C ana di an lul in e r al o gistVol. 16, pp. 9-15 (1978)

COLLOFORM AND FRAMBOIDAL PYRITE FROM THE CARIBOU DEPOSITNEW BRUNSWIGK*

T. T. CHENMineral Sciences Laboratories, CANMET, Department of Energy, Mines and Resources,

Ottawa, Ontario, KIA OGl

Arsrnecr

Pyrito masses from the Caribou deposit, NewBrunswick, commonly exhibit both colloform andframboidal textures which occur as one of thefollowing: (1) colloform bodies containing discreteinclusions of framboids; (2) colloform bodies andframboids occurring in different parts of the samepyrite mass; and (3) framboidal microcrysts whichappear to be related to the oonstituents of colloformbodies by gradational textural variations. These oc-currences suggest that the colloform and framboidaltexfures were formed by similar processes. Texturesintermediate between discrete framboids and dis-sominated euhedral pyrite crystals are a{so common.The presence of these intermediate textures, ancl theclose association of framboids and euhedral pyntecrystals, suggest that the framboids in this ore wereformed as chemical preoipitates.

SoMMAIRB

hs amas de pyrite du gisement Caribou (NouveauBrunswick) pr6sentent habituellement les texturescolloforme et framboidale que lnon retrouve sousl'une ou fautre des formes suivantes: (1) desamas colloformes contenatrt des inclusions discon-tinues de framboides; (2) ales amas oolloformes etframboides dans diff6rentes par.ties de la m6memasse de pyrite et (3) des micreristaux fram-boidaux qui semblent re1i6s aux 6l6ments constitutifsdes amas colloformes grdce i des transitions tex'turales. Ces exemples nous laissent supposer que lestextures colloforme et framboidale se sont form6esde fagon semblable. Ic d6veloppement habituel detextures interm6dia.ires entre les framboides dis-continus et les cristaux tliss6min6s idiomorphes depyrite ot l'association 6troite de cristaux de pyriteframboides et idiomolphes portent i croire que lesframboides de ce minerai ont 6t6 form6s par la pr6-cipitation chimique.

(Traduit par Ia R6daction)

INTRoDUcTIoN

Much has been written about pyrite framboidsand colloform textures (e.9. Love & Amstutz

*Minorals Research Program, Processing Contribu-tion No. 31.

1966; Roedder 1968; Kalliokoski & Cathles1969 ; Rickar d'J'97 A; Kalliokoski L97 4; Boctor etal. I976). Despite the common coexistence of thetwo textures, especially in stratiform massivesulfide depositso the incorporation of framboidsinto colloiorm structures (Rust 1935; Ramdohr1953; Papunen 1966) is apparently rare. -

The mineralogy of the Caribou stratiformmassive sulfide deposit, New Brunswick, is be-ing studied in detail, with the work directed to-wards improving metal recoveries from thesefine-grained sulfide ores. The Caribou sulfidescommonly exhibit colloform and framboidaltextures:

-of 101 polished sections etched with

1:1 HNOs, 66 were found to contain framboids,mostly in pyrite-rich ores. Inclusions of fram-boids in micro.colloform structures are common.

Cenmou DsPosrr

The Caribou deposit is about 50 km west ofBathurst in northern New Brunswick. Threemajor, en-echelon lenses of massive sulfidesform an almost continuous' stratigraphicallyconformable core zone enclosed within a foot-wall of graphitic schist and argillite, and a hang-ingwall of metavolcanic, muscovite schist rrhichare part of the Ordovician Tetagouche Group.The sulfide zone has a combined length of morethan 1500 m, and much of it is 25 to 30 m thick.Ore teserves are about 45 million tonnes of 4.5VoZn, L.7Vo Pb, O,47Vo C\, and 44 g Ag/t.

The sulfide zone is a steeply plunging syn-form. The hangingwall contact for the most partsharply abuts ihe massive sulfides, but there isalso some interfingering. In marked contrast,the abundance of sulfides decreases gpduallytoward the footwall graphitic schist. The matrixwithin this section of less-abundant sulfides hasbeen recognized by Roscoe (L97L) to consist pre-dominantly of metamorphosed tuff and argillite.

Sulfide foliations and micro-foldings are com-mon in specimens from this deposit @avis 1-972).Howevei some sulfides also shorr original tex-tures, such as colloform and framboidal textures.Framboidal pyrite had been reported previously

9

10 THB CANADIAN MINERALOGIST

Frc. 1. Colloform pyrite showing rhythmic bands of galena (grey) and chalcopyrite 0ight erey).Fto.?. Pyrite aggregate showing framboidal texture after etching with 1:l HNO3. Lower left corner uo-

otched.Fro. 3. Inclusions of framboids (Cha.ttanooga type) in euhedral pyrite crystals in a chloritis matrix.

Etched with 1:1 HNO8.Frc. 4. lramboidal aggregate showing overgrowt! zones resulting in coarser microcrysts. Etched with

1:1 HNOg.F1o..5.. 'luhgdrafl'

nVlte- gvstals showing reliot framboidal texture resulting from overgrowth. Matrix issphalerite Grey) and chlorite (dark). Etched with'l:l HNOg.

Frc. 6. Broken fragmerils of colloform pyrite, with bands of galena and non-sulfideq sqrtain,ing discreteinclusions of framboids in a matrix oi sphalerite (grey) ana chloritic material (dark). ntciea witnl:1 HNOg.

COLLOFORM AND FRAMBOIDAL PYRITE 1 1

Frc. 7. Reniform pyrite (with zones of galena, chalcopyrite, and a non-sulfide mineral) also showing fram-boidal texture (marked). Etched with 1:1 HNOg.

Frc. 8. Colloform and framboidal (marked, the whc{e zono on the right margin) textures occurring in dif-ferent parts of the same pyrite mass, possibly implying coeval and closely related formation mechan-isms for tle two textures. Etched with 1: 1 HNOg.

Frc.9. Colloform pyrite containing framboids and carbonate minetal (dark zones, dissolved). Enlargedfrom part of Figure 8.

Fro. 10. Colloform texture showing coars€r peripheral euhedral crystals, and also showing framboids(marked, upper right corner) within the colloform body. Etched witl 1:1 HNOa.

Frc. 11. Part of a colloform structure consisting of intergrowth of chalcopyrite (main light grey matrix)and pyrite (microcrystals). Colloform structure contains framboids (marked) and shows gradationaltexturil variations 6etween framboidal microcrysts and the constituents (1.e. pyrite microcrystals) ofthe colloform body. Carbonate (dark, dissolved), sphalerite (grey). Etched with 1:1 HNOg.

Frc. 12. The non-framboidal, extremely fine-gra.ined pyrite aggregates show subrounded gross outlinesand dusty appearanoes resembling those of framboids. Sphalerite (grey), silicates (dark).

L2 TTIE CANADIaN

by Roscoe (1971) and Davis (1972). Accordingto Davis, deformed pyrite framboids parallel thefoliation. The colloform bodies reported in thepresent study are usually in brecciated fragmentsin the ore.

Corr,oFonu eNp FneMsoDAL TEXTuRES

Pyrite is the principal constituent of the collo-form bodies, but bands of galena, chalcoppite,sphalerite, and silicates are also common (Fig.1). Some colloforrr bodies consist of extremelyfine-grained chalcopyrite and pyrite intergrowthswhich alternate with bands of silicates; othercolloform bodies consist of: (a) rhythmic bandsof sphalerite and chalcopyrite; (b) mapetitevirith discontinuous concentric bands of pyrite;(c) translucent gangue minerals with rhythmiczones either of chalcopyrite or of gangue min-erals. The colloform texture of some pwite isevident simply because of systematic viriationsin pyrite grain sizes. Coarser, euhedral pyritecrystals occur at the peripheries of some collo-fonn bodies. In a few cases, colloform bands areintemrpted by euhedral magnetite or arsenopy-rite inclusions, but there is no distortion of theenveloping bands.

Much of the pyrite in the ore zone at Caribouconsists of microscopically dusty aggregates inwhich framboids are visible after etching (Fig.2). Framboids commonly occur as aggregates inmatrix of sphalerite, silicates (chlorite?) and, lesscommonly, in chalcopyrite. Clusters and irreg-ular bands of framboids occur in some massivepyrite. Some disseminated pyrite microcrystals15-15 pm) have relict framboidal cores; otherframboids occur as discrete spheroids in a ma-trix of chlorite or within euhedral mapetite andpyrite @ig. 3).

Although framboids up to 9O p,m in diameterhave- been observed, most are 5 to 30 pm. Eachconsists of numerous approximately eQuidimen-sional polyhedral microcrysts, usually less than1 pm though some framboids have microcrystsup to 4 pm. The arrangement of tle microcrystswithin framboids varies from random, linear, orpolygonal to concentric. The interstices betweenmicrocrysts, seen after etching are filled withpydte. In the same polished sections, someframboids appear as discrete spheroids and haveordered internal arrangements similar to tleChattanooga type reported by Love & Amstutz(1969), whereas others have an out€r zone ofcoarsef, elongate microcrysts, corresponding tothe Plotz-Antachajra type (Love & Amstutz1969). The former type is more abundant. How-ever, gradational textural variation is cornmon

MINERALOGIST

between the two fires. Some framboids haveovergrowths of coarser microcrysts, broader in-ter-microcryst spaces (Fig. 4), or a brownishrim (seen after etching) around the outermostzone of microcrysts. Other framboids are grossly"euhedral" in outline, with a relict framboidaltexture (Frg. 5). The textures observed are, ingeneral, similar to those reported by Love &Amsturz (1966, L969), Kalliokoski & Cathles(L969), Dais (1972) and Ostwald & England(1e77).

The micro-colloform structures and fr"m-boids from the Caribou deposit commonly areassociated as one of the following: (1) colloformbodies containing discrete inclusions of fram-boids (Fig. 6); (2) colloform bodies and fram-boids occurring in different parts of the samepyrite mass (Figs. 7, 8, 9, 10); and (3) fram-boidal microcrysts which appear to be relatedto the constituents of colloform bodies by grada-tional textural variations (Fig. 11). The infer-ence from tlese occurrences is that the collo-form and framboidal textures were formed ap-proximately coevally by similar and closelyrelated processes.

DtscussroN

Pyrite framboids are common constituents inorganic-rich sediments that range from Precam-brian to Recent. The possible biogenic origin offramboids has been discussed by many writers;although there is evidence that some framboidsmay be fossils (Neves & Sullivan 1964; Honjoet al. 1965; Javor & Mountjoy L976), framboidalpyrite has also been formed by purely inorganicsynthesis @erner L969;Fanand 19701, Sunagawaet al. L97I; Sweeney & Kaplan 1973).

Electron probe analyses indicate that fram-boidal aggregates @e 46.7, S 53.5 and As 0.2wt. Vo) from the Caribou deposit have the sarnechemical composition as the adjacent coarsereuhedral pyrite crystals. I{owever, the originalcompositions of framboids were not necessarilyFeSr, and microcryst shapes do not necessarilyrepresent original forms, particularly since someof the ore shows effects of metamorphism(Roscoe 1971; Davis L972), The presence offramboidal inclusions in pyrite crystals or collo-form bodies might suggest that the ftamboidsare fossils (Figs. 3, 6). However, according toLove & Amstutz (1966), the internal axrange-ment of framboids is usually crystallographicand does not conform with the strusture of anyknown bacterial cell or colony. In addition, theinclusions may indicate only that the framboidsformed slightly earlier than the other pyrite tex-

COLLOFORM AND FRAMBOIDAL PYRITB

Fro. 13. Framboidal pyrite and disseminated pyrite crystals in a matrix of metasediment. The close as-sociation suggests tlat the fiamboidal microcrysts and the disseminated pyrite microcrystals wereforrned by the same proces6.

Frc. 14. Pyrite microcrystals showing textural variations from framboidal to non-framboidal aggrega-tions in a matrix of metasediment.

Fro. 15. Pyrite microcrystals showing textures from framboid-like aggregates to "euhedral" aggiregates, ina matrix of motasediment.

Frc. 16. Pyrite microcrystals showing non-framboidal aggregations in a matrix of metasediment.

Frc. 17. 'Euhedral' aggregates of pyrite with numerous sericite(?) 'oinclusions" in a matrix of metase-diment.

Frc. 18. Pyrite showing intermediate texture between frarntoids and euhedral crystals in a matrix of spha-lerite. Etched with 1:1 HNog.

13

L4 THE cANADIAN

tures, a case similar to that reported by Papunen(1966). The following textural relationships im-ply chemical precipitation: (1) in the ore zones,some extremely fine-grained pyrite aggregates,with subrounded gross outline and dusty ap-pearance, resemble those of framboids; how-ever, etching reveals that the aggregates arenon-framboidal, irregular grains (Fig. l2). (2)Nearly all micro-colloform structures in thisore are associated with framboids. In addition.some framboidal microcrysts and the constitu-ents of colloform bodies appear to be related bygradational textural variations (Fig. 11). (3)Disseminated euhedral pyrite crystals usuallyoccur within a few pm from discrete framboids;textures intermediate between these two firesare common. The intermediate textures suggestsimilar formation mechanisms.for (a) the fram-boidal microcrysts and the disseminated pyritemicrocrystals (Figs. 13, 1 ); (b) the framboid-like pyrite microcrystal aggregates and the"euhedral" aggregates of pyrite microcrystals(Fies. 15-17); (c) the framboid-rich layer andtho pyrite microcrystal-rich layer, commonlyrelated to each other by gradational texturalvariation; and (d) tle coarser euhedral pyritecrystals and the fine-grained pyrite microcrys-tals (Fig. 18).

Recrystallization of framboids might resultin tle formation of single euhedral pyrite crys-tals (Love & Amstutz 1966). In the Caribou de-posit, some disseminated euhedral pyrite micro-crystals (5-15 pm), which occur in thin lensesor lamellae, have relict framboidal cores. Otherframboids have overgrowths (or recrystalliza-tion?) textures distinguishable either by growthzones (seen after etching) around microcrysts(Fig. 4), or by a compact outer zone of coarserelongated microcrysts which form a "euhedral"gross outline (Fig. 5). The original textures (Figs.75,76,18) described above show loosely packedequidimensional microcrystal aggregates with"euhedral" gross outlines, both before and afteretching. The close association of disseminatedeuhedral pyrite crystals and framboids (Figs.15, t6, 18) indicates that these disseminated eu-hedral pyrite crystals are original textures. Con-sequently, textural relationships (1) to (3)above are considered to indicate that the fram-boids in this ore were formed by chemical preci-pitation. The framboids in colloform pyritemight be formed by colloidal precipitation @ust1935; Ramdohr 1953; Fapunen 1966). However,colloform and framboidal textures also crystal-lize from supersaturated solutions (Roedder1.968; Farrand, I97O).

Within a ferv millimeters of the framboidsand pyrite crystals in a chiloritic matrix are some

IVIINBRALOGIST

marcasite, needle-Iike rutile and ilmenite crys-tals. Marcasite is generally considered to bestable, and Ti to be mobile, only in acidic en-vironments, whereas framboidal pyrite is usual-ly synthesized in neutral or slightly alkaline en-vironments. Boctor et al. (L976) suggest that thecoexistence of marcasite and framboidal pyriteis probably due to non-equilibrium or local he-terogeneity of the chemical environment. Anindication that this type of environment mayhave existed for the Caribou ore is that nativeiron has been found With pyrite, magnetite andilmenite in a silicate matrix in the ore zone.

AcrNowr-eocEMENTs

The author wishes to thank Drs. L. J. Cabri,D. C. Harris, J. L. Jambor, W. Petruk, and Mr.S. Kaiman for valuable suggestions and dis-cussions. Dr. J. Kalliokoski (Michigan Tech.Univ.) read an earlier version of this manuscripr.

RmrnsNcss

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HoNro, S., FrscHrR, A. G. & GennrsoN, R. (1965):Geopetal pyrite in fine-erained limestones. J.Sed. Petrology 35, 480-488.

Jevo& B. J. & MouNr.rov, E. W. (1976): Late Pro-terozoic microbiota of the Miette Group, south-ern British Columbia. Geology 4, lLl-119.

KAr.r.roKosK, I. (t974): Pyrite framboid: animal,vegetable, or mineral? Geology 2, 26-n.

& Cersr,rs, L. (1969): Morphology, modeof formation, and dia€Bnetic changes in fram-boids. Bull, Geol. Soc. Finland 4L, 125-133.

Low' L. G. & Atusruz, G. C. (1966): Revtew ofmicroscopic pyrite from the Devonian Chattanoo-ga Shale and Ra:nmolsberg Banderz. Fortschr.Mineral. 43,273-309.

& - (1969): Framboidal pyrite in twoandesites. Neues Jahrb. Mineral. Monatsh. No. 3,97-108.

Nnws, R. & Swr,rvlll, H. J. (1964): Modification

COLLOFOR}I AND ERAIVTBOIDAL PYRTTB 15

of fossil spore exines associated with the presen@of pyrite crystals. Micropaleontology 10, M3-452.

OsrwAlo, J. & ENcr.exo, B. M. (1977): Notes onframboidal pyrito from Allandale, New SouthWales, Australiu Mineralium Deposita t2, tll-1 16.

PeruNrr, H. (1966): Framboidal texture of thepyritic layer found in a peat bog in SE-Finland.C.R, Soc. Geol. Finland, No. 38, Ll7-I25.

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RoBooER, E. (1968): The noncolloidal origin of

"colloform" textures in sphalerite otes. Econ.Geol,63, 45t41I.

RoscoB, W. E. (1971): Geology of the Caribou de-posit, Bathurst, New Brunswick, Can. l, EarthSci.8, 7125-1736.

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Received October 1976; revised manuscript acceptedAugust 1.977.