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 tectonomagmatic 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) Epithermal Geothermal systems Mesothermal
Expressions in surface Emission of hot water : geysers Depending of the H20 vapor pressure in the reservoir
Pools with thermal fluids 60m - 74 C - ph=5 - CO 2 - 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 (ve) (wr) qz ca ill qz ill ca ill op Changes of structure, mineralogy, rock chemistry when physicochemical conditions of the environment are modified in presence of fluids. 80µm 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 Typically : open system (movement of solutions) Diagenesis, metamorphism : rock alteration by hot water in equilibrium with the surrounding rocks («closed system»)
What are the alteration controls? - Fluid composition - Rock chemistry -Temperature - (Pressure) - Fluid/rock - Duration...
Fluids involved : not pure water (dissolved materials : salts, gases ) Influence of the fluid chemistry
Origin? d 18 O = (( 18 O/ 16 O) sample - ( 18 O/ 16 O) sw ) *1000/ ( 18 O/ 16 O) sw
As a function of lithology, temperature...
As a function of mineralogy
Formation of alteration minerals and their zoning Same assemblages observed in numerous hydrothermal systems Zoning of hydrothermal parageneses example of porphyry copper (Creasey, 1959)
Early parageneses - 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)
Late hydrothermal 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
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
Why do we study clay minerals?? Very reactive minerals Potential indicators of paleoconditions Temperature Cathelineau et Nieva, (1985) Structural criteria (interstratification, crystallinity, polytypism ) Chemical criteria (tetrahedral, octahedral occupancies )
In hydrothermal systems Clays minerals : Important neoformed minerals ill chl chl ill Result from : - Recrystallization processes - Instability of other silicate minerals op qz - Direct precipitation from solutions
...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, fo 2 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
(CaO+Na2O+K2O*100)/( ox. tot) AlIV ex. chlorite geothermometer 1.3 1.2 1.1 chlorites 1.6 1.4 1.2 1 0.8 0.6 0.4 Cathelineau et Nieva (1985) 1 0.9 0.8 0.7 0.6 0.5 0.4 C/S Cathelineau et Nieva (1985) St Martin Chipilapa Chipilapa C/S 100 150 200 250 300 350 400 Température 0.2 0 100 150 200 250 300 Température!
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
In fossil systems... - 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 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... Ill/Sm Smectite 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
General functionning of a hydrothermal system : Governed by dissipation processes of the thermal energy generated by the magmatic intrusions 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 hydrothermal systems... 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. 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 hydrothermal systems... 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. 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
Methodological approach -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
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 Temperature, F/R, nature of fluids, permeability, evolution of the systems Systèmes Situation actuelle Age max. St Martin Fossile Oligocène Actif Chipilapa (localement Ahuachapan fossile) < 16000 ans Nature des fluides Contexte lithologique Volcanosédimentaire Météor. Non Météor. Réservoir échant. V dom., L dom Volcanosédimentaire Milos Actif Pléistocène Métamorphique Marins V dom. < 600000 ans, à Volcanosédimentaire Bouillante Actif préciser Mixtes Non Situation struct. 3-4 km prof. 0-2.5 km prof. 0-1.3 km prof. Surface + Prof