On Types of Endogenous Zoning in Gold-Ore Deposits

D. A. Timofeevskii (TsNIGRI)

Among the generalizing works on questions of endogenous zoning and the character of its manifestation, the works of S. S. Smirnov (1937), V. I. Smirnov (1957, 1960, 1963, 1965), A. D. Levitskii (1963), E. A. Radkevich (1959), Ya. Kutin (1965), Ch. F. Park (1963), and others are known to be of great value. These and other investigators have considered the types and order of manifestation of zoning both on the regional scale (zoning of regions, belts, districts) and of individual ore bodies (Smirnov, 1957, 1960), and, to a lesser degree, of deposits and ore fields. It should be noted that in these works certain specific features of the zoning of gold-ore deposits were not taken into account, although the basic principles developed in them are confirmed by the available materials on the zoning of gold deposits.

In the present communication, questions are touched upon only insofar as they relate to the types of zoning of hydrothermal gold-ore deposits proper.

Manifestations of endogenous zoning in gold-ore deposits may be grouped as follows (taking into account the classifications of V. I. Smirnov, E. A. Radkevich, Ya. Kutin, and others; see Table 1).

1. Single-stage zoning—monoascendent, in the terminology of Ya. Kutin; partly depositional zoning, according to Yu. A. Bilibin (1954), reflecting a regular change in the material composition of ores in one or several ore bodies deposited from solutions of a single stage of mineralization. It is subdivided into zoning of solution evolution and reaction zoning.

1. Evolution of solutions (properly physicochemical), caused chiefly by changes in temperature, acidity–alkalinity, the activity of ions in the ore-forming solutions, and, to a lesser degree, pressure and other factors of a physicochemical nature. Often the most important of these factors operated jointly.

Zoning of “solution evolution” may, with a certain degree of conditionality (according to the leading role of one or another factor that caused its formation), be divided into “temperature,” “depositional,” “pressure-gradient,” and “mixing of hydrothermal solutions” with vadose waters.

a. Temperature zoning, caused mainly by a decrease in the temperature of the ore-forming solution and by a change in geo-

Fig. 1. Diagram of the distribution of mineralization in the Kuranakh and Novo-El’dgin ore fields of the Aldan gold-ore district: 1—areas of development of the pyrite association; 2—the same, of the pyrite–arsenopyrite association; 3—the same, of the pyrite–chalcopyrite association; 4—areas of development of the sulfoantimonite association; 5—ore bodies not affected by vertical zoning; carbonate mineral association.

[Label on the diagram: “Content of gold, silver, and electrical [[unclear: resistance/conductivity]].”]

Central shaft
NE

Fig. 2. Diagram of the distribution of various associations in the plane of the veins of the Novo-Sukhotskoe deposit, after D. A. Timofeev: 1 — quartz–tourmaline mineral association; 2 — quartz–pyrite–sphalerite association; 3 — quartz–pyrite association; 4 — areas of pyrrhotite development; 5 — ore-mineral association; 6 — pinches of ore bodies (not to scale); 7 — emplacement. Quartz–pyrite.

thermal zones around cooling magmatic massifs with which the mineralization is genetically associated.

Such single-stage zoning is indicated at the Moina deposit in Tasmania (Williams, 1958, Park, 1963). Here tin-tungsten ores, located near the contacts, are replaced, with increasing distance from the intrusive massif, by lead-zinc, copper, and gold ores.

The vertical zoning of some deposits of the Eastern Sayan, expressed in the replacement at depth of quartz ores with complex sulfide mineralization by pyrite-quartz ores, is also regarded by V. V. Levitskii (1966) as temperature zoning.

G. M. Birovlyanskii (1954) notes temperature zoning for the early associations of the Kochkar ore field; however, the question here remains debatable. According to unpublished data of Yu. I. Novozhilov and V. M. Yanovskii, the zoning of the early stages of this deposit can more likely be attributed to lithological or depositional zoning.

b. Zoning of deposition (after Yu. A. Bilibin), caused by the evolution of the composition of solutions in connection with changes in the concentration or activity of ions, the decomposition of complex compounds, and, to a lesser extent, changes in pressure and other factors of a physicochemical character.

Such zoning at the Darasun deposit (Timofeevskii, 1967) was manifested during the formation of ore complexes of some of the seven principal stages of mineralization. On the western flank of the ore field, the composition of the productive associations of the pyrrhotite-tetrahedrite-bournonite-chalcopyrite (5th) stage of mineralization changes. In veins at depths below 210 m, a pyrrhotite-chalcopyrite association with cubanite is developed (Fig. 1), rather than the pyrite-tetrahedrite-bournonite-chalcopyrite association characteristic of the entire ore field. On the western flank of the ore field, the rocks enclosing the ore bodies—gabbroids—are enriched in titanomagnetite and magnetite; during their hydrothermal alteration, iron from these rocks was taken up by the hydrothermal solutions, and, with a sulfur deficiency, pyrrhotite rather than pyrite developed, as in other parts of the ore field.

At this deposit, opened by mine workings to a depth of more than 600 m, the vertical zoning of deposition is expressed in an increase with depth in the role of pyrite in the pyrite-arsenopyrite association; in a decrease in the content of lead sulfo-antimonides owing to an increase in antimonite (at a depth of 300–400 meters in the Glavnaya, N.-Kuznetsovskaya, and other veins); and in an increase with depth in the role of quartz at the expense of sulfides. All this is consistent with the notion of a decrease in the acidity of solutions as they rise into the upper zones.

In other regions of the USSR, single-stage zoning of the depositional type is noted in some gold deposits of Central Asia (Koroleva et al., 1966), the Middle Vitim mountain country (Zagoskin, 1963), and the Caucasus (Magakyan, 1963, et al.). Among foreign deposits, the following deserve mention: Kolar (India), whose quartz gold-bearing veins, poor in sulfides and without significant changes in composition, are traced to depths of 3300 m (Rozhkov, 1966), with only a gradual increase in the fineness of gold from 800 to 930 (Park, 1963); and Cripple Creek (Colorado), in whose ore shoots, traced to depths of about 700 m, depositional zoning is also indicated, expressed in a decrease in the gold content of the ores with depth and an increase in the amount of sphalerite in them.

c. Zoning of pressure drop and mixing of hydrothermal waters with vado-

…by vadose waters has been revealed by studies by N. V. Petrovskaya, P. S. Bernstein, M. G. Andreeva, and others at the Balei deposit of Eastern Transbaikalia. Here, under near-surface conditions of deposit formation, zoning was sharply manifested in a decrease in gold content with depth, together with other changes in mineralization. Such zoning is explained by an abrupt change in thermodynamic conditions—especially the pressure drop accompanied by boiling—in the zone where deep-seated solutions mixed with vadose waters; it arose against the background of a pulsatory ore process and the formation of multistage zoning associated with structural factors, including the heterogeneous structure of blocks with varied mineralization developed within them.

Zoning of similar genesis probably also occurred in a number of other near-surface deposits of the epithermal type, for example, in Central Chukotka (Sidorov, 1965).

1. Reactional or lithological zoning (composition of rocks, according to V. I. Smirnov); the principal factor responsible for its formation is the interaction of hydrothermal solutions with the enclosing rocks.

At the Berikul deposit in Martaiga (Timofeevsky, 1952), a replacement is observed of quartz and quartz-sulfide veins, occurring in porphyries of the Martaiga Formation, by sulfide bodies (the Khatim bodies) occurring among Cambrian limestones.¹ At the Olkhovka deposit (Eastern Sayan), lithological zoning is expressed in the replacement of quartz veins (poor in sulfides), confined to silicate rocks, by sulfide bodies occurring among carbonate rocks. The formation of sulfide bodies is explained by a decrease in the acidity of hydrothermal solutions as they passed through carbonate magnesium-bearing rocks, and by the dissolution of silica in the latter with the formation of serpentine (Timofeevsky, 1950). Lithological control in the distribution of mineralization of one type or another has also been described at other deposits of the Eastern Sayan (Levitsky, 1966).

The lithological factor played a major role in the formation of zoning at the deposits of Aldan, Lebedinoe, and others (Petrovskaya, Kazarinov, 1952, and others); at the Berezovsk deposit in the Urals (Borodaevskaya, Borodaevsky, 1947; Samartsev, 1957); and at Mother Lode in California (Connolly, Gilluly, Ross, 1937).

From all the examples cited it is evident that the lithological composition of the enclosing rocks, in the formation of zoning, is in a number of cases a very significant factor.

II. Multistage zoning (pulsatory, according to S. S. Smirnov, 1937; staged, according to V. I. Smirnov, 1965, and others; polyascendent, according to Kutin, 1965, and others) is characterized by a regular distribution in space of zones of development of mineral associations of different stages.

Among the manifestations of this class of zoning, two types may be distinguished:

1. Pulsatory-structural zoning (zoning of repeated tectonic rupture and tectonic opening, according to V. I. Smirnov), caused by the successive opening of fractures (often with enlargement) or by the emergence of new rupture disturbances during the process of ore formation.

1. Pulsational physicochemical, caused by changes in physicochemical conditions, chiefly temperature, the pH of solutions, and, to a lesser degree, pressure gradients, mobility of components, and other factors of a physicochemical character.

In its pure form, pulsational-structural zoning is manifested rarely; it is known at the Radlik deposit in Czechoslovakia (Magavek, 1963). In the Sisler vein, which fills a feathering fracture of a large fault, in the part adjoining the fault, over a distance of 200 m, carbonates predominate; in the central part—quartz; and in the northern part, removed from the fault, over a distance of 200–300 m, sulfides of copper, lead, and antimony associated with gold are developed. Such zoning, also manifested in other veins, is explained by the successive growth northward of ore-hosting fractures feathering the fault.¹

In the gold deposits of Siberia, elements of structural zoning are observed in deposits of the Middle Vitim mountain region (Zagoskin, 1963), Central Asia (Koroleva et al., 1966), and other places.

An example of pulsational physicochemical zoning, in whose formation structural factors are not leading, may be the zoning of the Darasun ore field, characterized by a concentric arrangement of zones of development of heterostadial mineral associations around pipe-shaped bodies of plagiogranite-porphyries and the associated explosion breccias (Timofeevskii, 1959, 1967, et al.).

In the central part of the Darasun ore field, early mineral associations are developed: quartz-tourmaline and quartz-pyrite (the latter also occurs in other parts of the ore field). This zone, partly overlapping it, is replaced by a zone of arsenopyrite mineralization of the 3rd stage (Fig. 2), the outer contours of whose distribution have not yet been established and are indicated only in the eastern part of the ore field. Galena-sphalerite, gold-bournonite-tetrahedrite-chalcopyrite mineral associations are developed in a zone situated closer to the center than the arsenopyrite zone, and partly overlap it. The outer boundary of distribution of the productive associations has been confidently established only on the eastern and northeastern flanks of the ore field.

Study of the character of the distribution of minerals and useful components in large exploited veins to depths of more than 600 m (Fig. 1) made it possible to identify in them not only horizontal but also vertical (direct and reverse) zoning, reflecting the concentric zoning of the ore field. It is established thereby that the contours of distribution of mineral associations of different stages of mineralization plunge from the central part of the ore field toward the periphery.

The genesis of such concentric (volumetric) zoning is conditioned by many factors, but the leading role among them, judging from all data, is played by changes in the acidity-alkalinity and temperature of solutions along their path away from the main feeder channel, fixed by the body of plagiogranite-porphyries with explosion breccias. Near the latter, during the early stages of the ore process, intense acid leaching, silicification, sericitization, tourmalinization, and other alterations associated with this process are established. It is also established that at the beginning of each stage of mineralization the acidity and temperature were higher and decreased toward the end of the stage. A general, clearly expressed tendency is revealed for temperature to fall from the early stages (380–400°C, for early quartz and tourmaline, according to determinations by Yu. V. Lyakhov by the homogenization method)

Fig. 2. Zonal-concentric distribution of mineral associations in the Darasun ore field, after D. A. Timofeevsky. 1 — plagiogranite-porphyries with explosion breccias; 2 — gold-bearing quartz–sulfide veins; 3 — rupture dislocations; 4 — zone of distribution of the quartz–tourmaline mineral association; 5 — zone of distribution of the pyrite–arsenopyrite mineral association; 6 — zone of distribution in veins of galena–sphalerite and tetrahedrite–bournonite–chalcopyrite mineral associations; 7 — zone of distribution of the sulfoantimonite mineral association. Quartz–pyrite and carbonate mineral associations are developed everywhere.

to late ones (the last stage—the carbonate stage—was deposited at a temperature below 100°C).

The role of structural factors is not decisive, since the zones of distribution of particular single-stage mineral complexes do not encompass the entire length of the vein fissures renewed at the corresponding stage, but only individual segments of the latter, lying within the contours of zones favorable in thermodynamic conditions for mineral formation. This is well reflected in the distribution of mineral complexes over the area of the ore field and within the veins, as well as within many ore shoots. The latter, according to the available materials, were opened up along their entire extent, while ore material was deposited during each stage of mineralization only in certain places of the shoots, linked with the general volumetric zonality of the ore field.

Physicochemical factors, including the geothermal gradient, also played a rather important role, according to I. V. Kunaev (1963), in the formation of multistage zonality in the deposits of Sikhote-Alin and in the regions of the lower Amur, and also, according to I. Kh. Khamrabaev (1963), in Uzbekistan during the formation of a number of deposits. In the first case, the gold–quartz deposits are located 1–6 km from granitoid massifs of Cenomanian age, nearer to which they

are replaced by gold-bearing quartz-arsenopyrite deposits, and then by quartz-molybdenite or quartz-wolframite deposits. In Uzbekistan, belts of gold-arsenopyrite-quartz veins are located at some distance from the parent Upper Carboniferous intrusions that cut through the Upper Silurian sandy-shaly sequence and are bordered along the contact by lenses of skarns with scheelite and molybdenum. In the outer zone, distant from the massif, beyond the belt of development of gold-bearing quartz veins, the latter are replaced by a zone of quartz-carbonate veins with sulfides of lead, zinc, and copper.

III. Zoning of a complex type, or complex zoning (after V. I. Smirnov), caused by a combination of multistage and single-stage zoning. Good examples of such zoning may be the Darasun and Balei deposits in Eastern Transbaikalia, deposits of the Middle Vitim (Kedrovskoe and others), the Kochkar deposit in the Urals, and deposits of Central Asia.

Table 1

Types of zoning of gold-ore deposits

Other types of zoning—such as those caused by the morphological features of ore-hosting structural elements, their intersection by rejuvenating structures, and other causes—are mainly local, although in a number of cases they are also of great importance in carrying out prospecting and exploration work.

The study of zoning, its genesis, and the regularities in the distribution of mineral paragenetic associations and gold makes it possible, in combination with other geological factors, to conduct prospecting and exploration work more effectively and to correctly assess the prospects of deposits and ore fields. A good example of the application, for practical purposes, of the identified regularities in the distribution of mineral associations is the Darasun ore field, in which, in accordance with the concentrically zonal distribution of gold-

... gold-bearing mineral associations, prospecting and exploration work and an assessment of the deposit’s prospects are carried out. At this deposit the interval of commercial mineralization is estimated at several hundred meters, and the depth of mineralization is highly variable. In the central parts of the ore field the productive parts of the ore bodies have been destroyed by erosion; in the zone where productive associations reach the surface, the depth of commercial mineralization varies depending on the erosional truncation of the productive parts of the veins; and in the peripheral parts of the ore field commercial mineralization does not reach the surface, many ore bodies are blind, and the depth of commercial mineralization is greatest.

REFERENCES

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1. Consequently, it is above all conditioned by phenomena of a structural order. —Ed. ↩↩