Conadian MineralogistVol. 25, pp.213-2.24 (1987)

ABSTRACT

The Daisy Creek prospect, located approximately 25 kmnorth of Thompson FaIIs, Montana, consists of strataboundCu-Pb-Ag mineralization hosted by Helikian quartzites ofthe Bonner Formation, Belt Supergroup. Hypogene miner-alization occurs in zones containing disseminated galena,chalcopyrite, argentiferous bornite, and argentiferous chal-cocite, which precipiUted during middle diagenesis withinapatite-rich, medium-grained, channel-facies quartzites.Oxidation of these sulfides occurred in two episodes, oneduring diagenesis under conditions of relatively low CO2and high Ba2+ activiry, and the other during supergeneweathering under conditioas ef higher CO2 and lowerBa2+ activity. In the copper zones, cupriferous goethite,acanthite and barite occur as products of diagenetic oxi-dation, whereas nativ€ silver, limonite and malachite occuras products of supergene weathering. ln tle lead-rich zone,diagenetic barite replaces galena, and hinsdalite@b,Sr)Al3POaSOa(OH)e, a raxe supergene mineral, fillsvugs and locally replaces supergene cenrssite and pyromor-phite where the lead zone overlaps P-anomalous strata.

Keywords: stratabound copper, hinsdalite, oxidationproducts, Montana, Bonner Formation, Belt Super-group.

SoMMAIRE

L'indice min€ralisd en Cu, Pb et Ag de Daisy Creek, situ6environ 25 km au nord de Thompson Falls, au Montana,est conforme aux quartzites d'6ge Hdlikien de la forma-tion Bonner, du supergroupe Belt. La mineratsation hypo-gdne en zones est formde de galbne, chalcopyrite, et bor-nite et chalcocite argentifbres, esplces qui se sont formdespendant Ia diagenbse'moyenne dans des quartzites d'unfacies de chenal, riches en apatite et d grain moyen. L'oxy-dation de ces sulfures s'est faite en deux stades, le premierlors de conditions diagdn6tiques de faible activitd de CO2et de forte activitd de ga2+, €t le second pendant le lessi-vage supergBne, i forte activitd de CO2 et d'une activitdrdduite de Ba2+ . Dans le zones riches en cuiwe, goethitecuprifbre, acanthite et barytine sont les produits d'oryda-tion diag6n&ique, tandis que argent natif, limonite et mala-chite sont les produits du lessivage. Dans la zone riche enplomb, la barytine diagdndtique remplace la galbne, et lahinsdalite @b,Sr)AI3PO4SO4(OH)5, espice superg&ne peucornmune, remplit des cavit€s et remplace ici et li cerus-site et pyromorphite supergbnes, li orl la zone riche enplomb coihcide avec les strates i teneur de phosphoreanomale.

(Iraduit par la R6daction)

HINSDALITE AND OTHER PRODUCTS OF OXIDATION AT THEDAISY CREEK STRATABOUND COPPER-SILVER PROSPECT,

NORTHWESTERN MONTANA

CLIFFORD R. STANLEYDepanment of Geological Sciences, University of British Colunbia, 6339 Storq Road,

Vancouver, British Columbia V6T 284

Mots-cl6s: gite de cuiwe stratiforme, hinsdalite' produitsd'oxydation, Montana, formation Bonner, supergroupeBelt.

INTRoDUcTIoN

The Daisy Creek prospect, located approximately25 km north of Thompson Falls, Montana, consistsof stratabound Cu-Pb-Ag mineralization hosted byHelikian quartzites of the Bonner Formation, BeltSupergroup. Sulfides occur a{i disseminations,clouds, and ponds adjacent to siltite beds within areduced (white to gxey), epsilon:cross-bedded, flu-vial channel (Stanley 1984). B-horizon soil samples,float-rock samples, and soil heavy-mineral concen-trates were examined to determine the relationshipbetween hypogene mineralization and the productsof supergene alteration.

X-ray powder diffraction, scanning electronmicroscopy, X-ray spectroscopy (energy dispersion)and results of geochemical analyses were used toidentify the varisus minslal phases. The products ofoxidation of disseminated, quartzite-cementinggalena, chalcopyrite, and argentiferous bornite andchalcocite consist of cupriferous goethite, barite,limgnils, acanthite, native silver, cerussite, pyromor-phite, malachite, chalcanthite a:rd chrysocolla. Inaddition, hinsdalite @b,Sr)A!POoSOo(OlI)0, a rareproduct of superg€ne alteration, has been identified.

GSOI,OGTCET SBTUNG

Within the Belt Basin of western Montana, twomajor quartzite formations are recognized. Theyounger of these quartzites, the Bonner Formation,is a fluvial, hematite-stained sandstone containingthe Daisy Creek Cu-Ag-Pb prospect. Sedimentaryfacies within the Bonner Formation include alluvial-apron conglomerates, which grade into braid-plain'channsl-;a.irs quartzites and distributary, sheet-flow- and overbank-facies siltite couplets in the DaisyCreek area (Fig. l; Winston et al. 1977, Newton1982, Stanley 1984).

At the Daisy Creek prospect, mineralization occurswithin a grcy to white, epsilon- and trougb-cross-bedded, feldspathic, vitreous to micaceous, fine- to

2t3

2t4

J@lo Valley

si(

(--njIt

Daby Cteek Prosp€ot *\

medium-grained, channel-facies orthoquartzite(informally, the upper cross-bedded member)interbedded with hematite-stained (pink, red, andpurple), flatJaminated, coarse-grained, mud-cracked, overbank-facies siltite couplets @ig. 2). Theupper cross-bedded member averages 30 m in thick-ness and contains more disseininated ferroan car-bonate cement and less ferric oxide cement than thesurrounding red-beds.

B-horizon soil samples collected at l5-m intervalsover the upper cross-bedded member exhibit elevatedconcentrations of aqua-regia-soluble Ca, Sr, Mg, Tiand P (Fie. 3) with respect to the soils developed overthe enclosing red-bds. These hielr concentrations areattributed to the abundance of l) carbonate cementfilling the voluminous pore-spaces of these quarlz-ites, and 2) heaW minerals (specifically, apatite,ilmenite and leucoxene) concentrated within thelower 15 m of this member. Highly anomalous Pconcentrations (0.12 to 0.25 wt.tlo P2O5) from thesestrata contrast with lower concentrations from thesurrounding red-beds (< 0.08 wt9o P2O5).

The concentrations of P in tle soil samples are vir-tually identical to those found in float-rock samplesfrom the upper cross-bedded member (up to 0.55wt.9o P2O5), and are extremely high for terrigenoussandstones relative to Pettijohn's estimate of globalaverage composition (0.02 wt9o PrOr for ortho-

THE CANADIAN MINERALOGIST

Flat LamLuted Quartzlts

Crossb€dded Quartzlte

Conglomsrate Pacrnan prospoct

o 2a 60 76 lO0

kn

Flh Or@kBlueblrd /Mona Llsa Pro8pgal

Wlao FEvot

Jocko Valley

quartzites: Pettijohn 1963).netic fractions of panned

detritally transported fromthe south: Winston et al. I

horizon soils above this contain up to 8590apatite, .most of which of subrounded,frosted (abraded), detrital grains up to

igneous fluorapatiteI mm in diameter (probably

CroekFqiltlEw Wlee Rlver

nonmag-from B-

source-rocks to. The other l59o of

the apatite grains arelar prisms and have anof 5:1. These crystals

, euhedral, acicu-width-to-length ratioare recently formed

carboUate-hydroxyapatite, they do notexhibit evidence of abrasion. suggests that atleast some of the apatite hassitz within the soil.

precipitated lz

Float-rock samples of the quartzite facieslrom the upper cross-bedded contain abun-dant erains of detrital They also commonly

nsist of imbricated,contain lag lamellae thatsubrounded, pale green, (significant amountsof Fe and Al), phosphatic u p t o l c m i nlongest dimension. grains of detritalmonazite also occur within -mineral lag laminae in the upper cross-bedded . The monaziteis not dissolvedby the aqua digestion and thusdoes not contribute to the P deter-mined.

The above evidence

400

900

200

t@

o

Olark Fork Flshlrap Oreak

Ftc. l. Regional cross-section showing sedimentary facies relationships within the Bonner Belt Supergroup.At Fishtrap Creek, the approximate location of the Daisy Creek prospect, the Bonner F is divided into5 informal members (lower, middle, and upper flat-laminated, fine-grained quartzites 1, Ybn3, and Ybn5) andlower and upper cross-bedded, medium-grained quartzites (Ybn2 and Ybna, respectively)

that l) the upper

Ftc. 2. Geological map oi the Daisy Creek prospect.Shaded areas contain malachite oxidation products infloat-rock chips vdthin the Cu-anomalous zone. Starsrepresent prospect pits over the Pb-anomalous zone.Bonner Formation members Ybn3 and Ybn5 are flat-laminated, fine-grained quartzites that enclose themedium-grained, epsilon- and trough-cross-beddedYbna quartzite member. Regionally, bedding strikesapproximately 165o and dips 40' SW.

cross-bedded member of the Bonner Formation, atleast within the areal confines of the soil survey (22soil lines traversing the stratigraphy across a strikelength of 8 km), contains abundant apatite, both inmud chips and as individual detrital grains, in itslower 15 metres, and 2) supergene rock-water reac-tions have produced solutions with high concentra-tions of phosphate.

Nlrune oF THE MrNaRALrzArroN

The extent of the sulfide mineralization at theDaisy Creek prospect has been defined by anomalousconcentrations of Cu, Pb and Ag in soil (Fig. 4). Theoverlapping geometry of these anomalous regionsdemonstrates that tle dqposit has a distinct geochem-ical zonation that cannot be related to differentialhydromorphic dispersion or downslope creep bause

2t5

I 1 : :*c;!?. {f,a},1 1 1 i:T::g:*::SA

3eq u

Pr{oSPHoRUS

Frc. 3. Soils with anomalous concentrations of phospho-rus over the Daisy Creek prospect. The outer contourencloses areas with greater than 0.07 wt'Vo P2O5,whereas the shaded areas have concentrations greaterthan 0.10 wt.9o P2O5. Threshold values were selectedusing probability plots. The P-anomalous zone approx-imately overlies the lower half of the upper cross'beddedmanber (Ybna).

slope orientations are inconsistent with the zone off-sets. In addition, several distinct forms of dissemi-nated limonite gos$ans within the float-rock samplesare recognizable as supergene products of galena,chalcopyrite, argentiferous bornite, and argentifer-ous chalcocite (Blanchard 1968). These have spatialdistributions that precisely match the pedogeochem-ically anomalous zones, suggesting that the zonationis controlled by the primary hypogene mineralogy(Stanley 1984).

The sulfide zonation (galena, chalcopyrite, argen-tiferous bornite, and argentiferous chalcocite,observed sequentially:from SSW to NNE) is identi-cal to that in zonatiOn:'models for many otherstratabound copper deposits (Brown 1971, Annels1974, Hayes 1984), and can be attributed to a stableequilibrium assemblag€ of these sulfides across arange of Eh and pH conditions (Ilayes 1984).

OXIDATION OF A STRATABOUND COPPER-SILVER PROSPECT, MONTANA

+.: : : : : :f f i ,t : 'r tsAs &

(wr.%)

2t6 TIIE CANADIAN MINERALOGIST

Frc. 5. Van der Veer diagram1952) of mineralprospect. Four periods ofminerals are recognized.diagenetic cubic pyrite, 2)replacement of pyrite bysulfide phases Galena,cocite) under relativelyOX, middle-diagenetic

IGD)

t

2orffiD)

i Va i 4

ormx) (80)

zot{tNo GSTlOlls0lDs

coPPm)rooFn NsLvB>o.7Fm, [t{IEAD )d'PPLI A

Fro, 4. Soil-grid concentratiotrs of Cu, Pb and Ag over theDaisy Creek prospect. The threshold values wereselected from probability plots and demonstrate theoverlapping relationships of anomalous areas.

Paragenetic relationships of the sulfides, asobserved in polished sections of float-rock samples,demonstrate that cubes of early diagenetic pyritewere replaced progressively by galena and chal-copyrite, bornite, and chalcocite (Fig. 5). This sug-gests that the metals migrated from north to southinto a reduced zone within the quartzites, where theyprecipitated as sulfides during early-middle diagen-esis (Stanley 1984).

PRoDUC"TS oF OXIDATIoN

formed sulfides, producing barite andcupriferous goethite, and 4) supergene oxidationof tle sulfide mlnerals andproducts. Abbreviations: PY

diagenetic oxidation-GN galena, CP

chalcopyrite, BN bornite, CC BAbarite, GOcupriferous goethite, AC CH chrysocolla, CK

limonite, AG nativechalcanthite, MA malachite,silver, HD hinsdalite, PM ite, and CE cerus-site. Limonite 4nnolated with two, or three stars

morphologies ofrefers to recognizablY dilimonite, which can be related to the original galena,chalcopyrite ortively.

mineralogy, respec-

SOI-. The oxidation that occurred atby the followingDaisy Creek can be

balanced equilibria:

& Van der Veerat the Daisy Creek

of economicare: l) ED, an early-

middlediageneticmore copper-richbornite and chal-

conditions, 3) MD-of these previously

* BaSOn

0.5H2O +

After sulfide deposition, two episodes of oxida- Ba2* + pbs + 26,, - Pb2+tion occurred. The first is characterized by the oxi-dation of galena, chalcopyrite and argentiferous bor-nite, foriing cupriferous goethit-e, barite and 9F9S., + 2B!+-1 a'7192acanthite. Evidence for the breakdown of chalcociteduring this episode of oxidation has not beenobserved.

Oxidation appears to have occurred sequentially,

Q+2 ;c )e - -2BaSOa+( l + + CuroFeO.OH

(8.75-y)Oz +AguoCu5FeSa + +0/2)B*+0.5H2O + (?-r-Y-2)e- -Cu

involving fustly Fd+ to Fd+, followed by Sz- to (5-r)Cu2+ + (l/2)AezS +H +

OXIDATION OF A STRATABOUND COPPER-SILVER PROSPECT' MONTANA

Evidence for tlese reactions can be seen in the tex-tures of sulfide and oxide phases in Figure 6. Chal-copyrite, after cubic pyrite, generally is rimmed byargentiferous bornite, and these sulfide aggregatesare oxidized to cupriferous goethite. Acanthite occursat the goethite-chalcopyrite replacement boundary,probably because Ag from the bornite migratedalong with the oxidation front and precipitated whenencountering 52- released during oxidation of thechalcopyrite. Barite commonly rims the cupriferousgoethite and fills cracks within the goethite createdby a volume loss during oxidation. These productsare considered to result from an oxidatiou eventcharacterized by relatively low CO2 (malachite isabsent) and high activity ofthe barium ion (the oxi-dation of zulfide liberated sulfate, which reacted withBa2+ and precipitated as barite), and whichoccurred after sulfide precipitation. Reactionproducts from this oxidation event are consideredto have formed when an oxidized groundwatermigrated into and reacted with the sulfide-bearingquartzites. These reaction products are mineralogi-cally, geochemically, and texturally distinct fromother products of oxidation.

A second, later oxidation event of supergeneaffinity, i.e., high (atmospheric) CO2 and low Ba2+activities (no barite is observed with limonite aftersulfides) is characterized by the development oflimoai1e, malachite, native silver, cerussite andpyromorphite (Fig. 5; Stanley 1984). Malachitegenerally occurs as flakes along bedding planes,defined by laminae of white mica within thechalcocite-bearing quartzites, and as coatings onfracture surfaces within the bornite-bearing quartz-ites. Native silver was observed in heavy-mineral con-centrates from soils from both of these zonm. Withinchalcopyrite-bearing quartzites, limonite andmalachite in a synplectitic intergrowth replacecupriferous goethite along fractures and are thechaxacteristic supergene phases. Minor chalcantliteand chrysocolla also occur sporadically within Cu-anomalous strata. Within the Pb-anomalous zone,cerussite and pyromorphite replace galena and fillcarbonate-vacated voids as exotic gossans withinfloat-rock samples.

The overlapping region of the Pb-anomalous zoneof the mineral occurrence and the P-anomalous partof the upper cross-bedded member (the lower 15 m)contains abundant cerussite and pyromorphite.These minerals generally ocsur as a replacement ofgalena, and as exotic gossans filling wgs formed bythe dissolution of a pre-existing carbonate cement.Gandolfi camera X-ray powderdiffraction patterns,scanning-electron photomicrographs (Fig. 7), andenergy-dispersion X-ray spectra (Fig. 8) of float-rocksamples indicate that hinsdalite @b,Sr)Al3POaSO4(OH)6, a rare supergene mineral, is also present.

Hinsdalite is known to occur at only a few otler

Frc. 6. SEM backscattered electron photogxaph ofcupriferous goethite (GO) replacing chalcopyrite (CP).Note the rim of acanthite surounding the replacementboundary, andttre barite @A) infi[ing oacks and rim-ming the cupriferous goettrite. The thin dark bandaround the goethite consists of goethite with a higherconcentration of Cu than the core, and is consideredoriginally to have been bornite that r€placed a chal-copyrite rim before oxidation took place. Scale bar zl0

ltfi.

localifies worldwide. These include: l) the Sylvesterand 4omet mines, Tasmania @irch 1977), where itrepla$es pyromorphite, 2) the Mineral Park mine,Arizoba (Wilkinson el a/. 1980), where it occurs withsuperlene chalcocite replacing hypogene sphalerite,3) thd Butte mine, Montana @oberts et al. L974r,

it occurs with supergene covellite replacingand 4) the Golden Fleece mine, near Hins-

dale, @alache et al. 1951), the type local-itv, it occurs abundantly within a quartz vein

barite, rhodoshrosite and sulfides.Creek, hinsdalite commonly lines vugs

the dissolution of carbonate cement withinthat contain the galena mineralization. It

also in a few samples coating and replacingand, less commonly, cerussite, both

of the supergene alteration of galena (Stan-). The hinedxlils occurs in clusters ofrhombohedra up to l0 micrometres longinto the open spaces of the wgs. Tabular,

Atleft t

ley

thatprisms of hinsdalite, an abundant form

2t8 TIIE CANADIAN MINERALOGIST

FIc. 7. Backscattered (A) and secondary (B) electron SEM photomicrographs ofddite (HD). Rhombohedral form is evident in mass of hinsdalite rimming voidin quartzite formed by dissolution of pre-existing carbonate cement. Scalf barl0 pm.

(Birch 1977). Energy-dispersion X-ray spectra (Fig.8) indicate that the hingdalils at Daisy Creek con-tains little or no Sr, a feature also observed for thehinsdalite at the Comet mine (Birch 1977).

OXIDATION OF A STRATABOI.IND COPPER-SILYER PROSPECT, MONTANA

Sampfe DC-7547

keV

Frc. 8, Energy-dispersion X-ray spectrum (175,@ counts) for hinsdalite from sam-ple DC-7547. No Sr peak is observable.

2t9

cupriferous goethite, acanthite and barite, and prob-ably occurred during middle diagenesis owing to theintroduction of an oxidizing brine into the sulfide-bearing strata. The second, related to supergeneprocesses, produced mainly limonite and malachitefrom the previously formed cupriferous goethite, aswell as native silver, cerussite and pyromorphite.

The occurrence of hinsdalite at the Daisy Creekprospect, Montana, and its similarity to other occur-rences worldwide, attest to its importance as a super-gene mineral that commonly replaces previouslyformed products of oxidation. At the Daisy Creekprospect, the occurrence of hinsdalite in the zone ofoverlapping anomalous concentrations of Pb andphosphate suggests that the major limiting controlon its precipitation may be the activity of phosphateand Pb species in meteoric waters. Finally, the typi-cally microscopic size of.hinsdalite at all localitiesleaves open the possibility that it is more cotnmonthan has previously been cousidered.

AcKNowLEDGElvrEt{Ts

This research is part of a larger project supportedby Anaconda Minerals Company, and a NaturalSciences and Engineering Research Council(NSERC) operating grant to Dr. A.J. Sinclair,University of British Columbia.

REFERENcES

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220

Srarlev, C.R. (1984): The Geologt and Geochemistryof the Daisy Creek Prospect, A StrataboundCopper-Silver Qccurrence in Western Montano.M.Sc. thesis, Univ. British Columbia, Vancouver,B.C.

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TIIE CANADIAN MINERALOGIST

WnsroN, D., [,eNce, LM.,D. (1977): Alluvial fan,base deposits of tle BeltMontana. Geol. Soc. Arner.,Field Trip Guide 5.

Received November 12,accepted July 31, 1986.

D. & Goor"ewsn,water, and zub'wave

near Missoula,Mountain &ct.

revised manuscript