The Yunnan(Dian)-Guizhou(Qian)-Guangxi(Gui)-area of southwestern China is well known as the “Golden Triangle” since this region contains the second-largest concentration of Carlin-type gold deposits in the world (Fig. 1a, b), with a total resource of > 800 tons of contained Au at Au an average grade of 4.5 g/t1–10. Precise geochronological data of mineralization within micro-fine disseminated rocks, such as the Carlin-type/like gold deposits are not always available, since they are generally lack datable minerals for conventional isotopic dating techniques5,11−14. However, with the progress of mineral separation and isotopically analytical technologies, as well as the development and utilization of new techniques, great progress has been made in constraint the metallogenic ages of Carlin-type gold deposits during the past decades4,14,15. For example, the metallogenic ages of Carlin-type gold deposits in Nevada, USA, have been well constraint on 42 − 36 Ma by applied Rb-Sr and 40Ar/39Ar dating methods of galkhaite and adularia, respectively11,16,17.
There is no galkhaite and adularia in Carlin-type gold deposits from the Dian-Qian-Gui area “Golden Triangle” have been reported, thus, the published isotopic dating results from this region (Fig. 1b) mainly derived from hydrothermal altered minerals and fluid inclusions Rb-Sr dating19–21,28, arsenopyrite, pyrite and pyrobitumen Re-Os dating6,8,22, hydrothermal rutile/monazite/calcite/apatite U-(Th)-Pb method4,9,15,23, zircon U-Th-He method29, sericite and illite 40Ar/39Ar dating27, and hydrothermal calcite Sm-Nd method24–26. In summary, the reported geochronological data suggested that the Dian-Qian-Gui “Golden Triangle” region underwent two independent gold mineralization events during the late Triassic to early Jurassic (230 ~ 195 Ma) and the late Jurassic to early Cretaceous (150 ~ 122 Ma).
As one of the typical Carlin-type gold deposit in Bama County, northwest Guangxi (Fig. 1c), precise mineralization age of the Liaotun gold deposit is still poorly constrained mainly because no suitable minerals can be selected for traditional isotopic dating methods. Gold deposit in the district is hosted by middle Permian Baifeng Formation (T2bf) that consists of mudstone, sandstone and siltstone, and the orebodies are mainly controlled by NW-trending or EW-trending faults (Fig. 1c). A felsic dike intruded Carboniferous limestone and Triassic sandstone along an ENE- to NE-trending fault and cut off the biggest orebody (No. I). The 40Ar/39Ar dating of muscovite phenocryst from this dike yielded a plateau age of 95.5 ± 0.7 Ma, which is interpreted as the upper limit of the metallogenic epoch18. Later, SIMS zircon U-Pb dating result shows that the Liaotun dike was emplaced at 97.2 ± 1.1 Ma (MSWD = 2.9), and the authors inferred that there is no genetic link between the felsic dike and Liaotun carlin-type gold deposit30. Therefore, in order to unravel the precise mineralization age of this gold deposit, there still needs more accurate direct metallogenic data.
The 40Ar/39Ar in vacuo progressive crushing technique for dating the ages of fluid inclusions has been improved and developed for thirty-five years31. This method has been widely applied to constrain the formation ages of hydrocarbon accumulation32,33, high-ultrahigh pressure retrograde metamorphism34,35, and especial on direct dating of hydrothermal deposit minerals (Cassiterite, sphalerite, wolframite) and gangue minerals like mineralization quartz vein36–43. Nevertheless, there is still no successful report by this technique on sediment-hosted Carlin-type gold deposits yet, although mineralization quartz veins with abundant fluid inclusions are widely developing in this type of ore deposit.
In this contribution, we chose the Liaotun Carlin-type gold deposit, northwest Guangxi, as the research object, applying the 40Ar/39Ar in vacuo progressive crushing dating technique on the main mineralization stage pyritized gold-bearing quartz vein for the first time (Fig. 2c). Based on a combined approach of fluid inclusions petrographic observation and micro-thermometric measurement, our study is try to decipher the origin of fluid flow and constrain the age of quartz vein formation by using a direct approach. Furthermore, we believe our attempt not only can evaluate the feasibility of 40Ar/39Ar dating by in vacuo progressive crushing of fluid inclusions, but also could exploit a new approach to constraint the mineralization age of the lack datable minerals Carlin-type/like gold deposits.
The Dian-Qian-Gui ore concentration area is restricted to the Devonian-Triassic Youjiang basin, which is bound to the northeast by the Ziyun-Du’an fault, to the northwest by the Mile-Shizong fault, and to the southeast by the Pingxiang fault (Fig. 1b), which separates the basin from the Cathaysia block1. It widely developed Au-As-Sb-Hg low-temperature hydrothermal deposits and contains one of the largest concentrations of Carlin-type gold deposits in the world1–3,8,10.
Evolution of the Youjiang Basin can be divided into six stages from Early Devonian to Cretaceous44, while gold deposits in this region mainly formed in the tectonic transformation period from Indosinian compression Orogeny to Yanshanian extensional tectonic environment, that is, the extension stage after the collision of Youjiang fold belt formed by the evolution of foreland basin in Youjiang Basin1,2,4,6,44. All gold deposits over the Golden Triangle are mainly hosted in the Permian limestone and volcaniclastic sedimentary rocks or Triassic siliciclastic rocks and carbonates and are structurally controlled by various folds and associated faults, likely produced during the Indosinian orogenic deformation1,45.
Geology of the Liaotun Deposits
The fault type Liaotun gold deposit, in Bama County, Northwest Guangxi, is a medium-sized Carlin-type gold deposit, which located on southwest margin of the isolated Longtian carbonate platform (Fig. 1b)18,30,46. The exposed sedimentary rocks in the platform are mainly limestone intercalated with dolomite of Carboniferous Du’an Formation (C1 − 2d) and Permian sponge reef limestone (Pbls). The strata around the platform is the Triassic Baifeng Formation (T2bf), which consists of interbedded deep-water basin facies sandstone and mudstone (Fig. 2a)18,30.
The study area contains well-developed faults and linear folds and individual gold orebodies are structurally controlled by high-angle faults. There are five NW-trending and four EW-trending faults have been recognized in the mine area (Fig. 1c). Among them, the NW-trending faults F1 and F2 are syn-sedimentary faults, while the NW-trending F4 and the EW-trending F5, F6, F9 faults are ore-controlling/bearing structures, hosting the I, III, IV, and V orebodies, respectively (Fig. 1c)18,46. A later Yanshanian (97 − 95 Ma) quartz porphyry veins intruded Carboniferous limestone and Triassic sandstone along an ENE- to NE-trending fault across the Longtian dome18,30.
The deposit consists of five orebodies and the largest orebody (No. I) is cut off by the later Yanshanian quartz porphyry vein from the middle and respectively named as I-1 and I-2 (Fig. 1c). Gold mineralization occurred in shallow part is oxidized ore, dominant by silicification and limonitization detrital quartz greywacke and cataclasite. Primary to semi-primary ore minerals in deeper part are disseminated pyrite and minor Arsenopyrite. Hydrothermal alterations associated with gold mineralization in the deposit have identified silicification, pyritization, arsenopyrite, (de)carbonatination, clayization and sulfidation. In additionally, the occurrence, textures, and mineral assemblages of the ores at Liaotun indicate that the main-mineralization stages can be further divided into: (1) decarbonination + silicification stage; (2) quartz + pyrite + arsenopyrite stage; (3) quartz + stibnite stage; (4) quartz + calcite + clayization stage18,46.
The NW-trending F4-controlling largest I orebody is 656 m long and average 9 m thick, with steep dip angles of 50° to 85° and an average grade of 1.62 g/t Au. The smaller III and V orebodies are controlled by EW-trending vertical F5 and F6, which are 230 to 194 m long and 7.20 to 1.16 m thick with average gold grades varying from7.33 to 0.34 g/t Au18,46. The V orebody is hosted in siltstone, mudstone and thick bedded sandstone at the second member of middle Triassic Baifeng Formation. In this orebody, the dominant ore are taupe and purplish red silicified fine-sandstone, cataclasite, crushed rock, minor silicified siltstone and bedded mud, and veinlet quartz usually can be observed locally (Fig. 1c). The ore structures are disseminated, spotted, micro-veined-network, brecciated, porous and earthy18. The sample LT19-1-2 used in this study for fluid inclusion in vacuo crushing 40Ar/39Ar dating was collected from the goaf of V orebody (Fig. 2b, c). It is a 0.5-2 cm wide pyritized gold-bearing quartz vein with grade of 4.02 g/t Au46.