Types of Hypogene Zoning of Mineralization in Precambrian Gold-Bearing Provinces

(Patom Highlands)

V. A. Buryak, N. P. Popov

(Institute of the Earth’s Crust, Siberian Branch, Academy of Sciences of the USSR; Irkutsk Geological Administration)

Within the Patom Highlands, on the basis of prospecting and exploration work carried out recently, depending on the genesis of the formations and their order, we distinguish three types of zoning:

1) regional zoning,
2) zoning of ore fields and deposits,
3) zoning of individual ore bodies.

The geological structure of the region includes metamorphosed Upper Proterozoic sandy-clayey and carbonate rocks of the Kadalikan, Bodaibo, and partly Ballaganakh subseries of the Patom series [4].

In tectonic terms, its central part is the large Bodaibo synclinorium, while the southwestern part embraces the southeastern limb of the Mama synclinorium.

Magmatic rocks are developed chiefly in the peripheral parts of the region and are represented by synorogenic granites and granite-pegmatites of Upper Proterozoic age and by postorogenic Paleozoic (?) granitoids.

The degree of metamorphism of the sedimentary sequences is sharply uneven and reveals a well-expressed regional zoning [2]. Our observations show that gold-ore mineralization likewise displays, in the character of its occurrence, regional zoning in the closest connection with metamorphism (Fig. 1).

1. Regional Zoning

Regional zoning is represented by the following facies mineral zones:

a) a zone of development of pegmatites, granite-pegmatites, and barren (“empty”) quartz veins; it spatially coincides with the amphibolite facies of regional metamorphism.

b) a zone of non-gold-bearing quartz veins with biotite and tourmaline, very weakly mineralized by pyrrhotite and chalcopyrite; it lies within the epidote-amphibolite facies of regional metamorphism.

To this type belong quartz-vein ore sources of placers in the basin of the Marakan River and the upper reaches of the Zhuya River. Their gold content very rarely reaches 0.2–1.0 g/t;

c) a zone of weakly gold-bearing quartz veins and increased gold-sulfide mineralization in the enclosing rocks; developed among deposits of the biotite-chlorite subfacies of the greenschist facies of metamorphism.

Vein formations within this zone are represented by quartz and quartz-carbonate veins with chlorite, biotite, apatite, pyrrhotite, pyrite, chalcopyrite, and other sulfides. The amount of ore minerals in them is extremely insignificant, and the gold content is poor, although individual samples show contents of up to 6–12 g/t. Sulfide mineralization is represented by pyrite and pyrrhotite with an insignificant admixture of arsenopyrite, galena, pentlandite, and sphalerite. Its gold content is more uniform than that of the quartz veins, although it is also weak. In individual areas the gold content is up to 10–15 g/t;

d) a zone of the most gold-bearing quartz veins with subordinate development of gold-sulfide mineralization is confined to weakly metamorphosed enclosing rocks (up to the sericite-chlorite subfacies of greenschist metamorphism). Most typical for it are fields of development of poor-sulfide quartz, quartz-carbonate, and quartz-albite veins. The usual ore mineral in them is pyrite; chalcopyrite, galena, and gray ores occur. Areas of increased ore mineralization in the enclosing rocks in this zone are of subordinate importance and are composed predominantly of pyrite with small amounts of galena, pyrrhotite, and chalcopyrite. The gold content both of the veins and of the pyritized enclosing rocks is weak, although in some samples it reaches several tens of grams per 1 ton.

It is important to emphasize that the noted dependence of the character of mineralization on the intensity of metamorphism of the enclosing rocks is jointly expressed and is strictly regular. All this probably indicates a close, possibly genetic, connection of ore formation with the processes of regional metamorphism.

The identification of regional zonality in the distribution of gold-ore mineralization is of great practical importance in evaluating the prospects of primary ore occurrences and of river valleys and basins for placer gold content.

2. Zoning of Ore Fields and Deposits

The following are distinguished: stadial and facies zoning, and zoning of the composition of the enclosing rocks.

a. Stadial zoning is manifested most fully. It is caused by the formation of gold-bearing quartz veins and gold-sulfide mineralization in the enclosing rocks at different stages of mineralization [3]. Moreover, gold-sulfide mineralization is predominantly developed at deeper levels in comparison with the quartz gold-bearing veins, which leads to a characteristic vertical zoning in the distribution of mineralization.

This regularity has been studied most fully at the “Sukhoi Log” deposit.

This deposit is confined to an anticlinal fold of sublatitudinal extent, in the core of which carbonaceous quartz-sericite phyllite-like schists crop out, while the limbs are composed of calcareous-clayey schists, calcareous siltstones, and limestones. The axial surface of the fold, in harmony with the general folded structure of the region, is overturned to the south. Its northern limb dips at angles of 25–30°, and the southern, overturned limb dips northward at angles of 40–70° (Figs. 2, 3). In the core of the fold the rocks are intensely broken by fractures of axial cleavage, po-

![Geological map and section with legend]

Fig. 1. 1 — Iligir Formation, 2 — Bodaibo Subseries, 3 — Kadalikan Subseries, 4 — Ballaganakh Series, 5 — Mama Series, 6 — Konkudero-Mamakan intrusive complex (Pz), 7 — Mama-Oron intrusive complex (Pt₃), 8 — tectonic disturbances, 9 — zone of a deep fault according to geological-geophysical data, 10 — zone of gold-bearing quartz veins with polymetallic mineralization and gold-bearing sulfide (pyrite) “mineralized zones,” 11 — zone of “barren” quartz veins (the enclosing rocks are weakly mineralized with pyrrhotite), 12 — zone of weakly gold-bearing quartz veins with pyrite-pyrrhotite mineralization (the enclosing rocks carry pyrite-pyrrhotite mineralization), 13 — zone of micaceous pegmatites.

Fig. 2. 1 — gray sandstones with subordinate interbeds of carbonaceous phyllites and siltstones; 2 — carbonaceous quartz-sericite-chlorite schists (phyllites) with rare interbeds of carbonaceous siltstones and sandstones; 3 — calcareous-clayey schists and siltstones; 4 — micaceous gray limestones with rare interbeds of calcareous schists; 5 — gray limestones; 6 — dark calcareous siltstones with interbeds of schists; 7 — carbonate mineralization (siderite, ankerite); 8 — pyrite mineralization; 9 — quartz veins with siderite and ankerite; 10 — quartz veins with calcite, pyrite, and gold; 11 — quartz veins with gold and ankerite; 12 — dark-gray quartz sandstones; 13 — interbedding of gray limestones with rare dark-gray schists; 14 — dark-gray quartz-sericite-chlorite schists with interbeds of dark-gray carbonaceous sandstones.

the surfaces of which, near the axis, cut the bedding, while on the limbs they practically coincide with the elements of the attitude of the bedding. In addition to axial cleavage, fractures of bedding-parallel and interbed cleavage are widely developed.

The ore mineralization of the deposit is represented by two types. At the surface, poor-sulfide quartz veins are developed; these were the object of study during the first stages of exploration of the ore field. They are represented by bodies cutting across the dip and concordant with the strike of the folded rocks, 1–1.5 m thick and 100–150 m long, rarely more. Their ore-hosting structure is axial cleavage with elements of a NNE strike, with a dip angle of 40–60°.

In the course of exploration drilling, at a depth of 40–50 m in the core of the anticlinal fold, a veinlet-disseminated sulfide zone was exposed; it does not crop out at the surface and consists predominantly of pyrite (Fig. 3). Disseminated pyrite crystals, veinlets, veinlet-like and irregular accumulations of pyrite crystals are distinguished; they resulted from a single sulfide stage of mineralization. The ore-hosting structures for the sulfide mineralization are fractures of bedding-parallel and interbed cleavage.

The quartz veins are younger formations than the pyritization zone. They clearly cut pyrite crystals and veinlets, becoming noticeably enriched in sulfide material at the points of intersection.

As a result of exploration work, the following has been established:

1. The quartz veins are not persistent down dip and pinch out rapidly. Their total thickness and number decrease rapidly with depth. Thus, at a depth of 50 m from the surface, their total thickness is three times less than at the surface, and at a depth of 100–150 m only isolated, thin quartz veinlets and stringers occur.

2. The intensity of the veinlet-disseminated pyrite mineralization gradually increases from a depth of 40–50 m, reaching a maximum at a depth of 100–150 m; the highest gold contents are also recorded there.

3. The gold content of the sulfide-mineralization zone is assessed as considerably higher than that of the fields of development of quartz veins.

Owing to this, a zonal spatial distribution of mineralization of different stages is observed: quartz veins tend toward the crest part of the anticlinal fold, toward the boundary of rocks with different physicomechanical properties (carbonaceous shales and limestones), while the veinlet-disseminated pyrite mineralization is localized considerably deeper, among intensely slaty carbonaceous shales (Fig. 3).

The described vertical zoning is also reflected in plan: the number of quartz veins and their thickness increase regularly westward, toward the periclinal closure of the anticlinal fold.

Similar zoning is noted at other deposits of the district (Mount Vysochaishii, the Vernyi pit, the Dogaldyn vein). Thus, at Mount Vysochaishii, an intense sulfide (pyrite-pyrrhotite) gold-bearing zone, occurring, as at Sukhoi Log, among carbonaceous phyllites in the gently dipping limb of an anticlinal fold, has been exposed by erosion and comes directly to the surface. However, quartz-vein mineralization is represented here by only three thin veinlets (up to 30 cm), which rapidly pinch out with depth.

In the valley of the Vernyi pit, in the core of an anticlinal fold composed of interbedded weakly carbonaceous sandstones, siltstones, and shales, surface workings expose a series of quartz veins, the number of which at depth, according to drilling data, decreases noticeably. At the same time, the intensity of sulfide (pyrite) veinlet-disseminated

Fig. 3. 1 — interbedding of calcareous siltstones, calcareous-clayey schists, and limestones; 2 — carbonaceous quartz-sericite schists (phyllites); 3 — quartz veins; 4 — zone of pyrite veinlet-disseminated mineralization; 5 — areas of enriched gold ores; 6 — core-drill holes.

Scale: 50–0–50 m.

Visible drill-hole labels: Borehole 5; Borehole 1; Borehole 2; Borehole 7; Borehole 8.

…mineralization, weakly manifested at the surface, on the contrary increases sharply, reaching a maximum in the interval of thin interbedding of carbonaceous phyllite-like schists and siltstones.

An analogous vertical zonality in the distribution of quartz veins and sulfide zones is noted in the basins of the Buzhuikhta and Bodaibo rivers. Thus, along the most studied Dogaldyn vein, which has a saddle-shaped character and is confined to the hinge of an anticline, intense gold-sulfide mineralization is observed only on the lower levels—at a depth of 100–150 m.

Thus, stadial zonality of ore occurrences in the Lena district is developed quite widely. The importance of studying it in evaluating ore occurrences is exceptional, since it makes it possible to conduct more purposeful searches for concealed ore mineralization and to assess with a greater degree of reliability the depth prospects of ore nodes identified in the course of geological-exploration work. Knowledge of the spatial relationships of quartz and sulfide mineralization in the vertical section makes it possible in some cases to judge the relative depth of erosion of ore nodes, which is extremely important in evaluating the placer gold potential of regions, individual river valleys, and basins.

b) Facies zonality (depositional zonality according to V. I. Smirnov, 1960). A striking example of the manifestation of depositional zonality is the Golets Vysochaishii deposit, represented by a zone of sulfide veinlet-disseminated mineralization confined to intensely schistose carbonaceous phyllites in the northern, gently dipping limb of an anticline (Fig. 4). Massive limestones with interbeds of calcareous siltstones crop out in the core of the fold. Upsection they are rather sharply replaced by carbonaceous phyllites, which gradually pass into gray sandstones and siltstones. The phyllites and siltstones, compressed between rocks that are considerably stronger and less yielding (limestones below and sandstones above, Fig. 4), are intensely fragmented by bedding and interbed cleavage. At the boundary between them a zone of increased schistosity is observed, caused by the sharp difference in the physico-mechanical properties of the rocks. The ore body is represented by a veinlet-disseminated zone of pyrite-pyrrhotite mineralization in carbonaceous phyllites. The ore-hosting and ore-controlling structures are fractures of interbed and bedding cleavage.

All morphological and mineralogical varieties of sulfides have gradual transitions into one another, a single assemblage of impurity elements, and were deposited during one stage of mineralization.

In their spatial distribution the following vertical zonality is observed (Fig. 4).

1. In the lower part of the mineralized zone, in the interval of transition from limestones to carbonaceous phyllites, pyrite dissemination is developed in the form of individual crystals and “dust-like” fine-crystalline formations of metasomatic appearance. Rarely, veinlet-like bedding-parallel forms of pyrite segregations are noted among thin interbeds of carbonaceous phyllites occurring in the upper part of the limestone-rock section.

2. In the zone of intense development of interbed-slip fractures, in the interval of rapid transition in the section from limestones to phyllites and among the phyllites themselves, pyrite mineralization is widely developed, represented by veinlet and veinlet-like forms.

3. Upsection, pyrite veinlet mineralization very gradually passes first into pyrite-pyrrhotite, and then into essentially pyrrhotite mineralization. Here the dominant mineral is pyrrhotite.

Fig. 4.
1 — clayey limestones; 2 — interbedding of clayey limestones and calcareous sandstones; 3 — clayey phyllites with rare interbeds of limestones; 4 — thin interbedding of siltstone phyllites and aleurolites; 5 — interbedding of gray sericitic and carbonaceous sandstones, aleurolites, and carbonaceous sandstones; 6 — sericitic sandstones with interbeds of gray phyllites; 7 — pyrite mineralization; 8 — pyrite-pyrrhotite mineralization; 9 — pyrrhotite mineralization; 10 — disseminated mineralization of veinlet pyrite; 11 — tectonic crushing zone; 12 — boundaries between zones of differing mineralization; 13 — boreholes.

Scale: 100–0–100 m.

1. Finally, the uppermost zone, confined to the gradual transition in the section from phyllites and siltstones to sandstones, is the zone of development of disseminated pyrite–pyrrhotite mineralization. It is represented by rare and small (5 mm × 2–5 mm) scattered crystals of pyrite and small lenses of pyrrhotite. In outward appearance the enclosing rocks are more massive and are only weakly disturbed by fractures of interbed and bedding cleavage. With this zone, in essence, the veinlet-disseminated mineralization of the Vysochaishii Golets deposit comes to an end. Above it there is observed the disseminated impregnation of pyrite crystals that is usual for the entire district.

Analogous zoning is noted at the Buzhuikhta ore occurrence. Here the enclosing rocks are likewise represented by frequent interbedding of carbonaceous phyllites with calcareous-carbonaceous phyllites and calcareous siltstones, enclosed in the section between massive limestones, while the formation of sulfide mineralization (pyrite-pyrrhotite) proceeded in a single prolonged stage.

The zoning of deposition in the examples described is apparently explained by a change in the regime of sulfur and oxygen in the mineralizing solutions under conditions of differing tectonic fragmentation of the enclosing medium [1]. The latter is due to the peculiarities of the geological structure of the ore fields and to sharp differences in the physico-mechanical properties of the rocks composing them.

3. Zoning of Ore Bodies

Almost everywhere, in the study of gold-ore occurrences, in addition to the described types of zoning, a zonal structure of individual ore bodies (sulfide veinlets and quartz veins) is observed. Thus, at the Sukhoi Log deposit, gold-sulfide veinlets and veinlet-like accumulations of pyrite crystals often have a zonal structure: the central parts of the veinlets are composed of coarse-crystalline massive pyrite or of an accumulation of individual crystals, whereas the marginal parts are composed of grayish-white translucent quartz. The quartz crosscutting veins of the deposit also often reveal a zonal structure. Their central parts are composed of milky-white quartz with rare disseminations of muscovite flakes and crystals of pyrite, galena, chalcopyrite, while the selvages are contaminated by an admixture of carbonaceous matter, arranged in the form of closely spaced thin dark bands parallel to the contact. Here also are observed relics of reworked carbonaceous rocks and redeposited magnesian-ferruginous carbonates from the enclosing rocks. Sulfides and gold are often concentrated toward the selvage parts of the veins.

Depending on the composition of the enclosing rocks, the following are distinguished:

1) veins with calcite;
2) veins with siderite, chlorite, ankerite, and gold;
3) essentially quartz veins with rare chlorite and albite.

The ore-hosting and ore-controlling structures for all types of veins are fractures of axial cleavage of the Sukholozh anticline. The differences in their mineralogical composition and gold content are apparently determined by the influence of the composition of the enclosing rocks (Fig. 2). Quartz veins with calcite and rare disseminations of pyrite are developed in the periclinal closure of the anticline and in its overturned limb among deposits of the Zhuya Formation. The latter is represented by interbedding of calcareous-clayey phyllitoid shales with calcareous siltstones and limestones. Quartz veins developed among deposits of quartz-sericite-chlorite carbonaceous shales are characterized by the presence of increased amounts of siderite and ankerite, chlorite, and sericite. Along with this, there is a noticeable increase in

their content of ore minerals (pyrite, galena, chalcopyrite, etc.) in comparison with the first type of veins.

Among the quartzite sandstones of the Valyukhta Formation, on the left bank of the Nygri River, veins crop out that are predominantly quartz, with a small amount of chlorite and, occasionally, albite. Sulfides occur extremely rarely. The gold content of the various types of quartz veins is also very characteristic. Whereas the quartz-calcite and essentially quartz veins, practically devoid of sulfides, show gold contents of up to 2–3 g/t, rarely more, the content in veins with carbonates and sulfides is elevated by a factor of 2–3 in comparison with them. Moreover, the morphology itself of the vein bodies in rocks of different composition is noteworthy. Among sandstones and limestones, persistent, extended vein bodies are almost unknown. As a rule, they often pinch out, become lens-shaped, sometimes forming either thick swellings (up to 3–4 m), or disappear entirely. Among monolithic sandstones, nonpersistent stockwork zones with indistinct contacts are formed by mutually intersecting veinlets. Quartz veins among schists are more persistent in their elements of occurrence and thickness, and may constitute more reliable ore bodies.

The described regularities are characteristic of the veins of the entire Lena gold-bearing district and have repeatedly been noted earlier by Yu. P. Kazakevich and S. D. Sher.

A similar influence of the composition of the host rocks is also observed on individual quartz veins when they intersect rocks of different composition. Thus, at the Golets Vysochaishii deposit, in outcrops, a sharp increase in the amount of calcite in small quartz veinlets was observed as they passed from siltstones and phyllites into massive limestones. In some cases, such veinlets in limestones acquired a purely calcite composition.

A characteristic influence of the composition of the host rocks in mineralized zones is seen in the morphology of individual sulfide bodies and in their localization. The dominant material in the sulfide zones (Golets Vysochaishii, Sukhoi Log, Buzhuikhta) is pyrite, which forms both disseminated and veinlet forms. In pelitolites and siltstones, pyrite disseminations, usually represented by distorted crystals, have sizes within 1–2 cm along an edge, whereas in sandstones they form regular crystallographic shapes, their sizes rarely exceed 0.5 cm, and in limestones they are in general extremely rare and occur as small crystals. Veinlet pyrite bodies with massive texture are likewise characteristic of monotonous schist sequences, or of sequences with thin interbedding of schists (phyllites), siltstones, and sandstones. In the latter case they are localized at the contact of fine-grained rock with coarser-grained rock, forming sharp contacts. In terrigenous rocks with calcareous cement and in bedded limestones, within sulfide mineralized zones, densely disseminated metasomatic veinlets with uneven and indistinct contacts most often form. In pelitomorphic calcareous schists, even bedding-parallel “dust-like,” densely disseminated bodies are formed, the crystalline structure of which is established only under the microscope.

Veinlet-like, densely disseminated pyrite bodies also form in fine-grained sandstones when they occur in thin interbedding with phyllites and siltstones (the Vernyi area). As a rule, massive limestones and medium-grained sandstones are an extremely unfavorable environment for the formation of sulfide veinlets.

These are the general features of the manifestation of zoning in gold-ore quartz and sulfide mineralizations, depending on the composition of the host rocks. The practical significance, for prospecting, exploration, and the determination of the potential of the region’s gold deposits, of studying and taking into account the influence of lithology on the character of the manifestation of gold-ore mineralizations can hardly be overestimated. The gold content of sulfide mineralized zones, depending on the composition of the host rocks and on the morphology of the sulfide bodies, is quite obvious. It may be regarded as established that the most promising and gold-bearing zones are those occurring within a sequence of carbonaceous shales and represented by disseminated-veinlet formations (the Sukhoi Log and Vysochaishii deposits).

In conclusion, it should be noted that the described types of manifestation of ore zoning, both regional and local in scale within individual ore fields and bodies, are of great significance for the practice of geological exploration work. As work experience shows, mapping of regional zoning makes it possible to predict with greater confidence the prospects of individual areas for lode and placer gold content. Taking into account the vertical gradient of zoning in sulfide and quartz mineralizations, it is possible to evaluate more reliably the prospects of individual ore and placer nodes. Therefore, the noted regularities in the zonal distribution of ore mineralizations in the Lena gold-bearing region require further, more detailed and more purposeful study.

REFERENCES

1. Betekhtin A. G. Article in the book Fundamental Problems in the Study of Magmatogenic Ore Deposits. Academy of Sciences of the USSR, 1953.

2. Buryak V. A. Dependence of the mineralization of ancient (Precambrian) formations on regional metamorphic zoning (Vitim-Patom Highlands). Doklady of the Academy of Sciences of the USSR, vol. 163, no. 2, 1965.

3. Buryak V. A. Interrelationship between gold-bearing quartz veins and gold-sulfide mineralization of the host rocks in deposits of the Precambrian gold-ore formation (Lena gold-bearing region). Doklady of the Academy of Sciences of the USSR, vol. 165, no. 5, 1965a.

4. Salop L. I. Geology of the Baikal Mountain Region. Nedra Publishing House, 1964.

5. Smirnov V. I. Types of hypogene zoning of hydrothermal ore bodies. In: Genetic Problems of Ores, Gosgeoltekhizdat, 1960.