Geology mineral chemistry and formation

Ore Geology Reviews

journalhomepage: www.elsevier.com/locate/oregeorev

Geology, mineral chemistry and formation conditions of calc-silicate minerals of Astamal Fe-LREE distal skarn deposit, Eastern Azarbaijan Province, NW Iran

a Saeid Baghban a ⁎ , Mohammad Reza Hosseinzadeh , Mohsen Moayyed ,

b Mir Ali Asghar Mokhtari c , Daniel Gregory

b Department of Earth Science, Faculty of Natural Science, Tabriz University, 5166616471 Tabriz, Iran Department of Geology, Faculty of Science, University of Zanjan, 45371-38791 Zanjan, Iran c ARC Centre of Excellence in Ore Deposits (CODES), School of Physical Sciences, University of Tasmania, Private Bag 79, Hobart, Tasmania 7001, Australia

article info

abstract

Article history: The Astamal Fe-LREE skarn deposit is the largest iron skarn in NW Iran. It is a unique case, as a distal skarn deposit Received 24 April 2014

which crops out approximately 600 m from the associated Oligo-Miocene granodioritic pluton. This deposit formed Received in revised form 13 December 2014

in the south-southwest of the pluton where fractures and faults within the Upper Cretaceous volcano-sedimentary Accepted 18 December 2014

Available online 23 December 2014 host rocks acted as conduits for the mineralizing fluids. The deposit contains three iron ore bodies: southern, north- ern and eastern. The main mineral assemblage within the ore zones is characterized by magnetite, pyrrhotite and

Keywords: pyrite, with lesser quantities of chalcopyrite, hematite, goethite and limonite. The skarn minerals predominantly Fe-LREE

comprise garnet, epidote, actinolite, calcite, quartz, clinopyroxene and chlorite (in order of abundance). Retrograde Distal skarn

alteration is strongly developed in the skarn zone where most of the garnet has been pervasively altered to second- Garnet

ary minerals (e.g. epidote, calcite and quartz) both in the rims and the cores whereas the majority of the Clinopyroxene

clinopyroxene has been replaced by a hydrous retrograde mineral assemblage (e.g. tremolite, actinolite and chlo- Epidote

rite). Garnets with andradite–grossular compositions are the dominant mineral in the skarn zone, which are gener- Astamal

ally isotropic with a narrow compositional range along the growth lines (Adr 94.3–64.5 Grs 21.9–2.7 Alm 11.1–0.2 ). These NW Iran

garnets are Fe-rich and have high Fe/(Fe + Al) ratios (between 0.96 and 0.78). Cu and Ni are enriched in the garnets. This suggests that these elements were enriched in the hydrothermal fluids from which the garnet precipitated. This

is supported by the presence of chalcopyrite and Ni-bearing massive magnetite in the study area, which also sug- gests that Cu and Ni were enriched in the late stage ore-bearing hydrothermal fluids. Clinopyroxene with a

hedenbergitic composition is generally homogenous and has particularly high Fe/Fe + Mg ratios (between 0.99 and 0.86) and is poor in TiO 2 , MnO and Cr 2 O 3 . High Zn concentrations were also detected in the clinopyroxenes (up to 1044 ppm), despite an absence of significant Zn mineralization (such as sphalerite) in the district. Therefore, it is believed that the proportion of Zn in the hydrothermal fluids decreased significantly from the time of clinopyroxene formation to the period of the sulfide deposition phase. Allanite and LREE-bearing epidotes are the main LREE bearing minerals in this deposit. The epidote is also Fe-rich with high Fe/(Fe + Al) ratios (between

0.32 and 0.44). Due to a lack of replacement texture between both garnet and clinopyroxene and garnet and actin- olite (which is formed by alteration of clinopyroxene), it is believed that these two minerals have grown simulta- neously and are coexisting minerals. In the Astamal skarn, these minerals can be stable and coexisting at

temperatures between 490 and 560 °C and LogƒO 2 = −16 to −31.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction The Astamal Fe-LREE skarn deposit is located in the Qara-Dagh–

Sabalan metallogenic belt in northwest Iran and is considered to be part of lesser Caucasus in the Alpine–Himalayan orogenic belt. This

⁎ Corresponding author at: Abrasan Street, Pezeshkan, plaque 1.17, 5156845714 Tabriz, area has long been of interest to geologists and explorers because of Iran.

E-mail addresses: Saeid.baghban_geomine@yahoo.com , SaeidBaghban@gmail.com the well developed alteration zones and high prospectivity for mineral-

(S. Baghban), MR-Hosseinzadeh@tabrizu.ac.ir (M.R. Hosseinzadeh), ization. Volcanic activity, numerous intrusions and the influence of as- Moayyed@Tabrizu.ac.Ir (M. Moayyed), Amokhtari@znu.ac.ir (M.A.A. Mokhtari),

sociated hydrothermal fluids have led to the formation of a wide ddg@utas.edu.au (D. Gregory).

variety of deposits.

http://dx.doi.org/10.1016/j.oregeorev.2014.12.016 0169-1368/© 2014 Elsevier B.V. All rights reserved.

80 S. Baghban et al. / Ore Geology Reviews 68 (2015) 79–96

S. Baghban et al. / Ore Geology Reviews 68 (2015) 79–96

Fig. 2. Geological map of the study area showing the Astamal skarn location and its relationship with Qara-Dagh batholith. Modified after Mokhtari and Hosseinzadeh (2013) .

The Qara-Dagh–Sabalan metallogenic belt and Caucasus Mountains 2004 ), Mivehrood ( Jamali et al., 2010 ), Zaglik ( Heydarzadeh, 2007 ), formed during the post-collisional stage, after the closure of the Neotethys

and Nabijan ( Baniadam, 2003 )).

Ocean. During the Cenozoic era, intense magmatism, with a peak in the Also, various types of mineralization formed within the Qara-Dagh

batholith such as: Aniq-Qarachilar gold–copper–molybdenum vein de- was emplaced. This batholith has been recognized to be responsible for

Eocene and Oligocene, occurred and the 1500 km 2 Qara-Dagh batholith

posit ( Mokhtari et al., 2014 ), the Agarak and Kadjaran copper–molybde- the widespread mineralization in the region ( Mehrpartou, 1997;

num deposits ( Zvezdov et al., 1993 ), the Zod epithermal gold deposit Mokhtari, 2009 ).

( Konstantinov et al., 2010; Kozerenko, 2004 ) and the Kapan, Alaverdi

A variety of deposits are associated with this batholith and contem- and Mehmana epithermal polymetallic deposits ( Mederer et al., 2014 ). poraneous intrusive bodies. These include: porphyry copper deposits

The Astamal–Nojehmehr alteration zone is one of the main features (Sungun ( Calagari, 2003, 2004 ), Saunajil ( Hosseinzadeh, 2008 ),

of this region. It consists of a widespread argillic, propylitic, sericitic and Haftcheshmeh ( Hassanpour et al., 2011 ) and Masjed-Daghi porphyry

silicic alterations spanning a length of approximately 45 km. The alter- copper–gold deposits ( Akbarpour, 2005 ); skarn deposits (Sungun Cop-

ation zone is also associated with a widespread Cu anomaly. per ( Calagari and Hosseinzadeh, 2006 ), Mazraeh Copper ( Mollai, 1993 ),

The Astamal Fe-LREE skarn deposit is one of the most significant Anjerd Copper ( Hosseinzadeh, 1999 ), Avan Copper–Iron ( Mokhtari,

deposits in this region. It was discovered two years ago and explora- 2009; Mokhtari et al., 2012 ), Astamal Iron ( Baghban, 2013; Mokhtari

tion is ongoing. It is the largest and richest iron deposit in northwest- and Hosseinzadeh, 2013 ), Gavdel Iron–Copper ( Mahmoudinia, 2013 ),

ern Iran ( Mokhtari and Hosseinzadeh, 2013 ) with an estimated 10 Kamtal Iron–Copper ( Mokhtari et al., 2013 ), Pahnavar Iron ( Mokhtari,

Mt resource of magnetite Fe ore with an average grade of approxi- 2012 )) and epithermal gold deposits (Masjed-Daghi ( Mohammadi,

mately 60%.

Fig. 1. Regional geological map of the Siahrood area showing the occurrence and distribution of metallic mineralization associated with Qara-Dagh intrusion (modified after Mehrpartou, 1997 ). The study area is within the quadrangle in the center of the map.

82 S. Baghban et al. / Ore Geology Reviews 68 (2015) 79–96

In this paper, local and regional geology, petrographic studies of basal conglomerate layer overlain by sandstone. The presence of skarn zones, mineral chemistry and formation conditions of calc-

Nomolite in these layers ( Mehrpartou, 1997 ) indicates a Mid-Eocene silicate minerals are presented.

age. Upper Eocene facies overlie Mid-Eocene volcano-sedimentary rocks and outcrop south of Astamal village. The Upper Eocene rocks

2. Geology have a submarine andesitic to basaltic composition and form the south- ern and northern limbs of the Avansar anticline ( Mehrpartou, 1997 ).

2.1. Regional geology The emplacement of Oligocene aged intrusive bodies played an im- portant role in this region ( Fig. 1 ). The Qara-Dagh batholith is one of the

A specific group or formation or unit has not been identified in the largest intrusions of Oligocene–Miocene age covering an area of Qara-Dagh area, so far. Lithostratigraphically, the oldest rocks are of

1500 km 2 . Its intrusion into the Upper Cretaceous volcano-sedimentary Upper Cretaceous age and consist of flysch type rocks and mafic to inter-

units resulted in widespread contact metamorphism and alteration. This mediate submarine volcanic rocks. The flysch rocks have been identified

batholith consists of gabbro, diorite, quartz-diorite to quartz-monzonite, over a 30 km strike length and with a width of at least 15 km. They are

granodiorite, monzogranite and granite porphyry ( Mokhtari, 2009 ). composed of micritic limestone, sandstone, shale and mudstone. These sedimentary rocks have been folded and the calcareous layers decrease

2.2. Local geology

in abundance from west to east. Submarine volcanic activity is characterized by rocks of mafic to intermediate composition (andesite,

Upper Cretaceous submarine andesitic rocks are the most prevalent basaltic andesite and pyroxene andesite) interlayered with the sedi-

units in the Astamal area ( Fig. 2 ). These rocks have porphyritic textures mentary sequence. The presence of Nummufallutia sp., Hetrohelicid and

with plagioclase phenocrysts and have been propylitically altered as a Globotruncana sp. in the lower part of this sedimentary unit suggests a

result of the Qara-Dagh batholith intrusion. The Upper Cretaceous sed- Santonian to Maastrichtian age ( Mehrpartou, 1997 ). Rocks of Paleocene

imentary sequence is widespread and consists of a flysch-type assem- age are poorly represented in this region. The presence of red sandstone

blage including alternating thin to medium bedded sandstone, shale, and microconglomerate layers at the base of this sequence exhibits

marl and conglomerate covered by thick bedded to massive limestone. epeirogenic movements of Laramian phase. Gray sandstone layers

This flysch sequence has been thermally metamorphosed due to the with limy interbeds progressively increase up section. The sandstone

Qara-Dagh batholith intrusion resulting in the development of skarn, is overlain by Paleocene submarine andesitic volcanic rocks and felsic

hornfels and marble ( Fig. 3 ). The iron mineralization predominantly oc- tuffs. The Paleocene and Upper Cretaceous rocks are unconformably

curs as high grade massive magnetite ore bodies at the Astamal deposit. overlain by Eocene strata. These Eocene rocks are 60 m thick with a

Pyrrhotite, pyrite, chalcopyrite and minor hematite, malachite, azurite

Fig. 3. Detailed geological map of the Astamal skarn deposit showing the distribution of ore bodies, host rocks, dykes, skarn and hornfels localities. The conceptual geometry of the ore body in the cross section is inferred from magnetic surveys discussed in the text.

S. Baghban et al. / Ore Geology Reviews 68 (2015) 79–96

and secondary iron oxide–hydroxide species accompany the magnetite geological cross-section of the ore body in Fig. 3 is also drawn based in the ore bodies. Four iron ore bodies have been identified in the depos-

on the magnetic investigations. The northern iron ore body it. The southern iron ore body has skarn ( Fig. 4

a, e) is hosted within calc-silicate rocks ( Fig. 4 b). It is the main ore body and occurs between two faults,

a) and the hornfels host-

(10 m × 50 m × 40 m) ( Fig. 4

hornfelsic rocks and is situated approximately 400 m north of the which demonstrate that the mineralization is controlled by fracturing

southern ore body. Ancient mining activity took place at a depth of and faulting. The existence of a consolidated hornfelsic component on

5 m on the eastern side of this ore body ( Fig. 4 e). The eastern iron ore the ore body is an indication of the low level erosion of the skarn

body is the last one at the Astamal deposit. It is approximately 1 km ( Fig. 4 c). The western iron ore body ( Fig. 4

east of the main ore body and is poorly exposed. Minor ancient mining 200 m west of the southern ore body and is only observed in subcrop.

d) is located approximately

activities were carried out at this deposit as well. In the eastern ore However, magnetic data ( Jafari and Esmailzadeh, 2011 ) suggests that

body, mineralization is either disseminated or occurs as low grade this ore body and the southern ore body are continuous at depth and

veins. LREE mineralization occurs in the deposit as LREE-bearing epi- form a single (200 m × 250 m × 70 m) massive ore zone ( Fig. 3 ). The

dotes. These minerals are most common in the skarn and the Fe ore

Fig. 4. (a) Astamal skarn deposit showing the location of southern and northern iron ore bodies and their tectonically-controlled structures (view to the northwest). (b) Residue of calc- silicate hornfels host rock surrounded by magnetite ore (view to the west). (c) Hornfelsic cap rock of the iron ore body displaying low erosion of the mineralized zone (view to the north- west). (d) Outcrop features of western iron ore body and thick bedded marbles (view to the east). (e) Northern iron ore body in the vicinity of a strike-slip fault, quartz andesite–dacite and diabase dykes (view to the north). (f) Euhedral coarse-grained garnets in calcite and epidote matrix. (g) Subhedral very coarse-grained garnets in epidote matrix. (h) Massive garnetite consisting of euhedral to subhedral coarse-grained garnets. (i) Silicified zone within the andesitic rocks containing pyrite and secondary iron hydroxide minerals (view to the west). Ab-

breviations: ad = quartz andesite–dacite dykes, db = diabasic dyke, K f u = Upper Cretaceous contact metamorphosed flysch type rocks (skarn and hornfels), K u v = Upper Cretaceous metasomatized andesitic rocks, Mb = marble, Cal = calcite, Ep = epidote, Grt = garnet.

84 S. Baghban et al. / Ore Geology Reviews 68 (2015) 79–96

zones. The skarn and the hornfels zones are the most important lithol- shown in Fig. 3 . The skarn mineralogy predominantly consists of garnet ogies in the study area and are well developed and have appreciable

pervasively altered to epidote and calcite. This mineral is observed as economic value. Due to the interfingering of skarn and hornfels, their

euhedral to subhedral coarse-grained crystals with brown, reddish separation is very difficult even on the small scale geological map

brown and gray colors ( Fig. 4 f–g) and in some cases is the only rock-

Fig. 5. Photomicrographs of Astamal skarn rock specimens: (a) Mylonitized skarn zone with brecciated garnet, epidote, calcite and quartz crystals. (b) Astamal skarn containing garnet and epidote crystals in calcite and quartz matrix. (c) Replacement of garnet by calcite and quartz, both in the rim and the core, exhibiting the common intense alteration. (d) Epidote occurring as intergranular open-space fill between coarse-grained euhedral garnet crystals. (e) Epidote pseudomorph after garnet with mosaic aggregate in calcite matrix. (f) Magnetite replacing garnet rims within the unmineralized zone. (g) REE mineralization occurring as allanite and LREE-bearing epidotes in the Astamal skarn (plane-polarized transmitted light). (h) Very strong epidote replacement due to the pervasive retrograde alteration in the skarn zone. (i) Fibrous and narrow acicular actinolite resulting from alteration of clinopyroxene. (j) Extensive actinolite formation in the skarn zone due to the pervasive retrograde alteration. (k) Elongated acicular tremolite aggregates with nematoblastic texture. (l) Late actinolite vein crosscutting the hornfels zone (plane-polarized transmitted light). (m) Occurrence of cordierite in fine-grained quartz matrix within sandstone layers and fine-grained epidote and opaque minerals within marl layers from the metasedimentary sequence (plane-polarized transmitted light). (n) As the same as panel m but in non-polarized transmitted light. (o) Sparse to locally abundant radial texture wollastonite in the marble zone. (p) Aggregation of plagioclase, actinolite, opaque minerals and remnant hornblende with glomeroporphyro-blastic tex- ture showing hornblende hornfels contact metamorphism in the hornfels zone. Abbreviations: Act = actinolite, Aln = allanite, Bt = biotite, Cal = calcite, Chl = chlorite, Crd = cordierite, Di = diopside, Ep = epidote, Grt = garnet, Hbl = hornblende, Opq = opaque minerals, Pl = plagioclase, Qz = quartz, Spn = sphene, Tr = tremolite, Wo = wollastonite. Abbreviations from Whitney and Evans (2010) .

S. Baghban et al. / Ore Geology Reviews 68 (2015) 79–96

Fig. 5 (continued).

forming mineral present ( Fig. 4 h). Epidote is relatively abundant in the sulfide mineralization and related supergene alteration is common district as well and often occurs interstitially between the garnet crys-

along the shear zones ( Fig. 4 i). Marble of considerable extent and thick- tals ( Fig. 4 g). Metasedimentary rocks are also observed near the skarn

ness occurs at the contact of the skarn–hornfels zone and the Upper Cre- and include bedded marl, sandstone and conglomerate.

taceous andesite ( Fig. 4 d). These cream to white colored thinly bedded, Unlike most skarn deposits, endoskarn is not observed at the

brecciated marbles are locally cross-cut by late coarse-grained calcite Astamal skarn. Significant fracturing and faulting extends along a NE–

veinlets.

SW trend from the Qara-Dagh batholith to the Astamal skarn aureole. Two generations of dykes cross-cut the volcanic and sedimentary It is suggested that the ore-bearing hydrothermal fluids moved from

rocks in the district ( Fig. 3 ). The first has a quartz-andesite to dacite the Qara-Dagh batholith along these porous linear features and formed

composition and has different directions. They are extremely altered the Astamal Fe-LREE skarn as a distal occurrence within the Upper Cre-

due to their intrusion prior to the emplacement of the Qara-Dagh bath- taceous volcano-sedimentary rocks, approximately 600 m from the

olith. The second generation of dykes has a gabbroic to dioritic compo- Qara-Dagh batholith ( Figs. 2 and 3 ). Silicic alteration accompanied by

sition and is relatively fresh with negligible alteration. They intruded

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86 S. Baghban et al. / Ore Geology Reviews 68 (2015) 79–96 For Evaluation Only.

along the NE–SW fractures after the emplacement of the Qara-Dagh

4. Petrography

batholith. The main mineral assemblage at the Astamal skarn is, in order of abundance, garnet, epidote, actinolite, calcite, quartz, clinopyroxene

3. Methodology and chlorite. These minerals have granoblastic, granonematoblastic, poikiloblastic, mega-porphyroblastic, and hornfelsic textures. In some

A total of 73 samples were collected from marble, skarn and hornfels areas these minerals have undergone cataclastic deformation and are rocks to undertake petrographic and mineral chemistry studies. Petro-

locally mylonitized ( Fig. 5 a).

graphic studies include identification of mineral assemblages, textures, alteration and metasomatic replacements. These were performed

using an Olympus BX60 microscope at the ore-deposit laboratory at

4.1. Garnet

the University of Tabriz. Based on the optical microscope observations,

11 relatively unaltered samples were selected for electron microprobe Garnet (5–50%) is the most abundant mineral in the Astamal skarn analysis. The major oxide compositions of the calc-silicate minerals

and is present as euhedral to subhedral fine to coarse-grained (0.5– (e.g. garnet and clinopyroxene) were determined at the mineralogy di-

1 cm) crystals, though they can be very coarse-grained (N3 cm) locally. vision of the Iranian Mineral Processing Research Center (IMPRC) using

The garnets are dominantly isotropic ( Fig. 5 b–c–d); however, the very

a Cameca SX-100 electron microprobe equipped with 5 wavelength- coarse-grained crystals show weak diffusion and irregular zoning. Com- dispersive crystal spectrometers, operating with a 3 μm beam diameter,

monly the garnet has been intensely retrograde altered to epidote, cal-

15 kV accelerating voltage, 15 nA sample current and 60 s counting cite and quartz. In rare cases the epidote forms pseudomorphs after time. Elements were calibrated against synthetic and natural standards.

garnet ( Fig. 5 e). Less altered garnets exhibit extensive fluid pressure Chemical formulae and end-member proportions for the minerals anal-

fracturing. The fractures are filled by late stage and retrograde mineral yses were calculated following the method of Deer et al. (1992) , and

assemblages that include calcite, quartz, epidote and chlorite ( Fig. 5 c). Fe +3 was calculated based on ideal stoichiometric composition. X-ray

Magnetite is also observed in the unmineralized skarn zone as replace- fluorescence microanalysis was completed to determine epidote chem-

ments in the garnet rims and fissures ( Fig. 5 f). istry at the Kansaran Binaloud Mineral Research Corporation using a

XGT-5000 Horiba (XRF). An accelerating voltage of 50 kV, incident X-

4.2. Epidotes

ray beam of 10 μm and counting time of 100 s were utilized in these analyses. The X-ray fluorescence was detected using an energy disper-

Epidote occurs as euhedral to subhedral crystals (20–60%, b0.1– sive high purity silicon detector cooled to −195.8 °C by liquid nitrogen.

1 mm) and is the most common retrograde calc-silicate mineral and is X-ray source and detection chamber were placed under vacuum in

also the most important product of garnet alteration. The epidote is ob- order to increase the X-ray beam intensity.

served in several forms, such as: (1) massive aggregates in which

Fig. 6. Paragenetic sequence of minerals present in skarn, hornfels and marmorized zones in the Astamal skarn area. The thickness of the horizontal bars is related to the relative abundance of the minerals.

S. Baghban et al. / Ore Geology Reviews 68 (2015) 79–96

Table 1 Representative electron microprobe (EPMA) data of garnets from Astamal skarn.

A 16 A 17 Comment

Sample A 12

Rim Core Rim Major oxides, weight percent

SiO 2 35.49 35.59 35.11 35.72 35.72 35.59 35.63 35.98 35.94 36.05 36.34 36.38 36.25 36.88 36.83 37.14 TiO 2 0.07 0.16 0.61 0.17 0.35 0.82 0.11 0.14 0.67 0.30 0.12 0.26 0.52 0.56 0.41 0.52 Al 2 O 3 0.88 1.20 1.37 0.61 0.83 1.62 1.32 1.70 1.94 2.71 2.98 3.26 4.11 4.53 3.55 3.98 Cr 2 O 3 0.02 0.00 0.01 0.03 0.01 0.03 0.02 0.00 0.06 0.04 0.02 0.02 0.00 0.00 0.01 0.03

FeO a 30.39 30.05 29.97 29.96 29.56 28.61 29.51 28.92 27.98 27.92 27.66 27.12 27.24 26.34 27.78 27.07 MnO

0.69 0.81 0.94 0.76 0.98 1.23 0.88 1.07 1.06 0.68 0.77 0.75 0.97 1.10 0.96 1.07 MgO

0.11 0.13 0.20 0.18 0.43 0.59 0.19 0.31 0.67 0.54 0.52 0.53 0.33 0.54 0.83 0.95 CaO

32.36 31.97 31.55 32.48 31.91 31.43 32.29 31.97 31.66 31.76 31.42 31.64 30.54 29.96 29.56 29.06 Na 2 O

0.01 0.00 0.02 0.00 0.03 0.03 0.02 0.01 0.02 0.01 0.16 0.05 0.05 0.09 0.05 0.07 K 2 O

0.01 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.06 0.06 0.03 0.04 Total

100.07 100.06 100.01 99.93 Number of ions on the basis of 12 oxygen atoms and garnet end-member normative calculation

Si 2.91 2.92 2.89 2.94 2.94 2.92 2.92 2.94 2.93 2.94 2.96 2.96 2.95 3.00 3.00 3.02 Ti

0.00 0.01 0.04 0.01 0.02 0.05 0.01 0.01 0.04 0.02 0.01 0.02 0.03 0.03 0.03 0.03 Al

0.09 0.12 0.13 0.06 0.08 0.16 0.13 0.16 0.19 0.26 0.29 0.31 0.39 0.43 0.34 0.38 Cr 3+ b 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Fe 2.07 2.02 2.00 2.05 2.00 1.91 2.01 1.94 1.86 1.83 1.75 1.73 1.63 1.49 1.60 1.50

Fe 2+ 0.01 0.05 0.06 0.01 0.03 0.05 0.01 0.04 0.05 0.07 0.13 0.11 0.23 0.30 0.29 0.35 Mn

0.05 0.06 0.07 0.05 0.07 0.09 0.06 0.07 0.07 0.05 0.05 0.05 0.07 0.08 0.07 0.07 Mg

0.01 0.02 0.02 0.02 0.05 0.07 0.02 0.04 0.08 0.07 0.06 0.06 0.04 0.07 0.10 0.12 Ca 2.85 2.81 2.78 2.86 2.81 2.76 2.84 2.80 2.77 2.77 2.74 2.76 2.66 2.61 2.58 2.53

Na 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.01 0.00 0.01 K

0.1 0.0 0.0 0.1 0.0 0.1 0.1 0.0 0.2 0.1 0.1 0.1 0.0 0.0 0.0 0.1 Fe/(Fe + Al)

0.824 0.797 0.840 0.818 X Fe 0.994

0.979 0.965 0.949 0.941 Trace elements, parts per million

bdl bdl bdl Sample

A 38 Comment

A 30

Rim Major oxides, weight percent

SiO 2 36.58 36.79 37.16 36.48 36.73 TiO 2 0.28 0.26 0.26 0.21 0.18 Al 2 O 3 3.58 3.91 4.28 4.53 5.25 Cr 2 O 3 0.06 0.05 0.02 0.09 0.00 FeO a 27.98 27.79 27.11 26.11 25.94

MnO 0.86 1.06 1.20 0.98 1.23 MgO

0.67 0.41 0.73 0.25 0.21 CaO

29.95 29.69 29.11 31.37 30.43 Na 2 O

0.02 0.03 0.12 0.03 0.03 K 2 O

0.00 0.01 0.00 0.00 0.04 Total

100.04 Number of ions on the basis of 12 oxygen atoms and garnet end-members normative calculation

Si 2.98 3.00 3.02 2.96 2.98 Ti

0.02 0.02 0.02 0.01 0.01 Al

0.34 0.38 0.41 0.43 0.50 Cr 3+ b 0.00 0.00 0.00 0.01 0.00

Fe 1.66 1.59 1.49 1.62 1.51 Fe 2+

Na 0.00 0.00 0.01 0.00 0.00 K

2.0 2.4 2.7 2.3 2.8 (continued on next page)

88 S. Baghban et al. / Ore Geology Reviews 68 (2015) 79–96

Table 1 (continued) Sample

A 38 Comment

A 30

Rim Pyrope

0.2 0.1 0.1 0.3 0.0 Fe/(Fe + Al)

0.986 Trace elements, parts per million

X Fe 0.959

254 bdl: below detection limit. a b Total iron as FeO. Recalculated from stoichiometry.

epidote has replaced both the rim and the core of the garnets ( Fig. 5 b), actinolite appears to have precipitated directly from hydrothermal (2) mosaic aggregates in which epidote is closely packed with a

fluids ( Fig. 5 l) that postdate the skarn formation. granoblastic texture ( Fig. 5

e) and (3) as open-space filling aggregates

in which epidote occurs as intergranular-masses which have filled voids between coarse-grained garnet crystals ( Fig. 5 d). A locally abun-

4.4. Calcite

dant allanite is distinguished from epidote by its brownish color, and is occasionally rimmed by epidote ( Fig. 5 g). A lack of metamictization

Three types of calcite are found at the Astamal skarn. Two formed ei- ther from metamorphism of the limestone or alteration of the garnet

and anastomosing cracks within the allanite exhibit its low radioactive element content. Extensive epidote alteration is common throughout

crystals ( Fig. 5 b–c). These are usually 15–20% of the rock and the crys- the skarn zone ( Fig. 5 h).

tals are b1 mm in diameter. Late stage calcite veins are also common in the marble zone and calcite crystals are generally 1 cm in diameter.

4.3. Tremolite–actinolite

4.5. Clinopyroxene

Tremolite and actinolite are products of clinopyroxene alteration Clinopyroxene crystals are commonly formed in the isochemical that occur both as fibrous and acicular forms ( Fig. 5

and prograde stages of iron skarn deposit formation. Clinopyroxene from 10–35% of the rock, range in size from b0.1–1 mm and have a

i, j). They range

results from the interactions of ore-bearing metasomatic fluids nematoblastic texture ( Fig. 5 k). In some areas, actinolite is observed as

with the calcareous host rocks and is found in association with gar- veinlets which are different from the metasomatic actinolite. The veinlet

net. Together clinopyroxene and garnet are usually the predominant

Fig. 7. Composition of Astamal skarn garnets in the andradite–grossularite–pyralspite ternary diagram. Arrows indicate the growth trends in the garnets from the core to the rim. Repre- sentative EPMA data are given in Table 1 .

S. Baghban et al. / Ore Geology Reviews 68 (2015) 79–96

Fig. 8. Comparison of the Fe/(Fe + Al) mole fraction of garnet from the Astamal skarn and some other skarn deposits. Adopted from Einaudi (1982) .

minerals in calcic iron skarn deposits. In the Astamal skarn,

4.6. Other minerals

clinopyroxene forms 0–3% of the rock and crystals are b0.1–

0.5 mm in diameter. The clinopyroxene has almost been completely Quartz is usually fine-grained and present as a replacement mineral destroyed by extensive retrograde alteration and has been replaced

in the rims of coarse-grained garnets and is believed to have formed by by hydrous calc-silicate minerals such as tremolite, actinolite,

alteration of the garnet and clinopyroxene ( Fig. 5 c). Occasionally, it fills chlorite and opaque minerals ( Fig. 5 k–i). When observed,

fractures and interstices as a retrograde anhedral mineral in the late- clinopyroxene occurs as relict crystals between the different retro-

stage mineralization. Chlorite is a retrograde alteration product of the grade assemblages.

garnet and actinolite that occurs as sheet-like rims around garnet and

Table 2 Electron microprobe (EPMA) data of clinopyroxenes from Astamal skarn.

Sample A 12

A 42 A 43 Comment

A 13

Core Rim Major oxides, weight percent

SiO 2 48.88 49.19 49.46 49.13 48.98 48.91 48.85 48.77 49.96 49.11 Al 2 O 3 0.03 0.05 0.21 0.00 0.04 0.09 0.07 0.11 0.12 0.16 TiO 2 0.67 0.81 0.99 1.3 1.27 1.7 1.33 0.74 1.85 0.17 Cr 2 O a 3 0.00 0.01 0.03 0.00 0.02 0.03 0.02 0.00 0.01 0.00

FeO 27.9 27.79 27.72 27.71 27.87 27.66 24.82 23.3 21.36 20.62 MnO

0.34 0.29 0.27 0.18 0.23 0.25 0.51 0.45 0.87 1.05 MgO

0.07 0.19 0.19 0.56 0.53 0.57 1.22 1.1 1.96 1.8 CaO

21.99 21.43 20.81 21.13 20.82 20.66 23.11 25.14 23.65 26.91 Na 2 O

0.09 0.09 0.01 0.00 0.03 0.00 0.03 0.36 0.29 0.47 K 2 O

0.00 0.07 0.23 0.03 0.04 0.03 0.09 0.21 0.16 0.05 Total

100.23 100.34 Number of ions on the basis of 6 oxygen atoms and clinopyroxene end-members normative calculation

Si 2.01 2.02 2.03 2.01 2.01 2.00 1.98 1.99 2.00 1.96 Al

0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Ti

0.03 0.04 0.05 0.06 0.06 0.08 0.06 0.03 0.088 0.01 Cr

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Fe 3+ b Fe 2+

Mn 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.03 0.04 Mg

0.00 0.01 0.01 0.03 0.03 0.03 0.07 0.07 0.12 0.11 Ca 0.97 0.94 0.92 0.93 0.92 0.91 1.00 1.08 1.01 1.15

Na 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.03 0.02 0.04 K

0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.01 0.00 Total

4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 X En

0.061 0.058 X Wo

0.409 0.321 X Fs

0.530 0.622 Fe/(Fe + Mg)

0.859 0.865 Mn/Fe

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 Trace elements, parts per million

423 254 bdl: below detection limit. a b Total iron as FeO. Recalculated from stoichiometry.

bdl

90 S. Baghban et al. / Ore Geology Reviews 68 (2015) 79–96

Fig. 9. Compositions of Astamal skarn clinopyroxenes in the ferrosilite–enstatite–wollastonite ternary diagram ( Deer et al., 1978 ). Arrows indicate the growth trends in the clinopyroxenes from the core to the rim. EPMA data are given in Table 3 .

actinolite ( Fig. 5 c). Fine-grained spindle-shape sphene ( Fig. 5 l) and tab- Mn, Mg, Cu and Ti contents increase and Al, Fe 3+ , Ca and Ni contents de- ular apatite are rare accessory minerals in the skarn zone. Cordierite

crease from the core to the rim. Calcium enrichment in the garnets is as- with low birefringence and butterfly twinning occurs in the matrix of

sociated with proximity to calcareous sedimentary host-rocks, fluid fine-grained quartz ( Fig. 5 m and n) in the metasedimentary rocks. Wol-

infiltration and chloritization of open cracks within the grains ( Hwang lastonite with parallel extinction, elongated and radial texture ( Fig. 5 o)

et al., 2003 ). Very low content of TiO 2 in the garnet crystals of at the can be observed in the marble. Biotite is rarely observed in the skarn

Astamal skarn implies a high activity of SiO 2 during their formation zone, but it is a common mineral in the metasomatized andesitic

( Dingwell and Brearley, 1985 ). These low values are a clear difference rocks where it occurs as fine-grained disseminatations along with trem-

between skarn garnets and igneous garnets. For example garnets of olite–actinolite and remnant hornblende. These are indicators of the

analcime-bearing syenitic bodies in the Kaleybar ( Ashrafi et al., 2009 ), hornblende hornfels metamorphic facies ( Fig. 5 p). In samples that

melanite-bearing volcanic rocks of Alberta ( Dingwell and Brearley, have been intensively altered, clay minerals can be observed on the

1985 ), garnets in the Rugged Mountain trachytic dykes of the Canadian mineral surfaces. Fig. 6 represents a paragenetic sequence of minerals

Cordillera ( Russell et al., 1999 ) and garnets in the Kaiserstuhl phonolitic in the Astamal skarn, hornfels and marmorized zones.

dykes of Germany ( Armbruster et al., 1998 ) all have relatively high TiO 2 Based on the observed mineral assemblages, it is believed that the

compared to skarn garnets. Some exceptions are present, including tita- host rock of the skarn zones has been chiefly composed of marl, calcar-

nium andradite (TiO 2 = 12.4%) from skarn of the Northern Red Sea Hill eous marl and argillaceous limestone.

5. Mineral chemistry

5.1. Garnet Microprobe analyses, formulae and end-member compositions of

garnet crystals are given in Table 1 . Most of the garnets are generally isotropic and no specific zoning is observed along the growth lines

(i.e. there is little variation in overall range of composition) most fall within a narrow compositional range (Adr 94.3–64.5 Grs 21.9–2.7 Alm 11.1–

0.2 ). The amount of spessartine is minor, the amount of pyrope is negli- gible (total less than 4 mol%) and the amount of uvarovite is usually below detection limits. Therefore, the garnets are classified as being on the andradite–grossular solid solution series with andradite as the dominant end-member ( Table 1 , Fig. 7 ).

The chemical formulae vary slightly and are as follows: Na

(1.49–2.07) Ni (0–0.12) Cr (0–0.01) Al (0.06–0.50) Ti (0–0.05) Si (2.89–3.02) O 12 .

Though there is no distinct zone boundaries there are minor compo- sitional variations from the core to the rim of the garnet. Silicon, Fe 2+

Fig. 10. Ca (afu) vs. Fe/(Fe + Mg) (atomic) for clinopyroxenes in the Astamal skarn.

After Papike et al. (1998) .

S. Baghban et al. / Ore Geology Reviews 68 (2015) 79–96

Table 3 Representative X-ray microanalysis of epidotes from Astamal skarn.

Comment LREE-bearing epidotes

LREE-free epidotes

Sample A12-1-2 A12-1-3

A42-2-1 A43-1-3 A43-1-4 Major oxides, weight percent

A12-1-4

A13-1-2

A13-2-2

A13-3-2

16-3-1

A16-4-1

A-42-1-1

SiO 2 38.05 38.60 36.10 35.95 36.53 37.05 39.11 38.63 38.56 39.94 38.52 38.58 TiO 2 0.06 0.00 0.00 0.03 0.03 0.03 0.01 0.12 0.11 0.08 0.05 0.05 Al 2 O 3 19.53 16.70 19.69 18.93 18.62 18.23 19.86 19.88 18.97 19.13 14.69 19.06 Cr 2 O 3 0.05 0.00 0.00 0.00 0.02 0.00 0.00 0.02 0.02 0.02 0.04 0.00 FeO a 14.26 12.96 13.71 14.40 12.58 12.89 14.71 13.79 14.64 14.25 16.33 14.27 MnO

0.12 0.00 0.17 0.23 0.14 0.18 0.14 0.17 0.16 0.10 0.76 0.05 MgO

0.10 0.19 0.03 0.15 0.19 0.00 0.07 0.07 0.58 0.74 0.40 0.48 CaO

20.93 20.37 21.70 20.61 21.56 22.66 22.64 22.08 22.34 22.45 23.93 22.92 P 2 O 5 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.16 0.03 1.41 0.00 SrO

0.05 0.04 0.08 0.08 0.06 0.05 0.09 0.08 0.06 0.06 0.05 0.05 Na 2 O

0.02 0.46 0.01 0.01 0.47 0.05 0.54 0.63 0.26 0.01 0.08 0.34 K 2 O

0.00 0.18 0.00 0.04 0.05 0.06 0.10 0.09 0.00 0.04 0.05 0.01 V 2 O 5 0.00 0.00 0.14 0.00 0.02 0.00 0.03 0.02 0.00 0.00 0.00 0.00 SO 3 0.00 0.14 0.00 0.00 0.04 0.00 0.08 0.08 0.00 0.06 0.04 0.06

NiO 0.00 0.00 0.00 0.00 0.04 0.00 0.00 0.00 0.00 0.00 0.00 0.00 CuO

0.00 0.00 0.00 0.00 0.02 0.00 0.00 0.02 0.00 0.00 0.00 0.01 La 2 O 3 2.53 3.70 2.16 2.76 2.40 1.92 0.00 0.00 0.00 0.00 0.00 0.00 CeO 2 0.50 1.52 1.15 2.41 1.34 1.10 0.00 0.00 0.00 0.00 0.00 0.00 PrO 2 0.06 0.38 0.24 0.33 0.50 0.62 0.00 0.00 0.00 0.00 0.00 0.00 Nd 2 O 3 0.06 0.32 0.04 0.16 0.49 0.42 0.00 0.00 0.00 0.00 0.00 0.00 Sm 2 O 3 0.00 0.51 0.00 0.14 0.73 0.26 0.00 0.00 0.00 0.00 0.00 0.00 Eu 2 O 3 0.00 0.24 0.00 0.14 0.25 0.14 0.00 0.00 0.00 0.00 0.00 0.00 Gd 2 O 3 0.00 0.17 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

H 2 O 3.67 3.55 4.83 3.62 3.94 4.33 2.61 4.32 4.15 3.09 3.65 4.12 Total

3.15 6.84 3.59 5.94 5.71 4.46 0.00 0.00 0.00 0.00 0.00 0.00 Number of ions on the basis of 12.5 oxygen atoms

8.000 8.000 8.000 Fe/(Fe + Al)

0.34 0.36 0.33 0.35 0.32 0.33 0.34 0.33 0.35 0.35 0.44 0.35 a Total iron as FeO.

in Sudan ( Huggins et al., 1977 ) and hydrothermal titanian andradite the clinopyroxenes from the Astamal skarn. Based on these data, the (TiO 2 = 2.0%) from Galore Creek in the Canadian Cordillera ( Russell

clinopyroxene has a hedenbergitic composition ( Figs. 9 and 10 ), in et al., 1999; Micko et al., 2014 ). The latter skarns are associated with al-

which hedenbergite values are slightly decreased and ferrosalite and wol- kali plutonic and volcanic rocks. Fig. 8 shows similarities and differences

lastonite proportions are slightly increased from the core to the rim between Astamal skarn garnets and the Cananea, Mission, Yerington,

( Fig. 9 ).

Bingham and Twin Butte skarn deposits. Similar to the garnets, the clinopyroxene is generally homogenous and has particularly high Fe/Fe + Mg ratios and low TiO 2 , MnO and Cr 2 O 3

5.2. Clinopyroxene ( Table 3 ). As shown in Fig. 11 , there is significantly lower contents of Ti (0.008–0.087 a.p.f.u), Cr (N0.001 a.p.f.u), Na (N0.036 a.p.f.u) and Al

Clinopyroxene has been intensely altered and is rarely observed as an (N0.007 a.p.f.u.) indicating that the clinopyroxene is of metamorphic ori- intact mineral. Therefore, only 4 relatively unaltered clinopyroxenes were

gin. A wide variety of ionic substitutions occur in the clinopyroxene. The able to be analyzed in this study. Table 2 presents microprobe analyses of

substitution of Fe and Mg by Mn is common because diopside–

92 S. Baghban et al. / Ore Geology Reviews 68 (2015) 79–96

Fig. 13. Bivariate diagram showing positive correlation between almandine and grossular values in the Astamal skarn garnets.

Fig. 11. Al vs. (Ti + Cr + Na) displaying metamorphic genesis for clinopyroxene crystals from the Astamal skarn ( Berger et al., 2005 ).

At the Astamal skarn retrograde alteration is well developed. This hedenbergite–johannsenite form a complete solid-solution field ( Deer

has caused most of the garnets to be extremely fractured and altered et al., 1992 ). However, because there is a very low abundance of Mg

to secondary minerals, both in the rims and the cores. Similarly almost and Mn at the Astamal skarn, hedenbergite is the dominant

all of the clinopyroxene has been replaced by hydrous retrograde calc- clinopyroxene.

silicate minerals. Brecciation of prograde mineral assemblages is com- The clinopyroxene composition at the Astamal skarn is as follows:

mon in skarn deposits (e.g., Einaudi et al., 1981; Meinert, 1992; Na

Ciobanu and Cook, 2004 ) and is likely the mechanism responsible for

initiation of pervasive infiltration of fluids causing the retrograde alter- Si (1.96–2.04) O 6 .

Ca 2+

(0.91–1.15) Mg (0–0.11) Zn (0–0.1) Fe (0.59–0.96) Fe (0–0.09) Al (0–0.01)

ation ( Gaspar et al., 2008 ).

Meinert et al. (2005) suggested that garnet/pyroxene ratio will in- Higher contents of Ca and Si in the clinopyroxenes rim is may be

crease toward the causative intrusion and the appearance (color and due to the intense retrograde alteration. Fig. 12 shows similarities

texture) of garnet and pyroxene will also change, moreover, the distal and differences between Astamal skarn clinopyroxenes and the

zones will be more hedenbergitic and johannsenitic than the proximal clinopyroxenes from the Cananea, Yerington, Bingham and Twin

zones. The Astamal clinopyroxene has a predominantly hedenbergite Butte skarn deposits.

composition which supports the assertion that it is a distal skarn. The Mn/Fe ratio and Zn content of pyroxene vary according type of

5.3. Epidote metal concentrated in the deposit ( Nakano et al., 1991 ). Most pyrox- enes of Cu–Fe skarn deposits are characterized by a low Mn/Fe ratio

Epidote group minerals associated with retrograde alteration are Fe- (b0.1) and a low Zn content (b200 ppm), whereas those of Pb–Zn de- rich with high Fe/(Fe + Al) ratio (between 0.32 and 0.44). Representa-

posits have a high Mn/Fe ratio and a high Zn content (N200 ppm) tive X-ray microanalysis data are presented in Table 3 .

( Nakano et al., 1991 ). Clinopyroxene crystals of the Astamal skarn have a negligible Mn/Fe ratio (up to 0.05) but high Zn content (up to 1044 ppm). Large amounts of Fe 2+ are found in hedenbergite and diva-

6. Discussion and interpretation lent cations such as Zn 2+ substitute for Fe 2+ . The lack of any Zn min- erals (such as sphalerite) in the deposit suggests that the Zn content

The Astamal skarn differs from the other skarn deposits associated of the hydrothermal fluids changed significantly between the time of with the Qara-Dagh batholith (except for Pahnavar Fe skarn;

clinopyroxene formation (prograde stage) and the time of sulfide depo- Mokhtari, 2012 ) in that it is a distal skarn. The mineralogy and the

sition (retrograde stage).

chemical compositions of skarn minerals in distal skarns tend to be con- The garnets are predominantly an andradite–grossular solid solu- trolled by the chemical composition of the host rocks rather than the as-

tion. The growth is characterized by a continuous outward decrease of sociated intrusive body.

X Adr and X Fe [= Fe/(Fe + Mg)] and an attendant increase of X Grs and

Fig. 12. Comparison of the Fe/(Fe + Al) mole fraction of clinopyroxene from the Astamal skarn and some other skarn deposits. Adopted from Einaudi (1982) .

S. Baghban et al. / Ore Geology Reviews 68 (2015) 79–96

Fig. 14. T–XCO 2 diagram in P fluid = 1000 bars (Adapted from Sweeney (1980) ). Gr 100 is pure Ca–Al garnet (grossular), Ad is pure Ca–Fe garnet (andradite) and Gr 20 –Gr 80 are andradite– grossular solid solution.

X Alm . Einaudi and Burt (1982) suggested that almandine content hedenbergite to actinolite, calcite and quartz is concomitant with in- increases with increasing substitution of Al for Fe 3+ (increasing Grs/

creasing XCO 2 ( Uchida, 1983 ). Therefore, XCO 2 values are relatively Adr; Fig. 13 ). The observed increase in almandine content toward the

high in both of prograde and retrograde alteration stages. Microprobe edge of the crystals could also be due to an increase in the Fe 2+ activity

analyses show the compositional variation of Astamal garnets are and a decrease in ƒO 2 during the garnet growth. Similar to Zn, Cu and Ni

Adr 94.3–65.4 Grs 21.9–2.7 Alm 11.1–0.2 . Thus, based on T-XCO 2 diagram are considerably enriched in the garnets. However, chalcopyrite and

( Fig. 14 ), the garnets are believed to have been formed at temperatures massive Ni-containing magnetite are also present in the Astamal

between 500 and 560 °C.

skarn; therefore, it is concluded that Cu and Ni were presented in both Garnet coexists with clinopyroxene in most of skarn deposits ( Rose of the early stage garnet-forming and the late stage ore-bearing hydro-

and Burt, 1979 ). Due to a lack of replacement textures between garnet thermal fluids.

and clinopyroxene (also, garnet and actinolite which is formed by alter- Garnet growth reflects the interplay of heating and fluid infiltration

ation of clinopyroxene), particularly low Mg/Mg + Fe ratios, low con- (e.g. Jamtveit, 1997; Meinert et al., 2005 ) of the host rocks. Coarse-

tent of TiO 2 , MnO and Cr 2 O 3 and enrichment of Fe and Ca content in grained garnets in skarn deposits are usually formed in the periphery of the associated intrusive bodies. However, Einaudi et al. (1981) sug- gested that dimensions of garnet grains are more associated with fluid flow rate and equilibrium condition between fluid flow and wall rock. Relatively high rates of fluid flow result in supersaturation of elements such as Fe, Mg, Al and Ca. In the magmatic hydrothermal environments situated distal to the intrusive bodies (such as the Astamal skarn), fluid

movement is relatively limited and consequently the degree of super- saturation is limited. In such conditions, crystals grow slowly and form very coarse-grained crystals.

Most of the garnets in the Astamal skarn are isotropic and there is no specific zoning along the growth lines. The formation of the hornfelsic cap rock ( Fig. 4

c) has probably prevented the penetration cold meteoric fluids into the system which may have resulted in formation of homogenous garnets. This is because the hornfelsic cap may have acted as a blanket that prevented the loss of heat from the system. Alternatively, the lack of significant zoning in the garnets can be explained by an increased dif- fusion rate within the mineral at high temperatures ( Dietvorst, 1982 ). The presence of high temperature minerals such as wollastonite and cor- dierite in the study area confirm that the garnets were formed at temper- atures greater than 550 °C, making this a viable reason for the observed homogeneity.

The stability field of the andradite-grossular solid solution at

XCO 2 = 0 to 1 is shown in Fig. 14 . In most skarns, XCO 2 vary from initial

values of 0.2 to later values of 0.05 ( Einaudi et al., 1981 ). However, these values can be slightly higher when wollastonite is present. Andradite is

Fig. 15. Comparison of the oxygen fugacity in formation of various kinds of skarn deposits hedenbergite, calcite and magnetite and the later decomposition of

stable in relatively low XCO 2 conditions, but its decomposition to

( Zhang and Saxena, 1991 ) displaying a moderate oxidation condition for Fe skarns.

94 S. Baghban et al. / Ore Geology Reviews 68 (2015) 79–96

Fig. 16. Log ƒO 2 –T diagram for andradite + quartz bulk composition + excess H 2 O at (a) P fluid = 2000 bars and (b) P fluid = 500 bars ( Liou, 1974 ).

garnet and clinopyroxene in the Astamel skarn we conclude that these secondary mineral assemblage, such as epidote, calcite and

quartz. The majority of the clinopyroxene has been replaced by moderate and temperature was relatively high ( Burt, 1972 ). Based on

two minerals have grown simultaneously. This suggests that ƒO 2 was

hydrous retrograde calc-silicate minerals such as tremolite, ac- these data, the formation temperature of andradite can also be consid-

tinolite and chlorite.

ered as the same for hedenbergite. Zhang and Saxena (1991) suggested (4) The garnets are generally isotropic with a narrow range of varia- that the oxidation capacity of skarns where an andradite and

tion in composition along the growth lines and are in andradite– hedenbergite assemblage is present decrease in the order Cu, Pb–Zn,

grossular solid-solution; containing less than 15 mol% (alman- Fe, Mo and W (Sn) ore deposits ( Fig. 15 ). Therefore, iron skarns are

dine + spessartine + pyrope) with high Fe/(Fe + Al) ratios. formed at moderate and/or relatively low oxidation conditions. Accord-

The almandine content increases with increasing Grs/Adr ratio ing to Rose and Burt (1979) , the andradite and hedenbergite end-

(substitution of Al for Fe 3+ ) in these garnets. Relatively high members coexist over only a limited range of oxygen fugacities, but

amounts of Cu and Ni within the garnets together with the pres- substitutions of Al 3+ for Fe 3+ and Mg 2+ and Mn 2+ for Fe 2+ allow the

ence of chalcopyrite and Ni-bearing magnetite in the deposit two minerals to exist over a wide range of oxygen fugacities in natural

suggest that Cu and Ni were presented in both of prograde and systems. As mentioned above, Al 3+ /Fe 3+ substitutions occurred in the

retrograde hydrothermal fluids. These minerals are generally Astamal skarn. The stability fields of these two minerals are illustrated

coarse to very coarse-grained, which suggests a low rate fluid in Fig. 16 . This diagram shows the andradite and the hedenbergite can

flow and non-supersaturation of Fe, Mg, Al and Ca in the Astamal

be stable and coexist at temperatures between 490 and 560 °C and

skarn.

LogƒO 2 = −16 to −31. (5) Similar to the garnets, the homogenous, hedenbergitic Compared with other skarn deposits in the Qara-Dagh area (and also

clinopyroxene has high Fe/Fe + Mg ratios and is poor in TiO 2 , many skarn deposits in the world), the Astamal skarn is unique in its

MnO and Cr 2 O 3 . The clinopyroxene also has elevated Zn content high level of LREE enrichment. This type of mineralization is found as

(up to 1044 ppm). This is in agreement with the structure of LREE-bearing epidote and allanite which are known to be major hosts

hedenbergite (with high Fe 2+ ), which can substitute divalent for trace and rare earth elements. Jansson and Allen (2013) also attrib-

cations such as Zn 2+ for Fe 2+ . A lack of Zn minerals (such as uted high values of LREEs in the Smaltarmossen iron skarn to the pres-

sphalerite) in the Astamal skarn, suggests the Zn content of hy- ence of allanite. The iron ore also displays LREE-enrichment. Whole-

drothermal fluids decreased significantly from the time of rock analyses show ( Table 4 ) that La, Ce and Pr contents are enriched

clinopyroxene formation to the later sulfide deposition phase. in the southern iron ore body, with values up to 676 ppm, 566 ppm

These high Zn in clinopyroxene concentrations are characteris- and 175 ppm, respectively. The other iron ore bodies have lower LREE

tics of pyroxenes from Pb–Zn distal skarn deposits. concentrations in comparison.

(6) The garnet and the clinopyroxene coexist and have grown simul- taneously. This is suggested by their compositional similarities

7. Conclusions and lack of any replacement texture between them. These min- erals can be stable and coexist at temperatures between 490

and 560 °C and LogƒO 2 = − 16 to − 31, suggesting that these (1) The Astamal Fe-LREE skarn is a distal skarn deposit. It was