Mineralization episode of porphyry copper deposits in the Jinshajiang-Red River mineralization belt: Re-Os dating

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1 192 Science in China Ser. D Earth Sciences 2005 Vol.48 No Mineralization episode of porphyry copper deposits in the Jinshajiang-Red River mineralization belt: Re-Os dating WANG Denghong 1, QU Wenjun 2, LI Zhiwei 3, YIN Hanlong 4 & CHEN Yuchuan 5 1. Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing , China; 2. National Center of Rock and Mineral Analysis, Chinese Academy of Geological Sciences, Beijing , China; 3. Resource Assessment Center of Yunnan, Kunming , China; 4. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing , China; 5. Chinese Academy of Geological Sciences, Beijing , China Correspondence should be addressed to Wang Denghong ( wangdenghong@sina.com) Received May 26, 2003 Abstract Re-Os isotopic dating for the molybdenites from the porphyry copper deposits of the Jinshajiang-Red River mineralization belt in Yunnan Province yields isochron ages of Ma for the Machangqing deposit and Ma for the Tongchang deposit. This result shows that both the Machangqing and the Tongchang porphyry Cu-Mo deposits from two different ore-fields formed simultaneously. This new data and the published Re-Os model ages of molybdenite (35.4 Ma, 35.9 Ma, 36.2 Ma) of the Yulong porphyry copper deposit in Tibet, which is located in the same Jinshajiang-Red River mineralization belt as the Machangqing deposit and the Tongchang deposit, suggest that these three Cenozoic porphyry copper deposits in the Jinshajiang-Red River mineralization belt were contemporary for their mineralization episode. That is to say, even their present locality is far away and nearly iso-distantly distributed, these three porphyry Cu(Mo) deposits belong to the same mineralization episode at the end of Eocene. Keywords: Re-Os isotopic dating, porphyry Cu deposits, Cenozoic, mineralization episode. DOI: /03yd0200 The Jinshajiang-Red River mineralization belt is one of the most important mineralization belts in China, and is also attractive to domestic and overseas geologists for its special tectonic setting. Both the largest copper deposit in China (the Yulong porphyry copper deposits in Tibet) and the largest lead-zinc deposit in China (the Jinding Pb-Zn polymetal deposit in Yunnan) are located in this mineralization belt. In addition, the largest germanium deposit in China (the Lincang Ge-U-Coal deposit in Yunnan) and the largest blue asbestos deposit in China (the Gaofengshi blue asbestos deposit in Yunnan) and the probably largest silver deposit in China (the Xiashai Ag-polymetal deposit in Sichuan) are also located nearby the Jinshajiang-Red River mineralization belt. All the abovementioned largest deposits formed in the Cenozoic Era. However, it is not clear at which stage of Cenozoic Era and in what kind of geological setting for these deposits to form. In this work, molybdenite samples from the Machangqing Cu-Mo deposit and the Tongchang Cu-Mo deposit were chosen for Re-Os isotopic measurements in order to define their mineralization ages. 1 Geological setting of mineralization The major porphyry copper deposits on south- Copyright by Science in China Press 2005

2 Mineralization episode of porphyry copper deposits 193 western China are mainly distributed along the Jinshajiang-Red River tectonic belt (fig. 1), which strikes northwestern and cut Mesozoic terrane in its northwestern part and cut metamorphic rocks of Precambrian in its southeastern part. The Jinshajiang-Red River tectonic belt is very active during its Cenozoic evolution history, featured by obvious slide and shear phenomena along this tectonic belt. All of the Yulong orefield, the Machangqing orefield and the Tongchang orefield are located along the Jinshajiang-Red River tectonic belt, with the Yulong orefield located in the northwestern sector, the Machangqing orefield located in the middle sector, while the Tongchang orefield in the southeastern sector. Among them, the Yulong ore- Fig. 1. Sketch map of the Jinshajiang-Red River mineralization belt, showing the distribution of major Cenozoic deposits. 1, Magmatic rocks; 2, metamorphic rocks; 3, Palaeozoic Erathem; 4, Mesozoic Erathem; 5, Cenozoic Erathem; 6, age-known lode gold deposit; 7, age-unknown lode gold deposit; 8, Cu deposit; 9, placer gold deposit; 10, lead-zinc deposit; 11, coal deposit; 12, Yulong orefield; 13, Machangqing orefield; 14, Tongchang orefield; 15, major fault belt. field were studied in detail, with a lot of deposits discovered there. Beside the Yulong porphyry Cu-polymetal deposit, there are other important large- to middle-scale porphyry deposits such as the Duoxiasongduo, Malasongduo, Mangzong, Narigongma, Mamupu, Xixingcuo, Gegongnong and the Xiariduo porphyry-type copper deposits. Magmatic rocks related to mineralization are mainly alkalic granitic porphyry. In the Yulong area, the major magmatic rocks are monzonite granitic porphyry, syenitic granite-porphyry, syenite-porphyry and porphyritic syenite. In the Machangqing area, the major magmatic rocks are monzonite-porphyry, syenite-porphyry and granite porphyry. In the Tongchang area, the major magmatic rocks include syenite-porphyry, quartz syenite and pyroxene syenite. Even the Machangqing deposits and the Tongchang deposit in Yunnan Province are small in size compared with the Yulong deposit, a lot of Cu-Au deposits and occurrences related to syenite porphyry have been discovered recently in the outside of Machangqing and Tongchang. Thus, the south sector of the Jinshajing-Red River mineralization belt has being taken as an important target for national geological investigation by Chinese Survey of Geology. As to the mineralization age of the porphyry copper deposits in the Jinshajiang-Red River belt, former researchers took the rock-forming age as the same age of ore-forming. However, because the methods applied by former researchers are mainly K-Ar method and other methods such as whole-rock Rb-Sr isochron method usually give a wide range of age (table 1), it is difficult to define accurately the age of mineralization. For example, the whole-rock K-Ar isotopic ages dated on the porphyry samples from the Yulong ore-field range from 64.3 Ma (sample from 452 m depth of the drill hole Zk5 in the Malasongduo deposit) to 16.5Ma (quartz monzonite-porphyry sample from periphery area of the Yulong deposit), with time space up to 50 million years. Re-Os method has been used by Tang Renli et al. (1995) to define the age of molybdenites selected from the Malasongduo porphyry copper deposit in the Yulong orefield, and yielding model ages range from 36.2 to 35.4Ma [1]. These ages could be representative for the mineraliza-

3 194 Science in China Ser. D Earth Sciences Table 1 Isotopic ages of the Cenozoic Cu-Mo-bearing porphyry and molybdenite from the Jinshajiang-Red River mineralization belt Area Ore field Location Rock type Sample Method Age/Ma Reference Tibet Yulong Angnongke Monzonite-porphyry K-feldspar K-Ar 49.2 [1] Tibet Yulong Angnongke Monzonite-porphyry K-feldspar K-Ar 32.4 [1] Tibet Yulong Duoxiasongduo Monzonite-porphyry Whole-rock Rb-Sr 51.6 [1] Tibet Yulong Duoxiasongduo Monzonite-porphyry K-feldspar K-Ar 30.9 [1] Tibet Yulong Duoxiasongduo Monzonite-porphyry K-feldspar K-Ar 27.8 [1] Tibet Yulong Malasongduo Porphyry-type Cu-Mo ores Molybdenite Re-Os 36.2 [1] Tibet Yulong Malasongduo Porphyry-type Cu-Mo ores Molybdenite Re-Os 35.9 [1] Tibet Yulong Malasongduo Porphyry-type Cu-Mo ores Molybdenite Re-Os 35.4 [1] Tibet Yulong Malasongduo Monzonite-porphyry K-feldspar K-Ar 36.4 [1] Tibet Yulong Mamupu Syenite porphyry Whole-rock Rb-Sr 27.4 [1] Tibet Yulong Mangzong Biotite monzonite-porphyry K-feldspar K-Ar 26.4 [1] Tibet Yulong Mangzong Biotite monzonite-porphyry K-feldspar K-Ar 26.4 [1] Tibet Yulong Mangzong Biotite monzonite-porphyry K-feldspar K-Ar 35.0 [1] Tibet Yulong Xixingcuo Ore-bearing porphyry K-feldspar K-Ar 42.7 [1] Tibet Yulong Yulong, periphery Quartz monzonite-porphyry (barren) K-feldspar K-Ar 42.7 [1] Tibet Yulong Yulong, periphery quartz monzonite-porphyry (barren) K-feldspar K-Ar 16.5 [1] Tibet Yulong Yulong Biotite monzonite-porphyry K-feldspar K-Ar 57.9 [1] Tibet Yulong Yulong, ZK107 Biotite monzonite-porphyry Biotite K-Ar 50.6 [1] Tibet Yulong Yulong, ZK107 Biotite monzonite-porphyry Biotite K-Ar 43.5 [1] Tibet Yulong Yulong, ZK107 Biotite monzonite-porphyry K-feldspar K-Ar 40.2 [1] Tibet Yulong NW side of Yulong Biotite monzonite-porphyry Biotite K-Ar 43.2 [1] Tibet Yulong NW side of Yulong Biotite monzonite-porphyry K-feldspar K-Ar 40.0 [1] Tibet Yulong Zanaga Biotite monzonite-porphyry K-feldspar K-Ar 35.9 [1] Tibet Yulong Zanaga Biotite monzonite-porphyry K-feldspar K-Ar 34.9 [1] Tibet Yulong Malasongduo Cu-Mo ores Molybdenite Re-Os 35.8 [2] Tibet Yulong Yulong Biotite monzonite-porphyry Whole-rock+minerals Rb-Sr 52.0 [3] Tibet Yulong Angnongke Monzonite-porphyry Zircon U-Pb 40.9 [3] Tibet Yulong Malasongduo ZK5-452,Ma-M Whole-rock K-Ar 64.3 [4] Tibet Yulong Malasongduo ZK1-331Ma-M Whole-rock K-Ar 56.9 [4] Yunnan Jinping Tongchang Rb-Sr 36 [5] Yunnan Xiangyun Machangqing Granitic porphyry Biotite K-Ar 48 [5] Yunnan Jinping Tongchang Amphibole syenite-porphyry Biotite K-Ar 36.0 [6,7] Yunnan Xiangyun Machangqing Monzonite-porphyry Biotite K-Ar 35.1 [6,7] Yunnan Xiangyun Machangqing Amphibole syenite K-feldspar K-Ar 29 [6,7] Yunnan Xiangyun Machangqing Ore-bearing porphyry Rb-Sr 34 [8] Yunnan Jinping Tongchang Quartz syenite Whole-rock+minerals Rb-Sr [9] Yunnan Jinping Tongchang, X30-13 Quartz syenite Whole-rock+minerals Rb-Sr 33.9 [9] Yunnan Jinping Tongchang, X30-14 Quartz syenite Whole-rock+minerals Rb-Sr 35.9 [9] Yunnan Jinping Tongchang, X30-22 Quartz syenite Whole-rock+minerals Rb-Sr 36.1 [9] Yunnan Xiangyun Machangqing Porphyritic granite Biotite K-Ar 64.8 [10] Yunnan Xiangyun Machangqing Porphyritic granite K-feldspar K-Ar 46.5 [10] Yunnan Xiangyun Machangqing Lamprophyre Biotite K-Ar 45.7 [10] Yunnan Xiangyun Machangqing Granitic porphyry Whole-rock Rb-Sr 36.3 [10] Yunnan Jinping Tongchang Porphyry-type Cu-Mo ore Molybdenite Re-Os this work Yunnan Xiangyun Machangqing Porphyry-type Cu-Mo ore Molybdenite Re-Os 33.9 this work tion age, suggesting that the Malasongduo porphyry copper deposit formed at the end of Eocene. These ages are obviously younger than the rock-forming age of 64.3Ma, strongly suggesting that the rock-forming age is different from ore-forming age and that it is necessary to define the ore-forming age by new methods. Re-Os isotopic dating on molybdenite is thus the preferred method to solve this problem, not only be-

4 Mineralization episode of porphyry copper deposits 195 cause this method is a well-rounded isotopic dating method directly on sulfides, but also because molybdenite is one of the major ore minerals in these porphyry Cu-Mo deposits. This work selected molybdenites from the Machangqing Cu-Mo deposit and the Tongchang Cu-Mo deposit to date their age by Re-Os isotopic dating method described by Du Andao et al. [2]. 2 Re-Os isotopic dating of molybdenite seven molybdenite samples from the Machangqing porphyry copper deposit and nine molybdenite samples from the Tongchang porphyry copper deposit were taken for analysis in the National Center of Rock and Mineral Analysis. The Re-Os isotopic dating method was described in detail by Du Andao et al.(1994) [4] and the results are listed in tables 2 and 3, while the Re-Os isotopic isochrons are shown in figs. 2 and 3. As illustrated in fig. 2, the seven molybdenite samples from the Machangqing deposit define a well-correlated straight line. The isochron age obtained is Ma, with MSWD equal to 1.07 and the initial 187 Os equal to , suggesting that the result is reliable. Thus, the ore-forming age of the Machangqing porphyry-type Cu-Mo deposit is nearly the same as those of the Malasongduo porphyry Cu-Mo deposit, suggesting that both of them were the product of the same minerlization event happening at the end of Eocene. The nine measured points also define a straight line when plotted in a 187 Os vs. 187 Re diagram (fig. 3) and resulted in an isotopic isochron age of Ma, with MSWD equal to 4.9, also suggesting that the Tongchang porphyry Cu-Mo deposit formed at the end of Eocene. Thus, the Re-Os isotopic dating results show that the Tongchang, the Machangqing and the Yulong porphyry Cu-Mo deposits formed contemporarily. 3 Discussion 3.1 Precise define for ore-forming age Even we have been informed that the porphyry Cu-Mo deposits distributed along the Jinshajiang-Red River mineralization belt formed in Cenozoic, it is not clear at which stage they formed or whether they formed at the same time or at different times. In order to define the relationship between these deposits located far away from each other spatially, this work has obtained precise ore-forming ages of 33.9 Ma and 34.4 Table 2 Re-Os isotopic age of molybdenites from the Machangqing (Jiudingshan) porphyry Cu-Mo deposits, Yunnan Sample No. Analytic No. Weight/g Re/ g g Re/ g g Os/ng g -1 Model age/ma JDS (0.6) 31.1(0.4) 18.0(0.1) 34.7(0.5) JDS (0.5) 32.8(0.3) 19.1(0.2) 34.9(0.5) JDS (0.3) 21.4(0.2) 12.6(0.1) 35.2(0.5) JDS (0.7) 40.1(0.4) 23.1(0.2) 34.6(0.5) JDS (0.4) 22.3(0.2) 12.8(0.1) 34.5(0.5) JDS (0.5) 29.3(0.3) 16.9(.01) 34.6(0.5) JDS (0.7) 38.6(0.4) 21.9(.02) 34.0(0.5) Analyzed by Qu Wenjun of the National Center of Rock and Mineral Analysis. Table 3 Re-Os isotopic age of molybdenites from the Tongchang porphyry Cu-Mo deposits, Yunnan Sample No. Analytic No. Weight/g Re/ g g Re/ g g Os/ng g -1 Model age/ma YTC (0.7) 36.2(0.4) 20.8(0.1) 34.5(0.6) YTC-1c (0.6) 35.3(0.4) 20.3(0.1) 34.5(0.6) YTC (0.9) 57.6(0.6) 32.9(0.2) 34.3(0.5) YTC (0.05) 2.92(0.03) 1.64(0.02) 33.8(0.6) YTC (2.1) 84.8(1.3) 48.0(0.4) 33.9(0.7) YTC-5X (0.07) 4.61(0.04) 2.75(0.02) 35.8(0.6) YTC-6X (0.4) 24.5(0.2) 14.2(0.1) 34.8(0.69) YTC-7C (0.03) 1.43(0.02) 0.92(0.01) 38.7(0.7) YTC-8C (1.2) 47.7(0.8) 27.7(0.2) 34.8(0.7) Analyzed by Qu Wenjun of the National Center of Rock and Mineral Analysis.

5 196 Science in China Ser. D Earth Sciences Fig. 2. Re-Os isochron diagram of molybdenites from the Machangqing (Jiudingshan) Cu-Mo deposit. Fig. 3. Re-Os isochron diagram of molybdenites from the Tongchang Cu-Mo deposit. Ma by Re-Os isotopic dating on molybdenites from the Machangqing and the Tongchang deposits respectively, suggesting an ore-forming episode of late Eocene. This is one of the most important stages for the formation of porphyry copper deposits around the world, for most of the porphyry copper deposits in south American such as those in Chile also formed at that time ( Ma [11] ). Although the porphyry copper deposits in the Gangdisi belt of Tibet formed in Cenozoic, it is also clear that, they are younger than those located in the Jinshajiang-Red River belt, with the Re-Os age of the Gangdisi molybdenites concentrated at 14 Ma [12]. 3.2 Relationship between rock-forming and oreforming Usually, porphyry-type deposits are the result of post-magmatic hydrothermal minealization, so the age of ore-forming must be younger than that of rock-forming. However, it is unclear what is the interval between the rock-forming age and the ore-forming age. If we take the Zircon U-Pb age of 40.9 Ma [3] as the rock-forming age of the porphyry in the Yulong orefield, and take the Re-Os isotopic age of 35.8 Ma as the ore-forming age (the average value of the four Yulong molybdenite Re-Os model ages as shown in table 1), then the interval between rock-forming age and ore-forming age is 5.1 million years. If we take the Rb-Sr isochron age of 36.3 Ma (table 1) as the rock-forming age of the Machangqing porphyry, and take the Re-Os isochron age of 33.9Ma as the ore-forming age, the interval between them is 2.4 million years. If we take the Rb-Sr isochron age of 36.1 Ma as the rock-forming age of the Tongchang porphyry (table 1), and take the Re-Os isochron age of 34.4Ma as the ore-forming age, then the interval between them is 1.72 million years. So, the interval between rock-forming age and ore-forming age is in agreement with the mineral resources of different deposits, with the Yulong deposit the largest, then the Machangqing deposit, but the Tongchang deposit the smallest. These results not only prove that the ore-forming age is younger than the rock-forming age, but also suggest that the size of deposit could be larger and larger as the time interval between rock-forming age and ore-forming age is larger. 3.3 Mineralization episode At present, the idea of mineralization concentration area is known to geologists in China, but the idea of mineralization concentration stage (the same concept as minerlization episode in this paper) is still in debate. The concept of mineralization episode here means a short stage with a lot of mineral deposits avalanche within that short interval of geological history. When studying the ore-forming age of the gold deposits located in the Sanjiang (Lancangjiang-Nujiang- Jinshajiang rivers) area, especially those distributed along the Daduhe River in Sichuan, the first author of this paper found that most lode gold deposits in the Sanjiang area formed at the age of 25 Ma, which represents the peak time of lode gold formation in the

6 Mineralization episode of porphyry copper deposits 197 Sanjiang area, and named such a short stage of strong mineralization as mineralization episode [13]. Whether such regularity of mineralization is suitable to other kinds of mineral deposits? The study of Re-Os isotopic dating on molybdenites from the Machangqing porphyry copper deposit and the Tongchang porphyry copper deposit in Yunnan demonstrates that the idea of mineralization episode is also suitable to porphyry Cu-Mo deposits. That is to say, the three major porphyry Cu-Mo deposits in the Yulong, Machangqing and Tongchang ore-fields, which were distributed along the Jinshajiang-Red River tectonic belt, formed at the same mineralization episode (from 36 Ma to 33 Ma). Thus, even if these three deposits were located spatially far away from each other (with the Yulong located about 650 km northwest of the Machangqing deposit, while the Tongchang was located about 400 km southeast of the Machangqing deposit), mineralization of porphyry Cu-Mo deposits was basically simultaneous. Such phenomena also exist in the Gangdisi belt of porphyry copper deposits in Tibet, where the major three porphyry Cu-Mo deposits (Chongjiang, Lakang e and Nanmu) formed at nearly the same time of 14 Ma, and the interval of mineralization continued less than 1 million years [12]. 3.4 Implication to the tectonic setting of mineralization The above-mentioned Re-Os isotopic dating results show that the porphyry Cu-Mo deposits in Yulong, Machangqing and Tongchang orefields formed at the same geological time of late Eocene. Then, did the mineralization events happen in these three different orefields respectively or in a once united ore concentration area and then dispersed during the late stage of large-scale slide and escape process? Is it possible for the Tongchang and the Machangqing deposits escaped from their original mineralzation area (may be in the same ore concentration area as Yulong)? It is important to solve these problems, not only for reconstructing the uplift history of the Qinghai-Tibet Plateau, but for influencing the geological work and exploration. As shown by published works [14 18], large-scale slide movement happened during the stage from Eogene to Miocene along the Jinshajing-Red River tectonic belt, peaked at about 23Ma, and the slide distance up to 1000 km (nearly equal to the present distance between the Yulong deposit in Tibet and the Tongchang deposit in Yunnan). So, it is possible for the porphyry copper deposits, distributed along the Jinshajiang-Red River tectonic-mineralization belt, formed earlier than the large-scale slide movement. It is the large-scale slide movement which escaped the deposits from their original site to the present three isolated orefields hundreds kilometers far away from each other. In view of prospecting, if the above opinion could be proven further, it is possible to discover more and more ore-bearing porphyries not only along the Jinshajiang-Red River mineralization belt but also peryphral the Jinshajiang-Red River tectonic belt. Of course, it needs more and more work to attest to and it also deserves attention in the future prospecting work. 4 Conclusion The Re-Os isotopic ages of molybdenites from the major three porphyry copper deposits located in the Jinshajiang-Red River tectonic belt are Ma at Yulong, 33.9 Ma at Machangqing and Ma at Tongchang, respectively. This result shows that the mineralization ages concentrated at nearly the same time (from 36.6 Ma to 33.9 Ma, with an interval of 2.7 Ma), even though these three deposits are located far away from each other along the 1000-km-long mineralization belt. This result also suggests that the mineralization event for the formation of the porphyry copper deposits along the Jinshajiang-Red River mineralization belt happened at the end of Eocene (according to the newly published Regional Stratum Geochronological Table of China in 2002, the boundary line between Eocene and Oligocene is set at 32 Ma). Thus, the result indicates that different deposits of the same type such as those porphyry copper deposits along the Jinshajiang-Red River tectonic belt could form at the same mineralization episode, while the continuing history of episodic mineralization might be less than 3 million years. Acknowledgements This work was supported by the State Key Basic Research and Development Program (Grant No ), the National Geological Survey Program (Grant No. K ) and the State Planning and Developing Commission (Grant No. S98-1).

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