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

Abstract

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.