GeolCarp_Vol65_No3_207

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, JUNE 2014, 65, 3, 207—225 doi: 10.2478/geoca-2014-0015

Introduction

Magmatic  activities  and  the  Alborz-Azerbaijan  mountain
range in the northern to northwestern parts of Iran are the re-
sults  of  the  collision  of  segments  of  Gondwanaland  and  the
Arabian plate with the Eurasian plate. Volcanic rocks occurred
mainly as a result of extension or tension related to the conti-
nental  rifting,  or  subduction  of  the  developed  oceanic  litho-
sphere  under  the  continental  lithosphere  (Dilek  et  al.  2010;
Tarkhani et al. 2010). Most of the important Cu, Cu-Fe and Fe
skarn in the world are related to subduction and collision pro-
ducing I-type magmatic activities (Atkinson & Einaudi 1978;
Einaudi  et  al.  1981;  Einaudi  &  Burt  1982;  Meinert  1992;
Groves  et  al.  1998;  Boztug˘  et  al.  2003;  Chen  et  al.  2007).
Skarn deposits in the Gharah Dagh Formation (part of West-
ern  Alborz),  northwestern  Iran,  are  the  result  of  extensive
I-type  calc-alkaline  and  alkaline  magmatic  activity  of  Late
Eocene—Oligocene  and  Oligo—Miocene  age  in  the  Ahar  re-
gion.  This  magmatic  activity  was  responsible  for  the  contact
metasomatic mineralization as well as the porphyry-type cop-
per  occurrences  in  NW  Iran.  As  Figure 1  shows,  the  Gharah
Dagh Formation is one of the important metallogenic prov-
inces  of  the  East  Mediterranean  Copper-Molybdenum  belt
(Bazin & Hübner 1969; Superceanu 1971; Rolland et al. 2009;
Jamali  et  al.  2010).  The  associated  intrusive  bodies  occur  as

Genetic relationships between skarn ore deposits and

magmatic activity in the Ahar region, Western Alborz,

NW Iran

HABIB MOLLAI

, GEORGIA PE-PIPER

and RAHIM DABIRI

Department of Geology, Mashhad Branch, Islamic Azad University, Mashhad, Iran;

mollai@mshdiau.ac.ir

Department of Geology, Saint Mary’s University, Halifax, Nova Scotia, Canada B3H 3C3

(Manuscript received January 27, 2013; accepted in revised form March 11, 2014)

Abstract: Paleocene to Oligocene tectonic processes in northwest Iran resulted in extensive I-type calc-alkaline and
alkaline magmatic activity in the Ahar region. Numerous skarn deposits formed in the contact between Upper Creta-
ceous impure carbonate rocks and Oligocene—Miocene plutonic rocks. This study presents new field observations of
skarns in the western Alborz range and is based on geochemistry of igneous rocks, mineralogy of the important skarn
deposits, and electron microprobe analyses of skarn minerals. These data are used to interpret the metasomatism during
sequential skarn formation and the geotectonic setting of the skarn ore deposit related igneous rocks. The skarns were
classified into exoskarn, endoskarn and ore skarn. Andraditic garnet is the main skarn mineral; the pyroxene belongs to
the diopside-hedenbergite series. The skarnification started with pluton emplacement and metamorphism of carbonate
rocks followed by prograde metasomatism and the formation of anhydrous minerals like garnet and pyroxene. The next
stage resulted in retro gradation of anhydrous minerals along with the formation of oxide minerals (magnetite and
hematite) followed by the formation of hydrosilicate minerals like epidote, actinolite, chlorite, quartz, sericite and
sulfide mineralization. In addition to Fe, Si and Mg, substantial amounts of Cu, along with volatile components such as
H

S and CO

were added to the skarn system. Skarn mineralogy and geochemistry of the igneous rocks indicate an

island arc or subduction-related origin of the Fe-Cu skarn deposit.

Key words: Late Cenozoic, granodiorite, magmatic, skarn, garnet, epidote, sulfide, Iran.

batholiths  as  well  as  stocks,  ranging  in  composition  from
quartz monzodiorite to granite, and the Ahar Batholith is the
largest early Oligocene intrusion in the Gharah Dagh—Tarom
plutonic belt (Lescuyer 1976). The important skarn deposits in
these  areas  are  Mazraeh  skarn,  Sungun  skarn  porphyry  and
Anjered  skarn  deposits.  The  Sungun  copper  skarn  porphyry
deposit located 85 kilometers west of Ahar town is associated
with a smaller diorite of Oligocene age (Fig. 2), whereas in the
Mazraeh and Anjered area several smaller skarn deposits, are
found at the margin of the Ahar Batholith.

The aim of this paper is to present new field observations

of skarns in the western Alborz of northwestern Iran; to de-
scribe  the  petrography,  mineralogy,  mineral  chemistry  and
geochemistry of the important related rocks; and to use these
data to interpret the sequence and the geotectonic setting of
skarn  formation.  In  addition  this  paper  documents  correla-
tions  between  intrusion  composition  and  the  metal  contents
of associated skarns.

Methods

Field work included geological mapping, delineating the

igneous bodies, the skarn and marble contact. Sampling
traverses were done across the skarn and host rocks. More

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than 100 samples were collected from different rock types and
locations.  Some  representative  samples  were  thin  and  pol-
ished-sectioned  and  examined  microscopically. Electron
probe micro-analysis (EPMA) was used to determine mineral
compositions  of  various  skarn  minerals  like  garnet,  epidote,
pyroxene  and  actinolite,  in  the  Iranian Mineral Processing
Research  Center (IMPRC),  Karaj  town.  Selected  samples  for
whole-rock  geochemical  analysis  were  pulverized  using  a
shatter box with an iron bowl. These powders were analysed
in  Activation  Laboratories  Canada  according  to  their  Code
4Litho  research  and  Code  4B1  packages,  which  combine
lithium metaborate/tetraborate fusion major element analysis
with a trace element ICP-MS package. Fluid inclusion stud-
ies were conducted on more than 50 doubly polished plates
using  a  Leitz  1350  heating  stage,  a  SGE  gas  flow  heating/
cooling  system  based  on  a  U.S.  Geological  Survey  design
(Hollister et al. 1981), and a Chaixmeca stage at the Fluid In-
clusion Laboratory of the Wadia Institute of Himalayan Geo-
logy,  Dehra  Dun,  India,  and  the  Department  of  Earth
Sciences, Indian  Institute of Technology, Mumbai, India.

Magmatic activities in the Ahar Region

A belt of skarn porphyry Cu deposits of late Tertiary age

extends from the Caucasus Mountains to the Alborz unit in
the  Azerbaijan  region  of  NW  Iran  and  includes  the  well-
known  Sungun,  Mazraeh,  Anjered  porphyry  skarn  deposits
(Mollai 1993). All of these deposits are related to the Ceno-
zoic magmatic activity; the Eocene—Oligocene period can be
considered as a metallogenic epoch that formed the Alborz-
Azerbaijan magmatic belt. Large Cu-Mo porphyry deposits,
Cu skarn occurrences, and Cu-Mo-Au porphyry – vein de-
posits in this area attest to the economic value and potential
of mineralization in this magmatic belt (Jamali et al. 2010).
  There  are  three  parallel  magmatic  arcs  in  the  northwest  of
Iran,  of  Cretaceous  and  Eocene—Miocene  to  Quaternary

Fig. 1. Map showing the Eastern Mediterranean Iranian—Alpian Copper-Molybdenum belt and the area studied (Modified after Superceanu 1971).

ages,  trending  in  a  NW—SE  direction  between  the  Main
Thrust  zone  in  the  southwest  and  the  Tabriz  Fault  in  the
northeast  (Azizi  &  Moinevaziri  2009).  Major  tectono-mag-
matic  events  in  northwestern  Iran  are  the  result  of  geody-
namic  evolution  of  Tethys  belt  that  formed  between  the
Arabian  and  Eurasian  plates  during  the  Early  Mesozoic  to
Late  Cenozoic  orogeny  (Aghanabati  1993).  Among  these
magmatic activities in the area of study, the Ahar Batholith
and the Sungun porphyry stock are the most important. The
host rocks have a calc-alkaline, I-type chemical composition
of  a  continental  arc  geotectonic  setting  (Fig. 2).  The  Ahar
batholith, which extends about 30 km E—W and is 3 to 10 km
wide from north to south, ranges from granite to granodiorite
(Fig. 2).  The  batholith  was  responsible  for  mineralization,
skarnification  and  hornfels  at  its  margins.  The  granodiorite
pluton has been affected by a number of later magmatic ac-
tivities  which  include  various  types  of  quartz,  aplite,  and
pegmatite  veins,  hypabyssal  and  mafic  dykes,  as  well  as
Quaternary volcanic products. There are more than five vol-
canic  masses  belonging  to  Quaternary  episodes  within  the
plutonic body of Oligocene age. These igneous activities do
not form a single large central volcano. Two mineralized ma-
fic dykes have been noted in the eastern part of Javanshykh
village. Malachite as a secondary mineral is distributed within
the main body in different forms (Fig. 3b).
The stock of Oligo-Miocene age, which ranges in composi-
tion  from  quartz  monzonite  to  granite,  hosting  the  Sungun
Copper  Porphyry  deposit  (Mehrpartou  &  Torkian  1994;
Hezarkhani  et  al.  1999;  Calagari  &  Hosseinzadeh  2006a)
shows a calc-alkaline, I-type chemical composition of a con-
tinental  arc  geotectonic  setting.  The  stock  is  located  about
100 km NE of Tabriz and 85 km NW of Ahar, crops out over
an area of about 1.5 by 2.3 km (Calagari 2004) (Fig. 2), and
intruded a series of Eocene arenaceous-argillaceous and Up-
per Cretaceous carbonate rock sequences. The stock consists
of  three  different  intrusive  phases:  (1)  monzonite-quartz
monzonite,  (2)  diorite-granodiorite  and  (3)  andesite  and  re-

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RELATIONSHIPS BETWEEN SKARN ORE DEPOSITS AND MAGMATISM IN WESTERN ALBORZ (NW IRAN)

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Fig. 2.

Part

the

Ahar

quadrangle

geological

map

(Geological

Survey

Iran,

1978),

showing

distribution

extrusive

and

intrusive

rocks

and

skarn

deposits

the

Ahar

region.

–

Mazraeh

Cu-Fe

and

Ghranagh

Daragh

skarn

deposit

which

has

elliptical

shape

striking

W—E

direction;

–

Anjered

skarn

deposit

looks

arc

striking

almost

N-S;

–

Geological

map

Sungun

representing

the

exploratory

drifts.

the

Sungu

skarn

porphyry

some

patches

skarn

occur

xenoliths

within

the

granodiorite

(modified

after

Mehrpartou

1993;

Mollai

1993,

2009;

Calagari

Hossainzadeh

2006).

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character of the Cretaceous carbonate rocks as well as the ig-
neous rocks are unrecognizable. The skarn deposit in the area
reveals that after a hot gaseous stage, there was hydrothermal
activity which resulted in the alteration of igneous rocks.

Table 1: Comparison of granodiorite related to the Cu-Mo and Cu-Fe skarn deposits of the Ahar region NW Iran, with the granodiorite re-
lated to the skarn deposit of other parts of the world.

Deposit

Size(t)

Grade

Metal

Associated

igneous rocks

Host rock

Early

minerals

Late

minerals

Ore minerals

Reference

Mazraeh
Cu, Fe

40.000 t

1.2 % Cu and
Fe

Cu and
Fe

Granodiorite

Impure
carbonate and
granodiorite

Pyroxene,
garnet

Epidote,
chlorite,
calcite,
sericite

Cp, py, mt and
hem

Mollai 1993,
2009

Sungun 1bt

0.62 % Cu and
0.01 % Mo

Cu and
Mo

Diorite/
granodiorite to
quartz-
monzonite

granodiorite to
monzonite and
impure
carbonate
rocks

Pyroxene,
garnet

Epidote,
chlorite,
calcite,
sericite

Cp, mo, py, bor
and chal

Etminan 2012

Anjerd

20,000 t

0.8 % Cu

Granodiorite
quartz-
monzonite

granodioritic
and impure
carbonate
rocks

Pyroxene,
garnet

Actinolite,
epidote,
chlorite,
calcite and
quartz

Cp, py, mt

Mollai
2009/unpubl.
proj.

Daiquiri,
Cuba

100 m.t.

Mainly Fe

Diorite,
Limestone

Diorite,
limestone
limestone
blocks

Pyroxene,
garnet

Ep, cal, qz

Mt, hem,
(py, cp,)

Lindgren &
Ross 1916

Peschansk
Ural

173 m.t.

50 % Fe;

Fe ,Cu Diorite

Tuff,
sandstone,
andesite,
limestone

Garnet,
pyroxene

Epidote,
chlorite,
calcite,
diopside,
sericite

Magnetite,
minor
chalcopyrite

Sokolov &
Grigorev 1977

Bagirkac 250,000 t

7 % Pb

Pb, Zn,
Cu

Granite,
granodiorite of
Eybek Pluton

Skarn after
calcareous,
marble bearing
schist

Wo, ad-gr,
di-hd, scp

Cal, tr

Sp, cp, gn, hem,
py

Dora 1971

Shinyama
japan

10,000,000 t

Fe 30 %–35 %
Fe Cu ore
30–35 %
Cu 1 %

Fe-cp,
py, sph

granodiorite of
Samli Pluton

Basaltic
andesite,
limestone

Ad-gr,
di-hd,

Act, qz,
tourmaline,
tr, cal, ep

Mt, cp,
cubanite, pyrrh,
sph,

Tsusue 1961;
Kaneda et al.
1978

Bingham
Utah

100,000,000 t

3.2% Cu,
0–03% Mo

Monzonite to
Quartz
monzonite

Calc-silicate
hornfels after
metamorphic
rocks

Ad-gr,
di-hd

Ep, cal, amp Cp, py, born,

Atkinson &
Einaudi 1978;
Sweeney 1980

Evciler
Kazdag

no estimate

Au (up to
14 ppm) in
pyrrhotite

Granodiorite to
quartz diorite of
Evciler Pluton

Hornfels after
metamorphic
rocks, skarn
after limestone

Ad,gr,
di-hd,
scp,py

Ep ,cal, amp,
chl, qz

Po, py, cp

Ozturk et al.
2005; Ozturk
et al. 2008

Shinyama
mine,
Japan

>10 m.t.

30–35% Fe,
0.1% Cu

Fe–Cu

Diorite-
granodiorite
dykes, stock

Basaltic
andesite,
dacite,
Permian black
slate,
limestone

Gr, di, fer

Epidote,
amphibole,
actinolite,
quartz,
magnetite;
in marble:
pageite,
tourmaline,
magnetite,
calcite,
phlogopite

Mt; minor cp,
cub, pyrrh, sph,
trace
pentlandite,
valleriite,
arsenopyrite,
comackinawite

Tsusue 1961;
Kaneda et al.
1978

Ayazmant
Ayvalik
Turkey

5,750,000 t

46% Fe and
0.6% Cu

Fe–Cu

Granodioritic to
monzodioritic
porphyries of
Kozak Plutonic
Complex

Hornfels after
regional
metamorphic
rocks with
carbonate
lenses and
intercalations,
skarn after
limestone
lenses

Di, ad-gr,
scp

Ep, amp, py,
or, chl, cal,
qz

Mt, cp, py, bn,
mo, go, hem,
po, gn, sp,
various Au-Ag-
Te-Se minerals

Tolga Oyman
2010

The structural set up of the Mazraeh mine is an elliptical

shaped mega enclave of meta-sedimentary rocks within the
Mazraeh granodiorite, with the 1.5 km long major axis strik-
ing E—W and the 1.0 km long minor axis running in N—S

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direction (Fig. 2a). The southern contact between the grano-
diorite  and  crystalline  limestone  is  steeply  dipping  (60°  to
70°S) and is sub-concordant, transecting the limestone beds
(striking  E-W,  dipping  S).  The  width  of  skarn  here  ranges
from  2  to  25 m,  except  where  a  granodiorite  tongue  cuts
across the limestone and the skarn width is 50 m. Exoskarn
is the principal skarn zone enclosed by the marmorized- and
endo-skarn zones. A vein of wollastonite with a NE-SW di-
rection and thickness of about 50 cm, occurs in the north east
of the mine within the crystalline limestone. Fig. 2 shows the
zonal  arrangement  of  skarn  deposits  (Mollai  1993).  The
mine is now (summer 2012) abandoned.

The Anjered Cu skarn deposit is located at the western limit

of  the  Ahar  batholith  (Fig. 2).  The  structural  set  up  of  the
Anjered  skarn  deposit  is  a  semi  elliptical  shape,  with  1 km
long major axis striking almost N—S along the contact of igne-
ous  rocks  and  500 m  thickness  of  minor  axis  running  in  a
W—E direction (Fig. 2b). In most of the places near the contact
with granodiorite, the skarn is covered by loose eroded ma-
terial. Therefore the size of the Anjered skarn in such places
may be underestimated. The remains of old tunnels in the con-
tact  between  the  granodiorite  and  impure  carbonate  rocks
show that mining activities have been carried out in the past.

The intrusion of Oligo-Miocene porphyry granodiorite of

Sungun into the impure limestone of Cretaceous rocks led to
the formation of the Cu-Mo skarn deposit, recrystallizing the
impure  limestone  and  hornfels  in  the  east  Sungun  river
(Fig. 2c). Skarn-type metasomatic alteration and mineraliza-
tion occurs along the contact between Upper Cretaceous im-
pure carbonates and an Oligo-Miocene Cu-bearing porphyry
stock. The structural set up of Sungun porphyry is a narrow
zone  with  the  thickness  of  55 m  to  60 m.  Both  endoskarn
and  exoskarn  are  developed  along  the  contact.  Exoskarn  is
the principal skarn zone enclosed by marmorized carbonates
and skarnoid hornfelses. Petrographic studies in the Sungun
area  show  that  the  mineralized  dikes  are  mainly  andesitic
and  are  related  to  the  diorite-granodiorite  intrusive  phase,
whereas the andesite dykes in Mazraeh are barren. Molybde-
num was concentrated at a very early stage in the evolution
of the hydrothermal system and copper somewhat later. Four
distinct  types  of  hypogene  alteration  are  easily  distinguished
as follows: 1) Potassic, 2) Potassic-phyllic, 3) Phyllic, 4) Pro-
pylitic  (Mehrpartou  &  Torkian  1994).  The  metamorphic
rocks along with bimetasomatic skarns occur in the Mazraeh
and  Sungun  skarn  deposits,  but  this  phenomenon  was  not
clearly observed in Anjered. The portion of metamorphic in-
terlayering of bimetasomatized zones varies in thickness from
70 to 120 m in Sungun (Calagari & Hosseinzadeh 2006b) and
50 to about 200 m in the Mazraeh skarn deposit. This zone in
both  skarn  deposits  lies  between  exoskarn  and  impure  Late
Cretaceous  carbonate  rocks.  These  rocks,  in  addition  to  be-
ing  thermally  metamorphosed,  have  been  bimetasomatized
and have also produced bimetasomatic skarn (Einaudi et al.
1981). At a relatively long distance from the contact of mag-
ma,  this  heat  source  caused  the  development  of  ne-grained
anhydrous calc-silicates (mainly isotropic garnet and pyrox-
ene)  within  the  clay-rich  inter-layers  in  the  impure  carbon-
ates.  Some  amounts  of  hydrous  silicates  like  biotite,
amphibole,  epidote  and  chlorite  are  present;  these  minerals

are the retrograde products. The thickness of these bimetaso-
matized layers never reaches more than 10 cm. In the north
and north east of the Mazraeh skarn deposit, various types of
folding are present, which are due to syntectonic conditions
in the impure carbonate rocks. In the bimetasomatic skarn,
brownish elephant skin structures occur as a result of deep
weathering of crystalline limestone leaving residual metaso-
matized minerals.

Petrography and mineralogy

Petrography and mineralogy of igneous rocks

Texturally the granodiorite is coarse grained and porphyritic.

Its  modal  composition  ranges  are  6—15 %  quartz,  24—43 %
plagioclase, 20—48 % K-feldspar, 0—10 % hornblende, 0—5 %
biotite,  and  0—3 %  each  of  apatite,  sphene  and  magnetite.
Micro  diorite  rock  contains  large  isolated  crystals  (pheno-
crysts)  of  plagioclase  and  hornblende  in  a  mass  of  fine  tex-
tured  crystals  in  which  they  develop  a  porphyritic  texture
(Fig. 4a—d).  Some  feldspars  display  poikilitic  textures  and
perthitic  and  myrmekitic  intergrowths  were  recognized  in
some  samples.  Alkali  feldspar  in  places  jackets  the  plagio-
clase  laths,  which  is  indicative  of  their  order  of  crystalliza-
tion.  Veins  of  both  quartz  and  calcite  cut  the  granodiorite.
According to XRD and microprobe analyses, the plagioclase
is the most abundant mineral and ranges from albite to oligo-
clase. K-feldspar has been confirmed as orthoclase and pla-
gioclase  crystals  are  lath-shaped,  unzoned  or  zoned  with
both albite-type and pericline twinning. Plagioclase shows a
paragenetic  relationship  with  sphene.  Feldspar  minerals  are
partially to entirely altered to sericite, and biotite has changed
to  chlorite,  muscovite  and  opaque  minerals.  (Fig. 4b,c).  Por-
phyritic  texture  indicates  that  a  magma  has  gone  through  a
two  stage  cooling  process.  Pyrite  and  chalcopyrite  are  the
most  abundant  sulphide  minerals  present  in  the  porphyritic
stock  of  Sungun.  The  quartz  grains  are  present  in  varying
sizes, some with inclusions of feldspar, biotite, apatite, sphene
and  magnetite.  Sphene  is  a  very  common  accessory  mineral,
occurring  as  anhedral  to  well  developed  euhedral  lozenge-
shaped  grains  with  high  relief  (Fig. 4d).  They  are  associated
with epidote, biotite, rutile, and opaque minerals.

Thin section studies of samples from the Sungun porphyry

host rocks show porphyritic texture, containing phenocrysts
of  plagioclase,  K-feldspar,  quartz  and  biotite.  In  some  al-
tered rocks feldspar minerals are partially to entirely altered
to sericite, and biotite has changed to chlorite, muscovite and
relics  of  opaque  minerals.  Thin  section  studies  show  two
types of opaque mineral, the first type is primary mineral and
the second one is the altered product of mafic minerals. Most
of opaque minerals in Sungun porphyry are sulphide miner-
als  (pyrite,  chalcopyrite,  molybdenite,  blende  and  galena)
whereas  the  opaque  minerals  in  Mazraeh  granodiorite  are
mainly magnetite. The mineralized dikes are mainly andesitic
and  are  related  to  the  diorite-granodiorite  intrusive  phase.
These  deposits  reveal  that  after  a  hot  gaseous  stage,  there
was hydrothermal activity which resulted in the alteration of
igneous rocks.

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Skarn mineralogy

In general the dominant calc silicate (skarn) minerals in

the area are garnet, calcite, pyroxene, actinolite and epidote,
which are accompanied by quartz, feldspar, minor vesuvian-
ite and hornblende. These skarn deposits can be petrologically
classified  into:  (I)  Exoskarn,  (II)  Endoskarn  and  (III)  Ore
skarn. Each of the skarn types can be further divided on the
basis  of  their  mineral  assemblage.  In  general,  the  rocks  in
this zone contain principally ne-grained granoblastic calcite
(50—98 %),  garnet  (0—35 %),  pyroxene  (0—10 %),  epidote
(0—5 %),  chlorite  (0—2 %),  clays  (1—3 %),  and  sulphides
( < 0.5 %) (Calagari & Hosseinzadeh 2006a).

(I) Exoskarn: The exoskarn is developed within the coun-

try  rock  and  the  bimetasomatic  skarn  (Einaudi  et  al.  1981;
Einaudi & Burt 1982; Kwak & Kwak 1987). The alteration
of the host rock (impure carbonate and igneous rocks) in the
Ahar region is marked by the formation of coarsely crystal-
line skarn bands due to the introduction of Si-, Al-, Fe-, and
Mg-rich fluids into the host rock. Metasomatism of carbonate
in  the  Ahar  region  produced  andradite-  grossular/pyroxene
exoskarn, as components of the prograde assemblage, and epi-
dote, tremolite/actinolite, chlorite and/or calcite and quartz as
components  of  the  retrograde  mineral  assemblage  with  few
grains of vesuvianite (Fig. 4a,b and c). The skarn shows por-
phyroblastic,  poikiloblastic  brecciation,  overprinting  and  in
some  cases  cataclastic  textures.  Pyroxene  shows  porphyro-
blastic texture with alteration to actinolite (Fig. 4e). There are
at  least  two  generations  for  most  of  the  minerals  especially
garnet,  quartz,  calcite,  chlorite,  magnetite  and  chalcopyrite.
For example, garnet occurs at least in three generations with
different  geological  and  optical  properties.  Garnets  range  in
size  from  0.1 mm  up  to  4 cm  in  diameter,  showing  fine  to
coarse grains of isotropic and concentric, oscillatory zoning of
anisotropic  garnet  (Fig. 4h).  Most  of  the  pyroxene  minerals
were replaced by garnet. Idiomorphic, twinned epidote occurs
in exoskarn as well as in endoskarn (Fig. 5e,f). Very well de-
veloped,  coarse  grains  of  wollastonite  occurring  as  a  vein
within  the  crystalline  limestone  in  the  north-east  of  the
Mazraeh skarn deposit show porphyryblastic texture (Fig. 4g).
Actinolite  mineral,  which  is  the  altered  product  of  pyroxene
and garnet minerals, is more predominant in the Anjered skarn
in comparison with other skarn deposits in the area. The alter-
ation has started from the borders of minerals, with the rem-
nants of pyroxene and garnet observed within the actinolite.

(II) Endoskarn: Along the contact with the exoskarn, re-

placement of granodiorite by massive epidote and minor gar-
net-pyroxene  over  widths  of  centimeters  to  0.5 m  may  result
in  complete  destruction  of  the  original  igneous  texture  and
mineralogy.  This  is  the  evidence  of  progressive  addition  of
calcium from country rocks into the intrusive magma and loss
of Al, Si and also Na. This skarn comprises: plagioclase, alkali
feldspar,  magnetite,  epidote,  biotite,  garnet,  pyroxene,  horn-
blende,  actinolite,  sericite,  siderite  and  opaque  minerals
(Fig. 5a and b). As in the exoskarn, two types of garnets are
observed  in  the  endoskarns:  subhedral  to  anhedral,  isotropic
garnets and larger, well developed, oscillatory-zoned anisotro-
pic garnets. Pyroxene is generally subhedral to euhedral, and
hedenbergitic  to  diopsidic  in  composition.  Endoskarn  forma-

tion began with epidotization, and was coincident with sericit-
ization  during  metasomatic  reactions.  This  zone  consists  of
medium to coarse and well developed-grained epidote accom-
panied  by  interstitial  quartz,  pyrite,  chalcopyrite,  and  iron
oxide  (Fig. 5e  and  f).  Where  the  alteration  has  changed  the
endoskarn, the remnants of garnet and pyroxene are observed
within the hydrothermal product minerals (Fig. 5c and d). Far-
ther  into  the  granite,  endoskarns  occur  only  as  disseminated
epidote skarns, and are enriched in garnet towards the impure
carbonate rock (Fig. 4f). The garnet-rich skarn predominantly
comprises  exoskarn.  However,  garnet  locally  developed  by
dissolution and replacement of primary igneous minerals, par-
ticularly  feldspar,  in  the  granodiorite;  such  garnet  is  rich  in
grossularite.  Sericitization  and  carbonitization  of  igneous
rocks  in  the  retrograde  stage  of  skarn  formation  lead  to  the
formation  of  typical  endoskarn  of  sericite-siderite,  feldspar,
and actinolite-epidote. Epidote skarn is the only predominant
Al-rich skarn in the study areas, and it reaches up to 80 % and
65 %  in  Sungun  and  Mazraeh  skarn  deposits  respectively
(Mollai  1993;  Calagari  &  Hosseinzadeh  2006a),  whereas  in
Anjerd skarn actinolite is more abundant.

(III) Ore skarn: The early-formed calc-silicate minerals

were  later  texturally  replaced  by  oxides  (magnetite,  hema-
tite),  followed  by  sulphides  (chalcopyrite,  pyrite,  covellite,
bornite,  galena,  sphalerite  and  molybdenite),  hydro  silicate
(actinolite,  epidote,  chlorite  and  sericite)  and  carbonates
(calcite, ankerite and siderite). Oxides in the Cu Sungun and
Anjered skarn deposits are not predominant minerals, in con-
trast to the Cu-Fe Mazraeh skarn deposit.

The ore-forming fluids were initially thought to be of mag-

matic  origin  only.  Sulphur  and  oxygen  are  among  the  most
important volatiles which play a significant role in the forma-
tion of hydrothennal sulphide and oxide deposits (Schwartz
1950).  Magnetite  shows  replacement  texture,  including
relicts  of  replaced  minerals  like  garnet,  calcite  and  some-
times  feldspar.  In  addition  magnetite  is  intercrystalline  as
well  as  intracrystalline  with  garnet.  Garnet  usually  shows
corrosion  boundaries  with  magnetite  depicting  reaction.  In
places,  magnetite  shows  cataclastic  texture  with  numerous
fractures,  which  are  filled  by  third  generation  minerals  like
quartz,  calcite  and  sometimes  chalcopyrite.  In  such  condi-
tions  we  can  see  various  veins  of  magnetite,  sulphide  and
quartz (Fig. 6a). The martitization of magnetite is quite com-
mon, and is due to the changes of oxygen fugacity (Fig. 6b).
It occur as lamellar plates along octahedral planes, fractures,
zonal  growth  planes  and  outer  margins,  where  hematite  is
comparatively thicker (Mollai et al. 2009). Where the degree
of martitization is extensive, magnetite occurs as relicts.

Chalcopyrite is the most common sulphide in these skarn

deposits, along with bornite, and molybdenite in the Sungun
deposit. In the Sungun porphyry, sulphides occurring within
the feldspar altered zone are disseminated, while in the phyllic
zone  they  occur  as  veins.  The  argillic  zone  has  low  grade
disseminated  ores.  More  specifically  the  metallic  ores
present, in decreasing order of abundance, are: chalcopyrite,
pyrite,  bornite,  molybdenite  and  pyrrhotite,  with  minor  cu-
banite (Mehrpartou 1993; Mollai 1993; Calagari & Hossein-
zadeh  2006a).  Chalcopyrite  associates  closely  with  bornite
and also partially to fully replaces magnetite, sometimes re-

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stitution of A1

for Fe

. In the veins the major cations are

and Fe

and the andradite component predominates

( > 99 %)  (Table 3).  A  notable  feature  of  this  vein  garnet  is
the  excess  of  Ca,  which  is  due  to  the  deficiency  in  Si  solid
solution  (Table 3).  The  Ayazmant  Fe-Cu  skarn  of  Turkey
and Kamaish Fe skarn of Japan are richer in grossularite in

Fig. 6. a – shows intense and
multiple

replacement

hand

spacemen  of  skarn  and  forma-
tion  of  ore  skarn.  No. 1  –
shows  the  remnant  of  brown
garnet  after  replacement  by
magnetite. No. 2 – shows mag-
netite  veins  replacing  garnet.
No. 3  –  shows  an  intense  re-
placement of early rocks by sul-
fide  and  quartz.  No. 4  –
mineralized  quartz  veins  be-
longing to the 4

stage of alter-

ation  or  hydrothermal  activity;
b  –  is  a  microphotograph  of  a
polished  section  showing  re-
placement  of  radial  magnetite
by  hematite  due  to  martitiza-
tion.  Blades  consist  of  martite
crystals (5—20 mm) which repre-
sent  the  hematitization  of  euhe-
dral magnetite. The replacement
process  includes  an  initial  iron
atom diffusion through the oxy-
gen  framework  leading  to
maghemite,  followed  by  inver-
sion  of  maghemite  to  hematite
and  development  of  distorted
octahedrons;  c  –  microphoto-
graph of polished section shows
replacement  of  magnetite  and
anhydrous  minerals  by  patches
of  chalcopyrite  along  with
bornite;  d  –  microphotograph
of  polished  section  shows  re-
placement  of  silicate  minerals

Table 2: Representative electron microprobe analyses of garnet in the Ahar region skarn deposits, NW Iran (in weight percent, 12 oxygen basis).

comparison  with  the  Ahar  skarn  deposits  (Fig. 7).  Garnet
with 30 to 99 % mole fraction of andradite shows mixed op-
tical properties in the main garnet mass. There is no appre-
ciable change in chemistry except antipathetic  behaviour  of
Al and Fe which could lead to zoning in garnets. The Al-Fe
variation  could  be  due  to  local  fluctuations  in  temperature

Samples

MZ551

MZ552

MZ553

MZ8121

MZ8122

MZ8123

311

312

841

842

SiO

35.37 35.32 35.58 36.75 35.54 36.28 33.95 34.04 39.15

33.92

7.37 6.50 7.09

15.66 9.24

11.55 0.88 0.02 0.04

0.99

21.29 22.21 20.66 10.69 20.38 16.27 29.84 31.26 31.09

30.26

MgO

0.24 0.20 0.23 0.18 0.19 0.25 0.03 0.04 0.00

0.00

MnO

0.67 0.76 0.61 3.11 3.18 3.48 0.43 0.32 0.32

0.40

CaO

35.04 35.05 35.83 33.26 31.47 32.08 34.87 34.61 34.40

34.48

Total

99.93 100.00 100.00 99.66 100.00 100.00 100.00 100.29 100.29 99.99

2.90 2.91 2.94 2.86 2.85 2.93 2.97 2.89 2.92

2.89

0.71 0.63 0.69 1.44 0.87 1.10 0.09 0.02 0.00

0.09

1.31 1.38 1.28 0.63 1.23 0.98 1.85 1.99 2.00

1.94

0.03 0.02 0.03 0.21 0.02 0.03 0.04 0.00 0.00

0.00

0.05 0.05 0.04 0.20 0.22 0.24 0.03 0.02 0.02

0.03

3.08 3.09 3.11 2.77 2.70 2.76 3.08 3.14 0.15

3.15

Total

8.07 7.08 8.08 8.11 7.89 8.03 8.06 8.06 8.08

8.10

Andra

0.67 0.71 0.65 0.30 0.60 0.46 0.96

99.68 0.99

0.96

Gross

0.31 0.27 0.33 0.57 0.32 0.45 0.02 0.00 0.00

0.03

Pyrope

0.02 0.02 0.02 0.13 0.08 0.09 0.02 0.01 0.01

0.01

by magnetite and chalcopyrite.

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Table 3: Representative electron microprobe analyses of vein garnet in the northeast Mazraeh Cu-Fe skarn deposit, NW Iran (in weight
percent, 12 oxygen basis).

Fig. 7. Ternary plot of garnet composition from the Ahar region in
comparison with skarn deposits in other parts of the world.

(Rose & Burt 1979). Optical anisotropy in garnet is due to
their departure from the cubic symmetry, as a result of partial
replacement of (SiO

) by (OH) to form hydrogarnet. Rose &

Burt (1979) suggested that anisotropic garnet will form due
to fluctuation in fluid composition resulting from variable
mixture with meteoric water. Zoned garnets from skarn de-
posits of the Ahar region do not show a systematic composi-
tional variation from the core to the rim of the crystal.

Epidote is one of the important products of hydrothermal

fluid that was rich in Al. This Al-rich fluid may have played
an important role in carrying sulphide ore solution. Fe

re-

placing Al ranges from 71—95 mol % (Table 4). The Mn

the epidote ranges between 0.7 and 5.35 mol % piedmontite,
whereas the epidote in the Ayazmant Fe-Cu skarn deposit in
Turkey  (Oyman  2010)  is  Fe-rich  with  Fe/(Fe + Al)  ratios
varying between 0.20 and 0.29. At Ayazmant, epidote result-
ing  from  late  replacement  of  grossular-rich  garnet  shows  a
higher Fe/(Fe + Al) ratio (mean 0.28) than that which replaces
actinolite-magnetite skarn with a mean value of 0.24. During
retrograde stages most of the anhydrous calc-silicate miner-
als like garnet and pyroxene were replaced by a series of hy-
drous  calc-silicates  (epidote,  tremolite-actinolite),  sulphides
(pyrite, chalcopyrite, galena, sphalerite and bornite), as well
as  oxides  and  carbonates  (calcite,  ankerite).  Processes  such
as hydrolysis, carbonation and sulphidation, due to relatively
low  temperature  hydrothermal  fluids,  were  responsible  for

Oxide/samples

Hyv1

Hyv2

Hyv3

Hyv4

Hyv6

Hyv8

SiO

33.54

33.75

33.79

33.73

33.59

33.8

0.21 1.34

0.49

0.2 0.75 1.55

31.27 29.93 31.12

31.1 30.57

29.22

MgO

0.03

0.00 0.00 0.00

0.00 0.00

MnO

0.55

0.41

0.60

0.54

0.49

0.5

CaO

34.4

34.56 34.01 34.43

34.21 34.90

Total

99.99

100.00

100.4

99.61

99.66

2.87 2.87 2.88

2.87

2.88 2.88

0.02

0.13 0.05 0.02

0.02 0.16

2.16 1.92 2.00

2.02 1.97

1.87

0.002

0.0

0.04

0.03 0.04

0.04

0.04 0.35

3.16

3.15

3.11

3.17

3.14

3.18

Total

8.19

8.10

8.09

8.12

8.04

8.11

Oxide/Samples

Mz332

Mz333

Mz337

SiO

37.41

37.8

37.05

21.280

22.27

20.45

14.84

13.93

14.99

MgO

0.01 0.03 0.03

MnO

1.15

0.2

0.09

CaO

21.98

23.14

23.42

TiO

0.20 0.02 0.35

0.00

0.02

0.0

0.00

0.0

0.00

0.28

0.24

Tota1

96.05

97.57

96.42

3.15

3.16

2.2

2.05

0.88 0.87

0.95

0.00

0.0

0.05

0.01

0.02

1.96

2.06

2.14

0.01

0.0

0.01

Total

8.25 8.39 8.31

Table 4: Representative electron microprobe analyses of epidote
from the Ahar region skarn deposits, NW Iran (in weight percent,
13 oxygen basis).

the formation of these mineral assemblages. A local increase
in f O

may have played an important role in the formation of

epidote (Perkins et al. 1986; Berman 1988), according to the
equation:

(Al,Fe)

(garnet)+

+ HCO

FeAl

(OH)(epidote) + CaCo

Pyroxene in Sungun occurs as fine to medium grained an-

hedral  to  subhedral  crystals  showing  decussate  texture.
Chemically the pyroxene belongs to the diopside-hedenberg-
ite  series,  with  38 mol %  Hd/(Hd + Di)  and  minor  Mn.  The
pyroxene  also  contains  some  Mn,  ranging  from  0.001  to
0.058 mol fraction, hence the other end member of pyroxene
is johansenite with a mol fraction of 0.032. In addition, the
presence  of  a  1.45 mol  fraction  of  Al  indicates  scapolite
(meionite) with the formula of Ca

. Tremolite

in the Sungun porphyry is mainly the alteration product of
pyroxene, according to the equation:

Ca(FeMg)

(clinopyroxene) + 2H

O + 3CO

(Mg,

Fe)

(OH)(Tremo-actinolite) + CaCO

+ 2SiO

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In contrast, in the Anjerd Cu-skarn and the Mazraeh skarn

deposits, tremolite is mainly the alteration product of garnet
as well as pyroxene. Moreover tremolite in the Sungun Por-
phyry and Mazreah is tremo-actinolite in composition,
whereas in the Anjerd Cu skarn it is mainly actinolite with
very minor amounts of tremolite (Mollaei et al. 2009).

Geochemistry of igneous rocks

For a better understanding of the genetic relation of skarn de-

posits and magmatic activity one needs to understand the geo-
chemistry and geotectonic setting of magmatic rocks. Table 5
shows the whole rock analysis of igneous rocks in the region.

The igneous rocks range from diorite and monzonite to

quartz monzodiorite and monzodiorite, syeno diorite and
granite compositions in the SiO

versus Na

O + K

O diagram

(Fig. 8). In the Harker major element diagrams, K

O and

increase with increasing SiO

whereas Fe

and CaO

decrease with increasing SiO

, and Na

O displays a more er-

ratic distribution (Fig. 9a,b,c and d). In the alkalis versus
SiO

diagram and in the AFM diagram (Fig. 9e and f), the

samples show a typical sub-alkaline and calc-alkaline trend
respectively. All the samples plot on the boundary between
the calc-alkaline and high-potassium calc-alkaline fields in-
dicating that K

O enrichment was already important at the

Sample

A11

A17

A18

Z28a

Z28b

Z28e

MZ52

MZ54

MZ78

MZ77

MZ310

MZ81

SiO

63.23 60.63 66.27 67.73 66.02 67.05 67.20 68.92 67.81 64.54 64.69 66.39

15.02 15.56 13.77 14.73 15.91 15.16 14.70 14.91 14.95 15.90 15.00 15.78

TiO

0.30 1.03 1.25 0.74 0.87 0.62 0.49 0.38 0.58 0.69 0.87 0.67

FeO

3.19 3.07 3.00 2.85 3.31 3.01 2.00 2.06 3.00 2.36 2.39 3.42

1.66 2.27 2.18 0.74 0.97 0.53 0.68 0.33 0.44 1.63 2.14 1.47

MgO

2.35 3.08 1.18 1.23 2.61 1.90 1.72 1.30 1.77 2.20 2.61 2.13

CaO

4.64 7.61 3.87 3.86 4.38 4.46 4.64 2.78 3.40 4.09 4.61 3.71

MnO

0.10 0.09 0.11 0.07 0.63 0.10 0.04 0.03 0.04 0.07 0.07 0.06

2.89 0.68 2.98 2.75 2.85 1.80 2.89 2.91 2.90 2.83 2.99 2.98

3.47 4.41 2.83 4.23 4.03 3.90 4.46 3.95 4.00 4.26 3.34 3.79

0.45 0.62 0.12 0.26 0.47 0.42 0.32 0.30 0.40 0.47 0.53 0.44

1.47 0.72 1.49 0.18 0.44 0.02 0.13 0.46 0.38 1.55 0.16 0.13

O+

0.90 0.28 0.56 0.72 0.00 0.71 0.57 0.49 0.50 0.10 0.77 0.78

O–

0.49 0.80 0.15 0.22 0.03 0.16 0.09 0.04 0.03 0.05 0.03 0.21

126

101

103

156

126

380

195

160

5171

116

152

446

161

143

190

201

120

103

105

100

114

893 337 476

266

720 675 446 563 576 602 859

828

139 140 108

133

139 137 106 128 111 118 121

116

303

296

139

151

120

139

161

149

144

122

155

149

557

824

438

618

753

658

627

672

667

779

692

733

224

153

126

200

168

151

144

194

208

192

212

110

beginning of the liquid line of descent. This suggests that the
K

O enrichment is source-inherited.

The Rb vs. Y + Nb (Fig. 9g) and Nb vs. Y (Fig. 9h) dis-

crimination diagrams both show that the igneous rocks have
an I-type granite origin. The Rb, Ba, and Sr ternary diagrams
show the differentiation trend in the Ahar Magmatic Com-

Table 5: Representative chemical analysis of major (wt. %) and trace elements (ppm) for igneous rocks of the Ahar region NW Iran.

Fig. 8. Plot of Na

O + K

O vs. SiO

for igneous rocks of the Ahar

region after Wilson (1989).

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Cu and Pb (Fig. 12). Copper in the Ahar batholith ranges
from 86 ppm to 5171 ppm with average of 607 ppm (Tables 5
and 6). This indicates the original magma was rich in Cu.

Discussion

Evolution of the skarn deposits

Skarn deposit mineralogy is spatially zoned with respect to

pluton contacts, host rock lithology, and (or) fluid pathways.
The  prograde  stage  is  temporally  and  spatially  divided  into
two sub-stages: (a) metamorphic—bimetasomatic (sub-stage I)
and  (b)  prograde  metasomatic  (sub-stage II).  Sub-stage I  be-
gan immediately after the intrusion of the pluton into the en-
closing  impure  carbonates.  Sub-stage II  commenced  with
segregation and evolution of a uid phase in the pluton and its
invasion into fractures and micro-fractures of the marmorized
and  skarnoid-hornfelsic  rocks  developed  during  sub-stage I.
From  texture  and  mineralogy  the  retrograde  metasomatic
stage  can  be  divided  into  two  discrete  sub-stages:  (a)  early
(sub-stage III)  and  (b)  late  (sub-stage IV).  During  sub-
stage III, the previously formed skarn zones were affected by
intense  multiple  hydro-fracturing  phases  in  the  Cu-bearing
stock. In addition to Fe, Si and Mg, substantial amounts of Cu,
Pb, Zn, along with volatile components such as H

S and CO

were  added  to  the  skarn  system.  Consequently  considerable
amounts  of  hydrous  calc-silicates  (epidote,  tremolite-actino-
lite),  sulphides  (pyrite,  chalcopyrite,  galena,  sphalerite,
bornite), oxides (magnetite, hematite) and carbonates (calcite,
ankerite)  replaced  the  anhydrous  calc-silicates.  Sub-stage IV
was concurrent with the incursion of relatively low tempera-
ture, more highly oxidizing uids into the skarn system, bring-
ing  about  partial  alteration  of  the  early-formed  calc-silicates
and developing a series of very ne-grained aggregates of chlo-
rite, clay, hematite and calcite.

In the Ahar region the processes that lead to the formation

of skarn deposits include three stages as follows (Table 7):

Table 6: Averages of major oxide concentrations for various
groups of rocks in the Ahar region, NW Iran. Numbers correspond
to the zones shown in Fig. 13.

Oxide

neo

ndo

. I

tern

. E

xos

. Ou

ter

lli

mest

SiO

6.87 52.92

20.24

36.70

28.66

18.11

15.11

15.85

1.38

7.80

3.76

5.37

TiO

0.68 1.00 0.16 0.92 0.18 0.26

FeO

2.50

2.38

14.55

5.65

11.75

1.05

1.30

4.72

50.64

15.52

42.30

1.38

MgO

2.01 2.09 1.52 4.29 1.74 1.33

CaO

4.34 11.29

1.52 23.60

1.50 43.52

MnO

0.15 0.30 4.91 0.56 1.26 0.12

2.73 1.82 0.29 0.35 0.23 0.62

3.89 3.35 0.22 0.60 0.46 0.67

0.39 0.50 3.50 0.71 2.05 0.18

L.O.I

0.59 1.29 1.54 1.02 0.25

27.10

(1) Emplacement of plutonic magma which leads to the

isochemical contact metamorphism;

(2) prograde metasomatic skarn formation as the pluton

cools and an ore fluid develops, and;

(3) retrograde alteration of earlier formed mineral assem-

blages,  leading  to  the  formation  of  hydrosilicate  minerals
along with ore deposition. The third stage can also be divided
into  two  sub-stages.  In  the  other  words,  the  total  stages  of
skarn deposit and related hydrothermal activities can be con-
sidered in five stages. Calagari & Hosseinzadeh (2006a) be-
lieved  that  the  skarnication  process  occurred  in  two  stages:
(1) prograde and (2) retrograde and each stage is temporally
and spatially divided into sub-stages.

In the Ahar region, (1) the first stage commenced with the

emplacement, consolidation and crystallization of the magma.
As crystallization progressed the volume of hydrothermal

Fig. 13. Spatial variation diagram for average major oxide content
in different rock types in skarns of the Ahar Batholith. 1 – Igneous
rock, 2 – Endoskarn, 3 – Internal ore, 4 – Exoskarn, 5 – Outer
ore, 6 – Crystalline limestone.

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The minerals in the skarn zone belong to the system CaO—

—Fe

—SiO

—H

O and the system CaO—MgO—FeO—

SiO

—H

O so that the final mineralogy is a combination of

the two series of reaction in the endoskarn.

Plagioclase   Epidote   Garnet
Biotite   Amphibole   Clinopyroxene
(3) The  third  stage  represents,  therefore,  the  culmination

of the prograde skarn formation and is followed by a drop in
temperature and beginning of the retrograde stages of skarn
formation. The crystallization of epidote, chlorite, tremolite—
actinolite,  and  sphene  characterize  this  initial  retrograde
stage. The chalcopyrite follows magnetite by replacing it and
the  earlier  minerals  such  as  garnet,  calcite,  and  hematite.
Processes  such  as  hydrolysis,  carbonation  and  sulphidation,
due to relatively low temperature hydrothermal fluids, were
responsible for the formation of these mineral assemblages.
Epidote is the most common alteration mineral, locally rang-
ing from 50 to 85 % in modal value. A local increase in f O

may have played an important role in the formation of epi-
dote (Perkins et al. 1986; Berman 1988):

garnet epidote
Tremolite—actinolite in this sub-stage was probably

formed by retrograde alteration of clinopyroxene (Zussman
et al. 1992). Such alteration is well illustrated where the rem-
nants of pyroxene and garnet are seen within the actinolite
minerals (Mollai et al. 2009):

clinopyroxene tremolite—actinolite
Thus, at least two paragenetic stages of skarn formation and

ore  deposition  have  been  recognized:  stage 2,  hedenbergitic
pyroxene ± garnet ± scapolite (meionite) ± quartz ± magnetite,
and stage 3 amphibole ± epidote ± chlorite ± quartz co ± calcite
and  pyrrhotite + chalcopyrite ± pyrite.  The  hydrous  skarn  as-
semblage (stage 3) replaced early-formed skarn assemblages.
This alteration is similar to the other Cu-Fe and Fe-Cu skarn
deposits  in  the  world  (Meinert  1992;  Kwak  1994;  Newberry
1998; Meinert et al. 2005; Yücel Öztürk et al. 2008).

(4) The magnetite-chalcopyrite metasomatism is followed

by epigenetic hydrothermal veins containing chalcopyrite,
bornite, covellite, cubanite, magnetite, quartz, calcite and
chlorite. The veins filled a system of conjugate fractures trans-
verse to the original bedding direction.

(5) The latest episode in the area is represented by the bar-

ren hydrothermal veins containing quartz, calcite and/or chlo-
rite veinlets and alteration of the existing low temperature
assemblage minerals to epidote, chlorite, and carbonates.

According to Einaudi et al. (1981), the distribution, miner-

alogy and metal ratios of skarns are quite variable, and may
be correlated to the types of magma, depth of formation, oxi-
dation state and distance from intrusion (Meinert 1995). The
different  oxygen  fugacities  within  the  magma  and  liberated
ore  solution  are  essential  for  the  development  of  a  metallo-
genic  province.  According  to  Einaudi  et  al.  (1981),  magne-
tite-rich skarn, equivalent to the Fe-Co skarn of Smirnov et
al. (1976) and Smirnov & Beus (1983), with significant Cu,
Co and Au content, was produced from more mafic igneous
rock  types  of  an  oceanic  island  arc.  A  hypabyssal  environ-
ment produces Fe, Cu, Mo, Pb and Zn skarn. Lithophile min-

eralized skarns like Sn, W, F, Li, Be, and B are confined to
belts of highly siliceous alkalic granites, whereas litho-chal-
cophile mineralization (W, Ho, Cu, Zn, Pb, Au, Hg, Sb) is
more typical of moderately siliceous magmas.

The arc-magmatic or subduction-related setting of these

granodioritic  rocks  are  indicated  by  their  major  and  trace-
element  geochemistry  (Figs. 8,  9,  10).  Magnetite-bearing
epidote-pyroxene, plagioclase, garnet endoskarn and garnet-
epidote  bearing  exoskarn  are  characteristic  of  island-arc
skarn (Einaudi et al. 1981). The andraditic garnet is the main
skarn  mineral  and  pyroxene  belongs  to  the  diopside-heden-
bergite series. Magnetite is the dominant primary iron oxide
mineral,  occurring  either  between  exoskarn  and  limestone  or
endoskarn  and  exoskarn.  Chalcopyrite  and  pyrite  are  the  im-
portant sulphide minerals, as in the Shinyama mine, Kamaishi
district, Japan, which is an island-arc type of skarn deposit.
The most characteristic retrograde minerals include epidote,
actinolite, chlorite, calcite and quartz. Copper skarns reported
from  oceanic  island  arc  settings  associated  with  quartz
monzonite to granodiorite plutons are characterized by high
garnet  to  pyroxene  ratios,  relatively  oxidized  assemblages
(andraditic garnet, diopside, pyroxene, magnetite and hema-
tite) and moderate to high sulphide content (Meinert 1984).
From  the  above  discussion  and  comparison,  it  can  be  con-
cluded  that  the  island-arc  setting  is  very  well  fitted  to  the
skarn deposits in the Ahar region and the ore solution related
to their magmatic origin.

Conclusions

The skarn deposits of the Ahar region can be classified

petrologically into endoskarn, exoskarn and ore skarn. Each
of these can be further subdivided on the basis of predomi-
nant  mineral  assemblage.  The  dominant  skarn  minerals  are
garnet,  calcite,  pyroxene,  actinolite  and  epidote  which  are
accompanied  by  quartz,  feldspar,  minor  vesuvianite  and
hornblende.  These  early-formed  calc-silicate  minerals  were
later texturally replaced by oxides (magnetite, hematite), sul-
phides  (chalcopyrite,  pyrite,  covellite,  bornite,  galena,
sphalerite)  and  carbonates  (calcite,  ankerite  and  siderite).
Field evidence, mineralogical and textural criteria and com-
positional data show five stages of skarn evolution. The first
stage  consists  of  plutonic  emplacement  and  iso-  chemical
metamorphism, followed by the prograde metasomatic stage,
marked by the growth of anhydrous minerals (pyroxene and
garnet). The third stage of the skarn formation is marked by
magnetite  replacing  anhydrous  calc-silicate  minerals.  The
fourth stage is marked by a drop in temperature and the be-
ginning  of  the  retrograde  changes.  Magnetite-chalcopyrite
metasomatism is followed by epigenetic hydrothermal veins
containing chalcopyrite, bornite, covellite, cubanite, magne-
tite, quartz, calcite and chlorite. The last stage is represented
by barren hydrothermal quartz veins, veinlets of calcite and/
or chlorite, and alteration of the existing low temperature as-
semblage minerals to epidote, chlorite and carbonates.

Spatial and temporal association of mineral deposits with

island-arc setting related magmatic activity in the Ahar re-
gion of NW Iran allow us to define metallogenic epochs and

Ca(Fe,Mg)

+2H

O+3CO

(Fe,Mg)

(OH)

+CaCO

+2SiO

(Fe,Al)

Si3O

+HCO

CaCO

+Ca

FeAl

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petrographical  to  geochemical  provinces  that  could  be  used
in mineral exploration. Temporal association of Cu-Mo, Cu
and  Cu-Fe  skarn  deposits  with  post-collisional  granitoids
suggest  a  metallogenic  epoch  during  the  Oligo-Miocene  in
northwest and central Iran (Sarcheshmeh copper porphyry in
Kerman). In this epoch, formation of Cu and Cu-Fe skarn de-
posits took place in the province of granodioritic intrusions,
and Cu-Mo deposits were formed in the province of monzo-
nitic  to  monzodioritic  intrusions.  The  most  significant  fea-
ture  assigned  to  the  island  arc  setting  and  post-  collision
granitoids in the Ahar region is that they are products of ho-
mogeneous to heterogeneous mixing melt formed in a single
tectonic setting. From the above discussion and comparison,
it  can  be  concluded  that  the  calc-alkaline,  volcanic  arc
geochemistry of the host granodiorite and the mineralogical
assemblages in the skarns suggest that the skarn deposits of
the Ahar region formed in an island-arc subduction setting.

Acknowledgments:  We  gratefully  acknowledge  the  finan-
cial  support  for  this  research  made  available  by  the  Vice
Presidency  of  Research  and  Technology,  Mashhad  Branch,
Islamic Azad University Mashhad, Iran. I would like to offer
special  thanks  to  my  friend,  Dr.  A.  Fazelli,  in  GSI,  Dr  K.
Rostamy  in  Germany  and  Eng.  R.  Sharifian  for  their  very
helpful suggestions and constructive criticisms.

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