Quantitative mapping and statistical evaluation of fracture minerals in the granitic bedrock at Forsmark, Sweden

Abstract

This article provides quantitative data on occurrences and amounts of fracture minerals that coat discrete fractures in granitic rock at the Forsmark site in Sweden. The data are useful for retardation modelling of radionuclide and other contaminants, and for groundwater composition calculations. In a unique campaign, 2071 open fractures in groundwater conducting rock have been mapped with respect to chlorite, calcite, and pyrite. In total 767 m of drill core has been studied from very shallow rock down to~1000 m depth. The occurrences of fracture minerals, their thicknesses, and their fractions of surface coverage have been recorded for up to eight layers for each fracture. Detection limits are, for each layer, 0.1 mm for the thickness and 1 % for the surface coverage, except for pyrite crystals where surface coverages down to 0.01 % are detectable. The abundance of data has permitted statistical treatment, using parametric and non-parametric methods. Parametric fittings have been made to log-normal, truncated log-normal, and beta distributions. Chlorite, calcite, and pyrite were found in 57 %, 52 %, and 10 % of all mapped fractures, respectively. The fracture mineral thickness was 0.1 mm for calcite, 0.2 mm for chlorite, and 2 μm for pyrite, as averaged over the fracture surface area. For 50 % and 99 % of all fractures the total fracture coating thickness was less than 0.1 mm and 1 mm, respectively, which is important for diffusion resistance estimates. Average surface coverages were 18 % for calcite, 38 % for chlorite, and 0.5 % for pyrite. These data may be used for calculating the reaction capacity of flow paths.

Present‐Day Stress Field Influences Bedrock Fracture Openness Deep Into the Subsurface

“…This observation raises the possibility that many of these fractures are inherited from prior deformation, affected by geologic heterogeneity (e.g., lithologic variation), or influenced by sources of stress we have not accounted for in our model. Many of the fractures contain secondary minerals, precipitated from hydrothermal fluids under antecedent conditions, but some fractures coated or filled by calcite, clay minerals, or Fe oxides indicate low‐temperature water‐rock interaction, possibly also during the Quaternary (Drake et al, 2009; Löfgren & Sidborn, 2016; Sandström & Tullborg, 2009). The good correspondence between failure proxies and F open indicates that the present‐day stress field contributes to reactivating and opening these existing fractures and allowing water‐rock interaction.…”

“…Our stress model includes simplifying assumptions about material properties (e.g., linear elasticity, homogeneity, and isotropy) and ambient regional stress (e.g., spatially uniform regional horizontal stresses within rock masses separated by the Singö deformation zone). The bedrock at Forsmark has variable lithology, multiple deformation zones, multiple generations of fracture fillings and coatings, and a time‐varying stress history that spans 1.89 Ga and most recently includes Quaternary glaciations and deglaciations (Drake et al, 2009; Hall et al, 2019; Löfgren & Sidborn, 2016; Lönnqvist & Hökmark, 2013; Martin, 2007; Stephens et al, 2015). Previous studies have indicated that increased pore pressure associated with glacially induced hydraulic jacking may open subhorizontal fractures with large apertures (>1 cm) as deep as 200 m (Lönnqvist & Hökmark, 2013) and may influence groundwater geochemistry and support microbial life as deep as 500 m (Tullborg et al, 2017).…”

Hydrologic issues associated with nuclear waste repositories

“…Secondary minerals may coat the fracture surface and other precipitates; the original rock matrix near the surface may be mechanically disturbed when the fracture formed and may be also chemically altered in various ways. These alterations seldom extend more than a few millimeters, but on occasion can extend several centimeters in granitoid rocks [ Löfgren and Sidborn , ]. The alterations are practically always as porous as, or more porous than the rock matrix at larger distances from the fracture face.…”

A Combined X-ray Absorption and Mössbauer Spectroscopy Study on Fe Valence and Secondary Mineralogy in Granitoid Fracture Networks: Implications for Geological Disposal of Spent Nuclear Fuels

“…At the two study sites (Laxemar and Forsmark), which are located on the east coast of Sweden, the bedrock is composed of Paleoproterozoic crystalline bedrock (Figure S1) that comprise a complex network of fractures. , The frequency of open fractures is generally higher in Laxemar than in Forsmark, higher in the upper part of the bedrock (0–100 m) than in the deeper part, and in Laxemar higher in the upper 10–15 m (nearly 3 open fractures per meter) than in the deeper counterparts (∼1–1.5/m) . The latter feature is due to stress relaxation in Holocene caused by unloading of the Weichselian continental ice sheet.…”