Texture of Ore Minerals; Importance of Studying Textures; Individual Grains Properties; Filling of voids; Texture Types; Genetically differentiated between Texture types; Secondary textures from replacement; Hypogene Texture; Supergene Texture; Primary texture formed from Melts; Primary texture of open-space deposition; Secondary textures from cooling; Secondary textures from deformation; TEXTURES OF ECONOMIC ORE DEPOSITS; Textures of Magmatic ores; Cumulus textures; Intergranular or intercumulus textures; Exsolution textures; Textures of hydrothermal ore deposits and skarns; Replacement textures; Open space filling textures; Textures characteristic of surfacial or near surface environments and processes; Criteria for identifying replacement textures; Vein and Veining have different Nature Features

1. TEXTURES OF ORE DEPOSITS AND
ASSOCIATED FEATURES
Prof. Dr. Hassan Z. Harraz
Geology Department, Faculty of Science, Tanta University
hharraz2006@yahoo.com
Spring 2020

2. Texture
❑Textural identification and interpretation for ore
deposits and associated gangue minerals are tools
necessary for understanding the processes
involved in the genesis of these deposits, which in
turn is very important for prospecting for other
similar economic bodies.
❑General physical appearance or character of a
rock, including aspects of geometry, components,
relationships between components or constituent
crystals.
➢size, shape, arrangement, crystallinity,
granulity, and fabric

3. ◼ Many textures when we observe in general, looks
like is formed for a specific process, but when
viewed in a smaller portion, will show the
difference in the process shown by the texture.
◼ Megascopic observations (hand specimens)
and microscopic texture in general will give the
same results, only depending on the scale of
observation alone:
➢Microscopic will show growth and mineral
associations are more complex than megascopic
observation of hand specimens.
➢Observations megascopic will be helpful before
making microscopic observations in detail.
Texture of Ore Minerals

4. Importance of Studying Textures❑Specifically, textural studies are useful for:
1) Understanding the timing of formation of the ore minerals relative to the host rocks and
their structures.
2) Determiningthe sequence of events or depositional history within an ore body.
3) Determining the rates of cooling or of ore mineral accumulation (in some cases).
4) Identifyingthe equilibrium mineral assemblages, which in turn are necessary for
understanding phase relations and the correct interpretationof geothermometricresults.
❑Texture can provide evidence:
1) The initial process of ore deposition
2) Equilibrium after ore deposition process metamorphism
3) The deformationprocess
4) Annealing (reinforcement)
5) Weathering because meteoricwater
6) At polymetallic minerals, textures reflect the order of formationof minerals and history after
formation
7) Morphology and inclusion patterns can indicate the condition of the high temperaturein the
initial deposition
8) The presence of mineral pyrrhotite show their equilibrium temperatureto intermediate
cooling time
9) Minor sulfate and native metals showed a decrease to the equilibrium temperature

5. Individual Grains Properties
I) Internal Grain Properties :
1) Twinning; controlled by the lattice arrangement
➢ Inversion twinning: spindle shaped lamellae (long,
straight, such as spindles)
➢ Pressure twinning: generally lamellae with a
uniform thickness, associated with bending, and
marks the beginning of recrystallization
➢ Growth twinning: a tangle of several lamellae
with different directions
2) Inclusions: liquid / gas, solid (primary, exsolutions,
replacement
3) Internal Reflection; internal color reflection
(e.g. Cassiterite, ruby silver, sphalerite, hematite).

6. II) External Grain Properties :
◆Grain shape; controlled by the crystal
structure itself or by the influence of the
surrounding crystal.
◆Euhedral: a perfect crystal shape
◆Subhedral: partially crystalline form
◆Anhedral: do not have a crystal form

7. II) External Grain Properties (Cont.):
➢ Skeletal crystals: formed due to rapid crystallization,
resulting in reduced supplies of atoms to form crystals, so
that will be generated dendritic crystalline form
➢ Poikiloblasts: large crystals, where the center is filled by
the inclusion of the same mineral with a smaller size.
Characterize the formation by a process metamorphism.
➢ Spheroidal grains: form of drop / droplet.
➢ Coarse grain size and the same can be produced from
primary deposition process or metamorphism.

8. III) Grain Bonding
❑Modest growth of grain that occurs because
of the deposition of continuous and slow
growth.
❑In extreme conditions, will have a smooth
surface, the grain boundary curves.
❑Simple grain growth can also be caused by a
recrystallization process.
❑Form of complex fabric commonly produced
by rapid deposition or superposition effects,
especially replacement.

9. ❑Full charging will provide a good
indication for the determination
paragenesis.
❑In an imperfect charging will cause
porosity, so it will be difficult when
making polish section with good quality.
IV) Filling of voids

10. Texture Types
❑Genetically differentiated :
➢Primary Texture: existing in a rock at a time
of its formation .
➢Secondary Texture: resulting from the
alteration of primary minerals.
➢Hypogene Texture: formed by precipitation
from generally ascending waters.
➢Supergene Texture: formed by generally
descending waters includes ores and minerals
formed by downward enrichment.

11. Melts
Secondary TexturesPrimary Textures
Replacements
Cooling
Deformation
Annealing/
metamorphism
Open Space
Deposition
Texture

12. Primary texture formed from Melts
❑ Euhedral ~ Subhedral crystals:
Because little disruption during the crystal growth face
Example: Chromite primary minerals, magnetite,
ilmenite and platinum
❑ skeletal crystals:
Because there is no interference when the growth,
especially in the rapid cooling of basalt; can whole or in
part is the crystallization of silica.
❑ Poikilitic crystal
The formation of oxides and silicates in reverse for their
simultaneous crystallization

13. • Formed in cavities (vugs) and vein open (open vein),
characterized by the shape of the surface of a perfect
crystal
• There is no interference when the crystal growth of the
fluid that fills the cavity
• Texture commonly encountered:
➢zoning
➢Colloform: for their colloidal deposition
➢ Banding : occur because of changes in physico-chemical env. When
mineralization occurs over time
➢ Comb structures, symetrically & rhythmically crustified : the
deposition of the hydrothermal solution open fissures
➢ Radiating ~ fibrous : fill open fracture
➢ Iron, manganese oxide & hydroxides often formed on the open
fracture for their meteoric water circulation (eg, goethite,
lepidocrocite, pyrolusite, cryptomelane). Can form a concentric,
fibrous and radiating
Primary texture of open-space deposition

14. Secondary textures from replacement
• Weathering
→ Organic material replaced by mineral sulfide (pyrite, marcasite, chalcocite) or oxides
(hematite, goethite, limonite, uranium minerals
• formed by prosses :
→ Dissolution ~ reprecipitation
→ Oxidation
→ Solid state diffusion
• The boundary between minerals that replace and replaced the usual
sharp or irregular (corroded)
• Fractures, cleavages and grain boundaries :
→ Is the result of chemical reactions on the surface of the crystal.
• Crystal structure :
→ Replacement parts or directions in direction crystallography
• Chemical composition:
→ The chemical composition can control the composition of the
phase that replaced it, both in the process of weathering or
hydrothermal

15. • Recrystalization
• Exsolution & Decomposition : diffusion, nucleation,
growth
→ marginal, lamellar, emulsoid, myrmekite
• Inversion
• Oxidation-Exsolution
• Reduction-Exsolution
• Thermal stress
Secondary textures from cooling

16. Secondary textures from deformation
◼ Twinning
◼ Curvature or offset of linear features
◼ Schlieren
◼ Brecciation, cataclasis

17. Special textures
• Framboids: in the form of aggregates of particles are spherical
• Oolitic: common in carbonate or iron and manganese ore
• Martitization: replacement of magnetite by hematite along the plane
parts (111)
• Bird eyes: pyrrhotite alteration characteristics to smooth joint between
pyrite and marcasite
• Flames : exsolution of pentlandite in pyrrhotite
• Starts: exsolved sphalerite in chalcopyrite

18. Minute inclusions of chalcopyrite
(yellow) in core (a growth zone)
of sphalerite (grey). Silver
Queen epithermal vein, central
B.C. Field width 0.2 mm

19. Electron microscope backscatter image of
zoned tetrahedrite. Silver Queen epithermal
vein, central B. C. Layers are enriched in Ag
relative to Cu. Grain is about 70 microns in
diameter
Intergrowth of bornite (orange), chalcocite
(white), covellite (blue) and hematite
(ragged laths in chalcocite-bornite.
Discovery zone, White Lake Copper, Kluane
Range, Yukon Terr. Ore host is Nikolai
basalt. Field width is 1.0 mm.

20. Granular stibnite, crossed nicols,
showing deformation twinning and
intense anisotropism. Ferguson Creek,
B.C. Field width is 0.8 mm
Bireflectance of covellite (dark blue
to pale blue). Note kinks across the
covellite laths. Location unknown.
Field width is 1. 6 mm

21. Galena (white), tetrahedrite (grey),
pyrargyrite (blue) and chalcopyrite
(yellow). Note the black triangular
cleavage pits in galena. Location
unknown. Field width is 0.4 mm
Red internal reflection of
cinnabar enclosing a twinned
crystal of stibnite (blue-grey)
under crossed nicols. Red Devil
mine, Alaska. Field width is 0.8
mm.

22. Marcasite stalactite long section showing
structure. Note central core of fine granular
marcasite and curved platelets surrounding
core. Structure is readily visible because of
variable tarnish on surface. Pine Point Mines.
Field width is 2 cm
Margin of a marcasite stalactite under
crossed nicols showing anisotropism of
marcasite blades and structure at
margin of stalactite. Pine Point Mines.
Field width is 2mm.

23. Cross section of sphalerite stalaclite.
Contrast of layers is a combination of
textural differences and slight variation in
the sphalerite composition. Pine Point Mine
Field width is 1. 0 cm
Ilmenite exsolution in magnetite.
Crystallographic texture

24. Colloform sphalerite. Contrast of layers is a
combination of textural differences and
slight variation in the sphalerite
composition. Pine Point Mines. Field width
is 2 mm
Sphalerite (grey) "stars" in
chalcopyrite (yellow). Probably
an exsolution texture.

25. Growth zoning in pyrite.
Nadina epithermal vein,
central B.C. Field width is
0.4 mm

26. Specularite (hematite).
Echo Bay, N. W. T.
Radiating sheets, plane
polarized light. Field width
is 1. 6 mm
Bornite (orange), digenite
(blue) and chalcocite (grey).
Rainy Hollow, B. C. Digenite
forms thin rim between
bornite and chalcocite. Field
width is 0.4 mm

27. Relict chalcopyrite (yellow) in
network of goethite (blue grey)
and malachite (grey-green). Note
some malachite rims chalcopyrite.
Field width is 1.6 mm.
Bornite (orange) partly
replaced by covellite (blue)
along fractures. Small white
grain is galena

28. Tennantite (grey-green), pyrargyrite
(blue) and chalcopyrite (yellow). Note
pale grey rim (tetrahedrite) between
pyrarg and tennantite, possibly a reaction
zone indicating replacement of Tn by
pyrarg with Cu and Fe in Tn left as
chalcopyrite. Field width is 0.4 mm.
Galena (white) fractured along
cleavage planes. Small grey blebs are
tetrahedrite. Venus vein, Yukon Terr.
Field width is 0.4 mm

29. Galena (Gn; white) deposited on sphalerite (Sl; grey) in Tri-State lead-zinc
ores. The ores exhibit open space filling textures in which sphalerite was
deposited first forming euhedral crystals upward into open space, and
galena was subsequently deposited on sphalerite with its base taking the
shape of the underlying sphalerite.
Illustration open space filling textures

30. Pyrite (Py; white) and marcasite (Mc; whitish yellow) deposited on sphalerite (Sl;
grey). Pyrite and marcasite have formed euhedral crystals upward into open space,
and take the shape of the underlying rotund sphalerite. Pyrite has formed cubes and
marcasite formed more elongate prismatic crystals upward into open space. Plastic
(P) fills the former open space. Picher field, Tri-State Lead-Zinc District, Missouri,
Oklahoma, and Kansas. Ore microscopy, reflected light, medium magnification.

31. Pyrite (Py; white) deposited on sphalerite (Sl; grey) taking the shape of the
earlier deposited sphalerite and forming it own euhedral cubic shape upward
into former open space. Chalcopyrite (Cp; deep yellow) was subsequently
deposited on the top of pyrite and sphalerite. Picher field, Tri-State Lead-Zinc
District, Missouri, Oklahoma, and Kansas. Ore microscopy, reflected light,
medium magnification.

32. Illustration replacement textures
Pyrite (Py; yellow) partially replaced by chalcocite (Cc; bluish grey). The Butte,
Montana copper ores illustrate typical replacement textures: 1) irregularly shaped
replacement remnants ("sea islands") of pyrite being replaced by chalcocite, 2)
caries texture in which chalcocite embayments into pyrite are concave with
respect to the host pyrite, and 3) vein texture in which chalcocite replacement
veins traverse pyrite. Holes in the polished section are black. Ore microscopy,
reflected light, low magnification

33. Bornite (Bo; blue) patches in chalcocite (Cc; grey) in Butte, Montana
copper ores. The bornite areas may represent irregularly shaped
replacement remnants ("sea islands") of bornite being replaced by
chalcocite. However, the smoothly rounded character of the margins of the
bornite areas suggests that they more likely have formed by exsolution out
of the chalcocite.

34. Abundant covellite (Cv) and minor chalcocite (Cc) in Butte, Montana
ores. Covellite exhibits a variety of colors due to its extreme reflective
pleochoism. Depending upon its crystal orientation, covellite may be
deep blue, medium blue, or light bluish grey.

35. TEXTURES OF ECONOMIC ORE DEPOSITS
I) Textures of Magmatic ores:
1) Cumulus textures.
2) Intergranular or intercumulus textures.
3) Exsolution textures.
II) Textures of hydrothermal ore deposits and skarns:
A) Replacement textures.
B) Open space filling textures.
III) Textures characteristic of surfacial or near surface
environments and processes.

36. I) Textures of Magmatic ores:
1) Cumulus textures:
❖ result from the settling of an ore deposit from a crystallizing magma.
❖ The most common example is chromite which occurs as a cumulus phase
relative to pyroxenes.
2) Intergranular or intercumulus textures:
❖ where the ore mineral occurs as an intergranular anhedral phase relative to
the other gangue minerals.
❖ In such cases, this ore mineral crystallizes late in the magmatic sequence
(relative to the other gangue minerals) so takes up the shape of the
intergranular spaces left behind.
Examples include numerous sulfides, in many cases crystallizing from
liquids immiscible with, and of lower melting point than the silicate
magma.
3) Exsolution textures:
❖ Where one phase separates from another as a result of incomplete miscibility
during cooling, and has a tendency to concentrate along certain
crystallographic directions (e.g. cleavage planes).
❖ Exsolution textures usually indicate a slow or intermediate cooling rate.
Examples include the occurrence of ferrian ilmenite in titanohematite or
ilmenite in ulvospinel.

37. Granobalstc texture with cumulate olivine and
chromite
Orthocumulate texture - Harzburgite with cumulate
olivine and chromite
Chalcopyrite blebs near the vein are a texture
known as "chalcopyrite disease”
Sphalerite (light grey) with abundant inclusions of chalcopyrite

38. Pyrrhotite-rich, magmatic sulfide blebs partly altered to
chalcopyrite (e.g., Little Stobie Mine, Sudbury)

39. a) Crystals accumulate by crystal settling
or simply form in place near the margins
of the magma chamber (known as in situ
crystallization). In this case plagioclase
crystals (white) accumulate in loose
mutual contact, and an intercumulus
liquid (red) fills the interstices.
b) Orthocumulate: intercumulus
liquid crystallizes to form additional
plagioclase rims plus other phases in
the interstitial volume (colored).
There is little or no exchange
between the intercumulus liquid and
the main chamber.
Development of cumulate textures typical of layered mafic intrusions.

40. Example of orthocumulate texture.
In this case, the cumulus mineral is orthopyroxene and it is surrounded by later-
forming intercumulus plagioclase (showing albite twinning).
Stillwater complex, Montana. Field width 5 mm.

41. Adcumulates: open-system exchange
between the intercumulus liquid and
the main chamber (plus compaction
of the cumulate pile) allows
components that would otherwise
create additional intercumulus
minerals to escape, and plagioclase
fills most of the available space. Some
liquid is trapped and forms the small
intercumulus pyroxenes, etc.
Heteradcumulates: The texture illustrated
above is common in the larger layered
intrusions. In the example illustrated, pyroxene
nucleates from the intercumulus liquid and
grows to form giant poikilitic crystals
(grapefruit size) enclosing numerous cumulus
plagioclase crystals. During growth of the
oikocrysts, communication between the
intercumulus liquid and main magma is
maintained.

42. Example of adcumulus texture. Anorthosite
from Bushveld Complex, SA

43. Massive pyrrhotite (Po; tan and yellow) and pentlandite (Pn; white) Sudbury
massive ores in reflected light under crossed polars.
Pentlandite grains are intergrown with pyrrhotite and constitute the main
nickel mineral for which these ores are mined.

44. Some of the ores from the Sudbury nickel district contain significant amounts
of cubanite (CuFe2S3).
Cubanite (Cb) forms a complete solid solution series with chalcopyrite (Cp),
and the single solid formed at high temperatures may exsolve with declining
temperatures.

45. II) Textures of hydrothermal ore deposits and skarns:
❑ Replacement is the process of almost simultaneous solution and deposition by
which a new mineral of partly or totally different chemical composition may grow
in the body of an old mineral or mineral aggregate.
❑ According to this definition, replacement is accompanied by very little or no
change in the volume of the rock. However, in practice, this process is
accompanied by expansion or contraction (and it has proven quite challenging to
write balanced chemical reactions representing replacement textures in which the
volume of the products and reactants is the same!).
❑ Replacement is more common at high T and P where open spaces are very limited
or unavailable, and fluid flow is rather difficult. It also depends largely on the
chemical composition and reactivity of both the host rock and the hydrothermal
solution.
In general, it has been observed that certain minerals replace others preferentially.
Accordingly, a set of "rules" has been proposed:
a) Sulfides replace gangue or ore minerals.
b) Gangue minerals replace host rock, but not the ore minerals.
c) Oxides replace host rock and gangue, but rarely replace sulfides.
A) Replacement textures:

47. Criteria for identifying replacement textures:
1) Pseudomorphs.
2) Widening of fractures (Fig. 1).
3) Vermicular unoriented intergrowths (Fig. 2).
4) Islands (of the host or replaced mineral) having the same optical orientation and
surrounded by the new mineral (Fig. 3).
5) Relicts (Fig. 4).
6) Cusp and caries texture: (host or replaced mineral). Cusps are relict
protuberances of the replaced mineral or host rock between “caries”. The caries
are embayed surfaces concave towards the replacing mineral into the replaced
one (Fig. 5).
7) Non-matching walls of a fracture. This is a feature common when replacement
works outward from a central fissure (compare with open space filling textures)
(Fig. 6).
8) The occurrence of one mineral crosscutting older structures.
9) Topotactic and epitactic replacement: Topotaxy is a process where the replacing
mineral overgrows the replaced one along certain crystallographic directions
controlled by the structure of the replaced mineral. Epitaxy is the same process
except that the structure of the replacing (new) mineral is not controlled by the
replaced mineral, but instead by other "matrix" minerals.

48. Criteria for identifying replacement textures:
10) Selective association: Since replacement is a chemical process, specific selective associations of pairs or
combinations of minerals can be expected. For example, chalcopyrite is more likely to replace bornite by
a change in the Cu/Fe ratio or in fS2 than it is to replace quartz. Therefore, the occurrence of minerals
with some chemical similarity in some textural relationship is often a good indication of replacement
(Fig. 7).
11) The presence of a depositional or paragenetic sequence in which minerals become progressively richer in
one or more elements (Fig. 8).
12) Gradational boundaries: In contrast to open space deposition which produces abrupt textures and
structures between the hydrothermally deposited minerals and their host rocks, replacement is often
accompanied by gradational boundaries between both minerals. Accordingly, gradational boundaries
are a good indication of replacement.
13) Deposition of one or more hydrothermal minerals along a clear alteration front.
14) Doubly terminating crystals: If a crystal grows within an open cavity, it is normally attached to one of the
walls of the fracture, and can develop crystal faces only on the other end (i.e. the one away from this
wall). In contrast, the process of replacement may result in the growth of euhedral crystals with well
developed faces on more than one end.
15) The lack of offset on a fracture intersected by the replacing mineral: In contrast to open space filling
which may be associated with displacement of a preexisting structure by the fracture being filled by
hydrothermal fluids, replacement across a preexisting structure will not be accompanied by such offset.
The same holds true for two intersecting fractures (Fig. 9).

49. Ground Preparation

50. Vein and Veining have different Nature Features

51. Fault Irregularities
Vein occupying a fault and
exhibiting Pinch and Swell structure
Pinch
(thinnings)
Swell
(thickenings)
Sigmoidal Vein
Vein occupying a fault and exhibiting Pinch and Swell structure,
giving rise to Ribbon Ore Shoots
Ribbon Ore Shoots: rising Pinch
and Swell structure in Vein
occupying a fault

53. B) Open space filling textures
❑ Open space filling is common at shallow depths where brittle rocks deform by
fracturing rather than by plastic flow. At these shallow depths, ore bearing
fluids may circulate freely within fractures, depositing ore and gangue
minerals when sudden or abrupt changes in P and/or T take place.
❑ As such, open space filling textures will be different from those resulting from
replacement, and a set of criteria may be used to identify this process.
❑ Nevertheless, many hydrothermal ore deposits form by the combined effects
of replacement and open space filling, which requires a lot of caution in
textural interpretation.

54. Criteria for identifying open space filling processes:
1) Many vugs and cavities
2) Coarsening of minerals from the walls of a vein to its center
3) Comb structure:
▪ Euhedral prismaticcrystals growing from opposite sides of a fissure symmetricallytowards
its center develop an interdigitated vuggy zone similar in appearance to that of the teeth of
a comb (Fig. 10).
▪ A piece of vein in which crystallizationof minerals (usually quartz) takes place as single
euhedral crystals growing towards the centre of the vein.
4) Crustification:Crustification results from a change in composition and/or physicochemical
conditions of the hydrothermalsolution, and is represented by layers of different mineralogies
one on top of the other.
5) Symmetricalbanding (Figs. 10 & 11)
6) Matching walls: If an open fissure has been filled without replacement, the outlines of opposite
walls should match (Fig. 10).
7) Cockade structure:
▪ Mineralizationwithin the open spaces of a breccia or any other fragmental rock will
commonlyproduce a special pattern of symmetricalbanding and crustification where each
opening acts as a center for sequential deposition (Fig. 12).
▪ A colloidal-liketexture in which mineral precipitation takes place around a megascopic
particle or piece of rock. Characterisitic in low T systems
8) Offset oblique structures (Fig. 13)
In addition to replacement and open space filing textures, very low temperature
hydrothermal deposits (epithermal and telethermal deposits) are often characterized by colloform
habits (Fig. 14) and banding described in the following section.

55. Crustiform banding
➢ Crustification :it is a characteristic
feature in the cavity filling deposits in
which the ore is build up in successive
layers or crusts by crust, where the
younger crust is deposited on an
older one
➢ The cause of such sequence is related
to the decreasing of the mineral
solubility in the solution in
accordance with the decrease in T, P,
where the least soluble mineral is
deposited first while the most soluble
mineral is deposited last

57. Fig-5: Drill core
sections showing the
different types of
mineralized veins

60. Sample opened completely
Geode
(piece of rock which
has a large void in it)
Sample opened up a little

61. Void-filling textures
Characteristic in low temperature, silica enriched
(epithermal type Au) deposits.
a) Banded texture:
Minerals are crystallised along certain zones or
bands which may or may not repeat symmetrically
Usually occurs in epithermal deposits.
b) Comb texture:
c) Cockard texture:
d) Colloform texture :
colloidal-like texture in which mineral precipitation
takes place in the form of symmetric or asymmetric
circular or elliptical zones around a microscocopic
particle. Characterisitic in low T systems

62. Fig.10
(Figs. 10 & 11)
Fig.13
Fig.12
Void

63. III) Textures characteristic of surfacial or near surface
environments and processes:
❑ Under surfacial conditions, ore minerals may be deposited from
colloidal solutions. A colloid is defined as a system consisting of two
phases; one diffused in the other. Colloidal particles range in size
between ions in a true solution and particles that are <10-3 cm in a
coarse suspension. The colloidal material may be solid, liquid or gas
dispersed in another solid, liquid or gas.
❑ Colloidal solutions believed to be responsible for the formation of
ore deposits usually consist of solids dispersed in liquids and are
called "sols". In such sols, colloidal particles commonly adsorb
either cations or anions, and thus acquire similar charges which
cause them to repel each other, preventing them from coagulation.
❑ If an electrolyte is added to such a sol, the colloidal particles are
neutralized and flocculate giving rise to a variety of textures which
include:
a) Botryoidal or Reniform aggregates.
b) Banding or very fine layering.

64. Botryoidal or
reniform
aggregates

65. Note:
These textures are broadly lumped as "colloform"
textures (Figs. 14 & 15). Because some colloform
textures were observed in some hydrothermal
deposits, it was believed that some hydrothermal
solutions were colloids. However, fluid inclusion
analysis showed that hydrothermal solutions are too
saline to have been in the colloidal state, and the term
"colloform" should be considered descriptive and non-
genetic.

66. In the surfacial environment, colloidal solutions are common.
Criteria used to identify a colloform texture as a product of
deposition from a colloidal solution include:
1)Shrinkage cracks: which develop due to dehydration of a
gel
2)Variable composition of bands and/or deposits: This
phenomenon is due to the ability of colloids to adsorb
different ions from their surroundings.
3)Non-crystalline structure: The occurrence of amorphous
"minerals" or mineraloids (e.g. opal) is an indication of
formation from a colloidal solution. However, such
mineraloids will tend to crystallize with time.
4)Spheroidal texture: Are rounded objects similar to
pisolites, which result from the low surface tension of a
colloid.
Criteria used to identify a colloform texture as a
product of deposition from a colloidal solution

68. GEOCHEMICAL TECHNIQUES
Definition:
Fluid inclusions are inclusions in minerals that are filled with fluid (gas
and/or liquid), and in a few cases minor amounts of one or more solid
phases. Fluid inclusions result from defects in the crystals during their
growth, which lead to the entrapment of fluid in their surroundings.
• Fluids trapped in small crystal imperfections
• Can reveal information about the nature of ore forming fluids ie
exceedingly strong brines form at depth indicating chloride in
hydrothermal solutions is a potent solvent of metals through the
formation of metal-chloride complex ions (ligands)
1- Fluid inclusions

69. Fluid Inclusion Microthermometry

70. Fluid Inclusions
• Formed during crystal growth and provide us with a
sample of the ore forming fluid
• Yield crucial geothermometric data and tell us about the
physical state of the fluid eg boiling
• Most fluid inclusion work carried out on transparent
minerals such as quartz, fluorite and sphalerite
• Principle matter is water and carbon dioxide.
• 4 groups of inclusions
– Type 1 – two phase, principally water with some vapour
– Type 2 – two phase, principally vapour with some water
– Type 3 – three phase, water-vapour-halite, contain daughter
mineral that have crystallised from solution
– Type 4 – CO2-rich inclusions, CO2 liquid.

71. Types of fluid inclusions
(a) Based on their origin, fluid inclusions are of three types:
(i) Primary: Are those fluid inclusions formed during the formation of the
enclosing crystal. Texturally, they tend to be solitary or isolated. When
analyzed, these inclusions yield information on the conditions of
formation or crystallization of the host mineral.
(ii)Secondary: These occur along "healed fractures", i.e. fractures that
develop after the formation of the host mineral, and trap fluid available
then. Texturally, secondary inclusions can be recognized by their
occurrence in trails or clusters that often cut across grain boundaries.
(iii) Pseudosecondary inclusions: These form when, during the
crystallization of the host mineral, fracturing takes place, and the fluid is
trapped in inclusions along these fractures. These inclusions will
therefore occur along trails that end abruptly against grain boundaries.
(b) Ore petrologist classifications: These are classifications based on the
number and type of phases most commonly detected in fluid inclusions. Two
examples of such classifications are shown in Fig. 10.
A- Nash and Theodore (1971)
B- Ahmad and Rose (1980)

72. Principles of fluid inclusion analysis
After fluids are trapped at a certain P and T in a mineral, that mineral will most likely undergo a stage of
cooling and decompression as this rock begins its journey to the surface of the earth. During these
stages, the fluids in the inclusions, which were most likely entrapped under supercriticalconditions (Fig.
8), will undergo phase changes when their uplift path intersects a phase boundary. Because fluid
inclusions are preserved in minerals that are considered relativelystrong, it is assumed that no changes
in mass or volume of the fluid inclusion take place during uplift. This means that the fluid inclusions
follow a path of constant density. Lines of constant density on a P-T diagram are known as "isochores".
Fluids of different compositions will have isochores with different slopes in P-T space.
For example: isochores of pure H2O are much steeper than those of CO2. Accordingly, during uplift,
water rich inclusions will intersect the liquid vapour curve from the liquid field, at which point a gas
bubble will appear. On the other hand, an inclusion filled with CO2 may appraoch the liquid - vapour
boundary from the vapour side (depending on the density), in which case the vapour bubble will shrink
as the vapour condenses into liquid.
If an inclusion contains a mixture of two fluids that are miscible only at high T, then upon cooling and
decompression, these components will unmix, and two separate phases (immisciblefluids) will appear
in the inclusion.
To study phase transitions in fluid inclusions, a doubly polished thin section is prepared, then broken
into small chips that are mounted on a special heating and freezing stage. These chips are then
observed while the stage is either heated or cooled, and the phase changes which take place during a
heating or cooling cycle are carefully recorded along with the temperatures at which such changes
occur.

73. Measurtements and inferences: (Fig. 9)
▪Temperature of entrapment
▪Homogenization temperature
▪"Pressure" correction
▪Decrepitation temperature
▪Freezing point depression

74. Information derived from fluid inclusions
(a) Composition of the fluids: The stage is cooled with the help of liquid N2 during which time phase
changes in the inclusion are carefully monitored. The temperatureat which the fluid in the inclusion
freezes is then recorded. Knowing the freezingpoints of pure H2O and CO2, the recorded freezing point
is "plugged"into an equation of state (one of the form: PV= nRT), and the freezingpoint depression for
the system in question is then directly related to the "impurities" in this system or the concentrationof
salts. Moreover, the inclusions can be physically "opened"or crushed, and the fluids can be chemically
analyzed.
(b) Temperatures: Microthermometrictechniques rely on heating two - phase inclusions on the stage
until they homogenize (i.e. change to one phase). The temperature at which homogenizationtakes
place can therefore be considered a "minimum T" for fluid entrapment. If the pressure of entrapment
(or mineral formation)is known by some independent means (e.g. phase relations or geobarometry),
then the temperatureof entrapment can be immediatelydeterminedby the intersection of the
isochore passing through the homogenization T with this pressure. The difference between the
homogenizationT and the T of entrapment is known as the "P correction" (even though it has nothing
to do with P!; Fig. 9).
Assumptions
Microthermometric studies rely heavily on the validity of two assumptions:
(a) no leaking or necking since entrapment. If an inclusion shows evidence of necking, it should not
be used for microthermometry.
(b) the inclusion followed a path of constant density along one of the isochores for the system in
question (Figs. 8 & 9).

75. 2- Phase diagrams
1- Ternary diagrams
2- P-T diagrams
3- T-X diagrams (or T - activity ratio diagrams).
4- P-X diagrams
5- T-fO2 diagrams
6- activity - activity diagrams
3- Geothermometry
Examples:
(a) Polymorphic transitions: e.g. -Qz -Qz, orthorhombic chalcocite
hexagonal chalcocite
(b) Incomplete miscibility and exsolution textures (e.g. ilmenite –
hematite & magnetite – ulvospinel pairs).
(c) Stable isotopes.
(d) Fluid inclusions (Figs. 9 and 11).
4- Geobarometry
Examples: Fe in Sphalerite (Fig. 12).

76. 5- Stable isotope studies
A- Oxygen and hydrogen isotopes
Factors controlling 18O and D values of minerals:
1) 18O and D values of the original rock prior to hyrothermal alteration, metamorphism,
or supergene enrichment.
2) 18O and D values of the fluids interacting with these rocks.
3) Temperatures at which oxygen isotope equilibration between the various phases took
place.
Ranges of 18O and D values for various types of fluids: Figure 13 is a plot of 18O vs. D for
various fluids. This figure shows that meteoric waters plot along a line depending on the latitude,
that metamorphic rocks are more enriched in 18O than meteoric water, but less enriched than
sediments.
Applications: Stable isotopes can therefore be used to determine the temperatures of
equilibration, or more importantly the source of the fluids. For example, it was thought that most
hydrothermal fluids were "juvenile", or magmatic in origin. However, O and H isotope analysis of
hyrothermal minerals has shown that such fluids have a meteoric water signature, and represent
in large part meteoric water that infiltrated to significant depths (where it may have mixed with
small amounts of magmatic, metamorphic or connate waters). This water was then heated,
causing it to rise towards the surface again. Fig. 14 shows the 18O and D values of hydrothermal
waters from a variety of ore deposits.

77. 5- Stable isotope studies

78. The two stable isotopes of S commonly analyzed for are 32S (which constitutes
95% of natural S) and 34S (4.21%). The S isotope standard is S in troilite in a meteorite
from Arizona. Fractionation of these two isotopes takes place either organically or
inorganically in nature. Organic fractionation takes place with the help of bacteria that
consumes sulfates and can break the 32S-O bond more easily than it can break the 34S-
O one. This process results in the production of H2S that is isotopically “light” (34S as
low as -75%°). Reaction of this H2S with other elements as Fe then produces
“bacteriogenic sulfides” characterized by being isotopically light. Note that one factor
complicating the interpretation of S-isotope data is that 34S of marine water has
fluctuated over time between +10 and +30%° (Fig. 15a).
Inorganic fractionation of S isotopes is more complex, but in general the heavy
isotope is more concentrated in sulfates than in sulfides, and in sites which are
characterized by greater bond strength. As with O and H isotopes, this fractionation
also depends on T. Other factors affecting the 34S include pH, fO2, and fS2. Analysis of S
isotopes in hydrothermal ore minerals has revealed that the isotopic signature of
these minerals is controlled primarily by that of the ore forming fluid rather than that
of the host rock. S – isotopes were instrumental to understanding the origins of the
ore – bearing fluids of the MV type Pb-Zn deposits (Fig. 15b).
B- Sulfur isotopes

79. The stable isotopes of carbon 13C and 12C are
commonly measured in labs, especially for carbonates.
The standard for this technique is that for C in CO2
released from a belemnite sample (PDB). 13C commonly
ranges from 0%° in sedimentary limestones to -7%° in
deep seated crustal rocks, -25%° in biogenic limestones.
Hydrothermal veins are characterized by values of 13C
between -4 and -12%°, and C isotopes are generally not
very useful in identifying the source of such fluid.
C- Carbon isotopes