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 SiO 2

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 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

Sericitization K-feldspar to sericite: 3 KAlSi 3 O H + > KAl 3 Si 3 O 10 (OH) SiO 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 % 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)

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, mol/kg water = 30 g/l 1 km 3 of pluton At 3 wt% H2O = kg rock ≈ kg water Can dissolve kg of SiO 2, or 10 6 m 3

Composition of hydrothermal fluids G.B. Arehart, Acidic water dissolve less SiO 2 pH changes can precipitate SiO 2

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, 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

Summary: deposits around a magmatic body

Around the pluton

Deposits by chemical reactions

Outside the pluton: skarn

In the pluton

pH control on solubility G.B. Arehart, High pH helps to dissolve SiO 2

In the pluton Loss of quartz => « syenites » (Episyenites)

Fedlspar alteration in the pluton K-feldspar to sericite: 3 KAlSi 3 O H + > KAl 3 Si 3 O 10 (OH) SiO K + Sericite to kaolin: 2 KAl 3 Si 3 O 10 (OH) H H 2 0 > 3 Al 2 Si 2 O 5 (OH) 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)