Mineralogy and Sr Isotope Characteristics of Dahua Stratified Tremolite Nephrite and Host Rocks, Guangxi Province, China

Abstract

The tremolite nephrite deposit in Dahua county, Hechi City, Guangxi province, China is a new genetic type of nephrite deposit. It is hosed by Mg-poor limestone (~1.30 wt.% MgO) and intruded by diabase (~45 wt.% SiO₂). The Mg and Si contents of these rocks are lower than those of the tremolite (58.18 wt.% SiO₂, 13.18 wt.% CaO, 24.16 wt.% MgO), indicating an obviously insufficient source for the metallogenic material that generated the deposit. In particular, some tremolite nephrite ore bodies have no clear contact metamorphism between the host and intrusive rocks, which have the characteristics of stratified mineralization (stratified tremolite nephrite). The origin and mineralization of stratified tremolite nephrite remain poorly constrained. To address this shortcoming, the mineralogy, geochemistry and Sr isotopic of host rock, altered marble, stratified tremolite nephrite and intrusive rock in the Dahua stratified tremolite nephrite deposit were studied. The results show: stratified tremolite nephrite mainly consists of aggregates of microcrystalline-cryptocrystalline tremolites with content exceeding 95%. The in situ rare earth elements (REEs) distribution pattern of hydrothermal calcite in the contact position between stratified tremolite nephrite and marble is similar to that of marine carbonate rock, showing obvious enrichment of HREE, which is different from calcite in limestone and marble. ⁸⁷Sr/⁸⁶Sr of stratified tremolite nephrite is relatively uniform, with an average value of 0.7070, within the range of Permian seawater. The mean value of Y/Ho in the hydrothermal calcite is 51.24, indicating that the marine fluid has not been impregnated by terrigenous materials. In summary, the hydrothermal fluid rich in Ca and Si is formed after marine carbonate rocks are altered by marine fluids. Hydrothermal fluids alter diabase rocks formed by altered minerals like titanite, chamosite, zoisite, etc. This process leads to the formation of metallogenic hydrothermal fluids abundant in Si, Ca, Fe and Mg. The metallogenic hydrothermal fluids migrate in faults and fractures of marble and crystallize to form tremolite nephrite under suitable ore-forming conditions.

1. Introduction

Tremolite[Ca₂(Mg_5.0–4.5Fe²⁺_0.0–0.5)Si₈O₂₂(OH)₂] nephrite is an aggregate of tremolite with an interwoven fibrous structure; its hardness is slightly lower than that of jade [1]. Tremolite nephrite is a mineral resource with a high economic value [2]. However, some traditional tremolite nephrite resources have been exhausted owing to a lack of exploration; therefore, the exploration and development of new tremolite nephrite mines is crucial to meet the needs of the market and consumers [3]. Since the discovery of a large tremolite nephrite deposit in Luodian, Guizhou, China, distinctive tremolite nephrite resources have also been discovered in Dahua Yantan, Guangxi, China [4,5,6]. Unlike typical marble-type tremolite nephrite deposits, where a sufficient source of ore-forming material for the deposit is present, the Dahua tremolite nephrite deposit developed in the contact zone between Mg-poor limestone host rock (~1.3 wt.% MgO) and an intrusive mafic rock (45~50 wt.% SiO₂) [7,8], and neither the host rock nor the intrusive rock could have supplied sufficient metallogenic material to generate the deposit. Therefore, the source of the ore-forming material and the formation process of the Dahua tremolite nephrite deposit are more complex than those of traditional tremolite nephrite deposits. The traditional marble-type tremolite nephrite mainly occurs in the contact zone between magnesite marble and intermediate-acid magmatic rocks. From wall rock to tremolite nephrite, there are obvious skarn alteration zones, such as serpentinization-diopside-tremolization. Many of the Dahua tremolite ore bodies are interspersed with altered marble, showing sharp contacts, and there is no direct evidence of metasomatism or metamorphism in the host rock [8,9,10]. This type of ore body has the metallogenic characteristics of a stratified deposit, and its genesis is poorly understood. Based on detailed mineralogical studies of host rock, altered marble, tremolite nephrite and intrusive rock, the metallogenic sources of this type of stratified tremolite nephrite deposit are discussed by means of electron probe, in situ rare earth elements of hydrothermal minerals and Sr isotopes of host rock, altered marble, stratified tremolite nephrite and intrusive rocks. The results improve our understanding of the Dahua tremolite nephrite deposit and provide a theoretical basis for further exploration and prospecting.

2. Geological Overview of Dahua Tremolite Nephrite

The study area is located in the southwestern and eastern parts of the Youjiang fold belt in the South China fold system, adjacent to the Yangtze platform and the upper Yangtze platform fold belt [4]. The mining area is located in the Duyangshan–Damingshan anticline of the Youjiang fold belt. Since the late Paleozoic, this area has been influenced by subduction and the closure of the Paleo-Tethys Ocean, Emeishan mantle plume activity, subduction of the Paleo-Pacific plate and uplift of the Qinghai–Tibet Plateau; consequently, multiple periods of tectono-magmatic activity are recorded [11]. In the Dahua tremolite nephrite mining area, the dip angles of rock stratum are generally large. There are large faults distributed, and the structural joints are also well developed. Various fault lines and folds are common and distributed in parallel, providing channels for hydrothermal flow after the magmatic period [3]. The strata in this area are from the Upper Devonian to the Middle Triassic, belonging to the Middle and Upper Paleozoic and part of the Mesozoic strata. The lithology of strata of different geologic age can be divided into: the Upper Devonian to the Lower Permian of the Middle and Upper Paleozoic, with marine carbonate; the Upper Permian at the end of the Upper Paleozoic, with littoral facies; the Middle Triassic of the Mesozoic, with clastic rocks of marine-terrestrial alternating facies. The emergence stratums related to tremolite nephrite formation are Maokou Formation and Qixia Formation of the Lower Permian, constructed by Marine carbonate rocks, and contain clastic rocks of the Middle Triassic and red clay minerals, and clastic deposits of the Quaternary [11]. The magmatic rocks in the Dahua tremolite nephrite mining area are the diabase from Hercynian to Indochinese. The diabase intrudes into marine carbonate rocks along the contact surface of the Maokou Formation and Qixia Formation of Lower Permian in the shape of bedrock, forming tremolite deposits in the outer contact zone (Figure 1) [12].

3. Deposit Geology and Occurrence Characteristics of Dahua Tremolite Nephrite

The samples (stratified tremolite nephrite) were mainly collected from the open pit mine on the east bank of Yantan Town Hydropower Station. The geographical coordinates are 107°31′9.75″ E, 24°2′14.96″ N, and the altitude is about 310 m. The dark gray limestone in the mining area is produced in layers, the strata strike northeast-southwest, the dip angle is 20°–90°, and the degree of weathering is moderate to strong (Figure 2a–d). The diabase is the intrusive rock of limestone (Figure 2a–c). In the contact zone between limestone and diabase, layered marble appears, the bedding direction of which is consistent with that of limestone (Figure 2d). The protolith of marble is limestone. The dip angle of the limestone is too small or too large, only marble occurs in the contact zone between diabase and limestone, and no stratified tremolite nephrite veins are found (Figure 2c). The dip angle of the limestone is between 60° and 70°, the diabase invades along the limestone layer, and stratified tremolite nephrite veins appear in the metamorphism of marble (Figure 2d). The stratified tremolite nephrite ore body is distributed in layers of different thickness, and the direction of stratified tremolite nephrite veins is consistent with the direction of the marble bedding. There is a sharp contact between the stratified tremolite nephrite ore body and the marble, and sometimes there are large cracks between the stratified tremolite nephrite veins and the marble (Figure 2e–g).

4. Sample Collection and Analysis Testing

According to the different rock types of the stratified tremolite nephrite metallogenic belt, seven samples were selected in this study, including four tremolite nephrite samples with marble, one diabase and two limestone blocks (Figure 3).

The observation of mineral optical properties of samples was completed by a Zeiss Axio Scope A microscope, in the Precision Instrument Laboratory of School of Earth Sciences, Guilin University of Technology. Mineral composition, geochemical characteristics of whole rock and Sr isotope measurement of samples were completed in the Key Laboratory of Guangxi Concealed Deposits of Guilin University of Technology. The instrument model used for testing the elemental content of minerals is a JXA-8230 electron probe microanalyzer. Test conditions: voltage 15 kV, current 20 nA, diameter of beam spot 1–5 μm. Testing is conducted in accordance with GB/T15074-2008 “General Principles for Electron Probe Quantitative Analysis Methods” [13]. The range of test elements is Be⁴-U⁹², with an analysis detection limit of 100 ppm (0.01%). The analysis accuracy for elements with abundance greater than 5% is ≤1%, and for elements with abundance less than 1%, it is ≤5%. Trace elements were measured and analyzed by an Inductively coupled plasma mass spectrometer (ICP-MS) from Agilent 7500cx (Agilent Technologies Inc., Santa Clara, CA, USA). The output wavelength of the laser was 193 nm, with an analysis accuracy of 2~5% and an error of less than 5%. Sr isotopes were tested by using a Neptune Plus Multi-Collector inductively coupled plasma mass spectrometer (MC-ICP-MS) produced by Thermo Fisher Scientific, North Ryde, NSW, Australia. The resolution of this instrument is higher than 8000 square peaks, and the test acceleration voltage is 10 Kv. The sensitivity is as follows: Li 10.6 ppm; Sr 24 ppm; Nd 37 ppm; Hf 37 ppm; Pb 40 ppm; U 40 ppm. With abundance sensitivity less than 0.5 ppm (RPQ), the external accuracy is as follows: ⁷Li/⁶Li 0.3‰; ⁸⁷Sr/⁸⁶Sr 10‰; ¹⁴³Nd/¹⁴⁴Nd 10‰; ¹⁷⁶Hf/¹⁷⁷Hf 10‰; ²⁰⁷Pb/²⁰⁶Pb 20‰; ²⁰⁶Pb/²⁰⁴Pb 100‰; ²³⁵U/²³⁸U 300‰; ²³⁴U/²³⁸U 0.2%.

5. Analysis Results

5.1. Petrography

5.1.1. Limestone and Marble

The host rock sample is pure limestone, consisting of tightly packed calcite (>99%) grains of 5–70 μm in diameter (Figure 4a). Elliptic quartz particles (<1%) appeared locally (Figure 4a). The marble is also mainly composed of calcite, indicating the rock is calcite marble. Calcite grains are ~0.2–0.5 mm in size and closely packed, have no preferred orientation and exhibit polysynthetic twinning and rhombohedral cleavage (Figure 4b). Calcite grains have high relief under plane-polarized light and high-order interference colors under cross-polarized light. The marble contains minor quartz grains of <0.2 mm in diameter and with relatively straight edges. In the contact position between marble and stratified tremolite nephrite, lamellar tremolite with a particle size less than 0.5 mm and blue-green interference color can be seen in the interstice of calcite particles (Figure 4c).

5.1.2. Diabase

Diabase is composed of porphyry and matrix (Figure 4d). The matrix in the diabase consists of small tabular plagioclase and heterogranular pyroxene, both of which are partly altered (Figure 4e). Plagioclase crystals are more euhedral than the pyroxene, and larger pyroxene crystals locally enclose feldspar, forming a poikilitic texture (Figure 4e). Pyroxene crystals are 400–1000 μm long and make up 45%–55% of the rock. The pyroxene has been locally altered to chlorite that has a dusty appearance with blurred grain boundaries (Figure 4f). Plagioclase crystals are 200–500 μm long and make up 35%–45% of the rock. The feldspar has been locally epidotized (Figure 4f). The diabase sample contains minor titanite, ilmenite, apatite and other minerals that together make up ~10% of the rock (Figure 4d–f).

5.1.3. Tremolite Nephrite

Stratified tremolite nephrite and marble show abrupt contacts (Figure 4g). Tremolite particles are microcrystalline-cryptocrystalline with good orientation and a particle size less than 0.01 mm (Figure 4g–i). The tremolite is colorless with moderate relief in the thin section and shows second-order blue to blue-green interference colors. The boundary outline between tremolite particles in tremolite nephrite is fuzzy and the crystal is small, so it forms a cryptocrystalline crystalloblastic texture (Figure 4g–i). The tremolite is about 95% in content and contains small amounts of calcite, quartz and chlorite. There are two types of calcite in stratified tremolite nephrite samples. The first is coarse-grained calcite with semi-self-propelled, a particle size > 0.5 mm, high-order white interference colors and polysynthetic twinning; this calcite appears near the contact position near marble (Cal-1 in Figure 4h). The second type of calcite is anhedral fine-grained calcite; it has a particle size < 0.2 mm, high-order white interference colors can be seen and it is distributed in the local area of stratified tremolite nephrite (Cal-2 in Figure 4i). In addition, quartz in tremolite nephrite with a particle size of less than 0.2 mm is grayish-dark gray under the polarizer (Qz-1 in Figure 4i). The calcite and quartz in stratified tremolite nephrite are different from those in limestone and marble in grain size or optical properties, indicating that they are of hydrothermal origin. Granular chlorite locally fills voids between calcite grains in marble or tremolite nephrite (Figure 4g,i). The chlorite is nearly colorless under plane-polarized light and shows a first-order gray interference color or a typical indigo blue-lilac interference color [14]. The chlorite grains are anhedral and <3 mm in size.

5.2. Electron Probe

5.2.1. Limestone and Marble

The EPMA data show that calcite is the main mineral in the limestone. The calcite is relatively pure with a CaO content of 55.71%–55.95 wt.%, it lacks SiO₂, and has low contents of MgO (0.45%–0.51 wt.%), which is close to the theoretical value of pure calcite. The SiO₂ content of the quartz in the limestone is ~95.53%, although little quartz is encountered, which is consistent with the petrographic observations. Quartz crystals fill interstitial spaces between calcite grains and are allotriomorphic (Figure 5a). The calcite and quartz in the limestone are relatively pure (

Table 1. Electron probe data of stratified tremolite nephrite, limestone, marble and diabase.

Sample	Na2O	SiO2	Al2O3	MgO	CaO	P2O5	FeO	V2O3	K2O	Cr2O3	MnO	TiO2	Total	Mineral Name
Limestone	0.00	0.00	0.00	0.51	55.71	0.10	0.01	0.00	0.00	0.00	0.04	0.00	56.36	Calcite
	0.01	0.00	0.00	0.45	55.95	0.02	0.00	0.00	0.00	0.00	0.11	0.00	56.55	Calcite
	0.02	95.53	0.03	0.01	0.17	0.00	0.00	0.00	0.00	0.02	0.02	0.02	95.82	Quartz
Marble	0.00	0.00	0.00	0.69	55.49	0.03	0.00	0.00	0.00	0.02	0.03	0.01	56.27	Calcite
	0.00	0.00	0.02	0.38	55.34	0.02	0.02	0.08	0.00	0.03	0.08	0.04	55.71	Calcite
	0.00	96.01	0.00	0.01	0.36	0.00	0.01	0.08	0.01	0.00	0.00	0.01	96.48	Quartz
Tremolite nephrite	0.09	58.79	0.29	24.12	13.52	0.02	0.81	0.03	0.06	0.02	0.11	0.03	97.88	Tremolite
	0.07	58.68	0.19	24.20	13.39	0.02	0.80	0.03	0.06	0.00	0.11	0.00	97.54	Tremolite
	0.08	59.46	0.35	24.08	12.38	0.03	0.92	0.08	0.05	0.03	0.18	0.03	97.69	Tremolite
	0.01	0.82	0.00	1.27	57.06	0.02	0.00	0.08	0.00	0.00	0.00	0.00	59.26	Calcite-1
	0.04	0.96	0.00	1.44	57.22	0.04	0.03	0.02	0.01	0.01	0.04	0.00	59.81	Calcite-1
	0.00	0.73	0.01	1.24	55.19	0.01	0.00	0.00	0.00	0.00	0.05	0.00	57.23	Calcite-2
	0.01	1.21	0.02	1.51	54.46	0.04	0.14	0.11	0.01	0.01	0.07	0.00	57.59	Calcite-2
	0.02	97.69	0.09	2.95	0.29	0.00	0.23	0.00	0.03	0.04	0.00	0.02	101.36	Quartz-1
	0.00	99.45	0.04	0.72	0.43	0.00	0.00	0.00	0.01	0.00	0.03	0.00	100.68	Quartz-1
	0.02	35.75	13.53	34.49	0.37	0.00	3.35	0.06	0.02	0.07	0.09	0.00	87.75	Penninite
	0.00	35.01	13.84	34.12	0.39	0.02	4.11	0	0.01	0.03	0.08	0.00	87.61	Penninite
Diabase	0.30	39.19	25.15	0.05	23.96	0.02	9.80	0.09	0.09	0.00	0.10	0.08	98.84	Anorthite
	11.66	67.43	20.62	0.04	1.81	0.00	0.33	0.00	0.24	0.01	0.01	0.01	102.16	Albite
	0.44	48.31	3.23	12.85	21.52	0.02	9.84	0.16	0.02	0.15	0.28	1.40	98.23	Augite
	0.41	48.56	4.01	12.26	21.81	0.03	10.53	0.17	0.01	0.02	0.28	2.04	100.12	Augite
	0.00	0.05	0.03	0.00	0.21	0.00	44.88	0.55	0.00	0.02	1.69	52.42	99.84	Ilmenite
	0.00	0.11	0.31	0.04	0.02	0.00	45.14	0.35	0.00	0.76	1.50	51.89	99.80	Ilmenite
	0.30	39.19	25.15	0.05	23.96	0.02	9.80	0.09	0.09	0.00	0.10	0.08	98.84	Epidote
	0.16	38.75	25.12	0.05	24.01	0.00	9.97	0.03	0.10	0.03	0.08	0.06	98.35	Epidote
	0.02	29.17	2.23	0.06	30.01	0.00	1.39	0.76	0.02	0.00	0.00	36.41	100.07	Titanite
	0.03	0.30	0.00	0.15	48.96	40.34	0.48	0.00	0.01	0.00	0.02	0.03	90.32	Apatite
	0.02	0.26	0.00	0.19	48.76	40.57	0.70	0.01	0.00	0.00	0.05	0.13	90.66	Apatite
	0.11	42.23	22.84	0.01	24.98	0.01	1.02	0.00	0.02	0.00	0.01	0.14	91.36	Zoisite
	0.23	42.35	22.23	0.10	24.00	0.02	1.78	0.02	0.03	0.02	0.04	0.17	90.99	Zoisite
	0.01	24.94	15.28	14.36	0.12	0.00	32.89	0.06	0.11	0.00	0.14	0.00	87.89	Chamosite
	0.12	23.25	15.87	18.84	0.16	0.00	28.88	0.04	0.19	0.00	0.44	0.00	87.79	Chamosite

). The chemical compositions and proportions of calcite and quartz in the marble are similar to those in the limestone, suggesting that the marble is the thermally metamorphosed equivalent of the limestone (Figure 5b; Table 1). In the contact position between marble and stratified tremolite nephrite, it can be seen that ore-forming hydrothermal fluid fills cracks or voids in marble to form tremolite (Figure 5c).

5.2.2. Diabase

(1) Clinopyroxene and Plagioclase

Electron probe data showed that the diabase mainly consists of clinopyroxene and plagioclase, but also contains a small amount of minerals such as titanite, ilmenite, apatite, zoisite and chamosite. According to the molar fraction of plagioclase end members and the An-Ab-Or classification of feldspar, the plagioclase in diabase is mainly composed of albite (Na_1.46Al_1.01Si_2.82O₈) and anorthite (N_a0.63K_0.42Ca_1.31Al_1.50Si_1.99O₈). Albite is disseminated in the anorthite, as shown by backscattering photographs (Figure 5d). The pyroxene in diabase is mainly augite (Ca_0.92Mg_0.72Fe_0.34Si_1.86Al_0.18O₆), according to the electron probe data and Wo-En-Fs pyroxene classification [15].

(2) Ilmenite

Backscattering image observation shows that ilmenite (Fe_0.95Ti_0.99Mn_0.04O₃) is mainly disseminated with a particle size of 200–500 μm. Ilmenite is embedded between plagioclase and pyroxene in idiomorphic plate texture and subhedral-xenomorphic granular texture (Figure 5d), or embedded in plagioclase in tiny xenomorphic granular crystals (Figure 5e). Most ilmenite has dissolution caves with wavy edges. This is mainly due to the late hydrothermal alteration after the formation of ilmenite (Figure 5e). The electron probe test data of ilmenite show that w(TiO₂) = 51.89%–52.42 wt.%, w(FeO) = 44.88%–45.14 wt.%, w(MnO) = 1.50%–1.69 wt.%. The content of MnO is high while MgO is low, which is consistent with the composition characteristics of ilmenite in basic rock [16].

(3) Titanite

Titanite (Ca_1.02Ti_0.91Fe_0.04Si_0.98O₅) forms cryptocrystalline aggregates and mainly distributed along the edges of ilmenite and its cracks (Figure 5e,f). The electron probe test showed that w(TiO₂) = 36.41 wt.%, w(CaO) = 30.01 wt.%, w (SiO₂) = 29.17 wt.%, w(FeO^T) = 1.39 wt.%, w(Al₂O₃) = 2.23 wt.%, which was in line with the theoretical value of titanite. The titanite is formed by hydrothermal alteration of ilmenite.

(4) Apatite

Apatite (Ca_5.68P_3.69O₁₂(OH)₇) is mostly idiomorphic-semi-idiomorphic granular or short columnar, distributed among augite particles, indicating that the apatite was formed earlier in a relatively stable environment (Figure 5f). Electron probe data of apatite show that w(CaO) = 48.76%–48.96 wt.%, w(P₂O₅) = 40.34%–40.57 wt.%, and total element content is 90.32%–90.66 wt.%. The content of constituent elements of apatite has little change, indicating that apatite was formed in the same ore-forming period. Compared with the standard content of apatite (CaO 55.38 wt.%, P₂O₅ 42.06 wt.%), both CaO and P₂O₅, are lower, indicating that apatite may contain more additional anions (F⁻, Cl⁻, OH⁻), which suggests that apatite was formed in the late magmatic activity [17].

(5) Chamosite

Although the Fe³⁺ contents of the chlorite crystals cannot be determined by EPMA, Fe³⁺ contents in chlorite are generally <5% of the total Fe content [18]. Therefore, we use the total Fe content to approximate the Fe²⁺ content of the chlorite. The number of cations in the chlorite was normalized to 14 oxygen atoms (Table 4). The Si⁴⁺ atom number (2.60–2.78) and Fe²⁺/R²⁺ value (0.46–0.56) in chlorite from diabase are within the range of chamosite [19]. The distribution of chamosite (Mg_2.75Fe_2.86Si_2.69Al_2.02O₁₀(OH)_3.26) in augite by the form of stellated or wormy indicates that chamosite is formed by alteration of augite (Figure 5e). Chamosite is in w (SiO₂) = 23.25%–24.94 wt.%, w (FeO^T) = 28.88%–32.89 wt.%, w (Al₂O₃) =15.28%–15.87 wt.%, w (MgO) = 14.36%–18.84 wt.%.

(6) Zoisite

In the backscatter photos, zoisite (Ca_2.27Al_2.28Si_3.59O₁₂(OH)) is distributed in the anorthite in the shape of pockmark, which indicates that the zoisite is altered from anorthite (Figure 5e). The content of SiO₂ in zoisite is 42.23%–42.35 wt.%, w(CaO) = 24.00%–24.98 wt.%, w(Al₂O₃) = 22.23%–22.84 wt.%, w(FeO^T) = 1.02%–1.78 wt.%. The relatively stable composition of zoisite indicates that it is a hydrothermal alteration product of the same period.

5.2.3. Tremolite Nephrite

(1) Tremolite

The main minerals of Dahua stratified tremolite nephrite are tremolite (Ca_1.87Na_0.04K_0.01)(Mg_5.10Fe_0.11)Si_8.34O₂₂(OH)₂), and calcite, quartz and chlorite are secondary minerals. According to the electron probe test results, the main chemical components of tremolite are SiO₂, MgO and CaO, and the minor chemical components are Al₂O₃, FeO, Na₂O, K₂O and MnO (Table 1). The content of tremolite SiO₂ is 58.68%–59.46 wt.%, with an average of 58.97 wt.%, the content of MgO is 24.08%–24.20 wt.%, with an average of 24.13 wt.%, and the content of CaO is 12.38%–13.52 wt.%, with an average of 13.10 wt.%, which is consistent with the theoretical value of tremolite composition (SiO₂: 59.17 wt.%, MgO: 24.81 wt.%, CaO: 13.80 wt.%).

(2) Calcite and Quartz

The calcite can be divided into primary and hydrothermal calcite, according to the EPMA data. The primary calcite is relatively pure, does not contain SiO₂, contains a small amount of MgO and its composition characteristics are consistent with those of calcite distributed in limestone and marble. The hydrothermal calcite has much higher contents of SiO₂ and MgO, SiO₂ (0.73%–1.21 wt.%) and MgO (1.24%–1.51 wt.%) (Cal-1 Cal-2 in Figure 5i; Table 1). The contents of MgO (0.72%–3.41 wt.%) and CaO (0.43%–2.07 wt.%) in hydrothermal quartz of tremolite nephrite are relatively high (Qz-1 in Figure 5i; Table 1).

(3) Penninite

The number of Si atoms (3.28–3.32) and the Fe²⁺/R²⁺ value (0.05–0.06) in chlorite from stratified tremolite nephrite are within the range of penninite (Mg_4.80Fe_0.29Si_3.30Al_1.51O₁₀(OH)_4.04) (Table 4) [19]. Penninite is Mg-rich chlorite, and its chemical composition is SiO₂:35.01%–35.75 wt.%, MgO:34.12%–34.49 wt.%, Al₂O₃: 13.53%–13.84 wt.%, FeO: 3.35%–4.11 wt.%. Penninite composition in marble and stratified tremolite nephrite is basically the same, indicating the same metallogenic source (Figure 5h,i).

5.3. In Situ Trace Element Analysis

In situ trace element analysis, the hydrothermal calcite in stratified tremolite nephrite, as well as calcite found in limestone and marble were conducted. The results showed that the distribution patterns of REEs in calcite from limestone and marble are quite similar [20]. This similarity suggests that the formation of marble occurs through the process of thermal metamorphism of limestone. During this metamorphic process, limestone undergoes changes in response to heat and pressure, transforming it into marble while retaining similar REE distribution patterns to those observed in calcite from limestone (Figure 6). Chondrite-normalized REEs patterns of hydrothermal calcite in stratified tremolite nephrite are quite different from those of calcite in limestone and marble (Figure 6). Chondrite-normalized REEs patterns of hydrothermal calcites in stratified tremolite nephrite display obviously negative Ce anomalies and moderately negative Eu negative anomalies, declined LREE (LREE: La-Eu) and enriched HREE (HREE: Gd-Lu) (Figure 6a). All calcites in the analyzed limestone and marble yield strong negative Ce and Eu anomalies with declined LREE and flat HREE. The REE-Y of hydrothermal calcite in stratified tremolite nephrite ranges from 5.12 × 10⁻⁶ to 13.24 × 10⁻⁶ (average REE-Y = 11.48 × 10⁻⁶) (

Table 2. REE content of calcite of hydrothermal origin in nephrite, in limestone and in marble.

Sample	Calcite in Tremolite Nephrite				Average	Calcite in Limestone		Average	Calcite in Marble		Average
La	0.90	1.51	2.77	2.40	1.89	3.57	3.42	3.49	8.95	5.25	7.10
Ce	0.28	0.41	0.79	0.92	0.60	1.15	1.19	1.17	2.76	1.61	2.18
Pr	0.15	0.28	0.44	0.45	0.33	0.60	0.53	0.57	1.46	0.92	1.19
Nd	0.58	1.65	2.24	2.07	1.63	2.29	1.98	2.14	6.03	3.90	4.97
Sm	0.19	0.39	0.50	0.56	0.41	0.46	0.37	0.41	1.08	0.74	0.91
Eu	0.07	0.15	0.17	0.13	0.13	0.10	0.07	0.08	0.16	0.06	0.11
Gd	0.29	0.97	0.78	0.78	0.71	0.46	0.34	0.40	1.09	0.68	0.89
Tb	0.11	0.18	0.16	0.17	0.16	0.08	0.06	0.07	0.18	0.12	0.15
Dy	0.77	1.55	1.44	1.35	1.28	0.50	0.34	0.42	1.17	0.71	0.94
Ho	0.28	0.36	0.34	0.34	0.33	0.12	0.08	0.10	0.27	0.17	0.22
Er	0.64	1.35	1.22	0.97	1.04	0.32	0.21	0.26	0.71	0.44	0.57
Tm	0.10	0.22	0.24	0.16	0.18	0.05	0.03	0.04	0.10	0.07	0.08
Yb	0.68	1.77	1.84	1.02	1.33	0.28	0.17	0.22	0.54	0.36	0.45
Lu	0.08	0.18	0.31	0.15	0.18	0.04	0.03	0.03	0.08	0.05	0.07
Y	13.72	20.12	17.76	16.74	17.08	5.88	3.69	4.79	13.18	7.83	10.51
∑(REE-Y)	5.12	10.98	13.24	11.48	10.20	10.00	8.82	9.41	24.57	15.07	19.82
δCe	0.16	0.15	0.16	0.20	0.17	0.18	0.19	0.19	0.17	0.17	0.17
δEu	0.72	0.71	0.84	0.62	0.72	0.67	0.56	0.61	0.45	0.25	0.35
Y/Ho	48.98	55.30	51.85	49.24	51.34	50.92	46.83	48.88	49.11	47.30	48.21
(La/Yb)N	1.00	1.18	1.08	1.69	1.24	9.30	14.73	12.02	11.98	10.47	11.23
(La/Sm)N	2.95	2.53	3.59	2.76	2.96	5.06	6.05	5.55	5.36	4.61	4.99
(Gd/Yb)N	0.45	0.45	0.35	0.64	0.47	1.40	1.70	1.55	1.68	1.57	1.63

). The average REE-Y values of calcite in limestone and marble are 9.41 × 10⁻⁶ and 19.82 × 10⁻⁶, respectively. It can be seen that the REE-Y content of hydrothermal calcite in stratified tremolite nephrite is basically the same as that in limestone, while the REE-Y content of calcite in marble is higher. Analysis results of REEs of hydrothermal calcite in stratified tremolite nephrite and calcite in limestone and marble were standardized using data from post-Archean shales in Australia (PASS), and compared with modern seawater sediments [21,22]. The standardized Rare Earth Elements (REEs) distribution pattern of calcite, as determined by PASS analysis, is in line with that of marine carbonate rocks. This suggests that the hydrothermal calcite identified in the stratified tremolite nephrite ore deposit is associated with marine carbonate materials. However, there does not appear to be a significant relationship between the hydrothermal calcite and limestone or marble.

5.4. Sr Isotope Characteristics

Sr has four isotopes in nature: ⁸⁴Sr, ⁸⁶Sr, ⁸⁷Sr and ⁸⁸Sr, of which ⁸⁷Sr is derived from the radioactive decay of ⁸⁷Rb, and the others are stable nuclides [23]. Since the mass number of strontium (Sr) isotopes is relatively large and the relative mass difference between different isotopes is small, the influence of changes in the physical and chemical conditions of the ore-forming solution on Sr can generally be ignored during the mineralization process. This means that the isotopic composition of strontium remains relatively constant during the formation of minerals in terms of its physical and chemical conditions [24]. The Sr isotope exchange between the ore-forming solution and the rock is slow, so ⁸⁷Sr/⁸⁶Sr in the mineral can indicate its source of mineralization when there is no element addition of Sr [24].

Sr isotope analysis results of Dahua stratified tremolite nephrite, limestone, marble and diabase are shown in

Table 3. Sr isotope test data of Dahua stratified tremolite nephrite, limestone, marble and diabase.

Rock Type	Sr (ppm)	Rb (ppm)	Rb/Sr	87Sr/86Sr	±2σ (×10−6)
Limestone	807.826	0.627	0.0008	0.7073	±13
Limestone	807.826	0.627	0.0008	0.7073	±12
Average value	807.826	0.627	0.0008	0.7073
Marble	733.854	0.057	0.0001	0.7075	±15
Marble	661.076	0.228	0.0003	0.7079	±14
Marble	781.969	0.086	0.0001	0.7072	±12
Average value	725.633	0.124	0.0002	0.7075
Tremolite nephrite	91.694	0.419	0.0046	0.7068	±9
Tremolite nephrite	73.196	0.493	0.0067	0.7069	±10
Tremolite nephrite	62.468	0.577	0.0092	0.7072	±11
Tremolite nephrite	75.850	0.519	0.0068	0.7070	±13
Tremolite nephrite	119.200	0.293	0.0025	0.7068	±10
Average value	84.482	0.460	0.0055	0.7069
Diabase	618.000	15.550	0.0252	0.7061	±13
Diabase	662.800	16.730	0.0252	0.7061	±10
Average value	640.400	16.140	0.0252	0.7061

. The Sr content in limestone is relatively high, with an average value of 807.826 × 10⁻⁶, the Rb content is extremely low, with an average value of 0.6, and the Rb/Sr ratio is average 0.0008. Therefore, the Sr isotope composition in limestone can approximately represent the Sr isotope composition of the ore-forming fluid during mineralization [25]. The average ⁸⁷Sr/⁸⁶Sr value of the limestone sample is 0.7073, which is consistent with the ⁸⁷Sr/⁸⁶Sr value 0.7068–0.7082 of Permian seawater, indicating that it is a Permian marine carbonate. The ⁸⁷Sr/⁸⁶Sr value range of marble is 0.7072–0.7079, with an average value of 0.7075. The content of Sr is close to that of limestone, with an average of 725.633 × 10⁻⁶, Rb mean of 0.124 × 10⁻⁶ and Rb/Sr mean of 0.0002. It can be seen that there was no contamination of extraneous Sr during the process of limestone metamorphism into marble.

The Rb content of stratified tremolite nephrite in Dahua is higher than that of limestone and marble, with an average value of 0.496 × 10⁻⁶. The Sr content is lower than that of limestone and marble, with an average of 75.786 × 10⁻⁶, so the average of Rb/Sr is 0.0068. The Sr isotope composition of stratified tremolite nephrite is relatively uniform, ranging from 0.7068 to 0.7072, with an average value of 0.7070. The average ⁸⁷Sr/⁸⁶Sr ratio of diabase is reported to be 0.7061, which is slightly higher than the value for the mantle of 0.7040 ± 0.002 [26]. This suggests that the diabase in the Dahua stratified tremolite nephrite ore belt might have experienced crustal contamination or crystallization differentiation processes. These processes can lead to changes in the isotopic composition of strontium in the diabase, resulting in a higher ⁸⁷Sr/⁸⁶Sr ratio.

6. Discussion

6.1. Analysis of Metallogenic Hydrothermal Fluid

The characteristic ⁸⁷Sr/⁸⁶Sr ratio of Sr released by a specific weathering mineral can vary in nature [27]. Different fluid activities in water bodies can lead to different ⁸⁷Sr/⁸⁶Sr values. In the case of stratified tremolite nephrite in Dahua, the ratio of ⁸⁷Sr/⁸⁶Sr is relatively stable. This stability suggests that there is no evidence of multistage hydrothermal mineralization in the formation of it. The range of ⁸⁷Sr/⁸⁶Sr of stratified tremolite nephrite is 0.7068–0.7072, with an average value of 0.7070, which is within the range of ⁸⁷Sr/⁸⁶Sr of Permian seawater (0.7066–0.7082), indicating that the hydrothermal solution has marine characteristics. There is a significant linear negative correlation between ⁸⁷Sr/⁸⁶Sr and 1/Sr (r = −0.45) (Figure 7) of stratified tremolite nephrite, which indicates that the ⁸⁷Sr/⁸⁶Sr of stratified tremolite nephrite is controlled by two sources [28]. In Figure 7, the ratio of ⁸⁷Sr/⁸⁶Sr obtained by extrapolating 1/Sr = 0 from stratified tremolite nephrite is close to the ratio of ⁸⁷Sr/⁸⁶Sr from diabase, which can explain that the exogenous Sr in the hydrothermal fluid comes from diabase. It is inferred that the metallogenic hydrothermal fluid with marine characteristics flows through the diabase and is influenced by the ⁸⁷Sr/⁸⁶Sr values of the diabase.

Y and Ho have the same charge and similar ionic radii [29]. The Y/Ho values of terrestrial clasts and chondrites typically exhibit a consistent range between 26 and 28 due to their similar chemical behavior in natural waters [30]. The Y/Ho values of seawater are usually higher (44–74), significantly higher than those of river water and terrigenous detritus [31]. Therefore, the Y/Ho ratio is an effective index to distinguish marine and non-marine sedimentary environments. In this study, the Y/Ho ratios of hydrothermal calcite in stratified tremolite nephrite (average Y/Ho 51.34), calcite in limestone (average Y/Ho 48.88) and calcite in marble (average Y/Ho 48.21) are all within the range of seawater. The average Y/Ho value in hydrothermal calcite is noticeably higher than that in calcite found in limestone and marble. This indicates that the calcite found in the stratified tremolite nephrite ore belt has formed in a marine sedimentary metallogenic environment and is not influenced by external sources. Furthermore, the hydrothermal sources of calcite in the stratified tremolite nephrite ore belt are not closely associated with limestone and marble.

6.2. Source of Ore-Forming Material

The contact position of stratified tremolite nephrite with marble is abrupt contact, and no alteration is found in marble, which indicates that stratified tremolite nephrite is formed by filling of hydrothermal solution rich in Si, Mg and Ca. REEs are high-field-strength elements and do not migrate in hydrothermal systems [29]. The content of REEs in hydrothermal fluid is very low, so the distribution pattern of REEs in hydrothermal calcite in stratified tremolite nephrite is mainly affected by altered rocks. The REEs of hydrothermal calcite in stratified tremolite nephrite are consistent with marine carbonate through normalized distribution patterns, but differ from calcite in limestone and marble [22]. Based on the information provided, it can be inferred that the source of alteration for stratified tremolite nephrite is marine carbonate. This conclusion is drawn from the fact that the distribution patterns of REEs in calcite from stratified tremolite nephrite are similar to those of calcite found in limestone and marble, both of which are derived from marine carbonate deposits. Furthermore, the consistent distribution of REEs in the hydrothermal calcite of stratified tremolite nephrite suggests that the hydrothermal mineralization process occurred synchronously. This means that the formation of hydrothermal calcite and the deposition of REEs in stratified tremolite nephrite likely occurred simultaneously and were influenced by the same geological processes.

Hiller et al. (1991) and Zang et al. (1995) proposed that w(Na₂O + K₂O + CaO) < 0.5% can be used as a standard index to assess whether chlorite composition has been affected by mixing [32,33]. In the contact zone, chlorite has w(Na₂O + K₂O + CaO) = 0.33–0.47 wt.% (Table 2 and

Table 4. Chlorite electron probe test data, structural formula and eigenvalue calculation (based on 14 atoms).

Sample	Chlorite in Marble Contact Zone		Chlorite in Tremolite Nephrite Contact Zone		Chlorite in Diabase
Na2O (wt.%)	0.00	0.00	0.02	0.00	0.00	0. 12
SiO2 (wt.%)	33.80	33.29	35.75	35.01	27.82	23.25
Al2O3 (wt.%)	14. 19	15.00	13.53	13.84	14.21	15.87
MgO (wt.%)	32.61	32.32	34.49	34.12	11.16	18.84
CaO (wt.%)	0.40	0.29	0.37	0.39	0.40	0. 16
P2O5 (wt.%)	0.00	0.00	0.00	0.02	0.02	0.00
FeO (wt.%)	6.20	7.30	3.35	4.11	33.24	28.88
V2O3 (wt.%)	0.04	0.00	0.06	0.00	0.00	0.04
K2O (wt.%)	0.03	0.04	0.02	0.01	0.01	0. 19
Cr2O3 (wt.%)	0.01	0.06	0.07	0.03	0.03	0.00
MnO (wt.%)	0.06	0. 11	0.09	0.08	0.08	0.44
TiO2 (wt.%)	0.02	0.00	0.00	0.00	0.00	0.00
Total (wt.%)	87.36	88.41	87.75	87.61	86.82	87.79
nSi4+	3.21	3. 15	3.32	3.28	2.78	2.57
nAl3+	1.59	1.67	1.48	1.53	2.00	2.06
nAlIV	0.77	0.85	0.68	0.72	1.22	1.43
nAlVI	0.82	0.82	0.80	0.83	0.78	0.63
nMg2+	4.65	4.58	4.81	4.79	1.00	1.21
nFe2+	0.49	0.58	0.26	0.32	3.05	2.66
nFe2+/R2+	0.10	0.11	0.05	0.06	0.75	0.69
w(CaO + K2O + Na2O)	0.43	0.33	0.41	0.40	0.23	0.47
nAl/n (Al + Mg + Fe)	0.24	0.24	0.23	0.23	0.27	0.26
d001/0.1 nm	14.24	14.23	14.26	14.25	14.13	14.12
T/°C	139	150	121	128	244	260

), indicating that the chlorite was not dissolved by late-stage hydrothermal fluids after mineralization. Chlorite in argillaceous rocks differs from that in mafic rocks. Chlorite in altered mafic rocks is relatively Al-poor [nAl/n(Al + Mg + Fe) < 0.35] [17]. The nAl/n(Al + Mg + Fe) ratio of chlorite in the present study varies from 0.23 to 0.27, suggesting that chlorite in the stratified tremolite nephrite metallogenic belt are all formed by basic rock alteration. The occurrence of large diabase intrusion in the Dahua stratified tremolite nephrite deposit suggests that the source of chlorite is related to diabase.

In the past few decades, many types of chlorite geothermometers have been established [34,35]. However, different types of chlorite have different thermometers. In this paper, the calculation formula d₀₀₁(0.1 nm) = 14.339 − 0.115 n(Al^IV) − 0.0201 n(Fe²⁺) modified by Nieto et al. (1997) is used to calculate the d₀₀₁ [36]. Then the formation temperature of chlorite is calculated based on the equation t/°C = (14.379 − d₀₀₁(0.1 nm))/0.001 proposed by Battaglia (1999) for the relationship between d₀₀₁ and temperature [37]. The calculation results of chlorite temperature are shown in Table 3. The mineralogical study of the chlorite in the contact zone between stratified tremolite nephrite and marble, as well as diabase, reveals that they all originate from diabase. In the diabase, there is the presence of chamosite (Fe-chlorite) at mineral-forming temperatures ranging from 242 to 255°C. In the contact zone, clinochlore (Mg-chlorite) is found with a metallogenic temperature range of 124–150°C.

The formation of chlorite is intricately linked to hydrothermal processes, particularly regarding the replacement of augite by chamosite (Fe-chlorite) in diabase. The metasomatic dissolution and metasomatic corrosion texture in augite show that the chamosite is generally transformed from augite, and the Fe and Mg components are not absorbed from the hydrothermal fluid. The alteration of anorthite to zoisite and albite, and ilmenite to titanite indicated that a certain amount of Ca and Si was present in the altered hydrothermal fluid. It is speculated that marine carbonate rocks were dissolved by marine fluids, forming ore-forming hydrothermal fluids rich in Ca and Si. The hydrothermal solution intruded into diabase and altered ilmenite, pyroxene and plagioclase, forming altered minerals such as titanite, chamosite and zoisite. After the hydrothermal intrusion into the diabase, Mg and Fe were extracted from plagioclase, pyroxene and other minerals to form the metallogenic hydrothermal fluid rich in Si, Ca, Mg and Fe. The metallogenic hydrothermal fluid migrates along the fracture of marble rock stratum and finally forms tremolite under suitable ore-forming conditions. Because of the decrease in hydrothermal temperature, the increase in oxygen fugacity and the enhancement of alkalinity, Fe is expelled during hydrothermal migration; therefore, the contact position between stratified tremolite nephrite and marble forms chlorite which is rich in Mg and poor in Fe.

7. Conclusions

(1)

The mineral composition, REE partition pattern and ⁸⁷Sr/⁸⁶Sr values of limestone and marble in the Dahua stratified tremolite nephrite metallogenic belt are similar, indicating that marble forms through thermal metamorphism of limestone. The intrusive rock in the metallogenic belt is diabase, which has undergone hydrothermal alteration, resulting in the formation of titanite, chamosite and zoisite from ilmenite, pyroxene and plagioclase, respectively.

(2)

The abrupt contact relationship between Dahua stratified tremolite nephrite and marble indicates that it is of hydrothermal origin, and the metallogenic hydrothermal fluid is rich in Si, Mg and Ca.

(3)

The hydrothermal calcite in stratified tremolite nephrite has a similar REE distribution pattern to marine carbonate rock, but it differs from limestone and marble. This suggests that limestone or marble is not the rock that underwent hydrothermal alteration.

(4)

This indicates that ⁸⁷Sr/⁸⁶Sr in Dahua stratified tremolite nephrite is controlled by two resources, one is marine carbonate rock with higher ⁸⁷Sr/⁸⁶Sr and the other is diabase with lower ⁸⁷Sr/⁸⁶Sr. The average value of Y/Ho in hydrothermal calcite in stratified tremolite nephrite is 51.34, which is within the range of seawater, indicating that the hydrothermal fluid is not contaminated by continental substances.

(5)

Based on the characteristics of diabase alteration, it can be inferred that carbonate alteration resulted in the formation of a hydrothermal solution rich in Ca and Si. This hydrothermal fluid flowed through the diabase, causing alteration of minerals such as ilmenite, pyroxenes and feldspar. As a result, elements like Si, Ca, Fe and Mg were enriched, leading to the formation of a metallogenic hydrothermal fluid. This fluid migrated along fissures and gradually crystallized under suitable ore-forming conditions, eventually forming tremolite.