On the Formation of the Ampalyk Iron Ore Deposit

E. A. Babina

(Presented by Professor A. M. Kuzmin)

The Ampalyk deposit consists of seven separate magnetic anomalies, elongated in a north-northwestern direction in the form of a belt 12 km long and 1–1.5 km wide. It is confined to the western contact of a granitoid intrusion with a sedimentary-effusive sequence of Silurian age. The latter is composed of basic effusives in the form of various porphyrites and diabases with interbeds of limestones, tuff sandstones, and tuff siltstones. This sequence dips steeply to the northeast. The intrusive rock complex at the deposit is represented by gabbro-gabbro-diorites, diorites, granites, and syenites, while at a depth of 450–700 m granites of alkaline composition occur. Of the entire variety of intrusive formations, the diorites directly in contact with the effusive-sedimentary sequence are the most widespread.

In the zone of contact between the intrusion and the effusive-sedimentary sequence, processes of recrystallization, hornfelsing, and skarn formation took place. As a result of these processes there arose marbled limestones, hornfelsed porphyrites, hornfelses, various skarns, more rarely quartzites, and magnetite ores. In the immediate contact zone there occur hornfelses and skarns, which toward the exocontact are replaced by quartzites and hornfelsed porphyrites. Among the hornfelses, according to the predominant mineral in their composition, quartz-plagioclase-biotite, hornblende-plagioclase, pyroxene-plagioclase, and pyroxene varieties are distinguished. By the same principle, among the skarns two groups have been distinguished:

1. Magnesian skarns (forsterite-phlogopite-serpentine, magnetite-serpentine).
2. Calcareous skarns (pyroxene, pyroxene-scapolite, pyroxene-garnet, garnet, pyroxene-magnetite, and amphibole-pyroxene). Magnetite ores are closely associated with the skarns.

The ore bodies have a submeridional strike and a steep 75–80-degree dip, varying from westerly to easterly. The contacts of the ore bodies with the rocks enclosing them are sharp. In plan, the form of the ore bodies is a complex configuration, generally concordant with the bedding of the enclosing rocks and with areas of skarns not replaced by magnetite.

At the deposit, two most promising sectors have been identified—the Central and the Northern. Each sector is characterized by two ore bodies—the Main and the Western—which are situated

almost parallel to one another, and have a configuration complicated by swells and constrictions, and sometimes by apophyses of intrusions. The largest is the Main ore body in the Central sector, lying conformably with the strike of the rocks of the enclosing sequence. Its length is 700 m, its width 50–60 m, and down dip the body has been traced for 450 m. In the northern sector the Main ore body is elongated in a meridional direction, with an inconsistent dip from the western flank on the north to the eastern flank on the south, with a dip angle of up to 75°. The western bodies of both sectors are of considerably smaller dimensions.

The structure of the ore body is determined, on the one hand, by the lithologic composition and structural-tectonic makeup of the sedimentary-effusive sequence, and on the other by selective replacement of the enclosing rocks. Skarns and hornfelses arose as a result of metasomatic replacement of porphyrites with interbeds of limestones, diorites, and gabbro, relics of which have been preserved among the skarns and ores.

In the skarn-ore formations a definite zonality is observed, caused by the decline in mineralization and in the intensity of replacement of the enclosing rocks with increasing distance from the intrusive contact, which serves as convincing evidence of the genetic connection of the deposit with the Olginsko-Ampalyk granitoid massif.

In the ore field there are widespread vein rocks of the intrusion, subdivided into pre-skarn-ore and post-skarn-ore rocks. The pre-skarn-ore dikes are composed of syenite porphyries, microsyenite porphyries, and diabases. The younger, post-skarn-ore dikes are represented by granite porphyries, granite aplites, and pegmatites.

The ores of the deposit are distinguished by the diversity of their mineralogical composition and are divided into magnetite, magnetite-sulfide, polymetallic, and arsenopyrite-cobalt-bearing ores. The first two varieties are the most widespread.

Magnetite ores are composed essentially of magnetite with minor disseminations of sulfides and occur in the form of small lens-shaped bodies of fine-grained composition. In drill-hole sections a gradual transition is clearly observed from massive (solid) magnetite ores to spotted and breccia-like varieties. As a rule, the ores are broken by a large number of fractures, along which amphibole, chlorite, carbonate, and sulfides readily develop. The sulfides in the magnetite ores are represented by pyrite, pyrrhotite, chalcopyrite, sphalerite, galena, arsenopyrite, and molybdenite.

Magnetite-sulfide ores are characterized by an elevated content of sulfides. Along with magnetite, pyrite and pyrrhotite are present here in significant quantities, with one or the other mineral predominating. They have fine- and medium-grained structures with spotted, breccia-like, and disseminated textures. Their mineralogical composition is considerably more complex; in decreasing order there are magnetite, pyrite, pyrrhotite, chalcopyrite, marcasite, melnikovite-pyrite.

Polymetallic mineralization is encountered somewhat separately in relation to the first two ore varieties and is localized in altered limestones and in hornfelses. In terms of structural-tectonic features it corresponds to breccia-like, veinlet, and disseminated types. Its composition includes sphalerite, galena, pale ore, chalcopyrite, pyrite, pyrrhotite, marcasite, arsenopyrite, molybdenite, bismuthinite, native gold, silver, and cerussite.

Arsenopyrite-cobalt-bearing mineralization has been encountered in drill-hole sections at anomalies II, III, and IV and is confined to pyroxene hornfelses and skarns. It does not form large accumulations,

and occurs in the form of disseminations, small nests, and veinlets. Disseminated texture is more common. Mineral composition: arsenopyrite, glaucodot, danaite, lellingite, safflorite, with a small amount of pyrite, magnetite, and molybdenite, among which isolated grains of nickeline, native gold, and copper are encountered.

From the brief characterization of the ores given above, it is evident that they are close to one another in structural-textural features and mineralogical composition. Some minerals included in the composition of the individual ore types have the same sequence of deposition in the overall process of ore formation.

The Ampalyk iron-ore deposit is a typical representative of the contact-metasomatic type; genetically and spatially it is associated with the Olgino-Ampalyk granitoid intrusion.

Formation of the deposit began with the emplacement of the first phase of the Olgino-Ampalyk intrusion. The emplacement of the intrusion was accompanied by high temperature and the release of a considerable amount of heat, which was experienced by the enclosing rocks. These abrupt changes in temperature created entirely new conditions of mineral formation in the enclosing rocks. Mineral components stable before the emplacement of the intrusion became unstable in the new physicochemical environment. Regrouping and recrystallization of minerals began, and hornfelses were formed.

Subsequently, contact metamorphism was accompanied by an influx of material and the formation of metasomatic hornfelses and skarns. The metasomatic hornfelses preceded the formation of the skarns, bordered them, and were, as it were, an early stage in the manifestation of metasomatism associated with the activity of the same postmagmatic solutions as the skarns. This is confirmed by the similarity of the mineralogical composition of the hornfelses and skarns, their close association, and their confinement to the same tectonic zones. The metasomatic hornfelses were formed at the expense of the same rocks as the skarns. In this process the original rocks lost a considerable amount of iron. As a rule, hornblende in them was replaced by pyroxene with the formation of a granoblastic and microgranoblastic structure. Magnetite disappears from the enclosing rocks, and in its place accumulations of leucoxene aggregates appear.

The principal role in the formation of the skarns, ore bodies, and various mineral associations at the deposit was played by replacement processes caused by the dissolution of one mineral and the deposition of another in its place, accompanied by reaction phenomena. Thus, the formation of metasomatic bodies, and chiefly of skarns, took place as a result of the chemical interaction of three media: two adjoining solid rocks (the rocks of the effusive-sedimentary sequence and the granitoid intrusion) and the solution impregnating them, with a certain role of infiltration. The skarn bodies that arose in this process possessed a regular structure, the main features of which were identified by D. S. Korzhinskii [4].

Metasomatic replacement, with a considerable influx of material, affected porphyrites, limestones, tuffaceous sandstones, tuffaceous shales, and also the solidified marginal parts of the first phase of the Olgino-Ampalyk intrusion. The close spatial and genetic connection of the deposit with the granitoid intrusion is obvious and requires no special proof. The source of the skarn- and ore-forming solutions was probably the same magmatic chamber that gave rise to the Olgino-Ampalyk intrusion.

Adhering to the viewpoint of the majority of researchers, we believe that the cause of the formation of solutions and the separation of ore-forming matter was the complex physicochemical processes that took place in the magma during its emplacement. A significant, and perhaps the principal, role in this was played by processes of interaction between the parental magma and the country rocks, as well as by processes of assimilation. Iron, in all probability, was removed from the enclosing rocks and, first of all, from the effusives.

The question of the sources of iron at the Ampalyk deposit remains not entirely clear. In the opinion of many scholars—M. A. Usov [9], F. N. Shakhov [8], and Yu. A. Kuznetsov [6]—for iron-ore contact-metasomatic deposits, iron is transported by postmagmatic solutions from the depths of the magmatic chamber during the postmagmatic stage of granitoid formation.

Kh. M. Abdullaev [1] believes that the iron could have been borrowed from the enclosing rocks, where it occurred in a state of disseminated impregnation among the effusives. In this case the iron must not only have been borrowed from the enclosing rocks, but also dissolved in the contaminated magma and then deposited by postmagmatic solutions.

V. G. Karel [5], agreeing with Kh. M. Abdullaev [2], believes that, during the removal of iron, the latter must be transformed into pyroxene-scapolite metasomatic rocks. The iron was borrowed from the enclosing rocks not by postmagmatic solutions, but by carbon dioxide–alkaline solutions of the magmatic stage of subalkaline rocks of the very latest stage of the Olginsk–Ampalyk intrusion. These solutions were generated simultaneously with the magma or ahead of it. Under conditions of the hypabyssal facies, magma crystallizes after the deposit has been formed; therefore, fragments of skarns occur in the intrusive rocks of the late phases. During assimilation of the roof by the cooling Olginsk–Ampalyk intrusion, ore solutions were separated.

The possible separation of ore-bearing solutions during assimilation of roof rocks by cooling intrusive masses is demonstrated by numerous experimental studies. Thus, for example, L. N. Ovchinnikov [7], together with L. I. Mettikh, when melting a silicate rock with limestone and subsequently crystallizing such a melt at a gradually decreasing temperature, obtained on the surface of the solidified melt a thin crust of magnetite, analogous to natural massive magnetite ore, and accumulations of hematite plates. In the ore crust that formed, the impurities present included copper, cobalt, nickel, tin, and certain other metals also extracted from the silicate rocks. This indicates that assimilation leads to the separation into an independent phase not only of iron, but also of other metals.

The separation of iron and accompanying metals from the ore-forming melt, their removal, and the formation of skarns and ores at the contact of the intrusion arise as a result of intensive assimilation processes. At the same time the composition of the parental magma changes, which leads to the formation of a great variety of erupted rocks, ranging from gabbro to alaskites, and of numerous schlieren segregations of hybrids for which it is sometimes difficult to find a name. This circumstance led M. A. Usov [9], A. N. Zavaritskii [3], and many others to the idea that the source of skarn- and ore-forming solutions may be acid magmas, while the intermediate composition of the intrusions is due to the assimilation of limestones.

Text in the figure

No.
Name of minerals

Hypogene epoch of mineralization

• Stage I: formation of hornfelses
• Stage II: formation of skarns
• diopside–scapolite
• garnet–pyroxene
• Stage III: magnetite
• Stage IV: sulfide–high-temperature
• Stage V: sulfide–low-temperature

Supergene epoch of mineralization

1. quartz
2. plagioclase
3. potassium feldspar
4. biotite
5. pyroxene
6. scapolite
7. garnet
8. olivine
9. apatite
10. zircon
11. sphene
12. magnetite
13. epidote
14. zoisite
15. arsenopyrite
16. lellingite
17. glaucodot
18. vauxite
19. safflorite
20. ordinary hornblende
21. actinolite
22. tremolite
23. pyrite
24. sphalerite
25. pyrrhotite
26. chalcopyrite
27. galena
28. pale ore
29. molybdenite
30. bismuthine
31. nickeline
32. antimony
33. native gold
34. native silver
35. native copper
36. prehnite
37. phlogopite
38. chlorite
39. sericite
40. muscovite
41. talc
42. calcite
43. dolomite
44. siderite
45. ankerite
46. marcasite
47. melanocerite–piolite
48. hematite
49. ilmenite
50. cerussite
51. cuprite
52. lepidocrocite

Fig. 1. Paragenetic scheme of mineral deposition at the Ampalyk iron-ore deposit

The formation of the Ampalyk deposit took place in two epochs: hypogene and supergene (Fig. 1)*. The hypogene epoch consists of five principal stages. Each subsequent stage is characterized by a decrease in temperature and by a change in the composition and concentration of the solutions that determined a particular mineral paragenesis.

With the emplacement of the Olginsko-Ampalyk intrusion, the effusive-sedimentary sequence hosting the deposit underwent thermal metamorphism. The result of the thermal effects of the intrusion was hornfelsing of the enclosing rocks and the formation of hornfelses of various compositions.

The stage of hornfels formation was followed, after an interval, by the skarn stage, characterized by different mineral parageneses. In this stage, metasomatic processes were widely manifested, as a result of which skarns varied in mineralogical composition.

The formation of skarns under different temperature conditions was reflected in their composition and structure. With increasing distance from the intrusion, the high-temperature magnesian, pyroxene, and pyroxene-scapolite skarns are replaced by garnet skarns.

The stage of magnetite mineralization is clearly superimposed on the skarns, and magnetite readily replaces the latter. Deposition of magnetite occurred over a broad temperature range, as evidenced by its three generations. However, the bulk of the magnetite was deposited in the magnetite stage after the skarn minerals and is characterized by a significant input of iron.

With a further decrease in temperature, sulfides began to be deposited instead of magnetite. The magnetite stage is succeeded by two sulfide stages—high- and low-temperature—which differ from one another in their mineral associations.

In the high-temperature sulfide stage, magnetite-sulfide ores with an admixture of sphalerite of an early generation were formed. The low-temperature sulfide stage began with the deposition of polymetallic and arsenic-cobalt-bearing mineralization and ended with the deposition of the main mass of sulfides and native metals. The low-temperature sulfide mineralization is superimposed on the magnetite ores.

The final stage in the formation of the deposit was the supergene epoch, characterized by oxidation and decomposition of the primary minerals.

From the foregoing, the following conclusions may be drawn.

1. As a result of the action of the Olginsko-Ampalyk intrusion on the effusive-sedimentary rocks, the latter underwent hornfelsing and skarnification, with subsequent superposition of magnetite and sulfide mineralization.

2. The polymetallic and arsenic-cobalt-bearing mineralizations are genetically related to the hydrothermal activity during the formation of the Olginsko-Ampalyk intrusion.

3. The sulfide mineralization at the deposit did not acquire independent significance. It is superimposed in relation to the magnetite mineralization.

REFERENCES

1. Kh. M. Abdullaev. The role of assimilation processes and the formation of intrusive complexes of Western Uzbekistan. Problems of Petrography and Mineralogy, vol. 1, Publishing House of the Academy of Sciences of the USSR, 1953.

* The paragenetic scheme of mineral deposition is given in abbreviated form.

1. Kh. M. Abdullaev. Genetic relationship of ore mineralization with granitoid intrusions. Gosgeoltekhizdat, 1954.

2. A. N. Zavaritskii. Mount Magnitnaya and its iron-ore deposits. Trudy GK, issue 122, part III, 1961.

3. D. S. Korzhinskii. Formation of contact deposits. Publishing House of the Academy of Sciences of the USSR, ser. 3, 1949.

4. V. G. Korel’. On iron-ore mineralization and genetic types of intrusions. Izvestiya of the Academy of Sciences of the USSR, geological series, No. 8, 1956.

5. Yu. A. Kuznetsov. Iron-ore mineralization and genetic types of intrusions. Izvestiya of the Academy of Sciences of the USSR, geological series, No. 2, 1955.

6. L. N. Ovchinnikov. Experimental studies in connection with the investigation of ore deposits. Trudy GGI, Ural Branch of the Academy of Sciences of the USSR, issue 26, Mineralogical collection No. 3, 1955.

7. F. N. Shakhov. On the theory of contact deposits. Trudy GGR ZS Branch of the Academy of Sciences of the USSR, vol. 1, 1947.

8. M. A. Usov. Iron-ore deposits of the West Siberian Territory. Publishing House of the Academy of Sciences of the USSR, 1937.