First Question:
Define the following terms
a)
Wall-rock
alterations
The country rocks enclosing or surrounding ore
deposits commonly show effects of reaction with the ore-bearing fluids. Such
reactions are manifested by "alteration haloes" (also known as alteration
zones, selvages or envelopes) which range in size from a few centimeters to
several kilometers. This may take the form of colour, textural, mineralogical
or chemical changes, or any combination of these. Wall-rock alteration is
useful for exploration and for understanding the processes of ore genesis.
b)
Kimberlites
Kimberlites
are the rock formation where diamonds can be found. The name "kimberlite"
comes from the town of Kimberly, in South Africa, considered, last century, as
the world centre for diamond mining. Kimberly was the site of the first
diamonds found in a rock. Kimberlites are a group of
volatile CO2-rich, potassic ultrabasic rocks with variable
composition of megacrysts in a fine grained groundmass. The megacrysts can
contain ilmentite, pyrope garnet, olivine clinopyroxene, phlogopite, enstatite
and chromite. The groundmass or matrix can contain olivine, phlogopite,
perovskite, spinel and diopside. A kimberlite is composed of three
parts: the roots, the diatreme and the crater.
c)
Laterite
In tropical
climates, where chemical weathering is high but erosion is low, thick soils
known as laterites develop. They contain
the clay mineral kaolinite and insoluble hydrated oxides of Al and Fe (which
makes them deep red). The downward
movement of percolating water and the upward movement of moisture during dry
spells, mobilises soluble ions in solution and transport fine grained particles
through the soil. The result is enriched
clay in the deeper layers and upper layers richer in Fe. Laterites developed mainly on Fe-rich rocks
(peridotite).
d)
Hydrofracturing
(overpressuring) – (simple illustration is needed)
At some stage, either early or late in
the crystallization sequence, granitic magma will become water-saturated,
resulting in the exsolution of an aqueous fluid to form a chemically distinct
phase in the silicate melt. This process is called H2O-saturation.
Because the aqueous fluid has a density lower than that of the granitic magma
it will tend to rise and concentrate in the roof, or carapace, of the magma
chamber. At shallow levels in the crust the volume change accompanying H2O
fluid production may be as much as 30%. This results in overpressuring
of the chamber interior and can cause brittle failure of the surrounding rocks
which named as hydrofracturing. This hydrofracturing usually forms
fractures with a steep dip that tend to emanate from zones of H2O
fluid production in the apical portions of the granite body and may propagate
into the country rock and even reach surface.
e)
Silicification
and silication – give an example of each.
Silicification
refers to the addition of silica as quartz or one of its polymorphs such as
chalcedony, opal, or jasper, while silication refers to the process of
conversion to, or replacement by, silicate minerals.
Silicification
as in the silicification of limestones:
2CaCO3
+ SiO2 + 4H+ ↔ 2Ca+2 + 2CO2 +SiO2
+2H2O
calcite
quartz
or as in the breakdown of the
orthopyroxene:
MgSiO3
+ CO2 ↔ SiO2 + MgCO3
opx
quartz magnesite
Silication
occurs in contact metamorphism by such reactions as:
CaCO3
+ SiO2 ↔ CaSiO3 + CO2
calcite silica
wollastonite
Second Question:
a)
Classification
of volcanogenic massive sulfide deposits (VMS).
VMS Deposits are
categorized into two groups:
There are two
types of classifications; the first is based on the types of metals, and the
second is based on the associated rock types.
Based on the
common metal, VMS can be classified into:
1. Cu-Zn; barite is absent
from the majority of these types of deposits. Deposits are concordant to
semi-concordant, massive iron-sulfide-rich bodies are commonly underlain by
vein systems comprising stringer ore, within volcanic sequences that consist of
mafic volcanic rocks with locally significant felsic and/or sedimentary rocks.
Some examples include Precambrian deposits like the Noranda massive sulfide
district. Others include Cyprus and Oman ophiolite sequences and the Besshi
deposits of Shikoku Island, Japan.
2. Zn-Pb-Cu; oxide facies iron
formation is associated with this group. Barite is abundant in some deposits
concentrated where Cu/(Zn+Pb) ratios are lowest. Examples include the Kuroko
deposits of Japan. Deposits are tabular, concordant, massive pyritic bodies,
typically underlain by less prominent stringer ore, in felsic volcanic
sequences.
Based on the
tectonic setting and rock association, the VMS can be classified into the following
groups:
1. Cyprus-type:
Cu-Zn: associated with spreading ocean or back-arc spreading ridges and with
basic volcanics usually ophiolites,
2. Besshi-type:
Cu-Pb: associated with the early part of the main calc-alkaline stage of island
arc formation, and associated with mafic to intermediate rock composition like
andesite and dacite.
3. Kuroko-type: Zn-Pb-Cu: associated with the latter stages
of island arc formation and more felsic volcanics.
b)
Give
short notes on the chemical composition and characteristics of magmatic
solutions.
(A)
Major Elements in Magmatic Solutions
(1)
Silica
is the main component of magmatic fluids in the form of H4SiO4,
which attains about 8 wt% at temperatures of 900 °C and pressures up to 7 kbar.
(2) Water can dissolve
significant amounts of other major elements, such as the alkali metals (Na and
K).
(3)
The materials precipitating from an aqueous solution at high P- T could have
the mineralogy of quartz + plagioclase + K feldspar in approximately equal
proportions.
(4)
At progressively lower pressures and temperatures, however, the total solute,
including the alkali metals (i.e. Na + K), content of the solution decreased
relative to silica.
(5)
Close to the surface, the products precipitating from an aqueous hydrothermal
solution comprise mainly silica.
(6)
Magmatic aqueous solutions can also transport significant amounts of Ca2+,
Mg2+, and Fe2+, as well as a variety of anionic
substances, in particular Cl-.
(7)
There are several others which are also commonly found in aqueous solutions,
including HS-, HCO3-, and SO42-.
(B)
Other Important Components of Magmatic Aqueous Solutions
In addition to the major constituents of the
magmatic solutions, the trace constituents cannot simply be dismissed,
especially in an ore-forming context, as it is these ingredients that
distinguish an ore-forming fluid from one that is likely to be barren.
Typically the solute content of magmatic fluids is
dominated by the alkali and alkali-earth metal cations and by chlorine as the
major ligand, although exceptions do occur. There is often a significant amount
of carbon dioxide associated with magmatic fluids. The amount of sulfur in
magmatic fluids is generally low, but this may reflect the fact that at high
crustal levels SO2 partitions into vapor phase on boiling.
c)
Talk
about the main differences between podiform and stratiform chromite deposits –
give an example of each.
Podiform chromitites are texturally distinct from
stratiform ones in the following features: 1. the former are commonly comprised
of coarse anhedral interlocking grains, whilst the latter commonly have smaller
euhedral grains. 2. The matrices of podiform chromitites are most commonly
composed of olivine more or less altered to serpentine, while the matrices in
stratiform chromitites are mainly plagioclase and pyroxenes. 3. Podiform
chromitites usually display nodular and orbicular textures, whilst stratiform
chromitites lack these textures. 4. Podiform chromitites characterized by low
TiO2 and Fe3+ content, while startiform chromitites have relatively
high contents of TiO2, (Al2O3+Cr2O3)
and Fe3+. 5. The Cr/Fe ratio is higher in chromite from podiform
deposits compared with most stratiform ones. For example Albanian podiform
chromite has a Cr/Fe ratio of 3:1 compared with 1.6:1 for South African
Bushveld chromite. As a consequence chromite from podiform deposits has been
traditionally preferred for metallurgical uses and remains the sole ore type
for refractories. 6. Podiform chromitite usually associated with upper most
part of the upper mantle in the ophiolite sequence like in Oman ophiolite and
many other ophiolites worldwide, while the stratiform chromitite usually
associated with mafic layered intrusions like the Bushveld complex in South
Africa.
Third Question:
a)
Talk
briefly about the physico-chemical factors affecting metal precipitation from
the hydrothermal solutions.
At shallow crustal levels ore deposition
will take place by open space filling, whereas deeper down where
porosity is restricted, replacement of existing minerals tends to occur.
The actual geological processes that affect ore deposition controls are
presented below:
(1) Temperature: Since
the stabilities of many metal-ligand complexes increase as a function of
temperature it is clear that cooling of a fluid will generally have the effect
of promoting ore deposition. Temperature decrease is particularly effective for
destabilizing metal-chloride complexes because their solubilities are much more
sensitive to temperature changes than are those of equivalent sulfide
complexes.
(2) Pressure: Pressure
variations do not dramatically affect the solubilities of metal-ligand
complexes although it is clear that pressure increases will lead to a volume
reduction which, in turn, promotes the dissociation of complexes to ionic
species. The decrease in pressure tends to favor an increase in solubility and,
therefore, works in the opposite sense to temperature.
(3) Phase separation (boiling and
effervescence): At
deeper levels in the crust a decrease in fluid pressure could be accompanied by
effervescence, or the transition from a single phase H2O-CO2
mixture to one where H2O and CO2 unmix. Both these
processes are potentially very important as mechanisms of precipitating metals
from ore-forming solutions because they dramatically change the prevailing
conditions under which metal-ligand complexes are stable.
(4) Fluid mixing/dilution: The
mixing of two fluids with distinct temperature, pH, and redox characteristics is
widely regarded as another important mechanism for reducing solubility in
ore-forming solutions and promoting metal precipitation. This is particularly
the case when a relatively hot, metal-charged ore fluid mingles with a cooler,
more dilute solution. Mixing of the two fluids would result in cooling of the
hotter with modification of the prevailing ore fluid properties and
destabilization of existing metal-ligand complexes.
(5)
Fluid/rock reactions (pH and Eh controls): The
interaction that occurs between a fluid and its wall rock promotes metal
precipitation because it is yet another process that changes the prevailing
fluid properties, especially in terms of acidity (pH) and redox state.
b)
Talk
about the mechanism of supergene enrichment process – draw a sketch.
(1)
Secondary enrichment takes place when the dissolved
metal ions which have been removed from the residual deposits come out of
solution.
(2)
When rainwater enters soils and rock it is oxidising
and slightly acidic, but as it interacts with organic matter and metal ions,
oxygen is used up and it becomes more reducing with depth. For example, chalcopyrite
near the surface will be oxidised to form Fe hydroxide and soluble Cu and sulfate
ions. The Fe hydroxide is a residual deposit composed largely of “rusty”
limonite and forms Fe cap or Gossans.
(3)
Some Cu precipitates as malachite, azurite or native
Cu, but most will stay in solution until it reaches the reducing environment
below the water table. In this environment, existing sulfide minerals (primary
ore minerals) do not break down, but can react chemically with the incoming
metal ions to produce secondary ore minerals.
(4)
Ag and Zn behave similarly to Cu - they will enter
into solution (be leached) from minerals where groundwater conditions are sulfate-bearing
and oxidising. As these pass down into reducing sulphide-bearing environments,
Zn remains fairly soluble, so will stay in solution while Cu and Ag will
precipitate out to form secondary enrichment deposits.
Fourth
Question:
a) Give short accounts on the general features used to
determine the paragenetic sequence of an associated group of minerals.
The
term "paragenesis", or "paragenetic sequence"
refers to the time-successive order of formation of a group of associated
minerals within a particular deposit. Following are the features used to
determine the paragenetic sequence of an associated group of minerals:
(1)
The shapes of
individual crystals and the nature of the contacts between adjacent grains. In
general, euhedral crystals have been interpreted as forming early and growing
unobstructed; grains with convex (curved outward) faced have been interpreted
as formed earlier than those with concave (curving inward) faces (see figure
below).
Figure showing
the sequence of minerals paragenesis with time.
(2)
Cross-cutting
relationships in mineralogical examination, just as in geological field
studies, are a key to paragenetic interpretation. The veinlet or other feature
that cross-cuts another is younger than that which it cuts across.
(3)
Deformational
episodes are often indicated by the presence of microfaults which offset bands
or veins of earlier formed minerals or by crushing of earlier grains that may
have been subsequently infilled by later minerals.
(4) Replacement
features are very useful in the determination of paragenesis; clearly the
mineral being replaced predates the one replacing it.
b) Compare between podiform and startiform chromite
deposits; give an example of each type. (5 marks).
Podiform chromitites are texturally
distinct from stratiform ones in the following features: 1. the former are
commonly comprised of coarse anhedral interlocking grains, whilst the latter
commonly have smaller euhedral grains. 2. The matrices of podiform chromitites
are most commonly composed of olivine more or less altered to serpentine, while
the matrices in stratiform chromitites are mainly plagioclase and pyroxenes. 3.
Podiform chromitites usually display nodular and orbicular textures, whilst
stratiform chromitites lack these textures. 4. Podiform chromitites
characterized by low TiO2 and Fe3+ content, while
startiform chromitites have relatively high contents of TiO2, (Al2O3+Cr2O3)
and Fe3+. 5. The Cr/Fe ratio is higher in chromite from podiform
deposits compared with most stratiform ones. For example Albanian podiform
chromite has a Cr/Fe ratio of 3:1 compared with 1.6:1 for South African
Bushveld chromite. As a consequence chromite from podiform deposits has been
traditionally preferred for metallurgical uses and remains the sole ore type
for refractories. 6. Podiform chromitite usually associated with upper most
part of the upper mantle in the ophiolite sequence like in Oman ophiolite and
many other ophiolites worldwide, while the stratiform chromitite usually
associated with mafic layered intrusions like the Bushveld complex in South
Africa.
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