Long‐term chemical evolution and modification of continental basement brines – a field study from the Schwarzwald, SW Germany

Abstract

Highly saline, deep‐seated basement brines are of major importance for ore‐forming processes, but their genesis is controversial. Based on studies of fluid inclusions from hydrothermal veins of various ages, we reconstruct the temporal evolution of continental basement fluids from the Variscan Schwarzwald (Germany). During the Carboniferous (vein type i), quartz–tourmaline veins precipitated from low‐salinity (<4.5wt% NaCl + CaCl2), high‐temperature (≤390°C) H2O‐NaCl‐(CO2‐CH4) fluids with Cl/Br mass ratios = 50–146. In the Permian (vein type ii), cooling of H2O‐NaCl‐(KCl‐CaCl2) metamorphic fluids (T ≤ 310°C, 2–4.5wt% NaCl + CaCl2, Cl/Br mass ratios = 90) leads to the precipitation of quartz‐Sb‐Au veins. Around the Triassic–Jurassic boundary (vein type iii), quartz–haematite veins formed from two distinct fluids: a low‐salinity fluid (similar to (ii)) and a high‐salinity fluid (T = 100–320°C, >20wt% NaCl + CaCl2, Cl/Br mass ratios = 60–110). Both fluids types were present during vein formation but did not mix with each other (because of hydrogeological reasons). Jurassic–Cretaceous veins (vein type iv) record fluid mixing between an older bittern brine (Cl/Br mass ratios ~80) and a younger halite dissolution brine (Cl/Br mass ratios >1000) of similar salinity, resulting in a mixed H2O‐NaCl‐CaCl2 brine (50–140°C, 23–26wt% NaCl + CaCl2, Cl/Br mass ratios = 80–520). During post‐Cretaceous times (vein type v), the opening of the Upper Rhine Graben and the concomitant juxtaposition of various aquifers, which enabled mixing of high‐ and low‐salinity fluids and resulted in vein formation (multicomponent fluid H2O‐NaCl‐CaCl2‐(SO4‐HCO3), 70–190°C, 5–25wt% NaCl‐CaCl2 and Cl/Br mass ratios = 2–140). The first occurrence of highly saline brines is recorded in veins that formed shortly after deposition of halite in the Muschelkalk Ocean above the basement, suggesting an external source of the brine's salinity. Hence, today's brines in the European basement probably developed from inherited evaporitic bittern brines. These were afterwards extensively modified by fluid–rock interaction on their migration paths through the crystalline basement and later by mixing with younger meteoric fluids and halite dissolution brines.

The Niederschlag fluorite-(barite) deposit, Erzgebirge/Germany—a fluid inclusion and trace element study

“…A notable exceptions are recent studies on the Freiberg district (Bauer et al 2019;Ostendorf et al 2019), where Sm-Nd geochronology and fluid inclusion analyses have illustrated that fluoritebarite mineralization is of Mesozoic age and related to the migration and mixing of shallow crustal, highly saline brines. This mineralization style is very similar to many other fluorite-barite veins known from Variscan basement exposures in Europe, with examples from Germany, France, Spain, Poland, Czech Republic, and Great Britain (Boiron et al 2010;Kraemer et al 2019;Muchez et al 2005;Walter et al 2016). The formation of this vein-style mineralization is typically related to the opening of the northern Atlantic in the Cretaceous (Walter et al 2018b) and that share fluid inclusion systematics (e.g., (Baatartsogt et al 2007;Walter et al 2017)).…”

Nature and Origin of Mineralizing Fluids in Hyperextensional Systems: The Case of Cretaceous Mg Metasomatism in the Pyrenees

“…The replacement of anhydrous minerals (dolomite, feldspars, and plagioclases) by hydrated minerals (talc and chlorite) increases the chlorinity during the alteration of the crystalline basement by fluids. This process has already been suggested in unconformity U-type deposits hosted in the Athabasca Basin in Canada [52,53], in the Modum Complex in Norway [39], and in the Schwarzwald district in Germany [54,55].…”

“…t1 ≠ t2 represents the fact that the circulation of the brines derived from the seawater evaporation and derived from the serpentinization should have been temporally disconnected in the deposit. 13 Geofluids ones ( [39,52,54,55,58,80]; Hovland et al, 2018). Carrying this idea a step further, Scribano et al [81] suggested that the formation of a certain subseafloor giant salt deposit in deep marine basins may be genetically related to the serpentinization of ultramafic rocks.…”

The Pb-Zn-Ag vein system at Montevecchio-Ingurtosu, southwestern Sardinia, Italy: A summary of previous knowledge and new mineralogical, fluid inclusion, and isotopic data

“…T h and Tm ice ranges, and the occurrence of CaCl 2 in the fluid) are similar to those of the fluid inclusions trapped in ore and gangue minerals in post-Variscan (i.e. Jurassic-Cretaceous) F-Ba-(Pb-Zn-Ag) deposits in Sardinia and Central-Western Europe (Bauer et al, 2019;Behr et al, 1987;Boni et al, 2002Boni et al, , 2009Cathelineau et al, 2012;Muchez et al 2005;Munoz et al, 1994;Walter et al, 2016). In particular, among the similarities between the Montevecchio and the Saint Salvy deposit (specifically for the zinciferous M4 stage, Munoz et al, 1994) Salvy (Munoz et al, 1994).…”