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© Ministry of Mines and

Northern Development Minerals and Mines Division

Ontario O N T A R I O G E O L O G I C A L S U R V E Y

G E O P H Y S I C A L / G E O C H E M I C A L S E R I E S

MAP B1260

TASHOTA-GERALDTON-LONGLAC AREA

I Airborne Electromagnetic Survey Totai intensity iUlagnetic Survey

DISTRICT OF THUNDER BAY 2? o n ST

z Scale: 1:20 000 O

Metre; 500 0 1 Kilometre ' 1 I H I • I • = 1

M i l . 1/4 0 1 /2 Wile g

N T S References: 531/9.521/8 O D M - G S C Aeromagnetic Map: 960G.2131G ODM-Qeolog ica l Compilation Map: 2102

•1989 Queens Printer (or Ontario. Printed in Ontario, Canada

81274 BlZttl pm^^ •efsW ^ ^ p ^ / ^ ^ ^ ^ ^ ^ ^ ^ - ' ^ ^ ^ ' ^ ^

j ̂̂̂̂^ ^ ^ 1̂ ^ ^ 'i^ ̂

^ Index Map ao 10 0 20 4o kilometres

LEGEND

E L E C T R O M A G N E T I C ANOMALY SYMBOLS

Anoma ly C o n d u c t a n c e Classi f icat ion

0 > 32 Siemens ^ 16 - 32 Siemens A N O M A L Y L E T T E R ^ 1 ^ IN-PHASE R E S P O N S E (ppm)

® 8 - 1 6 Siemens A P P A R E N T D E P T H (m) 3 5 1 7 Q U A D R A T U R E R E S P O N S E (ppm)

Q 4 - B Siemens 0 2 - 4 Siemens

Q 1 - 2 Siemens

< 1 siemen

^ very weak conductor ^ (quadrature anomaly only) ^ Igh frequency anomalies are distinguished from low n riiltiirfti Rf»*innn«;p frequency anomalies by plotting an aster isk(») in the U L-Ulturai Mesponse apparent depth quadrant of the anamaly symbol. (1 siemen in SI units = 1 mho)

MAGNETIC CONTOURS I 500 nT Contour _ P 50 nT Contour W- l O n T Contour ,

Magnetic depression Cn~jD (1 Gamma = 1 nanoTesIa in SI units)

Mean magnetometer sensor alt i tude 45 metres Mean electromagnet ic sensor alt i tude 30 metres Mean fl ight line spacing 200 metres Fl ight l ines 12630 < - . - ^

' 5460 •

1000 I 1 1 1 1 A E R O D A T H E M S Y S T E M R E S P O N S E ^ / ^ ^

V E R T I C A L H A L F - P L A N E j> fi^jLj^

/ / / /\ / / y

' V yi'P'*'/ o - 1 - Conduciance i B i s m B n B )

. r / y ^ w y g ' ^ / , , I I 10 100 1000

IN-PHASE RESPONSE (ppm)

DESCRIPTIVE NOTES The helicopter is equipped with the Aerodat ElectnDmagnetic System which measures in-phase and quadrature com­ponents at four frequencies, employing Iwo vertical coaxial coil pairs operating at 935 and 4600 Hz and two copianar coil pairs operating at 4175 Hz and 32 kHz, The EM data are sampled at 0.1 second intervals employing a 0.1 sec­ond time constant. In addition to bedrock conductors, the helicopter electromagnetic system responds to conduc­tive overburden and near-surface horizontal conducting layers. Discrimination of conductors is based upon the ratio of in-phase to quadrature responses, variation of response with frequency, magnetic correlation, and variation of anomaly shape of the two coil configucations together with conductor pattern and topography. Conductance and depth estimates are determined, assuming a vertical half-plane model in free-space, at 935 Hz (signal to noise per­mitting) where conductive overburden effects are minimal. In addition, a more representative value is achieved at a lower frequency since a larger portion of a conductor is energized. Other pehpheral information le.g. 60 Hz monitor and Radar Altimeter) is recorded. The response diagram in the legend is derived from the mode results presented in Figure 3.3.2 on page 60 in Ghosh (1972). The aeromagnetic total field contours are generated from digitally recorded data from an Aerodat/Scintrex High Sen­sitivity Optically Pumped Cesium Magnetometer and the sampling inten/al is 0.2 seconds. The magnetic data are (i) corrected for diurnal variations by subtraction of the digitally recorded base station magnetic daia and (ii) inter­polated onto a regular 0.25 cm grid at the map scale of 1 20 000 using a cubic spline technique. No correction has been made for the regional variation of the earth's magnetic field. A Syledis SR-3 radar navigation system is used tor both navigation and flight path recovery. The aircraft position is derived several limes per second to a relative accuracy of less than 10 metres. Absolute positional accuracy of the presented data with respect to the photomosaic base maps should generally be within 25 metres.

— INTERPRETATION REFERENCES — Ghosh, M K. 1972: Interpretation of Airborne EM Measurements Based on Thin Sheet Models; unpublished Ph. D. Thesis. University of Toronto.

CREDIT Data compilation and drafting were done by Aerodat Limited, Toronto, Ontario, June to December 1988. The ^otomosaic base maps were prepared by Herdes and Herdes Consultants, Toronto, using Ontario Ministry of Natural Resources 1:15 840 aerial pfratographs which were controlled by reference to 1:50 000 topographic maps published by Energy, Mines and Resources. Ottawa. Every possible effort has been made to ensure the accuracy of the information presented on this map, however, the Ontario Ministry of Northern Development and Mines does not assume any liability for errors that occur. Users may wish to verify the analog records on file for a limited time at the Resident Regional Geologist's office nearest the map area

The Survey was funded by the Ontario Government Northern Development Fund.

This map is published with the permission o( V .G. Milne, Director, Ontario Geological Survey. Magnetic declination in the area was approximately 3°27'W in November 1988. Issued 1969.

H| Information from this publication may be quoted if credit is given. It is recommended that reference be made in the following form:

B ONTARIO GEOLOGICAL SURVEY 1989 Airborne Electromagncric and Total Intensity Magnetic Survey. Tashota-Geraldton-Longlac Area. District of

H j Thunder Bay by Aerodat Limited for (he Ontario Geological Survey. Geophysical/Geochemical Series. Map 81260 B e Scale 1:20 000. Survey and Compilation, June lo December 1988.

K NOTE: A set of 1:31 680 scale maps showing topography, flight l i n ^ and electromagnetic results^c-wk companies this set of maps and is available free of charge with the purchase of this set of maps.