J. Cent. South Univ. Technol. (2007)01-0104-07

DOI: 10.1007/s11771-007-0021-4               

 Isotope systematics and metallogenetic age of Zhuanghe gold deposit, Liaoning province, China

WEI Jun-hao(魏俊浩), TAN Wen-juan(谭文娟), GUO Da-zhao(郭大招),

TAN Jun(谭 俊), LI Yan-hua(李闫华), YAN Yun-fei(鄢云飞)

(Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China)

Abstract: Based on the metallogenetic geology conditions, the H, O, C and S isotopic compositions were measured by MAT-251 mass spectrometer, and Pb isotope and Rb-Sr dating were carried with MAT-261 multi-acceptor mass spectrometer. The results show that the δ¹⁸O values of gold-bearing vein quartz from different levels are 1.19%-1.42%. The calculated δ¹⁸O values of ore fluids are 0.55%-0.78%, and δD values are from –8.64% to –6.66%. The calculated values of by the δ³⁴S_py values in quartz veins display sulfur isotope compositions from -0.053% to +0.413%. Carbon isotope compositions of carbonates are from -0.612% to 0.140%. The mole ratios of ²⁰⁶Pb to ²⁰⁴Pb, ²⁰⁷Pb to ²⁰⁴Pb and ²⁰⁸Pb to ²⁰⁴Pb in auriferous quartz vein are 16.987-17.545, 15.342-15.623, and 38.254-38.744, respectively. The age of the Zhuanghe gold deposit determined by Rb-Sr isochron of the fluid inclusions in quartzes is (143.0±5.8) Ma. These isotopic data suggest that the metallogenetic fluids are generated from magmatic hydrotherm and the origin of ore-forming matters is related to the deep-derived magmatic activities. Meanwhile, the metallogenetic epoch of the Zhuanghe gold deposit is in Yanshanian period.

Key words: isotope systematics; metallogenetic age; Zhuanghe gold deposit

1 Introduction

The Zhuanghe gold deposit is located 4 km east of Zhuanghe city, Liaoning Province.  From 1975 to 1978, the 103th Team of Liaoning Nonferrous Metal Geological Prospecting Bureau had explored for gold deposit in the Zhuanghe zone, and initially supplied with a cumulative gold reserve of 5 t. L? et al ^[1] suggested that the mineralization was related to magmatic activities in Yanshanian period. Archaeanic and Paleoproterozoic metamorphic basement probably provided a part of metallogenic matter, but the metals may have originated from magmatic fluids in Yanshanian period and the deposit belonged to a moderate temperature magmatic hydrothermal deposit. However, only geological and macroscopic comparisons have been done and no metallogenetic suggestions reported.

Based on the geological characteristics of the Zhuanghe gold deposit, the source of the ore-forming fluids and the metallogenetic age were studied by using oxygen, hydrogen, sulfur, carbon and lead isotope measurements and Rb-Sr isotopic dating.

2 Geological background

The Zhuanghe area is situated in the eastern part of the Tanlu fault zone and in the southern part of Liaoning Province. The presences of the strata primarily encom- pass the Archeanic Anshan Group, Paleoproterozoic Liaohe Group, Sinian to Cambrian System and Cretaceous System. The distribution belongs to the typical binary platform facies, i.e. the Anshan Group is the foundation bed, Sinian to Cambrian age rocks constitute the sedimentary cover and sedimentary basins formed by Cretaceous System occur along the faults. The dominant lithologies of the Anshan Group are compound of biotite-amphibole-plagiogneiss, biotite-monzogneiss, biotite-plagiogneiss, amphibole plagiogneiss interbedded with fine-grained plagio-amphibolite, biotite- metagranulitite, leucogranulitite and magnetic quartzite. The Liaohe Group is distributed a little in this area, including dolomitic marble containing siliceous banding of the Dashiqiao Formation, phyllitic-biotite schist and biotite-quartz schist of the Gaixian Formation. Sinian System only comprises Lower Diaoyutai, Nanfen and Qiaotou Formations, which are carbonate-shale construction.

In addition, a wide range of magmatic rocks also occur, such as the Zhuanghe lithosome consisting of granodiorite, quartz diorite and porphyritic granite, and granitic batholith and medium-basic to medium-acid dike rocks in Indosinian-Yanshanian period like granite- porphyry, fine-grained diorite, lamprophyre dikes and diabase-porphyrite.

Local stratigraphy within the Zhuanghe gold deposit consists mainly of the Archeanic Anshan Group, Paleoproterozoic metamorphic rocks as foundation bed, epimetamorphic rocks of Sinian System, Neoproterozoic Erathem as cover and Quaternary System, which are distributed in the east-north, west, south and lowland part, respectively. 

Within the mining district, three major events are identified. The thrusting brittle-ductile décollements extensively spread stepping arrays along northwest direction in the plane due to the segmentation of north-east-trending faults. The folding mainly appears north-north-east-striking. The deposit in this study is situated in the core zone of an anticline. The major faults are east-north-, north-north-east- and north-north-west- trending. Small amounts of granodiorite-porphyry that outcrop at the mining surface display the vein occurrence, but the lamprophyre dikes are only discovered in the tunnel. The dykes at depth also fully produce from the log of drill hole.

The mineralization types of gold are mainly the auriferous quartz veins and gold bearing altered rocks, which are closely associated with faults and form the east, west and south ore belts. The auriferous quartz veins occur predominantly in the eastern ore belt with the length of 20 to 140 m, up to the thickness of 5 to 6 m. The quartz veins in the southern ore belt present the relatively small scale, having 20 to 80 m in length, without economical value at present. In the western ore belt, the gold-bearing altered rocks appear relatively large scale; for example, two ore belts have 700 and    1 200 m in length, respectively. These belts strike northeast and dip at 40? to 60? towards the south-east. The gold-bearing quartz veins in this belt prolong several meters with west-north-trending, but verge 100 to 200 m in the section having west-south dip, about 50? dip angle. In a word, the ore bodies in the quartz veins present lens-like shapes, discontinuous and small extents, but in the gold-bearing altered rocks the ore bodies have stable attitude and larger scale where some stretch 400 m or even more. The attitude of gold orebodies is consistent with that of the gold-bearing quartz veins and tectonic altered rocks (Fig.1). In Fig.1, Q denotes quaternary; Z₁q, Z₁n and Z₁d denote Qiaotou formation, Nanfen formation

Fig.1 Geological sketch map of Zhuanghe gold deposit

 1－Granodiorite-porphyry dyke; 2－Anticline and syncline; 3－Overfold; 4－Compress-shear fracture; 5－Tension-shear fracture; 6－Unknown fracture; 7－Geologic boundary; 8－Unconformity geologic boundary; 9－Au-bearing quartz vein and its number; 10－Boundary of mineralization zone and its digit; 11－Ore vein zone(27^#-29^#)

and Diaoyutai formation in lower Sinian, respectively; Ar denotes Archeozoic gneiss. Two types of gold ore have been distinguished: the quartz vein ore and the altered rock ore. Ore minerals in the quartz veins include native gold, electrum, pyrite, chalcopyrite, galena, sphalerite and bornite, and gangue minerals consist of quartz, sericite, potassium feldspar and calcite. The ore minerals exhibit idiomorphic to xenomorphic granular, metasomatic solution, cataclastic and exsolution textures, and the ore assemblages have disseminated stockwork and taxitic structures in hand specimens. In comparison with the quartz vein ore, native gold, pyrite, chalcopyrite and galena make up the ore minerals of the altered rock ore, but vein minerals contain quartz, microcline, orthoclase, sericite, calcite, chlorite and epidosite.

According to analysis of the coring and tunnel observation combining with the thin sections studies, four stages of mineralization are recognized. Stage 1 is represented by quartz veins filling the faults and some altered rock bands, such as orthoclase, microcline; Stage 2 is composed of the medium to coarse-grained cubic pyrite; Stage 3 is characterized by native gold, pyrite, chalcopyrite, galena and sphalerite, and is the main gold formation period in the four metallogenic stages of the Zhuanghe gold deposit; Stage 4 is marked by the formation of carbonate and goethite. Hydrothermal replacement is intense and common alterations contain silicification, pyritization, potassium alteration, sodium alteration, sericitization, chloritization and carbonation. However, owing to the lower level of exploitation, the distribution of the alteration has not been identified.

4 Sampling and analytical method

All samples used in this study were sampled from the eastern ore belt where the quartz veins display extensively. The unaltered samples were collected from the auriferous quartz vein in Stage 3 at the first, third, forth and sixth mine levels for hydrogen, oxygen and sulfur isotope analyses. The carbon isotope samples were picked from Stage 3 gold-bearing quartz-carbonate veins. And the gold-bearing quartz veins from the second and fifth mine levels, the granite-porphyry veins from underground pits and Archeozoic metamorphic rocks in the prospecting trenches were sampled for lead isotope analyses. Pb isotope analyses were done on pyrite separates and whole-rock samples. In addition, the samples of Stage 3 auriferous quartz vein for Rb-Sr isotope dating were collected from the same ore body at different elevations so as to gain the perfect isotopic age with variable isotopic compositions.

In the laboratory, samples were crushed into various granularities in accordance with the testing requirement, and pure pyrite separates were hand-picked under a binocular microscope to make sure the purity of single mineral of 99%. All samples were analyzed for their isotopic values at the Yichang Institute of Geology and Mineral Resources, Chinese Academy of Geological Sciences. Analyses were conducted using a MAT-251 mass spectrometer for hydrogen, oxygen, sulfur and carbon isotopes. Isotopic data were given in standard delta notation relative to the Standard Mean Ocean Water (SMOW) for oxygen and hydrogen, the Canyon Diablo Triolite (CDT) standard for sulfur, the Peedee Belemnite limestone (PDB) standard for carbon. i.e. δX=((R_sp/R_st)-1)×100%(X denotes ¹⁸O, D, ³⁴S; R_sp denotes isotope ratio of sample, R_st denotes isotope ratio of standard material). The Rb-Sr and lead isotopes for this studied gold deposit were all measured using a MAT-261 multi-acceptor mass spectrometer. The measurement error of mole ratio of ⁸⁷Rb to ⁸⁶Sr is less than 2%. The Rb-Sr isochronal age of the Zhuanghe gold deposit was recalculated using the ISOPLOT (2.9 version) program^[2].

5 Results

Oxygen and hydrogen isotope results are listed in Table 1.

Table 1 Oxygen and hydrogen isotope data from Stage 3 quartz veins in Zhuanghe gold deposit

For the calculation of δ¹⁸O values of fluid in equilibrium with measured quartz, the quartz-water fractionation equation being used is =3.306×10^-6T^-2–2.71^[3]. The measured homogeneous temperature of the fluid inclusions in the quartz is 255 ℃, but it need to be emended by the pressure, so the actual temperature is T_t= 603 K. The δ¹⁸O value of quartz ranges from 1.19% to 1.42% (mean of 1.29%), with calculated between 0.55% and 0.78% (mean of 0.65%). Hydrogen isotopic analyses of the fluid inclusions in

quartz have yielded δD values of -6.66% to -8.64% (Table 1). Their compositions are shown in Fig.2. 

Fig.2 Diagram of δD versus for fluid inclusions in quartzes

The δ³⁴S_py values are within the range of 0.057% to 0.523% (mean of 0.255%) determined on the 14 pyrite samples from Stage 3 auriferous quartz veins. The calculated values range from -0.053% to   +0.413% (mean of 0.145%) (Table 2), and are plotted in Fig.3. The isotopic data have a narrow range with the obvious pyramidal shape and peak values of 0 to 0.3%.

Table 2 Sulfur isotope compositions from Stage 3 quartz veins in Zhuanghe gold deposit

Lead isotope ratios of 20 samples are listed in Table 3. These data are obtained from pyrite in the auriferous quartz veins, granite-porphyry veins and Archeozoic metamorphic rock of the Zhuanghe gold deposit with the majority within restricted ranges. The plots of mole ratio of ²⁰⁷Pb to ²⁰⁴Pb (n(²⁰⁷Pb)/n(²⁰⁴Pb)) and mole ratio of ²⁰⁸Pb to ²⁰⁴Pb (n(²⁰⁸Pb)/n(²⁰⁴Pb)) versus mole ratio of ²⁰⁶Pb to ²⁰⁴Pb (n(²⁰⁶Pb)/n(²⁰⁴Pb)) are shown in Fig.4.

Fig.3 Sulfur isotope distribution pattern of pyrites

Fig.4 Correlativity of lead isotope compositions of pyrites and rocks in Zhuanghe gold deposit

(a) n(²⁰⁷Pb)/n(²⁰⁴Pb) vs n(²⁰⁶Pb)/n(²⁰⁴Pb);(b) n(²⁰⁸Pb)/n(²⁰⁴Pb) vs n(²⁰⁶Pb)/n(²⁰⁴Pb)

Calcite in the Stage 3 quartz-carbonate veins has a total range of δ¹³C values from -0.612% to -0.140%, with a mean of -0.362% (Table 4).

Rb and Sr concentrations of 7 samples were examined from the fluid inclusions of pure quartz minerals in the auriferous quartz veins (Table 5). The mass fraction of Rb is between 0.166 7×10^-6 and

Table 3 Lead isotope compositions in Zhuanghe gold deposit

Table 4 Carbon isotope compositions of calcite in Stage 3 from auriferous quartz-carbonate veins in Zhuanghe gold deposit

2.231 0×10^-6, and mass fraction of Sr varies from 2.332×10^-6 to 4.462×10^-6. The n(⁸⁷Rb)/n(⁸⁶Sr) ratios vary over a wide range between 0.048 5 and 1.518 0, and the scatters observed reflect probably homogeneity of the initial Sr isotope compositions of (0.721 32±     0.000 06)-(0.724 39±0.000 03). Seven samples yielded the Rb-Sr age of (143.0±5.8) Ma, with a mean square of weighed deviate (MSWD) of 2.67 and initial n(⁸⁷Sr)/n(⁸⁶Sr) ratio of 0.721 298 (Fig.5).

6 Discussion

The isotope data play an important role in tracing the metallogenic fluids source and confirming the mineralization physical-chemical conditions^[4-6].

Fig.5 Rb-Sr isochron of fluid inclusions in quartz from auriferous quartz veins of Zhuanghe gold deposit

6.1 Oxygen and hydrogen isotopes

Different fluids vary in the characters of hydrogen and oxygen, so the origin of the fluids is often confirmed on the basis of the constitution of water in the fluid inclusions^[7]. The calculated δ¹⁸O is 0.55% to 0.78%, and δD of -6.66% to -8.64%. These values are similar to those of magmatic fluid^[8]. As can be seen from Fig.2, the calculated δ¹⁸O and δD values plot near the field of primary magmatic water and metamorphic water area, nearer to the former. Consequently, it suggests that the source of ore-forming fluid is primarily magmatic fluid.

Table 5 Rb-Sr isotope compositions of fluid inclusions in quartz from auriferous quartz veins in Zhuanghe gold deposit

6.2 Sulfur isotope

The S isotope composition may be used to conclude the source of metallogenic matter. Generally, sulfur of this isotopic composition may originate directly from magmatic fluids or indirectly from leaching or desulfidation of primary magmatic sulfide minerals, or average crustal sulfur. In the gold deposit, the gold is associated with the metallic sulfides, so the variety of sulfur isotopic content reflects not only the source of the sulfur, but also the migration and precipitation mechanism of ore-forming element. Many reports indicate that the isotopic composition of hydrothermal minerals is decided on the δ³⁴S values of region and the ore-forming condition. In the moderate–low temperature hydrothermal deposit, the δ³⁴S values of sulfuretted hydrogen can substitute approximatively the sulfur isotope of region. The calculated values are gained by the coefficient of fractionation between pyrite

and sulfuretted hydrogen, i.e. =0.4×10⁶T^-2[7].

In the Zhuanghe gold deposit, the compositions of pyrites in the Au-bearing quartz veins and the calculated

values are relatively uniform, indicating an isotopic uniform sulfur source (shown in Table 2 and Fig.3). The sulfur isotope compositions of region display a relatively tight cluster, ranging from -0.053% to +0.413%. This characteristic range of δ³⁴S is similar with the meteoric sulfur, which is interpreted to indicate that the sulfur isotope in this deposit has the feature of deep sulfur, and the source of ore-forming matters is deep magmatic fluids.

6.3 Lead isotope

Lead isotope analyses of separate minerals and whole rocks aim to trace metal sources in ore deposits and infer source reservoirs^[9]. Summing up the data in Table 3 and the aforementioned lead isotope correlativity plots (Fig.4), it suggests that the ore lead and granite- porphyry veins were derived from a similar magmatic source, and further indicates that the origin of ore-

forming fluid and mineralization are closely associated with granite-porphyry veins. However there are obvious differences in lead isotopic characters between ore and Archeozoic metamorphic rocks.

6.4 Carbon isotope

Carbon source has significance in discussing on ore sources. Carbon in hydrothermal fluids may originate from decarbonation or dissolution of preexisting carbonates, from magmatic sources, and/or from the oxidation or hydrolysis of reduced carbon in sedimentary or metamorphic rocks. These carbon reservoirs have different carbon isotope compositions. At present, according to the study on the inclusions of the mantle-derived rock, the deep-derived carbon isotopic composition is confirmed. The δ¹³C range of -0.2% to -1.0% (majority in the range of -0.5% to -0.9%) stands for the isotopic composition of the deep-derived magmatic carbon^[10-11]. The δ¹³C values of carbonates (calcite) in the Zhuanghe gold deposit range from  -0.612% to -0.140% (mean of -0.362%; shown in Table 4), which are consistent with the δ¹³C values of magmatic and mantle CO₂ that have values between -0.7% and -0.2%, with a mean of -0.5%^[12]. The uniform and negative carbon isotope compositions of carbonates in the Zhuanghe gold deposit would suggest that the source for carbonic species in the ore fluid is from the deep magma.

6.5 Rb-Sr dating of fluid inclusions

The ore-forming age of the Zhuanghe gold deposit was obtained by Rb-Sr dating of the fluid inclusions in quartz. The primary fluid inclusions in quartz crystals of the auriferous quartz veins represent the trapped mineralizing fluid components, so they are considered as the ideal samples for studying the age of ore formation^[13-14]. All samples were collected from the gold-bearing quartz veins formed at the main minerallization stage, but distributed in different  locations in order to assess the range of ore mineralization. Seven sample data points without the anomalous values, make up a well fitting Rb-Sr isotime line with MSWD of 2.67 and yield an age of (143.0±5.8) Ma (Fig.5). This represents the metallogenetic epoch of the Zhuanghe gold deposit is in Yanshanian period.

7 Conclusions

1) The δ¹⁸O values of ore-forming fluid are calculated at 0.55% to 0.78%, and δD values are from -6.66% to -8.64%, indicative of a magmatic origin of the ore-forming fluid.

2) The δ³⁴S values of gold-related sulfide (-0.053% to 0.413%, with a mean of 0.145%) consist with the meteoric sulfur characters and may reflect metallogenetic sulfur source of the deep magmatic sulfur.

3) Lead isotope analysis demonstrates that the lead isotopic compositions of ore and granite-porphyry veins have the comparability, and further suggest that ore-forming materials are related to the magmatic activities.

4) The δ¹³C values of carbonate (-0.612% to -0.140%, with a mean of -0.362%) may indicate that the most likely source for the ore fluid is deep magma, associated with the deep-derived magmatic activities.

5) Rb-Sr isochronal age of (143.0±5.8) Ma of fluid inclusions of quartz minerals in the auriferous quartz veins represents that the metallogenetic age of the Zhuanghe gold deposit is in Yanshanian period.    

This study was supported by the Open laboratory of deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences and Yichang Institute of Geology and Mineral Resources, Chinese Academy of Geological Sciences.

References

[1] L? Yi-feng. The prospecting and forecasting studies on the district of Guanjiashan to Heidao in Zhuanghe County, Liaoning Province[R]. Wuhan: China University of Geosciences, 1994.(in Chinese)

[2] LUDWIG R K. ISOPLOT: A Plotting and Regression Program for Radiogenic-Isotope Data (version 2.9)[M]. Berkeley: US Geological Survey, 1996.

[3] ZHANG Li-gang, LIU Jing-xiu. Oxygen isotope fractionation in quartz-water-salt system[J]. Economic Geology, 1989, 84: 1643-1650.

[4] JIA Y, LI X, KERRICH R. Stable isotope (O, H, S, C, and N) systematics of quartz vein systems in the turbidite-hosted Central and North Deborah gold deposits of the Bendigo gold field, Central Vivtoria, Australia: constraints on the origin of ore-forming fluids[J]. Economic Geology, 2001, 96: 705-721.

[5] S?NCHEZ-ESPA?A F J, VELASCO F, BOYCE A J, et al. Source and evolution of ore-forming hydrothermal fluids in the northern Iberian pyrite belt massive sulphide deposits (SW Spain): evidence from fluid inclusions and stable isotopes[J]. Mineralium Deposita, 2003, 38: 519-537.

[6] CHEN Jiang-feng, YU Gang, XUE Chun-ji, et al. Pb isotope geoch emistry of the Pb-Zn-Au ore enrichment area in the east Liaoning rift zone[J]. Science in China (Series D), 2004, 34(5): 404-411.(in Chinese).

[7] ZHENG Yong-fei, CHEN Jian-feng. Stable Isotope Geochemistry[M]. Beijing: Science Press, 2000: 143-245.(in Chinese).

[8] TAYLOR H P. The application of oxygen and hydrogen isotope studies to problems of hydrothermal alteration and ore deposition[J]. Economic Geology, 1974, 69: 843-883.

[9] CHIARADIA M, FONTBOT? L. Separate lead isotope analyses of leachate and residue rock fractions: implications for metal source tracing in ore deposit studies[J]. Mineralium Deposita, 2003, 38:185-195.

[10] WEI Jun-hao. Comagmatic Evolution and Gold Mineralization—An Example of Wulong Gold Mine[D]. Guiyang: Institute of Geochemistry Chinese Academy of Sciences, 2001.(in Chinese)

[11] LIU Shao-bo, WANG Lian-kui, ZHANG Sheng. Summarization and discussion of the carbon isotopic source tracing in hydrothermal gold deposits[J]. Geology-Geochemistry, 1996, 24(4): 21-24.(in Chinese).

[12] DEINES P, HARRIS J W, GURNEY J J. The carbon isotopic composition and nitrogen content of lithospheric and asthenospheric diamonds from the Jagersfontein and KoffiEontein kimberlites, South Africa[J].Geochimica et Cosmochimica Acta, 1991, 53: 2615-2626.

[13] WEI Jun-hao, LIU Cong-qiang, TANG Hong-feng. Rb-Sr and U-Pb isotopic systematics of pyrite and granite in Liaodong gold province, North China: Implication for the age and genesis of a gold deposit[J]. Geochemical Journal, 2003, 37: 567-577.

[14] WEI Jun-hao, QIU Xiao-ping, GUO Da-zhao, et al. Geochemistry of ore fluids and Rb-Sr isotopic dating for the Wulong gold deposit in Liaoning, China[J]. Acta Geologica Sinica, 2004, 78: 1267-1274.

(Edited by YANG You-ping)

Foundation item: Project(20040491502) supported by the Doctoral Education Program Fund of Ministry of Education, China

Received date: 2006-05-12; Accepted date: 2006-07-18

Corresponding author: WEI Jun-hao, Professor; Tel: +86-27-62329563; E-mail: junhaow@163.com