Influence of fluid infiltration on the carbon and oxygen isotopic compositions of calcite from Khoud copper skarn deposit, west of Yazd

Document Type : Research Article

Authors

• Azam Zahedi ¹
• Mohammad Boomeri ²
• Mohammad Ali Mackizadeh ³

¹ Assistant Professor, Department of Geology, Faculty of Sciences, University of Gonabad, Gonabad, Iran

² Professor, Department of Geology, Faculty of Sciences, University of Sistan and Baluchestan, Zahedan, Iran

³ Associate Professor, Department of Geology, Faculty of Sciences, University of Isfahan, Isfahan, Iran

10.22067/econg.2023.81026.1069

Abstract

The Khoud copper skarn ore deposit, located about 50 km northwest of the Taft City in Yazd province, is situated within the middle part of the Urumieh-Dokhtar magmatic belt. The geology of this ore deposit is dominated by upper triassic carbonate units that were intruded by an Oligo-Miocene granodiorite porphyry. Garnet and garnet-pyroxene skarn zones had replaced marble and limestone in the contact zone with the granodiorite porphyry. In this paper, δ¹⁸O and δ¹³C values of calcites from limestone (sedimentary calcite), marble (metamorphic calcite), and skarn calcite have been reported and interpreted. The data shows that systematic depletion of ¹⁸O and ¹³C has occurred in the metamorphic and skarn calcites. The skarn calcites show the highest depletion compared to calcite in marble and limestone. Calculated models showed that this systematic isotopic depletion in the study calcites can be attributed to a magmatic fluid infiltration and fluid-induced decarbonation. Depletion in ¹⁸O and ¹³C of various calcite types of Khoud deposit can be explained by magmatic fluid (δ¹⁸O_fluid = +9.0‰) that reacted/interacted with unaltered carbonate rocks at temperatures of less than 400^oC with X_(CO2) lower than 0.05 and W/R ratio of 25 to 50%.
 
Introduction
The Khoud copper skarn deposit is located 50 km west of Yazd in the middle part of the Urmia Dokhtar magmatic belt. This magmatic belt is known for porphyry Cu, skarn Cu and epithermal Au ore deposits. The study area is dominated by Mesozoic marble and carbonate rocks that intruded a granodiorite porphyry and host skarn Cu mineralization. The mineralization occurs as massive sulfides such as chalcopyrite, bornite, pyrite and pyrrhotite replacement within marble and skarn. Supergene minerals occur as Cu carbonates and iron hydroxides. The sedimentary, metamorphic, metasomatic and Cu mineralization in the Khoud may be representative different fluids and indicate that the study area had experienced variable water-rock interaction. Shieh and Taylor (1969) made the first carbon and oxygen stable isotopes studies of skarn systems and showed that calcite in skarn zones (skarn calcite) was sharply depleted in ¹⁸O and ¹³C relative to calcites in marble (marble or metamorphic calcite) and limestone (sedimentary calcite). In the last few decades, the ¹⁸O and ¹³C data on the skarns and carbonates of metamorphic aureoles has been subject of many studies (for example, Shin and Lee, 2003; Boomeri et al., 2010; Orhan et al., 2011; Demir et al., 2017). The objective of this paper is to apply carbon and oxygen isotopic studies of carbonate minerals in the metamorphic-metasomatic aureole of the Khoud intrusions to understand nature of fluids and estimate water-rock ratio by mass balance calculation and modelling.
 
Method
Approximately 20 mg of powdered samples of granite rocks were reacted in F₂ gas, in a nickel tube at 500° C for twelve hours to produce O₂ gas. This gas was finally converted into carbon dioxide gas in a graphite furnace at a temperature of 700^o C and was collected by a pump and a liquid nitrogen trap.
Approximately 20 mg of powdered calcites was decomposed in pure phosphoric acid at 25°C and the released carbon dioxide gas was used for isotopic analysis. The released carbon dioxide gas was collected in a liquid nitrogen trap. Carbon dioxide gas was then separated from water vapor by replacing the dry ice-acetone trap. Carbon dioxide gas isotope measurements were performed using a mass spectrometer using a Finnigan MAT 250 Mass Spectrometer of the Akita University.
 
Results and Discussion
Silicates
The δ¹⁸O values of the granodiorite porphyry range from 11.3 to 12.8 ‰ (Table 2). These δ¹⁸O values are higher than those of the I-type granitoids (e.g., Taylor and Sheppard, 1986; +8.0 to +10.0 ‰). The higher δ¹⁸O values could indicate crustal contamination in magma or direct exchanges between granitic melt and metamorphic sedimentary rocks (Taylor and Sheppard, 1986). The alteration and weathering are another processes to change and increase δ¹⁸O value of the granodiorite porphyry. Maximum homogenization temperature of fluid inclusions in garnets of the study area is 361 ^oC which with pressure correction the maximum temperature of hydrothermal fluids is 400 ^oC (Zahedi et al., 2014). Therefore, the δ¹⁸O value of a fluid in equilibrium with the Khoud granodiorite porphyry can be calculated at temperature of 400 ^oC (according to the fluid inclusion in garnet) and the oxygen isotopic fractionation factor of O'Neil and Taylor (1967) or 1000Ln_{plagioclase (An=30 %) -water}=2.68(1000000/T²)-3.29. For granite,  is close to  (Taylor, 1978). The δ¹⁸O value calculated for this fluid ranges from 8.7 to 10.2 ‰ (Table 2), that is in the range of a magmatic water.
 
Carbonates
d¹⁸O and d¹³C values of various calcite types from the Khoud deposit are shown in Table 3 and are drawn in Figure 5. The d¹⁸O and d¹³C values of limestone calcites vary from 23.6 to 24.7 ‰ and 2.2 to 2.4 ‰, respectively. The d¹⁸O and d¹³C values of marble calcites range from 15.5 to 19.0 ‰ and -0.7 to 0.0 ‰, respectively. The d¹⁸O and d¹³C values of skarn calcite changes from 12.2 to 13.4 ‰ and -2.9 to -0.4 ‰, respectively. The d ¹⁸O and d¹³C vales of the sedimentary calcites are in range of normal marine sedimentary calcites, the marble calcites have the lower value, the skarn calcites are strongly depleted in d ¹⁸O relative to limestone and marble calcites. The coupled ¹⁸O-¹³C depletions are observed in many skarn systems involving carbonate (Shin and Lee, 2003; Taylor and O'Neil, 1977). The depletions have been made by batch and Rayleigh decarbonation and infiltration. In the skarn systems, d ¹⁸O and d ¹³C depletions by batch volatilization or decarbonation are low while d¹³C depletion by Rayleigh volatilization is high. The isotopic effects by Rayleigh volatilization are illustrated in Fig. 4. This illustration shows trends of Rayleigh volatilization. The d ¹⁸O and d ¹³C values of marble and skarn limestones shown in Fig. 4 shift around from Rayleigh decarbonation trends.
In fact, the large isotopic shifts in d¹⁸O and d¹³C of skarn calcites from Khoud deposit can be caused by infiltration processes. Significant infiltration fluids could also cause large d¹⁸O and d¹³C depletions in skarn calcites. There are models that can be used to express the nature and rate of isotopic change of carbonate host rocks as a function of progressive increase in water-rock ratio where temperature and X_(CO2) are known (Boomeri et al., 2010). These models are widely used to interpret changes in d¹⁸O and d¹³C values in carbonate rocks in metasomatic-metamorphic halos, for example by (Shin and Lee, 2003). The mass balance equations of closed and open systems from Taylor and O'Neil, 1977 and isotopic fractionation factors from Friedman and O^' Neil, 1977 for calcite-water and calcite-CO₂ was used to model the isotopic variation of carbonate rocks in Khoud deposit.
The d¹⁸O and d¹³C variation curve diagrams shown in figure 5 are the result of interaction between the hydrothermal solutions (δ¹³C = -8.0 ‰ and δ¹⁸O = +9.0‰) and the fresh limestone (δ¹⁸O=25.0 ‰, δ¹³C = 3‰) at temperatures of 250° to 400° C in open systems with X_(CO2) = 0.02 and water/rock ratio of 0 to 100 % for calcites. Therefore, the low d¹⁸O and d¹³C value of skarn calcite can be interpreted by infiltration of the magmatic water in the calcic skarn. The amount of d¹⁸O and d¹³C depends on several factors such as initial isotopic composition of rock, fluid, temperature, water-rock ratio and X_(CO2). As Figure 5 shows the d¹⁸O and d¹³C depletion could mainly be caused by magmatic fluids (d¹⁸O =9.0‰) that reacted/interacted with unaltered limestone rocks at temperatures of 400^oC with X_(CO2) =0.02 and water/rock ratio of 20 to 50%.
 
Acknowledgments
The isotopic analysis of the samples was done using the laboratory equipment of Akita University, Japan. The authors sincerely thank and appreciate the cooperation of Prof. Ishiyama, from the Department of Geosciences, Akita University, Japan.

Keywords

• Isotopic depletion
• C and O Stable isotopes
• Skarn
• Khoud
• Yazd

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