ON THE PATTERNS OF LOCALIZATION OF HYPERGENE NICKEL DEPOSITS

A. G. Bakirov

(Presented by Professor A. M. Kuzmin)

Nickel deposits associated with weathering crusts of ultrabasic rocks are known in various countries of the world. Numerous studies have been devoted to the mineralogy and geochemistry of nickel-bearing crusts. The questions of their geology and of the patterns in the distribution of the deposits mentioned are less developed; the author devotes his article to these questions.

The known nickel-bearing weathering crusts of hyperbasites formed in the Mesozoic and Cenozoic. In some countries (Indonesia, the Philippines, Venezuela, and others) crust formation leading to the accumulation of nickel is taking place at the present time as well. In the Urals, Cuba, and in some other regions, Cenozoic weathering processes are observed to be superimposed on more ancient crusts.

In the profiles of the most fully developed Mesozoic crusts, three zones are clearly distinguished from top to bottom: the first of them is a goethite-hydrogoethite (ocher) zone, the second is a zone of secondary ferruginous silicates (nontronite), and the third is a zone of primary magnesian and magnesian-ferruginous silicates, altered to one degree or another by weathering—the zone of leached serpentinites. Gradual transitions are most often observed between the zones mentioned. In the upper horizons of modern weathering crusts we see accumulations of goethite-hydrogoethite formations, which downward in section pass into disintegrating serpentinites. Ochers and ferruginous silicates arising during the weathering of ultrabasic rocks form blankets on hyperbasites or else fillings of fractures in the crushed parent substratum.

Areas of preserved Mesozoic nickel-bearing weathering crust are in most cases confined to modern watersheds coinciding with fragments of ancient watersheds, dissected and prepared by erosion, within which processes of chemical transformation of rocks formerly took place. The hypsometric elevations of areas with the aforementioned crusts of the blanket and blanket-fracture types most often lie within 300–400 m. Nickel-bearing weathering crust forming in the present epoch in young mountain regions, sometimes superimposed on more ancient crusts, is often found at elevations reaching 1200 m and more (Cuba, Brazil, New Caledonia, the Philippines).

Nickel deposits associated with Mesozoic weathering crust of the blanket and blanket-fracture types in many cases tend to be situated in the zone of transition from denuded moun-

structures to lowlands filled with Cenozoic sediments. The width of this zone is on the order of 50–70 km. Such a picture is observed in Kazakhstan, in the Salair, in the Urals, in regions of Poland, the USA, and other regions. Some ore-bearing areas associated with weathering crusts of different ages tend toward the margins of depressions and superposed troughs within folded structures,^1 located both on continents and on islands. As a consequence, there are cases of close adjacency of nickel deposits to lakes and artificial reservoirs (the Urals, Ukraine, regions of Indonesia, Madagascar, Venezuela, Greece, and other countries).

In the above examples, the areas of development of nickel deposits are situated in zones of transition from regions that experienced upward movements in the Neogene–Anthropogene to regions characterized by downward movements. Hence there is, as it were, a certain relative stability of the mentioned areas, localized at the junction of domains contrasting in the sign of neotectonic movements and in orography. In this connection, the expression concerning the stabilizing role of depressions in the preservation of weathering crusts adjacent to them is quite appropriate. Nickel deposits associated with islands and peninsulas, in most cases, are located near the seacoast, within a zone 5–15 km wide and sometimes 50–60 km wide (Cuba, New Caledonia, Sulawesi, Tasmania, Haiti, Greece, and others). The examples cited illustrate the connection of deposits with zones delimiting regions that also differ in their orographic and neotectonic features.

The analysis carried out by the author of the distribution of hypergene nickel and iron–nickel deposits of the world has shown that many of them fall within discontinuous belts associated with zones of certain parallels. Their latitudes are as follows: 50° N, 41–44° N, 20–22° N. Such belt-like distribution is to some extent connected with climatic zonality, which manifested itself during the formation of weathering crusts in the Mesozoic and Cenozoic [4, 6]. In the author’s opinion, structural factors also evidently influenced the belt-like distribution of the deposits. In the literature [1, 5], latitudinal zones of planetary scale, about 50–80 km wide, are noted. We shall indicate some of them, which are associated with the geographic ring of the Subarctic (50° N), the critical parallels (35°, 62° N, the equator). Li Si-tuan [3] distinguished in China three latitudinal tectonic zones, which, according to some researchers, have planetary significance. They are: the Yinshan zone (between 40–42° N), the Qinling zone (between 34–35° N), and the Nanling zone (between 23–26° N). Many of the above-listed zones are tectonically mobile and weakened.

Comparisons carried out by the author have shown that nickel-bearing regions of the world are often associated precisely with the above-mentioned latitudinal zones or are located in their immediate vicinity. Thus, the deposits of Kazakhstan (Charskoye, Gornostayevskoye, and the Ekibastuz–Bayanaul group), of the Southern Urals (Kempirsai and Buryktal), of Poland (Ząbkowice), and of the GDR (the region of the Saxon Granulite Mountains) tend toward the geographic ring of the Subarctic. Near the critical parallel we see the Serov deposit of the Urals. Within the latitudes of the Yinshan zone are nickel-bearing areas of the Caucasus, Bulgaria, Yugoslavia, Albania, and the Oregon–California grouping of the USA. The Webster deposit (USA) and iron–nickel occurrences in Japan may be associated with the Qinling zone, and the Qinzhou nickel-bearing area (China) with the Nanling zone. At the equator we see a grouping of nickel deposits on the island of Sulawesi (Celebes).

^1 This regularity was first noted in the Urals by I. I. Ginzburg [2].

Fig. 1. Geological map of the Zarkaman deposit. Compiled by V. G. Sazantsev.
1 — Quaternary system. Recent division. Alluvial–proluvial deposits: boulders, pebbles, gravel, sands, clays;
2 — Cretaceous system. Upper division. Samar suite: conglomerates with pebbles of limestones and sandstones with calcareous–sandy cement, with interbeds and lenses of marls;
3 — Jurassic system. Upper division. [[unclear: suite name]] suite: sandstones, gravelites, quartz conglomerates and monotonous fine-grained sediments;
4 — Middle Ordovician diorite-porphyrites;
5 — intrusions of the Kanansai complex: granodiorites, adamellites, quartz monzonites, quartz syenites;
6, 7 — dike complex: 6 — leucocratic granite-aplites, 7 — microgranites;
8 — skarn–ore zones;
9 — boundary of unconformable occurrence of Cretaceous deposits;
10 — faults;
11 — bedding elements of rocks;
12 — gold placers;
13 — exploratory adits;
14 — ancient workings (10th–11th centuries A.D.);
15 — core-drilling boreholes.

The nodal character of localization² of many nickel-bearing regions of the world and of individual deposits is revealed. Its essence lies in the confinement of the latter to nodes of intersection of structures of different orientation, among which latitudinal and sublatitudinal dislocations of Precambrian and younger age are widespread and have undergone prolonged and inherited development³. Thus, hyperbasites with nickel-bearing weathering crusts of Cuba, the Dominican Republic, Venezuela, Puerto Rico, Tasmania, Ukraine (the Devladovo and Nikopol groups), and the Caucasus are confined to zones of obvious faults of the above-mentioned orientation complicated by transverse structures. Many nickel deposits are associated with zones of concealed latitudinal and sublatitudinal faults that intersect structures of other trends or are themselves intersected by the latter (the Urals, Kazakhstan, Madagascar, the USA, China, Brazil, and some other regions). This is evidenced, in addition to geophysical data, by the occurrence of the ore accumulations mentioned in the same zones together with superposed synclines, chains of granite bodies, gabbroids, alkaline rocks, and also by other features by which concealed faults are usually marked in an erosional section.

The island of New Caledonia, with its accumulations of nickel ores, and the largest Brazilian deposit near Niquelândia are associated with large, long-developing latitudinal and sublatitudinal uplifts. New Caledonia lies on the western flank of a seismic zone extending from west to east. Characteristic is the confinement of nickel deposits to islands complicated by transverse structures and, as a result, having a block-like structure (Cuba, Sulawesi, partly New Caledonia). In regions of development of weathering crusts located in various countries, the author knows of four hyperbasite complexes that, in erosional section, have a ring-shaped or approximately ring-shaped form. They are located in the Urals (the Buruktal–Akkarga and Kiembay complexes), in the GDR (in the area of the Saxon granulite mountains), and in the USA (near Webster in North Carolina). All of them are nickel-bearing to one degree or another. At the same time, the largest ore accumulations in scale predominantly gravitate toward the southern parts of the complexes mentioned, as the most tectonically complicated. In the ancient strata of Brazil and Ukraine, nickel-bearing hyperbasite massifs frame synclines composed of Proterozoic rocks.

In Brazil, nickel deposits are known that are chiefly associated with the weathering crust of hyperbasites and, more rarely, endogenous ones. They form several separate groups, the localization of which, in the author’s opinion, was influenced by concealed “translucent” latitudinal and sublatitudinal structures. As an example one may cite the grouping of nickel deposits located east of Belo Horizonte (Fig. 1)⁴. The belt of their development is latitudinal. It, in turn, is included in a zone extending from west to east and encompassing the area of submarine volcanic centers, the Vitória Bank, the belt of granite development, and a segment of the valley of the Rio Grande River. A latitudinal structure is present, having different forms of manifestation in its individual parts and being ore-controlling.

² In tectonic nodes an environment was created that favored the development of increased fracturing, the formation and preservation of deep roots of the weathering crust.

³ Latitudinal and sublatitudinal dislocations are developed in the system of planetary fracturing. They are due to rotational forces of the Earth, which caused extension or compression of individual zones of the Earth’s crust.

⁴ The figure gives a scheme compiled on the basis of the geological map of Brazil [7].

Within hyperbasite massifs, other conditions being equal, ore-bearing sectors of the weathering crust tend toward the most fractured zones, toward places where faults intersect. In this connection, the important ore-controlling role of various zones of increased fracturing becomes apparent: dike, endocontact, near-flank, fissure-vein, and other zones manifested in the hyperbasite substrate, and their nodes. Owing to complicating transverse structures (disjunctive, folded), nickel-bearing ultrabasic-rock massifs often have a belt-like structure (Urals, Ukraine, Salair, China, Brazil, Poland).

At present, the important role of small post-ore grabens in preserving blocks of nickel-bearing crust has been clearly established (various regions of the USSR, Poland, the GDR, Albania, the USA). There prove to be many more of these tectonic forms at deposits than had previously been assumed. Strongly fragmented hyperbasite massifs located in areas of possible development of weathering crusts, having apophyses and tectonic contacts with enclosing rocks, are potentially nickel-bearing. They often lie discordantly relative to the dominant structures.

The nodal character of the localization of nickel-bearing districts and individual deposits, with the important role of latitudinal and sublatitudinal dislocations in tectonic nodes, also finds its morphological expression. Thus, hyperbasites with ore-bearing weathering crust are often confined to sectors of latitudinal bends: of islands (Japan), of folded belts (USSR, Poland), of individual hyperbasite massifs and their boundaries (USSR and other countries). Nickel-bearing districts and deposits are sometimes localized at sites of latitudinal expansions (“bulges”) of folded belts and geological bodies, in sectors of constrictions of hyperbasite massifs caused by transverse folds or disjunctives. Some nickel deposits tend toward the re-entrant angles of folded belts (Urals, Salair) or else are located within latitudinal zones that include the re-entrant angles of island contours (Cuba, Haiti). In this case, deep faults, disjunctives, dikes, axes of troughs and synclines, chains of islands of archipelagoes, and other geological and geomorphological objects elongated in one direction are often “aimed” at nickel deposits (Fig. 2). In the same role as distinctive “ore indicators” there appear latitudinal segments of certain river valleys and of the sea coastline.

The above-mentioned morphological features of the structures of nickel-bearing districts may be used in forecasting the latter.

LITERATURE

1. P. S. Voronov. On the role of morphostructures of the Arctic and Antarctic in revealing certain regularities of the structural plan of the Earth. Informational Bulletin of the Scientific Research Institute of Arctic Geology, issue 23, 1961.

2. I. I. Ginzburg et al. Ancient weathering crust on the ultrabasic rocks of the Urals. Part 1. Types and morphology of the ancient weathering crust. Transactions of the Institute of Geological Sciences, Academy of Sciences of the USSR, issue 80 (1), 1941.

3. Li Sy-guan. Geology of China. Moscow, 1952.

4. V. P. Petrov. Geological-mineralogical investigations of the Ural white clays and some conclusions on the mineralogy and genesis of clays in general. Transactions of the Institute of Geological Sciences, Academy of Sciences of the USSR, issue 95, 1948.

5. M. V. Stovas. Critical Parallels of the Terrestrial Ellipsoid. Leningrad, 1951.

6. N. M. Strakhov. Foundations of the Theory of Lithogenesis. Vol. 1, Publishing House of the Academy of Sciences of the USSR, 1962.

7. Mapa geologico do Brasil. Ministério da agricultura departamento nacional da producao mineral divisao de geologia e mineralogia, 1960.