Groundwater in fractured crystalline rocks, the Clara mine, Black Forest (Germany)

Abstract

The chemical composition of water sampled in a 700 m deep underground barite-fluorite mine in the crystalline basement of the Black Forest area (SW Germany) varies systematically with depth and the length of flow paths trough, the fracture porosity of the gneiss matrix. Calcium and sulfate increase as a result of a combined sulfide oxidation and plagioclase alteration reaction. The gneiss contains andesine-plagioclase (An 20 -An 40 ) and is rich in primary sulfide. As an effect of Ca and SO 4 release by the prime water-rock reaction, dissolved oxygen decreases and the waters become more reduced. The waters have Cl/Br mass ratios of about 50, which is very close to that of experimentally leached gneiss powders indicating that the rock matrix is the source of the halogens. The waters are undersaturated with respect to calcite in the upper parts of the mine. With increasing reaction progress, calcite saturation is reached and carbonate forms as a reaction product of the prime reaction that also controls the partial pressure of CO 2 to progressively lower values. The chemical evolution of groundwater in fractured basement of the Clara mine suggests that the partial pressure of CO 2 is an internally buffered parameter rather than a controlling external variable.

Aquifer geochemistry of crystalline rocks and Quaternary deposits in a high altitude alpine environment (Kauner Valley, Austria)

“…Na and K) in such aquifers (e.g. Bucher et al, 2009;Hölting and Colde wey, 2013). On the other hand, non-silicate minerals, in particular carbonates, sulphides and sulphates, which occur as accessory minerals disseminated in the rock and / or as fracture coatings and fillings, often have a larger impact on groundwater chemistry than the main rock-forming silicate minerals due to their higher reactivity (e.g.…”

Negative Ce anomalies in Mn oxides: The role of Ce4+ mobility during water–mineral interaction

“…The geological setting, mineralogy, and stable isotope geochemistry of the primary hydrothermal fluorite vein have been described elsewhere (Huck, 1984;Baatartsogt et al, 2007;Pfaff et al, 2009). The host rock consists mainly of gneisses of the Central Black Forest Gneiss complex and is composed of plagioclase, K-feldspar, quartz, biotite, amphibole, garnet (locally), and accessory minerals (Bucher et al, 2009). The gneiss is partially overlain by Triassic sandstone.…”

“…Both paragneiss and orthogneiss occur, which are intensely folded and fractured and have locally been intruded by late Variscan granitic dikes (Huck, 1984). Fractures may be filled with sulfide mineralization or weathering products of the gneiss (Bucher et al, 2009). In the vicinity of the hydrothermal mineralization, the host rock is pervasively altered.…”

“…In the vicinity of the hydrothermal mineralization, the host rock is pervasively altered. Detailed geological maps and cross section are given in Huck (1984) and Bucher et al (2009). The mineral sample material (67 samples) covers (1) primary mineralization from the fluorite vein and (2) secondary fillings of geologically young fracture networks and open fissures, where reaction between meteoric water and the primary vein minerals resulted in mobilization and redistribution of material.…”

Disposal systems evaluations and tool development : Engineered Barrier System (EBS) evaluation.

“…CO 2 -rich recharge from surface water has initially low pH, typically around 6 but as low as 5 (Bucher, Zhu, and Stober, 2008). In deeper systems dominated by calcium-rich saline fluids, both carbonate-solubility constraints and silicate reactions act to remove bicarbonate ions as precipitated calcium-and magnesium-carbonates, often adjusting the pH to levels greater than 9 (Frape et al, 2003).…”