Study on the Occurrence States and Enrichment Mechanisms of the Dispersed Elements Ga, Ge, and In in the Chipu Pb-Zn Deposit, Sichuan Province, China

Abstract

The dispersed elements Ga, Ge, and In are crucial strategic mineral resources often enriched in Pb-Zn deposits. The Chipu Pb-Zn deposit, located on the western edge of the Yangtze Block, lies to the north of the Sichuan-Yunnan-Guizhou (SYG) Pb-Zn metallogenic province with large amounts of Emeishan basalt. Based on trace element and in situ sulfur isotope analyses by (LA)-ICP-MS, sphalerite is the main carrier mineral for Ga (17~420 ppm), Ge (3.87~444 ppm), and In (31~720 ppm). Ga or Ge correlate significantly with Cu, while In substitutes for Zn in sphalerite alongside Fe. Key substitution reactions include Ga³⁺ + Cu⁺ ↔ 2Zn²⁺, Ge⁴⁺ + 2Cu⁺ ↔ 3Zn²⁺, and 2In³⁺ + Fe²⁺ ↔ 4Zn²⁺. Sphalerite crystallized at medium to low temperatures (114–195 °C). Sulfide δ³⁴S values (+3.48 to +24.74‰) suggest sulfur mainly originated from Dengying Formation marine sulfates via thermochemical sulfate reduction (TSR). Metal-bearing fluid release at 30 Ma post-Emeishan mantle plume activity (261–257 Ma) coincides with the Chipu deposit’s mineralization period (230–200 Ma), suggesting the Chipu deposit is associated with Emeishan plume activity. The magmatic activity drove basinal brine circulation, extracting In from intermediate-felsic igneous rocks and metamorphic basement. Elevated temperatures promoted the coupling of Fe and In into sphalerite, causing anomalous In enrichment.

1. Introduction

Gallium (Ga), germanium (Ge), and indium (In), as key dispersed elements with unique characteristics, are vital for the growth of diverse emerging industries [1,2]. Nonetheless, the challenge lies in adequately enriching dispersed elements and forming independent dispersed element deposits (with rare exceptions) due to the low average abundances of these elements in the Earth’s crust (mostly at 10⁻⁶~10⁻⁹ level) [3,4,5]. It has been proven that the Pb-Zn deposits are enriched in high-grade dispersed Ga, Ge, and In [6,7]. Studies over the past few decades have shown that the ore minerals found in different Pb-Zn deposits (such as MVT, Skarn, Epithermal, VMS, and SEDEX) contain sufficiently high concentrations of Ga, Ge, and In, primarily carried by sphalerite [8,9,10,11,12,13].

Situated on the southwestern margin of the Yangtze Block, the Sichuan-Yunnan-Guizhou (SYG) Pb-Zn metallogenic province forms a crucial segment of the extensive low-temperature metallogenic domain in Southwest China, encompassing more than 400 Pb-Zn deposits. Reports indicate that numerous ore deposits in the area, which are key sources of Pb, Zn, and Ag in China [14,15], are also associated with dispersed elements like Ga and Ge (e.g., Tianbaoshan; Niujiaotang; Huize; Maliping) [16,17,18,19,20].

The Chipu Pb-Zn deposit is located in the northern part of the SYG Pb-Zn metallogenic province. Earlier research primarily focused on the fundamental geological features, geochronology, isotope geochemistry, and ore-forming fluid in the deposit [21,22,23,24,25,26,27]; the potential value of dispersed elements within the deposit has yet to be assessed. Here, we employed electron microprobe analysis (EMPA) and in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to methodically analyze trace elements and sulfur isotopes in typical sulfides from the deposit. The results suggest an enrichment of Ga and Ge in the Chipu deposit, with an unusual enrichment in In. This research focuses on the occurrence mechanisms of Ga, Ge, and In within the Chipu Pb-Zn deposit and reveals the characteristics of the ore-forming fluid in the Chipu Pb-Zn deposit.

2. Regional Geology

Located in the western Yangtze Block, the SYG Pb-Zn metallogenic province serves as a transition area linking the Gondwana paleocontinent and the Laurasia paleocontinent, flanked by the Songpan-Ganzi Orogenic belts in the north, the Sanjiang Orogen belts in the southwest, and the Cathaysia Block in the southeast (Figure 1a) [25,26,28,29,30]. The area’s strata are largely intact, predominantly consisting of the basement and sedimentary cover rocks. The Pb-Zn deposits in the region are predominantly found in carbonates from the Sinian to Permian periods [30,31,32]. The tectonic development in the area is remarkable, with its basement tectonics being E-W-trending and consisting of a series of faults and folds, while the cover is characterized by deep and large faults and retrograde fold tectonics, and the deposits are mainly distributed within the Xiaojiang Fault and its sub-fractures on the eastern side [25]. Four distinct faults, namely the Anninghe Fault, the Xiaojiang Fault, the Shizong-Shuicheng Fault, and the Ziyun-Yadu Fault, are intricately linked to the Pb-Zn mineralization (Figure 1b) [19]. These faults provide an important tectonic background for the mineralization and may serve as a conduit for the transport and enrichment of the mineral matter. The magmatic rocks in the region are characterized by both intrusive and extrusive types. During the Indosinian period, the area was marked by extensive development of the Emeishan basalts [14,30,33], with eruption ages ranging from approximately 261 Ma to 257 Ma [34].

Over four hundred Pb-Zn deposits, including Huize, Tianbaoshan, Daliangzi, and Maliping, are found in the SYG Pb-Zn metallogenic region (Figure 1b) [30,36]. Known for their significant reserves and high-grade ores, these deposits account for as much as 30% of China’s Pb and Zn resources [28,34,37]. For example, the Huize large Pb-Zn deposit has reserves of up to 5 Mt Pb + Zn, and the ore grade ranges from 25% to 30%. In addition, there are a lot of large–medium–small-sized Pb-Zn deposits with Pb + Zn reserves between 0.6 and 2.0 Mt, with ore grades between 8% and 10% [25,28,37].

3. Deposit Geology

The Chipu Pb-Zn deposit is located in Ganluo County, Sichuan Province. It is situated in the northern segment of the SYG Pb-Zn mineralization province and along the Ganluo River Fault, which is part of the eastern branch of the N-S-trending Xiaojiang Fault system [25,27,38]. The exposed strata are mainly composed of Sinian Dengying Formation dolomite, lower Cambrian Qiongzhusi Formation carbonaceous siltstone, middle to upper Cambrian dolomite and dolomitic siltstone, Ordovician dolomite and sandstone, and Silurian siltstone intercalated with limestone. The contact between the Dengying Formation and the Cambrian System is characterized by an unconformity. A well-developed interlayer fracture zone exists between them, which provides favorable space for mineralization, with sphalerite and galena veins often filled within it.

The main structures with the mining region include the Malaha inverted anticline and the Malaha inverted fault, which trends in the NNW axial direction. The majority of ore bodies found within the mining region are situated in the dolomite of the Dengying Formation in the eastern flank of the Malaha inverted anticline, and most of them are stratiform and/or lenticular along the fracture zone (Figure 2) [25,27,38,39]. The occurrence of ore bodies was found to be basically consistent with that of the ore-bearing wall rock, with a dip of 10~50° [23,24]. Furthermore, the mining region features secondary faults that trend in the NW and NNW directions. These post-mineralization faults exert a certain degree of disruption on the ore body. The ore body contains Pb reserves of 0.3 Mt and Zn reserves of 0.12 Mt, with average grades of 7.4% and 3.1%, respectively, while the combined Pb + Zn grade can reach 10%~22% in certain areas [25,27,39].

4. Characteristics and Sequences of Mineralization

Pyrite, sphalerite, and galena dominate as the primary ore minerals in the deposit, with dolomite, quartz, and a minor amount of calcite being typical gangue minerals. Based on the paragenetic textures and mineral interconnections, the hydrothermal mineralization period of the Chipu deposit is segmented into three stages: the pyrite stage (Stage Ⅰ), the sphalerite–galena–pyrite stage (Stage Ⅱ), and the galena stage (Stage Ⅲ) (Figure 3 and Figure 4).

During the pyrite stage (Stage Ⅰ), the mineral assemblage is primarily composed of fine-grained quartz (Ⅰ-Qz) and abundant coarse-grained, euhedral to subhedral pyrite (Ⅰ-Py), which has experienced partial dissolution by later fluids or been replaced by subsequent sulfide minerals (Figure 3a,d,e). During the main ore stage, the sphalerite–galena–pyrite stage (Stage Ⅱ), the mineral assemblage is primarily composed of fine to coarse-grained sphalerite (Ⅱ-Sp), subhedral galena (Ⅱ-Gn), dolomite (Ⅱ-Dol), and fine-grained pyrite (Ⅱ-Py) (Figure 3b,c,e–g). The coprecipitation of Ⅱ-Dol, Ⅱ-Sp, and Ⅱ-Gn is distinctly marked by the presence of mutual growth boundaries (Figure 3f,g). It is noticeable that II-Py is dispersed across the surface of II-Sp or manifests as a fracture filling (Figure 3g–i). Furthermore, Figure 3f illustrates the frequent occurrence of rhythmic banding in Ⅱ-Sp crystals, characterized by a distinct color alternation between light yellow and dark brown, which corresponds to fluctuations in trace element concentrations. The galena stage (Stage Ⅲ) is characterized by anhedral or vein-like galena (Ⅲ-Gn) (Figure 3h,i), in which the vein-like Ⅲ-Gn cuts through the coarse-grained sphalerite, which hosts the fine-grained pyrite (Figure 3i). The detailed mineral paragenesis textures and mineral interconnections are shown in Figure 3 and Figure 4.

5. Sampling and Analytical Methods

5.1. Sample Description

All samples in this study were collected from ore bodies in the Chipu No. 1 and No. 2 ore zone, with all specimens being sulfide-bearing ore samples. The sulfide’s structural and mineralogical properties were ascertained through an in-depth petrographic study of 18 polished sections and thin sections of typical lead-zinc ores from the Chipu deposit. Hence, the initial examination of these samples took place on a hand specimen scale, followed by the confirmation of mineral types and structures using the optical microscope, and culminating in the analysis of trace elements and in situ sulfur isotopes.

This research involved the detailed trace element analysis, yielding 57 points for sphalerite, 10 for pyrite, and 5 for single-mineral galena, as well as in situ sulfur isotope testing at 30 points for sphalerite, pyrite, and galena, all conducted on 18 lead-zinc ore specimens from the Chipu Pb-Zn deposit.

5.2. (LA)-ICP-MS Sulfides Trace Element Analysis

The trace element concentrations within sphalerite and pyrite were identified at Guangzhou Tuoyan Analytical Technology Co., Ltd., (Guangzhou, China), employing a NWR 193 nm ArF Excimer laser-ablation system coupled to an iCAP RQ (ICPMS). The laser emitted a fluence of 3.5 J/cm², featuring a 6 Hz repetition rate, a spot size of 30 μm, and a 40 s analysis period, succeeded by a 40 s background evaluation. Two standard blocks (NIST 610 and GSE-2G) and a MASS-1 sulfide standard were succeeded by 5 to 8 unidentified samples. In the case of sphalerite and pyrite, measurements were taken of 21 isotopes, cumulatively amounting to a dwell time of 210 ms: ³⁴S, ⁵¹V, ⁵³Cr, ⁵⁵Mn, ⁵⁷Fe, ⁵⁹Co, ⁶⁰Ni, ⁶⁵Cu, ⁶⁶Zn, ⁷¹Ga, ⁷³Ge, ⁷⁵As, ⁷⁷Se, ¹⁰⁷Ag, ¹¹¹Cd, ¹¹⁵In, ¹¹⁸Sn, ¹²¹Sb, ¹²⁶Te, ²⁰⁵Tl, and ²⁰⁸Pb. Calibration of ³⁴S, ⁵¹V, ⁵³Cr, ⁵⁵Mn, ⁵⁷Fe, ⁵⁹Co, ⁶⁰Ni, ⁶⁵Cu, ⁶⁶Zn, ⁷¹Ga, ⁷³Ge, ⁷⁵As, ⁷⁷Se, ¹⁰⁷Ag, ¹¹¹Cd, ¹¹⁵In,¹¹⁸Sn, ¹²¹Sb, ¹²⁶Te, ²⁰⁵Tl, and ²⁰⁸Pb primarily employed the MASS-1 sulfide standard (Fe = 15.6%) [40]. To prevent the mutual interference between In and Sn isotopes from affecting the test data during the analysis of sphalerite, the data were corrected following the method provided by [41]. The detailed correction process is described in Table S2. The corrected In concentrations were consistent with the measured In concentrations, confirming that the measured In concentrations reliably represent the actual In concentrations in sphalerite.

The preliminary treatment for the trace element testing of galena by ICP-MS was completed at Wuhan Shangpu Analytical Technology Co., Ltd. (Wuhan, China), and the experiment was conducted at the Analytical Testing Research Center of the Beijing Research Institute of Uranium Nuclear Geology (Beijing, China). The trace elements analyzed included Mn, Fe, Co, Ni, Cu, Ga, Ge, In, Cd, Tl, As, Se, Te, Ag, Sn, Sb, V, Cr, Zn, Pb, etc. The analysis began by selecting single-mineral galena and grinding it to 200 mesh, then placing the samples in a sealed digestion vessel, dissolving them with hydrofluoric acid and nitric acid, and heating on an electric hot plate to completely evaporate the hydrofluoric acid, followed by sealed dissolution with nitric acid. After appropriate dilution, the content of trace elements in the samples was directly determined using the external standard method of ICP-MS.

The trace element mapping of sphalerite were ascertained through a NWR 193 nm ArF Excimer laser-ablation system coupled to an iCAP RQ ICPMS at the Guangzhou Tuoyan Analytical Technology Co., Ltd. (Guangzhou, China). The examination of the sample included assessing 32 isotopes, noting the dwell times in milliseconds as follows, cumulatively amounting to a dwell time of 160 ms: ²⁷Al (5), ²⁹Si (5), ³⁹K (5), ⁴³Ca (5), ⁵⁷Fe (5), ⁷¹Ga (5), ⁷³Ge (5), ⁸²Se (5), ⁸⁵Rb (5), ⁸⁹Y (5), ⁹¹Zr (5), ⁹³Nb (5), ¹¹¹Cd (5), ¹¹⁸Sn (5), ¹²⁶Te (5), ¹³³Cs (5), ¹³⁹La (5), ¹⁴⁰Ce (5), ¹⁴¹Pr (5), ¹⁴⁶Nd (5), ¹⁴⁷Sm (5), ¹⁵³Eu (5), ¹⁵⁷Gd (5), ¹⁵⁹Tb (5), ¹⁶³Dy (5), ¹⁶⁵Ho (5), ¹⁶⁶Er (5), ¹⁶⁹Tm (5), ¹⁷²Yb (5), ¹⁷⁵Lu (5), ¹⁷⁸Hf (5), ¹⁸¹Ta (5), ¹⁸²W (5), ¹⁸⁵Re (5), and ²⁰⁵Tl (5). The laser emitted a fluence of 3.5 J/cm², repeated at a rate of 25 Hz, and had a 5 μm spot size, equivalent to a scanning speed of 15 μm/s. Calibration of the trace elements was conducted using the NIST 610 as an external benchmark.

5.3. In Situ Sulfur Isotope Analysis

The in situ sulfur isotope analysis of sulfide was performed with LA-MC-ICP-MS at Guangzhou Tuoyan Analytical Technology Co., Ltd., (Guangzhou, China). The employed laser-stripping apparatus was a 193 nm excimer laser-stripping system (RESOlution M50-LR, ASI), produced by ASI, Australia. The sulfur isotope testing was performed with a high-resolution multi-collector inductively coupled plasma mass spectrometer (Nu Plasma 1700 MC-ICP-MS) produced by Nu Instruments, UK. The laser exhibited an energy density (fluence) of 3.6 J/cm², a frequency of 3 Hz, a spot size ranging from 25 to 37 μm, and a single-point stripping mode. The mode used for gathering data was TRA mode, with an integration duration of 0.2 s, a background capture time of 30 s, a sample integration period of 50 s, and a purge duration of 75 s. Other parameters of the instrument were set as follows: RF power at 1300 W; Neb atomization gas injection at 0.8 mL/min; plasma gas injection at 13 L/min. All the internal benchmarks of S isotopes were ascertained using gas-stable isotope mass spectrometry (GSIMS) or MC-ICP-MS with solution injection, employing IAEA-S-1, IAEA-S-2, and IAEA-S-3 (Ag₂S powder) as the benchmark samples for this analysis. The detailed analytical methods used are described in [42,43,44].

6. Results

6.1. Trace Element Compositions of Sulfides

The trace element compositions of sphalerite, pyrite, and galena in the Chipu deposit, including Fe, Mn, Cu, Ga, Ge, In, Cd, Ag, As, Sn, Sb, Tl, Bi, Rb, and Pb, with the specific findings are presented in

Table 1. Trace elements in sulfide minerals from the Chipu Pb-Zn deposit (ppm).

Sphalerite	Mn	Fe	Co	Cu	Ga	Ge	As	Ag	Cd	In	Sn	Sb	Pb
CP-01A-2	319	282	0.26	1045	66	123	434	30	1006	166	7.64	454	38
	357	531	0.94	1881	84	213	111	37	992	182	8.79	895	76
	352	463	0.80	1292	56	162	68	27	1034	205	5.70	531	45
CP-01A-3	344	490	-	1809	59	158	116	38	998	277	5.09	915	101
	335	587	-	1375	98	195	56	31	984	472	18	333	28
	336	653	0.28	1573	155	395	22	19	959	552	24	59	38
	342	859	1.88	1255	115	64	36	14	972	679	130	209	15
	348	862	2.56	1804	223	60	89	18	984	720	90	588	75
	348	695	2.30	2309	420	120	119	28	999	562	42	848	82
CP-01A-4	372	528	2.30	779	41	15	50	14	1001	249	4.79	449	122
	367	517	2.50	1352	29	50	109	21	967	260	0.95	855	220
CP-01B-2	344	937	1.68	1438	88	100	113	31	899	31	1.45	1022	183
	334	888	1.61	1940	17	154	137	21	886	48	1.53	1388	224
	323	847	1.83	4711	171	444	399	57	861	90	1.77	3337	1005
	326	730	1.20	467	75	78	8.51	5.07	894	107	14	69	14
	319	557	1.24	706	175	71	5.99	5.53	929	176	51	68	2.43
CP-02B-1	351	640	0.89	735	23	5.21	17	12	975	400	94	194	192
	349	598	0.59	2841	85	48	165	43	985	452	121	1703	485
	370	1154	0.71	6381	127	241	326	90	993	516	45	4087	889
	351	561	0.36	2186	152	22	84	19	1020	400	9.54	1486	265
	355	483	0.59	661	46	3.87	18	8.77	1145	280	83	158	84
CP-03	96	2272	1.69	950	36	80	27	34	868	606	13	146	141
	94	2251	1.73	1392	43	295	22	19	835	628	4.12	55	22
	97	2200	1.63	798	32	17	28	23	879	618	24	117	12
	105	2179	1.69	2154	132	121	109	52	935	541	109	759	43
	109	2169	1.50	1019	100	51	27	15	995	406	146	184	8.45
	98	2261	1.47	1475	25	325	44	37	851	514	3.75	141	18
	100	2219	1.77	1343	43	278	27	25	869	593	30	85	15
	104	2186	1.67	1313	30	142	83	44	873	610	47	301	59
	109	2284	2.03	713	50	21	24	17	977	458	38	147	16
	118	2160	0.95	1516	235	109	70	41	1079	297	59	466	28
Pyrite	Mn	Fe	Co	Cu	Ga	Ge	As	Ag	Cd	In	Sn	Sb	Pb
CP-01A-2	-	460000	21	344	0.49	93	909	3.15	4.35	5.16	33	85	23
CP-01A-3	174	460000	-	215	0.02	5.55	4269	8.88	0.47	-	-	190	280
CP-01B-2	7.51	460000	68	1702	2.88	37	653	61	39	3.83	1.51	755	11383
CP-02A	342	460000	7.36	183	-	4.53	4409	40	1.81	-	-	335	1598
CP-02B	12	460000	373	281	0.05	5.85	2743	31	0.59	-	-	122	1177
CP-03	-	460000	0.86	1468	0.69	12	446	125	0.34	-	-	833	828
Galena	Cu	Co	Ga	Cd	In	Tl	Re	Sb	Bi	Rb	V	Pb	W
CP-01A	189	0.10	0.49	8.59	0.12	7.99	-	1001	5.11	0.30	71	/	0.13
CP-01B	20	0.72	0.21	14	0.50	7.84	-	771	4.99	0.10	5.73	/	0.06
CP-02A	85	0.21	0.16	6.34	0.15	7.97	-	2800	5.28	0.12	26	/	0.11
CP-02B	53	0.14	0.40	8.28	0.19	7.27	-	661	4.82	0.13	58	/	0.05
CP-03	40	0.07	0.48	13	0.05	9.42	-	683	5.35	0.10	70	/	0.05

“-” means concentration lower than detection limits; “/” means concentration higher than detection limits.

and Figure 5 (the raw data are listed in Table S1). Gallium is mainly found in sphalerite (Figure 5), which has a high level content of 17~420 ppm. In contrast, pyrite and galena show lower concentrations of Ga, with 0.16~0.49 ppm in galena, and the highest Ga content in pyrite is 2.88 ppm. Sphalerite contains the highest concentration of germanium, with concentrations between 3.87 and 444 ppm, potentially attributable to the irregular dispersal of Ge within the sphalerite. The Ge content in pyrite is 4.53~93 ppm, and that in galena is lower than the detection limit. The indium shows significantly higher concentrations in sphalerite, mainly between 31 and 720 ppm. The levels of In in galena and pyrite are notably low, with the highest content in galena being 0.50 ppm, with the majority of other elements being below the detection limit, while the highest indium content in pyrite is 5.16 ppm.

Apart from the dispersed Ga, Ge, and In, the analytical examination further shows that the patterns of enrichment for other trace elements in sulfides are markedly distinct. Sphalerite is predominantly enriched with Mn, Cu, Cd, and Sn, with respective average concentrations of 272 ppm, 1670 ppm, 963 ppm, and 40 ppm. The pyrite is predominantly enriched with As, with an average content of 2238 ppm. Galena shows a relative enrichment in Sb, Tl, and V, with respective average concentrations of 1183 ppm, 8.10 ppm, and 46 ppm. In summary, sphalerite predominantly serves as the primary carrier for the dispersed Ga, Ge, and In in the Chipu deposit.

6.2. Sulfur Isotopic Compositions

The S isotope results for different sulfides in the Chipu deposit are listed in

Table 2. LA-MC-ICP-MS in situ S-isotope composition of various sulfides from the Chipu Pb-Zn deposit.

Sample No.	Measured Mineral	δ34SV-CDT (‰)	2SE
CP-01A-2	sphalerite	+24.13	0.18
CP-01A-2	sphalerite	+23.83	0.19
CP-01A-3	sphalerite	+24.74	0.18
CP-02A-1	sphalerite	+24.31	0.18
CP-02A-1	sphalerite	+23.06	0.21
CP-01B-2	sphalerite	+19.64	0.20
CP-02B-3	sphalerite	+17.72	0.21
CP-01-1	sphalerite	+16.20	0.19
CP-01-5	sphalerite	+17.50	0.24
CP-02-3	sphalerite	+20.15	0.20
CP-03-1	sphalerite	+24.11	0.18
CP-03-1	sphalerite	+18.30	0.19
CP-01A-3	sphalerite	+12.37	0.21
CP-01B-2	sphalerite	+14.70	0.21
CP-02B-2	sphalerite	+8.40	0.20
CP-02B-2	sphalerite	+10.51	0.19
CP-02B-3	sphalerite	+14.84	0.19
CP-01-1	sphalerite	+13.02	0.20
CP-02-2	sphalerite	+11.23	0.18
CP-02-3	sphalerite	+8.66	0.85
CP-01A-1	galena	+13.40	0.40
CP-01B-1	galena	+13.36	0.27
CP-02B-1	galena	+13.23	0.23
CP-02-1	galena	+13.31	0.24
CP-03-1	galena	+16.37	0.24
CP-02B-1	pyrite	+10.47	0.23
CP-01-5	pyrite	+3.48	0.25
CP-02-3	pyrite	+4.26	0.24
CP-02-3	pyrite	+5.09	0.23

. Across twenty samples of sphalerite, the δ³⁴S_V-CDT values span from +8.40‰ to +24.74‰, averaging at +17.37‰. The δ³⁴S_V-CDT values of the five galena samples ranged from +13.23 to +16.37‰, averaging at +13.93‰. The δ³⁴S_V-CDT values of the four pyrite samples ranged widely between +3.48 and +10.47‰, averaging +5.82‰ (Table 2).

7. Discussion

7.1. Distributions and Substitutions of Trace Elements in Sphalerite

As mentioned above, the sphalerite in the Chipu deposit predominantly concentrates Ga, Ge, and In (Figure 5). The trace elements within sphalerite are predominantly present as solid solutions, manifesting as nanoparticles or mineral microinclusions [45,46]. The fluctuations in LA-ICP-MS signals from time-resolved depth profiles can be used to effectively determine the occurrence states of trace elements. Typically, a “flat” signal spectrum signifies either a solid solution or a uniformly distributed nanoparticle, whereas strong signal fluctuations always indicate an uneven distribution of mineral microinclusions or nanoparticles [17,45,47,48].

This research reveals that most trace elements, including Mn, Ga, Ge, In, Fe, Cu, As, Ag, Sn, Sb, Pb, and Cd, display a level profile (as shown in Figure 6), indicating their existence in solid solutions or uniformly distributed nanoparticles. The mapping image of a single sphalerite grain (Figure 7) reveals distinct zoning patterns among critical dispersed metals. Ga and Ge exhibit heterogeneous distributions, with their enrichment zones closely overlapping those of Cu and displaying concentric zoning patterns concentrated in the core and rim domains of the sphalerite, corresponding to its dark-brown colored regions. This spatial correspondence suggests a strong correlation among Ga, Ge, and Cu. In contrast, In shows a relatively homogeneous distribution that aligns spatially with Fe-enriched regions, indicating a robust positive correlation between In and Fe.

The frequent occurrence of coupling substitution in sphalerite is attributed to the similarity in the radii of Ga³⁺ (0.62 Å), Ge⁴⁺ (0.53 Å), In³⁺ (0.81 Å), Fe²⁺ (0.72 Å), Cu⁺ (0.96 Å), Sn⁴⁺ (0.71 Å), and Sn²⁺ (0.93 Å) with that of Zn²⁺ (0.74 Å) [11,44]. A variety of possible substitution mechanisms for the incorporation of Ga, Ge, and In into the sphalerite lattice have been proposed, such as Ga³⁺ + (Ag⁺, Cu⁺) ↔ 2Zn²⁺, Ge⁴⁺ + 2(Ag⁺, Cu⁺) ↔ 3Zn²⁺, In³⁺ + Cu⁺ ↔ 2Zn²⁺, In³⁺ + Sn²⁺ + (Ag⁺, Cu⁺) ↔ 3Zn²⁺, and In³⁺ + Sn⁴⁺ + (Ag⁺, Cu⁺) + □ ↔ 4Zn²⁺ [7,17,47,49,50,51,52,53,54]. In instances of copper-deficient sphalerite (0.29~19.20 ppm), Ge can be incorporated into the sphalerite through a coupled substitution with Pb²⁺ [55].

As shown in Figure 8a,b, the Chipu sphalerite exhibits elevated levels of Ga, Ge, and Cu relative to other deposits, along with Ga/Cu = 1 and Ge/Cu = 1. This is akin to the coupled substitution of Ga and Ge with Cu+ observed in other deposits [56,57]. Based on the spatial correlations revealed by mapping images among Ga, Ge, and Cu, coupled with the “flat” signal of Ga and Ge observed in time-resolved depth profiles, we propose that these elements primarily incorporate into the sphalerite lattice via isomorphic substitution, indicative of the substitution mechanism involving Ga³⁺ + Cu⁺ ↔ 2Zn²⁺ and Ge⁴⁺ + 2Cu⁺ ↔ 2Zn²⁺. Notably, the marked association between Cu + Ag and Ga + Ge + Sn + Sb, following an equality line, implies a probable integration of these elements into sphalerite via coupled substitution (Figure 8e).

In is strongly correlated with Fe (Figure 7 and Figure 8c), and negatively correlated with Cu (Figure 8d), indicating that In enters the sphalerite lattice mainly through coupling with Fe²⁺ and that the coupling substitution mechanism probably occurs through 2In³⁺ + Fe²⁺ ↔ 4Zn²⁺. A significant positive correlation exists between Cu + Ag and Ga + Ge + Sn+ Sb (Figure 8e), with the addition of In strengthens the relationship of element coupling at a ratio of (Ga + Ge + In + Sn + Sb)/(Cu + Ag) =1 (Figure 8f), indicating that the multi-elemental coupling mechanism also promotes the entry of In into sphalerite.

7.2. Precipitation Temperature of Sphalerite

The trace element composition of sphalerite is closely linked with its precipitation temperature [58,59,60]. Various geothermometry methods for sphalerite have been proposed [59,60,61]. In this research, taking into account the precision and applicability of various geothermometers, the geothermometry developed by [60] was employed to determine the precipitation temperature of sphalerite in the Chipu deposit. The calculation method of geological thermometers is proposed as the following equation:

T(°C) = − (54.4 ± 7.3) · PC1* + (208 ± 10)

(1)

$$\mathrm{P}\mathrm{C}{1}^{*}=\ln \left[\right({\mathrm{C}}_{\mathrm{Ga}}^{0.22}·{\mathrm{C}}_{\mathrm{Ge}}^{0.22})/({\mathrm{C}}_{\mathrm{Fe}}^{0.37}·{\mathrm{C}}_{\mathrm{Mn}}^{0.20}·{\mathrm{C}}_{\mathrm{In}}^{0.11}\left)\right]$$

(2)

in which C_Ga, C_Ge, C_In, and C_Mn are given in ppm, and C_Fe is given in wt.%. The results are shown in

Table 3. Calculation results of the sphalerite mineralization temperature of the Chipu Pb-Zn deposit.

Sample	PC1*	T1/°C	T2/°C	T3/°C	T4/°C	Tmax/°C	Tmin/°C	Tmean/°C
CP-01A-2	1.6	120	143	100	123	143	100	122
	1.5	126	148	106	128	148	106	127
	1.4	133	153	113	133	153	113	133
CP-01A-3	1.3	135	155	115	135	155	115	135
	1.4	133	153	113	133	153	113	133
	1.6	121	144	101	124	144	101	122
	1.0	157	172	137	152	172	137	154
	1.1	150	166	130	146	166	130	148
	1.5	125	147	105	127	147	105	126
CP-01A-4	0.7	174	184	154	164	184	154	169
	0.9	163	176	143	156	176	143	159
CP-01B-2	1.3	136	155	116	135	155	116	136
	1.0	154	169	134	149	169	134	151
	1.7	111	136	91	116	136	91	114
	1.2	144	161	124	141	161	124	142
	1.4	130	151	110	131	151	110	131
CP-02B-1	0.2	203	207	183	187	207	183	195
	1.0	155	170	135	150	170	135	152
	1.2	144	161	124	141	161	124	143
	1.0	155	170	135	150	170	135	153
	0.5	189	196	169	176	196	169	183
CP-03	0.7	176	186	156	166	186	156	171
	1.0	155	170	135	150	170	135	153
	0.3	198	202	178	182	202	178	190
	1.1	152	168	132	148	168	132	150
	0.8	166	178	146	158	178	146	162
	0.9	161	174	141	154	174	141	158
	1.0	157	171	137	151	171	137	154
	0.8	170	182	150	162	182	150	166
	0.5	189	196	169	176	196	169	182
	1.2	143	161	123	141	161	123	142

.

The sphalerite mineralization temperatures range from 114 °C to 195 °C (Table 3; Figure 9a), consistent with the homogenization temperature range (109–248 °C) measured by fluid inclusions within quartz in previous studies (109~248 °C) [22]. In summary, the mineralization of the deposit generally occurred under medium to low temperatures. Furthermore, as temperature decreases, the concentrations of Ga and Ge in sphalerite show varying degrees of increase, while the concentration of In exhibits no discernible trend with temperature variation (Figure 9b–d). Temperature serves as a critical factor controlling the enrichment of Ga and Ge in sphalerite. Although the detailed processes governing the enrichment of these scattered elements (Ga and Ge) in sphalerite remain unclear, numerous studies have demonstrated that their enrichment (100–200° C) is closely associated with medium- to low-temperature mineralizing fluids [7,50]. In the Sichuan-Yunnan-Guizhou region, the Late Permian Emeishan mantle plume magmatic activity generated extensively distributed flood basalts with thicknesses ranging from hundreds to thousands of meters, along with substantial accumulations of mafic magmatic residues retained in the deep crust [62,63]. The Emeishan basalts exhibit elevated Ge (1.0 × 10⁻⁶~2.3 × 10⁻⁶) and Ga (20 × 10⁻⁶~25 × 10⁻⁶) contents [62,64]. These elements (Ge and Ga) within the basalts and deep crustal mafic magmatic residues could be leached by hydrothermal activities during metamorphism and alteration processes. Consequently, the Ga and Ge in sphalerite from the Chipu deposit likely originated from the Emeishan basalts, with the enrichment of these elements in sphalerite resulting from the extraction of source layers by mineralizing fluids (basinal brines).

7.3. Sources of Sulfur

In the Chipu deposit, the predominant sulfides are sphalerite, pyrite, and galena (Figure 3), with a lack of sulfate minerals. Consequently, the sulfur isotopic signatures of sulfides can be used to approximate the isotopic compositions of the ore-forming fluid [65,66]. In the Chipu deposit, the sulfide δ³⁴S measurements during the ore-stage range between +3.48‰ and +24.74‰, with an overall average of approximately +15.19‰ for all the sulfides. In the SYG metallogenic province, the reduced sulfur in the majority of Pb-Zn deposits is typically believed to originate from marine sulfates (e.g., Huize; Tianqiao; Maoping; Zhugongtan) [36,56,67,68]. Several processes can convert marine sulfates into H₂S, predominantly through bacterial sulfate reduction (BSR) and thermochemical sulfate reduction (TSR) [66]. BSR is mainly characterized by large negative δ³⁴S values [69], and TSR is typically characterized by positive δ³⁴S values [70]. Moreover, all δ³⁴S measurements of the sulfate in the Chipu deposit are positive and the formation temperature of the sphalerite ranges 122.25 to 206.55 °C, which is more favorable for TSR (commonly >130 °C) [71,72,73] than for BSR (generally 50~70 °C, with a maximum of 110 °C) [71,74]. Consequently, we infer that the reduced S in the Chipu deposit is sourced from marine sulfates in the mineral-bearing carbonates, through the process of TSR (Figure 10).

It has been reported that the δ³⁴S measurements of marine sulfates within the Dengying Formation range from +20.2‰ to +38.7‰ (Figure 10) [75]. It is hypothesized that the TSR process involving reduced sulfur and marine sulfates results in sulfur isotope fractionation of about 20‰ at 100 °C, 15‰ at 150 °C, and 10‰ at 200 °C [71]. Through TSR at 200 °C, which corresponds to the mean temperature of the ore-forming fluid, marine sulfates are expected to yield reduced sulfur with δ³⁴S values between +10‰ and +30‰, aligning with the S isotopic composition of the Chipu sulfides (Figure 10). Consequently, the sulfur in the Chipu deposit’s sulfides is believed to have originated from marine sulfates in the Dengying Formation, with reduced sulfur from TSR potentially contributing to the mineralization process and the formation of metal sulfides.

7.4. The Extraordinary Indium Enrichment in Sphalerite and Exploration Potential

Our study revealed that the sphalerite found in the Chipu deposit contains Ga and Ge levels akin to those in other deposits. However, the In content in the Chipu sphalerite is exceptionally high, ranging between 31 and 720 ppm (averaging 390 ppm), significantly surpassing the levels found in other Pb-Zn deposits within the SYG Pb-Zn metallogenic province (e.g., Daliangzi; Huize; Maoping; Tianbaoshan) (Figure 11) [17,19,20,76]. The Pb-Zn ore body of this deposit is found in the dolomite stratum of the Dengying Formation of the Upper Sinian Series, close to the basement stratum, and is obviously controlled by the local geological structure. In addition, some studies have reported that intermediate-felsic igneous rocks are easily enriched in In, with In content reaching up to 0.26 ppm [4,18].

Indium is predominantly hosted by sphalerite, which constitutes 95% of the global indium reserves [8]. The three main factors controlling the entry of In into sphalerite are as follows: (1) the ionic radius and similar ionic radius, which are more conducive to coupling substitution between elements [11]; (2) the ore-forming fluid temperature, where an increase in temperature enhances In enrichment [77]; (3) the enrichment of elements in sphalerite.

The main mineralization ages based on sphalerite and calcite dating in the Pb-Zn deposits along the western margin of the Yangtze Block range from 232 to 192 Ma (Figure 12). The Emeishan large igneous province is situated on the western margin of the Yangtze Block and features a large amount of Emeishan basalt, which is a product of mantle plume magmatic activity that occurred between 261 and 257 Ma [33,34]. The mineralization period of the Chipu Pb-Zn deposit is estimated to be between 230 and 200 Ma [78], which is subsequent to the mantle plume magmatic activity linked to the Emeishan large igneous province. However, paleomagnetic dating of six major Pb-Zn deposits across North America has revealed that Pb-Zn mineralization could span a duration of up to 25 Ma [5]. Thermal simulation studies on the evolution of the underplated Emeishan basalt indicate that the release of metal-containing fluid started to release 30 Ma post-underplating initiation, aligning with the age records of the Chipu deposit in this area. The simulation studies on the evolution of Emeishan basalt underplating suggest that the release of ore-forming fluid from the crystallizing underplated basalts can last for more than 100 Ma, encompassing nearly the entire range of SYG Pb-Zn mineralization age data [79]. Additionally, the reported metallogenic depth of the Chipu Pb-Zn deposit primarily ranges from 0.54 to 1.42 km [39]. If only the geothermal gradient (30 °C/km) is considered as the thermal source for metallogenesis, the theoretical mineralization temperature would be far below 100 °C, which is significantly lower than the actual mineralization temperature range (114–195 °C). Therefore, the Emeishan mantle plume activity likely served as the primary thermal energy source for mineralization, providing the majority of the thermal energy required for basinal brine circulation. Thus, the Pb-Zn mineralization along the western margin of the Yangtze Block is likely associated with the mantle plume magmatic activity that characterized the Emeishan large igneous province [33]. Furthermore, a mantle upwelling in the Emeishan large igneous province led to crustal stretching and increased heat flow. This deep mantle activity also stimulated fluid circulation in the shallow crust, which in turn facilitated the formation of the Chipu deposit. Like the Huayuan Zn-Pb orefield, its origin is associated with the Cambrian mantle upwelling that occurred in the northeast region of Gondwana [80].

As mentioned above, the magmatic activity linked to the Emeishan mantle plume could have facilitated the circulation of basinal brines, which in turn may have led to the extraction of In from the intermediate-felsic igneous rocks of the metamorphic basement. The increased presence of In in sphalerite is due to the similarity in geochemical properties between In and Zn, where elevated temperatures also enhance the synergistic incorporation of Fe and In into sphalerite. The In-enriched basinal brines, transported upward along the deep Malaha fault, accumulated in the high-porosity and high-permeability dolomite strata of the Dengying Formation, leading to the formation of the Chipu deposit with an abnormal enrichment of dispersed In. Furthermore, this study has positive implications for the exploration potential of Pb-Zn deposits with the same high In content characteristics on the western margin of the Yangtze Block.

8. Conclusions

(1)

Within the Chipu Pb-Zn deposit, sphalerite is mainly enriched in Mn, Cu, Ga, Ge, In, Cd, and Sn. Pyrite is mainly enriched in As. Galena is relatively enriched in Sb, Tl, and V. The dispersed Ga, Ge, and In are primarily enriched in sphalerite, and the substitution mechanisms include Ga³⁺ + Cu⁺ ↔ 2Zn²⁺, Ge⁴⁺ + 2Cu⁺ ↔ 3Zn²⁺, and 2In³⁺ + Fe²⁺ ↔ 4Zn²⁺.

(2)

Geological features, geochemical studies, and sphalerite geothermometry results reveal that the mineralization processes in the Chipu deposit occurred within a range of medium to low temperatures (114–195 °C).

(3)

In situ sulfur isotopes from sphalerite, pyrite, and galena in the Chipu Pb-Zn deposit exhibit δ³⁴S values ranging from +3.48 to +24.74‰, suggesting that marine sulfate present in the ore-bearing strata of the Dengying Formation is the predominant source of sulfur for the Chipu deposit.

(4)

The abnormal accumulation of In in sphalerite from the Chipu deposit may be related to the circulation of basinal brine promoted by the magmatic activity associated with the Emeishan mantle plume, and the basinal brine extracted significant amounts of In from the intermediate-felsic igneous rocks in the metamorphic basement.