torud

The study area is located in the south of Damghan, 160 km south of Shahrood, and 17 to 30 km south of Torud village. The area geologically, lies in the Cenozoic magmatic belt, a part of the Alpine-Himalayan belt, in the north of the structural zone of Central Iran (Aghanabati, 2004). The Cenozoic magmatic belt has been studied by many researchers (e.g., Ghorbani, 2005; Khajehzadeh, 2009; Mardani-Beldaji, 2011; Tayefi, 2014; Yousefi, 2017). The volcanic rocks in the southern part of the Torud area have not been comprehensively studied. Therefore, it requires a detailed study. So, for the purpose of this study attempt has been made to investigate and to study the nature of magma, tectonic setting, and the petrogenesis of the volcanic rocks using the geochemical data of the whole rock. Also, the results of this study have been compared with some areas belonging to the Cenozoic Era located in the north of the structural zone of Central Iran.

Regional Geology:

The area under study in the Torud-Moalleman magmatic belt belongs to the Chah-Shirin-Sabzevar-Khaf magmatic complex, located in the western part of this magmatic complex. In this magmatic belt, the Eocene volcanic rocks, including the main volume of igneous rocks are basic to acidic in composition. The predominant rocks are basaltic to intermediate rocks. The Torud-Moalleman magmatic belt is mainly composed of volcanic rocks with a lithological composition consisting of olivine-basalt, basalt, andesite, and dacite rocks and their pyroclastic equivalents, as well as plastic and limestone interlayers.

During field surveying, 50 samples of the volcanic rocks with the least alteration were collected. From these samples, 30 thin sections were prepared for microscopic studies and 11 samples were selected for ICP-MS geochemical analyses for minor elements and XRF for major elements and were sent to ACME Laboratory in Vancouver (Canada). GCDkit, Excel, and Corel Draw software were used to check the results obtained from the whole rock chemistry analyses and drawing diagrams.

Petrography:

The study rocks include volcanic rocks ranging from andesite to basalt. The basalts are dark gray to black in color with glomeroporphyritic, microlithic, sieve, and trachytic textures containing plagioclase and clinopyroxene as the main minerals. These minerals along with olivine and magnetite can also be seen in the form of microcrystals in the background of the rock, and their accumulation have created the glomeroporphyritic texture in these rocks. Secondary minerals are chlorite, iron oxide, zeolite (natrolite and analcime), calcite, and gypsum, filling the holes.The andesites are light gray to slightly dark with porphyritic and glomeroporphyritic textures and are dominated by amphibole (green and brown hornblende), plagioclase, and clinopyroxene as the main, biotite, iron oxide, sphene, and zircon. as the minor, as well as sericite, chlorite, calcite, and epidote as the secondary minerals.

Whole Rocks Chemistry :

The data obtained from the whole rock geochemical analyses display that the volcanic samples of the Torud area are classified as the andesite and basalt, placed mostly in the range of calc-alkaline series (medium potassium). LREE and LILE enrichment, HREE and HFSE depletion and Nb, Ta, and Ti negative anomalies of these rocks point to their formation in subduction zones. Also, as the tectonic diagrams display the rocks belong to the active continental margin. The rocks under study have mostly mantle origin and are derived from an enriched lithospheric mantle. The flat HREE patterns also show that melting occurred in the mantle, above the stability field of garnet. Therefore, the parent magmas were formed by the melting of spinel lherzolite at a depth of 80 to 100 km and evolved due to fractional crystallization as well as contamination caused by subducted sediments and the continental crust.

The rare elements pattern of the study rocks in spider diagrams show the cogenesis of these rocks and the role of differential crystallization as the main mechanism of their formation. Based on geochemical data, the study samples and compared volcanic rocks share similar characteristics. Therefore, the normalized REE patterns with chondrite (Nakamura, 1974) NMORB (Sun and McDonough, 1989) and MORB (Pearce, 1983), indicate the enrichment of LREEs (such as La, Ce) and LILEs (e.g., Ba, K, U, Pb, Cs) compared to HREEs and HFSEs (i.e. Nb, Ta, Ti, P) indicating that the rocks under study were formed in the active continental arc margin. The samples have no negative anomaly of Eu. Volcanic rocks with the age of late Eocene and Oligo-Miocene and basaltic to trachy-basaltic composition range from alkaline to sub-alkaline rocks and volcanic rocks with the age of middle Eocene with andesite to trachy-andesite composition have the nature of calc-alkaline.

The volcanic rocks in the south of Torud, with calc-alkaline and medium potassium nature, are mainly composed of basalt and andesite characterized by LREE enrichment, negative Nb-Ta-Ti anomaly, and the high ratio of LILE/HFSE. These characteristics point to the formation of these rocks in the subduction zones.The rocks under investigation have low SiO2 content, high amounts of Sr, no significant Eu anomaly, and Mg# content greater than 40. These geochemical features indicate a mantle source for the studied volcanic rocks. The changes of Rb/Y versus Nb/Y show the enrichment by subduction components or crustal contamination in the magmatic evolution of these rocks. Based on the geochemical investigations, the productive magma originated from a spinel lherzolitic source at a depth of about 80 to 100 km; during the ascent of magma, as a result of fractional crystallization and contamination, the magma derived from the mantle has been enriched and gave rise to lithological diversity.

In the north of Torud and the eastern part of the Trood- Chah Shirin magmatic arc, the Late Eocene- Early Oligocene subvolcanic rocks with andesite, trachyandesite and dacite composition intruded the Eocene volcanic- pyroclastic sequence. Mineralogically, they are dominated by plagioclase, hornblende, biotite, quartz and magnetite with microlithic porphyry and hyallomicrolithic porphyric textures. Plagioclase with polysynthetic twinning and oscillatory zoning is the most abundant mineral that sometimes having sieve texture, dissolution, and corroded margins. Biotite and amphibole phenocrysts are idiomorphs to subidiomprph with opacitic margins. Quartz phenocrysts sometimes have corrosion gulf. Microscopic evidence, including non- equilibrium textures (oscillatory zoning and sieve texture of plagioclases), dissolution, and corrosion of crystals point to assimilation and magmatic contamination. The geochemical signatures of the rocks studied including enrichment in LILEs (especially Rb, K, Th) and LREEs relative to HREEs and HFSEs (Ta, Nb, Ti), a calc- alkaline affinity and their position on discrimination tectonomagmatic diagrams, display that these rocks derived from a subduction- related environment. These mentioned signatures with a differentiated pattern of rare earth elements without Eu anomaly, depletion in Y and Yb, and high ratio of Sr/Y and La/Yb show these rocks belong to high silica adakites. The parent magma of adakitic rocks in this region originated from partial melting of subducted oceanic crust (in garnet stability depth) associated with fractional crystallization and assimilation processes.

Band-e-Ghichy copper deposit is located in the northern margin of the Central Iran structural zone, about 120 km south of Shahrood and 70 km south east of Torud. Copper mineralization occurred in the rock units including sandstone, conglomerate and siltstone with the Oligocene age. Mineralization has occurred in the form of stratiform (syngenetic), stratabound (epigenetic) and supergen (the surface processes). The minerals forming the deposit include sulfide (chalcocite, covellite, bornite, chalcopyrite and pyrite) and carbonate (malachite and azurite) minerals. The main mineralization forms in the reduction zone as well as in the red formations and is controlled by permeability, the content of organic materials and sulfides in the host rock. Due to the expansion of Eocene andesitic lavas in the adjacent area of the deposit and the presence of volcanic fragments containing copper-bearing minerals in host conglomerate units, it can be concluded that the source of copper in the region is attributed to volcanic units. Based on geochemical studies, sandstone of the region have a felsic to intermediate source rocks and the copper element in the base metals has the highest production coefficient (4 to 8 weight percent) and shows the highest correlation with silver. According to the basic characteristics of Band-e-Ghichy copper deposit such as host rock, mineralogy, structure and texture, companion elements, depositional environment and important mineralization factors, this deposit can be considered as a copper deposit with a sedimentary host and red bed type.

Kuh Zar Au-Cu deposit is located in the central part of the Torud-Chah Shirin Volcanic-Plutonic Belt, 100 km southeast of the city of Damghan. Mineralization including quartz-base metal veins are common throughout this Cenozoic volcano-plutonic belt (Liaghat et al., 2008; Mehrabi and Ghasemi Siani, 2010). The major part of the study area is covered with Cenozoic pyroclastic and volcanic rocks that are intruded by subvolcanic rocks. This paper aims to study the geological, geochemical and petrogenesis of the area using exploration keys for new mineral deposits in the Torud-Chah Shirin zone.

To better understand the geological units and identify the alteration zones of the area, 200 rock samples were collected from the field and 132 thin sections with 15 polished thin sections were prepared for petrography and mineralization studies. Ten samples of intrusions with the least alteration were analyzed using the XRF at the East Amethyst Laboratory in Mashhad, Iran. These samples were also analyzed for trace and rare earth elements using ICP-MS, following a lithium metaborate/tetraborate fusion in the Acme Analytical Laboratories Ltd, Vancouver, Canada. 137 geochemistry samples were prepared by the chip composite method of alteration and mineralization zones and were analyzed in the Acme laboratory by Aqua Regia AQ250.

The geology of the area consists of pyroclastic (crystal tuff) and volcanic rocks with andesite and latite composition, which were intruded by subvolcanic intrusive rocks with porphyritic texture and monzonitic composition. Monzonite rocks were intruded by younger subvolcanic units with dioritic composition. The intrusion of monzonitic pluton and stocks led to the formation of QSP, propylitic, carbonate and silicification-tourmaline broad alteration zones in the area. Monzonite rocks accompanied with disseminated mineralization of about 1 to 10% of pyrite and these sulfides have been converted to secondary iron oxides such as goethite, hematite and limonite. Lithogeochemical exploration revealed Au (up to 598 ppb), Ag (up to 3747 ppb), Cu (up to 679 ppm), Pb (up to 1427 ppm) and Zn (up to 1013 ppm) anomalies. Based on geochemical studies, intrusive rocks have characteristics of high-K Calc-alkaline to slightly shoshonitic and they are within metaluminous to the slightly peraluminous range. Enrichment of LREE versus HREE, enrichment of LILE and depletion in HFSE indicate that the magma was formed in the subduction zones. The negative Eu anomaly is due to the presence of plagioclase as a residual mineral in the magma source. The parent magma is probably formed by the partial melting of amphibolites. The presence of monzonite porphyry source rock, QSP and propylitic alterations, pyrite disseminated mineralization and geochemical anomalies of Au and Cu in the Kuh Zar deposit represents Au-Cu porphyry mineralization in the area.

Tectonic setting discrimination diagrams (Pearce et al., 1984) show that subvolcanic rocks plot almost on the fields of the volcanic arc granites (VAG). In the Rb/Zr vs. Nb diagram from (Brown et al., 1984), the samples are plotted in the field of primitive island arc/continental margin arc. The Torud-Chah Shirin Belt is a part of the Alborz magmatic assemblage (AMA). The AMA has been interpreted to represent the subduction of the Neo Tethyan oceanic lithosphere beneath the Central Iranian continental microplate and the subsequent continental collision of the Arabian and Iranian microplates in the late Cretaceous-early Cenozoic (Berberian and Berberian, 1981; Berberian et al., 1982; Alavi, 1994; Golonka, 2004).
Acknowledgement: This study has been supported by the Research Foundation of the Ferdowsi University of Mashhad, Iran (Project No. 27126.3). The authors would like to acknowledge the East Amethyst Laboratory for XRF analysis. We also thank the Gold Company of Iran for providing conditions for camping and accommodation.

The Chahmora copper deposit is located at South west of Shahrud, within the Torud-Chahshirin magmatic arc. Mineralization in the Chahmora area occurred within volcanic units of Eocene. Based on field and laboratory investigations, the outcropped rocks in the Chahmora deposit are andesite, andesite – basalt, trachy andesite, trachy andesite basalt and several small exposures of pyroclastic rocks such as agglomerate.­ Basic to intermediate sub-volcanic bodies intruded Eocene volcanic-pyroclastic sequences. The rocks are high-K, calc-alkaline to shoshonitic in nature, and are formed at a magmatic arc setting in a subduction zone. The host rocks have been affected by silicification, carbonatization, sericitization and chloritization. The textures and structures of mineralization are vein-veinlet, replacement and open space filling. According to the mineralography studies, main minerals of copper are chalcocite, chalcopyrite, covellite, digenite, cuprite, malachite and rare native copper together with hematite. Chalcocite and malachite are the most abundant minerals. Geochemical studies indicate that copper has only relative correlation with silver (R=0.894) and arsenic (R=0.520).Since silver has not founded as an independent crystalline phase, therefore copper was replaced by silver in chalcocite. . Fluid inclusion studies on trapped fluids in quartz and calcite show average homogenization temperature of 200-220ºC and fluids salinity degree of 0/97-1/37 and 3/67-4/07 %wt NaCl. Copper mineralization in the Chahmora deposit has similarities in mineralogy, host rock, texture, structure and geometry with manto-type and volcanic red bed copper deposits.

The area that contains studied gemstones is located in 19 km South of Torud and 135 kilometer Southeast of Damghan. The quickest and most cost-effective processing method for gemstones in this area is using grind discs in the abrasion and forming stage with the 120 mesh and dry method in polishing stage. The concentration of iron oxide and the presence of calcite and clay minerals made the variation of colors in Turod jasperits. Also Based on ICP-OES analysis, the color of chrysoprases could be related to presence of Ni, pinkish orange agates is due to Fe, Co, Mn and V; and gray colors in agates are related to Cr and Ti. For coloring treatment of yellow agates, cream jasperits, blood stones and opals, heating is the best method. Gems with the texture of micro-spherulitic and micro-variolitic are the most susceptible ones for coloring with chemical method.

The Torud - Chah Shirin volcanic-plutonic complex is located in the western part of Sabzevar Metallogenic Belt and in the south of Moaleman city in the Semnan province. The presence of several mineral occurrences, especially base metal veins of epithermal origin has increased the economic importance of the magmatic complex for geological studies. The intrusionof igneous acidic to intermediate bodies in the volcanic rocks of the area has caused different alternation and mineralization in some parts of the area. In this research, various images processing methods such as false color composites (FCC), band ratios (BR) and spectral angle mapping (SAM) were performed on ASTER L1B VNIR+SWIR dataset, for discrimination of alteration zones in the Torud-Chah shirin magmatic arc. Alteration minerals like kaolinite, illite alunite, pyrophyllite, sericite, chlorite, epidote and calcite, which are associated with argillic, advanced argillic, phyllic and prophyllitic zones were recognized by processing aster dataset and highlighted altered area throughout the range. According to the field studies and XRD analysis, accuracy the results of spectral angle mapping and Band Ratio Logical Operator Algorithms evaluated by confusion matrix and kappa coefficient. Accuracy assessment shows an overall accuracy of 72% and 68% and a kappa coefficient of 0.627 and 0.6 respectively for spectralangle mapping and Band Ratio Logical Operator Algorithms for enhancing argillic and phyllic alteration zones in the study area. Therefore, the results show spectral angle mapping method achieved better results in compared to Band Ratio Logical Operator Algorithms.

The Torud-Chah Shirin volcanic-intrusive arc, in the south of Kavir-e-Chah Jam depression (SW of Damghan), hosted many Pb, Zn, Cu, Ag and Au occurrences and deposits. Cheshmeh Hafez (Pb-Zn±Cu±Ag±Au) and Challu (Cu-Au±Ag) polymetallic vein-type ore deposits are the major one located in the central part. The Tertiary calc-alkaline volcanism and related mineralization occurred along the main Anjillo fault in the north and the Torud fault in the south with NW-SE trend. The exposed rocks in the Torud-Chah Shirin consist of volcaniclastic sequence composed of siltstone, thin bedded sandstone, lapilli tuff and volcano breccias and mostly andesitic and andesitic-basalt flows. Main host rocks at Cheshmeh Hafez are andesite and andesitic-basalt, while in Challu are trachy-andesite and basaltic trachyandesite, hosted polymetal hydrothermal mineralization in the area. Mineralization in the Cheshmeh Hafez occurred in three main stage includes stage 1) vein and disseminated pyrite and chalcopyrite hosted in quartz with homogenization temperature of 188 to 238°C and salinity of 6 to 14 wt.% NaCl equiv, stage 2) main sulfide mineralization with vein, disseminated and brecciate texture hosted in quartz with homogenization temperature of 243 to 276°C and salinity of 11 to 18 wt.% NaCl equiv and stage 3) replacement mineralization and vein-type (quartz and barren calcite) with homogenization temperature of 148 to 185°C and salinity of 4 to 11 wt.% NaCl equiv. Mineralization in the Challu district with low expanse and simple mineralogy occurred at two stage composed of stage 1) vein and veinlet pre-mineralization stage includes specularite, magnetite, chalcopyrite and pyrite (I) hosted in quartz with homogenization temperature of 332 to 356°C and salinity of 7 to 11 wt.% NaCl equiv and stage 2) brecciate and stock-work post-mineralization stage includes pyrite (II), bornite, tetrahedrite, vein-type calcite and chlorite with homogenization temperature of 290 to 322°C and salinity of 3 to 7 wt.% NaCl equiv. According to the recent studies, it can be inferred that with distance increasing from Challu district to Chesmeh Hafez mining area, homogenization temperatures (Th), salinity and depth of mineralization decreases gradually and mineralization at Challu and Cheshmeh Hafez districts have characteristics of an individual mineralization system that caused by mixing of hydrothermal fluid with meteoric water.