Soil Hg–As–Sb anomalies and associated trace elements as indicators of geothermal fluid migration

Abstract

Areas with high-temperature geothermal activity commonly exhibit anomalous enrichment of trace elements in near-surface soils. In this study, 167 soil samples were collected from the Yangbajing geothermal field, Tibet, to investigate the enrichment characteristics, spatial distribution, and indicative significance of six trace elements, namely Hg, As, Sb, Bi, Cu, and Mn. The results show that Hg, As, Sb, and Bi are enriched relative to the Upper Continental Crust baseline, whereas Cu and Mn show weak enrichment or relative depletion. Among these elements, Hg, As, and Sb display the most distinct anomaly patterns and are the most effective soil geochemical indicators of geothermal activity in the study area. The spatial distribution of trace-element anomalies shows clear element-specific zoning. Hg anomalies are mainly distributed in Zones I and II and are closely associated with high-temperature geothermal manifestations and fault-controlled upflow. In contrast, As and Sb anomalies are concentrated in Zones II and III, reflecting liquid-phase transport during geothermal fluid migration, cooling, mixing with shallow cold water, and subsequent adsorption or precipitation. Mn mainly occurs as a weak peripheral anomaly and is related to low-temperature discharge conditions, whereas Bi and Cu show localized enrichment associated with alteration minerals near the margins of geothermal fluid pathways. The migration and retention of these elements are jointly controlled by fault structures, temperature gradients, fluid mixing, and alteration mineralogy. Hg is preferentially transported in the vapor phase along deep faults and retained in kaolinite-rich alteration zones, while As, Sb, and Mn are mainly associated with liquid-phase geothermal fluids and are enriched in shallow or peripheral zones through cooling, adsorption, and precipitation. Comparison of anomaly distributions before and after geothermal development indicates that Hg, As, and Sb respond to changes in geothermal water level, discharge pathways, and fluid migration conditions. Overall, the coupled spatial patterns of Hg, As, Sb, Bi, Cu, and Mn provide a useful soil geochemical framework for identifying geothermal anomaly zones and evaluating the evolution of geothermal fluid pathways during exploitation and recovery. Hg is the most sensitive indicator of high-temperature vapor-phase upflow, As and Sb indicate medium-temperature liquid-phase migration, Mn provides supplementary evidence for low-temperature peripheral discharge, and Bi and Cu mainly reflect local alteration near geothermal fluid channels.