FEATURES OF THE CRYSTAL MORPHOLOGY AND THERMOELECTRIC PROPERTIES OF PYRITES FROM GOLD ORE DEPOSITS OF VARIOUS TYPES

A. Ya. PSHENICHKIN, A. F. KOROBEINIKOV, A. V. MATYUSHEVSKII

(Presented by Professor A. M. Kuzmin)

Pyrite is one of the common sulfide minerals of gold ore deposits of various types. In terms of the time of its formation, in each deposit there appears from one to several generations of pyrite, corresponding to definite stages and phases of mineral deposition. It occurs in gold-bearing quartz veins, veinlets, and hydrothermally altered rocks. Therefore, in order to identify the typomorphic features of pyrites from various types of gold ore deposits, the morphology of crystals and the thermoelectric properties of pyrites from quartz veins and near-ore altered rocks of the Kommunarovskoe, Balakhchinskoe, Olkhovskoe, Tardanskoe, Berezovskoe, Lyubavinskoe, and Mnogovershinnoe deposits of Siberia, the Urals, and the Far East were studied. The geological features of the deposits have been considered in sufficient detail in the literature, and therefore we do not dwell on this question here. It should only be noted that the Kommunarovskoe, Olkhovskoe, Balakhchinskoe, and Tardanskoe deposits are situated in the structural-facies zone of the Salair cycle of consolidation of the Sayan–Altai folded region; Berezovskoe—in the Hercynides of the Urals; Lyubavinskoe—in the zone of Mesozoic activation of ancient folded structures of Transbaikalia; and Mnogovershinnoe is associated with Meso-Cenozoic volcanism of the Pacific folded belt. The deposits belong to vein, stockwork (Kommunarovskoe, Balakhchinskoe, Berezovskoe, Lyubavinskoe, Mnogovershinnoe), and contact-metasomatic types (Tardanskoe, Olkhovskoe) and are predominantly confined to volcanogenic-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 located in near-contact zones or endocontacts of granodiorite intrusions of intermediate and shallow depths (down to 5–1.5 km).

On the basis of the character of the near-ore processes (development of beresites, albitites, muscovite metasomatites) and homogenization of gas-liquid inclusions in quartz (240–460°C), the mineral-forming processes of the deposits studied belong to high- and medium-temperature ones.

A number of works [1, 2, 5, 9, and others] have been devoted to the study of the morphology of pyrite crystals in connection with the conditions of their formation. However, for gold ore—

Fig. 1. Morphological types of pyrite crystals from gold-ore veins and near-ore altered rocks:
\(a\{100\}\); \(e\{210\}\); \(o\{111\}\); \(s\{321\}\); \(s'\{[[unclear]]\}\); \(d\{110\}\); \(n\{211\}\).

deposits, such information is extremely limited. Therefore the authors attempted at least partially to fill this gap. As a result of a detailed study of the morphological features of pyrite crystals from the indicated deposits, the presence of seven simple forms was established: \(a\) (100), \(e\) (210), \(o\) (111), \(d\) (110), \(n\) (211), \(s\) (321), \(s^{1}\) (312), which occur in the following combinations: (100), (111); (100), (111), (211); (100), (321); (100), (210); (100), (210), (111); (100), (210), (111), (321); (100), (210), (321); (100), (210), (111), (211); (100), (210), (111), (321), (312); (210), (111); (210), (111), (321); (210), (111), (321), (312); (210), (110), and as simple forms: (100), (210), (111). Depending on the degree of development of particular forms, 40 morphological types of crystals are distinguished (Fig. 1). Studies have established that, in different deposits, the number of combination types of pyrite crystals varies. For example, in the quartz-ore veins of the Kommunarovskoe, Ol'khovskoe, Tardanskoe, and Lyubavinskoe gold-ore fields, the largest number of combination types of crystals has been recorded—23, 22, 24, and 20, respectively (Fig. 2). By contrast, the pyrites of hydrothermally

Fig. 2. Frequency of occurrence of pyrite crystal types in quartz veins and altered enclosing rocks near the ore: quartz veins — 1 — predominant type, 2 — widespread type, 3 — sporadically occurring type; enclosing rocks — 1 — predominant type, 2 — widespread type, 3 — sporadically occurring type.

altered enclosing rocks (beresite–listvenites) of these same ore fields are sharply impoverished in combination types. The pyrites of the gold-ore veins of the Balakhchino, Berezovskoe, and Mnogovershinnoe deposits are also poor in combination types, and the number of combinations of pyrite crystals in both the veins and the enclosing rocks is almost the same (10–9, 8–8, 6–5, respectively; Fig. 2).

The frequency of occurrence of the different types of pyrite crystals from the deposits described is shown in Fig. 2. From it one can see a regularity in the distribution of pyrite crystals, expressed in the periodic appearance of crystals corresponding to serial numbers 1, 9, 16, 25, and 33. These numbers correspond to the beginning of the horizontal rows of the scheme of morphological types of crystals in Fig. 1. As Figs. 1 and 2 show, the most highly developed at the deposits are pyrite crystals of the first three vertical rows, represented by the simple forms (100) and (210) and by the combinations (100), (210), and (111). These crystal types are common both to gold-quartz veins and to

hydrothermally altered rocks. The more complex forms of pyrite crystals are specific (typomorphic) for each deposit (Figs. 1–3). The graph of the development of faces in combinations of pyrite crystals from various deposits (Fig. 3) also shows that the most highly developed pyrite forms for gold–quartz veins are the forms (100), (210), and partly (111), whereas for the enclosing hydrothermally altered rocks they are (100) and (210).

Fig. 3. Graph of the development of faces in combinations of pyrite crystals (A—in quartz veins; B—in near-ore altered rocks). Deposits: 1—Lyubavinskoye, 2—Tardan, 3—Berezovskoye, 4—Balakhchinskoye, 5—Kommunarovskoye, 6—Olkhovskoye, 7—Mnogovershinnoye.

At the Balakhchino, Berezovskoye, and Mnogovershinnoye deposits, pyrites from both quartz veins and hydrothermally altered wall rocks are characterized by almost identical development of faces in the crystals (Fig. 3). This feature of the pyrites is apparently explained by the similar conditions of their formation, i.e., by their nearly simultaneous origin both in the quartz veins and in the enclosing rocks. The identical set of impurity elements in the pyrites of the veins and of the hydrothermally altered wall rocks also points to this (Fig. 4).

Fig. 4. Distribution of impurity elements in pyrites of near-ore altered rocks and quartz veins (quartz veins / enclosing rocks).

Let us briefly consider the crystallomorphological features of pyrites from various gold-ore deposits.

Form a (100) is the most widespread both in quartz veins and in the enclosing rocks of all deposits, with the exception of Berezovskoe. But this crystal form of pyrite is most characteristic of the Kommunarovskoe, Balakhchinskoe, and Mnogovershinnoe deposits. The (100) faces of pyrite crystals are almost always covered with combination striation (according to the classification [31]) parallel to the axis of the \([001]\) zone, especially in pyrites from altered wall rocks. Step-like combination striation with polygonal growth layers often appears on the (100) faces, especially in crystals of the combination (100), (111). The accretion of layers proceeds either from one or from several centers. In the latter case, positive and negative forms of relief of the (100) faces may arise. Very rarely are the (100) faces of pyrite even, lustrous, and without even traces of combination striation. Sometimes vicinals and subindividuals of the cube and of the combination (100) with (210) appear on the (100) faces (Mnogovershinnoe, Olkhovskoe deposits). In this case the striation on the faces of the subindividual and of the host crystal, as a rule, coincides.

Form e (210) in its development is subordinate to the cube, but, like the cube, is predominant relative to the other forms and determines the appearance of pyrite crystals of all types of deposits. It is, however, especially characteristic of pyrites of the veins and beresites of the Berezovskoe deposit (Fig. 2). The (210) faces are usually even, lustrous, without traces of combination striation, but sometimes they are covered with sharp vertical striation, both continuous (Balakhchinskoe, Kommunarovskoe, Lyubavinskoe, Mnogovershinnoe) and discontinuous (Olkhovskoe). As a rule, the density of the striation increases toward the central vertical edges of the pyrite crystals. In cases where the pyrite crystals correspond to the 36th type (Fig. 1), step-like combination striation with polygonal or triangular growth layers appears on the (210) faces. In polygonal layers, some layer lines are parallel to the vertical edge of the (210) face, while others (as in the case of triangular layers) form an angle of 40–45° with this edge. Pyrite crystals from the Berezovskoe deposit are characterized by a distinctive striation of the (210) faces. On the (210) faces there is horizontal striation or a combination of horizontal and vertical striation, as a result of layer-by-layer growth of the faces. Often the striation on the (210) face assumes a very complex pattern owing to vicinal formations. In the general case, horizontal striation predominates.

Form o (111) occurs in many crystals of pyrites from gold-ore deposits. However, in terms of their development the (111) faces are much inferior to the (100) and (210) faces. In the quartz veins of the Berezovskoe, Balakhchinskoe, and Mnogovershinnoe deposits, the (111) face usually only truncates the apices of crystals at the exits of crystallographic axes of the third order. At the Kommunar, Olkhovka, and Lyubov deposits, the (111) face of pyrite crystals often dominates over the others, and the crystals acquire an octahedral habit. In the skarn–gold-ore Tardan deposit, the (111) face is developed equally with the (100) and (210) faces of pyrite crystals. In the hydrothermally altered enclosing rocks of all types of deposits, the (111) face in pyrite crystals has a sharply subordinate significance or is absent altogether. The surfaces of the (111) faces are usually even, lustrous, without traces of striation. Only on the most highly developed (111) faces do growth triangles appear, with the formation of complex patterns due to multicenter growth of the faces. The orientation of the lines of the layers forming the triang—

growth lines relative to the octahedron edges is regular: the angles between the octahedron edges and the lines of the growth triangles are 90 and 30°, respectively. Vicinals often appear on the (111) faces, which is especially characteristic of pyrites from the Kommunar ore field.

Form $s$ (321) appears rarely, although it occurs in 12 types of combinations: 8, 13, 14, 15, 22, 23, 24, 30, 31, 32, 39, and 40 (Fig. 1). In most gold-ore deposits, in pyrites of quartz–gold-ore veins the (321) face is weakly developed, and only in the beresites of the Berezovskoe deposit does the (321) face, in combination with the (312) face (types 32, 40, Fig. 1), occur rather frequently. At the Balachino and Mnogovershinnoe deposits, the (321) face has not been observed in pyrites.

Form $n$ (211) is among the rare ones and occurs only in three types of combinations: 7, 21, and 30 (Berezovskoe, Balachinskoe, and Lyubavinskoe deposits). Within the Berezovskoe and Balachinskoe deposits, the (211) face in combination with (110) and (111) forms the 7th type of pyrite crystals and occurs both in quartz–ore veins and in the enclosing beresites.

Form $d$ (110) was encountered in pyrite crystals only from the Kommunar ore field and is most likely a dissolution form.

The thermoelectric properties (TEF) of pyrites have been studied by many researchers in connection with mineral diagnostics [4] and in metallogenic studies [6, 7, 8]. Studies have shown that pyrites “pure” with respect to impurities possess hole ($p$) conductivity, whereas with impurities of $Ti^{+3}$, $Ni^{+3}$, $Cr^{+3}$, $V^{+3}$, $Sn^{+4}$, which isomorphically replace divalent iron, pyrite acquires electron ($n$) conductivity. Conversely, impurities of monovalent metals ($Au^{+1}$, $Ag^{+1}$, $Cu^{+1}$, $Tl^{+1}$) enhance the effect of hole conductivity.

Taking into account the circumstance that the possibility of isomorphic incorporation of trivalent metals into the pyrite lattice increases with increasing temperature of mineral formation (and, consequently, the role of electron conductivity of the crystals also increases), many authors have come to the conclusion that the TEF of pyrites can be used for paleotemperature purposes. In this connection, in order to identify the features of pyrites from different types of gold-ore deposits, and to establish geochemical characteristics distinguishing pyrites of some deposits from others, in addition to morphology and impurity elements of pyrites, their thermoelectric properties were studied.

The TEF of pyrites was measured on an apparatus assembled according to the scheme of V. G. Prokhorov [7]. The EMF was measured with an R-325/U millivoltmeter. Needle-shaped copper electrodes were used. The TEF of all pyrites was measured at a temperature of 250°C. The temperature on the hot electrode was determined using a chromel–copel thermocouple. The measurement results were compiled into histograms of the TEF of pyrites from various gold deposits (Fig. 5). The histograms show that pyrites from veins and enclosing rocks possess electron ($n$) and hole ($p$) conductivity. The only exceptions are pyrites of the Lyubavinskoe deposit, which possess essentially only electron conductivity, and of the Mnogovershinnoe deposit, which possess only hole conductivity. The TEF histograms of pyrites are characterized by multiple peaks, which apparently reflects the presence of several generations of pyrite in many gold-ore deposits.

Let us briefly consider the TEF of pyrites from individual deposits. Pyrites of the Lyubavinskoe deposit possess only electron conductivity, and hole conductivity is manifested in individual crystals. On the histogram, a bimodal character is noticeable in the TEF distribution and especially

Fig. 5. Histogram of the thermoelectric power of pyrites. Deposits: L — Lyubavinskoe, T — Tardan, B — Berezovskoe, K — Kommunar, Bl — Balakchinskoe, O — Olkhovskoe, M — Mnogovershinnoe; 1 — in quartz veins, 2 — in wall-rock altered rocks.

for pyrites of altered country rocks, which most likely indicates 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. The modal value corresponds to 55–60 and 70–75 mV in quartz veins and to 15–20, 55–60 mV in country rocks; the scatter is from 10 to 80 mV. The pyrites of the Tardan skarn–gold-ore deposit possess hole and electron conductivity. For \(n\)-pyrites there is a weakly expressed bimodality of the TEP histogram; the modal value corresponds to the intervals 20–25 and 60–65 mV, with a scatter from 10 to 85 mV. In \(p\)-pyrites an asymmetric unimodal distribution of TEP is observed; the modal value corresponds to the interval 10–15 mV, with a scatter from 10 to 60 mV. The pyrites of the Berezovskoe deposit exhibit both electron and hole conductivity. In the enclosing beresites, mainly \(p\)-pyrites are identified, the character of whose TEP distribution is close to lognormal. The modal value corresponds to the interval 35–40 mV, with a scatter from 10 to 70 mV. In pyrites from the enclosing beresites, electron conductivity is found only in crystal chips, which apparently can be explained by disruption of the crystal structure. Vein pyrites of the Berezovskoe deposit are characterized by hole and electron conductivity. In \(n\)-pyrites an indistinctly expressed trimodality is recorded; the histogram of the distri-

distribution of the thermoelectric power, whose modal values are 5–10, 30–35, and 60–65 mV, with a spread from 10 to 80 mV. For \(p\)-pyrites a two-peaked character is typical. The modal values of the thermoelectric power are 5–10 and 30–35 mV, with a spread from 5 to 75 mV. The multi-peaked character of the thermoelectric-power histograms of pyrites is evidently due to the presence of pyrites of different generations.

Investigation of the thermoelectric power of pyrites of different habit from the beresites of the Berezovsk deposit showed that the majority of crystals of cubic and pentagonal-dodecahedral form possess hole conductivity (Fig. 6). Electron conductivity was recorded mainly on fractures of the crystals. By contrast, pyrites of quartz veins have mainly a cubic habit and predominant electron conductivity.

Fig. 6. Character of the thermoelectric power of pentagonal dodecahedra (A) and hexahedra (B) of the Berezovsk deposit: 1 — in quartz veins, 2 — in near-ore altered enclosing rocks.

In the Balakhchinsk deposit both \(n\)- and \(p\)-pyrites were found. In the enclosing beresites \(p\)-pyrites predominate; the histogram of the thermoelectric-power distribution of these is characterized by a single peak. The modal value of the thermoelectric power of the pyrites corresponds to the interval 20–25 mV, with a spread from 10 to 65 mV. For vein \(n\)-pyrites of the Balakhchinsk deposit, a two-peaked histogram of the thermoelectric-power distribution is characteristic. The modal values are −5–10 and 35–40 mV, with a spread from 5 to 60 mV. In \(p\)-pyrites of quartz veins, the character of the thermoelectric-power distribution is similar to that of \(p\)-pyrites from beresites.

Pyrites of the Kommunar ore field possess electron and hole conductivity. The character of the thermoelectric-power distribution in the histogram tends toward a left-asymmetric single peak. In pyrites from quartz veins, \(n\)- and \(p\)-conductivity is manifested; for \(n\)-pyrites the modal value is 15–20 mV, with a spread from 10 to 80 mV, and for \(p\)-pyrites it is 20–25 mV, with a spread from 10 to 65 mV. In \(n\)-pyrites of the enclosing rocks the modal value is 20–25 mV, and in \(p\)-pyrites it is 15–20 mV.

For the Olkhovsk deposit, both \(n\)-pyrites and \(p\)-pyrites are characteristic. In quartz-ore veins \(p\)-pyrites predominate, and the thermoelectric-power distribution in the histogram is defined by a single-peaked left-asymmetric curve. The modal value of the thermoelectric power of these pyrites corresponds to 15–20 mV, with a spread from 10 to 60 mV. For \(n\)-pyrites of the veins, the modal value of the thermoelectric power corresponds to 20–25 mV, with a spread of 10–50 mV. Pyrites from the enclosing rocks are also characterized by electron and hole conductivity. The modal value of the thermoelectric power for \(n\)-pyrites is 10–15 and 30–35 mV, with a spread of 5–50 mV; for \(p\)-pyrites it is 25–30 mV, with a spread of 10–65 mV.

Pyrites of the Mnogovershinnoe near-surface deposit are characterized only by hole conductivity. The thermoelectric-power distribution in the histogram is single-peaked and left-asymmetric. The modal value

the thermoelectric power in vein pyrites is 25–30 mV, scatter 20–70 mV; in propylites and quartzites—30–35 mV, scatter 15–50 mV. Thus, the close physicochemical conditions of deposition of pyrites in veins and in the enclosing hydrothermally altered rocks are emphasized.

Conclusions

On the basis of the study of the crystallomorphology, thermoelectric properties, and impurity elements of pyrites from gold-ore deposits of various types, the following general conclusions may be drawn.

1. The typical forms of pyrites from gold-ore deposits are (100), (210), (111); (321), (312), (211), and (110) occur rarely. Most often, pyrite crystals are represented as combinations of (100), (210), (111) and as simple forms (100) and (210) (i.e., crystals of the first three vertical rows of Fig. 1). These types of crystals are typical for gold–quartz veins and near-ore altered rocks. The typomorphic forms of pyrite crystals for each deposit individually will be the most complex combinations of forms (beginning with the 4th–5th vertical row of Fig. 1).

2. For pyrites of the gold–quartz veins of all the deposits studied, faces (100) and (210) are characteristic, and for some deposits (Kommunar, Tardan, Olkhovka)—(111). The most developed faces of pyrites of near-ore altered rocks are likewise (100) and (210). The habit of pyrite crystals is determined by the degree of development of these faces.

3. The faces (100) and (210) of pyrite crystals from gold-ore deposits are complicated by vertical combinational or stepped combinational striation with layered growth of the faces. Growth triangles are often observed on the face (111).

4. In the Berezovskoe, Balakhchinskoe, and Mnogovershinnoe gold-ore deposits, the formation of pyrites in veins and hydrothermally altered country rocks apparently took place under similar thermodynamic conditions, as indicated by the identical degree of development of faces in combinations of crystals (Fig. 3), the identical number of morphological types of pyrite crystals in veins and hydrothermally altered country rocks (Fig. 2), and the similar character of the distribution of impurity elements in them (Fig. 4).

5. Pyrites from gold-ore deposits of various types possess both electron and hole conductivity, and only at the Lyubavinskoe and Mnogovershinnoe deposits has only electron and only hole conductivity, respectively, been established. The presence in one deposit of pyrites with electron and hole conductivity may indicate differences in the thermodynamic conditions of their formation at different stages of the hydrothermal process.

6. Pyrites in the form (100) and (210) from the enclosing beresites of the Berezovskoe deposit are characterized by identical hole conductivity, whereas pyrites of the same form from veins are characterized by electron conductivity. Consequently, the nature of the thermoelectric power of pyrites from one deposit does not depend on the form of the crystals of one generation under study.

7. In chips of pyrite crystals with hole conductivity, electron conductivity is often manifested. Apparently, the nature of the thermoelectric power of pyrite crystals depends on the character and degree of deformation of the crystals.

8. From the nature of the thermoelectric power of pyrites from gold-ore deposits, determining the temperature of mineral formation without taking other factors into account is extremely difficult.

1. A study of the distribution of impurity elements in pyrites from gold–quartz veins and host rocks showed that, for pyrites of the enclosing rocks, a predominance of nickel over cobalt is characteristic, whereas for vein pyrites the reverse is true.

2. The investigations carried out showed that pyrites from most hydrothermally altered rocks of gold-ore deposits of various types have morphological features and physical properties close to those of pyrites from gold–quartz veins; i.e., their formation took place under similar conditions and, probably, during the ore stage of mineral formation.

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