Geological and isotopic evidence for magmatic-hydrothermal

ARTICLE
Geological and isotopic evidence for magmatic-hydrothermal origin of the Ag –Pb –Zn deposits in the Lengshuikeng District,east-central China
Changming Wang &Da Zhang &Ganguo Wu &
M.Santosh &Jing Zhang &Yigan Xu &Yaoyao Zhang
Received:7August 2012/Accepted:27March 2014/Published online:8April 2014#Springer-Verlag Berlin Heidelberg 2014
Abstract The Lengshuikeng ore district in east-central China has an ore reserve of ∼43Mt with an average grade of 204.53g/t Ag and 4.63%Pb+Zn.Based on contrasting geological characteristics,the mineralization in the Lengshuikeng ore district can be divided into porphyry-hosted and stratabound types.The porphyry-hosted minerali-zation is distributed in and around the Lengshuikeng granite porphyry and shows a distinct alteration zoning including minor chloritization and sericitization in the proximal zone;sericitization,silicification,and carbonatization in the periph-eral zone;and sericitization and carbonatization in the distal zone.The stratabound mineralization occurs in volcano-sedimentary rocks at ∼100–400m depth without obvious zoning of alterations and ore minerals.Porphyry-hosted and stratabound mineralization are both characterized by early-stage pyrite –chalcopyrite –sphalerite,middle-stage acanthite –native silver –galena –sphalerite,and late-stage pyrite –quartz –calcite.The δ34S values of pyrite,sphalerite,and galena in the ores range from −3.8to +6.9‰with an average of +2.0‰.The C –O isotope values of siderite,calcite,and dolomite range from −7.2to −1.5‰with an average of −4.4‰(V-PDB)and from +10.9to +19.5‰with an average of +14.8‰
(V-SMOW),respectively.Hydrogen,oxygen,and carbon iso-topes indicate that the hydrothermal fluids were derived main-ly from meteoric water,with addition of minor amounts of magmatic water.Geochronology employing LA –ICP –MS analyses of zircons from a quartz syenite porphyry yielded a weighted mean 206Pb/238U age of 136.3±0.8Ma considered as the emplacement age of the porphyry.Rb –Sr dating of sphalerite from the main ore stage yielded an age of 126.9±7.1Ma,marking the time of mineralization.The Lengshuikeng mineralization classifies as an epithermal Ag –Pb –Zn deposit.
Keywords Stable isotope .Geochemistry .Porphyry .Stratabound .Ag –Pb –Zn .Lengshuikeng
Introduction
The Lengshuikeng ore district,located in the Jiangxi Province of east-central China (Fig.1a ),contains more than 50ore bodies belonging to seven deposits hosted in granite porphyry,pyroclastic,and carbonate rocks.The ore reserves in Lengshuikeng have been estimated at ∼43Mt with average grades of 2.11%Pb,2.61%Zn,204.53g/t Ag,0.08g/t Au,and 0.01%Cd.The ores can be grouped into two types:(1)porphyry-hosted (Yinluling,Baojia,and Yinzhushan)and (2)stratabound (Xiabao,Yinkeng,Yinglin,and Xiaoyuan).The porphyry-hosted mineralization is distributed within and around the Lengshuikeng granite porphyry,whereas the stratabound mineralization occurs in volcano-sedimentary rocks at ∼100–400m depth.The spatial distribution of the porphyry-hosted and stratabound ore bodies,their mineral constituents,and the zoning of alteration assemblages are markedly different from those of typical porphyry deposits.
Editorial handling:T.Bissig and G.Beaudoin
C.Wang (*):
D.Zhang :G.Wu :M.Santosh :J.Zhang :Y .Xu :Y .Zhang
State Key Laboratory of Geological Processes and Mineral
Resources,China University of Geosciences,No.29,Xueyuan Road,Beijing 100083,People ’s Republic of China e-mail:wcm233@
Y .Xu
No.912Geological Surveying Team,Bureau of Geology and Mineral Exploration and Development,Yingtan 334000,China
Miner Deposita (2014)49:733–749DOI 10.1007/s00126-014-0521-8
Over the past several decades,research in the Lengshuikeng ore district was focused on geological characteristics,mineral-ization,wall rock alteration,fluid inclusions,and geochemistry of the porphyry-hosted deposits (Deng 1991;Meng et al.2007;Wang et al.2010c ).Recently,stratabound Ag –Pb –Zn deposits have been found beneath the porphyry-hosted deposits,occur-ring in volcano-sedimentary rocks of the E ’huling Formation.This paper reports the geologic and isotopic characteristics of the porphyry-hosted and stratabound ores,in combination with those from Chinese literature.The regional and the
Lengshuikeng Ag –Pb –Zn ore district geologies,including the occurrence of ore bodies,and associated hydrothermal alter-ation are described,followed by a discussion of the origin of the Ag –Pb –Zn deposits.
Regional geological setting
The Lengshuikeng ore district is located in northeastern Jiangxi Province,east-central China,close to the Shaoxing
–
Fig.1a Location map for the Lengshuikeng District.b Sketch map showing the Late Mesozoic volcanic-intrusive complex belt in SE China.c Sketch map showing the regional geology of the Lengshuikeng ore district and Dexing ore field (modified after Jiang et al.2011).Major faults in the study area:Shi –Hang Shiwandashan –Hangzhou as collision-induced suture zone between the Cathaysia and Yangtze blocks,SJ Shaoxing –Jiangshan,ZD Zhenghe –Dapu
Jiangshan(SJ)fault between the Cathaysia and Yangtze blocks(Fig.1b).This fault represents the eastern part of the Shiwandashan–Hangzhou(Shi–Hang)fault zone and is con-sidered to mark the collisional suture zone in SE China(Yao et al.2011).Geophysical and remote sensing data suggest that the SJ fault extends deep into the lower crust and upper mantle (Fig.1c;Wang et al.2010c).The collisional amalgamation of the Cathaysia and Yangtze blocks into the proto“South China”Block(SCB)began in the early Qingbaikou period (∼950±50Ma)and was completed by the end of the Jinning Orogeny at ca.850Ma(Shu and Charvet1996;Charvet et al. 1996;Li et al.2008,2010;Zhang and Zheng2013).During the Sinian(ca.850–600Ma),the Yangtze and Cathaysia blocks were rifted.In the Caledonian(600–405Ma),the Yangtze and Cathaysia blocks collided for a second time and reunited within the SCB(Zhou et al.2002;Wang et al.2010a, b).During the Variscan(405–270Ma),extension within the SCB resulted in Paleozoic intracontinental rifts.The Indosinian collision between the South China and North China blocks occurred from the Palaeozoic to Late Triassic(ca.270–208Ma;Wang et al.2013a,2014a). Magmatic activity during208to180Ma is documented by bimodal magmatism in southeastern Hunan Province and the A-type granitic magmatism in southern Jiangxi Province(Zhao et al.1998;Yu et al.2006,2010).From Triassic to Early Jurassic(180–145Ma),most areas of the Wuyi Mountains in the northeastern part of the Cathaysia Block(Fig.1b)were folded and uplifted before undergoing extensional collapse.The region ex-perienced Early Cretaceous granite magmatism(145–100Ma) and the formation of large-scale Late Cretaceous–Paleogene red bed sedimentary basins between100and70Ma(Jahn 1974;Jahn et al.1990;Zhou et al.2006;Shu et al.2009;Wang et al.2014b).
The Precambrian units include greenschist facies metamorphic rocks dated at1,000–800Ma(Fig.1c;Li et al.1996;Shu and Charvet1996;Zhou et al.2002; Shu et al.2009).The Cambrian and Ordovician rocks are mainly sandstone,mudstone,and carbonaceous mudstone. Silurian sedimentary rocks are absent.The Devonian, Carboniferous,and Permian strata comprise shallow marine to littoral facies clastic rocks,limestone,and dolomite.The Lower Triassic series consists of muddy limestone and shale. Middle Triassic strata are absent in most areas of the Wuyi Mountains.
The Wuyi Mountains are largely composed of Late Mesozoic volcanic rocks and associated clastic strata (Fig.1c).Lower Jurassic strata include conglomerate and coarse arkosic sandstone,quartz sandstone,and siltstone with carbonaceous mudstone and coal-bed intercalations.Middle Jurassic strata are composed of terrestrial clastic rocks and bimodal volcanic rocks.Upper Jurassic strata comprise andes-ite and rhyolitic tuffs and tuffaceous siltstone(Liu1985;Ye 1987).Lower Cretaceous strata include rhyolitic welded tuffs with basalt intercalations(Yu et al.2006).Upper Cretaceous siltstone and mudstone are intercalated with gypsum-bearing layers and basalt,the latter with an age of105–98Ma(Yu et al.2001).Paleogene strata include coarse clastic rocks,siltstone,and mudstone with inter-calated gypsum and oil-bearing shale.Neogene silt-stones locally overlie the Paleogene rocks.
Southeast China is characterized by extensive magmatism, which formed a belt of volcanic-intrusive complexes(Fig.1b). Two major tectono-magmatic periods have been recognized in the Wuyi Mountains:the Indosinian and the Yanshanian.The Indosinian magmatic period lasted from240to208Ma(Xie et al.2006).The Yanshanian igneous rocks formed during two main stages of Early Yanshanian(208–145Ma)and Late Yanshanian(145–90Ma)and are characterized by abundant rhyolitic volcanic rocks and highly aluminous granitoids(Jahn et al.1976;Chen1999;Li2000;Deng et al.2010,2011,2014; Zhou et al.2006;Zhao et al.2012).
Geology of the Lengshuikeng ore district
Deposit geology
The stratigraphic sequence in the Lengshuikeng District com-prises the Jurassic Daguding and E’huling Formations.The Daguding Formation is composed of andesite and rhyolitic tuffs and tuffaceous siltstone.The E’huling Formation is composed of tuffs,rhyolite,tuffaceous siltstone,sandstone, and manganese-and iron-rich carbonates,which are the main host of the stratabound ores.
The NE-striking F1fault dipping toward NW(Figs.2and 3a)comprises the northern segment of the Hushi fault.The most prominent structural feature in the Lengshuikeng District is F2reverse fault(Figs.2and3a).The stratabound fracture along the manganese-and iron-rich carbonate strata(Fig.3a) was cemented by later ore sulfide minerals and hydrothermal alteration minerals.
Middle Jurassic and Early Cretaceous magmatic rocks are exposed in the Lengshuikeng District(Fig.2).The Jurassic igneous rocks are mainly granitic including the Yinluling, Yinzhushan,Biaojia,and Yinglin porphyries.The Yinzhushan granite porphyry has been dated as162–159Ma (Meng et al.2007;Zuo et al.2010).The Early Cretaceous rocks include quartz syenite porphyry,rhyolite porphyry, alkali-feldspar granite porphyry,and mafic dykes(Fig.2). The quartz syenite porphyries crop out in the southeastern and northwestern parts of the Lengshikeng District(Fig.2),
with widely dispersed chloritization,sericitization,silicifica-tion,and carbonatization,with associated pyrite.Wang et al.(2010c )and Meng et al.(2007)reported that rhyolite porphyry and alkali-feldspar granite porphyry cut all the granite por-phyry and pyroclastic carbonate ore-hosted rocks (Fig.3)as well as the quartz syenite porphyry.
The least hydrothermally altered granite porphyries contain phenocrysts (15–35%)of quartz,plagioclase,K-feldspar,and biotite in a groundmass (65–85%)of subhedral K-feldspar,quartz,plagioclase,and minor biotite.Accessory minerals (∼1%)are mainly magnetite,zircon,and apatite.Most quartz crystals are xenomorphic and exhibit undulose extinction.The quartz syenite porphyries contain phenocrysts (33–55%)of K-feldspar (∼20%),biotite (∼10%),and plagioclase (∼8%)in a groundmass (45–67%)of subhedral K-feldspar,biotite,and quartz.K-feldspar phenocrysts are euhedral to subhedral,have a grain size of 2–5mm,and are locally replaced by sericite.Plagioclase phenocrysts are euhedral to subhedral,3–6mm in size,with evidence of weak silicification and sericitization.Biotite phenocrysts are 0.5–1.0mm in size and show alteration to chlorite and carbonate.
Ore bodies and wall rock alteration
Among the various ore deposits,the Baojia porphyry-hosted deposit is the most important,accounting for 52%of the total ore reserve in the Lengshuikeng District (Fig.2).
The porphyry-hosted ore bodies (Fig.3a )are associated with NNE-striking F 2reverse faults or comprise of fracture fillings in veins and breccias.Associated with the porphyry-hosted ore bodies is a distinct zoning of alteration and ore minerals,both vertically and laterally (Fig.3b,c ).An inner (or proximal)zone in the granite porphyry is characterized by enrichments in lead,zinc (with grade Pb+Zn >5%)with minor disseminated chalcopyrite and pyrite,and with minor chlorite and sericite alteration.The intermediate zone surrounds the inner zone near the contact between the granite porphyry and country rocks.This intermediate zone exhibits strong sericitization,carbonatization,and silicification,and in some cases,high-grade native silver mineralization (Ag >200g/t),with minor galena –sphalerite vein mineralization (Fig.4a ).The outer peripheral (or distal)zone is hosted by volcano-sedimentary country rocks to the granite porphyry and is de-fined by weakly developed silver,galena,sphalerite veins (Ag <100g/t;Pb+Zn <2%),and vein sericite and carbonate.Stratabound mineralization is hosted by manganese –iron carbonate layers of the E ’huling Formation (∼5.0–33.1-m thickness)between tuffaceous sandstone and rhyolitic crystal tuff (Jiangxi Bureau of Geology and Mineral Exploration and Development (JBGMED)1982;Meng et al.2007).These rocks are characterized by high-grade native silver minerali-zation (Ag >200g/t),lead,zinc (with grade Pb+Zn >5%)as veins and breccias,with minor vein chlorite,sericite,and carbonate (Fig.4b,c ).Ore mineralogy and paragenesis
The mineralization in stratabound and porphyry-hosted ores can be divided into three stages (Fig.5):stage 1,pyrite –chalcopyrite –sphalerite;stage 2,silver minerals –galena –sphalerite;and stage 3,pyrite –quartz –calcite.The mineral assemblage of stage 1is dominantly pyrite and Fe-rich sphal-erite,with small amounts of chalcopyrite,cubanite,galena,arsenopyrite,pyrrhotite,and minor quartz (Fig.6a –c ).Stage 1mineralization was accompanied by chloritization and sericitization,replacing K-feldspar and plagioclase crystals in rhyolitic crystal tuff and granite porphyry.Siderite is intergrown with sphalerite but occurs mostly as overgrowths on sphalerite or as monomineralic cement in breccias and thin veinlets.Stage 2was the principal stage of silver –lead –zinc mineralization.This stage is characterized by sericitization and carbonatization,and minor chloritization.The silver –lead –zinc minerals of stage 2fill the manganese –
iron
Fig.2Geological map of the Lengshuikeng Ag –Pb –Zn ore district (after JBGMED 1982).Sections a and b are shown in Fig.3a –c
carbonate stratabound fractures.The dominant silver minerals are acanthite (Ag 2S)and native silver,which occur in fissures within manganese –iron carbonate or in the intergranular space between manganese –iron carbonate and early sulfides.In ad-dition,fine-grained canfieldite (Ag 8SnS 6),proustite (Ag 3AsS 3),aerosite (Ag 3SbS 3),Ag-bearing tetrahedrite (Cu 12Sb 4S 13),and kustelite (Ag,Au)occur in the intergranular pores of manganese –iron carbonate or as inclusions in galena,sphalerite,and other sulfides.The galena is euhedral and coarse-grained (Fig.6e ).V eins,veinlets,or disseminated silver minerals –gale-na –sphalerite are disseminated in the intergranular pores of manganese –iron carbonate (Fig.6f –h ).Stage 1pyrite is cut by galena and sphalerite vein (Fig.6b ),sphalerite surrounded by galena (Fig.4d ),and sphalerite cut by ankerite –galena vein.
In
Fig.3Geological features along sections a and b of the Baojiao deposit in the Lengshuikeng ore district (after JBGMED 1982):a relationship of ore bodies,b alteration zoning,and c mineralized zone
stage 3,ore minerals are dominated by pyrite,with lesser galena,pyrrhotite,and arsenopyrite.Calcite,quartz,and quartz –pyrite are characterized by open-space filling textures such as comb,veins,and veinlets and cut the stage 2galena or sphalerite veins (Figs.4e,f and 6d ).
Sampling and analytical procedure for stable and radiogenic isotopes
Samples were collected from ore materials at the −80,−120,−152,and −160m levels of underground workings,
outcrops
Fig.4Macroscopic features of samples selected for ore mineralogy and paragenesis studies.a Porphyry-hosted ore with vein galena and with chloritization and sericitization.b Brecciated stratabound-type ore.c Stratabound-type ore with vein sphalerite and galena and with chloritization,sericitization,and carbonation (Lu et al.2012).d Sphalerite phase (stage 1)surrounded by galena (stage 2)(Lu et al.2012).e Pyrite (stage 1)intersected by quartz vein (stage 3).g Disseminated ore intersected by pyrite vein (stage 3).Sp sphalerite,Gn galena,Py pyrite,Chl chloritization,Ser sericitization,Cbn carbonation,and Qz quartz
(Fig.2)at ∼160m.s.l.elevations,and two drill holes in the Lengshuikeng ore district.Pure mineral concentrates from the porphyry-hosted and stratabound ores and wallrocks were prepared using a combination of heavy liquid and magnetic-and hand-picking techniques,and these were then checked by X-ray diffraction to ensure mineral purity.
Sulfur isotope analyses of sphalerite,galena,and pyrite were carried out at the Resource and Environment Analysis Centre of the Geochemistry Institute,Chinese Academy of Sciences.Pyrite,galena,and sphalerite were combusted with CuO at 1,000°C,and the sulfur isotopic compositions was determined on a MAT-253mass spectrometer.The sulfur isotopic compositions were reported relative to the Canyon Diablo Triolite (VCDT)standard.Routine analytical precision for standards material was ±0.2‰.
Rhodochrosite,siderite,calcite,and ankerite were handpicked under a stereomicroscope and washed with dis-tilled water.Carbon and oxygen isotopic measurements were made in the Environmental Isotope Geochemistry Laboratory of the Institute of Geology and Geophysics,Chinese Academy of Science.About 150μg was reacted with phosphoric acid at 72°C for 6h using a GasBench II (Thermo-Finnigan).The
CO 2produced was analyzed for carbon and oxygen isotope ratios using a MAT-253isotope ratio mass spectrometer.The carbon and oxygen isotope measurements have a precision better than 0.1‰.Accuracy and precision were routinely checked by running the carbonate standard NBS-19after every six measurements of the samples.Carbon isotope ratios are reported relative to Peedee belemnite (V-PDB),and oxy-gen isotope ratios are reported relative to standard mean ocean water (V-SMOW).
Rb –Sr analyses were performed at the Laboratory for Radiogenic Isotope Geochemistry (LRIG),Institute of Geology and Geophysics,Chinese Academy of Sciences (Li et al.2006).Sphalerite grains in Teflon®vessels,washed ultrasonically in analysis-grade alcohol and millipore water,were dissolved using 0.3ml 3N HNO 3and 0.1ml HF at 120°C.Rb and Sr were separated using ion exchange col-umns.Rb and Sr concentrations were measured by isotope dilution,and a mixed 87Rb/84Sr spike solution was used.Isotopic ratios of Rb and Sr were measured on an IsoProbe-T mass spectrometer at the LRIG.Correction of mass frac-tionation for Sr isotopic ratios was based on an 88Sr/86Sr value of 8.37521.Repeated measurements of NBS987Sr standard solution gave an average value of 87Sr/86Sr ratio of 0.710250±31(2σ).
One quartz syenite porphyry sample in the Lengshuikeng District (Fig.2)was selected for U –Pb zircon dating by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA –(MC)–ICP –MS)at the Tianjin Institute of Geology and Mineral Resources,Tianjin,China (Li et al.2009).The zircon was ablated with a NUP193-FX ArF Excimer laser using a spot diameter of 35μm,with constant 13–14J/cm 2energy density and a frequency of 8–10Hz.The ablated material was carried in He into the plasma source of a GVI.All measurements were made in static mode,using Faraday cups for 238U,232Th,208Pb,206Pb,207Pb,and an ion-counting channel for 204Pb.A common Pb correction is achieved by using the measured 204Pb and assuming an initial Pb composition from Stacey and Kramers (1975).
Analytical results Sulfur isotope data
The δ34S values determined from 20sulfide samples collected in this study and δ34S data from previous works in the same area (Meng et al.2007;Xu et al.2001;No 912Geological Surveying Team (NGST)1997)are listed in Table 1and plotted in Fig.7.The δ34S values of sulfides in ores range from −3.8to 6.9‰with an average of 2.0‰.However,the majority of sulfide minerals have δ34S values between −1
and
Fig.5Mineral paragenesis of the porphyry-hosted and stratabound ores in the Lengshuikeng ore district showing mineral assemblages
4‰.The ranges of δ34S values of sulfides in the porphyry-hosted and stratabound ores from −3.8to 6.9‰(average 2.3‰)and −2.4to 4.9‰(average 1.7‰),respectively.The δ34S values of pyrite in the porphyry-hosted ores in pyrite –chalcopyrite –sphalerite (stage 1)range from 2.2to 4.9‰(average 3.9‰).However,the δ34S values of pyrite,sphaler-ite,and galena in the porphyry-hosted ores in silver minerals –
galena –sphalerite (stage 2)range from −0.4to 3.1‰(average 2.0‰),0.9to 4.3‰(average 2.8‰),and −2.4to 2.8‰(average −0.4‰),respectively.The δ34S values of pyrite,sphalerite,and galena in the stratabound ores in pyrite –chal-copyrite –sphalerite (stage 1)range from 2.3to 4.0‰(average 3.3‰),3.5to 6.9‰(average 5.0‰),and 3.2‰,respectively.However,the δ34S values of pyrite,sphalerite,and galena
in
Fig.6Photomicrographs
showing the salient mineralogical and textural aspects.a Pyrite disseminated in porphyry-hosted ore and replacing K-feldspar crystals.b Granite porphyry fragments cemented by sulfide minerals with brecciated vein structure and pyrite breccias intersected by galena and sphalerite vein (Su 2013).c Chalcopyrite,galena,and
sphalerite in rhyolitic crystal tuff,with chloritization and
sericitization.d Galena with “crumpled ”texture and galena and sphalerite intersected by quartz vein.e Dolomite replaced by sphalerite and sphalerite surrounded by galena.f
Aggregate of acanthite in the intergranular pores of ferromanganese carbonate (Lu et al.2012).g Subhedral native silver occurring in the intergranular pores of ferromanganese carbonate
(Lu et al.2012).h Native silver vein in ferromanganese carbonate (Lu et al.2012).Aca acanthite,Cpy chalcopyrite,Dol dolomite,Fer ferromanganese carbonate,Gn galena,Ksp K-feldspar,Py pyrite,Qz quartz,Slv native silver,and Sp sphalerite
the stratabound ores in silver minerals–galena–sphalerite (stage2)range from2.3to2.8‰(average2.5‰),0.9to 3.8‰(average2.2‰),and−3.8to2.4‰(average0.1‰), respectively.
Carbon and oxygen isotope data
The carbon and oxygen isotope values of carbonate minerals were determined for nine hydrothermal siderite samples col-lected from the stratabound ores and crystal tuffs with weak alteration,two calcite vein samples,five ankerite vein sam-ples,and eight manganese–iron carbonate samples(rhodo-chrosite and siderite)from the volcano-sedimentary strata. Carbon and oxygen isotopic compositions are listed in Table2and plotted in Fig.8.
Theδ13C PDB values in rhodochrosite and siderite samples from the volcano-sedimentary strata vary from−7.0to−2.4‰(Table2).Theδ13C PDB values of hydrothermal carbonates in siderite,calcite,and ankerite samples vary from−7.2to −2.2‰,from−3.3to−1.5‰,and from−3.9to−2.0‰,
Table1Sulfur isotopic compositions of sulfide minerals from the Lengshuikeng Ag–Pb–Zn ore district
Sample no.Mineral Stageδ34S(‰)Sample no.Mineral Stageδ34S(‰)Sample no.Mineral Stageδ34S(‰)
Stratabound ores Stratabound ores Porphyry-hosted ores
ZK504Py1 3.7PD152-4Gn2 1.1ZK10412-1b Py2 1.9
Zk13Py1 4.0ZK136-1Gn2−1.4L14c Py2 2.0
ZK410-2Py1 3.0LSK-74a Gn2 1.8L15c Py2 1.7
No12Py1 2.3N4-8-102b Gn20.1L16c Py2 2.4
ZK515Py1 3.2S4-0-28b Gn20.2L26c Py2 1.5 LSK-9a Py1 3.5LSK-101a Gn2 2.0126-2-1a Py2−0.4 LSK-9-1a Py1 3.6LSK-102a Gn2 2.4130N-4a Sp2 4.3 LSK-41a Py1 2.7So-8-11b Gn2−1.7130S-1a Sp2 2.9 LSK-42a Py1 3.6N4-8-77b Gn2−0.1L17c Sp2 3.2
ZK136-1Sp1 4.3N4-8-13b Gn2 1.1L18c Sp2 2.9
PD160-6Sp1 5.8N4-8-96b Gn2−0.9L19c Sp2 2.6
PD80-10Sp1 5.1N8-4-20b Gn20.2L20c Sp2 1.9
Zk197Sp1 3.5So-12-23b Gn2−3.8LSK-77a Sp2 4.6
No7Sp1 5.2N4-8-102b Py2 2.7LSK-78a Sp2 3.6
PD152-4Sp1 4.6N8-8-26b Py2 2.3L27c Sp2 1.5
ZK515Sp1 6.9So-8-11b Py2 2.3L28c Sp20.9 LSK-101a Sp1 4.6Porphyry-hosted ores130N-4a Gn2 1.2
PD80-13Sp1 5.0L1c Py1 4.9130S-1a Gn20.1 LSK-74a Sp1 4.8L2c Py1 4.9LSK-77a Gn2 1.6
ZK515Gn1 3.2L3c Py1 4.1L21c Gn2 2.8
N4-8-77b Py1 3.0L4c Py1 4.1L22c Gn2−0.3
N8-0-150b Py1 4.0L5c Py1 3.6L23c Gn2−0.9
ZK136-1Py2 2.8L6c Py1 3.6L24c Gn2−1.7
So-8-11b Sp2 2.3L7c Py1 2.2L25c Gn2−2.3
N4-8-77b Sp2 2.8L8c Py2 3.1L29c Gn2−0.1
N4-8-102b Sp2 1.0L9c Py2 3.0L30c Gn2−0.2 LSK-102a Sp2 3.8L10c Py2 2.6L31c Gn2−0.4
So-12-23b Sp20.9L11c Py2 2.5L32c Gn2−0.9
N4-8-77b Gn2−0.8L12c Py2 2.3L33c Gn2−1.1
Zk29Gn2 1.6L13c Py2 2.3L34c Gn2−1.2
Zk311Gn2−0.1ZK10010-718b Py2 2.3L35c Gn2−2.4
Zk198-1Gn20.2ZK10010-719b Py2 1.4
Sp sphalerite,Gn galena,Py pyrite
a Meng et al.(2007)
b Xu et al.(2001)
c NGST(1997)
isotopic composition of sulfide
minerals in the Lengshuikeng ore
district.a All sulfide minerals.b
Sulfide minerals in porphyry-
hosted ore.c Sulfide minerals in
stratabound ore
Table2Carbon and oxygen isotopic compositions of carbonate minerals from the Lengshuikeng Ag–Pb–Zn ore district.Theδ18O SMOW values were calculated fromδ18O PDB values using the formula:δ18O SMOW=1.03091×δ18O+30.91(González and Lohmann1985)
Sampling location Sample no.Lithology Mineralδ13C PDBδ18O SMOW ∼152m elev.PD152-3Manganese–iron carbonate Rhodochrosite−3.311.6
∼160m elev.160-4Manganese–iron carbonate Rhodochrosite−2.418.0
∼120m elev.PD120-9Manganese–iron carbonate Siderite−3.914.6
∼152m elev.PD152-11Manganese–iron carbonate Siderite−5.313.0 13703drill hole Zk315Manganese–iron carbonate Siderite−2.716.8 15150drill hole Zk18Manganese–iron carbonate Siderite−5.914.9
∼152m elev.PD152-10Manganese–iron carbonate Siderite−5.513.4 13704drill hole Zk198-1Manganese–iron carbonate Siderite−7.019.5
∼160m elev.160-3Crystal tuff Hydrothermal siderite−2.217.9 13213drill hole ZK513-1Crystal tuff Hydrothermal siderite−3.917.5 15150drill hole Zk32Crystal tuff Hydrothermal siderite−4.213.5
∼120m elev.PD120-5Crystal tuff Hydrothermal siderite−5.917.5
∼160m elev.160-6Stratabound ore Hydrothermal siderite−3.816.8
∼120m elev.No5Stratabound ore Hydrothermal siderite−5.413.6
∼120m elev.PD120-6Stratabound ore Hydrothermal siderite−5.913.5
∼152m elev.PD152-4Stratabound ore Hydrothermal siderite−3.613.7 13213drill hole ZK509Stratabound ore Hydrothermal siderite−7.212.8 15150drill hole Zk33Ankerite vein Ankerite−3.313.1 15150drill hole Zk34Ankerite vein Ankerite−2.013.5 15151drill hole ZK136Ankerite vein Ankerite−3.313.9 15151drill hole Zk138-2Ankerite vein Ankerite−3.911.0 13703drill hole Zk309Ankerite vein Ankerite−3.012.4 15151drill hole ZK138-3Calcite vein Calcite−1.510.9 13703drill hole Zk313Calcite vein Calcite−3.311.9 Surface a83Carboniferous Limestone0.619.6
a NGST(1997)
respectively (Table 2).The δ18O SMOW values in rhodochrosite and siderite samples from the volcano-sedimentary strata vary from 11.6to 19.5‰and from 10.9to 11.9‰,respectively (Table 2).The δ18O SMOW values of hydrothermal carbonates in siderite,calcite,and ankerite samples vary from 12.9to 17.9‰,from 10.9to 11.9‰,and from 11.0to 13.9‰.One Carboniferous limestone sample is isotopically heavy (19.6‰;NGST 1997).
Rubidium and strontium isotope data
The Rb –Sr analytical data of sphalerite are given in Table 3.The sphalerite was separated from the manganese –iron car-bonate ores in the silver minerals –galena –sphalerite stage.Data regression for isochron ages and weighted mean values were performed using the ISOPLOT software (Ludwig 2001),with 2%error for 87Rb/86Sr ratios and 0.05%error for 87
Sr/86Sr at the 95%confidence level.The analytical data for sphalerite yielded an age of 126.9±7.1Ma (Fig.9)with an initial 87Rb/86Sr ratio of 0.71490(mean square weighted deviation (MSWD)=0.94).
Zircon U –Pb geochronology
Measured 206Pb/238U ages from individual zircons are shown in Fig.10,and the analytical results of LA –ICP –MS U –Pb dating are listed in Table 4.For the quartz syenite porphyry (sample T10),analyses from 20spots cluster close to the concordia,yielding a weighted mean 206Pb/238U age of 136.31±0.81Ma (2σ,MSWD=1.3;Fig.10).
Discussion
Ages of magmatism and mineralization
The results presented in this study together with those from previous investigations suggest multistage magmatism in the Lengshuikeng and adjacent regions.The magmatic activity took place principally during three periods,in the Jurassic,earlier Early Cretaceous,and later Early Cretaceous.
The Jurassic magmatism in the Lengshuikeng District is represented by the emplacement age of the granite
porphyry,
Fig.8Plots of calculated δ18O SMOW versus δ13C PDB from various samples in the Lengshuikeng ore district.Carbonate fields are from previous studies.The data are from four different materials including marine carbonate (Baker and Fallick 1989;Hoefs 1997),continental carbonate (Hoefs 1997),sedimentary organic matter carbon (Hodson 1977;Hoefs 1997),and magma-mantle carbonate (Taylor et al.1967;Valley 1986;Ray et al.1999).This plot offers information about various processes of CO 2and carbonate ions including meteoric water influence,
sea water penetration,sediment contamination,and high temperature influence,low temperature alteration (Deines 1989;Demrny and Harangi 1996;Demeny et al.1998;Hoernle et al.2002),decarboxylation and oxidation (Hofmann and Bernasconi 1998),decarbonate and carbon-ate dissolution (Lorrain et al.2003),crystallization differentiation with no significantly influence on the oxygen,and carbon isotopic composition (Santos and Clayton 1995;Bindeman 2008)
Table 3Rb –Sr isotopic
compositions of sphalerite in the Lengshuikeng Ag –Pb –Zn ore district
Sample no.Mineral Rb (μg/g)Sr (μg/g)87
Rb/86Sr
87
Sr/86Sr(±2σ)
I sr PD152-7Sphalerite 1.390 4.0800.98910.716557±340.71435ZK205Sphalerite 0.0960.9720.28660.715641±380.71500ZK513-1Sphalerite 2.060 1.820 3.27260.720846±220.71354No4
Sphalerite
0.960
3.960
0.70190.716056±18
0.71449。