Geology Reference
In-Depth Information
and Seltrust 2). The juvenile lapillis (usually <2 cm) are characterized by por-
phyritic texture with phenocrystal (up to 8 mm) to microphenocrystal olivine
(0.25
fine-grained glassy groundmass. Lapillis in some pipes
comprise laths of phlogopite, while in others, are characterized by the presence of
phenocrysts of leucite and clinopyroxene in a groundmass of leucite, clinopyroxene
and phlogopite. Because of glassy and vesicular nature of the juvenile lapillis, they
suggested that most of the volcanoclastic rocks to be pyroclastic tuffs.
The magmatic rocks occurring within the pipe-like intrusions are characterized by
macrocrystic or porphyritic texture with olivine set in a
0.5 mm) set in a
-
finer groundmass. In case of
some magmatic rocks, the groundmass is constituted of phlogopite and clinopyrox-
ene, whereas the other variety is constituted of leucite, clinopyroxene, phlogopite,
potassic richterite, sanidine, opaque minerals, perovskite, apatite and glass.
Phlogopites are TiO
2
-rich (5
9 wt%) and relatively poor in Al
2
O
3
(4
11.5
-
-
Al
2
O
3
). The Na
2
O content (0.3
1.3 wt%) is similar to other lamproites, but
fluorine
-
content is signi
5 wt%). Olivines are forsteritic with Mg/(Mg + Fe) ratio
ranging from 0.8 to 0.934.
Clinopyroxenes are diopsidic with variable amounts of TiO
2
(0.4
cant (1
-
-
2.7 wt%),
Al
2
O
3
(0.4
-
2.2 wt%) and Na
2
O (0.2
-
0.9 wt%). The amphiboles are mainly potassic
richeterites (4.0
9.2 wt% K
2
O). The
groundmass spinels are titano-magnetites sometimes with chromite-rich core.
Leucites are nearly isotropic and in some instances are replaced by sanidine and
analcite. Primary sanidine (FeO: 2.1 wt%, Na
2
O: 2.7 wt%) occurs as a coarse-grained
mineral. Ilmenite (<0.1 mm) may be a primary or xenocrystal phase. Composition of
primary glass is similar to phlogopite but having lower MgO (10 wt%) and higher
FeO (20
-
6.3 wt% TiO
2
, 4.8
-
9.5 wt% FeO and 2.7
-
22 wt%) content.
Chemical analyses of the rocks are summarized in Table
4.7
, which suggest that
lamproites have variable K
2
O content (sometimes higher than 5 wt%) and high
K
2
O/Na
2
O ratio (3
-
5). There are however, rocks with high Na
2
O content
(>0.48 %). These are the rocks, where leucite is replaced by analcite and /or
nepheline. Scott-Smith et al. supported the mechanism of Gupta and Fyfe (1975)
regarding the alteration of leucite to analcite. The rocks have MgO content varying
between 8.62 and 12.09 wt% and is higher than that of Al
2
O
3
(7.69
-
9.51 wt%).
The olivine lamproites from northwestern part yielded a lot of diamonds and are
similar in mineralogical compositions to the lamproites from Prairie Creek,
Ellendale and Argyle. Diamonds in these rocks are considered to be xenocrystic.
Chemical analyses of lamproitic rocks from Kapamba are given in Table
4.7
.
-
4.3.4 Leucite Lamproites from Pniel, Post Masburg,
Swartruggens, South Africa
Phlogopite and leucite-bearing dykes have been described in association with
diamond-bearing kimberlites at the Hellam Mine from Swartruggens (33
°
54
′
S, 18
°
58
′
E), South Africa by Skinner and Scott (1979). These rocks are characterized by
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