APPLICATION OF MICROSTRUCTURAL ANALYSIS AND STUDIES OF THE PHYSICOMECHANICAL PROPERTIES OF ROCKS IN THE STUDY OF GOLD ORE DEPOSITS

N. A. ROZANOVA (TsNIGRI)

Microstructural analysis and the study of the physicomechanical properties of rocks can be carried out only as part of an overall complex of geological-structural investigations. Both methods constitute part of investigations aimed at clarifying the conditions of localization of mineralization, and they also help to solve complex questions concerning the structure of deposits and ore fields unambiguously.

Both types of investigation, to a greater extent than any others, depend on the thoroughness and correctness of sample selection, on knowledge of the specific geological setting of the sampling site, and on a previously considered direction of research. Both methods are based on the use of knowledge of the fundamentals of other sciences, chiefly various branches of physics.

I. Microstructural analysis

In studying the structure of a deposit, microstructural analysis rendered us substantial assistance in deciphering movements along ore-hosting fractures and faults, in determining the relative age of two intersecting geological bodies, and in studying elements of the prototectonics of intrusive massifs.

1. Deciphering the nature of an ore-hosting fracture and the movements along it. In studying the structure of the Central gold-ore deposit, which occurs in granodiorites, convincing evidence was needed for the joint nature of the ore-hosting fractures. Using oriented samples of granodiorites from the contact of the ore-hosting fracture and from locations away from it, an analysis was made of the orientation of the optical axes of quartz, which showed a sharp difference. A diagram compiled from measurements of quartz from granodiorites taken away from the ore veins shows the absence of any orientation in the arrangement of the optical axes of quartz. In the diagram compiled for granodiorites from the contact of the ore-hosting fracture, the position of maxima of the optical axes of quartz characterizes the rocks as an S-tectonite. It follows from this that the ore-hosting fracture is a shear fracture. In addition, the orientation of the optical axes of quartz in the granodiorites from the contact of the ore-hosting fracture (azimuth of dip SE 165°∠48°) is close to the orientation of the slickenside grooves (azimuth of dip SE 160°∠50°) on the plane of the ore-hosting fracture.

The data of microstructural analysis were also used in resolving the question of the direction of displacements during the formation of ore veins. On the orientation diagram of the optical axes of quartz in granodiorites from the contacts of ore veins, the planes of ore-bearing fractures and dikes of diabase porphyrites were plotted (in two principal positions each). In Fig. 1 it is seen that maximum I of the orientation of the optical axes of quartz is located near the planes reflecting the attitude of the dikes, and coincides with the line of conjugation of the ore vein and the dike of diabase porphyrites. Such an arrangement of the optical axes of quartz indicates movement of the rock blocks enclosing the ore veins parallel to the line of conjugation of the diabase-porphyrite dike and the ore vein. Thus, the dikes of diabase porphyrites existed at the moment when the ore veins were formed, i.e., the dikes are pre-ore.

Fig. 1. Orientation diagram of the optical axes of quartz in granodiorites taken at the contact of an ore vein. Measurements 211; \<0.5—1—2—3—4—5\<%. Orientation of the optical axes of quartz: maximum I (azimuth of dip SW 165∠48°) coincides with the line of conjugation of the ore vein with dikes of spessartites, microdiorites, and diabase porphyrites; maximum II (azimuth of dip SW 65∠78°), by its position, is close to the plane of a major fracture; maximum III (azimuth of dip SE 182∠88°) coincides with a system of dikes of microdiorites, spessartites, and diabase porphyrites. 1 — ore vein; 2 — dikes of diabase porphyrites, spessartites, microdiorites; 3 — fault.

2. Study of elements of prototectonics. For the granodiorites of the Tsentralninsky massif, in which the gold mineralization occurs, as in other intrusive massifs, an oriented arrangement of dark-colored schlieren, mica plates, and line-...

...the elongation of hornblende phenocrysts. Often such an orientation [3], without any evidence, is attributed to elements of prototectonics and is considered decisive in the localization of mineralization. However, the orientation of the indicated elements is not always related to the primary fabric of the intrusions and may be caused by dynamometamorphic phenomena. For the Tsentralnoe deposit, many investigators [8] believed that the factor determining the localization of mineralization was fracturing caused by the primary orientation of the granodiorites. In structural studies at this deposit, the author obtained data on the confinement of mineralization to fractures associated with the formation of a large fault. In order to resolve correctly the questions of localization of gold mineralization, it was necessary to check the genesis of the fracturing. A comparison was made between the orientation of the optical axes of quartz in the granodiorites and the attitude of the ore veins. It should be recalled that quartz, which crystallizes in granodiorites as one of the last minerals, cannot acquire an orientation during the process of magma flow. Under the influence of dynamometamorphism, however, its grains become oriented according to their internal structure, which is expressed in a regular arrangement of their optical axes. At the same time, the possibility that a tectonic orientation is superimposed on the primary one [3] is not excluded. Thus, if the spatial arrangement of the ore-bearing fractures coincides with the orientation of the optical axes of quartz, and does not tend toward the orientation of the elements of prototectonics, then one may confidently speak of the decisive role, in the localization of mineralization, of fracturing associated with the formation of a large fault. The orientation of the optical axes of quartz in granodiorites (Fig. 1) taken from areas of development of ore veins showed, as noted earlier, a regular arrangement of the optical axes of quartz. Maximum I in the orientation of the optical axes of quartz coincides with the direction of the lines of junction of the ore vein with a dike of basic rocks and with slickenside grooves on the planes of the ore-bearing fracture. The orientation of the optical axes of quartz of maximum II is close to the position of the plane of the large fault and reflects displacements associated with movement along this fault. Maximum III of the orientation of the optical axes of quartz in the granodiorites likewise characterizes displacement associated with the constraining influence of pre-ore dikes of intermediate and basic rocks. The spatial arrangement of the ore-bearing fractures of the Tsentralnaya vein is close to the position of maximum I in the orientation of the optical axes of quartz and does not coincide with the orientation of schlieren of dark-colored minerals in the granodiorites. The data obtained confirm the confinement of the ore-bearing fractures to directions determined by manifestations of dynamometamorphism.

II. Physicomechanical Properties of Rocks

The physicomechanical properties of rocks (in the present case, effective porosity and elastic properties), as has long been demonstrated [1, 2, 4], exert a substantial influence on the formation of structural types of ore fields and deposits, and on the course of physicochemical processes taking place before and during ore formation.

A. Effective Porosity

Data from studies of the effective porosity of rocks hosting gold-ore deposits were used in determining the most favorable conditions for the localization of gold mineralization, and also as an additional criterion in determining the relative age of intersecting geological bodies and as a criterion unambiguously indicating

the presence of changes in the structure of the rock under the influence of various factors (dynamometamorphism, hydrothermal metamorphism, etc.).

1. Change in the magnitude of effective porosity, reflecting changes in the structure of the rock under the influence of hydrothermal metamorphism and dynamometamorphism, was studied by us at the Tsentralnoe and Natalievskoe gold-ore deposits.

At the Tsentralnoe deposit, the effective porosity of unaltered granodiorites ranges from 0.59 to 0.91%, and that of unaltered rocks composing dikes (diabase porphyrites, spessartites, and microdiorites) from 0.35 to 0.5%. The effective porosity of the same rocks, but hydrothermally altered, is already, respectively, up to 3.2% and up to 1.52%. The effective porosity of mylonitized rocks of the same types is still higher and reaches 3.47—6.7% (Fig. 2). Study of the Natalievskoe deposit (Fig. 3) likewise showed a difference in the magnitude of effective porosity between unaltered rocks and rocks altered by various processes.

Fig. 2. Effective porosity of rocks enclosing the Tsentralnoe deposit. 1 — diabase porphyrites, spessartites, microdiorites; 2 — granodiorites; 3 — hydrothermally altered rocks; 4 — mylonitized rocks.

The effective porosity of unaltered limestones and diorites is 0.08—0.09%; for skarnized diorites it is somewhat higher—0.12—0.26%; the effective porosity of garnet-pyroxene skarns is still higher and amounts to 0.42—0.51%. The increase in the magnitude of effective porosity during skarnization is caused by loosening of the skarnized rocks, which is determined by the very nature of the skarn-forming process. An increase in effective porosity is also caused by epidotization of rocks: the effective porosity of epidotized garnet-pyroxene skarns reaches 0.59—0.69%. At the same time, the effective porosity of silicified garnet-pyroxene skarns decreases to 0.27%. During silicification of skarns, healing and clogging of pores by newly—

Fig. 3. Effective porosity of rocks enclosing the Natal’evskoe deposit.
Vertical axis: Effective porosity, in weight percent.
1 — limestones; 2 — diorites; 3 — skarnified diorites; 4 — garnet–pyroxene skarns; 5 — tremolite–diopside skarns; 6 — epidotized rocks; 7 — silicified rocks.

formations of quartz, which naturally led to a decrease in porosity. A decrease in the value of effective porosity during silicification [5] can occur only under conditions of low-temperature solutions, when SiO₂ is deposited without the removal of other components (for example, CaCO₃) into the adjacent rocks. In high-temperature solutions, along with the deposition of silica, leaching of other components would inevitably take place and, accordingly, loosening of the structure of the altered rock and an increase in porosity. The greatest value of effective porosity was established for epidotized tremolite-diopside skarns—0.75% (in unaltered rocks it is only 0.46%).

The data presented clearly showed a direct dependence of the value of effective porosity on the character of rock alteration, which is also widely known at deposits of other useful minerals in various regions of the USSR [4, 5].

2. The use of data on effective porosity as an additional indicator for identifying the conditions most optimal for the localization of mineralization, in our case—for establishing the horizon of favorable rocks, can be carried out only in conjunction with an analysis of all the geological-structural features of the deposit and the elastic properties of the rocks enclosing the mineralization. In studying the conditions of localization of mineralization at the Natal’evskoe deposit, investigations of effective porosity along a section rich in gold and also containing barren rocks (from bottom to top) showed the following:

1) spotted garnet-pyroxene skarns, with areas of limestones, garnet, and nests of sulfides, epidotized; effective porosity—2.04%;

2) coarse-grained, epidotized limestones, with veinlets of garnet, epidote, and ore minerals; effective porosity—0.66%;

3) limestones with veinlets and nests of garnet, containing crystals of pyrite; effective porosity—0.42%;

4) medium-grained garnet-pyroxene skarns, with pyrite along fractures; effective porosity—0.59%;

5) medium-grained, silicified garnet-pyroxene skarns, with accumulations of garnet; effective porosity—0.27%;

6) skarnified diorites with garnet, medium-grained; effective porosity—0.51%;

7) unaltered, dense diorites; effective porosity—0.09%.

The highest gold content is observed in the skarnified diorites, whose effective porosity is not maximal within the section considered. A distinctive feature of the area of gold localization is a sharp change in the value of effective porosity, caused by the occurrence above the skarnified diorites of less porous rocks, unaltered diorites, i.e., by the presence of a kind of screen controlling the placement of mineralization. Such examples are also known at other gold-ore deposits [7].

3. The use of data from investigations of the effective porosity of rocks as an additional indicator in determining the relative age of intersecting geological bodies is based on observations of an increase in effective porosity in relatively older bodies and an expansion of the halo of increased porosity on approaching pre-ore bodies or structures. Thus, at the Andreevskoe deposit the effective porosity of diabases at the contact of an ore vein is 2.4 and even up to 4.5%. At the same time, the effective porosity of diabases from the same dikes, but taken at a distance of 3–5 m from the ore vein, is only 0.8–1.3%. The study of diabases with increased

...values of effective porosity showed that they are significantly hydrothermally altered, and that sericite, epidote, and carbonates play an important role in their composition. With increasing distance from the ore vein, the degree of hydrothermal alteration in the rock decreases, and at a distance of 3.5 m they acquire a normal, unaltered appearance. The material presented above clearly indicates the pre-ore age of the dikes of diabase.

The expansion of the aureole of increased porosity of the rocks enclosing the mineralization, on approaching pre-ore structures or bodies, was widely observed by us at the gold-ore deposits of the Mariinsk taiga in places where gold-ore veins approach dikes of intermediate and basic composition. In this case, the outlines of the aureoles of increased porosity strictly coincide with the hydrothermal alteration of the rocks.

In conclusion, it is necessary to note the substantial role, in localizing mineralization, of the ratio between the values of effective and total porosity of the rocks enclosing the mineralization. This ratio depends entirely on the presence in the rocks of closed and open pores and determines the permeability of the rocks. In the case where the effective porosity is equal to the total porosity, all pores are open and permeability is greatest [4]. When both open and closed pores are present, permeability is determined mainly by the magnitude of the effective porosity.

B. Elastic properties of rocks

Studies of the elastic properties (modulus of elasticity and Poisson’s ratio) of the rocks enclosing mineralization make it possible to identify types of rocks of differing brittleness, i.e., to detect the rocks most favorable for the development of fracturing and, consequently, for the localization of mineralization. The greater the value of the modulus of elasticity of rocks, the more brittle the rocks are, the more strongly fracturing is manifested in them, and the greater the possibilities for localization of mineralization.

1. At the Natal’evskoe deposit, occurring among limestones, skarns, skarned diorites, and diorites, the industrial mineralization tends predominantly toward skarned rocks and skarns. This distribution of gold, along with the geological-structural features of the deposit and the high porosity of the rocks, is also determined by their elastic properties. We did not have our own investigations of the elastic properties of the rocks; all data were borrowed from the work of the Laboratory for the Study of Physicomechanical Properties of Rocks of IGEM, Academy of Sciences of the USSR [1, 5]. The elastic properties of rocks close to the host rocks of the Natal’evskoe deposit are characterized by the following values:

As established [1, 5], high values of the modulus of elasticity with low values of Poisson’s ratio (the ratio of longitudinal and transverse deformations) for skarns characterize these rocks as brittle, more brittle than granodiorites, granodiorite-porphyries, and limestones. Under shearing stresses in an environment of omnidirectional compression, skarns will behave more brittly than any of the indicated rocks, whereas limestones will deform plastically. As indicated above, during the skarnification of rocks at the Natal’evskoe de...

in the deposit there occurred an increase in effective porosity and a loosening of their structure, which characterizes the process of skarnification in other deposits as well. At the same time, however, an increase in the modulus of elasticity and a decrease in Poisson’s ratio are observed. This relationship of the physicomechanical properties of rocks during skarnification is explained [5] by the formation of minerals with a high specific gravity (with a greater density of crystalline structure). In addition, such an increase in the elastic properties of skarns substantially offsets the decrease in the modulus of elasticity caused by loosening of the rock structure as a result of increased porosity.

It is worth dwelling briefly also on the absence of gold mineralization in the silicified garnet-pyroxene skarns of this section of the Natal’evskoe deposit. The silicified garnet-pyroxene skarns are distinguished by a compact fabric and monolithic character, and fracturing is almost undeveloped in them. This is partly explained by a certain increase in the modulus of elasticity and Poisson’s ratio during silicification of the skarns, in connection with a decrease in the magnitude of porosity (through the sealing and filling of pores in the skarns by quartz) and the development of the quartz itself.

On the basis of the foregoing, and relying on data on the elastic properties of the rocks, the preferential localization of gold mineralization at the Natal’evskoe deposit in skarns and skarnified diorites is entirely understandable.

Fig. 4. Diagram of the arrangement of ore-bearing fractures in granodiorites and in a dike composed of microdiorites.
1—granodiorites; 2—microdiorites; 3—ore vein; 4—hydrothermal alteration of rocks; 5—thin shear fractures; 6—elements of occurrence.

2. On the southern flank of the Tsentral’noe deposit, mineralization is observed to be confined to fractures in granodiorites (Fig. 4), which, on approaching the pre-ore dike of microdiorites, are cut off and traced again beyond the dike. Upon careful study of the dike, numerous thin shear fractures are observed in it along the continuation of the ore vein, oriented at an acute angle to the ore vein in the granodiorites. The fractures have a very limited extent, raz—

are confined only within the body of the dike and are filled with quartz, sulfides, and even gold. During the formation of the ore vein in the granodiorites, no rupture occurred in the continuity of the microdiorite dike. The stresses that led to the formation of the ore-hosting fracture in the granodiorites were insufficient to form the same kind of fracture in the microdiorite dike, which is determined by differences in the physicomechanical properties of these two types of rocks. The dominant role in this case was played by the magnitude of the modulus of elasticity, which in the dioritic rocks is approximately twice as high as the modulus of elasticity of the granodiorites, respectively \(8.7—13.0 \times 10^5\) and \(5.15 \times 10^5\) kg/cm\(^2\) [1].

In conclusion, it should be noted that the examples cited above of the application of microstructural analysis and the study of the physicomechanical properties of rocks enclosing mineralization, in our view, convincingly prove the effectiveness of these methods in resolving questions concerning the structural features of deposits and the localization of mineralization. It seems to us that the indicated methods of supplementary investigation should become as obligatory as the study of the mineral composition of ores, fracturing, etc.

REFERENCES

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