Genesis of fahlore in the Tianbaoshan lead–zinc deposit,Sichuan Province,China:a scanning electron microscopy–energy dispersive spectroscopy study

Yusi Hu·Lin Ye·Zhenli Li·Zhilong Huang·Jiawei Zhang

Abstract The Tianbaoshan deposit,located in the southwestern part of the Yangtze Block,is a representative Pb–Zn deposit in the Sichuan–Yunnan–Guizhou Pb–Zn metallogenic province.The Pb–Zn orebodies are hosted in the upper Sinian Dengying Formation dolostone.The predominant minerals are sphalerite,galena,pyrite,chalcopyrite,quartz,and calcite with minor arsenopyrite,fahlore,and dolomite.The deposit is characterized by relatively strong Cu mineralization.However,the relationship between Pb–Zn and Cu mineralization is unknown.We analyzed the mineralogy and composition of fahlore,chalcopyrite,arsenopyrite,sphalerite,and galena using scanning electron microscopy–energy dispersive spectroscopy,with the aim of providing new evidence for the genesis of the Pb–Zn–(Cu)ore.The results show that the Cu ore in the deposit is dominated by chalcopyrite and fahlore,both of which formed before or during the Pb–Zn ore-forming stage.The fahlore showed dramatic compositional variation and was characterized by negative correlations between Ag and Cu,and between As and Sb,suggesting substitution of Ag for Cu,and that As and Sb substitute in the same site in the fahlore lattice.Based on backscattered electron images and composition,the fahlore was divided into two types.Type I fahlore crystallized early and is characterized by enrichment of Cu and depletion in Ag and Sb.Type II fahlore formed after Type I,and is rich in Ag and poor in Cu and As.Moreover,galena and fahlore are the host minerals of Ag.The variation of valence state with As host mineral—from fahlore to arsenopyrite—indicates the metallogenic environment changed from relatively oxidizing to reducing with a high pH.In the light of Gibbs energies of reciprocal reactions and isotherms for cation exchange,the composition of the fahlore implies its ore-forming temperature was lower than 220°C,corresponding with typical Mississippi Valley-type(MVT)deposits.Based on the geologic character and geochemical data of this deposit,we suggest that the Tianbaoshan deposit belongs to the MVT deposit category.

Keywords Fahlore ·Tianbaoshan Pb–Zn deposit·SEM–EDS·MVT deposit

1 Introduction

The Tianbaoshan Pb–Zn deposit,a typical Pb–Zn deposit in the Sichuan–Yunnan–Guizhou(SYG)metallogenic province,is located in the southwestern part of the Yangtze Block.It contains more than 2.6 million tons of Pb–Zn metal reserves(Cromie et al.1996;Wang et al.2010).There has been much research on the geologic features,ore-controlling factors,geological exploration,and geochemical characteristics of the deposit(Wang 1992;Wang et al.2000;Feng et al.2009;Cai 2012;Cheng 2013;He et al.2016;Sun et al.2016;Ye et al.2016).However,the sources of the ore-forming materials and the genetic classi fi cation of this deposit are still unclear and controversial.Over the past several decades,there have been three proposals for the ore-forming sources,including:the upper crust and orogenic belt(Li 2003;Zhang 2008;Yu 2014),the dolomite of the upper Sinian Dengying Formation(Tu 2014),and the dolomite of the Sinian and basement rocks(Wang et al.2000;Zhou et al.2013a).Several genetic models have been proposed for the deposit,including:karst cave fi llings(e.g.,Wang 1985),groundwater hydrothermal deposit(e.g.,Cheng 2013),SYG-type deposit(e.g.,Huang et al.2004;Zhou et al.2013a),and Mississippi-Valley-type(MVT)deposit(Wang 1992;Wang et al.2000,2010;Feng et al.2009;Cai 2012;Yu 2014;Liu 2015;Yu et al.2015;Ye et al.2016).Recently,Cu mineralization in the Pb–Zn orebody was explored in the mine area at a deeper level.Although there has been some research on the mineralogy(Tu 2014)and geochemistry(Sun et al.2016)of the Cubearing minerals from the copper mineralization,the relationship between Cu and Pb–Zn mineralization is still unclear.In this study,microscopic observation,backscattered electron(BSE)imagery,and in-situ analysis of Cubearing minerals were carried out to investigate the relationship between the Cu and Pb–Zn mineralization.These data will provide a better understanding of the ore-forming process and the genesis of Pb,Zn,and Cu in the Tianbaoshan Pb–Zn deposit.

2 Geologic setting

2.1 Regional geology

The Yangtze Block is near the southwestern margin of the South China Craton and bounded by the Cathysia Block to the southeast,the Qinling–Dabie Orogenic Belt to the north,the Tibetan Plateau to the west,and the Indochina Block to the south.The Yangtze Block is dominated by a Mesoproterozoic to Early Neoproterozoic basement overlain by Middle Neoproterozoic weakly metamorphosed strata,Late Neoproterozoic(Sinian)unmetamorphosed rocks,and Phanerozoic cover(Jin 2008).

One of the signi fi cant features of the western Yangtze Block is that a mantle plume erupted and formed the Emeishan large igneous province around 260 Ma,covering an area of more than 250,000 km2(Zhou et al.2002).The igneous province is dominantly composed of volcanic rocks known as Emeishan fl ood basalts,interlayered with Permian limestone,with numerous ma fi c–ultrama fi c intrusions along major faults(Zhou et al.2008).

In the SYG triangle district,the basement is composed of the Mesoproterozoic to Neoproterozoic Kunyang Group.The Kunyang mainly consists of sandstone,siltstone,slate,shale,dolostone,and minor tuffaceous volcanic rocks with a total thickness of about 20 km(Li et al.1984),and has experienced greenschist facies metamorphism. The overlying sedimentary strata are composed of Neoproterozoic to Middle Triassic submarine carbonate and clastic sedimentary sequences in a passive continental margin and Late Triassic to Cenozoic terrigenous sedimentary sequences(Liu and Lin 1999;Yan et al.2003).

The fault systems within the SYG triangle district are very complex.Three regional fault belts extend into basement rocks:the NW-trending Weining–Shuicheng,the N–S-trending Anninghe,and the NE-trending Mile–Shizong control the distribution of the Pb–Zn deposits.These long-lived fault belts have been activated and reactivated by numerous tectonic events e.g.,the Hercynian,Indosinian,and Yanshanian orogenic events.The area also contains numerous NE-and NW-trending secondary faults and thrust-fold belts(Wang 1992;Zhou et al.2013a).

More than 400 carbonate-hosted Pb–Zn deposits distributed in the SYG triangular area exceed 170,000 km2.These deposits are epigenetic,and they are hosted in Sinian to Lower Permian carbonate rocks,especially in Sinian and Early Carboniferous dolostone(Liu and Lin 1999;Huang et al.2004;Jin 2008).The deposits are characterized by irregular orebodies with simple mineralogy,weak wallrock alteration,and a high grade of Pb and Zn associated with the enrichment of Ag,Ge,Cd,Ga,and In.Recent Sm–Nd dating of hydrothermal calcite/ fl uorite and Rb–Sr dating of sphalerite indicate that the carbonate hosted Pb–Zn deposits formed between 226 and 191 Ma.This suggests that Pb–Zn mineralization in the SYG district is related to the Indosinian Orogeny(Li et al.2007;Lin et al.2010;Zhou et al.2013b,c;Zhang et al.2015).Additional details have been described in Zhou et al.(2013a)and Wang et al.(2000).

2.2 Ore deposit setting

The Tianbaoshan Pb–Zn deposit,including the Tianbao and Xinshan ore blocks,is in the northwestern part of the SYG metallogenic province(Fig.1).It has an estimated original resource of 2.6 Mt Pb and Zn with grades of 1.28 wt%–2.50 wt%Pb,7.76 wt%–10.10 wt%Zn,and 96.30 g/t Ag(Wang et al.2000).Faults and folds are intensively developed in the ore district;the main faults are branch faults(F1)of the Anninghe tectonic belt and NW-trending fractures(F2).

In the ore area,exposed strata include the Upper Sinian Dengying Formation dolostone,the Middle Cambrian Xiwangmiao Formation clastic rock enriched black shale,and the Upper Triassic Baiguowan Formation continental sandy shale.

Fig.1 Sketch geological map of the Tianbaoshan Pb–Zn deposit.Modi fi ed from Zhou et al.(2013a)

Major ore bodies occur in dolostone of the Dengying Formation and are structurally controlled by concealed fractures.Underground mining and exploratory drilling provide excellent access to three Pb–Zn orebodies.The No.I orebody is the largest with a depth of 400 m,length of 285 m,and width of 2.1–50 m.The mineralization is associated with silici fi cation and carbonate hydrothermal alteration.The ores are divided into massive,disseminated,and banded types and are predominately sul fi des,including sphalerite, galena, pyrite, chalcopyrite, and minor arsenopyrite.Calcite,dolomite,and quartz are the common gangue minerals.

Copper mineralization occurs within the No.I Pb–Zn orebody between depths of 2014 and 2064 m,with a length of 20–30 m,and width generally between 10 and 20 m,but extending up to 40 m;and is wrapped by the Pb–Zn ore body(Fig.2).There is no clean boundary between the Cu and Pb–Zn ores,but rather a gradual transition.The Cu ores are composed of chalcopyrite,sphalerite,fahlore,and minor arsenopyrite and galena.As in the Pb–Zn orebodies,the primary gangue minerals are calcite,dolomite,and quartz. Most chalcopyrite observed was massive(Fig.3a,b);disseminated and pisolitic textures(Fig.3c)were also occasionally observed.Sphalerite ingrowth—common with chalcopyrite—was mainly observed as veins(Fig.3b)and with pisolitic texture(Fig.3c).

3 Sampling and analytical techniques

Fig.2 Geologic pro fi le of the Tianbaoshan Pb–Zn deposit.Modi fi ed from Zhou et al.(2013a)

Ore samples from the Cu mineralization were collected from the 2064 m level of the Tianbaoshan deposit.Textural and energy dispersive spectroscopy(EDS)analyses of the samples were performed using a fi eld emission scanning electron microscope(SEM)(JSM-6460lv,JEOL,Japan)in combination with EDS(TEAM Apex XL,EDAX,America)at the State Key Laboratory of Ore Deposit Geochemistry,Institute of Geochemistry,Chinese Academy of Sciences.The beam current was 10 nA with an accelerating voltage of 25 kV.BSE images,panchromatic cathodoluminescence(CL)images,and qualitative EDS analysis of ore minerals were obtained in this research.

Fig.3 Copper ore samples in situ,in hand specimen,under microscopic observation,and as backscattered electron images.a The deep massive Cu mineralization is wrapped by the lead–zinc orebody;b,c sample-scale images show massive chalcopyrite(Ccp)replaced by and intergrown with sphalerite(Sp);d–f crystal-scale images display Sp cut cross Ccp and/or within massive Ccp,in addition to some fahlore(Fah)veinlets and arsenopyrite(Apy)distributed in Sp;g–i scanning electron microscopy–energy dispersive spectroscopy images reveal two types of Fah with Sp,Ccp,Apy,and galena(Gn)

4 Results

4.1 Microscopic observation

Microscopic observation and BSE images show that the ore mineral samples of the copper orebodies of the Tianbaoshan Pb–Zn deposit were dominated by chalcopyrite and sphalerite with minorfahlore and arsenopyrite(Fig.3d–i).Chalcopyrite was brilliant yellow in re fl ected light and was divided into three types:massive,emulsion,and veinlet.Massive chalcopyrite(Type I Cp)was observed to coexist with sphalerite(Fig.3e).Emulsion(droplet)chalcopyrite(Type II Cp)was fi ne-grained(＜ 0.2 μm),and randomly distributed within sphalerite to form ‘‘chalcopyrite disease’’(Fig.3d–f).Chalcopyrite veinlets(Type III Cp)cut across sphalerite(Fig.3f).The sphalerite was dark gray in color,and cut across chalcopyrite or was wrapped by the massive chalcopyrite.In addition,some veinlets of fahlore and arsenopyrite were observed in sphalerite(Fig.3d–i).Two types of fahlore were observed:Type Iisdark-gray to gray with xenomorphic granular texture and was observed mainly in the core of the fahlore,while Type II is light-gray to gray and was observed on the edges of the fahlore.Fahlore grains were enclosed in chalcopyrite or cut by sphalerite with chalcopyriteveinlets.Xenomorphicgalenawas intergrown with fahlore.Arsenopyrite was bright white with high re fl ectance,euhedral–subhedral in shape,and observed as vein assemblages cutting across the fahlore and sphalerite.

4.2 Results of SEM–EDS analysis

Results of SEM–EDS analyses of fahlore and arsenopyrite in three samples from the Cu mineralization are listed in Tables 1 and 2.BSE imagery shows massively distributed chalcopyrite with an anhedral crystal structure(Fig.3g–i),wrapping fahlore,arsenopyrite,and sphalerite droplets.EDS results indicate that the chalcopyrite is composed of 32.41 wt%–34.03 wt%S,30.91 wt%–32.23 wt%Fe,and Ag,Sb,Fe,and Cu.The EDS analysis of arsenopyrite revealed S,As,and Fe,at 14.81–22.35,39.02–50.59,and 9.26 wt%–36.19 wt%, respectively. Some of the arsenopyrite contained Co at 3.09 wt%–18.28 wt%.34.61 wt%–35.48 wt% Cu,arelatively homogeneous composition. Sampled sphalerite was 64.02 wt%–67.90 wt%Zn and 31.09 wt%–32.27 wt%S.Cu was detected in two spots,with content less than 3.00%,due to randomly distributed chalcopyrite inclusions in sphalerite.The samples from the Cu mineralization contained minor galena,the composition of which was homogeneous:S(1.20%–12.60%)and Pb(66.22%–84.21%),with minor

Table 1 The SEM–EDS results of fahlore composition

Table 2 The SEM–EDS results of arsenopyrite composition

The BSE images show compositionally zoned fahlore from less dense cores(Type I)surrounded by more dense rims(Type II).SEM–EDS analysis identi fi ed the main elements as S,Ag,Sb,Fe,Cu,Zn,and As.The composition of S in fahlore samples was relatively homogeneous,varying between 24.09%and 27.77%,while Ag,Sb,Fe,Cu,Zn,and As were heterogeneous,ranging from 3.05%to 10.58%,14.12%to 26.06%,2.37%to 7.04%,31.03%to 37.58%,4.12%to 6.02%,and 1.17%to 11.51%,respectively.The two types of fahlore had different compositional ranges,especially with regard to As,Ag,and Sb.The ranges were 4.89%–11.51%,3.50%–6.50%,and 14.12%–19.50%,respectively,in the Type I fahlore(Fig.4a);and 1.17%–7.40%,6.76%–10.58%,and 18.85%–26.06%,respectively,in the Type II fahlore(Fig.4b).

5 Discussion

5.1 Mineral paragenesis

Fig.4 Scanning electron microscopy–energy dispersive spectroscopy image of the two types of fahlore

At present,this deposit is not well studied.The relationship between the Cu and Pb–Zn mineralization is still unclear.Based on limited mineralogical research,three ore-forming stages are recognized,including a sedimentary–diagenetic stage,a hydrothermal mineralization stage,and a supergene oxidation stage(Tu 2014;Wu et al.2016).It is suggested that chalcopyrite replaced the sphalerite and formed at the post-hydrothermal stage and that there was no genetic relationship between Cu and Pb–Zn mineralization.However,S and Pb isotopes indicate that the Cu and Pb–Zn ores had a similar ore-forming metal source and formed during the hydrothermal mineralization stage(Sun et al.2016).In fact,Cu mineralization being enclosed by the Pb–Zn orebody(Fig.2),the sphalerite vein texture crossing and wrapping massive chalcopyrite at the scale of the hand specimen(Fig.3b,c),and sphalerite crossing and wrapping chalcopyrite grainswith metasomatic-relict texture on the microscale together suggest that massive chalcopyrite formed before sphalerite in the Cu mineralization.Previous studies have indicated that the ‘‘chalcopyrite disease’’ within sphalerite formed through exsolution(Hutchison and Scott 1981;Bortnikov et al.1991),and considered that chalcopyrite and sphalerite formed at the same metallogenic stage.In addition,microscopic observation and BSE images show fahlore coexisting with chalcopyrite crossing through sphalerite and chalcopyrite with a vein texture,which suggests that the fahlore and vein chalcopyrite formed later than sphalerite.The chalcopyrite,sphalerite,and fahlore crossing each other indicates that they formed at the same metallogenic stage,while the arsenopyrite assemblage along fahlore distributed or dispersed in chalcopyrite suggests that it formed at the last stage.In conclusion,according to the deposit geology,hand specimen structure,microscopic observation and BSE images,the mineral paragenesis of the copper mineralization in the Tianbaoshan deposit is massive chalcopyrite→sphalerite+emulsion chalcopyrite→vein chalcopyrite+fahlore→arsenopyrite.

5.2 Compositional characteristics and signi fi cance of the fahlore

The compositional variation of fahlore is a sensitive indicator of mineralization environment(Johnson et al.1986;Yan et al.1994;Xu 2005).Generally,the formula of fahlore is III(Cu,Ag)TGR IV[Cu2/3(Fe,Zn,Mn,Cd,Hg,Pb)]III(Sb,As,Bi)SM(S,Se)(Sack and Loucks 1985;O’Leary and Sack 1987;Johnson et al.1988;Huang 2000).The negative correlation between(Ag+Fe+Zn)and Cu in the Cu-bearing samples from the Cu mineralization,suggests that Ag,Fe,and Zn are isomorphic in fahlore(Tatsuka and Nowacki 1973;Huang 2000;Xu 2005).Figure 6b suggests a strongly negative correlation between Ag and Cu,indicating possible direct substitution of Ag with Cu;the inverse correlation between As and Sb(Fig.6d)also indicates that As substitutes for Sb in fahlore.Due to the incompatibility between Ag and As,Zn and As,and Ag and Zn in the fahlore crystal lattice,Ag for Cu and As for Sb are coupled energetically(Sack and Ebel 2006).EDS element mapping shows that the color variation of fahlore in BSE images relates to Cu,Ag,Sb,and As(Fig.5).The Type I fahlore is the product of early crystallization with high Cu content but low Ag and Sb,while the Type II fahlore is enriched in Ag and Sb and depleted in Cu and As as the product of late crystallization.The range of Ag content(3.50%–10.58%),and the correlation between Ag and Sb/(Sb+As)(Fig.6d,e),suggests that the Ag content is related to Sb rather than to Fe and Zn(Fig.6f).Compared with Type I fahlore,the contents of Ag and Sb in Type II fahlore samples were higher due to decreasing crystallization temperature from the early to the late stage.Previous studies concluded that fahlore containing high Sb within the crystal structure could incorporate more Ag atoms compared with low-Sb fahlore(Wu and Petersen 1977;Yan et al.1994).In addition,analysis of major and trace elements in sul fi de minerals has shown that galena is more enriched in Ag compared to sphalerite in the deposit(Ye et al.2016).Consequently,it is suggested that galena and fahlore are the main carrier minerals of Ag in the Tianbaoshan deposit.

The correlation between Sb and Sb/As(Fig.6g)shows a series change.The wide range of Sb/As ratio(0.21–19.51),suggests its turbulent mineralization environment—the early,Type I fahlore with low Sb;and the late,Type II with low As.Previous studies have shown that Sb being mostly replaced by As in fahlore is indicative of a relatively oxidizing environment,whereas reducing environments and high pH promote the substitution of Sb for As(Johnson et al.1986;Yan et al.1994).The negative correlation between As and S in arsenopyrite(Fig.6h)indicates substitution of As for S(Liu et al.1980;Yan et al.1994;Wang et al.2012)with a reducing environment.In conclusion,the variation in composition and the valence state of As suggests the transition from relatively oxidizing to reducing,high pH environment between the period of fahlore formation to that of arsenopyrite.

5.3 Ore-forming temperature

The geology,hand specimen texture,and microscopic observation of the Tianbaoshan deposit suggest fahlore,chalcopyrite, and sphalerite formed in the same hydrothermal mineralization stage.Previous research proposed that Gibbs energies of reciprocal reactions in fahlore can be established from constraints on isotherms for cation exchange,and the incompatibility between Ag and As in fahlore has the largest reciprocal energies resulting in a major impact on the distribution of fahlore composition in nature(Sack et al.2005).Sack and Ebel(1993)established the thermodynamic model using the calculation of miscibility gaps for Zn-and Fe-bearing fahlores,and 170,200,250,300,and 400°C isotherms for fahlore composition(Sack et al.2005).The present data of fahlore from the Cu ores fall under the 160 °C isotherm(Fig.7a)and 170 °C isotherm(Fig.7b).However,there is no buffering Ag(e.g.,pyrargyrite,miargyrite)in copper ores,and the temperature estimate of 160–170 °C is only the minimum temperature.As mentioned above,the Cu-and Pb–Zn ores belong to one hydrothermal system and are the products of the same mineralization but different ore-forming stages(Sun et al.2016).Therefore,the ore-forming temperature of Pb–Zn ores could approximately represent that of Cu ores.Apart from this,the temperature of quartz fl uid inclusions from the Tianbaoshan deposit range from 120 to 220°C(Yu 2014;Yu et al.2015),and the ore-forming temperature calculated by trace elements in sphalerite ranged from 150 to 200°C.Thus,the Tianbaoshan deposit is a low-temperature deposit,similar to other Pb–Zn deposits in the SYG district,China(Zhang 2008;Li et al.2016).The lowtemperature condition resulted in the wide range of compositions between the two types of fahlore with rapid crystallization.Under the low-temperature condition,the concentration of elements in arsenopyrite that formed later than fahlore varied in a wide range,leading to the heterogeneous composition of the arsenopyrite.

Fig.5 Mapping of the copper ore sample

5.4 Ore genesis

In general,there is little Cu mineralization in MVT deposits(Wilkinson 2014),although it has been discovered in the Wieloch deposit in Germany(Pfaff et al.2009);the MVT deposit of the Capricorn orogenic belt in western Australia(Muhling et al.2012);and the Maoping(Zhou and Li 2005;Wei et al.2015;),Fule(Li et al.2015),and Tianqiao deposits(Zhang 2005)in the SYG district,South China,with chalcopyrite,fahlore,and malachite being the common Cu minerals in those deposits.Moreover,research has shown(1)an ore-forming temperature range of 160–220 °C;(2)sphalerite characterized by enrichment of Cd and Ge and depletion of Fe,Mn,In,Sn,and Co(Ye et al.2011,2016),characteristic of MVT deposits;and(3)Cu and Pb–Zn both derived from basement rocks(Sun et al.2016).In combination with geology and geochemical evidence,this suggests that the Tianbaoshan deposit is an MVT deposit.

6 Conclusions

1. The primary Cu ore in the Tianbaoshan deposit is dominated by chalcopyrite and fahlore,and formed before or during the Pb–Zn ore-forming stage.

2. The wide range of compositions of fahlore displayed a negative correlation between Ag and Cu,indicating Ag substitution for Cu.Meanwhile,the inverse correlation between As and Sb suggests that As replaced Sb in fahlore.The fahlore was divided into two types,Type I and Type II;Type I was dark in BSE image,characterized by high Cu and low Ag and Sb concentrations,and was the product of early crystallization.Type II,on the other hand,was light in BSE image and is the expression of late crystallization with high Ag and low Cu and As concentrations.Ag content increased with Sb content,suggesting that the high-Sb late fahlore is also enriched in Ag.Researchers maintain that galena and fahlore are the primary Agbearing ores.

Fig.6 The relationship between fahlore and arsenopyrite

Fig.7 Different temperature isotherms for fahlore composition.a Modi fi ed from Sack and Ebel(2006);b modi fi ed from Sack et al.(2005)

3. In arsenopyrite,As replaces S as As3-,while As3+exists in fahlore.High pH is bene fi cial to the substitution of Sb for As,suggesting that the metallogenic environment changed from relatively oxidizing to reducing with high pH between the formation of fahlore and arsenopyrite.

4. The ore-forming temperature of fahlore from the Tianbaoshan deposit is below 220°C,indicating that chalcopyrite and fahlore formed at low temperature.According to the mineralogy and geochemistry of the Tianbaoshan deposit,it is suggested that the deposit is an MVT deposit.

AcknowledgementsThis research project was jointly supported by the State Key Program of National Natural Science Foundation of China (No. 41430315), the National ‘973 Project’ (No.2014CB440900),theNationalKeyR&D Program ofChina(2017YFC0602502),and the Guizhou Scienti fi c and Technology Planing Project(QKHPTRC[2018]5626).We would like to thank Dr.Shaohua Dong(SEM Lab.)for her assistance in SEM analysis.