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The origin and evolution of skarn-forming fluids from the Phu Lon deposit, northern Loei Fold Belt, Thailand: Evidence from fluid inclusion and sulfur isotope studies
Journal of Asian Earth Sciences 34 (2009) 624–633
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The origin and evolution of skarn-forming fluids from the Phu Lon deposit, northern Loei Fold Belt, Thailand: Evidence from fluid inclusion and sulfur isotope studies Teera Kamvong *, Khin Zaw CODES ARC Centre of Excellence in Ore Deposits, University of Tasmania, Tasmania 7001, Australia
a r t i c l e
i n f o
Article history: Received 15 February 2008 Received in revised form 5 July 2008 Accepted 1 September 2008
Keywords: Loei Fold Belt Thailand Phu Lon Calcic skarn Oxidized skarn
a b s t r a c t The Phu Lon skarn Cu–Au deposit is located in the northern Loei Fold Belt (LFB), Thailand. It is hosted by Devonian volcano-sedimentary sequences intercalated with limestone and marble units, intruded by diorite and quartz monzonite porphyries. Phu Lon is a calcic skarn with both endoskarn and exoskarn facies. In both skarn facies, andradite and diopside comprise the main prograde skarn minerals, whereas epidote, chlorite, tremolite, actinolite and calcite are the principal retrograde skarn minerals. Four types of fluid inclusions in garnet were distinguished: (1) liquid-rich inclusions; (2) daughter mineral-bearing inclusions; (3) salt-saturated inclusions; and (4) vapor-rich inclusions. Epidote contains only one type of fluid inclusion: liquid-rich inclusions. Fluid inclusions associated with garnet (prograde skarn stage) display high homogenization temperatures and moderate salinities (421.6–468.5 °C; 17.4– 23.1 wt% NaCl equiv.). By contrast, fluid inclusions associated with epidote (retrograde skarn stage) record lower homogenization temperatures and salinities (350.9–399.8 °C; 0.5–8 wt% NaCl equiv.). These data suggest a possible mixing of saline magmatic fluids with external, dilute fluid sources (e.g., meteoric fluids), as the system cooled. Some fluid inclusions in garnet contain hematite daughters, suggesting an oxidizing magmatic environment. Sulfur isotope determinations on sulfide minerals from both the prograde and retrograde stages show a uniform and narrow range of d34S values ( 2.6 to 1.1 ‰ d34S), suggesting that the ore-forming fluid contained sulfur of orthomagmatic origin. Overall, the Phu Lon deposit is interpreted as an oxidized Cu–Au skarn based on the mineralogy and fluid inclusion characteristics. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Most skarn deposits generally form as a result of metasomatism at or near the contact between predominantly carbonate-rich rocks and intrusions, or in carbonate veins along faults or fractures (Meinert et al., 2005). Typically, they are characterized by two main alteration styles based on the separate precipitation of calcsilicate minerals: (1) early prograde assemblage (garnet and pyroxene) and (2) later retrograde assemblage (epidote, chlorite and amphibole). Studies on fluid inclusions and stable isotopes of skarn deposits indicate that high-temperature (400–600 °C) and saline (10– 60 wt% NaCl equiv.) magmatic fluids are mainly responsible for their formation (e.g., Meinert et al., 2003; Baker et al., 2004). These authors suggested that (1) the fluids responsible for the formation of early and late skarn alteration stages came directly from crystallizing magmas and (2) the metal transport and mineralization processes of skarn systems are primarily controlled by magmatic fluids during the retrograde stage. However, some stable isotope and fluid inclusion data from the skarn deposits (e.g., Haynes and * Corresponding author. Tel.: +61 3 62262376; fax: +61 3 62267662. E-mail addresses: [email protected], [email protected] (T. Kamvong). 1367-9120/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2008.09.004
Kesler, 1988; Bowman, 1998; Rubin and Kyle, 1998; Lu et al., 2003) have shown that different sources of fluids (e.g., meteoric water) may be involved during the late skarn stage. Thus, it appears that both magmatic fluids and meteoric water can play a role in the formation of skarns and their associated ores (e.g., Einaudi et al., 1981). In this paper, we report the first study of skarn-hosted fluid inclusions and sulfur isotopes from the Phu Lon Cu–Au skarn deposit, northern Loei Fold Belt, Thailand. Our aim is to provide some insights into the fluid evolution of the Phu Lon skarn deposit that can be applied to give a better understanding of the mechanisms of skarn genesis. 2. Geological framework 2.1. Geological setting The Phu Lon Cu–Au skarn deposit is located in the town of Sang Khom, on the right bank of the Mekong River. Geologically, it is situated in the northern Loei Fold Belt (LFB), which lies along the western margin of the Indochina terrane (Fig. 1B). The LFB hosts several economic ore deposits (Fig. 1C), such as Chatree (epithermal Au–Ag), Puthep 1 (porphyry-related skarn Cu), Phu Thap Fah
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Fig. 1. (A) Map of Asia showing the tectonic plates and their relative motion (see arrows). Small box shows location of B. (B) Map of mainland Southeast Asia showing the major terranes, suture zones, faults and active subduction zones, modified from Searle (2006). ASRR, Ailao Shan-Red River. Box shows location of C. (C) Map showing the location of major ore deposits along the Loei Fold Belt, Truongson Fold Belt and Sukhothai Fold Belt. All base maps are after http://www.ngdc.noaa.gov/mgg/image/.
(Au skarn), Phu Kham (porphyry-related skarn Cu–Au) and Phu Lon (Cu–Au skarn). The LFB records a complex history of subduction, collision, accretion and deformation along the length of the western margin of the Indochina terrane. Its origin and history can be traced back to the Devonian when the Indochina terrane was rifted away from northwestern Gondwana (Metcalfe, 2005). Subsequently, the Shan-Thai terrane, an allochthonous continental microplate from northeastern Gondwana, rifted and separated in the Early Permian (Metcalfe, 2005). After drifting northwards, these tectonic terranes appear to have collided and amalgamated during the Triassic (Indosinian Orogeny; Mitchell, 1981; Metcalfe, 1995, 1996; Khin Zaw, 2007; Meffre et al., 2008), forming the LFB of central Thailand. However, the precise collision, amalgamation and accretion history remains unclear and is still controversial (Carter et al., 2001). The Phu Lon deposit has a resource of 5.4 Mt, grading 2.4% Cu and 0.64 g/t Au (supplementary appendices on CD-ROM of Meinert et al., 2005). It is hosted by a Devonian volcano-sedimentary sequence intercalated with limestone and marble units (Fig. 2). These rocks are underlain by Silurian-Devonian greenschist-facies metasedimentary rocks and are overlain by Permian-Triassic units of intermediate to felsic volcanic rocks. The stratified sequence of volcano-sedimentary units was intruded by porphyries, including diorite and quartz monzonite. These intrusions contain pyroxene, hornblende, biotite, plagioclase, alkali-feldspar, quartz, magnetite
and titanite and have geochemical affinities to fractionated and oxidized I-type calc-alkaline magmas (Kamvong and Khin Zaw, 2005). LA-ICPMS U-Pb dating of zircon from quartz monzonite phases indicates an age of 244 ± 4 Ma (Kamvong and Khin Zaw, 2005). 2.2. Skarn formation and mineralization According to the classification schemes of Einaudi et al. (1981) and Meinert (2000), the skarn at Phu Lon is a Cu skarn with byproduct Au. The deposit is a calcic skarn, rich in andradite–diopside–epidote–amphibole assemblages and can be subdivided into exoskarn and endoskarn zones (Kamvong and Khin Zaw, 2005). Fig. 3 illustrates a simplified paragenetic sequence for the skarn mineral assemblages and ore sulfides within the Phu Lon deposit, previously outlined by Pisutha-Arnond et al. (1993), Sitthithaworn et al. (1993) and Kamvong and Khin Zaw (2005). The endoskarn formed as irregular pods and discordant veins and is limited to the margin of the Phu Lon porphyries (both diorite and quartz monzonite). The endoskarn mainly consists of pale brown to colorless, fine- to coarse-grained garnet and subordinate pyroxene, epidote, actinolite, calcite, chlorite, quartz, magnetite, pyrite, chalcopyrite and pseudomorphs of the original feldspar grains (Fig. 4A and B). The massive bodies of exoskarn are divided into prograde (Fig. 4C) and retrograde (Fig. 4D) skarn assemblages.
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Fig. 2. Simplified geologic map (modified from Kaewsang (1995)) of the Phu Lon deposit, showing distribution of rock units and drill hole locations.
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Fig. 3. Generalized paragenetic diagram for skarn minerals, sulfides and gold at the Phu Lon deposit (modified from Sitthithaworn (1989) and Kamvong and Zaw (2005)).
Prograde skarn alteration dominantly comprises massive brown andradite and diopside, which precipitated contemporaneously with the magnetite mineralization. The retrograde skarn assemblage that overprints the prograde skarn minerals is recorded by the formation of hydrous minerals (e.g., epidote, chlorite, amphibole), calcite, quartz and sulfides (pyrite, chalcopyrite, sphalerite, bornite, molybdenite, marcasite). Sitthithaworn (1989) indicated that free gold grains formed as small inclusions in chalcopyrite, pyrite and sphalerite in the retrograde skarn alteration. Such evidence indicates that the bulk-metal budget at Phu Lon is related to the retrograde skarn stages. 3. Sampling and methods In order to ensure that the fluid inclusions are genetically related to the skarn genesis, only samples containing primary fluid inclusions in skarn minerals from the prograde and retrograde stages were analyzed. The best preserved fluid inclusions were found in the coarse-grained garnet and epidote of these skarn
stages. These garnet- and epidote-hosted primary fluid inclusions can provide constraints on the temperature, pressure and composition of skarn-forming fluids (Klemd, 2004; Meinert et al., 2005). Microthermometry of fluid inclusions was conducted on a Linkam MDS 600 stage, with a temperature range between 200 and 600 °C, mounted on an Olympus BX60 microscope. The temperature of the stage was calibrated using a set of Syn-Flinc synthetic fluid inclusions. The precision of measured temperatures is ±1.0 °C for heating and ±0.3 °C for freezing (Rae et al., 2003). Salinity estimates are reported in weight percent NaCl equivalent and were determined from the final melting temperatures of ice (e.g., Bodnar, 1993; Rusk et al., 2008). Laser Raman spectroscopic data were obtained at Geoscience Australia using a Dilor SuperLabram laser Raman microprobe. A total of 12 significant coarse-grained sulfides (sphalerite, pyrite, chalcopyrite) from the prograde and retrograde stages were analyzed for their sulfur isotope compositions at the Central Science Laboratory (CSL), University of Tasmania. The method was carried out on mineral separates drilled from samples of coarse-
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Fig. 4. Photomicrographs of representative samples from the Phu Lon deposit, showing details of skarn mineral assemblages. (A and B) Diopside and andradite replacing intrusive porphyry in the endoskarn unit (sample PL5-17 at 116 m). Note the original magmatic textures and minerals are totally obliterated. (C) Diopside replacing and overprinting andradite in the prograde skarn alteration stage (sample PL5-3 at 15.7 m). Andradite also shows concentric zoning. (D) Epidote together with chlorite replacing andradite in the retrograde skarn alteration stage (sample PL5-16 at 114.6 m).
grained sulfides, using the procedure of Robinson and Kusakabe (1975). Results are expressed in the d34S (‰) relative to the Canyon Diablo Troilite (CDT). Analytical accuracy is ±0.1‰. 4. Results and interpretation 4.1. Fluid inclusions 4.1.1. Classification Four types of fluid inclusions in garnet and epidote were distinguished in terms of number, nature and proportion of phases at room temperature (Kamvong et al., 2006). Epidote only contained one type of fluid inclusions. Type I inclusions contain two phases (liquid + vapor) and are liquid-rich at room temperature. These inclusions are found in both the prograde (Fig. 5A and B) and retrograde skarn stages (Fig. 5C–E). They range in size from <5 lm to 60 lm, have irregular, elongate and oblate shapes and typically occur along growth zones in garnet and epidote, consistent with a primary origin. Type II inclusions contain three phases (liquid + vapor + solid phases; (Fig. 5F–I) and are characterized by the presence of small opaque and translucent daughter phases identified
by Laser Raman analysis as hematite and calcite, respectively. Type II inclusions were also observed in prograde stages. They range in size from <5 lm to 30 lm and generally occur in bands parallel to the optically visible zonation of garnets. These fluid inclusions are also interpreted to be primary. Type III inclusions are rare, salt-saturated inclusions and contain liquid, vapor and halite (Fig. 5J). They range in size up to 50 lm, have negative crystal shapes and occur as both isolated inclusions and small inclusions on garnet growth zones. Type IV inclusions are vapor-rich without soluble solid phases (Fig. 5K–M). These inclusions range in size up to 20 lm and are found within garnet growth zones. 4.1.2. Microthermometry Due to the rare occurrence of Types III and IV fluid inclusions, microthermometric data have not been collected for these inclusions. Thus, only Types I and II inclusions were investigated. Heating and freezing experiments were made to determine the homogenization (Th(LV ? L)) and final ice melting temperatures (Tm) of Types I and II inclusions from epidote (retrograde stage) and garnet (prograde stage), respectively. Results of microthermometry are shown graphically in Fig. 6. Table 1 is the summary
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Fig. 5. Photomicrographs of skarn fluid inclusions trapped in garnet and epidote. (A–E) Type I inclusions (liquid + vapor). Fluid inclusions in a and b occur along zonation parallel bands in garnet (sample PL5-47 at 238.5 m). (C–E) Primary fluid inclusions trapped in epidote (sample PL5-10 at 36.4 m). (F–I) Type II inclusions (liquid + vapor + solid phases) contain small opaque and translucent daughter phases. Laser Raman analysis identified these solid phases as hematite (opaque) and calcite (translucent solid phases). (J) Type III inclusions are salt-saturated fluid inclusions with halite daughter crystals (sample PL5-47 at 238.5 m). (K–M) Type IV vapor-rich inclusions. Fluid inclusions in (K) and (M) formed on garnet growth zones. Dashed line represents growth zonation of garnet. Abbreviations: cal, calcite; hl, halite; hem, hematite; L, liquid; V, vapor.
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Fig. 6. Frequency histograms of homogenization temperatures (Th(LV ? L)) and salinities (wt% NaCl equiv) for the Types II and I fluid inclusions in garnet (prograde skarn) and epidote (retrograde skarn).
Table 1 Summary of microthermometric data from fluid inclusions hosted in garnet and epidote from the Phu Lon skarn deposit, Thailand Sample
Host mineral
Prograde skarn stage PL5-47 Garnet PL5-34 Garnet PL5-18 Garnet Retrograde skarn stage PL5-10 Epidote
FI type
Th(LV ? L)
Tm (ice)
Range
Mean
n
II II II Total
434.2–468.5 425.3–460.0 396.7–421.6 421.6–468.5
448.2 447.8 410.3 433.5
5 3 5 13
I
350.9–399.8
377.2
7
Salinity
Range 21.1 20.3 18.7 21.1
to to to to
5.1 to
14.0 13.6 14.8 13.6 0.3
Mean
n
Range
Mean
n
17.8 16.9 16.2 17.1
4 2 3 9
23.1–17.8 22.6–17.4 21.5–18.5 23.1–17.4
20.7 20.0 19.6 20.2
4 2 3 9
1.4
7
0.53–8.0
2.33
7
Notes: Type I = liquid + vapor; Type II = liquid > vapor + solid daughter phases (hematite and calcite); Th(LV ? L) = temperature of homogenization into liquid phase; Tm (ice) = temperature of final ice melting; temperatures in °C units; salinities in wt% NaCl equiv.
of microthermometric data for fluid inclusions in this study. Measured temperatures of homogenization for Type II fluid inclusions in the prograde stage range from 421.6 to 468.5 °C, whereas Type I fluid inclusions in the retrograde stage show homogenization temperatures in the range between 350.9 and 399.8 °C (Table 1; Fig. 6). Daughter minerals that were observed in the Type II inclusions apparently do not dissolve upon heating over 470 °C, suggesting that they were accidentally trapped rather than precipitated and are not considered to be true daughter minerals. Final ice melting temperatures for Type II inclusions in the prograde stage and Type I fluid inclusions in the retrograde stage range from 13.6 to 21.1 °C and 0.3 to 5.1 °C, respectively (Table 1; Fig. 6). Apparent salinities for Types II and I fluid inclusions in the prograde and retrograde stages are 17.4–23.1 wt% NaCl equivalent and 0.5–8 wt% NaCl equivalent, respectively. Eutectic temperatures (Te) for Type II inclusions in the prograde stage range
from 40 to 50 °C, which are below the eutectic temperature for the H2O–NaCl and H2O–NaCl–KCl systems (Hall et al., 1988), suggesting the presence of additional components such as CaCl2, MgCl2 and FeCl2. 4.1.3. Comparison with other deposits The fluid inclusion data derived from garnet are broadly similar to those documented for other skarn deposits such as the Santa Rita porphyry Cu skarn (Ahmad and Rose, 1980) and the Mines Gaspé Cu skarn (Shelton, 1983) (Fig. 7). The existence of hematite phases in fluid inclusions and the high homogenization temperatures in the prograde skarn are also similar to conditions documented for prograde fluid events in other Cu–Au skarn deposits (e.g., Ahmad and Rose, 1980; Myers, 1990). However, the presence of halite in Type III inclusions may have locally contributed to the higher salinity during the prograde skarn stage although no heat-
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Fig. 7. Plot of measured salinities versus homogenization temperatures for skarn fluid inclusions from Phu Lon together with data from other Cu–Au skarn deposits (modified from Franchini et al. (2000)). Homogenization temperature–salinity fields are from Theodore and Blake (1978) for Copper Canyon, Ahmad and Rose (1980) for Santa Rita, Shelton (1983) for Mines Gaspé and Meinert et al. (1997) for Big Gossan. The gray box illustrates temperature–salinity trends or fluid evolution paths as a result of different geological processes (modified from Shepherd et al. (1985)).
ing/freezing experiments were undertaken for these inclusions due to their rarity. Moreover, the lower homogenization temperatures and lower salinities of fluid inclusions in epidote are similar to the retrograde stage in most skarn systems (e.g., Singoyi and Khin Zaw, 2001; Meinert et al., 2003). 4.2. Sulfur isotopes The majority of d34S values for sulfides associated with both prograde and retrograde skarn at Phu Lon fall in the range 2.6 to 1.1 ‰, with a mean of 1.9 ‰ d34S (Table 2). Seven chalcopyrite samples have d34S values between 2.6 and 2.1 ‰, three sphalerite samples have d34S values of 1.2 to 1.1‰, and two pyrite samples have d34S values of 1.9 to 1.4‰. Based on these results, there is no evidence that the sulfides vary systematically in isotopic composition with respect to skarn alteration stages. Instead, most of the d34S values for sulfides have a remarkable homogeneity (Table 2). Such evidence may indicate the lack of
Table 2 Sulfur isotope compositions of ore sulfides from the Phu Lon deposits Sample no.
Drill hole and depth
Mineral
Stage
PL5-7 PL5-9 PL5-12a PL5-13 PL5-22 PL5-27 PL5-30 PL5-38 PL5-39 PL5-41 PL5-48 PL5-49
DDH-PL5, DDH-PL5, DDH-PL5, DDH-PL5, DDH-PL5, DDH-PL5, DDH-PL5, DDH-PL5, DDH-PL5, DDH-PL5, DDH-PL5, DDH-PL5,
Pyrite Chalcopyrite Chalcopyrite Chalcopyrite Chalcopyrite Chalcopyrite Sphalerite Sphalerite Chalcopyrite Chalcopyrite Pyrite Sphalerite
Retrograde Retrograde Prograde Retrograde Prograde Prograde Retrograde Prograde Retrograde Retrograde Retrograde Marble
23 m 25 m 57.1 m 84 m 145.1 m 184.9 m 192.5 m 210.2 m 217 m 218.1 m 240.2 m 338.9 m
d34S ‰ (CDT) 1.9 2.6 2.4 2.4 2.5 2.6 1.1 1.1 2.1 2.4 1.4 1.2
Fig. 8. Total range of d34S values from the Phu Lon deposit in comparison to values from other Cu skarns (Taylor, 1987), Mine Gaspé (Shelton and Rye, 1982), Big Gossan (Prendergast et al., 2005) and Wanagon (Prendergast et al., 2005).
significant contamination or assimilation of country-rock sulfur. The d34S compositions at Phu Lon are similar to other major
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Fig. 9. Schematic paragenetic diagram of mineralized skarn relative to interpreted fluid paths of the Phu Lon deposit. It also shows possible skarn formation temperatures. Mineral abbreviations: act, actinolite; adr, andradite; cal, calcite; chl, chlorite; di, diopside; ep, epidote; qtz, quartz.
Cu–Au skarn deposits (Fig. 8) although the values are generally more negative. These more negative d34S values may reflect a more oxidized environment of the host rocks at Phu Lon (cf., Ohmoto and Rye, 1979; Salier et al., 2005). Overall, the range of d34S values for all sulfides analyzed from the Phu Lon deposit is very narrow ( 2.6 to 1.1‰), suggesting only one well-homogenized source of sulfur, largely of magmatic origin (e.g., Shimazaki and Yamamoto, 1979, 1983; Shelton, 1983; Shimazaki and Sakai, 1984; Ohmoto and Goldhaber, 1997).
tinued to cool (e.g., Shelton, 1983; Einaudi et al., 1981; Haynes and Kesler, 1988; Meinert et al., 1997; Bowman, 1998; Figs. 7 and 9). According to Laser Raman data, fluid inclusions in garnet contain hematite daughters. Such characteristics show evidence of an oxidizing environment during fluid inclusion entrapment. The oxidizing condition of ore formation was also recorded in other skarn systems (e.g., Khin Zaw and Singoyi, 2000).
5. Discussion
Salinities and homogenization temperatures from the two most abundant fluid inclusion populations at the Phu Lon Cu–Au deposit are consistent with a model of fluid mixing between two endmember fluids combining to form an oxidized Cu–Au skarn. Higher-temperature higher-salinity fluids representative of magmatic fluids are trapped within prograde skarn mineral assemblages. Lower-temperature lower-salinity fluids, displaying a possible meteoric affinity are trapped within the retrograde skarn assemblages. Additional data consistent with an oxidized skarn model are hematite-bearing fluid inclusions and oxidized mineral assemblages comprised of andradite, magnetite and hematite. The uniform and narrow range of d34S values suggests a well-homogenized possibly igneous sulfur source.
Field observations and fluid inclusion and sulfur isotope studies indicate that the Phu Lon skarn deposit was formed during multiple geological events similar to other Cu–Au skarn systems (e.g., Einaudi et al., 1981; Shelton, 1983; Meinert et al., 1997; Forster et al., 2004; Fig. 9). Emplacement of the Phu Lon intrusive porphyries during the Early Triassic was followed by expulsion of a juvenile magmatic-hydrothermal brine into a zone of highly fractured rocks in and around the intrusions. The magmatic fluid was likely channeled from the intrusions through the marble and migrated along permeable zones, reacting with the host rocks (i.e., limestone) to form prograde skarn assemblages. During the retrograde skarn stage, the formation of epidote and other retrograde minerals may have resulted from mixing of magmatic fluids with meteoric water input (e.g., Einaudi et al., 1981; Haynes and Kesler, 1988; Bowman, 1998). At Phu Lon, the high temperatures (up to 468 °C) and moderate salinity (up to 23 wt% NaCl equiv.) of fluid inclusions in prograde skarn-stage garnet indicate that the fluid inclusions represent the composition of magmatic fluids after reaction with the calcareous wall rocks (cf., Franchini et al., 2000). By contrast, fluid inclusions in epidote (retrograde skarn stage) are both low in temperatures and salinities as low as 0.5–8 wt% NaCl equivalent. Such characteristics may have resulted from mixing between magmatic fluids and external fluid sources (e.g., meteoric waters), causing more dilute fluids, lower temperatures and lower salinities, as the system con-
6. Conclusions
Acknowledgements This research has been carried out as a part of the project ‘‘Geochronology, Metallogenesis and Deposit Styles of the Loei Foldbelt in Thailand and Laos PDR” lead by Khin Zaw and financially supported from the Australian Research Council (ARC), Kingsgate Consolidated Ltd., Oxiana Ltd. (now OZ Minerals Ltd.) and PanAust Ltd. We thank Ross Large for making the manuscript more robust. Valuable suggestions by Daniel Marshall and an anonymous reviewer are greatly appreciated. Thanks also go to Terry Mernagh from Geoscience Australia for assisting with Laser Raman analysis of fluid inclusions. Department of Primary Industries and Mines (DPIM) in Thailand is thanked for permission to sample the drill
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cores. Editorial acknowledged.
handling
by
Ian
Metcalfe
is
gratefully
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Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jaes
The origin and evolution of skarn-forming fluids from the Phu Lon deposit, northern Loei Fold Belt, Thailand: Evidence from fluid inclusion and sulfur isotope studies Teera Kamvong *, Khin Zaw CODES ARC Centre of Excellence in Ore Deposits, University of Tasmania, Tasmania 7001, Australia
a r t i c l e
i n f o
Article history: Received 15 February 2008 Received in revised form 5 July 2008 Accepted 1 September 2008
Keywords: Loei Fold Belt Thailand Phu Lon Calcic skarn Oxidized skarn
a b s t r a c t The Phu Lon skarn Cu–Au deposit is located in the northern Loei Fold Belt (LFB), Thailand. It is hosted by Devonian volcano-sedimentary sequences intercalated with limestone and marble units, intruded by diorite and quartz monzonite porphyries. Phu Lon is a calcic skarn with both endoskarn and exoskarn facies. In both skarn facies, andradite and diopside comprise the main prograde skarn minerals, whereas epidote, chlorite, tremolite, actinolite and calcite are the principal retrograde skarn minerals. Four types of fluid inclusions in garnet were distinguished: (1) liquid-rich inclusions; (2) daughter mineral-bearing inclusions; (3) salt-saturated inclusions; and (4) vapor-rich inclusions. Epidote contains only one type of fluid inclusion: liquid-rich inclusions. Fluid inclusions associated with garnet (prograde skarn stage) display high homogenization temperatures and moderate salinities (421.6–468.5 °C; 17.4– 23.1 wt% NaCl equiv.). By contrast, fluid inclusions associated with epidote (retrograde skarn stage) record lower homogenization temperatures and salinities (350.9–399.8 °C; 0.5–8 wt% NaCl equiv.). These data suggest a possible mixing of saline magmatic fluids with external, dilute fluid sources (e.g., meteoric fluids), as the system cooled. Some fluid inclusions in garnet contain hematite daughters, suggesting an oxidizing magmatic environment. Sulfur isotope determinations on sulfide minerals from both the prograde and retrograde stages show a uniform and narrow range of d34S values ( 2.6 to 1.1 ‰ d34S), suggesting that the ore-forming fluid contained sulfur of orthomagmatic origin. Overall, the Phu Lon deposit is interpreted as an oxidized Cu–Au skarn based on the mineralogy and fluid inclusion characteristics. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Most skarn deposits generally form as a result of metasomatism at or near the contact between predominantly carbonate-rich rocks and intrusions, or in carbonate veins along faults or fractures (Meinert et al., 2005). Typically, they are characterized by two main alteration styles based on the separate precipitation of calcsilicate minerals: (1) early prograde assemblage (garnet and pyroxene) and (2) later retrograde assemblage (epidote, chlorite and amphibole). Studies on fluid inclusions and stable isotopes of skarn deposits indicate that high-temperature (400–600 °C) and saline (10– 60 wt% NaCl equiv.) magmatic fluids are mainly responsible for their formation (e.g., Meinert et al., 2003; Baker et al., 2004). These authors suggested that (1) the fluids responsible for the formation of early and late skarn alteration stages came directly from crystallizing magmas and (2) the metal transport and mineralization processes of skarn systems are primarily controlled by magmatic fluids during the retrograde stage. However, some stable isotope and fluid inclusion data from the skarn deposits (e.g., Haynes and * Corresponding author. Tel.: +61 3 62262376; fax: +61 3 62267662. E-mail addresses: [email protected], [email protected] (T. Kamvong). 1367-9120/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2008.09.004
Kesler, 1988; Bowman, 1998; Rubin and Kyle, 1998; Lu et al., 2003) have shown that different sources of fluids (e.g., meteoric water) may be involved during the late skarn stage. Thus, it appears that both magmatic fluids and meteoric water can play a role in the formation of skarns and their associated ores (e.g., Einaudi et al., 1981). In this paper, we report the first study of skarn-hosted fluid inclusions and sulfur isotopes from the Phu Lon Cu–Au skarn deposit, northern Loei Fold Belt, Thailand. Our aim is to provide some insights into the fluid evolution of the Phu Lon skarn deposit that can be applied to give a better understanding of the mechanisms of skarn genesis. 2. Geological framework 2.1. Geological setting The Phu Lon Cu–Au skarn deposit is located in the town of Sang Khom, on the right bank of the Mekong River. Geologically, it is situated in the northern Loei Fold Belt (LFB), which lies along the western margin of the Indochina terrane (Fig. 1B). The LFB hosts several economic ore deposits (Fig. 1C), such as Chatree (epithermal Au–Ag), Puthep 1 (porphyry-related skarn Cu), Phu Thap Fah
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Fig. 1. (A) Map of Asia showing the tectonic plates and their relative motion (see arrows). Small box shows location of B. (B) Map of mainland Southeast Asia showing the major terranes, suture zones, faults and active subduction zones, modified from Searle (2006). ASRR, Ailao Shan-Red River. Box shows location of C. (C) Map showing the location of major ore deposits along the Loei Fold Belt, Truongson Fold Belt and Sukhothai Fold Belt. All base maps are after http://www.ngdc.noaa.gov/mgg/image/.
(Au skarn), Phu Kham (porphyry-related skarn Cu–Au) and Phu Lon (Cu–Au skarn). The LFB records a complex history of subduction, collision, accretion and deformation along the length of the western margin of the Indochina terrane. Its origin and history can be traced back to the Devonian when the Indochina terrane was rifted away from northwestern Gondwana (Metcalfe, 2005). Subsequently, the Shan-Thai terrane, an allochthonous continental microplate from northeastern Gondwana, rifted and separated in the Early Permian (Metcalfe, 2005). After drifting northwards, these tectonic terranes appear to have collided and amalgamated during the Triassic (Indosinian Orogeny; Mitchell, 1981; Metcalfe, 1995, 1996; Khin Zaw, 2007; Meffre et al., 2008), forming the LFB of central Thailand. However, the precise collision, amalgamation and accretion history remains unclear and is still controversial (Carter et al., 2001). The Phu Lon deposit has a resource of 5.4 Mt, grading 2.4% Cu and 0.64 g/t Au (supplementary appendices on CD-ROM of Meinert et al., 2005). It is hosted by a Devonian volcano-sedimentary sequence intercalated with limestone and marble units (Fig. 2). These rocks are underlain by Silurian-Devonian greenschist-facies metasedimentary rocks and are overlain by Permian-Triassic units of intermediate to felsic volcanic rocks. The stratified sequence of volcano-sedimentary units was intruded by porphyries, including diorite and quartz monzonite. These intrusions contain pyroxene, hornblende, biotite, plagioclase, alkali-feldspar, quartz, magnetite
and titanite and have geochemical affinities to fractionated and oxidized I-type calc-alkaline magmas (Kamvong and Khin Zaw, 2005). LA-ICPMS U-Pb dating of zircon from quartz monzonite phases indicates an age of 244 ± 4 Ma (Kamvong and Khin Zaw, 2005). 2.2. Skarn formation and mineralization According to the classification schemes of Einaudi et al. (1981) and Meinert (2000), the skarn at Phu Lon is a Cu skarn with byproduct Au. The deposit is a calcic skarn, rich in andradite–diopside–epidote–amphibole assemblages and can be subdivided into exoskarn and endoskarn zones (Kamvong and Khin Zaw, 2005). Fig. 3 illustrates a simplified paragenetic sequence for the skarn mineral assemblages and ore sulfides within the Phu Lon deposit, previously outlined by Pisutha-Arnond et al. (1993), Sitthithaworn et al. (1993) and Kamvong and Khin Zaw (2005). The endoskarn formed as irregular pods and discordant veins and is limited to the margin of the Phu Lon porphyries (both diorite and quartz monzonite). The endoskarn mainly consists of pale brown to colorless, fine- to coarse-grained garnet and subordinate pyroxene, epidote, actinolite, calcite, chlorite, quartz, magnetite, pyrite, chalcopyrite and pseudomorphs of the original feldspar grains (Fig. 4A and B). The massive bodies of exoskarn are divided into prograde (Fig. 4C) and retrograde (Fig. 4D) skarn assemblages.
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Fig. 2. Simplified geologic map (modified from Kaewsang (1995)) of the Phu Lon deposit, showing distribution of rock units and drill hole locations.
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Fig. 3. Generalized paragenetic diagram for skarn minerals, sulfides and gold at the Phu Lon deposit (modified from Sitthithaworn (1989) and Kamvong and Zaw (2005)).
Prograde skarn alteration dominantly comprises massive brown andradite and diopside, which precipitated contemporaneously with the magnetite mineralization. The retrograde skarn assemblage that overprints the prograde skarn minerals is recorded by the formation of hydrous minerals (e.g., epidote, chlorite, amphibole), calcite, quartz and sulfides (pyrite, chalcopyrite, sphalerite, bornite, molybdenite, marcasite). Sitthithaworn (1989) indicated that free gold grains formed as small inclusions in chalcopyrite, pyrite and sphalerite in the retrograde skarn alteration. Such evidence indicates that the bulk-metal budget at Phu Lon is related to the retrograde skarn stages. 3. Sampling and methods In order to ensure that the fluid inclusions are genetically related to the skarn genesis, only samples containing primary fluid inclusions in skarn minerals from the prograde and retrograde stages were analyzed. The best preserved fluid inclusions were found in the coarse-grained garnet and epidote of these skarn
stages. These garnet- and epidote-hosted primary fluid inclusions can provide constraints on the temperature, pressure and composition of skarn-forming fluids (Klemd, 2004; Meinert et al., 2005). Microthermometry of fluid inclusions was conducted on a Linkam MDS 600 stage, with a temperature range between 200 and 600 °C, mounted on an Olympus BX60 microscope. The temperature of the stage was calibrated using a set of Syn-Flinc synthetic fluid inclusions. The precision of measured temperatures is ±1.0 °C for heating and ±0.3 °C for freezing (Rae et al., 2003). Salinity estimates are reported in weight percent NaCl equivalent and were determined from the final melting temperatures of ice (e.g., Bodnar, 1993; Rusk et al., 2008). Laser Raman spectroscopic data were obtained at Geoscience Australia using a Dilor SuperLabram laser Raman microprobe. A total of 12 significant coarse-grained sulfides (sphalerite, pyrite, chalcopyrite) from the prograde and retrograde stages were analyzed for their sulfur isotope compositions at the Central Science Laboratory (CSL), University of Tasmania. The method was carried out on mineral separates drilled from samples of coarse-
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Fig. 4. Photomicrographs of representative samples from the Phu Lon deposit, showing details of skarn mineral assemblages. (A and B) Diopside and andradite replacing intrusive porphyry in the endoskarn unit (sample PL5-17 at 116 m). Note the original magmatic textures and minerals are totally obliterated. (C) Diopside replacing and overprinting andradite in the prograde skarn alteration stage (sample PL5-3 at 15.7 m). Andradite also shows concentric zoning. (D) Epidote together with chlorite replacing andradite in the retrograde skarn alteration stage (sample PL5-16 at 114.6 m).
grained sulfides, using the procedure of Robinson and Kusakabe (1975). Results are expressed in the d34S (‰) relative to the Canyon Diablo Troilite (CDT). Analytical accuracy is ±0.1‰. 4. Results and interpretation 4.1. Fluid inclusions 4.1.1. Classification Four types of fluid inclusions in garnet and epidote were distinguished in terms of number, nature and proportion of phases at room temperature (Kamvong et al., 2006). Epidote only contained one type of fluid inclusions. Type I inclusions contain two phases (liquid + vapor) and are liquid-rich at room temperature. These inclusions are found in both the prograde (Fig. 5A and B) and retrograde skarn stages (Fig. 5C–E). They range in size from <5 lm to 60 lm, have irregular, elongate and oblate shapes and typically occur along growth zones in garnet and epidote, consistent with a primary origin. Type II inclusions contain three phases (liquid + vapor + solid phases; (Fig. 5F–I) and are characterized by the presence of small opaque and translucent daughter phases identified
by Laser Raman analysis as hematite and calcite, respectively. Type II inclusions were also observed in prograde stages. They range in size from <5 lm to 30 lm and generally occur in bands parallel to the optically visible zonation of garnets. These fluid inclusions are also interpreted to be primary. Type III inclusions are rare, salt-saturated inclusions and contain liquid, vapor and halite (Fig. 5J). They range in size up to 50 lm, have negative crystal shapes and occur as both isolated inclusions and small inclusions on garnet growth zones. Type IV inclusions are vapor-rich without soluble solid phases (Fig. 5K–M). These inclusions range in size up to 20 lm and are found within garnet growth zones. 4.1.2. Microthermometry Due to the rare occurrence of Types III and IV fluid inclusions, microthermometric data have not been collected for these inclusions. Thus, only Types I and II inclusions were investigated. Heating and freezing experiments were made to determine the homogenization (Th(LV ? L)) and final ice melting temperatures (Tm) of Types I and II inclusions from epidote (retrograde stage) and garnet (prograde stage), respectively. Results of microthermometry are shown graphically in Fig. 6. Table 1 is the summary
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Fig. 5. Photomicrographs of skarn fluid inclusions trapped in garnet and epidote. (A–E) Type I inclusions (liquid + vapor). Fluid inclusions in a and b occur along zonation parallel bands in garnet (sample PL5-47 at 238.5 m). (C–E) Primary fluid inclusions trapped in epidote (sample PL5-10 at 36.4 m). (F–I) Type II inclusions (liquid + vapor + solid phases) contain small opaque and translucent daughter phases. Laser Raman analysis identified these solid phases as hematite (opaque) and calcite (translucent solid phases). (J) Type III inclusions are salt-saturated fluid inclusions with halite daughter crystals (sample PL5-47 at 238.5 m). (K–M) Type IV vapor-rich inclusions. Fluid inclusions in (K) and (M) formed on garnet growth zones. Dashed line represents growth zonation of garnet. Abbreviations: cal, calcite; hl, halite; hem, hematite; L, liquid; V, vapor.
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Fig. 6. Frequency histograms of homogenization temperatures (Th(LV ? L)) and salinities (wt% NaCl equiv) for the Types II and I fluid inclusions in garnet (prograde skarn) and epidote (retrograde skarn).
Table 1 Summary of microthermometric data from fluid inclusions hosted in garnet and epidote from the Phu Lon skarn deposit, Thailand Sample
Host mineral
Prograde skarn stage PL5-47 Garnet PL5-34 Garnet PL5-18 Garnet Retrograde skarn stage PL5-10 Epidote
FI type
Th(LV ? L)
Tm (ice)
Range
Mean
n
II II II Total
434.2–468.5 425.3–460.0 396.7–421.6 421.6–468.5
448.2 447.8 410.3 433.5
5 3 5 13
I
350.9–399.8
377.2
7
Salinity
Range 21.1 20.3 18.7 21.1
to to to to
5.1 to
14.0 13.6 14.8 13.6 0.3
Mean
n
Range
Mean
n
17.8 16.9 16.2 17.1
4 2 3 9
23.1–17.8 22.6–17.4 21.5–18.5 23.1–17.4
20.7 20.0 19.6 20.2
4 2 3 9
1.4
7
0.53–8.0
2.33
7
Notes: Type I = liquid + vapor; Type II = liquid > vapor + solid daughter phases (hematite and calcite); Th(LV ? L) = temperature of homogenization into liquid phase; Tm (ice) = temperature of final ice melting; temperatures in °C units; salinities in wt% NaCl equiv.
of microthermometric data for fluid inclusions in this study. Measured temperatures of homogenization for Type II fluid inclusions in the prograde stage range from 421.6 to 468.5 °C, whereas Type I fluid inclusions in the retrograde stage show homogenization temperatures in the range between 350.9 and 399.8 °C (Table 1; Fig. 6). Daughter minerals that were observed in the Type II inclusions apparently do not dissolve upon heating over 470 °C, suggesting that they were accidentally trapped rather than precipitated and are not considered to be true daughter minerals. Final ice melting temperatures for Type II inclusions in the prograde stage and Type I fluid inclusions in the retrograde stage range from 13.6 to 21.1 °C and 0.3 to 5.1 °C, respectively (Table 1; Fig. 6). Apparent salinities for Types II and I fluid inclusions in the prograde and retrograde stages are 17.4–23.1 wt% NaCl equivalent and 0.5–8 wt% NaCl equivalent, respectively. Eutectic temperatures (Te) for Type II inclusions in the prograde stage range
from 40 to 50 °C, which are below the eutectic temperature for the H2O–NaCl and H2O–NaCl–KCl systems (Hall et al., 1988), suggesting the presence of additional components such as CaCl2, MgCl2 and FeCl2. 4.1.3. Comparison with other deposits The fluid inclusion data derived from garnet are broadly similar to those documented for other skarn deposits such as the Santa Rita porphyry Cu skarn (Ahmad and Rose, 1980) and the Mines Gaspé Cu skarn (Shelton, 1983) (Fig. 7). The existence of hematite phases in fluid inclusions and the high homogenization temperatures in the prograde skarn are also similar to conditions documented for prograde fluid events in other Cu–Au skarn deposits (e.g., Ahmad and Rose, 1980; Myers, 1990). However, the presence of halite in Type III inclusions may have locally contributed to the higher salinity during the prograde skarn stage although no heat-
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Fig. 7. Plot of measured salinities versus homogenization temperatures for skarn fluid inclusions from Phu Lon together with data from other Cu–Au skarn deposits (modified from Franchini et al. (2000)). Homogenization temperature–salinity fields are from Theodore and Blake (1978) for Copper Canyon, Ahmad and Rose (1980) for Santa Rita, Shelton (1983) for Mines Gaspé and Meinert et al. (1997) for Big Gossan. The gray box illustrates temperature–salinity trends or fluid evolution paths as a result of different geological processes (modified from Shepherd et al. (1985)).
ing/freezing experiments were undertaken for these inclusions due to their rarity. Moreover, the lower homogenization temperatures and lower salinities of fluid inclusions in epidote are similar to the retrograde stage in most skarn systems (e.g., Singoyi and Khin Zaw, 2001; Meinert et al., 2003). 4.2. Sulfur isotopes The majority of d34S values for sulfides associated with both prograde and retrograde skarn at Phu Lon fall in the range 2.6 to 1.1 ‰, with a mean of 1.9 ‰ d34S (Table 2). Seven chalcopyrite samples have d34S values between 2.6 and 2.1 ‰, three sphalerite samples have d34S values of 1.2 to 1.1‰, and two pyrite samples have d34S values of 1.9 to 1.4‰. Based on these results, there is no evidence that the sulfides vary systematically in isotopic composition with respect to skarn alteration stages. Instead, most of the d34S values for sulfides have a remarkable homogeneity (Table 2). Such evidence may indicate the lack of
Table 2 Sulfur isotope compositions of ore sulfides from the Phu Lon deposits Sample no.
Drill hole and depth
Mineral
Stage
PL5-7 PL5-9 PL5-12a PL5-13 PL5-22 PL5-27 PL5-30 PL5-38 PL5-39 PL5-41 PL5-48 PL5-49
DDH-PL5, DDH-PL5, DDH-PL5, DDH-PL5, DDH-PL5, DDH-PL5, DDH-PL5, DDH-PL5, DDH-PL5, DDH-PL5, DDH-PL5, DDH-PL5,
Pyrite Chalcopyrite Chalcopyrite Chalcopyrite Chalcopyrite Chalcopyrite Sphalerite Sphalerite Chalcopyrite Chalcopyrite Pyrite Sphalerite
Retrograde Retrograde Prograde Retrograde Prograde Prograde Retrograde Prograde Retrograde Retrograde Retrograde Marble
23 m 25 m 57.1 m 84 m 145.1 m 184.9 m 192.5 m 210.2 m 217 m 218.1 m 240.2 m 338.9 m
d34S ‰ (CDT) 1.9 2.6 2.4 2.4 2.5 2.6 1.1 1.1 2.1 2.4 1.4 1.2
Fig. 8. Total range of d34S values from the Phu Lon deposit in comparison to values from other Cu skarns (Taylor, 1987), Mine Gaspé (Shelton and Rye, 1982), Big Gossan (Prendergast et al., 2005) and Wanagon (Prendergast et al., 2005).
significant contamination or assimilation of country-rock sulfur. The d34S compositions at Phu Lon are similar to other major
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Fig. 9. Schematic paragenetic diagram of mineralized skarn relative to interpreted fluid paths of the Phu Lon deposit. It also shows possible skarn formation temperatures. Mineral abbreviations: act, actinolite; adr, andradite; cal, calcite; chl, chlorite; di, diopside; ep, epidote; qtz, quartz.
Cu–Au skarn deposits (Fig. 8) although the values are generally more negative. These more negative d34S values may reflect a more oxidized environment of the host rocks at Phu Lon (cf., Ohmoto and Rye, 1979; Salier et al., 2005). Overall, the range of d34S values for all sulfides analyzed from the Phu Lon deposit is very narrow ( 2.6 to 1.1‰), suggesting only one well-homogenized source of sulfur, largely of magmatic origin (e.g., Shimazaki and Yamamoto, 1979, 1983; Shelton, 1983; Shimazaki and Sakai, 1984; Ohmoto and Goldhaber, 1997).
tinued to cool (e.g., Shelton, 1983; Einaudi et al., 1981; Haynes and Kesler, 1988; Meinert et al., 1997; Bowman, 1998; Figs. 7 and 9). According to Laser Raman data, fluid inclusions in garnet contain hematite daughters. Such characteristics show evidence of an oxidizing environment during fluid inclusion entrapment. The oxidizing condition of ore formation was also recorded in other skarn systems (e.g., Khin Zaw and Singoyi, 2000).
5. Discussion
Salinities and homogenization temperatures from the two most abundant fluid inclusion populations at the Phu Lon Cu–Au deposit are consistent with a model of fluid mixing between two endmember fluids combining to form an oxidized Cu–Au skarn. Higher-temperature higher-salinity fluids representative of magmatic fluids are trapped within prograde skarn mineral assemblages. Lower-temperature lower-salinity fluids, displaying a possible meteoric affinity are trapped within the retrograde skarn assemblages. Additional data consistent with an oxidized skarn model are hematite-bearing fluid inclusions and oxidized mineral assemblages comprised of andradite, magnetite and hematite. The uniform and narrow range of d34S values suggests a well-homogenized possibly igneous sulfur source.
Field observations and fluid inclusion and sulfur isotope studies indicate that the Phu Lon skarn deposit was formed during multiple geological events similar to other Cu–Au skarn systems (e.g., Einaudi et al., 1981; Shelton, 1983; Meinert et al., 1997; Forster et al., 2004; Fig. 9). Emplacement of the Phu Lon intrusive porphyries during the Early Triassic was followed by expulsion of a juvenile magmatic-hydrothermal brine into a zone of highly fractured rocks in and around the intrusions. The magmatic fluid was likely channeled from the intrusions through the marble and migrated along permeable zones, reacting with the host rocks (i.e., limestone) to form prograde skarn assemblages. During the retrograde skarn stage, the formation of epidote and other retrograde minerals may have resulted from mixing of magmatic fluids with meteoric water input (e.g., Einaudi et al., 1981; Haynes and Kesler, 1988; Bowman, 1998). At Phu Lon, the high temperatures (up to 468 °C) and moderate salinity (up to 23 wt% NaCl equiv.) of fluid inclusions in prograde skarn-stage garnet indicate that the fluid inclusions represent the composition of magmatic fluids after reaction with the calcareous wall rocks (cf., Franchini et al., 2000). By contrast, fluid inclusions in epidote (retrograde skarn stage) are both low in temperatures and salinities as low as 0.5–8 wt% NaCl equivalent. Such characteristics may have resulted from mixing between magmatic fluids and external fluid sources (e.g., meteoric waters), causing more dilute fluids, lower temperatures and lower salinities, as the system con-
6. Conclusions
Acknowledgements This research has been carried out as a part of the project ‘‘Geochronology, Metallogenesis and Deposit Styles of the Loei Foldbelt in Thailand and Laos PDR” lead by Khin Zaw and financially supported from the Australian Research Council (ARC), Kingsgate Consolidated Ltd., Oxiana Ltd. (now OZ Minerals Ltd.) and PanAust Ltd. We thank Ross Large for making the manuscript more robust. Valuable suggestions by Daniel Marshall and an anonymous reviewer are greatly appreciated. Thanks also go to Terry Mernagh from Geoscience Australia for assisting with Laser Raman analysis of fluid inclusions. Department of Primary Industries and Mines (DPIM) in Thailand is thanked for permission to sample the drill
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cores. Editorial acknowledged.
handling
by
Ian
Metcalfe
is
gratefully
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