فصلنامه زمین شناسی اقتصادی
سال دوازدهم شماره 4 (پیاپی 27، زمستان 1399)

The Tighanab area is located in the Southern Khorasan province and 104km south-east of Sarbisheh, in the eastern part of Sistan suture zone (Tirrul et al., 1983). The Sistan suture zone has formed as a result of collision between the Lut and Afghan blokcs and its closure time is related to upper Cretaceous era (Bröcker et al., 2013). Eocene-Oligocene magmatism in eastern Iran (Lut-Sistan) crop out as volcanic rocks, pyroclastic and subvolcanic rocks (Pang et al., 2013) which have caused skarn mineralization in some parts. The relationship between skarn mineralization and adakites has been discussed by various researchers (Lei et al., 2018). Skarn deposits and their associated Cenozoic plutonic rocks in Iran, have outcrops in northwest, central and southeast of the Urumiyeh-Dokhtar magmatic belt, Sabzevar-Dorouneh magmatic belt and the eastern Iran magmatic belt (Sepidbar et al., 2017). The Tighanab subvolcanic bodies play an important role in skarn mineralization.This research study is carried out for studying petrography, geochemistry and tectonic setting of subvolcanic bodies and their role in skarn mineralization since geochemistry and petrology of the mentioned masses have not been studied.

This research is based on field observations, thin sections, polished thin section studies and chemical analysis of samples. In this regard, 90 thin sections were prepared and studied by microscope. Then, 11 samples of subvolcanic rocks with the least alteration were selected. Then they were crushed and powdered. Next, they were analyzed by the ICP-ES method for major elements and the ICP-MS method for trace and rare earth elements. The magnetic susceptibility of the samples was measured by SM20 magnetic sensitivity device at university of Birjand.

The study area is located in the eastern part of the Sistan suture zone and the Mahirud geological map (1:100000). Quartzdioritic subvolcanic rocks intruded the Paleocene-Eocene limestone and sandstone and formed iron skarn mineralization. The main textures in quartz diorite porphyry are porphyry with microgranular groundmass and poikilitic. Plagioclase, hornblende and quartz are the main constitutes of these rocks. Plagioclase phenocrysts have polysynthetic twinnig, zoning and resorption rim and are andesine and rarely oligoclase based on extinction angle. Different geochemical diagrams show correlation between the Tighanab igneous rocks and intrusions associated with iron skarns. Geochemical features as mean of SiO 2 (64.48%), Al 2 O 3 (16.68%), Sr(470ppm), Y(8.9ppm), Sr/Y(55.58), Yb(0.89ppm) and poor negative anomaly of Eu are representative of high silica adakitic features for these rocks. The amount of Mg#(55.48-68.1), Sr/Y(mean55.58), Th/La(mean0.32), La/Yb N .(4.2) and Th(mean1.8ppm) indicate oceanic crust melting with garnet-amphibolite composition to generation of adakitic magma.

Field evidence, mineralogy, and magnetic susceptibility measurements show that granitoids of the Tighanab area belong to the magnetite series. Based on tectonic discrimination diagrams, the intermediate samples of the Tighanab area are located in the range of VAG and VAG + Syn-COLG. The studied rocks show depletion of HFSE such as Ti, P, Nb, Yb, Y and enrichment in LILE that indicates their association with the subduction environment. Negative anomaly of HFSE may be a result of contamination of magma by crustal materials during ascent and emplacement in subduction zones. Comparison of some major and trace elements of Tighanab samples with adakites indicated that these rocks have high silica adakitic nature. Geochemical evidence shows that the studied rocks are similar to the rocks associated with iron skarns. Some geochemical characteristics such as HREE and HFSE depletion, high Sr, Sr/Y and (Gd/Yb) N >1 and poor negative anomaly of Eu in the studied samples, indicate that the adakitic magma has been formed at pressures above the plagioclase stability. The geochemical characteristics of the studied samples, such as low Y and high Sr/Y ratio, indicate the presence of garnet in the origin of these rocks (Mao et al., 2018). Trace and rare element diagrams show that adikatic magma of the Tighanab area subvolcanic rocks have been produced by melting of the oceanic slab. Adakitic rocks of the Tighanab area have been formed from a source with 10 to 25% garnet amphibolites composition.

The Baout antimony deposit is located 80 km west of Zahedan. Antimony occurs as a trace element in Earth crust, introduced in many minerals, especially sulfides and sulfosalts and occurs as small high grade ore deposits in different parts of the earth. Antimony mineralization in Iran is mainly in the form of hydrothermal veins associated with volcanic and plutonic activities. The Sistan suture zone (SSZ) in east and southeast of Iran hosts high-grade Sb-veins in several areas from north to south such as Sefidabeh, Baout, Lakhshak, Sefidsang and Shurchah (Boomeri et al., 2018). The aim of this paper is petrography and mineralogy of the host rocks and ore-bearing veins, loss and gain of various elements, especially antimony and related elements in alteration zones, and investigation of fluid inclusions in quartz associated with stibnite 

Geology:

The Baout is located in the SSZ and consists of Cretaceous ophiolitic rocks, Eocene flysch sedimentary rocks (turbidite), Oligo-Miocene intermediate dikes and recent sediments. The ophiolites and flyschs are metamorphosed and altered and host several NE quartz-stibnite veins. The area is a shear zone and has been affected by strike-slip faults. The NE faults are dominant in the mineralized area.

Method and material:

20 thin sections and 10 polish and 6 thin-polish sections were examined by polarizing microscope under transmission and reflected light for petrography, mineralogy and alteration and mineralization studies. A few samples from the fresh and altered rocks were analyzed by XRF and ICP-MS for major, trace and rare earth elements, respectively. Sb ores were analyzed by ICP-OES to study and interpret grade and variation of Sb, Cu, Au, As, Ag, and Zn. After optical observations three representative samples from Baout were chosen for subsequent micro thermometric measurements. The micro thermometric measurements were carried out by Linkham THMS600 heating-freezing stage (-196 to +600˚C) at Iran processing research center.

Petrography The igneous rocks in the Baout rock are serpentinized harzburgite, gabbro, diorite, basalt, diabase and dacite. These rocks mainly contain plagioclase with or without clinopyroxene, amphibole and biotite. The turbiditic rocks are sandstone, siltstone and metamorphosed shale (phyllite). Limestone and list waenite are other rocks of the area.

Alteration and Mineralization:

The host rocks including igneous rocks in the study area are extensively altered. The propylitic alteration occurs in the mafic rocks and sericitic alteration in the turbiditic rocks. The propylitic alteration is characterized by quartz, actinolite, epidote and calcite. Quartz and calcite are dominant secondary minerals that occur as vein, veinlets, and open space infillings in the host rocks. Serpentinization and list waenitization occur in harzburgite. Mass changes of altered igneous rocks are calculated by the Isocon method (Grant, 2005).  The altered rocks are depleted relative to less-altered rocks of Baout from mobile elements of Sb and As while they are enriched by SiO2 and immobile elements such as Pb. However, the hosrocks in the Baout area have more Sb than equal rocks from non-mineralized area of the Kurin to the south.The Sb mineralization is structurally controlled and occurs as NE quartz-stibnite veins. The stibnite is the most abundant sulfide and ore mineral and it occurs as open space filling mainly later than quartz.  There are also locally variable amounts of valentinite, senarmontite and stibiconite calcite, and iron oxides in the veins. The Sb grade is mainly high and reaches up to more than 30 wt. %. Other anomalous elements are Au, Pb, Zn, As and Cu

Fluid inclusion:

Fluid inclusions in quartz from the Baout area are primary, secondary, and pseudo-secondary in type. The fluid inclusion homogenization temperature and salinity range from 130 to 215˚ C and 2.07 to 3.06 wt. % NaCl eq., respectively. They all fall within the range of those from epithermal ore deposits and metamorphic waters.

The Sb mineralization in Baout occurs as quartz-stibnite veins. The ophiolitic and flysch units are host of the veins. The oldest veins are non-mineralized quartz veins followed by quartz-stibnite and carbonate veins, respectively. These veins are structurally controlled by NE Faults. The altered rocks are more depleted of Sb and As, and enriched in Pb and SiO2 as compared with less altered rocks. According to homogenization temperatures and salinities of liquid-rich two-phase primary fluid inclusions in quartz, Sb mineralization was formed by metamorphic hydrothermal solutions.

Two fundamental goals are followed in this paper: 1- Active neotectonics of the Kopeh Dagh Mountains particularly in its central part that is called the Bakharden-Quchan Zone in NE Iran for special features of faulting and role of faults within this zone in the collision between Arabia-Eurasia plates. 2- Seismicity hazards of faulting to recognize the relationship between asperities and earthquakes through analyzing the correlation of fractal dimension and b-value parameters. The Kopeh Dagh Mountain is accommodating a large portion of northward motion of central Iran with respect to Eurasia, involving a major right- lateral strike-slip fault system in its central part (the Bakharden-Quchan Zone). This fault system corresponds to the northeastern boundary of the Arabia-Eurasia collision and can be considered to be a lithospheric scale tectonic feature. The Kopeh Dagh Mountain forms a linear intercontinental fold-thrust belt trending NW-SE between the stable Turan platform and central Iran (Afshar Harb, 1979; Hollingsworth et al., 2006; Shabanian et al., 2009; Shahidi et al., 2013).

This research uses both historical and instrumental seismicity data along with observations from Landsat 7 satellite imageries, topographic data (SRTM), field observations and mathematical fractal dimension (D) model plus integral mathematical functions to find a logical correlation between tectonic movements, asperities and earthquakes in different active zones.

There is an array of active right-lateral strike-slip faults in the central part of the Kopeh Dagh Mountain which obliquely cut the range and produce offsets of several Kilometers in the geological structures. These faults all end in thrusting and link to blind faults, revealed by the uplifts and incision of the Late Quaternary terraces. These faults have rotated around their vertical axes and can account for several Kilometers of the N-S shortening. They are responsible for major destructive earthquakes in both 19th and 20th centuries and represent important seismic hazards for populous regions of NE Iran. These faults also require several Kilometers along-strike extension that is taken up by the westward component of motion between south Caspian sea basin, Shahrood fault system and both Eurasia and central Iran (Hollingsworth et al., 2006; Shabanian et al., 2009; Bretis et al., 2012).

The Bakharden-Quchan faults have identifiable ends, where they turn into thrusting and link to blind faults. The fault changing mechanism to reverse has caused increase of stress, shortening by thrusting in their end bending. Structural relation faults between this zone and the Binaloud Mountain through Meshkan transfer zone which is the major motion engine of this zone to put it constantly under neotectonic stresses for convergence of Arabia-Eurasia plates since the last Alpine orogeny phase. Most of the seismic activities of this zone could provide us with precious data on crust tension distribution through microseismic and computing parameters of b-value, fractal dimension (D) and mapping of local stresses. In neotectonic active zone b.

The Sari Gunay veining and breccia epithermal gold mineralization is situated between the Urumieh-Dokhtar magmatic belts and the Sanandaj-Sirjan metamorphic zone in central-NW Iran. The Sari Gunay gold deposit is hosted by a middle Miocene volcanic complex that has been formed in the two Sari Gunay and Agh Dagh hills with ~2 km distance. The Sari Gunay volcanic complex consists of dacite to rhyolite volcanics and its coeval volcaniclastic rocks. There are some published data on the Sary Gunay ore deposit (e.g. Richards et al., 2006), while mineral chemistry of silicate and sulfide minerals have not been studied previously. The main goal of the present investigation is to determine type of mineralization based on detailed mineralogy, mineral chemistry, and fluid inclusion evidence and previously published data by Richards et al. (2006).

A total of 300 samples were collected systematically from 25 drill cores and outcrops.
A total of 100 samples from different mineralization veins were selected for optical microscopy and after comprehensive study by stereomicroscope that was carried out at the Kharazmi University and Iranian Mineral Processing Research Center (IMPRC). The selected mineral phases were analyzed by an Electron Microprobe Analysis (EPMA) Cameca X-100 with 20 kV and 20 nA, with a beam diameter of 5 μm at the IMPRC. Micro thermometric analyses were carried out on 10 doubly polished thin sections from breccia quartz-tourmaline and quartz-pyrite-arsenic sulfides-stibnite and quartz-tourmaline veins using a Linkam THMS 600 freezing-heating stage, mounted on a ZEISS Axioplan2 research microscope at the IMPRC.

Field geology and petrographic observations indicate that veining and breccia ore mineralization in the Sary Gunay ore deposit have occurred in deferent levels including quartz-magnetite-sulfide veinlet in the deeper levels and brecciated quartz-tourmaline-sulfide veins in the shallow levels. Several high-grade gold-bearing veins and veinlets of quartz-pyrite-stibnite-realgar-orpiment with diverse abundance ratio have formed within, and finally silver-bearing quartz-base metals veins have been formed outward of the hydrothermal system. EPMA data indicate that gold has occurred in arsenian pyrite as solid solution and very fine inclusions. Stibnite, realgar and orpiment exhibits extensive range in As/Sb substitution. Hg-bearing minerals have been detected in stibnite and arsenian sulfide minerals and also rutile has been detected in pyrite by EPMA. According to EPMA evidence, all tourmalines are alkaline belonging to dravite-type which show hydrothermal origin of quartz-tourmaline breccia veins. Fluid inclusions in the first stage have homogenization to a liquid in the range of 320° to 380°C, corresponding to salinities of 35 to 45 wt. % NaCl equivalent. Moreover, fluid inclusions in quartz-tourmaline veins show homogenization to a liquid in the range of 203° to 398°C, corresponding to salinities of 31.43 to 45.01 wt. % NaCl equivalent based on Sterner et al. (1988). Fluid inclusions in quartz-pyrite-stibnite veins homogenized to a liquid between 200° and 339°C, with salinities of 1.70 and 11.74 wt. % NaCl equivalent, and finally base metal veins were formed by fluid with 165° and 230°C, with salinities of 1 and 7.20 wt. % NaCl equivalent based on Bodnar (1993).

Textural relationships and microscopic features allowed us to recognize five stages of veining; (1) quartz-magnetite-sulfide, followed by (2) quartz-tourmaline breccia, (3) quartz-pyrite-gold-stibnite, (4) quartz-pyrite-stibnite-realgar-orpiment-gold and (5) late Ag-bearing quartz-calcite-pyrite-galena-sphalerite. There is evidence of As/Sb substitution in stibnite-realgar-orpiment minerals. Moderate temperature and salinity features, presence of V and L rich in association with L+V fluid inclusion types, variation in fluid composition, and pressure fluctuation of the mineralizing fluid during the main stage of gold mineralization are the main highlights of the Sari Gunay epithermal deposit, whereas high salinity and temperatures with first quartz-sulfide-magnetite veins are consistent with porphyry ore mineralization in depth. Possibly rapid variations in the fluid chemistry and availability of enough As and Sb in the solution are responsible for As/Sb substitution, indicating that gold mineralization has occurred approximately at 250°C, which is supported by fluid inclusion data. A large As/Sb substitution range has also been reported by Mehrabi et al. (1999) in the Zarshuran ore deposit. In this condition, gold has occurred in mineral structure defected in arsenian pyrite due to substitution of Fe with large As ion. There are differences in core and rims of pyrite crystals on BSE images, reflecting lower As and higher S contents in the core of pyrite grains. Compositional zoning that has been found in pyrite represents rapid evolving conditions during ore mineral precipitation, probably due to episodic hydrothermal fluid degassing. The correlation between gold content and degree of As-enrichment in arsenian pyrite could indicate that gold has precipitated from hydrothermal fluids on to the As-rich growth surfaces of pyrite (e.g. Cepedal et al., 2008). Decrease of temperature and salinity during paragenitic sequences are consistent with fluid mixing with meteoric water and following fluid dilution. We can then conclude that the occurrence of porphyry-epithermal veins in the Sary Gunay deposit is due to the presence of a fault system under the aquifer causing sudden depressurization and gradual mixing with shallow water. During temperature and pressure decrease gold was precipitated in the main stage of epithermal gold mineralization evidenced by extensive Au-As-Sb-Fe substitution in stibnite-realgar-orpiment-pyrite minerals.

Cenozoic magmatism in Central Iran has caused formation of contact metmorphed rocks especially skarns (Calagari and Hosseinzadeh, 2006, Karimzadeh Somarain and Moayyed, 2002). The skarns consist of valuable ore deposits. The Shahrak mining area is located on the border of Central Iran and Sanandaj-Sirjan zone (SSZ). This mining area includes 9 iron ore deposits. The Sarab 3 iron ore deposit is located to the south of them. The volcanic rocks of the study area include dacite, andesite, rhyolite and andesitic basalt has occurred during the Eocene period. The intrusive rocks of the study area include post early Miocene diorite- granodiorite, diorite and granite. The iron mineralization stage has formed in limestone-dolomite contact with intrusive igneous rocks (diorite- granodiorite and diorite) as a skarn deposit. The main ore of the Sarab 3 iron ore deposit is the magnetite and hematite. Limonite and goethite, pyrite, pyrrhotite and chalcopyrite can also be seen. The ore deposit geometry is characterized by massive to lens-like shape.

In addition to study of drilling cores, 70 samples were taken from them and the mine pit of the Sarab 3 iron ore deposit in order to study thin sections, thin-polish section, fluids inclusion and sulfur stable isotope. Finally, 20 samples were selected and studied at the Bu Ali Sina University of Hamedan. Fluid inclusion studies were performed on six doubly polished thin sections. These samples were taken from calcite in magnetite hosts. Measuring the temperature parameters was carried out at the mineralogical laboratory of Iran Mineral Processing Research Center to assist the Stage: THMS600 with Linkam model on ZEISS microscope. The temperature range is -196 to +600°C. The machine also has two controllers, heating (TP94) and cooling (LNP), a nitrogen tank (for the nitrogen pump for freezing) and a water tank (for cooling the device in high temperature). Calibration of Stage in heating has a precision of ± 0.6°C which was carried out with cesium nitrate with a melting point of 414 °C and freezing was carried out at a precision of ± 0.2 ° with a standard N-hexane material with a melting point of -94.3°C. Five sulfide samples were selected from an open pit of the Sarab 3 iron ore deposit and the isotopic ratio of their sulfur was measured at the Isotope Lab of the University of Queens, Canada.

During the retrograde mineralization stage in the Sarab 3 iron ore deposit, the effects of the remaining fluid on the skarn rocks and adjacent hornfels result in release of calcium from the skarn and then transport of volatile matter into it. At this stage, the fluid is barren and it has a lower temperature and salinity than its original state. As a result of retrograde reactions, the replacement of high calcium calc-silicate minerals with a series of lower calcium minerals occurs. Also, some amounts of dissolved calcium are combined with carbonate ions in the fluid and thus calcite is formed in the faults and microfractures. Study of the fluid inclusions in the Sarab 3 iron ore deposit shows that the manufacturer fluids have been related to the retrograde phase and have lower salinity and temperature. The study of sulfur stable isotopes in the Sarab 3 iron ore deposit shows that sulfur may have been derived from one of these two sources: It has been created directly from the magmatic differentiation fluid or by the dissolution of previously sulfide igneous sources. The values of δ34S of mineralization fluids have been calculated from the Pyrite-H2S separation factor (Ohmoto and Rey, 1979), assuming that H2S is the most important sulfur compound in the mineralization fluid. Considering the amount of δ34S in the Sarab 3 iron ore deposit (3 to 3.6 permil) it can be stated that all of them can be attributed to hydrothermal fluids with magmatic sources. Also the amount of δ34S of H2S in the fluid equilibrated with sulfides of the Sarab 3 iron ore deposit was close to zero (0.8-1.9‰).

Due to the emplacement of intrusive bodies in the limestone-dolomite of the Qom formation with Oligo-Miocene age, the skarn mineralization has occurred in the Sarab 3 Iron ore deposit. The study of sulfur stable isotopes on pyrite in magnetite ore, has shown the source of mineralization fluids to be derived from magma. Skarns have been formed in several stages the last of which is the retrograde stage. Retrograde fluids have been overprinted on ore and affected it. Calcite veins and sulfides have been formed in the retrograde stage in the Sarab 3 iron ore deposit. In this study it was found that the temperature of fluid in the Sarab 3 iron ore deposit was about 115-324ºC and the salinity was about 0.4-35 wt.% NaCl.

Extensive Eocene–Oligocene magmatic rocks in the Lut–Sistan region, eastern Iran crop out as a huge magmatic province (Pang et al., 2013). Skarnification in Paleocene- Eocene limestones in the Sistan suture zone is very popular (e.g. Nakhaei et al., 2013; Nakhaei et al., 2015; Zarrinkoub et al., 2011) and the Kalate Shab skarn is one of them. Geological and geochemical studies and the results of magnetic measurements in the area of interest and its applicability in exploration of other potential iron deposits in the neighboring areas (Saadat, 2016) are of interest. Metamorphic (the Siah Kamar Skarn) edge of the Mount Rigi granitoid intrusion is calcium type (Biabangard et al., 2015). This skarn is located 105 kilometers east of Sarbisheh, north of the Kalate Shab village. This area is a part of 1: 100,000 geological map of Mahirud (Guillou et al., 1981), with 60° 31' to 60° 35' longitudes, and 32° 21' to 32° 26' latitudes, in the southern Khorasan province (Figure 1), and in the northern part of the Sistan suture zone (Tirrul et al., 1983) in the east of Iran. The Sistan suture zone represents a deformed accretionary prism that was emplaced during the destruction of a small Neotethyan ocean basin, referred to as the Sistan Ocean, which once separated the Lut and Afghan continental blocks from each other (Tirrul et al., 1983). Late Cretaceous adakitic granodiorites and Early Eocene A-type granites have been emplaced in the suture (Zarrinkoub et al., 2012). This was followed by widespread Eocene–Oligocene calc-alkaline volcanic activity in the suture zone and to the west in the Lut block (Pang et al., 2013). Oligomiocene intrusive and sub volcanic bodies (Guillou et al., 1981) have intruded into the sedimentary units and caused skarnification in the north of the Kalate Shab.

This study was done based on 140 thin sections, 1 polished section and 5 polished thin sections with 3 XRD analysis at the University of Birjand laboratory. 3 samples for fluid inclusions have been studied in the laboratory of the Payame Noor University of Mashhad. Microprobe analysis on 4 samples have been done at the Laboratory of Iran Mineral Processing Research Center.

Intrusive and sub volcanic bodies with composition of diorite and quartz diorite have intruded into limestone and have produced Fe skarn in the Kalate Shab area. Mineralogical evidence suggests two stages of progressive and retrograde metamorphism in the region. Microprobe analysis of minerals in the skarn zone shows that garnets are andradite and pyroxene is of diopside- hedenbergite type. Average salinity and temperature of fluid based on micro thermometric data in the Kalate Shab are 13.2 wt% and 222oC, respectively. Magmatic and meteoric waters mixing and chemical changes in carbonate host rock are the main factors for genesis of Fe deposit.

An intermediate magma has intruded into the Paleocene- Eocene limestone, and has caused Fe- skarnification in the Kalate Shab region. The intrusive and sub volcanic rocks are diorite, quartz diorite, porphyritic quartz diorite and porphyritic diorite. Skarnification has occurred as exo skarn with pyroxene, garnet, idocrase, epidote and magnetite minerals. Pyroxenes are diopside- hedenbergite type and garnets are andradite based on EPMA and XRD analyses. Micro thermometric data in the Kalate Shab skarn show temperatures ranging from 171 to 286 ° C and salinity from 11.81 to 14.77%. Petrological studies show that the magnetite formation has occurred in the final stage of skarnification.

Malek Ghasemi and Karimzadeh Somarin (2005) reported that Chromite deposits in Iran occur in Paleozoic and Mesozoic ophiolite complexes in association with serpentinite and serpentinized peridotites and dunites (Ghazi et al., 2004; Shafaii Moghadam and Stern, 2014). There are more than 74 chromite deposits that have been explored in these complexes and they are mainly of alpine-type (Ghorbani, 2013). These ophiolite complexes are part of the Tethyan belts which link to other Asian ophiolite belts such as Pakistan and Tibet in the east as well as ophiolites in the Mediterranean region such as Turkey, Troodos, Greek, and east Europe in the west (Yaghoubpur and Hassannejhad, 2006; Hassanipak and Ghazi, 2000).New data in the current research study are used to infer the geology, mineralization, mineralogy, mineral chemistry and origin of the Qaranaz-Alamkandi chromite.

After preparing 72 samples from the study area, microscopic studies were carried out on 18 thin sections and 23 polished-thin sections for recognition of the microscopic features of the host rock as well as the mineralogy and texture of the ore body. Then, two chromite samples were analyzed at the Iran Mineral Processing Research Center, Karaj, Iran using electron microprobe and scanning electron microscope (SEM) methods.

The Qaranaz-Alamkandi chromite occurrence is located in the west of the Zanjan province and in the northern part of the Sanandaj-Sirjan zone. This area is composed of ultramafic sequences associated with Precambrian metamorphosed rocks such as amphibolite, marble, granitic gneiss and schist.According to petrographic studies rock units in the Qranaz-Alamkandi area consist of serpentinized harzburgite, serpentinized lherzolite, serpentinized dunite, serpentinite, amphibolite, amphibole schist, gneissic granite and mica shcist. This study show that the peridotitic rocks in this region include dunite, harzburgite and lherzolite. Olivine, orthopyroxene and lesser amounts of clinopyroxene associated with secondary minerals (such as serpentine, chlorite and calcite) and opaque minerals (chromite and magnetite) are the main minerals in peridotites.Mineral chemistry of olivines in the peridotites shows magnesium rich olivine with forsterite composition, slightly tending to chrysolite. The composition of olivines falls in the olivine spinel mantle array. Moreover, the olivines of dunites are comparable with those from the oceanic supra-subduction zone peridotites.Clinopyroxenes and orthopyroxenes are Fe-Mg-Ca rich. Furthermore, clinopyroxenes show augite composition and are mainly of the calcium- magnesium type. Orthopyroxenes show mainly bronzite and minor samples showing hypersthene composition. The composition of clinopyroxenes is similar to those of boninites and arc related magmas. This result and the given fact of the low contents of TiO2 and high contents of SiO2 in the structural formula of the pyroxenes suggest that the pyroxenes of the study area are comparable with those from the subduction tectonic settings.Chromite mineralization in the Qaranaz-Alamkandi area has occurred within the ultramafic rocks with serpentinized harzburgite and serpentinite composition. Due to the limited expansion of the peridotitic host rocks, chromite mineralization is also limited and it has occurred as lenses with maximum length up to two meters and one meter width. Chromite mineralization has occurred as massive, disseminated, lenzoid and vein- veinlets form in this area.Mineral chemistry of Chrome spinels from the Qaranaz-Alamkandi area indicate that the chromite samples plot within the ophiolite complexes and high- magnesium chromite field (Leblanc and Nicolas, 1992), which classifies them as podiform chromite deposits in terms of mineralization type (Arai et al., 2004).Chromite mineralization in the Qaranaz-Alamkandi area indicates an Alpine type deposit which is enriched in Cr and Mg and depleted in Ti. The Qaranaz-Alamkandi chromite mineralization has been formed from boninitic magmas which were derived from the subduction process in a supra-subduction zone and fore-arc tectonic settings (Ahrabian, 2018).

Separation of geochemical anomalies from the background has always been a major concern of exploration geochemistry. The search for methods that can make this analysis quantitative and objective aims not only at the reduction of but also at providing an automatic routine in exploration, assisting the interpretation and production of geochemical maps (Nazarpour et al., 2015). The Malayer-Isfahan metallogenic belt with the north-west-south-east trend is the largest and most important Pb-Zn belt of MVT type in Iran (Rajabi et al., 2012). In this study, separation of Pb and Zn geochemical anomalies was performed using the methods named further in the study area.

1. Classical statistics Various statistical methods have been used to process geochemical data in order to determine threshold values. Statistical quantities, such as the mean, standard deviation (SDEV) and percentiles, have been used to define thresholds for separating anomalies form the background. For example, geochemical anomalies have been defined as values greater than a threshold value defined as the 75th or 85th percentile, and Mean+SDEV or Mean+nSDEV (Nazarpour et al., 2015). The boxplot and median+2MAD techniques of the EDA approach have been widely used to separate geochemical anomalies from the background. In exploratory data analysis (EDA) of geochemical exploration data, the median+2MAD value was originally used to identify extreme values and serve as a threshold for further inspection of large data sets (Carranza, 2009). The MAD (is the median of absolute deviations of individual dataset values (Xi) from the median of all dataset values (Tukey, 1977). 2. Multifractal models Fractal and multifractal models have also been applied to separate anomalies from background values. These methods are gradually being adopted as effective and efficient means to analyze spatial structures in metallic geochemical systems (Cheng et al., 1994). The concentration-number (C-N), concentration-area (C-A) multifractal methods have been used for delineation and description of relations among mineralogical, geochemical and geological features based on surface and subsurface data (Nazarpour et al., 2015). Fractal/multi-fractal models consist of frequency distribution and spatial self-similar or self-affine characteristics of geochemical variables. These fractal/multifractal models have been demonstrated to be effective tools for decomposing geological complexes and mixed geochemical populations and to recognize weak geochemical anomalies hidden within strong geochemical background (Cheng et al., 1994).3. Singularity Index (SI) The Singularity technique is another important process for fractal/multifractal modeling of geochemical data (Zuo et al., 2015). This technique is defined as the characterization of the anomalous behavior of singular physical processes that often result in anomalous amounts of energy release or material accumulation within a narrow spatial–temporal interval. The Singularity can be estimated from observed element concentration within small neighborhoods based on the following equation (Cheng, 2007): (1) The Singularity Index is a powerful tool to identify weak anomalies, but it is influenced by the selection of the window size. (Zuo et al., 2015).

In this study, a total of 19946 stream sediment geochemical samples were analyzed using the ICP-MS and XRF methods. In the maps derived from the Singularity Index (SI) the higher accuracy of this method compared to other applied methods was employed. Therefore, the hidden and weak anomalies are better represented, and a better overlap with limestone as the major host rock of Pb and Zn deposits (MVT type) in the study area were observed. A comparison among all of the applied methods indicates that the concentration of Pb and Zn increased toward the and south east and northwest parts, respectively. In these regions there is a high potential for the occurrence of promising mining areas. Moreover, the obtained Pb and Zn anomalies have a good correlation with the exposure of limestone in the study area.