COMMONWEALTH OF AUSTRALIA

DEPARTMENT OF NATIONAL DEVELOPMENT

BUREAU OF MINERAL RESOURCES,GEOLOGY AND GEOPHYSICS.

RECORDS1956, No.59

PRELIMINARY REPORT

on

AEROMAGNETIC SURVEYS FOR IRON ORE IN TASMANIA

by

W.D. PARKINSON

COMMONWEALTH OF AUSTRALIA

DEPARTIENT OF NATIONAL DEVELOPMENT

BUREAU OF MINERAL RESOURCES,GEOLOGY AND GEOPHYSICS.

RECORDS

1956, No.59

PRELIMINARY REPORT

ON

AEROMAGNETIO SURVEYS FOR IRON ORE IN TASMANIA

by

W.D. PARKINSON.

li)

CONTENTS.

page

1. GENERAL

2. OPERATIONS^ 1 .

3. INSTRUMENTS^ 1.

4. RESULTS

HaMpshire.-Blythe River^1.

(b) Nelson River^ 2.

0) Rio Tinto-Rocky River^2.

CONCLUSIONS.^ 3-

ILLUSCR,TION.

Plate 1. LOCALITY MAP

COPY

The report stated that "Two anomalies wererecorded on the left bank of the Nelson Riverabout 4 miles from its mouth." The ma ,) showsthat one of these anomalies, whilst extendingacross the river, is actually centn-xl on theright (northern) bank. The reason for thisapparently erroneous staLement is that thepositions of the anomalies as quoted in thepreliminary report were derived from Checkpoints only, without the aid of strip photography.The possible eri.or in estimating .:ositions fromcheck points is greater than the distance fromthe centre of the anomaly to the river (about1/5 mile).

The report refers to an anomaly just off thecoast north of .Aindown Point. The reason whythis anomaly is no shown on the map is thatthe aircraft usually goes at least a mile pastthe end of one flight line before turning on tothe next, and the anomaly was presumably recordedduring such a turn. Contours are plotted onlyup to the ends of the flight lines.

1. GENERAL.

ACroMagnetic Surveys were made over three small areasin north-western Tasmania by aircraft VH-MIN between 30thApril and 4th May, 1956.^They were made as a result ofan application by the Director of Mines, Hobart, and theareas were suggested by him.^Details of localities and apreliminary account of the results are given below.

2. OPERATIONS.

The three areas were surveyed on flight lines at half-mile separation and running approximately east and west.^The

. system of gridding was closed by flying a tie system on thewestern and eastern sides of each area.^Control of altitudeWas difficult because of the rough terrain, but the heightabove ground was kept between 500 and 1000 feet whereverpossible.^No attempt Was made to follow the topography whenflying across narrow river gorges which are common in allareas.^Aerial photographs were used for navigation.^•

The party engaged on the surveys consisted or Dr.W.D.Park-inson, Senior Geophysicist in charge of the party, and Messrs.F.J.Merrick, A.Turpie and. 4.,Irving of^and CaptainK.Duffield, F/O &Worley and Flight Engineer A.M.Kean of T

3. INS22.UMENTS.

The total intensity of the magnetic field was measuredcontinuously over flight lines and tie lines with an AN/AS-1fluxgate magnetometer, and was recorded by a Speedomex recorder .

A sensitivity of 250 gammas per inch of recorder chart Was usedexcept for the central part of the Rio Tinto-Rocky River area •for which the sensitivity- was reduced to 630 gammas per inch.A test was made for heading error and this was found to have amaximum value of^13 gammas without compensation.^In viewof the low sensitivities used, no compensation was attemptedand nr, corrections need be made.

The height above terrain was continuously recorded by anSTR-30A radioaltimeter with an Esterline-Angus recorder. Verticalphotography was taken continuously with a strip camera in orderto fix the position of flight lines and magnetic features. Thisfilm is not available at the time of writing this report sopositions referred to below are only approximate.

4. RESULTS.

A sketch map of Tasmania showing the approximate positionsof the three areas surveyed is given in Plate 1.

(a) Hampshire-Blythe River: The area surveyed extends fromthe coastline between Burnie and Penguin in a south-westerly direction to latitude 41 ° 15 i.e. about 6mile north of the town of Hampshire.^The area coveredby the city of Burnie itself was not surveyed.

Flight lines wer directed approximately east andwest at half-mile spacing.^The terrain consists mainlyof a low plateau with narrow. river gorges cut into it.A height of 500 feet above the plateau was maintained bythe aircraft during the survey.

Apart from the northernmostmile or two, the entirearea is magnetically disturbed.^The most conspicuous.features are El large dome of increased magnetic field

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intensity in the eastern part of the area, and a seriesof sharp anomalies in the south-west corner.^The"dome" is roughly circular with a diameter of about8 miles.^It is centred between Natone and Camena.The amplitude of this rather large-scale featureis about 1000 gammas above the level of the undisturbedfield.^.Superimposed on this are several sharpanomalies, sf the order of one mile in lineardimensions.^These appear to be of restricted extent,since they rarely appear on Successive flight lines.The largest of them is about 1300 gammas above the fieldin the immediate vicinity and it is about half a mileacross.^It appears to coincide with a known depositof inon ore about a mile south-west of Ouprona.

In the south-west of the area, an anomaly appearson every flight line:^The indications form acontinuous line roughly following the Emu Bay Railwayfrom just north of Hampshire to Highclere.^Theiramplitudes vary from 500 to 1000 gamma and their widthsfrom half a mile to a mile and a half.

0)^Nelson River: This area extends from parallel 41 ° 05' Sto 41° 13' S and from meridian 144°405' E to the coast.It is on the north-west coast of Tasmania, just south ofthe mouth of the Arthur River.^It was surveyed by eastand west flight lines spaced half a mile apart, and bytwo tie sys6ems, one near the coast and one near theeastern edge.^The terrain is generally fairly flat,except in the east where river valleys are a fewhundred feet deep.^Most of the area is heavily timbered.Flight lines were surveyed at a height of 500 feet aboveterrain, except that no attempt was made to allow fornarrow river valleys.^The area, in general, ismagnetically quiet.^An anoaly in the south-east cornerof the area, i.e.. about 6 miles east of Temma, has anamplitude of some 300 gammas.^Two anomalies wererecorded on the left bank of the Nelson River about 4miles from its mouth.^They are a mile apart and ofamplitude 200 and 250 gammas.^However, they were notrecorded on the two intermediate flight lines.^Theonly other anomaly recorded is just off the coast northof Sundown Point.^None of the anomalies is of asignificant amplitude.

o)^Rio Tinto-Rocky River: A rectangular area about 11 milesby 24 miles Was designated for survey.^It is boundedby parallels 41° 23' S and 41 ° 45' S and meridians 144 °44' E and 144° 53' E, i.e. roughly between the 30vageand Pieman Rivers stretching eastward to the MeredithRange.^The country is very rough topographically, withmany river gorges several hundred feet in depth.^Muchof it, particularly the low-lying part, is heavilytimbered.

The area was surveyed on east and west flight linesat half-mile spacing, and these were controlled by twotie systems along the eastern and western edges of thearea.^Generally the height above terrain was between500 and 1000 feet, but it was not possible to keep theaircraft at a constant height above the ground.^In thecentral part of the area, in which the flight lines crossthe northern port of the Meredith Range, every flightline was flown in a westerly direction, so that the

/3

aircraft could fly downhill rather than uphill.^Thesouth-east corner of the area was not surveyed, besausethe topography W3S too rough for safe flying at a^-reasonable altitude.^The southern flight lines wereextended for some distance to the west because ananomaly was recorded on the extreme south-western partof the area.

Most of the recorded magnetic anomalies lie ona broad band running from north-east to south-westfollowing roughly th:; ftratah-Corinna road and itsextension to the.south-west.^Some groups of theseanomalies lie on lines which cfoss several adjacentf4ght lines.^The largest anomaly in this group has atotal intensity rising to almost 18,000 gammas abovethe normal field. -

Another group of anomalies was recorded near theRocky River, but in the most intense of these the fieldrises to only -3,500 gammas above normal.^In the extremesouth-west of the area is an txtensive anomaly withmaxima up to 1,300 gammas above normal.^This anomalyextends almost to the coast.

5.^C0NC.-LT..16'107S.

The survey has revealed some small but very intenseanomalies in the Rio-Tinto-Rocky River area but the natureof these will not be kitown until the final map of totalmagnetic intensity has been prepared.^There is an interestingpattern of magnetic field variations in the Hampshire-BlyteRiver area which will assist in the interpretation of the geologyof that area and may possibly indicate the presence of aditionalsmall iron-ore deposits.^No anomalies of any significancewere recorded in the Nelson River area.

The reduction of the results and the prepantion ofappropriate maps will be undertaken by the B.E.R. ReductionsGroup shortly.