On the Significance of Accessory Minerals for the Study of Igneous Rocks

A. M. Kuzmin, T. I. Poluektova

The enormous role and great significance of accessory minerals for solving a number of important theoretical and practical problems are becoming quite evident today. Questions connected with revealing the regularities in the distribution of accessory minerals in intrusions, as well as the features of the concentration of rare elements in rocks, are acquiring ever greater urgency.

Before turning to a consideration of the fields of application of accessory minerals, let us dwell on the meaning of the term “accessory minerals.”

There are several definitions of the concept “accessory mineral.” Thus, H. Rosenbusch [25, 38] calls “characteristic special admixtures” in a rock accessory minerals. Among them in intrusive rocks he includes apatite, zircon, titanite, magnetite, hematite, orthite, monazite, garnet, tourmaline, topaz, fluorite, sulfides, and other minerals. All the minerals listed crystallized from the cooling magma at earlier stages, and therefore the most perfect (idiomorphic) crystal forms were characteristic of them. I. A. Preobrazhenskii [22] believes that minerals should be called accessories if they are present in rocks in such small quantities that the use of concentration methods is required for their study. A similar definition is also given by foreign scholars [37].

The presence of accessory minerals in rocks in small quantities is a very characteristic feature of them and is taken as a basis by the majority of geologists. However, I. A. Preobrazhenskii’s definition does not take into account their second important property—the qualitative aspect—and does not draw attention to the features of the entry into their composition of rare and radioactive elements that are present in the rock in insignificant quantities. In the Geological Dictionary (1955) the following definition of an accessory mineral is given: “Accessory minerals are minerals that form a quantitatively negligible, and qualitatively often characteristic, admixture in rocks.” With such a formulation of the concept—

What has just been said is part of “Rosenbusch’s rule” concerning the order of crystallization of minerals in intrusive rocks and for a long time remained unshakable. This “rule,” with respect to accessory minerals, is recognized in explicit or hidden form by many specialists in accessories, despite the fact that other opinions have been expressed, according to which accessory minerals in the stage of magma formation are the last (A. N. Zavaritskii, 1955) and are even considered metasomatic.

“accessory minerals” coincides in the main with the definition proposed by V. K. Monich [21] and I. E. Smorchkov [30]. I. Kh. Khamrabaev [32], in considering questions of the petrologic-geochemical features of magmatic and ore complexes of western Uzbekistan, classifies as accessories “...rare minerals of the magmatic or late-magmatic stages, formed by minor elements or by an excess portion of the principal rock-forming elements.” In this interpretation of the concept “accessory minerals” there are also a number of shortcomings. For example, there is no definiteness in the content of the term “rare minerals”—it is unclear whether what is meant here is their rare distribution, or whether, by virtue of the peculiarities of their chemical composition, they were rare.

In 1965, in Moscow, at a conference on the problem “Accessory minerals in solving questions of the origin and metallogeny of magmatic complexes,” O. I. Matkovsky [19] gave the following definition of the concept “accessory mineral”: “Accessory minerals are minerals that usually form a quantitatively insignificant, but qualitatively characteristic, constituent part of rocks; they contain accessory chemical elements which, under certain physicochemical conditions, are not dispersed in the lattices of the principal minerals, but even in insignificant quantities in the mineral-forming medium (with possible partial dispersion of some of them in the principal minerals) become saturating components and crystallize in the form of independent accessory minerals” [19].

The majority of investigators, and we as well, accept the formulation of the concept “accessory mineral” proposed by O. I. Matkovsky. V. V. Lyakhovich adds to this definition the two most characteristic features of accessory minerals: 1. The use of special methods of concentration for their study. 2. The extremely insignificant sizes of the grains of the accessory minerals themselves—tenths and hundredths of a millimeter.

The range of questions solved with the aid of accessory minerals is defined fairly clearly in works [9, 24, 29, 30]. Let us consider the principal ones.

1. The presence of identical accessory minerals and trace elements in the intrusive rocks of a complex may testify, along with other geologic-petrographic features, to their comagmatic nature. Thus, M. G. Rub [29], studying the accessory minerals of the intrusive rocks of the Myao-Chan complex, came to the conclusion that the rocks were derived from a common parental intrusion. This principle is also widely used for solving the question of the comagmatic nature of effusive, subvolcanic, and intrusive rocks [29]. An extensive literature is devoted to the problem of the comagmatic nature of various intrusive formations, in particular the collection published under the editorship of V. S. Koptev-Dvornikov and M. G. Rub [29].

2. Accessory minerals objectively make it possible to solve the question of the relationship of mineralization to a magmatic complex. The solution of this question has not only scientific but also great practical significance [1, 9, 24, 26, 27, 28]. M. G. Rub, using the example of the study of the interrelation of vein rocks with mineralization in the southwestern part of Primorsky Krai [26, 28], and also of the granitoids of the Myao-Chan intrusive complex and the postmagmatic formations associated with them [28], came to the conclusion that “...the presence in magmatic rocks and the postmagmatic formations accompanying them of common accessory minerals containing trace elements of the same type testifies, along with other facts, to their connection.”

A. V. Rabinovich [24], studying the regularities in the distribution of accessory minerals in granitoids of various metallogenic zones, distinguished 12 types of magmatic rocks (zircon, zircon-fluorite, zircon-columbite, etc.) and established that certain ore occurrences are paragenetically associated with a definite type of these rocks. Thus, in the intrusive rocks of Eastern Transbaikalia, with which tin-tungsten mineralization is associated, the apatite-monazite type is observed, whereas the intrusive rocks of Rudny Altai, with which polymetallic mineralization is paragenetically associated, belong to the zircon type.

G. D. Afanas’ev [1] also considers that the principal criteria determining possible genetic links between metalliferousness and certain magmatic complexes, along with other features, include “the specificity of the material composition of magmatic rocks and ore occurrences, taking into account accessory minerals and trace elements.” These views are shared by F. K. Shipulin [34], E. P. Izokh [7], I. M. Volovikova [6], and others.

1. A rather important role is assigned to accessory minerals in the correlation and age subdivision of intrusions. I. Kh. Khamrabaev [31] points out that “…all granitoid complexes of different ages and different regions have a definite paragenetic association of accessories.” In subsequent works, I. Kh. Khamrabaev [32], in setting forth principles for the correlation of magmatic rocks, attaches great importance, along with general geological criteria, to petrographic-mineralogical criteria—in particular, to the characteristic association of accessory minerals and their quantitative ratios. The idea of comparing intrusive rocks according to the characteristic features and assemblage of accessory minerals can be found in the works of V. V. Lyakhovich and others [13, 17], and in reports devoted to the 10th anniversary of the accessory-mineral laboratory of IMGRE. At the meeting in Moscow held in 1965, it was shown quite convincingly that “on the basis of distinguishing types of associations of accessory minerals, and also data on the content of trace elements both in rock-forming and in accessory minerals, it is possible to correlate magmatic complexes that are difficult to subdivide on the basis of geological data.” It should be noted that, for identifying similar or different features of any group of intrusive formations, use is made, as a rule, of the species composition of the accessories. Insufficient attention has so far been paid to the peculiarities of crystal morphology. At present, the morphology of zircon has been studied in comparatively great detail among the accessory minerals. G. Hoppe [36] also believes that “…for purposes of correlation, apatite and its morphological features are of no less importance than zircon….” The need for a thorough study of the morphological features of crystals of accessory minerals is indicated by V. V. Lyakhovich and other researchers.

2. In a number of cases, certain radioactive accessory minerals—zircon, thorite, orthite, monazite—are used to determine the absolute age of granitoids. It is known that determining the age of igneous rocks is one of the most complex problems in geology. Potassium minerals used for these purposes often underestimate the absolute age of rocks, since they are not infrequently products of potassium metasomatism, and during deformation of potassic-sodic alkaline rocks they may lose part of their radiogenic argon [11]. The formation of most radioactive accessory minerals, as noted by a number of authors, is closely connected with the formation of the rock [16, 30]. Among foreign researchers, determination of the absolute age of intrusive rocks from pleochroic halos in biotites observed around zircon, monazite, xenotime, and others is widely used.

1. The study of the regularities in the distribution of accessory minerals in individual intrusive bodies makes it possible to reveal a number of interesting features. I. E. Smorchkov [30], studying the regularities of the vertical distribution of accessory minerals in a granodiorite massif, established that there are more accessory minerals in the deeper parts of the intrusion than in the upper parts. The amount of apatite in the massif decreases from top to bottom. Zircons from different parts of the massif differ sharply not only in the form but also in the color of their crystals. An increase in the amount of accessory minerals in the apical part of massifs is noted in the works of A. A. Konok and S. D. Turovskii [8], and V. N. Krivolutskaya [10]. V. V. Lyakhovich and A. D. Chervinskaya [14], studying the features of the vertical distribution of accessory minerals in the Eldzhurtin massif, came to the conclusion that in the vertical section of the massif the main mass of accessory minerals is distributed rather uniformly, without being concentrated in any noticeable quantities in its lower or apical parts. Fluctuations in the content of such accessory minerals as fluorite, tourmaline, as well as ore minerals, are explained by postmagmatic processes, which manifested themselves with varying intensity in different parts of the massif. A. I. Bazhenov [2, 3, 4] established that in the vertical section of the Elekmonar granitoid massif sphene is distributed unevenly, while the radioactivity of sphene and zircon increases regularly from the lower parts of the massif toward the apical ones.

Establishing the regularities in the vertical distribution of accessory minerals in an intrusion would in a number of cases make it possible to judge the depth of the erosional cut.

E. D. Semenova, using the example of the study of granites of the Verkh-Iset massif, came to the conclusion that in the endocontact facies there is more sphene and apatite than in the central parts of the massif. A similar regularity has also been established for magnetite, rutile, fluorite, tourmaline, and ilmenite [8, 12]. The radioactivity of accessory zircon from hybrid rocks developed in the endocontact parts of granites is higher than that of rocks developed in the central parts of the massif [3]. The opposite dependence of radioactivity has been noted for sphene from hybridized rocks of endocontact areas [4].

1. Studies by a number of authors have shown that accessory minerals are very sensitive to metasomatic processes [5, 15, 18, 23]. A. Kh. Khasanov and N. I. Krivoshchekova [33], using the example of the granitoids of the Southern Gissar, showed the sequence of manifestation in granitoids of postmagmatic metasomatic processes, which are reflected in the alteration, and sometimes abrupt replacement, of some accessory minerals by others. Thus, during the greisenization of granites, sphene changes into rutile according to the following scheme: sphene → leucoxene → brookite → rutile. V. K. Monich and G. N. Gogel [20] came to similar conclusions; they established that the accessory minerals of iron and titanium are very sensitive to metasomatic processes, and the products of their alteration may serve as a prospecting indicator for greisens.

2. Finally, V. A. Ermolaev [4] successfully used the morphology of accessory zircon not only for the correlation of the granitoids of Transbaikalia, but also for establishing the role and source of individual magmatic massifs in the feeding (formation) of zircon–ilmenite placers.

Thus, the examples cited above show how accessory minerals make it possible to solve practical problems. Their further study is necessary along the directions already outlined, with the application of more advanced methods for studying their composition and the laws of quantitative distribution in magmatic rocks with depth, in order to solve a number of

problems connected with the origin of the magma itself and its role in creating deposits of useful minerals and their distribution in the Earth’s crust.

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