2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Features of pyropes kelyphitization in E. Pryazovian kimberlites

Vyshnevskyi O.A.

N.P. Semenenko Institute of geochemistry, mineralogy and ore formation NAS of Ukraine, Kyiv, Ukraine

vyshnevskyy@igmof.gov.ua

Kelyphitization – reactionary substitution of pyropes by secondary minerals aggregate – is a very characteristic phenomenon for kimberlitic rocks, independently of their age and geographical position.  It has a certain features in different kimberlite pipes. In one diatremes a reactional rim on garnets are very rare and has a minimum thickness. In others, vice versa, all the pyropes have powerful shells and frequently are fully replaced by kelyphite. In compound diatremes, in rocks of different stages of intrusion, garnets, as a rule, replaced by kelyphite differently. It is known phenomenon of some pipes in which kelyphitic shells are usually developed on garnets in kimberlite groundmass and, at the same time, absent on garnets in mantle xenoliths. The observed distinctions in garnets kelyphitization in different kimberlite pipes are, more credible than all, a result of different conditions of formation and subsequent evolution of composing them rocks.

The feature of E. Pryazovian kimberlites is their strong alteration. They do no contain practically fresh olivine and unaltered xenoliths of mantle rocks. Garnets kelyphitization is also unusual in these kimberlites.

Visual study of representative collection showed absence of classic kelyphitic shells on pyropes from “Pivdenna” kimberlite pipe. Only rarely on garnet grains a thin rims of chlorite like matter are present. They can be relicts of strongly altered reactional rims. At the same time there is a numerous group of pyropes in which substitution products fill the inner parts of their grains. The new growth mineral aggregates form inside garnets sections of irregular shape and branchy veins reaching the crystals surface.  In a whole the substitution products in such pyropes can occupy from 5 to 30 % of their volume.

Detailed microprobe study of six samples of “kelyphitized” Cr-pyropes   (3,9-7,6 % Cr₂O₃) showed a very simple mineral composition of their substitution products. They are composed mainly from platy and fibrous crystals of silicate phase among which skeletal, isometric and irregular micrograins (1-15 μm) of opaque oxide mineral are placed. Content of the last vary from sample to sample in wide range – from rare crystallites till 5-7 % of kelyphite volume. Sometimes in kelyphite matter small relicts (5-30 μm in size) of unaltered garnet are present.

Silicate part of studied kelyphites is monomineralic and presented by magnesium (mg = 0,75-0,86) chrome rich chlorite (pennine - clinochlore). Chemical composition of chlorites is approximately the similar in all analyzed samples. But it varies strongly within each separate sample (Al₂O₃ – 8,5-13,5 %, Cr₂O₃ – 4-8 %, FeO – 10-14 %, MgO – 26-30 %).  Thus, sections without oxide phase enriched by chromium and magnesium, while sections with it lesser in Cr2O3 and MgO, but richer in SiO₂.

Oxide mineral in garnets substitution products is chrome spinel. By analogy with chlorite its chemical composition approximately similar in all samples, but varies in each of them (Al₂O₃ – 37-45 %, Cr₂O₃ – 18-24 %, FeO – 11-14 %, MgO – 18-24 %). Any correlation between chrome spinel chemistry and morphology was not detected. 

Obtained information about morphological peculiarities, mineral composition and mineral chemistry of the studied kelyphites allow us to propose two versions of their origin. Due to the first version primarily at the depth garnets replaced by assemblage of primary kelyphitic minerals – chrome spinel ± phlogopite ± clinopyroxene ± orthopyroxene. Later, on the last stages of diatreme development, indicated silicates fully substitute by chlorite, while chrome spinel is not changes. The second possible version suppose direct substitution of garnets by chlorite and chrome spinel, or only by chlorite, from which spinel phase appears later,  in low-depth but still high temperature conditions. In the both cases imperfect sections of garnets was replaced.

Both of these scenarios foresee aggressive influence of enriched by alkalies, water and other volatile components kimberlitic magma on garnets. Other mantle derived minerals containing in this rocks was influenced similarly that had negative affect on their safety. Possibly this is a reason why pryazovian kimberlites are diamonds free, while on the row of indirect signs they belongs to the category of low diamondiferous.