Zirconium minerals from kimberlites of Novolaspinsk pipe and associated dike (East Peri-Azovian)

Tsymbal S.N. *, Kremenetsky А.А. **, Sobolev V.B. ***, Tsymbal Y.S.*

 

* Institute of geochemistry, mineralogy and ore formation of N.P.Semenenko, NAS of Ukraine, Kiev, Ukraine; ** Institute of mineralogy, geochemistry and crystallochemistry of rare elements, RAS, Moscow, Russia; *** Technical centre NAS of Ukraine, Kiev, Ukraine

 

tsymbal@igmof.gov.ua

 

The Novolaspinsk pipe and associated  kimberlite dike are situated in the eastern part of the Near Azovian megablock of the Ukrainian Shield. At current erosional level the pipe has the shape of ellipse of 100 х 40 m in size. The dike with thickness from 5 to 10 m and more is traced to the southwest from the pipe on almost 300 m. This pipe is studied by drilling up to the depth more than 100 m and dike up to 75 m. Host rocks are represented by granites, grano-syenites and syenites of Paleoproterozoic Khlebodarovsk complex. Kimberlites are represented by micaceous (phlogopite) varieties of diatremic and hypabyssal facies. Eruptive breccias of kimberlites are developed in marginal parts of the pipe, massive porphyraceous kimberlites are in the central part of the pipe and dike. Near to the surface both varieties of kimberlites are strongly modified by the imposed processes of weathering, that considerably complicates their studying. Among xenogenic minerals of deep paragenesises olivine (completely replaced by serpentine and calcite), phlogopite, picroilmenite, chromespinelides and pyrope are commonly found both in the pipe and dike. Much minor abundances show chromediopside and amphibole of pargasitic composition. According to ICP MS data Zr reaches 392-580 ppm in kimberlites from the Novolaspinsk pipe and 425-494 ppm in similar dike.

Rather large (to 2-3 mm) zircons of "kimberlite" type are established in concentration fractions of the studied kimberlites. As a rule, it is found as colourless or light pink grains of irregular shape, their fragments and chips. The composition of macrocrystic zircon is close to the theoretical. According to the microprobe analysis the contents of the main mineral forming components are following (%): ZrO₂ ‑ 65,0-66,3; SiO₂ ‑ 32,0-32,7; HfO₂ ‑ 1,2-1,8. In some zircons such admixtures as FeO (to 0,08 %), Y₂O₃ (to 0,07 %), TiO₂ (to 0,05 %) and CaO (to 0,03 %) are established. The presence of U (4-60 ppm), Th (1-42 ppm) and REE (11,3-37,5 ppm) are also identified by LA-ICP MS method. Among REE heavy lanthanides (especially Dy, Er and Yb) prevail, and among light lanthanides Ce and Sm are presented at a small amount. REE distribution pattern normalized on chondrite shows the presence of distinct positive Ce anomaly. This anomaly is interpreted by some researchers as indicator of crystallization at low fugacity of oxygen (Belousova, et.al., 1998).

The values of ¹⁷⁶Hf/¹⁷⁷Hf ratio in zircons from Novolaspinsk kimberlite pipe and similar dike vary within 0,282704-0,282799 and 0,282737-0,282767 accordingly. The similarity between values of ¹⁷⁶Hf/¹⁷⁷Hf ratio in different grains of zircon sampled from the pipe and dike allows us to assume, that these grains should not be interpreted as xenocrysts but phenocrysts that were formed directly from protokimberlitiс melt. The ages of the zircons studied differ essentially from the age of kimberlites hosting them. The ages of kimberlites are defined by field geological evidences (end of middle - the beginning of late Devonian) and by Rb-Sr isochronous dating (380-391 million years) (Tsymbal, et.al., 2007). Different grains of zircon have isotopic ages ranging from 382 ± 33 to 469 ± 25 million years (measured by microprobe SHRIMP-II in the Center of isotopic investigations, St. Petersburg, Russia).

On many composition features zircons from Devonian kimberlites of Novolaspinsk pipe and dike are similar to those zircons from kimberlites of other regions, described by Kresten P., et.al. (1975), Krasnobayev A.A. (1979), Belousova E.A., et.al. (1998), Downes P.J., et.at. (2006).

Zircon megacrysts commonly have reactionary rims of grey-yellow and grey-white colour. The thickness of rims varies considerably between different and even the same grains of zircon. Most preserved zircon rim have two distinctly allocated parts, external and internal, that differ in structure and composition. The external part of rim is comprised mainly by baddeleyite crystals, at interstices of which diopside grains, sometimes amphibole, phlogopite and calcite are developed. The internal parts of rim are usually microcrystalline in nature. The composition of the zircon rims is characterized by wide variations of components, but similarly to the zircon grains such components as ZrO₂, SiO₂ and HfO₂ are also predominant in the rims. But at the same time the relations between these components in the rim are different than in zircon. ZrO₂ varies from 54,3 to 77,5 % in the rims, SiO₂ from 33 to 8,5 % and HfO₂ from 1,2 to 2 %. Unlike zircon the rims contain much more FeO (0,3-2,5 %), TiO₂ (0,1-3,0 %), CaO (0,25-8,0 %), MgO (0,1-1,6 %) and Y₂O₃ (0,2-1,8 %). They are also more enriched in REE (1,3-3,5 %). Among REE light lanthanides (Ce, Nd, Sm) prevail, and among heavy lanthanides Dy, Gd and Ho are presented at small amount.

The analysis of data available on composition of internal part of reactionary rims shows, that at its formation after zircon, in one case occurs partial loss of ZrO₂ with preservation of high values of SiO₂ contents but in other case occurs progressing loss of SiO₂ with associated raising ZrO₂. Therewith many components are introduced from a residual kimberlite melt, undersaturated in SiO₂ and enriched in alkalis, titanium, REE and carbonate component.

Baddeleyite is one of the most abundant minerals in the external part of reactionary rims. It forms fine short-length (to 10-20 micron) or very elongated (to 50-100 micron) idiomorphic crystals which are mostly oriented perpendicularly to contours of zircon grains and very often parallel to each other. Some of them originate near to the internal part of rim and "ingrow" into the whole length of external part of rim. Crystals show zonal structure. They have dark central parts dark and light marginal parts in back-scattered electron images. In addition, the central parts of crystals do not show any luminescence effects presented, but marginal parts show intensive luminescence in bluish-green colors. Microprobe studying of baddeleyites has shown, that their central and marginal parts are considerably different in composition. ZrO₂ varies from 90,6 to 95,6 % and TiO₂ from 1,6 to 6,2 % in the central part of crystals. Distinct inverse relation between ZrO₂ and TiO₂ is also observed. ZrO₂ makes 93-96 % and TiO₂ 0,5-3,4 % in the central part of crystals and there is no any correlation between them. Baddeleyite constantly shows the presence of FeO from 0,2 to 0,8 %. The central parts of baddeleyite crystals usually contain more FeO than its marginal parts. For baddeleyite rather high HfO₂ ‑ 1,8-2,5 % is typical, whereas in zircon on which baddeleyite is developed HfO₂ reaches 1,25-1,84 %. The value of Zr/Hf ratio ranges from 33 to 42 in the central parts of baddeleyite crystals. Rarely some admixtures of SiO₂ (to 0,1-0,2 %) and CaO (usually less than 0,1 %, and rarely to 0,20-0,25 %) are found at marginal parts of baddeleyite crystals. Ta₂O₅ (up to 0.20-0.25 %) and ThO₂ (up to 0,06-0,08 %) are established in more than half of studied crystals. Almost all crystals show the presence of admixture MgO (from 0,01 to 0,1 %, rarely to 0,3 %) and only some of them contain Y₂O₃ (0,01-0,06 %). REE are also established in small amounts.

The described baddeleyite has originated as a result of reaction between macrocrystic zircon and kimberlite melt undersaturated in SiO₂ and enriched in carbonate component, alkalis and other fluids. The silica released caused the formation of silicate minerals (diopside, phlogopite and amphibole) associated with baddeleyite. Some features of composition of these silicate minerals allow us to make a conclusion, that these processes occurred at low Р-Т parameters at the stage of crystallization of residual kimberlitic melt.

In Novolaspinsk pipe baddeleyite, zirconolite and calzirtite are established in massive kimberlites with porphyritic structure. These minerals have late-magmatic origin and were formed at crystallization of residual kimberlitic melt at hypabyssal conditions.

The baddeleyite is crystallized as post perovskite mineral, but before formation of apatite and calcite. It mainly forms idiomorphic microphenocrysts of homogeneous structure or intergrowths between several (up to 10) crystals of elongated-prismatic shapes with distinct apexes. The baddeleyite has persistent composition that does not depend on its position relative to associated minerals. Following values are identified for baddeleyite (%): ZrO₂ – 94,1-97,5; HfO₂ ‑ 0,6-2,0; TiO₂ ‑ 0,4-3,0; FeO ‑ 0,4-1,3. Some baddeleyites have raised contents of CaO (to 0,5 %), SiO₂ (to 0,4 %), MgO (sometimes up to 0,3 %) and ThO₂ (to 0,07 %). Admixtures of REE and Ta₂O₅ are established only in some crystals by microprobe. Baddeleyite shows intensive luminescence in bluish-green color and whole absence of zonal structure. The baddeleyites from kimberlitic rock mass is different from that of reactionary rims formed after zircon macrocrysts in its much higher ZrO₂, FeO and CaO and lower HfO₂, TiO₂, REE, Ta₂O₅.

Zirconolite is a rare mineral of rock mass of Novolaspinsk kimberlite pipe. It is found in the form of idiomorphic phenocrysts of the micron sizes or their inclusions in the perovskites, considerably replaced by sphene and manganous ilmenite. Its composition varies within following limits (%): ZrO₂ ‑ 42,8-48,1; TiO₂ ‑ 34,3-36,4; CaO ‑ 11,2-12,3; FeO ‑ 5,5-6,0. Among constant admixtures present are MgO (0,8-0,85 %), ThO₂ (0,4-0,5 %), Nb₂O₅ (0,3-0,4 %), Ta₂O₅ (0,17-0,35 %), SiO₂ (0,1-0,2 %). The REE values reach 1,0-1,5 %, with light lanthanides considerably prevailing over the heavy ones.

Calzirtite is rarely found in rock mass of Novolaspinsk kimberlite  pipe and, as a rule, occurs in the form of idiomorphic phenocrysts of less than 10 micron in size. These phenocrysts are commonly situated at boundary between perovskite crystals and calcite segregations but sometimes form intergrowths with baddeleyite. The composition of calzirtite composition is close to the theoretical one. The contents of mineral forming components are following (%): ZrO₂ ‑ 66,6-69,96; CaO ‑ 12,15-16,94; TiO₂ ‑ 13,85-16,93. Raised values are also established for HfO₂ (0,89-1,33 %), FeO (0,61-1,67 %), MgO (0,23-0,36 %) and SiO₂ (0,04-0,43). Despite high TiO₂, calzirtite almost completely does not show any presence of such admixtures as Nb₂O₅, Ta₂O₅ and REE.

  

References:

Belousova E.A., Griffin W.L., Pearson N.J. Trace element composition and cathodoluminescence properties of Southern African kimberlitic zircons // Miner. Magazine. 1998. Vol. 62 (3). P. 355-366.

Downes P.J, Griffin B.J., Griffin W.L. Mineral chemistry and zircon geochronology of xenocrysts and altered mantle and crustal xenoliths from the Aries micaceous kimberlite: Constraints on the composition and age of the central Kimberley Craton, Western Australia // Lithos. 2006. P. 1-24.

Krasnobayev A.A. Mineralogicval-geochemical features of zircon from kimberlites and problems of their origin // Internat. Geology Rev. 1979. Vol. 22. P. 1199-1209. (in Russian).

Kresten P. Fels P. and Berggren G. Kimberlitic zircons – a possible aid in prospecting for kimberlites // Mineral. Deposita. 1975. Vol. 10. № 1. P. 47-56.

Tsymbal S.N., Kremenetsky A.A. Strekozov S.N.,Bondarenko V.A. The age of kimberlites Peri-Azovian geoblocks of the Ukrainian Shield (according to geological and isotopic data) // In Sat: Alkaline magmatic and ore of the Earth. Kiev. 2007. P. 245-248. (in Russian).