and factoranalysis usingthe Statview programme. Values of majorand minorelements and 0 and C isotopes are listedin. Table 1. Elemental values corrected to ...
GEOCHEMISTRY OF GOLD·BEARING CARBONATES, BEACONSFIELD GOLD MINE, TASMANIA, AUSTRALIA IC.PrasadaRao and zMohammad H. Adabi
'School of Earth Sciences, University of Tasmania, GPO Box 252-79, Hobart, Tasmania, Australia 7001 2School of Earth Sciences, Shahid Beheshti University, Tehran, Iran ABSTRACT: The major carbonates minerals in the Beaconsfield gold mine are calcite, dolomite and magnesian ankerite. Gold occurs in magnesian ankerite. Ankerite has high Ca, moderate Fe, Mg and Mn and low Na and Sr. The values of Ca and Mg and total amounts of Mg, Fe and Mn, and values of Fe, Mn and Sr are strongly correlated. This is due to the two layered structure ofmagnesian ankerite comprised of Mg, Fe and Mn carbonites and Ca carbonites. The elemental and isotopic compositions of magnesian ankerite are related to ordering, substitution of elements, salinity, redox potential, dissolution and reprecipitation, temperature, composition of fluids, environmental setting and gold mineralization. Ca, Sr and Na are derived from dissolution of Ordovician limestone. Mg is derived from leaching of ultramafics. Fe and Mn are leached from both ultramafics and clay minerals. Meteoric water was high in Ca, Fe, Mg and Mn concentrations and formed magnesian ankerite in a reducing burial environment. The heat source was possibly deep burial depth. tectonic hot spots and a few granite intrusions east of Beaconsfield. Gold and ankerite were precipitated in alkaline conditions. Gold and magnesium in ankerite are derived from the leaching of Cambrian ultramafic rocks during the Devonian by the passage of meteoric fluids through tectonically affected Ordovician carbonates.
INTRODUCTION Gold is associated with quartz, carbonites and pyrite at the Beaconsfield gold mine (Hills 1998). Recent studies have documented that gold is mainly in carbonites as gold precipitates with carbonites in alkaline reducing environments (Rao and Adabi 1998). The elements Ca, Mg,Fe, Mo, Sr andNa in thecarbonites fraction areessential to understand carbonites mineralogy, composition of fluids, salinity, redox potential, environmental setting, substitution of elements, temperatures and gold mineralization (Rao 1996). Comparisonofelemental composition withoxygen andcarbonisotopic composition enables a better understanding of carbonites geochemical processes (Veizer 1983; Rao 1991). In this paper,majorandminorelements ofgoldbearingcarbonates at the Beaconsfield gold mineare presented and compared with oxygen and carbon isotopic compositions in order to understand the origin of the carbonates and the geochemical controls on the gold mineralization at the Beaconsfield area, to ensureoptimum recovery in future goldproduction.
Baillie 1992), is overlain by the fault bounded Andersons Creek Mafic-Ultramatic Complex and marine siliciclastic sediments (Dally's siltstone) of Cambrian age (Hills 1998). The Cambrian sequence is followed unconfonnably by the Ordovician shallow marine siliciclastic and limestone sequences, consisting of Blyth's Creek Formation, Cabbage Tree Formation and Flowery GullyFormation (Fig. 1). The Cabbage Tree Formation, consisting of Lower and Upper Transition beds, and to someextentthe lowerFloweryGully Limestone (a correlative of the Gordon Limestone) host the
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GEOLOGICAL SETTING The Beaconsfield gold mineis located in northern Tasmania, Australia (Fig. 1). Gold mineralization occurs within the quartz-carbonites Tasmania Reef, and is usually associated with sulfides (McClenaghan 1994; Hills 1998). The Tasmanian Reef is a fissure reef, with an overall length of about 395 m, striking about N50° E witha dip of 50-600 SE (Russell 1995; Hills 1998). The reef width varies from 2 em to 8 m, averaging 2.5 m (McClenaghan 1994; Russell 1995). The Beaconsfield gold deposits occur within the Lower to MiddlePaleozoic sequenceof the Beaconsfield Block(Elliott et al. 1993), boundedby the Precambrian BadgerHeadBlock to the west and the River Tamar to the east (Fig.l). The Badger Head Block, consisting of quartzite (powell and Carbonates and Evaporites, v, 15, no. 1,2000, p. 7-17.
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62.8 71.2 7fl.24 92.16 5704 70.64 46.72
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Depth
IJDH
Sl.1',() 96695 84195 95775 123215 68215 99275 39115 2503 74195 78135 6127 6195 22(,5 12(,.\15 2185 123915 15.1080 71875 61795 82175 21475 31395 54115 1267.1 I.1WI
2JW 6591 2567 3577 3543 8.1735 59115 1:1.\.1.1 30195 60.195 95735 3U195 121069 12.1149 19217 1.10509 53609 14812 130249 92029
IlLV. Srppn 1 REV, Nnppu REV. Feppu 1 REV. Mnppn OI80%"I'BJ ()IJC%nI'BD 49 16 56 54 44 46 46 45 30 I
47 28 6 lJ.16
2729 .16 146 51 63 73 41 74 1l!4 911 85 82
152106 359241 153973 1182.51 122412 133697 119591 140536 83722 4291 118902 91364 9397 78.18 7259 130275 2199 125420 153325 191564 148260 I37S08 58867 6.1914 75159 43163
2117 156 158 58 147 121 128 136 197 172 218 114 55 71 154 131 20 151 211 306 242 285 248 303 245 323
9
14943 27596 16220 10542 12007 13420 11192 13610 7834 341 100TI 8842 900
1465 7993 12673 210 11912 14451 21139 13693 9433 3004 2400 3719 2035
-1.1.4.1 -8.50 -13.52 ·14.25 -13.64 -13.54 -14.14 ·13.48 ·13.66 -14.30 -14.19 -13.68 ·13.82 -17.69 -17.70 -13.21 ·7.79 ·14.2J -14.65 -13.96 -13.90 ·12.63 -14.60 -15.43 ·15.72 -15.00
·5.14 ·0.90 ·5.00 -4.50 -4.96 -5.29 -4.60 ·4.74 -4.87 -4.35 ·4.58 -4.9.1 ·5.2.1 -.1.67
.1.90 -5.14 -0.4.1 -4.74 ·3.98 -4.79 -4.60 .1.93 -1.42 ·1.29 ·1.68 -1.56
GEOCHEMISTRY OF GOLD-BEARING CARBONATES, BEACONSFIELD GOLD MINE, TASMANIA, AUSTRALIA Table 1.· Valuesof oxygen and carbon isotopes and majorand minorelements in totalsample and in the carbonites fraction (prefix REV) and depth, carbonites% and insoluble residue%. (continued) MIl pplll
RFY,('a'7r· Il"V.Mg"" REY.Srppm Rfiv.Nappm REV.Fcppm REV. Mnppm bl80%ol'lll hIJC%l.I'BIJ
667 156.1 1O'J.1 2877 21.1 79
IU.5 .188 .19.1 '11.7 s.q
nM
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1
6
+
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2
.8
RevMn%
RevFe% 8
+
~5
+
ec ~ 4
>
++
6
10 12 8 6 Rev Fe% + Rev Mn%
14
16
18
0 0
Y = .082x + 159, r 2 = .592
+ 10
20
50 30 40 REV.5rppm
60
70
80
Figure 7. Variation between values ofMg versusFe (7A),Mn(7B), total ofFeandMn (7C) andSr(7D) inthecarbonitesfraction (prefix REV)of ankerite from theBeaconsfield goldmine. See textfor details.
12
RAO AND ADABI thanthoseof Mg (Fig. 7) as Mg valuesare dependent on the amounts of Fe and Mn incorporated in the magnesian ankerite layer.
(0.4% to 17.0%) are strongly correlated with Ca values (Fig. 60). Mostsample pointsfallabovethepureankerite line(Fig. 60) because of thelowerconcentrations ofFe and Mn thanin pure ankerite and the occurrence of appreciable Mg concentrations. Thus, the Beaconsfield carbonates are magnesian ankerite. The Sr values in magnesian ankerite increase withincreasing Ca values (Fig.6E). Pure magnesian ankerite hasabout60 ppm Sr (Fig. 6E),similar to theordered stoichiometric dolomite which also has 60 ppm Sr (Vahrenkamp and Swart 1990). The Na values are unrelated to Ca values (Fig. 6F).
Total Mg, Fe, and Mn Values vs. Other Elements The total concentrations of Mg, Fe and Mn increase with increasing amounts of Ca(Fig.3),Mg (Fig.8A),Fe (Fig.8B), Mn (Fig. 8C),totalconcentrations of Fe and Mn (Fig.80) and Sr values (Fig. 8E),and are unrelated to Na values (Fig. 8F). The correlation coefficients (Table 2) and regression values (Fig. 8AtoE) between elements andtotalvalues ofMg,Feand Mn arebetterrelatedandleastscattered thanthoseof Ca (Fig. 6 A to F) and Mg (Fig.7A to 0). This is due to the ordered structure of magnesian ankerite which consists Mg, Fe and Mn carbonites layer and a Ca carbonites layer in the Beaconsfield samples.
Mg vs, Other Elements The Mg values are stronglycorrelated withCa (0.97, Table2; Fig. 6A). The Mg values, like Ca, increase with increasing concentrations of Fe (Fig. 7B), Mn (Fig. 7B), the total amounts of Fe and Mn (Fig.7C) andSr (Fig. Fig. 70), andare unrelated to Na values. The regression values between elements and Ca (Fig. 6) are betterrelated and less scattered 8r.-~~~~--~~-:--~--,.
16
7
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2
2.5 5 7.5 10 12.5 15 17.5 20 22.5 25 Rev Mg% + Rev Fe% + Rev Mn%
c
y ; .658x- .609, r 2 = .957
+
o ++ o 2.5
5 7.5 10 12.5 15 17.5 20 22.5 25 Rev Mg% + Rev Fe% + Rev Mn%
C
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~ 12
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~~
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y; .059x + .073, r 2 = .736
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y = .717x- .536. r 2 = .965
o ++ o
2.5 5 7.5 10 12.5 15 17.5 20 22.5 25 Rev Mg% + Rev Fe% + Rev Mn% +
6
1200
+
2.5 5 7.5 10 12.5 15 17.5 20 22.5 25 Rev Mg% + Rev Fe% + Rev Mn%
F +
1000 60
[50 ~ 40
> ~
§.
800
Z" ;:;
600
c,
30
~
+ 400
+
20
+
200 +
y = 2.265x+ 6.464,r 2 = .587
+
2.5 5 7.5 10 12.5 15 17.5 20 22.5 25 Rev Mg% + Rev Fe% + Rev Mn%
Figure 8. Variation between values of total Mg. Fe and Mnversus Mg (Fig. BA). Fe (Fig. BB), Mn (Fig. 8e), total of Fe and Mn (Fig. 8D),Sr(Fig. 8E)andNa (Fig. 8F)inthe carbonitesfraction (prefix REV) ofankeritefromtheBeaconsfield goldmine.
See textfor details. 13
GEOCHEMISTRY OF GOLD-BEARING CARBONAlES, BEACONSFIELD GOLD MINE, TASMANIA, AUSTRALIA
A
80
+
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+
+ +
60
+
5. 50
50
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~ 30
52
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+
10
= .502
2
4
6 8 10 RevFe%
C
+
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14
o
16
=.482
.4
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.8 1 RevMn%
1.2
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....
D
70
+
60
60
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~ 30
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+
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+
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80
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2
4
6 8 10 12 Rev Fe% + Rev Mn%
14
16
18
0
200
400 600 800 REV.Nappm
1000
1200
Figure 9. Variation ofvalues ofSr versus Fe (Fig. 9A),Mn(Fig. 9B),total ofFe andMn andSr (Fig. 9C) andNa (Fig. 9D)in the carbonites fraction (prefix REV) ofankerite from theBeaconsfield goldmine. See textfor details.
7), and total values of Mg, Fe and Mn (Fig. 8) and other elements. Since the amountof Ca in the magnesian ankerite is themaximum expected valueof about25%or a proportions thereof relativeto Mg content, the loss of Ca is minorin the samples studied. Thus, Ca is not appreciably substituted by otherelements. Instead, lowerconcentrations of Fe and Mn (up to 18%; Fig. 6D) in the samples studied than in pure ankerite (25%) indicate that Fe and Mn in the magnesian ankerite are substituted mainly by Mg. Thus,high Mg values occurin the Beaconsfield ankerite.
DISCUSSION The elemental and isotopic composmons in magnesian ankerite in the Beaconsfield gold mine are related to: 1) ordering of ankerite, 2) substitution of elements, 3) salinity, 4) redox potential, 5) dissolution and reprecipitation, 6) temperatures , 7) composition of fluids, 8) environmental setting and 9) gold mineralization.
Ordering of Ankerite The strong correlation between Ca and Mg (0.97), total amounts of Mg, Fe and Mn (0.96), Fe, Mn and Sr values (fable 2) indicate good ordering of magnesian ankerite. In dolomite, Sr values decrease with increasing ordering (Vahrenkamp and Swart1990)andwell-ordered dolomite has about60ppmSr(Fig.6E). A Srvalueofabout60ppmin pure magnesian ankerite and a similarproportion between Sr and Ca values in the Beaconsfield gold mine confirm that the structure of magnesian ankerite is well ordered. Increasing values of Sr with increasing concentrations of Fe (Fig. 9A), Mn (Fig. 9B) and total amounts of Fe and Mn (9C) also document that the ankeritestructure is well orderedand that Sr,Fe andMnconcentrations areequilibrium values. Na (Fig. 9D)is unrelated to Sr becauseNa values depictlowsalinity of fluid.
16 14 12
~1O Q.I
J.1.
8
Q.I
0::;
6 4
+
2
y == 7.989x + 1.842, r 2 == .665 + .2
.4
.6
.8 1 RevMn%
1.2
1.4
1.6
1.8
Substitution of Elements Substitution of elements leads to loss and gain of elements. Positivecorrelations existsbetween Ca(Fig.3 and6),Mg(Fig.
Figure 10.Variation ofvalues ofFe andMn in the carbonites fraction (prefix REV) in the ankerite from the Beaconsfield goldmine. 14
RAO AND ADABI Salinity Na values increase with increasing salinity (Landand Hoops 1973; Veizer1983; Rao 1996). Thus,Na contentis higherin carbonates formed in marine and hypersaline solutions relativeto non-marine ones. The low Na valuesand lack of correlation between Na values and marine elements such as Ca, Mg and Sr (fable 2) in the Beaconsfield carbonites indicate that the solutions were non-marine. Since burial connatefluids are saline and have high Na values, meteoric water reacted with the Beaconsfield carbonates in a burial setting. Redox Potential The Fe and Mn concentrations in carbonates are sensitive to redox potential and high values of these elements enter carbonates in a reducing environment (Lohmann 1988; Veizer 1983; Rao 1990). Reducing environments occur in shallow to deep burial environments. Shallow burial environments occur during early diagenesis, whereas deep burial is common during late diagenesis. Magnesian ankerite fromBeaconsfield wasformedduring theDevonian in buried and fractured Ordovician carbonates. Meteoric water has
1200
A
00til
600
> ~
~
+
400
+
+ +
200 -,
1200
+.
++
+ +
'+f'
+
0 -16
-15.5
-15
B
y
-14.5 -14 d180
Composition of Fluid
+ + ,,~+ + + +++
-13.5
-13
The elemental compositions of magnesian ankerite and their linearrelationships between different elements (Figs. 6, 7 and 8) indicate that the fluid had high Ca (25%), moderate Fe (15%), Mg (7.5%) and Mn (1.7%) andlow Na(200 ppm)and Sr (75ppm)concentrations. The Ca values are themaximum values expected inankerite dueto an abundance of Ca in fluid from the dissolution of Ordovician limestones that host the goldreef. SinceFe, Mgand Mn concentrations are lowin the Tasmanian Ordovician host carbonites (Rao 1990), these elements are derived from leaching and dissolution of noncarbonites rocks in the area by hotmeteoric fluids. Ultramafic rocks common in thesubsurface sequence in theBeaconsfield area contain largeamounts ofMg (MgOrangesfrom 27.4%to 38.4%; mean33.9%), moderate Fe (peO + Fep3 rangesfrom 2.3% to 11 %; mean 6.8%) and low Mn (Mn0 ranges from 0.05% to 0.18%; mean0.1%) and traceamounts ofNa (Gee and Legge 1979). Thus, Mg is mainly derived from the leaching of ultramafics, whereas Fe and Mn originated from both ultramafics and host siliciclastic sediments that contain clays. TheSrconcentrations arederivedfromhostOrdovician
-12.5
= 172.023x + 943.719, r 2 = .585 +
1000
E 800
00 ;.,IJ ~
600 +
400 +
+ ++
+ +++ + +:Ift.+ + o + -5.5 -5 -4.5 -4 -3.5 -3 d13C
200
Thevariation of 5180 values fromabout-12.8to -15.5%oPOB (Fig. l1A) reveal that the Beaconsfield carbonates reacted with hot meteoric water and deviated appreciably from Ordovician marine(-5%oPOB) and meteoric (-7%oPOB; Rao 1990) calcitevalues. The variation of BI3C values fromabout -1.3to -5.3%oPOB (Fig. lIB) isduetoappreciable dissolution of Ordovician carbonites that originally had BI 3C values of about 2%0 POB (Roo 1990). Na values decrease with increasingly negative BI 3C values because the meteoric water thatreactedwith the carbonites had low Na and BI 3C values. The dissolution of Ordovician carbonites releasedCa. Sr and Na into the fluid. This fluidmixedwithanotherfluidhighin Fe, Mn and Mg, by leaching of ultramafics and siliciclastic sediments, and formed magnesian ankerite.
The average BI80 valueof -15%0 POBof ankerite (Fig. lIA), substituted in the calcite BI80 temperature equation, considering a dolomite to calcitefraction of 3 and BISO water value of -50/00 SMOW of Ordovician meteoric water (Rao 1990), indicates that the fluid temperature was about lWOC during ankerite formation and gold mineralization.
E 800 Z
Dissolution and Reprecipitation
Fluid Temperatures +
1000
higherconcentrations of Fe and Mn and lowerconcentrations of Nathanseawater. Thus,largeconcentrations of Feand Mn (Fig. 10) entered the carbonites lattice in a reducing burial environment by thepassage of meteoric fluids. Highvalues of Fe and Mn and increasing concentrations of these elements withincreasing Ca. Mg and totalamounts of Mg,Fe and Mn values in the Beaconsfield samples indicate that magnesian ankerite formed by meteoric fluids in a burial environment.
+
-2.5
-2 -1.5
-1
Figure 11. Variation cfvalues ofNa versus oxygen (Fig.11A) and carbon (11B) isotopes. 15
GEOCHEMISTRY OF GOLD-BEARING CARBONATES, BEACONSFIELD GOLDMINE, TASMANIA, AUSTRALIA limestones (250 ppm to 1,500ppm; average about 500 ppm;
exploration. The majorfeatures of ankeriteare:
Rao 1990). SinceNa contentis lowin the ultramafic rocks at 1) Ankerite has highCa, moderate Fe, Mg and Mn andlowNa andSr content Sincetheamountof Mg is higherthanin pure ankerite, the ankerite in the Beaconsfield gold mine is a magnesian ankerite.
Beaconsfield, Na values are derived from host Ordovician limestones (50 ppm to 300 ppm; mean 120 ppm; Rao 1990) and clays. Environmental Setting
The Beaconsfield area is affected by faulting through successive compressional and extensional regimes (Hills 1998). The Ordovician rocks and the underlying Cambrian sediments and ultramafics were emplaced by west directed thrusting against the Badger Head Block (Fig. 1). The Au mineralization coincided with compressional deformation related to the Tabberabberan Orogeny (Russell 1995; Hills 1998). Meteoric water passed through the tectonically affectedrocks at Beaconsfield and incorporated high Fe, Mg and Mn values in a reducing burial environment to form magnesian ankerite. The heat sourcewas possiblyfrom deep burial, tectonic hot spots and granite intrusions. It is postulated by some researchers that the gold bearing fluids were originated either from the leaching of ultramafics (Hills 1998) or fromgraniticbatholiths, locatedabout50 km east of the Beaconsfield area, by passing through ultramafics and associated rocks along Devonian faults and fractures (eg., Bottrillet al. 1992;Keele et al. 1994). Major, trace and rare earth elements datarelatedto granitic andultramafic rocksare needed to resolve the source of gold in the fluid in the Beaconsfield area. Oxygen and carbon isotope studies of Beaconsfield carbonates (Rao and Adabi 1998) indicate magmatic fluids did notreactwiththecarbonates. Instead. hot meteoric fluids reacted with carbonates and ultramafics. Gold Mineralization
In the Beaconsfield gold mine-goldis associated with quartz, carbonites and pyrite. Quartz and carbonites are abundant, whereas pyrite is minor. Quartz is crystalline (megaquartz) and was formed by silicification. Extensive silicification is due to acidic meteoric fluids that caused dissolution and replacement of carbonites. Sincelargevolumes of carbonites dissolve during silicification, acidic fluids become alkaline and precipitate ankerite. Fe and Mn enter the carbonites latticeina reducingenvironment Therefore, the formation of abundant magnesian ankerite is due to reducing alkaline conditions in a burial environment. In alkaline conditions, gold is precipitated (Seward 1973; Foster 1993; Gray 1997) along with ankerite. Thus, important factors in gold mineralization are favorable carbonites host rocks, the presence of alkaline and reducing conditions and the formation of magnesian ankerite. CONCLUSIONS Gold occurs in the Beaconsfield gold mine in magnesian ankerite. Thus, a knowledge of the formation of ankerite is important in understanding the origin of gold and for 16
2) The valuesof Ca and Mg and total amountsof Mg and Fe, Mn and Sr are strongly correlated with each other due to the two layeredstructure of magnesian ankerite.
3) The elemental and isotopic compositions of magnesian ankerite are related to ordering, substitution of elements, salinity, redox potential, dissolution and reprecipitation, temperature, composition of fluids, environmental settingand gold mineralization. 4) Elements in magnesian ankerite are derivedfromdifferent sources by percolating hot meteoric water. Ca, Sr and Na
originated from the dissolution of Ordovician limestones, whereas Mg is derived by theleachingof ultramafics. Fe and Mn were probably leached from both ultramafics and clay minerals. 5) Magnesian ankerite precipitated with gold in alkaline conditions. Since magnesium in the ankerite and possibly gold were probably derived from the leachingof Cambrian ultramafics, recognition of magnesian ankerite and its geochemical characteristics are important in understanding gold mineralization.
ACKNOWLEDGMENTS We are grateful to the University of Tasmania for financial support and Beaconsfield Mine Joint Venture for allowing access to relevant information and giving permission to publishthispaper. Wehavebenefited greatlyfromdiscussion with Peter Hills (ChiefGeologist) about different aspectsof the deposits. The authors are thankful to Philip Robinson, Peter Traill and Zohreh Amini for technical assistance in the operation of the AAS andpreparation of samples and to Keith Harrisand ChristineCookefor isotopeanalysis. REFERENCES BOITRILL, R.S., HUSTON. D.L., TAHERI. J., and ZAW, K., 1992. Goldin Tasmania: Bulletinofthe Geological Survey of TasnuvUa,v. 70,p. 24-47. ELUOIT, C.G., WOODWARD, N.B., and GRAY. D.R., 1993, Complex regional fault history of the Badger Head region, northern Tasmania, Australia: Australian Journal of Earth Sciences. v, 40, p. 155-168. FOSTER, R.P.• 1993, GoldMetallogeny and Exploration. Chapman and Hall, London, 432 p. GEE. R.D. and LEGGE, P.J., 1979, Geological Atlas 1 MileSeries. Sheet 30 (8215). Beaconsfield: Explanatory Report, Department of Mines, Tasmania, 127p. GRAY, D.J.• 1997, Geochemistry of Gold: Master of Economic Geology Course work Manual 11. CODES, University of
RAO AND ADABI Tasmania, Australia, p. 149-154. IllLLS, P.B., 1982, The geology of the Lower and Middle Paleozoic rocks of Flowery Gully, Northern Tasmania, unpublished B.S. Honors thesis, University of Tasmania, Hobart. IllLLS, P.B., 1998, Tasmania gold deposit, Beaconsfield. In Berkman, D.A. and Mackenzie, D.H., eds., Geology of Australian and Papua New Guinean Mineral Deposits, p. 467-472. KEELE, R.A., TAYLOR, B., and DAVIDSON, G.1., 1994, Relationships between Devonian thrusting and gold mineralization in northern Tasmania Contentious Issues in Tasmanian Geology: a Symposium: Geological Society of Australia,Tasmanian Division, p. 59-60. LAND, L.S. and HOOPS, G.K., 1973, Sodium in carbonate sediments and rocks: a possible index to the salinity of diagenetic solutions: JournalofSedimeniary Petrology, v. 43, p.614-617. LOHMANN, KC., 1988, Geochemical patterns of meteoric diagenetic systems and their application to studies of paleokarst. In James, N.P. and Choquette, P.W., eds., Paleokarst. Springer, New York, p. 58-80. MCCLENAGHAN, M.P.; 1994, A summary of the Beaconsfield, Lefroy, Back Creek and Gladstone goldfields: Mineral ResourceofTasmania, p. 1-39. POWELL, C.MCA. andBAlLIlE, P.W., 1992, Tectonic affinityof the Mathinna Group in the Lachlan Fold Belt: Tectonophysics, v, 214, p. 193-209. RAO, C.P., 1989, Geochemistry of the Gordon Limestone (Ordovician), Mole Creek, Tasmania: AustralianJournal of EarthSciences, v, 36, p. 65-71. RAO, C.P., 1990, Marine to mixing zone dolomitization in peritidal carbonates: The Gordon Group (Ordovician), Mole Creek, Tasmania, Australia: Carbonates andEvaporites, v, 5, p. 153178. RAO, C.P., 1991, Geochemical differences between tropical (Ordovician), temperate (Recent and Pleistocene) and subpolar (Permian) carbonates, Tasmania, Australia: Carbonates and Evaporites, v, 6, p. 83-106.
17
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Received: March 24,1999 Accepred:JuneI9,1999
