Influence of the Composition of the Host Rocks on the Character of Ore-Mineral Associations in Subvolcanic Gold and Silver Ore Occurrences of the Eastern USSR

Yu. S. Berman (TsNIGRI)

The formation of vein minerals in many ore bodies proceeds in close dependence on the composition of the host rocks. The preferential development of gold-bearing carbonate veins in intermediate and basic effusive rocks (Pepenvem), the occurrence of prehnite veinlets in quartz veins hosted by andesites and andesite-basalts, and their absence in rocks of more acidic composition (Valunistyi), the formation of adularia selvages predominantly in rocks of increased potassic content (dacites at Valunistyi), and the deposition of scheelite in gold-bearing quartz veins located in metamorphic rocks where they intersect actinolite schists all indicate that extensive chemical exchange takes place between hydrothermal solutions and the wall rocks.

One of the regular features of near-surface ore formation is the wide development of accompanying hydrothermal alterations, the character of which depends to a large extent on the composition of the initial rocks. Assigning decisive importance to this latter factor in combination with depth of formation, N. I. Nakovnik (1966) presents a scheme of vertical zoning of postmagmatic metasomatism as a function of increasing basicity of the educts. Secondary quartzites are thereby formed after acidic, and more rarely intermediate, effusives and their pyroclastics.

Hydrothermal alteration under near-surface conditions is characterized, as a rule, by a reduction with depth in the total zone of alteration and by the gradual disappearance of horizontal zoning. This is probably connected above all with an increase in the monolithic character of the rocks with depth, which is recorded by a decrease in the percentage of porosity (Naboko, 1963). It is not excluded that the reduction in the width of the zone at depth is caused by an increase in the aggressiveness of the solutions themselves. The alteration zones acquire a fissure-adjacent character, and in a number of cases the newly formed mineral associations ignore the original composition of the rocks. Thus, for example, at the Kavralyansk ore occurrences, various facies of secondary quartzites are widely developed areally in acidic effusives, whereas in the underlying andesites the secondary quartzites form narrow linear zones amid fields of propylitic alteration, marking the principal faults; that is, a blocking effect of the mineral associations that have arisen takes place: a kind of selvage is formed, hindering further chemical exchange with the surrounding rocks.

The character of an ore mineral association depends on many causes: the composition of the ore-forming solutions, the depth of ore deposition, distance from the parent magmatic center, the degree of permeability of the ore-hosting structures, and a number of other causes, among which the composition of the enclosing rocks is undoubtedly important. The latter factor acquires a significant role under conditions of near-surface ore formation. Therefore, in his genetic classification of epithermal deposits, W. Lindgren specifically noted gold-quartz veins in rhyolites and in andesites.

In subvolcanic ore occurrences and deposits of gold and silver in the eastern USSR, four ore mineral associations are clearly traceable, each of which, in general, corresponds to a stage of ore deposition. Usually, in individual ore bodies one of the ore mineral associations predominates, determining, within the present erosion level, the industrial value of the object. Therefore, the ore association also characterizes the mineral type of the ore occurrence. Accordingly, four mineral types are distinguished: gold-argentite (Valunistyi, Khakandzha); gold-pyrargyrite (Sopka Rudnaya, Pepenveem); gold-galena-chalcopyrite-sphalerite with variable sulfide ratios (Agatovskoe, Kavralyanskoe, Ogancha); gold-pyrite-arsenopyrite (gold-bearing veins of such composition occur within the ore fields of Pepenveem and Sopka Rudnaya).

To examine the distribution of mineral types as a function of the composition of the enclosing rocks, analyses of the latter, recalculated by the method of A. N. Zavaritskii (Fig. 1), were plotted on a diagram for 12 subvolcanic ore occurrences and deposits of gold and silver of the Northeast and Far East of the USSR. More than one third of the analyses are from the author’s collection (for 5 ore occurrences); the remainder were taken from the materials of L. N. Plyashkevich, A. A. Sidorov, I. M. Speranskaya, and others. In processing the analyses, two assumptions were made. Within the limits of ore fields, unaltered rocks were often absent, and in that case fresh rocks outside the ore field were taken as the educt, or weakly altered rocks were used, provided the alkali ratio was more or less normal in comparison with the corresponding rock types according to Daly.

A rather strict regional regularity is indicated, consisting in the fact that the first two types are localized only in effusives of acidic composition, sometimes with increased trachytoid character, whereas the last two mineral types are chiefly associated with intermediate and, more rarely, basic rocks, also often of increased alkalinity (Fig. 1). Such a distribution appears even in individual ore fields for bodies with different mineralization. Thus, at the Pepenveem ore occurrence, ore bodies with gold-galena-chalcopyrite-sphalerite mineralization are known in andesites, whereas the main gold-pyrargyrite mineralization is developed in liparites. At the Valunistyi ore occurrence, adularia-quartz veins with gold-argentite mineralization are localized in fluidal dacites, while weakly gold-bearing quartz veins with galena, sphalerite, and chalcopyrite are present in andesites and andesite-basalts.

The predominant development of properly silver mineral associations in acidic rocks, and of gold associations with sulfides in rocks of intermediate and basic composition, is possibly connected with partial borrowing of ore material from the enclosing rocks themselves, since Clarke contents of copper, arsenic, and zinc are somewhat higher in rocks of basic composition. On the other hand, a metamorphizing influence of the enclosing medium on the solutions is more probable. Gold-pyrite-

Fig. 1. Composition of the rocks hosting ore bodies with gold–argentite and gold–pyrargyrite mineralization (vectors plotted with a solid line) and gold–galena–chalcopyrite–sphalerite and gold–pyrite–arsenopyrite mineralization (vectors plotted with a dashed line)

arsenopyrite and gold–galena–chalcopyrite–sphalerite mineral associations, according to the values of the conventional ionization potentials of their constituent minerals, are located in the acidic and neutral fields, whereas the ore minerals composing the gold–pyrargyrite and gold–argentite mineral associations are acidophobic. Therefore, a change in the pH of the solution due to the acid–base characteristics of the host medium leads to the crystallization of particular mineral associations.

The determining influence of the host-rock factor is apparently characteristic only of sufficiently high levels of ore bodies, where the rocks themselves are more fractured and have greater porosity. The widely known change in ore mineralization with depth (usually from gold–silver to polymetallic with gold) is caused by the vertical zonality of deposition and, possibly, by stage zonality.

Thus, using the example of subvolcanic gold and silver deposits, it is possible to trace the evolution of forms of facies zonality: from zonality in the composition of the host rocks at the upper levels to zonality of deposition at deeper levels of ore bodies. If these considerations are correct, then in the future the correspondence between the character of the ore association and the composition of the host rocks, in terms of the observed regularity, may serve as an indicator for indirectly determining the depth of the erosion level.