Tennantite–Tetrahedrite-Series Minerals and Related Pyrite in the Nibao Carlin-Type Gold Deposit, Guizhou, SW China

: A number of sediment-hosted, Carlin-type/-like gold deposits are distributed in the Youjiang basin of SW China. The gold ores are characterized by high As, Hg, and Sb contents but with low base metal contents (Cu+Pb+Zn < 500–1000 ppm). The Nibao deposit is unique among these gold deposits by having tennantite–tetrahedrite-series minerals in its ores. The deposit is also unique in being primarily hosted in the relatively unreactive siliceous pyroclastic rocks, unlike classic Carlin-type gold deposits that are hosted in carbonates or calcareous clastic rocks. In this study, we have identiﬁed tennantite-(Zn), tennantite-(Hg), and tetrahedrite-(Zn) from the tennantite–tetrahedrite-series mineral assemblage. The tennantite-(Zn) can be further divided into two sub-types of Tn-(Zn)-I; and Tn-(Zn)-II;. Tn-(Zn)-I; usually occurs in the core of a Tennantite– tetrahedrite composite and appears the darkest under the SEM image, whereas Tn-(Zn)-II overgrows on Tn-(Zn)-I and is overgrown by tetrahedrite-(Zn). Tennantite-(Hg) occasionally occurs as inclusions near the uneven boundary between Tn-(Zn)-I and Tn-(Zn)-II. An appreciable amount of Au (up to 3540 ppm) resides in the tennantite–tetrahedrite-series minerals, indicating that the latter is a major Au host at Nibao. The coexistence of tennantite–tetrahedrite-series minerals and Au-bearing pyrite indicates the Nibao ore ﬂuids were more oxidized than the Carlin-type ore ﬂuids. The tennantite– tetrahedrite series at Nibao evolved from Tn-(Zn)-I through Tn-(Zn)-II to tetrahedrite-(Zn), which is likely caused by Sb accumulation in the ore ﬂuids. This indicates that the Nibao ore ﬂuids may have become more reduced and less acidic during Au precipitation.

In the Carlin-type gold deposits of the Youjiang basin, recent in situ analytical studies have demonstrated that the ore-stage pyrite is rich in Cu (≈800 ppm; Figure 2; [11,[33][34][35]), and the Cu is suggested to have been leached and transported together with Au in the basin [36]. However, the occurrence of Cu minerals, such as those of the tennantite-tetrahedrite-series, is confined only to a few deposits such as Nibao, and little is known about the chemistry or origin of Cu minerals in these deposits in the Youjiang basin.
In the Carlin-type gold deposits of the Youjiang basin, recent in situ analytical studies have demonstrated that the ore-stage pyrite is rich in Cu (≈800 ppm; Figure 2; [11,[33][34][35]), and the Cu is suggested to have been leached and transported together with Au in the basin [36]. However, the occurrence of Cu minerals, such as those of the tennantitetetrahedrite-series, is confined only to a few deposits such as Nibao, and little is known about the chemistry or origin of Cu minerals in these deposits in the Youjiang basin.

Regional Geology
The Youjiang basin is situated on the southwestern (SW) margin of the Yangtze craton ( Figure 1A). The basin is bordered by the Mile-Shizong fault in the northwest, the Ziyun-Yadu fault in the northeast, the Red River fault in the southwest, and the Pingxiang-Nanning fault in the southeast ( Figure 1B). The Yangtze craton is interpreted by many authors to have amalgamated with the Cathaysia block to form South China along the Jiangnan orogen at ca. 830 Ma [43,44]. Devonian rifting of the SW margin of the Yangtze craton may have formed the Youjiang basin and produced a series of NWand NE-striking high-angle basinal faults that controlled the subsequent sedimentation, deformation, and magmatism [3,16,[45][46][47].
The continuous Devonian rifting led to the opening of the Devonian-Carboniferous Ailaoshan-Song Ma Paleo-Tethyan ocean branch, and the subsequent extensive deposition of calcareous sandstone, siltstone, shale, and carbonate [47][48][49][50]. These rocks host the majority of Carlin-type gold deposits in the Youjiang basin [16]. Following the passivemargin sedimentation, the basin was deformed during the Early Triassic Indochina orogeny, which was caused by the collision between the Indochina and South China blocks [51]. In the late Mesozoic, the Paleo-Pacific subduction beneath South China had likely produced numerous NE-trending folds and faults in the Youjiang basin [52].

Deposit Geology
The Nibao gold deposit is situated on the edge of the ELIP flood basalt in the Youjiang basin ( Figure 1B), and it hosts a resource (measured plus indicated) of over 60 t Au at ≈2.0 g/t [18,35,55]. The Au mineralization age determined by apatite Th-Pb dating is ≈141 Ma [17]. Gold mineralization is dominantly fault-controlled and partly stratabound at Nibao ( [55,56]; Figure 3).
Local stratigraphic sequence is dominated by carbonate and pyroclastic rocks. The oldest sequence revealed by drilling is the Middle Permian Maokou Formation (Fm.) limestone (> 100 m thick), which is overlain by the silicified and brecciated Dachang Bed (≈42 m) [55][56][57]. Above that lies the Emeishan Formation, which consists of conglomerate, sandstone, siltstone, shale, and coal at the bottom and ELIP basaltic pyroclastic rocks at the top ( Figure 4A-E). In turn, this is overlain by the Longtan Fm. (≈20 m) interbedded siltstone, mudstone, coal, and limestone. The Emeishan and Longtan Formations are a minor ore host (Orebodies I, II, IV, V and VI). Tennantite-tetrahedrite-series minerals are mainly hosted in the ELIP basaltic pyroclastic rocks of the Emeishan Fm. and siltstone and limestone of Longtan Fm. The Middle Triassic Guanling Formation is distributed in SE Nibao and composed of thick limestone and dolostone (≈460 m). Among these sequences, the Emeishan Formation and Dachang Bed are the main ore host ( [56][57][58]; Figure 3B).
Dominant ore-controlling structures at Nibao are the ENE-trending F1 thrust fault, which may have formed by the Indochina orogeny [55], which has developed to be a brecciated alteration zone by later tectono-hydrothermal events ( Figures 3B and 4F). Brecciated rocks in F1 thrust fault host the majority of ores (Orebodies III). The F1 thrusting may have also formed the NE-trending Erlongqiangbao anticline. The NS-to NW-striking faults at Nibao may have formed by an NE-SW-directed compressional event [55]. Geologic mapping and drilling have yet to identify any igneous intrusions at Nibao.
At Nibao, ore-related alteration is pervasive and includes a combination of decarbonate, silicic, sulfide, argillic, and dolomite styles [35,57]. Decarbonation (carbonate dissolution) is caused by the calcareous nature of the ore host and the acidic ore-forming fluids ( [11,56,57]; Figure 4G). Silicification replaced carbonates in the rocks, and sulfidation involved the sulfide/sulfate precipitation from the H 2 S-bearing ore fluids when they reacted with Fe-bearing wallrocks [21]. Argillization has formed clay minerals such as illite and kaolinite in the ores, whilst dolomitization is manifested by ore-cutting Fe-dolomite veins. At Nibao, ore-related alteration is pervasive and includes a combination of decarbonate, silicic, sulfide, argillic, and dolomite styles [35,57]. Decarbonation (carbonate dissolution) is caused by the calcareous nature of the ore host and the acidic ore-forming

Sampling and Analytical Methods
In this study, sixty samples were collected from the Nibao Orebodies Ⅰ, Ⅱ, Ⅲ , and Ⅴ ( Figure 3B). The samples were prepared into thin sections to conduct optical microscopic observations and SEM textural and mineralogical analyses. Subsequently, six samples with tennantite-tetrahedrite-series minerals were selected for the EPMA and LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) analyses.
A ∑IGMA Field Emission SEM (Carl Zeiss Microscope Co., Ltd., Oberkochen, Germany) at the Guangxi Key Laboratory of Exploration for Hidden Metallic Ore Deposits (Guilin University of Technology) was used for mineral identification and semi-quantitative mineral geochemical analysis. High-resolution SEM imaging was also conducted to observe textural and mineral paragenetic relations and to provide a guide for positioning the EPMA spots.
The EPMA data were collected with a JXA8230 EPMA (Japan Electron Optics Lab., Tokyo, Japan) at the same laboratory as the SEM analysis. The instrument was operated under the conditions of 15 kV, 20 nA, 2-5 μm-diameter beam size, and measurement of time 10 s (sample) and 5 s (background). The following standards were used: bismuth selenide for Se, arsenopyrite for As, galena for Pb, Greenote (a standard from the Structure Probe, Inc., West Chester, USA) for Cd, manganese oxide for Mn, skutterudite for Co,

Sampling and Analytical Methods
In this study, sixty samples were collected from the Nibao Orebodies I, II, III and V ( Figure 3B). The samples were prepared into thin sections to conduct optical microscopic observations and SEM textural and mineralogical analyses. Subsequently, six samples with tennantite-tetrahedrite-series minerals were selected for the EPMA and LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) analyses.
A ∑IGMA Field Emission SEM (Carl Zeiss Microscope Co., Ltd., Oberkochen, Germany) at the Guangxi Key Laboratory of Exploration for Hidden Metallic Ore Deposits (Guilin University of Technology) was used for mineral identification and semi-quantitative mineral geochemical analysis. High-resolution SEM imaging was also conducted to observe textural and mineral paragenetic relations and to provide a guide for positioning the EPMA spots.
LA-ICP-MS pyrite trace element spot analysis and mapping were performed at the Centre for Ore Deposit and Earth Sciences (CODES), University of Tasmania. The instrument consists of a Coherent 193 nm ArF gas-charged excimer laser (Coherent Inc., Wilsonville, OR, USA) and an Agilent 7700 quadrupole ICP-MS (Agilent Tech., Palo Alto, CA, USA). Detailed analytical procedure and operating conditions are as described by Danyushevsky et al. (2011) [59] and Steadman et al. (2015) [60]. The beam sizes chosen (19 to 39 µm) in the spot analyses were dependent on the pyrite size and texture. The spot was pre-ablated with five pulses to eliminate surface contamination. The background was measured for 30 s (laser-off) prior to an 11-60 s measurement of the sample (laser-on). Helium was used as a carrier gas (flux rate: 0.71 L/min). Element mapping was conducted under similar conditions as spot analysis except with a smaller beam diameter (10 or 15 µm) and a 10 Hz pulse rate. A series of parallel lines were ablated at a speed of 10 µm/s. Then, the data obtained from parallel ablation lines were used to create trace element images, as described in detail by Steadman et al. (2013) [61].

Textures and EMPA Spot Analyses of Tennantite-Tetrahedrite-Series Minerals
Tennantite-tetrahedrite-series minerals in our samples show a wide range of textures and chemical compositions. Three minerals were recognized, i.e., tennantite-(Zn), tennantite-(Hg), and tetrahedrite-(Zn). Tennantite-(Zn) can be further divided into Tn-(Zn)-I and Tn-(Zn)-II (as described below). A paragenetic assemblage associated with the gold mineralization, involving arsenian pyrite, arsenopyrite, chalcopyrite, sphalerite, tennantitetetrahedrite-series minerals, reaglar-orpiment, quartz, illite, and kaolinite, is present in Figure 5 [17,35,56]). The EMPA spot results, including those of Tn-(Zn)-I (n = 14), Tn-(Zn)-II (n = 124), tennantite-(Hg) (n = 5), and tetrahedrite-(Zn) (n = 35), are summarized in Table 1, with the complete dataset given in Supplementary Table S1.  [60]. The beam sizes chosen (19 to 39 μm) in the spot analyses were dependent on the pyrite size and texture. The spot was preablated with five pulses to eliminate surface contamination. The background was measured for 30 s (laser-off) prior to an 11-60 s measurement of the sample (laser-on). Helium was used as a carrier gas (flux rate: 0.71 L/min). Element mapping was conducted under similar conditions as spot analysis except with a smaller beam diameter (10 or 15 μm) and a 10 Hz pulse rate. A series of parallel lines were ablated at a speed of 10 μm/s. Then, the data obtained from parallel ablation lines were used to create trace element images, as described in detail by Steadman et al. (2013) [61].

EMPA (Tennantite-Tetrahedrite-Pyrite) and LA-ICP-MS (Pyrite) Elemental Maps
In the EMPA elemental maps (Figure 7), chemical variations from core to the rim are clear in the tennantite-tetrahedrite-pyrite assemblage. The irregularly-shaped pyrite core (2-4 µm) is rich in S, Co, and Ni, but depleted in As, whereas As is rich outside the core. Compared with the coexisting pyrite, the tennantite-tetrahedrite-series minerals have relatively high contents of As, Sb, Cu, Zn, and Hg. From the core through inner rim to outer rim of these minerals, the As content decreases while the Sb content increases. Copper, Zn, Hg, and Mo are present throughout the whole tennantite-tetrahedrite.

EMPA (Tennantite-Tetrahedrite-Pyrite) and LA-ICP-MS (Pyrite) Elemental Maps
In the EMPA elemental maps (Figure 7), chemical variations from core to the rim are clear in the tennantite-tetrahedrite-pyrite assemblage. The irregularly-shaped pyrite core (2-4 μm) is rich in S, Co, and Ni, but depleted in As, whereas As is rich outside the core. Compared with the coexisting pyrite, the tennantite-tetrahedrite-series minerals have relatively high contents of As, Sb, Cu, Zn, and Hg. From the core through inner rim to outer rim of these minerals, the As content decreases while the Sb content increases. Copper, Zn, Hg, and Mo are present throughout the whole tennantite-tetrahedrite.  A typical Carlin-type Au-bearing pyrite [20] with an As-poor core and As-rich rim was selected for LA-ICP-MS trace element mapping. The result shows the core has very low Au content, whereas the narrow rim has the highest Au content (Figure 8). Copper (100-500 ppm), As (5000-10,000 ppm), Se (10-100 ppm), Ag (10-50 ppm), Sb (100-1000 ppm), Au (10-100 ppm), Hg (1000-10,000 cps), Tl (10-100 ppm), Pb (10-100 ppm), and Bi (1-5 ppm) are rich in the ore rim, while Co (10-100 ppm) and Ni (10-100 ppm) are concentrated in the porous core. Zinc (1-5 ppm) and Mo (1-5 ppm) are rich in part of the pyrite rim.

Discussion
At Nibao, several lines of evidence support that the tennantite-tetrahedrite-series minerals are ore-related and probably formed during the main-ore to late-ore stage. (1) Samples with tennantite-tetrahedrite-series minerals are from the Au orebodies; (2) Tennantite-tetrahedrite-series minerals contain varying amounts of Au (Table 1); (3) Coarse-grained tennantite-tetrahedrite-series minerals often contain ore-stage pyrite inclusions ( Figure 6C); (4) As observed by   [56], tetrahedrite at Nibao coexists with hydrothermal apatite; (5) Copper precipitation coincided with Au precipitation in the auriferous pyrite grains (Figure 8). Previous studies show that the Cu background concentration in the ELIP basalt is relatively high, e.g., up to 165 ppm Cu in NE Yunnan [62]. The preore sedimentary/diagenetic pyrite Cu contents in the pyroclastic-related Au deposits are higher than those of the carbonate-or fine-clastic sediment-relate Au deposits (Figure 2), whereas the syn-ore pyrite Cu contents in these two types of deposits are largely similar. Therefore, it is likely that the Cu in the pyroclastics was leached into the ore fluids and then precipitated in the ore-stage (arseno)pyrite and (mainly) independent Cu minerals, e.g., the tennantite-tetrahedrite-series minerals.
In Carlin-type gold deposits, pyrite and arsenopyrite are the main Au ore minerals, with the gold being structurally bound (Au + ) within the crystal lattice [3,11,15,16]. At Nibao, our result shows that Au not only occurs in pyrite and arsenopyrite but also resides in tennantite-tetrahedrite-series minerals (up to 0.35 wt %, Table 1). In hydrothermal deposits, boiling is usually considered as an important mechanism for Au precipitation, but Au-bearing minerals (pyrite, arsenopyrite, and tennantite-tetrahedrite-series minerals) at Nibao are mostly associated with liquid-rich inclusions [24] rather than aqueous liquidrich inclusions coexisting with vapor-rich inclusions in the quartz, calcite, and fluorite. At Nibao, the ore host usually contains relicts of ferroan carbonate ( Figure 6D,E), indicating that gold-bearing minerals likely precipitated from ore fluids by sulfidation of the ore host, and/or mixing with Fe-rich fluids produced in the alteration zones nearby [16,21,36]. This conclusion is also supported by the lack of As and Au correlation in the tennantitetetrahedrite-series minerals ( Figure 9A). Although it is well known that Au is closely related to As in Carlin-type gold deposits [58,63,64], and pyrite with high As content can host more Au [64], intense fluid-rock interactions may change the ratio of As and Au and affect their partitioning into the tennantite-tetrahedrite-series minerals. the pyroclastic host rocks at Nibao, cf. the carbonate and fine clastic rocks (organic matter rich) that host the majority of Carlin-type gold deposits in the region. As shown in Figures 6A and 9B, the core-rim texture of tennantite-tetrahedrite composite indicates that the ore fluids evolved from the Tn-(Zn)-Ⅰ through Tn-(Zn)-Ⅱ to tetrahedrite-(Zn) precipitation. During the Tn-(Zn)-Ⅰ and Tn-(Zn)-Ⅱ precipitation, arsenopyrite rarely precipitated because As occurs mainly as As 3+ in Tn-(Zn)-Ⅰ and Tn-(Zn)-Ⅱ, indicating that the early-ore fluids are relatively oxidized [66,75]. During the subsequent tetrahedrite-(Zn) precipitation ( Figure 9B), As in Tn-(Zn)-Ⅰ and Tn-(Zn)-Ⅱ was largely replaced by Sb (average 21.4 wt %, Table 1), indicating that the ore fluids became more reduced [75]. This ore fluid redox conversion may have been a result of intense water-rock interactions and meteoric water incursion, which are both critical mechanism for Au precipitation [7,11,16]. Meanwhile, the pH value may have also modified the compositions of the tennantite-(Zn) and tetrahedrite-(Zn). High pH was reported to favor tetrahedrite formation [75,76], which implies a pH increase during the ore-fluid evolution at Nibao.

Conclusions
In this study, tennantite-tetrahedrite-series minerals are recognized to be ore-related in the Nibao deposit, and they probably formed at the main-ore to late-ore stage. The oreforming fluids at Nibao may have had higher fO2 than the Carlin-type deposits (carbonate-/fine-clastic-hosted) in the Youjiang basin of SW China. From tennantite-(Zn) to tetrahedrite-(Zn) precipitation, the ore-forming fluids likely evolved to be more reduced and with higher pH.
Compared with porphyry mineral systems, the f O 2 condition of Carlin-type/-like gold deposits in the Youjiang basin is still poorly constrained due to the lack of suitable mineral assemblage. Xie et al. (2017) [74] revealed that ore fluids with unusually low f O 2 would enable the uncommon native Sb precipitation at the late-stage Au mineralization, e.g., at the Paiting Carlin-type deposit (carbonate-/fine clastic-hosted) in Guizhou. Unlike Paiting, a main-to late-ore stage mineral at Nibao is tennantite-tetrahedrite series rather than native Sb, indicating that the main-to late-ore fluids at Nibao may have had higher f O 2 than that of the fine clastic-hosted Carlin-type deposits in the region. This conclusion is also supported by the gypsum occurrence in the Nibao orebodies [56]. The higher f O 2 condition may have been attributed to the absence of reductants (e.g., organic matter) in the pyroclastic host rocks at Nibao, cf. the carbonate and fine clastic rocks (organic matter rich) that host the majority of Carlin-type gold deposits in the region.
As shown in Figures 6A and 9B, the core-rim texture of tennantite-tetrahedrite composite indicates that the ore fluids evolved from the Tn-(Zn)-I through Tn-(Zn)-II to tetrahedrite-(Zn) precipitation. During the Tn-(Zn)-I and Tn-(Zn)-II precipitation, arsenopyrite rarely precipitated because As occurs mainly as As 3+ in Tn-(Zn)-I and Tn-(Zn)-II, indicating that the early-ore fluids are relatively oxidized [66,75]. During the subsequent tetrahedrite-(Zn) precipitation ( Figure 9B), As in Tn-(Zn)-I and Tn-(Zn)-II was largely replaced by Sb (average 21.4 wt %, Table 1), indicating that the ore fluids became more reduced [75]. This ore fluid redox conversion may have been a result of intense water-rock interactions and meteoric water incursion, which are both critical mechanism for Au precipitation [7,11,16]. Meanwhile, the pH value may have also modified the compositions of the tennantite-(Zn) and tetrahedrite-(Zn). High pH was reported to favor tetrahedrite formation [75,76], which implies a pH increase during the ore-fluid evolution at Nibao.

Conclusions
In this study, tennantite-tetrahedrite-series minerals are recognized to be ore-related in the Nibao deposit, and they probably formed at the main-ore to late-ore stage. The ore-forming fluids at Nibao may have had higher f O 2 than the Carlin-type deposits (carbonate-/fineclastic-hosted) in the Youjiang basin of SW China. From tennantite-(Zn) to tetrahedrite-(Zn) precipitation, the ore-forming fluids likely evolved to be more reduced and with higher pH.
Author Contributions: Y.X. designed the study concept and revised the manuscript; D.W.; J.A.S. contributed to the analysis, data interpretation and manuscript preparation; D.W., Z.X., X.L., Q.T., and L.B. collected the samples; D.W., Z.X., and Q.T. constructed the geologic maps. All authors have read and agreed to the published version of the manuscript.