Criteria for Forecasting Gold Ore Deposits of the Post-Geosynclinal Activation Stage

Based on the Example of the Kuznetsk Alatau and Central Transbaikalia

B. D. Vasil’ev, G. V. Shubin

(Presented by Professor A. M. Kuz’min)

The main epoch of gold mineralization in the Kuznetsk Alatau and Central Transbaikalia coincides with the stage of tectonic activation of the region in the post-geosynclinal stage of development and is characterized by the formation of similar types of gold-ore formations regularly succeeding one another in space. It is precisely this, despite the sharply different ages of the main epochs of gold mineralization in such distant regions, that makes it possible to formulate general criteria for forecasting gold ore deposits arising at a definite stage in the development of a tectonic structure and regularly fitting into that structure. In the tectonic structure of each of the regions under consideration, two structural levels are clearly distinguished: a lower, geosynclinal-folded level with batholithic granitoids (the Martaiga complex of the Kuznetsk Alatau and the Kyra complex of Central Transbaikalia), and an upper transitional structural level (the volcanogenic-sedimentary Devonian of the Kuznetsk Alatau and the volcanogenic-sedimentary Mesozoic of Central Transbaikalia). The development of the structure of the upper level is associated with the formation of post-batholithic gold-bearing intrusive bodies (the Kiya gabbro-syenite complex of the Kuznetsk Alatau and intrusive bodies of the volcano-plutonic formation of Central Transbaikalia).

The tectonic structure of the upper level forms against the background of a developing extensive domal uplift, with the formation of large longitudinal and smaller transverse zones of deep-seated origin, creating the typical horst–graben structure of the level. In the present-day erosional section, within the folded-block structure of these regions, narrow (10–12 km) elongated grabens are clearly distinguished, in which the upper structural complex has been preserved from denudation, and broader (40–60 km) horsts, where erosion has exposed the lower structural complex. Deep faults bounding the large blocks (the margins of grabens, etc.), as a rule, are not ore-controlling. Ore-controlling zones in the present-day erosional section occur chiefly in uplifted, considerably eroded blocks (horsts), are for the most part transverse relative to the strike of the grabens, and control magmatism in plutonic form that preceded mineralization. Most often, in such zones, gold mineralization is spatially combined with stocks of rocks of increased basicity and alkalinity or with fields of development of dikes of basic composition.

Within deep-seated ore-controlling zones there is a distinct longitudinal zonality in the distribution of different types of mineralization and ore-localizing structures. Thus, in the Kuznetsk Alatau, with increasing distance from areas composed of Devonian rocks, and with the transition along the zone to areas with a greater erosional truncation of the crushing zone, polymetallic ores are replaced by irregularly shaped metasomatic quartz–carbonate bodies with gold–bismuth mineralization, and then by systems of quartz veins with gold–arsenic mineralization. In Central Transbaikalia, with increasing distance from depression structures along sublatitudinal zones of increased fracturing, a change of different types of gold-ore mineralization is observed. Thus, in the Tura–Il’in district, near-surface gold–silver mineralization is replaced by stockwork-disseminated mineralization of the gold–arsenic formation; next come zones of gold–bismuth metasomatic mineralization, which are replaced by typical vein formations. In the Verkhne-Onon district, gold–silver mineralization, through quartz–bismuth mineralization, is replaced by gold–quartz–sulfide mineralization. The existing horizontal zonality is a reflection, at the present erosional level, of the vertical (depth) zonality of deposition and depends on the depth of formation of the deposits relative to the day surface during the epoch of mineralization, i.e., on the position of the roof of the volcanogenic Devonian in the Kuznetsk Alatau and of the roof of the volcanogenic Mesozoic in Central Transbaikalia. Since the position of the roof is always reconstructed with greater difficulty than the position of the footwall of the transitional structural complex, in metallogenic studies it is possible to reconstruct the position of the footwall and determine the depth of occurrence of deposits relative to the footwall of this structural complex. Thus, deposits of the gold–arsenic formation are localized in the form of persistent suites of quartz veins at depths of 1.5–2.5 km below the footwall of the volcanogenic complex, whereas deposits of the gold–bismuth formation in the form of metasomatic bodies occur, respectively, at depths of only 0.8–1.0 km below this footwall. In the Kuznetsk Alatau, gold mineralization is replaced upward by polymetallic mineralization with fluorite, localized in the form of thick crushing zones cemented mainly by carbonates and cutting not only the underlying Devonian but also the Devonian formations proper. Similar relationships of fluorite–polymetallic mineralization with Jurassic–Cretaceous sequences may also be noted for some districts of Transbaikalia.

It should be noted that the more intense the folded structure of the transitional complex, the smaller the interval in the change of mineralization types along the zone controlling the mineralization. In the Kuznetsk Alatau, as an area of more rigid folding of the transitional structural complex, the dips are steeper, and therefore the horizontal zonality is clearly expressed in the change of different ore formations and ore-localizing structures over a relatively short distance (an interval of 5–6 km). In Central Transbaikalia the folds of the upper level are gentler, and the horizontal zonality is expressed less clearly: the change of mineral associations here is to a considerable degree connected with the successive development of ore-localizing structures and is complicated by pulsational zonality.

Along the zone between nodes with different types of mineralization, there are areas with a transitional character in the distribution of ore and associated elements in noncommercial concentrations (geochemical background).

The nodal distribution of deposits (and ore occurrences) of different types along the zone and in the vertical section is determined by the depth of formation (deposition) of the given type of mineralization, correspond-

to the critical thermodynamic state of the ore-bearing solution (chiefly pressure) and to the nature of the development of ore-localizing structures (vertical structural zoning).

In its movement toward the surface, the ore-bearing solution passes through several critical levels, each of which corresponds to a definite type of mineralization and of ore-localizing structures.

In connection with the dome-like character of the arch uplift and, correspondingly, the different depth of the erosional section in the center of the uplift and on the flanks, the zoning in the distribution of mineralization types is manifested not only along ore-controlling structures transverse to the long axis of the uplift, but also in the successive replacement, from the center of the uplift toward its flanks (along the long axis), of zones that are deeply eroded and characterized by a “deep” type of mineralization by zones that are less deeply eroded and therefore characterized by less “deep” types of mineralization; this is especially well expressed in the Verkhne-Onon district of Central Transbaikalia.

The regularities noted above in the distribution of different types of gold mineralization of the transitional stage of the geotectonic development of regions may be regarded as criteria for forecasting gold-ore deposits in the corresponding regions. The schematic program of forecasting work in accordance with this is as follows.

1. Identification and tracing of zones of deep-seated emplacement within uplifted blocks (horsts) by the presence of manifestations of magmatism of the epoch of post-geosynclinal activation in the form of stocks, dike fields of great extent, and subvolcanic bodies. Of particular interest are sectors of zones saturated with bodies of basic composition.

2. Reconstruction of the position of the roof (or at least the floor) of the sedimentary-volcanogenic transitional complex along the identified zone of deep-seated emplacement.

3. Identification of horizontal (longitudinal) zoning in the distribution of known ore occurrences and deposits of various types of mineralization.

4. Identification of vertical zoning and the depth of formation of various types of mineralization, taking into account the present erosional section.

5. Determination of the spacing of the change in mineralization types along a specific zone, taking into account the intensity of the folded structure of the transitional complex and the depth of dissection of the relief.

6. Delineation of areas of 1–2 km² that are prospective for mineralization of a definite type.

7. Testing of the delineated areas by metallometry for elements associated with gold is sufficiently effective and should precede detailed prospecting in covered areas. In open areas it is advisable to carry out geochemical investigations to reveal primary dispersion halos. In work on the prognostic evaluation of areas and deposits it should be borne in mind that, in addition to the regional zoning noted above, within ore fields there is a distinct vertical zoning in the succession of facies of hydrothermal alteration of the enclosing rocks, especially among volcanics at shallow depths of deposit formation. Above typical quartz-vein formations with gold-bismuth mineralization there is formed, successively:

1) a veinlet-metasomatic type of gold mineralization with intense silicification, albitization, and actinolitization;

2) a disseminated-stockwork type of mineralization with predominant sericitization, beresitization, and carbonatization;

3) a disseminated–veinlet type of mineralization with intense preceding siliceous metasomatism, with the development of chalcedony-like quartz, argillization, and adularization (up to the formation of massive metasomatic quartzites).

The vertical range in the succession of mineralization types is 100–150 m, so that, on the whole, the column is no less than 300–400 m vertically.

Thus, for a prognostic assessment at depth of particular ore fields, the identified regularities in the succession of types of metasomatic alteration of rocks and of the structurally and morphologically associated types of mineralization can be used.