THE CANADIAN MINERALOGIST Journal of the MineralogicalAssociation of Canada
Volume 25
Pafi 2
JI.JNE 1987
Canadian Mineralogist Vol.25, pp. 201-2ll (1987)
WOODHOUSEITEAND SVANBERGITEIN HYDROTHERMALORE DEPOSITS: PRODUCTSOF APATITE DESTRUCTIONDURINGADVANCED ARGILLIGALTERATION ROGER E. STOFFREGEN* axo CHARLES N. ALPERS*'{' Departmmt of Geology and Geophysics, University of Colifurnia, Berkeley, California 94720, U.S.A.
SOMMAIRE
ABSTRACT
Woodhous6ite et svanbergite,identifi6es d ce jour dans environ deux douzaines d'endroits, ont 6td trouvdes dans trois gits min€raux hydrothermaux, caractdrisdspar une alt6ration argilac€eavanc6e.Ces deux espbces,de composition sulfate-phogphate d'aluminium (APS), sont isotypes de I'alunite (R3n). Elles rdpondent i la formule g6n6avecR = ca raliseeiRAl3@odr *lsodt-loH)e-r.(llzo)o ou Sr et x < 0.5. A Summiwille (Colorado), la svanbergite setrouve dans un glte cuivre-or €pithermal, of elle sepr6setrte, avec kaolinite et alunite hypog0nes, dans la partie supdrieuredu grsement,tandis que la woodhous€iteserencontre I desniveaux inf6rieurs, oir la pyrophyllite est localement abondante. Dans le gite de La Granja (Pdrou), la woodhousdite se pr6senteavec de la pyrophyllite dans le porphyre cupriflre, of elle aurail remplac6 I'apatile. Le porphyre cuprifBre de La Escondida (Chili) contient des solutions solideswoodhous€ite-svanbergite,les uneshypog0nes,les autres supergOnes,i en juger d'aprbs les critbres texturaux, Des spdcimensd'alunite de Summitville, La Escondida, et plusieursautresgltes 6tudi€s,contiennentun constituant APS, dont la teneur en P2O5atteint2, lclo en poids, avec l.l29o en poids de SrO. Ceschiffres reflEtent la substitution accoupl6e K+ * SOa2-= $12+ 4 pQo3dans la structure cristalline de l'alunite. Les phasesAPS se seraient form€es par remplacementde l'apatite dans le milieu acide et riche en sulfate, caract6ristique d'une alt6ration argilac€eavanc€e.Cesphrasesseraientpass6esinapKeywords: svanbogite, woodhouseite,alteration (advanced perguesdans plusieurs examplesd'une telle alteration de roches h6tes porteuses d'apatite. argillic), alunite, apatite, ore deposits,
Woodhouseite and svaubergite,previously documented from roughly two dozen localities, have been found with advanced argillic alteration in three hydrothermal ore deposits. These aluminum phosphate-sllfate (APS) minerals are isostructural with alunite (R3m), with the generalized formula RAl3(POdl +r(SO/1-r(OH)6-r. (HzO)r, where R is Ca or Sr, and x is less than 0.5. At Summitville, Colorado, an epithermal gold*opper deposit, svanbergiteoccurs with hypogenekaolinite and alunite in the upper portions of the deposit, and woodhouseite is observedat deeperlevels,wherepyrophyllite is locally abundant. In the porphyry-copper deposit at La Granja @eru), woodhouseiteoccurswitl pyrophyllite and appearsto have replacedapatite. The porphyry-copperdepositat La Escondida (Chile) contains woodhouseite-svanbergitesolid solutions, some hypogene, others supergene,as judged from textual criteria. Alunite specimensfrom Summitville, La Escondida, and severalother localities investigatedin this study contain someAPS component, lvith up to 2.41 wt. q0 P2O5and l.l2wt.tlo SrO, reflectingtie coupledsubstitur*"tion K+ + SO42-= (Ca,Sr)z+ + POo3-io gt. t*t ture. The APS phasesare consideredto form by replacement of apatite in the acidic, sulfate-rich environment ttrat characterizesadvauccdargillic alteration. Their occurrence probably has beenoverlooked in loany areasshowing such alteration of apatite-bearing host rocks.
xPresedt address: Depafiment of Geological Sciences, Southern Methodist Urlivssity, Dallas, Texas 75275, U.S.A. *lPresent adrlress:Departrnentof GeologicalSciences,1006 C.C. Little Building, University of Michigan, Ann Arbor, Mchigan 48109-1063,U.S.A.
(Traduit par la Rddaction) Mots-clds: svanbergite,woodhousdite, alt6ration argilac€e avancde,alunite, apatite, gftes mindraux.
201
202
TI{E CANADIAN MINERALOGIST
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 asa vein mineral or alteration product of feldin the literature. This is in contrast to numerous spars and micas in sulfur-rich, highly oxidized enstudieson phosphatemineralsin 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 excessnegativecharge, 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 subphase.Apatite also ocbursas a tracephasein the zone stitution of potassicalteration developedin porphyry-copper deposits(e.9., Gustafson & Hunt 1975, Reynolds (l) 1985), and it can occur with sericite in vein minerSOr2-+ R+ = Pgo:- .u 4z+ alization (e.9., Kelly & Rye 1979).However, apatite has not beenreported in the "advanced argillic" alteration zone, generallycharacterizedby 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 acidIn the APS minerals woodhouseite aad svanberic, sulfur-rich environmentrequiredfor this type of gite, half of the SOo2-groups in the alunite strucalteration. 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 coupledsubstitumineral, 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 samesymmetry as'alunite (space advanced argillic zones of three hydrothermal ore group R3m) and only slightly modified a and c deposits.Specifically, we considerwoodhouseiteand 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 solutionsbetweenthesetwo minerals In addition to variation in ttre amount of Sr and and membersof the crandallite-goyazite series,for Ca in the R site, we haveobservedsubstantialdeviwhich r= 1. Formulae for theseand other alumiation from a 1:l ratio of sulfateto phosphatein the num phosphate-sulfatesare listed in Table l. Xsite. An increasein phosphateaboveonemole per Woodhouseite, svanbergite,and the other alumi- formula unit at the expenseof sulfate requires furnum 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 abbreviation "APS" to designatesolid solutions between TABLE 1. ALUI{TNUI,IPHOSPHATE(SULFATE) END-I.If, I'tsER COMPOSITIONS all end-membercompositionslisted in Table l. INTRoDUCTIoN
OccunnsNcBs Svanber.glie
Sr At3 ( P 0 4 ) ( s 0 4 ) ( 0 H ) 5
tloodhouselte
Ca Al3 ( P 0 4 ) ( s 0 4 ) ( 0 H ) 6
Htn6daLl,te
Pb Ar3 ( P o 4 ) ( s o 4 ) ( 0 H ) 6
Coyazlte
sr A13 ( P 0 4 ) 2 ( 0 H ) 5 . H 2 0
CrandaLLtbo
ca Al-3 ( P 0 4 ) 2 ( 0 H ) 5 . H 2 0
Gorcelxlte
Ba 413 ( P 0 4 ) 2 ( 0 u ) 5 . H 2 0
Florenclte
C e A 1 3 ( P 0 4 ) 2( 0 H ) 6
There'are approximately two dozen published localities for svanbergite and woodhouseite (Table 2). Most of these occurrenc€sare in intermediategrade aluminousmetamorphicrocks, in which the APS minerals are associatedwith quartz, pyrophyllite, andalusite, alunite, and other phosphate minerals such as lazulite and augelite (e.9., Ygberg 1945,Switzer 1949,Wise 1975).The APS minerals also occur in bauxite (Shalit 1965,Bulgakova 1973), sedimentaryphosphate deposits (Jiang el al. 1984), kaolinite clay$tone$(Goldbery 1978) and quartz-
203
WOODHOUSEITE AND SVANBERGITE IN ORE DEPOSITS
apatite veins (Morton 1961),though thesesettings are geologically unrelated to the hydrothermal environment of interest in our study. The aluminum-rich rocks in which APS minerals
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generally occur are mineralogically similar to the advancedargillic assemblage(Meyer& Hemley 1967) well developed in some porphyry-copper deposits @eane& Titley 1981),and also associatedwith some epithermal gold deposits and polymetallic vein systems. This alteration assemblageis characterizedby extensivehydrolytic leaching of alkali cations and may contain alunite, kaolinite and diasporein lowertemperatureoccurrences,or pyrophyllite, andalusite and, more rarely, corundum from higher-temperature occurrencs (e.9., Gustafson& Hunt 195). The mineralogical and chemical similarity between the advancedargillic alteration assemblage and the bauxites and intermediate-gradealuminous metamorphic rocks discussedabovesuggeststhat the APS minerals might be expected to occur in association with advansed argillic alteration in hydrothermal ore deposits. We have observedthe APS minerals in three ore deposits: the Summitville gold-copper deposit (Colorado), the porphyry-copper deposit at La Escondida (Chile), and the porphyry-copper deposit at La Granja (Peru). The APS mineralsoccur mainly within zonesof advancedargillic alteration and are generallyassociatedwith pyrite and covellite, indicative of a high fugacity of sulfur @rimhall & Ghiorso 1983).In addition to thesedepositsand the localities in Table 2, we are awareof the following unpublished descriptionsof hypogeneAPS minerals in ore deposits: svanbergite has been identified at the Golden Wonder mine in Colorado (pers. comm., P.E. Billings 1983), svanbergiteoccurs with uraninite, xenotime, pyrite and copper sulfides in the B.J. Lake area of the McArthur River Projecf Saskatchewan(Costello 1985),woodhouseiteoccurs at Cerro Verde, Peru @ers.comm., E. Cedillo 1984), and Lacy (1%9) observedapatite replacedby alunite "probably of a high phosphorusvariety" at Cerro de Pasco, Peru.
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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 Aluminum phosphate-sulfate minerals are common in the Summiwille deposit, but occur in only minor amounts. They range in composition from nearly pure $1 fs high-Qa varieties, and occur over a depth rauge of 800 metres. Hinsdalite (Pb,Sr)Alr(PO+)(SOr(OH)6 also occurs locally in the deposit. The APS minerals at Summitville ocsur outside the zone of most intense alteration and mineralization. This innermost zone, characterizedby "vuggy silica" rock, is the result of intense hypogene acidsulfate attack, which has removed coarsepotassium feldspar phenocrysts, leaving a nearly monomineralic quartz rock crowded with coarsevoids. Adjacent to this zone, a rock containing predominantly quartz and alunite, with minor kaolinite, forms a lto 2-metrehalo, with kaolinite more abundant than alunite beyond this distance. At geater depth, kaolinite becomesmore abundant or entirely supplants alunite in the interior zone of alteration around the vuggy silica (Stoffregen 1985). APS minerals occur in the quaxtz-alunite and quartz-kaolinite zones at Summitviue. Where associatedwith alunite, the APS minerals ocsur as 2.0-to 60-pmpseudocubicgrains generally enclosed by alunite (Fig. l), in nearly perfect optical continuity with the enclosing erain. In the quartz-kaolinite
zone, the APS minerals occw in qlusters of 10- to 30-pm pseudocubes,scatteredin inegularly shapgd 0.2- to 0.5-mm patchesof fine-grained kaolinite. In both zones pyrite is a cornmon accessorymineral, and covellite occursin trace dmounts. APS minerals in thesetwo zonesare rich in the svanbergitecomponent (Sr>Ca) but show considerablecompositional variability, as discussedbelow. In contrast to thesetwo 6Des of occurrences,APS minerals close to end-member woodhouseite have been observedin one sample from a deep drill-hole at Summitville, approximately 750 metresbelow the maiu ore-zone. In this sample, the APS minerals occur as coarseblades (up to 250 pm) and are intergrown with pyrite, chalcopyrite, tennantite, and minor amounts of sphaleritecontaining 0.7 I 0.1 forms 0.5- to l'cm veinwt.9o Fe. This assemblage lets that cut intensely sericitized rockLa Granja, Peru The APS mineral observed at La Granja is high in calcium, approachingend-memberwoodhouseite. It occursin associationwith qvafiz. pyrite, pyrophyllite and andalusite. Minor amounts of apatite occur locally, and appearto havebeenreplacedby anhedral erains ( < 10pm) of woodhouseite.Andalusite in the La Granja samplesis invariably broken and enclosed in pyrophyllite mattes,strongly suggestingrefograde
205
WOODHOUSEITE AND SVANBERGITE IN ORE DEPOSITS
reaction to produce pyrophyllite from the highertemperature andalusite-bearing assemblage. The woodhouseitegrains occur in contact with pyrophyllite, and the two phases are interpreted to have formed in equilibrium based on tfiis textural criterion. La Escondida, Chile
TABI.E 3.
K20 Na20 CaO Sr0 Bao ce2o3 41203 SOq p265., H 2 Ot '
COIIPOSIrICNS OF AIT'MINI'}'
.12 n.d. 6.08 9.7O .90 .q0 35.2\ 10 . 19 23.25 13.50
n,d n.d
3.7\ 15.79 .28 3q.43 'il.08 12 . 8 0
PEOSPEAB-SI'LrA]T
1.00 .l'l 10,18 2.\4 2,56 n.d. 35.87 16.2t 18.41 13.10
n.d. n.d. 9.65 5.75 r.14 n.a. 35.79 1?.60 17.91 12.90
UINDSA',S
,21 n.d. 3.08 15.2\ .50 .13 34.37 16.63 17.06 12.30
At La Escondida, APS minerals are probably of 100.38 99,38 100.?4 99.94 99.52 both hypogeneand supergeneorigin. The probable hypogene ocqurence consists of grains 5 to 50 pm K n.d, .01t1 n.d. .091 rn size, intermediate in composition between svan.019 Na n.d. n.d. .011t n.d, n.d. bergite and woodhouseite,and intergrown with diaca .473 .297 .776 .729 .2\5 sr .408 .678 .100 .235 .656 spore. An extensive zone of hypogene aluniteBa .026 .008 .070 ,031 .015 pyrophyllite-diaspore alteration is spatially Ca o.d. .020 .002 n,a. .00? Ar 2 . 9 7 5 3 . 0 1 4 3 . 0 0 5 3 . 0 1 0 3 ,009 associatedwith the APS minerals on a broad scale s .555 .615 .869 .932 .92'l P ^. 1,\32 1.398 1.110 1.072 1.076 @rimhall el a/. 1985,Alpers 1986),but no grain conoHz' 6,535 6.320 6.200 6.019 6.092 tacts between pyrophyllite or alunite and the APS minerals have been observed. 1/ reletrt based on obeev€d , H2O @lcutaled Elues of gtrontl@, In contrast, several finer-grained occurrencesof (9@ text). phosphorE, ud au.lfu ^_ @lolu, z/ Eolss 0H @lqlatod bassd on [elAht HrO aboye. 5 APS minerals at La Escondida are consideredto be of supergeneorigin. In drill interceptsin the leached Des@lpgton ol Mples. capping zone, cryptocrystalline material intermedi1 . srcltvlll€ aapla 266-273, ol€Etton 3544 e. e@pIo 281-401 , eLeEtt@ 3553 n. ate in composition betweenwoodhouseiteand svan- 2. Slmtivllle Sleliyllle oupLe l4-254?, eleEt!.@ 2827 n. 3. bergite fills late fractures and occupies more than 4. La OreJa sople 6-120; DDlt-6, 120 n. depth La Es@ndtda s@pt€ 1067-C-159; DDH-9?, 278 n. depth. 5. 3090 of some fist-sized core samples. Fine-grained n.d. - aqg detgoledi detootlon ltott ls €stlEaisd ai .05 f,etght t oxlde @nponent. APS mineralsin thezone of supergenecopperenrichn.a. - not ulyzed for. ment are spatially associatedwitl supergenecopper sulfides (maiuly "sooty chalcocite", a fine-grained mixture of djurleite, digenite and anilite: Alpers 1986), which rim and replace hypogene sulfide Q6mpositional variations in the molar ratios POo3/SOa2- and C*+ /Sf+ from selected Sumnitville grahs. 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 samCneMrcAL CoMPosrrroN ples from La Granja and someof the samplesfrom Summitville. Woodhouseite and svanbergitehave been chemiThe deepersamplesat 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 higherAn acceleratingvoltageof 15kY wasused,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.Standardsincluded Marysvalealunite, 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 occurusedto analyzeall alunite samplesand standardsto renceas well. Thesesamplesshow distinct chemical minimize problems with vaporization of sodium and differencesfrom the inferred lower-temperaturesampotassium. ples,including higher contentsof calcium (somesamTable 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 9.58wt.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 concontentsof from 0.1 to 0.2 formula units. Molar t^ining appcsinlle potassium,suggestingan 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 woodCerium 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 hasroughly
2M
TIIE CANADIAN MINERALOGIST
Alunlte
xar.(soo)r(ott)u AVERAGEGRAIN COMPOSITIONS ESCONDIDA S U M M I T V I L L E. v r * a
Woodhouselte
Svanberglte
caars(soo)(eoo)(on)u
aa
o"'a'"
srar.(soo)(eoo)(or)u
(t otvr aa
Goyazlte
Crandallltg
xro srarr(no4).(ox)r.
caar. (noo),(ox)r.nro
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 sametlin section all have the samesymbol.
equimolar Ca2* and Sr2*. Other LREE are probably presentin small amounts in thesephases,as indicated by whole-rock X-ray-fluorescence data on La Escondida samples sentaining abundant APS minerals; neodymium and praseodymiumvaluesare up to 25 times background levels. Qualitative analysis by electron microprobe indicates that lead and arsenic contents are low to negligible in woodhouseite-svanbergite solid solutions from all three localities studied. The H2O contents listed in Table 2 were computed using the observedmolar ratios of Sr2* to Ca2+ and of POo3-to SOo2-.APS minerals with high Ca2* have relatively high water contents by weight owing to the lower molecular weight of Ca2" compared with Sr2*. Phosphatein excessof one mole per formula unit also increaseswater content, becausethe additional negativechargecreatedby this increaseis assumedto be balanced by the addition of H+. This implies that the amount of HrO per formula unit should equal the amount of phosphate in excessof I mole per formula unit, neglecting the influence of cerium. This relation is expressedby the general formula RAl3(POtl +r(SOrr-r(OH)6-,. (H2O)", which is approximately followed in all the compositionslisted in Table 3.
MrscnrltrY
BsrwEsN THE APS MINERALS AND ALUNITE
Another point to be addressedis t}te extentof solid solution between the APS minerals and alunite. Although whole-rock compositions indicate that alunite may contain appreciablephosphorus, strontium, and lessercalcium(e.g., Schraderl9l3' Frondel 1958,Kashkai 1970),thesedata are not reliable becausethey may reflect the presenceof APS grains within the analyzed samples. Significant solidsolution betweenAPS minerals and alunite hasbeen documentedby Wise (1975),who observeda "strontian natroalunite" with 25 mole cloPoos-, in substitution for SO2-and a similar amount of Sr2* for Na+. In addition, natroalunite with up to 18 mole 9o calcium and sigrificant phosphate has been reported from South Africa (Schochet sl, 1985,in prep.). In contrast to these appreciable substitutions in natroalunite, the alunite from Summitvilleshowslow phosphate, calcium, and strontium contents (sample l, Table 4). Alunite from La Escondidais somewhat richer in phosphorus and strontium than tfie Summitville samples (sample 2, Table 4). It is interesting to noti that the molar amount of POar
207
WOODHOUSEITE AND SVANBERGITE IN ORE DEPOSITS TABI.E 4.
CETMICAL COI.IPOSITION OT ALI]NITE
KzO Na20 Cao Sr0 BaO A1>0r so; " Pr0c Hro'1/
7,\9 2.U1 n.d. .05 n.a. 3't.3\ 38.44 .23 13.20
9 . 18 1.44 n.d. .34 n.d. 36.80 38.67 .30 13.05
10.79 o.d. n.d. .38 n.d. 36.70 37.O',t .63 13 . 0 5
10,811 .19 n.d. .31 n.d. 36.92 36.911 .86 13 . 0 5
9.66 1.04 .05 n.d, .20 37,13 37.96 .05 13 . 10
TotaL
99.16
99,78
98.62
9 9 . 11
99.19
K Ia
.321
Sr BA AI
s P
a1
2/
.002 n.a. 3 . 0 17 1. 9 ? 8 . 0 13 6.0U8
.805 .192 n.d. .01q n.d. 2.980 1.994 . 0 18 5.981
.961 o.d. n.al. .016 n.d. 3.021 1.94q .036 6.080
.952 .024 n.d. .O12 n.d. 2.998 1.91 .0119 6.003
.853 .139 .003 n.d. .005 3.029 1,975 .002 6.03q
The substitution of Sr and Ca into alunite and the occurrence of strontium-bearing APS minerals within the advanced argillic zone have important implications for patterns of whole-rock abundance of elements developed around hydrothermal ore deposits. Such patterns are commonly used in exploration for baseand preciousmetal deposits(e.g., Govett 1983). In particular, the Rb/Sr and K/Ca ratios are profoundly altered within the zone of advanced argillic alteration relative to patterns observedin the potassic,phyllic and propylitic zones (Schwartz 1982,Alpers & Stoffregen, in prep.).
250"C 4
..t ..
'rf \. \ .,. l
1^/, uetgllt t H2o elcuratod as ln Tabte 2. z/ Eo16s 0H €lalated as ln Tablo 2. Doeorlptlon l. 2. 3. {. 5. n.d. n.a.
of
@plee.
almlte 255-629. Su@liytlle 1067-C-298. Ee@ndlda almtte La Toua aLatte. MaygELo aLult€. aluile. Coldfteld - trot det€ctedi delssblon llolLe - noi aalyzed for.
6
ln Table 2.
d onC'| -9 I
\t .l
wood 4
\...lio \ '/ \1/ .' l i o \--J
t
.'
/
I I I
i, t\ ttl
a | a l
||
.-..e---
i '..6, v'. 1!
o o a
E
o I
I
II"F
..lD-
!
II... in these samples,as well as in other analyzed samlli ples of alunite, exceeds in all sases the molar alun I kao I musc 1.. Sr2* + Ca2* content. This leaves uncompensated [l'i tt some of the additional negative charge introduced B Al I by the phosphatesubstitution. This discrepancymay ttl t l' . | | ,l , be explained by the presenceof minor amounts of other cations in the R sitg (e.g., PtF*), or perhaps 5 4 3 by the protonation of some hydroxyl groups in the alunite. To provide a basisof comparisonfor theseresults, Frc. 3. Plot of stability fields of selectedalteration minerals we havealso analyzedalunite from three other localin the systemCaO-K2O-AI2O3-SiO2-SO3-P2O5-H2O ities: La Tolfa (Italy), Marysvale (Utah) and Goldin terms of pH and total dissolvedphosphateat 250oC and 40 bars (vapor saturation). The pH boundaries field (Nevada).Of these,the first two are believed between alunite, kaolinite, muscovite and potassium to have formed at or near the surface, not directly feldspar in the presenceof quartz are computed using relatedto precious."tu1 *inslalization (Lombardi data from Helgesonet al. (1978)un6 z55uminga log & Barbieri 1983,Cunninghan et al. 1984)'whereas potassiummolality of -3.0, and a log total sultotal the third is a gold-sulfosalt deposit similar to Sumfate molality of -1.5. The apatite saturation-lineand mitville (Ashley 1974).We have not detectedAPS the pH dependenceof H3POa dissociation are comminerals in any of thesesamples.However, the La puted using data from Barner & Scheuerman(1978), Tolfa and Marysvalealunite hassignificant 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 indicates the pH for equal activities of H3PO4 and Table 4). These compositional data are similar to H2PO|; line B represents the PH value for equal whole-rock chemical data on monomineralic alunite activities of H2POa- and HPOa'. An approximate rock from both La Tolfa and Marysvale (pers. lower bound on woodhouseite stability is indicated by Barcomm., C.G. Cunningham1984,Lombardi & the short dashedlines. In constructing this boundary, bieri 1983)and indisate that the whole rock SrO and provision was made for complexing of calcium and P2O5valuesprobably reflect the presenceof these potassiumwith sulfate, using data from Wolery (1983). elements in alunite. Goldfield alunite (sample 5, The location of this boundary relativeto the other fields Table 4) conlainsup to 0.2 wt.9o BaO but no detecshown on the figure is inferred from observednatural table strontium and only 0.05 wt.9o P2O5. as discusseditr the text. assemblages,
pH
208
TTIE CANADIAN MINERALOGIST
oF FoRMATIoN PARAcENESIS AND CoNDITToNS APS minerals are believedto form by replacement of apatite, or in responseto apatite dissolution, in the low-pH environmentthat characterizesadvanced argillic alteration. An antipathetic relationship betweenapatite and the APS phasesis particularly well illustrated in the Summitville deposit. Here primary apatite is common as an accessoryphasein the fresh host-rock, whereit occursin euhedralprismatic grains up to 2 mm in length. These apatite grains also occur in the distal, montmorillonitedominated zone of clay alteration around tle wggy silica zone, and locally in the adjacent illitic alteration zode. Apatite is absentfrom the more proximal guartz-kaolinite and quartz-alunite zones, as well as the vuggy silica alteration zone. As noted above, the APS minerals occur in the quartz-kaolinite and quartz-alunite zones, as well as in association with intense sericitic alteration at greater depths in the deposit.APS mineralshave not beennotedrin association with illitic or montmorillonitic alteration at Summitville. Theseobservations,together with thermodynamic data on hydroxyapatite, alunite, kaolinite, muscovite, potassium feldspar and various aqueouscomplexes,are usedto construct a diagram showing the inferred field of stability of woodhouseiteas a function of pH and total dissolved phosphate (Fig. 3). This diagram pertains to a temperature of 250"C, the estimatedtemperature of vein mineralization in the upp€r part of the Summitville deposit (Stoffregen 1985), and to the vapor-saturation pressureat this temperature (40 bars). The pH boundaries betweenalteration mineralsare computedusing a log total potassiummolality of -3.0, and a log total sulfate molality of -1.5. The apatite field is bounded by the apatite-dissolution reaction
ca5@or3(olD * 7H+ = 5 C{ + 3 H2PO;+ H2O
a)
assuming a log total calcium molality of -5.0. Altlough the potential effects of fluoride and chloride on apatite stability are not considered, Figure 3 neverthelessprovides a useful representationofthe observed natural assemblages. No thermodynamic data exist for woodhouseite or svanbergiteat any temperature. However, some generalconstraintscanbe placedon the stability field of the APS mineralsbasedon the petrological observations described above. This is illustrated by the dashedline on Figure 3, which provides an approximate stability-boundary for woodhouseite in terms of total dissolvedphosphate and pH. Figure 3 indi-
catesthat woodhouseite can coexist with alunite or kaolinite or both, in accord with the assemblages describedabove, and also with muscovite, as is suggestedby the associationof woodhouseitewith sericitic alteration at deptl in the Summitville deposit. Figure 3 also indicates that apatite may coexist with muscovite, in agreementwith the association of apatite with illitic alteration at Summitville and also with the reported association of apatite with muscovite in vein mineralization at Panasqueira, Portugal (Kelly & Rye 1979).However, apatite cannot coexist with kaolinite or alunite for the conditions assumedin Figure 3. This is in accord with the observedreplacementor dissolution of apatite that has accompaniedadvancedargillic alteration in the deposits studied. The replacementof apatite by APS mineralsin the pyrophyllite zone at La Granja suggeststhat similar arguments apply at somewhathigber temperatures. Basedon data from Hemley et al. (1980)on the system AI2O3-SiO2-H2O, the pyrophyllite-bearing assemblageat La Granja probably formed between 260 and 350'C. The paucity of alunite in this deposit (Schwartz 1982)may also reflect a somewhathigher temperature of alteration (c/ Hemley et al. 1969). SUMMARYaxo Cotqcr,usroN Aluminum phosphate-sulfate minerals that are isostruc{ural with alunite but contain Sfr and Ca2* in place of K+ and Na+ have been reported from approximately two dozen localities to date. Previously dcctibed.occurrencesare mainly in aluminumrich intermediate-grade metamorphic rocks. This report describes three new oscurences of these !'APS" minerals, two from porphyry-copper deposits(La Escondida,Chile and La Grania' Peru)' and one from a lode gold - quartz - alunite deposit Colorado). In all thee occutrences, minerals occur within zones of advanced the argillic ; at both Summitville and La Esconincludes abundant kaolinite, alunite, and dida pyrophyllite, whereasat La Granja, Iocally and andalusiteare contmon, but alunite component occws in alunite from both and La Escondidadeposits,and also from Marysvale and La Tolfa. SrO values ln o f 0 . 1 to 0.30 wt.Vo, and P2O5values of 0.30 to 0.60 9o are common in thesesamples,and maximum of t.l2 wt.9o SrO and2.4l wt.9o P2O5 observedin a sample of alunite from La have This is in contrast to the more extensive ion of natroalunite with APS rrinerals in andalusite- and sillimaniteassemblagesreported by Wise (1975) and et al. (1985, in prep.). minerals occur in trace amounts in all three
An the
WOODHOUSEITE AND SVANBERGITE IN ORE DEPOSITS
209
depositsstudied in this repolt. Their lack of distinc- BornqeLLY, T. Q97A: A review of tlte mineralsof the alunite-jarosite, beudantite, and plumbogummite tive optical properties suggeststhat they may have gxoup$.J. Res.U.S. Geol.Surv.41213-216. been overlooked in other deposits s6nfaining advancedargillic alteration, particularly those with apatite-bearing host rocks. Our observations indi- Bnruset,I,,G.H, Jn., ALpERs,C.N. & CumnNcnau, A.B. (1985):Analysis of supergeneore-forming cate that apatite is unstable in this setting, and is processesand ground-watersolutetransport using either dissolved or replaced by the APS minerals. massbalanceprinciples.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 oreof the advancedargillic alterforming consequences ation processin hypogeneenvironmentsby magACKNowLEDGEIVIENTS matic gascontaminationof meteoricfluids. Ecor. Geol,18,73-90. The authors thank the Anaconda Minerals Company, 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 permissionto publish theseresults. Bill of near-ore alteration of country rocks. Geol. Evr. C&asriSSSR,Sev. Nieman of Anaconda and Jim Bratt of Utah have Polqn. Iskop. Sev.-Vostoke. Urala,Geol.Knof. Kome..4SSR2, 514(in Russ.). been especiallysupportive of geological researchat Summitville and La Escondida, respectively. Don Rebal, also of Utah International, provided prelimi- Bwoarove,,A.P. (193): Epigeneticsvanbergitein the weatheringcrust of tle Lebedinskdeposit (Kursk nary identification of APS minerals from La Esconmagneticanomaly).Zap. Vses,Mineral. Obshchqt. dida. We also thank J. Arias, M. Barton, G.H Brim102,702-7W(in Russ.). hall, P. Cernf, R.R. Loucks and an anonymous reviewer for their commentsop the paper, and J.L. Cosrsl.r,o,K.D. (1985):A Mineralogical and PetrographicStudy of UraniumMineroliutionfrom the Jambor and A.H. Clark, who called a number of B.J. Loke Area of the McArthar River Proiect, additional referenceson the APS minerals to our SaskatchewanB.Sc.tlesis, Queen'sUniv., Kingattention. Use of tle electron microprobe in the ston, Ont. Department of Geology and Geophysics,University of California, Berkeley, was made possible by LBL Cur.rnnucsau, C.G., Rve,R.O.,,Srevrn,T.A. & Mrn$art # 72575@to I.S.E. Carmichael. Samplesfrom r.renr,H.H. (1984):Origins and explorationsigLa Granja were kindly provided by Michael O. nificance of replacement and vein-type alunite Schwartz, those from La Tolfa and other European depositsin the Marysvalevolcanic field, westcenlocalitie by Carl Francis of the Harvard Mineralogtral Utah. Econ. Geol.79' 50-71. ical Museum, and those from Goldfield by Roger Ashley of the U.S. GeoloeicalSurveyat Menlo Park. CyR, J.8., Pnass,R.B. & ScnoE-rEr,T.G. (1984): Geologyand mineralizationat Equity silver mine. We also thank C.G. Cunninebam for providing Econ. Geol. 19, 947-968. unpublished data on alunite from Marysvale. The U.S. Geological Survey assistedin preparation of ps M.F.F. (1983):EstudoterLrMA,W.N. & RevMAo, figures and tables. modindmicotedrico aplicado d g€nesee ds alteraq6esdehidroxifosfatosnaturais:fosfatoslaterlticos REFERENCES de Jandi6(PA) e Pirocaua(MA). Rev.Brasil. GeociAncias13, 4l-51. Ar.peRs,C.N. (198?: Geochemicaland GeornorphologicalDynamicsof SupergeneCopperSalfide Ore FrcuernsDo Golrss,C.S. (1958):On a Sr and Al basic Formation and Presemation at La Escondida, phosphate-sulfate closeto svanbergite,occurringin Antofugosto,C&i/e.Ph.D. Thesis,Univ. California. a Portuguesebauxitic clay. Mem. Noticias, Mus. Berkeley, Calif. Lab. Mineral. Geol. Univ. Coimbra6,29-39. Asnlry, R.P. (1974):Goldfield minilg disrist. .fz Fuar. J. & Ntrc, M. (1977)zSvanbergitefrom t}te Radjou iron ore deposit (Syria). Mineral. Polonica E' Guidebookto tlte Geologyof Four TertiaryVolcanic Centers in Central Nevada. Nevada Bur. Mines 69:73. Geol. Rep. 19,49-66. FnoNosI-,C. (1958):Geochemicalenrichmentof stronBanNen,I{.8. & ScsnuBrurrer,R.Y. (1978): Thertium in mineralsof the alunite structuretyp€. Geol. mochemicalProperties of Inorganic Substances. (abstr.). Soc.Amer. Bull. 69, 1567-1568 Springer-Verlag,New York. Gr.aorovsrrr,A.K. & Ksnemsov, V.N. (1968):SvanBnaxs,R.E. & Tnr-ny,S.R. (1981):Porphyry copper bergite varieties in iron ores and bauxites of the deposits.II. Hydrothermalalterationand mineraliKursk magneticanomalyarea.Mineral. Sb.Sverd. z.ation.Econ. Geol., 75thAnnlv. Yol.,235-269, no. 8, ll0-t22 (in Russ.).
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THE CANADIAN MINERALOGIST
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