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Tin exploration at Mount Isa — A case history
Journal of Geochemical Exploration, 22 (1984) 71--82
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71
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
T I N E X P L O R A T I O N A T M O U N T ISA - - A C A S E H I S T O R Y
C.A.J. T O W S E Y
I,* and D.J. P A T T E R S O N
2
i Mining Division, M o u n t Isa Mines Limited, Mount Isa, Qld. 4825 (Australia) 2 Research and Development Division, Mount Isa Mines Limited, M o u n t Isa, Qld. 4825 (Australia) (Received M a y 25, 1983; revised and accepted December 12, 1983)
ABSTRACT Towsey, C.A.J. and Patterson, D.J., 1984. Tin exploration at M o u n t Isa -- a case history. J. Geochem. Explor., 22: 71--82. Minor tin, tungsten, tantalum and beryllium mineralization occurs near Mount Isa in pegmatites along the eastern rim of the Queen Elizabeth pluton, a multi-phase pluton in the Proterozoic Sybella Granite batholith o f northwest Queensland. The Queen Elizabeth pluton and adjacent metamorphic rocks were explored by Mount Isa Mines Limited during 1980--1983 to assess the tin potential of the area. An orientation study was carried out over an existing tin deposit, using streamsediment, soil and rock-chip sampling, magnetometer and scintillometer surveys and percussion drilling. This defined the geophysical and geochemical features of the deposit, and indicated that monazite and cassiterite had related distributions and could be used as heavy-mineral tracers. Broader exploration used stream~ediment and rock-chip sampling, and airborne magnetic and gamma-ray spectrometry surveys. The eastern rim of the Queen Elizabeth pluton was found to be enriched in thorium and tin, and an area of approximately 2000 x 400 m with anomalously high tin values in bedrock was delineated in a part of the pluton not previously known to be mineralized. An economic assessment led to relinquishment of the area at that stage. Although no new areas of economic mineralization were found, the study indicated that in this region the thorium gamma-ray response can be used in regional airborne surveys to define areas of prime interest for tin exploration, and that monazite in stream sediments can be used as a heavy-mineral tracer for pegmatitic tin mineralization.
INTRODUCTION
The area of study is located southwest o f Mount Isa in northwest Queensland (Fig. 1), and was taken up under an Authority to Prospect by Mount Isa Mines Limited from 1980 to 1983 to examine base metal targets in the sedimentary units and the tin-tantalum-tungsten association in and around the granite. The region is semi-arid, with annual rainfall of around 300 mm occurring in the m o n s o o n season from December to March, and mid-summer *Present address: Exploration Department, BHP Minerals, 344 Queen Street, Brisbane, Qld. 4000, Australia 0375-6742/84/$03.00
© 1984 Elsevier Science Publishers B.V.
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temperature maxima of 43 ° C. The topography is of moderate relief. The major stratigraphic units present in the region (Fig. 2) are the May Downs Gneiss, the Mount Guide Quartzite, the Eastern Creek Volcanics, the Judenan Beds and the Mount Isa Group. Part of the sequence is intruded by a pluton of the composite Sybella Granite batholith, and by several types of pegmatites (Fig. 2). Detailed descriptions of the stratigraphic units are given by Wilson (1972) and Hill et al. (1975). Only the Eastern Creek Volcanics, the Judenan Beds and the granitic rocks were studied in detail during the exploration programme.
73
Fig. 2. Simplified geological map of area of investigation, showing the Queen Elizabeth pluton of the Sybella Granite in relation to metasedimentary and metavolcanic units, and locations of tin prospects.
The Mount Isa area falls within the Isa Uplands o f Grimes (1979, 1980) and has been subjected to several cycles of deep weathering since the late Mesozoic. Metamorphic rocks and the eastern and northern portions of the granite crop o u t strongly in the study area, and generally show little alteration within a few tens of centimetres of the surface of boulders. In the centre of the pluton the granite is deeply weathered and covered b y a 1--2 m blanket of quartz-microcline gravel. In the west and southwest, the granite
74 also crops out strongly, but appears to have been close to an old weathering surface, as ridges are c o m m o n l y deeply weathered with patchy development of kaolinitic, ferruginous and silicified zones. Relics of silica-cemented arkose and titaniferous terrazzo-type silcrete occur sporadically in this area. Eastern Creek Volcanics This is a repetitive sequence of interbedded siltstones, quartzites and basic volcanics of continental tholeiitic affinities. West of the Mount Isa Fault, the sequence has been regionally metamorphosed to the amphibolite facies of an Abukuma-type metamorphism. Pressures causing this metamorphism have been estimated in the range of 3--5 kbar, with temperatures in the range 500--600°C (Mount Isa Mines Limited, unpublished work). One retrograde event has been recognized. At least three, and possibly four, deformational events have been recognized in mapping west of the Mount Isa Fault. Detailed studies of the regional deformation are being undertaken at James Cook University of North Queensland, and the deformation will not be discussed further here. As a result of the metamorphism, the Eastern Creek Volcanics in the study area are represented by foliated hornblende and actinolite amphibolites, minor diopside pyroxenites, muscovite-biotite-sillimanite schists, and micaceous quartzites. Judenan Beds A sequence of quartzites, micaceous quartzites, and mica schists grading from mica-cordierite through to mica-sillimanite schists form this unit west of the Mount Isa Fault. Minor amphibolites and biotite~hlorite schists are present. They have been correlated in part with the very weakly metamorphosed units of the basal Mount Isa Group east of the Mount Isa Fault by workers from the Bureau of Mineral Resources, and the Geological Survey of Queensland (Carter et al., 1961). Chlorite, muscovite, biotite, and sillimanite isograds were defined in the sequence, with an increase in grade from east to west, as also observed b y Wilson (1972) in areas lying north of the prospect area. The Sybella Granite The Sybella Granite is a composite batholith occupying a meridional belt 170 km long, to the west of Mount Isa. Pegmatites containing minor raremetal mineralization (Be, Ta, Nb, Sn, W) occur sporadically throughout the Sybella Granite b u t are best developed at the northern and eastern margins of an eUipsoidal b o d y of granite, informally k n o w n as the Queen Elizabeth pluton, cropping o u t a b o u t 12 km southwest of Mount Isa. Petrographic features of the granite and pegmatities have been described b y Wilson (1972) and Hill et al. (1975). The core of the Queen Elizabeth pluton is a foliated
75 biotite-hornblende granite (Table 1), with a high accessory mineral content. Near the centre of the pluton relict igneous textures are moderately well preserved, with original Carlsbad twinning and rods and beads of microperthite preserved in microcline potassium feldspar. Rocks near the margin are more recrystaUized and consist of lenticles of quartz, polyhedral microcline and albite-rimmed oligoclase, wrapped by discontinuous foliae of biotite (fluorine-bearing annite) and hornblende (hastingsite). Accessory minerals are typically concentrated in mafic foliae. TABLE 1 Typical modal analyses of granite phases in the Queen Elizabeth pluton
Quartz Microcline
Plagioclase Myrmekite Hornblende
Biotite Muscovite
Biotite-hornblende granite
Biotite granite
(vol. %)
(vol. %)
30.6 19.1 25.2 1.0 5.5 16.0 --
38.6 35.6 22.1 0.4 2.8 0.3
Accessories
Titanite
Allanite Apatite Zircon Fluorite Opaques
1.6 0.4 0.2
0.2
0.1
0.1 0.1
0.1
A foliated leucocratic biotite granite crops o u t at the northern end of the pluton. This is depleted in b o t h mafic and accessory minerals by comparison with the biotite-hornblende granite (Table 1), but in some places contains traces of hornblende and allanite. Contact relationships with the main part of the pluton are poorly known, but the biotite granite appears to be a separate intrusive phase, probably related to the Sybella microgranite which crops o u t further north in the Mount Isa 1:100 000 Sheet area (Hill et al., 1975). Cropping out along the eastern margin o f the Queen Elizabeth pluton, and approximately coincident with an arcuate thorium anomaly (Fig. 3), is a belt of foliated biotite-hornblende granite which is finer grained than that in the central part of the pluton. It is generally muscovite-bearing, and contains numerous aplitic and microgranite dykes, and pegmatitic veins and pods. Although petrographically similar to the main part of the pluton, this appears to be a separate and previously unmapped intrusive phase. Titanite and allanite are characteristic accessory minerals in hornblendebearing phases of the Queen EliZabeth pluton. Titanite c o m m o n l y contains
76
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Fig. 3. Simplified map of the Queen Elizabeth pluton and the surrounding Eastern Creek Volcanics, showing magnetic field, thorium distribution and contoured tin distribution.
relics of hemo-ilmenite and is probably of metamorphic derivation. Primary allanite is enriched in thorium, but much displays a partial metamict character. Alteration of allanite is accompanied by addition of potassium and chlorine and removal of thorium. The thorium may have been redeposited in nearby pegmatites, or in uranium-thorium minerals which occur in the Eastern Creek Volcanics to the north of the area (Brooks, 1960}.
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Pegmatites Mineralized pegmatites in the area have distinct structural and mineralogical features. Beryl (sporadically altered to bertrandite) occurs in pegmatites at the northern and southern ends of the ellipsoidal Queen Elizabeth pluton, and is characteristically accompanied by abundant tourmaline. Cassiterite occurs only in pegmatites along the eastern margin of the pluton. It is characteristically accompanied by monazite and minor to trace amounts of tourmaline. Detailed studies on the pegmatites, mainly relating to the economic aspects of beryl mineralization were conducted by Shepherd (1946), Brooks (1957}, and Brooks and Shipway (1960). Previous work b y Mount Isa Mines Limited (O'Dea, 1964) showed albite to be present in many pegmatites, and recent work has shown that only the stanniferous pegmatites contain dominant albite, almost always as replacement of pink potassium feldspar. This has enabled usage of a simple test for discriminating between stanniferous and non-stanniferous pegmatites. Samples taken in the field were stained in the laboratory, using the standard HF-sodium cobaltinitrite test for potassium feldspar. The absence of tourmaline in the stanniferous pegmatites was also used as a field discriminator. EXPLORATION PROGRAMME Tin mineralization has been k n o w n in the area for 50 years (Shepherd, 1938). This exploration programme started with an orientation study over two existing tin deposits on the eastern margin of the Sybella Granite, the "Little Leslie" and "Queen Elizabeth" deposits. Both deposits have been gouged to shallow depths over the last 50 years but only small tonnages of tin ore have been produced. The "Little Leslie" deposit has the best exposure and has recorded grades as high as 8% Sn (Power, 1938). Detailed channel sampling of the deposit showed grades in remaining exposed faces to be only 0.3% Sn. Mapping showed that the tin occurred as cassiterite in muscovite greisen selvages, and as disseminations in quartz-albite-muscovite pegmatites.
Orientation study An orientation study conducted over the "Little Leslie" deposit consisted of detailed lode sampling, soil-sampling, stream sediment sampling, a magnetometer survey and a scintillometer study. Short percussion holes were drilled under the surface outcrop. This study established the levels of tin likely to be encountered in the regional survey and the geophysical response of the pegmatites. The magnetic response was weak, despite the occurrence of accessory magnetite in the granite pegmatites. The high gamma count was assumed to reflect the K,feldspar content of the granite, and the presence of
78 monazite and allanite in both the pegmatite and granite respectively. Results of stream-sediment sampling are discussed below.
Regional geophysical study A contract aerial survey was flown combining magnetics and gamma-ray spectometry. The aerial magnetic data was required as part of the regional mapping programme to delineate both the amphibolites and the granite contact. The gamma-ray spectrometry was flown in an attempt to delineate the pegmatite bodies, either as potassium lows (due to the replacement of K-feldspar by albite) or as thorium highs (due to the high monazite content), or as combinations of both. Consequently, the survey specification included total count, potassium (as 4°K), uranium (as 214Bi) and thorium (as 2°ST1) channels, and ratios of U/Th, K/Th, K/U to discriminate the mineralogical responses. As large targets were sought, and the known pegmatites cropped o u t parallel to the granite margin and parallel to the regional meridional strike, east--west flight lines were specified at 400 m spacings with orthogonal tielines at a 3-km spacing. Instrument cycle-times and flight speeds resulted in data points approximately every 50 linear metres. As anticipated from the orientation study, the SybeUa Granite showed up as a magnetic low and a potassium high in comparison to the Eastern Creek Volcanics (Fig. 3). Under the main granite outcrop, the eastern margin of the granite dips steeply west as shown by the magnetic signature of the Eastern Creek Volcanics under the granite. The margin in the southern part of the area dips westwards at a very shallow angle, as indicated by the magnetics in Fig. 3. This attitude of the granite margin was later confirmed by a regional gravity survey. An unexpected result of the radiometric survey was the enrichment in thorium along the eastern margin of the granite {Fig. 3). This is regarded as a discrete phase of the granite, probably later than the main body, although until the survey was flown this phase had not been discerned in mapping. Slight variations in topography and grainsize of the granite were k n o w n in the rim, and a distinct vegetation anomaly was noted, defined by the appearance of prolific growth of western silver b o x (Eucalyptus pruinosa), contrasting with turpentine (Acacia chisholmii) on the Eastern Creek Volcanics, and snappy gum (Eucalyptus brevifolia) and grey b o x (Eucalyptus argillacea) in the main b o d y of the granite. These variations were originally thought to be contact effects, b u t are n o w interpreted as due to a discrete intrusive granitic phase. Mineralogical examination of the granite suggested that the thorium anomaly was due to the presence of allanite rather than monazite. The monazite-bearing pegmatites were not detected in the airborne radiometric survey, probably due to the wide flight-line spacing, and no new areas of monazite concentration were discovered.
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Stream-sediment mineralogy During the orientation study in the area, a heavy-mineral survey of material collected from major and minor drainage channels around the Little Leslie prospect, was used to identify the heavy-mineral suites present and the scale of alluvial dispersion. Samples of panned heavy-mineral concentrates were split into five magnetic fractions using a hand magnet and Frantz isodynamic separator, and minerals were identified by a combination of optical microscopy and X-ray diffraction, and in the case of cassiterite by a confirmatory HCI-Zn "tinning" test. Suites related to granite, pegmatite and metamorphic rocks were identified in the area (Table 2). In granite areas the heavy mineral content of the sediments is low, and consists principally of euhedral zircon with minor to trace amounts of other components. In areas of metamorphic rocks the total heavy-mineral content TABLE 2 Heavy-mineral suites in stream sediments from the Little Leslie Mine area Suite
Major ( 1 )
Minor (1)
Trace (1)
A: Granitederived (2)
Zircon
monazite (4) hornblende ilmenite/hematite magnetite (oxidized)
fluorite cassiterite (4) garnet (almandine) (4) tourmaline rutile epidote xenotime
B: Pegmatitederived (3)
cassiterite monazite
ilmenite/hematite magnetite (oxidized) fluorite
epidote tourmaline
C: Metamorphicderived
ilmenite/hematite magnetite (fresh) hornblende
epidote anthophyllite tourmaline
chlorite garnet (spessartine) diopside apatite
Notes: (1) Mineral abundances were estimated visually and assigned to one of the following classes: Major -- greater than 5% by volume of total heavy fraction; Minor -- 1--5% by volume; Trace -- less than 1% by volume. (2) Titanite and allanite are major heavy accessory minerals in the Sybella Granite, but are destroyed during weathering and were not recorded in stream sediments. (3) A cassiterite concentrate from the Little Leslie prospect contained in addition trace amounts of wolframite, columbite, futile, pyrite and chalcopyrite. (4) Cassiterite and garnet were not observed and monazite was only rarely observed in samples elsewhere in the Sybella Granite. These minerals, when present in Suite A, are probably derived from minor pegmatitic veinlets in the border zone near the Little Leslie prospect.
80 is high and is dominated by ilmenite-hematite, hornblende and fresh magnetite shed from metamorphosed, predominantly basaltic rocks in the Eastern Creek Volcanics. The relatively high tourmaline content of this suite is derived from pervasive development of tourmaline in metasediments along the eastern margin of the Sybella Granite and from local tourmalinization of metasedimentary and metavolcanic rocks around pegmatite bodies. The Little Leslie stanniferous pegmatite is characterized by high contents of cassiterite and monazite shed into nearby stream sediments, other minerals in the pegmatite suite being non-diagnostic. This heavy-mineral signature was detected only within minor drainage channels and at their confluence with major channels. Elsewhere in the main drainage channels material of the metamorphic suite obscures the mineralogical indicators of mineralization. A broader stream-sediment survey over the remainder of the pluton detected areas of detrital monazite, but no detrital cassiterite was observed outside the area of the orientation survey.
Bedrock geochemistry The newly recognized late phase of the granite, enriched in volatiles and residual trace elements, could be a favourable host for pegmatites and the deposition of tin-tantalum-tungsten minerals. Consequently, an outcrop sampling programme was undertaken on a random pattern, to sample part of the main granite b o d y and the anomalous thorium-rich rim. Samples of approximately 2 kg were taken at each outcrop, and later crushed, split and
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Fig. 4. Cumulative frequency distribution o f bedrock tin values, showing lognormal distribution, and t w o distinct populations. Threshold was taken as 10 ppm Sn.
81 assayed for tin, tantalum, tungsten and niobium. Individual elements were assessed statistically using cumulative frequency curves (Lepeltier, 1969) to obtain rapid workable estimates of background and threshold values and the number of populations present. The eastern rim of the Queen Elizabeth pluton was shown to be anomalously high in tin, and the sampling indicated an area some 2000 m long south of the Queen Elizabeth mine that contained values in excess of 40 ppm Sn. A much smaller tungsten anomaly was outlined within the anomalous tin zone. The tin values showed t w o distinct populations (Fig. 4), and a threshold of 10 ppm Sn was indicated. Values less than 10 ppm Sn were assumed to represent the background population in the unmineralized portion of the main granite. Contour plots of tin values were plotted at intervals of 10 ppm and are shown in Fig. 3. Tungsten and niobium values plotted as single populations, although an excess of high values of tungsten was present. Tantalum values were t o o low to be assessed statistically, most being below the limit of detection. ECONOMIC ASSESSMENT
After completion of the program described above, the third year of tenure of the Authority to Prospect had begun, and the economic possibilities of the area were critically reviewed. The price of tin had dropped substantially since the c o m m e n c e m e n t of exploration, and production quotas were being levied on existing Australian producers, so the possibility of market entry for a new producer was in doubt. In addition, although Mount Isa has an existing infrastructure base, it is poorly placed for export, being over 900 km b y rail from the nearest major port. These factors indicated that any potential tin o r e b o d y would have to be of high grade and high tonnage to justify development. An assessment of the size of the tin-anomalous zone along the eastern margin of the Queen Elizabeth pluton, the size of the k n o w n pegmatites and their recorded average grade of a b o u t 0.3% Sn indicated that the probable tonnage potential in the area would not make a viable target, despite inferred favourable metallurgical characteristics. It was consequently decided not to drill the indicated tin targets, and the area was relinquished. CONCLUSIONS
Although no areas of economic mineralization were discovered, tin mineralization in the area was shown to be spatially related to a thorium-, tin- and tungsten-enriched phase of the Sybella Granite along the eastern rim of the Queen Elizabeth pluton. An airborne radiometric survey rapidly delineated areas for follow-up bedrock geochemical sampling, and this technique could prove useful elsewhere in the region. Mineralogical studies showed that monazite and cassiterite occur together in mineralized
82
pegmatites in the area, and that stanniferous pegmatites are rich in albite and poor in tourmaline in comparison to beryl-bearing pegmatites. Monazite and cassiterite in the heavy-mineral fraction of stream sediments can be used as tracers of bedrock tin mineralization. ACKNOWLEDGEMENTS
We thank the management of Mount Isa Mines Limited for permission to publish the results obtained during exploration. We also acknowledge the assistance and co-operation of Chief Geologist, Rennie Blair, and Senior Geologist--Surface, Colin Robertson. The valuable contribution to the study by John Poole, Horst Schmidt and Kim Willcock is acknowledged. Basil Cinelli and Narelle Ward assisted with drafting and preparation of the manuscript. The contract geophysical survey was flown by Geometrics International Corp. and interpreted by Dr. David Leaman of Leaman Geophysics Pty Ltd. REFERENCES Brooks, J.H., 1957. Big Beryl Mine, Mount Isa, Cloncurry mineral field. Queensl. Gov. Min. J., 58: 605. Brooks, J.H., 1960. The uranium deposits oi" northwestern Queensland. Geol. Surv. Queensl., Publ. 297. Brooks, J.H. and Shipway, C.H., 1960. Mica Creek pegmatites, Mount Isa, northwestern Queensland. Queensl. Gov. Min. J., October 20: 511--521. Carter, E.K., Brooks, J.H. and Walker, K.R., 1961. The Precambrian mineral belt of northwestern Queensland. Bur. Miner. Resour., Geol. Geophys., Bull. 51. Collins, W.J., Beams, S.D., White, A.J.R. and Chappell, B.W., 1982. Nature and origin of A-type granites with particular reference to southeastern Australia. Contrib. Mineral. Petrol., 80 : 189--200. Grimes, K.G., 1979. The stratigraphic sequence of old land surfaces in northern Queensland. B M R J. Aust. Geol. Geophys., 4 : 33--46. Grimes, K.G., 1980. The Tertiary geology of northern Queensland. In: R.A. Henderson and P.J. Stephenson (Editors), The Geology and Geophysics of Northeastern Australia. Geological Society of Australia Incorporated, Queensland Division, Brisbane, pp. 329--347. Hill, R.M., Wilson, I.H. and Derrick, G.M., 1975. Geology of the Mount Isa 1:100 000 sheet area, Northwest Queensland. Bur. Miner. Resour., Geol. Geophys., Record 1975/175. Lepeltier, C., 1969. A simplified statistical treatment of geochemical data by graphical representation. Econ. Geol., 64: 538--550. O'Dea, T., 1964. The investigation of the Mica Creek pegmatites. Mount Isa Mines Limited, Tech. Rep. No. GEO-68 (unpubl.). Power, R., 1938. Tin at Mount ha. Queensl. Gov. Min. J., February 15: 39. Shepherd, S.R.L., 1938. Mica Creek tin deposit near Mount ha. Queensl. Gov. Min. J., March 15: 95--96. Shepherd, S.R.L., 1946. Big Beryl Mine. Queensl. Gov. Min. J., 47: 51. Wilson, C.J.L., 1972. The stretigraphic and metamorphic sequence west of Mount ha, and associated igneous intrusions. Proc. Australas. Inst. Min. Metall., 243: 27--42.
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Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
T I N E X P L O R A T I O N A T M O U N T ISA - - A C A S E H I S T O R Y
C.A.J. T O W S E Y
I,* and D.J. P A T T E R S O N
2
i Mining Division, M o u n t Isa Mines Limited, Mount Isa, Qld. 4825 (Australia) 2 Research and Development Division, Mount Isa Mines Limited, M o u n t Isa, Qld. 4825 (Australia) (Received M a y 25, 1983; revised and accepted December 12, 1983)
ABSTRACT Towsey, C.A.J. and Patterson, D.J., 1984. Tin exploration at M o u n t Isa -- a case history. J. Geochem. Explor., 22: 71--82. Minor tin, tungsten, tantalum and beryllium mineralization occurs near Mount Isa in pegmatites along the eastern rim of the Queen Elizabeth pluton, a multi-phase pluton in the Proterozoic Sybella Granite batholith o f northwest Queensland. The Queen Elizabeth pluton and adjacent metamorphic rocks were explored by Mount Isa Mines Limited during 1980--1983 to assess the tin potential of the area. An orientation study was carried out over an existing tin deposit, using streamsediment, soil and rock-chip sampling, magnetometer and scintillometer surveys and percussion drilling. This defined the geophysical and geochemical features of the deposit, and indicated that monazite and cassiterite had related distributions and could be used as heavy-mineral tracers. Broader exploration used stream~ediment and rock-chip sampling, and airborne magnetic and gamma-ray spectrometry surveys. The eastern rim of the Queen Elizabeth pluton was found to be enriched in thorium and tin, and an area of approximately 2000 x 400 m with anomalously high tin values in bedrock was delineated in a part of the pluton not previously known to be mineralized. An economic assessment led to relinquishment of the area at that stage. Although no new areas of economic mineralization were found, the study indicated that in this region the thorium gamma-ray response can be used in regional airborne surveys to define areas of prime interest for tin exploration, and that monazite in stream sediments can be used as a heavy-mineral tracer for pegmatitic tin mineralization.
INTRODUCTION
The area of study is located southwest o f Mount Isa in northwest Queensland (Fig. 1), and was taken up under an Authority to Prospect by Mount Isa Mines Limited from 1980 to 1983 to examine base metal targets in the sedimentary units and the tin-tantalum-tungsten association in and around the granite. The region is semi-arid, with annual rainfall of around 300 mm occurring in the m o n s o o n season from December to March, and mid-summer *Present address: Exploration Department, BHP Minerals, 344 Queen Street, Brisbane, Qld. 4000, Australia 0375-6742/84/$03.00
© 1984 Elsevier Science Publishers B.V.
72 Scole of kilometres 20
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Fig. 1. L o c a t i o n o f area o f investigation.
temperature maxima of 43 ° C. The topography is of moderate relief. The major stratigraphic units present in the region (Fig. 2) are the May Downs Gneiss, the Mount Guide Quartzite, the Eastern Creek Volcanics, the Judenan Beds and the Mount Isa Group. Part of the sequence is intruded by a pluton of the composite Sybella Granite batholith, and by several types of pegmatites (Fig. 2). Detailed descriptions of the stratigraphic units are given by Wilson (1972) and Hill et al. (1975). Only the Eastern Creek Volcanics, the Judenan Beds and the granitic rocks were studied in detail during the exploration programme.
73
Fig. 2. Simplified geological map of area of investigation, showing the Queen Elizabeth pluton of the Sybella Granite in relation to metasedimentary and metavolcanic units, and locations of tin prospects.
The Mount Isa area falls within the Isa Uplands o f Grimes (1979, 1980) and has been subjected to several cycles of deep weathering since the late Mesozoic. Metamorphic rocks and the eastern and northern portions of the granite crop o u t strongly in the study area, and generally show little alteration within a few tens of centimetres of the surface of boulders. In the centre of the pluton the granite is deeply weathered and covered b y a 1--2 m blanket of quartz-microcline gravel. In the west and southwest, the granite
74 also crops out strongly, but appears to have been close to an old weathering surface, as ridges are c o m m o n l y deeply weathered with patchy development of kaolinitic, ferruginous and silicified zones. Relics of silica-cemented arkose and titaniferous terrazzo-type silcrete occur sporadically in this area. Eastern Creek Volcanics This is a repetitive sequence of interbedded siltstones, quartzites and basic volcanics of continental tholeiitic affinities. West of the Mount Isa Fault, the sequence has been regionally metamorphosed to the amphibolite facies of an Abukuma-type metamorphism. Pressures causing this metamorphism have been estimated in the range of 3--5 kbar, with temperatures in the range 500--600°C (Mount Isa Mines Limited, unpublished work). One retrograde event has been recognized. At least three, and possibly four, deformational events have been recognized in mapping west of the Mount Isa Fault. Detailed studies of the regional deformation are being undertaken at James Cook University of North Queensland, and the deformation will not be discussed further here. As a result of the metamorphism, the Eastern Creek Volcanics in the study area are represented by foliated hornblende and actinolite amphibolites, minor diopside pyroxenites, muscovite-biotite-sillimanite schists, and micaceous quartzites. Judenan Beds A sequence of quartzites, micaceous quartzites, and mica schists grading from mica-cordierite through to mica-sillimanite schists form this unit west of the Mount Isa Fault. Minor amphibolites and biotite~hlorite schists are present. They have been correlated in part with the very weakly metamorphosed units of the basal Mount Isa Group east of the Mount Isa Fault by workers from the Bureau of Mineral Resources, and the Geological Survey of Queensland (Carter et al., 1961). Chlorite, muscovite, biotite, and sillimanite isograds were defined in the sequence, with an increase in grade from east to west, as also observed b y Wilson (1972) in areas lying north of the prospect area. The Sybella Granite The Sybella Granite is a composite batholith occupying a meridional belt 170 km long, to the west of Mount Isa. Pegmatites containing minor raremetal mineralization (Be, Ta, Nb, Sn, W) occur sporadically throughout the Sybella Granite b u t are best developed at the northern and eastern margins of an eUipsoidal b o d y of granite, informally k n o w n as the Queen Elizabeth pluton, cropping o u t a b o u t 12 km southwest of Mount Isa. Petrographic features of the granite and pegmatities have been described b y Wilson (1972) and Hill et al. (1975). The core of the Queen Elizabeth pluton is a foliated
75 biotite-hornblende granite (Table 1), with a high accessory mineral content. Near the centre of the pluton relict igneous textures are moderately well preserved, with original Carlsbad twinning and rods and beads of microperthite preserved in microcline potassium feldspar. Rocks near the margin are more recrystaUized and consist of lenticles of quartz, polyhedral microcline and albite-rimmed oligoclase, wrapped by discontinuous foliae of biotite (fluorine-bearing annite) and hornblende (hastingsite). Accessory minerals are typically concentrated in mafic foliae. TABLE 1 Typical modal analyses of granite phases in the Queen Elizabeth pluton
Quartz Microcline
Plagioclase Myrmekite Hornblende
Biotite Muscovite
Biotite-hornblende granite
Biotite granite
(vol. %)
(vol. %)
30.6 19.1 25.2 1.0 5.5 16.0 --
38.6 35.6 22.1 0.4 2.8 0.3
Accessories
Titanite
Allanite Apatite Zircon Fluorite Opaques
1.6 0.4 0.2
0.2
0.1
0.1 0.1
0.1
A foliated leucocratic biotite granite crops o u t at the northern end of the pluton. This is depleted in b o t h mafic and accessory minerals by comparison with the biotite-hornblende granite (Table 1), but in some places contains traces of hornblende and allanite. Contact relationships with the main part of the pluton are poorly known, but the biotite granite appears to be a separate intrusive phase, probably related to the Sybella microgranite which crops o u t further north in the Mount Isa 1:100 000 Sheet area (Hill et al., 1975). Cropping out along the eastern margin o f the Queen Elizabeth pluton, and approximately coincident with an arcuate thorium anomaly (Fig. 3), is a belt of foliated biotite-hornblende granite which is finer grained than that in the central part of the pluton. It is generally muscovite-bearing, and contains numerous aplitic and microgranite dykes, and pegmatitic veins and pods. Although petrographically similar to the main part of the pluton, this appears to be a separate and previously unmapped intrusive phase. Titanite and allanite are characteristic accessory minerals in hornblendebearing phases of the Queen EliZabeth pluton. Titanite c o m m o n l y contains
76
/
/
J
TIN C( ~
-
B
-
GRANI" PEGM~ >~ 150 I ~2100
I
Scale o
Fig. 3. Simplified map of the Queen Elizabeth pluton and the surrounding Eastern Creek Volcanics, showing magnetic field, thorium distribution and contoured tin distribution.
relics of hemo-ilmenite and is probably of metamorphic derivation. Primary allanite is enriched in thorium, but much displays a partial metamict character. Alteration of allanite is accompanied by addition of potassium and chlorine and removal of thorium. The thorium may have been redeposited in nearby pegmatites, or in uranium-thorium minerals which occur in the Eastern Creek Volcanics to the north of the area (Brooks, 1960}.
77
Pegmatites Mineralized pegmatites in the area have distinct structural and mineralogical features. Beryl (sporadically altered to bertrandite) occurs in pegmatites at the northern and southern ends of the ellipsoidal Queen Elizabeth pluton, and is characteristically accompanied by abundant tourmaline. Cassiterite occurs only in pegmatites along the eastern margin of the pluton. It is characteristically accompanied by monazite and minor to trace amounts of tourmaline. Detailed studies on the pegmatites, mainly relating to the economic aspects of beryl mineralization were conducted by Shepherd (1946), Brooks (1957}, and Brooks and Shipway (1960). Previous work b y Mount Isa Mines Limited (O'Dea, 1964) showed albite to be present in many pegmatites, and recent work has shown that only the stanniferous pegmatites contain dominant albite, almost always as replacement of pink potassium feldspar. This has enabled usage of a simple test for discriminating between stanniferous and non-stanniferous pegmatites. Samples taken in the field were stained in the laboratory, using the standard HF-sodium cobaltinitrite test for potassium feldspar. The absence of tourmaline in the stanniferous pegmatites was also used as a field discriminator. EXPLORATION PROGRAMME Tin mineralization has been k n o w n in the area for 50 years (Shepherd, 1938). This exploration programme started with an orientation study over two existing tin deposits on the eastern margin of the Sybella Granite, the "Little Leslie" and "Queen Elizabeth" deposits. Both deposits have been gouged to shallow depths over the last 50 years but only small tonnages of tin ore have been produced. The "Little Leslie" deposit has the best exposure and has recorded grades as high as 8% Sn (Power, 1938). Detailed channel sampling of the deposit showed grades in remaining exposed faces to be only 0.3% Sn. Mapping showed that the tin occurred as cassiterite in muscovite greisen selvages, and as disseminations in quartz-albite-muscovite pegmatites.
Orientation study An orientation study conducted over the "Little Leslie" deposit consisted of detailed lode sampling, soil-sampling, stream sediment sampling, a magnetometer survey and a scintillometer study. Short percussion holes were drilled under the surface outcrop. This study established the levels of tin likely to be encountered in the regional survey and the geophysical response of the pegmatites. The magnetic response was weak, despite the occurrence of accessory magnetite in the granite pegmatites. The high gamma count was assumed to reflect the K,feldspar content of the granite, and the presence of
78 monazite and allanite in both the pegmatite and granite respectively. Results of stream-sediment sampling are discussed below.
Regional geophysical study A contract aerial survey was flown combining magnetics and gamma-ray spectometry. The aerial magnetic data was required as part of the regional mapping programme to delineate both the amphibolites and the granite contact. The gamma-ray spectrometry was flown in an attempt to delineate the pegmatite bodies, either as potassium lows (due to the replacement of K-feldspar by albite) or as thorium highs (due to the high monazite content), or as combinations of both. Consequently, the survey specification included total count, potassium (as 4°K), uranium (as 214Bi) and thorium (as 2°ST1) channels, and ratios of U/Th, K/Th, K/U to discriminate the mineralogical responses. As large targets were sought, and the known pegmatites cropped o u t parallel to the granite margin and parallel to the regional meridional strike, east--west flight lines were specified at 400 m spacings with orthogonal tielines at a 3-km spacing. Instrument cycle-times and flight speeds resulted in data points approximately every 50 linear metres. As anticipated from the orientation study, the SybeUa Granite showed up as a magnetic low and a potassium high in comparison to the Eastern Creek Volcanics (Fig. 3). Under the main granite outcrop, the eastern margin of the granite dips steeply west as shown by the magnetic signature of the Eastern Creek Volcanics under the granite. The margin in the southern part of the area dips westwards at a very shallow angle, as indicated by the magnetics in Fig. 3. This attitude of the granite margin was later confirmed by a regional gravity survey. An unexpected result of the radiometric survey was the enrichment in thorium along the eastern margin of the granite {Fig. 3). This is regarded as a discrete phase of the granite, probably later than the main body, although until the survey was flown this phase had not been discerned in mapping. Slight variations in topography and grainsize of the granite were k n o w n in the rim, and a distinct vegetation anomaly was noted, defined by the appearance of prolific growth of western silver b o x (Eucalyptus pruinosa), contrasting with turpentine (Acacia chisholmii) on the Eastern Creek Volcanics, and snappy gum (Eucalyptus brevifolia) and grey b o x (Eucalyptus argillacea) in the main b o d y of the granite. These variations were originally thought to be contact effects, b u t are n o w interpreted as due to a discrete intrusive granitic phase. Mineralogical examination of the granite suggested that the thorium anomaly was due to the presence of allanite rather than monazite. The monazite-bearing pegmatites were not detected in the airborne radiometric survey, probably due to the wide flight-line spacing, and no new areas of monazite concentration were discovered.
79
Stream-sediment mineralogy During the orientation study in the area, a heavy-mineral survey of material collected from major and minor drainage channels around the Little Leslie prospect, was used to identify the heavy-mineral suites present and the scale of alluvial dispersion. Samples of panned heavy-mineral concentrates were split into five magnetic fractions using a hand magnet and Frantz isodynamic separator, and minerals were identified by a combination of optical microscopy and X-ray diffraction, and in the case of cassiterite by a confirmatory HCI-Zn "tinning" test. Suites related to granite, pegmatite and metamorphic rocks were identified in the area (Table 2). In granite areas the heavy mineral content of the sediments is low, and consists principally of euhedral zircon with minor to trace amounts of other components. In areas of metamorphic rocks the total heavy-mineral content TABLE 2 Heavy-mineral suites in stream sediments from the Little Leslie Mine area Suite
Major ( 1 )
Minor (1)
Trace (1)
A: Granitederived (2)
Zircon
monazite (4) hornblende ilmenite/hematite magnetite (oxidized)
fluorite cassiterite (4) garnet (almandine) (4) tourmaline rutile epidote xenotime
B: Pegmatitederived (3)
cassiterite monazite
ilmenite/hematite magnetite (oxidized) fluorite
epidote tourmaline
C: Metamorphicderived
ilmenite/hematite magnetite (fresh) hornblende
epidote anthophyllite tourmaline
chlorite garnet (spessartine) diopside apatite
Notes: (1) Mineral abundances were estimated visually and assigned to one of the following classes: Major -- greater than 5% by volume of total heavy fraction; Minor -- 1--5% by volume; Trace -- less than 1% by volume. (2) Titanite and allanite are major heavy accessory minerals in the Sybella Granite, but are destroyed during weathering and were not recorded in stream sediments. (3) A cassiterite concentrate from the Little Leslie prospect contained in addition trace amounts of wolframite, columbite, futile, pyrite and chalcopyrite. (4) Cassiterite and garnet were not observed and monazite was only rarely observed in samples elsewhere in the Sybella Granite. These minerals, when present in Suite A, are probably derived from minor pegmatitic veinlets in the border zone near the Little Leslie prospect.
80 is high and is dominated by ilmenite-hematite, hornblende and fresh magnetite shed from metamorphosed, predominantly basaltic rocks in the Eastern Creek Volcanics. The relatively high tourmaline content of this suite is derived from pervasive development of tourmaline in metasediments along the eastern margin of the Sybella Granite and from local tourmalinization of metasedimentary and metavolcanic rocks around pegmatite bodies. The Little Leslie stanniferous pegmatite is characterized by high contents of cassiterite and monazite shed into nearby stream sediments, other minerals in the pegmatite suite being non-diagnostic. This heavy-mineral signature was detected only within minor drainage channels and at their confluence with major channels. Elsewhere in the main drainage channels material of the metamorphic suite obscures the mineralogical indicators of mineralization. A broader stream-sediment survey over the remainder of the pluton detected areas of detrital monazite, but no detrital cassiterite was observed outside the area of the orientation survey.
Bedrock geochemistry The newly recognized late phase of the granite, enriched in volatiles and residual trace elements, could be a favourable host for pegmatites and the deposition of tin-tantalum-tungsten minerals. Consequently, an outcrop sampling programme was undertaken on a random pattern, to sample part of the main granite b o d y and the anomalous thorium-rich rim. Samples of approximately 2 kg were taken at each outcrop, and later crushed, split and
tOO
.J
, o~
./ c£ c£ _J cf
~_ l0
/ fo
Z
/
/"
/ 0'01
I0
90
99'99
CUMULATIVE FREQUENCY (%)
Fig. 4. Cumulative frequency distribution o f bedrock tin values, showing lognormal distribution, and t w o distinct populations. Threshold was taken as 10 ppm Sn.
81 assayed for tin, tantalum, tungsten and niobium. Individual elements were assessed statistically using cumulative frequency curves (Lepeltier, 1969) to obtain rapid workable estimates of background and threshold values and the number of populations present. The eastern rim of the Queen Elizabeth pluton was shown to be anomalously high in tin, and the sampling indicated an area some 2000 m long south of the Queen Elizabeth mine that contained values in excess of 40 ppm Sn. A much smaller tungsten anomaly was outlined within the anomalous tin zone. The tin values showed t w o distinct populations (Fig. 4), and a threshold of 10 ppm Sn was indicated. Values less than 10 ppm Sn were assumed to represent the background population in the unmineralized portion of the main granite. Contour plots of tin values were plotted at intervals of 10 ppm and are shown in Fig. 3. Tungsten and niobium values plotted as single populations, although an excess of high values of tungsten was present. Tantalum values were t o o low to be assessed statistically, most being below the limit of detection. ECONOMIC ASSESSMENT
After completion of the program described above, the third year of tenure of the Authority to Prospect had begun, and the economic possibilities of the area were critically reviewed. The price of tin had dropped substantially since the c o m m e n c e m e n t of exploration, and production quotas were being levied on existing Australian producers, so the possibility of market entry for a new producer was in doubt. In addition, although Mount Isa has an existing infrastructure base, it is poorly placed for export, being over 900 km b y rail from the nearest major port. These factors indicated that any potential tin o r e b o d y would have to be of high grade and high tonnage to justify development. An assessment of the size of the tin-anomalous zone along the eastern margin of the Queen Elizabeth pluton, the size of the k n o w n pegmatites and their recorded average grade of a b o u t 0.3% Sn indicated that the probable tonnage potential in the area would not make a viable target, despite inferred favourable metallurgical characteristics. It was consequently decided not to drill the indicated tin targets, and the area was relinquished. CONCLUSIONS
Although no areas of economic mineralization were discovered, tin mineralization in the area was shown to be spatially related to a thorium-, tin- and tungsten-enriched phase of the Sybella Granite along the eastern rim of the Queen Elizabeth pluton. An airborne radiometric survey rapidly delineated areas for follow-up bedrock geochemical sampling, and this technique could prove useful elsewhere in the region. Mineralogical studies showed that monazite and cassiterite occur together in mineralized
82
pegmatites in the area, and that stanniferous pegmatites are rich in albite and poor in tourmaline in comparison to beryl-bearing pegmatites. Monazite and cassiterite in the heavy-mineral fraction of stream sediments can be used as tracers of bedrock tin mineralization. ACKNOWLEDGEMENTS
We thank the management of Mount Isa Mines Limited for permission to publish the results obtained during exploration. We also acknowledge the assistance and co-operation of Chief Geologist, Rennie Blair, and Senior Geologist--Surface, Colin Robertson. The valuable contribution to the study by John Poole, Horst Schmidt and Kim Willcock is acknowledged. Basil Cinelli and Narelle Ward assisted with drafting and preparation of the manuscript. The contract geophysical survey was flown by Geometrics International Corp. and interpreted by Dr. David Leaman of Leaman Geophysics Pty Ltd. REFERENCES Brooks, J.H., 1957. Big Beryl Mine, Mount Isa, Cloncurry mineral field. Queensl. Gov. Min. J., 58: 605. Brooks, J.H., 1960. The uranium deposits oi" northwestern Queensland. Geol. Surv. Queensl., Publ. 297. Brooks, J.H. and Shipway, C.H., 1960. Mica Creek pegmatites, Mount Isa, northwestern Queensland. Queensl. Gov. Min. J., October 20: 511--521. Carter, E.K., Brooks, J.H. and Walker, K.R., 1961. The Precambrian mineral belt of northwestern Queensland. Bur. Miner. Resour., Geol. Geophys., Bull. 51. Collins, W.J., Beams, S.D., White, A.J.R. and Chappell, B.W., 1982. Nature and origin of A-type granites with particular reference to southeastern Australia. Contrib. Mineral. Petrol., 80 : 189--200. Grimes, K.G., 1979. The stratigraphic sequence of old land surfaces in northern Queensland. B M R J. Aust. Geol. Geophys., 4 : 33--46. Grimes, K.G., 1980. The Tertiary geology of northern Queensland. In: R.A. Henderson and P.J. Stephenson (Editors), The Geology and Geophysics of Northeastern Australia. Geological Society of Australia Incorporated, Queensland Division, Brisbane, pp. 329--347. Hill, R.M., Wilson, I.H. and Derrick, G.M., 1975. Geology of the Mount Isa 1:100 000 sheet area, Northwest Queensland. Bur. Miner. Resour., Geol. Geophys., Record 1975/175. Lepeltier, C., 1969. A simplified statistical treatment of geochemical data by graphical representation. Econ. Geol., 64: 538--550. O'Dea, T., 1964. The investigation of the Mica Creek pegmatites. Mount Isa Mines Limited, Tech. Rep. No. GEO-68 (unpubl.). Power, R., 1938. Tin at Mount ha. Queensl. Gov. Min. J., February 15: 39. Shepherd, S.R.L., 1938. Mica Creek tin deposit near Mount ha. Queensl. Gov. Min. J., March 15: 95--96. Shepherd, S.R.L., 1946. Big Beryl Mine. Queensl. Gov. Min. J., 47: 51. Wilson, C.J.L., 1972. The stretigraphic and metamorphic sequence west of Mount ha, and associated igneous intrusions. Proc. Australas. Inst. Min. Metall., 243: 27--42.