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

Outline of presentation

1.Introduction:Jharia Coal Field2.Geological Setting3. Igneous intrusions in Gondwana Coal 4.Intrusions in jharia coal field5.Petrography & Geochemistry of Intrusives6.Significance of intrusions7.Qualitative changes due to the intrusion impact8.Conclusion9.References

Coal in India: A View

 Coal deposits in India are of two distinct geological ages.

1. Gondwana Coal(Bituminous Type)

2. Tertiary Coal (Lignite)

The Indian coal is broadly classified into two types – Coking and Non-Coking.

Coking CoalPrime Coking - Mainly used for metallurgical

purpose.Medium Coking - Mainly used in steel

industry.Semi Coking - Mainly in cement, fertilizer

and sponge iron industries.

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.

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.

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.

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

Table 1 : Stratigraphic set up of Jharia Coalfield (after Fox, 1930, Chandra, 1992) showing the status of the igneous intrusives.

Age Formation LithologyJurassic or TertiaryLower Jurassic  Upper Permian   Middle Permian   Lower Permian    Upper Carboniferous   Archean

   Raniganj   Barren Measures   Barakar    Talchir

Dolerite dykesMica 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

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

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.

3.Igneous intrusions in Gondwana Coal

In Gondwana mainly two types of igneous intrusive forms are encountered.

1. Dykes (Discordant )

2. Sills(Concordant)

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

Fig 3.DykesMafic dyke

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

Sill spreads parallel to the bedding planes of the rocks, hence concordant in nature.

mudstone

sandstone

limestone

Fig 4. Sill

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.

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

anastomisingsystems of intrusive branches.The dolerite with limited effect on coal occurs

as dykes which are confined mainly to the western part of the coalfield.

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.

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.

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.

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--lamprophyreb. Leucite--feldspar--biotite---olivine•lamprophyrec. Quartz-•feldspar--biotite--lamprophyred. 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.

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).

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.

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.

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.

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.

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;).

Fig 6.Contact between Carbonaceous shale & Lamprophyre dyke

FIG 7 .MAFIC DYKE

Dyke

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).

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.

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.

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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