Download presentation
Presentation is loading. Please wait.
Published byDarwin Congleton Modified over 10 years ago
1
Sub-solidus evolution
2
Mineral transformations Secondary minerals Fluids expulsion and movement –Pegmatite/aplite veins –Mineralized veins Hydrothermal alteration –Episyenites, endoskarns, greisens –Exoskarns
3
Mineral transformations Polymorphs Exsolutions (solvus)
4
Phase diagram for SiO 2
5
Feldspar solvus
6
Perthites
7
Opx-Cpx exsolution
8
Secondary minerals « Autometamorphism »
9
Water-saturated solidus (granites)
10
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)
11
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 Bt Chl
12
Sericitization K-feldspar to sericite: 3 KAlSi 3 O 8 + 2 H + > KAl 3 Si 3 O 10 (OH) 2 + 6 SiO 2 + 2 K +
13
Saussuritization Dolerite from ODP leg 180 (sea of Java)
14
Olivine with iddingsite alteration
15
Calcite vein
16
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
17
Hydrothermal circulations Most of the water in hydrothermal systems comes from meteoric, surface waters (cf. O isotopes, G214)
18
Effect of free, hot water Overpressure, fractures, etc. Very aggressive solvent! Aplite/pegmatite veins
19
Pegmatites recording the same strain pattern as ductile structures Cape de Creus, Spain
20
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 km 3 of pluton At 3 wt% H2O = 2.7 10 12 kg rock ≈ 10 11 kg water Can dissolve 3 10 9 kg of SiO 2, or 10 6 m 3
21
Composition of hydrothermal fluids G.B. Arehart, http://equinox.unr.edu/homepage/arehart/Courses/713/Syllabus.htm Acidic water dissolve less SiO 2 pH changes can precipitate SiO 2
22
Evidence for Si-rich hydrothermal fluids Tatio hydrothermal field, Peru
23
Network of pegmatites/apl ite dykes
24
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
25
Analysis of hydrothermal fluids from inclusions in pegmatites
26
Gold-quartz veins See economic geology (GEOL344)
27
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…)
28
Barberton gold fields
29
Hydrothermal modifications of rocks Around the intrusion –Exoskarns, etc. In the intrusive rocks –Episyenites –Endoskarns, greisens
30
Summary: deposits around a magmatic body
31
Around the pluton
32
Deposits by chemical reactions
33
Outside the pluton: skarn
36
In the pluton
37
pH control on solubility G.B. Arehart, http://equinox.unr.edu/homepage/arehart/Courses/713/Syllabus.htm High pH helps to dissolve SiO 2
38
In the pluton Loss of quartz => « syenites » (Episyenites)
39
Fedlspar alteration in the pluton K-feldspar to sericite: 3 KAlSi 3 O 8 + 2 H + > KAl 3 Si 3 O 10 (OH) 2 + 6 SiO 2 + 2 K + Sericite to kaolin: 2 KAl 3 Si 3 O 10 (OH) 2 + 2 H + + 3 H 2 0 > 3 Al 2 Si 2 O 5 (OH) 4 + 2 K + Requires acidic fluids!
40
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)
Similar presentations
© 2025 SlidePlayer.com. Inc.
All rights reserved.