EXPERIMENTAL STUDY OF THE THERMAL METAMORPHISM OF QUARTZ ASSOCIATED WITH THE INTRUSION OF POST-ORE DIKES IN THE SARALINSK ORE FIELD

V. G. PROKHOROV
(Krasnoyarsk Branch of SNIIGGiMS)

A distinctive feature of the Saralinsk gold-quartz deposit is the abundance, within the ore field, of rocks of the dike series. Especially widespread among them are post-ore hyperbasite dikes, which cut the quartz veins in a direction close to normal. The thickness of these dikes ranges from fractions of a meter to 20–40 meters. A characteristic feature of the occurrence of the dikes is their persistence and parallelism. Observation in mine workings of contacts between dikes and quartz veins indicates their post-ore intrusion.

The intrusion of dikes into the ore-bearing sequence with already formed gold-quartz veins evidently could not have occurred without leaving traces; however, for the Saralinsk deposit there is practically no information on the scale, or even qualitative observations, of thermal metamorphism of vein material near the dikes.

In the present work an attempt is made to estimate the scale of thermal metamorphism by studying a thermophysical model of a dike that intruded a quartz vein.

From the theoretical point of view, heating of rocks by the heat of a cooling dike may be represented as a nonstationary (unsteady) heat flow propagating from a cooling plate into an infinite space.

Knowing the temperature of dike intrusion, its thickness and heat capacity, and specifying the heating time, one can calculate the temperature at any point in the space around the dike. The solution may be found from the heat-transfer equation for a cooling body—the Fourier equation.

\[ \frac{dT}{dt} = a \frac{d^{2}T}{dx^{2}}, \]

where \(a\) is the coefficient of heat transfer.

This solution will be approximate owing to the nonconstancy of the properties of the dike material and of the enclosing rocks with time \((t)\) (depending on the temperature \(T\), thermal conductivity and heat capacity will change).

A problem sufficiently complex for calculation becomes practically insoluble for direct mathematical computation if one takes into account that the thermal conductivity of the rocks enclosing the quartz vein differs from the thermal conductivity of quartz. In this case the heat flux from the dike will not be uniform, and the equation will have to be solved in a more general form,

\[ \frac{dT}{dt} = a \frac{d^{2}T}{dx^{2}} + a \frac{d^{2}T}{dy^{2}} \]

with two or even three variable values of the thermal-conductivity coefficient. These difficulties can be overcome to a considerable extent by using thermal modeling, the prospects and possibilities of which for solving the most complex thermophysical problems are highly regarded by specialists (L. R. Ingersoll et al., 1959; A. P. Dudarev, 1965).

We investigated the thermal model of a dike cooling in a quartz vein intruded by it. The scale of the model is 1 : 100. The dike model is made of an asbestos-cement plate with a built-in electric heater. Knowing the weight of the plate and heater, their specific heat capacities, the heating temperature, and the current strength, one can simulate the process

Fig. 1. Model of a quartz vein with an intruded dike:
1 — asbestos-cement plate with an electric heater simulating the dike;
2 — quartz plate;
3 — host rocks;
4 — thermal insulation;
5 — thermocouples.

of intrusion of “dikes” of different thicknesses and different temperatures. The quartz vein is a quartz plate about 2 cm thick, with recesses drilled for placing thermocouples. The plate can be placed between two layers of any material simulating the rocks of the hanging and footwall sides (Fig. 1).

Experimental setup. The “dike model” is heated to a specified temperature and then brought into contact with the quartz vein. Heating is carried out—

continues over a calculated time equivalent, at the scale of the experiment, to the thermal flux of a dike of a definite thickness.

The heating time is calculated from the following data. The heating area of the dike is \(2 \times 5\ \text{cm}^2\), the dike thickness is \(1\ \text{cm}\), the heat capacity of the diabase porphyrite is taken as \(0.3\ \text{cal}/\text{g}\cdot\text{deg}\), and the specific weight is \(3\ \text{g}/\text{cm}^3\). The amount of heat contained in the dike is:

\(Q = h \cdot s \cdot d \cdot C \cdot T = 1 \times 2 \times 5 \times 0.3 \times 3 \times 700\) (at an intrusion temperature of \(700^\circ\)). \(Q = 6300\). Let us take into account the amount of heat contained in the model heated to \(700^\circ\). The weight of the model is \(12\ \text{g}\), the total heat capacity is \(0.18\ \text{cal}/\text{g}\cdot\text{deg}\), \(Q\) of the model \(= 0.18 \times 12 \times 700 = 1500\ \text{cal}\). Heat losses through the lateral planes of the dike model not in contact with the vein amount to \(8\%\).

The amount of heat that must be supplied additionally in order to imitate the thermal flux from a dike \(1\ \text{m}\) thick will be:

\[ Q_{\text{calc.}} = 6300 - 1500 + (6300 + 1500) \times 8\% = 5430\ \text{cal}. \]

With an electric-heater resistance of \(16\ \Omega\), a current of \(4\ \text{A}\), the heating time of the dike, after bringing it into contact with the quartz vein, will be determined from the Joule–Lenz formula and is equal to \(5430 : (0.24 \times 4^2 \times 16) = 88\ \text{s}\).

Table 1

Table 2

In an analogous way, we calculate the heating time for other intrusion temperatures.

From the moment the dike is brought into contact with the vein, after a time interval of 1 minute, the temperature of heating of the quartz vein is recorded at eight points located along the normal to the dike contact at distances from 1 to 8 cm.

The data on the heating temperature as a function of the “dike” temperature, the heating time (i.e., the dike thickness), and the distance from the contact are given in Table 2.

On the basis of the experimental data summarized in Table 2, a series of graphs was constructed, relating the heating temperature of the quartz vein at different distances from the contact to the intrusion temperature of the dike (Fig. 2)

Fig. 2. Graph of the dependence of the minimum heating temperature of the quartz vein \((t_{\mathrm{h}})\) on the intrusion temperature of the dike \((T_0)\), distance from the contact \((x)\), for a dike thickness of 5 m.

and on the dike thickness (Fig. 3). All constructions were made on a logarithmic scale; therefore the exponential laws of heat transfer assume on the graphs a linear form, convenient for extrapolation into the region of higher temperatures. Such extrapolation is necessary, since under the experimental conditions we could reliably make measurements at a “dike” temperature of no more than \(600^\circ\) (at \(700^\circ\), overheating of the heating element was observed). The clear linearity of the graphs in the interval from 200 to \(600^\circ\) makes it possible, without fear of introducing any significant distortions into the actual picture, to extrapolate the experimental data to the temperature range \(1000\)—\(1200^\circ\).

Using the data from the experimental graphs and extrapolation, a summary diagram was constructed for the dependence of the heating temperature of the quartz vein on the intrusion temperature of the dike, the dike thickness, and the distance from the contact (Fig. 3).

Verification of the Thermal Model

Direct measurement of the temperature of basic-composition lavas at the earth’s surface gives values within the range 1050–1200°. The onset of melting of diabase is 1150°; complete melting occurs at 1300°. Free flow of magma of diabase composition is observed at 1225°. With slow cooling of the melt, supercooling may be observed, in which mobility is retained down to 1050° (Lovring, 1958). For our calculation we adopted a temperature of diabase magma equal to 1200°. Calculations

Fig. 3. Graph of the dependence of the minimum heating temperature of a quartz vein \((t_{\mathrm{pr}})\) on the temperature of dike intrusion \((T_0)\), distance from the contact \((x)\), and dike thickness \((M)\).

of heating temperature were carried out for portions of quartz veins situated at different distances from contacts, in which the temperatures of metamorphism were determined by the decrepitation method for quartz and by Callery’s sulfide geothermometer method.

1. Comparison of calculated data with decrepitation temperatures. For the Saralinsk deposit, we established that in direct contact with dikes the quartz contains no gas–liquid inclusions. Away from the dikes, such inclusions are detected by the decrepitation method and when examining chips of quartz.

under the microscope at magnifications of 400–800×. The small size of the inclusions makes it difficult to determine the homogenization temperature. Judging by the degree to which the inclusions are filled by the liquid phase, the quartz may be classified as relatively low-temperature; this is also supported by decrepitation data—mass cracking of the quartz begins at a temperature of 180–200°.

Quartz samples taken from the vein in the immediate vicinity of the contact, as a rule, do not reveal the presence of inclusions on heating; the latter, evidently, were destroyed as a result of heating of the vein material by the intruded dike.

Table 3 below gives calculated data for the heating of portions of the quartz vein and the decrepitation temperature of the quartz. The heating temperature was calculated from diagram 3.

Table 3

2. Comparison of calculated data with the temperature of sphalerite deposition. The iron content of sphalerite in the deposit varies widely, from fractions of a percent to 10%. The more iron-rich sphalerites are spatially confined to the contacts of the dikes or occur near them. We explain this regularity as a consequence of recrystallization of the primary low-temperature sphalerite under the influence of the heat of the dikes. The possibility of such recrystallization, during which more iron-rich sphalerite was formed, is confirmed experimentally.

The content of isomorphous iron in sphalerite was determined by chemical analysis. Sphalerites associated with pyrrhotite and pyrite were studied. The latter association occurs considerably more often in the deposit. According to Kullerud’s data, the temperature determined for the pyrite–sphalerite association may be regarded as minimal or underestimated (G. Kullerud, 1959); taking this into account, and also the critical remarks concerning the method in general that have appeared recently (N. A. Godovikov, A. B. Ptitsyn, 1966), we consider it possible to use the data of the method to reveal the temperature gradients that characterized sphalerite deposition in different parts of the deposit. The data comparing temperatures determined by Kullerud’s method and the calculated heating temperatures are given in Table 4.

As is evident from the comparison, higher heating temperatures correspond to higher temperatures of sphalerite deposition determined by Kullerud’s method. The primary sphalerite apparently was deposited at temperatures close to 100°. Such sphalerite occurs in parts of veins where the thermal influence of the dikes was practically not manifested.

3. Comparison of calculated data with occurrences of metamorphic associations.

Near the contact of dikes with quartz veins, pyrrhotite is noted in the latter, together with pyrite, in subordinate quantity. The absence of the pyrite–pyrrhotite association away from the contacts testifies in favor of the metamorphic origin of pyrrhotite through the decomposition of pyrite under the influence of the heat of the dikes. Mac Kinstry (1959) estimates the temperature of the transition pyrite → pyrrhotite at a sulfur pressure of 1 atm as equal to

Table 4

689°. The temperature of dissociation increases to 743° at 12.2 atm of sulfur. The low content of pyrite in the quartz vein, the absence of any significant accumulation of newly formed sulfides associated with the intrusion of the dikes, as well as the absence of sulfide mineralization in the dikes themselves, gives grounds to suppose that there was no introduction of sulfur with the intrusion of the dikes, and that the sulfur pressure was determined only by the elasticity of vapors over the sulfides, i.e., at temperatures below the dissociation of pyrite into pyrrhotite and sulfur it was immeasurably small, remaining very low even at higher temperatures. According to Udnintseva and Chukhrov (1941), a noticeable process of dissociation of pyrite into sulfur and pyrrhotite at low (mm Hg) sulfur pressures appears from 400°. Taking into account the duration of heating of the rocks around the dikes, amounting to months and even many years, it may be assumed that the process of partial transition of pyrite into pyrrhotite could have proceeded at temperatures close to, and even somewhat lower than, 400°. At the deposit the pyrrhotite–pyrite association was noted in areas whose calculated heating temperatures range from 1000 to 400°.

4. Determination of the duration of heating.

The experimental data obtained in the study of the model make it possible to estimate the heating time of the quartz vein. The calculations are based on the so-called “law of heat-transfer time,” which is formulated as follows. The time required for any two points to reach one and the same temperature is proportional to the squares of their distances from the heat source. Thus, if in our model a point 3 cm away from the “dike,” having a temperature of 600°, is heated to 150° after 16 minutes, then a quartz vein at a distance of 3 m will be heated to this temperature by a heat source of the same intensity after \(16 \times 10^4\) minutes, or 110 days.

A diagram of the dependence of the heating time to 100° for zones at various distances from the contact and for different dike thicknesses is given in Fig. 4. Similar diagrams may be constructed for other heating temperatures.

As can be seen from the diagram, at a distance from the contact with a dike intruded at a temperature of 1200°, equal to 2 meters, the vein reaches a temperature of 100° after 10 days. With a dike thickness of 5 m, the outermost point of the 100° zone will be 7 meters from the contact. It will reach

this temperature after 140 days. Cooling of a vein segment 2 meters from the contact to a temperature of 100° will occur after 270 days.

1. Evaluation of the influence of thermal metamorphism on the redistribution of gold.

Fig. 4. Diagram of the heating time of a quartz vein above 100° near a diabase-composition dike at the intrusion temperature of the latter of 1200°. \(x\) is the distance from the contact with the dike; \(m\) is the thickness of the dike.

As early as 1930, G. Berg established that finely dispersed gold in pyrite, invisible under the microscope, coarsens after heating the sample to 300–400° and becomes visible. Subsequently this effect was observed by many investigators. The influence of metamorphic heating on the size of gold particles in pyritic ores of the Ural deposits was studied by Kreiter (1941). According to Kreiter’s data, for South Ural deposits not affected by metamorphism (Sibai, Blyava), the extraction of gold from ores by the cyanidation method does not exceed 30–40%, whereas for Middle Ural intensely metamorphosed deposits gold is extracted by the same method at 95%. Kreiter finds the explanation for this in the dispersed character of gold in the unmetamorphosed deposits. In the process of metamorphic alteration of the ores of the Middle Ural deposits, the gold became coarser.

Experimental studies by N. N. Maslyanitskii (1940), O. E. Zvyagintsev, and I. A. Paulsen (1940) established that gold precipitated from solutions together with pyrite usually has submicroscopic grain sizes. Extraction of such gold by amalgamation and cyanidation gives a low yield of metal. Gold deposited on previously formed pyrite forms coarser segregations. Judging from G. Berg’s experiments, gold coarsens during subsequent metamorphism of sulfides.

Having estimated the temperature of thermal heating of the deposit caused by the intrusion of dikes, it would be interesting to establish to what extent this heating affected, and whether it affected at all, the redistribution of gold in the veins.

The problem posed cannot be solved by considering the change in gold content with distance from a single dike taken separately, since the unevenness in the distribution of gold along the vein undoubtedly existed before the intrusion of the dikes, and heating only complicated the picture. The problem can be solved statistically, taking into account the redistribution of gold over the entire deposit, with consideration of dozens of contact zones.

For this purpose, we calculated the heating temperatures of the vein throughout the deposit and established the relationship between gold content and heating temperature. To exclude the influence of other factors, we selected sections of veins occurring only under identical geological conditions, namely veins whose hanging wall and footwall are represented by schists.

A total of 487 assay analyses were processed. The data of the statistical processing are given in Table 5.

Table 5

Thus, the gold content in the area of heating below 200° remains constant, drops sharply in the heating zone of 200—400°, and enrichment occurs in the zone of the highest temperatures, i.e., in the area of immediate contact. The mechanism of gold redistribution in the high-temperature zone is not entirely clear to us, but apparently it is related to the coarsening of gold observed when polished sections of pyrite are heated. It is likely that redeposition of gold at the contacts is accompanied by a change in its grain size. It cannot be ruled out that fluctuations in gold recovery at the mine’s concentrating plant, which forced the enterprise to abandon the more economical cyanidation process, were associated with the mining of ore blocks heated to varying degrees by the heat of post-ore dikes.

This conclusion may be of serious interest not only from the theoretical but also from the practical point of view, and, it seems, deserves verification.

In conclusion, let us dwell on certain questions concerning the possibility of applying thermal modeling. The model we used makes it possible to estimate the temperature of heating of rocks near an intruded dike, provided that the heating proceeded without mass transfer. If the calculated heating temperatures are close to the temperatures determined using other geological thermometers, this will indicate that the formation of the dike took place rapidly and that only a small amount of molten material passed through the considered part of the section, i.e., that we are dealing with the upper parts of the dike. It is precisely such a conclusion that we can mo-

...than is done for the hyperbasite dikes that cut the Saralinsk ore field. If the calculated temperatures prove to be significantly higher, the conclusion may be drawn that we are dealing with deep horizons of the dike, or else that the dike was a feeder channel through which a larger intrusive body was supplied.

The thermal model makes it possible to estimate the time scale of metamorphism. In principle, the model can also be used to solve the inverse problem, i.e., to determine the temperature of emplacement of the dike from the temperature of metamorphic alteration of the rocks near the contacts. By carrying out experiments in a closed space, it will also be possible to estimate the influence of mass transfer associated with the “distillation” of pore waters from the enclosing rocks.

Undoubtedly, the “model” presented permits only an approximate estimate of the temperatures of rock heating. It does not take into account many factors (the heats of crystallization and melting of the dike material), but even such a model makes it possible to draw a number of interesting geological conclusions that shed light on the genesis of the deposit.

REFERENCES

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Godovikov N. A., Ptitsyn A. B. Introduction of iron into sphalerite during hydrothermal recrystallization. Dokl. AN SSSR, vol. 166, No. 1, 1966.

Zvyagintsev O. E., Paulsen I. A. Toward a theory of the formation of vein gold deposits. Dokl. AN SSSR, vol. 26, No. 7, 1940.

Ingersoll L. R., Zobel O. J., Ingersoll A. K. Heat conduction and its applications to geology. Moscow–Leningrad: Mashgiz, 1959.

Kullerud G. Sulfide systems as geological thermometers. In: Geochemical Investigations, IL, 1961.

Lovering G. S. Temperatures near and within intrusions. Problems of Ore Deposits, IL, 1958.

Maslyanitskii N. N. The form of association of gold with minerals in sulfide ores of nonferrous metals. ONTI, 1940.

Udintseva V. S., Chufarov T. I. Kinetics of pyrite dissociation. Journal of Applied Chemistry, vol. 14, 1941, No. 1.