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.
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.
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.
Enrichment
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.
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.
Below: Native copper, ASARCO Ray Copper Mine, Pinal County, Arizona.
References:
Romantic Copper its Lure and Lore, Ira B. Joralemon, D. Appleton-Century Company, New York, 1942
"Black
smoker"
hydrothermal
vents
http://www.oceansonline.com/hydrothe.htm
Voisey's
Bay
http://sci.uwaterloo.ca/earth/waton/voisey.html
Copper
-
the
Red
Metal
http://sciborg.uwaterloo.ca/earth/whaton/s03_copper_red.html
Cyprus
-
Island
of
Copper
(Metropolitan
Museum
of
Art)
http://www.metmuseum.org/toah/hd/cyco/hd_cyco.htm