These slides will tell the reader an short view about the effect of intrusions in coal horizons as well as intrusives charactrisation.

1. Role of Igneous Intrusives on Coal
bearing rocks in Jharia.
Presented By:
Shivam Sachan
Dept. Of AGL
I.S.M.Dhanbad
8051051503

2. Outline of presentation
1.Introduction:Jharia Coal Field
2.Geological Setting
3. Igneous intrusions in Gondwana Coal
4.Intrusions in jharia coal field
5.Petrography & Geochemistry of Intrusives
6.Significance of intrusions
7.Qualitative changes due to the intrusion impact
8.Conclusion
9.References

3. Coal in India: A View
 Coal deposits in India are of two distinct geological
ages.
 1. Gondwana Coal(Bituminous Type)
 2. Tertiary Coal (Lignite)

4.  The Indian coal is broadly classified into two types –
Coking and Non-Coking.
 Coking Coal
 Prime Coking - Mainly used for metallurgical
purpose.
 Medium Coking - Mainly used in steel industry.
 Semi Coking - Mainly in cement, fertilizer and
sponge iron industries.

5.  Non Coking Coal: Non- coking coal comprises
lion’s share of Indian coal. Based on Useful Heat
Value (UHV), it is classified into grades A to G for
commercial use.
 A to C grades are considered as Superior and are
used in cement, fertilizer and sponge iron industries.
 D to G grade, available in almost in all the coalfields,
is considered as Inferior and is mostly used in power
sector.

6.  Lignite: It is commonly known as brown coal and
is classified into grades A to C on the basis of Gross
Calorific Value as per the requirement of the
industries. It is considered as apt fuel for power
generation especially due to its low ash content.

7. 1.Introduction:Jharia Coal Field
 Jharia is famous for its rich coal resources, used to
make coke. Jharia plays a very important role in the
economy and development of Dhanbad City, and can
be considered as a part of Dhanbad City.
 The coal field lies in the Damodar River Valley, and
covers about 110 square miles (280 square km), and
produces bituminous coal suitable for coke.
 The field is roughly sickle shaped.

8. Fig 1.Structural and stratigraphic map of Jharia coalfield,
India (after Sengupta, 1980)

9. Table 1 : Stratigraphic set up of Jharia Coalfield (after Fox, 1930, Chandra,
1992) showing the status of the igneous intrusives.
Age Formation Lithology
Jurassic or Tertiary
Lower Jurassic
Upper Permian
Middle Permian
Lower Permian
Upper Carboniferous
Archean
Raniganj
Barren Measures
Barakar
Talchir
Dolerite dykes
Mica lamprophyre dykes and sills
Fine grained feldspathic sandstone, shales with coal
seams.
Buff colored sandstone, shale and carbonaceous
shales
Buff colored coarse and medium grained feldspathic
sandstone, grit, shale and carbonaceous shales, coal
seams.
Greenish shale and fine grained sandstone
Metamorphics

10. 2.Geological Setting
 Gondwana Supergroup evolved through a process of
deepening into basins and by vertical accretion of
sediments.
 Sedimentary rocks were deposited over Archaean
metamorphic (Proterozoic) in a sickle-shaped east-
west trending basin (BCCL, 1997).
 Basin is surrounded on all sides by horsts of
Precambrian gneisses (Ghosh 1999).
fig 2.Extension of C.C. provide accomodation space

11.  Structurally it is a large elongated structural basin
which includes a number of small open and elongate
domes and basins trending east-west parallel to that
of the major basin .
 The basin has an oval outcrop with an average dip of
5° to 10° (as per Coal Atlas ofIndia, 1993, the general
dip of the formation is 10° to 15°) towards the basin
centre.
 This coalfield is traversed by a number of major and
minor faults.

12. 3.Igneous intrusions in Gondwana Coal
 In Gondwana mainly two types of igneous intrusive forms
are encountered.
 1. Dykes (Discordant )
 2. Sills(Concordant)

13. DYKES
• They are discordant
• Cut across the bedding of the rocks in which
they intrude
• Vertical to steeply inclined and sheetlike
body (extensive in lateral dimension)
• Thickness vary widely from an inch upto
hundred of feet
• Injected through fractures, joints, and weak
planes

14.  Fig 3.Dykes
Mafic
dyke

15. SILLS
 Sills are relatively thin tabular sheetlike body that
penetrates parallel to the bedding planes.
 Laterally it may extends for 100s of km and upto
10 km in width.
 Lateral extend mainly depends on the hydrostatic
force, temperature, degree of fluidity or viscosity,
weight of overlying sediment column.
 Since basic magma are more fluid then acidic
magma- mostly sills are made up of gabbros,
dolerites and basalts

16.  Sill spreads parallel to the bedding planes of
the rocks, hence concordant in nature.
mudstone
sandstone
limestone
Fig 4. Sill

17. 4.Igneous intrusion in Jharia
 Two types of igneous intrusives are met within the
Jharia Coalfield.
 1. The mica peridotite/Lamprophyre sills.
 2. Dolerite dykes.
 Lamprophyres are the most notable igneous
intrusions in the area although intrusions of dolerite
dikes also occur.

18. Modes of occurrence
 Regular vertical dikes with well-defined walls.
 Dikes of irregular nature, which commonly pinch out
or bulge giving a lenticular shape.
 Sills of irregular nature.
 Complex and irregular intrusions, with
anastomising
 systems of intrusive branches.
 The dolerite with limited effect on coal occurs as
dykes which are confined mainly to the western part
of the coalfield.

19.  These intrusives are widely regarded as a part of the
Mesozoic alkaline and Rajmahal flood basalt
magmatism in the Eastern Indian shield (Srivastava
et al., 2009).
 The mica peridotite occurs as dykes and sills all over
the coalfield and has devolatilised the coal reserves
extensively forming what is known as Jhama or
natural coke.

20. Lamprophyres
 Van Gumbel (1874) coined the word LAMPROPHYRE
 Lampros = glistening porphyry (shining rock)
 Lamprophyre is a hypabyssal, mafic, potassic (mostly),
rock dominated by hydrous mafic silicates (amphiboles
&micas) and essentially has a porphyritic texture.
 Hydrous mafic silicates can occur both as phenocrysts as
well as groundmass phases.
 Feldspar is essentially confined to the groundmass.
 Ocellar structure is common.

21.  Lamprophyres can be of three major groups:
I. Calc-alkaline lamprophyres (minette, kersantite,
vogesite & spessartite). It Occurs in subduction zone
environment.
II. Alkaline lamprophyres (sannaite, camptonite &
monchiquite).
 III. Melilitic (or ultramafic lamprophyres) (alnoite,
polzenite)
 II and III are confined to intraplate and rift
environments.
 Lamprophyres occur as sills, dykes.

22. 5.Petrography & Geochemistry of Intrusives
 From the standpoint of the mineralogical assemblages,
the lamprophyres of the area can be grouped
as follows:
 a.Feldspar--phlogopite---olivine--lamprophyre
 b. Leucite--feldspar--biotite---olivine•lamprophyre
 c. Quartz-•feldspar--biotite--lamprophyre
 d. Quartz--feldspar--phlogopite--olivine•lamprophyre
 Intrusives in the Jharia coalfield have mineralogy, dominated
by olivine and phlogopite, with and zoned apatite.
 Accessory mineral phases are clinopyroxene, K-feldspar,
spinel, perovskite, ilmenite, rutile etc.

23.  Accessory to minor phases include clinopyroxene
(titanian diopside to aegirine-augite), K-feldspar
(Or> 96), variable spinel (up to 47% Cr2O3),
perovskite, niobian ilmenite, niobian rutile, sphene
and Ba-poor priderite.
 The whole rocks show low Al2O3 (5–7%) combined
with high K2O (4–5%), MgO (up to 30%) and Cr (up
to 1500 ppm), plus extreme TiO2 (up to 8.5%), Sr, Zr
(up to 6000 ppm each), Ba (up to 8000 ppm), and
LREE (Ce up to 3000 ppm).

24.  Geochemical studies on Precambrian mafic dyke
swarms in the Chotanagpur gneissic complex
(basement in the Damodar Valley) indicate a
derivation from a lithospheric extensional
environment (Kumar and Ahmad, 2007).
 Therefore, the alkaline potassic magmatism in the
Jharia region is clearly sited in a “thin spot” - a
region of pre-existing lithospheric extension.

25. Microphotograph of Intrusives
 Fig. 5 (A and B) Microphotographs
(in uncrossed polars) depicting
various textural aspects of Jharia
alkaline intrsuives:
 (A) Phenocrystal olivine (Ol)
surrounding by a groundmass rich
in mica (P) (JH6/4)
 (B) Two generations of phlogopite–
phenocrystal as well as fine grained
tabular laths in the groundmass–
are a conspicuous feature in JH6/3.
Euhedral to subhedral groundmass
olivines are also conspicuous;
Width of A &B photographs is 3.1
mm.
 (C) Back scattered electron (BSE)
image showing abundance of
phlogopite (P) together with apatite
(Ap) in the groundmass
 (D) BSE of needle shaped apatite
crystals in abundance in the
groundmass.

26. 6.significance of intrusion on the Gondwana
stratigraphy, coal bearing rocks
 In Indian coalfields huge amounts of natural coke
have been produced due to magmatic intrusions.
 Jharia Coalfield in eastern part of India alone
contains approximately 2000 Mt of baked coking
coal as a consequence of these intrusions.
 Apparently, during the Lower Cretaceous period, a
vast volcanic eruption took place, during which
innumerable dykes and sills of dolerite and mica
peridotite were injected into the coal-bearing strata.

27.  As a consequence of the exposure of coals to elevated
heat, minerals such as biotite, olivine, tridymite,
cristobalite, mullite, calcite, siderite, ankerite, pyrite,
etc. were produced, sometimes embedded in glassy
matrix (Singh, 1998).
 In the vicinity of the dikes, the first apparent changes
in the seam is the development of innumerable fine,
cleat-like fractures, which make the coal much more
friable than it is normally.

28.  within a short distance, the coal suddenly becomes
massive and compact without retaining any trace of
original coal lamination or jointing.
 Intrusive dykes punctured the coalseams and
induced a large-scale baking effect in addition to
mechanical destruction and the ash content of the
coked coals increased due to varying degree of
distillation (Pascoe, 1959; Merritt, 1990; Querol et
al., 1997;).

29.  Fig 6.Contact between Carbonaceous shale & Lamprophyre dyke

30. F I G 7 . M A F I C D Y K E

31. 7.Qualitative changes due to the intrusion impact
 The coking quality of the coals diminishes towards the
contact boundary. When the distance from the intrusion
contact increases, less pronounced changes in physico-
chemical and petrographic properties are observed.
 The brilliant lustre of the normal coal is totally
destroyed.
 In contradistinction, where a sill intrudes the coal
bearing strata at some distance above or below, it causes
improvement in the rank of coal through the effect of
gentle roasting (Raistrick and Marshall,1939).

32.  Near the intrusion-coal contacts, the coals became
coked giving rise to natural coke and also partition
rocks (shales and sandstones mostly) underwent
alterations and produced para-lava (Singh, 1998).
 These igneous intrusive units have caused partial or
total burning of valuable coking coal seams.

33. 8.Conclusion
 Jhariya coal field is traversed by maily two types of
igneous intrusives 1.Dolerites 2.ultrabasics.
 The effects of igneous intrusives on coal seams can be
either constructive or destructive relative to the use of
coal, depending upon the nature of intrusion reflected by
the various constituents of the coal.
 Dolerite dykes are encountered on the western part of the
coal field while central and eastern part of the coal field
is affected by ultramafics.
 Igneous intrusive units have caused partial or total
burning of valuable coking coal seams,converting more
than 1200 million tonnes of coal into jhama and ball coal.

34. 9.References
 1. Acharya, S.K., 2000. Coal and Lignite Resources of
India: An Overview. Geological Society of India,
Bangalore, ISBN No. 8185867429, pp:50
 Cretaceous potassic intrusives with affinities to
aillikites from Jharia area: Magmatic expression of
metasomatically veined and thinned lithospheric
mantle beneath Singhbhum Craton, Eastern India
Rajesh K. Srivastava, N.V. Chalapathi Rao , Anup K.
Sinha

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