Rock types − properties, alteration, volumetric proportions and thickness of the subordinate rock amphibolite

5.2.3 Rock types − properties, alteration, volumetric proportions and

Anomalously high uranium contents and natural exposure rates are restricted to the Group D rocks (Table 5-2), i.e. pegmatitic granite and pegmatite (101061) and fine- to medium granite (111058).

This observation supports the correlation between these rock types and an increased natural gamma radiation that has been observed in the integrated single-hole interpretation work. These observations need to be considered when the source of anomalies in the uranium content of fracture coatings and groundwater are assessed. However, Cambrian to Lower Ordovician oil shale, which covered the Precambrian crystalline rocks at Forsmark during the earlier part of the Phanerozoic (see section 3.1), and which is known to be uranium-rich where preserved in fault-controlled outliers in southern Sweden /Andersson et al. 1985/, is also a potential ultimate source of uranium. It should be noted that the natural exposure rate in pegmatitic granite and pegmatite (101061) varies much less than the uranium content because of the contribution of potassium and thorium to the natural exposure rate.

Alteration

The type and degree of bedrock alteration, within and outside possible deformation zones, were presented on a borehole by borehole basis in Appendix 7 in /Stephens et al. 2007/. The red-staining of fracture minerals or the walls to fractures (Figure 5-6a) is by far the most abundant type of alteration at Forsmark, and is mapped and referred to as oxidation. The major mineralogical changes in such altered rock are a fine dissemination of hematite inside and along boundaries of plagioclase grains, an almost complete saussuritisation of plagioclase and a chloritisation of biotite /Sandström Figure 5‑5. QAP (F = 0) modal classification of all the analysed intrusive rock samples at the Forsmark site (Groups B, C and D). The classification is based on /Streckeisen 1976/. Groups B, C and D are defined in Table 5-1. Nearly 70% of the borehole samples shown on this diagram come from the local model volume. By contrast, there is no such focus for the surface samples; over 75% of these samples lie outside the local model volume. Notwithstanding this discrepancy, the trends are identical in both sample sets.

100 90

90

90

80

80

80 70

70

70 60

60

60 50

50

50 40

40

40 30

30

30 20

20

20 10

10

10 100

Alkali

feldspar (A) Plagioclase

feldspar (P) Quartz (Q)

Tonalite Grano-

diorite Granite

(syeno- granite)

Quartz Quartz syenite

Syenite Monzonite monzonite (monzo- granite)

Quartz monzodiorite Quartz monzogabbro

Monzodiorite

Monzogabbro Quartz diorite Quartz gabbro Quartz anorthosite

Diorite Gabbro Anorthosite

Group D

Group C Borehole sample Group B

Surface sample Field that contains altered Group B rocks

In particularly the north-eastern part of the target volume, the Group B granitic rocks are affected by an alteration that has been mapped and referred to as albitization. On a mesoscopic scale, this altera-tion is recognised by a whitening of the feldspar in the granitic rocks (Figure 5-6b). Compared with the unaltered equivalents, it is characterised by a deficiency of K-feldspar relative to plagioclase and an increased content of quartz (Figure 5-5 and Table 5-2). Plagioclase is generally more albite-rich than in the unaltered equivalent, but is still not pure albite /Petersson et al. 2005/. The natural exposure rate is lower in these altered granitic rocks relative to that in the fresh equivalents (Table 5-2). This reduction is related to a reduced content of potassium, which is coupled to the removal of K-feldspar during alteration. Since the alteration shows no simple relationship to fracture zones, is absent in some of the younger rocks and occurs as contact rims adjacent to amphibolite (Figure 5-6c), it is inferred to be a syn-magmatic or syn-metamorphic feature, triggered by the heat supply provided by the younger amphibolites and even Group C metagranitoids /Stephens et al. 2007/.

Vuggy rock, which formed by the selective dissolution of quartz, corresponds to the alteration more generally referred to in the literature as episyenitisation /Möller et al. 2003/. In the alteration zones, the quartz dissolution, often combined with strong oxidation, affected all rock types indiscriminately (Figure 5-6d), but the texture of the host rock was maintained. Individual occurrences are typically a few metres in borehole length, although one occurrence along borehole KFM02A is approximately 50 m /Möller et al. 2003/. Virtually all occurrences occur inside or immediately adjacent to fracture zones /Stephens et al. 2007/. The conspicuous occurrence in KFM02A has been modelled /Stephens et al. 2007/ as a narrow, steeply inclined alteration pipe that links two gently dipping fracture zones (zones ZFMA2 and ZFMA3). It is inferred that this alteration took place after the rock mass had entered the brittle regime, i.e. after 1.8−1.7 Ga (see section 3.1), but under the influence of hot hydrothermal fluids at temperatures corresponding to greenschist facies conditions. It gave rise to significant changes in the physical properties of the host rock, including decreased density, porosity and resistivity.

Figure 5‑6. Principal types of hydrothermal alteration at Forsmark. a) Fine dissemination of hematite and red staining in the altered wall rock adjacent to a fracture filled with quartz at surface excavation AFM001243. b) Albitized and metamorphosed granite in borehole KFM06A in the interval c. 906–917 m.

c) Albitized and metamorphosed granite along the contact to an amphibolite dyke in a tectonically banded sequence at Klubbudden (see Figure 5-2). d) Strongly altered and vuggy metagranite in borehole KFM02A in the interval c. 282–293 m. The incoherent section (in plastic casing) is a strongly altered amphibolite that has been modified to a rock composed of chlorite, albite, hematite, Ti-oxide and quartz.

c d

a b

Albitized

Volumetric proportions

Quantitative estimates (volume %) of the proportions of different rock types from close to the surface down, in some cases, to c. 1,000 m depth have been calculated on a borehole by borehole basis and presented in the form of histograms and summary tables in successive model versions and stages /SKB 2005a, 2006a, Appendix 4 in Stephens et al. 2007/. Only the data from cored boreholes, which have a varied bearing and inclination, have been used. However, no consideration of the orientation of rock contacts and, thereby, the true thickness of rock intersections has been carried out in this analysis.

On the basis of the pre-conditions that the borehole is greater than 200 m in length and that it is situated entirely in both the local model volume and the Forsmark tectonic lens, estimates of the proportions of different rock types have been determined for the volume that has been selected as a potential repository. Metagranite (101057 or 101058) with a range between 68 and 83%, a mean value of 75% and a standard deviation of 5%, dominates inside this volume (Table 5-3). Along three of these cored boreholes (KFM06A, KFM08C and KFM08D) as well as borehole KFM06C, which are all in the north-eastern part of the target volume, these granitic rocks are affected, in part, by the type of alteration referred to as albitization (see above). The volumetric proportions of the subordinate rock types in this volume are also provided in Table 5-3.

Thickness of amphibolite

Although the subordinate rock amphibolite is clearly affected by ductile deformation and is, by definition, metamorphic in character, this rock is inferred to have intruded originally as dykes.

Amphibolite occurs as narrow, dyke-like tabular bodies and irregular inclusions that are elongate in the direction of the mineral stretching lineation (see also section 5.2.4). Due to the low content or absence of quartz in this rock type, it requires special treatment in the thermal modelling work (see chapter 6).

On the basis of the borehole length and the orientation of the contacts of amphibolites in twenty-one cored boreholes, estimates of true thickness have been calculated (Appendix 5 in /Stephens et al.

2007/). Although some bodies are more than a few metres in thickness and, locally (e.g. KFM06C, KFM08D), are some tens of metres thick, most are inferred to be minor rock occurrences, i.e. thin geological entities. The thicker bodies in boreholes KFM06C and KFM08D occur in different parts of the fine-grained and partly albitized granitic rocks in the north-eastern part of the target volume. An amphibolite with similar thickness is also exposed at the ground surface in this area. Stochastic simula-tions of the subordinate rocks in the target volume, including amphibolite, are presented in chapter 6.

5.2.4 Ductile deformation