On the Mechanisms of Vein Formation in Quartz–Gold Ore Deposits

P. F. Ivankin, K. R. Rabinovich, V. N. Akchurina (SNIIGGIMS)

Ideas about the mechanisms of hydrothermal vein formation have been obtained by studying the morphology of veins and their spatial relationships, as well as the regularities of branching, pinching out, and conjugation of veins with one another. Necessary information is also provided by the study of the composition, internal structure, textural and structural features, and zoning of veins. The authors have analyzed and compared such data for several vein gold-ore deposits of Western Siberia, Transbaikalia, Kazakhstan, and the Urals. They also took into account the materials of numerous studies of vein deposits, the results of which are examined in detail in the monographs of N. I. Borodaevsky [1] and F. N. Shakhov [7].

The principal structural-morphological unit of a vein gold-ore deposit is a bundle of veins, representing a system of veins that diverge and branch upward from a single center common to them [2, 3, 4, 5]. The structural-morphological features of such systems indicate the hydrodynamic unity of the flow of vein-forming solutions and its multistream branching during movement from below upward. The branching of vein systems is associated with the property of a hydrothermal fluid to expand in volume and disperse in space as it penetrates from a region of high external pressure (the deep levels of a deposit) into a region of relatively low pressure (the middle and upper levels of a deposit).

This phenomenon is especially typical of subvolcanic gold deposits [7, 8, 9]. At such deposits as Balei, Kliuchi, Darasun (Transbaikalia), Dealul Crucii, Baia Sprie (Romania), Cripple Creek (USA), and Waihi (New Zealand), ore veins repeatedly branch and, upward, often pass into zones of veinlet and stockwork ores. By contrast, in the deeper deposits of the plutonic group, changes in external pressure are appreciable only over comparatively large vertical distances, and therefore branching of the vein system is detected only by comparing levels far removed from one another, i.e., in especially deeply explored deposits (Dzhetygara, Kochkar, Bestyube, Olkhovskoe, and others). The general decrease in external pressure toward the surface determines, during the formation of a deposit, not only an upward increase in the number of vein bodies.

but also an increase in the total length of the veins and the amount of quartz. Thus, for example, in the northern part of Balei, between levels I and VII the total length of veins increases from 234 m to 2640 m, while the volume of vein quartz increases toward the surface by a factor of 4–5. In the Dzhetygarin vein cluster (central sector), the volume of quartz located in the interval from the surface to a depth of 100 m amounts to about 50% of the total volume of quartz in this vein cluster. With depth its amount decreases and reaches, in the interval from 100 m to 200 m, 30%; in the interval from 200 m to 300 m, 10–15%; and, finally, in the interval from 300 to 400 m, less than 10% of the reserves of quartz ore. Calculations for the Bestyube deposit (central sector) showed that from the surface to a depth of 340 m there is a decrease in the number of ore veins by a factor of 4–5, in the total length of veins by a factor of 7, and in the total amount of vein material by a factor of 7–8.

In the system of a vein cluster, one or several large (trunk) veins are usually distinguished, marking the main jets of the hydrothermal flow, from which smaller veins and apophyses branch off. This phenomenon of a vein cluster is also characteristic of individual large veins; it is expressed, on longitudinal projections of veins, as a fan-shaped widening of the outline of the vein body. At the same time, the horizontal length of the vein on the upper levels of a deposit increases many times in comparison with its length on the lower ones. The angles of divergence of the vein outline upward vary under different conditions, and they determine the variety of forms of longitudinal projections of veins and, ultimately, the variety of forms of the vein-forming flow in a fracture.

The features of the morphology and structure of vein systems and of individual veins indicate centrifugal development of the hydrothermal process. In this case, vein formation by no means occurred through passive filling of cavities by fluids, as is assumed by the suction hypothesis. On the contrary, the structural–morphological features of veins and vein clusters speak in favor of the hydrothermal-solution column possessing, relative to the general lithostatic pressure, excess energy sufficient for active “conquest of space” in compressed fractured-porous rocks that, in the general case, did not contain open cavities. As observations of frontal terminations of veins and their apophyses show, the walls of compressed fractures were pushed apart by the solution through hydraulic fracturing to limits equilibrating the excess pressure of the solution. Therefore the thicknesses of veins are regular, since they are causally related to the excess pressure of the solutions and to the stress-tectonic state of the enclosing rocks during the period of vein formation. Changes in vein thickness (and sometimes significant ones) generally indicate changing ratios between the internal pressure of the fluid and the external pressure on the fracture walls. The confinement of veins only to fractures of a definite orientation may indicate that injection of the hydrothermal fluid proceeded according to the principle of least expenditure of energy. The vein-hosting fractures were less loaded, while other pre-ore fractures proved “energetically unfavorable” for penetration of the solution into them: the pressure on the walls of such fractures exceeded the internal pressure in the fluid. The great thickness of trunk veins is determined by the greatest internal pressure in the main jet of the hydrothermal flow. At certain distances from the main jet, the excess pressure in the lateral jets of the fluid fell so much that hydraulic fracturing of compressed fractures became impossible, and therefore the formation of apophyses ceased. The gradual pinching out of a vein upward to its complete disappearance (the most widespread type of vein termination) also

indicates a gradual decrease of pressure in the fluid to the minimum at which hydrofracturing of compressed fractures ceased.

The method of formation of quartz veins considered by the authors is fully consistent with the concept of a significant density of the vein-forming fluid. This is also indicated by the following facts: textural effects of the injection of an unconsolidated viscous vein substance into fractures; active mechanical action of the fluid on fracture walls; impressions of fracture irregularities on the contact surface of veins; the presence, in the vein mass, of suspended angular fragments of the wall rocks; cases of rotation and transport over considerable distances of such fragments during injection of the vein substance; melting of fragments of a large clast during penetration of the vein substance into fractures cutting it; deformation, during fluid movement, of small plates of schistose rocks; pinching-off of the vein and formation of quartz veinlets; and the sealing of large gas bubbles in quartz. These features of veins, in the authors’ opinion, exclude the possibility of representing the vein-forming solutions as traditional “dilute true solutions.”

At present, among researchers of vein deposits, the prevailing opinion is that hydrothermal solutions entered fractures from a deep-seated source in a multi-act, pulsational manner. A comparative analysis of data on the discontinuity of mineral deposition in deposits shows that vein swarms are formed, as a rule, during three- and fourfold pulsation of ore-bearing sources in approximately the following sequence: intrusion of acid melts; metasomatic alteration of near-fracture rocks (listvenitization, beresitization, tourmalinization, kaolinization, etc.); intrusion of intermediate and basic dikes; injection of auriferous siliceous solutions. However, there is no direct evidence that deep-seated pulsation also occurred during the formation of the productive quartz veins themselves. Let us consider this question in more detail.

In gold-bearing veins, up to 4–6 mineral associations are usually distinguished, forming in a definite age sequence [6]. At first the main mass of “barren” quartz crystallizes, making up 80–95% of the volume of the veins. It is often accompanied by albite, scheelite, pyrite, arsenopyrite, and a small amount of gold. This early-crystallizing part of the solution was characterized by relative poverty in volatiles. Then the productive quartz crystallizes (usually of several generations), enriched in gas-liquid inclusions and containing carbonates, sulfides, sulfosalts and tellurides of metals, as well as the main mass of gold. The concluding stage of mineral formation is usually represented by quartz-carbonate associations. In some cases the productive veins are weakly differentiated (Kommunar, Dzhelambet), in others they are fully differentiated (Darasun, Balei, Olkhovskoe, Sarala, Bestyube, etc.). At the same time, a dependence is observed between the number of different-time mineral associations (stages of mineralization) in veins and the degree of their distance from the ore-bearing source. Ore bodies situated not far from the source that generated them are comparatively homogeneous. Relatively far-removed veins are distinguished by a great variety of mineral associations. Thus, in the Kommunar ore field, stockwork ores in diorites are represented essentially by a single mineral association, whereas quartz veins located above the stockworks (Zh. Yanvarskaya) or in their upper horizons consist of several different-time associations. The same tendency is also observed for the central part of Dzhelambet. The number of stages of mineralization

also depends on the depth of formation of deposits: in near-surface deposits up to 7–8 or more stages of mineralization are manifested (Balei); in deep-seated deposits the number of stages of vein formation is small.

The products of the late stages, and the principal mass of gold associated with them, were distributed unevenly within the vein cluster, which led to the formation of various kinds of zoning in mineralization and to the formation of ore shoots. This phenomenon has been described in detail in the literature [1, 7]. Let us note one general regularity in the spatial distribution of mineralization, namely: a decrease with depth in the productivity of veins. This regularity is established for many deposits deeply opened by mine workings and fully contoured, formed in different geological settings, and therefore it cannot be due only to local causes. We have seen that with depth in vein systems there is a regular decrease in the number of ore bodies, the total volume of vein material, and also in reserves and average gold contents in the ores. Along with this there is a decrease in the role of productive mineral associations and a reduction in the number of stages of mineralization. The upper parts, most expanded within the volume of the zone of a vein cluster, prove to be maximally productive. The same phenomenon is also characteristic of individual large and well-explored bodies of a vein cluster. As can be seen from the attached graphs, changes with depth in the horizontal length of veins and in average gold contents are directly interrelated (Fig. 1).

These and other features of the process of vein formation, in the authors’ opinion, reflect not so much pulsation of a deep-seated source as the phase separation of a multicomponent hydrothermal fluid in the course of its movement from the root parts of vein clusters to the frontal parts. The sequence of stages of vein formation is determined ultimately by the order of crystallization of the separated phases. In turn, the sequence of crystallization of hydrothermal phases is regulated by their physicochemical properties; at first, predominantly siliceous gels solidify, forming the main part of the vein, then complex multicomponent alkaline-sulfide-siliceous (relatively mobile) phases crystallize, and, finally, carbonic-acid aqueous residual solutions. The advance of the later phases of the solution onto the silica that was hardening and the crystallization of these phases occur under different tectonic conditions and lead to the formation of widespread types of textures of vein ores. At the same time, the late phases (stages) of the solution actively affected the earlier formed products of veins and the wall rocks, causing in them recrystallization, dissolution, and metasomatic alteration.

From the positions under consideration, many aspects of vein formation that are unsatisfactorily illuminated by the theory of deep pulsation of ore-bearing solutions find an explanation. Thus, in particular, questions remain unresolved by this theory as to why the influx of solutions from a deep-seated source during vein formation proper always begins with portions of relatively immobile solutions and ends with portions of mobile solutions enriched in volatiles and mineralizers (with deep pulsation one would expect, first of all, the departure from the source of the most volatile compounds); why the products of the later gold-bearing stages occupy the upper, most expanded parts within the zone of vein clusters, while with depth they diminish and completely wedge out. Evidently, the constant confinement of productive mineral associations to the upper parts of veins and vein clusters and the dying out of gold content with depth occur because,

Fig. 1. Graphs illustrating the character of change with depth in gold content (1), horizontal length (2), and thickness (3) of several deeply exposed veins (in conventional units): I—Yanvarskaya vein (Kommunar), II—vein No. 5 and III—Korennaya vein (Dzhetygara), IV—Troitskaya vein (Stepnyak), V—Ayaganovskaya I vein (Akzhal).

that the readily mobile productive phases, separated out in the process of the upward movement of the hydrothermal column, remained longer in a liquid state (whereas the substantially siliceous phases solidified), and this promoted their further upward migration. The spatial coincidence of the portions of vein systems that are expanded in volume with the zone of maximum productivity of mineralization, and the direct relation between the horizontal length of the vein and the average gold content in the ores, indicate an internal dependence between the differentiation of the fluid and the morphology of its column.

Thus, it is not deep pulsation of ore-bearing solutions, but phase separation and kinetic differentiation during the injection of a complex vein-forming solution that are imprinted in the uneven distribution, within veins, of products of different-time stages of mineralization. Therefore, it may be thought that the textures of vein quartz reflect a complex precrystallization differentiation of solutions and a definite sequence of phase crystallization. For example, the banded and rhythmically banded textural patterns widely distributed in gold-bearing quartz veins may characterize the separation of the solution into immiscible liquid phases (liquation) and the nearly simultaneous crystallization of the separated phases. At the same time, the concentrically zonal textures of veins, caused by repeated alternation of concentric zones of different composition around a single center (a fragment of wall rock), will reflect the separation of the solution at the site of its occurrence, without substantial migration in space. Conversely, banded textures should indicate liquation separation of a dense solution during its considerable movement (injection) along fractures. Veinlet and breccia textures (cementation of fragments of oreless quartz by later productive quartz) indicate a certain interruption in the crystallization of the separated phases and the thrusting over the early (earlier-solidifying quartz) of siliceous phases more complex in composition, enriched in sulfides and gold, more mobile, and crystallizing later. Some drusy cavities in a quartz vein may arise as a result of the sealing of gas-liquid phases by rapidly solidifying viscous siliceous gel. Dissemination of minerals of late stages in oreless quartz, evidently, is by no means always metasomatic in nature. Probably more often than is usually believed, such dissemination has a droplet-emulsion origin.

REFERENCES

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4. Bulyannikov V. A. Morphogenetic features of the Olkhovskoe ore field. Proceedings of the Tomsk Polytechnic Institute, vol. 134, 1968.

5. Ivankin L. F., Rabinovich K. R., Akchurina V. N. Morphogenesis of a quartz vein. In: Geology and Mineral Resources of Transbaikalia, Chita, 1967.

6. Petrovskaya N. V. Character of gold-bearing mineral associations in the formation of gold ores of the USSR. In: Genetic Problems of Ores. Moscow, Gosgeoltekhizdat, 1960.

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8. Shneiderkhen G. Ore Deposits. Moscow, IL, 1958.

9. Shcherbakov Yu. G. On certain features of gold-ore deposits of Transylvania. Geologiya i geofizika, 1960, no. 6.