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

Abstract

Hydrogen and oxygen isotopes of ore-forming fluid of nephrite deposits have always been changing due to mixings between different fluids and oxygen isotope exchanges between the ore-forming fluid and country rocks, resulting in that the tremolite (or actinolite) has to constantly re-establish new isotope fractionation equilibriums with the dynamic fluid, which is of great significance to understand the genesis of hydrogen and oxygen isotopes of nephrite. Based on this, Taylor’s closed model and fluid mixing model are used to unravel the control of multi-stage evolution of ore-forming fluid on the δD and δ¹⁸O of nephrite. Although Taylor’s closed model is conducive to interpreting the genesis of nephrite with light δD and δ¹⁸O, such as Vitim nephrite, Russia, and Chuncheon nephrite, South Korea, it is unable to be effectively used in other nephrite. The fluid mixing model can quantitatively constrain proportions of different fluids during different ore-forming stages. Multiple solutions of ore-forming fluids of carbonate rock-related nephrite result from the absence of external constraints, such as isotope compositions of intrusive rocks, carbonate rocks, and meteoric water. Due to the generally heavy δ¹⁸O of country rocks, a small amount of meteoric water that enters the hydrothermal system in the later ore-forming stage is insufficient to offset the δ¹⁸O increment of nephrite caused by the oxygen isotope exchange between country rocks and water, which should be responsible for the abnormal heavy δ¹⁸O of Luodian nephrite, Dahua nephrite, Sanchakou nephrite, Xiaomeiling nephrite, etc., and not metamorphic water dominating their formation.

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 principally composed of tremolite and 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 has been very prevalent to adopt the δD and δ¹⁸O of nephrite to invert the origin of the ore-forming fluid [2,3,4,5,6,7]. S-type nephrite (also called apoultrabasic-nephrite [6] or ortho-nephrite [8]) is occurred in serpentinite (or serpentinite mélange). It has a wide range of δD and δ¹⁸O [9,10] and is distinguished by high contents of Cr, Co, and Ni [8,11,12]. The sources of the ore-forming fluid are complex, involving the metamorphic water, seawater, and meteoric water [10]. For C-type nephrite (also known as D-type nephrite [13], apocarbonate-nephrite [6], or para-nephrite [8]) that is associated with carbonate rocks, the δD and δ¹⁸O of nephrite from the same deposit are relatively convergent (Figure 1a) [2,4,5,7,14], but there are significant differences in δD and δ¹⁸O among different nephrite deposits, resulting in the diversity of interpretation on their ore-forming fluids.

Traditionally, metamorphic water is considered as the source of ore-forming fluid of Chinese nephrite with heavy δ¹⁸O, such as Sanchakou nephrite (δ¹⁸O = +11.4–+12.6‰) [15], Dahua nephrite (δ¹⁸O = +10–+12.3‰) [16], and Luodian nephrite (δ¹⁸O = +14.1–+17.7‰) [17] (

Table 1. Hydrogen and oxygen isotope compositions of different nephrite and the ore-forming fluids (‰).

Nephrite Deposit	Sample No.	δD	δ18O	δDH2O	δ18OH2O			References
					350 °C	400 °C	450 °C
Xiaomeiling, Liyang, Jiangsu, China	LY-Q-13	−109.0	8.9	−87.3	9.64	10.10	10.42	This study
	LY-Q-14	−109.8	9.8	−88.1	10.54	11.00	11.32
	LY-Q-16	−110.3	9.4	−88.6	10.14	10.60	10.92
	LY-HL-3	−107.7	9.5	−86.0	10.24	10.70	11.02
	LY-HL-7	−109.2	9.3	−87.5	10.04	10.50	10.82
	LY-HL-10	−110.5	9.1	−88.8	9.84	10.30	10.62
	LY-QB-3	−112.6	10.0	−90.9	10.74	11.20	11.52
	LY-QB-12	−112.2	9.5	−90.5	10.24	10.70	11.02
	LY-QB-21	−113.1	9.7	−91.4	10.44	10.90	11.22
					330 °C	390 °C	450 °C
Pishan, Xinjiang, China	392-1	−89.9	4.99	−68.2	5.48	6.11	6.51	[18]
	392-2	−81.8	4.27	−60.2	4.76	5.39	5.79
	392-3	−84.0	2.67	−62.3	3.16	3.79	4.19
	392-4	−82.4	3.63	−60.7	4.12	4.75	5.15
	392-5	−82.4	3.89	−60.7	4.38	5.01	5.41
	392-6	−83.0	3.92	−61.3	4.41	5.04	5.44
	392-7	−81.6	4.24	−59.9	4.73	5.36	5.76
	392-8	−81.0	3.50	−59.3	3.99	4.62	5.02
Yingelike, Ruoqiang, Xinjiang, China	H-3	−74.9	5.3	−53.2	5.79	6.42	6.82	[4]
	H-4	−78.0	7.4	−56.3	7.89	8.52	8.92
	H-5	−86.7	5.4	−65.0	5.89	6.52	6.92
					293 °C	350 °C	400 °C
Alamas, Xinjiang, China	ABY-1	−86.7	3.8	−65.0	3.7	4.54	5.00	[3]
	ABY-1	−83.0	3.2	−61.3	3.1	3.94	4.40
	AQB-1	−93.1	6.1	−71.4	6.0	6.84	7.30
	AQB-2	−89.0	4.6	−67.3	4.5	5.34	5.80
	AQB-3	−85.1	3.5	−63.4	3.4	4.24	4.70
	AQB-4	−85.9	3.6	−64.2	3.5	4.34	4.80
	AQB-6	−94.7	6.2	−73.0	6.1	6.94	7.40
	AQY-1	−90.2	4.1	−68.5	4.0	4.84	5.30
	AQY-2	−85.0	3.6	−63.3	3.5	4.34	4.80
	AQY-3	−91.6	4.9	−69.9	4.8	5.64	6.10
	AQY-4	−90.4	4.8	−68.7	4.7	5.54	6.00
	AQY-5	−86.2	3.8	−64.5	3.7	4.54	5.00
					330 °C	350 °C	430 °C
Yurungkash River, Xinjiang, China	ZY2	−71.8	5.2	−51.4	5.7	6.0	6.6	[19]
	ZY3	−67.3	3.7	−46.8	4.2	4.4	5.1
Karakash River, Xinjiang, China	QZY4	−72.4	5.6	−52.5	6.1	6.3	7.0
	QZY5	−55.7	1.1	−34.9	1.6	1.8	2.5
	QZY6	−71.4	5.0	−51.0	5.5	5.7	6.4
	QZY7	−65.7	2.9	−45.2	3.4	3.6	4.3
	MY1	−68.7	3.2	−48.3	3.7	4.0	4.6
	MY2	−63.3	2.4	−42.7	2.9	3.1	3.8
	MY3	−69.3	4.5	−48.9	5.0	5.3	5.9
	MY4	−67.1	3.1	−46.6	3.6	3.8	4.5
					330 °C	390 °C	450 °C
Karakash River, Xinjiang, China	MYH1	−97	0.8	−75.4	1.3	1.9	2.3	[2]
	MYH2	−93	7.0	−70.8	7.5	8.1	8.5
	MYH3	−107	4.6	−84.9	5.1	5.7	6.1
	MYH4	−80	2.7	−57.9	3.2	3.9	4.2
	MYH6	−63	5.0	−40.9	5.5	6.2	6.6
	MYH7	−67	7.3	−45.3	7.8	8.4	8.8
	MYH10	−46	4.9	−24.6	5.4	6.0	6.4
	MYH11	−28	3.6	−5.8	4.1	4.7	5.1
	MYH21	−39	5.4	−17.7	5.9	6.6	7.0
	MYH24	−106	3.5	−84.5	4.0	4.6	5.0
	MYH25	−41	3.7	−19.4	4.1	4.8	5.2
	MYH30	−80	2.7	−58.4	3.2	3.8	4.2
	MYH31	−85	7.6	−63.3	8.1	8.8	9.1
	MYH32	−85	1.5	−63.1	2.0	2.6	3.0
	MYH33	−86	1.1	−64.3	1.5	2.2	2.6
	MYH34	−77	6.6	−55.3	7.1	7.8	8.2
	MYH35	−91	2.0	−69.2	2.5	3.2	3.6
	MYH36	−107	2.1	−85.2	2.6	3.2	3.6
	MYH37	−91	3.4	−69.6	3.9	4.5	4.9
	MYH38	−87	3.6	−64.8	4.1	4.7	5.1
	MYH39	−58	6.7	−36.6	7.1	7.8	8.2
	MYH40	−77	3.9	−54.8	4.4	5.0	5.4
	MYH41	−78	3.8	−55.8	4.2	4.9	5.3
	MYH42	−57	4.3	−35.7	4.8	5.4	5.8
	MYH43	−108	2.9	−86.1	3.4	4.0	4.4
	MYH44	−86	3.0	−64.5	3.5	4.1	4.5
	MYH50	−85	4.9	−63.5	5.4	6.0	6.4
	MYH51	−100	2.2	−77.9	2.7	3.3	3.7
	MYH52	−79	7.9	−56.8	8.4	9.0	9.4
	MYH54	−88	2.5	−66.0	3.0	3.7	4.1
	MYH55	−103	6.0	−81.1	6.4	7.1	7.5
	MYH56	−95	3.5	−73.2	4.0	4.6	5.0
	MYH57	−109	4.3	−87.1	4.8	5.5	5.9
	MYH58	−72	4.0	−50.1	4.5	5.1	5.5
	MYH59	−93	1.6	−70.9	2.1	2.7	3.1
					350 °C	400 °C	450 °C
Dahua, Guangxi, China	D-4	−76.9	12.3	−55.2	13.04	13.50	13.82	[20]
	D-8	−79.8	10.5	−58.1	11.24	11.70	12.02
Luodian, Guizhou, China	LDS		15.8		16.54	17.00	17.32	[21]
	LDW1		16.4		17.14	17.60	17.92
	LDW5		15.4		16.14	16.60	16.92
	LDGW-1		17.7		18.44	18.90	19.22
	LDW6		15.2		15.94	16.40	16.72
	LDLG4		15.8		16.54	17.00	17.32
Luodian, Guizhou, China	LMBT-13-2		15.3		16.04	16.50	16.82	[22]
	LMTC08-7		14.3		15.04	15.50	15.82
	11KY080		15.6		16.34	16.80	17.12
	11KY181		16.5		17.24	17.70	18.02
	ETC05-2-5		14.7		15.44	15.90	16.22
	ETC05-2-6		14.5		15.24	15.70	16.02
	ETC05-4		14.1		14.84	15.30	15.62
	ETC05-6		14.6		15.34	15.80	16.12
	ETC05-8		15.5		16.24	16.70	17.02
	ETC05-23		16.3		17.04	17.50	17.82
					350 °C	400 °C	450 °C
Sanchakou, Qinghai, China	QH-176	−86	11.4	−64.3	12.13	12.60	13.35	[23]
	QH-177	−87	12.3	−65.3	13.03	13.50	14.25
	QH-001	−78	12.2	−56.3	12.93	13.50	14.15
	QHSH-001	−84	12.6	−62.3	13.33	13.80	14.55
					330 °C	390 °C	450 °C
Xiuyan placer nephrite, Liaoning, China	LHM15-1	−88.23	8.40	−66.53	8.89	9.52	9.92	[24]
	LHM15-2	−75.2	8.50	−53.5	8.99	9.62	10.02
	LHM15-3	−93.29	8.80	−71.59	9.29	9.92	10.32
	LHM15-4	−94.95	9.30	−73.25	9.80	10.42	10.82
	LHM15-5	−78.51	10.60	−56.8	11.09	11.72	12.12
	LHM15-6	−93.78	8.20	−72.07	8.69	9.32	9.72
	LHM15-7	−86.58	8.00	−64.88	8.49	9.12	9.52
					350 °C	400 °C	450 °C
Xiuyan, Liaoning, China	Y-1	−70	10.0	−48	10.7	11.20	11.52	[25]
	Y-3	−74	9.3	−52	10	10.50	10.82
	G-3	−74	8.5	−52	9.2	9.70	10.02
	G-6	−72	8.1	−50	8.8	9.30	9.62
	W-1	−70	13.3	−48	14	14.50	14.82
	W-2	−73	11.7	−51	12.4	12.90	13.22
	S-1a	−76	10.4	−54	11.1	11.60	11.92
	S-1b	−72	10.3	−50	11	11.50	11.82
	S-3a	−74	9.1	−52	9.8	10.30	10.62
	S-3b	−70	9	−48	9.7	10.20	10.52
Chuncheon, South Korea	NE1	−108	−8.7	−86.3	−7.96	−7.50	−7.18	[5]
	NE2	−114	−8.4	−92.3	−7.66	−7.20	−6.88
	NE3	−105	−9.9	−83.3	−9.16	−8.70	−8.38
	NE4	−107	−9	−85.3	−8.26	−7.80	−7.48
	NE5	−108	−8.2	−86.3	−7.46	−7.00	−6.68
	NE6	−112	−8.6	−90.3	−7.86	−7.40	−7.08
	NE7	−109	−8.9	−87.3	−8.16	−7.70	−7.38
	NE8	−110	−9.3	−88.3	−8.56	−8.10	−7.78
	NE9	−109	−9.2	−87.3	−8.46	−8.00	−7.68
Złoty Stok, Poland	A	−76.4	10.2	−54.7	10.94	11.40	11.72	[26]
	B	−76.2	8.3	−54.5	9.04	9.50	9.82
	C	−77.2	10.4	−55.5	11.14	11.60	11.92
	D	−74.6	10.2	−52.9	10.94	11.40	11.72
Kavokta, Lower Ollomi, Golyube, Vitim, Russia		−119.3	−15.52	−97.6	−14.78	−14.32	−14.00	[6]
		−178.5	−17.24	−156.8	−16.50	−16.04	−15.72
		−133.2	−14.93	−111.5	−14.19	−13.73	−13.41
Voimakan, Vitim, Russia	V1-14		−18.5		−17.76	−17.30	−16.98	[27]
	PK-1		−18.8		−18.06	−17.60	−17.28
	PK-3		−18.8		−18.06	−17.60	−17.28

The δD_H2O and δ¹⁸O_H2O that are not given in references can be calculated by the hydrogen and oxygen isotope equilibrium fractionation equations between tremolite and water. Oxygen isotope equilibrium fractionation equation is 1000lnα_Tr-Water = 3.95 × 10⁶/T² − 8.28 × 10³/T + 2.38 [28]. Hydrogen isotope equilibrium fractionation equation is 10³lnα_Tr-Water = −21.7 ± 2 [29]. T is the absolute temperature.

and Figure 1). However, this assertion is not supported by the geological settings. They are formed by contact metasomatism between basic rocks and carbonate rocks; therefore, the genesis of their δ¹⁸O anomalies 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. However, so far, the research on isotope compositions of parent rocks lags behind that of nephrite. 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 nephrite deposits only by the δD and δ¹⁸O of tremolite. These questions motivate us to discuss the couplings between fluids, country rocks, and tremolite, and analyse the source and evolution of ore-forming fluids of nephrite deposits.

2. Methodology

Xiaomeiling nephrite discovered in Xiaomeiling village, Jiangsu Province is formed by the contact metasomatism between Miaoxi granite porphyry and Qixia Formation limestone [11]. Qixia Formation limestone and Miaoxi granite porphyry were also collected and tested in order to reveal the influence of the parent rocks on the hydrogen and oxygen isotope compositions of Xiaomeiling nephrite. Xiaomeiling nephrite, Qixia Formation limestone, and Miaoxi granite porphyry were ground to ~200 mesh powders for oxygen isotope tests. Samples were reacted with pure BrF₅ at 500 to 680 °C for 14 h to release O₂ and impurities. The O₂ reacted with a graphite at 700 °C, and the released CO₂ was collected by the freezing method. MAT253 mass spectrometry at Analytical Laboratory Beijing Research Institute of Uranium Geology was used to test oxygen isotopes of samples. The GBW-04409 and GBW-04410 were used as oxygen isotope standard reference materials, and the analytical error was less than ±0.2‰ [30].

In the hydrogen isotope test, Miaoxi granite porphyry and Xiaomeiling nephrite were ground to 40–60 mesh powders, and then samples were stored at 105 °C for more than 4 h and followed by the release of H-bearing gas, such as H₂O and H₂, from inclusions by heating samples at 1400 °C in a ceramic tube. The H-bearing gas had a reduction reaction with a glassy carbon at high temperatures, reducing the H-bearing gas to H₂. Pure helium gas flow brought the H₂ into the MAT253 mass spectrometer to analyze hydrogen isotopes, the GBW-04401 and GBW-04402 were used as hydrogen isotope standard reference materials, and the analytical error was less than ±1‰ [30].

3. δD and δ¹⁸O of Different Rocks in C-Type Nephrite Deposits

3.1. δD and δ¹⁸O of C-Type Nephrite

Although the contact metasomatism between intrusive rocks and carbonate rocks dominates the formation of most C-type nephrite (

Table 2. Ore genesis and parent rocks of different C-type nephrite deposits.

Nephrite Deposit	Ore Genesis	Intrusive Rock	Carbonate Rock	References
Xiaomeiling, Jiangsu, China	Contact metasomatism	Miaoxi granite porphyry	Qixia Formation limestone	This study
Pishan, Xinjiang, China		Quartz diorite	Dolomitic marble	[18]
Yingelike, Ruoqiang, Xinjiang, China		Granitoids	Dolomitic marble	[4]
Alamas, Xinjiang, China		Granitoids	Dolomitic marble	[3]
Dahua, Guangxi, China		Diabase	Sidazhai Formation limestone	[20]
Luodian, Guizhou, China		Diabase	Sidazhai Formation limestone	[21]
Sanchakou, Qinghai, China		Gabbro	Qingbanshisuzhan Formation of Wanbaogou Group	[23]
Chuncheon, South Korea		Chuncheon granite	Dolomitic marble	[5]
Złoty Stok, Poland		Granitoids	Dolomitic marble	[26]
Kavokta, Lower Ollomi, Golyube, Vitim, Russia		Granite	Dolomitic marble	[6]
Voimakan, Vitim, Russia		Vitimkan intrusive complex	Dolomite marble	[27]
Xiuyan, Liaoning, China	Metamorphism		Dolomite marble	[25]

Sidazhai Formation is mainly composed of limestone and siliceous rock [20]. Qingbanshisuzhan Formation of Wanbaogou Group is mainly composed of siliceous banded marble [23]. Vitimkan intrusive complex is mainly composed of granites, granodiorites, diorites, aplites, and pegmatites [27].

), such as C-type nephrite widely distributed in the southern margin of Tarim Basin (Table 1) [3,4,18,31], Sanchakou nephrite [15], Xiaomeiling nephrite [11], Vitim nephrite, Russia [6,32], Chuncheon nephrite, South Korea [5], Złoty Stok nephrite, Poland [26], Dahua nephrite [16], Luodian nephrite [17], etc., significant differences in the δD and δ¹⁸O between different nephrite 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 [6,32], 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 nephrite (Figure 1a), making them highly recognizable. In addition, the decarbonation of dolomite may also promote the δ¹⁸O depletion of Vitim nephrite [27]. 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 3. Oxygen isotope compositions of intrusive rocks and siliceous rocks in different nephrite deposits (‰).

Nephrite Deposit	Sample No.	Rock	δD	δ18O	References
Xiaomeiling, Jiangsu, China	γ-3	Miaoxi granite porphyry	−90.8	3.6	This study
	γ-4		−85.2	7.3
	γ-5		−85.0	7.0
Khaita, Vitim, Russia		Granite		4.16	[6]
Kavokta, Vitim, Russia				9.93
Luodian, Guizhou, China	11YK2100	Siliceous rock		22.4	[22]

Due to its proximity to the contact zone, γ-3 was affected by meteoric water, so the δD and δ¹⁸O of γ-3 are lighter than those of unaltered γ-4 and γ-5.

). The anomaly is difficult to get a reasonable explanation 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 those 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-forming processes. There is a significant difference between Xinjiang primary nephrite (Pishan nephrite, Alamas nephrite, and Yinggelike nephrite) and Xinjiang secondary nephrite produced from Yurungkash and Karakash River. The δD and δ¹⁸O values of Xinjiang primary nephrite 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]. The range of δD and δ¹⁸O of secondary nephrite 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 nephrite (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 nephrite deposits are considered to be products of regional metamorphism, such as Xiuyan primary nephrite [33], Mastabia nephrite [34], Scortaseo nephrite [35], Luanchuan nephrite [36], and Longxi nephrite. Previous research suggested that the ore-forming fluid of Xiuyan primary nephrite could be the Si-rich hydrotherm produced by regional metamorphism and migmatization [33,37]. Metamorphic water can be responsible for Xiuyan primary nephrite 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 in 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.

3.2. δD 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 nephrite (Table 3 and

Table 4. Oxygen isotope compositions of carbonate rocks in different nephrite deposits (‰).

Nephrite Deposit	Sample No.	δ18O	Sample No.	δ18O*	Reference
Xiaomeiling, Jiangsu, China	P-1	19.5	C-2	15.2	This study
	P-3	20.7	C-3	14.8
	P-5	19.9	C-4	16.4
Chuncheon, South Korea	D-1	18.2	D-18	2.4	[5]
	D-2	18.0	D-19	4.1
	D-3	15.0	D-20	3.8
	D-4	14.3	D-21	7.4
	D-5	18.2
	D-6	13.7
	D-7	15.1
	D-8	15.9
	D-9	14.7
	D-10	17.6
	D-11	15.5
	D-12	15.5
	D-13	16.9
	D-14	14.0
	D-15	14.4
	D-16	17.8
	D-17	14.2
Golyube, Vitim, Russia		28.4		22.36	[6]
Lower Ollomi, Vitim, Russia		29.26
		26.65
		28.40
Voimakan, Vitim, Russia	KP-81-1-3	26.1			[27]
Xiuyan, Liaoning, China	9872	22.6			[39]
Alamas, Xinjiang, China	S-1	6.1
Luodian, Guizhou, China	EC-4B2	20.2			[22]

δ¹⁸O* is the δ¹⁸O of carbonate rock after the oxygen isotope exchange between the carbonate rock and fluids. For Alamas nephrite, it cannot determine whether the +6.1‰ of S-1 is the δ¹⁸O value after the oxygen isotope exchange.

). The heavy δ¹⁸O anomalies of nephrite 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‰ [38], and the δ¹⁸O of intrusive rocks associated with the ore formation of nephrite is not significantly different from the most plutonic granitic rocks (+7 to +10‰) (Table 3) [1].

Generally, country rocks of carbonate rocks associated with the ore formation of nephrite have quite heavy δ¹⁸O (Table 4), which is in accord with carbonate rocks formed in the geological history [40]. 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 P-5) far away from the mining pit (Table 4). 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 [6,27], the δ¹⁸O of newly formed calcite is lighter (Table 4) [6]. In the area of Passo Bondolo, the δ¹⁸O of dolomite in metasomatic veins decreases from +23‰ before exchange to +12.5‰ after exchange [41]. 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 the tremolite 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 corresponding carbonate rocks [40], making siliceous rocks another important material source for heavy δ¹⁸O nephrite. 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 3 and Table 4) [22]. 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 5. Silicon isotope compositions of nephrite, diabase, and siliceous rock in the Luodian deposit (‰).

Nephrite Deposit	Sample No.	Rocks	δ30Si	References
Luodian, Guizhou, China	11YK2100	Siliceous rock	1.4	[22]
	LMBT-13-2	Nephrite	1.3
	LMTC08-7		1.2
	11KY080		1.2
	11KY181		1.1
	ETC05-2-5		1.4
	ETC05-2-6		1.7
	ETC05-4		1.4
	ETC05-6		1.3
	ETC05-8		1.1
	ETC05-23		1.2
Luodian, Guizhou, China	LDS	Nephrite	0.3	[21]
	LDW1		0.8
	LDW5		0.7
	LDGW-1		0.3
	LDW6		0.7
	LDLG4		0.8
	LKT001-C	Siliceous rock	0.5
	LKT002		0.4
	LJGL-1	Diabase	0.2
	LJGL-2		0.0
	LJGL-4		0.1

) [22], 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 nephrite. 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.

4. Multi-Stage Water/Rock Interactions

4.1. 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 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 6. Oxygen isotope compositions of W² and Tr* at different temperatures (‰).

Stage	Mineral-Water	δ18Oi of Country Rocks		δ18O of W2
			350 °C	400 °C	450 °C
I	Qtz-W1	22.4	17.10	18.34	19.34
	Dol-W1	20.0	13.76	14.94	15.88
	Cal-W1	20.0	16.23	17.25	18.07
				δ18O of Tr*
II	W2-Tr		16.36	17.14	17.83
	W2-Tr		13.02	13.73	14.36
	W2-Tr		15.50	16.05	16.56

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 3), +20.0‰ (approaching the average δ¹⁸O value of P series marbles in Xiaomeiling nephrite deposit), and +20.0‰ (for the comparison with nephrite 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 [42], 1000lnα_Dol-Water = 3.2 × 10⁶/T² − 2.00 [43], and 1000lnα_Cal-Water = 2.78 × 10⁶/T² − 3.39 [44], respectively.

). 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 °C, 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 6), which are generally higher than δ¹⁸O values of ore-forming fluids calculated by the δ¹⁸O values of tremolite (Table 1).

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 tremolite has isotope exchange reactions with W², the δ¹⁸O values of tremolite (Tr*) after the reaction with W² under three country rocks conditions are +16.36‰, +13.02‰, and +15.50‰, respectively (Table 6). The genesis of almost all nephrite with heavy δ¹⁸O that have been found so far can be explained by δ¹⁸O values of Tr*. And yet, the δ¹⁸O values of most nephrite 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 6), 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 the whole hydrothermal fluid will be closer to that of initial magmatic water with the increase in unexchanged magmatic water, which is one possible reason for the δ¹⁸O of most nephrite associated with granitic rocks being 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.

4.2. Taylor’s Closed Model

Yui and Kwon (2002) used Taylor’s 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 nephrite is not significantly affected by meteoric water, the ore-forming system for most nephrite should be relatively closed. Consequently, Taylor’s 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):

$$\mathrm{W}/\mathrm{R}={\displaystyle \frac{{\mathrm{\delta }}_{\mathrm{T}\mathrm{r}}^{\mathrm{f}}-{\mathrm{\delta }}_{\mathrm{T}\mathrm{r}}^{\mathrm{i}}}{{\mathrm{\delta }}_{{\mathrm{H}}_2\mathrm{O}}^{\mathrm{i}}-\left({\mathrm{\delta }}_{\mathrm{T}\mathrm{r}}^{\mathrm{f}}-∆\right)}}$$

(1)

where i is the initial value, ${\mathrm{\delta }}_{{\mathrm{H}}_2\mathrm{O}}^{\mathrm{i}}$ is the δD and δ¹⁸O of meteoric water, ${\mathrm{\delta }}_{\mathrm{T}\mathrm{r}}^{\mathrm{i}}$ is the δD and δ¹⁸O of Tr*, f is the final value after exchange, Δ = ${\mathrm{\delta }}_{\mathrm{T}\mathrm{r}}^{\mathrm{f}}-{\mathrm{\delta }}_{{\mathrm{H}}_2\mathrm{O}}^{\mathrm{f}}$, 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 2b,d). 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 nephrite deposits (Figure 2d), but the δD values of these nephrite deposits 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, the δD values of these nephrite deposits are between the W^4-2 straight line and the W^4-3 straight line, implying that the effect of meteoric water on these nephrite deposits 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 2c). 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. Without considering the decarbonation of dolomite [27], meteoric water far exceeding several times the mass of nephrite is a necessary condition for causing the extreme δ¹⁸O depletion of nephrite. 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 [45] (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 ${\mathrm{\delta }}_{{\mathrm{H}}_2\mathrm{O}}^{\mathrm{i}}$ 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.

4.3. 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 unexchanged 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:

$${\mathrm{W}}^1{\mathrm{\delta }}_{{\mathrm{H}}_2\mathrm{O}}^1+{\mathrm{W}}^2{\mathrm{\delta }}_{{\mathrm{H}}_2\mathrm{O}}^2+\cdots {\mathrm{W}}^{\mathrm{n}}{\mathrm{\delta }}_{{\mathrm{H}}_2\mathrm{O}}^{\mathrm{n}}={\mathrm{W}}^{\mathrm{n}+1}{\mathrm{\delta }}_{{\mathrm{H}}_2\mathrm{O}}^{\mathrm{n}+1}$$

(2)

where ${\mathrm{\delta }}_{{\mathrm{H}}_2\mathrm{O}}$ 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 heavy 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 them 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, their δD 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 close to one-third for Dahua nephrite and Sanchakou nephrite (Figure 3b), respectively, which means that a small amount of meteoric water mixed into the ore-forming 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 one can only roughly know that the total amount of meteoric water is close to eight 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 and close to W^4-7, 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 nephrite with ore-forming fluids close to magmatic water (Figure 3d). First, assuming that the formation of these nephrite deposits 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 4). 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 and Yinggelike nephrite, the amount of meteoric water is between one-half and one-third 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.

5. 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 nephrite 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’s 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’s 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 unexchanged 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 of unexchanged initial fluid 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 them. 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.