Day 4, hydrothermal deposits, part 3

Continuing this series of geology in general and more specifically in Guatemala, will keep today with the question of hydrothermal deposits, baseline for gold exploration.

Volcanic vent

Volcanic Vent, detail

volcan_fuego_1

Volcàn de Fuego, Acatenango, Guatemala, in full eruption

Volcanic vents:

When an explosive volcanic activity tubular openings practice, the materials expelled may fall back or be drawn back into the opening, forming a gap with angular spaces between the fragments. These openings are excellent transport conduits for mineralizing solution from which can be formed by filling cavities deposits or deposits of substitution. Guatemala has 23 volcanoes, the most active are in the south central area of the country, among which the Fuego and Pacaya volcano.

Rock alteration openings:

The rocky walls that have been altered by solutions turn out to be (after practicing them appropriate tests) generally more porous than undisturbed rocks and allow entry of mineralizing solutions.

Factors affecting the deposition

The modes of formation of minerals of different deposit types are discussed on pages. Those related to deposition from hydrothermal solutions are predominantly chemical changes in the solutions, and reactions between the solutions or rock walls reefs materials, temperature and pressure changes.

Changes and chemical reactions

In its long journey upstream, mineralizadoras solutions must inevitably experience a chemical change by its reaction with the rocks they pass through. The alkaline silicate rocks make them more alkaline. The hydrogen ion concentration (pH) may determine when the reaction occurs with rocks or deposition.

In the substitution, the substitution can occur naturally old minerals by new ones, only by reaction between the solution and a solid. Reactive rock walls, such as limestone, in disequilibrium with solutions produce a rapid chemical change accompanied by deposition.

Temperature and pressure

Back-Connection-Temperature-Pressure-Indicator

Temperature and Pressure

The most important factors that cause hydrothermal deposition from solutions are changes in temperature and pressure. In general, a temperature drop decreases solubility and precipitation occurs.

Hydrothermal solutions begin their journey with the heat provided by the magma, heat lost by going slowly through the rocks. The temperature drop depends on the rate of heat loss in the rocky walls, which in turn depends on the amount of solution being crossed, for exothermic reactions mainly of the rocky wall ability to absorb heat. The higher the thermal diffusivity of a rock more rapidly absorb the heat and the greater the drop in temperature in the solutions. In the early stages of circulation with cold rock walls, the temperature drop will be relatively quick, but the continuous flow of warm solutions wall rocks to reach the temperature of the solution, at which point decrease heat loss.

The nature of the rock openings also affects heat loss. The rapid flow through a fissure with straight walls will cause less loss of heat creep openings complicated by a gap having large specific surface area, where the initial temperature drop will be fast. However, once heated, the gap will not absorb much heat from the solution. The greater the volume of new solution passes through a given point the greater the heat input again, and the slower the temperature decrease of the solutions. Thus, a crack with constrictions and open areas characteristic, the temperature of the solution in portions descend less narrow than larger ones. These characteristics are also important in the identification and localization of mineral deposition.

Solutions start at high pressures in the deep where they originate. Its path upward through low pressure zones, and usually accompanied by a pressure drop also causing precipitation. But there are other factors that can determine changes in pressure. The narrowness of the ducts, the partial filling mineral deposition, or obstacles, can cause excessive pressure. The exhaust of the solutions more open spaces narrows above the pressure lowers and causes deposition. Thus, changing the physical character of the openings through which pass the solution play an important role to determine and locate the deposition of minerals from the hydrothermal solution.

Alteration of the rock.

Hydrothermal mineral deposits are usually accompanied by a strip of wall rock alteration, visible to the naked eye. For example, in a fissure vein, alteration zone parallel to the walls of fissure, of relatively uniform width and amplitude variations in the volume of the reef. Disturbance intensity also depends on the rock wall and the chemical, temperature and pressure of the mineralizing solution. If the veins are closely spaced together, the alteration halo of a vein can be confused with that of another, and the space between them is entirely altered. This is particularly striking in the case of “porphyry copper” where the host rock between the numerous small veins which intersect each other have been intensely altered to distances that reach hundreds of meters.

Texturas_minerales

Changing rock textures, detail

The nature of the alteration somewhat varies also with the kind of rock, but surprisingly, in the case of a copper deposit mesothermal the product of disruption of a quartz monzonite resembles a derivative or a shale diorite crystalline. With the exception most rocks limestone and quartzite, the final product is a rock alteration mostly formed part by sericite and quartz. The feldspar minerals and micas primary foerromagnésicos become sericite and silica are generally added. This is called “sericitación”.

Recent work has led to the discovery that many of the “porphyry copper” outer alteration zone is characterized by argillic alteration ie the formation of clay minerals such as dickite and montmorillonite.

Location of hydrothermal mineralization.

The cause of the location of the hydrothermal deposits has, simultaneously, scientific interest and practical importance. Naturally, varies in each district, and may be due to one or more factors that have acted together. Most of the time depends on the chemical and physical nature of the host rock, the structural features of the intrusion depth of the formation, the movements in the rock openings or a combination of these factors. In some cases is clearly defined because of the location, in others it is enigma.

Intrusions

As most hydrothermal solutions are magmatic source, the mother intrusion situation can determine the location of the ore. The domes or intrusions may apophysis in locating the mineral in its vicinity; volcanoes can do the same.

Character of the host rock

Hydrothermal deposits can form in any type of host rock, but some of them influence the deposition more than others. For filling cavities deposits, the location of the opening, rather than the nature of the rock container, which is located the mineral, although physical and chemical nature of the host rock can determine the location and method of the cavity. For example, crumble brittle rocks more easily than non-brittle, and therefore localized fractures and gaps; carbonate rocks allow the formation of openings for the solution. Apart from that a rock can be chemically favorable mineral deposition can not occur unless there are openings in the rock that provide sites for filling cavities or allow input substitution solutions. Permeability is required, and it can provide the original pore space, fusibility, the cleavage planes of minerals, gaps, joints, small fractures and other factors. The influence of the host rock from the ore in the location can be, therefore, chemically or physically, or both at once. Also the specific surface area is an important factor.

Structural features

The structural features are important locators hydrothermal deposits. The cracks serve to, sites themselves as well as conduits for fluid displacement rock minerals susceptible to substitution. The intersection of cracks in rocks favorable search is used in substitution deposits. Both are necessary. There may be a favorable limestone substitution, but substitution may occur if any solutions it can not reach it, and vice versa: there may be a crack pipe excellent, but if your walls are unfavorable, there will be no substitution. It is necessary coincidence of both. Thus, in many mining districts, even small cracks are followed with holes to reach favorable layers already known, hoping to discover exploitable deposits at such locations.

Multiple fissures and shear zones located mineral deposits in a similar way to the fissures.

Fisurales intersections are particularly favorable sites to mineral deposition. The folds together and inclined and you drag folds were important deposits locators for substitution. The gaps are very favorable sites for both cavity filling deposits and for substitution. Sedimentation due to traits such as bedding planes, rolling continuous permeable layers, uneven ground overlying impermeable layers can influence the deposit location, providing conduits for mineralizadoras solutions.

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