Paragenetic Relationships and Sequence of Mineral Formation in the Sarala Gold-Ore Deposits in the Kuznetsk Alatau

A. E. Miroshnikov, V. G. Prokhorov (SNIIGGiMS)

The quartz-vein deposits of the Sarala gold-ore field are located in the watershed part of the Kuznetsk Alatau. The ore field is composed of an Upper Precambrian–Lower Cambrian volcanogenic-sedimentary sequence, deformed into a dome-shaped asymmetric anticlinal fold with an axis plunging northward toward the Ararat granitoid massif. The deposits are confined to the northern part of the anticline and occupy bedding-plane separation structures in the hinge of the fold, as well as concentric and radial shear fractures, whose origin is associated with the period of formation of the brachyanticlinal structure of the ore field. These structures sometimes contain porphyrite sills, which are generally concordant with the quartz veins. In some areas the porphyrite sills are cut by the latter and have undergone near-ore alteration expressed in chloritization and silicification. The quartz veins, in turn, are cut by numerous post-ore submeridional dikes of diabase, gabbro-diabase, and diabase porphyrites, which occupy vaulted rupture fractures. At contacts with the latter, the quartz veins undergo contact metamorphism [1, 10, 13].

In terms of mineral composition, the deposits of the Sarala district belong to the low-sulfide quartz-gold-ore formation, whose place in the established genetic classifications is in the mesothermal type of hydrothermal deposits. The ores are based on quartz, the amount of which ranges from 90 to almost 100%. Among the accessory minerals are pyrite, scheelite, calcite, ankerite, and albite; galena, sphalerite, chalcopyrite, arsenopyrite, pyrrhotite, sericite, and chlorites are widespread in small quantities; pale ores, silver sulfides, apatite, rutile, native silver, bismuth, and some other minerals occur sporadically. Gold in the quartz veins is present in native form and in sulfides. The fineness of gold ranges from 500–600 to 900; high-silver gold and electrum are rarely encountered.

In the low-sulfide ores of the Sarala deposits, crosscutting relationships of different-stage mineral assemblages of hydro-

of the hydrothermal stage in the form of veinlets and veins, with the exception of a single example of the intersection of a vein, composed of sulfide-free dark-gray quartz, by a vein of light-gray quartz with sulfides at the Andreevskoe deposit, cited by A. M. Khazagarov [15]. But even this example can be explained merely by the simple superposition of sulfides on zones of cataclased quartz, in which the latter, in connection with this, underwent recrystallization and lightening. More often what is observed is simple spatial coexistence, the superposition of disseminations of minerals of later stages on earlier vein filling.

Therefore different investigators of the vein gold–quartz deposits of the Sarala ore field, using only structural analysis of the ores, outline sharply different numbers of mineralization stages.

The differences in the conclusions obtained when using one and the same method of study prompted us, in order to verify them, to apply a number of additional independent methods of investigation. For these purposes the authors attempted to consider the features of the distribution of typomorphic characteristics of the minerals of the Sarala deposits, taking the character of their distribution as a genetic characteristic of the process of mineral formation. As typomorphic characteristics, the contents of trace elements, thermoelectric properties, decrepitation and homogenization temperatures, the composition of gas–liquid inclusions, and luminescence were considered.

The occurrence of a trace element in the host mineral is determined by the processes of formation of the mineral. An element may be captured during the period of crystallization of the mineral or introduced by solutions of subsequent superimposed stages of ore formation and deposited on an earlier-formed mineral.

Considering the character of the distribution of a trace element in the host mineral as a genetic characteristic of the process of mineral formation [6, 9, etc.], several types of distributions can be distinguished that correspond to features of the process.

In the case where deposition of the host mineral and the impurity took place during one and the same period from one and the same solution, a uniform distribution of the impurity is characteristic. The content of the trace element in the host mineral is marked by constancy and small deviations from the mean value, i.e., the dispersion of impurity concentrations is insignificant. The distribution of the trace element obeys a definite law and is reflected by a single-peaked distribution histogram. Trace elements deposited from a single mineralizing solution together in one and the same stage of the process show the presence of a correlation relationship. At the same time, a positive correlation between trace elements indicates simultaneous deposition of the elements and of the carrier minerals of these elements, whereas a negative one indicates sequential deposition in one or different stages of the process. The absence of relationships between them indicates that they are products of different stages of the process [9, 17].

If accumulation of a trace element took place in two or more stages, there will be superposition of different distributions of one and the same element, and the concentration-distribution graphs will acquire heterogeneity, two or more peaks [2, 5, 16, 18].

Thus, the study of the character of the distribution of various elements in one and the same mineral provides considerable information in the geochemical study of mineral parageneses.

The character of the distribution of trace elements in pyrite from the Kaskadnoe deposit is presented in the form of histograms, based on data from quantitative spectral analysis of 266 samples for lead, silver, copper,

nickel, manganese, and, for arsenic, according to semiquantitative spectral analysis of the same samples (Figs. 1, 2). The choice of precisely these elements is due to the fact that some of them are regarded as hydrothermal companions of gold (lead, silver, arsenic) [7, 11, 12], while others are widely distributed in the host rocks of the ore field (manganese, nickel, copper).

Fig. 1. Histograms of the distribution of elements in pyrite of the Kaskadnoe deposit according to quantitative spectral analysis (analyst A. D. Titarenko, Kuznetsk Branch of SNIIGGiMS)

Along the abscissa axis in the graphs, concentration classes are plotted at logarithmically equal intervals; along the ordinate axis—the number of samples that showed the given concentration.

Qualitatively, judging from the configuration of the histograms, the distributions of manganese, copper, and nickel may be assessed as lognormal. A study of the agreement of the distribution laws with the logarithmic-normal law, carried out using statistical goodness-of-fit criteria—the coefficients of skewness and kurtosis, and Pearson’s constant—showed [14] that the distribution of manganese corresponds to the logarithmic-normal distribution; the distributions of copper and nickel are also close to lognormal, differing only slightly in “steepness.”

Lead and silver give distributions with a clearly expressed bimodality, which is a consequence of the superposition of two types of distributions with shifted modes [16].

Fig. 2. Histogram of the distribution of arsenic in pyrite of the Kaskadnoe deposit, according to data from approximate-quantitative spectral analysis
(analyst A. F. Zheltova, KO SNIIGGiMS)

To study the features of these distributions, the approximate method of truncations was used [4, 16]. The significance of differences in their mean values and variances was determined using the Fisher and Student criteria [3, 18]. Each of the truncated distributions was tested for agreement with a lognormal distribution using Pearson constants.

The calculations performed [14] confirm the existence, for lead and silver, of the superposition of two types of distributions. At the same time, only the right-hand distributions of both elements agree with the lognormal law; the left-hand distributions do not correspond to it.

A distribution with two modes, one in the region of low concentrations and the other in the region of higher concentrations, is also given by arsenic. A weakly expressed third peak in the interval 0.2–0.5% is due to the irregularity of the distribution and requires smoothing, since testing the hypothesis of a difference in variances by the Fisher criterion gives no basis for separating it.

Thus, according to the characteristics of the distribution, three groups of elements are distinguished. The first group includes manganese, which has a single-peaked lognormal distribution. The second group consists of copper and nickel, which have symmetrical single-peaked distributions that differ somewhat from a lognormal distribution in their “steepness.” And finally, lead, silver, and arsenic have two-peaked distributions with two superposed types of distributions and form a third separate group of elements.

The study of the correlation relationship between the elements studied—

by quantitative spectral analysis showed that only between lead and silver is there a substantial positive correlation. Correlation relationships between other pairs of elements are absent [14].

A bimodality expressed with varying distinctness is also revealed in studying the distribution of gold in the ores [8]. Comparison of parameter estimates for the left- and right-truncated distributions in several samples showed their significant differences, and graphical studies of their agreement with theoretical distribution laws by means of probability templates indicated their closeness to the lognormal law. This makes it possible to assign gold to the group of elements characterized by bimodal distributions.

The separation of impurity elements into the same groups according to the character of the distributions is also observed in quartz (Fig. 3), according to data from semiquantitative spectral analyses, when the order of concentrations is divided into a smaller number of intervals.

Relying on the considerations presented above concerning the relationship between the character of the distribution of impurity elements in the host mineral and its history of formation, and on the obtained results of studying the features

Fig. 3. Histograms of the distribution of elements in quartz of the Kaskadnoe deposit according to data from semiquantitative spectral analysis (analyst A. F. Zheltova, KO SNIGGIMS)

From the distribution of trace elements in pyrite and quartz and of gold in the ores, the following genetic conclusions may be drawn:

1) the uniform distribution of trace elements in pyrite and quartz indicates a similar or identical history of their formation in the ore bodies;

2) the accumulation of manganese and of the main amount of copper and nickel occurred simultaneously with the deposition of quartz and pyrite, which is expressed in the homogeneous distribution of these elements in the minerals;

3) the accumulation of lead, silver, arsenic, and gold took place during two periods of the mineral-forming process, as indicated by their heterogeneous, bimodal distribution;

4) the two distinguished periods differ in the elements accumulated and in character, since among the elements of the first of them there are no correlation relationships, whereas among the elements of the second a direct positive correlation is established, with no relationship between the elements of the first and second periods.

Such relationships among elements in minerals and among the minerals themselves could have arisen under the following sequence of processes.

In the first stage of mineral formation, quartz and pyrite were deposited with enrichment in manganese and small amounts of copper, nickel, lead, silver, arsenic, and gold. The formation of a large quantity of quartz occurred through the filling of opening cavities with silica, partly introduced by hydrothermal solutions and partly borrowed from the enclosing rocks. In zones of crushing, quartz was deposited, replacing fragments of rocks. The formation of pyrite proceeded mainly through the borrowing of iron from the rocks enclosing the quartz vein. Trace elements were likewise borrowed from the enclosing rocks. Owing to their unequal amounts in the rocks, they produced different and non-correlating distributions.

The enrichment of quartz and pyrite in lead, silver, arsenic, and gold, and the formation of the proper minerals of these elements, occurred during the superposition of the second, polymetallic stage of mineralization. The introduction of these elements determined the appearance of second maxima in their distributions. The absence of introduction of manganese and of substantial introduction of nickel and copper preserved the primary distribution of these elements in quartz and pyrite. The hydrothermal solutions of the second stage apparently did not contain the necessary material and did not entail the formation of new pyrite and significant quantities of quartz; they merely promoted some recrystallization of them.

This is confirmed by data from the study of the thermoelectric properties of pyrite and the decrepitation temperatures of quartz, pyrite, and other minerals.

The histogram (Fig. 4) shows the distribution of the magnitudes of the thermoelectric forces of pyrites from various veins of the Sarala ore field. This summary histogram was constructed on the basis of 3822 measurements made on 484 monomineralic samples of pyrite. As is evident from the histogram, the overwhelming part of the pyrite of the deposits of the ore field has hole conductivity, which characterizes it as a relatively low-temperature formation. An insignificant amount of pyrite possessing electron conductivity is the product of recrystallization of early pyrite under the influence of the high-temperature hydrotherms of the polymetallic stage. This is indicated by the uneven character of the distribution of areas with electron conductivity within the main mass of hole-type pyrite and by the absence of new crystals and aggregates composed entirely of pyrite with electron conducti-

…as is the case during the deposition of pyrite from high-temperature solutions.

Fig. 4. Composite histogram of the distribution of TEP values in pyrites of deposits of the Sarala ore field

Electronic conductivity, moreover, is observed in pyrites heated by the heat of post-ore dikes, where, under conditions of high-temperature contact metamorphism, recrystallization of pyrite and its alteration took place, up to transformation into pyrrhotite.

The recrystallization of pyrite under the influence of the heat of solutions of the polymetallic stage and the heat of post-ore dikes is also confirmed by decrepitation data. For pyrite, the presence is established of specimens with inclusions decrepitating in the low-temperature region (up to 100°C); specimens with inclusions breaking down in the high-temperature region (300–400°C); and specimens—the latter being the most numerous—with inclusions decrepitating over a broad temperature range from 100 to 400°C.

Low-temperature pyrites are pyrites from parts of veins remote from contacts with post-ore dikes and not affected by superimposed processes of the polymetallic stage of mineralization. In them, inclusions have been preserved that reflect the temperature of formation of early metasomatic pyrite. Pyrites with inclusions decrepitating over a broad temperature range are usually found adjacent to minerals of the superimposed polymetallic stage. High-temperature pyrites are situated near contacts with post-ore dikes.

Quartz of the deposits of the Sarala ore field is characterized by the same decrepitation data (Fig. 5), and consequently by an analogous history of formation and existence.

Fig. 5. Composite decrepitation diagrams for quartz of different types: I — primary low-temperature quartz (from 8 samples), II — recrystallized quartz from vein areas with superimposed polymetallic mineralization (from 5 samples), III — metamorphosed quartz from contacts of veins with post-ore dikes (from 7 samples).

Additionally drawing on information on the distribution in the deposits and on data concerning the temperature intervals of decrepitation of sphalerite, chalcopyrite, galena, and arsenopyrite, and also taking into account the character of luminescence and the distribution of yttrium and phosphorus in calcites, we have represented the sequence of mineral formation and the history of development of the deposits of the Sarala ore field in the form of a paragenetic scheme (Fig. 6).

STAGE OF MINERAL FORMATION

Hydrothermal: metasomatic stage; polymetallic stage. Metamorphogenic. Hypergene.

Minerals: ore-hosting; principal \(>1\%\); \(0.1\text{–}1\%\); accessory \(<0.1\%\).
Elements: Si, Fe, S, Ca, Zn, Pb, As, Ag, Cu, Mn, U.

Legend

1 — primary deposition of material;
2 — redeposition;
3 — metamorphism and recrystallization with addition;
4 — metamorphism without addition;
5 — boundary of mineral stability.

Fig. 6. Scheme of the sequence of mineral formation in the deposits of the Sarala ore field, according to data from the study of typomorphic features of minerals. 1 — primary deposition of material, 2 — redeposition, 3 — metamorphism and recrystallization with addition, 4 — metamorphism without addition, 5 — boundary of mineral stability.

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