Chemical variations of chromium spinel in Medeksky, Medvezhiy Log, Malaya Shita and Tartay differentiated ultrabasic bodies, Eastern Sayan Mountains
Benedyuk Y. P.
A.P. Vinogradov Institute of Geochemistry SB RAS, Irkutsk, Russia
benedyuk@igc.irk.ru
Chromium spinel is a typical mineral of basic and ultrabasic rocks. It holds information on the features of melt formation, it composition, nature and type of mineralization due to various chemical composition and early crystallization from melt.
The composition of chromium spinel was analyzed in four differentiated ultrabasic intrusions, e.g. Medeksky, Medvezhiy Log, Malaya Shita and Tartay. Cu, Ni, EPG-mineralization is associated with the massifs situated in the middle of the Eastern Sayan Mountains, between the Biryusa and Uda Rivers. The intrusions occur within the Alhadyrsky terrane which are in folded frame of the Siberian Platform foundation. Host rocks are primarily sedimentary metamorphosed and volcanogenic rocks, such as gneisses, schists, marbles, amphibolites, etc [1]. The intrusions are referred to the Barbitaysky intrusive complex of ultrabasic rocks derived by picritic magmas. The complex is dated as Neoproterozoic. Investigations of ultrabasic rocks accessory minerals composition were carried out only in south-east and middle part of The Alhadyrsky terrane [2, 3].
Compositions of chromium spinel of intrusion-forming rocks (dunite, wehrlite, plagiowehrlite and olivine gabbro) were examined by electron-probe micro analyzer Superprobe JXA-8200 (JEOL).
In the massifs studied chromium spinel occurs in all rock types in the form of rounded grains and crystals of octahedral habit. Chromium spinel makes up 5 % of rocks. There are two types of chromium spinel reflecting the order of crystallizations [2]: (I) smaller (0.05 – 0.25 mm) rounded or octahedral inclusions in olivine and (II) large (0.25 – 1 mm) irregular shape grains in rock-forming minerals interstices. Some look as homogeneous, zonal grains of chromium spinel and some others show the structures of ilmenite solid solution decay [2].
Chromian spinel in the intrusions of the Medeksky massif have highest Cr2O3 (23.3–57.5 mass. %, average 41.7 mass. %), relatively high MgO (2.1–14.1 mass. %, average 8.4 mass. %), Al2O3 (1.4–31 mass. %, average 19.4 масс. %) and FeOtot (20.2–48.7 mass. %, average 28.2 mass %). Chromium spinel of The Medvezhiy Log massif has highest content of Al2O3 (5–36.3 mass. %, average 21.3 mass. %), MgO (2.5–16.5 mass. %, average 8.4 mass. %) and FeOtot (18.3–65.3 mass. %, average 33 mass. %) and quite high content of Cr2O3 (17–50.2 mass. %, average 35.1 mass. %). Chromium spinel of The Malaya Shita have relatively low content of Al2O3 (9.2–31.2 mass. % , average 19.3 mass. %), MgO (2.2–11.4 mass. %, average 5.5 масс. %), Cr2O3 (18.3–41.2 mass. %, average 32 mass. %) and high FeOtot (20.5–57 mass. %, average 38 mass. %). Chromium spinel of The Tartay massif has the following content: 5.4–27.6 mass. % Al2O3 (average 19.9 mass. %); 4.1–10.1 mass. % MgO (average 6.5 mass. %); 33.7–53.3 mass. % Cr2O3 (average 40.3 mass. %); 23.1–40.7 mass. % FeOtot (average 30.6 mass. %).
Projection on the chromite (Cr) – picrochromite (Pc) – hercynite (Hc) – spinel (Sp) plane (Fig.c) shows that spinel widely varies in composition from chromite sensu stricto (FeCr2O4) to spinel sensu stricto (MgAl2O4).
According to data presented above chromium spinel isomorphism follows the scheme Cr⇄Al⇄Fe3+ and Mg⇄Fe2+.
According to reference [5] chromium spinel of layered intrusions shows high TiO2 and Fe2O3 tending to ferrichromite – chromium magnetite – magnetite trend on Cr–Al–(Fe3++2Ti) plot. Such a pattern of distribution is due to a regular increase of iron index of chromium spinel during substitution by the scheme Al→Cr→Fe3+. However, such law is not typical for chromium spinels of researching intrusions. There are three trends of spinel on the ternary plot: (a) chromium spinel – ferrichromite – chromium magnetite, (b) less chromium spine – ferrichromite – chromium magnetite and (c) alumochromite – ferrichromite –chromium magnetite.
The large concentration of points is observed at the chromium spinel-alumochromite border. Points of the chromium spinels (II) (inclusions in olivine) also located in this fields. This fact allows to consider spinel – alumochromite border as field of early magmatic cumulates.
It is a matter of common knowledge that during basic-ultrabasic magma differentiation Cr always enriches early differentiates, concentrating in spinel group minerals, but Ti enriches the latest differentiates of basic-ultrabasic liquids during the magmatic process [6]. The fact suggests that trends in ternary plot represent distribution of different crystallization stages of chromium spinels. Thus, (a) chromium spinel – ferrichromite – chromium magnetite trend was formed by the earliest cumulates, (b) less chromium spinel – ferrichromite – chromium magnetite trend has been formed by later ones and (c) alumochromite – ferrichromite –chromium magnetite trend shows the composition of the latest species. These trends appeared to each of the intrusions differently.
Composition of chromium spinel of Medeksky massif corresponds to alumochromite and ferrichromite (Fig. a). Single grains comply with chromium spinel sensu stricto and chromium magnetite. Trend b) looks most clearly, trends (a) and (c) are less distinct.
Composition of chromium spinel of Medvezhiy Log massif varies from chromium spinel sensu stricto and alumochromite to feerichromite and chromium magnetite (Fig. b). Trends (b) and (c) look most clearly, trend (a) is less clear.
Composition of chromium spinel of Malaya Shita massif is similar to that of Medvezhiy Log, forming the range from chromium spinel sensu stricto to chromium magnetite (Fig. c). Trend (c) and part of trend (a) are belomg to this intrusion.
F
ig.
Ternary plot of trivalent cation content of each intrusions cr-sp.
Fields of [7]: 1 – spinel, 2 – chromium spinel,
3 – alumochromite, 4 – chromite, 5 – ferrichromite, 6 – chromium magnetite, 7 – magnetite, 8 – alumomagnetite, 9 – ferrispinel
Composition of chromium spinel of Tartay corresponds to chromium spinel sensu stricto, alumochromite and ferrichromite (Fig. d), forming a clear trend (b) and illegible trends (a) and (c).
According to the data presented in Fig. it is concluded that the intrusions were derived by similar processes and under similar conditions.
As mentioned above there are zonal grains of chromium spinel along with homogeneous ones in rocks of intrusions. It is known that zoning of crystals may vary in nature. On the one hand, zoning may be due to sequential crystallization of complex oxide phases from the changing composition of crystallized melt or in consequence with the reaction between formed crystals and intercumulus liquid in a prolonged crystallization [5]. On the other hand, zoning formed due to metamorphic processes, in particular serpentinization, which is accompanied by removal of Cr2O3, Al2O3, MgO from grain margins with a simultaneous enrichment with FeO and TiO2 [5].
Based on the zonal chromium spinel some types of geochemical zoning are identified.
(I). Increase of Cr content, decrease of Al content with a constant sum of (Fe3++2Ti). Such zoning occurs in chromium spinel of the Medeksky, Medvezhiy Log and Malaya Shita intrusions. The crystals with such zoning occur in dunite, wherlite and olivine gabbro. Decrease of Al2O3 content from core to margin of a grain may be caused by its removal from the liquid during pyroxene and plagioclase mass crystallization [5].
(II). Decrease of Cr content, increase of Al with almost constant sum of (Fe3++2Ti). Such zoning occurs in chromium spinel of the Medvezhiy Log and Tartay intrusions. The grains with such zoning occur in dunite and wherlite. Decrease of Cr2O3 content and increase Al2O3 content from core to margin of a crystal is explained by a general tendency of increasing Al2O3 relative to Cr2O3 in the residual melt [5].
(III). Increase of Cr content, decrease sum of (Fe3++2Ti) with minor variations in Al. The zoning occurs in chromium spinel of the Medeksky, Medvezhiy Log and Malaya Shita massifs. The species with such zoning occur in wherlite, plagiowherlite and olivine gabbro. Decrease of FeO, Fe2O3 and TiO2 content may be due to post-cumulus reaction between chromium spinel, intercumulus liquid and primary silicates [5].
(IV). Decrease of Cr content with variations of Al and sum of (Fe3++2Ti). This type of zoning is observed in a single grain of chromium spinel from wherlite of the Tartay intrusion. It was found that FeO, Fe2O3 and TiO2 content increases and Cr2O3 and Al2O3 decreases in chromium spinel with decreasiong temperature [5].
The compositions of chromium spinel of Tokty-Oi intrusion are plotted on the ternary plot for comparison. This zoning is produced by metamorphic alteration, in particular serpentinization of ultramafic rocks.
Conclusions:
1. Probably similarity of the trends indicates the similar physical and chemical conditions of the massif formation.
2. It is likely that different trends of chromium spinel reflect the change of physical and chemical parameters of the mineral forming environment during crystallization process.
3. Zoning of chromium spinel has magmatic origin.
References:
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