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TABLE I. Alunite samples investigated in this study
Sample no.Depositand Genetic
location environmentSample description,
occurrenceDeposit age Sampte preparation,
size fraction
AAR3(8-5) Riaza alunite-kaolinite deposit,central Spain
MAD/FM Kiraly Hill kaolinite Steam-heateddeposit, Mad,southern TokajMtns, Hungary
AK92/A19 Akahvahigh- Magmatic-sulfidation hydrothermalepithermaldeposit, centralHokkaido, Japan
ALRDG/CG Alunite Ridge Magmatic-alunite deposit, steamsouthwest ofMarysvale, Utah
F in e-gr ained rhombohedral 13.5alunite with associated halloysite- 13.9kaolinite; as porcelaneous veins, pods,and large replacement deposits inSilurian black shale
Rhombohedral crystals in kaolinite-richaltered zone associated withepithemtal Auquartz veins andsilicasinter or silicified sediments; alteredMiocene andesitehost rock
Coarsely crystalline, tabular or bladed 9.7pink alunite with dickite. diaspore,pyrophyllite and zunyite in theadvanced argillic zone that surrounds
Au-mineralized vuggy silica; asreplacements orfissure and cavityfilling in Miocene andesite
Plumose aggregates oflight pink alunite 13.6 0.6crystals up to 1 cm long, in massive 14.3 0.6aluntte veins up to 20 m wideand 100 mdeep; crosscutting Oligoceneintermediate composition volcanic rock
5.2
4.7
23.1
-2.0
Alunite crystals or crystafragments, 5 to 6 sizfraction
25 to 50 mesh crystals ocrystal fragments, treatto removeFe oxide andhydroxide coatings
25 to 50 mesh crystals ocrystal fragments, treatto removeFe oxkle andhydroxide
25 to 50 mesh crystals ocrystal fragments
same as above, treated dilute
1
*
By previous studies, seeTable 3 for alunite ages determined in this study
3
Ages byE. H. McKee (USGS, written comm.. 1991) on and size fractions, respectively
4
No ages were available for Hill, but K-Ar ages for similar deposits km west of Kiraly Hill range between 10.4 and 10.9 Ma
5
Fe oxide treatment: reaction et during5 mins ofup to 1 g ofsample with4 ml 0.3MNa citrate, 0.5ml 1M Na bicarbonate and0.1 g Na dithianke
HF treatment: reaction at room temperature during 12Omins of 0.6 g ofsample with 100 ml of concentrated hydrofluortc acid: distilled
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F I G. 1. Scanning electron images of typical tine-grained rhombohedral alunite of supergene origin (A. scale bar
= 10 and coarse-grained tabular and bladed alunit e of magmatic-hydrothermal origin (B. scale bar = 500
Samples from the Riaza alunite-kaolinite deposit. central Spain. and Akaiwa high-sulfidation epithermal deposi t.
central Hokkaido. Japan. respectively.
cristobalite diaspore dickite goethite, halloysite. hematite. i l l i t e ,
jarosite, kaol ini te quar tz pyrophyllite smectite sulfide minerals
( mainly pyr i te)native sulfur, woodhouseite-svanbergite. and zun-yite Alunite is rarely the pure potassium endmember. and partial
substitution of Na for K (up to greater than 95 Na) IS common
in all alumte forming environments (Stoffregen and Cygan. 1990).
Other spec ie s that may subs ti tu te for K inc lude ammonium
Altaner et al.. 1988). hydronium Ripmeester et al.,
1986. Alpers et al.. 1992). and or which may be
accompanied by substitutionof for (Stoffregen and Alp-
ers, 1987. Li et al.. 1992)
The effect of alunite sample impurities on K-Ar analysesisvari-
able It may be of limited significancei f there i s little substitution
ofNa forK ( K content up to 9. 4 wt%) and the contaminant phasesare non K-bearing minerals such as quartz or kaolinlte. It may be
problematic however, if the contaminant i s illite ofsignificantly
older age (e g . old detrital illite mixed with young impure aluni tei n hypersalinelakesof southern AustraliaBird et al.. 1990; Alpers
et al.) 1992). ori f the sample contains alunite or Jarosite of different
ages (e g , older hydrothermal alunite partly replaced by younger
superggene alumte and Jaroslte at the Goldtield. Nevada. high-sulfida-
tion eplthermal Au deposit: Keith et al.. 1979: Vasconcelos et
these cases preparation of mineral separatesof high purity
is of great Importance to obtain reliable results. In our experience.
the most significant step in obtaining pure alunite separates i s collec-
tion in the field of the best possible specimens In the case of hypo-
gene alunl te in ore deposits, this commonly implies collection of
samples from drill core, below the oxidation zone In areas with
supergene or steam-heated alteration the best specimens are com-
monly found in powdery to porcelaneous veins or cavity-fillings.
The minerals associated with alunite mentioned above are typi-
callyfine graIned and Intergrown. Sample preparation method\ based
on physical properties such
specified gr av it y magnetic susceptibil-
i ty or shape a r e therefore. of limited use The most effective meth-
ods involve taking advantage of differences in particle size and
chemically removing contaminant phases. A summary of these meth-
ods is provided in Table 2. The samples selected in this study were
all of high initial purity and required minor sample preparation.
This consisted mainly of separation of individual crystals or Crystal
clusters, light crushing, when necessary to reduce grain size, and
sieving to extract the 25-50 mesh fraction (-0.7 to 0.3 mm. respec-
tively: Table I ). The supergene alunite sample analyzed (AAR3 )corresponds to a 5-8 km size fraction prepared by elutriation and
centrifugation to remove minor amounts of very fine (
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45.30 T. I taya A. Arribas, J r , a nd T Okada
0.1
0.01
1000
100
0.1
0
Temperature
0.1
0.01
10
1
0.1
2. Argon release patterns of samples investigated in this study (A-E), including data for analytical blanks See Table for sample information and Tables Al and A2 in Appendix for results of individual measurements.
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m e n t a l of II
J. and E.
and text ures : tie of
P. H.. T. A.. and PR . of
to the of
and West Africa.
D . Rve R. P. and A
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of by up to
TABLE I. Step-heating argon isotope data for samples investigated in this study
338.8+-4.o
369.4 3.6 026
0.03
TABLE I. Continued
266.9 5 .0 4.11 0.05
14.53
A2. Step-heating argon isotope forblanks