Mineralogical peculiarities of carbonatites of the Chagatay complex

(western Uzbekistan)

Divaev F.K., Golovko A.V., Golovko D.P.

GP “Centralnaya GGE” The State Geological Committee of Uzbekistan Republic,

Samarkand, Uzbekistan

The rocks of Chagatay trachyte-carbonatite complex form swarm of contiguous dikes and volcanic pipes, which are distributed within basin of Chagatai Creek (the complex was called by creek’s name) on the northern slopes of southern Nuratau Mts. Enclosing rocks are presented mainly by terrigenous formations of Djazbulak and Nakrut suites of the Silurian age (sandstones, siltstones, sheets, tufaceous sandstones) with rare tectonic wedges of Ordovician terrigenous Badamchala suite and Cambrian Kalsara suite of essential carbonate composition. Besides carbonatites and trachytes within descripted territory among magmatic formations rare separated Silurian diabase dikes of Malgusar complex and Triassic camptonites of southern Tien-Shan complex are developed.

šššššššššššššš In composition of Chagatay complex carbonatites are prevailed. They form dikes (thickness 0.5 – 5 m, extension 20 – 1000m, strike 20-30œ, dip of high angles, rare 60-70œ, total near 60 dikes and two volcanic diatremes (dimensions: 8x170m and 200x250m). Trachytes are of single dikes (total 3 dikes), their thickness 0.5-1.5m, and extension up to 100m. They are close associated with carbonatite dikes. Carbonatites are both rocks of considerably calcite composition, so-called sevites, and rocks of transitional carbonate-silicate composition, so-called casenites, ringites and melilitic carbonatites (alvikites). They differ between them both on features of mineralogical composition and on structural-textural indications. Contacts between carbonatites varieties are both sharp and gradational. Judge by available interrelations, the earliest formations are fine-grained vitreous eruptive breccias of carbonatites, which form diatremes and dikes. They were dissected by fine- and medium-grained alvikite and sevite dikes. Interrelations between them are reflected in geological outcrops. Interrelations of carbonatites and trachytes didn’t establish. Both dikes and diatremes have clear intrusive contacts with enclosing rocks and contact alterations are manifested very weakly in interval 0.1-0.5 m. Alterations are manifested by poor hornfels and finely squamosed biotite. Metasomatic alterations of enclosing rocks are manifested poorly as well (weak albitization and limonitization in intervals 0.1-0.7 m.

šššššššššššššš 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.

šššššššššššššš 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 didn’t establish. Some times dike’ composition changes gradually along the strike or from center to border.

šššššššššššššš 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). Pipe’s 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 don’t dissolve. Then we arrive to the conclusion that assimilation of limestones by carbonatites didn’t occur. This fact can be explained by very quick carbonatites magma’s 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.

šššššššššššššš Calcite forms large tabular and prismatic grains often with rhombohedral ends. Dimensions of grains are up to 1-3 mm on elongation with characteristic polysynthetic twinning and glomeroblastic intergrowths of small xenomorphic tabular grains as well. Large tablets of calcite often content poikilitic inclusions of idiomorphic grains of pyroxene, apatite and magnetite. Isotopic composition of carbon in calcite from the Chagatay carbonatites is varied from -4.3 up to -5,2‰, what is characteristic for mantle depth of forming.

šššššššššššššš 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 dimensions 0.5-2 mm. Mineral color is chiefly brown, rare – yellowish-green. In coarse grains color can be zonal: in the central parts coloration is more intensive than in marginal ones. Coarse grains of garnet substitute by pale-green chlorite.

Biotite is characteristic by idiomorphic flakes up to 1 mm long. They have greenish-brown and brown colors. Partly biotite substitutes by chlorite. Contents of garnet and biotite are in inversely proportional dependence: the more garnet, the less biotite and conversely: the more biotite, the less garnet, and some times in certain cases garnet disappears completely.

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 0.1 x 0.5 mm up to 0.5 x 2.5 mm. Visually melilite crystals have honey-yellow color, in thin section they are colorless, yellowish-brown and dark brown. Often crystals have subparallel orientation. Chemical composition of melilite is corresponded to ferriferous ocermanite with subordinated quantity of gehlenite minal.

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 (0.5-1.5 mm), which contain abundant poikilitic inclusions of idiomorphic crystals of melilite, pyroxene, garnet, apatite and magnetite.

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.

5.                Apatite-magnetite carbonatites are characterized by more small-granular texture (up to fine-grained rocks). They often form eruptive brecciated dikes, rare small pipes (diameter 5-10 m), which are overfilled with xenoliths both enclosing sedimentary rocks and carbonatites of former injection phases. Carbonatites contain calcite (40-50%), magnetite (15-25%), apatite (15-20%), pyroxene (5-10%), biotite (1-2%) and secondary chlorite and albite (2-3%). In contrast to the third type of carbonatites in this type garnet forms fine (0.n mm) and finest (0.0n mm) grains equally spaced in all over the rock. As a poikilitic inclusion garnet intercalates within more coarse (up to 0.5-1.0 mm) plate grains in calcite. As a rule garnet was substituted completely by brown isotropic hydrogarnet (hibschite).

Pyroxene (pale greenish enstatite-hedenbergite-diopside) forms prismatic grains (up to 0.2-0.3 mm in elongation).

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 (up to 2.5 mm across). Its composition is pyrope-spessartine-almandine; 2 - in biotite-pyroxene-garnet carbonatites garnet is represented by abundant small grains of grossular-andradite composition equally spaced within rock’ mass; 3 - in brecciated carbonatites garnet is of spessartine-grossular-almandine composition.

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 of grains. Crystals’ color is peculiar: red-meat of various tints, rare grey-greenish, yellow and marshy-brown.

After results of microprobe analyses apatite composition varies from fluorine apatite to francolite.

Zircon is represented by rare colorless prismatic crystals (0.05 x 0.3 mm). In zircon there are admixtures: Hf, Nb, Ta, La, Ce, Pr.

Pyrite. Main 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 tiny (<0.1 mm) grains of framboidal (spheroidal) pyrite. Often pyrite is oxidized and covered with film of iron hydroxide. It is observed holopseudomorph of iron hydroxide on pyrite. Often pyrite forms inclusions in magnetite, but some times contrary are observed magnetite inclusions in pyrite. Rare inclusions of pyrrhotite in pyrite are observed as well.

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.

Galena forms growths with sphalerite and acantite. Often along the margins of galena grains are observed rims of chalcopyrite, chalcocite and covellite.

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 (0.2-0.4 mm across). Colors vary from bright orange transparent up to dark cherry, opaque.

Muassanite is observed as fragments of irregular forms, some times with melted surface, light green or bluish green colors. Dimensions: 0.1-0.3 mm.

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 2 mm across. It is observed as cement in interstitials of non-metallic minerals. Isotopic composition of carbon in graphite varies from -4.3 – 8.21‰. This fact definitely points out mantle genesis of graphite.

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 ¹05-1000008-7938