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

?