## 3 Sampling and Analytical Methods
### Analytical Methods  
(1) Petrography  
Veins filled with organic matter, pyrite, and clay minerals that were produced by the thin polished sections, were selected for petrographic and mineralogical studies using optical microscopy (transmitted and reflected light) and scanning electron microscopy (SEM) in the backscattered electron mode equipped with an energy dispersive X-ray spectrometer (EDS). The SEM-EDS uses a Nova Nano SEM450 \(\backslash\) TESCAN-VEGA \(\backslash\) LMU SEM with a LINK-ISISX alpha particle spectrometer. The SEM analysis was operated at 15 kV in 150 high-vacuum modes, with manual aperture and 4.5 beam spot sizes. All of the SEM thin sections were coated with thin carbon to ensure surface conductivity.  
(2) Geochemical composition  
The chemical compositions of the selected grains under the thin sections were determined using a JXA-8100 electron microprobe analysis (EMPA). The analytical conditions were a 15 kV acceleration voltage, a 20 nA probe current, a 1-5 microns beam diameter depending on the grain size, a 10 s counting time for the major elements, 20 s for the minor elements, and 40 s for the trace elements.  
The samples were powdered below 200 meshes in an agate mill for the whole-rock geochemical composition analysis. The major compositions were measured using AB104L X-ray Fluorescence (XRF) on fused glass beads, and the particular experiment procedures are referred to in reference [49; 50]. The trace element compositions were analyzed using the Nexion 300d plasma mass spectrometer (ICP-MS); the detailed steps are illustrated in reference [51; 52; 53; 54; 55]. The chemical assay only qualified if the relative standard variation (GBW04131, GBW04132, GBW04133) between the duplicate samples was less than 5% and the accuracy of the CRM was greater than 99.9%.  
(3) U and Pb isotope and dating  
The U and Pb isotopes were analyzed using an ISOPROBE-T Thermal Ionization mass spectrometer (TIMS). The samples were crushed and milled to fine-grained sand before splitting with a micro splitter to yield representative 5 g fractions for dissolution. The approach of [PERSON] et al. (1985) was used in that all of the samples were completely dissolved in hot HF-HNO3-HCIO4 at 1 atm before small (2-20 \(\upmu\)L) liquid aliquots of the 50 mL total solution were employed for U and Pb purification using conventional HBr (Pb) and HNO\({}_{3}\) (U) ion exchange column techniques [56]. The U and Pb concentrations within the aliquots were determined by isotope dilution mass spectrometry before the isotopic compositionswere determined using TIMS. The Pb isotope ratios were corrected for mass discrimination and laboratory blanks, and the uncertainties vary somewhat from sample to sample but are on average 0.01% (on a 2\(\sigma\) basis) for 206 Pb/204 Pb, 207 Pb/204 Pb, and 208 Pb/204 Pb, and \(\sim\)0.15% for 238U/204 Pb and 235U/204 Pb. The acceptable precision for Certified Reference Material (CRM) analyses is 208 Pb/204 Pb = 36.690 \(\pm\) 0.004, 207 Pb/204 Pb = 15.489 \(\pm\) 0.001, and 206 Pb/204 Pb = 16.936 \(\pm\) 0.002 (for NBS9810) and 235U/238U = 1.022220 \(\pm\) 0.00015 (for UTB500) [55; 57]. Uncertainties and error correlations for the blank-corrected isotope ratios (all blank corrections were negligible) and the resulting concentrations were determined using the equations of [PERSON] (1980, 1984) [58; 59]. All of the uncertainties are given at the 95% confidence level, and the ore isotopic ratios (206 Pb/238U and 207 Pb/235U) were used to estimate ages using ISOPLOT [60].  
Ra226 was conducted on the GMX-50A-Plus High-Purity Germanium Gamma Spectrometer by the standard of GB/T 11743_2013 (Determination of radionuclides in soil by gamma spectrometry).  
All of these experiments above were performed at the Geological Analysis and Testing Center, Beijing Research Institute of Uranium Geology (BRIUC), Beijing, China.