Siklus Batuan, Kontrak Kuliah, Kelulusan, & Praktikum Magma, Diferensiasi, Viskositas, Gas dan Komposisi Batuan Beku, Intrusi Dalam & Karakteristiknya Batuan Gunung Api, Intrusi Dangkal & Karakteristiknya Batuan Gunung Api, Lelehan, Letusan & Karakternya Klasifikasi Batuan Beku & Gunung Api Batuan Alterasi
Batuan Sedimen, Jenis & Proses Pembentukan Batuan Sedimen Silisiklastika Batuan Sedimen Karbonat Klasifikasi Batuan Sedimen Batuan Metamorf, Jenis & Proses Pembentukan Batuan Metamorf Derajat Rendah, Menengah & Tinggi Klasifikasi Batuan Metamorf
Batuan Alterasi
Definition of alteration. [... in rocks] Any change in the mineralogic composition of a rock brought about by physical or chemical means, esp. by the action of hydrothermal solutions; also, a secondary, i.e., supergene, change in a rock or mineral.
[... in rocks] Any change in the mineralogic composition of a rock brought about by physical or chemical means, esp. by the action of hydrothermal solutions; also, a secondary, i.e., supergene, change in a rock or mineral. Alteration is sometimes considered as a phase of metamorphism, but is usually distinguished from it because of being milder and more localized than metamorphism is generally thought to be.
Ref: AGI[... in minerals] A change in the chemical composition and/or atomic structure of a mineral brought about by chemical attack or changing physical conditions.
Fig. 8. Diagram from Cann(1979) tries to indicate how the minerals in a basalt affected by hydrothermal activity contribute to the secondary phases. At low temperatures it is mainly the basalt glass and olivine which are unstable and contribute to brownstone facies minerals, but plagioclase and then augite and iron oxide become progressively involved at higher grades until the whole rock recrystallises.
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
Existence of a hydrothermal system
1. Temperature
Temperaturesignificant hydrothermal
is the mostinfactor
alterationbecausechemicalelevatedalso
mostreactions
of therequire
temperaturesminerals
andare
thermodynamicallyat high temperatures.
stable
Factors affecting hydrothermal alteration
2. Permeability
Permeability of the rocks controls
the access of thermal fluids, which
cause hydrothermal alteration ofofthe rocks and precipitation
secondaryspaces.
minerals in open
Rocks which haveor
veryare
restrictedpermeabilityimpervious
completelyto fluid will be only
slightly altered.
3. Pressure
Unlikeexceed
metamorphic environm. Pressures seldom
200 bars.
Main effect of pressure is an indirect one in that it
controls depth at which boiling.
4. Initial rock composition
The chemical composition of the host rockformdetermines the availability of components to
alteration minerals.
•Glass easily susceptible than the crystalline rock
•Reverse of the Bowen's reaction series
5. Fluid composition
The pH and the composition
of the fluidrate
greatly
determine and the
typesminerals
ofto
hydrothermal
be formed in a
geothermal system.
•Samples for petrographic,
petrochemistry and dating
require fresh samples.
•Altered samples are alreadyobliterated and the alteration
productszeolites.
are mainly clays,
Sampling in the field
Sampling at the rig
-Cuttings samples are taken after every 2 m
-Cores are cut mainly during exploration wells.
Analytical methods
1. Binocular microscope
Preliminary analysis is
done at the rig site by use
of a binocular microscope.
(pyrite, quartz, calcite,
epidote)
2. Petrographic
Representative
samples
microscope
arethinselected
sectionsand
prepared
forstudies.
petrographic
3. X-Ray Diffractometer
The X-Ray Diffractometer is
used to identify individual
minerals especially clays
and zeolites.
Clays (OW-903, 1137-1135 masl)1100
1000
900
800
700
600
500
400
300
200
100
0
2.2 3 4 5 6 7 8 9 10 11 12 13
Diffractograms
Lin
(Cps)
d=
31,6
0
d=
29,0
95
d=
15,4
73
d=
13,5
58
d=
10,0
52
d=
8,9
51
d=
7,7
13
d=
7,1
31
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
(a) Direct deposition
Sulphur
Precipitated from saturated ed in veins and vesicles
Quartz vein
solution and deposit-
(b) 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,
microcline
Adularia
Magnetite Pyrite, sphene, haematite
Process takes place at margins of
geothermal field
Acidified steam condensate dissolve primary
mineralsformed.
without replacing voids thus
(c) Leaching