Magnetic field analysis of Morro do Leme nickel deposit

Vinicius Hector Abud Louro1, Marta Silvia Maria Mantovani1, and Vanessa Biondo Ribeiro1

ABSTRACT

The Morro do Leme laterite nickel deposit lies inside thewestern border of the Parecis Basin (Brazil). This deposit ischaracterized by high concentrations of lateritic Ni (about1.8%) and anomalous contents of Pd, Au, Cu, Na, Co, Zn,and Pt in a peridotite and dunite layered intrusion. Besidesthe existence of geochemical and drilling data, the 3D dis-tribution in the subsurface of this layered intrusion is stillunknown. An airborne magnetic survey revealed threeeast–west elongated magnetic anomalies, characterized by asignificant remanent magnetization. The sources of theseanomalies were delimitated laterally and had their depthsestimated between 90 and 150 m, using techniques that usederivatives. Further, the total magnetization direction wasobtained from a distortion analysis of the magnetic anoma-lies. All these data were united in an initial model for the 3Dinversion of the magnetic data. The total and induced mag-netization directions were attributed to the inverted model of0.12 (SI) susceptibility, allowing indirect estimation of theremanence. The model, defined by the depth, the inversion,and the remanence estimates, linked the intrusion to ana-logue events in the Rondonian-San Ignácio Province. Theresults indicated that to explore for laterite Ni, the best lo-cations are the southern part of the main anomaly and in thecover above the two smaller anomalies, whereas to explorefor Pd, Au, Cu, Na, Co, Zn, and/or Pt, the indicated region isthe central portion of the main anomaly.

INTRODUCTION

Nickel laterite deposits are derived from the chemical alterationof olivine-bearing mafic and ultramafic rocks such as dunites andolivine-pyroxene peridotites (Brand et al., 1998).

In the west portion of the Mato Grosso state (Brazil) is the Co-modoro Nickel District, characterized by the deposits of Morro doLeme and Morro do Sem Boné, with known occurrences of lateriticnickel. The Morro do Leme total reserves are 14,306,000 t with Niconcentrations of 1.8% (Nunes, 2000).The Morro do Leme deposit encompasses three hills comprised

essentially of dunites and peridotites covered by a laterite layer.Nunes (2000) indicates that the main Ni concentration is locatedin this layer, the thickness of which ranges from 20 to 40 m. Indeeper horizons (approximately 150 m deep), there are intercalatedmagmatic concentrations of sulfides and chromites, presentinganomalous values for Pd, Au, Cu, Na, Co, Zn, and Pt.This work analyzed the magnetic field of the deposit considering

previous geologic surface mapping and borehole data from itssoutheastern portion. A procedure using enhanced horizontal deriv-atives (EHDs) (Fedi and Florio, 2001), to estimate the source borderand, further, its depth — named here as the EHD-depth — isbased on Hsu et al. (1998). The MaxiMin technique (Fedi et al.,1994) used to estimate the angles of the magnetization components.Using these estimates, we composed an initial model for a furtherinversion of the magnetic data to determine the susceptibility.The magnetization components (geomagnetic and directions esti-mated through the MaxiMin), when applied to the distribution ofsusceptibility, generated a scenario in which it was possible toindicate an apparent remanent magnetization that explained theanomaly.This analysis and its results evidenced regional magnetic field

features for the time of crystallization of the deposit’s protolithalong the best exploration zones for the laterite Ni and for the Pd,Au, Cu, Na, Co, Zn, and Pt-rich horizons.

GEOLOGIC CONTEXT

In the Alto Guaporé belt from the Rondonian-San Ignácio Igne-ous Province (RSIP), lies the Alto Guaporé Sequence, characterizedby Phanerozoic sediments; such types of sediments usually havevery low magnetic susceptibility (10−5 to 10−4 SI) (Telford et al.,

Manuscript received by the Editor 17 March 2014; revised manuscript received 16 August 2014; published online 28 October 2014.1Universidade de São Paulo, Instituto de Astronomia, Geofísica e Ciências Atmosféricas, São Paulo, Brazil. E-mail: vilouro@usp.br; msmmanto@usp.br;

vanessa.ribeiro@usp.br.© 2014 Society of Exploration Geophysicists. All rights reserved.

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GEOPHYSICS, VOL. 79, NO. 6 (NOVEMBER-DECEMBER 2014); P. K1–K9, 9 FIGS., 1 TABLE.10.1190/GEO2014-0131.1

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1990) and host the Morro do Leme deposit. Nunes (2000) affirms,based on borehole geologic and geochemical loggings, that it waspossible to identify the Nova Brasilândia metavolcano-plutonic-sedimentary sequence (NBS) underneath the Alto Guaporé. TheNBS belongs to the Sunsás Province (Tassinari and Macambira,1999), and it is marked by psamopelitic and chemical-exhalativerocks, sea-bottom and pyroclastic metabasalts, whose average mag-netic susceptibility can be assumed as 10−3 SI (Telford et al., 1990).Morro do Leme encompasses three hills (Morro 1 — the main

intrusion, Morro 2 and 3; Figure 1), which geologically is part of theCacoal basic-ultrabasic intrusive suite, made up of dunites and peri-dotites from the Cacoal Intrusive Suite of Mesoproterozoic age(1372� 21 Ma, by Rb-Sr) (Teixeira and Tassinari, 1977). Telfordet al. (1990) indicate a range of magnetic susceptibility from 0.09 to0.20 (SI) for these lithologies.Nunes (2000) performs petrographic analysis in drill cores that

indicates the presence of serpentinized peridotites and dunites, witha primary mineral paragenesis formed by olivine intensely replacedby serpentine in fractures and venules.In the central portion of Morro 1, the drill logs from Morro do

Leme indicated the top of the fresh rock at 30 m depth (Nunes,2000). At deeper horizons, approximately 150 m, geochemicalanalyses from Nunes (2000) in drill logs found intercalated layersof chromites and sulfides enriched in Pd, Au, Cu, Na, Co, Zn,and Pt.

MAGNETIC FIELD DATA AND PROCESSING

In 2006, the Brazilian Geological Service (CPRM) acquired anairborne magnetic and gamma-ray spectrometric survey over thenorthwest portion of Mato Grosso state and the southeast portionof Rondônia state. This survey used a line spacing of 500 m with

control lines at spacing of 10,000 m. The flight height was main-tained at about 100 m and the aircraft speed at 280 km∕h, resultingin an along-line measurement spacing of 7.8 m on average. The datawere gridded with the minimum curvature method with a cell size of100 m (one-fifth of the line spacing) and an internal tension of 0.25(Ribeiro and Mantovani, 2011). The International GeomagneticReference Field (IGRF) calculated for this region at the time ofthe survey showed an inclination of −8.5°, a declination of −12.9°(347.1°), and an intensity of 23,843 nT.The separation of the local from the regional components of the

magnetic field was done using the spectral analysis method pro-posed by Spector and Grant (1970). This technique calculates theradially averaged power spectrum using the fast Fourier transform(FFT). Once calculated, it is possible to see the characteristic behav-ior of each component of the magnetic field: Those related to smallwavenumbers represent regional structures, and those related tohigher wavenumbers indicate local anomalies and noise.Analyzing the regional power spectrum (Figure 2a), it was pos-

sible to observe the band dominated by regional structures (insidethe black box). This band varied from 0 to 0.64 cycles∕km, whichprovided the cut-off wavelength value for the high-pass filtering ofthe regional magnetic field (9.888 km). The Figure 2b and 2c showsthe unfiltered total and high-pass-filtered residual magnetic field,respectively.After the regional-residual separation, the residual magnetic field

(RMF) was limited to the Morro do Leme region. In this local grid(Figure 3), it is possible to observe a well-defined anomaly asso-ciated to the main outcrop from the deposit. With the help of theoutcrops’ contours, it was possible to identify two other anomaliesrelated to their specific bodies. The magnetic anomalies fromMorrodo Leme show lows to the north, which in the southern hemisphereindicates the presence of significant remanent magnetization. The

amplitudes on Figure 3 range from −512 to676 nT.

Estimating the total magnetizationdirection

In cases of significant remanent magnetiza-tion, knowing the components of the source ofmagnetization (total, induced, and remanent) be-comes essential for a successful modeling and/orinversion. Fedi et al. (1994) present a techniqueto analyze the distortion of magnetic anomaliesthat have been reduced to the magnetic pole(RTP) using a frequency domain filter:

RTPðρ; θÞ¼ f½sinð IHÞ þ i cosðIHÞ cosðDH − θÞ�× ½sin ðITÞ þ i cos ðITÞ cosðDT − θÞ�g−1;

(1)

where ρ and θ are the polar representations fromthe frequency coordinates in the frequency do-main, i is the imaginary term, IH and DH are theinclination and declination of the induced field(from the IGRF), and IT and DT are the totalmagnetization vector angles for inclination anddeclination.Figure 1. Geologic map of the Morro do Leme deposit (extracted from Nunes, 2000).

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These authors indicated that the grid resulting from RTP filtering,when using the proper IT and DT , will result in smaller negativeamplitudes, and these negative amplitudes occur over a smallerareal extent when compared with the amplitudes and areas obtainedfor other directions of the total magnetization vector.Cordani and Shukowsky (2009) call this procedure the MaxiMin

technique. They also implement it in a MATLAB algorithm. Thisalgorithm selects 30 pairs of inclination and declination angles andperforms RTP filtering from the inserted grid (in the case, the RMF)with each pair. The resulting grid that presents the most negativevalues is discarded, and a new iteration is initiated. This processis repeated until the 30 pairs of values do not differ from each othermore than a predefined error (2° in this work) or the process reachesa predefined maximum number of iterations (4000).Figure 4a shows the convergence of the best (when the negative

amplitudes of the reduced field are minimized) and worst (when thenegative amplitudes of the reduced field are not minimized) solu-tions for the reduction at each iteration. It took 182 iterations toconverge the pairs of inclination and declination to the predefinederror of 2° (Figure 4b).The resultant map, from the MaxiMin filtering technique, indi-

cates that the three anomalies have very similardirections of total magnetization because the sameRTP that resulted in the pair of total inclinationand declination, 58.1° and −133.3° (226.7°),reduced the three anomalies to the magnetic polesuccessfully as shown in Figure 5. The differencebetween the angles obtained in the MaxiMinprocess and the ones indicated by the IGRF forthe time of the survey confirmed the presenceof a significant remanent magnetization. Therecovered angles represented the vector sum ofthe induced and the remanent magnetizations andhad a crucial role in the inversion and modelingstages.The MaxiMin results showed higher values

concentrated inside the contours of the geologicbounds indicated by Nunes (2000). Such behav-ior was expected for a successful use of the RTPtechnique with higher positive amplitudes centeredover the assumed source of magnetic signal. Twosmaller signals between Morro 1 and 3, and 1 and2 are displayed in Figure 5. Their low amplitudesmay represent smaller magnetic sources or, pos-sibly, numerical effects of the RTP filtering. Thefirst hypothesis is considered more probable onceit is possible to distinguish its representation inRMF (Figure 3) map; however, the interpretationof these anomalies would be difficult due their sizein relation to the survey’s line spacing.

Estimating the spatial limits

To define the lateral limits of the magneticstructures, we used the EHD method (Fedi andFlorio, 2001). EHD presents good accuracy toestimate lateral boundaries of the source of agiven anomaly (assuming there are vertical con-tacts between lithologies), although it does notunduly introduce noise due to the derivative

creation process and does not have a strong dependence on effectsrelated to remanent magnetization.In this technique, Fedi and Florio (2001) define a new quantity ϕ,

which is the sum of all the vertical derivatives:

ϕðx; y; z0Þ ¼ fðx; y; z0Þ þ fð1Þðx; y; z0Þþ fð2Þðx; y; z0Þþ · · · þfðnÞðx; y; z0Þ; (2)

where fðx; y; z0Þ is the RTP and fðnÞðx; y; z0Þ is its vertical deriv-atives of order n. It is important to indicate that the physical dimen-sion of ϕ depends directly on the order n used. In Morro do Leme,we used the n ¼ 5, so the dimension of ϕ is given as nT and nT∕mand nT∕m2 and nT∕m3 and nT∕m4 and nT∕m5, being hereafter de-noted as α.According to Fedi and Florio (2001), using vertical derivatives

shall enhance dramatically the noise in the magnetic signal and,for this reason, they suggest a different strategy to avoid part ofthis noise. The strategy is called the integrated second verticalderivative (ISVD) and consists of calculating the componentsfðnÞðx; y; z0Þ from equation 2 as follows: (1) When n is odd,

Figure 2. (a) Power spectrum of the regional magnetic field from Morro do Leme de-noting the frequency band associated with the regional field, (b) unfiltered total mag-netic field, and (c) residual (high-pass >0.64 cycles∕km) magnetic field.

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integrate the field by using a frequency-domain operator and(2) after integrating the field or when n is even, compute the secondvertical derivative by summing its second horizontal derivatives (us-ing a finite-difference algorithm) as in the Laplace equation:

∂2f∂z2

¼ −�∂2f∂x2

þ ∂2f∂y2

�: (3)

Thus, the EHD filter occurs from

EHDðx; yÞ ¼��

∂ϕ∂x

�2

þ�∂ϕ∂y

�2�1

2

; (4)

where n is the order of the EHD. The order of the EHD is defined bythe vertical derivative applied in ϕðx; y; z0Þ.The EHD filtering characterized the main anomaly of Morro do

Leme with an east–west elongation (Figure 6), also showing better-defined north–northwest boundaries (by higher values in the EHDsignal). This behavior may be interpreted as a more vertical limit inthe north part of the body.A method for using the EHD to estimate the depth was proposed

by the same authors, based on the enhanced analytic signal (Hsuet al., 1998). For practical reasons, this extrapolation shall be re-ferred to as the EHD-depth in this work, as it was not formallynamed in Fedi and Florio (2001). The EHD-depth method estimatesthe depth d of the top of the lateral limits of the source, where theEHD presents points of maxima (Fedi and Florio, 2001):

EHD−depthðx;yÞ¼ jEHDnðx;y;0Þj∕jEHDnþ1ðx;yÞj: (5)

The EHD-depth method indicated depths (Figure 7) for the top ofthe contact between the mafic lithologies (dunites and olivine-pyroxene peridotites) and the host rocks (Nova Brasilândia meta-sediments and Alto Guaporé sediments/soils) varying from 72 to190 m (Figure 7) — depending on their location. Its central partshows points of contact between the intrusion and host rock withdepths from 90 to 120 m, unlike the borehole data from Nunes(2000) that showed the laterite layer beginning at 5 m and extendingfrom 35 to 55 m depth in the southwest portion of the deposit inholes FS-01 and FS-04.Such difference can be attributed to two main aspects: (1) consid-

ering the use of 100 m grid cells, it is not possible to identify depthsshallower than 30 m (based on experimental data using syntheticmodels) and (2) the definition of fresh rock due to weatheringand/or serpentinization of olivine as indicated by Nunes (2000),may not correspond to the depth of weathering magnetite. It alsoindicated that the borders of the western and northern portion ofthe source of the anomaly are at shallower depths than in the easternand southern.

Figure 3. RMF of Morro do Leme deposit. The bold curves re-present the superficial geologic boundaries of the deposit definedby Nunes (2000; Figure 1). The contour lines have a spacing of150 nT.

Figure 4. (a) Graph showing the convergence of the solutions of theobjective function as a function of the iteration number. (b) Graph ofinclination versus declination indicating the position of the individ-ual and best solutions along with the location of the induced field.

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Modeling and inversion of magnetic field data

After estimating the spatial features and the directions of the totaland the induced magnetizations, it was possible to compose aninitial model to be used as the starting point to the inversion of themagnetic data. This model was conceived through the use of themagnetic data along with prior geologic and borehole informationfrom Nunes (2000).The initial model was composed through a modeling algorithm

based on Barnett (1976). The angles of induced magnetization (de-fined by the IGRF) and of the total magnetization (defined by theMaxiMin technique) were attributed to the model after the geometryof the model was specified. The synthetic magnetic signal shall bemodeled so it has the overall behavior of the real signal, but withoutthe compromise of an exact adjustment. Its features have an averagesusceptibility of 0.15 (SI): induced magnetization: inclination−8.5°, declination −12.9° (347.1°), and intensity 2.28 A:m−1; totalmagnetization: inclination 58.1°; declination 226.7°; and, as a firstguess, intensity 2.5 A:m−1.The inversion of the magnetic data was performed with the algo-

rithms from Li and Oldenburg (1996, 2003) apply in the UBC-GIFMAG3D software. These inversion algorithms take into account themagnetization observed for the ith cell, given by

Ji ¼ κi:H0; (6)

where κi is the magnetic susceptibility for the ith cell and H0 is theearth’s magnetic field at the cell position. The algorithm seeks tominimize the objective function:

ϕ ¼ ϕd þ μ:ϕm; (7)

where ϕd and ϕm are the data misfit and regularization functions,respectively, and μ is the regularization parameter.

The inverted model (Figure 8) was consistent with the resultsfrom the magnetic techniques and geologic data. The recovered sus-ceptibility contrast varied from 0.10 to 0.13 (SI) for the main body,being a plausible value for dunites and peridotites (Telford et al.,1990), if we ignore the effects of self-demagnetization commonlyobserved in rocks whose magnetic susceptibility is above 0.1 (Guo

Figure 5. RTP filtering with the MaxiMin technique of the Morrodo Leme deposit. The contour lines are spaced at 150 nT.

Figure 6. EHD map of the Morro do Leme Deposit. The contourlines are spaced at 0.6 α∕m.

Figure 7. EHD-depth of the Morro do Leme deposit. The blackcontour indicates the superficial geologic boundaries of the deposit.

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et al., 2001). This model shows an expected east–west elongationspread along about 4 km and a 1 km north–south distribution. Thevertical features interpreted in the EHD and EHD-depth results werealso corroborated by the inversion: a steeply boundary in its northside and a smoother one in its southern, considering the first 1000 mwhere the magnetic inversion is effectively more reliable.The two other features were found to have a reduced contrast of

0.03 (SI), although they have the same lithology as the main body(Nunes, 2000). This effect can be credited to their size (smaller than700 m in both directions) that limited their sampling to one flightline each and inevitably had a larger portion of their total volumeaffected by weathering.The residue (Figure 9c) from the inversion’s magnetic signal

(Figure 9a) showed a good approximation to the original data (Fig-ure 9b). More than 90% of the residue was less than 5 nT, with themaximum residue being of 41 nT (less than 4% of the total ampli-tude of the RMF, Figure 9d). This small residue indicated that themodel presented in Figure 8 is a valid mathematical solution forthe geophysical problem from Morro do Leme. Because its shape,depths, and magnetic susceptibility contrast agree with the resultsfrom prior magnetic techniques and geologic investigations, it ispossible to consider it a valid and reliable representation of the dis-tribution of magnetic minerals from the protolith of Morro do Leme.Using the inverted model as a pattern of distribution of magnetic

susceptibility in the subsurface (assuming the the host rock’s mag-netic susceptibility is negligible) and applying to it the estimatedtotal and induced magnetization features, it was possible to modela magnetic field similar to the original one shown in Figure 9aby a simple variation of the intensity of the total magnetization(Louro, 2013). It also allowed estimating the average remanentmagnetization of the Morro do Leme anomalies with a Köenigs-berger ratio (remanent magnetization/induced magnetization) of1.82 — reinforcing the predominance of the remanence over theinduced magnetization. Its components are presented in Table 1.The reverse polarity seen in Morro do Leme’s magnetic anoma-

lies is also found in other location in the RSIP, such as the smallerSão José anomalies (Ribeiro et al., 2013a), Aguapeí, Sertãozinho,Nova (Ribeiro et al., 2013b), Rio Jauru (Cotis et al., 2013), Indiavaíand Figueira Branca (Louro et al., 2013) among others, all locatedsouth of Morro do Leme. The similar polarity suggests that theycould have been intruded into the crust in a relative short timewindow.Considering the datings from the Figueira Branca Suite (1426�

8 Ma, SHRIMP zircon age) (Teixeira et al., 2011) and Morrodo Leme (1372� 21 Ma, by Rb-Sr — Cacoal Intrusive Suite)(Teixeira and Tassinari, 1977), and the proximity of the averageremanent magnetization between both (Morro do Leme: Table 1;Figueira Branca Suite: inclination 50.7°, declination 209.8°, α95° ¼8.0°) (estimated by D’Agrella-Filho et al., 2012), it is also possibleto stipulate whether the magnetic field stood the same in this period— with some small variations — or it suffered an even number ofreversals (to present the same polarity) and established coincidentlyat a similar condition to the one at the crystallization of FigueiraBranca when Morro do Leme crystallized. The association andproximity of the ages of both intrusions and the comparison of theirmagnetic anomalies to others from the RSIP (Louro et al., 2013)indicate a tectonically active period during the Santa Helena Orog-eny as reported by Ruiz et al. (2005), with several intrusions beinginjected in the crust.

Figure 8. Inversion model of the Morro do Leme deposit viewedfrom: (a) south, (b) west, (c) top, (d) perspective with north–southand east–west cuts, and, in the inset, (e) a similar perspectiveview with a contrast of susceptibility cut-off between 0.09 and0.13 (SI).

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Taking into account the presented geologic background, our re-sults, and their implications, some speculations can be made as tothe exploration potential of Morro do Leme: The estimated depths(ignoring the limitations related to the grid cell size) reveal a varia-tion of the top of the fresh mafic rock, being shallower in the centraland west zones of the anomaly and deeper in its eastern portion. The3D model shows a steeper border in the north side of the anomaly,whereas the south side has a smoother behavior. The model alsorevealed the concentration of higher susceptibilities in the centerand north areas of the model.Thus, the described features permit us to infer zones for explo-

ration in Morro do Leme. Considering analogous cases of Archeanto Palaeozoic layered peridotite/dunite intrusions in cratonic ter-ranes, containing lateritic Ni deposits formed by weathering of ser-pentinized peridotite, such as the Murrin Murrin deposit (WesternAustralia) (Gaudin et al., 2005), Niquelândia and Barro Alto (Bra-zil), Musongati (Burundi) andWingellina (Australia) (Butt and Clu-zel, 2013), the best portions for Ni exploration are those possessinga thicker weathered-rock package. So, the deeper west and south ofMorro 1, 2, and 3 anomalies fit this criterion.The Pd, Au, Cu, Na, Co, Zn, and/or Pt exploration in Morro do

Leme is normally concentrated on the sulfide/chromite horizonswithin the layered peridotite intrusion, as in similar cases fromMuni Muni (Western Australia) (Hoatson and Keays, 1989), Lang-muir (Western Australia) (Green and Naldrett, 1981), and Muskox(Canada) (Barnes and Francis, 1995). Borehole data from Nunes(2000) indicate that in the southwest area of the deposits, these layersare located at 150-m below the surface. Assuming that these layersare present in the whole extension of the intrusive body, the best op-tion for exploring for these elements should be the central and easternpart of Morro 1, where the depth to the fresh rock and, consequently,to its sulfide/chromite layers is smaller due to the absence of lateriticcover unrelated to the weathering of Morro do Leme.In an ideal exploratory scenario, this interpretation must be linked

and confirmed with different geophysical methods such as electro-magnetism and seismics and, of course, with drilling in the centralportion of Morro 1, 2, and 3.

CONCLUSIONS

The Morro do Leme deposit in the southwest of the AmazonCraton has an already known potential for laterite nickel and pos-sibly for other elements as Pd, Au, Cu, Na, Co, Zn, and Pt. Themagnetic field of its region has three reverse-polarized anomalies,with possible sources of 4 by 1 km and about 700 m of diameter forthe others.

Figure 9. (a) RMF from Morro do Leme deposit, (b) magnetic fieldfrom the inverted model, (c) residue from both grids, and (d) residuehistogram indicating the frequency of values.

Table 1. Average magnetic features of the Morro do Lemedeposit: inclinations, declinations, and intensities from theinduced, remanent, and total magnetization vectors.

Morro do Leme deposit

Magnetization vector Inclination Declination Intensity (A.m−1)

Induced −8.5° 347.1° 2.3

Total 58.1° 226.7° 2.4

Remanent 41.8° 193.5° 3.5

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The analysis of Morro do Leme’s magnetic field permitted us toestimate the spatial features of the deposit’s protolith — where thecontrast of magnetic susceptibility with the host rock is more sig-nificant. Its EHD revealed an east–west elongation for the mainanomaly — as expected, observing the RMF. It also showed a well-defined vertical border in the north portion of the main anomaly anda possible smoother dip in its southern portion for the smaller am-plitudes of EHD in this portion. The depths of these borders wereestimated principally between 90 and 150 m, where they were in-terpreted as the upper limit of the fresh rock without effects ofweathering and/or serpentinization.Through the MaxiMin technique, we could estimate the average

directions of total magnetization. An initial model was createdbased on the limits indicated by the EHD and the average suscep-tibility expected for peridotites (0.15 SI). The inversion model wasconsistent physically and geologically fit the scenario and tectonicshosting the deposit.The three inverted bodies showed two trends of susceptibilities

contrasts: 0.13 and 0.03 (SI). These different behaviors can be as-sociated with the size of the anomalies that, while the larger mainbody was correctly represented with the expected contrast for dun-ites and peridotites, the smaller bodies were subsampled in the east–west direction, occupying only one possible flight line. This matterallied to the weathering effects in a larger portion of the smallerintrusions (compared to their total volumes) may have resultedin smaller contrasts of magnetization in the inversion process.The inverted magnetic susceptibility multiplied by the IGRF

gives the induced field vector. The directions of total magnetizationdefined by the MaxiMin method were applied to the inverted modelalong the induced field vector. The total magnetization intensity wasmodeled with these features fixed (susceptibility, induced vector,and total inclination and declination). When the modeled field andthe real adjusted within a margin of 10% of error, the intensity oftotal magnetization was defined and, by vector subtraction, the aver-age remanent magnetization. This process together with isotopicdata, indicated that the magnetic field between 1426� 8 Ma and1372� 21 Ma were very similar.Finally, the deposit offers different possibilities of exploration.

The geophysical data indicated that to explore the laterite Ni, thebest locations are the southern part of Morro 1 and over the Morro2 and 3, where the weathering is thicker; whereas to best exploit thePd, Au, Cu, Na, Co, Zn, and/or Pt, the indicated region should bethe central and north portion of Morro 1. Although very good resultshave been obtained through the magnetic method, these findingneed to be confirmed with more drilling and the use of a comple-mentary geophysical method to precisely determine the top and bot-tom of the protolith.

ACKNOWLEDGMENTS

The authors would like to thank the CPRM for releasing the datafrom the airborne survey and the National Council of Scientific andTechnological Development for granting the Ph.D. scholarships.We also would like to thank the reviewers and editors whose ques-tions and suggestions contributed to a significant improvement ofthis paper.

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