CLASSIFICATION OF GOLD-BEARING ORE-MAGMATIC SYSTEMS OF THE GRANITOID SERIES

P. F. Ivankin, K. R. Rabinovich (SNIIGGiMS)

The present level of our knowledge about the forms of association between gold mineralization and magmatism has been attained thanks to the widely known works of W. Emmons, V. Lindgren, V. A. Obruchev, Yu. A. Bilibin, F. N. Shakhov, T. M. Dembo, A. Ya. Bulynnikov, P. N. Kropotkin, M. B. and N. I. Borodaevsky, and many others, in which the great diversity of geological settings for the formation of magmatogenic gold-ore deposits and the diversity of the very links between gold mineralization and magmas have been revealed. Analyzing the data of these studies and new materials, the authors have concluded that it is necessary to distinguish three series of gold-ore formations, associated respectively with derivative hyperbasitic, basaltoid, and granitoid magmas [1]. In each series, the observed spatial-structural and genetic relationships between magmatic rocks and ores are diverse; therefore, in metallogenic investigations and prospecting-exploration work it is necessary to distinguish types of these relationships. In the present communication an attempt is made to classify the various typical cases of the association of gold mineralization with magmatic rocks for the granitoid series of gold-ore formations. For this purpose the authors have generalized data on several deeply exposed gold-ore fields of Western Siberia, Kazakhstan, and the Urals, and have traced the relationships of ore and magmatic bodies in space from the present-day surface down to the maximum explored depths.

It is well known from practice that any potentially gold-bearing magmatic complex, or an isolated large pluton, is accompanied by commercial gold mineralization only in certain of its parts or nodes, and such parts are extremely small in comparison with the dimensions of the complex or pluton. This leads to the need to regard gold-ore fields as parts of a magmatic complex or of an individual pluton. At the same time, it is noteworthy that these parts of a potentially ore-bearing complex are always specific in composition, phase-facies features, relationship to the enclosing rocks of the framework, and other properties; this provides grounds for considering them as a certain, to some extent isolated system, and for finding the causes of the local separation of gold-bearing solutions in the distinctive development of this system, this specific segment.

of the magmatic complex. Such, in the final analysis, a petrological approach to the ore field makes it possible to make more concrete our ideas about the causes, time, and place of the generation of gold-bearing solutions, about the character of their separation from granitoid plutons and the paths of their migration, than do the usual research methods, which take into account the spatial-structural relations of mineralization to the magmatic body, the phases and stages of ore formation, and the time of mineralization relative to thermal metamorphism and the appearance of vein intrusions, establishing only the general connection between mineralization and magmatism. In this respect, the morphogenesis of the gold-ore field becomes the necessary link in research that ensures a solution to the problem of the space of the ore-magmatic process, proceeding under conditions of changing temperatures and pressures. Volumetric mapping of ore districts, in particular, makes it possible to identify the root parts of ore fields, the specific features of the phase-facies structure of the gold-bearing pluton in these parts of the granitoid complex, and to find the distinctive geological-petrological features of a given gold-bearing ore-magmatic system as compared with other non-ore-bearing parts of the same complex. Thus, we arrive at the concept of an ore-magmatic system and at the distinction of types of such systems by studying the phase-facies structure of ore-bearing plutons and by volumetric mapping of gold-ore fields associated with these plutons.

In the regions mentioned, the following types of gold-bearing ore-magmatic systems may be distinguished for granitoid batholiths of certain formations:

1. Aureoles of quartz veins of granitoid acmoliths (rootless).
2. Aureoles (and stockworks) of quartz veins with roots within diorite sill-laccoliths, formed in the do-batholithic stage.
3. Aureoles of mineralized skarns and hornfelses around diorite satellites (stocks and protrusions of batholiths).
4. Bundles of quartz veins and vein intrusions with roots in the endocontacts of “variegated” batholiths.
5. Bundles of quartz veins with roots within diorite stocks (diapir plutons).
6. Bundles of quartz veins and fully differentiated dikes with deep roots:
   a) situated in the deep parts of exposed batholiths, both “variegated” and homogeneous granodioritic;
   b) situated in the apical (?) parts of concealed batholiths.

The accompanying Table 1 summarizes the principal features of these systems and gives examples that have been studied. In the authors’ opinion, the six types of ore-magmatic systems distinguished do not exhaust the entire diversity of relations between gold mineralization and granitoid magmatism, but they characterize the principal forms of these relations. Analyzing the various features of the systems considered, one may make several general observations on the genesis of gold-ore deposits of the plutonic cycle. The separation of solutions from granitoid magmas and the formation of gold-bearing mineral masses take place during the stage of early magmatism and progressive metamorphism of rocks, during the period of formation of the batholithic bodies proper, and in the post-batholithic stage of the development of formations of “variegated” batholiths and granodiorites [2]. The most important features and conditions of granitoid magmatism that promote the accumulation of large quantities of gold in certain parts of the magmatic complex and its subsequent deposition in favorable structures are the following:

1) the hybrid character of palingenic magma, corresponding in composition to diorite or gabbro-diorite, during the magmatic replacement of various iron–magnesian sedimentary-volcanogenic and intrusive rocks—products of basaltic magmas of preceding cycles;

2) the completeness of differentiation of hybrid magmas, with the appearance of polar petrographic rock types (for example, gabbro and plagiogranite, diorite-porphyrite and syenite-porphyry, etc.);

3) significant upward displacement of hybrid magmas in the meso- and hypabyssal zones of the Earth’s crust, facilitating magma differentiation, the accumulation of mineralizers in the frontal parts of magmatic columns, separation at different levels, and concentrated movement of solutions; magmas that have not been displaced, even with high primary gold content, apparently do not create ore-magmatic systems;

4) assimilation of carbonate rocks by magmas, which is one of the causes of their differentiation;

5) interaction of mobile structural-tectonic elements with magmas during the period of their origin, displacement, and differentiation, which largely determines the development of the ore-magmatic system.

Various combinations of these factors, manifested under specific favorable geological-structural conditions, lead to the emergence of the distinguished gold-bearing ore-magmatic systems.

Further work on the typification and classification of gold-bearing systems, the deepening of our knowledge of the relationships between ores and magmas in typical systems, and the development of questions concerning their petrogeochemical vertical zonality, the regularities of formation, and the structural control of mineralization appear to be urgent above all for improving methods of prospecting and evaluation at depth of gold-ore deposits.

REFERENCES

1. Ivankin P. F., Rabinovich K. R. On two series of gold-ore formations and certain features of the relationship of gold mineralization to magmatism. Izv. Tomsk Polytechnic Institute, vol. 134, 1968.

2. Kuznetsov Yu. A. Main Types of Magmatic Formations. Moscow, Nedra Publishing House, 1964.

Types of Gold-Ore–Magmatic Systems of the Granitoid Series

Table 1