Introduction

An element of the Western Tasmanian RegionalMinerals Program involves definition of graniteaureoles in western and northwestern Tasmania andtheir possible relationships with mineralisation,prospectivity and further exploration.

Some previous attempts have been made to definegranitoid forms in Tasmania, the first being Leaman etal. (1980), with more comprehensive treatments byLeaman and Richardson (1989) (partly revised asLeaman and Richardson, 1992), and a detailed analysis of a section of central western Tasmania (Leaman andWebster, 2002). These outlines have been based ongravity data available at the time, with someconstraints applied using magnetic data, especiallywhere alteration-related anomalies assist definition ofthe granitoid roof. The granitoids of King Island wereincluded in the 1980 analysis but have not beensubsequently examined. The Beulah Granite, based onpresumed exposures in the Beulah–Paradise area asshown on regional maps since about 1960, wasassessed en passant as part of the regional studiesundertaken for the Mt Read Volcanics Project; theresults were reported by Leaman and Richardson(1989). Neither the Beulah Granite, nor the bodies onKing Island, have ever been examined individually orin detail.

There have been various reasons for this situation.

The Beulah Granite has not been obviously associatedwith rich or large deposits, or even many mineraldeposits. There has been, consequently, littleexploration pressure and the geophysical database isof regional standard only.

King Island, however, was far removed from theregional studies in northwest Tasmania and thenetwork of gravity and magnetic profiles examined for the other projects. Further, and most crucially, therewas either little confidence in the database on the

island or the data coverage was extremely patchy.Public domain gravity data available in 1986–1989, forexample, were widely spaced and indicated littlevariation in values which might correspond togranitoid relief. There was no pattern which might beassociated with major, and especially siliceous,granitoids as on nearby Three Hummock Island. Thiswas an unexpected situation given the extant regionalmapping (Department of Mines 1:250 000 scale) orgeneral outlines such as Turner (1990).

The present review seeks to redress both situations and to define the forms of any plutons present within thelimits set by available data and the time allocated foranalysis. The data provide the greatest constraint onthe interpretation offered.

King Island

Several granitoids have been mapped on King Island.The oldest suite (approximately 730 Ma) is exposedalong the west coast and near Cape Wickham at thenorthern tip of the island. This late Proterozoic suiteincludes a porphyritic biotite adamellite (dominantlithology) and minor biotite granodiorite, aneven-grained biotite adamellite, a biotite-muscovitegranite, and sundry aplites and pegmatites. Noinformation on the physical properties of these rocks isavailable.

The Devonian–Carboniferous granitoids are exposedin the eastern parts of the island, near Grassy(granodiorite), Bold Head (adamellite), and MountCounsel (granite). Little is known about these intrusive rocks (e.g. Camacho, 1989) as emphasis has beenplaced on associated alteration and mineralisation inthe Grassy–Bold Head area.

Regional gravity data have been available since 1967(University of Tasmania projects) but, as explained byLeaman (1992), there were base and correctionuncertainties. All older data were acquired in an erawhen base maps and elevation controls were very

Tasmanian Geological Survey Record 2003/06 1

Western Tasmanian Regional Minerals Program

The form of the King Islandand Beulah granites

D. E. Leaman

DEPARTMENT ofINFRASTRUCTURE,

ENERGY and RESOURCES

Tasmania

Mineral Resources Tasmania

Tasmanian Geological Survey

Record 2003/06

limited. Geopeko Limited augmented this database in1982 but, again, there were problems in unification and reference. Surveys were individual entities withdifferent control elements. Leaman (1992) wasengaged to assess this situation, correct it if possible,and make recommendations. Geopeko subsequentlycompleted an additional survey and the entiredatabase was reduced from original observationsheets and integrated to within reasonable tolerances— estimated at the time to be of the order of 1 mgal inreduced Bouguer anomaly. It was then interpretedregionally in the particular context of an explorationlicence in the central south of the island (Leaman,1993). The database was further upgraded in 1995 by auniversity project team.

There is now a fair regional coverage of the entireisland at standards which are acceptable. Survey crosschecks and repeated station occupation lends someconfidence to the entire data set although there areclearly some stations which might be suspect.Unfortunately most of the coverage is in the southeastquadrant of the island and there are large gaps in otherregions. These gaps inevitably mean thatinterpretation and resolution is limited in those zones.

The partial island interpretation of Leaman (1993)indicated that there was considerable relief on thegranite surfaces, with interpretation problemscompounded by the distribution of, and consequentdensity contrasts associated with, the twoPrecambrian rock suites on King Island; more or lessmetamorphosed. The 1993 study presumed somegross physical properties for the granitoids(Precambrian and Devonian– Carboniferous) on thebasis that the older rocks were dominated bygranodiorite (approximately 2.71 t/m3) and theyounger were slightly mafic granites (2.65 t/m3). Nointernal discrimination was attempted. The twoPrecambrian series were assigned nominal densities of about 2.69 t/m3 and 2.77 t/m3. These values appearednecessary to generate some of the local andnear-surface contrasts.

The present study has benefited from the 1995improvement in the database and its reliability, andsome reconsideration of the likely bulk densities givenfurther information about the actual rock typespresent. It must be stressed that no statisticallysignificant sampling of rock types and properties is yetavailable and this interpretation therefore depends onthe interplay of several, quite critical, assumptions.

These assumptions were finalised after preliminarymodelling of the entire array of profiles used.

1. The dominant Precambrian granitoid is adamellitic and has a density of the order of 2.65 t/m3 or less.

2. Other local variations in the Precambrian granitoidcomplex may range from 2.69–2.71 t/m3.

3. The largest Devonian–Carboniferous granitoidshave a bulk density of about 2.62–2.64 t/m3 and are

typically represented by the Sea Elephant Pluton(Mt Counsel) and Bold Head Pluton.

4. The Grassy Granodiorite must possess a densityconsistent with such a description (2.70–2.71 t/m3),but the responses imply a relatively insignificantvolume of material. This option was tested andfound to be the case. The granodiorite has beenintruded by another adamellite/granite.

It is clear that any modelling of the gravity field, ormagnetic field, on King Island is dependent on suchassumptions and the consistent implications of anarray of interpreted sections. The interpretation isconsistent with the parameters defined by Leaman(1993) for a valid potential field interpretation. Theseparameters do not ensure correctness, simply a validand internally consistent assessment. Modelling usingtwo-dimensional methods has been undertaken on aninterlocked set of E–W, N–S and variably orientatedprofiles which reflect the availability of coverage. Alllines north of about 5 268 000 mN contain gaps andill-defined segments. The array methods usedminimise the possible deficiencies in the assessmentbut cannot eradicate them. Doubts about densities ofall parts of the geology affect estimates of interfacedepths but not the gross shapes of structures.Consequently the interpretation provided is the bestpossible with the current data and control information. The shape of the plutons is considered valid but thedepth to roof of the main plutons is rated as theminimum likely. Tests using a range of realistic bulkdensity contrasts suggested that the inferredmaximum depths to granitoid may be doubled. Thesevalues are not considered likely.

The interpretation of depth to granitoid is shown in Figure 1. This interpretation is considered the bestpossible using the present data set, and 2D methods,and could be used as feedstock for a 3D modellingassessment provided more information about densitycontrasts was provided. Further evaluation is notjustified until the database is further improved andreliable rock property information is available.

Preliminary modelling of the data indicated a regionaltrend from south to north. The nature of the crustalcomponent was estimated by sampling profiles whichtraversed at least two exposed plutons. The SeaElephant Pluton was used as reference anchor andconfirmed by responses at Cape Wickham. All modelshave then allowed for these trends and been adjustedfor them. This factor has never been incorporated inany previous modelling on King Island. This approachmust yield an improved interpretation, but notnecessarily a totally valid one in terms of granite depthdue to problems with ill-defined contrasts.

The new interpretation generally confirms theimplication of the 1980 study in that the bulk of theisland is underlain by granitoids at depths of around600 to 1600 metres. The Devonian–Carboniferousplutons have intruded the Precambrian bodies and thelatter are decidedly granodioritic in the central west

Tasmanian Geological Survey Record 2003/06 2

Figure 1

Tasmanian Geological Survey Record 2003/06 3

3000

KING ISLANDDEVONIAN GRANITE ISOBATHS

Depth in metres

20001000500

4000

100 100

500

1000

2000

3000

3000

9000

4000

50

0

100

61

60

59

58

57

5560000 mN

23

00

00 m

E

24

25

but adamellitic in the north. The form of the structuralcontours indicates several discrete plutons of whichthe largest is the Sea Elephant Pluton. The Grassy andBold Head plutons are much smaller and the greaterpart of the volume, at depth, of the Grassy Pluton is avery siliceous granite (density approximately2.60–2.62 t/m3). The volume of granodiorite isminimal. This light pluton extends to shallow depth alittle west of Mt Stanley and a little north of YarraCreek at Lymwood. An additional small pluton ispresent north of Pearshape. The eastern limit of theseplutons is quite well defined east of Grassy and YarraCreek and the margin dips very steeply to great depthinland of Bold Head and Naracoopa.

The western margin of the Devonian complex isill-defined, partly due to the effect of the oldergranitoid complex and the difficulty of discriminatingthe various plutons, and the lack of data in the west ofthe island. The margin of either, or both, plutoncomplexes is probably well offshore although it maybe very close to the coast near Currie. Figure 1 indicates the western margin of the Devonian granites based ontheir correlation from the eastern exposures andconsistent properties, as well as definite contrastswithin the areas of exposed Precambrian granites. Thepresent interpretation is constrained by those areas inwhich data coverage and responses, and granitoidcomposition, allow identification of contrast orcontacts. Not all profiles examined provided clearindications of limits because of variation in contrastsand chemistry within the Precambrian complex itself.It may well be that some of the granite exposuresassigned a Precambrian age are in fact Devonian butdetailed field work would be required to confirm ordeny this possibility. The intrusive interfingering ofthese bodies cannot be resolved with extant surface orgeophysical data and the western limit of theDevonian granites, as shown in Figure 1, should beaccepted only as an indication of form. Theinterpretation of roof depth is much more reliablealthough, of course, either granitoid family may havebeen described in the west of the island.

It is clear that the east–west step in the gravity fieldacross the island north of Naracoopa reflects a generalchange in granitoid relief and depth, from relativelyshallow related to the Sea Elephant Pluton, tomoderately deep related to the other plutons.

It is recommended that some minor mapping beundertaken to define the internal Precambrianboundaries and the various compositions of theexposed plutons. Rock property determinations arerequired for all rock types. Additional gravitycoverage is required in many areas and all data shouldthen be re-checked for consistency. The subtlety ofmany deviations in the gravity field, and the inferenceof a broad, generally shallow pluton complex, meansthat the quality and coverage of the database must beimproved if the interpretation is to be further detailedor extended.

Magnetic data have not been examined in detailbecause of time constraints but preliminary reviews,especially in the Grassy region, do indicate that thisdatabase can be used to trace elements of the granitoidboundaries, constrain roof depth estimates, and mapmafic or altered rock units.

Beulah

An interpretation of the form of the Beulah Granite,based on a series of regional profiles, was provided byLeaman and Richardson (1989). It was noted that thename was inappropriate, as the exposed material wasthen described by other departmental staff as an‘I-type’ granodiorite. Little published material orcomment is available about the very limited, andpresumed, granitic exposures. More recent worksuggests that the rocks are possibly andesitic. Leamanand Richardson (1989) noted the difficulty ofinterpreting this intrusion in the absence of freshsampling and property determinations, especially asthe regional models suggested that the intrusionpossessed the density properties of a granite oradamellite and not a granodiorite. Further, everysection which sampled the Beulah Granite did so in amanner which required a complex interbalancing ofthick Cambrian sections and the pluton. Given thatlittle was known about the pluton or its properties, and the scale of the overlying sections was also uncertainlydefined, the interpretation offered was necessarilycrude. It was, however, the first attempt to suggest thegeneral distribution and depth of any intrusion in theBeulah–Sheffield region and the inferred propertiesimplied that the body was Devonian in age.

This last assumption might now be challenged as therecent work of Leaman and Webster (2002) revealedthat large bodies of Cambrian granite may also existand each such intrusion should be dated (also J.Everard pers. comm., and property determinationsfrom southwest Tasmania). This report deals only with the form of any pluton in the Sheffield area and cannotresolve any aspect of its age. It may be remarked thatthe interpretation offered in Leaman and Richardson(1989) is very similar to that of Leaman and Webster(2002) for the hidden Cambrian granite within theWest Coast Range, in that the plutons appear to form ageneral basement for the volcano-sedimentarysequences and may not intrude them. Not enough isyet known to form any conclusion. These commentsare relevant to the following discussion as the actualexposures in the Beulah area, probably of andesite, arealmost certainly Cambrian in age.

The present analysis sought to uti l ise anyimprovements in either geophysical data coverage ormapping and was focussed directly on the regionpreviously interpreted as roof to the granite.Unfortunately this region has only partly benefitedfrom recent mapping programs and the gravitydatabase is much the same as in 1986–1988 when lastreviewed. Leaman and Richardson (1989)foreshadowed an improvement in interpretation with

Tasmanian Geological Survey Record 2003/06 4

Figure 2

Tasmanian Geological Survey Record 2003/06 5

BEULAH AREADEVONIAN GRANITE ISOBATHS

Depth in kilometres

42

43

00

00 m

E

44

45

5400000 mN

39

41

42

43

44

45

-9

-4-3

-2

-1

-1

-1-1

-2

-2

-2

the availability of the new residual separation process,then under test, and it is the residual data which havebeen utilised for the present work. This data formatcannot recover problems in quality of individualstations or lack of coverage, but it does reduce manyaspects of uncertainty and ambiguity.

The resulting interpretation, now in a format for test inthree-dimensional analysis, is provided in Figure 2.

The new and more detailed specific treatment hasshown that the pluton is much more restricted thanpreviously thought but the boomerang shape has beenconfirmed with some caveats. The most reliable part ofthe interpretation is the limb of intrusion extendingENE from Nietta to Lower Barrington. Thegeophysical responses are better defined in this areathan between Barrington and Mt Roland and this maybe due to differences in granitoid composition, depth,or thickness of surrounding Cambrian section. There is little doubt that a large pluton is present; the gravityfield cannot be explained in any other way or by anyother material, or lack of material. In all sectionssampled no correlation could be found betweenexposed lithologies, or any likely to be at shallowdepth, and required or observed responses. Thematerial, previously mapped as granite, is associatedwith positive Bouguer anomalies with stronggradients indicating that the source is local and ofrelatively high contrast. This is consistent with thedescription of the rocks as andesite.

The regional presentation of the observed Bougueranomalies and the residual Bouguer anomalies appearto provide little indication of a hidden pluton. Theresidual values provide a hint of the body, itsshallowest projection, as a negative response is present and cannot be associated with any surface or nearsurface unit or sequence. This is a cautionary note:qualitative inspection of data which does not considerthe implications of actual contrasts or sequencethicknesses may be misleading. The process ofmodelling, which demands a quantitative assessmentof interacting contrasts, soon exposes what is possibleand what is required. This comment is includedbecause the reality of the granitoid is not in question,although the gravity field does not display the samecharacter as associated with exposed granitoidselsewhere in western Tasmania. Similar doubts havebeen expressed about the body of granite inferredbeneath the West Coast Range (Leaman and Webster,2002) as the gravity responses appear different incharacter. In each case, Beulah and West Coast Range,the field responses are determined by the relief andsize of cupola projections, the general depth of the roof, and the composition and density of the covering rocks.

The arms of the intrusion appear to intrude segmentsof the Cambrian sequences and extend to quite shallow depths (of the order of one kilometre) in two areas. It isunlikely that the intrusions extend to depths of lessthan 750 metres anywhere based on the existing data.The implied density of the granite is of the order of

2.62–2.63 t/m3, as inferred by Leaman and Richardson(1989) regionally.

The interpretation is weakest in the area south ofSheffield where there are very large gaps in the data.Whilst every attempt has been made to select profileswhich maximise data availability, some critical areascannot be reviewed with assurance.

It is recommended that some minor ground reviews beundertaken in the areas north of Barrington and east ofSheffield to check if any evidence exists of distantmetamorphism which might be consistent with thepresence of a large pluton.

Any further interpretation in this area could be moreintensively supported by available magnetic data, alsomoderately dated, but really requires some infill of the gravity database. A nominal two kilometre spacing inthe large gaps present would transform interpretivepossibilities. The region further west and northwesthas been covered with a nominal one kilometrespacing. Further consideration and mapping of theCambrian rocks, including details of likely sequencecontent and thickness, would also assist futuremodelling. The model provided could be used asfeedstock for a three-dimensional analysis but this isnot warranted without some improvement in datacoverage and sequence knowledge.

References

CAMACHO, A. 1989. Mid-Palaeozoic granitoids, King Island,in: BURRETT, C. F.; MARTIN, E. L. (ed.). Geology andMineral Resources of Tasmania. Special PublicationGeological Society of Australia 15:256–257.

LEAMAN, D. E. 1992. King Island. Review of gravity andmagnetic data. Unpublished Report by Leaman Geophysics for Geopeko Limited.

LEAMAN, D. E. 1993. Preliminary interpretation, gravity andmagnetic data, EL 26/92, Pegarah, King Island, for Geopeko.Leaman Geophysics [TCR 94-3557A].

LEAMAN, D. E.; RICHARDSON, R. G. 1989. The granites of west and north-west Tasmania — a geophysical interpretation.Bulletin Geological Survey Tasmania 66.

LEAMAN, D. E.; RICHARDSON, R. G. 1992. A geophysicalmodel of the major Tasmanian granitoids. ReportDepartment of Mines Tasmania 1992/11.

LEAMAN, D. E.; WEBSTER , S. S. 2002. Quantitativeinterpretation of magnetic and gravity data for theWestern Tasmanian Regional Minerals Program. ReportMineral Resources Tasmania 2002/15.

LEAMAN, D. E.; RICHARDSON, R. G.; SHIRLEY, J. E. 1980.Tasmania — the gravity field and its interpretation.Unpublished Report Department of Mines Tasmania 1980/36.

TURNER, N. J. 1990. Late Proterozoic of northwest Tasmania— regional geology and mineral deposits, in: HUGHES,F. E. (ed.). Geology and mineral deposits of Australia andPapua-New Guinea. Monograph Serial AustralasianInstitute of Mining and Metallurgy 17:1169–1174.

[1 April 2003]

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