Germanium and its isotopes as indicators of hydrogeochemical conditions in a terrestrial geothermal system (Karkonosze granitoid, Sudetes, Poland)

Abstract

The germanium content and its isotope ratio in thermal waters, aquifer rocks and main silicate minerals in a terrestrial groundwater system located in a granitic pluton (Karkonosze granitoid, Sudetes, Poland) have been investigated. This is the first extensive Ge isotope study carried out in both groundwater and aquifer rocks to better understand previously the identified enrichment of groundwater relative to rocks. This research also provides the first data on δ^74/70Ge for hybrid acid rocks formed by mixing magmas from two different sources (mantle- and crustal-derived) and main silicate minerals. The Ge isotopic composition in different facies of granite (porphyritic, equigranular) is comparable (δ⁷⁴/⁷⁰Ge of 0.43–1.23‰) to its hybrid rocks (quartz diorite–granodiorite, microgranular magmatic enclaves, composite dykes) – 0.79–1.27‰. However, the range of variability of this feature is quite wide. The minerals have δ^74/70Ge values of 1.01–1.04‰ in quartz, 0.84–0.90‰ in alkali feldspars, 0.76–0.88‰ in plagioclase, and 0.36–0.39‰ in biotite. The thermal waters are enriched in heavy Ge isotopes (δ^74/70Ge of 1.21–2.78‰) relative to the aquifer rocks (δ^74/70Ge of 0.43–1.27‰). Currently, the most likely explanation for this is the influence of secondary mineral phases formed in the geothermal granitic system. This comprises the preferential incorporation and adsorption of light Ge isotopes in/onto iron oxide/hydroxides and clay minerals. The other theoretical explanation, i.e., sulfide formation, is unlikely due to the unfavourable hydrogeochemical conditions in the studied system. The δ^74/70Ge values of thermal waters are similar to those of previously studied continental geothermal waters (1.65–3.29‰; Siebert et al., 2011) and fresh groundwater (2.24–4.02‰; Baronas et al., 2020), confirming that groundwater enrichment in heavy Ge isotopes relative to aquifer rocks may be widespread in the upper continental crust. Variation in the isotopic composition of the studied thermal waters is a result of the aquifer rock mineralogy and, in some cases, mixing of the old deep-circulating thermal water with the shallow modern cold groundwater. The general similarities in the chemical composition of the studied waters are due to the influence of porphyritic granite, which is the dominant rock type in this aquifer system. Some differences in chemical composition, including differences in the Ge isotopic composition, are related to the local distribution of hybrid rocks and sulfides. The δ^74/70Ge and Ge/Si ratios have been shown to be sensitive indicators of local hydrogeochemical conditions in an alimentation zone and a near-borehole zone of particular water intakes.