THERMOELECTRIC PROPERTIES OF PYRITES FROM SOME GOLD ORE DEPOSITS OF VARIOUS TYPES

A. Ya. Pshenichkin

(Presented by Professor A. M. Kuzmina)

S. I. Golosov (1939) was the first to draw attention to the presence of thermoelectric properties in pyrites and to the possibility of using the thermoelectric effect for mineral diagnostics. Subsequently, F. Smith (1947), G. A. Gorbatov (1957), G. I. Knyazev, V. K. Kudelya (1960), Prokhorov (1966, 1970), and others developed a method for determining the thermo-EMF in pyrites and certain other semiconducting sulfides and revealed the possibility of applying the thermoelectric properties of pyrites in metallogenic studies. The thermoelectromotive force is a sensitive typomorphic property of pyrites and other ore minerals—semiconductors—and in recent years has been widely used in the study of minerals from ore deposits.

Pyrite belongs to the class of impurity semiconductors, and “pure” pyrites possess hole ($P$) conductivity [4]. If pyrite contains an admixture of elements with a valence higher than 2 ($\mathrm{Ti}^{+3}$, $\mathrm{Ni}^{+3}$, $\mathrm{Cr}^{+3}$, $\mathrm{Sn}^{+4}$), which isomorphically replace iron ($\mathrm{Fe}^{+2}$), then the pyrite acquires electron ($N$) conductivity. Divalent ions entering isomorphically into the lattice do not change the intrinsic hole conductivity of pyrite, whereas monovalent ions ($\mathrm{Au}^{+1}$, $\mathrm{Ag}^{+1}$, $\mathrm{Cu}^{+1}$, $\mathrm{Tl}^{+1}$) enhance the effect of hole conductivity.

In view of the fact that the possibility of isomorphic incorporation of the above elements occurs at high temperatures, it should be expected that among pyrites of high-temperature deposits pyrites with electron conductivity will predominate and, conversely, among deposits of low-temperature, especially sedimentary, origin, pyrites with hole conductivity will be widespread.

Pyrite is one of the common sulfide minerals of gold ore deposits of various types. In each deposit, in terms of the time of its formation, from one to several generations of pyrite are manifested, corresponding to definite stages and phases of mineral formation. It occurs in auriferous quartz veins, veinlets, and hydrothermally altered rocks. Therefore, in order to identify features of pyrite associated with gold mineralization, to establish geochemical differences and criteria that make it possible to distinguish pyrites of some gold ore deposits from others, the thermoelectric properties of pyrites from quartz veins and near-ore altered rocks of the Kommunarovskoye, Balakchinskoye, Olkhovskoye, Tardanskoye, Berezovskoye, Lyubavinskoye, and Mnogovershinnoye

gold-ore deposits of Siberia, the Urals, and the Far East. The geological features of the deposits have been considered in sufficient detail in the literature, and therefore we shall not dwell on this question here. It should only be noted that the Kommunarovskoye, Olkhovskoye, Balakhchinskoye, and Tardanskoye deposits are located in the structural-facies zone of the Salair cycle of consolidation of the Sayan-Altai folded region; Berezovskoye is in the Hercynides of the Urals; Lyubavinskoye is in the zone of Mesozoic activation of the ancient folded structures of Transbaikalia; and Mnogovershinnoye is associated with Meso-Cenozoic volcanism of the Pacific folded belt. The deposits belong to vein, stockwork (Kommunarovskoye, Balakhchinskoye, Berezovskoye, Lyubavinskoye, Mnogovershinnoye) and contact-metasomatic (Tardanskoye, Olkhovskoye) types and are predominantly confined to volcano-sedimentary strata of various ages, broken into blocks by longitudinal and transverse faults and intruded by granitoid intrusions. The gold-ore fields have a folded-block structure and are confined to nodes of greatest complication of folded and fault structures. The deposits are spatially and, apparently, genetically associated with granitoid magmatism and are situated in the near-contact zones or endocontacts of granodiorite intrusions of intermediate and shallow depths (down to 5—1.5 km). By the character of the near-ore processes (development of beresites, albitites, muscovite metasomatites) and the homogenization of gas-liquid inclusions in quartz (240—460°C), the mineral-formation processes of the deposits studied belong to high- and medium-temperature ones.

The study of the thermoelectric power of pyrites was carried out on an apparatus assembled according to the scheme of V. G. Prokhorov [4]. The emf was measured with a milliammeter-nanovoltmeter (type R-325). Copper electrodes of needle shape were used, the design of which makes it possible to make measurements both in individual crystals (down to 1 mm) and in hand specimens and polished sections. The thermoelectric power of all pyrites was measured at a constant temperature (250°C). The temperature was determined with the aid of a chromel-copel thermocouple.

Figure 1 shows histograms of the thermoelectric power of pyrites from the deposits studied, from which it is evident that pyrites of ore veins and near-ore altered rocks possess both electron ($P$) and hole ($P$) conductivity. The exceptions are the pyrites of the Lyubavinskoye deposit, which have mainly only electron conductivity, and of the Mnogovershinnoye deposit, which has only hole conductivity.

The electron and hole conductivity of pyrites from quartz veins and near-ore altered rocks occurring in a single deposit apparently indicates sharply different thermodynamic conditions of crystal growth, two sharply different-temperature associations—low-temperature (150—200°C) for $p$-pyrites and high-temperature (more than 250—300°C) for $n$-pyrites [4].

The histograms of the thermoelectric power of pyrites are characterized by multipeakedness (especially for $n$-pyrites), which indicates the presence of several generations of pyrites in gold-ore deposits, reflecting the multistage, multiphase nature of their formation.

In the character of the distribution of thermoelectric power (Fig. 1), pyrites of different deposits show substantial differences and a certain commonality. Thus, in the Lyubavinskoye deposit, mainly $n$-pyrites are encountered, whereas in Mnogovershinnoye—$p$-pyrites. Moreover, for the Lyubavinskoye pyrites the histogram shows a two-peaked distribution of thermoelectric power, especially for pyrites of altered wall rocks, which most likely indicates

Fig. 1. Histogram of the thermoelectric power of pyrites (deposits: L—Lyubavinskoye, T—Tardan, K—Kommunarovskoye, Bl—Balakhchinskoye, O—Olkhovskoye, M—Mnogovershinnoye), 1—from quartz veins; 2—from near-ore altered rocks.

two generations of pyrites: an early one, associated with the formation of hydrothermally altered rocks, and a late one, associated with the formation of quartz-ore veins. In the \(p\)-pyrites of the Mnogovershinnoe deposit from quartz veins and near-ore altered rocks, the distribution of thermo-emf shows a left-asymmetric single peak and almost identical modal values of thermo-emf (30–35 mV). This thereby emphasizes the similar physicochemical conditions of pyrite deposition in the veins and in the enclosing hydrothermally altered rocks.

For pyrites from the enclosing rocks of the Berezovskoe and Balakhchinskoe deposits, mainly hole conductivity is characteristic; this is probably caused by the same degree of alteration of the enclosing rocks (beresitization). In the Berezovskoe pyrites from the enclosing rocks, electron conductivity is manifested mainly only in crystal fractures, which, apparently, can be explained by disruption of the crystal structure. Pyrites from the quartz veins of the Tardanskoe, Berezovskoe, Balakhchinskoe, Kommunarovskoe, and Olkhovskoe deposits have both \(n\)- and \(p\)-conductivity; for the Kommunarovskoe pyrites, \(p\)-conductivity is characteristic.

The modal values of thermo-emf of pyrites for quartz veins and near-ore altered rocks, although different for different deposits, are on the whole close and, on average, are 10–25 mV for \(n\)-pyrites and 20–40 mV for \(p\)-pyrites.

In order to determine the influence of the form of pyrite crystals on thermoelectric properties, large pyrite crystals of cubic and pentagonal-dodecahedral habit from the Berezovskoe deposit were studied (because large crystals were absent from the other deposits, such studies were not carried out). It was established that most crystals of cubic and pentagonal-dodecahedral habit from near-ore altered rocks possess hole conductivity (Fig. 2). Electron conductivity was recorded mainly

Fig. 2. Character of the thermo-emf of pentagonal dodecahedra (a) and cubes (b) from the Berezovskoe deposit: 1 — from quartz veins; 2 — from near-ore altered rocks

in crystal fractures. Conversely, pyrites of quartz veins have, as a rule, a cubic habit and predominant electron conductivity; i.e., the character of the thermo-emf of pyrites from one deposit does not depend on the form of the studied crystals of one generation.

On the basis of the study of the thermoelectric properties of pyrites from gold-ore deposits of various types, the following conclusions can be drawn.

1. Pyrites of gold-ore deposits possess electron and hole conductivity, and only at the Lyubavinskoe and Mnogovershin—

deposits, only electronic and only hole conductivity, respectively, has been established.

1. The presence, in one deposit, of pyrites with electronic and hole conductivity possibly indicates differences in the thermodynamic conditions of their formation at different stages of the hydrothermal process.

2. In the character of the distribution of thermoelectric-power values of pyrites, a multi-peaked character is observed (especially in $n$-pyrites), which apparently reflects the presence of several generations of pyrites.

3. In chips of pyrite crystals with hole conductivity, electronic conductivity is manifested more often, i.e., the character of the thermoelectric power of pyrite crystals depends on the character and degree of deformation of the crystals.

4. Pyrites in the forms (100) and (210) from the enclosing beresites of the Berezovskoe deposit are characterized by the same hole conductivity, whereas pyrites of the same forms from veins are characterized by electronic conductivity. Consequently, the character of the thermoelectric power of pyrites of a single deposit does not depend on the form of the investigated crystals of one generation.

5. Judging from the character of the thermoelectric power of pyrites of gold-ore deposits, it is difficult to infer the temperature of mineral formation without taking other factors into account.

6. The studies carried out showed that pyrites of most hydrothermally altered rocks of gold-ore deposits of various types have physical properties close to those of pyrites of gold-quartz veins, i.e., their formation occurred under similar conditions and, probably, during the ore stage of mineral formation.

REFERENCES

1. S. I. Golosov. Possibilities of applying the thermoelectric effect for the diagnostics of ore minerals. — In the book: Sov. geol., vol. 9, 10, 1939.

2. G. A. Gorbatov. Thermoelectric properties of pyrite and galena and a possible relation to the temperature of mineral formation. Methods of investigation of mineral raw materials. Transactions of VIMS, Moscow, 1957.

3. G. I. Knyazev, V. K. Kudelya. Semiconductor properties of galenas and pyrites as a criterion of ore-formation conditions. Publishing House “Naukova Dumka,” Kiev, 1969.

4. V. G. Prokhorov. Pyrite. Krasnoyarsk, 1970.

5. V. G. Prokhorov, A. D. Titarenko, I. A. Khayretdinov. Features of pyrites of gold-ore deposits of the south of Krasnoyarsk Krai. Materials of a geological conference. Krasnoyarsk, 1966.

6. F. G. Smith. The pyrit geothermometr. “Econom. Geol.,” v. 42, No. 6, 1947.