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Granite- and gabbrodiorite-associated skarn deposits of NW Iran
Ore Geology Reviews 20 (2002) 127 – 138 www.elsevier.com/locate/oregeorev
Granite- and gabbrodiorite-associated skarn deposits of NW Iran A. Karimzadeh Somarin *, M. Moayyed Department of Geology, Faculty of Natural Sciences, University of Tabriz, Tabriz, Iran Received 17 December 2001; accepted 16 May 2002
Abstract Field and laboratory studies show that there are two types of skarn deposits in NW Iran: granite-associated (type I) and gabbrodiorite-associated (type II). Granite-associated deposits are accompanied by Cu and Fe mineralisation, whereas Mn and Fe are the main ore metals in gabbrodiorite-associated skarn deposits. There are some differences in the mineralogy of these skarn deposits. Bixbyite, piemontite and Cr-bearing garnet are found only in gabbrodiorite-associated skarns, whereas idocrase occurs only in granite-associated deposits. Type II skarns show exoskarn features, whereas some type I skarns have developed endoskarn as well. Evidence of boiling of hydrothermal fluid can be seen in both types and seems to be a common mechanism of mineral deposition. Gabbrodiorite-associated skarns show higher fO2 than granite-associated deposits. Based on mineralogical and textural evidence, mineralisation in both groups has started from about 550 jC. Early formed anhydrous minerals have begun to be replaced by hydrous minerals from about 400 jC. It seems that due to low fluid content in the gabbrodioritic magma, heated meteoritic water in the surrounding volcanoclastic and tuffaceous rocks was the main source of hydrothermal solution in the gabbrodiorite-associated skarn system. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Iran; Granite; Gabbrodiorite; Skarn
1. Introduction The most common plutons associated with skarn deposits are notably hydrous intermediate to felsic intermediate rocks (e.g. quartz diorites and granodiorites). In comparison, dry basic and ultrabasic rocks (e.g. gabbros and norites) rarely yield skarn deposits. There are many skarn deposits in NW Iran. Based on the associated pluton, these deposits can be divided into two types: (1) Granite-associated, Cu-bearing skarn deposits (type I), which include Anjerd (Cu), Mazraeh (Cu– *
Corresponding author. E-mail address: [email protected] (A. Karimzadeh Somarin).
Fe), Sungun (Cu), Zand Abad (Cu – Mo), Javan Shakhe (Cu), Gudul (Cu), all in Ahar region, and Kharvana (Cu, Au) in Kharvana area, Pahnavar (Cu) in Jolfa area and Pasveh (Cu) in Piranshahr area. (2) Gabbrodiorite-associated, Mn-bearing skarn deposits (type II), which include Tikmeh Dash (Fe – Mn) and small deposits in Bostan Abad area. This study was designed to (1) evaluate and clarify the main differences of two types of skarn deposits in NW Iran and (2) investigate geochemical features of the Tikmeh Dash gabbrodioritic pluton which has formed a rare Mn-bearing skarn. Mazraeh Cu – Fe skarn and Sungun Cu skarn porphyry deposits from type I and Tikmeh Dash Fe –Mn skarn from type II are discussed in this paper. Mazraeh and Sungun
0169-1368/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 6 8 ( 0 2 ) 0 0 0 6 8 - 9
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Fig. 1. Schematic geological map of NW Iran (modified after Aghanabati, 1996) showing the locations of the three studied deposits.
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Fig. 2. Detailed geological map of the Sungun area.
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deposits are currently mined, but there is no mining activity at Tikmeh Dash.
2. Geological setting Both Sungun and Mazraeh skarn deposits occur in the Ahar region (Fig. 1). This region is part of the Alpine – Euroasia metallogenic belt that extends from Greece to NW Iran. The porphyry, skarn and veintype mineralisations in this region are attributed to the Pyrenean orogenic phase (Bazin and Hubner, 1969). The Sungun Cu porphyry skarn deposit is the result of intrusion of an Oligomiocene pluton into the Oligocene acid-intermediate volcanic rocks, Eocene terrigenous rocks and Cretaceous limestone (Fig. 2). Average composition of the intrusive rocks is mon-
zodiorite. This deposit contains > 500 Mtons of sulfide reserves grading 0.76% Cu and f0.01% Mo (Hezarkhani and Williamz-Jones, 1998). Mazraeh skarn is located about 30 km southeast of Sungun. This skarn has formed in the contact of the Mazraeh granodioritic pluton (Oligocene in age) with the Cretaceous limestone (Fig. 3). The Mazraeh deposit contains about 400 000 tons sulfide reserves grading 1.2% Cu (unpublished data of the Iranian Copper). Based on the Chappell and White (1974) classification, both Mazraeh and Sungun plutons are type I. Tikmeh Dash skarn is located about 160 km south of the Sungun deposit in Bostan Abad area (Fig. 1). In this area, intrusion of the Tikmeh Dash pluton (Oligocene in age) into the Eocene volcanic and volcanoclastic rocks has formed some skarn deposits (Fig. 4).
Fig. 3. Detailed geological map of the Mazraeh area.
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Fig. 4. Detailed geological map of the Tikmeh Dash area.
The largest deposit is the Tikmeh Dash Fe – Mn skarn. The composition of this pluton ranges from gabbro to diorite. The Tikmeh Dash skarn is located in the Bostan Abad – Mianeh Mn metallogenetic province where many syngenetic and epigenetic Mn deposits are found.
3. Skarn mineralogy 3.1. Sungun Cu skarn porphyry Sungun deposit shows features of both porphyry (e.g. alteration type and pattern) and skarn deposits. The characteristic Cu skarn mineralogy has been developed in a 150-m zone in the contact of a subvolcanic pluton with limestone. A notable macroscopic feature of the skarn zone is the high abundance of garnet and idocrase, which gives a dark brown to red-brown color to this zone. The garnet-rich zone is called garnet skarn herein. Locally, epidote and pyroxene contents are high, forming zones which
are called epidote skarn and pyroxene skarn, respectively. Malachite and azurite have covered the skarn zone. Field evidence shows that Sungun skarn is an exoskarn (i.e. has developed in the carbonate wall rock). Pyroxene skarn is dark gray to grayish green in handspecimen and contains magnetite and sulfides. Chalcopyrite occurs as disseminated grains and the chalcopyrite-to-pyrite ratio is higher in pyroxene skarn than in garnet skarn. Garnet veinlets have cut the pyroxene zone suggesting later formation of garnet skarn. Similar paragenetic relationships have been reported from Palo Verde and Twin Butts (Einaudi, 1982b). In the later stages of skarn formation, pyroxene has been replaced by epidote, chlorite, calcite, magnetite, quartz, pyrite, actinolite and tremolite. Garnet skarn occurs in the vicinity of the Sungun pluton. Locally, in some handspecimens, it forms layers alternating with pyroxene skarn suggesting stratigraphic control of skarn formation (Meinert, personal communication). There are two types of garnet:
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(1) dark red-brown andradite-rich garnet with up to 15% Fe, accompanying ore minerals and (2) light brown-yellow grossular-rich garnet with up to 7.6% Fe (Karimzadeh Somarin, in press). Similar to pyroxene, garnet has been replaced by paragenetically young minerals such as calcite, quartz and magnetite. Epidote skarn overlaps both pyroxene and garnet skarns. Commonly, epidote, along with other latestage minerals, replaces pyroxene and garnet, and is replaced by chlorite, calcite and quartz. Sulfides commonly occur in this skarn. Scapolite and sphene are accessory minerals. The formation of idocrase, actinolite and chlorite postdates that of epidote. The ore minerals in the Sungun deposit include pyrite, chalcopyrite, bornite and magnetite. Locally, hematite has replaced magnetite. Paragenetically, sulfide formation postdates that of oxides. 3.2. Mazraeh Cu –Fe skarn Skarn mineralogy of Mazraeh is similar to that of Sungun. The main difference is that Mazraeh shows both exoskarn and endoskarn (see below) features, whereas endoskarn has not developed at Sungun. Pyroxene skarn forms a small zone (about a few meters) in which pyroxene has been intensively replaced by garnet, calcite, quartz and young hydrous minerals. There are two types of garnet in garnet skarn: red-brown garnet with up to 18.4% Fe, associated with ore minerals and yellow-green garnet with up to 8.3% Fe in garnetised limestone (Karimzadeh Somarin, in press). In some deposits (e.g. Carr Fork; Atkinson and Einaudi, 1978), garnetisation has mainly occurred along the hydrothermal veins and, approaching the pluton, this alteration has been developed within a few meters of the veins and preferentially in marble. At Mazraeh, garnetisation has occurred in both marble and pluton and the gradual alteration of granodiorite to garnet can easily be seen over a distance of a meter. In places where there is a direct contact between granodiorite and garnet skarn zone, there is doubt as to whether this zone is exoskarn (i.e. the result of replacement of limestone by garnet) or endoskarn (i.e. the result of replacement of granodiorite by garnet). It seems that garnet skarn is mainly exoskarn and this is suggested by the presence of a layering structure inherited from the original limestone. However, locally, garnet skarn
has been developed by replacement of granodiorite minerals such as feldspars by garnet. The ore minerals in garnet skarn include magnetite, hematite, pyrite, chalcopyrite, bornite, covellite and chalcocite. Paragenetically, magnetite is the earliest ore mineral and has partly formed due to garnet alteration. Epidote skarn is composed of epidote, chlorite, actinolite, tremolite, scapolite, sphene, calcite, quartz, pyrite, chalcopyrite and magnetite. 3.3. Tikmeh Dash Fe –Mn skarn Because of the presence of Mn as a dominant ore metal and the mafic to intermediate composition of the intrusive rock, Tikmeh Dash skarn is one of the rare skarn types in the world. It is the result of intrusion of a gabbrodioritic pluton into carbonatecemented volcanoclastic rocks (mainly tuff). This skarn has developed mainly in the surrounding rocks, and only late-stage veins of epidote, calcite, tremolite and actinolite are found in the pluton. Therefore, it is considered as an exoskarn. Tikmeh Dash skarn is similar in mineralogy to other skarns of NW Iran. However, it differs in its geochemical conditions of formation and its ore minerals and their abundances. Pyroxene is rare partly due to intensive replacement by late-stage minerals such as chlorite, calcite, quartz and hematite. Based on optical properties, pyroxene composition is variable in the diopside – hedenbergite series. It is possible that some pyroxenes contain Mn (Abrecht, 1985, 1988). Garnet skarn is seen mainly in the vicinity of the gabbrodioritic pluton. There are some garnets in the epidote skarn away (about 600 m) from the pluton. There is a notable change in garnet color from dark red-brown (andradite-rich with up to 11% Fe) in the vicinity of the pluton to light brown-yellow (grossular-rich with up to 7.1% Fe) in the epidote skarn (Karimzadeh Somarin, in press). Trace element composition of garnets from Tikmeh Dash differs from that of Sungun and Mazraeh. Tikmeh Dash garnet contains much less Cu than garnets of Sungun and Mazraeh (Karimzadeh Somarin, in press). Other minerals of garnet skarn include epidote, calcite, quartz, biotite, chlorite, pyroxene, pyrite, chalcopyrite, hematite and bixbyite. Most of these minerals (except pyroxene) are paragenetically younger than garnet. Commonly, replace-
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Table 1 Chemical composition of the Tikmeh Dash gabbrodiorite 1
2
3
4
5
6
7
8
9
10
SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total
58.91 0.66 15.88 2.16 4.19 0.09 4.19 7.75 3.44 1.02 0.14 98.43
58.15 0.69 16.29 2.19 4.95 0.13 4.74 6.78 2.71 1.88 0.15 98.66
58.82 0.73 15.80 2.23 4.68 0.15 4.14 7.39 3.07 1.91 0.15 99.07
55.75 0.84 15.75 2.34 5.64 0.17 4.14 8.39 3.00 1.67 0.24 97.93
54.91 0.84 16.95 2.34 5.62 0.18 3.99 8.26 3.77 1.26 0.20 98.32
54.72 0.80 17.40 3.44 4.45 0.17 3.79 8.10 3.58 1.20 0.19 98.46
56.39 0.78 15.93 3.30 4.53 0.16 3.89 7.92 2.82 1.57 0.22 98.12
53.90 0.83 16.61 3.43 4.91 0.15 4.80 8.84 2.74 1.26 0.20 98.34
58.33 0.66 14.84 3.06 4.02 0.12 4.55 6.69 2.60 1.80 0.14 97.36
55.86 0.83 15.76 2.33 5.65 0.02 4.41 8.04 3.04 1.67 0.24 97.85
Q Or Ab An Di Hy Mt Ilm Ap
13.00 6.03 29.11 24.88 10.23 10.47 3.13 1.25 0.33
12.40 11.11 22.93 26.73 4.81 15.85 3.17 1.31 0.35
12.00 11.29 25.98 23.69 9.74 11.40 3.23 1.39 0.35
8.43 9.87 25.39 24.58 12.67 11.46 3.39 1.60 0.57
4.77 7.45 31.90 25.60 11.52 11.62 3.39 1.60 0.47
7.25 7.09 30.29 27.86 8.95 9.45 4.99 1.51 0.45
12.26 9.28 23.86 26.17 9.42 9.74 4.78 1.48 0.53
8.15 7.45 23.19 29.30 10.68 11.90 4.97 1.58 0.47
15.67 10.64 22.00 23.50 7.04 11.94 4.44 1.25 0.33
8.42 9.87 25.72 24.42 11.34 12.56 3.38 1.58 0.57
Cl S Rb Sr V Y Zr Zn Ba Co Cr Cu Nb Ni Pb
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
438 18 32 296 112 15 105 42 142 18 57 8 9 11 3
780 11 40 288 110 17 96 112 372 22 66 62 10 8 11
1003 26 39 285 112 16 104 93 353 18 85 33 9 21 7
1300 31 41 382 121 16 105 77 488 17 93 37 11 15 4
700 22 24 382 134 17 88 85 465 19 82 18 8 19 8
704 21 26 379 132 15 86 86 468 19 80 21 7 20 8
1333 33 39 387 126 16 106 79 491 18 94 39 11 16 3
89 12 28 365 150 14 94 80 468 23 63 31 8 15 10
800 12 42 285 112 16 95 110 363 21 68 66 9 8 10
1212 34 41 390 129 16 98 288 507 17 98 55 7 15 11
ND ND 45 440 200 ND 100 ND 300 45 200 100 ND 160 8
ND ND 100 800 100 ND 260 ND 650 10 50 35 ND 55 15
130 260 90 375 135 33 165 70 425 25 100 55 20 75 13
70 58 220 250 17 13 210 45 1220 2.4 20 13 24 1 48
200 123 21 190 264 25 105 86 160 47 114 110 9.5 76 7.8
Oxides and trace elements are in wt.% and ppm, respectively. Trace element data for mafic and intermediate rocks, crust, granite and basalt are from Krauskopf and Bird (1995). ND: no data; analyses determined by XRF; (1) Ti, (2) T2, (3) T3, (4) T5, (5) T6, (6) T7, (7) T8, (8) T9, (9) T10, (10) T4, (11) mafic rock, (12) intermediate rocks, (13) crust, (14) granite and (15) basalt.
ment of andradite by calcite+quartz+hematite can be seen in thin section. Garnet color in thin section varies from colorless to pale brown. However, there are some greenish grains of possibly uvarovite [Ca3Cr2 (SiO4)3] or uvarovite – andradite solid solutions (e.g. Cr-bearing andradite).
Epidote skarn has been developed away from the pluton within the volcanoclastic wall rock. Locally, piemontite (Mn-bearing epidote) occurs in this skarn. Spessartite [Mn3Al2 (SiO4)3] has been reported from some other Mn-bearing deposits (Varentsov, 1996); however, it is not seen at Tikmeh Dash.
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Bixbyite and hematite are the main ore minerals. Bixbyite occurs as flakes up to a few centimeters across. Hematite mainly occurs as specularite. These minerals form ore masses of up to a few meters in length and a meter in thickness. Quartz crystals (up to 4 cm in length) are found in these masses. Chalcopyrite is disseminated in the epidote skarn and it is commonly replaced by digenite and malachite.
4. Geochemistry of Tikmeh Dash pluton Chemical composition of Tikmeh Dash pluton is shown in Table 1. SiO2 content varies between 53.90% and 58.91% (i.e. in wt.%). TiO2 content is low (maximum 0.84%). The analysed samples show normative quartz from 4.77% to 13%. Normative hypersthene content varies from 9.45 to 18.85. Tikmeh Dash pluton plots in the fields of gabbro and diorite in the classification diagram of De La Roche et al. (1986) (Fig. 5). The average content of MnO is 0.13%, which is close to that in intermediate rocks (0.15%; Krauskopf and Bird, 1995). This shows that despite forming a
Mn-bearing skarn, the Tikmeh Dash pluton is not enriched in Mn. It seems that the surrounding volcanoclastic and tuffaceous rocks were the main source of Mn. This is supported by the presence of epigenetic and syngenetic Mn occurrences in these rocks at other localities. The Tikmeh Dash pluton has Rb, Zr, Co, Cr, Cu, Nb, Pb and, partly, Ba concentrations similar to those of mafic or intermediate igneous rocks (Table 1), whereas V and Y contents are more similar to those of crust and granite, respectively. It seems that high Cl contents (up to 1333 ppm) of Tikmeh Dash pluton is notable. It is much higher than average Cl content of crust, granite and basalt. S concentration varies from 11 to 34 ppm, which is much lower than its average concentration in crust, granite and basalt. Sr concentration ranges from 285 to 390 ppm, which is lower than its average concentration in mafic and intermediate rocks. This possibly suggests that Tikmeh Dash pluton has originated from a Sr-depleted source. Zn concentrations range widely from 42 to 288 ppm. Ni contents range between 8 and 21 ppm, which is much lower than its average values in both mafic and intermediate rocks. Ni contents are even lower than
Fig. 5. Classification diagram of De La Roche et al. (1986) in which Tikmeh Dash data plots in the fields of gabbro and diorite.
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its average value in the crust, possibly suggesting a Ni-depleted source for Tikmeh Dash pluton.
5. Fluid inclusion studies Morphological studies of fluid inclusions were carried out on several minerals of the studied deposits. The results are discussed below. 5.1. Fluid inclusions of Sungun deposit There are three types of fluid inclusions in garnet: (1) primary fluid inclusions, two phase (liquid+ vapor), up to 3 Am in size; (2) pseudosecondary fluid inclusions, two phases (liquid+vapor), up to 4 Am in size; (3) secondary fluid inclusions, two phases (liquid+vapor), up to 2 Am in size. Garnet mineralisation has occurred at an early stage at a temperature range of 320 – 550 jC and salinity of up to 60 wt.% equivalent NaCl (Hezarkhani and Williamz-Jones, 1998). There are some primary two-phase (liquid+vapor) fluid inclusions in idocrase and calcite. In epidote, there are some primary fluid inclusions up to 15 Am in which the vapor volume varies from 1/4 to 3/4 of the fluid inclusion volume. Occasionally, four-phase (liquid+vapor+halite+sylvite) fluid inclusions are seen in epidote, suggesting a salinity of >21 wt.% equivalent NaCl (Roedder, 1984). It seems that hydrothermal solution has boiled during the late stage of alteration (i.e. during epidote and hydrous minerals formation). At this stage, formation temperature has decreased to 240 jC (Hezarkhani and Williamz-Jones, 1998). The boiling has caused (1) variation in vapor size of the primary fluid inclusions and (2) increasing salinity from <21 to >21 wt.% equivalent NaCl at late stage. 5.2. Fluid inclusions of Mazraeh deposit Irregular, randomly distributed, primary two-phase (liquid+vapor) fluid inclusions can be seen in garnet. The vapor volume is about 1/3 of the fluid inclusion
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volume, and the size of fluid inclusions reaches 7 Am. In one sample, multiphase (vapor+liquid+sylvite+halite) fluid inclusions were seen in pyroxene, suggesting high salinity of hydrothermal solution at an early stage of pyroxene (and possibly garnet) formation. Similar high salinities have been reported from other skarn deposits (Atkinson and Einaudi, 1978; Ahmad and Rose, 1980; Einaudi, 1982a). Two-phase (vapor+ liquid) primary and pseudosecondary fluid inclusions are common in late-stage calcite, which suggests that salinity has decreased during the late stages of mineralisation. Homogenisation temperature of mineralised quartz veins and skarn minerals varies from 312 to 470 jC. Salinity ranges from 5.9 to 63 wt.% equivalent NaCl (Mollai, 1993). 5.3. Fluid inclusions of Tikmeh Dash skarn Two types of fluid inclusions are seen in garnet: (1) two-phase (vapor+liquid) primary fluid inclusions; vapor size is 1/2 to 1/4 of the fluid inclusion volume; (2) four-phase (vapor+liquid+halite+sylvite) primary fluid inclusions. Two-phase (vapor+liquid) and three-phase (vapor+ liquid+halite) primary fluid inclusions were found in quartz crystals filling fractures in garnets. Change in the size of vapor volume and coexistence of two- and four-phase fluid inclusions are thought to be the result of the boiling of hydrothermal solution during garnet formation at an early stage. The presence of two-phase (vapor+liquid) fluid inclusions in late-stage minerals (e.g. calcite and epidote) suggests decreasing salinity at this stage.
6. Geochemical formation conditions Since all studied minerals lie in the system Ca – Fe– Si– C – O –H, Fig. 6 is used to determine probable geochemical conditions of skarn formation. It should be mentioned that this diagram is based on 0.5 kbar pressure and XCO2=0.1 [i.e. CO2/(CO2+H2O)=0.1] (Einaudi, 1982b). These conditions may not be exactly the same as those at the studied deposits, but
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Fig. 6. Temperature – log fO2 diagram at 500 bar and XCO2=0.1 for the system Ca – Fe – Si – C – O – H (modified after Einaudi, 1982a). Ad=andradite, Cc=calcite, Fa=fayalite, Hd=hedenbergite, Hm=hematite, Mt=magnetite, Qz=quartz, Sd=siderite and Wo=wollastonite.
the presence of porphyry texture and subvolcanic features in the studied plutons suggests similar pressure. Also, the presence of calcareous wall rock (limestone at Sungun and Mazraeh and carbonatecemented volcanoclastic rocks at Tikmeh Dash) suggests XCO2f0.1. Hedenbergite, relative to its alteration products, is stable at lower fO2 (Fig. 6). It is replaced by andradite+quartz+calcite at temperatures higher than 385 – 430 jC (depending on fO2). It is replaced by calcite+quartz+magnetite at lower temperatures. In the studied deposits, hedenbergite has been mainly replaced by the latter assemblage. Therefore, the temperature of this alteration is considered to be about 400 jC. Andradite shows high stability field from T=400 to 700 jC and log fO2= 15 to 25. It is replaced by wollastonite+magnetite at T>700 jC and by hedenbergite+wollastonite at T>550 jC (depending on fO2) where quartz is available. Such alterations
were not found at the studied skarns, suggesting a formation temperature of <550 jC. The main difference of the Tikmeh Dash skarn conditions from the Mazraeh and Sungun skarns is fO2. As mentioned above, formation temperatures of these skarns are similar. Therefore, it can be deduced that due to high abundances of hematite and absence of magnetite at Tikmeh Dash (in contrast to the studied Cu-bearing skarns), fO2 was higher at Tikmeh Dash relative to those at Mazraeh and Sungun. Replacement of andradite by hematite+ quartz+calcite at Tikmeh Dash and by magnetite+ quartz+calcite at Sungun and Mazraeh suggests: (1) this alteration has occurred at a temperature of about 400 jC and (2) fO2 was higher at Tikmeh Dash not only during oxide mineralisation but also during garnet alteration.
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7. Discussion Skarns are generally associated with intermediate to felsic intermediate plutons. There are two reasons: (1) These magmas commonly contain higher concentrations of skarn ore metals than other rock types and can be the source of these metals. (2) They can evolve more volatile matter by which ore materials are transferred from the source magma to the wall rock. On this basis, although gabbrodioritic magma can act as a source of ore metals, since it cannot develop high fluid volume, it is not suitable to produce skarn mineralisation. However, at Tikmeh Dash, skarn mineralisation has occurred at the contact of a gabbrodioritic pluton with volcanoclastic and tuffaceous rocks. It seems that these wall rocks had an important role in the formation of the Tikmeh Dash skarn, The reasons are follows. (1) Due to high permeability and porosity, the wall rocks could have contained a large volume of meteoric water which formed a fluid circulation cell upon intrusion of the Tikmeh Dash magma. This fluid could have acted as the hydrothermal solution at Tikmeh Dash. This may be supported by (a) the exoskarn feature of the Tikmeh Dash skarn and (b) occurrence of late-stage hydrous minerals in the epidote zone away from the pluton. However, isotope data of dD and d18O are needed to prove the source of hydrothermal fluid. (2) Due to their calcareous composition, these rocks had a main role in precipitation of skarn and ore minerals.
8. Conclusion Although gabbrodioritic plutons generally are not suitable for skarn mineralisation, they can form skarns under special circumstances. Tikmeh Dash skarn is a rare example of this type of skarn deposit. It seems that calcareous volcanoclastic and tuffaceous wall rocks had a significant role in the formation of this skarn. This deposit differs from granite-associated skarns in the dominant ore metals, fO2 and, partly,
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mineralogy. In both types, skarn formation has started from about 550 jC and the alteration of anhydrous minerals and formation of hydrous minerals have begun from about 400 jC.
Acknowledgements This contribution is based on field and laboratory studies carried out at the University of Tabriz. Funding for this project was provided by the Research Office at the University of Tabriz, and we wish to acknowledge the generous support from all the staff of this office. Dr. N. Stephenson is thanked for proofreading and valuable comments. The manuscript benefited greatly from reviews by L. Meinert, H. Fo¨rster and Y. Zhao. G. Hosainzadeh and N. Mosaiebzadeh are thanked for their technical assistance.
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and mineralization in the Sungun porphyry copper deposit, Iran: evidence from fluid inclusions and stable isotopes. Econ. Geol. 93, 651 – 670. Karimzadeh Somarin, A., in press. Garnet composition as an indicator of Cu mineralization: evidence from skarn deposits of NW Iran. Econ. Geol. Krauskopf, K.B., Bird, D.K., 1995. Introduction to Geochemistry McGraw-Hill, Singapore, 647 pp.
Mollai, H., 1993. Petrochemistry and genesis of the granodiorite and associated iron – copper skarn deposit of Mazraeh, Ahar, East Azarbaijan, Iran. Unpublished PhD thesis. Roedder, E., 1984. Fluid inclusions. Mineral. Soc. Am. Rev. Mineral. 12, 644 pp. Varentsov, I.M., 1996. Manganese Ores of Supergene Zone: Geochemistry of Formation. Kluwer Academic Publishing, Dordrecht, The Netherlands, 342 pp.
Granite- and gabbrodiorite-associated skarn deposits of NW Iran A. Karimzadeh Somarin *, M. Moayyed Department of Geology, Faculty of Natural Sciences, University of Tabriz, Tabriz, Iran Received 17 December 2001; accepted 16 May 2002
Abstract Field and laboratory studies show that there are two types of skarn deposits in NW Iran: granite-associated (type I) and gabbrodiorite-associated (type II). Granite-associated deposits are accompanied by Cu and Fe mineralisation, whereas Mn and Fe are the main ore metals in gabbrodiorite-associated skarn deposits. There are some differences in the mineralogy of these skarn deposits. Bixbyite, piemontite and Cr-bearing garnet are found only in gabbrodiorite-associated skarns, whereas idocrase occurs only in granite-associated deposits. Type II skarns show exoskarn features, whereas some type I skarns have developed endoskarn as well. Evidence of boiling of hydrothermal fluid can be seen in both types and seems to be a common mechanism of mineral deposition. Gabbrodiorite-associated skarns show higher fO2 than granite-associated deposits. Based on mineralogical and textural evidence, mineralisation in both groups has started from about 550 jC. Early formed anhydrous minerals have begun to be replaced by hydrous minerals from about 400 jC. It seems that due to low fluid content in the gabbrodioritic magma, heated meteoritic water in the surrounding volcanoclastic and tuffaceous rocks was the main source of hydrothermal solution in the gabbrodiorite-associated skarn system. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Iran; Granite; Gabbrodiorite; Skarn
1. Introduction The most common plutons associated with skarn deposits are notably hydrous intermediate to felsic intermediate rocks (e.g. quartz diorites and granodiorites). In comparison, dry basic and ultrabasic rocks (e.g. gabbros and norites) rarely yield skarn deposits. There are many skarn deposits in NW Iran. Based on the associated pluton, these deposits can be divided into two types: (1) Granite-associated, Cu-bearing skarn deposits (type I), which include Anjerd (Cu), Mazraeh (Cu– *
Corresponding author. E-mail address: [email protected] (A. Karimzadeh Somarin).
Fe), Sungun (Cu), Zand Abad (Cu – Mo), Javan Shakhe (Cu), Gudul (Cu), all in Ahar region, and Kharvana (Cu, Au) in Kharvana area, Pahnavar (Cu) in Jolfa area and Pasveh (Cu) in Piranshahr area. (2) Gabbrodiorite-associated, Mn-bearing skarn deposits (type II), which include Tikmeh Dash (Fe – Mn) and small deposits in Bostan Abad area. This study was designed to (1) evaluate and clarify the main differences of two types of skarn deposits in NW Iran and (2) investigate geochemical features of the Tikmeh Dash gabbrodioritic pluton which has formed a rare Mn-bearing skarn. Mazraeh Cu – Fe skarn and Sungun Cu skarn porphyry deposits from type I and Tikmeh Dash Fe –Mn skarn from type II are discussed in this paper. Mazraeh and Sungun
0169-1368/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 6 8 ( 0 2 ) 0 0 0 6 8 - 9
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Fig. 1. Schematic geological map of NW Iran (modified after Aghanabati, 1996) showing the locations of the three studied deposits.
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Fig. 2. Detailed geological map of the Sungun area.
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deposits are currently mined, but there is no mining activity at Tikmeh Dash.
2. Geological setting Both Sungun and Mazraeh skarn deposits occur in the Ahar region (Fig. 1). This region is part of the Alpine – Euroasia metallogenic belt that extends from Greece to NW Iran. The porphyry, skarn and veintype mineralisations in this region are attributed to the Pyrenean orogenic phase (Bazin and Hubner, 1969). The Sungun Cu porphyry skarn deposit is the result of intrusion of an Oligomiocene pluton into the Oligocene acid-intermediate volcanic rocks, Eocene terrigenous rocks and Cretaceous limestone (Fig. 2). Average composition of the intrusive rocks is mon-
zodiorite. This deposit contains > 500 Mtons of sulfide reserves grading 0.76% Cu and f0.01% Mo (Hezarkhani and Williamz-Jones, 1998). Mazraeh skarn is located about 30 km southeast of Sungun. This skarn has formed in the contact of the Mazraeh granodioritic pluton (Oligocene in age) with the Cretaceous limestone (Fig. 3). The Mazraeh deposit contains about 400 000 tons sulfide reserves grading 1.2% Cu (unpublished data of the Iranian Copper). Based on the Chappell and White (1974) classification, both Mazraeh and Sungun plutons are type I. Tikmeh Dash skarn is located about 160 km south of the Sungun deposit in Bostan Abad area (Fig. 1). In this area, intrusion of the Tikmeh Dash pluton (Oligocene in age) into the Eocene volcanic and volcanoclastic rocks has formed some skarn deposits (Fig. 4).
Fig. 3. Detailed geological map of the Mazraeh area.
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Fig. 4. Detailed geological map of the Tikmeh Dash area.
The largest deposit is the Tikmeh Dash Fe – Mn skarn. The composition of this pluton ranges from gabbro to diorite. The Tikmeh Dash skarn is located in the Bostan Abad – Mianeh Mn metallogenetic province where many syngenetic and epigenetic Mn deposits are found.
3. Skarn mineralogy 3.1. Sungun Cu skarn porphyry Sungun deposit shows features of both porphyry (e.g. alteration type and pattern) and skarn deposits. The characteristic Cu skarn mineralogy has been developed in a 150-m zone in the contact of a subvolcanic pluton with limestone. A notable macroscopic feature of the skarn zone is the high abundance of garnet and idocrase, which gives a dark brown to red-brown color to this zone. The garnet-rich zone is called garnet skarn herein. Locally, epidote and pyroxene contents are high, forming zones which
are called epidote skarn and pyroxene skarn, respectively. Malachite and azurite have covered the skarn zone. Field evidence shows that Sungun skarn is an exoskarn (i.e. has developed in the carbonate wall rock). Pyroxene skarn is dark gray to grayish green in handspecimen and contains magnetite and sulfides. Chalcopyrite occurs as disseminated grains and the chalcopyrite-to-pyrite ratio is higher in pyroxene skarn than in garnet skarn. Garnet veinlets have cut the pyroxene zone suggesting later formation of garnet skarn. Similar paragenetic relationships have been reported from Palo Verde and Twin Butts (Einaudi, 1982b). In the later stages of skarn formation, pyroxene has been replaced by epidote, chlorite, calcite, magnetite, quartz, pyrite, actinolite and tremolite. Garnet skarn occurs in the vicinity of the Sungun pluton. Locally, in some handspecimens, it forms layers alternating with pyroxene skarn suggesting stratigraphic control of skarn formation (Meinert, personal communication). There are two types of garnet:
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(1) dark red-brown andradite-rich garnet with up to 15% Fe, accompanying ore minerals and (2) light brown-yellow grossular-rich garnet with up to 7.6% Fe (Karimzadeh Somarin, in press). Similar to pyroxene, garnet has been replaced by paragenetically young minerals such as calcite, quartz and magnetite. Epidote skarn overlaps both pyroxene and garnet skarns. Commonly, epidote, along with other latestage minerals, replaces pyroxene and garnet, and is replaced by chlorite, calcite and quartz. Sulfides commonly occur in this skarn. Scapolite and sphene are accessory minerals. The formation of idocrase, actinolite and chlorite postdates that of epidote. The ore minerals in the Sungun deposit include pyrite, chalcopyrite, bornite and magnetite. Locally, hematite has replaced magnetite. Paragenetically, sulfide formation postdates that of oxides. 3.2. Mazraeh Cu –Fe skarn Skarn mineralogy of Mazraeh is similar to that of Sungun. The main difference is that Mazraeh shows both exoskarn and endoskarn (see below) features, whereas endoskarn has not developed at Sungun. Pyroxene skarn forms a small zone (about a few meters) in which pyroxene has been intensively replaced by garnet, calcite, quartz and young hydrous minerals. There are two types of garnet in garnet skarn: red-brown garnet with up to 18.4% Fe, associated with ore minerals and yellow-green garnet with up to 8.3% Fe in garnetised limestone (Karimzadeh Somarin, in press). In some deposits (e.g. Carr Fork; Atkinson and Einaudi, 1978), garnetisation has mainly occurred along the hydrothermal veins and, approaching the pluton, this alteration has been developed within a few meters of the veins and preferentially in marble. At Mazraeh, garnetisation has occurred in both marble and pluton and the gradual alteration of granodiorite to garnet can easily be seen over a distance of a meter. In places where there is a direct contact between granodiorite and garnet skarn zone, there is doubt as to whether this zone is exoskarn (i.e. the result of replacement of limestone by garnet) or endoskarn (i.e. the result of replacement of granodiorite by garnet). It seems that garnet skarn is mainly exoskarn and this is suggested by the presence of a layering structure inherited from the original limestone. However, locally, garnet skarn
has been developed by replacement of granodiorite minerals such as feldspars by garnet. The ore minerals in garnet skarn include magnetite, hematite, pyrite, chalcopyrite, bornite, covellite and chalcocite. Paragenetically, magnetite is the earliest ore mineral and has partly formed due to garnet alteration. Epidote skarn is composed of epidote, chlorite, actinolite, tremolite, scapolite, sphene, calcite, quartz, pyrite, chalcopyrite and magnetite. 3.3. Tikmeh Dash Fe –Mn skarn Because of the presence of Mn as a dominant ore metal and the mafic to intermediate composition of the intrusive rock, Tikmeh Dash skarn is one of the rare skarn types in the world. It is the result of intrusion of a gabbrodioritic pluton into carbonatecemented volcanoclastic rocks (mainly tuff). This skarn has developed mainly in the surrounding rocks, and only late-stage veins of epidote, calcite, tremolite and actinolite are found in the pluton. Therefore, it is considered as an exoskarn. Tikmeh Dash skarn is similar in mineralogy to other skarns of NW Iran. However, it differs in its geochemical conditions of formation and its ore minerals and their abundances. Pyroxene is rare partly due to intensive replacement by late-stage minerals such as chlorite, calcite, quartz and hematite. Based on optical properties, pyroxene composition is variable in the diopside – hedenbergite series. It is possible that some pyroxenes contain Mn (Abrecht, 1985, 1988). Garnet skarn is seen mainly in the vicinity of the gabbrodioritic pluton. There are some garnets in the epidote skarn away (about 600 m) from the pluton. There is a notable change in garnet color from dark red-brown (andradite-rich with up to 11% Fe) in the vicinity of the pluton to light brown-yellow (grossular-rich with up to 7.1% Fe) in the epidote skarn (Karimzadeh Somarin, in press). Trace element composition of garnets from Tikmeh Dash differs from that of Sungun and Mazraeh. Tikmeh Dash garnet contains much less Cu than garnets of Sungun and Mazraeh (Karimzadeh Somarin, in press). Other minerals of garnet skarn include epidote, calcite, quartz, biotite, chlorite, pyroxene, pyrite, chalcopyrite, hematite and bixbyite. Most of these minerals (except pyroxene) are paragenetically younger than garnet. Commonly, replace-
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Table 1 Chemical composition of the Tikmeh Dash gabbrodiorite 1
2
3
4
5
6
7
8
9
10
SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total
58.91 0.66 15.88 2.16 4.19 0.09 4.19 7.75 3.44 1.02 0.14 98.43
58.15 0.69 16.29 2.19 4.95 0.13 4.74 6.78 2.71 1.88 0.15 98.66
58.82 0.73 15.80 2.23 4.68 0.15 4.14 7.39 3.07 1.91 0.15 99.07
55.75 0.84 15.75 2.34 5.64 0.17 4.14 8.39 3.00 1.67 0.24 97.93
54.91 0.84 16.95 2.34 5.62 0.18 3.99 8.26 3.77 1.26 0.20 98.32
54.72 0.80 17.40 3.44 4.45 0.17 3.79 8.10 3.58 1.20 0.19 98.46
56.39 0.78 15.93 3.30 4.53 0.16 3.89 7.92 2.82 1.57 0.22 98.12
53.90 0.83 16.61 3.43 4.91 0.15 4.80 8.84 2.74 1.26 0.20 98.34
58.33 0.66 14.84 3.06 4.02 0.12 4.55 6.69 2.60 1.80 0.14 97.36
55.86 0.83 15.76 2.33 5.65 0.02 4.41 8.04 3.04 1.67 0.24 97.85
Q Or Ab An Di Hy Mt Ilm Ap
13.00 6.03 29.11 24.88 10.23 10.47 3.13 1.25 0.33
12.40 11.11 22.93 26.73 4.81 15.85 3.17 1.31 0.35
12.00 11.29 25.98 23.69 9.74 11.40 3.23 1.39 0.35
8.43 9.87 25.39 24.58 12.67 11.46 3.39 1.60 0.57
4.77 7.45 31.90 25.60 11.52 11.62 3.39 1.60 0.47
7.25 7.09 30.29 27.86 8.95 9.45 4.99 1.51 0.45
12.26 9.28 23.86 26.17 9.42 9.74 4.78 1.48 0.53
8.15 7.45 23.19 29.30 10.68 11.90 4.97 1.58 0.47
15.67 10.64 22.00 23.50 7.04 11.94 4.44 1.25 0.33
8.42 9.87 25.72 24.42 11.34 12.56 3.38 1.58 0.57
Cl S Rb Sr V Y Zr Zn Ba Co Cr Cu Nb Ni Pb
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
438 18 32 296 112 15 105 42 142 18 57 8 9 11 3
780 11 40 288 110 17 96 112 372 22 66 62 10 8 11
1003 26 39 285 112 16 104 93 353 18 85 33 9 21 7
1300 31 41 382 121 16 105 77 488 17 93 37 11 15 4
700 22 24 382 134 17 88 85 465 19 82 18 8 19 8
704 21 26 379 132 15 86 86 468 19 80 21 7 20 8
1333 33 39 387 126 16 106 79 491 18 94 39 11 16 3
89 12 28 365 150 14 94 80 468 23 63 31 8 15 10
800 12 42 285 112 16 95 110 363 21 68 66 9 8 10
1212 34 41 390 129 16 98 288 507 17 98 55 7 15 11
ND ND 45 440 200 ND 100 ND 300 45 200 100 ND 160 8
ND ND 100 800 100 ND 260 ND 650 10 50 35 ND 55 15
130 260 90 375 135 33 165 70 425 25 100 55 20 75 13
70 58 220 250 17 13 210 45 1220 2.4 20 13 24 1 48
200 123 21 190 264 25 105 86 160 47 114 110 9.5 76 7.8
Oxides and trace elements are in wt.% and ppm, respectively. Trace element data for mafic and intermediate rocks, crust, granite and basalt are from Krauskopf and Bird (1995). ND: no data; analyses determined by XRF; (1) Ti, (2) T2, (3) T3, (4) T5, (5) T6, (6) T7, (7) T8, (8) T9, (9) T10, (10) T4, (11) mafic rock, (12) intermediate rocks, (13) crust, (14) granite and (15) basalt.
ment of andradite by calcite+quartz+hematite can be seen in thin section. Garnet color in thin section varies from colorless to pale brown. However, there are some greenish grains of possibly uvarovite [Ca3Cr2 (SiO4)3] or uvarovite – andradite solid solutions (e.g. Cr-bearing andradite).
Epidote skarn has been developed away from the pluton within the volcanoclastic wall rock. Locally, piemontite (Mn-bearing epidote) occurs in this skarn. Spessartite [Mn3Al2 (SiO4)3] has been reported from some other Mn-bearing deposits (Varentsov, 1996); however, it is not seen at Tikmeh Dash.
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Bixbyite and hematite are the main ore minerals. Bixbyite occurs as flakes up to a few centimeters across. Hematite mainly occurs as specularite. These minerals form ore masses of up to a few meters in length and a meter in thickness. Quartz crystals (up to 4 cm in length) are found in these masses. Chalcopyrite is disseminated in the epidote skarn and it is commonly replaced by digenite and malachite.
4. Geochemistry of Tikmeh Dash pluton Chemical composition of Tikmeh Dash pluton is shown in Table 1. SiO2 content varies between 53.90% and 58.91% (i.e. in wt.%). TiO2 content is low (maximum 0.84%). The analysed samples show normative quartz from 4.77% to 13%. Normative hypersthene content varies from 9.45 to 18.85. Tikmeh Dash pluton plots in the fields of gabbro and diorite in the classification diagram of De La Roche et al. (1986) (Fig. 5). The average content of MnO is 0.13%, which is close to that in intermediate rocks (0.15%; Krauskopf and Bird, 1995). This shows that despite forming a
Mn-bearing skarn, the Tikmeh Dash pluton is not enriched in Mn. It seems that the surrounding volcanoclastic and tuffaceous rocks were the main source of Mn. This is supported by the presence of epigenetic and syngenetic Mn occurrences in these rocks at other localities. The Tikmeh Dash pluton has Rb, Zr, Co, Cr, Cu, Nb, Pb and, partly, Ba concentrations similar to those of mafic or intermediate igneous rocks (Table 1), whereas V and Y contents are more similar to those of crust and granite, respectively. It seems that high Cl contents (up to 1333 ppm) of Tikmeh Dash pluton is notable. It is much higher than average Cl content of crust, granite and basalt. S concentration varies from 11 to 34 ppm, which is much lower than its average concentration in crust, granite and basalt. Sr concentration ranges from 285 to 390 ppm, which is lower than its average concentration in mafic and intermediate rocks. This possibly suggests that Tikmeh Dash pluton has originated from a Sr-depleted source. Zn concentrations range widely from 42 to 288 ppm. Ni contents range between 8 and 21 ppm, which is much lower than its average values in both mafic and intermediate rocks. Ni contents are even lower than
Fig. 5. Classification diagram of De La Roche et al. (1986) in which Tikmeh Dash data plots in the fields of gabbro and diorite.
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its average value in the crust, possibly suggesting a Ni-depleted source for Tikmeh Dash pluton.
5. Fluid inclusion studies Morphological studies of fluid inclusions were carried out on several minerals of the studied deposits. The results are discussed below. 5.1. Fluid inclusions of Sungun deposit There are three types of fluid inclusions in garnet: (1) primary fluid inclusions, two phase (liquid+ vapor), up to 3 Am in size; (2) pseudosecondary fluid inclusions, two phases (liquid+vapor), up to 4 Am in size; (3) secondary fluid inclusions, two phases (liquid+vapor), up to 2 Am in size. Garnet mineralisation has occurred at an early stage at a temperature range of 320 – 550 jC and salinity of up to 60 wt.% equivalent NaCl (Hezarkhani and Williamz-Jones, 1998). There are some primary two-phase (liquid+vapor) fluid inclusions in idocrase and calcite. In epidote, there are some primary fluid inclusions up to 15 Am in which the vapor volume varies from 1/4 to 3/4 of the fluid inclusion volume. Occasionally, four-phase (liquid+vapor+halite+sylvite) fluid inclusions are seen in epidote, suggesting a salinity of >21 wt.% equivalent NaCl (Roedder, 1984). It seems that hydrothermal solution has boiled during the late stage of alteration (i.e. during epidote and hydrous minerals formation). At this stage, formation temperature has decreased to 240 jC (Hezarkhani and Williamz-Jones, 1998). The boiling has caused (1) variation in vapor size of the primary fluid inclusions and (2) increasing salinity from <21 to >21 wt.% equivalent NaCl at late stage. 5.2. Fluid inclusions of Mazraeh deposit Irregular, randomly distributed, primary two-phase (liquid+vapor) fluid inclusions can be seen in garnet. The vapor volume is about 1/3 of the fluid inclusion
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volume, and the size of fluid inclusions reaches 7 Am. In one sample, multiphase (vapor+liquid+sylvite+halite) fluid inclusions were seen in pyroxene, suggesting high salinity of hydrothermal solution at an early stage of pyroxene (and possibly garnet) formation. Similar high salinities have been reported from other skarn deposits (Atkinson and Einaudi, 1978; Ahmad and Rose, 1980; Einaudi, 1982a). Two-phase (vapor+ liquid) primary and pseudosecondary fluid inclusions are common in late-stage calcite, which suggests that salinity has decreased during the late stages of mineralisation. Homogenisation temperature of mineralised quartz veins and skarn minerals varies from 312 to 470 jC. Salinity ranges from 5.9 to 63 wt.% equivalent NaCl (Mollai, 1993). 5.3. Fluid inclusions of Tikmeh Dash skarn Two types of fluid inclusions are seen in garnet: (1) two-phase (vapor+liquid) primary fluid inclusions; vapor size is 1/2 to 1/4 of the fluid inclusion volume; (2) four-phase (vapor+liquid+halite+sylvite) primary fluid inclusions. Two-phase (vapor+liquid) and three-phase (vapor+ liquid+halite) primary fluid inclusions were found in quartz crystals filling fractures in garnets. Change in the size of vapor volume and coexistence of two- and four-phase fluid inclusions are thought to be the result of the boiling of hydrothermal solution during garnet formation at an early stage. The presence of two-phase (vapor+liquid) fluid inclusions in late-stage minerals (e.g. calcite and epidote) suggests decreasing salinity at this stage.
6. Geochemical formation conditions Since all studied minerals lie in the system Ca – Fe– Si– C – O –H, Fig. 6 is used to determine probable geochemical conditions of skarn formation. It should be mentioned that this diagram is based on 0.5 kbar pressure and XCO2=0.1 [i.e. CO2/(CO2+H2O)=0.1] (Einaudi, 1982b). These conditions may not be exactly the same as those at the studied deposits, but
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Fig. 6. Temperature – log fO2 diagram at 500 bar and XCO2=0.1 for the system Ca – Fe – Si – C – O – H (modified after Einaudi, 1982a). Ad=andradite, Cc=calcite, Fa=fayalite, Hd=hedenbergite, Hm=hematite, Mt=magnetite, Qz=quartz, Sd=siderite and Wo=wollastonite.
the presence of porphyry texture and subvolcanic features in the studied plutons suggests similar pressure. Also, the presence of calcareous wall rock (limestone at Sungun and Mazraeh and carbonatecemented volcanoclastic rocks at Tikmeh Dash) suggests XCO2f0.1. Hedenbergite, relative to its alteration products, is stable at lower fO2 (Fig. 6). It is replaced by andradite+quartz+calcite at temperatures higher than 385 – 430 jC (depending on fO2). It is replaced by calcite+quartz+magnetite at lower temperatures. In the studied deposits, hedenbergite has been mainly replaced by the latter assemblage. Therefore, the temperature of this alteration is considered to be about 400 jC. Andradite shows high stability field from T=400 to 700 jC and log fO2= 15 to 25. It is replaced by wollastonite+magnetite at T>700 jC and by hedenbergite+wollastonite at T>550 jC (depending on fO2) where quartz is available. Such alterations
were not found at the studied skarns, suggesting a formation temperature of <550 jC. The main difference of the Tikmeh Dash skarn conditions from the Mazraeh and Sungun skarns is fO2. As mentioned above, formation temperatures of these skarns are similar. Therefore, it can be deduced that due to high abundances of hematite and absence of magnetite at Tikmeh Dash (in contrast to the studied Cu-bearing skarns), fO2 was higher at Tikmeh Dash relative to those at Mazraeh and Sungun. Replacement of andradite by hematite+ quartz+calcite at Tikmeh Dash and by magnetite+ quartz+calcite at Sungun and Mazraeh suggests: (1) this alteration has occurred at a temperature of about 400 jC and (2) fO2 was higher at Tikmeh Dash not only during oxide mineralisation but also during garnet alteration.
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7. Discussion Skarns are generally associated with intermediate to felsic intermediate plutons. There are two reasons: (1) These magmas commonly contain higher concentrations of skarn ore metals than other rock types and can be the source of these metals. (2) They can evolve more volatile matter by which ore materials are transferred from the source magma to the wall rock. On this basis, although gabbrodioritic magma can act as a source of ore metals, since it cannot develop high fluid volume, it is not suitable to produce skarn mineralisation. However, at Tikmeh Dash, skarn mineralisation has occurred at the contact of a gabbrodioritic pluton with volcanoclastic and tuffaceous rocks. It seems that these wall rocks had an important role in the formation of the Tikmeh Dash skarn, The reasons are follows. (1) Due to high permeability and porosity, the wall rocks could have contained a large volume of meteoric water which formed a fluid circulation cell upon intrusion of the Tikmeh Dash magma. This fluid could have acted as the hydrothermal solution at Tikmeh Dash. This may be supported by (a) the exoskarn feature of the Tikmeh Dash skarn and (b) occurrence of late-stage hydrous minerals in the epidote zone away from the pluton. However, isotope data of dD and d18O are needed to prove the source of hydrothermal fluid. (2) Due to their calcareous composition, these rocks had a main role in precipitation of skarn and ore minerals.
8. Conclusion Although gabbrodioritic plutons generally are not suitable for skarn mineralisation, they can form skarns under special circumstances. Tikmeh Dash skarn is a rare example of this type of skarn deposit. It seems that calcareous volcanoclastic and tuffaceous wall rocks had a significant role in the formation of this skarn. This deposit differs from granite-associated skarns in the dominant ore metals, fO2 and, partly,
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mineralogy. In both types, skarn formation has started from about 550 jC and the alteration of anhydrous minerals and formation of hydrous minerals have begun from about 400 jC.
Acknowledgements This contribution is based on field and laboratory studies carried out at the University of Tabriz. Funding for this project was provided by the Research Office at the University of Tabriz, and we wish to acknowledge the generous support from all the staff of this office. Dr. N. Stephenson is thanked for proofreading and valuable comments. The manuscript benefited greatly from reviews by L. Meinert, H. Fo¨rster and Y. Zhao. G. Hosainzadeh and N. Mosaiebzadeh are thanked for their technical assistance.
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