Sub-solidus evolution. Mineral transformations Secondary minerals Fluids expulsion and movement...

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)