REY and Trace Element Chemistry of Fluorite from Post-Variscan Hydrothermal Veins in Paleozoic Units of the North German Basin

Abstract

Hydrothermal fluorites from Paleozoic sedimentary rocks and volcanic units in the North German Basin (NGB) have been investigated to create a petrographic and geochemical inventory-with particular focus on strategic elements such as rare earth elements (REE)-and to uncover possible links between the post-Variscan hydrothermal mineralization in the NGB and bordering areas such as the Harz Mountains and Flechtingen Calvörde Block (FCB). Fluorites from ten localities underwent a detailed petrographic examination, including SEM-BSE/CL imagery, and were compositionally analysed using LA-ICP-MS. Overall, REY concentrations are comparatively low in fluorite from all investigated areas-the median sum of REY ranges from 0.3 to 176 ppm. Europium CN anomalies are slightly negative or absent, indicating that either the formation fluid experienced temperatures above 250 • C or that fluid-rock interactions and REE enrichment was likely controlled by the source rock (i.e., volcanic) composition and complexation processes. Fluorites from the Altmark-Brandenburg Basin (ABB) and the Lower Saxony Basin (LSB) display distinctly different REY CN signatures, suggesting that fluid compositions and genetic processes such as fluid-rock interaction differed significantly between the two areas. Complex growth zones and REY CN signatures in fluorite from the ABB and the FCB reflect geochemical variability due to adsorption processes and intrinsic crystallographic controls and imply that they are genetically related. Two petrographically and geochemically distinct generations are observed: Fluorite I-light SEM shades, relatively enriched in LREE; Fluorite II-darker SEM shades, comparatively depleted LREE, slightly higher HREE concentrations. These fluorite generations represent zoned (or cyclical) growth within a single progressive hydrothermal event and do not reflect a secondary remobilization process. We demonstrate that increasing Tb/La ratios and decreasing La/Ho ratios can be the result of continuous zoned growth during a single mineralizing event, with significant compositional variations on a micron-scale. This has implications for the interpretation of such trends and hence the inferred genetic evolution of fluorite that displays such geochemical patterns. The complex micro-scale intergrowth of these generations stresses the need for detailed petrographic investigations when geochemical data are collected and interpreted for mineral exploration.

Stylolite-controlled diagenesis of a mudstone carbonate reservoir: A case study from the Zechstein_2_Carbonate (Central European Basin, NW Germany)

“…Fluorite is one of the last diagenetic phases to precipitate in the Ca2, indicating fluid cooling (Richardson and Holland, 1979), and is most probably associated with the uprising of heated hydrothermal brines (see Warren, 2016). Both Duschl Nadoll et al (2018) use REE values from fluorite in the LSB to suggest that precipitation originated from remobilised basinal brines of sedimentary origin. This may suggest that lateral fluid flow has also occurred from basinal sediments in the PB, likely between anhydrite layers, to supply late-stage sources of fluorite and celestine.…”

Metal budget and origin of aqueous brines depositing deep-seated Zn-Pb mineralization linked to hydrocarbon reservoirs, North German Basin

“…Up to several meter-thick mineralization zones, observed in the drill cores, are hosted by the Ca2 unit or occur at the contact between the Ca2 dolomite and Upper Carboniferous siliciclastic rocks. The ore zones and fracture-fill mineralization are well characterized and consist of sphalerite, galena, pyrite, fluorite and barite (Duschl et al 2016;Nadoll et al 2018Nadoll et al , 2019Sośnicka and Lüders 2019;Knorsch et al 2020). The mineralization in this area coincides with the presence of hydrocarbon and locally prominent CO 2 -rich gas reservoirs (e.g., Lüders et al, 2021;Sośnicka and Lüders, 2021).…”

Trace element composition of fluorite from the Chumathang pegmatite deposit, eastern Ladakh, India

“…The trace element and REY characteristics of hydrothermal solutions are primarily controlled by the physicochemical conditions that exist during migration and deposition (Bau & Möller, 1992; Schwinn & Markl, 2005). The trace elements (including REY) concentration in fluorite have been, therefore, used considerably to understand the genesis of hydrothermal deposits in different geological environments (Azizi et al, 2017; Bau & Dulski, 1996; Nadoll et al, 2018; Pei et al, 2022; Sasmaz et al, 2018; Schwinn & Markl, 2005; Souissi et al, 2010; Tang et al, 2013). During fluorite formation, terbium (Tb) and lanthanum (La) show significant degrees of differentiation in all fluorite and as a result, Tb/La and Tb/Ca ratios of fluorites are widely used to discriminate their depositional environments and degree of fractionation (Möller & Morteani, 1983).…”