Acta Geochim (2018) 37(4):614–631 https://doi.org/10.1007/s11631-017-0257-0 ORIGINAL ARTICLE Genesis of the Nanyangtian scheelite deposit in southeastern Yunnan Province, China: evidence from mineral chemistry, fluid inclusions, and C–O isotopes 1 1 1 1 1 Qianru Cai • Yongfeng Yan • Guangshu Yang • Fuju Jia • Chao Li Received: 19 April 2017 / Revised: 22 November 2017 / Accepted: 26 December 2017 / Published online: 16 January 2018 Ó Science Press, Institute of Geochemistry, CAS and Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract The Nanyangtian skarn-type scheelite deposit is the formation of the Nanyangtian deposit is related to an important part of the Laojunshan W–Sn polymetallic Laojunshan granitic magmatism. metallogenic region in southeastern Yunnan Province, China. The deposit comprises multiple scheelite ore bodies; Keywords Fluid inclusion Á C–O isotopes Á Skarn Á multilayer skarn-type scheelite ore bodies are dominant, Scheelite deposit Á Nanyangtian with a small amount of quartz vein-type ore bodies. Skarn minerals include diopside, hedenbergite, grossular, and epidote. Three mineralization stages exist: skarn, quartz– 1 Introduction scheelite, and calcite. The homogenization temperatures of fluid inclusions in hydrothermal minerals that formed in The Laojunshan W–Sn metallogenic district is an impor- different paragenetic phases were measured as follows: tant mineral resource base in Yunnan Province. Various 221–423 °C (early skarn stage), 177–260 °C (quartz– W–Sn polymetallic deposits surround the Laojunshan scheelite stage), and 173–227 °C (late calcite stage). The granitic pluton: the Nanyangtian-Chayeshan W metallo- measured salinity of fluid inclusions ranged from 0.18% to genic belt in the east, the Dazhushan-Xinzhai Sn–W met- 16.34% NaCleqv (skarn stage), 0.35%–7.17% NaCleqv allogenic belt in the north, and the Manjiazhai-Tongjie Zn– (quartz–scheelite stage), and 0.35%–2.24% NaCleqv (late Sn–In metallogenic belt in the southwest. Ten years ago, a calcite vein stage). Laser Raman spectroscopic studies on large Nanyangtian scheelite deposit containing high-grade fluid inclusions in the three stages showed H2O as the main ore with considerable prospecting potential was discov- component, with N2 present in minor amounts. Minor ered; it has since been the subject of several studies. Pre- amounts of CH4 were found in the quartz–scheelite stage. It vious research has focused on the geological setting, was observed that the homogenization temperature gradu- geochemistry, and fluid inclusions of minerals in this ally reduced from the early to the late mineralization deposit and has produced a detailed chronology of events 13 stages; moreover, d CPDB values for ore-bearing skarn in (Zeng et al. 1999; Shi et al. 2011; Wang et al. 2012; Feng the mineralization period ranged from - 5.7% to - 6.9% et al. 2011a, b), thereby paving the way for further 18 and the corresponding d OSMOW values ranged from 5.8% prospecting. to 9.1%, implying that the ore-forming fluid was mainly Previous work in the field of fluid inclusions includes: sourced from magmatic water with a minor amount of Feng et al. (2011a, b) work on quartz and garnet in the meteoric water. Collectively, the evidence indicates that Nanyangtian scheelite deposit; Li (2013) research of fluid inclusions in quartz, garnet, and calcite in the Yanggouhe deposit; and Zhang et al.’s (2014) study of inclusions in skarn mineral and quartz in the Guanfang scheelite deposit & Yongfeng Yan using a thermometer and laser Raman spectrometer. Pre- [email protected] vious research on the source of ore-forming fluids and ore 1 Kunming University of Science and Technology, genesis by fluid inclusion analysis has not been compre- Kunming 650093, China hensive. As gangue minerals and ore minerals have 123 Acta Geochim (2018) 37(4):614–631 615 different characteristics, the condition of ore minerals The study area is in the southeastern Yunnan–western cannot be gleaned by analyzing gangue (Campbell and Guixi metallogenic belt, which lies to the west of the Robinson-Cock 1987; Campbell and Panter 1990; Gia- Nanling W–Sn polymetalloic metallogenic belt. Laojun- mello et al. 1992). All three publications mentioned above shan therefore experiences frequent geologic activity, focused on the characteristics of fluid inclusions in gangue contains many active faults, and has excellent metallogenic minerals rather than in ore; by analyzing fluid inclusions of conditions. ore minerals, the current work aims to provide more rele- Because of a lack of upper Ordovician, Silurian, Juras- vant information on the properties of metallogenic fluids. sic, and Cretaceous strata, other strata are fully exposed in Following a description of the geological setting and the area. According to lithology, the Laojunshan rock body lithology of the Nanyangtian scheelite deposit, we discuss can be divided into seven intrusive bodies: the Tuantian skarn mineralogical characteristics and the associated medium- to fine-grained granitic, Laochengpo fine-grained evolutionary process. Next, we report data from fluid granitic, Huashitou two-mica adamellite, Malutang med- inclusions in gangue and ore minerals, focusing particu- ium- to fine-grained two-mica adamellite, Taipingtou larly on the analysis of scheelite fluid inclusions. Finally, medium- to fine-grained two-mica adamellite, Qingjiao information about the ore-forming fluid and a logical micrograin two-mica adamellite, and Kouha granite-por- explanation of the genesis of the Nanyangtian skarn phyry intrusions. The Cambrian strata are exposed in the scheelite deposit are provided. external contact zone of the Laojunshan rock body, which is mainly composed of schist, plagioclasite hornblende 1.1 Regional geology schist, amphibolite, and carbonate. Under magmatic hydrothermalism, carbonate altered to stratiform skarn, in Laojunshan is located at the intersection of the peri-Pacific which the scheelite ore body occurs. and Tethyan Himalayan tectonic syntaxes and at the The Cambrian strata comprise the Chongzhuang For- junction of the South China fold system, Ailaoshan fold mation in the Lower Cambrian series ( 1” ch), and the ” system, North Vietnam Block, and Yangtze Plate (Fig. 1). Longha ( 2” l) and Tianpeng formations ( 2t) in the Middle Fig. 1 Schematic tectonic map (a) and regional geological map (b) (after Zijin Tungsten Industry Group Co. LTD 2014) 123 616 Acta Geochim (2018) 37(4):614–631 Cambrian series. The Chongzhuang Formation exists with direction of motion (from southeast to northwest). It is mainly in the Nanyangtian mining area and can be classi- thought that the nappe fault may have been formed by the fied into three lithologic members: the lower Geling section decoupling detachment of the Nanwenhe metamorphic ” 1 ( 1ch ), comprising granite gneiss and plagioclase gneiss; core complex. The Nanyangtian sliding fracture is the ” 2 the middle Nanyangtian Section ( 1ch ), comprising ver- highest sliding fracture within the nappe structure and is delite biotite granulite, biotite plagioclase gneiss, two-mica exposed to the east of the mining area near Nanyangtian. schist, and tremolite-zoisite-diopside skarn; and the upper The southwest section of the fault is intruded by Dulong ” 3 Taiyangping section ( 1ch ), comprising muscovite mon- super-unit granite; the upper wall of the fault relates to the zonitic gneiss. The Longha Formation is exposed mainly in Laochengpo intrusion and contains Nanwenhe sequence Wenshan and Maguan as a hoary-white medium-embedded gneissic fine-grained granite. The major fracture surface dolomite marble with a middle layer containing a small has strong mylonitization, and a quartz vein is sheared into amount of phyllite and a bottom layer containing mus- a ‘‘Z’’ fold and a rootless fold by the fault. The hanging covite schist. The Tianpeng Formation occurs exclusively wall mainly consists of schist within the Chongzhuang in the Dulong mining area. Its bottom layer is gneiss; its Formation, and the cleavage tendency is southeast. The middle layer contains ore-bearing rocks mainly comprising schist and gneiss have a small asymmetric fold plunging actinolite, diopside, and chlorited skarn; and the rocks in its toward 200°–220° at an angle of 8°–15°. upper stratum are dolomitic phyllite, mica quartz schist, The Yanshanian Laojunshan granitic pluton outcrops to and interbedded marble (Jia 2010). the east of the mining area. Local igneous activity exhibits The Wenshan-Malipo Fault lies to the north of the multiphase and multistage characteristics. Evolutionary Laojunshan granite body, the Maguan-Dulong Fault lies to periods (from ancient times to the current day) have the south, and the Nanwenhe Fault is an almost east–west resulted in three rock types: medium-coarse monzogranite, direction. The secondary fault of Laojunshan provides medium-fine monzogranite, and granite porphyry (the conditions suitable for W–Sn ore formation. Regional spread of which is reduced from the first rock body to the metamorphism is strong, owing to multistage tectonism. A third, with the late-stage rock body intruding into the early low-temperature regional dynamo metamorphism of gran- rock body). The main part of the outcropped bedrock in the ulite and schist occurred during the Caledonian-Indosinian. mining area is gneissic fine-grained granite porphyry with a In addition, contact metamorphism occurred with the mineral composition of 40% plagioclase, 40% quartz, and intrusion of Laojunshan granite during the Yanshanian, 10% biotite. The gneiss composition