Cochise College Student Papers in Geology
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Roger Weller,
geology instructor
wellerr@cochise.edu
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
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.
(http://www.dmtcalaska.org/course_dev/explogeo/class08/notes08.html#Hydrothermal)”
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.
Reference sites
http://www.dmtcalaska.org/course_dev/explogeo/class08/notes08.html#Hydrothermal
http://www.dmtcalaska.org/course_dev/explogeo/class08/notes08.html#Hydrothermal