# Hydrogeochemistry of Fault-Related Hot Springs in the Qaidam Basin, China
## 1 Introduction  
Hot springs are the windows between the deep and shallow parts of the earth's crust, which are exposed mostly along the fracture zones and are also related to deep groundwater circulation systems. Therefore, the water chemistry and isotopic characteristics of hot springs reflect a variety of information regarding the geological structure, the stratigraphic lithology, and water-rock interaction during formation [1]. The distribution of a few hot springs is controlled by regional major fractures, particularly the active major fractures. One reason for this is the permeability of the fractures. Another significant factor is that large regional fractures cut deeper. Meanwhile, the fracture zone rocks are broken, resulting in the formation of fissures that provide space and channels for groundwater storage and transportation [2]. Due to the different hydrogeological conditions of the fracture zone, geothermal fluids located at different parts of the fracture zone often exhibit different water circulation characteristics and water-rock interactions in regional deep and large fractures, resulting in different hydrochemical and thermal storage characteristics of the exposed hot springs [3]. Hot springs frequently circulate to depths of several kilometers, using the head of the water as a driving force, migrating upward along channels formed by angled, highly permeable fracture zones and becoming exposed to the surface [4,5]. Based on the components of He, Ne, Ar, N2, and CO\({}_{2}\) in hot springs, the distribution of the tectonic background, the degassing of Earth, the distribution of the crust-mantle heat flow, and other geological hot topics have been studied [6,7]. Several scholars have studied the relationship between the spatial and temporal variations in the helium and carbon isotopes of hot spring gases and fracture activities in non-volcanic fracture regions. It has been revealed that regions with strong fracture activities serve as good channels for deep material transport. The regions with high hot spring gas isotope ratios within active fracture zones usually exhibit significantly high-conductivity and low-velocity zones at their depths [8, 9, 10]. Therefore, helium isotope and carbon isotope analyses of hot spring gases in active fracture zones allow for the quantitative characterization of the mantle-origin mixture of helium and carbon dioxide in different ratios. Hot springs are naturally exposed underground hot water resources and serve as a kind of mineral resource with both natural and social significance. Therefore, hot springs are considered important material resources for the economic and social development of a particular region. Consequently, the hydrogeochemical characteristics of hot springs have been of interest to scholars in recent years. Several scholars have combined hydrogeochemical methods with gas and isotopic geochemical methods to study the source of underground hot water and the mixing of water from different sources [11, 12, 13]. The Altyn Tagh fault zone and the East Kunlun fault belt intersect at the southwest rim of the Qaidam Basin, a region rich in mineral resources and containing hot springs exposed along the fracture zones, including the highly active Aiken spring. Previous studies focused solely on the amount of water gushing from the hot springs, leaving out further investigation and analysis into the hot springs' genesis, chemical composition, and underlying formation mechanism. Therefore, the main study objects of this study were four typical hot springs distributed in this region of the fracture zone. The study looked at the ionic and isotopic properties of hot springs, as well as the geochemical properties of hot spring gases and the regional geological structure, the hot springs' recharge source, the recharge elevation, the water-rock interaction, the thermal storage temperature, and the circulation depth.