The δ 65 Cu values of Cu (II) minerals

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fractionation, phase separation among vapor, brine and sulfide, and partitioning of Cu isotopes between fluid and mineral during sulfide precipitation (Graham et al., 2004; Markl et al., 2006; Seo et al., 2007; Li et al., 2010; Maher et al., 2011;

Berkenbosch et al., 2015; Savage et al., 2015).

Gerel and Munkhtsengel (2005) proposed a genetic model for the Erdenetiin Ovoo deposit including multiple partial melting of the upper mantle to produce the wide span of composition of host rocks from diorite to granite. They suggested that multiple partial melting and intrusion led to at least three distinguishable intrusive rocks with variable compositions, textures and ages(Koval et al., 1982). Therefore, the δ⁶⁵Cu values of chalcopyrite in the Erdenetiin Ovoo deposit may be interpreted to be affected by different magma or hydrothermal fluid sources with different Cu isotope compositions. However, it is unclear whether the variable δ⁶⁵Cu values of chalcopyrite in the Erdenetiin Ovoo deposit are controlled by heterogeneous Cu sources due to a lack of distinct evidence. Maher et al. (2011) noted that the explanations for a larger δ⁶⁵Cu value range of hypogene minerals in some ore deposits vary slightly depending on the case study and deposit types, and the reason is not fully understood. Further research is needed to interpret this large variation of δ⁶⁵Cu values in chalcopyrite from the Erdenetiin Ovoo deposit.

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4.72 ‰ to 9.05 ‰ (avg. 6.88 ‰), respectively (Table 2). When secondary Cu minerals are produced from the mineralizing fluid under low-temperature conditions, the magnitude of Cu isotope fractionation varies depending on the mineral phase (Zhu et al., 2002; Ehrlich et al., 2004). As mentioned above, the lighter Cu isotope can be enriched in the secondary Cu sulfide minerals by up to 3 ‰ compared to the solution from which precipitate (Larson et al., 2003; Ehrlich et al., 2004; Mathur et al., 2005; Braxton & Mathur, 2011). By contrast, the secondary Cu (II) minerals where Cu exists as Cu²⁺ show much smaller Cu isotope fractionation (Maréchal &

Sheppard, 2002; Ehrlich et al., 2004). In an experimental study, Maréchal and Sheppard (2002) reported that the Cu isotope fractionation between Cu²⁺ ion in solution and malachite is 0.20 ‰ to 0.38 ‰ at 30 °C.

The characteristics of the δ⁶⁵Cu values of Cu (II) minerals in the Erdenetiin Ovoo deposit indicate that they were precipitated from fluid with a different δ⁶⁵Cu value and/or at different timing in the supergene environment. Because of the small Cu isotope fractionation during the precipitation of Cu (II) minerals, they appear to reflect the δ⁶⁵Cu value of the mineralizing solution. These results imply that chrysocolla might be precipitated from the solution with lower δ⁶⁵Cu values, whereas malachite and azurite formed from the ⁶⁵Cu-enriched solution.

As the secondary Cu sulfide minerals precipitate from the fluid in the supergene environment, the δ⁶⁵Cu value of the fluid becomes heavier due to the preferential partitioning of the lighter Cu isotope (⁶³Cu) into the mineral phase. If secondary Cu (II) minerals are precipitated from single solution, the deposition of chrysocolla with a lower δ⁶⁵Cu value could occur at relatively early stage of fluid evolution, when the value of δ⁶⁵Cu is sufficiently low. Then, malachite and azurite would be precipitated from the fluid with increasing δ⁶⁵Cu value due to the partitioning of the lighter Cu

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However, it is not plausible that the δ⁶⁵Cu values of secondary Cu (II) minerals in the Erdenetiin Ovoo deposit could be affected by the evolution of a single mineralizing fluid. The natural variation of Cu isotope composition in supergene ore minerals has been reported to range from -16.5 ‰ to 12 ‰, much larger than the measured Cu isotope fractionation factors in experimental studies (e.g., Larson et al., 2003; Mathur et al., 2005; Mathur et al., 2009). The greater variation of δ⁶⁵Cu values observed in natural ore minerals has been attributed to various factors, such as multiple cycles of redox reactions, fractionation processes among dissolved species within a fluid and between dissolved and solid phases during precipitation, Rayleigh fractionation in a closed system, fractionation behavior in an open system and ligand- bonding characteristics (Markl et al., 2006; Asael et al., 2009; Sherman, 2013;

Moynier et al., 2017).

The δ⁶⁵Cu values of Cu (I) and Cu (II) mineral pairs can be helpful for tracing the signature of Cu transport at the deposit scale (Markl et al., 2006; Asael et al., 2007; Mathur et al., 2009). Mathur et al. (2009) suggested that the significance of Cu transport in the supergene environment can be examined by measuring the δ⁶⁵Cu values of Cu (I) and Cu (II) mineral pairs. If the difference between the δ⁶⁵Cu values between Cu (II) and Cu (I) minerals is positive (ΔCu (II) mineral – Cu (I) mineral > 0), there is an insignificant mobilization of Cu. This indicates an occurrence of a rough mass balance during the reactions, and the products exist near the reaction site (Mathur et al., 2009). By contrast, no mass balance occurs and most Cu is transported laterally when the ΔCu (II) mineral – Cu (I) mineral value is negative(Mathur et al., 2009).

The positive difference in δ⁶⁵Cu values between Cu (II) and Cu (I) minerals (Table 2) therefore suggests an insignificant transport of Cu in the Erdenetiin Ovoo

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deposit, implying that the precipitation of Cu (II) minerals by the redox reactions occurs adjacent to the site where the reactions occur. Thus, the redox reactions occur very quickly, and there is insufficient time for the transport of Cu toward the distant area. In this case, the border between the leach cap and enrichment zone tends to vary slightly (Mathur et al., 2009). However, it is hard to identify this relationship because the leach cap zone has been already eroded in the Erdenetiin Ovoo deposit.