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Home , Alunite, Jarosite

American Mineralogist, Volume 75, pages 1176-1 181, 1990

Origin of - and -group minerals in the Mt. Leyshon epithermal gold deposif northeast Queensland,Australia KurH M. Scorr CSIRO Division of Exploration Geoscience,P.O. Box 136, North Ryde, New South Wales 2113, Australia

Ansrrucr The origin of alunite-jarosite family minerals, found in surface samples from the Mt. Leyshon Au deposit, northeast Queensland,has been determined using electron micro- probe and textural data. The base-metal contents of the alunite-group minerals provide significant additional information to help evaluate whether they formed during hypogene or weathering processes. K- and Na-rich members of the alunite group (KNa)Al3(SOo)r(OH)u generallyform as hydrothermal minerals in the acid- epithermal system at Mt. kyshon. Becauseof their highJevel formation they may be affected by subsequent weathering, during which they may incorporate Ag, Bi, Cu, and Pb derived from .Pb-rich members of the alunite group [, PbAI3H(PO4),(OH)., and hinsdalite, PbAI.(POJ(SO.)- (OH).] that form during weathering derive componentsfrom both sulfidesand apatite. Jarosite-groupminerals, (K,Na)Fe, (SOo),(OH)u, that replacealunite-gloup minerals dur- ing weathering retain the Na-K ratio of the parent; substantial Al is also retained by the resultant jarosite. that directly replace have low Al contents and Na-K ratios that differ from those of any alunite-group minerals presentwithin the sample.

IrvrnonucrroN ameter -2 km) Permian volcanic complex at the bound- Minerals in the alunite-jarosite family have the general ary of the Ordovician-Devonian Ravenswood Grano- formula AB3(XO.)r(OH). where A is a large cation (like diorite and metasediments of the Carboniferous- K*, Na*, NHf,, Pb2*, and Ca'z*)held in l2-fold coordi- Ordovician Puddler Creek Formation (Fig. l). nation. B sites are occupied mainly by trivalent Al or Fe In outcrop the complex shows zones characterizedby for alunite and jarosite groups, respectively. Cu and Zn strong development of kaolinite, oxides,and alunite- may also be minor but important components in the B and jarosite-group minerals within a broader iron-oxide- sites. The (XOo) sites are usually occupied by SO?-, jarosite zone and peripheral chlorite-feldspar zone. Au POf-, or AsOl-. occursin outcrop in the alunite zone (Scott, 1988).Below Extensive solid solution occurs within all three sites the zone of weathering the mineralization consists of gold (Scott, I 987) with that betweenalunite, KAlj (SOo),(OH)6, and electrum associated with pyrite and Cu + Pb + Zn and natroalunite, NaAl3(SOo)r(OH)6,and their Fe ana- + Bi sulfides within pervasive sericitic alteration (Mor- logues,i.e., jarosite, KFe, (SOo),(OH)., and natrojarosite, rison et al., 1987). NaFe, (SOo),(OH)u, being particularly well studied exper- Becausethey have been observed only in the weathered imentally (e.g.,Parker, 1962;Brophy and Sheridan,1965). top 60 m of the deposit, the alunite-jarosite minerals have Pb-rich members of the alunite-jarosite family generally been considered to be weathering products (Morrison et have some trivalent anions presentwithin the (XO4) sites al., 1987). K-Ar dating of surficial alunite has suggested to balancethe effect ofthe divalent cations in the A site, that such formation occurred 2-3 Ma (Bird et al., 1987). e.g., hinsdalite, PbAl, (PO.XSO.XOH)6. However, this study of the distribution of the alunite- Alunite-group minerals are often formed under acid jarosite minerals at Mt. kyshon and their textural and oxidizing conditions in hypogene porphyry Cu and epi- compositional featuressuggests that many of the thermal Au deposits (e.g., Knight, 1977). However such originally had a hydrothermal origin. minerals may also form during weathering of sulfides in thesetypes of deposits(e.g., Bladh, 1982).Thus in weath- Snupr-ns AND METHODS ered environments the origin of alunite-group minerals A total of 3 I 6 sarnples was collected from the surface may not be obvious. of the deposit prior to the commencement of mining in Minerals in the alunite-jarosite family are well devel- 1986. Of these samples, 126 contain members of the al- oped in outcropping volcanic brecciasat the Mt. kyshon unite-jarosite family. Becausealunite-group minerals were Au deposit, I l0 km southwest of Townsville, northeast generally discolored by admixed jarosite, or , or Queensland. The breccias occur in a roughly circular (di- both, X-ray diffractometry was used to distinguish the 0003-004v90/0910-l I 76$02.00 It7 6 SCOTT: ORIGIN OF ALUNITE AND JAROSITE tt'77

-t

MAtll010 OPENPII MTLEYSHON

0____:io0m

@ A,lu^tle l(\\ Nqtroolunitc v qnd Nq-qlunitc st.uot ]--.--Rood/trock

r:.-n Altercd ond U-.J brecciotcd volcqnics

Fig. 1. Extentof alterationand brecciationin outcropat Mt. Fig.2. Distribution of alunite-groupminerals, Mt. Leyshon. Leyshonand location of samplesanalyzed. alunite and jarosite groups (cf. Scott, 1987). Fifty-nine content (Table l) and 30 pm-sized spherulesof Cu-rich samples(including 24 &aing alunite-jarosites)were an- goethite. Although jarosite-group minerals are also pres- alyzed chemically. On the basis of the mineralogical and ent in some of thesealunite-bearing samples, no jarosite- chemical data, 184 electron microprobe analyses were group minerals are present in samples 4305, 4389, and made on 15 alunite-jarosite-bearingsamples from the al- 4409 (Table l). tered volcanics (Fig. l). Sodian alunite (i.e., alunite with geater than 100/oof A Sampleswere analyzed using a Camebax electron mi- sites occupied by Na) and natroalunite have a more re- croprobe operating at 20-kV accelerating voltage with a stricted distribution than alunite but are especially well beam current of l0 nA. The beam was defocusedto a developed on the southern flank of Mt. kyshon (Fig. 2). diameter of 20 pm to avoidthermal damageduringcount- Between 30 and 530/oof their A sites are occupied by Na ing periods of5 s. By arrangingthe counting sequenceso (Table l). The Fe contentsof sodian alunite blebs in 4382 that the thermally mobile major elements(Na, K, Al, and are generally similar to those in alunites. However, the S) were counted first and less abundant elements later, high Fe content of natroalunite from sample 4304 prob- consistent analytical results were obtained. No observable ably reflects its intergrowth with sodian jarosites and na- beam damage was done to the sample under these con- ditions. Oxide totals were generally lower than expected for stoichiometric alunite-jarosites owing to the porous nature of many samplesor the possibility of unanalyzed components, e.g., NHo*, HrO* in A sites (Altaner et al., 1988;Ripmeester et al., 1986).Therefore, mineral com- positions were calculated on the basis of 2 mol (XOo) as recommendedby Scott (1987).

OccunnnNcE oF ALUNTTE-cRoup MTNERALS Members of the alunite group presentin outcrop at Mt. Leyshon can be divided into K-, Na-, and Pb-rich vari- eties. Alunites [i.e., with compositions close to KAl3 (SOo), (OH).] are particularly well developedabout Mt. Leyshon (Fig. 2) and are characterizedby up to l0o/oNa in A sites and Fe upto l2o/oin B sites (Table l). In some samples (4183,4229, and 5021) alunite occursas 100-pm blebs replacing feldspar. Alunite from sample 4409 contains appreciable Ag and Bi and is coated by iron oxides. In samples4305 and 4389 it occurs as veins up to 5 mm thick. Alunite veins in sample 4389 contain -100 pyrite grains pm across (Fig. 3). The alunite in Fig. 3. Pyrite G;ray) rimmed by copper (white) in al- sample 4305 is green and contains a relatively high Cu unite vein (black) (BSE image, sample 4389). I 178 SCOTT: ORIGIN OF ALUNITE AND JAROSITE

MAIN OPENPIT MI LEYSHON

0-___ ,!00m

@ Jorcsite ((\t Sodion jorosrte i t\ * qnd notrororosite i.\ { -'-i \ "--- Strcom j r-- Rood/trqck ( |- Atteredond ! l-J brcccioted volconics

Fig. 5. Distributionofjarosite-group minerals, Mt. I-eyshon. Fig. 4. Plurnbogummitevein (white) cutting hematite cast afterpyrite @SEimage, sample 4382). OccunnnNcE oF JARosrrE-cRoLrp MTNERAT-s trojarosite. Significant POooccurs in the XOo sites in so- Jarosite-groupminerals have a more extensive distri- dian alunites and natroalunite from 4382and4304. bution than alunite-group minerals within the weathered Pb-rich members of the alunite group are also phos- rocks of the volcanic complex at Mt. kyshon (Fig. 5). phate-rich (Table l), with compositions varying between Jarosite [i.e., KFe, (SOJr(OtD.] contains up to l3oloNa plumbogummite, PbAl3H(POo)r(OH)6, and hinsdalite, in A sites with 40 to 640/oof such sitesbeing occupied by PbAl3(SO4)eO4)(OH)6,and have been identified within Na in sodianjarosite and natrojarosite,NaFer(SOo)r(OH)u the alunite zones at Mt. Leyshon and northeast of Mt. (Table 2). The jarosite-group minerals frequently occur Mawe. These Pb-rich minerals contain significant Ca and admixed with muscovite and goethite or replacing alunite Zn and generally are present as veins or blebs closely (Fig. 6). When jarosite replacesalunite, it retains the Na-K associatedwith iron oxides (Fig. 4). The plumbogummite ratio ofthe original alunite, e.g.,both alunitesandjarosites and hinsdalite in sample 4409 have Bi contents up to 700 are Na-poor in sample 4229 and Na-rich in sample 4304 ppm (Table l). (Tables I and 2). Thus the distribution ofNa-rich jarosites

TABLE1. Compositionsof alunite-groupminerals at Mt. Leyshon

No. A Site XO. Site Trace-element @ntents of anal- so. Poo Cu Zn Sample yses (v{t%) (a)Alunite 4188' 5 1.00 0.02 0.01 0 2.48 0.35 0.01 0 1.98 0.02 <0.01 0.10 0.04 4229' I 0.86 0.10 0.01 0 2.59 0.34 0.01 0 1.98 0.02 <0.01 0.07 0.05 4305 7 0.98 0.02 0.01 0 2.63 0.14 0.05 0 2.00 0 <0.01 0.76 0.02 4350' 7 0.94 0.01 0.01 0 0 2.3i! 0.32 0.01 0 1.99 0.01 <0.01 0.07 <0.01 4389 6 0.94 0.08 00 2.67 0.10 0.01 0 1.99 0.01 <0.01 0.12 0.05 4409 8 0.91 0.06 0 0.02 2.79 0.23 0.01 0 1.97 0.03 0.77 0.07 0.05 5021' 8 0.88 0.03 0.01 0.02 0 2.90 0.08 00 1.99 0.01 0.64 0.02 0.o2 (b) Natroaluniteand sodian alunite 4304- s 0.29 0.s3 00 1.67 1.32 0 0 't.92 0.08 <0.01 0.02 0.02 4382" 8 0.51 0.30 0.01 0 2.64 o.27 0.01 0 1.90 0.10 <0.01 0.07 <0.01 4382' 2 0.41 0.38 o0 2.74 0.26 0.02 0 1.91 0.09 <0.01 0.08 <0.01 (c)Plumbogummite and hinsdalite 4382', 10 0.03 0.01 0.03 0.73 o.22 2.78 0.10 0.01 0 0.11 1.89 0.07 0.05 4404' 5 0.07 0 0.01 0.87 2.88 0.09 0.01 0.02 0.47 1.53 0.03 0.19 4404' 1 0.40 0.03 0.02 0.50 2.70 0.27 0.01 0.02 1.09 0.91 0.07 0.19 4409 6 0.06 0.01 0.01 0.82 0.09t 2.87 0.12 0 0.02 0.16 1.84 <0.01 0.18 4409 6 0.18 0 0.01 0.69 0.11t 2.87 0.14 0 0.02 0.42 1.58 <0.01 0.18 4409 4 0.31 0.01 0.01 0.52 0.101 2.88 0.14 0 0.02 0.69 1.31 <0.01 0.16 /Vote.'Nosignificant AsOi was found.Compositions are based on the generalformula AB36OTXOH)6 with XOnnormalized to 2. 'Jarosite€roupminerals also present in th6s€samples. '- Ag : 470,Bi:220 ppmalso present. t Bi : 700 ppmalso present. SCOTT: ORIGIN OF ALUNITE AND JAROSITE rt79 ' t:.'t

'f,.F - Fig. 6. Weatheringof pyrite (white) leadingto the develop- ment of voids (black)and jarosite Gray) replacementof alunite Fig. 7. Jarosite (white) casts after pyrite. Muscovite Gray) (dark gray)(BSE image, sample 4229'1. and quartz Olack) (BSE image, sample 431l). correspondsclosely with the distribution of Na-rich alu- nites at Mt. Leyshon (cf. Figs. 2 and,5). However, where DrscussroN there is no physical contact betweenalunite and jarosite, In the advancedargillic alteration zoneof an epithermal the Na-K ratios may be quite dissimilar (as in samples Au deposit, alunite, kaolinite, and gold are associated. 4350 and 4382, Tables 1 and 2). Such acid-sulfateepithermal depositsalso tend to be Cu- The Al content of the jarosite-group minerals may be rich, to contain bismuthinite, and to have alunite replac- quite high, especiallywhen they replacealunites (e.g., sam- ing potassium feldspar (Hayba et al., 1986; Stoftegen, ple 4304). However, wherejarosites directly replacepyrite 1987).These features all occur at Mt. kyshon (e.g.,Mor- (Fig. 7), Al within B sitesis low (sample4311, Table 2). rison et al., 1987), although previous workers have con- No Pb-rich jarosite-group minerals have been found (Ta- sidered the alunite and kaolinite to be derived by weath- ble 2). ering.

TABLE2. Compositionsof iarosite-groupminerals at Mt. Leyshon No. of A site XO1site anal- Sample yses Ca SO. POo AsO. zon€' (a)Jarosite 4229 3 0.72 0.13 0 0 2.77 0.16 0 0 1.99 0.01 OA 4229 3 0.78 0.08 0 0 2.3s 0.68 0 0.01 1.97 0.03 OA 4240 6 0.80 0.06 0.10 0.04 2.02 0.93 0.04 0.01 1.59 0.38 0.03 J 4350 I 0.82 0.09 0 0 0 2.58 0.36 0 0 1.99 0.01 OA 4369 8 0.90 0.01 0.01 0.01 0.01 2.85 0.13 0 0 1.90 0.10 OJ 4382 5 0.83 0.01 0 0 2.67 0.23 o.o2 0.01 1.98 0.02 OA 4412 5 0.89 0.08 0.01 0 0 2.U 0.16 0 0.01 1.92 0.08 OJ 4412 10 0.93 0.03 0.01 0 0 2.69 0.30 0 0.01 1.89 0.11 OJ (b)Natroiarosite and sodian iarosite 4262 5 0.52 0.40 0 0 0 2.82 0j2 0 0 1.91 0.08 0.01 A 4262 5 0.52 0.45 0 0 0 2.87 0.07 0 0.01 1.98 0.02 OA 4304 3 0.47 0.42 0 0 2.67 0.34 0 0.01 1.97 o.Crit OA 4304 1 0.33 0.47 0 0 2.58 0.59 0 0 1.91 0.08 OA 4304 2 0.30 0.48 0 0 1.99 0.92 0 0 1.95 0.05 0A 4304 6 0.28 0.51 0 0 1.79 1.32 0 0 1.93 0.07 OA 4311 6 0.51 0.48 0 0 0 2.96 0.04 0 0 1.97 0.03 OA 4311 3 0.49 0.52 0 0 0 2.98 0.02 0 0 1.97 0.03 OA 4311 5 0.40 0.57 0 0 0 2.98 0.O2 0 0 1.97 0.03 OA 4311 2 0.31 0.64 0 0 0 2.99 0.01 0 0 1.97 0.03 OA Note.'Compositionsare based on the generaltormula AB3(XO1)IOH)6 with XO4normalized to 2. 'A : Alunite-and iarosite-group minerals present. J : Jarosite€roupminerals only. " 0.01Ag in this analysis. I 180 SCOTT: ORIGIN OF ALUNITE AND JAROSITE

Outcrop from the highly altered and weathered area ering sulfides.Thus Pb- and P-rich alunite-groupminerals centeredon Mt. lryshon, where Au is also present(Scott, may have originally formed as Ca-rich members of the 1988), contains alunite intimately associatedwith goe- alunite group (e.g., as at Summitville; Stof- thite, but jarosite is absent,e.g., samples 4305, 4409 (Ta- fregenand Alpers, I 9 87) with subsequentweathering lead- ble I, Fig. 1). The work of Bladh (1982) indicatesthat ing to the incorporation of Pb and other base metals. such assemblagesreflect oxidation of pyrite under quite Alternatively, plumbogummite and hinsdalite may have Al-rich conditions and are likely to occur during the formed directly by acid attack on apatite during weath- weatheringof advancedargillic alteration assemblages.A ering of Pb-rich sulfides. Becausesuch sulfides are inti- K-Ar age of -2-3 Ma is reported for the analysisof two mately associatedwith Au at Mt. kyshon (Morrison et such samples (Bird et al., 1987). However when one of al., I 987), either hypothesissatisfactorily accounts for the thesesamples was dated,using the Rb-Sr method, a Perm- restriction ofthe lead phosphatesto the alunite zone. ian age(260 Ma) was obtained (Whitford et al., 1988).If Jarosite-groupminerals replace alunites in some cases both thesedeterminations are correct, this feature is best (Fig. 6) with simple one-for-one substitution of Fe for Al, explained by weatheringoforiginal hypogenealunite dur- and the retention of some Al and the Na-K ratio of the ing which the K-Ar systematicsare affectedbut the Rb- original alunite. In contrast, jarosite formed by replace- Sr systematicsare not. ment of pyrite (Fig. 7) has a low Al content. Sample4389 containspyrite within an alunite vein (Fig. 3). The alunite has a d3aSvalue : 9.87o, whereas the Coxcr,usroNs coexistingpyrite has d3aS: 8.8V* (A. S. Andrew, personal K- and Na-rich alunite-gtoup minerals originally formed communication, 1987).Bird et al. (1989) regardthis as as hydrothermal alteration minerals at Mt. kyshon. Be- indicating that the alunite is secondary,i.e., derived from causeof their formation closeto the original surface,i.e., weathering sulfides(cf. Jensenet al., l97l). However be- above the water table, they were susceptibleto modifi- causethis alunite also has an old Rb-Sr age (D. J. Whit- cation during subsequentweathering. During suchweath- ford, personal communication, 1987) the more likely ex- ering they may have incorporated Ag, Bi, Cu, and Pb planation is that the alunite is formed by oxidation of HrS derived from the breakdown ofsulfides. above the water table (cf. Hayba et al., 1986) and is hy- Pb- and P-rich alunite-group mineral formation may pogene. have required the destruction of both apatite and Pb-rich In addition, base-metalcontents of alunites also appear sulfide. Such specializedconditions may be reflected by to differ dependingupon the extent ofexposure to weath- the restricted distribution of plumbogummite and hins- ering. Those alunites least likely to have been affected by dalite at Mt. Leyshon. weathering(i.e., those still bearing sulfides,e.9.,4229 and Jarosite-groupminerals at Mt. kyshon formed either 4389) tend to have base-metalcontents <0.2 wto/owhereas by replacement of alunites, retaining the Na-K ratio of weathering-affectedalunites (i.e., those with intimately the parental alunite and generally some Al, or directly associatedgoethite, e.g., 4305 and 4409)have higher base- from pyrite, resulting in only a low Al content. metal contents(>0.6 wto/0,Table l). The actual basemet- als presentmay be either Cu or Pb + Bi + Ag, suggesting AcxNowr,nocnnnNrs contributions from Cu-bearing sulfidesor Pb-Bi-bearing Ltd. is for accessto the deposit and for sulfosalts,respectively. Both types ofsulfides present Pan Australian Mining thanked are facilirating discussions with slaffmembers, particularly Jim Morrison, Ian in unweatheredrocks at depth (Morrison et al., 1987). Hodkinson, and John Downing. Anita Andrew and David Whitford The low base-metalcontents of natroalunite and sodian (CSIRO) are also thanked for their isotopic analysis ofalunite from sample alunite in 4344 and, 4382 suggestthat these occurences 4389 and for stimulating discussions about the origin ofal'nite. Electron are also hypogene.Furthermore, Na-rich members of the microprobe samples were prepared by Manal Kassis, and assistance in was provided by David French and group electron microprobe determinations alunite can only form from solutions with very high Ken Kinealy, all of the North Ryde laboratories of the CSIRO Division Na-K ratios (Parker, 1962);such solutions are unlikely to of Exploration Geoscience. Comments by Geoff Plumlee and an anony- be extensively developed during weathering of the mus- mous reviewer have also helped to improve the paper. covite-rich rocks in the Au-rich zone at Mt. Leyshon. However, high Na-K ratios are retained when alunite is RnrnnrNcns crrED jarosite altered to during weathering(see below). Altaner, S.P.,Firzpatrick, J.J., Krohn, M.D., Bethke, P.M., Hayba, D.O., The origin of plumbogummite and hinsdalite is not so Goss,J.A., and Brown, Z.A. (1988) Ammonium in alunites.American easily evaluated.Stoffregen and Alpers (1987) have found Mineralogist 73, | 45-l 52. P-rich members of the alunite group that forme