Survey of aerial geophysicss anomalies with magnetometry method, RS and IP in Rashm – Semnan Province region

Author Eng. Naser Abedian
Published At 14 July 2009


GSI launched an ground geophysics group during a 20 days trip to the region in order to geophysics operation in methods of magnetometry and RS and IP in Rashm Region, located in Semnan Province for survey of results concordance and some geophysical surveys with methods of areal and ground types.


Generalities Introduction GSI launched an ground geophysics group during a 20 days trip to the region in order to geophysics operation in methods of magnetometry and RS and IP in Rashm Region, located in Semnan Province for survey of results concordance and some geophysical surveys with methods of areal and ground types. In this launch, more than other writers which had cooperation to the equip, are abbas Bagheri, Hossein Iranshahi, Ebrahim Turk and Faramarz Allah Verdi with much thanks giving. Totally 12,916 stations were surveyed with magnetometry method and 1,000 stations in method of IP and RS. Purpose of these studies in the region is concordance of gathered results in geophysics in methods of aerial and ground and finally geophysical exploration in the area. Regarding geophysical surveys in last year, by methods of magnetometry and EM by equip of aerial geophysics in GSI was done in the region. After mapping data, a vast area in shape of that is gained in aerial magnetic maps and map of electrical conductance. In order to ground geophysics operation, first with magnetometry and then by surveys of dipole – dipole, in order to deep surveys was examined in aspect of their quality.   Geographical position and communication ways From geographical aspect, Rashm region is situated in Semnan Province. There are two roads to access this region. One of them is road of Damghan- Jandagh that we can reach to area by that. This path passes through some villages of Kala, Saleh Abad, Soltanieh, Alian and Yazdan Abad and it joins to Jandag from Moaleman way. Rashm village is situated in path of this road and survey region, west of Rashm village. Other communication way is from Rashm and Moaleman to Semnan Province that after passing through site area in defending industry, it joins to Moaleman and then after to Rashm. The half of this road is asphalt and the rest is in second or third degree soil way. From these two roads, jeep way is separated and we can access from this way to other points. Geology of studied area (derived from report 1:250,000 Torud-Moaleman) Studied area is situated in quadrangle of Torud with a scale of 1:250,000 (Houshmandzade et al., 1357) in Semnan Province and it is a member of central Iran structure (Nabavi, 1355). This zone which is oldest location of Iran plate, had platformic characteristics in Paleozoic and early Mesozoic but it converted to an active orogenic zone late Mesozoic and Cenozoic (Stöcklin, 1986). This zone have many extensive activities in tertiary that its product is seen in Torud region- volcanic belt- Torud intrusive- Chah Shirin which is as a upland with a length of 100m and a width more than 10km with a trend of NE-SW (parallel to Torud and Enjilou faults) in northern margin of central Iran desert. Coordination of quadrangle is as follows: A: 35˚ 18΄ 06˝ N-54˚ 14΄ 14˝ E B: 35˚ 18΄ 54˝ N-54˚ 14΄ 17˝ E C: 35˚ 18΄ 08˝ N-54˚ 16΄ 01˝ E D: 35˚ 18΄ 57˝ N-54˚ 15΄ 60˝ E Area of extent is about 4km2. Geological units are as follows: Unit Qal : this unit includes alluvial sediments of river bed. Unit Q+1: This unit includes flat terrace alluvial deposits and or alluvial fans. Unit Eda: this unit is green dacite and light brown, dacite, andesite and quartz-bearing andesite. Gypsum- bearing marl (Mmg): this unit includes some colorful sequences in color of grey red to lighoream which gypsum layers are seen between them. This unit has extent in south of the area. Stratigraphy of various regions is done by a quadrangle with a scale of 1:250,000 of Torud and other studies. Precambrian, Paleozoic and Mesozoic oldest studied rocks are attributed to Precambrian which have outcrop in east of the area and they are formed from gneiss, amphibolites and micaschist and they are replaced by phyllite and quartz gradually upwards. Middle Cambrian chert-bearing crystalline dolomites have been formed after crystalline set of oldest bed rock some out crops in the region. Lithologic units and beds belonged to Paleozoic and Mesozoic (before Aptian) which have outcrop in some parts of east, west and center of 1:250,000 map of Torud and their outcrops are characterized. That they are metamorphic and they have been deformed from types of schist, marble metamorphic sand and metamorphic volcanic above units are equivalent to metamorphic formations of Niur, Dadeha, Sibzar and Bahram (Cambrian- Devonian) formations of Sardar and Jamal (carboniferous – Permian), Sorkh shale and Shotori formations (Triassic) and Nayband and Shemshak formations (Jurassic). Cretaceous sediments are with angular unconformity with toleration of metamorphism in older deposits. Cretaceous units include clastic red limestone and shale.   Cenozoic Tertiary- Quaternary Tertiary is begun with beginning of volcanic activities in the region. Sediments in Eocene period are accompanied with some thick sequences from volcanic rocks. Depression in basin floor and marine transgression from Eocene are begun from a conglomeratic unit which has variable thickness. Then nummulite- bearing marine sediments is caused after conglomerate which has sometimes a thickness about 3,500m in this time, sea floor is stormy and it is consequently undergone of volcanic activities. Movements of Enjilou and Torud faults results in cause of separated basins in fault area and remnant sequences in these basins have much difference to each other, so deposits related to tertiary are divided into three groups in the region: Nummulite- bearing marine sediments are to north Enjilou fault which have some horizons of conglomerate, tuff, shale and marl. Marine sediments are to south Torud fault which includes volcanic and sedimentary rocks and it has outcrop in trial of Rashm and Chah Shirin Faults. Volcanic rocks which have extent between Enjilou and Torud faults (Torud-Chah Shirin range) which have been formed from very extensive and various lithologic units. Their thickness is different in various points and it has been formed from volcanic, lava, tuff and marl which are situated on older metamorphic rocks and cretaceous sediments as unconformity. Eshraghi and Jafarian (1374) have separated rocks in Torud-Chah Shirin into two groups: -          Detrital- volcanic rocks -          Shallow and dome shape intrusive rocks from late Eocene- early Oligocene According to his mind, lithologic composition in the set has Eocene detrital-volcanic units from andesite- andesibasalt, trachy basalt, trachyte, basalt, dacite and rhyolite are mentioned that tuff beds, sandstone, siltstone and conglomerate beds are caused among them. These rocks are mainly in contact to cold water and in marginal environments to shallow depths (lagoonal). Dome shape and shallow intrusive rocks are with composition of andesite-porphyry to diorite. In this group of rocks, we can imply to dome shape dacite porphyry bodies-after early Oligocene, tertiary deposits are almost equal in Torud area and it includes marine and continental regimes. Difference in these deposits is shallow in various points and it can be separated into LRF and Qom Fm. and URF after Miocene – Pliocene deposits are more than conglomerate, gypsum- bearing marls are formed which are covered with unconformity on all of older formations. Torud Area is from oldest geological times and it is seeing some magmatic activities. These activities are done in fist and second Era gradually and it reaches to its highest amount in third era. Magmatic activities in the region are begun in Silurian which is accompanied with extrusion of andesite lava. Magmatic activities do not extend in second era and tertiary is a pick for magmatic activities in the region that its products (tuff and intrusive bodies) cover all over the area. First stage of activities is in middle Eocene which teaches to highest amount in thick andesite lava and pyroclastic materials and from late Eocene. Intrusive bodies which are seen in the areas that is as dike, sill and small intrusive bodies (with dominant composition of diorite and less amount of granite and granodiorite). They pass all of sequences up to Oligocene. Chemical composition of rocks is in type of calc alkaline in all of phases and some rocks with different compositions are observed. Types of rocks are andesite, andesi basalt, trachy andesite and dacite but volume of andesite products are very much in comparison with other products. In Torud area, two separated groups of metamorphic rocks have outcrops. First group is metamorphic rocks related to Percambrian and it includes a set from schist and gneiss and amphibolites which have out crop in east of area. Second group is Cambrian deposits to before Aptian (lower cretaceous) which have been deformed up to schist facieses (Houshmandzadeh et al., 1357 and Ashraghi and Jafarian, 1374 and Eshraghi, 1374). Ecologists have divided these deposits in maps into two chronologic ages of before and after Cenozoic into two groups of Paleozoic and Mesozoic. From ore deposits pertaining to Paleozoic era, we can imply to lead and zinc ore deposits of Anarou and metamorphic dolomites of Bahram Fm.. From Mesozoic ore deposits, in set of Torud-Chah Shirin, we can imply to Pb and Ag ore deposits of Khanjar to host rock with cretaceous limestone. According to Borna and Eshgh Abadi from khanjar ore deposit are under influence of tertiary diorite intrusive bodies which have been formed with injection to cretaceous limestone. Ore deposits and mineral trace from Cenozoic are divided into follow types: Scarns in Torud – Chah Shirin set is divided into two groups of iron-metal bearing skarns and we can imply to 5 mountains of skarns which is after intrusion of five mount. Granodiorite intrusive body into tuff- pyroclastic units from middle Eocene that main mineral is magnetite and little amount of olegists. Host rock of mineral material is tuff and middle Eocene volcanic. From basic metals skarns, we can imply to Sorb Tangeh in south of Kuhzar village which have been formed due to penetration of granodiorite bodies into volcanic units from middle Eocene. Main minerals include galena, pyrite, chalcopyrite, bornite and tetrahedrite (Borna and Eshghabadi, 1376). -          Siliceous veins poor from gold: This group of vein ore deposits are extracted in an area as copper and or lead and zine ore deposits in past and they are divided into two groups. First group is some ore deposits that their metal is particular and they are seriously poor from gold. This group of ore deposits is named from ore deposit of Chah Musa with a gold of 200 ppb and Gholeh Sukhteh with 10 ppb from gold. Second group are ore deposits that their metal is copper, lead and in some veins as zinc. Gold amount of these ore deposits are 1 to 2 gr/tonne that we can name Cheshmeh Hafez ore deposits, Zereshkkuh and Abolhassani. Host rock of these two groups are from are deposits of gold in area of andesite-dacite sets. These ore deposits are formed likely after penetration of diorite-granodiorite bodies into volcanic units from middle Eocene and their related fluids to them. -          Siliceous veins reach from gold: These ore deposits are with respect to gold economical value in comparison with other ore deposits in the region. Based on last studies, mentioned ore deposits are divided into two groups. First vein is copper-gold siliceous veins second group is siliceous lend, zinc, copper, silver and gold veins. Mineralogy of Cu-Au siliceous veins are simple and it includes chalcopyrite, gyrate, olygist and gold. Sample of this type is mineral trace of Darestan and Baghou. Mineralogy of siliceous veins is from Pb, Zn, Cu and Au as complicated and it includes gallen, sphalerite, pyrite, chalcopyrite and sulfusalts (like tetrahedrite).   For example, we can imply to Gondi ore deposit from this type of ore deposits that mean of Au is 20 gr/tones in its veins studies and present evidences are representative of formation of above ore deposits and their relation to semi- deep acidic bodies and hydrothermal systems are resulted from them. Non- metallic and pelacers ore deposits More than mentioned ore deposit, other ore deposits have been formed in Torud area in Cenozoic Era and so we can imply to Firuzeh Baghou mine in porphyry bodies as semi deep in Baghou. Reservoirs of industrial soil – Gandi kaolinite, Rashm bentonite and industrial soil of Susan are formed due to allocthonous alterations (diagenetic). Weathering and erosion in vein ore deposits and veinlet of Cu-Au have caused gold-bearing placers in the area. A vast extent of ancient mining is in order to gold extraction in sediments and young quaternary alluviums around Baghou (contact area of Chah Alikhan in east and chemistry village in west of Baghou) is done. Probable of porphyry ore deposits in Chah Shirin Torud complex In volcanic – Torud – Chah Shirin well have numerous outcrop from porphyry bodies these bodies have been studied too much due to alteration and some of other properties of them for porphyry copper reservoirs. We can imply to baghou, Ghale Sukhteh, Chalou and Chah Musa porphyry. Most of studies are on Baghou porphyry which is based on mineralization studies in Baghou region in type of porphyry which is based on mineralization studies in Baghou region in type of porphyry Cu-Au.l other porphyry bodies are not studied too much. From above states, we can state that before Cenozoic ore deposits are mainly from lead and zinc ore deposits with host of dolomite and limestone which have been influenced in sequences before cretaceous. Major ore deposits ore with respect to little reservoir, mineralogy (lack of precious metals especially gold) and locating in high regions and due to lack of suitable access way (Borna and Eshgh Abadi, 1376). From Cenozoic period ore deposits, siliceous veins with basic metals-gold is due to gold presence with explorative and economical value number of good-bearing veins in Torud. Chah shirin complex and high grade of some of them is especially. Baghou in northern side and Gondi in southern side is indicative of high potential gad in this volcanic – intrusive stripe from tertiary so upcoming studies can be reach from tertiary so upcoming studies can be reach from gold on siliceous veins and semi-deep bodies in the region that gold generation is concentrated related to each other. Theory of geophysics methods Theory and used geophysical methods in the region explorative are explain as follows. Magnetometry method Study of earth magnetism is oldest geophysics branch. For first time, Gilbert indicated that earth magnetic field has a general trend of N-S near to earth torsional axis. Since then some considerable promotions are carried out in field of machine construction and interpretation of measurements in this method. In magnetic methods, usually total or vertical field are measured. Regarding magnetic field has two poles and also trends, so interpretation of related maps are complicated rather than other methods. On the other hand, in comparison to most of geophysical methods, field measurements are very simple and cheap and there are no long and complicated corrections in their readings. Earth magnetic field is related to geophysical exploration and they are formed three parts. -          Main field, even though it is not fixed to time, it is calmly changed and its origin is internal and it forms about 90% earth magnetic field. -          External field is a small part of main field that its origin is outside of the earth and it is changed relatively calmly, change which is a part of periodic and accidentally part (related to diurnal changes and annually sun and diurnal in the moon. -          Changes in main field, usually but not always are less than main field and it is relatively  fixed to time and place and it is caused due to local magnetic anomalies near to earth crust surface. These changes form explorative geophysics. If any object is located in earth field of F, in this case, a magnetic field of J (induced magnetism) are induced into the object which we will have: J = KF That K is magnetism coefficient (magnetic induce). Objects are divided into three groups in terms of coefficient of K. 1.     K0, paramagnetism. Some elements like nickel and calcium and … this trace is decreased to temperature. 3.     K >>0, ferromagnetic, mainly with iron oxides. 4.     Magnetic induce, an important variable in magnetism and it has same role that density has in gravitation interpretation. Although big changes in K amounts is even for a special rock and a vast overlap is seen between various types, sedimentary rocks have lowest and igneous rocks have highest mean of induction. In each case, only induction depends to ferromagnetic minerals that they are mainly manitite and sometimes ilemnite or pyrotite (some rocks like gabbro, pyroxenite, basalt and andesite have high magnetic properties). Mainly it is possible that minerals with negative induction are measured by detailed magnetism are place determined, even though these negative amounts are small. Also we should remind that some of iron- bearing minerals are a little magnetic. Rocks and minerals are divided into three groups from magnetism aspect, diamagnetic (without magnetism), paramagnetism (with magnetism when it exposed to magnetic field) and ferromagnetic (with magnetism).   Parameter for measurement of magnetic properties is magnetic induction which is represented in terms of emu for various minerals in follow table. Measurement machines are divided into three types in this method, variations of magnetic, magnetometer in type of flux-gate (Shar Gate), nuclear accelerator and rhobidium steam. Most modern type of this machine is in type of smartmag, made by scintrex co, in Canada country which works by cesium uapor. This machine has very high fidelity and it was in level of 0.01 gamma and it has most abundant usage for anceology discovering. Measurement unit of magnetic field intensity is gamma or NT. Magnetometers with less precise and fidelity are very appropriate for easy mineral operations. نوع میانگین خودپذیرى 106* نوع میانگین خودپذیرى 106* هماتیت 550 کرومیت 600 مانیتیت 500000 لیمونیت 220 زغال سنگ 1- کوارتز 1- Analysis methods for magnetic data In order to preparing of data, first, all of coordinations are surveyed by GPS on profiles. Then all of data are analyzed on maps by software of GEO SOFT. For better interpretation of data, some various analytical methods were used. For example, method of trial of behind is more suitable in order to decrease of noise traces in the surface and better display, while derivation maps are more suitable in order more distinction of superficial anomalies. First and second derivation maps rehears anomaly changes and intensity of anomaly changes so superficial anomalies have more intensive changes and they will be more obvious in this manner, besides probability of deep anomaly and or continuing of these anomalies is not rejected and hence trail of behind is prepared in this case. For easy case, we can assume that receiver of magnetometer is higher than current level. Map of decrease to pole is gained in order to determine anomaly location or with respect to geographical position and or with containing amounts of declination, incloration in the region and doing of corrections. Induced polarization (1P). For first time, in 1940 decade, IP was used for exploration of rock bodies especially for dispersed sulfides (desiminated). In decade of 1960, this method was used for ground mineral exploration operations extensively. Conrad schlomberge was first man who reported presence of induced polarization phenomenon. Experimental experiences has shown when current is in type of DC and or AC with very low

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