Welcome to today’s lecture.
To recapitulate we have been discussing about
the magmatic
hydrothermal deposits and essentially the
hydrothermal deposits which result from felsic
magma magmatism which give rise to rocks of
broadly granitic composition arranging
from granite to monzonit monzo granite diorite
kind of composition.
So, these rocks as a
whole the magma at the hydrothermal system
that are associated with these rocks, they
give rise to a spectrum of deposits- the porphyry
deposits which are distributed mostly
in the convergent tectonic settings.
We will discuss the another class of deposits
another
type of deposit that are associated with felsic
magma, but a little different type of a
situation here the deposits.
resulting from the interaction of this felsic
magma felsic
intrusive body within the crust where there
are some pre-existing sedimentary sequences
and here we broadly categorize them into something
like the 
contact
metamorphic or contact metasomatic deposits.
The mostly dominant ones are generally
the Skarn deposits.
In the literature, this skarn deposits have
been divided into 2 as the
reaction skarns and replacements skarns, but
we shall not be going by this kind of
classification.
In general, this skarn deposits result when
a felsic intrusive body comes in
contact with the rocks through which it intrudes
and is emplaces in the that in the
surrounding of them and then here this deposits
are observed to be confined to specific
lithologic horizon.
So, that is why you can call them broadly
as strata bound 
and they
are the class of deposit are the important
sources of tin tungsten mainly.
They do also
result iron skarn, iron gold tin tungsten
and molybdenum gold etcetera sometimes some
rare earth mineral also.
Let us first have a look on what has been
the popular model for the formation of this
skarn deposits, or this skarn process.
This skarn process could be visualized by
looking at
these 3 diagrams the number 1 the first one
which was existing in any particular domain
in the crust, which has been intruded by a
felsic magma, also let me point out here they
are far more evolved compared to the kind
of magma which give rise to porphyry
deposits, quite enriched with many of the
volatiles for example, boron lithium and many
other volatiles including the ore forming
elements.
So, when this particular intrusive body is
emplaced within a sequence of these rocks
then
first thing it does is that it drives out
the pore fluid or any of the lower temperature
even
pore fluid from these sediments and then gives
rise to the skarn formation.
The contact
metamorphic formation and which forms something
we can call as a metamorphic
aureole.
In this metamorphic aureole this argillaceous
units are converted to the
hornfelse.
With typical skarn minerals as we know them
the garnet the other minerals - this
essentially hornfelse, containing some ferromagnetic
minerals and then this is converted
into.
So, in the very fast so within that and it
reacts with the and also some typical
mineral like so the wollastonite and dioside
type of mineral which are also which forms
along the carbon in the carbonate rock contact.
So, essentially this dotted boundary it shows
that it is converted into a metamorphic
aureole thereby creating some change,
basically they make the conditions very
conducive for the fluid to be further acted
upon.
So, in a stay and also it this is a
schematically as i shown as the host the magmatic
body has caught up a fragment of the
sediment as a xenolith here.
Now, in the second stage the magma when it
crystallizes.
So, it eventually expels or
evolves or exhales a magmatic volatile phase
as exactly happens in the porphyry system.
So, this fluid is now channelized through
the metamorphic or you will which was created
by the initial phase of the contact phenomena
through this, and then the horizons mainly
the calcareous horizons which were converted
into the skarn, to the contact aureole are
acted upon by the fluid which are evolving
from this specializing of much of the pluton
has been crystallized much of the magmatic
body is crystallized and the fluid is evolving
and escaping through because this contact
are providing the suitable channel wave for
the fluid to flow through the country rock.
And then this zone which are shown as hatched
here are basically the mineralization
zone which is forming at this skarn, mineralization
in the form of the tin tungsten the
oxides or the tongue straights of the mostly
between tungsten and from there or minerals.
So, these skarns, these zones the mineralized
zones which we can call them which are
occurring on the other side of the intrusive
contact contoured into say the country rock
will call them as the exoskarn.
And sometimes also we do get the path of the
granite inclusive itself being mineralized.
So, in that in that place in call in that
case we call them as the endoskarn, and this
is this
gives rise to we can say that this coincides
with the very, very initial phase or the phase
in which a magmatic volatile is actually is
playing a dominant role and we causing the
mineralization.
The early phase mineralization is supposed
to be the condition is also
supposed to be manifested in them and then
now after the pluton is cooled down and the
system has retrograded, it is a stage in which
there will be retrograde cooling and that
attribute cooling will allow meteoric water
to infiltrate and then alter the originally
formed skarns and then the a this skarm minerals
can get altered, the hornfels and the
other part of the system.
And we also do see some later stage of mineralization
in the form of metal sulfides
which actually overlaps or which later over
printing the earlier mineralization episode
and still preserve.
If we look at.
So, the source of the fluid in this kind of
skarn is of course, a dominantly a magmatic
source is always identifiable.
This diagram has come from one of the most
studied of this
skarn deposits of the world this has come
from Dachang deposit in China and here is
the
fluid inclusion data from the different parts
of the deposit.
So, as stated this part is the representing
the temperature even going even well above
within 600 degree centigrade or so, and the
high saline, of course, the salinity part
is not
shown, and here it is mostly down on quartz,
and this is the stage A where you have
grossular garnet, vesuvianite and quartz diopside
plagioclase and the this is the stage in
which the fluid is retaining it is original
high temperature magmatic characteristic,
and
now with the evolution of this skarn system
and the later on coming to the later stage
this
stage 2 where we can call them as depending
on the proximity here, any of the deposits
which are forming, for example, in the later
stage away from the intrusive contact this
will be called as the distal skarns, which
will be having the major component of the
later
mineralization in the form of sulfides and
the ones which will be very close or to the
winter ship contact will be call them as the
proximal skarn and the ones which are away
from the intrusive contact will be called
as the distance skarn and the stage 2 will
be with
the sulfide precipitation and here they meant
the fluid inclusions have been studied from
both from casita rate as well as skarn quartz
you could see a good degree of overlap in
the temperature range studied from these 2
minerals, thus indicating or essentially
establishing the coeval or the coeval nature
or the same time depletion of both quartz
and
casita rate at the same time so this is so
here if this is also corroborated from many
other
types of studies, but this gives a very good
idea as to what happens in a typical skarn
system as far as if fluid evolution is concerned
and parallely with the changing pattern of
mineralization for the this proximal and the
distance skarn.
So, as far as this skarn deposits are concerned
although all over the world they are
categorized into different tectonic - they
are observed in to be occurring in a different
tectonic regimes.
So, this is essentially is a high steep subduction
oceanic type of
tectonic setting, which is observed to be
giving rise to iron copper and gold dominantly
iron copper and gold skarns.
We do have a kind of subduction against the
continental
lithosphere this would possibly this is moderate
dip which gives rise to tungsten
molybdenum and sometimes in the disturb in
the parts is zinc and lead also.
So, this possibly will explained why we do
sometimes have a bit of a problem in
categorizing the deposits says whether it
is a porphyry tin or a skarn, but they can
very
well be distinguished on the basis of the
exact morphology the type of the intrusive
and
the exact mode of occurrence of the mineralization
which is stratabound in case of the
skarn deposits.
Represents some different other areas then
the Bolivian area this account
this is arising dominate mainly out of the
continental type subduction.
These this situation which is called corresponds
to a transitional low angle subduction
gives rise to the molybdenum tungsten bismuth
sink fluorine kind of skarn, a monzo
granite and also these are the ones which
the once which is porphyry molybdenum and
project.
And they are in the last one which can also
be there this skarns which is the
continental rifting in one of the diagrams
also we saw the domains on the technique the
crustal domains in which during the rifting
time there are implement of magma and
essentially it is a kind of a rifting setup
in which.
So, these magma are mostly very and very evolved
and alkali type of magma - felsic
magma, which give rise to the tin deposits,
the tin skarns and they are more richer, say
you could see here some of them also do have
uranium, beryllium and lithium and the
ones which have the maximum concentration
of the elements like lithium and beryllium
and these also do have sometimes development
of tourmaline in the country rock
because of this boron metasomatism.
So, with that brief discussion about the skarn
we come to the ones which is the
pegmatitic environment.
Pegmatitic environment generally are the pegmatites
are
generally the rich source of tin tantalum,
niobium, cesium and sometimes uranium as
well is one particular example that is considered.
The salient features here that they are
they occur from near surface to as deep as
11 kilometer depths there was some there is
some kind of a classification scheme is also
followed for the different type of depth
range.
But essentially the pegmatites they can be
told is representing the extreme
situation of a felsic, of a fluid saturated
felsic magma, that evolves in such a way,
that
the last stage the residual melt almost get
changed to a to almost like a fluid like mass
and they and they get separated out of the
parent crystallizing granitic pluton and they
form veins.
So, pegmatite said essentially seen is occurring
as veins in countries in
different types of rocks mostly the country
rocks has a gneissose country rock or any
kind of a country rock in which they occur
is veins.
And in some in some in situations
they do have been labelled as metamorphic
the only example of it being one of the
deeper pigmented body occurring at the Rossing
pegmatite in namibia which is also is a
source of uranium.
So, represent the extreme case of fractionation
of water which melt from parent granitic
melt and they are rich in the volatiles the
large ion lithophile elements the high field
strength elements and they occur as veins
traverse into fractures - weak planes they
are
often zoned depending on the kind of a country
rock and depending on the compositional
characteristics and the rate at which these
pegmatites are crystallizing.
They give rise to
zone pegmatites.
So, essentially since the pegmatites are essentially
characterized by
very, very coarse-grained nature and then
the constituent minerals like quartz products
feldspar and mica.
So, pegmatites are actually in fact, the most
important and the wide sources of industrial
mineral like mica.
The example is this kind of mica bearing pegmatites
are available in
plenty in the in the Bihar mica belt within
the total recognize a complex, which will
see
them in our discussion on the mineral potentials
of the Indian subcontinent.
And they are
also rich sources of graphite.
They occur as extremely coarse grained and
sometimes
these are the pegmatites which give rise to
the gem variety of many of the minerals like
beryl, tourmaline which are much sought after
and they are the most and the richest
sources of this kind of gemstones in many
parts of the world including India.
So, with the pegmatites, many of the others,
features of uncertain affiliation we now we
now closed we now complete the discussion
of the hydrothermal systems or the
hydrothermal mineralization associated with
felsic megmatism and we will switch over
to a situation which is another class of deposits,
where the situation is a little bit
different.
So, we will be switching over to this particular
class of, but a very, very important class
of hydrothermal deposits which are the lode
type gold deposits in many older cationic
blocks in the world like the Abitibi province
in Canada, Dharwar Cotrona in India
Yilgarn block in western Australia, Barberton
mountain range in South Africa and so on.
So, these are the deposits which are the low
type gold deposits in granite green stone
terrain, these are widely believed to be a
metamorphic hydrothermal system.
So, we will
see these deposits and see the role of metamorphism
in giving rise to or in giving rise to
such kind of a hydrothermal system.
So, these are the deposits which are very,
very extensively studied all across the
continents and are the rich sources of gold
all over the world including the richest gold
deposit in India, which is the Kolar gold
deposit which is from where the substantial
amount of gold has been produced in the country
right at this moment also there are
other occurrences where gold is being produced.
So, let us see these deposits now before
you I am presenting a model.
In this model what we see here is that there
are some
granitic body which are shown which are basically
the syn kinematic granitoid.
What is shown here is high amphibolite faces
metamorphism parcel melting and absent
of parcel melting is absent or minor this
is essentially undergoing metamorphism without
it and it and is also associated with some
granitic emplacement the granite coming in
from deeper regions deeper sources.
Now what exactly is happening here is that
it is
being shown that this mass of rock, they are
undergoing high temperature low pressure
metamorphism and reading this metamorphic
process of metamorphism there is
devolatilization of this rock which is undergoing
metamorphism and there is generation
of copious amount of volatiles which are essentially
carbon dioxide and water in
dominantly water and charged with variable
concentration of carbon dioxide in this fluid
Now remember, recall the diagram which we
discussed in the in one of the previous
classes while discussing the role of pressure.
And the fluid which is being generated
here are experiencing high pressure, in excess
of what is actually expected a lithostatic
pressure here, because this fluid is somehow
not being able to escape because of the low
permeability of the rocks which are lying
above and also that it is kind of confined
by a
zone of, where it is acting as kind of a lid
which is for which the fluid is accumulating
over here.
Now, what is represented is these vertical
structures are the ones which are the crustal
scale shear zones or the faults, which are
created through the process of deformation
through the process of deformation of the
crust of the lithosphere.
They are deep seated
deep seated a fracture or the shear zones
and now they become the channels through
which this fluid could escape, because once
this crustal skill weak zones are created
they
are the zones in which the pressure is reduced.
And so, this fluid which is present in
much high pressure will always be channelled
through these kinds of crustal scale
structures.
Now in this fluid on it is way up, on it is
upward migrational upward journey is likely
to,
because this is a this is just a cartoon that
is being shown to represent the diversity
in the
host rock type for the occurrence of these
gold deposits as observed in many different
parts of the continents,
for example, here we have a mafic body which
is a
unmineralized iron rich tholeiite, and this
fluid could possibly may be passing through
such kind of rock origin.
And this is this horizon is a chemogenic sediment
which is a
blended iron formation the fluid could also
encounter a horizon like this, there could
be
some carbonaceous sediments which will be
having a environment which will be
reducing and also the fluid might get ponded
with in that kind of a horizon here other
than the fact then these as discussed before.
These crustal skull shear zone themselves
will be the locales of deposition of this
huge
amount of silica that is dissolved in this
fluid which is generated as a very high-pressure
temperature conditions and will be present
as what we call as mega veins or lodes and
the way we see them in different parts that
they constitute.
So, what we observe
basically.
So, coming to the situation is to how, what
kind of mythology or what kind of
mode of occurrence we see for these gold deposits.
So, number one is, that they might occur as
lodes means whatever is present in a shear
zone which could be running for several kilometers
in it is several tens of meters and a
100s of meters or in kilometer in it is strike
length and is of substantial crustal depth
extension and will be characterized by complicated
intense deformation signatures of
deformation and shearing mostly their shear
zones and we see them that they are present
as gold quartz lodes.
So, here we get free gold deposited along
with the quartz they could we get them as
gold
quartz lode.
The second other possibility number 2 we get
them as loads with quartz gold
also sometimes some amount of sulphide minerals
like pyrite sno pyrite also associated
with them.
We get them as within the bended iron formation
i will just discuss about the
where from this banded iron formation could
have come.
Now, this with replacing the iron oxide phases
in the banded iron formation and
formation of the sulfides and these sulfides
being the host of the gold which is liberated
within this from the fluid which traverses
through this and encounters the banded iron
formation.
This mafic body may also act as a trap so
the gold could also be present as
deposited within this deformed mafic body
the gold could also represent in the
carbonaceous a sediment.
So, the situation is that this particular
fluid which is liberated
from this from the dehydration of metamorphism
of this rock is also charged with is
basically is enriched with the precious metal
in substantial amount and is transporting
the
gold as we just discussed that this kind of
gold where substantial amount of carbon
dioxide is dissolved might have acted or might
have enhanced the gold solubility and
would have transported the gold in this fluid.
We will continue discussing on this very important
class of deposit which are the low
type gold deposits in green stone terrains
any across the continents and very popularly
being now known as originic gold.
And we will continue discussing this continue
we
discuss in the next class.
Thank you.
