sub-solidus evolution. mineral transformations secondary minerals fluids expulsion and movement...
TRANSCRIPT
Sub-solidus evolution
• Mineral transformations
• Secondary minerals
• Fluids expulsion and movement– Pegmatite/aplite veins– Mineralized veins
• Hydrothermal alteration– Episyenites, endoskarns, greisens– Exoskarns
Mineral transformations
• Polymorphs
• Exsolutions (solvus)
Phase diagram for
SiO2
Stishovite
Coesite
- quartz
- quartz
Liquid
TridymiteCristobalite
600 1000 1400 1800 2200 2600
2
4
6
8
10
Pre
ssur
e (G
Pa)
Temperature oC
Feldspar solvus
Perthites
Opx-Cpx exsolution
Secondary minerals
• « Autometamorphism »
Water-saturated solidus (granites)
Secondary minerals
• Px => Amp => Bt
• Px, Amp, Bt => chlorite (phyllosilicate)
• K-feldspar, feldspathoids => sericite (fine white mica)
• Ca-plagioclase => saussurite (epidote)
• Olivine => serpentine (complex phyllosilicate), iddingsite (a mixture of various Fe-Mg silicates)
Figure 3-20. a. Pyroxene largely replaced by hornblende. Some pyroxene remains as light areas (Pyx) in the hornblende core. Width 1 mm. b. Chlorite (green) replaces biotite (dark brown) at the rim and along cleavages. Tonalite. San Diego, CA. Width 0.3 mm. © John Winter and Prentice Hall.
Pyx
Hbl
BtChl
Sericitization
K-feldspar to sericite:
3 KAlSi3O8 + 2 H+ > KAl3Si3O10(OH)2 + 6 SiO2 + 2 K+
Saussuritization
Dolerite from ODP leg 180 (sea of Java)
Olivine with iddingsite alteration
Calcite vein
Fluid expulsion
• Typical water contents: 2-4% in a granite
• Water content of a biotite: ~2 %
• Biotite: max. 5-10 % of the rock
Excess water = ?
+ meteoric water also feeding the hydrothermal system
Hydrothermal circulations
Most of the water in hydrothermal systems comes from meteoric, surface waters (cf. O isotopes, G214)
300
o
200o
meteoricwater flow
steam and hot waterrainsinter and
hydrothermal ores
magma
volcanicdeposits
older bedrock
Effect of free, hot water
• Overpressure, fractures, etc.
• Very aggressive solvent!
• Aplite/pegmatite veins
Pegmatites recording the same strain pattern as ductile structures
Cape de Creus, Spain
Quartz solubility in hydrothermal fluids
G.B. Arehart, http://equinox.unr.edu/homepage/arehart/Courses/713/Syllabus.htm
0.5 mol/kg water= 30 g/l
1 km3 of plutonAt 3 wt% H2O= 2.7 1012 kg rock≈ 1011 kg waterCan dissolve 3 109 kg of SiO2, or 106 m3
Evidence for Si-rich hydrothermal fluids
Tatio hydrothermal field, Peru
Network of pegmatites/apl
ite dykes
Mineralized veins
• Very incompatible elements (large ions, typically) concentrated in last liquids, then in fluids
• The same elements are leached from an already cooled rock (igneous intrusion or its wall-rock)
• Precipitate with hydrothermal veins
Analysis of hydrothermal fluids from inclusions in pegmatites
Gold-quartz veins
• See economic geology (GEOL344)
pH control on solubility
G.B. Arehart, http://equinox.unr.edu/homepage/arehart/Courses/713/Syllabus.htm
Changes of pH can precipitate ore bodies:
•mixing with acid groundwater
•Interaction with rocks of very different chemistry (e.g., carbonates, very mafic rocks…)
Barberton gold fields
Hydrothermal modifications of rocks
• Around the intrusion– Exoskarns, etc.
• In the intrusive rocks– Episyenites– Endoskarns, greisens
Around the pluton
Deposits by chemical reactions
Outside the pluton: skarn
In the pluton
pH control on solubility
G.B. Arehart, http://equinox.unr.edu/homepage/arehart/Courses/713/Syllabus.htm
High pH helps to dissolve SiO2
In the plutonLoss of quartz => « syenites »(Episyenites)
Fedlspar alteration in the pluton
• K-feldspar to sericite:
3 KAlSi3O8 + 2 H+ > KAl3Si3O10(OH)2 + 6 SiO2 + 2 K+
• Sericite to kaolin:
2 KAl3Si3O10(OH)2 + 2 H+ + 3 H20
> 3 Al2Si2O5(OH)4 + 2 K+
Requires acidic fluids!
In the pluton
• Episyenites are plutonic rocks from which the quartz has been dissolved away (therefore, they become syenites) (high pH)
• Greisens are plutonic rocks where the feldspar has been transformed into clays (kaolinite) by hydrothermal reactions(low pH)