On the Composition of the Gabbro-Syenite Complex in the Southern Part of the Kondoma Iron Ore Zone
Abstract Generated abstract
This paper examines the gabbro-syenite magmatic complex of the Kochura area in the southern Kondoma iron ore zone, where iron ore deposits are associated with syenite intrusions. On the basis of petrographic mapping, petrochemical analyses, and trace-element data, the study distinguishes gabbro and gabbro-porphyrites, monzonites, calc-alkaline and alkaline syenites, and later diorite and diabase dikes, and describes the internal structure of the Kochurino massif. The geochemical and chemical patterns, including shared accessory-element assemblages, transitional monzonites, elevated potassium among alkalis, and iron-rich syenitic varieties, are interpreted as evidence that the gabbroid and syenite rocks are comagmatic differentiates of a basaltic magma with a trachytic differentiation trend. The Kochura complex is also compared with the Tashtagol area, with differences attributed partly to deeper erosional exposure.
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ON THE COMPOSITION OF THE GABBRO-SYENITE COMPLEX IN THE SOUTHERN PART OF THE KONDOMA IRON ORE ZONE
G. M. Ivanova
(Presented by the Joint Scientific Seminar of the Geological Prospecting Faculty)
The gabbro-syenite complex, with which the iron ore deposits of the Kondoma group are associated, is located within the territory of Mountain Shoria, within the Kondoma graben-syncline. The rocks of the complex are represented by surface exposures of five syenite massifs (Kochura, Tashtagol, Shalym, Taenzin, and Sheregesh), extending as a chain in a northeastern direction. The intrusive rocks discussed in this article are situated in the southern part of the Kondoma iron ore zone, on the watershed of the left tributaries of the Kondoma River: the Kochura, Chernaya Soldatka, and Belaya Soldatka streams. This area is called the Kochura area.
The first systematic studies of the geology of the region were carried out by M. A. Usov, N. A. Batov, A. K. Kyuze, K. T. Tyumentsev, K. V. Radugin, T. V. Pyatnitskaya, A. S. Mukhin, and others. Later, more detailed geological mapping, geological prospecting, and thematic work was conducted by G. L. Pospelov, A. G. Volodin, V. I. Kaminskaya, I. I. Zabolotnikova, N. V. Monkevich, V. P. Bondarets, and others.
Despite the fact that iron ore deposits in the region are associated with syenite intrusions, detailed petrographic maps of the syenite massifs have not yet been compiled here; the numerous rock varieties of the complex have not been studied, and the genetic relationship between the rocks of the gabbroid and syenite series has not been proven.
As a result of petrographic studies in 1963–64, the author characterized the petrochemical and geochemical features of the iron-bearing magmatic complex within the indicated area and detailed the geological-petrographic structure of the Kochura massif.
Principal Elements of the Geology and Petrography of the Magmatic Complex in the Kochura Area
The host rocks of the intrusive formations of the complex in the area are metamorphic schists, tuffs, and basic effusives with numerous interbeds and lenses of limestones. These effusive-sedimentary formations, assigned to the Kondoma series of the Lower Cambrian, constitute here the northwestern limb of the Tashtagol syncline.
In terms of mineralogical and chemical composition, three groups of rocks are distinguished in the magmatic complex of the area, which are spatially closely associated with one another: 1. Gabbro and pyroxene, pyroxene–hornblende, and, more rarely, plagioclase gabbro-porphyrites. 2. Monzonites, lime-alkaline and alkaline syenites. 3. Dike formations of diorites and diabases.
Fig. 1. Schematic petrographic map of the Kochurino massif. 1—alkaline leucocratic microsyenites; 2—alkaline leucocratic syenite-porphyries; 3—alkaline leucocratic medium-grained syenites; 4—alkaline hornblende syenites; 5—lime-alkaline leucocratic syenite-porphyries; 6—lime-alkaline leucocratic medium-grained syenites; 7—lime-alkaline hornblende syenites; 8—monzonites; 9—gabbro; 10—gabbro-porphyrites: pyroxene, pyroxene–hornblende; 11—effusives and their tuffs of acidic composition with limestone lenses and interbeds; 12—schistose sandstones and siltstones with limestone lenses; 13—metamorphic schists after effusives and their tuffs of basic composition, with limestone lenses and interbeds; 14—limestone lenses; 15—ore; 16—zones of cataclasis in syenites; 17—line of tectonic disturbance; 18—boundaries between petrographic varieties of rocks.
The rocks of the first group, represented by gabbro-porphyrites, make up a stock-like body situated at the northwestern end of the Kochurino syenite massif. Contacts of the porphyrites with the syenites are encountered both as sharp and as indistinct. Cases of potassium metasomatism of the gabbro-porphyrites are observed; their hornfelsing is sometimes manifested. Gabbro has been found southwest of the Kochurino syenite massif on the left slope of the Kobarzinka River.
The rocks of the second group compose the Kochurino syenite massif, which is a stock-like body of northeastern strike with a total area of 11–12 km² and a maximum length of 6.5–7 km.
and a width of 2 km. In addition to this large stock, a number of small bodies of microsyenitic composition, with areas from 0.40 km² to 0.005 km² and lenticular in plan, have been encountered to the east of it. Between the main body and the aforementioned small syenite outcrops, syenites are also exposed by columnar drilling at considerable depth (70, 98, 293 m). This gives grounds to suppose that the small syenite bodies are vein-like apophyses of the main syenite body.
Among the rocks of the second group, the following varieties may be distinguished by mineral composition: monzonites, calc-alkaline hornblende and leucocratic syenites, alkaline leucocratic and hornblende syenites. By structural features they include microsyenites, microsyenite-porphyries, syenite-porphyries, and medium-grained varieties of syenites.
The characteristic features of the structure of the Kochurino massif are as follows: 1. The central part of the massif is composed of calc-alkaline syenites. 2. Monzonites occur as relicts of small size in the central part of the field of leucocratic calc-alkaline syenites. Hornblende calc-alkaline syenites are associated with them. 3. The southwestern and northeastern parts of the massif are composed of leucocratic and hornblende alkaline syenites. At the same time, alkaline hornblende syenites are present at the boundary of alkaline and calc-alkaline syenites. 4. The transition from alkaline varieties to calc-alkaline ones is gradual and is expressed in the appearance and increase in plagioclase content from 0–10% to 30%. 5. The transition from monzonites to calc-alkaline syenites is likewise gradual, expressed in a decrease in the amount of mafic minerals and plagioclase. 6. In the peripheral parts of the massif, syenite-porphyries occur. The small bodies east of the main syenite body are represented by microsyenites. This indicates that the intrusive rocks here are exposed in their apical part. 7. Among the rock varieties of the Kochurino massif, quartz-bearing varieties are absent, unlike the more northerly Tashtagol, Shalym, and Sheregesh massifs. 8. The Kochurino massif formed in a hypabyssal setting, as evidenced by the uneven grain size of the rocks, the presence of porphyritic structures, and insignificant contact alteration of the enclosing rocks. 9. The syenites of the massif at the northeastern end and partly in the northwestern part are intensely altered by cataclasis processes associated with disjunctive disturbances.
The third group of rocks comprises dike formations, namely diorites and diabases. Their thickness most often does not exceed 3–4 m. The younger among them are the diabases.
Geochemical Features of the Complex
A general geochemical feature of the complex is the presence, in the syenite and gabbroid groups of rocks, of the same elements—Be, Ga, Co, Zr, Pb, Cu, Ni, Zn, Ba, Ti, Sr, Mn, V—and the absence of such elements as W, Hf, Ge, In, Cd, Mo, As, Nb, Sn, Hg, Ag, Sb, Ta, Te, U, Th, Zn, La, Ce, Pt, Li.
Characteristic of all the rocks is an elevated content of Ba, Ti, Sr, Mn, and V in relation to the other elements; moreover, the content of V, Mn, and Ti in gabbro and gabbro-porphyrites is somewhat higher than in monzonites and syenites (Figs. 2, 3, 4, 5). This is explained by the fact that these elements V, Mn, Ti enter as isomorphic admixtures into iron-
ferro-magnesian silicates—pyroxene, hornblende, and biotite. However, the content of these elements relative to clarkes in all rocks of the complex has one and the same feature: Be and Mn are present in amounts exceeding clarke values, while the content of Ti and V is below clarke. Ni and Co serve as indicators of early crystallization of rocks; their content gradually decreases from gabbro and gabbro-porphyrites to monzonites, and then to lime-alkaline and alkaline syenites.
This is explained by the fact that these elements, in terms of ionic radius, have an affinity with Fe and Mg and therefore occur as an isomorphic admixture in ferro-magnesian silicates. However, their content in gabbro and gabbro-porphyrites is below clarke values, while in po-
in rocks of the syenite group their content is close to, or only slightly below, the average values for rocks of this composition.
Lead is present in all rocks of the complex; moreover, its content in gabbro and gabbro-porphyrites is somewhat higher than Clarke values, while in monzonites and syenites it is lower. Its absolute contents in rocks of alkaline composition are higher than in basic rocks, which is apparently explained by the affinity of its ionic radius to potassium and by its accumulation in rocks of a late stage of magma differentiation.
The beryllium contents in rocks of both groups slightly exceed their Clarke contents.
Gallium and copper are present in all rocks in approximately equal amounts and are approximately equal to Clarke values.
Of the rare elements, only Sc and Zr were found in all rock varieties; their contents are below, or only slightly exceed, Clarke values.
Thus, on the basis of the foregoing, the following conclusions may be drawn:
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For all rocks of the complex in the Kochurinsky area, the absence of rare-earth and rare elements is characteristic, with the exception of Sc and Zr, as well as the absence of typical elements of acid magma—W, Mo. In the rocks, elements of the iron group (Ni, Cr, Mn, V, Co) and of the group of dispersed elements predominate. This indicates that all the rocks described above are differentiates of magma of basaltic composition.
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Both groups of rocks of the syenite and gabbroid series are similar in the presence and absence in them of the same elements, and also in the elevated content in all rocks of Ba, Be, Ti, V, Sr, Mn relative to other elements.
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Rocks of syenite and gabbroid composition, and the transitional variety between them—monzonites—are characterized by the same content of a whole series of elements relative to their Clarke values. Thus, the contents of Mn and Be exceed Clarke values; Ga and Cu are everywhere present in approximately equal amounts, close to Clarke values; and V, Ti, Zr are below Clarke values.
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The presence of Co and Ni in syenites and monzonites above the average value for rocks of this composition, and reduced amounts in basic rocks, makes their absolute contents in all rocks similar.
All the enumerated conclusions once again confirm the comagmatic nature of the rocks of the gabbroid and syenite series, which are differentiates of a single magmatic source.
The indicated geochemical features of the gabbro-syenite complex in the Kochurinsky area are very close to those at the Tashtagol mine area, considered earlier by G. M. Ivanova.
Petrochemistry of the complex
All rocks of the complex are divided into two groups—basic gabbro-basaltic rocks and rocks of the monzonite-diorite, calc-alkaline, and alkaline series. Rocks of basic composition, petrographically characterized as gabbro and gabbro-porphyrites, are chemically close to gabbro and basalts, with a value of \(Q\) of 4.12 and −6.8 (Table 1, Fig. 6). The value of \(n\) in the basic rocks is 72.4 and 63.4 instead of 79–80 for average gabbro and basalts according to Daly, which indicates a somewhat high role of potassium among the alkalis in these rocks.
In the rocks of the complex in the area, transitional varieties occur—monzonites, linking the groups of basic and calc-alkaline—
Table 1
| Constituent | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| SiO₂ | 53.04 | 49.30 | 55.0 | 56.10 | 59.1 | 59.62 | 56.96 | 57.82 | 63.72 | 59.56 | 64.34 | 65.25 | 60.62 | 62.86 | 60.97 | 63.19 | 62.40 | 64.01 | 62.74 |
| TiO₂ | 0.8 | 0.85 | 0.75 | 1.00 | 0.82 | 0.35 | 0.39 | 0.38 | 0.15 | 0.11 | 0.17 | 0.23 | 0.13 | 0.20 | 0.15 | 0.14 | 0.04 | 0.05 | 0.19 |
| Al₂O₃ | 13.32 | 18.69 | 19.40 | 17.08 | 16.67 | 20.78 | 17.56 | 18.84 | 18.96 | 19.48 | 18.45 | 18.56 | 18.77 | 19.30 | 19.80 | 18.23 | 19.55 | 19.38 | 19.55 |
| Fe₂O₃ | 7.58 | 2.60 | 1.83 | 2.30 | 2.40 | 4.07 | 1.83 | 4.03 | 3.18 | 2.93 | 2.77 | 1.97 | 2.38 | 1.65 | 2.35 | 2.30 | 0.79 | 0.56 | 0.70 |
| FeO | 1.82 | 6.00 | 7.5 | 4.64 | 3.34 | 0.22 | 4.11 | 1.64 | trace | 3.28 | 0.37 | 0.58 | 3.45 | 2.30 | 1.03 | 0.3 | 1.27 | 1.65 | 1.68 |
| MnO | 0.09 | 0.10 | 0.01 | 0.10 | 0.10 | 0.31 | 0.02 | 0.02 | 0.09 | 0.01 | 0.07 | 0.10 | 0.01 | 0.02 | 0.11 | 0.10 | 0.09 | 0.04 | 0.05 |
| MgO | 7.03 | 6.70 | 3.08 | 3.33 | 3.10 | 0.52 | 2.03 | 1.34 | 0.19 | 0.58 | 0.08 | 0.06 | 0.58 | 0.42 | 0.31 | 0.29 | 0.47 | 0.19 | 0.22 |
| CaO | 6.69 | 10.5 | 3.90 | 6.00 | 4.00 | 0.52 | 4.90 | 4.30 | 0.33 | 1.60 | trace | 0.13 | 2.10 | 1.30 | 2.68 | 1.55 | 1.90 | 0.95 | 1.19 |
| Na₂O | 2.82 | 2.60 | 5.37 | 4.06 | 3.82 | 4.62 | 4.00 | 5.48 | 5.56 | 5.91 | 4.78 | 5.03 | 5.35 | 5.30 | 4.41 | 4.24 | 5.75 | 5.85 | 6.36 |
| K₂O | 2.45 | 1.50 | 2.01 | 3.80 | 4.11 | 6.28 | 3.70 | 5.13 | 5.47 | 5.17 | 6.31 | 5.71 | 5.12 | 5.92 | 6.75 | 6.99 | 6.91 | 6.83 | 6.60 |
| P₂O₅ | 0.15 | 0.48 | 0.33 | 0.50 | 0.33 | 0.259 | 0.16 | 0.03 | 0.06 | 0.03 | 0.08 | 0.08 | 0.09 | 0.08 | 0.069 | 0.07 | 0.066 | 0.062 | 0.055 |
| mn | — | — | 2.98 | — | — | trace | 4.95 | 1.44 | 0.86 | 2.17 | 0.97 | 0.77 | 1.68 | 1.27 | 0.89 | 1.72 | 0.49 | 0.43 | 0.95 |
| H₂O | — | 1.84 | 0.10 | 1.00 | 1.23 | — | 0.24 | 0.18 | — | trace | — | — | 0.32 | 0.14 | — | — | — | — | — |
| a | 9.42 | 8.3 | 12.26 | 14.65 | 14.70 | 19.22 | 15.09 | 19.90 | 19.70 | 20.80 | 19.47 | 19.3 | 19.62 | 20.20 | 20.18 | 20.00 | 22.76 | 22.65 | 23.8 |
| c | 3.91 | 8.9 | 4.96 | 4.31 | 4.07 | 0.61 | 4.98 | 2.88 | 0.30 | 2.02 | 0 | 0.20 | 2.66 | 1.56 | 3.48 | 2.00 | 1.83 | 1.14 | 1.30 |
| b | 27.34 | 23.2 | 17.38 | 15.32 | 11.65 | 12.28 | 10.93 | 9.87 | 9.00 | 8.35 | 7.98 | 7.50 | 7.22 | 6.74 | 4.12 | 3.90 | 3.99 | 3.23 | 2.70 |
| s | 59.32 | 59.1 | 65.40 | 65.70 | 69.58 | 67.89 | 69.00 | 67.35 | 71.00 | 68.83 | 72.55 | 73.0 | 70.50 | 71.60 | 72.28 | 74.10 | 71.42 | 72.98 | 72.20 |
| f′ | 29.62 | 35.8 | 41.70 | 42.30 | 46.25 | 32.78 | 53.42 | 51.60 | 29.6 | 68.10 | 34.75 | 30.30 | 75.53 | 53.50 | 74.14 | 58.90 | 60.17 | 64.57 | 81.27 |
| m′ | 42.24 | 51.3 | 31.10 | 37.40 | 46.06 | 7.22 | 34.16 | 23.40 | 22.2 | 12.00 | 2.54 | 1.80 | 14.00 | 10.50 | 12.07 | 12.50 | 22.70 | 10.12 | 13.67 |
| c′ | 28.16 | 12.9 | 27.20 | 20.28 | 7.69 | 60.00 | 12.42 | 25.00 | 47.4 | 19.20 | 62.71 | 67.90 | 10.47 | 36.00 | 13.79 | 28.60 | 17.13 | 25.31 | 5.06 |
| a′ | 63.4 | 72.4 | 75.50 | 62.00 | 56.20 | 52.48 | 62.40 | 62.40 | 60.8 | 63.40 | 53.47 | 57.0 | 61.40 | 57.40 | 50.0 | 47.90 | 55.80 | 56.64 | 59.40 |
| n | 1.3 | 1.2 | 1.20 | 1.37 | 1.03 | 0.40 | 0.51 | 0.50 | 0.94 | 0.12 | 0.18 | 0.27 | 0.16 | 0.29 | 0.19 | 0.20 | 0.03 | 0.04 | 0.26 |
| t | 23.3 | 9.9 | 9.17 | 12.69 | 18.00 | 28.89 | 15.20 | 30.60 | 29.6 | 30.40 | 28.81 | 22.0 | 28.70 | 21.20 | 48.27 | 50.00 | 19.52 | 15.19 | 20.25 |
| φ | −4.12 | −6.8 | +1.32 | −2.19 | +5.78 | −3.27 | +2.84 | −7.98 | +2.3 | −5.96 | −6.16 | +7.2 | −0.90 | +1.14 | 0.60 | +6.20 | −4.51 | +0.32 | −4.5 |
| Q | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — |
1, 2 — gabbro, labradorite-pyroxene; 3, 4 — monzonites; 5, 7 — calcareous-alkaline syenites; 6, 8, 9, 10, 11, 12, 13, 14 — weakly alkaline and alkaline syenites; 15, 16, 17, 18, 19 — alkaline [[unclear: kaimevye]] syenites with elevated silica content.
alkaline composition. In chemistry they are close to diorite and monzonite varieties. Thus, a connection is indicated between the basic and alkaline differentiates of the magma through its differentiation.
The limestone-alkaline syenites of the complex are close in chemical composition to Daly’s average limestone-alkaline syenites and to average limestone-alkaline hornblende syenites.
Fig. 6. Petrochemical diagram of the compositions of intrusive rocks of the Kochura area after N. A. Zavaritsky. Points of chemical compositions correspond to Table 1.
The connecting link between the limestone-alkaline varieties and the alkaline ones is represented by weakly alkaline syenites, which in chemical composition vary from trachyandesites to keratophyres. The alkaline varieties of syenites, in chemical composition, correspond to the average alkaline syenite of the world, nordmarkite, pulaskite, trachyte, and bostonite. Part of the alkaline syenites is distinguished by a peculiar composition. Namely: these rocks fall in the field of the diagram higher and farther to the right than the alkaline varieties of the noted rocks are located, i.e., in the area of aplite rocks with a higher silica content (\(\mathrm{SiO}_2 = 60, 97\%\) —
64.01%) and a high content of aluminosilicates ($\mathrm{Al_2O_3} = 18.9\%$—19.8%). In these rocks, as a rule, dark-colored minerals are absent. In terms of potassium and sodium content they belong to the potassium series and are characterized by an $n'$ value from 47.9 to 59.40. In weight percent the $\mathrm{K_2O}$ content is 6.9–6.5%, with $\mathrm{Na_2O}$ content from 4.2% to 6.3%. The somewhat high presence of $\mathrm{Na_2O}$ is due to the development of later albitization.
The value of $Q$ in all varieties of syenites ranges from $+7.2$ to $-7.98$. Varieties with a high undersaturation in silica are absent here.
The alumina content in the syenites, in weight percent, ranges from 16.67% to 20.78%. Some of the rocks, according to calculations by the method of A. N. Zavaritsky, are oversaturated with alumina; this is probably connected with the development of sericite in the rocks.
The magnitude of the FeO:MgO ratio in the monzonites and syenites of the Kochura area, as also in the syenites of the Tashtagol massif, is very high. This once again proves that the varieties described above are differentiates of basaltic magma.
Thus, gabbro, gabbro-porphyrites, monzonites, and syenites are comagmatic and belong to successive phases in the formation of the complex. This is indicated not only by the spatial association of rocks of the gabbroid and syenite series, and by the presence of rocks of intermediate composition—monzonites—but also by the distinctive chemistry of the rocks of the complex, expressed in the relatively high role of potassium among the alkalies in all the rocks described, the increased iron content of the monzonites and syenites, and the enrichment of the rocks of the complex in the same accessory elements characteristic of magmas of basic composition, with the absence in them of microelements of granitic magma.
Conclusion
The iron-ore-bearing gabbro-syenite magmatic complex in the Kochura area, as in the region of the Tashtagol mine, developed in an area of completed folding, but in the geology of the massifs of these areas there are certain peculiarities connected with the greater erosional truncation in the area of the Kochura River. If in the Tashtagol massif the set of varieties among the rocks of the syenite series consists mainly of alkaline and, to a lesser degree, calcareous-alkaline syenites, the latter often appearing here at depth, then in the Kochura area, apparently owing to the greater erosional cut, the main field of the massif is already represented by calcareous-alkaline syenites, among which hornblende syenites and monzonites also occur. Alkaline syenites, as well as fine-grained and porphyritic varieties of rocks in the Kochura area, are observed more rarely than in the Tashtagol area, which also indicates the presence here of a deeper erosional cut.
In their chemistry, the intrusive rocks of the Kochura area show a tendency toward accumulation of alkalies and a certain decrease in silica content in comparison with the Tashtagol rocks.
However, despite these peculiarities, the rocks of the complex in both areas have many common features in the behavior of rare and dispersed elements, and in the petrographic and chemical compositions of the rocks. Thus, signs of petrographic similarity between the basic gabbroid and syenite rocks, established earlier in the area of the Tashtagol mine (G. M. Ivanova, 1966), were again clearly manifested in the Kochura area as well. These features, discussed in detail above, indicate that the formation of the complex is connected with the intrusion of basaltic magma having a trachytic trend of differentiation.