Ore Geology Reviews 53 (2013) 50–62

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Ore Geology Reviews

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The origin of the world class tin-polymetallic deposits in the Gejiu district, SW China:Constraints from metal zoning characteristics and 40Ar–39Ar geochronology

Yanbo Cheng a,b,c,⁎, Jingwen Mao c, Zhaoshan Chang b, Franco Pirajno d

a Faculty of Earth Science and Mineral Resources, China University of Geosciences, Beijing, 100083, Chinab School of Earth and Environmental Sciences, James Cook University, Townsville, 4811, Australiac MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing, 100037, Chinad School of Earth and Environment, University of Western Australia, 35 Stirling Highway, Crawley WA 6008, Australia

⁎ Corresponding author at: Faculty of Earth Science aUniversity of Geosciences, Beijing, 100083, China. Tel.:

E-mail address: chamboll@yahoo.cn (Y. Cheng).

0169-1368/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.oregeorev.2012.12.008

a b s t r a c t

a r t i c l e i n f o

Article history:Received 20 August 2012Received in revised form 4 December 2012Accepted 5 December 2012Available online 31 December 2012

Keywords:Deposit geologyMetal zoning40Ar–39Ar datingOre genesisGejiu

The Gejiu tin-polymetallic deposits in the Western Cathaysia Block of South China comprise the world's largestprimary tin district, with a total resource of approximately 300 million metric ton ores, at an average grade of1 wt percent Sn. Tin polymetallic mineralization occurs in five deposits and has four ore types, i.e., greisen,skarn, stratabound cassiterite-sulfide (mostly oxidized) and vein type ore. In each deposit the orebodies typicallyoccur in an extensive hydrothermal system centered on a shallow Late Cretaceous granitoid cupola.Metal zoningiswell developed both vertically and horizontally over the entire district, fromW+Be+Bi±Mo±Snores insidegranite intrusions, to Sn+Cu-dominated ores at intrusionmargins and farther out to Pb+Zn deposits in the sur-rounding host carbonate. This zoning pattern is similar to that of other hydrothermal deposits in other parts ofthe world, indicating a close genetic relationship between magmatism and mineralization. In this paper, wedated thirteen mica samples from all types of mineralization and from the five deposits in the Gejiu district.The ages range from77.4±0.6 Ma to 95.3±0.7 Ma and are similar to the existing zircon U–Pb age of the graniticintrusions (77.4±2.5–85.8±0.6), indicating a genetic relationship between the mineralization and the intru-sions. Geological characteristics, metal zoning patterns and new geochronological data all indicate that thetin-polymetallic ores in theGejiu district are hydrothermal in origin and are genetically related to thenearby graniticintrusions. It is unlikely that the deposits are syngenetic, as has been proposed in recent years.

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1. Introduction

TheGejiu tin-polymetallic ore district is located approximately severaltens of kilometers southeast of Gejiu city, Yunnan Province, SWChina. It isone of the largest and oldest mining districts in the world. It has beenmined since the Han dynasty (202 B.C. to 220 A.D.), intermittently formore than 2000 years. The Gejiu district comprises five Sn–Cu–Pb–Znpolymetallic deposits, namely Malage, Songshujiao, Gaosong, Laochangand Kafang, containing approximately 300 Mt of Sn ores at an averagegrade of 1% Sn, another 300 Mt of Cu ores averaging 2% Cu, and 400 Mtof Pb–Zn ores with an average grade of 7% Pb+Zn. It is the largest tindistrict in the world. Exploration in this district is still finding moreores to this day.

Tin polymetallic ores in the Gejiu district, characterized by extensiveskarn alteration, multiple mineralization styles and well defined metalzoning around granitic cupolas, have long been considered geneticallyrelated to Cretaceous granites (e.g., 308 Geological Party, 1984; Luo,1995; Mao et al., 2008; Peng, 1985; Zhao and Li, 1987; Zhuang et al.,

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1996). Theories based on this understanding have contributed a greatdeal to exploration efforts (e.g., 308 Geological Party, 1984; Jianget al., 1997; Peng, 1985; Zhuang et al., 1996). However, as manyorebodies are approximately stratabound, some authors have challengedthis view, arguing that the oresmay be syngenetic (Jin, 1991; Peng, 1992;Zhou, 1988). Metacolloidal textures and resembling oolites aggregateshave been found in some of the orebodies, leading to the speculationof a submarine exhalation hypothesis (Zhang et al., 2004; Zhou et al.,1997, 1999). Based on K–Ar, Ar–Ar and Pb–Pb dating on differentminerals (see below for details), it has been argued that Sn polymetallicmineralization may be of VHMS style and/or SEDEX deposits (Li et al.,2006, 2009; Qian et al., 2011a, 2011b; Qin and Li, 2008; Qin et al., 2006).

Dating is one of themost powerful tools that place constraints on thegenesis of ore. However, the above-mentionedmineralisation ages havea wide range and are somewhat inconsistent. Qin et al. (2006) obtainedseveral mica 40Ar–39Ar ages ranging from 83.23±2.07 to 205.11±4.38 Ma, while another group of ~200 to ~240 Ma ages were obtainedthrough the use of Pb–Pb methods on sulfides. Qin et al. (2006) and Liet al. (2009) reported K–Ar ages of 43.49±0.87 Ma to 186.01±3.72 Ma for cassiterite. These data were viewed as important evidencefor the syngenetic model (Li et al., 2009; Qin and Li, 2008; Qin et al.,2006), as the ores were considered to be of Triassic ages.

51Y. Cheng et al. / Ore Geology Reviews 53 (2013) 50–62

In this study,we document the geological characteristics of the Gejiudistrict, demonstrating the clear metal zoning, both vertically and hori-zontally, around granitic intrusions in all deposits in the Gejiu district.Moreover, due to lack of precise and systematic mineralization dating,we have also completed Ar–Ar dating of 13 representative muscoviteand phlogopite samples from all ore types to constrain the age of min-eralization. Comparing these dating results with U–Pb zircon ages ofthe granitic intrusions in the Gejiu district provides further constraintson the ore genesis.

2. Regional geologic setting

The world class Gejiu tin polymetallic ore district is located on thewestern margin of the South China Block, adjacent to the YangtzeCraton in the north and the Sanjiang Fold Belt in the west (Fig. 1).It is bounded by the Mile–Shizong Fault to the north, the regionalstrike–slip Ailaoshan–Honghe Fault to the west, and the NorthVietnam Block to the south. The Gejiu District is ~1600 km2 inarea and was a tectonic sag or depression for much of its geological his-tory. Cambrian to Quaternary rock successions are well-preserved(mostly not exposed and not shown in Fig. 1), and the late Triassic toCretaceous strata are preferentially exposed near and at the surface dueto uplift associated with Yanshanian (Mesozoic) tectonic movements

Fig. 1. A simplified geological map of the Gejiu ore district (modified fromMao et al., 2008). Tfrom Cheng and Mao (2010). Some samples in the eastern sector were from underground

(308 Geological Party, 1984). Most of the outcrops in theGejiu area con-sist of Middle Triassic Gejiu Formation carbonate (>3000 m thick), andMiddle Triassic Falang Formation fine-grained clastic sediment and car-bonatewith interlayeringmafic lavas (1800 to 2800 m thick; Qin and Li,2008). Numerous faults are present in the Gejiu region, including theNNE-trending Longchahe, Jiaodingshan and Yangjiatian faults, theNE-trending Baishachong fault and the NS-trending Gejiu fault. TheGejiu fault, which is the dominant structure in the Gejiu District, di-vides the Gejiu area into its eastern and western sectors (Fig. 1),with the major deposits in the eastern sector. The Gejiu batholith isa prominent igneous body in this area, composed of Mesozoic gab-bro, mafic microgranular enclaves (MMEs; Cheng et al., 2012), por-phyritic biotite granite, equigranular biotite granite, syenites andmafic dykes. Mafic-intermediate rocks and alkaline rocks occur mostlyin the western sector, while granitoids are distributed throughout theregion. Granites in the vicinity of orebodies have intense greisen alter-ation, albite alteration and K-feldspar alteration, with or without skarnsin carbonate wallrocks.

3. Characteristics of the Gejiu granitic batholith

The Gejiu granitic batholith has an outcrop area of ~300 km2 and isone of the largest intrusions in the western Cathaysia Block (Fig. 1),

he triangles and data represent the sample locations and U–Pb zircon ages of intrusionsmine workings.

52 Y. Cheng et al. / Ore Geology Reviews 53 (2013) 50–62

intruding the Mid-Triassic sedimentary rocks. Based on texture and min-eralogy, the Gejiu granitic batholith comprises six phases (308 GeologicalParty, 1984; Cheng andMao, 2010; Dai, 1996; Li, 1985; Lu, 1987; Zhuanget al., 1996), which are summarized below.

Phase 1 is represented by porphyritic biotite granite with largeeuhedral K-feldspar phenocrysts that mostly measure 3–5 cm in length,but thatmay reach up to 10 cm. Phase 1 is represented by the Longchahestock in the western sector. The rock-forming minerals include plagio-clase (~40%), quartz (15–20 vol.%), K-feldspar (10–25 vol.%) and biotite(10–20 vol.%). Accessory minerals are magnetite, titanite, apatite, zirconand allanite. Phase 2 is fine-grained porphyritic biotite granite, typicallydistributed around the Baishaya area, which is close to the Longchahestock. The size and shape of phenocrysts vary in this phase. The majorminerals are K-feldspar (~40 vol.%), plagioclase (~25 vol.%), quartz(~25 vol.%) and biotite (5–10 vol.%). The accessory minerals are thesame as those from Phase 1. Phase 3 is porphyritic biotite granite withmedium-grained euhedral to subhedral K-feldspar phenocrysts, whichis best represented in theMalage, Songshujiao and Laochangoredeposits.Phenocryst size is generally between 1 and 3 cm,with larger phenocrystsreaching 6 cm. Minerals include plagioclase (~25 vol.%), K-feldspar(10–20 vol.%), quartz (~30 vol.%) and biotite (~10 vol.%), with magne-tite, zircon, apatite, titanite, fluorite, allanite and tourmaline as accesso-ry minerals. Phase 4 is a coarse tomedium-grained equigranular biotitegranite, mostly distributed in the Laochang ore deposit and in theShenxianshui, Baishachong and Kele areas. The major minerals includequartz (>30 vol.%), K-feldspar (~40 vol.%), plagioclase (~20 vol.%) andbiotite (~5 vol.%). Accessory minerals include zircon, magnetite, mona-zite, apatite and minor titanite. Phase 5 is medium to fine-grainedleucogranite in the Xinshan area of the Kafang deposit. Rocks of thisphase are fine-grained equigranular and light in color, containing littleor no biotite, and with a high content of quartz (~40 vol.%), K-feldspar(~35 vol.%) and plagioclase (~30 vol.%). Accessory minerals includezircon, apatite and monazite. Phase 6 refers to the fine-grained equi-granular granitic dyke swarm and small stocks around granite margins,especially around the Baishachong and Shenxianshui stocks. The majorminerals are quartz (~60 to ~70 vol.%) and K-feldspar (~ 25 vol.%), withminor occurrences of plagioclase, muscovite, zircon.

Recent studies have revealed that the Gejiu granite batholith formedbetween ~85 Ma and 77 Ma, is transitional frommetaluminous to weak-ly peraluminous and is high-K and alkali-rich (Cheng and Mao, 2010).Geochemical and Sr–Nd–Hf isotopic data suggest that the Gejiu graniticmagma experienced high degrees of fractional crystallization followingits formation due to the partial melting of the Mesoproterozoic crustwith minor input from mantle-derived magmas (Cheng and Mao,2010). The geochemical data indicate that degrees of fractional crystal-lization are distinctive from each other and that the later phases tend tobe more evolved. Mineralization is spatially related to highly evolvedgranitic phases.

4. Ore deposit geology

4.1. Malage Sn–Cu deposit

The Malage deposit is located in the northernmost part of the Gejiudistrict and occurs over approximately 40 km2. Thedeposit ismostly un-derlain by theMid-Triassic Gejiu Formation,which reaches a thickness ofmore than 1000 m and consists of limestone, dolomitic limestone, anddolomite. The NW-trending Masong anticline and its associated faultsare the major ore-controlling structures of the Malage ore (Sun et al.,1987). The faults in Malage deposit are considered crucial for the ore-forming fluid migration and ore genesis as they affected the interactionbetween fault deformation and fluid flow (Jiang et al., 1997). The LateCretaceous granites, the Baishachong equigranular granite in the northand the Beipaotai porphyritic granite in the south, are themain igneousrocks in this area. Although they have different petrological and geo-chemical compositions, they are believed to be derived from the same

magma chamber, with different degrees of fractionation (Cheng andMao, 2010; Lu, 1987).

TheMalage deposit is Sn–Cu-dominant, with varying amounts of Pb,Zn,W,Mo and Bemineralization. Two types of ore styles have been rec-ognized in this deposit (Fig. 2). The first is skarn, located in the contactareas between porphyritic granite and carbonate wallrocks. Tin and Cugrades range from 0.02% to 0.05% and 0.3% to 1.56%, respectively (308Geological Party, 1984). Ore minerals include pyrrhotite, chalcopyrite,pyrite, beryl, bismuth, molybdenite, scheelite, sphalerite, galena, cassit-erite and arsenopyrite. These ore minerals commonly occur as dissem-inations and/or as veins in skarn. The orebodies are generally lenticular,centered on Beipaotai porphyritic granite and are characterized by aclear metal zoning (308 Geological Party, 1984). The second type ofore style found in the Malage deposit is stratabound ore (weathered),examples of this ore style are present in some layers of various sedi-mentary rocks and are commonly distal to granite, with an average tingrade of 2.39% (308 Geological Party, 1984). Tin is the major metallicelement in this stratabound ore and is generally associatedwith varyingamounts of Cu, Pb, Zn, In and Bi, with a few gangue minerals of quartz.The shape of these orebodies is quite complicated, including stratabound,vein and tabular. Alteration in the Malage deposit is extensive, withskarn being the major alteration type, comprising garnet, pyroxene,tremolite, actinolite, wollastonite, epidote and phlogopite. However,other alteration types also occur, including alkali metasomatism(potassic and sodic), greisen (quartz-muscovite), sericite and chlo-rite alteration of the intrusive rocks.

4.2. Songshujiao Sn–Pb deposit

The Songshujiao Sn–Pb deposit is located to the southeast of theMalage deposit and to the northeast of the Gaosong deposit (Fig. 1).The Mid-Triassic Gejiu Formation in this deposit consists of dolomite,interbedded dolomitic limestone and limestone. The NNE-trendingWuzishan anticline and the NW-trending Baishachong fault, togetherwith various folds and fractures, are the main structural componentsin the area of the Songshujiao deposit. Centered on granitic “humps”of the batholith are well developed fault slip zones between layers ofdifferent rock types (Fig. 2) that provide critical conduits for graniticmagma-derived hydrothermal fluids and space for the precipitation ofore minerals. Three groups of faults occur in the Songshujiao depositthat trend NW, NE and E-W. The fault zones are usually approximately3 m wide and are infilled with small amounts of oxidized ores. Igneousrock in the Songshujiao deposit is porphyritic biotite granite. Skarnhas commonly developed around the contact areas between graniteand carbonate wallrocks.

Two types of mineralization are recognizable in the Songshujiao de-posit. Skarn Sn–Cu ores have developed mainly along the contact areasbetween porphyritic granite and Gejiu Formation carbonates. This typeof ore occurs as lenses, strata-bound bodies or as veins, and it is gener-ally distributed around granitic cupolas. Thus, the size of the orebodiesgenerally varies from ~5 to ~50 m thick (locally up to ~100 m thick),and from 100 m to 500 m long. The skarn minerals include pyroxene,scapolite and garnet. The oreminerals are pyrrhotite, chalcopyrite, arse-nopyrite, marmatite, pyrite, magnetite, cassiterite, scheelite, and mo-lybdenite. The deposit contains Sn, Cu, Zn and W with minor amountsof In, Bi and Ag. Skarn type ore constitutes approximately 66% of theSn and ~80% of the Cu resources in the Songshujiao deposit.

Stratabound Sn–Pb ore, the second type of ore, is mainly hostedin interbedded limestone and dolomite and is distal from the granite(Fig. 2). Lead ore of this type accounts for approximately 60%–70% ofthe total Pb resources in the Songshujiao deposit. The occurrence ofsuch ores is relatively complex and commonly controlled by fault, foldand/or fault slip structures. The strike length of the stratabound orebodiesis 20–50 m,with a few reachingmore than 100 m; their thickness is com-monly >20 m. Most of these ores are weathered, containing limonite

Fig. 2. Cross sections of the five deposits in the Gejiu district that show the spatial relationship of granite, orebodies and host rocks (modified from Mao et al., 2008).

53Y. Cheng et al. / Ore Geology Reviews 53 (2013) 50–62

(~ 70%), calcite (~ 15%), and quartz (~ 5%), withminor concentrationsof fluorite, cassiterite, pyrite, arsenopyrite, cerussite and malachite.

4.3. Gaosong Sn–Pb–Zn deposit

The Gejiu Formation in Gaosong consists of the following threeunits: 1) the Bainidong Unit, consisting primarily of light gray togray limestone, with minor banded and lenticular calcareous dolo-mite; 2) the Malage Unit, consisting primarily of dark gray to graylimey dolomite, with minor dolomitic limestone; 3) the KafangUnit, which is mainly composed of gray to light gray limestone andlimey dolomite interlayers. The Kafang Unit is the main host of theGaosong mineralisation. The 8-km long NE-trending Lutangba faultand its associated subsidiary faults are the major structures control-ling ore. They cut the Kafang Unit, acting as major conduits forore-forming fluids. Greisen (quartz–muscovite) alteration, musco-vite, sericite and skarn minerals have developed intensively on themargins of the granitic cupola and in the contact zone between graniteand carbonate.

Two types of ore occur in Gaosong. Type 1 skarn-sulfide ore oc-curs along the contact zones between granite and limestone and/ordolomite, with disseminated, veinlet and massive ores hosted byskarn. The primary skarn minerals are pyroxene, garnet and scapo-lite. The sulfides are associated with retrograde skarn minerals(actinolite, tremolite and chlorite). The major ore minerals includearsenopyrite, pyrrhotite, chalcopyrite and marmatite, with lesseramounts of cassiterite, pyrite, scheelite, native bismuth, molybdenite

and magnetite. The average Sn grade is ~0.5–1%, with the highestgrade up to 3%.

Type 2 ores are weathered and have complex orebody shapes, in-cluding stratabound, irregular banded, lenticular and veinlet. Theorebodies occur in different layers that are controlled by fault slipzones and fractures (Fig. 2) and typically consist of several ore layersat different levels, generally 3–5 layers, locally up to 8–9 layers. Type2 orebodies can be tens to hundreds of meters long and 100–200 mwide. The weathered ores consist of limonite, hematite, goethiteand clay minerals, with minor amounts of cassiterite, marmatite, an-glesite, pyrolusite and malachite. Gangue minerals include primarilyscorodite, siderite, calcite, quartz, phlogopite, fluorite, tourmaline,chlorite, garnet, tremolite, pyroxene, with minor amounts of plagio-clase, jarosite and kaolinite. The average Sn grade is ~2%.

4.4. Laochang Sn–W–Cu polymetallic deposit

The Laochang Sn–W–Cu polymetallic deposit is situated betweenthe Gaosong deposit in the north and the Kafang deposit in thesouth. It is the most important deposit in the Gejiu district, containing~50% of the tin resources of the whole district. The Laochang depositis bounded by the Beiyinshan fault in the north and the Laoxiongdongfault in the south. In Laochang, six EW-trending faults provided con-duits for the ore forming fluids and the space for the precipitationof ore minerals. Six NE- and/or NW-trending faults cut the Laochangdeposit. The stratigraphy is similar to the deposits described above(Fig. 1), and the interlayer zones are the same ore-hosting locations.

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Equigranular and porphyritic granites are the major igneous rocks inthis area, which intruded into the Mid-Triassic Gejiu Formation. Themajor mineralization styles in the Laochang deposit are skarn ores,stratabound ores and carbonate hosted veins (Fig. 2).

Skarn ores include skarn W ores, skarn-sulfide Sn ores and Cuores, associated with minor amounts of Bi, In and Ga mineralization.Tin in skarns accounts for ~60% of all the Sn resources in Laochang.The distribution of orebodies is influenced by the geometry of the granitecontact areas, with most ores occurring around granitic “humps”.Major metallic minerals are pyrrhotite, arsenopyrite, chalcopyrite,pyrite, marmatite, cassiterite and scheelite. Major gangue mineralsare pyroxene, garnet, plagioclase, fluorite, phlogopite, quartz andchlorite. Small amounts of ores are weathered to an assemblage oflimonite, goethite, malachite and plumbojarosite, and these are asso-ciated with sericite, phlogopite, muscovite, quartz and calcite.

Stratabound ores occur in the interlayer zones of limestone and dolo-mite, and these types of ores contain approximately 25%of the total Sn re-sources in Laochang. The ores have been strongly weathered, currentlyprimarily composed of cassiterite, hematite, limonite, goethite,malachite,scorodite, conichalcite, anglesite, cerusite, plumbojarosite and wadite,withminor amounts of arsenopyrite, pyrite, chalcopyrite, sphalerite, gale-na and marmatite.

Vein-type mineralization comprising tourmaline–quartz veins,tourmaline–K feldspar–skarn veins, tourmaline–skarn–cassiteriteveins, and tourmaline–phlogopite veins in Gejiu Formation carbon-ate. The scale of mineralization and vein distribution is influencedby host rock type, stratigraphy, structure, and association with near-by granite (Cheng et al., 2012). The host rocks are folded and faultedlimestone and dolomite of the Mid-Triassic Gejiu Formation. The orezone has an approximately rhombic shape, bordered by theHuangnidongfault in the east, the Aotoushan fault in the west, the Meiyuchongfault in the north and the Longshupo fault in the south. Granite bod-ies occur 200–1000 m beneath the surface and are aligned along a NEtrend beneath the ore veins. The dimensions of the veins vary inlength from several centimeters to more than 200 meters and inwidth from several millimeters to approximately 1 meter. Tin is themajor metal produced from these ore veins and the average gradesof Sn, Be and WO3 of the vein-type orebodies are 0.42%, 0.13%, and0.11%, respectively.

4.5. Kafang Cu–Sn deposit

The Kafang Cu–Sn deposit is located in the southernmost part of theGejiu district (Fig. 1), to the east of the Gejiu fault and between the paral-lel EW-striking Laoxiongdong andXianrendong Faults. Basaltic lavaflowswith a thickness of 60–100 m are intercalated with the Gejiu Formationcarbonate beds (Fig. 2). The Xinshan granitic pluton is fine-grainedequigranular biotite granite intruded into theGejiu Formation carbonatesand the interbedded lava. Three types of ores are recognizable in Kafang(Fig. 2).

Type 1 skarn Cu–Sn–Mo–W–Au–Bi polymetallic ores occur in thecontact areas between Xinshan granite and carbonate wallrocks (Fig. 2).Ore minerals include magnetite, chalcopyrite, pyrrhotite and cassiterite,and minor minerals include pyrite, scheelite, native bismuth, nativegold, sphalerite, galena, molybdenite, wolframite and arsenopyrite.Skarn minerals include garnet, pyroxene, epidote, actinolite, tremolite,sericite, chlorite, calcite and quartz.

Type 2 stratabound Cu-dominant ores are hosted in the basaltic lavas.The orebodies are ~200–400 m long and up to 10 m wide. The gangueminerals include actinolite, phlogopite and tremolite, with minoramounts of pyroxene, calcite and fluorite. The ore minerals are pri-marily chalcopyrite, pyrite, arsenopyrite, molybdenite and pyrrho-tite. The orebodies are roughly sheet-like or tabular.

Type 3 stratabound Cu–Sn–Pb–Zn polymetallic ores are hosted inlimestone and/or dolomitic limestone layers up to 2000 m from thegranitic pluton. The metallic minerals include pyrrhotite, cassiterite

and pyrite, and the gangue minerals are variable amounts of quartz,tourmaline, tremolite and fluorite. The contact areas between orebodiesand host rocks are sharp. Localized ore veins, emanating from the mainorebodies, cut the host rock. Alteration around these orebodies is minor,but some banded skarns (garnet, pyroxene, wollastonite and calcite)occur parallel to the bedding of the host rocks and parallel to theorebodies; these are better developed closer to the orebodies.

5. Metal zoning

The mineral deposits in the Geiju district are distinctly zoned out-ward fromW+Be+Bi±Mo±Sn deposits in the granite cuppolas, toSn+Cu-dominated deposits at the granite contact zones, and Pb+Zndeposits in the surrounding host rocks (Fig. 3). This metal zoning isdeveloped both vertically and horizontally (Fig. 3).

(1) Inner W–Be–Bi± Mo±Sn zoneThis zone mostly occurs in the interior of granite (endogranitic)or its margins. The size of the W, Be, Bi, Mo orebodies are usu-ally limited. Examples of this style of mineralization includethe Malage and Laochang Be deposits, the Xinshan W depositand the Xinshan Bi deposit. Tungsten is generally more abun-dant and more widespread than Be and Bi (308 GeologicalParty, 1984). Be–Bi mineralization is most common aroundequigranular (non-porphyritic) granite, whereas W (scheelite)is developed in both equigranular and porphyritic granites(Fig. 4). Gangue minerals include large quartz/feldspar crystals,garnet and pyroxene.

(2) Middle Cu–Sn zoneCopper and Sn are themost abundant metals in this zone, whichhosts almost 90% of the Cu–Sn resources in the Gejiu area. Thetin and Cu orebodies generally occur together, although the Cuores are generally located closer to the granites than the Snores (Fig. 3). The ore minerals are mainly cassiterite and chalco-pyrite, occurring locally primarily with pyrrhotite, pyrite, arse-nopyrite, molybdenite and with minor amounts of sphaleriteand galnea. Prograde and retrograde skarn minerals are repre-sented by garnet, pyroxene, wollastonite, tremolite, epidote, chlo-rite and phlogopite in this zone (Fig. 4).

(3) Distal Pb–Zn zoneOutwards from the Cu–Sn zone is a zone with dominant Pband Zn sulfides (Fig. 3) that is closer to the surface and distalto granite. Examples include the Malage Pb–Zn deposit, theSongshujiao Pb deposit and the Laochang Pb deposit. Fieldevidence indicates that there were at least two episodes ofPb mineralization (308 Geological Party, 1984), with theabove-mentioned deposits belonging to the early stages. A laterPb±Zn mineralization stage exists as veins in both wall rocksand granites with no Sn mineralization (Fig. 4) (308 GeologicalParty, 1984).

6. Ar–Ar dating techniques and results

6.1. Analytical techniques

For our study, thirteen samples containing 0.18 to 0.28 mm micagrains were wrapped in aluminum foil, loaded into an aluminumtube, then sealed into a quartz bottle and irradiated for 24 h in theSwimming Pool Nuclear Reactor at the Chinese Institute of AtomicEnergy in Beijing by fast neutrons with a flux of 2.2464×1018 ncm−2. The monitor used in this analysis was an internal standard ofFangshan biotite (ZBH-25) with an age of 132.7±1.2 Ma and a potassi-um content of 7.579±0.030 wt.% (Wang, 1983). The measured isotopicratioswere corrected formass discrimination, atmospheric argon compo-nents, blanks and irradiation-induced mass interference. The correctionfactors of interfering isotopes produced during irradiation were deter-mined by an analysis of irradiated pure K2SO4 and CaF2. Their values

Fig. 3. Horizontal and vertical metal zoning patterns in the Gejiu district (modified from 308 Geological Party, 1984). a-Horizontal zoning of metallic elements in the whole easternmining district. The zoning pattern is similar in individual deposits, which center on granitic cupolas in each deposit. b-Plan map showing horizontal metal zoning in the Malagedeposits (above) and the cross section along A-A' showing vertical zoning (below). c-Vertical metal zoning pattern in the Gaosong deposit.

55Y. Cheng et al. / Ore Geology Reviews 53 (2013) 50–62

corresponded to the following: (36Ar/37Ar) Ca=0.000271; (40Ar/39Ar)k=0.00703; (39Ar/37Ar) Ca=0.000652. The 40Ar/39Ar step-heating anal-ysis was performed at theME Key Laboratory of Oregenic Belt of CrustalEvolution in Peking University and the detailed experimental process hasbeen previously reported (Gong et al., 2008). The 40Ar/36Ar vs 39Ar/36Arisochron diagramwas defined by using the ISOPLOT version 3.0 programof Ludwig (2003).

6.2. Results

The 40Ar/39Ar analytical results are summarized in Table 1 and thedetails are listed in Appendix 1 and Appendix 2, which are availableas a digital supplement to this paper. In this study, apparent agesobtained from 40Ar/39Ar analyses at low temperatures are not consideredto have geological significance because of the low percentage of 39Arkreleased (Guo et al., 2011), which was likely caused by the initial loss ofsmall quantities of Ar from the edges of mineral grains (Hanson et al.,1975). Plateau ages of this study were determined from six or more con-tiguous steps that comprise >75% of the total 39Ar released. The uncer-tainties of the ages are reported at a 95% confidence level (2σ).

Two muscovite samples were collected from the Malage deposit forAr–Ar analysis to constrain the age of the weathered ore (sample 8255)and the retrograde-skarn ore (sample 8253). 40Ar/39Ar stepwise heatinganalyses of muscovite over the higher temperature intervals yieldeduniform and remarkably flat 40Ar/39Ar age spectra with plateau agesfor 90.3% of the total 39Ar released (Fig. 6), indicating the absence of ex-cess argon or any diffusive argon loss. Six plateau ages were obtainedfrom sample 8255 at temperatures between 1000 and 1400 °C. These6 pieces of data are consistent with the release of 90.2% of the total39Ar. The 95.4±0.7 Ma and 89.7±0.7 Ma ages for samples 8253and 8255, respectively, are believed to be reliable estimates of the

crystallization age of muscovite from the oxidized ore and skarnore in the Malage deposit.

One muscovite sample (sample SSJ-01) from greisen ore and onephlogopite sample (sample 7060) from skarn ore in Songshujiao de-posit were collected. Ten plateau ages were obtained from sampleSSJ-01 at temperatures between 900 and 1400 °C, and these constitutea flat age plateau comprising 98.8% 39Ark released. Eight plateau ageswere obtained from sample 7060 at temperatures between 1000and 1400 °C with 91.7% 39Ark released. The plateau ages of 85.5±0.7 Ma and 92.3±0.6 Ma are believed to be the best estimates of theage of the greisen ore and the skarn ore in the Songshujiao deposit,respectively.

Twomuscovite samples from Gaosong deposit were collected torepresent the retrograde skarn ore (sample GS-01) and weatheredore (sample 7091). Eight plateau ages were obtained from sample7091 at temperatures between 1000 and 1350 °C, with 96.2% 39Arkreleased, yielding a well-defined plateau age of 85.6±0.7 Ma. Twelvestep-heating experiments at temperatures ranging from 850 °C to1400 °C were performed for sample GS-01; of these, seven stepsduring 950 °C to 1250 °C constituted a flat age plateau encompassing94% 39Ark released and yielded a well-defined plateau age of 84.3±0.6 Ma.

Four muscovite samples, including samples 8145 and 8187 fromgreisen ores in different locations, sample 8180 from oxidized ore, andsample 8120 from vein ore, were collected from the Laochang depositto represent four differentmineralization types. Greisen samples yieldedwell-defined plateau ages of 85.6±0.6 Ma and 87.4±0.6 Ma. Sample8145 was analyzed at a temperature between 950 °C to 1250 °C with79.3% 39Ark released, and sample 8187 was analyzed at a temperaturebetween 950 °C to 1400 °C with 89.2% 39Ark released. Seven plateauages were obtained from the oxidized ore sample (sample 8180) at tem-peratures between 1000 and 1300 °C with 84.4% 39Ark released to yield a

Fig. 4. Photographs showing field characteristics, mineral associations, mica analyzed in this study and their relationships with other intergrowth minerals from the Gejiu ore district.

56 Y. Cheng et al. / Ore Geology Reviews 53 (2013) 50–62

plateau age of 77.4±0.6 Ma. The plateau age of the vein ore (sample8120) yielded 87.5±0.6 Ma, based on 76.4% 39Ark released between950 °C to 1400 °C. As shown in Appendix 2, the isochron ages and

inverse isochron ages of these samples are indistinguishable fromtheir plateau ages, and we therefore believe that these 40Ar/39Ardates are reliable.

Table 140Ar–39Ar dating results of Gejiu tin–copper deposits.

Sampleno.

Samplerepresentative

Sample locations Minerals Age Error

8255 Oxidized ore Malage deposit Muscovite 89.6 0.658253 Skarn ore Malage deposit Muscovite 95.3 0.66SSJ-01 Greisen ore Songshujiao deposit Muscovite 85.2 0.657060 Skarn ore Songshujiao deposit Phlogopite 92.6 0.63GS-01 Skarn ore Gaosong deposit Muscovite 84.3 0.597091 Oxidized ore Gaosong deposit Muscovite 85.4 0.668145 Greisen ore Laochang deposit Muscovite 85.5 0.578187 Greisen ore Laochang deposit Muscovite 87.4 0.588180 Oxidized ore Laochang deposit Muscovite 77.4 0.578120 Vein ore Laochang deposit Muscovite 87.7 0.63ZKF08-49 Greisen ore Kafang deposit Muscovite 79.7 0.57ZKF08-29 Basalt host

stratiform oreKafang deposit Phlogopite 79.8 0.47

ZKF08-62 Mineralized basalt Kafang deposit Phlogopite 85.6 0.63

57Y. Cheng et al. / Ore Geology Reviews 53 (2013) 50–62

One muscovite sample and two phlogopite samples were collectedfrom the Kafang deposit to constrain the age of greisen ore (sampleZKF08-49), basalt host stratabound ore (sample ZKF08-29) and alteredbasalt (sample ZKF08-62). The plateau ages of these samples are79.5±0.6 Ma, 79.6±0.5 Ma and 85.5±0.6 Ma, respectively. Tem-peratures were relatively high (>850 °C) and the39Ark released atmore than 80%. These three plateau ages agree with their isochronand inverse isochron ages, indicating that they are reliable and of highquality.

7. Discussions and conclusions

7.1. Timing of mineralization

The 13 representative muscovite/phlogopite samples from differ-ent mineralization environments in this study show excellent agree-ment between the plateau age, the isochron age and the inverseisochron age, within the applicable analytical uncertainty, indicatingthe absence of excess argon or any diffusive argon loss (Appendix 1and 2). Therefore, the 13 well-defined plateau ages, from 77.4±0.6 Ma and 95.3±0.7 Ma, that we report are accurate and believedto be a reliable estimate for the crystallization time of these mica(Fig. 5).

As shown in published works (308 Geological party, 1984; Chengand Mao, 2010; Cheng and Mao, 2012; Dai, 1996; Li, 1985; Li et al.,2011; Lu, 1987; Mo, 2006; Zhuang et al., 1996), diverse textures, min-eral associations, major and trace elements and isotopic compositionsof granites in the Gejiu district indicate multi-stage crystal fraction-ation. However, as opposed to the intrusive rocks in the western sec-tor that are commonly exposed and whose petrochemical features areavailable for study, exposures of granites in the eastern part of theGejiu mining district are limited (Fig. 1). The majority of the mineral-ization associated with the granitic cupolas is well below the surfaceand can be observed only in drill cores or in mining adits, thereby pre-cluding a direct understanding of the granite-phase variations. Existinghigh precision zircon U–Pb LA-ICP-MS/SHRIMP dating results revealthat the crystallization age of the granites is Late Cretaceous (~78 Mato ~85 Ma, Cheng and Mao, 2010; Cheng et al., 2012), which is approxi-mately consistent with the mineralization–alteration Ar–Ar age of themica samples (~78 Ma to ~95 Ma) presented in this study (Fig. 6).

Although our mineralization ages are not completely consistentwith magmatism dating results, this is not unusual. For example,long-lived hydrothermal systems can last for several tens of millionyears, such as in La Escondida, Chile (Padilla-Garza et al., 2004),

Chuquicamata, Chile (Ballard et al., 2001), El Teniente (Maksaevet al., 2004), Río Blanco, Argentina (Deckart et al., 2005), Bajo de laAlumbrera, Argentina (Harris et al., 2008), Fresnillo, Mexico (Velador etal., 2010), and Laramide, Mexico (Barra et al., 2005). In this case, asthe Gejiu granitic batholith is made up of multiple phases, eachwith a corresponding long-lived magma chamber related to miner-alization is possible. Therefore, it would be reasonable to assumethat these different phases were accompanied by multiple alter-ation–mineralization events. Moreover, the Mesozoic magmatismand mineralization events in the western Cathaysia Block occurredfrom approximately 100 Ma to 80 Ma (Cheng and Mao, 2010; Maoet al., 2008), which supports the contention that the mineralizationages of the Sn–Cu ores in the Gejiu district lie in the range of~77 Ma to ~95 Ma, as measured by this work.

7.2. Implications of metal zonings

Metal/mineral/alteration zoning of granite-related ore systemshas been recognized since the 1920s (Emmons, 1924; Park, 1955;Wong, 1920) and has been identified by many scientists all overthe world. This is exemplified by Sn–W deposits from around theworld, including in the Bolivian tin deposits (Ahlfeld, 1941; Lehmann,1990), the Cornwall Sn polymetallic district (Bromley and Holl, 1986),the Erzgebrige Sn deposits (Stemprok, 2003), and in the Dachang Sndistrict (Chen and Li, 1993). Metal zoning also found with other ores,such as Au-polymetallic deposits (James and Baker, 2001; Markowskiet al., 2006; Meinert et al., 1997; So et al., 1998) and Pb–Zn–Ag deposits(Baumgartner et al., 2008; Newberry et al., 1991; Plumlee andWhitehourse-Veaux, 1994). For the origin of these metal zoningsand their implications, Ashley and Plimer (1989) re-examined thecontroversial Zn/Sn skarn deposits in eastern Australia and provedthat the mineralization of these deposits is closely associated withgranites, emphasizing that there was no possibility that the oreswere of syngenetic origin. Newberry et al. (1991) further strength-ened the theory that the genesis of the Pb–Zn–Ag zoning of theDarwin deposit in California should be attributed to the variationsof temperature, pH and oxidation state of the granitic magma-derived ore-forming fluids. Temperature changes of hydrothermalfluids are very important for the precipitation of ore minerals andtherefore metal zoning, as revealed by several studies (Pirajno,1992; Wu and Jin, 1993). Audétat et al. (2000) suggested that themetal zoning of Sn–W deposits around the Mole Granite reflectedboth compositional variations of the source fluid and sequentialmetal precipitation. For the Ertzgebirge Sn-bearing granite batholith,Stemprok (2003) proposed that the tectonics along the granite con-tact areas and several deep-seated fault zones were also importantfor the mobilization of the hydrothermal fluids and distribution ofthe ore zoning around the granite pluton. Thus, we can concludethat the above-mentioned metal/alteration zonings are geneticallyrelated to intrusions (mostly granitic) and are influenced by the na-ture of the hydrothermal fluids and surrounding geologicalcharacteristics.

Fig. 3 shows that metal zoning occurs both vertically and laterallyaway from the various granitic bodies in the Gejiu Sn-polymetallicdistrict. The general sequence of zoning is upward and outwardfrom W–Be–Bi±Mo±Sn to Cu–Sn to Pb–Zn at the periphery. Miner-alization types change consistently upward, from disseminated ingranite outward to skarn along the contact area with carbonatebeds, to oxidized ores and eventually to veins in limestone units distalto granites. This pattern is similar to those established for the Corn-wall W–Bi–Sn–Cu–Pb–Zn–Ag ore district in SW England (Bromleyand Holl, 1986). The zonings suggest a close genetic relationshipbetween magmatism and mineralization. The metal zonings arebelieved to have been caused by the progressive cooling ofmagma-derived hydrothermal fluids, perhaps combined with fluid–

Fig. 5. 40Ar/39Ar age spectra of muscovite and phlogopite separate from the Gejiu tin polymetallic district, SW China. Data uncertainty is given as 2σ.

58 Y. Cheng et al. / Ore Geology Reviews 53 (2013) 50–62

rock interaction or fluid–fluid mixing, as they move upward and out-ward away from the magmatic source.

7.3. Genetic implications and exploration significance

Controversies are long-standing about the genesis of the Gejiu Sn–Cu polymetallic district, with arguments focusing on whether the

deposits are of the SEDEX type or granitic intrusion-related (308Geological Party, 1984; Mao et al., 2008; Qian et al., 2011a, 2011b; Qinet al., 2006; Yu et al., 1988; Zhang et al., 2003, 2005; Zhou et al., 1997,1999; Zhuang et al., 1996; and the references therein). The geneticlink between magmatism and mineralization is demonstrated bythe distinctive spatial association and zoning around granitic cu-polas and their greisen-style alteration with Sn–W mineralization

Fig. 6. Spatial–temporal relationships between granites and orebodies in the Gejiu mining district (modified from Cheng et al., 2008).

59Y. Cheng et al. / Ore Geology Reviews 53 (2013) 50–62

that leads outward to a sequence of exoskarns with metal associa-tions from proximal to distal. Importantly, the granite-mineral sys-tem genetic coupling is supported by systematic geochronologicconstraints from the Ar–Ar ages in this study. The age data show amineralization age of 77 Ma to 95 Ma, much younger than the age ofthe host Mid-Triassic Gejiu Formation sedimentary carbonate rocksand correlating closely with the timing of the granite intrusions(Cheng and Mao, 2010; Fig. 6). We conclude that the Sn–Cupolymetallic ores of the Gejiu ore district are of hydrothermal originand granite-related.

Step T (°C) (40Ar/39Ar)m (36Ar/39Ar)m (37Ar/39Ar)m

7060, Skarn ore sample from Songshujiao deposit, J=0.0054971 850 45.6398 0.1185 5.99362 900 14.5381 0.0272 0.68433 950 11.3942 0.0102 0.17714 1000 10.6827 0.0039 0.15925 1050 10.4156 0.0028 0.08556 1100 10.1969 0.0022 0.08127 1150 10.6082 0.0035 0.06358 1200 10.3956 0.0030 0.05539 1250 10.7584 0.0042 0.009310 1300 12.3698 0.0098 0.615111 1400 69.2145 0.2074 27.4870

7091, Oxidized ore sample from Gaosong deposit, J=0.0052431 850 61.0124 0.1695 16.32432 900 26.5211 0.0476 3.91723 950 16.9420 0.0293 1.76034 1000 12.2552 0.0103 0.54525 1050 12.6975 0.0118 0.78096 1100 11.8796 0.0090 0.38067 1150 12.5284 0.0112 0.37478 1200 13.3860 0.0141 0.39839 1250 11.0045 0.0059 0.218110 1300 10.2680 0.0036 0.092511 1350 10.6706 0.0049 0.710912 1400 49.6609 0.1676 15.5659

7120, Vein ore sample from Laochang deposit, J=0.0051541 850 15.6048 0.0285 3.23902 900 11.3162 0.0149 1.64403 950 10.4444 0.0028 0.23554 1000 11.0829 0.0051 0.34885 1050 12.6834 0.0103 0.31976 1100 11.2582 0.0056 0.17997 1150 10.9487 0.0044 0.06248 1200 9.7553 0.0003 0.20569 1250 9.9861 0.0012 0.064210 1300 10.9591 0.0012 0.098611 1400 10.6666 0.0050 0.2396

Appendix 1

Acknowledgments

Two anonymous reviewers are thanked for their professional reviewof the paper. We appreciate the assistance of Jiyao Tu, Rongshuang Sunand Jianqing Ji with 40Ar–39Ar dating analysis in Peking University,China. We also want to express our thanks to Guopei Mo, Xiang Tong,Junde Wu and other local geologists from the Yunnan Tin Group fortheir assistance during our field work. Xiaolong Li and Juan Zhangfrom China University of Geosciences (Beijing) are thanked for theirhelp on field works. This study was supported by the National Natural

(38Ar/39Ar)m 40Ar*/39Ar 39Ar (×10-15 mol) 40Ar* (%)

0.0753 11.1265 2.55 24.280.0223 6.5491 21.30 45.030.0233 8.3985 92.20 73.700.0203 9.5358 143.00 89.250.0205 9.5881 198.00 92.050.0208 9.5591 246.00 93.740.0203 9.5780 270.00 90.280.0192 9.5195 324.00 91.570.0126 9.5189 83.40 88.480.0205 9.5334 22.90 77.040.0842 10.3227 1.14 14.65

0.2839 12.3585 2.62 20.040.0575 12.8023 8.49 48.150.0165 8.4300 18.10 49.700.0169 9.2606 60.90 75.540.0156 9.2655 39.70 72.930.0167 9.2580 75.10 77.910.0175 9.2586 73.20 73.880.0170 9.2523 143.00 69.100.0155 9.2818 136.00 84.330.0166 9.2144 208.00 89.730.0211 9.2921 60.60 87.040.1510 1.3889 2.05 2.77

0.0454 7.4492 12.40 47.640.0217 7.0471 19.30 62.210.0134 9.6240 84.40 92.130.0108 9.5918 84.30 86.530.0131 9.6491 102.00 76.060.0123 9.6205 75.70 85.440.0116 9.6615 211.00 88.240.0106 9.6754 112.00 99.170.0124 9.6313 317.00 96.440.0126 10.6202 197.00 96.900.0117 9.2183 76.70 86.41

(continued on next page)

(continued)

Step T (°C) (40Ar/39Ar)m (36Ar/39Ar)m (37Ar/39Ar)m (38Ar/39Ar)m 40Ar*/39Ar 39Ar (×10-15 mol) 40Ar* (%)

8145, Greisen ore from Laochang deposit, J=0.0051421 850 9.8028 0.0108 1.1735 0.0368 6.7067 38.00 68.362 900 9.3168 0.0033 0.4415 0.0156 8.3833 91.00 89.953 950 10.4176 0.0032 0.3639 0.0128 9.5044 134.00 91.214 1000 10.4723 0.0037 0.1551 0.0145 9.3971 184.00 89.725 1050 10.2108 0.0026 0.1100 0.0127 9.4482 259.00 92.536 1100 10.2029 0.0025 0.1810 0.0128 9.4887 175.00 92.997 1150 10.0673 0.0023 0.1937 0.0118 9.3898 197.00 93.268 1200 10.2788 0.0030 0.1517 0.0132 9.4070 227.00 91.519 1250 10.6010 0.0040 0.1653 0.0124 9.4293 183.00 88.9410 1300 9.6347 0.0034 0.2908 0.0128 8.6552 107.00 89.8211 1400 9.6189 0.0040 0.2452 0.0130 8.4614 118.00 87.95

8180, Oxidized ore from Laochang deposit, J=0.0051231 850 12.0702 0.0095 5.3797 0.0254 9.7123 7.38 80.182 900 16.5634 0.0214 3.1615 0.0100 10.5227 13.30 63.403 950 12.1315 0.0122 1.2944 0.0032 8.6178 26.60 70.984 1000 9.9469 0.0050 0.7022 0.0147 8.5317 41.00 85.735 1050 10.0040 0.0048 0.5209 0.0150 8.6190 73.10 86.136 1100 8.8585 0.0011 0.3333 0.0127 8.5512 94.60 96.517 1150 9.0491 0.0016 0.2558 0.0129 8.5824 105.00 94.838 1200 9.1658 0.0020 0.4217 0.0169 8.5944 60.30 93.749 1250 9.9655 0.0054 1.0112 0.0147 8.4653 47.80 84.8910 1300 11.8393 0.0114 0.5829 0.0151 8.5152 46.20 71.9011 1400 12.7722 0.0106 0.7804 0.0172 9.7066 39.10 75.96

8187, Greisen ore from Laochang deposit, J=0.005501

Appendix 1 (continued)

60 Y. Cheng et al. / Ore Geology Reviews 53 (2013) 50–62

Appendix 2

61Y. Cheng et al. / Ore Geology Reviews 53 (2013) 50–62

62 Y. Cheng et al. / Ore Geology Reviews 53 (2013) 50–62

Science Foundation of China (40930419), the Special Research Fundingfor the Public Benefits Sponsored by MLR (200911007–12), the Re-search Programof Yunnan Tin Group (2010-04A), and the FundamentalResearch Funds for the Central Universities (2-9-2010-21).

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