Geological Structures of Vein Deposits Favorable for the Occurrence of Gold Nuggets

M. N. Al'bov (SGI)

In the gold-bearing districts of the Urals, Siberia, and the northeast of the USSR, many places are known where gold nuggets of various weights have been found and continue to be found at the present time. The predominant share of nuggets is extracted during the working of eluvial gold placers. More rarely, cases are known of the recovery of gold nuggets from ore veins, although the origin of nuggets in eluvium through the destruction of outcrops of vein deposits is beyond doubt.

The practice of working placers and ore veins shows that gold nuggets occur in nature in groups under identical geological conditions, sometimes several dozens at a time. For example, during the working of the Kashcheevskaya vein (Leninskii mine, Miass, Southern Urals), down to a depth of 35 m, 34 nuggets were recovered. The opening by a shaft, located there, of the Kolyushchenskaya vein down to a depth of 40 m yielded 20 nuggets. At the Tyelginsky mine, nuggets occurred in stopes along the vein every 0.2–0.3 m, with a total weight of up to 40 kg.

In the same groups, nuggets pass from vein outcrops into eluvial placers. During the working of three eluvial placers at the same Leninskii mine in Miass, respectively 199, 95, and 67 nuggets were recovered. Three hundred gold nuggets with a total weight of 80 kg were found in the trenches of the eluvial placer along the Gar'-Vtoraya River (the “Oktyabr'skii” mine of the Amurzoloto trust, between the Zeya and Selemdzha rivers).

It follows from this that, when one nugget is found in a placer or in a vein, there is a high probability of finding an entire group of similar nuggets under the same geological conditions. Generalization of the established positions of gold nuggets in the geological structures of many vein deposits of the Southern Urals made it possible to determine the following four features of their occurrence.

1. The mutual intersection or close position of vein milky-white, weakly gold-bearing quartz with gold-bearing quartz containing sulfides. It follows from this that gold nuggets are characteristic of deposits with a complex and prolonged process of vein formation, in which different generations of vein filling are clearly manifested.

1. The morphology of ore veins, their structure, and the tectonics of sectors of vein deposits with coarse gold are favorable for secondary gold enrichment. This is evidenced by the combination or mutual intersection of veins with gentle and steep dips; the confinement of ore veins to synclinal folds of the enclosing rocks or to false synclinal sectors; the joining of ore veins downward along the dip; and gently dipping post-ore displacements. Consequently, gold nuggets are characteristic of those structural-tectonic sectors of vein deposits that are typical for the localization of ore shoots of supergene origin.

2. The superposition of an ancient weathering crust, or of its linear pocket, on the above-mentioned structural-tectonic sectors of vein deposits. All nuggets known in the Urals have been found in stopes within the zone of complete or active oxidation of quartz-sulfide veins, where, in addition to remnants of sulfides, iron hydroxides are invariably present. There is not a single Ural mine where gold nuggets have been found at a depth of more than 100 m from the ground surface, in the zone of primary quartz-sulfide ores. It follows from this that the presence of an ancient weathering crust is a favorable condition for the formation of nuggets; that they are genetically associated with this crust and arise within it under the influence of supergene processes. It may be added that the largest mass of native gold in Australia (the Holtermann plate, weighing 285 kg and 1.42 m long) was extracted in 1872 from the oxidation zone of a quartz vein.

3. A higher fineness of nuggets, and of coarse gold in general, compared with the average fineness of the metal in the deposit. Often the fineness of nuggets from ore veins is even higher than the average fineness of the metal from the oxidized ores of the given deposit.

In mining quartz gold-bearing veins in the zone of primary ores, unusually rich accumulations of native gold are sometimes encountered, with contents amounting to kilograms of metal per tonne of ore. Such finds, however, are relatively rare. Possibly this is connected with the considerably lesser development of mine workings at deep levels compared with the oxidation zone. The following structures of vein deposits are favorable for finding gold nuggets.

1. Gentle dip of veins and their displacers. Sectors of veins with a gentle dip, or even with horizontal occurrence, caught by a linear pocket of the weathering crust, are highly favorable for sharp secondary gold enrichment. At the Dzhetygara deposit (N. Kazakhstan), one of the quartz veins in the field of mine 3, at a depth of 20–25 m, had horizontal occurrence, lay in the oxidation zone, and was exceptionally rich in gold. Assays of individual stope samples reached 1 kg/t.

Widely distributed gently dipping (horizontal) displacements of many steeply dipping ore veins have an analogous effect. Such displacements usually shift the veins for short distances. Being within the limits of a linear pocket of the weathering crust, gently dipping displacements filled with vein clay form a screen that delays deeper penetration of oxidation processes. Above such a screen, sharply expressed secondary gold enrichment often arises.

At the Berezovsky gold mine (Urals), the Nadezhnaya dike is cut by a post-ore gently dipping displacer with a displacement along it of 3–5 m. Cleanup mining of quartz-sulfide veins below the displacer established in them an industrial, but moderate, gold content, and in the composition of the ores—quartz and sulfides with almost complete

absence of limonite. As soon as mining operations along these same veins moved upward and crossed the plane of the dislocator, the gold content in the ore increased sharply. The sulfides disappeared and their place was taken by limonite. In the faces, small gold nuggets weighing from 1 to 10 g and more were encountered.

At the South Chelyabinsk deposit, large nuggets with a total weight of up to 120 kg were mined by prospectors at the intersections of three systems of fractures: “undercutting” veins (strike 150–160°, dip 50°), “pillars” (veins with a strike of 120° and vertical dip), and “breaks.” The latter are the youngest dislocators, not gold-bearing, with a strike of 310–320° and a dip from 15 to 40° (Fig. 1). According to available data, large nuggets and many dozens of smaller nuggets lay on the hanging wall of the “breaks,” “as if on a shelf.” No nuggets were mined in the footwall of the “breaks” or in the undercutting veins beneath the breaks.

Fig. 1. Schematic block diagram of the tectonics of ore veins and the position of nuggets at the South Chelyabinsk deposit:
1—“undercutting” vein; 2—“pillar”; 3—“break.” Nuggets are indicated by asterisks.

2. Folding of ore veins. Many ore veins occurring conformably in folded sedimentary and metamorphic rocks acquire the form of folds of the country rocks, synclines and anticlines. Sections of veins occupying the bottom of such a syncline, within a thick ancient weathering crust, are, as a rule, sharply enriched in gold redeposited in the oxidation zone.

The most striking example of secondary gold enrichment of a synclinal fold is the Smolensk bush vein of the Nepryakhin deposit in the Miass district of the Southern Urals. In 1872, from the bottom of a synclinal fold of a sulfide zinc-copper vein with a conformably occurring quartz vein, from a depth of 75 m, in the zone of active sulfide oxidation, according to available data, about 300 kg of gold was extracted (M. N. Al’bov, 1960).

In the area of the same Nepryakhin ore field, the Nikolaevskaya bush vein is known. This is a thin quartz vein with a vertical

dip, occupying a fissure along the axial plane of a compressed synclinal fold in deeply weathered chlorite schists, was mined along strike for 100 m and down dip for 25 m. About half of the gold recovered consisted of nuggets weighing from several grams to 1 kilogram.

3. Close occurrence of quartz and sulfide veins. A favorable condition for finding gold nuggets is the close occurrence of quartz veins with sulfide veins or with zones of abundant pyrite dissemination in the country rocks.

The segment of the Kolyuchenskaya vein mentioned above, from which 20 nuggets were recovered, represented a concordant contact of a quartz vein with a thin sulfide vein. Other segments of the same vein, where along strike the quartz and sulfide veins diverge, contain no nuggets. The Smolenskaya stockwork vein of the Nepryakhin deposit, described above, is another example of the close occurrence of pyrite and quartz veins. At the Dzhetygarin gold-ore deposit, large gold nuggets were extracted from the Belaya vein in the area where it intersects a thick quartz-sulfide gold-bearing vein.

These and other examples show that gold nuggets often occur in the zone of active oxidation of sulfides, if veins of gold-bearing quartz are also present in the same area.

Fig. 2. Geological structures of quartz-sulfide veins favorable for finding gold nuggets

Upper row: horizontal occurrence of a vein; displacement of a steeply dipping vein; close occurrence of a quartz-sulfide and a sulfide vein.
Middle row: synclinal occurrence of a vein; veins in the axial plane of a syncline; intersection of bands of pyritized rocks by a quartz-sulfide vein.
Lower row: vertical displacements of a gently dipping vein; intersection of veins.
1 — gold nuggets; 2 — quartz-sulfide veins; 3 — sulfide veins, pyritized layers; 4 — limonite; 5 — ancient weathering crust; 6 — groundwater level.

4. Intersections and displacements of steeply dipping veins. Examples of the occurrence of gold nuggets along the lines of intersection of two veins with opposing dips are not rare.

In the Kochkar ore field (Southern Urals), in the Diana vein, a sharp enrichment in gold at the 72 m level is confined to the junction of two veins into one down their dip. N. K. Vysotskii (1900), in his monograph on Kochkar, gives a description of the Bazhukovskaya vein, which at a depth of 42 m joined with a vein parallel to it. The gold content at the place where the veins joined exceeded the average by a factor of 25. In the Elizavetinskaya vein, in the same Kochkar area, at a depth of 58 m, a gold nugget weighing 200 g was found at the place where a thin quartz stringer joined the vein. At the Kashcheevskoe deposit (Miass district of the Southern Urals), 34 nuggets were recovered from depths of 12 to 35 m from the lines of intersection of gently dipping and steeply dipping quartz-sulfide veins.

The intersection by a quartz vein of zones of sulfide dissemination in the zone of active oxidation of sulfides is likewise favorable for the occurrence of nuggets.

Figure 2 schematically shows geological structures of quartz-sulfide veins favorable for the occurrence of gold nuggets in them. Naturally, such structures may be destroyed by erosion, and then the nuggets are projected onto the present-day ground surface, forming an eluvial placer.

The search for nuggets should begin with a detailed study of the geological structure of the site, with precise geometrization—by the method of contour lines—on mine-surveying plans and sections of the strike and dip of quartz-sulfide veins, zones of sulfide dissemination, and tectonic displacements, especially post-ore ones. Nuggets are confined to lines of mutual intersection of two planes (veins), or to the plane of conformable contact between quartz and sulfide veins.