Rock units and possible deformation zones in the sub‑surface realm Geological, geophysical and radar logs

Table 5‑1. Major groups of rocks and rock units at Forsmark based on /Stephens et al. 2005/. SKB rock codes that distinguish different rock types within a rock unit are shown in brackets. The alteration code 104 for albitization is also included.

Rock groups Rock units on the bedrock geological map

All rocks are affected by brittle deformation. The fractures generally cut the boundaries between the different rock types. The boundaries are predominantly not fractured.

Rocks in Group D are affected only partly by ductile deformation and metamorphism.

Group D • Fine- to medium-grained granite and aplite (111058). Pegmatitic granite and pegmatite (101061).

Age partly 1.85 Ga.

Variable age relationships with respect to Group C. Occur as dykes and minor bodies that are commonly discordant and, locally, strongly discordant to ductile deformation in older rocks.

Rocks in Group C are affected by penetrative ductile deformation under lower amphibolite-facies metamorphic conditions.

Group C • Fine- to medium-grained granodiorite, tonalite and subordinate granite (101051).

Age 1.86 Ga.

Occur as lenses and dykes in Groups A and B. Intruded after some ductile deformation in the rocks belonging to Groups A and B with weakly discordant contacts to ductile deformation in these older rocks.

Rocks in Groups A and B are affected by penetrative ductile deformation under amphibolite-facies metamorphic conditions.

Group B • Biotite-bearing granite (to granodiorite) (101057) and aplitic granite (101058), both with amphibolite (102017) as dykes and irregular inclusions. Local albitization (104) of granitic rocks. Age 1.87 to 1.86 Ga.

• Tonalite to granodiorite (101054) with amphibolite (102017) enclaves. Granodiorite (101056).

Age 1.88 Ga.

• Ultramafic rock (101004). Gabbro, diorite and quartz diorite (101033). Age 1.89 Ga.

Group A • Sulphide mineralisation, possibly epigenetic (109010).

• Volcanic rock (103076), calc-silicate rock (108019) and iron oxide mineralisation (109014).

Subordinate sedimentary rocks (106001). Age 1.89 Ga or older.

Metamorphosed, medium-grained granite (to granodiorite), dated to 1.87 Ga, is dominant inside the target area. It is suggested that the occasional granodioritic composition is caused by an incipient stage of the alteration referred to as albitization (see section 5.2.3) and that the rock was initially granitic in composition throughout the target area. In the north-eastern part of this area, close to Asphällsfjärden and Klubbudden (Figure 5-2), fine-grained granitic rocks that, in part, also show alteration referred to as albitization are conspicuous. Subordinate rock types throughout the target area are pegmatitic granite/pegmatite, amphibolite, metamorphosed fine- to medium-grained granitoid, and fine- to medium-grained granite that shows little effect of metamorphism. The subordinate rocks occur as dyke-like bodies and small irregular intrusions. They range in age from 1.87 to 1.85 Ga.

Exploration potential

An assessment of the potential of the Forsmark area for exploration after metallic and industrial min-eral deposits has been presented in /Lindroos et al. 2004/. A potential for iron oxide minmin-eralisation and possibly base metals was recognised to the south-west of the candidate area, predominantly in the felsic to intermediate metavolcanic rocks (Figure 5-2). However, the small iron mineralisations in the Forsmark area have no economic value and this judgement was also deemed to be valid in a long-term perspective /Lindroos et al. 2004/.

5.2.2 Rock units and possible deformation zones in the sub‑surface realm

the oriented images of the borehole provided by the Borehole Image Processing System (BIPS).

The terminology and procedures used in the acquisition of fracture data are described in /SKB 2005a, p. 194/. Significant changes in the documentation of data relevant to fractures, including aperture size and the recognition of sealed fracture networks, occurred after the mapping of the earliest boreholes KFM01A, KFM02A, KFM03A and KFM03B /Olofsson et al. 2007, p. 14/.

Since no routine was developed to measure linear structural features in a systematic manner in the boreholes, ductile linear fabric data from depth are lacking. Shear striae along fault planes in deformation zones (see section 5.2.6) were measured with the help of fracture orientation data from Boremap and a drill core holder, which allowed the drill core to be positioned in a correct manner in 3D space. The uncertainties in borehole orientation data, which involve uncertainties in both the orientation of boreholes and BIPS images, have been evaluated in /Munier and Stigsson 2007/. They are addressed briefly in section 2.3.

Standard geophysical and radar logs complement the oriented image logs generated along each borehole with the help of BIPS. A summary of the data that have been acquired in the geophysical logging work is presented in /SKB 2005a, p. 192–194/. A combination of some of the geophysical data (e.g. density, natural gamma radiation) with relevant petrophysical data (see section 5.2.3) provides support to the mapping of the bedrock in the boreholes, especially in the percussion boreholes, where drill core is absent. A special study that evaluated the geological interpretation of radar anomalies along possible deformation zones /Carlsten 2007/ showed that this geophysical tool is able to detect rock contacts more efficiently than it does broken fractures, crush rock or breccias.

Furthermore, the method is not effective for the detection of unbroken fractures, sealed fracture networks, alteration and ductile structures. A careful assessment of the geological character under consideration needs to be made before any conclusions are drawn from these data that concern the orientation of a deformation zone.

The geological mapping of the boreholes and the geophysical and radar logging programmes gener-ate primary sub-surface data that bear on the geological features; rock type, rock alteration, ductile deformation and brittle deformation. Ninety data reports have been produced (see chapter 2 and Table 1 in Appendix 3). These programmes have provided the input to the single-hole interpretations.

Single-hole interpretation − rock units and possible deformation zones

The single-hole interpretation of a borehole provides an integrated synthesis of the geological and geophysical information from the borehole. The results of this synthesis are presented in nineteen data reports, all produced by the same working team. A description of the procedures adopted during the various stages of the single-hole interpretation work is provided in section 3.3 in /Stephens et al.

2007/.

During the first stage of the single-hole interpretation, rock units and possible deformation zones in each borehole were identified and described. Furthermore, the interpretation of each geological feature was assigned a level of confidence. Key data, which are predominantly geological in character, and the results of the single-hole interpretation have been presented in the form of WellCad diagrams for each of the twenty-one cored boreholes addressed in stage 2.2 (Appendix 3 in /Stephens et al. 2007/) and for the remaining four cored boreholes in /Stephens et al. 2008/.

One example is presented in Figure 5-3. The second stage of the single-hole interpretation work has involved a more detailed description of the characteristics of the possible deformation zones that were recognised with high confidence. Adjustments of boundaries to rock units and possible deformation zones during the modelling work, as well as the identification of possible minor zones in connection with a data reappraisal at a higher level of resolution are both documented in Table 3-2 in /Stephens et al. 2007/.

Rock units (RU) have been defined primarily on the basis of the composition, grain size and the inferred relative age of the dominant rock type, i.e. in the same manner as that used for the surface

Figure 5‑3. WellCad diagram for the cored borehole KFM08A, showing a suite of key base geological and geophysical data that have been used to identify rock units and possible deformation zones in the single-hole interpretation of boresingle-holes (see also /Stephens et al. 2007/).

Pegmatite, pegmatitic granite

Granite, granodiorite and tonalite, metamorphic, fine- to medium-grained Granite, metamorphic, aplitic

Granite to granodiorite, metamorphic, medium-grained Amphibolite

Felsic to intermediate volcanic rock, metamorphic

High confidence Medium confidence High confidence

ROCK UNIT DEFORMATION ZONE

ROCK ALTERATION Albitization Quartz dissolution Oxidized Coordinate System RT90-RHB70 Length [m] 1001.190

Title GEOLOGY KFM08A

Diameter [mm] 77

Elevation [m.a.s.l.ToC] 2.49

Borehole KFM08A

Inclination [°] -60.84

Site FORSMARK

ROCK TYPE FORSMARK

Northing [m] 6700494.49

Easting [m] 1631197.06

Date of mapping 2005-05-11 10:43:00 Drilling Stop Date 2005-03-31 10:40:00 Plot Date 2007-03-04 22:14:36 Bearing [°] 321.00 Drilling Start Date 2004-09-13 11:03:00

AlterationRock LengthBH

(m.f.ToC) Elevation

(m.b.s.l.) Rock Type

(> 1m)

OccurrenceRock (< 1m)

RockUnit (SHI)

Sealed Network (fract/1m)

0 150

Sealed Fractures (fract/1m)

0 20

OpenCrush (fract/1m)

0 10

Open + Partly Open Fract (fract/1m)

0 20

Fracture Width (mm)

0 30

Fracture Aperture (mm)

0 20

Resistivity Foc 300 cm

(ohm*m) 1100000

Deformation Zone(SHI)

0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900

950 1000

0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750

RU1

RU2

RU3 RU4

DZ1

DZ2 DZ3

DZ4

DZ5

DZ6 DZ7

Fifteen cored boreholes intersect, along their entire length, the bedrock that is typical of that mapped inside the tectonic lens at the surface (Figure 5-2). Furthermore, the long boreholes KFM01A, KFM02A and KFM03A confirm that the rock unit dominated by medium-grained metagranite (101057) at the surface (Figure 5-2) continues to at least 1,000 m depth. Four boreholes (KFM06A, KFM06C, KFM08C and KFM08D) in the north-eastern part of the target volume (Figure 5-2) contain significant proportions of metagranite that is also affected by the alteration referred to as albitization (101057_104 or 101058_104). The upper part of several boreholes inside the target volume contains an increased frequency of fractures with conspicuous apertures (Figure 5-4a), even though these rock units are situated predominantly outside possible deformation zones.

Five cored boreholes (KFM04A, KFM06C, KFM07A, KFM08A and KFM09A) provide constraints on the continuation at depth of the rock units that are marginal to the metagranite inside the target volume (Figure 5-2). For example, the fine-grained metagranite and metavolcanic rocks that occur at the surface between the nuclear power plant and SFR are encountered at c. 615−710 m depth along borehole KFM08A (Figure 5-3). Two boreholes (KFM11A and KFM12A) also confirm how rock units that were recognised during the geological mapping outside the tectonic lens can be followed a considerable distance at depth as steeply dipping, geological entities.

The possible deformation zones (DZ) identified inside the target volume are solely brittle in character, i.e. they are possible fracture zones. These zones have been defined in the single-hole interpretation work primarily with the help of the geological and geophysical data sets fracture frequency, rock alteration and focused resistivity. Other features, which have assisted in their identification, include the occurrence of low radar amplitude anomalies in the borehole radar data, low magnetic susceptibility and the occurrence of caliper anomalies.

Sealed fractures and sealed fracture networks, the presence of well-defined steeply and gently dipping fracture sets, and an alteration that involved red-staining of fracture minerals or fracture walls dominate the possible fracture zones inside the target volume (see, for example, Figure 5-4b).

Relative to the bedrock outside possible deformation zones, open and partly open fractures also increase in occurrence along these zones (see, for example, Figure 5-3). By contrast, possible deformation zones in the four cored boreholes KFM02A, KFM02B, KFM03A and KFM03B, to the south-east of the target volume (Figure 5-2), contain a conspicuous occurrence of open and partly open fractures that dip gently. Brittle deformation and fracture mineralogy along cored boreholes are addressed in more detail in section 5.2.5, and the character and kinematics of deformation zones are presented in section 5.2.6.

Figure 5‑4. a) High frequency of sub-horizontal to gently dipping fractures and some hematite staining in the borehole interval c. 182−193 m along RU1 in KFM01A. The fractures intersect this sub-vertical borehole at a high angle. Virtually all fractures are interpreted as open. This section lies outside a possible

b a

5.2.3 Rock types − properties, alteration, volumetric proportions and