ON THE GEOLOGY OF THE ILYA GOLD ORE DEPOSIT

(EASTERN TRANSBAIKALIA)

A. M. Kuzmin, G. V. Shubin

The Ilya gold ore deposit is confined to a major fracture of northeastern strike, which, within Chita Oblast, occurs in the area of development of granitoid rocks of the Daurian intrusive complex and of widely developed effusive and dike rocks of acidic and intermediate composition.

Investigations carried out in 1959–1960 provided us with new materials on various questions concerning the structure of the fracture zone, the character of tectonites, the regularities of fracture tectonics, the conditions of localization of gold mineralization, and hydrothermal alteration of the enclosing rocks.

Within the Ilya deposit, biotite granites, locally porphyritic, are widely developed, and more rarely associated with them are thin dikes of aplites and aplite-pegmatites. As the history of formation of the area in this part of Eastern Transbaikalia shows, after the emplacement of the indicated granites and aplites, whose formation took place from the residual magma of the granites following its recrystallization, powerful dislocations occurred, leading to the formation of broad and numerous zones of crushing and fracturing.

One of such zones, extending in a northeastern direction along the middle course of the Ilya River, within the Ilya ore field is represented by granites dislocated to varying degrees, among which cataclastic granites, cataclasites, and tectonic breccias may be distinguished. Between these varieties of rocks disturbed by crushing there are gradual transitions. Cataclasites are most extensively developed; within their field the breccias often have the form of irregular lenses, bands, and areas of various sizes and outlines elongated in a northeastern direction. Cataclastic or weakly dislocated granites are represented within the general field of crushing by small remnants with fanciful outlines in plan.

Field observations of the character of manifestation of cataclasis and mylonitization, as well as petrographic study of the tectonites of the fracture zone, indicate their clearly tectonic origin, associated with repeated movements along the zone of the Ilya fracture. Often these movements manifested themselves locally in different places of the fracture zone, affecting individual parts of it, which along strike not infrequently are gradually replaced by completely undislocated rocks.

In places where the dislocation of the granitic rocks was most intense, zones of coarse mylonitization and typical tectonic breccias were formed, in which both relatively large fragments and their cement are granitic. Among the breccia fragments one can distinguish individual grains of quartz and feldspar; porphyritic grayish, grayish-green, and cataclased granites altered to varying degrees; granitic cataclasites; aplites; and aplite-pegmatites. The development of breccias and other varieties of tectonites is not confined solely to the area of the deposit; they can be traced beyond its limits both to the northeast and to the southwest of the deposit.

After the consolidation of the granitic intrusion, the formation of vein rocks—aplites and aplite-pegmatites—and the establishment of the fault zone, the latter left its imprint on the entire subsequent history of the development of magmatic and hydrothermal manifestations, including the character of localization of the mineralization.

The presence of the fault zone, with its intensely crushed rocks, was a favorable condition for the manifestation of hydrothermal processes expressed in chloritization, sericitization, and beresitization.

The further history of the deposit’s development was marked by the outpouring of acidic effusives represented by felsites, felsite-porphyries, and their tuffs. These formations are widely developed within Mount Grisheevskaya and have a blanket-like character, filling a relatively small steeply sloping valley incised into the granites. This supposition is confirmed by the relationships between outcrops of felsites and granites in the relief, as well as by drilling data.

The next stage in the history of the fault zone was expressed in the emplacement of dikes and stock-like bodies of small intrusions represented by quartz porphyries, diorites, diorite-porphyries, and their vein derivatives—microdiorite-porphyrites. The formation of these bodies was accompanied by new tectonic movements, expressed in the widespread development of mylonitization and crushing in a number of areas of the felsite cover.

Hydrothermal manifestations due to the emplacement of diorites and diorite-porphyries were expressed in silicification, tourmalinization, and pyritization. As detailed sampling of the altered areas, diorite-porphyries, and pyrites has shown, gold was also deposited in association with these hydrothermal processes.

The final stage in the formation of the deposit was preceded by new strong tectonic dislocations, expressed in the formation of small zones of crumpling represented by strongly sheared and crushed material of cataclasites and breccias. The manifestation of these crumpling zones was marked by new hydrothermal processes, which led to the formation of quartz-carbonate-sulfide mineralization of the stockwork type. At this stage of mineralization such sulfides as arsenopyrite, pale ores, sphalerite, galena, molybdenite, and visible gold were deposited.

Analyzing the general course of development of the magmatism of small intrusions and hydrothermal manifestations, the following conclusions may be drawn: 1. The study of the chemical characteristics and behavior of trace elements in the small intrusions indicates a connection of the indicated bodies with manifestations of intrusive activity from a single parent source. In this process, the evolution of the magma over time proceeded from acidic to basic. 2. The intensity of rock alteration is directly dependent not only on the nature of the solutions and their character, but also on the degree of dislocation of the original rocks. In particular, zones of strong hydrothermal alteration are characterized precisely by the greatest manifestation of dislocation—

metamorphism, within which the character of the hydrothermal alterations distinctly has metasomatic zonality. 3. The first generation of gold mineralization is closely associated with the processes of silicification, tourmalinization, and pyritization; the second generation is closely associated with the quartz–carbonate–sulfide stage of mineralization.

One of the principal questions in the study of the deposit is the question of the character and regularities of distribution of the useful component and of the further prospects of the object under study.

In order to present a picture of the distribution and possible concentration of gold within the limits of the Ilya ore field, it is necessary to consider two main factors: 1) the petrographic-mineralogical factor and 2) the geological-tectonic factor.

The study of the petrography, mineralogy, character of hydrothermal manifestations and alterations of the rocks of the deposit and, finally, the results of sampling in heavy-mineral, metallometric, and gold-metric surveys within the ore field indicate not only the general prospects of the zones of beresitization and tourmalinization, but also make it possible to outline a number of areas for their further study. In addition, the character of transport and the assemblage of minerals in the valley of the Kurlukta River, northeast of the deposit, indicate the possible presence here of a placer, which may be of interest for the discovery of gold.

It is interesting to note the fact that gold is present in the felsites of Mount Grishchevskaya. The study of the relationships of diorite-porphyrites with felsites, and of the character of the contact alterations, expressed in crushing, silicification, carbonatization, and sulfidization with the presence of gold-bearing pyrite, shows that in a number of cases these processes led to enrichment of the felsites in gold.

In the character of the distribution of gold at the Ilya deposit, two principal conditions should be noted, arising from the tectonic specificity of the fault zone as a whole and from the regularities of fracture tectonics. Each of these conditions influences in its own way the character of gold distribution.

First, the structural setting of the fault, with its intensely crushed rocks and good circulation within them of ore-bearing solutions, creates conditions favorable for dispersion of the mineralization, which we also observe when analyzing the sampling within the quarries and throughout the entire ore field of the deposit.

Such a picture, as the practice of studying tectonic zones has shown, is characteristic mainly of the upper horizons of faults with the development of stockwork-type mineralization, belonging to the apical part of a deposit.

On the other hand, analysis of the fracture tectonics showed that the two identified systems of zones of intense fracturing, of NE 30–70° and NW 310–340° strike, are of great importance in the process of localization of mineralization. This is expressed in the fact that the ore shoots of the deposit are confined to the places of intersection of the above-mentioned zones.

It should be said here that at the deposit, in addition to the noted structural control, the mineralization represented by the quartz–carbonate–sulfide stage is confined to places where ore-bearing fractures are bounded by claylike mylonite of fold zones. Finally, a concentration of mineralization is also noted along a thick dike of quartz porphyry confined to fractures of northeastern strike, which are leading ones in the distribution of mineralization. Characteristically, these mineralized areas are small in size and rapidly pinch out down the dip.

In conclusion, we note that an analysis of the characteristics of the deposit, the nature of the manifestation of hydrothermal processes, and the regularities in the distribution of mineralization lead us to the conclusion that the Ilya gold-ore deposit is of the stockwork type and of hydrothermal genesis. The stockwork character is especially evident in the upper horizons of the deposit, where mining and exploration work is currently being carried out. Since we are dealing with the apical part of the deposit, the question arises that, with depth, the dispersed ore zones may decrease in size, but at the same time prove to be more enriched in gold.

Such an approach to the type of deposit makes it possible to search for enriched areas not only at the present horizon of exploration work, which can be identified only by detailed sampling, but also at depth, with accompanying structural analysis of the ore field in order to identify tectonic structures favorable for the penetration of solutions and the localization of ore accumulation.