Geology Reference
In-Depth Information
The lamprophyres intrude mostly the Barakar formation as subvertical dykes
trending predominantly east
northeast to west-southwest (Raja Rao 1987).
According to Basu et al. (1997), the dykes bear various contact relations with the
country rocks comprising shale, sandstone and coal seams. The nature of contact
may be sharp, bulbous and sometimes of tapering forms in pro
-
le sections. There
may be dykes with angular offsets, branching and veined types. They incorporate
xenoliths of the country rocks or ultrama
c rocks of mantle origin. Rarely, they
occur as sills and at times exhibit circular pipe-like cross sections. The coal seams
in contact with the intrusives are indurated and transformed into coke. Some of the
dykes are vesicular with amygdules of carbonate minerals, secondary silica, zeolite
etc. Macroscopic bandings are common especially in the olivine lamproites with
sharp or gradational inner contacts with high concentration of phenocrysts, xeno-
crysts or xenoliths in the central part. Basu et al. (1997) found that in some of the
dykes there is a systematic disposition of
fine and more glassy layers, and the
lamproitic dykes indicate
flow differentiation. Occasionally conjugate intersecting
dyke swarms are also noticed. The dolerite dykes, whenever present cut across
the entire sequence of the Gondwana formation with strikes varying between
north
-
south and northwest
-
southeast. The dykes often extend into the Precambrian
basement rocks.
Detailed petrological study indicates a systematic mineralogical variation rep-
resenting a continuous spectrum of ultrapotassic rocks ranging from olivine lam-
proite through lamproite to minettes.
The minettes are reported mostly from the western part of the east Bokaro
coal
eld (West of Gomia, Swang, Gobindpur, etc.). They display porphyritic to
panidiomorphic textures with phenocrysts of alkali feldspar, phlogopite, olivine
(pseudomorphed by carbonates) and slender prisms of apatite set in a glassy matrix
that contains
finer crystals of alkali feldspar, olivine, zoned phlogopite, and opaque
phases like magnetite, ilmenite and sulphides of Ni and Cu.
The olivine lamproites generally contain megacrysts and possible xenocrysts of
phlogopite and olivine. In some samples large needles of ilmenite are set in a glassy
groundmass which contains
finer crystals of phlogopite, apatite and carbonates.
Some of the olivines are pseudomorphed by magnesian carbonates, iron carbonates,
silica and rare serpentine. In other samples they are surrounded by phlogopites
exhibiting ocellar texture. The opaque minerals include magnetite, pyrite, arseno-
pyrite, pentlandite, millerite, ilmenite, priderite and native gold. The olivine-leucite
lamproite differs from the olivine lamproite in that it contains leucite both as a
phenocrystal as well as groundmass phase. Distinctive leucite twining is sometimes
preserved in the usually altered leucite crystals. Serpentinized olivine xenocrysts are
occasionally found to be rimmed by leucite crystals with thin intervening zone of
phlogopite. The leucite lamproite contains leucite as a major phase both in the form
of phenocrysts and in the groundmass. Minor phases include olivine and apatite set
in a glassy matrix. The leucite phenocrysts usually contain inclusions of glass and
opaque oxides. Some of the olivine lamproite (with or without leucite) dykes
occurring at Kalyani, Bokaro and Jarangdih Mines display excellent
flow layers
along the margins. They are essentially glassy layers with small opaques and
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