Redeposition of Gold and Quartz During the Processes of Formation of Gold-Ore Deposits

N. V. PETROVSKAYA (TsNIGRI)

The problem of the linkage of hydrothermal ore formation with metamorphogenic phenomena has long attracted the attention of geologists. It has been considered in various aspects, but chiefly in connection with questions of rock metamorphism and its role in the localization of ore mineralization, of wall-rock alterations—primarily pre-ore alterations—and of post-ore metamorphism. Syn-ore metamorphogenic phenomena affecting the characteristics of the ore bodies themselves have, in effect, fallen outside the field of view of researchers. Mentions of the possibility of their development occur in individual works [10, 8], but their features are not analyzed.

The materials of our observations at various gold-ore deposits of the Soviet Union make it possible to assert that “intra-ore” transformations of mineral aggregates developed widely, under different geological conditions, acquiring in the process traits of definite distinctions. They arose as a consequence of the coexistence of nonequilibrium systems: solutions—mineral aggregates, especially at the beginning of each new stage of ore formation [6]. Their expression is provided by signs of deformation, recrystallization, dissolution and redeposition of minerals, and their metasomatic replacement. Without setting ourselves the task of characterizing all these heterogeneous phenomena, we shall dwell only on that part of them which relates to intermineralization transformations of aggregates of vein quartz and native gold.

Ore-bearing quartz in all gold-ore deposits bears traces of more or less strong, often repeated, changes in the structures of aggregates. Among them it is not always easy to recognize transformations that arose before the completion of the ore process. The most clearly manifested are the signs of intra-ore dynamic effects, since the zones of fracturing that arise in vein quartz are associated with manifestations of later sulfide mineralization. This feature was noted long ago and has been described by many researchers.

Using examples from ore fields of the Yenisei Ridge, Western Uzbekistan, and some others, it has been possible to show that the distribution of later sulfide and gold mineralization is also influenced by unevenly manifested plastic deformations of quartz, which testifies to their intermineralization development [5, 1]. Similar cor-

are characteristic of deposits of formations of relatively great depths. In deposits formed at shallow depths, only intermineralization fracturing of quartz aggregates, disintegration of grains, and local crushing are manifested.

The development of deformations, especially fracturing, was everywhere accompanied by redeposition of quartz. Where these phenomena were on a small scale, only the healing of fine cracks in quartz aggregates occurred. The studies of G. G. Lemmlein [2] showed that such healing took place by dendritic crystallization of quartz, which led to the encapsulation in it of abundant gas–liquid inclusions. Often the healed cracks can be seen in thin sections only by the chains of such inclusions, since the orientation of the grains of newly formed quartz is completely identical with the orientation of the enclosing quartz grain. A characteristic feature of intermineralization healed cracks, distinguishing them from similar post-ore fracturing, is the variability of the thicknesses and structure of the newly formed veinlets. Transitions are observed from capillary cracks, recorded, as indicated above, by chains of secondary gas–liquid inclusions, to veinlets up to 0.1–1.0 mm thick, in which the quartz preserves the orientation of the “host” crystal but is sharply depleted in gas–liquid inclusions. These formations, usually called “pseudoveinlets,” as the width of the crack increases acquire an indistinct comb structure (Fig. 1) and then pass into typical comb veinlets, also visible macroscopically.

The connection of newly formed quartz with phenomena of dissolution and redeposition of material from earlier quartz aggregates is demonstrated by the following data. Not only pseudoveinlets but also veinlets of comb quartz are developed within portions of deformed bodies of early quartz and disappear away from them. In a number of deposits of Transbaikalia, the Yenisei Ridge, and the Urals, it has been noted that in such portions cracks cutting the early quartz bear traces of corrosion of the walls, up to the formation of slit-like cavities. Various stages can be traced of subsequent regeneration of chips of early quartz grains and of the formation on them of drusy crusts of redeposited quartz (Fig. 1, b). Similar crusts form in irregular cavities, in place of leached fragments of silicate rocks cemented by aggregates of early quartz and located along the paths of circulation of late solutions. Evidently, the activity of such solutions was sufficiently intense.

The data cited do not exclude the possibility that part of the comb quartz formed at the expense of a new supply of silica by solutions, including silica leached from more deeply situated quartz bodies and quartz-rich rocks. It is probable that the phenomena under consideration were conjugate, and the role of each of them could vary under different conditions.

The following facts speak for the intra-ore development of quartz redeposition: a) sulfides belonging to late mineral parageneses are not infrequently associated with veinlets of redeposited comb quartz; b) elevated concentrations not only of sulfides but also of gold are connected with areas of their distribution; veinlets of comb quartz in many deposits are one of the indicators of increased gold content of ore-bearing sections; there is reason to attribute to similar indicators the intensive development of pseudoveinlets in fractured early quartz as well; c) in individual cases textures are observed of repeated intersections of some veinlets of redeposited quartz by others, and also by veinlets of carbonate and sulfide–carbonate composition. Such textures are usually regarded as an indicator of multi-

Fig. 1. Redeposited quartz in gold-quartz veins of the Yenisei Ridge:
a) indistinctly comb-like veinlets in strongly deformed early quartz: transmitted light, crossed nicols, magnification 6;
b) drusy crusts on the walls of leaching cavities in a quartz vein. Photographic specimens, magnification 6.

...of the stages of ore deposition, but, taking into account the data cited above, they may rather be regarded as indications of repeated redeposition of the mineral matter.

In deposits formed under different conditions, the redeposition of quartz is manifested unequally. In ore fields belonging to the formation of great depths (districts of the Yenisei Ridge, etc.), there are mainly pseudoveinlets and rarer thin, indistinctly comb-like veinlets, only locally acquiring a clear comb-like structure. In deposits formed under conditions of intermediate depths (Klyuchevskoye and Darasunskoye in Transbaikalia, Lebedinoye on the Aldan, etc.), the role of comb-like quartz segregations increases sharply, the thickness of its veinlets grows, and drusy cavities acquire characteristic significance. The products of redeposition and deposition from new

...portions of solutions are difficult to distinguish here. In “epithermal” deposits, typical small comb-like veinlets of redeposition are widely developed; they cut across banded metacolloidal zones of early quartz and disappear beyond them. Along with this, patch-like and original rounded segregations of fine-grained and indistinctly comb-like quartz are found, with which redeposited gold is locally associated. The formation of such segregations can be explained on the basis of concepts concerning the special features of local areas of “soaking” in capillary-porous media; special studies have shown that such areas tend to acquire a spherical form and that intensive cracking, dispersion, and redeposition of mineral matter develop in them [9].

The features observed in deposits of different types, indicating that periods of maximum quartz redeposition were confined to the beginning of major stages of ore formation, served as the basis for the conclusion regarding the cyclic development of the multistage ore process [6].

Of particular interest is the question of the endogenous redeposition of gold and its possible influence on the localization of rich ores. It is important to clarify the relation to this phenomenon of gold segregations of different ages.

In the opinion of most researchers, including the author [4], two principal generations of gold are usually present in gold-ore deposits: the first forms finely dispersed disseminations in sulfides of the early association, while the second—in the form of coarser segregations—accompanies the late sulfide association. There is reason to believe that both generations of gold underwent dissolution and redeposition. The indications of the development of regrouping processes in gold segregations of different ages are different.

Direct studies of the morphology and distribution of early finely dispersed gold, possible only with the aid of an electron microscope, are at an initial stage; but even the limited materials show that, alongside uniformly scattered ultrafine particles of gold, their groups are observed, tending toward microcracks that cut the mineral grains. The nonuniform distribution of fine gold in sulfides, with elevated contents in fractured areas, is also revealed by analysis of monomineral samples. In interpreting such relationships, one cannot fail to take into account that not all dispersed gold belongs to the early generation, and that late segregations, under certain conditions, may be represented by submicroscopic, unevenly distributed particles. The following data make it possible to assume intermineralization regrouping of finely dispersed gold. The ability of gold to diffuse in solid crystalline bodies and in solutions filling capillary cracks has been proved experimentally [11, 12]. The relative ease of dissolution of ultrafine gold particles under the action of alkaline solutions is also well known. In light of these data, the development of processes of gold redeposition under natural conditions, at changing temperatures and pressures and with a variable acid–alkali regime, appears more than probable. Convincing evidence of gold redeposition was obtained in the study of gold ores of the Balei district of Transbaikalia. Here, veinlets of redeposited comb quartz locally intersect accumulations of early finely dispersed gold, becoming enriched in coarser gold; the latter disappears with distance from the intersection areas (Fig. 2). Apparently, the enlargement of gold particles, which was assumed ...

as a consequence of post-ore metamorphism [3], characterizes also inter-mineralization transformations of the ore substance.

Indirect evidence for the assumption of a substantial role of the phenomena considered is provided by the dependence of the development of late gold segregations on the position of sites of concentration of early auriferous sulfides. As noted earlier [7], late mineral parageneses are rich in gold only where they are spatially associated with segregations of sulfides of the early association. Such relationships have been noted in deposits of the Yenisei Ridge, the Klyuchevskoe ore field in Transbaikalia, and elsewhere. Calculations show that simple regrouping of early, finely dispersed gold in sulfides could have created sharply elevated gold concentrations in many local areas. The latter usually have considerably smaller dimensions than the zones of development of the early auriferous sulfide association. In other cases the values of the contents prove to be disproportionate, and there are grounds for assuming a late introduction of gold by solutions. Under these conditions the redeposition of gold, like that of quartz, was probably accompanied by the activity of new portions of solutions in areas where such solutions were undersaturated with gold.

If the regrouping of finely dispersed early gold can be judged mainly from indirect indications, the dissolution and redeposition of later, coarser gold are proved by data from direct observations. On the surface of gold grains (from deposits of Central Aldan, the Southern Urals, Transbaikalia, and other regions) we found pits of natural etching, distributed very unevenly. They are especially distinct on the surface oriented parallel to the (III) crystal grains of gold, where characteristic trigonal depressions can be seen. With more intense dissolution they become polygonal and lose their sharp outlines, taking on the appearance of isometric pits. Similar relationships were observed in the etching of gold under laboratory conditions. From the hypergene corrosion of gold, which usually leads to the development of generally matte surfaces of gold grains, the pits of intra-ore etching differ in their greater regularity of form, uneven distribution, in places a stepped, regenerated surface of the walls (noticeable at high magnifications), and the absence of association with segregations of hypergene minerals.

Fig. 2. Redeposited gold in cracks (1) and in veinlets of comb quartz (2), cutting a narrow zone of accumulations of dust-like early gold (3), situated in banded chalcedony-like quartz (4). Also visible are irregularly shaped areas of recrystallization of quartz with enlarged inclusions of gold (5). Balei ore field, deep horizons. Transparent section, without analyzer, magn. 15.

Features of dissolution and redeposition of gold are observed in the same or nearby portions of ore bodies. Newly formed gold is comparatively easily distinguishable in those cases where it is represented by overgrowths on corroded gold grains; they have the appearance of

irregular warty or moss-like neogenic formations, or well-defined crystallites, dendrites, and wires. Their dimensions are disproportionate to the size of the gold grain being overgrown and vary within wide limits, from the first microns to the first millimeters. A characteristic tendency is observed for the newly formed gold crystallites to assume a regular orientation, depending on the crystallographic orientation of the surface of the gold grains on which they grow, i.e., obeying the laws of autoepitaxy. On the surface of gold grains retaining signs of dendritic structure, the finest overgrowths (fractions of a micron) likewise of dendritic structure have been detected by electron-microscopy methods (Fig. 3). Hypergene gold does not form such overgrowths.

The redeposition of late, comparatively coarse gold, judging from the available data, developed locally, and its scale was considerably inferior to the scale of regrouping of finely dispersed early gold; possibly it affected the configuration of gold-enriched areas and influenced their internal structure, increasing the irregularity of gold distribution. If the supposition is correct according to which the coarse segregations of gold of the second productive stage were products of redeposition of earlier, finely dispersed gold, then it must be acknowledged that the redeposition of gold developed repeatedly, recurring at different stages of the ore process.

Fig. 3. Dendrite-like neogenic formations of redeposited gold (1) on the surface of face (III) of a tabular gold crystal (2). Balei ore field, Transbaikalia. Photograph taken by K. E. Frolova in the electron-microscopy laboratory of IGEM, Academy of Sciences of the USSR. Cellulose-carbon replica, magnification 4000

Dissolved gold was then deposited, primarily, on the surfaces of preserved gold grains situated along the paths of movement of the solutions. With prolonged development of such processes they could lead to the formation of the richest ore shoots and gold nuggets within them. The stated hypothesis is consistent with the facts of the presence in nuggets of inclusions of semitransparent comb-like or crested quartz possessing the characteristic features of redeposited formations, as well as with the peculiarities of the internal structure of nuggets, with traces of relict inclusions of individual gold grains. Thus

This explanation removes many contradictions that arise in attempts to relate the genesis of gold nuggets to hypergene processes.

New formations of gold in deposits of genetically different types are not identical in form: in deposits of formations of great and intermediate depths they are represented by simple crystals and their intergrowths (octahedra and elongated rhombic dodecahedra predominate), whereas in “epithermal” ores they occur as the finest dendrites—moss-like and flat—and also as complex, often flattened crystals according to (III).

We have limited our task to the consideration and comparative analysis of specific features that make it possible to recognize the products of intermineralization transformations of minerals, since it is precisely in this area that a definite deficiency of knowledge is felt, under which clear evidence is sometimes replaced by general reasoning about the probability of development of presumed phenomena. Studies of the mechanism of the processes of dissolution and redeposition of gold and quartz, as well as their physicochemical characterization, remain tasks for future work. Further accumulation of materials will make it possible to characterize more fully the significance of the phenomena considered and to approach determination of the scale of their development under different geological conditions.

REFERENCES

1. Badalova R. P. On quartz and its metamorphism in gold-bearing veins of Western Uzbekistan. In: Problems of Mineralogy and Geochemistry. Nauka Publishing House, Tashkent, 1964.

2. Lemmlein G. G. The process of healing of cracks in a crystal and the transformation of the forms of cavities of secondary liquid inclusions. Doklady AN SSSR, vol. 78, no. 4, 1951.

3. Kreiter V. M. Sizes of gold particles as an indicator of post-ore metamorphism. Izvestiya AN SSSR, Geological Series, no. 1, 1948.

4. Petrovskaya N. V., Timofeevskii D. A., Dembo T. M., Bernshtein P. S., Borishanskaya S. S., Babintsev N. I. On the question of the time of gold deposition in ore deposits. Transactions of NIGRIzoloto, issue 16, 1947.

5. Petrovskaya N. V. Some features of intra-ore metamorphism of gold-quartz formations, with the deposits of the Yenisei Ridge as an example. Transactions of NIGRIzoloto, issue 21, 1956.

6. Petrovskaya N. V. The character of gold-bearing mineral associations and formations of gold ores of the USSR. Reports of Soviet geologists at the XXI Session of the International Geological Congress. GONTI, Moscow, 1960.

7. Petrovskaya N. V. Mineral associations in gold-ore deposits of the Soviet Union. Transactions of the Central Mining-Exploration Institute, issue 76, 1967.

8. Popkov N. N. On generations of gold in hydrothermal deposits of the Northwestern Tien Shan. Proceedings of the Kirghiz Department of the All-Union Mineralogical Society, no. 5, 1965.

9. Postel’nikov L. L. On the nature of granitic bodies and the role of hydrothermal leaching during their formation. In: Physicochemical Problems of the Formation of Rocks and Ores. Moscow, Publishing House of the Academy of Sciences of the USSR, 1963.

10. Unksov V. A., Ivanova T. N. On the mineralogy of the Balei gold-ore deposit. Soviet Geology, 1945, no. 7.

11. Clark L. A. The Fe-As-S system: Phase relations and application, pt I–II. Economic Geology, vol. 55, no. 7–8, 1960.

12. Okkerse B. Self-Diffussion of gold. Physical Review, vol. 103, no. 5, 1956.