Sb-Bi Alloys and Ag-Cu-Pb-Sb-Bi Sulphosalts in the Jialong Cu-Sn Deposit in North Guangxi , South China

Although native bismuth is a relatively common mineral, native antimony is less abundant, and Sb-Bi alloys are relatively rare phases in Nature. Sb-Bi alloys and Ag-Cu-Pb-Sb-Bi sulphosalts have been discovered in the Jialong vein-type Cu-Sn deposit in North Guangxi, South China. The Jialong deposit is hosted by schist within the contact zone of a Neoproterozoic granite. Four stages of ore formation are recognised, with the Sb-Bi alloyand sulphosalt-bearing assemblage formed during the third stage. Sulphosalts include Pb-Bi-Ag sulphosalts (pavonite), Sb-Bi sulphosalts (tintinaite, terrywallaceite), and Sb sulphosalt (ullmanite, freibergite, bournonite). Grains of Sb-Bi alloy measure 2–20 μm in diameter, show rounded margins and occur together with galena along the edges or internal fissures of sulphosalts. The Sb-Bi alloys do not coexist with bismuthinite, BiS (an unnamed mineral), or with native bismuth. Two phases of Sb-Bi alloys are identified based on back-scattered electron image observations and electron microprobe analysis. The textural and thermodynamic relationships indicate that Phase I was formed before Phase II. Phase I contains high Sb (69.15–80.12 wt %) and lower Bi (18.01–27.85 wt %), while Phase II contains low Sb (0.89–25.24 wt %) and high Bi (72.95–98.89 wt %). Cooling in the range of 270–400 ◦C and decreasing sulphur fugacity promote precipitation of Sb-Bi alloys and sulphosalts during the late stage of incursion of Sband Bi-bearing magmatic hydrothermal fluids.


Introduction
Bismuth-bearing minerals, including native bismuth, bismuth sulphosalts, and bismuth chalcogenides, occur in a variety of deposits [1], and have attracted significant interest as a source of this useful metal.Bi and Sb are group V elements in the periodic table and occur within a range of sulphosalt minerals [2].The complex substitution mechanisms of sulphosalts [3,4] are of particular geological interest for what they can reveal about the physico-chemical conditions of mineral formation [5][6][7][8][9].Metallic Bi minerals include native bismuth, with some Pd-Bi (e.g., froodite PdBi 2 , [10,11]) and Au-Bi (e.g., maldonite Au 2 Bi, and Au 0.53 Bi 0.47 [12]) alloys.However, Sb-Bi alloys are relatively rarely formed by geological processes.
Much of the research on Sb-Bi alloys has involved materials science applications [13].The reported occurrences of Sb-Bi alloys include antimonian bismuth in the Loddiswell quartz carbonate vein mine, southwest England [14], the Tanco pegmatite, Canada [15], the Skrytoe scheelite skarn deposit, Russia [16] and the Katerina coal mine in the Czech Republic [17], bismuthian antimony in the Viitaniemi pegmatite, Finland [18], and from the Sulitjelma VMS-type deposit in Norway [19].Thus, due to their limited geologic occurrence, the composition and mineralisation conditions of Sb-Bi alloys are poorly studied.
The Jialong deposit is a small Sn-Cu deposit with metal reserves of 12,806 t Cu and 3000 t Sn [20] in North Guangxi Province, South China.This vein-type deposit is indium-bearing with abnormally high Bi and Sb contents (showing maximum Bi and Sb contents of 2010 ppm and 1155 ppm, respectively), which were observed during a recent study of indium mineralisation in the deposit (Jianping Liu' unpublished data).After further mineralogical investigation, Sb-Bi alloys and several Ag-Cu-Pb-Sb-Bi sulphosalts were discovered.Therefore, this deposit allows us to assess how Sb-Bi alloys were formed during the mineralisation processes.In this study, mineralogical analysis of ores in the Jialong Sn-Cu deposit was first performed, after which the mineral chemistry of the Sb-Bi alloys and Ag-Cu-Pb-Sb-Bi sulphosalts were obtained.We use these data to discuss the formation conditions of the Sb-Bi alloys.

Geological Setting
The Jialong deposit is located in the Jiuwandashan-Yuanbaoshan region (JYR) in the southwest corner of the Jiangnan Orogenic Belt (Figure 1a) [21].The regional stratigraphy of the JYR comprises, in geochronological order, of the Paleoproterozoic Sibao Group, composed of regional low-grade metamorphic sandstone, sandy slate, phyllite, and mica schist; the Mesoproterozoic Danzhou Group, composed of metamorphic siltstone and phyllite; the Neoproterozoic Sinian sandstone, composed of siltstone and sandy slate; and Devonian limestone [22] (Figure 1b).The Sibao Group including the Yuxi, Jiuxiao and Baidingyan Formations (from top to bottom), is a thick flysch formation of hemipelagic and pelagic facies terrigenous clastic sedimentary rocks interlayered with ultramafic volcanic and intrusive rocks [23].Granites are widespread in the JYR, intruding into the Sibao Group, and underlain by the Danzhou Group [23].Two types of granite are identified [24]: (1) 835-820 Ma granodiorite intrusions with minimal surface outcrops; and (2) 810-800 Ma biotite granite intrusions with widespread outcrop across the district (Figures 1b and 2).Geochemical features show that the granites are strongly peraluminous S-type granites with A/CNK > 1.1, and a CIPW (Cross, Iddings, Pirsson, Washington) normative corundum content of 1.53-3.85vol % [24].There are more than 30 Sn-polymetallic deposits in the JYR (Figure 1b), with a total Sn reserve of 0.2 Mt [21,22].These Sn deposits are hosted in the metamorphic rocks of the Sibao Group, and are related to type 2 granites [21].
The Jialong deposit is a vein-type deposit and is situated in the northeast of the Yuanbaoshan intrusion (Figures 1b and 2a).Contact metamorphism is local and only weakly affects the mining area.The deposit is hosted by the Yuxi Formation, which is composed of muscovite-quartz and chlorite-muscovite-quartz schist interlayered with leptinite and two-mica schist [20].Orebodies occur in a silicified zone between the leptinite and muscovite schist of the Yuxi Formation (Figure 2a).The mineralised silicified zone is approximately 500-600 m from the granite intrusion and approximately 1 km in length and 30-50 m in width, with an inclination of 45 • -60 • , and a steep dip of 55 • -60 • [20].Seven orebodies (Nos.1-7) are outlined; the first three are the major orebodies.The number 3 ore-body, the largest in the deposit, is 677 m long and 3.0-4.5 m thick, extending 442 m along the dip [20].Orebodies are layer-like, lamellar, and lenticular with grades of 0.45-0.70wt % Cu and 0.2-0.3wt % Sn [20].Ore structures of the Jialong deposit comprise irregular veinlets (Figure 3a) and veins with small amounts of massive ore (Figure 3b).[25]), showing the Jiuwandashan-Yuanbaoshan Sn-polymetallic metallogenic area located in the southwestern corner of the Jiangnan Orogen Belt; (b) Simplified geological map of the Jiuwandashan-Yuanbaoshan region, showing the distribution of Sn-polymetallic deposits and the location of the Jialong deposit (after reference [23]).[25]), showing the Jiuwandashan-Yuanbaoshan Sn-polymetallic metallogenic area located in the southwestern corner of the Jiangnan Orogen Belt; (b) Simplified geological map of the Jiuwandashan-Yuanbaoshan region, showing the distribution of Sn-polymetallic deposits and the location of the Jialong deposit (after reference [23]).[20]); (b) Sketch of a representative section of the vein-following drift at 440 m above sea level (after reference [20]), showing the mineralisation pattern of the Jialong deposit.

Sampling and Analytical Methods
In our previous study, more than 20 ore samples were collected from the No. 3 ore-body, and 10 polished sections were prepared for microscopic observation and electron-probe microanalyses.
The ore minerals and textures were identified using standard reflected-light microscopy techniques.Analysis of chemical compositions of minerals and X-ray mapping were performed using a Shimadzu EPMA-1720H (Tokyo, Japan) electron-probe microanalyser (EPMA) housed at the School of Geosciences and Info-physics, Central South University (Changsha, China).The BSE images were obtained under 15 kV accelerating voltage and 0.5 nA beam current.Each analysis used a 15 kV accelerating voltage, a 10 nA beam current, and a 1-μm diameter electron beam.Count times were 10 seconds each for peak and background.The X-ray lines, crystals used for analysis, standards, and detection limits are listed in Table 1.The resulting data were processed using the atomic number (Z), absorption (A), and fluorescence (F) effects (ZAF) correction method.X-ray element mapping was performed at a 15 kV accelerating voltage, and 60 nA beam current.

Sampling and Analytical Methods
In our previous study, more than 20 ore samples were collected from the No. 3 ore-body, and 10 polished sections were prepared for microscopic observation and electron-probe microanalyses.
The ore minerals and textures were identified using standard reflected-light microscopy techniques.Analysis of chemical compositions of minerals and X-ray mapping were performed using a Shimadzu EPMA-1720H (Tokyo, Japan) electron-probe microanalyser (EPMA) housed at the School of Geosciences and Info-physics, Central South University (Changsha, China).The BSE images were obtained under 15 kV accelerating voltage and 0.5 nA beam current.Each analysis used a 15 kV accelerating voltage, a 10 nA beam current, and a 1-μm diameter electron beam.Count times were 10 seconds each for peak and background.The X-ray lines, crystals used for analysis, standards, and detection limits are listed in Table 1.The resulting data were processed using the atomic number (Z), absorption (A), and fluorescence (F) effects (ZAF) correction method.X-ray element mapping was performed at a 15 kV accelerating voltage, and 60 nA beam current.

Sampling and Analytical Methods
In our previous study, more than 20 ore samples were collected from the No. 3 ore-body, and 10 polished sections were prepared for microscopic observation and electron-probe microanalyses.
The ore minerals and textures were identified using standard reflected-light microscopy techniques.Analysis of chemical compositions of minerals and X-ray mapping were performed using a Shimadzu EPMA-1720H (Tokyo, Japan) electron-probe microanalyser (EPMA) housed at the School of Geosciences and Info-physics, Central South University (Changsha, China).The BSE images were obtained under 15 kV accelerating voltage and 0.5 nA beam current.Each analysis used a 15 kV accelerating voltage, a 10 nA beam current, and a 1-µm diameter electron beam.Count times were 10 seconds each for peak and background.The X-ray lines, crystals used for analysis, standards, and detection limits are listed in Table 1.The resulting data were processed using the atomic number (Z), Minerals 2018, 8, 26 5 of 20 absorption (A), and fluorescence (F) effects (ZAF) correction method.X-ray element mapping was performed at a 15 kV accelerating voltage, and 60 nA beam current.

Mineralogy and Mineral Chemistry
Based on microscopic observations and EPMA analyses, more than 27 minerals were identified.The paragenetic sequence of the minerals is summarised in

Mineralogy and Mineral Chemistry
Based on microscopic observations and EPMA analyses, more than 27 minerals were identified.The paragenetic sequence of the minerals is summarised in

Sb-Bi Alloys and Native Bismuth
Sb-Bi alloys display intergrowth with galena and sulphosalts, but are not intergrown with bismuthinite or native bismuth.The alloys grains are small and irregular and range from 1 μm to 20 μm in diameter (Figure 5).Two phases (I and II) of Sb-Bi alloys are identified based on

Sb-Bi Alloys and Native Bismuth
Sb-Bi alloys display intergrowth with galena and sulphosalts, but are not intergrown with bismuthinite or native bismuth.The alloys grains are small and irregular and range from 1 µm to 20 µm in diameter (Figure 5).Two phases (I and II) of Sb-Bi alloys are identified based on microscopic observations, back-scattered electron images (Figures 5 and 6), and EPMA analysis (Table 2).Phase I is accompanied by galena I (see Section 4.3 for a description of the two types of galena), tintinaite, and bournonite (Figure 6), and is replaced by Phase II (Figure 5c).Phase I contains high Sb (69.15-77.68wt %) and low Bi (18.01-27.85wt %), while Phase II contains low Sb (0.89-25.24 wt %) and high Bi (72.95-98.89wt %), displaying a gap in composition.In addition, Sb-Bi alloys contain trace amounts of Cu (<0.03-1.51wt %), Fe (0.07-1.48 wt %) and S (<0.05-0.24wt %).We infer from mineral textures that Sb-Bi alloys formed after galena I (Figure 5a-d), and galena I formed after the sulphosalts (Figure 5d).The native bismuth comprises large grains ranging from 20-80 µm, which occur with pavonite, freibergite, galena II (see Section 4.3), and bismuthinite (Figure 7).The EPMA data shows that native bismuth is nearly pure (Table 2).

Tintinaite
Tintinaite is widely distributed in the Jialong ores.It predominantly occurs together with Sb-Bi alloys, and a small amount is found with native bismuth.Grains are variable in diameter, ranging from several μm to more than 500 μm, and are euhedral to subhedral (Figure 9).Tintinaite composition varies and can be divided into two types based on EPMA data (Table 4): Cu-bearing (Type-I) and Cu-poor (Type-II).Type-I contains 1.08-1.33

Tintinaite
Tintinaite is widely distributed in the Jialong ores.It predominantly occurs together with Sb-Bi alloys, and a small amount is found with native bismuth.Grains are variable in diameter, ranging from several µm to more than 500 µm, and are euhedral to subhedral (Figure 9).Tintinaite composition varies and can be divided into two types based on EPMA data (Table 4): Cu-bearing (Type-I) and Cu-poor (Type-II).Type-I contains 1.08-1.33

Freibergite
Freibergite is abundant within the Jialong ores and occurs together with native bismuth.Freibergite grains are irregularly shaped and range from several µm to 100 µm in diameter.Texturally, freibergite is incorporated in, but formed earlier than, galena II (Figure 11).Freibergite (Table 6

Ullmannite
Ullmannite is scarce in the Jialong ore samples and occurs with galena II and native bismuth in the interstices of pyrrhotite crystals (Figure 12).Ullmannite has a higher reflectance than galena and a lower hardness than pyrrhotite.The analyses shows amounts of 0.42-0.87wt % Fe, 0.43-1.64wt % Bi, 1.01-1.16wt % As and 0.20-0.40wt % Se (Table 7), with an average chemical formula of  "-" below detection limits.

Ullmannite
Ullmannite is scarce in the Jialong ore samples and occurs with galena II and native bismuth in the interstices of pyrrhotite crystals (Figure 12).Ullmannite has a higher reflectance than galena and a lower hardness than pyrrhotite.The analyses shows amounts of 0.42-0.87wt % Fe, 0.43-1.64wt % Bi, 1.01-1.16wt % As and 0.20-0.40wt % Se (Table 7), with an average chemical formula of Ni 0.97 Fe 0.02 Sb 0.97 As 0.03 Bi 0.01 S 1.00 Se 0.01 .

Galena
Galena is widely distributed in the Jialong ore samples and mostly comprises anhedral grains ranging from several to tens of micrometres.According to mineral assemblages and chemical compositions, two types of galena are identified (galena I and galena II).Galena I is intergrown with Sb-Bi alloys and contains 0.30-0.47wt % Bi and 0.87-1.49wt % Se, while galena II occurs with bismuthinite and native bismuth, and contains 1.66-2.80wt % Bi and 1.92-3.14wt % Se (Table 9).In addition, galena II has more Ag than galena I.The data show some substitutions of Bi and Ag for 2 Pb.

Galena
Galena is widely distributed in the Jialong ore samples and mostly comprises anhedral grains ranging from several to tens of micrometres.According to mineral assemblages and chemical compositions, two types of galena are identified (galena I and galena II).Galena I is intergrown with Sb-Bi alloys and contains 0.30-0.47wt % Bi and 0.87-1.49wt % Se, while galena II occurs with bismuthinite and native bismuth, and contains 1.66-2.80wt % Bi and 1.92-3.14wt % Se (Table 9).In addition, galena II has more Ag than galena I.The data show some substitutions of Bi and Ag for 2 Pb.

Bismuthinite and Unnamed BiS
Bismuthinite is associated with native bismuth, galena II, bismuthinite and unnamed BiS, all of which appear filling cavities between chalcopyrite and gangue minerals (Figure 15).Galena II, bismuthinite, and unnamed BiS have similar reflectance values (Figure 15).Based on BSE images, the paragenetic sequence of sulphides is: chalcopyrite→bismuthinite→galena II→unnamed BiS→native bismuth.(Figure 15b).The bismuthinite grains, around 40 µm in size, are incorporated in galena II, and fissures in bismuthinite are filled by native bismuth.The bismuthinite contains trace amounts of 4.3.4.Nisbite Nisbite is a trace mineral in the Jialong ores that coexists with Sb-Bi alloys and native bismuth.It occurs as small grains of 3-20 µm in diameter.Nisbite is surrounded by native bismuth, galena (I or II) or Sb-Bi alloys in complex aggregates that infill interstitial cavities between pyrrhotite grains.The pyrrhotite is partially corroded and infilled by native bismuth (Figure 16a), which also infills galena I and Sb-Bi alloys (Figure 16b).Nisbite contains 18.41-19.46wt % Ni and 77.70-79.82wt % Sb with trace amounts of Fe (0.75-2.03 wt %) (Table 12), and an average chemical formula of Ni 0.99 Fe 0.09 Sb

Nisbite
Nisbite is a trace mineral in the Jialong ores that coexists with Sb-Bi alloys and native bismuth.It occurs as small grains of 3-20 μm in diameter.Nisbite is surrounded by native bismuth, galena (I or II) or Sb-Bi alloys in complex aggregates that infill interstitial cavities between pyrrhotite grains.The pyrrhotite is partially corroded and infilled by native bismuth (Figure 16a), which also infills galena I and Sb-Bi alloys (Figure 16b).Nisbite contains 18.41-19.46wt % Ni and 77.70-79.82wt % Sb with trace amounts of Fe (0.75-2.03 wt %) (Table 12), and an average chemical formula of Ni0.99Fe0.09Sb1.90S0.02.
The Jialong is a rare deposit containing a high abundance of Sb-Bi alloys.The Sb-Bi alloys contain variable Sb and Bi contents ranging from 20.72 wt % to 98.89 wt % Bi and 0.89 wt % to 77.68 wt % Sb.Two phases of Sb-Bi alloys separated by a composition gap are identified (Figure 17a).However, thermodynamic calculations show that Sb-Bi solid solution is a continuous solution [44] that includes the compositions of Phase I and Phase II (Figure 17b).The crystallisation temperature during cooling is higher for Phase I (Sb-rich member) than Phase II (Bi-rich member) (Figure 17b).Phase I is formed earlier than Phase II according to textural analysis (Figure 5c); the earlier formation also indicates higher formation temperatures for Phase I.Because continuous slow cooling would precipitate the full range of solid solutions, the presence of two phases implies multiple steps of non-continuous temperature decrease during formation.
The Jialong is a rare deposit containing a high abundance of Sb-Bi alloys.The Sb-Bi alloys contain variable Sb and Bi contents ranging from 20.72 wt % to 98.89 wt % Bi and 0.89 wt % to 77.68 wt % Sb.Two phases of Sb-Bi alloys separated by a composition gap are identified (Figure 17a).However, thermodynamic calculations show that Sb-Bi solid solution is a continuous solution [44] that includes the compositions of Phase I and Phase II (Figure 17b).The crystallisation temperature during cooling is higher for Phase I (Sb-rich member) than Phase II (Bi-rich member) (Figure 17b).Phase I is formed earlier than Phase II according to textural analysis (Figure 5c); the earlier formation also indicates higher formation temperatures for Phase I.Because continuous slow cooling would precipitate the full range of solid solutions, the presence of two phases implies multiple steps of non-continuous temperature decrease during formation.showing the compositional gap between them.The data of the Loddiswell mine and the Tanco pegmatite are from [14] and [15], respectively; (b) Sb-Bi phase diagram (after [45,46]), showing the temperature and composition gap between Phase I and Phase II.Note that the temperatures that stabilise the Sb-Bi alloys in the pure Sb-Bi system in laboratory conditions are higher than those for the natural metallogenic system.

Formation of Sb-Bi Alloys and Sulphosalts during Mineralisation
The Cu-Sn orebodies in the Jialong deposit are located in the external contact zone of the Yuanbaoshan granitic intrusion.All orebodies are less than 600 m from the intrusion boundary.They strike northwest and are controlled by faults.Some studies suggest that ore genesis is related to the presence of ultramafic rocks [47][48][49][50] in the JYR.However, because the orebodies are present in the contact zone of granite intrusions, the genesis of the Cu-Sn mineralisation system is linked to the granite intrusions [21,22,51].
The analysis of ore petrology in this study indicates that the Jialong deposit experienced four mineralisation stages.The Sb-Bi alloys and sulphosalts were formed in Stage III, and infilled cavities in chalcopyrite and pyrrhotite, while bismuthinite, unnamed BiS, and native bismuth formed in Stage IV.Our petrographic evidence shows that the Sb-Bi minerals were formed during the late stages of magmatic hydrothermal evolution.This situation is consistent with studies on other Sb-Bi mineral-bearing deposits [7].In addition, Sb-Bi alloys are not intergrown with bismuthinite, unnamed BiS, or native bismuth in the Jialong deposit, which implies that Sb-Bi alloys and native bismuth were formed at different times.
The Sb-Bi phase diagram shows that the formation temperature of Sb-Bi alloys is higher than that of native bismuth (Figure 17b).Experiments and evidence from many geological deposits indicate that Bi sulphosalts formed at temperatures of >400-300 °C [1,4,9], while native bismuth and  [14,15], respectively; (b) Sb-Bi phase diagram (after [45,46]), showing the temperature and composition gap between Phase I and Phase II.Note that the temperatures that stabilise the Sb-Bi alloys in the pure Sb-Bi system in laboratory conditions are higher than those for the natural metallogenic system.

Formation of Sb-Bi Alloys and Sulphosalts during Mineralisation
The Cu-Sn orebodies in the Jialong deposit are located in the external contact zone of the Yuanbaoshan granitic intrusion.All orebodies are less than 600 m from the intrusion boundary.They strike northwest and are controlled by faults.Some studies suggest that ore genesis is related to the presence of ultramafic rocks [47][48][49][50] in the JYR.However, because the orebodies are present in the contact zone of granite intrusions, the genesis of the Cu-Sn mineralisation system is linked to the granite intrusions [21,22,51].
The analysis of ore petrology in this study indicates that the Jialong deposit experienced four mineralisation stages.The Sb-Bi alloys and sulphosalts were formed in Stage III, and infilled cavities in chalcopyrite and pyrrhotite, while bismuthinite, unnamed BiS, and native bismuth formed in Stage IV.Our petrographic evidence shows that the Sb-Bi minerals were formed during the late stages of magmatic hydrothermal evolution.This situation is consistent with studies on other Sb-Bi mineral-bearing deposits [7].In addition, Sb-Bi alloys are not intergrown with bismuthinite, unnamed BiS, or native bismuth in the Jialong deposit, which implies that Sb-Bi alloys and native bismuth were formed at different times.
The Sb-Bi phase diagram shows that the formation temperature of Sb-Bi alloys is higher than that of native bismuth (Figure 17b).Experiments and evidence from many geological deposits indicate that Bi sulphosalts formed at temperatures of >400-300 • C [1,4,9], while native bismuth and bismuthinite formed below 271 • C. Thus, sulphosalts and Sb-Bi alloys formed at temperatures of 271 to >400 • C.
Additional evidence shows that the Bi-bearing galena in Stages III and IV formed at higher temperatures than pure galena [1].Sb-Bi mineral precipitation is affected by temperature, sulphur fugacity, oxygen fugacity, pH, and the solubility of Bi and Sb ions [7].The precipitation of many sulphides and sulphosalts during the late third stage depleted the fluid of S. This low f S2 fluid could then precipitate Sb-Bi alloys and trace amounts of nisbite at high temperature.After formation of Sb-Bi alloys, a later mineralisation stage formed bismuthinite, galena II, unnamed BiS, and native bismuth.The formation of additional sulphur minerals indicates that the fourth stage experienced decreasing f S2 , which could support the deposition of native bismuth at lower temperature.

Conclusions
(1) The Sb-Bi alloys in the Jialong Cu-Sn deposit are 1-20 µm and intergrown with earlier-formed sulphosalts.The alloys formed during the late stages of hydrothermal mineralisation.
(3) Two phases of Sb-Bi alloys were identified.The textural relationship indicate that Phase I was formed before Phase II.The composition of Phase I is dominated by Sb, while that of Phase II is dominated by Bi.It is inferred that Sb-Bi alloys formed in a high temperature (271-400 • C) and low f S2 environment.The presence of the two phases may indicate multiple steps of cooling during mineralisation.
(4) The Jialong deposit is related to granite intrusions.Sulphosalts, Sb-Bi alloys, bismuthinite, and native bismuth were precipitated from the late Sb-Bi-rich fluid during cooling and fluctuating f S2 conditions.

Figure 1 .
Figure 1.(a) Map of the tectonic units in south China (after reference [25]), showing the Jiuwandashan-Yuanbaoshan Sn-polymetallic metallogenic area located in the southwestern corner of the Jiangnan Orogen Belt; (b) Simplified geological map of the Jiuwandashan-Yuanbaoshan region, showing the distribution of Sn-polymetallic deposits and the location of the Jialong deposit (after reference [23]).

Figure 1 .
Figure 1.(a) Map of the tectonic units in south China (after reference [25]), showing the Jiuwandashan-Yuanbaoshan Sn-polymetallic metallogenic area located in the southwestern corner of the Jiangnan Orogen Belt; (b) Simplified geological map of the Jiuwandashan-Yuanbaoshan region, showing the distribution of Sn-polymetallic deposits and the location of the Jialong deposit (after reference [23]).

Figure 2 .
Figure 2. (a) Simplified geological map of the Jialong deposit (after reference [20]); (b) Sketch of a representative section of the vein-following drift at 440 m above sea level (after reference [20]), showing the mineralisation pattern of the Jialong deposit.

Figure 2 .of 20 Figure 2 .
Figure 2. (a) Simplified geological map of the Jialong deposit (after reference [20]); (b) Sketch of a representative section of the vein-following drift at 440 m above sea level (after reference [20]), showing the mineralisation pattern of the Jialong deposit.

Figure 4 .
Figure 4. Paragenetic sequence of minerals observed in the Jialong deposit.

Figure 4 .
Figure 4. Paragenetic sequence of minerals observed in the Jialong deposit.

Figure 5 .
Figure 5. Reflected light photomicrograph (a) and back-scattered electron (BSE) images (b-f) of Sb-Bi alloys and associated minerals.(a) Sb-Bi alloys and galena I (Gn-I) infilling gaps between stannite (Stn) and tintinaite (Tnt), showing the different reflectance of Phase I (BS-I) and Phase II (BS-II) of the Sb-Bi alloys; (b) enlarged BSE image of Figure 5a, showing the different brightness of the two phases of Sb-Bi alloys; (c) Phase I and Phase II infilling galena I, with growth of Phase II on the irregular surfaces of Phase I; (d) Phase II infilling galena I, together displacing stannite and tintinaite; (e) Phase II of Sb-Bi alloy and galena I infill on the surface of tintinaite and bournonite (Bnn); and (f) Phase II and galena I deposition on the surface of tintinaite.

Figure 5 .
Figure 5. Reflected light photomicrograph (a) and back-scattered electron (BSE) images (b-f) of Sb-Bi alloys and associated minerals.(a) Sb-Bi alloys and galena I (Gn-I) infilling gaps between stannite (Stn) and tintinaite (Tnt), showing the different reflectance of Phase I (BS-I) and Phase II (BS-II) of the Sb-Bi alloys; (b) enlarged BSE image of Figure 5a, showing the different brightness of the two phases of Sb-Bi alloys; (c) Phase I and Phase II infilling galena I, with growth of Phase II on the irregular surfaces of Phase I; (d) Phase II infilling galena I, together displacing stannite and tintinaite; (e) Phase II of Sb-Bi alloy and galena I infill on the surface of tintinaite and bournonite (Bnn); and (f) Phase II and galena I deposition on the surface of tintinaite.

Figure 6 .
Figure 6.(a) Back-scattered electron images of Sb-Bi alloys and associated galena and bournonite (Bnn), showing Sb-Bi alloys and galena I infill along the edge and internal fissures of bournonite, and the later formation of Sb-Bi alloys and galena compared to bournonite; (b) enlarged image of Figure 6a, showing Sb-Bi alloys and galena I (Gn-I) infilling bournonite, and the different brightness of Phase I (BS-I) and Phase II (BS-II); (c) X-ray element-distribution map of Bi for the area of Figure 6b, showing Bi content from different Sb-Bi alloys.The Bi content of Phase I is lower than that of Phase II, and Bi content of the Phase II grains is variable; and (d) X-ray element-distribution map of Sb for the area of Figure 6b, showing a higher Sb content in Phase I than in Phase II.

Figure 6 .of 20 Figure 6 .
Figure 6.(a) Back-scattered electron images of Sb-Bi alloys and associated galena and bournonite (Bnn), showing Sb-Bi alloys and galena I infill along the edge and internal fissures of bournonite, and the later formation of Sb-Bi alloys and galena compared to bournonite; (b) enlarged image of Figure 6a, showing Sb-Bi alloys and galena I (Gn-I) infilling bournonite, and the different brightness of Phase I (BS-I) and Phase II (BS-II); (c) X-ray element-distribution map of Bi for the area of Figure 6b, showing Bi content from different Sb-Bi alloys.The Bi content of Phase I is lower than that of Phase II, and Bi content of the Phase II grains is variable; and (d) X-ray element-distribution map of Sb for the area of Figure 6b, showing a higher Sb content in Phase I than in Phase II.

Figure 8 .
Figure 8.(a) Reflected light photomicrograph of pavonite (Pv) occurring with bismuthinite (Bmt), galena II (Gn-II), and native Bi.Pavonite and associated minerals occur as infilling gangue minerals; (b) enlarged BSE image of Figure 8a, indicating earlier formation of pavonite than galena II and native bismuth.

Figure 8 .
Figure 8.(a) Reflected light photomicrograph of pavonite (Pv) occurring with bismuthinite (Bmt), galena II (Gn-II), and native Bi.Pavonite and associated minerals occur as infilling gangue minerals; (b) enlarged BSE image of Figure 8a, indicating earlier formation of pavonite than galena II and native bismuth.

Figure 9 .
Figure 9. Back-scattered electron image showing euhedral to subhedral tintinaite crystal (Tnt) infilling arsenopyrite (Apy) and pyrrhotite (Po), and partially replaced by galena I (Gn-I) and Sb-Bi alloys along the edges and internal fissures of tintinaite and gudmundite (Gd).

Figure 9 .
Figure 9. Back-scattered electron image showing euhedral to subhedral tintinaite crystal (Tnt) infilling arsenopyrite (Apy) and pyrrhotite (Po), and partially replaced by galena I (Gn-I) and Sb-Bi alloys along the edges and internal fissures of tintinaite and gudmundite (Gd).

20 Figure 11 .
Figure 11.Reflected light photomicrograph of freibergite (Fr) associated with galena II (Gn-II) and native bismuth (Bi), infilling cavities between pyrrhotite (Po) crystals and partially replacing them.The image shows freibergite grains enclosed by galena II.

Figure 11 .
Figure 11.Reflected light photomicrograph of freibergite (Fr) associated with galena II (Gn-II) and native bismuth (Bi), infilling cavities between pyrrhotite (Po) crystals and partially replacing them.The image shows freibergite grains enclosed by galena II.

Figure 11 .
Figure 11.Reflected light photomicrograph of freibergite (Fr) associated with galena II (Gn-II) and native bismuth (Bi), infilling cavities between pyrrhotite (Po) crystals and partially replacing them.The image shows freibergite grains enclosed by galena II.

Figure 13 .
Figure 13.Back-scattered electron images of bournonite and associated minerals.(a) Bournonite (Bnn) infilled by Sb-Bi alloys and galena I along edges and internal fissures; and (b) irregular bournonite replaced or infilled by galena I and Sb-Bi alloys.

Figure 13 .
Figure 13.Back-scattered electron images of bournonite and associated minerals.(a) Bournonite (Bnn) infilled by Sb-Bi alloys and galena I along edges and internal fissures; and (b) irregular bournonite replaced or infilled by galena I and Sb-Bi alloys.

Figure 17 .
Figure 17.(a) In a Diagram of Bi (apfu) versus Sb (apfu) for the Sb-Bi alloys (Phase I and Phase II)showing the compositional gap between them.The data of the Loddiswell mine and the Tanco pegmatite are from[14] and[15], respectively; (b) Sb-Bi phase diagram (after[45,46]), showing the temperature and composition gap between Phase I and Phase II.Note that the temperatures that stabilise the Sb-Bi alloys in the pure Sb-Bi system in laboratory conditions are higher than those for the natural metallogenic system.

Figure 17 .
Figure 17.(a) In a Diagram of Bi (apfu) versus Sb (apfu) for the Sb-Bi alloys (Phase I and Phase II) showing the compositional gap between them.The data of the Loddiswell mine and the Tanco pegmatite are from [14,15], respectively; (b) Sb-Bi phase diagram (after[45,46]), showing the temperature and composition gap between Phase I and Phase II.Note that the temperatures that stabilise the Sb-Bi alloys in the pure Sb-Bi system in laboratory conditions are higher than those for the natural metallogenic system.

Table 1 .
Electron-probe microanalyser (EPMA) analysis conditions used in the study.

Table 1 .
Electron-probe microanalyser (EPMA) analysis conditions used in the study.

Table 2 .
EPMA data for Sb-Bi alloys and native bismuth.

Table 2 .
EPMA data for Sb-Bi alloys and native bismuth.

Table 2 .
EPMA data for Sb-Bi alloys and native bismuth.