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Recognizing some basic elements of local geologic character, and knowing some elemental concepts of hydrothermal water movement offer a baseline for understanding mineral deposition and thus gold prospecting opportunities in Guatemala.

Volcanic vent

Volcanic Vent, detail

volcan_fuego_1

Volcàn de Fuego, Acatenango, Guatemala, in full eruption

Volcanic Vents:

Following explosive volcanic activity, expelled material may fall back or be drawn back into the vent, forming a plug where angular spaces exist between rock fragments. This porous matrix forms an excellent conduit for the upward passage of mineralizing solutions that fill cavities through deposition or through substitution in neighboring rock materials. Guatemala has 23 volcanoes; the most active of which (presently) are in the south central area of the country, and include Fuego, Pacaya, and Santa Maria. Historically, however, volcanic action has been ongoing here for millions of years as the Cocos Plate sub-ducts below the Central American Plate. Numerous primary, secondary, and tertiary volcanic vents have been formed, filled, buried, and subsequently exhumed over this time span, and these, at least initially, have offered avenues for the upward migration of hydrothermal, mineral rich, solutions.

Rock Alteration:

The rocky walls of structural conduits (volcanic in origin or otherwise) are altered by solution passage and may become generally more porous than undisturbed rock. This enables increased entry and passage of mineralizing solutions.

Hydrothermal mineral deposits are usually accompanied by a bounding margin of rock wall alteration, often visible to the naked eye. In a fissure vein, for example, a relatively uniform width zone of alteration parallel to the walls of the fissures may be observed and the intensity of alteration will depend not only on rock wall character but the chemical composition, temperature, and pressure of the mineralizing solution. If veins are spaced closely together, the alteration “halo” of one vein may be confused or overlap with that of another, and the space between them entirely changed. This is particularly striking in the case of “porphyry copper” where the host rock between numerous small veins that intersect each other have been intensely altered to distances extending more than a hundred meters.

The nature of alteration also varies with the kind of rock. In the case of the cited mesothermal copper example above, the product of disruption of a quartz monzonite resembles a crystalline derivative or a shale diorite. In most limestone and quartzite rocks, however, the final altered product is mainly characterized by sericite and quartz. The feldspar minerals and ferromagnesium micas primary become sericite and silica. This is called “sericitation”. Recent work has also led to the realization that much of the outer alteration zone in “porphyry copper” is characterized by argillic alteration, i.e. the formation of clay minerals such as dickite and montmorillonite.

Factors Affecting Deposition

The modes of formation of minerals of different types are discussed elsewhere. Those related to interaction with hydrothermal solutions though, result from chemical changes in the solutions themselves, and to reactions between the solutions and rock wall materials as temperature and pressure of the solutions change.

Changes and Chemical reactions

During the long journey upwards from the molten magmatic mass below, mineralized solutions inevitably experience chemical change due to reaction with the rocks through which they pass. In alkaline silicate rocks they become more alkaline and the hydrogen ion concentration (pH) determines when reaction occurs with rocks or when deposition will or will not occur. Acidic environments cause similar though different reactions.

In the case of substitution, this can occur only when reaction between the solution and a solid results in partial dissolution followed by subsequent deposition in resulting cavities. Reactive rock walls, such as limestone, in disequilibrium with solutions produce a rapid chemical change accompanied by deposition.

Temperature and Pressure Influences

Back-Connection-Temperature-Pressure-Indicator

Temperature and Pressure

The most important factors causing hydrothermal deposition from solutions are changes in temperature and pressure. In general, a temperature or pressure drop decreases solubility and precipitation is more likely to occur.

Hydrothermal solutions begin their journey with heat provided by the magma, and this heat is subsequently lost by passing slowly through the rocks. Actual rate of temperature decrease in turn depends on the amount of solution being moved and also the ability of different rock types to absorb heat. The higher the degree of thermal dissipation in a particular rock the greater the drop in temperature in the solutions and the greater the likelihood of deposition. In the initial stages of circulation through cold rock the temperature drop will be relatively rapid; but with the continuous flow of heated material the rock walls rise to or near the temperature of the solution. At this point, of course, heat loss decreases or is minimal and deposition less likely.

The character of rock structure and pore geometry also affects heat loss. Rapid flow through a fissure with straight walls will cause less loss of heat. Similarly, slow movement through material with a large surface area will suffer a high initial temperature drop but, once heated, the surface area will not absorb much heat from the solution. These properties of rock structure and solution flow rate are also important in identifying and finding potential mineral deposition.

Solutions at depth begin, generally, at high pressures. Upward movement is also usually accompanied by a pressure decrease which may also result in mineral precipitation. Other factors, besides depth can also influence changes in pressure. These include narrowness of passageways, partial filling of conduits by prior mineral deposition, and other obstacles which can serve to maintain or even increase initial pressure. Exhaustion or dissipation of solution pressure due to variation in rock environment may lower pressure and initiate deposition. The changing physical character of the openings through which the solution passes plays an important role in determining and locating mineral deposition originating from movement of hydrothermal solutions.

Texturas_minerales

Changing rock textures, detail

Location of Hydrothermal Mineralization.

The nature and location of hydrothermal deposits has, simultaneously, both scientific interest and practical importance. Of course, variations occur from one district to another and may be due to one or several factors acting in concert. Most of the time, however, these variations depends on the chemical and physical nature of the host rock, the structural features of the intrusion depth at formation, the rates of solution movement in the rock openings or a combination of all the above. In some cases it may be immediately and clearly defined because of location, in others it may be completely enigmatic.

Intrusions

As most hydrothermal solutions are magmatic in origin, the source intrusion location can determine the location of the ore. Understanding dome or intrusions distribution facilitates locating minerals in the vicinity. Understanding paleo-volcanism and ancient volcano locations can do the same.

Character of the Host Rock.

Hydrothermal deposits can form in any type of host rock, but some are more favorable than others. For deposition to occur in cavities, predicting the likely location of openings, rather than the nature of the rock container itself, may be more useful. For example, brittle rocks crumble more easily than non-brittle rocks, and therefore localized fractures and gaps are more likely. Carbonate rocks, on the other hand, favor the formation of openings through solution as long as conduit paths exist. Any rock type can be chemically favorable to mineral deposition but not unless there are openings in the rock to provide sites for filling or to allow introduction of substitution solutions. Permeability is required; whether related to original pore space, fusibility, cleavage planes of minerals, gaps, joints, small fractures or other factors. Physical and chemical character of the host rock is therefore critical. Thus, in many mining districts, even small cracks and openings are followed to reach favorable layers already known, hoping to discover other exploitable deposits along the way.

Multiple Fissures and Shear Zone Locations

Fissure intersections are particularly favorable sites for mineral deposition. Thus folds axes and inclines associated with drag folds are important deposit locators for substitution. Created gaps are very favorable sites for both cavity filling deposits and for substitution. Other characteristics of sedimentation such as bedding planes, rolling, continuously permeable layers, or, for example uneven ground overlying impermeable layers can influence deposit location; all providing pathways for mineralized solutions.

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