Constraints on Ore Genesis from Trace Ore Mineralogy: A New Occurrence of Kupčíkite and Paděraite from the Zhibula Cu Skarn Deposit, Southern Tibet

Abstract

Mineral assemblages containing Cu-Bi sulfosalts, Bi chalcogenides, and Ag-(Au) tellurides have been identified in the mid-Miocene Zhibula Cu skarn deposit, Gangdese Belt, southern Tibet. Different mineral assemblages from three locations in the deposit, including proximal massive garnet skarn, proximal retrogressed pyroxene-dominant skarn in contact with marble, and distal banded garnet–pyroxene skarn hosted in marble, are studied to constrain the evolution of the mineralization. Hypogene bornite contains elevated Bi (mean 6.73 wt.%) and co-exists in proximal andradite skarn with a second bornite with far lower Bi content, carrollite, Au-Ag tellurides (hessite, petzite), and wittichenite. This assemblage indicates formation at relatively high temperatures (>400 °C) and high f_S2 and f_Te2 during prograde-stage mineralization. Assemblages of Bi sulfosalts (wittichenite, aikinite, kupčíkite, and paděraite) and bismuth chalcogenides (e.g., tetradymite) in proximal pyroxene skarn are also indicative of formation at relatively high temperatures, but at relatively lower f_Te2 and f_S2 conditions. Within the reduced distal skarn (chalcopyrite–pyrrhotite-bearing) in marble, cobalt, and nickel occur as discrete minerals: cobaltite, melonite and cobaltic pentlandite. The trace ore mineral signature of the Zhibula skarn and the distributions of precious and critical trace elements such as Ag, Au, Co, Te, Se, and Bi support an evolving magmatic–hydrothermal system in which different parts of the deposit each define ore formation at distinct local physicochemical conditions. This is the first report of kupčíkite and paděraite from a Chinese location. Their compositions are comparable to other occurrences, but conspicuously, they do not form nanoscale intergrowths with one another.

1. Introduction

Copper skarns commonly contain anomalous concentrations of precious metals (Au and Ag) and may also be markedly enriched in one or more elements considered critical, including Co, Bi, Se, and Te [1]. This relative enrichment may be expressed by characteristic ore mineral assemblages that include a range of tellurides, selenides, and Bi-Pb and Cu-Bi sulfosalts. The presence of these minerals and their prevailing associations may provide valuable additional constraints on ore genesis, including estimates of the temperatures and pressures of ore-forming fluids, as well as relevant fugacities (e.g., f_Se2, f_S2, and f_Te2) [2].

Telluride minerals are widespread in many types of ore deposits and can provide valuable physicochemical constraints on mineralization [2,3]. Tellurides have also been proposed as a tool to decipher orefield zonation in porphyry–epithermal systems, e.g., in the Larga orefield, Romania [3]. Bismuth chalcogenides and some related tellurides when forming Bi-Te-bearing assemblages have low melting points and thus represent ideal melt scavengers that can play a significant role in gold enrichment processes [4].

Sulfosalts of the cuprobismutite homologous series are scarce in nature. The series includes cuprobismutite, hodrushite, and kupčíkite, with the ideal formulae Cu₈AgBi₁₃S₂₄, Cu₈Bi₁₂S₂₂, and Cu₈Bi₁₀S₂₀, respectively ([5,6] and references therein). Paděraite, Cu₆AgPbBi₁₂S₂₂, is a closely related phase [7]. These minerals are reported from only a limited number of occurrences in mineralized veins and skarns, e.g., from the Felbertal metamorphogenic scheelite deposit, Austria [5,8]; the Băiţa Bihor Cu and Ocna de Fier Fe-Cu skarns, Romania [6,9]; the Swartberg rare metal pegmatite, South Africa [10]; and in Slovakia [11], Poland [12], and the high-grade Obari Au-Cu-Bi deposit, Japan [13]. Makovicky [14] postulated the general formula for these minerals as Cu₄Me_2(N-1)+1Bi₄S_2N+8, where Me is Bi and Ag, and N is the order number for each homologue type. However, subsequent study of natural samples revealed a greater compositional complexity due to substitutions of several other elements, notably Fe, Pb and Ag. The substitution mechanisms and site occupancies for these elements and the formation conditions of related species remain imperfectly constrained [9].

The Gangdese Belt, southern Tibet, hosts a large and growing number of ore deposits associated with episodes of magmatic activity spanning the Jurassic to Miocene (e.g., [15,16]). Porphyry Cu-Mo and Cu-Au deposits and associated skarns formed during this age range are the most economically significant types of mineralization. Until recently, there had been little detailed study of these deposits with respect to the distribution and ore mineralogy of minor components such as Co, Se, Bi, and Te [17,18]. This is despite the significance of such elements for interpretation of deposits related to magmatic–hydrothermal events. This contribution focuses on the occurrence of ore minerals and assemblages from the Zhibula Cu skarn that include several critical trace elements (Bi, Co, Te, Au, Ag, and Se). An appreciation of these minerals permits us to constrain the ore genesis of the deposit. Detailed mineral characterization also allows us to offer insights that may assist the potential recovery of these value-adding components.

2. Geological Setting

The Zhibula Cu skarn deposit has a reserve of 0.32 Mt Cu @ 1.64%. It is located approximately 2 km south of the Qulong porphyry Cu-Mo deposit in the Gangdese Belt, southern Tibet (Figure 1). Skarn orebodies occur as layers and less lenticular bodies within marble and tuffs of the 3 km-thick Yeba Formation, which has a U-Pb zircon age of 174.4 ± 1.7 Ma [19]. Skarn emplacement was controlled by the contacts between tuff and marble, as well as by fractures in the tuff. Outcropping intrusive rocks have not been identified in the mine area except for a few monzogranite and granodiorite dikes in deeper exploration drillholes. These granitoids have zircon U-Pb ages of ~17 Ma, which are concordant with those measured in minerals from the Qulong deposit [20]. Comparable molybdenite Re-Os ages (16–17 Ma; [21]) and zircon Hf-O isotopic signatures [22] for Zhibula and Qulong are also noted, strongly suggesting that skarns and porphyry mineralization share a common magmatic source and comprise a single large magmatic–hydrothermal ore system [22].

Isotope data for sulfur (δ³⁴S −0.1 to −6.8 ‰), hydrogen (δD_H2O −91 to −159 ‰), and oxygen (δ¹⁸O_H2O 1.5 to 9.2 ‰) further support an interpretation in which the Zhibula mineralization is related to magmatic–hydrothermal fluids. Fluid inclusion data confirm that ore-forming fluids changed from relatively high temperature and high salinity to low temperature, moderate salinity during transition from the prograde to retrograde stage [22].

Insights into skarn formation, particularly from the trace element signatures of garnet, were reported by Xu et al. [23]. Although minor endoskarn is noted in granodiorite, few skarns develop at the contacts between the Yeba Formation and granitoids. Instead, they typically occur within the Yeba Formation, with marble and tuff units as the dominant protoliths. A combination of different protoliths and variation in the local physicochemical environment resulted in skarns of different appearance and composition, as well as the observed zonation at the orefield scale. Garnet is the dominant skarn mineral. The trace element endowment of garnet, particularly the rare earth element signatures, but also other elements (e.g., W, Sn, As, and Mo), changes in both space and time, as a function of multiple factors, e.g., mineralogy of co-crystallizing phases, variation in X(CO₂), salinity, and proximity to local fluid sources.

Copper skarns are primarily comprised of chalcopyrite and bornite. Other trace ore minerals include magnetite, sphalerite, galena, scheelite, molybdenite, Cu-Bi sulfosalts, Bi chalcogenides, and Au-Ag tellurides. Solid solution between scheelite and powellite (up to 80 mol.% powellite) is considered the retrogressed product of the replacement of garnet. A detailed micro- to nanoscale study of scheelite–powellite aggregates showed that non-linear thermodynamics governing the patterning in non-ideal solid solution may account for the formation of distinct patterning domains within the Mo-rich areas of the aggregates. The sharpest contrast in chemical composition of the Mo-rich scheelite is also recognizable by variation in the growth directions [24].

3. Sampling and Analytical Methodology

Detailed skarn mineralogy and associated mineralization has been depicted by Xu et al. [23]. We focus on three types of mineralized skarn here: distal banded garnet–pyroxene skarn in marble (e.g., sample 321), proximal massive garnet skarn (e.g., sample 374), and proximal retrogressed pyroxene-dominated skarn (e.g., sample 180). For locations and short descriptions of these samples, the reader is referred to Figure 1 and

Table 1. Overview of samples studied.

Skarn Type	Sample ID	Drill Hole	Depth (m)	Main Skarn Assemblage							Minor Skarn Assemblage	Ore Minerals
				Grt	Px	Czo	Act	Chl	Qz	Cal		Sulfides	Oxides
Distal skarn (banded Grt-Px in marble)	321	1611	321	xx	x	x			x	xxx	Wo, Ap, Ttn, Ves	Pyh, Ccp, Gn, Sp, Pn
	323	1611	323	xx	x				x	xxx	Wo, Ap, An, Ves	Pyh, Ccp, Gn, Sp, Pn
Proximal skarn (massive Grt)	372	1611	372	xxx	x		x	x	x	x	Ap	Ccp, Bn, Mol, Cli, Gn, Sp, Py	Mag, Hem, Sch
	374	1611	374	xxx	x		x		x	x	Ap	Ccp, Bn, Cc, Sp, Gn, Ptz, Hes, Wtc, Cli
	8	quarry	-	xxx	x			x	x	x	Ap	Bn, Ccp, Mol, Cc, Gn, Hes, Ptz, El, Cli	Sch
Proximal skarn (retrogressed Px in contact with marble)	180	1611	180	x	xxxx	x		x	xx	xxx	Ap, Wo	Ccp, Bn, Wtc, Aik, Ttd, Pde, Kup	Mag, Hem, Sch
	355	1215	355	x	xx	xxx	xx	xx	xxx	xx	Ap	Ccp, Sp, Gn

Abbreviations: Act—actinolite; Aik—aikinite; An—anorthite; Ap—apatite; Bn—bornite; Cal—calcite; Cc—chalcocite; Ccp—chalcopyrite; Chl—chlorite; Cli—carrollite; Czo—clinozoisite; El—electrum; Gn—galena; Hem—hematite; Hes—hessite; Kup—kupčíkite; Mag—magnetite; Mol—molybdenite; Pde—paděraite; Ptz—petzite; Pwl—powellite; Px—pyroxene; Py—pyrite; Pyh—pyrrhotite; Qz—quartz; Sp—sphalerite; Ttd—tetradymite; Ttn—titanite; Ves—vesuvianite; Wo—wollastonite; Wtc—wittichenite. xxx—major; xx—minor; x—trace.

.

Samples were prepared as 1 inch-diameter polished blocks and thin sections. All analytical work was performed at Adelaide Microscopy, University of Adelaide. Imaging was performed in reflected light and using an FEI Quanta 450 scanning electron microscope (SEM) (FEI, Hillsboro, OR, USA) equipped with energy-dispersive X-ray spectrometry and backscatter electron (BSE) imaging capabilities. BSE imaging was performed at a 20 kV accelerating voltage and 10 nA beam current. Quantitative mineral compositional data was obtained on a Cameca SX-Five electron probe microanalyzer (EPMA) (Cameca Instruments Inc., Fitchburg, WI, USA) and a Resonetics M-50-LR 193-nm Excimer laser microprobe (Resonetics, Nashua, NH, USA) coupled with an Agilent 7700cx quadrupole ICP-MS (LA-ICP-MS) (Agilent, Santa Clara, CA, USA).

Foils for nanoscale study were prepared using an FEI-Helios nanoLab (FEI, Hillsboro, OR, USA) focused ion beam–scanning electron microscopy (FIB–SEM). Transmission electron microscopy (TEM) imaging and selected area electron diffraction (SAED) patterns were obtained using a Philips CM200 microscope (Amsterdam, The Netherlands), equipped with a LaB6 source and Gatan Orius digital camera (Gatan Inc., Pleasanton, CA, USA), and operated at 200 kV. Data processing was carried out using DigitalMicrograph™ 3.11.1 (Gatan Inc., Pleasanton, CA, USA) software for indexation of electron diffractions. High-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) studies were performed using an FEI Titan Themis (FEI, Hillsboro, OR, USA). TIA software (v4.7.2) was used for STEM image processing. WinWulff© (JCrystalSoft, Livermore, CA, USA) was used to index diffraction patterns. CrystalMaker^® (v10.5.7) (CrystalMaker Software Ltd., Kidlington, UK) was used to generate crystal structure models, and image simulations were created using STEM for xHREMTM software (v 4.1) (HREM Research, Tokyo, Japan).

4. Results

4.1. Ore Petrography and Mineralogy

Minor sulfides, chalcopyrite associated with pyrrhotite, occur in the distal skarn formed within altered marble with a weak banding expressed by different proportions of grossular (Gr_~50) and diopside (Di_~80) (Figure 2a). In contrast, the proximal skarn represented by massive andradite garnet skarn is typified by the presence of chalcopyrite and bornite, as well as subordinate Au-Ag-Bi-Te and Co minerals, which occur as disseminations and/or larger patches (Figure 2b,c), as well as veinlets.

Bismuth sulfosalts and tellurides are most abundant throughout the proximal pyroxene-dominant, garnet skarn that experienced strong retrogression marked by changes from Mg–pyroxene (diopside) to Mn- and Fe-rich pyroxene (johannsenite–hedenbergite) and replacement of andradite (Figure 2d). Bismuth minerals occur as tiny inclusions in the garnet or as larger patches along the contact between the two silicates. The same Bi minerals are also abundant in areas with higher concentrations of magnetite (Figure 2e). Chalcopyrite is also present throughout the magnetite. In detail, zoned garnet shows reworking of smaller particles of Bi minerals along the zones, into coarser patches, and across zones (Figure 2f). Magnetite also displays abundant Bi minerals within growth zones, outside from grain cores that are mottled with inclusions of chalcopyrite (Figure 2g). Coarser, euhedral tetradymite (Bi₂Te₂S) occurs at contacts between chalcopyrite and magnetite (Figure 2h).

Distal skarn pyrrhotite occurs together with chalcopyrite within calcite (Figure 3a). In detail, inclusions of diopside are noted within the sulfides (Figure 3b). Pentlandite is present as minor lamellae within pyrrhotite (Figure 3b). Note the flame-like morphology of pentlandite indicative of exsolution from solid solution (Figure 3c). Trace sphalerite and seleniferous galena (clausthalite?) are also noted along the margins or within pyrrhotite (Figure 3d,e).

In the proximal garnet skarn, carrollite (CuCo₂S₄) accompanies bornite (Figure 4a–c) and is the only confirmed cobalt mineral. Locally, carrollite is replaced by late chalcocite and trace galena (Figure 4a). Marginal or grain boundary replacement of bornite by lamellar chalcocite is noted (Figure 4b,c). Bornite occurs as blebs within chalcopyrite. Particularly interesting are binary blebs of bornite and a Bi-rich bornite-like phase containing ~6.7 wt.% Bi that appears brighter on BSE images and has a light reddish-brown color in reflected light (Figure 4d–f).

Abundant tiny Ag-Au tellurides and the common Cu-Bi sulfosalt wittichenite (Cu₃BiS₃) are observed as inclusions within Cu-(Fe) sulfides from the proximal garnet skarn (Figure 5). The Ag-Au tellurides are dominated by hessite (Ag₂Te) with subordinate petzite (Ag₃AuTe₂) and electrum (Au,Ag). Hessite, generally accompanied by petzite, displays a close relationship with bornite and wittichenite (Figure 5a–c), and locally is associated with molybdenite. Rare melonite (NiTe₂), together with tsumoite (BiTe), and hessite were also observed within wittichenite (Figure 5d). Minor sphalerite and galena together with Ag-Au-Bi minerals can form along the margin of bornite grains (Figure 5e–f). Notable is the presence of petzite and hessite as filaments and patchy inclusions within wittichenite (Figure 5f).

In the proximal retrogressed pyroxene skarn, the main Bi minerals are wittichenite and aikinite (CuPbBiS₃) (Figure 6). These phases occur together at contacts between chalcopyrite and magnetite (Figure 6a). Dense fields of micron- to submicron-scale lamellae of tetradymite form inner parts of the wittichenite enclosed in magnetite (Figure 6b). These textures are likely indicative of tetradymite exsolution from a broad wittichenite solid solution. Wittichenite and aikinite form along magnetite margins; adjacent tetradymite is present as a separate, coarser (~50 μm-long) lamella (Figure 6c,d). Such textural relationships indicate co-crystallization between the two Bi minerals and their host magnetite and chalcopyrite.

4.2. Paděraite and Kupčíkite: Micron to Nanoscale Characterization

Two rare Bi-Cu-(Pb) sulfosalts, kupčíkite and paděraite, are observed within the retrogressed skarn enriched in associations of Bi minerals (Figure 7). The two rare sulfosalts occur as discrete grains, oriented at ~90° to one another at the edge of a larger lamella of tetradymite (Figure 7a). Paděraite is euhedral, whereas kupčíkite is marginally replaced by wittichenite (Figure 7b). This also displays a scalloped boundary with tetradymite. Paděraite is also replaced by wittichenite, as observed from marginal relationships (Figure 7c). Slightly ragged boundaries between the two sulfosalts are also observed (Figure 7d). Tetradymite is present as a small sigmoid-shaped inclusion on the boundary of paděraite, whereas wittichenite is observed on the side of the kupčíkite. Taken together, these textural relationships indicate reworking of primary paděraite–kupčíkite + tetradymite associations (Figure 7a,b) with superimposed replacement by wittichenite (Figure 7c,d).

Nanoscale study was carried out on several S/TEM foils to assess the identity of the two rare sulfosalts (Figure 8). Two of these foils comprise kupčíkite with a rim of wittichenite towards the boundary with magnetite at depth (Figure 8a) and paděraite alone (Figure 8b). The [010] zone axis was targeted in both sulfosalts, as this is the best orientation to assess their crystal-structure building modules using HR TEM [6].

Paděraite from a third foil was tilted only close to [010], as shown in the selected area electron diffraction (SAED) in Figure 8c, but could not be imaged. Kupčíkite from the foil in Figure 8a was identified from the SAED on the [010] zone axis (Figure 8d). This could be imaged only in BF-TEM mode (Figure 8e) as this zone axis was out of reach for the tilt allowed on the S/TEM microscope. The BF-TEM image shows regular ~12 Å repeats corresponding to the c parameter of kupčíkite (indexing using Topa et al. [8]). Paděraite from the foil in Figure 8b was identified from the SAED and HAADF STEM imaging on the [110] zone axis (Figure 8f,g). The image shows regular ~27 Å repeats corresponding to the c parameter of paděraite using one of the structures of Topa and Makovicky [25]. Wittichenite was also identified from SAED and HAADF STEM imaging on the [100] zone axis (Figure 8h,i). The crystal structure model for wittichenite (overlay on image in Figure 8i) and crystal model and STEM simulation for paděraite on the [110] zone axis (Figure 8j) show a good match with the images.

4.3. Compositional Data

4.3.1. EPMA Data

EPMA data for the Bi-rich bornite, Cu-Bi sulfosalts, Bi chalcogenides, and Au-Ag tellurides are listed in

Table 2. Summary EPMA data for Bi-rich and normal bornite in the Zhibula Cu skarn.

	Bi-Rich Bornite-like Phase (wt.%)				Normal Bornite (wt.%)
	Sample 374 (n = 3)				Sample 374 (n = 13)				Sample 8 (n = 29)				Sample 180 (n = 3)				Sample 372 (n = 2)
	1	2	3	Mean	Max.	Min.	SD	Mean	Max.	Min.	SD	Mean	Max.	Min.	SD	Mean	Max.	Min.	SD	Mean
Cu	54.6	55.24	54.77	54.87	64.50	61.39	0.73	63.55	63.90	62.50	0.37	63.35	60.86	60.59	0.11	60.71	62.93	62.52	0.20	62.72
Ag	0.09	0.09	0.07	0.08	0.20	<dl	0.06	0.03	0.23	<dl	0.04	0.12	0.06	<dl	0.00	0.02	0.10	0.09	0.00	0.10
Fe	14.53	14.12	13.68	14.11	11.59	10.27	0.31	11.31	11.48	10.78	0.17	11.19	15.23	14.76	0.20	14.96	11.65	11.64	0.00	11.64
Pb	<dl	0.07	<dl	0.02	0.10	<dl	0.02	0.04	0.25	<dl	0.05	0.05	<dl	<dl	-	<dl	0.09	<dl	0.03	0.05
Bi	6.47	6.82	6.9	6.73	0.51	0.12	0.10	0.20	0.31	0.11	0.04	0.18	0.30	0.25	0.02	0.27	0.39	0.25	0.07	0.32
Co	26.6	26.5	26.32	0.01	0.03	<dl	0.01	0.01	0.04	<dl	0.01	0.01	0.17	0.08	0.04	0.14	0.02	<dl	0.01	0.01
S	0.07	0.07	0.09	26.47	25.64	23.38	0.51	25.02	27.74	25.77	0.35	26.35	26.83	26.22	0.27	26.60	26.23	26.13	0.05	26.18
Se	<dl	0.02	0.02	0.08	0.09	<dl	0.01	0.04	0.09	<dl	0.02	0.03	0.05	<dl	0.00	0.02	0.06	<dl	0.02	0.03
Total	102.35	102.91	101.83	102.36	101.14	98.59	0.74	100.22	102.08	99.91	0.62	101.27	102.61	102.52	0.04	102.58	101.34	100.77	0.28	101.06
Formula (to 10 atoms)
Cu	4.336	4.383	4.391	4.370	5.167	4.938	0.058	5.038	4.978	4.820	0.031	4.931	4.672	4.632	0.017	4.650	4.901	4.894	0.004	4.898
Ag	0.004	0.004	0.004	0.004	0.009	-	0.003	0.002	0.011	-	0.002	0.005	0.003	-	0.000	0.001	0.005	0.004	0.000	0.004
Pb	-	0.002	-	0.001	0.002	-	0.001	0.001	0.006	-	0.001	0.001	-	-	-	-	0.002	-	0.000	0.001
Fe	1.313	1.275	1.248	1.279	1.058	0.926	0.029	1.020	1.009	0.950	0.012	0.991	1.331	1.286	0.020	1.303	1.037	1.032	0.002	1.035
Bi	0.156	0.165	0.168	0.163	0.012	0.003	0.002	0.005	0.007	0.003	0.001	0.004	0.007	0.006	0.000	0.006	0.009	0.006	0.002	0.008
Total M	5.809	5.828	5.811	5.816	6.232	5.998	0.055	6.065	5.991	5.779	0.037	5.933	6.010	5.934	0.035	5.960	5.946	5.945	0.001	5.945
S	4.186	4.167	4.183	4.179	3.997	3.768	0.054	3.932	4.221	4.006	0.038	4.065	4.066	3.990	0.034	4.039	4.055	4.050	0.003	4.053
Se	0.004	0.005	0.006	0.005	0.006	-	0.001	0.003	0.006	-	0.001	0.002	0.003	-	0.000	0.001	0.004	-	0.000	0.002
S(+Se)	4.191	4.172	4.189	4.184	4.002	3.768	0.055	3.935	4.221	4.009	0.037	4.067	4.066	3.990	0.035	4.040	4.055	4.054	0.001	4.055
M/S	1.386	1.397	1.387	1.390	1.654	1.499	0.037	1.542	1.495	1.369	0.022	1.459	1.506	1.460	0.022	1.476	1.467	1.466	0.001	1.466

Note: <dl, below minimum limit of detection. For detailed data, see Table S1.

,

Table 3. Summary EPMA data for common Cu-Bi sulfosalts (wittichenite and aikinite) in the Zhibula Cu skarn.

	Wittichenite (Sample 374, n = 4)				Wittichenite (Sample 180, n = 10)				Aikinite (Sample 180, n = 11)
	Max.	Min.	SD	Mean	Max.	Min.	SD	Mean	Max.	Min.	SD	Mean
Cu	39.80	38.79	0.38	39.35	38.03	36.88	0.39	38.00	10.98	10.54	0.13	10.75
Ag	0.50	0.30	0.07	0.39	0.22	<dl	0.05	0.19	<dl	<dl	-	<dl
Fe	1.16	0.60	0.22	0.83	0.87	0.54	0.14	0.77	0.86	0.07	0.29	0.53
Cd	0.09	0.06	0.01	0.08	0.06	0.04	0.01	0.04	0.14	0.05	0.04	0.04
Pb	<dl	<dl	-	<dl	<dl	<dl	-	<dl	36.22	35.11	0.37	35.53
Mn	<dl	<dl	-	<dl	0.08	<dl	0.02	0.02	0.03	<dl	0.01	0.01
Bi	40.42	39.15	0.51	39.55	41.89	40.57	0.44	40.90	37.56	36.07	0.46	37.01
S	20.12	19.80	0.12	19.94	20.45	19.39	0.30	20.06	16.91	15.70	0.39	16.25
Te	<dl	<dl	-	<dl	<dl	<dl	-	<dl	0.02	<dl	0.01	0.01
Se	0.11	<dl	0.02	0.07	1.04	0.09	0.25	0.26	1.07	0.32	0.29	0.67
Total	101.44	98.97	0.98	100.18	101.45	98.41	0.87	99.87	102.11	99.59	0.91	100.64
	Formula (to 7 atoms)								Formula (to 6 atoms)
Cu	3.004	2.977	0.011	2.989	2.922	2.870	0.016	2.923	1.157	0.947	0.078	1.050
Ag	0.022	0.013	0.003	0.017	0.010	-	0.002	0.009	-	-	-	-
Pb	-	-	-	-	-	-	-	-	1.185	0.978	0.080	1.064
Fe	0.100	0.053	0.018	0.072	0.075	0.047	0.012	0.033	0.101	0.008	0.033	0.041
Cd	0.004	0.003	0.001	0.003	0.003	0.002	0.000	0.002	0.008	0.003	0.002	0.002
Cu+Ag+Fe	3.126	3.047	0.029	3.078	2.986	2.875	0.031	2.965	1.202	1.007	0.075	1.091
Pb+Cd	0.004	0.003	0.001	0.003	0.003	0.002	0.000	0.002	1.188	0.984	0.081	1.066
Bi	0.925	0.904	0.008	0.914	0.989	0.959	0.011	0.957	1.204	1.010	0.084	1.099
Bi+Sb+As	0.925	0.904	0.008	0.914	0.989	0.959	0.011	0.957	1.204	1.010	0.084	1.099
Total M	4.034	3.968	0.026	3.994	3.947	3.867	0.023	3.924	3.549	3.022	0.235	3.256
S	3.028	2.963	0.025	3.002	3.120	2.988	0.035	3.060	3.470	2.871	0.264	3.145
Te	-	-	-	-	-	-	-	-	0.001	-	0.000	0.001
Se	0.007	-	0.001	0.005	0.065	0.006	0.015	0.016	0.092	0.027	0.024	0.053
S(+Te+Se)	3.032	2.966	0.026	3.006	3.133	3.053	0.023	3.076	3.507	2.943	0.264	3.198
Charge M	5.946	5.850	0.042	5.896	5.922	5.808	0.037	5.874	7.145	6.071	0.478	6.562
Charge S	6.063	5.932	0.051	6.012	6.266	6.106	0.046	6.152	7.014	5.885	0.528	6.396
mean	5.965	5.939	0.011	5.954	6.057	6.014	0.013	6.013	7.037	6.005	0.502	6.479
diff.	0.014	−0.204	0.091	−0.116	−0.185	−0.437	0.079	−0.278	0.292	0.004	0.087	0.166
diff (%)	0.2	−3.4	1.5	−1.9	−3.1	−7.2	1.3	−4.6	4.8	0.1	1.5	2.6

Note: <dl, below minimum limit of detection. For detailed data, see Table S2.

,

Table 4. Summary EPMA data for trace Cu-Bi sulfosalts (kupčíkite and paděraite) in the Zhibula Cu skarn (sample 180).

	Pb	Bi	Cu	Ag	Fe	Cd	S	Se	Te	Total	Pb	Bi	Ag	Cu	Fe	Cd	S	Se	Te	eV%
	Kupčíkite										Formula (to 19 atoms)
Ku-1	0.46	64.90	13.33	0.21	2.33	0.23	19.81	0.63	<dl	101.9	0.035	4.941	0.031	3.337	0.664	0.033	9.832	0.127	-	−1.3
Ku-2	0.52	64.37	13.09	0.19	2.14	0.29	19.72	0.65	<dl	100.97	0.040	4.949	0.028	3.310	0.616	0.041	9.883	0.132	-	−2.3
Ku-3	0.26	64.17	13.39	0.11	2.00	0.27	19.57	0.68	<dl	100.45	0.020	4.956	0.016	3.400	0.578	0.039	9.851	0.139	-	−2.1
Ku-4	<dl	64.22	13.08	0.13	2.19	0.34	19.66	0.70	<dl	100.32	-	4.954	0.019	3.318	0.632	0.049	9.885	0.143	-	−2.5
Ku-5	0.21	64.21	13.73	0.19	2.17	0.26	19.61	0.64	<dl	101.02	0.016	4.918	0.028	3.458	0.622	0.037	9.791	0.130	-	−1.3
Ku-6	<dl	63.82	13.52	<dl	2.32	0.20	19.45	0.71	<dl	100.02	-	4.929	-	3.434	0.671	0.029	9.792	0.145	-	−1.3
Ku-7	0.39	63.95	13.68	0.15	2.26	0.22	19.77	0.63	<dl	101.05	0.030	4.879	0.022	3.432	0.645	0.031	9.832	0.127	-	−2.1
Ku-8	0.04	64.06	13.54	0.14	2.21	0.22	19.67	0.60	<dl	100.48	0.003	4.920	0.021	3.420	0.635	0.031	9.848	0.122	-	−2.0
Ku-9	<dl	64.39	13.35	0.09	2.34	0.26	19.46	0.68	<dl	100.57	-	4.966	0.013	3.386	0.675	0.037	9.783	0.139	-	−0.6
Ku-10	<dl	64.21	13.70	0.08	2.53	0.10	19.71	0.70	<dl	101.03	-	4.892	0.012	3.432	0.721	0.014	9.788	0.141	-	−1.4
Ku-11	0.23	64.64	13.23	<dl	2.39	0.12	19.57	0.75	<dl	100.93	0.018	4.970	-	3.345	0.688	0.017	9.809	0.153	-	−1.1
Ku-12	0.51	63.34	13.27	<dl	1.94	0.06	19.34	0.63	<dl	99.09	0.040	4.961	-	3.418	0.569	0.009	9.873	0.131	-	−2.4
180Ku-13	0.77	63.99	13.45	<dl	1.76	0.11	18.98	0.79	<dl	99.85	0.061	5.032	-	3.478	0.518	0.016	9.730	0.164	-	−0.1
Ku-14	0.68	63.30	13.69	<dl	1.51	0.14	18.90	0.77	<dl	98.99	0.054	5.008	-	3.562	0.447	0.021	9.747	0.161	-	−0.9
Ku-15	0.06	63.09	13.41	0.13	1.34	0.08	18.85	0.72	<dl	97.68	0.005	5.048	0.020	3.529	0.401	0.012	9.832	0.152	-	−2.2
Ku-16	0.85	63.77	13.79	<dl	1.51	0.17	19.32	0.79	<dl	100.2	0.067	4.966	-	3.532	0.440	0.025	9.808	0.163	-	−2.3
Ku-17	0.08	63.29	13.07	0.10	1.93	0.11	18.92	0.70	<dl	98.2	0.006	5.028	0.015	3.415	0.574	0.016	9.798	0.147	-	−0.9
Ku-18	0.14	64.28	13.99	<dl	1.97	0.09	19.67	0.72	<dl	100.86	0.011	4.923	-	3.523	0.565	0.013	9.820	0.146	-	−2.3
Min.	<dl	63.09	13.07	<dl	1.34	0.06	18.85	0.60	<dl	97.68		4.879		3.310	0.401	0.009	9.730	0.122	-	−2.5
Max.	0.85	64.90	13.99	0.21	2.53	0.34	19.81	0.79	<dl	101.90	0.067	5.048	0.031	3.562	0.721	0.049	9.885	0.164	-	−0.1
Mean	0.29	64.00	13.46	0.08	2.05	0.18	19.44	0.69		100.20	0.02	4.96	0.01	3.43	0.59	0.03	9.82	0.14		−1.6
SD	0.26	0.48	0.26	0.04	0.32	0.08	0.31	0.06		1.05	0.02	0.05	0.01	0.07	0.09	0.01	0.04	0.01		0.7
	Paděraite										Formula (to 42 atoms)
Pad-1	7.25	62.26	11.48	0.36	0.51	0.12	18.64	0.98	0.13	101.73	1.310	11.153	0.125	6.762	0.342	0.040	21.765	0.465	0.038	−1.8
Pad-2	7.29	61.92	11.58	0.28	0.39	0.05	18.57	0.96	0.18	101.22	1.324	11.146	0.098	6.855	0.263	0.017	21.789	0.457	0.053	−2.3
Pad-3	7.58	61.70	11.66	0.30	0.38	0.09	18.31	1.02	0.19	101.23	1.383	11.159	0.105	6.935	0.257	0.030	21.586	0.488	0.056	−0.9
Pad-4	7.35	61.07	11.35	0.27	0.68	0.09	18.32	1.09	0.15	100.37	1.344	11.075	0.095	6.769	0.461	0.030	21.657	0.523	0.045	−1.6
Pad-5	7.25	61.87	11.36	0.25	0.47	0.09	18.08	1.01	0.16	100.54	1.337	11.311	0.089	6.829	0.322	0.031	21.546	0.489	0.048	0.1
Pad-6	7.58	61.76	11.16	0.24	0.47	0.12	18.48	0.98	0.22	101.01	1.384	11.182	0.084	6.645	0.318	0.040	21.811	0.470	0.065	−2.1
Pad-7	7.12	61.70	11.20	0.25	1.09	0.07	18.06	0.99	0.08	100.56	1.306	11.224	0.088	6.700	0.742	0.024	21.415	0.477	0.024	1.7
Pad-8	6.92	61.91	12.87	0.21	0.50	<dl	18.14	0.92	0.18	101.65	1.250	11.090	0.073	7.581	0.335	-	21.181	0.436	0.053	1.7
Pad-9	6.85	61.93	12.47	0.25	0.27	<dl	18.45	0.97	0.16	101.35	1.238	11.095	0.087	7.347	0.181	-	21.546	0.460	0.047	−1.3
Pad-10	7.37	61.15	12.80	0.11	0.39	0.06	18.64	0.97	0.16	101.65	1.319	10.846	0.038	7.466	0.259	0.020	21.551	0.455	0.046	−2.0
Pad-11	7.26	61.15	12.44	0.28	0.45	0.05	18.45	0.96	0.16	101.20	1.310	10.940	0.097	7.318	0.301	0.017	21.515	0.455	0.047	−1.2
Pad-12	6.93	61.22	12.56	0.29	0.42	0.11	18.18	1.02	0.19	100.92	1.258	11.018	0.101	7.433	0.283	0.037	21.328	0.486	0.056	0.0
Min.	6.85	61.07	11.16	0.11	0.27	<dl	18.06	0.92	0.08	100.37	1.238	10.846	0.038	6.645	0.181	-	21.181	0.436	0.024	−2.3
Max.	7.58	62.26	12.87	0.36	1.09	0.12	18.64	1.09	0.22	101.73	1.384	11.311	0.125	7.581	0.742	0.040	21.811	0.523	0.065	1.7
Mean	7.23	61.64	11.91	0.26	0.50	0.07	18.36	0.99	0.16	101.12	1.314	11.103	0.090	7.053	0.339	0.024	21.558	0.472	0.048	−0.8
SD	0.23	0.37	0.63	0.06	0.20	0.03	0.20	0.04	0.03	0.44	0.045	0.120	0.020	0.330	0.137	0.009	0.180	0.022	0.010	1.3

Note: <dl, below minimum limit of detection.

,

Table 5. Summary EPMA data for Bi chalcogenides in the Zhibula Cu skarn.

	Cu	Ag	Cd	Pb	Bi	S	Te	Se	Total	Cu	Ag	Pb	Cd	Cu+Ag	Pb+Cd	Bi	Total M	S	Te	Se	S(+Te+Se)
	Tetradymite (sample 180, n = 15)									Formula (to 5 atoms)
Max.	0.13	0.22	0.14	0.13	59.51	5.05	37.21	2.17	101.65	0.015	0.014	0.004	0.009	0.022	0.009	1.971	1.983	1.090	2.012	0.191	3.077
Min.	<dl	<dl	<dl	<dl	56.83	4.16	34.33	0.74	98.51	-	-	-	-	-	-	1.910	1.923	0.917	1.862	0.065	3.017
SD	0.02	0.04	0.03	0.03	0.73	0.28	0.80	0.40	0.99	0.003	0.003	0.001	0.002	0.005	0.002	0.014	0.014	0.052	0.042	0.036	0.014
Mean	0.08	0.06	0.04	0.05	58.39	4.69	35.48	1.44	100.24	0.009	0.004	0.002	0.003	0.012	0.004	1.928	1.945	1.010	1.919	0.126	3.055

Note: <dl, below minimum limit of detection. For detailed data, see Table S3.

and

Table 6. Summary EPMA data for Au-Ag tellurides in the Zhibula Cu skarn.

	S	Se	Cu	Ag	Hg	Te	Bi	Au	Total	S	Se	Te	S+Se+Te	Cu	Ag	Hg	Bi	Au	Total M
	Hessite (sample 8, n = 7)									Formula (to 3 atoms)
Max.	0.11	0.19	0.53	63.82	0.22	40.83	0.07	0.32	102.53	0.011	0.008	1.097	1.111	0.028	1.988	0.004	0.001	0.006	2.005
Min.	0.04	0.00	<dl	59.35	<dl	37.37	<dl	<dl	100.53	0.005	0.002	0.984	0.995	-	1.886	-	-	-	1.886
SD	0.02	0.05	0.11	1.49	0.04	1.12	0.01	0.09	0.69	0.002	0.002	0.037	0.038	0.009	0.032	0.001	0.000	0.001	0.038
Mean	0.07	0.08	0.36	62.93	0.10	38.20	0.02	0.07	101.73	0.007	0.003	1.008	1.019	0.014	1.964	0.002	-	0.001	1.981
	Hessite (sample 374, n = 4)									Formula (to 3 atoms)
Max.	0.20	0.26	<dl	64.14	0.22	37.91	0.07	0.34	102.82	0.021	0.011	0.991	1.016	-	2.000	0.004	0.001	0.006	2.007
Min.	0.12	0.12	<dl	63.36	0.12	36.23	<dl	<dl	100.38	0.012	0.005	0.967	0.993	-	1.978	0.002	-	-	1.984
SD	0.03	0.06	-	0.28	0.04	0.69	0.00	0.07	0.93	0.003	0.003	0.010	0.010	-	0.011	0.001	0.000	0.002	0.010
Mean	0.16	0.19	<dl	63.75	0.15	37.19	0.02	0.18	101.64	0.017	0.008	0.981	1.005	-	1.989	0.003	0.000	0.003	1.995

Note: <dl, below minimum limit of detection. For detailed data, see Table S4.

. Data for other sulfides are given in Tables S1–S7.

Our EPMA dataset indicates that most bornite is stoichiometric Cu₅FeS₄, albeit with detectable levels of Bi (mean 0.20 wt.%), Ag (mean 0.12 wt.%), Co (~0.17 wt.%), and Se (~0.09 wt.%) (Table 2). The Bi-rich bornite-like phase occurring in sample 374 (Figure 4d–f) is relatively Fe-rich, Cu-poor, and contains a mean Bi content of 6.73 wt.%, giving the empirical composition Cu_4.37Bi_0.16Fe_1.28S_4.18 (Table 2). Both chalcopyrite (Cu_0.98–1.02Fe_0.98–1.01S_1.97–2.01) and chalcocite (Cu_1.85Fe_0.06S_1.09) are stoichiometric and host comparable contents of Bi (0.06–0.16 wt.%), Ag (<dl~0.16 wt.%), Se (<dl~0.08 wt.%) and Co (<dl~0.10 wt.%) (Table S1).

Compositional data for all Cu-Bi-(Pb) sulfosalts are listed in Table 3 and Table 4 and plotted in Figure 6e (wittichenite and aikinite) and Figure 9 (kupčíkite and paděraite).

Wittichenite (Cu_2.92–2.99Fe_0.03–0.07Bi_0.91–0.96 S_3.00–3.06) and aikinite (Cu_1.05Fe_0.04Pb_1.06Bi_1.10 S_3.16) contain detectable Fe (average contents 0.77 wt.% and 0.53 wt.%, respectively). Selenium contents are also quite high in both wittichenite (0.05–1.04 wt.%) and aikinite (0.32–1.07 wt.%). Tellurium concentrations are below minimum limits of detection in both minerals. Wittichenite contains measurable Ag concentrations in some analyses (<dl~0.50 wt.%), whereas Ag is below the minimum detection limit in all analyzed aikinites.

Kupčíkite is a scarce mineral with fewer than 10 localities noted to date. Analytical data for kupčíkite (Table 4) give the empirical formula (Cu_3.43Fe_0.59)_4.02(Bi_4.96Pb_0.02Cd_0.03Ag_0.01)_5.02(S_9.82Se_0.14)_9.96 (n = 18). Iron (1.34–2.53 wt.%) is an essential component in kupčíkite. Kupčíkite displays variable contents of Pb (<dl to 0.68 wt.%), but Cd (0.06–0.34 wt.%) is relatively steady. Kupčíkite also contains relatively high, consistent concentrations of Se (0.60–0.79 wt.%), but no Te.

Paděraite contains Pb (average 7.23 wt.%), Ag (average 0.26 wt.%), Cd (average 0.07 wt.%) and minor Fe (average 0.50 wt.%). It also has Se contents (average 0.99 wt.%) that are comparable with kupčíkite: and trace amounts of Te (average 0.16 wt.%). Analysis of paděraite gives the empirical formula Cu₇(Cu_0.05Ag_0.09Fe_0.34Pb_1.31Cd_0.02Bi_11.10)_12.91(S_21.56Se_0.47Te_0.05)_22.08 (n = 12; Table 4).

EPMA data for tetradymite are listed in Table 5. Tetradymite is stoichiometric Bi_1.93Te_1.92S_1.01Se_0.13 (15 data) with detectable Cu (0.08 wt.%), Ag (0.06 wt.%), Cd (0.04 wt.%), and Pb (0.05 wt.%). The ratios of Bi/(Te+Se+S) for Zhibula tetradymite are 0.62–0.65, as plotted in Figure 6f. We note the presence of 0.74–2.17 wt.% Se in Zhibula tetradymite.

Silver–(Au) tellurides are dominated by hessite (Table 6). Analysis of hessite gives the formula Ag_1.97Te_1.00S_0.01Se_0.01 (n = 11). Traces of Cu (average 0.17 wt.%), Hg (average 0.12 wt.%), Bi (average 0.02 wt.%), Se (average 0.12 wt.%), and Au (average 0.11 wt.%) are noted. Despite its relative abundance in the samples, the fine grain size of petzite prohibited acquisition of high-quality compositional data.

Carrollite associated with bornite contains 12.16–18.18 wt.% Cu, 26.07–36.24 wt.% Co, 9.91–11.25 wt.% Ni, and 0.05–4.07 wt.% Fe, giving the calculated formula Cu_0.59–0.90 Fe_0.00–0.23 Co_1.39–1.88 Ni_0.52–0.60 S_3.97–4.02 (Table S5). Analyzed grains also contain measurable concentrations of Bi (0.10–0.17 wt.%), Te (0.02–0.07 wt.%), and Se (<dl~0.08 wt.%).

Pyrrhotite contains measurable Ni (mean 0.83 wt.%) and Co (mean 0.14 wt.%) (Table S6). Pyrrhotite is non-stoichiometric and has a generic formula of Fe_1-xS (0 < x < 0.125; e.g., [26]). The x value of Zhibula pyrrhotite averages 0.108. Exsolved pentlandite in pyrrhotite has low Fe (mean 35.23 wt.%) and high Ni (mean 26.94 wt.%) with a mean formula of Fe_4.86Ni_3.54Co_0.35S_8.23. Pentlandite contains minor Co (1.30–3.22 wt.%), Se (0.05–0.09 wt.%), and negligible Te (<dl~0.03 wt.%).

Galena contains significant concentrations of Ag (mean 1.14 wt.%) and Bi (mean 2.31 wt.%), as well as detectable Se (mean 0.20 wt.%), Cu (mean 0.25 wt.%), Te (mean 0.05 wt.%), and Cd (mean 0.05 wt.%) (Table S7). Small grain size hampered efforts to obtain reliable compositional data for seleniferous galena (galena–clausthalite solid solution; Figure 3e).

Two markedly different types of sphalerite are observed (Table S7). The first (in sample 321) is characterized by high Fe (average 12.59 wt.%) and Cd (average 1.34 wt.%), low Zn (average 51.24 wt.%), and measurable average contents of Co (0.08 wt.%) and Ni (0.05 wt.%). The second features low Fe (mean 1.24 wt.%) and Cd (mean 0.22 wt.%), high Zn (mean 64.01 wt.%), with detectable Co (mean 0.26 wt.%), traces of In (mean 0.02 wt.%), and Hg (mean 0.05 wt.%), but without detectable Ni.

4.3.2. LA-ICP-MS Trace Element Data

In an attempt to corroborate the concentrations of elements of interest in common sulfides, trace element analysis by LA-ICP-MS was undertaken on selected grains of chalcopyrite and bornite (Table S8).

Bornite from two samples confirms the notable enrichment in Bi (846–27,467 ppm, average 7351 ppm), Ag (264–901ppm, average 462 ppm), Se (143–543 ppm, average 302 ppm), and Te (3.0–283 ppm, average 63 ppm). Two samples (374 and 8) display distinct Bi and Se contents (Figure 10a,b). Additionally, a positive correlation between Ag and Te is noted (Figure 10c).

In comparison, chalcopyrite shows much lower concentrations of Bi (<dl~75 ppm, average 8.8 ppm), Ag (0.17–14 ppm, average 2.8 ppm), Te (0.31–5.0 ppm, average 1.8 ppm), and Se (90–234 ppm, average 139 ppm). We note, however, higher In concentrations in chalcopyrite (1.7–6.3 ppm) than in bornite (0.03–3.3 ppm).

5. Discussion

5.1. A New Skarn Occurrence of Kupčíkite and Paděraite, the First Report from China

The present study introduces a new occurrence of kupčíkite and paděraite, minerals that are for the first time reported from a Chinese locality. Kupčíkite is a member of the cuprobismutite homologous series of copper–bismuth sulfosalts [8,14]. Paděraite is not a member of the cuprobismutite homologous series, but is chemically and structurally related to it [6,9,25,27]. Structural relationships between paděraite and the cuprobismutite homologous series were postulated by Mumme [27], and later illustrated as nanoscale intergrowths between paděraite and cuprobismutite in Ciobanu et al. [6]. Cook and Ciobanu [9] postulated that paděraite belonged to a closely related homologous series with potential links to cuprobismutite series via the substitution Cu + Bi → 2Pb. Topa and Makovicky [25] discussed hypothetical intergrowths between paděraite and cuprobismutite homologues.

The cuprobismutite homologous series, Cu₈Me_4(N–1)+2^(quasi)octBi₈^sq.pyrS_4N+16, (oct = octahedral; sq. pyr. = square pyramidal; where Me = Bi, Ag, Fe; [5,14]) comprises three named minerals with layered structures and predictable compositional variation via the homologue number N = (N1 + N2)/2; N1 and N2 relate to the incremental quasi-octahedral layer. Kupčíkite, Cu_3.4Fe_0.6Bi₅S₁₀, was first described from type locality (TL), the Felbertal scheelite deposit, Hohe Tauern, Austria [8] and is the simplest N = (1,1) homologue (Figure 9). This corresponds structurally to the synthetic phase Cu₄Bi₅S₁₀ of Mariolacos [28]. It differs, however, from the synthetic equivalent in that it also contains Fe, an element that seems to be essential for mineral stability in the geological environment. Kupčíkite contains more Fe than other cuprobismutite homologues, but typically only minor Ag or Cd, if any. Compositional data for kupčíkite from black shale-hosted hydrothermal Ni-Bi-As mineralization at Čierna Lehota, Slovakia show a significant Pb content (2.73 wt.%), yielding the formula (Cu_3.92Fe_0.24Bi_4.60Pb_0.19Ag_0.04Sb_0.01S_9.58) [11]. Kupčíkite from granitic pegmatites in Karkonosze Massif, southwestern Poland has traces of Zn (~0.20 wt.%) (Cu_6.67Fe_1.22Zn_0.10Bi_9.99S_20.03) [12]. The Zhibuka kupčíkite is stoichiometric with Fe content (Fe = 0.59 apfu), comparable to the TL specimen, but also contains Se (0.14 apfu).

Paděraite was first described by Mumme and Žák [7] from Băiţa Bihor (Romania) with the empirical composition Cu_5.9Ag_1.3Pb_1.6Bi_11.2S₂₂. Mumme [27] proposed the structural formula Cu₆AgPbBi₁₂S₂₂. Paděraites from Swartberg (South Africa) and Ocna de Fier (Romania) yield different empirical formulae: Cu_7.36Pb_1.31Bi_11.32S₂₂ and Cu_7.11Ag_0.36Pb_1.2Bi_11.28S_22.05, respectively [9,10]. Due to the different Cu and Ag contents, Topa and Makovicky [25] distinguished between Ag-bearing paděraite (Băiţa Bihor) and Ag-free, Cu-enriched paděraite (Swartberg). Their structural formula can be expressed as Cu₇(X_0.33Pb_1.33Bi_11.33) _∑13S₂₂, where X is either Cu or Ag. Zhibula paděraite (Figure 9; Table 4) is also Ag-bearing, albeit with much lower Ag than the examples from Baita Bihor or Ocna de Fier, i.e., Ag = 0.09 apfu, compared to 0.2–0.3 and 0.3–0.4 apfu, respectively. It also contains relatively high content of Se (0.47 apfu) and minor Te (0.05 apfu). Notably, Zhibula paděraite differs from the other three specimens in that it contains Fe (0.34 apfu), an element that requires further constraints in terms of how it is incorporated into the crystal structure.

The interest in these rare minerals stems from the fact they are often intergrown with one another and with other Bi sulfosalts, particularly in skarn deposits rich in Bi minerals, such as those from Baita Bihor and Ocna de Fier [5,6,9,25]. In such skarn occurrences, the two minerals are enclosed within members of the bismuthinite derivative series, either aikinite or bismuthinite, respectively. Co-crystallization between cuprobismutite homologues is supported by micron and nanoscale intergrowths observed at Ocna de Fier [6,9], whereas Topa and Makovicky [25] propose replacement of hodrushite by paděraite in the Baita Bihor example.

The Zhibula skarn is outstanding in that kupčíkite and paděraite, although associated, do not display such lamellar intergrowths with one another at either the micron- or nanoscale (Figure 7 and Figure 8). Secondly, they are not associated with one of the bismuthinite derivatives (only aikinite is present at Zhibula), but instead co-crystallize with tetradymite (Figure 7a). Superimposed overprinting is evidenced by the observed replacement of wittichenite (Figure 7c) and reshaping of mutual phase boundaries (scalloped and ragged morphologies; Figure 7b,d). This implies that kupčíkite and paděraite can be attributed to the prograde (diopside–andradite) skarn stage rather than the retrograde skarn stage (hedenbergite–johannsenite) in proximal skarn from Zhibula. Temperatures in the range 400–600 °C were constrained from phase associations for this stage and are corroborated by fluid inclusion data for the Zhibula skarn (405–667 °C; [22]). Likewise, the Ocna de Fier skarn records temperatures of 400–600 °C and ~371 °C during prograde and retrograde stages, respectively, as estimated from skarn associations [29].

Although few studies have mentioned the formation environments of Cu-Bi sulfosalts and associated paděraite, we note their occurrence is generally in deposits characterized by relatively high temperatures, including metamorphosed W deposits [8], granite-related Cu-rich veinlets, and Li-Be-bearing granitic pegmatite at Swartberg ([10] and references therein). Furthermore, based on fluid inclusion data (310–390 °C) for quartz from the Obari mine, Japan, formation of cuprobismutite-group minerals took place above 300 °C [13].

5.2. Genetic Constraints—Trace Minerals and Metallogenic Implications

Exotic trace mineralogy comprising Bi-Te-Au associations are typical of many deposits [3] and are common in skarn deposits of various types [30]. In the latter case, skarn orefield zonation can lead to specific ore assemblages, for example, as observed at Ocna de Fier [29,31]. This is also the case of the Zhibula skarn where proximal and distal skarn differ in terms of skarn mineralogy [23] and can be enriched in a range of exotic trace minerals, also including the W-Mo associations in the distal skarn reported by Xu et al. [24].

Mineralogical variability within calcic skarn deposits is an intrinsic characteristic of metasomatism driving the interaction between magma-derived fluids and carbonaceous protoliths ([29] and references therein). Therefore, skarn mineral associations will vary as a function of different protoliths, fluid components, and physicochemical conditions (X_CO2, T, P, pH, etc.) (e.g., [32,33,34]). Similarly, ore mineral assemblages and accompanying trace mineral signatures are also likely to exhibit variation in different skarns. Based on prior results, including the work of Xu et al. [23], we here address the diversity of mineralization styles observed at Zhibula and discuss the local environments represented by the three distinct types of skarn ores.

The results presented here show that the proximal skarns (garnet- and pyroxene-dominant) host distinct ore and trace mineral associations. They both share Cu sulfides and trace Bi-Te minerals, although of different speciation. The distal skarn has its own distinct characteristics. The schematic in Figure 11 shows the formation of the three associations, “A1”, “A2” and “A3,” interpreted in terms of evolving f_Te2–f_S2 conditions.

5.2.1. Au-Ag Telluride Associations in Bornite–Chalcopyrite Ores

A first association “A1” (column in Figure 11) comprises bornite and chalcopyrite with minor carrollite, chalcocite, wittichenite, molybdenite, several tellurides (tsumoite, melonite, hessite, and petzite), and electrum (Figure 2b,c, Figure 4 and Figure 5). This is constrained at relatively high T, at least 400 °C, from conditions interpreted for host proximal garnet skarn [22] and f_O2–f_S2 stability within the magnetite–pyrite field. Considering phase stabilities for tellurides from Afifi et al. [2], the field for association “A1” at 400 °C is constrained from the stability fields of Au tellurides versus that of native gold, as well as Ag and Ni telluride versus sulfide, and the presence of galena instead of altaite (Figure 11a). Overall, the co-existing mineral assemblage “A1,” consisting of bornite chalcopyrite, petzite–hessite, magnetite–hematite, and wittichenite, suggests that they formed at relatively high f_Te2, f_S2, and f_O2 conditions (Figure 4 and Figure 11) [35].

Trace element signatures and textural relationships between Cu-Fe sulfides and co-existing species further assist in characterizing association “A1.” Bornite has long been reported to accommodate Bi (~0.20 wt.%) and Ag (0.2–2.0 wt.%) (e.g., [36,37,38]). In Zhibula, bornite carries comparable concentrations of Bi (1021–21,419 ppm) and Ag (264–901 ppm), consistent with a hypogene origin and analogous to other comparable occurrences. Bismuth and Se, particularly the Bi in bornite, display two distinct groups (Figure 10a,b), probably indicating the variation in formation temperatures, as higher Bi incorporation into bornite occurs under elevated temperatures. Considering the intergrowth textures among bornite, chalcopyrite, wittichenite, and hessite (Figure 4 and Figure 5), they probably represent a hypogene assemblage formed during the early high-temperature prograde skarn stage. Carrollite is likely also part of this stage, whereas chalcocite and galena are attributable to the retrograde stage (Figure 4a–c).

The unusual Bi-rich bornite-like phase (average 6.73 wt.%) observed in minor amounts at Zhibula stands out against the “normal” low-Bi (0.20 wt.%) variety (Figure 4d–f). Similar textures are reported from the Yangzhaiyu Au deposit, China, in which the brighter patches in bornite contain up to 7.7 wt.% Bi [39]. These findings are, however, in agreement with the experimental work of Sugaki et al. [40], which showed that bornite occupies a relatively extensive field of solid solution at higher temperatures and can incorporate as much as 17.2 wt.% Bi at 420 °C and 11.4 wt.% Bi at 300 °C. Moreover, Nanri et al. [41] reported that bornite solid solution can reach a maximum of 18.2 wt.% Bi at 500 °C. The observations are also consistent with fluid inclusion data from co-existing quartz (250–420 °C, mean ~300 °C) [22]. The mechanism by which the two compositionally distinct bornites co-exist is, however, unclear at present, and may possibly reflect unmixing of two distinct bornite superstructures during cooling.

5.2.2. Bismuth Sulfosalt and Tellurides in Magnetite–Chalcopyrite Ore Prograde to Retrograde Skarn Transition

Minor and trace minerals comprising Cu-Bi sulfosalts (wittichenite, aikinite, kupčíkite, and paděraite), and tetradymite (Figure 6 and Figure 7), hereafter called assemblage “A2” (column in Figure 11), are hosted within magnetite–chalcopyrite ore recording lower temperatures and a reducing event from higher to lower f_O2, from diopside–andradite to hedenbergite–johannsenite skarn (Figure 2d–h; [23]). Such an overprint is also expressed by a coarsening of the Bi minerals within zoned garnet or new growth within magnetite, from mottled chalcopyrite cores to marginal inclusions of Bi minerals (Figure 2f,g). Coarsening of Bi minerals is also accommodated along mutual boundaries between magnetite and chalcopyrite (e.g., the tetradymite in Figure 2g). We note the presence of Se in tetradymite (0.74–2.17 wt.%), as well as kupčíkite and paděraite.

Assemblage “A2” is shown within a narrow stability field formed at lower temperature (~300 °C) and f_Te2 (disappearance of Au-Ag tellurides and all other tellurides except tetradymite), but higher f_S2 than assemblage “A1” (Figure 11b). Nonetheless, as discussed above, early paděraite and kupčíkite could have formed during prograde formation of diopside–andradite skarn at ~400 °C.

5.2.3. Pyrrhotite–Chalcopyrite in Distal Skarn

A further reducing skarn assemblage comprising grossular garnet, vesuvianite, wollastonite, and anorthite, which hosts pyrrhotite and chalcopyrite [23], hereafter called association “A3” (Figure 11b), is represented by the distal skarn. Association “A3” is estimated to form at the lowest f_O2, f_S2, and f_Te2 conditions. Exsolution of lamellar pentlandite from pyrrhotite is apparently favored by cooling from high temperatures (e.g., [42]), so we used a temperature of 300 °C for the phase diagram shown in Figure 11b).

Although the mineral assemblages are different in each of the three skarn types, the primary fluids and their sources are likely to be the same. For example, Co and Ni crystallize as carrollite and melonite in bornite–chalcopyrite ores from andradite skarn, but are instead incorporated into high Co- and Ni-bearing pyrrhotite (mean 0.83 wt.% and 0.14 wt.%, respectively) and pyrrhotite (<dl~0.15 wt.% Co and 0.63–0.91 wt.% Ni) in the distal (banded grossular) skarn.

In summary, the complex and varied ore mineral assemblages observed in the Zhibula Cu skarn (Bi-bearing bornite, Au-Ag tellurides, Cu-Bi sulfosalts, Bi chalcogenides, Co-Ni sulfides, and scheelite–powellite) reflect a complex magmatic–hydrothermal system, closely related to the nearby Qulong porphyry Cu-Mo deposit [22,23,24]. The Zhibula deposit has a characteristic metallogenic signature (Cu, Au, Ag, Te, Se, Bi, Co, W, and Mo). These elements are common accessories in base metal skarns and occur within a variety of mineral assemblages depending on the prevailing wall rocks and local physicochemical conditions. Mineralogical investigation that is sufficiently detailed to establish the mode of occurrence of minor minerals and their contained elements of interest is not only helpful to indicate how the ore was formed and evolved but also provides fundamental information necessary to achieve recovery and optimal beneficiation of these critical metals as by-products.

6. Summary and Conclusions

The Zhibula ±Cu skarn contains a group of conspicuous trace ore minerals, including high Bi-bearing bornite (average 6.73 wt.%), Cu-Bi sulfosalts (wittichenite, aikinite, kupčíkite, and paděraite), Au-Ag tellurides (hessite, petzite), and Bi chalcogenides (tetradymite). During a decrease in temperature from 400 °C to 300 °C, the stability fields for these Au and Ag tellurides changed and f_Te2 was reduced, which can be clearly noted by phase equilibrium between AuTe₂ and gold. Similarly, logf_S2 decreases from −7.4 to −11.4, as marked by the relative prevalence of pyrrhotite relative to pyrite. Overall, the co-existing mineral assemblage “A1,” consisting of bornite–chalcopyrite, petzite–hessite, magnetite (±hematite), and wittichenite, suggests that they formed in relatively high f_Te2, f_S2, and f_O2 conditions. In contrast, the abundant Bi-Te associations in magnetite–chalcopyrite ore (association “A2”) have formed during the retrograde sulfidation of pyroxene-dominant skarn at lower f_Te2 and higher f_S2.

Kupčíkite (Cu_3.43Fe_0.59)_4.02(Bi_4.96Pb_0.02Cd_0.03Ag_0.01)_5.02(S_9.82Se_0.14)_9.96 and paděraite (Cu₇(Cu_0.05Ag_0.09 Fe_0.34Pb_1.31Cd_0.02Bi_11.10)_12.91(S_21.56Se_0.47Te_0.05)_22.08 from Zhibula skarn do not form nanoscale intergrowths with one another.

The most reduced assemblage is chalcopyrite and pyrrhotite within the distal skarn at the contact with marble (association “A3”). Cobalt and nickel occur as discrete minerals: cobaltite, melonite, and pentlandite. The precious metal association is restricted to bornite–chalcopyrite ores.

These trace ore mineral signatures (assemblages, their compositions and mutual textures) suggest precipitation from a high-temperature magmatic–hydrothermal system. Together with the skarn assemblages, trace ore mineralogy supports a genetic relationship with the nearby Qulong porphyry Cu-Mo deposit and represents evidence for an evolution of the ore-forming fluids from high f_S2–f_Te2 conditions, through a moderate S-fugacity, towards relatively reduced, lower f_S2, f_O2, and f_Te2 conditions. The observed evolution also corresponds to a diverse suite of local environments controlled by lithology and structure.