ALTERATION OF GRANITE AND MONZONITE FROM THE NORTHERN BLACK FOREST, GERMANY – EVIDENCE FROM HYDROTHERMAL EXPERIMENTS AT 70°C AND 200°C

Abstract

In order to better understand their alteration processes, granites and monzonites from the northern Black Forest have been reacted as rock powder with bi-distilled water at temperatures of 70°C and additionally as cylindrical rock samples with a 2-molal NaCl-H2O solution at 200 °C in autoclaves for 36 days. Rock powder after the 70°C experiments was not investigated but the resulting leaching fluids of the low temperature experiments showed enhanced concentrations of K, Na, Ca and other parameters, with higher TDS being recorded by the monzonites. Thus, even by low temperature experiments alteration reactions could be observed. The main minerals affected by dissolution during the high temperature experiments are plagioclase and K-feldspar, quartz, biotite and chlorite, irrespective of the rock type. In the marginal zones of the rock samples, K-feldpar is partly replaced by albite, whereas biotite reacted to chlorite and hematite. Precipitates on the sample surfaces include illite, hematite, and analcime. The solutions after the high temperature experiments showed enhanced concentrations of K, Ca, Si and other parameters, not present at the beginning of the experiments and are thus originating from alteration and partial dissolution of plagioclase, K-feldspar, quartz, biotite, and chlorite. The solutions were partially saturated, partially undersaturated with respect to quartz. Certain elements (e.g., Na, Fe, Al) were subsequently incorporated into the solids precipitated during the experiments or adsorbed onto mineral surfaces and cannot be considered as conservative. The alteration process is simulated by using stability diagrams.