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
Original sulphide sources of mineralization.
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,
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.
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,
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
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.
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: Vanadinite (lead chlorine vanadate),
Grey Horse Mine, Pinal County, Arizona.
Below: Vanadinite (lead chlorine vanadate),
Grey Horse Mine, Pinal County, Arizona.
Below: Copper in gypsum, Tohono O'odham First
Nation, Pima County, Arizona.
Below: Native copper, ASARCO Ray Copper Mine,
Pinal County, Arizona.
Below: Lavender Pit, Bisbee, Arizona
Below: Gossan underground in the Queen Mine,
Romantic Copper its Lure and Lore, Ira B. Joralemon, D. Appleton-Century
Company, New York, 1942
"Black smoker" hydrothermal vents
Copper - the Red Metal
Cyprus - Island of Copper (Metropolitan Museum of Art)