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The behaviour of trace and rare earth elements (REE) during hydrothermal alteration in the Rangan area (Central Iran)
Journal of Asian Earth Sciences 34 (2009) 123–134
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The behaviour of trace and rare earth elements (REE) during hydrothermal alteration in the Rangan area (Central Iran) A. Parsapoor *, M. Khalili, M.A. Mackizadeh Department of Geology, The University of Isfahan, Isfahan, Iran
a r t i c l e
i n f o
Article history: Received 20 September 2007 Received in revised form 31 March 2008 Accepted 15 April 2008
Keywords: Hydrothermal alteration Trace elements Isfahan Central Iran
a b s t r a c t The rhyolitic dome in the Rangan area has been subjected to hydrothermal alterations by two different systems, (1) A fossil magmatic–hydrothermal system with a powerful thermal engine of a deep monzodioritic magma, (2) An active hydrothermal system dominated by meteoric water. Based on mineralogical and geochemical studies, three different alteration facies have been identified (phyllic, advanced argillic and silicic) with notable differences in REE and other trace elements behaviour. In the phyllic alteration zone with assemblage minerals such as sericite, pyrite, quartz, kaolinite, LREE are relatively depleted whereas HREE are enriched. The advanced argillic zone is identified by the presence of alunite–jarosite and pyrophyllite as well as immobility of LREE and depletion in HREE. In the silicic zone, most of LREE are depleted but HREE patterns are unchanged compared to their fresh rock equivalents. All the REE fractionation ratios (La/Yb)cn, (La/Sm)cn, (Tb/Yb)cn, (Ce/Ce*)cn and (Eu/Eu*)cn are low in the phyllic altered facies. (Eu/Eu*)cn in both advanced and silicic facies is low too. In all alteration zones, high field strength elements (HFSE) (e.g. Ti, Zr, Nb) are depleted whereas transition elements (e.g. V, Cr, Co, Ni, Fe) are enriched. Geochemically speaking, trace and rare earth elements behave highly selective in different facies. Ó 2008 Published by Elsevier Ltd.
1. Introduction Rare earth elements (REE) were often accepted as rather immobile elements, but more recent studies have shown that they can be mobilized by hydrothermal fluid circulation (Alderton et al., 1980; Michard and Albarede, 1986; Palacios et al., 1986). Lately, numerous studies have been carried out on the geochemistry of REE in hydrothermal systems (Lottermoser, 1990; Hopf, 1993; Arribas et al., 1995; Fulignati and Sbrana, 1998; Terakado and Fujitani, 1998; Chang-bock et al., 2002). Felsche and Herrmann (1978) found that most REE are transported in alkaline solutions as carbonate, sulphate or fluorine complexes. Some other workers concluded that the REE mobility is significantly controlled by the availability of complexing ions such as CO3 2 ; PO4 3 ; F ; SO4 2 and Cl, as well as low pH and high rock/water ratio (Michard, 1989; Wood, 1990; Haas et al., 1995). According to Pirajno (1992) and Takahashi et al. (2004) heavy rare earth elements (HREE) form more stable complexes with some ligands and stay in solution longer than light rare earth elements (LREE). Therefore, they tend to concentrate in later products of hydrothermal systems. In this context, some investigators focused on REE behaviour in different alteration facies. Taylor and Fryer * Corresponding author. Tel.: +98 311 7932153; fax: +98 311 7932152. E-mail address: [email protected] (A. Parsapoor). 1367-9120/$ - see front matter Ó 2008 Published by Elsevier Ltd. doi:10.1016/j.jseaes.2008.04.005
(1980) identified multiple-stage hydrothermal altered zones in the porphyry copper deposits in Turkey and stated that REE distribution in unaltered and altered rocks can be taken as an evidence of changing fluid conditions from dominantly magmatic to dominantly meteoric. The works of Taylor and Fryer (1982, 1983) demonstrate that in the potassic alteration of a porphyry system, LREE are relatively immobile whereas HREE are strongly depleted. Also, from propylitic to phyllic alteration or with decreasing K+ activity and increasing H+ metasomatism an overall depletion of REE is noticed. This reduction is more pronounced for LREE than for HREE. In contrast, Terakado and Fujitani (1998) came to the conclusion that under strongly acidic hydrothermal conditions, some REE are retained in the rocks and the alunite in their study area is characterized by LREE enrichment. Fulignati et al. (1999) studied REE distribution in the alteration facies of the magmatic–hydrothermal system of La Fossa Vulcano (Aeolian Islands, Italy) and found remarkable differences in the REE behaviour in different alteration facies. These elements are strongly depleted in silicic and advanced argillic rocks whereas they are relatively immobile in intermediate argillic, phyllic and propylitic zones. Also, a number of studies have been published pertaining to the significance of REE for exploration (Arvanitidis and Richard, 1986; Ganzeyev et al., 1987; Whitford et al., 1988; Qi-Cong and Cong-Qiang, 2002; Weimin et al., 2003). Based on
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these studies, the ore bodies prospective for metal mineralization might have a distinctive REE signature and therefore, they may have some applications for exploration. On this account, it appears that the REE behaviour in an aqueous or hydrothermal environment is complex and no simple rules can be established for their mobility during hydrothermal processes. In the present study, we document the effect of hydrothermal alteration on the rhyolitic – dacitic dome southwest of Ardestan, some 70 Km northeast of Isfahan, Iran (Fig. 1), which is supposed to be an epithermal system, investigate the behaviour of REE and ore forming elements in different alteration facies and determine a tentative pattern for their mobility in the study area. 2. Geological setting According to Stocklin (1968), Rangan area is situated in the Central Iranian tectono-sedimentary unit, forming a part of Tertiary Sahand-Bazman or Uromieh-Dokhtar volcanic belt (Fig. 1). The Laramide orogeny (late Cretaceous) created a regional unconformity at the base of Eocene deposits throughout a vast part of Iran. Following this orogeny, and as a result of extensional movements, active basins formed in which a great thickness of basaltic lava to acidic flows and pyroclastic rocks were deposited. Meanwhile, a series of fracturing and folding events were produced by the Laramide orogeny. Flexural slip and drag folds are the main fold types of the area. The Marbine-Rangan fault, trending NW-SE and crosscutting the main Qom-Zefreh fault (QZF) (Fig. 2), had a significant role in creating the structural framework of the region (Emami and Radfar, 2000). The predominant rock types of the area are andesite, dacite, rhyolite, tuff and ignimbrite with a minor amount of basalt, all related to the Eocene volcanic activity. The intrusive suites are composed of diorite to monzodiorite and have successively intruded into the volcanic rocks during the post- Oligocene period. The oldest exposed rocks in the studied area are quartz sandstones, black shales and yellow to grayish dolomitic limestones of the Triassic period followed by Jurassic shales and lower Cretaceous limestones. Meanwhile, a dome-shaped rhyolitic lava developed during Eocene time. This unit trends NW-SE parallel to the main direction of the QZF and has subjected to hydrothermal alteration. Two different systems have been involved in its formation: (1) a fossil magmatic–hydrothermal system, which is a major contributor to the system. This system has provided a powerful thermal engine
generated by consolidation of a deep monzodioritic magma and has driven hydrothermal circulation which has led to the alteration of dacitic dome during post-Oligocene (Fig. 2). Incidentally, the Jurassic shales and Cretaceous limestones have been locally metamorphosed into hornfels and skarn. (2) An active low-T hydrothermal system evidenced by the hot springs that were responsible for precipitation of calcium carbonate (Travertine) along the western margin of rhyolitic dome (Fig. 2). The tectonic movements and the QZF in particular, have had a major role in the development of an active meteoric system and in the infiltration of meteoric waters into the rhyolitic rocks of the study area. Based on field observations, as well as mineralogical and geochemical studies, three different hydrothermal facies (phyllic, advanced argillic and silicic types) are recognized in the Rangan area. The general features of these zones are briefly described in the following sections. 3. Sampling and analytical methods 3.1. Sample collection A total of 70 rock samples were taken. Several unaltered rocks were collected from fresh surface outcrops in order to determine the nature of the original rocks. Characteristics such as grain size, mineralogy, colour, freshness or alteration of samples were described in the field (Fig. 2). 3.2. Analytical methods The mineral assemblages were analyzed by optical methods in combination with X-ray (powder) diffraction analysis at the Center of Analysis and Test in the University of Isfahan. Several samples of each alteration zone as well as unaltered samples were crushed to less than 200 meshes by using a steel mortar. Major elements were determined by X-Ray Fluorescence Spectrometry (XRF). Sample powders were digested by hydrofluoric and nitric acid (1:1). REE and trace elements were analyzed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) by Amdel Ltd. in Australia. For quality control, the International Standards AMH-1 and OU-3 were used as standard samples. The mineral composition of the advanced argillic mineralogical assemblage was determined using a Cameca SX50 electron microprobe at the University of Oklahoma, Norman (USA). Analytical conditions for the sulphates were 20 kV accelerating voltage, 10 nA beam current and 20 lm defocussed spot size. All elements were counted for 30 s on peak except for Fe (45 s), Mn (45 s) and Sr (60 s). These methods yielded minimum detection levels in the 0.02–0.05 wt% oxide range for all components except for Ba (0.08 wt% BaO), Sr (0.08 wt% SrO) and S (0.08 wt% SO3) calculated at 3-sigma above mean background. The results of the analyses were presented in Tables 1 and 2. 4. Field relations and petrographic studies Based on field studies and their petrographic characteristics, the parent rhyolitic rocks of the study area, have been subjected to post magmatic–hydrothermal alteration processes resulting in phyllic, advanced argillic and silicic facies. 4.1. Field relations
Fig. 1. Geographical map of Iran and study area in the Uromieh-Dokhtar belt.
Different hydrothermally altered zones in the study area display various colours. The phyllic alteration is widespread in the east of the rhyolitic dome unit (Fig. 2). This altered zone is characterized
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Fig. 2. Geological map of the Rangan area (after Emami and Radfar, 2000). The figure also shows the location of the samples and their geological relationships and hydrothermal alteration facies.
by the presence of abundant pyrite, giving a greenish colour to the rocks (Fig. 3a). The advanced argillic alteration zone extends from the south to the north of the dome. The zone varies in colour from creamish to yellow-pea in the southern parts (Figs 2 and 3b) to whitish in the north (Figs 2 and 3c). Whereas vast quantities of iron – bearing minerals are distributed in the southern margin, the northern portion of this zone is essentially associated with clay minerals. The silicic zone, which is dark red in colour (Figs 2 and 3c), runs parallel to the marginal zone of the advanced argillic fa-
cies. A spatial relationship between the different alteration facies and the intrusion mass (diorite to monzodiorite) which is known as the engine of the fossil hydrothermal system in the study area is shown in Fig. 2. 4.2. Petrographic study On the base of optical study, in less altered rocks, quartz and sanidine are the predominant phases. The quartz and alkali feld-
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Table 1 Representative chemical analyses from different hydrothermally altered and unaltered rocks Sample
Unaltered
Phyllic alteration
RUR1
Advance argillitic alteration
Silicic alteration
RSE1
RSE2
RSE3
RAA1
RAA2
RAA3
RAA4
RSI1
RSI2
Major elements (Wt%) 71.98 SiO2 15.32 Al2O3 1.36 Fe2O3 CaO 0.78 0.76 Na2O 6.09 K2O MgO 0.76 MnO 0.018 0.409 TiO2 0.064 P2O5 L.O.I 1.89 Total 99.43
79.94 11.63 1.16 0.15 5.89 0.14 0.09 0.015 0.176 0.049 0.72 99.96
74.15 14.33 1.55 0.03 1.55 5.15 0.67 0.038 0.196 0.04 1.56 99.26
69.82 18.59 1.93 0.02 0.24 5.45 0.77 0.007 0.648 0.047 2.27 99.79
71.95 23.65 0.23 0.2 0.06 0.02 0.07 0.003 0.877 0.042 2.76 99.86
21.06 7.25 48.15 0.13 0.53 5.8 0.06 0.086 0.001 0.226 3.56 86.85
20.03 7.04 49.97 0.16 0.64 7.53 0.01 0.033 0.034 0.37 3.26 89.07
22.09 7.47 42.47 0.5 0.88 8.42 0.01 0.017 0.022 0.286 4.08 86.24
93.42 1.26 2.72 0.4 0.51 0.11 0.7 0.026 0.006 0.022 0.31 99.48
85.82 8.98 1 2.28 0.04 2.66 0.45 0.05 0.438 0.037 2.04 99.75
Trace elements (ppm) S 2836 Cl 110 V 46 Cr 1 Co 4 Ni 10 Cu 30 Zn 65 Rb 242 Sr 723 Y 25 Nb 13 Ba 873 W <1 Pb 478 Th 21 u 6 Zr 240 As 27 Sb <1
244 70 31 1 7 11 4 28 10 700 25 8 8 <1 18 10 5 133 72 <1
5091 13 27 1 2 10 1 21 158 690 20 15 313 <1 24 14 3 124 25 <1
402 547 79 34 2 16 3 15 217 644 71 19 317 <1 17 24 6 292 5 <1
534 10 176 8 5 8 0 11 11 733 29 4 89 <1 18 3 6 125 21 <1
114337 285 41 1 2 10 64 125 36 1110 13 2 966 1 455 93 5 122 537 25
105439 350 73 1 1 16 39 141 86 1620 13 9 959 <1 562 124 4 246 1096 38
125454 212 27 1 1 14 594 166 159 3168 18 8 954 <1 1723 56 5 319 2910 444
430 3337 35 1 7 9 30 17 12 679 6 7 115 <1 12 5 3 110 38 <1
216 12 58 39 5 13 6 13 113 661 29 8 155 <1 16 9 9 134 9 <1
REE (ppm) La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
27.4 33 6.75 24 4.7 1.06 2.26 0.4 1.56 0.31 0.87 0.13 0.8 0.14
25 32.4 2.54 9.21 1.99 0.37 1.16 0.29 1.33 0.34 0.99 0.19 1.16 0.19
25.9 38 5.23 17.2 2.99 0.46 1.47 0.29 1.4 0.36 1.14 0.19 1.5 0.26
32.7 40.1 11.3 44.3 9.67 1.36 4.62 0.88 3.54 0.72 1.86 0.2 1.32 0.16
21 26 3.36 14 3.22 0.81 1.23 0.19 0.8 0.21 0.58 0.09 0.53 0.08
15.6 19.3 2.01 6.5 0.95 0.24 0.52 0.1 0.41 0.1 0.28 0.05< 0.26 0.04
137 167 10.4 19.7 2.63 0.52 2.02 0.29 1.22 0.24 0.75 0.11 0.75 0.13
24.6 31.4 1.77 7.55 2.15 0.68 1.01 0.16 0.54 0.09 0.19 0.05< 0.12 0.02<
13.5 15.9 0.06 0.3 0.11 0.03 0.06 0.03 0.22 0.09 0.43 0.1 0.87 0.18
23.4 35.6 5.9 22.1 3.95 0.63 1.67 0.3 1.25 0.24 0.66 0.08 0.47 0.06
RREE (La/Yb)cn (La/Sm)cn (Tb/Yb)cn (Eu/Eu*)cn (Ce/Ce*)cn Fe/Na
103.38 24.06 3.76 2.27 0.99 0.59 1.68
77.16 15.46 8.1 1.13 0.74 0.99 0.185
96.39 12.4 5.6 0.88 0.67 0.8 0.943
152.73 17.78 2.18 3.03 0.62 0.51 7.58
72.1 28.4 4.21 1.63 1.24 0.76 3.6
46.31 43.01 10.6 1.74 1.04 0.84 85.7
342.76 131.07 33.63 1.76 0.69 1.08 73.66
70.26 146.2 7.38 6.02 1.41 1.66 45.5
31.88 11.51 82 0.15 1.13 4.262 5.03
96.31 15.46 3.8 3.71 0.75 0.74 23.58
spar grains are characterized by rounded margins and frequently gulf corrosion. The gulf texture is possibly formed by the intrusion of basic and younger dioritic magma into the older and felsic rhyolitic one (Fig. 4a and b). Locally, the studied rocks are defined by abundant sericite (Fig. 4c) and large quantities of very fine grained quartz with mosaic texture (Fig. 4d). Massive jarosite along with sericite form the matrix of some rocks (Fig. 4e). Less commonly, the rims of jarosite are replaced by brownish fine grained aggregates of goethite as a consequence of progressive oxidation (Fig. 4f and g).
The alteration facies are characterized by various mineral assemblages. Phyllic alteration facies (QSP) comprises fine grained quartzsericite-pyrite with kaolinite, albite and small amount of K-feldspar. This facies changes to advanced argillic facies that is mostly composed of abundant alunite–jarosite, goethite, pyrophyllite, diaspor, barite and tourmaline (Fig. 4h and i). Due to strong acid sulphate alteration, K- feldspar and pyrite in the host rocks were altered and released K, Al and Fe, facilitating the formation of alunite–jarosite. Amorph silica, owing to the increased H+-metasomatism, was transported upward and deposited over the whole rock (Fig. 4j).
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A. Parsapoor et al. / Journal of Asian Earth Sciences 34 (2009) 123–134 Table 2 Electron microprobe analyses of the alunite–jarosite (advanced argillic facies) Sample oxides (Wt%)
JM1
JM2
JM3
JM4
JM5
JM6
JM7
JM8
JM9
JM10
Al2O3 Fe2O3 MnO MgO Na2O K2O CaO SrO BaO P2O5 SO3 Cl O@Cl Total
7.73 36.12 0.00 0.00 0.09 8.42 0.04 0.73 0.35 2.21 32.09 0.05 0.01 87.83
8.74 35.20 0.02 0.00 0.13 8.52 0.04 0.49 0.29 2.02 31.11 0.05 0.01 86.62
6.90 35.52 0.03 0.00 0.14 8.58 0.02 0.60 0.29 2.04 32.27 0.03 0.01 88.40
7.86 36.16 0.00 0.00 0.15 8.28 0.03 0.53 0.26 2.32 30.46 0.04 0.01 86.09
8.43 35.58 0.01 0.00 0.10 8.61 0.01 0.68 0.27 2.23 31.99 0.05 0.01 87.93
8.43 36.42 0.01 0.01 0.11 7.71 0.03 0.70 0.27 1.94 28.39 0.05 0.01 84.05
9.18 34.88 0.01 0.02 0.15 8.01 0.00 0.41 0.27 1.78 29.70 0.04 0.01 84.05
8.99 34.26 0.01 0.00 0.12 8.62 0.04 0.59 0.29 1.96 32.03 0.07 0.02 86.96
11.56 31.59 0.00 0.00 0.12 8.85 0.02 0.48 0.18 1.67 32.81 0.06 0.01 87.35
16.64 25.53 0.00 0.00 0.21 9.26 0.02 0.47 0.15 1.28 34.59 0.10 0.02 88.23
Base on 8 oxygen ions Al Fe3+ Mn Mg Na K Ca Sr Ba P S Cl Sum
0.730 2.176 0.000 0.000 0.015 0.860 0.004 0.034 0.011 0.150 1.927 0.007 5.905
0.836 2.150 0.002 0.000 0.020 0.882 0.004 0.023 0.009 0.139 1.895 0.007 5.960
0.830 2.117 0.002 0.000 0.021 0.867 0.002 0.028 0.009 0.137 1.918 0.004 5.929
0.761 2.232 0.000 0.000 0.023 0.866 0.002 0.025 0.008 0.161 1.876 0.006 5.956
0.792 2.136 0.001 0.000 0.015 0.876 0.001 0.032 0.008 0.151 1.915 0.007 5.925
0.843 2.326 0.001 0.001 0.018 0.835 0.003 0.034 0.009 0.139 1.808 0.008 6.017
0.903 2.189 0.001 0.002 0.024 0.852 0.000 0.020 0.009 0.126 1.859 0.005 5.985
0.851 2.071 0.00 0.00 0.019 0.884 0.003 0.027 0.009 0.133 1.931 0.009 5.929
1.073 1.873 0.000 0.000 0.019 0.890 0.002 0.022 0.006 0.111 1.940 0.008 5.935
1.485 1.454 0.000 0.000 0.031 0.895 0.002 0.021 0.004 0.082 1.965 0.013 5.940
Fig. 3. Field photographs of (a) phyllic alteration (QSP), (b) advanced argillic alteration (AA) and (c) silicic alteration (SI).
5. Geochemical studies Hydrothermal alteration, according to Pirajno (1992), is a very complex process involving mineralogical, chemical and textural changes resulting from the interaction of hot aqueous fluids with the wall rock. The observed geochemical features in the studied rocks will be discussed at some length below.
In the phyllic alteration, the occurrence of minerals such as sericite and pyrite may be developed by destabilization of feldspars in the presence of H+, OH, K+ and HS. Subsequent to this event, advanced argillic alteration in the Rangan deposits appears to dominate the original rhyolitic rocks, suggesting the prevalence of a high sulphide-acid system in the study area. The extreme acid leaching of the original rocks produced silicic alteration facies
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Fig. 4. Photomicrograhs of sub-volcanic rhyolites showing (a) sanidine and quartz, (b) gulf corrosion in alkali feldspar, sericitic-silicic volcanic rocks, (c) phyllic zone (quartz, sericite, pyrite), (d) fine grained quartz with mosaic texture, jarositic-sericitic volcanics, (e) sericite graded into jarosite, (f) relicts of pyrite, (g) rims of jarosite replaced by the fine grained aggregates of goethite, (h) jarositic volcanic rocks, which wholly composed of yellow-pea microcrystalline jarosite, (i) pyrophyllite as a dominant phase, (j) silicification in silicic alteration.
(Stoffregen, 1987), which usually occurs in the inner and towards the marginal zone of advanced argillic alteration facies of high sulphidation epithermal systems (Hayba et al., 1985; Arribas et al.,
1995). Silicic alteration derives from a nearly total leaching of the original rocks by extremely acid fluids with a pH <2 (Stoffregen, 1987).
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5.1. Major elements The contents of some major elements in different altered and unaltered zones have been determined here in order to investigate their relationship: the SiO2 wt% is high in rhyolite (>71 wt%). Its content in silicic alteration zone is much higher (>93 wt%) than the unaltered zone. The lowest amount of SiO2 is found in the advanced argillic zone with approximately 20 wt%. Except for the advanced argillic samples, the level of K2O decreased from unaltered to altered rocks. The enrichment of K2O in advanced argillic alteration rocks is due to the presence of alunite–jarosite in these rocks. The MgO content decreases from unaltered towards altered zones. From the fresh rock to the advanced argillic alteration, the content of TiO2 decreases from 0.4% to 0.006% (RSI1) (Table 1). The remaining major elements do not show a regular pattern. 5.2. Rare earth and other trace elements In order to investigate the enrichment of light REE (LREE) over heavy REE (HREE) in the studied alteration zones, the (La/Yb)cn ratio was calculated. Fractionation among LREE expressed as (La/ Sm)cn and between middle REE (MREE) and HREE is shown as (Tb/Yb)cn (Table 1). The abundances of these elements were normalized to the chondritic values, according to Sun and McDonough (1989). The geochemical behaviour of REE and their ratios will be discussed in the following sections. 5.2.1. Phyllic alteration As it was noted before, the rocks of phyllic alteration are composed of kaolinite, pyrite, sericite, albite and quartz mineral assemblage (Fig. 5a). The REE ratios (La/Yb)cn, (La/Sm)cn and (Tb/Yb)cn were employed to demonstrate the REE fractionation. These ratios and the values of (Ce/Ce*)cn and (Eu/Eu*)cn are also low for the studied phyllic altered rocks (Fig. 6). Two samples (RSE1, RSE2) of these rocks are relatively depleted in LREE and MREE but are enriched in HREE (Fig. 7a). 5.2.2. Advanced argillic alteration The samples characterized by advanced argillic alteration based on XRD analyses are composed mainly of jarosite (RAA2, RAA3 and RAA4) and pyrophylite (RAA1) (Fig. 5b and c). The (La/Yb)cn and (Tb/Yb)cn ratios in the studied samples are high, whereas the (La/Sm)cn, (Ce/Ce*)cn and (Eu/Eu*)cn are relatively low (Fig. 6). The chondrite-normalized REE patterns (Fig. 7b) indicate that, except for La, Ce and Pr (in sample RAA3), LREE are relatively immobile in the rocks of this zone and depleted in MREE and HREE. This behaviour may be due to the entrance of these elements in the lattice of alunite–jarosite (Arribas et al., 1995; Fulignati et al., 1999). LREE can, in fact, substitute potassium in the large radius cations (A) of the alunite–jarosite formula (Scott, 1987). The formula is written as: [AB3 (XO4)2(OH)6] in which A: Na, U, K, Ag, NH4, Pb, Ca, Ba and S. B: Al, Fe, Cu and Zn. (X): P, S. Large-ion lithophile elements (LILE), such as Rb, Sr and Ba, display very different features. 5.2.3. Silicic alteration The rocks of the silicic alteration facies, owing to the leaching process and mobilization of all elements, are composed of residual silica (Fig. 5d). These rocks tend towards low ratios in terms of their (La/Yb)cn, (Tb/Yb)cn and (Eu/Eu*)cn ratios and high values of (La/Sm)cn and (Ce/Ce*)cn (Fig. 6). The positive (Ce/Ce*)cn anomalies may be due to predominance of Ce+4 over Ce+3 in the studied rocks. Unique among REE are Eu and Ce, which may in certain physicochemical environments occur as Eu+2 and Ce+4. The Eu+2 has essentially the same radius as Ca+2 and therefore, it should occur
Fig. 5. The results of XRD analyses for (a) phyllic alteration (Ab: albite, Ka: kaolinite, Se: sericite, py: pyrite and Qz: quartz), (b and c) advanced argillic alteration (Ja: jarosite, Pp: pyrophyllite and Qz: quartz), (d) silicic alteration (Qz: quartz).
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(La/Sm)Cn
(La/Yb)Cn
Unaltered rocks phyllic alteration
Advanced argillic alteration Silicic alteration 0
25
50
75
100
0
50
100
(Tb/Yb)Cn
150
(Ce/Ce*)cn
Unaltered rocks phyllic alteration
Advanced argillic alteration Silicic alteration 0
1
2
3
4
5
6
7
0
1
2
3
4
5
(Eu/Eu*)cn
Unaltered rocks phyllic alteration
Advanced argillic alteration Silicic alteration 0
0.5
1
1.5
Fig. 6. Variations in REE ratios (La/Sm, La/Yb, Tb/Yb)cn displaying the REE fractionation in the hydrothermal altered rocks facies compared to unaltered equivalent original pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rocks. The (Eu/Eu*)cn and (Ce/Ce*)cn ratios show the variation of the Eu and Ce anomalies respectively, ðEu=Eu ¼ Eu= SmN GdN Þ and ðCe=Ce ¼ Ce= LaN PrN Þ.
naturally in Ca+2 minerals. Under oxidizing conditions Ce+3 is oxidized to Ce+4, which is less soluble than the trivalent REE (Crozaz et al., 2003). In this state Ce+4 precipitates and therefore exhibits a distinctive positive Ce anomaly in altered rocks relative to other REE. The behaviour of REE in silicic altered samples (RSI1 and RSI2), in comparison to other kinds of alteration is different. In this alteration (particularly in RSI1), a strong reduction is observed in the amount of LREE (except for La and Ce) and MREE, but HREE are subjected to less mobility (Fig. 7c). This notable low abundance of REE is readily explained by the high acidity and abundant Cl, F and SO4 2 complexing ions in the hydrothermal fluids (Michard, 1989; Wood, 1990; Lottermoser, 1992). According to Michard (1989) and Lottermoser (1992), the high concentration of Cl in hydrothermal fluid is a good complexing agent for Eu, and may be responsible for negative Eu anomaly. Alternatively, depletion in Eu may be enhanced by dissolution of Eu-enriched minerals (Lewis et al., 1997). A summarizing of the mineralogy and other characteristics of the alteration types are given in Table 3. 5.3. Fe/Na relationship and barium mobility According to Brookins (1987) barite (BaSO4) is the major carrier of barium in sedimentary environments. Barite can dis-
solve under strongly acidic (pH <2) hydrothermal conditions, but it is insoluble over wide Eh-pH range (Fig. 8). He also pointed out that pyrite is decomposed to form Fe+2 under oxidizing conditions but it can be stable over wide pH range under reducing conditions. Based on these observations and using Fe/Na ratio vs. Ba abundances, Eh-pH conditions have been determined for Roseki deposits of Japan (Terakado and Fujitani, 1998). For the purpose of this study, Fe/Na vs. Ba concentration was applied for the K-depleted (K <0.2 wt%) and K-enriched (K >5 wt%) Rangan deposits. A positive correlation is observed between Fe/Na vs. Ba in K-depleted rocks (Fig. 9a), whereas in the K-enriched deposits at constant value of Ba in sericitic (315 ppm) and advanced argillic facies (950 ppm) the Fe/Na ratio increases (Fig. 9b). Uncertainties remain on interactions of these elements in hydrothermal solutions. The behaviour of K+ in a hydrothermal system is function of temperature, composition of rocks being leached and water–mineral reaction. In advanced argillic facies, K+ is fixed in jarosite under strongly acidic conditions (pH <2) (Dutrizac and Jambor, 2000) and in a moderately acidic environment (pH4), it is replaced by Na+. Under relatively high temperature, Na in comparison to K, has more tendency to be fixed in alunite structure (Stoffregen and Cygon, 1990). Based on these observations as well as the composition of the studied alunite–jarosite, it is therefore
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Fig. 7. Chondrite-normalized REE patterns of unaltered rocks (RUR1) and hydrothermally altered products, (a) phyllic alteration (RSE1 and RSE2), (b) advanced argillic alteration (RAA1, RAA2, RAA3 and RAA4), (c) silicic alteration (RSI1 and RSI2).
Table 3 The summarizing table of the REE behaviour in the hydrothermal alteration facies of the Rangan area (Central Iran) Alteration facies
Phyllic
Advanced argillic
Silicic
pH Temperature LREE MREE HREE Dominating complexing agent Mineral assemblages REE bearing mineral phase (La/Yb)cn (La/Sm)cn (Tb/Yb)cn (Ce/Ce*)cn (Eu/Eu*)cn
Near neutral 150–200 °C Depleted Depleted Enrichment HSAlbite, pyrite, sericite, kaolinite, quartz Sericite, kaolinite Low Low Low Low Low
2
<2 <100 °C Strongly depleted Strongly depleted Less mobility SO4 2 , F Amorphous silica None Low High Low High Low
The temperatures and pH data are taken from Gioncada et al. (1995).
likely that the advanced argillic zone in the study area formed under considerably low pH, relatively oxidizing conditions and low temperature. 5.4. The behaviour of the REE and other trace elements during hydrothermal alteration 5.4.1. The isocon method Direct comparison of element concentrations cannot be a reliable indication of element mobility in altered and unaltered rocks. Therefore, the isocon diagram can be applied for this purpose (Grant, 1986). In the present paper, we adopted a constant
volume; therefore, the isocon equation will be CA = (qO/qA) CO and the enrichment and depletion of various elements are given using the equation: ðDC i =C i Þ 100 ¼ ðqA =qO ÞðC Ai =C Oi Þ 1. The geometry of diagram (Fig. 10) is such that the elements below the straight isocon are depleted whereas those above are enriched by the hydrothermal alteration. In the phyllic alteration we obtain that some of the oxides and elements such as CaO, P2O5, S, Cu, Zn, Rb, Ba, Pb, Th, Zr and U are depleted and the remaining components are enriched (Fig. 10a) whereas, in the advanced argillic alteration there is a strong enrichment in most elements (e.g. S, Cl, Sr, Ba, La, Ce and Th). This supports the idea of Terakado and Fujitani (1998) that jarosite may be a good
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Fig. 8. Eh-pH diagram for the systems Fe-S-O-H and Ba-S-O-H-C (after Brookins, 1987). Assumed activities of dissolved species are Fe = 106, S = 103, C = 103. Note the Eh-pH conditions required for the formation of jarosite.
repository for the above- mentioned elements. Also, the concentration of Pb, Zn, Cu and As in the advanced argillic zones is high (Fig. 10b). This feature may be due to the chalcophile nature of these elements as well as sulphur enrichment of this altered facies. During hydrothermal processes, the abundances of Cr and Ni commonly do not vary significantly. In the silicic alteration zone except for SiO2, Fe2O3, CaO, MnO, Co, Cr, Cl, V, Ni, Sr and U, the other elements and oxides show a clear reduction (Fig. 10c). 6. Conclusion Judging from the available analytical data, the following model has been derived to explain the formation of the hydrothermal system identified on the Rangan rhyolitic dome, and the REE behaviour in particular. During post-Oligocene period, the hydrothermal fluids originated from the near-by diorite intrusion related to monzodioritic magmatism, representing fluids which probably separated from the dioritic melt. Such fluids are expected to interact with rhyolitic unit, producing different hydrothermal alteration facies including phyllic, advanced argillic and silicic zones. The phyllic alteration zone is identified by the mineral assemblage of quartz + pyrite + sericite as well as albite, K-feldspar and kaolinite. HREE are immobile, MREE and LREE are relatively depleted in this facies. Such feature can be explained by the presence of quartz which is considered to have fixed HREE in its structure. The major constituents of the advanced argillic facies are alunite–jarosite and pyrophyllite which are accompanied by goethite, barite, tourmaline and quartz. The immobility of LREE along with depletion in MREE and HREE is mainly due to the presence of sec-
Fig. 9. Fe/Na vs. Ba concentration in the K-depleted rocks (a) and K-enriched rocks (b) of the Rangan deposits.
ondary minerals (e.g. alunite–jarosite) which are known as hosts for the LREE enriched phase. A strong LREE depletion is observed in the silicic facies mineralogically composed of silica, barite and gypsum. MREE and HREE are subjected to less mobility. This behaviour may be caused by the entrance of these elements into the silica lattice. The relative immobility of REE in the phyllic facies can be related to the higher pH conditions and lower water/rock ratio. Also, the presence of sericite and quartz in this facies may be a good candidate to fix the REE into their structures. The relevant LREE fractionation, with respect to MREE and HREE in the silicic facies, may be explained by high acidity and abundance of SO4 2 ion in solution. Cl and F can be responsible in REE complexing in acidic condition. Finally, the study of hydrothermal alteration in the Rangan area shows that except for the advanced argillic zone, which displays enrichment in some trace elements (e.g. Th, Pb, Ba, Cu, Zn, As and Sr), significant mineralization is not observed in the altered zones. The available data in the present study show that both leaching and precipitation are major processes for the Rangan deposits and its related alteration facies, and point to the selective nature of some trace elements even under strongly acidic hydrothermal conditions. Also, the results of this work may be useful for better understanding of the surficial alteration mechanism which develops in the high-sulfidation hydrothermal system.
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Fig. 10. Isocon diagrams of the three different alteration facies identified in the superficial hydrothermalised zones of Rangan and histograms showing gains and losses of selected elements during hydrothermal alteration as calculated from isocon diagram method. (a) phyllic, (b) advanced argillic (except for RAA1 sample), (c) silicic alteration. Values of +4 to +3 = 75–100%, +3 to 2 = 50–75%, +2 to +1 = 25–50%, 1 to 0 = 0–25% and 0 = immobile elements. Values of 0 to 1 = losses of 0–25%, 1 to 2 = 25–50%, 2 to 3 = 50–75% and 3 to 4 = 75–100%.
Acknowledgments
References
This paper has greatly benefited from critical comments, suggestions and reviews by Dr. M. Smith, Dr. D. Huston and in particular Dr. P. Fulignati who are warmly thanked for their interest. Also, we appreciate Dr.G. Morgan of the University of Oklahoma, Norman (USA) and the staff of the Center of Analysis and Test of the University of Isfahan who conducted a number of microprobe and XRF chemical analyses.
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Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jaes
The behaviour of trace and rare earth elements (REE) during hydrothermal alteration in the Rangan area (Central Iran) A. Parsapoor *, M. Khalili, M.A. Mackizadeh Department of Geology, The University of Isfahan, Isfahan, Iran
a r t i c l e
i n f o
Article history: Received 20 September 2007 Received in revised form 31 March 2008 Accepted 15 April 2008
Keywords: Hydrothermal alteration Trace elements Isfahan Central Iran
a b s t r a c t The rhyolitic dome in the Rangan area has been subjected to hydrothermal alterations by two different systems, (1) A fossil magmatic–hydrothermal system with a powerful thermal engine of a deep monzodioritic magma, (2) An active hydrothermal system dominated by meteoric water. Based on mineralogical and geochemical studies, three different alteration facies have been identified (phyllic, advanced argillic and silicic) with notable differences in REE and other trace elements behaviour. In the phyllic alteration zone with assemblage minerals such as sericite, pyrite, quartz, kaolinite, LREE are relatively depleted whereas HREE are enriched. The advanced argillic zone is identified by the presence of alunite–jarosite and pyrophyllite as well as immobility of LREE and depletion in HREE. In the silicic zone, most of LREE are depleted but HREE patterns are unchanged compared to their fresh rock equivalents. All the REE fractionation ratios (La/Yb)cn, (La/Sm)cn, (Tb/Yb)cn, (Ce/Ce*)cn and (Eu/Eu*)cn are low in the phyllic altered facies. (Eu/Eu*)cn in both advanced and silicic facies is low too. In all alteration zones, high field strength elements (HFSE) (e.g. Ti, Zr, Nb) are depleted whereas transition elements (e.g. V, Cr, Co, Ni, Fe) are enriched. Geochemically speaking, trace and rare earth elements behave highly selective in different facies. Ó 2008 Published by Elsevier Ltd.
1. Introduction Rare earth elements (REE) were often accepted as rather immobile elements, but more recent studies have shown that they can be mobilized by hydrothermal fluid circulation (Alderton et al., 1980; Michard and Albarede, 1986; Palacios et al., 1986). Lately, numerous studies have been carried out on the geochemistry of REE in hydrothermal systems (Lottermoser, 1990; Hopf, 1993; Arribas et al., 1995; Fulignati and Sbrana, 1998; Terakado and Fujitani, 1998; Chang-bock et al., 2002). Felsche and Herrmann (1978) found that most REE are transported in alkaline solutions as carbonate, sulphate or fluorine complexes. Some other workers concluded that the REE mobility is significantly controlled by the availability of complexing ions such as CO3 2 ; PO4 3 ; F ; SO4 2 and Cl, as well as low pH and high rock/water ratio (Michard, 1989; Wood, 1990; Haas et al., 1995). According to Pirajno (1992) and Takahashi et al. (2004) heavy rare earth elements (HREE) form more stable complexes with some ligands and stay in solution longer than light rare earth elements (LREE). Therefore, they tend to concentrate in later products of hydrothermal systems. In this context, some investigators focused on REE behaviour in different alteration facies. Taylor and Fryer * Corresponding author. Tel.: +98 311 7932153; fax: +98 311 7932152. E-mail address: [email protected] (A. Parsapoor). 1367-9120/$ - see front matter Ó 2008 Published by Elsevier Ltd. doi:10.1016/j.jseaes.2008.04.005
(1980) identified multiple-stage hydrothermal altered zones in the porphyry copper deposits in Turkey and stated that REE distribution in unaltered and altered rocks can be taken as an evidence of changing fluid conditions from dominantly magmatic to dominantly meteoric. The works of Taylor and Fryer (1982, 1983) demonstrate that in the potassic alteration of a porphyry system, LREE are relatively immobile whereas HREE are strongly depleted. Also, from propylitic to phyllic alteration or with decreasing K+ activity and increasing H+ metasomatism an overall depletion of REE is noticed. This reduction is more pronounced for LREE than for HREE. In contrast, Terakado and Fujitani (1998) came to the conclusion that under strongly acidic hydrothermal conditions, some REE are retained in the rocks and the alunite in their study area is characterized by LREE enrichment. Fulignati et al. (1999) studied REE distribution in the alteration facies of the magmatic–hydrothermal system of La Fossa Vulcano (Aeolian Islands, Italy) and found remarkable differences in the REE behaviour in different alteration facies. These elements are strongly depleted in silicic and advanced argillic rocks whereas they are relatively immobile in intermediate argillic, phyllic and propylitic zones. Also, a number of studies have been published pertaining to the significance of REE for exploration (Arvanitidis and Richard, 1986; Ganzeyev et al., 1987; Whitford et al., 1988; Qi-Cong and Cong-Qiang, 2002; Weimin et al., 2003). Based on
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these studies, the ore bodies prospective for metal mineralization might have a distinctive REE signature and therefore, they may have some applications for exploration. On this account, it appears that the REE behaviour in an aqueous or hydrothermal environment is complex and no simple rules can be established for their mobility during hydrothermal processes. In the present study, we document the effect of hydrothermal alteration on the rhyolitic – dacitic dome southwest of Ardestan, some 70 Km northeast of Isfahan, Iran (Fig. 1), which is supposed to be an epithermal system, investigate the behaviour of REE and ore forming elements in different alteration facies and determine a tentative pattern for their mobility in the study area. 2. Geological setting According to Stocklin (1968), Rangan area is situated in the Central Iranian tectono-sedimentary unit, forming a part of Tertiary Sahand-Bazman or Uromieh-Dokhtar volcanic belt (Fig. 1). The Laramide orogeny (late Cretaceous) created a regional unconformity at the base of Eocene deposits throughout a vast part of Iran. Following this orogeny, and as a result of extensional movements, active basins formed in which a great thickness of basaltic lava to acidic flows and pyroclastic rocks were deposited. Meanwhile, a series of fracturing and folding events were produced by the Laramide orogeny. Flexural slip and drag folds are the main fold types of the area. The Marbine-Rangan fault, trending NW-SE and crosscutting the main Qom-Zefreh fault (QZF) (Fig. 2), had a significant role in creating the structural framework of the region (Emami and Radfar, 2000). The predominant rock types of the area are andesite, dacite, rhyolite, tuff and ignimbrite with a minor amount of basalt, all related to the Eocene volcanic activity. The intrusive suites are composed of diorite to monzodiorite and have successively intruded into the volcanic rocks during the post- Oligocene period. The oldest exposed rocks in the studied area are quartz sandstones, black shales and yellow to grayish dolomitic limestones of the Triassic period followed by Jurassic shales and lower Cretaceous limestones. Meanwhile, a dome-shaped rhyolitic lava developed during Eocene time. This unit trends NW-SE parallel to the main direction of the QZF and has subjected to hydrothermal alteration. Two different systems have been involved in its formation: (1) a fossil magmatic–hydrothermal system, which is a major contributor to the system. This system has provided a powerful thermal engine
generated by consolidation of a deep monzodioritic magma and has driven hydrothermal circulation which has led to the alteration of dacitic dome during post-Oligocene (Fig. 2). Incidentally, the Jurassic shales and Cretaceous limestones have been locally metamorphosed into hornfels and skarn. (2) An active low-T hydrothermal system evidenced by the hot springs that were responsible for precipitation of calcium carbonate (Travertine) along the western margin of rhyolitic dome (Fig. 2). The tectonic movements and the QZF in particular, have had a major role in the development of an active meteoric system and in the infiltration of meteoric waters into the rhyolitic rocks of the study area. Based on field observations, as well as mineralogical and geochemical studies, three different hydrothermal facies (phyllic, advanced argillic and silicic types) are recognized in the Rangan area. The general features of these zones are briefly described in the following sections. 3. Sampling and analytical methods 3.1. Sample collection A total of 70 rock samples were taken. Several unaltered rocks were collected from fresh surface outcrops in order to determine the nature of the original rocks. Characteristics such as grain size, mineralogy, colour, freshness or alteration of samples were described in the field (Fig. 2). 3.2. Analytical methods The mineral assemblages were analyzed by optical methods in combination with X-ray (powder) diffraction analysis at the Center of Analysis and Test in the University of Isfahan. Several samples of each alteration zone as well as unaltered samples were crushed to less than 200 meshes by using a steel mortar. Major elements were determined by X-Ray Fluorescence Spectrometry (XRF). Sample powders were digested by hydrofluoric and nitric acid (1:1). REE and trace elements were analyzed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) by Amdel Ltd. in Australia. For quality control, the International Standards AMH-1 and OU-3 were used as standard samples. The mineral composition of the advanced argillic mineralogical assemblage was determined using a Cameca SX50 electron microprobe at the University of Oklahoma, Norman (USA). Analytical conditions for the sulphates were 20 kV accelerating voltage, 10 nA beam current and 20 lm defocussed spot size. All elements were counted for 30 s on peak except for Fe (45 s), Mn (45 s) and Sr (60 s). These methods yielded minimum detection levels in the 0.02–0.05 wt% oxide range for all components except for Ba (0.08 wt% BaO), Sr (0.08 wt% SrO) and S (0.08 wt% SO3) calculated at 3-sigma above mean background. The results of the analyses were presented in Tables 1 and 2. 4. Field relations and petrographic studies Based on field studies and their petrographic characteristics, the parent rhyolitic rocks of the study area, have been subjected to post magmatic–hydrothermal alteration processes resulting in phyllic, advanced argillic and silicic facies. 4.1. Field relations
Fig. 1. Geographical map of Iran and study area in the Uromieh-Dokhtar belt.
Different hydrothermally altered zones in the study area display various colours. The phyllic alteration is widespread in the east of the rhyolitic dome unit (Fig. 2). This altered zone is characterized
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Fig. 2. Geological map of the Rangan area (after Emami and Radfar, 2000). The figure also shows the location of the samples and their geological relationships and hydrothermal alteration facies.
by the presence of abundant pyrite, giving a greenish colour to the rocks (Fig. 3a). The advanced argillic alteration zone extends from the south to the north of the dome. The zone varies in colour from creamish to yellow-pea in the southern parts (Figs 2 and 3b) to whitish in the north (Figs 2 and 3c). Whereas vast quantities of iron – bearing minerals are distributed in the southern margin, the northern portion of this zone is essentially associated with clay minerals. The silicic zone, which is dark red in colour (Figs 2 and 3c), runs parallel to the marginal zone of the advanced argillic fa-
cies. A spatial relationship between the different alteration facies and the intrusion mass (diorite to monzodiorite) which is known as the engine of the fossil hydrothermal system in the study area is shown in Fig. 2. 4.2. Petrographic study On the base of optical study, in less altered rocks, quartz and sanidine are the predominant phases. The quartz and alkali feld-
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Table 1 Representative chemical analyses from different hydrothermally altered and unaltered rocks Sample
Unaltered
Phyllic alteration
RUR1
Advance argillitic alteration
Silicic alteration
RSE1
RSE2
RSE3
RAA1
RAA2
RAA3
RAA4
RSI1
RSI2
Major elements (Wt%) 71.98 SiO2 15.32 Al2O3 1.36 Fe2O3 CaO 0.78 0.76 Na2O 6.09 K2O MgO 0.76 MnO 0.018 0.409 TiO2 0.064 P2O5 L.O.I 1.89 Total 99.43
79.94 11.63 1.16 0.15 5.89 0.14 0.09 0.015 0.176 0.049 0.72 99.96
74.15 14.33 1.55 0.03 1.55 5.15 0.67 0.038 0.196 0.04 1.56 99.26
69.82 18.59 1.93 0.02 0.24 5.45 0.77 0.007 0.648 0.047 2.27 99.79
71.95 23.65 0.23 0.2 0.06 0.02 0.07 0.003 0.877 0.042 2.76 99.86
21.06 7.25 48.15 0.13 0.53 5.8 0.06 0.086 0.001 0.226 3.56 86.85
20.03 7.04 49.97 0.16 0.64 7.53 0.01 0.033 0.034 0.37 3.26 89.07
22.09 7.47 42.47 0.5 0.88 8.42 0.01 0.017 0.022 0.286 4.08 86.24
93.42 1.26 2.72 0.4 0.51 0.11 0.7 0.026 0.006 0.022 0.31 99.48
85.82 8.98 1 2.28 0.04 2.66 0.45 0.05 0.438 0.037 2.04 99.75
Trace elements (ppm) S 2836 Cl 110 V 46 Cr 1 Co 4 Ni 10 Cu 30 Zn 65 Rb 242 Sr 723 Y 25 Nb 13 Ba 873 W <1 Pb 478 Th 21 u 6 Zr 240 As 27 Sb <1
244 70 31 1 7 11 4 28 10 700 25 8 8 <1 18 10 5 133 72 <1
5091 13 27 1 2 10 1 21 158 690 20 15 313 <1 24 14 3 124 25 <1
402 547 79 34 2 16 3 15 217 644 71 19 317 <1 17 24 6 292 5 <1
534 10 176 8 5 8 0 11 11 733 29 4 89 <1 18 3 6 125 21 <1
114337 285 41 1 2 10 64 125 36 1110 13 2 966 1 455 93 5 122 537 25
105439 350 73 1 1 16 39 141 86 1620 13 9 959 <1 562 124 4 246 1096 38
125454 212 27 1 1 14 594 166 159 3168 18 8 954 <1 1723 56 5 319 2910 444
430 3337 35 1 7 9 30 17 12 679 6 7 115 <1 12 5 3 110 38 <1
216 12 58 39 5 13 6 13 113 661 29 8 155 <1 16 9 9 134 9 <1
REE (ppm) La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
27.4 33 6.75 24 4.7 1.06 2.26 0.4 1.56 0.31 0.87 0.13 0.8 0.14
25 32.4 2.54 9.21 1.99 0.37 1.16 0.29 1.33 0.34 0.99 0.19 1.16 0.19
25.9 38 5.23 17.2 2.99 0.46 1.47 0.29 1.4 0.36 1.14 0.19 1.5 0.26
32.7 40.1 11.3 44.3 9.67 1.36 4.62 0.88 3.54 0.72 1.86 0.2 1.32 0.16
21 26 3.36 14 3.22 0.81 1.23 0.19 0.8 0.21 0.58 0.09 0.53 0.08
15.6 19.3 2.01 6.5 0.95 0.24 0.52 0.1 0.41 0.1 0.28 0.05< 0.26 0.04
137 167 10.4 19.7 2.63 0.52 2.02 0.29 1.22 0.24 0.75 0.11 0.75 0.13
24.6 31.4 1.77 7.55 2.15 0.68 1.01 0.16 0.54 0.09 0.19 0.05< 0.12 0.02<
13.5 15.9 0.06 0.3 0.11 0.03 0.06 0.03 0.22 0.09 0.43 0.1 0.87 0.18
23.4 35.6 5.9 22.1 3.95 0.63 1.67 0.3 1.25 0.24 0.66 0.08 0.47 0.06
RREE (La/Yb)cn (La/Sm)cn (Tb/Yb)cn (Eu/Eu*)cn (Ce/Ce*)cn Fe/Na
103.38 24.06 3.76 2.27 0.99 0.59 1.68
77.16 15.46 8.1 1.13 0.74 0.99 0.185
96.39 12.4 5.6 0.88 0.67 0.8 0.943
152.73 17.78 2.18 3.03 0.62 0.51 7.58
72.1 28.4 4.21 1.63 1.24 0.76 3.6
46.31 43.01 10.6 1.74 1.04 0.84 85.7
342.76 131.07 33.63 1.76 0.69 1.08 73.66
70.26 146.2 7.38 6.02 1.41 1.66 45.5
31.88 11.51 82 0.15 1.13 4.262 5.03
96.31 15.46 3.8 3.71 0.75 0.74 23.58
spar grains are characterized by rounded margins and frequently gulf corrosion. The gulf texture is possibly formed by the intrusion of basic and younger dioritic magma into the older and felsic rhyolitic one (Fig. 4a and b). Locally, the studied rocks are defined by abundant sericite (Fig. 4c) and large quantities of very fine grained quartz with mosaic texture (Fig. 4d). Massive jarosite along with sericite form the matrix of some rocks (Fig. 4e). Less commonly, the rims of jarosite are replaced by brownish fine grained aggregates of goethite as a consequence of progressive oxidation (Fig. 4f and g).
The alteration facies are characterized by various mineral assemblages. Phyllic alteration facies (QSP) comprises fine grained quartzsericite-pyrite with kaolinite, albite and small amount of K-feldspar. This facies changes to advanced argillic facies that is mostly composed of abundant alunite–jarosite, goethite, pyrophyllite, diaspor, barite and tourmaline (Fig. 4h and i). Due to strong acid sulphate alteration, K- feldspar and pyrite in the host rocks were altered and released K, Al and Fe, facilitating the formation of alunite–jarosite. Amorph silica, owing to the increased H+-metasomatism, was transported upward and deposited over the whole rock (Fig. 4j).
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A. Parsapoor et al. / Journal of Asian Earth Sciences 34 (2009) 123–134 Table 2 Electron microprobe analyses of the alunite–jarosite (advanced argillic facies) Sample oxides (Wt%)
JM1
JM2
JM3
JM4
JM5
JM6
JM7
JM8
JM9
JM10
Al2O3 Fe2O3 MnO MgO Na2O K2O CaO SrO BaO P2O5 SO3 Cl O@Cl Total
7.73 36.12 0.00 0.00 0.09 8.42 0.04 0.73 0.35 2.21 32.09 0.05 0.01 87.83
8.74 35.20 0.02 0.00 0.13 8.52 0.04 0.49 0.29 2.02 31.11 0.05 0.01 86.62
6.90 35.52 0.03 0.00 0.14 8.58 0.02 0.60 0.29 2.04 32.27 0.03 0.01 88.40
7.86 36.16 0.00 0.00 0.15 8.28 0.03 0.53 0.26 2.32 30.46 0.04 0.01 86.09
8.43 35.58 0.01 0.00 0.10 8.61 0.01 0.68 0.27 2.23 31.99 0.05 0.01 87.93
8.43 36.42 0.01 0.01 0.11 7.71 0.03 0.70 0.27 1.94 28.39 0.05 0.01 84.05
9.18 34.88 0.01 0.02 0.15 8.01 0.00 0.41 0.27 1.78 29.70 0.04 0.01 84.05
8.99 34.26 0.01 0.00 0.12 8.62 0.04 0.59 0.29 1.96 32.03 0.07 0.02 86.96
11.56 31.59 0.00 0.00 0.12 8.85 0.02 0.48 0.18 1.67 32.81 0.06 0.01 87.35
16.64 25.53 0.00 0.00 0.21 9.26 0.02 0.47 0.15 1.28 34.59 0.10 0.02 88.23
Base on 8 oxygen ions Al Fe3+ Mn Mg Na K Ca Sr Ba P S Cl Sum
0.730 2.176 0.000 0.000 0.015 0.860 0.004 0.034 0.011 0.150 1.927 0.007 5.905
0.836 2.150 0.002 0.000 0.020 0.882 0.004 0.023 0.009 0.139 1.895 0.007 5.960
0.830 2.117 0.002 0.000 0.021 0.867 0.002 0.028 0.009 0.137 1.918 0.004 5.929
0.761 2.232 0.000 0.000 0.023 0.866 0.002 0.025 0.008 0.161 1.876 0.006 5.956
0.792 2.136 0.001 0.000 0.015 0.876 0.001 0.032 0.008 0.151 1.915 0.007 5.925
0.843 2.326 0.001 0.001 0.018 0.835 0.003 0.034 0.009 0.139 1.808 0.008 6.017
0.903 2.189 0.001 0.002 0.024 0.852 0.000 0.020 0.009 0.126 1.859 0.005 5.985
0.851 2.071 0.00 0.00 0.019 0.884 0.003 0.027 0.009 0.133 1.931 0.009 5.929
1.073 1.873 0.000 0.000 0.019 0.890 0.002 0.022 0.006 0.111 1.940 0.008 5.935
1.485 1.454 0.000 0.000 0.031 0.895 0.002 0.021 0.004 0.082 1.965 0.013 5.940
Fig. 3. Field photographs of (a) phyllic alteration (QSP), (b) advanced argillic alteration (AA) and (c) silicic alteration (SI).
5. Geochemical studies Hydrothermal alteration, according to Pirajno (1992), is a very complex process involving mineralogical, chemical and textural changes resulting from the interaction of hot aqueous fluids with the wall rock. The observed geochemical features in the studied rocks will be discussed at some length below.
In the phyllic alteration, the occurrence of minerals such as sericite and pyrite may be developed by destabilization of feldspars in the presence of H+, OH, K+ and HS. Subsequent to this event, advanced argillic alteration in the Rangan deposits appears to dominate the original rhyolitic rocks, suggesting the prevalence of a high sulphide-acid system in the study area. The extreme acid leaching of the original rocks produced silicic alteration facies
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Fig. 4. Photomicrograhs of sub-volcanic rhyolites showing (a) sanidine and quartz, (b) gulf corrosion in alkali feldspar, sericitic-silicic volcanic rocks, (c) phyllic zone (quartz, sericite, pyrite), (d) fine grained quartz with mosaic texture, jarositic-sericitic volcanics, (e) sericite graded into jarosite, (f) relicts of pyrite, (g) rims of jarosite replaced by the fine grained aggregates of goethite, (h) jarositic volcanic rocks, which wholly composed of yellow-pea microcrystalline jarosite, (i) pyrophyllite as a dominant phase, (j) silicification in silicic alteration.
(Stoffregen, 1987), which usually occurs in the inner and towards the marginal zone of advanced argillic alteration facies of high sulphidation epithermal systems (Hayba et al., 1985; Arribas et al.,
1995). Silicic alteration derives from a nearly total leaching of the original rocks by extremely acid fluids with a pH <2 (Stoffregen, 1987).
A. Parsapoor et al. / Journal of Asian Earth Sciences 34 (2009) 123–134
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5.1. Major elements The contents of some major elements in different altered and unaltered zones have been determined here in order to investigate their relationship: the SiO2 wt% is high in rhyolite (>71 wt%). Its content in silicic alteration zone is much higher (>93 wt%) than the unaltered zone. The lowest amount of SiO2 is found in the advanced argillic zone with approximately 20 wt%. Except for the advanced argillic samples, the level of K2O decreased from unaltered to altered rocks. The enrichment of K2O in advanced argillic alteration rocks is due to the presence of alunite–jarosite in these rocks. The MgO content decreases from unaltered towards altered zones. From the fresh rock to the advanced argillic alteration, the content of TiO2 decreases from 0.4% to 0.006% (RSI1) (Table 1). The remaining major elements do not show a regular pattern. 5.2. Rare earth and other trace elements In order to investigate the enrichment of light REE (LREE) over heavy REE (HREE) in the studied alteration zones, the (La/Yb)cn ratio was calculated. Fractionation among LREE expressed as (La/ Sm)cn and between middle REE (MREE) and HREE is shown as (Tb/Yb)cn (Table 1). The abundances of these elements were normalized to the chondritic values, according to Sun and McDonough (1989). The geochemical behaviour of REE and their ratios will be discussed in the following sections. 5.2.1. Phyllic alteration As it was noted before, the rocks of phyllic alteration are composed of kaolinite, pyrite, sericite, albite and quartz mineral assemblage (Fig. 5a). The REE ratios (La/Yb)cn, (La/Sm)cn and (Tb/Yb)cn were employed to demonstrate the REE fractionation. These ratios and the values of (Ce/Ce*)cn and (Eu/Eu*)cn are also low for the studied phyllic altered rocks (Fig. 6). Two samples (RSE1, RSE2) of these rocks are relatively depleted in LREE and MREE but are enriched in HREE (Fig. 7a). 5.2.2. Advanced argillic alteration The samples characterized by advanced argillic alteration based on XRD analyses are composed mainly of jarosite (RAA2, RAA3 and RAA4) and pyrophylite (RAA1) (Fig. 5b and c). The (La/Yb)cn and (Tb/Yb)cn ratios in the studied samples are high, whereas the (La/Sm)cn, (Ce/Ce*)cn and (Eu/Eu*)cn are relatively low (Fig. 6). The chondrite-normalized REE patterns (Fig. 7b) indicate that, except for La, Ce and Pr (in sample RAA3), LREE are relatively immobile in the rocks of this zone and depleted in MREE and HREE. This behaviour may be due to the entrance of these elements in the lattice of alunite–jarosite (Arribas et al., 1995; Fulignati et al., 1999). LREE can, in fact, substitute potassium in the large radius cations (A) of the alunite–jarosite formula (Scott, 1987). The formula is written as: [AB3 (XO4)2(OH)6] in which A: Na, U, K, Ag, NH4, Pb, Ca, Ba and S. B: Al, Fe, Cu and Zn. (X): P, S. Large-ion lithophile elements (LILE), such as Rb, Sr and Ba, display very different features. 5.2.3. Silicic alteration The rocks of the silicic alteration facies, owing to the leaching process and mobilization of all elements, are composed of residual silica (Fig. 5d). These rocks tend towards low ratios in terms of their (La/Yb)cn, (Tb/Yb)cn and (Eu/Eu*)cn ratios and high values of (La/Sm)cn and (Ce/Ce*)cn (Fig. 6). The positive (Ce/Ce*)cn anomalies may be due to predominance of Ce+4 over Ce+3 in the studied rocks. Unique among REE are Eu and Ce, which may in certain physicochemical environments occur as Eu+2 and Ce+4. The Eu+2 has essentially the same radius as Ca+2 and therefore, it should occur
Fig. 5. The results of XRD analyses for (a) phyllic alteration (Ab: albite, Ka: kaolinite, Se: sericite, py: pyrite and Qz: quartz), (b and c) advanced argillic alteration (Ja: jarosite, Pp: pyrophyllite and Qz: quartz), (d) silicic alteration (Qz: quartz).
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(La/Sm)Cn
(La/Yb)Cn
Unaltered rocks phyllic alteration
Advanced argillic alteration Silicic alteration 0
25
50
75
100
0
50
100
(Tb/Yb)Cn
150
(Ce/Ce*)cn
Unaltered rocks phyllic alteration
Advanced argillic alteration Silicic alteration 0
1
2
3
4
5
6
7
0
1
2
3
4
5
(Eu/Eu*)cn
Unaltered rocks phyllic alteration
Advanced argillic alteration Silicic alteration 0
0.5
1
1.5
Fig. 6. Variations in REE ratios (La/Sm, La/Yb, Tb/Yb)cn displaying the REE fractionation in the hydrothermal altered rocks facies compared to unaltered equivalent original pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rocks. The (Eu/Eu*)cn and (Ce/Ce*)cn ratios show the variation of the Eu and Ce anomalies respectively, ðEu=Eu ¼ Eu= SmN GdN Þ and ðCe=Ce ¼ Ce= LaN PrN Þ.
naturally in Ca+2 minerals. Under oxidizing conditions Ce+3 is oxidized to Ce+4, which is less soluble than the trivalent REE (Crozaz et al., 2003). In this state Ce+4 precipitates and therefore exhibits a distinctive positive Ce anomaly in altered rocks relative to other REE. The behaviour of REE in silicic altered samples (RSI1 and RSI2), in comparison to other kinds of alteration is different. In this alteration (particularly in RSI1), a strong reduction is observed in the amount of LREE (except for La and Ce) and MREE, but HREE are subjected to less mobility (Fig. 7c). This notable low abundance of REE is readily explained by the high acidity and abundant Cl, F and SO4 2 complexing ions in the hydrothermal fluids (Michard, 1989; Wood, 1990; Lottermoser, 1992). According to Michard (1989) and Lottermoser (1992), the high concentration of Cl in hydrothermal fluid is a good complexing agent for Eu, and may be responsible for negative Eu anomaly. Alternatively, depletion in Eu may be enhanced by dissolution of Eu-enriched minerals (Lewis et al., 1997). A summarizing of the mineralogy and other characteristics of the alteration types are given in Table 3. 5.3. Fe/Na relationship and barium mobility According to Brookins (1987) barite (BaSO4) is the major carrier of barium in sedimentary environments. Barite can dis-
solve under strongly acidic (pH <2) hydrothermal conditions, but it is insoluble over wide Eh-pH range (Fig. 8). He also pointed out that pyrite is decomposed to form Fe+2 under oxidizing conditions but it can be stable over wide pH range under reducing conditions. Based on these observations and using Fe/Na ratio vs. Ba abundances, Eh-pH conditions have been determined for Roseki deposits of Japan (Terakado and Fujitani, 1998). For the purpose of this study, Fe/Na vs. Ba concentration was applied for the K-depleted (K <0.2 wt%) and K-enriched (K >5 wt%) Rangan deposits. A positive correlation is observed between Fe/Na vs. Ba in K-depleted rocks (Fig. 9a), whereas in the K-enriched deposits at constant value of Ba in sericitic (315 ppm) and advanced argillic facies (950 ppm) the Fe/Na ratio increases (Fig. 9b). Uncertainties remain on interactions of these elements in hydrothermal solutions. The behaviour of K+ in a hydrothermal system is function of temperature, composition of rocks being leached and water–mineral reaction. In advanced argillic facies, K+ is fixed in jarosite under strongly acidic conditions (pH <2) (Dutrizac and Jambor, 2000) and in a moderately acidic environment (pH4), it is replaced by Na+. Under relatively high temperature, Na in comparison to K, has more tendency to be fixed in alunite structure (Stoffregen and Cygon, 1990). Based on these observations as well as the composition of the studied alunite–jarosite, it is therefore
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Fig. 7. Chondrite-normalized REE patterns of unaltered rocks (RUR1) and hydrothermally altered products, (a) phyllic alteration (RSE1 and RSE2), (b) advanced argillic alteration (RAA1, RAA2, RAA3 and RAA4), (c) silicic alteration (RSI1 and RSI2).
Table 3 The summarizing table of the REE behaviour in the hydrothermal alteration facies of the Rangan area (Central Iran) Alteration facies
Phyllic
Advanced argillic
Silicic
pH Temperature LREE MREE HREE Dominating complexing agent Mineral assemblages REE bearing mineral phase (La/Yb)cn (La/Sm)cn (Tb/Yb)cn (Ce/Ce*)cn (Eu/Eu*)cn
Near neutral 150–200 °C Depleted Depleted Enrichment HSAlbite, pyrite, sericite, kaolinite, quartz Sericite, kaolinite Low Low Low Low Low
2
<2 <100 °C Strongly depleted Strongly depleted Less mobility SO4 2 , F Amorphous silica None Low High Low High Low
The temperatures and pH data are taken from Gioncada et al. (1995).
likely that the advanced argillic zone in the study area formed under considerably low pH, relatively oxidizing conditions and low temperature. 5.4. The behaviour of the REE and other trace elements during hydrothermal alteration 5.4.1. The isocon method Direct comparison of element concentrations cannot be a reliable indication of element mobility in altered and unaltered rocks. Therefore, the isocon diagram can be applied for this purpose (Grant, 1986). In the present paper, we adopted a constant
volume; therefore, the isocon equation will be CA = (qO/qA) CO and the enrichment and depletion of various elements are given using the equation: ðDC i =C i Þ 100 ¼ ðqA =qO ÞðC Ai =C Oi Þ 1. The geometry of diagram (Fig. 10) is such that the elements below the straight isocon are depleted whereas those above are enriched by the hydrothermal alteration. In the phyllic alteration we obtain that some of the oxides and elements such as CaO, P2O5, S, Cu, Zn, Rb, Ba, Pb, Th, Zr and U are depleted and the remaining components are enriched (Fig. 10a) whereas, in the advanced argillic alteration there is a strong enrichment in most elements (e.g. S, Cl, Sr, Ba, La, Ce and Th). This supports the idea of Terakado and Fujitani (1998) that jarosite may be a good
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Fig. 8. Eh-pH diagram for the systems Fe-S-O-H and Ba-S-O-H-C (after Brookins, 1987). Assumed activities of dissolved species are Fe = 106, S = 103, C = 103. Note the Eh-pH conditions required for the formation of jarosite.
repository for the above- mentioned elements. Also, the concentration of Pb, Zn, Cu and As in the advanced argillic zones is high (Fig. 10b). This feature may be due to the chalcophile nature of these elements as well as sulphur enrichment of this altered facies. During hydrothermal processes, the abundances of Cr and Ni commonly do not vary significantly. In the silicic alteration zone except for SiO2, Fe2O3, CaO, MnO, Co, Cr, Cl, V, Ni, Sr and U, the other elements and oxides show a clear reduction (Fig. 10c). 6. Conclusion Judging from the available analytical data, the following model has been derived to explain the formation of the hydrothermal system identified on the Rangan rhyolitic dome, and the REE behaviour in particular. During post-Oligocene period, the hydrothermal fluids originated from the near-by diorite intrusion related to monzodioritic magmatism, representing fluids which probably separated from the dioritic melt. Such fluids are expected to interact with rhyolitic unit, producing different hydrothermal alteration facies including phyllic, advanced argillic and silicic zones. The phyllic alteration zone is identified by the mineral assemblage of quartz + pyrite + sericite as well as albite, K-feldspar and kaolinite. HREE are immobile, MREE and LREE are relatively depleted in this facies. Such feature can be explained by the presence of quartz which is considered to have fixed HREE in its structure. The major constituents of the advanced argillic facies are alunite–jarosite and pyrophyllite which are accompanied by goethite, barite, tourmaline and quartz. The immobility of LREE along with depletion in MREE and HREE is mainly due to the presence of sec-
Fig. 9. Fe/Na vs. Ba concentration in the K-depleted rocks (a) and K-enriched rocks (b) of the Rangan deposits.
ondary minerals (e.g. alunite–jarosite) which are known as hosts for the LREE enriched phase. A strong LREE depletion is observed in the silicic facies mineralogically composed of silica, barite and gypsum. MREE and HREE are subjected to less mobility. This behaviour may be caused by the entrance of these elements into the silica lattice. The relative immobility of REE in the phyllic facies can be related to the higher pH conditions and lower water/rock ratio. Also, the presence of sericite and quartz in this facies may be a good candidate to fix the REE into their structures. The relevant LREE fractionation, with respect to MREE and HREE in the silicic facies, may be explained by high acidity and abundance of SO4 2 ion in solution. Cl and F can be responsible in REE complexing in acidic condition. Finally, the study of hydrothermal alteration in the Rangan area shows that except for the advanced argillic zone, which displays enrichment in some trace elements (e.g. Th, Pb, Ba, Cu, Zn, As and Sr), significant mineralization is not observed in the altered zones. The available data in the present study show that both leaching and precipitation are major processes for the Rangan deposits and its related alteration facies, and point to the selective nature of some trace elements even under strongly acidic hydrothermal conditions. Also, the results of this work may be useful for better understanding of the surficial alteration mechanism which develops in the high-sulfidation hydrothermal system.
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Fig. 10. Isocon diagrams of the three different alteration facies identified in the superficial hydrothermalised zones of Rangan and histograms showing gains and losses of selected elements during hydrothermal alteration as calculated from isocon diagram method. (a) phyllic, (b) advanced argillic (except for RAA1 sample), (c) silicic alteration. Values of +4 to +3 = 75–100%, +3 to 2 = 50–75%, +2 to +1 = 25–50%, 1 to 0 = 0–25% and 0 = immobile elements. Values of 0 to 1 = losses of 0–25%, 1 to 2 = 25–50%, 2 to 3 = 50–75% and 3 to 4 = 75–100%.
Acknowledgments
References
This paper has greatly benefited from critical comments, suggestions and reviews by Dr. M. Smith, Dr. D. Huston and in particular Dr. P. Fulignati who are warmly thanked for their interest. Also, we appreciate Dr.G. Morgan of the University of Oklahoma, Norman (USA) and the staff of the Center of Analysis and Test of the University of Isfahan who conducted a number of microprobe and XRF chemical analyses.
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