hydrothermal alteration mineralogy - the argeo

Sustainable Development Goals Short Course II on Exploration and Development of Geothermal Resources

Lucy Njue [email protected]

Hydrothermal Alteration Mineralogy

10/11/17

SDG Short Course II on Exploration and Development of Geothermal ResourcesOrganzied by UNU-GTP, GDC and KenGen | Lake Bogoria and Lake Naivasha | Nov 9-

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PRESENTATION OUTLINE

• Introduction• What is hydrothermal alteration mineralogy• Sampling methods and analytical methods• Alteration types• Factors affecting hydrothermal alteration• Application of hydrothermal alteration


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A mineral is naturally occurringinorganic solid that possesses anorderly internal structure and adefinite chemical composition.

Normally crystalline and hasformed as a result of geologicprocesses.

Mineral

Quartz (SiO2)

Orthoclase(KAlSi3O8)


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3,500 known minerals

“rock-forming minerals” quartz,feldspars, micas, amphiboles,pyroxenes and olivine.

Main rock forming minerals

Others include: oxides,sulphides, carbonates and nativeelements etc.

Amphibole

Pyrite


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A rock is an aggregate of one or more minerals,naturally occurring crystalline substances withdefined properties.

Igneous rocksSedimentaryMetamorphic rocks

Rock

Types of rocksGranite


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Rock alteration simply means changing themineralogy of the rock.

Rock alteration

AlteredFresh


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HYDROTHERMAL ALTERATION A change in mineralogy as a result of interaction of therock with hot water fluids (hydrothermal fluids).


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SAMPLING• In the field, altered zones are sampled for further XRD analyses

• In geothermal drilling, cuttings samples are taken after every 2m interval.

• Cores are cut mainly during exploration wells or when needarise.


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ANALYTICAL METHODS-1

Binocular/stereo microscopePreliminary analysis is done at the rig site by use of a stereo microscope.

In the field use of hand lens


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Petrographic microscopeRepresentative samples selected andprepared for petrographic studies.

1. Confirm the rock type(s) and thealteration minerals

2. Additional alteration

3. Mineralogical evolution of the alterationminerals.


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X-Ray DiffractometerThe X-Ray Diffractometer is used to identify individual minerals especially clays and zeolites.

I nci dent X-ray（λ

） Dif f

ract

ed X

-ray

θ θ

d

Bragg' s Equati on2dSi nθ=nλ

X-ray DiffractometerClays (OW-903, 1137-1135 masl)

Lin (C

ps)

0

100

200

300

400

500

600

700

800

900

1000

1100

2.2 3 4 5 6 7 8 9 10 11 12 13

d=15,4

73

d=13,5

58

d=31,6

0

d=29,0

95

d=8,95

1

d=7,71

3

d=7,13

1d=10,0

52


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ANALYTICAL METHODS 4Fluid inclusion analysis

Fluid inclusions are small portions of fluid,which are trapped in a solid crystal as itgrew or recrystallized.

Quartz and calcite crystals are prepared forfluid inclusion analysis.

Th-Temperature of homogenization-fluidinclusions

Tm-Temperature of melting-melt inclusions


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ANALYTICAL METHODS 5

Electron Microprobe analysis Permits non destructive major and trace elements analysis ofsmall areas.

Minerals not identified by methods above are easily picked.

• Solid solution chemical formula (Plagioclase series)

• Mole fractions of e.g.- Garnet andradite and grossular; Xan: and Xgro 0.64 and 0.36- Epidote epidote and clinozoisite Xepi: and Xclir 0.20 and 0.80


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ALTERATION TYPES-1(a) Direct deposition

Quartz vein

Precipitated from saturated solution and deposited in veins and vesicles

Sulphur


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ALTERATION TYPES-2(b) Replacement

Primary phases Alteration productsVolcanic glass Zeolites, clays, quartz, calciteOlivine Chlorite, actinolite, hematite, clay

minerals

Pyroxenes, amphiboles Chlorite, illite, quartz, pyrite, calcite

Ca-plagioclase Calcite, albite, adularia, quartz, illite,epidote, sphene

Sanidine, orthoclase, microcline Adularia

Magnetite Pyrite, sphene, haematiteQuartz No alteration


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ALTERATION TYPES-3(c) Leaching

Acidified steam condensate dissolve primaryminerals without replacing thus voids are formed.


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ALTERATION TYPES-4(d) Ejecta

Bladed euhedral crystals of calcite form in theturbulence of boiling


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FACTORS CONTROLLING ALTERATION-1

1. TemperatureTemperature is the mostsignificant factor inhydrothermal alteration.Most chemical reactionsrequire elevated temps.Alt. min. are thermo-dynamically stable at hightemperatures.


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FACTORS CONTROLLING ALTERATION-22. PermeabilityPermeability of the rocks controlsthe access of thermal fluids,which cause hydrothermalalteration of the rocks andprecipitation of secondaryminerals in open spaces.

Rocks which have very restrictedpermeability or are completelyimpervious to fluid will be onlyslightly altered.

Crystalline


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FACTORS AFFECTING ALTERATION-33. PressureUnlike metamorphic environments, Pressuresseldom exceed 200 bars.

Main effect of pressureis an indirect one in thatit controls depth atwhich boiling.


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FACTORS AFFECTING ALTERATION-44. Initial rock compositionThe chemical composition of the host rock determines theavailability of components to form alteration minerals.

•Glass easily susceptible than the crystalline rock•Reverse of the Bowen's reaction series


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FACTORS AFFECTING ALTERATION-5

5. Fluid compositionThe pH and the composition of the fluid greatlydetermine rate and the types of hydrothermalminerals to be formed.Acidic e.g. leachingNeutral e.g. replacementBasic e.g. deposition


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APPLICATION-1

1. GeothermometersAlteration minerals e.g. Zeolites, Clays, Prehnite,Epidote are useful as geothermometers

Minerals Tmin(0C) Tmax(0C)Zeolites 40 120Quartz 180 300Calcite 50 300Illite 230 300Albite 120 150Epidote 230 300


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APPLICATION -2

2. Setting production casing depths

Temperature ranges of minerals especiallyphyllosilicates and calcsilicates assist inestimating subsurface temperatures hencesetting of production casing depths.-Epidote-Chlorite


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APPLICATION-3

3. Permeability indicators

Alteration minerals e.g. quartz, anhydrite,wairakite calcite and pyrite are used asindicators of high permeability.


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APPLICATION-4

4. Understand the nature of the reservoir

• Determine the upflow, outflow and marginal zones

• Determine structures that control the geothermal fluids

• Predicting possible boiling and or high gas zones duringdrilling-bladed calcite

• Identifying past fluctuations in the thermal system


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APPLICATION-5

5. Predicting scaling and corrosion tendencies

• pH of the hydrothermal fluids control alterationminerals

• Bladed calcite indicate boiling hence possibility ofcalcite scaling and corrosion


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APPLICATION-6

6. Identifying structures

Leakages occur at the pointof weakness, hence easy toidentify structures e.g. Pakavolcano.


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APPLICATION-7

6. Source of economic minerals

Gold,silver,sulphur etc.


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CONCLUSIONSHydrothermal Alteration mineralogy

1. Setting the correct production casing depth

2. Accurately site best location

3. Location of the inflow, upflow and the outflow.

4. Good producer wells >10 MWe

5. Temporal changes have occurred in the field.

hydrothermal alteration mineralogy - the argeo

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