INTRUSIVE ROCKS IN THE ALGUY ORE FIELD

A. I. Shevelev

(Presented by Professor A. M. Kuzmin)

The Alguy ore field, which includes the large Alguy deposit of low-iron talc and separate talc occurrences, is associated with deposits of the West Siberian suite of the Upper Proterozoic, represented mainly by dolomites. The ore field extends in a northeastern direction, in accordance with the strike of the dolomites, for a distance of 3.5–4.0 km and is confined to a system of northeast-striking faults dipping almost vertically (the Alguy tectonic zone).

In the area of the deposit there are three intrusive bodies that cut through the carbonate deposits of the West Siberian suite and are arranged linearly within the ore field—from northeast to southwest: a) the marginal part of the Tygertysh intrusion of porphyritic granites; b) a stock of plagiogranites 1.5–2.0 km from the Tygertysh intrusion; c) the Alguy intrusion of quartz diorites in the southwestern part of the ore field.

a) The porphyritic granites within the ore field are represented by the southwestern margin of the large Tygertysh massif, composed of rocks of the plagiogranite–adamelite–granite series [2]. Studies carried out mainly within the eastern slope of the Kuznetsk Alatau have made it possible to assign the Tygertysh intrusion to the intrusive magmatic formation of Ordovician age [2, 8], to the complex of typical fissure, multiphase intrusions accompanying the closure of first-order structures in the Kuznetsk Alatau [2]. The absolute age of the intrusion is, according to D. I. Musatov [8], 445–460 million years, and according to V. S. Meleshchenko [6], 480–490 million years.

No bedrock outcrops of granite have been encountered within the ore field; they form continuous stone streams covering the slopes and summits of the ridge. The granites are light gray, sometimes pinkish, with a porphyritic structure and a hypidiomorphic-granular groundmass. Mineral composition: plagioclase ~40%, K-feldspar ~35%, quartz ~20–

25%, biotite—up to 5%, hornblende—single grains; secondary minerals—sericite, chlorite; accessory minerals—zircon, apatite, magnetite. The main mass of the granite is composed of grains 0.5–0.8 mm in size, of quartz and K-feldspar; plagioclase is present in smaller amounts. The porphyritic segregations are represented by plagioclase, K-feldspar, and quartz.

Plagioclase of oligoclase composition (No. 18–23) occurs in tabular segregations up to 3–4 mm long. Its grains are often corroded by fine-grained K-feldspar, which also penetrates the plagioclase in the form of veinlets. Many plagioclase grains are albitized in their marginal parts, while in the central parts they are sericitized; the distribution of sericite aggregates emphasizes the zonal and polysynthetic structure of the plagioclase.

K-feldspar in porphyritic segregations, up to 8 mm long, has a patchy microperthitic structure. In the K-feldspar, inclusions of quartz of rounded or irregular form are common. Some crystals undergo albitization, as a result of which albite rims individual grains and penetrates inward along cracks, forming replacement perthites.

Quartz is represented by grains of pseudo-square, rounded, or irregular form, up to 3–4 mm in size. The quartz contains veinlet-like, as well as separate small inclusions of K-feldspar confined to microcracks.

Biotite occurs as small (up to 0.04 mm) and larger (0.4–0.8 mm) flakes and plates. Coarse-platy biotite is predominantly isometric in form; fine-platy varieties are usually elongated and are arranged along the boundaries of plagioclase and quartz grains, and also penetrate into them along cracks or cleavage, forming bay-like shapes. Biotite is often chloritized, and sometimes very strongly, up to complete replacement.

b) A stock of plagiogranites measuring 300×600 m has an oval form, oriented in a northeastern direction. The plagiogranites of the stock, in contrast to the granites of the Tygertish intrusion, are characterized by a finer-grained structure. Porphyritic segregations of plagioclase, K-feldspar, and quartz are represented by grains 1.5–2.0 mm in size. The mineral composition of the rock also differs somewhat, since the plagioclase content reaches ~45%; K-feldspar ~30%, quartz ~35%, biotite ~3%. The composition of the plagioclase is more basic—No. 20–25. In the plagiogranite, processes of K-feldspathization of plagioclase, albitization of plagioclase and K-feldspar, and chloritization of biotite are also developed. At the surface, the plagiogranites are intensely destroyed, up to grus, and impregnated with iron hydroxides.

c) The Alguy intrusion of quartz diorites has a stock-like form, measuring 1.5×2.0 km, slightly elongated in a northeastern direction. The absolute age of the quartz diorites, determined at the Central Laboratory of ZSGU by the potassium-argon method, is 460 million years; however, geologists who carried out geological mapping in the region (V. P. Marushchenko) take its age to be Devonian by analogy with the Kazyr intrusion of quartz diorites.

The quartz diorites are light gray, medium-grained, with a hypidiomorphic-granular structure and massive texture. Composition: plagioclase ~60–65%, K-feldspar ~5–8%, quartz ~15–20%, hornblende ~7–10%, biotite ~1–5%, pyroxene—single grains; secondary minerals—sericite, saussurite, chlorite, epidote, zoisite; accessory minerals—sphene, zircon, apatite.

Plagioclase of oligoclase-andesine composition (No. 25–35) occurs in prismatic segregations up to 3–4 mm long; many grains have a polysynthetic and zonal structure. The marginal parts of the grains are often albitized and surrounded by a rim of light albite, while the central parts are saussuritized and sericitized.

Potash feldspar occurs as small (up to 0.6 mm) isometric and irregular grains that corrode plagioclase and quartz, and also penetrate into them in the form of veinlets.

Quartz occurs in xenomorphic grains of irregular or isometric shape, up to 2 mm in size.

Hornblende ($\mathrm{Ng}=1.655\pm0.003$, $\mathrm{Nm}=1.648\pm0.003$, $\mathrm{Np}=1.638\pm0.003$, $c\mathrm{Ng}=19$—$20^\circ$; $2V=-76^\circ$) forms prismatic grains 0.5—2.0 mm in size. Hornblende crystals occur both in fairly regular forms and with very intricate outlines, owing to their development along microfractures in plagioclase, quartz, or along the boundaries of these grains; the form of hornblende segregations is determined by the character of the fractures in which it develops.

Biotite ($\mathrm{Ng}=\mathrm{Nm}=1.624\pm0.003$, $a\mathrm{Ng}=0^\circ$) occurs in isometric platy segregations up to 2 mm in size; it is usually associated with accumulations of hornblende and replaces it; in addition, fine-scaly biotite develops along microfractures in plagioclase and quartz.

In general, the dark-colored minerals are grouped in quartz diorites in the form of subparallel bands up to 1—2 cm wide, or as accumulations of rounded, oval shape, ranging in size from several centimeters to 0.5—1.0 m, oriented along their elongation in one direction.

The grains of hornblende and biotite are chloritized, sometimes very intensely; in addition, epidote and zoisite develop on hornblende.

The intrusive rocks described are characterized by complex interrelations among their constituent minerals, which may indicate a complex path of formation of these intrusions, proceeding in at least two stages: a) magmatic crystallization of the rock-forming minerals, b) autometasomatic or postmagmatic crystallization. In the first stage, plagioclase, quartz, part of the potash feldspar, and the dark-colored minerals formed. Postmagmatic crystallization is characterized by the manifestation, in granites and quartz diorites, of hydrothermal-metasomatic processes, the sequence of which may be represented as follows: potash feldspathization—albitization—magnesian metasomatism (formation of biotite, hornblende)—chloritization of dark-colored minerals—sericitization and saussuritization of plagioclases.

Processes of potash feldspathization in intrusive rocks have been noted by a number of researchers, including A. I. Volzhenkov [1], D. S. Korzhinskii [3], K. Menert [7], V. I. Sotnikov and A. P. Berzina [9], A. M. Kuzmin [5], and others.

Potash feldspathization in the granites and quartz diorites of the district was developed rather widely and took place under conditions of an alkaline medium, with an influx of potassium and mobility of such elements as sodium, calcium, aluminum, and silicon. As the alkalinity of the solution decreased in the course of potash feldspathization, albitization of feldspars began, which is “the most typical process of the leaching stage and occurs under conditions of decreasing alkalinity” [3].

The appearance of magnesian metasomatism in the intrusive rocks is associated with weakened zones that facilitated the penetration of solutions. Evidence of this may be the association of the dark-colored minerals with microfractures in the minerals, as well as the formation of banded accumulations of hornblende in the quartz diorites. Magnesian metasomatism proceeded with an increase in the alkalinity of the solutions due to their enrichment in potassium during albitization and in the presence of magnesium.

The final stage of the postmagmatic process was expressed in the chloritization of biotite and hornblende, the epidotization of hornblende, and the sericitization and saussuritization of plagioclases.

Summarizing the above, it can be seen that the intrusive bodies described have a number of common features and peculiarities. Spatially they are confined to the Alguy tectonic zone, which determined their form, oriented lengthwise along the zone; this is especially clearly expressed for the plagiogranite stock and the intrusion of quartz diorites, which are fully contoured within the area. All the intrusive rocks are characterized by the common occurrence in them of processes of potassium feldspathization, albitization, and magnesial metasomatism, which took place in the postmagmatic stage and should be regarded as a single process developing over time. The rocks contain, in almost identical quantities, the same accessory elements: Ni, Co, Pb, Cu, Ga, Zr, Cr, Ti, Ba, Sr, V.

Near the intrusive bodies, an analogous character of alteration of the enclosing dolomites is established; this is clearly revealed for the plagiogranite stock and the intrusion of quartz diorites. With increasing distance from the intrusive bodies, an exocontact metasomatic zonality is revealed, expressed in diopsidization, tremolitization, talc alteration, and silicification of the dolomites.

According to the classification of Yu. A. Kuznetsov [4], the intrusive rocks described belong to the series of volcanogenic intrusive formations. All the features identified make it possible to regard the intrusive bodies not as formations of different ages, but as belonging to a single Ordovician Tigertysh magmatic complex, in which the Alguy intrusion of quartz diorites represents one of the phases.

Dikes are fairly widely developed in the area, forming crosscutting and, in considerable number, interstratal bodies. The composition of the dikes is quite varied: diabases, diabase porphyries, gabbro-diabases, diorite porphyries. The dike facies is the concluding phase in the formation of the magmatic complex.

REFERENCES

1. Volzhenkov A. I. Reaction relationships accompanying the crystallization of granitic magma. — Transactions of the Institute of Geological Sciences, issue 21. Petrographic series No. 7, Academy of Sciences of the USSR, 1940.

2. Ivanova T. N. The history of geological development and magmatism of the Kuznetsk Alatau in the Paleozoic. — In: Problems of Magmatism, Metamorphism, and Ore Formation, Gosgeoltekhizdat, Moscow, 1963.

3. Korzhinskii D. S. An outline of metasomatic processes. — In: Basic Problems in the Study of Magmatogenic Ore Deposits. 2nd ed. Academy of Sciences of the USSR, 1955.

4. Kuznetsov Yu. A. The Principal Types of Magmatic Formations. Nedra, 1964.

5. Kuzmin A. M. Mineralogical-petrographic features of the granitoids of Mount Mokhnatukha (Rudny Altai). — Geology and Geophysics, 1968, No. 3.

6. Meleshchenko V. S., Yanov E. N., Kazakov I. N. Principal features of the tectonics of the Sayan-Altai folded region. — Materials on the Geology and Mineral Resources of Eastern Siberia, VSEGEI, issue 32, 1960.

7. Menert K. New Contributions to the Problem of Granites. IL, Moscow, 1963.

8. Musatov D. I. Intrusive magmatism of the eastern slope of the Kuznetsk Alatau. — In: Materials on the Geology and Mineral Resources of Krasnoyarsk Krai, issue 1, Krasnoyarsk Book Publishing House, 1961.

9. Sotnikov V. I., Berzina A. P. Features of the process of metasomatism at the Sorsk deposit. — Geology and Geophysics, 1966, No. 9.