Wall-Rock Alterations at Some Mercury Deposits of the Southeastern Altai

V. N. Troshchenko

(Presented at the scientific seminar of the Department of Mineralogy, Petrography, and Useful Minerals)

Wall-rock alterations are those hydrothermal changes that occur in the process of mineralization as a result of reactions of ore-forming solutions with the enclosing rocks. Later, in particular productive, stages of mineralization may take place among rocks that have already been altered [2].

The most typical wall-rock alterations at mercury deposits are silicification and carbonatization [1, 13]. In addition, for a number of mercury deposits situated among basic or ultrabasic rocks, wall-rock listvenitization is noted [11].

All these alterations are also observed at the mercury deposits of the Gorny Altai. However, listvenites closely associated with mercury ores at some deposits are at present regarded not as wall-rock altered rocks, but as pre-ore formations not connected with the processes of ore formation [8]. Only dolomitization and silicification are recognized as indisputable wall-rock alterations [8].

The material presented below testifies to the close connection of listvenitization with the processes of ore formation.

Among the mercury deposits of the Gorny Altai, a group may be distinguished territorially that is located along the northern margin of the Chuya Steppe and includes the well-known Chagan-Uzun deposit. This deposit and the smaller ones situated near it have many features in common in their mineralogical composition, wall-rock alterations, and the relation of mineralization to tectonic structures.

The area in which the deposits of this group occur is situated at the junction of large structural elements of the Gorny Altai—the Altai-Sayan structural-facies zone, the Baratal horst, and the Chuya-Anui synclinorium. This accounts for the presence here, in contact within a comparatively small area, of many and varied stratigraphic complexes. Exposed here are thick limestones with siltstones of the Baratal Formation of the Sinian, porphyrites and pyroclasts of the ancient porphyrite formation of the Lower Cambrian interbedded with limestones, graywackes, conglomerates, and limestones of the Kurai Formation \((Cm_3—O_1)\), crystalline schists and gneisses of a metamorphic sequence, possibly formed as a result of metamorph—

... of sandy-clayey deposits of the Cambrian–Ordovician [6], predominantly tuffaceous deposits of the Ongudai ($D_2^1$) and Akkaya ($D_3^1$) suites, sandy-shaly strata of the Middle Kurai ($D_2^2$) and Chagan-Uzun ($D_2^2—D_3^1$) suites. Finally, coal-bearing deposits ($C_3—P_1$) and sandy-clayey Tertiary deposits with interbeds of brown coals are developed to a minor extent. The fault zones contain tectonic blocks of serpentinized peridotites.

The mercury deposits are grouped within a narrow belt that bends around, from the north, the intermontane depression of the Chuya Steppe superimposed on the Chuya-Anui synclinorium. The general layout of the deposits is controlled by large disjunctive structures—the Kurai fault zone of northwestern trend and a younger, approximately latitudinal fault zone cutting it, which borders the Chuya Steppe from the north. The ore bodies of the deposits are situated in zones of crushing and fracturing, and at the intersections of tectonic disturbances.

The ore veins of the mercury deposits of the group described occur in many lithological varieties of the enclosing rocks: in serpentinites, limestones of the Baratal and Kurai formations, in porphyrites and pyroclasts of the ancient porphyrite formation, and in tuffites of the Ongudai and Akkaya suites.

The mineralogical composition of the ores, as is generally characteristic of mercury deposits, is not complex.

The ore mineral is cinnabar. Pyrite is widely developed; less commonly marcasite and arsenopyrite occur, and in individual cases chalcopyrite, bornite, chalcocite, and galena are observed. At the Chagan-Uzun deposit, in addition, small amounts of antimonite, pale ores, orpiment, and realgar are closely associated with cinnabar. Among the vein minerals the dominant ones are carbonates and quartz; quartz predominates in veins occurring among tuffites, porphyrites, and siliceous rocks, whereas carbonates dominate in veins cutting substantially carbonate rocks. Similar regularities are also observed at many other mercury deposits [13]. In one case, barite veins are considerably developed, in which cinnabar is deposited along cracks. In the remaining deposits, barite is either absent or, as at Chagan-Uzun, only slightly developed. Chalcedony is frequently encountered.

Wall-rock alterations of the enclosing rocks are widely developed, considerably exceeding the distribution of the ore mineralization itself, and include processes of dolomitization, silicification, and listvenitization. It should be noted that, along with the wall-rock silicification, a much more widely developed earlier silicification is controlled by the same tectonic structures. At the same time, the wall-rock alterations accompanying the mercury ores are sometimes superimposed on brecciated silicified rocks.

Ore veinlets and disseminations, in the overwhelming majority of cases, are situated in deeply altered rocks.

As already mentioned, mercury ores are often located in disjunctives cutting serpentinites, which are thereby subjected to listvenitization. Unaltered serpentinites consist almost entirely of serpentine and usually have a mesh structure. There are accumulations of small grains of magnetite and chromite; rare veinlets of chrysotile asbestos and individual flakes of prochlorite occur. With approach to the zone of listvenitization, individual veinlets and accumulations of fine-grained dolomite replacing serpentine first appear; then carbonatization increases, and at the same time increasingly frequent...

still individual flakes and accumulations of fine-scaled talc. In parallel with these changes, pyrite, fuchsite, and arsenopyrite appear in ever-increasing quantities. The increase in the number of newly formed minerals leads to the fact that serpentine among them is preserved only as isolated relics, densely penetrated by thin veinlets of carbonates and talc. Finally, serpentinite is completely transformed into white, grayish-white, and sometimes greenish listvenite, consisting chiefly of breunnerite, dolomite, calcite, and quartz, with disseminated pyrite, fuchsite, arsenopyrite, and a small admixture of talc.

Hypergene decomposition of the iron-bearing breunnerite and dolomite, with the formation of limonite, causes the outcrops of listvenites to be colored reddish-brown and yellowish-brown; the intensity of these tones depends on the proportions of dolomite and breunnerite in the rock composition—varieties enriched in breunnerite are colored more strongly than dolomitic ones, which is evidently connected with the greater iron content in breunnerite. Under the microscope, segregations of limonite along cracks in the rock, along grain boundaries, and along the cleavage of carbonates are clearly visible.

In microscopic study of listvenites, the mesh structure preserved from the replaced serpentinite is often observed; breunnerite and dolomite, with small grains of quartz and flakes of talc, participate in its structure. It is often possible to observe how the relict mesh structure, composed of breunnerite, is destroyed when it is replaced by hypidiomorphic-granular dolomite, which indicates a somewhat earlier crystallization of breunnerite. This does not contradict the usual sequence of carbonate crystallization from iron-rich to less ferruginous varieties [14]. The carbonate aggregates of the listvenites are cut by veinlets mainly of quartz or of carbonate composition; by their mutual intersections several stages of their formation are established. The earliest are dolomite veinlets, in the central parts of which fine-grained quartz sometimes appears, heavily contaminated by minute inclusions. They are cut by quartz-dolomite veinlets composed of fine-grained quartz with abundant contaminating inclusions. The dolomite grains here tend toward the peripheral parts of the veinlets. Sometimes in the central parts of these veinlets there appear aggregates of isometric-mosaic quartz free of inclusions. At the walls of the veinlets, individual inclusions of breunnerite or dolomite composition, sometimes containing pyrite grains, can be observed in the quartz mass. These inclusions indicate replacement of the wall material by vein minerals [12]. Still younger veinlets, cutting the dolomite and quartz-dolomite ones, are composed of mosaic quartz, comparatively clean of inclusions, with chalcedony, calcite, and cinnabar. Cinnabar and calcite fill small residual cavities in the central parts of these veinlets. Sometimes cinnabar and calcite replace quartz, as evidenced by traces of corrosion of quartz grains. Cinnabar often also replaces calcite. Calcite sometimes forms independent veinlets containing cinnabar xenomerphic with respect to it. There are cases of cinnabar deposition along cracks in calcite veins (Fig. 1). In this case the cinnabar is accompanied by quartz and replaces finely crushed calcite, leaving only more or less large fragments in the form of “unsupported inclusions.”

The stibnite accompanying cinnabar is usually replaced by it. In addition to thin cutting veinlets, the quartz and quartz-chalcedony cinnabar-bearing aggregates form isometric accumulations within the carbonate mass of the listvenite, probably produced by replacement.

Along with veinlets of simple structure, zonal veinlets are also encountered, in which, from the walls toward the axial part, the zones are arranged symmetrically: dolomite, then quartz–dolomite, and finally quartz with chalcedony.

In the axial part of such veinlets one can observe residual cavities, rimmed by drusy aggregates of quartz and filled with calcite and cinnabar with antimonite.

Fig. 1. Sketch of a polished section. Cinnabar (black) replaces carbonate in a calcite vein along the crushing zone. ×54.

Sometimes it may be observed that one of the central zones, or the filling of the central cavity of a vein, gives off a branch which, in the form of an independent veinlet, cuts across the peripheral zones and passes into the enclosing rock.

The observations cited indicate that ore formation took place simultaneously with tectonic movements, with the opening of ever new fissures that served as pathways for the penetration of ore-forming solutions. The latter, throughout the entire period of formation of the ore-bearing listvenites, apparently did not cease their circulation for a prolonged time and underwent a continuous series of chemical changes. This was reflected in the successive replacement of some mineral associations by others, in which continuity is observed from early segregations to late ones.

The association of cinnabar with quartz in the listvenites once again emphasizes the close connection between listvenitization and ore deposition, since the segregation of quartz in the final stages of the process is generally characteristic of listvenitization [9]. Thus, these observations indicate that, in the present case, listvenites should be regarded as wall-rock formations closely connected with ore formation, and not as pre-ore formations having no relation to ore deposition [8].

Rocks of the most varied composition undergo wall-rock dolomitization almost throughout the entire extent of the ore-controlling disjunctives. Near the disjunctive zones the rocks are sometimes completely replaced by iron-bearing dolomite, the outcrops of which have a yellowish-brown color owing to the separation of limonite during weathering of the carbonate. With increasing distance toward the enclosing rocks, dolomitization gradually weakens until it disappears completely. Microscopic study establishes that, at a distance from the mineralization-controlling fracture, dolomite first appears in the form of cutting thin veinlets and isometric accumulations replacing the minerals of the rock, the density of which may increase up to complete replacement of the rock by dolomite. At the same time, in siliceous rocks, enlargement of quartz grains is observed near such accumulations. This phenomenon is evidently connected with the process of collective recrystallization that developed under conditions of dissolution and removal of silica during replacement by dolomite [4, 5].

In dolomitized rocks there is usually a noticeable dissemination of pyrite. Sometimes dolomite is associated with ankerite partially replacing it.

Cinnabar-bearing veinlets cutting dolomitized rocks usually have a quartz–chalcedony, and sometimes a quartz–calcite, composition.

If mineralization is emplaced in limestones, quartz may be entirely absent, and calcite becomes the principal vein mineral. In tuffaceous rocks, the main mass of cinnabar is concentrated not in veinlets, but is distributed in the selvages as disseminations. In this case the cinnabar disseminations are closely associated with isometric aggregates of fine-grained mosaic quartz and chalcedony.

In such cases it is often possible to observe traces of recrystallization of the fine-grained quartz of the surrounding groundmass of the rock, with coarsening of the grain; from this it follows that these cinnabar-bearing aggregates formed by the filling of leaching cavities.

On the other hand, the presence of pseudomorphic, after large grains of the rock, aggregates of fine-grained quartz and cinnabar (Fig. 2) indicates that, in addition, ore deposition occurred by replacement.

Fig. 2. Sketch of a thin section. Pseudomorphic aggregates (A) of small grains of quartz and cinnabar. ×54.

Fig. 3. Sketch of the face of a clearing.
1—listvenitized serpentinite; 2—tuffaceous sandstone; 3—dolomitized tuffaceous sandstone; 4—zones of dissolution; 5—post-mineralization disjunctive fracture.

Cinnabar is usually xenomorphic in relation to the quartz associated with it and sometimes replaces it. Calcite, by the time of its deposition, is younger than quartz, since its veinlets cut the quartz ones. Cinnabar in calcite veinlets, in most cases, is xenomorphic in relation to calcite and sometimes replaces it.

As the study of various mercury deposits of the group described has shown, the paragenetic schemes for the formation of their ores differ somewhat in detail, but for all of them, to a first approximation, the general sequence of mineral formation is preserved: ferruginous carbonates with pyrite — dolomite — quartz — calcite, cinnabar.

The data presented also indicate that, during the formation of the mercury ores of our deposits, the processes of filling cavities did not play as great a role as is usually assigned to them [8], but were greatly subordinate, in degree of development, to replacement processes.

Along the ore-controlling disjunctive fractures, heterogeneous rocks are often brought into contact, in which the near-ore alterations, evidently depending on the original composition of the rocks, manifested themselves differently, and their products are linked by mutual transitions. One such case is shown in Fig. 3, where a transition is shown from listvenites, formed at the expense of alteration of serpentinites, to dolomites and, further, to dolomitized tuffaceous sandstones of the Akkaya Formation, which through an ore-controlling disjunctive fracture are in contact with serpentinites. List—

the listvenites here have a breunnerite–dolomite composition and, to the naked eye, pass imperceptibly into tuffaceous sandstone that has been almost entirely replaced by dolomite. Farther on, the degree of dolomitization weakens and, in this way, there is a gradual transition to unaltered tuffaceous sandstones. Under microscopic study, the contact between the listvenites and the dolomitized tuffaceous sandstones is clearly distinguished, since the dolomites that have replaced the tuffaceous sandstone contain inclusions of relict quartz grains and are clouded by fine inclusions of clay particles.

Another figure shows a case of transition of listvenites, through a silicification zone, into dolomitized limestones (Fig. 4). Evidently, this is a case of interaction between limestone and serpentinite treated by hydrothermal solutions [2].

Macroscopic study, as well as microscopic study, did not make it possible to note distinct boundaries between unaltered and dolomitized limestone, between dolomitized limestone and the silicification zone, and between the latter and the listvenites. The limestones are almost completely replaced by a hypidiomorphic-granular aggregate of dolomite, in which individual grains of ankerite partly replaced by dolomite are contained. In addition, there is disseminated pyrite.

Fig. 4. Sketch of an outcrop.
1—limestone; 2—dolomitized limestone; 3—silicification zone; 4—listvenite.

In the silicification zone, the dolomitized limestone undergoes partial replacement by quartz, which develops in the form of isometric-mosaic aggregates that also contain chalcedony. The rock is cut by a network of quartz and quartz-calcite veinlets. Together with silicification, the amount of disseminated pyrite increases, and sometimes thin veinlets of it appear. The zone of silicified dolomitized limestones gradually merges with the listvenite. Separation can be made only by the presence in the listvenites of breunnerite, talc, and fuchsite, and by textural features (relict looplike patterns).

To judge the chemical changes that occurred in rocks subjected to treatment by ore-forming solutions, chemical analyses were made in the chemical laboratory of ZSGU in Novokuznetsk of samples of tuffaceous sandstones, listvenites, and serpentinites; use was also made of the results of analyses contained in the report of geologist Yu. F. Sechkin. These latter analyses were performed on samples of altered and unaltered limestones taken from the outcrop shown in Fig. 4. The petrochemical recalculation of the analyses was carried out with allowance for the porosity of altered and unaltered rocks [10] for different types of wall-rock alteration—dolomitization of tuffaceous sandstones (Table 1), listvenitization of serpentinites (Table 2), and dolomitization and silicification of limestones (Table 3).

Comparison of the recalculation results shows that, in the case of hydrothermal wall-rock treatment of tuffaceous sandstones, removal of

Table 1

Table 2

significant quantities of silica and the introduction of calcium and carbon dioxide. Iron, alumina, and sodium are also removed from the original rock. The contents of magnesium, manganese, and phosphorus remain almost unchanged. Small amounts of titanium and potassium are introduced.

Table 3

During listvenitization of serpentinites, silicon, iron, and, in addition, magnesium are also removed in significant quantities. Carbon dioxide and calcium are introduced in large quantities, and alumina, as well as a little potassium and titanium, in considerably smaller quantities.

Alterations of limestones, as was shown, take place in two stages. At first, during dolomitization, the small quantities of silicon contained in the unaltered limestone are almost completely removed, as is a little calcium. Magnesium is introduced in large quantities, and iron and a little alumina in smaller quantities. In the second stage—silicification—the removal of large quantities of calcium and the introduction of iron continue, but at the same time magnesium is removed in large quantities and much silicon is introduced.

These data make it possible to judge changes in the composition of ore-forming solutions as they pass through different rocks and interact with them. The interaction of the solutions with limestones led to their enrichment in carbon dioxide and calcium, while interaction with silicate rocks of basic and intermediate composition led to their saturation with silica, magnesium, and iron. The entry of carbon dioxide- and calcium-bearing solutions into serpentinites caused their listvenitization [1, 2]. The presence of calcium probably accounts for the presence of dolomite in the composition of our listvenites, by which they differ somewhat from ordinary ones [3, 1, 9]. Treatment of the sandy-clayey, pyroclastic rocks and limestones by these same solutions led to dolomitization. At the same time, further saturation of the solutions with silica took place, which led to its subsequent precipitation, under favorable conditions, in the form of quartz. In cases where hydrothermal solutions penetrated along

at the contacts of heterogeneous rocks, diffusion metasomatism possibly occurred [7]. Thus, where serpentinites came into contact with limestones, calcium and carbon dioxide could have diffused from the latter into the serpentinites, while magnesium, iron, and silica diffused in the opposite direction; this led to listvenitization on the one hand and to dolomitization and silicification on the other.

The pyritization of hydrothermally altered rocks indicates that the solutions were saturated with hydrogen sulfide.

A comparison of the quantities of substances given up by the rocks to the solutions and left by the solutions in the rocks during processes of near-ore alteration (Tables 1, 2, 3) suggests that the sources saturating the solutions with silica, carbon dioxide, calcium, magnesium, and iron were the thick and compositionally varied rock sequences that make up this area.

The presence in them of mercury, antimony, arsenic, titanium, and potassium must evidently be attributed to some more distant sources, possibly magmatic ones. The latter may serve as an indication of a connection between the hydrothermal solutions and acid magmas [1].

The formation of carbonates, listvenitization, and deposition of cinnabar are usually associated with the activity of alkaline hydrothermal solutions [2, 11]. Finds of marcasite and barite in the ore assemblage and the close association of cinnabar with quartz and chalcedony indicate oxidation of the solutions, which is evidently connected with the near-surface conditions under which the ores formed [2, 15].

Conclusions

The formation of the mercury ores of the deposits described is closely connected with dolomitization, listvenitization, and silicification of the rocks, and this should be used as a guide in prospecting for mercury mineralization in this district.

Ore formation took place under conditions of frequent tectonic movements, with prolonged circulation of ore-forming solutions. The latter probably had an alkaline reaction and entered into active chemical interactions with the enclosing rocks.

At the same time, processes of metasomatic replacement and hydrothermal leaching were extensively developed, leading to an increase in the porosity of the altered rocks.

As is usual, at least for most mercury deposits [13], the productive stages of mineralization followed the preliminary preparation of the rocks by hydrothermal solutions.

References

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