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Allanite, a member of the epidote supergroup, is a widespread rare earth element (REE)-rich accessory mineral in the late Archean Closepet batholith (Dharwar craton, India). It is commonly associated with titanite. Previously recognized shear zones served as preferential paths for magma and later fluids. As a response to hydrothermal activity, allanite exhibits complex alteration textures, geochemical features, and breakdown products that vary across the batholith. In the central zone, allanite displays the largest variations. It has decomposed into secondary allanite, bastnäsite, chlorite, thorite, calcite, pyrite, and galena. In the southern zone, magmatic allanite core is preserved. The alteration products in the marginal regions are limited to secondary allanite, bastnäsite, chlorite, thorite, and synchysite. The breakdown products and textural features of allanite in the northern intrusions differ strongly from those in the other zones of the Closepet batholith and are limited to secondary allanite and chamosite. However, nanoscale element remobilization at the interface between allanite and titanite is evident. The observed texture in allanite indicates a complete dissolution–reprecipitation process. The chemical variations and differences in alteration products after allanite indicate that the fluid composition changed along the Closepet granitoid. The fluids that altered the allanite were most likely F-, Cl-, and CO 2 -rich and alkaline but eventually became acidic. When the chlorine-bearing fluids reached the northern zone, the concentrations or active contributions of CO 2 , F and H 2 S were very low. The alteration products (bastnäsite, chlorite, and thorite) indicate a rather low-temperature fluid. [ABSTRACT FROM AUTHOR]

Abstract: Zircon and monazite from the Stolpen granite and related rhyolitic dyke show magmatic as well as post-magmatic textures. The magmatic textures are overprinted by post-magmatic fluid interaction which is indicated by elevated contents of Y, P, Th, and U in altered zircon domains. This can be observed both in the granite and rhyolite; however these processes were less intensive in the rhyolite. Patchy zoning in primary monazite from the granite also suggests fluid-induced alterations. High Ca, Pb, and Si contents and depletion in Th, as well as porous, spongy textures in monazite from the rhyolite are results of fluid-induced alterations that caused also crystallization of cheralite. Zircon and monazite from the granite are accompanied by fluorite, Y-rich and Nb-rich minerals. The alteration of primary, magmatic accessory minerals and formation of secondary minerals is interpreted as related to magmatic fluids, which origin cannot be defined precisely on the base of the present data, however their composition suggests some mantle contribution. The presence of fluorine and CO2 which promoted the transport of some HFSE and LILE accounts for their enrichment in the accessory minerals. Fluids that affected the accessory minerals from the rhyolite show different features (silica- and Ca-enrichment) and most probably came from another source. [Copyright &y& Elsevier]

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 δ74/70Ge 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 δ74/70Ge 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 (δ74/70Ge of 1.21–2.78‰) relative to the aquifer rocks (δ74/70Ge 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 δ74/70Ge 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 δ74/70Ge 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. • New data on Ge isotopes in geothermal waters, aquifer rocks and silicate minerals. • Geothermal waters markedly enriched in heavy Ge isotopes relative to aquifer rocks. • Secondary solids responsible for groundwater enrichment in heavy Ge isotopes. • Ge isotope and Ge/Si ratio indicate water mixing and processes in near-borehole zones. [ABSTRACT FROM AUTHOR]