PHYSICOCHEMICAL PATTERNS OF THE ORE-FORMING PROCESS AT GOLD–MOLYBDENUM DEPOSITS IN EASTERN TRANSBAIKALIA

Yu. V. LYAKHOV, A. V. PIZNYUR (LGU)

The ore-forming process at most molybdenum and gold-ore deposits (quartz–tourmaline ore formation) of Eastern Transbaikalia was characterized by a rather prolonged, multistage history of development. Usually it proceeded in four to five stages of mineralization, during which paragenetic associations arose in a strictly definite sequence, stable both for molybdenum and for gold-ore deposits [5]. For the group of deposits under consideration, the following scheme of stages is the most general: 1) quartz–molybdenite; 2) quartz–tourmaline; 3) pyrite–arsenopyrite–quartz; 4) gold–polymetallic; 5) quartz–carbonate stages. The only exception is the earliest, quartz–molybdenite stage, which is weakly manifested in the composition of gold–molybdenum deposits and is completely absent in gold-ore deposits proper. The fourth (polymetallic) stage behaves in a completely opposite way: the degree of its development regularly decreases from gold-ore to molybdenum deposits.

The commonality of the scheme of ore-formation stages at gold and molybdenum deposits apparently indicates that they belong to a single genetic group of endogenous formations, which is well confirmed by data from mineral thermobarometric analyses of inclusions of mineral-forming media in minerals from these deposits. On the basis of material on the thermobarometry of inclusions in minerals from molybdenite (Zhireken, Davenda), gold–molybdenum (Klyuchevskoye, Shakhtaminskoye, Golgotaiskoye, Shirginskoye), and gold-ore (Darasunskoye, Chernozilunikhinskoye, Kazakovskoye, Fatimovskoye, Maiskoye, Lyubavinskoye) deposits, it appears possible to assert that both hydrothermal and pneumatolytic solutions participated in their formation. At the same time, the mineral-forming activity of the latter was manifested exclusively in the early (pneumatolytic–hydrothermal) period of the ore process. This period encompassed the quartz–molybdenite and quartz–tourmaline stages. At some deposits, the emplacement of a dike complex (Davenda, Klyuchi, Zhireken) or the formation of explosive breccias, which usually preceded the quartz–tourmaline stage (Zhireken, Darasun), dates precisely to this time. At individual deposits (Darasunskoye, Chernozilunikhinskoye)

the mineral-forming role of gases was also noted at the beginning of the third stage of early sulfides, which, apparently, is explained by the location of these objects near paragenetically related magmatic formations. Each of the listed stages at the indicated objects began its development exclusively under conditions of the gaseous state of the mineral-forming solutions, which, gradually becoming denser as the temperature decreased, were regularly transformed into high-temperature hydrotherms. In the quartz–molybdenite stage these solutions at the molybdenum deposits proper (Zhireken) were characterized by a high content of sodium and potassium chlorides, whereas at the gold–molybdenum Golgotai deposit the carbon dioxide component predominated in them. In the second, quartz–tourmaline, stage the gaseous solutions were characterized chiefly by a carbon dioxide composition (Chernozipunikhinskoye, Kazakovskoye, Fatimovskoye). The development of the third stage of early sulfides for most deposits (with the exception of the Darasunskoye and Chernozipunikhinskoye deposits) marked the beginning of the hydrothermal period proper of their development. It also included the subsequent gold–polymetallic stage and the quartz–carbonate stage that completed the process. The solutions of this period of ore formation, judging from data on aqueous extracts, were predominantly of a hydrocarbonate–calcium–magnesium character.

No less clearly, the similarity of the ore process for the deposits under consideration is also indicated by a comparison of the temperature and pressure of the solutions in the corresponding stages. The highest-temperature conditions (above 300–350°C) are noted for the quartz–molybdenite stage. At the same time, molybdenite deposition was confined to a rather narrow temperature interval: 420–370 (Zhireken), 380–350 (Shirga), 370–325 (Golgotai), 390–360°C (Klyuchi). The pressure during this period apparently reached 1000 atm. The quartz–tourmaline stage was also characterized by high-temperature conditions, on the order of 450–290°C. The exception is the Zhireken deposit, where temperatures of 530–380°C were recorded for this stage. The pressure during this stage apparently exceeded 1000 atm. High-temperature conditions are also characteristic of the stage of early sulfides (from 420–230°C at Zhireken to 290–240°C at the Lyubavinskoye deposit). However, at most deposits this stage developed within a narrower temperature range, on the order of 350–250°C. The pressure, in this case, evidently reached 800–1000 atm. The most stable for all deposits was the temperature interval of deposition of the gold–polymetallic association—280–200°C: Lyubavinskoye—260–205; Maiskoye—235–215; Fatimovskoye—255–210; Kazakovskoye—250–210; Chernozipunikhinskoye—295–235; Darasunskoye—280–190; Klyuchevskoye—280–240; Davendinskoye—270–220; Golgotaiskoye—255–200; Shirginskoye—250–210°C. The pressure during this period is estimated in the range of 560–750 atm. The quartz–carbonate stage, concluding for all deposits, developed over a broad temperature range, encompassing medium- and mainly low-temperature conditions from 220–200°C to 80–50°C. In this case the pressure averaged 200–400 atm.

The patterns noted above in the development of the ore-forming process at the deposits of the gold–molybdenum series are especially clearly visible from Fig. 1. Comparison of the schemes of ore-forming processes by a graphical method in the temperature–relative time system (in characterizing the evolution of the aggregate state of the solution) makes it possible to identify not only the general features of their development, but also, on the basis

Comparative Characteristics of the Stages of Formation of Gold and Molybdenum Deposits

Depth of formation of deposits, km (and the present erosional level)

Temperature, \(T^\circ\mathrm{C}\)

Molybdenum deposits

• Molybdenum-porphyry
  Tyrnyauz deposit
• Molybdenum-skarn
  Dzhidinskoe deposit
• Molybdenum-quartz
  [[unclear: deposit name]]

Gold–molybdenum deposits

• Molybdenite–quartz
  Shakhtama deposit
• Molybdenite–quartz
  Naplyvno-Kvarstevoe [[unclear]]
• Gold–quartz–sulfide
  Tolstakoe deposit
• Gold–sulfide
  Klyuchevskoe deposit
• Gold–polymetallic
  Darasun deposit
• Gold–polymetallic
  [[unclear: deposit name]]
• Gold–polymetallic
  [[unclear: deposit name]]

Mineral stages shown in the diagram

• Quartz–molybdenite
• Quartz–molybdenite–scheelite
• Quartz–molybdenite with pyrrhotite
• Quartz–molybdenite with wolframite
• Quartz–scheelite–molybdenite
• Quartz–molybdenite with pyrite
• Quartz–molybdenite–arsenopyrite
• Quartz–molybdenite
• Quartz–pyrite
• Quartz–molybdenite–galena
• Quartz–pyrite
• Quartz–pyrite–arsenopyrite
• Quartz–pyrite
• Quartz–molybdenite–pyrite
• Quartz–gold–pyrite
• Quartz–pyrite
• Quartz–gold–pyrite
• Quartz–molybdenite
• Quartz–sulfide
• Quartz–molybdenite
• Quartz–scheelite
• Quartz–molybdenite
• Quartz–pyrite
• Quartz–polymetallic
• Quartz–pyrite–arsenopyrite
• Quartz–gold–pyrite
• Quartz–polymetallic with pyrrhotite
• Quartz–polymetallic with pyrrhotite
• Quartz–scheelite
• Quartz–scheelite–molybdenite

Optimum time

Fig. 1. Aggregate states of solutions:
1 — gaseous; 2 — first condensation (critical); 3 — liquid; 4 — composition [[unclear]]; 6 — optimum conditions for molybdenite precipitation; 7 — optimum [[unclear]].

[[Figure]] 1. [[unclear: mineralization stages]] and the temperature regime of deposits in Eastern Transbaikalia

Temperature scale: \(T^\circ\mathrm{C}\): 500, 400, 300, 200, 100, 30.

Relative time →

Gold

• Magmatic
• postmagmatic
• telemagmatic
• Gold
• arsenopyrite–quartz
• pyrite–quartz
• pyrrhotite
• quartz
• Gold
• arsenopyrite–quartz
• marcasite
• Gold
• arsenopyrite–quartz
• marcasite
• Gold
• gold–quartz
• gold–quartz
• polymetallic
• Gold
• gold–quartz
• gold–quartz
• pyrrhotite

Visible stage and mineral labels in the diagram

• quartz–pyrrhotite
• arsenopyrite–marcasite
• arsenopyrite–pyrite–marcasite
• quartz–pyrrhotite
• arsenopyrite–marcasite
• quartz–pyrrhotite
• arsenopyrite–marcasite
• galena–sphalerite
• quartz–pyrrhotite–sphalerite
• quartz–carbonate
• gold–quartz–sulfide
• quartz–carbonate
• quartz–pyrrhotite
• pyrite–marcasite
• gold–quartz
• pyrite–marcasite
• pyrite–marcasite
• gold–quartz
• gold–quartz
• pyrite–marcasite
• quartz–pyrrhotite
• galena–sphalerite
• quartz–pyrrhotite
• quartz–carbonate
• galena–sphalerite
• pyrite–marcasite
• quartz–pyrrhotite
• gold–quartz
• gold–quartz–sulfide
• quartz–carbonate
• quartz–carbonate
• chalcedony–carbonate
• quartz–carbonate
• chalcedony–carbonate

Legend

1. [[unclear: blank/outlined intervals]]
2. presumed correlation of mineralization stages;
3. [[unclear: hatched symbol]]
4. [[unclear: stippled symbol]]

the latter to raise the question of introducing certain changes into the scheme of the ore process of one deposit or another. Such correction applies especially to the number of stages distinguished. As is known, stages of mineralization are often distinguished solely on the basis of age relationships among mineral complexes. At the same time, some researchers, ignoring the possibility of tectonic movements within a stage, regard each intersection of veinlets of different mineral composition—and often of quite similar mineral composition—as a sufficient basis for distinguishing an independent stage. Such an approach, based only on mineralogical observations, leads to the construction of highly detailed paleoseismic schemes for the ore process (which is also very important), but not to the identification of its stages in the sense of the manifestation of mineral-forming activity by different portions of solutions sharply differing in composition and thermodynamic characteristics. As the experience of integrated mineralogical-geochemical and mineral-thermometric study of deposits has shown, for many of them, owing to the reasons noted above, a considerably larger number of stages of mineralization had previously been distinguished than actually existed. In view of the foregoing, and on the basis of the commonality of the patterns of development of the ore process established for the deposits of the group under consideration, it seems possible to raise the question of reducing the number of mineralization stages at the Darasun gold-ore deposit. In particular, the validity of distinguishing, for this deposit, the quartz-pyrite and pyrite-arsenopyrite-quartz periods of mineral formation as separate stages [5] is doubtful, since the composition of the mineral associations corresponding to them hardly permits one to speak of a substantial difference in the chemistry of the ore-forming solutions. The closeness of the cation-anion composition of aqueous extracts from inclusions in quartz from both associations testifies to the same. The thermometric characteristics of the relics of these solutions likewise indicate a very similar regime of temperature conditions for the development of the quartz-pyrite (410–345°C) and pyrite-arsenopyrite-quartz (397–275°C) mineral associations. Quartz, which preceded and later accompanied the deposition of pyrite, and then of pyrite and arsenopyrite, began to be deposited from gaseous solutions, the transformation of which into a liquid state for different parts of the deposit falls within the temperature interval 395–370°C. The successive deposition of pyrite and, with some overlap, arsenopyrite took place under the conditions of a hydrothermal system. The mineral-forming activity of this portion of solutions ended with the deposition of carbonate at a temperature of about 315–275°C. The existing signs of tectonic movements at the boundary between the deposition of early pyrite and arsenopyrite should apparently be regarded as the result of intramineralization phenomena, which could not have created pathways for the inflow of a qualitatively new portion of solutions. The same should be noted with respect to the successively formed mineral associations of galena-sphalerite, chalcopyrite-bournonite, and quartz-sulfoantimonite compositions. All of them are characterized by a rather similar (differing only quantitatively) set of minerals, which does not allow one to speak of a sharp difference in the chemistry of the ore-forming solutions. The minerals of these associations developed under close-temperature (280–180°C) conditions of a hydrothermal system. Apparently, in this case as well, one should speak of three successively formed mineral parageneses, the deposition of which took place in one stage from

a single portion of hydrothermal solutions, against a background of periodic intrastage movements.

In the course of a comparative analysis of ore-formation processes at the deposits under consideration, it is clearly established that at most of them, after the quartz–tourmaline stage, there usually developed a stage of early sulfides (quartz, pyrite, arsenopyrite), which preceded the gold–polymetallic stage. According to the information available at present, the Davendinskoe, Golgotaiskoe, and Shirginskoe deposits constitute exceptions in this respect. The absence of this stage in the ore-formation scheme for the latter two deposits—which, it should be noted, have been studied insufficiently fully—is apparently caused by its relatively weak development. As for the Davendinskoe deposit, in its ores the corresponding mineral paragenesis, in composition and position in the scheme, is known; however, it is combined together with the gold–polymetallic paragenesis into a single stage, as an earlier one [1]. The temperature characteristic available for it (340–270°C) makes it possible to compare it with the corresponding pyrite–quartz stage at other deposits. It is possible that, with more detailed studies of the Davendinskoe deposit, a body of data sufficient for distinguishing an independent stage of early sulfides in the ore-formation scheme will nevertheless be identified.

On the basis of the comparison made of the thermodynamic conditions of development of the ore process for a whole series of gold-ore, gold–molybdenum, and molybdenum deposits of Eastern Transbaikalia, it may be said that all of them, regardless of their position in different structural-facies zones of the region, belong to a single genetic group of high-temperature pneumatolytic–hydrothermal formations of medium depths, which developed in a very similar thermodynamic regime. If the beginning of the ore process is characterized by a sharp overlap of the temperature conditions of successively developing stages (the first three stages), then its second half is characterized by a certain separation of them and by extension in temperature terms. The end of the ore process took place under conditions of a relatively calm decrease in temperatures and pressures.

In connection with the above, it seems appropriate to touch upon the question of the assignment of the deposits under consideration to one or another ore formation. As follows from a number of works by V. A. Kuznetsov [4], E. A. Radkevich, and R. M. Konstantinov [2], ore formations are currently understood by most researchers as “groups of deposits with mineral associations of similar composition, recurring in a definite sequence at all deposits of a given formation and formed under similar geological conditions, regardless of the time of formation” ([2], p. 11). The comparison made of the ore-formation processes for deposits of gold and molybdenum in Eastern Transbaikalia makes it possible to unite them into a single group of deposits, which quite legitimately fits within the concept of a single ore formation. In this connection, the opinion of some researchers [3] that the deposits considered belong to numerous different ore formations (see Fig. 1) should become the subject of further discussion. In our opinion, the similarity in the composition of the mineral associations, in the sequence of their appearance, and in the thermodynamic regime of formation of the noted deposits makes it possible to assign them to a single gold–molybdenum ore formation. For most of them a spatial connection is noted, and for many a paragenetic connection is also demonstrated, with a definite complex of rocks of the dike series and small intrusions.

References

1. Druzhinin A. V. Collection “Structures of Ore Fields.” Moscow, 1960.

2. Konstantinov R. M., Zharikov V. A., Omelyanenko B. I., Petrovskaya N. V., Shatalov E. T. Study of the regularities in the distribution of mineralization in metallogenic investigations of ore districts. Moscow, “Nedra,” 1965.

3. Konstantinov R. M. Collection “Endogenous Ore Formations of Siberia and the Far East.” Moscow, “Nauka,” 1966.

4. Kuznetsov V. A. Collection “Endogenous Ore Formations of Siberia and the Far East.” Moscow, “Nauka,” 1966.

5. Timofeevsky D. A. Questions of the geology and genesis of certain lead-zinc deposits of Eastern Transbaikalia. Proceedings of IGEM, issue 83, 1963.