Hydrothermal circulation cells: In such a cell water circulates down into crust, gets heated and rises. On the temperature increase path generally mineral solubility will increase, and more soluble material will be selectively leached from the rock the fluid passes through. On the temperature decrease path, the opposite will happen. Most of this transport appears to be along fractures. A much slower fluid flow path is through the bulk of the rock. The circulation can be driven by topography or by a shallow heat source ( usually magmatic). Black smokers on oceanic spreading ridges are a spectacular surface example of such a hydrothermal circulation cell.

Veins in cores from the basement rocks of the Savannah River Plant in South Carolina. In these two cores you can see two different vein complexes cutting the rock. To the left the white and green material obliquely cutting the core is quartz and epidote the precipitated in the fractures when they opened up. If you look carefully you can see how the quartz is in the interior, and the epidote tends to be on the exterior of the vein suggesting an evoluton of the fluids moving through these cracks. You can also see that multiple cracks exist suggesting this complex opened up and sealed with hydryothermal precipitates several time. In the core to the right you see pink zeolite crystals in the vein that point inwards to the void which is still left. Note how this vein is discordant to the layering in the enclosing gneiss. Zeolites are hydrothermal index minerals giving some idea to the conditions of formation. They also have useful industrial filtering properties.

What causes mineral precipitation in hot waters circulating in a geologic setting?

• temperature decrease.
• pressure decrease due to faulting/cracking.
• mixing of waters with different composition.
• contact with reactive rock (especially carbonates).

Zonation of hydrothermal deposits.

Common minerals in veins:

• quartz and calcite very common.
• as a function of the specific geochemistry can have exotic/unusual minerals.
• sulfides also common.
• networks of veins can create ore bodies.

Dilational vs. replacement origin of veins.

In what settings would you expect in hydrothermal activity and associated rocks to be common?

• generally in areas of igneous activity.
• specifically in rift zones and arcs.
• in mountain belts.
• along deep fault zones that can tap deep and hotter fluids.

Chemosynthesis and associated biotic communities.

Hydrothermal activity, open systems and the rock cycle.

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