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CHAPTER 5

CHAPTER 5. GEOTHERMOMETRY GEOBAROMETRY PARAGENESIS ZONING DATING OF MINERALS. GEOTHERMOMETRY. The estimation of the temperature of formation of a mineral or a mineral assemblage is called geothermometry.

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CHAPTER 5

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  1. CHAPTER 5 GEOTHERMOMETRY GEOBAROMETRY PARAGENESIS ZONING DATING OF MINERALS

  2. GEOTHERMOMETRY • The estimation of the temperature of formation of a mineral or a mineral assemblage is called geothermometry. • Ore and gangue minerals in mineral deposits are deposited at various temperatures and pressures. • The temperatures range from atmospheric (about 20C to 50C) to magmatic (up to 900C).

  3. GEOTHERMOMETRY • Methods of geothermometry may be divided into two: • qualitative estimates • quantitative estimates.

  4. GEOTHERMOMETRY Qualitative estimates: These methods indicate rangestemperatures orrelative temperatures • Mineral Assemblages • Mineral Textures • Color Changes

  5. GEOTHERMOMETRY Quantitative estimates These methods indicate definite temperatures or ranges of temperatures. • Direct temperature measurements • Fluid inclusions • Inversion points • Exsolution textures • Mineral synthesis & melting temperatures • Stable isotope studies • Thermoluminescence • Electrical conductivity

  6. GEOTHERMOMETRY:Qualitative Estimates • Mineral Assemblages • Assemblages of minerals are used to indicate relative temperatures such as: • very high • magmatic:chromite-olivine, magnetite-ilmenite, pentlandite-pyrrhotite-chalcopyrite • pegmatitic:orthoclase-quartz-beryl-columbite • high • cassiterite-wolframite-topaz-tourmaline • medium • quartz-pyrite-chalcopyrite-sphalerite-galena • low • cinnabar-pyrite-stibnite

  7. GEOTHERMOMETRY: Qualitative Estimates Mineral Textures • Textures and growth habits of some minerals may indicate relative temperatures. e.g: • pyrite has • pyritohedron form at higher temperatures, • octahedron form at medium temperatures, • cubicform at low temperatures, and • colloform at very low temperatures. • cassiterite is • pyramidal or short prismatic at high temperatures, • needle-like at medium temperatures, and • botryoidal at low temperatures.

  8. GEOTHERMOMETRY: Qualitative Estimates Color Changes • Color changes in minerals may indicate temperatures of formation • Color of sphalerite ranges • from blue-black • through honey-brown • to light yellow • as temperature and iron content decreases. • Smoky quartz and amethyst lose color at around 250C, so their presence indicate temperatures below 250C. • Fluorite loses color at around 175C, so colored fluorites indicate lower temperatures. • None of these changes, however, is reliable enough to be of significant use.

  9. GEOTHERMOMETRY:Quantitative Estimates Direct Temperature Measurements • The measurement of the temperatures of lavas, fumaroles, and hot springs yields maximum temperatures of formation for the minerals transported in them. • Temperatures of about 1200C have been recorded for basaltic lavas (Kilauea, Hawaii), and about 800C-900C have been estimated for rhyolitic lavas (Santorini, Greece). • Crystallization of minerals occurs principally between 870C and 650C. • The gas temperatures of fumaroles indicate maximum temperatures for fumarolic minerals. Recorded temperatures are about 600C- 700C. At Katmai, Alaska, magnetite has been deposited from a fumarole at 645C.

  10. GEOTHERMOMETRY:Quantitative Estimates Fluid Inclusions • Give reliable estimates on  temperatures of mineral deposition • Estimation of temperatures using fluid inclusions is based on the following assumptions: • Fluid inclusions in minerals were formed during the deposition of the minerals, • the contents of inclusions were residues from the original solutions • the content was homogeneous before cooling, but during cooling it was separated into liquids, gases, and solids. In order to bring them into a homogeneous state, they are heated in a heating stage under microscope; the temperature at which minor or subordinate phases disappear (liquid will expand to fill the inclusion, liquid will dissolve gaseous and/or solid phases, gas will absorb the liquid fraction), is called the temperature of homogenization. This temperature is accepted as the minimum temperature of formation.

  11. GEOTHERMOMETRY:Quantitative Estimates Inversion Points • Certain minerals undergo internal structural changes at definite temperatures and pressures. • The phenomenon of one chemical compound occurring in more than one structural form is called polymorphism. • The temperatures and pressures at which such changes occur are called inversion points.

  12. GEOTHERMOMETRY:Quantitative Estimates Inversion Points Low Quartz (SiO2) = High Quartz [573C and 1 atm ] Chalcopyrite (Tetragonal, low) = Chalcopyrite (Cubic, high) [525C] Chalcocite (Orthorhombic, low) = Chalcocite (Hexagonal, high) [105C] • high temperature polymorph minimum temp. of formation. • low temperature polymorph maximum temp. of formation

  13. GEOTHERMOMETRY:Quantitative Estimates Exsolution Textures • At high temperatures, mineral structures are disordered, thus several molecules may be accommodated in the structure. • As temperature decreases, the order increases, and the molecules that do not fit the structure are forced out. • This phenomenon is called exsolution or unmixing. • Forced out material tend to accumulate along structurally controlled directions, forming lamellae (ilmenite-magnetite), blades (bornite-chalcopyrite), or blebs (chalcopyrite-sphalerite)

  14. Blebs (sphalerite-chalcopyrite) Blades (bornite-chalcopyrite) Lamella (ilmenite-magnetite) GEOTHERMOMETRY:Quantitative Estimates Exsolution Textures

  15. GEOTHERMOMETRY:Quantitative Estimates Exsolution Textures • Exsolution textures are observed under microscope. • The presence of exsolution textures indicates a minimum temperature of formation. • Some common exsolution pairs are Bornite-Chalcopyrite 300C-500C Sphalerite-Chalcopyrite 350C-650C Chalcopyrite-Cubanite 235C-450C Magnetite-Ilmenite 600C-800C Chalcopyrite-Pyrrhotite 250C-600C.

  16. GEOTHERMOMETRY:Quantitative Estimates Mineral Synthesis and Melting Temperatures Mineral Synthesis • Sulfide minerals may be synthesized in the laboratory under various conditions • For this purpose, the components of a mineral are mixed in stoichometric proportions and they are heated in a closed container. • The lowest temperature at which the mineral forms is accepted as the minimum temperature of formation. • However, these values are valid for pure systems. During the formation of mineral deposits, several other components and phases affect the formation of minerals. As a result, these values may only be used as suggestions.

  17. GEOTHERMOMETRY:Quantitative Estimates Mineral Synthesis and Melting Temperatures Melting Temperatures • The temperature and pressure at which a mineral melts are assumed as the upper limits of stability for that mineral. Melting points are useful to establish maximum temperatures of formation. • But, these are usually of limited value for estimating actual temperatures. • If the melting point is very high (Chalcocite-1130C, Galena-1112C, Sphalerite 1850C) it cannot be used. • if the melting point is low (Realgar-320C, Native Bismuth-271C)  it will set an actual upper limit for the temperature of formation.

  18. GEOTHERMOMETRY:Quantitative Estimates Mineral Synthesis and Melting Temperatures Melting Temperatures • Melting temperatures of individual minerals may be high, but the melting temperatures for mixtures of minerals may be lower, if they have common components • Such temperatures are called eutectic temperatures. • For example, • the melting temperature of argentite =840C • the melting temperature of chalcocite =1130C • the binary eutectic temperature for argentite-chalcocite= 675C. • Eutectic temperatures are very useful, since they provide estimates for assemblages, and they closely resemble natural systems.

  19. 1130 C TC Liquid 840 C eutectic Liquid + Cu2S Liquid + Ag2S 675 C 675 C Ag2S + Cu2S Ag2S Cu2S Eutectic Temperatures

  20. GEOTHERMOMETRY:Quantitative Estimates Stable Isotope Studies • Oxygen isotope ratios (18O/16O) are temperature-dependent, and they are used as geothermometers for pairs of minerals. • The ratios are obtained by analyzing the samples by a mass spectrometer. • These are recalculated into 18O values using the formula:

  21. GEOTHERMOMETRY:Quantitative Estimates Stable Isotope Studies (18O/16O )sample - (18O/16O)SMOW 18O (‰) = ---------------------------------------------- x 1000 (18O/16O)SMOW • Experimentally it is found that log 18O = A(1/T2) + B where A and B are system dependent constants. • Calibration curves are obtained for quartz-magnetite, quartz-ilmenite, quartz-muscovite, quartz-calcite, quartz-alkali feldspar, etc

  22. Qtz-mag Qtz-musc 1 - 2 Qtz-alk. Felds. Temperature 1= 18O for the first mineral of the mineral pair (e.g. Qtz) 2= 18O for the first mineral of the mineral pair (e.g. mag) GEOTHERMOMETRY:Quantitative Estimates

  23. GEOTHERMOMETRY:Quantitative Estimates Thermoluminescence • Thermoluminescence is the property of a substance to emit visible light when it is heated. • Different minerals will give characteristic glow curves which show the intensity and the spectral distribution of luminescence as a function of temperature. • These are recorded as a sample is heated slowly while it is watched by a photomultiplier tube. Once a sample has been heated, its thermoluminescence is drained permanently the sample will not glow again when reheated

  24. GEOTHERMOMETRY:Quantitative Estimates Thermoluminescence • Limestones at Gilman, Colorado, gave glow peaks at 235C and 330C. • The low temperature peak was absent from dolomitized material • Both peaks were absent very close to ore • This may mean thatdolomitized rocks were heated above 235Candvery close to ore, temperatures were above 330C.

  25. GEOTHERMOMETRY:Quantitative Estimates Electrical Conductivity • Crystals that formed at high temperatures have higher electrical conductivities. • Thus, it is assumed that the electrical conductivities of minerals may indicate the temperature of formation, that is the higher the conductivity, the higher the temperature of formation. • The method still needs more detailed work and experimentation.

  26. GEOBAROMETRY • The estimation of the pressure of formation of a mineral or a mineral assemblage is called geobarometry. • The pressures range from atmospheric (1 bar) to very high (12 kilobars). • Methods of Geobarometry • Ionic Substitutions • Fluid Inclusions

  27. GEOBAROMETRY Ionic Substitutions • Coupled substitution of silica-alumina and alkalies in white mica (sericite-phengite) lattices are pressure controlled. This results in a systematic change in bo cell dimension. Measurement of bo by x-ray techniques is promising to be a geobarometer. • Another example is FeS content in sphalerite, in equilibrium with pyrrhotite and pyrite. Amount of FeS in sphalerite increases with pressure

  28. GEOBAROMETRY Fluid Inclusions • Homogenization temperature is a function of pressure. • The pressure of formation may be estimated using the equipment known as nomograms.

  29. PARAGENETIC SEQUENCE &ZONING

  30. PARAGENESIS • In a mineral deposit, minerals form an assemblage and display a sequence of mineralization. For this concept, we use the term PARAGENESIS. • Paragenesis means • the time sequence of mineral deposition (definition preferred in North and South America), • the association of minerals having a common origin (definition preferred in Europe).

  31. PARAGENESIS • The associations may be classified as hypogene or supergene, or according to nature of the fluid. • Some common associations are • gold-quartz, • sphalerite-galena-barite- fluorite, • cassiterite-tourmaline-topaz, • pyrite-chalcopyrite, magnetite-apatite, and • pyrite-chalcopyrite-bornite-molybdenite.

  32. PARAGENESIS • The order of deposition is determined by studying mineral assemblages under microscope. • Samples are prepared as polished sections, and a reflected light microscope is used. • Textures and structural relationships between minerals are used to decide which mineral is formed earlier. • The relative ages of each mineral pair is noted and plotted on a mineral-mineral diagram. • Then the interpretations are plotted on a time-mineral diagram, called a paragenetic diagram.

  33. PARAGENESIS EXAMPLE • observations:the assemblage consists of two generations of pyrite and quartz, sphalerite, chalcopyrite and galena. • pyrite and quartz are intergrown (simultaneous) • sphalerite replaces pyrite • chalcopyrite crosses sphalerite • galena crosses chalcopyrite • interpretations: Early Pyrite & Quartz Sphalerite Chalcopyrite Late Galena

  34. PARAGENESIS • The ore microscopy studies and field observations throughout the world has established a general sequence of mineral deposition. • The associations from early to late may be grouped as • Cr-Fe-Ti-oxides • Fe-Ni-Co-sulfides/arsenides • Sn-W-Mo • Zn-Cu-Pb-Ag-Fe-S • Cu-Pb-Zn-Ag-Sb-As-S (sulfosalts) • Au-Ag • Sb-Hg-S

  35. ZONING • Zoning is the three-dimensional distribution of minerals or mineral deposits with respect to each other. • Zoning patterns are shown as mineralogic or chemical element changes along vertical and horizontal traverses. • Ideally, as the ore-bearing fluid moves vertically and horizontally, physical and chemical changes cause deposition of different assemblages at different points in a three-dimensional space. • This causes a concentric zoning for most epigenetic deposits.

  36. ZONING • Epigenetic Hydrothermal Zoning is of 3-types: • Regional Zoning • District Zoning • Ore Body Zoning

  37. ZONING Regional Zoning • occurs on a very large scale, often corresponding to large sections of orogenic belts and their foreland. • The zoning pattern may be defined by the configuration of a number of mining districts. • The scale may be in kilometers to tens of kilometers. • Examples: • the Variscan Metallogenic Province of the Western Europe. • a number of provinces described from the circum-Pacific orogenic belts. • the Andes (South America)

  38. ZONING District Zoning • defined by a number of mines in a district • The scale may be in kilometers. The example is Cornwall-Devon region in southwestern England, where Sn-W-Mo, Cu, Pb-Zn-Ag, and Fe districts are zoned around a center of intrusions • Zoning of this type is most clearly displayed where the mineralization is of considerable vertical extent and was formed at depth where changes in the pressure and temperature gradients were very gradual. If deposition took place near to the surface, then steep temperature gradients may have caused superimposition of what would, at deeper levels, be distinct zones.

  39. Cu BARREN Fe Pb-Zn-Ag Fe Fe Pb-Zn-Ag Cu Cu Sn-W-Mo GRANITE Sn-W-Mo District Zoning Cu

  40. ZONING Ore Body Zoning • Defined by mineralogic or chemical changes within a mineral deposit • The scale may be in meters to hundreds of meters • E.g. Emperor Gold Mine, Fiji

  41. DATING MINERAL DEPOSITS • When orebodies form part of a stratigraphical succession, such as the Mesozoic ironstones of north-western Europe, their age is not in dispute. • Similarly the ages of orthomagmatic deposits may be fixed almost as certainly if their parent pluton can be well dated. • On the other hand, epigenetic deposits may be very difficult to date, especially if deposits have resulted from polyphase mineralization, with epochs of mineralization being separated by intervals in excess of 100 Ma. • Three main lines of evidence can be used: the field data, radiometric and palaeomagnetic age determinations: • field evidence • radiometric dating • palaeomagnetic dating

  42. DATING MINERAL DEPOSITS Radiometric Dating • either ore minerals (suitable for age dating, e.g. Uraninite) or wall rock alteration products (micas, feldspars, clay minerals) are used. • In the latter case, the wall rock alteration is coeval with some of or all the mineralization. • Age dating is based on the radioactive decay of an unstable parent isotope to a more stable isotope of a different element (daughter)

  43. Table 5.1. Major isotopic dating methods.

  44. DATING MINERAL DEPOSITS Paleomagnetic Dating • The successful palaeomagnetic dating of ore deposits depends on a number of factors. Some of the most important are: • the development of magnetic minerals in a deposit or its wall rocks during one of the principal phases of mineralization; • a lack of complete oxidation or alteration, which may be accompanied by overprinting with a later period of magnetization • Magnetite and hematite are the two principal carriers of magnetization in rocks; this is true also for ore deposits.

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