On the Study of Zonation in Quartz-Gold Ore Deposits: The Sarala Ore Field as an Example

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

Quartz veins carrying gold mineralization within the Sarala ore field, in terms of structural-geological features, belong to those that fill fracture fissures—radial and concentric—and sublatitudinal delamination structures. The veins that fill delamination fractures are of the greatest industrial interest; such are the veins of the Vstrechnoye, Kaskadnoye, and Andreevskoye deposits. The quartz veins of the named deposits consist of separate lenses, terminating down-dip and along strike in thin stringers, branching and pinching out in the altered enclosing rocks.

A characteristic feature of the linkage of individual lenses is their overlapping one another. Each new lens, with increasing depth and with advance from west to east, begins on the hanging-wall side of the pinching-out lens. The regular arrangement of the lenses relative to one another and their similar internal structure provide grounds for assuming that their formation was due to similar geochemical processes periodically repeated in the deposits. We find confirmation of this assumption in the features of the distribution of trace elements in the quartz and pyrite of individual lenses composing the Kaskadnoye deposit and other veins of the ore field, and also in the values of the temperatures of deposition of ore minerals.

Determination of the contents of trace elements in monomineralic quartz and pyrite was carried out by the method of emission spectral analysis. For quartz, a procedure of semiquantitative analysis was adopted, with a relative determination error of ±50–100%; for pyrite, a procedure of quantitative analysis with a relative error of ±50–20%. The establishment of features of the distribution of elements by individual levels, lenses, and deposits was carried out by constructing histograms, on the basis of which the variance of concentrations, the distribution law, and the modal and geometric-mean contents were determined. The histograms were constructed on a logarithmic scale according to the method described and substantiated earlier (Prokhorov, Miroshnikov, 1968).

Taking into account the great extent of the lenses in the dip direction, samples were grouped by individual levels; for each level—

the mean content of the zone was calculated. Such grouping made it possible to elucidate the features of the vertical zonation of the distribution of elements in individual lenses and in the deposit as a whole.

The results of studying the distribution of some elements in the quartz of different lenses of the Kaskadnoye deposit are shown in Fig. 1. For compactness and greater clarity, the largest mean horizontal contents of all elements in the graphs are denoted by segments of equal maximum length; smaller contents are shown as fractions of this value. As can be seen from the graphs, for lenses exposed along dip and strike over their entire extent (I, X), the greatest enrichment of quartz in lead, silver, copper, and zinc is observed in the upper and middle parts. This feature is also retained for lenses V and XI, which are exposed only partially, and may be used in estimating the probable depth extent of the unexposed parts of the lenses. As microscopic study of the quartz shows, the main part of the impurity elements in it is associated with the presence of microinclusions of pyrite, sphalerite, galena, arsenopyrite, and chalcopyrite. Polymetallic mineralization in the deposit was superimposed in character. The minerals of this stage were deposited in already formed quartz lenses, in which, of the sulfides, only early pyrite had been developed.

In each lens, zones of maximum enrichment of quartz in impurity elements are distinguished vertically. Below this zone, deposition of the elements apparently proceeded weakly, owing to the still undisturbed carrying capacity of the hydrothermal solutions; above it, only those insignificant quantities of the elements were deposited that remained in the solutions after passing through the zone of active precipitation. For different elements, the zones of maximum enrichment are located at somewhat different levels. Closest to the upper boundary of the pinching-out of the lenses are the zones of enrichment in lead; somewhat lower are the zones of maximum enrichment in silver; still lower—in zinc. According to V. I. Smirnov’s classification (1965), zonation of this kind belongs to depositional zonation and is caused by the different mobility of elements that migrated and precipitated simultaneously.

The common character of the zonal deposition of lead, silver, and zinc in lenses lying at different hypsometric levels indicates the same nature of the change in the carrying capacity of the hydrothermal solutions of the polymetallic stage as they passed through the quartz lenses. At the same time, the similar contents of these elements in different lenses give grounds to suppose that the superimposed mineralization in each lens arose as a result of the penetration into it of “fresh,” “independent” portions of hydrothermal solutions with similar or identical initial composition, and that each individual lens, forming part of the complex Kaskadnaya vein, may be regarded as a separate deposit with a similar history of formation.

Manganese is distributed differently in the quartz of the deposit: its content increases with depth in each lens individually and in the deposit as a whole. Only in isolated cases, for example in lens No. 1, is the upper part enriched in manganese. This is determined by the occurrence of this part of the lens in the oxidation zone, where accumulation of iron and manganese hydroxides takes place in the form of films and coatings, from which it is impossible to clean the quartz when separating the monomineral fraction. The increase in manganese content in quartz with depth corresponds to the case of joint deposition of the impurity and the host mineral. As is known, entry of impurities into the lattice of the host mineral during joint deposition occurs more freely and completely at higher—

Fig. 1. Change in the content of trace elements by minerals of individual lenses of the Kaskadnoe deposit

...at lower temperatures and pressures, which, in general, corresponds to the great depths at which the vein was formed.

For lens XI, over a distance of 300 m down dip, the manganese content increases from 0.002 to 0.3%. Over a distance of 280 m down dip of lens V, the manganese content in quartz increases from 0.015 to 0.19%. In the deposit as a whole, the manganese content in quartz from horizon 4 to horizon 14 (approximately 700 m down dip and 400 m vertically) increases from 0.016 to 0.214%, i.e., by a factor of 13.

The distribution of trace elements in the quartz of other veins of the ore field is characterized by the following features.

1. The fluctuation in average contents of trace elements in the quartz of different veins amounts to 1–2 orders of magnitude; the enrichment in arsenic, lead, copper, and manganese is the most contrasting.

2. The highest concentrations of elements of the polymetallic stage of mineralization—lead, zinc, silver, arsenic—are observed for quartz from split-off veins. Differences in the degree of enrichment of quartz from radial and concentric veins are insignificant and are mainly deter-

are distinguished by the spatial position of the veins relative to the contact with the Ararat granitoid massif, which adjoins the ore field from the north. All veins lying in the exocontact zone of the massif at a distance of up to 1–1.5 km from it, and on the northwestern flank of the ore field, where, judging by individual apophyses, the named massif comes close to the surface, although it has not been exposed by erosion, are distinctly enriched in arsenic, silver, lead, and zinc.

1. Manganese is distributed differently. The maximum enrichment of quartz in manganese is observed for the veins of the southern and central parts of the ore field, where its average content in individual veins ranges from 0.1 to 0.4%.

For the Kaskadnoye deposit, such a manganese content is observed at a level 300–400 m below the surface. The similarity in the character of the distribution of trace elements in quartz, regardless of the structural type of the veins, indicates that all of them were formed from a single source under similar physicochemical processes accompanying the deposition of quartz in differently oriented fracture systems. The enrichment of quartz in manganese depended only on the depth of vein formation, increasing with rising pressure and temperature. Thus, the greater enrichment in manganese of the quartz of the veins in the central part of the ore field may be regarded as an indicator of the greater depth of their formation. By analogy with the Kaskadnoye vein, it should be assumed that the depth of the erosional truncation in the central part of the ore field (the core of the brachyanticline) exceeds by no less than 400 m the erosional truncation of the northern part of the ore field.

The greater erosional truncation of the central part of the ore field is also confirmed by geological data. In the central part of the field the veins occur in rocks of the Middle Sarala Formation, in its upper and lower subformations; on the northern and northwestern flanks they occur in contact with the Upper Sarala and within the Upper Sarala Formation itself. The thickness of the Upper Sarala Formation, which has been completely cut off in the central part of the ore field, according to geological constructions carried out by V. N. Lyakhnitskii in 1956, is 800–1000 m. The thickness of the upper subformation of the Middle Sarala Formation reaches 1000 m. Thus, on the basis of general geological considerations, we may suppose that the formation of the quartz veins now exposed at the surface in the central part of the ore field took place at a depth more than 2 km greater than the depth of formation of the veins of the northern and northwestern flanks. However, we believe that this value is, of course, overestimated. The basis for the latter view is the presence of regularities in the distribution of veins that make up conjugate fracture systems, which were initiated during the folding of the rocks of the ore field into a brachyanticlinal fold. By the time the fractures began to be filled with the material of the quartz veins, the rocks had been gathered into a pronounced fold, and, evidently, the rising central part of the ore field could already have been substantially eroded. The additional erosion of the central part of the ore field, which we have determined from the degree of manganese enrichment of the quartz to be 400 m, took place after the formation of the quartz veins, with the continuing uplift of the core of the brachyanticlinal fold.

As already noted, the zonality in the distribution of the elements of the polymetallic stage of mineralization is opposite in character to the distribution of manganese: the veins of the northern and northwestern flanks of the ore field are enriched in these elements. The latter evidently indicates that the veins of this part of the field were located closer to the source of mineralization,

At present it is difficult to answer the question of whether the hydrotherms of the polymetallic stage were juvenile waters—products of the differentiation of the Ararat pluton—or whether they were due to processes that accompanied the granitization of rocks in this center; whether the introduction of elements of the polymetallic stage of mineralization is connected with their separation from the magmatic center or whether they were carried out from the enclosing rocks. One thing is clear: as the contact with the massif is approached, the intensity of the superimposed polymetallic mineralization increases and changes qualitatively toward a predominance of copper-arsenic mineralization. The replacement of galena and galena-sphalerite mineralization by copper-arsenic mineralization also proceeds at depth, which is observed at the Kaskadnoe deposit. The presence of such zonality makes it theoretically possible to regard veins in which only galena is found at the surface as potentially the richest.

Given a sufficient extent of the vein, below purely galena mineralization one should expect galena-sphalerite, then sphalerite-chalcopyrite, and then chalcopyrite-arsenopyrite mineralization. Since all sulfide associations in the quartz veins of the Sarala ore field are accompanied to one degree or another by gold, the prospects of such veins are the most favorable. In practice, however, it is always more advantageous to begin development immediately from the richest horizons, and these, as a rule, are horizons with complex associations of sulfide minerals.

This last consideration compels special attention to be paid to the fields of distribution of veins tending toward the contact with the Ararat massif or its apophyses.

In concluding the discussion of the features of zonality in the distribution of trace elements in the quartz of the gold-ore veins of the Sarala ore field, the following conclusions may be drawn.

1. The study of the distribution of lead, zinc, silver, copper, arsenic, and manganese in quartz made it possible to establish a clear vertical zonality of deposition within individual lenses composing the Kaskadnoe deposit, and for the Kaskadnoe deposit as a whole.

2. The zonality is manifested in a greater enrichment of the upper horizons of the lenses in lead, silver, and zinc; the lower horizons are enriched in arsenic and copper. The manganese content increases with depth.

3. The difference in the character of the distribution of manganese and of the elements of the polymetal group gives grounds to assert that their deposition occurred during different stages of mineralization.

4. The manganese content in quartz may be used as an indicator of the depth of formation of a quartz vein.

5. The regular change in the content in quartz of elements of the polymetal group within individual lenses makes it possible to assess the prospects of still unopened parts of quartz veins.

6. Since productive gold mineralization is associated with the polymetallic stage of mineralization, the prospectivity of veins is determined by an elevated content in quartz and pyrite of lead, silver, arsenic, and zinc, and by their zonal distribution along the strike and dip of the vein.

7. The study of zonality in the distribution of trace elements in quartz and pyrite, according to data from approximate quantitative analysis, may be recommended for establishing the prospects for the extension of mineralization to depth within individual quartz veins, and also for assessing the prospectivity of ore fields.

References

Prokhorov V. G., Miroshnikov A. E. On the question of the accumulation and processing of spectral analyses in geological investigations. Lithology and Mineral Resources, 1968, No. 3.

Smirnov V. I. Geology of Mineral Deposits. Moscow: Nedra, 1965.