On the Development of Plumose Structures

A. I. Aleksandrov

In recent years, plumose fractures have often been described or mentioned in the geological literature; the explanation of their origin was developed in the works of Academician A. A. Polkanov, who had been studying questions of the occurrence and significance of this type of fracture since 1920. A. A. Polkanov described a considerable number of examples of the development of plumose fractures, mainly from regions of the Kola Peninsula [4; 5; 6]. Plumose (diagonal, flexural) fractures, in the opinion of A. A. Polkanov and other investigators, arise under the action of two oppositely directed forces and characterize the initial stages of zones of mylonitization. Experimental work on the origin of plumose fractures, carried out by a number of investigators, has shown that they are formed during opposing movements of blocks.

Despite the widespread development of plumose fractures in nature, up to the present time their study has not received due attention. Meanwhile, the detection of these fractures makes it possible to determine easily the direction of stresses and displacements. Our investigations within the Taldy granite pluton in Gorny Altai showed that plumose fractures are widely developed in the shire zone, and that their study is of great importance. In some cases the confinement of mineralization to plumose fractures is well established. For example, in the area of the Ulysh key, in the system of the Karakol River, among hornfels we were able to observe a quartz–hornblende veinlet confined to a fracture of this type. Figure 1 gives a diagram of the structure of this veinlet. The veinlet, 0.8 cm thick, cuts the hornfels almost perpendicular to their relict banding. Before the formation of the veinlet, mineralization had occurred in the hornfels parallel to the banding; in the specimens, a plumose structure of rapidly wedging-out veinlets is clearly visible. Microscopic study showed that, in the first stage of the origin of the plumose fractures, the hornfels were silicified, epidotized, and much sericite and a small amount of tourmaline, actinolitic hornblende, and sulfides developed in them. The continuing stress led to rupture in the hornfels, and in the resulting fracture a veinlet formed, having sharp contacts with the enclosing rock. First of all, quartz and actinolite formed in the veinlet; in places the actinolite is confined to the boundaries of quartz grains. It should be noted that during this period part of the quartz, hornblende, and epidote penetrated into the wall rocks, chiefly along the mineralized bands. Simultaneously with the formation of the veinlet, sulfides were deposited in it. Thus, here, on a small scale, we clearly have two stages in the formation of fractures: 1) the formation of plumose fractures and their mineralization, and 2) the occurrence of a rupture and the formation of a quartz–amphibole veinlet.

A classic example of the development of plumose structures is the structure of the ore field of the Mulchikha deposit. During prospecting work in 1943, A. M. Prusevich and O. G. Konovalova noticed-

that the ore bodies have a complex plumose structure. Further study established that the ore field of the deposit has an original

Fig. 1. Plumose structure in hornfels with a quartz–hornblende veinlet in the center. Area of the Karakol deposit. Sample A-132.

1—quartz–hornblende veinlet; 2—silicified and sericitized areas; 3—hornfels.

plumose structure, partially described by us [1]. Prospecting work has revealed several zones of plumose structure, located at various

Fig. 2. Structure of the ore field of the Mulchikha deposit (schematic).

1—granite-porphyries; 2—quartz veins; 3—joints of parting, sharply expressed; 4—joints of parting, weakly expressed.

at distances from one another. The total width of the ore field in the investigated part of the deposit exceeds 0.3 km (Fig. 2). The structure of individual zones is shown in Figs. 3 and 4; from these it is evident that the short quartz veinlets

Fig. 4. Structure of individual parts of zone No. 1, Mulchikhinskoye deposit.
1—quartz veins, 2—granite porphyries.

of small thickness (from fractions of a cm to 20 cm, rarely more) are arranged at an angle to the strike of the zone itself. In individual areas where a rupture has occurred, the quartz veinlets merge and form a thick vein, usually confined to the center of the zone and having a strike parallel to the ore zone. From this vein (Fig. 5), small quartz veinlets extend on both sides at an angle, emphasizing the primary plumose structure. This pattern exactly recalls the veinlet described by us above (Fig. 1).

Fig. 3. Structure of zone No. 4, Mulchikhinskoye deposit.
1—quartz veins, 2—granite porphyries.

The ore field of the Mulchikhinskoye deposit lies among granite porphyries, passing into aplite-like granites and equigranular granites, and represents the apical (marginal) part of the Talytsky granite pluton.

Fig. 5. Structure of zone No. 1.
1—granite porphyries, 2—quartz veins.

In the granite-porphyries within the limits of the ore field of the deposit, jointing cracks are developed, shown in Fig. 3. The most sharply expressed cracks have the following attitudes: a) strike NW 280–290°, with a dip to the SW at an angle of 65–75°; b) strike NW 320–355°, with a dip to the SW at an angle of 60–75°; c) (horizontal) strike NE 10–25°, with a dip to the SE at an angle of 10–25°. In addition, there are indistinctly expressed jointing cracks with a strike of NE 50–60° and a dip to the SE at an angle of about 85°, and a weakly developed system of cracks with a strike close to latitudinal and an almost vertical dip. The latter system of cracks is well developed in the even-grained granites situated south of the deposit.

Mineralization is confined to individual zones of plumose structure (Fig. 2), the width of which ranges from 1.2 m to 8 and even 10 m. The mineralized zones have a strike of NW 325–350°, more often 330°, and partly coincide with the system of NW-striking jointing cracks, but differ from it in dip angle. Whereas the jointing dips to the SW at an angle of 60–75°, the ore zones have a NE dip at an angle from 80° to vertical. The strike and dip of the quartz veinlets within the zones coincide in general outline with the system of cracks of NW 280–290° direction. But the veinlets often have the form of an integral sign, and, consequently, in detail their attitudes differ from those of the jointing cracks.

The pattern formed by the quartz veinlets, both in plan and in vertical section, has a plumose structure. The veinlets overlap one another en echelon, and at the same time an en-echelon structure is also observed in individual zones. The en-echelon structure of the zones and the deviation of individual segments to the right relative to one another is explained as follows. It has been established experimentally that when blocks move relatively toward one another, cracks arise in the rocks, situated at an acute angle (12–17°) to the direction of movement. These cracks in the zone of disturbance are displaced to the right. In our case, with small movements of the eastern block to the NW, cracks formed at an acute angle to the movement, and these caused the en-echelon structure of the ore-bearing plumose zones. Along with this, from analysis of the deformation ellipsoid it is evident that relative movements could also have taken place in other directions; for example, the NW parts of the plumose zones, under the action of forces, could have shifted to the SW, as shown by arrows on the diagram in Fig. 3. These movements caused the deviation of individual segments of the zones to the left, which is also observed here and there in the field of the Mul’chikhin deposit.

The strike of the plumose zones generally coincides with the direction of schistosity, relict banding, and tectonic disturbances passing through the hornfelses and schists of the green-violet formation. A disturbance zone of NW 340° strike passes through the hornfelses to the east, and a second crumpling zone was recorded by us in the granites 10 km west of the Mul’chikhin deposit. Analysis of the structure of the ore field shows that the forces were directed from SW to NW along an azimuth of 280–300°. With such an arrangement of the forces, as is seen in Fig. 2, the shearing stresses will be oriented along azimuths NW 330–350° and NE 50–60°. The NW 280–300° rupture cracks show that the predominant movement was in a NW direction. The deformation ellipsoid was apparently arranged in such a way that its long axis was oriented almost vertically, with a slight inclination to the north. With such an arrangement, the relative movement of the eastern block will be directed somewhat downward, perpendicular to the short rupture cracks, while the western block will move correspondingly somewhat upward. The presence of a considerable number of zones shows that these mov—

The displacements of the blocks were differential. The vertical orientation of the long axis of the ellipsoid is proved by the fact that the angle between the system of jointing and the zones of mineralization generally does not exceed 30°. We note that movement along these zones was exceptionally small (within tens of centimeters, rarely meters). The zones of plumose structure have not been traced to their pinching out, while individual observations give grounds to say that along strike they are at least 2.5 km long.

In concluding the description of the structure of the ore field of the Mulchikhinskoye deposit, we must emphasize that establishing the plumose structure of the zones made it possible to revise the assessment of the deposit and to change the direction of exploration work, which yielded positive results.

The study of the form of the Talitsa massif leads us to suppose that the magma was intruded along a fault zone of complex plumose structure. On the geological sketch map (Fig. 6) several belts of hornfels xenoliths are visible, arranged along an azimuth of NE 10–30°. In places there are deep “bays” of shales, having an orientation analogous to that of the xenoliths.

Fig. 6. Structure of the Talitsa pluton in Northwestern Altai (schematic):

1—porphyritic granites; 2—aplite-like granites; 3—granodiorites and tonalites; 4—gabbro-diorites; 5—green-violet formation; 6—tectonic zones.

Such an arrangement of the shales and hornfels xenoliths, as well as the sharp turns of the eastern contact of the pluton, can be explained by the confinement of the granites to a tectonic zone of complex structure. This question is treated in greater detail in another of our works.

The plumose structure of a pluton will apparently be clearly visible only in the upper zones of the massif, where complete assimilation of the enclosing rocks has not taken place; therefore, the deeper the erosional cut, the less noticeable the original plumose structure of the zone will be.

The association of the Talitsa pluton with a tectonic zone is demonstrated by the schistosity of the granodiorites before the formation of the Talitsa intrusion, by the occurrence of the pluton in rocks of the green-violet formation of the Lower Silurian, which had been metamorphosed and had acquired their present appearance even before the deposition of the Anui Formation of the Upper Silurian [8]. Finally, the association of Upper Emsian intrusions with tectonic zones (faults) in Western Altai and Kalba was noted by V. P. Nekhoroshev [3]. Similar conclusions were drawn by Yu. A. Kuznetsov from observations of intrusive rocks in northwestern Altai. Yu. A. Speit reached the same conclusion in studying the Alei pluton, composed of rocks of the Zmeinogorsk intrusive complex.

Zones of plumose structure are known in other regions as well; for example, the ore bodies of the Yuzhno-Mokhatukhinskoe, Chernovinskoe, and other deposits (Kazakhstan) are plumose structures. Especially demonstrative is the ore field of the Yuzhno-Mokhatukhinskoe deposit, in which the ore veins occur in a belt of greisenized granites. The greisen belt, about 25 m thick, is elongated along an azimuth of NW 75°, while the quartz ore-bearing veins occur along azimuths from 10 to 50°; that is, with the strike of the belt of greisenized granites they form an angle of 25 to 65°. Such an arrangement of the veins can be explained by the plumose structure of the zone.

It is possible that zones of plumose structure are developed at some deposits of the Sinyushinsky granite massif, located within Gorny Altai. Unfortunately, the study of the ore fields in most of these deposits has been entirely insufficient. Plumose structures were noted by A. A. Polkanov [6] for the disthene deposit of Terbeostrov in Northern Karelia, and plumose fractures of a vein on Telyachy Island, in the area of Shueretskaya station. G. L. Pospelov [7] noted the development of plumose structures in the area of the Dashkesan and Ulen deposits.

Alongside relatively small zones, in our opinion, there also exist large-scale plumose fractures, to which bodies of igneous rocks are in some cases confined. Above we noted the development of a tectonic zone of complex structure, to which the Talitsa granite massif is confined. From this point of view, the granodiorite massifs located within Katun are of interest. Yu. A. Kuznetsov [2], who studied these massifs, writes: “It is curious to note that many granodiorite bodies occupy the place of brachyanticlines, and the orientation of the long axis is always subordinate to the latitudinal structures of the continent, whereas the area of distribution of the plutons in the general plan appears as a broad meridional belt, approximately parallel to the powerful disjunctives that frame the Devonian grabens.” Such an arrangement of intrusive bodies between fault zones suggests to us rather a plumose structure.

Some conclusions

Plumose structures are fairly widely developed in nature. In geological investigations and in the study of structures of ore fields of deposits, insufficient attention has been paid to them, especially in regional investigations in areas with well-pronounced horsts and grabens. Establishing a plumose structure may help in solving many problems of geology, in particular in understanding the genesis of structures.

ore fields of deposits and the directions of movement of individual blocks of the Earth’s crust.

References

1. Aleksandrov A. I. — The Talitsa granite pluton, Sov. Geol., No. 27, 1947.

2. Kuznetsov Yu. A. — Geological structure of the central part of Gorny Altai, Materials on the Geology of the West Siberian Territory, No. 41, 1937.

3. Nekhoroshev V. P. — New data on the geology of Altai, Bolshoi Altai, vol. I, 1934.

4. Polkanov A. A. — A geological-petrographic outline of the northeastern part of the Kola Peninsula, part I, Izv. AN SSSR, 1935.

5. Polkanov A. A. — Geological investigations in regions of magmatic and metamorphic rocks, GONTI, 1934.

6. Polkanov A. A. — Plumose cracks and questions of mineral paragenesis and petrology, Problems of Mineralogy, Geochemistry, and Petrography, AN SSSR, 1946.

7. Pospelov G. L. — Materials on the tectonics of intrusions, Izv. TPI, vol. 62, 61, 1944.

8. Usov M. A. — The source of metasomatic changes in the Lower Silurian sequence of the West Siberian Territory, Izv. AN SSSR, No. 2–3, 1936.