Multiple Solutions of Ore-Forming Fluids of Carbonate Rock-Related Nephrites Constrained by the Hydrogen and Oxygen Isotopes

1. Introduction

Hydrogen and oxygen isotopes of rocks and minerals are generally useful as isotopic tracers in analyzing the source and evolution of geological fluid [1]. Nephrite, a metamorphic rock composed of tremolite (or actinolite), has a natural advantage of using the δD and δ¹⁸O to discuss the origin of its ore-forming fluid. In the past two decades, it is very prevalent to adopt the δD and δ¹⁸O of nephrite to invert the origin of ore-forming fluid [2,3,4,5,6,7]. S-type nephrite occurred in serpentinite (or serpentinite mélange) has a wide range of δD and δ¹⁸O [8,9], and sources of the ore-forming fluid are complex, involving the metamorphic water, seawater and meteoric water [9]. For C-type primary nephrites that is associated with carbonate rocks, the δD and δ¹⁸O of nephrite from the same deposit are relatively convergent (Figure 1a) [2,4,5,7,10], but there are significant differences in δD and δ¹⁸O among different nephrites, resulting in the diversity of interpretation on their ore-forming fluids [7,10].

Traditionally, metamorphic water is considered as the source of ore-forming fluid of nephrites with heavy δ¹⁸O, such as Sanchakou nephrite (δ¹⁸O=+11.4-+12.6‰) [11], Dahua nephrite (δ¹⁸O=+10-+12.3‰) [12] and Luodian nephrite (δ¹⁸O=+14.1-+17.7‰) [13] (Table 1 and Figure 1). However, this assertion is not supported by the geological settings. These nephrites are formed by contact metasomatism between basic rocks and carbonate rocks, therefore the genesis of δ¹⁸O anomaly of these nephrite is still a controversial issue. In addition, hydrogen and oxygen isotope compositions of parent rocks of nephrite are of considerable importance, because any composition in the system must be accounted for during the evolution of ore-forming fluids, otherwise it is difficult to determine actual causes of δD and δ¹⁸O anomaly of nephrite, much less to trace sources of ore-forming fluids. But so far, researches on isotope compositions of parent rocks lag behind that of nephrites. Understanding how parent rocks control the isotope compositions of nephrite can contribute to the constraint of the origin and evolution of ore-forming fluids. Correspondingly, it is not comprehensive to determine the ore-forming fluids of nephrites only by the δD and δ¹⁸O of tremolite. These questions motivate us to discuss the couplings between fluids, country rocks and tremolite, and re-deconstruct the source and evolution of ore-forming fluids of nephrites.

2.δ. D and δ¹⁸O of Different Rocks in C-Type Nephrite Deposits

2.1. δ. D and δ¹⁸O of C-Type Nephrite

Although the contact metasomatism between intrusive rocks and carbonate rocks dominates the formation of most C-type nephrites, such as C-type nephrites widely distributed in the southern margin of Tarim Basin [3,4,25,26], Sanchakou nephrite [11], Xiaomeiling nephrite [27], Vitim nephrite [6,28], Chuncheon nephrite [5], Złoty Stok nephrite [22], Dahua nephrite [12], Luodian nephrite [13], etc, significant differences in the δD and δ¹⁸O between different nephrites suggest that their ore-forming fluids have different evolutionary processes.

Meteoric water is considered to be participated in the ore-forming processes of Vitim nephrite [6,28], Chuncheon nephrite [5] and Xiaomeiling nephrite (Figure 1b), and their δD and δ¹⁸O domains are independent of each other and do not overlap with other nephrites (Figure 1a), making them highly recognizable. Paradoxically, the δD of Xiaomeiling nephrite is much the same as that of Chuncheon nephrite, which could be attributed to meteoric water participating in the growth of tremolite, but the δ¹⁸O far exceeding that of its parent rock (Miaoxi granite porphyry) indicates the existence of other heavy δ¹⁸O material in the Xiaomeiling nephrite deposit (Figure 1a and Table 1). The anomaly is difficult to be explained from previous studies about the ore-forming fluid of nephrite.

For Pishan nephrite, Alamas nephrite, Yinggelike nephrite and Złoty Stok nephrite, their δD and δ¹⁸O domains and their ore-forming fluids are concentrated in and around that of most plutonic granitic rocks and that of magmatic water (Figure 1), indicating that magmatic water from intrusive rocks plays an important role in their ore formation processes. There is a significant difference between Xinjiang primary nephrites (Pishan nephrite, Alamas nephrite and Yinggelike nephrite) and Xinjiang secondary nephrites produced from Yurungkash and Karakash River. The δD and δ¹⁸O of Xinjiang primary nephrites are slightly less than those of most plutonic granitic rocks [1], and the range of ore-forming fluids is relatively narrow and closer to magmatic water (Figure 1) [3,4,26]. The range of δD and δ¹⁸O of secondary nephrites is much broader (Figure 1) [2]. indicating that the weathering related to low-temperature water-rock interactions may dominate the evolution of δD and δ¹⁸O of secondary nephrites (Figure 1). The δ¹⁸O range of Xiuyan placer nephrite overlaps with that of Xiuyan primary nephrite, while its δD is generally lighter (Figure 1), which could be the result of weathering. Consequently, the influence of Earth’s surface geochemical processes on the δD and δ¹⁸O of secondary nephrite is also a noteworthy issue.

Some nephrites are considered to be products of regional metamorphism, such as Xiuyan primary nephrite [29], Mastabia nephrite [30], Scortaseo nephrite [31], Luanchuan nephrite and Longxi nephrite. Previous researches suggested that the ore-forming fluid of Xiuyan primary nephrite could be the Si-rich hydrotherm produced by regional metamorphism and migmatitization [29,32]. Metamorphic water can be responsible for Xiuyan primary nephrites with heavy δ¹⁸O (Figure 1), but the source of metamorphic water with heavy δ¹⁸O directs the crux of the problem to the country rock.

The ore-forming fluid hardly has the hydrogen isotope exchange with country rocks of carbonate rocks without hydrogen element, implying that the process controlling the δD and δ¹⁸O evolution of tremolite is not synchronous. The δD evolution of ore-forming fluid in the hydrothermal system can be considered as a process only controlled by the initial and external fluid, and with the increase of the proportion of external fluid, the δD of final ore-forming fluid evolves towards the external fluid. Vitim nephrite and Chuncheon nephrite are typical examples of the ore-forming fluid evolving towards meteoric water.

2.2. δ. D and δ¹⁸O of Intrusive Rocks and Country Rocks

The fluid, no matter what its source, first reacts with country rocks, then tremolite crystallizes from the fluid, which determines an important significance of country rocks in constraining intermediate states of fluid evolution. As a consequence, it is necessary to investigate isotope compositions of intrusive rocks, carbonate rocks and siliceous rocks associated with the ore formation of nephrites (Table 2 and 3). The heavy δ¹⁸O anomalies of nephrites are difficult to be explained by the δ¹⁸O of basic rocks and granitic rocks, because the δ¹⁸O of basic rocks is between +5.5 and +7.4‰ [33] and the δ¹⁸O of intrusive rocks associated with the ore formation of nephrites is not significantly different from the most plutonic granitic rocks (+7 to +10‰) (Table 2) [1].

Generally, country rocks of carbonate rocks associated with the ore formation of nephrites have quite heavy δ¹⁸O (Table 3), which is in accord with carbonate rocks formed in the geological history [35]. The δ¹⁸O of marble metamorphosed from Qixia Formation limestone exceeds that of Xiaomeiling nephrite and the δ¹⁸O of marble (C-2, C-3 and C-4) at the mining pit is lighter than that of marbles (P-1, P-3 and P5) far away from the mining pit (Table 3). For Chuncheon nephrite, the δ¹⁸O range of dolomite without oxygen isotope exchanges in the mining area is between +13.7 and +18.2‰, while that of dolomite after exchanges is between +2.4 and +7.4‰ [5]. Compared with the dolomite in Vitim nephrite deposit, the δ¹⁸O of newly formed calcite is lighter (Table 3) [10]. In the area of Passo Bondolo, the δ¹⁸O of dolomite in metasomatic veins decreases from +23‰ before exchange to +12.5‰ after exchange [36]. Oxygen isotope exchanges between carbonate rocks and water can cause the δ¹⁸O depletion of carbonate rocks and the δ¹⁸O enrichment of fluid, and it is conceivable that tremolites crystallized from the fluid will inherit the heavy δ¹⁸O characteristic of carbonate rocks.

One particular concern is that the δ¹⁸O of siliceous rocks is heavier than that of the contemporaneous carbonate rocks [35], making siliceous rocks another important material source for heavy δ¹⁸O nephrites. The δ¹⁸O of siliceous rock (11YK2100) in contact with Luodian nephrite is up to +22.4‰ and slightly heavier than the EC-4B2 carbonate rock in contact with it (Table 2 and 3) [18]. In addition, the δ³⁰Si range of Luodian nephrite coincides exactly with that of siliceous rocks in carbonate rocks but distinctly differ from that of diabase in the mining area (Table 4) [18], which illustrate that the siliceous rock is the vital source of Si element for Luodian nephrite. In field, Luodian nephrite is always interbedded with banded siliceous rocks in carbonate rocks, while carbonate rocks without siliceous rocks often do not produce nephrites. In similar fashion, the δ¹⁸O enrichment of Dahua nephrite is also possibly related to siliceous rocks in carbonate rocks. Therefore, the abnormal heavy δ¹⁸O of Luodian nephrite and Dahua nephrite should arise out of complex oxygen isotope exchanges between carbonate rock, siliceous rock, water and tremolite.

3. Multi-Stage Water/Rock Interactions

3.1. Isotope Exchanges Between Water and Minerals

Before the crystallization of tremolite, the isotope exchange between the initial fluid (W¹) and country rocks is the first predictable process (I). There is only the oxygen isotope exchange for the initial fluid, no the hydrogen isotope exchange, and the initial fluid can be magmatic water, metamorphic water or any other hydrothermal fluid. Equilibrium fractionation equations between mineral and water can be used to calculate the δ¹⁸O of fluid (W²) after oxygen isotope exchanges (Table 5). If the δ¹⁸O of initial fluid is lighter than that of country rock, the δ¹⁸O of W² only depends on reaction temperatures and the δ¹⁸O of country rock. When the temperature is 350℃, the δ¹⁸O of W² calculated by oxygen isotope equilibrium fractionation equations of quartz-water, dolomite-water and calcite-water is +17.10‰, +13.76‰ and +16.23‰, respectively (Table 5), which are generally higher than δ¹⁸O values of ore-forming fluids calculated by the δ¹⁸O values of tremolite (Table 1).

W² is the fluid after the oxygen isotope exchange between W¹ and country rocks. The δ¹⁸O of W¹ is set to the left boundary of magma water (+5.5‰). The δ¹⁸O and δD of Tr* can be calculated by the isotope equilibrium fractionation equation between W² and tremolite. Dolomite, limestone and siliceous rock are three kinds of country rocks that are most closely related to the formation of nephrite, so dolomite, calcite and quartz as main minerals of these rocks can be used for the calculation of oxygen isotope exchange reactions. The initial δ¹⁸O values of quartz, calcite and dolomite are set to +22.4‰ (the δ¹⁸O value of siliceous rock in Luodian nephrite deposit in Table 2), +20.0‰ (approaching the average δ¹⁸O value of P series marbles in Xiaomeiling nephrite deposit) and +20.0‰ (for the comparison with nephrites related to the limestone), respectively. Oxygen isotope equilibrium fractionation equations of quartz-water (Qtz-Water), dolomite-water (Dol-Water) and calcite-water (Cal-Water) are 1000lnα_Qtz-Water=3.38×10⁶/T²-3.40 [37], 1000lnα_Dol-Water=3.2×10⁶/T²-2.00 [38] and 1000lnα_Cal-Water=2.78×10⁶/T²-3.39 [39], respectively.

The second predictable process (Ⅱ) is the crystallization of tremolite from W², and isotope compositions of tremolite are bound to be affected by W². Assuming that all the initial fluid is converted to W² and then tremolites have isotope exchange reactions with W², the δ¹⁸O values of tremolites (Tr*) after the reaction with W² under three country rocks conditions are +16.36‰, +13.02‰ and +15.50‰, respectively (Table 5). The genesis of almost all nephrites with heavy δ¹⁸O that have been found so far can be explained by δ¹⁸O values of Tr*. And yet, the δ¹⁸O values of most nephrites are always lighter than that of Tr*, indicating that there are other factors controlling isotopic compositions of tremolite.

The δ¹⁸O of Tr*_Dol-Water-Tr is significantly lighter than that of Tr*_Qtz-Water-Tr and Tr*_Cal-Water-Tr at the same temperature (Table 5), which could be an important reason why the δ¹⁸O of nephrite associated with dolomite is relatively light, such as Alamas nephrite, Yinggelike nephrite and Pishan nephrite (Figure 1a), whereas the δ¹⁸O of nephrite associated with limestone and siliceous rock is always heavier, such as Dahua nephrite, Xiaomeiling nephrite and Luodian nephrite (Table 1 and Figure 1a). The δ¹⁸O range of Luodian nephrite highly coincides with that of Tr*_Qtz-water-Tr (+16.36‰) (Figure 2b), indicating that almost all of magmatic water from diabase has oxygen isotope exchanges with country rocks. Due to the fact that basic rocks can only release a small amount of magmatic water, country rocks with heavy δ¹⁸O can fully exchange with the magmatic water, which is also conducive to explaining why the δ¹⁸O enrichment is a common feature of Dahua nephrite, Luodian nephrite and Sanchakou nephrite.

Different from basic rocks, granitic rocks can produce more magmatic water, but perhaps not all of magmatic water can have oxygen isotope exchanges with country rocks. The δ¹⁸O of whole hydrothermal fluid will be closer to that of initial magmatic water with the increase of unexchanged magmatic water, which is one possible reason for the δ¹⁸O of most nephrites associated with granitic rocks less than the δ¹⁸O of Tr*. Meteoric water is another important factor that can cause the evolution of δD and δ¹⁸O of ore-forming fluid towards low values, such as Chuncheon nephrite and Vitim nephrite. If δD and δ¹⁸O of unaltered Miaoxi granite porphyry are regarded as isotope compositions of initial fluid (W¹), the Tr* should be at the dotted circle (Figure 2a). The evolution direction from the dotted circle to Xiaomeiling nephrite is in sync with the evolution of Miaoxi granite porphyry altered by meteoric water, indicating that meteoric water did participate in the formation of Xiaomeiling nephrite. Therefore, both the impact of internal and external fluids on the evolution of ore-forming fluid and the isotope re-equilibration between the mixed fluid and tremolite should not be ignored.

3.2. Taylor Closed Model

Yui and Kwon (2002) used Taylor open model to discuss the significance of water/rock interaction for the origin of Chuncheon nephrite. Results suggest that Chuncheon nephrite is formed at high W/R ratios and low X_CO2 (<0.01-0.1), and the ore-formation fluid is the circulating meteoric water induced by Chuncheon granite, which provides a new insight into the contribution of water/rock interaction to the abnormal δ¹⁸O and δD of nephrite. In view of the fact that most of nephrites are not significantly affected by meteoric water, the ore-forming system for most of nephrites should be relatively closed. Consequently, Taylor closed model can be used to discuss the evolution of ore-forming fluid of nephrite whose δ¹⁸O value is lower than that of Tr*. Assuming all meteoric water enters the closed hydrothermal system at once, the interactions between Tr* and meteoric water can be expressed by the following equation [1], which is the third water/rock interaction (Ⅲ):

$$\mathrm{W}/\mathrm{R}={\displaystyle \frac{{\delta }_{Tr}^f-{\delta }_{Tr}^i}{{\delta }_{H_{2}O}^i-\left({\delta }_{Tr}^f-∆\right)}}$$

(1)

where i is the initial value, ${\delta }_{H_{2}O}^i$ is the δD and δ¹⁸O of meteoric water, ${\delta }_{Tr}^i$ is the δD and δ¹⁸O of Tr*, f is the final value after exchange, Δ=${\delta }_{Tr}^f-{\delta }_{H_{2}O}^f$, W and R are the atom percents of oxygen and hydrogen of meteoric water and tremolite in the total system, respectively. Based on Equation (1), the evolution curve of δD and δ¹⁸O of Tr* can be obtained at different W/R ratios (Figure 2).

The δ¹⁸O of Xiaomeiling nephrite indicates that W/R ratios are between 0.1 and 0.5, much higher than 0.01 reflected by its δD, revealing the inconsistency of W/R ratios in explaining the interaction between meteoric water and Tr* (Figure 2a). The W/R ratios reflected by the δ¹⁸O of Sanchakou nephrite, Dahua nephrite and Złoty Stok nephrite are between 0.1 and 0.5, indicating that a proportion of meteoric water is relatively low in the whole system (Figure 2a and 2d). The W/R ratios reflected by the δ¹⁸O of Alamas nephrite, Pishan nephrite and Yinggelike nephrite (Xinjiang primary nephrite) are between 0.5 and 1, indicating that meteoric water plays an important role in the formation of these nephrites (Figure 2b, 2c and 2d), but the δD values of these nephrites are incapable of accurately constraining the W/R ratios due to a lack of δD of initial fluid and meteoric water. On the other hand, δD values of these nephrites are between the W^4-2 straight line and the W^4-3 straight line, implying that the effect of meteoric water on these nephrites should be not significant. If meteoric water participates in the ore formation of Xiuyan nephrite, the W/R ratios will be between 0 and 0.5 (Figure 3c). The W/R ratios of Chuncheon nephrite are more than 5 and there is no inconsistency in the revelation of δ¹⁸O and δD on W/R ratios (Figure 2d), indicating that the ore-forming fluid is mainly composed of meteoric water. Moreover, the range of δD and δ¹⁸O of meteoric water during the formation of Chuncheon nephrite is likely to be between W^4-3 and W^4-3 in the meteoric water line [40] (Figure 2d). The W/R ratios of Vitim nephrite are relatively discrete, indicating that the range of δD and δ¹⁸O of meteoric water is relatively broad.

An implicit condition in Equation (1) is that meteoric water is the only fluid participating in the water-Tr* interactions, which is responsible for the inconsistency of W/R ratio. In fact, it is difficult to determine that all the initial fluid has oxygen isotope exchanges with country rocks. If there is still part of the initial fluid without oxygen isotope exchanges in the hydrothermal system, and/or not all of W² transforms into the constitution water of tremolite in the II process, mixings between the residual initial fluid, W² and meteoric water will inevitably cause the ${\delta }_{H_{2}O}^i$ in Equation (1) to no longer be just meteoric water, but a mixed fluid, which inevitably has an impact on the subsequent evolution of δ¹⁸O and δD of tremolite. As a consequence, if there is no way to determine the intermediate states of ore-forming fluid, the evolution of ore-forming fluid of nephrite cannot be truly reflected.

3.3. Fluid Mixing Model

Except isotope exchanges between fluids and country rocks, another process that can effectively affect the δD and δ¹⁸O of tremolite is the mixing between different fluids. From magmatic to meteoric water, the evolution of ore-forming fluid can be divided into two mixing processes. The first process (Ⅰ) is the mixing between the residual initial fluid (W¹) and exchanged fluid (W)² at high temperatures, producing the first mixed fluid (W³). The δD and δ¹⁸O of W³ are only affected by the mixing of W¹ and W² due to the absence of meteoric water in the hydrothermal system. W² has continuously mixed with the residual initial fluid since its formation. The second process (Ⅱ) is the mixing between W³ and meteoric water (W⁴) at relatively low temperatures, which produces the second mixed fluid (W⁵). The Ⅱ process can be regarded as the last effective mixing between different fluids and the isotope re-equilibration between late tremolites and W⁵ determines the δD and δ¹⁸O of nephrite. Based on the principle of mass balance, the δD and δ¹⁸O of mixed fluid can be calculated as follows:

$$W^1{\delta }_{H_{2}O}^1+W^2{\delta }_{H_{2}O}^2+\cdots W^n{\delta }_{H_{2}O}^n=W^{n+1}{\delta }_{H_{2}O}^{n+1}$$

(2)

where ${\delta }_{H_{2}O}$ is the isotope value and W is the mass of fluid in the system.

If W^4-6 is taken as a starting point, extending towards coordinates of ore-forming fluids of Xiaomeiling nephrite and intersecting with the W¹-W² fluid mixing line (Figure 3a), the range of intersection points represents W¹/W² ratios in the Ⅰ mixing process. The W¹/W² ratios of Xiaomeiling nephrite approach 1, indicating only half of magmatic water from Miaoxi granite porphyry had the oxygen isotope exchange with Qixia Formation limestone (Figure 3a). As a consequence, the δ¹⁸O of Xiaomeiling nephrite much less than that of Tr^*_Cal-Water-Tr (+15.50‰) should be attributed to the residual magmatic water, not meteoric water. The less than 0.1 of W⁴/W³ ratios indicate that the amount of meteoric water is only one-tenth of that of initial magmatic water at most, but it is this less than 10% of meteoric water that significantly reduces the δD of Xiaomeiling nephrite, resulting in the δD almost at the same level with that of Chuncheon nephrite (Figure 1a). If meteoric water in Equation (2) shifts from W^4-6 to W^4-3, intersection points will shift to W², and the contribution of residual magmatic water to the δ¹⁸O of nephrite will decrease, but the contribution of meteoric water to the δ¹⁸O and δD of nephrite is just the opposite.

It is almost certain that oxygen isotope exchanges between country rocks, fluid and tremolite give rise to the high δ¹⁸O of Sanchakou nephrite, Luodian nephrite and Dahua nephrite. However, due to the δD of their ore-forming fluids overlapped with that of magmatic water, there are two possible situations that need to be discussed. Assuming that the formation of these three nephrites is irrelevant to meteoric water, W¹/W² ratios of Luodian nephrite are a little more than 0.1 (Figure 3b), indicating that the vast majority of magmatic water from basic rocks had oxygen isotope exchanges with country rocks, whereas W¹/W² ratios of Dahua nephrite and Sanchakou nephrite indicate that about one-third to one-half of magmatic water had the oxygen isotope exchange (Figure 3b). If meteoric water participates in their formation, the δD of these nephrites will decrease with the increase of W⁴/W³ ratios, unless the δD of meteoric water during the crystallization of tremolite is consistent with that of magmatic water. If that happens, the proportion of meteoric water in W⁵ is slightly less than 10% for Luodian nephrite and slightly more than 10% for Dahua nephrite and Sanchakou nephrite (Figure 3b), respectively, which means that a small amount of meteoric water mixed into the system is insufficient to offset the δ¹⁸O increment of nephrite caused by the oxygen isotope exchange between country rocks and magmatic water.

It is difficult to determine W¹/W² ratios of Vitim nephrite and Chuncheon nephrite, and can only roughly know that the total amount of meteoric water is almost ten times that of magmatic water for Chuncheon nephrite and more than ten times for Vitim nephrite, respectively (Figure 3c). If the slope from W¹ to the coordinates of these ore-forming fluids is less than that of meteoric water line, the intersection point of extension line and meteoric water line can represent the lower limit of meteoric water during the crystallization of late tremolite, and the extension line from W² to the coordinates of these ore-forming fluids can be used to determine the upper limit. The range of meteoric water during the ore formation of Chuncheon nephrite is relatively narrow, while that of Vitim nephrite is much broader (Figure 3c). In fact, the absence of δD and δ¹⁸O of intrusive rocks and δ¹⁸O of carbonate rocks make it impossible to determine isotope compositions of W¹ and W² as well as meteoric water.

Multiple solutions are complicated for nephrites with ore-forming fluids close to magmatic water (Figure 3d). First, assuming that the formation of these nephrites has no relevance to meteoric water, W¹/W² ratios of Yinggelike nephrite are between 1 and 100 (Figure 3d), indicating that the residual magmatic water accounts for a high proportion in the ore-forming fluid. The W² is the main component of ore-forming fluids of Xiuyan primary nephrite and Złoty Stok nephrite, but a wide range of W¹/W² ratios indicates that W¹ is not evenly mixed with W² (Figure 3d). Pishan nephrite and most of Alamas nephrite are situated on the left side of magmatic water (Figure 3d), but it is not certain whether this is caused by meteoric water, the low δ¹⁸O intrusive rocks or low δ¹⁸O carbonate rocks (Table 3). On the other hand, if the δD range of meteoric water participating in the growth of late tremolite is consistent with that of magmatic water, there are few differences in the δD between the final hydrothermal fluid and magmatic water. The W⁴/W³ ratios of Pishan nephrite are close to 1 (Figure 3d), indicating that the amount of meteoric water approaches that of initial magmatic water . For Alamas nephrite, Pinshan nephrite and Yinggelike nephrite, the amount of meteoric water is less than half of W⁵, while W⁴/W³ ratios of Xiuyan primary nephrite and Złoty Stok nephrite are lower (Figure 3d), implying that only a large amount of meteoric water can offset the impact of residual magmatic water and the oxygen isotope exchange between magmatic water and country rocks on the δ¹⁸O of nephrite, otherwise it is easy to cause a misjudgment that magmatic water or metamorphic water is the source of their ore-forming fluid.

4. Conclusions

Hydrogen and oxygen isotope compositions of nephrite are results of multi-stage water/rock interactions and the δ¹⁸O evolution of nephrite is not synchronous with the δD evolution. Multiple solutions of ore-forming fluids of C-type nephrites result from a lack of constraints about isotope compositions of parent rocks and meteoric water during the growth of late tremolite. In this study, Taylor closed model and fluid mixing model are used to reveal the intermediate processes of evolution of ore-forming fluid of nephrite. Due to the influence of multicomponent fluids on the δD and δ¹⁸O of ore-forming fluid of nephrite, the application range of Taylor closed model is limited. Fluid mixing model can calculate the proportion between different fluids step by step and is conducive to quantitatively constraining the evolution of ore-forming fluid of primary nephrite.

Oxygen isotope exchanges between the magmatic water from basic rocks and country rocks with heavy δ¹⁸O should be responsible for the formation of nephrite with heavy δ¹⁸O, such as Sanchakou nephrite, Luodian nephrite, Dahua nephrite etc. In the absence of meteoric water, the ratio of initial fluid to exchanged fluid in the first mixed fluid determines the δ¹⁸O shift direction of nephrite. For Luodian nephrite, the initial magmatic water from basic rocks has almost completely oxygen isotope exchange with country rocks, while the proportions for Sanchakou nephrite and Dahua nephrite are increased to about one-third to one-half of the initial magmatic water. If the δD of meteoric water is consistent with that of magmatic water, it is difficult to determine whether meteoric water participates in the formation of these nephrites. In such circumstances, only when the δ¹⁸O of ore-forming fluid of nephrite is less than the left boundary of magmatic water, otherwise any δ¹⁸O value from W² to the left boundary of magmatic water hardly indicates the existence of meteoric water in the hydrothermal system. Due to meteoric water in the hydrothermal fluid exceeding several times that of the initial magmatic water, the influence of meteoric water masks the oxygen isotope exchange between magmatic water and country rocks during the ore-forming processes of Vitim nephrite and Chuncheon nephrite.