Experimental alteration of granitic rocks: Implications for the evolution of geothermal brines in the Upper Rhine Graben, Germany

• Published in: Geothermics 2020-11, Vol.88, p.101903, Article 101903
• Main Authors: Drüppel, Kirsten, Stober, Ingrid, Grimmer, Jens C., Mertz-Kraus, Regina
• Format: Article
• Language: English
• Subjects: Dissolution-precipitation; Geothermal power; Granitic reservoir rocks; Hydrothermal experiments; Lithium; Rhine graben; Water-rock interaction
• ISSN: 0375-6505; 1879-3576
• DOI: 10.1016/j.geothermics.2020.101903

•Element concentrations in fluids of granite-water leaching and alteration experiments can be linked to the dissolution processes.•Using hydrochemical data of the experiments, it is possible to predict probable alteration products and paths at low and high temperatures.•Li is mainly contained in phyllosilicates of granitic rocks (highest concentrations in muscovite, biotite) as and in fluid-inclusions in quartz.•Biotite is the main source of lithium in deep crystalline basement waters.•Passive increase of Cl and Li by precipitation of secondary smectite/chlorite explains the correlation between Cl and Li of continental brines. Geothermal energy exploitation in the Upper Rhine Graben (URG) chiefly has targeted faults and fractures within or connected with the crystalline basement, where hot fluids of c. 200 °C circulate at depths of c. 5 km. Formation fluids of the crystalline basement are highly saline, NaCl-dominated brines, whereas shallow crystalline basement water (< 500 m) is only weakly mineralized. In order to contribute to a better understanding of redistribution of elements in fluids and alteration phases during fluid-rock interaction processes we conducted leaching and alteration experiments with granites and monzonites from the Schwarzwald at 70 °C (rock powder, pure water) and 200 °C (texturally intact rock cylinders, 2m-NaCl), respectively, inspired by the P-T-x conditions reported for the URG. The trace element concentrations of the leachates are hereby related to the composition and stability of minerals in the rocks and can be directly linked to the proposed and observed dissolution processes at the different temperatures. In experiments with pure water at 70 °C, representing the recharge or infiltration conditions, water-rock interaction mobilized K, Cl, Fe, Mn, Mg, Cu, Zn, Cr, Sc, V, and Li (dissolution of biotite/chlorite) and Si, Al, K, Ca, Na, Ba, Sc, Pb, Sr, Rb, and Eu (dissolution of plagioclase, K-feldspar, and quartz) into the fluid. According to calculated activity diagrams, alteration at these temperatures may result in the formation of illite, kaolinite, Ca-beidellite, and stilbite. In experiments at higher temperatures of 200 °C, according to reservoir conditions, the net output of most elements was increased, but especially of those contained in feldspar, i.e. K, Ca, Al, Si, Ba, Rb, Cs, and Pb, suggesting that fluid-rock interaction is dominated by feldspar dissolution. This alteration is followed by albitization of feldspar s