Mineralogical peculiarities of carbonatites of the Chagatay complex
Divaev F.K., Golovko A.V., Golovko D.P.
GP Centralnaya GGE The State Geological Committee of
The rocks of
Chagatay trachyte-carbonatite complex form swarm of contiguous dikes and
volcanic pipes, which are distributed within
In composition of Chagatay complex carbonatites are
prevailed. They form dikes (thickness 0.5
The main rock-forming minerals of Chagatay complex carbonatites are the following: calcite, pyroxene, biotite, garnet, melilite, apatite, magnetite and secondary minerals amphibole, chlorite, albite, potash feldspar, hematite, quartz, montmorillonite.
By texture and structure indications and features of mineral composition seven types of carbonatites are distinguished: 1 - brecciated carbonatites with vitreous cement. They fill Taty pipe and some small pipes; 2 - biotite-garnet-pyroxene ones, 3 - garnet-biotite; 4 - garnet-pyroxene-melilite; 5 brecciated, rare massive dikes and small pipes of apatite-magnetite carbonatites; 6 - sevites (carbonatites of considerably calcite composition); 7 - albitizated, orthoclasizated, chloritizated carbonatites, which fill Chagatay diatremes and some dikes. These seven types of carbonatites are distinguished very relative, because clear intrusive contacts were established only for vitreous brecciated carbonatites of Taty pipe and for considerably calcite carbonatites (sevites). Contacts between other types didnt establish. Some times dike composition changes gradually along the strike or from center to border.
According to classic description of Bregger and Eccerman the former five types of Chagatay carbonatites are corresponded to so-named transitional silicate-carbonaceous rocks casenites and ringites, the sixth to sevites. Visually carbonatites of the former five types are the massive, compact; fine- and medium-grained rocks of dark grey and black color resembled diabases and pyroxenites. Only carbonatites of essential calcite composition are colored light-grey and brownish-pinky.
1. Brecciated carbonatites with vitreous cement fill the largest volcanic pipe Taty (after denomination of settlement, which is located near the pipe). Pipes body is made of eruptive breccia, which consists of rounded, oval and sharply angular fragments of enclosing rocks (sandstones, siltstones, flints, carbon-bearing shales, limestones, and marbles). It is characteristic, that small sharply angular fragments of white limestones retain their clear outlines and dont dissolve. Then we arrive to the conclusion that assimilation of limestones by carbonatites didnt occur. This fact can be explained by very quick carbonatites magmas intrusion and cooling. Breccia was cemented by vitreous basic mass with rare lath-like crystals of calcite. Composition of vitreous cemented mass was analyzed using electron microprobe and it is corresponded to biotite-garnet-pyroxene carbonatites.
2. Biotite-garnet-pyroxene carbonatites consist of calcite (30-45%), pyroxene (20-30%), garnet (10-15%), biotite (5-10%), magnetite (5-10%), apatite (0.5-2.5%), chlorite (1-5%), albite and orthoclase (1-5%).
Pyroxene is represented by columnar and keg-like crystals of greenish-grey diopside (Fig. 24) with mixture of hedenbergite (7-20%) and enstatite (1-15%) molecules. Rarely it is found like prismatic-oblong crystals of ferruginous aegirine-augite (acmite) with mixture of ferrosilite and johannsenite minals (up to 12.2 and 3.2% correspondingly).
Garnet forms xenomorphic, rarely hexagonal crystals of yellowish-brown color (andradite) with anomalous interference colors. It has been substituted intensively for earthy isotropic hydrogarnet (hibschite).
Biotite forms fine xenomorphic flakes, with pleochroism from brownish-green up to grayish-yellowish color within some dikes and from light-brown up to dark-brown within another ones.
Chlorite forms finely squamosed light-green aggregate as a pseudomorph of pyroxene, partly substitutes biotite flakes.
Albite and orthoclase occur sporadically in rare thin section as xenomorphic transparent grains.
forms large tabular and prismatic grains often with rhombohedral ends.
Dimensions of grains are up to 1-
Texture of rock is typical poikilitic.
3. Garnet-biotitic carbonatites include: calcite (30-40%), garnet (5-40%), biotite (10-40%), magnetite (2-10%), apatite (1-3%), chlorite (1-10%), albite (0-2%). Depending on correlation of calcite and femic minerals these rocks can be divided into leucocratic and melanocratic varieties.
Garnet forms coarse porphyry idiomorphic grains of
Biotite is characteristic by idiomorphic flakes up to
4. Garnet-pyroxene-melilitic carbonatites form up to half a volume of mapping dikes.
In outward appearance the melilite carbonatites are fine-grained massive rocks, from light to dark grey-green color. Chiefly they include: calcite, melilite, pyroxene, magnetite, garnet and apatite with insignificant admixture of secondary albite, chlorite, calcite, hibschite and limonite.
Melilite contents in rock vary from 5 up to 40%. It
forms oblong-prismatic crystals with square cross sections of dimensions: from
Pyroxene diopside-hedenbergite has a pale grey-greenish color. Its quantity in rock varies from 5 up to 30%. Content of pyroxene is in inversely proportion dependence on melilite quantity. In chemical composition diopside minal is prevailed. Contents of hedenbergite and tchermakite (oligoclase) minals are subordinated. Admixtures of enstatite and aegirine are insignificant.
Calcite (20-40%) forms grains of two generations as
well. The first one magmatic generation include coarse polysynthetic twinned
tabular grains (0.5-
The second generation is autometasomatic. It is represented by fine-grained aggregate, substituted by the veinlets and spots of primary tabular calcite grains.
Rock texture is poikilitic, partly trachytoid, complicated by microgranoblastic texture.
carbonatites are characterized by more small-granular texture (up to fine-grained
rocks). They often form eruptive brecciated dikes, rare small pipes (diameter
Pyroxene (pale greenish
enstatite-hedenbergite-diopside) forms prismatic grains (up to 0.2-
6. Sevites (carbonatites of considerable calcite composition) are the least distributed rocks of the Chagatay complex. As a rule they form compound dikes, rare small stock-like bodies. To all appearance sevites are the new varieties of carbonatites. Indicative of this fact are their intrusive contacts with chilling zones in contacts with former phases of carbonatites injections.
In samples were established the following accessory minerals: garnet, apatite, zircon, rutile, leucoxene, barite, pyrite, magnetite and chromo-spinel. All these minerals have through wide-spread occurrence and were established single finds of zircon, periclase, moissanite, corundum, iocite, arsenopyrite, galena, gold, graphite, diamond.
Garnet forms three
varieties: 1 - in garnet-biotite carbonatites it forms coarse porphyric grains
Apatite is represented by
crystals of mainly hexagon-prismatic habit and their fragments. Often are found
crystals of flattened tabular habit, distorted crystals and columnar aggregates
After results of microprobe analyses apatite composition varies from fluorine apatite to francolite.
Zircon is represented by
rare colorless prismatic crystals (0.05 x
part of pyrite occurs in electro-magnetic fraction. It is represented by
crystals of cubic habit and their fragments. Pentagonal-dodecahedrons, compound
combinations and their distorted crystals are met rare. It is observed very
Magnetite forms octahedron and hipidiomorphic grains, often appear inclusions within pyrite and hematite, intensive substitute with hematite and non-metallic minerals, so that only outward rim was remained. Grains of magnetite are of zonal structure: the central part is formed with titan magnetite (up to 17.7% TiO2) and marginal zones with titan-bearing magnetite (up to 2.1% TiO2). The central part is enriched with Al, Mn, Mg, Cr, V and Zn.
Ilmenite is presented as idiomorphic plates in inclusions within titan magnetite and forms exsolution phenomena. As a rule it intensive substitutes by hematite.
Hematite. It is observed three varieties: 1. independent grains fragments, 2. pseudomorphoses after magnetite, 3. diffused hematite in rocks fragments.
Spinel composition is very unusual and is characteristic for all types of carbonatites of the Chagatay complex. It is corresponded to magnochromite: high chromous (57-59% Cr2O3) and magnesium (6.6-9.5% MgO). Presence of chromiumless noble spinel within brecciated carbonatites can be explained its xenogenous origin.
Corundum is met as small fragments (0.05-0.5mm) of white, blue, sky-blue, rare pink colors.
Rutile forms rounded and oval grains (0.2-
Muassanite is observed as fragments of
irregular forms, some times with melted surface, light green or bluish green
colors. Dimensions: 0.1-
Silver is presented practically in all samples, in shape of fine wires and rounded-flat grains. Its contents in rock are ranged usually from 1.5 up to 3.5 g/t. In silver composition were established heightened contents of platinoides (Pt up to 300g/t, Pd up to 200g/t and Ir up to 0.6 g/t).
Gold is observed as single grains in majority of samples. Often it forms intergrowths with hessite and fahlerz. Dimensions of gold particles: from 0.05 up to 0.1mm. Their forms are pellets and interstitial; colour yellow, dark yellow.
Graphite forms separate
flakes of black colour with greasy luster, up to
Diamonds are represented mainly aggregates of octahedron crystals and intergrowths of grains with vague manifested crystallographic and skeletal forms, dimension of which are 0.01-0.05mm (Fig. 33-34). Rare are met octahedron monocrystals with plane crystal faces and sharp apexes and edges of crystals. Their dimensions 0/02-0/05 mm. Colour is mainly light grey with light greenish shade. Some times are met grains lighter and very rare near black. Rarely was it observed dark micro inclusions, which are located mainly within apex of crystals. Brilliant lustre. Hardness is more 9.5, which was determined on hardness standard. Determination of diamond was confirmed by debaegram.
The presence of native elements within the Chagatay carbonatites indicated of low sulfur and oxygen potentials and, correspondingly, of sharp reduction conditions within magmatic chamber.
The work was accomplished by financially supported
by INTAS Fond น05-1000008-7938