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Malmgeologi
Some definitions etc:
Ore - (economic term) Concentration of ore minerals ,which can be mined with a profit.
Mineralization - (general term) Concentration of valuable ore minerals.
Example
Copper ore:
100 years ago
today
5 wt.% Cu
0.5 wt.% Cu (5000 ppm).
This is equivalent with a 10fold decrease in concentration.
At the same time the global ore reserve of Cu has
increased with a factor from 100 to 1000 (ore reserve is an
equivalent of the amount of known ore ,which remains to
be mined) .
Different kinds of ore:
Elements: Gold (Au), Silver (Ag), Copper (Cu), Platinum (Pt), Palladium (Pd) etc.
Sulfide ores:
Cu - in chalcopyrite (CuFeS2), chalcocite (Cu2S) etc.
Zn - in sphalerite ((Zn,Fe)S)
Pb - in galena (PbS)
Gold and/or silver are almost always extracted as byproducts during sulfide ore mining. Often ,the sulfide ore
minerals are intergrown with each other and form
so-called complex ores.
galena
chalcopyrite and sphalerite
Oxide ore:s
Fe - in magnetite (Fe3O4) or hematite (Fe2O3)
Cr - in chromite (FeCr2O4)
Mn - in several oxides
Ti - in ilmenite (FeTiO3) and/or ulvospinel
(Fe2TiO4)
Al - in bauxite. Approximately (AlO(OH))
hematite
magnetite
ilmenite
chromite
bauxite
Formation of an ore can best be understood
in terms of understanding of the
mechanisms of:
Source
Transport
Deposition
Example 1
Let’s try to understand it!
The concentrations of Cr and Ni vary strongly between
different types of magmatic rocks.
On the average:
In ultramafic rocks (rock of mantle-origin):
Cr - 3000 ppm
Ni - 2000 ppm
In basalt:
Cr - 270 ppm
Ni - 130 ppm
In granite:
Cr - 10 ppm
Ni - 10 ppm
All Cr- and Ni -ores of planet Earth are confined to
ultramafic rocks illustrating the importance of correctly
understood source .
Example 2
The solubility of Fe2+ (ferrous iron) in seawater is much higher than the
solubility of Fe3+ (ferric iron). Fe2+ is stable in a reduced environment, Fe3+ in
an oxidizing environment.
The early Earth atmosphere contained no free oxygen and was thus strongly
reducing. Under these circumstances the oceans contained a lot of dissolved
iron. When life and photosynthesis commenced some 3.5 billion years (3.5
Ga) ago the action of bluegreen algae in the shallow waters of the oceans
resulted in oxidation of Fe2+ to Fe3+, followed by precipitation of iron oxide
(magnetite or hematite, depending on local circumstances). Enormous
amounts of sedimentary iron ores formed (the so called Banded Iron
Formation (BIF) in the time interval 3.8 - 1.7 Ga, with a strong maximum at
2.2 Ga.
This demonstrated the importance of solubility (Fe2+ under reducing
conditions) and concentration (precipitation of iron-oxide on the ocean floor).
Example 3
Formation of a placer deposit of gold shows the importance of
instantaneous deposition. Particles of gold are extracted from the
source rocks during weathering and erosion (of e.g. volcanic rocks
charged with numerous small gold-quartz veins.) The heavy gold
particles are transported in streams and rapids downstream from
the source region. In rapids the water has enough power to
transport both gold and gravel, but whenever the stream slackens
the gold particles settle on the floor of the stream channel, forming
a placer deposit.
In this way the classical gold-fields of California (1849)
and SE Australia (1851) were formed.
The process of exploration for a mining company includes a
number of steps at successively increasing costs. After each
step the operation is analysed and can be stopped at short
notice. In principle:
Regional scale exploration (large tracts can be covered at a low
cost/km2): Airborne geophysics, geochemistry etc.
Local scale exploration: Geological mapping, detailed
geophysics (from the ground), drilling to get cores of ore and
country rocks etc. (much more expensive)
Trial mining operation.
Mineral resources and igneous processes
•
Examples of igneous mineral resources
• Magmatic segregation
– Separation of heavy minerals that crystallize early or
enrichment of rare elements in the residual melt
• Diamonds
– Originate at great depths
– Crystals are disseminated in ultramafic rock called
kimberlite
Example
The Bushveld ultramafic layered complex.
Many geologists say this is the most impressive geological formation of all.
It covers the central parts of RSA, has a surface extension of about 300x300 km
and is the major source of both Cr and Platinum Group Elements (PGE) on
Earth. Around 2 Ga ago a big meteorite struck planet Earth on
a ”Protocontinent” which much later became South Africa. Before impact, the
heavenly body broke into three pieces which struck down close to each other,
forming a three-lobed structure. The crust cracked all the way down to the
mantle and heavy molten magma welled up. Subsequently a big magma
chamber formed at a depth of a few km and the magma started to slowly cool.
The first minerals to crystallize were heavy chromite (FeCr2O4), pyroxene etc.,
which sank to the floor of the magma chamber forming bands of alternating
black chromite and yellow pyroxene (see the ore samples showing
pseudostratification!!). Individual less than meter-thick layers can be followed for
up to 90 km in Bushveld. The whole layered complex is saucer-shaped because
it is so heavy compared with the surrounding continental granites and gneisses
that the central portion sags down.
Example
Example
Example
Kimberlites
Kimberlite is a type of potassic volcanic rock best known for diamonds occurence. It is
named after the town of Kimberley in South Africa.
Kimberlite occurs in the Earth's crust in vertical structures known as kimberlite pipes. The
consensus on kimberlites is that they are formed deep within the mantle. Formation occurs
at depths between 150 and 450 kilometres, from anomalously enriched exotic mantle
compositions, and are erupted rapidly and violently, often with considerable carbon dioxide
and other volatile components. It is this depth of melting and generation which makes
kimberlites prone to hosting diamond xenocrysts.
Open pit mine near Yakutsk (Siberia)
Example
Pegmatites
Example
Porphyry copper ores
These subvolcanic sulfide ores are typically confined to active subduction zones (geologically
young rocks) and are especially abundant around the Pacific Ocean (the Girdle of Fire).
They are the biggest copper ores and in order to understand their genesis the amount of
copper they contain must be explained.
In principle:
Basalt (Cu-rich rock type) dominates among rock types on the ocean floor. When the waterrich ocean plate goes down the subduction zone it eventually loses its water (as well as
copper!) which become incorporated in the melts which form above the subduction zone
and which subsequently results in widespread volcanism in the island arcs and continents
surrounding the Pacific Ocean. Eventually a series of Cu-rich rock types form, for example
up in the Andes, of which porphyry copper ores are the most prominent. Thus, the huge
amount of copper present in these ores can be explained if we allow for all the copper of the
ocean floor to be processed and concentrated along a few narrow strips, along the periphery
of the ocean! This is the reason why Chile is the number one copper exporter of the world by
far, and it also explains why the Bingham Canyon (outside Salt Lake City), with a diameter of
4 km and a depth of 800 m, is the biggest man-made hole in the ground.
Example
Porphyry copper ores
Bingham Canyon open-pit mine
Mineral resources and igneous processes
• Hydrothermal solutions
– Among the best known and important ore deposits
– Majority originate from hot, metal-rich fluids that are
remnants of late-stage magmatic processes
– Move along fractures, cools, and precipitates the metallic
ions to produce vein deposits
Hydrothermal deposits often occur with igneous rocks
VMS deposits
VMS stands for Volcanogenic Massive Sulfide deposits. The word ”Massive”
means that the content of sulfide minerals (mostly pyrite) is much more than
50% giving the ore a massive appearance. They are believed to be associated
with volcanoes and subduction and are consequently found:
1) in geologically young rocks along the periphery of the Pacific Ocean (The
Girdle of Fire) etc.
2) associated with fold belts in older geological formations.
Formation of a typical VMS deposit can be explained as follows: On the flank
of an active volcano at the coast, groundwater is forced upwards-outwards to
escape the heat. Water has to be replaced, and water from the ocean is forced
down into the sediments and volcanic rocks nearby. Convection of hot water
starts and since this water is charged with salts from the ocean, it has the
power to dissolve and pick up heavy metals from the porous rock-pile it
encounters. As the hot circulating water, driven by the volcanic heat,
encounters the ocean floor from below (in the form of a hot spring or Black
Smoker!) it suddenly cools from some 300-500°C to almost zero. This forces
the heavy elements to precipitate out as sulfides and a compact ore deposit
will form with high contents of Cu, Zn, Pb etc. These ores are relatively
common, found on all continents and represent the whole history of planet
Earth.
Sedimentary deposits
A)
B)
C)
D)
E)
Banded Iron Formation
Placer deposits
Weathering deposits
Deep ocean precipitation
Supergene enrichment
BIF
These ores were already discussed above in connection with solubility and
oxidation of Fe2+ to Fe3+ (and insoluable Fe-oxide) in the early oceans.
However, a few points deserve to be added. BIFs have been found on all
continents with ”old” enough rocks. The names differ but the formations in
principle always consist of a package of alternating iron-oxide(s) and quartz
layers. The packages may contain thousands of layers and be several
hundred kms in the longest direction. They are often referred to as taconite
in North America, itabarite in Brazil, hematite-quartzite in India, banded
jaspilite in Australia, banded ironstone in South Africa, and quartz-banded
ore in Scandinavia.
One popular idea concerning the actual banding is that the bluegreen algae
oxygen-poisoned themselves when their concentration exceeded a certain
limit and they became almost extinct. Thus iron oxide precipitation stopped
and the conditions became reducing again. Eventually, algae production
started again and the whole process was repeated. And so on.
Placer deposits
•
Placer deposits
• Placers – deposits formed when heavy metals are mechanically
concentrated by currents
• Involve heavy and durable minerals
• Examples include
– Gold
– Platinum
– Diamonds
Weathering and ore deposits
•
•
Secondary enrichment - concentrating metals into economically
valuable concentrations
Bauxite
• Principal ore of aluminum
• Forms in rainy tropical climates from chemical weathering and
the removal of undesirable elements by leaching
Mineral resources and metamorphic rocks
• Regional metamorphism can also generate useful deposits
• Talc
• Graphite
Who is responsible?!
Thanks for your attention!!!