Physical Geology lecture outline - hydrothermal rocks

These are rocks that form by minerals precipitating out of hot waters that carry the dissolved ions needed for that mineral to grow. Volumetrically they are a minor proportion of all rocks, but they have a much greater significance than represented by that small fraction.

Pure visible gold in a hydrothermal quartz vein from California. Most gold is of a hydrothermal origin. Image from: James St. John, Wikipedia, https://commons.wikimedia.org/wiki/File:Mother_Lode_Gold_OreHarvard_mine_quartz-gold_vein.jpg .

Hydrothermal circulation cells: In such a cell water seeps down into crust, through the rocks, gets heated and rises. On the descent path where temperature increases mineral solubility will increase, and more soluble mineral material will be selectively leached/altered from the rock the fluid passes through. During temperature decrease along the ascent path, the opposite will happen. Most of this transport appears to be along fractures. The circulation can be driven by topography or by a shallow heat source (usually magma bodies). A much slower fluid flow path is through the bulk of the rock (through grain boundaries and microcracks).

Schematic image of hydrothermal circulation cell beneath Lassen Peak in California that results in surface hot springs. The fluids are primarily moving along a host of fractures in the rock. Image source: http://pubs.usgs.gov/fs/2002/fs101-02/

Black smokers on oceanic spreading ridges are a spectacular surface example of such a hydrothermal circulation cell. USGS site with more information and photos of black smokers.

Chemosynthesis and associated biotic communities.

U-Tube video on submarine hot springs from Woodshole Oceanographic Institute.

 


Mammoth Springs, Yellowstone hotspot: This is a more obvious surface manifestation of a hydrothermal circulation system, where hydrothermal deposits have built up. In this case the mineral is carbonate. Note the distinct lips that grow up forming dams for the blue pools. What causes the build up as this specific spot? As the water spills over this lip carbon dioxide can escape from it, which cause a reduction in pH, which in turn promotes carbonate precipitation. Hot volcanic rocks or magma at depth, part of the caldera system that makes up Yellowstone, drives this and several other geyser fields.

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 (the distinctive green mineral) tends to be on the exterior of the vein suggesting an evolution 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 with a complex chemistry and provide information as to the conditions of formation. They also have useful industrial filtering properties.


Hydrothermal breccia in Baraboo quartzite of Wisconsin. The reddish angular pieces are broken up quartzite (metamorphosed sandstone). Out of hot waters between the breccia clasts quartz has precipitated. Three pockets of quartz crystals can be seen. This is where not enough mineral matter precipitated to entirely fill in the void space. Fuids under pressure can aid the fracturing process and may have contributed to the formation of the breccia in the first place.


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

Vertical zonation of a mineral assemblage in a hydrothermal deposits due to temperature gradient.

What are common minerals in veins?

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

Hydrothermal activity, open systems and the rock cycle.


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