Crustal magnetization beneath the aulneau and sabaskong batholiths, kenora and fort frances districts, Ontario, Canada

Geoexploration, 25 (1988) 61-89 Elsevier Science Publishers B.V., Amsterdam

61

-

Printed

in The Netherlands

Crustal Magnetization Beneath the Aulneau and Sabaskong Batholiths, Kenora and Fort Frances Districts, Ontario, Canada* D.H. HALL and T.W. MILLAR*’ Geophysics Section, Department of Geological Sciences, The University of Manitoba, Winnipeg, Man. R3T 2N2 (Canada) Centre for Precambrian Studies, The linioersity of Manitoba, Winnipeg, Man. RYT 2N2 (Canada } (Received

February 10.1987; accepted after revision September

25,1987)

ABSTRACT Hall, D.H. and Millar, T.W., 1988. Crustal magnetization beneath the Aulneau and Sabaskong batholiths, Kenora and Fort Frances districts, Ontario, Canada. Geoexploration. 25: 61-89. Gravity survey and aeromagnetic data over the Aulneau and Sabaskong batholiths, in the Kenora-Fort Frances districts of northwestern Ontario, have been modelled using crustal seismic surveys in the area as constraints. A crustal section was constructed from these results extending from the surface to the uppermost portion of the lower crustal layer. The batholiths are found to be relatively thin (average thickness 7 and 5 km, respectively). A prominent magnetic anomaly is centred in the contact zone between the batholiths, and overlaps both of them. Its source is interpreted as an intrusion in the lower part of the upper crustal layer. Surface measurements of magnetization and anomaly interpretation show that most of the magnetization of the Sabaskong body is concentrated in a linear central zone which extends through the body. An improved method of combining measurements of magnetization of rock samples with magnetic anomaly interpretation allows a more reliable calculation of the percent magnetite equivalent for buried rock units.

INTRODUCTION

The Aulneau and Sabaskong batholiths lie in northwestern Ontario, between Kenora and the U.S. boundary (Fig. 1) , and form part of the Wabigoon belt in the Superior province of the Canadian shield. They cover areas of 1000 and 2500 km2, respectively. Both are surrounded by archean volcanic and metasedimentary rocks except in Sabaskong Bay, where the two bodies either *Presented at the CSEG-GGU Meeting, Calgary, Alberta, May 1985. Precambrian No. 134. *‘Present address: St. Boniface Hospital, Winnipeg, Man. R2H 2A6 (Canada).

0016-7142/88/$03.50

Q 1988 Elsevier Science Publishers

B.V.

Studies Paper

62

Fig. 1, Location map of traverses and areas sampled for measurement See explanation of numbering in text.

of surface magnetization.

come into contact or are separated by a number of smaller intrusions. The Aulneau batholith is a multiphase body, including mainly trondhjemite, granodiorite, and quartz monzonite phases (Zielke, 1974, pp. 156-161, p. 184). The geology of the Sabaskong batholith is not known in comparable detail, but Blackburn (1976 f mapped a small area (containing appro~ima~ly 10% of the batholith ) and found that it consists principally of felsic plutonic rocks with trondhjemitic and granodioritic types predominant. Both batholiths are shown on Davies and Pryslak’s (1967) Kenora-Fort Frances sheet, based on compilation and reconnaissance work. Zielke (1974) describes two important intrusions (The Morson and the Painted Rock Island plutons) in the area of contact between the batholiths, Both batholiths have magnetic anomaly trends over them (Fig. 2 ) which are of suf~cient area1 extent to be possibly the reflection of deep crustal mag-

63

%,

94030’

0

94000'

5

10

15

2c

25

km

Fig. 2. Contour plot of the digitized aeromagnetic data upward continued to 3 km above ground level for display purposes. l-2,3-4,5-6 are traverses along which modelling was conducted; A and 3 are the peaks of the two principal anomalies over the Aulneau and Sabaskong batholiths. Contour values represent total anomalous field in units of 1000 nT relative to a base level of 60,000 nT.

netization. The present delling combined with measurements are used pretation and estimates Previous geophysical

paper reports on new interpretations based on mosurface measurement of magnetization. The surface in a novel way so as to provide constraints on interof magnetization at depth.

work

Several geophysical crustal studies have been published on the Aulneau area. These include seismic methods (Brown et al., 1977; Green et al., 1978, 1979)) magnetic methods (Hall, 1968; Hall and Stephenson, 1976; Hall et al., 1979)) and gravity methods (Brisbin and Green, 1980). These earlier interpretations

64

have been reviewed (Hall and Brisbin, 1982) in light of other crustal studies of the Superior province in Manitoba and northwestern Ontario. AEROMAGNETIC DATA

Anomaly interpretation in the present paper is based on data derived from aeromagnetic maps published by the Geological Survey of Canada (1961,1969, 1984). The mapping was done with continuous recording along flight lines with 0.30 km ground clearance and approximately 1 km line spacing. The maps published in 1961 were at a scale of 1:63,360. Those in 1979 were compilations of the above at 1:253,440. In 1984, the compilations were recontoured and colour-plotted at 1:1,000,000. The study area is shown in Figs. 1,2 and 3 with the outlines of the batholiths shown on Fig. 3. The long-wavelength anomaly field can be displayed adequately over the area using a dataset derived from Geological Survey of Canada (1961) maps, digitizing them on a 3-km grid. This spacing allows definition without aliasing of anomalies with wavelengths greater than the Nyquist wavelength of 6 km. The two principal magnetic anomalies (A and B, Figs. 2 and 3 ) over the Aulneau-Sabaskong area can be defined and interpreted without distortion using the digital data obtained as described. Some of the modelling of source areas for these anomalies was done on profiles digitized with closer spacing as described in later sections. Fig. 2 is a contour plot of the digital data smoothed by upward continuation to 3 km above ground surface. Fig. 3 is a gray-level plot of the same data. Anomaly A is elliptical in shape with a north-south major axis and is centred on Painted Rock Island in the contact area between the two batholiths. Its width (or wavelength) is about 50 km. It overlaps about two-thirds of the Aulneau batholith and one-half of the Sabaskong. Anomaly B is a long linear anomaly trending NE-SW across the Sabaskong body. Its highest intensity lies over Mathieu township, south of Sabaskong Bay. Qualitative survey of the aeromagnetic

field

Aulneau batholith. The field (apart

from anomaly A and two smaller anomalies) is relatively featureless as compared with that over the Sabaskong batholith or the metavolcanic and metasedimentary rocks surrounding the two batholiths. There are weak trends in the anomaly pattern in the NW-SE and NE-SW directions (Figs. 2 and 3 and Geological Survey of Canada, 1961,1969, 1984). Sabaskong

batholith. Anomaly A overlaps addition, anomaly trend B crosses the central (roughly linear and striking N65 “E for over mental structure of the Sabaskong batholith

part of the Sabaskong area. In part of the batholith. This trend 50 km) may reflect the fundabecause it parallels other trends

65

Fig. 3. Gray level of plot of the digitized aeromagnetic data upward continued to 3 km above ground level for display purposes. A and B are the peaks of the two principal anomalies over the Aulneau and Sabaskong batholiths, the outlines of which are shown. Symbols are: “-‘I represents values below 422 nT; “blank” represents 422-476 nT; ‘<.I’represents 476-530 nT; “M” represents 530-584 nT; “W represents 584-698 nT, and “ + ” represents values above 698 nT. All values are relative to a base level of 60,000 n?‘.

66

in Fig. 2 such as the southwestern boundary of the batholith, the southwestern margin of anomaly A (Figs. 2 and 3)) and a belt of mafic to intermediate volcanics (described by Blackburn, 1976) adjacent to it. In addition, major lineaments (Davies and Pryslack, 1967) in the batholith also trend in the same direction. This comparison of the aeromagnetic fields over the two batholiths shows that there are more localized magnetic contrasts in the Sabaskong area than in the Aulneau. This might be the signature of a greater degree of near-surface geological heterogeneity in the Sabaskong area. Trend B is terminated by a broad anomaly low at 94’ W. North of 49 ’ latitude this low strikes north-south and as will be shown below separates the Sabaskong batholith into two regions, each marked by a different magnetic character. The two batholiths have one characteristic of their signatures in common; the magnetic trends within the Sabaskong body, and also its margins, parallel the weaker trends discernible in the magnetic signature of the Aulneau body. The principal aim of the present paper is to interpret anomalies A and B. The combination of anomaly interpretation with surface magnetization measurements is an important part of the approach used. SURFACE MAGNETIZATION

Types of measurement made Remanent and induced magnetization of surface rocks on the Aulneau batholith have been measured and published in earlier years (Hall, 1968; Hall et al., 1979; see also Hall and Brisbin, 1982). More recently, measurements of induced and remanent magnetization in the central region of anomaly A, and induced magnetization on the Sabaskong batholith have been made and are reported for the first time in the present paper. All measurements are summarized in Table I and are referred to traverses or areas in Fig. 1. The numbering of traverses in the present paper begins at 7 to conform with the traverse numbering in an earlier paper (by Hall et al., 1979). Magnetization values from that publication are given in items 7, 8 and 9 of Table I. All values are means for the particular traverse or area. Sampling for items 7 to 9 consists of collecting on average two oriented drill cores from each of several sites along the traverse. Magnetic susceptibilities for the cores were measured with a portable susceptibility meter. The drill cores were each cut into three cylindrical samples for measurement of natural remanent magnetization (NRM) with a Schonstedt digital spinner magnetometer. Sampling for item 10 consisted of in-situ susceptibility measurements with a Kappameter bridge at sites along the traverse (see also Table II). At each site an average of five determinations were made. For item 11, an in-situ Scintrex SM-5 susceptibility meter was used

67 TABLE I List of measurements

referring to traverses

Average induced magnetization I( A/M 1

Magnetization projected on F J( A/m )

50

0.09

0.11

Aulneau batholith near its northeastern margin

96

0.12

0.15

Aulneau batholith and contact area with Sabaskong batholith (area beneath anomaly A)

51

0.10

0.13

0.11

0.14

17 sites 79 observations

0.16

020*

33 sites 682 observations

0.12

0.15*’

0.13

0.17

0.04

0.05

No. of samples

Traverse or area no. and length or area

Rock unit

7

(15 kmi

metavolcanics metasediments

8 (15 km) 9 120 km)

and

839 averaged

average for Aulneau batholith

10 (37 km)

Sabaskong

11 (600 km’ )

Sabaskong

10,ll

average*’ for Sabaskong batholith

averaged 10.11

or areas in Fig. 1

batholith batholith

metavolcanics and metasediments adjacent to the Sabaskong batholith

9 sites 72 observations

8,9, 10,ll combined

average *’ for both batholiths

0.11

0.14

7, 10, 11 combined

average*’ for adjacent rock units, both batholiths

0.08

0.10

*‘Except for traverse 9, these are calculated using r= 0.26 (the value derived from traverse 9 ) in eq. 11. *‘Averages are weighted with numbers of sites. in addition to the Kappameter. Larger numbers of determinations (Table III) were made at each site than were made in items 7 to 10, allowing the calculation of the standard deviation of the measurements at each site, and thus confidence limits for average values of magnetization for groups of sites could be assigned. This treatment of the data for area 11 is a novel approach in interpretation, in combining surface measurements of magnetization with anomaly interpretation in the Sabaskong area. These results will be discussed in later sections of the paper.

68 TABLE II Induced ma~etization, Sabaskong bathoiit~, traverse 10 Site 81-OOl*’ 81-002*’ 81-003 81-004 81-005 81-006 81-007 81-008 81-009 81-010 81-011 81-012 81-013 81-014 81-015 81-016 81-017 81-018 81-019 81-020 81-021

K,, x lo3

0.08 0.02 0.08 0.34 0.16 0.08 0.33 1.04 0.015 0 0.49 0.11 1.99 0.41 0.35 0.10 0.16 0.069 1.15 0.29 0.09 0.03 0.09 0.07

Ks,lO" = (K,,X47r) 0.96

0.27 1.03 4.24 2.03 0.96 4.10 13.02 0.19 0 6.18 1.42 25.0 A 5.10 B 4.38 1.30 2.00 A 0.07 B 14.5 A 3.65 B 1.12 0.38 1.19 0.87

IsI(A/m) (=K,,xO.6,10:‘) 0.04 0.01 0.05 0.20 0.10 0.05 0.20 0.62 0.01 0 0.30 0.07 1.19 0.24 0.21 0.06 0.10 0.04 0.69 0.17 0.05 0.02 0.06 0.04

*‘On surrounding rock units.

Surface mugnet~zatio~ be~utk the peak of a~rna~y A (truuerse 9) As described in the previous section, the induced magnetization (1) and the remanent magnetization (8) were measured for this traverse. Several new parameters are introduced as indicators of the effectiveness of rock units as sources of magnetic anomalies. These are required because, as is often found for rock units in the northwest Ontario area, the directions of g are widely scattered for different parts of the unit (see Fig. 4 and Hall, 1968)) and their mean effect must be determined. Let 5‘ be a unit vector parallel to r’, the geomagnetic field. s therefore represents the direction of i, the induced magnetization of a magnetic unit. It also represents approximately the direction of the field component measured in most aeroma~etic surveys, as in the case for the data used in this paper. This direction is a suitable one onto which to resolve the magnetization of a magnetic unit in order to develop a parameter which indicates the anomaly-

69

TABLE III Induced magnetization, Sabaskong batholith, area 11 Site

N

k,” x 10:’

ks, x lo”

IS, (A/m)

82-OOl*’ 82-002 82-003 82-004 82-005 82-006 82-007 82-008 82-009 82-010 82-011 82-012 82-013 82-014 82-015*’ 82-016*’ 82-017*’ 82-018*’ 82-019 82-020 82-021 82-022

10

20 41 10 9 11 7 16 10 13 9 19 10 10 7 6 11 16 38 21 22 26

0.03 t 0.03 0.28 k 0.08 0.07 i 0.08 0.09 i 0.08 0.12+0.10 0.20 * 0.09 0.11~0.04 0.09 + 0.06 0 0.26? 0.32 0.16 + 0.23 0.15 fO.18 0.08 ? 0.03 0.28 + 0.26 0.05 & 0.03 0.11 kO.05 0.07 i 0.04 0.03 t 0.03 0.17kO.15 0.39 * 0.09 0.21+0.12 0.55 + 0.25

0.34 ? 0.38 3.49 i 1.0 0.84 i 1.04 1.17i 1.01 1.51 i 1.28 2.54 ?C1.13 1.37kO.53 1.14+0.71 0 3.24k4.00 2.08? 2.89 1.87i2.31 1.04 * 0.43 3.54i 3.21 0.61 i 0.32 1.43 + 0.57 0.83 -I 0.48 0.34 + 0.37 2.13? 1.93 4.85 & 1.16 2.70+ 1.5 6.87i3.14

0.01 -t 0.02 0.17 kO.05 0.04 f 0.05 0.06 + 0.05 0.07 f 0.06 0.12 * 0.05 0.07 2 0.03 0.05 * 0.03 0 0.16+0.19 0.10~0.14 0.09+0.11 0.055 0.02 0.17?0.15 0.03 t 0.02 0.07 + 0.03 0.04 t 0.02 0.02 i 0.02 0.10i0.09 0.23 f 0.05 0.13 i 0.07 0.33 * 0.15

83-001 83-002 83-003 83-004 83-005 83-006 83-007 83-008 83-009 83-010 83-011 83-012 83-013 83-014 83-015 83-016 83-017

70 51 16 22 24 9 11 25 27 10 38 13 14 10 8 18 24

0.11 0.18 0.16 0.11 0.10 0.11 0.10 0.42 0.40 0 0.28 0 0.42 0.41 0.25 0.19 0.18

1.346+ 1.197 2.29 2 1.48 1.96 ? 1.30 1.37 +2.93 1.31 k1.26 1.40 k2.03 1.26 il.49 5.33 i 1.79 5.07 2 7.73 0 3.47 12.36 0 5.30 k2.37 5.15 k1.92 3.14 t2.77 2.37 +-1.61 2.20 +2.00

0.06 i 0.06 0.11* 0.07 0.09 + 0.06 0.06?0.14 0.06 + 0.06 0.07 + 1.10 0.06 & 0.07 0.25 It 0.09 0.24 & 0.37 0 0.17io.11 0 0.25kO.11 0.25 + 0.09 0.15io.13 0.11 kO.08 0.11 io.10

to.095 to.12 iO.10 rf-0.23 +0.10 kO.16 kO.12 kO.14 +0.62 +0.19 kO.19 io.15 kO.22 kO.13 +0.16

*‘Stations that are on surrounding rock units not part of the Sabaskong batholith.

70

producing capacity of that rock unit. The induced part of the magnetization f lies along s, and R (which is generally not parallel to the present-day geomagnetic field) lies at some angle (which we will define as y) to that field. By standard vector relations: &OS-l

RS

RS

(1)

where R = 1Z?1 and similarly for other vectors. Thus a parameters, parallel to s, with:

J=I+R

cosy

(2)

is the indicator that we require to measure the effectiveness of rock units as sources of magnetic anomalies. The parameter s is a suitable one for all magnetic latitudes. It parallels I’, the geomagnetic field, and the component of the magnetization of the source area lying in this direction makes the principal contribution to the anomaly, in all magnetic latitudes. An anomaly is composed of the sum of the contributions of Nelements within the source, the magnetization of the jth element being I,+ &. Thus J, the average over all elements, is the relevant parameter, and: (3) We may divide eq. 3 into two parts. The first is simply E the average induced magnetization; and the second (JR) represents the remanent magnetization: L$

,c I, J

(4)

1

and: ~~ =~

,~ J-1

Rj COSYj= R COSY

(5) (6)

In order to calculate J we must be able to average, using samples from a rock unit, the following: 1, R, and R cosy. The first two can be measured from any suite of samples, but the last can be measured only if oriented samples have been collected, as for traverse 9 (this section). However, the method can be greatly extended if two new parameters to estimate the degree of scatter of remanence directions in rock units are introduced. Then it is possible to use the many large collections of nonoriented samples that are available. This approach could make possible the widespread application of the interpretation method used in the present paper.

71

Proposed new parameters. ‘I’he first is defined as follows: k= t

Rj

Rj=R

COSJ’,/z

COSJ’/R

(7)

jxl

j=l

Thus from eq. 5:

&=; g R,=kR J 1

Example of application of k. If we do not have oriented samples, we cannot measure JR. But we can measure R in such a case. Thus if we can estimate k, we can estimate JR using non-oriented samples. Since we can determine Ffrom these, we could then estimate J. The parameter k can be estimated for hypothetical cases, and also for rock units from which oriented samples have been collected and compilations of k for various rock units can be made. As an example of a hypothetical case consider moments of equal magnitude (R, = Rz = ... = R) distributed randomly with their directions in the range 0 I y I n/2. In this case the parameter k is given by k=2/n =0.64. Traverse 9 (this section) is a case in which k can be calculated rather than estimated. The second proposed new parameter is defined as follows: r=J,/t

(9)

Thus JR=r< and if the induced magnetization of a rock unit is known, then ria can be estimated whenever r can be estimated even if oriented samples are not available, for example in the case where Tis found using an in-situ susceptibility meter. It should be noted that the quantity r is not the same as the Koenigsberger ratio. The average Koenigsberger ratio, Q, for a rock unit is given by:

Q= (R/T)

(10)

The quantity r is a more useful parameter than &Iin estimating the effectiveness of a rock unit as a source area for a magnetic anomaly if its remanance directions are scattered. Substituting eq. 9 into eq. 6: J=r(l+r)

(11)

Along traverse 9 (covering the region beneath the peak of anomaly A) the magnitudes and orientations of the natural remanent magnetization l? were determined for 50 cores, two to each of 25 sites. These values are plotted on Fig. 4. For an s orientation with declination D = 4’ and inclination i= 79 O,the value of y was calculated for each core and & = R cosy (eq. 5 ) was found to be JR = 0.0264 + 0.0176 A/m (90% confidence ) .

0’

N

90”E

Fig. 4. Declinations and inclinations of I? for samples along traverse 9. Boxed, heavy dots represent reversed moments relative to the vector 5%

E was found to be: R= 0.20 2 0.04 A/m (90% confidence). Thus k =J’/Z?= 0.13, and @= 2.0 + 0.4 ( 90% confidence). It was also found for traverse 9 that: 1~0.102 1: 0.131 A/m Thus $, an indicator of a rock unit’s effectiveness 2 and 3) is, for this traverse: J=T+Js

as an anomaly source ( eqs.

=0.128 A/m

The ratio I”is given by &/E

and thus

For traverse

into eq. 11 gives:

9, substituting

J= 1.26 T

(12)

Surface ~ag~etizat~o~ of the Sa~~kong ~ut~lith This area is covered by a more dense network of measurements than was the Aulneau. These are of induced magnetization, and no remanent magnetizations have yet been measured in the area. Measurements were made with a portable in-situ susceptibility meter. Results are listed in Tables II and III and are from traverse 10, Fig. 1 (sites 81-001 to 81-021) and area 11, Fig. 1 (sites 82-001 to 82-022, and 83-001 to 81-017). In Tables II and III sufficient numbers of determinations per site were available to allow the inclusion of standard deviations for each of the site means. Fig. 5 shows the locations of the sites.

73

The means of the magnetization at the sites are plotted and contoured on Fig. 6. From Fig. 6 we see that the different character evident in the magnetic field over the Sabaskong batholith west of 94’ 00’ long as compared with the portion east of 94”OO’ (Figs. 2 and 3) is also present in the surface susceptibility measurements. The largest area1 coverage was obtained in the western portion. Here we see (Fig. 6) that magnetization is not evenly distributed through the body but is concentrated mainly in a central zone, which lies close to the trend of anomaly B. The Sabaskong batholith west of94”UO’ Contrast with surrounding rock units. This portion of the batholith is sampled sufficiently well to be the best area in which to examine the question of relative mean averages for rock units on and off the batholith. The 682 observations on the batholith constitute a sample of its magnetization. The average is 0.12 rt 0.11 A/m (mean ‘_ standard deviation). For the 50 observations on surrounding metavolcanic and metasedimentary units the average is 0.032 Ifr:0.017 A/m (mean t standard deviation). The batholith average In with its 90% confidence interval is given by: 1, =0.12 t 0.01 A/m where: 0.01 A/m=The non-batholith

(Koch and Link, 1981) average &with

its 90% confidence

interval

is:

1, = 0.032 S 0.004 A/m where: 0.004 A,m=v Thus, the portion of the Sabaskong batholith samples in area 11, in common with other large granitic bodies in northwestern Ontario (Hall, 1968; Hall et al., 1979) has higher average magnetization than surrounding rock units. Concentration of magnetization. In Fig. 6, a contour map of induced magnetization in the Sabaskong batholith is shown. The values that are contoured are site means, and these values have large standard deviations as may be seen from Table III. Therefore, the statistical significance of the contouring should be examined. Considering the observations in sites with means above 0.20 A/m

74

0.03

0.'

Fig. 6. Values of I, induced magnetization

i

i

O.,?

\ /;?’ in A/m, in area 11, on the Sabaskong

batholith.

d

as one sample, those between 0.10 and 0.20 A/m as another, and those with means below 0.10 A/m as a third sample, the batholith means with their 90% confidence limits can be calculated. They are: above 0.20 A/m, &= 0.26 ? 0.02 A/m; between 0.10 and 0.20 A/m, 1; = 0.12 -+0.01 A/m; below 0.10 A/m, Is= 0.06 -t 0.01 A/m. These means are well separated at the 90% confidence level, confirming the statistical validity of the contouring and the inference that magnetization is concentrated in the central zone of this portion of the batholith. It should be emphasized that remanent magnetizations of surface rocks were not measured, and the above results are for induced magnetization only. As has been shown in earlier sections, the effective total magnetization can be estimated for the area. The Sabaskong batholith east of 94” 00’ contrasted with surrounding rock units. This portion of the batholith is less well sampled for surface magnetization than the western portion examined above. Let us, however, look at the data. Using all observations on this portion of the batholith listed in Table II (N= 19 ) : & = 0.17 -+0.01 (90% confidence level). This average is close to but significantly different from that for the western portion of the batholith. For those off the batholith (N= 4 ) : J0 = 0.039 -+0.022 (90% confidence level). The confidence levels do not overlap and the data support the finding in the previous section, that the average magnetization of the batholith is greater than that for surrounding rock units. A comparison of the average magnetizations of the eastern and western portions of the batholith points to a division of the batholith into an eastern and a western portion. SUMMARY OF SURFACE MAGNETIZATION

The parameters of surface magnetization for rock units in the area are given in Table I. The traverses or areas represented in the table are numbered from 7 to 11 so that the numbering matches that used in an earlier paper (Hall et al., 1979 ) . The table presents J, the average magnetizations projected on I’, of traverses or areas of the Aulneau and Sabaskong batholiths and metavolcanic and metasedimentary rock units adjacent to them. The values of Jare calculated from measured induced and remanent magnetizations on traverse 9 (on the Aulneau batholith and in the contact area with the Sabaskong batholith) and for the other traverses or areas from eq. 12, which was the value of r derived from traverse 9. The data for the values tabulated in the rcolumn of Table I for the Sabaskong batholith and adjacent metavolcanic-metasedimentary rock units are derived from a sufficient number of samples to allow a statistical evaluation of the significance of the difference between ffor the batholith and that for the

surrounding rock units. This difference was found to be significant at the 90% confidence level (see the previous section) _The Aulneau body was not as well sampled, so a precise statistical test cannot be made. But it is reasonable to assume because of the similar nature of the two batholiths, that the calculated differences are likewise significant. From the last two rows of Table I we see that for the surface of the two bodies averaged, the magnetization contrast is &=I 0.04 A/m. The value for the Sabaskong is dJ~O.12 and for the Aulneau, dJ=O,O2. We can draw the conclusion that the magnetization contrast projected on 13 at the surface of these batholiths with surrounding metavolcanic-metasedimentary rock units is statistically significant. The contrast is small, with the batholiths averaging a magnetization of 0.04 A/m above t,hat of the surroun~ng rock units. MODELLING

Gravity interpretation A detailed gravity survey was conducted over the Aulneau batholith by Brisbin (1974). A significant gravity low over the Aulneau was found, and a subsurface model (considering a single-layer source only) for the source area was published by Brisbin and Green (1980). This model indicated that the source area lies in the upper few kilometres of the subsurface. Their inte~retation suggests that the batholith does not extend below the gravit*y source area. The gravity source areas shown in Figs. 7 and 8 are derived by a combined gravity and magnetic interpretation program described in the following section. A twolayer model for the source area was used. For the gravity part of the interpretation, the result closely resembles that found by Brisbin and Green (1980)) who found that the gravity source area lies above 7 km depth in the crust.

Magnetic interpretation Anomaly A. Figs. 7 and 8 show two profiles across anomaly A. The profiles were digitized from 1:63,360 aeromagneti~ sheets at 20 nT intervals and at maxima and minima. Average spacing of the data points is about 850 m, and ground clearance approximately 300 m. Magnetic and gravity modelhng were carried out using a program designed to accommodate either of these fields. A Talwani algorithm, with correction for end effects (24 D) was used. The best fit gravity and magnetic fields were found by a Marquardt algorithm. In these calculations, several bodies can be modelled simultaneously. A two-layer structure was modelled, with the top of the upper layer at the surface and constrained by surface geology. The lower layer could have its up-

78 -

MODEL FIELD OBSERVED FIELD

l

60.700

lo-15 -

20 km

Rl

_--t?2

----

20

t

Fig. 7. Model for line 5-6 (Fig. 2) based on gravity and magnetic anomalies. The area stippled with “v’s” is the layer within the Aulneau body which is the gravity anomaly source area. It is also weakly magnetic. The crosses represent an area of critic-~~odio~tic intrusions. Crosshatched is the principal magnetic source area. The interval RI-R2 is a transition layer between the upper and lower crustal zones as found from seismic surveys (see review by Hall and Brisbin, 1982 1. The wavy symbols represent metavolcanic and metasedimentary rocks. Model and observed magnetic anomalies are shown. The observed field is plotted in 20-nT intervals. (3)N 0

(4)s 10

20

30

40

50

60

km

0 5

5

IO

10

15

15

20 km

--_

-

g&p-

-C-L--*

__

R*

j-20 km

Fig. 8. Model for line 3-4 (Fig. 2) based on gravity and magnetic anomalies. Descriptive for Fig. 7 apply here also.

comments

per surface constrained by the bottom of the layer above or lie some distance below it. Best-fit magnetization and density contrasts were also sought for both layers. Results are as follows. The gravity source area is confined entirely to the upper layer, the confi~ration of which is shown in Figs. 7 and 8. The

79

density contrast assuming uniform density in the layer is - 0.15 t/m” ( = g/cm3 1. The gravity part of the interpretation confirms the model produced by Brisbin and Green (1980)) that the gravity source area is confined to a few kilometres below the surface. Earlier interpretations suggest that the source area for the magnetic anomaly lies at greater depth than the gravity sources (Hall, 1968; Hall and Stephenson, 1976; Hall and Brisbin, 1982). It is possible to derive source area cross-sections for a range of depths to the source varying from surface to midcrustal depths. However, if the source area is made to extend to the surface, the required magnetization contrast (0.5 A/m) is greater than observed in measured rock magnetizations at the surface. Furthermore, shallow models are not conformable with the section of the gravity source area. A conformable configuration can be achieved for a magnetic source lying immediately below the gravity source, defining a two-layer body which has an acceptable match of model and observed fields (Figs. 7 and 8). In this solution the best-fit magnetic source area was found to extend to a depth about equal to that of the bottom of the upper crustal layer as defined by Hall and Hajnal (1973 ) and Green et al. (1978) from crustal seismic surveys. If a magnetization contrast of 0.05 A/m ( average measured value at the surface ) is used for the upper layer of the source body the lower layer of the body has a magnetization contrast of 0.80 A/m with that of the surrounding rocks, and is directed parallel to p. The reasons for accepting the above model (a two-layer source area for the magnetic and gravity anomalies) are as follows. First, the lower layer when constrained to fit against the upper layer produces a close fit of model and observed fields (Fig. 7). Second, the lower surface of the model lies at a depth which is close to the transition zone between the upper and the lower crust in the area as found by seismic crustal studies. For a discussion of the seismic crustal model, see Hall and Brisbin (1982, p. 2052). Thus the gravity anomaly over the Aulneau batholith and the magnetic anomaly A, centred in the contact zone of the Aulneau and Sabaskong batholiths and extending over much of both bodies, are interpreted as a two-layered body extending through the upper crustal layer. Equivalent magnetite content A relationship betweenp, the volume percent of magnetite content of a rock and I, its induced magnetization in A/m, (with F in nT) is as follows: p=g

10.83

(13)

This relationship corresponds to fig. 25-3 in Lindsley et al. (1966). The figure embodies values of weight percent of magnetite and magnetic susceptibility measured using a large number of rock samples containing magnetite as the

80

magnetic mineral. If we agree to call this a “standard” group of samples, then the p vs. I curve for any other group of samples can be compared to this “standard” group whether the magnetic mineral is magnetite which differs from that in the “standard” group or a different magnetic mineral. If these differences are known, corrections of the value of p derived from eq. 13, the “equivalent magnetite content”, can be made. There would be value in the International Association for Geomagnetism and Aeronomy (IAGA) establishing a carefully chosen dataset giving p as a function of I to act as an internationally recognized standard. For the area studied in the present paper, F= 60000 nT. Thus eq. 13 becomes: p = 0.56 IO.“’

(14)

The methods developed in the present paper are designed to derive more reliable values than in the past of the percentage of magnetic minerals in a subsurface rock unit. This percentage is in turn an input to petrological studies. This combination might establish the nature of the source area. INTERPRETATION

OF CRUSTAL STRUCTURE

Previous crustal geophysical studies in the area suggest a model for crustal structure in the Aulneau-Sabaskong area. The results of these studies have been reviewed by Hall and Brisbin (1982). Along with results of the present paper they lead to the models of crustal structure described below. Anomaly A

The source region for magnetic anomaly A and the associated gravity anomaly (the latter is not centred over anomaly A but lies over the Aulneau batholith, and overlaps A) is a two-layered structure (Figs. 7,8 and 9). The physical property values, given in an earlier section, are listed in Table IV. The upper layer will be discussed in a later section. Let us begin with considering the lower layer, the interpretation of which requires detailed discussion. Lower layer of the source region

The interpreted contrast in magnetization is 0.80 A/m, (Table IV ) in a direction parallel to p. This means that for the source region, the magnitude of its interpreted magnetization, s,, is 0.90 A/m, given by the interpreted contrast plus the averaged value of J for the upper layer (Table I, J for 8,9, 10 and 11 combined). Because the direction of s, is found to be parallel to I’, one of the two following possibilities holds. (1) I? is scattered, but its mean direction is that of p. Then the r value of the rock unit is the relevant parameter in the calculation of I (eq. 9) to obtain the volume percent of magnetite equivalent (p) .

81

AULNEAU (3)N 0

%%? I 20 ’

SABASKONG (4)s 40

0

60Km 0

20 Km

20 Km

0

Fig. 9. Idealized layer and body structure beneath anomaly A, drawn from Fig. 8. Aeromagnetic coverage extends across the figure, and gravity coverage extends south from (3) for 38 km. The Aulneau batholith extends 25 km from (3); the rest of the section covers the contact zone and the Sabaskong batholith. RI and R2 are the seismic discontinuities shown on Figs. 7 and 8. Symbols such as @ represent layers and bodies discussed in the text and Table IV.

is strongly parallel to p. Then the relevant nigsberger ratio (eq. 10). For both cases J, = I+ R. For case (1) we have: (2)

l?

Jm = (l+r)l

parameter

is Q, the Koe-

(15)

and for case ( 2 ):

Jr,,=O+QU

(16)

If we take a typical value of r to be r=0.26 (eq. 12) and of Q to be 1.25, the average value for northwestern Ontario, (Hall et al., 1979, p. 1768)) t.hen we have for the two cases the following. For case (l), J=1.261, and 1=0.794& and for case (Z),J=2.251, and I= 0.4445. For J= 0.90 A/m, I lies between the limits 0.71 A/m and 0.40 A/m for cases (1) and (2 ) , respectively. From eq. 14 we find that the corresponding values of p are 0.42% and 0.26%. If the remanent magnetization of the lower layer of the source is viscous in origin, we can draw on previous work on surface samples. We have data on a rock unit near the area which possesses magnetization that has been shown to have viscous remanent magnetization (Coles, 1973; Hall et al., 1979). This is the English River Batholithic belt. The vector & was found to be scattered in direction, as in case (1)) eq. 15. If remanent magnetization in the lower layer of the source beneath the Aulneau-Sabaskong area is similarly caused, as is usually assumed for long-wavelen~h anomalies with deep sources (which anomaly A is), then we would expect the magnetization of the lower layer of the source region to be governed by eq. 15, and therefore the volume percent magnetite equivalent would be near the 0.4% level.

2.80 t/m” cafe. from if)

3.1 t/m” talc. from ( f)

2

1

2.2-4.0 A/m talc. from (c )

l.l-1.8% talc, from Cd)

the values in 2 may apply here rather than in that layer

5 A/m

-

-

would be 2.80 t/m,’ if similar to (6)

3

-

2.80 t/m”

4

7.1 km/set

6.90 km/set

6.19 km/see

6.90 km/set talc. from (b)

metavolcanic rocks

and

and other

Granulite-facies or amphibolite-facies metamorphic rocks

Amphibolite-facies metamorphic rocks or intermediate to basic igneous rocks

Similar to ( 6 ) or acid igneous rocks

Amphibolite-facies metamorphic rocks or intermediate to basic igneous rocks or similar to (3) but with a concentration of magnetic minerals

and trondjhemite

Granodiorite

6.19 km/set

0.09% talc. from (d) 0.30.4% talc. from (d)

0.11 A/m

0.14 A/m

2.65t/m,’

5

Greenschist-facies metasedimentary

6.19 km/set

0.07% talc. from (d 1

0.4-0.7 A/m talc. from (c)

0.08 A/m

0.10 A/m

2.80 t/m”

6

Interpreted

V

P

rock type

surveys or calculations

(f)

anomaly A. from geophysical

(e)

0.90 A/m

I

JorJ,for (2) and (4)

cr

Unit

Cd)

(bl

(cl

rock types for the source region beneath

(al

Physical property values and interpreted constraints described in the text

TABLE IV

83

The gravity data of Brisbin and Green (1980) were used for the gravity modelling in the present paper. These data cover the Aulneau area, the contact zone, and a small portion of the northern part of the Sabaskong (see Fig. 10). ( See Fig. 9 for location of these zones on line 3-4. ) The upper layer of the source region is found in this paper to agree closely in thickness, shape and density contrast with that found by Brisbin and Green (1980) using the gravity anomaly alone and a 3-D Talwani inversion. The layer is surrounded by metavolcanic and metasedimentary rocks, the average densities of the surrounding rocks and the layer being 2.80 t/m3 and 2.65t/m3, respectively. The upper layer is found from surface geological studies to be granitic and granodioritic in composition. This is consistent with the densities found from geophysical interpretation, and from the magnetization found from measurements on surface rock samples and anomaly interpretation. From Table I the mean value of I for this layer is 0.11 A/m. Using eq. 14 we find that p = 0.090%. The general structure beneath anomaly A The source region extends to 17-20 km depth, and is roughly elliptical in plan as shown by the aeromagnetic contours (Figs. 2 and 3 ) . Two vertical sections of the source region are given in Figs. 7 and 8 (see Fig. 2 for locations). The principal subsurface regions that figure in the interpretation are numbered on Fig. 9. Their physical properties as derived from geophysical surveys in the area or calculated from them are tabulated in Table IV, and the general structure inferred from this paper combined with all geophysical surveys in the region that provide information on anomaly A is discussed below. Unit 1. Seismic properties of this layer (which is a part of the lower crustal layer ) were determined by Hall and Hajnal (1969,1973 ) , Gurbuz (1970 ) , and by Green et al. (1978). These properties were interpreted in terms of rock type by Christensen and Fountain (1975). P-wave velocity ( V) as determined by the authors mentioned above averages to 7.1 km/s as given in Table IV. From that we can suggest a value for density in this unit using V and Birch’s law (Bott, 1982, p. 71) , as given in the table. Magnetic properties of the lower crust in Manitoba and northwestern Ontario were derived by Hall (1974) in a study of long-wavelength magnetic anomalies. His derived value for J, is approximately 5 A/m, and values of I andp are calculated using eqs. 1516 and 14 and are given in Table IV. The rock types consistent with the seismic interpretation of Hall and Hajnal (1969,1973) and Gurbuz (1970) were taken from the experimental work of Christensen and Fountain (1975)) and are included in the table. In addition, the last-mentioned authors conclude that metamorphic rocks are more likely to occur in the continental lower crust than common igneous rocks, which may be unstable there (Christensen and Fountain, 1975, p. 234). Unit 2. This unit is interpreted as the uppermost layer of the lower crust (Hall and Brisbin, 1982, pp. 2051-2053) and was named by them the “mid-

84

crustal layer”. The P-wave velocity of this layer is known from Green et al. (1979) to be 6.90 km/s. This value agrees with that derived by Gurbuz (1970) (see Hall and Brisbin, 1982, p. 2052). Using Birch’s law (Bott, 1982, p. 71) a density of g = 2.80 t/m3 is suggested. These values lead to the interpreted rock types in Table IV for this layer and the reasons for that inberpretation are given by Hall and Brisbin (1982, p. 2052 ) . Unit 3. These are better discussed after the other bodies. Unit 4. In plan, the body is elliptical in shape. In section we see from Figs. 7 and 8, remembering that depth determinations can be uncertain to 10% or more, that it is reasonable to take the bottom of unit 4 to lie at the Rl discontinuity, as is done in Fig. 9. There are two main interpretations of this unit. (a) It is a distinct intrusion, of a rock type differing from that of the surrounding unit 3. It has, however, the same density 0 = 2.80 t/m” as unit 3. If Birch’s law (Bott, 1982, p. 71) applies at such shallow depths in the crust the corresponding value of V would be 6.90 km/s as given in Table IV. It is unlikely that the value would be this high in the upper part of the body because pores and cracks are likely to persist to depths of several kilometres. These conditions affect seismic velocity much more than they affect density. The Birch value for V can nevertheless indicate rock type. The values of V and (7 are identical with those derived for the mid-crustal layer (unit 2), and thus unit 4 lies in the same group of possible rock types listed for unit 2 in Table IV. Unit 4 could be an intrusion of the same magma that formed unit 2, although the interpretation does not require it. There should be good seismic reflections generated at the lower boundaries of the portions of the Aulneau and Sabaskong bodies and their contact zone which are underlain by unit 4 because of the large velocity and density contrasts. The only seismic reflection survey in the area (Green et al., 1979) was designed to cover the Aulneau body. Consequently, only three recording points (80, 84 and 69) cover the area underlain by unit 4, which was discovered by magnetic methods after the seismic survey was completed. The expected reflection times from the top of this body range from 5.3 s to 5.5 s (for 5 and 7 km depths, respectively) and coincide with reflections interpreted by Green et al. (1979, p. 309) as reflections from near-vertical fault planes. Consequently, reflections from the interface between units 4 and 5, if present, lie in a complex part, of the record and cannot be expected to be identified and resolved from such a small group of recordings. A detailed seismic profile could be recorded using marine seismic techniques in Lake of the Woods over the central area of unit 4 to remedy this deficiency. (b) Alternatively, unit 4 might be part of a layer identical to unit 3, except that it represents a concentration of magnetic minerals. Large concentrations are observed in surface rock units in the area. Most of the magnetization at the surface of the Sabaskong is contained in an area 10 x 30 km, for example. However, whether or not concentrations of the size of unit 4 occur within oth-

85

erwise uniform rock units is unknown. From what is known (see Hall, 1968; Hall et al., 1979; Dunlop, 1979) it is unclear whether a concentration with such a large volume could occur in a layer consisting of metavolcanic and metasedimentary rocks. Unit 3. Seismic velocities determined in the Aulneau area indicate that V= 6.19 km/s through unit 3 on the north end of Fig. 9. Thus those velocities within unit 3 that have been determined are close to those found for the metavolcanic and metasedimentary rocks surrounding the Aulneau and Sabaskong bodies (units 6). This would suggest that the units 3 are similar or identical in rock type to bodies 6. If not, they must be composed of acid igneous rocks. Unit 5. This unit (Fig. 9) includes the Aulneau and Sabaskong batholiths and the contact zone between them along traverse 3-4 (Fig. 2). Its surface outline is shown on Fig. 3. All of the traverse is covered by aeromagnetic data and about 40% of it by gravity data. The latter covers a small portion of the Sabaskong body. Traverse 5-6 (Figs. 2 and 7) provides a section of unit 5 across the Aulneau area. All of that traverse is covered by gravity and aeromagnetic data. Thus the joint interpretation of gravity and aeromagnetic data used here applies to the Aulneau and to a portion of the Sabaskong batholith. The aeromagnetic data extend the interpretation to the main portion of the Sabaskong body. A principal conclusion is that the Sabaskong is thinner than the Aulneau ( 7 km and 5 km, respectively, on average). Values of cr, J and I are given in earlier sections and in Table IV. The value of V was found by seismic surveys in the area to be 6.19 km/s (see Hall and Brisbin, 1982; Green et al., 1978). The percent magnetite equivalent can be calculated from I using eq. 14. These values are shown in Table IV. Unit 6. The values in Table IV for o and V are obtained from the gravity and seismic survey interpretations referred to in earlier sections. The value of J in Table IV is from Table I. I andp are calculated from eqs. 12 and 14. Development of the crust in the survey area The simplest sequence of events to explain all of the physical property values and interpreted rock types listed in Table IV is as follows. Unit 2 is mapped over a wide area (Hall and Brisbin, 1982, p. 2052) and must mark a widespread event in the evolution of the Archean crust. Units 3 and 6 are similar to each other and of the same origin, subject to different depths and the corresponding pressure-temperature conditions in a volcanic-sedimentary pile. The remaining units are successive stages in a differentiation process, which the magnetic source area, unit 4, and the two batholiths, Aulneau and Sabaskong, were part of. Intrusions in the contact area between the two batholiths are probably part of this process. The sequence of intrusions seem to be closely related to the development of unit 2. If

86

it is, this means that unit 2 was developed along with local upward intrusions during some episode of crustal development. The percent magnetite equivalent decreases upwards in the crust, through units 1,2, 4 and 5. This sequence leads from the lower crust to the batholiths at the surface. The volcanic-sedimentary pile has a low percent magnetite equivalent.

This anomaly trend coincides with a belt of high values of I lying within a 5 km wide zone (compare Figs. 2 and 6). From Fig. 2 we see that the anomaly trend and presumably the zone of magnetization extend for 50 km or more across the Sabaskong bathoIith. This trend lies in the centre of the batholith and has a NE-SW orientation. It is most strongly expressed between 94’ 15’ and 94’ 00’. This trend may mark a fracture zone into which an intrusion of a rock type different from and more basic than the average constitution of the Sabaskong batholith took place. It parallels one of the two principal directions (NE-SW and NW-SE) which are evident in the contours of magnetic inten-

0

” f

-

“0

2

4

6

2

4

6

8 IO 12 DISTANCE (km)

8 DISTANCE

IO

I2

14

16

18

14

I6

18

(km)

Fig. 10. Aeromagnetic profile across anomaly B and derived model assuming lateral variation in J, (model magnetization) estimated from surface measurements of Z (induced magnetization). J,,,=Z(l-l-Q).

87

sity (Fig. 2)) in airphoto lineaments (Davies and Pryslak, 1967)) and in the boundaries of the Sabaskong body (Figs. 1 and 3). The zone contains most of the cumulative total of induced magnetization measured during the course of the work reported for area 11 in the present paper (see Fig. 6). Values of I in the belt range up to 0.2-0.3 A/m (Fig. 6). For Q in the l-2 range, as indicated by measurements on samples from traverse 9, the model magnetization (J,) could be up to 0.9 A/m. A profile of aeromagnetic data (Geological Survey of Canada, 1961) along line l-2, crossing the belt, was modelled with model magnetization J,, using estimated average values of J, as determined from measured values of I at the surface for several intervals along the profile 1-2, using the multi-body 24 D Talwani method described above. Fig. 10 shows a 4-km wide zone extending below anomaly trend B to about 5 km depth. This means, using the thickness of the Sabaskong body in Fig. 9 (5 km), that the derived model for anomaly B represents a magnetic source area extending through the whole thickness of the Sabaskong batholith. Using eq. 14, if I= 0.3 A/m, the equivalent magnetite content of the zone is 0.2% by volume. CONCLUSIONS

(1) Methods of using measurements of magnetization for sampled rock units are refined in the course of the research for this paper. A parameter (r) is introduced which relates the magnitude of induced magnetization (1) to the remanent magnetization (I?), giving the proportionality between R and I. This parameter is a measure of the anomaly-producing capacity of the rock unit. If r can be estimated for rock units, the more rapidly measureable quantity I can be used to estimate R. In turn, s,, the magnetization derived for a buried rock unit from anomaly interpretation (model magnetization) can be used to estimate the value of I. The latter is an important quantity in the identification of possible rock type for a buried unit. ( 2 ) A standard relationship between I and p (the percent magnetite equivalent) and the methods described above are used to estimate I from the model magnetization for buried units. This has led to improved interpreted values of percent magnetite equivalent. (3 ) An interpretation method which treats magnetic and gravity anomalies together combined with crustal seismology produces the following interpretation. (a) The Aulneau and Sabaskong batholiths are the principal rock units over which long-wavelength gravity anomalies occur. Gravity surveys extend over the Aulneau body but only over a small part of the Sabaskong (Brisbin, 1974). They do not have a great vertical extent (from the surface to 7 km on average for the Aulneau, and to 5 km on average for the Sabaskong, confirming the

88

conclusions of Brisbin and Green, 1980). Magnetic interpretation (Fig. 9) shows that the 5 km thickness extends through the contact zone and across the Sabaskong body. Below these depths is a transition to rock units which are more basic in composition. (b ) These two batholiths follow the common pattern in archean terranes, having higher average concentrations of magnetization than surrounding metasedimentary and metavolcanic rocks. (4) Using a combination of magnetic, gravity and seismic interpretation a crustal section extending from the surface downwards into the uppermost part of the lower crustal layer has been constructed. Compositions of rock units corresponding to this section have been proposed (Table IV, Fig. 9, and accompanying text). One feature of these proposed rock units is that the percent magnetite equivalent decreases systematically upwards through the system of proposed units. (5 ) The principal source of the main magnetic anomaly (A) in the study area is interpreted as an intrusion in the lower part of the upper crustal layer (Table IV and Fig. 9). (6) In the Sabaskong batholith most of the magnetization is found to be concentrated in a central zone which extends through the entire thickness of the batholith. ( 7 ) Comparison with the interpretation of Brisbin and Green (1980 ) shows that 2 1 -D gravity modelling (used in the present paper, and employing their data) produces results comparable to their 3-D modelling method. In their work, the gravity source was treated as a one-layer body, and in the present paper as a two-layer body. ACKNOWLEDGEMENTS

The research leading to this paper was supported by a grant from the Natural Sciences and Engineering Research Council of Canada. Mr. R. Seabrook, Mr. I.A. Noble and Mr. T. Salvail assisted in data processing. The Manitoba Department of Energy and Mines loaned a Kappameter for the project. Mrs. E.B. Hall assisted in the measurement of magnetization on the Sabaskong batholith, and Mrs. S. Fay with the layout of tables and manuscript.

REFERENCES Blackburn, C.E., 1976. Geology of the Off Lake-Burditt Lake Area, District of Rainy River. Geoscience Report 140, Ontario Division of Mines, 62 pp. Bott, M.H.P., 1982. The Interior of the Earth. Elsevier, Amsterdam, 2nd ed., 404 pp. Brisbin, W.C., 1974. Aulneau batholith project: gravity studies. Centre for Precambrian Studies, Annu. Rept. 1974. The University of Manitoba, Winnipeg, Man., pp. 185-186.

89

Brisbin, WC. and Green, A.G., 1980. Gravity model of the Aulneau batholith, northwestern Ontario. Can. J. Earth Sci., 17: 968-977. Brown, R.J., Friesen, G.H., Hall, D.H. and Stephenson, O.G., 1977. Weightedvertical stacking in crustal seismic reflection studies on the Canadian shield. Geophys. Prospect., 25 (2) : 251-268. Christensen, N.I. and Fountain, D.M., 1975. Constitution of the lower continental crust based on experimental studies of seismic velocities in granulite. Geol. Sot. Am. Bull., 86: 227-236. Coles, R.L., 1973. Relationships between Measured Rock Magnetizations and Interpretations of Longer Wavelength Anomalies in the Superior Province of the Canadian Shield. Ph. D. thesis, The University of Manitoba, Winnipeg, Man., 215 pp. Davies, J.C. and Pryslak, A.P., 1967. Kenora-Fort Frances Sheet (Map 2115). Geological Compilation Series, Kenora Rainy River Districts, Ontario Department of Mines. Dunlop, David J., 1979. A regional paleomagnetic study of Archean rocks from the Superior Geotraverse area, northwestern Ontario. Can. J. Earth Sci., 16 (10): 1906-1919. Geological Survey of Canada, 1961. Geophysical Series: 1182G, 1183G, 1184G, 1185G. 1175G, 1176G, 1177G, 1178G. Geological Survey of Canada, 1969. Geophysics Paper 7092G, 7106G. Geological Survey of Canada, 1984. Magnetic Anomaly Map NM-15. Green, A.G., Hall, D.H. and Stephenson, O.G., 1978. A sub-critical seismic crustal reflection survey over the Aulneau batholith, Kenora region, Ontario. Can. J. Earth Sci., 15 (2): 301-315. Green, A.G., Anderson, N.L. and Stephenson, O.G., 1979. An expanding spread seismic reflection survey across the Snake Bay-Kakagi Lake greenstone belt, northwestern Ontario. Can. J. Earth Sci., 16 (8): 1599-1612. Gurbuz, B., 1970. A study of the Earth’s crust and upper mantle using travel times and spectrum characteristics of body waves. Bull. Seismol. Sot. Am., 60 (6): 1921-1935. Hall, D.H., 1968. Regional magnetic anomalies, magnetic units, and crustal structure in the Kenora District of Ontario. Can. J. Earth Sci., 5: 1277-1296. Hall, D.H., 1974. Long-wavelength aeromagnetic anomalies and deep crustal magnetization in Manitoba and northwestern Ontario, Canada. J. Geophys., 40: 403-430. Hall, D.H. and Brisbin, W.C., 1982. Overview of regional geophysical studies in Manitoba and northwestern Ontario. Can. J. Earth Sci., 19 (11): 2049-2059. Hall, D.H. and Hajnal, Z., 1969. Crustal structure of northwestern Ontario. Refraction seismology. Can. J. Earth Sci., 6: 81-99. Hall, D.H. and Hajnal, Z., 1973. Deep seismic crustal studies in Manitoba. Bull. Seismol. Sot. Am., 63 (3): 885-910. Hall, D.H. and Stephenson, O.G., 1976. Interpretation of Thompson-Lynn Lake seismic refraction survey. Centre for Precambrian Studies, Annu. Rept. 1976. The University of Manitoba, Winnipeg, Man., pp. 108-111. Hall, D.H., Coles, R.L. and Hall, J.M., 1979. The distribution of surface magnetization in the English River and Kenora subprovinces of the Archean shield in Manitoba and Ontario. Can. J. Earth Sci., 16 (9): 1764-1777. Koch, Geo. S. Jr. and Link, R.R., 1981. Statistical Analysis of Geological Data. Dover Publications, New York, N.Y. 850 pp. Lindsley, D.H. et al., 1966. Magnetic properties of rocks and minerals. In: S.P. Clark Jr. (Editor), Handbook of Physical Constants. The Geological Society of America, New York, N.Y., pp. 543-552. Zielke, D.V., 1973. Aulneau batholith project: geological investigations. Centre for Precambrian Studies, Annu. Rept. 1973. The University of Manitoba, Winnipeg, Man., pp. 60-73. Zielke, D.V., 1974. Aulneau batholith project: geology and petrology. Centre for Precambrian Studies, Annu. Rept. 1974. The University of Manitoba, Winnipeg, Man., pp. 150-184.