AN INTRODUCTION TO ORE MICROSCOPY
- 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.