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.

Hydrothermal circulation in a continental setting – Minerals form in

hydrothermal veins, disseminated deposits

and grade into pegmatites.

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

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.

Hydrothermal veins

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

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

Fig. 19.1, page 415 See Table 19.2, page 417 The diagram shows some of the main A.

categories of sulfide mineral associations in continental settings: Porphyry copper – chalcopyrite, other copper sulfides and molybdenite, near the top of a felsic B.

C.

D.

E.

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)

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.

Secondary or Supergene Hydrothermal Minerals 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 416.

Figure 19.3, page 418 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.

Oxygen rich environments Oxygen poor environments 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. Minerals shown are: Chalcocite Native copper Covellite Cuprite Malachite Which would you expect to form in alkaline, highly oxidized waters?