Student Papers in Geology-Cochise College
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Roger Weller, geology instructor

Hydrothermal Alteration
Micah Hill
Physical Geology
Spring 2006

Hydrothermal Alteration


    Hydrothermal alteration occurs when the minerals in the old rock are changed because a change in the conditions. These changes could be in temperature, pressure, chemical conditions, or any number of these. Hydrothermal Alteration is the change of a mineral composition when it has come in contact with hot water fluids called, hydrothermal fluids.

    The fluids carry metals in solution, either from a nearby igneous source, or from leaching out of some nearby rocks.  Hydrothermal alteration is a common phenomenon in a wide variety of geologic environments, including fault zones and explosive volcanic features. The below photograph shows how erratic these flows can be and after years of exposure, the weathering process that has weathered these minerals to their current form.


     Hydrothermal fluids are not well understood but geologists have come to the conclusion that there are three possibilities that exist. The first source being Magmatic rocks. Magmatic rocks exsolve water and are called juvenile water during the final stages of cooling. The author could not find any adequate pictures of these rocks.

     The excess water circulates through the surrounding rocks and may scavenge and transport metals to sites where they can be precipitated as ore minerals. 

    The excess water circulates through the surrounding rocks and may scavenge and transport metals to sites where they can be precipitated as ore minerals. 

    The second source being large amounts of hydrothermal fluids being swept along a convective path, with the analogy referred to as a pot of boiling water with the hottest water directly above the hottest source, rising rapidly.


    Below is a picture of what once was moving hydrothermal fluids that finally settled.


     Hydrothermal fluids also flow around fractures and faults. Veins form where the fluids flow through larger, open space fractures and precipitate mineralization along the walls of the fracture, and eventually completely fill it as shown in the picture below where a hole has been filled.



     Below is a picture of a hydrothermal area where leaching of the soft chalk like material, has dug out trenches.





The white chalky appearance of the landscape in the below picture, was caused by Volcanic eruptions in the area where materials came in contact with hot boiling Hydrothermal fluids, changing the mineralogy of the rock and thus changing its composition to this white chalky appearance.

     All but one of the mineral formations seen in the above pictures was taken in Santa Cruz County of Southern Arizona United States. “All the formations are sericitic in origin. Sericitic alteration alters the rock to the mineral sericite, which is a very fine-grained white mica.  It typically forms by the decomposition of feldspars, so it replaces feldspar.  In the field, its presence in a rock can be detected by the softness of the rock, as it is easily scratch able. It also has a rather greasy feel (when present in abundance), and its color is white, yellowish, golden brown or greenish.  Sericitic alteration implies low pH (acidic) conditions.
Alteration consisting of sericite + quartz is called “phyllic” alteration.  Phyllic alteration associated with porphyry copper deposits may contain appreciable quantities of fine-grained, disseminated pyrite which is directly associated with the alteration event. (”

Below the author has scratched in the word Soft, to display how fine gained and chalk like the surface of the particular area is.

     After many of years of erosion and weathering, moisture has leaked through cracks in the landscape of this area and dug out trenches and caverns. Above the small cavern, lies the chalky white landscape seen in the above photos. Many small animals have made their homes in the resulting cavern spaces as seen in this photograph.


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