1. HYDROTHERMAL MINERALS
Dr. Arifudin Idrus
Department of Geological Engineering
Gadjah Mada University

2. Hydrothermal minerals
When hot magma resides in the crust, water in the surrounding rock is heated and begins to convect. As it travels, the hot water dissolves materials (ions) from the surrounding rock and carries them to new locations. If the conditions of the water change (temperature, pressure, pH, oxygen content), new minerals will precipitate.

3. Hydrothermal circulation in a continental setting –
Minerals form in hydrothermal veins,
disseminated deposits and grade into pegmatites.

4. A hot spring on the surface is a sign of deep hydrothermal circulation.

5. Hydrothermal circulation on the ocean floor.
Seawater circulates through the ridge basalt. Most minerals form when the hot water exits into the cold deep ocean water. Sulfide minerals form “black smokers”, sulfates form “white smokers”. Minerals are typically volcanogenic massive sulfides.

6. Hydrothermal veins

7. The dark colored layers are chalcopyrite, sphalerite and galena – valuable ore minerals
The white layers are quartz – a useless “gangue” mineral which must be removed

8. Porphyry copper replacement deposit – copper minerals are deposited in fractured, altered igneous rock

9. See Table 19.2, page 381
The diagram shows some of the main categories of sulfide mineral associations in continental settings:
Porphyry copper – chalcopyrite, other copper sulfides and molybdenite, near the top of a felsic igneous intrusion
Hydrothermal vein with chalcopyrite, galena and sphalerite
Galena and sphalerite in limestones, typically with dolomite
Low temperature (epithermal) gold, silver, cinnabar vein
Low temperature (epithermal gold, cinnabar deposit)
Fig. 19.1, page 379

10. Fluid inclusions record mineral and fluid temperature
Fluid inclusions record mineral and fluid temperature. Minerals entrap fluid as they grow. When the minerals cool, the fluid contracts, forming a bubble of gas. By heating the mineral until the bubble disappears (until the fluid reaches its original volume) you can estimate the temperature of entrapment.

11. The silicates in igneous rocks (feldspar, hornblende and micas) are altered by reaction with hydrothermal solutions to form characteristic alteration minerals:
Propylite (chlorite and epidote form)
Argillite (clay minerals form)
Sericite (mica forms from clays)
Fig. 19.2, page 380.

12. Sulfide minerals are unstable in the presence of oxygenated groundwater. Primary (hypogene) sulfides react to form secondary (supergene) sulfides, and then supergene oxygen-bearing minerals such as oxides, carbonates, sulfates, and phosphates, depending on the anions that are available in the groundwater. At the surface, red/orange colored iron oxides (gossan) are left behind and become a marker for sulfide mineral prospecting.
See Table 19.3, page 382 for names of some minerals in the oxidized part of the supergene zone.
Figure 19.3, page 382

13. Which would you expect to form in alkaline, highly oxidized waters?
The stability of supergene hydrothermal minerals is typically shown on a plot of Eh (a measure of the availability of oxygen) versus pH (concentration of hydrogen ions, or acidity). The concept of this phase diagram is the same as that of a plot of pressure versus temperature.
Oxygen rich
environments
Oxygen poor environments
Minerals shown are:
Chalcocite
Native copper
Covellite
Cuprite
Malachite
Which would you expect to form in alkaline, highly oxidized waters?