نشریه علوم زمین
پیاپی 58 (زمستان 1384)

Gold bearing metamorphosed and deformed rocks (of green schist facies) are exposed in NE-SW trending shear zone, about 25 km southwest of Sagez in Kordestan province.The protolith ore, supposed to be mafic and mostly acidic volcanic and sedimentary rocks (phyllite, re-crystallized limestone, metaferrodolomite), was intruded by several granitic plutons. Shallow to moderately dipping mylonite foliation towards NW and stretching lineation plunging to the N20ºE on the foliations imply that mylonites and ultra-mylonites were produced during oblique reverse left lateral tectonic regime. Hydrothermal alteration is both spatially and temporally associated with development of ductile shear zone.The major types of alteration include sericitization, chloritization, silicification, sulfidation and carbonatization. Among them, sercitization and chloritization are pervasive; whereas, silisification, sulfidation and carbonatization are usually limited to the inner zone of altered shear zone and in close relation with the intensity of deformation. Two planar fabrics (S and C) of different morphology are recognized in the shear zone of Kervian gold field. These fabrics appear contemporary with deformation in Kervian's ductile shear zone.The main mineralization zone, with lensoidal geometry, is about 2500 m long and 60 m wide, on average. The ores, predominantly auriferous hydrothermaly altered mylonites, have been over printed by intense hydrothermal alteration. Gold, existing along the shear fabrics either in the native form or in composition with sulfide such as pyrite and arseno- pyrite, was deposited due to ductile shearing and strong alteration.Final emplacement of gold is an integral part of hydrothermal alteration system, i.e., the major auriferous minerals were deposited in association with hydrothermal alteration. Formation and occurrence of gold field are controlled by ductile shear zone, intensity of deformation and associated hydrothermal alteration. Based on present evidences and comparison with other gold types in the World, the characteristics of Kervian gold deposit are similar to ductile shear zone type gold mineralization.

During Campanian-Maestrichtian an association of Rudist-rich thrived sedimet was formed along the south eastern Neotethys margin. This rock unit is called Tarbur Formation in the Zagros mountains. The main extension of the formation is in the interior Fars; however, it has slim outcrops in Khorramabad area. Tarbur Formation conformably overlies Amiran Formation and is disconformably underlain by the Kashkan Formation. Paleontological study indicates that the age of Tarbur Formation in the study area is the Middle Maestrichtian. Facies analysis and petrographic study led to the recognition of four microfacies of the low-angle open shelf margin. Furthermore, this formation was deposited during a regressive phase in a warm and low energy shallow marine environment.

In model–based inversion, a geological model is compared to seismic data. Then, the result of comparison between real and modeled data is used to iteratively update the model to have a better match of seismic data. This method is intuitively very appealing since it avoids the direct inversion of the seismic data itself. On the other hand, it may be possible to come up with a model that matches the data very well, but it is incorrect. There are two ways in which constraints may be used. One way is to consider the additional information as a soft constraint, meaning that the initial impedance is a separate piece of information which is added to the seismic trace with some weighting of two. This approach is called “stochastic”. The second method is to consider additional information as a hard constraint that sets absolute boundaries on how far the final answer may deviate from the initial model. This approach is called “hard constraint”. In the model-based inversion, average block size and the number of iterations are crucial. Using of average block size greater than seismic sample interval is recommended. The main cause of this idea is the fact that there is some noise in the data. If the number of iterations sets to large numbers, the results will be better. The problem here is computational time. By investigation on Ab-Teymur 3-D seismic data, the following results were obtained: proper values for constraint limit, average block size and number of iterations are 35%, 8 ms and at least 8, respectively. Two low impedance regions were detected that could possibly have the potential of hydrocarbon by using model-based inversion with the mentioned parameters for Sarvak formation.

The Ardakan barite mine, located in 37 km northeast of Ardakan city in the Central Iran, occurs in limestone and dolomitic rocks of the Permian. Secondary copper minerals of the mine include Cu-sulfides, Cu-carbonates and Cu-oxides. In the studied samples, association of pyrite with chalcopyrite and differences in their electric potentials caused a series of galvanic reactions and chalcopyrite alteration. In a chemical battery of pyrite-chalcopyrite, pyrite with higher electric potential remains fresh as a cathode and chalcopyrite with lower electric potential alters to another sulfide phase as an anode. The final product of the chemical reaction is the formation of secondary copper sulfides. Two stages of alteration are observable in the chalcopyrite alteration sequence. The first stage is the alteration of chalcopyrite to covellite, led to a decrease of Fe content with a simultaneous increase of Cu and S. The second stage is the alteration of covellite to anilite with an enrichment of Cu and a corresponding decrease in S. Copper bearing acid solutions were neutralized by carbonate host rock which resulted in the deposition of Cu-carbonates in the ore body.

The Bagh Khoshk copper deposit, situated in 145 km southwest of Kerman, is in the Orumiyeh-Dokhtar zone. The major trend of joints is NE–SW in the mine district. Major rock units in the study are volcano-sedimentary and volcanic rocks such as tuffs, basalts, basaltic andesite, andesite and rhyolite of the Eocene age. Furthermore, other rocks outcropped in the area are gabbrodiorite, dolerite, diorite, quartz-diorite, tonalite and granodiorite of the Oligo-Miocene age. Different alterations of potassic, phyllic, argllic and propylitic types are observed in the area. The ore-paragnesis includes pyrite, chalcopyrite, bronite, covelite, chalcocite, cuprite, tenorite, malachite and minerals such as magnetite and hematite. Using 264 rock-samples, lithochemical exploration shows a high Cu-anomalous zone in the central part of tonalite intrusion (Cu content 0.003 % to 1.06%). Based on these data and particularly the alteration patterns, the Bagh Khoshk Cu-deposit could be compared with porphyry–type model. From the geophysical studies and drilling data in 1971, the estimated Cu-reservoir was reported 24 Mt with average grade of 0.27% Cu.

The Jajarm bauxite deposit is so far the biggest known bauxite deposit in Iran. This deposit is located in structural zone of Alburz. In this area two bauxite horizons, called A and B, with two stratigraphic gaps can be found. The A horizon is underlained by carbonate rocks of Mobarak Formation and covered by Nesen Formation. The B horizon that constitutes the Jajarm bauxite deposit overlies the dolomite of Elika Formation of the Upper Triassic age and covered by the Shemshak Formation. Sharp contact between bauxite deposit and overlaying as well as underlying units indicate an independent source for bauxite deposit. One of the most important characteristics of Jajarm bauxite deposit is its asymmetrical morphology along the tectonic structure of the area. The geologic setting is striking16 kilometers in E-W direction. In addition to major tectonic elements parallel to the strike, there are a number of minor north-south faults in the area that have tectonically divided the mine area into four regions. Based on these studies the tectonic activities have important role in mineralogy and improving the quality of bauxite deposit. Most of the karsts and sinkholes are along the fault planes and show a positive relationship with thickness and bauxite quality. Hard bauxite and diaspore formation instead of bohemite and gibbsite as well as the repetition of different bauxite layers are the result of these tectonic activities. Therefore, the hard diasporic bauxite has played very important role in the process of alumina production. Constant profiles of: Kaolinite clay-Hard bauxite- Clayey soft bauxite and Kaolinitic clay, from top to bottom can be seen in different karst and drill holes, respectively. But the nature of upper top kaolinite is different from the bottom one. Mineralogy and variation in chemical composition (Al2O3=30% to 60% and SiO2 between 5% to 39%) are other special characteristics of Jajarm bauxite deposit.Based on the exploration activities 22 million tones of diasporic bauxite, up to 250 meters of depth along the layer of ore deposit and Al2O3 content range between 47% to 48% with about 10% of SiO2, have been proved.

To investigate Nessen formation sediments in Alborz, two stratigraphic sections were studied in Central Alborz region, Noshahr (Menjeer) and Amol.The objective of the research was to determine the sedimentary environments and the sequences of Nessen formation, therefore, 380 thin sections were prepared. Petrographic studies of thin–bedded limstone, marl, shally limestone and Nessen formation shales have resulted in the recognition of two groups of detrital and carbonated facies. Detrital facies are related to deeper marine environment (basinal facies) and in some cases have skeletal fragments of the open sea. Investigation of limstones from Nessen formation resulted in recognation of seven microfacies of three groups: open marine environments, barrier environment, lagoonal to tidal flat environment.Carbonate facies were deposited in marine environment and on carbonate ramps while detrital facies deposited in distal ramps regions. The carbonate sequence of Nessen formation consists of three upward shallowing megacycles. Each of these megacycles has been made of some regressional microcycles in metric scale (parasequences). Regressional cycles in million-year scale with extentional phases, regional subsequences and sea-level fluctuations are mostly related to the sea-surface fluctuations made by tectonic activities and through gradual up-coming (ascending) and sudden down-going (descending) of the sea-surface, while the microcycles (parasequences) have probably originated from cyclic subsidence made by sea-surface fluctuations due to glaciers or Milanchovich climatic systems and accompanied by gradual descending and sudden ascending of the relative sea-level.

Alteration zones with their extended halos are considered as greater targets than ore zones in exploration. Therefore, an attempt was made to study these zones and their respective anomalies by integration method for base metals, radioactive elements and precious metals in Mianeh, East Azarbaijan. Alteration zones in the region were recognized by principal component analysis (PCA), ETM image and band ratio techniques. In a later stage, these zones were precisely detected by integration of radiometric data (eU, eTh, eK) and airborne magnetometric technique. By means of PCA, the linear combination of the four parameters was presented and used for detecting the alteration zones. The results reveal that the combined application of the three methods can be used successfully as an optimized technique in detecting true alteration zones and delineating false anomalies.

The Kashaf-rud formation is a sequence of siliciclastic sediments outcropped in the southeast of Kopeh-dagh region. Its lithology consists of conglomerate, sandstone, limy sandstone and shale. However, Tapeh Nader area presents different lithology, comprising sandy limestone, limy sandstone and shale with belemnite, ammonite, barchiopoda, gastropada, bivalve, bryozoa, ostracoda, coral, and crinoid. The genus of these carbonate rocks are biosparite or bio oo sparite that shows coastal environments in this area, whereas, original facies of Kashaf-rud formation is detrital facies. Many evidences display that turbidity currents were efficient on the creation of these sediments, i.e., turbiditic classic sediments, massive sandstone, pebbly sandstone, and conglomerate. The most important fossil in this formation is Ammonites, of which 13 genera and 18 species were recognized and classified in two biozones (Garantiana and Parkinsoni zones).

Kareh Bas fault system with a total length of more than 200 km is a nearly N-S trending transverse fault system in Zagros fold and thrust belt. This system is situated about 40 km in the west of Shiraz. Having a right-lateral strike slip fault system and based on its geometry, it consists of six structural segments.In this paper for the first time, the type and attitude of Mountain Front Fault (MFF) have been determined by data integration and Kareh Bas fault system, that caused approximately 109 km strike separation of MFF, has also been introduced as a transcurrent fault system. In addition, the role of this fault system in the basin partitioning, thickness and facies controlling of sediments since Middle Cretaceous is discussed. Finally, fractal dimensions have been analyzed on spot space image maps.

Famenin–Ghahavand- Kaboudrahang plain is located in the north of Hamedan province in Iran. The thickness of aquifer in the area is about 70-100 meters and its exploitation has been more than aquifer capacity. Annual falling of water table is about 2.5-3m. At least 19 small and large sinkholes have been formed in the plain during the past 10 years and this is a problem for inhabitants of the area. In this study, field investigation, analysis of water samples and calculation of free carbon dioxide were carried out and the characteristic features of the sinkholes recorded. Results show that the presence of specific conditions of limy bedrock and high amount of free carbon dioxide (more than 1500 mg/l) in groundwater are the main causes for the development of karst in the area. Furthermore, the presence of large holes, water channels, joints and fracture systems in the bedrock and water pumping are the main sources for the creation of sinkholes, whereas other factors such as sand-washing of layers, gas release from wells and fast falling of water table are minor effective causes. Therefore, to stop this phenomenon, drilling in bed rock should be prevented, suitable gravel pack should be used and all wells with high water yield form bedrock should be filled.

Alamgholagh Mountain is located in NW of Hamedan city. It comprises diorite to syenite and quartiziferous syenite rocks near its climax. The Pre-Jurassic ignimbrite and rhyolite host Baba-Ali ore body. This ore body contains magnetite, hematite and trace amounts of pyrite and chalcopyrite. Textural studies with reflecting microscope show some primary and replacement textures. SEM studies reveal parallel thin plates in magnetite crystals and bacterial activity on pyrite crystals.Spider REE diagrams show noticeable different patterns for ore body amphiboles and country rocks. In the ore body, the concentration of elements fluctuates alternately, however, in the country rocks decrease from La to Lu and show a negative anomaly in Eu values. Some of the calculated REE ratios for the ore body and country rock are different and suggest a different genesis.

The Molataleb granitoid is located in the north west of Aligoudarz and at a distance of about 20 km of the Zagros Thrust Fault within the Sanandaj-Sirjan zone. It intruded into the Triassic and Jurassic metasediments during Laramide orogeny. The intrusion has thermally metamorphosed the country rocks up to albite-epidote hornfels facies. Lithologically, the pluton is composed mainly of granite and granodiorite of S-type. The granitoid plutons ocassionally show concentration of economic concentrations with special geochemical patterns. Geochemical investigation on Molataleb granite, e.g., the quantities of Li and Ni in biotite, Pb in K-feldspars and distribution of Mg/Li, K/Rb and Sr/Rb indicates that the Molataleb granitoid is barren with respect to economic elements.

Kuhbanan fault is located between Tabas and Yazd blocks in the southeast margin of Central Iran microplate, extending from the northwest of Kerman to the east of Bafgh, with 300 km length and NW-SE strike. The fault is one of the most seismic structural trends in the region. The latest seismic activity of the fault occurred on the seventh of October, 2003 with earthquakes of magnitude of 3.8 on Richter scale over the Kuhbanan segment of the fault. Kuhbanan earthquake fault consists of several segments. Movement in the southeast and central segments has been dextral strike-slip with a large reverse component and in the northwest segments dextral strike-slip with a large normal component during Plio-Quaternary.Morphotectonic effects of the active kuhbanan fault in mountain fronts of Kerman-Kuhbanan range and west Bahabad mountain range are landform features such as young and prominent fault scarps, linear mountain fronts, well  defined triangular facets, narrow V-shape valleys with steep longitudinal profiles, steep slopes of valleys and mountains, long drainage basins, deflected and offset streams, pressure ridges, shutter ridges, linear ridges, linear valleys, springs, sag ponds, young and small pull-apart basins along the fault. Therefore, with respect to the variety and numerous landform features, kuhbanan fault is introduced as a special model of morphotectonic landforms in Iran.Study of morphotectonic landforms and calculation of geomorphic indices such as, SmF, Facet%, VF V, SL, K and HY in the 32 fronts and 33 rivers with similar rock resistance along the fault segments, indicate that the degree of tectonic activity has been different in the fault segments during the Plio-Quaternary times. Therefore, the fault segments can be classified as very active, active, moderately active and with low tectonic activity.Based on the landforms and morphometric indices obtained along the Kuhbnan fault, the fault segments are classified as follows:1) The first class segments: Gisk Dartangel, NW-Dehzou, Khan-Makan Toghrolijard, Charmis segments that have the highest tectonic activity. 2) the second class segments: Khanok, Rigabad, Dahou, Katgouyh, Gazouyh, Apang-Sarbagh, Dezouyh, SE Gavar, Dargazri-Darhod, Pasando-Babkham, Fakhrabad, Bonestan, Khoshabad, Hossianabad I., II, Kuh-e-Sorkh I, II, III, Kuh-e-segoush, segments have been active. 3) The third class segments: West Katgo, Abbasabad, south-east part of Dargas=Godgas, Gavar and Kuhbanan segments have low activity. 4) The fourth class segments: Rushk, Dasht-e-Khan, Babrouyeh and Asadabad segments have the lowest tectonic activity.It should be noted that based on morphotectonic data the most active and active segments are coincided with earthquake fault segments. However, earthquakes have not occurred over some active segments. It seems that earthquakes might occur over these segments in the future, or active morphotectonic data might not match the seismic activity of fault segments. Nevertheless, it is obvious that an appraisal of seismic activity of faults, based on the geomorphic indices and landforms features, must be taken into account with care.

Historical background and instrumentally located earthquakes as well as the geological evidences all suggest that Alborz region in northern Iran is one of the seismically active regions in the Middle East. In 1996, as a part of national seismic network, the Geophysics Institute of Tehran University deployed a telemetric seismic network in Alborz region mainly to monitor local earthquakes. Relying on the records obtained during 1996-2005, several seismically active areas with the following aspects could be recognized. The epicenters of local earthquakes are in good agreement with the location of major faults as well as the regional tectonic settings. The distribution of earthquakes in eastern and western parts of the region are consistent with the related major faults. A kind of seismic quiescence exists in the central part of the Alborz around the Tehran city. A similar situation could be observed along the south eastern extension of Rudbar fault system. Most of the recorded earthquakes have shallow depths indicating that the seismic activity is mainly taking place in upper crust and the seismogenic layer has a thickness of about 20 km. Taking into account the historical background and the present situation, the occurrence of a major earthquake in Alborz in the vicinity of Tehran is not far from expectation.