MORPHOLOGICAL FEATURES OF PYRITES FROM THE LYUBAVINSKOYE AND NIKOLAYEVSKOYE GOLD ORE DEPOSITS (CENTRAL TRANSBAIKALIA)

G. V. Shubin, S. A. Chubarov, A. Ya. Pshenichkin

(Presented by Professor A. M. Kuzmin)

Pyrite is one of the most widespread minerals in the composition of the quartz veins of the Nikolayevskoye and Lyubavinskoye deposits. It occurs either as disseminations of individual crystals and their druses, or in the form of irregular nests and thin veinlets. Usually pyrite has well-formed crystals with clearly expressed combinational features.

Depending on the number of simple forms entering into the combination and on the degree of their development, 18 morphological types of pyrite crystals are distinguished (Fig. 1). As detailed mineralogical studies show, pyrite deposition took place in four of the six stages of the ore process established for the indicated deposits (Table 1).

Pyrite of the first generation, closely associated with the arsenopyrite of the earliest arsenopyrite stage, preceding the formation of the quartz veins, occurs as individual grains sporadically scattered in the enclosing rocks. The crystals of this pyrite (Table 1) are represented mainly in the form {100} (type 1), less often {210} (type 15), or by a combination of the indicated forms, with a sharp predominance of the faces {100} or {210} (types 5, 13).

Pyrite of the second generation is associated with dark-gray quartz and arsenopyrite of the third quartz-sulfide stage, which form dark bands against the background of milky-white quartz of the second stage and impart a banded structure to the veins. Pyrite of this generation occurs both as simple forms {100} and {210}, and as combinations {100} + {111}; {100′} + {210}; {111} + {210}; {100} + {111} + {210} (types 1, 2, 5, 6, 7, 10, 11, 12, 13, 14, 15, 16).

Fig. 1. Morphological types of pyrite crystals from the Lyubavinskoye and Nikolayevskoye deposits

$a\{100\},\ o\{111\},$

$e\{210\},\ d\{321\}.$

Table 1

Frequency of occurrence of morphological types of pyrite forms by generations
(in percent)

Pyrite of the third generation is paragenetically associated with arsenopyrite, galena, and other minerals of the gold-sulfide stage and is characterized by an extremely great variety of forms (1–9, 12–14, 16–18).

Pyrite of the fourth generation occurs as monomineralic veinlets and in calcite as a fine sprinkling on the faces of crystals of the latter, forming well-developed crystals $\{100\}$ and $\{210\}$ and combinations: $\{100\}+\{210\}$, $\{100\}+\{111\}$, $\{111\}+\{210\}$, $\{100\}+\{111\}+\{210\}$

with the most developed face \(\{100\}\) or \(\{210\}\) (types 1, 2, 5, 6, 13, 14, 15, 16).

On the basis of the study of pyrite forms, a table was compiled of the relative frequency of occurrence of various combinations by generations corresponding to different stages of the ore process (Table 1). As can be seen from the table, crystals of types 1, 2, 5, and 13 are most widespread and occur in all generations, thus being persistent, although their frequency of occurrence differs in each generation. At the same time, such morphological types of crystals as 3, 4, 8, 9, 10, 11, 17, and 18 (Table 1) are specific only to certain generations; moreover, types 3, 4, 8, 9, 17, and 18, characteristic of the gold–sulfide association, may serve as one of the criteria of the gold content of ore bodies. As is evident from Table 1, at the beginning of the ore process pyrite yielded a small number of combinations; then this number increased, reaching a maximum in the gold–sulfide stage, and toward the end of the ore process the number of combinations again decreased.

Fig. 2. Degree of development (stability) of simple pyrite forms by generations

For each generation of pyrite, the degree of stability \(T\) (development) of the various simple forms was calculated (Fig. 2). Thus, for the first generation, 54% of the pyrite crystals are represented by the form \(\{100\}\) and 32% by the combination \(\{100\} + \{210\}\) (type 5), with a sharp predominance of cube faces. Consequently, for the first generation the leading form is \(\{100\}\), amounting to 83%, with an insignificant influence of the form \(\{210\}\) (17%) and with the complete absence of \(\{111\}\) and \(\{321\}\). In the second generation the degree of stability of \(\{100\}\) decreases to 61.4%, \(\{210\}\) increases to 35.6%, and the form \(\{111\}\) appears, amounting to 3%. The third generation is characterized by a decrease in the stability of \(\{100\}\) to 51.6%, and of \(\{210\}\) to 24.1%, with a simultaneous increase of \(\{111\}\) to 24.1% and the appearance of the faces \(\{321\}\) to 0.2%. Pyrite of the fourth generation is represented to an equal degree by the forms \(\{100\}\) and \(\{210\}\) (49% each), with a minor participation in the faceting of the form \(\{111\}\), amounting to 2%. In general, for the Nikolaevskoye and Lyubavinskoye deposits, the average stability of pyrite forms shows that the most developed morphologically is \(\{100\}\)—61.6%, followed by \(\{210\}\)—30.8%, \(\{111\}\)—7.5%, and \(\{321\}\)—0.1%. There is an opinion [1] that the forms \(\{210\}\) and \(\{321\}\) are characteristic of pyrite from medium-temperature deposits, whereas the forms \(\{100\}\) and \(\{111\}\) indicate high-temperature formation of pyrite. Taking into account that the degree of development of the forms \(\{100\}\) and \(\{111\}\) for the pyrites of the Lyubavinskoye and Nikolaevskoye deposits is 69.1%, it may be said that they formed in high-temperature stages of the ore process, which generally does not contradict the data of thermobarometry and observations of the character of the decomposition of solid solutions of sulfides [2]. The decrease in the degree of stability of the form \(\{100\}\) from pyrites of early generations to pyrites of late generations, with a simultaneous increase—

…role of {210} indicates a gradual decrease in temperature toward the end of the ore process. At the same time, the decrease in the role of the {210} form, with a simultaneous increase in the influence of the {111} form in the third generation, may indicate a temporary rise in temperature in the middle stage of the ore process, which is also confirmed by the formation of tourmaline in the fourth quartz–tourmaline stage, immediately preceding the deposition of pyrite of the third generation.

Comparison of the results for the quantitative distribution of different morphological types of crystals by generation (Table 1) with the data of spectral analyses of pyrites from different stages (Fig. 3) shows that pyrite of the third generation, characterized by the greatest diversity of isomorphic impurities, also has the maximum number of combination forms; at the same time, pyrite of the first and fourth generations, which contains a minimal number of impurity elements, also has a smaller number of simple forms and their combinations. The comparison of the degree of development of simple forms with the data on the distribution of isomorphic impurities (Fig. 3) in pyrites of different generations allows the following to be stated.

1. In the case of an increase in the content of such elements as cobalt, nickel, and copper, an increase is observed in the development of the forms {111} and {321}, with a noticeable decrease in the influence of the forms {100} and {210}. With the disappearance of the indicated impurities from the composition of the pyrites, the forms {111} and {321} also disappear completely.

2. Pyrites containing an elevated titanium content are characterized by a weakening of the influence of the form {210}.

3. With an increase in the content of isomorphic arsenic in pyrites, accompanied by a simultaneous decrease in the zinc content, a distinct increase is observed in the degree of stability of the form {100}.

Fig. 3. Composition of isomorphic impurities in pyrites of different generations

Conclusions

The combinational features of pyrite depend on the composition of the ore-forming solutions and, probably, on the content of particular isomorphic impurities.

The degree of stability of simple forms in pyrite may indirectly, taking other criteria into account, indicate the temperature conditions of crystallization.

The study and systematization of the combinational features of pyrite in mineral associations may serve as one of the criteria for distinguishing individual stages of mineral formation.

References

1. I. N. Sitkovsky, G. K. Abdullaev. Morphological features of pyrite crystals from the Bitti-Bulak deposit. — Transactions of the All-Union Mineralogical Society, Series 2, Vol. 86, Issue 4, 1957.

2. G. V. Shubin, A. V. Matsyushevsky. Genetic features of the Lyubavinskoye gold-ore deposit, Central Transbaikalia. — Proceedings of the Tomsk Polytechnic Institute, Vol. 134, 1967.