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A
New Data about Alkaline-Ultrabasic Volcanism in the Kimberlite Province
Lunda (Angola)
Podvysotskiy V.T.,
Ostafiichuk N.N.
State technological
university, Zhytomyr, Ukraine
vt_podvysotski@mail.ru
The kimberlite province
Lunda, where the basic diamond resources of the Republic Angola are
concentrated, is located in north-eastern part of the country. More than
70 kimberlite bodies which form four the kimberlite fields are here
exposed; information about it with description of features of kimberlite
occurrences and description of rocks is resulted in a number of literary
sources (Makkenda, 1989; Nosyko, Rotman, 2003 and others.). The data
about the presence in this region of other types of rocks of the
alkaline-ultrabasic or the alkaline-basic row of diatreme facies we did
not meet.
We first conduct the field
and laboratory-analytical study of pipe bodies of picritic porphyrites
and alkaline basaltoids, which were exposed at verification of
aeromagnetic anomalies of pipe type in 2007 by the company of the ATS
BHP Billiton. The licensed area of works is located between the known
kimberlite fields Katoka and Kamutue. One body of picritic porphyrites
was drilled to the depth 120 meters and primarily rocks visually were
defined as kimberlites. However, as turn out soon, a complete absence of
minerals-satellites of diamonds, deep mantle inclusions, phlogopite and
other signs allowed to define them as picritic porphyrites by analogy
with such north of the Yakut diamond-bearing province. It is the dark
dense very strong rocks of afyric and small-porphyric structure with
insignificant content of xenoliths of granite-
gneisses. Olivine
and pseudomorphoses on it are idiomorphic; the oval impregnations of the
first generations which are characteristic for kimberlites, are
practically absent. The results of chemical analyses show the difference
of these rocks from kimberlites on content of oxides of potassium,
magnesium and some petrochemical correlations.
As it marked in many
literary sources, on ancient platforms and shields, at least in the
Yakut diamond-bearing province, among kimberlites and relative rocks to
them a few rock facies on the depths of melts forming in a mantle are
selected: diamond - pyrope, pyrope and picritic. All of them in the form
of pipes are present within the limits of diamond-bearing regions. The
industrial deposits of diamonds are related to kimberlites of diamond -
pyrope deep facies only; in kimberlites of pyrope facies diamonds, as
well as pyropes of diamond association, are very rare. To picritic deep
facies the picritic porphyrites in which pyrope is absent belong, but
noticeable part is acted by pyroxenes.
To in relation to the least
deep facies - alkaline-feldspar, subalkaline basalts belong. Such rocks
are exposed on six anomalies of pipe type, to the depths 50-75 meters
they are drilled. They, as well as picrites, first for this region are
described and studied. Rocks are characterized by the grayish
brown-brown colouring due to high content of the potassium field
spar-microcline, is partial as a result of oxidization of magnetite. A
structure is a microporphiritic quite often, texture is breccia, taxic
and massive. In a bulk, except for feldspars, pseudomorphoses on
idiomorphic olivine, pyroxenes, and also pulverescent magnetite which is
visible and stipulate
heightened magnetic of
rocks and appearance of anomaly is present. Large xenolithes of granites
are registered in separate intervals. For the described rocks their
intensive hydrothermal-metasomatic transformation the degree of
manifestation of which with a depth diminishes is characteristic. The
post-magmatic processing of rocks is expressed both in development of
the secondary serpentine -
carbonate and talc -
chlorite
mineralization on the chaotically located areas of rock and in formation
of hydrothermal veins, built by serpentine, by calcite, by talc and
chlorite in one or another combinations and in the form of aggregates.
Especially high intensity of metasomatic, vein hydrothermal is
registered in overhead parts of cuts, to the depths 20-25 meters, as a
result quite often appeared pseudobreccia texture of rock. Although
petrogeochemical descriptions of the described rocks are considerably
distorted, however some features of primary composition of rocks are
looked over. On basic petrochemical parameters - correlation of alkalis
and silica, coefficients of magnesialty and ferruginousity and etc -
described basaltoids belong to relatively the potassium high-magnesia
and high-titanic varieties, that to derivative to the alkaline
olivine-basalt series. The presence of porphyry excretions of potassic
feldspar allow, apparently, to deliver rocks from some bodies of
basaltoids to trachyte-basalt. About bedding of the examined rocks as
pipes can testify, foremost, as character of anomalies (local
isometric), breccia texture, so intensive development of hydrothermal-metasomatic
mineralization, that is characteristic both for basaltic and kimberlite
dyatrem. It is explained that the most intensive manifestation of
hydrothermal-metasomatic processes in overhead parts of diatreme more
high fluid –saturated in top, head levels of magmatic columns. Age of
introduction of picrites and alkaline basaltoids is not simply
determined, because in the studied bodies is blocked them only by the
quaternary deposits by thickness to 10-15 meters. Judging on their on
the whole in relation to cainotype to the look, on core from more deep
levels of mining holes, they scarcely are more ancient than the
kimberlite province Lunda, age of which, as is generally known, early
Cretaceous.
Works on verification of
aeromagnetic anomalies in the same period expose also a kimberlite pipe
first for this area, and which is drilled by a mining hole to the depth
220 м. Rocks are represented mainly by one variety – middle-fragmental
kimberlite breccia. Autolithic formations in a breccia are absent.
Fragmental material is represented by xenolithes of containing rocks of
basement. Relative inclusions in the breccia are of the same type, this,
mainly, mantle lherzolites with saved relicts, quite often with almost
non-replaced, by the grains of
chrome-diopside,
diopside, pyrope and phlogopite. Very characteristic for described
kimberlites is unusually the high content of minerals-satellites of
diamond. Pyrope have the different colour varieties, quite often in
kelyphitic thin borders, picroilmenite monolithic and aggregate
structure. Table of contents of phlogopite from moderate to a some
hightened. Olivine and serpentine pseudomorphoses on it are mainly
rounded-oval form. The hydrothermal vein mineralization is developed in
kimberlites very poorly.
As follows from literary
sources in the diamond-bearing regions of Yakutia among basic rocks
bedding in the earth's crust in the form of pipes next to kimberlite
diatremes, only insignificant their part belongs to the tholeitic
differences, the main part of them belongs
to the subalkaline
row. The signs of high alkalinity of the rocks studied by us testify to
the high depth of origin of their melts in a mantle and, consequently,
they are associated , probably, to those blocks of lithosphere and those
tectonic structures, that kimberlites of this region.
References :
Makkenda А. The geology of
diamond deposits of northeast of Angola: Author's abstract diss. cand.
geol.-min. sciences. М., 1989. 19 p. [in Russian]
Nosyko S.F., Rotman А.Ya.
Specific of kimberlite manifestations and prospects of diamond-bearing
of northeast of Angola. Problem of prognostication, searches and study
of mineral deposits on the threshold of ХХ1c. Voronezh, 2003.Pp.102-108.
[in Russian] |