## 2 Study Area and Data
### Geochemical Data  
The datasets used in this study were geochemical analyses of 4959 stream sediment samples taken at a density of 5.55 points per 1 km\({}^{2}\) by the Geological Survey Institute of Qinghai Province (Figure 2). The concentrations of 15 elements (Au, Sn, Ag, As, Sb, Bi, Co, Cu, La, Pb, Zn, W, Mo, Nb, and Cd) were measured in each sample. The samples were obtained through multi-pit combination sampling and were mainly collected from  
Figure 1: Geological map of the study area, modified from [43].  
the debris materials of the bedrock composition in the catchment area, as well as medium- and coarse-grained sand in the stream sediments. The methods used to analyze the concentration of heavy metals include atomic emission spectrometry (AES) for Au, Ag, and Sn; atomic fluorescence spectrometry (AFS) for As, Sb, and Bi; atomic absorption spectrometry (AAS) for Cu, Pb, Zn, Co, and Ni; and polarography (POL) for W and Mo.  
The elemental concentrations are summarized in Table 1. The coefficient of variation (CV) is expressed as the ratio of the standard deviation to the mean and is an important parameter that reflects the homogenization of element distribution. The elements with CV \(>\) 1, from largest to smallest, are Bi, W, Sb, As, Ag, Sn, Au, Cu, Pb, and Mo. Larger CV represents more inhomogeneous elemental concentrations. The higher the coefficient of variation, the greater the level of dispersion around the mean. We performed a logarithmic transformation on the 15 elements and plotted the log-frequency distribution histogram in the study area (Figure 3); therefore, we found that most elements tend to be lognormally distributed.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Element** & **Mean** & **Median** & **Standard Deviation** & **Skewness** & **Kurtosis** & **Coefficient of Variation** \\ \hline Au & 1.42 & 1.20 & 1.81 & 25.07 & 793.15 & 1.27 \\ Sn & 2.31 & 1.70 & 3.21 & 10.35 & 134.68 & 1.39 \\ Ag & 76.31 & 41.00 & 117.35 & 6.77 & 67.02 & 1.54 \\ As & 13.07 & 8.10 & 21.89 & 13.21 & 303.48 & 1.68 \\ Sb & 0.82 & 0.47 & 1.45 & 14.56 & 372.59 & 1.88 \\ Bi & 0.37 & 0.17 & 1.00 & 13.68 & 256.65 & 2.50 \\ Co & 7.07 & 6.40 & 3.44 & 3.07 & 24.09 & 0.48 \\ Cu & 16.05 & 12.10 & 20.44 & 15.15 & 330.62 & 1.27 \\ La & 13.73 & 12.00 & 8.30 & 8.99 & 192.65 & 0.61 \\ Pb & 17.00 & 12.80 & 21.29 & 13.35 & 314.69 & 1.25 \\ Zn & 48.29 & 40.40 & 32.06 & 4.27 & 27.61 & 0.66 \\ W & 2.74 & 1.70 & 5.99 & 18.99 & 528.95 & 2.22 \\ Mo & 1.20 & 0.96 & 1.36 & 9.31 & 117.22 & 1.17 \\ Nb & 3.93 & 3.30 & 2.26 & 4.53 & 37.90 & 0.59 \\ Cd & 0.15 & 0.10 & 0.18 & 5.28 & 49.30 & 0.85 \\ \hline \end{tabular}
\end{table}
Table 1: Main statistical results of the stream-sediment elements in the study area.  
Figure 2: Map of stream sediment geochemical sampling points.  
Figure 3: Log-frequency distribution histogram of the 15 elements in the study area: (**a**) Au, (**b**) Sn, (**c**) Ag, (**d**) As, (**e**) Sb, (**f**) Bi, (**g**) Co, (**h**) Cu, (**i**) La, (**j**) Pb, (**k**) Zn, (**l**) W, (**m**) Mo, (**n**) Nb, and (**o**) Cd.  
In addition, we compared the average concentrations of seven mineralized elements in the widely distributed bedrocks with those in the corresponding overlaying stream sediments (Figure 4). The element concentrations in the bedrocks and their corresponding overlaying stream sediments are very close; in particular, the two kinds of concentrations in the upper Triassic Flashan Formation almost coincide. The element concentrations show strong correlations between the bedrocks and their corresponding overlying stream sediments.