Distribution and existing state of indium in the Gejiu Tin polymetallic deposit，Yunnan Province，SW China

Yubang Li·Yan Tao·Feilin Zhu·Mingyang Liao· Feng Xiong·Xianze Deng

Distribution and existing state of indium in the Gejiu Tin polymetallic deposit，Yunnan Province，SW China

Yubang Li1，2·Yan Tao1·Feilin Zhu3·Mingyang Liao1，2· Feng Xiong1，2·Xianze Deng1，2

The Gejiu tin polymetallic deposit，located in the southwest margin of the South China Belt bordering on the Yangtze Block，is hosted in Triassic carbonates.This study shows that there is an indium enrichment in the ores. Indium mainly exists in sphalerite-rich massive sulfide ores with a concentration of 198.3—1570 ppm；the indium concentration in sphalerite ranges from 493.3 up to 4781 ppm.The inferred indium reserve in the Gejiu Sn deposit is over 4000 t，indicating that the Gejiu tin deposit is a giant In-bearing Sn-polymetallic deposit，similar to the Dachang and Dulong in southwest China.Sphalerite with a high Fe value（8.05—13.27 wt%）is the predominant indium carrier mineral；meanwhile chalcopyrite plays the secondary indium carrier role.EPMA mapping result shows that indium is likely distributed homogeneously and presented as a substitution for Zn in sphalerite.

Indium·Sphalerite·Existing states·Gejiu· Sn-polymetallic deposit

1 Introduction

Indium is a rare metal and has a wide application for hightech equipment manufacture including solar cells，displays of LCDs（liquid crystal display），semiconductors，low temperature fusible alloys and solders（Stevens and White 1990；Schwartz-Schampera and Herzig 2002；Sinclair et al. 2006）.Two major primary sources for industrial indium are by-products of the massive sulfide deposits and graniterelated vein，skarn and disseminated deposits，where minor amounts of indium are extracted from zinc concentrates of In—Sn-bearing base metal ores（Ishihara et al.2006，2011；Ishihara and Matsueda 2011）.Indium occurs as several individual mineral phases，including native indium（In），Indite（FeIn2S4）， Roquesite（CuInS2）， Sakuraiite［（Cu，Fe）2Zn（In，Sn）S4］，Damiaoite（PtIn2），Yixunnite（AgInS），Dzhalindite［In（OH）3］，etc.These indium minerals carry the most indium concentration，but they are rare in nature（Liu et al.1984）.So far，there has been no report of indium independent ore deposits.Even though some of the worldwide indium-bearing deposits，such as the Mount Pleasant deposit（Sinclair et al.2006），Toyoha deposit（Shimizu and Morishita 2012；Shimizu and Kato 1991）and Freiberg district（Seifert and Sandmann 2006），have indium minerals or indium solid solutions within sphalerite，they make a smaller contribution to the indium reserve than sphalerite，due to the small amount in the indium-bearing deposits.Nearly 90%of the indium resources were produced by sphalerite and several Sn-bearing sulfate minerals，for example stannite and cernyite（Tu et al.2004）.

In China，indium-bearing deposits are mainly located in thesouthwestandalmostalloftheseindium-bearingdeposits are Sn-rich Pb—Zn—Cu hydrothermal ore deposits.Thus，this traitiscalledindiummetallogenicspecialization（Zhangetal. 1998，2003）.Dachang and Dulong Sn-polymetallic oredepositsarefamousindium-bearingdepositswiththeindium reserve over 6000 t and 5000 t，respectively（Li et al.2010；Ishihara et al.2011）.Other small scale indium-bearing Snpolymetallic deposits such as Meng'entaolegai，Dajing and Jinziwo produce 400 t to over 768 t indium reserves（Zhang et al.2005；Ishihara et al.2008）.

The world class Gejiu Sn-polymetallic deposit has been estimated to have metal reserves with 300 million metric tonnes（Mt）of tin ores in a grade of 1.0%Sn，300 Mt of copper ores in a grade of 2.0%Cu，400 Mt of lead—zinc ores in a grade of 7.0%Pb+Zn，and more than 20 other metals including W，In，Be，Bi，Mo，Cd，Ta，Fe，Au，Ag，REEs（308 Geological Party 1984；Zhuanget al.1996）.Previous studies on the indium potential in the Gejiu deposit point out that indium is developed in the oxidized ores in Malage（308 Geological Party 1984；Zhang et al.2002），and inferred the indium reserve is 4000 t（Zhang et al.2005）.Cassiterite，whichisthepredominantSnmineralcarryingindiumfrom25 to 50 ppm（Tu et al.2004），should not be the main indium carrierinthe Gejiudeposit.Littleresearchonotherminerals，except cassiterite and oxidized ores in Malage，has been reported in order to discover the real indium carrier.As a consequence，which minerals are indium carriers and what theindiummineralizationmechanisminthecarrieris，arestill the open questions in Gejiu.In this study，samples of the different types of ores and the main sulfide minerals were analyzed，with the purpose of ascertaining the indium carrier minerals，indium distribution characters in the ores and the existingstate ofindiumintheGejiuSn-polymetallicdeposit.

2 Geological setting

The Gejiu mining district is located in the southeast Yunnan Province，approximately 200 km southeast of Kunming.Tectonically，it is located on the western margin the Cathaysian Block（alternatively called the South China Fold Belt）and adjacent to the Yangtze Craton in the north（Zhuang et al.1996）and the Ailaoshan metamorphic belt in the south（Fig.1）.This region has experienced a long period of tectonic evolution and has developed various sizes of faults and fold systems.The NS-trending Gejiu fault divides the district into eastern and western parts.The ore deposits are located in the eastern part，such as Kafang，Laochang，Songshujiao and Malage，where the structures include several NS-，NNE-and EW-trending faults，and the NNE-trending Wuzhishan anticline，which controlled the location and configuration of orebodies in the eastern Gejiu district（308 Geological Party 1984）.In the western part，widespread Yanshannian granites and Triassic lavas are outcropped（Fig.1）.The sedimentary formations exposed in the Gejiu area are mainly Permain to Triassic strata，including carbonates，clastic rocks and interbedded mafic lavas.In the mines，the Lower Triassic Feixianguan Formationarecomprisedofsandstonesandmudstone（～173—389 m），the Lower Triassic Yongningzhen Formationofmarlstonesinterlayeredwithmudstones（～408—457 m）and the Middle Triassic Gejiu Formation ofpurecarbonateinterbeddedwithmaficlavas（～1400—4200 m）.The Middle Triassic Falang Formation（～1800—2800 m）overlaying the Gejiu Formation consists of mudstone，limestone，shales，interbedded volcanics and，it is in conformity and overlaid by the Upper Triassic Niaoge Formation（～300 m）of fine-grain sandstones，silty shales and mudstones.The carbonate rocks and interbeded Triassic mafic lavas of the Gejiu Formation and Falang Formation are the main ore-host strata（308 Geological Party 1984；Zhuang et al.1996）.

During the Mesozoic，there was a big event of magma emplacement in addition to the Triassic lavas.Gabbro，porphyritic biotite granite，equigranular biotite granite，alkali granite，alkaline rocks and mafic dikes are outcropped widely in Gejiu.The Zircon U—Pb age of the Granites detected by Cheng et al.（2009）argued that the magmatism took place in a short period from 76 to 85 Ma. The granites have a close relationship with the metallization in the region（308 Geological Party 1984；Mao et al. 2008）.

3 Geology of ore deposit

3.1 Kafang Cu-Sn deposit

The Kafang deposit is located in the southernmost part of the Gejiu district，between the parallel EW-striking Laoxiongdong fault and Xianrendong fault，and bordered by the Gejiu fault on the western margin（Fig.1）.The Gejiu Formation carbonates dominate the country's rocks and a basaltic lava layer with a thickness of 60—100 m is imbedded concordantly with the carbonate strata.Later，the Yanshanian Xinshan granite pluton，which has the characteristics of fine-grain to medium-grain equigranular biotite granite，broke into the overlying Gejiu Formation and basaltic lava.Ores in the Kafang include four types.

Type 1 is the skarn Cu—Sn—Mo—W—Bi polymetallic ore，which occurs in the contact zone between the Xinshan granite and carbonates.The ore assemblage includes chalcopyrite，pyrrhotite，cassiterite and sphalerite，and minorpyrite，molybdenite，wolframite，arsenopyrite，galena，native bismuth and native gold.Gangue minerals are comprised of garnet，pyroxene，actinolite，epidote，tremolite，sericite，calcite and quartz.

Fig.1 Geological map of the Gejiu area，Yunnan Province（after Mao et al.2008；Cheng and Mao 2010）.SCB South China Block；NCB North China Block；YC Yangtze Block

Type 2 is the stratabound Cu-dominant ore（Fig.3b），hosted by the basaltic lava in Kafang（Fig.2）.Primary minerals are chalcopyrite，pyrrhotite，pyrite，arsenopyrite and molybdenite with the gangue minerals of phlogopite，tremolite，actinolite，epidote and chlorite.Orebody shapes are mostly stratiform and sheet-like due to the controlling by the basaltic lava and carbonate wall rocks.

Type 3 is the stratiform Cu—Sn—Pb—Zn sulfide ores，which existed in the limestone or dolomitic limestone above the granite pluton in Kafang.The primary ore minerals of these sulfide ores mainly consist of pyrrhotite，chalcopyrite，pyrite and arsenopyrite，and the gangue minerals include tourmaline，tremolite and fluorite.Orebody shapes vary from lenticular to vein-like，and they cut through the wall rocks or deposit as stratiform shapes along with the strata.

Type 4 is the interstratified oxidized ore，which occurs in levels several tens to hundreds of meters above the granite contact zone，is shaped in composite multilayers， lenticular and irregular appearances.These ores are mainly composed of limonite，hematite，cassiterite，cerussite and covellite，with a few gangue minerals of quartz，calcite and fluorite（308 Geological Party 1984；Zhuang et al.1996；Luo and Fang 2011；Qian et al.2011）.

3.2 Laochang Sn-W-Cu polymetallic deposit

The Laochang deposit located between south Kafang and north Songshujiao.It is the most valuable deposit，containing～55%of the reserve of tin metal and about 40% of the total metal reserve in the whole Gejiu district（308 Geological Party 1984）.The EW-trending Laoxiongdong fault and Beiyinshan fault are the southern and northern borders of Laochang.Several other small faults，in addition to the Laoxiongdong and Beiyinshan faults，restrain theore-body occurrences and provide conduits for ore-forming fluids.Igneous rocks intruding into the Gejiu Formation carbonates are equigranular and porphyritic granites.The three main ore types in Laochang are skarn type sulfide ores，vein ores and stratabound oxidized ores（Fig.2）.

Skarn type sulfide ores are the main ores in Laochang and occur in the contact zone between granite and carbonate.This kind of ore is also located in the country rocks above the contact zone and are shaped as vein，lenticular and layer-like sulfide ore-bodies.Skarn ores produced most of Sn metal resources in Laochang.The main ore minerals are pyrrhotite，arsenopyrite，chalcopyrite，sphalerite，cassiterite and scheelite，and are associated with gangue minerals of garnet，diopside，plagioclase，fluorite，quartz and chlorite.

The vein-type ore is another main mineralization type in Laochang，and can be divided into tourmaline—quartz veins，tourmaline—K feldspar—skarn veins，tourmaline—skarn—cassiterite veins，and tourmaline—phlogopite veins. These veins are distributed in the limestone or dolomite of the Gejiu Formation carbonates with the scale of veins varying from centimeters of thickness by meters of length to tens centimeters of thickness by hundred meters of length.Many small-scale veins crosscut each other，forming the stockwork ore bodies（Fig.2）.The vein-type ores are mainly produced for Sn meta，l as well as minor amounts of byproducts including Be，W，B，Li，Rb，Nb，Ta and REE metals.The average grades of Sn，Be and WO3in the vein-type ores are 0.42%，0.13%and 0.11%，respectively.

Stratabound oxidized ores are located in the interlayer of the limestone or dolomitic carbonates and are strongly weathered from the original sulfide ores.Ore mineral assemblage includes cassiterite，hematite，limonite，goethite，conichalcite，anglesite，malachite，cerussite and wadite，and also minor amounts of arsenopyrite，pyrite，chalcopyrite，galena and sphalerite.

3.3 Songshujiao Sn-Pb deposit

The Songshujiao deposit is located in the north of Laochang and northeast of Gejiu city.Rock units in the Songshujiao include the Mid-Triassic Gejiu Formation and the underlying Later Yanshannian porphyritic biotite granite.The NNE-trending Wuzishan anticline and the NW-trending Baishachong fault，with small-scale faults and folds，make up the geological structure.Two types of ores are developed in the Songshujiao deposit：the skarn type Sn—Cu sulfide ores and the stratabound Sn—Pb ores（Fig.2）.

The skarn type Sn—Cu sulfide ores occur in the contact zones between the granite and carbonates.This type of ore provides more than 60%of the Sn and～80%of the Cu resources in Songshujiao.The primary ore minerals are chalcopyrite，arsenopyrite，sphalerite，pyrite，magnetite，cassiterite，scheelite and molybdenite，and the associated skarn minerals of garnet，pyroxene and scapolite.The sizes of ore-bodies vary from～5 to～20 m in thickness by～200 to～500 m in length and are shaped lenticular and layer-like.

Stratabound Sn—Pb sulfide ores are mostly hosted by the limestone and dolomite of the Gejiu Formation.The occurrences of the ore-bodies are generally lenticular，layer-like and banded.Most of the ores occurred in the interbeded of the limestone and dolomitic limestone，while some of these ores were deposited in the shear fracture zones of the carbonates.The stratabound orebody thickness is～20 to～50 m and the strike length is commonly～300 to～500 m，with a few ore-bodies stretching to 1500 m.Most of these ores are weathered and form oxidized ores with the mineral assemblage of limonite（～70%），calcite（～15%）andquartz（～5%），and combined with minor minerals of fluorite，pyrite，arsenopyrite，cassiterite，cerussiteand malachite.

3.4 Malage Sn-Cu deposit

The Malage Sn—Cu deposit is located in the northernmost part of the Gejiu district and adjacent to the Songshujiao deposit in the south.Strata in the Malage are comprised of Mid-Triassic Falang Formation limestone，shales with a thickness of 400 m and the underlying Gejiu Formation carbonate，with the thickness of over 1000 m（308 Geological Party 1984）.The igneous rocks of the Malage are Later Cretaceous Baishachong equigranular granite in the north and Beipaotai porphyritic granite in the south. Although，these two granite intrusions show different petrological characteristics，they are believed to be from the same magma chamber with different fractionation degrees（Cheng and Mao 2010）.The skarn type Cu—Sn ores and the stratabound oxidized ores are the main two types of ores in the Malage（Fig.2）.

The skarn type ore bodies occurred in the contact zone between the porphyritic granite and carbonate wallrocks and were generally shaped as lenticular，with ore minerals including pyrrhotite，chalcopyrite，pyrite，molybdenite，galena，sphalerite，cassiterite，asenopyrite and scheelite. The Sn and Cu grades of skarn ores ranged from 0.02%to 0.05%and 0.3%to 1.56%，respectively（308 Geological Party 1984；Cheng et al.2013）.

The stratabound oxidized ore bodies shaped as vein，lenticular and irregular were hosted in the carbonate country rocks and controlled by the faults and folds in the Malage deposit.These ores were weathered from skarn ores；thus，the Sn grade increased with the average of 2.39%，and were associated with different Cu，Pb，In and Bi metallization extents（308 Geological Party 1984）.The ore minerals include limonite，hematite，magnetite，cassiterite，malachite，cerussite，bismuthinite，scheelite and hydrozincite，with gangue minerals of fluorite and minor amounts of skarn minerals.

4 Sampling and analysis

4.1 Sampling

Sampling works were carried out in the Kafang，Laochang and Songshujiao deposits covering variable types of ores. In the Kafang deposit，samples（Table 1）were collected in the Qianjin tunnel，including the basaltic lava hosted in the stratabound Cu-domiant ores（Fig.3b）from the No.1—9 lode，stratiform sulfide ores from the No.1—10 lode on the 1700 m level mining platform（samples named with 1-10-3 and 1-10-6）and the skarn type sulfide ores from the No. 1—15 lode on the 1600 m level mining platform（samples named with 1-15-3 and 1-15-5a）.Sampling in Laochang was focused on the skarn type Cu—Pb—Zn polymetallic massive sulfide ores（Fig.3c，d）from the granite-carbonate contact zone and the stratabound oxidized ores（Fig.3a）overlying the contact zone.In the Songshujiao deposit，samples were collected from three different level mining platforms.The skarn type sulfide ores（Table 1）were collected from the No.1—6 lode on the 1540 m level platform and the stratabound-oxidized ores were collected from the carbonate country rocks several meters higher above the No.1—6 lode.On the 1720 and 1920 m level platforms，the collected samples were stratabound Pb—Zn sulfide ores（Fig.3e）from the No.10—14 lode and the stratabound Zn—Cu sulfide ores（Fig.3f），respectively.

4.2 Analysis

A detailed bulk ores and individual minerals chemical analysis and an EMPA mapping on the sphalerite were carried out in order to clarify the indium content and distribution characteristics.To prevent contamination of the chemical analysis due to the use of a mental mortar，all the sulfide ores，oxidized ores and sulfide minerals were crushed manually with an agate mortar.

4.2.1 Bulk ores and individual mineral geochemistry analysis

Samples of In-barren sulfide ores（Table 1）and oxidized ores（Table 3）were analyzed at the ALS Laboratory Group，Guangzhou，China.To begin with，these samples were dissolved by HNO3and HClO4，and then redissolved by HF.After that，there volume was constrained with HCl，then the sample solutions were analyzed via ICP-MS. Those In-rich sulfide ores and individual minerals from Tables 2 and 5 were dissolved by HF and HNO3following the solution process of Qi et al.（2000）and were then tested by ICP-MS in National Research Center for Geoanalysis（Beijing）.

4.2.2 EMPA mapping scan

Elements mapping on sphalerite by EMPA was conducted at Institute of Geochemistry，Chinese Academy of Sciences under the condition of 25 kV voltage，10 nA electricity and 1 μm beam diameter with a dot density of 1 spot per square micrometer.

5 Results

5.1 Indium in sulfide ores

The chemical composition analysis results of sulfide ores（Tables 1 and 2）show varying indium values in thedifferent ore districts.In the Kafang ore district，the indium concentration ranges from 0.1 to 65.8 ppm，with a mean value of 13.4 ppm（n=8）.Ni and Bi are enriched with mean values of 208.5 and 1305.5 ppm，respectively.The indium grade of sulfide ores of Laochang varies from 3.2 to 85.2 ppm with a mean value of 38.1 ppm（n=5），which isa little higher than those from Kafang.However，the mean value of Ni and Bi are lower in contrast with those from Kafang.The sulfide samples from Songshujiao show different In concentration levels：sulfide ores with less sphalerites contain relatively low In value，which range from 2 to 90.4 ppm with an average of 23.7 ppm（n=6；Table 1）. The other ores are the most In-rich sulfide ores with indium value varying from 198.3 ppm to 1570 ppm （Table 2）. These In-enriched ores are also associated with relatively high Cd concentrations，which range from 564.7 to 1627 ppm.The reason for indium abnormal enrichment is that the sulfide ores mineral assemblage has a large proportion of sphalerite.

Table 1 Geochemical bulk analyses of polymetallic In-barren sulfide ores from Gejiu

Fig.3 Representative specimen photographs of the various types of ores in Gejiu. Spl sphalerite；Po pyrrihotite；Ccp chalcopyrite；Fl fluorite；Apy arsenopyrite；Mar marble

5.2 Indium in oxidized ores

The oxidized ore bodies shaped as simple layer，composite multilayers，lenticular and irregular appearance are widely spread in the Gejiu district.The composition of oxidized ores varies in different ore districts.The oxidized ores in Kafang have high Cu，but low indium（308 Geological Party 1984）.The oxidized ores in Laochang have low indium at the 1 ppm level（Table 3）.The oxidized ores from Songshujiao have a higher indium concentration，which ranges from 2.4 to 21.5 ppm （Table 3）whereas oxidized ores at the Malage have the most abundant indium due to the appearance of dzhalindite［In（OH）3］，which is an indium hypergene mineral.The ores are predominated by iron oxides（hematite，turgite）and iron hydroxides（goethite，hydrogoethite），with a relatively high concentration of indium （308 Geological Party 1984）.The hematitedominant oxidized ores contain indium，ranging from 7 to 1480 ppm，while goethite-dominant oxidized ores have indium ranging from 25 to 930 ppm（Zhang et al.2002）. Only oxidized ores in Malage with indium enrichment contribute to the indium reserve in Gejiu.

Table 2 Geochemical bulk analyses of In-enriched sulfide ores from Songshujiao

5.3 Indium in sulfide minerals

In order to determine the indium distribution in the different kinds of sulfides，sphalerite，pyrite，pyrrhotite，chalcopyrite，arsenopyrite and galena were separated manually from the sulfide ores under a binoscope.Analytical results show that indium is distributed quite differently among these sulfides（Table 4）.Sphalerite is the most important indium carrier and accounts for most of the indium resource in Gejiu.However，chalcopyrite，pyrrhotite，pyrite，arsenopyrite and galena contain relatively low indium concentrations.

5.3.1 Sphalerite

Previous literature shows sphalerite to be one of the most predominant indium-bearing minerals in the tin-base metal deposits and indium to occur directly within the crystal lattice of sphalerite（Schwartz-Schampera and Herzig 1999；Sinclair et al.2006；Ishihara et al.2006；Ishihara and Endo 2007；Cook et al.2009）.Sphalerites are almost opaque and contain indium values from 434 to 4781 ppm，with an average of 1182 ppm（Table 5）in the sulfide ores of the Gejiu.Fe contents in the sphalrite are from 8.05 to 13.27 wt%.Besides，sphalerites in the Gejiu deposit characteristically have high Cu，Mn and Cd values.Dill et al.（2013）reported that In-bearing sphalerites always carry a certain amount of Cd，whose value ranges between 0.2 and 0.6 wt%，indium will be more enriched in sphalerite called‘‘Indium window''，which has Cd values in the Gejiu sphalerites ranging from 0.25 to 0.41 wt%.

5.3.2 Chalcopyrite

The separated mineral analytical results（Table 5）reveal that chalcopyrites have a minor amount of indium in the Gejiu deposit.The indium contents varied from 52.6 to 266.3 ppm with an average of 138.1 ppm（n=7）.Concentrations of indium in the chalcopyrite were much lower than that of the co-existing sphalerite in the same sample，suggesting there was an indium fractionation during the sulfide mineralization and that indium preferred to join with the sphalerite（Cook et al.2011）.However，the chalcopyrite presented an indium resource potential to some extent，as its indium value was much higher than thatof the other sulfides（Table 4），and the chalcopyrite contributed a very considerable amount of Cu metal resources to the Gejiu district.

Table 4 Indium and Cd concentration of the sulfide minerals in Gejiu

5.3.3 Pyrrhotite

Pyrrhotite carries negligible indium ranging from 0.1 to 19.6 ppm with an average of 3.9 ppm（Table 4），in the ores，with some even co-existing in the same ores with sphalerite.This implies that indium does not tend to be incorporated into the pyrrhotite lattice.Most of the pyrrhotites occured as coarse grains in the sulfide ores（Fig.3b，e，f），and many fine or tiny pyrrhotite grains were enclosed in the sphalerite，combining with chalcopyrite and galena as inclusions（Fig.6a，b），in the whole Gejiu district.

5.3.4 Pyrite，galena and arsenopyrite

Pyrite generally picked from the skarn type sulfide ores（Fig.3d）in Laochang contained indium ranging from 0.1 to 40.8 ppm，with an average of 10.5 ppm（Table 4）.The pyrites were mostly euhedral without the zonations or bands associated with anhedral sphalerites in the skarn ore in Laochang.The indium content of galena ranged from 13.7 to 15.4 ppm，and the arsenopyrite carried an even lower indium concentration，which varied from 3.5 to 6.5 ppm（Table 4）.These make a negligible contribution to indium reserve in Gejiu district.

5.4 The existing state of indium

5.4.1 Existing state of indium in sphalerite

The distribution of indium in the sulfide minerals shows that sphaleriteisthemostsignificantindiumcarriermineralinthe Gejiu tin deposit.Microphysiography research，including ore specimens and polished sections observation by optical microscope，SEM and EPMA，illustrate that the sphalerites in the sulfide ores are almost opaque without oscillated zone structure，and quite a few sphalerites include many spotted pyrrhotite，chalcopyrite and galena inclusions.

EPMA Mapping scan analysis on the sphalerite grain shows that indium is possibly homogeneous（Fig.4）in the sphalerite without an indium abnormal enrichment point or area.The Zn，S and Fe distribution images（Fig.4）furtherconfirm that sphalerite has no oscillated zonations，unlike those In-bearing sphalerites characterized by oscillatory zonations with alternating high-In and low-In bands at the Toyoha deposit（Shimizu and Morishita 2012）.The EPMA mapping result indicates that indium is likely isomorphously in the sphalerite.However，more work should be done in the future to prove and confirm this characteristic，due to the limited precision of the EPMA mapping.

Fig.4 Mapping analysis images of sphalerite by eletron microprobe showing the distribution characters of Zn，S，Fe，Cd and In

Fig.5 Binary diagrams of In versus Zn（a），Cd（b），Fe（c）and Cd versus Zn（d）for the indium-bearing sulfide ores from Gejiu，Dachang and Dulong（Data of Dulong and Dachang are from Ishihara et al.2011 and Li et al.2010）

5.4.2 Independent indium mineral

We used detailed microscopic and SEM observation to make sure that there were no occurrences of indium independent minerals in the primary sulfide ores.Dzhalindite［In（OH）3］occurred as an indium hypergene mineral in the oxidized zone of the Malage deposit，however，there were no indium minerals in the oxidized ores from Kafang，Laochang and Songshujiao（308 Geological Party 1984）. Oxidized ores in Malage were weathered from the primary sulfide ores.During the oxidization and weathering，indium could be leached out into the supergene geochemical cycles，accompanied by major elements such as S，As，Fe，Cu，Pb and Zn and minor elements including Ga，Ge， Co and Ni in the primary sulfide minerals to form a series of hypergene minerals（308 Geological Party 1984；Zhang et al.2002）.

6 Discussion

6.1 Indium enrichment characters in sulfide ores

Fig.6 Photomicrographs of In-bearing sulfide ores polished sections In Gejiu.Spl sphalerite；Po pyrrihotite；Ccp chalcopyrite；Fl fluorite；Gn Galena

Based on the indium concentrations of variable sulfide ores including the skarn type and the stratabound type，sulfide ores can be divided into In-barren（Table 1）and In-rich sulfide ores（Table 2）.Most of the In-rich sulfide ores were discovered in Songshujiao；they were caused by the large proportion of sphalerites that existed in the ores.The indium of the sulfide ores in Gejiu，Dulong and Dachang against the common elements，including Zn，Cd and Fe，are plotted，in addition to Cd against Zn，in Fig.5.No matter whether in the In-rich sulfide ores or in In-barren sulfide ores，indium and Zn show a positive correlation and have similar characteristics present in both Dulong and Dachang（Fig.5a）.Commonly the 1000×In/Zn ratio is used as a parameterfordistinguishingthedegreeofindium contribution by sphalerite in ores（Ishihara et al.2011），while the lower 1000×In/Zn value is used to distinguish the indium contribution by sphalerite in sulfide ores.The average of the ten In-rich sulfide ores from Songshujiao（Table 4）is 557 ppm（n=10）and the mean 1000×In/Zn ratio is 4.8.However，the 1000×In/Zn ratio of In-barren sulfide ores（Table 1），whose indium average value is 23.2 ppm（n=19），is much higher than that of the In-rich ores in Gejiu.The ores with a minor amount of sphalerites and other sulfide minerals containing a certain amount of indium probably slightly increase the ratio.The average 1000×In/Zn ratios of the In-bearing ores from Dachang and Dulong are 1.9 and 6.0（Data from Li et al.2010 and Ishihara et al.2011），and they are similar to that of Gejiu. Previous research（Zhang et al.1998；Li et al.2010；Ishihara et al.2011）has proven that the indium reserve of Dachang and Dulong mainly exist in In-bearing sphalerites. All the evidence suggests that sphalerite makes the largest contribution to the indium reserve of these In-bearing deposits in southwest China.

Indium has no direct correlation to Fe in the sulfide ores（Fig.5c）.On the contrary，indium looks to be correlated with Cd；most of the Cd/In ratios vary from 0.1 to 10（Fig.5b）in the three giant In-bearing deposits in southwest China.Sphalerites from In-bearing deposits always contain a relatively high amount of Cd.This is probably due to the fact that indium and Cd are chalcophile elements with similar atomic radii（the atomic radii of In and Cd are 167 and 151 pm，respectively），and they have similar geochemical behaviors.The In-bearing deposits also show an extremelypositivecorrelationbetweenZnandCd（Fig.5d）.Sulfides in the ores，except sphalerite，are short of Cd concentration（Table 4），and the positive correlation between Cd and Zn is reflected by the sphalerite in the sulfide ores.In general，sphalerites carry Cd as isomorphous by substituting Fe for Zn（Liu et al.2010）.In addition，indium and Cd presenting a positive correlation will point to the fact that they possibly go into sphalerite together during the sulfide mineralization.

6.2 Indium enrichment in sphalerite

Indium is a common trace element in sphalerite and often associated with Cd and Ge（Johan 1988）.Black sphalerites typically contain a variety of mineral inclusions in Gejiu. The most common mineral inclusions are pyrrhotite，chalcopyrite，galena and a few tiny grains of cassiterite in sphalerite.Those chalcopyrite inclusions（Fig.6c，d）arereferred to as the‘‘chalcopyrite disease''structure，as described by Barton and Bethke（1987），and some of the pyrrhotites associated with the chalcopyrites present in the orientation（Fig.6a）are developed along a favorite horizon in the sphalerite（Fig.6b）.

The ICP-MS tests reveal that sphalerite has a relative high Cu concentration，with an average of 6905 ppm（Table 5），due to the chalcopyrite appearances as inclusions in sphalerite.It is as follows：at the beginning of the sphalerite crystallization，indium and Cu went into the sphalerite lattice by substituting Zn to form a In—Cubearing sphalerite solid solution.When the physical and chemical conditions changed，such as the temperature dropping the after sphalerite crystalized，the Cu in the sphalerite lattice separated out and formed inclusions（chalcopyrite），whereas the indium stayed in the sphalerite. It is unclear whether or not these inclusions，featured as crystals，attempted to achieve phase equilibrium with their external and internal environments.

7 Conclusions

（1） This study confirms that the Gejiu Sn-polymetallic deposit is an indium-bearing tin deposit，similar to the Dachang and Dulong in southwest China，and shows giant indium potential，with over 4000 t of indium reserve.

（2） The indium resource is mainly reserved in the stratabound Zn-enriched，skarn Zn-enriched sulfide ores in Gejiu.Oxidized ores with dzhalindite in Malage provide a certain amount of indium reserve because of the limited oxidized ore resources.Cudominant ores in Kafang make a small contribution to the indium reserve.

（3） Sphalerite is the most predominant indium carrier，whilechalcopyriteplaysasubordinateroleintheGejiu deposit.Detailed EPMA examinations indicate that indium is likely present as isomorphous in sphalerite.

AcknowledgmentsWe appreciate Hu Jing and Zheng Wenqin of State Key Laboratory of Ore Deposit Geochemistry for their assistance in the ICP-MS and EPMA analyses.This study was supported by the CAS/SAFEA International Partnership Program for Creative Research Teams（KZZD-EW-TZ-20），the National 973 Program of China（2015CB452603）and China Geology Survey（12120113078200）.

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10.1007/s11631-015-0061-7

25 November 2014/Revised：15 March 2015/Accepted：9 June 2015/Published online：26 June 2015

✉ Yan Tao

taoyan@vip.gyig.ac.cn

1State Key Laboratory of Ore Deposit Geochemistry，Institute of Geochemistry，Chinese Academy of Sciences，Guiyang 550180，China

2University of Chinese Academy of Sciences，Beijing 100049，China

3College of Earth Sciences，Chengdu University of Technology，Chengdu 610059，China

©Science Press，Institute of Geochemistry，CAS and Springer-Verlag Berlin Heidelberg 2015