GOLD-BEARING GABBRO-DIORITE COMPLEX OF GORNAYA SHORIA

V. N. Salyomatin (TPI)

The area under consideration is located on the border of Kemerovo Oblast and Krasnoyarsk Krai, in the southeastern part of Gornaya Shoria. As early as the beginning of the last century, the area attracted the attention of researchers because of its rich placer gold deposits. In 1831, almost simultaneously, gold-bearing placers were discovered on the Petrópavlovka River, the right tributary of the Bazas River, and on the Fedorovka River, the left tributary of the Orton River. Later, placer mining continued in Soviet times along the tributaries of the rivers mentioned, in a number of other valleys, and on the remaining pillars from earlier workings. During this period, individual gold-bearing quartz veins were identified and explored. They are confined to exposures of gabbro-diorite massifs forming a series of small stock-like bodies. Thus, for example, in the area of the Bazas gabbro-diorite massif, about 20 quartz veins with an uneven distribution of gold have been explored. Let us consider the geological position of this magmatic complex.

In the area, carbonate-terrigenous deposits of the Kabyrza Formation of the Upper Proterozoic are widespread, as are effusive-sedimentary deposits of the Ust-Anzas Formation of the Upper Proterozoic—Lower Cambrian, and deposits of the Usa Formation of the Lower Cambrian, composed of black limestones with archaeocyathid fauna.

All the above-listed deposits are cut by intrusive formations occurring as small, roughly isometric massifs—the Ortotag, Bazas, Chezim, and Fedorov massifs. They are represented by a diverse association of rocks and are united into the gabbro-diorite complex.

The Ortotag massif is composed mainly of rocks of basic composition. The dominant position here is occupied by pyroxene-hornblende gabbros, hornblende gabbros, norites, and gabbro-norites. All of them are characterized by gabbroic or hypidiomorphic-granular textures. They consist of labradorite No. 60–62, usually strongly saussuritized; colorless monoclinic pyroxene, represented by augite \((2V = +58^\circ,\ \angle C : Ng = 40^\circ,\ Ng - Np = 0.022)\); pigeonite \((2V = +40^\circ,\ \angle C : Ng = 38^\circ,\ Ng - Np = 0.015)\); and green hornblende \((2V = -76—-78^\circ,\ \angle C : Ng = 12—20^\circ,\ Ng - Np = 0.014)\). In the gabbro-norites, hypersthene is observed \((2V = -56^\circ,\ Ng - Np = 0.010)\), and in the norites, enstatite \((2V = +66—76^\circ,\ Ng - Np = 0.009)\).

In the southwestern endocontact belt, melanocratic coarse-grained monomineralic pyroxenites are widespread, consisting almost entirely of a single rhombic pyroxene, and hornblendites—giant-grained rocks composed of 90% hornblende. The latter form irregular nests, lenses, and veinlets among the gabbro. Less commonly, pyroxenites and hornblendites form bands in the gabbro with distinct sharp boundaries, with thicknesses from 1–2 to 10 cm. Gabbro with a banded texture occurs most often in the central parts of the massif.

Within the indicated massif, diorites are weakly developed; they are observed in its eastern and northeastern parts. These are gray, greenish-gray, sometimes dark-gray, medium-grained and even-grained rocks of dioritic texture.

Plagioclase is strongly altered. In places, fine polysynthetic twinning has been preserved. In composition the mineral belongs to andesine No. 30–40. Hornblende belongs to the ordinary green variety. Its properties are analogous to those of the rocks described above. Quartz is always xenomorphic and amounts to 3–5%. Accessories are observed as separate grains of apatite.

Among the diorites, dike and vein rocks occur comparatively rarely, represented by aplites and granite-aplites; in the gabbro, small veins of pegmatite are encountered.

The Bazasskii massif is located 1 km northeast of the Ortotagarskii massif. They are separated by a belt of marbles, quartzites, and skarned rocks. The petrographic composition of the Bazasskii massif differs from that of the Ortotagarskii by the presence of quartz and quartz-bearing diorites, granodiorites, granites and plagiogranites, and quartz veins. A major difference lies in the spatial distribution of the rocks. Diorites, quartz diorites, and quartz-bearing diorites are widespread throughout the Bazasskii massif. Among them, in significantly smaller amounts, occur gabbro, norites, and gabbro-norites, which occupy small areas. The apical parts of the massif are characterized by a large number of dikes, irregularly shaped nests, lenses, and veinlets of aplites, plagiogranites, granites, granodiorites, hornblendites, and pegmatites. In the northeastern part of the massif, a rather large number of thin quartz gold-bearing veins with sulfide mineralization has been found.

Quartz hornblende diorites are most often light-gray, medium-grained, and even-grained. Macroscopically they are indistinguishable from normal diorites. The texture is dioritic or hypidiomorphic, sometimes porphyritic. The mineralogical composition is as follows: plagioclase (40–45%), green hornblende (35–40%), quartz (10–15%), accessory and secondary minerals (5–7%).

Plagioclase No. 35–40 is observed as tabular grains, densely replaced by sericite and clayey material. Hornblende forms compact grains or intergrowths of finer grains. It is replaced by chlorite, epidote, and biotite. Its optical properties are: \(2V = -74\text{–}88^\circ\), \(\angle C : Ng = 13\text{–}16^\circ\). Quartz, in xenomorphic grains and their aggregates, occupies the interstices between minerals and sometimes forms inclusions in them.

Accessories are represented by small grains of apatite, zircon, and ore mineral.

In melanocratic diorites, the amount of hornblende reaches 60–70%, plagioclase 10–20%, quartz 5–8%, and secondary and accessory minerals 10–15%. Their properties are the same as in the quartz diorites. The gabbroic rocks of the Bazasskii massif are represented by the same varieties of rocks as those of the Ortotagarskii massif. More characteristic

the structure becomes porphyritic. Gabbro-norites are much more widespread. These are dark-gray, medium-grained rocks, often with schlieren-like segregations of dark-colored minerals. Composition of the rock: pyroxenes (30–50%), hornblende (10–20%), plagioclase (20–30%). Accessory minerals are quartz, chlorite, epidote, and an ore mineral.

In the gabbro-norites, among the rhombic pyroxenes, hypersthene is observed—\(2V = -58–60^\circ\)—or bronzite—\(2V = -82–84^\circ\).

Monoclinic pyroxenes occur in smaller amounts and are represented by diopside with \(2V = +62–65^\circ\), \(\angle C : Ng = 40^\circ\), \(Ng - Np = 0.025\). Green hornblende is observed in the form of tabular grains; it often intergrows with pyroxenes or forms rims around them. Its properties vary within the following limits: \(2V = -75^\circ\), \(\angle : Ng = 18–28^\circ\). Plagioclase is usually replaced to one degree or another by saussurite and has polysynthetic twins. It belongs to labradorite No. 50–52. In some samples the plagioclase is more acidic, No. 45. Quartz in the rock is either entirely absent or, in xenomorphic grains, occupies the spaces between other minerals.

The rocks of the dike group are in many respects analogous to the Orto-Tas rocks. The acidic differentiates—hornblende granites and plagiogranites—are characterized by the same compositions and properties of their constituent components.

On the left bank of the Fedorovka River, in its lower course, lies the Chezim intrusive massif. It is characterized by a variegated petrographic composition. Its structure includes granites, syenites, granosyenites, quartz syenites, monzonites, essexites, gabbro, and gabbro-anorthosites.

The more acidic varieties of rocks are distributed mainly in the central parts.

The granites have the following mineral composition: plagioclase (25–30%), microcline (20–35%), quartz (20–25%), biotite, hornblende (5–10%). Accessories are represented by magnetite, zircon, and occasionally sphene. The microstructure is granitic.

Syenite-diorites occur in the northeastern endocontact part of the massif. Their mineral composition is: plagioclase (30–40%), K-feldspar (25–30%), quartz (5–10%), hornblende, biotite (10–15%), accessories (2–5%).

Monzonites and essexites are also confined to the endocontact parts of the massif. Essexites consist of basic plagioclase (60–70%), pyroxene, biotite, hornblende (15–20%), and K-feldspar (3–5%). Their structure is hypidiomorphic or ophitic.

Olivine gabbro and gabbro-anorthosites are confined to the western and northwestern marginal parts of the massif. They consist of basic plagioclase, monoclinic pyroxene, and olivine. Under the microscope the structure is gabbroic, more often hypidiomorphic, grading into ophitic.

At the contact of the Chezim massif with the enclosing rocks there are areas of pyroxene, pyroxene-forsterite, magnetite, and pyroxene-garnet skarns. In three areas, the Tebinskaya party in 1964–1965 exposed disseminated and massive magnetite ores. In individual skarn areas, sampling revealed a small gold content.

Southeast of the Chezim massif there is another massif, unnamed. It is separated from the Chezim massif by a narrow (about 300 m) belt of deposits of the Ust-Anzas Formation. The massif is composed of granites, plagiogranites, and granodiorites. Its structure is very uniform, without definite regularities. Predominant

pink granites are coarse-grained, massive biotite–hornblende rocks. Dikes of basic composition develop along the marginal parts of the massif.

In the headwaters of the Fedorovka River lies the Fedorovsky massif. Granites, plagiogranites, and granodiorites also take part in its structure. Rocks of basic composition form small dikes, chiefly in the apical parts. In the northern exocontact zone of the massif, skarns of diopside–garnet and epidote–garnet composition develop.

A characteristic feature in this area is the spatial association of gold-bearing placers with certain intrusive massifs. These include the Bazassky and Fedorovsky massifs and the massif located southeast of Chezimskoe. In the northeastern exocontact of the Bazassky massif, the rich gold-bearing placer of the Petropavlovka River was worked. Placer gold was also mined along a spring flowing in the northeastern endocontact part of the massif. It is precisely in this part of the massif, as already mentioned above, that quartz veins with sulfide mineralization are developed. The quartz in these veins is represented by two generations. First-generation quartz is milky white, of massive texture, granular, and barren. Second-generation quartz is gray, brecciated, and contains disseminations of sulfides.

The ore minerals are represented by pyrite, chalcopyrite, sphalerite, and galena. They form scattered disseminations of individual grains and rarer nest-like accumulations in quartz. Among secondary minerals, limonite, chalcocite, and covellite are present. Corrosion, emulsion, and colloform microstructures of the mineral aggregates are characteristic.

Previous investigators described giant crystals of pyrite in the veins (up to several centimeters across) and rutile.

The richest placer was situated along the Fedorovka River. It has been worked to a considerable extent. Recently the question has been raised of working the placer and the old dumps by a mechanized dredging method.

From all that has been set forth above, the following conclusions may be drawn:

1. The placer gold deposits in the area described are confined to the zone of development of the ancient gabbro–diorite complex.

2. The richest placers begin at massifs of diorites, granodiorites, plagiogranites, and granites, among which ore occurrences in the form of quartz veins are observed.

3. The sources of the placer gold are evidently quartz veins with sulfide mineralization, formed at the final stage of massif formation, as well as skarns developing at the contact of acidic and intermediate rocks with the enclosing rocks.