BEHAVIOR OF GOLD IN PRIMARY HALOS OF SOME VEIN GOLD ORE DEPOSITS

N. V. Roslyakova, N. A. Roslyakov, V. G. Zvyagin
(IGiG, Siberian Branch, Academy of Sciences of the USSR, Darasun Deposit)

The behavior of gold in the primary halos of gold-ore deposits has been poorly studied. The most complete and interesting data in this respect are those given by A. Fakhri [8] on the primary halos of gold-ore deposits in Kazakhstan. There is information on the distribution of gold in the primary halos of deposits in Transbaikalia [5] and the state of Montana [10]. It has been established that gold-ore bodies are usually accompanied by gold halos of considerable size—up to 50 m, according to Safronov et al. [5]. Only halos with contents of this element of \(n \cdot 10^{-6}\%\) and higher have been subjected to detailed study. Halos of lower orders were not considered, which is partly explained by the absence, until comparatively recently, of an easily accessible method for determining near-Clarke concentrations of gold.

Using an improved spectrochemical method for determining gold [9], with a sensitivity of \(2.0 \cdot 10^{-8}\%\) and an accuracy of \(\pm 25\%\) (analyst V. G. Tsimbalist, IGiG), we obtained material on the distribution of this element in the enclosing rocks of several vein-type deposits. The most detailed work was carried out at the Darasun deposit in Eastern Transbaikalia and the Berikul deposit in the Kuznetsk Alatau.

Samples were taken at various levels from veins and their immediate contacts, at distances of 0.25, 0.5, 1, 2, 5, and 10 m from the veins and, farther on where possible, every 10 m toward their hanging and foot walls. In addition, about 200 samples were taken from rocks of the ore fields and almost as many at distances of 5–25 km from them.

The geological structure of the Berikul and Darasun deposits has been discussed in considerable detail in the literature. Works by A. Ya. Bulynnikov [1], V. K. Monich [2], D. A. Timofeevsky [6, 7], and many others are devoted to this subject. Common to these deposits is the vein character of the gold mineralization of a moderately sulfide formation (after N. V. Petrovskaya [3]). The veins are of small thickness (on average 15–35 cm), steeply dipping, with a columnar distribution of gold. The gold-ore columns usually coincide with sulfide ones.

These deposits differ mainly in the composition of the enclosing and near-vein altered rocks.

The host rocks of the Berikul deposit are plagioclase and hornblende porphyries and their tuffs. The near-vein alteration of these rocks was manifested in the development of carbonate, sericite, chlorite, pyrite, rutile, and quartz, sometimes with the formation of rocks close in composition to beresites. The most intense alterations are observed on both sides of the veins at a distance not exceeding their thickness.

The veins of the Darasun deposit are situated in intrusive rocks of varied composition: from granites to gabbro, predominantly in granodiorites. The near-vein altered rocks are represented chiefly by beresites, whose thickness is 1–2 m. These veins, in general, are steeper and richer in sulfides than the veins of the Berikul deposit.

Despite differences in the geological structure of the deposits under consideration, the distribution of gold in their primary halos is identical.

Gold has been found in all types of host rocks. Its background content at a distance of hundreds of meters from the deposit varies within the range of \(10^{-7}\%\) and very rarely amounts to \((1—2)\cdot 10^{-6}\%\).

As one approaches the deposits and then the ore bodies, the gold content in the rocks increases. Three orders of primary halos are clearly distinguished: 1) halos of the ore field, 2) halos near veins, or effective halos according to Fakhri [8], and 3) negative halos in the exocontacts of veins.

Halos of the ore field cover the entire area of the deposit. They are genetically connected with the manifestation of regional hydrothermal metamorphism, expressed mainly in the process of pyritization and partial alteration of mafic minerals and plagioclases. The gold content in the rocks of the ore field is an order of magnitude or more higher than the content in analogous rocks of the district, and usually amounts to \(n \cdot 10^{-6}\%\). At the same time, the distribution of gold becomes uneven; fluctuations from \(n \cdot 10^{-7}\%\) to \(n \cdot 10^{-5}\%\) are observed. The principal carrier of gold is pyrite, in which it amounts to \(n \cdot 10^{-5}\%\). The data cited indicate that, along with possible redistribution of gold, there was also introduction of it by solutions that caused regional hydrothermal metamorphism, thereby creating halos of the ore field.

Effective halos are characterized by a gold content an order of magnitude higher than in the halos of the ore field, i.e., they differ from background contents by two orders of magnitude. Their dimensions toward the hanging and footwall sides of the veins are approximately the same and are determined chiefly by the composition of the host rocks. In general, they are up to 5 m in porphyries and gabbro (Fig. 1), and up to 15–20 m in granodiorites (Fig. 2).

In the rocks of effective halos, in contrast to halos of the ore field, intense carbonatization and sericitization are observed, and chlorite appears. Segregations of ore minerals are 2–3 times larger, but they still are distributed throughout the entire mass of the rock. In granodiorites, enlargement of quartz is also observed; it grows onto sulfide grains, almost without corroding the latter. The principal carriers of gold here too are sulfides, where it amounts to the first grams per tonne.

Microscopic and spectrochemical studies showed that alteration of the host rocks in the direction from effective halos to halos of the ore field is gradual, waning, with a more or less simultaneous decrease in gold content. Consequently,

In-figure labels:

Gold content, %

Vein 223, Vongusskaya

Profile 1

1; 2; 3

Profile 2

Fig. 1. Primary halos of gold in gabbro. Darasun deposit

1. Gabbro. 2. Wall-rock altered gabbro. 3. Gold-bearing vein.

Fig. 2. Primary halos of gold in granodiorites. Darasun deposit

1. Granodiorite. 2. Granodiorite altered near a vein. 3. Gold-bearing vein.

alterations of the enclosing rocks and the effective halos accompanying them, as well as the halos of the ore field, are the product of a single process of hydrothermal metamorphism with the introduction of gold. The more active penetration of gold-bearing solutions was favored by weakened zones, later manifested as vein fractures, as evidenced by the constant presence of effective halos around veins. The dependence of the dimensions of primary halos on the composition of the enclosing rocks is, in all probability, due to their differing permeability.

Negative halos develop along the vein walls. The gold content in them is sharply reduced in comparison with the effective halos and approaches the background level (Figs. 1, 2). At the same time, the dimensions of the negative halos depend not only on the composition of the enclosing rocks, as is the case with effective halos, but also on the dip angle of the veins. Thus, other conditions being equal, the flatter the vein, the wider the negative halo on its footwall side. In porphyries it reaches 0.5 m, and in granodiorites, 2 m.

The portions of the enclosing rocks corresponding to negative halos are the most strongly hydrothermally altered. Here sericitization, carbonatization, and other alteration processes of the enclosing rocks, characteristic of the halos of the ore field and of effective halos, reach a maximum. At the same time, in negative halos there are changes of an entirely new character, inherent only in these portions of the rocks. These include the formation of isolated quartz, quartz-carbonate, and quartz-carbonate-sulfide aggregates. Thus, whereas in the porphyries of the Berikul deposit, within effective halos, sulfides—in particular pyrite—are disseminated through the rock, in negative halos they are confined to quartz-carbonate aggregates, occurring between their grains. The dimensions of sulfide segregations increase and often amount to 1.2–1.5 mm. In granodiorites, within negative halos, the grain size of quartz increases many times in isolated quartz aggregates (up to 0.5 mm). The amount of pyrite (sometimes arsenopyrite) is approximately the same as in effective halos. But there, where it is accompanied by coarse-grained quartz aggregates, its grains are to a considerable degree corroded or redeposited.

The gold content in sulfides is an order of magnitude lower than its content in the sulfides of effective halos.

Chemical analyses of the enclosing rocks showed that, in areas of negative gold halos, there is a decrease in the contents of other elements as well, such as silica, manganese, iron, magnesium, sodium, and calcium. CO₂, potassium, and sulfide sulfur accumulate here.

All this indicates an intense regrouping of elements during the formation of negative gold halos, caused by the combined effect on the enclosing rock of hydrothermal metamorphism, which affected the entire ore field, and superimposed hydrothermal alteration connected with the process of vein filling. Gold, silica, and other elements were leached from the near-contact part of the veins by solutions moving along the vein fracture and, apparently, were deposited in the vein cavities. Confirmation of the latter is the certain dependence we have noted between the gold content in the vein and the size and contrast of the negative halos. The richer the vein is in gold, the more distinctly the negative halos are expressed (Fig. 3).

The presence of negative gold halos can also be observed on the graphs presented by A. Fakhri [8] and N. I. Safronov et al. [5]. Consequently, the occurrence of negative halos is characteristic not only of the deposits we have considered, but evidently has a broad

Fig. 3. Graph showing the dependence of the width and contrast of negative halos of gold on its content in the vein

distribution. It may be considered that the redeposition of gold during the formation of veins, discussed in the report by N. V. Petrovskaya [4], begins from the moment negative halos arise.

All the other most widespread ore elements (arsenic, lead, zinc, copper, bismuth, nickel, cobalt, etc.) in primary halos of all three orders behave more or less similarly to gold (Fig. 4).

Thus, on the basis of the material presented, the following conclusions may be drawn:

1. Gold-ore deposits are accompanied by primary halos that arose in the process of areal hydrothermal metamorphism, with an influx of gold.

Fig. 4. Distribution of gold, lead, and zinc in the primary halos of the Berikul deposit

1. In the immediate exocontacts of veins, negative halos arise as a result of the action of ore-forming solutions on the zones of effective halos during the filling of the veins.

2. The study of primary halos of gold is of great practical importance. It makes it possible to identify promising areas at the prospecting stage, to delineate individual ore zones and bodies within these areas, and, on the basis of negative and effective halos, to evaluate the prospects of veins at the stage of detailed prospecting.

REFERENCES

1. Bulynnikov A. Ya. Gold-ore formations and gold-bearing provinces of the Altai-Sayan mountainous region. Proceedings of Tomsk University, 1948.

2. Monich V. K. On the petrology of the area of the Berikul mine in Kuznetsk Alatau. Proceedings of TSU, vol. 93, 1938.

3. Petrovskaya N. V. The character of gold-bearing mineral associations and formations of gold ores of the USSR. In: Genetic Problems of Ores. International Geological Congress, 21st Session. Reports of Soviet Geologists. Problem 16, 1960.

4. Petrovskaya N. V. Redistribution of gold and quartz during the processes of formation of gold-ore deposits (report at the Second Interuniversity Conference on the Study of Gold Deposits of Siberia, Tomsk, 1968).

5. Safronov N. I., Polikarpochkin V. V., Uttof A. A. Spectro-goldometric surveying as a method of prospecting for gold-ore deposits not accompanied by mechanical halos (placers). ONTI, exchange of experience, VITR, issue 35, 1960.

6. Timofeevsky D. A., Shcheglov P. I., Borishanskaya S. S. The Berikul gold-ore deposit. Geology of the Principal Gold-Ore Deposits of the USSR, vol. V, 1952.

1. Timofeevskii, D. A. Patterns of localization and prospecting criteria for mineralization of the Darasun type in Eastern Transbaikalia. Transactions of TsNITRI, issue 41, 1962.

2. Fakhri, A. Zoning of primary halos of a gold-ore deposit and the migration capacity of elements. Moscow University Bulletin, Series IV, Geology, No. 5, 1965.

3. Tsimbalist, V. G. Determination of microquantities of gold in various geological objects. Geology and Geophysics, in press.

4. Mantei, Erwin J., Brownlow, Arthur H. Variation in gold content of minerals of the Marysville quartz diorite stock, Montana. Geoch. et cosmochim. acta, 1967, 31, No. 2.