Extraordinary Copper Minerals from Milpillas, Mexico

Write By: tmmadmin Published In: Classic Mineral Localities Created Date: 2014-12-23 Hits: 4900 Comment: 0


2" Azurite with Malachite, Milpillas Mine, Cuitaca,
Mun. de Santa Cruz, Sonora, Mexico

Outstanding azurite crystals and malachite pseudomorphs from the Milpillas copper mine, 30 km NW of Cananea, in Cuitaca, Mun. de Santa Cruz, Sonora, Mexico, began appearing on the mineral market in 2006. At their best, the specimens from this new classic locale are right up there with the best crystals from Bisbee and Morenci, Arizona or Tsumeb, Namibia. Three different minerals from this mine have captivated the mineral market: electric blue azurites; velvety, dark green malachite pseudomorphs (replacements) after azurite; and brilliant green brochantite in long, slender, acicular crystals.


3.1" Azurite with Malachite, Milpillas Mine, Cuitaca,
Mun. de Santa Cruz, Sonora, Mexico

LONGEVITY OF THE MINE
When they opened the mine in 2006, Industrias Peñoles reported that the expected life was 11 years. However, I've heard that the oxidized zone will be exhausted by late 2012, which will end the run of collector quality specimens. In addition, it bears mentioning that all specimens have to be smuggled out of the mine. The mine owners are very unhappy with miners who collect specimens, and recently they have been really cracking down on them. To sum this situation up, mineralogist Peter Megaw said that, "In 10 years these will be hard to find classics... so load up now while you can!" Thomas Moore added that "…collectors thus are advised to choose their Milpillas "keepers" without too much dithering."

MINERALS FOUND


1.8" Azurite, Milpillas Mine, Cuitaca, Mun. de Santa Cruz, Sonora, Mexico

Azurite
The best azurite specimens from the Milpillas Mine come close to being the world's finest in quality and in sheer beauty, often equaling those from famed localities such as Tsumeb, Bisbee, Morocco and Morenci. They feature highly lustrous, mirror-faced crystals of excellent dark blue color. The largest crystals to date are about 4.7" (12 cm). They typically occur in groups from small miniature to cabinet size. Crystal habits include distinctive blocky types with a rectangular cross section, sometimes with serrated edges, as well as blocky truncated crystals.


1.8" Azurite, Milpillas Mine, Cuitaca, Mun. de
Santa Cruz, Sonora, Mexico

Most azurite specimens from Milpillas have a characteristic electric blue color which is the result of a unique geological sequence of development. The specimens grew first as azurite crystals, which were later changed into malachite by a process called replacement pseudomorphism, wherein each azurite molecule Cu3(CO3)2(OH)2 loses a copper atom, thus changing the azurite into Cu2(CO3)2(OH)2 - malachite. The final step in the process occurred when an outer layer of azurite overgrew the malachite. The unique brilliant electric blue color and high luster flash of Milpillas azurite is the result of light passing through the outer layer of azurite, where it reflects off the malachite, and backlights through the azurite.


1.8" Azurite, Milpillas Mine, Cuitaca, Mun. de Santa
Cruz, Sonora, Mexico. Note the malachite underlying
the thin coating of azurite on the right side.

Some Milpillas azurite specimens display flower-shaped combinations up to 1" (2.5cm) across, which typically lie flat contrasting with dark green malachite; others have dome-like stepped crystal clusters of azurite on malachite.


2" Malachite pseudomorphs of azurite, Milpillas Mine,
Cuitaca, Mun. de Santa Cruz, Sonora, Mexico

Malachite pseudomorphs after azurite
Malachite pseudomorphs from Milpillas can be quite sharp, often stand up straight, and frequently display dark velvety forest-green surfaces with a shimmering luster. Primary malachite (not pseudomorphed) occurs, but is rarely seen in collector specimens.


2" Malachite pseudomorphs of azurite, Milpillas Mine,
Cuitaca, Mun. de Santa Cruz, Sonora, Mexico

The pseudomorphed malachite specimens are often found as floaters (with no attachment points), consisting of sharp, bladed pseudocrystals to 1" (2.5cm), usually as low-relief clusters, some with small bits of matrix, rarely to 7" (18cm) across.


2" Malachite pseudomorphs of azurite, Milpillas Mine,
Cuitaca, Mun. de Santa Cruz, Sonora, Mexico

Brochantite
Outstanding brochantite specimens found in seams and mugs in massive cuprite and chrysocolla began to emerge at Milpillas in 2010, as mining moved downward in the oxidized ore bodies. The crystals range from 0.5 to 1" (1.3-2.5cm needles to crystals that rarely reached 2" (5 cm long), typically forming hairy mats in a saturated electric green color.


2" Brochantite, Milpillas Mine, Cuitaca, Mun.
de Santa Cruz, Sonora, Mexico


2" Brochantite, Milpillas Mine, Cuitaca, Mun. de
Santa Cruz, Sonora, Mexico


Brochantite crystals to 0.4", Milpillas Mine, Cuitaca,
Mun. de Santa Cruz, Sonora, Mexico

Other minerals
Other minerals found at Milpillas include barite, chalcocite, chrysocolla, copper, cuprite, and plancheite.

GEOLOGY
The Milpillas copper deposit is part of the Cananea mining district, said to be the largest porphyry copper district in North America. The Milpillas Mine is a huge underground excavation which mines a manto deposit of the copper-rich mineral, chalcocite. Manto orebodies are irregular to rod-shaped ore occurrences which usually occur in horizontal layers. The orebody is covered by 800 to 2,400 feet (250-720m) of gravels. Most of the copper carbonate oxide ores which produce specimens occur in the top 650 feet (200m) of the deposit. Below the oxide ores is a low grade chalcocite-bornite mix which will be exploited after the upper deposits have been mined out. Collector quality secondary copper minerals occur in small pockets and narrow seams (commonly filled with white, sticky clay) in the brecciated volcanic country rocks in and around the oxidized zone in the mine's upper levels. Each pocket is unique, so the specimens from each vug are subtly distinct. Solitary crystals are most common, though they are also found as solid linings on the walls of vugs. Most Milpillas azurite specimens are damaged, often heavily, either because they grew in contact with the walls of the pocket, or due to damage from ore blasting. Hurried collecting and inadequate protection during transportation also take their toll.


Cross-section through the Milpillas Mine, from
Berrocal-Lopez, et al., 2009. Scale is approximately
2,300 feet vertically (700m) and 5,000 feet horizontally
(1500m). It is the copper carbonate-oxide bodies (oxides
de cobre) which produce collectable azurite, malachite
and brochantite specimens.


The Milpillas Mine is located about 40 miles south
of the U.SA-Mexico border

HISTORY AND OPERATIONS
The Milpillas deposit was not discovered until the late 20th century because it is hidden underground, with no surface outcrops. Signs of a possible deposit were found in the 1970s, which led to drilling in the 1970s and 80s that outlined the deposit. Ironically, this float was not from the Milpillas deposit. Industrias Peñoles acquired the property in 1998, followed by more extensive drilling which showed that the project would be profitable if combined with an on-site copper solvent extraction plant. Mining began in 2006, when the plant was ready. By the end of 2011, mining of the copper oxide orebodies was nearly complete, and after they were gone, mining of chalcocite ores would begin.


Aerial view of the Milpillas Mine complex

THE UNDERGROUND MINE
To enter the Milpillas Mine, you pass through a very large concrete-lined portal. Huge trucks roar up the steep, wide, branching roads from below, carrying ore to the nearby extraction plant. The loud roar of the diesel engines and the diesel fumes are almost overwhelming. Loaded trucks coming up from below have the right of way, so the empty trucks going down have to swerve into side tunnels to wait while the loaded trucks growl upwards in the earsplitting din and murky light.


Aerial view of the Milpillas Mine, showing the
copper extraction plant

VISITING THE MINE
Visitors are rarely allowed underground at the Milpillas Mine, and the few who do get in are monitored very closely at all times. Industrias Peñoles does not allow crystal collectors or buyers, and deals harshly with miners who are caught collecting crystals or smuggling them out for sale.


2" Azurite with malachite, Milpillas Mine,
Cuitaca, Mun. de Santa Cruz, Sonora, Mexico

CONCLUSION
The exceptional specimens of azurite, malachite pseudomorphs and brochantite that have come from the Milpillas Mine have gained wide recognition as world-class specimens, and are clearly the finest of their kind available on today's mineral market. Many already consider the mine a contemporary classic. When the flow of specimens stops and the word "contemporary" is dropped, Milpillas will have earned its classic rating, and will take its rightful place in mineralogical history as the source of the most brilliant azurite crystals, the finest malachite pseudos, and the best brochantite specimens ever found.

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