FMF - Friends of Minerals Forum, discussion and message board

[Paul Biehler]

I have been wondering why we don't see great Azurite and Malachite from the large copper deposits in Chile? What are the geological reasons? If there are Azurite and Malachite from Chile where would I be able to see them? Thank You Paul

[alfredo]

Part of the reason is that copper carbonates like azurite and malachite rarely form in abundance unless there is an abundant source of carbonate - ie. calcite and/or dolomite - in the primary sulphide ores. If calcite and dolomite are scarce, one ends up with copper sulphates, like the abundant antlerite in Chuquicamata, rather than azurte and malachite.

[Jean Sendero]

As Alfredo said, the development of malachite and azurite is dependent on the host rock into which the porphyry will intrude and form the deposits. In Chile, most of the porphyry Cu are formed in the Paleocene or Oligocene or Miocene volcanic arcs. The intrusions are in placed generally in coeval volcanic rocks or older volcaniclastic or intrusive rocks. Hence these two hosts are carbonate poor hence the rare occurrence of malachite and azurite.

In what the miners call the "oxide zone" located just above the secondary enriched sulphide zone, the most common secondary "green minerals" are brochantite, atacamite and chrysocolla. Additionally, due to the rare open space filling in porphyry Cu, most of these minerals will be deposited along fracture planes and very very rarely in cavities to form nice crystals. MinDat has a very nice photograph of brochantite on chrysocolla from Cerro Colorado located east of Iquique.

Few specimens make their way to the market from these deposits for a couple of reasons. First is the "mineral collecting" culture is poorly developed to un-existant in Chile. Hence, it is rarely recognized by those who work in these giant open pits that these can be valuable. Second, the mining companies, for multiple safety reasons, make it very difficult to retrieve specimens during the mining process.

Later this weekend, I will post a few photos illustrating some nice brochantite in cavity within massive enargite veins from Cerro Colorado, some cuprite and native copper from Cerro Colorado and some typical atacamite from Minera Spence.

Jean

[Paul Biehler]

Thank you so much Alfredo and Jean. I thought if might be partly do to the Andes being volcanic. I appreciate such fine explanations. Maybe you can help with another question. Why do so many fine minerals come from desert regions? Is it the geology or just easy to prospect there, or both.

Thank You again Paul

[Riccardo Modanesi]

Hi to everybody!
maybe the reason why malachite and azurite are so scarce is to be researched not in a theoretical lack of oxygene, but in a more likely lack of carbon. Atacamite, for example, IS a well-oxygenated copper mineral and IS abundant in the Atacama desert in Chile (its name comes from there!), but it is a copper phosphate! Take it just as a personal theory, but, as we say in my country, "two plus two doesn't give five"!
Greetings from Italy by Riccardo.

[Joaquin Montoro]

Sorry Riccardo, but atacamite is not a copper phosphate...it is a basic chloride.
Cu2(OH)3Cl

[Roger Warin]

About the origin of secondary copper minerals.
Ore generally consist of copper sulfides. In the superficial areas of the deposit, a hydrothermal weathering occurs. These chemical reactions set in motion copper cations. These can then be combined with other anions forming another insoluble compound. Another mineral species crystallizes.
Depending on conditions, cations combine with carbonates to form especially malachite and / or azurite according to environmental conditions (pH, etc.). In arid areas, the lack of water disrupts this process. Others anions as chlorides can then play a role
At Tsumeb, probably one of the best places for azurite, these crystals can be found even under 1000 m of rock as it exists on this site a big fault for the passage of water at great depths.
Roger.

[Jean Sendero]

Well, some interesting comments above that may need some clarity.

This topic is very very well documented and well understood by the geoscientists. I would refer to most of you, to a very good little article, and simple enough for everyone to understand its content. the reference is below:

Chavez, W.X., 2000, Supergene Oxidation of Copper Deposits: Zoning and distribution of Copper Oxides Minerals. SEG newsletter, No 41, p. 1-13. (you may be able to find it on the internet)

Below I have extracted one figure that explains a few things. This article refers to porphyry copper style mineralization but the physical and chemical reactions and conditions are similar to any other secondary mineral deposit. (i.e Mapimi, Sta Eulalia, Tsumeb, etc...)The rule is simple, pyrite is oxidized, creates some acid, mixed with water and leach the metals, travel downwards until the right conditions are met to precipitate the minerals. It is more complex but this is my one line explanation.

Below I have inserted photos progressing down into the weathering profile. 1) leach cap, (pyrite gone), 2) a photo of precipitated secondary minerals in the oxidized zone (in this case atacamite, veins and along fractures), 3) More advance oxidation with Cuprite, and 4) and one with the secondary sulphide mineralization (Chalcocite coating pyrite) at the water table.

Cheers

jean

[Riccardo Modanesi]

Hi Joaquin!
Of course you are right: this time I mistook it by thinking of a copper phosphate, therefore I apologize for my mistake; however turquoise and ceruleite (resp. copper phosphate and copper arseniate) ARE common in the Atacama Desert as well! And this time I'm sure: they ARE oxygenated copper minerals!
Greetings from Italy by Riccardo.

[Paul Biehler]

Thank you again for a great explanation. My understanding is much more complete.

Paul