Revista de la Asociación Geológica Argentina

Tapiolita-(Fe): paragénesis y composición química en pegmatitas de la provincia pegmatítica Pampeana

2026-06-01T15:25:23+00:00

Se describen la paragénesis y composición química de la mayoría de las manifestaciones de tapiolita-(Fe) en yacimientos de la Provincia Pegmatítica Pampeana. El mineral se encuentra excepcionalmente en cristales idiomorfos; más comúnmente se presenta asociado con minerales del grupo de la columbita o del grupo de la wodginita mostrando evidencias texturales de reemplazo por estas fases. Las composiciones químicas muestran rangos expresados en %p. de Ta₂O₅ comprendidos entre 86,22 y 72,11, FeO de 14,03 a 10,46, y de Fe₂O_3calc. desde 2,06 a 0,00. Los elementos menores tienen contenidos medios (%p.) de: Nb₂O₅ (2,40), TiO₂ (0,74), SnO₂ (0,21) y MnO (1,15). Los elementos traza tienen contenidos ≤ 0,5 % p. o son ínfimos. Los valores de Ta# (Ta / (Ta + Nb) at) de tapiolita-(Fe) varían de 0,99 a 0,84 y muestran muy buen agrupamiento en la mayoría de las pegmatitas. Siempre los valores de Ta# de tapiolita-(Fe) son más altos que en los análisis de los minerales del grupo de la columbita y similares a los del grupo de la wodginita asociados, mientras que los valores de Mn# son inferiores a los de ambos grupos. El origen de la tapiolita-(Fe), en la mayoría de las diferentes paragénesis estudiadas, se adscribe a la cristalización magmática tardía y en desequilibrio de un fundido pegmatítico evolucionado, enriquecido en fundentes y elementos incompatibles.

Hydrothermal alteration and exploration implications of the Laguna Amarilla high-sulfidation epithermal prospect, Los Menucos District, Río Negro Province, Argentina.

2026-06-19T00:29:02+00:00

The Laguna Amarilla prospect, located in the Los Menucos mining district (Río Negro Province), is one of six high-sulfidation epithermal prospects recognized in the central sector of the North Patagonian Massif. It developed over a volcanic and subvolcanic succession of felsic to intermediate composition, comprising phenodacitic lapilli tuffs, phenorhyolitic tuffs, monomictic breccias, and subvolcanic bodies. The concordia age obtained (279.6 ± 6.3 Ma) for the phenodacitic lapilli tuff is consistent with the Gondwanan magmatism recorded in the region, although two grains yielding ages near the Triassic-Jurassic boundary indicate a younger magmatic event, correlatable with the host rocks of the Cerro La Mina and Iván projects.

Integration of petrography, VNIR-SWIR reflectance spectroscopy, scanning electron microscopy, and X-ray diffraction allowed the identification of three hydrothermal alteration assemblages: i) advanced argillic (high-crystallinity kaolinite + dickite + quartz ± pyrophyllite ± diaspore); ii) intermediate argillic (illite + montmorillonite ± quartz ± low-crystallinity kaolinite); and iii) propylitic (montmorillonite ± chlorite ± epidote ± carbonate). Their spatial distribution defines an advanced argillic core with vuggy silica textures, grading laterally into intermediate argillic domains. Au anomalies are concentrated within this core, particularly where dickite and vuggy silica textures are developed. The Kaolinite Crystallinity Index proved to be the most effective spectral tool for vectoring toward mineralization. Finally, the presence of pyrophyllite and diaspore suggests a deep erosional level of the system and potential preservation of economic mineralization at shallow depth.

Sedimentology of a rock avalanche older than 5.4 ka in the Sierra San Bernardo, Chubut, Argentina

2026-09-28T12:31:22+00:00

This study characterizes a mass movement deposit exposed in the Matasiete Canyon (Sierra San Bernardo, Chubut). Lithofacies analysis and structural survey allow the deposit to be interpreted as the result of a rock avalanche (RA) emplaced prior to 5401 cal yr BP (Middle Holocene). The deposit is composed of breccias of claystones, tuffs, and conglomeradic sandstones of the Matasiete Formation, organized into facies consistent with the classical carapace-body-base model, displaying diagnostic features of cataclastic laminar flows (high fragmentation and differential shearing), such a jigsaw and survivor clasts, boudin-like structures, R and R’ shears, folds, and normal faults. Two main shear zones control the architecture of the deposit (PCPA and ZCPB) and indicate a transport direction toward the WSW. The absence of surface morphological features associated with the deposit hinders direct interpretation of the landslide (emplacement type, volume, and extent), although climatic conditions (intense precipitation) are considered the main triggering factor.

U–Pb Geochronology of the Sierra del Mayal Subvolcanic Bodies: Implications for Late Cretaceous–Paleogene Magmatism and the Structural Evolution of the Internal Sector of the Chos Malal Fold-and-Thrust Belt, Neuquén, Argentina

2026-07-27T19:55:49+00:00

We present two new U–Pb zircon ages from andesitic sills exposed in the Sierra del Mayal, within the internal sector of the Chos Malal Fold-and-Thrust Belt (FTB), Neuquén Province, Argentina, integrated with petrographic, geochemical, and structural data. These igneous bodies, previously dated at 39.7 ± 0.2 Ma by whole-rock Ar–Ar geochronology, yielded U–Pb zircon ages of 61.91 ± 0.63 Ma and 60.23 ± 0.26 Ma, placing their emplacement in the middle Paleocene (Selandian) and providing a more robust chronological constraint than previous age determinations. Petrographically, the rocks are porphyritic andesites composed of plagioclase phenocrysts, amphibole, and subordinate biotite set in a microgranular groundmass. Their geochemical signature indicates a calc-alkaline arc affinity, characterized by enrichment in incompatible elements and negative Nb, Ta, and Ti anomalies. The zircon population also records inherited Permian grains (~280 Ma), attributed to the Choiyoi Group, suggesting crustal assimilation during magma evolution. The emplacement of the intrusive bodies was strongly controlled by both structural and lithological factors, being concentrated along the axis of the Mayal Syncline, which they offset, and preferentially intruding mechanically weak units of the Agrio and Huitrín formations. These new results, integrated with regional thermochronological and geological data, refine the evolutionary model for the internal (western) sector of the Chos Malal FTB, supporting the recognition of at least two contractional deformation stages: one between the Late Cretaceous and the Paleocene, and a second during the middle Miocene.