ON THE PETROGRAPHY OF THE GABBRO–SYENITE COMPLEX IN THE SHALYMSKY AREA OF THE KONDOMA IRON-ORE DISTRICT

T. M. IVANOVA

(Presented by the scientific seminar of the Department of Petrography)

The Shalymsky area encompasses the watersheds of the Shalym, B. Kamenushka, and Taenza rivers and occupies a middle position between the Tashtagol and Sheregesh areas of the Kondoma iron-ore district in Gornaya Shoria.

The intrusive formations of the gabbro–syenite complex of this area are represented by two large syenitic massifs—the Shalymsky and Taenzinsky—and by a number of stock-like bodies of gabbro and gabbro-porphyrites. The enclosing rocks of the complex are effusive-sedimentary rocks of the Mundybash Formation of the Middle Cambrian. Lithologically, they are represented by albitophyres and their tuffs with lenses and interbeds of limestones and sandstones. Quite often these effusive-tuffaceous formations have been strongly altered as a result of dynamometamorphism into porphyroids and metamorphic schists. Structurally, the enclosing rocks of the Middle Cambrian in the area make up the second structural level and form an anticlinal fold, near the crest of which the Shalymskoye deposit is situated.

The intrusive formations of the gabbro–syenite complex in this area were formed in three successive phases: phase I—gabbro, pyroxene gabbro-porphyrites; phase II—syenites of various varieties; phase III—vein formations (dikes—diabases, diorites, quartz porphyries).

Rocks of intrusive phase I. Intrusive phase I is represented by a stock-like body of gabbro encountered in the Chernichny area by boreholes 186, 291, 299, 283, 272, and by stocks of pyroxene gabbro-porphyrites traced to a depth of 250 m by boreholes 250 and 252. The largest are two bodies (0.35–0.4 km²), having in plan a somewhat elongated submeridional and sublatitudinal strike and situated in the upper reaches of the Summarny Spring and the B. Kamenushka River, in direct contact with the syenites of the Shalymsky massif in its northern part.

The intrusive character of these porphyrites is observed here quite clearly: namely, they cut the effusive-sedimentary formations of the Middle Cambrian and, in turn, are cut by the intrusion of syenites. The same is observed with respect to rocks of gabbroic composition. Contacts of the gabbroid bodies with the enclosing rocks are most often...

clear-cut with chilled zones; only occasionally is slight hornfelsing observed.

Gabbro in the area occurs in two varieties—porphyritic and coarse-grained. Moreover, the porphyritic gabbro is confined mainly to the marginal parts of the body, and the coarse-grained gabbro to the central parts.

Coarse-grained gabbro has a panidiomorphic-granular structure and the following mineral composition: monoclinic pyroxene—40%, basic plagioclase—60%. The pyroxene is represented by broad tabular colorless grains with the following optical constants: \(Ng = 1.695 \pm 0.002\); \(Nm = 1.627 \pm 0.002\); \(Np = 1.672 \pm 0.002\); \(CNg = 48^\circ\); \(2V = +60^\circ\), which corresponds to augite. The plagioclase usually has an elongate-tabular form and twinned structure. In optical properties (\(Ng = 1.567 \pm 0.002\); \(Nm = 1.562 \pm 0.002\); \(Np = 1.559 \pm 0.002\); \(2V = +80^\circ\)) it belongs to labradorite No. 60. Plagioclase and pyroxene possess approximately the same degree of idiomorphism with respect to one another, indicating their simultaneous separation from the magmatic melt at the moment of crystallization of the rock. Among accessory minerals in the rock, magnetite, apatite, and more rarely sphene are observed.

In gabbro of porphyritic structure, the phenocrysts are represented by grains of plagioclase of the same composition as in the coarse-grained varieties described above. The groundmass of the rock has a sideronitic structure and consists of plagioclase No. 60, pyroxene-augite, and magnetite. The quantitative mineral composition of the porphyritic gabbro is as follows: plagioclase—35%, pyroxene—40%, magnetite—25%. Magnetite, represented by small grains, is as a rule xenomorphic with respect to plagioclase and pyroxene; at the same time it is clearly visible that it occupies the spaces between grains of the indicated minerals, as though cementing them. At the same time, the grains of plagioclase and pyroxene possess the same degree of idiomorphism with respect to one another. Among secondary postmagmatic minerals there occur actinolite, biotite, sericite, and saussurite. As a rule, secondary amphibole-actinolite develops after pyroxene, with the following optical properties: \(Ng = 1.667 \pm 0.002\); \(Nm = 1.658 \pm 0.002\); \(Np = 1.647 \pm 0.002\); \(CNg = 14^\circ\); \(2V = -64^\circ\), as well as biotite. Sericite and saussurite usually develop after plagioclase. The most intense secondary alterations in the gabbro appeared under the influence of the intrusion of syenites, which can be clearly observed in boreholes No. 286 and also No. 299. Here the gabbros, at the contact with the syenites, undergo complete metasomatic replacement with the formation of a rock composed almost entirely of calcite.

Pyroxene gabbro-porphyrites have a porphyritic structure with pyroxene segregations up to 7–8 mm in size, constituting 35–40% of the total mass of the rock. Optical properties of the pyroxene: \(Ng = 1.698 \pm 0.002\); \(Nm = 1.678 \pm 0.002\); \(Np = 1.675 \pm 0.002\); \(CNg = 48^\circ\); \(2V = 44^\circ\). The groundmass has a hypidiomorphic-granular, sometimes gabbroic, structure with grains of plagioclase No. 60–63 and pyroxene. For the most part it is packed with secondary minerals—biotite, actinolite, epidote, carbonate; sometimes a large amount of chlorite develops. The pyroxene of the porphyritic segregations is often replaced by amphibole-actinolite with the following optical properties: \(Ng = 1.663 \pm 0.002\); \(Nm = 1.648 \pm 0.002\); \(Np = 1.643 \pm 0.002\); \(CNg = 14^\circ\), \(2V = -64^\circ\). In addition to actinolite, biotite also develops in the form of a fine-scaly aggregate.

Rocks of the second intrusive phase (syenitoids). The principal constituent part of the complex in the Shalym area is syenites, represented by two parallel massifs: the western—Shalymsky—and the eastern—Taenzinsky. The distance between them

on average is 0.9–1.2 km. Both syenite outcrops, in all probability, join at depth, forming a single stock-like body. This supposition is confirmed by their petrographic and petrochemical composition, as well as by their structural position. Thus, in the near-contact part of the western (Shalymsky) syenite outcrop, fine-grained alkaline syenites are observed. Similar syenites were encountered at a depth of 300 m in borehole No. 252, located 850 m east of this contact. At the western contact of the eastern—Taenzinsky—syenite outcrop, fine-grained alkaline syenites are also present. Thus, between the two large syenite outcrops there is observed, apparently, a cupola-like roof of the syenite body. The general dip of the body and of most apophyses is eastward, with angles from 60 to 90°. Thus, the emplacement of magma of syenitic composition probably proceeded in a direction from east to west.

In plan, the western—Shalymsky—massif has a complexly outlined form of submeridional extent, with a total area of 8.2 km², a maximum length of 5.5 km and a width of 2.5 km. The crested roof of this massif is especially clearly visible in the area of the Shalymsky deposit, situated near the northwestern end of the Shalymsky massif. Here the iron-ore deposit is underlain at a depth of 150–250 m by an intrusion of syenites with apophyses extending upward from it, with which the ore bodies, subordinate to the same structures, are usually associated. The Taenzinsky massif is situated east of the Shalymsky one, on the watershed of the Taenza and B. Kamenushka rivers. It has a somewhat irregular form, also of submeridional extent, with a total area of 3.7 km², a maximum length of 4.5 km and a width of 3 km.

Contacts of syenites with the enclosing rocks are of various kinds: both sharp, chilled contacts and diffuse ones with uneven boundaries due to potassium metasomatism. Skarnification is observed rather often; both exoskarns and endoskarns occur. Judging from the position of the contacts of the syenite massifs and their offshoots, it is evident that the emplacement of the syenite mass was controlled chiefly by eastern and latitudinal fracture zones of brecciation.

The internal structure of the syenite body is in accordance with its external form. In particular, in the near-contact parts, as a rule, fine-grained alkaline syenites occur; with increasing distance from the contact the grain size gradually increases, and the composition also changes somewhat—plagioclase and hornblende appear. The rock texture becomes coarse-grained, sometimes porphyritic. Thus, at the contact with the enclosing rocks, rapid cooling of the melt and chilling of the syenites took place. The western and eastern (Shalymsky and Taenzinsky) massifs differ somewhat in the varieties of syenites composing them. Thus, the Shalymsky massif is composed mainly of alkaline and lime-alkaline hornblende syenites, whereas the Taenzinsky massif is represented chiefly by leucocratic lime-alkaline syenites, in the field of which hornblende lime-alkaline varieties occur as patches. Besides these varieties, alkaline syenites are also present here, though in insignificant quantity.

Alkaline quartz syenites under the microscope display a texture close to aplitic. Their mineralogical composition is as follows: K-feldspar—80–85%, quartz—10–15%, plagioclase—0–5%. Potassium feldspar is represented by tabular grains 2–3 mm in size with optical properties: \(N_g = 1.527 \pm 0.002\); \(N_m = 1.525 \pm 0.002\); \(N_p = 1.521 \pm 0.002\); \(2V = -83^\circ\), which corresponds to potassium-sodium

composition, with a content of about 25% albite molecule. Plagioclase, present in some varieties of syenites in amounts from 2 to 5%, has the following optical constants: Ng = 1.543 ± 0.002; Np = 1.535 ± 0.002; 2V = +80°, which corresponds to albite No. 10. Among the accessory minerals the usual ones are magnetite, sphene, and more rarely apatite. Secondary alterations are expressed in the intensive replacement of plagioclase by sericite. K-feldspar, in turn, acquires a brown color at the expense of pelite, albite, and carbonate.

Varieties of the quartz alkali syenite described above are, on the one hand, types that contain no free silica at all, and, on the other hand, syenites transitional to leucocratic granosyenites, with a quartz content up to 20–25%. Quartz-free syenites within the Shalymsky area occur in insignificant quantities. Granosyenites are observed mainly in certain peripheral parts of the Shalymsky syenite massif. The potassium feldspar in these varieties is of the same composition as in the quartz alkali syenites described earlier.

Lime-alkaline leucocratic quartz syenites are developed over the area of the Taenzinsky massif and make up a large part of it. Their average mineralogical composition is: K-feldspar—60%, plagioclase—30%, quartz—7–10%. Under the microscope a hypidiomorphic-granular texture is revealed, with the following sequence of idiomorphism: plagioclase, potassium feldspar, quartz. Plagioclase forms elongated tabular crystals up to 7–8 mm in size. Many of the plagioclase grains are overgrown by a rim of K-feldspar. The optical properties of the plagioclase are as follows: Ng = 1.546 ± 0.002; Nm = 1.542 ± 0.002; Np = 1.537 ± 0.002; 2V = +80°, which corresponds to oligoclase No. 13. Potassium feldspar is represented by elongate-tabular grains xenomorphic with respect to plagioclase, with optical properties: Ng = 1.527 ± 0.002; Nm = 1.525 ± 0.002; Np = 1.521 ± 0.002; 2V = −83°, which corresponds to a mineral with a content of 25% albite molecule. Quartz is usually xenomorphic with respect to plagioclase and K-feldspar and, as a rule, fills the interstices between them. Among the accessory minerals the usual ones are sphene, magnetite, apatite, and sometimes zircon. Secondary alterations in the rock are expressed in sericitization, albitization, pelitization, and weak carbonatization.

Lime-alkaline hornblende quartz syenites constitute a significant part of the area of the Shalymsky massif and a small area in the central part of the Taenzinsky massif, occurring there within the field of lime-alkaline leucocratic syenites. The average mineralogical composition of the rock is: potassium feldspar—50%, plagioclase—30%, quartz—10%, hornblende—8–10%. The texture of the rock is hypidiomorphic-granular, with the following sequence of idiomorphism: hornblende, plagioclase, potassium feldspar, quartz. The hornblende is ordinary hornblende, represented by short-prismatic crystals with rhombic cross-section; it is pleochroic from light green along Ng to colorless along Np. Its optical properties are as follows: Ng = 1.660 ± 0.002; Nm = 1.651 ± 0.002; Np = 1.643 ± 0.002; CNg = 25°; 2V = −80°. Plagioclase occurs as elongate-tabular crystals up to 6–7 mm in size with polysynthetic twinning. The plagioclase is usually intensively replaced by sericite and saussurite, so that only the outer rim of the grains is preserved, evidently substantially deacidified, showing optical properties: Ng = 1.544 ± 0.002; Nm = 1.541 ± 0.002; Np = 1.538 ± 0.002; 2V =

+80°, which corresponds to oligoclase No. 13. Potassium feldspar is represented by short-tabular grains of brownish color due to pelitization; its optical constants are: Ng = 1.527 ± 0.002; Nm = 1.525 ± 0.002; Np = 1.521 ± 0.002; 2V = −80°, which corresponds to a mineral with a content of 25% of the albite molecule. Quartz is usually xenomorphic in relation to the other minerals and is represented by irregularly shaped grains filling the spaces between them. At the contact with potash feldspar, quartz sometimes forms pegmatitic intergrowths. Secondary alterations in the hornblende calc-alkaline syenite are expressed in the replacement of hornblende by chlorite, biotite, and epidote. Sericite, saussurite, and carbonate develop abundantly after plagioclase. Potash feldspar is often replaced by albite, with the formation of microperthite and antiperthite, and also by carbonate. Accessory minerals are represented by apatite, magnetite, sphene, and zircon.

The contacts between the individual varieties of syenites are mostly gradual and indistinct.

Rocks of the III intrusive phase (dikes). Among the dike formations in the area one can distinguish pre-ore and post-ore dikes. The former include dikes of alkaline microsyenites; the latter include hornblende diorites, diabases and diabase porphyrites, as well as hornblendites.

Alkaline microsyenites are relatively limited in distribution and are usually represented by dikes 0.4–0.5 m thick, rarely up to 1 m. Their strike varies within rather wide limits—from sublatitudinal to submeridional; the dip is steep. They occur mainly within the Shalymsky ore field, in particular in the quarry, along the Chernichnyi stream and the B. Kamenushka River.

Alkaline microsyenites have a tabular-grained texture, represented by disorderly arranged grains of potash feldspar 1–1.5 mm in size. The optical properties of the potash feldspar are: 2V = −82°; Ng = 1.528 ± 0.002; Nm = 1.525 ± 0.002; Np = 1.522 ± 0.002, which corresponds to a mineral with a content of 26% of the albite molecule. Sometimes the rock has a porphyritic texture with sparse porphyritic segregations of potash feldspar grains 2–3 mm in size, with the same properties as in the groundmass. Secondary minerals are represented by sericite, albite, and, more rarely, carbonate. Among the accessory minerals, magnetite and apatite occur abundantly.

Of the post-ore vein formations, dikes of hornblende diorites are the most widespread. They are observed over the entire area of the Shalymsky area, but are especially numerous within the ore field, where they cut the ore bodies. Their strike is sublatitudinal, their dip is steep, and their thickness ranges from 0.5 to 10–15 m.

Diorites have a hypidiomorphic-granular texture and the following quantitative mineralogical composition: plagioclase—40%, hornblende—40%, potash feldspar—10%, quartz—10%. Hornblende is represented by elongated-prismatic grains with distinct idiomorphism relative to the other minerals. Its grain size is 2–3 mm. The pleochroism of hornblende is well expressed: from dark brown along Ng to light yellow along Np. The mineral has the following optical properties: CNg = 26°; 2V = −80°; Ng = 1.665 ± 0.002; Nm = 1.652 ± 0.002; Np = 1.645 ± 0.002. Plagioclase forms elongated tabular grains 1.5–2 mm in size with polysynthetic twinning. Determination of its optical constants is difficult because of its almost complete replacement by postmagmatic mine-

frames. Thus, very often only a relict form and fragments along the grain periphery remain of the plagioclase grains. Potassium feldspar is sharply xenomorphic in relation to the preceding minerals. Its optical properties are as follows: \(2V = -79^\circ\); \(Ng = 1.526 \pm 0.002\); \(Nm = 1.525 \pm 0.002\); \(Np = 1.521 \pm 0.002\), which corresponds to a mineral with a content of 23% albite molecule. Quartz is the last to separate from the melt, as is indicated by the xenomorphic form of its grains. It usually has irregular outlines and is located in the interstices between crystals of other minerals. Secondary alteration of the diorite has manifested itself very actively. Thus, hornblende has been replaced to a considerable degree by chlorite and epidote. Sericite, epidote, and zoisite develop abundantly after plagioclase. The best preserved of the rock-forming minerals is potassium feldspar; it undergoes only partial albitization and pelitization. Among accessory minerals there is much leucoxene, and magnetite is encountered.

Somewhat less widespread, in comparison with the diorites, in the Shalymsky area are dikes of diabases and diabase porphyrites. Their strike is latitudinal, their dip steep, and their thickness varies from 0.5 m to 8 m. Diabase porphyrites and diabases have porphyritic or ophitic structures and a massive texture. In the case of the diabase porphyrites, porphyritic phenocrysts up to 3–3.5 mm in size and making up 15–25% of the total mass are represented by plagioclase. Under the microscope the following mineral composition is observed in such rocks: plagioclase—50%, pyroxene—40%, quartz—10%, which corresponds to quartz diabase. The plagioclase-labradorite is intensely altered by secondary processes. Thus, among postmagmatic minerals, epidote, sericite, and zoisite develop abundantly after it. Pyroxene is represented by small grains, usually pinched between plagioclase crystals. The optical properties of the pyroxene are as follows: \(CNg = 38^\circ\); \(2V = +44^\circ\); \(Ng = 1.690 \pm 0.002\); \(Nm = 1.674 \pm 0.002\); \(Np = 1.667 \pm 0.002\), which corresponds to augite. Among secondary minerals, epidote, actinolite, and chlorite develop after it. Quartz usually occurs as small grains of isometric form, xenomorphic in relation to the plagioclases and pyroxenes. Accessory minerals are represented by magnetite, leucoxene, and apatite.

Hornblendites have been encountered on the watershed of the Bolshaya Kamenushka and Telbes rivers. The rock has a porphyroid structure with hornblende phenocrysts 2–2.5 mm in size. These grains often have, as it were, broken, split ends. The optical properties of the hornblende are as follows: \(CNg = 24^\circ\); \(2V = -80^\circ\); \(Ng = 1.661 \pm 0.002\); \(Nm = 1.653 \pm 0.002\); \(Np = 1.642 \pm 0.002\). It is necessary to note the clearly observed pleochroism in it, from green by \(Ng\) to light yellow by \(Np\). The groundmass of the rock usually has a tabular-granular structure and consists of hornblende with the same optical properties as the hornblendes in the porphyritic phenocrysts. In addition, up to 10% plagioclase occurs in the groundmass. Secondary alteration of the rock is expressed in partial replacement of hornblende by chlorite and carbonate. The plagioclase is altered considerably more strongly; sericite and zoisite develop abundantly after it. Epidote forms small nest-like accumulations, in which it is usually represented by columnar grains of yellowish color.

In conclusion, it should be noted that the dike formations of the Shalymsky area, both pre-ore and post-ore, usually have sharp contacts with chilled zones; cases of slight hornfelsing are sometimes observed. At the same time, a significant part of the dikes, for the most part post-ore (hornblende diorites, diabases), devel—

developed directly in the ore zone of the Shalymsky deposit, and also near it. These dikes quite often cut through both skarn formations and ores. It is possible that these late-ore dikes arose in a tectonically active inherited zone, which had previously served as an area of localization of iron mineralization. Thus, the presence of burrow-hornblende diorites and diabases, in all probability, can be used as a prospecting indicator for identifying new areas of iron mineralization.

Thus, within the Shalymsky area of the Kondoma iron-ore district, a gabbro-syenite magmatic complex is distinguished quite clearly; its formation took place in three successive stages, each of which is associated with the intrusion, in the form of successive portions, of basaltic magma differentiated to varying degrees. The first phase of intrusion—the gabbroid phase—is associated with the formation in the area of bodies of gabbro and gabbro-porphyrites. The second phase—the syenitoid phase—led to the formation of two large syenite massifs, the Shalymsky and Taenzinsky, with which the iron mineralization of the area is directly connected. And finally, with the last, third phase, characterized by the intrusion of vein formations—dikes—the development of the complex considered by us was completed, after which the connection with the parent magmatic center had already ceased.