BASIC DATA ON THE NATURE AND REGULARITIES OF GOLD MINERALIZATION WITHIN THE WESTERN PART OF THE IROKINDA-KINDIKAN ORE FIELD

V. A. RUBANOV

(Presented by the scientific seminar of the gold laboratory)

The Irokinda-Kindikan ore field is located in the central part of the South Muya Range of the Olekma-Vitim mountain region, within a large tectonic uplift of the Archean basement known as the South Muya block.

It is situated in immediate proximity to a major regional fault zone, called the Kilyan tectonic zone, which runs along the western margin of the South Muya block and separates the Archean rocks composing it from Proterozoic formations.

The geological structure of the ore field includes metamorphic rocks represented by various types of migmatites and gneisses containing thin interbeds of amphibolites, crystalline schists, and skarnized limestones. These rocks are assembled into large folds complicated by higher-order folding, and are cut by numerous ruptural disturbances, some of which are filled by igneous rocks of the dike facies and by gold-bearing quartz veins.

The gold-ore quartz veins have a rather complex morphology, characterized by swellings and constrictions of the ore bodies, the presence of apophyses, and are accompanied by thick zones of hydrothermally altered rocks.

The character of the sulfide mineralization accompanying the gold mineralization makes it possible to assign the quartz veins of the studied area to the formation of low-sulfide ores, considered by N. V. Petrovskaya as formations of comparatively great depths. Ore shoots in such veins, according to the same author, can be traced, with interruptions, for many hundreds of meters, which significantly increases the prospects of the veins being explored in the ore field.

The most productive and extended quartz veins occupy splay fractures of the Kilyan tectonic zone and occur in tectonic junctions and blocks formed by the conjunction of ore-hosting structures with ruptural tectonic disturbances of sublatitudinal strike and steep dip.

The most gold-enriched portions of quartz veins occur at the intersections of the latter with interbeds of garnet-pyroxene paragneisses, amphibolites, and skarnized limestones.

The regularities presented indicate the determining role of structural and lithological factors in the localization of gold mineralization.

Near-ore alterations are manifested in silicification, carbonatization, sericitization, and chloritization of the host rocks. Sulfidization, in the form of fine disseminations of pyrite, has a narrowly localized character and is usually established directly near the contact of the host rocks with quartz veins. In the spatial distribution of mineralogical neoformations arising during processes of ore metasomatism, a certain zonality is observed. The chemistry of the process of near-ore alterations indicates intensive reworking of the host rocks and a high degree of mobility of many primary components. The nature of migration of the principal rock-forming oxides and trace elements differs in different types of host rocks. Thus, in garnet-pyroxene paragneisses, import tendencies predominate in the migration of matter, characterizing these rocks as a favorable medium for the deposition of matter from hydrothermal solutions; in garnet-mica gneisses and migmatites, by contrast, removal tendencies predominate, indicating a less favorable medium for the precipitation of the components contained in the solution, which ultimately form the ore bodies.

It should be noted that the validity of these conclusions is confirmed by facts of lithological control revealed during geological mapping, which established that quartz veins in most cases occur in paragneisses, whereas, when they enter garnet-mica gneisses and migmatites, they markedly reduce their gold content and pinch out. This gives grounds to believe that one of the possible causes of the lithological control of mineralization within the Irokinda-Kindikan ore field is differences in the physicochemical properties of the rocks.

The genetic position of the deposits of the Irokinda-Kindikan ore field is not entirely clear; however, the presence of a large halo of near-ore alterations in the host rocks, the predominance of massive ore textures in the mineral formations of the veins, together with the almost complete absence of colloform zonal formations, the presence in the ores of temperature-indicator minerals (cubanite), and other features make it possible to assign the productive quartz veins of the ore field to hydrothermal deposits of the medium-temperature type.