Magmatism and Gold Mineralization of the Lyubavinskoye Ore Field (Central Transbaikalia)

V. M. Volkov

(Presented by Professor A. M. Kuzmin)

Within the Lyubavinskoye ore field, intrusive rocks are widely distributed; by age and morphological features they can be divided into two groups: Triassic granitoids and Middle–Upper Jurassic dike rocks.

The granitoids are represented by a number of small stocks composed of porphyritic granodiorites and granites, which are probably apophyses of the Khamaro-Tyrin massif, located on the eastern flank of the ore field and assigned by N. I. Tikhomirov to the Kyrin complex, widely developed in Transbaikalia [5]. Its absolute age is 188–191 million years [4].

The Middle–Upper Jurassic dike complex, in turn, according to petrographic composition, conditions of occurrence, and age, is subdivided into a group of acidic rocks (granite-porphyries, felsite-porphyries, albitophyres) and a group of rocks of the lamprophyre series (quartz porphyrites, diorite-porphyrites, microdiorites, and spessartites).

Granite-porphyries and felsite-porphyries form a broad belt elongated in a latitudinal direction, dipping southward at an angle of 65–70°, and represented by a series of closely spaced dike bodies of varying thickness, often connected with one another. Albitophyres compose thick (5–30 m) and extended steeply dipping dike bodies of east-southeastern strike.

Rocks of the lamprophyre series are represented by numerous dikes of small thickness and extent, predominantly of east-northeastern strike, clearly cutting across the belt of acidic rocks. The age relationships of dikes within this group have not been clearly established, but there are cases of xenoliths of quartz porphyrites being found in dikes of microdioritic composition. In relation to the gold mineralization, these bodies are pre-ore.

This paper does not provide a detailed petrographic and petrochemical characterization of the magmatic rocks, nor a geological description of the deposit, since this has been done in previous works [6, 7]. It should only be noted that the rocks of the dike complex of the Lyubavinskoye ore field show a certain similarity to the rocks of the Sokhondinsky complex [5].

Postmagmatic activity at the Lyubavinskoye deposit was expressed in the formation of numerous, differently oriented quartz veins with a small content of sulfides and gold. The morphology of the veins is complex, with bends along strike and dip and sharp fluctuations in thickness. The ore textures are banded, reticulate, or brecciated. Analysis of paragenetic associations and their relationships made it possible to distinguish five stages of mineral formation [6]: arsenopyrite, quartz, quartz-sulfide, gold-sulfide, and quartz-carbonate. Among the ore minerals, arsenopyrite and pyrite are the most widespread; sphalerite, galena, chalcopyrite, gold, boulangerite, tetrahedrite, antimonite, and pyrrhotite occur in considerably smaller amounts. Scheelite has been found in some veins, especially on the western flank of the deposit; in addition, the quartz stage of the ore process is constantly accompanied by a small amount of wolframite. Near-ore alterations are expressed in sulfidization, sericitization, chloritization, silicification, and carbonatization. The width of the halo of rock alteration is 1–1.5 m. On the whole, the deposit may be assigned to the low-sulfide formation of the gold-sulfide-quartz series.

One of the important questions in the study of deposits is the consideration of genetic links between mineralization and intrusive formations. Among the principal criteria currently used to demonstrate a connection between mineralization and magmatism, in the present case the following should be indicated [3]:

a) a close spatial and age relationship of mineralization with intrusive and dike formations proper;

b) comparable depths of formation of intrusions and ore occurrences;

c) recurring associations of certain ore formations with certain magmatic formations;

d) the presence of ore minerals in intrusive rocks as accessory minerals;

e) geochemical kinship between intrusive rocks proper and postmagmatic formations.

Below, manifestations of these features at the Lyubavinskoye deposit are considered. During detailed mapping, it was established that the Lyubavinskoye gold-ore field extends as a narrow belt along a zone of increased fracturing in a Permian sandstone-shale sequence. From the south this belt is bounded by a dike belt of granite porphyries that screens the ore bodies, and from the north by a fault accompanied by lamprophyre dikes and probably representing the principal ore-controlling structure [6]. The zone of increased fracturing is possibly the surface expression of a long-lived deep fault. It controls the distribution within the Lyubavinskoye ore field of both magmatic formations and ore bodies. The dike complex of rocks of the Lyubavinskoye ore field, as noted above, is Middle–Upper Jurassic in age. Of all varieties, the youngest are the rocks of the lamprophyre series; immediately following them are the gold-bearing quartz veins. At the same time, the occurrence of quartz pebbles with gold in the Valanzhinsky conglomerates [1] indicates an Upper Jurassic age of the mineralization.

The rocks of the dike complex of the Lyubavinskoye ore field are hypabyssal formations, which is confirmed by their porphyritic structure, the variegation of their petrographic composition, and their morphological features. Analysis of mineral associations, taking into account the structural features of the deposit, the morphology of the ore bodies, their structural-textural characteristics, the nature and degree of wall-rock alterations, as well as thermobarometric data, makes it possible to conclude that the quartz–gold-ore veins of the deposit formed at depths of 1–1.5 km.

The confinement of gold mineralization to dike rocks has been discussed in the works of many researchers [2, 3]. In particular, for Transbaikalia the connection of gold-ore deposits with intrusive complexes similar to the dikes of the Lyubavinskoye ore field is established rather often [5].

The study of accessory minerals of the intrusive rocks of the Lyubavinskoye ore field showed that the lamprophyre dikes are characterized by the constant presence of a noticeable amount of arsenopyrite, pyrite, and chalcopyrite, and of single grains of sphalerite, galena, and gold. All of them take part in the ore process.

Analysis of the geochemical features of the ore process makes it possible to distinguish the following groups of elements [6]:

1. Mineral-forming: Au, As, W, Mo, Sb, Pb, Cu,
2. Specific: Sn, W, Hg, Cd, In, Bi, Cr, Go, Sr, Zn.
3. Through-going: Pb, Mo, Cu, Zn, Ag, Sb, Mn, Co, Ti, As, Ba.

In the dike rocks, spectral analysis revealed a suite of trace elements similar to the suite of elements of the ore process, with the exception of Nb, Hf, and Ge, noted only in the magmatic rocks, and W and In, present in the ores. In addition, as shown by the study of mineral associations and wall-vein alterations, throughout all stages of mineral formation the solutions were substantially potassic, weakly alkaline or neutral, which is demonstrated by the presence of carbonates in the veins, the formation of potassic mica—sericite, and also by a considerable introduction of potassium into the host rocks during wall-rock alterations. At the same time, the study of the dike rocks indicates the manifestation of intensive potassic autometasomatism of dikes of the lamprophyre series.

All the above makes it possible to conclude that there is a paragenetic connection (in the sense of E. T. Shatalov [3]) between gold mineralization and the lamprophyre series of rocks of the dike complex. This circumstance, taking into account their close spatial relationship, has practical significance as a reliable prospecting criterion for gold mineralization.

REFERENCES

1. N. N. Bindeman, Yu. V. Sokolova. On the question of the age of small intrusions and gold mineralization in the area of the Lyubavinsko-Khaverginsky ore cluster. — Transactions of TsNIGRI, issue 50, 1962.

2. M. I. Konychev. Relationships of gold mineralization with dikes of intrusive rocks in the Yana-Kolyma folded zone. “Geologiya i razvedka,” 1971, No. 2.

3. V. S. Koptev-Dvornikov, M. G. Rub. Criteria for the connection of mineralization with intrusive complexes. — In: Criteria for the connection of mineralization with magmatism as applied to the study of ore districts. “Nedra,” Moscow, 1965.

4. N. I. Polevaya, N. I. Tikhomirov. Age of some intrusive complexes of Transbaikalia according to geological and radiological data. — In: Materials on the petrology of the granitoids of Transbaikalia, 1962.

1. N. I. Tikhomirov et al. Intrusive Complexes of Transbaikalia. Nedra, Moscow, 1964.

2. G. V. Shubin, S. A. Chubarov et al. Structure of the ore field and genesis of the ores of the Lyubavinskoye deposit (Transbaikalia). — In: Geology of Gold-Ore Deposits of Siberia. Nauka, 1970.

3. G. V. Shubin, V. M. Volkov. The dike complex of rocks of the Lyubavinskaya ore zone (Central Transbaikalia). — Izv. TPI, vol. 217, 1971.