Controls of Ore Localization
Structural Controls of Ore Localization
Structural controls on mineralization are evident in almost every type of ore deposit. Faults or other permeable features, either primary or superimposed, tap a supply of mineral-bearing fluid and allow it to migrate into a "trap" where it cools and precipitates the mineral content or, more commonly, where it has sufficient time to react with and replace receptive country rocks. It is therefore important to recognize different types of structtiral features which are present in rocks, and how the development of these structures can influence ore deposition either directly or indirectly. Natural forces, such as heat and pressure, can occur on any scale, large or small. The same forces can cause "deformation" of the rocks, which includes:
Nearly all hydrothermal deposits exhibit some degree of structural control on mineralization. Structures (fractures, faults or folds) which form prior to a mineralizing event are referred to as "pre-mineral" (Fig 1). Geologists are keenly interested in premineral structures because these structures influence the localization of ore by hydrothermal fluids utilizing these pathways. By mapping these structures and projecting the geometry in the subsurface, new ore deposits may be discovered. Structures which form after a mineralizing event, and hence may be responsible for offset or removal of mineralized zones, are referred to as "post-mineral". In some cases the formation of structures and mineralization appear to be nearly synchronous (Fig 2). In these situations, shearing was probably ongoing during the mineralization event. This is evidenced by ore minerals localized along a fault plane which are deformed.
Fractures and fault zones provide excellent pathways for hydrothermal fluids to circulate through. Open-space filling has long been recognized as the primary method of vein formation. The formation of breccia and gouge due to the grinding action of the rocks adjacent to the fault plane increases the 'structural porosity', which in turn increases the permeability. Under certain conditions, breccia or gouge may itself provide the host for mineralization. Intersections of structural features often are better locations to prospect for mineralization, especially where the structures are high angle. It is thought that the intersection of high angle structures provides pathways for fluids from deep sources to move closer to the surface.
In most epigenetic ore deposits, structures superimposed on the rocks have exerted a great influence upon the path of circulation followed by the orebearing fluids. Faults and folds are probably the most common secondary structures, though breccia zones, pipes, and other features are locally of great significance. Because fault surfaces are uneven, movement along a fault will produce breccia and gouge. A zone of fine-grained gouge will frequently hinder the circulation of fluids, either along or across a fault. On the other hand, coarse, clean breccia, containing a minimum of powdered rock material, results in a considerable increase in permeability, especially in brittle rocks that are fractured under light loads (Lovering, 1942). Accordingly, faults of minor displacement may be much better hosts for ore solutions than faults of large magnitude, which are more likely to develop gouge. As a general rule, then, tight fractures filled with gouge are less favorable places for ore deposition than the more open fractures.
Veins form along cracks or fissure zones in the earth's crust, and fault planes are especially favorable loci. They are either the simple filling of open .fissures or the replaced country rock along a narrow but permeable fracture.
Physico-chemical Controls of Ore Localization:
The movement of fluids under-ground is controlled by permeability, which in turn is a function of the physico-chemical characteristics of the rock plus the elements of superimposed structure.
The structures and textures that control ore deposition can best be described as primary or superimposed (secondary), according to whether they were formed at the same time as the rock mass or were formed later. In certain types of ore deposits, the primary controls are dominant; in others, superimposed features, such as faults, are the only basic controls
of ore deposition. Establishing a physical control and differentiating between the two types are fundamental problems in the exploitation of any mineralized district.
Primary structures and textures of both igneous and sedimentary rocks commonly control the distribution of ore-bearing fluids, and hence the localization of the ores. Any textural or structural feature that influences the porosity and permeability of a rock may control the deposition of ores, and as a result, the variety of primary controls is practically unlimited. A few of the most obvious primary structural controls are:
1. Permeable (elastic) limestone or dolomite, especially where it is dammed by impermeable cap rocks;
2. Well-sorted conglomerates that permit easy circulation of ore-bearing fluids; 3. broken and permeable tops of lava flows, which also permit ready circulation of ore bearing fluids;
3. Permeable sandstones, especially channel sands and beach deposits.
Dolomites and dolomitic limestones are ordinarily somewhat more permeable and porous than pure limestones, and for this reason dolomites permit mineralizing solutions to circulate more readily than do limestones. Hence many geologists believe that dolomite is more likely to be a host for ore.
Concentrations of ore are found in conglomerate beds between lava flows and in the fragmental, vesicular surface layers of individual flows. These regions have been favorable for ore deposition because of their extremely high permeabilities.
Permeable zones formed by channel deposits and sands interbedded with siltstones are often mineralized.
Chemical Controls of Ore Deposition:
The simple existence of a favorable structural environment, and even the presence in this structure of the ore-bearing fluids, does not necessarily mean that ore will be deposited. Ore-bearing fluids react continuously with the wall rocks, and they are constantly changing in character as well as changing the character of the material they traverse. Moreover, factors such as reductions in temperature and pressure may bring about chemical reactions or decrease solubilities and contribute to the deposition of ore minerals. In many places geologists are unable to explain why certain beds are mineralized and other beds, both above and below the ore horizon, are barren. These barren beds may have compositions and physical characteristics identical with the mineralized strata.
Even though we understand little about chemical controls, we can often demonstrate the existence of such controls.
Much of the ground preparation that takes place prior to the introduction and deposition of ores is really chemical. Silicification, dolomitization, and recrystallization are all chemical processes, and even much of the brecciated ground was made brittle by chemical reaction with earlier solutions.
One explanation advanced for the common localization of ores in carbonate rocks beneath relatively impermeable covers is that ascending orebearing fluids are impounded and forced to move laterally in the more permeable carbonate rocks. Since carbonates are permeable and are chemically favorable host rocks, the additional migration through them that results from forced lateral movement allows ample contact for chemical reaction to take place, which results in the precipitation of ore minerals. Carbonates are termed chemically reactive rocks because they break down readily in the presence of acids and because they are relatively soluble in water.
The effects of temperature and pressure are important in the deposition of ores. Solubilities of many substances increase in direct proportion to the temperature of the solution. As a result, cooling solutions will precipitate any materials whose saturation values have been exceeded and it is very probable that some ore bearing fluids travel away from the source and deposit the metals as soon as they reach a zone of reduced temperature, regardless of the type of host rock. A reduction in pressure may have the similar effect.
Solubility of an ore mineral may be dependent upon the concentration of dissolved volatiles, such as H2S or CO2; hence a reduction in pressure will allow these gases to leave the solution, causing a concomitant precipitation of the ore minerals. The importance of such a mechanism is debatable, but it is probably operative in open-fissure deposits. Solutions ascending through veins will naturally undergo a drop in pressure, and the deposition of ores may be dependent upon such a factor.
The stability of a solution may be determined by the conditions of pH and the oxidation potential of the environment, changes in which could cause precipitation of the dissolved material. The ability of an environment to supply or accept electrons will determine the valence state of any ion present, and the valence state may in turn determine whether the ion can remain in solution. For example, iron in simple solution is quite soluble in the ferrous state and nearly insoluble (except at low pH) when oxidized to the ferric state.
Stratigraphic Controls of Ore Localization:
Many ore deposits occur exclusively in a given stratigraphic horizon. This is particularly true of ores of sedimentary origin. Many epigenetic ores are also confined to particular strata. Such deposits are collectively referred to as stratabound deposits. Whereas such localization is quite obvious and understandable in the case of sedimentary ores, in the case of epigenetic ores it depends upon a multiplicity of factors.
The reason why one rock is more receptive to ore than another is not always evident. On theoretical grounds, two conditions would be expected to be favorable:
(a) Permeability, in order to allow passage solutions, and
(b) Chemical reactivity, in order to induce precipitation of ore-minerals.
The two conditions may be combined in the case of a soluble rock through which solutions can eat their own way by chemical reaction. Permeability may be either a primary property the rock, as in sandstones, conglomerates, or vesicular lava-tops, or may be imposed by fracturing or shearing. Whether physical properties (especially permeability) or chemical properties (especially reactivity will be the predominating influence is rarely predictable in advance of exploration. Thus, if a porphyry and a limestone occur together, ore may favor the porphyry, because of more open fractures in it, or it may favor the limestone, which it finds more hospitable chemically. Although limestones are normally very receptive to ore, there are cases in which the ore shuns them and deposits in rocks which might ordinarily be considered as poor hosts. Eg at Mount Isa (Queensand), the great lead-zinc deposits occur in shale while limestones in the region have not been mineralized.
There is some indication that certain host-rocks show a preference for specific metals. Limestone is especially hospitable to lead and zinc but relatively unreceptive to gold. Quartzite is also a good carrier of lead-zinc ores in some districts.
The rocks most receptive to gold seem to be those which contain chlorite or other minerals of similar composition, although chlorite in the immediate vicinity of the ore is often altered to sericite. There are more gold deposits in chloritic slates and phyllites and in basic to intermediate igneous rocks than in quartzites, rhyolites, or limestones.
Susceptibility to replacement is often a matter of delicate if not obscure control. Why replacement should, for example, single out certain beds within apparently uniform limestones is a question that has aroused much inquiry but has received no conclusive answer, at least none that applicable to the general case
It is probable, however, that the mineralogy and texture of the rock, though important, do not tell the whole story, and that the manner in which individual beds behaved during folding may hold part of the secret. Delicate differences in relative competence could control the manner in which individual beds are prepared to receive ore solutions.
Competent vs. Incompetent Formations:
In some districts, at least, competent rocks are more hospitable hosts to ore than incompetent ones, and surely this is what would be expected from their mode of failure in fracturing. "Competent" as the term is used here, refers to rocks that are relatively strong but, when they do fail, break as though they were brittle material. "Incompetent" refers to rocks which are weak and have a tendency to deform plastically or by flow. Under most conditions, quartzites, conglomerates, and fresh igneous rocks are competent. Incompetent are shales, slates, schists, and limestones; also igneous rocks that have been altered to sericite, chlorite or serpentine. These generalizations, however, are subject to some modifications with varying circumstances. In the first place, competence is a relative matter. A limestone behaves as a competent rock; an identical limestone between beds of quartzite is likely to behave incompetently. Furthermore, the manner of failure depends in some degree on the manner in which the rocks were deformed. A limestone under light load may behave as a competent rock, but under high confining pressure, especially in the presence of solvent and with the rate of deformation slow enough to give time for recrystallization, it may behave as a very incompetent material.
Competent rocks, in addition to their tendency to fail by fracture rather than by shear, have an advantage in that they yield to fracture such a way as to provide permeable channelways. Their strength tends to prevent fractures from squeezing shut, and, if they do succumb to failure adjacent to fracture-walls, it is by producing a jumble of fragments which presents large surfaces to ore depositing solutions. When competent rocks shatter, they produce either a network of interconnected cracks or a permeable breccia free from gouge. Furthermore, since the shearing angle decreases with increasing brittleness, a shear-fracture passing from semi-plastic into brittle rock is deflected toward the plane of maximum normal stress and therefore toward an attitude more favorable to opening by the movement that initiated the shear.
In spite of the superior mode of failure of competent rocks, it is not everywhere true that they are the most hospitable to ore deposition.
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