On False Intersections of Hydrothermal Ore Bodies by Dikes of Igneous Rocks
Abstract Generated abstract
This article examines the problem of determining age relationships between hydrothermal ore bodies and igneous dikes, focusing on cases where apparent dike intersections of ore bodies may be misleading. Drawing on examples from gold, polymetallic, lead-zinc, and pyrite deposits, especially in the Kuznetsk Alatau and other Soviet ore fields, it identifies diagnostic criteria for “false intersections,” including ore veinlets cutting dikes, hydrothermal alteration of dikes near ore bodies, absence of contact metamorphism in ores, and dike control of ore morphology. The paper argues that such relationships are most characteristic of basic-composition dikes emplaced before mineralization and can be explained by contrasts in strength, elasticity, porosity, and permeability between dikes and host rocks. It concludes that false intersections are more common than previously assumed and must be considered in reconstructing the sequence of magmatic and hydrothermal events.
Full Text
ON FALSE INTERSECTIONS OF HYDROTHERMAL ORE BODIES BY DIKES OF IGNEOUS ROCKS
V. I. Bazhenov
(Presented by the scientific seminar of the Department of MGPI)
As is known, an important element in the structure of the ore fields of many endogenous deposits is dikes of igneous rocks of various composition. Often they play an important ore-controlling role, an example of which may be, for instance, the dikes of the Berezovskoe gold-ore deposit in the Urals. In many other cases the ore-controlling role of dikes has not manifested itself so clearly. Therefore, the correct resolution of the question of the age relationships between dikes and ores acquires enormous importance. It makes it possible to reconstruct the general sequence of geological events during the formation of ore deposits and to establish the time of emplacement of the dikes. Along with this, solving the indicated question also has exceptionally important theoretical significance. It makes it possible to reconstruct the process of development of magmatism and to show the place of hydrothermal mineralization in the course of the formation of intrusive massifs, and to help in the study of the forms of genetic connection between mineralization and magmatic activity, etc.
Without dwelling on general theoretical questions concerning the relationships of dikes and ores and the place of dikes in the general postmagmatic process, the present article proposes to consider certain features of the study of age relationships between dikes and hydrothermal mineralization.
There is a widely held opinion that a fully reliable criterion for resolving the question of the relative age of dikes and ores is the intersection of some mineral formations by others. Thus, for example, the intersection of dikes by ores indicates the pre-ore age of the dikes, while the opposite relationships, i.e., the intersection of ores by dikes, according to such views, speak in favor of the post-ore age of the dikes. In the first case, the pre-ore age of the dikes raises no doubts, and such a structural criterion proves entirely reliable. In the second case, however, i.e., when ores are intersected by dikes, the matter is considerably more complicated. In some hydrothermal deposits such intersections turn out to be false, distorting the true relationships between dikes and mineralization. Therefore, the presence of intersections of ore bodies by dikes of igneous rocks cannot serve as indisputable proof of the post-ore age of the dikes, and the question of their relative age requires more detailed investigations. The possibility of the formation of false intersections was first established by F. I. Wolfson (1955) for the relationships of ores with dikes of basic composition.
D. V. Lewis (Lewis, 1955) noted that incompetent dikes may play the role of barriers to mineralization and be mistakenly taken for post-ore formations.
Description of examples of false intersections of dikes and ores
By a false intersection we understand such relationships between dikes of igneous rocks and hydrothermal ore bodies in which visible intersections of ores by pre-ore dikes are produced. In this case their age relationships are represented in reverse form, i.e., the conclusion is drawn that the dike formations are post-ore.
As is known, the emplacement of vein rocks associated with one or another magmatic complex takes place in two stages. The dikes of the first stage include dikes of acidic composition: aplites, pegmatites, vein granites, and others. The dikes of the second stage include granite porphyries, diabases, diorite and diabase porphyrites, lamprophyres, and others.
A review of the literature and our own observations in the gold-ore fields of Western Siberia show that dikes of the first stage do not create false intersections with ore bodies. In deposits of gold, copper, polymetallic, and lead-zinc ore formations, they are usually pre-ore and show normal intersections with ore veins.
All known cases of false intersections are characteristic exclusively of dikes of basic composition belonging to the second stage.
Such relationships prove to be extremely characteristic of the gold-ore deposits of the Kuznetsk Alatau.
In the Sarala ore field, quartz veins occur within a complex effusive-sedimentary sequence. Numerous dikes of diabases and diabase porphyrites intersect the ore bodies at various angles. The presence of such visible intersections gave grounds for considering the dikes to be post-ore formations (Bulybnikov, 1929, 1948; Novoselov, 1939; Usov, 1917). A detailed study of the relationships between dikes and quartz veins made it possible to establish that most of the more numerous and more massive dikes, in terms of their relative age, are pre-ore, and their intersections of quartz veins are false intersections (Bazhenov, 1960). Later generations of dikes are intra-ore or post-ore. In the case of false intersections, the boundaries of pre-ore dikes with quartz veins are even. Only occasionally are splitting and replacement of the dike by vein quartz observed. In this case their contacts acquire extremely sinuous, whimsical outlines. Unreplaced relics of dike rocks are preserved within the quartz vein itself. In some pre-ore dikes, along the continuation of quartz veins, quartz veinlets are observed, often with sulfide mineralization. Hydrothermal alterations are observed in the dikes, entirely analogous to the wall-rock alterations of the enclosing rocks in contact with quartz veins. Alterations in the dikes are confined to those areas that adjoin the quartz veins or are situated on their continuation. Sometimes a “spreading” of the quartz veins along contacts with dikes is observed. A characteristic feature of false intersections is the absence of manifestations of contact metamorphism of the ores in the exocontact of the dikes.
In the Central ore field, gold-bearing quartz veins occur among intrusive rocks of the Tsentralninsky granodiorite massif. The vein rocks are represented by dikes of spessartites, microdiorites, and diorite porphyrites. The presence of visible intersections of veins by dikes served as the basis for ideas about the post-ore age—
dike expansion (Bulynnikov, 1948; Timofeevsky). Later studies showed that these intersections are false, and the age of the dikes came to be determined as pre-ore (Rozanova, 1959). This is supported by the presence of intersections of the dikes by quartz veinlets or, more rarely, by veins. Hydrothermal alteration of the host rocks of the veins is often manifested, and is spatially confined to areas adjoining the quartz veins. As quartz veins approach a dike, they not infrequently markedly decrease in thickness, even to the point of complete wedging out. Sometimes a “spreading” of the vein along the contacts of the dike is established.
Analogous relationships are established between dikes and quartz veins in the Komsomolsk and Berikul ore fields. The principal arguments in favor of a pre-ore age of the dikes are intersections, in some areas, of the dikes by veins or veinlets of vein quartz, and the widespread development of hydrothermal alterations of the dikes near quartz veins.
Examples of false intersections are also known in the literature for a whole series of other deposits of various metals.
Yu. A. Averin (1960) notes the presence of false intersections for the Chadak gold-ore deposit, where such relationships are produced by gold-bearing quartz veins and dikes of diabasic porphyrites of the first generation. Hydrothermal alterations of the dikes, the presence of pockets and thin veinlets of ore and skarn minerals in the dikes, locally observed weak skarnization of the dikes, and the frequently encountered shielding of rich gold mineralization by dikes speak in favor of a pre-ore and pre-skarn age of the dikes.
S. D. Turovsky (1951, 1959) and I. K. Davletov (1957) describe cases of false intersections of porphyrite and lamprophyre dikes with lead-zinc ores in the deposits of Northern Kirghizia. A large number of features convincingly testify in favor of a pre-ore age. S. D. Turovsky (1959) considers the cause of the formation of false intersections to be the chemical inertness of the porphyrites, their high plasticity and viscosity.
A. A. Garmash, G. A. Safonov, and Z. V. Sidorenko (1956), as well as M. P. Astaf’ev, V. P. Dmitriev, and A. V. Faleichik (1958), in describing the relationships of crosscutting microdiabase and diabase dikes with polymetallic ores in the Zmeinogorsk district of Rudny Altai, note cases in which ore stringers intersect dikes in individual areas, mineralization is controlled by the contacts of diabase dikes, and hydrothermal alteration of the vein rocks appears near ore bodies, etc.
Analogous relationships are established by A. I. Schmidt (1959) in ore occurrences of the Southern Kurosan in the Southern Urals. Here false intersections of dikes of basic composition with pyrite mineralization are noted. Arguments in favor of such an assumption are the facts of veinlets “following along” dikes, the spreading of veinlets along dike contacts, and intersections of basic dikes by veinlets.
The material presented shows that false intersections are not a rare exception, but represent a rather widespread type of relationship between dikes and hydrothermal mineralization. At present, on the basis of an analysis of factual material, an attempt can be made to formulate certain criteria for recognizing them.
- The presence, in individual areas, of intersections of dikes by ore veins or veinlets composed of the same mineral associations as the ores. Not infrequently, disseminations and small veinlets of sulfides in a dike may testify in favor of a pre-ore age. However, in the latter case it is necessary to exercise a certain
caution, since ore mineralization may be genetically connected with gas-aqueous solutions—derivatives of the mafic magma that formed the vein rocks—or with solutions that arose as a result of secondary mobilization of material.
Of great importance for recognizing false intersections is the spatial distribution of veinlets and ore disseminations in dikes that are spatially associated with the areas where dikes intersect ore bodies.
It is interesting to note that sometimes a gradual transition is established from false intersections of ores by dikes to normal intersections of the same dikes by quartz veins. It is not uncommon to observe that the contours of a pre-ore dike are noticeably corroded by ore, with the formation of irregular, strongly corroded boundaries. Cases of shielding of ores and quartz veinlets by dikes, and the “flowing” of ores along their contacts, also argue in favor of the pre-ore age of the dikes.
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Manifestations of hydrothermal alteration of dikes in the areas where they intersect ore bodies. It is important to emphasize that the character of near-ore alterations of vein rocks fully corresponds to their general trend in the enclosing rocks. When studying hydrothermal processes in dikes, it is also necessary to keep in mind the possibility of the manifestation of autometamorphism.
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Alterations of ores in the exocontacts of dikes are extremely characteristic of their post-ore varieties. In this case, distinct contact metamorphism is manifested in the ores. Thus, for example, in the moderately sulfidic veins of Sarala, in contact with post-ore dikes of mafic composition, dissociation of pyrite is manifested with the formation of pyrrhotite, decomposition of pale ores with the formation of arsenopyrite and chalcopyrite (Novoselov, 1939). Here, recrystallization of sphalerite, galena, and chalcopyrite with the formation of granoblastic aggregates is also noted. At the Butte deposit, in contact with post-ore rhyolite dikes, newly formed chalcopyrite at the expense of chalcocite and pyrite is noted (Sales, Meyer, 1951). The formation of magnetite at the expense of pyrite decomposition is also noted (Salop, 1954), along with a number of other features. In any case, it may be considered that in the exocontacts of post-ore dikes a distinct metamorphism is established, accompanied by a change in the mineralogical composition, the structural-textural features of the ore, and a change in the sequence of mineral formation in altered and unaltered ores.
It is quite natural that the absence of contact metamorphism of ores in contact with dikes argues in favor of their pre-ore age.
- A characteristic feature of many false intersections is the influence of pre-ore dikes on the morphology of ore bodies, as well as on the distribution of gold and various mineral associations. It is often noted that, on both sides of a pre-ore dike in its false intersection of ore bodies, the latter are characterized by different thickness, metal content, and distribution of mineral associations. It is often established that significant accumulations and enriched areas with increased vein thickness are located only at one contact of the dike, whereas on its other side the quartz vein is represented by a thin, barren quartz veinlet bearing no mineralization. Such a picture was often noted in the Sarala ore field. Evidently, pre-ore dikes played the role of a screen for ore-bearing solutions. The same regularity was also established by Yu. A. Averin (1960) for the Chadak gold-ore deposit.
This picture can be regarded only as indirect evidence of false intersections, and, in combination with other features, it serves as an argument in favor of the pre-ore age of the dikes.
Some Physicomechanical Properties of Rocks and the Formation of False Intersections
We have already noted that there is no single opinion on the question of the formation of false intersections. Thus, Yu. A. Averin (1960) explains them by the high plasticity of vein rocks, while S. D. Turovskii (1959) believes that an important role here is played not only by the physicomechanical properties of rocks (plasticity, viscosity), but also by the chemical inertness of porphyrites. However, it is difficult to speak of the chemical inertness of porphyrites. As O. P. Gorkovoi and I. M. Mirkhodzhiev (1960) noted, in many deposits of Central Asia it is precisely porphyrite dikes that are the most favorable medium for ore deposition.
Let us consider certain physicomechanical properties of rocks which, in our opinion, could explain the conditions and causes of the formation of false intersections. These properties are given in Table 1.
From an analysis of the data presented, it is not difficult to conclude that dikes of basic composition in the deposits noted are distinguished by a considerably lower effective porosity in comparison with the enclosing rocks. The difference reaches 200–500%.
We do not have data on other physicomechanical properties of rocks in the ore fields noted. However, the order of these values may be judged from the data of B. P. Belikov (1961), which are given by him for other regions of the Soviet Union.
| $\sigma$ comp. | Modulus of elasticity, $E\ 10^5\ \mathrm{kg/cm^2}$ | $\mu$ | |
|---|---|---|---|
| Granodiorite | 2530 | 5.81 | |
| Granodiorite | 2500 | 6.62 | 0.23 |
| Diabase | 3090 | 10.02 | 0.26 |
| Diabase | 3300 | 12.91 |
Table 1
Data on the effective porosity of vein rocks and the rocks enclosing them
| Deposit | Effective porosity, % | Note |
|---|---|---|
| Central deposit. Granodiorite | 0.59–0.91 | after N. A. Rozanova (1963) |
| Central deposit. Spessartites, microdiorites | 0.3–0.5 | ibid. |
| Central deposit. Granodiorites | 0.449–1.845 | after V. I. Bazhenov |
| Central deposit. Spessartites, microdiorites | 0.161–0.389 | ibid. |
| Komsomol deposit. Diorites | 0.277–0.691 | ibid. |
| Komsomol deposit. Diabases | 0.020–0.372 | ibid. |
| Andreevskoe deposit. Saralin ore field, effusive rocks | up to 2.5 | after N. A. Rozanova (1963) |
| Andreevskoe deposit. Saralin ore field, diabases | 0.8–1.3 | ibid. |
These quantities also indicate that there is a substantial difference between intrusive and vein rocks in their strength and elasticity.
The data presented make it possible to explain the formation of false intersections of ore bodies by dikes of melanocratic rocks from the standpoint of their physicomechanical properties. Under comparatively weak tectonic stresses, cases are quite possible in which their magnitude proves sufficient for the formation of fractures in the surrounding rocks, but insufficient for the destruction of vein rocks, as a result of which the dikes turn out to be either completely undisturbed or only slightly disturbed in comparison with the surrounding rocks. Differences among rocks in the magnitude of effective porosity also play an enormous role in the formation of false intersections. Naturally, the most permeable to the movement of hydrothermal solutions are rocks possessing the highest value of effective porosity, whereas vein rocks prove to be extremely weakly permeable to solutions. As a result, the vein rocks are not captured by hydrothermal processes and are not subjected to mineralization, causing the appearance of false intersections and, not infrequently, shielding the mineralization.
Fig. 1. Stages in the formation of false intersections: a — formation of a nonmineralized ore-hosting fracture; b — displacement of the nonmineralized fracture along a transverse disjunctive; c — intrusion of a pre-ore dike; d — formation of the ore body.
The possibility is also not excluded of the formation of false intersections as a result of a definite sequence of geological events, which is shown in Fig. 1. In this case, the inception and formation of ore-hosting fractures take place in the pre-dike stage of development of the structure of the ore field. In subsequent stages, this nonmineralized fracture is cut and displaced by cutting ruptures, along which dikes of igneous rocks are intruded. Hydrothermal mineralization affects the segments of fractures located on both sides of the pre-ore dike. Owing to the different permeability of the massive dikes and the fractured surrounding rocks, the mineralization does not affect the dike, thus giving rise to the impression that the dikes are of younger age.
Conclusion
In the preceding sections we have described examples of false intersections of ore bodies by dikes of igneous rocks for deposits
various ore formations belonging to different genetic types. A characteristic feature is the circumstance that false intersections are inherent only in the relationships of hydrothermal ore bodies with dikes of basic-composition rocks occurring among less competent rocks. In none of the cases studied do acidic-composition dikes create false intersections. Nor are there any indications in the literature of their presence. Apparently, this is explained by the fact that these dikes are distinguished by lower strength in comparison with basic-composition dikes and by a higher value of effective porosity. These characteristics prove to be quite commensurate with the characteristics of the surrounding country rocks (granites, granodiorites, diorites, effusives, etc.), as a result of which favorable preconditions are not created for the formation of false intersections.
Thus, it may be concluded that false intersections are not a rare exception. They are quite regular and, apparently, more widespread than has hitherto been assumed, manifestations of the relationships of vein rocks with hydrothermal mineralization. They are expressed the more clearly, the greater the difference in the physicomechanical properties of the vein rocks and the rocks enclosing them.
LITERATURE
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