by Andrei KARAKIN, Dr. Sc. (Phys. & Math.), All-Russia Research Institute of Geologic, Geophysical and Geochemical Systems
How do mineral deposits take body and form? If we get an answer to this question, we shall be able to make mineral prospecting much more reliable and cost effective. But this problem is still mooted.
Our planet has a fairly complex structure, with the inner and outer core in the center, and the mantle and the lithosphere above. The outer part of the lithosphere, the crust, is about 35 to 40 km thick in continental zones (which is thinner than an apple's skin by comparison); in its turn, the crust is broken down into lower, middle and upper regions. These are a medium where all the various fluids-gases, liquids (water, oil, solutions) and melts keep circulating.
Such substances may be of abysmal or surface origin, that is juvenile and meteoric, respectively. Geologists have been debating for decades which of these two prevail. It is important to know that so as to elucidate, among other things, the hydrocarbon formative mechanism. They, who content that juvenile fluids are predominant, say: petroleum and other liquids rise from the earth's interior. But their opponents claim: petroleum, the "black gold", comes from organic matter residues. Scientists differ about ore deposits, too. Most of them argue that ores have got into the crust from plutonic depths together with melts or have appeared as a result of action of surface waters in the process of deposition and redeposition of sedimentary rock. According to another viewpoint, the movement of aqueous fluids in the cool upper crust has contributed to the concentration of mineral compounds. We might as well add that many geoecological problems are related to this very phenomenon. For instance, toxic substances released during petroleum and gas mining pollute the surface layers of the earth crust and escape into the atmosphere. And moisture penetrating into minute cracks of hard rock erodes it and can even touch off an earthquake if fractures and fissures develop.
At first sight it seems easy to identify which of the above-named fluids is a dominant one if we compare
Articles in this rubric reflect the opinion of the author. - Ed .
Fluids migrating from a fracture into the environment: a) consolidation phase, b) dilatation phase.
their circulation intensity. The "victors" in this "competition" will be in a majority. The rate of juvenile fluids is calculated thus: the volumes of the hydrosphere (for liquids) and the atmosphere (for gases) is divided by the time that our planet has been in existence in its present form (ca. 3 billion years).
As to the meteoric fluids, we can learn about them, say, by the resurrection time for dead petroleum and gas deposits (dozens of years). Now, another example: thermal (heat) convection* in the fissures and pores of rock. Melts (intrusions), upon entering the crust, create a high temperature gradient (differential) causing convective motions. These can be identified by characteristic aureoles (haloes) on the ground surface near volcanoes and magmatic chambers. In the regions of mid-oceanic ridges, where the partly melted asthenosphere** comes closest to the ocean floor, the effects of the water's convection are likewise observed. Its pulsations break through into oceanic water at large. Looking through a bathyscaph's portholes we can see white and black hot streams rising from the bottom, the "smokers" (their color depends on minerals dissolved in water); this process occurs in consequence of the turbulent (random) mode of heat convection***.
Estimates show: the intensity of thermal convection (and of other meteoric processes as well) is thought to be three orders as high as that of juvenile processes, as calculated by the method described above. However, there is some discrepancy in the rates of juvenile and meteoric fluids, and at this stage we cannot reduce them to a common denomi-
* Heat (thermal) convection - transfer of macroscopic particles in a medium under the effect of temperature differentials resulting in the transport of heat and substances dissolved in water. - Ed.
** Asthenosphere - a weaker stratum in the upper mantle, the underside of the lithosphere. The asthenosphere-in some of its parts anyway-is thought to be partly molten. - Ed.
*** See: A. Lisitsyn, A. Sagalevich, "Breakthrough Discovery of the Century", Science in Russia, No. 1, 2001; - Ed.
nator, i.e. obtain median velocity rates by direct instrumental methods. On the other hand, the available indirect methods are not reliable and cogent enough and thus should be upgraded. That is why this problem is still around. Let us consider one of the possible approaches toward its solution.
Earth studies involve a good many methods in different fields whereby a subject of inquiry is viewed as a whole. Superdeep wells have revolutionized the knowledge on the structure of the upper mantle and have also shown that we know all too little about what literally lies under our feet. However, the number of wells probing into the upper crust is all too small; and technical difficulties do not enable us to reach deeper than 12 km either.
The scale-related factor, too, should be considered: lab data on samples may not reflect the true situation over long stretches. All the more so, such data can hardly apply to the geologic time of respective processes.
The indirect methods of upper crust studies are concerned with all related phenomena under different physical conditions over definite periods of the geologic history of the earth with the focus on most intensive processes (in fact, not many at all). It is all-important to get to know the possible consequences thereof. Although this approach cannot guarantee absolute reliability, it comes closer to the actual truth.
The object of our inquiry is the upper crust, a 10 - 15 km stratum otherwise known as ore-deposition sphere contain that it does all the available mineral deposits. The rock, very weak here, is destroyed even by minor shear forces. This is why the upper (and partially, the middle) crust abounds in numerous fissures and fractures of different dimensions. Listric (curved) fractures represent a peculiar type. They go from the terrestrial surface nearly vertically at first, then spread horizontally, and reach occasionally into the jointy (fractured) layers. The latter are remarkable for lower seismic rates and enhanced electric conductivity. Seismic waves reflected from the upper and lower boundaries of a stratum like that may travel in it over long distances without attenuating (like in a waveguide).
Analogous structures in the ocean let through acoustic waves, and radio waves in the atmosphere (both are used for long-range communication). But if we take the upper crust of the earth, the waveguide strata there are rich in fluids, largely in aqueous solutions of minerals and in hydrocarbons. The overall amount of moisture there is very high and is comparable to the volume of the World Ocean, which certainly affects all geological processes. Rule-of-thumb and direct numerical estimates tell us: gaseous fluids travel from a depth of about 7 km (that is from the level within reach of research wells) up to the surface for about 400 years, while water propagates at rates 10 to 102 as slow. Thus it takes a short time, i.e. 103 to 104 years, for the entire mass of free water to rise up from the upper crust. But waveguides will disappear should the fissures close up. This does not occur, however. Why?
The answer is quite obvious: fluids keep circulating in the bowels of the earth. And it is the waveguides that operate here as "perpetual motion" of sorts (in existence ever since the contemporary earth crust and the ocean came to be formed). If some of such
Physical processes within fissures.
waveguides disappeared, others replaced them, or else one and the same layer became filled with fluids against.
Now let's look into the mechanics of waveguides. Convective processes in the mantle supply energy to sustain all processes occurring deep within. This energy is sufficiently high so as to give rise to fissures and fractures, and to pump in any fluids. And it initiates self-sustained oscillations* of the waveguides as well. Such motions occur in a cycle comprising two phases-tension and compression of pores within jointy media. In course of the tensile phase the pressure in the pores falls and fluids come to be sucked in; during the compressive phase the sucked-in fluids are ejected. That is to say, the waveguide substance is like a sponge now contracting and squeezing the liquid out, now relaxing and absorbing it again.
Fluids are drawn into a waveguide rather fast. And the routes of their travelings in free voids (cavities) are always new in every particular cycle (due to the irreversibility of changes), though distanced not far off from the previous pathways. The substance is squeezed out of the porous layer at a much slower pace, and it travels all through the waveguide. The time of a cycle like that ranges from 103 to 104 years.
Many geologists question the efficacy of this mechanism, for at depth of ca. 10 km there could never be any free cavity not filled with water oozing from above. Yet such cavities can appear during seismic shocks in corresponding foci lying not deep beneath. It is these processes that stimulate the activity of the "blood circulatory" system of the earth. Slight and moderate quakes are rather frequent and they sustain this very process.
Harder rocks lie over the softer "sponge". They move along it like bricks on an oiled surface, but these movements differ in their rates (even though the acting horizontal momentum is constant). The cumulative oscillations of many such interconnected "bricks" are far more complex than in the case of one single "brick". Such kind of construction can be an analog of actual processes in the upper crust of the earth. Mathematical model studies have shown that waveguides can endure for a long time and not violate the laws of physics and mechanics, or conflict with the actual evidence.
Thermal convection, diffusion, osmosis** as well as other physico-chemical, chemical and electrokinetic processes are likewise capable of transforming and transferring the crustal matter. Yet the radius of their action is not large as a rule. Besides, in a jointy medium elastic (strain) energy turns into a crushing force, which subsequently breaks up into heat and short-wave radiation. These are sufficient conditions for the birth of metallic clusters***, the macroscopic nuggets. The latter are dissolved in water within fissures and thus migrate through the earth crust. The formation and evolution of clusters is one possible way for the concentration and deposition of ore matter in the process of circulation of fluids in the upper crust.
* Self-sustained (self-excited) oscillations take place in an oscillatory (resonant) system in the absence of external effects owing to an active element within this system; this element makes up for the inevitable energy losses, and it does it through energy supply from a constant power source outside. - Ed.
** Osmosis - one-way diffusion of a solvent across a semipermeable membrane separating solution from pure solvent or solution of lower concentration. - Ed .
*** Clusters-here unstable congregations of a small number of molecules or atoms. - Ed.
Movements of fluids and formation of hydrocarbon deposits in the subduction zones. Waveguides are the motive force here. Formed in the frontal part of an underthrusting plate are gas deposits predominantly; petroleum deposits originate in the central, and bitumen deposits, in the rear part.
A comparative study of the aforementioned processes has shown that fluid flows move fastest if assisted by fractures and waveguides. This mechanism, in fact, accounts for a high pressure gradient commensurate with the gravitational pull; this force ejects the substance to long distances, into zones of sedimentation, differentiation and accumulation. Let us stress it again: this transpires as a result of multiple repeat oscillation cycles.
Now we have come up to the question of how gas and oil-bearing deposits are formed. One possible scenario of this process is visualized as follows. Suppose fluids are squeezed out upwards during a waveguide's compression phase. Moving up, they can bump into impermeable anticlinal* structures (traps) and generate anomalously high pressures. Since such saturated hydrocarbons happen to be present in fluid streams, conditions are on hand for the formation of respective deposits.
Yet another example, that of subduction zones where oceanic lithospheric plates underthrust the continental ones. Drawn thither are sedimentary layers as well, accumulating under the continental firmament**. Huge masses of mother-bed-of-oil rocks come to be concentrated there; these contain organic matter subsequently converted to hydrocarbons. The deeper sedimentary strata are drawn in, the higher the temperature and the longer their "maturation time". The "black gold" thus formed will contain more of the light and gaseous cuts.
A similar pattern holds for the formation of ore and gas-hydrate (solid gas) deposits. Such mechanisms are realized wherever there are corresponding fluids, tectonic movements, fractures and other permeable zones, and also in the presence of moderate seismicity (for it speeds up chemical and physicochemical interconversions).
If the body of anticlinal structures is cut by a network of ruptures, the fluids getting into them rise upwards to escape in gas outbursts or through mud volcanoes. This is why mud volcanoes crop up along faults and fractures, and on the shelf slope where rocks are squashed, deformed and destroyed in the end.
So, the key points of mineral deposits genesis are related to the circulation of fluids and the regime of this circulation. These substances, however, are like the two-faced Janus as to their role in the geologic history of the earth's crust: on the one hand, building up the underground mineral wealth and thus playing a creative role, and triggering earthquakes, on the other, which is a destructive role. Getting into the beak (end) of a fissure, even a small amount of fluid can unleash a force many times stronger than the motive force of fluids themselves.
* Anticlinal - with reference to anticline, a rock formation in which the layers slope downward from the crest in opposite directions. - Ed.
** See: D. Rundkvist et al., "Geodynamics of the 21st Century", Science in Russia, No. 6, 1998; I. Rezanov, "How Mountains Are Formed", Science in Russia, No. 5, 2002; V. Trubitsyn, "Global Plate Tectonics", Science in Russia, No. 2, 2003. - Ed .
Gas-hydrates accumulating on oceanic shelves may pose even a worse ecological hazard. These substances convert readily into gas with a change in thermodynamic conditions. An abrupt change of the climate could be the worst-case scenario, for global climatic warming can alter the structure of ocean currents and cause a release of methane into the atmosphere to aggravate the hothouse effect still more. Interacting, all these processes can snawball into an avalanche.
Mother Nature is a very flighty and extravagant lady, so to speak. She lets the lion's share of hydrocarbons escape into the atmosphere, with only a smaller part deposited under neath. The icebound Antarctic and Greenland are a spectacular example. The interconversion and migration of substances proceed there like elsewhere on the other continents. But rising up, under the effect of subzero temperatures the gases turn into a solid state near the terrestrial surface to make rocks impermeable. And here our "extravagant lady" behaves like a "covetous knight": the fluids that cannot seep outside remain locked in underground treasuries. Consequently, both the Antarctic and Greenland may store fabulous deposits of hydrocarbons much above all the explored ones. In theory at least.
An important side of the problem we are discussing is connected with the disposal of radioactive, industrial and other toxic wastes. In case of sudden, unpredictable disasters the accumulation of such wastes can badly pollute extensive areas. And we should remember that the decay time of radioactive elements is very long. Consequently, adequate geoecological activities should cover a period (up to 10,000 years) longer than the life span of many generations. Yet given the present level of science and engineering, geological arrays are not oriented to long-range targets like that.
And here our knowledge on the fluid regimen of the earth's upper crust can be quite helpful. For instance, nuclear power stations and burial grounds of radioactive wastes should best be sited in areas where fluid streams flow downward (of course, we should determine well in advance how long a given cycle of waveguide oscillations is going to last). In that case outbursts of nuclides would be drawn downward.
Needless to say, large-scale studies should be launched to lend more credence to our conclusions. What with the global scope of the problems at hand, it would be advisable to tackle them cooperatively within the framework of international projects.
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