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Journal of Applied Geophysics 1420 (2003) xxx–xxx
OOFIntegrated magnetic studies of the El Romeral iron-ore deposit,
Chile: implications for ore genesis and modeling of
magnetic anomalies
L.M. Alva-Valdiviaa,*, M.L. Rivas-Sancheza, A. Goguitchaichvilia,J. Urrutia-Fucugauchia, A. Gonzaleza, W. Vivallob
a Instituto de Geofısica, Universidad Nacional Autonoma de Mexico, Ciudad Universitaria, Del. Coyoacan, Mexico D.F. 04510, MexicobServicio Nacional de Geologia y Mineria, Chile
Received 9 April 2002; accepted 19 May 2003
RNCORRECTED PAbstract
Mineralogical and rock-magnetic studies of iron ores and host rocks in El Romeral Mine are carried out to characterize the
magnetic mineralogy and the processes that affect the natural remanent magnetization during emplacement and evolution of the
iron-ore deposit. Extremely important is the identification of magnetic mineralogical composition (magnetite and/or
titanomagnetite, hematite and/or titanohematite, and titanomaghemite) and grain size. These data permit investigation of
magnetic domain state and magnetization acquisition processes and to assess their significance as a source of magnetic
anomalies. Chemical remanent magnetization (CRM) seems to be present in most of investigated ore and wall-rock samples,
substituting completely or partially the original thermoremanent magnetization (TRM). Magnetite (or Ti-poor titanomagnetite)
and titanohematite are commonly found in the ores. Although hematite may carry a stable CRM, no secondary components are
detected above 580 jC, which probably attests that oxidation occurred soon after the extrusion and cooling of the ore-bearing
magma.
The microscopy study under reflected light shows that magnetic carriers are mainly titanomagnetite with significant amounts
of ilmenite–hematite minerals. Magmatic titanomagnetite, found in igneous rocks, shows trellis texture, which is compatible
with high temperature (deuteric) oxy-exsolution processes. Hydrothermal alteration in ore deposits is indicated by goethite and
hematite oxide minerals. Grain sizes range from a few microns to >100 Am, and possible magnetic states from single to
multidomain, in agreement with hysteresis measurements. Thermal spectra, continuous susceptibility measurements, and
isothermal remanent magnetization acquisition suggest a predominance of spinels as magnetic carriers, most probably
titanomagnetites with low-Ti content.
For quantitative modeling of the magnetic anomaly, we used data on bulk susceptibility and natural remanent intensity for
quantifying the relative contributions of induced and remanent magnetization components, and this allows greater control of the
geometry of source bodies. The position and geometry of these magnetic sources are shown as ENE-striking tabular bodies, one
steeply inclined (75j) to the south and another lying horizontal.
D 2003 Published by Elsevier B.V.
UKeywords: Rock magnetism; Magnetic anomalies; Microscopy; Romeral iron deposit; Chile* Corresponding author.
0926-9851/03/$ - see front matter D 2003 Published by Elsevier B.V.
doi:10.1016/S0926-9851(03)00043-0
E-mail address: [email protected] (L.M. Alva-Valdivia).
APPGEO-01420
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UNCORREC
1. Introduction
It is increasingly recognized that information on
rock-magnetic properties assists in optimizing mag-
netic anomaly interpretations, especially for iron
ores and related igneous rocks. The intensity of
remanent magnetism varies greatly in these rocks
and Fe-ores and it may exceed the intensity of
induced magnetization by factors greater than 10
(Alva-Valdivia et al., 1991, 2001; Alva-Valdivia and
Urrutia-Fucugauchi, 1995; DuBois, 1962, 1963).
Some authors have focused on different aspects,
e.g., relationships between petrology and observed
aeromagnetic anomalies and physical properties (to
define geological and structural units and the map-
ping of these units; Skilbrei et al., 1991) and to
establish a correspondence between magnetic pe-
trology and rock-magnetic properties (Henkel,
1994).
Microscopy and rock-magnetic properties appear
as appropriate tools for investigating Chilean iron-
ore deposits. If the ores were initially lava flows
of magnetite, they would have cooled rapidly and
acquired a stable thermoremanent magnetization
(TRM). Different forms of thermochemical pro-
cesses, which may occur during and after em-
placement of iron ores, may result in recrystalliza-
tion of oxide minerals. Hence a TRM may be
partially or completely replaced by CRM. A hydro-
thermal origin of the ores at relatively low temper-
atures would entail acquisition of CRM rather than
TRM.
Rock-magnetic parameters represent the link be-
tween geophysical anomalies of potential fields and
geology in terms of structure and lithology. These
properties were successfully used for quantitative
interpretation of the magnetic anomaly by providing
adequate control of the geometry of source bodies,
and thereby considerably reducing the ambiguity in
such interpretations. The aim of this paper is to
examine the results of microscopy and rock magne-
tism in the El Romeral Mine of the Chilean iron belt
(Fig. 1), in order to evaluate factors like type, grade,
grain size, mineralogy, size and depth, shape and
attitude of the ore bodies. They are valuable tools to
investigate the origin of the iron mineralization, and
helpful for the exploration–exploitation program
planning.
ED PROOF
2. Geology and mineralization
Mining activity in El Romeral started at the begin-
ning of the 20th century, reaching 3 Mt of 64% iron-
ore production in 1964. Up to 1995 the total produc-
tion was around 130 Mt. The exploited area covers
approximately 1900 m N–S by 1000 m E–W, with an
altitude ranging from 130 to 460 m. Several geologic
studies have been made since 1975, mainly as unpub-
lished Internal Reports for the Companıa Minera del
Pacifico and Theses. There are two hypotheses to
explain the origin of this Fe-ore. The first, and older
one proposed by Bookstrom (1977), suggests a hy-
drothermal process as the source of the ore. The
second one, proposed by several authors, suggests
that the iron ore is closely related to direct injection of
Fe-ore magma (Dobbs and Henriquez, 1988; Frietsch,
1978; Nystrom and Henriquez, 1994).
The Chilean iron belt (see Nystrom and Henri-
quez, 1994) consists of several large high-grade ore
bodies and many medium-sized and small deposits of
apatite iron ore. Our sampling concentrated at the El
Romeral iron-ore deposit, which is one of the largest
still being mined. The deposit was formed in a
magmatic arc along the western margin of the Lower
Cretaceous shallow-marine basin (Fig. 1; Espinoza,
1990).
The Cretaceous volcanic rocks and intrusives host-
ing the iron deposits are interpreted as co-magmatic or
as derived from different parent magmas. The volca-
nic sequence is dominated by basaltic to andesitic lava
flows, and most of the intrusives are dioritic bodies of
a large batholith, apparently younger than the lavas.
The ores are coeval with the volcanic rocks according
to Espinoza (1984) and Oyarzun and Frutos (1984),
but Ruiz et al. (1965, 1968) consider them somewhat
younger. Available radiometric K–Ar whole-rock
ages from rocks associated with the ores from four
deposits fall in the range 100 to 128 Ma (Zentilli,
1974; Pichon, 1981; Montecinos, 1983).
El Romeral is composed of two major bodies
(known as Cuerpo Principal and Romeral Norte) partly
delimited by zones of faulting and mylonitization, and
three small bodies. Cuerpo Principal is lenticular, with
horizontal dimensions of 850� 250 m, and a depth
extent of up to 600 m. Romeral Norte is a strongly
deformed andmartitized ore body, 300 by 50–120m in
plan and with a depth extent up to 240 m.
UNCORRECTED PROOF
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Fig. 1. Simplified regional map of the study area showing the location of the El Romeral iron-ore deposit, Chile. Geological map of the El
Romeral District. Black dots indicate the paleomagnetic sampling sites.
L.M. Alva-Valdivia et al. / Journal of Applied Geophysics 1420 (2003) xxx–xxx 3
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L.M. Alva-Valdivia et al. / Journal of Applied Geophysics 1420 (2003) xxx–xxx4
Four sites were collected at Cuerpo Norte as
follows (Fig. 1): site RMM is a small massive
magnetite body of irregular shape hosted in actino-
litized rocks. It is cut by a pegmatitic K-feldspar,
quartz and tourmaline dike; site RBL is a disseminat-
ed low-grade ore, hosted in an strongly actinolitized
rock with disseminated pyrite–chalcopyrite, showing
weak to moderate foliation; site RF is disseminated
Fe-ore located close to the fault that marks the western
boundary of high-grade ore. It is located in a zone of
high silicification and many quartz veins, together
with many veinlets of disseminated pyrite–chalcopy-
rite; site RGW is a granodioritic body of medium
grain-size showing silicified fractures in moderate
quantity. It is on the western border of the deposit
and does not show mineralization.
Another three sites are in the southern sector of the
open pit mine (Fig. 1): site RD is a dioritic fine grain-
size dike with some disseminated pyrite and highly
fractures filled with epidote. It has a NE strike and
cuts the mineralization; site RMB is a brecciated iron
ore with abundant pyrite in disseminations and veins;
site RCE is the host rock to the east side of the
deposit. It is strongly silicified and actinolitized with
variable disseminated magnetite content; Finally, site
RGE is a granodioritic body of medium grain-size and
regular amount of fractures filled with silica. It lies
towards the west boundary of the deposit and does not
show mineralization.
C194195
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ORRE3. Methods of studyWe collected 48 drill-core samples from eight
sites of host-rocks and Fe-ore from the cut exposures
in the open-pit mine of El Romeral (Fig. 1). Six
cores were taken at each site with a portable gaso-
line-powered drill, and oriented with an inclinometer
UNCFig. 2. Representative photomicrographs of the host rocks and iron ore
Ab= albite, Ac = actinolite, Pi = pyrite, Pr = pyrrothite, T = transparent min
disseminated in an actinolite mesostasis. In minor proportion are tabular
flow direction. (B) Again sample RBL but in transmitted light shows the
(C) shows ilmenite segregated in actinolite and quartz filling veins in magn
small holes partly or completely filled by pyrrothite (D). Sample RD (E) sh
fracturing affect the magnetite, the fractures were later filled by transpare
grains. Sample RCE (F) shows a single ilmenite crystal with hematite la
hematite lamellae along the crystallographic planes of ilmenite. Sample R
angles along their straight contacts.
ED PROOF
and magnetic compass (using a 50-cm extension in
order to diminish the effect of the highly magnetic
iron ore).
Remanent magnetization was measured with a
Molspin spinner magnetometer. Separation of second-
ary magnetization components from the characteristic
component was accomplished by thermal demagneti-
zation up to 550 jC using a noninductive Schonstedt
furnace. Room-temperature low-field magnetic sus-
ceptibility k was measured with an MS-2 Bartington
susceptibility meter. Continuous susceptibility meas-
urements (i.e., k–T low- and high-temperature curves)
were obtained using a Highmoor instrument. Isother-
mal remanent magnetization (IRM) acquisition curves
with 10–14 steps to a maximum field of 700 mTwere
obtained with a pulse magnetizer in selected samples.
Stepwise alternating field (AF) demagnetization of the
IRMs (8–10 steps up to a maximum field of 170 mT)
was accomplished with a Schonstedt demagnetization
apparatus.
Magnetic hysteresis loops were measured with a
‘MicroMag’ alternating gradient force magnetometer.
Associated IRM acquisition and DC back-field de-
magnetization curves were also acquired with this
instrument. Opaque and transparent minerals were
examined under reflected and transmitted light to
determine composition, texture type, degree of alter-
ation and mineralogical association.
4. Microscopy
Petrography and oxide mineral observations were
done with an ore microscope using thin-polished
sections, and oil immersion, for a characteristic sam-
ple from each site. Direct observation of both trans-
parent and opaque minerals may assist in estimating
the origin of mineralization and magnetization, be-
. Labels are as follows: M=magnetite, H = hematite, I = ilmenite,
eral. Sample RBL (A) shows magnetite xenoblasts homogeneously
ilmenite crystals into the actinolite. Both minerals show a preferred
120j interfacial angles for the albite–actinolite crystals. Sample RF
etite. Another view is of massive pyrite filling holes in actinolite and
ows skeletal crystals of magnetite within transparent mineral. Highly
nt minerals. It is possible to see corroded borders in the magnetite
mellae. Sample RGE (G) shows a hexagonal ilmenite crystal with
MM (H) shows magnetite crystals joined in groups forming 120j
UNCORRECTED PROOF
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t1.1t1.2
t1.3
t1.4t1.5t1.6t1.7t1.8t1.9t1.10t1.11
t1.12
L.M. Alva-Valdivia et al. / Journal of Applied Geophysics 1420 (2003) xxx–xxx6
REC
cause we still have no clear magnetic indication to
distinguish between TRM and CRM.
A summary of the observations with transmitted
and reflected light is shown in Fig. 2. Almost all the
sampled rocks show the effects of metamorphism in
varying degrees. Some samples contain pyrrothite,
which may present ferrimagnetic properties and a total
magnetization up to 10 times larger than those samples
containing magnetite (Henkel, 1994). The main mag-
netic minerals are titanomagnetites (titanohematite)
associated with exsoluted ilmenite (hematite) formed
as a result of oxidation. These intergrowths typically
develop at temperatures higher than 600 jC (Haggerty,
1976) and, consequently, the natural remanent magne-
tization (NRM) carried by these samples is thermo-
remanent (TRM) magnetization. It should be noted
that the size of the magnetic grains observed is often
too small and the petrological/optical observations of
larger ferromagnetic minerals may not completely
correlate with the magnetic behavior of the whole
rock.
Hydrothermal alteration has played an important
role in the El Romeral deposit, producing magnetite,
quartz, epidote, calcite, actinolite and chlorite. The
main objective of microscopic observations was to
evaluate how magnetic grains were affected by hy-
drothermal alteration and the different stages of meta-
morphic processes.
Some samples (RMB, RF and RGE) were analyzed
in the scanning electron microprobe (EPMA) and the
scanning electron microscope (SEM) in order to
determine the composition and mineral textural rela-
tionships. In general, the magnetite ore has Fe 2 + and
Fe 3 + and in order of decreasing proportions: V, Ti
UNCORTable 1
Rock magnetic parameters of host and minerals from El Romeral iron-ore
Site no. N/R D I K A95 M
RD 4/2 20.1 � 35.8 91.8 9.6
RMB 3/3 356.9 8.0 29.0 23.3 1
RGW 6/4 256.9 � 2.0 9.9 30.7
RCE 4/2 223.0 � 16.0 20.8 27.7
RGE 4/2 182.5 34.5 15.4 23.8
RF 4/2 328.5 � 25.4 12.1 27.5
RBL 4/2 27.2 � 19.3 23.4 19.4 1
RMM 3/3 29.3 � 15.3 39.5 41.0
N/R, number of samples used/rejected for site mean calculation; D/I and M
A95, confidence parameters; k, initial susceptibility; Q, Konigsberger fact
Ms, magnetization ratio.
and Co. Ilmenite is mainly composed of Fe 2 +, Ti and
Mn. Sphene has Ti, Ca and Si as the main constitu-
ents, with minor amounts of Mn and Al. Finally,
actinolite is formed of Si, Mg, Ca and Fe.
ED PROOF
5. Magnetic measurements
Rock-magnetic experiments were accomplished to
identify the magnetic carriers responsible for the
remanent magnetization and acquire information
about their magnetic stability. These experiments
included NRM and susceptibility measurements; step-
wise alternating field and thermal demagnetization of
NRM; continuous susceptibility measurements; IRM
acquisition and AF demagnetization of saturation
IRM (SIRM); and hysteresis experiments.
5.1. NRM intensity, susceptibility, and Koenigsberger
ratio (Q)
NRM intensity (Mo), susceptibility (k) at room
temperature, and the Koenigsberger ratio Q =Mo/kH
(calculated for the present geomagnetic field) are
shown in Table 1. Koenigsberger ratios attest the
comparative importance of remanent and induced
magnetization, with remanence dominant for Q>1.
Variations in Mo and k are due mostly to two factors,
different initial volume contents of magnetite, and
development of weakly magnetic or nonmagnetic
phases produced during oxidation or alteration of
the rocks.
Magnetic carriers in the diorite (RD) and diorite
(RGW) intrusive rocks are associated with the
deposit
o (mA/m) k (10� 3 SI) Q Hcr/Hc Mr/Ms
2.47 1.34 0.09 2.22 0.002
8,969.70 1105.41 0.88 3.23 0.036
141.96 26.28 0.28 7.0 0.023
25.63 2.54 0.52 4.64 0.116
292.88 15.30 0.98 4.04 0.068
7430.01 215.83 1.76 5.5 0.019
2,962.30 430.21 1.54 5.14 0.020
8999.00 1919.25 0.24 4.8 0.007
o, direction and intensity of natural remanent magnetization; K and
or using H= 0.24611, Oe = 19.58 A/m. Hcr/Hc, coercivity ratio; Mr/
T
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L.M. Alva-Valdivia et al. / Journal of Applied Geophysics 1420 (2003) xxx–xxx 7
lowest Q values ( < 0.3), indicating a predominance
of multidomain (MD) carriers (Table 1; Fig. 3).
Relatively high Q ratios (average f 1), for diorite
(RGE), may indirectly attest a TRM origin for the
magnetization (Goguitchaichvili, 1995). As noted in
the last section, the dominant carriers are Ti-poor
titanomagnetites, with some deuteric oxy-exsolution
products. The actinolite host rock (RCE) with no
Fe-ore association exhibits medium Q values (av-
erage f 0.52). Sites RD and RCE have the lowest
susceptibilities (1.34–2.54� 10� 3 SI) associated
with relatively low–medium NRM intensities,
resulting in low–medium Q ratios. For these sites,
the NRMs have low to intermediate coercivity and
low to medium directional stability under AF
demagnetization.
The brecciated, disseminated and massive iron-ore
samples have the highest NRM intensity values (up to
19 A/m, site RMB) and the highest susceptibilities (up
to 1920� 10� 3 SI, site RMM), with high to low Q
ratios, suggesting a wide range of magnetite grain
sizes carrying NRM. Bulk properties like IRM acqui-
sition indicate saturation in low fields (Fig. 4). These
experiments reflect mainly the coarse MD grains, but
finer pseudo-single-domain (PSD) to single-domain
(SD) grains must also contribute to the NRM. These
finer-grained minerals probably account for the mod-
erate coercivity, detected during AF demagnetization
of IRM (Fig. 5). Although (titano)hematite was di-
UNCORREC322
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340Fig. 3. Mean values of natural remanent magnetization vs. magnetic
susceptibility for each site.
ED PROOF
rectly observed under reflected light microscopy, its
magnetic signal seems to be minor.
The NRM directions for sites RD, RMB, RF, RBL
and RMM are broadly of normal polarity whereas
RCE and RGe have reverse polarity NRMs, RGW has
intermediate directions, suggesting the presence of
components with both polarities. Overall, the NRM
directions suggest the occurrence of at least one
geomagnetic reversal during the magmatic hydrother-
mal evolution of the deposit. For the highly magnetic
rock types, the scatter of directions may also reflect
inhomogeneous internal (self-demagnetizing) fields.
5.2. Alternating field and thermal demagnetizations
Both alternating field demagnetization using a
laboratory made AF-demagnetizer and stepwise ther-
mal demagnetization up to 550 jC were carried out.
The low-field susceptibility at room temperature was
measured after each step of thermal demagnetization
with a Bartington susceptibility meter. Using principal
component analysis (Kirschvink, 1980), the directions
of characteristic magnetization were determined. Nev-
ertheless the carefully sampling procedure, directions
for each sampled site show high within-site angular
dispersion and the site-mean directions are scattered.
This is attributed to possible chemical remagnetization
and not to secular variation of the Earth’s magnetic
field. Another possible source of discordant directions
may result from small physical movement of the ores
during faulting or mining. Naturally, we always tried
to find the more stable (in situ) outcrops for sampling.
Vector plots (Zijderveld diagrams) show different
behaviors for each rock type. In general, two magne-
tization components are found (Fig. 5). The charac-
teristic remanence is defined above 450 jC or 20 mT.
The low temperature (200–500 jC) or low stability
(up to 10 mT) component is well defined for massive
Fe samples (RMB, RMM, RBL) as well as for the
granodiorite (RCE) sample, and may also correspond
to a chemical overprint. Alternatively, some partial
thermoremanence (pTRM) could be acquired from
about 350 jC (the temperature of hydrothermalism)
to ambient temperature. This pTRM may persist to
higher demagnetization temperatures if magnetization
is carried by PSD or MD particles (Shcherbakova et
al., 1996; Goguitchaichvili et al., 1997). In general,
the behavior of all samples yields reasonably linear
UNCORRECTED PROOF
ARTICLE IN PRESS
Fig. 4. Representative isothermal remanent magnetization (IRM) acquisition curves, and normalized intensity diagram for AF demagnetization
of the IRM for different groups of rocks.
L.M. Alva-Valdivia et al. / Journal of Applied Geophysics 1420 (2003) xxx–xxx8
RRECTED PROOF
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Fig. 5. Orthogonal vector plots of remanent magnetization (Zijderveld, 1967) for representative samples. The numbers refer to the temperatures
(magnetic field) in jC (mT). o—projections onto the horizontal plane; �—projections onto the vertical plane.
L.M. Alva-Valdivia et al. / Journal of Applied Geophysics 1420 (2003) xxx–xxx 9
UNCOand stable magnetic components. A major part of
magnetization is removed above 450 jC or 20 mT,
which probably points to the presence of magmatic or
hydrothermal (titano)magnetite as well as (titano)he-
matite, as observed under reflected light microscopy.
5.3. Continuous susceptibility measurements
One sample from each site was heated up to 600 jCat a heating rate 20 jC/min and then cooled at the same
rate. Curie temperatures were determined by Prevot et
al.’s (1983) method. Alternatively, low-temperature
(from about � 185 jC to room temperature) suscepti-
bility was recorded using the same apparatus.
Three typical curve shapes were observed during
low-temperature susceptibility experiments (Fig. 6).
Curve on sample RD1-Z shows a monotonic decrease
from near � 175 jC, which probably indicate to
Verwey transition, characteristic of almost pure mag-
netite. Ozdemir and Dunlop (1993) shows that this
transition may be largely suppressed for the titanomag-
netites with variable titanium content. Alternatively,
UNCORRECTED PROOF
ARTICLE IN PRESS
Fig. 6. Low-temperature k–T curves.
Fig. 7. High-temperature k–T curves showing Curie temperatures close to 580 jC. These suggest that the main carrier of magnetization is
almost pure magnetite, which is consistent with microscopic observations. The arrows indicate the heating and cooling curves.
L.M. Alva-Valdivia et al. / Journal of Applied Geophysics 1420 (2003) xxx–xxx10
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L.M. Alva-Valdivia et al. / Journal of Applied Geophysics 1420 (2003) xxx–xxx 11
samples RGW4-Z and RMB6-Y show a similar behav-
ior, with a displacement of the inflection point about 25
jC in temperature. They may also correspond to non-
stoichiometric (partially oxidized) magnetite. In one
case, a quite well-defined minimum was detected
(sample RGW4-Z) at f� 115 jC and maximum at
about � 90 jC. This temperature is substantially dif-
ferent fromMorin transition observed for pure hematite
(around � 15 and � 143 jC, respectively, Dunlop andOzdemir, 1997). Only tentatively, we can attribute this
behavior to the presence of mixture of some (titano)-
hematites and (titano)magnetites. Rock-magnetic liter-
ature about the interpretation of low-temperature
continuous susceptibility curves from natural samples
is still needed (e.g., Radhakrishnamurty et al., 1981;
Senanayake and McElhinny, 1981; Urrutia-Fucugau-
chi et al., 1984). Ozdemir (1987) and Dunlop and
UNCORRECT
Fig. 8. Hysteresis loops without paramagnetic correction for typical sample
(RGE), actinolitized host rock (RCE) and dioritic dike (RD).
OOF
Ozdemir (1997) show results from chemically well-
known synthetic (titano)magnetites.
Analogous high-temperature susceptibility experi-
ments as well indicate, in all cases, the presence of Ti-
poor titanomagnetites (Fig. 7). A few sites show
evidence of two ferrimagnetic phases during heating
(samples RMM and RF). The lower Curie point ranges
between 350 and 420 jC, and the highest one is about
580 jC. The cooling curves show only a single phase,
with a Curie temperature close to that of magnetite.
Irreversible k–T curves can be explained by titanoma-
ghemite, which presumably changed into magnetite
(Readman and O’Reilly, 1970; Ozdemir, 1987) during
heating.
The behavior possibly corresponds to small PSD to
MD magnetic grains (Dunlop and Ozdemir, 1997).
Susceptibility values are useful to define the contribu-
ED PR
s. Rock samples and corresponding sites are Fe-ore (RBL), intrusive
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L.M. Alva-Valdivia et al. / Journal of Applied Geophysics 1420 (2003) xxx–xxx12
NCORREC
tion of hematite relative to magnetite, for example, sites
RMB, RF, RBL and RMM show the highest suscepti-
bility values (Table 1), suggesting that the contribution
of hematite is negligible. Here, again, hematite may
coexist judging from susceptibility curves but its con-
tribution is not significant, and could be caused by
thermal drift.
5.4. IRM acquisition and AF demagnetization of SIRM
To determine the isothermal remanent magnetiza-
tion acquisition curves, previously AF demagnetized
samples from all sites were subjected to magnetic
fields along one axis. The curves were found to be
quite similar in all cases (Fig. 4). Saturation is reached
at relatively low fields (200–400 mT), which points to
spinels as remanence carriers. Hematite, although
observed directly, seems to be masked by a strong
ferrimagnetic phase.
AF demagnetization curves of SIRM yield median
destructive fields (MDF) in the range 20–30 mT,
which is compatible with the coercivity of PSD
particles (Dunlop, 1981). Intrusives and altered rocks
seem to have slightly higher coercivity. Fe-ores show
medium MDF (about 30 mT), which correspond to
‘small’ PSD to MD magnetic grains.
5.5. Hysteresis experiments
Hysteresis measurements at room temperature
were performed on samples from all studied units in
fields up to 1 T. The saturation remanent magnetiza-
tion ( Jrs), the saturation magnetization ( Js) and coer-
cive force (Hc) were calculated after correction for the
paramagnetic contribution. The coercivity of rema-
nence (Hcr) was determined by applying progressively
increasing backfield after saturation. Some typical
hysteresis plots are shown in Fig. 8. Hysteresis
parameter plots indicate that almost all samples fall
in PSD grain size region (Day et al., 1977). The Hcr/
Hc ratio ranges from 2.22 to 7.0, and Mrs/Ms varies
between 0.002 and 0.116.
Fig. 9. Aeromagnetic anomaly map. Notice the presence of a huge
dipolar anomaly positioned over the mineralized area. Position of
the profile is shown. Values are given in nanoteslas.
U6. Quantitative modeling of magnetic anomalies
The magnetic anomaly corresponds to a large nor-
mally polarized dipole oriented almost north–south
PROOF
(strike = 185j) that is situated over two major mineral-
ized bodies (Fig. 9). For modeling of the anomaly, we
selected one profile (A–AV) oriented following the
dipolar anomaly field. The dipolar anomaly is charac-
terized by a low/high, with peak-to-peak amplitude of
about 5500 nT. The method used to calculate the
magnetic model is based on the methods of Talwani
et al. (1958) and Talwani and Heirtzler (1964) for two-
dimensional polygonal bodies (Talwani, 1965), which
makes use of the algorithms described in Won and
Bevis (1987). The software used is GM-SYS from
Northwest Geophysical Associates.
We used the measured magnetic properties of the
rock-units sampled to constrain the interpretation. We
analyzed several polygonal models with various com-
binations of magnetic properties, and the best fit
corresponds to two large tabular bodies. One is located
beneath the magnetic low, and another smaller body is
postulated to fit the northern part of the profile (Fig.
10). The rock-magnetic parameters for the proposed
bodies were those measured in samples of site RMB.
The model (that produced an acceptable fit) required a
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Fig. 10. Observed magnetic anomaly and quantitative model. Local
geomagnetic field parameters are as follows: declination D = 3.42j,inclination I =� 28.22j, and total magnetic intensity F= 24611 nT.
We used 2D bodies that extend infinitely along-strike perpendicular
to this profile. Magnetic data for both bodies are as follows: NRM
declination = 357j, NRM inclination = 8j, remanent intensity = 19
A/m and magnetic susceptibility = 1.11 SI.
L.M. Alva-Valdivia et al. / Journal of Applied Geophysics 1420 (2003) xxx–xxx 13
CORRECsusceptibility in the order of 1.11 SI, remanent intensity
of 19.0 A/m, and direction of remanence of D = 357j,I = 8j, for both bodies. The values of susceptibility
postulated are high because normally they increase
with depth because of lower alteration. This suggests
that the anomaly is generated by approximately equal
contributions from induced and remanent magnetiza-
tions (Q = 0.88). The ambiguity of the modeling pro-
cess has been constrained, decreasing notably the
uncertainty in the size, depth and geometry of the
proposed bodies. The fitted curve shows almost no
deviation from the observed one. The small deviations
may be accounted for by considering that there are
many small outcrops of Fe-ore. Therefore, we consider
our model representative for the main source bodies
that produce the magnetic anomaly.
N 522523
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U7. Results and discussion
The study of the El Romeral deposit provides
evidence that the iron ore was neither purely hydro-
thermal, nor volcanic in origin, as previous workers
ED PROOF
have proposed. The ores were formed from magnetite-
rich (magmas?), hydrothermally altered and intruded
at an indicated crustal depth in excess of 600 m. Thus,
a rather complex sequence of events has occurred to
produce this deposit.
Microscopic examination of the El Romeral ore
samples indicates they are composed basically of
magnetite and (titano)hematite, with minor amounts
of apatite, actinolite, pyrrothite, calcite, epidote and
quartz. Hematite results from the first stage of meta-
somatism of original magnetite, and also probably
occurred during the hydrothermal event. Microscopic
observations suggest that an andesite protolith was
intruded by a diorite, producing amphibolite facies
contact metamorphism (supported by the recrystalli-
zation of sodic plagioclase to albite, garnet formation,
and replacement of pyroxene by hornblende). During
subsequent tensional shearing with fracturing, the
metamorphosed andesite was intruded by hydrother-
mal solutions that deposited magnetite, followed by
sulfides (sequentially: pyrite–pyrrothite–chalcopy-
rite). The end stages of the hydrothermal process
result in deposition of quartz, epidote, actinolite and
chlorite (greenschist facies).
Generally speaking, possible significant differen-
ces are found in the directions of remanence carried
by magnetite (RBL, RF, RMB, RMM) and hematite
(RCE, RGE, RGW), which suggests that the hydro-
thermal event occurred some considerable time after
the extrusion and cooling.
Although observed directly, (titano)hematite does
not seem to represent a major contributor to total
remanence. In most cases, the magnetic signal is
dominated by a ‘near magnetite’ phase, which seems
to have a wide range of PSD toMDmagnetic structure.
Thus, the dominant magnetic carriers in studied ores
correspond to magnetite characterized by highest val-
ues of rock magnetic properties observed in the various
lithologies examined, remanent magnetization intensi-
ty (18.9 A/m), high magnetic susceptibility (1.92 SI),
0.09–1.76 Q coefficients, and low to medium coerciv-
ity under AF demagnetization.
Fig. 10 summarizes the quantitative interpretation
of the observed magnetic anomaly. For the calculated
anomaly, we used the intensity and direction of the
NRM, and the induced intensity. When the remanence
component is lower than the induced component, the
corresponding Q coefficients are 0.09–0.98, lower
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572573574575576577578579580581582583584585586587588589590591592593594595596597598599600601602
L.M. Alva-Valdivia et al. / Journal of Applied Geophysics 1420 (2003) xxx–xxx14
than unity. When the remanence component is greater
than the induced component, the corresponding Q
coefficients are 1.54–1.76, greater than unity. The
position and the geometry of the main magnetic
source bodies are shown as irregular tabular bodies,
the larger one steeply inclined (75j) to the south, and
the smaller one lying almost horizontal. This coin-
cides with proposals for these bodies made from field
geology observations and drill-hole data.
The intensity of remanence contribution is signif-
icant in the interpretation of magnetic anomalies,
particularly for cases where the remanent component
has high intensity (Alva-Valdivia et al., 1991, 1998;
Alva-Valdivia and Urrutia-Fucugauchi, 1995). For
highly magnetic rocks, such as Fe-ore deposits, its
influence is stronger. The dipolar anomaly can be
modelled by two prismatic magnetite-rich bodies that
closely follow the general attitude of the rock units.
Application of rock magnetic properties can be
used for interpretation of magnetic patterns and con-
tacts. When combined with field checks of the source
rocks of magnetic anomalies, the magnetic interpreta-
tion can provide a pseudo-geological map that aids in
geological mapping, making it more effective and less
time-consuming. We strongly suggest that such inter-
pretation of the aeromagnetic data ought to be per-
formed together with photogeological analysis before
beginning the geological fieldwork.
C 603604605606607608609 RE8. Uncited referencesDeer et al., 1992
Espinoza, 1996
R 610611612613614615616617618619620621622623624625626UNCOAcknowledgements
We are grateful for the kind cooperation of
Leonardo Vergara and Mario Rojo engineering staff
Exploration Division of the Companıa Minera del
Pacifico. This research was funded by the Instituto
Panamericano de Geografia e Historia (IPGH) and
CONACyT project nos. 32756-T and J32727-T.
Experiments k vs. high T were performed by J.
Morales. Great improvements were done thanks to the
revision of Prof. D. Clark and an anonymous
reviewer.
ED PROOF
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