Textures of ores

1. Presented by
Kavya Mol K S

2. DEFINITION
 The spatial relationship between grains.
 The word “texture” is derived from the Latin word textura, meaning
“web” or texto, meaning “to weave”.
 Barton (1991) has astutely noted that "the interpretation of textures is
simultaneously one of the most difficult and important aspects of the
study of rocks and ores."
 The recognition and interpretation of textures is often the most
important step in understanding the origin and post depositional
history of an ore.

3.  It may provide information on ;-
 Initial ore deposition
 Postdeposition or metamorphism
 Deformation
 Annealing
Primary
Secondary
Types

4. PRIMARY TEXTURES
 Primary textures of ore minerals formed from melts.
 Primary textures of open space deposition.
SECONDARY TEXTURES
 Secondary textures resulting from replacement.
 Secondary textures resulting from cooling.
 Secondary textures resulting from deformation.
 Secondary textures resulting from annealing.

5. PRIMARY TEXTURES OF ORE MINERALS FORMED
FROM MELTS
 The growth of ore minerals in silicate melts generally results in the
development of euhedral to subhedral crystals, because there is little
obstruction to the growth of faces.
 This can be seen in primary chromite, magnetite, ilmenite and
platinum often occur as well-developed equant euhedral interspersed
in the plagioclase, olivine, and pyroxene of the host rock.
 Unobstructed growth, especially in rapidly cooled basalts,
sometimes results in the formation of skeletal crystals that may be
wholly or partially contained within subsequently solidified glasses
or crystallizing silicates.

6. PRIMARY TEXTURES OF OPEN-SPACE DEPOSITION
 The initial deposition and growth of the ore and gangue minerals in
many deposits occurs in open spaces in vugs or fractures developed by
dissolution or during faulting.
 The initial formation of the crystals in open space is often still
evidenced by the presence of well-developed;-
 Crystal faces (Fig: 1.a and 1.b).
 Crystals that exhibit growth-zoning (Fig: 2).
 Colloform or zoned monomineralic bands (Fig: 3 a- c).

7. 1 (b) The face of the first generation of
sphalerite (containing numerou s dark fluid
inclusions).
1 (a) Well-developed freestanding crystals
such as these 1-2 μm diameter sphalerites are
generally good evidence of growth in open
spaces.

8. Fig : 2 Growth zoning in a single crystal
of sphalerite
Fig: 3 (a) Concentric growth banding in sphalerite viewed
in transmitted light through a doubly polished thin section.
Black areas are pyrite; white areas are dolomite .
Fig; 3 (c) Growth zoning in pyrite showing
radial and concentric development
Fig: 3 b Colloform growth banding in sphalerite seen
in reflected light.

9.  All of these features result from unobstructed growth of minerals
into fluid-filled voids in the ore-forming fluids and the physico-
chemical environment of mineralization with time.
 Deposition from hydrothermal solutions in open fissures can result
in comb structures and in symmetrically and rhythmically
crustified veins (Fig: 4 a and 4 b).
 Movement along such a vein during or after ore formation may
cause brecciation resulting in breccia ore.

10. Fig 4 a) comb structures Fig 4 b) crustified veins

11. SECONDARY TEXTURES RESULTING FROM
REPLACEMENT
 Replacement of one ore mineral by another or by a mineral formed during
weathering is common in many types of ores.
 The most easily recognized replacement textures are:
 Those in which organic materials, such as wood fragments (Fig: 5 a).
 Fossil shells have been pseudomorphed by metal sulfides (commonly
pyrite, marcasite,chalcocite) or oxides (commonly hematite, goethite,
"limonite," uranium minerals) (Fig: 5 b).
 Pyrite cubes and marcasite laths that have been replaced by iron oxides
during weathering (Fig: 5 c).

12. Fig 5 (a) Cellular structure in coal
material replaced by pyrite.
Fig 5(b) Pseudomorphous replacement of euhedral
marcasite (white) crystals by goethite (light gray).
Fig 5 c) Pyrite cubes have been converted into
goethite during weathering with the formation of a
relatively solid outer rim and a porous interior.

13.  Replacement may result from one or more of the following processes :
(I) dissolution and subsequent reprecipitation.
(2) oxidation.
(3) solid state diffusion.
 The boundary between the replaced and the replacing mineral is commonly
either sharp and irregular or diffuse.
 Replacement textures depend chiefly for their development on three features
of the phase being replaced:
(I ) The surfaces available for reaction.
(2) The crystal structures of the original and secondary minerals
(3) The chemical compositions of the original mineral and the reactive fluid.

14.  Any channel between grains or through grains is a prime site for initiation of
the replacement process.
 Replacement along grain boundaries or internal channel ways very often
appears in the form of thin laths or equant crystals of the replacing phase
projecting into the host.
 In the early stages of the process, replacement may be readily identified,
because much of the original phase remains and the original grain
boundaries, fractures, or cleavages are still visible (Fig: 6 a and 6 b).
 In more advanced stages, the original phase may be reduced to "islands"
now left in a matrix of the secondary minerals (Fig: 6 c).
1.Crystal Surfaces

15. Fig: 6 (a) Covellite replacing chalcopyrite (white)
along grain boundaries.
Fig: 6 (b) Chalcocite (medium gray) in
replacing galena (light gray) along grain
boundaries and cleavages.
Fig 6 (c) Galena (white) replaced by cerussite, light
gray) from the margins of the original gray.

16. The crystal structure of the phase being replaced may control replacement.
Because this determines cleavage directions or because diffusion can take
place more readily along certain crystallographic directions.
The chemical composition of the primary phase may control the composition
of the phase that replaces it.
An example of replacement is the open void "boxwork" texture composed
of cellular crisscross laths of goethite, hematite, and sometimes pyrite,
as are found in gossans (Fig: 8).
2.Crystal Structures
3.Chemical Composition

17. Fig : 8 - Boxwork texture of laths of hematite and goethite
with residual pyrite in a gossan.

18. SECONDARY TEXTURES RESULTING FROM
COOLING
 Most ores form at elevated temperatures and have undergone cooling
over temperature ranges that may be retain their original composition
and texture through the cooling episode.
 Examples are, less than 100°C for many Pb-Zn ores in carbonates but
as much as l000°C for Fe-Ni-Cu ores in ultramafic rocks.
 The textural effects resulting from cooling includes;

19. 1. Recrystallization
 Reequilibration of ores on cooling is usually accompanied to some
degree by recrystallization of the primary minerals.
 This effect that may or may not leave any vestige of the original
texture.
2. Exsolution and Decomposition
 One phase is expelled from the other.
 Results from diffusion, nucleation of crystallites and growth of
crystallites.
 Exsolution itself is a form of decomposition.

20. 3. Inversion
 Inversion of a mineral from one structural form to another of the
same composition is not often easily discernible texturally but may
produce characteristic twinning.
 Some high-temperature phases always invert so rapidly on cooling
that only the low temperature forms are observed (e.g., troilite,
chalcocite, acanthite).
4. Thermal stress
 Most ore minerals have approximately the same coefficient of
thermal expansion.
 Thus, mono or polymineralic masses suffer little induced strain on
cooling.

21. SECONDARY TEXTURES RESULTING FROM
DEFORMATION
 Many ores contain textural evidence of deformation.
 The evidence ranges from minor pressure-induced twinning to complete
cataclasis.
 The degree to which individual mineral grains both respond to and
preserve deformational effects ranges widely.
 It depends on;
 The mineral
 The rate of strain
 The nature of the deformation
 The associated minerals
 The temperature at the time of deformation
 The post deformational history

22.  Many native metals, sulfosalts, and copper and silver sulfides
deform most readily.
 Copper-iron sulphides and monosulphides, less readily.
 The softer minerals deform most readily, but they also recrystallize
most readily, so that the deformational effects are obliterated before
those in more refractory minerals.
 Specific deformation features commonly observed include
Twinning, Kinkbanding, Pressure Lamellae.

23.  Twinning, kinkbanding, and pressure lamellae occur in ores subjected
to any degree of deformation.
 It can even be artificially introduced into some of the softer minerals
by rough treatment of specimens.
 Twinning may occur in minerals during initial growth, during
structural inversion on cooling or as a result of deformation.
 Ramdohr (1969)suggests that the three major types of twinning can be
distinguished as follows:
 Growth
 Inversion
 Deformation
Twinning, Kinkbanding, Pressure Lamellae

24.  Growth
Occurs as lamellar twins of irregular width that are unevenly
distributed and may be strongly interwoven.
 Inversion
Commonly occurs as spindle-shaped and intergrown networks not
parallel throughout grains.
 Deformation
Occurs as uniformly thick lamellae, commonly associated with
bending, cataclasis, and incipient recrystallization, with lamellae
often passing through adjacent grains.

25. Fig: 9 Growth twin of wolframite
Fig: 10 Deformation twins in pyrrhotite

26.  Kinkband deformation predominates below 300°C, whereas above
this temperature twinning is common.
 Pressure-induced twins and pressure lamellae (Fig: 11) often
terminate in regions of brittle fracture and crumpling, or very fine-
grained regions in which crushed grains have recrystallized.
Fig: 11 Deformation twinning induced in specular hematite.

27. Schlieren
 Deformed ores often contain zones along which shearing has
occurred, such zones, known as schlieren.
 The ore minerals may be pulverized and smeared out parallel to
the direction of movement.
 The schlieren are usually planar features in which the ore
minerals are very fine grained (sometimes recrystallized) relative
to the surrounding rock.

28. Brecciation, Cataclasis, and Durchbewegung
 Evidenced by fracturing or brecciation of ore and gangue minerals,
especially of those that are harder and more brittle, such as pyrite,
chromite, and magnetite.
 The amount of brecciation depends on both the degree of deformation
and the mineralogy of the ore.
 Minor brecciation grades into complex cataclasis with an increasing
degree of fragmentation and disorientation (Figure 12); this
penetrative deformation has been termed Durchbewegung (literally
"move through") by Vokes(1969).

29. Fig: 12 Durchbewegung texture developed in intensely deformed pyrrhotite. The black
silicate inclusions were stretched and rotated as the pyrrhotite matrix flowed under
pressure.

30. SECONDARY TEXTURES RESULTING FROM ANNEALING
 Many ores have been subjected to slow cooling or have passed one or more
times through metamorphic cycles in which they were subjected to
prolonged periods of slow heating and slow cooling.
 The textural effects of this treatment may be indiscernible, or may lead to
distinctive annealed grain boundaries or even to the growth of
porphyroblastic crystals.
 The annealing effects of the slow cooling of ores after deposition or slow
heating during metamorphism can significantly alter the original textures.
 Both cooling and metamorphism are prolonged annealing processes.

31.  The most characteristic feature of annealing is recrystallization to
minimize the areas of grain surfaces and interfacial tension through
the development of roughly equant grains with 120° interfacial
angles (Fig:13).
 During the annealing process, small grains are resorbed at the
expense of larger ones; however, small grains of minor phases may
remain trapped as lens-like bodies along the grain boundaries of
larger grains.
 The reequilibration that results from annealing can produce either
zoned overgrowths on grains or the homogenization of grains
containing primary growth zoning.

32. Fig : 13 Annealed texture of recrystallized monomineralic pyrite sample

33. TEXTURES OF PLACER GRAINS
 Placer deposits are composed of the grains of ore minerals that have
been weathered out of pre-existing rocks.
 The mineral grains generally contain internal textures that reflect
their initial origins but marginal and external textures that reflect the
abrasion and weathering to which they have been subjected.

34. SPECIAL TEXTURES
 Framboids : It is an depositional textures the aggregates of spherical
particles often seen in pyrite and in uraninite.
 Oolitic textures : familiar from carbonate rocks but also found in
iron and manganese ores.
 Martitization: It is an replacement textures, the replacement of magnetite
by hematite along cleavage (111) directions.
 Birds eye texture : the characteristic alteration of pyrrhotite to a fine
mixture of pyrite and marcasite results in this texture.
 Some exsolution textures are particularly characteristic, such as the flames
of pentlandite in pyrrhotite and the stars of exsolved sphalerite found in
some chalcopyrite.

35. Fig: 14, Well-developed "bird's-eye" texture
composed of pyrite and marcasite, formed along a
fracture during weathering of pyrrhotite.
Fig: 15, Sphalerite "star" in chalcopyrite.

36. IMPORTANCE OF STUDYING TEXTURES
 Textural studies are useful for:
 Understanding the timing of formation of the ore minerals
relative to the host rocks and their structures.
 Determining the sequence of events or depositional history
within an ore body.
 Determining the rates of cooling or of ore mineral accumulation.
 Identifying the equilibrium mineral assemblages, which in turn
are necessary for understanding phase relations and the correct
interpretation of geothermometric results.

37. Conclusion
Rarely does a single texture provide
unequivocal evidence regarding the origin or history of an ore
deposit. Commonly, a variety of textures representing different
episodes in the development and subsequent history of a
deposit are observed.

38. REFERENCES
 Craig and Vaughan (1940), Ore Microscopy and Ore
Petrography, John Wiley and Sons, Ore Mineral
Textures, pp: 108 – 140.
 Roger Taylor (2009), Ore Textures, Springer, pp: 163 –
175.