Magnetites of Different Ages from the Lebedskoye Deposit as Indicators of the Conditions of Its Formation

N. F. Stolbova

(Presented by the scientific seminar of the Department of Petrography)

The Lebedskoye deposit consists of skarns with productive gold–bismuth–telluride mineralization. In addition to gold-ore mineralization, the skarns are accompanied by magnetite mineralization, as well as albite–prehnite, microcline, quartz–hematite, quartz–epidote, quartz–sulfide, and carbonate–sulfide mineral associations (their names are given according to the predominant minerals). On the basis of the textural-structural interrelations of the minerals of the listed associations, the sequence of their formation is established in the order in which they are given above. At different stages in the formation of post-skarn mineralization, magnetite appears. Thus, magnetites are widely represented in magnetite ore bodies and recrystallized skarns, which we described earlier [1]. In addition, it occurs in the quartz–hematite association and in the carbonate–sulfide association.

Naturally, the physicochemical parameters of the medium in which magnetite formed left their imprint on certain of its features. Changes in these features in magnetites formed at different stages of the formation of the deposit will indicate a change in one or another set of parameters. Thus, comparison of different magnetites will help to detail the dynamics of the post-skarn process. Taking into account that gold forms before carbonate–sulfide mineralization, the conditions of its genesis can be refined.

Comparison of the geological and mineralogical features of different magnetites shows that the formation of the earliest magnetites took place under conditions of abundant, quiet inflow of components into the surrounding solution and of their high chemical activity. This is indicated by the abundant accumulation of solid masses of magnetites-I, their very large crystals, and the absence of traces of tectonic crushing. During crystal growth, the physicochemical conditions of the medium underwent frequent but insignificant changes, which is expressed in their fine but frequent zoning. Brittle deformations of the rocks are not characteristic of this time of magnetite formation. Later, under conditions of decreasing temperatures, their role increases, which is expressed in the appearance of veins, veinlets, breccia-like textures, and crystals with a blocky structure in magnetites-II, III, IV. The change in the character of the solution during crystal growth is more substantial (their individual zones differ more clearly from one another in the intensity of interaction

Table 1

Geological-mineralogical features of magnetites

with standard reagents and conc. HCl). The formation of magnetites-II, III, and IV apparently takes place without an additional supply of iron, by borrowing it from the surrounding iron-bearing minerals. Thus, magnetite-II forms only within skarns with magnetite-I, while magnetite-III develops after hematite, and magnetite-IV after pyrite. This is possible under conditions of a changing oxidation-reduction potential of the solutions. The sequence of replacement of some iron-bearing minerals by others and the character of the replacements presented in Table 1 (magnetite-I — magnetite-II — hematite — magnetite-III — pyrite — magnetite-IV) indicate a regular increase in the partial pressure of oxygen and an increase in the reducing potential toward the middle of the post-skarn process, and a decrease of these quantities toward its end.

The data of chemical analysis of magnetites-I and II indicate an abundance of components participating in the formation of magnetite, and their comparatively high contents. This testifies to a sufficiently high activity in the solution, in addition to iron, of such components as aluminum, calcium, magnesium, manganese, titanium, and chromium. It is of interest to note that during the formation of magnetite-I, Al, Ti, Cr, substituting for \(Fe^{3+}\) in the crystal structure of the mineral, played a large role, whereas during the formation of magnetite-II, Ca, Mg, Mn, substituting in the crystal for \(Fe^{2+}\).

Table 2

Results of chemical analyses of magnetites

Analysis of Table 3 shows that the calculated crystallochemical formulas of magnetites I and II have a regular deviation from the theoretical ones. Such deviations are often encountered in nature and are explained by

Table 3

Data from recalculation of the chemical components of magnetites I and II to a crystallochemical formula

they (Diar, 1965) as the characteristic feature of magnetite to contain in its structure oriented inclusions of hematite, hercynite, spinel, and other minerals. The ability to contain such inclusions is most characteristic of the earliest, high-temperature magnetites. With decreasing temperature, magnetites are freed of accessory minerals (which is evident when the crystallochemical formulas of magnetite-I and II are compared) and then become completely freed of them. Thus, in magnetite-III the chemical analysis reveals only FeO—29.37, Fe₂O₃—71.76, whereas in magnetite-IV FeO—24.73 and Fe₂O₃—75.27. The excess of Fe₂O₃, 12%, in magnetite-III is due to relics of the hematite-I being replaced, and the excess of 25.8% Fe₂O₃ in magnetite-IV to the replacing hematite-II.

Data of spectral analysis (Table 4) of the magnetites also emphasize the change in the amount and composition of microcomponents entering into the magnetites.

Table 4

Averaged results of spectral analysis of magnetites
in wt. % × 100

In successively forming magnetites, the content of siderophile components regularly decreases and the content of chalcophile components increases. According to activation-analysis data, the gold content in magnetite IV exceeds its amount in magnetite III by more than 100 times. The increase in the activity of chalcophile elements and gold, especially in the interval between magnetite III and IV, is associated with a rather intensive change in the physicochemical parameters of the medium. In particular, one may speak of a change in pH toward increased alkalinity. This is indicated by the results of analysis of suspensions of mineral associations with different magnetites, their conditional ionization potentials, and the chlorine content in them (Table 5).

Table 5

Acid–alkaline characteristics of mineral associations

Apparently, against the background of decreasing temperatures and increasing brittle deformations, the pressure also changes. Its decrease is indicated indirectly by the decreasing values of the microhardness and specific gravity of magnetite, as well as by certain changes in the parameter of the crystal lattice (Table 6).

Table 6

Physical parameters of magnetites

Analysis of the conditions of formation of magnetites of different times makes it possible to interpret the genesis of gold. Taking into account that gold deposition occurs after magnetite III and before magnetite IV, one may speak of lowered temperature and pressure during this period and of conditions of brittle deformation of the rocks. At this time the solution undergoes a change toward increased alkalinity, a decrease in reducing potential and in the partial pressure of oxygen, as well as an increase in the activity of chalcophile elements.

LITERATURE

1. Il’enok S. S., Stolbova N. F. Origin and composition of garnets of the Lebedskoye deposit. New data on the geology and geography of Kuzbass and Altai, Novokuznetsk, 1968.

2. Deer W. A., Howie R. A., Zussman J. Rock-forming minerals, vol. V. “Mir,” 1965.

3. Zharikov V. A. Skarn deposits. In: “Genesis of Endogenous Ore Deposits.” “Nedra,” 1968.