Oxidized zone minerals

Wednesday, March 1, 2006

Peter Russell

The oxidized zone of ore deposits has fascinated me for many years because of the process of their formation and the resulting reddish brown rocks. The colourful rocks are a signal to prospectors that economic ore cannot be far away. Stories abound of rusty outcrops luring prospectors across the southwestern U.S.A. to grub in the rocks to find a fortune, or a flicker of hope, which faded rapidly. Many generated a flurry of penny stocks, which changed from promise to worthless paper.

Voisey's Bay nickel deposit in Labrador was found in 1993 by Albert Chislett and Chris Verbiski of Archean Resources, a small St. John's, Newfoundland based company. These observant Newfoundlanders clued in on a rusty outcrop while flying into Nain after a summer of prospecting. They encouraged the helicopter pilot to land on an outcrop so that they could collect samples. These were analyzed and proved to contain nickel. The finding at Voisey's Bay shows that using your eyes may still lead to a bonanza, if you are lucky to find rich ore samples when you step on the ground. The ultimate extraction of the ore became entangled in litigation and politics. INCO's Voisey's Bay deposit will finally produce nickel in 2006 after huge amounts of money were spent on acquisition and exploration and agreements with governments of Newfoundland and the first nations.

Oxidized zone mineral deposits produce spectacular colourful mineral specimens, though the fact that they occur near the surface means that it is a depleting resource. My interest was sparked by the variety of colourful mineral samples, which became available from Tsumeb, Namibia in the late 60's and 70's.

Below: Cross section of an oxidized sulphide deposit.

Cross section of an oxidized sulphide deposit

Original sulphide sources of mineralization. 
Black Smokers.

In order for an oxidized deposit to be formed, an original sulphide ore body must be present. The sulphide ore body may be formed by cold alkaline oxidizing seawater migrating through sediment and other rock beneath the sea. This seawater circulates and turns into a hydrothermal fluid near a magmatic heat source from an active ocean ridge. This type of deposit is being formed today off the coast of Vancouver Island on the Juan de Fuca Ridge. This enriches the concentration of metals and hydrogen sulphide. These minerals precipitate out of solution at huge areas of chimney-like vents called "black smokers" on the sea floor. The hot water containing sulphides forms chimneys and showers the surrounding area with a rain of particles as the water cools from about 400-300 degrees C. The mounds of sulphide rich material are then covered with other rocks and eventually moves to the surface over millions of years allowing oxidation and enrichment to begin.

Below: Gossan at the Lavender Pit overlook, Bisbee Arizona.


Porphyry Copper Deposits
A Porphyry Copper Deposit derives its name from a porphyritic stock located at the center of the mineral deposit. A stock results from a cylindrical mass of magma, which moves up through the Earth's crust underneath a stratovolcano and cools. Stratovolcanoes are formed of a mixture of lava flows and fragmentary ejected material. Mount Fuji in Japan, Mount Rainier in the U.S.A.and Vesuvius in Italy (see this isssue) are examples of stratovolcanoes. In a porphyritic rock, some of the minerals are very large crystals (up to 10 cm in length) and the rest are microscopic. In the ore deposits we generally find that the upper parts of the stratovolcano have been eroded away. The surrounding rock, which has been intruded, is often metamorphosed by heat and pressure. During metamorphism, sulphide minerals form in the rocks surrounding the stock or magma chamber. An enriched mineral blanket or oxidized zone will then form near the surface of these deposits. The porphyritic stock at the center of the system may not contain enough of the copper minerals to be an ore deposit. The rock that surrounds the stock however may be rich in copper mineralization.

Below: Eh-pH diagram for a copper deposit. Oxidation and reduction (Redox) reactions play an important role in the geochemical processes that produce enrichment of ores. Eh is a measure of reduction potential. Redox reactions are reactions in which electrons are transferred. The species receiving electrons is reduced, that donating electrons is oxidized. The black dotted line is the watertable. The reaction stops here and the sulphide ore is enriched.

Eh-pH diagram for a copper deposit

The porphyritic stock is the engine that allows the development of the minerals. The ore minerals are found in a series of zones radiating outwards from the stock. Each of these zones contains a specific suite of minerals. These minerals include azurite, malachite, gold, silver, chalcocite, and chalcopyrite.

Oxidation and Reduction
Chemical reactions in the upper part of a sulphide ore deposit begin when naturally acidic rainwater and oxygen dissolved in groundwater attacks pyrite or other sulphides. The absorption of oxygen by pyrite causes it to change into iron oxyhydroxides and sulphuric acid. This acid works its way down through the ore-body taking copper, lead and other elements with it. Masses of spongy insoluble limonite (insoluble ferric-oxy-hydroxides) are produced. The rusty mass of limonite is characteristic of ores containing pyrite. German miners called this material "iron hat." The equivalent term in English is gossan. 
Once the iron sulphide oxidizes and generates sulphuric acid and limonite, the acid reacts with other sulphide minerals. Copper sulphide reacts forming copper sulphate. On contact with carbonate ions from a source such as limestone, the copper sulphate reacts to form the copper carbonates, malachite and azurite.

Native gold present in the sulphide ore will not be affected by the chemical reactions and would be left behind in the gossan.

When the acid solution enriched with copper arrives at the water table, the descending water loses oxygen and the oxidation of sulphides cannot continue, unless Fe3+ions are present. Fe3+ may oxidize FeS2 forming Fe2+ and more sulphuric acid. The dissolved copper sulphate interacts with copper sulphides and enriches them forming minerals such as bornite, chalcocite and covellite. The copper content of chalcopyrite is 34% and that of covellite 66%. Enrichment takes away copper from the upper part of an ore body and drops it off at the water table. This process is called supergene enrichment. The oxidized zone and zone of enrichment may contain profitable mineralization. The original sulphide bearing rock may not have contained sufficient ore to pay for deep mining, pumping of water and treatment of the ore.

Other sulphide minerals such as sphalerite (zinc sulphide) and galena (lead sulphide) are broken down in the oxidized zone. Galena will form cerrusite, anglesite and wulfenite. Sphalerite forms smithsonite.
Oxidized zones are found in many deposits, frequently found in arid areas of the world including the United States, Mexico, Peru, Chile and Africa. The oxidized zone of mineral deposits found in the Arizona and New Mexico are around 122 metres or 400 feet deep.

Cyprus - The Copper Island
Cyprus is the island, which gave its name to the metal. The word copper comes from cuprum, the Roman name for Cyprian metal. The first copper found was native copper, which could easily be fashioned into useful objects. Mining began in the 4th millennium B.C. The islanders then discovered that green stones (malachite - copper carbonate) when heated in a fire produced copper metal. They then learned to collect blue-green water seeping from the rocks, containing copper sulphate and process this to produce the metal. The Romans used copper sulphate from mines, where they would collect the water as it dripped through cracks in the rock. This was a labour intensive process and they used slave labour to dig the tunnels just below the water table and collect the copper solution in jars. The water was evaporated and the material was used for medicine, pigments and other copper products. In AD 162 Galen, geographer and personal physician to the Roman emperor Marcus Aurelius, visited mines of Cyprus in search of hydrated sulphates of copper, zinc, and iron, which were used extensively in medicines at the time. He wrote a journal of his visit with a description of the mines and tools used in mining.

Skouriotissa is a copper mine in a hilly region on the foothills of the Troodos mountains in Cyprus. It is one of the oldest copper mines on the island. The ancient Romans leased the mines to the highest bidder. In 12 BC rights to mines in the area went to King Herod, who was allowed to keep half the profits. This area is still producing copper from sulphide minerals found in pillow lavas.

Rio Tinto, Spain.
The Moors who occupied Spain in the Middle Ages found out that copper could be extracted from the sulphide ore. The ore was crushed and water percolating through the mass would produce copper sulphate. This technique is called "heap leaching" and is still in use today. The Rio Tinto area is highly polluted by natural weathering of the primary minerals plus the spoils produced from hundreds of years of mining.

Copper extraction at the Asarco Ray Copper Mine, Hayden Arizona.

Below: Acid leaching of copper ore, Ray Copper Mine, Arizona. All water on the mine property must remain there or be allowed to evaporate, with no runoff.

Acid leaching of copper ore

Oxide leaching and solvent extraction. 
Copper ore is piled onto a thick high-density polyethylene liner. Sprinklers are placed in the surface to spray a weak acid solution onto the pile. This dissolves the copper in the ore. The copper bearing solutions are collected and pumped to an extraction plant where an organic extractant removes the copper from solution. The resulting solution is then transferred to the electro-winning process, where copper is plated out as a cathode. The leaching of the oxide ores is relatively easy by using sulphuric acid. Leaching of sulphide materials requires a chemical oxidizing agent - ferric ions (Fe3+). These ions are generated by reactions with the atmospheric oxygen. Oxidation can be assisted by either pressure (as in an autoclave) or more commonly with bacteria. Sulphuric acid is not the only reagent that can dissolve copper from a concentrate.

BHP Billiton has patented a process using ammonia to dissolve part of the copper concentrate. This process is used at the Coloso Plant in Chile.

Below: Chalcanthite


Below: Vanadinite (lead chlorine vanadate), Grey Horse Mine, Pinal County, Arizona.


Below: Copper in gypsum, Tohono O'odham First Nation, Pima County, Arizona.

Copper in gypsum

Below: Native copper, ASARCO Ray Copper Mine, Pinal County, Arizona.

Native copper

Below: Lavender Pit, Bisbee, Arizona
Lavender Pit
Below: Gossan underground in the Queen Mine, Bisbee, Arizona.
Gossan underground


Romantic Copper its Lure and Lore, Ira B. Joralemon, D. Appleton-Century Company, New York, 1942

"Black smoker" hydrothermal vents

Voisey's Bay 

Copper - the Red Metal

Cyprus - Island of Copper (Metropolitan Museum of Art)