Download K-feldspar, feldspathoids

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)
Stishovite
Pressure (GPa)
Phase
diagram for
SiO2
10
8
6
Coesite
4
2
- quartz
- quartz
Liquid
Cristobalite
Tridymite
600
1000
1400
1800
2200
Temperature oC
2600
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
Chl
Bt
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
rain
sinter and
hydrothermal ores
steam and hot water
o
0
20
older bedrock
300 o
volcanic
deposits
meteoric
water flow
magma
Most of the water in hydrothermal systems comes
from meteoric, surface waters (cf. O isotopes,
G214)
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
0.5 mol/kg water
= 30 g/l
1 km3 of pluton
At 3 wt% H2O
= 2.7 1012 kg rock
≈ 1011 kg water
Can dissolve 3 109 kg
of SiO2, or 106 m3
G.B. Arehart, http://equinox.unr.edu/homepage/arehart/Courses/713/Syllabus.htm
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
Changes of pH can
precipitate ore bodies:
•mixing with acid
groundwater
•Interaction with rocks
of very different
chemistry (e.g.,
carbonates, very mafic
rocks…)
G.B. Arehart, http://equinox.unr.edu/homepage/arehart/Courses/713/Syllabus.htm
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
High pH helps
to dissolve
SiO2
G.B. Arehart, http://equinox.unr.edu/homepage/arehart/Courses/713/Syllabus.htm
In the pluton
Loss 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)