BUREAU OF MINERAL RESOURCES, GEOLOGY AND GEOPHYSICS

REPORT 217

BMR MICROFORM MF105

ABSTRACTS OF 8TH BMR S~WOSIUM,

CANBERRA, 1-2 MAY 1979

DEPARTMENT OF NATIONAL DEVELOPMENT

Minister: The Hon. K.E. Newman, M.P.Secretary: A.J. Woods

BUREAU OF MINERAL RESOTJRCES, GEOLOGY AND GEOPHYSICS

Acting Director: L.W. WilliamsActing Assistant Director, Operations Branch: A.R. Jensen

Published for the Bureau of Mineral Resources, Geology andGeophysics by the Australian Government Publishing Service

© Commonwealth of Australia, 1979

B~m Symposia are held annually to report resultsor pr'ogress of proje~ts considered particularly relevant tothe Australian mining industry, and to provide a forum fori~.1dustry commer:t on BMR' s activities.

The E:'>th Symposium was opened by Mr W.M. Lonie,President, Austr~lasian Institute of MIning and Metallurgy.

Most of the BMR programs reported ori at thesymposium and in these abstracts have involved the collaborativeeffort of colleagues, many of whom have assisted in thepreparation of the abstract or the presentation. The abstractsare, however, attributed only to the speakers.

CONTENTS

Page

J.B. Willcox - Petroleum prospectivity of the Australian 1

marginal plateaus.

G.D. Karner - Mechanism for offshore sedimentA.ry 'Jasin 3

formation.

B.R. Senior - structure, hydrodynamics and hydrocarbon 4

potential 01' the central ErOIIls.nga Basin, Q1J.eensland.

B.M. Radke - Diagenetic history ef epeiric carbonates, 5Georgina Basin: the economic implications.

K.S. Jackson - Petroleu~ geochemistry applied to exploration. 6

A. Renwick (Chairman) - Taking publications into the Eighties. 7

M.J. Jackson - M.ineral potential of the Officer Basin, 8Western Australia.

N.F. Exon - Investigation of offshore mineral resources in 9the South Pacific under UN auspices.

D. Perkin - Predicting the location, grade and tonnages of 10

Australia's future tin orebodies.

J.H.C. Bain - Some new ideas on the age and origin of the 11

Etheridge Goldfield, Queensland.

G.M. Derrick - Geology and mineral potential of red-bed and 12

associated environments in the Mount Oxide Region,

Northwest Queensland.

P. Wilkes .- The characteristics and int~e:rrY''?tation of 14

geophysical anomalies in the Cobar area, NSW.

D.M. Finlayson - Crustal profiles in the Lachlan Fold Belt 15from explosion seismology, and the Palaeozoic evolution

of southeastern Australia.

John Ferguson - Strangways cryptoexplosio~1. structure NT - 16terrestrial or extra-terrestrial origin?

D. Denham - The state of stress in the Australian continent. 18

R.D. Shaw - Basement evolution and mineral potential of the 19

Alice Springs area.

L.P. Black - New developments in geochronology.

B.S. Oversby - Late ?alaeozoic ignimbrite volcanism a~d

mineralisation in northeastern Queensland.

D.E. Mackenzie - Revised Proterozoic stratigraphy and

economic potential of the western Georgetown Inlier,north Queensland.

Page

20

21

22

1 .

Petroleum Prospectivity of the Australian Marginal Plateaus

Speaker - J.B. Willcox

The Au~tralian continental margin has an area ofabout 5.5 106 km , which is 65 percent of the area of thecontinent itself. Almost half consists of mid-slopefeatures - the sixteen marginal plateaus, terraces, and rises.These features, which are largely unexplored for petroleum,lie jn medium water depths of between 500 and 2800 m;several are within rang2 of current drilling and well­completion technology.

The western, southern and eastern margins of theAustralian continent are believed to have formed in responseto five discrete episodes of continental rifting, whichculminated ip seafloor spreading, commencing in differentareas at tin. ranging from Late Jurassic to Paleocene. Mostof the mid-slope features (hereafter termed 'plateaus') areregarded as blocks of continental crust which have foundered,mainly since the final stage of continental breakup andformation of ocea~ic crust in the adjacent basins.

Most of the Australian plateaus are associatedwith 'pull-apart basins', in which numerous normal faultstrend subparallel to the cOEtinental margin. Three majorsedimentary sequences can usually be distinguished: a pre­rift sequence which is extensively faulted; a rift-stagesequence which is typically fluvio-deltaic and in some areasis faulted near the oceanward margin of the basin; and apost-breakup sequence, generally comprising a marinetransgressive mudstone, overlain by clastic and pelagiccarbonates. Distinctive 'rift-onset' a~d 'breakup'unconformities generally separate these sequences. Therift-stage sediments may contain significant quantities ofhumic source material and reservoir sandstones. ~1arine

incursions during the latter part of the rift stage, and thefinal marine transgression associated with breakup, appear tohave led to the deposition of suitable petroleum source andcap rocks in many areas. Total sediment thicknesses under theAustralian plateaus range from about 1000 to 9000 m, but inmany areas the post-breakup sequence is thir' .

Because little exploration activity has been undertakenin basins of the pull-apart type, their petroleum potential andthe nature of the hydrocarbons are virtually unknown.Theoretically these basins may be expected to have had highgeothermal gradients at the inception of seafloor spreading,and source rocks may well have matured with less overburden thanwould normally be required.

2.

Tne most prospective areas - the Exmrut!J Plateau,and Carnarvon and Rowley Terraces - appear to be underlain byseveral thousand metres of mainly fluvio-deltaic rift-stagesediments, which provide reservoirs and gas-prone source beds.A transgrAssive marine mudstone could provide the main sourcebeds and seal. The Queensland and Townsville Troughs, and adepocentre flar~ing the Marion Plateau, are also consideredfairly prospeci~ive. The Ceduna Terrace is probably underlainby Aptian-Albian marine source rocks, and hence also warrantsfurther exploration. Rift basins which may theoretically occurin the South Tasmania Rise, Lord Howe Rise, Kenn Plateau,Eastern Fields Plateau. Other areas off northeastern Australiarequire further surveying, as they may provide long-termprospects.

3.Mec~anism for offshore sedimentary basin formation

Speaker -. G. D. Karner

Cross-spectral techniques have been used toanalyse the relationship between free-air gravity andbathymeT.ry of approximately 100 profiles over the continentalmargins of ffistern USA, South West Africa, Lord Howe Riseand the Queexlsland Plateau. These margins are passivecontinental margins in the plate tectonic model, but differin age of creation and sediment content.

The resultant filters or transfer functions havebeen used to evaluate the state of isostasy at these margins.The east USA coast can be best explained by a fle)_ure modelwith an equivalent clastic thickness, De, in the range 10 to20 kms, South West Africa can be best explained by 5 <. De'< 10 kms.The Coral Sea-Lord Howe Rise can be equally explained by eithero < De < 5 kms or any Airy model with a depth of compensation of30 kms. It should be noted that an Airy model is equivalentto a flexure model with a rigidi~y of zero.

Three factors control the degree of flexure: theage of the margin; the age and amount of sediment at the margin;and the thermal his~or2r 2~ the margin. The observed differencebetvfeen margins is explainea ~y a simple model in which thelithosphere cools after initi~l ~ifting, increasing the elasticthickness. Thus, the overall response to the sedi~ents at an oldmargin is as a relatively thick plate and at a young margin as avery thin plate or Airy model, d8pending on the age distributionof the sedime~ts.

The resulting transfer function at each margin cantherefore bp explained in terms of a simple thermal and mechanicalmodel for the evolution of the margins. A model of basin formationhas been constructed based on the rigidity cr elastic thicl<ncss/age information for oceanic lithosphere. Using this model,speculations concerning sedimEnt distribution, thicknecs, andheat flow within the forming basin can be made.

4.

structure, Hydrodynamics and Hydrocarbon Potential of

the Central Eromanga Basin, Queensland

Speaker - B.R. Senior

The hydrocarbon potential of the Eromanga BasJlJ.sequence in southwest Queensland is evaluated in terms ofsource rock geochemistry, structure, and groundwaterhydrodynamics. Interbedied coal, carbonaceous mudstone,and siltstcne ,vithin the Jurassic and Lower Cretaceousare fair to very good source rocks for oil. Approximatelyhalf of the central Eromanga B8sin sequ8Dce is marginallymat~re or mature, depending on thickness and variations ingeothermal gradients. Linear features interpreted fromLANDSAT imagery and aerial photographs indicate theprobable presence of major fracture systems as some featurescoincide with faults delineated by seismic surveys. Ground­water flow provides a mechanism for moving indigenous(Eromanga Basin) or derived (Cooper or Adavale Basins)hydrocarbons through the region. Faults with displacementsof 15 m or more, whicr. impede groundwater £low, could trapand protect petroleum from the effects of predominantlysouthwesterly groundwater movement.

5.Diagenetic history of epeiric carbonates, Georgina

Basin: the economic implic~tions

Speaker - B.M. Radke

Porosity and permeability are significant factorsinfluencin~ the er-onomic potential of carbonates. Throughoutcarbonat~ diagenefj is, . the patterns of porosity and permeabilityare contlnually cnanglng and consequently, trends in themigration of pore fluids are also modified.

. Epeiric carbonates of the Ninmaroo Formation (UpperCambrlan - Lower Ordovician) in the Georgina Basin, centra.lAustralia, were modified throughout diagenesis by interact_onwith saline fluids. This has superi~posed on the caroonatesediments, marine sabkha diagenesis, late diagenetic dolomitisation,sulphide mineralisation, and subsequent oedolomitisation duringero.sion.

In the Late Cambrian and Early Ordovician, a shallowsea extended into central Australia as an epicontiner..talembayment. Ninmaroo sedimentation was in both tidal and nontidalenvironments in an extensive belt adjacent to normal-marine tidalconditions, and delineated by a seaw~rd barrier complex ofcarbonate sand shoals. Channels through this barrier extendedweakened tidal effects to the periphery of an epeiric region,where a patchwork pattern of semi-emergent shoals was transitionalwith marginal emergent pavements. Indurated pavements, algalmounds and shoals of carbonate sand constituted a varied but low­relief, submerged topography.

Sabkhas developed around low-relief emergent shoals andwere characterised by gypsum precipitation within, and dolomitisationof, the sediments. Nodules and laminae of anhydrite precipitatedwhe.re extreme temperatures and salinities prevailed, and in areasof prolonged emergence, porosities were increased by the dissolutionof sulphates and carbonates with the development of k2rst.

These Ninmaroo environments migrated seaward across aflat shelf, producing progradational offlap in response to a slowrelative drop in sea level. With a subsequent relative rise of sealevel, transgressive deposition prevailed until excessive sedimentaccumulation rates again produced offl9.p of lithofacies.

During burial and structural deformation, hydrocarbonswere introduced with the updip migration of basinal brines whichproduced extensive dolomitisation and minor emplacement of sulphates.Where hydrocarbons had already accumulated and occluded porosity,interaction between host carbonates and dolomitising fluids wasprevented. Subsequent faulting produced near-vertical conduits forthe escape of deeper and warmer brines which precipitated saddledolomite, pyrite, fluorite, galena and sphalerite in the fault zonesand along the more permeable strata. vmere the sequence has beenexhumed by subsequent erosion, relict sulphates have been redissolvedand su2.phides oxidj.sed in meteoric waters. The interaction ofresultant sulphate-enriched waters has produced near-surfacededolomitisation.

6.

Petroleum geochGmistry 8pplied to exploration

Speaker - K.S. Jackson

As petroleum exploration costs continue to soarboth in onshors and offshore basins, the industryneeds to consider all technical too13 at its disposal.PetroleLilll geochemistry has emerged in the past few years~s a relatively inexpensive answer to many explorationproblems. Varied 2naJytiGal procedures, including gasand liquid chromatography, pyrolysis, organic extractionand stable isotopes, have been developed to answer thequestions of source potential, organic maturation, kerogentypes, hydrocarbon migration, and hydrocarbon-sourcecorrelations. While exploration histories froTh Canada andthe USA have shown how geochemistry has been integratedinto the overall exploration program, it is now possible toreport on geochemical data and their application to explorationin .veral of Australia t S sed5_mentary basills. Two currentge r =' _emical projects will be discussed. A joint BlVlR/CSIRO/GSU study on the Permian rocks of the Denison Trough, BowenBasin, illustrates how sou~ce rock and maturation studies havehighlighted the significance of the Cattle Creek and ReidsDome Beds as source units. A different appro88h was taken ina joint SADME/BMR study on the Jurassic Eromanga sequence andTriassic rocks of the Pedirka Basin. Using classical sourcerock methods and isotopic correlation procedures, the Jurassicand Triassic rocks in the Poolowanna well have been studied inan attempt to correlate theI!1 with the Jurassic reservoiredPoolowanna oil.

7.

PANEL DISCUSSION

SYNOPSIS

Taking pUblications into the Eighties

The panel will comprise a chairman and five members:four of whom are BMR editor~, the other two representauthor Branches.

The panelfs presentation will start \rlth a briefintroduction by the chairman in which he will outline thehistoric backgro'.-:nd of BMR publishing; the phi.losophy andits product. It will then be shown how, in some areas,change of emphasis in BMR programs has necessitated a changein the earlier pattern of BMR publishing; and two editorswill elaborate on the publisherfs response to the changeddemand. In contrast, another panel member will show how,while the program thrust and data to be presented by hisBranch have changed little over the years, chauges inpresentation have nevertheless been needed. Two editors willagain indicate how the publishers have attempted to achievethese changes, and at the same time, to achieve economieswhich maintain the price of the publications at an acceptablelevel. The panelfs presentation will conclude with adescription by an editor of BMRfs attempts to reach a wider,mainly non-scientific audience through its publishing program.

The panel will answer questions and discuss pointsof cr~ticism and suggestions made by members of the audience.

At the end of the discussion, the chairman will sumup and hazard some opinions as to the likely direction of BMRpublishing in the next decade.

8.

Mineral p£~ential of the Officer £asin,Western Aust~alia

Speaker - M.J. Jackson

The Officer Basin is an intracratonic sedimentarvbasin st:cetching from the Gi bson Desert in Western Australiato the easte~n part of the Great Victoria Desert in SouthAustralia. The Western Australian portion contains a poorlyexpofled , little known sedimentary sequence of Proterozoic andPhanerozoic age. The area was geologically mapPGd by a jointBMR/nSWA party between -j 970 and 1974. To complement the fie ldwork. reconnaissance gravity and airborne magnetic surveys, anda ground seismic, gravity and magnetJ-c sur':ey, have been carriedout. This work showed that the basin contains a Proterozoicsequ\.:nce up to about 10 km thick overLain by a Ph2.nerozoicsequence (incJ.udir.g Cambro--OrdovicJan, Permian, and Cretaceousrocks) up to 1.5 km thick, but the sparse information availableindicated the are0 had very low mineral potential. F(~ instance~

although suitable structures and porous units were foundpetroleum source rocks and cap rocks appeared to be lacking.The Permian sequence appeared to contain little carbonaceousmaterial.

However, drilling and field investigations by theSouth Australian Department of Mines and Energy in 1978 haveupgraded the petroleum potential of the eastern part of theOfficer Basin in South Australia. The ?Cambrian ObservatoryHill Beds in Wilkinson 1 contain anomalously high quantitiesof organic material that is suitable for hydrocartcm generation.Based on lithological and microfossil cnmparisons the sequencesin the eastern and western parts of the b8sin can now becorrelated. The Observatory Hill Beds (of the Eastern Ofi'icerBasin) are probably equivalent to the Babbagoola and BrOwileBeds in the Western Australian part of the basin. Thepresence of attract1ve structures and good porosities inWestern Australia is now complemented by at least someindications of a possible source rock.

9.Investigation of offshore mineral resources

in the South Pacific under UN auspices

Speaker - N.F. Exon

In 1972 a number of Paciftc countries formed aCommittee for Co-ordination of Joi: : Brospectin& for MineralResources in South Pacific Offshore Areas (CCOPjSOPAC), withUnited Nation3' support. With a small dedicated staff, ashoestring budget, and enthusiastic local support, muchoff3hore work was done in the region, demonstrating that alarger ,)peration was feasible and worthvrhile. By late 1978CCOP/SOPAC had nine member countries, and had producedabout 30 publications on offshore geology and mineral potential.Financial support had come from the United Nations~ membercountries, and a number of donor countries including Australia.In January 1979 a major three-year Project - Investigation ofMineral Potential jf the South Pacific - commenced with abudget of $US4 000 000 provided by member COLITltries and theUnited Nations Development Program. Work is roughly equallydistributed in the waters of eight member countries (NewZealand, although a member, is excluded from the Project),and will involve 6 full-time professional and technical staff,and more than 2 years of ship-time.

The eight project countries can be divided into twodistinct geological groups: those of island-arc origin (PapuaNew Guinea, Solomon Islands, New Hebrides, Fiji and Tonga) , andthose forming parts of seamount chains (Samoa, Cook Islands,and GJlbert Islands). The island-arc countries have a land 2area ,,)f 525 000 km2 , a population density averaging 7.5 per km ,and extensive shallow seas arolilln much of their coasts. Theseamount-chain countries have a land a~ea of 4300 km2 , apopulation density averaging 65 per km , and are surrounded bythe deep ocean. The preliminary CCOP/SOPAC program showed thatboth groups have offshore pot~ntial for manganese nodules,phosphate, construction materials and precious corals. Theisland-arc countries also have potential for offshore petroleum,bauxite, detrital minerals, and perhaps metalliferous mudsassociated with seafloor spreading centres.

The three-year project will give a much better ideaof the offshore potential of the various countries and willprovide useful environmental and bathymetric information, anda much better catalogue of resources of construction materialswhich are important for the smaller, more densely populatedislands.

'10 .

•Predicting the location, grade and tonnages of

Australia's futur'e ti,n orebodies

Speaker - D. Perkin

Models of tin deposits of specific genetic typesand typical ore grades are paired with different kinds andsizes of mining operations with tlleir characteristic costs,via a computer program. It is possible then to constructcurves on price/grade graphs equating revenue at the mine withthe operating cost, from which a notional cut-off-grade forany assumed tin metal price can be derived. More importantly,a curve showing the rate of return consistent with 'normal'profit (assumed to be 9% in real dollar terms) may be derived,resulting in the establishment of th.e desired minimum averagegrade for any specific type of (profitable) mining operation,given a relatively stable medium-term tin price. Conversely,for a tin deposit with known grade and to~age, the minimumaverage -Sin price necessary for profitable operation may bederived C~C2 the general type and size of operation is specified.

Reserves are defined as identified economic resourcesand hence, from the graphs, it is possible to establish minimumgrades aLd tonnages of potentially profitable tin deposits,and thus establish an 2ggregate tin resource inventory.

The paper concludes that there are areas withinAustralian tin-fields where accumulations of tin exist atgrades hitherto thought too low. Because of the relative worldscarcity of tin and consequent high prices coupled withAustralia's natural endowment of tin mineralisation, theremay be many economic tin deposits yet to be found.

11.Some new ideas on the age and or~gin of the

Etheridge Goldfield, Queensland

Speaker - J.B.C. Bain

The vein deposits of the Etheridge Goldfield yieldedat least 23 000 kg of gold-silver bullion oetween 1869 and1940. Poorly documented and little studied, the deposits havegenerally been regarded as Precamhrian in age, and to have beenintroduced directly by the Precambrian granitoids andmetabasic ~ocks with which they are closely related spatially.

Recent field research suggests that the deposits areof late Palaeo2oic age but have formed by amultistageprocessthat began with the eruption of submarine basalts in early ormiddle rrotero2oic times. Whilst the volcanic pile cooled onthe seafloo~ base and precious metals could have been leachedfrom the pillows anJ deposited as sulphides in interpillowpositions and in fractures in the volcanics. These would havebeen easily mobilised by subsequent amphibolite-faciesmetamorphisms (550-700oC, 3-4 kb) which were accompanied bymoderate to strOl'lg deformation? the formation of penetrativefoliation, FInd emplacement of granitoids at about 1570 m.y.and 1470 m.y. B.P. The locally high carbon ~ontent of enclosingmetasediments may have llelped fix the precious metals instructurally favourable sites, possibly in ore-grade concentrations.However, it wes probably not until these enriched 20nes werelocally subjected to prolonged (about 50 m.y.) and intensehydrothermal activity during the late Palaeo2oic that most of theexisting deposits formed. The loci of such activity appear tohave been keels of Protero2oic metamorphic and granitic rockso\Terlying a concealed late Palaeoz~ic granitoid batholith. Theseare represented by embayments into a regional graVity low thatcorresponds with the extent of the late Palaeo2oic igneous rocks.The thickness of the roof over the hypothetical batholith inthese zones, and the presence of deep vertical or steeplydipping fractures in it, such as those in well-jointed, strainedgranitoids like the Fursayth Granite, cculd have providedmineralising hydrothermal fluids with access to higher, coolerlevels and suitable depositional sites.

fhe formation of ore with easily extractable goldresulted from oxidation enrichment of the upper near-surfaceparts of the deposits during Caino2oic time.

12.

Geology and mineral potential of Red Bed andassociated environments iD the Mount Oxide

Region, Northwest Queensland

Speaker - G.M. Derrick

The Mount Oxide area, 200 km north of Mount Isa,links Lawn Hill and Mount Isa geology. Field investigationsin 1978 established a relatively simple, unifying stratigraphythat can be traced from Lawn Hill south to Mount Isa and eastto the Kalkadoon-Lp.ichhardt basement block.

From the base, eastern CreEk Volcanics and MyallySubgroup sandstones are overlain by the Quilalar Formation, asandstone-dolomite unit equivalent to the Mary Kathleen Groupto the east. A major regional unconformity separates QuilalarFormation from the Fiery Creek Volcanics, which in turn isoverlain unconformably by sandstone and siltstone of the SurpriseCreek Formation; the McNamara Group (broadly equivalent to MountIsa Group) overlies Surprise Creek Formation, usuallyunconformably, and contains the Lady Loretta Pb-Zn deposit.

Near Mount Oxide, Eastern Creek Volcanics are recordedwest of the Mount Gordon Fault Zone in the cores of large domes.Thickness and facies variations in the volcanics and MyallySubgroup define the Leichhardt River F'ault Trough, a rift-likefeature bounded in the east by the Quilal~r Arch, and in the westby the Mount Gordon Arch, a monoclinal flexure extending northwardsfrom near Mount Isa. Westwards across the Mount Gordon ArchMyally Subgroup feldspathic sandstones grade into ferruginousdolomitic sandstone and siltstone, mudstone, and dolomite - atypical red-bed association deposited in a very shallow?intracratonic sea. Westwards across the Mount Gordon Arch theQuilalar Formation is similarly enriched in ferruginous siltstone,mudstone and dolomite, and shallow water, possibly intertidalto supratidal~ features such as stromatolites, halite casts, andpossible nodular pseudomorphs after anhydrite. Around the flanksand upon the Mount Gordon Arch thin regolithic conglomerates ofthe Myally Subgroup, Surprise Creek Formation, and McNamara Groupare preserved.

Basal McNamara Group sequences become thinner and morearenaceous adjacent to the Mount Gordon Arch, and gypsumpseudomorphs in the basal Torpedo Creek Quartzite reflect adiagenetic sulphate overprint near the palaeohigh. YoungerMcNamara Group sequences are less influenced by the Mount GordonArch, and extensive dolomitic and locally evaporitic peritidalenvironments characterised by an abundant stromatolite biotawere formed west of the Arch.

13.

Copper mineralisation, reminiscent of that atRedbank, is present in trachyte-rhyolite breccia and dykeswhich intrude the Myally-Quilalar red beds. Copper is alsowidely distributed in breccia zones in feldspathicferruginous sandstone and siltstone; it infills simplevertical shears in flat-lying sandstone (e.g., near McNamara'smine), or forms in complexly faulted breccia zones, as at Mammoth.

The Mammoth deposit is located in upper Myally Subgroupadjacent to the Mount Gordon Arch, below a thick regolithicbreccia of the Surprise Creek Formation and younger carbonaceoussiltstone, quartzite, and dolomite. Although much of the Mammothcopper mineralisation may be related to late-stage cross-faulting,the tectono-sedimentary setting is considered favourable forsyngenetic to diagenetic copper mineralisation. Synsedimentarypyrite suggests either that other red-bed zones in the area mayalso be pyritic at depth, or that within a mainly oxidisingshallow-water environment there existed local zones of seawatersulphate reduction.

14.

The characteristics and interpretation cf geophysical

anomalies in the Cobar Area, NSW

Speaker - P. Wilkes

From 1978 BMR has studied the characteristics andinterpretation of gravimetric, magnetic, and electricalanomalies in the Cobar area using experimental airborne, groundand drillhole surveys, laboratory rock property measurements,data processing, and modelling. Regional gravity anomaliesin the Cobar area reflect the density contrast between Ordovicianbasement and younger rocks. Interpretation of the regional .gravity provides possible models of the Cobar trough. Detailedgravity surveys frequently highlight local lithological contrastswhich can assist in mapping areas of poor outcrop. Small,discrete gravity anomalies are observed over most mineraldeposits in the area, and are useful for selecting drill targetsand evaluating magnetic or electrical anomalies.

Regional magnetic anomalies in the Cobar area resultfrom basement structure, the distribution of basic volcanics,weakly magnetic zones in argillaceous rocks, and areas whereextensive surficial maghemite has developed. Each of the regionalanomalies has a distinctive characteristic which can be recognisedon the basis of source geometry. The magnetic anomalies associatedwith basement structure support interpretation based on gravitymodelling, and the weakly magnetic zones in argillaceous rocksmay be associated with mineralised rock units. High resolutionairborne and carborne surveys over discrete anomalies related tomineral deposits, basic plugs, or maghemite concentrations offerhope that 3-D modelling, spectral studies, and amplitudeattenuation characteristics can aid in the discriminatingbetween different sources. Model studies, and the results ofairborne and ground TEM surveys show that a strong response tothick surface conductive zones is observed throughout most ofthe Cobar area. Only very small EM anomalies can be expected frombedrock conductors which do not have any expression in the weatheredzone. MO~8l and field studies indicate that discrimination ofbedrock and surface conductors can be aided by a careful choiceof survey parameters. A study of the application of resistivity andIP methods in the Cobar area shows that significant IP sourcesonly occur in unweathered bedrock. However, the dilution of IPeffects by the conductive overburden is marked, and meaningfulinterpretation of IP data in this area requires the overburdeneffects to be taken into account. With careful interpretation ofresistivity and IP data, electrical contrasts ~etween differentrock types can be identified and used for mapplng. The IPresponse of mineral deposits in buried b~dro?k is likely to be .small but diagnostic - if careful attentlon lS taken of ~he deslgnand interpretation of surveys. Tests of the magnetometrlcresistivity method suggest that this method may prove to be aneffective and rapid tool to aid mapping in areas of little outcrop.

15.Crustal profiles in the Lachlan Fold Belt from

explosion seismology, and the Palaeozoic

evolution of southeastern Australia

Speaker - D.M. Finlayson

Since 1976 BMR has made seismic recordings alongfour profiles radiating from the Dartmouth Dam site innortheastern Victoria; Dartmouth to Merimbula, Dartmouth toMarulan, Dartmouth to Dubbo, and Dartmouth to Condobolin.Interpretation of the data to satisfy both the kinematic anddynamic characteristics on the s,eismic record sections ind.icatesa) that the crust under the Lachlan Fold Belt is neither uniformnor homogeneous, b) that velocity=depth functions arecharacterised by transitions rather than discontinuities,c) that the crust may contain one or more low-velocity zones,d) that the crustal thickness may exceed 50 km in places, ande) that the greatest crustal thickness is under the highesttopography,decreasing towards the Murray Basin.

Geological and geochemical evidence now suggeststhat a thick continental-style crust must have existed priorto the Palaeo2oic evolution of the Lachlan Fold Belt. It issuggested that the crustal structure evolved during more thanone tectonic episode and may have analogues in the Appalachiansand western South America, rather than in the oceanic island-arcregions of the western Pacific.

16.

Strangways crrptoexplosion structure NT ­

terrestrial or extra-terrestrial origin?

Speaker - John Ferguson

The StrangNays cryptoexplosion structure comprisesa circular feature containing abundant evidence of shockmetamorphism. The core consists of granite gneiss about 5 kmradius, surrounded by a collar of Proterozoic quartzite andsiltstone. No definitely coherent bedrock is exposed in thecore: exposures are breccid, commonly highly shocked, ormelt rock rich in clasts. In different areas the clasts maybe all of gneiss, or a mixture of gneiss and sediments. Inthe collar, which is about 5 km wide, qU:irtzite forms ridgesof steeply dipping, outward-facing strat~, which commonly extendoutward into overturned flaps, presumabJy lying on poorly exposedsiltstone. In one sector the outermost quartzite forms a flapof nearly horizontal overturned beds 4 km wide, making thediameter of the disturbance about 22 km. The innermost quartziteis mainly breccia both in the strike rid~es and flaps; brecciais minor in the outer quartzite ridges. Shatter fracturing -more commonly intersecting sets of striated cleavage surfacesthan well-formed cones - is well developed in the inner quartziteof the collar, but rare toward the outside. Anomalous featuresare small patches or house-sized blocks of at most weakly shockedquartzite resting on gneiss up to 1.5 km inside the normal contact,and rare blocks of shocked gneiss as far as 0.5 km outside. Inthe northwest corner of the structure a flat-lying carbonate unitis found which appears to post-date the explosive event and ispossible Cambrian Tindall Limestone.

The basic structure is similar to the Vredefort Domein South Africa, but the melt rock and the flaps indicate ashallower level of exposure. The ncc~rrence of the flaps atabout the same elevation as the base of the melt layer is surprlSlng.It is not clear whether they represent features of the collar rocksoutside the central uplift or of the floor inside.

The presence of shatter-cones, diaplectic glass and planardecorations in quartz indicate a high-degree of shock metamorphism.Extrapclation of static experimental data suggest pressures couldhave been as high as 100 kb. To achieve pressure on this scale athigh levels in the crust would require the impact of a meteorite orcomet; the chemistry of the shattered and melted rocks is howeverenigmatic. Relative to the undisturbed rocks they show enrichmentin the compatible elements Ni, Co, Cr and V and in the incompatibleelements Ba, La, Ce, P, and Ti. Large-scale introduction of K hasalso taken place. Although the compatible elements could beattributed to volatilisation of a meteorite during impact, theintroduction of K and the incompatible elements would not support suchan origin. If the shock-metamorphism observed is to be attributed

17.

to a terrestrial orlgln a violently explosive crypto-volcanicevent would be the most likely cause. The chemical changesrecorded would suggest involvement of a volatile-enrichedalkaline ultramafic magma. From a terrestrial argument thebiggest difficulty would be in the generation of excessivelyhigh overpressures, as suggested by the experimental work.

The e'~cessively high overpressures suggested by theexperimental work represent the greatest difficulty to aterrestrial origin for the structure.

18.

The state of stress in the Australian continent

Speaker - D. Denham

Evidence from earthquake focal mechanisms,overcoring measurements, and surface deformations indicatesthat the Australian continent is in 8 state of substantialcompression.

Reliable focal mechanism~ are now available frolntwelve earthquakes that have occurred in several parts ofthe continent since 1967. Each of these mechanisms indicatesthat the faulting associated with the earthquakes was causedby compressive stress acting close to horizontal.

Overcoring measurements made in underground minesand close to the surface in quarry floors or on rock outcropsalso indicate compressive stress in all areas.

Near the site of the 1968 Meckering earthquake,shallow overcoring measurements «10 m) were carried outto compare the in-situ observations with the earthquake focalmechanism and the surface faulting. The measurements were madein competent granite at seven locations along a 200 km 'north­south traverse. The results indicate a high regional compressivestress acting about 77°E of N which agrees well with the 91 GEof N direction for the pressure axis obtained from the earthquakefocal mechanism. The highest stress (23 MPa) was measured atthe site furthest north from the epicent~e and the lowest stressclose to the epicentre, where the maximum principal stress wasabout 4 MPa.

During 1978 five sites in NSW were tested at depthsranging from 3-9 m. At each site the stress measured wascompressive. Near the coast at Milton and Moruya the axes o£maximum compression were north-south, but at the other sitesthe maximum stress was close to east-west. The highest values(~20 MPa) were obtained in Silurian granite at Tocumwal andBerrigan.

19.

Basement evolution and mineral potential

of the Alice Springs area

Speaker - R.D. Shawo

A simplified model is presented of the stratigraphicand tectonic evolution in the basement rocks in the Alice Springs1:250 000 Sheet area based on the concept of three broad lithologicalgroupi~~=, termed Divisions.

In this model Division I consists chiefly of basic andacid granulites and is interpreted as a bimodal se~~c~ce ofvolcanics and intrusives introduced dvrirlg a phase ~of Qrrestedeast-striking rift development on an ensialic crust.

Later widespread subsidence resulted in two, possiblyconnected, east-trending troughs flanking the metavolcanic beltrepresented by Division I rocks. The sOLtthern trough was filled byacid volcanic and vol-caniclastic rocks, now represented bywell-layered quartzofeldspathic gneisses, tentatively assigned tounits of the lower part of Division 11. In contrast a moreargillaceous sequence containing only minor volcanic rocks, andmore typical of the upper part of Division 11, was deposited inthe northern trough. The quartzofeldspathic gneisses of lowerDivision III appear to be much thinner.

The sedimentary cycle was completed by deposition ofmore mature quartz sandstone and argillite belonging to Division Ill.

Tectonism at about 1800 m.y. resulted in met~morphism upto granulite grade, widespread migmatisation, and minor granite emplace­ment. Metamorphism and deformation re-occurred at 1700-1600m.y., possibly at 1500-1400 m.y., at1100-1000 m.y., and at 400-300m.y. Granites were emplaced during the 1700-1600 m.y., and somegranite emplacement accompanied the 1100-1000 m.y. metamorphism.Overthrusting, possibly at 1600 m.y., and certainly at 300 m.y.,was synchronous with metamorphism.

The Woolanga Lineament, which cuts the main east-westtectonic trend at a high angle, is thought to be a deep-crustalfracture responsible for localisation of the potassic-ultramaficMordor Igneous Complex and the Mudtank Carbonatite during periodsof stability about 1090 m.y. and 730 m.y. ago, respectively.

Arunta rocks have some potential for Oonagalabi-typeZn-Pb-Cu deposits. Various schist zones are prospective forgold, lead, copper, and possibly uranium. The zone along~ide theWoolanga Lineament is a possible locus for kimberlite, whlch maycontain diamonds, and for carbonatite which may contain economicconcentrations of niobium, rare earth elements, zircon, apatite,and vermiculite. Other minerals of interest in the region includemica in the Harts Range pegmatites, and ruby, some of which is of gemquality, occurring in amphibole-plagioclase pods in the Mount RiddochAmphibolite.

20.

New developments iI2; geochronology

Speaker - L.P. Black

In only the simplest of geological terrains willdifferent dating techniques yield the same age for a particularrock. With increasing knowledge, however, this apparentdisadvantage is generally proving to be a help rather than ahandicap, for it makes possible the identification and datingof different events during the history of that rock.

In the Mount Isa region, for example, detailedstudi.es have shown that Rb-Sr total-rock ages for igneous rocksare generally significantly younger than U-Pb zircon ages,which usually represent the time of primary crystallisation.The two methods thus provide a stratigraphic framework forthe area. Farther east, in the Georgetown Inlier, it has beenshown that different Rb-Sr total-rock ages can be derived fromDcks of the same stratigraphic age. These ages correlate withtemporally distinct deformational-metamorphic events spread overat least 1000 m.y. The data indicate that the criticalstructural requisite for resetting Rb-Sr total-rock ages is thedevelopment of a penetrative schistosity. This resetting isindependent of metamorphic grade .- e.g., it occurs in granuliterocks in the Arunta Block, in amphibolite-facies rocks in theGeorgetown Inlier and the Arunta Block, and in greenschist faciesrocks in western Tasmania and the Tennant Creek and Mount Isaregions. Elsewhere in the world it has been documented in rockswhich have not even reached greenschist conditions.

The Sm-Nd dating technique, which has recently beendeveloped overseas, is currently being established in the jointANU-BMR laboratory. Like the U-Pb zircon method, this techniquecan also provide the primary crystallisation age of an igneousrock in a geolog1cally complex terrain. Provided the samplesare properly collected, metamorphism, and even alteration,weathering, erosion, and deposition as sediment apparently donot disturb the original Sm-Nd systematics. Consequently, thetechnique will become increasingly important in Australia. Itsmain limitation is that it can be applied only to relatively oldrocks - i.e., older than about 1000 m.y ..

21 .

Late Palaeo2oic ignimbrite volcanism and mineralisation

in northeastern Queensland

Speaker - B.3. Oversby

During Carboniferous and Permian time the Georgetown­Chi1lagoe district of northeastern Queensland was the site ofan extensive post-orogenic (or transitional) calc-alkalinevolcanic field (Newcastle Range-Featherbed Field). This wascharacterised by eruptions of voluminous, mainly rhyolitic,ignimbrites. Similar fields also occurred in northern CapeYork Peninsula and Torres Strait (Janet Ranges-Torres StraitField), and in the Bowen area (Bulgonunna Field). Ignimbriteeruptions in these fields were commonly accompanied by the developmentof subsidence structures which appear to have been representedat the surface by relatively gentle depressions. Structural formwas emphasised at depth as underlying magma chambers stopedtheir way upwards; further emphasis occurred as a consequence oflate-stage settling-in when magma chambers solidified. Some of thesubsidence structures are large, and irregular in plan; thesestructures, which are commonly composite, are regarded asvolcano-tectonic features. Smaller second-order features aremore-or-less circular to elongate in plan.

Uranium, tin, and porphyry-copper-like depos1tsare associated with late Palaeo20ic ignimbrites in northeasternQueensland, Undiscovered deposits, possible of large size, shouldoccur preferentially in the more complex and longlived parts ofsubsidence structures. One area in the postulated Featherbedvolcano-tec·tonic structure is considered to be especiallyprospective. However, most deposits will not be easy to find;their discovery will depend on fuller understanding of therelative importance of different subsidence structures, so thatthose with suitable charactetristics can be .'3elected for detailedexploration in preference to others. Fuller understanding canbe achieved in part by relating ignimbrite units to their sourceeruptive centres, and by elucidation of the structure of volcanicsequences at the surface, and at depth.

22.

Revised Proterozoic stratigraEhy and economic

potential of the western Georgetown Inlier,

north Queensland

Speaker - D.E. Mackenzie

Geological work by White & others in 1956-58 resultedin the division of the Pr~cambrian sequence of the westernGeorgetown Inlier i.nto fljur Proteroz:cic units Robertson Hivel"Metamorphics, and Berne;ker Creek, Etheridge, and Langdon RiverFormations, and a posp~bly Archaean basement (Einaslei~l

Metamorphics) .

More recent joint BMR-GSQ field work has shown thatall of the previously narn~d units form ~ continuous sequence,the Etheridge Group, which is made up er the Bernecker Creek,Robertson River, Townley, Helirnan, Cand~ow, Langdon RiverFormations and the Einasleigh Metamorphics. The RobertsonRiver Meta.morphics is the high-grade (eastern) part of themore extensive, dominantly pelitic Robertson River Formation.The Einasleigh Metamorphics contain higher grade equivalentsof the calcareous Bernecker Creek Formation and also grades intothe lowermost(?) part of the BJbertson River Formation, but thestratigraphy of the Einasleigh Metamorphics is incompletelyresolved. A marked angular unconformity separates the EtheridgeGrqup from the overlying Malacura and Yarman Formations (new units),which are in turn separated from the Proterozoic (previouslyconsidered to be Permian-Carboniferous) Croydon Volcanics -partly by faults, and partly by an angular unconformity. Eachunit is described briefly.

The Einasleigh Metamorphics contain stratabound Pb-Znand Cu deposits spatially related to a change from calcareous topelitic lithologies; th~(s the Bernecker Creek Formation­Robertson River FormatioLl contact, as well as calcareous partsof the latter unit, may also be prospective. Maximum developmentof volcanic rocks is in the middle Robertson River Formation,wheremetabasalts and metadolerites are locally associated withminor Cu-Ag-Au mineralisation. The overlying more quartzoseTownley Formation may contain some volcanic detritus, and, aswell as several Ag-Pb-Zn mines and prospects, there are signsof apparently stratigraphically controlled Pb-Zn geochemicalanoinalies. Parts of the Candlow Formation, a shallow-watersiltstone - fine sandstone unit, are very carbonaceous, also

commonly pyritic, and locally dolomitic, all reminiscent ofevaporiti.c conditions. The unit is very poorly ~xposed~

but several minor Pb-Zn gossans, generally assoclated wlthintraformational(7) breccias,are known.