The study of mineralogy
and ore deposits can at large be attributed to hydrothermal alteration
processes where the chemical alteration results in many metallically
rich chemical compositions: gold, quartz, tin, etc. This hydrothermal
alteration process mainly combines tectonics, volcanism, and heated
water. The basic mineralogy of rock is altered as a result of condition
changes in the temperature, pressure, or chemical composition/makeup.
This has shaped things such as the California gold rush and has been
an economic boom since its discovery.
The Hydrothermal Alteration Process
Hot water or “hydrothermal fluids” pass through a nearby igneous rocks fractures
or porous spaces within the rock, altering the chemical composition
(Adams). This chemical alteration can be the result of “adding, removing,
or the redistribution of the chemical components” (Adams). These chemical
components I have mentions entail the basic makeup of the rock. For
example, the chemical composition of kaolinite is (Al4Si4O10 (OH)8)
according to the “Alteration Chemistry” handout distributed in the classroom
materials, while the chemical composition before it was altered is KALSI3O8
An example of a hydrothermal system and its circulation. From
“The Blue Planet” by Brian J. Skinner (1995). Also taken from Williams,
Curtis “Hydrothermal Alteration and Mineral Deposits.” (2002).
Typically, these “hydrothermal fluids” or “aqueous solutions” carry many metals
in addition to “silicates and other non-metallic materials” (Jones and
Hutton). The different igneous rock compositions can range in a variety
of minerals, when the water that is heated by a nearby magmatic chamber
rises in temperature and alters the nearby igneous rocks the hydrothermal
solution then becomes mineral rich. This mineral-rich solution rises,
meandering its way through fractures or cracks in the rock cooling as
it moves and dissolving other minerals on its path, once this solution
has cooled in the fracture of the rock- creating veins.
Black smoker from the mid-Atlantic Ridge This is an example of hydrothermal alteration releases in the ocean.
The energy behind the hydrothermal alteration process is the “geothermal cell”
(Jones and Hutton 2000). A “geothermal cell” is the place in which the
water is heated (Jones and Hutton 2000). Typically, from the source
the cold water moves through the fractures and cracks within the rock
until it is heated. As stated earlier, it is heated by a nearby magmatic
chamber. The heated water solution then passes through the rocks dissolving
metallic ions and other minerals, therefore altering the chemical composition
and makeup of the rock.
Typically, the hydrothermal solutions have a high saline content; therefore the
movements of these fluids alter the rock. With the minerals varying
conditions: temperature, pressure, pH and Eh if the conditions change
a condition will therefore change. This may then cause the rock to react
with nearby materials. According to Jones and Hutton “the temperatures
at which the minerals are formed range from 50 to 650°C.” These highly
varying temperatures create for a highly conducive altering agent. “The
movement of these hydrothermal fluids in the crust of the Earth is known
as “hydrothermal convection.” (McCaffrey and Pavey). “The reasoning
behind this terminology is the root meanings of the word: hydro meaning
water, thermal meaning heat and convection meaning transfer of heat
by physical movement of material” (McCaffrey and Pavey).
Hydrothermal Ore Formation
An ore is a rock that is rich in the metals, often times metal. The formation
of hydrothermal ores is attributed to the hydrothermal solution or fluids
that filter metals and minerals from the rock. These metals are then
deposited into fracture and cracks within the rock called: veins. Fractures
are a result of things such as water freezing and expanding. Fractures
can be caused by seismic activity, when the ground shifts fractures
and fissures are left in the bedrock and other components that make
up the crust. The continental crust is composed of granitic rocks, whereas
the oceanic crust is composed mainly of basaltic rock.
This animated figure has been taken from (McCaffrey and Pavey).
This animated figure has been taken from (McCaffrey and Pavey).
Some necessary factors for development of hydrothermal ore deposition include:
water source, source of ore components, and transport of ore constituents,
permeability, cause, and ore deposition. Each of these factors strongly
influences the hydrothermal process.
The main sources of water which will eventually move to a place to where it will
be heated by the nearby magmatic chamber include: “surface water, including
groundwater, referred to as meteoric waters; sea water; connate water
or water enclosed in the rocks at the time of formation; metamorphic
water; and, magmatic water (from magma)” (Jones and Hutton 2000). Meteoric
waters can be collected from precipitation factors such as snow, rain,
etc; whereas, “formation water that has been trapped with a specific
area; such as pores of sediments”(McCaffrey and Pavey). This water will
eventually become highly concentrated with minerals and metals that
it dissolves as it moves through the igneous rocks.
“Figure 16.24b A magmatic ore deposit. Layers of pure chromite (black) enclosed
in layers of plagioclase, settled out during the crystallization of
the bushveld igneous complex. This unusually fine outcrop is located
at Dwars River in South Africa” www.usd.edu/esci/figures
Ore metals are commonly derived from Earths crust. In addition to the already
available source of ore constituents is the high salinity that is assists
the transport of the ore constituents. The hydrothermal fluids, as a
result of the highly concentrated minerals (Cl, F, and CO2), easily
carry the highly concentrated metals. The wasy sin which these metallic
ions are typically transported is through “complex ions” (McCaffrey
and Pavey). “A complex ion is defined as a single chemical species made
up of an unusual combination of two or more atoms” (McCaffrey and Pavey).
These complex ions prevent deposition of the metal during the move.
In contrast, the simple ions easily loose the metal during the transport.
This process can only happen if the solution can move through the rock; therefore
permeability plays a key role in the movement of the hydrothermal solutions.
These solutions can move through passageways such as pores, cracks,
fractures, etc. It is essential for the success of hydrothermal alterations.
Deposition of the ore minerals is usually due to (1) temperature decrease-
cooling, (2) decrease in pressure, (3) change in composition of hydrothermal
solutions. A temperature decrease is a result of cooling of the hydrothermal
fluids. While a decrease in pressure can be the result of many factors.
Concentrations of these metals in a constrained space equivocates an ore. Deposition
of the ore can be sub aerial, seafloor, found in a fracture, or a rock.
Hydrothermal faulting may develop breccias and gouge. This is a form
of mineralization and alteration that takes place where there is a wealth
of fine grained veinlets. Mineral zoning patterns often times develop
near ore deposits as a result of changes in temperature, the chemical
composition of the fluid, and gas content.
“Figure 16.23 Ore formed by metamorphism. Ore of the Tem-Piute Mine, Nevada.
White is calcite, purple is fluorite. Ore minerals visible are sphalerite
(brown, lower left), pyrite (gold) and scheelite (CaWO4), the sugary
pale brown mineral upper left and lower right. Scheelite is an important
tungsten ore mineral” www.usd.edu/esci/figures
Vein and Skarn Deposits
Veins as mentioned several times above are the most common fashion in which the
hydrothermal concentrated material cools. Hydrothermal ore is formed
when the cracks, faults, and fractures are filled. Most commonly they
appear in volcanic arcs and collision terrains. The reason behind this
is that the magmas circulate the fluid moving combining with the additional
stress resulting in a major fracture. The fracture is then filled with
the hydrothermal solution cooling at some point. The metals found in
the veins are typically found in the crust and maybe the source of the
”Vein and disseminated chalcopyrite in quartzite” figure
taken from www.zambia-mining.com/
The large skarn deposits form as a result of fluid replacing the rock. Often
times the rocks are made of limestone.
(McCaffrey and Pavey).
Figure taken from http://www.davidkjoyceminerals.com/graphics/841.jpg6.1
Gold is known as an epithermal deposit because it is found at a very
shallow depth. With ore deposits there are two types of classifications:
principal and secondary. A primary ore is made primarily of one main
metallic component. Gold is an example of a primary ore because it
is made primarily of gold; however have components such as silver
can be found. Another classification is a gangue mineral. Gangue minerals
generally include quartz, calcite and other such minerals as kaolinite
and chlorite. Gold is thought to precipitate from groundwater near
in areas nearby a hot spring. These deposits are commonly found in
volcanic arc regions such as the Sierra Nevada region.
How this all relates to the Sierra Nevada Region
While visiting beautiful areas such as Blue Chert and the Kaolinite mill I was
intrigued by how heated water could chemically alter rocks to make such
beautiful things. With the seismic activity often causing the faults
and fissure for the altered rock to eventually fill; tectonics too has
its hand in it. Tectonically the subduction of a plate will force water
down to eventually be heated. Often times even the magma is closer to
the surface and can heat a greater amount of water. Hot Springs are
a great example of magmatic pocket that is closer to the surface; as
a result the water is heated and emission of sulfur and CO2 are greater.
Other landforms that can be attributed to hydrothermal alterations are
the Inyo Craters. The groundwater circulated through the magma as it
was rising resulting in steam charged explosive eruptions.
Bodie is another great example of how this process can affect life: people were
driven to the adventure and possible money they could find. With the
potential money came thieves, prostitutes, and outcasts. This now abandoned
town that had been converted into a preserve shares an interesting birds-eye-view
to California’s past.
Adams, David. Delta Mine Training Center http://www.dmtcalaska.org/course_dev/explogeo/class08/notes08.html
Barnes, Hubert L., Geochemistry of Hydrothermal Ore Deposits, 1 – 13, 303 – 307,
435 – 448, 1997.
Bove, Dana J., Compositional Changes Induced by Hydrothermal Alteration at the
Mountain Alunite Deposit, Lake City, Colorado, U.S.G.S. Bulletin 1936.
Gonchar, G.G., Fluids in the Crust: Equilibrium and Transport Properties, 1 –
Geological Association of Canada., Alteration and Alteration Processes Associated
with Ore – Forming Systems, 1 – 43, 315 – 339, 1994.
Jessey, Dr., Theories of Ore Genesis GSC433 Lecture. http://geology.csupomona . edu/drjessey/class/GSC433/Genesis.htm
Jones and Hutton., University of Wollongong, GEOS102 Ore Bodies 3- Hydrothermal
Deposits Lecture http://cedir.uow.edu.au/Projects/GEOS102/lectures/ach6.html . 2000.
Kirkemo, Harold and William L. Newman and Roger P. Ashley. “Gold”. U.S. Geological
Survey. << http://pubs.usgs.gov/gip/prospect1/goldgip.html >> 1997.
Lamber, David., The Field Guide to Geology. New York. Facts on File. 1988.
McCaffrey and Pavey, Lecture 1 - Ores and Ore Minerals, 5.2 Criteria
Ore Formation, Lecture 6 - Ores formed by Hydrothermal Processes II:
Intracrustal Deposits. http://www.dur.ac.uk/juliette.pavey/geology/lectoutline.htm
Pirajno, Franco., Hydrothermal Mineral Deposits, 22, 33, 42 – 44, 101, 110 –
Schafersman, Steven D., Miami University. GLG 111 Chapter 21: Geological Resources.
Lecture Outline. http://www.utpb.edu/SCIMATH/schafersman/geology/phy-geol/lecture-notes/ch21-resources.html.
Skinner, Brain J., The Blue Planet: An Introduction to Earth System Science,
419 – 425.
Williams, Curtis. Hydrothermal Alteration and Mineral Deposits. http://www.indiana. edu/~ sierra/papers/williams.html.
Materials collected from G188 binder. Compiled by John Rupp, Michael Hamburger,
and Assistant Instructor. May 10-25, 2003.
Field Work from Sierra Nevada Region, Geology G188. Compiled by Megan Patterson.
May 10-25, 2003.
Sites only used for images/ figures