URANIUM MINERALIZATION IN THE RUM JUNGLE-ALLIGATOR RIVERS PROVINCE, NORTHERN TERRITORY, AUSTRALIA*

R . G . DODSON. R.S. NEED H A M , P. G . WILKES, R.W. PAGE, P . G . S M A R T . A . L . W A T C H M A N Bureau o f Mineral Resources. Geology and Geophysics, Department of Minerals and Energy. Canberra, Australia

Absnacr

URANIUM MINERALIZATION IN T H E R U M I U N G L E - A L U G A T O R RIVERS PROVINCE, NORTHERN TERRITORY, A U S T R A L I A .

Renewed tnteretf in titanium exploration in northern Aut i ta l i a in Ihe late I960* led to the dUcovety o f one o f the m o i l important uranium fields In the world. T o obtain an undemanding o f the geo log ica l letting o f the uranium depoi l t i . and to provide guide l ine i for future explorat ion, the Bureau o f Minera l Retoutcei (BMR) undertook an invettlgation which included l e m l - d e t a i l e d mapping, aided by geophyi lca) tu rvcy i . itotopic dating, and itratigraphic dr i l l ing . From the retulti to far obtained and from the important data made avai lable by mining compante i engaged in exploration In Ihe region, a pattern o f uranium mineral izat ion can be recognized , particular ly In relation to the Lower Proterosoic tedlmentation o f the region.

I N T R O D U C T I O N

In a r ev i ew of the^more impor tant env i ronments of A u s t r a l i a n u r a n i u m depos i t s , Dodson pointed out that ' F u t u r e exp l o ra t i on of the A l l i g a t o r R i v e r s a r ea w i l l be great ly ass i s t ed if the genes is of

u r a n i u m ore i s e s t ab l i shed . 1 S ince d e l i v e r y f>t the paper (September 5

1970), s e m i - d e t a i l e d geo log ica l mapping " and geophys ica l s u r v e y s by the B u r e a u of M i n e r a l R e s o u r c e s ( B M R ) , and invest igat ions by min ing companies , have added cons ide rab l y to our knowledge of the geo log ica l se t t ing of these depos i ts . A l though c e r t a i n assumpt ions cannot be fu l ly substant ia ted at present , we bel ieve that we a r e c l o s e to unders tand ing the o r i g i n of the o r e . C o n v e r s e l y , c e r t a i n hypotheses put f o r w a r d i n the past appear l e ss l i k e l y as new data a r e obtained.

F o l l o w i n g the d i s cove r y of u r a n i u m at R u m Jung le in 1949, the K a t h e r i n e - D a r w i n r eg i on was mapped at r e conna i ssance sca le by B M R . The survey es tab l i shed the ma in out l ines of the r eg i ona l geology, and ident i f ied the most favourable env i r onments for u r a n i u m . The r e s u l t i n g 1:250 000 geo log ica l maps f o rmed the bas i s for more deta i led mapping and the p lann ing of exp l o ra t i on when in te res t i n u r a n i u m was r e v i v ed at the c l o s e of the 1960s. In 1970, f o l l ow ing the

• Publiihed by permirj ion o f the Director . Bureau o f Minera l Reaourcei. Geology and G e o p h y i i c i . Canberra. Austral ia .

551

DODSON et at.

d iscove ry of h igh-grade u r a n i u m ore i n the A l l i g a t o r R i v e r s a r ea about 250 k m east -nor theast of R u m Jung l e , a s tar t was made on s e m i -deta i led geo log ica l mapping a ided by geophys i ca l su rveys , i so top ic dat ing, and s t r a t i g r a p h i c d r i l l i n g . The a r ea be ing mapped c o n s i s t s of the whole of the A l l i g a t o r R i v e r 1:250 000 Sheet a r ea and pa r t s of the Cobourg P e n i n s u l a , J u n c t i o n Bay, M t l i n g i m b i , and Mount E v e l y n Sheet-a r eas ( F i g . 1.) A s i m i l a r but more r e s t r i c t e d p r o g r a m at R u m Jung l e commenced in 1972. The a i m of the cur ren t p r o g r a m i s to de te rmine the geo log ica l se t t ing of the newly d i s cove r ed u r a n i u m depos i t s , to c o r r e l a t e the geology of the u ran i f e r ous a r e a s wi th each other , and to e s t a b l i s h the o r i g i n of the m i n e r a l i z a t i o n .

G E O L O G Y O F T H E U R A N I U M D E P O S I T S

W i t h the except ion of a few m i n o r p r o s p e c t s , a l l known u r a n i u m depos i ts of the p rov ince o c cu r i n L o w e r P r o t e r o z o i c sed iments of the P ine C r e e k geosync l ine . The deposi ts a r e concentrated in three a r ea s : R u m Jung l e , South A l l i g a t o r R i v e r v a l l e y , and an a r e a between the South A l l i g a t o r R i v e r and Cooper C r e e k , known for convenience as the A l l i g a t o r R i v e r s a r e a . P e r h a p s the most important c lue to the o r i g i n of u r a n i u m i n th is p rov ince that has r e s u l t e d f r o m the p resen t inves t i ga t ion i s the recogn i t i on of a s t r a t i g r a p h i c con t ro l on the d i s t r i bu t i on of u r a n i u m . In the A l l i g a t o r R i v e r s a r ea , both the u r a n i u m depos i ts and untested u r a n i u m prospec t s l i e i n the same sequence, and there a r e sound r easons for c o r r e l a t i n g the s u c c e s s i o n at the A l l i g a t o r R i v e r s a r ea wi th those^>f the South A l l i g a t o r R i v e r va l l e y and R u m J u n g l e . Both the R u m Jung le and South A l l i g a t o r R i v e r va l l e y ' depos i ts have been des c r i b ed in de ta i l , and a r e there fore only br ie f l y out l ined in th is paper . The more r e c en t l y d i s cove r ed deposi ts of the A l l i g a t o r R i v e r s a r ea a r e l e ss w e l l known; de ta i l s of the i r env i ronment a r e only now becoming ava i lab le as geo l og i ca l and geophys i ca l invest igat ions in the a r e a p r o g r e s s .

Rum Jung l e

L o w e r P r o t e r o z o i c sed iments a r e draped a round a co r e of the P ine C r e e k Geophys i ca l basement , the late A r c h a e a n R u m Jung le C o m p l e x . The sed iments dip s teep ly away f r o m the complex , i nd i ca t ­ing p o s t - A r c h a e a n updoming of the complex at the c lose of sed imenta t ion in the P ine C r e ek Geosync l ine . F u r t h e r ev idence of th i s updoming has been p rov ided by the d i scove ry of a shea r ed contact between the R u m Jung l e Comp l ex and the L o w e r P r o t e r o z o i c sed iments (K. Johnson , p e r s . c o m m . , 1974).

R h o d e s * 1 d i s t ingu ished s i x major un i t s i n the southern par t of the R u m Jung le Comp l ex , in o rder of d e c r e a s i n g age: S ch i s t s and gne isses , grani te gne i ss , me tad io r l t e , c oa r s e gran i te , la rge fe ldspar gran i te , and l eucoc ra t i c g ran i te . Banded Ironstone and quar t z i t c have a l so been

! A E A - S M - 1 8 3 ^ 8 555

r e c o r d e d i n the complex . The u r a n i u m contents of these r o c k s range f r o m a ^gw 2.8-4.5 p p m in the metad io r i t e to 10-26 p p m i n the l eucoc ra t i c grani te . R b - S r dat ing of the l eucoc ra t i c grani te has y i e lded a whole-rock i s o c h r o n o f about 2400 m . y . , and th ree z i r c o n s f r om different par ts of the comp lex y i e lded an ex t rapo la ted U - P b age of around 2550 m.y. Ev idence of p o s t - A r c h a e a n ac t i v i t y i n the complex i s p rov ided by the presence of qua r t z - t ou rma l i n e v e i n s rad ia t ing f r o m cen t r es i n the comp lex and invading the o v e r l y i n g Lower P r o t e r o z o i c r o c k s (K. J o h n s o n , p e r s . c o m m . , 1974).

The complex i s cut by the G i an t ' s Ree f Fau l t , which s t r i k e s northeast and has a h o r i z o n t a l d i sp lacement of about 6 k m .

D u r i n g the L o w e r P r o t e r o z o i c , s ed imen ts of the Ba t che l o r G joyp (Beestons F o r m a t i o n , C e l i a Do lomi te , C r a t e r F o r m a t i o n , and Cooma l i e Do lomi t e ; see F i g . 2) were depos i ted against and a round the R u m Jung l e Comp lex . The Ba t che l o r G roup sed iments were i n tu rn o v e r l a i n by s ed imen ts of the Goodpa r l a Group , r epresen ted at R u m Jung l e by the Golden Dyke F o r m a t i o n and the A c a c i a Gap tongue, which u n d e r l i e s and in t e r f l nge r s wi th the Go lden Dyke F o r m a t i o n . The Ba t che l o r Group sed imen ts we re deposi ted i n l i t t o r a l to n e r i t l c condi t ions; the o v e r l y i n g Goodpa r l a Group sed imen ts were deposi ted in a sha l l ow m a r i n e , pa r t l y eux intc env i ronment . Both the B a t c h e l o r Group and the Goodpar la Group sed iments were o v e r l a in by the F l n n i s R i v e r G r o u p , r epresen ted i n the R u m Jung l e a r ea by the No l ten ius F o r m a t i o n . The B u r r e l l C r e e k F o r m a t i o n i s a l a t e r a l f a d e s v a r i a t i o n of the No l t en ius F o r m a t i o n .

The five s m a l l u r a n i u m depos i ts were in Lower P r o t e r o z o i c s c h i s t , s i l t s t one , and shale of the Golden Dyke F o r m a t i o n of the Good ­p a r l a Group . The lodes were located in s h e a r s , f au l t s , and, at R u m Jung l e C r e ek South, In a tight fo ld , the host r o ck be ing carbonaceous s ed imen ts o r c h l o r i t l c s la te , near the contact wi th the under l y ing C o o m a l i e D o l o m i t e .

South A l l i g a t o r R i v e r va l l e y

The L o w e r P r o t e r o z o i c s ed imen ts of the South A l l i g a t o r R i v e r va l l ey a r ea a r e l i t ho l o g i ca l l y s i m i l a r to and a r e s t r a t i g r a p h i c a l l y re la tab le to the L o w e r P r o t e r o z o i c sed iments of the R u m Jung le a r e a .

Basement r o c k s of the Pine C r e e k Geosync l ine a r e not exposed i n the va l l e y . The Stag C r e e k V o l c a n i c s , tentat ive ly ass igned to the A r chaean by Walpo le et a l . , a r e cons ide r ed in the l ight of recent f i e l d work to be L o w e r P r o t e r o z o i c ; the unit o c c u r s i n sequence w i th Lower P r o t e r o z o i c sed iments at the top of the Masson F o r m a t i o n , under l y ing the Mount P a r t r i d g e F o r m a t i o n , both of the Goodpa r l a Group . In the South A l l i g a t o r R i v e r va l l e y the Mount P a r t r i d g e F o r m a t i o n i s o v e r ­l a in by the South A l l i g a t o r G r o u p , cons i s t ing of the Koo l p in F o r m a t i o n

5 5 6 D O D S O N e t a l . 1AEA-SM-183/28 557

and i t s f ac i es va r i an t , the Gerow ie C h e r t , and the o v e r l y i n g F i s h e r C r e e k S i l t s tone .

A l l o w i n g for l a t e r a l fac ies changes, the L o w e r P r o t e r o z o i c success i ons at R u m Jung le and the South A l l i g a t o r R i v e r va l l ey a r e s i m i l a r . K. Johnson ( p e r s . c omm. , 1974) be l i e ves that sed iments of the Ba t che l o r G roup lens out w i th in a short d i s tance of the R u m Jung l e Complex , and a r e interbedded wi th sed iments of the Masson F o r m a t i o n and Golden Dyke F o r m a t i o n , as shown i n F i g u r e 2. In the v i c i n i t y of Mundogie H i l l north of the South A l l i g a t o r va l l e y , the Masson F o r m a t i o n i s o v e r l a i n by the Stag C r e e k V o l c a n i c s and The Mount P a r t r i d g e F o r m a t i o n , which i s o v e r l a i n by the Koo l p in F o r m a t i o n , a unit s i m i l a r l i tho l og i ca l l y to the Golden Dyke F o r m a t i o n .

The u r a n i u m depos i ts of the va l l ey cons is ted of th i r teen s m a l l but most ly h igh-grade deposi ts d i s t r i bu ted a long a 19km sect ion of a fault zone a long the South A l l i g a t o r R i v e r v a l l e y . A l m o s t a l l the u r a n i u m ore i n the Lower P r o t e r o z o i c r o c k s i s in steeply d ipp ing Koo lp in F o r m a t i o n b lack carbonaceous sha l e . U r a n i u m i s a l so conta ined i n v o l c a n i c s and sandstone of the C a r p e n t a r i a n E d i t h R i v e r V o l c a n i c s at Corona t i on H i l l and Saddle R idge , and i n sandstone of the C a r p e n t a r i a n Kombo lg i e F o r m a t i o n at Pa l e t t e . Where pitchblende o c c u r s in the sandstone,the host rock i s most ly i n contact wi th carbonaceous sha le of the K o o l p i n F o r m a t i o n . The lodes a r e i n the f o r m of v e ins , s t r i n g e r s , o r pods located in shea r s and c r o s s - f r a c t u r e s a long the ma in fault zone, which s t r i k e s nor th-nor thwest .

A l l l g a t o r R i v e r s a r e a

The geology of the A l l i g a t o r R i v e r s a r ea i s dominated by two large comp l exes - the Nanambu and N imbuwah Comp l ex e s - su r r ounded by Lower F r o t e r o z o i c me tamorph i c r o cks which grade into unmetamorphos* sed imentary units of the Goodpar la and South A l l i g a t o r G r o u p s . R b - S r geo-chrono l og i ca l s tudies by B M R have demonstrated that the geo log ica l h is tor ic of the two comp l exes a r e di f ferent . The Nanambu Comp l ex ( F i g . 3) extends over about 3500 sq k m in the centre of the a r ea and cons i s t s of a c o r e of g ran i to id r o c k s , l eucogne iss , and mlgmat i t e , mant led by L o w e r P r o t e r o z o i c sed iments . It i s sur rounded by metamorphosed L o w e r P r o t e r o z o i c s ed imen t s . Me tamorph i c grade dec reases away f r o m the complex , and a r k o s e , sandstone, and s i l t s tone a r e exposed southwest of J i m J i m C r e e k . Isotopic dat ing has r evea l ed that the g ran i t o i d core of the complex i s A r chaean , suggest ing that the L o w e r P r o t e r o z o i c r o c k s that now f o r m the outer mantle of the complex were draped a round a basement h igh . The ant iquity of par t of the g ran i t o i d co r e of the Nanambu Complex has been e s t ab l i shed by R b - S r t o t a l - r o c k i s o c h r o n s , the oldest of which i s 2520^30 m.y . ( in i t i a l S r 87/Sr 86 = 0.704-0.001) . Other pa r t s of the complex y i e l d i s o c h r o n ages of a round 1800 and 2000 m.y. , but with r a t h e r high i n i t i a l S r 87/Sr 86 r a t i o s (up to 0.78). The L o w e r P r o t e r o z o i c ages r epresen t p a r t i a l o r complete r ese t t ing of the

558 DOOSON et «|. 1AEA-SM-183/28

tota l r o c k s at the t ime of the 1800 m.y. m e t a m o r p h i s m . M i c a ages , obtained by K - A r and R b - S r methods, a l so re f l ec t the 1800 m.y . event.

The N imbuwah Complex to the northeast extends over about 10 000 sq k m . Its nor theastern and southern extents a r e masked by C a r p e n t a r i a n and younger s ed imen t s ; i t s t rue extent may exceed 20 000 sq k m . Walpo le et a l . a s c r i b e d an A r c h a e a n age to s ch i s t and q u a r t z i t e ( M y r a F a l l s Metamorph ics )^which c rop out ma in ly east of the E a s t A l l i g a t o r R i v e r . Needham et a l . , however , have shown these r o cks to be metamorphosed and m y l o n i t i z e d equiva lents of the L o w e r P r o t e r o z o i c South A l l i g a t o r G r o u p , and that in m igmat i z ed f o r m they consti tute the outer zones of the N imbuwah Complex . The complex has been d i v ided into zones which represent i n c r e a s i n g degrees of m t g m a t l z -at ion and/or mobi l i t y t owards the centre . The zonat ion r ep r e s en t s a c l a s s i c a l gradat ion of migmat i te types f r o m inc ip i en t l y m i g m a t i z e d s c h i s t s to comple te l y homogenized and m o b i l i z e d g ran i to id ana^exites in the cen t r e . A l l of the migmat i te types d e s c r i b e d by Mehner t have been r ecogn i z ed . South A l l i g a t o r G roup sed imen ts have been ful ly i n c o r ­porated into the outer zones of the N imbuwah Comp l ex , and i n the M y r a F a l l s In l i e r c ross -bedded quar t z i t e , carbonaceous s c h i s t , do l oml t t c m a r b l e , and amphibo l i t e wi th in banded and l i t - p a r - l i t gne i ss masses a r e cons ide r ed to be me tamorph i c equiva lents of the Koo lp in F o r m a t i o n . The inner zones of the comp lex may a l so have been f o r m e d by m i gma t i z a t i on of L o w e r P r o t e r o z o i c un i t s wh i ch have been highly m i gma t i z ed . F i e l d ev idence i s not conc lus i ve , however; the N imbuwah Comp l ex cou ld have an A r c h a e a n core l i k e the Nanambu C o m p l e x . A n 1840 m . y . R b - S r t o t a l -r o ck i s o c h r o n age obtained for the N imbuwah Complex r ep r e s en t s the t ime at wh i ch Rb and S r became f ixed i n the r o cks shor t l y a f ter the peak of the m i gma t i z a t i on and m e t a m o r p h i s m .

A l t e r e d p ink biot ite g ran i t es , a l l anomalous ly r ad i oac t i v e , are thought to be anatect ic and produced by m o b i l i z a t i o n of m i g m a developed in the cent re of the N imbuwah Complex d u r i n g m i gma t i z a t i on . They are probab ly comagmat i c w i th the E d i t h R i v e r V o l c a n i c s , a s e r i e s of a c i d v o l c a n i c s best exposed i n the South A l l i g a t o r va l l e y where they uncon -fo rmab ly o ve r l i e fo lded South A l l i g a t o r G roup s ed imen t s . The a c i d igneous r o c k s mark the last Lower P r o t e r o z o i c event.

L o w e r P r o t e r o z o i c s ed imen ts of the A l l i g a t o r R i v e r s a r e a compr i s e the Masson and Mount P a r t r i d g e F o r m a t i o n s of the Goodpa r l a Group and the Koo l p in F o r m a t i o n equivalent and F i s h e r C r e e k S i l t ­stone of the South A l l i g a t o r G roup ( F i g . 2).

M a s s o n F o r m a t i o n r o c k s a r e the o ldest sed iments i n the P i n e C r e e k Geosync l ine , and c rop out between the South A l l i g a t o r R i v e r and M c K l n l a y R i v e r and Mount Bundey. The f o rma t i on i s p redominant l y quar t z g reywacke , carbonaceous s i l t s t one , c a l c a r eous s i l t s tone , and cong lomerate about 3000 m th i ck .

560 DODSON «t a l .

Arkosic sandstone, siltstone. and conglomerate of the Mount Partridge Formation conformably overlie the Masson Formation. The Mount Partridge Formation appears to be wedge-shaped, and has a maximum thickness of about 3000 m; deposition was probably confined to the west and southwest flanks of the Archaean core of the Nanambu Complex. The formation thins out westwards, and west of the South Alligator River valley is continuous with the Coirwong Greywacke.

The South Alligator Group unconformably overlies the Goodparla Group and appears to onlap the Archaean core, although this relation is not clearly observable in the field. The Koolpin Formation metamorphic equivalent of the Alligator Rivers area also differs lithologlcally from the Koolpin Formation type section of the South Alligator Valley; it is composed of chlonte-muscovite schist, carbonaceous schist, amphibolite, quartzlte and gneiss with lenses of massive carbonate rock near the base of the sequence.

The Fisher Creek Siltstone gradationally overlies Koolpin Formation and its equivalent. It Is a homogeneous sequence of siltstone and arenaceous siltstone, metamorphosed to phyllite and schist north of J im J i m Creek. The Fisher Creek Siltstone is the youngest Lower Proterozoic unit to be deposited in the Alligator Rivers and South Alligator River valley areas, and is believed to be the lateral equivalent of the Burrell Creek Siltstone and Noltenius Formation farther west.

Two series of basic igneous rocks intrude Lower Proterozoic rocks: the Zamu Complex diorite, dolerite, and differentiates which were emplaced as numerous sil ls prior to migmatization, and subsequently altered in places to amphibolite, and the Oenpelli Dolerite 1 . emplaced as a large undulating flat-lying discordant sheet soon after metamorphism and migmatization. The absence of hornfelsing adjacent to the dolerite where it intrudes the inner zones of the Nimbuwah Complex suggests that the Oenpelli Dolerite was emplaced before the migmatite complex had cooled.

Phonolttic and sodic trachyte dyke swarms intrude one small area, near Mudglnberri homestead in the Nanambu Complex (Mudgtnberri Phonoltte 1).

The Carpentarian Kombolgle Formation sediments unconformably overlie Lower Proterozoic rocks, and form the most prominent feature in the region, the Arnhem Land Plateau. The Kombolgle Formation consists of lower (about 300 m) and upper (about 150 m) medium to coarse sandstone units separated by the basic Nungbalgarri Volcanic Member which consists of one or more flows with minor intercalcated

sediments. The volcanic member ranges from 0-70 m in thickness. The upper sandstone unit includes a thin (up to 3 m) volcanic layer comprising

In lot ma 1 name.

I A U - S M - 1 8 3 / 7 3

tuff, tuffaceous siltstone, and basalt known as the Gilruth Volcanic Member 1 . The Carpentarian plateau sandstone covers most of the north­east, east, and southeast of the region mapped.

Post-Carpentarian rocks in the region cover extensive areas. They include terrestrial Mesozolc sediments which form low plateau in the northern part and underlie Cainozolc sand plains in the centre. A recent coastal submergence has resulted in extensive alluvial accumulation along major rivers; the flood plains are seasonally inundated by brackish water up to 100 km inland during the wet season.

During parts of 1971 and 1972, BMR made an airborne magnetic and gamma-ray spectrometry survey of the Alligator Rivers area . The airborne geophysics was designed to assist the semidetailed geological mapping and the uranium search in the area particularly where the surface geology is covered by superficial deposits (see Figs 1, 3). It was hoped that the radiometric and magnetic characteristics of Individual rock units would prove sufficiently distinctive to allow recognition of hidden rock units. The survey was of a regional nature; east-west lines were flown 150 metres above the ground with a spacing of 1.5 km.

Many of the magnetic anomalies are of shallow or surface origin, and their correlation with geological units has been helpful in tracing these units beneath the extensive overburden which covers much of the area to the west and north of the Arnhem Land Plateau escarpment. The more magnetic of the geological units appear to be the Stag Creek Volcanics, Koolpin Formation, Koolpin Formation equivalent, Zamu Complex dolerite. and Oenpelli Dolerite.

The magnetic data show that the Nanambu Complex is only weakly magnetic (anomalies generally less than 30 gammas). This complex is surrounded by rocks of Koolpin Formation equivalent, and locally by Oenpelli Dolerite. Both of these exhibit stronger magnetic expression (anomalies up to 300 gammas). Mapping these magnetic features permits indirect interpretation of parts of the margin of the complex. Magnetic interpretation has also indicated the likelihood of two further areas of Nanambu Complex to the north of the main mass, and this has been confirmed by shallow drilling. The Nimbuwah Complex, however, has a very variable and complex magnetic expression, and its extent seems impossible to determine from the magnetic and radiometric data.

The Koolpin Formation equivalent exhibits very variable magnetic expression, with anomaly amplitudes ranging from less than 10 gammas (In the Myra Falls Inlier) to possibly as high as 300 gammas (to the weal of Koongarra.) This makes it difficult to trace its full extent, and also to see whether It is continuous with the Koolpin Formation.

562 D O D S O N e l j l .

D o l e r i t e s appear to be much more w idespread than o r i g i n a l l y mapped. Magnet ic anoma l i es a r e assoc ia t ed w i th both the Z a m u Comp l ex do l e r i t e and the Oenpe l l i Do l e r i t e . In terpre ta t ion of the magnet ic data ind ica tes that many of the sur face exposures a r e p a r t s of l a r g e r continuous do ler i te bodies i n the subsur face .

The a i rbo rne r a d i o m e t r i c r e s u l t s have ident i f ied a r eas of h igher than average rad i oac t i v i t y , and p rov ided an ind i ca t i on of the p r e ­dominant s ou r c e s of the rad ioac t i v i t y - i . e . , whether due to u r a n i u m , t h o r i u m , o r p o t a s s i u m .

The r a d i o m e t r i c anoma l i e s of highest ampl i tude were r e co rded over three of the major u r a n i u m deposi ts (Ranger 1, Nabar l ek , and Koongar ra ) , v a r i o u s u r a n i u m p r o s p e c t s , grani te masse s , and a large h i l l of Mount P a r t r i d g e F o r m a t i o n (Mount Basedow) . The B M R survey did not fly d i r ec t l y o v e r the J a b i l u k a u ran ium deposit , which appears to have been too fa r f r om the nearest l ines to be detectable .

F o r t y anoma l i es were c l a s s i f i e d as ' u r a n i u m a n o m a l i e s ' . F o r th i s purpose a u r a n i u m anomaly was def ined a s one where p o t a s s i u m was not the major s ource and the u r a n i u m - t o - t h o r i u m r a t i o was roughly two o r g r ea t e r (assuming e q u i l i b r i u m to be achieved in both the u r a n i u m and t h o r i u m decay s e r i e s ) . The host r o c k s for these ' u r a n i u m a n o m a l i e s ' a r e Nanambu Complex , N imbuwah Complex , Koo l p in F o r m a t i o n equiva lent . Mount P a r t r i d g e F o r m a t i o n , and l a t e r i t e s a s so c i a t ed wi th the v o l can i c m e m b e r s of the Kombo lg l e F o r m a t i o n .

A l l the grani te m a s s e s a r e anomalous ly rad i oac t i v e , p o t a s s i u m and t h o r i u m being the major s o u r c e s . S i m i l a r l y , the anomalous r a d i o ­a c t i v i t y of the large h i l l of Mount P a r t r i d g e F o r m a t i o n , r e f e r r e d to e a r l i e r , i s due to p o t a s s i u m and t h o r i u m .

The sandstone of the Kombo lg l e F o r m a t i o n i s v e r y low in r a d i o ­ac t i v i t y . However , c e r t a i n l a t e r i t e s that developed down-slope f r o m the Nungba l ga r r i V o l c a n i c s and the G i l r u t h V o l c a n i c M e m b e r are somewhat rad i oac t i v e , w i th modera te ly high u r a n i u m - t o - t h o r i u m r a t i o s . It i s c o n s i d e r e d that these a r e only sur face f ea tures , and that the l a t e r i t e i s a concentrat ing agent for the u r a n i u m ,

j The A l l i g a t o r R i v e r s u r a n i u m depos i ts were br ie f l y d e s c r i b e d by Dodson i n 1971. S ince then exp l o ra t i on has p r o v e d extensions of known lodes, and much has been learnt about the geology of the deposi ts through

d r i l l i n g and t r ench ing . F r o m nor th to south the deposi ts a r e : Naba r l e k (Queensland Mines Ltd ) , J a b i l u k a (Pancont inenta l M i n i n g L td ) , Ranger 1 (Peko W a l l s e n d - E . Z . Indus t r i e s Ltd ) , and Koonga r r a (Noranda Ltd ) .

16 17 Nabar l ek ' c o n s i s t s of two h igh-grade l enso id lodes composed

of o re r i c h e r than 10 percent U„0_, sur rounded by a zone of l owe r - g rade

IAEA -SM-183/28 563

ore 230 m long and up to 20 m wide. M a s s i v e pi tchblende coated wi th secondary m i n e r a l s was in te rsec ted at 1.5 m below the sur f ace , and the ore extends to a depth of about 70 m, l ens ing out above a thick f la t -l y ing dyke of Oenpe l l i Do l e r i t e . D i s s em ina t ed u r a n i u m m i n e r a l s o c cu r a long s t r i k e for a tota l of about 270 m. The lode s t r i k e s nor th -nor thwes t , p a r a l l e l to the s ch l s t o s l t y of the host r o ck ( qua r t z - ch l o r i t e -muscov i t e sch is t of the Koo l p in F o r m a t i o n equivalent ) , and dtps to the east at 30-45°.

In two deep d r i l l - h o l e s at Nabar lek , grani te was in t e r s ec t ed about 500 m below the sur face .

Nabar l ek has es t imated r e s e r v e s of 9540 tonnes U 3 0 g (402 000 tonnes of o r e of an average grade of 2.37 percent Ug0g)«

17

Jab i l uka i s s i tuated about BO km MUttnPMt I N a b a r l e k . c lose to the A r n h e m Land F l a t eau . Fancont inenta l M i n i n g L td announced the d i scove ry of J a b i l u k a 1 i n J a n u a r y 1973. The lode t rends west-southwest away f r om the A r n h e m Land P la teau and d ips south at about 45°. The ore zone i s up to 30 m th ick and about 150 m long , and extends to a depth of about 105 m. P r i m a r y ore i s p i tchblende in q u a r t z - c h l o r i t e carbonaceous sch i s t of the Koo l p in F o r m a t i o n equivalent . P y r i t e and cha lcopyr i t e are minor const i tuents in the lode, and gold i s l o c a l l y present . Secondary

u r a n i u m ore Is d i s s em ina t ed through a n a r r o w zone In the Kombo lg l e F o r m a t i o n sed iments , poss i b l y deposi ted by hypogene en r i chmen t .

In September 1973 Pancont inenta l M i n i n g L t d announced the d i s cove r y of J a b i l u k a 2, about 480 m to the east of J a b i l u k a 1 and beneath the Kombo lg i e F o r m a t i o n sed iments of the A r n h e m Land P la t eau . J a b i l u k a 2 i s s i m i l a r i n geo log ica l se t t ing to J a b i l u k a 1 but it i s much l a r g e r : the ore zone i s up to 49 m th i ck , and at depths of between 61 m and 192 m below the su r f a ce . The length of J a b i l u k a 2 has not been ful ly tes ted .

Ore r e s e r v e s announced by Pancont inenta l M i n i n g L t d on 20 December 1973 were : J a b i l u k a 1-3490 tonnes UgO . ; J a b i l u k a 2-19 580 toiinos U 3 0 g .

A n anomalous zone in the same exp l o ra t i on l i c ence a r ea known am P rospec t 7J i s as yet not fu l ly tes ted . Secondary u ran ium m i n e r a l s a r e present at the sur face and gold values as high a s 23 g/tonne were i i - c i i i d i - i l i n the o r e .

18 17 Range r 1 ' c ons i s t s of s i x r a d i o m e t r i c anoma l i es grouped i n

a roughly nor th - t r end ing a r c - l i k e zone about 6.5 k m long and up to 1 k m wide, approx imate l y 25 k m south of J a b i l u k a . Of the s i x anoma l i e s , NI»H. 1 and 2 have been fu l l y tes ted; ore-grade u r a n i u m has been i n t e r ­sec ted at anoma l i es 2, 4, and 5. The host r o c k s at Ranger 1 a r e known i n Mm company geo log is ts a s the 'M ine S e r i e s ' o r 'M ine Sequence 1 - a

564 DODSON et a l .

sequence r ead i l y d i v i s i b l e into upper and l ower un i t s . The M ine S e r i e s i s cons ide r ed by B M R geo log is ts to be p a r t of the Koo lp in F o r m a t i o n equiva lent . The L o w e r M ine S e r i e s cons i s t s of b i o t t t e - f e l dspar -quar t z s c h i s t o v e r l a i n by a d iscont inuous band of do lomi te and b io t i t e -quar t z s ch i s t . In the m i n e r a l i z e d par t , the s ch i s t i s c h l o r i t i z e d . Carbonaceous s c h i s t i s l o c a l l y p resen t i n the sequence. The Mine S e r i e s i s intruded by pegmat i te and do l e r i t e , both of wh i ch a r e heav i ly a l t e r ed in the m i n e r a l i z e d p a r t s .

The u r a n i u m i s p resen t mos t l y a s p r i m a r y p i t c h b l e n d e - r l c h c h l o r i t i c ve in le ts In f i l l i ng c r a c k s and f i s s u r e s , ma in l y i n the L o w e r Mine S e r i e s but a l s o in the Upper M ine S e r i e s . In the Upper Mine S e r i e s , u ran i f e r ous ve in l e t s tend to be concentrated a round f ragments of b r e c c i a t ed ch l o r i t e s c h i s t . P i tchblende i s a l s o present as f ine g r a i n s d i s s em ina t ed through ve ins and in m i n e r a l i z e d zones in c h l o r i t i c host r o c k s . Copper , l ead , and gold a r e e r r a t i c a l l y d i s t r i bu t ed through the o r e . Ore r e s e r v e s for anoma l i e s 1 and 3 announced by Peko W a l l s e n d L td (1972-73 Annua l Repor t ) c o n f i r m e d an e a r l i e r e s t ima t ed f i gure (1972) of 100 350 tonnes U 3 O g . O n 29 M a r c h 1974 Peko W a l l s e n d L t d announced an i n c r e a s e i n the r e s e r v e s to 100 350 tonnes ('..(tg.

19 17 The Koonga r r a ' deposi t i s l ocated about 20 k m sou th - sou th ­

west o f Ranger 1. The deposi t l i e s c lose to the southern s ide of the Mount B r o c k m a n mass i f , an o u t l i e r of Kombo lg i e F o r m a t i o n sandstone. The m i n e r a l i z e d zones a r e roughly con formable wi th the sch i s tos l t y of the host r o ck and p a r a l l e l to a major r e v e r s e fault d ipp ing at about 60° to the southeast. The fault plane co inc ides wi th the southeastern m a r g i n of the Mount B r o c k m a n mass i f , and has brought the L o w e r P r o t e r o z o i c host r o ck into contact wi th u n m i n e r a l i z e d Kombo lg i e F o r m a t i o n sandstone . The fau l ted contact i s intense ly b r e c c i a t ed and m a r k s the foo twal l of the m i n e r a l i z e d zone; the hang ingwa l l i s a carbonaceous h o r i z o n , some 70 m above and near ly p a r a l l e l to the faul t . The orebod ies cons is t of a s e r i e s of en eche lon zones of d i s semina ted u r a n i u m m i n e r a l s . The zones enc lose cores of h i ghe r - g rade o r e . Host r o c k to the o re i s L o w e r P r o t e r o z o i c q u a r t z - c h l o r i t e - m u s c o v l t e s ch i s t of the Koo lp in F o r m a t i o n equiva lent . Graphi te and garnet a r e a c c e s s o r y m i n e r a l s . In the p r i m a r y ore the u r a n i u m m i n e r a l i s p i tchblende; m i n o r p y r i t e , cha l copy r i t e , galena, and a t r a ce of gold a r e assoc ia t ed wi th the o r e .

T o date no announcement has been made on the o re r e s e r v e s of K o o n g a r r a .

G E N E S I S O F T H E O R E

The c u r r e n t B M R p r o g r a m has shown that metased iments o r i g i n a l l y cons ide r ed to be A r c h a e a n a rc the metamorphosed equivalent of L o w e r P r o t e r o z o i c s ed imen ts (South A l l i g a t o r Group) exposed i n the

South A l l i g a t o r R i v e r v a l l e y . F i e l d Invest igat ions have p rov ided evidence

IAEA -SM-183/28 565

for the c o r r e l a t i o n of the Koo l p in F o r m a t i o n , host r o ck to most of the u r a n i u m deposi ts i n the South A l l i g a t o r R i v e r v a l l e y , w i th the Koo l p in F o r m a t i o n equivalent , wh i ch i s the host r o ck to the u r a n i u m depos i ts at Nabar l ek , J a b i l u k a , Range r , and Koonga r r a (See F i g . 3.) and a l s o , we bel ieve, wi th the Golden Dyke F o r m a t i o n , host r o ck to the u r a n i u m depos i ts at R u m J u n g l e . The concentrat ion of u r a n i u m in the Koo l p in F o r m a t i o n and i t s c o r r e l a t i v e s suggests a syngenet ic enr i chment of u r a n i u m d u r i n g sed imentat ion . The source of u r a n i u m i s not known, but it i s a s s u m e d that the provenance was suf f i c i ent ly en r i ched in u r a n i u m to make u r a n i u m ava i lab le d u r i n g sed imenta t i on . (Archaean r o c k s at R u m Jung le , f r o m which some of the sed iment was de r i v ed , have anomalous ly h i gh u r a n i u m content, rang ing f r o m 2.8 to 28 p p m U - O . ). The h i gh carbonaceous content of the Koo l p in F o r m a t i o n and equiva lents (upto 30 percent ) would prov ide a r e ­ducing env i ronment , d u r i n g sed imenta t i on , capable of p r e c i p i t a t i n g u r a n i u m .

A s s u m i n g syngenet ic enr i chment of u r a n i u m i n favourable un i t s In the Lower P r o t e r o z o i c sequence, there r e m a i n s an important c ons id e ra t i on : the m e c h a n i s m respons ib l e for the concentrat ion of u r a n i u m into o re depos i t s .

The orebod ies occupy fau l t s , s h e a r s , and b r e c c i a s o r , a s at R u m Jung le C r e ek South depos i t , a r e conta ined i n a f o ld . T h i s r e l a t i on may be due to the squeez ing-out of m i n e r a l i z e d so lut ions f r o m a sed imentary p i l e tnto-pentres of l ow s t r e s s d u r i n g a tec ton ic event as suggested by Hube r t s . Howeve r , the p r o x i m i t y of the major depos i ts to the mant led gne iss domes suggest that m i n e r a l i z a t i o n was a l s o r e l a t ed to orogenic ac t iv i ty respons ib le for the re juvenat ion of A r c h a e a n igneous masse s , and the i n t rus i on of other igneous masses at about 1800 m.y . Ex tens i ve zones of a l t e ra t i on , most ly c h l o r i t i z a t i o n and s e r i c l t l z a t i o n , around the deposi ts of the A l l i g a t o r R i v e r s a r e a i n p a r t i c u l a r indicate

a meso the rma l m i n e r a l i z a t i o n t empera tu re range . U r a n i u m - l e a d ages and lead isotopic compos i t i ons of o re f r o m the A l l i g a t o r R i v e r s a r e a point to at least four p i tchblende ages; 1880, 1700, 900, and 500 m.y . The ages support the suggest ion of syngenet ic depos i t i on of u r a n i u m i n I lin Lower P r o t e r o z o i c sed iments before 1880 m.y . and u r a n i u m concentrat ion at 1700 m.y . , pe rhaps by r e m o b i l i z a t i o n of p r e - e x i s t i n g u r a n i u m accumula t i ons , fo l lowed by p e r i o d s of l ead l o s s at 900 m.y .

' i 500 m.y . - ages wh i ch cannot be re la ted to an igneous o r tectonic ovmit. ( H i l l s & R i c h a r d s , p e r s . c o m m . , 1974)

The spa t i a l d i s t r i bu t i on of u r a n i u m depos i ts and p r o spec t s I m l i h e Nanambu and R u m Jungle C o m p l e x e s suggests a

r e l a t i on between the m i n e r a l i z a t i o n and a major igneous phase (1700-IHOO m.y.) r e spons ib l e for the r enewa l of ac t i v i t y i n the p r e - e x i s t i n g \ i i luioun masses and for the emplacement of la rge g ran i t i c masses i n

1 h " p rov ince and e l sewhere in nor thern A u s t r a l i a . The nature of th i s i • l a l Ion Is not known, but Judging f r om the compara t i v e l y l ow grade of

U v h o i l i r i m i l a l t e ra t ion in ore z •» throughout the p rov ince , we

D O D S O N el a l .

consider that the mineralizing solutions did not originate from the granitic masses. More probably the mineralizing solutions were fluids Isolated by lithlfication of the Lower Proterozoic sediments and mobilized during the Igneous and contemporaneous tectonic phases.

The uranium deposits of the South Alligator River valley occur close to, and in some places in, volcanics and sediments of the Edith River Volcanics. The uranium content of these volcanic rocks has been considered as a possible source of the ore In this area. However, the Edith River Volcanics could not have been responsible for the formation of deposits elsewhere in the province. We believe therefore that the part played by the Edith River Volcanics in uranium mineralization is small, if not Insignificant,

During the current investigations, attention has been paid to the Kombolgle Formation, a massive Carpentarian sedimentary sequence which might be considered as a source rock from which uranium was leached and then precipitated in a suitable environment. As previously stated, the Kombolgle Formation consists of lower and upper sandstone units separated by the basic Nungbalgarri Volcanic Member composed of interbedded volcanics and sediments. The upper sandstone unit includes a thin volcanic unit, the Gilruth Volcanic Member. The sandstone components of the Kombolgle Formation are composed of compact quartz sandstone and tnterbeds of pebble conglomerate, both being characteristically low in radioactivity. The Nungbalgarri Volcanic Member Is also of low radioactivity. However, significant radiometric anomalies have been recorded over laterite developed locally down-slope from the volcanic members. Anomalous radioactivity is • feature common to most laterites in the Rum Jungle-South Alligator River valley and the Alligator Rivers a r e a .

Concentration of ore near the present-day land surface at some deposits in the Rum Jungle area and at Saddle Ridge in the South Alligator River valley suggests that supergene enrichment has taken

place locally, probably by solution and accretion as the surface has been lowered by erosion. At the other deposits, such as Palette in the South Alligator River valley, uranium in the overlying Kombolgie Formation sediments is believed to be a product of hypogene enrichment.

We concluded that the following sequence of events has been responsible for the formation of the uranium deposits of the Rum Jungle-Alligator Rivers Province:

(1) Syngenetic enrichment of uranium In carbonaceous sediments during the Lower Proterozoic.

(2) Concentration of ore-grade deposits of uranium minerals In favourable places from mineralized solutions mobilized during a major Igneous/tectonic event.

(3) Relocation of uranium by circulating solutions through leaching and redeposition not far away.

II I I I II I N i I >

( I ) D O D S O N . * . C . , " W o * . . . M i M M t .1 h - m . t t — ..I - ^ p o i i l . " . Uranium Proapecliag Handhoo* (BOWIE. S . H . U . . DAV IS M ' - t i l | M , , , r . . - A I • i - .pon.oted Ad» Study lartitat* am

Melhodi o f ftoipeclliif lot Umm Mtwiafcl . \m»mm I91|>. hMl M M . htoUI I . . U M M k M ( l t T t ) 33-44. [*) N F E D H A M . I . f . . S M A R T , f A PNpBjBj * • * > * % Alllgataa Rlvar Party H . T . 1911, i n c . Bar. Miner.

Rearm. l . u n p u M i . t - " 131 N E E D H A M . R .S . , W I U t B , f. (• IMI I I t o , , w A t < MMAN A. I. . Alligator R i v e n Region

Environmental r a t i findu> a M a d * Ra t . Mt-at - e t a * . (I913/20B) MpybHar tN . (4 ] S M A R T . P . O . . WILKCB. P . O . N I I IR IAM H a H A M IIMAN. A . L . . The Gaology and Geophyi lc i

o f Ihe Alligator R i v e n Region O n BBJBJBJSjfjSj (51 MORS FALL . K . R . , W I I K R . P O , l>nlr*ula Alligator Rlvt i and Mount Fvelyn (par!) Airborne

Magnetic and Radiometric l a rva y IB1I/1, Re * . Bar. M I M I . Aunt, (in preparallon). (61 WALPOLE . I. P . , C R O H N . P. W, . D U N N P.I . RANMAI M A . . Otology o( Ihe Ka lher lne -Denr ln

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Austral ia . Melbourne. Auit i alailan I M M r I IMS) IK - i l , 19) PRICIIARD, C . I . , "Uranium or* d t p m l l i i>t i t - *o»th Al l lgahH R lva r ' , Oto logy o f Australian Ore

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(10 ) T A T I O R . I., ' O r i g i n and contro l , o f uranium rnine iaHiat ina in tha l o a th All igator V a l l e y " . Proc. Symp. Uranium In Aastr all a. Mt f co t r a t Atrt ia lat laa I M M (1MB) 3B-44.

(11 ) RHODES. 1. M. . Tha Gtoksgtt a I R a l t t M i i M p * o f tha Ram I angle ( o m p t e a . Northern Territory. B « . Miner . I n w . A tat. Rep. B 9 ( I M S ) .

[12 ] H E W . K . f t . . R H O M B , I. M . . T a o c M m . araalarn and pMaaUum < or- i n t t a two . in ( r a n i t o and g n e i m s o f (ha R a m hakght Coanajhti. N T . . A ta t i a l i a . fcon. Gaol 4 [ J U R M i BBB-11.

(13 ) RICHARDS. L B . , BEBflT. H. . RHOPES . I . M . . Isotopic and lead-a lpha agai o l a m . Aurtrallaa i l r c o n i . Gao l . Soe. A m i . 13 1 (19SS) S9 -9S .

(14 ) N E E D H A M . R. S . . S M A R T . P. C . . W A T C H M A N . A . I . . Regional Oto logy o f I he Alligator Rive* Uranium Province. Northern Terr itory. Austral ia . Rec. Bur. Miner Reaour. 1913/S4.

(15 ) M E H N E R T . K .R . . Mlgmat l te i and the Origin o f Granit ic Rocto. EltrvWr. Ami t t r dam ( IMS ) . ( I S ) A N T H O N Y , P . , Nabarlek. Spacia l i i t Group in the Oene . l i ot Ore D t p o i l l i . Symp. Gao l . Soc. A m i .

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Sydney ri9l3) unpubll ihad. (30 ] ROBERTS, W . M . B . , Do loml l lxat lon and Ihe gene iU o f Ihe Woodcutteri b rad - t lnc prospect. Northarn

T t n l t o r y , Austral ia . Miner . Dtpot i ta (1973) 835 -56 . [21) BARNES. P . O . . ! • Uranium Protpectlng Handbook (BOWIE, S . H . U . . DAV IS . M . . O S T U . D . . Etta).

(Proc. NATO- iponaor td Adv. Study tottilute on Methods o f Protpectlng for Uranium Minerals . London.

1911). Inn M i a . Meta l ) . . London (1972 ) 45 .

D I S C U S S I O N

H .H . ADLER: How do you suppose uranium could have been "squeezed out" or concentrated from sediments with a presumed concentration of 1% U or less to such a pure and highly concentrated pitchblende deposit as that at Narbalek without leaving any dispersion of uranium in the nearby environment?

R .G . DODSON: I believe that Narbalek is a classic example of relocation (and concentration) of uranium within a source rock. Uraniferoua solutions deposited pitchblende in a favourable structure, which appear to he • slip