Pattern of Gold Distribution in the Veins of the Nikolaevskoye Deposit

(Central Transbaikalia)

S. A. Chubarov

(Presented by Professor A. M. Kuzmin)

The Nikolaevskoye gold-ore deposit is located in the south of Central Transbaikalia and is confined to the eastern flank of the Lyubavinskoye ore field, the main features of whose geological structure were described by G. V. Shubin and others [2]. The structure of the Nikolaevskoye deposit includes Permian sedimentary-metamorphic deposits intruded by a series of sublatitudinal dikes of quartz porphyries, calcareous-alkaline syenites, diorite-porphyrites, and microdiorites. Both the sedimentary-metamorphic and the igneous rocks of the deposit are cut by numerous disjunctive disturbances, among which sublatitudinal thrust faults dipping steeply to the north play a special role. In the hanging walls of the indicated thrust faults, numerous feathering fractures are developed, of which gently dipping tensional fractures are the principal ore-localizing structures.

The Nikolaevskaya-2 vein has been explored in the greatest detail at the deposit, and it is on its example that we examine the pattern of gold distribution. The attitude elements of the vein are not constant: the strike azimuth is 250–330°, and the dip is in northern rhumbs at an angle of 20–45°. The vein is characterized by sharp bends both along strike and along dip, and it is readily established that here there is not a curvature of the ore-hosting fracture, but a phenomenon of transition from one fracture to another. The complexity of the ore-hosting structure naturally affected the behavior of the thickness, which varies from 0.05 to 4.0 m; at the same time, in the plane of the vein, along with pinches, significant swellings are observed. Among the swellings two groups are clearly distinguished: some are oriented along the strike of the vein (the axial line is almost horizontal); others incline eastward at an angle of 30–45° (Fig. 1). Horizontally oriented swellings are confined to places of flexure—

the smoothing of sublatitudinal intervals of ore-hosting fractures. In accordance with the known regularities of fracture opening [1], the formation of horizontally oriented swells can be explained by upthrow movements along steeply dipping sublatitudinal disjunctions. Swells of the vein that dip eastward are confined to intervals of the vein having a NW strike. The formation of these swells took place under conditions of shear displacements along sublatitudinal intervals of the ore-hosting fractures. These displacements are also confirmed by the development, along the main ore-hosting structure, of subsidiary fractures, both shear (azimuth of strike 250°) and tensional (azimuth of strike 340–350°). Numerous veinlets and short but thick apophyses are confined to the tensional fractures.

Fig. 1. Plan of isolines of gold content and vein thickness of Nikolaevskaya-2 (projection onto the horizontal plane). Gold content: 1—low, 2—medium, 3—high; 4—isolines of thickness at 1 m intervals.

The distribution of gold in the veins is highly uneven. As can be seen on the plan of content isolines (Fig. 1), against the background of low-grade ores three more or less persistent ore shoots stand out, gently dipping westward. The ore shoots distinctly cut across both the horizontally oriented and the inclined swells of the veins. This circumstance may indicate the absence of a correlation between vein thickness and the gold content in it, which is also established by the method of mathematical statistics (the correlation coefficient is 0.14). At the same time, nests of the richest ores within the contour of the ore shoots spatially tend toward the intersections of the above-described swells, toward areas of sharp fluctuations in thickness and/or toward areas where the elements of occurrence change.

In general, it is established that areas where the morphology of the vein becomes more complicated (regardless of the magnitude of its thickness) have increased gold content. This circumstance is probably connected with the fact that areas of complicated morphology are characterized by an increased density of intra-ore fracturing (Fig. 2), which in turn correlates rather reliably with gold content (the correlation coefficient is 0.57).

As observations show, gold in the vein is controlled by thin shear fractures of sublatitudinal strike, being confined to intergranular spaces and small ruptures along the periphery of the indicated shear fractures. The spatial orientation of the considered intra-ore shear fractures (azimuth of strike 250–270°) and extension fractures (azimuth—

Fig. 2. Graph of changes in thickness, gold content, and fracture density along the strike of the Nikolaevskaya-2 vein. 1 — vein thickness in dm; 2 — gold content in arbitrary units; 3 — fracture density; one graph unit corresponds to ten fractures per 0.32 m².

strike 340–350°) repeat the orientation of the quartz veinlets feathering the vein. This may indicate that the formation of both these and the others proceeded under the same deformation pattern, i.e., during strike-slip movements along the main ore-hosting structure. As the studies of V. F. Chernyshev [1] have shown, in the case of displacements along curvilinear faults, intervals of fractures close to the direction of displacement are characterized by the development, on their flanks, of feathering rupture fractures, whereas shear feathering fractures develop in areas whose strike forms a certain angle with the direction of displacement. This, probably, can also explain the fact that in sections of veins having a northwestern strike, the intensity of manifestation of shear intra-ore fractures is much higher than in sections of near-latitudinal strike. This circumstance was the decisive factor in the localization of ore shoots, spatially gravitating toward those sections of the vein where the strike changes from latitudinal to northwestern. Within the ore shoots, the increase in the intensity of quartz fracturing in segments of the vein characterized by sharp variability in thickness can be explained as follows. During movements along the vein, slip planes develop predominantly along its weakened contacts. If the vein contact is rather even, then no major crushing of the quartz occurs; if, however, the contact is uneven, the slip planes develop directly in the quartz, cutting off protrusions of the vein, as a result of which the quartz here is strongly crushed, which also promotes circulation of solutions and localization of mineralization.

Conclusions

The irregularity in the distribution of gold in the Nikolaevskaya-2 vein is caused by the different intensity of intra-ore fracturing of the quartz. Increased intra-ore fracturing, which controls gold mineralization, develops in sections of curvature of the vein along the strike.

earlier, which also determines the position of the ore shoots. Within the ore shoots, pockets of the richest veins are confined to places of abrupt changes in thickness, where the intensity of intra-ore fracturing is higher than in the ore shoot as a whole.

The identified regularities in the distribution of gold should also hold for other veins of the deposit that have structural features in common with the Nikolaevskaya-2 vein.

REFERENCES

1. V. F. Chernyshev. On regularities in the position of feathering shear and tension fractures. In: Problems in the Study of the Structures of Ore Fields and Deposits. Proceedings of the Institute of Geology, Academy of Sciences of the USSR, issue 162, ore-deposit series, No. 17, 1956.

2. G. V. Shubin, S. A. Chubarov, A. V. Matsyushevsky, A. I. Volkova. Structure of the ore field and genesis of the ores of the Lyubavinskoye deposit (Transbaikalia). In: Geology of Gold-Ore Deposits of Siberia. Nauka Publishing House, Novosibirsk, 1970.