Browsing by Subject "Faults (Geology)--Honduras"

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Geology of the Agalteca magnetite skarn deposit, central Honduras
(1983) Logan, William Stevenson; Kyle, J. Richard
The Agalteca iron skarn deposit, central Honduras, is localized along the contact between a plagioclase-pyroxene diorite stock of Laramide(?) age and the Albian-Aptian Atima Formation. The Atima consists of micritic and biomicritic limestone interbedded with yellow and gray shale. It is underlain by the Jurassic-early Cretaceous Todos Santos Formation which consists of conglomerate, sandstone and shale derived from a nearby metamorphic highland; it is overlain by the late Cretaceous Valle de Angeles Group redbeds. These units are unconformably overlain by the predominantly Miocene Padre Miguel Group rhyolitic ignimbrites and recent stream terrace deposits. Rhyolite porphyry dikes intrude the Mesozoic sediments and the diorite pluton. The deposit lies on the northeastern flank of the Montaña de Comayagua structural belt. This transpressional feature is marked by N6OW-trending right-lateral transcurrent faults, west-northwest-trending thrust faults, northeast-trending normal and transcurrent faults and N6OW listric normal faults. Kink-style folds are conspicuous in thinly bedded limestone of the Atima Formation. Intrusion of the diorite into the Atima Formation resulted in initial contact metamorphism and development of reaction skarn and hornfels. Subsequent magnesium-rich metasomatism resulted in the formation of vesuvianite, chlorite, quartz and hematite. Simultaneously, endoskarn composed of pyroxene + plagioclase ± epidote ± prehnite was formed in the diorite along the intrusive contact. With rising temperatures, earlier-formed exoskarn was converted to grandite (Ad₁₅₋₈₅) and clinopyroxene. Retrograde introduction of large quantities of iron resulted in alteration to epidote, ferroactinolite, iron-chlorite and finally to magnetite. Lastly, calcite partially replaced many of the earlier phases. The diorite also underwent extensive alteration, including albitization and later sericitization of the plagioclase, and replacement of pyroxene by chlorite, epidote and late calcite. Hydrothermal alteration of the proximal diorite involved a gain in iron, and therefore cannot be considered a source for the iron. The source was probably residual magmatic fluids and/or leaching of mafic minerals from the diorite at depth, with a possible minor contribution from the overlying redbeds. Early magnesian metasomatism was widespread and originated from crystallization of the magma near the intrusive margin. Later iron-rich metasomatism was localized by fractures and faults that allowed trapped residual fluids to escape. Magnetite mineralization was also favored by a high initial carbonate content of the host rock