The Canadian Mineralogist

Home , Augelite, Beudantite, Svanbergite, Woodhouseite

THE CANADIAN MINERALOGIST

Journal of the MineralogicalAssociation of Canada

Volume 25 JI.JNE 1987 Pafi 2

Canadian Mineralogist Vol.25, pp. 201-2ll (1987)

WOODHOUSEITEAND SVANBERGITEIN HYDROTHERMALORE DEPOSITS: PRODUCTSOF APATITE DESTRUCTIONDURING ADVANCED ARGILLIGALTERATION

ROGER E. STOFFREGEN* axo CHARLES N. ALPERS*'{' Departmmt of Geology and Geophysics, University of Colifurnia, Berkeley, California 94720, U.S.A.

ABSTRACT SOMMAIRE

Woodhouseite and svaubergite,previously documented Woodhous6ite et svanbergite, identifi6es d ce jour dans from roughly two dozen localities, have been found with environ deux douzaines d'endroits, ont 6td trouvdes dans advanced argillic alteration in three hydrothermal ore trois gits min€raux hydrothermaux, caractdrisdspar une deposits. These aluminum phosphate-sllfate (APS) alt6ration argilac€eavanc6e. Ces deux espbces,de compo- minerals are isostructural with alunite (R3m), with the sition sulfate-phogphate d'aluminium (APS), sont isoty- generalized formula RAl3(POdl +r(SO/1-r(OH)6-r. pes de I'alunite (R3n). Elles rdpondent i la formule g6n6- = (HzO)r, where R is Ca or Sr, and x is less than 0.5. At raliseeiRAl3@odr *lsodt-loH)e-r.(llzo)o avecR ca Summitville, Colorado, an epithermal gold*opper deposit, ou Sr et x < 0.5. A Summiwille (Colorado), la svanbergite svanbergiteoccurs with hypogenekaolinite and alunite in setrouve dans un glte cuivre-or €pithermal, of elle sepr6- the upper portions of the deposit, and woodhouseite is setrte, avec kaolinite et alunite hypog0nes, dans la partie observedat deeperlevels, where pyrophyllite is locally abun- supdrieuredu grsement,tandis que la woodhous€itese ren- dant. In the porphyry-copper deposit at La Granja @eru), contre I des niveaux inf6rieurs, oir la pyrophyllite est loca- woodhouseiteoccurs witl pyrophyllite and appearsto have lement abondante. Dans le gite de La Granja (Pdrou), la replacedapatite. The porphyry-copper depositat La Escon- woodhousdite se pr6sente avec de la pyrophyllite dans le dida (Chile) contains woodhouseite-svanbergitesolid solu- porphyre cupriflre, of elle aurail remplac6 I'apatile. Le tions, some hypogene, others supergene,as judged from porphyre cuprifBre de La Escondida (Chili) contient des textual criteria. Alunite specimensfrom Summitville, La solutions solideswoodhous€ite-svanbergite, les uneshypo- Escondida, and severalother localities investigated in this g0nes,les autres supergOnes,i en juger d'aprbs les critbres study contain someAPS component, lvith up to 2.41 wt. q0 texturaux, Des spdcimens d'alunite de Summitville, La P2O5and l.l2wt.tlo SrO, reflectingtie coupledsubstitu- Escondida, et plusieursautres gltes 6tudi€s,contiennent un tion K+ + SO42-= (Ca,Sr)z+ + POo3-io gt. t*t r*"- constituant APS, dont la teneur en P2O5atteint2, lclo en ture. The APS phasesare consideredto form by replace- poids, avec l.l29o en poids de SrO. Ceschiffres reflEtent ment of apatite in the acidic, sulfate-rich environment ttrat la substitution accoupl6e K+ * SOa2-= $12+ 4 pQo3- characterizesadvauccd argillic alteration. Their occurrence dans la structure cristalline de l'alunite. Les phasesAPS probably has been overlooked in loany areasshowing such se seraient form€es par remplacement de l'apatite dans le alteration of apatite-bearing host rocks. milieu acide et riche en sulfate, caract6ristique d'une alt6- ration argilac€eavanc€e. Ces phrases seraient pass6es inap- Keywords: svanbogite, woodhouseite,alteration (advanced perguesdans plusieurs examples d'une telle alteration de argillic), alunite, apatite, ore deposits, roches h6tes porteuses d'apatite.

xPresedt address: Depafiment of Geological Sciences, Southern Methodist Urlivssity, Dallas, Texas 75275, (Traduit par la Rddaction) U.S.A. *lPresent adrlress:Departrnent of GeologicalSciences, 1006 C.C. Little Building, University of Michigan, Ann Arbor, Mots-clds: svanbergite,woodhousdite, alt6ration argilac€e Mchigan 48109-1063,U.S.A. avancde, alunite, apatite, gftes mindraux. 201 202 TI{E CANADIAN MINERALOGIST

INTRoDUCTIoN and X i-s either sulfur, phosphorus, or arsenic (Botinelly 1970. Perhapsthe most common of these The behavior of phosphate minerals during minerals is alunite KAl3 (SOr2(OH)u, which occurs hydrothermal alteration has receivedlittle attention mainly as a vein mineral or alteration product of feld- in the literature. This is in contrast to numerous spars and micas in sulfur-rich, highly oxidized en- studieson phosphateminerals in the weatheringcycle vironments (Hemley et al. 1969). (e.g., Nriagu 1976,Vieillard et al. 1979,de Lima & Substitution of one mole of POa3-for SO42-in Reymio 1983),and on the complex mineralogy of alunite createsan excessnegative charge, which can phosphate-richpegmatites (e.9., Moore 1973).Apa- be compensatedby substitution of an.equimolar tite is the most common phosphate mineral in igne- amount of a divalent cation on the R position. Our ous rocks, where it generally occrus as an accessory findings suggesta limited degreeof this coupled sub- phase.Apatite also ocbursas a trace phasein the zone stitution of potassic alteration developedin porphyry-copper deposits(e.9., Gustafson & Hunt 1975, Reynolds 1985), and it can occur with sericite in vein miner- SOr2-+ R+ = Pgo:- .u 4z+ (l) alization (e.9., Kelly & Rye 1979).However, apatite has not beenreported in the "advanced argillic" alteration zone, generallycharacterized by alunite and in samplesof natural alunite formed at temperatures aluminosilicate minerals (Meyer & Hemley 1967). below 300oC, although at higher temperaturesa This suggests that apatite may be dissolved or more extensive solid-solution may exist. replacedby another phosphatemineral in the acid- In the APS minerals woodhouseite aad svanber- ic, sulfur-rich environmentrequired for this type of gite, half of the SOo2-groups in the alunite struc- alteration. ture are replaced by POa3-,and all of the monovalent cations are replaced by the divalent cations We discuss here the occurrence of such a Ca2+and Sr2*, respectively.This coupledsubstitu- mineral, with the generalized chemical formula tion has little effect on crystal structure, as these RAl3(P04)r+"(S04)r-x(OH)e-".(HzO),, in the minerals have the same symmetry as'alunite (space advanced argillic zones of three hydrothermal ore group R3m) and only slightly modified a and c deposits.Specifically, we considerwoodhouseite and parameters,as noted by Pabst (1947),Kato (197L, svanbergite, the Ca and Sr end-membersfor which 1977)and Kato & Miura (1977). .lr= 0, and solid solutions betweenthese two minerals In addition to variation in ttre amount of Sr and and members of the crandallite-goyazite series,for Ca in the R site, we haveobserved substantial devi- which r= 1. Formulae for theseand other alumi- ation from a 1:l ratio of sulfateto phosphatein the num phosphate-sulfatesare listed in Table l. Xsite. An increasein phosphateabove one mole per Woodhouseite, svanbergite,and the other alumi- formula unit at the expenseof sulfate requires fur- num phosphate-sulfate(APS) minffals listed in ther compensationof charges,which can be achieved Table I belong to an isostructural group of minerals either by the addition of a trivalent cation, such as that have the generalizedformula R 43(XOJz(O}I)0, cerium, or by protonation of someof tlte hydroxyl where R is a monovalent, divalent, or trivalent ca- groups in the structure. Where phosphatecompletely tion, A is a trivalent cation, generally Al3+ or Fd+, replacessulfate, the end membersgoyazite, crandallite and florencite are formed (formulae listed in Table l). In this communication we usethe abbrevi- TABLE 1. ALUI{TNUI,IPHOSPHATE (SULFATE) ation "APS" to designatesolid solutions between END-I.If,I'tsER COMPOSITIONS all end-membercompositions listed in Table l.

OccunnsNcBs

Svanber.glie Sr At3 (P04) (s04) (0H)5 There'are approximately two dozen published tloodhouselte Ca Al3 (P04) (s04) (0H)6 localities for svanbergite and woodhouseite (Table 2). Most of these occurrenc€sare in intermediate- (Po4) (so4) (0H)6 Htn6daLl,te Pb Ar3 grade aluminous metamorphicrocks, in which the Coyazlte sr A13 (P04)2 (0H)5.H20 APS minerals are associatedwith quartz, pyrophyllite, andalusite, alunite, and other phosphate CrandaLLtbo ca Al-3 (P04)2 (0H)5.H20 minerals such as lazulite and augelite (e.9., Ygberg Gorcelxlte Ba 413 (P04)2 (0u)5.H20 1945,Switzer 1949,Wise 1975).The APS minerals also occur in bauxite (Shalit 1965,Bulgakova 1973), Florenclte Ce A13 (P04)2(0H)6 sedimentary phosphate deposits (Jiang el al. 1984), kaolinite clay$tone$ (Goldbery 1978) and quartz- WOODHOUSEITE AND SVANBERGITE IN ORE DEPOSITS 203 apatite veins (Morton 1961),though thesesettings generally occur are mineralogically similar to the are geologically unrelated to the hydrothermal advancedargillic assemblage(Meyer & Hemley 1967) environment of interest in our study. well developed in some porphyry-copper deposits The aluminum-rich rocks in which APS minerals @eane& Titley 1981),and also associatedwith some epithermal gold deposits and polymetallic vein sys- tems. This alteration assemblageis characterizedby extensive hydrolytic leaching of alkali cations and may contain alunite, kaolinite and diasporein lower- temperatureoccurrences, or pyrophyllite, andalusite a, mffi-TmEmTmE (mffi) ocfl88ilcBs and, more rarely, corundum from higher-tempera- 1. @pio! ntn€, rd, sv qtr, ed, pf], ala, L@n (1937) (e.9., he 6h!y, rl, n@eto6 pnos- Itso (19?5) ture occurrencs Gustafson & Hunt 195). The phale olnaals mineralogical and chemical similarity between the 2. Clltreut1, !9 qtz, &d, prl, kao, Lacol. (1918) advancedargillic alteration assemblageand the baux- $6ns€t-Lot re, dls, orn, op, br!, ites and intermediate-gradealuminous metamorphic 3. Clro@lols U.at sv oao, py, @bs fuhnoY o! al. rocks discussedabove suggests that the APS minerals rft., ussR ( 1 9?3) might be expected to occur in association with 4. hv6r e&lulto aY qtr, prl Sitzer (1949) dhe, Hafrorn€, advansed argillic alteration in hydrothermal ore ileE&, Usa deposits. 5, Eqully allrer av qfz, !oa, d@, Cr 6t aL. (1984) d€Falt, almt@-a111@t€8. We have observedthe APS minerals in three ore bltlsh 6l@b1a (ap)3/, no, n6€roE Eulfldea deposits: the Summitville gold-copper deposit

6. hntr a1!a1, u$n sv nFoi (Kts)3/ xlktltna 6t al. (Colorado), the porphyry-copper deposit at La ( I 965) Escondida (Chile), and the porphyry-copper deposit ?. tursjdd€tA, q9z, end, prt Yabra (1945) Xeded, Xoday at La Granja (Peru). The APS mineralsoccur mainly

8. k Es6nd1&, ch116 rd-sv qtz, d8, prl, thla aludy within zonesof advancedargillic alteration and are qls generallyassociated with pyrite and covellite, indica- 9. La osnje, Peru rd prl, dd, (ap)3/ lhle aiudy dnor py tive of a high fugacity of sulfur @rimhall & Ghiorso

10. o]degrrd€n, S.ilonat rd fhtt, 9lz, lap)3/ krton (1961) 1983).In addition to thesedeposits and the locali-

11. s@tivtlle, rd, ala, ke, pt ties in Table 2, we are aware of the following unpub- _ thla aludy Colda6, USA d-av Blnor gr rae 3{, len lished descriptionsof hypogeneAPS minerals in ore deposits: svanbergite has been identified at the 12. Vllrefa: for@ilon, 9Y $-M, ri, pfl, P@baton & hF lal Co6ty. ks, loa, @n atdoeur (19?1) Golden Wonder mine in Colorado (pers. comm., dlfrnta, ugt P.E. Billings 1983), svanbergiteoccurs with ura- 13. t6tEaa, tuden 61 q$r, heo Y8bra (1945) ninite, xenotime, pyrite and copper sulfides in the l!. Aor& @B!f, USR av qtz, 'hyf@!@nl &kolo% & k&, py, opr rt, $l.rya81ns (1971) B.J. Lake area of the McArthur River Projecf loFr, bft Saskatchewan(Costello 1985),woodhouseite occurs B. Lq-TEWEMTME (8PERGEM) OCCURRENCS at Cerro Verde, Peru @ers.comm., E. Cedillo 1984), 1. Andofl@- al uurlie olay ll&elfdo fue! ht$€de, (ln hrat 6lnk) 0968) and Lacy (1%9) observedapatite replacedby alunite hrlu&l "probably of a high phosphorusvariety" at Cerro 2. Bru9rlok [o. 6 d Sblm (196?) nlne, n6{ bwrlok de Pasco, Peru.

3. (hbsy, UsR sv bulge u11! (1965)

!. &ovleesko&, 0SSR sy 1 hbov (196?) IABL6 2 5, k E6@nd1&. Chllo rd-!v Tnla stdy Pootnot€s obl@olt€,n al@ 1/ 6, gdb, (1973) rd. r@dleWelte Lebdl@k dopatt, a" ho, n€a, tul$ko* - Kask d&6t,1o Uuxlt€ CladbElt I & sv sEntErSttg @@1.y, USR tueboY (1968) rd-sv . r@dhous€lt€-swbgrglte 9o11d golutlon

C. LK CEMAil ORICXN 2/ Mlneral abbrgYlatlo@:

1 . Mul Pfovln@, rd-aY et al. (1984) Qtn ale - alulte hen - hsnatlte Olm al@, pt qd - &dalwlte Ja - Jaroette - - 2. Blaok gb]@, USB av (ap)3/ cua, pr &ddor & <gktl ap apatlt€ l€o kaolltrlt€ ( 19?0) bft - ba.lt€ Kfe - K-feld8paf €1 - €lelle kyn - kyette (1951) 3. roajre, rota, d-sy .1 Tercra @bs - o*bonateg oEq - EW@V1i€ Btszll @n - car4ollto frl. - pyrophylllt€ - ceruslte py - pyrlte 4. ffitsh @, rd eldbq (1978) cgr trqav, ke1 op - ohal@pyrlbe pyroo - pyrolorphlbe eud - cradalllte qtz - qurtz - rt - rutlLe 5. rc@1b1€vsk sy PL rE trhetao€ et al orn @rud@ . - - bh deptt, ( 19?6) ov @velllte s@r 6@rodlt9 bn6ta Ustn, U$n dlF - d16por9 ger - gealcltE d@ - [email protected] Sr-bar - strontie barit€ 5. hdJou Fo d6!os1t, 6r, LOO, @I FlJal & Itoo (19??) - glbbslte ten - t€milttto strte Bl.b gor - gor@1xltE tow - tow@lln€ rhll - whttlocklie 7. Stfd8, ghlna av h ar d. (1982)

8' ged@, &l@ at ? Jle8 ot d. (1984) 3/ ( ) donotso .ep1a@Eont by APS @tneral or croas-cuttin8 Y61a r€latloGhlp. 2M TITE CANADIAN MINERALOGIST

FIc. l. Photomicrograph of APS mineral (abeled APS) enclosedin bladed alunite from Summitville, Colorado. This grain contains subequalmolar proportions of calcium and strontium. Note the pseudocubichabit, which is typical of the APS minerals at Summitvile,

Summitville, Colorado zone, the APS minerals occw in qlusters of 10- to 30-pm pseudocubes,scattered in inegularly shapgd Aluminum phosphate-sulfate minerals are com- 0.2- to 0.5-mm patchesof fine-grained kaolinite. In mon in the Summiwille deposit, but occur in only both zones pyrite is a cornmon accessorymineral, minor amounts. They range in composition from and covellite occurs in trace dmounts. APS minerals nearly pure $1 fs high-Qa varieties, and occur over in these two zones are rich in the svanbergite com- a depth rauge of 800 metres. Hinsdalite ponent (Sr>Ca) but show considerablecomposi- (Pb,Sr)Alr(PO+)(SOr(OH)6 also occurs locally in tional variability, as discussedbelow. the deposit. In contrast to thesetwo 6Des of occurrences,APS The APS minerals at Summitville ocsur outside minerals close to end-member woodhouseite have the zone of most intense alteration and mineraliza- been observedin one sample from a deep drill-hole tion. This innermost zone, characterizedby "vuggy at Summitville, approximately 750 metresbelow the silica" rock, is the result of intense hypogene acid- maiu ore-zone. In this sample, the APS minerals sulfate attack, which has removed coarsepotassium occur as coarseblades (up to 250 pm) and are inter- feldspar phenocrysts, leaving a nearly monominer- grown with pyrite, chalcopyrite, tennantite, and alic quartz rock crowded with coarse voids. Adja- minor amounts of sphaleritecontaining 0.7 I 0.1 cent to this zone, a rock containing predominantly wt.9o Fe. This assemblageforms 0.5- to l'cm vein- quartz and alunite, with minor kaolinite, forms a l- lets that cut intensely sericitized rock- to 2-metre halo, with kaolinite more abundant than alunite beyond this distance. At geater depth, La Granja, Peru kaolinite becomesmore abundant or entirely sup- plants alunite in the interior zone of alteration The APS mineral observed at La Granja is high around the vuggy silica (Stoffregen 1985). in calcium, approachingend-member woodhouseite. APS minerals occur in the quaxtz-alunite and It occursin associationwith qvafiz. pyrite, pyrophyl- quartz-kaolinite zones at Summitviue. Where lite and andalusite. Minor amounts of apatite occur associatedwith alunite, the APS minerals ocsur as locally, and appearto havebeen replaced by anhedral 2.0-to 60-pm pseudocubicgrains generally enclosed erains ( < 10 pm) of woodhouseite.Andalusite in the by alunite (Fig. l), in nearly perfect optical continuity La Granja samplesis invariably broken and enclosed with the enclosing erain. In the quartz-kaolinite in pyrophyllite mattes, strongly suggestingrefograde WOODHOUSEITE AND SVANBERGITE IN ORE DEPOSITS 205 produce pyrophyllite reaction to from the higher- TABI.E 3. COIIPOSIrICNS OF AIT'MINI'}' PEOSPEAB-SI'LrA]T UINDSA',S temperature andalusite-bearing assemblage. The woodhouseitegrains occur in contact with pyrophyl- K20 .12 n,d 1.00 n.d. ,21 lite, and the two phases are interpreted to have Na20 n.d. n.d .l'l n.d. n.d. formed in equilibrium based on tfiis textural CaO 6.08 3.7\ 10,18 9.65 3.08 Sr0 9.7O 15.79 2.\4 5.75 15.2\ criterion. Bao .90 .28 2,56 r.14 .50 ce2o3 .q0 n.d. n.a. .13 41203 35.2\ 'il.083q.43 35.87 35.79 34.37 La Escondida, Chile SOq 10.1 9 16.2t 1?.60 16.63 p265., 23.25 18.41 17.91 17.06 H2Ot' 13.50 12.80 13.10 12.90 12.30 At La Escondida, APS minerals are probably of 99,38 100.38 99.94 100.?4 both hypogene and supergeneorigin. The probable 99.52 hypogene ocqurence consists of grains 5 to 50 pm rn size, intermediate in composition between svan- K .01t1 n.d, .091 n.d. .019 Na n.d. n.d. .011t n.d, n.d. bergite and woodhouseite,and intergrown with dia- ca .473 .297 .776 .729 .2\5 spore. An extensive zone of hypogene alunite- sr .408 .678 .100 .235 .656 Ba .026 .008 .070 ,031 .015 pyrophyllite-diaspore alteration is spatially Ca .020 o.d. .002 n,a. .00? Ar 3.014 3.005 3.010 2.975 3,009 associatedwith the APS minerals on a broad scale s .555 .615 .869 .932 .92'l @rimhall el a/. 1985,Alpers 1986),but no grain con- P ^. 1,\32 1.398 1.110 1.072 1.076 oH z' 6,535 tacts between pyrophyllite or alunite and the APS 6.320 6.200 6.019 6.092 minerals have been observed. 1/ reletrt , H2O @lcutaled based on obeev€d Elues of gtrontl@, In contrast, several finer-grained occurrencesof ^_ @lolu, phosphorE, ud au.lfu (9@ text). z/ APS minerals at La Escondida are consideredto be Eolss 0H @lqlatod bassd on [elAht 5 HrO aboye. of supergeneorigin. In drill intercepts in the leached Des@lpgton ol Mples. capping zone, cryptocrystalline material intermedi- 1 . srcltvlll€ aapla 266-273, ol€Etton 3544 e. ate in composition betweenwoodhouseite and svan- 2. Slmtivllle e@pIo 281-401 , eLeEtt@ 3553 n. 3. Sleliyllle oupLe l4-254?, eleEt!.@ 2827 n. bergite fills late fractures and occupies more than 4. La OreJa sople 6-120; DDlt-6, 120 n. depth 3090 5. La Es@ndtda s@pt€ 1067-C-159; DDH-9?, 278 n. depth. of some fist-sized core samples. Fine-grained n.d. - aqg detgoledi detootlon ltott ls €stlEaisd ai .05 APS mineralsin thezone of supergenecopper enrich- f,etght t oxlde @nponent. n.a. - ment are spatially associatedwitl supergenecopper not ulyzed for. sulfides (maiuly "sooty chalcocite", a fine-grained mixture of djurleite, digenite and anilite: Alpers Q6mpositional variations in the molar ratios POo3- 1986), which rim and replace hypogene sulfide grahs. /SOa2- and C*+ /Sf+ from selected Sumnitville and La Escondida samplesare illustrated in Figure 2. This plot doesnot take into account the presence of Baz* and Cd+, which are signifisanl in the sam- CneMrcAL CoMPosrrroN ples from La Granja and some of the samplesfrom Summitville. Woodhouseite and svanbergitehave been chemi- The deepersamples at Summitville (e.9., sample cally analyzed with the ARL 8-channel electron 3, Table 3) are from a part of the deposit in which microprobe at the University of California, Berkeley. pyrophyltte is locally abundant, suggestinga higher- An acceleratingvoltage of 15 kY was used,with sam- temperature origln than for the shallower APS ple current rarrglngfrom 0.012 to 0.0?2 microam- minerals, which are accompanied by alunite and peres.Standards included Marysvale alunite, natural kaolinite. The La Granja samples (e.g., sample 4, brazilianite, and synthetic anhydrite, celestite and Table 3) occur with abundant pyrophyllite and thus barite. A diffuse beam (10 to 30 pm in diameter)was are likely to represent a higher-temperature occur- usedto analyze all alunite samplesand standardsto rence as well. Thesesamples show distinct chemical minimize problems with vaporization of sodium and differencesfrom the inferred lower-temperaturesam- potassium. ples, including higher contentsof calcium (somesam- Table 3 lists averagecompositions of three APS ples approach end-memberwoodhouseite), and hieh grains from the Summitville deposit, and one grain barium contents,with amaximum of 9.58 wt.9o BaO each from La Granja and La Escondida. Most of in one sample from La Granja. The deep APS thesegrainsshow variation in calsium and strontium minsral5 at Summitville are also noteworthy in con- contentsof from 0.1 to 0.2 formula units. Molar t^ining appcsinlle potassium,suggesting an increase ratios of phosphateto sulfate are nearly constant in in the extent of solid solution between the APS most grains, and range from a minimum value of minerals and alunite at higher temperature. l:1, the expectedvalue for stoichiometric wood- Cerium occurs in trace amount in some s?mples, houseite or svanbergiteCIable 1), to a maximum of to a maximum value of 3.59 wt.9o Ce2O,noted in 1.5:0.5, observed in some Summitville samples. a shallow occurrenceat Sumnritville that has roughly 2M TIIE CANADIAN MINERALOGIST

Alunlte xar.(soo)r(ott)u AVERAGEGRAIN COMPOSITIONS

ESCONDIDA SUMMITVILLE.vr*a

Woodhouselte Svanberglte caars(soo)(eoo)(on)u srar.(soo)(eoo)(or)u o"'a'" aa (t otvr aa

Crandallltg Goyazlte caar. (noo),(ox)r.nro srarr(no4).(ox)r.xro

Frc.2. Compositional data on APS minerals from Summitville (solid symbols)and La Escondida (open symbols). Individual symbols represent averagecompositions for single gains. Grains from within the same tlin section all have the samesymbol.

equimolar Ca2* and Sr2*. Other LREE are proba- MrscnrltrY BsrwEsN THE APS MINERALS bly presentin small amounts in thesephases, as indi- AND ALUNITE cated by whole-rock X-ray-fluorescence data on La Escondida samples sentaining abundant APS Another point to be addressedis t}te extentof solid minerals; neodymium and praseodymiumvalues are solution between the APS minerals and alunite. up to 25 times background levels. Qualitative anal- Although whole-rock compositions indicate that ysis by electron microprobe indicates that lead and alunite may contain appreciablephosphorus, stron- arsenic contents are low to negligible in tium, and lessercalcium (e.g., Schraderl9l3' Fron- woodhouseite-svanbergite solid solutions from all del 1958,Kashkai 1970),these data are not reliable three localities studied. becausethey may reflect the presenceof APS grains The H2O contents listed in Table 2 were com- within the analyzed samples. Significant solid- puted using the observedmolar ratios of Sr2* to solution betweenAPS minerals and alunite has been Ca2+ and of POo3-to SOo2-.APS minerals with documentedby Wise (1975),who observeda "stron- high Ca2* have relatively high water contents by tian natroalunite" with 25 mole cloPoos-, in sub- weight owing to the lower molecular weight of Ca2" stitution for SO2-and a similar amount of Sr2* for compared with Sr2*. Phosphatein excessof one Na+. In addition, natroalunite with up to 18 mole mole per formula unit also increaseswater content, 9o calcium and sigrificant phosphate has been becausethe additional negativecharge created by this reported from South Africa (Schochet sl, 1985,in increaseis assumedto be balanced by the addition -prep.). of H+. This implies that the amount of HrO per In contrast to these appreciable substitutions in formula unit should equal the amount of phosphate natroalunite, the alunite from Summitvilleshowslow in excessof I mole per formula unit, neglecting the phosphate, calcium, and strontium contents (sam- influence of cerium. This relation is expressedby the ple l, Table 4). Alunite from La Escondidais some- general formula RAl3(POtl +r(SOrr-r(OH)6-,. what richer in phosphorus and strontium than tfie (H2O)", which is approximately followed in all the Summitville samples (sample 2, Table 4). It is compositionslisted in Table 3. interesting to noti that the molar amount of POar WOODHOUSEITE AND SVANBERGITE IN ORE DEPOSITS 207

TABI.E 4. CETMICAL COI.IPOSITION OT ALI]NITE The substitution of Sr and Ca into alunite and the occurrence of strontium-bearing APS minerals KzO 7,\9 9.18 10.79 10,811 9.66 within the advanced argillic zone have important Na20 2.U1 1.44 o.d. .19 1.04 for patterns of whole-rock abundance Cao n.d. n.d. n.d. n.d. .05 implications Sr0 .05 .34 .38 .31 n.d, of elements developed around hydrothermal ore BaO n.a. n.d. n.d. n.d. .20 patterns used in ex- A1>0r 3't.3\ 36.80 36.70 36.92 37,13 deposits. Such are commonly so; " 38.44 38.67 37.O',t 36.911 37.96 ploration for baseand precious metal deposits(e.g., Pr0c .23 .30 .63 .86 .05 Hro'1/ 13.20 13.05 13.05 13.05 13.1 0 Govett 1983). In particular, the Rb/Sr and K/Ca ratios are profoundly altered within the zone of TotaL 99.16 99,78 98.62 99.11 99.19 advanced argillic alteration relative to patterns observedin the potassic,phyllic and propylitic zones K .805 .961 .952 .853 (Schwartz 1982,Alpers & Stoffregen, in prep.). Ia .321 .192 o.d. .024 .139 n.d. n.al. n.d. .003 Sr .002 .01q .016 .O12 n.d. BA n.a. n.d. n.d. n.d. .005 AI 3.017 2.980 3.021 2.998 3.029 s 1.9?8 1.994 1.94q 1.91 1,975 250"C P .013 .018 .036 .0119 .002 a1 2/ 6.0U8 5.981 6.080 6.003 6.03q 4 ..t .. I 'rf 1^/, uetgllt t H2o elcuratod as ln Tabte 2. \. z/ Eo16s 0H €lalated as ln Tablo 2. I \l.,. Doeorlptlon of @plee. I \t .l l. Su@liytlle almlte 255-629. 2. Ee@ndlda almtte 1067-C-298. wood i, La Toua aLatte. t\ 3. d ttl o {. MaygELo aLult€. / 4 .---..e-- o 5. Coldfteld aluile. o- a | a n.d. - trot det€ctedi delssblon llolLe 6 ln Table 2. a l iv-'..6, E n.a. - noi aalyzed for. .' | n t | '. 1! o \...lio C'| \/1.'\/'lio I ! -9 \--J I ..lD- I II"F in these samples, as well as in other analyzed sam- II... ples of alunite, exceeds in all sases the molar lli 1.. alun I kao I musc Sr2* + Ca2* content. This leaves uncompensated [l'i tt some of the additional negative charge introduced Al I B by the phosphatesubstitution. This discrepancymay ttl t-l' be explained by the presenceof minor amounts of . | | ,l , other cations in the R sitg (e.g., PtF*), or perhaps 3 4 5 by the protonation of some hydroxyl groups in the alunite. pH To provide a basisof comparisonfor theseresults, Frc. 3. Plot of stability fields of selectedalteration minerals we have also analyzedalunite from three other local- in the systemCaO-K2O-AI2O3-SiO2-SO3-P2O5-H2O ities: La Tolfa (Italy), Marysvale (Utah) and Gold- in terms of pH and total dissolvedphosphate at 250oC field (Nevada).Of these,the first two are believed and 40 bars (vapor saturation). The pH boundaries to have formed at or near the surface, not directly between alunite, kaolinite, muscovite and potassium relatedto precious."tu1 *inslalization (Lombardi feldspar in the presenceof quartz are computed using & Barbieri 1983,Cunninghan et al. 1984)'whereas data from Helgesonet al. (1978)un6 z55uminga log potassiummolality of -3.0, and a log total sul- the third is a gold-sulfosalt deposit similar to Sum- total fate molality of -1.5. The apatite saturation-lineand (Ashley APS mitville 1974).We have not detected the pH dependenceof H3POa dissociation are com- minerals in any of these samples. However, the La puted using data from Barner & Scheuerman(1978), Tolfa and Marysvalealunite has significant and fairly Wolery (1983)and Vieillard & Tardy (1985),and an consistentSrO and Pp, contents (samples3 and 4, assumedlog total calcium molality of -5.0. Line A indi- Table 4). These compositional data are similar to cates the pH for equal activities of H3PO4 and whole-rock chemical data on monomineralic alunite H2PO|; line B represents the PH value for equal rock from both La Tolfa and Marysvale (pers. activities of H2POa- and HPOa'. An approximate comm., C.G. Cunningham1984, Lombardi & Bar- lower bound on woodhouseite stability is indicated by this boundary, bieri 1983)and indisate that the whole rock SrO and the short dashedlines. In constructing provision was made for complexing of calcium and probably presenceof these P2O5values reflect the potassium with sulfate, using data from Wolery (1983). elements in alunite. Goldfield alunite (sample 5, The location of this boundary relative to the other fields Table 4) conlainsup to 0.2 wt.9o BaO but no detec- shown on the figure is inferred from observednatural table strontium and only 0.05 wt.9o P2O5. assemblages,as discusseditr the text. 208 TTIE CANADIAN MINERALOGIST

PARAcENESISAND CoNDITToNSoF FoRMATIoN catesthat woodhouseite can coexist with alunite or kaolinite or both, in accord with the assemblages describedabove, and also with muscovite, as is sug- APS minerals are believedto form by replacement gestedby the associationof woodhouseitewith seri- of apatite, or in responseto apatite dissolution, in citic alteration at deptl in the Summitville deposit. the low-pH environmentthat characterizesadvanced Figure 3 also indicates that apatite may coexist argillic alteration. An antipathetic relationship with muscovite, in agreement with the association between apatite and the APS phasesis particularly of apatite with illitic alteration at Summitville and well illustrated in the Summitville deposit. Here also with the reported association of apatite with primary apatite is common as an accessoryphase in muscovite in vein mineralization at Panasqueira, the fresh host-rock, where it occursin euhedral pris- Portugal (Kelly & Rye 1979).However, apatite can- matic grains up to 2 mm in length. These apatite not coexist with kaolinite or alunite for the condi- grains also occur in the distal, montmorillonite- tions assumedin Figure 3. This is in accord with the dominated zone of clay alteration around tle wggy observedreplacement or dissolution of apatite that silica zone, and locally in the adjacent illitic altera- has accompaniedadvanced argillic alteration in the tion zode. Apatite is absentfrom the more proximal deposits studied. guartz-kaolinite and quartz-alunite zones, as well The replacementof apatite by APS minerals in the as the vuggy silica alteration zone. As noted above, pyrophyllite zone at La Granja suggeststhat similar the APS minerals occur in the quartz-kaolinite and arguments apply at somewhat higber temperatures. quartz-alunite zones, as well as in association with Basedon data from Hemley et al. (1980)on the sys- intense sericitic alteration at greater depths in the tem AI2O3-SiO2-H2O, the pyrophyllite-bearing deposit. APS mineralshave not beennotedrin associ- assemblageat La Granja probably formed between ation with illitic or montmorillonitic alteration at 260 and 350'C. The paucity of alunite in this deposit Summitville. (Schwartz 1982)may also reflect a somewhathigher Theseobservations, together with thermodynamic temperature of alteration (c/ Hemley et al. 1969). data on hydroxyapatite, alunite, kaolinite, muscovite, potassium feldspar and various aqueous com- SUMMARYaxo Cotqcr,usroN plexes, are used to construct a diagram showing the inferred field of stability of woodhouseiteas a func- Aluminum phosphate-sulfate minerals that are tion of pH and total dissolved phosphate (Fig. 3). isostruc{ural with alunite but contain Sfr and Ca2* This diagram pertains to a temperature of 250"C, in place of K+ and Na+ have been reported from the estimated temperature of vein mineralization in approximately two dozen localities to date. Previ- the upp€r part of the Summitville deposit (Stoffre- ously dcctibed.occurrencesare mainly in aluminum- gen 1985), and to the vapor-saturation pressure at rich intermediate-grade metamorphic rocks. This this temperature (40 bars). The pH boundaries report describes three new oscurences of these betweenalteration minerals are computedusing a log !'APS" minerals, two from porphyry-copper total potassium molality of -3.0, and a log total sul- deposits(La Escondida,Chile and La Grania' Peru)' fate molality of -1.5. The apatite field is bounded and one from a lode gold - quartz - alunite deposit by the apatite-dissolution reaction Colorado). In all thee occutrences, the minerals occur within zones of advanced argillic ; at both Summitville and La Escon- ca5@or3(olD * 7H+ = dida includes abundant kaolinite, alunite, and 5 C{ + 3 H2PO;+ H2O a) Iocally pyrophyllite, whereasat La Granja, and andalusiteare contmon, but alunite

assuming a log total calcium molality of -5.0. An component occws in alunite from both Altlough the potential effects of fluoride and chlo- the and La Escondidadeposits, and also ride on apatite stability are not considered, Figure ln from Marysvale and La Tolfa. SrO values 3 neverthelessprovides a useful representationofthe of 0.1 to 0.30 wt.Vo, and P2O5values of 0.30 to observed natural assemblages. 0.60 9o are common in thesesamples, and maxi- No thermodynamic data exist for woodhouseite mum of t.l2 wt.9o SrO and2.4l wt.9oP2O5 or svanbergite at any temperature. However, some have observedin a sample of alunite from La generalconstraints can be placedon the stability field This is in contrast to the more extensive of the APS minerals basedon the petrological obser- ion of natroalunite with APS rrinerals in vations described above. This is illustrated by the andalusite- and sillimanite- dashedline on Figure 3, which provides an approxi- assemblagesreported by Wise (1975) and mate stability-boundary for woodhouseite in terms et al. (1985, in prep.). of total dissolved phosphate and pH. Figure 3 indi- minerals occur in trace amounts in all three WOODHOUSEITE AND SVANBERGITE IN ORE DEPOSITS 209 depositsstudied in this repolt. Their lack of distinc- BornqeLLY,T. Q97A: A review of tlte minerals of the tive optical properties suggeststhat they may have alunite-jarosite, beudantite, and plumbogummite been overlooked in other deposits s6nfaining gxoup$.J. Res.U.S. Geol.Surv.41213-216. advancedargillic alteration, particularly those with apatite-bearing host rocks. Our observations indi- Bnruset,I,,G.H, Jn., ALpERs,C.N. & CumnNcnau, (1985):Analysis of supergeneore-forming cate that is unstable in this setting, and is A.B. apatite processesand ground-watersolute transport using either dissolved or replaced by the APS minerals. massbalance principles. Econ, Geol, 8O'lU-1256. This is in accord with existing thermodyamic data on apatite stability at 250"C. & Gnronso,M.S. (1983): Origin and ore- forming consequencesof the advancedargillic alter- ACKNowLEDGEIVIENTS ation processin hypogeneenvironments by mag- matic gas contaminationof meteoricfluids. Ecor. Geol,18,73-90. The authors thank the Anaconda Minerals Com- pany, Galatic Minerals Inc., Utah International Inc., Buraxov, V.V., Bureuova, V.A. & NKrrsNKo,I.P and Getty Minerals for sponsoringthis researchaud (1973):New data on svanbergitizationas a process for granting permission to publish theseresults. Bill of near-ore alteration of country rocks. Geol. Nieman of Anaconda and Jim Bratt of Utah have Polqn. Iskop. Sev.-Vostoke.Evr. C&asriSSSR ,Sev. been especially supportive of geological researchat Urala,Geol. Knof. Kome..4SSR2, 514(in Russ.). Summitville and La Escondida, respectively. Don Rebal, also of Utah International, provided prelimi- Bwoarove,, A.P. (193): Epigeneticsvanbergite in the nary identification of APS minerals from La Escon- weatheringcrust of tle Lebedinskdeposit (Kursk magnetic Zap. Vses,Mineral. Obshchqt. dida. We also thank J. Arias, M. Barton, G.H Brim- anomaly). 102,702-7W(in Russ.). hall, P. Cernf, R.R. Loucks and an anonymous reviewer for their comments op the paper, and J.L. Cosrsl.r,o,K.D. (1985): A Mineralogical and Petro- Jambor and A.H. Clark, who called a number of graphic Study of Uranium Mineroliutionfrom the additional referenceson the APS minerals to our B.J. Loke Area of the McArthar River Proiect, attention. Use of tle electron microprobe in the SaskatchewanB.Sc. tlesis, Queen'sUniv., King- Department of Geology and Geophysics,University ston, Ont. of California, Berkeley, was made possible by LBL R.O.,,Srevrn, T.A. & Mrn- # 72575@to I.S.E. Carmichael. Samples Cur.rnnucsau,C.G., Rve, $art from (1984):Origins and explorationsig- provided r.renr,H.H. La Granja were kindly by Michael O. nificance of replacement and vein-type alunite Schwartz, those from La Tolfa and other European depositsin the Marysvalevolcanic field, west cen- localitie by Carl Francis of the Harvard Mineralog- tral Utah. Econ. Geol.79' 50-71. ical Museum, and those from Goldfield by Roger Ashley of the U.S. GeoloeicalSurvey at Menlo Park. CyR, J.8., Pnass,R.B. & ScnoE-rEr,T.G. (1984): We also thank C.G. Cunninebam for providing Geologyand mineralization at Equity silver mine. -968. unpublished data on alunite from Marysvale. The Econ. 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