case history delineating hydrothermal stockwork copper

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GEOPHYSICS, VOL. 74, NO. 5 SEPTEMBER-OCTOBER 2009; P. B167–B181, 13 FIGS., 2 TABLES.10.1190/1.3174394

ase History

elineating hydrothermal stockwork copper deposits usingontrolled-source and radio-magnetotelluric methods: Aase study from northeast Iran

ehrdad Bastani1, Alireza Malehmir2, Nazli Ismail2, Laust B. Pedersen2, and Farhang Hedjazi3

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ABSTRACT

Radio- and controlled-source-tensor magnetotelluric RMTand CSTMT methods are used to target hydrothermal veins ofcopper mineralization. The data were acquired along six east-west- and three north-south-trending profiles, covering an area ofabout 500400 m2. The tensor RMT data were collected in the10–250-kHz frequency band. A double horizontal magnetic di-pole transmitter in the 4–12.5-kHz frequency range allowed usto constrain the deeper parts of the resistivity models better. Toobtain optimum field parameters, ground magnetic profiling wasconducted prior to the RMT and CSTMT surveys. Although thestudy area in Iran is remote, a number of radio transmitters withacceptable signal-to-noise ratio were utilized. The 2D inversionof RMT data led to unstable resistivity models with large data

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isfits. Thus, the RMT data were used to complement and ana-yze the near-surface resistivity anomalies observed in the 2DSTMT models. Analyses of strike and dimensionality from theSTMT data suggests that the low-resistivity structures areainly three dimensional; therefore, 2D inversion of determi-

ant data is chosen. Independent 2D inversion models of the de-erminant CSTMT data along crossing profiles are in good agree-

ent. Known copper mineralization is imaged well in theSTMT models. The thinning of the conductive overburden cor-

elates very well with magnetic highs, indicating the bedrock isesistive and magnetic. In this sense, the magnetic and electro-agnetic fields complement each other.Analysis of the 2D resis-

ivity models indicates the volcanic rock deepens at the center ofhe study area. This zone is associated with a magnetic low andherefore is recommended for detailed exploration work.

INTRODUCTION

Economic mineral deposits usually consist of conducting sulfideo oxidized metallic minerals, which interact significantly with elec-romagnetic EM signals. Therefore, EM methods are important ex-loration tools for detecting metallic minerals. Induced-polarizationIP or controlled-source audiomagnetotelluric CSAMT methodslso provide good images of mineral deposits e.g., Tuncer et al.,006; however, data acquisition is cumbersome and expensive. Forhese reasons, these methods are unsuitable for reconnaissance sur-eys and exploration of small targets. Because of weak naturalource fields and strong noise in the 1–500-kHz frequency band,

Manuscript received by the Editor 22 December 2008; revised manuscript1Geological Survey of Sweden SGU, Uppsala, Sweden. E-mail: mehrdad2Uppsala University, Department of Earth Sciences, Uppsala, Sweden

eo.uu.se.3Kahanroba Engineering Company, Tehran, Iran. E-mail: kahanroba@kah2009 Society of Exploration Geophysicists.All rights reserved.

rospecting for natural resources in the upper few hundred meters ofhe earth’s crust using natural plane-wave sources is difficult Peder-en et al., 2006.

Measurements of the signal from distant radio transmitters in the0–250-kHz frequency range with the radio magnetotelluric RMTechnique can provide useful information about near-surface resis-ivity structures. In the 1–10-kHz frequency range, it is better to useontrolled sources situated sufficiently far away from the measuringoint so their EM fields satisfy plane-wave assumptions. The con-rolled-source tensor-magnetotelluric CSTMT technique offers aost-effective, single discipline from which detailed electrical-resis-

d 3 May 2009; published online 1 September [email protected]: [email protected]; [email protected]; laust.pedersen@

com.

SEG license or copyright; see Terms of Use at http://segdl.org/

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B168 Bastani et al.

ivity models can be obtained Pedersen et al., 2005.The RMT technique in its original and scalar forms was first ap-

lied by Müller and his group and has been used mainly in hydrogeo-hysical applications e.g., Turberg et al., 1994; Stiefelhagen andüller, 1997; Pedersen et al., 2005. There are only a few published

ccounts of the application of controlled-source EM methods inineral exploration e.g., White et al., 2000; Garcia et al., 2003.he CSTMT technique at its present stage of development has noteen used for mineral exploration.

The Chah-Mussi mining area, located about 500 km east of Te-ran, Iran Figure 1, was chosen for this study. The area has cap-ured increased attention because of newly discovered large coppereposits and frequent occurrences of small base metals. Mineraliza-ions include chalcopyrite and bornite within a phyllite and porphy-itic alteration zone. The host rocks are porphyritic andesites. Inerms of mineralization, the Chah-Mussi mining area is divided intowo parts: highland and foreland Figure 1. The highland orebodiesre the focus of this study.

To investigate the depth extent of existing ores in the Chah-Mussiining area in the past, geophysical measurements such as IP, self-

otential SP, and direct-current DC geoelectrics were conducted.owever, because of the complexity and small thickness of mineral-

zed veins, no useful interpretation was made available. Kahanrobangineering Co. has conducted exploration and limited mining ac-

ivity. A sharp change in thickness and continuity of exposed miner-lized zones with depth has been a matter of debate among mine ge-logists. To extend mining activities toward depth, a better under-tanding and reconsideration of the continuation of exposed ores isecessary.

The main objectives of this study are 1 to collect CSTMT andMT data to examine their application as a tool for a detailed miner-l exploration program, 2 to help delineate possible continuation ofxisting orebodies laterally and vertically, and 3 to study the RMTignals in northeast Iran. We show how a combination of CSTMTnd RMT techniques with ground magnetics and surface geologicbservations improves our understanding of the nature of a complex,hallow copper mineralization in the study area.

AlluviumAlteration ofand andesitDacite breccTuff, andesivolcanic breBiotite horndacite porpBiotite hornandesite po

Activecopper mine

Closed

Fault

igure 1. Regional geologic setting of the Chah-ussi mining area in northeast Iran simplified

rom Emam-Jomeh, personal communication,006. This study focuses on the highland miningrea rectangle, the main geologic map. Arrowshow locations of major copper mineralizationones.


LOCAL GEOLOGIC SETTING

Detailed information about the geologic setting of the Chah-ussi mining area is scarce. The description given here is a summa-

y of our observations during data acquisition in addition to thosehared with us from the mine geologist Emam Jomeh, personalommunication, 2006.

The Chah-Mussi copper deposits are situated within a wide10–15-km northeast-southwest-directed package of volcanicocks Figure 1. The main copper deposits, including those in theighland and foreland, are within a unit of porphyry biotite-horn-lende andesite, mainly covered by Holocene alluvium. Metallicineralizations are copper 1%–18%, lead 5–15%, and zinc 2%–

% but are limited to veins from a few centimeters to occasionally0–15 m in thickness. The highland orebodies are mainly copper,anging from 0.5% as infiltrated malachite to 5% when sulfide min-rals such as rich chalcosite are present.

It is generally believed that the copper mineralizations are the re-ult of a major hydrothermal activity after the volcanic rocksormed. They are usually observed within the existing faults andracture zones in direct association with the porphyry biotite-horn-lende andesite unit. Occasionally they appear as sparse veins butre usually stockwork connected veins. Thus so far, tectonic stud-es have provided the most useful guide for the exploration programn the highland area. In the central part of the highland, the currentocus of exploration, there is little or no surface expression of theost rock porphyry biotite-hornblende andesite unit. It is coveredither by the holocene alluvium or is altered and fractured extensive-y so that it cannot be recognized easily from the alluviums, whichre composed mainly of similar rocks.

EM METHODS USED IN THIS STUDY

Bastani 2001 presents a detailed description of the EnviroMTnstrument that we used in this study to conduct RMT and CSTMT

easurements. With both methods, two horizontal electric-fieldomponents Ex,Ey and three magnetic-field componentsHx,Hy,Hz are measured on the ground. Cantwell 1960 shows that

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CSTMT and RMT in mineral exploration B169

hese components are related through the earth’s electrical resistivi-y, so in the frequency domain we can write

Ex

EyZHx

Hy 1

nd

HzTTHx

Hy, 2

here Z and T are magnetotelluric transfer functions. The complexmpedance tensor Z is given by

ZZxx Zxy

Zyx Zyy, 3

Processingunit

N

E

W

NES

W

AF box

S

Hz

HxHy

+

+

+

4

9

2

7

3

65

1

8

12V 12V 12V

12V

Transmitterbox

GPSantenna

Modemantenna Loop

antennas

Connectorsto the box

To GPS

To modem

12-batteries

Mast supports

Masts tofix the loop

V

c)

a)

igure 2. Field setup of the EnviroMT system at the receiver and traneal setup at a receiver site, c schematic of the double horizontal mtudy area see Figure 3.


nd T is the complex geomagnetic transfer function or as we call ithe tipper vector, defined by T A,BT. The superscript T denotesransposition. These transfer functions contain useful informationbout the resistivity structures below the measuring point.

MT

The source for RMT measurements are fixed transmitters used forommunication as well as long-wavelength LW radio transmis-ion. These transmitters normally are vertical electric dipoles Ped-rsen et al., 2006. The wavelength of the source field ranges from0 km at 15 kHz to 1 km at 300 kHz. The corresponding penetra-ion depth in the ground in this frequency range for a resistivity of0,000 ohm-m varies from about 400 to 90 m. Thus, to a very goodpproximation, the wavelength is infinitely large compared to theenetration depth.

The RMT field layout is shown in Figure 2a and b. The signal-to-oise ratio S/N is largely a function of transmitter strength and the

AF box

Processingunit

GPSantenna

Magnetometer

)

GPSantenna

12-batteries

Loopantennas

Loopantennas

Masts tofix the loops

V

)

sites. a Schematic of the EnviroMT setup at a receiver site, b thec dipole, and d the transmitter site in the field at location S2 in the

b

s

d

smitteragneti


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P

M

R

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B170 Bastani et al.

istance to the transmitter as well as the bandwidth used. The noiseevel is estimated using the tensor very-low-frequency VLF tech-ique described by Pedersen et al. 1994. The electric and magneticoise levels are estimated from the median filtered power of the hori-ontal fields. The S/N for each frequency in the band of interest ishen defined as the ratio between the horizontal power and the esti-

ated noise power. An adjustable magnetic S/N threshold can besed to detect the radio transmitters automatically. Transmitter azi-uths are measured from the magnetic north toward the east and can

e obtained by rotating the coordinate system about a vertical axisuch that the total magnetic field power is minimized.

To estimate the transfer functions, we use a band-averaging tech-ique e.g., Sims et al., 1971; Gamble et al., 1979; Pedersen, 1982 innarrow band of one octave. The main band is split into nine over-

apping subbands with a bandwidth of one octave in which the trans-er functions are assumed to be constant. The method described byedersen 1982 is used to estimate errors of the transfer functions.

STMT

The amplitude of natural EM fields in the 1–10-kHz frequencyange usually is very low. Therefore, the signal from natural sourcesn this band is unpredictable and cannot be used routinely Vozoff,987; Smith and Jenkins, 1998, although studies by Garcia andones 2008 show that with very long time series, stable estimatesan be obtained using a wavelet transform technique. The EM noisen this frequency range may become so high that it dominates theatural source fields Szarka, 1987; Qian and Pedersen, 1992; con-equently, audiomagnetotelluric AMT data quality may degradeonsiderably. To overcome the noise problem, Goldstein and Strang-ay 1975 originally used the CSAMT technique with a grounded

lectric dipole source. If the transmitter is located sufficiently farrom the receiver, such that far-field or plane-wave conditions pre-ail, then plane-wave theory is valid for CSAMT data. Furthermore,

able 1. Data-acquisition parameters.

arameter Data

agnetic

Number of profiles 19

Sampling spacing 5 m

Total length 6.5 km

MT



Total length 4.5 km

Available transmitters 12 S /N1

STMT



Total length 4.5 km

Frequencies used 4.0, 6.25, 8.0, 1


stimation of the transfer functions is simpler and more straightfor-ard in the CSAMT method. However, with this technique, it maye difficult to define criteria for far-field conditions Wannamaker,997; Pedersen et al., 2005.

The EnviroMT system employs a double horizontal magnetic di-ole source Figure 2c for the CSTMT measurements. In shallowxperiments, it is advantageous to use horizontal magnetic dipolesecause they are safer and much easier to install, and their range isufficient to cover distances up to several hundred meters. Magneticipoles also have little coupling to nearby conductive structuresompared to electric dipoles; therefore, they are expected to provideetter plane-wave conditions than electric dipoles.

A picture of the transmitter configuration taken during the fieldampaign is shown in Figure 2d. The two horizontal magnetic di-oles are set up approximately perpendicular to each other. Two peo-le can set up the transmitter in about 30 minutes. The signal sources phase locked to an external global-positioning-system GPSlock for synchronization with the receiver, which is equipped with aimilar GPS clock. The transmitter is controlled remotely by a radioodem from the receiver site. With a relatively small transmitter fed

y three car batteries, it is quite fast to measure at distances up tobout 600 m.

In the CSTMT processing technique, unique transfer functionsan be defined at the selected transmitter frequencies because theransmitter consists of two independent, coincident, horizontal di-ole coils Bastani, 2001.Aset of transmitter frequencies can be se-ected in the CSTMT measuring mode. The selection depends on re-eiver-transmitter distance and penetration depth. The S/N increasesonsiderably by stacking the amplitudes of the measured field com-onents in the time or frequency domain. Phase stability is vital inhis technique and is provided by GPS-controlled clocks with ahort-term accuracy of 109 and a proper triggering system.

FIELD DATA ACQUISITION

Prior to the RMT and CSTMT surveys, a studyincluding geologic field observations, rock sam-pling for susceptibility measurements, andground magnetics was conducted to obtain opti-mum field parameters. Table 1 shows details ofthe data acquisition. The preliminary study pro-vided useful information to choose locations ofthe RMT and CSTMT profiles. Figure 3a showsthe locations of RMT and CSTMT profiles super-imposed on the total-field magnetic map. Theprofile directions were chosen to cross main mag-netic features. A few profiles i.e., profiles 1, 4,and 6 cross known mineralization zones to pro-vide a ground to find correlation with models andto help make a more precise interpretation. Cur-rent mining activity in the study area did not al-low a more regular design of profiles.

The RMT and CSTMT data were acquired atthe same locations Figure 3a. The CSTMTtransmitter frequencies were selected based onresults from previous vertical electrical sound-ings Table 2 and a series of preliminary testmeasurements. The resistivity of the host rockcontrols the selection of the transmitter frequen-cies far-field criteria; see Wannamaker 1997

nd 12.5 KHz

0 dB

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nd, accordingly, the penetration depth. The RMT measurements onhe outcrops of porphyritic andesite gave the highest resistivity500 ohm-m. The receiver-transmitter distance controls the S/Nnd, according to Pedersen et al. 2005, has a maximum range of00 m for the EnviroMT system. Based on these controlling limits,he minimum transmitter frequency was set. To avoid the near-field

S1

S2

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2

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Profile 3

Profile6

47990

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47950

47940

47920

47890

47880

47830

47690

Profile 4

Profile6

Profile 1

Profile 1

Profile 4

Profile 2

Profile 3

a)

b)

Log f =0.5Scale

Cu

igure 3. a Ground magnetic map of the study area shown with theb The real part of the induction arrows for the selected CSTMT freqap of the study area.


ffect, we set up the double transmitter at two locations, S1 and S2see Figure 3a. Location S1 is for profiles 1–3. The selected trans-itter frequencies were 4.0, 6.25, 8.0, 10.0, and12.5 kHz. With aagnetic dipole source, near-field effects appear as decreasing resis-

ivity and increasing phase at the lower frequencies Li and Peder-en, 1991a.

EnviroMT, VLF, Mag

Crossing profile

4:26

5

P1:34

P3:14

P8:0

7P8

:12

Profile 5

Profile7

Profile8

50 m

48200

48150

48120

48100

48080

48060

48050

48040

8020

Source location

50m

Profile 5

Profile8

Current miningactivity

50 m

50m

Log f = 3.8

Cu

n of the RMT and CSTMT profiles and transmitter sites S1 and S2.s of 6.25 and 12.5 kHz, projected onto a grayscale ground magnetic

1:25

P

P3:

P7:0

2P7

:11

P7:1

5

448010

Profile7

4.1

locatiouencie


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Data quality

We set a low S/N threshold of 10 dB to identifymost available radio transmitters. Figure 4 showsthe RMT signal quality along profile 1. The num-ber of selected radio transmitter varies from 10 to16 Figure 4a. This is partly a function of back-ground noise and partly the changes in thewaveguide’s electrical properties Vallée et al.,1992a, 1992b; Pedersen et al., 2006. The VLFtransmitters 10–30 kHz and transmitters in the150–180-kHz frequency range were rather stableduring the entire measurement period Figure4a. A gap in the spectra between 30 and 140 kHzcan be observed clearly in Figure 4d and e. A nar-rower gap 80–110 kHz is reported by Pedersenet al. 2006 in data acquired in a few areas in Eu-rope. The variation in the S/N shown in Figure 4bis nearly constant 10–28 dB along the profile

ck, andivity values, personal

nt resistivityhm-m

150

0–110

8–50

along Profile 1

0 40 50 60

number

0 40 50 60number

105

equency (Hz)

number0 40 50 60

510equency (Hz)

ies, b variation of transmitters’S/N, c direction to the selected ra-variation of direction to the transmitters versus transmitter frequen-

able 2. List of average susceptibility, apparent resistivity of host roopper mineralization used for final interpretations. Apparent-resisthown are compiled from geoelectrical vertical sounding (F. Hedjaziommunication, 2006).

ockSusceptibility

SINumber of

samplesAppare

o

iotite-hornblende andesiteorphyry host rock

800–1000105 5

xidized-sulfidic porphyryopper ore Cu 1.5%

15–50105 7 5

assive sulfidic copperre Cu 3%

5–10105 5 1

arth’s total magneticeld strength

48,100 nT

agnetic inclination 53.9°

agnetic declination 3.8°

300

200

100

0

Trans.freq.(kHz)

Transmitter information

0 10 20 3

Station

S/N(dB)

40

30

20

10

200

0 10 20 3Station

104

40

30

20

10

S/N(dB)

Transmitter fr

Dirtotrans()

100

0

Station0 10 20 3

o

Dirtotrans()

10 4

200

100

0

Transmitter fr

o

a)

b)

c)

d)

e)

igure 4. RMT signals along profile 1. a Variations of transmitter frequencio transmitter, d variation of S/N versus transmitter frequencies, and eies.


atu

fTot

fift6

D

tsh

Na

Fa


nd varies slightly with the changing number of transmitters. The es-imated direction to the transmitters along profile 1 is shown in Fig-re 4c.

Figure 4d and e shows the S/N and direction to the transmitter as aunction of transmitter frequency at a single station, respectively.he S/N shows some variation for each frequency; that is a functionf frequency stability of the transmitter and sampling frequency ofhe system.

Figure 5a-e shows an example of CSTMT spectra in the measuredve channels. The data belong to a 6.25-kHz CSTMT transmitterrequency at station 33 on profile 8. The amplitudes are normalizedo the maximum in the spectrum for each channel. The strong peak at.25 kHz demonstrates the benefit of the phase-locked technique.

1

2

-10

-10Normalized

amplitudes(dB)

a)

1

2

-10

-10Normalized

amplitudes(dB)

b)

1

2

-10

-10

Normalized

amplitudes(dB)

c)

1

2

-10

-10

Normalized

amplitudes(dB)

d)

1

2Normalized

amplitudes(dB)

110

-102

10

-10

e)

igure 5. An example of spectra from profile 8, indicating a strong CSnd e Ey.


RESULTS

imensionality analysis

There are several properties of the MT transfer functions to checkhe dimensionality of conductivity structures. One very useful mea-ure of dimensionality is the skew Reddy et al., 1977; Ting and Ho-mann, 1981; Li and Pedersen, 1991b, defined as

SZxxZyyZxy Zyx

. 4

ote that the skew is rotationally invariant and equal to zero for 1Dnd 2D structures.

x

y

z

x

y

30

ency (Hz)

410

510

ignal generated at 6.25 kHz observed at a Hx, b Hy, c Hz, d Ex,

H

H

H

E

E

1

Frequ

TMT s


datsps

T

iifbqstliI

ts

aQtas

setmsvtemt

faafr

Fsvstm

B174 Bastani et al.

Another measure is the estimated strikes along a given profile. Aetailed discussion about strike estimation can be found in Zhang etl. 1987 and Linde and Pedersen 2004. The impedance data athree neighboring frequencies were used to estimate the regionaltrike at the center frequency. We used a similar approach to that pro-osed by Zhang et al. 1987 to check the best choice of regionaltrike that satisfies the 2D assumption with a 3D local distortion.

he objective function Q, given by

Q

j1

N Zxx,j Zyx,j2

xx,j2

Zyy,j Zxy,j2

yy,j2

N2, 5

s minimized at each center frequency for a given regional strike us-ng a weighted least-squares method to estimate and , which areunctions of electrical distortion parameters. The value N is the num-er of data points in the window in this case, three neighboring fre-uencies, N12; six complex numbers, and xx,j and yy,j are thetandard deviation of Zyx,j and Zxy,j, respectively. A 4% error floorhreshold is applied to the data errors. The Q-values were then calcu-ated for each choice of regional strikes an interval of 15° was usedn this study for each frequency along all stations on a given profile.f the estimated Q-values decrease considerably for a certain direc-

12.5

10

8

100

Frequency(kHz)

12.5

10

8

100

12.5

10

8

100

12.5

10

8

100

Frequency(kHz)

Frequency(kHz)

Frequency(kHz)

igure 6. Estimated Q-values for sets of regionaltrikes changing between 0° and 90° with an inter-al of 15° along profile 1. The bottom-left panelhows the strikes using Zhang et al. 1987; the bot-om-right panel shows the estimated skews for di-

ensionality analysis.


ion, then the corresponding regional strike is used to rotate the mea-urements along that profile.

The strikes were analyzed just for the CSTMT data. The Q-valuenalysis is shown along profile 1 Figure 6. Generally, the estimated-values are large — greater than 2.0. At the two ends of the profile,

he estimated values are considerably large for all regional strikesnd are smallest at distances between 300 and 400 m. The regionaltrike of 30° has the smallest Q-value.

The estimated strikes and calculated skews along profile 1 arehown at the bottom of Figure 6. The estimated strikes have consid-rable scatter and do not reflect any well-defined direction. The scat-ering suggests that the resistivity structures may be one or three di-

ensional. We use estimated skews to rule out one of the two. Thekew has extremely large values at the two ends of the profile. Theseariations correlate well with those seen in the Q-values. Therefore,he conductivity structures are probably three dimensional at bothnds of profile 1 and to some extent one or two dimensional in theiddle. The estimated Q-values along the other profiles show that

here are no preferred strike directions.Strikes and skews were estimated along all profiles but are shown

or profiles 4–7 in Figure 7. The estimated strikes along profiles 4, 6,nd 7 show a larger scatter than profile 5 and do not coincide withny preferred direction. The estimated skews indicate strong 3D ef-ects along profiles 4, 6, and 7, which is in good agreement with theesults obtained from the strike analysis. In general, the high skew

80

60

40

20

0

0.5

0.4

0.3

0.2

0.1

0

6543210300 400 500

trike Skew

0 15

30 45

( )

nce (m) SquarerootofnormQ-value

60 75

Profile 1o o

o o

o o

o

612.5 5

410 3

28 1

0100 200 300 400 500

S quarerootofnormQ-value

6543210300 400 500

SquarerootofnormQ-value 6

12.5 54

10 32

8 10100 200 300 400 500


6543210300 400 500

SquarerootofnormQ-value 6

12.5 54

10 32

8 10100 200 300 400 500


300 400 500

12.5

10

8

100 200 300 400 500

Frequency(kHz)

Frequency(kHz)

Frequency(kHz)

Frequency(kHz)

Distance (m)

nce (m) Distance (m)



200

S

Dista

200

200

200

Dista

Dista

Dista


vais

mdiChsCfvrtpp

2

citt

m2mcpr

piscariTCstm

masTaml


alues 0.3 indicate a strong 3D effect. Overall, the skew analysislso indicates that the structures can be considered three dimensionaln the west and become reasonably one dimensional in the east of thetudy area.

Further dimensionality analysis was conducted using the verticalagnetic field Parkinson, 1962. Figure 3b shows the real part of in-

uction arrows superimposed onto the total-field magnetic map. Thenduction arrows are shown for two selected frequencies in theSTMT band i.e., 6.25 and 12.5 kHz. The sign of induction arrowsas been reversed to point to the conductor and facilitate a compari-on with the resistivity models.Along profile 1, data from the lowestSTMT frequency 6.25 kHz are missing. The induction arrows

or the two frequencies correlate well see Figure 3b and indicateery complex resistivity/conductivity structures. The induction ar-ows at 12.5 kHz at the first half of profile 1 and entire profile 2 pointo a low-resistivity feature see the inversion results that lies almostarallel to both profiles. The induction arrows from 6.25 kHz alongrofile 2 confirm the same direction.

D inversion

Results from the dimensionality analysis demonstrate that theonductivity structures are mainly three dimensional; therefore, 2Dnversion of the determinant data along the profiles is preferablePedersen and Engels, 2005. The 2D models based on inversion ofhe determinant data show better resolution than those derived fromhe combined data set using transverse electric TE and transverse

20 40 60 80 100 120 140 160 180

Profile 4

Profile 5

Profile 6

Profile 7

( )o

80

60

40

20

0

Strike Ske

50 109 159 209 259 390

Frequency(kHz)

Distance (m) Distan

50 100 150 200

50 109 159

80

60

40

20

0

( )o

( )o

12.5

6.25

4

8

12.5

6.25

4

8

2

12.5

6.25

4

8

2

12.5

6.25

4

8

12.5

6.25

4

8

Strike Ske

( )oStrike Ske

50 100 150 200

80

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Distance (m) Distan

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Frequency(kHz)

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Frequency(kHz)


agnetic TM modes either separately or jointly Pedersen et al.,005. Traditionally, one important use of the TM mode aloneBoerner et al., 1999 is to suppress 3D effects. However, the deter-inant has the advantage of retaining the ability of the TE mode to

ouple well to isolated conductors and to some extent retaining theroperty of the TM mode to model the boundaries where the majoresistivity changes occur.

A 2D inversion of the RMT and CSTMT determinant data waserformed using a modified version of the REBOCC program Sir-punvaraporn and Egbert, 2000.A1000-ohm-m homogeneous half-pace with a cell width of 5 m and a logarithmically increasing verti-al thickness was used as the starting model to invert both data setslong all of the profiles. The 2D inversion of determinant RMT dataesulted in very unstable models and large rms data fits 8, whichs mainly the result of too few radio transmitters in the study area.herefore, we present the results of 2D inversion of determinantSTMT data. The RMT data at the most stable frequencies are

hown on top of each CSTMT model to find the correlation betweenhe resistivity anomalies in the RMT data and those in the CSTMT

odels.Figure 8a and b shows the RMT data and the resistivity-depthodel obtained from 2D inversion of the determinant CSTMT data

long profile 1. The highest CSTMT frequency 12.5 kHz corre-ponds approximately to the lowest RMT frequency 14.4 kHz.he number of transmitters is reduced considerably between 260nd 370 m. The CSTMT model with a very low rms data fit 0.95ainly shows a more resistive feature 1000 ohm-m at depth over-

ain by an overburden with considerable lateral resistivity and thick-

0.5

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0259 390

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0 160 180

Figure 7. The estimated strikes and skews alongprofiles 4–7, indicating that the conductivity struc-tures can be mainly considered three dimensional.

w

ce (m)209

w

w

w

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150

0 120 14

ce (m)

ce (m)

ce (m)


n2ttsfi

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dd

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3fimmt

2

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B176 Bastani et al.

ess variations see Figure 8b. A very low resistivity zone about5 ohm-m at a depth of about 5–10 m and 380 m distance alonghe profile is resolved clearly. The location coincides exactly withhe exposed copper mineralization see Figures 3a and 8b. The re-istive structure is very close to the surface at the two ends of the pro-le.Variations of the RMT apparent resistivity correlate well with the

esistivity variations resolved in the CSTMT model. The low-appar-nt-resistivity anomaly in the RMT data is observed in all frequen-ies at 380 m distance along the profile. The RMT anomaly coin-ides with the location of exposed copper mineralization and a low-esistivity anomaly in the CSTMT model see Figures 3a and 8b.he increasing RMT resistivities at both ends of the profile and high-

esistivity anomalies at 460 and 500 m also show strong correlationsetween the measured RMT data and the modeled CSTMT data. TheSTMT model shows a lower-resistivity zone at depth in the0–210-m interval along the profile caused by the near-field effectsee the Discussion section. At this interval along the profile, theistance to the transmitter is the shortest see Figure 3a, which mayictate the near-field regime.

0

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D

1.0 1.2 1.4 1.6 1.8

rms = 0.95

b)

0 40 80 120 160 200D

Profile6

101

102

103

Appresistivity(ohm-m)

a)Determinant

igure 8. a RMT apparent-resistivity data along profile 1 and b 2Dllustrate the position of the crossing profiles see Figure 3a. The sho


The RMT data and 2D inversion results of determinant CSTMTata along profile 2 Figure 9a and b reveal almost the same trend asbserved along profile 1. A less-resistive layer with lateral changesverlies a more resistive feature resistivity 1000 ohm-m at aepth of about 50 m. The most distinct low-resistivity anomaly inhe CSTMT model at 30 m distance corresponds to a very-low-resis-ivity anomaly on the RMT data at the location of exposed copper

ineralization see Figure 9b.The CSTMT resistivity models and the RMT data along profiles

–8 are presented in Figure 9c and Figures 10 and 11. One can alsond a considerable correlation between the CSTMT models and theeasured RMT resistivity data along these profiles. Overall, theain trend seen on all of the CSTMT models is a highly resistive fea-

ure at depth that is overlain by a lower-resistivity layer/overburden.

DISCUSSION

D resistivity models and data fit

We used an error floor of 2% on the impedance and 1.1° on phaseuring the inversion. The overall rms data fit for each resistivityodel is shown at the bottom-left corner of the inversion model

280 320 360 400 440 480 520

(m)

.0 2.2 2.4 2.6 2.8 3.0

Ωm)

280 320 360 400 440 480 520(m)

Profile7

Profile8

Copper

14 kHz20 kHz160 kHz226 kHz

(

ivity models from inverting determinant CSTMT data. Dashed linesshows the location of the exposed copper mineralization zone.

240

istance

2

Log ρ10

240istance

resistrt arrow


Fdc

FoS8


0 40 80 120 160

Depth(m)

Distance (m) Distance (m) Distance (m)

a)

0

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rms = 2.68

0 40 80 120 160 200 240 280

rms = 1.2024

e)

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Profile7

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Profile6

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Copper

rms = 2.01

101

102

103





1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

Determinant Determinant Determinant

log10ρ (Ω m)

Copper

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b) d) f)

Depth(m)

Depth(m)

igure 9. RMT apparent resistivity data along profiles a 2, c 3, and e 4, and 2D resistivity models from inversion of determinant CSTMTata of the same profiles b, d, and f. Dashed lines illustrate the position of the crossing profiles see Figure 3a. The short arrow shows the lo-ation of the exposed copper mineralization zone. See the text for a detailed description of the results.

0

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b)

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0 40 80 120 160 200Distance (m)

a)

rms = 0.976

d)

c)

0 40 80 120 160rms = 0.953

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f)

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e)

Profi

le4

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le1

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le2

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le4

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le1

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le3

Borehole

101

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1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0




Determinant Determinant Determinant

Log10ρ (Ω m)

0

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0 40 80 120 160

igure 10. RMT apparent resistivity data along profiles a 5, c 6, e 7, and 2D resistivity models from inversion of determinant CSTMT dataf the same profiles b, d, and f. Dashed lines illustrate the position of the crossing profiles with respect to the current profiles see Figure 3a.haded rectangle in profile 6 marks the location of the borehole; the transparent white zone indicates a column of copper mineralization down to0-m depth. The darker area marks a nonmineralization zone. See the text for a description of the results.

Downloaded 02 Sep 2009 to 130.238.140.101. Redistribution subject to SEG license or copyright; see Terms of Use at http://segdl.org/

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e.g., Figure 8b. Except for profiles 2 and 3, the rms data fit is closeo one, indicating a good fit. To provide a more detailed account ofata fit, the measured and modeled CSTMT data along profile 4 areresented in Figure 12. The relative difference between the modelednd measured phases is within 1 standard deviation. The resistivi-y and phase data fit along profile 4 are much better after 100 m dis-ance. The reason for this may be the presence of stronger 3D effectst the beginning of profile 4 see the estimated skews for profile 4 inigure 7.

vidence of near field on CSTMT data

Profiles 1 and 6 cross each other at 170 m distance Figure 3. Onemportant feature to compare between CSTMT models along pro-les 1 and 6 is the presence of the low-resistivity zone at depth ob-erved in the CSTMT model along profile 1 see Figure 8b; distancesf 110–210 m. The CSTMT model along profile 6 Figure 10d athe crossing point resolves a resistor at depth 50 m. The receiv-r-transmitter separation in profile 6 is about 100 m longer than thene for profile 1 at the same point. Therefore, because of shorter sep-ration in profile 1 and use of a magnetic-dipole source, the near-

0

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140rms = 0.9558

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a)

Profile3

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App

resistivity(ohm-m)

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0 30 60 90Distance (m)

14 kHz20 kHz

160 kHz226 kHz

Determinant

Log 10

ρ(Ω

m)

0 30 60 90Distance (m)

Profile1

igure 11. a RMT apparent-resistivity data along profile 8; b 2Desistivity model from inverting determinant CSTMT data. Dashed


eld effect causes the low-resistivity artifact in the model.At the oth-r crossing points, e.g., profile 1 with 7 and 8, the RMT data andSTMT models show a reasonable correlation, even in the estimat-d resistivities. Therefore, we believe the near-field effect has dis-urbed only parts of the data along profile 1 and has not been extend-d to the other profiles.

orrelation between resistivity models and magneticeld data

Knowledge about magnetic susceptibility variations is limited tofew samples taken from the host volcanic and mineralized rocks

Table 2. Therefore, no modeling has been attempted with the ac-uired magnetic data. Instead, we have made an effort to explain theorrelation between the variation in the total magnetic field and theesistivity models along the measured profiles.

In general, there is good correlation between interpreted resistiveost volcanic rocks and the highs in the magnetic field. In all resistiv-ty models, the resistive bedrock deepens to the south where the

agnetic field has a decreasing trend see Figure 3a. To the north,ll of the models show an outcropping resistive host rock that fitsell with the magnetic highs seen at the end of all profiles. For exam-le, in the resistivity model along profile 1 Figure 8b, the resistiveedrock deepens in the middle and is very close to the surface at bothnds. Looking at the total magnetic field map along the same profileFigure 3a, one notices that the total magnetic field shows two highst both ends and a minimum in the middle. The measured suscepti-ilities of the host and the mineralized rocks verify the same trend.he presence of the high magnetic feature that lies almost parallel torofile 1 may generate artifacts in the 2D resistivity models.

ineralization potential

The RMT data and 2D CSTMT resistivity models show their po-ential for exploration of hydrothermal stockwork copper mineral-zation in small prospects in Iran. The VLF and radio transmittersave a stable S/N over a period of a few hours. Some shallow low-re-istivity anomalies that are considered as mineralization potential,.g., along profile 1, also appear in the RMT apparent-resistivityata. A 3D view of the CSTMT models is shown in Figure 13. TheSTMT models agree well at points where profiles cross each other

see also Figure 3a. Note that the measurements at the crossingoints were done independently and at different times.

Apart from sparse, localized low-resistivity zones with some po-ential for targeting mineralization zones, our overall interpretationf mineral potential in the highland area is that the central part of therea with an associated magnetic low can be targeted for detailed ex-loration work e.g., drilling. Recently drilled boreholes down to20 m depth near the crossing point between profiles 4 and 6 Figure0c and d indicate good correspondence with the models obtainedrom the CSTMT data. Preliminary geologic logs indicate that ahick column 50 m of malachite with a high-grade copper con-ent 3% gradually diminishes at a depth of about 80 m see Fig-re 10d.
ines illustrate the position of the crossing profiles see Figure 3a.

F


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0 20 40 60 80 100120140160180200220240260280

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1.01.21.41.61.82.02.22.42.62.83.0

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1.01.21.41.61.82.02.22.42.62.83.0

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−3.0−2.5−2.0−1.5−1.0−0.50.00.51.01.52.02.53.0

Line 4: Observed resistivity Line 4: Observed phase

Frequency(kHz)

Line 4: Calculated resistivity

Frequency(kHz)

Frequency(kHz)

Distance (m)Line 4: Calculated phase

Distance (m) Distance (m)Line 4: Resistivity difference Line 4: Phase difference

Frequency(kHz)

Frequency(kHz)

Distance (m) Distance (m)

a) b)

c) d)

e) f)

Wei

ghte

dm

isfit

Wei

ghte

dm

isfit

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0ρ a

(Ωm

)

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igure 12. Measured and estimated CSTMT apparent resistivities, phases, and their relative difference errors along profile 4.

Downloaded 02 Sep 2009 to 130.238.140.101. Redistribution subject to SEG license or copyright; see Terms of Use at http://segdl.org/

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CONCLUSIONS

This study shows that CSTMT and RMT methods can provide aast, reliable means for mineral exploration. The collected data are inhe frequency range of 4–250 kHz. The number of radio transmit-ers with acceptable S/N is sufficient to study the shallower-resistivi-y structures qualitatively and to confirm low-resistivity anomalieseen in CSTMT resistivity models. The remotely controlled doubleorizontal magnetic dipole source provides strong EM signals atower frequencies for deeper penetration. The CSTMT data haveeen used for strike and dimensionality analysis, indicating a mainlyD structure. No preferred directions along the measured profilesould be identified to perform TE, TM, or joint modeling. Conse-uently, 2D modeling of the determinant resistivity data was per-ormed to reduce 3D effects. The reasonably small overall rms datats along almost all of the profiles are good measures of confidence

n the resulting 2D models.The data coverage is such that 2D models along crossing profiles

an be compared and correlated with each other. At the crossingoints, the models generally show the same resistivity features. Theutcrops of the resistive volcanic host rock at several places alongnd close to the measured profiles help to calibrate the modeled re-istivities against the measured high-frequency RMT data. The 2Desistivity models show a moderate- to low-resistivity zone near on-oing mining activities. This correlates well with the type of copperineralization seen in the area low-resistivity malachitic.Analysis

f the 2D resistivity models indicates that the volcanic rock deepenst the center of the study area where profiles 1 and 4 cross profiles 6nd 7. This zone is associated with a magnetic low and therefore isecommended for detailed exploration work. Preliminary drillingesults confirm parts of the interpretation presented in this study.urther work is ongoing for joint modeling of magnetic and EM dataith the help of additional physical rock property measurements.

ACKNOWLEDGMENTS

Kahanroba Engineering Co., and in particular F. Hedjazi and F.ehdizadeh, are thanked for their support during the field campaign

nd for permission to release the data. Acknowledgments go to A.mamjomeh and the Chah-Mussi prospecting and mining teams,

Profile 1

Profile 2

Profile 3

Profile 4

Profile 6Profile 7

Profile 8

50m

100m

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2log ρ (Ωm)10

igure 13. The 3D views of the 2D resistivity models from the CSTMo the north. We suggest the central part of the study area for detailehat includes a drilling pattern.


who shared their knowledge about the geology ofthe study area with us. We highly appreciate theconstructive comments made by two anonymousreviewers and the associate editor.

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0

a with a vieworation work

.8 3.

T datd expl


T

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Z


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case history delineating hydrothermal stockwork copper

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