INFLUENCE OF THE pH OF THE MINERAL-FORMING ENVIRONMENT ON THE PETROCHEMICAL AND GEOCHEMICAL CHARACTERISTICS OF MAGNETITES

N. F. STOLBOVA

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

Studying the geology and mineralogy of the Lebed skarn–gold-ore deposit (Gorny Altai) and using a number of criteria, we distinguished numerous mineral associations and traced their replacement by one another in space and in time. Mineral associations forming under identical temperature conditions (according to data from homogenization of gas–liquid inclusions) were combined into stages: pre-skarn metasomatism, the skarn stage, the recrystallization stage, the hydrothermal quartz stage, and the hydrothermal carbonate stage (Stolbova, 1970). Taking into account the characteristics of the mineral associations that replace one another over time—namely, the chemistry of replacement of some minerals by others, the change in the conditional ionization potential of successively forming minerals, and also the pH of suspensions of individual minerals and their associations—we were able to trace the evolution of the pH of the postmagmatic solution from stage to stage (Stolbova, 1970).

Chemical-analysis data

Results of semiquantitative spectral analysis

Its general features are as follows. Toward the end of the skarn stage there is a gradual decrease in the alkalinity of the solutions. It temporarily ceases and even increases somewhat during the recrystallization stage. In the hydrothermal quartz stage, the alkalinity of the solutions increases, ceasing toward its end. At the end of the postmagmatic process, in the hydrothermal-carbonate stage, a tendency toward neutralization of the solutions is observed.

At all stages of the postmagmatic process except the first, which exhibit different acidity–alkalinity regimes, magnetites are formed (Stolbova, 1970). In a number of properties these magnetites differ sharply (their characteristic features are presented in Table 2). In this connection the question arises: does the pH of the mineral-forming environment affect the chemical composition of magnetites and the composition of their trace elements?

To answer the question posed, we turned to data from chemical analyses of magnetites, to the available semiquantitative spectral analyses, and considered them with allowance for the acid–base properties of the components found. The latter, as is known, are determined by electronegativity values, which are given in the work of A. E. Fersman (1962).

Tables 1 and 2 below present data from chemical and spectral analyses of magnetites. It is evident from them that both the petrogenic components of magnetites and the trace elements in them change their quantitative value from stage to stage. The set of trace elements also changes in its qualitative composition.

In order to trace the quantitative changes of elements as a function of their acid–base properties, we shall use graphical constructions. Along the abscissa axis we shall place the petrogenic components found in magnetites I and II in the order of their increasing electronegativity, i.e., in the order of their decreasing basicity. Along the ordinate axis we shall plot values reflecting the quantitative changes of the petrogenic components in magnetites I and II. The quantitative changes are determined by the formula

Table 1

[[unclear: beginning of caption]] analysis of magnetites

Table 2

analysis of magnetites in wt. %, 100,000

in magnetites (average of five)

\[ P_i^{\mathrm{II}} - P_i^{\mathrm{I}} \left/ \frac{P_i^{\mathrm{II}} + P_i^{\mathrm{I}}}{2} \right., \]

where \(P_i^{\mathrm{I}}\) and \(P_i^{\mathrm{II}}\) are the contents of the \(i\)-th element in magnetite I and magnetite II, respectively.

The relationship obtained in graph 1 indicates that, in magnetite II, relative to magnetite I, there are more strong bases and fewer weak ones. This may indicate, respectively, a greater activity of these components in the solution and, consequently, an increasing alkalinity of the solution.

Let us consider, in the same relationship, the changes in the contents of impurity elements in magnetites I and II. To do this, on the graph we shall place the impurity elements along the abscissa in order of their increasing electronegativity, and along the ordinate we shall plot the values of the quantitative changes in the contents of impurity elements of magnetite II relative to magnetite I. The resulting relationship is presented in graph 2. It, like the preceding one, indicates an increasing alkalinity of the solution.

The results of analysis of the two graphs are analogous and agree well with the proposition that the formation of magnetite II takes place under conditions of increasing alkalinity.

Let us try, in the same relationships, to trace the change in the contents of impurity elements in magnetite III relative to magnetite II and of magnetite IV relative to magnetite III. They are presented in graphs 3 and 4.

Fig. 1. Graph of the change in the contents of impurity elements of magnetite II relative to magnetite I

Fig. 2. Graph of the change in the contents of petrogenic components of magnetite II relative to magnetite I

Fig. 3. Graph of the change in the contents of trace elements in magnetite III relative to magnetite II

Fig. 4. Graph of the change in the contents of trace elements in magnetite IV relative to magnetite III

The changes in the contents of trace elements, as is evident from the graphs, are such that they indicate an increase in the role of components with more acidic properties during the formation of magnetite III and their decrease during the formation of magnetite IV. This also agrees well with other data on the change in the pH regime toward increased alkalinity in the quartz stage and its decrease in the carbonate stage.

Thus, the first attempt to trace the influence of the pH of the mineral-forming medium on the features of the chemical composition and the composition of trace elements proved positive. From the foregoing it follows that a change in the pH regime toward increased alkalinity leads to an increase in the content, in the mineral lattice, of elements with more basic properties, to the retention in them of trace elements likewise with more basic properties, and vice versa.

REFERENCES

1. A. E. Povarenykh. On the use of the electronegativity of elements in crystal chemistry and mineralogy. Transactions of the Ukrainian Branch of the All-Union Mineralogical Society. Issue 1, Kiev, 1962.

2. N. F. Stolbova. Mineralogy and genesis of the Lebed’ gold-bearing skarn zone. Dissertation for the degree of Candidate of Geological-Mineralogical Sciences. Tomsk, TPI, 1970.