DISTRIBUTION OF GOLD AND TRACE ELEMENTS IN ORE MINERALS OF THE BERIKUL DEPOSIT

N. V. ROSLYAKOVA (IGiG, Siberian Branch, Academy of Sciences of the USSR)

The behavior of gold in the process of ore formation, and in particular in the process of vein filling, still remains to a considerable extent unclear. Determining the position of gold in ores is one of the ways of approaching the solution of this question. To this end, we carried out a study of the distribution of gold and trace elements in the principal ore minerals of the Berikul deposit.

The Staryi Berikul gold-ore deposit is represented by a rather large, gently dipping (on average 10–20°) vein that dips to the southeast and is confined to the plane of a northwest-trending thrust, and by a series (about 60) of steeply dipping (35–50°) northwestward veins that filled feathering tension fractures occurring both in the footwall and in the hanging wall of the gently dipping vein.

Vein formation took place in five stages. The first (pre-ore) stage is represented by quartz, which makes up a considerable part of the veins and was deposited at temperatures above 400°C. Associated with it (in order of abundance) are chlorite, rutile, and hematite. The second stage is pyrite–pyrrhotite, with quartz and arsenopyrite. The homogenization temperature of gas–liquid inclusions in quartz of the second stage is 390–315°C. The third stage is quartz–pyrite–arsenopyrite, with parankerite. Gas–liquid inclusions in the quartz accompanying the deposition of pyrite and arsenopyrite homogenize at 380–300°C, and in the quartz that completes this stage of mineralization, at 270–240°C. In the fourth stage the productive carbonate–sulfide association was deposited: chalcopyrite, sphalerite, galena, tennantite, tetrahedrite (?), chabazite, silver, gold, calcite, quartz, and a small amount of pyrrhotite, pyrite, and arsenopyrite. The formation temperature was 230–170°. Gold was deposited at 190–170°C. The fifth stage is represented by post-ore calcite, quartz, chlorite, and sericite; the temperature was below 150°C.

Cobalt–nickel mineralization is not considered in this report.

The distribution of gold and trace elements was studied in pyrrhotite, pyrite, arsenopyrite, chalcopyrite, sphalerite, galena, quartz, parankerite, and calcite, selected from various veins and levels of the deposit. Gold was determined by neutron-activation (L. K. Pavlova, IGiG, Siberian Branch, Academy of Sciences of the USSR) and spectrochemical (V. G. Tsimbalist, IGiG, Siberian Branch, Academy of Sciences of the USSR) methods; trace elements, by approximate quantitative spectral analysis (L. G. Izyumova, IGiG, Siberian Branch, Academy of Sciences of the USSR).

Minerals differing both in composition and in stages contain gold in different amounts. The greatest amounts are in sulfides of stages III and IV. In pyrite from near-ore altered rocks it amounts to 0.7 g/t; in pyrrhotite of stage II, 0.014 g/t; in pyrite and arsenopyrite (stage III), from tens to hundreds of g/t. In minerals of stage IV, gold, along with that dispersed in other minerals (up to tens of g/t), already forms comparatively large (up to 0.4 mm) independent segregations.

The trace elements found in the ore minerals are given in Table 1. In addition, germanium and niobium were encountered in quartz of stage III, and molybdenum in sphalerite.

It is evident from the table that the number and content of trace elements increase from the initial stages to the final ones, where they already form independent minerals: native silver and schapbachite. Nickel and cobalt, and to some extent copper, constitute an exception, showing the opposite tendency.

Since the association of trace elements with particular minerals is determined to a significant degree by crystal-chemical factors in the differentiation of elements, their behavior in the process of mineral formation is most clearly seen when several generations of one mineral are considered. In our case these are pyrite and quartz.

If in quartz of stage I only 9 elements were found, then in stage IV the entire list of them was already present. It proved interesting that a new trace element found in quartz of any given stage, in comparison with the preceding one, is encountered in sulfides one stage earlier.

V. V. Shcherbina and K. I. Yakubovich [9], following A. E. Fersman, consider that for most substances isomorphous miscibility increases with rising temperature. Early generations of any mineral contain more impurities, both qualitatively and quantitatively, than later ones. At the same time, these authors note: “Undoubtedly, first of all, the presence of certain elements (in the mineral.—N. R.) and their contents depend on the presence and concentration of these elements in the original solution...”. Consequently, one may think that if all the elements we found in minerals of stage IV had been present in the solutions that produced the minerals of the initial stages, they apparently would have entered the latter, perhaps in greater amounts. The difficulty in resolving this question arises from the fact that the forms in which trace elements enter minerals have remained unstudied. Nevertheless, the distribution of trace elements in the ore minerals of the Berikul deposit that we have established can be explained by the fact that the amount and content of most elements in the ore-forming solutions evolved toward an increase from the early stages to the late ones. In this process, the appearance of a new element in the solution was first recorded by the sulfides and only, apparently, when its concentration in the solution had increased sufficiently was it taken up by quartz as well.

With regard to gold, one may think that its content in the solution, at least up to stage III of mineralization, was relatively low. This is indicated by the very small gold contents in pyrrhotite ($n \cdot 10^{-6}$—$n \cdot 10^{-5}\%$), which was deposited among the first of the sulfides. The temperature of formation of pyrrhotite differs little from that of pyrite and arsenopyrite of stage III (in both cases above 300°C). But in the latter the gold content amounts to tens and even hundreds of grams per ton. At the Darasun deposit, according to G. P. Volarovich [2] and D. A. Timofeevsky, and at the Sovetskoe deposit, according to O. E. Zvyagintsev [3], in pyrite deposited among the earliest sulfides, gold is associated to a lesser degree than with other sulfides.

Table 1

Content of gold and trace elements in the leading ore minerals of the Berikul deposit

At the same time, there are examples in which pyrrhotite contains gold in significant quantities, not yielding to pyrite \((n \cdot 10^{-4}—n \cdot 10^{-3}\%,\) according to the data of M. S. Sakharova for the same Darasun deposit). Apparently, it may be assumed that the change in gold content from stage to stage in ore-forming solutions is widespread and, as the examples cited show, it increases in the direction of mineral formation, with the exception of its very latest stages.

The interrelations among trace elements in individual minerals can, to some extent, be judged from the correlation coefficients between them. Thus, in pyrrhotite (stage II) a high direct correlation is observed between lead, bismuth, copper, and silver; between cadmium and zinc, antimony and nickel. In pyrite (stage III)—between titanium and zirconium, cadmium and zinc, cadmium and silver. In arsenopyrite—between lead and zinc, titanium and cobalt; copper, silver, and zinc with manganese and cadmium; nickel and titanium; an inverse relationship of nickel with cobalt. In sphalerite—between bismuth and silver. In galena, cadmium correlates well with copper and zinc, while an inverse relationship is manifested for bismuth with antimony. In addition, an increased direct dependence is indicated here between the contents of gold and antimony and an inverse dependence between gold and bismuth.

In the spatial distribution of trace elements, which was studied separately in pyrite, arsenopyrite, sphalerite, and galena, a certain regularity is also indicated.

In pyrite and arsenopyrite, within the ore column the contents of some elements (lead, bismuth, zinc, cadmium, antimony, and titanium) increase with depth, while those of others (nickel, cobalt, silver, copper, and manganese) decrease. When the contents of trace elements are compared in the upper parts of columns of steeply dipping veins along the plane of the thrust (the gently dipping vein), it turns out that the amount of nickel, cobalt, silver, and copper increases in the southeastern direction of the ore field, while lead, bismuth, zinc, cadmium, and antimony increase in the northwestern direction. The total content of trace elements in pyrite and arsenopyrite increases somewhat in the northwestern direction (Fig. 1).

In sphalerite and galena (stage IV), within the ore column the content of most elements increases upward with approach to the plane of the gently dipping vein. The content of these same elements increases in the southeastern direction. Nickel behaves in the opposite way. Some of the elements (gallium, manganese, indium, copper, antimony, vanadium, and titanium) are distributed uniformly. From Fig. 1 it is evident that the increase in the total content of trace elements in minerals of stage IV is expressed much more sharply than in stage III.

What, then, accounts for the distribution of trace elements that we have established?

The behavior of isomorphic impurities in sphalerite during cocystallization in open systems, considered by V. N. Anfilogov et al. [1], makes it possible to conclude that, during crystallization from a solution of one stage of some mineral, the first portions of it take up the largest amounts of impurities. In addition, L. N. Ovchinnikov [6] notes that the mobility of trace elements, owing to their low concentration in solution, is considerably less than the mobility of the principal ore-forming elements. On the basis of these two works, it may be considered that an increase in the contents of trace elements in one and the same mineral of one stage should be observed in the direction from which the solutions that deposited this mineral came.

If we now return to the spatial distribution of trace elements in the ore minerals of Berikul, it may be said that in the initial period of formation of the deposit, including stage III, the solut—

ores, generally moving from below, entered the vein fractures literally from everywhere. This is natural, since all sorts of fissures and pores in the enclosing rocks, and the vein fractures themselves, while not yet filled, contributed to such a phenomenon. Some advantage, especially in stage III, was enjoyed by the movement of solutions containing lead, bismuth, zinc, cadmium, and antimony from the northwest, along the fractured zone accompanying the thrust. V. K. Monich [5] also spoke of this same direction when studying the sulfide deposits of Berikul. In stage IV the direction of movement of the solutions was different. The solutions moved chiefly

Fig. 1. Spatial distribution of the total content of trace elements in minerals of stages III and IV; I—arsenopyrite, II—pyrite, III—sphalerite, IV—galena.

from the southeast along the fractured zone accompanying the thrust, and entered those parts of the veins that had undergone brecciation. And indeed, since the intensity of fracturing of the enclosing porphyrites increases with approach to the plane of the gently dipping vein (thrust), and the vein fractures themselves by that time had been filled with minerals of the earlier stages, then in the areas adjoining the gently dipping vein the solutions could move more rapidly. Having reached the steeply dipping vein, they could move through the newly formed cavities in it upward and downward from the thrust plane, i.e., into the veins of the hanging and footwall sides of the gently dipping vein. This is also consistent with the distribution of stage IV minerals (sphalerite, galena, gold) in the steeply dipping veins.

Proceeding from the fact that the gold-bearing solutions (stage IV) moved from the southeast, and taking into account the general structure of the deposit, one should recognize the possibility of finding ore veins in a southeasterly direction as far as the thrust is manifested.

On the basis of the regular change in trace-element contents over comparatively small intervals, the increase in their contents toward the end of the mineral-formation process, the silver-to-gold ratio of more than 1, and also the repeated manifestation of intramineralization movements [4, 8, 7], it may be said that the ores of the studied part of the Berikul deposit formed at comparatively shallow depth.

References

1. Anfilogov V. N., Belov B. I., Troshin Yu. P. On the behavior of isomorphic impurities during cocrystallization in open systems. Geochemistry, 1966, No. 2.

2. Volarovich G. P. On the geological structure of the Darasun gold-bearing district of Eastern Transbaikalia. Proceedings of the VNIGRI Institute, issue 16, 1947.

3. Zvyagintsev O. E. Geochemistry of Gold. Publishing House of the Academy of Sciences of the USSR, 1941.

4. Kuznetsov K. F. Rare and dispersed elements in the ores of polymetallic deposits of the Nerchinsk-Zavod group of Eastern Transbaikalia. Proceedings of IMGRE, issue 2, 1959.

5. Monich V. K. Pyrite deposits of Berikul. Proceedings of TTU, vol. 96, 1939.

6. Ovchinnikov L. N. Trace elements as indicators of ore-formation processes and the use of the regularities of their distribution in prospecting and exploration of ore deposits. In: Chemistry of the Earth’s Crust, vol. 2, Nauka Publishing House, 1964.

7. Shakhov F. N. Ore Textures. Nauka Publishing House, 1961.

8. Shcherbina V. V. On the geochemical significance of the quantitative ratio of silver to gold. Geochemistry, 1956, No. 3.

9. Shcherbina V. V., Yakubovich K. I. Limits of isomorphic miscibility as a function of genetic conditions. Proceedings of the Geochemical Conference “Chemistry of the Earth’s Crust,” vol. 1, 1963.