to high fractionation in LREE patterns. These geochemical signatures point out to the subduction-related calc-alkaline nature of these rocks and their similarity to volcanic rocks of continental arcs or convergent margins (Yu et al., 2017). Pb enrichment and low values of Nb/La, Nb/U and Ce/Pb ratios reveal that crustal contamination has played an important role in magma evolution (Srivastava and Singh, 2004; Furman, 2007). The large volume of hydrous fluids coming from the subducted slab rather than sediments have caused enrichment and metasomatism of the subcontinental lithospheric mantle source. The geochemical characteristics of the studied rocks suggest that the parental magma have been derived from partial melting of a metasomatized spinel lherzolite of lithospheric mantle, which was previously modified by dehydration of a subducting slab. The tectonic environment, in which these rocks were formed has probably been a volcanic arc. Subduction of oceanic crust around the Central-East Iranian Microcontinent (CEIM) is the most reasonable mechanism which can be used to explain enrichment in volatiles of the mantle, and the calc-alkaline magmatism of the study area in Eocene times.

Subduction-related magmas are characterized by enrichment of large ion lithophile elements (LILEs), light rare earth elements (LREEs) and depletion in high field strength elements (HFSEs) (Harangi et al., 2007). These geochemical signatures of magmatic rocks are commonly explained by the addition of hydrous fluids from subducting oceanic lithosphere combined with the flux of melts from subducted sediments to the mantle wedge, lowering the mantle solidus and leading to magma generation (Aydınçakır, 2016). Asthenospheric mantle, subcontinental lithospheric mantle and/or lower crust may be the principal source of these rocks (Eyuboglu et al., 2018). In addition, magma differentiation processes, such as fractional crystallization, crustal contamination, and magma mixing may also play an important role in the genesis of these rocks.This research study presents new petrological and geochemical data from the volcanic rocks with NW–SE trending, which are situated in the northwestern margin of the Central –East Iranian Microcontinent (CEIM) (south-east of Khur, Isfahan Province) which have been formed during the peak activity of Eocene. Study of this typical small volume subduction- related magmatism will be useful in understanding the origin and geological evolution of the Central Iran in Cenozoic.

The petrographic investigations on Eocene volcanic rocks from the SE of Khur area were carried out with an optical microscope (Olympus-BH2) in the petrology Laboratory of the University of Isfahan, Iran. Major and trace element concentrations of samples from whole- rocks were obtained by a combination of inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) at the Als Chemex Laboratory of Ireland. The chemical compositions of 4 samples (B865, B866, B867, and B868) were determined by Neutron Activation Analysis (NAA) in the Isfahan Activation Center. The detection limit was 0.01% for all major element oxides and 0.01 ppm for rare earth elements. Mineral abbreviations were adopted from Whitney and Evans (2010).

Eocene volcanic rocks with trachy-basalt and trachy-basaltic andesite composition are exposed in the northwestern part of the Central-East Iranian Microcontinent (CEIM) (SE of Khur, Isfahan Province, Central Iran). These rocks which have a dominant northwest-southeast trend crosscut the Cretaceous sedimentary rocks.Petrography and mineral chemistry analyses indicate that the predominant rock-forming minerals of volcanic rocks are olivine, plagioclase, clinopyroxene and orthopyroxene. Phenocrysts set in a fine to medium grained matrix of the same minerals plus sanidine with minor amounts of opaque minerals. Secondary minerals are chlorite and calcite. The most common textures of these rocks are porphyritic, microlitic porphyritic, poikiolitic and glomeroporphyritic. Geochemical analyses of whole rock samples show that these rocks have been enriched in alkalies and large ion lithophile elements (Cs, K, Rb, Sr, Ba,), and have been depleted in high field strength elements (HFSE) (Ta, Nb, Ti). All samples indicate moderate to high fractionation in LREE patterns. These geochemical signatures point out to the subduction-related calc-alkaline nature of these rocks and their similarity to volcanic rocks of continental arcs or convergent margins (Yu et al., 2017). Pb enrichment and low values of Nb/La, Nb/U and Ce/Pb ratios reveal that crustal contamination has played an important role in magma evolution (Srivastava and Singh, 2004; Furman, 2007). The large volume of hydrous fluids coming from the subducted slab rather than sediments have caused enrichment and metasomatism of the subcontinental lithospheric mantle source. The geochemical characteristics of the studied rocks suggest that the parental magma have been derived from partial melting of a metasomatized spinel lherzolite of lithospheric mantle, which was previously modified by dehydration of a subducting slab. The tectonic environment, in which these rocks were formed has probably been a volcanic arc. Subduction of oceanic crust around the Central-East Iranian Microcontinent (CEIM) is the most reasonable mechanism which can be used to explain enrichment in volatiles of the mantle, and the calc-alkaline magmatism of the study area in Eocene times.

The Kahnouj Fe-Ti ore district is located 25 km southeast of Kahnouj city associated with the large gabbro intrusion of the Kahnouj ophiolitic complex. This ophiolite is one of the largest ophiolite assemblages of Iran (SE Iran), and part of neo-tethyan ophiolites (Kananian et al., 2001; Ghasemi Siani et al., 2021b). The Dar Gaz district is located in the middle part of Kahnouj ophiolitic complex and it is classified as the main ortomagmatic Fe-Ti ore mineralization. Although the geothermobarometric of iron-titanium oxide minerals in the Dar Gaz district has been studied by Karimi Shahraki et al. (2019), the geothermometry of silicate minerals (especially ferromagnesian) in the Dar Gaz gabrroic rocks has not been performed. Therefore, the main aim of this study is to determine the crystallization temperature and replacement of gabbroic rocks hosting Fe-Ti mineralization of the Dar Gaz district, using geothermometry of ferromagnesian silicate mineral.

A total of 100 thin-polish sections from different parts of the mining area were prepared and studied at the Iran Mineral Processing Research Center (IMPRC) and the Kharazmi University of Tehran with a Zeiss Axioplan 2 microscope. In order to achieve the temperature conditions of gabbroic rocks formation, 64 points (20 points of olivine, 13 points of clinopyroxene, 3 points of orthopyroxene, 14 points of plagioclase and 14 points of amphibole) from ferrogabbro to coarse-grained pyroxene-hornblende gabbro, 42 points (11 points of olivine, 10 points of clinopyroxene, 1 point of orthopyroxene, 8 points of plagioclase and 12 points of amphibole) from pyroxene-hornblende to fine-grained olivine gabbro, 30 points (12 points of clinopyroxene, 10 points of plagioclase, 8 points of amphibole) from fine-grained hornblende gabbro and 20 points (3 points of clinopyroxene, 11 points of plagioclase and 5 points of amphibole) from the diabasic dike of the Dar Gaz district were analyzed using CAMECA SX 100 electron microscopy (EPMA) with 20 kV and 20 nA conditions in the IMPRC.

The mafic rocks of the Dar Gaz district include ferrogabbro to coarse-grained pyroxene-hornblende gabbro, fine-grained pyroxene-hornblende gabbro, hornblende gabbro and diabasic dikes. Ferrogabbro to coarse-grained pyroxene-hornblende gabbro is one of the most important host rocks for Fe-Ti mineralization in the district.According to the thermo-barometers, the formation temperature and pressure of gabbroic rocks in the Dar Gaz district are in the range of 750 to 1258°C and a pressure of 2.5 and 6 kbars (clinopyroxene and amphibole barometers), and dibasic dikes are in the range of 700 to 1145°C and a pressure of 2.5 and 6 kbars were obtained. The highest crystallization temperature related to fine-grained pyroxen-hornblende gabbro unit (754 to 1258 °C) is the base of the sequence.The ascending of asthenosphere in the back-arc tectonic settings are from a magmatic chamber with a depth of about 15.34 to 21.20 km, and a pressure of about 4 to 8 kbars upwards. The average geometric results of pyroxene-ilmenite mineral pair geothermometry and pyroxene geothermometer of these rocks, their equilibrium temperature was determined between 901 to 1228°C, which is close to the magmatic temperatures.With comparison of temperature (700 to 1258°C), pressure (4 to 8 kbars) and oxygen fugacity (-19.25 to -25.25 bars) obtained for gabbroid rocks hosting Fe-Ti oxide mineralization with the temperatures obtained from ilmenite and titanomagnetite by Karimi Shahraki et al. (2019), it can be concluded that oxide mineralization is classified as orthomagmatic and occurs during the replacement, cooling and fraction of basic magma and formation of gabbroid intrusion associated with fractional crystallization.

Thermometry of pyroxenes at 2.5 kbars pressure indicates a temperature of 750 to 1258 °C for gabbroid bodies and 700 to 1145 °C for diabaic dikes. Thermometry of plagioclase and hornblende-plagioclase at 6 kbars pressure for coarse-grained ferrogabbro, fine-grained pyroxene-hornblende gabbro, hornblende gabbro and diabasic dikes are 868, 884, 776 and 784 °C, respectively. Amphibole thermometers at 6 kbar pressure for coarse-grained ferrogabbro, fine-grained pyroxene-hornblende gabbro, hornblende gabbro and diabasic dikes are 911, 948, 937 and 946°C, respectively. Comparison of temperature, pressure and high oxygen fugacity values obtained for gabbroic rocks and ilmenite and titanium magnetite ores of the Dar Gaz district, indicating oxidation conditions associated with fractional crystallization is the main factor for control of orthomagmatic mineralization in the back-arc environment.