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GOLD-BISMUTH-COPPER MINERALISATION
IN THE TENNANT CREEK DISTRICT,
NORTHERN TERRITORY, AUSTRALIA.
by
Ross R. Large
Thesis submitted for the degree of Doctor of Philosophy, Geology Department, University of New England, Armidale, New South Wales.
January, 1974.
I certify that this thesis has been composed by myself and
that it is not substantially the same as any other which may have
already been.submitted at another University.
I certify that any help received in preparing this thesis,
and all sources used, have been acknowledged in this thesis •
. R.~ .. ~.~:.
TABLE OF CONTENTS
GOLD-BISMUTH-COPPER MINERALISATION IN THE TENNANT CREEK DISTRICT,
NORTHERN TERRITORY, AUSTRALIA
ABSTRACT
INTRODUCTION
ACKNOWLEDGEMEI:i!TS
PART I: REGIONAL GEOLOGY AND ~~INERALISATION
CHAPTER 1:
l.l
1.2
1.3
1.4
CHAPTER 2:
2~1
2.2
GEOLOGY OF TENNANT CREEK DISTRICT
INTRODUCTION
STRATIGRAPHY OF THE WARRAMUNGA GROUP
1.2.1 Whippet Formation
1.2.2 Bernborough Formation
1.2.3 Carraman Formation
INTRUSIVE ROCKS
1.3.1 Rhyolitic Porphyries
l. 3. 2 Granites
1.3.3 Basic Intrusives
BROAD STRUCTURAL FEATURES
REGIONAL ASPECTS OF MINERALISATION
INTRODUCTION
LITHOLOGICAL ASSOCIATIONS
2.2.1 Magnetite-Hematite Bodies (ironstones)
2.2.2 Stratigraphic Controls
2.2.3 Rhyolitic Porphyries and Mineralisation
Page No.
(i)
(v)
(vii)
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l
3
3
5
6
11
11
12
13
13
15
15
18
18
20
22
PART II:
CHAPTER
CHAPTER
2.3
2.4
STRUCTURAL ASSOCIATIONS
2.3.1 Folds
2.3.2 Faults and Shears
PREVIOUS ORE GENESIS THEORIES
2.4.1 Igneous Rhyolitic Porphyries as the Source
2.4.2 Remobilised and Retextured Sediments as the Source
2.4.3 Gabbroic and Doleritic Intrusives as
Page No.
24
24
25
27
27
28
the Source 30
2.5 CONCLUSION TO CHAPTER 2 30
THE JUNO GOLD-BISMUTH MINE 32
3: LOCAL GEOLOGY 33
3.1 STRUCTURAL SETTING 33
3.2 STRATIGRAPHY 35
3.2.1 Feldspathic Greywacke-Shale Turbidite Unit 35
3.2.2 Hematite Shale sub Unit 38
3.2.3 Tuffaceous Greywacke-Shale Turbidite Unit 43
3.2.4 Laminated Magnetic Shales 45
3.3 SEDIMENTATION AND DIAGENESIS 45
4: MINERAL ZONATION 53
4.1 GANGUE MINERALS 53
4.1.1 Envelope of Chloritised Sediments 53
4.1. 2 Mineralised Sediment Zone 55
4.1. 3 Magnetite-Chlorite Zone 56
4.1.4 Talc-Magnetite Zone 56
4.1. 5 Dolomite Zone 57
4.2 ORE MINERALS 58
4.3
4.4
CHAPTER 5:
5.1
5.2
5.3
5.4
5.5
5.6
5 . 7
CHAPTER 6:
6.1
6.2
6.3
6.4
CHEMISTRY OF ZONATION
4.3.1 Rock Chemistry
4.3.2 Mineral Chemistry
ALTERATION ZONE BELOW MINERAL BODY
MINERAL TEXTURES
W\GNETITE
5.1.1 Magnetite in _the Magnetite-Chlorite
Page No.
61
61
68
74
80
80
Zone 80
5.1.2 Discussion on the Morphologies of Massive Magnetite 83
5.1.3 Magnetite in the Mineralised Sediment Zone 87
5.1.4 Magnetite in the Talc-Magnetite Zone 88
HEMATITE
DOLOMITE
JASPER
PYRITE
ORE MINERALS
5.6.1 Gold
5.6.2 Bismuth sulphosalts
5.6.3 Chalcopyrite
CONCLUSION TO CHAPTER 5
ISOTOPE GEOLOGY
INTRODUCTION
ANALYTICAL PROCEDURES
ISOTOPIC NOTATION
CARBON AND OXYGEN ISOTOPE STUDY OF CARBONATES
88
89
95
95
96
96
96
97
98
101
101
102
102
103
6.5
6 . 6
CHAPTER 7:
7.1
7.2
7.3
7.4
7.5
7.6
6.4.1 Results
6.4.2 Oxygen Isotope Composition of the Hydrothermal Solutions
6.4.3 Carbon Isotope Composition of the Hydrothermal Solutions
SULPHUR ISOTOPE STUDY ON SULPHIDES
6.5.1 Results
6.5.2 Discussion of Sulphur Isotope Fractionation in the Mineral Body
6.5.3 source of the sulphur
SUMMARY TO CHAPTER 6
THE SELENIFEROUS BISMUTH SULPHOSALTS
INTRODUCTION
PREVIOUS STUDIES OF Bi-Cu-Pb SULPHOSALTS
Page No.
103
106
109
110
llO
114
ll7
ll8
120
120
121
7.2.1 Pb-Bi Sulphosalts 121
7.2.2 Bi-Cu Sulphosalts 123
7.2.3 Bi-Pb-Cu sulphosalts 123
7.2.4 Selenium Bearing Bismuth Sulphosalts 125
ELECTRON MICROPROBE PROCEDURES
JUNITE
7.4.1 Physical Properties
7.4.2 Chemical Composition
7.4.3 X-ray Investigations
'WITTITE'
7.5.1 Physical Properties
7.5.2 Chemical Composition
7.5 .3 X-ray Investigations
SELENIFEROUS HEYROVSKYITE
7.6.1 Physical Properties
7.6.2 Chemical Composition
127
129
129
130
134
134
134
135
138
138
140
140
7.7
7.8
CHAPTER 8:
8.1
8.2
8.3
8.4
8.5
Page No.
7.6.3 X-ray Investigations 143
7.6.4 Significance of the Mineral Intergrowth 143
BISMUTHINITE-AIKINITE SERIES
7.7.1 Physical Properties
7.7.2 Chemical Composition
7.7.3 X-ray Investigations
SUMMARY TO CHAPTER 7
THE PROCESS OF MINERAL FORMATION AT JUNO
INTRODUCTION
METASOMATIC ZONATION
CONSTANCY OF VOLUME
GEOTHERMOMETRY
8.4.1 Isotope Results
8.4.2 Iron Content of Talc
8.4.3 Thermal Gradients
CHEMISTRY OF MINERAL DEPOSITION
8.5.1 Chemical Changes
8.5.2 Thermodynamic Considerations of Mineral Stabilities
8.5.3 A Chemical Hydrothermal Model ·for Gangue Zonation
8.5.4 pH Drive - The Cause of Mineral Zonation and Ore Formation
8.5.5 Vertical Hydrothermal Acid-Base Differentiation
144
144
148
148
148
150
150
152
156
160
160
162
162
163
164
166
177
182
184
8 . 6 TRANSPORT, DEPOSITION AND ZONATION OF ORE COMPONENTS 187
187
190
191
192
8.6.1 Gold
8.6.2 Copper
8.6.3 Bismuth, Lead and Selenium
8.6.4 Explanation of Ore Zonation
8.7 THE MECHANISM OF REPLACEM~NT
PART III: OTHER SELECTED ORE DEPOSITS IN THE DISTRICT
CH,t\PTER 9:
9.1
9.2
9.3
9.4
9.5
CHAPTER 10:
10.1
10.2
10.3
THE GECKO COPPER-BISMUTH MINE .
INTRODUCTION
LOCAL GEOLOGY
9.2.1 Stratigraphy
9.2.2 Chemistry of Sediments
9.2.3 Lode Types
THE ANOMALY 2 MINERAL BODY
9.3.1 Mineral Zones and Textures
9.3.2 Ore Metal Zonation
THE ANOMALY 3 MINERAL BODY
9.4.1 Introduction
9.4.2 Mineral Zones and Textures
MINERAL FORMATION AT GECKO
9.5.1 Introduction
9.5.2 Cause of the pH Drive At Gecko
COPPER, BISMUTH AND GOLD MINERALISATION AT WARREGO MINE
INTRODUCTION
LOCAL GEOLOGY
10.2.1 Stratigraphy
10.2.2 Structure
GANGUE MINERAL ZONATION AND TEXTURES
10.3.1 Massive Magnetite Lode .
10.3.2 Quartz-Magnetite Lode
10.3.3 Magnetite-Chlorite Lode
10. 3.4 Wall Rock Alteratien
Page No.
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201
201
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202
207
207
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214
214
214
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218
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221
222
222
222
223
224
225
226
227
227
227
10.4 ORE MINERAL ZONATION
10.5 MINERAL FORMATION AT WARREGO
CHAPTER 11: THE EXPLORER 46 GOLD-BISMUTH PROSPECT
ll.l INTRODUCTION
11.2 LOCAL GEOLOGY
11.3 MINERAL ZONATION AND TEXTURES
11.3 .l Magnetite-Pyrite Lode
ll. 3. 2 Dolomite Lode
ll. 3. 3 Metal Distribution
11.4 MINERAL FORMATION AT EXPLORER 46
PART IV: SUMMARY AND DISCUSSION
CHAPTER 12:
12.1
12.2
12.3
12.4
SUMMARY OF ORE FORMATION AT TENNANT CREEK
AGE OF MINERALISATION
SOURCE OF HYDROTHERMAL SOLUTION AND ORE COMPONENTS
CAUSES OF ORE LOCALISATION
METASOMATIC ZONATION AND PRECIPITATION OF ORE COMPONENTS
12.5 THE SIGNIFICANCE OF THIS INVESTIGATION IN TERMS OF FUTURE EXPLORATION
REFERENCES
APPENDIX A
APPENDIX B
APPENDIX C
Procedure used in calculation of the normative mineralogy
Additional Geochemical data on Carraman Formation greywackes and shales
Rock Samples lodged at University of New England and Geopeko Ltd., Tennant Creek
Page No.
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230
231
231
231
232
232
233
233
233
235
235
236
244
244
246
250
276
279
280
Table No.
1.1
1.2
2.la
2.lb
3.1
LIST OF TABLES
Main rock units in the Tennant Creek Area.
Stratigraphic section in the Whippet loc~lity.
Production figures for operating mines at Tennant Creek to June 1973.
Reserves for operating mines at Tennant Creek at June 1973.
Stratigraphy at Juno.
3.2 Analyses and norms of greywacke-shale sediments at Juno.
3.3 Analyses and norms of iron and magnesium-rich
Page No.
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4
16
17
36
37
rocks at Juno. 41
4.1 Analyses and norms of lode rocks and alteration zones along DDH 800/128. 62
4.2 Analyses along DDH 800/128 expressed in cation gram equivalents per 1000 cc of rock. 66
4.3 Semi-quantitative microprobe analyses of chlorites and tales from Juno. 70
4.4 X-ray powder data on Juno talc compared with data presented by Gruner (1934 and 1944). 73
4.5 Analyses of unaltered, leached and chloritised sediments below Juno orebody. 76
4.6 Trace analyses of tuffaceous greywacke-shales below the Juno orebody. 78
5.1 Microprobe analyses of dolomite phases within the banded dolomites. 91
6.1 Carbon and oxygen isotope data for Juno carbonates. 104
6.2 o18o composition of hydrothermal solutions ' in equilibrium with Juno envelope dolomites and bif calcites. 107
6.3 Sulphur isotope data on Juno sulphides. 111
7 . 1 Phases in the Pb-Bi-S system. 122
7. 2 Phases in the Pb-Bi-Cu-s system. 124
7 . 3 Previously reported seleniferous bismuth sulphosalts. 125
7.4 Microprobe analyses of junite. 131
7.5 X-ray powder .data on junite . 132
Table No .
7.6
7.7
7 . 8
7 . 9
7.10
7.11
7 . 12
7.13
8.1
8.2
8.3
X-ray powder data on junite compared with other bismuth sulphosalts.
Microprobe analyses of Juno 'Wittite'.
X-ray powder data on Juno 'Wittite'.
Microprobe analyses of bismuth sulphosalts in sample R27797 .
Chemical formulae of bismuth sulphosalts in sample R27796.
X-ray powder data on sample R27796.
Microprobe analyses of aikinite members from Juno.
X-ray powder data for members of the bismuthiniteaikinite series.
Difference in gram equivalents per 1000 cc between the lode rocks along DDH 800/128 and the 'average rock composition'.
Summary of chemical changes in the alter ation channel and l ode zones at Juno .
Important aqueous equilibria involved in the stability of silicate, oxide and carbonate minerals at Juno (after Helgeson, 1969).
8.4 Examples of equal volume replacement reactions which may have occurred during metasomatic zonation at
8 . 5
9 . 1
9 . 2
9. 3
9.4
Juno.
Equilibrium reactions involving sulphur.
Stratigraphic column at Gecko.
Analyses and norms of sedimentary rocks at Gecko .
Analyses and norms of lode types at Gecko, anomaly 2 .
Gains and losses of cations per 1000 cc involved in lode formati on at anomaly 2 .
Page No.
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136
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143
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165
167
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219
Figure No.
Ll
2 . 1
2.2
LIST OF FIGURES
Locality map of Tennant Creek Goldfield
Regional geology of Tennant Creek district.
Geological map of the Peko-Juno area.
Association of mineralisation and rock lithologies in the Peko-Juno area.
2.3 Relationship between porphyries and ironstones (quartz-jasper hematite bodies) in the Jubilee area.
3.1
3.2
3.3
4.1
4.2
4 . 3
4.4
4 . 5
4 . 6
4.7
4.8
4 . 9
4 . 10
4 . 11
4.12
Geological plan of 700 level, Juno Mine.
Drill intersections of the Hematite Shale in the western end of the 700 level workings .
Sedimentary rocks from Juno and Gecko plotted in an Al203-K20-(Mg0+Fe0) triangular diagram.
Geological section through the centre of number 2 orebody, 750 East, Juno Mine.
Contoured assay cross section 675 East.
Contoured assay cross section 700 East.
Contoured assay cross section 725 East.
Contoured assay cross section 750 East.
Diamond drill pattern on section 750 East.
Metal distribution within number 2 orebody.
Ore element relationships along diamond drill holes 800/128 and 700/90 .
Relationship between zoning and ore mineralogy at Juno .
Uranium distribution on a portion of section 700 East (compare with gold, bismuth and copper distribution in Fig. 14b) .
Position of diamond drill hole 800/128 projected onto a generalised section through number 2 orebody, J uno Mine.
. . . . 4+ 3+ 3+ 2+ 2+ 2+
Preceding Page No .
(vi) Pocket at back
of thesis 20
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53
58
58
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61
Varlatlon ln Sl , Al , Fe , Fe , Mg , Ca H+ and Fe3+; Fe2+ expressed in gram equi valents per 1000 cc, a long DDH 800/128 . 66
Figure No.
4.13
4.14
Juno chlorites and talc plotted in the classification diagram of Hey (1954).
(a) Variation in Fe+Mn/Fe+Mn+Mg ratio for chlorite and talc along DDH 800/128, determined by microprobe analyses and from bulk rock analyses.
(b) Variation in Fe+Mn/Fe+Mn+Mg ratio for chlorite along DDH 800/170, determined by X-ray diffraction
Preceding Page No.
70
procedures. 71
4.15
4.16
4.17
6.1
6.2
6.3
Variation in intensity of the basal reflections 004/006 for talc, from samples along DDH 700/165 passing from the inner edge to the outer edge of the talc-magnetite zone.
Alteration zone below the number 2 orebody, depicted by deep diamond drilling.
Frequency histograms of copper content of (a) greywacke-shale turbidites in the Juno-Peko region, (b) leached greywacke-shale turbidites in the alteration channel below Juno, and (c) chloritised greywacke-shale turbidites in the alteration channel below Juno.
Location of samples used in the carbon and oxygen isotope investigation plotted on a generalised section of 2 orebody.
A plot of o18o against o13c for the calcites and dolomites from Juno Mine.
o1Bo and o13c of dolomites plotted against vertical height in the envelope of the Juno mineral body.
6.4 Sulphur isotope results in the various zones at
6.5
7.1
7.2
7.3
Juno.
Variation of o34s in sulphides along diamond drill holes 800/128 and 700/165.
Known minerals in the Bi2 (s,se)3 - Pb(S,Se) -Cu2(S,Se) system together with the Juno microprobe analyses.
A portion of the system PbS-Bi2s3-Bi2se3 •
(a) Variation in microhardness (VHN) of junite. (b) Variation in microhardness (VHN) of 'wittite
A' and 'wittite B' and probable correlation with copper content.
73
74
78
103
104
105
112
114
125
126
131
Figure No.
8.1
8.2
8.3
8 . 4
8.5
8.6
8 . 7
9 . 1
9.4
9.5
10 . 1
10.2
l-0.3
10.4
11.1
Preceding Page No.
Equilibrium between muscovite and quartz in aqueous solution at (a) 250°C, [K] = 10-2M (b) 250°C, [K) = 1o-1M (c) 300°C, [K) = 10-2M. 169
Equilibrium curves bet:ween quartz and the alumino silicates -muscovite, K-feldspar, Mg-chlorite and Fe,Mg-chlorite, in a solution at 250°C with [K+] = 10-2M, [Mg2+) = 10-3M and [Fe2+] = 10-3M. 170
Ferric chloride complexes in equilibrium with magne-tite at 250°C and [Fe2+J = 10-JM, [Cl-] = 1o0 • 5M. 173
Stability fields of hematite, magnetite and amorphous ferric hydroxide at 250°C in a solution containing [Fe2+] = 10-3M and [cl-] = lo0 · 5M. 176
Equilibrium curves to represent the dissociation of talc and dolomite at 250°C in a solution containing [Ca2+J = 1o-2M, [~C03 ] = 10-2M and [H4Si04 J = lo-2.1M. 178
Stability fields of gangue minerals involved in ore formation at Juno .in a solution at 250°C containing the components listed on the following page. 178
Stabilit~ fields in the Fe-S-0 system at 250°C with ~s = 10- M and Fe2+ = 1o-3M. 182
Magnetic contour pattern in the Gecko area showing the position of anomaly centres with respect to the projected subsurface geology. 202
Geological plan of 2 level, Gecko Mine, anomaly 2. 209
Geological cross section of Gecko Mine, anomaly 2, 9180 East . 209
Contoured assay cross section, Gecko Mine, anomaly 2,· 9180 East. 214
Generalised geological cross section, Gecko, anomaly 3 . 215
Geological cross section, Warrego Mine, 8240 North. 224
Contoured assay cross section, Warrego Mine, 8240 North . 229
Contoured assay cross section, Warrego Mine, 8180 North. 229
Sketch relating the structure and metal zonation to the path of the mineralising solutions at Warrego Mine . 229
Generalised geological cross section, Explorer 46 prospect . 233
Plate No.
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12
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LIST OF PLATES
Preceding Page No.
Bernborough and Whippet Formation Lithologies. 5
Sedimentary rocks typical of Carraman Formation. 7
Porphyritic rocks in the Warramunga Geosyncline. 10
Intrusive, rhyolite porphyries south of Bernborough Workings. 12
Banded iron formations at Juno. 39
Other sediments at Juno. 42
Opaque minerals in the tuffaceous greywacke-shale. 44
Sedimentary features indicative of tectonic dewatering. 50
Progressive chloritisation of sedimentary rocks adjacent to lode at Juno (crossed nicols). 54
Crack patterns developed in the massive magnetite - Juno Mine.
Grain boundary textures o! massive magnetite (etched with HI for 20 seconds) - Juno Mine.
Micro-colloform textures in the magnetitechlorite zone - Juno Mine.
Macro-colloform textures in the magnetite-chlorite
82
82
82
zone - Juno Mine. 8 2
Spherulitic magnetite in sample R27893 (etched with HI for 35 seconds) - Juno Mine. 82
Forms of platy magnetite - Juno Mine. 82
Zoned magnetite - Juno Mine. 82
Textures in the mineralised sediment zone -Juno Mine. 87
Textures in the talc-magnetite zone - Juno Mine. 88
Textural features of banded dolomites - Juno Mine.
Textural features of jasper in the dolomite zone - Juno Mine.
Textural associations of gold in the massive
89
95
magnetite zone - Juno Mine. 96
Textural associations of the bismuth sulphosalts in the massive magnetite zone - Juno Mine. 96
Pla.te No.
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24
25
26
27
28
29
30
31
32
33
34
35
36
37
Bismuth sulphosalts from Juno Mine.
Exsolution-type structures in the bismuthiniteaikinite minerals from sample R27799 - Juno Mine .
Sedimentary host rocks at Gecko.
Replacement textures in the breccia-conglomerate - Gecko, anomaly 2.
Textural features of the hematite lode rocks -Gecko, anomaly 2 .
Structures in the quartz-hematite lode rock -Gecko, anomaly 2 .
Textural features of the magnetite lode (I) -Gecko, anomaly 2.
Textural features of the magnetite lode (II) -Gecko, anomaly 2.
Textural features of magnetite lode (III) -Gecko, anomaly 3 .
Sulphides at Gecko, anomaly 2.
Mineral textures in the Gecko, anomaly 3 lode .
Colloform textures in the magnetite lode -Gecko, anomaly 3.
Textural features of the massive magnetite lode at Warrego.
Textures of pyrite and marcasite in the magnetite lode at Warrego.
Textures in the Explorer 46 lode rocks.
Preceding Page No.
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(i)
ABSTRACT
Gold, bismuth and copper mineralisation at Tennant Creek occurs
in transgressive magnetite- and hematite-rich lodes within the Carraman
Formation of the Lower Proterozoic Warramunga Group.
Rocks of the Warramunga Group are dominantly felsic greywackes
and shales with features indicative of turbidity current deposition. These
are interbedded with massive pyroclastic rocks, rhyolitic lavas, preconsoli
dation slump breccias and minor lenses of banded iron formation.
The magnetite-hematite lodes (locally referred to as ironstones)
have an ellipsoidal to pipe-like shape commonly flattened in the direction
of the regional east-west cleavage. They are typically localised in
small anticlinal structures within the greywacke-shale turbidites adjacent
to thin lenses of hematite-chlorite-calcite bearing banded iron formations
or hematite-rich shales. A smaller number of mineralised ironstones have
replaced the preconsolidation slump breccia horizons within the felsic
sediment pile.
Silicate, oxide and carbonate gangue minerals within the lode
structures are grouped into a series of compositionally distinct zones
which exhibit sharp contacts against one another and the enclosing
country rocks , Massive magnetite (>80%) and Fe-Mg chlorite (<20%)
commonly constitute the core of the mineralised lode and are surrounded
by var i ous umbrella-shaped zones. These may be: talc-magnet.ite; dolomite;
chloritised sediments, as at J·uno Mine, or quartz-hematite; hematite
magnetite; hematite-chlorite; chloritised sediments, as at Gecko Mine.
The magnesium content of the chlorites (dominantly ripidolites) increases
from the base, to the top, of the lode structures.
(ii)
The chemical and mineralogical characteristics of these zones
indicate contemporaneous formation and growth, resulting from the flow of
hydrothermal solutions which reacted with the host rocks and suffered
continual, and systematic, changes in chemistry. A zone of intense
chloritisation extends below each of the orebodies and constitutes what
is thought to be a channel of hydrothermal alteration.
This investigation deals with the structure, mineralogical
constitution, mineral zoning, textures, and origin of the lode rocks in
the three largest operating mines in the goldfield, namely the Juno, Gecko
and Warrego deposits.
At the Juno and Warrego mines, gold, bismuth and copper
mineralisation has been shown to occur in three overlapping zones within
the magnetite-rich lodes e Gold is concentrated at depth, and is overlain
above by an umbrella shaped zone rich in bismuth sulphosalts. Chalco
pyrite is concentrated at the top of the lode structures enveloping the
bismuth zone. At Gecko (anomaly 2), bismuth and copper show a similar
vertical zonation but gold is completely lacking. Within the bismuth zone
at Juno, the sulphur/selenium and bismuth/ lead ratios of the bismuth
sulphosalts increase from its inner edge (overlapping the gold zone) to
its outer edge (overlapping the copper zone).
The most common bismuth sulphosalt at Juno is junite, a new
mineral, unique to Juno, which has been shown by microprobe analysis to
have the formula Bi8PbJCu2 (S,Se) 16 , containing 3.8 to 11.6 wt.% selenium.
Junite is easily distinguished from other lead-bismuth sulphosalts by its
characteristic x-ray powder pattern. The second most abundant bismuth
sulphosalt has a composition close to Bi10Pbg(S,Se) 23 and may be equivalent
to the mineral wittite , previously reported from Falun, Sweden by
Johansson in 1924 . Other selenium bearing sulphosalts at Juno include
heyrovskyite and members of the aikinite-bismuthinite series.
Colloform textures occur throughout all the lode struct ures in
(iii)
the goldfield, strongly indicating that replacement of the sediment host
rocks to form the ironstone bodies was achieved by the processes of
dispersive metasomatism. Replacement of this type involves the gradual
hydrolysis 2+ ' h Mg 1.n t e
2+ of the host sediments and permits ready exchange of Fe and
hydrothermal solutions with si4+, Al3+, Na+ and K+ in the
hydrolysed sediment matrix. Most of the magnetite in the central lode
zones replaced needle-shaped a - FeO(OH) and S - Fe203.H20 crystal forms
which probably developed from ageing of ferric hydroxide gels.
Thermodynamic considerations of gangue mineral stabilities in
hydrothermal solutions of the type which may have caused mineralisation
at Tennant Creek, suggests that the solutions were initially acidic in
1 • I ' I 2+ 2+ ( 1 nature and capable of each1.ng the base catl.ons Fe and Mg p us ore
metals) from the Carraman sediments at depth. There is evidence of leach
ing of this type in the lowest parts of the Juno hydrothermal channel.
The process of metasomatic lode formation was most probably initiated by
interaction of rising, chloride-rich, hydrothermal solutions with the
calcite bearing banded_ iron formation or with particularly porous horizons
containing abundant pore fluids (e.g., preconsolidation slump breccias)
leading to an increase in solution pH and f0 2 which caused the deposition
of amorphous, hydrated, ferric oxides.
Under the influence of increasing solution p~the gangue minerals
at Juno, were deposited in the order:- iron-rich chlorite at depth,
followed by hematite, magnetite, talc and dolomite, to form a well zoned
lode structur e . The progressive drop in f02 associated with the deposition
of magnetite probably resulted in the increase in sulphur/selenium and
sulphur/metal ratios of sulphides passing up the Juno lode structure, and
may have also lead to the zonal distribution of gold, bismuth and copper .
Sulphur isotope studies at Juno lend support to this proposal.
The 16o;18o and
12c;13c ratios in dolomites from the outer
envelope zone at Junm-eempa:t;.jJ;~.l.e-wd:.·t-h-a-mcrgma or conna· e orlg-:i: · 32 34 .
-~~-:the.-hyd-r;:et.herma · rl1i1'4d&o The S/ . S ratl.o of syngenetic sulphides in
(iv)
the tuffaceous greywackes and vein sulphides in the hydrothermal channel,
below the orebody, supports a proposal that the sediments of the Lower
Carraman Formation provided the source for the sulphur. These sediments
also contain sufficient trace quantities of gold, bismuth and copper to
constitute a source for the ore components.
Connate wate.rs (pore water and interlayer water) released from
the argillaceous sediments in the vicinity of granitic and rhyolitic
porphyry intrusions provides the most probable source for the hydrothermal
solutions. Such solutions moved upwards continually leaching iron,
magnesium and ore elements from the sediments in their path and were
eventually channelled into low pressure anticlinal sites and deposited
their metal load in favourable structural-lithological traps.
