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aligned phlogopite phenocrysts. The central part of some of the dykes (e.g. those in
the Kalyani mines) have a coarse-grained core (about 65
75 cm thick) composed of
olivine, leucite phlogopite, apatite, opaque minerals and vesicles
-
filled with cryp-
tocrystalline silica and zeolite. The olivine lamproites and their leucite-bearing
variants contain large olivine xenocrysts (up to 1 cm
×
0.5 cm). The rocks in
general contain
fine specks of native gold and sulphides, mostly arsenopyrite,
chalcopyrite, pentlandite and millerite. Oxide phases include magnetite, ilmenite
and priderite. Nodules and xenoliths of various sizes (up to about 2 cm diameter)
and shapes (pear, discoid, etc.) have been noted in some samples. Olivine xeno-
crysts are common, but rare occurrence of xenoliths, comprising an aggregate of
olivine, orthopyroxene (harzburgite) and chromian spinel is also noted.
A set of dolerite dykes of much larger dimensions (up to about 12 m thick and
more than 2.5 km long along strike length) than the lamproites, traverse through the
Bokaro basin.
Analyses of some of the rocks representing a large array of lamproitic rocks from
the Bokaro basin are given in Table
4.2
. Extensive alteration in the form of car-
bonation, silici
cation, formation of zeolites that
fill the vesicles and devitri
cation
poses some dif
culty in evaluating the chemical data. Moreover, there are frequent
changes in composition from one unit to the other even within a single dyke. There
is also some overlapping composition among these units. The presence of car-
bonates is often reflected in higher concentrations of CaO and MgO, though MgO
content also depends on the presence of phlogopite lamproite magma, where H
2
O
plays a dominant role compared to CO
2
. High K
2
O values usually re
ect the
presence of phlogopite, leucite and in some instances, alkali feldspar (viz. minette
with K
2
O/Na
2
O > 4). Some of the rocks that have been identi
ed as olivine lam-
proites are low in K
2
O. The concentration of MgO may also be as high as 20 wt%
compared to olivine lamproites sensu stricto (Rock 1987). This is attributed to the
originally high modal percentage of olivine, now almost entirely replaced and
pseudomorphed by magnesite and/or silica. In some extreme cases the olivine
megacrysts are thoroughly carbonated along with groundmass (viz. olivine lam-
proites from Pichari). These rocks are now rich in magnesium carbonates (MgO:
13.10
21.32 % and CO
2
: 17.64
23.56 %) with variable SiO
2
(25.30
34.10 %),
-
-
-
CaO (4.35
2.60 %).
Characteristically the lamproitic rocks are high in incompatible elements viz. Ba
(up to >1,000 ppm), Sr (up to 0.44 %) and Rb (up to 500 ppm). The chondrite
normalized REE patterns of different varieties of lamproites are shown in Fig.
4.4
.
High concentration of REE (up to 1139.4 ppm.) and signi
9.00 %), TiO
2
(5.50
7.30 %) and K
2
O contents (2.01
-
-
-
cant enrichment of LREE
[(La/Lu)
n
up to 16
5.60]. The highly fractionated rare earth pattern (La/Lu)
n
(up to
165) suggests that they were derived by low degree of mantle melting (Paul and Potts
1981). The signi
-
cant positive correlation (r = 0.938) between (Gd/Lu)n and TiO
2
suggests that fractional crystallization of major phases such as phlogopite and per-
haps to some extent the minor phases like rutile, ilmenite etc. had important effect.
The
87
Sr/
86
Sr ratios in the samples range from 0.70394 to 0.71544. This ratio in
three Raniganj lamprophyres ranges between 0.70493 and 0.71067 (Middlemost
et al. 1988), but in seven Jharia ultrapotassic ultrama
c rocks the
87
Sr/
86
Sr ratio
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