فصلنامه زمین شناسی اقتصادی
سال دوازدهم شماره 2 (پیاپی 25، تابستان 1399)

The Zarshuran gold deposit is located in the Takht-e-Soleyman complex, NW Iran. It is formed in the Iman Khan NW-trending anticline (Mehrabi et al., 1999; Samimi, 1992). The rock units at Zarshuran mainly consist of Precambrian sequence (Iman Khan schist, Chaldaq limestone and Zarshuran black shale) which are overlain by Cambro-Ordovician limestone and Oligo-Miocene formations. Gold mineralization in the Zarshuran area occurs associated with jasperoid alteration within the Zarshuran black shale and Chaldaq carbonate rocks (Bazdar et al., 2015). The deposition and transportation of mineralized fluids from buried igneous rocks, faulting and fractures in basement and covered units, physico-chemical exchange in site of deposition, and hydrothermal alterations are the most important controls on gold mineralization at Zarshuran. There is a structural control on mineralization and ore deposition with respect to the alignment of gold mineralization along the NW- and NE-trending faults at Zarshuran. The mechanism of deformation in the studied area has been controlled by two sets of main fault systems. The early system is the NW-trending steeply-dipping right-lateral strike-slip faults with a reverse component and the late systems are the NE-trending faults that have been displaced by the early system. In this study, the relationship between fractures and structural lineaments with gold mineralization in the Zarshuran gold deposit was studied by remote sensing methods and fractal analysis. Then, development of extension and compressive structures of the Imam Khananticline in the right-lateral shear zone and role of fractures related folding in mineralization were investigated. It revealed that the spatial distribution of the fractures systems in folding structures is related to the kinematic evolution and geometry of folding.

Relations between mineralization and fracture systems are interpreted using field observations, extraction of lineaments and fractal analysis in this study. The lineaments map of the study area were extracted using appropriate algorithms of spatial data (STA algorithm), using Landsat 8 satellite (ETM+ sensor) images by semi-automatic method. Also, fractal dimensions in six parts of the area were calculated for faults and structural lineaments using the square method, log-log charts and related fractal analysis. A total of 75 samples were collected from trenches and surface outcrops. Then, analysis was performed for measurement of base and precious metal assays by inductively coupled plasma-optical emission spectroscopy (ICP-OES) in the Kanpazhuh laboratory (Tehran, Iran). Geochemical anomalies were reported by field observations and geological maps, and the extension and compressive structures associated to right-lateral shear zones were studied after determining the areas with high fractures density. Then, the relations between mineralization and transverse and longitudinal fractures related to folding in the Iman Khan anticline are investigated and a schematic model of the ore formation related to the folded structure are presented.

The study area is first divided into six parts (a to f) in order to calculate the fractal dimensions of the lineaments and faults. The most frequent trends of the lineaments and faults is at about N20E to N75E and N30E to N85E, respectively. In order to study the relation between fractal dimensions of faults and lineaments with the distribution of gold in study area, the fault and lineament maps are somehow combined with the gold distribution map. In parts of the study area where the fractal dimensions of faults and lineaments show high values, gold concentration is also high. This indicates a structural relationship between the density of lineaments and faults of the study area with distribution of gold mineralization. The most important fault structure affecting the study area is the Takab fault zone with NNW-SSE trend and a length of about 80 km. The Takab fault zone and its related fractures play a major role in transport of hydrothermal fluids, and the biometric data show that these activities have continued from Miocene to date (Mehrabi et al., 1999; Daliran, 2008). The right-lateral strike-slip movement of the Takab fault has led to displacement of the rock units in study area. Also, this right-lateral movements caused formation of fractures with different trends and compressive and extensional structures as folding, reverse faulting and normal fractures. The Iman Khan anticline hosting the Zarshuran gold deposit is one of the main structures related to the Takab fault movement. The fractures related to this folded structure have played an important role in transporting the hydrothermal fluids. The fracture systems related to the Iman Khan anticline, is generally of longitudinal and transverse type. Therefore, after the slab break-off event and formation of a compressive structure due to the strike-slip movement of the Takab fault, the faults served as pathways to the ore-forming fluids. All evidences shows that the longitudinal fractures have an important role in the mineralization along the Iman Khan anticline, although the thickness of the mineralized zone increases at the intersection of longitudinal and transverse fractures.

Ophiolites are a set of oceanic rocks with different appearance and mineralogy in the world's largest orogenic belt, from the Alpine to the Himalayas. The ophiolites of Iran are also located in this belt. Among ophiolites in Iran, the Nehbandan Ophiolitic Complex in the east of the country is of great importance. The complete ophiolitic sequence consists of two sets. The first is the crust sequence, gabbro, diabase and basalt, and the second is a mantle sequence or peridotites, both of which are sequences in the Nehbandan Ophiolitic Complex. The main purpose of this research study is mantle section. There are three study areas, located near the city of Nehbandan: Kalateh-Shahpori, Qadam Gah peridotites that are about 30 km northwest of the city of Nehbandan near the Chahar Farsang village and the third area is located between the Khansharaf village and Nasfandeh Kuh area that is 10 km east of Nehbandan.

In this lithological and mineralogical research study, thin and polished sections were prepared from samples. The thin sections were analyzed by polarizing OLYMPUS microscope BH-2 and the polished sections were analyzed by the OLYMPUS BX-60 reflecting microscope. A CAMECA SX100 electron probe microanalyzer was used to determine the chemical composition of the minerals in samples. The analytical condition include 15 kV and 20 nA rays with periods of 10 to 30 seconds at peaks for different minerals that are analyzed at the electron probe microanalysis center in the University Of Toulouse, France. The stoichiometry of minerals was used to calculate the amount of Fe3+ for access to the structural formula of minerals (Droop, 1987.

In terms of petrography, the Kalateh Shahpori peridotites are of the Harzburgite type and the Nasfandeh Kuh peridotites are of the Lherzolite type. The Qadam Gah peridotites are both geographically and petrographically indicative of the state of transition between the two other regions. The mineralogy of the Kalateh Shahpori peridotites is composed of olivine (Fo91), orthopyroxene (En90 Fs9), and the Cr-spinel is of the high Cr type. The Nasfandeh Kuh peridotites have olivine minerals that are Chrysolite (Fo89), orthopyroxene (En89 Fs9) and (En86 Fs10), clinopyroxene (En46 Fs5 Wo49) and, the Cr-spinel is of the high Al type. The Qadam Gah peridotites are composed of olivine (Fo90), orthopyroxene (En89 Fs9.5), clinopyroxene (En47 Fs3 Wo50) and, the Cr-spinel is of the medium Cr type.According to geochemical data and petrogenesis, the Kalateh Shahpori harzburgites are of the supra-subduction zone type in the forearc basin. The Nasfandeh Kuh Lherzolites are of the middle-oceanic type. The Lherzolites of Qadam Gah have the same characteristics of both regions in terms of the formation environment. However, they are much more similar to the middle-oceanic peridotites. The degree of partial melting of the peridotite has a direct relationship with the Cr content and it has an inverse relationship with the Al2O3 content in the chromium-spinel of the peridotite (Hellebrand et al., 2001). Probably, these lherzolites formed due to the re-fertilization of harzburgites (Monsef et al., 2018). Accordingly, Kalateh-Shahpori harzburgites with 20% partial melting are of high-grade, and the Nasfandeh Kuh Lherzolites with 5% partial melting are of the low grade type. The herzolites of the Qadam Gah are approximately 11% partial melting and are located between the Kalateh Shahpori peridotites and the Nasfandeh Kuh peridotites. The high degree of melting in the Harzburgites may indicate their remelting in the fluid environment because the hydrosis condition increases the degree of partial melting of peridotite (Hirose and Kawamoto, 1995). The Cr# in Cr-spinel, and the Mg# in olivine of the peridotites indicate the presence of at least 3 types of peridotites in the Nehbandan Ophiolitic Complex. According to mineralogy, petrography, geochemistry, and petrogenesis studies of the peridotites in the Nehbandan ophiolitic complex, it is recommended to explore possible chromite deposits, high melting and supra-subduction harzburgite zones such as Kalateh Shahpori harzburgites which should be considered to be the first priority. Then the peridotites of transition regions such as Qadam Gah should be at second priority and finally the low melting middle-oceanic lherzolites such as the Nasfandeh Kuh shuld be considered to be the third priority.

Sediment-hosted stratabound copper (SSC) deposits are bodies of disseminated, cementing, and lesser veinlet hosted copper minerals that are peneconformable with their sedi mentary or metasedimentary host rocks (Hayes et al., 2015). These deposits have been termed sediment hosted stratiform Cu, sedimentary rock-hosted stratiform Cu, Cu shale, Cu sandstone, cupriferous sandstones, sandstone Cu, red-bed Cu, Kupferschiefer type Cu, marine paralic type Cu, reduced-facies Cu, or Revett Cu deposits (Cox et al., 2003, 2007; Hitzman et al., 2005; Hayes et al., 2015). SSC deposits occur in three subtypes divided by host lithology and the corresponding type of reductant that precipitated sulfur and Cu from warm, oxidized, metals-transporting, sedimentary brines. These types are as follows: (1) reduced-facies type; (2) sandstone-type (Revett); and (3) red-bed type. These deposits have been formed during the middle-late Paleoproterozoic to Tertiary.There are several SSC deposits in the Avaj Zanjan Tabriz Khoy area in the northwestern Iran that are hosted by grey sandstone units of the Upper Red Formation (URF). The Tasouj, Tazekand, Nahand Ivand, Ortasou, Chehrabad, Halab, Zagelou and Avaj are the main important deposits in this area. These deposits predominantly consist of bedding-parallel replacement and disseminated Cu–Pb–Zn sulfides, roughly concordant with the stratification. The average Cu, Pb and Zn content of these deposits are ~1.5, 2 and ~1 wt.%, respectively.Apart from small scale geological maps of the area, i.e., 1:250,000 geological maps of Takab (Alavi and Omidi, 1976), 1:100,000 geological maps of Mahneshan (Lotfi, 2001) and a number of unpublished Pb–Zn–Cu exploration reports, no other work has been reported prior to this research study on Pb–Zn–Cu mineralization at Chehrabad. The present paper provides an overview of the geological framework and the mineralization characteristics of the Chehrabad deposit with an application to ore genesis. Identification of these characteristics can be used as an exploration model for this type of Pb–Zn–Cu mineralization in this area and elsewhere.

Detailed field work has been carried out at different scales in the Chehrabad area. During the field works, detailed stratigraphic sections were measured, sampled and described. In addition, the color of the sandstone layers and the presence of fossil woods were scanned during the field work. About 23 polished thin and thin sections from host rocks and mineralized layers were studied by conventional petrographic and mineralogic methods at the University of Zanjan in Iran.

The Pb–Zn–Cu deposit at Chehrabad, 75 km northwest of Zanjan, is located in the Central Iranian zone. Rock units exposed in this area belong to the URF, and consist of alternations of red and green marl intercalated with red to grey, medium- to thick-bedded sandstone. In this area, URF has 980 m thickness and consists of four main parts. These parts, from bottom to top, consist of 1- alternation of gypsiferous green marls along with gypsium and salt layers (235 m), 2- red marls intercalated with grey and red sandstones (445 m), 3- alternation of red and green marls intercalated with sandstones (145 m), and 4- alternation of green marls and green siltstones (155 m). The Pb–Zn and Cu mineralization in the Chehrabad deposit has occurred in grey sandstone units of the second part of the URF. Mineralization has often been formed around and within the fragments of the plant fossils, in the form of disseminated and the solution seems to have been sulfides. Based on field studies, mineralization at the Chehrabad deposit has occurred in two horizons of reduced-grey sandstones, H-A and H-B, with about 4 and 6 m thickness and about 200 and 1000 m length, respectively. These horizons contain red oxidized zone, bleached zone and mineralized reduced zone with the latter being located within the bleached zone.The red oxidized zone consists of red marl and sandstone layers containing iron oxides which are located adjacent to the reduced horizons. The red color of this zone has been caused by the presence of iron oxides around the grains. The oxidized pyrite crystals are the main important minerals in this zone. The bleached zone is a part of sandstone sequences that have undergone changes in their color due to the alteration processes. Grey and green colors in this zone have occurred due to the presence of organic materials and diagenetic pyrites. Mineralization in the reduced zone has occurred within the organic materials bearing bleached zones. Plant debris, plant fossils, diagenetic pyrites and permeability of host rocks have the most important roles for the Pb-Zn and Cu mineralization at the Chehrabad deposit.Galena, sphalerite, chalcocite, pyrite and chalcopyrite along with minor Ag-bearing sulfides (mckinstryite, stromeyrite) are the main ore minerals at the Chehrabad deposit. Cerussite, malachite, azurite, covellite, atacamite, vanadinite, and goethite are formed during supergene processes. Disseminated and cemented textures along with lens-shaped, solution seems, replacement, vein-veinlet, and framboidal are the main ore textures at the Chehrabad deposit. Based on the tectonic setting, host rock, geometry, presence of plant fossils, ore structure and texture and mineralogy, it can be concluded that the Chehrabad deposit is a sediment-hosted Redbed type Cu deposit.

The investigated area in northeastern Iran that is known as the Tarik Darreh arsenopyrite-Au-W prospecting target is situated in the Kopeh Dagh zone (Fig. 1). Most of the study area is covered with black slate rocks which most authors have referred to as Upper Triassic with Norian age (e.g., Behroozi et al., 1993). Plutonic rocks with gabbro, diorite, and quartz diorite and monzodiorite composition were introduced in the slate rocks. The previous studies in the Tarik Darreh are the preliminary report of arsenopyrite with Au-W quartz lode mineralization carried out by Taghizadeh (1965). Since then, the Geological Survey of Iran (e.g., Alavi Naini and Mossavi-Khorzughi, 2006) a few geologic companies and the universities (Shafi Niya, 2002) and Ghavi et al. (2013) have been studying in the area. In the present study, we provide more information, complete with new information about the geology of the Tarik Darreh area.

Expansive field geology surveying to draw a geology map in 1:8000 scale and sampling from plutonic rocks was performed and two main fault system were distinguished in Tarik Darreh (Figs. 2 and 3). Thin section microscopic studies were carried out following field investigations. Magnetic susceptibility was measured with a portable Model KT-10 and GMS-2 Fugro instrument (precision of 0.0001 SI units).Sixteen samples of intrusive bodies and dykes were analyzed for major and trace elements using 4E-Reserch package (INAA, FUS ICP-OMS, ICP-MS) at the Acme Lab (Canada). Gabbroic units were selected for U-Pb geochronological study. In the heavy liquid process, zircon crystals were separated, hand-picked under microscope. Zircon grains for Th-U-Pb dating were examined by an Agilent 7700 quadrupole ICPMS at the University of Tasmania (Australia).

Plutonic rocks in the Tarik Darreh area are in the form of bodies and dykes. The plutonic bodies are mostly gabbro, gabbro diorite, diorite (hornblende pyroxene diorite, biotite hornblende diorite and hornblende biotite diorite), biotite hornblende monzodiorite, quartz diorite, and quartz monzonite in composition. However, it is diorite with granular textures as a common rock (Figs. 4 and 6). Other rocks as dyke have more hornblende diorite composition with porphyritic textures. Common mafic minerals in the plutonic rocks in the Tarik Darreh include pyroxene, hornblende and biotite. The alteration assemblages of uralite, chlorite, calcite and sericite are very common gabbroic-diorite rocks (Fig. 5). The plutonic rocks have SiO2 with values in the range of 46 to 56% (Table 1). Therefore, these rocks are intermediate in composition.  Most of these rocks also have high K-calc-alkaline composition with metaluminous characteristic (Fig. 7).Enrichment in LREE relative to HREE, low La/By ratio, enrichment in LILE and negative anomaly of Eu, Ti, Zr, Y and Ba are shared in all of the studied plutonic rocks and dykes (Figs. 8 and 9). All the plutonic rocks have low values of magnetic susceptibility (less than 3×10-3 SI). Two main systems of fault have affected the area (Fig. 10). Plutonic rocks from the Tarik Darreh area are placed within the volcanic arc to within plate’s environment in tectonic setting discrimination diagrams (Fig. 11).Zircon U-Pb dating results on the gabbroic rocks are shown in Table 2. Zircon grains through cathodoluminescence imaging show euhedral, clear crystals with no visible heritage cores. However, some of them have inclusions. Zircon U-Pb dating indicates that the gabbroic rocks formed at 215.5±0.9 Ma (late Triassic) (Fig. 12). Thus, it is obvious that the country rock (called Miankuhi Formation), must be older than the late Triassic plutonic rocks rather than Norian age as previously thought.The variation of major elements with SiO2 in the plutonic rocks from Tarik Darreh (Fig. 13) indicates that the primary magma of these plutonic rocks underwent fractional crystallization from gabbro to quartz monzodiorite. Geochemical characteristics such as low ratios of (La/Yb)N (<13), high contents of MgO (7%), high Mg# (57) and high Sr (on average 572 ppm) indicate that the magma of the studied rocks have probably originated from partial melting of garnet-free source from mantle or low continental crust. However, the presence of slate as country rocks suggest possible assimilation of country rocks that indicate processes such as assimilation were more likely to be responsible for the evolution of plutonic rocks in the Tarik Darreh area.

Trace elements including As, Sb, Bi, Ga, Ge, In, Hg, Cd, Tl, Se and REEs have special applications in various industries due to their physical and chemical properties. Ore deposits of these metals have not occurred in the Earth's crust, and these elements are mainly hosted in sulfide minerals of Cu, Pb and Zn (Hall and Heyl, 1968; Song and Tan, 1996; Ye et al., 2011; George et al., 2015). Elika Formation (middle Triassic) in central Alborz is host of several carbonate rock-hosted fluorite deposits such as Kamarposht, Pachi-Miana, Shashroodbar and Era (Alirezaee, 1989; Rastad and Shariatmadar, 2001; Rajabi et al., 2013; Vahabzadeh et al., 2014; Zabihitabar and Shafiei, 2014; Mehraban et al. 2016; Nabiloo et al. 2017). Despite previous valuable studies in these deposits, the value of the presence of trace elements in the galena of these deposits has not yet been documented.

For the current research study, sampling from 3 flourite mines including Kamarposht, Pachi-Miana and Era was carried out to collect 26 pure galena grains separated from various fluorite ore-types. The samples were analyzed for trace elements at the ICP-MS at Act Labs Ltd., Canada.

The highest concentrations of trace elements in galena samples were obtained for Sb (with mean 692 ppm and maximum concentration of 2531 ppm) and Ag (with a mean of 24.28 ppm and a maximum concentration of 2531 ppm). The lowest values ​​were obtained for Bi (mean 0.04 ppm), Se (average 1.89 ppm) and Ga (mean 0.9 ppm) and Tl (mean 0.3 ppm). Hg (0.06 – 10 ppm), Cd (1.14 – 23 ppm) and As (0.1 – 36 ppm) exhibited a wide range of concentrations.The comparison of the trace elements concentration in the studied galena samples with those of the MVT, SEDEX, Irish-type deposits shows that the concentrations of Sb, Tl, Hg, Se and Cu in the studied galena are close to the values ​​for MVT deposits, whereas the studied galena samples are much poorer in Ag, Bi, Cd and As than those of the MVT deposits. There is meaningful relationship between concentrations of some trace elements such as Tl– Ag (r=0.82), Tl– Cu (r=0.71), Ag–Sb (r=0.66), Cu– Ag, Ag –  As, Cu– As, Sb–  Cu, Hg – Zn, As – Sb, Hg –  Cd, Zn – Cu (0.3 < r < 0.6).

Our data revealed that galena samples are relatively rich in Sb (up to 2581 ppm with an average ~ 620 ppm) and Ag (up to 70 ppm with an average ~ 30 ppm), whereas they are poor in other trace elements. Inter-element relationships in galena show strong correlation between Sb – Ag (r≥0.65) and moderate correlation between Ag – As, Ag –Cu as well as Hg –Zn (0.4<r<0.6). Based on our data, high concentration of Sb and Ag in galena could be related to the presence of special minerals (e.g., tetrahedrite, stephanite, diaphorite, twinnite) as inclusion in the host galena, whereas the occurrence of not very high concentrations of Cd and Hg and meaningful relationship with Zn concentrations in galena could be due to the presence of inclusions of sphalerite (ZnS) and polhemusite (ZnHgS) in the galena. Due to the large presence of galena (several tens of thousands) in the studied fluorite mines, and the relatively high concentration of Sb and Ag in the galena samples of these deposits, an assessment of the economic recovery on a laboratory scale of these elements is suggested.

The Shahzade-Ali Akbar area located at 68 Km south of Isfahan is located in the southern zone of the Cimmerian Sanandaj-Sirjan block as a part of the northern shelf of the Neo-Tethyan Ocean (Stampli and Borel, 2002). This area lies in the Shahreza-Abadeh-Hambast belt and is well-known for its classic Permian-Triassic outcrops. The Lower Triassic carbonate rocks have hosted several interlayer igneous horizons. The composition of these rocks varies from olivine basalt to quartz basalt and their hypabyssal equivalents. Plagioclase (labradorite), clinopyroxene (augite), olivine, amphibole and quartz are major and ilmenite and titanomagnetite are minor minerals. The main objective of this research study is to investigate the geological, geochemical and petrogenesis of igneous rocks using mineral chemistry.

More than 50 samples representing whole units were selected in order to identify the geological setting of the igneous horizon, and their thin sections were prepared. Minerals and textures of rocks were studied by using polarizing microscope (Olympus BH-2). Then, 6 samples were selected for mineral chemistry and their mineral compositions were determined by electron microprobe at the Naruto University, Japan. The EPMA (Jeol- JXA-8800R) was used at operating conditions of 15 kV, 20 nA. Minpet software and spread sheet have been used for mineral chemistry studies and mineral formula calculations.

Based on the field observations, igneous units that are 120 centimeters to 10 meters thick have basaltic composition and are interlayered with Lower Triassic carbonate rocks. Microscopic study showed that these rocks are composed of plagioclase, clinopyroxene, olivine, amphibole, quartz and opaque minerals (ilmenite and titanomagnetite) and have porphyritic, ophitic, intersertal to intergranular textures. These rocks have undergone alterations and secondary minerals are widespread. EPMA analyses show andesine to labradorite composition, clinopyroxene (augite) and amphibole (edenite). In the Q-J diagram (Morimoto et al., 1988), all clinopyroxenes are located in the Mg-Fe-Ca (Quad) field. In the Wo-En-Fs diagram (Beccaluva et al., 1989), clinopyroxens show augitic with lessor amounts of diopside composition.  According to clinopyroxene chemistry diagrams such as Si O2 vs. Al2O3 and Ti vs. Al (Le Bas, 1962), the samples belong to sub-alkaline series. Discriminate diagrams such as Ti O2 vs. Al2O3 (Le Bas, 1962), Ti vs. Ca+Na and Ti vs. Al (Leterrier et al., 198a2) are used for identification of magma affinity. These diagrams show that the studied rocks are tholeiitic. The rocks under study demonstrate the MORB feature on tectonic discrimination diagrams (TiO2-SiO2/100-Na2O, Beccaluva et al., 1989) In the 2Ti+Cr+AlVI vs. Na+AlIV diagram (Morimoto et al., 1988) all clinopyroxenes are located below the Fe3+=0 line that indicates low oxygen fugacity during crystallization (Schweitzer et al., 1979). In Helz (1973) diagrams, the pressure and percentage of magma water estimated to be 2 to 10 Kbar pressure and about 10% water content. In YPT vs. XPT diagrams (Soesoo, 1997) the temperatures and the pressure of clinopyroxene crystallization are about 1150-1200 ◦C and 2-10 Kbar respectively.

The studied area had been a part of the Cimmeride microcontinent (Horacek et al., 2007) which had begun separating from the northern margin of Gondwana during Triassic time (Şengör, 1984), and traversed north to the southern Eurasian border (Stampli and Borel, 2002).In this area, several interlayer igneous rocks with basaltic composition are seen with Lower Triassic carbonate rocks. Based on the chemical composition of pyroxenes, the magma has sub-alkaline and tholeiitic affinity. The crystallization of ilmenite-titanomgenetite and diagram of clinopyroxenes crystallization conditions illustrate the low level of oxygen fugacity in the formation of the rocks under discussion. The pressure of magma crystallization is estimated to be between 2 and 10 kb, and the magmatic water content is about 10%. The studied rocks show MORB characteristics. Interlayering with lower Triassic sediments, sub-alkaline nature of the magma, low level oxygen fugacity during crystallization and geotectonic environment, suggest that the rocks have been formed in the early stages of the opening of the oceanic crust. A process that has led to the formation of the Neo-Tethys Ocean in the later stages.