Turquoise from ancient Mesoamerica and Southwestern
US/Northern Mexico has been mined for over a thousand years, beginning with the
Aztecs and Mayans, followed by the Native Americans and pioneers near the turn of
the 20th century, and is still being mined today. However, the formation of
this highly sought-after gem mineral has remained controversial since its
discovery.
Turquoise mines are
found in the drier regions
of Mexico and the U.S.
Two hypotheses have been proposed: (1) the hydrothermal
or hypogene hypothesis suggests that turquoise was formed by hot fluids flowing
upwards from depth, and (2) the supergene hypothesis suggests that turquoise
formed from cool meteoric water (rain) flowing downward through the rocks that
host the gem mineral.
Turquoise mines in the Cerrillos, New Mexico, area are classic examples of
turquoise deposits throughout the southwestern United States. These are generally
located within 100 meters of the surface and are associated with the most altered
areas of the porphyrytic quartz monzonite plutons [solidified granite-like magma,
initially a subsurface intrusion into older formations, subsequently altered by
geologic processes and now near-surface or exposed].
These plutons have several distinctive zones of alteration which include propylitic
[altered andesites]and argillic [whitish clay-like (kaolinite)] zones. Samples
were collected from each zone to better understand the fluid-rock interaction.
Based on detailed petrography, electron microprobe and x-ray diffraction data,
the deposits were formed during the waning stages of hydrothermal activity.
Hydrothermal convection of the ground water interacting with the plutons leached
P, Cu, Al, and Fe [phosphorus, copper, aluminum and iron]. These fluids moved to
shallower depths depositing turquoise along fractures. The data also show that
supergene processes are slowly altering the turquoise deposits of the Cerrillos
area and eventually gem-quality turquoise from the Southwest United States will
become more rare.
Study of the geologic features of the turquoise deposits
throughout the world reveals that this mineral has three modes of occurence.
Type 1
The turquoise is associated with Cu-rich porphyritic igneous rocks. This type
represents the majority of the commerically important turquoise deposits, such
as those of Nishapur (Persia), Turkey, and the principal deposits of the
southwestern USA, particularly those of Cerrillos Hills and the Burro Mountains.
A timeline schematic representing
the intrusion of porphyritic igneous rocks into sedimentary country rock.
In general, this is the process that is responsible for the creation of
the Cerrillos Hills
Type 2
The turquoise occurs in sedimentary or metamorphic rocks near or at the contact
with igneous intrusives. The mines in the Mojave Desert and several in the
Esmeralda-Nye County region of Nevada, most notably the Smith Black Matrix Mine,
are examples of this type of deposit.
Type 3
The turquoise is present in a non-igneous matrix (usually sandstone or shale) and
has no apparent genetic connection with any igneous body. Deposits that occur in
the Sinai Desert, Egypt, and Australia are the principal representatives of this
type of turquoise deposit.
The geology of the Cerrillos area.
The wide yellow strip running from center left to lower right (alluvium)
is the channel of the Galisteo River. The narrow yellow strip (center)
is the San Marcos Arroyo. The globs of dark red represent the Cerrillos
intrusives, the geologic structure that is the hills. Light red is the Mancos Formation.
Light blue is the Mesa Verde Formation
and dark blue is the Galisteo Formation.
Precipitation from ascending solutions of magmatic origin, the components being
supplied wholly by these solutions.
Hypothesis 2
Formation dependent upon alteration of sedimentary country rock by magmatic
fluids. The components of turquoise are supplied through breaking down of minerals
in the country rock.
Hypothesis 3
Precipitation from meteoric waters that descend through the sedimentary country
rock, leaching the elemental constituents from rocks near the surface. All the
components of turquoise are supplied by minerals in the sedimentary country rock.
This model represents the hydrothermal convection that was
produced when the intrusion came in contact with the water table.
Country rock refers to the geology that was
there before a geologic process happened. Thirty-five to thirty million years ago
magma intruded into the Cerrillos-area country rock, and by heat and other
processes altered it. Different kinds of alteration may occur near a magmatic
porphyry intrustion (see the illustration above). Common alteration types include:
The Potassic Alteration Zone, which is late magmatic and occurs at the core of
the alteration event. It is characterized by the formation of secondary
K-silicates [K = Potassium = "Potassic"]. This zone is dominated by feldspars,
biotite and phlogopite, with anhydrite, gypsum, vermiculites and chlorite.
The Argillic Zone, which is perhaps the most common alteration type encountered
in the western United States. The alteration of plagioclase produces kaolinite,
illite, smectite, mixed layer illite/smectite, and halloysite. Calcite can
also be present. These are all species of clay that are very sensitive to
environmental conditions.
The common alteration types in the Propylitic Zone are chlorite, epidote,
zeolites, montmorillonite, illite, and carbonate.
Mineral Paragenesis.
Relative timing for the creation of different minerals.
Note that turquoise is a later, cooler product of the hydrothermal processes.
FEATURES OF TURQUOISE DEPOSITS IN SOUTHWESTERN USA
The turquoise is confined to the altered portion of igneous rocks rich in
K-feldspar, illite, and quartz.
Turquoise ranges in color from blue to green and in composition from Fe-poor
(1% by weight Fe2O3 = blue turquoise) to Fe-rich (5% by weight Fe2O3 = green
turquoise).
Apatite, pyrite, and copper sulfides are usually present below the turquoise
deposits.
The turquoise deposits are invariably near the surface. For example,
turquoise is seldom found below 100 feet from the current surface.
The turquoise is absent in highly oxidized zones and alters to pyrophyllite.
The "Blue Bell" turquoise workings (a privately
owned patent claim) in the southeast quadrant of the Cerrillos Hills. The zones
of alteration are clearly visible.
A turquoise deposit situated near the center of the Cerrillos
Hills.
The turquoise mineral has been deposited in a fracture in the phyllic zone host
rock, adjacent to the zone of supergene alteration.
The alteration of turquoise at the Tiffany
Mine, a privately owned patent claim in the northeast quadrant of the
Cerrillos Hills.
This layer of turquoise originally filled a fracture in
the host rock.It has been exposed for only a few years, and already it is
showing significant alteration to pyrophillite.
Comparison of the structures of Turquoise and
Chalcosiderite at the molecular level.
Turquoise is a hydrous aluminum phosphate. Chalcosiderite
is a hydrous iron phosphate. These nearly identical minerals vary only by the
"B" component of the formula above.
When turquoise is present in a highly oxidized zone,
such as being exposed at the surface, it alters to prophyllite. Compare the
x-ray diffractogram signature of turquoise (upper chart) with that of
prophyllite-turquoise (lower chart).
Turquoise in the Southwestern United States occurs within monzonite
porphyries or is genetically related to igneous activity. The turquoise deposits
are invariably near the surface, seldom found below 100 feet from the current
surface. Apatite, pyrite, and copper sulfides are usually present below the
turquoise deposits.
Known members of the turquoise group of minerals include Planerite (Al),
Turquoise (Cu-Al), Faustite (Zn-Al), Aheylite (Fe2+, Al), Coeruleolactite
(Ca-Al), and Chalcosiderite (Cu-Fe3+). However, the majority of the turquoise
found in the Southwest US ranges in color from blue to green and in composition
from Fe-poor (1% by weight Fe2O3 = blue turquoise) to Fe-rich (5% by weight
Fe2O3 = green turquoise.)
Based on detailed paragenesis, turquoise is a late stage hydrothermal
mineral that occurs in fractures and veins and is associated with illite and
quartz. The source of the hydrothermal fluids was likely groundwater and
meteoric waters that were heated during interaction with the sulfide-rich plutons
at depth. The interaction of these fluids with Cu-rich and Fe-rich sulfides
produced acidic fluids that were capable of leaching the elemental constituents
(Cu, Al, Fe, P) necessary to form turquoise. As these fluids ascended and
cooled they precipitated turquoise at shallower depths.
Turquoise is likely not a supergene mineral because turquoise is absent in
highly fractured and oxidized zones where meteoric fluids permeated the plutonic
rocks. When turquoise is found in these oxidized zones it is chalky, bleached,
and is invariably altered to pyrophyllite. Therefore, supergene processes are
slowly altering the turquoise deposits of the Cerrillos area and eventually gem
quality turquoise from the Southwestern United States will become more rare.
The turquoise group of minerals
Micropictographs of the mineral sources of the
components of turquoise.
Expressed in the manner of a cookbook, turquoise is simply...
oxidized hydrogen (water)
aluminum
oxidized phosphorus (phosphate)
with a dash of copper [or iron] for color.
Cu Al6 (PO4)4 (OH)8 · 4(H2O)
[In some older publications the chemical formula
of turquoise was reported with 5 water molecules,
or 4-5 water molecules. The number of water
molecules is now (2002) settled as 4.]
This website is maintained by the Cerrillos
Hills Park Coalition
and is dedicated to the creation, enhancement and stewardship
of an historical, recreational, and cultural open space in
the
Cerrillos Hills, Santa Fe County, New Mexico, USA
Type 1
