ON THE AGE OF LEAD MINERALIZATION ON THE EASTERN SLOPE OF THE KUZNETSK ALATAU

R. M. Gol’d and N. K. Grigor’ev

(Presented by Professor A. M. Kuz’mina)

Within the territory of the eastern slope of the Kuznetsk Alatau, a number of occurrences of mineralization and commercial lead deposits are known. The age of these deposits and their genetic relationship with particular magmatic complexes have so far remained controversial. In addition to the available geological data on this question, the authors attempted to carry out certain geochronological studies. In particular, the isotopic composition of lead was determined in several ore occurrences with galena, and their age was estimated, which makes it possible to clarify the relationship of specific ore occurrences with particular magmatic complexes.

In the summer of 1963, in order to carry out the above-mentioned studies, the authors performed special field work to collect material. In the territory of the Khakass Autonomous Oblast, 7 samples were selected, descriptions of which are given below.

Sample No. 205. Galena from the Darya skarn ore occurrence of the Karysh group, located on the right side of the valley of the Karysh River. Scheelite mineralization is confined to skarns developed at the contact of the Ulen’-Tuim (Tuimo-Karysh) granodiorite massif with the enclosing marbles of the Boi intrusion (?). In one of the areas, a quartz–calcite vein is noted, containing marcasite, chalcopyrite, and galena, and cutting the skarns. In the area there are a number of porphyrite dikes, the origin of which is probably associated with Devonian magmatism. The sample consists of a breccia of skarns cemented by white calcite with marcasite and galena. The latter sometimes forms granular aggregates, separated as veinlets or lens-like accumulations.

Sample No. 206. Galena from the beresitization zone in the Kolodzhul’ massif of adamellites. The massif is exposed in the middle course of the river of the same name. Four km east of the mouth of the Glukhaya River, which flows into the Kaladzhul’ River, a beresitization zone was found, which is cut by thin quartz–carbonate veinlets (1–5 cm thick) with pyrite, chalcopyrite, and galena. The sample was taken from these veinlets. It should be noted that the massif contains a large number of dikes of Devonian porphyrites. The Kolodzhul’ massif itself cuts through rocks of the faunistically characterized Middle Cambrian.

Sample No. 212 was taken on the right side of the Kara-Tege River valley, 3 km below the mouth of the Buzurgata brook, which flows into the Kara-Tege River. Gale-

here it is found as a fine dissemination or, more rarely, as inclusions up to 5 mm across in silicified marbles of the Karateg suite of Upper Proterozoic age. The latter in this area consists of remnants of the roof penetrated by the Ulen’-Tuim granodiorite massif, and is often silicified. In the immediate vicinity of the sampling site there are two dikes of syenite porphyries up to 10 m thick. The age of the dikes is tentatively considered to be Devonian.

Sample No. 214. The sampling point is located on the left side of the valley of the Glukhaya River, 1.5 km west of its headwaters. In the conglomerate unit of the lower part of the Koshkulak essentially effusive suite, an elevated lead content was found, up to 0.01%. The Koshkulak suite lies on an eroded surface of faunistically characterized rocks of the Middle Cambrian. The Kolodzhul intrusion cuts rocks of the Koshkulak suite. The age of this suite has not yet been precisely established, and its upper boundary is determined by the fact that Devonian effusives unconformably overlap the rocks of the Kolodzhul intrusion.

Sample No. 215. Galena from mineralization near the settlement of Karasuka (Batenevsky Range). In the area there are a number of dikes of syenite porphyries cutting carbonate strata. The mineralization is confined mainly to the calcite cement of tectonic breccia of limestones. In addition to galena and sphalerite, pyrite and chalcopyrite occur more rarely. The sample was taken from the oxidation zone.

Sample No. 215a. An anglesite–cerussite crust on galena from sample No. 215.

Sample No. 216. Native lead, collected from panned concentrates of rocks of the Ulen’-Tuim massif (middle course of the Bely Iyus River in the vicinity of the settlement of Mendolya). Lead is encountered in all varieties of rocks of this massif in the form of scales and grains of elongated and irregular shape. The grain size is from 0.08 to 1 mm. In some panned concentrates lead is found together with native zinc. The material for analysis was supplied by E. V. Vladimirova, and a detailed description of it is contained in the paper [2] published in this same collection.

Isotopic analysis in all samples was carried out for PbJ+ on a mass spectrometer with a furnace source. To obtain lead iodide, the mineral was dissolved in nitric acid. The solution was evaporated to dryness, and the dry residue was dissolved in bidistilled water. The solution was placed in a centrifuge tube, and KJ was added to it. The resulting precipitate of lead iodide was centrifuged and washed several times with ethyl alcohol. From the tube, the lead iodide was applied with a capillary to the ribbon of the ion-source furnace of the mass spectrometer.

In the treatment of sample No. 214, its decomposition differed somewhat from that described above. A 1-g charge of finely ground porphyrite was treated in a fluoroplastic cup with a mixture of hydrofluoric and oxalic acids. The dry residue was dissolved in 5N nitric acid. Lead from the nitric-acid solution was extracted with sodium diethyldithiocarbamate, followed by purification with a solution of dithizone in chloroform. After reextraction of the lead with a weak solution of nitric acid (dilution 1:500), nitric-acid copper was added to the latter as a carrier. The separation of lead and copper iodides was carried out according to the procedure described above. The results of the isotopic analyses are given in Table 1.

The age of galenas and native lead was estimated according to the table proposed by A. P. Vinogradov [1], who statistically processed

Table 1

Results of isotopic analyses and the age of lead from the Vostochnyi mine of the Kuznetsk Alatau

Note. δ is the relative error, calculated from a single quantity of analyses, in %.

compiled data from isotopic analyses of about 200 galenas of different ages, drew up a table, and in it showed the isotopic composition of ordinary lead characteristic of each cycle of tectogenesis.

Methods for calculating age from the isotopic composition of lead, proposed by Alpher, Russell, Damon, Houtermans, Tugarinov, and Sobotovich, lead to considerable errors for such comparatively young ore occurrences as those we are studying.

If the age of galena with a lead isotopic composition characteristic of the Variscan folding according to A. P. Vinogradov \((\mathrm{Pb}^{204}=1;\ \mathrm{Pb}^{206}=18.16;\ \mathrm{Pb}^{207}=15.70;\ \mathrm{Pb}^{208}=38.17)\) is calculated by the formulas proposed by the above-mentioned authors, it proves to be 400–350 million years. A. P. Vinogradov himself determines the age of the Variscan cycle within the limits of 200–260 million years. The most suitable for calculating the age of post-Proterozoic lead minerals should probably be considered the equations proposed by Cumming [7]:

\[ t_{206}=6.51\cdot 10^9 \lg \frac{29.82-x}{11.02}\ \text{years}, \]

\[ t_{208}=20.04\cdot 10^9 \lg \frac{79.90-z}{40.8}\ \text{years}, \]

where

\(t_{206}\) and \(t_{208}\) are the age in years, determined respectively from the ratios \(\mathrm{Pb}^{206}/\mathrm{Pb}^{204}\) and \(\mathrm{Pb}^{208}/\mathrm{Pb}^{204}\);

\(x\) and \(z\) are the ratios in the lead being determined, respectively

\[ \mathrm{Pb}^{206}/\mathrm{Pb}^{204}\ \text{and}\ \mathrm{Pb}^{208}/\mathrm{Pb}^{204}. \]

The authors calculated the age from the isotopic ratios given by A. P. Vinogradov, using Cumming’s equations. It turned out that, for post-Proterozoic galenas, the results differ little from A. P. Vinogradov’s data. The results of the comparison are given in Table 2.

Table 2

Comparison of ages according to A. P. Vinogradov and according to Cumming

The age of the ore occurrences of the Khakass Autonomous Oblast, calculated according to Cumming’s equations, is given in Table 1.

Among the analyzed samples, two groups may be distinguished. The first group should include lead with an Upper Caledonian–Lower Variscan age. The absolute age of galenas from these samples lies within the range 266–353 million years. The ore mineralizations from which the samples were taken are probably associated with the Devonian magmatic complex, identified on the eastern slope of the Kuznetsk Alatau by many researchers [3; 4; 5]. This group includes samples Nos. 205, 206, 212, 215, and 215a.

The data obtained by us do not contradict the geological position of the formations with which the lead mineralization is associated. Some doubt may be raised by the age of galena in the Karateg marbles (sample No. 212). As already indicated, in this area the Karateg sequence is in contact with the rocks of the Ulen-Tuim massif. The silicification of the marbles is associated with contact-metasomatic processes that took place during the emplacement of the Ulen-Tuim intrusion. At first glance it might seem that the galena also appeared as a result of these processes. In that case its age should be more ancient, since the Cambrian age of the Ulen-Tuim intrusion is almost beyond doubt. But it should be noted that the galena and the elevated lead content, according to the data of spectral analyses, clearly gravitate toward the zone of development of Devonian dikes of syenite-porphyries. In other parts of the contact of the Karateg marbles with the Ulen-Tuim intrusive rocks, where silicification is also observed, no elevated concentrations of lead have been found. All of the above makes it possible to assume that the lead in this sample is associated with the Devonian dikes of syenite-porphyries.

The second group should include samples with a more ancient age. First, this is native lead from the rocks of the Ulen-Tuim massif (sample No. 216), whose age, according to A. P. Vinogradov’s table, is Lower Caledonian. According to Cumming’s equations, the absolute age of this lead is 390–470 million years. It should be noted that the native lead was taken from crushed rock with a significant content of pyrochlore [2], and therefore its age may be “rejuvenated” because of some addition of radiogenic lead. The data obtained for native lead from the rocks of the Ulen-Tuim intrusion do not contradict its Cambrian age. Second, this same group includes lead from the Kashkulak Formation. The Kashkulak Formation contains thick sequences of effusives, and the occurrence of lead in it may be associated with hydrothermal processes taking place in the lavas. In the course of field work the authors often observed, in the Kashkulak effusives, hydrothermal formations with copper minerals, which are most likely associated with the hydrothermal processes that accompanied the formation of the effusive sequences. The possibility is not excluded that the lead here has the same origin. The age of this lead, according to A. P. Vinogradov’s table, is Lower Caledonian, and according to Cumming’s equations, 420–530 million years. The lead in the mineralizations of the second group is probably associated with the Salair magmatic complex, identified by many researchers on the eastern slope of the Kuznetsk Alatau.

Of great interest is the analysis of galena and of the anglesite-cerussite crust on it from the Batenyov Ridge (samples No. 215 and No. 215a). As is evident from Table 1, the anglesite-cerussite aggregate is somewhat enriched in the isotopes Pb^206 and Pb^207. This may be the result of two causes. First, the introduction into the oxidation zone of uranium-decay products, which is seemingly indicated by the relative depletion in the isotope Pb^208. Second, the possibility is not excluded of a shift in the isotopic composition of lead during the transition from sulfide to sulfate and carbonate. This also is not contradicted by the relative depletion in Pb^208 isotopes, since work carried out by I. E. Starik and other authors [7] on the leaching of lead from minerals shows that, under certain conditions, Pb^206 and Pb^207 may prove to be more mobile than Pb^208. Both of these causes, when appropriate studies are carried out, will help to study more fully the geochemical processes in the oxidation zone. Unfortunately,

the material available to us proved insufficient for deeper and well-founded conclusions.

Conclusions

Determination of the absolute age of ore lead from ore occurrences on the eastern slope of the Kuznetsk Alatau has shown that two groups can be distinguished among them. These are mineralizations of Devonian age, probably associated with the Devonian magmatic complex, and more ancient mineralizations of Cambrian age, genetically connected with the Salair magmatic formations of this region. Noteworthy is the difference in the isotopic composition of lead in galena and in the anglesite–cerussite crust that appears on it in the oxidation zone. Further investigations in this direction will help to study more fully the processes in the oxidation zone.

References

1. A. P. Vinogradov. Proceedings of the session of the Department of Chemical Sciences of the Academy of Sciences of the USSR on the peaceful use of atomic energy. Publishing House of the Academy of Sciences of the USSR, Moscow, 1955.

2. E. V. Vladimirova. On the occurrence of native lead and zinc in the territory of the Kuznetsk Alatau. The present collection.

3. A. P. Dodin. Intrusive complexes of the Kuznetsk Alatau and Gornaya Shoria. Information Collection of the West Siberian Geological Administration, No. 4, 1956.

4. Yu. A. Kuznetsov. On the intrusions of the Kuznetsk Alatau and their ore potential. Bulletin of the ZSGRT, issue 34, 1932.

5. Yu. A. Kuznetsov, G. A. Ivankin. Magmatism of the eastern slope of the Kuznetsk Alatau. Geology of the USSR, vol. XV, Krasnoyarsk Territory, 1961.

6. R. Russell, R. Farquhar. Lead Isotopes in Geology. IL, Moscow, 1962.

7. I. E. Starik. Nuclear Geochronology. Publishing House of the Academy of Sciences of the USSR, Leningrad, 1961.