Day 20, Geology of Motagua River II

The next article is about The relations between the Maya Block and Motagua Suture which  is very important to find out where the alluvial gold of the Motagua River could be formed or where did it come. Of course, in this report it is very advisable to understand geological therms like mèlanges or harzburgites… The gold is waiting for us to be recovered, let`s see where the relationship with other rocks is taking place. Our dredges, highbankers and all people is waiting.



AsthenoLithoThe geology of the Motagua suture zone has been explained in terms of Mesozoic oceanic lithosphere (there used to be an ocean before land was formed from the age Mesozoic) , island-arc lithosphere (then an island was born)  and subduction mélanges (and the collision of the fault in a violent way produce mèlanges, mèlanges are lots of diferent types of rocks in small sizes putting together) that were emplaced over older high-grade basement of the Chuacús complex (Maya block) to the north, and the Las Ovejas complex (Chortis block) to the south. The suture zone developed during one or several collisions of the Maya block, Chortis block, terranes of southern Mexico, and the Cuban arc. Geologic investigations along a N-S cross section from the Chixoy dam to the Pachalum-Granados area in Central Guatemala revealed important geologic features and field relations, which constrain models for the geologic evolution of the Motagua suture.


Harzgurgite, sample

It has long been argued that large harzburgite bodies (The igneus rock, harzburgite, is a variety of peridotite  consisting mostly of the two minerals, olivine and low-calcium (Ca) pyroxene; it is named for occurrences in the Harz Mountains of Germany) in Baja Verapaz (island arc lithosphere?) have been emplaced over Upper Cretaceous and older sedimentary rocks of the Maya block. But the northern (Chixoy dam) and southern (Río Negro) contacts are vertical faults that separate the harzburgites from Upper Cretaceous carbonates (Cobán). These faults, which are probably associated with the Tertiary Motagua fault system, obliterated original structures. No overthrust or metamorphic sole was identified in Central Guatemala. South, in the Cubulco quadrangle, vertical faults put the Cobán limestone in contact with the Carboniferous-Permian Santa Rosa group, in which crinoids were observed.

There is a continuous southwards transition between the Santa Rosa group and Chuacús complex garnet-bearing metasediments. The transition occurs along a narrow band a few kilometers thick (perpendicular to the fabric) in the Rabinal-Cubulco area, which contains abundant reverse mylonitic zones analogous to those composing the Baja Verapaz shear zone (BVsz) in the Salamá quadrangle. The continuity suggests para-allochthony of the blocks on each side of the BajaVerapaces, and suggests that the Chuacús complex is a metamorphosed margin of the Maya block, not an allochthonous terrane.

The kinematic indicators in the BVsz indicate it played a key role exhuming high-grade Chuacús rocks. These include (only northernmost occurrence is reported): amphibolites in Cubulco, migmatites in the Plan de Sánchez area, and eclogites in Tuncaj and Ixchel. The foliation of the Chuacús complex is for the most part uniform, strike being consistently NNE-WSW, oblique to the southern margin of the Maya block. This feature may be the result of metamorphic recrystallization between two major E-W (current coordinates) left-lateral shear zones located roughly where the active BVsz and Motagua are today, and may indicate that collision that triggered orogeny was oblique.

sierra de chacus

Sierra de Chuacùs

The southern flank of the sierra de Chuacús exposes the geologic relations between the Chuacús complex and the Motagua mélange, abundant eclogite occurrences, and polymetamorhphic gneisses of the Chuacús. The later are mainly exposed in the El Chol area, where banded gneisses contain several generations of folds, pegmatite intrusions, and amphibolites with eclogite relics. Several geologic features indicate a common late-stage metamorphic event for the Chuacús complex and Motagua mélange: antigorite-schist mélange on the southern flank of the sierra de Chuacús was intruded by Chuacús pegmatites developing metasomatic rinds; metamorphic grade and dominant foliation is similar in both. Ultramafic talc+phlogopite+actinolite bands interlayered in eclogitic Chuacús gneisses were observed in Agua Caliente. They are interpreted as ultramafic bodies introduced into the Chuacús from a mantle wedge during continental subduction. Severe metasomatism accounts of current mineralogy of ultramafic bands. The above features allow concluding that the Chuacús complex is a continental margin of the Maya block, whose metamorphic history is coeval and strongly linked to the formation and evolution of Motagua Suture.


As we see, there are many phenomena that relate Motagua suture or failure with the Mayan block. But … also we can see many geological terms that have been used in the study. For most of the people these terms seem strange but as many sciences geology uses specific terms.

When we talk of terms like Mesozoic, paleozoico, Carboniferous, Permian and vocabulary well, we refer to geological ages. A geological age may take millions of years, then we can see that the process of faulting, rocks, metals, and other elements is time consuming. And gold is a mineral that requires millions of years to form and after several processes as seen previously.


Marble Quarry, near to Motagua River

According to the study, the block development Mayan equally to the Motagua fault, with a history of the metamorphic process or metamorphism. Hence in the Motagua has walls of melanges, rocks and serpentinites, slate, limestone, marble, cancagua. The large part of the area of the river has metamorphic rocks though other types (minor) but also important to understand where the gold is bringing the river.

In our next articles we will see how the metamorphic, igneous, intrusive, sedimentary and plutonic relate to gold. The gold has been there waiting for us and it has been several millions passed by.

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