Page 178 - Microtectonics

6.2 · Veins 167

Fig. 6.9. Detail from an antitaxial shear vein of quartz and calcite (only quartz visible). The contact with the wall rock is parallel to the
long axis of the photograph. Bands of solid inclusions trend from top left to bottom right; each band probably represents a separate
crack-seal event. Jogs in the bands are thought to indicate the opening direction of the vein (horizontal). Elongate quartz crystals are
oblique to both the solid inclusion bands and the opening direction of the vein, and trend from bottom left to top right. Orobic Alps, Italy.
Width of view 5 mm. CPL

may or may not track the opening direction of a vein as
discussed in Sect. 6.2.1.
The nature of the interaction of fluids and growing
fibres or elongate crystals in veins is still unclear (Bons
2000; Oliver and Bons 2001; Means and Li 2001; Hilgers
and Urai 2002). In some cases, the crystals may grow from
a fluid migrating through a central channel (syntaxial
veins) or along the vein-wall contacts (antitaxial and
unitaxial veins), but lack of gradients in composition of
vein crystals along the length of the vein makes this un-
likely in many cases, especially for fibres in antitaxial
veins. Antitaxial vein fibres are usually strictly symmet-
ric around the median line, something not to be expected
in the case of repeated cracking and sealing of vein con-
tacts (P. Bons, pers. comm.). Such veins also normally lack
inclusion bands. Growth from a fluid introduced through
the porous wall rock may therefore be responsible, al-
though the exact mechanism of precipitation is unclear
Fig. 6.10. Various types of solid and fluid inclusions in a fibrous (Means and Li 2001). Such veins could develop without a
vein. None of the inclusions is necessarily parallel to crystal through-going open crack along the vein-wall, the walls
boundaries in the vein. Surfaces of fluid inclusions are usually par-
allel to the vein wall, but solid inclusions may be both parallel to being pushed apart by the growing fibres. This process
the opening direction of the vein (inclusion trails) or in planes is known as Taber growth and the resulting veins as Taber
parallel to the vein wall (inclusion bands) veins (Taber 1916, 1918; Means and Li 2001).