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CET/UWA Microstructure Course

Veins

See and Bons 2000

- Terms relating to the shape of crystals in veins
 

Fibrous: High to extreme length/width ratio of grains (>10 ... >100)  Fibrous shape not determined by crystal habit  Fibrous shape independent of crystallographic orientation of grains  Shape of all grains identical  All fibres parallel  No nucleation during growth          (Antitaxial calcite vein in carbonaceous shales, Arkaroola, South Australia)

 

NB. A fibrous texture can be formed by fibrous sub-grains or twins, while the true grains may not really be fibrous. Here you see a section perpendicular to the fibres in an antitaxial vein. One grain covers most of the image. The fibres are much smaller and are defined by sub-grain and possibly twin-boundaries.     (Antitaxial calcite vein in carbonaceous shales, Arkaroola, South Australia)

Elongate blocky: Low to high length/width ratio of grains (<10) Elongate shape not determined by crystal habit  Fibrous shape often related to crystallographic orientation of grains  Not all grains have identical shape  Long axes of grains in approximately same direction No nucleation during growth (Syntaxial/asymmetric quartz vein from Cape Liptrap, Victoria, Australia)

Blocky:   None of the specific characteristics of fibrous or elongate blocky textures, in particular: Often continuous nucleation No elongate shape and/or shape preferred orientation of crystals (Calcite vein in carbonaceous shales, Arkaroola, South Australia)

Stretched: Elongate crystals  Parts of pre-existing grain at both ends of crystals Often "radiator" structures and/or jogs on grain boundaries   (Calcite vein in carbonaceous silt stone and shales, Arkaroola, South Australia)

Slicken-fibres: Fibrous or elongate blocky crystals  Long axis of crystals at low angle or parallel to vein wall   (Calcite vein in carbonaceous shales, Arkaroola, South Australia)

- Terms relating to site(s) of precipitation during growth of a vein
 

Syntaxial veins:

• Growth persistently occurs at the same plane
• Growth occurs in the middle of the vein
• Growth often commences by syntaxial (crystallographic sense) overgrowth of wall rock grains
• Often elongate blocky
• Oldest precipitate is on vein - wall rock contact; youngest on median plane
• Individual crystals do not extend across median plane

(Asymmetric antitaxial quartz vein from BIFs in Hammersley Ranges, W.Australia)

Antitaxial veins:

• Growth persistently occurs at the same planes
• Growth occurs on the two vein - wall rock contact surfaces
• Mineral(s) growing in the vein are often absent in, or a minor constituent of the wall rock
• Often fibrous
• Youngest precipitate is on vein - wall rock contact; oldest on median plane
• Individual crystals extend across median plane

Ataxial or stretching veins:

• Growth occurs at various sites (cracks) over time
• Mineral(s) growing in the vein are often major constituent of the wall rock
• Stretched crystals
• -> No consistent variation from young to old precipitate
• -> No median plane
• -> Individual crystals extend from wall to wall of the vein

Asymmetric veins:

• Growth commences by ataxial growth (usually, but can be syn/antitaxial)
• One side of the vein becomes preferred growth plane
• Often elongate blocky
• -> Veins are asymmetric

Replacement veins:

• Vein precipitate is not in newly created space, but replaces pre-existing minerals.
• Usually vague edges
• Inclusions of pre-existing grains often remain
• Crystal shape often blocky or determined in shape and size by pre-existing texture

Composite veins: Combination of syntaxial growth on both margins of vein and syntaxial growth in centre of vein.  Usually different minerals forming syntaxial and antitaxial parts (Calcite + quartz vein in carbonaceous shales from Arkaroola, South Australia)

(Stylolite in calcite vein in carbonaceous shales from Arkaroola, South Australia)

Stylolites:

Stylolites are in a way the opposite of veins (hence the term 'anti-crack' which is some used).

• Continuous / repetative dissolution on a plane
• Accumulation of insoluble material (often dark/opaque) on stylolite
• Saw-teeth shape develops, due to differences in dissolution rate on either side of stylolite
• Saw-teeth indicate the direction of shortening

- Tracking of opening trajectory

The opening trajectory is the path that two, originally adjacent, points on the opposite vein walls travelled relatively to each other as the vein grew. Fibres & elongate blocky crystals often track the opening trajectory, but not always completely (=partial tracking).

Ghost fibres may some give a more reliable indication of the opening trajectory than normal fibres. Ghost fibres are trails of a different mineral growing off a specific point (grain) on the wall rock.

- Veins and structural analysis

The wide variety of internal structures of veins, vein shapes and vein arrangements make veins useful structures for structural analysis. Quite often the elongate blocky or fibrous crystals in a vein allow us to determine the whole history of the formation of a vein, giving insight in the deformation history of its host rock. The micro-structures should of course be correctly interpreted (syntaxial or antitatxial, partial or complete tracking, etc.).

Veins also often form in arrays. The left image shows a group of veins that originally formed at a small angle with the horizontal. Interaction between the veins caused them to merge into one horizontal vein with wall rock inclusions. The right image shows a set of sigmoidal veins. The veins did not form all at the same time and are in different stages of development (see film below). Sigmoidal vein arrays are often useful kinematic indicators.

 

- Sigmoidal veins Opening (widening) of a vein is in maximum instantaneous stretching direction opening trajectory records kinematics of deformation Propagation of vein-tips parallel to maximum instantaneous shortening direction  Opening of vein parallel to maximum instantaneous extension direction  Vein rotates when deformation is non-coaxial Sigmoidal veins form

A rigid object (e.g. pyrite crystal) disturbs the stress and strain field around it during deformation. On the sides of the object normal to maximum compression, differential stress and pressure are highest (high strain areas). On the sides of the object normal to minimum compression, differential stress and pressure are lowest (low strain areas). Difference in pressure can lead to material transport from strain cap to pressure shadow or pressure fringe (alternatively called strain shadow and strain fringe). See beautiful natural and numerical examples at Daniel Koehns old PhD home page or at a local mirror of his numerical simulations
 
 

Pressure fringe of fibrous quartz around a concretion of iron ore in a BIF-chert from the Hamersley ore province, Pilbara, West Australia. Width of view 2.3 mm, crossed polars.

Quartz + mica pressure shadow adjacent to a quartz porphyroclast (on right, grey grain with inclusions) in a quartz-mica schist from Nooldoonooldoona Waterhole, S.W. Mount Painter Inlier, Arkaroola, South Australia. Width of view 3.2 mm, crossed polars. Note the sharp boundary of the pressure fringe, in contrast to the vague boundary of the pressure shadow

- Fringes versus shadows

 

distributed precipitation in low pressure area: pressure shadow usually blocky texture  non-distinct boundary of pressure shadow  similar to replacement veins

localised precipitation in low pressure area:  pressure fringe usually fibrous or elongate blocky texture sharp edge of pressure fringe  similar to syntaxial/antitaxial/composite veins

NOTE: later recrystallisation may produce a blocky texture in a pressure fringe, making it look like a pressure shadow.

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- Syntaxial versus antitaxial fringes

 

Syntaxial fringe:  precipitation is on outside of pressure fringe: between fringe & wall rock  precipitate can be same mineral as core object with crystallographic continuity between object and fringe  relatively uncommon

Antitaxial fringe: precipitation is on inside of pressure fringe: between object & fringe  precipitate usually different mineral as core object relatively common (typically pyrite with quartz and/or calcite fringe)

Notice that at the syn-/antitaxial terminology for veins and fringes seems inconsistent. Reason:

• in veins, the wall rock is reference material
• in syntaxial veins crystals grow syntaxially from wall rock grains
• in antitaxial veins crystals grow antitaxially towards wall rock
• in fringes, the core object is reference material
• in syntaxial fringes crystals grow syntaxially from core object
• in antitaxial fringes crystals grow antitaxially towards core object

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- Displacement versus face-controlled growth

Displacement-controlled growth:

• Growth direction of fringe crystals (fibres) = opening direction

Face-controlled growth:

• Growth direction of fringe crystals (fibres) = normal to object surface

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- Deforming versus non-deforming fringes

The fringe mineral can be very strong compared to the surrounding material and behave as rigid material. The texture in the fringes is preserved. The fringe gets deformed if the fringe mineral is not effectively rigid.

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- Pressure shadows/fringes in boudin necks

Pressure shadows and fringes also occur in boudin-necks of competent layers or in between parts of broken rigid (but brittle) crystals.

(Quartz fringe in broken lump of iron ore in BIF from Hammersley Ranges, Western Australia)
 
 

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- Pressure shadows/fringes and structural analysis

As with veins, the shape of pressure shadows / fringes and the internal texture of fringes often provide excellent information about the kinetics of deformation during their formation:

• degree of non-coaxiality of deformation
• amount of deformation (finite strain)
• possible changes in deformation kinetics

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The crack-seal mechanism

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The crack-seal mechanism (Ramsay 1980) is the favoured mechanism for about all veins with elongate crystals. In this model growth occurs in many repeated small increments: crack-seal events:

• Crack event -> opening of narrow open crack, filled with fluid
• Seal event -> Precipitation fills (=seals) the crack again

Most telling microstructural indicators of crack-seal mechanism are regularly spaced trails of small inclusions (typically small micas or pieces of wall rock or fluid inclusions). Opening per crack event is generally in the order of 10 mm.

Elongate blocky and stretching textures are very well explained by, and often show evidence of crack-seal growth (inclusions, radiator structures)

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- Crack-seal mechanism, pressure and fluid flow

The crack-seal cycle involves the buildup of fluid pressure to enable fracturing (Crack). Then increased permeability allows fluid flow and material transport (Seal) and the fluid pressure drops.

• Presence of cracks -> fluid can flow through crack network
• Presence of cracks -> brittle failure (often extensional)
• Extensional failure -> high fluid pressure & low differential stress

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- Fibrous textures and the crack-seal mechanism: some questions

How does material get to vein?
• percolating fluid: precipitating in and clogging of cracks
• diffusional transport: local material
Which textures develop ?
• growth of crystals into open cracks is crystallographically determined, resulting in elongate blocky textures and a preferred crystallographic orientation
• to get fibrous texture:
• isotropic growth (special conditions, sub-grains or twins)
• growth is not in open crack, but on wall rock - fibre tip contact (*)
Why often tracking of opening trajectory ?
• tracking capability is a function of growth anisotropy (crystallography), vein wall geometry (roughness) and crack width.
• if crack width ->0 best tracking (*)
How to get symmetric opening in antitaxial vein ?
• Simultaneous or alternating failure on both sides of vein is difficult to imagine. More likely to see one side winning and get asymmetric elongate blocky vein
• Diffusional transport to and precipitation on surface of vein gives symmetric antitaxial vein (*)
(*) These observations suggest that the crack seal mechanism is not the only mechanism: it best explains the formation of syntaxial or asymmetric elongate blocky veins. Fibrous antitaxial veins may form without repeated cracking and sealing, but by continuous growth on the surface of the vein, with diffusional transport of material to the vein.

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