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Magnetic anomalies resulting frombaked sediments over burnt coalseams in southern New ZealandJ.K. Lindqvist a , T. Hatherton b & T.C. Mumme ba New Zealand Geological Survey , Department of Scientific andIndustrial Research , Private Bag, Dunedin , New Zealandb Geophysics Division , Department of Scientific and IndustrialResearch , P.O. Box 1320, Wellington , New ZealandPublished online: 06 Feb 2012.

To cite this article: J.K. Lindqvist , T. Hatherton & T.C. Mumme (1985) Magnetic anomaliesresulting from baked sediments over burnt coal seams in southern New Zealand, New ZealandJournal of Geology and Geophysics, 28:3, 405-412, DOI: 10.1080/00288306.1985.10421195

To link to this article: http://dx.doi.org/10.1080/00288306.1985.10421195

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405New Zealand Journal of Geology and Geophysics, 1985, Vol. 28: 405-4120028-8306/85/2803-0405$2.50/0 Crown copyright 1985Magnetic anomalies resulting from baked sediments over burntcoal seams in southern New Zealand

INTRODUCTION

Abstract Baked and fused sediments over burntlignite seams in Southland and Otago have totalmagnetic intensities of up to 8 A/m, and the edgesof seams having residual clinker can thus be locatedby magnetic surveys if the seams are covered bycolluvium. Minerals causing the magnetisation ofthe clinkers appear to include both pyrrhotite andsolid solutions of the ulvospinel-magnetite series.

J. K. LINDQVISTNew Zealand Geological SurveyDepartment of Scientific and Industrial ResearchPrivate BagDunedin, New ZealandT.HATHERTONT. C. MUMMEGeophysics DivisionDepartment of Scientific and Industrial ResearchP.O. Box 1320Wellington, New Zealand

Keywordsminerals

coal; magnetic surveys; magnetic

In the exposures opened up on the southern mar-gin of the present New Vale Coal Company minein the Waimumu deposit, Quaternary quartzosegravels and overlying loess as well as fused Tertiarysilty-clay overburden have been incorporated intothe clinker. The clinker rubble appears to have pro-gressively collapsed as a welded mass as the seamburned back into the hillside; at this locality theclinker is up to 3 m thick.

Lignite, when exposed to the atmosphere, ismoderately prone to spontaneous combustion, soit can be assumed that burning in any area occurredsoon after local erosional events during terracedevelopment in the late Quaternary. From a studyof paleosecular magnetic variation records in clinkerin the Powder River Basin, Wyoming, burning ratesgreater than 500 m/lOoo years were inferred (Jones-Fahley & Geissman 1982).

Major (1978) suggested that magnetic anomaliesresulting from the clinker associated with burnt coalseams might be of use in coal exploration. Thissuggestion has been tested in Southland over partsof the Croydon, Waimumu, and Ashers-Waitunadeposits (Fig. 1). Initially, the magnetic propertiesofa few samples of clinker from the New Vale minewere measured (9286-9289, Table 1) to confirm thatthe magnetic intensities of the clinkers were suffi-cient to produce observable magnetic anomalies.

Exposures of clinker occur at numerous localitiesin Southland and Central Otago and have beennoted in the Gore region by Wood (1956) and ina small lignite measures outlier north of Clintonby Bishop (1965). Fused material is particularlycommon in the Waimumu and Croydon lignitedeposits, and an area of clinker along the edge ofthe main (W6) seam at Waimumu was mapped byMcKellar (1969). Previously, in the Waikaia area40 km northwest of Gore, clinker and associatedhardened shales (porcellanites) had been misiden-tified by MacPherson (1935) and Healy (1936) asbasalts and lava-baked sediments.

In appearance, the clinker varies from vesicularor scoriaceous to dense and vitreous. Ropy flowagestructures and aligned lenticular gas or steam vesi-cles are common. Colour varies from vitreous darkgreenish grey or dark purple grey to reddish brownon weathered surfaces.

Received 2 August 1984; accepted 27 May 1985

FIELD SURVEYSTraverses were conducted across operating andabandoned mining areas in the Waimumu (3 trav-erses), Croydon (1 traverse), and Ashers-Waituna(2 traverses) deposits. The stratigraphy and struc-ture in these areas is well known from drilling dataand surface exposures (Lindqvist 1980a, b).

A Geometries Model G-826 proton precessionmagnetometer was used to measure the total mag-netic force. The sensor was operated at the groundsurface or at a height of 2.2 m depending on theintensities and gradients of intensity being meas-ured. Readings were taken at 5, 10, or 20 m inter-vals depending on the rate of variation in magneticforce between stations.

Waimumu depositTraverse 1; New Vale mine (Fig. 2)This traverse was made across an area of clinkernear the southern margin of the present mineworkings in an 18 m thick lignite seam dipping west

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406 New Zealand Joumal of Geology and Geophysics, 1985, Vol. 28

I Central Otago Coal Fields I

o Coal-bearing basins~ Major lignite deposits[!] Active surface mines

o 50km, I

(8 ')}Eas:~rn Southlandr;~:N~A~ KaitangataCoal Eeld . V . Coal Field

Kai Point Mine ,WANGALOA

.....~~

.-J ~Harliwich Mine

I'ROXBURGH

J1

SOUTH ISLAND

Fig. 1 Locations of the larger lignite deposits of eastern Southland and eastern and Central Otago.

at approximately 6. In addition to a strong anomalyrecorded over areas of exposed clinker (grid ref.F45/819428*), a second anomaly was located to theAll grid references are to sheets of the NZMS 260,

1:50 000 topographic map series.

west of the mine exposures. This anomaly is inter-preted to correspond with the unexposed strati-graphically higher W7 lignite bed, typically 1-2 mthick, which has been logged in drillholes elsewherein the deposit.

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Lindqvist et al.-Magnetic anomalies, Southland coal 407

D Declination of remanent magnetisationI Inclination of remanent magnetisationJR Intensity of remanent magnetisationXMagnetic susceptibility

clinkerTable 1 MagnetisationSouthland/Otago.

Specimen D(0) In9283.1 4 -76

.2 356 -67

.3 5 -68

.4 9 -69

.5 5 -68

.6 1 -669284.1 10 - 10

.2 13 -57

.3 3 -50

.4 353 -50

.5 350 -51

.6 0 -58

.7 356 -55

.8 357 -459285.1 310 -39

.2 298 -409286.19287.19288.19289.19652.1 54 -709653.1 37 -719654.1 9 -669655.19656.19657.19658.19659.1

.29660.19661.1

of

JR (Azrn)

1.60

0.10 }0.0070.030.110.490.174.653.265.434.360.110.493.323.95}1.104.808.587.123.891.100.743.661.690.410.463.730.38}0.410.610.10

samples,

X(SI units)

4.58 X 10-4

2.37 X IO- J

1.43 X IO-J

3.28 X IO- J2.49 X IO-J6.15 X 10-31.90 X 10-3

9.53 X 10-4

9.53 X 10-4

9.53 X 10-42.46 X 10-3

1.43 X IO-J1.43 X IO-J5.38 X 10-3

< 10-4

1.43 X 10-3

9.53 X 10-4

Traverse 3: Falconer's farm (north) (Fig. 4)This traverse was made at right angles to strikeadjacent to an old lignite pit (grid ref F45/824455).No clinker is exposed and no magnetic anomalywas recorded.

Croydon depositTraverse 4: Copeland's lignite pit (Fig. 5)This traverse was made near the northwesternmargin of the Croydon deposit, at right angles tostrike, intersecting the abandoned Copeland's lig-nite pit in the 20 m thick C6 lignite seam (grid ref.F45/8665 17). At this locality, the surface stratastrike north-south and dip east at 25-35. A strongmagnetic anomaly is associated with clinkerexposed in the 6-10 m high walls ofthe mine cut-ting. A second anomaly located on the westernmargin of the traverse possibly corresponds withthe burnt edge of seam C5. Lignite C5 is elsewhereencountered in drillholes 10-20 m beneath C6 andreaches 3-4 m thickness. The detailed structure isprobably complicated by high-angle thrusting alongthe Bushy Park Fault (McKellar 1969).

Ashers-Waituna .depositTraverses 5 and 6: Asher's Siding (Fig. 6)Traverses 5 and 6 were made across the alluvium-covered erosion edge of bed KI, 50 m and 100 msouth of the abandoned Diamond lignite mine atAsher's Siding (grid ref F47/766066). Traces ofburning (reddish brown staining) have beenobserved previously in drainage cuttings throughthe base of bed KI, southeast of the old pit. Smallanomalies ofaround 50 nT in each of the two trav-erses are interpreted as recording minor amountsof clinker beneath the alluvium. In both traverses,the magnetometer sensor was mounted on the fullrod extension, 2.2 m above ground level.

MAGNETIC PROPERTIES OF THECLINKERS

Traverse 2: Falconer's farm (south) (Fig. 3)North of traverse I, 800 m along strike from theNew Vale mine, a strong set of anomalies wasrecorded over a rubbly clinker topography formedby burning back of the W6 seam at the base of theterrace edge (grid ref. F45/819443). Subsidiaryanomalies correspond with rubble mounds. Themost eastward "highs" are either a record of clinkerat the base of W6 or the 2-3 m thick lignite bedusually found 5-15 m beneath W6.

Oriented and non-oriented samples of fused clinkerand baked siltstone/sandstone were collected fromoutcrops ofbumt country rock overlying coal seamsat four mines in Southland and Otago. The minesfrom which samples were collected are:Sample numbers Mine NZMS 260 grid ref

9283-9289 New Vale F45/8194309652-9656 Eden Leith H41/7426779657-9659 Kai Point H46/6913279660-9661 Harliwich G43/214193

Six samples (9283-9285 and 9652-9654) wereoriented.

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408 New Zealand Joumal of Geology and Geophysics, 1985, Vol. 28

62000--

EAST

/'~'"'".

Lindqvist et al.-Magnetic anomalies, Southland coal 409

Fig.3 Total magnetic force pro-file and geological cross section,Falconer's farm (south) traverse,Waimumu deposit.

62000, ,

61500

\i\

i=.sCll~

.2o

..,CllCc>

'"E~oI-

61000

60 500

- ....... --". 0

".

;,,'u:/ '

1\t l!

Ii

"'

W EST EAST

lign ite W 6--......-.

. -. ? T}.J'?' ~. _ 1I: 1I 1l:: .!.: ~-,.U I Ud::Jt:' U I YY U

o 100 200 300

D istance (m)

61000, ,

i=.sCllo.

_ a_a _a_ a _._ ._ r_._.___ ~__

EAST

~- -- ~= -= ---~- -;------:-- ..

old open cast pit,lig ni~W~

u 60500'"CllC

~ I W ESTE~oI-

Fig.4 Total magnetic force pro-file and geological cross section,Falconer's farm (north) traverse,Waimumu deposit.

o 100 200Distance (m)

3 00

one specimen, 9284.7, exhibited an intensity decaythat could be ascribed to a thermally discrete com-ponent of magnetisation, with a blocking temper-ature confined to a small range between 450C and500C. The steady decay of intensity on increasingtemperature, observed in most specimens, indi-cates that no unique magnetic mineral is respon-sible for their magnetisation; rather, they containa mixture of magnetic minerals, possibly of variouscompositions of the ulvospinel-magnetite solidsolution series.

The directions of rnagnetisation for the New Valespecimens were stable to a demagnetisation tem-perature of 450C, though after 500C only two of

the nine specimens demagnetised had directionsunchanged from those of their NRM. The apparentstability of the directions on demagnetisation, andthe close agreement of these with the present-dayaxial dipole field direction, suggest that the mag-netic vectors were completely reset at the time ofburning, thought to have been in the late Quater-nary (Lindqvist et al. 1983). Further, the stabledirections to 450C indicate that temperatures inexcess of this value must have been reached duringthe bum.

The decay of intensity of some specimens fromEden Leith, Kai Point, and Harliwich appear toindicate that two quite different minerals are pres-

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410 New Zealand Journal of Geology and Geophysics, 1985, Vol. 28

WEST

~~._...~ ../ .

n ), ~.\ / \ -." "" ' -.../ . r:

Fig.5 Total magnetic force pro-file and geological cross section,Copeland's lignite pit on thewestern margin of the Croydondeposit.

EAST

Copeland pit.lign ite C6~? ligni te C5

61000

;:::.s

411

-l600400

Temperature (Oe)200

(A)

New Vale

....o 0.6

--...,.....,...

I---...,

Lindqvist et al.-Magnetic anomalies, Southland coal

Fig.7 Normalised thermal 1.0~ 0-=;;;;:;:g--0_09284.7demagnetisation curves ofclinker~\..\...

412 New Zealand Journal of Geology and Geophysics, 1985, Vol. 28

N

~660.1600

600

600

N

/

600

o 9284.6

O~9657.1

mental iron formed by reduction during burning(as in a blast furnace), the high measured remanentmagnetism of the Southland and Otago samples,up to 8.58 Aim, appears to be accounted for bytitanomagnetite and pyrrhotite, although the pres-ence of the latter is inferred from demagnetisationcharacteristics only.

The results of the magnetometer traversesreported here demonstrate that it is possible tolocate the burnt edge of a lignite bed. The methodwould appear to have application in coalexploration.

ACKNOWLEDGMENTSThe authors are grateful to Dr G. A. Challis, New ZealandGeological Survey, for her efforts to resolve the difficultproblem of the nature of the magnetic minerals in theclinkers.

Fig. 8 Directions of magnetisation during thermaldemagnetisation for selected specimens from easternSouthland and eastern and Central Otago. Solid circlesindicate normal magnetisation, i.e., S seeking pole down-wards. 9283.4, 9284.6, and 9653.1 are oriented speci-mens. The directional movements of specimens 9657.1and 9660.1 are relative only but are included to show themagnetic behaviour of specimens from Kai Point andHarliwich mines.

Electron probe examination of the samplesfailed to positively identify magnetite. Thisis due to the extremely small size of indivi-dual grains ( < 1 urn), The diameter of theelectron beam used in analysis is approxi-mately 3 urn, so that it proved impossible toget an analysis of pure material. A compositeanalysis of the spongy aggregates gives anaverage analysis: Si02 56.17%, Al203 13.28%,Ti02 14.06%, FeO 12.01%, MnO 0.23%, MgO1.18%, CaO 0.16%, Na20 0.92%, K20 1.19%.This analysis suggests a mixture of ilmenite,magnetite, quartz, feldspar, and clay minerals.

DISCUSSION AND CONCLUSIONSThe presence of strong anomalies of up to 1450 nTassociated with the Southland clinker deposits isconsistent with the measured magnetic intensitiesof the clinkers. In two traverses (Fig. 4 and 7),anomalies were interpreted to correspond with theburnt edges of obscured lignite seams.

Although Major (1978) suggested that the mag-netic characteristics of clinker can result from ele-

REFERENCESBishop, D. G. 1965: The geology of the Clinton district,

South Otago. Transactions of the Royal Society ofNew Zealand 2: 205-230.

Healy, J. 1936:Whakaea Subdivision. 30th Annual Reportof the Geological Survey 1935-36: 6-9.

Jones-Fahley, A. H.; Geissman, J. W. 1982: Brunhes andMatuyama Epoch paleosecular variation recordsin clinker, Powder River Basin, and implicationfor rates of Quaternary burning ofcoals. AmericanGeophysical Union. Fall, 1982 meeting. A-bstract.

Lindqvist, J. K. 1980a: Resources of Eastern SouthlandCoal Field: Waimumu and Croydon sector coalestimates. New Zealand Geological Survey reportM98.- 1980b: Resources of Eastern Southland CoalField: Kapuka (Ashers) sector coal estimates. NewZealand Geological Survey report M99.

Lindqvist, J. K.; Hatherton, T.; Mumme, T. C. 1983:Magnetic anomalies above burnt coal seams havepotential application in exploration of CentralOtago and Southland lignite deposits. New ZealandGeological Survey report M~33.

McKellar, I. C. 1969: Sheet S169-Winton. Geologicalmap of New Zealand 1:63360. Wellington,Department of Scientific and Industrial Research.

MacPherson, E .0. 1935: Waikaia Subdivision. 29thAnnual Report ofthe Geological Survey 1934-1935:6-8.

Major, M. W. 1978: Coal geophysics: A need for tech-nology transfer and research. In: Voelker, R. M.ed. Proceedings, Coal Seam Discontinuities Sym-posium. D'Appolonia. Pittsburgh, Pennsylvania.

Wood, B. L. 1956: The geology of the Gore Subdivision.New Zealand Geological Survey bulletin 53. Wel-lington, Department of Scientific and IndustrialResearch.

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