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Palynologic evidence for iron-oxide ash fall at La Perla, an Oligocene
Kiruna-type iron ore deposit in northern Mexico
Rodolfo Corona-Esquivelab; Enrique Martínez-Hernándeza; Fernando Henríquezc; Jan Olov Nyströmd;
Jordi Tritllaef
a Instituto de Geología, Universidad Nacional Autónoma de México, México D.F., Mexico b Escuela
Superior de Ingeniería y Arquitectura, Instituto Politécnico Nacional, México D.F., Mexico c
Departamento de Ingeniería en Minas, Universidad de Santiago de Chile, Santiago, Chile d Swedish
Museum of Natural History, Stockholm, Sweden e Centro de Geociencias, Universidad Nacional
Autónoma de México, Querétaro, Mexico f Grupo de Disciplinas Geolólogicas, Repsol Exploración,
Madrid, Spain
Online publication date: 26 November 2010
To cite this Article Corona-Esquivel, Rodolfo , Martínez-Hernández, Enrique , Henríquez, Fernando , Nyström, Jan Olov
and Tritlla, Jordi(2010) 'Palynologic evidence for iron-oxide ash fall at La Perla, an Oligocene Kiruna-type iron ore
deposit in northern Mexico', GFF, 132: 3, 173 — 181
To link to this Article: DOI: 10.1080/11035897.2010.519048
URL: http://dx.doi.org/10.1080/11035897.2010.519048
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Palynologic evidence for iron-oxide ash fall at La Perla, an Oligocene
Kiruna-type iron ore deposit in northern Mexico
RODOLFO CORONA-ESQUIVEL
1,2
, ENRIQUE MARTI
´NEZ-HERNA
´NDEZ
1
,
FERNANDO HENRI
´QUEZ
3
, JAN OLOV NYSTRO
¨M
4
and JORDI TRITLLA
5,6
Corona-Esquivel, R., Martı
´nez-Herna
´ndez, E., Henrı
´quez, F., Nystro
¨m, J.O. & Tritlla, J., 2010: Palynologic evidence for
iron-oxide ash fall at La Perla, an Oligocene Kiruna-type iron ore deposit in northern Mexico. GFF, Vol. 132 (Pt. 3– 4,
September–December), pp. 173–181. Stockholm. ISSN 1103-5897.
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 1508C, probably as volcanic ash that
captured wind-blown pollen.
Keywords: Pollen, fungal spores, apatite iron ore, Kiruna-type, hematite, magnetite, ash fall, Oligocene,
Paleogene, Neogene, Mexico.
1
Instituto de Geologı
´a, Universidad Nacional Auto
´noma de Me
´xico, Ciudad Universitaria 04510, Coyoaca
´n,
Me
´xico D.F., Mexico; rcorona@servidor.unam.mx, emar@servidor.unam.mx
2
Escuela Superior de Ingenierı
´a y Arquitectura, Instituto Polite
´cnico Nacional, Avenida Ticoman 600, C.P.
07340, Me
´xico D.F., Mexico
3
Universidad de Santiago de Chile, Departamento de Ingenierı
´a en Minas, Casilla 10233, Santiago, Chile;
fernando.henriquez@usach.cl
4
Swedish Museum of Natural History, SE-10405 Stockholm, Sweden; jan.nystrom@nrm.se
5
Centro de Geociencias, Universidad Nacional Auto
´noma de Me
´xico, Campus Juriquilla, 76230 Quere
´taro,
Mexico; jtritlla@gmail.com
6
Grupo de Disciplinas Geolo
´logicas, Repsol Exploracio
´n, Paseo de la Castellana 278, 28046 Madrid, Spain
Manuscript received 23 October 2009. Revised manuscript accepted 23 August 2010.
Introduction
La Perla is an Oligocene iron ore deposit of Kiruna-type situated
in northern Mexico. Kiruna-type ores consist of magnetite, or
less commonly hematite, and small amounts of apatite and
silicate gangue minerals. These ores are associated with volcanic
and subvolcanic rocks of intermediate to moderately silicic
composition. Ore districts with deposits unaffected by
deformation and strong metamorphism contain orebodies of
different primary structure and texture. Such variation is found
at La Perla, where one variety of ore is of special genetic
interest: powdery iron oxide with small amounts of pollen.
The origin of magnetite-apatite deposits of Kiruna-type has been
controversial for more than a century. At present the discussion is
focused on El Laco, a Pliocene volcanic complex in the Chilean
Andes where the best preserved deposit of this ore type is found.
The traditional interpretation is that the orebodies at El Laco
formed from extremely iron-rich magmas erupted onto the surface
or crystallized at shallow depth (Park 1961; Henrı
´quez & Martin
1978; Nystro
¨m&Henrı
´quez 1994; Naslund et al. 2002; Henrı
´quez
et al. 2003). Other authors argue that the ore was deposited from
hydrothermal-metasomatic fluids that replaced preexisting silicate
rocks (Rhodes et al. 1999; Rhodes & Oreskes 1999; Sillitoe &
Burrows 2002).
Magmatic and hydrothermal processes have also been proposed
for the formation of the La Perla deposit. Ca
´rdenas-Vargas & Del
Castillo-Garcı
´a (1964) concluded that the iron ore originated from
iron-rich liquids derived by liquid immiscibility; a gaseous phase
generated the powdery ore. Van Allen (1978) suggested a
hydrothermal replacement model for the ore: fluids rich in Fe,
GFF
volume 132 (2010), pp. 173–181. Article
Downloaded By: [Naturhistoriska Riksmuseet] At: 06:46 30 November 2010
F and P reacted with and dissolved glassy volcanic rock, depositing
iron oxide;the highly porous nature of the powderyore is attributed
to loss of volume during the replacement.
The purpose of this paper is to describe an extensive, many
meters thick, partly stratified bed of friable hematite-magnetite
ore at La Perla that contains well-preserved fossil palynomorphs
(pollen and fungal spores), and to discuss how the character of
the powdery ore and the palynomorphs constrain the deposi-
tional environment and formation of the ore.
La Perla iron ore deposit
The La Perla ore deposit is located in the state of Chihuahua
(Fig. 1; coordinates 2881805100 N, 10483304600 W). It is situated in
a broad valley within Sierra de Mesten
˜as, at an altitude of ca.
1550 m above sea level. The iron ore is hosted by a 250 m thick
sequence of rhyodacitic lavas called the La Perla Formation, that
constitutes the lower part of a 560 m thick pile of Oligocene
volcanic rocks, mainly lavas of rhyolitic to dacitic and trachytic
compositions (Campbell 1977; Van Allen 1978; Corona-
Esquivel et al. 2003). The rocks are subalkaline to alkaline
with geochemical features reflecting slight crustal contami-
nation that suggest formation during extension in an intraplate
setting (Jacinto-Estanes 2005).
Before mining started at La Perla in 1953, the deposit was
known as “Cerro del Fierro” (Iron Mountain). It was a hill rising
70 m above the surrounding alluvial plain. The original ore
deposit was 1200 m long, 700 m wide and 180m thick in its
central part, and the total reserves were estimated at 87 million
metric tons of iron ore with 57.2 wt.% equivalent Fe (Ramirez-
Lara 1973). The exploitable reserves remaining at present
exceed 5 million tons. Open-pit mining and diamond drilling
revealed that the original deposit was composed of many
subhorizontal to gently dipping lenticular orebodies, the largest
being more than 400 m long and 50 m thick (Fig. 2). Individual
orebodies were dipping away from the central part of the
deposit, accounting for the morphology of the hill. There is no
evidence that the structural pattern is caused by doming.
The orebodies occur intercalated in the upper part of the LaPerla
Formation.No radiometric age determination has been reported for
the mineralization. The La Perla rhyodacite has been dated to
between 31.5^0.7 and 31.8^0.5 Ma using the K/Ar method
(Campbell 1977), i.e. it is early Oligocene in age. The intercalation
of the ore in the lava sequence suggests that they are coeval and
that, therefore, this age is also valid for the ore.
The main ore types at La Perla are massive ore, ore breccia and
powdery ore (Fig. 2), the latter being the topic of this study. The
massive ore that made up the predominant part of the unexploited
deposit has been removed by mining. It is reported to have had
sharp contacts with the underlying and overlying rhyodacites. The
massive ore was largely composed of fine-grained hematite, but
widespread relicts suggest that magnetite was the main primary
phase. Part of the ore was scoriaceous and looked like a vesicular
basalt, according to Ca
´rdenas-Vargas & Del Castillo-Garcı
´a
(1964); they also documented a columnar texture later found to be
common at El Laco (Henrı
´quez & Martin 1978). Van Allen (1978)
reported flow banding in the massive ore. Apatite visible to the
unaided eye was scarce, and only found as light yellow to white
prisms up to a few millimeters long. The ore breccia consists of
similar iron oxide, scarce apatite, and varying amounts of angular
rhyodacite fragments. Pyroxene has been reported as a rare
mineral. Quartz, calcite and fluorite are seemingly late phases.
The powdery ore occurs as at least three horizontal to gently
dipping bedsof stratified, friableiron oxide (Fig. 3) that are thickest
in the northern and southern parts of the deposit. The most
extensive bed is more than 300 m long. In the north, the powdery
ore forms a thick bed (.10 m) at the bottom of the open pit (Fig. 4).
Continued mining has shown that the lower part of this ore bed is a
breccia very rich in unmineralized fragments and blocks of
rhyodacite. The proportion of rhyodacite in the ore increases
irregularly with depth. The contact between the ore breccia and
underlying rhyodacite of the La Perla Formation is unexposed.
Methods
This study is based on 20 samples of powdery ore collected
during November 2005 in the cross section depicted in Fig. 4,
and 15 additional samples from 2008. Special care was taken to
avoid contamination of the unconsolidated and friable ore by
material introduced during mining. The idea to search for pollen
in the ore came after the first samples had been collected.
Palynological processing of these samples at Instituto de
Geologı
´a de la Universidad Nacional Auto
´noma de Me
´xico
resulted in the discovery of pollen. In June 2008, the pollen-
bearing sampled section (Fig. 4) had been removed by mining.
Pollen was found again in powdery ore from a new section about
40 m east of the original sampling site, in the inferred
continuation of the pollen-bearing strata.
Palynological processing
An extraction procedure was necessary due to the scarceness of
pollen in the ore. 250 g ofsample wasadded to distilled water with
a detergent.After vigorous stirring and sedimentation for 24 hours,
still floating fine material was removed and new water was added.
After stirring, the water and suspended fine particles were decanted
into another beaker, discarding the heavy residue on the bottom.
This procedure was repeatedseveral times until only clay- and silt-
sized particles were suspended in the water. The suspended fine
fraction was concentrated by centrifugation. The average amount
of fine fraction obtained from a sample was ca. 30 cm
3
(wet).
The recovered fine material was submitted to acetolysis,
following standard procedures (Jackson 1999). It was treated
with a 9:1 mixture of acetic anhydride and concentrated sulfuric
acid. After heating at 708C for 12 minutes, the fine material had
Fig. 1. Location of the La Perla iron ore deposit in Chihuahua, northern
Mexico.
174 Corona-Esquivel et al.: Palynologic evidence for iron-oxide ash fall at La Perla GFF 132 (2010)
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dissolved, leaving only a small organic residue, including pollen,
if present, in the sample. The organic material was mounted on
glass slides, using Canada balsam and hydroxyethyl cellulose as
an immersion medium, and studied with an optical microscope.
Mineralogical study
The mineralogy, texture and crystal habit in the samples of
powdery ore from La Perla were studied with a binocular
microscope. SEM photos of representative material were taken
using a HitachiS-4300 scanning electron microscope (accelerating
voltage ¼6 kV; beam current 10 mA), at the Swedish Museum of
Natural History in Stockholm.
Powdery ore
The beds of powdery ore show variations with regard to texture,
grain size, composition, percentage of included silicate material,
and resistance to erosion. Stratification is common, especially in
the ore bed in the northern side of the open pit (Figs. 4– 5). The
stratification varies fromcoarseto fine. Asa rule, the finer the grain
size, the thinner the strata. Cross-bedding is reported, but the
observations are too few to establish a consistent dip direction. The
ore lacks discernible stratification in some places, especially in the
eastern side of the open pit.
The powdery ore is a friable, locally unconsolidated, aggregate
of iron oxide, either hematite or martitized magnetite. It was often
difficult to determine the nature of fine-grained iron oxide with
certainty in the field. Readily visible crystals consist of tabular
hematite (Fig. 6). Inspection under microscope shows that all the
samples, except two, are composed of seemingly unaltered
hematite; control with X-ray diffraction reveals no trace of
magnetite in the samples. The hematite plates are subhedral to
euhedral with well-developed crystal faces, and most of them
range from c. 0.05 mm to 3 mm in size. An incipient to partial
corrosion of the crystals is common. Some ore layers or parts
thereof are unconsolidated (Fig. 7A), whereas others are friable
open frameworks of platy crystals (Fig. 7B – C). The hematite
plates lack preferred orientation, and are seemingly piled upon
each other.
The two exceptional samples come from two different beds of
powdery ore: the thick ore bed in the northern part of the open pit
and another bed in the southwestern part (Malvinas). The samples
consist of martitized magnetite of octahedral habit (Fig. 7D),
without visible hematite plates.
Apatite is absent, or present only in small amounts, in the
powdery ore (Fig. 7A– D). However, a few samples are relatively
rich in apatite. The mineral occurs as tiny euhedral needles
invisible to the unaided eye (up to 0.1 mm long). Many needles
Fig. 2. The partly exploited La Perla iron deposit in March 1976 (after unpublished cross sections and a mine map modified by Van Allen 1978).
The pollen-bearing ore bed treated in this study is projected as a short thick line on the inset map. Available maps and sections of the iron
mineralization are based on ore grade (% Fe) and physical nature (hardness): as used in the mine, massive ore requires dynamite during mining,
whereas powdery ore can be loaded as it is. Thus, the massive ore of the sections includes powdery ore (in the sense used in our study) of moderate
to low friability.
GFF 132 (2010) Corona-Esquivel et al.: Palynologic evidence for iron-oxide ash fall at La Perla 175
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project from hematite crystals, and form radiating clusters in
apatite-rich parts of the ore. Some needles end with a hollow point.
Other minerals in the ore are quartz, kaolinite and small
amounts of K-feldspar and smectite, derived from altered
rhyodacite. Some of the ore is incipiently to partly cemented by
quartz (Fig. 7C), or calcite; fluorite is also found. The lower part
of the thick ore bed in the north is locally silicified, with loss of its
original texture and pore space. The silicification is gradual and
has a patchy distribution. The rhyodacite hosting the ore is
marked by silicification and kaolinization of varying intensity,
and local concentrations of fluorite (Ca
´rdenas-Vargas & Del
Castillo-Garcı
´a 1964; Van Allen 1978).
Angular fragments of partly to strongly altered rhyodacite with
knife-sharp contacts are common in the powdery ore. They show
a general increase in size and abundance towards the bottom of
the thick ore bed. However, in the northeastern wall large angular
blocks of rhyodacite occur near the top of the bed.
Type section with pollen. – A vertical, 105 m long cross section
through the thick bed of powdery ore in the northern part of the
open pit, where pollen was discovered, illustrates the variation
within the powdery ore (Fig. 4). The .10 m thick ore bed is
overlain concordantly by 12 m of rhyodacite, that in turn is
covered discordantly by 30 m of arkose. During sampling, the
ore bed was subdivided into four layers, based on differences in
texture, abundance of rock fragments, and friability. The
lateral extension of the layers were not determined due to
discontinuous outcrop and difficult access. Figure 4 shows
the wall of the open cut in November 2005, when the original 20
samples were collected, and the lower part of the section had not
yet been exposed by mining. From bottom to top the layers are:
Fig. 3. Pollen-bearing, stratified powdery ore composed of
submillimeter-sized hematite crystals in the northern part of the open
pit at La Perla. The vertical flows of ore at the left side of the photo,
caused by rain water, illustrate the poorly consolidated nature of the
powdery ore.
Fig. 6. Variation of grain size in stratified powdery ore in the northern
part of the open pit. The coarse-grained ore below the bright red grooves
at the top is composed of 0.1–0.8 cm large hematite plates without
preferred orientation.
Fig. 4. The type section of powdery ore in the northern part of the open
pit where pollen was first found (Layers 1 –4 are indicated at the left
side).
Fig. 5. Stratified powdery orein the northern part of theopen pit (cf. Fig 4).
176 Corona-Esquivel et al.: Palynologic evidence for iron-oxide ash fall at La Perla GFF 132 (2010)
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Layer 1 is a chaotic breccia with a matrix of powdery ore.
Angular to subangular blocks, as large as 60 – 80 cm, and smaller
fragments of altered rock are abundant, though irregularly
distributed in the ore. They make up between 10 and 90% of the
volume of the layer. The iron-oxide matrix is fine- to medium-
grained with 0.2 to 1.5-mm-long hematite plates. The latter
typically have a rounded, corroded appearance. The ore lacks
stratification, and the friability is low to moderate. During
the sampling the exposed part of Layer 1 was ca. 5 m thick;
subsequent mining uncovered 12 m more without reaching
its base.
Layer 2 ranges considerably in thickness, from 1.5 to 5 m. It
rests concordantly on Layer 1, with an irregular contact surface.
The ore is stratified on the 1–2 cm scale and fine-grained, with
up to 2 mm large crystals of iron oxide. It is highly friable and can
be dug out by hand. Rock fragments occur in highly variable
amounts and sizes. Locally they make up 40% of the volume and
attain sizes of 20 –40 cm.
Pollen was discovered in ore from Layer 2, and the subsequent
search for more pollen consumed most of the samples from this
layer. The iron oxide is martitized magnetite in some of the
sampled ore (Fig. 7D). Other parts of Layer 2 consist of platy
hematite (Fig. 7B –C). The nature of the iron oxide was not
systematically determined in the field due to difficult access and
small grain size, and could not be clarified on the second visit
since mining had removed the type section.
Layer 3 lies concordantly on Layer 2 and has a uniform
thickness between 1.5 and 2 m. The stratification is more
pronounced and finer here, on the 0.5–1 cm scale. The ore is
very friable and, in part, unconsolidated. The iron oxide is
euhedral hematite, predominantly of submillimeter-size, alter-
nating with thin strata of coarser grain size. Tiny apatite needles
(up to 0.1 mm long) are more common in samples from this layer
than in the other layers. Angular, ca. 2-cm-large rock fragments
occur in a 15 to 20-cm-thick part of the layer.
Layer 4 has a thickness between 1 and 2 m, and is covered
concordantly by a rhyodacite flow. A disconformity at the base of
the layer is the reason for itsseparation from similar ore in Layer 3.
The friable, stratified ore is characterized by tabular hematite
crystals ranging in size from 0.1 to 2 mm. Cross-bedding is
observed locally, and ca. 3 to 4-cm-large rock fragments are
common.
Palynomorphs found in the powdery ore
Out of the 20 samples collected in the type section, only 5 were
productive (one of them was richer in clay fraction and
contained much more pollen). These samples nevertheless host a
well-preserved flora dominated by angiosperm pollen but
gymnosperm pollen is also present. Furthermore, two genus of
fungal spores have been identified in the palynological
assemblages. Recent study of the palynomorphs recovered
from the samples collected in June 2008 revealed the same taxa,
with the addition of the genus Typha.
The exines of the pollen grains have a light yellow color and thus
show no signs of thermal alteration. Some pollen grains have a
reddish stain caused by impregnation of hematite or goethite.
The palynological assemblages comprise palynomorphs
which range in age from the Paleogene to the present. However,
presence of two index fungal spores indicates that the minimum
age of the fossil flora is Upper Oligocene or Miocene.
Fungal spores
Fungal spores from two genera, Frasnacritetrus and Dyctios-
porites, were found in the samples from Layer 2. They are good
index genera, with age ranges from the Eocene to the Miocene.
The scarcity of material does not permit species differentiation
and further chronostratigraphic refinement.
Frasnacritetrus (Taugourdeau 1968; Saxena & Sarkar 1986),
morphologically similar to F. indicus from the Cenozoic of
India, was described by Saxena & Khare (1992). However, its
hyphea and the central body are much smaller (Fig. 8A). This
genus was abundant during the Paleogene – Neogene, with
different species reported from Oligocene, Miocene and
Pliocene formations.
Dictyosporites morularis (Salard-Cheboldaeff & Locquin
1980; Kalgutkar & Jansonius 2000) (Fig. 8B). This fungal spore
was reported initially from an Oligocene formation in
Cameroon, Africa. In later reports the genus Dictyosporites
comprises other species of Eocene, Oligocene and Miocene age
(Kalgutkar & Jansonius 2000).
Pollen
The most common pollen grains in the analyzed samples belong
to wind-pollinated plants of the Betulaceae family. They include
Alnipollenites (Fig. 8C), Triatriopollenites (Fig. 8D), and
Triporopollenites (Fig. 8E). Different species of these genera
appeared during the Late Cretaceous, and became abundant
during the Paleogene in Laurasia. At present, Betulaceae is well-
represented in temperate plant communities.
Chenopodipollis, produced by wind-pollinated plants such as
Chenopodiaceae and Amaranthaceae, is well-represented in the
samples (Fig. 8F). The oldest reported Chenopodipollis is from
the Paleocene of North America and it became more common
with the change of climate from the Oligocene on. In Mexico,
the oldest finds are of Eocene age (Martı
´nez & Ramirez 2006).
The Compositae family is represented by three morphological
types belonging to Tubulifloridites (Fig. 8G – I). This taxa first
appeared in Laurasia during the Eocene (Cross & Martı
´nez
1980), and became more common during the Oligocene-
Miocene (Martı
´nez & Ramirez 2006). The diversity of
Compositae pollen in the samples indicates that the maximum
age of the La Perla deposit is Oligocene, when these plants
probably were abundant in the region.
The form genus Momipites is represented by a few pollen
grains in the samples (Fig. 8J). It belongs to the group Coryloides
sensu Nichols (1973), with species that are indistinguishable
from the present genera Engelhardia and Alfaroa. Trees of these
genera are characteristic of cloud forests in Mexico and Central
America. Momipites species were widespread in North America
and Europe from the Paleocene to the Miocene.
Myrtaceidites pollen occurs in all of the pollen-bearing samples
(Fig. 8K). The affinities of this form genus have been the subject
of much discussion. Krutzsch (1969) concluded that this form
belongs to the Myrtaceae family. The oldest occurrences reported
are from the Late Cretaceous of Africa (Muller 1968). In the
U.S.A. Myrtaceidites is reported from the Eocene (Elsik &
Dilcher 1974), and in Mexico from the Eocene – Oligocene
boundary (Martı
´nez & Ramı
´rez 1999).
Quercoidites pollen is found in all the pollen-bearing samples
(Fig. 8L, M and Q), suggesting that this genus was widespread in
the region. Quercoidites was very abundant during the
GFF 132 (2010) Corona-Esquivel et al.: Palynologic evidence for iron-oxide ash fall at La Perla 177
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Oligocene in U.S.A. (Elsik 1978). This genus is also common in
Mexico from the Neogene on. At present, oaks (Quercus spp.)
grow in mountain forests in Mexico.
The form genus Rugulitriporites is found in only one
sample (Fig. 8N). It is associated with Bursera, which is common
in Paleogene basin deposits in Mexico (Martı
´nez & Ramı
´rez
2006). Rugulitriporites grew in tropical deciduous forests.
Salixipollenites (Fig. 8O –P), with features similar to pollen of
Salix, is reported from Cenozoic basins from the Paleocene on in
North America (Srivastava 1972). However, Muller (1981),
quoting Graham and Jarzen (1969), considered that Salix first
appeared during the Oligocene.
Pollen of the genus Typha were found in the samples from
June 2008, studied recently. Present-day species of this genus
are aquatic, living in wetland habitats.
Gymnosperms are represented in the samples by two groups of
conifers. The first group consists of Pinus (Fig. 9A – B) which
suggests that conifer forests grew nearby at higher elevation.
This genus has existed since the Cretaceous and is known in
Mexico from the Miocene on (Martı
´nez & Ramı
´rez 1998). At
present, Pinus dominates temperate forest at altitudes above
3000 m. The second conifer group is represented by Cupressa-
cites which occurs in two different forms with probable affinities
to Cupresaceae and Taxodiaceae (Fig. 9C – D). These families
have a reported age range from the Cretaceous to the present.
In Mexico, they have been common in basins since the Paleogene.
Other taxa found in the samples are Graminidites (Fig. 9E),
Liliacidites (Fig. 9F –G), Rhoipites (Fig. 9H–I), and Stria-
tricolporites (Fig. 9J).
Discussion
Several lines of evidence strongly suggest an ash-fall
origin of the powdery iron ore at La Perla. The most important are:
1. the presence of well-preserved fossil palynomorphs in the ore;
2. the morphology, structure and texture of the orebodies;
3. the physical nature of the iron-oxide aggregate constituting
the ore; and
4. the composition of the ore: euhedral crystals of hematite or
martitized magnetite; these two phases do not occur together
in the same layer or part thereof.
Fig. 7. SEM images of powdery ore from La Perla. A. Subhedral to euhedral hematite plates in unconsolidated ore (sample M-1). B. Locally
corroded hematite plates and small amounts of apatite in friable ore (LAP03-08). C. Euhedral hematite plates, stubby quartz crystals and an apatite
prism in partly cemented ore (LAP02-08). D. Martitized magnetite of octahedral habit in unconsolidated ore (Pb9737).
178 Corona-Esquivel et al.: Palynologic evidence for iron-oxide ash fall at La Perla GFF 132 (2010)
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The Miocene minimum age of the palynomorphs is in good
agreement with the Early Oligocene radiometric age of the
volcanic rock hosting the ore, and rules out contamination with
present-day material during mining. The light yellow color of the
exines of the palynomorphs in the La Perla samples demonstrate
that they are unaffected by thermal alteration. On heating, exines of
pollen and spores become light brown in the 100–1508Crange,
showing a progressive darkening in color with temperature due to
carbonization, until they become almost black close to 2008C, and
are then destroyed at higher temperatures (Gray & Boucot 1975;
Hemsley et al. 1996). Thus, the temperature of the iron-oxide
matrix of the pollen has not exceeded 1508C since its deposition.
The beds of powdery ore occur intercalated in a sequence of
lavas and clastic rocks. Exposed contacts with the host rock are
razor-sharp and concordant, and rock fragments in the ore lack
iron mineralization. Morphologically, the subhorizontal ore beds
resemble deposits laid down by sedimentary processes.
The stratification of the ore, with prominent size-sorting and
even cross-bedding, indicates particulate transport and depo-
sition. However, the euhedral habit characterizing a large
proportion of the iron-oxide crystals constituting the ore, and its
composition (either hematite or martitized magnetite, not both),
are inconsistent with an epiclastic character. The crystals have
not been rounded by abrasion. Some hematite ore samples
contain crystals with partly rounded faces, but other features
reveal that these crystals are marked by corrosion. In addition,
there is no evidence that the iron-oxide beds are resedimented
deposits.
The friable or unconsolidated nature of the powdery ore and its
high porosity rule out formation by replacement of preexisting
silicate rock. Such a process would fill open spaces between the
constituent phases and destroy textures. The silicified ore at the
Fig. 8. Photomicrographs of fungal spores (A–
B) and pollen (C–Q) from powdery ore at La
Perla (the scale in Aapplies to all photos).
A.Frasnacritetrus.B.Dictyosporites morularis.
C.Alnipollenites.D.Triatriopollenites.
E.Triporopollenites.F.Chenopodipollis.G–I.
Tubulifloridites.J.Momipites.K.Myrtaceidites.
L–M.Quercoidites.N.Rugulitriporites.O–P.
Salixipollenites.Q.Quercoidites.
GFF 132 (2010) Corona-Esquivel et al.: Palynologic evidence for iron-oxide ash fall at La Perla 179
Downloaded By: [Naturhistoriska Riksmuseet] At: 06:46 30 November 2010
base of the section is a good illustration: it is dense and nonporous,
and its original texture has been erased or remains as relict
patches. It is also difficult to imagine a hydrothermal process that
would deposit extensive subhorizontal beds of stratified, size-
sorted hematite or magnetite, made up of crystal aggregates that
remain unconsolidated. The presence of pollen in the ore means
that the ore must have formed at the surface, assuming that
the palynomorphs were not introduced at a later stage of the
geologic history. The low-temperature environment (,1508C)
inferred from the well-preserved palynomorphs is consistent
with the kaolinite-quartz-dominated alteration of the rhyodacite
occurring as fragments in the ore and hosting the orebodies.
The fact that a bed of powdery ore or part of it is composed
either of hematite plates, or martitized magnetite octahedra,
shows that both minerals are primary phases. The simplest and
most likely explanation is that the ore beds were deposited by
ash fall (or perhaps by surge in the case of Layer 4; cf. Cas &
Wright 1987). In fact, the deposits would be regarded as tuffs (or
ash) by any geologist familiar with volcanic rocks, were it not
for their unusual composition. Formation of hematite requires a
relatively high oxygen fugacity, consistent with crystallization
at shallow depth or subaerially; magnetite forms at a lower
fugacity. This suggests a volcanic setting with fluctuations in
oxygen fugacity during episodic eruptions of partly crystallized
and highly gas-charged iron-oxide magma. The observations of
scoriaceous ore that looked like a vesicular basalt (Ca
´rdenas-
Vargas & Del Castillo-Garcı
´a 1964), and flow banding (Van
Allen 1978), support a volcanic setting. The exploited lenticular
orebodies with these features were probably lava flows.
Thus, the available evidence suggests that the powdery ore is a
pyroclastic deposit: a poorly consolidated crystal tuff. The well-
developed hematite crystals which compose the friable open
framework of most powdery ore (Fig. 7B –C) did not grow
in situ; the crystals in unconsolidated parts show no sign of
having grown attached to a surface. They were deposited. Wet
iron-oxide ash might have captured pollen and spores if the ash
fall was accompanied by rain. A more likely alternative is that
the iron oxide and wind-blown pollen were deposited in an
environment with temporary pools or small lakes. This
interpretation is supported by the occurrence of clay-rich strata
in the powdery ore (Pe
´rez-Segura 1982), the observation that the
sample with most pollen also contains more clay than the other
samples, and the presence of Typha pollen. The association of
pollen and clay argues against the possibility that the pollen was
transported into the ore at a later stage by circulating water.
Unconsolidated powdery iron-oxide ore has been described
from other iron ore deposits in volcanic settings, in Mexico
(Cerro de Mercado; Lyons 1988) and Chile (El Laco; Nystro
¨m&
Fig. 9. Photomicrographs of pollen from powdery ore at La Perla (the scale in Aapplies to all photos). A–B.Pinus.C–D. Cupresaceae and
Taxodiaceae. E.Graminidites.F–G.Liliacidites.H–I.Rhoipites.J.Striatricolporites.
180 Corona-Esquivel et al.: Palynologic evidence for iron-oxide ash fall at La Perla GFF 132 (2010)
Downloaded By: [Naturhistoriska Riksmuseet] At: 06:46 30 November 2010
Henrı
´quez 1994; Naslund et al. 2002). These deposits are
stratified and size-sorted, or internally structureless. They are
characterized by euhedral crystals of magnetite or hematite. The
cited authors argued that the iron oxide was deposited as
volcanic ash. This suggestion is strongly supported by the
existence of very similar pyroclastic ore at La Perla.
Acknowledgements.–This study was financed by Programa de Apoyo a Proyectos de
Investigacio
´n e Innovacio
´n Tecnolo
´gica de la Universidad Naciona
´l Auto
´noma de Me
´xico
(PAPIT-UNAM) research projects IN-123202-2 and IN-115706-3. Grupo Acerero del Norte
(GAN) kindly supplied facilities for the mine work and lodging at La Perla. We thank
Marcos Escudero Cha
´vez, Toma
´s Quintana Fierro, Emilio Torres de la Torre, Claudia Sosa
de la Rosa, Hugo Alarco
´n Talavera and Carlos Nun
˜ez Alfaro for assistance in the mine, and
Vivi Vajda and an unknown reviewer for criticism that improved the manuscript.
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