J. Cent. South Univ. Technol. (2007)04-0568-07

DOI: 10.1007/s11771-007-0109-x

Bainiuchang super-large silver-polymetallic ore deposit related to granitic magmatism in Mengzi, Yunnan

LIU Ji-shun(刘继顺)¹, ZHANG Hong-pei(张洪培)^{1, 2}, OUYANG Yu-fei(欧阳玉飞)¹, ZHANG Cai-hua(张彩华)²

(1. School of Geoscience and Environmental Engineering, Central South University,Changsha 410083, China;

2. Mengzi Minning and Metallurgy Co. Ltd., Mengzi County, Yunnan 661100, China)

________________________________________________________________________________

Key words:

Bainiuchang; super-large silver-polymetallic deposit; granite; granitic-porphyry; tin ore；

________________________________________________________________________________

1 Introduction

Bainiuchang silver-polymetallic ore deposit in Mengzi County is one of the super-large deposits^[1] discovered in Yunnan in 1980s. It is located in the southeast Yunnan tin-polymetallic metallogenic belt. There are three granitic intrusions, named Gejiu, Bozhushan and Dulong  Laijunshan granites, which are almost equidistant outcropping in the southeast of Yunnan. The granites and the surrounding strata host three super-large tin (silver) polymetallic ore deposits. The Gejiu deposit mainly hosts Sn and Pb, and Dulong deposit mainly occurs Sn and Zn ore. The submitted probable reserves and inferred resources of the Bainiuchang deposit located at 7 km northwest Bozhushan ganite give priority to Ag-Pb-Zn. There are mainly two views to interpret the genesis of the Bainiuchang deposit at present. Some geologists regard the Bainiuchang deposit as an exhalative sedimentary mineral deposit^[2-4]. Others think that the deposit is formed by exhalative sedimentary superimposed by the granitic magamatism of the Yanshan period^[5-6]. Few people emphasize the importance of the granite. The tinprospecting potential for the deposit has been greatly underestimated. However, after preliminary research on the Bainiuchang deposit, it is discovered that the deposit is distinctly controlled by granites, and many evidences are conflicting with the exhalative sedimentary origin^[7].

In this paper, the genetic relationship between granitic rocks and the deposit was studied, and the feasibility of tin prospecting was reviewed.

2 Geologic characteristics of Bainiuchang deposit

2.1 Geological setting

Bainiushan-Bozhushan area, which is the important part of Sn polymetallic ore belt in the southeast of Yunnan, lies in the southeast folded belt, west-extended section of South China fold system^[8-10]. The exposed formation consists of Paleozoic Cambrian clastic rocks with carbonate, Lower Ordovician carbonate with clastic rocks, Devonian carbonate with clastic rocks, Carbonic carbonate and Permian carbonate-clastic rocks with basic volcanic rocks. No much Mesozoic group occurs in the district only with Triassic clastic rocks with carbonate formation (shown in Fig.1).

Fig. 1 Geological sketch of Bainiuchang-Bozhushan area

1—Quaternary; 2—Tertiary; 3—Upper Permian basalt; 4—Permian; 5—Carboniferous; 6—Devonian; 7—Ordovician; 8—Cambrian; 9—Granite of late Yanshan period; 10— Fault; 11—Unconformity; 12—Geological boundary; 13—Anticline axis; 14—Syncline axis; 15— Bainiuchang ore field; 16—Deposit/mineral occurrence

The main format of regional tectonics belongs to fold-fault system composed of a series of fold, compressive and compress-shear faults, on the north-east structural line. Regionally magma acted intensively. The eruption of basic magma in Hercynian period and the acidic magmatic intrusion in the late Yanshan period have formed upper Permian Ermaishan basalt in the east and Bozhushan granite in the southeast. The concealed granite, granite-porphyry, monzonite vein and some sporadically distributing dolerite are also discovered in the Bainiuchang orefield.

The major mineralization elements in the area include W, Sn, Fe, As, Pb, Zn, Cu, Ag and Sb, which occur around the Bozhushan granite and in the exocontact of the Bainiuchang concealed granite.

2.2 Deposit geology

The strata in Bainiuchang orefield are clastic rock-carbonite rock construction of lower Devonian and lower-middle Cambrian. The sedimentary sequence from top to bottom is as follows: 

1) Lower devonian consists of clastic rocks- carbonate rocks of Bajiaojing formation (D₁b), Pojiao formation (D₁p), and Posongchong formation (D₁ps).

2) Middle Cambrian consists of Longha formation (? ₂l), Tianpeng formation (? ₂t) and Dayakou formation (? ₂d). Longha formation is mainly composed of a set of dolomite formed coastal supralittoral zone under lagoon environment. Tianpeng formation is composed of the rock association of limestone and siltite formed under the coastal tidal flat environment. Dayakou formation is composed of silty limestone.

3) Lower Cambrian consists of Dazhai formation (?₁d) and Chongzhuang formation (?₁ch). Dazhai formation is mainly composed of silty porphyry and limestone formed in the upper part of neritic continental shelf, the central and lower section of coastal zone. Chongzhuang formation is composed of fine gravel thin silty slate and carbonaceous sil formed under neritic contimental shelf environment.

The ore-bearing strata mainly focus on the intermediate zone between the upper section of Tianpeng formation and the lower section of Longha formation.

The format of the field structure belongs to fold-fault system, mainly on the NWW-trending structural line, the NE- and NS-trending folds and faults interpenetrate in the main line, which is inconsistent with the NE-trending regional tectonics. Yuanbaoshan synclinorium, Awei anticline, Bainiuchang syncline and Chuanxindong anticline make up of the western section of Bainiuchang-Luchachong brachy-anticlinorium, which is obviously superimposed over the NE-trending Bainiuchang-Malutang anticline.

The NW- and NWW-trending faults include F₂,  F₃,  F₄,  F₅,  F₆,  F₇,  F₈. Among them, F₄, F₅,  F₆,  F₇ and F₈  are all high-angle normal faults, converging at F₃. F₃, a lower-angle denudational fault^[11], generally striking by 200?-230? and dipping 10?-35?, is the host space of the main orebody V₁. The upper part serves as boundary between Tianpeng formation and Longha formation. The lower section penetrates into the strata of Tianpeng formation.

The tension belt occurs in county rocks above the Bainiuchang concealed granite in the south of Awei-Duimenshan and also develops in the marble, skarn, hornstone and altered dolomite in the contact zone. The thickness of the tension belt ranges from 200 to 400 m. The cracks have tensional character distributing closely and filling with sulfide vein. The surface of cracks is uneven, zigzag and irregular. The fissure dips by 60?-90?, with multiple directions of dip.

The deposit is divided into five ore blocks, i.e. Baiyang, Duimenshan, Miwei, Chuanxindong and Awei. The main orebody (V₁) is bedded and slightly tilting. The body is 4.84 km long, averaged at 5.89 m thick. The max tilting depth extends 2.5 km. The resources of it take up over 98% of the whole deposit, including 1.10 Mt of Pb resource, 1.72 Mt of Zn, 6 470 t of Ag and 86 kt of Sn. Other orebodies, which lie over or under the V₁ orebody, are mainly discovered by single drilling. The scale is very small and the grade is lower. The main orebody V₁ is controlled by F₃ fault, trending by 192?-239?, dipping by 15?-30?. Seen from the section, it slowly changes wave-like.

The main ore structures range from disseminated structure, massive structure, veinlet-vein structure and banded structure to brecciform structure. The ore textures include colloform texture, granular texture, metasomatic texture, exsolution texture, poikilitic texture, corrugation structure, mortar texture and mylonitic texture.

About 53 ore minerals were found in the deposit, mainly including pyrite, marcasite, pyrrhotite, arsenopyrite, sphalerite-marmatite, galena, boulangerite and cassiterite. The silver minerals are freibergite, phyargyrite, and miargyrite. The crystallization sequence for these minerals from early to late is as follows: assiterite-quartz association, arsenopyrite-pyrite- pyrrhotite association and chalcopyrite-marmatite-quartz association, pyrite-galena-sphalerite-pyrrhotite-quartz- calcite association (with stannite and chlorite),  pyrite-marcasite-galena-sphalerite-calcite paragenetic association, stibnite-quartz-calcite association, and ferrodolomite and manganese dolomite association.

The ores are chemically rich in Si, Fe and S, and poor in K, P, Mn and Ti. They also enrich Pb, Zn, Ag, Sn, As, Cu, Ga, In and Gd. The V₁ orebody contains 1.56% of Pb, 2.46% of Zn, 92 g/t of Ag and 0.12% of Sn.

3 Petrology of granitic rocks

3.1 Distribution of granitic rocks

The Bainiuchang orefield lies on the trend line of the Bozhushan granite. In the exploration stage, the concealed granite was respectively discovered at 1 345 m level and 1 396 m level in ZK126-10 and ZK130-11 drill hole in the south of Awei ore block. Recently, the concealed granite is discovered in another six drill holes at the depth of 1 250-1 420 m in the south of the orefield and the north of Awei ore block. So, the controlled Bainiuchang concealed granite NWW-trending, is over  2 100 m long, over 1 000 m wide. On surface, the granite-porphyry veins expose in Yutang-Chaigoupo and Yuanbaoshan-Duimenshan; the large-scale granite- porphyry veins and monzonite veins occur in the deep section of Chuanxintong and Duimenshan ore blocks. The existing project record shows that both the granite rock mass and granite-porphyry veins strike closely NW trending, north-uplift and south-falling, just as F₃ fault does.

3.2 Petrology and geochemistry characteristics

3.2.1 Concealed granite

The concealed granite intrudes the strata of the carbonate rocks and clastic rocks of Tianpeng formation (? ₂t) and lower strata. The granite is composed of biotite adamellite, with fine granular-coarse granitic texture. The major rock-forming minerals consist of 30%-45% of K-feldspar (including orthoclase and perthite), 25%-30% of plagioclase, 20%-35% of quartz, 5%-10% of biolite. The accessory minerals include apatite, zircon and tourmaline. The rocks host many metallic minerals such as scheelite, cassiterite, chalcopyrite, arsenopyrite, galena, marmatite, pyrrhotite and pyrite.

The rocks have strong alkalic characteristics, rich in Na and K. The content of K₂O amounts to 5.28% , and that of Na₂O takes up 2.90% on average, w(K₂O)/w(Na₂O)＞1(shown in Table 1)^[10]. A/NCK= 0.93-1.07, so, it belongs to Al-supersaturation series—normal series. The initial value of m(Sr⁸⁷)/m(Sr⁸⁶) equals 0.714 4. The total amounts of REE of the rocks are higher than that of normal granites, and the former is rich in LREE and shows obviously Eu-negative anomaly (shown in Table 2 and Fig.2). The genetic type of the granite belongs to S-type tin-bearing granite^[12-14]. The Rb-Sr isochron age of the granite is 80.17 Ma^[10], suggesting that the granite formed during the late Yanshan period.

Table 1 Compositions of granitic rocks in Bainiuchang orefield(w_B/%)

Table 2 RAEE and trace element compositions of granitic rocks and ores in Bainiuchang orefield(mg/g)

Fig.2 Chondrite-normalized REE patterns of granite, granite-porphyry and ore in Bainiuchang orefield (Chondrite values are from Boynton, 1984^[14])

In sum, all Bainiuchang granite, Gejiu granite^[15] and Dulong Laojunshan granite^[16] are formed in the same period, and  have the same characteristics on petrology and rock geochemistry approximately.

3.2.2 Granite-Porphyry

Those rocks are light-ruby, grey or grayish brown, having porphyritic texture. The phenolcryst is composed of 1%-10% of quartz, 2%-10% of K-feldspar and 1%-3% of biotite. The matrix is fine-granular granitic texture. The matrix minerals consist of K-feldspar, quartz, biotite and litte apatite and zircon. The major metallic minerals include pyrite, pyrrhotite, sphalerite, galena and cassiterite. These metallic minerals usually make up metallic sulphide ore veins.

The rocks are chemically enriched in K and depleted in Na. Those rocks belong to Al- supersaturation.

The rocks suffur from alteration intensively. The feldspar phenolcrysts are kaolinised and the matrixes are replaced by sericite, chlorite and carbonate.

3.2.3 Monzonite

The monzonites are only discovered in the section of Duimenshan from Line63 to Line102. Those rocks occur as veins. The normal thickness ranges from 1.28 to 7.17 m. They have monzonitic texture or porphyritic- like texture. The major minerals include plagioclase, K-feldspar, biotite and chlorite. Intensive alterations such as chloritization, carbonation and metallic sulphide mineralization, also occur in these rocks. 

4 Ore-controlling of granitic intrusions

4.1 Source of ore-forming materials

The major ore minerals in orebodies such as pyrite, pyrrhotie, sphalerite, galena, arsenopyrite, chalcopyrite and cassiterite are also discovered in granite and granite-porphyry. The granite-porphyry has strong mineralization and locally forms industrial orebodies. The average content of the ore-forming elements Cu, Pb, Zn, Ag, Sn, As and Sb (shown in Table 2) in the granite is 3.49-100 times higher than that of normal granites (from Vinogradov, 1962). The average content of Sn is 46.05×10^-6, and 15.35 times that of normal granites. The average content of Sn of tin-bearing granites^[17] in China ranges from 20×10^-6 to 213×10^-6. So, Bainiuchang granite may be one of tin-bearing granites.

The δS³⁴ of ores, distributing like a tower in the frequency bar chart (shown in Fig.3), is limited to a scope from -4.00‰ to +6.00‰ and the δS³⁴ of pyrite (3.16‰) is higher than that of galena (2.06‰),  which is higher than that of sphalerite (1.62‰), suggesting that S in sulphide originates from the deep magma.

Fig.3 Histogram of sulphur isotopes of metal minerals in Bainiuchang ore deposit

Ⅰ—Pyrite; Ⅱ—Phrrhotite; Ⅲ—Sphalerite; Ⅳ—Galena; Ⅴ—Chalcopyrite; Ⅵ—Boulangrite; Ⅶ—Arsenopyrite

The REE pattern of ores is similar to that of the concealed granite and the granite-porphyry (shown in Fig.3). The lines on the REE pattern figure sharply incline right. Both those ores and rocks show obviously Eu-negative anomaly, indicating that the ores may be cognate with granite and granite-porphyry.

4.2 Spacial zoning of alteration and mineralization

From the center (taking the concealed granite as the alteration center) to the outside strata composed of the clastic rock-carbonatite, the alteration was divided into contact metamorphic zone, silication zone and carbonation zone. The contact metamorphic zone, at the normal thickness of about 300 m, has obvious skarnization, hornfelsing and marbleization.

The mineralization of Cu, Sn, Pb, Zn, and Ag occurred in the contact metamorphic zone, among those metals, the dominated minerals are Cu and Sn. The silication zone and carbonation zone lie to the outside of the contact metamorphic zone, having Cu, Pb, Zn, Ag and Sb mineralization. From the contact zone to clastic rock-carbonatite strata, the following mineralized elements occur in sequence(Sn→Cu→Zn→Pb→Ag→ Sb).

 V₁ is a Zn-Pb-Ag–Sn composite orebody. The proved copper orebody lies under V₁, on the top margin of which lies the independent silver orebody. The independent Sb orebody lies in the exocontact of V₁. As far as V₁ is concerned, the value of ω(Zn)/ω(Zn +Pb) in the upper section is lower than 0.60 (ω(Zn)/ω(Zn +Pb)＜0.60), on the other hand, ω(Zn)/ω(Zn +Pb) in the lying section is higher than 0.60. For instance, ω(Zn)/    ω(Zn +Pb) in the upper Baiyang section is 0.58 and than in the lower section is 0.68; ω(Zn)/ω(Zn+Pb) in Duimenshan section equals 0.54, however in V₃ ore body, which lies just under V₁, (ω(Zn)/ω(Zn+Pb)=0.77) reaches 0.77. Closer to the concealed granite,  Zn-Pb-Ag- Sn composite orebody has more zinc hosts.

4.3 Ore-forming temperature and pressure

The ore-forming temperature of Bainiuchang ranges from 52 ℃ to 520 ℃, and gradually decreases from Awei through Chuanxindong, Baiyang, Miwei, and Maoshandong.

There exists a pressure gradient around the concealed granite^[10]. The diagenetic pressure of the concealed granite is focused at 150-250 MPa. The ore-forming pressure in the contact metamorphic zone is 160-280 MPa. The pressure forming the V₁ orebody ranges from 43 to 160 MPa above the contact metamorphic zone. However, the antimony-forming pressure is lower, limited to 42-70 MPa in Maoshandong far from the concealed granite.

4.4 Relationship between granitic intrusive and ore-bearing structure

The ore-bearing fault (F₃) is formed by magmatic intrusion and Bainiuchang-Luchachong brachy- anticlinorium in the district responding to the magmatic emplacement and uplift^[11]. The attitude of the ore-bearing fault (F₃) is concordant with that of the granite-porphyry veins and the top surface of the Bainiuchang concealed granite. A part of mineralizing fluid fills the tension belt, which consists of dense fissures above the concealed granite, to form veinlet ores. 

Mentioned above evidences indicate that the deposit is closely related to the granite. Based on characteristics of the deposit, it may be classified to the skarn type^[18]. As most other skarn deposits, the zonation pattern of the deposit is a guide to mineral deposit exploration^{[13, 19]}.

5 Prospecting and test result

Based on the research results on the relationship between Bainiuchang deposit and granitic rocks, and the alteration and mineralization zoning, it was proposed that the enriched and thick Zn-Pb-Ag composite orebodies could be discovered along granite-porphyry veins, the independent Zn or Cu orebodies could be found under the Zn-Pb-Ag composite orebodies, and the independent tin ore Cu orebodies may occur in the contact between the concealed granite intrusion and sedimentary rocks.

In 2005, the south Bainiuchang orefield and the north Awei were circled as prospecting targets for tin, copper and zinc orebodies. After carrying out hybrid-source magnetotellurics electrical conductive imaging system measuring in the two areas, drill holes were designed to test obtained anomalies. In result, great development for ore prospecting was made in the two areas. A tin ore-body (22.85 m thick grading 1.331% Sn, accompanying with 0.15% WO₃) and two copper ore-bodies (one 12 m thick with 0.60% Cu, another 11 m thick with 0.63% Cu) are found in the area (Ⅰ). In the area (Ⅱ), a independent tin ore-body (9.90 m thick with 0.718% Sn) and a biggish zinc tin ore-body (11.4 m thick with 6.23% Zn) are discovered. The concealed granite is found at the bottom of each of drill holes.

6 Conclusions

1) The Bainiuchang concealed granite is over 2 000 m long and 1 000 m wide. It is formed in late Yanshan period. From the petrology and geochemistry characteristics, it belongs to the tin-bearing granite.

2) The Bainiuchang deposit is closely related to the granitic rocks in space and genesis. The granitic magmatism provides material source and physical-chemical conditions for the minerization, and controls the spacial distribution of the deposit.

3) Besides the silver-lead-zinc orebodies, there are prospecting potentials of independent tin and copper orebodies in the Bainiuchang ore field.

4) Bainiuchang is a large-super Pb-Zn-Ag composite deposit. At the same time, it indicates a better prospect for hunting large-scale tin deposit and copper deposit for tin and copper ore prospecting.

References

[1] LUO Jun-lie. The minerogenetic characteristics of super-large deposits in Yunnan[J]. Yunnan Geology, 1995, 14(4): 276-80. (in Chinese)

[2] CHEN Xue-ming, LIN Zong, XIE Fu-chang. Geological and geochemical characteristics of the Bainiuchang super-large silver polymentallic deposit of superimposed mineralization, Yunnan Province[J]. Scientia Geologica Sinica, 1998, 33(1): 115-124. (in Chinese)

[3] ZHOU Jian-ping, XU Ke-qin, HUA Ren-min, et al. Characteristics and genesis of exhalative sedimentary massive sulfides in southeastern Yunnan Province[J]. Acta Mineralogical Sinica, 1998, 18(2):158-168. (in Chinese)

[4] DENG Jun, CHEN Xue-ming, RAO Yi-qun, et al. Two kinds of compaction fluid systems of the Nanling area[J]. Geoscience, 2004, 18(1): 1-7. (in Chinese)

[5] HUA Ren-min, ZHU Jin-chu, ZHAO Yi-ying, et al. Preliminary studying on metallogenetic series of nonferrous metal deposits in Youjiang fold belt[J]. Geological Journal of China Universities, 1997, 3(2): 183-190. (in Chinese)

[6] ZHU Chao-hui, ZHANG Qian, HE Yu-liang. The characteristics of mineralizing elements of the Bainiuchang silver Polymetallic deposit in southeastern Yunnan[J]. Bulletin of Mineralogy, Petrology and Geochemistry, 2005, 24(4): 327-332. (in Chinese)

[7] LIU Ji-shun, GAO Zhen-guan, DENG Gong-quan, et al. One potential superlarge Pb-Zn ore occurrence with Himalayan thermal brine genesis—Wuqia Region, Xinjiang, China[J]. Journal of Central South University of Technology, 2002, 9(1): 41-46.

[8] YANG Shi-yu. Characteristics of distribution in time and its metallogenic model[J]. Scientia Geologica Sinica, 1990(2): 137-148. (in Chinese)

[9] QIN De-xian, TAN Shu-cheng, FAN Zhu-guo, et al. Geotectonic evolution and tin-polymentallic metallogenesis in Gejiu-Dachang area[J]. Acta Minerralogica Sinica, 2004, 24(2): 117-123. (in Chinese)

[10] GAO Zi-ying. On the Mineralization of Bainiuchang Silver Polymetallic Deposit in Mengzi[M]. Kunming: Yunnan Science and Technology Press, 1998. (in Chinese)

[11] LIU Ji-shun, ZHANG Hong-pei, FANG Wei-xuan et al. Problems pertaining to Bainiuchang silver, polymetallic deposit in Mengzi country, Yunan[J]. Engineering Science, 2005, (Supp.): 238-244. (in Chinese)

[12] BREITER K, SOKOLOVA M, SOKOL A. Geochemical speciabization of the tin-bearing granitoid massifs of NW Bohemia[J]. Mineralium Deposita, 1991, 26(4): 298-306.

[13] BAY G E, WEBSTER I C L, BALLANTYNE S B, et al. The geochemistry of three Tin-bearing skarns and their related platonic rocks, Atlin, northern British Cdumbia[J]. Economic Geology, 2000, 95(6): 1349-1365.

[14] CHEN Jun, WANG He-nian. Geochemistry[M]. Beijing: Science Press, 2004. (in Chinese)

[15] YU Chong-wen, TANG Yuan-jun, SHI Ping-fang, et al. Dynamical System of Mineralization in Gejiu tin Polymetallic Deposit[M]. Wuhan: China University of Geosciences Press, 1988. (in Chinese)

[16] GUAN Rong-sheng. An approach of mineralization of granite mass in the structure tectonicmagmatic belt in southeast of Yunnan[J]. Journal of Minerology and Ptrology, 1991, 11(1): 92-101. (in Chinese)

[17] LI Xi-ji, YANG Zhuang, SHI Lin, et al. Tin deposits of China[C]// SONG Shu-he, et al. Mineral Deposits of China. Beijing: Geological Publishing House, 2: 105-188. (in Chinese)

[18] EINAUDI M T, BURT D M. A special issue devoted to skarn deposits; Instruction—terrninology, classification, and composition of skarn deposits[J]. Bulletin of the Society of Economic Geologists, 1982, 77(4): 745-754.

[19] MEINERT L D. Application of skarn deposit zonation models to mineral exploration[J]. Exploration and Mining Geology, 1997(6): 195-208.

______________________

Foundation item: Project(40072032) supported by the National Natural Science Foundation of China; Project(2004YX06) supported by the Yunnan Province-Institutes/Universities’ Science and Technology Cooperation Project Item

Received date: 2006-12-24; Accepted date: 2007-02-27

Corresponding author: LIU Ji-shun, PhD, Professor; Tel: +86-731-8877706; E-mail: jsliu@mail.csu.edu.cn

(Edited by YANG Hua)