Application of Potassium Radiometric Surveying in the Study of Ore-Controlling Factors within the Darasun Ore Field

L. A. Zashchinsky, V. A. Kortunov, G. G. Nomokonova,
T. I. Turysheva (TPI)

At present, considerable progress has been achieved in mapping fracture tectonics in the area of the Darasun ore field. In addition to the usual methods of tectonic mapping, good results in this direction are provided by electrical profiling and high-precision magnetic surveying, which reliably identify the position of faults in plan. However, well-known difficulties in tectonic mapping are caused by determining the age and depth of fracture tectonics, as well as the nature of secondary processes (hydrothermal activity, potassium metasomatism, etc.) manifested along certain fracture systems. In particular, during geological surveys and geological mapping, solving these questions requires the driving of numerous mine workings, which make possible direct petrographic-mineralogical observations; this significantly increases the cost of investigations.

Our work in 1967–1968 established the possibility of detecting manifestations of potassium metasomatism along tectonic disturbances by means of one of the lithogeochemical methods—potassium radiometric surveying.

The physical-geological prerequisites for solving this problem are as follows.

The process of granitization, especially characteristic of the field of development of amphibolites and gabbroids of the ancient metamorphic complex, developed in the northern and northwestern parts of the Darasun ore field, was not ubiquitous but selective.

The movement of emanations from the magmatic source that deposited quartz–potassium feldspar material took place along weakened zones in the body of the metamorphic massif. These zones, having increased permeability, were apparently linearly elongated zones of increased fracturing of tectonic origin. Potassium was released from the emanations as feldspars (mainly microcline) and, possibly, micas, characteristic of the material composition of veins and lenticular injection bodies, schlieren-like segregations,

individual accumulations and coatings of grains of xenogeneic minerals developing along fractures.

During destruction of the massif in the course of denudation, the products of mechanical and chemical breakdown of the rocks of these zones entered the eluvial-deluvial layer, being redistributed in its vertical section according to the size of the fragments. Potassium, being part of minerals relatively resistant to the action of agents of chemical weathering, migrated mainly, probably, in mechanical dispersion halos. With more complete chemical decomposition of these minerals, potassium could pass into aqueous solutions; however, forming readily soluble compounds, it was undoubtedly carried away into the area of groundwater discharge.

All this gives grounds to hope that tectonic disturbances along which the process of potassium metasomatism manifested itself sufficiently intensively may be marked by mechanical dispersion halos of potassium minerals that are not strongly displaced relative to the outcrops of these disturbances.

By the present time these predictions have been confirmed by practical data. The results of determining potassium during a lithogeochemical survey of one of the areas of the ore field showed that elevated concentrations of this element are indeed observed in zones of development of acidic dikes and tectonic disturbances.

The survey was carried out along the profiles shown in Fig. 1. Samples were taken from the layer of subsoil grus at a depth of 20–30 cm. The sample weight and the method of preparing them for spectral analysis corresponded to the instructions for lithogeochemical surveys [2]. The sample fraction with grain size less than 0.5 mm was ground to powder. Before spectral analysis, samples weighing 2–10 g were used for radiometric determination of potassium in them.

We used the radiometric method for determining potassium for the following reasons. It is very inexpensive and, at the same time, the sensitivity of measurements to potassium content is on the order of 1%, which is sufficient for identifying areas with intensive manifestation of the process of potassium metasomatism. Errors caused by anomalous contents of uranium and thorium in the samples arise rather rarely; moreover, the very increase in the contents of these elements, easily established by the radiometric method, is of considerable geological interest. The method is very simple and does not require highly qualified operators.

Measurement of the equivalent potassium content was carried out by the beta method on a DP-100 laboratory setup with an SGS-6 counter. The sensitivity of the method was determined using standards. The count rates of pulses, calculated for clarke contents of radioactive elements in soils, are given below:

The increased sensitivity of the setup to potassium radiation made it possible to use the equivalent potassium content as an estimate of the true content, since the effect from a uranium-thorium concentration 3–4 times higher than the clarke value is commensurate with the measurement error.

Samples and standards were measured under identical geometric conditions. At potassium concentrations up to 15%, no pulse miscounting by the setup is observed, and the calibration graph is a straight line.

tion. The function for reducing radiation to the saturated layer was determined experimentally. Samples were measured for 5 minutes, the natural background for 10 minutes, and standards for 30 minutes. The equivalent potassium contents were calculated on a slide rule of the logarithmic type, but with functional scales. The root-mean-square error of the measurements is: for samples weighing more than 5 g—0.4% potassium; for samples weighing less than 2 g—1.6% potassium.

The analyses carried out showed that the background potassium content for the survey area ranges from 1 to 3%; anomalous values plotted on the plan (Fig. 1) are at least 6%, and there are a number of anomalous points where the potassium content exceeds 10%.

When the factual potassium data are plotted on the geochemical map of the area, compiled by Yu. A. Aferov and N. M. Kashtanova, several zones of elevated potassium content are revealed, which trend toward the western half of the work area.

The most clearly recorded by elevated potassium content are disturbances located between the zero and minus-first adits, and intersecting profiles from 74 to 102.

Anomalous potassium values are associated with pickets located at the points where the profiles intersect the above-mentioned tectonic disturbances, or with pickets located near the intersection points. There is a small number of points with anomalous contents shifted relative to these disturbances, mainly toward lower hypsometric elevations of the daytime surface relief. A number of points with elevated potassium values are associated with exposures of large dikes of acidic composition and stock-like granitoid massifs localized, according to Yu. A. Aferov and N. M. Kashtanova, among amphibolites and gabbroids that make up the predominant part of the survey area.

A few isolated points with anomalous potassium concentrations, noted in the field of gabbroid development, may be connected both with the destruction of boulders of potassium-bearing rocks that migrated downslope for considerable distances from their bedrock exposures, and with the presence of tectonic disturbances not detected by the geological survey, especially small ones.

Attention is drawn to the association of points with high potassium contents with the system of northeast-trending disturbances. Tectonic zones of northwestern orientation are marked by such points only in areas of intersection with zones of the above-mentioned system. It is also characteristic that anomalous points are often localized on continuations of structural lines mapped on the map. This may make it possible to refine the results of the geological survey.

The results of the potassium survey make it possible to put forward certain assumptions regarding the age and depth of the main systems of tectonic disturbances in the area. Since the survey area is 3–4 km removed from the main fields of development of granitoid massifs, it is obvious that here one should assume an ascending-flank movement of its emanations, relative to the center of the magmatic focus, which caused the phenomenon of potassium metasomatism. If the process of granitization is assigned to the phenomena of magmatic metasomatism according to the classification of D. S. Korzhinsky, and if it is taken into account that the process took place at an erosion-cut level significantly higher than the present one, it is easy to suppose that the greatest pressure gradients, favoring the movement of emanations, occurred in the vertical direction. However, even if the flank displacement of emanations is considered commensurate with the vertical, the depth of the supply channels along which the pro-

Fig. 1. 1 — alluvial deposits; 2 — sandstones, felsite-porphyries, and plagiogranite-porphyries; 3 — medium-grained, fine-grained, biotite leucocratic granites; granodiorites and hybrid granites; 4 — hornblende gabbro and gabbro-amphibolites, partly granitized; 5 — carbonatization, silicification, and tourmalinization; 6 — beresitization, pyritization, and fluoritization; 7 — quartz-sulfide veins; 8 — established and inferred tectonic faults; 9 — zones of crushing; 10 — established and inferred rock boundaries; 11 — dip angles; 12 — anomalous potassium values \(>6\%\).

the movement occurred, is determined by the first kilometer. It is precisely this value of the minimum depth of propagation of fracture zones of northeastern orientation that should be regarded as probable. It may be assumed that this system of tectonic disturbances has a minimum age corresponding to the time of manifestation of the last stage of granitization at Darasun. The absolute age of the latter, apparently, can be determined by the potassium method [1]. As for the system of disturbances of northwestern orientation, the absence of indications of the manifestation of granitization along it, according to the potassium survey data, may, of course, be regarded as evidence of its younger age, although the work we carried out cannot serve as decisive proof of the latter. Assessing the results of the potassium survey from a purely methodological point of view, it should be stated that under the conditions of Darasun, at least in the fields of development of rocks of the ancient metamorphic sequence, it makes it possible to determine the manifestation, in covered areas, of the process of potassium metasomatism along tectonic zones; provides additional data for tracing them; and indicates the relative depth and relative age of tectonic disturbances.

REFERENCES

1. Zakirova F. S. Study of Certain Physical Properties of Potassium Minerals and Rocks for the Purpose of Developing a New Method of Absolute Geochronology. Author’s abstract of a dissertation for the degree of Candidate of Technical Sciences. Tomsk, 1966.

2. Instructions on Geochemical Methods of Prospecting for Ore Deposits, “Nedra,” 1965.