American Mineralogist, Volume 59, pages 302-306, 1974

Hypogeneand SupergeneGobalt-Gopper SLlfides, CarrizalAlto, Atacama,Chile

AreN H. Crlnrc Departmentof GeologicalSciences, Queen's Uniuersity, Kingston, Ontario, Canada

Abstract

The supergene replacement of an intermediate member of the linnaeite-carrollite series by djurleite in the Carrizal Alto deposits has resulted in the development of microscopic inter- mediate zones consisting of essentially stoichiometric carrollite (Co,cuS+) and lesser amounts of a cobalt sulfide mineral. The latter, with the approximate composition CoonuS, exhibits physical properties and an X-ray powder pattern corresponding to synthetic F Cor-"S. It clearly formed at normal temperatures, despite the experimental evidence for the instability of the B-phasebelow ca 460"C.

Introduction oxidation, sensu stricto, are observedin the hand- but heterogenite and Microscopicexamination of a hand-specimenfrom specimen under discussion, (cf Clark, the dumps of the now-abandoned,but once highly cuprian heterogeniteof oxidation origin productive, Canizal Alto copper camp, Atacama 1970) havebeen confirmed as thin coatingson sev- Province,northern Chile (Lat. 28" 5.5'S; Long. 70' eral other ore specimensfrom the Carrizal Alto 55'W; Ruiz, 1965,pp.2tl-214), hasrevealed the dumps. occurrenceof an unusual associationof sulfide min- HyPogeneAssemblage erals whose compositionsessentially conform to the systemCo-Cu-S. The effectsof supergenealteration The hypogenevein material is almost monomin- of a cobalt-copper sulfide are well shown in this eralic, consistingof a sulfide of the linnaeite series specimen,which containsa phasesimilar or identical associated with very minor amounts of pyrite, to synthetic hexagonal (F) Cot ,S, perhaps cor- chalcopyrite,and bornite. The dominant constitnent respondingto the poorly-definedand generally dis- forms coarseintergrowths of blocky, subhedralgrains credited mineral, jaipurite (uiz, Palache, Berman, exhibiting an irregular, rectangularcleavage. Grains and Frondel,1944) . not in immediate contact with djurleite contain no The cobalt-rich minerals were emplaced as nar- oriented incluslons which could be consideredsug- row (1-12 mm) veins in altered dioritic country gestive of exsolution, and the sparseinclusions of rock, and are now preservedlargely as fragmentsin bornite and pyrite have irregular, bleb-like forms. late-stage calcite-ankerite vein material. The ore The linnaeite-typephase is completely isotropic, mineral assemblageis remarkably poor in iron and and exhibits a slightly pinkish-white color in plane- nickel, and is quite distinct from the pyrrhotite- polarized reflected light in air and oil; it takes an pyrite-chalcopyrite-arsenopyriteassociation which excellentpolish. Its Vickers micro-indentationhard- constituted much of the ore worked at this camp. ness,determined for 15 grains with a Leitz Duri- Its preciseprovenance in the mine is unknown. ment instrumentand a 100p load, shows(Table 1) Grains of the hypogeneore minerals are locally a restrictedrange from 390 to 425 (avenge, 405). cross-cut and mantled by "massive chalcocite," The reflectancesof six grains at the four standard which formed during an episodeof supergenesulfide wavelengths(Table 1) were determinedin air with enrichmentante-dating the local develoirmentof the a Vickers-Bnr digital microphotometer,equipped AtacamaPediplain in the Upper Miocene (Sillitoe, with a continuous-bandinterference filter and a Mortimer,and Clark, 1968;Clark et al,1967), and photomultiplier detector, and using the calibrated consistsentirely of djurleite. No effectsof supergene Carl Zeisssilicon carbide and tungstencarbide stan- 302 COBALT.COPPER SULFIDES, ATACAMA, CHILE 303

dards issued by the I.M.A. Commissionon Ore Tlsre l. Physical Properties of Carrollites A and B and Microscopy. No bireflectancewas 6Co'-"S 'weredetected and only minor reflectance difterences found between Refleclance (:) the grains {in air) examined(Table 1). 47om 546m 589m 65om

The foregoing physical properties are in general 4l.l- 43.2- 43.9- 45.3- 390- 40S (100p) agreementwith those reported for linnaeite-group 4r.5 43.3 44 5 45.7 425 carrollile B 40.1- 4r.5- 42.7- 43.2- 3?3 385 (100p) minerals,although quantitative values for reflectance 4r.o 42.O 43,2 43.7 387

and 34.8 36.0 34.0 30.8 hardnessdo not appear to have been recorded 218 (lop) for analyzedintermediate members of the linnaeite- 42.4 40.7 39.2 36.1 245 carrollite series.The reflectancedispersion curve of the Carrizal Alto mineral lies at lower valuesbut is determinationsof the cell parametersof analyzed similar in form to thosefound for siegeniteby Petruk, intermediatemembers of the linnaeite-carrollitesub- Harris, and Stewart (1969) and for polydymite by seriesappear to have been reported, but the deter- E. A. J. Burke (Uytenbogaardtand Burke, 1,969.\. mined parameter is significantly greater than that Previous records of the micro-indentationhardness reported for syntheticCozCuSr by Bouchard, Russo, of linnaeite-type minerals (Uytenbogaardt and andWald ( 1965;see below). Burke, 1969) show wide variationsfor each sub- For the purposeof discussion,this hypogenephase species,perhaps as a result of compositionalvaria- will hereinafterbe referred to as A", tions in the specimensexamined. "carrollite al- though it may be more preciselycharacterized as a Electron microprobe analysisconfirms the identity cobaltiancarrollite. of this mineral as a memberof the linnaeite-carrollite solid solutionseries, as definedby Tarr (1935) and Supergene Assemblage by Vokes (1967). Using as standardssyntheric The minor sup€rgene djurleite (confirmed by chalcocite (Cua6eS), covellite (Cul.nsS), metallic X-ray powder study) in the hand-specimenunder cobalt, and a natural cobaltianpentlandite (composi- discussiondeveloped largely through replacementof tion Co6.2Ni1.2Fe1.eSs)from Outokumpu,Finland, a carrollite A. Where grains of the hypogenesulfide composition of Co6.7eCuo.22S1.2,or (Co,Cu)353.62 are surroundedby djurleite, the mutual contactsare was determined(Table 2); only tracesof nickel and highly irregular and, in severalareas, these minerals iron are present.No variationsin Co:Cu ratio could are separatedby an intermediatephase. The latter be detected within individual grains or between is optically similar to the original carrollite, being grains. No indication of the presenceof a second isotropic and pinkish-whitein color, but is detectably phase intergrown with the thiospinel was detected darker in both air and oil. It forms rims up to 2.5 during the microprobe and optical study. Examina- mm in width, with microscopically-sharpcontacts tion using a Nomanki interference-contrastattach- against the djurleite and, probably, against carrol- ment revealedno zonal or irregular hardnesshetero- lite A. It everywherecontains small (<0.25 mm), geneities.The sulfur content is apparently distinctly and irregularly-vermicular inclusions of a second, lower than that required to satisfythe formula M3Sa, optically-distinct,anisotropic mineral. lending support to the suggestionsof Tarr (1935), Darnley and Killingworth (1962\, and Vokes Carrollite B (1967), amongothers, that somenatural linnaeites Microprobe analysis (Table 2) of the supergene are sulfur-deficient.The experimental evidence for isotropic mineral reveals it to have a composition discretesolid solution in the thiospinel phasein the systemCo-S is, however,ambiguous (Elliot, 1965), and there is apparentlyno information on the phase TlsI-r 2. Compositions of Carrollites A and B and pCo'-"S relationships in this region of the ternary system cocus Total Co-Cu-S. Mi neral weight pe! X-ray powder patterns of this mineral are of the cent carrolliie linnaeitetype, and a unit-cell edgeof 9.468(5) A A 39. O 99.4 was obtained,using an X-ray diffractometer (C\rKa carrollite B 3e.2 20.5 41.5 100.2 gCol-xS radiation;), = 1.5418 A), and metallic silicon 63.80 !0.24 99.85 (5.4306 A) as an internal standard.No previous brobably i.n associated carlollite. ALAN H. CLARK closelyapproaching stoichiometric CozCuS+, the gen- is lessreflectant and appreciablysofter than the en- erally acceptedformula for carrollite sensustricto; it closingcarrollite, and took a mediocrepolish. Aniso' will hereinbe termed"carrollite B." The anisotropic tropism is intense, with purplish-blue colors de- phaseis a cobalt sulfide (Table 2), apparentlyfree velopedin most grainsat the positionsof maximum of copperand other metals. illumination.A poor cleavageor parting in one di- Carrollite B is slightly softer and less reflectant rection is shown by severalgrains; no twinning is than carrolliteA (Table 1), the reflectancedisper- observed.The micro-indentationhardness (Table sion curve retainingthe form of that of other lin- 1) for this phase, determinedfrom four of the naeite-typeminerals. Solid solution of copper in larger grainsusing a lop load, is in the tange224- linnaeite is inferred to causesignificant decreases in 245. However, thesevalues should be regardedas reflectanceand micro-indentationhardness, at least approximate,and perhapsoverestimated, because of over the compositionalrange representedby the the highly irregular contactswith the enclosingcar- Car.izal Alto phases.This conclusionis in agreement rollite. Values for Ro' and R, were determinedfor with the data compiled for the thiospinel minerals six grains;the extremereadings are given in Table 1. by Vaughan,Burns, and Burns (197I), who inter- Electron microprobe analysis of several of the pret the trendsin physicalproperties in the linnaeite- small inclusions, using natural cobalt pentlandite carrollite seriesas resulting from increasingelectron and metallic cobalt as standards, shows that the occupancyof the anti-bonding,o*molecular orbitals mineral contains,at the most, only traces of copper with increasingcopper substitution. (max. 0.2 percent). Sincewavelength scans reveal In contrast to the hypogene phase, carrollite B an apparentenrichment in copper in the immediate showsno significantsulfur-deficiency. That naturally- vicinity of the contactswith the carrollite, it is con- occurring carrollite attains a copper-cobalt atomic cluded that the mineral is essentiallycopper-free' ratio of 1 :2 is implied by the work of Darnley and The almost identical cobalt and sulfur contents Killingworth (1962) and Richards (1965, in, Note- determinedfor three grains were averagedto yield baart and Yink, 1972). Vaughanet d (1971) sug- (Table 2) a compositionof Coo.seS(omitting the gestthat more copper-richcompositions cannot exist, Cu); the departurefrom stoichiometryshould be owingto the instabilityof the Cu(II) oxidationstate. regarded as an approximate estimate. However, it The X-ray powderpattern of carrolliteB is again is clear that this mineral containsless cobalt than of the linnaeitetype, and yields a unit-cell edgeof cobaltpentlandite, and more than linnaeite.The dis- 9.480(5) A (114.6 mm camera) in reasonable tinctly anisotropicnature of the phaseprecludes its agreementwith the valuereported for an unanalyzed mis-identification as either of those two isotropic specimenof this mineral by R. M. Thompson (rlr minerals.or ascattierite. Berry and Thompson, 1962). If the cell edge of On this basis,it was tentativelyinferred that the 9.435 A found for linnaeiteby L. G. Beny (in Berry Carizal Alto mineral correspondsto the fairly well- and Thompson,1962) correspondsto pure Co3Sa, defined syntheticphase, hexagonal Cor-,S (Lund- it would appear that there is a positive relationship qvist and Westgren,1938; Rosenqvist' 1954). Kuz- betweenunit-cell parameterand copper contentin netsov and co-workers(Elliot, 1965) confirm that the linnaeite-carrollitesub-series. This trend approxi- this "B-phase"possesses a NiAs+ype structure,and mately parallels that found by Bouchard et al a solid solution range, comparable to that of pyr- (1965) for syntheticthiospinels, but lies at signifi- rhotite, resulting from omissionof cobalt atoms. At cantly higher levels.This apparentdiscordance be- leastone superstructurc(B'), reflectingthe ordering tween the cell dimensionsof natural and svnthetic of lattice vacancies,has been reported (Kuznetsov phasesis unexplained. et al., JCDPS X-ray Powder Data File Card no. 19-366) for the composition "CoS1.6e7".X-ray Anisotrapic, Blue-Gray, Cobalt Sulfide powderinvestigation of the inclusions,using a 114.6 The anisotropicmineral occurring as inclusionsin mm camera,was hamperedby their small size and carrollite B exhibits a distinctly bluish-gray.color in irregular form, but severalsmall samplesdrilled from air, and is slightlydarker in oil-immersion.It is mod- the polished section, when combined, yielded a erately bireflectantin air and distinctly so in oil. powderpattern showing the major lines of carrollite Reflectionpleochroism, from mediumbluish-gray to with severaladditional reflections. The latter (Table whitish-gray,is faint in air and in oil. The mineral 3) correspondclosely with the most intenselines of COBALT-COPPER SULFIDES, ATACAMA, CHILE 305

BCol-,S (NiAs-structure)as reportedby Kuznetsov TeaI-r 3. Partial X-Ray Powder Data for Natural and et al. (JCPDS Card no. 19-365). Although several Synthetic Co_,S lines characteristicof the B/ superstructurewould col-xs, calriza] Alto 9co1-"s] B'col-xs2 presumablybe maskedby stronger carrollite reflec- (synrhetic) tions, the relativeintensities of the additional d(obs.)R rl aR r hkl aB r ilt lines, 5.40 2 IO2 and 4,50 2 103 the spacings,particularly of the two lowest- 3.20 4 2IL 2.94 25 2.911 20 100 2.59L 20 3OO,2O4 anglelines, would appearto rule out the presenceof 2.749 2 2I) 2.54 2.543 20 101 2.523 35 303,205 the modification. It is concluded,therefore, that the 2.254 4 2I5 2.195 4 305 anisotropicmineral representsthe proper. 1.950 t0o 1.945 100 ro2 I.931 too 00A,206 B-phase 1.454 4 4t2 Microprobe analysisof the djurleite enclosingthe I 792 2 413.405 I.690 t-694 t5 110 1.744 4 500 1.679 35 330 intermediate intergrowths described above show it 1.495 10 101 I.4!2 10 I12,2Ot to containonly tracesof cobalt (ca 0.2 wt percent), r.299 10 004 1.280 r.276 20 242 and that only within 50 ,t.mof the inner contact. I 200 5 211 additional 1.I89 l0 t04 r. tt8 5 203 1. ro8 5 2IO Discussion I 033 40 I. O€3 t0 2tr 1 030 55 tr4 In the specimenunder discussion,the supergene I.016 45 55 212 l"*"o -xPD -relarive replacementof an intermediatemember of the lin- aoru t9-,55, s!d 19-366; ihLensiries esrimred visual naeite-carrolliteseries has led to the developmentof a more copper-rich carrollite, approachingstoichio- metric Co:CuS+ in composition. The excesscobalt and no significant changes in H2/HrS ratio (or has been locally concentratedas a copper-free, or sulfur fugacity) were detected in low tempera- copper-poor,cobalt sulfide, with compositional and ture runs near this composition.At 400'C, Ros- crystallographicproperties which match those of the enqvist observedthe gradual breakdown of Co1-,S BCol-"S phase in the systemCo-S. The lack of sig- to CogSr in runs of up to a month's duration, an nificant compositionalvariations in theseminerals is adequate demonstration of the instability of the interpreted to indicate that, although they have B-phase at that temperature, although, earlier, formed at an intermediatestage of a supergene,sul- Lundqvist and Westgren(1938) had inferred that fide-enrichmentprocass-under conditions charac- the "CoSr-," phase exhibits only polymorphic terized by the development and preservation of changesat low temperatures. definitely metastable,and compositionally variable The formation of Cor-,S, rather than cobalt phases (cl Clark and Sillitoe, 1971)-they con- pentlanditeor linnaeite,at temperatureswell below stitute an assemblagestable under essentiallynor- 460"C in the wholly unmetamorphosedCarrizal mal temperaturesand at atmosphericpressure. The Alto deposit, is clearly in conflict with the phase sharp composition discontinuity between carrollite relationshipspresented by Rosenqvist.The small B and djurleite may indicate that the compositionof amounts of copper which may be present in the the former ropresentsthe copper-rich limit of the mineral would probably be insufficient to affect linnaeite series, at least at low temperatures(cf significantlythe stability relationshipsof the B-phase. Vaughanet al,197l). The possibility remains that the B-phaseagain be- The cobalt sulfide mineral in this assemblageis comes stable at some temperaturebelow 40O'C. consideredto representa natural analogueof the However,the evidencepresented here for the natural BCol-,Sphase. The originaldescription of the "CoS', occurrence of BCol ,S will require amplification mineral,jaipurite, by Mallet (1880, cited in Palache, beforethe mineralname jaipurite can be considered Berman, and Frondel, 1944, p. Za\ was sketchy, worthy of reassessment.Unless new studiesof the lacking adequateinformation on physicalproperties type-materialreveal the existenceof the BCol-,S and composition.As a consequence,jaipurite has phase,the Carrizal Alto cobalt sulfide should prob- been generallydiscredited as a natural species.This ably be consideredto be a new mineral species. caution has apparentlybeen justified by Rosenqvist's (1954) conclusionthat Co1_,S(his D-phase)is un- Acknowledgments stable in the system Co-S below ca 460oC. This phase Field work in Chile was carried out in cooperation with did not persist in experimental runs of the Instituto de Investigaciones Geol6gicas de Chile, and appropriatecomposition between 450"C and 400.C, supported by the National ResearchCouncil of Canada. 305 ALAN H. CLARK

References System of Mineralogy, 7th ed., vol. l, Elements, Sulfides, Sulfosalts, Oxides. John Wiley and Sons, New York. Btnnv, L. G., lxo R. M. TnoupsoN (1962) X-ray powder Pernux, W., D. C. Hrnnrs, exr J. M. Srewmr (1969) data lor ore min:erals: the Peacock Atlas. Geol. Soc. Langisite, a new mineral, and the rare minerals cobalt Am. Mem. t5,281pp. pentlandite, siegenite, parkerite and bravoite from the Bouor.lno, R. J., P. A. Russo, eNo A. W.lro (1965) Prepa- Langis mine, Cobalt-Gowganda area, Ontario. Can. ration and electrical properties of some thiospinels. Inorg. Mineral. 9, pt. 5, 597-616. Chem.4,685-688. RosrNqvrst, T. (1954) A thermodynamic study of the Crenx, A. H. (1970) Schulzenite, cuprian heterogenite, iron, cobalt, and nickel sulfides.J. Iron Steel Inst,176, from Mina Pabell6n, Copiap6, Chile. Mineral. Mag. 37, 37-54. 943-944. Rurz, C. FurrEn (1965) Geolosia y yacimientos metaliferos A. E. S. MeyEn, C. Monrrurn, R. H. Snrrroe, de Chile. Inst. Invest. Geol., Santiago, 305 pp. R. U. Coorr, ,rNo N; J. SNErrrNc (1957) Implications Srrrrroe, R. H., C. Monrrunn, lt.tp A. H. Crlnr (1968) of the isotopic ages of ignimbrite flows, southern Atacama A chronology of landform evolution and supergene al- Desert, Chile. N at ure, 215, 723-7 24. teration, southern Atacama Desert, Chile, Trans. Inst. lxo R. H. Snrrroe (1971) Cuprian galena solid Mining Metal. 77, Bl66-159. solutions, Zapallar mining district, Atacama, Chile. Amer. Tlnn, W. A. (1935) The linnaeite group of cobalt-nickel- Mineral. 56, 2142-2145. iron-copper sulfides. Amer. Mineral. 20, 69-80. DARNLEv,A. G., eNp P. J. KnuNcwoRTH (1962) Identifi- UyrBNsocernor, W., eNn E. A. J. Bunxn (1971) Tables lor cation of carrollite from Chibuluma by X-ray scanning Microscopic ldentification ol Ore Minerals. 2nd Rev. microanalyser. Trans. Inst. Mining Metal.72, 165-168. Ed., Elsevier, Amsterdam, 430 pp. (1971) Errror, R. P. (1965) Constitution ol Binary Alloys, lst VrucnlN, D. J., R. G. BunNs, lNo V. E. BunNs Supplement. McGraw-Hill, New York, 877 pp. Geochemistry and bonding of thiospinel minerals. Geo- chim. Cosmochim.Acta, 35, 365-38l. Luxoqvrsr, D., eNo A. WBsrcnnN (1938) Rijntgenun- Vorus, P. M. (1957) Linnaeite from the Precambrian tersuchung des Systems Co-S. Z. Anorg, Chem. 239, E5-88. Raipas Group of Finnmark, Norway. Mineral. Depta, 2, lt-25. Noreaernr, C. W., lNo B. W. Vnr (1972\ Ore minerals of the Zambian Copperbelt. Geol. Mijnb. 51, 337-345. Manuscript receioed,lune 28, 1973; accepted P.rrecue, C., H. BenuAN, AND C. FnoNosr (1944) The for publication,October 31, 1973.