Download HYDROTHERMAL ALTERATION MINERALOGY

Survey
yes no Was this document useful for you?
   Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Transcript
HYDROTHERMAL ALTERATION
MINERALOGY
By
John Lagat
Presentation outline
zIntroduction
zWhat
is hydrothermal alteration mineralogy
zSampling methods
zAnalytical methods
zAlteration types
zFactors affecting hydrothermal alteration
zPrimary minerals and their alteration products
zApplication of alteration in geothermal systems
zApplication of hydrothermal alteration in ore
deposits
zProblems with hydrothermal alteration minerals
z Case study: Olkaria Domes field
Introduction-1 (Geology terminology)
Mineral
A mineral is defined as
an
element
or
chemical
compound
that
is
normally
crystalline and has
formed as a result of
geologic processes.
Introduction-2 (Geology terminology)
Main rock forming minerals
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 “rock-forming minerals”
minerals and include quartz,
feldspars, micas, amphiboles, pyroxenes and
olivine.
Others
include:
Fe-Mg,
oxides,
carbonates and native elements.
sulphides,
Introduction-3 (Geology terminology)
Rock
By definition, a rock is
an aggregate of one or
more minerals, naturally
occurring
crystalline
substances with defined
properties.
Types of rocks
Igneous rocks
Sedimentary
Metamorphic rocks
Hydrothermal alteration 1
Rock alteration
Rock alteration simply
mineralogy of the rock.
means
changing
the
Hydrothermal alteration 2
Hydrothermal alteration
Hydrothermal alteration is a change in mineralogy
as a result of interaction of of the rock with hot
water fluids (hydrothermal fluids).
Hydrothermal alteration 3
Existence of a hydrothermal system
•Heat source
•Recharge fluids
•Permeable rocks
•Enough time for heat transfer
•Return path to the surface
Sampling 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 rig
-Cuttings samples are taken after every 2 m
-Cores are cut mainly during exploration wells.
Analytical methods-1
1. Binocular microscope
Preliminary analysis is
done at the rig site by use
of a binocular microscope.
(pyrite, quartz, calcite,
epidote)
2. Petrographic microscope
Representative
samples
are
selected and thin
sections
prepared
for
petrographic
studies.
Analytical methods-2
3. X-Ray Diffractometer
The X-Ray Diffractometer is
used to identify individual
minerals especially clays
and zeolites.
d=15,473
Clays (OW-903, 1137-1135 masl)
1100
d=13,558
d=31,60
1000
900
800
600
d=29,095
d=8,951
300
d=7,131
400
d=7,713
500
d=10,052
Lin (Cps)
700
200
100
0
2.2
3
4
5
6
7
8
9
10
11
Diffractograms
12
13
Analytical methods-3
4. Fluid inclusion analysis
Fluid inclusions are small
portions of fluid, which
are trapped in a solid
crystal as it grew or
recrystallized.
Alteration types-1
(a) Direct deposition
Sulphur
Quartz vein
Precipitated from saturated solution and deposited in veins and vesicles
Alteration types-2
Replacement
Primary phases
Alteration products
Volcanic glass
Zeolites, clays, quartz, calcite
Olivine
Chlorite, actinolite, hematite, clays
Pyroxenes, amphiboles Chlorite, illite, quartz, pyrite,
calcite
Ca-plagioclase
Calcite, albite, adularia, quartz,
illite, epidote, sphene
Sanidine, orthoclase, Adularia
microcline
Magnetite
Pyrite, sphene, haematite
Alteration types-3
(c) Leaching
Process takes place
geothermal field
at
margins
of
Acidified steam condensate dissolve primary
minerals without replacing voids thus
formed.
Alteration types-4
(d) Ejecta
Bladed euhedral crystals of calcite form in the
turbulence of boiling
Factors affecting hydrothermal alteration-1
1. 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 alteration-2
2. 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 hydr. alt-3
3. Pressure
Unlike 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 hydr alt-4
4. Initial rock composition
The 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 alteration-5
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 systems-1
1. Geothermometers
Alteration mins e.g.
zeolites,
clays,
prehnite,
epidote
and many others
are
useful
as
geother-mometers
MINERA LS
C halcedony
Mordenite
C alcite
Pyrite
C hlorite
Illite
A lbite
A dularia
Q uartz
Sphene
Wairakite
Prehnite
Epidote
Biotite
A ctinolite
G arnet
TEMPERATU RE °C
100
200
300
Application of alt. min in geothermal systems-2
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 systems-3
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 systems-4
4. Understand the nature of the reservoir
9Determine the uplflow, outflow and marginal zones
9Determine structures that control the geothermal
fluids
9Predicting possible boiling and or high gas zones
during drilling-bladed calcite
9Identifying past fluctuations in the thermal system
Application of alt. min in geothermal systems-5
5. Predicting scaling and and corrosion tendencies
9Bladed calcite indicate boiling hence possibility
of calcite scaling and corrosion
9Formation of certain alteration minerals depend
on the pH of the hydrothermal fluids
Application of alt. min in ore deposits
Source of economic minerals
9Veins form where the fluids flow through larger,
open space fractures and precipitate mineralization
along the walls of the fracture, eventually filling it
completely
Problems with hydrothermal alteration minerals
Corrosion and
systems
scaling problems in geothermal
Hydrothermal min. zonations
2100
OW -903
OW -902
1900
OW -901
UNALTERED ZONE
1700
+
+
C
10 0 °
C
125°
150°C
1500
ZEOLITE-CHLORITE ZONE
17 5°C
200 °C
1300
225°C
230°C
2 40
°C
24 5
°C
ILLITE-CHLORITE ZONE
1100
2 48
°C
900
Depth(masl)
27 5°C
700
E
500
E
OT
P ID
300°C
300
32 5°C
340 °C
100
-100
GARNET-BIOTITE-ACTINOLITE ZONE
-300
LIT
-I L
NE
ZO
E
IT
OR
HL
C
E-
x
Case example Domes OW-902
2100
OW -903
O W-902
1900
1700
OW-90 1
100°C
°C
125
150°C
1500
175°C
200°C
Chl orite isograd
1300
225°C
230°C
240
°C
ad
isogr
Illite
1100
ote
Epid
rad
isog
245°C
24
8°
C
900
Depth (m.a.s.l)
275°C
700
500
Garnet isograd
300°C
300
325° C
340°C
100
?
-300
e
rad
og
e is
o lit
tin
Ac
-100
Case example Domes OW-901
2100
OW -903
O W-902
1900
1700
OW-90 1
100°C
°C
125
150°C
1500
175°C
200°C
Chl orite isograd
1300
225°C
230°C
240
°C
ad
isogr
Illite
1100
ote
Epid
rad
isog
245°C
24
8°
C
900
Depth (m.a.s.l)
275°C
700
500
Garnet isograd
300°C
300
325° C
340°C
100
?
-300
e
rad
og
e is
o lit
tin
Ac
-100
Similar