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)