Some Features of the Occurrence of Products of Hydrothermal Metamorphism in Carbonate Rocks

L. P. Rikhvanov

(Presented by Professor A. M. Kuzmin)

This article is devoted to the characterization of a distinctive near-ore alteration and ore mineralization in terrigenous-carbonate rocks. In the area of interest to us, rocks of the Lower Ordovician and Devonian are developed.

The Lower Ordovician deposits are represented by a carbonate-terrigenous sequence 400–500 meters thick. It includes limestones, siltstones, sandstones, tuff-gravelites, and tuff-conglomerates. The upper part of the sequence has a substantially carbonate composition, while the lower part is terrigenous. Above, with angular and azimuthal unconformity, the Ordovician deposits are overlain by Devonian deposits.

Devonian deposits are widely developed and have an effusive-sedimentary and sedimentary character. They are clearly subdivided into three sequences, differing in the composition of the effusives and in the amount of sedimentary material in the section. The lower sequence is characterized by the broad development of sedimentary rocks and by a basaltic composition of the effusives. Its thickness is 150–170 m. The middle sequence is composed chiefly of acidic effusives. Its thickness does not exceed 450 m. The upper sequence is represented by red-colored conglomerates, gravelites, sandstones, and siltstones. Its thickness is more than 130 m.

Intrusive formations are represented as small bodies of granitoids, dikes, and bodies of diabase porphyrites and microgabbro-diorite porphyrites. Intrusive rocks of acidic composition cut through the deposits of the lower and middle Devonian sequences; their relationships with the upper sequence have not been clarified. Rocks of basic composition are later than the granitoids. The structural pattern of the area is determined by the combination of several systems of dislocations. The leading role is played by a deep-seated fault of ancient inception. The rocks within this structure are intensely fractured. The following principal systems of fractures are distinguished: 1) dip azimuth 65° ∠80°; 2) dip azimuth 248° ∠85°; 3) a system of bedding-parallel fractures with a gentle dip azimuth of 170° ∠20–30°.

Products of hydrothermal metamorphism are widely developed within the area. Areal and near-ore metasomatites are distinguished. Areal metasomatites include products of hydrothermal metamorphism that form broad halos of distribution. They develop chiefly in acidic volcanics and their tuffs, tuff-sandstones, tuff-gravelites, and also in sandstones and limestones.

The end products of hydrothermal reworking acquire an effusive-like appearance. Only with detailed study of sections is it possible to determine the primary composition of the rock. Of the intrusive forma-

granitoids are subject to this type of alteration, whereas the basic intrusive formations have not experienced its effect. The distribution of hydrothermally altered rocks of this type is controlled by a deep fault.

Areal metasomatites contain more uranium in comparison with unaltered rocks, which indicates redistribution of uranium in the latter and enrichment of the altered rocks with this element.

Near-ore altered rocks are specific to the area and are observed only in association with Lower Ordovician limestones. They form a vein-like body elongated along a system of northwest-trending brecciation zones, which are subsidiary with respect to the deep fault.

The formations composing the body are metasomatic rocks of carbonate composition, among which, even macroscopically, on the basis of color and texture, separate zones can be distinguished (from the periphery toward the center of the body):

Zone 0. Massive, dense, aphanitic limestones, gray with a reddish tint. The rock structure is oolitic. The limestones contain fauna of brachiopods, trilobites, and oncolites. The composition of the limestones includes calcite 85–90%; quartz 2–3%; pelitic material 10–12%; and organic matter 1–2%. Among ore minerals, pyrite and chalcopyrite are rarely observed.

Zone I. Dark-gray crystalline limestones, massive, dense. The rock structure is granoblastic. The composition of this zone is analogous to the preceding one. Among ore minerals, chalcopyrite and pyrite up to 3–4% are present. Vein mineralization is represented by calcite. The thickness of the zone is about 6–8 m.

Zone II. A dark-gray crystalline rock with a granoblastic structure, consisting of dolomite 40–50%, calcite 30–40%, ankerite 5–6%, quartz 5–10%, hydromica 15–20%, and organic matter 5–6%. Ore mineralization is represented by chalcopyrite and pyrite up to 5%. At the same time, the formation of dolomite after calcite is clearly visible. New formations of quartz and hydromica are observed in places of the most intense dolomitization. The thickness of the zone is 0.5–0.7 m.

Zone III. The macroscopic appearance and structure of the rocks are analogous to zone II. Its mineralogical composition is as follows: ankerite 50%; dolomite 30%; calcite 10%; quartz 2–3%; hydromica 2–4%; and organic matter up to 5%; barite is noted. The amount of chalcopyrite increases sharply (up to 20%); pyrite is preserved only as relicts. The thickness of the zone is 0.4–0.6 m.

Zone IV. A greenish-gray, massive, dense crystalline rock with a granoblastic structure, consisting of ankerite 60%, dolomite 10%, quartz 15–20%, hydromicas 5–10%, and organic matter 5–6%. Relicts of calcite and dolomite grains are often observed in the ankerite. The development of quartz–hydromica neomineralizations is observed along small systems of cracks cutting the dolomite, calcite, and ankerite formations. Among ore minerals in this zone there are chalcopyrite (up to 8–10%), nasturan, galena, pale ore, sphalerite; relicts of pyrite are preserved, and skuttersudite may be present. In addition, a large number of veinlets of ankerite, barite, and quartz are developed. The thickness of the zone is 0.3–0.4 m.

Zone V. Represented by massive, dense rocks, predominantly of red tones. The rock structure is breccia-like and taxitic. Composition of the zone: chalcedony-like quartz 40–50%; ankerite 15–20%; sericite 15–20%. Chalcopyrite, nasturan, galena, pale ore, and sphalerite are developed in significant amounts. The thickness of the zone is 0.1–0.2 m. The boundaries of the zones are indistinct. They are drawn according to changes in textural—

of structural pattern, the character of coloration, and the predominance of certain complexes of newly formed minerals. A similar spatial arrangement of individual zones of altered rocks has been noted at a number of polymetallic, mercury, and gold–silver deposits.

Nasturan mineralization, as indicated above, is encountered in the two inner zones. The contours of ore mineralization coincide with areas of intense ankeritization and silicification. Nasturan is a fairly widespread, but extremely unevenly distributed, ore mineral. In most cases it is confined to veinlet-like accumulations of chalcopyrite. The ore texture is disseminated, more rarely veinlet-like. Nasturan has distinct rounded botryoidal formations with syneresis cracks. Individual nasturan botryoids, joining with one another, form accumulations of irregular shape or vein-like aggregates. The dimensions of individual botryoids do not exceed 0.010 mm. Nasturan is present in the form of two varieties. Nasturan I has large botryoids and most often forms accumulations and vein-like bodies. Nasturan II has botryoids 3–4 times smaller than those of the first variety and is evenly disseminated, without forming accumulations.

Radiographs obtained on nitrocellulose film show that alpha-particle tracks from nasturan I are distributed exclusively along the boundary of aggregates of chalcopyrite and ankerite, whereas alpha tracks from nasturan II occur in chalcopyrite.

The data presented indicate a close association of nasturan with chalcopyrite. Deposition of nasturan took place in parallel with chalcopyrite, which continued to crystallize even after deposition of the main mass of nasturan (nasturan I).