Welcome to today’s lecture, we had a brief
look at the deposits, mineral deposits of
important metals like chromium, platinum group
of metals nickel, associated with mafic
ultramafic magmatism and we looked at the
general characteristics, the tectonic
affiliation, the age distribution and got
an overview, basic idea about these deposits
and
as I explained that, there could be many ideas,
many new ideas coming up because of
many observational facts, many things which
remain unexplained or many new
experimental results coming, they can always
be looked at a higher level.
But within this limitation of this particular
lecture series, we looked at the general
characteristics of these deposits, which are
associated with mafic tetramafic magmatism.
Now, we move on to an interesting class of
mineral deposits, which are associated with
also ultramafic magmatism, but they generally
are the rich sources of the gem, of the
precious stone which is diamond.
So, we will move on to the diamond deposits,
which
are associated with ultramafic rocks and the
diamonds generally are very, very
specifically associated with the type of rock,
which are the kimberlites.
So, have a look
at them, the classification there of diamondiferous
ultramafic rocks.
The diamondiferous ultramafic rocks, generally
can be classified into 3 groups, the
group one kimberlites: dominantly olivine
macrocrysts with pyrope, diopside, phlogopite
and enstatite and chromite.
The group 2 kimberlites: Dominantly phlogopite
in an olivine
mica groundmass and the Lamproite is a Dominantly
titanium bearing phlogopite,
titanium, potassium richterite and olivine,
diopside and sanidine.
So, essentially the
ultramafic rocks, would be generated by the
melting of mantle rocks, as we know, but
these constitute a very special group of ultramafic
rocks in having hydrous mineral like
phlogopite, which is a biotite, and that is
how it makes them, to be a very special type
of
rock considering their direct origin from
the mantle.and also, the expression, because
they do sometimes contain diamond and the
occurrence of diamond in these kimberlites,
is very much in the form of a the very disseminated.
Their concentration will be far low.
About hardly a few carats in a ton of rock,
a carat is 200 milligram and then, more
importantly they do will have both the gem
and the non-gem variety diamond.
And the
ones which are the gem variety and depending
on the high carat value, there very
precious to us, they are present in many of
the older cratonic areas of the world.
We just have a look at this diagram which
gives us an idea, about the conditions in
which
the diamond might form.
To note an important point here, the diamond
crystals that we
see within the kimberlite, are not a product
of crystallization from the ultramafic melt.
Because a diamond as we know, is stable only
at a very high-pressure temperature
condition and we have seen phase diagrams
like this will be graphite and diamond.
So,
generally diamond will always be stable at
very, very high pressure condition the
pressure temperature conditions and if the
conditions changes the diamond can always
get reversed to graphite.
Now, this diagram essentially shows a very
thickened part of the is a lithosphere, where
a
very thick continental crust is shown, with
this deep with this gray color and the graphite
diamond
line, the curve is also shown, with respect
to the isotherms 900 1200 1500
degree celsius and as you could see, the depth
goes to as high as 600 kilometer, below
the surface within the mantle, this is the
part of the upper mantle and also, we see
the
eclogite stability field, this is the stability
fielder peridotite the harzburgite and dunite.
And from the diamond shape, we could see here
that within this region, which is
essentially a reduced part in the mantle - part
of the asthenosphere.
Here, this is the actually the part in which
the diamond is stable, because the zone below
this is slightly more oxidized, with the carbon
oxide layer and the fugacity of oxygen,
which is shown here I could see that it is
less it is much greater than the fayalite
magnetite quartz buffer whereas, the oxidation
state in this region of the a semisphere is
2 to is 10 to the power minus 2 to 10 to the
power minus 1 or an FMQ, this fayalite
magnetite quartz buffer almost like 1 to 2
orders magnitude less, in terms of the oxygen
fugacity.
So, that essentially helps the carbon, to
be present in the form of diamond in this
particular region and this essentially the
diamond stability field, below which the
diamond is not stable, what is shown here
in this black lines on the black thick lines,
they
represent the kimberlite in the form of pipes,
which will be seeing in the short while now.
So, they arein the form of dyke.
So, they move through that they just include
through, the melts are generated here and
the melts migrate through the crust and in
the form of dykes or pipes rather and they
are
exposed on the surface, and these are the
kimberlitic melt or the kimberlite, the melt,
which is generated within these layers when
they move up, they are very likely to catch
or they have these diamonds at the quarter
fragments and can carry them or transport
them, to the within the crust which we find
them.
So, there are many studies which is done on
them, this diamond is found out to be older
than the kimberlite and there are many evidence
which are there, to suggest that the
diamond which is present in this kimberlite
pipe and not actually product of
crystallization from the kimberlitic melt,
a kimberlitic melt definitely comes from a
carbon di oxide rich part of the mantle.
This diagram shows, something here which is
the gray part, which is known as the
diamond window and that correlates with what
exactly we just have seen in the diagram,
where we require a thickened part of the lithosphere,
where the geotherm, should be
shallow.
Like the this these dotted series are blotter
lines over here, at the geotherm in
the in the lithosphere continental, geotherm
the lithosphere this geotherm which is pretty
shallow, which is about 35 degrees per kilometer
will be only possible, when we have a
much thicker crust above compared to the areas
where the crust is thinner, where the
geothermal gradient is supposed to be much
steeper.
So, this is the diamond graphite stability
field, on the pressure temperature diagram
and
the intersection of the pressure temperature
this diamond stability field, with this the
geotherm shown on this 4 series of lines,
this grey part is only the labeled as a diamond
window, this the areas or the region in which
diamond is stable and this is a typical
morphology, typical shape of a diamond bearing
kimberlite, which is the - this part is
known as the diatreme and this is the kimberlite
breccias here and then it is extruded at
the surface here.
It is very interesting situation because,
sometimes we call these is a representing
something like a phreatomagmatic process here,
because when this diamond bearing
kimberlite, is intruding through the crust,
it could always heat up the groundwater.
So,
here, the depth is through shown in terms
of a kilometer, this is 0 this is up to 2-kilometer
depth is the top surface of the kimberlite
pipe, which is has a base in the form of the
sill
and is pretty interesting and complicated,
in the subsurface structure.
And the more interesting fact is, at this
such kind of kimberlite pipe they are occurring
in
groups or in terms of in the form of swarms,
many such dykes occurring in any particular
part in the crust and this heated water, actually
causes the rocks to fragment and so, that
is why, the breccia kind of a zone is very
common to many of the kimberlites and they
are, extruded at the surface which fall with
material and the alteration there are some
later sediments, which are also deposited
here some tuffaceous or some kind of material,
these are the these sediments also get fragmented
and they fall back on this material, they
do represent very interesting geomorphic features
as well.
Now, what could be basically be told about
such a situation that we get diamond in them,
if we the diamond is sourced from a very great
depth and it traverses through several
100s of kilometers on it is way up, and then
encounters lower pressure temperature
conditions, then it is very likely that the
diamond would get converted to graphite in
this
process, and we would not be having any ground
diamond available within this
kimberlite.
So, it does not happen.
So, that led us to believe, these kind of
a kimberlite
pipes, actually move up very fast within the
earth’s crust, sometimes the movement of
can be as fast as 70 meters per second, kind
of rate which which is calculator, visualized
for the rate of a ascent or the rate of propagation
of such kind of kimberlite pipes.
So, this is how the situation with this kimberlite
of the diamond deposits, these deposits
are plentily available in many of the cratonic
blocks, including the older cratons, in India,
in Canada in, Australia the many of the famous
diamond fields are there, like the Ekati
diamond field in Canada, the Argyll diamond
field in Australia and they are very each
sources of diamond.
We will get back to this again, when we discuss
about exploration
for diamond.
Now, we move on to another class of a deposits
which has to be of course, included in
this particular category, that we are discussing
when we discuss about the mafic
automafic clocks and they are the Carbonatites,
they are a very special type of a magma,
but they represent a very special type of
magma, a magma which is essentially being
carbonate melt and they give rise to rocks,
which are carbonates with calcite and
dolomite and they also do occur, as pipes
steeply plunging stalks up to 3 kilometer
in
diameter and these Carbonatites, occur in
many parts of the world one of the 2 important
occurrences, that we could cite is the Mountain
pass, in California united states and the
Bayan Obo deposit, in inner Mongolia, which
is one of the major resources.
So, this Carbonatites the their economical
is very, very significant, they contain very
high concentrations of the metals, which are
the light rare earth (LREE) elements and the
high field strength elements, the light rare
earth elements are the lanthanum, samarium
neodymium, gadolinium etc, with some of the
high field strength elements also and like
some of the minerals, they contain the carbonate
minerals, carbonate and the phosphate
minerals, is the Bastnaesite is a lanthanum,
cerium, carbonate with chlorine and
hydrogen this in the hydroxyl site, parisite
with calcium, cerium, lanthanum and
carbonate monazite, which we all know it is
a phosphate of thorium, one of the major
minerals of thorium.
So, they come from these Carbonatites.
The Carbonatites, generally are a result of
very low degree of a partial, melting of a
mantle rocks where the mantle rocks is rich
in carbon dioxide, by some process and the
rocks melt at about 2.5 gigapascal, corresponding
to a 90 kilometer depth and such kind
of Carbonatites and such kind of a generation
of this kind of Carbonatite melt is usually
expected, where we have a continental rift
zones, like their presence in some of the
prominent reef zones, only has given this
idea.
This diagram actually, represent in the Mountain
Pass deposit where you could see that,
it is one of the large-scale ore from this
particular deposited is the Mountain Pass
deposit.
Within the gneissose country rock, what they
generally observable is?
That they do have
multiple generations of such, Carbonatite
bodies present in them and there are different,
they can be differentiated into like Monazite
Carbonatite, Bastnaesite dolomite
carbonotite, Bastnaesite dolomite calcite
Carbonatite, Bastnaesite calcite carbonatite..
So, these they do represent multiple episodes
of injection, of the generation of this kind
of this carbonatitic body, and interesting
is that that the ores in this particular class,
are
clearly primary magmatic, they crystallized
from the magma and their subtle differences
there in the host ore minerals.
In Bayan Obo although it is an iron oxide
fluorite arginine
all their type of rock, hosted within the
larger Carbonatite intrusion, and a Carbonatites
are of multiple generation, and rear earth
element concentration up to 0.5 percent.
The way the metal rocks melt in low degree
of partial melting, of about 3 percent this
can have a cabonatitic melt, which could be
could have about 0.5 percent the total rare
earth element concentration, and when we get
them in the deposit, they get further
enriched to almost going up to 2 percent.
So, that must be there must be some kind of
a
differences, in fertilization differentiation
process, which is responsible in making these
or further enrichment of this rare earth element
in them.
So, that brings us to the close of our discussion
on deposits, which are resulting out of
the direct orthomagmatic processes, although
in situations like what we just discussed
in
a Carbonatite, it cannot be essentially called
as a fully dry magma, because even in case
of a kimberlite, which is which can also be
cannot be also be called as a dry magma in
the strict sense.
Because, the presence of many hydrous minerals
and what we have seen
in case of the Carbonatite also, there are
mineral phases which have fluorine and
hydroxyl, as in their in the the crystal sides,
to make up some of the points.
So, generally whenever we are discussing about
these deposits, it is the age distribution,
the tectonic settings, which becomes the important
attributes of these deposits.
In the
ultramafic complexes, we have seen that any
simple interpretation on the tectonic setup,
that or the tectonic control of this mineralization
does not seem to be very straight
forward, which is true, for cases that we
have already discussed and even in case of
the
the platinum group of metal enrich we under
the sulfide faces, they are all controlled
by
processes, which all sometimes seem to be
pretty hybrid and not exactly a very simple
sequence of crystallization evolution, that
is expected in any crystallizing magma
chamber, controlled by many factors like magma
mixing etc, coming in.
So, magmatism is ineffective and what we have
to sum up, we have seen that magmatism
is an effective and an efficient mechanism
of enrichment of many important metals, and
also precious stone is diamond, which we can
go on to even calculate, by using simple
governing equations, taking the distribution
coefficient of a metals, between Silicate
melt
and solid or a Silicate melt in the sulfide
melt, and can understand the basics to make
an
appreciation, as to why such kind of enrichment
takes place.
Although some broad generalization could be
made, for these deposits deviation from the
theme is a rule rather than the exception,
because as we have seen in many such cases,
we always see observations which are starting
from the very, situation in the chromite
deposits, many a times the simple models fail
to explain many features and thus models
need to be involve for mixing of magma sources
tectonic environment, mix tectonic
environment like, both lifting and the subduction
type and deposits which are associated
with older greenstone belts, are considerably
modified.In the very beginning what we
discussed that, we do see present day ore
forming processes in many well defined
tectonic zones, in the lithosphere as expressed
in the seafloor or in many of the
continental interiors, rift zones and so on.
The older ones, like we see an edge distribution
they go all the magmatic deposits that,
we have discussed so far bearing a 1 or 2
of them, most of them are in the old
Precambrian terrains, where they have undergone
multiple phases of deformation,
metamorphism, remobilization of the ore body,
thus making things a little bit more
complicated, for explaining all those deposit
formation through very simple process of
magnetic differences in revolution.
So, we had a brief overview of, mineral deposits
which result from the magmatic process, and
we have restricted ourselves in the
beginning, to the orthomagmatic process and
magma which are essentially to start with
basaltic composition and dryand the mineral
deposit formation, in a broad sense could
be
explained by the process of the magmatic differentiation,
which involves crystallization
to give rise to a different members, in a
basaltic magma and also the exsolution of
a of a
magmatic sulfide component, which selectively
enriches many of the important metals,
like the like nickel and the platinum group
of metals.
So, with this now, we conclude the discussion
on the magmatic deposits, the
orthomagmatic deposits and we move on to discuss
another important process, which is
the sedimentary process and we see that, how
we can discuss or how the sedimentary
process the in terms of it is efficiency?
How they are important in formation of the
mineral deposits?
So, we move on to sedimentary process, and
the resultant deposits.
In the classification
scheme that we proposed in the beginning,
we broadly divided the sedimentary deposits
into 2 classes, the deposits resulting from
process of clastic sedimentation; that means,
the erosion and transportation of a fragments
of rock in form of clastic sediments, and
a
deposition in the depositional basins, which
could be a marine basin or could be a
continent in a intercontinental basin or somewhere
in the continental margin.
And then, the other class which are the chemogenic
sediments or the chemical sediments,
and we will first take up to begin with, the
deposits which are resulting from the
chemical process, chemical sedimentation and
within this, we have a very important
class of deposits of metal, important metal
which is iron and manganese both of them are
abundant metals.
So, the major source of iron and manganese
in the present day, come
from this type of ore deposits, which is arising
out of the chemogenic sediments, in
ancient late Archean to early proterozoic
sedimentary basins, distributed in many parts
of
the world like the Superior province in Canada,
the Quadrilatero Ferrifero peripheral in
Brazil, the Transvaal super group in South
Africa.
The Singhbum-Orissa iron ore craton in India,
the there are other sedimentary basins,
like the in the central India and the Bastar
craton and one the most important and the
most rich one is the Hamersley basin in Western
Australia.
So, these are the examples, in
which we get prolific development of this
kind of iron formations, in these basins which
constitute the majority or the bulk, of the
iron oreresources of the world.
A little less significant in this class, are
the iron stones which form mostly in the
Mesozoic time.
We will not be considering them in details,
because they are not the
sources of iron, they are mostly used for
different other purposes and then the Neogene
manganese deposits, which are mostly in and
around the black sea region in Ukraine,
they also constitute a major manganese resonant
land-based manganese resource of the
world.
So, we will first take up the Precambrian
banded iron formation or the BIF, to begin
with
when we are saying, it is banded iron formation,
it does not represent any stratigraphic
unit, as we know them formation, but it is
actually used for denoting a type of rock,
which we basically name, as banded iron formation
and such kind of banded iron
information deposits, say a invariably associated
with also occurrences of manganese
along with them.
As we come to this Precambrian iron, deposits
the branded iron foundations, we can
classify them to 3 broad categories the Algoma
type, the Superior type and the Rapitan
type, the Algoma type - they are associated
with the old greenstone belts and they are
the
oldest, allowing to middle to Late Archaean,
the Superior type which are mostly come
from the type area in the Lake Superior region
in Canada and there the Late Archaean to
Mid Proterozoic Basins and the Rapitan type,
also is from the Rapitan basin in Canada,
they are the Late Proterozoic or the youngest
amongst the banded iron information in the
series.
As far as the banded iron formations are concerned,
they are definitely very, very
interesting class of deposits - iron being
one of the abundant metals and also as we
saw
before, they do from their deposits are huge
in quantity, billions of tons.
This photograph
is from Hamersley Basin, where you could see
such type of a iron ore is occurring almost
like, constituting the entire hill and some
such occurrences are very common in many
parts of the world including, the iron ore
craton in IndiaThis is an exposure scale
photograph of the typical banded iron formation.
So, we call them, the term banded iron
formation has come from the very appearance,
in which there are iron rich and Silica rich
layers which are present in the ore.
When you could see from the photograph of
taken from Hamersley Basin, or even on
these exposures can photograph.
So, this is one of the curiosities, so far
as this deposites
are concerned.
So, this banded iron formation, they mainly
they are the BIF.
So, they occur in Archean and Proterozoic
sedimentary basins, in many parts of the
world as I just mentioned, and they constitute
many different types of mostly where they
are named in different parts, the way there
way they occurred in this kind of banded
forms, they are known as the BHJ or the in
the form of banded hematite jasper.
The jasper is basically the iron rich 
Cryptocrystalline Silica., They are also present
in the
form of BHQ as banded hematite quartzite,
they 
could be as BMQ banded
magnetitequartzite.
They are named in different they are called
by different names, in
different parts of the world for example,
they are called Itabirite in Brazil and the
area of
quadrilateral peripheral that is just mentioned,
what we see the dominant mineralogy, in
most of the cases whether it is a Algoma type
the mineralogy is dominated by mostly,
Magnetite and in the Superior type and Rapitan
type, the mineralogy is dominated by
Hematite, maybe, it has to it basically because,
of the fact that the with increasing
oxidations the Superior type in the Rapitan
type, by the time they formed the partial
crystals of oxygen in the atmosphere was quite
higher compared to the time, during
which these Algomatite was formed and these
Algoma type, which formed mostly as a
part of the these green stone, where we get
a very great sequence of the volcano
sedimentary sequence starting with very mafic
high MgO, rich komatiites to he give rise
to the Tholeiitic basalts and the Chemogenic
sediments, Clastic sediments and the
bimodal volcanism and so on.
So, many of these members of this the lower
member, of
these green stones they are associated with
this magnetite mineralization, in the form
of
this banded iron formation.
So, we will discuss about this banded iron
formation, their characteristics, the possible
origin and many of their characteristics in
the next class.
Thank you very much.
