Rodolfo Corona-Esquivel

New geological and geochronological data of the Placer de Guadalupe uplift, Mexico: a new piece of the Late Triassic–Jurassic Nazas Arc?

Basement exposed in the Placer de Guadalupe–Plomosas uplift in northern Mexico provides important clues for the geologic evolution of the region. The stratigraphic units form stacked thrust sheets of psammitic and calcareous formations, interlayered with magmatic rock. The eastern calcareous and quartzite formations exhibit structures associated with ductile deformation, whereas the upper stratigraphic units only contain structures formed via younger brittle deformation. Porphyry interlayered in the upper Plomosas Formation has a U-Pb zircon age of 171 ± 1 Ma. This age is consistent with its stratigraphic position, interbedded quartzarenites with a maximum depositional age of ~168 Ma. Granite flakes within the Horquilla Formation are dated at 209 ± 3 Ma, and the La Viñata quartzite exhibits a maximum age of ~193 Ma. The Upper Plomosas Formation correlates well with the arc-related Middle Jurassic Nazas Formation of northeastern Mexico, constituting the first report of a Jurassic continental margin arc outcrop in the ‘Central Mexican Gap zone’. We document Late Norian to Bajocian ages for the stratigraphic units cropping out in the Placer de Guadalupe area. The Jurassic age cluster indicates that the Nazas Arc magmatism in the region occurred during the Late Triassic and ended in the Middle Jurassic times. Permian ages previously assigned to these rocks and the occurrence of a Permo–Triassic deformation event have to be dismissed.

Palynologic evidence for iron-oxide ash fall at La Perla, an Oligocene Kiruna-type iron ore deposit in northern Mexico

La Perla is an Oligocene deposit of apatite iron ore located in northern Mexico. The main ore types are massive ore, ore breccia and powdery ore. The latter is of special genetic interest because it contains well-preserved palynomorphs; fossil pollen representing several plant families growing in the region of investigation during the late Paleogene-Neogene; the assemblages include angiosperm and gymnosperm pollen grains, and also fossil fungal spores from two genera, Frasnacritetrus and Dyctiosporites, indicative of Eocene to Miocene age. The beds of powdery ore are stratified and size-sorted, but in some places there is no discernible stratification. The ore consists of a friable open framework of anhedral to euhedral hematite plates, or less commonly, martitized magnetite octahedra. Locally, the ore is even unconsolidated. The ore minerals show no abrasive rounding or other epiclastic features, and the high porosity of the iron-oxide crystal aggregate embedding the palynomorphs rules out formation by hydrothermal deposition or replacement. The exines of the palynomorphs have a light yellow color which demonstrates that they are unaffected by thermal alteration. This shows that the pollen-bearing powdery ore was deposited at a temperature below 150°C, probably as volcanic ash that captured wind-blown pollen.

Fluid origin of the Ixtacamaxtitlan hydrothermal deposits, Puebla State, Mexico

The hydrothermal deposit of Ixtacamaxtitlan (Puebla, Mexico) is made up by a succession, from bottom to top, of quartz veins and stockwork enclosed in a porphyritic subvolcanic body, a kaolinitized rhyolitic tuff and a layered opal deposit. This vertical arrangement coupled with the distribution of the different alteration assemblages lead to the interpretation of the whole as a low-sulfidation epithermal deposit. The fluid inclusion study carried on the veins and the stockwork along with the stable isotopic analyses performed on the kaolinitized bodies helped us to propose two major hydrothermal events that occurred in the area: an early event, characterized by hot, hypersaline fluids (up to 280 jC and 36 wt.% NaCl eq.) closely associated with a potassic alteration episode; and a late event, distinguished by cooler and dilute fluids (up to 150 jC and 4 wt.% NaCl eq.), associated with propylitic and quartz-sericite alterations at depth and acid-sulfate alteration close to the paleosurface due to steam-heated phreatic waters. Post-trapping changes found affecting the primary fluid inclusions (hook-shaped morphologies) suggest that there was an uplifting period between the two stages. D 2003 Elsevier Science B.V. All rights reserved.