Some Features of the Distribution of Trace Elements in Pyrites and the Bulk Sulfide Fraction from the Halos of the Baley Gold Ore Field

P. A. Kitaev, A. A. Nesterova, V. N. Sarapulova (Institute of Geology, Siberian Branch, Academy of Sciences of the USSR)

Identifying the regularities in the distribution of sulfides and of trace elements within them, besides its significance for elucidating ore-formation processes, is of great practical interest, since under certain conditions analysis of the sulfide fraction can serve as an important prospecting technique in geochemical exploration. In this connection, in the study of the primary dispersion halos of the Baley ore field [2, 6], we investigated the sulfides developed in the halos.

The deposit is represented by a series of extended ore zones filled with veins of chalcedony-like quartz. The amount of sulfides is small (up to 1.5%); the most widely distributed are pyrite and marcasite; to a lesser extent chalcopyrite, arsenopyrite, antimonite, pyrargyrite, pale ores, and other minerals occurring still more rarely are present.

The study of the sulfides was carried out along a transverse section passing through the central, most gold-enriched part of the ore field. In this section the ore bodies do not crop out at the surface. Samples were taken from mine workings and from drill cores exposing the supra-ore zone, the ore-mineralized zone, and the lower, weakly gold-bearing part of the deposit to a depth of as much as 700 m from the surface. The sample weight was 1–2 kg; the sampling interval was 5–10 m; in all, about 250 samples were taken. The sulfide fraction was separated by carrying out a series of successive operations: crushing to −0.5 m, washing on a pan to a gray concentrate, separation of the sulfide portion in a heavy liquid (bromoform, specific gravity 2.93), and selection of monomineral fractions under a binocular microscope.

Semiquantitative spectral analysis of monomineral samples, as well as of the sulfide fraction as a whole, was performed—apart from gold—for the trace elements identified in the halos of the Baley ore field [6] and for a number of other elements whose presence is possible in near-surface gold-ore deposits of another type [1]: Ag, As, Sb, Pb, Cu, Zn, Co, Ni, Tl, Ge, Mo, In, Te, Cd, Bi, Sn, Hg. The sensitivity of determination of the elements was: Ag, Mo—0.0001%; Sn—

0.0005%; Au, Pb, Cu, Co, Ni, Cd, Ga, Ge, In—0.001%; Hg—0.005%; As, Zn, Tl, Te—0.01%. Determination of the relative amount of sulfide minerals at different vertical levels was carried out using specially taken large-weight samples (up to 10 kg); their distribution across the strike of the ore bodies was determined from polished sections and thin sections under the microscope.

Some features of the distribution of sulfide minerals in halos

The bulk of the sulfide portion in the heavy fractions obtained is represented by pyrite and, to a lesser extent, by arsenopyrite. In many samples antimonite is present; sphalerite and galena occur as individual grains. Other sulfide minerals among those mentioned according to the mineralogical studies of the ores [5], including silver minerals, were not detected in the sample wash concentrates. Native gold was likewise not found, which indicates that in the halos it occurs almost exclusively as very fine segregations. Some sulfide minerals (cinnabar, silver minerals, etc.) may be absent from the heavy fractions of the wash concentrates because, during crushing of the samples, they are overground and are washed away during panning.

A characteristic feature is the change in the amounts of the principal minerals (pyrite and arsenopyrite) and in the ratio between them vertically. This ratio changes toward a decrease in the amount of pyrite and, conversely, an increase in arsenopyrite upward (Table 1, Fig. 1).

Table 1.

Number of grains of pyrite and arsenopyrite
in the heavy fraction of wash concentrates¹
(in % of the total number of grains)

Fig. 1. Results of quantitative mineralogical analysis of wash concentrates of samples taken along a borehole intersecting ore zone 1
1—ore bodies; 2, 3—curves of variation of arsenopyrite and pyrite

Away from the ore bodies (across the strike), pyrite is distributed over fairly considerable distances (up to 100 m and more). At the lower and middle levels, arsenopyrite forms narrow halos accompanying the ore bodies, whereas in the upper part of the deposit the arsenopyrite halos broaden considerably.

¹ The sampling sites are shown in Figs. 2–5.

The character of the distribution of sulfides correlates with the distribution of elements in the halos: at the deposit, arsenic “caps” above the ore bodies are clearly observed [6]. At the Baley deposit the As/S ratio increases with approach to the surface. It should be noted that these data agree with the conclusions of G. P. Taylor [10], obtained by him in studying the quantitative and qualitative composition of sulfides at many deposits of different genetic types. According to this researcher, the As/S ratio increases from the group of high-temperature deposits to low-temperature ones.

Trace elements in pyrites

Trace elements were determined in the most widely distributed sulfide mineral—pyrite. Determination in other minerals was hindered by the difficulty of separating them in the quantities required for analysis. In this connection, the collective fraction of sulfide minerals was analyzed. At the sensitivity of the analyses indicated above, the presence of the following trace elements in pyrites was established: Au, Ag, As, Sb, Pb, Cu, Zn, Co, Ni, Bi, and Tl. The following circumstance must be noted. According to mineralogical studies [5], several generations of pyrite, associated with different stages of mineralization, are distinguished at the Baley deposits. These generations may differ in their content of trace elements. Because of the similarity of their morphological features, there is no possibility of separating pyrite by generations when isolating it from crushed fractions. Therefore, the contents of trace elements are given as a whole for a mixture in which different generations may be present. However, this circumstance, as will be shown below, does not prevent the identification of general spatial variations in the contents of elements in pyrites.

Data on the average contents of elements in pyrites, calculated for identical vertical intervals, are presented in Tables 2 and 3. Table 2 contains the elements detected only in part of the analyzed samples (Au, Tl, and Bi). Their content in pyrites is characterized by frequency of occurrence and by the ranges of values in significant samples. Table 3 includes the remaining elements, which were recorded in all analyzed pyrite samples. The necessary explanations and the revealed regularities are given below.

Table 2

Content of gold, thallium, and bismuth in pyrites

Table 3

Content of trace elements in pyrites (in \(10^{-3}\%\))

Gold, by ordinary spectral analysis (not specialized for this element), is detected only in a small proportion of the samples. Consequently, its content in pyrites is for the most part less than 0.001% (\(10\ \text{g/t}\)), although on the whole it is considerably higher than in the halos. The presence of silver is recorded in all pyrite samples, whereas in the enclosing rocks it is recorded only in a small part of them. In the distribution of gold and silver concentrations in pyrites the following regularities are observed. Gold is most often recorded within the zone of optimal mineralization. The maximum amounts of silver are also confined to this zone. Above and below it (in the supra-ore and sub-ore parts), the concentrations of these elements in pyrites are considerably lower. The contents of Au and Ag decrease with increasing distance away from the veins (Fig. 2). The value of the Ag/Au ratio¹ in pyrites (Table 4) is considerably lower than in the rocks as a whole. In the latter, the Ag/Au ratio is usually greater

Fig. 2. Distribution of gold and silver concentrations in pyrites.

1 — loose deposits; sedimentary rocks: 2 — with a predominance of sandstones; 3 — conglomerates; 4 — granitoids; 5 — quartz veins and veinlets; 6 — tectonic disturbances; 7 — rock boundaries; 8 — samples with elevated silver concentrations (\(>5 \cdot 10^{-4}\%\)); 9 — samples in which gold was detected (at an analytical sensitivity of \(10\ \text{g/t}\)); 10 — pyrite sampling sites.

1 (up to 50 and more) [3]. It follows from this that Ag, apparently, is present in the halos mainly in other forms, probably, to a considerable degree, as independent silver minerals (pyrargyrite) and a silver-rich variety of fahlores (freibergite).

Table 4.

Ratios of pairs of elements in pyrites

A characteristic feature of the pyrites of the Balei deposits is the high content of arsenic and antimony. These elements were found in all pyrite samples, with As concentrations sometimes reaching 1–3%, and Sb 0.1%. The highest concentrations of As and Sb, unlike Au and Ag, are confined to the upper horizons (above the zone of optimal mineralization).

Figure 3 shows the dispersion halos of antimony, constructed from the data of analyses of pyrite and of the heavy fraction as a whole, representing the collective fraction of sulfides and certain accessory minerals. As is evident from Fig. 3, the maximum concentrations of antimony in pyrites are observed, first, in the upper part of the section and, second, at a distance from the ore veins. Near the ore veins and on the lower horizons, the antimony content in pyrites decreases. A similar distribution pattern was noted for Sb and As in the rocks [6]. The distribution of Sb in the collective sulfide fraction—which, in addition to pyrite, as indicated above, contains arsenopyrite, antimonite, and individual grains of other sulfides and sulfosalts—is of an entirely different character. The maximum Sb concentrations are observed near the ore veins, and no significant decrease in content with depth is observed. This halo is superimposed on the halo constructed from pyrites and cuts across it. This is consistent with the fact that antimonite is associated with a later antimonite stage of mineralization [5]. No regular vertical changes in the Sb/As ratio in pyrites (Table 4) are observed, although it is possible that at the very top (0–100 m) the value of the ratio is somewhat higher. It should be noted that the distribution features of As and Sb in the pyrites of the Balei ore field agree with some data available in the literature. Thus, in the pyrites of high-temperature deposits formed at great depths, the contents of As and Sb are insignificant. In this respect, for example, the data on hypothermal gold-ore deposits of Canada are characteristic [7, 11]. The concentrations of As and Sb increase in deposits formed at shallower depths and at lower temperatures [9, 10]. At Balei this tendency is expressed within a single deposit.

A feature of the distribution of copper in pyrites is the presence of elevated concentrations in the upper and middle parts of the deposits (Fig. 4). In contrast to copper, the pyrites of the upper horizons are characterized by the lowest lead contents (Fig. 5). Within the zone

Fig. 3. Dispersion halos of antimony in pyrites and in the bulk sulfide fraction as a whole

1–7—see Fig. 2; 8—elevated concentrations of antimony in the bulk sulfide fraction of samples. Antimony content in pyrites: 9—0.05–0.1% and more; 10—0.01–0.05%; 11—sampling sites; 12—sites where large samples were taken

Fig. 4. Distribution of elevated copper concentrations in pyrites

1–7—see Fig. 2; 8—samples with elevated (>0.01%) copper contents; 9—sampling sites

![Figure 5]

Fig. 5. Distribution of elevated concentrations of lead in pyrites

1–7—see Fig. 2; 8—samples with elevated (>0.01%) lead contents; 9—sampling sites

elevated concentrations of Cu and Pb are observed at some distance from the veins, across their strike. In general, the distribution of copper and lead in pyrites repeats their distribution in the rocks [6]. The exception is the decrease in Pb concentrations in the upper horizons, which deserves special attention, since, in studying the halos in the indicated places, the presence of clearly expressed Pb halos was noted. Apparently, in the upper horizons Pb is present in the halos in some other form. The value of the Cu/Pb ratio in pyrites is clearly shifted in the upper horizons in favor of copper.

For Co and Ni it is characteristic that their concentrations increase noticeably with depth, reaching maximum values in the lowest horizons (Table 3). The Co/Ni ratio also increases with depth (Table 4). It is known [4, 8, 11] that the contents of Co and Ni and the Co/Ni ratio increase from low-temperature to high-temperature ones, and within a single deposit—from near-surface to deeper horizons (in connection with an increase in temperature in this direction). In the present case this general regularity is manifested. It should be noted that the contents of Co and Ni in the pyrites of the Baley deposits are comparatively low. According to Hawley [9], in pyrites of high-temperature gold-ore veins of Canada, Co contents range from 0.001% to 0.1% (on average 0.02%), and Ni—0.005–0.1% (on average 0.04%). In low-temperature (125–200°) varieties of pyrite, according to the data of the same investigator, Co contents decrease, amounting, for Co, to 0.008–0.006% (occasionally up to 0.33%), and for Ni—to 0.013–0.25%. In pyrites of low-temperature gold-quartz veins of Mexico, Ni and Co were not detected.

Conclusions

The regular position of areas of elevated contents of trace elements in pyrites with respect to the zone of maximum con-

concentrations of gold in the ores may be interpreted from the standpoint of the syngenetic character of the main mass of pyrite with respect to gold mineralization.

The superposition of later mineralization is manifested, for example, in the antimony halos constructed from analyses of the bulk sulfide fraction; these occupy a cross-cutting position relative to the halos identified from pyrites.

Analysis of pyrites makes it possible to obtain additional information about halos. Thus, elevated concentrations of Co, Ni, Tl, and Bi have been identified in the halos.

The characteristics of the distribution of trace elements in pyrites can be used in prospecting work. The zonality phenomena noted above may serve to determine the position of zones of optimal mineralization.

REFERENCES

1. N. A. Kitaev, V. V. Polikarpochkin. Features of the primary dispersion halos of the Baley ore field and their use in prospecting for epithermal gold deposits. In: “Ore Potential of Volcanogenic Formations of the Northeast and Far East.” Magadan, 1967.

2. N. A. Kitaev, A. M. Naryshev, V. V. Polikarpochkin. Primary mercury dispersion halos in the Baley gold-ore field. (In press. In: “Problems of the Metallogeny of Mercury.”)

3. N. A. Kitaev, V. N. Sarapulova, I. Ya. Korotaeva. The gold–silver ratio in primary halos of the Baley gold-ore field (abstracts of a report). In: “Geology and Exploration of Mineral Deposits of Transbaikalia.” Chita, 1968.

4. L. N. Ovchinnikov. Trace elements as indicators of ore-formation processes and the use of regularities in their distribution in prospecting and exploration of ore deposits. In: “Chemistry of the Earth’s Crust,” vol. 11, 1964.

5. N. V. Petrovskaya, P. S. Bernshtein, S. G. Mirchink, M. G. Andreeva. Geological structure, mineralogy, and genetic features of gold-ore deposits of the Baley ore field (Eastern Transbaikalia). Transactions of TsNIGRI, issue 45, part II, 1961.

6. V. V. Polikarpochkin, N. A. Kitaev, V. N. Sarapulova. Structure and vertical zonality of primary dispersion halos of the Baley gold-ore field. “Geochemistry,” 1965, No. 8.

7. P. E. Auger. Zoning and district variations of the minor elements in pyrite of Canadian gold deposits. Economic Geology, vol. XXXVI, No. 4, 1941.

8. H. Byfrlykke. The cobalt content of pyrite from Norwegian nickel ores. Chem. Abs., vol. 40, 1946.

9. I. E. Hawley. Spectrographis studies of pyrite in some Eastern Canadian gold mines. Economis Geology, vol. 47, 1952.

10. H. P. Taylor jr. Evolution of chemical composition of orebearing solutions dependent on the same source. In the proceedings of the Conference “Problems of Postmagmatic Ore Formation,” vol. 1, 1963.

11. M. Fleischer. Minor elements in some sulfide minerals. Economic Geology. Fiftieth Anniversary volume 1905–1955. Part II, Lancaster, 1955.

1. The ratio was calculated from samples with significant values for both elements. ↩