Types of Iron–Titanium Mineralization in Mafic Rocks of the Patyn Complex

S. S. Il’enok*

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

Several occurrences of iron–titanium mineralization are known in the territory of Mountain Shoria. All of them are confined to exposures of gabbro massifs, which occur as the mountains Patyn, Bol’shaya Kul’-Taiga, Kupcha, Kalbuk, and a number of others. These massifs are elongated in the form of a chain of arcuate shape and mark the manifestation of a major fault which, in the eastern part of the district, adjoins the tectonic zones bounding the Tashtyp graben. The age of these massifs is Devonian, as is emphasized not only by the structural-tectonic features of their position, but also by indications of a direct connection with effusive rocks, among which interbeds of sedimentary rocks with Devonian flora occur.

Disseminated iron–titanium mineralization has also been encountered among the gabbroic rocks of the Kondoma iron-bearing complex. By the absolute age of the rocks, this complex is likewise assigned to the Devonian.

Each of the gabbro massifs has certain individual features due to differences in form, structure, and facies characteristics. This is reflected in the character and type of the iron–titanium mineralization. The latter is manifested on the most significant scale in the massifs of Mount Patyn and Bol’shaya Kul’-Taiga.

Types of Iron–Titanium Mineralization

The gabbro massifs of the Patyn magmatic complex have a funnel-shaped form and a complex layered character. The mineral composition of the contact-metasomatic aureoles, belonging to the gehlenite–monticellite facies according to D. S. Korzhinskii, confirms the near-surface conditions of their formation. Within individual horizons of the massifs the following rock types are observed: forellensteins, pyroxenites, kosvites, olivine and normal gabbros, norites, titanomagnetite gabbros, pegmatoid gabbros, leucocratic gabbros, and anorthosites. The highest contents of ore minerals are observed in titanomagnetite and pegmatoid gabbros, as well as in kosvites. As a result of prospecting and exploration work for titanium ores in the Mount Patyn area, 13 thick horizons of titanomagnetite gabbros have been exposed, although their total number is much greater. The thickest horizon of ore-bearing gabbros is located in the center of the massif and reaches

50 m thick. Together with weakly mineralized rocks, this thickness may be increased to 60–80 m.

Petrographic and geological data indicate a connection between the mineralization and the accumulation of an ore residue during crystallization of the magma and its interstitial intrusion among olivine and olivine-free gabbro, and also, in part, pyroxenites. At the same time, the paths of penetration of the melt were also zones along which block movements of the solidifying rocks (schlieren) occurred. Therefore the mineralized gabbros always bear traces of cataclasis. The ore minerals are situated not only between the grains of the rock, with the formation of a sideronitic texture, but also penetrate along cracks in plagioclase and pyroxene.

The following types of mineralization may be distinguished within the noted massifs: a) iron–titanium, b) iron–titanium–sulfide, c) sulfide.

Iron–titanium mineralization. It occurs in disseminated and massive types. To the disseminated type we assign the occurrence of ore minerals as separate grains and as their accumulations, with formation of a sideronitic texture. A moderate content of ore minerals does not ensure the coalescence of ore accumulations into a continuous mass. They are developed as small nests, usually distributed uniformly in the rock mass. Against the background of the rock most enriched in ore minerals, continuous accumulations of the latter are developed. They form lens-shaped nests elongated in accordance with the trachytoid structure of the rock. Rare resorbed inclusions of non-ore minerals occur within these nests.

The composition of the disseminated mineralization differs in the quantitative proportions of its constituent minerals. The principal ore minerals are titanomagnetite, ilmenite, maghemite, and hematite; among fine inclusions, aluminous and titaniferous spinels and rutile are observed. It should be noted that within the complex of ore minerals there are primary products of separation, products of decomposition of a solid solution, and, finally, products of oxidation and alteration of the primary minerals.

Around grains and aggregates of ore minerals, which usually compose sideronitic textures, reaction rims of brown hornblende amphibole are always observed, either alone or together with biotite. Traces of resorption of the ore minerals are evident. The deepest embayments and signs of splitting are manifested in magnetite; ilmenite is better preserved. Such reaction rims indicate that, with the deposition of ore minerals, the content of volatile components, alkalis, and silica in the residual melt increased strongly. This led to cessation of deposition of ore minerals and to the formation of iron- and titanium-bearing silicates, whose composition also includes volatile components. The features of the ratio of sodium and potassium contents in the residual melts caused the appearance not only of sodium-bearing amphibole, but also of the potassic mica—biotite. Another feature of the disseminated mineralized gabbros is their high apatite content. The content of this mineral reaches from 5–7% to 15%. High apatite contents are also characteristic of ore pyroxenites. As for the ore minerals, their content varies within different limits, more often amounting to 10–15%, more rarely up to 20–30%.

Iron–titanium minerals in separate parts of the massif are accompanied by sulfides—usually pyrite, pyrrhotite, chalcopyrite, and sphalerite. The sulfides form fine disseminations; their segregations are later than those of the iron–titanium minerals.

According to the composition and the features of the relationships among the minerals, disseminated ore-bearing formations may be subdivided into several types:

1. Substantially titanomagnetite. They contain a fine network of ilmenite lamellae having the character of exsolution of a solid solution.

2. Titanomagnetite–ilmenite. In addition to titanomagnetite with fine inclusions of ilmenite lamellae, there are coarse plates of the latter intergrown with the former. These are separate segregations of ilmenite formed contemporaneously with titanomagnetite. There are varieties of ores in which ilmenite forms independent grains.

3. Ilmenite–titanomagnetite. Ore-bearing rocks of this type show transitions to the type described above. They are distinguished by a considerable content of independent ilmenite grains, ranging from proportions equal to those of titanomagnetite to predominance over the latter.

4. Ilmenite. They form bands of small thickness. The ilmenite content in them ranges from 3–4% to 15–20%.

The first type of mineralization occurs in the upper horizons of the Patyn massif. The remaining types are widespread both on Patyn and Bolshaya Kul-Taiga, as well as in other massifs of this complex.

Pegmatoid gabbros are distinguished by coarse grain size. The grain size of the principal rock-forming minerals—labradorite and titan-augite—reaches 2–3 cm. The ore-mineral grains have the same dimensions. Here, ilmenite forms both independent grains of isometric outlines and a system of parallel broad plates within titanomagnetite. These plates have uniform thickness and cut across the magnetite grain to its boundary. The size of the plates increases toward the marginal part of the magnetite grains. Within the magnetite, fine chains of spinel are also visible.

Pegmatoid gabbros usually form thin injections parallel to the layering of the rocks of the massif. The thickness of horizons of these rocks is often small: from 2–3 cm to 20–30 cm, 1.5 m. In the northern part of the massif there are horizons of coarse-grained pegmatoid gabbros with thicknesses from 20–30 m to 50 m.

Massive iron–titanium ores have been noted both in the area of Mount Patyn and in the area of Bolshaya Kul-Taiga. In addition to large grains of titanomagnetite containing ilmenite lamellae as a result of exsolution of a solid solution, and also large independent grains of ilmenite, there are veinlets of magnetite and ilmenite here cutting the earlier minerals. Hematite is developed after magnetite.

Iron–titanium–sulfide mineralization. This type of mineralization is weakly developed. Ore accumulations form small lenses filling fractures arranged perpendicular to the banding and trachytoid texture of the rocks. This indicates their later age with respect to the iron–titanium mineralization. In the lenses, pyrite is observed, making up 60–70% of the total amount of minerals, magnetite, and chalcopyrite. At the boundary between magnetite and pyrite, traces of corrosion of the former mineral by the latter are noticeable.

Sulfide mineralization. It is manifested as mineralization of varied genesis. Thus, we have distinguished: a) disseminated sulfides of magmatic origin in mafic rocks; b) disseminated sulfides in melilite–nepheline–garnet rocks; c) sulfides of emanation origin in endo- and exocontact zones, with dissemination and nest-like accumulations.

Sulfides of magmatic origin usually form very fine dissemination among gabbro, leucocratic rocks—gabbro-anorthosites, and also in melanocratic gabbros and pyroxenites. Dissem-

this mineralization is not sufficiently abundant and is detected with difficulty. The sulfides are not accompanied by secondary hydrothermal minerals. Among the sulfides are observed pyrrhotite, pyrite, marcasite, chalcopyrite, and covellite. Marcasite and covellite are secondary minerals. The grains and globules of sulfides are probably connected with liquation of the silicate melt during its cooling and crystallization.

Among the melilite–nepheline–garnet rocks in the southern part of the massif, a fine, dense impregnation of sulfides is encountered. These rocks form dikes and vein-like and lens-like segregations among calcite rocks. At the same time, the sulfide impregnation is confined only to the indicated alkaline rocks and does not extend into the formations enclosing them. The presence of this impregnation even in small veinlets is characteristic. All this indicates their close genetic connection with the alkaline melt that gave rise to rocks of melilite–nepheline–garnet composition. Among the sulfides are observed abundant alabandite, pyrrhotite, pyrite, and rare molybdenite.

In the southern part of the Mount Patyn massif, a broad field of sulfide mineralization has been encountered, embracing the near-contact parts of the intrusive rocks as well as the enclosing rocks. Together with the sulfides, there is formation of vesuvianite, scapolite, albite, calcite, wollastonite, biotite, actinolite, and chlorite. Among the sulfides, pyrrhotite, pyrite, alabandite, pentlandite, millerite, hauchecornite, sphalerite, chalcopyrite, and rare molybdenite are developed. Of the hypergene minerals, marcasite, gypsum, and minerals of the limonite group occur. It should be noted that within the massif, in its upper horizons, small nests of sulfides in the form of pyrite and pyrrhotite are occasionally encountered. They are always surrounded by a zone of vesuvianite developed at the expense of basic plagioclase.

On the question of the origin of iron–titanium mineralization

The origin of such mineralization has been considered in a number of works. Vogt (1927) attached primary importance, in the formation of ore magma, to crystallization differentiation and gravitation. A. N. Zavaritsky (1937) regarded the titanomagnetite deposits of the Urals as fusive formations of a late-magmatic stage. I. I. Malyshev (1957), on the basis of the features of occurrence and of the structures and textures of the ores, assigned them to residual pegmatitic melts of basic magma. B. Asklund (1959), A. P. Lebedev and O. A. Bogatikov (1963, 1966), and a number of others assign leading importance to phenomena of liquation.

The petrographic features of the Patyn complex show a noticeable sequence in the crystallization of minerals and the appearance of ore iron–titanium formations at the concluding stage of rock formation. If one examines in detail the structure of the gabbroic rocks with which the ore formations are associated, what is most conspicuous is above all the early separation of basic plagioclase relative to pyroxene. Moreover, pyroxene often forms lens-like accumulations or, at times, veinlets cutting the plagioclases. This indicates migration of the melt during crystallization. Such a process explains the segregation of pyroxenites in the upper parts of individual gabbroic horizons.

Following pyroxene (titan-augite), the separation of ore minerals and apatite took place. Incidentally, during this period, after crystallization of the basic rock-forming minerals in the form of plagioclase, olivine, and pyroxene, which formed the framework of the rock, block movements occurred, and the melt, enriched in ore components, volatiles, and alkalis and under increased hydrostatic pressure, penetrated along interstices and partly along fractures, forming horizons enriched ...

ore minerals. This is confirmed not only by the confinement of enriched horizons to cataclastic rocks, but also by the local presence of such zones of cataclasis and ore deposition cutting across the parallel textures of the rocks.

A significant role in the course of differentiation of basaltic magma, and also in determining the mineralogical composition of the ore minerals, has recently been ascribed to oxygen. Particularly interesting work in this direction has been done by Kennedy (1948, 1955), Osborn (1959), Yoder and Tilley (1962), and others. Osborn notes that the direction of differentiation of basaltic magma, with the formation of either acid melts or iron-rich residues, is determined by the partial pressure of oxygen. Undoubtedly the role of oxygen in this is great, but the process cannot be considered from the standpoint of oxygen alone, since a variety of components participate in it and different external conditions are manifested.

The sequence of crystallization of the ore minerals, the degree to which titanium entered into the isomorphous mixture of titanomagnetite and crichtonite, and the appearance of exsolution structures in solid solutions were determined by the temperature regime, especially by the rate of cooling and, apparently, by the regime of volatile components.

In our earlier works, a feature of the differentiation of the Patyn magma was noted, expressed in a continuous increase in the content of feldspathic lime (according to the petrochemical characteristics of A. N. Zavaritskii). It was noted that this phenomenon is associated with assimilation by the magma of limestones, among which the massif is situated. The manifestation of this process is indicated by the features of the composition and structure of the rocks of the southern near-contact zone of the massif of Mount Patyn, where distinctive lime-alkaline rocks occur, close to turjaites.

This assimilation led to the separation of an ore melt owing to an increase in the role of ferric iron, which limited the entry of iron into the crystal lattice of iron–magnesian silicates. Part of the lime also went into the formation of apatite. Among the local differentiates there are rocks belonging to the ijolites. A certain increase in the role of alkalis in the process of ore deposition is seen in the formation, at late stages after the separation of the ore minerals, of dark-colored silicates containing alkalis. At the same time, the generally low content of alkalis and water does not provide the high mobility of iron characteristic of the formation of monzonite–syenite and granitoid complexes.

The role of oxygen changed continuously during crystallization of the ore residue. After crystallization of titanomagnetite and ilmenite there came a stage of a kind of post-ore alteration, in which the increased role of active oxygen was expressed in the oxidation of previously formed minerals and the appearance of maghemite, hematite, and rutile. In this case the indicated process was not accompanied by any noticeable hydrothermal alteration of the minerals, which indicates the absence of the influence of water.

In individual parts of the massifs, alteration of the rocks was manifested, accompanied by saussuritization, sometimes scapolitization of plagioclase, and the development of actinolite after dark-colored minerals. In this case, sphene is observed developing along the margins of titanomagnetite grains. Ilmenite remains more stable and is preserved even with complete replacement of titanomagnetite.