DOI:10.19762/j.cnki.dizhixuebao. 2021104
 
Sources of ore- forming fluids and metals of the Pangkuam Cu- Au deposit, Laos: evidence from H- O- He- Ar- C- S- Pb isotopes
Author NameAffiliationE-mail
CHEN Xiaofeng 1) Institute of Mineral Resources and Geology, China Nonferrous Metals (Guilin) Geology and Mining Co. ,Ltd. , Guilin, Guangxi, 541004
2) School of Land and Resources, China West Normal University, Nanchong, Sichuan,637009 
cxf514a@163.com 
ZHAO Yanpeng 1) Institute of Mineral Resources and Geology, China Nonferrous Metals (Guilin) Geology and Mining Co. ,Ltd. , Guilin, Guangxi, 541004  
ZHANG Qingwei 3) College of Earth Sciences, Guilin University of Technology, Guilin, Guangxi, 541004  
DONG Aiguo 4) Institute of Earth Sciences, China University of Geosciences (Beijing), Beijing, 100083
5) Key Laboratory of Deep- Earth Dynamics of Ministry of Natural Resources, Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037 
 
HU Qiaofan 1) Institute of Mineral Resources and Geology, China Nonferrous Metals (Guilin) Geology and Mining Co. ,Ltd. , Guilin, Guangxi, 541004  
BAI Ling’an 3) College of Earth Sciences, Guilin University of Technology, Guilin, Guangxi, 541004  
WU Jiwei 1) Institute of Mineral Resources and Geology, China Nonferrous Metals (Guilin) Geology and Mining Co. ,Ltd. , Guilin, Guangxi, 541004  
Abstract:The newly discovered Pangkuam deposit in NW Laos is a large- size copper- gold deposit from the Luang Prabang- Loei metallogenic belt. Most of the skarn and orebodies are hosted in andesite and the zonations of skarn minerals, unlike as in typical skarn- type- deposits,are scarce inthe Pangkuam deposit. Thus, identifyingthe sources of ore- forming fluid and metals, and clarifying metallogenic process of the Pangkuam copper- gold deposit is essential for the subsequent exploration of large copper- gold deposits in the Luang Prabang- Loei metallogenic belt. The δ D and δ 18 O of fluid inclusions for quartz from copper- gold mineralization stages range from -110‰ to -90‰ and -1. 5‰ to 7. 1‰ respectively, and the lower δ D values match the higher δ 18 O. The ore- forming fluid enclosed in ore pyrite have 3He/4He of 0. 41 Ra to 3. 43 Ra (mostly<1 Ra) and 40 Ar/ 36 Ar of 314. 8 to 362. 4. The H- O- He- Ar isotope results suggest that the ore- forming fluid of the Pankuam deposit is derived from the mixing of magmatic fluid (part or in whole from the mantle) and meteoric water with low δ D, and the latter have a larger proportion of mass in ore- forming fluid. Some meteoric water component of ore- forming fluid have obviously exchanged oxygen isotope with wallrock before mixing. Except for one sample with high δ 34 S value (8. 1‰), the rest δ 34 S values of the sulfide (-0. 9‰ to 1. 5‰) fall in the range of mantle sulfur isotopes compositions. 〖JP〗The result of calculations of sulfur isotope equilibrium fractionation between pyrite and coexisting chalcopyrite from the sample No. BK64, and kinetic fractionation theory both show that the sulfur of pyrite with δ 34 S of 8. 1‰ is notderived from hydrothermal fluid, likely from wallrock. The carbon isotope compositions of hydrothermal calcite ( δ 13 C: -3. 1‰~2. 5‰; δ 18 O: 26. 0‰~28. 4‰) indicate their carbon source from limestone in Pangkuam district. The limestone dissolution process proved a pathway for sedimentogenesis carbon and sulfur being taken into ore- forming fluids. The 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb of ore pyrites, ranging respectively within 17. 9284~18. 7756, 15. 5336~15. 6651 and 37. 9125~38. 8090, are similar to that of Late Permian- Middle Triassic continental arc volcanic- intrusive rocks in Loei and Trungson fold belts, and lie between Indosinian basalts and S- type granite and granite- related deposits in Thailand and Laos, which suggest that Pb inPangkuam depositis derived from mantle and upper crust. According to the S- Pb- He- Ar isotope results and petrogeochemistry studies related to skarn- type Cu- Au deposits in Luang Prabang- Loei metallogenic belt, we conclude that Cu and Au of the Pangkuam depositare mainly derived from the mantle. On base of comparative studies of S- Pb- H- O isotopes and the salinity of fluid inclusions for skarn minerals between Pangkuam deposit and other typical skarn- type Cu- Au deposits, and the previous experiments results that the reaction between the volcanic gas hydrothermal fluids and andesite could form prograde metamorphic skarn under high- temperature conditions, we suggest that theore- bearing magmatic fluid of Pangkuam skarn- type Cu- Au deposit dominated by gas- phase rises along the fault to replace the andesite or marble, and undergoes multiple geological processes, such as fluid mixing, boiling and ore precipitation.
keywords:skarn  copper- gold deposit  H- O- He- Ar- C- S- Pb isotope  Luang Prabang- Loei metallogenic belt  Pangkuam, Laos
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