Genesis and geodynamic setting of the Nanyangtian tungsten deposit, SW China: Constraints from structural deformation, geochronology, and S–O isotope data

Abstract

Southeastern (SE) Yunnan contains one of the most economically important Mesozoic W–Sn metallogenic belts in South China. The belt comprises three well-known W–Sn ore-concentration areas, namely, the Gejiu, Bainiuchang, and Laojunshan areas from west to east. Numerous skarn W–Sn deposits have been identified in association with Late Cretaceous granites in these areas, although the genesis and geodynamics of the deposits remain uncertain. The Nanyangtian W deposit, which is located in the southern part of Youjiang Basin and the eastern margin of the Laojunshan granitic pluton, consists of skarn and superimposed scheelite–mica–quartz (SMQ) vein mineralization. Multiple stages of structural deformation and mineralization, and geochronological data indicate the Nanyangtian deposit contains important clues to the genesis and geodynamics of W mineralization in SE Yunnan. Structural analysis of the Song Chay dome allows four stages of deformation to be identified from early to late, namely, north-directed detachment (D1), northwest-directed thrusting (D2), dextral transtensional faulting (D3), and late sinistral strike-slip faulting (D4). Laser ablation–inductively coupled plasma–mass spectrometry U–Pb dating of zircons from plagio-amphibolite and altered granitoid yielded ages of 220.37 ± 0.59 and 162.5 ± 3.0 Ma, respectively, indicating the timings of regional detachment deformation (D1) with related metamorphism, and the emplacement of coeval granite related to skarn W mineralization, respectively. 40Ar–39Ar isotope dating of muscovites from Laochengpo gneissic granite yielded well-defined plateau ages of 224.12 ± 0.35 and 141.2 ± 0.2 Ma, indicating the timing of D1 deformation and Early Cretaceous tectono-thermal event, respectively. 40Ar–39Ar isotope dating of biotite from an SMQ vein within the deposit yielded a well-defined plateau age of 97.81 ± 0.70 Ma, which is interpreted to represent the timing of SMQ vein W mineralization. δ34S values of sulfide in skarn ores range from 7.46‰ to 9.55‰, and δ18O values of skarn-type scheelite vary from 2.9‰ to 4.0‰. However, δ18O values obtained for vein-type scheelite range between 4.4‰ and 5.7‰, indicating a different material source of W mineralization from that of the skarn ores. Based on our analysis of ore-controlling structures, isotopes, and geochronology, we propose that the inter-layer fractures that formed during D2 controlled the Late Jurassic skarn mineralization and associated granitic magmatism, and that Late Cretaceous SMQ vein mineralization resulted from dextral transtension (D3). Integration of the sequence of deformation and the two periods of W mineralization in combination with well-defined geochronological constraints indicates four stages of tectonism and mineralization in SE Yunnan: Late Triassic–Early Jurassic D1 deformation and early Sn mineralization, late Early–Late Jurassic D2 deformation and subsequent skarn mineralization and associated granitic magmatism, Cretaceous D3 deformation and associated W and late Sn polymetallic mineralization within a regional extensional setting, and Cenozoic post-mineralization strike-slip D4 deformation. By combining our new results with other research findings from SE Yunnan and South China, the inferred sequence suggests that the multiple stages of deformation and mineralization were influenced by geodynamic processes involving the Indochina and Paleo-Pacific Plates with the South China Block.