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Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

 A New Data about Alkaline-Ultrabasic Volcanism in the Kimberlite Province Lunda (Angola)

Podvysotskiy V.T., Ostafiichuk N.N.

State technological university, Zhytomyr, Ukraine



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]