Metallogeny and tectonic development of the tasman fold belt system in tasmania

Ore Geology Reviews, 1 (1986) 153--201 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

153

METALLOGENY AND TECTONIC DEVELOPMENT OF THE TASMAN FOLD BELT SYSTEM IN TASMANIA P . L . F . C O L L I N S a n d E. W I L L I A M S

Tasmania Geological Survey, Departmenl of Mines, p.o. Box 56. Rosny Park, Tasm. 7018 (Australia) (Accepted for publication January 13, 1986 )

Abstract Collins, P.L.F. and Williams, E., 1986. Metallogeny and tectonic development of the Tasman Fold Belt System in Tasmania. In: E. Scheibner (Editor). Metallogeny and Tectonic Development of Eastern Australia. Ore Geol. Rev., 1 : 153--201. In western Tasmania, Precambrian sedimentary sequences form the basement for narrow trough accumulations of Eocambrian and younger sequences. The main trough, the meridional Dundas Trough, is flanked to the west by the Rocky Cape region of Precambrian rocks within which major, apparently stratiform, exhalative magnetite--pyrite deposits are intercalated with metabasaltic volcanics and ultramafic bodies. The Eocambrian--Cambrian troughs apparently developed during extension of Precambrian continental crust. Early shallow-water deposition includes thick dolomite units in some troughs. Deepening of the troughs was accompanied by turbidite sedimentation, with minor limestone, and submarine basaltic volcanism with associated minor disseminated native copper. Ultramafic and related igneous rocks were tectonically emplaced in some troughs during a mild compressional phase. They contain only minor platinoids, copper--nickel sulphides and asbestos, but are source rocks for Tertiary secondary deposits of platinoids, chromite and lateritic nickel. In the Dundas Trough, Eocambrian--Early Cambrian rocks are separated by an inferred erosional surface from structurally conformable overlying Middle to Late Cambrian fossiliferous turbidite sequences. The structural conformity continues through overlying Ordovician to Early Devonian terrestrial and shallowmarine stable shelf deposits. A considerable pile of probable Middle Cambrian felsic volcanics accumulated between the sedimentary deposits of the Dundas Trough and the Tyennan region of Precambrian rocks to the east. A lava-dominated belt within the volcanics hosts major volcanogenic massive sulphide deposits, including those of the exhalative type, which in the south are enriched in copper, gold and silver, whereas in the north they are rich in zinc, lead, copper, gold and silver. Cambrian movements along faults near the margin of the Tyennan region resulted in erosion of the mineralized volcanics, locally exposing sub-volcanic granitoids. Above the local unconformities occur unmineralized volcaniclastic sequences that pass conformably into Ordovician to Early Devonian shelf deposits. Ordovician limestone locally hosts stratabound disseminated and veined base metal sulphide deposits. Pre-Middle Devonian rocks of western Tasmania differ, for most part, from those in the northeast where deeper marine turbidite quartz-wacke sequences were deposited during the Ordovician and Early Devonian. The Eocambrian to Early Devonian rocks of Tasmania were extensively deformed in the mid-Devonian. The Precambrian regions of western Tasmania behaved as relatively competent blocks controlling early fold patterns. In northeastern Tasmania, folding is of similar age but resulted from movements inconsistent with those affecting rocks of equivalent age in western Tasmania. The final metallogenic event is associated with high-level granitoid masses emplaced throughout Tasmania (luring the Middle to Late Devonian. In northeastern Tasmania, extensive I-type granodiorite and 0169-1368,"86/$03.50

,7, 1986 Elsevier Science Publishers B.V.

!5 S-type granite, with alkali-feldspar granites, are associated witi~ mainly endogranitic slanniferous ~;~t:::~l)s and w o l f r a m i t e +-cassiterite vein deposits. In contrast, scheelite-bem'ing skarns and cassiterite sta~mite p y r r h o t i t e carbonate replacement deposits are d o m i n a n t in western Tasmania, associated main!:, with S-type granites. Several argentiferous lead-- zinc vein deposits occur in haloes around tin --I,ungsten ch.posits. -\ n u m b e r of gold deposits are apparently associated with I-type granodiorite, but some haw~ ~',m.erlain genesis. ]'he contrasting regions of western and northeastern Tasmania have probably been brought togeth,:f by lateral m o v e m e n t along an inferred fracture. Flat-lying, Late Carboniferous and younger deposits ,est on the older rocks, and the only k n o w n post-Devonian primary mineralization is gold associated with C r e t a ceous syenite.

Introduction Tasmania forms a very small part of the Tasman fold belt system of Palaeozoic-Mesozoic age, but has an extraordinary variety of ore deposits, which is enhanced by the unusual form and mineralogy of several of the larger deposits (e.g., Savage River, Mt. Lyell, Renison}. All known primary metallic mineralization occurs in rocks ranging in age from Late Proterozoic to Late Devonian (except for minor Cretaceous(?} gold), but there are considerable differences in the geology and metallogenesis of western Tasmania compared to that of northeastern Tasmania, east of the Tamar River [ 500430* ]. *Coordinates refer to the map grid of Fig. I (easting, then northing).

In western Tasmania, narrow troughs developed between and within Precambrian regions in Eocambrian (?)--Cambrian times. The Precambrian regions became geanticlines during early trough deposition, and later acted as blocks whilst the younger rocks.were folded in a number of directions during the period of major deformation of Devonian age. In contrast, the folded sedimentary rocks of northeastern Tasmania are of Early Ordovician and Early Devonian age and exhibit comparatively simple Devonian folding. Granitoid plutons were intruded in the Late Proterozoic and Cambrian in western Tasmania, but the most substantial granitoid masses were emplaced within the folded rocks throughout Tasmania from about 375 to 335 Ma ago. Later, Tasmania was part of a craton and

Peter Collins graduated f r o m the University of Tasmania, Australia in 1973 and joined the Geological Survey of the Tasmanian Department of Mines. He received his doctorate from the same university in 1984. His principal research interests are in economic geology, particularly the genesis of tin and tungsten deposits and their associated granitoids, and has a special interest in fluid inclusion research. Present address: Tasmania Geological Survey, Department of Mines, P.O. Box 56, Rosny Park, Tasm. 7018, Australia.

E m y r Williams received his doctorate from Nottingham University, England in 1951. Before joining the Geological Survey of the Tasmanian Mines Department in 1963 he had worked for the British Overseas Geological Surveys and taught at th Universities of Nottingham and Tasmania. His varied research interests range from sedimentology to structural geology and tectonics. Present address: Tasmania Geological Survey, Department of Mines, P.O. Box 56, Rosny Park, Tasm. 7018, Australia.

155

prolonged erosion of the Devonian granitoids and older rocks was followed by deposition of flat-lying Late Carboniferous and younger beds, which have undergone epeirogenic deformation only. The heterogeneity in the pre-Carboniferous geology and tectonic development of Tasmania is also reflected in the metallogenesis. In western Tasmania there are three main metallogenic epochs which occurred late in the Proterozoic, during the Cambrian, and late in the Devonian. The two early phases are dominated by volcanogenic massive sulphide and oxide deposits, whereas the later Devonian phase is dominated by graniterelated deposits. In eastern Tasmania there is a single later Devonian metallogenic epoch associated with granite emplacement. In reviewing the metallogenesis of the Ta~manian segment of the Tasman Fold Belt System, a series of schematic east--west geological sections are used to illustrate the various metallogenic events and diversity of mineral deposits. Representations for western Tasmania are based on the metallogenically important Dundas Trough and associated rocks. A comprehensive legend for all sections accompanies the final diagram of the series (Fig. 11), which is a revision of similar but less elaborate schematic sections presented in Collins (1978) and Solomon (1981). Proterozoic framework

Tyennan and Forth regions The largest area of Precambrian basement is the Tyennan region, extending from the southwest coast to the Central Highlands. It consists generally of sequences of interbedded siltstone and ortho-quartzite, metamorphosed to upper greenschist facies during the Late Proterozoic Frenchman Orogeny which is defined by multiple folding and an early main period of metamorphism (Spry, in Spry and Banks, 1962, pp. 107--126). In the Cradle Mountain area [415385] (Gee et al., 1970), pre-kinematic almandine garnet

growth developed in the main phase of metamorphism, which accompanied early folding. Later folding was associated with a minor metamorphic event and regional transposition. A similar sequence of events has been recorded throughout the Tyennan region, but at Raglan Range [400335] {Gee, 1963) metamorphism continued after the earlier folding and involved the development of kyanitc and eclogite (Boulter and Raheim, 1974). Farther south, at Strathgordon [422265] (Boulter, 1972) metamorphism reached a maximum during the earlier folding dated by Rb--Sr methods at 780+23 to 8 1 5 + 6 6 " Ma (Raheim and Compston, 1977; Turner, 1982). Rocks similar to those of the Tyennan region constitute the small Forth region on the north coast [430435] (Burns, 1965). Quartzite, schist, conglomerate and amphibolite have been mechanically interlayered, though some layering may be bedding. All early deformation event was accompanied by metamorphism to almandine garnet grade, and minor metamorphism occurred during later folding. The deformation and metamorphism is considered to be of the Frenchman Orogeny. At the western margin of the Forth region are comparatively unmetamorphosed but folded turbidite q u a r t z - w a c k e successions that have been thrust onto the metamorphic rocks. The relatively unmetamorphosed rocks are correlated with Precambrian formations constituting the Badger Head region [ 475440 ], and identical sequences of the Burnie Formation in the R o c k y Cape region, 5 km to the west.

Rocky Cape region Geology The northwestern area of the extensive Precambrian R o c k y Cape region is underlain by the R o c k y Cape Group, which is, in general, separated by an 8--15 km wide belt of metamorphic rocks from the Burnie *All ages have b e e n re-calculated to c o n f o r m with c o n v e n t i o n o f Steiger a n d J~iger ( 1977 ).

156 Formation and its southern correlate, the Oonah Formation, at the eastern margin of the region. The Rocky Cape Group (Gee, 1971) exhibits broad, c o m m o n l y symmetrical folds of 1.5 km half-wave length and greater, and consists of two formations of supermature ortho-quartzite, each more than 1200 m thick, and formations of laminated mudstone. In contrast, the Burnie Formation (Gee, 1977) is comparatively complexly deformed with a five-phase development of co-axial flexural folds, and it consists of more than 5000 m of turbidite quartz--wacke sequences and rare occurrences of pillowed spilitic basalt. In the south of the Rocky Cape region [345390] the contrasting formations are not separated by the metamorphic belt for the boundary between them is 5---6 km west of the belt {Turner, 1984). At Mt. Donaldson [339392] a basal sequence of the Oonah Formation, which includes dolomite and basic metavolcanic horizons, rests unconformably on correlates of the Rocky Cape Group. The metamorphic belt, known as the Arthur Lineament (Banks, 1965) extends southwesterly from Wynyard [395463] to the west coast (Figs. 1 and 2). Many rockunits within the Arthur Lineament, attained biotite and amphibolite grades of metamorphism, and are derived from adjacent sedimentary rocks of the Rocky Cape Group, Burnie and Oonah Formations, and dolerite intrusions which occur t h r o u g h o u t the region, for there are transitional boundaries between the metamorphic belt and the flanking rocks (McNeil, 1961; Longman and Matthews, 1962; Blissett, 1962; Gee, 1977). The Arthur Lineament also contains schists that have been derived from ultramafic as well as mafic volcanic and intrusive rocks (Matzat, 1984). The original igneous rocks are t h o u g h t to be related to a zone of extension along the western margin of a developing basin in which accumulated sequences of the Burnie and Oonah Formations. Also included within the Arthur Lineament are extensive magnesite deposits, derived from magnesium metasomatism of dolomite though the origin of

the alteration is uncertain (e.g. Mare Creek, Savage River and Arthur River deposits: Frost, 1982; Frost and Matzat, 1984 ;, The deformation of rocks of t,he Rocky Cape region and development of the Arthur Lineament constitute the Late Proterozoic Penguin Orogeny (Spry, m Spry and Banks, 1962, p. 124; Gee, 1977) and may be associated with emplacement of an S-type granitoid on western King Island (Eigs. 1 and 2) that has a minimum isotopic age of about 720 Ma (McDougall and Leggo. 1965; I. McDougall, pers. commun., 1983). The age of the effects of this period of deformation is given by K--Ar dates of 744+22 Ma for amphibolite from the Arthur Lineament at Savage River (Coleman, 1975; Turner, 1982); 725+35 Ma for dolerite intrusions within the Burnie Formation which are involved in the folding (Richards, in Crook, 1980ak and 6 3 0 - 6 9 0 Ma for slates of the Burnie and Oonah Formations and the Rocky Cape Group (Adams et al., 1985).

Volcanogenic massive sulphide-oxide deposits Within the Arthur Lineament, at Savage River [348403], unusual deposits of magnetite, pyrite and silicate ore are mined for magnetite (crude ore resource in excess of 130 Mt, Table 1). The Savage River deposits have been considerably deformed and metamorphosed and the ore recrystallized during the Late Proterozoic Penguin Orogeny, but rhythmically banded magnetite and pyrite is preserved locally. The ore appears to have been stratiform sheets or lenses intercalated with a pillowed, tholeiitic meta-basalt and meta-sediment sequence that has been intruded by co-magmatic tholeiitic dykes and sills (Coleman, 1975; Pratt and Girschik, 1976; Matzat, 1984). The volcanogenic exhalative(?) massive sulphide--oxide deposits at Savage River are the largest of several magnetite deposits in the Arthur Lineament (Figs. 1 and 2) and represent the oldest metallogenic epoch in Tasmania.

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(%)

Cu

(central deposit) (north deposit)

Grade

Published reserves/resource

deposits in western Tasmania

Au

PR PR Pit PR GR ~ GR

reserves; GR = (indicated)

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PR PR

Category

geological

2 3 3 4 4 5

1 1

Sotlrce

Data sources: 1 = Savage River Mines, Pets. eommun., 1985; 2 = Annual reports Renison Goldfields Consolidated Ltd.; Reid, 1975- 3 = Annual reports EZ Industries L t d . ; E l e c t r o l y t i c Z i n c C o . A / a s i a L t d . . p e r s . c o m m u n . , 1 9 8 5 ; p r o d u c t i o n t o J u n e , 1 9 8 5 : 4 = A n n u a l r e p o r t s A b e r f o y l e L t d . ; 5 = C o l l i n s e t a[., 1 9 8 1 .

C u r r e n t p u b l i s h e d r e s e r v e / r e s o u r c e e s t i m a t e s c a t e g o r i s e d as P R = p r o v e . d / m e a s u r e d resotLrce. 4 A d d i t i o n a l 4 M t o f i n f e r r e d ore,, A b e r f o y l e L t d . I n t e r i m R e p o r t , 1 9 8 5 . C h e s t e r d e p o s i t m i n e d o n l y f o r s u l p h u r ; r e s o u r c e e s t i m a t e f o r _2- 2 0 % s u l p h u r .

1.16 0.74 0.42 0.44

(%)

Cu

Grade"

to 1984

mine.

45.8

FeO (%)

C o - o r d i n a t e s in [ 1 r e f e r t o F i g . 1: * = o p e r a t i n g 2 Grade of ore treated; Mt = million tonncs.

• • • •

Cambrian (Mt. Read Voleanics)

Late Proterozoie (Rocky Cape region) * Savage River ]348403]

Mr"

Production

and ore reserve estimates for principal volcanogenic sulphide and sulpilide--oxide

Metallogenic epoch Mine/deposit [co-ords] '

Production

TABLE

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Early

Palaeozoic

troughs

Eocambrian and Cambrian sediments and volcanics collected in a number of apparently distinct troughs in western and northern Tasmania (e.g. Smithton, Dundas and Adamsfield Troughs; Fig. 1) though three of the troughs (Dundas, Fossey Mountain, and Dial Range) may have been contiguous. These three troughs are also of greatest metallogenic interest, particularly the Dundas Trough, for they have associated felsic--intermediate volcanics (e.g. Mt. Read Volcanics) that host several major volcanogenic massive-sulphide deposits.

Dundas Trough and associated rocks A northerly trending, 20---30 km wide trough of Eocambrian and Cambrian rocks is situated in western Tasmania, flanked by the R o c k y Cape and Tyennan regions, and containing belts of distinctive rock units parallel to its margins (Fig. 1).

Early trough development The western boundary of the Dundas Trough is an inferred angular landscape unconformity though it is c o m m o n l y masked by young faults (Williams, 1978; Brown, 1980). The unconformity is also a structural hiatus (Brown, 1985), for in the Pieman River bed [362375] the basement, which consists of the Oonah Formation, has structural characteristics (e.g. disturbed isoclinal folds) that

developed during the Precambrian Penguin Orogeny, whereas the overlying Eocambrian(?) Success Creek Group displays comparatively simple, open folds of younger age. The Success Creek Group (Fig. 3; Taylor, 1954; Brown, 1986) consists of a 50 m thick basal diamictite followed conformably by 550 m of shallow-water sandstone, a 75 m thick unit of laminated mudstone and siltstone displaying soft-sediment deformation related to subaqueous landslides, 100 m thick succession of siltstone with minor sandstone and calcareous siltstone, and some 50 m of interbedded hematitic chert and mudstone with minor lithic-wacke and conglomerate (Newnham, 1975). Dolomite units, 5---.15 m thick, are locally interbedded with the upper members of the group {e.g. Renison Bell. Stanley River) and apparently formed in a littoral environment (Collins, 1972). The Success Creek Group is unfossiliferous, apart from acritarchs (Vidal, in Cooper and Grindley, 1982) and rare stromatolites in carbonate lenses (Brown, 1986), and is apparently restricted in its distribution to between Renison Bell [369371] and Stanley River [ 3 5 6 3 8 3 ] . The Success Creek Group is followed conformably by the Crimson Creek Formation (Fig. 3) which, apart from the presence of acritarchs (Vidal, in Cooper and (;rindley, 1982), also is unfossiliferous. The formation (Taylor, 1954; Blissett, 1962; Brown, 1986), which is up to 5000 m thick, consists mainly of laminated siltstone and mudstone with turbidites, mafic volcaniclastic lithic-wacke,

167

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Fig. 3. Schematic section of Eocambrian(?)--Early Cambrian trough deposits of shallow-marine sequences containing dolomite (e.g., Success Creek Group), and deep marine sequences including tholeiitic basalt and minor limestone (e.g., Crimson Creek Formation) unconformably overlying the Proterozoic basement (cf. Fig. 2 ). Native copper occurs rarely in basalt. For legend see Fig. 11. marie t u f f horizons and tholeiitic basalt flows, and minor, but locally a b u n d a n t limestone such as at Mt. Lindsay [ 3 6 0 3 8 1 ] , Mt. Ramsay and Luina. Basalt and mafic volcanic detritus with some feldspathic lithic-wacke are more a b u n d an t toward the nor t he r n end of the trough, and near Luina [ 3 6 7 4 0 5 ] , southwest of Mt. Bischoff, is a massive and pillowed, spilitic, tholeiitic basalt unit in excess of 350 m thick (Collins, 1983). At Mt. Bischoff [ 3 7 5 4 1 2 ] , rocks similar to the Oonah F o r m a t i o n form an inlier which is partly surrounded by unfossiliferous Eocambrian or Cambrian sequences. The c o n t a c t is a fault zone (Groves, 1971), which was probably active during deposition, for large blocks o f d e f o r m e d Precambrian rock are f o u n d slumped into the basal Eocambrian sediments. Near Zeehan [ 3 6 2 3 6 1 ] , 1 0 k i n south of the Pieman River, the Success G r o u p is absent, and the Crimson Creek F o r m a t i o n overlies the Oonah F o r m a t i o n with a relationship believed to be similar to that at Mt. Bischoff. Early deposition in the Dundas Trough, in a shallow-marine environment, s m o o t h e d the uneven basement consisting of the Oonah F o rm a tio n , and accompanied extension of the continental basement resulting in occasional submarine slides in deposits at the margins of the deepening trough (Brown, 1980). The extensional phase culminated in outpourings

of tholeiitic basalt of the Crimson Creek Form at i on, with m axi m um activity at the nort hern end of the trough. The mafic volcanism occurred in a marine environment, but apparently intercontinental rather than oceanic (Collins, 1983). Ultramafic masses and associated m ineraliza tion

Extension of the basement was succeeded by a mild compressional phase during which portions of ultramafic and mafic igneous complexes were steeply upthrust into, but caused little disturbance of, the sedimentary rocks and basalt of the Crimson Creek Formation (Fig. 4). The ultramafic complexes were re-emplaced along a meridional zone that extends from south of Macquarie Harbour [361285] to Mt. Cleveland [ 3 6 0 4 1 0 ] . Some of the ultramafic igneous rocks may be comagmatic with earlier e x t r u d e d mafic lavas of the Crimson Creek F o r m a t i o n , and the largest mass, the Heazlewood River Complex near Mt. Cleveland, coincides geographically with the thickest of the mafic volcanic piles (Fig. 1). The lleazlewood River Complex [360401] is a fault-bounded layered mass comprised of orthopyroxenite, peridotite and dunite (Rubenach, 1973). Farther south, the Serpentine ttill Complex [370368] (Brown et al.. in press) is an Alaskan-type complex which

168

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consists of multiple intrusions that formed a layered, dominantly orthopyroxene-rich sequence, a dominantly olivine-rich sequence and a gabbroic unit. The mafic and ultramafic masses locally contain minor nickel and copper sulphides, asbestos (e.g. Serpentine Hill) and chromite, but are noted for platinoid minerals recovered mainly from Tertiary alluvial deposits. The magmatic and associated deposits of the mafic and ultramafic masses represent a minor, Eocambrian (?)--Early Cambrian metallogenic epoch (Fig. 4). Heazlewoodite and pentlandite appear as syn/post-serpentinization disseminations or ramifying veinlets within the ultramafic masses at Heazlewood River [360409] and Trial Harbour [349356] (Williams, 1958). At Cuni [ 3 6 5 3 6 8 ] , near Zeehan, small, highgrade lenses of banded pentlandite .... pyrrhotite--chalcopyrite and millerite--chalcopyrite--pyrite ore (av. grade 10% Ni, 5% Cu) lie in the footwall of a dolerite intrusive, of probably Cambrian age (Fig. 4; Williams, 1958; Blissett, 1962), though it is apparently unrelated to the nearby Serpentine Hill ultramafic mass (Rubenach, 1974). Although there is only one recorded occurrence of primary platinoid-group minerals in a "seam" in serpentinite (Caudrys prospect; Reid, 1920), the ultramafic masses have yielded considerable quantities of platinoids

(iridosmine--rutheniridosmine and osmiridium; Ford, 1981) that have been concentrated mainly in Tertiary alluvial sediments. Total production is in excess o f 880 kg of "osmiridium", a b o u t half of which has been won from the Heazlewood district (Elliston, 1953; Hughes, 1965).

Dundas Group and correlates At Dundas [ 3 7 0 3 6 5 ] , to the east of Zeehan, the Crimson Creek Formation is followed with structural conformity by the fossiliferous Dundas Group (Fig. 5). The boundary is either along young faults or marked by ultramafic emplacements, but in the Dundas area fragments of the faultbounded ultramafic complex have been recovered from basal beds of the adjacent Dundas Group (Rubenach, 1974), and erosion is inferred (Fig. 5). Apparently the mild compressional phase that introduced the ultramafic masses was sufficient to change the configuration of the floor of the trough so that submarine erosion exposed the ultramafic bodies, which then contributed detritus to the overlying Middle Cambrian sediment. The Dundas Group (Elliston, 1954; Blissett, 1962; Brown, 1980), which is a b o u t 3800 m thick, consists of laminated siltstone and mudstone with either interlayered mudflow conglomerate and felsic volcanic rocks or turbiditic chert--conglomerate sequences, but

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Fig. 5. S c h e m a t i c s e c t i o n o f Middle a n d L a t e C a m b r i a n s e d i m e n t a r y s e q u e n c e s (e.g., D u n d a s G r o u p ) overlying, w i t h s t r u c t u r a l c o n f o r m i t y , early t r o u g h deposits, t h o u g h t h e r e is an inferred erosional surface a b o v e some u l t r a m a f i c c o m p l e x e s ; a n d Mt. Read V o l c a n i c s w i t h associated s u b - v o l c a n i c granitoids a n d v o l c a n o g e n i c massive s u l p h i d e deposits (el. Fig. 4). V o l c a n o g e n i c ore deposits: I = massive s p h a l e r i t e - - g a l e n a - - c h a l c o p y r i t e - - p y r i t e w i t h gold a n d silver (e.g., R o s e b e r y , Que River); 2 = d i s s e m i n a t e d p y r i t e - - c h a l c o p y r i t e w i t h m i n o r gold and silver (e.g., Mt. Lyell). F o r legend see Fig. 11.

is a p p a r e n t l y devoid o f m a j o r c a r b o n a t e units. T h e oldest fossils r e c o r d e d are o f earlyMiddle Cambrian (Opik, 1956), whereas the youngest beds have yielded late-Late Cambrian fossils (Jago, 1 9 7 3 ) . T h e u p p e r cong l o m e r a t e beds of the D u n d a s G r o u p contain m e t a q u a r t z i t e clasts similar to those o f the basal beds o f the O w e n C o n g l o m e r a t e at the western margin o f the T y e n n a n region. T h e eastern b o u n d a r i e s o f the D u n d a s G r o u p and its correlates are usually n o r t h e r l y t r e n d i n g faults or structural highs o f older rocks, including a c o r r e l a t e o f the P r e c a m b r i a n O o n a h F o r m a t i o n near Mt. D u n d a s [ 3 7 2 3 6 0 ] (Fig. 9, section A--B; Elliston, 1954; T u r n e r , 1 9 7 9 ) , which has a K--Ar age o f 684-+10 Ma {Adams et al., 1985).

Mt. Read Volcanics Geology T h e c o m p r e s s i o n a l phase t h a t i n t r o d u c e d the u l t r a m a f i c masses m a y also have initiated the onset o f volcanism responsible for a considerable a c c u m u l a t i o n o f d o m i n a n t l y acid to i n t e r m e d i a t e volcanic rocks o f the Mt. Read Volcanics flanking the western margin o f the

T y e n n a n Geanticline (Figs. 1 and 5). The 1 0 - - 1 5 km wide volcanic belt ( C a m p a n a and King, 1963; S o l o m o n , 1964) e x t e n d s f r o m the west coast, at Elliott Bay [ 3 8 0 2 4 0 ] to n o r t h o f Que River [ 3 9 0 3 9 5 ] , and m a y be divided into two m a j o r lithofacies ( C o r b e t t . 1981). T h e r e is a central belt o f r h y o l i t i c - - d a c i t i c - andesitic, subaerial and s u b a q u e o u s volcanics in which lavas, intrusives, ash flows and o t h e r pyroclast.ics are c o m m o n , b u t lacks sedim e n t a r y rocks. T h e volcanics are associated with granitoid intrusions, such as at Mt. Murchison [ 3 8 8 3 7 3 ] w h e r e K - - A r dating o f h o r n b l e n d e indicates a m i n i m u m age o f 512-538 Ma (MeDougall and Leggo, 1965; I. McDougall, pers. c o m m u n . , 1983). U--Pb zircon dating gives a c o m p a r a b l e age o f 510 Ma for a similar granitoid at Mt. D a n v i n [ 3 8 3 3 1 8 ] (Adams et al., 1985), and a maxim u m age o f a b o u t 540 Ma for lavas near Mt. Lyell (Black and Adams, 1980). A Cambrian age o f 5 4 0 ± 3 0 Ma has also been o b t a i n e d for hangingwall daeitic volcanics at the Que River mine (Whitford et al., 1983)• T h e central s e q u e n c e is f l a n k e d on the western side by v o l c a n o - s e d i m e n t a r y m a r i n e sequences o f i n t e r b e d d e d tuff, t u r b i d i t e

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volcanic lithic-wacke, quartz-wacke, siltstone and shale, with sill-like intrusiw~s of quartz feldspar porphyry. At the northern end of the Mt. Read volcanic belt, central sequence volcanics are overlain by the Que River Beds that is the basal unit of the western sequence in the Que River area (Collins ct al., 1981), which contain agnostid trilobites of the Ptychagnostus nathorsti Zones of the Middle Cambrian (Gee ct al., 1970). The contact between the western sequence and the central belt generally is poorly exposed but apparently is abrupt throughout its length, with no interfingering (Corbett, 1979, 1981; Collins et al., 1981). The contact shows conflicting stratigraphic relationships along strike, but the pronounced linearity of the central belt and abruptness of the contact suggests tectonic control accounting for the apparent ponding of the central lava-dominated sequence (Corbett, 1981). Farther to the west, the sedimentary rocks of the western sequence grade into, and are probable facies

equivalents of the fossiliferous Middle-LaW Cambrian turbiditic successions of the Dundas Group (Blissett, 1962; Corbett. i 9 8 i . 198..-I: Brown, 1986}. The bulk of the Mt. Read volcanism appm'-. ently occurred during Dundas Group time,, i.¢.. Middle and Late Cambrian (Corbett, 1981), but the age of the earliest eruptions is uncer, rain. The Mt,. Read volcanic rocks, however, appear to be younger than the Crimson Creek Formation which is devoid of acid-intermediate volcanic material, but cont aiz~s abun-. dant tholeiitic basalt flows and det:ritus (Brown, 1986). The central volcanic sequence is unfossiliferous but at Comstock [3833451 near Queenstown an inferred unconformity separates it from an overlying voleaniclastic sequence (Fig. 6; Tyndall Group, C(:)rbett et al., 1974) containing fossiliferous, marine, bioclastic limestone of late-Middle or earlyLate Cambrian age (Jago et al., 1972; Jago, 1979). The angular unconformity is devel-

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171 oped along the belt, and at Mt. Darwin [383318] a correlate of the Tyndall Group unconformably truncates central belt rocks and the Darwin Granite (White, 1975; Corbett, 1979), indicating considerable erosion of the earlier volcanics. The Tyndall Group has a conformable relationship with the overlying Owen Conglomerate (Fig. 6). At Red Hills [382360] (Corbett, 1975b), correlates of the Tyndall Group are conformably overlain by a fossiliferous Late Cambrian turbidite quartz-wacke sequence which passes into overlying shallow-marine and terrestrial siliciclastic Owen Conglomerate, followed conformably by Ordovician and Silurian deposits. The later beds lapped onto the Cambrian geanticlines as well as being deposited on the filled Cambrian troughs {Fig. 6).

Volcanogenic massiue sulphide deposits The Mt. Read Voh,'anics is host to several major, volcanogenic massive sulphide deposits that dominate the Cambrian metallogenic epoch. Virtually all of the known volcanogenic mineralization is confined to the central belt of massive volcanics (Corbett, 1981), and is distributed ow~,r some 150 km between Elliott Bay in the south and Hellyer in the north (Fig. 1). The volcanogenic deposits generally occur either as stratiform lenses of Zn, Pb and Cu-sulphides with Ag and Au (e.g. Rosebery, Que River) or as lenticular zones of disseminated Cu-sulphides with minor Ag and Au (e.g. Mr. Lyell) (Figs. 1, 5 and "Fable 1) but the host assemblages and depositional environments vary along the belt. South of the Henty Fault [380365], in the Queenstown area, the bulk of the sulphide mineralization is copper-rich and dominated by large, concordant, diffusely bounded lenses of disseminated pyrite--chalcopyrite ore dispersed within altered felsic volcanics at Mt. Lyell [382343] (Reid, 1975; Cox, 1981; Walshe and Solomon, 1981; Table 1). There are some ten disseminated deposits confined to the middle and upper parts of the 800 m thick east-facing mine sequence and they are

thought to have formed from ore fluids circulating within and replacing more permeable units in the host sequence (Walshe and Solomon, 1981) Associated, small, exhalative stratiform massive sulphide deposits occur at or near the stratigraphic top of the host volcanic pile (e.g. The Blow, Comstock) and similar thin conformable units are dispersed within the mine sequence (Cox, 1981). The host volcanics apparently accumulated largely in a terrestrial or sub-aerial environment close to a major eruptiw~ centre, though the presence of exhalative massive sulphide units within the mine sequence indicate a subaqueous environment during at least some of the depositional history (Cox, 1981; Walshe and Solomon, 1981). In the Mt. Jukes- Mt. Darwin area [385325] to the south of Mt. Lyell, there are several small disseminated and veined Cusulphide deposits, but stratiform style mineralization apparently is absent. The mineralization is similar to that at Mt. Lyell, below the main syngenetic deposits, and the Jukes--Darwin deposits have been interpreted as " r o o t systems" that were exposed during a period of erosion in the Late Cambrian (Corbett, 1981). Farther south, in the Elliott Bay area [380250] several small disseminated Cu and bedded, massive sulphide Pb--ZnAg--Au deposits occur within sul)aerial and submarine felsic volcanics that are correlated with the central belt of the Mt. Read Volcanits to the north (Large and Wilson, 1982). North of Mt. Lyell, and east of the Henty Fault, are several prospects that have explored disseminations and veinlets of chalcopyrite and pyrite with associated hematite and magnetite in locally altered felsic volcanics, such as at Red Hills [382366] and Lake Dora [387355]. They may also be " r o o t systems" similar to the Jukes--Darwin mineralization (Corbett, 1981), but at Red Hills, a small, black shale-hosted pyrite-sphalerite massive sulphide lcns overlies the disseminated copper mineralization. In the northern part of the Mt. Read Volcanics, north and west of the Henty Fault,

172 the volcanogenic deposits are dominantly large, sub-aqueous, exhalative, stratiform massive sulphide deposits that are enriched in Pb and Zn as well as Cu, Ag and Au. The largest is at Rosebery [379375] tTable 1), where an east-facing, stratiform, polymetallic sulphide deposit and overlying black shale were deposited in a marine environment (Brathwaite, 1974; Green et al.. 1981). The succeeding massive pyroclastics were also deposited under sub-aqueous conditions, b u t the altered footwall ash-flow tuffs apparently are of terrestrial origin, indicating marked subsidence prior to ore deposition {Green et al., 1981), similar to, but not as dramatic as, the rapid vertical movements envisaged by Cox {1981) to have occurred during accumulation of the mine sequence volcanics at Mt. Lyetl. The Hercules deposit [ 3 7 7 3 6 8 ] , located 8 km along strike south of Rosebery, comprises a series of tightly folded pods and lenses of polymetallic massive sulphides, with a mineralogy and east-facing host sequence similar to that at Rosebery {Burton, 1975; Green et al.~ 1981; Table 1). Several small sulphide deposits (disseminated pyrite, massive pyrite--sphalerite--galena ~ occur between Hercules and Rosebery, at the same stratigraphic horizon (Fig. 1; Green et al., 1981). In the R o s e b e r y - H e r c u l e s area generally, alteration of the footwall pyroclastics is very extensive and persists into the lower parts of the overlying massive pyroclastics (Green et al., 1981), consistent with a continuation of hydrothermal activity long after ore deposition (cf. Green et al., 1983; Date et al., 1983). North of Rosebery, the ore horizon is truncated by a meridional fault {Green et al., 1981), but may reappear at The Pinnacles pyrite--galena--sphalerite--silver-gold deposits [378385] {Collins et al., 1981) located a farther 10 km north. Between The Pinnacles and Rosebery is the Chester massive pyrite deposit [378380] that differs from other deposits in the area and is inferred to be at a slightly higher stratigraphic level {Green et

al., 1981). It consists of interbedded pyrite and chert, disturbed by later deformation. and is devoid of base metals and gold. but contains minor silver (Collins et ai.. ! 9 8 i : Table 1 ). The Que River volcanogenic massiw.' sulphide ore b o d y and the polymetallic deposit at the Hellyer prospect are only 3 km apart and located at the northern end of the Mt. Read Volcanics. The Que River Zn- P b - C u Ag--Au deposit [391394] consists ol: a series of discrete, conformable near-vertical lenses hosted by a west-facing sequence of variably altered dacitic pyroclastics and lavas (Wallace and Green, 1982; Wallace, 1984; Whitford, 1984}. There are two main ore lenses: an eastern, Cu-rich, pyritic lens with minor chalcopyrite, sphalerite and galena occurring in veins and disseminations in porphyritic dacite overlain at its northern end by a small massive sulphide lens; and a larger western (overlying), massive, banded sphalerite ..... pyrite--galena lens (Wallace and Green, 1982). Que River is a typical volcanogenic, exhalative deposit, though a feeder zone has not been detected, and apparently was deposited contemporaneously with the enclosing w)lcanits in a shallow, subaqueous environment proximal to a volcanic vent {Wallace and Green, 1982). The dacitic volcanics of the mine sequence occur within an ~,xtensive, dominantly andesitic volcanic succession that also hosts the large Hellyer deposit [393396] which has a mineralogy and grade similar to Que River ore (Table 1). However, at Hellyer the conformable massive sulphide lens and underlying stringer {feeder?) zone are within a sub-horizontal andesitic and basaltic volcanic sequence, contrasting with the w:rtical disposition at Que River (Sise and Jack, 1984). A feature of the mineralized volcanic belt is an apparent clustering of the largest deposits {i.e. Mt. Lyell field, Rosebery--Hercules, Que River--Hellyer) at regular intervals along the belt (Fig. 1). There also appears to be a correlation between the size of a deposit and distance to the nearest deposit {Solomon,

173

1976) which has been attributed to convective circulation cells of sea water driven by Cambrian sub-volcanic plutons (e.g. Solomon, 1981). Another feature, noted by Campana et al. (1958), is that north of the H e n t y Fault the deposits are dominantly zinc and lead rich (except for the Chester deposit), whereas to the south they are dominantly copper and iron rich (Fig. 1). It is also noted (R.R. Large, pers. commun., 1983) that all major bedded volcanogenic deposits in the Mt. Read Volcanics are substantially enriched in silver and gold relative to volcanogenic ore deposits generally (cf. Table 1, and L y d o n , 1984, table 1).

Fossey Mountain and Dial Range troughs The rock units of the Dundas Trough continue north into the 30 km wide, easterly directed Fossey Mountain Trough (Jennings, 1958, 1963, 1979), situated between the Forth and Badger Head regions of Precambrian rocks at the north coast and the Tyennan region to the south (Fig. 1}. Accumulations of volcanic rocks, similar to those of the Mt. Read Volcanic belt contain minor volcanogenic base-metal deposits, and are probably associated with three small granitoid plutons that are at the southern margin of the trough [e.g. 422396]. To the north, the volcanic rocks are in part transitional to sparsely fossiliferous Late Cambrian turbidite lithic-wacke sequences of some 800 m in thickness. Farther north 400--500 m thick mafic volcanic lenses overlie thick chert successions. The chert and mafic volcanic formations of the Fossey Mountain Trough extend into the Dial Range Trough [423440] (Burns, 1965}, which is northerly trending and only 5 km wide at the north coast where it is situated between the R o c k y Cape and Forth regions (Fig. 1}. Conformably overlying the mafic volcanic rocks is a fossiliferous dominantly mudstone sequence of Middle Cambrian age (Palmer, in Burns, 1965, p. 47), and a similar fossiliferous sequence of 1000 m thickness structurally underlying the chert

horizon is of late-Middle Cambrian age ((~pik, in Burns, 1965, p. 34). A large andesitic body, the Lobster Creek Volcanics, occurs in the central region of the trough and appears to be a late stage intrusion (Cooper and Grindley, 1982). Deposition in the Dial Range Trough closed with the accumulation of 150 m of breccia containing fragments up to 120 m long of various lithologies set in a matrix of turbidite lithic-wacke and conglomerate. Instability of depositional conditions, as indicated by the breccia, may have been associated with uplifts of large regions of the trough deposits, which contributed chert detritus to thick terrestrial and shallowmarine conglomerate that accumulated to the west. The siliciclastic conglomerate is correlated with the Owen Conglomerate of Queenstown and it unconformably overlies beds of the Dial Range Trough and the Precambrian rocks constituting the surrounding geanticlines. Correlates of the Owen Conglomerate also rest unconformably on the Fossey Mountain Trough deposits, but there they were derived mainly from Precambrian rocks of the Tyennan Geanticline.

Smithton Trough In northwest Tasmania, west of the Arthur Lineament is a triangular area of Eocambrian(?} and Cambrian volcano-sedimentary deposits, which filled the Smithton Trough (Fig. 1; Lennox et al., 1982). The earliest deposits are of interbedded shallow-marine, stromatolite-bearing dolomite, oolitic limestone, chert, carbonaceous siltstone and dolomitic breccia, with locally developed basal siliciclastic conglomerate and sandstone, which rest with angular u n c o n f o r m i t y on the folded Precambrian Rocky Cape Group. The beds are followed conformably by up to 750 m of tholeiitic metavolcanics including massive and pillowed basalt flows, hyaloclastites, subaqueous chill breccias and mafic volcaniclastic sediments (Baillie and Crawford, 1984). Native copper occurs locally as fine disseminations in the basalt, such as

1 "7 4

at Copper Point near Smithton 1343479]. A second, thinner dolomite unit with minor shale and chert (Large, 1982} is thought to conformably overlie the mafic volcanic suite and is in turn believed to be succeeded conformably by 600 m of fossiliferous interbedded quartzwacke and siltstone, which have yielded faunas indicating a late-Middle to early-Late Cambrian age (Jago, 1971, 1976). The beds of the Smithton Trough usually show open folds with northerly trending gently plunging hinges. The basal dolomite and associated beds of the Smithton Trough are correlated with the Success Creek Group at the base of the Dundas Trough (Williams, 1978), and the youngest sedimentary rocks are equivalent in age to the Dundas Group (Figs. 1, 3, and 5).

Adamsfield Trough The northerly directed Adamsfield Trough is situated at the southeastern margin of the Tyennan region, which forms its western boundary (Fig. 1). The trough is bounded in the southeast by a Cambrian geanticline constituted of probable Precambrian dolomite and quartzite sequences and to the east it disappears beneath a cover of Permian and younger rocks in which windows show underlying Ordovician beds as well as rocks similar to those of the geanticline. The rock units of the trough (Brown et al., 1982; Turner et al., 1985) are in belts parallel to the northerly trending boundary with the Tyennan region. The oldest deposits are at the western margin where they rest with angular u n c o n f o r m i t y on the Precambrian basement. These basal beds include unfossiliferous turbidite sequences with detritus from the Tyennan Geanticline. A northerly trending, fault-bounded ultramafic body (Varne and Brown, 1978} occurs within the unfossiliferous rocks, but its emplacement preceded deposition of fossiliferous mudstone of Middle or early-Late Cambrian age (Quilty, 1971) which contains ultramafic detritus. To the east, the unfossiliferous beds of the

Adamsfield Trough are overlain with a.,~ inferred angular u n c o n f o r m i t y by ~l 500 m thick, fossiliferous, siliciclastic iate-Middle Cambrian succession (Corbett, 1975a; Brown et al., 1982), and farther east, correlates ,~J~ the Owen Conglomerate include fossiliferou:; siliciclastic turbidite beds of Late Cambriar~ age which rest with angular unconformity on older rocks of the Adamsfield Trough. The Adamsfield ultramafic ~.'omplex [446270], an alpine-type complex (Varne and Brown, 1978), is the other main source of platinoid-group minerals in Tasmania {Fig. 4; Hughes, 1965). The minerals are of similar composition to platinoids from the Heazlewood district and are also associated with dunite, but here there appears to have been a two-stage concentration in Tertiary alluvial deposits with some derived from Palaeozoic sediments unconformably overlying the ultramafic complex as well as directly from parental ultramafic rocks (Ford_, 1981).

Other Early Palaeozoic troughs In southeast King Island [255565], a 200 m thick basal sequence consists of diamictite, overlain by interbedded dolomite, dolomitic siltstone and mudstone. This is followed by a thick mafic volcanic suite that includes tholeiitic and picritic pillowed basalt (Scott, 1951; Solomon, 1969). In the Beaconsfield and Port Sorell areas Eocambrian(?) and Cambrian beds dip and face away from the fault bounded Precambrian Badger Head region IGee and Legge, 1974}. On the western flank, at Port Sorell [465440], occurs more than 700 m of interbedded unfossiliferous turbidite lithic-wacke, chert and dolomite. Around Beaconsfield [483438], on the eastern flank, structural slices include fossiliferous turbidite lithic.wacke sequences of late-Middle Cambrian (Jago, 1980), minor andesite, an ultramafic portion of a once floored gabbro chamber (Green, 1959; Gee and Legge. 1974} emplaced in Cambrian rocks, and correlates of the Owen Conglomerate containing heavy

175 minerals (Green, 1959) resulting from erosion of the ultramafic complex (Fig. 1 and Fig. 9, section E--F). At Andersons Creek [480328], near Beaconsfield, serpentinized ultramafics containing up to 0.7% Ni are capped by nickeliferous laterite (Noldart, 1975}, and the same complex yielded chromian spinel that has been concentrated in Early Tertiary paralic sediments (Summons et al., 1981). Cambrian deformation The main E o c a m b r i a n - C a m b r i a n trough in western Tasmania, the Dundas Trough, and the probably contiguous Dial Range Trough to the north formed along the northerly trending boundary between comparatively unmetamorphosed Precambrian rocks of the Rocky Cape Geanticline to the west, and Precambrian metamorphic rocks of the Tyennan and Forth Geanticlines to the east (Fig. 1; Williams, 1978). Remnants of Rocky Cape rock-types at the eastern margin of the troughs indicate the existence of the Precambrian boundary. Early deposition within the deepening Dundas Trough occurred during extension of the continental basement, and culminated in outpourings of basaltic lava. Later, portions of ultramafic intrusions were upthrust into undeformed Crimson Creek Formation sediment during a mild compressional phase that was sufficient to change the configuration of the floor of the trough, for submarine erosion apparently exposed the ultramafic bodies. The same compressional phase may haw~ heralded the volcanism responsible for the bulk of the Mt. Read volcanic belt, and may also have continued t h r o u g h o u t their accumulation (see Williams, 1978; Corbett, 1984), for reverse movements along the westerly dipping Great Lyell Fault [380350] (Fig. 6 and Fig. 9, section C--D) governed Late Cambrian deformation and deposition at the margin of the Tyennan Geanticline during the waning stages of volcanism. Late in the Cambrian, an elongate basin

formed within the mineralized rocks of the Mt. Read Volcanic belt on the eastern side of the Great Lyell Fault (Reid, 1975). The basin filled with volcaniclastic sediments, locally overlain by turbidite quartz-wacke sequences (Corbett, 1975b), and followed by shallowmarine Owen Conglomerate derived from the emerging Tyennan Geanticline. The angular u n c o n f o r m i t y at the base of the basin deposits is known as the Jukesian Unconformity (Corbett et a].. 1974}, and subsequent movements along the Great Lyell Fault caused drag-folds in the lower beds of the, Owen Conglomerate, resulting in another marked angular unconformity, the Haulage Unconformity (Reid, 1975), between the upper and lower beds (Fig. 6). The deformation along the margin of the Tyennan Geanticline did not extend over the Dundas Trough for within the trough it is represented by only a gradual influx into time-equivalent deposits of clasts derived from the rocks of the Tyennan region. The presence of ultramafic masses and the substantial belt of Mt. Read Volcanics haw~ encouraged interpretations involving subduction within the Dundas Trough and the probably contiguous Dial Range Trough. A model involving a westward-plunging zone of subduction between the volcanic belt and the Tyennan Geanticline (Solomon and Griffiths, 1972) has been shown to be incorrect (Corbett et al., 1972) because of the complete absence of any trench deposits at the postulated site of subduction. An eastward-plunging subduction zone at the western boundary of the volcanic belt has also been considered a possibility (Corbett et al., 1972; Solomon and Griffiths, 1974}, but has been shown to be unacceptable because similar Precambrian Rocky Cape sequences are present on either side of the relatively simply deformed deposits of the Dundas and Dial Range Troughs (Williams, 1978). Recently, the metamorphic rocks of the laterally persistent Arthur Lineament were interpreted as belonging to a basement of the Rocky Cape region, which was believed

176

to be one of two blocks involved in collision and development of a subduction complex consisting of the Burnie Formation and th0 fillings of the E o c a m b r i a n - C a m b r i a n troughs to the east (Crook, 1980a, b). Such a model, however, is untenable for there is a 100 Ma discrepancy between the age of deformation of the Burnie Formation and the age of the bulk of the Mr. Read Volcanics as indicated by radiometric data {Black and Adams, 1980: Cooper and Grindley, 1982). Also, there is a considerable structural hiatus between the Precambrian rocks of the R o c k y Cape region and the E o c a m b r i a n - C a m b r i a n sequences of the troughs. Furthermore, the boundaries of the Arthur Lineament with the adjacent rocks are transitional. The known geological relationships indicate that the Dundas Trough, and the probably contiguous Dial Range Trough, developed in a narrow rift zone (Campana and King, 1963; Williams, 1978), possibly as a "failed-arm" rift {Brown et al., 1980) with periods of comparatively minor compression associated with emplacement of ultramafic bodies and possibly with the generation of the Mt. Read Volcanics. Reconstruction of the areal distribution of rock-units during the Early Palaeozoic in Tasmania, by unfolding and unflattening the deformed troughs {Williams, 1983), indicates that the width of the Dundas Trough and associated rocks at probable maximum extension was of the order of only 70 km. In the Smithton (Baillie and Crawford, 1984), Dial Range and Fossey Mountain Troughs (Jennings, 1979) early basement extension was probably marked by outpourings of basaltic lavas on to earlier trough deposits. The areal association between the a c i d - i n t e r m e d i a t e volcanic rocks and the Tyennan Geanticline is similar in both the Fossey Mountain and Dundas Troughs. Throughout the Dial Range, Fossey Mountain and Adamsfield Troughs, and in the Port Sorell--Beaconsfield area, Late Cambrian movements associated with the emergence of the Tyennan Geanticline resulted in deposi-

tion of terrestrial and shallow-marine correlates of the Owen Conglomeratv.. ttsuall.v with angular u n c o n f o r m i t y (~:I ~:h~:. ,.mderiying rocks (Fig. 6) Tho widespread ~-ffo~:L ,.~' these movements indicates that Precambrian rocks of the Cambrian geanticlines ~.ontinm, beneath the Eocambrian and Cambrian trough accumulations. K--Ar and R b - S r whole-rock determina. tions of many Cambrian rocks, including granitoids at the southern margin ot' th0 Fossey Mountain Trough give an Ordovician age of 450--490 Ma, which may reflect the influence of either the Late Cambrian mow:ments, an Ordovician thermal event or widespread, later Devonian deformation {McDougall and Leggo, 1965; I. McDougall, pers. commun., 1983; Adams et al., 1985).

Ordovician--Devonian transgressive and shelf deposits in western Tasmania, the regionally developed Owen Conglomerate and its correlates rest conformably on deposits filling local basins that developed on the surface of the tilted and eroded underlying rocks. These basal beds include volcaniclastic rocks and turbidite quartz-wacke sequences, which rarely contain fossiliferous Late Cambrian horizons. The siliclastic Owen ConglOmerate and its correlates, of probable Late Cambrian to Early Ordovician age, consist of terrestrial fans of conglomerate, and shallow-marine conglomerate and quartz sandstone (Banks, in Spry and Banks, 1962, pp. 147--176), which attain a thickness of some 1200 m at Mr. Owen, east of the Great Lyell Fault {Wade and Solomon, 1958), and more than 1500 m near Adamsfield (Corbett, 1975a). The transgressive siliciclastic deposits are followed conformably by the Early to Late Ordovician Gordon Limestone {Fig. 7: Banks and Burrett, 1980). The limestone deposits reach a thickness of 2000 m near Adamsfield (Corbett and Banks, 1974), and generally accumulated under warm peritidal conditions, though a deep-water carbonate, probably

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turbiditic, sequence has been recorded near Surprise Bay 1.172473] (Burrett et al., 1984). The Gordon Limestone locally contains stratabound, disseminated and veined basemetal sulphides that may represent a minor Ordovician metallogenic event (Fig. 7). Finegrained galena, sphalerite and pyrite, believed to be of possible syngenetic or early diagenetic origin (Ellis, 1984), occur at some Pb--Zn deposits in the Zeehan area (e.g. Oceana mine [362357] which has an estimated resource of 2 Mt at 8.6~ Pb, 4.0~, Zn and 69 g t -I Ag; Ellis, 1984). At Bubs Hill [399335], sphalerite and galena occur in irregular veins and patches apparently confined to particular stratigraphic horizons in the limestone (Growls, 1972a). The limestone deposits are succeeded conformably by dominantly shallow-marine quartz sandstone and mudstone, and minor

limestone, of the Early Silurian to Early Devonian Eldon Group and its correlates (Fig. 7), which are up to 1800 m thick at the site of the Dundas Trough (Banks, in Spry and Banks, 1962, pp. 177--187). At Point Hibbs [280357] deformed limestone of Middle to Late Siegenian age (Flood, 1974), contains Devonian conodonts of a colour indicative of low temperatures, and also reworked Ordovician conodonts. The older conodonts are presumably derived from nearby Gordon Limestone and their colour suggests temperatures of about 300°C (Burrett, 1984}. Evidently, limited tectonic activity occurred before deposition of the limestone at Point Hibbs, which was accompanied by erosion of older beds that had been subjected to a widespread thermal event. A large number of isotopic ages obtained from the Eldon Group and older rocks range

178

within 400--420 Ma, and may, in part, reflect the effects of the later Devonian deformation (Adams et al., 1985).

near Scamander [606409J: on the east coast. mudstone layers in the more arenaceous so. quences to the east have yielded graptolite~ of middle-Early Devonian age (Rick;~rds .~nti Banks, 1979)•

Folded sedimentary rocks of northeastern Tasmania

Devonian deformation The Owen Conglomerate and younger conformable strata of western Tasmania differ, for most part, from the folded Mathinna Beds of comparable age east of the Tamar River (Figs• 1 and 8). The Mathinna Beds, which underlie large regions of eastern Tasmania and the Furneaux Group of Islands, consist of interbedded mudstone, siltstone and quartzwacke, transported by turbidity currents from the western margin (Williams, 1959; Marshall, 1969). At Turquoise Bluff [505456], near the eastern margin of the Tertiary Tamar trough, Early Ordovician graptolites have been recovered from dominantly mudstone successions (Banks and Smith, 1968), whereas

The Eocambrian--Cambrian to Early Devonian rocks of Tasmania are extensively deformed by parallel folds• In the north of western Tasmania, at Eugenana [442433], undisturbed, spore-rich, late-Middle Dewmian, terrestrial cavern fillings contain blocks of deformed Gordon Limestone that have collapsed from the cavern walls (Fig. 8; Balme, 1960; Burns, 1965). Evidently, deformation of the Gordon Limestone, and the beds of Silurian to Early Devonian age which follow conformably, terminated before accumulation of the speleologic deposits. In northeastern Tasmania, near St.. Marys ' " -~;::"~$'!"i'c'~d .'.: :":.:,::: C . .::,~.: i.:i::~L'::.. : -..:

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179 [395600], folded middle-Early Devonian Mathinna Beds are separated by an angular u n c o n f o r m i t y from overlying acid-pyroclastic deposits of the St. Marys Porphyrite {Fig. 8) that has a R b - S r isotopic age of 388+1 Ma {Turner et al., 1986). Thus the deformation in northeast Tasmania is apparently the same age as that affecting the Early Devonian and older rocks of western Tasmania. The widespread Devonian deformation has been correlated with the Tabberabberan Orogeny of eastern Australia (Browne, 1949) and has been described in many studies (e.g. Carey, 1953; Bradley, 1956; Jennings, 1958, 1963; Jennings et al., 1959; Solomon, in Spry and Banks, 1962; Solomon, 1965; Burns, 1965; Williams, 1978; Seymour, 1981). Western Tasmania

During the Devonian deformation, which is expressed by two main phases of folding, the Cambrian Geanticlines of western Tasmania behaved as relatively competent blocks {Williams, 1978). In the early phase, folds developed in zones of closure between converging blocks resulting in fold patterns of the northerly West Coast Range/Valentines Peak trend* in the west {Fig. 9, sections A--B and C--D) and the easterly Loongana/Wilmot trend in the north. The dominantly later phase arcuate folds in northern Tasmania are of the northwesterly to northerly Deloraine/ Railton trend, and in the west, are of the northwesterly Zeehan/Gormanston trend extending from within the Rocky Cape Block to within the Tyennan Block, which also yielded in a narrow zone and behaved as two blocks I Fig. 1). '['he first-order folds of the northerly West Coast Range/Valentines Peak trend have half wave-lengths varying from 2 km at the margin of the Tyennan Block to 15 km to the west. Westerly dipping axial surfaces, as at Mt. Farrell 1.387380] (Barton et al., 1966), and associated reverse faults and thrusts, such as *See Fig 1 for Iocationof fold trends.

at Tyndall Range [360382] (Corbett, 1975b) and Dial Range [445423] (Burns, 1965), indicate that the folds resulted from tectonic transportation from the west. Folds of the easterly Loongana/Wilmot trend in northern Tasmania are symmetrical and have a half wave-length of some 5 km (Jennings, 1963). In the late fold phase of the Deloraine/ Railton trend of northern Tasmania, the firstorder folds have a half wave-length of about 2.5 km (Jennings, 1963; Gee and Legge, 1974), and they resulted from transportation from the northeast for they exhibit asymmetry with northeasterly dipping axial surfaces, associated cleavage and thrusts. The thrusts form an imbricate system at the eastern margin of the Badger Head Block (Fig. 9, section E--F). The Precambrian rocks of the Tyennan Block near Queenstown [3853,12] {Solomon, 1965; Cox, 1.981) yielded in a narrow zone along the northwesterly Zeehan/ Gormanston trend during the later phase o1" folding and became deformed together with the overlying Owen Conglomerate, associated conformable strata and the Great Lyell Fault in the 6 km wide, fault-bounded Linda disturbance (Fig. 1). '['his zone of deformation extends to the southeast and to the northwest, where it broadens around Zeehan (Blissett, 1962) and continues into the Rocky Cape Block. In the Black Bluff region [410414] relationships between interfering structures suggest that the oldest folds arc of the easterly Loongana/Wilmot trend, with those of the northerly West Coast Range/Valentines Peak trend later, followed by folds of the northwesterly Deloraine/Railton trend (Seymour, 1981 ). Northeastern Tasmania

Folds in the Mathinna Beds tGee and Legge, 1974; Williams, 19781 trend northwesterly with hinge lines either horizontal or plunging gently to the southeast (Fig. 1). First-order folds have a half wave-length of about 20 km. The folds typically are long-

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WEST COAST RANGE

390350

Fig. 9. Vertical geological s e c t i o n s ( c o - o r d i n a t e s of e n d p o i n t s refer to Fig. 1 ). S e c t i o n A - - B : D u n d a s T r o u g h , b a s e d o n Blissett { 1 9 6 2 ) and B r o w n (1986). Section C - - D : Mt. Read volcanic belt, based o n C o r b e t t e t a ] . ( 1 9 7 4 ) . S e c t i o n E - - F : D e v o n i a n d e f o r m a t i o n o f d o m i n a n t l y later g e n e r a t i o n D e l o r a i n e / R a i l t o n t r e n d at Beaconsfield, based o n (:lee anti Legge !'1974). S e c t i o n (;--.!t: M a t h i n n a Beds d e f o r m a t i o n , based o n Gee a n d Legge ( 1 9 7 4 ) and Marshall ( 1 9 6 9 ) .

~

.

~ ~ , ~ , , . ,

PEAKED HILL I

LY|LL FAULT

GREAT

~ R , ' ~ : X ~ ' ~ , ~ ' , "

i~-,,~,~.~;.,.

C

374349

F ~ ~ . . ~ _ _ _ ~ _ ' ~ _ ~ 7 = = = - . i ~ f = _ ~ : - = j - ~ m ~ : = r : ! ~ ~ 6 " - - . I ~ ~ - . -~-~."--~-:i '~-~u-~-~ = ' : ~,~ ~ , ~ / ~ ~ ~ - -

I

E

455434

I

[A"

MT DUNDAS

B MISERY HILL ,,

A

ZEEHAN ,,

374361

360363

O

F~

181

limbed with narrow hinge zones, and they are c o m m o n l y asymmetrical with axial surfaces dipping to the southwest indicating tectonic transportation from the southwest. This direction of movement contrasts with the transportation from the northeast, which resulted in folds of similar age west of the Tamar River (Fig. 9, sections E--F and G--H). Devonian granitoid emplacement and associated ores During the Middle to Late Devonian, substantial post-kinematic granitoids with narrow contact aureoles were emplaced at relatively shallow depths within the folded rocks throughout Tasmania. The granitoids and associated t i n - t u n g s t e n , silver-lead--zinc and probably also gold deposits constitute the final, major metallogenic epoch in the develo p m e n t of the Tasman Fold Belt System in Tasmania (Fig. 10). Although the granitoids are widespread, there are significant differences between western and northeastern Tasmania in the age and composition of the granitoids and in mineralization styles assoelated with their emplacement.

Devonian grani toids The relatively large granitoid masses in eastern Tasmania (i.e. Blue Tier Batholith, Scottsdale Batholith) contain several distinctiw~ plutons composed mainly of l-type hornblende--biotite granodiorite and S-type garnet--l)iotite granite and biotite granite (Figs. 1 and 10; Cocker, 1977, 1982; Groves, 1977; McClenaghan et al., 1982; Higgins et al.. 1985). They have minimum isotopic ages ranging from a b o u t 348 to 389 Ma (McDougall and Leggo, 1965; Cocker, 1982; I. McDougall, pers. commun., 1 9 8 3 ) a n d are regarded as significantly older than the granitoids in western Tasmania which range from a b o u t 332 to 367 Ma (McDougall and Leggo. 1965; Brooks and Compston, 1965; Brooks, 1966; I. McDougall, pets. commun., 1983). The western granitoids are composed

dominantly of S-type biotite granite, though I-type biotite--hornblende granodiorite crops out in southeast King Island (Figs. 1 and 10). The granitoids of northeastern Tasmania appear to have been passively emplaced by diapiric intrusion and roof-lifting with minimal assimilation (Cocker. 1977, 1982; Groves, 1977), though local folding resulted from granitoid intrusion in a number of areas. such as at Bridport [533463] (Marshall. 1969) and Dianas Basin [607420] (Gee and Groves, 1971). Foliations, defined by mineral alignments, are c o m m o n within the granitoid masses (McClenaghan et al., 1982) and in some localities resulted from post-intrusion regional stresses, whereas in other localities appear to be related to flow during ernplacement. In northeastern Tasmania generally, intrusion of sheet-like bodies of granodiorite was followed by emplacement of granite plutons. In the composite Blue Tier Batholith, the granodiorite and granite plutons have distinctive mineralogic, al, geochemical and Sr isotopic compositions, which indicate derivation from separate melts formed from Proterozoic source rocks (Cocker, 1977, 1982; McClenaghan et al., 1982). The granodiorite may have resulted from crystallization of a basic parental magma, which caused melting of crustal sedimentary rocks giving rise to adamellite (McClenaghan, 1984}. This contrasts with previous suggestions (e.g. McCarthy and Groves, 1979) that the Blue Tier Batholith formed by m-situ fractional crystallisation of a single "adamellitic" parent. The l-type granodiorite of the Blue Tier Batholith is associated, near St. Marys [ 3 9 5 6 0 0 ] , with a dacitic ignimbritc, intracaldera sheet, which is part of the St. Marys Porphyrite and rests with angular unconformity upon folded Early Devonian Mathinna Beds (Turner et al.. 1986). The granite plutons in northeastern Tasmania have been divided into two predominant rock types: biotite adamellite and younger, more silicic, biotite and biotitemuscovite alkali-feldspar granite (Fig. 1;

I b2

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Fig. 10. Schematic section of mineralization associated with emplacement of Middle and Late Devonian granitoids throughout Tasmania ( ci. Fig. 8). For legend see Fig. 11. Representative mines and deposits: i Zeehan field: argentiferous galena and sphah;rite in veins; 2 = Interview River: wolframite and scheelite in quartz--.greisen veins; 3 = South Heemskirk: cassiterite in quartz--tourmaline--topaz alteration veins, pipes and breccias; 4 ~. Kara mine: scheelite in skarn: 5 7 = Stormont: gold--bismuth skarn (5), cassiterite, wolframite, molybdenite and bismuthinite in quartz veins as at Shepherd and Murphy and All Nations 16, 7). "wrigglitt," skarn at Moina (6), and peripheral Ag--. Pb-..Zn veins in Moina district I 7); 5 :o Tasmania, Beaconsfield: auriferous quartz reefs; 9 = Aberfoyle and Storeys Creek: cassiterite and wolframite in quartz veins, and Great Pyramid: cassiterite in quartz vein stockwork; 10 = district zoning at Scamander; I I = Mathinna-Mangana: auriferous quartz reefs; 1 2 = Mt.Paris: cassiterit~ in greisens and veins; 13 = Anchor, Blue Tier: cassiterite in greisens and veins within earlier granitoids; 14 ~ King Island: scheelite in skarn; 1 5 ==Renison: cassiterite in massive pyrrhotite carbonate replacement bodies; 1 6 = Cleveland: cassiterite, stannite and chalcopyrite in carbonate replacement lenses; ] 7 = Foley zone, Cleveland: wolframite, cassiterite, molybdenite and fluorite in quartz vein stockwork; 18 = Balfour: cassiterite in quartz vein stockwork and chalcopyrite in fracture zones; 1 9 : Mr. Bischoff: cassiterite in carbonate replacement, bodies and a!tered porphyry dykes; 2 0 = Oakleigh Creek: wolframite and cassiterite in quartz veins•

G r o v e s , 1 9 7 7 ; M c C l e n a g h a n e t al., 1 9 8 2 ; M c C l e n a g h a n a n d W i l l i a m s , 1 9 8 2 } . T h e alkalif e l d s p a r g r a n i t e s w e r e e m p l a c e d as s m a l l d o m a l b o d i e s a n d a p p e a r t o haw? b e e n d e r i v e d by crystal fractionation of parental adamellite ( M c C l e n a g h a n a n d W i l l i a m s , 1 9 8 2 ) , a n d haw, been subjected to varying degrees of pervasive h y d r o t h e r m a l a l t e r a t i o n {e.g. C o c k e r , 1 9 8 2 ) . T h e y e x h i b i t several characteristics of the "metallogenically specialized granites" of

T i s c h e n d o r f { 1 9 7 7 ; e.g. e n r i c h e d in F, R b , Li, U, T h ) a n d in t h e B l u e T i e r r e g i o n are genetically linked with tin (- t u n g s t e n ) mineralization. T h e g r a n i t o i d s o f w e s t e r n T a s m a n i a are petrologically and g e o c h e m i c a l l y similar to, and m o s t likely have a similar origin to the northeastern granitoids, though they have not b e e n s t u d i e d in t h e s a m e d e g r e e o f d e t a i l , rrhe western granitoids characteristically are

183 related to several quartz--feldspar p o r p h y r y intrusive systems that extend for considerable distances from source granitoids and c o m m o n l y are associated with major tin deposits as at Mt. Bischoff [376412], (Growls et al.. 1972), Renison Bell 1369372] (Patterson et al., 1981), Cleveland [365406] and Zeehan 1360360] (Fig. 10).

Granitoid-related mineralization Tin and tungsten deposits Primary tin and tungsten deposits occur in rocks ranging in age from Late Proterozoic to Late Devonian (Table 2), and are spatially and genetically associated with emplacement of the Middle to Late Devonian granitoids (Figs. 1 and 101. The distribution of t i n - t u n g s t e n deposits presumably reflects a subsurface granitoid structure, and interpretation of gravity data (Leaman et al., 1980) indicate that the outcropping granitoids are only the roof projections of much larger " b a t h o l i t h s " (map inset, Fig. 1). Most deposits occur within the 1 km subsurface contour of the gravity-derived " b a t h o l i t h s " and clearly delineate two tin tungsten metallogenic districts: northeastern Tasmania and western Tasmania, but each with separate styles of mineralization (Fig. 1 and Table 2). In northeastern Tasmania, tin and tungsten mineralization appears predominantly as disseminated cassiterite in greisenized granite (e.g. Blue Tier district) and as dilational quartz---wolframite + cassiterite vein systems (e.g. Aberfoyle, Storeys Creek). In contrast, tin and tungsten skarn and replacement deposits are d o m i n a n t in western Tasmania, reflecting the high incidence of carbonate rocks in the Late Proterozoic to mid-Palaeozoic sedimentary sequences (e.g. Renison, Cleveland, Mr. Bischoff, King island). Greisen and vein-style deposits are also found in western Tasmania but are relatively minor. Most. endogranitic tin deposits are in small, domal bodies of alkali-feldspar granite in the Blue Tier Batholith, and localised at the southern margin of the l leemskirk Granite in

western Tasmania (Fig. 11. They are generally low-grade ( < 0 . 5 % Sn) and of small to moderate tonnage (e.g. total resource at the Anchor mine, Blue Tier is about 5.5 Mt averaging 0.25% Sn; Table 2). In the Blue Tier district, cassiterite is disseminated within greisenized biotite (muscovite) alkali feldspar granite which was emplaced into earlier and much larger bodies of biotite adamellite tGroves, 1977: McClenaghan et al., 1982; McClenaghan and Williams, 1982). The greisenized granite and greisens generally are confined to irregularities in the upper surfaces of the alkali-feldspar granites in structural traps adjacent to overlying impervious biotite granite and have an autometasomatic rather than post-magmatic origin, such as at the Anchor mine [585435] (Fig. 10; Groves and Taylor, 1973: Growls, 1977). Elsewhere in the Blue Tier Batholith there are small, post-magmatic deposits of cassiterite in greisenized granite adjacent to narrow greisen veins t)enetrating alkali-feldspar granite in the Mt. Paris area [564,137] (Jack, 1966) and in older biotite granite at the Cambria mine [580,134] !Green, 197.1). Similar deposits are found in some of the western granitoids, such as the South Bischoff mine [370,101] (Jack and (]roves. 1965 ~. In the Heemskirk Granite, layered, biotite granite ( " r e d " granite) apparently has been intruded by a sill-like body of more alkaline, biotite--muscovite granite ( " w h i t e " granite) (Klominsky, 1972; Wells, 1978). Tin mineralization associated with the "'white" granite is found as disseminated cassiterite in greisen veins, pipes and irregular zones of quartztopaz- tourmaline alteration in argillized granite, such as at the Federation mine 13,18359] (Fig. 10; Wells, 19781, and i n p o l y metallic sulphide assemblages at. Sweeneys and Globe mines 1351358] (Wells, 1978: Roberts, 1984). Breccias that developed in some of t.he larger veins/pipes within the granite, contain exotic fragments of hornfels indicating close proximity to the roof of the granitoid.

V V V V

G G

M.-L. D e v o n i a n ( g r a n i t e ) h o s t Anchor [ 585435] Ro?,'al Gco.rge [ 5 7 4 3 6 8 ]

Sk Sk

C-r C-r C-r Sk C-r

Eocamllrian--E. Cambrian host * Renk~on [369371 ] * Cleveland [365406] Q u e e n Hill/Severn [ 3 6 0 3 6 0 ] * King Is. m i n e s [ 2 5 0 2 6 2 / 5 ] Razorback [368363[

Ordovieian--E. Devonian host Moina [ 4 2 0 4 0 7 ] * Kara [398425] Aberfoyle [ 5633fl7] Storeys Creek [561390] Greaf Pyramid [599413 | Shepherd attd Murphy [420407]

V

Sk

C-r

'Fype '~

St. Dizier [ 3 4 4 3 6 8 ] Oaldeigh Creek [419374]

Late Proterozoic host Mt. Bisehoff [37641 2]

DEPOSITS

| co-ords] ~

TIN/TUNGSTEN

Mine/deposit

0,2 0..I

0.23

0,2

1.94 [). 1 (i

0.9l 0.18

O. 15

9.77 0.1

0.61 2.1 1.1

1.23 0.70

1.56

Sn (%)

Grade 2

0.11

O. 18 0.28 [.09

0.61

0 4

WO (%)

to 1984

8.70 5.51

0.02

5.59

Mt

Production

0.31

Cu (%)

Production and ore reserve estimates for principal Devonian granitoid related deposits in Tasmania

'FABLE 2

3.5 0.16

3 0.08

26 1.6

16.7 0.26 7.3 7.14 0.24

4.73 5 0.1

Mt

0.28 0.6

0.33 0.21

0.1

l.l

1.06 0.77 0.7

0.62 0.5

Sn (%)

Grade

tP.;t7

0.I 0.8

1.01

1.2

WO ~ (%)

Published reserves/resource

0.34

Cu (%)

(;R PR

GR PR

GR' PR

PR ~ PR GR PR PR

GR GR PR

Category

3, ! ] i2

iiJ

7 4 [,5,8 4,5,8

3 5 5 6 4

I. 2 3 4

Source

oc

VEIN DEPOSITS

REEFS

1 ~._

Ag

1.09 0.30 0.02 0.08

506 370 427

0.30 0.43 0.63

24 26 14 31

AU (g t-l)

(420) (475) (480) (284) (273) 143

(0.5) (0.4) (0.3) (0.2) (0.2) 0.04

(et-‘)

11.4 7.3

Id.9

(9.7) (10.1) (8.7) (5.3) (7.5) 5.3

Pb (90)

4.8 7.3

1.0

0.22

(0.4) (0.1)

CU (W)

(0.6) (0.1)

Sl-I (B)

1

0.07

0.03

24

4

Ag

12.3

11.5

~~-9et

318

52

‘) 1

(8)

cu

4.8

8.8

Zn (F/o)

0.24

0.32

cu (8)

PR

PR

GR

18,19 18 18 18

13 13 13 13 13 13 14 15 15.16 17

’ Mine/deposits in northeastern Tasmania in italics. * = operating mine. Coordinates in [ ] refer to Fig. 1. ‘Grade of ore treated. recovered grade in italics. Grade and tonneage for the Zeehan mines in ( ) are estimates based on concentrate sales. ‘Current published reservelresourcr estimates catcgorised as PR = proved/measured + probable/indicated reserves or mineable reserves; GR = geological resource. Genetic types of tin/tungsten deposits are: C-r = carbonate-replacement; G = greisen: Sk = skam: 11 = single. sheeted or stockwork vein. ’ Additional 16.95 Mt of possible ore. Renison Goldfields Consolidated Ltd. 1982 Annual Report. ’ Moina skarn also contains 18% CaF,: Wright and Smyth. 1982. Data SCAUC~S: 1 = Production compiled from Groves et al., 1972; 2 = annual reports Metals Exploration Ltd.; 3 = annual reports Rcnison Ltd. and Renison Goldfields Consolidated Ltd.; 4 = annual reports Direclor of Mines, Tasmania; 5 = annual reports Aberfoyle Ltd.: 6 = annual reports Peko Wallsend Ltd.: 7 = Wright and Smyth. 1982: 8 = Blissett. 1959: 9 = Ruxton and Plummcr. 1984; 10 = *Jennings. 1979: 11 = Thomas, 1953; 12 = Reid and Henderson. 1929: Noldart, 1968: 13 = compiled from production data in Blissrtt.. 1962 (see note 2): 14 = Green. in Baillie and Corbett, 1985; 15 = Collins et al.. 1981; 16 = Electrolytic Zinc Co. A/Asia Ltd.. pus. commun.. 1985: 17 = compiled from Cottlc. 1953; 18 = Noldart and Threader. 1965: 19 = Min. J.. 304 (781%). 1985: p. 331.

Tasmania (Beaconsfield) [484438] .Vew Golden Gale (Mathinna) [574406] Tnsmar~ian Co%wJs (Mathinna) [ 574407 Lefro?;(7 mines) [5004501

GOLD

Zeehan Montana [361363] Spray (Zeehan) [3593581 Zeehan Western 13613631 Oonah (Zeehan) [3593621 Zeehan Queen [3603601 Montana (Zeehan) [359365J Queensberry [3663451 North Mt. Farrell 1385379 I New North Ml. Farrell [3863801 Magnet [37041OJ

SILVER-LEAD--ZINC

186

Quartz--wolframite-- cassiterile vein deposits, with only minor sulphides, occur in Preeambrian to Late Devonian rocks throughout Tasmania, but they are relatively small (e.g. Aberfoyle, the largest, yielded about 20,000 t Sn, 6000 t W()~ ; and nearby Storeys Creek about 2000 t Sn, 12,000 t WO.~:Table 2). Most deposits form discrete veins or sheeted-vein systems penetrating the country rock overlying altered granitic cupolas. They include the sheet-vein systems in Mathinna Beds at Aberfoyle [5633871 and Storeys Creek [561390] (Fig. 10; Kingsbury, 1965), several parallel veins in skarn and Ordovician elastic sediments at Shepherd and Murphy, Moina [420407[ (Fig. 10; Jennings, 1965) and a single quartz--wolframite vein in metamorphosed Precambrian rocks at Oakleigh Creek [419374] (Fig. 10). Granitic cupolas are also inferred to underly some stockwork vein tin deposits, such as at Great Pyramid 1599413] {Groves, 1972b; R u x t o n and Plummer, 1984) and Balfour 13234291, but at Cleveland [3654061 W.---Sn--Mo--Bi stockwork vein mineralization (Foley zone) is centred on the top of an altered quartz-feldspar porphyry dyke in the footwall of stratabound, cassiterite-sulphide deposits (Fig. 10; Collins, 1981, 1983). Discrete veins haw.~ also been mined within granite, such as the quartz-..wolframite-.-scheelite veins at Interview River [324398] (Fig. 10; Henderson, 1943). Of the primary tin deposits, the largest and most significant are the stratabound cassiterite--stannite-pyrrhotite ores which formed by replacement of carbonate beds in Precambrian--Cambrian sedimentary sequences (e.g. Renison, Mt. Bischoff. Cleveland, Queen Hill: Fig. 10 and Tabh-, 2). In addition, pyrrhotite-cassiterite ore has replaced talcose dolomite at the contact between an ultramafic mass and Dundas Group sedimentary rocks at Razorback [368363], near Zeehan (Blissett and Gulline, 1961), where the carbonate is thought to be related to serpentinization of the ultramafic mass.

At Mt. Bischoff [376412], ,-assiterlu: occurs in stratabound lenses or' massive pyrrhotite which has replaced .i. ~ioiomite bed within Preeambrian sedimentary ro,::k~, (correlated with the ()onah F<~rmation.'. adjacent to altered, cassiteri te.-bearing, Devonian quartz--feldspar porphyry dykes (Groves et al., 1972). The large, stratabound, eassiterite-bearing massive pyrrhotite deposits at Renison Bell [369371.] haw~ replac.ed dolomite horizons within the Eocambrian(?) Success Creek Group, adjacent, to a major fault (Federal-Bassett structure) wi~ich also contains vein mineralization with a mineralogy similar to the stratabound ore (Patterson et al., 1981). The Cleveland tin.-. copper deposit [3654061 comprises several stratiform lenses of pyrrhotite-ca:;siterite stannite--chalcopyrite mineralization that has replaced limestone within a succession of marine basalt, siltstone and volcanie-lithic turbidites correlated with the Early Cambrian(?) Crimson Creek Formation (Collins, 1981, 1983). In contrast to the other replacement tin deposits, the Cleveland ore and the Queen Hill deposits at Zeehan [3603601 are comparatively enriched in copper (e.g. 0.31% Cu at. Cleveland). These deposits are m sequences dominated by marie volcanies and the copper is believed to have been leached from basaltic rocks (Collins, 1983). Although altered feldspar--quartz porphyry dykes are found at, or in (:lose proximity to, each of the stratabound tin deposits, and biotite granite (and altered granite) crops out nearby, they are all relatively remote from granitoids. Assuming a granitic source for the mineralizing fluids, then the metals (and sulphur) have been transported m solution over considerable vertical distances, possibly as much as 1 kin. This remoteness from a granitoid, proximity to the Mr. Read Volcanics and an association of some. deposits with marie volcanies, has led some authors to suggest a sedimentary--hydrothermal or voleanogenic origin for these deposits (e.g. Hutchinson, 1979, 1981; Lehman and Schneider, 1981). ttowever, the ~:ontrasting

187 stratigraphic relationships and environments of accumulation of the host rocks, combined with mineralogic, isotopic and structural data (Patterson et al., 1981; Collins, 1981), clearly indicate a granite-associated, epigenetic, replacement origin for the stratabound tin ores. Of the primary tungsten deposits, the most important are the scheelite-bearing exoskarns, including the Grassy and Bold Head deposits on King Island (Figs. 1 and 10) which contain in excess of 130,000 t WO3. At Grassy [ 2 5 0 5 6 2 ] , scheelite is mainly in andradite skarn in a contact metamorphosed Eocambrian--Cambrian mafic volcanic-sedimentary sequence (similar to the Crimson Creek Formation), on the flank of an l-type biotite--hornblende granodiorite (Edwards et al., 1956; Danielson, 1975). At Bold Head [ 2 5 0 5 6 5 ] , a b o u t 2 k m north of Grassy, a similar but smaller skarn deposit is adjacent to biotite adamellite (Large, 1972; Danielson, 1975). The King island deposits carry minor molybdenite and the scheelite contains 2.0-2.5% Mo (Danielson, 1975). The Kara deposits [ 3 9 8 4 2 5 ] , located about 20 km south of Burnie, m'e in Ordovician limestone that has been replaced by scheelite-bearing magnetiterich skarn, occurring as roof pendants above biotite granite (Fig. 10). At each of these deposits the contiguous granite is essentially unaltered, and contains magnetite as an accessory phase (Solomon, 1981). In addition to the skarn and replacement deposits described above, several other tin and tungsten-bearing skarns have formed adjacent to granitoids in western Tasmania, but they are generally low grade {i.e. < 0.5% Sn) and much of the tin is in minerals other than cassiterite. Examples include the unusual magnetite- fluorite "wrigglite" skarn in Ordovician limestone at Moina [420407] (Fig. 10; Kwak and Askins, 1981; Wright and Smyth, 1982) and neart)y gold-bismuth skarn at Stormont {Fig. 10); a complex cassiteritebearing magnetite---garnet skarn replacing caltic marble in correlates of the Crimson Creek Formation at Mt. Lindsay [3603811 (Kwak,

1983; Eadington and Kinealy, 1983) and a similar deposit nearby at Stanley Reward 1356383] which is in Success Creek Group correlates; and others at St. Dizier [ 3 4 4 3 6 8 ] . Mt. Ramsay [372394], Tenth Legion [354361] and Mt. Youngbuck [ 3 5 6 4 0 3 ] , all in EocambrianI?)--Early Cambrian sedimentary sequences.

Silver--lead--zinc deposits Several exogranitic tin and tungsten deposits are clearly zoned (e.g. Cleveland, Fig. 10) and have outer haloes of argentiferous lead and zinc-sulphide vein mineralization. The best known is the Zeehan field, which yielded about 0.2 Mt Pb and 826,000 kg .-\g (Table 2) but district zoning has also been defined at Mt. Bischoff ((;roves et al.. 1972). Cleveland (Collins, 1981, 19831 and Moina (Jennings, 1965) in western Tasmania. and at Scamander in eastern Tasmania iGroves. 1972b; R u x t o n and Plummer, 1984; Fig. 10). The Zeehan field has been the subject of much discussion and interpretation (cf.. Ward, 1911; Both and \Villiams, 1968; Both et al., 1969; Solomon, 1981), but it is now thought that. most of the lead--zinc veins are genetically associated with, and centred on the Queen Hill/Severn cassiterite-sulphide replacement deposit [3603601. and that west to east zoning of pyritic to sideritic ores away from the Heemskirk Granite reflects composition of the local host rock, with sideritic gangue ores due to dissolution of Ordovician limestone which underlies much of the eastern part of the f M d (Solomon, 1981). However. interpretation of the Zeehan field is further complicated by the detection of possible syngenetic Pb- Zn sulphide mineralization in some sections of the Ordovician limestone (e.g. Ellis, 1984 ~. The Mt. Bischoff and Cleveland tin deposits each have haloes of base-metal sulphide mineralization, and mid-way between them is the small, t)ut rich Magnet Ag--Pb...Zn mine, (Cox, 1975; Table 2). At Magnet 1370410]. sphalerite, argentiferous galena and manganosiderite filled open fractures along a fault

!88 boundary between an upthrust Cambrian mafic/ultramafic mass (Magnet " d y k e " ) a n d correlates of the Crimson Creek Formation. An enigmatic style of mineralization occurs within the Mt. Read Volcanics, near Mt. Farrell [386378], where there is a suite of small deposits along a 14 km long northnortheast trending fracture zone, that is the probable northern extension of the Henty Fault {Fig. 1). On a regional scale, the deposits are stratabound, being confined to a 700 m thick mudstone, volcanic lithic-wacke and tuff sequence (Farrell Slate) and include the North Mt. Farrell and New North Mr. Farrell argentiferous galena--sphalerite--chalco-pyrite--siderite deposits which yielded about 9 5 , 0 0 0 t Pb and 3 1 0 , 0 0 0 k g Ag, and are similar in mineralogy and grade to granitoido related vein deposits (e.g. Zeehan, Magnet; Table 2). Most of the deposits are locally transgressive, occurring as lenticular, bifurcating veins which acutely cross-cut the host sedimentary and tuffaceous rocks i Burton, 1965; Collins et al., 1981), though in some deposits the mineralization appears to be locally stratiform. A Cambrian volcanogenic origin has been suggested (e.g. Solomon, 1981; Polya et al., 1984), but t h e y are most likely associated with emplacement of the Devonian granitoids.

Gold deposits Primary gold mineralization is largely restricted to eastern and northern Tasmania. The gold deposits rarely are spatially associated with the Middle-Late Devonian granitoids (Fig. 1), though there is an apparent genetic relationship between the gold mineralization and hornblende-bearing granodiorites (Klominsky and Groves, 1970). At several localities, g o l d - q u a r t z veins occur within, or adjacent to biotite--hornblende granodiorite, such as at Lisle [528435] and Golconda [525444] (Klominsky and Groves,

1970; Groves, 1977). Other deposits occur as quartz reefs occupying foliation surfaces, bedding and joints along a 70 km belt in the Mathinna Beds from Mangana 1574395] north through Alberton [566430] iNoldart and Threader, 1965) between the more granodioritic parts of the Scottsdale and Blue Tier Batholiths (Figs. 1 and 10}. Although a genetic relationship with granodiorite appears probable for most of the gold deposits, it is inconclusive, because some of the larger deposits are remote not only from exposed granitoid but also from gravityderived, subsurface projections of a granitoidcrust surface (Fig. 1). These deposits include the auriferous quartz reefs filling shear zones at Lefroy [500450] and at the Tasmania mine near Beaconsfield [484438], which produced over 26,000 kg Au (Table 2 and Fig. 10; Noldart and Threader, 1965). They occur in Ordovician sedimentary rocks and are most likely of Middle to Late Devonian age, but a granitic source for the mineralization is uncertain. Post-Devonian development

Tamar fracture system From the preceding discussion, it is clear that there is a considerable difference between the pre-Carboniferous geology and metallogenic development of western Tasmania and the region to the east of the Tamar River [500430]. The Early Ordovician and Early Devonian deeper marine turbidite quartz-wacke sequences of the Mathinna Beds of northeast Tasmania, east of the Tamar River, contrast with successions of similar age range in western Tasmania, which are essentially shelf deposits of conglomerate, limestone and quartz sandstone. West of the Tamar River, the main folds resulted from tectonic transportation from the northeast,

Fig. :l 1. Schematic geological section of Tasmania and mineral deposits associated with the various metallogenic episodes (cf. Figs. 2--8, and 10). Late Carboniferous and younger sequences, resting unconformably on the preCarboniferous rocks, contain coal and oil shale, minor primary gold associated with Cretaceous syenite, and widespread Tertiary alluvial deposits of tin, gold and "osmiridium" derived from older primary sources.

189

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190

whereas folds in northeastern Tasmania developed during tectonic transportation from the southwest. Furthermore, isotopic ages of the Devonian granitoid emplacements of western Tasmania are generally younger than those of the granitoid masses of northeastern Tasmania. The abrupt change in sedimentary rock types and structural characteristics and the different ages of granitoid intrusion indicate that the Tamar River is the site of an inferred fracture along which lateral movements brought together the contrasting regions (Williams, 1978). The inferred fracture at the Tamar River is buried beneath a trough containing up to 1500 m of flat-lying Permian and younger rocks {Fig. 11; Gee and Legge, 1974). A model based on seismic and gravity investigations indicates that the Tamar structure extends through the crust (Richardson, 1980), and that there are significant changes in magneto-variations across the structure (W.D. Parkinson, pers. commun., 1985). The general distribution of the contrasting rock types both in outcrop and from intersections in bore-holes through the cover of Permian and younger rocks (e.g. Clarke and Farmer, 1983) suggests that the Tamar fracture system probably continues to the southeast, passing between Maria Island 1590280] and Hobart [ 5 2 5 2 5 5 ] .

Post-Carboniferous epeirogenesis Tasmania became part of a craton and prolonged erosion of the granitoids and older rocks was followed by deposition of the Late Carboniferous--Late Triassic Parmeener Super-Group, which has been intruded by substantial Jurassic dolerite sheets (Fig. 11). These rocks, together with Cretaceous and Cainozoic accumulations are flat-lying and have undergone epeirogenic deformation only. Coal and oil shale occur at several horizons in the Permo-Triassic sediments (Fig. 11), and fossil tin placer deposits have been found in Permian basal conglomerate overlying Devonian granite at Royal George [570368]

(Reid and Henderson, 1929). The only posLDevonian primary mineralization is gold ass()elated with high-level Cretaceous syenite intrusions at Cygnet [5082181 i Fig. 1 I. Farmer, 1981). Tertiary alluvial tm det)osit, are spread throughout Tasmania. hut predon~. inantly in the northeast, including the large Briseis deposit at Derby [5674461 i Jennings. 1965). Alluvial and eluvia] det)osits of gold (Noldart and Threader. 1965), ~:hromite (Summons et al., t 9 8 1 ) and "()smiridium" (Elliston, 1953; Hughes, 1965), and lateritic nickel (Noldart, 1975) derived locally from the older primary deposits are widespread (Figs. 1 and 11).

Summary The Tasmanian segment of the Tasman Fold Belt System has a diverse geology and an extraordinary variety of metallic mineral deposits (Table 3). Most major deposits occur in western Tasmania and are dominated by volcanogenic, zinc--lead--copper-silver-gold deposits associated with the Middle to Late. Cambrian Mt. Read Volcanics, and tin/tungsten carbonate replacement/skarn deposits related to emplacement of Middle to Late Devonian granitoids. In western Tasmania, Precambrian sedimentary sequences form the basement for the ensuing accumulation of Eocambrian and younger sequences. A central region (Tyennan region) of Precambrian sedimentary rocks, that were deformed and metamorphosed during the Late Proterozoic Frenchman Orogeny (about 800 Ma), is surrounded by younger and comparatively unmetamorphosed sedimentary sequences (e.g. R o c k y Cape region) that were extensively folded in thE: Late Proterozoic Penguin Orogeny (about 700--750 Ma). The eastern part of the Rocky Cape region contains several ultramafic bodies and basaltic volcanic piles (e.g. Savage River}. which are thought to be related to a zone of extension along the western margin of a developing basin in which accumulated the youngest of the. Precambrian sediments (i.,.'. Burnic

191 and Oonah Formations). Large, apparently stratiform exhalative, massive magnetite-pyrite deposits (e.g. Savage River) are intercalated within the metabasaltic sequences. The volcanogenic deposits and associated mafic volcanics, which represent a Late Proterozoic metallogenic epoch, have been intensely deformed and occur within a north-northeast trending belt of metamorphic rocks (Arthur Lineament) that developed during the Penguin Orogeny. Eocambrian--Cambrian sediments and volcanics accumulated in several distinct troughs in western and northern Tasmania (e.g. Smithton, Dundas and Adamsfield Troughs). The troughs apparently developed during extension of Precambrian continental crust. The main trough, the Dundas Trough, and the probably contiguous Dial Range Trough to the north, developed along a northerly trending boundary between the Rocky Cape and Tyennan geanticlines. Early trough deposition was characterised by shallow-water sedimentation including thick dolomite units in some troughs (e.g. Smithton and Dundas Troughs). Deepening of the troughs resulted in widespread turbidite sedimentation, with minor limestone, and was accompanied by submarine basaltic volcanism which was most active in the northern troughs. Although disseminated natiw~ copper is the only primary mineralization associated with the mafic volcanics, they have apparently been a source of copper for later hydrothermal activity associated with Devonian granitoids. A mild compressional phase resulted in tectonie emplacement of ultramafic and related igneous rocks, some of which are apparently comagmatic with the earlier extruded basalts. They are regarded as upthrust fragments of crustal cumulates and fall along meridional zones in the Dundas and Adamsfield Troughs and at Beaconsfield. Mineralization associated with the ultramafic rocks is generally of minor importance, with only the asbestos deposits at Serpentine Hill and magmatic(?} copper--nickel sulphide

deposits in dolerite at Cuni having achieved any significant production. The ultramafics are, however, the source rocks for significant Tertiary alluvial deposits of platinoids (e.g. Heazlewood, Adamsfield) and also chromite and lateritic nickel (e.g. Beaconsfield). Of the minerals in the parent ultramafic, only chromite appears to be a magmatic crystallization product, while asbestos and concentrations of nickel sulphides and platinoids probably result from serpentinization. The basaltic volcanism and upthrust fragments of ultramafic complexes, and their associated mineral deposits represent an Eocambrian (?)--Early Cambrian metallogenic epoch. It is similar to the Late Proterozoic metallogenic epoch in that similar rock types accumulated in zones of crustal extension, but their associated mineral deposits are strikingly dissimilar. The Eocambrian(?)--Early Cambrian sequences are succeeded conformably by Middle to Late Cambrian, fossiliferous turbidite sequences (e.g. Dundas Group), though in the Dundas Trough an inferred erosional level separates the Eocambrian(?)-Early Cambrian and Middle Cambrian sequences. The structural conformity continues through overlying Ordovician--Early Devonian terrestrial and shallow-marine stable shelf sediments. Along the western and northern margins of the Tyennan geanticline, there accumulated a considerable pile of felsic volcanics of probable Middle-Late Cambrian age, though economically the most important are the Mt. Read Volcanics associated with the Dundas Trough. The 10--15 km wide Mt. Read Volcanics belt contains a central unit of rhyolitic---andesitic, subaerial--subaqueous volcanics that are dominated by lavas and pyroclastics, with associated sub-volcanic granitoids. This is flanked abruptly, to the west, by a volcanosedimentary marine sequence which is dominated by volcanic--lithic turbidites and water-lain tuffs that are apparently facies equivalents of the Middle to Late Cambrian Dundas Group farther to the west.

Devonian

Carboniferous

Permian

Triassic

Jurassic

...................

Cretaceous

..................

NE Tasm. Dacitic ignimbrite. inlra caldcru sh~!,:t

.................. U n c o n f o r m i t y

stone

Local cave deposits in G o r d o n L i m e -

Shallow glaeio-marine mudstone, sandstone, m i n o r oil-shale, t e r r e s t r i a l coal measures Widespread glaciation (tillite)

T e r r e s t r i a l s a n d s t o n e , coal m e a s u r e s

Erosion ...................

Shallow-marine sand Basalt lava flows Terrestrial clay, sand, gravel

Tertiary

................... E r o s i o n ...................

Fluvial a n d coastal deposits W i d e s p r e a d glaciation

Quaternary

a n d volcanic activity

Sedimentation

Age

NE Tasm. G r a n i t o i d intrusions (S and l - t y p e . m i n o r alkali feldspar ~ranitc l

OROGENY (correlated with Tabberabberan o f E. A u s t r a h a )

Granitoid intrusions (high level, m a i n l y S-type)

Lateral movement along Tamar fracture system

Formation of troughs and separation of Australia and Antarctica S y e n i t e i n t r u s i o n (SE T a s m . ) Epeirogenic deformation Intrusion of dolerite

T e c t o n i c a n d i g n e o u s activity .................................

S u m m a r y of m e t a n o g e n i c a n d t e c t o n i c d e v e l o p m e n t o f t h e T a s m a n i a n s e g m e n t of t h e T a s m a n F o l d Belt S y s t e m

TABLE 3

Granitoid-related mineralization Sn g r e i s e n ( A n c h o r ) ; Sn, W v e i n ( A b e r f o y l e , S t o r e y s Ck, Oakleigh Ck, I n t e r v i e w R . ) : W s k a r n ( K i n g Is., K a r a ) ; Sn c a r b o n a t e - r e p l a c e m e n t ( R e n i s o n , C l e v e l a n d , Mt. B i s c h o f f ) ; At--. Pb • Zn v e i n ( Z e e h a n , Mt. Farrell. Magnet); Au reef (Beaconsfield, Lcfroy, Mathinna)

Sn in " f o s s i l " p l a c e r s

A u in s y e n i t e

Alluvial Sn, A u , Os-lr; C h r o m i t e , l a t e r i t i e Ni

Mineralization

cD

Erosion ............

?==?===

Rhyolitie~andesitic, subacrial-subaqueous voleanies and vnlcaniclastics (Mt. Read Voleanies)

M e t a m o r p h o s e d s u c c e s s i o n s d e r i v e d froHl shallow-marine quartz sandstone, mudstone (Tyennan region)

Turbidite quartzwaeke, mudstone, minor basaltic volcanies, dolomite--magnesite (Burnie, Oonah Formations) Shallow-marine quartz sandstone, mudstone (Rocky Cape Group)

Deeper marine mudstone, turbidite lithic-wacke, basaltic volcanics, chert. limestone (Crimson Creek Formation) Shallow-marine quartz sandstone. dolomite ................. Unconformity .................

.........

Mudstone, turbidite lithic-wacke, chert conglomerate (Dundas Group)

Shallow marine limestone (Gordon Mlldslone, minor Limestone) Shallow marine and turbidite terrestrial quartz quarlzwacke sandstone. -----= .. )= = = . =o - ~ conglomerate Local turbidite quartzwacke and volcanicvolcaniclastie sequences --- U n c o n f o r m i t y ---

Deep marine turbidite quarlzwaeke, mudslorle

........ Structural/metamorphic hiatus ........ I"RENCHMAN OROGENY Mafic intrusions (now amphibolite). eclogite

................ Structural hiatus ................ P E N G U I N O R O G E N Y --- l o c a l m e t a m o r p h i s m (Arthur Lineament), granitoid intrusion ( S - t y p e , K i n g Is.) Dolerite intrusion, mafie- ultramafic emplacement

Development of narrow depositional troughs during tension

Local erosion Emplaeement of mafie--ultramafic masses during mild compression

Subvolcanic granitnids and porphyritic i n t r u s i o n s i n Mr. R e a d V o l e a n i c s

Local deformation associate.d with emergence of Wyennan geanticline

Thermal event

P r e - C a r b o n i f e r o u s d a t a f o r w e s t e r n T a s m a n i a is s h o w n i n n o r m a l t y p e a n d f o r n o r t h t : a s t T a s m a n i a in i t a l i c s .

Late Proterozoie

Eocambrian

Cambrian

Ordovician

Silurian

Shallow marine quartz sandstone, mudstone, m i n o r limestone

Voleanogcnic massive magnetite-pyrite -- associated with basaltic voleanism (Savage River)

Cu in b a s a l t

O s - - l r , Ni s u l p h i d e s , C r i n s e r p e n t i n i z e d ultramafics; Cu--Ni sulphides in gabbro (Cuni)

Voleanogenie massive sulphide deposits D i s s e m i n a t e d C u - - A g - - A u (Mt. L y e l l ) ; massive, bedded Zn--Pb--Cu--Ag--Au (Rosebery, Hercules, Que River, H e l l y e r ) in Mt. R e a d V o l c a n i c s

Stratabound veined and disseminated Pb Zn s u l p h i d e s ( Z e e h a n ) in Gordon Limestone

tD CO

i 94 The lava-dominated central belt v()hranics, which apparently accumulated in a meridional rift-like structure, host several major volcanogenie massive sulphide deposits that dominate a Middle Cambrian metallogenic epo(rh. -\t the southern end of the belt, the volcanics are thought to be largely terrestrial, and contain the disseminated copper-.- silver gold deposits at Mt. Lyell. The Mt. Lyell ores apparently formed from convective fluids circulating within and replacing permeable units in the volcanic pile, though there are also exhalative deposits that occur mainly at the stratigraphic top of the mineralized vol(rani(:s. To the north, the central belt volcanics accumulated in both terrestrial and sub-aqueous environments in which there are typical stratiform, exhalative massive sulphide deposits that are characterized by a b u n d a n t Zn and Pb as well as relatively abundant Ag, Au and Cu. They include the sediment-hosted deposits at Rosebery and Hercules and the volcanic-hosted deposits at Que River and Hellyer, and also the barren massive pyrite deposit at Chester. The central belt voleanics were locally eroded during Cambrian mow~ments along faults near and parallel to the Tyennan geanti(:line (e.g. Great Lyell Fault). In the Jukes .... Darwin area this erosion also exposed the subvolcanic Darwin Granite and removed the bulk of several Mt. Lyell-type subsurface deposits. Above these local unconformities there occur unmineralized, late-Middle to early-Late Cambrian volcaniclastic sequences that pass conformably upward into the Ordovieian-Early Devonian shelf deposits. The Ordovician Gordon Limestone. locally hosts stratabound disseminated and veined base metal sulphides that may represent an Ordovician metallogenic event, with accompanying thermal activity. Such an event may also be reflected in Ordovician isotopic ages (450--490 Ma) obtained for many Cambrian rocks. Sedimentation in western Tasmania was interrupted by mid-Devonian deformation that extensively deformed the Eoeambrian-Early Devonian rocks, whilst the Preeambrian

regions behaved as relat}veiy ~.,.,mpeLen,. blocks controlling early fold pat!erns. !i-~ northeastern Tasmania, (Icep marina i.urbidit(. quartz-wacke sequences were deposit.ed durin~ the Ordovician and Early I)evonian }mr f,,r most part there is apparently no .-,imilarity between these sediments and the dominantly terrestrial and shallow-marine shelf deposits of equivalent age in western Tasmania, Folding of the eastern Tasmania sedimentary rocks is of similar age, but resulted from movements diametrically opl)osite i o those affecting pre-Middle Devonian rocks, m western Tasmania. The most widespread and diverse :nineralization is that which is related to the emplacement of substantial, high-lewd granitoid masses within the folded rocks throughout Tasmania during the Middle-.-Late Devonian (389--332 Ma). The granitoids and related tin- tungsten, silver lead--zinc and probably also gold mineralization constitute a later Devonian metallogenic epoch, which is the final metallogenic episode in the development of the Tasman fold belt system m Tasmania. The dominantly S-type granites in western Tasmania are slightly younger than the extensiw~ I-type granodiorites and S-type granites, with associated metallogenically specialized alkali-feldspar granites, in eastern Tasmania, and have different genetic types of associated tin and tungsten mineralization. In northeastern Tasmania, it is dominated by endogranitic stanniferous greisens te.g. Anchor) and quartz--wolframite--eassiterite veir, deposits in or near apophyses of altered alkali-feldspar glcanite (e.g. Aberfoyle, Storeys Creek, Great Pyramid). In contrast, scheelite-bearing skarns (e.g. King Island, Kara) and cassiterite---stannite--pyrrhotite carbonate replacement deposits le.g. Renison, Cleveland, Mt. Bisehofft. and several low-grade skarns, are dominant in western Tasmania, reflecting the relatiw~ abundance of carbonate units within the lateLate Proterozoic to Early Cambrian seque, nces. Although there are small greisen and vein deposits in western Tasmania (e.g. Federation. Interview River, Oakleigh Creek) there are

195

no known skarn or replacement deposits in the sedimentary rocks in eastern Tasmania. Most argentiferous lead--zinc vein deposits are in zones peripheral to, and apparently related to formation of several t i n - t u n g s t e n deposits (e.g. Zeehan, Moina, Scamander districts), but the enigmatic Mt. Farrell deposits are remote from known tin deposits and may represent a separate base metal mineralization phase. The tin--tungsten and silver lead--zinc deposits are in rocks ranging in age from Late Proterozoic to Late Devonian, but their spatial distribution, and also that of the related Devonian granitoids, is confined mainly to the limits of subsurface projections of parent "batholiths". This is not the case, however, for auriferous veins and shear zones which also are believed to have formed during the Devonian metallogenic epoch. Most gold deposits in northeastern Tasmania are apparently associated with I-type granodiorites, but some of the largest deposits (e.g. Beaconsfield) are strikingly remote from a granitoid source and their genesis is uncertain. Several mineral deposits in western Tasmania show no clear relationship to any of the metallogenic units, and their genesis remains uncertain. They include a number of copper deposits dispersed throughout the Precambrian regions (e.g. Balfour), and small gold deposits near Corinna, which are also in Precambrian rocks. The contrasting regions of western and northeastern Tasmania appear to have been brought into juxtaposition by lateral movement along a probable fracture at the Tamar River. Erosion of the Devonian granitoids and older rocks was followed by deposition of the fiat-lying Late Carboniferous and younger deposits. The only known post-Devonian primary mineralization is gold associated with Cretaceous syenite.

Acknowledgements Constructive comments on the manuscript were provided by M.R. Banks and R.R. Large, University of Tasmania. The diagrams were

drafted by J.H. Clarke, G.J. Dickens, D.M. Hardy and A.J. Hollick; and the manuscript was typed by A.E. Taylor. The paper is published with the permission of the Director of Mines, Tasmania.

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