Clay minerals : paleo-conditions and dynamic evolution of hydrothermal systems
examples of volcanic contexts related to subduction zones
P. Patrier Mas (D. Beaufort, D. Guisseau, A. Mas, P. Papapanagiotou...)
Summary
Introduction (definitions, generalities)
1- Alteration parageneses
2- Clay alterations used as paleocondition indicators - limitations
3- General functioning of geothermal systems
4- Methodology to use clays as a guide
5- Some examples
6- Clay genesis and transformations
7- Signature of clay minerals
Conclusion
Hydrothermal systems...
- What is a hydrothermal system ?
- In which contexts ?
- Why do we study clay minerals ?
A hydrothermal system …
Geological system characterized by (lateral and/or vertical) circulations of hot fluids with variable temperatures and
pressures under the Earth surface. The fluid temperature must be warmer (5°C or more) than the surrounding
environment
These systems can be active or fossil (activity long enough to generate anomalous concentrations in metals = ore
deposits…)
Occur mainly in surfical part of earth crust where the tectono-magmatic activity is able to generate a thermal source and to
enhance important transfers of fluids between very contrasted formations (in terms of T, chemistry…)
Distribution of active hydrothermal (geothermal) systems
- Mobile fluid phase (density gradient), occurrence of fracturation
- Occurrence of a heat source
Cycles of deep descent,heating, rising
Terminology used ...
1 – Epithermal system : metal
deposits formed at depth lower than 1500m and for
50 < T < 200 °C
2 – Mesothermal system : Metal deposit formed at
intermediate depth (1500-4500 m) and for
200 < T < 400 °C
3 – Hypothermal system: Metal deposits formed at depth > 4500 m and for
400 < T < 600 °C Clay minerals : 1 – 2 (max : 300°C)
Geochemical studies of fossil (or extinct) epi-mesothermal system indicate that the majority of hydrothermal ore
deposits hosted by volcanic rocks or their subjacent plutonic suites were formed within systems of similar size, chemistry and behaviour, as well as similar geologic settings to those we
see active today, i.e. geothermal systems (Henley and Ellis, 1983)
Geothermal systems
Epithermal
Mesothermal
Emission of hot water : geysers
Depending of the H20 vapor pressure in the reservoir
Expressions in surface…
Pools with thermal fluids
60m - 74°C - pH=5 - CO2 - Au, Ag, Hg, Ar, S, Th, Sn
Rising of thermal waters
Silica deposits + Ar, Sb, Mo, W, Fe
+ algue dev.
Emission of vapors : fumerolles
Taupo area
« mud pots »
Kaolinite + opale CT + Quartz + pyrite pH =2.5 100°C
High temperatures even at the surface (Milos)
Expression in depth
Occurrence of veins (« filled fractures »)
± transformation at the wall rock
(vein type alteration)
+ Alteration of the bulk rock
(pervasive alteration )
Why do we study hydrothermal systems ?
Environments of economic interest
Duration of the activity is long enough to concentrate metal
deposits :ore deposits
Geothermal energy
(1970-1980...)
Summary
Introduction (definitions, generalities)
1-Alteration parageneses
2- Alterations used as paleocondition indicators - limitations
3- General functionning of geothermal systems
4- Methodology to use clays as a guide
5- Some examples
6- Clay genesis and transformations
7- Signature of clay minerals
Conclusion
Hydrothermal alteration ...
- Definition ?
- What are the controls ?
- Why do we study hydrothermal alteration ?
Definition of hydrothermal alteration
Changes of structure, mineralogy, rock chemistry… when physico-
chemical conditions of the environment are modified in
presence of fluids.
80µm
qz
(wr) (ve)
ca
ill
ill ca
op
ill
qz
Rock alteration where solutions of higher temperature than that expected from the geothermal
gradient in a given area interacts locally with the surrounding
rocks (Utada, 1980)
Differs from metamorphism by...
- The quantity of fluids involved
- An important thermodynamic instability linked to the tectono-magmatic activity
- The amplitude of disequilibrium between fluids and rocks
- The duration of phenomena, reaction kinetics
- The amount of clay phases
Diagenesis, metamorphism : rock alteration by hot water in equilibrium with the surrounding rocks (« closed system »)
Typically : open system (movement of solutions)
What are the alteration controls ?
- Fluid composition
- Rock chemistry
-Temperature
- (Pressure)
- Fluid/rock
- Duration ...
Influence of the fluid chemistry
Fluids involved : not pure water
(dissolved materials : salts, gases…)
d18O = ((18O/16O)sample - (18O/16O)sw) *1000/ (18O/16O)sw
Origin ?
As a function of lithology, temperature...
As a function of mineralogy
Same assemblages observed in numerous hydrothermal systems
Zoning of hydrothermal parageneses
example of porphyry copper
(Creasey, 1959)
Formation of alteration minerals and their zoning
- Propylitic alteration : Chl., epid., Alb., Carb.
T>250°C
low F/R, fluids located in the rock porosity
Large distribution
- Potassic alteration : K Felsp, Biot., « Sericite », Chl., Qz
T>320°C
Magmatic fluids
Deeper parts (host rock + intrusion)
Early parageneses
- Phyllic alteration : Qz., « Sericite », Pyr., Chl., Illite.
T>220°C
High F/R, Important magmatic contribution, located at the top of the systems
- Argillic alteration : Kaol., Sme., « Chl. »
T<200°C
high F/R, important meteoric contribution
Late hydrothermal parageneses...
Summary
Introduction (definitions, generalities)
1- Alteration parageneses
2- Clay alterations used as paleocondition indicators - limitations
3- General functionning of geothermal systems
4- Methodology to use clays as a guide
5- Some examples
6- Clay genesis and transformations
7- Signature of clay minerals
Conclusion
Hydrothermal alteration can help to...
- Localize permeable zones, characterize the hydrodynamic of the system
- Precise the fluid composition
- Evaluate qualitatively the well production rate (active syst.)
- Precise the evolution of the field during time…
- Understand the mineralization processes
- General modelling of hydrothermal systems
- Exploration optimisation
Structural criteria (interstratification, crystallinity, polytypism…)
Chemical criteria (tetrahedral, octahedral occupancies…)
Potential indicators of paleoconditions
Temperature
Why do we study clay minerals ??
Very reactive minerals
Cathelineau et Nieva,
(1985)
In hydrothermal systems…
- Direct precipitation from solutions
ill
qz
ill
chl
chl
op
Clays minerals :
Important neoformed minerals
Result from :
- Recrystallization processes - Instability of other silicate
minerals
...Relationships clay minerals - temperature
...Relationships cla minerals - temperature
At the mineral scale Geothermometry :
The dioctahedral clay sequence – The trioctahedral clay sequence
Structural criteria : interstratifications, cristallinity, polytypes…
Smectite : max 120°C
I/S R=1 120-180°C - I/S max 200-220°C
Disordered C/S up to 240°C
Illite, Chlorite : 220°C min.
Chemical criteria
Mainly used for chlorites (tetrahedral + octahedral occupancies)
To use clay minerals as a geothermometer
You have to be sure that
1- Other variables involved in the functionning of the system must be controlled
2- Clay minerals have to be well characterized : chemical and structural homogeneous phases
...
difficult to validate
Use of clay minerals as geothermometer
An illustration : geothermometer based on chemistry
1- Chemistry of a mineral is also a function of the rock chemistry, fluids chemistry, F/R, fO2 … Need a strong control of all these variables
2- Chemical analyses of pure phases : size, mixing and frequent mixed-layering
3- Based on a structural formula : different from a real distribution
ex. chlorite geothermometer
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
100 150 200 250 300
Température
(CaO
+N
a2
O+
K2
O*
10
0)/
( o
x.
tot)
Cathelineau et Nieva (1985)
!
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
100 150 200 250 300 350 400
Température
AlI
V Cathelineau et
Nieva (1985)
St Martin
Chipilapa
Chipilapa C/S
chlorites
C/S
Use of clay minerals in geothermometry
Equilibrium state : often discussed
Mixed layered clays = metastable transitory state (Jiang et al., 1994 ; Essene et Peacor, 1995…)
Occurrences controlled by kinetic factors
Not only temperature
F/R, time…
= dynamic of the system
- Depends of too many parameters to be an efficient geothermometer
- Chemical heterogeneity is a better indicator of the reaction rates
As crystallization occurs during disequilibrium stage, heterogeneity is max. at t = 0.
In fossil systems ...
In active systems ...
Frequently :
- Clay minerals are not in agreement with the temperatures measured in the wells (generally lower than their stability domain)
- Numerous anomalies of clay minerals distribution (compared
to the classical scheme)
Ulumbu (Kasbani et al., 1998)
In active systems ...
Smectite Ill/Sm
Aluto-Langano (Ethiopie)
(Teklemariam et al., 1996) Sumikawa (Japon)
(Inoue et al., 1999)
How can we explain the difficult to transpose the parageneses defined in fossil systems to active and young systems ?
Summary
Introduction (definitions, generalities)
1-Alteration parageneses
2- Clay alterations used as paleocondition indicators - limitations
3- General functionning of geothermal systems
4- Methodology to use clays as a guide
5- Some examples
6- Clay genesis and transformations
7- Signature of clay minerals
Conclusion
1) In a first time, main thermal transfers are conductive (heat diffusion).
This first stage results in zonation of alteration products parallel to isotherms. At this stage, alteration is controlled most by the nature of the rock than by the nature of the fluids. F/R is low (fluids involved are interstitial fluids trapped in the rock porosity)
General functionning of a hydrothermal system :
Governed by dissipation processes of the thermal energy generated by the magmatic intrusions
2) In a second time, main heat transfers are convective tranfers. Such transfers are associated with the opening of the system (in relationships with fracturing and influx of meteoric or marine fluids)
This heat transfer process is mainly efficient at the top of magmatic plugs where the fracturation rate is the most important. At this stage, reactions are mainly controlled by the fluids percolating in the system.
General functionning of hydrothermal systems ...
3) Alterations associated with these fluids circulations will seal the system : this results in conductive heat transfers (mineral zonation) again until the fracture network is not reactivated
2) In a second time, main heat transfers are convective tranfers. Such transfers are associated with the opening of the system (in relationships with fracturing and influx of meteoric or marine fluids)
This heat transfer process is mainly efficient at the top of magmatic plugs where the fracturation rate is the most important. At this stage, reactions are mainly controlled by the fluids percolating in the system.
General functionning of hydrothermal systems ...
3) Alterations associated with these fluids circulations will seal the system : this results in conductive heat transfers (mineral zonation) again until the fracture network is not reactivated .
General functionning of a hydrothermal system ...
1- on the overall activity of the system (100,000-1My), heat transfers are mainly conductive
2- Convective heat transfers are effective during each events stimulating the fracture network
3- Fluid /rock interactions are multiple, sporadic and localized
4- System dominated by the rock system dominated by fluids : alternating control
Summary
Introduction (definitions, generalities)
1-Alteration parageneses
2- Clay alterations used as paleocondition indicators - limitations
3- General functionning of geothermal systems
4- Methodology to use clays as a guide
5- Some examples
6- Clay genesis and transformations
7- Signature of clay minerals
Conclusion
-Acquire a good knowledge of the sample petrography separate the different hydrothermal parageneses Classical techniques + Separation and identification of different clay populations - Acquire mineralogical data on the whole alteration phases and primary minerals precise the mineral reaction rate, conditions of nucleation/growth Ex : textural properties (crystal size, habits) and microstructural properties (order/disorder, shift from equilibrium, cristallinity index)… Crystal-chemistry techniques - SEM - Coupling with fluids (isotopes, FI in fossil parts) - Integrate all these data in the dynamics of these sytems
Methodological approach
Summary
Introduction (definitions, generalities)
1-Alteration parageneses
2- Alterations used as paleocondition indicators - limitations
3- General functionning of geothermal systems
4- Methodology to use clays as a guide
5- Some examples
6- Clay genesis and transformations
7- Signature of clay minerals
Conclusion
Some examples of hydrothermal systems
Systèmes
Situation
actuelle Age max.
Contexte
lithologique
Nature
des
fluides
Réservoir
échant.
Situation
struct.
St Martin Fossile Oligocène
Volcano-
sédimentaire Météor. Non
3-4 km
prof.
Chipilapa
Ahuachapan
Actif
(localement
fossile) < 16000 ans
Volcano-
sédimentaire Météor.
V dom.,
L dom
0-2.5 km
prof.
Milos Actif Pléistocène Métamorphique Marins V dom.
0-1.3 km
prof.
Bouillante Actif
< 600000
ans, à
préciser
Volcano-
sédimentaire Mixtes Non
Surface +
Prof
Temperature, F/R, nature of fluids, permeability, evolution of the systems
