Types of Gold-Ore Mineralization in the Daurian Zone of Central Transbaikalia

G. V. Shubin (TPI)

This paper presents material obtained by us beginning in 1959 during large-scale geological-structural mapping at the Ilyinsky, Dybyksinsky, Lyubavinsky, and Khaverginsky deposits. An attempt is made to consider this material together with an analysis of the types of gold-ore mineralization of the indicated deposits.

By a type one should evidently understand such mineralization as is typical for a group of deposits characterized by productive mineral associations similar in composition, stable, and by specific wall-rock alterations formed under similar geological conditions.

Taking the above into account, we propose distinguishing the following types of gold-ore mineralization in the region under study:

1. Gold–quartz–sulfide (Lyubavinsky and Khaverginsky deposits).
2. Gold–sericite–sulfide (Ilyinsky deposit).
3. Gold–albite–actinolite–bismuthinite (Dybyksinsky deposit).

The systematization presented for the largest gold-ore deposits of the region makes it possible, in a certain way, to reassess the enormous number of gold ore occurrences found in the areas of the above-mentioned deposits.

We shall now consider some specific and common features of the indicated types of gold-ore mineralization.

The gold–quartz–sulfide type is characterized by two mineral subtypes: arsenopyrite and quartz–carbonate–sulfide. The productive association is represented by arsenopyrite, galena, boulangerite, and tetrahedrite; to a lesser extent also by sphalerite, pyrrhotite, and chalcopyrite. The specific elements of the ore process are Bi, Sn, W, Hg, Cd, In, Cr, Ga, Sr.

Wall-rock alterations were expressed in silicification, more rarely sericitization and chloritization; in places the development of tourmaline is noted. A vein type of mineralization is characteristic [12].

The gold–sericite–sulfide type is characterized by two mineral subtypes: sericite–pyrite, in rare cases with tourmaline, and quartz–carbonate–sulfide. The productive association, apart from

pyrite, tetrahedrite, and chalcopyrite, and is represented to a lesser extent also by arsenopyrite, sphalerite, and galena. The specific elements of the ore process are Bi, Sn, and Ag. The wall-rock alterations are expressed in sericitization, berezitization, silicification, chloritization, and tourmalinization. A stockwork-disseminated type of mineralization is characteristic [5].

The gold–albite–actinolite–bismuth type is characterized by a productive association represented by bismuth, more rarely by chalcopyrite, arsenopyrite, and molybdenite. The specific elements of the ore process are Ag, Zn, Pb, Sn, and Sb. The wall-rock alterations are expressed in actinolitization, albitization, phlogopitization, apatization, and silicification. A metasomatic (Dybyksa) and a vein (Slavyanka ore occurrence) type of mineralization are characteristic [11]. The composition of the mineral associations, the metasomatic character of their manifestation, and the confinement of the mineralization to distinctive pre-ore and wall-rock alterations make it possible to raise the question of distinguishing a new and independent type of gold-ore deposits [6, 11].

The gold-ore deposits of the region under consideration formed during the period of Mesozoic activation of Paleozoic structures, corresponding to an independent structural element of the Earth’s crust known to us as the “Sibiretin structure.” Development of the deposits took place within the volcanogenic-sedimentary Mesozoic structural stage. A close connection is noted between the mineralization and the post-batholithic Middle–Upper Jurassic complex of dike rocks.

Gold-ore deposits are closely confined to major tectonic zones of increased fracturing that mark deep faults at the surface. Within such zones there is a manifestation of longitudinal horizontal and corresponding vertical zoning of ore-localizing structures and mineral types [1]. Thus, the stockwork-disseminated mineralization of the Il’inskoe deposit proper, of the gold–sericite–sulfide type, is replaced westward, along the strike of the tectonic zone, by the metasomatic type of gold–albite–actinolite–bismuth mineralization of the Dybyksinskoe deposit and then passes into the vein type of mineralization (the Slavyanka ore occurrence). At the Lyubavinskoe quartz-vein deposit, horizontal zoning within the tectonic zone is expressed in the westward replacement of gold–arsenopyrite ores (Nikolaevskii sector) by gold–polysulfide mineralization (Bol’she-Fedorovskii sector). This horizontal zoning reflects the vertical zoning observed within the noted deposits and their individual ore bodies.

Both longitudinal horizontal and vertical zoning may be regarded as zoning caused by the imbrication of an ore belt as a result of tectonic deformations, leading to the successive development of ore-localizing structures along the tectonic zone. The observed pattern of interrelation between structures and mineral types for deposits of the Daurian zone has much in common with a similar interrelation for deposits of the Kuznetsk Alatau [1]. This circumstance makes it possible to raise the question of the existence of general regularities in the formation and distribution of mineralization in regions of different age.

The types distinguished are characterized by medium- to high-temperature mineral associations. For example, the study at the Lyubavinskoe deposit of two decomposition systems—sphalerite–chalcopyrite–pyrrhotite and chalcopyrite–sphalerite—makes it possible to estimate the temperature of mineral formation within the range from 300 to 470°. Materials on thermobarometry

inclusions in minerals, carried out for the deposits in 1968 by Yu. V. Lyakhov and A. V. Piznyur, indicate temperatures on the order of 205–260° and even up to 450° for quartz-tourmaline formations.

The mineral associations described were formed, according to the depth criteria of D. I. Gorzhevsky and V. N. Kozerenko [2], at a depth of 1–2 km under alkaline-environment conditions [12], with transport of the ore component in complex compounds of the type \(K[Me(S_2O_3)_2]\) and \(Na[Me(Cl,F)_4]\). There is continuity between the autometasomatic processes of the intrusive complex and the subsequent postmagmatic solutions, which were predominantly alkaline in character. This conclusion is supported by the study of near-ore alterations and the composition of ore veins, where such minerals as albite, anorthoclase, sericite, chlorite, carbonates, and apatite are noted. All these minerals readily develop in dike rocks during autometasomatism, not to mention the accessory minerals (arsenopyrite, pyrite, gold, apatite, etc.) of intrusive rocks, which to a considerable extent are the principal components of the ore bodies.

Analysis of all the events makes it possible to note that the alkaline nature of the early stages of the ore process was influenced to a certain degree by alkaline processes of autometasomatism of intrusive formations. The development of these processes led to subsequent ore formation. Further study of such phenomena will open broad possibilities for analyzing the genetic relationships of mineralization and magmatism.

The complexity of the tectono-magmatic development of the region during the period of Mesozoic activation naturally also influenced the evolution of ore-bearing solutions, which, evidently, led to the formation of rather “variegated” mineral types. A. D. Shcheglov [13] is entirely correct in noting that in such “activation” structures deposits of new mineral types may be identified (for example, the Dybyksinskoye deposit). Despite all this complexity, a fairly clear connection is established among individual deposits through mineral subtypes. And, finally, all the distinguished types of gold-ore mineralization, taken together, may be united into a single genetic group of medium- to high-temperature hydrothermal deposits; into a single ore complex associated with a complex of small intrusions of dioritic composition or, possibly, as is now inclined to be considered, with the volcano-plutonic formation of Central Transbaikalia. At the same time, all these formations arose under extremely similar geological conditions.

The materials we have considered may subsequently be used as a basis for systematizing deposits and distinguishing gold-ore formations of the region. For this purpose, using the above-mentioned deposits and their types of gold-ore mineralization as examples, it is evidently necessary to take into account: 1) the principal minerals that predominate in the ore bodies of a deposit or give it industrial value [3]; 2) minerals that determine the productive association (mineral type); 3) near-ore alterations specific to these deposits.

The significance and principles of using the first criterion, i.e., the composition of the principal minerals, in determining the names of formations are given in considerable detail in the works of V. A. Kuznetsov [4], R. M. Konstantinov [3], and others.

At present, for the gold-ore deposits of the Daurian zone, the basic (principal) minerals determining the names of formations are considered to be gold, quartz, and tourmaline. Proceeding from this, within the Daurian zone a gold-quartz formation is distinguished, rela-

the Lyubavinskoye and Khaverginskoye deposits belong here, and the Ilyinskoye and Dybyksinskoye deposits belong to the quartz-gold-tourmaline formation.

It seems to us that such an approach is insufficient. In fact, tourmaline also occurs in deposits assigned to the gold-quartz formation. Further, quartz is not everywhere unambiguous as one of the principal minerals. If for the Lyubavinskoye and Khaverginskoye deposits it may fully be considered the principal mineral, then for the Ilyinskoye and Dybyksinskoye deposits quartz does not play a strictly determining role. Moreover, the “scope” of the gold-quartz formation is so indefinite that an extremely large number of different gold-ore deposits are often assigned to it, which may lead to the effacement of a whole series of important features of endogenous mineralization that are specific in a metallogenic respect. Suffice it to say that there are tendencies toward distinguishing, for Central Transbaikalia in general, a single gold-tourmaline formation for all deposits [10]. As can be seen, quartz here no longer figures in the name of the formation, which is not entirely justified; but tourmaline remains, and another extreme appears. It seems to us that tourmaline can hardly be regarded as the mineral determining the type of formation for deposits of the Daurian zone, as I. A. Preobrazhenskii pointed out for this district as early as 1945 [7], noting that in most cases the coexistence of gold and tourmaline may testify to the “accidental” joint occurrence of these minerals.

Thus, in our opinion, the formations being distinguished do not fully reflect the genesis of the deposits under study. This difficulty can evidently be overcome if, in distinguishing formations, we rely on the totality of all the criteria indicated above. This will make it possible to reflect the specificity of the formations being distinguished.

In studying deposits, for the purpose of subdividing them into formations, the establishment of productive mineral associations (mineral type), which determine the time and place of appearance of the industrially valuable mineral and serve as a reliable criterion for distinguishing mineral types of definite ore formations, is of great importance. The subtypes distinguished by D. A. Timofeevskii [9] for deposits of the Darasun type may serve as a vivid example of this. It must be noted that here the matter is not any paragenetic association, but specifically a “productive” one [8], which must be stable for a definite formation. Mineral types may be the connecting link between ore formations that formed under similar geological-structural conditions. In this case, throughgoing mineral types must be noted.

Considering the deposits of the Daurian zone with respect to the principal minerals of the productive associations, it may be noted that for the Lyubavinskoye and Khaverginskoye deposits such minerals are arsenopyrite, galena, boulangerite, and tetrahedrite; for the Ilyinskoye deposit—pyrite, tetrahedrite, and chalcopyrite; and for the Dybyksinskoye deposit—bismuthinite, more rarely chalcopyrite.

Now concerning the last and no less important criterion—wall-rock alteration. What is meant here are specific wall-rock alterations characteristic only of a definite type of mineralization. Thus, for the Ilyinskoye deposit, sericitization and berezitization are specific and characteristic [5]. For the Dybyksinskoye deposit, albitization and actinolitization are typical [11]. For the remaining deposits, processes of silicification are characteristic, and even these on a very small scale.

All the above-mentioned circumstances, as it seems to us, must be taken into account when speaking of the gold-ore formations of the Daurian zone, where, in this respect, essentially only the first steps are being taken. At present the question of developing genetic criteria for formational analysis on a geochemical basis for various types of endogenous mineralization is becoming acute.

REFERENCES

1. Vasil’ev B. D., Shubin G. V. General features of the zonality of gold mineralization, based on the example of certain districts of the Kuznetsk Alatau and Central Transbaikalia. In: Problems of the Geology of Gold Deposits of Siberia. Publishing House of Tomsk State University, 1968.

2. Gorzhevskii D. I., Kozerenko V. N. Facies of depth of magmatic rocks and magmatogenic deposits of useful minerals. Sov. Geol., 1965, No. 8.

3. Konstantinov R. M. Methods of study and criteria for distinguishing magmatogenic ore facies in large-scale metallogenic investigations (based on the example of Eastern Transbaikalia). In: Endogenous Ore Formations of Siberia and the Far East. Nauka Publishing House, 1966.

4. Kuznetsov V. A. Genetic groups and formations of ore deposits and their significance for metallogenic analysis. In: Endogenous Ore Formations of Siberia and the Far East. Nauka Publishing House, 1966.

5. Kuz’min A. M., Shubin G. V. The Il’inskoe deposit (Eastern Transbaikalia). Publishing House of Tomsk State University, 1968.

6. Korobeinikov A. F. Contact-metasomatic and hydrothermal formations of the Kommunar gold-ore field. In: Problems of the Geology of Gold Deposits of Siberia. Publishing House of Tomsk State University, 1968.

7. Preobrazhenskii I. A. On a quartz–tourmaline gold-bearing formation. Izv. AN SSSR, Geological Series, No. 4, 1945.

8. Petrovskaya N. V. On productive mineral associations in gold-ore deposits. ZVMO, vol. 84, issue 3, 1955.

9. Timofeevskii D. A. On types of mineralization of Jurassic gold-ore deposits. Proceedings of the Third Scientific Conference of ZabNII. Geology and Exploration of Mineral Deposits of Transbaikalia, Chita, 1968.

10. Fogel’man N. A. Geological conditions of formation of the Tura–Il’inskii gold-bearing district of Central Transbaikalia and assessment of its prospects. Transactions of TsNIGRI, issue 63, 1965.

11. Shubin G. V. Processes of hydrothermal alteration and ore deposition at the Dybyksinskoe gold deposit (Eastern Transbaikalia). In: Problems of the Geology of Ore Deposits of Transbaikalia, Chita, 1963.

12. Shubin G. V., Matsoshevskii A. V. Genetic features of the Lyubavinskoe gold-ore deposit. In: Problems of the Geology of Gold Deposits of Siberia. Publishing House of Tomsk State University, 1968.

13. Shcheglov A. D. Mesozoic endogenous ore formations of activated folded regions of Transbaikalia. In: Endogenous Ore Formations of Siberia and the Far East. Nauka Publishing House, 1966.