ARTICLE

Guoliang Ma ˘ Georges Beaudoin ˘ Sijing Qi ˘ Ying Li

Geology and geochemistry of the Changba SEDEX Pb-Zn deposit, Qinlingorogenic belt, China

Received: 7 August 2003 / Accepted: 20 February 2004 / Published online: 16 April 2004� Springer-Verlag 2004

Abstract The Changba Pb-Zn SEDEX deposit occurs inthe Middle Devonian sequence of the Anjiaca Formationof the Western Qinling Hercynian Orogen in the Gansu

characterized by the coexistence of two types of deposits:1) stratiform SEDEX deposits hosted by fine-grainedclastic rocks in the northern part of the belt; and2) stratabound deposits hosted by chert in the southernpart of the belt. The Changba deposit is an example ofstratiform SEDEX whereas the Dengjiashan deposit isan example of the stratabound deposits (Fig. 1).

Changba is the largest, and economically mostimportant, deposit in the Qinling belt with total Pb +Zn metal reserves of 11.2 million tonnes. It was dis-covered in the early 1960s and exploited during the1980s. Between the middle 1970s and middle 1980s, theChangba deposit was regarded as a Mississippi Valley-Type deposit based on Pb and S isotope compositionsand fluid inclusion data (Wang 1984). A SEDEX originbased on exhalative hydrothermal features was proposedby Qi and Li (1993) and Wang (1996), reinterpretationof the regional tectonic, magmatic, sedimentary andmetamorphic history of the region, and comparison withSEDEX deposits in other parts of the world (Large1983; Goodfellow et al. 1993; Lydon 1996). This paperdescribes the geology and geochemistry of the Changbadeposit that indicates an origin by exhalation of basinalbrines during Mid-Devonian subsidence of the Qinlingbasin.

Regional geological setting

Qinling is an east-west orogenic belt at the contact be-tween the North China and the Yangtze cratons (Wang1982; Fig. 1). The Middle Paleozoic collision of theNorth China block and the South Qinling along theShangdan suture, and the Late Triassic collision betweenthe South Qinling and the Yangtze Craton along theMianlue suture resulted in uplift of the Proterozoic andPaleozoic sedimentary rocks to form the Qinling Range(Meng and Zhang 2000). The northern belt was thesouthernmost active continental margin of the NorthChina Craton, whereas the southern belt was thenorthernmost passive margin of the Yangtze Cratonbefore their Middle Paleozoic collision (Meng andZhang 2000). The northern belt comprises a Proterozoicmetamorphic complex of metagraywackes, marbles andmetatholeiites (Zhang et al. 1994). It also contains asuite of Late Proterozoic and Cambro – Ordovicianophiolites (Zhang 1988), dominated by tholeiitic spiliteswith several horizons of quartz keratophyre, graywacke,marble and siliceous rock. These ophiolites are inter-preted to have formed in an island-arc or marginal-seabasin environment (Zhang 1988; Zhang 1993).

In the southern belt, the Lower – Middle Proterozoicmetamorphic complex is a metasedimentary sequence ofparagneiss and marble, overlain by a Middle to UpperProterozoic complex of volcanic arc succession evolvingfrom low-K tholeiite through calc-alkaline to alkalinebasalt. The Sinian (800 – 700 Ma) to Silurian carbonate,shale and sandstone were deposited on a carbonateplatform. The Cambrian rocks are mainly carbona-

ceous, rich in P, Mo, V and Cu, and contain alkalinevolcanic rocks accompanied by coeval swarms of alka-line dikes (Gao et al. 1995). The Siluro-Devonian rocksform a flysch sequence of metagraywacke, slate, phylliteand carbonate that indicates large-scale subsidence andrapid sedimentation (Zhang 1988).

The Devonian sediments were deposited in subordi-nate fault basins during N-S extension of an intracra-tonic basin (Zhang 1988). Devonian rocks locallyunconformably overlie the Lower Paleozoic-Sinianrocks. Several centers of subsidence are distributed fromwest to east and are filled by thick flysch sandstone-shale-carbonate sequences (Du 1986). Water depth isinterpreted to increase from south to north where therecognized paleo-environments range from beach, tidalflat, inner shelf, carbonate platform, outer shelf to deep-water transitional zones of shelf to continental slope(Liu 1990; Zeng 1992). The outer shelf facies in turn maybe separated into smaller sub-basins. Within each sub-basin, sedimentary facies range from coarse-grainedclastic limestone and bio-clastic limestone to fine-grained clastic-argillaceous-carbonate turbidite facies,corresponding to shallow and deep-water environments,respectively. In places, Devonian units contain corals,brachiopods and echinoderms, which may constitute areef facies at the margin of the carbonate platform. Thespatial and temporal variation of facies indicates thatMiddle Devonian sedimentation was accompanied bytectonic movements and development of a complexinternal basin structure. In contrast, the overlying UpperDevonian lithofacies assemblage indicates a return torelatively stable conditions with deposition of a up to5000 m thick sequence of turbidites. Carboniferous-Triassic sediments are found only in very small basins.They consist of shallow-marine limestone with rarecontinental clastic sediments and coal beds.

The Pb-Zn deposits are mainly hosted in the Devo-nian sequence, although some smaller deposits are hos-ted in Upper Silurian and Lower Carboniferous strata.The larger deposits are in Middle Devonian rocks, whichoutcropcrop out widely and vary greatly in lithology andfacies in the belt. Large sulfide deposits occur mainly atthe edge of sub-basin carbonate banks. In cross-section,the mineralized horizons grade laterally from clastic-carbonate rocks to turbidite, suggesting that minerali-zation formed in transitional environments betweenstable shallow-water and unstable deeper water basins(Liu 1990; Zeng 1992).

The metallogenic belt is a polyphased fold systembounded by a series of E-W regional faults (Zhang 1988,1993; Xu 1988). These ductile-brittle faults developedover an extended period from the Proterozoic to theMesozoic to form a foreland fold and thrust belt. Thetight, isoclinal or overturned folds have axial planes thatdip north and strike E-W. Secondary faults between themajor faults form two groups: 1) tensile-shear or com-pressive-shear axial or interlayer faults parallel to themajor faults; and 2) late tensile faults at high angles tothe major E-W faults.

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The metallogenic belt is interpreted to have been azone of active fault-controlled or down-warped basins(Qi and Li 1993) controlled by regional strike slip faultsand growth anticlines, separated by submarine uplifts(Zhang 1988; Yang, 1991). From south to north, paleo-environments include shallow seas, confined carbonateplatforms, marginal reefs to deep-water facies (Du1986). Sub-basins of deep-water sedimentary rocks ap-pear to be controlled by intersection of major regionalfaults with subordinate N–S faults. Laterally, these sub-basins show rapid lithofacies changes from platformfacies with thick-bedded dolomitic limestone or bio-clastic limestone to basin facies with thick argillaceous-fine clastic rock and turbidite.

Magmatic rocks include granodiorite batholiths,intermediate-acid or intermediate-basic plutonic com-plexes, with minor ultramafic stocks and dikes (Shangand Yan 1988). The intrusions are concentrated near theShangdan suture and decrease in abundance southward.Caledonian ultramafic and mafic plutons occur in east-ern Qinling near the Shangdan suture. A Sm-Nd iso-chron age of 402 Ma for an ultramafic rock (Qi and Li1993) and a whole rock Rb-Sr isochron age of 487 Mafor a mafic complex (Shang and Yan 1988) were ob-tained. These ultramafic rocks are interpreted to be relicsof ancient seafloor and the mafic rocks were intrudedduring Caledonian orogenic movement in Qinling (Qiand Li 1993). The most widespread granodiorite

batholiths were intruded during the Late Triassic Indo-sinian-Yanshan movement when the North China Cra-ton and the Yangtze Craton collided, closing the QinlingSea. Three whole rock Rb-Sr isochron ages range from219 to 275 Ma, whereas 14 K-Ar model ages range from103 to 293 Ma (Qi and Li 1993) therefore indicatingwidespread K-Ar resetting.

The sedimentary sequence underwent a long periodof dynamothermic lower greenschist facies regionalmetamorphism during the Indosinian orogeny, accom-panied by extensive deformation. One biotite 39Ar/40Arage in upper greenschist facies biotite schist yields 314.6± 0.8 Ma (Xu, 1988). K-Ar model ages for biotite in thewestern Qinling range from 176 to 194 Ma (n =7, Qiand Li 1993) suggests that the K-Ar system was reset atthat time.

Geology of the Changba deposit

The Changba deposit is divided into the Changba andthe Lijiagou segments by the NE-trending F1 fault(Fig. 2). Mineralized rocks at Changba crop out in aWNW-ESE zone about 2000 m long and 500 m wide(Fig. 2). To date, 134 sulfide lenses have been found, thelargest four, Changba-I, Changba-II, Lijiagou-I andLijiagou-II, accounting for 93% of the total metal re-serves of the deposit. The Changba deposit contains80.4 Mt of ore with average grades of 1.32% Pb and7.04% Zn in the Changba segment and 62.1 Mt of orewith average grade of 1.31% Pb and 7.34% Zn inthe Lijiagou segment. The Pb+Zn metal containedis 11.2 Mt with a (Zn +Pb)/Zn ratio of 1.2. Other

Fig. 2 Geologic map of the Changba Pb-Zn deposit (after 706Team of Northwest Metal Corp.). Orebodies: Ch-I: Changba-I,Ch-II: Changba-II; Li-I: Lijiagou-I; Li-II: Lijiagou-II. Cross-section A-A’ shown on Fig. 3

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elements of economic interest include Ag (5–30 g/t) andCd (0.038–0.007%).

The Changba orebodies are in an overturned, south-dipping sequence with stratigraphic polarity towards thenorth (Fig. 3). Although there are some faults strikingWNW-ESE and NW, most faults strike NE. The F1fault is a post-ore, reverse fault that displaced and sep-arated sulfide ore-bodies into the Changba and the Li-jiagou segments (Fig. 2).

The Changba Devonian rocks comprise two for-mations, the Lower Devonian Wangjiashan Formationand the overlying Middle Devonian Anjiacha Forma-tion. The Wangjiashan Formation includes a sedimen-tary cycle from basal sandstone through fine-grainedclastic to carbonate rocks with an average thickness of870 m (Qi and Li 1993). The Anjiacha Formation(D2a) hosts almost all of the Pb-Zn ore-bodies (Figs. 3and 4). It is composed of fine-grained clastic rocksintercalated with limestone and dolomite. Althoughfossils are rare, stratigraphic correlations with strata tothe east suggest it is Eifelian in age (Du 1986). TheAnjiacha Formation consists of five units (Fig. 4). Thebasal unit comprises laminated limestone, carbona-ceous limestone, marble, quartz marble and minorbiotite quartz schist (D2a

1). It is overlain by biotite

quartz schist with minor carbonaceous marble (D2a2)

that, in turn, is overlain by thick massive and lami-nated dolomite of unit D2a

3. The dolomite unit isoverlain by medium-bedded marble and quartz marblewith minor calcite-biotite quartzite (D2a

4), which con-tain the Changba-I and Lijiagou-II orebodies. Theupper unit is comprised of quartzite and quartz schistintercalated with biotite quartz schist (D2a

5) that hostthe Changba-II, VIII, and Lijiagou-I orebodies. TheAnjiacha Formation contains some primary sedimen-tary fabrics, such as slumps, intraclastic textures andbedding. The marble layer lacks fossils, but containssome mudstone and siliciclastic intraclasts.

The Changba host rocks are cut by two graniticplutons. The Huangzhuguan batholith, about 500 mnorth of the orebodies, is a zoned intrusion comprising arim of pyroxenite and a central zone of granodiorite thatyielded a Rb-Sr isochron age of 243 ± 11 Ma (Qi and Li1993). The metamorphic aureole of this intermediate-mafic complex contains disseminated galena. TheChangba two-micas granite, which is 500 m to thesouthwest of the orebodies (Fig. 2), exhibits no obviouszonation but shows a foliation similar to that of theregional host rocks. Mica K-Ar model ages range be-tween 196 Ma and 201 Ma (Qi and Li 1993).

Fig. 3 Geological cross section,A-A’ (see Fig. 2), in theChangba Pb-Zn deposit (after706 Team of Northwest MetalCorp.). Stratigraphic units(Dxa

y, described in Fig. 4) areoverturned with polarity to theN-NE

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Of the 51 orebodies in the Changba segment, 32 arehosted by biotite quartz schist and quartz schist in theupper D2a

5 unit, with the remaining 19 orebodies hostedby marble and dolomite in the underlying D2a

4 unit.Most orebodies are 10–20 m thick massive sulfide layerswith up to 1200 m of lateral extension. However, theChangba-II orebody, which accounts for more than 70%of the metal reserves, is lens-shaped and occurs concor-dantly in biotite quartz schist, trending in a general EWdirection, with a 60� to 90� S dip, a length of 380 m and athickness up to 49 m (Figs. 2 and 3). The sulfide layersare interlayered with schist and marble. The schists areweakly silicified. The Changba-II orebody is composedof sulfide layers intercalated with biotite quartz schistand quartz albitite and baritite stratigraphically under-lain by a lens of brecciated rocks (Fig. 3).

Bedded sulfides

Changba sulfide lenses are composed of sphalerite,pyrite and galena, and minor arsenopyrite, pyrrhotiteand boulangerite, with rare chalcopyrite, in a gangue ofwidespread biotite, muscovite, sericite and chlorite, andless abundant quartz, barite, albite, K-feldspar, calcite,dolomite, actinolite and epidote. The Changba-II bed-ded sulfides exhibits a mineral zonation from the bottomupward: bedded sphalerite ore intercalated with quartzalbitite, interbedded pyrite and massive sphalerite ore,and bedded sphalerite ore intercalated with beddedbarite at the top.

In the lower part of the orebody, the quartz albitite iscomprised of albite (50–80%) and quartz, with minorbarite, calcite, biotite, pyrite and sphalerite and acces-

sory tourmaline and apatite. The homogranular anhe-dral albite has an average diameter of 20 lm, and isintergrown with quartz. Barite and calcite form anhedralgrains from 0.05 to 0.1 mm. The bedded and laminatedstructures are caused by the intercalation of 1–2 cm al-bite and quartz beds and 1–2 mm thick laminae ofsphalerite and pyrite (Fig. 5A). Microprobe analysisindicates that it is nearly pure albite with very low CaOand K2O content (Table 1).

The beds and laminae are parallel to bedding in thewall rocks. The contacts between sulfide- rich beds andsulfide- poor beds are abrupt or gradational. Com-monly, bedding is defined by variation in the content ofpyrite, sphalerite, albite and barite. Fragmental fabric inthe sulfide beds is comparable to similar clastic texturesin the biotite quartz schists. Pyrite, quartz and albiteclasts are usually cemented by fine-grained sphaleritewith minor galena. Framboidal pyrite is found in thebedded sulfides and sedimentary interbeds, and fram-boids may be metamorphosed into pentagonal ordodecahedral pyrite aggregates with inclusions ofsphalerite and quartz.

Barite is more common in the upper part of the or-ebodies and is interlayered with sulfide beds, 1 to 20 cmthick, in gradational contact (Fig. 5B). The sphaleritebeds are composed of fine-grained, light-colored sphal-erite and minor coarse-grained barite and calcite. Thebarite beds are comprised of coarse-grained barite withminor fine-grained sphalerite and coarse-grained calcite.

Breccia lens

A 30 m thick lens of brecciated and altered rocks occursto the south, in the stratigraphic footwall of the over-turned Changba-II sulfide orebody (Fig. 3). The contactwith the overlying orebody is abrupt or gradational. Thebreccia consists of grey-white angular fragments (up to20 cm by 4 cm in size, Fig. 5C and D) of quartz albitite(30 – 90% Ab) cemented by quartz and coarse-grainedpyrite, sphalerite, galena and pyrrhotite with up to 3%of tourmaline. Remnants of grey biotite-quartz schists inthe core of some albitite fragments (Fig. 5D) indicatethat albitite formed by the alteration of the host biotite-quartz schist.

Geochemistry of the Changba deposit

Analytical methods

Sulfur isotope analysis were carried out in the Instituteof Mineral Deposits in Beijing. Pure barite was reducedwith graphite to BaS and then precipitated as Ag2S byreaction with AgNO3. Sulfide samples were combustedin the presence of excess CuO in vacuo to produce SO2.Analyses were carried out using a MAT 252 isotoperatio mass spectrometer. The precision of analysis is0.3&, and the values are reported relative to the Canon

Fig. 4 Stratigraphic column of the Anjiacha Formation in theChangba deposit (with unit thickness)

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Diablo Troilite (CDT). Lead isotope analysis were per-formed by the Yichang Institute of Geology and MineralResources, Hubei Province. The galena was fused inphosphoric acid, Pb was separated by anion-cation ex-change in HCl media, and subsequently analyzed using aMAT 261 mass spectrometer. The results are standard-ized to the Broken Hill No.1 standard with 207Pb/204Pb= 15.389, 206Pb/204Pb = 16.003 and 208Pb/204Pb =35.657. Strontium was separated by standard ion ex-change techniques. Mass spectrometry was carried outwith a MAT 261 at the Mineral Deposit Institute of theAcademy of Geosciences in Beijing. The data were

normalized to 87Sr/86Sr = 8.37521 and the precision isbetter than 0.00001. Argon isotope composition of albitewas determined in the Institute of Geology, the ChineseAcademy of Sciences. Albite from the Changba depositwas purified to more than 99%. The samples wereirradiated with appropriate flux monitors and analyzedfor Ar isotope composition using a MAT 261 massspectrometer. Major and trace element analysis wereperformed by ICP in the Institute of Mineral Deposit ofthe Academy of Geosciences in Beijing. Rare earth ele-ment analysis were carried out by instrumental neutronactivation at the Geochemical Institute of ChineseAcademy of Sciences in Guiyang Province, China.

Sulfur isotopes

The S isotope composition were determined for sulfidesand barite in the ores and the adjacent wall rocks. Sulfurisotope compositions range from +9.9 to +25.4& for

Fig. 5 Fabric of ores and rocks in the Changba deposit. A Beddedwhite-grey quartz albitite interlayered with sphalerite laminas.BGrey-white bedded barite interlayered with dark-grey sphaleritelaminas. C Quartz albitite breccia consisting of white grey pebblesand grey stockwerk composed of sphalerite, pyrite and quartzveins. D Quartz albitite breccia with altered relics of biotitequartzite. (Abbreviations: Sp, sphalerite ore; Ab, quartz albitite;Ba, baritite; Bi, biotite quartzite)

Table 1 Chemical composition (wt. %) a of albite in the Changba-II orebody

Sample Description SiO2 Al2O3 TiO2 CaO MgO Na2O K2O Total

P-6 Brecciated 68.88 20.95 0.00 0.00 0.00 11.48 0.00 101.31P-7 Brecciated 68.67 20.19 0.00 0.05 0.00 11.94 0.00 100.85Z-38 Bedded 69.82 19.92 0.00 0.00 0.06 11.30 0.00 101.10Z-41 Bedded 69.39 18.30 0.00 0.00 0.02 12.29 0.02 100.02

a Microprobe analysis was performed in the Institute of Geochemistry of the Chinese Academy of Science in Guiyang Province, China

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Table 2 Sulfur isotope compositions of sulfides and barite in the Changba deposits

Sample No. Description d 34S (&)

Barite Pyrite Sphalerite Galena

K012 Bedded ore in Ch-II 24.3K015 Bedded ore in Ch-II 24.8K017 Bedded ore in Ch-II 31.5K021 Bedded ore in Ch-II 24.2K023 Bedded ore in Ch-II 21.4K024 Bedded ore in Ch-II 20.8K041 Bedded ore in Ch-II 20.5 15.4K044 Bedded ore in Ch-II 23.0 21.4 19.2K045 Bedded ore in Ch-II 23.9 21.9K048 Massive ore in Ch-II 23.8 21.9 19.5K056 Massive ore in Ch-II 24.0 23.4 18.6K058 Massive ore in Ch-II 21.1 18.1K060 Bedded ore in Ch-II 24.2 22.8K062 Bedded ore in Ch-II 23.9 23.0K065 Bedded ore in Ch-II 19.8 18.0K066 Massive ore in Ch-II 24.2 22.4 21.3K067 Massive ore in Ch-II 23.0 15.9K071 Bedded ore in Ch-II 24.1 18.8K072 Massive ore in Ch-II 21.8 16.9K076 Bedded ore in Ch-II 23.4 22.0K080 Massive ore in Ch-II 20.0 18.9K083 Massive ore in Ch-II 24.2 22.9K085 Bedded ore in Ch-II 22.8 21.7 19.7K086 Bedded ore in Ch-II 21.7 20.5 18.2K089 Massive ore in Ch-II 23.6 20.4K090 Bedded ore in Ch-II 22.1 22.4K092 Massive ore in Ch-II 22.4 20.7K093 Massive ore in Ch-II 23.0 15.9K096 Bedded ore in Ch-II 24.1 18.8K098 Bedded ore in Ch-II 21.8 16.9K099 Bedded ore in Ch-II 18.8 11.0 17.1K102 Massive ore in Ch-II 23.4 22.0 22.3K105 Massive ore in Ch-II 18.8 16.3K106 Bedded ore in Ch-II 17.6 9.9 14.1K110 Massive ore in Ch-II 11.0 8.9K112 Bedded ore in Ch-II 20.6 15.1K114 Massive ore in Ch-II 22.8 18.5 18.1K115 Bedded ore in Ch-II 19.0 17.6K118 Bedded ore in Ch-II 22.8 20.6K121 Bedded ore in Ch-II 28.1K122 Massive ore in Ch-II 25.9K126 Massive ore in Ch-II 25.8K127 Bedded ore in Ch-II 21.5K132 Bedded ore in Ch-II 18.5K133 Massive ore in Ch-II 26.1K135 Bedded ore in Ch-II 17.8K136 Massive ore in Ch-II 17.7K138 Bedded ore in Ch-II 16.2K146 Massive ore in Ch-II 26.3K148 Massive ore in Ch-II 19.2K166 Breccia lens in Ch-II 29.3K167 Breccia lens in Ch-II 21.5K169 Breccia lens in Ch-II 27.1K170 Breccia lens in Ch-II 25.1K173 Breccia lens in Ch-II 24.7K182 Breccia lens in Ch-II 24.5K186 Breccia lens in Ch-II 21.4K187 Breccia lens in Ch-II 21.1K188 Breccia lens in Ch-II 23.6K193 Breccia lens in Ch-II 22.5K194 Breccia lens in Ch-II 19.9K197 Breccia lens in Ch-II 22.1K198 Bedded ore in Li-I 20.2K201 Breccia lens in Ch-II 19.4K202 Breccia lens in Ch-II 18.1

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sphalerite (n = 40, average = + 20.7&), from +8.1 to+29.3& for pyrite (n = 60, average = +21.4&) andfrom +8.9 to +24.2& for galena (n =30, average = +18.2&, Table 2 and Fig. 6A). The S isotope composi-tion of barite ranges from +20.8 to +31.5&, with anaverage d34S of +24.5& (n = 6, Fig. 6A). Analysis ofsulfides and barite in equilibrium indicates that inmost cases their S isotope compositions vary as follows:

d34S barite> d34Spyrite > d34Ssphalerite > d34Sgalena. Incontrast, d34Sgalena ranges from –0.4 to +1.2& in themetamorphic aureole of theHuangzhuguan intermediate-mafic complex (Qi andLi 1993). The d34Spyrite ranges from+8.1 to +10.6& (n = 3, average = +9.7&) in thefootwall biotite quartz schist and from 11.1 to +14.7&(n = 4, average = + 12.4&) in the hangingwall biotitequartz schist (Fig. 6B). The S isotope composition of the

Fig. 6 Histogram of sulfurisotope compositions in theChangba deposit. A) Sulfidesand barite in Ch-II; B) Pyrite inhost biotite quartz schist

Table 2 (Contd.)

Sample No. Description d 34S (&)

Barite Pyrite Sphalerite Galena

K204 Breccia lens in Ch-II 25.1K206 Breccia lens in Ch-II 25.4K210 Breccia lens in Ch-II 25.1K213 Breccia lens in Ch-II 16.5K216 Breccia lens in Ch-II 23.0K218 Breccia lens in Ch-II 23.5K219 Breccia lens in Ch-II 22.2K230 Breccia lens in Ch-II 22.8K234 Breccia lens in Ch-II 22.9K235 Breccia lens in Ch-II 21.6K237 Breccia lens in Ch-II 24.1K239 Breccia lens in Ch-II 23.6K240 Breccia lens in Ch-II 17.7K243 Breccia lens in Ch-II 24.2K244 Breccia lens in Ch-II 27.2K248 Breccia lens in Ch-II 20.6K249 Breccia lens in Ch-II 24.3K252 Breccia lens in Ch-II 19.5K267 Breccia lens in Ch-II 19.3K269 Breccia lens in Ch-II 22.0K281 Breccia lens in Ch-II 15.7K312 Hanging wall biotite quartz schist Ch-II 14.7K315 Hanging wall biotite quartz schist Ch-II 12.2K318 Hanging wall biotite quartz schist Ch-II 11.1K319 Hanging wall biotite quartz schist Ch-II 11.6K330 Footwall biotite quartz schist Ch-II 10.6K335 Footwall biotite quartz schist Ch-II 9.8K336 Footwall biotite quartz schist Ch-II 8.1

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Table 3 Lead isotopecompositions of galena in theChangba deposit

No. Location Mineral 208Pb/204Pb 207Pb/204Pb 206Pb/204Pb

K058 Massive ore in Ch-II Galena 38.072 15.572 17.884K060 Bedded ore in Ch-II Galena 38.123 15.420 17.928K062 Bedded ore in Ch-II Galena 38.314 15.628 18.051K083 Massive ore in Ch-II Galena 38.036 15.507 17.928K085 Bedded ore in Ch-II Galena 38.014 15.493 17.957K086 Bedded ore in Ch-II Galena 38.299 15.679 18.015K167 Brecciated ore in Ch-II Galena 38.336 15.599 18.066K188 Brecciated ore in Ch-II Galena 38.139 15.642 18.219K202 Brecciated ore in Ch-II Galena 38.785 15.837 18.452K184 Bedded ore in Li-I Galena 38.153 15.686 18.153K198 Bedded ore in Li-I Galena 38.733 15.874 18.459

Fig. 7 Pb isotope compositionof galena from the Changbadeposit. A 207Pb/204Pb vs206Pb/204Pb. B 208Pb/204Pb vs206Pb/204Pb. The growth curveswith tick marks every 400 Maare from Zartman and Doe(1981)

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sulfides in the ores is systematically heavier than those ofthe host rocks (Fig. 6). There is a large variation of d34Svalues for sulfides and barite, with a maximum value of+31.5&, higher than the values of +17 to +25& forDevonian seawater (Claypool et al., 1980).

Lead isotopes

Pb isotope ratios from 11 galenas range from 17.847 to18.459 for 206Pb/204Pb, 15.420 to 15.874 for 207Pb/204Pband 38.014 to 38.785 for 208Pb/204Pb (Table 3). Fig. 7Ashows that the Pb isotope compositions form an array athigh angle to the Orogene curve with most data close tothe Orogene curve for Phanerozoic Pb. The arrayspreads from values typical of the Mantle reservoir tovalues more radiogenic than the Upper Crustal reservoir(Doe and Zartman 1979). In Fig. 7B, the Pb isotopedata form an array with distinct 208Pb/204Pb enrichment,suggesting that the lead in the ores have a significantLower Crustal Pb component.

REE

Table 4 lists the REE composition and the chondritenormalized REE ratios of seventeen samples comprisingmassive sulfide ore, bedded sulfide ore, breccia ore, quartzalbitite, marble, dolostone, footwall biotite quartz schistnear Changba-II and hangingwall biotite quartz schistfar away from sulfide orebodies. Fig. 8 shows theREE of the ores and country rocks. The patternsshow fractionated REE with (La/Lu)N >1 (Table 4).Most of them exhibit fractionated LREE patterns(except two quartz albitite samples), and flat HREEpatterns. Another important similarity among the sam-ples is the slight depletion (0.72 – 0.98) in Ce (Table 4).The biotite quartz schist has the highest REE content,with average value of SREE of 194 ppm. Marble anddolostone have the lowest REEs content (average3 ppm) and the most depleted in HREE (average(La/Lu)N of 4.40). Massive sulfide ores have intermedi-ate SREE (average = 21 ppm) between the carbonaterocks and the hangingwall and footwall biotite quartzschists and less fractionated in HREE (average (La/Lu)Nof 1.52). The REEs contents of breccia lens, quartzalbitite and bedded ore are close to the value of biotitequartz schists. All biotite quartz schists are depleted inEu, while other samples display positive or negative Euanomalies. Quartz albite has similar REE pattern as thebiotite quartz schist, except for the fact that it is lower inLa + Ce content.

Strontium and argon isotopes

Six samples of the interlayered sulfide ores and rocks(one bedded quartz albitite, one bedded baritite, twobedded sphalerite ore, one bedded pyrite ore and one T

able

4REEcomposition(ppm)oforesandrocksin

theChangba-IIorebody

Description

#La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

La/Lu

La/Sm

Gd/Lu

Eua

Ceb

TotalREE

Hangingwallschist

K312

39.95

73.61

8.00

33.94

6.76

1.33

5.41

0.92

5.24

1.05

3.01

0.47

2.99

0.45

1.32

1.05

1.10

0.96

0.89

183.13

K318

41.92

80.98

7.58

31.79

5.92

1.03

4.42

0.67

4.03

0.81

2.35

0.37

2.53

0.38

1.65

1.26

1.07

0.88

0.98

184.77

Footw

allschist

K335

45.60

89.80

8.85

38.70

7.98

1.42

5.96

0.95

6.33

1.17

3.09

0.28

2.76

0.40

1.71

1.02

1.38

0.90

0.97

213.29

Dolostone

K060

20.55

39.17

4.09

17.26

3.30

0.51

0.40

0.42

2.23

0.46

1.34

0.21

1.34

0.21

1.50

1.11

0.18

1.24

0.93

91.48

K062

28.02

51.56

5.72

23.99

4.22

0.96

3.29

0.58

3.52

0.73

2.21

0.36

0.32

0.36

1.17

1.18

0.85

1.13

0.88

125.82

K085

36.30

67.00

7.18

30.10

5.74

1.00

4.26

0.76

5.41

0.94

2.81

0.19

2.52

0.38

1.43

1.13

1.03

0.88

0.90

164.59

K086

37.10

66.30

5.89

27.50

4.69

0.90

3.25

0.56

3.25

0.65

1.71

0.15

1.68

0.26

2.14

1.41

1.15

1.00

0.95

153.89

Quartzalbitite

K128

23.19

57.38

8.97

38.01

6.07

1.41

3.51

0.64

2.72

0.70

1.58

0.34

2.04

0.33

1.05

0.68

0.98

1.31

0.85

146.89

K131

12.33

34.01

5.79

25.93

5.23

0.82

3.45

0.81

3.45

0.92

1.98

0.35

2.39

0.39

0.47

0.42

0.82

0.84

0.83

97.85

Breccia

lens

K188

28.70

51.20

4.94

19.30

3.11

0.61

2.18

0.37

1.95

0.40

1.10

0.31

1.28

0.23

1.87

1.64

0.87

1.02

0.92

115.68

K202

38.30

69.70

7.08

29.70

5.74

1.42

4.07

0.64

4.07

0.80

2.05

0.46

1.86

0.29

1.98

1.19

1.30

1.28

0.91

166.18

Bedded

ore

K118

25.90

46.60

4.48

16.30

2.19

0.34

1.08

0.18

1.16

0.27

1.75

0.05

1.07

0.16

2.43

2.11

0.62

0.93

0.93

101.53

Massiveore

K058

4.88

8.89

1.15

3.80

0.68

0.12

0.49

0.08

0.46

0.10

0.29

0.05

0.30

0.06

1.28

1.29

0.79

0.94

0.82

21.33

K083

4.70

7.80

1.20

3.40

0.79

0.50

0.65

0.10

0.58

0.12

0.31

0.50

0.30

0.04

1.76

1.06

1.50

3.06

0.72

20.99

Marble

K139

0.63

0.92

0.08

0.23

0.05

0.01

0.03

0.004

0.02

0.01

0.02

0.002

0.01

0.002

4.76

2.40

1.25

1.56

0.83

2.02

K142

0.54

1.04

0.11

0.29

0.06

0.01

0.04

0.01

0.03

0.01

0.02

0.002

0.01

0.002

4.04

1.57

1.71

1.27

0.94

2.16

K143

1.01

1.91

0.24

0.94

0.23

0.06

0.18

0.05

0.14

0.05

0.07

0.06

0.13

--

0.78

-1.29

0.84

5.07

aEu=

2·Eu/(Sm

+Gd)

bCe=

2·Ce/(La+

Pr)

390

massive sphalerite ore) from the Changba-II orebodywere analyzed for their whole-rock Rb-Sr isotope com-positions. The data show a linear relation and yield aRb-Sr isochron age of 389 ± 14 Ma (Table 5, Fig. 9).This age is higher, but within error, than Rb-Sr isochronages for the host biotite quartz schist, which ranges from382 to 342 Ma (He 1987). The initial 87Sr /86Sr of 0.7101is slightly higher than those of Devonian limestone in theQinling orogenic belt (0.7082 – 0.7095, Qi and Li 1993).This indicates that the major source of the Sr in thehydrothermal fluids is seawater, with some admixing ofradiogenic Sr.

Albite from the bedded quartz albitite shows a seriesof Ar39-Ar40 step ages, with a large plateau defined by

65% of the total 39Ar released yielding an age of 352.8± 3.5 Ma (Table 6, Fig. 10). A younger age at 184.7 ±3.3 Ma for the initial 15% released 39Ar (Fig. 10) likelyrecords a Jurassic thermal event.

Discussion

In the Qinling polymetallic belt, the Pb-Zn sulfidedeposits form beds and laminated lenses that are strat-abound within Middle Devonian strata. It is one ofseveral SEDEX deposits (Red Dog, USA; Tom andJason, Canada; Mirgalimsai and Tekeli, Kazakhtsan;Meggen and Rammelsberg, Germany) formed between

Fig. 8 REE composition ofsulfide ores and wall rocks inthe Changba deposit (chondriteREE compositions fromHenderson 1984). Solid lines:massive ore, bedded ore, breccialens, and quartz albitite; shortdashed lines: marble anddolostone; dashed and dottedlines: hanging wall and footwallbiotite quartz schist. Samplesare listed in Table 4

Table 5 Rb-Sr isotopecomposition of ores and rocksin the Changba-II orebody

Sample Description Rb (ppm) Sr (ppm) 87Rb/86Rb 87Sr/86Sr

K062 Bedded pyrite ore 119.370 1182.393 0.2912 0.71238K021 Bedded barite 28.891 3067.163 0.0272 0.70960K083 Massive sphalerite ore 25.625 31.821 2.3250 0.72280K146 Massive sphalerite ore 25.577 32.132 2.2983 0.72295K098 Bedded sphalerite ore 9.545 266.057 0.1035 0.71083K115 Bedded quartz albitite 71.161 600.180 0.3420 0.71184

391

the middle Devonian and the Mississippian (Turner1992). The orebodies show a characteristic upward se-quence from a lowermost breccia zone through inter-layered sphalerite ore and quartz albitite, massivesphalerite and pyrite ores to bedded and laminatedsphalerite ores and barite. This asymmetrical sequenceresembles, for example, the zoning in other sediment-hosted Pb-Zn deposits such as Tom (Goodfellow andRhodes 1990), Sullivan (Hamilton et al. 1982; Lydonet al. 2000) and Red Dog (Moore et al. 1986). The well-ordered sequence of albite-sulfides-barite orebodies atChangba suggests episodic exhalation of hydrothermalfluids in the early and middle stages of mineralization,and waning hydrothermal input towards the top of themineralized sequence. The increase in abundance ofbedded barite towards the top of the Changba-II ore-body may indicate that the sedimentary environmentevolved from reducing in the early stage to more oxi-dizing towards the end of mineralization. Mineralizationformed over a vertical interval of 500 m in the AnjiachaFormation, of which the lower 200 m concentratedabout half of the total thickness of all the orebodies(Fig. 4). This stacking of sulfide lenses over a strati-graphic interval is also characteristic of several otherSEDEX deposits (Goodfellow et al. 1993).

The breccia lens at the base of the Changba-II ore-body is reminiscent of fragmental rocks are associated

with SEDEX deposits such as the Sullivan (Andersonand Hoy 2000), the Tom (Goodfellow and Rhodes 1990)and Jason deposits (Turner 1990). Tourmalinization ofthe breccia lens bears similarities to tourmalinizedfragmentals in the Belt-Purcell basin and to mud vol-canoes in the Black sea (Slack et al. 1998). Cementationof breccia fragments by sulfides indicates that the brecciawas infiltrated by the hydrothermal fluids. The Changbadeposit, however, lacks the discordant vent complex thatis characteristic of several SEDEX deposits such as theRammelsberg (Hannak 1981; Large and Walcher 1999),Sullivan (Hamilton et al. 1982), Tom (Goodfellow andRhodes 1990) and Jason (Turner 1990).

Source of sulfur

The S isotope composition of sulfides in the ore rangesfrom +8.9 to +29.3&, whereas d34S values of barite atChangba range from +20.8 to +31.5&, at the higherend of the range of typical Devonian seawater sulfate(from +17 to +25&, Claypool et al. 1980). Barite d34Sas high as +31.5& perhaps indicates some restriction inopen sea sulfate replenishment in the Changba sub-ba-sin, therefore causing enrichment in 34S in the residualsulfate during bacterial sulfate reduction (BSR). Frac-tionation (D sulfate-sulfide) during BSR is highly variablewith values ranging from 20 to 70&, and averagingabout 50& (Ohmoto 1992). Using the lower fraction-ation value (20&) and the Changba barite d34S values,BSR of Changba Devonian seawater sulfate would formsedimentary sulfides with maximum d34S values from+0.8 to +11.5&. These d34S values are comparablewith footwall biotite quartz schist pyrite values (+8.1 to+10.6&). Ore sulfide d34S values display a wide rangefrom values typical of footwall biotite quartz schists upto values (+29.3&) similar to those measured in theoverlying barite (up to +31.5&).

The hanging wall biotite quartz schist has higherpyrite d34S values (+11.1 to 14.7&) than the footwallbiotite quartz schist. This increase in sedimentary pyrited34S values from footwall to hanging wall schists impliesthat 34S enrichment in the Changba sulfate-restrictedsub-basin by BSR continued after formation ofChangba ore. Using the higher d34S value for hanging-wall quartz biotite schist (+14.7&) and the minimum

Fig. 9 Rb-Sr isochron for hydrothermal sediments in the Changbadeposit, from data in Table 5

Table 640Ar-39Ar data of albite in bedded quartz albitite in the Changba-II orebody

Step T (�C) (40Ar/39Ar)m (36Ar/39Ar)m (37Ar/39Ar)m (38Ar/39Ar)m39ArK (40Ar/39ArK)

39ArK(%) Age(10-12mol) (Ma)

1 500 35.79 0.0874 0.6569 0.0301 0.78 10.09 15.32 184.72 650 32.33 0.0689 0.3457 0.0216 0.47 12.04 9.23 218.23 760 38.27 0.0765 0.4096 0.0357 0.42 15.76 8.25 280.64 900 23.85 0.0135 0.1883 0.0177 2.06 19.86 40.5 346.95 1000 43.54 0.0799 0.3419 0.0272 0.49 20.10 9.63 350.16 1150 50.19 0.1011 0.5274 0.0225 0.45 20.51 8.84 356.97 1300 97.22 0.2593 0.6459 0.0648 0.23 20.98 4.52 364.88 1500 123.2 0.2321 2.1594 0.0446 0.19 55.20 3.73 835.9

392

BSR fractionation (20&), the minimum Changba sea-water sulfate d34S value would have been about +35&,however, this value is not recorded in sulfate. It is pos-sible that extensive BSR of seawater sulfate eventuallyformed the Changba ore sulfides with bulk d34S valuesup to +29.3&, but this interpretation is unlikely be-cause a water column dominated by reduced sulfur isincompatible with formation of barite above the sulfidelens, and would result in residual sulfate with minimumd34S values of +50&.

It is more likely, therefore, that the high d34S valuesof ore sulfides formed as a result of influx of heavyhydrothermal sulfur into the Changba sub-basin. Theultimate source of this heavy sulfur must be Devonian orolder seawater sulfate. One possible mechanism to formsulfides with d34S values similar to those of the initialsulfate is quantitative thermochemical sulfate reduction(TSR), perhaps from reduction by iron of seawater orsediment porewater along the fluid flow path. Anothersulfate source may be reduction of disseminated orbedded sulfate within the underlying sedimentary se-quence. The range of ore sulfides d34S values from thatof BSR pyrite in the footwall to that of barite mayrepresent mixing between sub-basin BSR and hydro-thermal TSR sulfides. This scenario is analogous to thatsuggested for the Sullivan deposit by Taylor andBeaudoin (2000) wherein heavier hydrothermal sulfide

was injected into the Sullivan sub-basin where it mixedwith BSR sulfides, therefore inducing the Main Bandsulfide accumulation.

Origin of hydrothermal fluids

Sulfur isotopes indicate that the dominant source ofreduced hydrothermal sulfur was through TSR of sea-water sulfate reduced during fluid transport. Pb isotopecompositions form a linear array (Fig. 7) close to theOrogene curve, and similar to that of the shale-hostedPb-Zn deposits in the Selwyn basin in northwest Canada(Godwin and Sinclair 1982). It can therefore be inferredthat Changba Pb was leached from a mixture of crustalleads comparable to the Orogene reservoir of Zartmanand Doe (1981). The Pb isotope composition of K-feldspar in the Huangzhuguan intrusive complex issimilar to that of the Changba galena-lead compositions,although it is less variable in composition, suggestingthat the leads in the plutonic rock were also from mixingcrustal rock reservoirs (Shang and Yan 1988). We de-duce, therefore, that the metals for the Changba sulfideswere derived from rocks underlying the orebodies.

The similarity of host rocks REE patterns with thosein bedded and massive sulfide ores suggests that thehydrothermal fluids leached REE from the underlying

Fig. 10 Ar39-Ar40 model agefor albite in banded quartzalbitite in the Changba depositfrom data in Table 6

393

clastic sedimentary rock sequence. The negative Ceanomaly for the sulfide ore and bedded albitite (Fig. 8),suggests that the hydrothermal fluid originated as oxicseawater trapped in, or entrained into, the sedimentaryrocks (Courtois and Treuil 1977). The initial Sr ratio(Sr87/Sr86)iof 0.7101 in sulfide ores (Fig. 9), is higherthan the values of 0.7076 – 0.7082 for Devonian marinecarbonates (Burke et al. 1982), implying that the Srisotope composition of Changba ores reflects a hydro-thermal fluid dominated by seawater that had leachedradiogenic Sr from underlying clastic sedimentary rocks.

Timing of mineralization

Timing of mineralization is recorded by hydrothermalsedimentation, including the formation of the sulfide orebodies, and the related hydrothermal alteration. Thewhole rock Rb-Sr isochron age for hydrothermal sedi-ments including bedded albitite, barite and sulfide oresof 389 ± 14 Ma (Table 5 and Fig. 9) is comparable,within error, to that of whole rock Rb-Sr isochron agesfor the regional ore-bearing strata (including biotitequartz schist, quartz schist and marble) that range from382 to 342 Ma (He 1987). The coincidence in age of hostrocks, massive and bedded sulfides, and barite suggeststhat Pb-Zn mineralization is broadly coincident in timewith sedimentation. The sedimentary bedding structuresof the ores are consistent with a syngenetic origin.

The Ar39/Ar40 plateau age of 352.8 ± 3.5 Ma and theAr39-Ar40 isochron age of 346.6 ± 6.4 Ma (Table 6 andFig. 10) for albite in bedded quartz albitite are at thelower end of the range of ages for the host rocks. Thisage could either be that of albite forming after the sulfideores, or that of thermal resetting of the Rb-Sr system atabout 350 Ma. This time coincides with the youngerages attributed to the collision between the North Chinaand Yangtze cratons (Meng and Zhang 2000).

Conclusion

The Changba stratiform Pb-Zn sulfide deposit bearssimilar characteristics to other SEDEX mineral deposits.The major orebodies consist of a bedded facies underlainby a breccia lens. The bedded facies is composed of in-terlayered quartz albitite, baritite, sulfide ore and mas-sive sulfide ore, whereas the breccia lens of albitized andtourmalinized fragments is cemented by sulfides. Sulfurisotopes suggest there were two sources of reduced sul-fur: one derived from the bacterial reduction of Devo-nian seawater sulfate and the second by influx of heavyhydrothermal sulfur, ultimately seawater reduced bythermochemical sulfate reduction. REE, Sr and Pb iso-tope data suggest that the hydrothermal fluids leachedmetals from the footwall sedimentary sequence.

Acknowledgements The work is supported by the Open Geochem-ical Lab of the China Academy of Sciences of China, the Natural

Sciences and Engineering Council of Canada and Universite Laval(Quebec, Canada). G.M. thanks Shaojun Zhong for his guidanceand support. The Changba Mine of the Baiyin Non Ferrous MetalsCorporation (BNMC) is thanked for access to the mines and theirvaluable materials. Comments by two anonymous reviewers, Dr.Karen Kelley, and Dr. R. Goldfarb and L. Meinert significantlyimproved this manuscript.

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