International Geology Review, Vol. 45, 2003, p. XXX–XXX.Copyright © 2003 by V. H. Winston & Son, Inc. All rights reserved.

Rock-Magnetic and Oxide Microscopic Studies of the El Laco Iron-Ore Deposits, Chilean Andes, and Implications for Magnetic Anomaly Modeling

L. M. ALVA-VALDIVIA,1 M. L. RIVAS, A. GOGUITCHAICHVILI, J. URRUTIA-FUCUGAUCHI,J. A. GONZALEZ, J. MORALES, S. GÓMEZ,

Laboratorio de Paleomagnetismo y Geofisica Nuclear, Instituto de Geofísica, Universidad Nacional Autonoma de Mexico,Del. Coyoacan 04510 D. F., Mexico

F. HENRÍQUEZ,Departamento de Ingeniería en Minas, Universidad de Santiago de Chile, Santiago, Chile

J. O. NYSTRÖM,Department of Mineralogy, Swedish Museum of Natural History, Stockholm, Sweden

AND R. H. NASLUND

Department of Geological Sciences, State University of New York, Binghamton, NY, 13902

Abstract

Microscopic and rock-magnetic studies of the ores and host rocks of the El Laco iron oxidedeposits permit us to characterize the magnetic mineralogy and the processes affecting naturalremanent magnetization (NRM) during emplacement and evolution of the deposits. Particular atten-tion was devoted to identifying the magnetic mineral composition (magnetite and/or titanomagnetite,and hematite and/or titanohematite, and titanomaghemite) and grain size variations of both ores andhost rock. Rock-magnetic data are used to clarify magnetic domain states and remanence acquisi-tion processes, and to assess their significance as a source of magnetic anomalies.

Microscopy under reflected light demonstrates that magnetic carriers are mainly magnetite,with significant amounts of ilmenite-hematite minerals. Magmatic titanomagnetites in the andesiticrocks show trellis textures, compatible with high-temperature oxy-exsolution processes. Supergenereactions in ore deposits under eruption conditions are indicated by goethite and hematite oxideminerals. Grain sizes range from a few microns to >100 µm. Hysteresis measurements point topseudo-single-domain states. Thermal spectra, continuous temperature-dependent susceptibilitymeasurements, and isothermal remanent magnetization (IRM) acquisition suggest predominance ofspinels (titanomagnetite or titanomaghemite) with low-Ti contents as magnetic carriers. Althoughthe presence of (titano)hematites is indicated by hysteresis and IRM studies, their contribution to thetotal remanence seems to be minor. The Fe-oxides in the ore are typically poor in Ti, whereas in therocks they are Ti-bearing.

For the modeling of the magnetic anomalies, we used data on bulk susceptibility and NRMintensity and direction in order to constrain the relative contributions of induced and remanentmagnetization components, and to obtain improved control regarding depth and geometry of sourcebodies. The deep magnetic source corresponds to an ENE-striking tabular body, steeply inclined65° to the north.

Introduction

THE EL LACO IRON DEPOSIT is located slightlybehind the Plio-Pleistocene volcanic arc that forms

the Central Andes in northern Chile (Fig. 1). Dis-crete bodies of massive magnetite have been inter-preted as volcanic flows and dikes (Naslund et al.,2002 and references therein), or as products ofmetasomatic replacement (Sillitoe and Burrows,2002 and references therein), or as hydrothermal inorigin (Rhodes et al., 1999; Hitzman, 2000).

1Corresponding author; email:lalva@tonatiuh.igeofcu.unam.mx

10020-6814/03/XXX/XXX-15 $10.00

2 ALVA-VALDIVIA ET AL.

Rock-magnetic parameters represent the linkbetween geophysical anomalies of potential fieldsand geology in terms of structure and lithology.These parameters are needed for quantitative inter-pretation of magnetic anomalies by providingconstraints on the geometry of the source bodies,thereby considerably reducing ambiguity in theinterpretations. Furthermore, the contribution ofremanent magnetism to magnetic anomalies is wellknown, using the information of rock magnetic prop-erties to assist in constraining magnetic anomalyinterpretations, especially for iron ores and associ-ated igneous rocks (Alva-Valdivia and Urrutia-Fucugauchi, 1995; Alva-Valdivia et al., 1991, 2001;Clark, 1997).

This purpose of this paper is to examine theresults of microscopic and rock magnetism studiesof samples from the El Laco iron ore deposits (Fig.1), in order to define factors such as ore type, grade,grain size, and mineralogy; as well as depth, shape,size, and orientation of the iron oxide ore body thatis the source of the magnetic anomaly. This informa-tion is useful for the exploration-exploitation pro-gram planning of the iron oxide ores and relateddeposits of Cu, Au, U, and REE minerals.

Our microscopic observations and rock magneticexperiments support the hypothesis of Naslund et al.

(2002), who concluded that the field associationsand structures and textures of fresh-looking ores androcks at El Laco indicate that the main, massive ore-bodies crystallized from volatile-rich, iron-oxidemagmas. Magnetite-apatite (magnetite-pyroxene)veins commonly associated with smaller orebodieswere produced at high temperature from magmaticfluids released from the crystallizing magmas. Thenature of volatile-rich magmas is that they transformtheir immediate host rock during emplacement andcrystallization. Besides, concentrations of Cu, Au,U, and REE are associated with many iron oxide–apatite ores, either within the iron-rich orebodies, oroccurring nearby as breccias, veins, or stockworks.From an exploration point of view, it is important todetermine if a Cu-(Au-U-REE) deposit is linkedwith the intrusion or eruption of iron oxide magma.

Geology and Mineralization

The El Laco iron deposits (Park, 1961; Ruiz etal., 1965; Haggerty, 1970; Frutos and Oyarzún,1975; Henríquez and Martin, 1978; Gardeweg andRamírez, 1985; Frutos et al., 1990; Nyström andHenríquez, 1994) are exposed on the flanks of acalc-alkaline volcanic complex at an altitude of4700 to 5300 m in the High Andes of northern Chile

FIG. 1. Simplified map of the study area showing the location of the iron ore deposits at El Laco, Chile. Numberedblack stars indicate the paleomagnetic sampling sites. Modified from Nyström and Henríquez (1994).

MICROSCOPIC AND ROCK-MAGNETIC STUDIES 3

at 23°48' S latitude and 67°30' W longitude (Fig. 1).The mineralization formed about 2 Ma, according tofission-track data for apatite in the ore (2.1 ± 0.1Ma; Maksaev et al., 1988) and a K-Ar whole-rockdating of an andesite from the host rock at the east-ern flank of Pico Laco (2.0 ± 0.3 Ma; Gardeweg andRamírez, 1985). Seven iron deposits are recognizedin the El Laco area (Fig. 1), with total estimatedresources of 500 million tons.

The El Laco magnetite-apatite ore deposits occuras massive, subhorizontal bodies of ore lava andpyroclastic material (Laco Norte, Laco Sur, SanVicente Alto, and San Vicente Bajo), dikes, and veincomplexes (Rodados Negros, Laquito, and CristalesGrandes). The original volcanic structures and tex-tures are generally preserved in the ores and theirhost rocks (Naslund et al., 2002). Surface and drillcore observations show the following upward suc-cession in the largest deposit (Laco Norte): andesitelava, pyroclastic ore, magnetite lava, a second pyro-clastic unit with fragments of massive ore, and asecond andesite flow. Mining at Laco Sur, anotherlarge deposit, reveals a section through ore lava, andmassive, locally stratified pyroclastic ore, crosscutby numerous vertical gas escape tubes. The struc-tures in the ore suggest vicinity to a volcanic vent(Nyström and Henríquez, 1994).

Hematite is generally an oxidation product aftermagnetite. Primary hematite occurs only in a fewplaces. Apatite, locally common in some intrusions,is an accessory phase in the ore lavas. The ores con-tain highly variable amounts of pyroxene occurringas thin elongated prisms, typically with dendriticbranching. The pyroxene is partly to completelyaltered to talc, opal, goethite, and smectite. Octa-hedral magnetite crystals coat open spaces; manyoctahedra are pitted by molds with a square crosssection after pyroxene. The orebodies emplacedbelow the surface are surrounded by thin metaso-matic halos composed of pyroxene, scapolite,quartz, and garnet. The volcanic complex includingthe ores is affected by locally strong hydrothermalalteration with sicilification and development ofgypsum, jarosite, and alunite (Nyström and Hen-ríquez, 1994).

Study Methods

We collected 110 drill-core samples from sevensites in the mined Laco Sur orebody and from sevensites of iron ore and host rocks cropping out at thesurface in other parts of the El Laco district (Fig. 1).

A portable gasoline-powered drill was used for thesampling, and the cores were oriented with an incli-nometer and both sun and magnetic compasses.

Remanent magnetization was measured witha Molspin spinner magnetometer. Separation ofsecondary magnetization components from the char-acteristic component was accomplished by stepwisethermal demagnetization up to 550°C using a non-inductive Schonstedt furnace, and in a few cases bystepwise alternating field (AF) demagnetization upto 100 mT. Room-temperature low-field magneticsusceptibility (k) was measured with an MS-2 Bar-tington susceptibility meter. Continuous suscepti-bility measurements (i.e., susceptibility vs. hightemperature curves) were obtained using a High-moor instrument. Isothermal remanent magnetiza-tion (IRM) acquisition curves with 10–14 steps to amaximum field of 3300 mT were obtained with apulse magnetizer on one standard paleomagneticsample per site. Demagnetization of the IRMs wasdone by step-wise thermal demagnetization, using8–10 steps up to a maximum temperature of 700°C.

Small chips of rocks were used for measurementof magnetic hysteresis parameters with the AGFM-MicroMag alternating gradient force magnetometer.Associated IRM acquisition and DC back-fielddemagnetization curves were also acquired with thisinstrument. Opaque and transparent minerals wereexamined under reflected and transmitted light todetermine mineralogical association, degree ofalteration, and textural type, and iron oxide compo-sitions were determined with an electron probemicroanalyzer (EPMA).

Electron Probe Microanalyzer and Opaque Oxide Microscopy

Polished thin sections were studied from all 14sampled sites, which can be divided into fourgroups: Laco Sur (sites 1 to 7), lower andesite flow(site 8), Laco Norte (sites 9 to 13), and upper andes-ite flows (site 14). Representative photomicrographsand EPMA images are given in Figures 2 and 3,respectively.

Laco Sur

The orebody consists largely of massive magne-tite with abundant filled fractures. The magnetiteshows an incipient to minor oxidation to hematite(Figs. 2A and 3C). The hematite is partly alteredto goethite.

4 ALVA-VALDIVIA ET AL.

The filled fractures correspond to veins withmagnetite-apatite, magnetite-pyroxene, or hematite,all commonly with later formed quartz. Some frac-tures are only partly filled. Apatite occurs as well-formed crystals projecting from the border into thefractures/veins. Small amounts of sulfides likepyrite are disseminated in the magnetite. Some frac-tures (probably faults) are partly filled with clayminerals and sericite.

Lower andesite flow The sampled lava in site 8 (Fig. 1) is a porphy-

ritic andesite with oligoclase-andesine and minordiopside and hyperstene in a dark glassy matrix.Minerals present in small amounts are ilmenite(containing <1% V, Mn, and traces of Co and Ni),titanomagnetite (<1% V, and traces of Co, Ni, Cr,and Mn), apatite, and zircon. Secondary minerals

are clay minerals, sericite, titanohematite (traces ofV, Ni, and Mn), and limonite.

A trellis texture (Haggerty, 1976) was observedin these rocks as an intergrowth of ilmenite lamellaein titanomagnetite grains. The titanomagnetite isfractured in contrast to the ilmenite, suggesting thatthe fracturing took place prior to the formation ofilmenite. The titanomagnetite is characteristicallyintergrown in graphic texture with diopside (Fig.2B). Some titanomagnetite is included in anhedralcrystals of pyroxene and plagioclase. Anhedral crys-tals of ilmenite are included in plagioclase, showingexsolved hematite in a sandwich texture (Fig. 3A,Haggerty, 1976). Ilmenite and titanomagnetite alsoform anhedral crystals evenly distributed into therock matrix.

Apatite occurs as euhedral inclusions mainly intitanomagnetite, pyroxene, and plagioclase (absent

FIG. 2. Representative reflected-light photomicrographs of iron ore and host rocks from El Laco. Symbols: A = apa-tite; F = iron phosphate; G = goethite; H = hematite; I = ilmenite; M = magnetite; TM = titanomagnetite. Texture descrip-tion and oxidation state follow Buddington and Lindsley (1964) and Haggerty (1976), respectively. A. Euhedralmagnetite crystals and ilmenite lamellae (trellis texture) in ore sample L027, Laco Sur; C2 oxidation state, width of field =81 µm. B. Titanomagnetite with hematite (left side) forming graphic texture with diopside (right side) in lower andesitesample L053, width of field = 525 µm. C. Magnetite extensively replaced by hematite, with goethite in the curved cracks,ore sample L059, Laco Norte; width of field = 525 µm. D. Phenocrysts of diopside with anhedral inclusions and pseudo-morphs of hematite, upper andesite sample L103; width of field = 525 µm.

MICROSCOPIC AND ROCK-MAGNETIC STUDIES 5

in the ilmenite), which suggests that the apatiteformed probably during the initial phase of crystalli-zation and magma solidification.

Laco Norte The ore in the sampled surface outcrops is

moderately to highly oxidized. The hematite contentis greater than 50%, and magnetite is present onlyas relicts in the hematite (Fig. 2C). Much of thehematite is altered to goethite (Fig. 3D). Cavities inhematite are filled by iron phosphate (Fig. 3B),apatite, quartz, barite, and clay minerals.

Upper andesite flowsThe lavas sampled in site 14 (Fig. 1) are porphy-

ritic andesites with oligoclase-andesine, diopside,and hyperstene. The pyroxenes and plagioclase con-tain abundant inclusions of titanohematite, contraryto the lower flow where these minerals have abun-

dant inclusions of ilmenite and scarce titanomagne-tite. This suggests that the upper andesite flowshave been affected by a strong oxidation thatresulted in a partial to complete replacement ofthe included titanomagnetite by titanohematite(Fig. 2D).

Magnetic Measurements

Rock-magnetic experiments were carried out toinvestigate the magnetic carriers of remanence, andto obtain information about their magnetic stability.These experiments included: NRM and initial sus-ceptibility measurements; stepwise alternating fieldand thermal demagnetization of NRM; continuoussusceptibility measurements; IRM acquisition andthermal demagnetization of the saturation IRM(SIRM); and hysteresis experiments.

FIG. 3. EPMA images of iron ore and host rocks from El Laco (abbreviations are the same as those used in Figure 2;scales are indicated on each image). A. Hematite and ilmenite showing sandwich texture (lower andesite, sample L053).B. Hematite with open space filled by iron phosphate (sample L074, Laco Norte). C. Massive porous ore consisting of anaggregate of magnetite crystals (and minor hematite) with interstitial apatite (sample L027, Laco Sur). D. Hematite withopen-space filling of botroydal goethite (sample L074, Laco Norte).

6 ALVA-VALDIVIA ET AL.

NRM intensity, susceptibility, and Koenigsberger ratio (Q)

Table 1 summarizes the values for NRM intensity(Mo), susceptibility (k) at room temperature, and theKoenigsberger ratio Q = Mo/kH (calculated for thepresent geomagnetic field). Koenigsberger ratioscondition the relative importance of remanent andinduced magnetization, being remanence dominantfor Q > 1. Variations in remanent intensity and sus-ceptibility depend especially on two factors: volumecontent of magnetite and development of weaklymagnetic or non-magnetic phases during oxidationor alteration of the rocks.

Magnetic carriers in the andesites (sites 8 and14) show similar, low Q values, suggesting a possi-ble predominance of multidomain (MD) carriers(Table 1). The iron ores from Laco Sur and LacoNorte (with the exception of site 1) have medium tohigh Q ratios, indicating a wide range of magnetitegrain sizes carrying the remanence.

Alternating field and thermal demagnetizationsBoth AF demagnetization and stepwise thermal

demagnetizations were carried out to study the vec-torial composition of the remanence. The low-fieldsusceptibility at room temperature was measuredafter each step of thermal demagnetization. Thedirections of characteristic magnetization werecalculated using principal components analysis(Kirschvink, 1980). These directions for each siteshow high within-site angular dispersion and thesite-mean directions are rather scattered. This mightbe attributed to potential chemical remagnetizationdue to oxidation activity and not to variations of theEarth’s magnetic field. Unusual directions may alsoresult from movements (faulting) during the deposi-tion of the ore, or disturbances caused by mining.

Vector plots show different behavior for iron oresand each rock type. In general, we found evidencefor multicomponent magnetization (Fig. 4). Thelow-temperature component is presumably due to a

TABLE 1. Rock Magnetic Parameters of Host Rocks and Iron Ore from El Laco, Chile1

Site Sample N/n Dec (°) Inc (°) K α95(°) Mo (A/m) k (SI) Q Areas

1 L001-L005 5/5 43.0 –27.8 9.7 25.9 0.48 1.24 0.02 Laco Sur

2 L006-L010 5/4 103.0 20 6.3 39.8 26018.25 1.37 1006 Laco Sur

3 L011-L015 5/5 138.2 –34.3 3.4 48.5 9.11 1.36 0.36 Laco Sur

4 L016-L021 6/4 140.0 –5.5 3.7 55.2 14.17 1.13 0.67 Laco Sur

5 L022-L028 7/3 176.5 –22.3 4.6 65.4 2.18 1.07 0.11 Laco Sur

6 L029-L036 8/8 277.3 –42 5.2 26.8 25.71 1.23 1.11 Laco Sur

7 L037-L043 7/4 189.4 57.1 2.7 71.1 0.11 0.01 0.35 Laco Sur

8 L044-L057 13/9 71.1 46.2 2.8 37.7 0.03 0.03 0.05 Lower andesite

9 L058-L063 6/3 170.0 –08.79 2886.38 0.17 928 Laco Norte

10 L064-L077 14/6 284.2 48.4 3.3 43.6 0.36 0.03 0.66 Laco Norte

11 L078-L083 6/1 173.0 16 770356.09 0.29 140994 Laco Norte

12 L084-L093 10/6 298.7 –26.9 3.7 40.2 2.19 0.26 0.45 Laco Norte

13 L094-L099 6/5 122.0 –2 7436.51 0.08 5244 Laco Norte

14 L100-L110 4/6 161.4 –30.2 5.6 31.1 0.02 0.03 0.04 Upper andesite

1N/n = total number of samples/number of samples used for site mean calculation; Dec adn Inc, and Mo = direction and intensity of mean natural remanent magnetization; k = initial susceptibility; K and ±95 = confidence parameters; Q = Königsberger factor.

MICROSCOPIC AND ROCK-MAGNETIC STUDIES 7

viscous component or to hydrothermal alteration.The characteristic remanence is defined above300°C or 20 mT. The low-temperature (200°–400°C) or low stability (up to 10 mT) component iswell defined for iron ore samples from Laco Sur(L008C and L034C) and Laco Norte (L098C). Alter-natively, some partial thermoremanence (pTRM)could be acquired from ca. 350°C (oxidation duringemplacement) to ambient temperature. This pTRMmay just be removed at higher temperatures if mag-netization is carried by pseudo-single-domain ormultidomain particles (Shcherbakova et al., 1996;

Goguitchaichvili et al., 1997). The larger part ofremanence is removed above 450°C or 20 mT, whichsuggests the presence of magmatic magnetitetogether with hematite, as observed by EPMA andmicroscopy in both ores and rocks.

Continuous susceptibility measurementsOne sample from each site was heated up to

650°C at a heating rate of 20°C/min and then cooledat the same rate. We resolved Curie temperaturesusing the method of Prévot et al. (1983). Four typi-cal curves corresponding to the Laco Sur (sample

FIG. 4. Orthogonal vector plots of remanent magnetization (Zijderveld, 1967) for representative samples of ore androck (see Table 1). The numbers refer to temperatures (magnetic field) in °C (mT). Projections onto the horizontal planeare marked by open circles; projections onto the vertical plane are denoted by crosses (x).

8 ALVA-VALDIVIA ET AL.

L024), lower andesite (sample L050), Laco Norte(sample L097), and upper andesite (sample L101)are given in Figure 5. The curves of the last threesites correspond to magnetite or Ti-poor titanomag-netite. The Laco Sur sample (L024) shows evidenceof two ferrimagnetic phases during heating. Thelower Curie point ranges from 400° to 450°C, andthe highest one is about 600°C. The cooling curvesshow only a single phase, with a Curie temperatureclose to that of magnetite. These irreversible k-Tcurves can be explained by the presence of tita-nomaghemite, which presumably changed into mag-netite (Readman and O’Reilly, 1970; Ozdemir,1987) during heating.

The heating and cooling behavior possibly corre-sponds to small pseudo-single-domain to multi-domain magnetic grains (Dunlop and Ozdemir,1997). The susceptibility values are useful in defin-ing the possible contribution of hematite relative to

magnetite. For example, sites 1 and 2 show the high-est susceptibility values (Table 1), suggesting thatthe contribution of hematite is negligible.

IRM acquisition and AF demagnetization of SIRMTo determine the IRM curve, previously AF

demagnetized samples from all sites were subjectedto magnetic fields along one axis. Two groups ofquite similar curves were found for the rocks andores, respectively. The andesites (Fig. 6, samplesL049 and L107) approximately reach saturation infields around 200–300 mT, whereas the iron oresfail to reach saturation in fields up to 3500 mT (Fig.6, samples L038 and L077). The saturation fieldspoint to a mixture of titanomagnetite and titanohe-matite as remanence carriers.

Thermal demagnetization curves of SIRM yieldmedian destructive temperatures (MDT) in therange 300–350°C for the andesitic rocks, and 400–

FIG. 5. Susceptibility vs. high-temperature curves showing Curie temperatures close to 580°C. These suggest that themain carrier of magnetization is almost pure magnetite, consistent with microscopic observations. The arrows indicatethe heating and cooling curves.

MICROSCOPIC AND ROCK-MAGNETIC STUDIES 9

FIG. 6. Representative isothermal remanent magnetization (IRM) acquisition curves, and corresponding curves forthermal demagnetization of the IRM for the studied areas.

10 ALVA-VALDIVIA ET AL.

450°C for the iron ores. The andesite curves corre-spond to titanomagnetites with a very well definedCurie point around 580°C. For iron ores, the curvessuggest the presence of titanohematite; the magne-tite of the Laco Sur sample (L038) shows two mag-netic phases, ~250°C and 680°C, which seem tocorrespond to goethite and hematite.

Bulk properties like IRM acquisition, which sat-urates in low fields (sample L049, left side), detectsonly coarse MD grains, but finer pseudo-single-domain (PSD) to single-domain (SD) grains mustalso contribute greatly to the NRM. These finergrains probably account for the moderate to highcoercivity detected during thermal demagnetizationof IRM (Fig. 6, right side diagrams).

Hysteresis experiments

Hysteresis measurements at room temperaturewere performed on all studied sampled sites infields up to 1.2 T. The saturation remanent magneti-zation (Jrs), the saturation magnetization (Js), and thecoercive force (Hc) were calculated after correctionfor the paramagnetic contribution. The coercivity ofremanence (Hcr) was determined by applying a pro-gressively increasing backfield after saturation.Typical hysteresis plots are shown in Figure 7. Twosamples from Laco Sur show the behavior of magne-tite (L006) and a mix of magnetite with majoramount of hematite (L042), respectively. A LacoNorte sample (L078) also shows magnetite andminor hematite mixed. The hysteresis parametersindicate that almost all samples fall in the PSD grainsize region (Fig. 8; Day et al., 1977).

Quantitative Modeling of Magnetic Anomaly

The magnetic anomaly corresponds to a largenormally polarized dipole oriented NE-SW that islocated over the orebodies (Figs. 1 and 9). Weselected one profile (A-A') across the maximum andminimum of the dipolar anomaly and orientedN25°E for quantitative modeling. The dipolar anom-aly presents an E-W elongated low and high, with apeak-to-peak amplitude of about 295 nT. To calcu-late the magnetic model we used the methods ofTalwani et al. (1959) and Talwani and Heirtzler(1964) for two-dimensional polygonal bodies (Tal-wani, 1965), and utilized the algorithms describedin Won and Bevis (1987). The software used for themodeling process is GM-SYS by Northwest Geo-physical Associates.

We used the measured magnetic properties of thesampled rock units to constrain the interpretation.

Different models with several combinations of mag-netic properties were tested, and the best fit corre-sponds to a large tabular orebody and two very smallbodies that reach the surface. The large orebody islocated beneath the magnetic high, and the smallone lies under the magnetic low (Figs. 9 and 10).The rock-magnetic parameters used for the pro-posed large body were the mean of the parametersobtained for the Laco Sur samples. The model thatproduced an acceptable fit is based on three bodies:(1) a large magnetite body, M, 26 km long and withsusceptibility on the order of 1.0601 SI, remanentintensity of 11.11 A/m, and direction of remanenceDec = 167°, and Inc = 46°; (2) two hematite bodies,H, with values of susceptibility = 0.1326 SI, rema-nent intensity = 9.713 A/m, and direction of rema-nence Dec = 299°, and Inc = –27°; and (3) theandesite host rock, A, with values of susceptibility =0.0338 SI, remanent intensity = 0.03 A/m, anddirection of remanence Dec= 71°, and Inc= 46°.These values were obtained by averaging data fromthe four different areas (magnetite = Laco Sur,hematite = Laco Norte, and andesite = the lower andupper andesites). The ambiguity of the modelingprocess is reduced considerably with regard to size,depth, and geometry of the proposed bodies. Weconsider that our approach accounts for the mainsource bodies of the magnetic anomaly.

Discussion and Conclusions

Our study of the El Laco deposits provides evi-dence that the iron ore shows volcanic features, asconcluded by Naslund et al. (2002) and referencestherein. EPMA and microscopic examination of thesurface samples from Laco Sur show that the oreconsists of magnetite incipiently altered to hematite,with small amounts of apatite in open spaces. Incontrast, the ore is considerably to be largely oxi-dized at Laco Norte. The reason for this difference isthat the Laco Sur samples were collected in arecently mined part of an orebody largely unaffectedby supergene processes, in contrast to the LacoNorte samples, which came from surface outcropsinfluenced by meteoric conditions. The observationsof ilmenite intergrowths in low-Ti titanomagnetitesshowing trellis (Fig. 2A), sandwich (Fig. 3A), andcomposite (Fig. 2B) textures (Buddington andLindsley, 1964) from the iron ores strongly suggeststhat the oxy-exsolution process was produced byoxidation at low to moderate pressures and above600°C, with the direct formation of titanohematites

MICROSCOPIC AND ROCK-MAGNETIC STUDIES 11

(Haggerty, 1976). These conditions suggest, regard-ing the origin, the possibility that the iron ores areprimary precipitates from the melt and/or resultfrom the exsolution processes. In addition, there isthe possibility of oxidation at low pressures and

between 400–600°C to yield cationic deficientspinels of the metastable titanomaghemite series,which subsequently inverts to members of thetitanohematite series (Fig. 2C), representing proba-bly the hydrothermal event.

FIG. 7. Hysteresis loops (uncorrected for paramagnetism) for representative ore samples (L042, Laco Sur; L078, LacoNorte; L006, Laco Sur), and the respective IRM acquisition curves.

12 ALVA-VALDIVIA ET AL.

Generally speaking, the characteristic directionsof magnetization show high within-site angular dis-persion and the mean site directions are quite scat-tered, which can be ascribed to possible oxidationduring the emplacement and not to changes of theEarth’s magnetic field, and/or the result of physicalmovement of the ores due to faulting or mining.Moreover, the scatter in directions of remanence

may in part result from hydrothermal activity afterextrusion and cooling of the orebodies.

The samples provided quite linear and stablemagnetic components in response to demagnetiza-tion (removing the remanence over 450°C or 20mT), which suggests the presence of magmatic mag-netite together with hematite, as observed bymicroscopy and EPMA.

The Curie points defined by susceptibility high-temperature experiments correspond to the mag-netic signal from a “near-magnetite” phase, whichseems to have a wide range of pseudo-single-domainmagnetic structures (Fig. 5). The linearity of thevector plots and the shape of the continuous suscep-tibility curves suggest the possible presence of a pri-mary phase of almost pure magnetite partly overlaidby a hydrothermal process represented by the tita-nomaghemite series.

The IRM acquisition curves suggest that satura-tion in low fields will detect the multidomain grains.However, finer pseudo-single-domain to single-domain grains that greatly contribute to the NRMare also detected during thermal demagnetization ofIRM (Fig. 6).

The hysteresis properties of the El Laco ores androcks show the response of Ti-poor titano(magne-tite), and more generally, a mixture of magnetite andhematite. The hysteresis parameters indicate that

FIG. 8. Magnetic hysteresis parameter ratios plotted in aDay diagram (Day et al., 1997).

FIG. 9. Aeromagnetic anomaly map. Note the presence of a well-defined dipolar anomaly located over the mineral-ized area. The position of the profile shown in Figure 10 is indicated. Values are given in nanoteslas. Modified fromSERNAGEOMIN (1992).

MICROSCOPIC AND ROCK-MAGNETIC STUDIES 13

almost all samples fall in a pseudo-single-domaingrain size region, with just two samples fromthe Laco Sur area falling in the multidomain region(Fig. 8).

In most cases, the dominant magnetic carriers inthe studied ores correspond to magnetite character-ized by the highest values of the rock magneticproperties—e.g., remanent magnetization intensity(25.703 A/m); high magnetic susceptibility (1.3559SI); 0.02 to 1.11 Q coefficients; and low to mediumcoercivity under AF demagnetization.

Figure 10 summarizes the quantitative interpre-tation of the observed magnetic anomaly. For thecalculated anomaly, we used the mean intensity anddirection of the NRM, and the induced intensity(mean of susceptibility values). The remanencecomponent is lower (greater) than the induced com-ponent, corresponding to Q coefficients lower(greater) than unity. The location and the geometryof the main magnetic source bodies are exposed asirregular tabular bodies, with the larger one steeplyinclined 65° to the north. This large body could beinterpreted to correspond to a magmatic (magnetite)iron oxide dike system, and the small ones to (hema-tite) iron oxide dikes. Both assumptions constitutestrong evidence in support of a magmatic origin forthis deposit. This model corresponds with modelsfor these bodies initially elaborated from geologicalfield observations in the mine area (massive tabularorebodies, crosscutting dikes and vein complexes).

Once again, we emphasize that the contributionof remanence intensity for the modeling of magneto-metric anomalies is significant, mainly for caseswhere the remanent component has high intensityand a direction different from the present-daygeomagnetic field (Urrutia-Fucugauchi, 1977; Alva-Valdivia and Urrutia-Fucugauchi, 1995, 1998;Alva-Valdivia et al., 1991). For extremely magneticiron ores, remanence effects are significant, andwhen they are used together with field confirmationsof the source rocks of these anomalies, can supply amore reliable geophysical model.

The ambiguity of the modeling process has beenconstrained, decreasing largely the uncertainty insize, depth, and geometry of the proposed bodies.We consider that our approach accurately retrievesthe main characteristics of the major source bodiesof the magnetic anomaly, and can be of great help inthe exploration-exploitation for similar iron oxideores and related mineralizations of Cu, Au, U, andREE.

Acknowledgments

We are grateful for the kind cooperation ofLeonardo Vergara and Mario Rojo, Exploration Divi-sion of the Compañía Minera del Pacifico. Thisresearch was funded by the CONACyT project No.32756-T and J32727-T.

FIG. 10. Observed and calculated magnetic anomalies, assuming two-dimensional bodies that extend infinitely alongstrike perpendicular to this profile. Local geomagnetic field parameters: declination D = –2.17°, inclination I = –20.15°,and total magnetic intensity F = 23635nT. Magnetic data for the modeled bodies are described in the text (see Fig. 9).

14 ALVA-VALDIVIA ET AL.

REFERENCES

Alva-Valdivia, L. and J. Urrutia-Fucugauchi, 1995, Rockmagnetic surveys in the iron ore deposit of El Encino,Mexico: Journal of South American Earth Science, v.8, p. 209–220.

______, 1998, Rock magnetic properties and ore micros-copy of the iron ore deposit of Las Truchas, Michoacan,Mexico: Journal of Applied Geophysics, v. 38, p. 277–299.

Alva-Valdivia, L., Urrutia-Fucugauchi, J., Böhnel, H., andMoran-Zenteno, D., 1991, Aeromagnetic anomaliesand paleomagnetism in Jalisco and Michoacan, south-ern Mexico continental margin, and their implicationsfor iron-ore deposits exploration: Tectonophysics, v.192, p. 169–190.

Alva-Valdivia, L. M., Urrutia-Fucugauchi, J., Goguitchai-chvili, A., and Vivallo, W., 2001, Rock–magnetismand ore microscopy of magnetite-apatite ore depositfrom Cerro de Mercado, Mexico. Earth, Planets, andSpace, v. 53, p. 181–192.

Buddington, A. F., and Lindsley, D. H., 1964, Iron-tita-nium oxide minerals and synthetic equivalents: Jour-nal of Petrology, v. 5, p. 310–357.

Clark, D. A., 1997, Magnetic petrophysics and magneticpetrology: Aids to geological interpretation of magneticsurveys: Journal of Australian Geology and Geophys-ics, v. 17, p. 83–103.

Day, R., Fuller, M., and Schmidt, V. A., 1977, Hysteresisproperties of titanomagnetites: Grain size and compo-sitional dependence: Physics of the Earth and Plane-tary Interiors, v. 13, p. 260–267.

Dunlop, D., and Ozdemir, O., 1997, Rock-magnetism, fun-damentals and frontiers: Cambridge, UK, CambrigeUniversity Press, 573 p.

Frutos, J., and Oyarzún, J., 1975, Tectonic and geochemi-cal evidence concerning the genesis of El Laco magne-tite lava flow deposits: Chile: Economic Geology, v. 70,p. 998–990.

Frutos, J., Oyarzún, J., Shiga, Y., and Alfaro, G., 1990,The El Laco magnetite lava flow deposits, nothernChile: An up-to-date review and new data, in Fontboté,L., Amstutz, G. C., Cardozo, M., Cedillo, E., and Fru-tos, J., eds., Stratabound ore deposits in the Andes:Berlin, Germany, Springer-Verlag, p. 681–690.

Gardeweg, M., and Ramírez, C. F., 1985, Hoja RioZapaleri, II Region de Antofagasta: Santiago, Chile,Servicio Nacional de Geología y Minería, CartaGeológica de Chile, no. 66, 89 p.

Goguitchaichvili, A., Sologashvili, D. Z., Prévot, M.,Calvo, M., Pavlenishvili, E. S., Maissuradze, G. M.,and Schnepp, E., 1997, Paleomagnetic and rock-mag-netic study of a Pliocene volcanic section in southGeorgia (Caucasus): Geologie en Mijnbouw, v. 76, p.135–143.

Haggerty, S. E., 1970, The Laco magnetite lava flow,Chile: Carnegie Institute Washington Year Book no.68, p. 329–330.

______, 1976, Oxidation of opaque mineral oxides inbasalts, in Rumble, D., ed., Oxide minerals: Mineral-ogical Society of America, Short Course Notes, v. 3, p.1–100.

Henríquez, F., and Martin, R. F., 1978, Crystal-growth tex-tures in magnetite flows and feeder dikes, El Laco,Chile: Canadian Mineralogist, v. 16, p. 581–589.

Hitzman, M. W., 2000. Iron oxide–Cu–Au deposits: What,where, when, and why, in Porter, T. M., ed., Hydrother-mal iron oxide cooper-gold and related deposits: A glo-bal perspective: Adelaide, Australia, AustralianMineralogical Foundation, p. 9–25.

Kirschvink, J. L., 1980, The least-square line and planeand analysis of palaeomagnetic data: GeophysicalJournal of the Royal Astronomical Society, v. 62, p.699–718.

Maksaev, V., Gardeweg, M., Ramírez, C. F., and Zentilli,M., 1988, Aplicación del método trazas de fisión (fis-sion track) a la datación de cuerpos de magnetita de ElLaco e Incahuasi en el altiplano de la región de Anto-fagasta: Congreso Geológico Chileno 5th, Santiago,1988, v. 1, p. B1–B23.

Naslund, H. R., Henríquez, F., Nyström, J. O., Vivallo, W.,and Dobbs, F. M., 2002, Magmatic iron ores and asso-ciated mineralisation: Examples from the Chilean highAndes and Coastal Cordillera, in Porter, T. M., ed.,Hydrothermal iron oxide copper-gold and relateddeposits: A global perspective, v. 2: Adelaide, Austra-lia, PGC Publishing, p. 207–226.

Nyström, J. O., and Henríquez, F., 1994, Magmatic fea-tures of iron ores of the Kiruna type in Chile and Swe-den: Ore textures and magnetite geochemistry:Economic Geology, v. 89, p. 820–839.

Ozdemir, O., 1987, Inversion of titanomaghemites: Phys-ics of the Earth and Planetary Interiors, v. 65, p. 125–136.

Park, C. F., 1961, A magnetite “flow” in northern Chile:Economic Geology, v. 80, p. 646–665.

Prévot, M., Mainkinen, E. A. Grommé, S., and Lecaille,A.,1983, High paleointensity of the geomagnetic fieldfrom thermomagnetic studies on rift valley pillowbasalts from the middle Atlantic ridge: Journal of Geo-physical Research, v. 88, p. 2316–2326.

Readman, P. W., and O’Reilly, W., 1970, The synthesisand inversion of nonstoichiometric titanomagnetites:Physics of the Earth and Planetary Interiors, v. 4, p.121–128.

Rhodes, A. L., Oreskes, N., and Sheets, S. A., 1999, Geol-ogy and rare earth element geochemistry of magnetitedeposits at El Laco, Chile, in Skinner, B. J., ed., Geol-ogy and ore deposits of the central Andes: Littleton,CO, Society of Economic Geologists, Special Publica-tion no. 7, p. 299–332.

MICROSCOPIC AND ROCK-MAGNETIC STUDIES 15

Ruiz, C., Aguirre, L., Corvalan, J., Klohn, C., Klohn, E.,and Levi, B., 1965, Geología y yacimientos metalíferosde Chile: Santiago, Chile, Instituto de InvestigacionesGeológicas, 386 p.

SERNAGEOMIN, 1992. Carta Magnética de Chile: HojasAntofagasta–Toconao 1:250 000: Santiago, Chile, Ser-vicio Nacional de Geología y Minería.

Shcherbakova, V. V., Shcherbakova, V. P., Schmidt, P. W.,and Prévot, M., 1996, On the effect of low temperaturedemagnetizations of TRMs and pTRMs: GeophysicalJournal International, v. 127, p. 379–386.

Sillitoe, R. H., and Burrows, D. R., 2002, New field evi-dence bearing on the origin of the El Laco magnetitedeposit, northern Chile: Economic Geology, v. 97, p.1101–1109.

Talwani, M., 1965, Computation with the help of a digitalcomputer of magnetic anomalies caused by bodies ofarbitrary shape: Geophysics, v. 20, p. 797–817.

Talwani, M., and Heirtzler, J. R., 1964, Computation ofmagnetic anomalies caused by two-dimensional bodiesof arbitrary shape, in Parks, G. A., ed., Computers in

the mineral industries, Part 1: Stanford, CA, StanfordUniversity Publications in the Geological Sciences,no. 9, p. 464–480.

Talwani, M., Worzel, J. L., and Landisman, M., 1959,Rapid gravity computations for two-dimensional bod-ies with application to the Mendocino submarine frac-ture zone: Journal of Geophysical Research, v. 64, p.49–59.

Urrutia-Fucugauchi, J., 1977, Importancia del magne-tismo remanente natural en la interpretación de anom-alías magnéticas: Boletin de la Asociación MexicanaGeofísicos de Exploración, v. 18, no. 4, p. 83–116.

Won, I. J., and Bevis, M., 1987, Computing the gravita-tional and magnetic anomalies due to a polygon: Algo-rithms and Fortran subroutines: Geophysics, v. 52, p.232–238.

Zijderveld, J. D. A., 1967, AC demagnetization of rocks:Analysis of results, in Collinson, D. W., Creer, K. M.,and Runcorn, S. K., eds., Methods in palaeomag-netism: Amsterdam., UK, Elsevier, p. 254–286.

All in-text references underlined in blue are linked to publications on ResearchGate, letting you access and read them immediately.