FEATURES OF THE GEOLOGICAL STRUCTURE OF THE ITATKA GOLD ORE DEPOSIT (EASTERN TRANSBAIKALIA)

K. M. Melnikova, L. P. Stafeeva, I. A. Shevyrev (MGRI),
V. S. Alkin (ChGU)

The Itatka gold ore deposit is located southwest of the Aldan Shield, within the gold–molybdenum belt (after S. S. Smirnov), and is confined to the western part of the Mogochinsk geoanticlinorium of northeastern, nearly latitudinal strike. In the core of the anticlinorium, deeply reworked Archean formations are exposed. Among the Archean rocks, V. P. Kirilyuk and V. S. Alkin (1965) distinguished three suites, differing in composition and degree of metamorphism: the Amazar, Amazarkan, and Yarka suites. In terms of composition and the nature of alternation of rocks, these suites are correlated with the Timpton (charnockite) and Dzheltulin series of the Aldan complex. They are characterized by linear folds of various trends. Tectonic movements of Late Paleozoic–Mesozoic time were expressed in the formation of extended fault zones and zones of fracturing, which divided the area into blocks. The Itatka depression, a graben filled with Cretaceous deposits, is also bounded by them; stocks of subvolcanic dacite porphyries are confined to the margins of the graben. By one of the faults—the Alekseev fault—the ore field is divided into two large blocks: the northwestern block (the Surmyanaya Gorka sector) and the southeastern block (the Maleevsky sector), which possess certain features of structure and material composition of the ores.

Stratified Archean formations and magmatic rocks of the Lower Proterozoic, Lower Mesozoic, and Upper Mesozoic intrusive complexes take part in the geological structure of the ore field (Fig. 1).

Among the Archean group of rocks the following varieties are distinguished: biotite–sillimanite–disthene, biotite–cordierite crystalline schists; garnet, garnet–biotite, and two-pyroxene gneisses; and, most widespread within the ore field, garnet plagiogranite-gneisses and granodiorite-gneisses, which are products of regional granitization of the Archean sequence. All of them are distinguished by a regressive trend of metamorphism. These rocks are intruded by Lower Proterozoic gabbro-amphibolites, partly transformed into magnesian and garnet–pyroxene skarns.

Mesozoic magmatism was manifested in the formation of intrusive rocks of the Lower Mesozoic (Amanan) and Upper Mesozoic (Amudzhikan) intrusive complexes. The first is represented by biotite and biotite–hornblende granites, granodiorites, and granosyenites (to monzonites), composing a fissure-type intrusive body elongated in a northeastern direction. The Amudzhikan complex is represented by granodiorite porphyries, low-quartz diorite porphyrites, microgranites, granite porphyries, hybrid porphyries, hornblende diorite porphyrites, and olivine-basalt porphyrites. In the Maleevsky area, moreover, a stock of dacite porphyrites is known, also assigned to the Amudzhikan complex. In places, dikes of hornfelsed microgranites and spessartites have been encountered, which we provisionally assign to pre-Mesozoic time.

The principal structural elements of the ore field are rupture dislocations of different trends that inherit more ancient structural elements. Within large blocks with relatively large displacement amplitudes along secondary ruptures, smaller blocks are formed.

The ore bodies in the Surmyanaya Gorka area are represented by comparatively persistent quartz veins and are confined mainly to the sublatitudinal–northeastern Main Vein Zone. The latter, in the central part of the deposit, is a thick (40–50 m) zone of crushing dipping to the northwest and southeast, containing veinlet-disseminated quartz–sulfide mineralization and gold-bearing quartz veins. The thickness of the ore veins varies from a few centimeters to 1.5–2 meters.

The ores of the Itaka deposit, according to the classification of N. V. Petrovskaya (1967), belong to the moderate-sulfide formation, in which the amount of sulfides is 10–30% of the ore material. The process of hydrothermal mineral formation at the deposit (see diagram) proceeded in five successive stages, which are clearly distinguished by the relationships of mineral aggregates of different ages: quartz–pyrite–molybdenite, quartz–arsenopyrite (1st productive), quartz–polymetallic (2nd productive), quartz–antimonite, and calcite–fluorite.

Each stage is characterized by its own hydrothermal-solution composition, sharply different from the others, and by corresponding alterations of the enclosing rocks.

Gold mineralization was formed mainly in the quartz–arsenopyrite and quartz–polymetallic stages. The ore veins of the first productive stage consist of cryptocrystalline dark-gray quartz with finely disseminated pyrite and arsenopyrite, with which the gold is associated. The chief vein mineral of the second productive stage is white saccharoidal medium- and coarse-crystalline quartz, grading to comb quartz.

The temperature of formation of quartz of the 1st productive stage, according to data of S. M. Ivasiv (LGU), is 300–290°. Deposition of ore quartz was preceded by quartz–sericite alteration of the enclosing rocks. In this stage two mineral associations are distinguished: quartz–gold–arsenopyrite and quartz–ankerite.

The second productive association—the quartz–polymetallic one—is associated with the formation of veins and veinlets of white saccharoidal quartz, passing in cavities and along vein selvages into semitransparent comb quartz with polymetals, sulfosalts, carbonates, and hematite. It is accompanied by early argillization and sericitization. Gold in this stage is associated with galena, chalcopyrite, gray ore,—

Fig. 1. Diagram of the geological structure of the ore field

1 — alluvial, deluvial, and deluvial–eluvial deposits, \(Q_{3-4}\); 2 — Lower Cretaceous volcanogenic–sedimentary deposits of the Itatka depression, \(C_{1,3}\); 3 — Archean granite-gneisses, \(A\); 4 — massive basaltic porphyrites and olivine basalts, \(Mz_2\); 5 — hybrid porphyries, \(Mz_2\); 6 — quartz diorites and diorite porphyrites, \(Mz_2\); 7 — lamprophyres, \(Mz_2\); 8 — lamprophyre porphyrites, \(Mz_2\); 9 — granodiorite-porphyries and spessartites, \(Mz_2\); 10 — pre-Mesozoic spessartites and gabbro-amphibolites, \(Pz_2\); 11 — granites, \(Pz_2\); 12 — gabbro-amphibolites and gabbro-amphibolites with aplite dikes; 13 — block-forming faults; 14 — smaller ruptures; 15 — gold-bearing quartz veins and near-vein alteration of rocks; 16 — elements of unconformity; 17 — fractures.

tennantite (principal minerals), enargite, bismuthinite, jamesonite, bournonite, altaite, and bornite (secondary minerals).

Gold of the 2nd productive association was found in macroscopically visible grains 1–3 mm in size, in paragenesis with white saccharoidal quartz and jamesonite. The temperature characteristic of the white saccharoidal quartz, determined by S. M. Ivasiv (Leningrad State University) by the method of homogenization of two-phase inclusions, is 230–220°C* (Main vein zone).

The temperature regime, higher at the beginning of each stage, gradually decreases toward its end. A feature of the Itatka deposit, distinguishing it from many similar medium-temperature gold deposits of Eastern Transbaikalia, is the absence of tourmaline in its ores and host rocks. This is apparently explained by the fact that the deposit formed below the depth level optimal for the deposition of large masses of tourmaline (N. V. Petrovskaya, 1967).

The principal ore-hosting structure of the deposit, Surmyanaya Hill—the Main vein zone, has been traced along strike for more than 2 km. At the surface it consists of broad, locally coalescing bands of metasomatically altered rocks (quartz–K-feldspar alteration, silicification, beresitization, early and late argillization, sericitization, and carbonatization). The width of individual bands of altered rocks ranges from 5–6 to 20–30 m.

The internal structure of the Main vein zone is fairly complex. On the hanging-wall and footwall sides it is bounded by extended tectonic seams with friction clay. The rocks of the footwall are, as a rule, more strongly crushed and silicified. The central part of the Main vein zone is filled by one or two veins of dark-gray cryptocrystalline quartz with fine sulfide disseminations and veinlets of white quartz, which cement fragments of fine-grained quartz and ankerite. In places, “rolled” fragments of the enclosing rocks and of quartz of earlier generations are very abundant.

The first tectonic displacements along the Main vein zone apparently arose simultaneously with the intrusion of the gabbroids. During subsequent tectonic stages this zone repeatedly opened slightly. Movements of Late Mesozoic age along it apparently had a reverse-strike-slip and normal-strike-slip character. This can be judged from the relationships of the principal faults with the fractures feathering them. Movements were repeatedly renewed during the process of ore formation and later. This is evidenced by the predominance of breccia-like and brecciated ore textures, as well as by the presence of slickensides with mineralized friction clay within them.

In addition to the Main vein zone, veins of northwestern (290–300°) trend are known at the deposit. Thus, in the southern part of the area two veins have been traced, dipping steeply to the southwest, one of them being controlled by the contact with a dike of hybrid porphyries. The veins are composed of fine-grained semitransparent gray quartz, differing not only in appearance but also in crystallization temperature from the quartz making up the 1st productive association of the Main vein zone. The homogenization temperature of gas-liquid inclusions from the ore-bearing quartz of the northwestern-trending veins is 290–285°C, and that of inclusions from the saccharoidal quartz of the vein zone is 270–230°C.

* Note. The homogenization temperature of gas-liquid inclusions is given without correction for pressure.

Several faults of submeridional strike have been identified at the deposit; they also play an essential role in the structure of the ore belt. These are the 1st and 2nd Western, the Central, and the Eastern faults in the Surmyanaya Gorka area, and two submeridional faults in the Maleevsky area. Block displacements during the pre-ore stage led to the partial opening of fractures supported by them, of northeastern strike, and to the intrusion of a series of dikes of lamprophyres and porphyrites. During the period of ore formation, these disturbances were inaccessible to hydrothermal solutions and screened them. In the post-ore period, movements along the near-meridional disturbances continued. Thus, the block of rocks enclosed between the 1st Western and Central faults was downthrown in pre-ore and intra-ore time. The main mass of low-temperature antimony ores is localized in it, the exploitation of which gave the Surmyanaya Gorka deposit its name. The block situated to the east was apparently uplifted and therefore contains almost no antimony ores. In this block the quartz–arsenopyrite mineral association predominates.

At the deposit, pulsational zoning has been established, closely connected with the zoning of deposition. It was caused by repeated partial opening of fractures and by the acid–alkaline evolution of solutions at different stages of mineralization. Considering changes in the composition of the predominant mineral associations vertically within one of the blocks, from the surface to the adit level, we see a replacement of quartz–antimony mineralization by quartz–polymetallic and quartz–arsenopyrite mineralization with early sulfides.

In the post-ore period, at least two major stages of tectonic movements are recorded for the Itatka deposit. The first of them led to erosion of the roof of the granodiorite massif, which supplied material for filling the Itatka depression in Cretaceous time. The second major uplift of the Itatka massif occurred in Cenozoic time. These movements led to the formation of a horst with the most uplifted central part—the summit of Surmyanaya Gorka—to the subsidence of the previously uplifted Maleevsky area, and to its displacement southeastward along the fault bounding the area on the east and separating it from the Itatka depression. Repeated tectonic movements led to the creation of the modern block structure of the ore field.

REFERENCES

Alkin V. S. and Kirilyuk V. P. On the stratigraphy of the Archean of the southwestern margin of the Aldan–Vitim Shield. Proceedings of the Transbaikal Department of the Geographical Society, vol. I, issue 4, 1965.

Petrovskaya N. V. Mineral associations in gold-ore deposits of the Soviet Union. Transactions of TsNIGRI, issue 76, 1967.

Scheme of the Sequence of Mineral Formation

Itatka Deposit

Legend:
■ Main minerals
▬ Rarely occurring minerals
▰ Ore minerals
⇔ Minerals not precisely established / not exactly determined in time