Red-staining of the wall rock and its influence on the reducing capacity around water conducting fractures

Red-staining and alteration of wall rock is common around water conducting fractures in the Laxemar–Simpevarp area (SE Sweden), which is currently being investigated by the Swedish Nuclear Fuel and Waste Management Co. (SKB) in common with many other places. Red-staining is often interpreted as a clear sign of oxidation but relevant analyses are seldom performed. The area is dominated by Palaeoproterozoic crystalline rocks ranging in composition from quartz monzodiorite to granite. In this study wall rock samples have been compared with reference samples from within 0.1 to 1 m of the red-stained rock, in order to describe mineralogical and geochemical changes but also changes in redox conditions. A methodology for tracing changes in mineralogy, mineral and whole rock chemistry and Fe³⁺/Fe_tot ratio in silicates and oxides in the red-stained wall rock and the reference rock is reported. The results show that the red-stained rock adjacent to the fractures displays major changes in mineralogy; biotite, plagioclase and magnetite have been altered and chlorite, K-feldspar, albite, sericite, prehnite, epidote and hematite have been formed. The changes in chemistry are however moderate; K-enrichment, Ca-depletion and constant Fe_tot are documented. The Fe³⁺/Fe_tot ratio in the oxide phase is higher in the red-stained samples whereas the Fe³⁺/Fe_tot ratio in the silicate phase is largely similar in the wall rock and the reference samples. Because most of the Fe is hosted in the silicate phase the decrease in reducing capacity (Fe²⁺), if any, in the red-stained wall rock is very small and not as high as macroscopic observations might suggest. Instead, the mineralogical changes in combination with the modest oxidation and formation of minute hematite grains in porous secondary minerals in pseudomorphs after plagioclase have produced the red-staining. Increased porosity is also characteristic for the red-stained rock. Moderate alteration in the macroscopically fresh reference rock shows that the hydrothermal alteration reaches further from the fracture than the red-staining. The extent of the red-staining can therefore not be used in the same way as the extent of the alteration adjacent to a fracture. The increase in porosity in the red-stained rock may result in enhanced retention of radio-nuclides due to an increased sorptivity and diffusion close to the fracture.