1. AN INTRODUCTION TO ORE MICROSCOPY
 Definition, classification & types of ores.
 Sample preparation & techniques of study.
 Properties of ore minerals.
Krishanu Nath,
Sr. Geologist,
STM II Division,
NER, GSI, Shillong.

2. Ore is rock that may be, is hoped to be, will be, is or has been
mined; and from which something of value may be (or has
been) extracted.
(Taylor, 1989, Ore reserves – a general overview. Mining Industry
International, vol. 990, pp. 5–12.)

3. Ore Minerals
Primary Ore/Hypogene
Deposits
Syngenetic
Ore
Epigenetic
Ore
Secondary Ore / Supergene
deposit
Residual
deposit
Placer
deposit
Mechanical/
Chemical
processes

4. Syngenetic mineral deposit is a deposit which formed at the same time
as the rocks that enclose it. Magmatic deposits are syngenetic in that
the ore minerals crystallize from the same liquid that produces the
silicate minerals which form the bulk of the intrusive - they crystallize
more or less simultaneously as the melt cools.
Brassy-yellow Chalcopyrite is
the primary copper-bearing
mineral in Porphyry deposits.

5. If a mineral deposit is formed much later than the rocks which hosts it, it is
said to be epigenetic.
The host rock must be lithified and brittle, creating open spaces when it
breaks open to let the hydrothermal solutions pass along the open space and
deposit or precipitate the ore and gangue minerals within the open spaces.
Development of Sulphide
Minerals within veins by
circulation of hydrothermal fluid.

6. Supergene Processes: The processes of enrichment are those that occur
relatively near the surface of the earth. These processes are predominantly
effected by circulation of meteoric water. Movement of the water table
upward or downward with respect to the ground surface.

7. Pyrite: FeS2 + 7O + H2O →FeSO4 + H2SO4
2FeSO4 + H2SO4 + O → Fe2(SO4)3 + H2O
Chalcopyrite: CuFeS2 + 2Fe2(SO4)3 → CuSO4 + 5FeSO4 + 2S
Covellite: CuS + Fe2(SO4)3 →2FeSO4 + S
Sphalerite: ZnS + 4Fe2(SO4)3 + H2O →ZnSO4 + 8FeSO4 + 4H2SO4
Galena: PbS + Fe2(SO4)3 + H2O + 3O →PbSO4 + 2FeSO4 + H2SO4

8. Bauxite deposits are formed chiefly by weathering of aluminous
rock; some have been transported to their present locations, but most
are residual accumulations from which most other constituents of the
parent rock, other than alumina, have been leached.
CO2 + H2O → H2CO3
H2CO3 → HCO3 - + H+
HCO3- → CO32- + H+
3KAlSi3O8 + 2H+ + 12H2O → KAl3Si3O10(OH)2 + 2K+ + 6Si(OH)4
Feldspar Illite
2 KAl3Si3O10(OH)2 +2H+ + 3H2O → 3Al2Si2O5(OH)4 + 2K+
Illite Kaolinite
Al2Si2O5(OH)4 + 5H2O → 2Al(OH)3 + 2Si(OH)4
Kaolinite Gibbsite
Kaolin Clay Pisolitic Bauxite

9. Placer Deposits: Placers may be defined as surficial mineral deposits formed
by the mechanical concentration of mineral particles from weathered debris.
The mechanical agent is usually fluvial, but can also be marine, aeolian,
lacustrine, and glacial, and the mineral is usually heavy metal, such as gold.
They are thus the deposits of gravel, sand or similar material resulting from
the crumbling and erosion of solid rocks containing valuable minerals that
have been derived from rocks or veins.

10. Volcanic hosted Massive Sulphide Deposit
Sedimentary Exhalative Deposit

11. Technology Methodology Analyses Results

12. An ore microscope differs from a petrographic one in that it has an
incident light source rather than a substage transmitting one, which
allows examination of polished surfaces of opaque minerals under
reflected light.

13. SAMPLE CUTTING AND MOUNTING
1
2
3
4
5
6

14. i) 600-mesh silicon carbide. ii) 15 μm diamond.
iii) 6 μm diamond. iv) 1 μm diamond.
QUALITY OF POLISH USING DIFFERENT MEDIUM

15. DOUBLY POLISHED THIN SECTION
Comparison of the structure visible in the same sample of sphalerite when viewed:
(a) in normal reflected light (b) in transmitted light

16. Chemical etching has been used for mineral identification. The electron
microprobe has largely replaced this method but it still has its uses in revealing
structure. Etching is suitable for detecting zoning, twinning and grain boundaries
and for enhancing mineral phases when modal analyses are required.
Etching may be used to enhance structures within grains, (a) a freshly polished pyrite
grain, (b) after etching with concentrated HNO3.
Etchants may also enhance the difference between hexagonal pyrrhotite (dark) and
monoclinic pyrrhotite (light). Identification of different phase.
Commonly used etchants conc. HNO3, ammonium dichromate-HCl solution,
H2SO4.
Complex Etching solutions:-
SnCl2- 1 part saturated solution to 1 part concentrated HCI.
KMn04-2.5 g in 100 cm3 H20; I part to I part concentrated HN03 ; I part to I part
concentrated H2S04; 1 part to 1 part KOH.

17. STUB FOR & GRAIN MOUNTING FOR EPMA STUDY

18. ADVANCE TECHNIQUES IN ORE MICROSCOPY
Electron Probe Micro Analyses
Raman Spectroscopy
Fluid Inclusion
Scanning Electron Microscope
Stable Isotope Study.
X-Ray Defraction

20. Colour:-A very small number of ore minerals are strongly and distinctively
coloured, but most are only weakly colored and may appear to the beginner
as white through various shades of gray.
IFD (Illuminator Field Diaphragm) and IAD (Illuminator Aperture
Diaphragm) opened wide .
 Low power objective .
High voltage for light of good intensity .
Polarizer inserted, Analyzer withdrawn.

21. The reflectance of a mineral may be quantitatively or qualitatively measured; in
other words, measured accurately or estimated by eye. The amount of light
which is reflected back to the observer can be influenced by a number of factors.
i. Crystallographic orientation of the polished surface
relative to the vibration directions of linearly polarized
incident light.
ii. Wavelength of light.
iii. Angle of incidence.
iv. Degree of polish.

22. The change of reflectance is a property termed bireflectance
(anisotropism), and the change of color (or tint) is a property called
reflection pleochroism.
All sections of the cubic minerals and basal sections of the hexagonal
and tetragonal crystals do not exhibit these properties.
The difference in maximum and minimum values of the percentage of
reflectance is the measure of the bireflectance.
strong bireflectance - graphite, molybdenite, covellite, stibnite, valleriite;
moderate bireflectance -marcasite, hematite. niccolite, cubanite, pyrrhotite;
weak bireflectance - ilmenit, arsenopyrite etc.

23. Polishing hardness is the resistance of a particular mineral to abrasion
during the polishing process. The fact that hard minerals are worn away
more slowly than soft minerals means that they may stand slightly above
the surfaces of softer grains in the section-an effect known as polishing
relief.
Scratch hardness is the relative amount of surface scratching and the
depth of scratches that cross grain.
Sphalerite
Pyrite
Mohs’ Scale of hardness for
ore minerals
Ag2S
PbS
CuFeS2
(Cu,Fe)12Sb4S13
NiAs
Fe3O4
FeTiO3

24. KALB LINE TEST

25. Cleavage and Parting: Feature arising from the arrangement of atoms of
the chemical composition of the mineral.
Cleavage or parting is seen in polished section as one or more sets of
parallel cracks, if three or more cleavage directions are present, parallel
rows of triangular pits maybe observed. Such pits are particularly
characteristic of galena but may also be observed in magnetite,
pentlandite, etc.

26. Zonal structure may be seen in ordinary or polarised light or it may require the sample
to be etched before it becomes apparent. It appears as concentric (internal) bands parallel
to the crystal faces or indicating the original crystal form.
1. Depositional pauses during growth.
2. Different growth rates with or without inclusions.
3. Chemical variations (often minor) during growth.
KIRUNA ZONING in FeS
Twinning: Three major types of twinning may be observed in ore minerals seen in polished
sections - growth, inversion and deformation. Twinning is best seen in anisotropic minerals
under crossed polars. In isotropic minerals it is generally not visible unless the surface is
etched.
Arrow Head Twinning in
Marcasite.

27. Epithermal Breccia: Clasts of Quartzite within iron rich hydrothermal solution.