Carbon–oxygen isotopic covariations of calcite from Langdu skarn copper deposit,China:implications for sulfde precipitation

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Carbon–oxygen isotopic covariations of calcite from Langdu skarn copper deposit,China:implications for sulfde precipitation

Tao Ren?Xingchun Zhang?Runsheng Han?Baohong Hou

The Langdu skarn copper deposit in the Zhongdian area,YunnanProvince,China,hasan average Cu grade of 6.49%.The deposit is related to a porphyry intrusion (～216 Ma),which was emplaced in the Upper Triassic sedimentary rocks of the Tumugou and Qugasi Formations. At the Langdu skarn copper deposit,carbon and oxygen isotope ratios of fresh limestones(δ18O=3.0–5.6‰relative to V-SMOW;δ13C=24.5–25.7‰ relative to PDB) and partly altered limestones(δ18O=27–7.2 to-1.9‰; δ13C=11.8–15.2‰)indicated that the deposit was a typical marine carbonate source.Oxygen and carbon isotope values for calcites formed at different hydrothermal stages are-9.1 to 0.2 and 10.1–16.3‰,respectively.Moreover, the carbon–oxygen isotopic composition of an ore-forming fuid(δ18O=5.0–9.5‰,δ13C=-7.3to-5.3‰)suggested the presence of magmatic water,which most likely came from the differentiation or melting of a homologous magma chamber.The deposition of Calcite I may arise from metasomatism inanopensystemwithaprogressivelydecreasingtemperature. Later,the minerals chalcopyrite,pyrrhotite,quartz and Calcite II were precipitated due to immiscibility.Water–rock interactioncouldpotentiallyberesponsibleforCalciteIIIprecipitation in the post-ore stage.

Stable isotope·Copper deposit·Skarn·Fluid evolution·Yunnan province

1 Introduction

For decades,stable isotopes have been successfully used in geochemical studies as powerful petrogenetic tracers and paleothermometers.Quantitative stable isotope models and their applicationshave become themainstayofinternational stable isotope geochemistry.Using these models,researchers worldwide have elucidated the formation mechanisms of numerous deposits(e.g.,Zheng 1990;Zheng and Hoefs 1993;PengandHu2001;Choietal.2003;Zhouetal.2013a, b,c).Carbon and oxygen isotopic covariation in carbonates associated with hydrothermal mineralization can supply useful mineralization information,such as temperature and isotopiccompositionofhydrothermalfuid,andthenatureof dissolved carbon species(e.g.,Rye and Williams 1981; Matsuhisaetal.1985;Zheng1990).Inthispaper,wepresent the frst analysis of the isotopic compositions of carbon and oxygeninlimestone,marble,silicate-bearinglimestone,and calciteformedatdifferentstagesinthe Langduskarncopper deposit,Yunnan Province,China.Using the theoretical models of Zheng(1990)and Zheng and Hoefs(1993)with homogenized temperature data derived from fuid inclusions,this study investigates the nature and evolution of the hydrothermalfuidandthemechanismofcalcite,sulfde,and quartz precipitation.

2 Geology of the Langdu skarn copper deposit

The Langdu skarn copper deposit is located about 50 km northeast of Xianggelila City,Yunnan Province,China.The deposit was discovered in 1964 by Team No.1 of the Regional Geological Survey,Bureau of Geology and Mineral Resources,Yunnan Province,China.Recently (particularly during 2006 and 2007),Yunnan Huaxi Mining Co.,Ltd.has discovered several copper-rich ore bodies in the contact zone of porphyry bodies and marbleized limestone to the north and west of the original Langdu skarn deposit,containing 104,030 tons of copper reserves and over 50,000 tons of copper metal resources.With ongoing exploration,more copper metal resources may be discovered in the Langdu skarn copper deposit area.

The outcropped strata in the Langdu deposit mainly include the Upper Triassic Qugasi and Tumugou Formations.The valley,a small part of the deposit area,is covered with Quaternary alluvial sediments with a thickness varying from 0 to 30 m.The Tumugou Formation is mainly in the southwestern corner of the district,which consists of slate,sericitic slate,meta-sandstone and limestone with a thickness of 1433 m.The Qugasi Formation is mainly exposed in the deposit area.The Qugasi Formation forms the core part of the Langdu anticline and consists of layered limestone,sericitic slate,fne meta-sandstone,with a total thickness varying from 487 to 2,683 m.Under the infuence of the porphyry intrusions,the clastic rock was hornfelsed to felsic hornfels and brecciated hornfels,and the limestone was metasomatized to skarn and marbleized in the contact zone.

The major structure of the deposit area includes the NW-trending Langdu anticline and some NW-trending and NE-trending faults.The NW-trending fractures are mainly inter-layer fractures along the striking of the Langdu anticline.A part of the NE-trending Bidu fault cuts through the core part of the Langdu anticline.The copper mineralization mainly occurs in the conjunction between the NW and NE trending faults,in the southwest limb of the Langdu anticline.

The known copper ore bodies occur mainly in the contact zone between the porphyry bodies and limestone of the Upper Triassic Qugasi Formation.Using Ar–Ar dating of biotite which had been separated from the intrusive rock, Zeng et al.(2004)obtained the age of 216.93 Ma.The only ore body that occurs within the monzonite porphyry is orebody III3.Currently,6 ore blocks and 21 ore bodies have been identifed in the deposits(Fig.1).Ore blocks I, III,and IV,which collectively contain 13 ore bodies,are under mining operation.

The Langdu deposit is characterized by multistage skarn formation and complex mineralization which result in a diversity of minerals.Dominant Ca–Fe anhydrous silicate minerals(andradite and hedenbergite)were formed during the prograde skarn stage(Ia).Electron microprobe analyses show that the composition of andradite varies from Ad57to Ad94,while that of pyroxene is Hd78–92Di3–11Jo5–11. Hydrous silicate minerals,including actinolite and grunerite,form during the retrograde stage(I),with later a deposition of minor Cu–Fe sulfde,quartz,and calcite. Chalcopyrite and pyrrhotite dominate the main ore-forming stage(II).These minerals occur as veins,veinlets or patches in the coarse veins of quartz or calcite,which contain an average copper grade of 6.49 wt%(exceeding 10 wt% in some localities).During the later ore-forming stage, chalcopyrite occurs as veins or veinlets that fll the fssures of marble,or as patches in Fe–dolomite coarse veins.Postore hydrothermal minerals(III)are prominent and occur with calcite,quartz,and chlorite.

3 Sampling and analytical methods

Based on its petrochemical characteristics,limestone is divided into two distinct groups:fresh limestone and silicate-bearing limestone.The fresh limestone is light gray and fne-grained,whereas silicate-bearing limestone is brown and coarse-grained and characterized by its high SiO2content and low CaO content.For analysis,21 grains of calcites were collected from the Langdu deposit.These calcites were classifed into Calcite I,Calcite II,and Calcite III,according to their geological characteristics and mineral assemblages.Calcite I is usually white,and flls the fractures of altered pyroxene or garnet in patches or veins (Fig.2a,b).Calcite II is generally milky and associated with quartz,pyrrhotite,and chalcopyrite(Fig.2c).Calcite III occurs as coarse veins(Fig.2d).Carbonate samples were crushed and reacted with phosphoric acid at 25°C, and the carbon and oxygen isotopes(δ13C and δ18O, respectively)in the liberated CO2gas were measured using a mass spectrometer with a precision of±0.2‰(Finnigan MAT252)at the State Key Laboratory of Environmental Geochemistry,Chinese Academy of Sciences.

4 Results and discussion

4.1 Results

The fresh limestones were more enriched in carbon and oxygen isotopic compositions (Table 1;δ13C=3.0–5.6‰,δ18O=24.5–25.7‰)in comparison to the marbles(δ13C=2.0–2.8‰,δ18O=24.9–25.4‰).Similarly,the δ13C and δ18O values of the marbles were higher than those of silicate-bearing carbonate rocks(δ13C= -7.2 to-1.9‰,δ18O=11.8–15.2‰).This decreasing trend of carbon and oxygen isotope ratios from limestones to silicate-bearing limestones indicates that the fresh limestones were modifed due to repeatedly interacting with a hydrothermal fuid with a lower carbon and oxygenisotope ratio.Different carbon and oxygen isotope ratios in marbles and silicate-bearing limestones are attributed to different water/rock ratios.As shown in Fig.3,the carbon and oxygen isotope values of fresh limestones are generally consistent with those of marine carbonates.Theoretical models indicate that marbles form at temperatures below 100°C and have water/rock ratios less than 0.5(Fig.4). This suggests that limestone recrystallized at low temperatures and its heat energy was most likely derived from nearby magmatic fuid/rocks.

Fig.1 a Simplifed geological map of the tectonic framework of the Sanjiang Tethys.b Simplifed geological map of the Langdu deposit, adapted from the detailed geological report of the Langdu skarn deposit

The carbon isotope values of the calcites(Calcite I and Calcite II)in the ore-forming stage were similar to those of calcites in porphyry copper deposits of the Zhongdian region,ranging from-9.1 to-7.6‰ (Table 1).These carbon isotopic compositions show that the carbon in the hydrothermal fuid may originate from deep regions of the earth(i.e.,from the mantle or lower crust)or from the mixture of carbonate and organic matter.However,its narrow range of δ13C values excludes the possibility of an organic origin.The oxygen isotopic compositions of theore-forming stage range from 10.1 to 12.6‰.Carbon and oxygen isotopic compositions of syn-ore and post-ore calcites(III)are signifcantly different from each other.In fact,the carbon and oxygen isotopic compositions of postore calcites(III)are similar to those of marine carbonates.

Fig.2 Specimen photos:a cavity in the garnet flled by patch calcite;b calcite veins/veinlets occurring in pyroxene fractures;c coarse veins of calcite,quartz,and sulfde;d coarse veins of barren-sulfde calcite

From the plot of δ13C versus δ18O(Fig.3),we observe that mostofthesamplesoriginatedfromtheigneousorsurrounding felds.The isotopic composition progressively increased from thesyn-orestage(IandII)tothepost-orestage(III).According to theoretical models of water/rock interactions,calcites that form in the syn-ore stage at temperatures ranging from 300 to 400°C(Fig.4)are consistent with the principal microthermometric results of fuid inclusions in sulfde-bearing quartz. The water/rock ratios are greater than 20(Fig.4),indicating that the isotopic compositions of these samples were largely affected by that of the hydrothermal fuid.The formation temperatures of the post-ore calcites vary from 200 to 300°C, and the water/rock ratios range from 2 to 10(Fig.4).These results suggest that both limestone and hydrothermal fuid contribute to the isotopic composition of Calcite III.

4.2 Origin of the hydrothermal fuid

The isotopic composition of the hydrothermal fuid, assuming that it existed in isotopic equilibrium with the hydrothermal minerals,can be calculated from the isotopic composition of the minerals,the fractionation equation at a given temperature,and the mineral precipitation temperature.The oxygen isotope fractionation equation between calcite and water occurs from 200 to 700°C(O’Neil et al. 1969),whereas the carbon isotope fractionation equation between CO2and calcite occurs from 0 to 700°C(Bottinga 1969).We assume that there is no fractionation between CO2and H2CO3in the calculation.

The isotopic compositions were calculated from measured isotope values.The precipitation temperatures of Calcites I and II taken from fuid inclusions(which refect the principal temperatures of fuid inclusions measured from quartz coexisting with calcite)range from 300 to 400°C.While calculating the relevant isotopic value of the hydrothermal fuid,the formation temperature of Calcite IIIis assumed as 200°C.Based on the aforementioned parameters,the stable isotopic compositions of Stage I, Stage II,and Stage III hydrothermal fuids that equilibrated with Calcite I,Calcite II,and Calcite III are δ18O= 5.0–9.5‰,δ13C=-6.7 to-5.5‰;δ18O=6.3–8.8‰, δ13C=-7.3 to -5.3‰; and δ18O=3.0–7.2‰, δ13C=-6.0 to 0.6‰,respectively.

Table 1 Oxygen and carbon isotope data and calculated compositions of equilibrium waters

Three types of fuids of different origins magmatic water, meteoric water,and seawater contributed to the formation of the hydrothermal minerals.Each fuid has a distinctive stable isotopic composition.The calculated stable isotopic compositions of ore-forming fuids in equilibrium with syn-ore calcite are consistent with the composition of magmatic water.High temperatures(300–400°C)and high salinity (10–20 equiv.wt%NaCl)also typify fuid inclusions.This suggests that the ore-forming fuid was magmatic water supplied from the Langdu intrusives or the igneous chamber. This means that the post-ore fuid is in fact more complicated than previously suspected and that the input of seawater cannot be excluded.

Fig.3 Carbon and oxygen isotopic compositions of the Langdu deposit.Carbon and oxygen isotope data of igneous calcite and marine carbonates are from Ray et al.(2000)and Zheng(2001), respectively

4.3 Fluid evolution,calcite and sulfde precipitation

The observed positive correlation between the δ13C and δ18O values of all calcites shown in Fig.3 can be explained by two processes:the mixing of two fuids with different isotopic compositions or the calcite precipitation,which is due to a temperature effect in the open system coupled with either CO2outgassing or fuid/rock interactions.

If fuid mixing were responsible for calcite precipitation, then two different fuids with distinct isotopic compositions and temperatures would have been observed.However,the δ13C values of the calcites(particularly those of Calcites I and II)do not to support this mechanism.Although we found no evidence of CO2-bearing fuid inclusions,there is a possibility that the outgassing of CO2led to calcite precipitation.This mechanism is possible because CO2concentration in hydrothermal fuid is below 1.0 mol and therefore,the observed three-phase inclusions should not exist(Zheng 1990).Since the carbon isotopic fractionation factor between calcite and CO2is negative(temperature＞200°C),CO2outgassing will inevitably deplete the quantity of carbon isotope in later-deposited calcites.On account of the high water/rock ratios of Calcite I and Calcite II,the observed isotopic signature of the various calcites cannot be reasonably explained by water/rock interactions.

The above analysis suggests that calcite precipitation in the Langdu deposit occurred in three stages under different precipitation formation mechanisms.Calcite I,which spatially coexists with skarn minerals,is depleted in δ13C and δ18O.Theoretical models indicate that Calcite I was formed at high water/rock ratios(＞50;see Fig.4),which may represent the primitive isotopic composition of the magmatic fuid.For metasomatism,one must assume decreasing temperature(contact metasomatic processes related to hydrothermal fuid and cool sedimentary rocks), which can cause Calcite I precipitation in an open system. The slightly lower δ13C values of Calcite II relative to Calcite I within the disseminated material or veinlets imply an identical isotope signature of carbon and oxygen in the fuid reservoir.The observed δ13C values can be readily explained by CO2outgassing,as the boiling phenomenon has been identifed in the coexisting quartz-hosted fuid inclusion.Since the oxygen in hydrothermal fuids generally exists as H2O,CO2outgassing and calcite precipitation will affect the carbon isotope composition insignifcantly. Theoretical models show that Calcite III was formed at lower temperatures and water/rock ratios than Calcites I and II were(Fig.4).In the case of Calcite III,where the δ13C and δ18O values were more enriched relative to Calcite I and Calcite II,water/rock interaction accompanied by a progressive decrease in temperature may have occurred at different water/rock ratios.

Fig.4 Covariation of carbon vs oxygen isotopic compositions during water/rock interactions involving limestone with δ13C=+4‰(PDB)and δ18O=+26‰ (V-SMOW),andafuidwith δ13C=-9‰ (PDB)and δ18O=+7‰ (V-SMOW).The dissolved carbon mainly exists as CO2at temperatures ranging from 100 to 450°C.The solid and dashed curves denote the closed and open systems,respectively

During phase separation,most of the volatile compounds(CO2,SO2,HCl)partitioned into the vapor phase, and the high-saline residual liquid may have retained most of the metal elements,including Cu,Zn,Pb,and Fe. Consequently,the fuid became supersaturated with metal ions Ca2+and Si4+.The subsequent decrease of pressure and temperature and increase of pH of the fuid resulted in the precipitation of a large amount of ore materials,which formed coarse calcite-quartz-sulfde veins.

5 Conclusion

The deposit was devoid of CO2-bearing fuid inclusions, and the modeled precipitation temperatures of various calcites in this study are consistent with microthermometry measurements of various fuid inclusions.Thus,the theoretically calculated CO2fraction of 0.1 mol percent is reasonable and the results obtained are credible.

AcknowledgmentsThis study was supported by the National Science Foundation of China(NSFC)project(40372049).The authors wish to thank C.Z.Yang(Branch of Mineral Resources Investigation, Yunnan Geological Survey,China)for their helpful discussions on the geology of the Zhongdian area and for information on the Langdu deposit.

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Received:12 March 2014/Revised:31 March 2014/Accepted:2 April 2014/Published online:16 December 2014 ?Science Press,Institute of Geochemistry,CAS and Springer-Verlag Berlin Heidelberg 2014

T.Ren(?)·R.Han

Faculty of Land and Resource Engineering,Kunming University of Science and Technology,Kunming 650093,Yunnan,China e-mail:rtao1982@126.com

X.Zhang

State Key Laboratory of Ore Deposit Geochemistry,Institute of Geochemistry,Chinese Academy of Science, Guiyang 550003,Guizhou,China

B.Hou

Minerals&Energy Division,Primary Industries and Resources SA,GPO Box 1671,Adelaide,SA 95000,Australia