By perusing the Blog postings readers probably
realize that I have a hankering for nice blue minerals, most of which are
associated with copper. My home states
of Kansas, Colorado, Wisconsin, South Dakota and Utah have supplied a few blue
minerals in my collection---well maybe not Kansas; however, Arizona is a state
I visit quite often and it supplies a plethora of blue minerals. Of course, Arizona has been, and is, a major
producer of copper. Locals like to brag
about the five Arizona Cs that supported the economy during the early years of
statehood; copper, cotton, citrus, climate and cattle. Today copper is still important although many
of the big open pit mines have closed down and environmental concerns and
litigations plague prospective new mines.
Cattle operations house only a fraction of the animals as were present a
century ago. Numerous other states now
produce more beef and/or milk than Arizona.
Cotton is still plentiful; however, it is a major user of water and I
presume production will decline as available water continues to decline. The demise of citrus crops, as far as I can
tell, is due to a vastly increased population and urban sprawl---cut down the
orchards and plant houses. Arizona’s
climate is what attracts current money to Arizona in the form of tourists,
snowbirds and retirees. However, with
the onset of, and continuing, global warming, the climate simply may get too
warm for the available resources. The
state is in a multi-year drought and climatologists are not looking for an
early relief. Water is becoming scarcer
every year although the developers seem to have problems digesting that tidbit
of information. I believe Maricopa
County (Phoenix) is the second fastest growing county in the US. Water is at a premium and fast relief is not
in site. Both Lake Powell and Lake Mead,
the large reservoirs on the Colorado River in the north are less than 50%
capacity. All of the western states in
the Colorado River Compact, plus Mexico, want a piece of the water action;
however, the fact remains that states are using the water faster than
replenishment.
Major planning must begin as soon as possible;
however, in Arizona the politics dictate an emphasis on other items and omit
the 900 pound gorilla lurking in the corner.
Back to copper.
I continue to try and better understand ore genesis but rely on the
great books, Mineralogy of Arizona 3rd
edition (Anthony and others, 1995) and The
Frugal Collector; Part 1 (Jones, 2011 and his numerous articles in Rocks and Gems) as my guides to the state’s mineral resources and their
genesis. It is my understanding that one
of the 3rd edition authors, Ray Grant, is working on a 4th
edition of the minerals book and that part two of Jones’ work is near the press.
One thing that I have learned, well sort of tried to
digest, is that the major copper deposits in Arizona have been/are found in
very low grade ore called porphyry copper deposits. Here the disseminated
copper makes up less than 1% of the total ore.
Anthony and others (1995) noted that these ore deposits are generally
found in the Basin and Range Physiographic Province, “are always associated
with an intrusive calc-alkalic porphyritic rock (distinctive difference in
grain sizes)…typically quartz monzonite (intrusive igneous rock with equal
amounts of orthoclase and plagioclase and 5-20% quartz) , tonalite (intrusive
igneous rock with major plagioclase and less than 10% orthoclase with more than
20% quartz), or granodiorite (intrusive igneous rock with major plagioclase,
some orthoclase and greater than 20% quartz)…ranging in age from late Mesozoic
through middle Tertiary…The major primary (hypogene) ore minerals [are]
chalcopyrite (CuFeS2), some bornite (Cu5FeS4) [while] pyrite is typically the most
abundant sulfide.” In some porphyry deposits,
weathering of the upper exposed surfaces allowed the formation of sulfuric acid
and iron and copper sulfates (especially from the pyrite) and percolated
downward where secondary ore sulfides were deposited, especially chalcocite (Cu2S),
in the supergene (layer around the water table). The collectable minerals of porphyry copper
deposits such as azurite, malachite, cuprite, chrysocolla and others were
deposited in the uppermost oxidized zone (above the supergene) or in deposits
peripheral to the major porphyry deposits.
Wow, it is much more complicated than this paragraphs indicates;
however, I would need an entire university course to really embed the
information in my mind (assuming I could kick out several gigabytes of unneeded
information).
The New Cornelia Mine is near the town of Ajo in
southwestern Arizona, and in fact, Ajo was a company town for the miners and is
the name typically associated with the mining district. The New Cornelia is a large porphyry copper
deposit and copper was mined in one of those huge open pit mines about 7600
feet across and over 1100 feet deep. I
find it amazing that in the initial mining “boom” in the mid-1800s, the miners
needed to ship the New Cornelia ore clear to Swansea, Wales, for processing! I have not been able to locate the exact
route the ore took on its journeys—but perhaps it was hauled by horses/mules to
the Sea of Cortez and then by ship to the United Kingdom. As one might suspect, the profit margin of
this low grade ore was nonexistent. In
fact, it was not until around 1915 that a smelter was constructed nearby and
miners begin to excavate the copper carbonates, for example azurite and
malachite, in the upper oxidized zone.
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The New Cornelia open pit mine (lower center) and
associated tailings (left) and ponds (upper right). The former company town of Ajo (3300 people
in 2010) is directly above the mine and is the closet US city to Organ Pipe Cactus
National Monument.
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During
this process of mining with steam shovels, the New Cornelia became the first, large
open pit mine in Arizona. But like most porphyry
copper deposits, the easily mined copper carbonates became exhausted and miners
then began to attack the underlying hypogene ore (chalcopyrite and bornite) by
the mid- 1920s. Unlike many porphyry
copper deposits, the New Cornelia did not contain a copper enriched supergene
layer. Mining continued until about 1983
when “union problems” and low commodity prices forced the closure. New Cornelia produced over 6.3 billion pounds
of copper (and some gold and silver) during its life (Arizona Department of
Mines and Mineral Resources, 2008).
MinDat lists 83 valid minerals known from the New
Cornelia including two, ajoite (K,Na)Cu7AlSi9O24(OH)6-3H2O)
and papagoite (CaCu[H3AlSi2O9]), where the
mine is the type locality. Both of these
rare minerals were/are collected from the upper oxidized zone.
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Blue-green mat of ajoite sprays coving an unknown
matrix---right side of specimen. Width
of specimen ~2.1 cm.
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I have a small specimen of ajoite, another one of
those blue-green copper silicates oxidized from the copper rich hypogene
minerals. Ajoite is a fairly rare
mineral best known from a few Arizona localities and South Africa. The Arizona specimens are usually sprays of
bladed prismatic crystals (elongated along the C Axis and small ~3-5 mm) and
flattened. The crystals on my specimen
are the flattened sprays that almost appear to be mats. However, the most sought after ajoite
specimens are from South Africa (Messina District) where mineral inclusions
occur in quartz crystals.
Crystals have a vitreous luster, are transparent in
individuals, a greenish-white streak, and a hardness of ~3.5 (Mohs). Ajoite may alter to another copper silicate, shattuckite [(Cu5(SiO3)4(OH)2],
and also may form from the alteration of
shattuckite.
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Photomicrographs of sprays of ajoite crystals. FOV ~1 cm.
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So, ajoite is one of those nice, blue, uncommon
minerals that I love for my collection.
I have not seen the quartz with included ajoite but am looking forward
to this observation. According to a
friend, there was a dealer at Tucson selling flats from South Africa---I missed
those.
REFERENCES
CITED
Anthony, J.W., S.A. Williams, R.A. Bideaux and R.W. Grant, 1995, Mineralogy of Arizona, 3rd Edition: The University of Arizona Press, Tucson.
Arizona Dept. of Mines and Mineral Resources, 2008: Arizona's
Metallic Resources Trends and Opportunities - 2008
Jones, Bob, 2011, The Frugal Collector, Part 1, 2011: Miller Media/Miller Magazines, Inc., Ventura.