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©2015 TJEAS Journal-2015-5-1/172-181
ISSN 2051-0853 ©2015 TJEAS
Investigated the occurrence of iron ore, magnetite,
carbonate sediments in the Oligocene North
Bijar(NW of Iran)
Maryam Ahankoub1, Fariba Jamshidi2, Reza sheikhi2
1. Assistance professor, Department of geology, Faculty of science, Payam- Noor University, Iran.
2. Department of geology, Faculty of science, Payam- Noor University, Iran
Corresponding author email: [email protected]
ABSTRACT: Studied area is located in NW of Central Iran Zone in Kurdistan Province, west of Shahrak City.
There are High potassic to Shoshonitic intrusion bodies of Miocene consisting of granite, diorite, monzodiorite, and
syenite that have been intruded into the sequence of Oligocene carbonate rocks. Petrography of these rocks show
that maior minerals are plagioclas, pyroxen, biotite, olivine, hornbland, feldspar potasic and quartz. Geochemical
investigation of the intrusion bodies show that it has been formed in Continental arc tectonic setting. The intrusion
of igneous rocks into the carbonate formation has caused occurrence Ca and Mg skarnes with Iron mineralization.
Shahrak Iron minerals have precious ore deposit because of the Iron̕ s High alloy, low amount of S, P and also Iron̕
s high reserve. Main ore deposit is iron with an average of 65 % with a small percentage (less than 5%) of sulfide
minerals of pyrite and chalcopyrite. Also there is no accessory mineral such as Cu and Au.
Key words: Iron Ore, Ca and Mg Skarn, Magnetite, Shahrak deposit, Kurdistan
INTRODUCTION
North Bijar skarn iron deposite is iron skarn with large store and valuable. Skarns are formed of Mg, Al
and Fe silicates minerals that resulting by metasomatism of the limestone and dolomite rocks and Replacement of
Si, Fe and Mg. Skarns, on the base of the conditions and How to formation, were divided to two groups, the
reaction skarns and Skarnoides (Einaudi, 1981).
Considering the importance of economic reserves of metals such as iron in countries development, study
and investigation about metal potential is very important.
In recent years, there has been done extensive investigation about pre-feasibility studies, detailed design
and description exploration studies of the iron deposit.
Following the conclusion of the contract of the Iran Steel Factory about Fe exploration, Iran Barite
Company have been investigate about discovery of the iron ore deposits and were identified three zone including:
a) Kerman - Bandar Abbas , B ) Khorasan , and c )Zanjan _ Hamedan .
The first report, about geological and iron mineralization in shahrak area, was presented by Pey Company
in 1978(Sheikhi, 1996).
After licens issuing of exploration, was prepared geological-mining map and then was drilled wells and
boreholes. Finally exploitation of Shahrak mine began in early 1994(Sheikhi, 1996).
Today Shahrak skarn Iron deposite is one of the largest deposits of Fe mines in the NW of the Iran, that
was utilize with storage of more than 20 million tones and over 50 m thickness.
In this study, we present new geochemistry data about ores and country rocks from the Shahrak skarn iron
deposit. Based on the results, we evaluate the source of ore-forming materials, and the geodynamic setting of ore
genesis.
Geological setting
The study area is located in the NW of Iran in Kurdestan province in part of Zanjan West Mountain. Zanjan
West Mountain is small part of the Soltanieh - Misho zone (Eftekharnzad, 1359) that base on the structure
geological divisions is located in central Iran zone. The major lithological units of the Shahrak are composed of
Tech J Engin & App Sci., 5 (1): 172-181, 2015
mezozoic metamorphic and sedimentary rocks units, and Cenozoic magmatic intrusions. The basement mainly
consists of the skarn iron deposits of Shahrak are located within the periphery of the west of Central Iran.
Regionally, the rocks in this area are dominated by sedimentary, metamorphic and igneous rocks. The rock units
oldest consists of metamorphosed shale, limestone and dolomites, with andesite bands that is covered by Eocene
volcanogenic thick layer sediments (conglomerate, sandstone, tuff and siltstone). Limestone Oligocene to early
Miocene was formed in low deep marine environment that is covered by Quaternary alluvial subsequently. The
most important deposits of iron has occurred as mass or dispersed within metamorphosed carbonate formations of
Oligocene (Figure 1).
Figure 1. geological map 1/20000 simplified the study area
The Shahrak Iron deposits have been located as strataform, lens, massive, disseminated, granoblast and
layer in limestone and dolomite Oligocene. The dominant basement rocks in this region are the Mesozoic rocks,
intruded by Cenozoic plutons comprising mostly of monzonite, monzodiorite, and diorite, and quartz–diorite,
granite, syenite. The genesis of these intrusions has been due to the subduction of the Paleo-thetys (Shaikhi,
1996). The major sedimentary unit in the mine is the Oligocen limestone. The main ore-controlling structure in the
mine is an anticline trending NW.
The ore bodies typically occur at the contact zones of the intrusion bodies and the limestone. From the
contact zone profile, the shape of the ore bodies is reconstructed as complex lenses with end-to-end discontinuity.
The serrated and interspersed features of the ore bodies are visible near their edges, with both inward and outward
contact zones. Field studies reveal a close relationship between the diorite, which at some places grades to diorite
porphyrite or monzodiorite, and the formation of the skarn and ores.
Alteration zones are clearly developed at the contact of the intrusive with the surrounding limestone, and
these zones can be divided into altered diorite zone, the endoskarn zone, the magnetite zone, the exoskarn zone
and the marble zone. In general, the endoskarn zone shows a wider distribution as compared to the exoskarn zone
which is narrow. The skarn mineral compositions basically include diopside, tremolite, actinolite, phlogopite, talc,
garnet, and epidote. The marble at the external contact zone often has been brecciated or altered by chlorite. The
main metallic minerals are magnetite, and a small amount of hematite, chalcopyrite, and pyrite. The content of
magnetite in the ores varies from 55 to 75%. The subhedral to anhedral coarse magnetite grains range from 0.15 to
0.4 mm in size. Under the influence of late hydrothermal alteration, the magnetite grains are replaced by pyrite,
calcite, and hematite, with the relict texture preserved in many cases. The pyrite is subhedral to idiomorphic and
generally ranges in content from 1 to 2%, and sometimes up to 5% in some domains.
Mineralogical and lithological
There is diversity of lithological composition in the study area, including
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Petrology and mineralogy intrusive bodies
Intrusive rocks has outcrop as large masses, dikes and small bodies that Petrographic and lithological study has
been done as follows:
Granites almost are coarse to medium granular including to 30% Qtz with 0.2 to 1 mm, 40 to 45 %
hypidiomorphic pertite with dimension to 2 mm, to7% plagioclase crystals with dimensions of to 2 mm with albite
twining, up 5% Or with dimensions of 1 mm. The pyroxene crystals are 15 to 20 %t to mafic crystals with
dimensions up to 1 mm, (Fig. 2a).
Granodiorite are bright color and porphyry texture. There are major minerals 20% K-feldspar phenocrysts
(0.5 to 1 mm), fine grain 60% plagioclase (< 0.3 mm) and 2%quartz (Fig. 2B).
Diorite are gray color and texture intergranular, which contain 60% plagioclase, and 30% prismatic crystals of
amphibole (Fig. 2 C).
Monzodiorite almost has outcrop in North West - South East area with gray color. The rocks are mediumto coarse-grained, with plagioclase, K-feldspar and Pyroxene as the major mineral phases and magnetite, zircon,
apatite, titanite and allanite as common accessory phases.
Syenite are gray to pink color body with well-defined magmatic textures. K-feldspar is often pertite. (Figure
2 d).
Skarn mineralogy
There were formed skarn by injected of the Intrusions into carbonate rocks (limestone and dolomite)
Oligocene and has created diversity skarns. According to the skarns classification by (Einaudi, 1981), both
endoskarns and exoskarns can be identified in the region. Two-stage progressive metamorphism and two-stage
retrograde metamorphism is distinguished in the skarn zone. First stage is formed garnet, phlogopite, and
pyroxene, and in the second is formed stage amphibole, talc, calcite and chlorite. There are some minerals such as
actinolite, chlorite, sericite, calcite, quartz and garnet in endoskarn. Main part of the skarn is exoskarn that has
diffrent thickness from 5 to 20 m and is located between crystalline limestone and endoskarn. There are observed
different skarns zone including garnet skarn, pyroxene skarn, amphibole skarn, plagioclase skarn, talc skarn and
phlogopite skarn.
Figure 2. A): phenocrysts of feldspar potassic with quartz crystals in granite, B): Am and Plag in granodiorite, C): Px, Am and
plage in the diorite, D) the granular texture of feldspar potassic and in syenite, (Kretz, 1983).
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Garnet skarn
with to have 5 m thickness is the closest skarn zone to iron ore deposite. There are paragenesis
mineralogy including garnet (grossular and andradite), chlorite, calcite and magnetite (Figure 3A). Garnet is often
zoned and may reach more than 80% volume in some samples and has andradite composition in central that
surrounded by grossular. Pyroxene skarn: is up 10 m thick that contain pyroxene (diopside and Hedenbergite)
Along with the chlorite, albite, epidote, amphibole and magnetite. Secondary minerals, chlorite, actinolite and
epidote occur as alteration products of pyroxene. (Fig. 3B).
Amphibole skarn
is large extended. There are tremolite, actinolite and pyrite crystals as Mineralogy paragenesis. actinolite
green crystals was formed broom and filament texture of the up to 3 mm that comprises up to 70% volume of
rock. There is chlorite as actinolite alteration often to 15% (Fig. 3 c, d).
phlogopite skarn
contain coarse grain green phlogopite and Magnetite. There are paragenesis of the minerals such as
calcite, diopside, epidote, amphibole, and pyrite.
Talc skarn
with small extended, is often in the southern study area. Studies of core drilling indicate that this zone is
located at deep. These rocks are green with soap tactility that has magnetite, talc, phlogopite, Pyrophilite as
milnerlaogy paragenesis.
Plagioclase skarn
is close to intrusion bodies and there are forms 35-40 percent plagioclase, quartz, 5-10%, 10-15% and 45% magnetite and chlorite as mineralogical paragenesis .
Figure 3. A): Garnet zoning (andradite in the center, grossular in the margin) in garnet skarn, B): px and magnetite in pyroxene
skarn, C): Am in amphibole skarn, D): coarse Act in amphibole skarn, (Kretz, 1983).
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Mineralization
Iron mineralization occurred in adjacent intrusive and over metamorphosed carbonate formations.
Magnetite is the most abundant of the constituent in ore deposits and has vast exposure in the superficial and deep
parts of the region. The ore zone consists of discontinuous that there is 55 to 75 % magnetite in mineralization
zone. These ore deposit has high economic value because iron high content, low content sulfur and phosphorus.
Magnetite
is developed idiomorphically. It is characterized by simple crystallographic forms: (111), (110) and (110) or
their combinations. Individuals of magnetite are of considerable size, appearing like porphyroblasts (idioblasts) in a
fine-grained mass of other ore and gangue minerals. Size varies on average between 1 and 10 mm.
There is replacement or granular texture. In Large part of the gang occure martitized. (Ramdhor, 1980) believes
that magnetite was affected by the influx of low temperature fluids and change to hematite along cleavage.
Magnetite mineralization is part of skarns zone that were formed by solution of iron-rich. Some evidence for the
presence of the solutions is consisting:
Magnetite is most important Fe minerals.
There are skarn minerals such as garnet, actinolite, phlogopite, talc and diopside with magnetite.
Magnetite crystals were formed as crystalline and massive.
Ore deposit is located within the skarn zone.
We are classified mineralization zone based on the paragenesis that consisting of:
1 - Magnetite upper zone (Fig. 4A), 2 – oxide zone (Fig. 4B) and 3 - Magnetite lower zone (Fig. 4 c, d). In table 1
have been shown to separation of the three types of iron ore deposit zone. There are sulfide minerals such as
pyrite 5%, chalcopyrite (up 2 percent), and iron oxides hematite and goethite. Pyrite Subidiomorphic to amorphous
crystals have been found with dimensions less than 1 mm that main texture is observed often as scattered and
fillers between of granular magnetite, cavities, fractures and boundaries of magnetite crystals. According to the
pyrite formation low temperature, and its presence in the study area, pyrite was formed after magnetite.
Figure 4. A): Magnetite in the upper zone, B): conversion of magnetite to hematite in the oxide zone, C and D): Pyrite in
magnetite margin
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Chalcopyrite has been found (with a frequency of <2%) as fine crystal less than 0.5 mm, often filling the
empty space in the margins of magnetite, pyrite and quartz gangue. That was formed in final stage of retrograde
metamorphic and related to the sulfide mineralization. In Table 2 are shown minerals paragenetic relations. There
has occurred some progressive and retrograde alteration that is given as follows:
Progressive metamorphism – metasomatism stage:
This stage has occurred after emplacement of the intrusive rocks in the vicinity of impure carbonate
formations during metamorphism and then was formed anhydrous silicate minerals of the fine-grained garnet,
pyroxene and phlogopite.
Progressive stages of metasomatism:
This stage has ocuured by arrival of magmatic hydrothermal fluids into the host rock and formation of the
anhydrides silicate minerals such as coarse garnet and pyroxene. Garnet and pyroxene formation temperature is
about 450 to 650 ° C (Rose and Burt, 1979). Al was added to the host rock by hydrothermal solutions (Einaudi,
1982a). Garnet was formed by combination calcium, iron and hydrothermal silica. There are andradite,
hedenbergite and magnetite by, mouth of high FeO and lack of silica (Deer and et al, 1962)
2Fe2O3 + 2FeO +5 SiO2 + 4CaCO3 = Ca3Fe2Si3O12 + CaFeSi2O6 + Fe3O4 + 4CO2
According to the lack of wollastonite, there is
SiO 2/Fe2O3 <3 (Deer and et al, 1962). There is not
exchange of wollastonite - magnetite or hedenbergite- wollastonite, which was shown garnet zone was formed in
less than 650 °C.
Table 1. Zone of Shahrak ore deposits
Clay Minerals
Kaolinite
-
Oxides minerals
Hematite, Martite,Geothite,Limonite ،
-
Els minerlas
Fe/
FeO
4.6
Hematite, Limonite,Geotite, Martite
،
Pyrite, ، pyrotite
5.48
2.3
S
15%<
15%<
15%<
Sulfid minerals
Pyrite
Pyrite,
Pyrothite ،
Fe- alloy
Major minerals
Magnetite>80%
Magnetite
Zone
Upper magnetite
Oxide
Magnetite
Lower magnetite
Ore deposite
type
68%<
Magnetite
45-65%
Magnetite
45-55%
Magnetite
Magnetite(A)
- Magnetite )B(
Martite
- Magnetite (C)
Martite
Table 2) Paragenetic diagram of the minerals existing in Shahrak area
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Figure 5. diagram Log fO2- T (Taylor and liou, 1978) to represent the range of stability of minerals in skarn paragenesis in
Shahrak
Early retrograde alteration stage
that was formed hydrolysis process and occurred alteration of the garnet to quartz, magnetite, hematite,
epidote, and pyroxene. Based on the formula provided by (Lentz et al, 1995), coexist magnetite and andradite
Hedenbergitet (± quartz and calcite), can be explained by the following equation.
CaFeSi2O6 + Ca3Fe2Si3O12 + 4CO2 = 4CaCO3 + Fe3O4 + 5SiO2
Usually magnetite, calcite and quartz are result of alteration andradite by decreasing temperature and increases of
the fugacity.
3Ca3Fe2Si3O12 + 9CO2 = 9CaCO3 + 9SiO2 + ½ O2 + 2Fe3O4
The Beginning of garnet-epidote alteration was occurred in less than 450 C .Replacement of calcite, quartz and
magnetite instead of andradite is an important retrograde reactions that occurs at low sulfide relatively condition in
andradite skarn (Deer and et al, 1962).
Late Retrograde stage
there were formed are comprised of fine-grained chlorite, calcite, quartz and clay minerals by alteration hydrous
and anhydrous calcareous silicate minerals by low-temperature fluids.
Physicochemical conditions of skarn formation
That indicates shahrak skarn is Ca-Mg skarn that contains phlogopite and talc (Taylor and Liou, 1978). On the
base of Paragenetic diagrams (Taylor and Liou, 1978) and. Stability limits mineralogy paragenesis state that
anhydrous minerals such as garnet and pyroxene was altered in 450 t0 650 C with oxygen fugacity 23-10 to 27-10
belongs.
Geochemistry of intrusion bodes
There is result of analyses 15 samples by XRF in table 3.
Table 3. Data of the XRF analyzes 15 samples of intrusion bodies Shahrak
Samples
syenite
syenite
syenite
diorite
diorite
diorite
monzodiorite
monzodiorite
monzodiorite
granodiorite
granodiorite
granodiorite
granite
granite
granite
SiO2
58.6
59.86
59.71
56.99
56.86
56.26
60.71
61.84
59.79
65.53
66.1
71.23
71.92
72.09
72.12
TiO2
0.72
0.75
0.76
0.72
0.67
0.71
0.75
0.73
0.71
0.61
0.75
0.68
0.71
0.65
0.71
Al2O3
15.11
14.78
14.91
15.78
15.8
16.11
18.02
18.04
18.07
15.43
15.74
14.96
14.83
14.98
14.85
Fe2O3
4.61
5.52
5.68
7.28
6.19
6.46
5.52
5.6
5.31
3.61
3.54
3.56
0.97
0.77
0.81
MnO
0.153
0.148
0.147
0.149
0.151
0.145
0.19
0.17
0.16
0.152
0.155
0.145
0.146
0.145
0.156
MgO
2.73
2.71
2.98
5.73
5.32
5.45
3.49
2.97
3.02
2.28
1.92
1.86
0.73
0.97
0.83
CaO
3.51
2.73
3.18
6.17
6.22
6.07
4.51
4.5
5.28
3.26
3.31
2.37
1.35
1.22
1.11
Na2O
4.5
5.4
5.42
3.07
2.51
2.63
4.02
3.82
4.01
3.97
4.12
3.21
3.34
4.14
4.15
K2O
5.12
5.31
5.31
2.56
2.29
2.17
3.58
3.16
3.93
2.81
3.54
3.89
5.09
5.23
5.98
P2O5
0.23
0.21
0.26
0.27
0.38
0.19
0.15
0.28
0.29
0.21
0.28
0.38
0.22
0.24
0.23
total
95.28
97.42
98.36
98.72
96.39
96.20
100.94
101.11
100.57
97.86
99.46
102.29
99.31
100.44
100.95
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Plotting the data in the diagram R1-R2 (De La Roche and et al, 1980) and the diagram Na2O + K2O versus
SiO2 proposed (Middelmost, 1985) revealed the variety of granite, diorite, granodiorite, monzonite to monzodiorite
and syenite rocks (Figure 6 A and B). The diagram FeO / MgO vs. SiO2 (Miyashiro, 1974) show belong to an
alkalic serie (7 a). The in the diagram SiO2 vs. K2O (Peccerillo and Taylor, 1976), display the range of K-rich calcalkaline to shoshonitic (Fig. 7 B). Whereas in the A/NK vs. A/CNK (Maniar and Piccoli, 1989), Shahrak sample are,
are metaluminous or weakly peraluminous (Fig. 7 c). Diagrams of the samples Geodynamic position displays to
belong to continental arc basalts (Mullen, 1983; Pearce and Cann, 1973) (Fig. 8 A and B).
Figure 6. R2-R1 and Na2O+K2O-SiO2 (De La Roche and et al, 1980; Middlemost, 1985) plots of the intrusion rocks
Figure 7. plots of the intrusion rocks in Feo/MgO- SiO2 (Miyashiro, 1974), B) SiO2 vs. K2O (Peccerillo and Taylor, 1976), c) A /
NK vs. A / CNK (Maniar and Piccoli, 1989)
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Figure 8. Plot of intrusion samples in diagrams of tectonic setting (Mullen, 1983; Pearce and cann, 1973).
Geochemistry of ore Magnetite
So far, there are many report that has been published about Fe- skarn (Einaudi, 1981; Sangster, 1969;
Sokolov and Grigorev, 1977; Grigoryev and et al, 1990). This type of iron deposits has high reserve and it is
economic value. We were take 60 sample from 10 deposit mass distinct exposures as systematically and were
performed atomic absorption and XRF analysis. The results obtained display in Table 4 as 10 samples (1 sample
for per deposit mass). Investigation about geochemistry ore samples shows similarity it to ore samples of the
Canada (Ray and Webster, 1998), Finland (Nicolas and Miller, 2005) (Table 5). Also there are similarity of the
amount of storage and accessory materials with ore deposits of Chile (Marschik and Leveille, 1998), Europe
(Polard, 2000) and Brazil (Bartonand et al , 1998) that suggests high ore reserves of the Iron with very low
amounts of copper and gold (Table 6).
Table 4. Tthe average results of 60 samples of the XRF Shahrak.
Wt%
SiO2
TiO2
Fe2O3
Al2O3
TFeO
MnO
MgO
CaO
Na2O
K2O
P2O5
L.O.I
Total
BH1
10.96
0.12
70.1
4.62
40.82
0.04
11
1.58
0
0
0.1
1
99.52
BH2
11.12
0.5
74.4
2.8
50
0.12
2.7
3.5
1.57
0.16
0.12
2.67
99.66
BH3
8.81
0.09
71.45
2.71
51
0.04
7.2
4.36
1.29
1.87
0.09
1.55
99.46
BH4
10
0.1
79.21
0.09
53
0.05
8.11
0.98
0
0
0.1
1
99.64
BH5
4.46
0.7
89.57
1.31
60.1
0.07
1.5
0.48
0
0
0.1
1.23
99.42
BH6
4.31
0.5
89.64
0.17
60.5
0
2.49
0.28
0.27
0.57
0
0.76
98.99
BH7
3.45
0.1
91.78
0.93
65.4
0
0.98
0.32
0
0
0
2.12
99.68
BH8
4.78
0.12
90.44
0.13
55.67
0
0.79
1.6
0
0
0
1.46
99.32
BH9
1.98
0.07
94
0.21
67.3
0.01
1.67
0.36
0
0
0.13
0.94
99.37
BH10
2.31
0.08
93.52
0.17
66.44
0.02
1.19
0.12
0
0
0.11
1.91
99.43
Table 5. Shahrak.ICP-MS analysis of samples of iron ore and compared to other ores
CO
15
11
n.a
Ni
35
5
110
V
450-1100
n.a
500-1300
As
n.d
29
n.a
Ag
0.1
0.1
n.a
Zn
45
30
40
Pb
13
15
n.d
Cu
33-520
27
20-500
Au
2
n.d
n.a
%Fe
51-58
n.a
32-70
Ore
Shahrak
Canada
Finland
Table 6. store Amount Shahrak iron ore and compared with those of different parts of the world
Area
Shahrak
Chilly
Europe
Brazil
%Cu
7
0.8-1
0.4
1.5
Au g/t
0.25
0.02
0.26
0.82
million ton Storage
20
366
300
170
accessories minerals
V, Ti, Mn, Cr
As,Mo,b,Zn
Mo
Mo, U
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CONCLUSION
Miocene intrusion bodies with different combinations injected into the Oligocene carbonate formation and
were formed skarn with Fe- mineralization. Iron skarn Mineralogy Paragenesis indicates the presence of two endo
and exo skarn . exoskarn is including garnet skarn zones , pyroxene skarn, skarn phlogopite, talc and amphibole
skarn and skarn skarn is plagioclase. The presence of garnet, pyroxene, phlogopite, talc, and plagioclase and
amphibole skarn type indicates the presence of both calcium and magnesium is in Shahrak area. Skarns consists
two stages progressive metamorphism in 650 º C and retrograde metamorphism at temperatures less than 450ºC.
Intrusion bodies are consisting of granite, granodiorite, monzonite to monzodiorite and syenite. These rocks belong
to K-rich calc-alkaline to shoshonitic series. These bodies are metaluminous to weakly peraluminous and belong to
the continental arc basalts. The most important Magnetite mineralization was occurred during progressive and
regressive metamorphism in exoskarn and endoskarn. That has Mineralogical paragenesis such as hematite,
limonite and goethite. The presence of low amounts of pyrite and chalcopyrite indicated sulfur low fugacity of the
hydrothermal solutions. Shahrak ore deposit is Iron deposits of the lack of gold and copper that has high economic
value by reserves of 20 million tons of magnetite.
Acknowledgments
This paper is part of the Shahrak Fe- mineralization and Tectonic Setting Project, which has been
supported by the University of payam-e-noor. The authors would like to thank the University of payam-e-noor for
supporting the fieldwork and analyses .We thank Mr.Shaykhi for Their help in the fieldwork and preparation of the
samples. Also we thank anonymous reviewers for critical comments in the first version of this manuscript.
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