by f. pirajno and n. g. adamides · (tm) and magnetic images. results of geological mapping were...

DEPARTMENT OF MINERALS AND ENERGY

GEOLOGICAL SURVEY OF WESTERN AUSTRALIA

REPORT60

GEOLOGY AND MINERALIZATIONOF THE PALAEOPROTEROZOIC

YERRIDA BASINWESTERN AUSTRALIA

by F. Pirajno and N. G. Adamides

GOVERNMENT OFWESTERN AUSTRALIA

GEOLOGICAL SURVEY OF WESTERN AUSTRALIA

REPORT 60

GEOLOGY AND MINERALIZATIONOF THE PALAEOPROTEROZOICYERRIDA BASIN, WESTERN AUSTRALIA

byF. Pirajno and N. G. Adamides

Perth 2000

MINISTER FOR MINESThe Hon. Norman Moore, MLC

DIRECTOR GENERALL. C. Ranford

DIRECTOR, GEOLOGICAL SURVEY OF WESTERN AUSTRALIADavid Blight

Copy editor: D. P. Reddy

REFERENCEThe recommended reference for this publication is:PIRAJNO, F., and ADAMIDES, N. G., 2000, Geology and mineralization of the Palaeoproterozoic Yerrida Basin, Western Australia:

Western Australia Geological Survey, Report 60, 43p.

National Library of AustraliaCataloguing-in-publication entry

Pirajno, Franco, 1939–.Geology and mineralization of the Palaeoproterozoic Yerrida Basin, Western Australia

Bibliography.ISBN 0 7309 6668 2

1. Geology, Structural — Western Australia — Yerrida Basin.2. Geology, Structural — Western Australia — Bryah Basin.3. Mines and mineral resources — Western Australia — Bryah Basin.4. Mines and mineral resources — Western Australia — Yerrida Basin.5. Yerrida Region (W.A.).6. Bryah Region (W.A.)I. Adamides, N. G.II. Geological Survey of Western AustraliaIII. Title. (Series: Report (Geological Survey of Western Australia); 60).

553.1099413

ISSN 0508–4741

Grid references in this publication refer to the Australian Geodetic Datum 1984 (AGD84)

Printed by Optima Press, Perth, Western Australia

Copies available from:Information CentreDepartment of Minerals and Energy100 Plain StreetEAST PERTH, WESTERN AUSTRALIA 6004Telephone: (08) 9222 3459 Facsimile: (08) 9222 3444www.dme.wa.gov.au

Cover photograph:Breakaway in a turbiditic arkose of the Doolgunna Formation, near the Goodin Find gold deposit.

iii

Contents

Abstract ................................................................................................................................................................. 1Introduction .......................................................................................................................................................... 2

Previous investigations ................................................................................................................................. 2Regional tectonic setting ...................................................................................................................................... 5Goodin Inlier ........................................................................................................................................................ 5Palaeoproterozoic Yerrida Group ........................................................................................................................ 6

Windplain Subgroup ..................................................................................................................................... 8Juderina Formation ................................................................................................................................ 8

Finlayson Member .......................................................................................................................... 9Bubble Well Member .................................................................................................................... 10Southwest contact of the Goodin Inlier with the Juderina Formation ......................................... 11

Johnson Cairn Formation ..................................................................................................................... 11Mooloogool Subgroup ................................................................................................................................ 12

Doolgunna Formation .......................................................................................................................... 12Thaduna Formation .............................................................................................................................. 14Killara Formation ................................................................................................................................. 16

Mafic intrusive rocks .................................................................................................................... 17Mafic extrusive rocks .................................................................................................................... 18Volcaniclastic rocks ...................................................................................................................... 19Bartle Member .............................................................................................................................. 19Geochemistry of the Killara Formation ........................................................................................ 21

Maraloou Formation ............................................................................................................................ 26Earaheedy Group ................................................................................................................................................ 29

Yelma Formation ........................................................................................................................................ 29Mount Leake Formation ............................................................................................................................. 32

Mafic dykes ........................................................................................................................................................ 32Quartz veins ........................................................................................................................................................ 33Metamorphism ................................................................................................................................................... 33Structure and deformation .................................................................................................................................. 33Mineralization .................................................................................................................................................... 34

Base metals .................................................................................................................................................. 34Precious metals ............................................................................................................................................ 37

Basin development and tectonic evolution ....................................................................................................... 37References .......................................................................................................................................................... 40

Appendix

Gazetteer of localities ......................................................................................................................................... 43

Plates

1. Interpreted geology of the Palaeoproterozoic Yerrida Basin2. Interpreted geology of the Palaeoproterozoic Yerrida Basin (cross sections)

Figures

1. Simplified geological map of the Bryah, Padbury, and Yerrida Basins ................................................... 32. New stratigraphy of the Yerrida Group, compared with previous stratigraphy of the

Glengarry Group ......................................................................................................................................... 43. Schematic relationships of stratigraphic units of the Yerrida Group ........................................................ 74. Summary log of drillhole QMW 83-1 ....................................................................................................... 75. Stratigraphy of the Windplain Subgroup on MARYMIA ............................................................................. 86. Stratigraphy of the Juderina Formation, near Mount Alice ...................................................................... 97. Quartz arenite with ripple marks from the Finlayson Member of the Juderina Formation .................... 108. Thick-bedded granite-derived quartz wacke of the Doolgunna Formation ............................................ 139. Rounded block of chert breccia in an unsorted matrix of diamictite of the Doolgunna Formation ...... 13

10. Soft-sediment deformation structures in rocks of the Thaduna Formation ............................................ 14

iv

11. Laminated turbiditic siltstone of the Thaduna Formation ....................................................................... 1512. Flame structures in lithic wacke of the Thaduna Formation ................................................................... 1513. Block of volcanic scoria in turbidite rocks of the Thaduna Formation .................................................. 1514. Idealized stratigraphy of the Killara and Maraloou Formations in drillhole KDD 1 ............................. 1615. Killara Formation mafic rocks ................................................................................................................. 1716. Bartle Member rocks ................................................................................................................................ 2017. Total alkali versus silica diagram for the Killara Formation .................................................................. 2518. Cationic plot showing predominant iron-rich tholeiite composition of the Killara Formation ............. 2519. Diagrams comparing the chemistry of the Narracoota and Killara Formations ..................................... 2520. Triangular total iron-– total alkali – magnesium oxide diagram ............................................................ 2621. Multi-element diagram of Killara Formation rocks and volcaniclastic rock, normalized to

N-type MORB .......................................................................................................................................... 2622. Multi-element diagram of Killara Formation and volcaniclastic rock, normalized to

continental crust ....................................................................................................................................... 2623. Chondrite-normalized rare earth element plot for dolerite (dark grey), tholeiitic basalt, and

volcaniclastic rock .................................................................................................................................... 2624. Discriminant triangular plot ..................................................................................................................... 2725. Chondrite-normalized rare earth element abundances in Bartle Member cherts and oceanic

hydrothermal water, and average of chondrite-normalized rare earth element abundances inchert layers of Indian banded iron-formations ........................................................................................ 27

26. Measured stratigraphic section at Mount Russell .................................................................................... 2827. Well-bedded siltstone of the Maraloou Formation ................................................................................. 2828. Mudcracks in thin-bedded siltstone of the Maraloou Formation ............................................................ 2929. Stratigraphy of the Yelma Formation ...................................................................................................... 3030. Stromatolite form Ephyaltes edingunnensis Grey 1994 in dolomitic rocks of the Yelma Formation ... 3131. Chert breccia derived from dolomite of the Yelma Formation ............................................................... 3132. Idealized section through drillholes GD-1, GD-2, and GD-3 ................................................................. 3633. Lead isotope plots showing: a) 207Pb/204Pb versus 206Pb/204Pb of Magellan ore and sulfide

samples from Pb–Zn prospects in the Yelma Formation; b) ore lead-isotope evolution curve ............. 3734. Tectonic setting for the Yerrida Basin and emplacement of the Killara Formation

continental tholeiite .................................................................................................................................. 3835. Model showing stages in the geodynamic evolution of the Yerrida Basin as a pull-apart structure ..... 39

Tables

1. Major- and trace-element analyses of Killara Formation rocks and mafic dykes in theGoodin Inlier ............................................................................................................................................ 22

2. Rare earth element analyses of dolerite and tholeiitic basalt of the Killara Formation ......................... 243. Whole-rock trace-element analyses of Bartle Member cherts ................................................................ 274. Trace-element analyses of gossans .......................................................................................................... 35

Digital data (in pocket)

Whole-rock geochemical analyses of Killara Formation rocks (killara.csv)

killara.csv

GEOCHEMICAL ANALYSES OF ROCKS FROM THE KILLARA FORMATION: For details of analytical techniques see text of Report 60,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,Sample,Easting,Northing,SiO2,TiO2,Al2O3,Fe2O3,FeO,MnO,MgO,CaO,Na2O,K2O,P2O5,S,LOI,TOTAL,Ag,As,Au,Ba,Bi,Cd,Ce,Co,Cr,Cu,Ga,Ge,La,Mo,Nb,Ni,Pb,Rb,Sb,Sc,Sn,Sr,Ta,Te,Th,U,V,W,Y,Zn,Zr,Pt,Pd,Ti,Hf,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er,Tm,Yb,Lu,Y,U,Th,Ta,Sc,Nb,La,Ce104309,699707,7091999,74.3,0.59,5.21,8.48,2.8,0.23,1.35,2.75,2.36,0.34,0.06,0.01,2.18,100.8,1,4,10,109,4,5,15,,4,44,8,3,12,2,7,9,4,5,5,,4,32,5,6,4,2,221,,14,38,59,,,,,,,,,,,,,,,,,,,,,,,,104311,698823,7088627,51.4,0.68,14.2,2.03,8.05,0.18,7.31,9.59,3.31,1.14,0.06,0.01,3.07,101.2,1,4,10,303,4,5,13,,78,146,14,3,8,2,7,87,5,47,4,,4,292,5,6,3,2,217,,17,70,71,,,,,,,,,,,,,,,,,,,,,,,,104312,699662,7089076,54,0.93,12.5,1.39,8.88,0.2,8.15,6.87,4.73,0.14,0.08,0.01,3.43,101.5,1,4,10,96,4,5,16,,76,117,12,3,8,2,7,58,5,4,4,,4,61,5,6,5,2,318,,17,89,86,,,,1.4,3.6,13.8,3.1,1.1,3.7,0.6,3.6,0.7,2.2,0.3,1.9,0.3,19.4,1.5,5.4,2.2,91.6,12.0,13.0,29.7104313,699655,7088614,53.3,0.98,13.4,1.83,8.7,0.17,5.91,8.35,4.74,0.48,0.08,0.01,3.08,101.2,1,4,10,396,4,5,23,,70,110,14,3,9,2,7,61,4,17,4,,4,284,5,6,6,2,335,,20,80,91,,,,,,,,,,,,,,,,,,,,,,,,104325,745758,7105369,52.2,1.21,13.2,1.53,10.1,0.18,5.95,9.57,2.51,0.59,0.13,0.07,2.8,100.2,1,4,10,336,4,5,21,,116,88,17,3,14,2,7,87,4,15,4,,4,174,5,6,2,2,275,,24,103,126,,,,2.1,4.9,19.1,4.3,1.6,4.9,0.9,4.9,0.9,2.8,0.4,2.6,0.4,23.9,0.5,2.5,1.1,72.8,11.2,19.6,40.9104327,748666,7104852,54,1.07,13.3,2.18,9.62,0.18,4.96,9.33,1.96,0.83,0.12,0.01,3.03,100.8,1,4,10,371,4,5,30,,66,84,17,3,15,2,7,50,6,20,4,,4,183,5,6,7,2,262,,22,91,121,,,,,,,,,,,,,,,,,,,,,,,,104329,747859,7106253,51.6,0.57,14.2,1.45,8.23,0.17,8.09,8.65,3.81,0.47,0.05,0.01,3.53,101.1,1,4,10,964,4,5,7,,143,111,8,3,5,2,7,89,4,13,4,,4,238,5,6,2,2,234,,13,74,42,,,,1.6,1.7,7.4,1.9,1.0,2.3,0.4,2.4,0.5,1.6,0.2,1.5,0.2,13.1,0.3,1.2,0.5,72.1,5.1,5.6,12.8104332,749422,7108071,47.1,0.65,14.5,1.18,7.17,0.15,6.62,18.3,0.46,0.33,0.06,0.01,4.05,100.7,1,4,20,328,4,5,8,,312,99,15,3,6,2,7,91,4,7,5,,4,27,5,6,2,2,232,,17,66,52,,,,,,,,,,,,,,,,,,,,,,,,104334,753276,7106149,51.3,1.16,12.7,1.32,11.1,0.2,6.15,11,1.68,0.39,0.11,0.25,2.71,100.1,1,4,10,142,4,5,20,,48,114,15,3,7,2,7,54,4,11,4,,4,148,5,6,3,2,314,,23,101,100,,,,1.3,3.7,15.8,3.7,1.3,4.7,0.8,4.5,0.9,2.7,0.4,2.4,0.4,21.8,1.0,2.5,1.3,81.7,11.5,13.2,29.7104340,788060,7095259,48.3,0.8,13.2,4.08,5.4,0.19,6.08,17.9,0.29,0.05,0.07,0.01,4.24,100.7,1,4,30,140,4,5,7,,211,117,14,3,5,2,7,77,4,2,4,,4,12,5,6,2,2,278,,17,51,59,,,,,,,,,,,,,,,,,,,,,,,,104342,791475,7098879,53.5,1.09,12.9,2,10.4,0.21,5.15,8.78,2.32,0.83,0.11,0.08,3.09,100.6,1,4,10,236,4,5,24,,16,115,16,3,9,2,8,44,8,23,4,,4,181,5,6,5,2,296,,25,98,117,,,,,,,,,,,,,,,,,,,,,,,,104345,794976,7106193,51.5,0.69,13.8,1.89,8.96,0.2,7.76,11.4,1.76,0.14,0.06,0.04,2.46,100.8,1,4,20,94,4,5,6,,134,125,13,3,5,2,7,83,4,2,4,,4,138,5,6,3,2,253,,16,79,52,,,,1.2,2.2,9.3,2.2,0.9,2.9,0.5,3.1,0.6,2.0,0.3,1.9,0.3,15.8,0.4,1.6,0.4,81.7,5.7,7.2,16.8104346,794976,7106193,51.4,0.68,13.8,2.66,8.01,0.18,7.86,10.5,2.15,0.12,0.06,0.01,3.13,100.7,1,4,20,229,4,5,6,,146,138,13,3,5,2,7,88,4,3,4,,4,166,5,6,2,2,251,,16,78,53,,,,,,,,,,,,,,,,,,,,,,,,104347,803377,7144814,51.9,0.64,13.6,2.25,7.89,0.17,8.25,11.1,1.72,0.61,0.06,0.03,2.75,101.1,1,4,10,148,4,5,11,,124,146,14,3,6,2,7,91,4,23,4,,4,147,5,6,3,2,224,,14,67,55,,,,,,,,,,,,,,,,,,,,,,,,104348,796687,7144966, , , ,68, ,0.1, , , , , , , , ,2,4,10,339,4,5,6,,36,55,5,3,8,2,7,42,4,2,4,,4,44,5,6,2,2,323,,12,284,28,,,,,,,,,,,,,,,,,,,,,,,,104349,795872,7145909,50.6,0.61,14.8,1.54,7.94,0.17,8.64,11.7,1.7,0.23,0.05,0.01,2.64,100.8,1,4,20,141,4,5,6,,112,129,12,3,5,2,7,118,4,5,4,,4,152,5,6,2,2,203,,13,64,46,,,,0.7,1.9,7.8,1.8,0.7,2.4,0.4,2.5,0.4,1.5,0.2,1.4,0.2,11.8,0.7,1.7,1.4,81.7,7.5,6.9,15.0104350,795872,7145909,49.9,0.6,14.6,2.18,7.34,0.18,8.34,11.8,1.81,0.22,0.05,0.01,3.47,100.6,1,4,10,169,4,5,11,,86,149,14,3,5,2,7,123,4,8,4,,4,174,5,6,2,2,208,,12,68,45,,,,,,,,,,,,,,,,,,,,,,,,104351,795056,7146851,53.5,1.11,12.7,1.88,10.3,0.21,5.33,8.54,2.44,1,0.11,0.1,3.61,101,1,4,10,372,4,5,18,,16,117,15,3,11,2,7,43,6,28,4,,4,198,5,6,5,2,294,,24,98,109,,,,,,,,,,,,,,,,,,,,,,,,104377,791772,7149696, , , ,72.9, ,0.1, , , , , , , , ,2,4,10,3324,4,5,24,,19,96,3,3,18,2,7,44,5,2,4,,4,54,5,6,2,2,174,,17,67,15,,,,,,,,,,,,,,,,,,,,,,,,104378,791772,7149696,51.8,0.95,13.7,2.82,9.05,0.2,7.22,10.6,1.59,0.26,0.09,0.01,3.06,101.5,1,4,20,342,4,5,10,,71,140,16,3,6,2,7,80,4,5,4,,4,114,5,6,3,2,284,,19,90,77,,,,,,,,,,,,,,,,,,,,,,,,104380,791772,7149696,51.7,1.05,13.2,1.88,10.4,0.2,6.38,10.2,1.74,0.33,0.1,0.17,3.02,100.5,2,4,80,136,4,5,14,,40,156,15,3,6,2,7,65,4,8,4,,4,132,5,6,3,2,281,,22,91,95,,,,,,,,,,,,,,,,,,,,,,,,109552,711350,7091198,,,,18.02,,0.02,,,,,,,,,7,7,10,981,4,5,6,,98,118,,3,6,9,7,111,8,2,4,,4,16,5,6,2,2,428,,8,204,63,,,,,,,,,,,,,,,,,,,,,,,,109553,711350,7091198,51.5,1.46,12.9,2.39,9.75,0.21,5.86,8.99,2.01,0.82,0.14,0.01,2.66,100,7,4,,484,4,5,27,,121,111,17,3,13,9,9,68,8,27,4,,4,177,5,6,4,2,359,,31,107,147,,,,,,,,,,,,,,,,,,,,,,,,109554,711380,7093045,56.4,0.98,12.9,1.32,7.55,0.22,5.22,6.82,5.56,0.45,0.09,0.13,1.42,99.9,6,4,,571,4,5,15,,74,115,10,3,7,7,7,52,6,10,4,,4,70,5,6,5,2,313,,19,64,89,,,,,,,,,,,,,,,,,,,,,,,,112685,747754,7150779,51.1,0.69,14.2,2.66,7.05,0.2,7.67,11.2,1.62,1.2,0.07,,,,1,

1

GSWA Report 60 Geology and mineralization of the Palaeoproterozoic Yerrida Basin

Geology and mineralization of thePalaeoproterozoic Yerrida Basin,

Western Australia

by

F. Pirajno and N. G. Adamides

AbstractThe Palaeoproterozoic Yerrida Basin is part of the Capricorn Orogen, a zone of low- to high-grademetamorphic rocks, magmatic belts, and low-grade volcano-sedimentary basins that were formed as aresult of the oblique collision between the Pilbara and Yilgarn Cratons at about 1.8 Ga. The YerridaBasin was probably formed at approximately 2.2 Ga and was affected by the Capricorn Orogeny. TheYerrida Basin has a faulted contact with the Bryah Basin in the west (Goodin Fault) and the MarymiaInlier in the north, and is unconformably overlain by rocks of the Earaheedy Basin in the east. Rocksthat have accumulated in the Yerrida Basin are assigned to the Yerrida Group (formerly part of theGlengarry Group).

The stratigraphy of the Yerrida Group is complicated by interdigitating relationships betweenthe component units. The Yerrida Group is divided into the Windplain and Mooloogool Subgroups.The Windplain Subgroup contains the Juderina and Johnson Cairn Formations, which include siliciclasticrocks, evaporites, argillites, and locally turbidites. The rocks of the Windplain Subgroup were depositedin a shallow epicontinental sea, locally with sabkha environments. The Mooloogool Subgroupwas deposited in a high-energy environment, probably in a widening rift structure, surrounded byuplifted Archaean rocks of the Marymia and Goodin Inliers. The Subgroup comprises four formations:the Doolgunna, Thaduna, Killara, and Maraloou Formations. The first two contain turbiditicrocks deposited in deepening rifts. The Killara Formation contains continental tholeiitic basaltsthat were erupted along northeasterly, easterly, and northwesterly trending structures, a few of whichcontain dolerite dykes. The Doolgunna, Thaduna, and Killara Formations interdigitate, suggesting thatsedimentation and volcanism were contemporaneous. The end of volcanism is marked by the depositionof chert units (Bartle Member), and sedimentological, textural, and petrographic data suggest that theywere formed in a playa lake environment, with hot springs. The final phase of rifting is represented bysulfidic shale and laminated siltstone of the Maraloou Formation, probably of lacustrine facies.

Known mineralization consists of shear zone-hosted copper deposits, quartz veins containing basemetals, black shale-hosted barium, copper, platinum-group elements, and a large lead–carbonatedeposit (Magellan) in the southeast. This lead deposit is present in the upper units of the JuderinaFormation and the lower units of the Yelma Formation (Earaheedy Group), which are preserved asscattered outliers in the region.

The geodynamic evolution of the Yerrida Basin is interpreted as having commenced as a pull-apartopening related to sinistral strike-slip faulting. Subsequent to this opening, a rift phase occurredduring which the continental tholeiitic lavas of the Killara Formation were erupted. The life of theYerrida Basin concluded with the formation of an intracontinental lake and the deposition of lacustrinesediments.

KEYWORDS: Palaeoproterozoic, Bryah Basin, Yerrida Basin, Earaheedy Basin, evaporites,stromatolites, tholeiite, basalt, rift basin, copper, lead, palladium, barium,mineralization, tectonics.

2

Pirajno and Adamides

IntroductionIn early 1994, fieldwork commenced on a project aimedat reappraising the geology and mineralization of thePalaeoproterozoic Glengarry Basin, as part of a programof new mapping initiatives by the Geological Survey ofWestern Australia (GSWA). The Glengarry Basin, asdefined by Gee and Grey (1993), was the western part ofthe greater Palaeoproterozoic Nabberu Province, which inthe east included the Earaheedy Basin (Bunting et al.,1977; Hall and Goode, 1978; Gee, 1990). This projectresulted in the recognition that the Glengarry Basin ofGee and Grey (1993) consists of three tectonic units: theBryah, Padbury, and Yerrida Basins (Fig. 1; Plate 1). As aresult, the volcano-sedimentary rocks of the GlengarryGroup are now divided into the Bryah and Yerrida Groups(Fig. 2), characterized not only by different lithologies, butalso by different regional structures, metamorphism, andmineral deposit types. Sediments and volcanic materialthat accumulated in the Yerrida Basin have been assignedto the Yerrida Group, which is discussed in detail below.The Yerrida Group is unconformably overlain by theEaraheedy Group of the Earaheedy Basin.

Work in the Yerrida Basin involved 1:25 000-scalemapping to produce 1:100 000 geological series mapsheets. Geological mapping was carried out using colouraerial photographs, available from the Department of LandAdministration (DOLA), and Landsat Thematic Mapper(TM) and magnetic images. Results of geological mappingwere integrated with petrographic and geochemicalstudies. During this work a total of 648 rock or coresamples were collected, of which 350 were made into thinand polished sections and 126 geochemically analysed. Inaddition, logging of 1000 m of diamond drillcore and visitsto prospects and old mines enhanced the knowledge ofthe geology of the area. Most of the mapped areaswere also included in a regional geochemical samplingprogram that covered the 1:250 000 PEAK HILL* andGLENGARRY sheets (Subramanya et al., 1995; Crawfordet al., 1996). Published geological maps at 1:100 000 scaleand accompanying Explanatory Notes that include partsof the Yerrida Basin comprise DOOLGUNNA (Adamides,1998), GLENGARRY (Pirajno et al., 1998a), MOOLOOGOOL(Pirajno et al., 1998b), MOUNT BARTLE (Dawes and Pirajno,1998), THADUNA (Pirajno and Adamides, 1998), CUNYU(Adamides et al., 1999), MARYMIA (Bagas, 1998), WILUNA(Langford and Liu, 1997), and MEREWETHER (Ferdinandoand Tetlaw, in prep.). The layout of these map sheets inrelation to the Yerrida Basin and adjacent tectonic units isshown in Figure 1.

Interim accounts of the tectonic evolution of the YerridaBasin and its structural and stratigraphic relations with theadjacent Bryah and Padbury Basins have been reported byPirajno et al. (1995, 1996) and Occhipinti et al. (1997).Models of the tectonic evolution of these basins have beendiscussed by Pirajno (1996) and Pirajno et al. (1998c).

In this Report the geology, stratigraphy, volcanicgeochemistry, and mineralization of the Yerrida Basin arediscussed. The Report concludes with a model of thegeodynamic evolution of the Yerrida Basin, based on thefield, geochemical, petrographic, and geophysical datapresented in this work.

Previous investigationsThe earliest account of the geology of the Yerrida Basinwas that of Talbot (1920, 1928) who examined thephysiography and regional geology of part of WesternAustralia between 1912 and 1914, over an area of180 000 km2. The geology was broadly subdivided intoareas of granite, greenstone, and younger sedimentaryunits. The Proterozoic sedimentary rocks between Wilunaand the Glengarry Range were collectively included in theNullagine Beds, which Talbot extended south fromNullagine and the Throssell Range. Talbot (1920, 1928)also provided detailed geological descriptions of rock unitsin the various areas, supplemented by petrographic workby R. A. Farquharson.

Sofoulis and Mabbutt (1963) described the geology ofthe same region and first recognized the unconformitybetween the ‘Nabberu Basin’ and Archaean basement.They separated the Archaean metamorphic belts into alower greenstone and an upper whitestone phase, whereasthe Proterozoic succession was collectively included in theNullagine system. Sofoulis and Mabbutt (1963) describedthe arenite outcrops in the area of Mount Alice, north ofWiluna, and concluded that the sediments were laid downin fairly shallow basins in the Archaean basement. Theboundaries of Sofoulis and Mabbutt (1963) wereextrapolated north by Horwitz (1966) on the 1:2 500 000State geological map. Horwitz (1966) considered the rockspresently included in the Earaheedy Group to be the oldestProterozoic sedimentary rocks, with the Glengarry Group†

being transgressive onto them, and equated with the Wylooand Manganese Groups of the eastern Pilbara region.However, he correctly mapped the unconformity betweenthe Bangemall Group rocks and the older units. Thegranite of the Goodin and Marymia Inliers was thoughtto intrude the Proterozoic sequence.

The interpretations of Sofoulis and Mabbutt (1963)remained on State geological maps until the work ofMacLeod (1970), who produced the first geological mapof the PEAK HILL 1:250 000 sheet. MacLeod (1970)speculated an Archaean age for the sedimentary unitsbetween the Goodin and Marymia Inliers. This was doneon the basis of lithological and structural similarities toYilgarn Craton clastic–volcanic sequences, and thepresence of gold mineralization and banded iron-formation (BIF). A Lower Proterozoic age, however, wasnot discounted, with the sedimentary rocks (e.g. BIF)tentatively being correlated with the Mount BruceSupergroup in the Ashburton region. MacLeod (1970)considered the structure of the Peak Hill area as a majorsynclinorium, with the granite of the Goodin and MarymiaInliers enclosing the Archaean sedimentary sequence(Peak Hill Schist and part of the Yerrida Group). The maficrocks of the Narracoota Formation (Bryah Group; Pirajno

* Capitalized names refer to standard 1:100 000 map sheets, unless otherwiseindicated.

† In this section we use the terminology established by previous workers;thus Glengarry Group or Basin collectively refer to the Bryah, Padbury,and Yerrida Groups or Basins.

3


Figure 1. Simplified geological map of the Bryah, Padbury, and Yerrida Basins (after Pirajno et al., 1998c)

MARYMIAINLIER Je

nkin

Fault

Goodi

n

Fault

119˚30'

Killara

Narracoota

30 km

Yandil

119˚30'

31.01.00

PEAK HILL

GLENGARRY

GOODININLIER

Merrie

Range

Fa

ultMurchisonFau

lt

25˚30'

26˚00'

26˚30'

NABBERU

WILUNA

118˚

30'

CAPRICORN OROGEN

YILGARNCRATON

Padbury, Bryah,and

Yerrida BasinsPILBARACRATON

120˚

30'

Three Rivers

ThadunaDoolgunna

Mount BartleMooloogoolGlengarry

Gabanintha Yanganoo Merewether

Cunyu

Merrie

Wiluna

Bryah

Marymia Fairbairn

FMP296a

Fault

Geological boundary

Microgabbro dyke

Bouguer gravity anomaly

Peak Hill Schist

YE

RR

IDA

GR

OU

P

Karalundi Formation

Padbury Group

Horseshoe and Ravelstone Formations

Narracoota Formation: mafic and ultramafic

Earaheedy Group

Homestead

schist, and metabasaltic hyaloclastite

Juderina and Johnson Cairn Formations

Maraloou Formation

Doolgunna and Thaduna Formationswith intercalated Killara Formation

WindplainSubgroup

MooloogoolSubgroup

Intercalated Killara andMaraloou Formations

GR

OU

PB

RYA

H

PALAEOPROTEROZOIC ARCHAEAN

Granite–greenstonebasement

4


et al., 2000) were similarly assigned an Archaean age andthought to intrude the sedimentary units. The sedimentaryrocks around Thaduna mine and to the south wereassigned to the Bangemall Group. Later, Horwitz (1975)assigned a Lower Proterozoic age to the whole of theGlengarry Group and correlated these rocks with similarlithologies in the Earaheedy Basin and, farther north, withthe Wyloo and Manganese Groups.

The work of Hall and Goode (1975, 1978) wasinstrumental in defining the Nabberu Basin. This basincontained the Earaheedy, Glengarry, and Padbury Sub-basins. Their geological map (Hall and Goode, 1978,fig. 2) presented the currently accepted tectonic andlithological framework, that is, the unconformablerelationships between the Archaean granitoids and latersedimentary units, and between the sedimentary units ofthe Glengarry and Bangemall Groups. Most of thesedimentary units of the Glengarry and Earaheedy Groups,with the exception of the Peak Hill Beds (later named thePeak Hill Metamorphics by Gee, 1979), are named in thiswork.

Figure 2. New stratigraphy of the Yerrida Group, compared with previous stratigraphy of the Glengarry Group

Gee (1987)P

AD

BU

RY

BA

SIN

/GR

OU

PG

LEN

GA

RR

Y B

AS

IN/G

RO

UP

Millidie Creek Formation

Robinson Range Formation

Wilthorpe Formation

Labouchere Formation

Horseshoe Formation

Narracoota Volcanics

Karalundi Formation

Doolgunna Formation

Johnson Cairn Shale

Thaduna Greywacke

Juderina Formation

Maraloou Formation

Crispin Conglomerate

Finlayson Sandstone

Peak Hill Metamorphics

GLE

NG

AR

RY

BA

SIN

/GR

OU

P

Maraloou Formation

Thaduna Greywacke

Narracoota Volcanics

Doolgunna Formation

Johnson Cairn Shale

Juderina Formation(Finlayson SandstoneMember)

*YE

RR

IDA

BA

SIN

/GR

OU

P

*Win

dpla

in S

ubgr

oup

*Moo

loog

ool S

ubgr

oup

Peak Hill Schist

*BR

YA

H B

AS

IN/G

RO

UP

*PA

DB

UR

Y B

AS

IN/G

RO

UP

TE

CT

ON

IC C

ON

TA

CT

Millidie Creek Formation

Wilthorpe Formation

Labouchere Formation

Horseshoe Formation

Karalundi Formation

*Ravelstone Formation

*Narracoota FormationMaraloou Formation

*Killarra Formation(*Bartle Member)

Doolgunna Formation

Thaduna Formation

Juderina Formation

FMP409 31.01.00

Gee and Grey (1993) Occhipinti et al. (1997); Pirajno et al. (1998c; this study)

Robinson RangeFormation

Unconformity

Unconformity

Johnson CairnFormation

NOTE: *New or redefined units (Occhipinti et al., 1997)

(Finlayson and BubbleWell Members)

ARCHAEAN BASEMENT

Bunting et al. (1977) suggested that there was anunconformity between a younger Earaheedy Group to theeast and an older Glengarry Group to the west, and thatthe Glengarry and Earaheedy Sub-basins should beelevated to basin status. Horwitz and Smith (1978)considered, on the basis of the presence of a common dykeswarm and the distribution of stresses in the Yilgarn andPilbara Cratons, that prior to the earliest sediment-ation, both Archaean blocks were part of a single craton.The authors also attributed a deep-water origin to theturbidite sequences of that part of the Glengarry Basin,now referred to as the Yerrida Basin, and described thepresence of olistostromes in this unit. They correlatedsome of these olistostromes with the Frere Formation ofthe Earaheedy Group, thus considering this group to beolder than the Glengarry Group.

The GSWA began systematic 1:250 000-scale mappingin 1975. The results were published in a series of papersand Explanatory Notes by Bunting et al. (1977), Gee(1979, 1987; PEAK HILL), Elias et al. (1982; GLENGARRY),Elias and Williams (1980; ROBINSON RANGE), and Elias and

5


The Capricorn Orogen is situated between the Pilbaraand Yilgarn Cratons, and can be traced for more than1000 km with northwesterly to westerly trends, forminga broad belt of deformed, low-grade volcano-sedimentarybelts, low- to high-grade metamorphic belts, and granitoidintrusions (Fig. 1). The Capricorn Orogeny occurred atc. 1.8 Ga and was the result of the collision between thePilbara and the Yilgarn Cratons (Occhipinti et al., 1999;Tyler, 1999). It involved the closure of an interveningocean, formation of a back-arc basin, and possibly theaccretion of microcontinental fragments (Tyler andThorne, 1990; Myers, 1993; Myers et al., 1996; Tyleret al., 1998; Occhipinti et al., 1999). Other tectonic units,such as the Archaean Narryer Terrane, Marymia Inlier,Sylvania Inlier, and parts of the Hamersley Basin, werealso affected by the Yilgarn–Pilbara collision (Tyler andThorne, 1990; Myers et al., 1996; Tyler et al., 1998). TheYerrida Basin may have formed, at approximately 2.2 Ga,as a pull-apart structure related to the development ofeasterly trending strike-slip movements and regionalsinistral faults. Alternatively, the basin may have beenformed as a passive rifted continental margin. These issuesare examined more fully in Basin development andtectonic evolution. The convergence between the Pilbaraand Yilgarn Cratons between 1.84 and 1.78 Ga (Tyler,1999) resulted in the deformation of the western andnorthern margins of the Yerrida Basin.

The Palaeoproterozoic sedimentary and volcano-sedimentary successions are unconformable on thenorthern margin of the Yilgarn Craton, whereas to thenorth and northwest they are either unconformablyoverlain by or faulted against rocks of the Bangemall andOfficer Basins and Archaean granitic inliers. Thesegranitic inliers comprise the Marymia Inlier in contactwith the Bryah, Yerrida, and Earaheedy Basins; theGoodin Inlier enclosed within the Yerrida Basin; and theMalmac Inlier, about 300 km to the east on the northernside of the Earaheedy Basin. The Goodin and MarymiaInliers partly controlled the sedimentation of theDoolgunna and Thaduna Formations of the Yerrida Group.

Goodin InlierThe Goodin Inlier (AgGl) is a fragment of Archaeangranitic basement, approximately 30 × 45 km, in thewestern part of the Yerrida Basin (Plate 1). The rocks ofthe inlier are unconformably overlain by the basal unitsof the Yerrida Group. The basal unconformity is wellexposed in the northwestern and southeastern parts of theinlier (Plate 1), where the granite is overlain by quartzarenite of the Juderina Formation (Finlayson Member).However, contacts in the southwest are complicated bytectonic slices of quartz arenite within the granite. Theinlier played a significant role in the evolution of theYerrida Basin.

It is not clear whether the Goodin Inlier is structurallypart of the Murchison Province, Southern Cross Provinceor Eastern Goldfields Province of the Yilgarn Craton. East-southeasterly trending dykes that intruded the inlier mayeither be part of the east-northeasterly trending dykeswarm in the Murchison Province (Myers and Hocking,

Bunting (1982; WILUNA). Gee (1990) presented a synthesisof the whole of the Nabberu Basin (which includes theBryah, Padbury, Yerrida, and Earaheedy Basins), whereasBunting (1986) integrated all investigations for theEaraheedy Basin.

The stratigraphy of the Glengarry Group was revisedand formalized by Gee (1979, 1987). Gee and Grey (1993)eventually elevated the Glengarry Sub-basin to basinstatus (see above). The new basin was defined toinclude the Padbury Sub-basin and the supersededGlengarry Sub-basin. The Earaheedy Sub-basin was alsoelevated to basin status. Gee and Grey (1993) alsodescribed the stratigraphy and structure of the GlengarryBasin, with emphasis on evaporites and stromatolites ofthe Juderina Formation.

Windh (1992) worked on the tectonics and metallo-genesis of the Glengarry Basin. This work envisaged thepresence of syndepositional faults during the formation ofthe Glengarry Basin as an intracratonic rift at about1.9 Ga. The intersection of the northward extension of theMurchison greenstone belt and the northward extensionsof the Wiluna greenstone belt was considered to be a locusof voluminous magmatism. Basin closure involved north–south crustal shortening (D1), tight folding, reversefaulting, and development of penetrative cleavage.

Grey (1994a) described new stromatolite taxa from theGlengarry Group. These are considered diagnostic of thisgroup and allow the Glengarry Group to be differentiatedfrom lithologically similar units of the Earaheedy Group.

In the early stages of the present study, Pirajno et al.(1996) subdivided the Glengarry Basin into three distinctbasins: the Padbury Basin in the west, Bryah Basin in thecentre, and Yerrida Basin in the east (Fig. 1).

Interim conclusions from the recent remapping of theYerrida Basin were presented by Pirajno and Occhipinti(1995), Pirajno and Davy (1996), Pirajno (1996), Pirajnoet al. (1996), Occhipinti et al. (1997), Pirajno and Preston(1998), Pirajno et al. (1998c) and Tyler et al. (1998); andin maps and Explanatory Notes by Adamides (1998),Bagas (1998), Dawes and Pirajno (1998), Pirajno andAdamides (1998), Pirajno and Occhipinti (1998), Pirajnoet al. (1998a,b,c), and Adamides et al. (1999). Krapez andMartin (1999) investigated the rift basins of the NabberuProvince using concepts of sequence stratigraphy.However, we disagree with the recently published modelof Krapez and Martin (1999) for the Yerrida Basin, whichis based on correlations and stratigraphic relationships thathave already been shown to be invalid (Occhipinti et al.,1997; Pirajno et al., 1995, 1996, 1998c).

Regional tectonic settingThe Yerrida Basin is situated along the northeastern marginof the Archaean Yilgarn Craton, and separates the Bryahand Padbury Basins in the west from the Earaheedy Basinto the east. Together these basins extend for approximately650 km in east–west to east-southeasterly directions,forming the southern part of the Capricorn Orogen(Fig. 1).

6


1998), or of the easterly trending swarms of the northernYilgarn Craton, some of which cut through rocks of theYerrida Group (Plate 1). In the former case, the inlierwould have undergone substantial rotation after theemplacement of the dykes; in the latter case, only a smallamount of rotation is implied. Mafic dykes are discussedfurther in Mafic dykes.

Granitic outcrops of the Goodin Inlier are commonlyweathered, with low breakaways and kaolinitized materialon the walls, and fresher rock on the floors. The bestgranitoid outcrops are around Utahlarba Spring andGranite Bore. One sample collected from the Granite Borearea was dated using the sensitive high-resolution ionmicroprobe (SHRIMP), yielding a zircon crystallizationage of 2624 ± 8 Ma (Nelson, 1997; Plate 1).

The inlier dominantly consists of medium-grainedmonzogranite, with average grain size of around 4–5 mm.Porphyritic varieties, with feldspar exceeding 8 mm insize, are locally present, as are micropegmatitic and apliticphases. The monzogranite contains K-feldspar (12 to29 vol.%), plagioclase (12 to 26 vol.%), quartz (26 to46 vol.%), and biotite (up to 9 vol.%) as main minerals.Accessory phases include epidote, muscovite, sericite,rutile, apatite, and zircon. In places, fluorite and allanitemay be present. Fluorite is a late-forming euhedral phasealong grain boundaries or infilling microfractures; allaniteis euhedral and typically zoned. Quartz is present insubhedral aggregates interstitial to the plagioclase, orforms recrystallized polygonal mosaics replacing thefeldspars. In the QAPF classification of Streickeisen(1976), these samples plot in the field of monzograniteto syenogranite.

With the exception of the tectonic contact at thesouthwestern margin of the Goodin Inlier and localdevelopment of foliation, the monzogranite is undeformed.Deformed varieties are characterized by the developmentof a foliated sericitic matrix enclosing lenticular feldsparand polycrystalline quartz, and complete destruction ofplagioclase and partial preservation of K-feldspar.

Near the mafic dykes, contact metamorphism of themonzogranite is characterized by strongly sericitizedfeldspar, locally resulting in the complete replacement ofthe mineral by fine-grained sericite. The feldspars arecommonly surrounded by micrographic reaction rims.

With the exception of a small outcrop of amphibolitein the northeastern parts of the inlier, no greenstone rocksare present. The amphibolite has a doleritic texture andcontains quartz, epidote, chlorite, and clinozoisitereplacing plagioclase, associated with skeletal crystals ofilmenite altered to titanite, and actinolite after clino-pyroxene. Micrographic intergrowths of quartz andplagioclase are interstitial to the other silicates, with theplagioclase altered to a mixture of euhedral clinozoisiteand quartz.

The Goodin Inlier influenced sedimentation patternsin the Yerrida Basin. As explained more fully below, theinlier is surrounded by clastic sedimentary rocks(Doolgunna and Thaduna Formations), whose provenanceindicate both granitic and sedimentary sources. The

sedimentary component includes stromatolitic cherts andevaporites, most probably sourced from the JuderinaFormation. This suggests that the inlier was uplifted afterthe deposition of the Juderina Formation and stripped ofits sedimentary cover (Fig. 1).

Palaeoproterozoic YerridaGroup

The Yerrida Group outcrops in the eastern and south-eastern parts of the former Glengarry Basin of Gee andGrey (1993), and includes siliciclastic rocks, carbonatesedimentary rocks, and mafic volcanic and hypabyssalrocks. The group unconformably overlies, or is in tectoniccontact with, Archaean granite–greenstone rocks of theYilgarn Craton and the Marymia and Goodin Inliers(Fig. 1, Plate 1). Its northwestern boundary with the BryahGroup is marked by the Goodin Fault. In the easternYerrida Basin, the Yerrida Group is unconformablyoverlain by the Palaeoproterozoic Earaheedy Group(Fig. 1, Plate 1).

The age of the Yerrida Group is constrained betweenc. 2.2 and c. 1.9 Ga. A Pb–Pb isochron of 2173 ± 64 Mawas obtained from stromatolitic carbonate rocks of theBubble Well Member (Windplain Subgroup in the lowerpart of the Yerrida Group; Fig. 2, Plate 1; Woodhead andHergt, 1997), which gives a depositional age for thestromatolitic carbonate rocks. In addition, Russell (1992)recorded anomalously high d13Ccarb of between +9 and+9.36 per mL for a carbonate from near the base of theBubble Well Member. These values are consistent with amajor positive d13Ccarb shift at 2200 ± 100 Ma in theFennoscandian Shield and elsewhere (Melezhik et al.,1997). Pb–Pb dating of stromatolitic carbonate rocksfrom the Yelma Formation gave ages of 2008 ± 68 Ma and1946 ± 71 Ma (Russell et al., 1994), whereas studies ofstromatolite taxa from the unconformably overlyingcarbonate rocks of the Yelma Formation (EaraheedyGroup) suggest ages of between 1.9 and 1.8 Ga (Grey,1994a).

The Yerrida Group is divided into the Windplain andMooloogool Subgroups, each the product of differenttectonic settings (Pirajno et al., 1995). Lithostratigraphicrelationships between units of these two subgroups areschematically shown in Figure 3. The Windplain Subgroupcontains the Juderina and Johnson Cairn Formations.The sedimentological characteristics are indicative of asag basin or pre-rift depression (Pirajno et al., 1995,1996). The Juderina Formation consists of siliciclastic,carbonate, and evaporite rocks and contains the Finlaysonand Bubble Well Members. Importantly, no basalconglomerates are present. The Finlayson Memberconsists of a thin (

7


and Marymia Inliers, and the Yilgarn Craton. Thisformation is conformably overlain by the Johnson CairnFormation, which is dominantly composed of argillaceousrocks.

Field and sedimentological evidence indicate that thesiliciclastic and evaporitic rocks of the WindplainSubgroup were deposited in supratidal and intertidalenvironments. It is also important to note that noconglomerates are present in the basal units and that themature quartz arenites of the Juderina Formation restunconformably on Archaean basement. This suggests thatthe Juderina sediments were sourced from a peneplainedarea. The unconformity between the basal WindplainSubgroup and the Archaean basement is essentiallyundisturbed along the southern and eastern margins of theYerrida Basin, but highly tectonized along the northernand western contacts with the Marymia Inlier and theBryah Group. A stratigraphic column of the WindplainSubgroup, derived from drillcore in the western part ofthe Yerrida Basin, is shown in Figure 4.

The Mooloogool Subgroup conformably overlies theWindplain Subgroup and was deposited in a rift-basin

setting (Pirajno et al., 1995, 1996). The MooloogoolSubgroup is divided into the Thaduna, Doolgunna, Killara,and Maraloou Formations. The sedimentary rocks ofthe Thaduna and Doolgunna Formations (conglomeratesand turbidite-facies rocks) were deposited in a high-energy setting and, as such, these rocks herald an abruptchange from the shallow, lower energy setting of theWindplain Subgroup. The distribution of these twoformations is centered around the Goodin Inlier, in a broadnortheasterly trending belt (Fig. 1; Plate 1). The Thadunaand Doolgunna Formations were largely sourced from the

Earaheedy Group

11.01.00

Maraloou Formation

Killara Formation

FMP187a

Bartle Member

Doolgunna Formation

Thaduna Formation

Johnson Cairn Formation

Geological boundaryFaultUnconformity

Juderina Formation

Finlayson Member

Bubble Well Member

Archaean basement

Figure 3. Schematic relationships of stratigraphic units ofthe Yerrida Group (vertical length reflects relativethicknesses)

Figure 4. Summary log of drillhole QMW 83-1 and details ofthe stratigraphy of the Juderina Formation (afterPirajno and Adamides, 1998; width of columnreflects relative resistance to weathering)

surface

Regolith

Red-brown siltstone

Altered mafic rock

Massive siltstone

(m)

100

200

300

372

JuderinaF

ormation

Finlayson

Mem

ber

Johnson Cairn

Form

ation

Archaean

greenstonebasem

ent

FMP288 31.01.00

EOH

0

Bubble W

ellM

ember

Black shale; locally contains pyrite

Massive arenite; shale interbeds

Pink and white dolomite

Interbedded purple siltstone and arenite

Unconformity(breccia zone)

Pyroxenite and peridotite

Microbial laminites and stromatolitic dolomite

8


Goodin and Marymia Inliers and they interdigitate,testifying to complex lateral-facies changes. TheDoolgunna Formation is a succession of conglomerates,turbidite-facies rocks, and diamictite units, the latter beingthe result of mass-wasting sourced from rocks overlyingand including the Goodin Inlier. These sedimentsaccumulated in a northeasterly trending graben-likestructure (Doolgunna graben; Pirajno, 1996; Pirajno andOcchipinti, 1998) on the eastern side of the Goodin Fault(Fig. 1). The Thaduna Formation is typically a turbiditicsuccession dominated by coarse- to fine-grained wackes,containing lithic fragments of volcanic rocks, shale, andsiltstone. Petrographic studies show that some of thesevolcanic rocks were sourced from Archaean greenstoneswithin the Marymia Inlier, and others from the Killara andNarracoota Formations. The Doolgunna and ThadunaFormations also interdigitate with the tholeiitic volcanicand intrusive rocks of the Killara Formation, indicatingthat volcanism and clastic sedimentation were concurrentin rift-related depocentres (Mooloogool rift; Pirajno,1996). In the east and southeast, tholeiitic rocks of theKillara Formation overlie and intrude the WindplainSubgroup, and are overlain by the Maraloou Formation.In the east, mafic sills of the Killara Formation intruderocks of the Juderina Formation.

The Maraloou Formation includes carbonaceousargillite, marl, dolostone, and minor chert, and itsdeposition represents a marked deepening of the basin.In the west, the contact between the Maraloou and KillaraFormations is transitional over a stratigraphic thick-ness of approximately 150 m, with the volcanic Killaracomponent consistently decreasing with stratigraphicheight. Peperite margins in the mafic lavas and sills of theKillara Formation indicate that the mafic magmas wereemplaced into or onto (or both) unconsolidated wetsediments of the Maraloou Formation. In the east,however, the Maraloou Formation is unconformable on theKillara Formation and conformable over the Thaduna andDoolgunna Formations. This may reflect onlap due tocontemporaneous block faulting in the eastern areas.

Windplain Subgroup

Juderina Formation

The Juderina Formation (Occhipinti et al., 1997) is apredominantly shallow water sequence of quartzsandstone, with lesser amounts of siltstone, chert breccia,and conglomerate, at the base of the Yerrida Group (Figs 2and 3). The formation is exposed around the margins ofthe Yerrida Basin, but is inferred to be present throughout,forming the floor of the basin. The thickness of theJuderina Formation varies considerably throughout theYerrida Basin. At the type locality, 3.5 km north ofJuderina Bore on DOOLGUNNA, it is only 30 m thick (Gee,1979), but in reference sections on MARYMIA andMOOLOOGOOL, it is between 300 and 2000 m thick (Figs 4and 5).

The Juderina Formation is in faulted or unconformablecontact with Archaean basement rocks, and conformablyoverlain by the Johnson Cairn Formation, with the contact

Figure 5. Stratigraphy of the WindplainSubgroup on MARYMIA (afterBagas, 1998; width of columnreflects relative resistance toweathering)

Win

dpla

in S

ubgr

oup

Moo

loog

ool

Sub

grou

p

John

son

Cai

rn F

orm

atio

n

2000

1500

1000

500

0

Conglomerate

11.01.00

Tha

duna

For

mat

ion

Jude

rina

For

mat

ion

YE

RR

IDA

GR

OU

P

LB66

Sandstone, sandy siltstone,

Greywacke, siltstone, and shale

Siltstone, shale, and sandy siltstone

siltstone, and shale

(m)

(GSWA730880)

Chert and chert breccia (silcrete)

9


defined by the top of the last quartz arenite unit. Theunconformity at the base of the Juderina Formation hasnot been observed in the eastern parts of the Yerrida Basin.However, Elias and Bunting (1982) described theunconformity 2 km west of Lanagan Bore on WILUNA.They reported an 8 m-thick lens of conglomerate andpebbly arenite, passing laterally and vertically into coarse-grained quartz arenite. Pebbles in the conglomerate consistmainly of vein quartz, with minor banded chert,presumably derived from the Archaean basement.

Elsewhere on WILUNA, the unconformity is marked bya band of thinly bedded granule conglomerate. Clastsconsist of quartz and jasper set in a matrix of poorly sortedand weakly kaolinitic quartz sandstone. The basal areniteunits are flaggy bedded and ripple marked. A generalizedsection through the Juderina Formation in the area 5 to10 km west and southwest of Mount Alice is shown inFigure 6.

The basal units of the Juderina Formation arecommonly mature quartz arenite and subordinate quartzsiltstone. These show sedimentary structures (ripplemarks, herringbone cross-laminations) indicative ofshallow-water sedimentation. Units that exhibit thesesedimentological characteristics are referred to as the

Finlayson Member. The Bubble Well Member is adistinctive unit of stromatolitic chert and chert breccia inthe middle of the formation. Both are discussed more fullybelow. In the eastern parts of the Yerrida Basin, theformation has been considerably thickened by theintrusion of dolerite sills.

Higher in the sequence, the Juderina Formationbecomes increasingly thicker bedded, and dominated bygrey quartz arenite, commonly with a purple hematiticsurface colouration. Sand grains in the upper quartz areniteare commonly more angular, and cross-bedding and ripplemarks are not as well developed as in the basal units. Thearenite is composed of packed aggregates of 0.3 – 0.4 mm,rounded to subrounded quartz grains with subordinateinterstitial kaolinite, and traces of rutile, zircon, andtourmaline. Approximately 2.5 km northwest of GoosieBore, the arenite contains nodules up to 10 cm in diameter,or tabular bodies (up to 30 cm long) with well-developedcubic voids, probably after halite.

Quartz arenite of the Juderina Formation is typicallyfine to medium grained and moderately sorted. Ferrug-inous variants contain white mica within the clay matrixand are associated with platelets of hematite. Siltstonelaminae are a few millimetres thick and comprise angularquartz averaging 0.1 mm in diameter enclosed in ahematitic matrix. The argillaceous laminae contain finedisseminations of iron hydroxides.

Thick-bedded quartz arenites, southeast of theGlengarry Range, form hills and ridges between swalesof siltstones and shales. Quartz arenite is medium grained,with angular grains, intraclast moulds, and abundant mudclasts. It is silica cemented and poorly sorted, and formsbeds commonly more than 1 m thick. Thicker beds havemassive bases, with laminations near the top, whereasthinner arenite units interbedded with the siltstones areparallel laminated. The quartz arenite contains localkaolinitized feldspars and sparse lithic clasts. Laminatedsiltstone contains sparse feldspar and weathered muscovitewith minor detrital tourmaline in a matrix of quartz andkaolinitic clays.

Shale and quartz siltstone form up to 30% of theJuderina Formation, but are commonly recessive. Thesiltstone varies from white to light grey and is typicallyfinely laminated. Siltstone consists of quartz grains(around 0.05 mm in diameter) in a matrix of very poorlycrystalline illite and kaolinite-group clays. The quartzshows irregular crystal boundaries and is partly diffusedinto the clay matrix. Local white mica is altered alonggrain margins to clays. Fine-grained cherty types arecharacterized by very finely crystalline interstitial clays,which display preferred orientation parallel to lamination,probably as a result of diagenetic compaction.

Finlayson Member

The Finlayson Member is at the base of the JuderinaFormation, close to the unconformity with the Archaeanbasement. The Finlayson Member is typically a cream-coloured, silica-cemented quartz arenite formingindividual beds a few centimetres to 1 m thick. Both

Figure 6. Stratigraphy of the Juderina Formation, near MountAlice (width of column reflects relative resistanceto weathering)

Cunyu Sill (dolerite)

~100 m

algal laminites

arenite interbed

BASEMENT

NGA97 11.01.00

Cross-bedded quartz areniteRipple-marked quartz arenite

Fin

lays

onM

embe

r

Up to200 m

Bub

ble

Wel

lM

embe

r

Up to150 m

Up to50 m

Couplets up to 30 cm of lithic sandstonefining upward to siltstone–shale

Ferruginous cross-bedded sandstone

Stromatolitic carbonate, chert breccia

Siltstone interbeds (up to tens of metres thick)(locally, may rest directly on basement)

Thick-bedded quartz sandstone, minordolomite lenses

Thick (up to 5 m) quartz arenite, siltstone interbeds; dewatering structures and halite moulds

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symmetrical and asymmetrical ripple marks are present.Ripple crests are straight, although sinusoidal forms arealso present. Wavelengths vary widely from a fewcentimetres to tens of centimetres. Small-scale ripples withflat tops (Fig. 7) suggest very shallow water to emergentconditions. Nodular chert concretions and halite mouldsare locally developed.

Quartz arenites of the Finlayson Member are char-acterized by very well sorted and well-rounded quartzgrains, 0.2 – 0.5 mm in diameter. Heavy mineralconcentrates from this unit contain subhedral zircon andtourmaline, associated with rounded brown biotite andrutile. Well-rounded zircon and tourmaline are sub-ordinate, suggesting that the basal quartz arenite is mainlyderived directly from erosion of granitic source areas, withlittle second-generation reworking.

Good exposures of the Finlayson Member are presentin the southern and southwestern part of the Yerrida Basin(Plate 1). Here, they are part of a broad, approximatelyeasterly trending, arcuate band of shelf-facies rocks thatlies unconformably on granite–greenstone rocks (Plate 1).On MOOLOOGOOL the quartz arenite dips between 5° and20° to the north, and contains well-preserved ripple marksand laminae of clay platelets. A limited number ofmeasurements (

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microbial-like laminae and fragmented laminated textureswith spherulitic or stellate pseudomorphs. The rockcontains microcrystalline and cryptocrystalline quartz,minor sericite, euhedral barite crystals, albite, apatite,and isolated K-feldspar crystals. Spherulitic pseudomorphsare up to 10 mm in diameter and consist of radiatingaggregates of microcrystalline and polycrystallinequartz, interpreted to be pseudomorphs after anhydrite.Unidentified radiating acicular or blade-like crystals couldbe pseudomorphed gypsum. El Tabakh et al. (1999)interpreted the Bubble Well Member evaporites as havingformed displacively within fine muds, during intermittentflooding and dry periods of a saline environment.Precipitation of evaporite minerals from interstitial brinesoccurred in the early diagenetic stages, before cementationof the sediments.

On CUNYU the Bubble Well Member is present aslenticular outcrops, commonly between basal arenite ofthe Finlayson Member and overlying thick beddedarenites. At the southern boundary of CUNYU, chert andchert breccia of the Bubble Well Member is both wellbedded, with subrounded bioherms, and a chaotic brecciafacies, with intervening thin semicontinuous chert bands.Stromatolitic bioherms are widespread in the cherts anddiagenetic chert nodules are abundant. The cherts are inturn underlain by cream-coloured and ripple-markedquartz arenite close to the basal contact.

In the western Yerrida Basin (Plate 1) on GLENGARRYand at a locality approximately 5 km south of KaralundiHomestead, the Bubble Well Member consists of siltstone,laminated chert, and a crudely bedded chert breccia. Thechert rocks contain bioherms of a new stromatolite form,Kussoidella (Gee and Grey, 1993). Parallel laminations,soft-sediment deformation structures, cross-bedding, andplanar bedding surfaces in the original sediment arecommonly preserved. The chertified sedimentary rock ischaracterized by cryptocrystalline and microcrystallinequartz, chalcedonic quartz, and minor iron oxides. Theupper and lower contacts of the Bubble Well Member havenot been directly observed, but about 3 km north ofReferendum Bore, the Bubble Well Member clearly liesabove the Finlayson Member.

Southwest contact of the Goodin Inlier with theJuderina Formation

A series of shear zones trend west-northwest, withmoderate dips (45° to 50°) to the north or south, on thesouthwestern margin of the Goodin Inlier, aroundUtahlarba Spring. Quartz veins are present, orientedoblique to the shearing. Thin outliers of laminated andlocally ripple marked quartz arenite are associated withthese shear zones. The contact with the underlying graniteis unconformable, starting with a basal unit of parallel-laminated arenite followed by cream-coloured, locallycross-bedded arenite. The rock is cut by steeply dippingto vertical longitudinal joints. This jointing becomesprogressively more closely spaced in a southerly directionacross the outlier and is associated with silicification andquartz veining, suggesting the presence of a faultedcontact.

Vesicular mafic rocks (possibly volcanic) are presentin a number of outcrops, extending northwest from about2 km east of Browny Bore. The presence of these rockssignifies magmatic activity contemporaneous with theearliest sedimentation of the Yerrida Basin in this area.This volcanism may be correlated with the KillaraFormation (see below), which includes similar mafic rocksexposed at higher levels in the Yerrida Group succession.The presence of these mafic volcanic rocks in the lowerunits of the Yerrida Group stratigraphy, coupled with theirabsence from the same basal units elsewhere, suggeststectonic activity and early development of magma-tappingfractures along the southern margin of the Goodin Inlier.

Johnson Cairn Formation

The Johnson Cairn Formation, originally the ‘JohnsonCairn Shale’ of Gee (1987) and redefined by Occhipintiet al. (1997), is a succession of varicoloured iron-richshale, with graded silty layers and thin dolomite bands.The formation overlies the Juderina Formation in the areasnortheast of the Thaduna copper mine and around theGoodin Inlier. The boundary with the underlying unit istaken as the topmost bed of quartz arenite. At the typearea, 13 km northeast of the Thaduna copper mine on theJohnson Cairn hill, the formation rests conformably on theJuderina Formation and consists of laminated, vari-coloured iron-rich shale interbedded with minor carbonate(Gee, 1987). The formation is up to 1250 m thick andconformably overlain by the Thaduna Formation of theMooloogool Subgroup (Bagas, 1998).

On MOUNT BARTLE the Johnson Cairn Formation is asequence of laminated purple- to cream-coloured siltstonesand is considerably thinner, probably not exceeding 100 m(Dawes and Pirajno, 1998). On MOOLOOGOOL the mostextensive outcrops form a broad east-northeasterlytrending belt in the central part of the map sheet, whereit is interbedded with mafic rocks of the Killara Formation.Outcrops of the Johnson Cairn Formation are also presentin the area around Mooloogool Homestead. Here, theJohnson Cairn Formation includes thinly bedded (up to0.3 m thick) argillaceous siltstones with local thindolomite beds and minor lithic and quartzose wacke. Thesiltstone consists of variable mixtures of kaolinite andillite, commonly associated with iron hydroxides. Fourkilometres west of Mooloogool Homestead, a moderatelysorted quartz wacke has rounded quartz grains in akaolinitic matrix, and is locally interbedded with hematiticshale. The quartz wacke consists of subangular tosubrounded polycrystalline quartz in a matrix of illite andkaolinite. The matrix also contains weathered white micaand rarer tourmaline and zircon. Arenite units spatiallyassociated with the siltstone consist of subangular strainedquartz and minor chert in a ferruginous clay matrix.Dolomitic siltstone consists of fine-grained micriticcarbonate enclosing detrital quartz grains. The carbonatedisplays millimetre-scale laminations. Outcrops south ofNorth Bore have oolith-like structures, 1.1 to 1.4 mm indiameter, composed of fine dolomite and opaque minerals.

Possible tuffaceous units are present 2 km west-southwest of Cave Hill. They are dark green-grey in colourand have a fine vesicular texture. The vesicles are filled

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with opaline or chalcedonic quartz. The matrix consistsof glassy material with abundant clays of the illite group.The fine spherulitic appearance is probably the result ofdevitrification of the glass. The matrix is locally replacedby carbonate.

In the western part of the Yerrida Basin (Plate 1), theJohnson Cairn Formation is dominantly argillaceous,which results in subdued outcrops. The contact with theenclosing units of the Windplain Subgroup is not exposed,but disconformable contacts with the overlying DoolgunnaFormation can be seen on DOOLGUNNA (Adamides, 1998).The lower contact with the Juderina Formation is probablytransitional. In the northwest, the Johnson Cairn Formationis in faulted contact with rocks of the Bryah Group.

On GLENGARRY rocks of the Johnson Cairn Formationare laminated, argillaceous, purple to grey siltstones, withsignificant interbedded lithic wacke. Thin beds of dolomiteare common. Samples collected from wells consist ofgreenish-grey, graded, mafic lithic arenite. The maincomponents are plagioclase and K-feldspar, associatedwith abundant amphibole. Epidote is present in the matrix,may be metamorphic, and is associated with minormuscovite and biotite. Quartz is present in minor amounts,as are sedimentary fragments, dominantly of chert andsiltstone. These lithic arenites can be attributed to the rapiderosion of a granite–greenstone-dominated terrain.Carbonate units, mainly dolomite and marl, are present inthe east where they are intruded by dolerite.

Mooloogool SubgroupThe Mooloogool Subgroup conformably overlies theWindplain Subgroup and contains four formations:Thaduna, Doolgunna, Killara, and Maraloou. Sedimento-logical characteristics suggest that it was deposited in arift-basin setting (Pirajno et al., 1995, 1996).

The sedimentary rocks of the Thaduna and DoolgunnaFormations (conglomerates, turbidite-facies rocks) weredeposited in a high-energy environment that indicates anabrupt change from the shallow and mature environmentof the Windplain Subgroup. These two formations forma broad northeasterly trending belt around the GoodinInlier (Fig. 1). The Goodin and Marymia Inliers werethe principal source for the sediments of the Thadunaand Doolgunna Formations. The two formations inter-digitate and, along with the mafic rocks of the KillaraFormation, indicate a complex depositional environmentin which sedimentation was taking place at the same timeas volcanism. The Doolgunna Formation containsconglomerates, turbidite rocks, and diamictite units thataccumulated in a graben-like depository (Doolgunnagraben; Pirajno 1996; Pirajno and Occhipinti, 1998). Thediamictites are the result of mass-wasting sourced fromrocks overlying and including the Goodin Inlier. TheThaduna Formation is a turbiditic succession of coarse tofine wackes, containing lithic fragments of volcanic rocks,shale, and siltstone. The volcanic rocks were sourced fromthe Archaean greenstones in the Marymia Inlier and fromthe Killara and Narracoota Formations. The KillaraFormation consists of lavas and sills of tholeiiticcomposition, emplaced in a continental environment.

The Maraloou Formation contains rhythmicallylaminated siltstone, sulfidic shale, marl, dolostone, andminor chert. The formation is indicative of a deepeningof the basin and an anoxic low-energy lacustrineenvironment.

Doolgunna Formation

The Doolgunna Formation (Occhipinti et al., 1997;originally ‘Doolgunna Arkose’ of Gee, 1979) is distributedmainly around the Goodin Inlier, except on its northernside where the formation locally interfingers with theThaduna Formation (see below). Kaolinitic wackes of theDoolgunna Formation and maroon-coloured, ferruginouslithic wackes of the Thaduna Formation interdigitate onTHADUNA (Pirajno and Adamides, 1998) and probablyon MOUNT BARTLE (Dawes and Pirajno, 1998). TheDoolgunna Formation is largely sourced from the GoodinInlier, based on its distribution around the inlier and thenature of the component clasts. The thickness of theDoolgunna Formation decreases away from the GoodinInlier.

The lower contact of the Doolgunna Formation withthe Johnson Cairn Formation is commonly marked by thefirst appearance of kaolinitic quartz wacke. The contactis disconformable northwest of the Goodin Inlier andgradational in the areas east of the inlier. On GLENGARRYthe lowest part of the Doolgunna Formation above theJohnson Cairn Formation is defined by a sequence offluviatile sandstones and pebble beds. The pebble beds areoverlain by diamictite units that define a zone at theboundary with the main arkosic wacke. South of theGoodin Inlier, on MOOLOOGOOL, the Doolgunna Formationis an arkosic wacke that appears conformable with theJohnson Cairn Formation. East of the Goodin Inlier, theformation is conformable with the Johnson CairnFormation and displays interfingering relationships withthe Thaduna Formation.

Northwest of the Goodin Inlier, the DoolgunnaFormation is faulted against the Karalundi Formation(Bryah Group) along the Goodin Fault (Fig. 1). East ofthe Goodin Inlier, on THADUNA, the formation is overlainunconformably by siltstones of the Maraloou Formation(Pirajno and Adamides, 1998), with the contact definedby a thin sequence of laminated chert and chert brecciain central THADUNA. On MOOLOOGOOL, south of the GoodinInlier, the formation is intercalated with the KillaraFormation and overlain, probably unconformably, bysiltstones of the Maraloou Formation.

The basal unit of the formation is probably of fluvialorigin, deposited during an initial deltaic period ofsedimentation. In the northeastern PART OF GLENGARRY itcomprises a sequence of arenites and associated pebblebeds. Bands of oligomictic conglomerate are present,almost exclusively comprising vein quartz with clasts upto several centimetres in size. They are subangular torounded and enclosed in a matrix of medium-grainedquartz arenite. Zones of strong silicification andbrecciation are associated with arenites and pebble beds,and probably testify to hydrothermal activity associatedwith faulting.

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The Doolgunna Formation is typically an arkosicturbidite sequence of whitish arenaceous wacke beds,commonly averaging 1 m in thickness, but locally up to3 m thick (Fig. 8). Rocks weather orange-brown andclosely resemble granite in their weathering character-istics. The beds consist of upward-fining sequences withevidence of intense scouring in the lower parts, andlaminated facies at the upper levels. The basal contacts aremarked by lags of pale, argillaceous rip-up clasts andpebbles. Cyclic units are commonly stacked in upward-thinning beds, with broad concave bases suggestive ofchannelized turbidite flow. Greenish-grey siltstonesseparate individual beds and these are often scoured outby the subsequent sedimentary units. Angular tosubrounded clasts, mainly of quartz and up to severalcentimetres across, are enclosed in an unsorted kaolinite-rich matrix. Preserved clasts most commonly consist ofquartz and chert; however, Gee (1987) mentioned theisolated preservation of identifiable granite clasts in thearea of John Bore. In rare cases, chert containing lath-likepseudomorphs, probably of evaporitic minerals, is alsopresent.

Rocks of the Doolgunna Formation consist ofsubangular quartz and feldspar grains set in a light-brownkaolinitic matrix. Minor white mica is present, in somecases altered to illite. In the weathered varieties,vermiform kaolinite or illite (or both) replace the feldspars.In the less-weathered samples of the formation, freshfeldspars are abundant, commonly consisting of micro-cline and orthoclase. Zircon is a common accessorymineral. The lithological character of the assemblagessuggests derivation from the erosion of granite basementrocks. This is further supported by the examination ofheavy mineral populations, which disclose predominantlyeuhedral and subhedral zircons, apatite, and tourmaline,in association with titanomagnetite.

A belt of diamictite units occupies a stratigraphicposition approximately in the middle of the formation andis particularly well developed from the area 8 km west ofCentre Pool Bore to 2 km south-southwest of MountLeake Bore on DOOLGUNNA (Adamides, 1998). This belt,

which may be up to 300 m thick, includes clasts andblocks, up to several metres in diameter, of banded,microbial chert, laminated quartz arenite, and chert brecciaof the Bubble Well Member of the Juderina Formation(Fig. 9). The blocks are enclosed in a white, sheared, siltykaolinitic matrix. This diamictite is interpreted as the resultof mass wasting, probably derived from the unroofing ofbasement and deposited in a deepening trough (Doolgunnatrough; Pirajno et al., 1995, 1996). Associated bands offine-grained granule conglomerate locally exhibit inverseto normal grading, suggesting a genesis by debris-flowprocesses.

A sequence of thinly bedded, fissile, purple-greysiltstones exposed on THADUNA has been assigned to theDoolgunna Formation because of spatial relationships(Pirajno and Adamides, 1998). The siltstones are in beds10 to 30 cm thick and show well-developed millimetre-scale parallel laminations and shallow-water ripple cross-laminations. They locally contain ellipsoidal diageneticironstone nodules up to 30 cm in diameter. Similarconcretions have been described in argillaceous units ofthe Thaduna Formation by Dawes and Pirajno (1998). Afew beds are disturbed by syndepositional faulting.

The Doolgunna Formation shows wide variation inthickness and lithology throughout the Yerrida Basin. Itis thickest in the northern part of the Goodin Inlier, whereit may be 5 km thick (Gee, 1987). South of the inlier, onMOOLOOGOOL (Pirajno et al., 1998b), it is considerablythinner and forms a narrow belt extending for approxi-mately 20 km eastwards from Rainlover Well. At theeastern end of this belt, the formation includes quartzsandstone with minor pebble beds and arkosic wacke,intercalated with dolerite sills (Killara Formation), and isin contact with rocks of the Thaduna Formation. East ofthe Goodin Inlier, the formation thins to a sequence ofkaolinitic wacke and shale. Beds are normally graded,with subangular quartz in a kaolinitic matrix. Coarserpebbly facies are present locally, with subrounded pebblesof quartz and chert infilling channels in the wacke.Individual wacke beds are commonly thin, in the rangeof 40 to 70 cm, locally thinning to 10 cm.

Figure 8. Thick-bedded granite-derived quartz wacke of theDoolgunna Formation, about 4 km east of JohnBore

Figure 9. Rounded block of chert breccia in an unsortedmatrix of diamictite of the Doolgunna Formation

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Thaduna Formation

The Thaduna Formation (redefined by Occhipinti et al.,1997) derives its name from the type area at the Thadunacopper mine. It is equivalent to the ‘Thaduna Beds’ ofMacLeod (1970) and the ‘Thaduna Greywacke’ of Gee(1979, 1987). Outcrops in the Bryah Basin that werepreviously included in the ‘Thaduna Greywacke’ are nowassigned to the Ravelstone and Narracoota Formations ofthe Bryah Group (Occhipinti et al., 1997; Pirajno et al.,2000). In areas north and east of the Goodin Inlier, theThaduna Formation forms the base of the MooloogoolSubgroup, where it conformably overlies the JohnsonCairn Formation. Its thickest development is in the areaof the Thaduna and Green Dragon copper mines onMARYMIA (Bagas, 1998). Southwest of the Thaduna mineit passes, in an interfingering relationship, into the thickturbidite wedge of the Doolgunna Formation (Adamides,1998). In the area of MOUNT BARTLE the formation isintercalated with the mafic rocks of the Killara Formation(Dawes and Pirajno, 1998), whereas similar intercalationsof lithic wacke and mafic rocks are noted on the southernmargin of THADUNA (Pirajno and Adamides, 1998).

These relationships suggest contemporaneous sedi-mentation with the Doolgunna and Killara Formations.The interfingering relationships in the lateral sense arerepeated in the vertical sense, with passage of thepredominantly lithic wacke of the Thaduna Formationupwards into the granite-derived wacke of the DoolgunnaFormation.

The Thaduna Formation is predominantly a sequenceof lithic sandstone, quartz wacke with intercalated siltstoneand shale, and subordinate dolomite. Blockley (1968),based on surface geology and drilling information at theThaduna mine, subdivided the formation at the type areainto two siltstone and two wacke units. The lower siltstoneunit, of undetermined thickness, is overlain by acoarsening-upward greywacke unit (the lower greywacke),estimated to be 750 m thick and composed of interbeddedcoarse-, medium-, and fine-grained lithic wacke. Theupper siltstone member is approximately 60 m thick andoverlain by the upper greywacke unit, about 90 m thick.Lithologically the two greywacke units are almostidentical, varying only in the higher proportion of shaleassociated with the lower greywacke.

The lithic sandstone commonly comprises beds,averaging 1 to 2 m in thickness, of grey hard rock, whichin the more even-grained varieties bears a superficialresemblance to dolerite. Elsewhere, outcrops are deeplyweathered and have a characteristic red-brown hematiticstaining, with primary mineralogy mainly destroyed.Individual wacke beds commonly have a scoured lowercontact with the underlying shale and display typicalfeatures of a Bouma sequence (Bouma, 1962). Associatedsiltstones are thinly bedded and commonly parallellaminated. They typically show a purple colouration as aresult of the abundance of finely divided iron oxides.

Lithic sandstone from the type area is composed of amixture of mainly angular and sparse rounded quartz,abundant potash feldspar (including microcline), albitizedplagioclase, epidote, and less abundant siltstone and sparse

carbonate grains. These are set in a fine-grained unsortedmatrix, which is commonly extensively chloritized.Titanomagnetite, with well-developed lamellae ofilmenite, is the main opaque mineral. Chlorite (apart fromits presence as separate clasts) with blue and purplebirefringence is also present in the matrix as a new phaseor a replacement of feldspar. Epidote, locally an importantconstituent, is commonly detrital. However, both chloriteand epidote also replace siltstone clasts, probably as aresult of hydrothermal alteration. Volcanic-derivedcomponents typically include numerous clasts of tholeiiticbasalt containing small plagioclase laths set in a chloriticmatrix.

Away from the type area, on DOOLGUNNA (Adamides,1998) and THADUNA (Pirajno and Adamides, 1998), theformation is a sequence of purple-grey weatheredlitharenite interbedded with thin, sandy, and silty unitswith parallel and convolute laminations (Fig. 10). Bothsingly and multiply graded layers are present. Theycommonly include lithic clasts up to several centimetresacross mixed with abundant rip-up clasts. These rocks arecharacterized by abundant fine-grained iron hydroxides,imparting a distinctive red-brown colour. They arecomposed predominantly of subrounded quartz, sub-ordinate fine-grained quartzite fragments, minor feldspar,locally granophyric siltstone fragments, and angular clastsof opaque minerals. The majority of feldspar is convertedto vermiform kaolinite, associated with minor illite. Lithicfragments are rarely preserved, being mostly altered toclays. The matrix is predominantly composed of kaolinite,which is in the form of acicular aggregates, probablypseudomorphing chlorite.

Elsewhere, the formation shows widespread interdigit-ation with both the Doolgunna and Killara Formations. OnMOUNT BARTLE (Dawes and Pirajno, 1998), the lower partof the Thaduna Formation is exposed around the DiamondWell anticline. In this area it is intercalated with rocksof both the Doolgunna and Killara Formations andincludes pale-coloured litharenites and associatedargillaceous rock types that form beds 15 cm to 1 m thick.

Figure 10. Soft-sediment deformation structures in rocks ofthe Thaduna Formation, about 8 km northeast ofNo. 8 Bore

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The wacke beds show typical turbidite features, withvariations across the beds from a basal granulestone unitthrough sandstone to an upper siltstone unit. Scour-and-fill structures are present in the base of the thicker units,with a conglomeratic lag of rounded quartz pebbles anda common easterly orientation of scour channels. Thewackes are composed of variable amounts of angular tosubrounded quartz, associated with chert and feldspar. Inplaces, chert forms an important component of the rock.

The lithic wacke typically forms beds varying inthickness from a few centimetres up to 2 m, and isintercalated with finely laminated siltstone. The wackecontains an abundance of sedimentary structures (cross-bedding, scours, slumps) consistent with a turbidite origin.Complete Bouma sequences (Bouma, 1962) are observedin some of the more complete turbidite units. Angular rip-up clasts of siltstone or wacke are abundant, particularlyin the lower parts of the wacke beds, and these vary insize upwards from a few millimetres to angular blocksseveral centimetres in size.

The siltstones interbedded with the wacke are parallel-laminated on a millimetre scale, and form beds averaging10 cm in thickness (Fig. 11). They commonly showparallel or small-scale cross-laminations and fine slumpstructures. Flame structures are also common where thesesiltstone are interbedded with lithic wacke (Fig. 12).Brecciation of the substrate and incorporation as rip-upclasts in lithic wacke is consistent with turbiditic activity.

In the upper levels of the Thaduna Formation onMOUNT BARTLE, a subfacies of the Thaduna Formationwacke is a mixed sequence of well-bedded quartzose andfeldspathic litharenite and wacke, locally subarkosic,associated with fine-grained laminated siltstones andshales. Flattened, discoidal, or ellipsoidal diageneticferruginous concretions, up to 80 by 30 cm, are locallypresent in the siltstones. Minor conglomerate is locallyassociated with this unit, reaching a thickness of up to40 cm and composed of predominantly subrounded androunded pebbles and cobbles set in a matrix of ferruginous

sandstone. Clast types in the conglomerate include greychert, ferruginous arenite, and pale feldspathic sandstone.

An outcrop on the southeastern side of the GoodinInlier, approximately 3.5 km north of Divide Bore (Plate 1;Pirajno et al., 1998b) consists of turbiditic quartz wacke,siltstone, laminated volcaniclastic mudstone, and thin bedsof grey volcanic ash. The volcaniclastic rock containsquartz, biotite, sericite, chalcedony, and kaolinitic claysreplacing crystal fragments and lithic fragments of basalticscoria (Fig. 13). Clay-replaced particles are eitherpyroclasts or crystals. The outcrop is interpreted as mass-flow deposits deposited in a trough formed by local upliftand subsidence during volcano-sedimentary processes.

Figure 11. Laminated turbiditic siltstone of the ThadunaFormation

Figure 12. Flame structures in lithic wacke of the ThadunaFormation

Figure 13. Block of volcanic scoria in turbidite rocks of theThaduna Formation (after Pirajno et al., 1998b)

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Killara Formation

The Killara Formation was originally part of the ‘doleritesills’ of Elias et al. (1982). Subsequently, all mafic rocksof the former ‘Glengarry Group’ (Bryah and YerridaGroups) were included in the ‘Narracoota Volcanics’ (Gee,1987; Gee and Grey, 1993). The ‘Narracoota Volcanics’have since been redefined (Occhipinti et al., 1997) andsubdivided into the Narracoota Formation in the BryahGroup (Pirajno et al., 1998c) and Killara Formation in theYerrida Group. Details of the geology, petrography,and geochemistry of the Killara Formation have beenreported by Dawes and Pirajno (1998) and Pirajno et al.(1998c).

The Killara Formation outcrops in the southern,eastern, and southeastern parts of the Yerrida Basin. Tothe east of the Goodin Inlier (Fig. 1), units of theKillara Formation interdigitate with the Johnson Cairn,Doolgunna, and Thaduna Formations. The formation alsointerdigitates with the base of, and is transitional to, theoverlying Maraloou Formation.

The Killara Formation forms a T-shaped zone acrossthe central and eastern parts of the Yerrida Basin (Plate 1and Fig. 1). The stem of the ‘T’ trends northwest and isaligned with Archaean greenstone units, and the top of the‘T’ has an east-northeast to northeast trend parallel to theGoodin Fault. The significance of the T-shape is discussedin Basin development and tectonic evolution. Thestratigraphic relationships of the Killara Formation withother units of the Yerrida Group are shown in Figure 2.

The Killara Formation consists of subalkaline intrusiveand extrusive rocks, with minor intercalations of chertifiedvolcaniclastic rocks. The thickness of the KillaraFormation is uncertain, but is estimated to be in the orderof 1000 m (Pirajno et al., 1995). The mafic rocks arecommonly unmetamorphosed, and flat lying or shallowdipping. They have tholeiitic to calc-alkaline basaltic andbasaltic andesite compositions, and were emplaced assubaerial and subaqueous lava flows, intrusive sheets, sills,and dykes (Pirajno et al., 1998c).

An important component of the Killara Formation isthe Bartle Member, consisting of chertified microbiallaminites with barite and anhydrite nodules, massive chert,and chert breccia. These units are stratigraphically at thetop of the Killara Formation and represent the end-phaseof volcanic activity. Relic textures in the Bartle Membercherts suggest that precursor rock types includedvolcaniclastics, hot spring-related chemical sedimentaryrocks, and evaporites (Pirajno and Grey, 1997; Dawes andPirajno, 1998).

The Killara Formation is interpreted to representcontinental volcanism associated with the rifting phase ofthe Yerrida Basin. Fifteen individual lava flows are presentin a 90 m-thick section of drillcore (see below) from anarea 3 km east of North Bore, indicating a high rate oferuption. Two km north of Desert Well, a diamonddrillhole (KDD 1; Bromley and Cull, 1985) intersected anundisturbed, flat-lying succession of tholeiitic basaltpillow lavas and dolerite sills, from about 319 m belowthe surface to the final depth of 503 m (Fig. 14). They

Figure 14. Idealized stratigraphy of the Ki

by f. pirajno and n. g. adamides · (tm) and magnetic images. results of geological mapping were...

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