Lithium possesses exceptionally high economic and strategic significance. China’s lithium resources rank second globally, predominantly found in salt lake brine. Recently, clay-type lithium deposits have been discovered in the Mahai Salt Lake within the Qaidam Basin, featuring a broad distribution area, stable production layers, and thick strata. This paper investigates the mineral sources and mining circumstances of the clay deposits in the study area through the principal trace elements and Sr and Li isotopes of the clay rocks. The study reveals that the sedimentation of the clay deposits in the study area is mainly semi-brine–brackish water phase sedimentation, formed in a shallow water environment of weak oxidation. A terrigenous source characterizes the clay sediment. The tectonic setting of the study area shares common traits with a continental island arc-active continental margin, and the abundant lithium clay rocks mainly receive contributions of feldspar, amphibole, pyroxene, etc. The primary sediment source of the clay deposits mainly stems from igneous and quartz sediment source areas. The ⁸⁷Sr/⁸⁶Sr value indicates that the Sr source of the clay rocks in the study area is the shallow over-pressured brine and Yuka River water, mainly derived from the weathering dissolution of silicate rocks. The δ⁷Li value indicates that the lithium in the lithium-rich clay rocks mainly originates from the release of lithium from silicate minerals in the surrounding mountain ranges, carried by Yuka River water, and from the supply of lithium from the deep Ca-Cl type water body. The research findings establish a foundation for the future exploration and development of clay-type lithium deposits in salt lake regions.

The Balun Mahai Basin (BLMH), located in the northern Qaidam Basin (QB), is endowed with substantial brine resources; however, the genetic mechanisms and potential of these brine resources remain inadequately understood. This study investigated the intercrystalline brine (inter-brine) in BLMH, performing a comprehensive geochemical analysis of elemental compositions and H-O-Sr isotopes. It evaluated the water source, solute origin, evolutionary process, and genetic model associated with this brine. Moreover, a mass balance equation based on the ⁸⁷Sr/⁸⁶Sr isotopic ratio was developed to quantitatively evaluate the contributions of Ca-Cl water and river water to the inter-brine in the study area. The results suggest that the hydrochemical type of inter-brine in the north part of BLMH is Cl-SO₄-type and in the south part is Ca-Cl-type. The solutes in brine are mainly derived from the dissolution of minerals such as halite, sylvite, and gypsum. The hydrochemical process of brine is controlled by evaporation concentration, water–rock interaction, and ion exchange interaction. Hydrogen and oxygen isotopes suggest that the inter-brine originates from atmospheric precipitation or ice melt water and has experienced intense evaporation concentration and water–rock interaction. The strontium isotopes suggest that the inter-brine was affected by the recharge and mixing of Ca-Cl water and river water, which controlled the spatial distribution and formation of brine hydrochemical types. The analysis of ionic ratios suggest that the inter-brine is derived from salt dissolution and filtration, characterized by poor sealing and short sealing time in the salt-bearing formation. The differences in hydrochemical types and spatial distribution between the north and the south are fundamentally related to the replenishment and mixing of these two sources, which can be summarized as mixed origin model of “dissolution and filtration replenishment + deep replenishment” in BLMH.