Morphology, Sizes, and Zonation of Magmatogenic Gold Ore Fields of the Root Type

P. F. Ivankin, V. A. Bulynnikov, K. R. Rabinovich (SNIIGGiMS)

On the basis of the nature of the relation of mineralization to the parent rocks, ore fields of halo and root types may be distinguished [1]. The present article considers the principal regularities in the morphology and vertical zonation of large and economically important fields of the root type. Within such fields, systems of hydrothermal bodies (veins, zones, stockworks) and vein intrusions form strongly branching ore–magmatic clusters that expand upward. The formation of cluster systems is due mainly to the laws governing the evolution of deep fluids, which tend to expand and disperse as they penetrate from regions of high pressure into upper levels, where the external pressure decreases.

Ore bodies and small intrusions in some cases are grouped into a single ore–magmatic cluster, while in others they form several spatially separate vein nodes with independent roots at depth. On this basis, fields are subdivided into single-root and multi-root (composite) fields. In relation to ore-bearing magmatic complexes and according to the position of the roots in the parent rocks, among gold-ore fields one distinguishes: (a) subvolcanic fields, consisting of two groups, and (b) plutonic fields, consisting of five groups. A characterization of the distinguished groups of fields is given in Table 1.

Single-root fields of the plutonic cycle are usually situated in regional zones of folding or in major faults and have a flat, more rarely a flattened conical, form. Regardless of the character of the occurrence of mineralization (veins, vein zones, bodies of massive or disseminated sulfide ores), the ore bodies in these fields converge or pinch out with depth in such a way that their inferred continuations pass into a single structural node—the root area. The latter usually has a small area and is spatially drawn toward places where rupture structures intersect or are conjugated.

The comparatively large and few ore bodies and vein intrusions of these fields are strongly elongated along strike or dip, branch weakly upward, and are inclined either to one side or fanlike within the plane of the zone. The length of single-root fields along strike usually reaches several kilometers, and their width is 0.2–0.3 km or more. These elongated, but small in area, fields are often large and very large in terms of metal reserves, since

as ore bodies and mineralized zones, are well sustained along strike and dip, have large thicknesses, and are not infrequently traced to depths of 2–3 km. The total volume of the figures of the fields down to the level of the presumed roots reaches 1 km³ and more (Bakyrchik).

Multirooted subvolcanic and plutonic fields are characterized by considerable areas of ore distribution, often reaching 5–10 km² and more; a large number of ore bodies and dikes grouped into separate isolated clusters; and a diversity of forms of these vein clusters. They are formed at places where major faults or fracture zones intersect, causing the emergence of several convergent but hydrodynamically unconnected magmatic and ore-feeding channels. Therefore, in studying the morphogenesis of multirooted fields, the elementary ore–magmatic cluster must be taken as the unit of investigation.

Analysis of the morphological features of individual vein clusters of multirooted fields indicates that they are characterized predominantly by conical and flattened-conical types of figures. A system of vein bodies developing in complex tectonic frameworks within a relatively homogeneous geological environment acquires a conical form (the Central sector of the Central field, the Eastern and Western sectors of the Darasun field, Dzhetygara, and others). Characteristic features are a comparatively uniform distribution of ore bodies over the entire area of the cluster, their differing spatial orientations, and an isometric or nearly isometric form. In those cases where mineralization develops in lithologically heterogeneous complexes, at rock contacts, or in shear zones where disturbances of one direction predominate, the form of the clusters is more often flattened-conical. Normal sections of these clusters at the upper levels have an irregular form elongated in the direction of the prevailing strike of the ore-hosting structures (the Olkhovskoe and Konstantinovskoe fields, zones I–III of Balei, Kommunar, and others). With depth, the horizontal extent of the vein systems decreases sharply, and the form of their sections at lower levels often becomes isometric. The flat type of figures in multirooted fields is characteristic mainly of ore veins located at a distance from the other systems of the field (Vasilyevskaya vein—1 of the Vasilyevskoe field), or of systems of veins developing in extended rupture disturbances and having an en echelon structure (the Oktyabrsky sector of the Central field).

The dimensions of multirooted fields vary within very considerable limits and, on the whole, increase from relatively shallow-rooted fields (Balei, Kommunar) to deep-rooted ones (Darasun, Bestyube, Central, Kochkar). Thus, the dimensions of normal sections of the studied vein clusters of conical, flattened-conical, and flat form at the upper levels vary as follows: length—from hundreds of meters to 2–2.5 km; width—from 100 m to 1–1.2 km. The areas of ore distribution accordingly vary from 0.03–0.1 km² to 1.0–1.5 km². At the same time, the ratio of the length of the indicated sections of the clusters to their width in conical figures does not reach 3, in flattened-conical figures it varies from 3 to 10, and in flat figures it exceeds 10.

The vertical extent of the fields studied (from the erosional surface to the level of the presumed roots) also varies within wide limits (Table 1). In the group of subvolcanic fields, the depth of distribution of gold mineralization varies from 0.4–0.6 km (Balei type) to 1.5 km and more in fields having a paragenetic connection with subvolcanic intrusions (Darasun type).

Table 1

Characteristics of magmatogenic gold-ore fields of the root type according to morphometric data

In fields of the plutonic cycle, the vertical extent of mineralization ranges from 0.5 km (shallow-rooted fields) to 2.0 km and more (deep-rooted fields). The vertical extent is minimal in fields whose roots lie within pre-batholithic diorite intrusions (stockwork ores of Kommunar), and increases sharply in vein fields associated with post-batholithic diorite stocks (Bestyube and others) and dikes of small intrusions (Kochkar, Tsentralnoye). The coefficient of vertical extent in fields of conical and flattened-conical form varies from 0.3 to 2.5, but more often ranges from 1 to 2. In fields of the flat type this ratio usually exceeds 2.5–3. With depth, the linear dimensions of the normal sections of the bundles regularly decrease, which leads to a considerable reduction on the lower horizons in the areas of distribution of mineralization and in their volumes.

The volumes of the figures of multi-rooted fields eroded to different degrees vary from 0.01 to 0.2 km³ in shallow-rooted fields (Balei, Kommunar) and from 0.1 to 1 km³ and more in deep-rooted fields (Darasun, Bestyube, Tsentralnoye, Kochkar). In individual multi-rooted fields, the maximum extent is more characteristic of flat and flattened-conical bundles, while the maximum width is characteristic of conical ones. There is no substantial difference in the volumes of the principal ore bundles of different morphological types in these fields.

Fig. 1. Longitudinal projections of some gold-ore vein deposits.
I — Olkhovskoye, II — Medvezhye, III — Maslovskoye (Kommunar), IV — Razdolninskoye, V — Dzhetygara; structural-morphological zones: 1 — frontal, 2 — perifrontal, 3 — near-root.

Gold-ore fields of the root type are characterized by vertical structural–morphological zonation, which is manifested most clearly in weakly eroded and fully explored vein clusters. At the present stage of study the following zones are distinguished: frontal, near-frontal, near-root, and root zones (Fig. 1).

The frontal zone is the area of mass pinching-out of vein bodies and attenuation of the ore process along the rise of the ore-hosting structures. It is represented most fully in weakly eroded “blind” deposits. The zone is characterized by a broad manifestation of hydrothermally altered rocks and by the predominance of disseminated, relatively low-temperature veinlet–disseminated mineralization of the final stages of the ore process. Large ore bodies with elevated gold concentrations are few in number and develop chiefly in extended rupture disturbances. The thicknesses of the ore bodies and the gold content within them increase with depth. The amount of gold in the zone is small and amounts to less than 5% of the total amount of metal in the cluster. The extent of the frontal zone along the rise of the cluster varies from several tens of meters to 0.4–0.5 km. At the same time, the greatest lengths of zones are characteristic of fields of the planar type, whereas relatively small ones are characteristic of conical and flattened-conical fields with strongly branching ore veins or with a stockwork type of mineralization (Kommunar, Dzhetygara). In the case of clearly expressed screening of the mineralization, the extent of the zone is sharply reduced (the Maslovskoe deposit of Kommunar). Paragenetic small intrusions usually extend beyond the front of mineralization.

The near-frontal zone occupies the greatest part of the volume of the ore–magmatic cluster and includes the main mass of the mineralization. Ore bodies and vein intrusions are numerous and occupy maximum areas in the upper parts. The ore veins have a complex form, are strongly branched, and are often accompanied by apophyses. They are characterized by the broad development of productive complexes and stages of ore formation. Therefore the ores of the near-frontal zone are distinguished by a great diversity of their constituent mineral components and often have high gold contents. The latter is often localized in the upper parts of ore veins in the form of ore shoots varying in shape and size. A considerable part of the gold is also concentrated in numerous small vein bodies and mineralized zones developed in the upper parts of the zone. With depth, the number of ore bodies, the volume of ore, and the average gold content decrease. The extent of the near-frontal zone along the rise of the cluster varies from several hundreds of meters to 1–1.5 km and more.

In the near-root zone there is a sharp reduction in the areas of distribution of mineralization, in the number of ore bodies, and in their average gold content. The vein bodies are spatially brought together and are localized in relatively large rupture structures. They have a simple structure and in the lower parts of the zone begin to pinch out. Apophyses and small veins almost completely disappear. The mineralogical composition of the ores becomes uniform and relatively simple, since with depth there is a reduction in the productive generations of quartz and in the stages of mineralization. Relatively high-temperature minerals are predominantly developed. The pinching-out of mineralization with depth occurs either through a reduction in the length and thickness of the ore bodies (Dzhetygara), or through replacement of the vein bodies down the dip (inclination) by stockworks (Dzhelambet). The extent of the near-root zone along the rise of the cluster may reach 0.5 km and more. The ore bodies of the zone are often insufficiently explored at depth. In these

cases its total extent is determined approximately—from the morphology of the entire vein bundle.

The root zone usually remains unopened by exploration workings. It is assumed that in this area all vein bodies, including the trunk bodies, completely pinch out.

In the gold-ore fields studied, the extent along the rise, the volume, and the ore content of the identified structural-morphological zones vary within wide limits. However, the proportional relationships of these parameters for different fields prove to be close and may be compared even between fields of deep-rooted and shallow-rooted types (Table 2). A preliminary analysis of the vertical structural-morphological

Table 2

zonation of some gold-ore fields of the subvolcanic cycle (Balei, Cripple Creek) indicates a considerable compression of their frontal and near-frontal zones and a great elongation of the root zone in comparison with plutonic fields.

References

1. P. F. Ivankin, K. R. Rabinovich, V. A. Bulyinnikov. Experience in grouping magmatogenic gold-ore fields by morphogenetic types. In: Geology and Mineral Resources of Transbaikalia, Chita, 1967.