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By
John Lagat
HYDROTHERMAL ALTERATION MINERALOGY
Presentation outlinePresentation outlineIntroductionIntroductionWhat is hydrothermal alteration mineralogyWhat is hydrothermal alteration mineralogySampling methodsSampling methodsAnalytical methodsAnalytical methodsAlteration typesAlteration typesFactors affecting hydrothermal alterationFactors affecting hydrothermal alterationPrimary minerals and their alteration productsPrimary minerals and their alteration productsApplication of alteration in geothermal systemsApplication of alteration in geothermal systemsApplication of hydrothermal alteration in ore Application of hydrothermal alteration in ore
depositsdepositsProblems with hydrothermal alteration mineralsProblems with hydrothermal alteration mineralsCase study: Olkaria Domes fieldCase study: Olkaria Domes field
IntroductionIntroduction--1 (Geology terminology)1 (Geology terminology)
A mineral is defined as an element or chemical compound that is normally crystalline and has formed as a result of geologic processes.
Mineral
IntroductionIntroduction--2 (Geology terminology)2 (Geology terminology)
Although any of hundreds of different minerals can group together to form a rock, only six are very common. These common minerals are referred to as the “rockrock--forming mineralsforming minerals” and include quartz, feldspars, micas, amphiboles, pyroxenes and olivine.
Main rock forming minerals
Others include: Fe-Mg, oxides, sulphides, carbonates and native elements.
IntroductionIntroduction--3 (Geology terminology)3 (Geology terminology)
By definition, a rock is an aggregate of one or more minerals, naturally occurring crystallinesubstances with defined properties.
Igneous rocksSedimentaryMetamorphic rocks
Rock
Types of rocks
Hydrothermal alteration 1Hydrothermal alteration 1
Rock alteration simply means changing the mineralogy of the rock.
Rock alteration
Hydrothermal alteration 2Hydrothermal alteration 2
Hydrothermal alteration is a change in mineralogy as a result of interaction of of the rock with hot water fluids (hydrothermal fluids).
Hydrothermal alteration
•Heat source•Recharge fluids•Permeable rocks•Enough time for heat transfer•Return path to the surface
Hydrothermal alteration 3Hydrothermal alteration 3Existence of a hydrothermal system
Sampling in the fieldSampling in the field
•Samples for petrographic, petrochemistry and dating require fresh samples.
•Altered samples are already obliterated and the alteration products are mainly clays, zeolites.
Sampling at the rigSampling at the rig
-Cuttings samples are taken after every 2 m
-Cores are cut mainly during exploration wells.
Analytical methodsAnalytical methods--11
1. Binocular microscope
2. Petrographic microscope
Preliminary analysis is done at the rig site by use of a binocular microscope. (pyrite, quartz, calcite, epidote)
Representative samples are selected and thin sections prepared for petrographic studies.
Analytical methodsAnalytical methods--22
Diffractograms
The X-Ray Diffractometer is used to identify individual minerals especially clays and zeolites.
Clays (OW-903, 1137-1135 masl)
Lin
(Cps
)
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
,473
d=13
,558
d=31
,60
d=29
,095
d=8,
951
d=7,
713
d=7,
131
d=10
,052
3. X-Ray Diffractometer
4. Fluid inclusion analysis
Fluid inclusions are small portions of fluid, which are trapped in a solid crystal as it grew or recrystallized.
Analytical methodsAnalytical methods--33
Alteration typesAlteration types--11
(a) Direct deposition
Sulphur Quartz veinPrecipitated from saturated solution and deposit-ed in veins and vesicles
Alteration typesAlteration types--22
ReplacementReplacement
Pyrite, sphene, haematiteMagnetite
AdulariaSanidine, orthoclase, microcline
Calcite, albite, adularia, quartz, illite, epidote, sphene
Ca-plagioclase
Chlorite, illite, quartz, pyrite, calcite
Pyroxenes, amphibolesChlorite, actinolite, hematite, claysOlivineZeolites, clays, quartz, calciteVolcanic glassAlteration productsPrimary phases
Alteration typesAlteration types--33(c) Leaching
Process takes place at margins of geothermal field
Acidified steam condensate dissolve primary minerals without replacing voids thus formed.
Alteration typesAlteration types--44(d) Ejecta
Bladed euhedral crystals of calcite form in the turbulence of boiling
Factors affecting hydrothermal alterationFactors affecting hydrothermal alteration--111. Temperature
Temperature is the most significant factor in hydrothermal alteration because most of the chemical reactions require elevated temperatures and also minerals are thermodynamically stable at high temperatures.
Factors affecting hydrothermal alterationFactors affecting hydrothermal alteration--222. Permeability
Permeability of the rocks controls the access of thermal fluids, which cause hydrothermal alteration of the rocks and precipitation of secondary minerals in open spaces.
Rocks which have very restricted permeability or are completely impervious to fluid will be only slightly altered.
Factors affecting Factors affecting hydrhydr. alt. alt--333. PressureUnlike metamorphic environm. Pressures seldom exceed 200 bars.
Main effect of pressure is an indirect one in that it controls depth at which boiling.
Factors affecting Factors affecting hydrhydr altalt--44
4. Initial rock compositionThe chemical composition of the host rock determines the availability of components to form alteration minerals.
•Glass easily susceptible than the crystalline rock•Reverse of the Bowen's reaction series
Factors affecting hydrothermal alterationFactors affecting hydrothermal alteration--55
5. Fluid composition
The pH and the composition of the fluid greatly determine rate and the types of hydrothermal minerals to be formed in a geothermal system.
Application of alt. mins in geothermal systemsApplication of alt. mins in geothermal systems--11
1. Geothermometers
Alteration mins e.g. zeolites, clays, prehnite, epidote and many others are useful as geother-mometers
Chalcedony
Mordenite
Calcite
Pyrite
Chlorite
Illite
Albite
Adularia
Quartz
Sphene
Wairakite
Prehnite
Epidote
Biotite
Actinolite
Garnet
MINERALS TEMPERATURE °C100 200 300
2. Permeability indicators
Alteration minerals e.g. quartz, anhydrite, wairakite abundant calcite and pyrite are used as indicators of high permeability.
Application of alt. min in geothermal systemsApplication of alt. min in geothermal systems--22
3. Setting production casing depths
Temperature ranges of minerals especially phylosilicates and calcsilicates assist in estimating subsurface temperatures hence setting of production casing depths.
Application of alt. min in geothermal systemsApplication of alt. min in geothermal systems--33
4. Understand the nature of the reservoirDetermine the uplflow, outflow and marginal zones
Application of alt. min in geothermal systemsApplication of alt. min in geothermal systems--44
Determine structures that control the geothermal fluids
Predicting possible boiling and or high gas zones during drilling-bladed calcite
Identifying past fluctuations in the thermal system
5. Predicting scaling and and corrosion tendencies
Formation of certain alteration minerals depend on the pH of the hydrothermal fluids
Application of alt. min in geothermal systemsApplication of alt. min in geothermal systems--55
Bladed calcite indicate boiling hence possibility of calcite scaling and corrosion
Application of alt. min in ore depositsApplication of alt. min in ore deposits
Veins form where the fluids flow through larger, open space fractures and precipitate mineralization along the walls of the fracture, eventually filling it completely
Source of economic minerals
Problems with hydrothermal alteration mineralsProblems with hydrothermal alteration minerals
Corrosion and scaling problems in geothermal systems
Hydrothermal min. zonationsHydrothermal min. zonations2100
1500
1100
900
300
-100
-300
1900
1700
1300
700
500
100
Depth(m
asl)
OW-901
OW-903
OW-902
+100°C
+125°C
150°C
175°C
x
225°C
200°C
230°C240°C240°C245°C
248°C
275°C
UNALTERED ZONE
ZEOLITE-CHLORITE ZONE
ILLITE-CHLORITE ZONE
EPIDOTE-ILLITE-CHLORITE ZONE
325°C
340°C
GARNET-BIOTITE-ACTINOLITE ZONE
300°C
Case example Domes OWCase example Domes OW--9029022100
1500
1100
900
300
-100
-300
1900
1700
1300
700
500
100
Dept
h(m
.a.s
.l)
OW-901
OW -903
OW-902
100°C
125°C
150°C
175°C
225°C
200°C
230°C240°C240°C
245°C248°C
275°C
300°C
325°C
340°C
Epidote isograd
Actinoliteisograde
Chlorite isograd
Illiteisograd
Garnet isograd
?
Case example Domes OW-9012100
1500
1100
900
300
-100
-300
1900
1700
1300
700
500
100
Dept
h(m
.a.s
.l)
OW-901
OW -903
OW-902
100°C
125°C
150°C
175°C
225°C
200°C
230°C240°C240°C
245°C248°C
275°C
300°C
325°C
340°C
Epidote isograd
Actinoliteisograde
Chlorite isograd
Illiteisograd
Garnet isograd
?