Page 174 - Microtectonics

6.2 · Veins 163
6.2 6.2
Veins

6.2.1
Crystals in Veins

Many dilatation sites such as veins or strain shadows con-
tain parallel oriented elongate crystals (Williams 1972b;
Durney and Ramsay 1973; Ramsay 1980b; Machel 1985;
Passchier 1982a; Bons 2000). This concerns naturally
Fig. 6.4. Three types of common crystal shapes in veins. Only fi-
elongate minerals such as asbestos, actinolite or mica but bres follow the opening trajectory of a vein that is indicated in the
more commonly minerals which do not normally have crystals as a dotted trail of solid inclusions
an elongate shape such as quartz, calcite, dolomite, feld-
spar, gypsum and anhydrite (Williams 1972b; Durney and and the veins as syntaxial fibre veins (Ramsay and Huber
Ramsay 1973; Ramsay 1980b; Machel 1985; Passchier 1982a). 1983) or syntaxial veins (Fig. 6.5a, ×Video 6.5ab). Syn-
The elongate habit of minerals in such veins appears to taxial veins are commonly asymmetric, i.e. with an off-
be due to a special growth mechanism (Figs. 6.4, 6.5). centred median line, and in extreme cases grow from one
Imagine a crack that forms in a crystalline aggregate of side of the vein only. Such unitaxial veins (Fisher and
quartz or calcite. Immediately on opening, new crystal- Bryne 1990; Köhn and Passchier 2000; Bons 2000; Hilgers
line material can be deposited on the existing crystals from et al. 2001; Oliver and Bons 2001) lack a median line
solution in the fluid. If neighbouring crystals grow at simi- (Fig. 6.5a).
lar rates, the overgrowth obtains an elongate shape; crys- Some veins are filled by growth of a mineral that is
tals that grow slowly because they have an unsuitable crys- not the main constituent of the wall rock, e.g. a calcite
tallographic orientation will become thin and end while vein in quartzite. In such cases, the growth usually oc-
1
their neighbours make contact . A process of growth com- curs along the contact of elongate grains or fibres and
petition can lead to aggregates of few equidimensional the wall rock, i.e. on both sides of the material in the vein
grains, but if the growth competition is suppressed, many (Fig. 6.5b). A weak median line defined by small equidi-
elongate grains will result. A gradient in shape can be en- mensional grains or fragments of the wall rock is nor-
visaged between fibres, elongate grains and equidimen- mally present in the centre of the fibrous aggregate, in-
sional or blocky grains (Fisher and Brantley 1992; Bons dicating the initial nucleation site of the vein filling. This
and Jessell 1997; Hilgers and Urai 2002; Fig. 6.4). Fibres type of growth towards the wall rock is termed antitaxial
are extremely elongate rod- or hose-shaped grains with growth, and the veins are called antitaxial fibre veins
parallel boundaries, a width of 10–350 µm and a length (Ramsay and Huber 1983) or antitaxial veins (Figs. 6.2,
width ratio up to 100. Elongate grains have a smaller 6.5b, ×Video 6.5ab). Material in antitaxial veins com-
length-width ratio and are tapering (Fig. 6.4). monly consists of fibres rather than elongate crystals, and
veins are commonly symmetric. Single fibres in antitaxial
6.2.2 veins can be continuous over the median line, in con-
The internal Structure of Veins trast to elongate grains or fibres in syntaxial veins. Com-
posite veins, in which an antitaxial vein segment is sand-
In the case of an isolated crack, fibrous growth can oc- wiched between syntaxial vein rims are also possible
cur into the widening void from both sides at the same (Fig. 6.5c, ×Video 6.5cd). Such a vein has three ‘median’
rate. This process can be referred to as bitaxial growth lines. In fact, most fibrous antitaxial calcite veins inves-
(Bons 2000; Hilgers et al. 2001). The result is a symmetric tigated to date have thin quartz-chlorite selvages along
vein with a central plane where deposition of new mate- the vein wall contact and could therefore be classified as
rial takes place. In thin section this plane appears as a composite veins (Hilgers and Urai 2002).
clear line known as the median line (Fig. 6.2). The me- Veins can be filled during a single event of gradual
dian line is often marked by small opaque grains and a growth of crystals into a fluid-filled cavity, (either in a
discontinuity in the elongate crystal fabric. Growth of vein opened by a single event, or one that is opening faster
elongate grains or fibres as described here from the vein than the crystal growth rate) or by periodic opening and
wall towards the vein centre is known as syntaxial growth, filling of a narrow cavity, here referred to as periodic
growth. If periodic growth happens by periodic sealing
and fracturing, the process is known as crack-seal growth
1 We have simplified most drawings in this section by omitting this (Ramsay 1980b). If growth, either periodic or continu-
tapering effect and imagine all fibres to grow at the same rate. ous, always occurs at the same site in the vein, e.g. the