itaya et al_1996 gca argon alunite

8/11/2019 Itaya Et Al_1996 GCA Argon Alunite

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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

itaya et al_1996 gca argon alunite

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