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Home , Acanthite, Nickeline, Pearceite, Proustite, Pyrargyrite, Rammelsbergite

667

The CanndianM ineralo gi st Vol. 32,pp. 667-679(1994)

POLYMETALLICAg-Te-BEARING PARAGENESIS OFTHE CERRO NEGRO DISTRICT. FAMATINARANGE, LA RIOJA.ARGENTINA

ISIDOROB. SCHALAMUK aNpM. AMELIA V. LOGAN* CONICET(Consejo Nacianal de InvestiqacionesCienttficas y Tdmicas),Instituto de RecursosMinerales, UniversidadNacionnl de La Plata, calle 47 N" 522, 1940In Plata, Argentina

ABSTRACT

Mineralization in the FamatinaRange, La Rioj4 Argentina is related to silicic to intermediatemagmatism of l,ower to Middle Plioceneage. This metallogenicsubprovince includes 1) -bearingdeposits with a very rich and complex Ag-Pb-Zn (Cu-Ni-Co-Te-Sb) paragenesis,2) Cu-Au districts, and 3) Cu-Mo districts. Detailed microscopic and geochemicalstudies of the Peregrinadeposit, of the first group, revealedthe occurrenceof tellurides suchas altaite and native tellurium. The paragenesisconsists of gold silver-bearingminerals (,argyrodite, miargyrite, , , ,freibergite), Co-Ni (-,, ,), base-metalsulfides (,, , ) and gangueminerals (, , barite and ). Textural and structural evidenceindicates that mineralizationtook place during three stagesfrom solutionsunder epithermalconditions. During the first stage,base-metal were depositedin a gangueof siderite.During the secondstage, gold was depositedat tle beginning,followed by the main depositionof silver antimonidesand arsenides,and Ni-Co-bearing mineralsin a gangue of quartz and ferroan rhodochrosite.Studies of fluid inclusionsin sphaleriteindicate a temperatureof formation ranging from 265" to 190"C and fluids with a salinity of 4.2 to 6.2 eq.VoNaCl. The third stageis characterizedby the depositionof scarce base-metalsulfides and Ni-Co-bearing minerals,and the main depositionof acanthite,silver, altaite,tellurium and marcasitein a gangueof calciteand quartz.

Keywords: silver, proustite,acanthite, freibergite, altaite, tellurium, Cerro Negro district, Peregrina,Famatina Range, Argentina.

Soruuann

La mindralisationdans la chainede Famatina,h La Rioja, en Argentine,est 1i6€A un magmatismefelsique i intermddiaire d'6ge pliocbne inf6rieur i inoyen. Cette province mdtallogdniquecomprend 1) des gisementsd'argent avec une paragenbse complexeet trbs riche en Ag-Pb-Zn-(Cu-Ni-Co-Te-Sb), 2) des districts enrichisen Cu-Au, et 3) d'autresenrichis en Cu-Mo. Nos 6tudespdtrographiques et g6ochimiquesddtailldes du gisementde Peregdn4 qui fait partie du premier groupe, r6vdlent la pr6sencede tellurures, dont altarle, et tellure natif, La paragenbsecontient de 1'or, des mindraux argentifbres (acanthite,argyrodite, miargyrite, pyrargyrite, proustite, st6phanite,freibergite), des min6raux i Ca-Ni (nickel-skutterudite, nickeline, rammelsbergite,safflodte), des sulfuresde m6tauxde base(sphal6rite, chalcopgite, pyrite, galbne)et des min6raux de gangue(sid6rite, quartz, et calcite).D'aprbs l'6vidence texturaleet structurale,la min6ralisationaurait eu lieu en trois stades,d partir de solutionssous conditions 6pitlermales. les sulires des m6tauxde basesont appamsau cours du premier stade,dans une ganguede siddrite.Au cours du second,I'or a d'abord 6t6 d6pos6,suivi par les antimoniureset les arsdniures d'argent, et par les min6rauxb Ni-Co dansune ganguede quaxtzet de rhodochrositeferrifbre. lrs inclusionsfluides dansla sphal6riteindiquent une temp6raturede formation entre 265 et 195'C, et une salinit6 enre 4.2 et 6.27ode NaCl (6.quivalent). Le troisiamestade est marqu6par la ddpositionde raressulfures de m6tauxde baseet de min6rauxtr Ni-Co, et de la plupart de l'acanthite,I'argent, l'altaite, le tellure et la marcasitedans une ganguede calcite et de quaxtz. (Traduit par la R6daction)

Mots-cl6s: argent, proustite, acanthite,freibergite, altai'te, tellure, district de Cerro Negro, Peregrina,chalne de Famatina, Argentine.

x Presentaddress: U.S. GeologicalSurvey, MS 959,National Center, Reston,Yigjna22092, U.S.A. 668 TIIE CANADIAN MINERALOGIST

.0 67?45' NCRUCIJAOA

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+ :.w.+ + + + t+ + +++ + ++ + ++ + i:+ + + /.+++ ' '; '-._ + * + * I%:: * J--- -\r- + + * +k * *'%."i1-,

Detriticol, cover.OUATERNARY / raun @." MogoteFormsliqn TERTIARY a Oro - .u B PotquioFormotion PERMIAN (Do, Elon Ef,Nuiorco Formotion0EVONIAN O# 74 *"nroPeinodoFormotion ORooVlC,o*

Ftc. 1. Geologicalsketch of the eastemside of the FamatinaRange showing the metallogenicsubprovince with Cu-Au, Cu-Mo andPb-Ag-Zn districts.Geology modified from variousregional geological studies: Bassi (1953), De AIba (1960, 1972)and Tumer (1962, 1,964, 197 1). Ag_Te MINERALZATION, ARGENTINA 669

INttooucrroN Although exploitation has been sporadic,and none of the mineshas been active in the last several Polymetallic and precious metal depositsoccur in decades.these areas are believed to still haveeconomic the FamatinaRange, province of La Rioja, Argentina potential.In the last few years,regional research as and define a polymetallic and monocyclicsub- well as detailed mapping, geophysicalstudies and province. Although the size of this metallogenic drilling of the Cu-Au and Cu-Mo mineralizations subprovinceis about200 km2,the majority of the min- have been completed (Marcos & Zarettini L981, eral concentrationsoccur in an areaof 20 km2 (Frg. l). M6ndez1981). Angelelli (1984),Schalamuk er a/. Three distinct zonesare recognized:1) the Pb-Ag-Zn (197'7),Bodenbender (1.913, 1.922) and Allchurch (Cu-Ni-Co-Te-Sb) depositsof the Caldera,Tigre and (1895) reportedinformation aboutthe Pb-Ag-Zn con- Cerro Negro districts (.a Viuda and Peregrinamines), centrationsin Famatina. which form a north-south-hendingband on tle east- em side of the FamatinaRange, 2) the Cu-Au districts ANaLvncaL Msrrrons known as La Mejicana, Los Bayos, Offir and Ampallado, located in the northwesternregion, Compositionsof altaite, tellurium, Pb(As,Sb) and the El Oro mine" in the southeasterncomer of the Te2.72-2.54,freibergite, sphaleriteand carbonateswere study areq and 3) the La EstrechuraCu-Mo district, determinedby the authorswith a CAMECA SX located in the northwesternregion. The regional 50 electron microprobe at the Comisi6n Nacional de zoning of Cu-Mo, Cu-Au and Pb-Ag-Zn Energia At6mica (CNEA) in BuenosAires. The ana- (Cu-Ni-Co-Te-Sb) assemblagesshows similarities lytical parametersare reported in footnotes to the to that shownby mineralizationin Colorado. Tables.Al1 analyseswere performedunder an acceler- The complexand interestingmineral paragenesisof ation voltage of 20 kV and a beamintensity of 20 nA, the Peregrinamine motivated this detailed mineralo- exceptfor the analysisof the Te-bearingminerals, in gical and geochemicalstudy, in which we soughtto which casethe beam intensity was 30 nA. In the identify the Te-bearingminerals. The veins in the analysesof altaite, native tellurium and freibergite, Peregrinamine that we studiedare locatedin the west- pure elementswere used as standardsfor Cu, Pb,7n, ern part of the Cerro Negro (Fig. 1). Bi, Te, Sb, Au and Ag, pyrite for S and Fe, and a syn- Some of the argentiferousdeposits were mined by thetic As-Se compoundfor As and Se. The following natives,then (1780-1810)by Spaniards(aic Jesuits), standardswere used in the analysesof sphaleriteand and also sporadicallyin the late 1800sand early carbonates:MnTiO2 for Mn, MgCO3for Mg, FeS2for 1900s.The major mining activity was concentratedin S and Fe, CaCO, for Ca, synthetic ZnS for Zn, and the Cerro Negro district, where the bonanzascould synthetic CdS for Cd. CO2 was calculatedby dif- contain 1G-12kg of silver per tonne.In the Peregrina ference.The following incorporatedprograms were mine, the was recoveredfrom open-castworkings used: CALIB (Calibration of standards),DECLAR and galleriesdown to 300 meters. (Declarationof analyticalconditions) and QUANTI

TABLE 1. RESIJLTS OFFTECTRON-MICROPROBEAI.IALYSBS OFALTAITE

,ilt%o To q.73 39.33 39.49 39.t3 39.U 39.47 3957 40.19 Pb 58.32 58.03 58.97 59.82 57.97 58.43 5E.32 60.r0 sb 0.30 0.?5 0.36 0.30 0.33 o2l 't oz7 Au 0.r2 As 0.trt 0.08 0.06 Se 0,19 0.04 0.08 0.19

Sum 9.69 nS3 9t.t6 9,33 n2A 9E.19 9E,8 1m.81

al% Te 5Lt3 5222 sLJg 5rA, 52.n S2.m 5Ll3 51.62 Pb 6,94 47.43 47,63 4f,23 6& 47.{t {tA 4754 sb 0.40 o.49 0,41 0,47 029 056 0.36 AI 0.r0 "t: Ag - 0.13 0.(B Se 0.39 0.(B o,rE 0.40

f.i&s ur€d (aralyzitrgrystals): ToLc (IJF), PbLc gJF), Sfc (PgD, Sbtc eED, tuIr (I19, A8,c egO, SeJ(cfiJF), d AsIa (iF). 670 TTIE CANADIAN MINERALOGIST

(Quantitativeanalysis). Linares 1975)is representedin the areaby the Mogote Data on trace elementsin sphaleriteand galena rhyodacite-dacitecomplex (Fig. 1). Theserocks gen- were obtained by neutron-activationanalyses by Dr. erally have a granophyrictexture madeup of large I.M. Cohenat the Comisi6nNacional de Energia phenocrystsof plagioclase, qvafiz and biotite in a At6mica, Ezeiza, Argentina. In order to avoid inter- microgranulargroundmass with tie samemineralo- ferencesproduced by some nuclides presentin the gical compositionas the phenocrysts,plus abundant matrix (6etZn znd65zn),samples were irradiatedfor a K-feldspar.At depth,the rhyodacitegrades into a gra- very short period of time to identify most of the ele- nodiorite in the areaof La Estrechuraand Ampallado ments. Irradiations were performedin the RA-3 (Marcos & Zanettini 1981).In many areas,intense atomic reactor at the Comisi6n Nacional de Energia hydrothermalalteration has been superimposedon the At6mica, Ezeiza, Argentina, with a flow of primary assemblages. l0l3 n/cm2.s,using thermal or epithermalneutrons Structurally, the Famatina Range is characterized accordingto the nuclide to be analyzed.Pure elements by the presenceof stepfaults and by large north-south were usedas materialsof reference.In somecases. the boundaryfaults. The altitude of the fault blocks samplesto be irradiatedwere coatedwith cadmium,in increasestoward the west. Their uplift persistedat order to absorbor minimize the flow of thermal neu- least into the Lower Pliocene(McBride 1972).T\e trons where a flow of epithermalneutrons was used. structureis the result of different tectonic movements; The measurementsof the sampleswere obtainedwith however, the FamatinaRange acquiredits present a Ge(Li) detectorof 27 cm3,with a resolutionof appearanceduring the AndeanOrogeny. The intrusion 3.2 keV f.or 1332keV, correspondingto the second of the Mogote rhyodacite-dacitecomplex took place peak of @Co,connected to a Packardmodel 46 multi- betweenAndean MovementsII and III (Marcos & channel analyzer,with 4096 channels.The time of Zanetnni 1981).The mineralizationin the Famatina decayrequired for the measurementsranged between Rangemay have occurredduring the earliestphase of a few minutes(In and Cd) and 24 hours (Tl). AndeanMovement III. Thesedeposits were formed Microthermometricstudies on sphaleritewere per- late in the orogeniccycle, during the subduction formed with a freezing-heating CHADOvIECA stage phase,in a compressionalenvironment in the back-arc (-192o to 600'C) at the Universityof Canariasby basin of the Tertiary magmaticarc. The veins were Dr. J. MangasViffuelas. The calibration curye was formed in secondaryfractures after the formation of obtainedby measuringphase-changes in solid stan- reversefaults, during tensionalrelief (V. Ramos,pers. dards (melting point) at atmosphericpressure. In the comm.). In the study area,east-west-striking fractures heatingprocess between +25o and 600'C, various coincide with the main mineralized structuresof La Merck synthetic standardswere used. Standardinclu- Mejicana,Offir and Montey (Cu-Au), and Caldera, sionsof CO2(-56.6'C) and water (0'C) wereused to Tigre and Cero Negro (Pb-Ag-Zn). The veins dip calibrate the freezing intervals. Salinities were calcu- between40o and 70o north and south.Northwest- lated from tablesof freezing-pointdepression. striking faults with variable dips (30-80') control the Temperaturesof homogenizationwere obtainedby Los Bayoszone of mineralizationand, to someextent, progressiveheating of the sample. the Caldera(Pb-Ag-Zn) and El Oro (Au-Cu) districts.

GsoLocrceLSsrm.rc Mnrnal ParecENEsIs

The sody area consistsof low-grade metamorphic Hydr othermal alt eration and sedimentaryrocks intruded by Devonianand Pliocene igneous rocks. The Negro Peinado Hydrothermal alteration has affected the different Formation, the best representedlithological unit, is a lithological units with varying degreesof intensity. In thick N-S-striking marine metasedimentarysequence the region, many major types of alterationwere identi- composedof siltstoneand subordinateshale and sand- fied by Marcos(1975) and Mdndez(1981), including stone, which locally show contact metamorphism. propylitic, argillic, alunitic, sericitic, potassicand sili- Regional metamorphismand deformation, related to cic. The last one is the most extensiveand affects all the CaledonianOrogeny, occurred during the Lower the units. Basedon the type of hydrotherrnalalteration Ordovician(480 + 15 Ma, Toselli 1978).The beds and the assemblages(ore and gangue),we show a slight schistosifyand a high degreeof silicifi- considerin this study that the Cu-Au and Pb--Ag-Zn catio:r. Biotite granite and hornblendegranodiorite of districts can be grouped into two types of envi- the Nuflorcobatholith (310 + 15 Ma, De Alba 1972) ronment.These correspond to the acid sulfateand adu- intruded the Ordovician metasedimentaryrocks. laria-sericite types of Heald et al. (1987). The La Permiancontinental sediments (Patquia Formation) Mejicana, Los Bayos and Offir Cu-Au deposits, overlie unconformablythe Negro PeinadoFormation. whosemost commonlyoccurring minerals are enar- The Lower to Middle Pliocenemagmatism (5.9 * gite, , chalcopyriteand pyrite in a gangueof 0.2 Ma, McBride 19721'5.7 t 0.2 Ma, Stipanicic& quartz and alunite, belong to the first type. The Ag_Te MINERALZATION, ARGENTINA 671 advancedargillic alterationin thesedistricts is charac- freibergite, and silver-bearingminerals like acanthite, terizedby the presenceofhypogene alunite, which in argyrodite and silver. Silver antimonidesand sulf- someareas is the dominantmineral. Chlorite is absent. arsenidesinclude miargyrite, pyrargyrite, Another characteristicfeature is the commonpresence -,proustite-pyrargyrite, stephanite and of free gold and gold-bearingminerals like freibergite. The Ni- and Co-bearingminerals include andgoldfieldite (Losada-Calder6n & Bloom 1990). nickel-skutterudite,nickeline, rammelsbergite and The adularia-sericitetype of depositis represented safflorite.The Te-bearingminerals are altaite,native by the La Viuda, Caldera,Cerro Negro and Tigre tellurium and an unknownarsenotelluride. The gmgue argentiferousdisfficts. These contain a large number mineralsare siderite,ferroan rhodochrosite,quartz, of speciesincluding: pyrite, chalcopyrite,sphalerite, bariteand calcite. For purposesofdiscussion, the min- wurtzite,galena, covellite, digenite, nickeline, nickel- eralshave beendivided into the following composi- skutterudite,rammelsbergite, safflorite, freibergite, tional groupswithout regard to abundanceor order of pyrargyrite, proustite, miargyrite, stephanite, deposition:(l) Ag-bearingminerals and gold, pearceite-polybasite,argyrodite, barite, acanthite, (2) Ni{o-bearing minerals,(3) Te-bearingminerals, native silver (identified microscopicallyand chemi- (4) other minerals and (5) gangueminerals. cally by Schalamuket al. 1977),altaite, narive Theseare described in the sectionsthat follow. tellurium and an unknown arsenotelluride(identified microscopicallyand by electron-microprobeanalysis Silver-bearingminerals and gold in this study) in a gangueof siderite, ferroan rhodochrosite,calcite and quartz.The sericitic alter- Gold occursas small grainsof 10-15 pm in size ation surroundsthe silicified zonesnear the veins. The included in quartz of the secondstage of rnnerubza- potassicalteration, essentially defined by the biotite - tion, and associatedwith the Te-bearingminerals in K-feldspar- quartzassemblage, is not lithologically the third stage. controlled and is superimposedon porphyry as well as Prousrtte appearstogether with pyrargpite, native granite and shale.The silicification occurredin at least silver and acanthite,and forms one of the main com- two stages.Propylitization is confinedto a localized ponentsof the Cerro Negro mineralization.It is domi- bandalong the easternboundary of the zone. nant over pyrargyrite. It occurs as small granular red aggegatesup to 1.5 cm in diameter.Microscopically, Mineralization it appearsas xenomorphicgrains that replacegalena, and also in veinletsthat cross-cutand replacefreiberg- The argentiferousdistricts of Famatina(Cerro ite, galena,siderite and, rarely, sphalerite (Fig.2A). It Negro, Calderaand Tigre) are typical epithermal may be intergrown with polybasiteand stephanite. deposits.In the caseof CerroNegro, the mineraliza- Lamellar twinning is common.A chemicalanalysis tion occursin veins.Ore shootsare associated with the revealsthe following composition:64.0 wt.VoAg, developmentof breccias.The host rocks consist of 13.370As, 18.87oS andtrace of Sb (Schalamuket al. Ordovicianshale. The main vein in the Peregrinamine 1977\. srikes N30'W anddips 70oNE,has a lengthof 350 m Pyrargyrite occursas irregular grainsin association and an averagethickness of 0.75 m. A brecciastruc- with proustite in prismatic and scalenohedralcrystals ture is common,consisting of angularand elongate 2-3 cm long. Veinlets 5 mm in width cross-cutprefer- clastsof light to dark grey shale,occasionally with entially sphaleriteand siderite.Pyrargyrite also fills fine banding, from 1 to 5 cm in length. Fine-grained cavitiesin associationwith pearceite-polybasite, sideriteforms the cementand enclosesthe sulfides. native silver and .The internal reflectionsare Cavities I to 3 cm in diameterare common:the silver- not very intense. bearingminerals occur in the largestones. The ore Acanthite is relatively abundant;two generations occursin two different forms: l) fine-grainedveinlets aredistinguished. The first one follows the deposition with siderite,pyrite, sphaleriteand galena,and of silver-bearingsulfosalts and replacesgalena, 2) coarse-grainedveinlets with ferroan rhodochrosite sphalerite,proustite and pyrargyrite,forming cusp- and silver-bearingminerals. Open-space-fi lling tex- and-cariestextures and occasionallyintergrowths turesare common,indicating a shallowenvironment (Fig. 2A). It is characterizedby its greenishgrey color of emplacement.They occurin two forms: l) as vugs in immersionoil, low hardness,poor polish and low and cavities,which are interpretedas incomplete anisotropy.A later generationappears in open spaces open-spacefilling, and 2) as crustifications,which and wgs in sideriteas elongatecrystals of up to 2 mm were formed owing to changesin the compositionof in length. The absenceof transformationtwinning and the mineralizing solutions.The earlier-formedcrystals the isotropy in many sectorsindicate that it was areenclosed by the later.Symmetrical banding also is formed at a temperaturebelow 179'C (Ramdohr present. 1980).Inclusions of nickeline were identified bv The hypogenemineralization is representedby reflectedlight microscopy. pyrite, chalcopyrite,marcasite, sphalerite, galena, ,Silyeroccurs as grains or platesup to 1.5 mm 672 TI{E CANADIAN MINERALOGIST

Ftc. 2. Photomicrographsof someof the ore minerals of the Peregrinamine showing: A) proustite (Prt), acanthite(Ac) and galena(Gn), B) native silver (Ag) and galena(cn), C) freibergite @r), stephanite(St), chalcopyrite(Cp) and galena(Gn), D) nickeline (Nic), rammelsbergrte(Ram) and galena(Gn), E) rammelsbergrte(Ram) and pararammetsbergite(Pram), and D altaite (Alt), galena(cn), pyrite (Py) and PbAsTe2(As). PhotomicrogaphsA, C, D, E and F taken in oil immersion, B takenin air. Crossednicols were usedin E.

across.In the first soge of mineralization,it occursin to be hypogene,although where it appearsas a thin associationwith ferroan rhodochrosite,galena, spha- film in associationwith quartzand limonite, it is lerite and acanthite(Fig. 2B). In the secondstage, it probably secondary. appearsmore commonly as teethup to 2 mm in width In general,a memberof thepearceite-polybasite in drusy siderite, and as dendritic masses.In most seriesappears as anhedralgrains that replace galena, cases,it displaysa cusp-and-cariestexture and as symplectitic intergowthswith galen4 and as vein- replacesacanthite. Bodenbender (1913) considered it lets. In openspace, it occursin vugs as tabularcrystals Ag_Te MINERALIZATION, ARGENTINA 673 up to 2 cm with the characteristicstriations and perfect freibergite. It is also found in associationwith (001) .Red internal reflections are abundant. pararammelsbergite(Fig. 2E). The textural evidence It has a poor polish, is etchedby a light source,and a indicatesthat there are at least two types of rammels- number of characteristicdots are formed. An X-ray- bergite.The youngerone is colloform, whereasthe diffraction QGD) analysisperformed by the powder older one replacesgalena and acanthite.In somesec- method with Cu cathodeand Ni filter by Schalamuk tions, polysynthetic twinning can be recognizedwith et al. (1977) showedthat the main lines are compar- crossednicols andprevious etching with nitric acid. able to a sampleof Aspen, Colorado(PDF 8-130), Nickelineis scarceand appearsas small compact indicating, therefore,that the mineral found in the massesin associationwith silver-bearingminerals. Peregrinamine correspondsto the arsenoanend- Usually it is enclosedin rammelsbergiteor safflorite member of the series (pearceite).A semiquantitative and displaysan atoll texture (Fig. 2D). spectrographicanalysis (Schalamuk et al. 1977) Nickel-skutteruditeis very rare and occurs as showedtraces of Ni, Ge, Sn and Pb. isolatedgrains up to 70 pm acrossenclosed in sideritic Freibergite occursas individual grains or as gangue.The zoning, which consistsof an increasein anhedralaggregates as well as in veinlets.It is charac- the Ni contenttoward the centerof the ,as terized by a grey color with an olive hue, excep- determinedspectrographically by Schalamukel a/. tionally with red internal reflections.It displayscusp- (1977), is also recognizableby fine differencesin and-cariestextures with galena.It is replacedby hardness. polybasite and locally by other silver-bearingminer- Sffiorite occursas very fine grains (smallerthan als. An elebtron-microprobeanalysis performed in the 60 pm). The characteristicstar-shaped twinning context of this study [CuKcl (analyzing : LiF), causedby an intergrowth of triplets is very common. AsZcr (IAP), SbLcr,SKcr, Aelo @ET), PbKu,ZnKa, It is generally associatedwith sphalerite,although it FeKo, Bi,La (-iF)l indicates the following composi- appearsalss as 6 rim lining smallgrains of nickeline. tion (averageof 15 analyses):16.28 wt.Vo Cu, 6.1.87o 4s,20.94VoSb, l3.62Vo5,28.917o Ag, l.46%oZn, Te-bearingminerals 6.377o Fe and6.20Vo Pb. Miargyrite appearseither a) in associationwith Altaite (PbTe), though rare, was identified micro. proustite and pyrargyrite, forming symplectitic inter- scopically by )(RD and electron-microprobeanalyses growths or as fine grainsup to 150 pm across,or b) in in some samplesof the Peregrinamine. It forms veinlets cross-cuttingferroan rhodochrosite.In oil anhedralgrains in aggregatesin associationand inter- immenion, it shows strong anisotropyand rare inter- grown with galenaand pynte (Fig. 2D. In oil immer- nal reflections. sion, it showsa white color and strongreflectivity, Stephaniteis scarceand appearsas irregular aggre- galena-likecleavage and isotropy.The associated gates.It usually forms intergrowthswith polybasite, galenadisplays cusp-and-caries texture and showsa proustite,pyrargyrite, argyrodite,freibergite and violet-grey color where in contactwith altaite. galena(Fig. 2C). Electron-microprobestudies of altaite performedby Argyrodite seemsintergrown with pearceite- the authorsduring this study (Table 1) indicate that it polybasite,in close associationwith stephanite, has a compositionvarying from 39.35to 39.84wt.Vo proustite,galena and, rarely, sphalerite,in grainsup to Te andfrom 58.03to 57.077oPb (PbTe1.o,- PbTe1.1). 60 pm in size.Argyrodite was determinedmicroscop! The most commonimFurity is Sb, with concentrations cally and by )(RD (Schalamuket al. 1977).A spectro- from 0.21 to 0.40Vo.These concentrations increase graphic analysisindicated more than 1000 ppm Ge togetherwith enrichmentin Te. Gold was found in and 100ppm Sn. The presenceof Ge and the very low quantitiesup to 0.127oand silver, up to 0.l9%o.Ge, Fe, concentrationof Sn iridicatethat the mineral in the Ni and Bi were also soughtbut not found. Peregrinamine correspondsto the Ge-rich memberof Tellurium, rare in natureand identified in only a the argyrodite(AgrGeS6) - canfieldite(AgsSnS6) few depositssuch as Cripple Creek, Colorado series(Schalamuk et al.1977\. (Guilbet & Park 1986) and Musariu in the Metaliferi Mountains,Romania (Berbeleac & David 1982),was Co-Ni-b earing minerals identified in the Peregrinamine during this study as blebs a few micrometersin size. They are located Rammelsbergiteis abundantamong the Ni-Co- along the rims of the altaite crystals; their identifica- bearing minerals. Microscopically, it shows different tion is only possiblethrough secondaryelectron forms, such as sheaf-like groups that enclose and imagesin scanningelectron microscopy (Fig. 3). replacenickeline (Fig. 2D) as a consequenceof an Electron-microprobeanalyses performed by the increaseof As in the solutions,colloform aggregates, authors(Table 2) revealedthe following composition: and skeletalcrystals in siderite. This arsenidealso 88.47to 97.24wt.Vo Te,9.l9 to 4.1.2VoPb, 0.47 to appearsas fringes or crowns that surroundgalena, l.95VoS and0.65 to 0.707oSb. sphalerite,prou$tite-pyrargyrite, polybasite and In this associationof Te-bearingmineralso an 674 T}IE CANADIAN MINERAIOGIST

Othersulfide minerals

Two types of sphalerite, coresponding to the fust and secondstages of mineralization,appear in grains up to 6 mm, in placesas rhombododecahedra;one is light, and the other is dark. Wurtzite appearsin coarser-grainedaggregates, usually zonedand with concentricstructures of the "schalenblende"type (Ramdohr 1980),formed by altematingsphalerite and wurtzite. Occasionally,the latter appearsas radiating acicularaggregates in concentricbands. The two habits may form a colloform fabric togetherwith mar- casite,pyrite andgalena. As a consequenceof crystal- lization from a gel, fine rhythmic structureswere formed. Sphaleriteshows yellowish brown internal ftc. 3. Scanningelectron micrograph showing blebs of tel- lurium (Te) in altaite (Alt) Secondaryelectron image, reflectionsand displayscusp-and-caries texture in scalebar 10pm. galena,being cross-cut by silversulfosalts and veinlets of freibergite. Electron-microprobeanalyses performed by the authorsallow the identification of two different kinds of sphalerite(Table 3). Type-I sphaleritehas a higher unknownmineral occurs as irregular aggregatesof contentof Fe andMn (2.58to 6.49mole 7oFeS" 0.06 20 pm in size that replacealtaite, displayingcusp-and- to 0.61 mole 7oMnS), whereasthe cadmiumconcen- cariestextures (Fig. 2F). It is characterizedby low tration is lower (0.24 to 0.59 mole VoCdS). Type-II reflectivity, strong (grey to greyish blue) sphaleriteis characterizedby a low concentrationof andstrong anisotropy (greyish brown to greyishblue;. Fe andMn (0.68-1.1 8 mole7o FeS" 0.09-{. 12 moleVo Electron-microprobeanalyses indicate that its formula MnS) and an enrichmentin Cd (0.74-1.03 mole Vo rangesfrom Pb(As,Sb)1.1Ter.22to Pb(As,Sb)r.osT%.s+. CdS) with respectto type I. The trend (Fig. a) suggests The Sb contentvaries from 0.25 up to 0.37 wt.%o, that whereasthe ore-forming solutions were progres- whereasthe Seconcentration reaches 0.167o. The gold sively depletedin Fe and Mn, they were becoming contentis ashigh as0.537o, whereas the silvercontent enrichedin Cd. Falling valuesofFeS in sphaleriteare is lessthan 0.085Vo. interpretedby Scott& Kissin (1973)as a consequence of an increaseof in the system.Neutron-activa- tion analysesindicate that the Ga and In contentsare similar in both types of sphalerite.Type-I sphalerite tends to have generallyhigher contentsof As and T1, whereasthe concentrationsof Ge are higher in type-Il sphalerite(Table 4). Fluid inclusionsin sphaleriteof TABLE 2. RESULTS OF ELBCTRON-MICROPROBE ANALYSES OF NATIYE TEIIURIUTTI tlte secondstage are composedof two phases,vapor and liquid. Neither daughtercrystals nor CO2(g)are No. present.The temperatureof homogenizationdeter- vL% Te 96.66 9724 8.41 95.91 Ib 259 4.t2 9.19 3,3't s o.47 1.95 031 sb o.73 0.70 0.65 0,66 AU As 0.u2 0.04 0.@ 0.15 Se a.u2 032 TSG 3. B&TS OF EIJCRON-MIq{oROBE ANALYSES OF SPHALM

Srm tos.t7 rv231 10028 1m.75 II fr% 8.?o h g8 6a65 6180 65.40 64J0 6:10 66.30 9532 Fe l.?! 1J0 4fi 0.6t 0.?l 0.64 0.46 97.fi 9491 K2. Un 035 O40 0.04 06 0m OG 0O6 Pb 1.61 2.8 5J1 L6 cd 0.40 035 0.16 059 0,49 ojl (16l s l.E4 7J8 t.23 s 33.m 33.10 n3A 3,.70 3210 9,9 Qq sb o.7l 0;12 0.66 0.@ 1m30 9&40 9,9 950 9&94 99.tr 9E13 Au - 0.q2 s@ AS o.q, 0.05 0.03 o.rE ffi% - a8 95:t2 95.94 v32l t7B n.q 9&19 fi32 Se 0.(B 0J1 FoS 2s8 2-91 6"49 l& l.l8 096 (168 tfts 052 0.61 016 0.12 0.11 o09 0.09 cds 0J9 0s4 024 l.o3 034 (L?6 0.90 Lin€s lsed (dslyring .ryrtals): TeJa (LiD, Pbrc (IlF), (PE'D, (LiF), Src @ET), SUa (PED, Auta C,in, Asad SeKc Il-& lYpe-I sdulsitq tr4-Ir': Ttle,tr splalaftq d ArIs (U9. Lto6 !s.d (@lyrtog qels): z!trd (uF), Fbra G.@, l&fa CJF)' cdra (PEI) ed strq (PBD. Ag_Te MINERALZATION, ARGENTINA 67s FeS

a TypeJSphalerire . TypeJISphalerite

MnS cds 50 mole 70 Ftc.4. Compositionofthe differenttypes ofZnS.

mined during this study rangesbetween 190 and to be idiomorphicand is closelyrelated to silver- 265"C, and the salinity, between4.2 and 6.2 eq. Vo bearingminerals such as proustite,pyrargyrite, poly- NaCl. basiteand freibergite, with which it may form a sym- Pyrtte,thefrst-formed metallic compound, is abun- plectitic intergrowth. The third stageof galenaoccurs dant in only a few samples,occurring as fine aggre- associatedwith pyrite and silver, and is replacedby gatesor as isolatedgrains. In somecases, it is almost altaite. Neutron-activationdata on galenaof the Cerro completelyreplaced, and isolated; seemingly suspend- Negro district indicatethat type-I galenacontains ed remnantsof pyrite crystalsform atoll structuresor 2 850 ppm of silver, whereasthat of type II contains remain as skeletalcrystals in sphalerite,galena and up to 3 680 ppm (Table5). Type-II galenais chalcopyrite. characterizedby an enrichmentin Bi, As, Hg and Cd Galena appearsas irregular aggregatesin associa- (180,640, 300 and70 ppm,respectively) with respect tion with sphaleriteand other sulfides in a gangueof to type-I galena.An electron-microprobeanalysis siderite and ferroan rhodochrosite.Three different forms of galenawere identified and correspondto three distinct stagesof deposition.The first stage occursin associationwith more or lesscontempo- raneoussphalerite, and the secondone is replacedby TABLE 5. RFSIJLTS OF NEIITRON-AC'ITVAIION sphalerite-wurtzite.In the latter case,the galenatends ANALYSFS OF GALENA

II ppn AC 2850 1680 3680 sb 1650 tw 14& Bi a 180 140 Se 44dd A8 68 550 ffi -23 FFI Ga lu 8A 26 t4.3 zts l?.E t6J Ge -<1<1 ln 5.6 43 72 42 28 5.6 68 In -<1<1 Oo 74 X3 A 2.& 210 180 tj Hg 45 N 300 Bt-<<34< cd 28 55 70 A8 630 ll0 198 2t0 U5 190 M 179UtE438.66J Sn 2535

tl-[!: TrDe.l @lqib, tr4-mr Typo-tr Bdd.aib I1: Tne-I galena.ID-IB: Type-trgstena. 676 THE CANADIAN MINERALOGIST

TAELE 6. RESIJLIS OF ELBCTRON-MICROPROBE ANALYSES OF CARBONATES

*tL% FeO 4r.60 u59 46.& 4tJO 4,& 23.fi 26,fi n.n 23.3A 25.80 Ivtno 15.80 14.90 1230 r5.80 15.00 3Lm 28.80 26.fi 31.00 2.4 ivrgo 2.70 2,& rn - 6.10 5a 5.q) 0J0 16.&) Cao 0r0 0.110 0.30 3.m 2,80 0.80 0.60 0J0 6.00 42n @z 38.15 37m 39.10 38.40 3730 38.10 38.l|{) 38.lm 3820 50.40

Sum 98.45 9.60 9.74 98.90 9pi0 99.80 99JO 98J0 9,& 99.d)

mole Vo Fe@3 8,9 7t.tt TI23 8.93 71.47 3'i.25 4.n 45,ffi 3527 52.4 Itdnco3 26,o 23.E8 ?!.30 ?5.r2 u.M 50J7 47.14 U.23 6.13 4.8E WcO3 4.8 4.17 1.98 - 9.89 851 9.81 0,74 v.rl &O: 0.33 0.u 0,49 4.96 4.49 1.30 0.98 0.83 8.93 854

Sl-S5: Siderite.R1-R3: femoanftodochosito, A ak€rite. Linosused (@ab"zrng systals): Fsrc GIF), lvtnKc (iF), MgKc (PE'I)' andcarc OAP)'

(performedwith the sameequipment and analytical of a microbreccia,which is maidy composedof frag- conditions as the analysesof Te-bearingminerals) mentsof sphalerite,siderite, proustite and galena. indicatedthat Te is presentin galenaof the third stage only (1 400 ppm). Gangueminerak Chnlcopyriteformed later than pyrite. It presentsno specialcharacteristics, except for a weak anisotropy.It Two forms of Fe-Mn carbonateswere recognized occurs as alhedral grains surroundedor cross-cutby on the basesof habitand composition, an -richone galenaand sphalerite,and it replacespyrite. It also (siderite)and a -richone (ferroanrhodo- occurs in veinlets or intergrown with freibergite and chrosite). Siderite occurs as massesof larger crystals polybasite. than ferroanrhodochrosite, forming bandsof 2 cmit Marcasite is lesscommon than pyrite, with which it thickness.The crystalshave a light brown,honey-like is associatedin well-developedtooth-like aggregates. color where fresh, and are dark where lightly altered. A later generationof marcasitecomprises the cement In the "La Viuda" deposit,siderite forrns bandswith a

FeCO3

M9CO3 MnCO3 50 mole% Ftc. 5. Compositionofthe carbonategangue minerals. Ag_Te MINERALZATION, ARGENTINA 677 fibrous texture. Ferroanrhodochrosite occurs as Quartz tendsto occur in fine veinletsin association rosette-likemasses with grains up to 2 mm, with a with fibrous sideriteor as fine crystalslining cavities. very light pinkisfubrown color. It may appearalone or Calcite is rare and occurs as fine aggregateswith with minor sulfides. The veins of the easternpart of the silver minerals. the Cerro Negro are richer in carbonatesthan those in the Peregrinamine. Electron-microprobestudies of PARAcEN:ETIcSpeupxcr thesecarbonates indicate that sideritecontains 41.646.8 wt.Io FeO. 12.3 to 15.87aMnO. and less The diverse , the geochemicalrelation- than 2.7VoMgO. In contrast,the rhodochrositecon- ships and the textural and structuralcharacteristics of tains only 25.8 to 26.5 wt.VoFeO, higher MnO the Pb-Ag-Zn (Cu-Ni{o-Te-Sb) suggestthat concentrations(26.6 to 3l.2%o),and MgO contentsup the mineralizationtook place in at leastthree stages to 6.17o,whereas the Ca contentis in generalless than Grg. 6).The first stageis characterizedby the deposi- l%oCaO (Table 6). In terms of FeCOr-MnCOr- tion of sphalerite,pyrite, chalcopyriteand siderite.The MgCO, (Fig. 5), two populationsof carbonatescan be main depositionof the Ag- and Ni{o-bearing miner- clearly distinguished.Ankerite occurs as crystalsof a als occurredduring the secondstage, with some con- few pm in size; the textural relationshipsindicate that tinuation into the third. The parageneticsequence of it crystallizedafter sideriteand ferroanrhodochrosite. the third stageconcludes with the depositionof Te. It Barite was identifiedin two forms. In the second should be noted that no zone of supergeneenrichment stage,it occursas platy crystalsfrom 0.5 to 1 mm in could be distinguished,and the zone of oxidationis length, andin the third stage,as smallergrains. insignificant.

Couclupntc REMARKS

PARAGENETICSEOUENCE The depositsof the easternside of the Famatina Rangeare epithermalin origin. On the basisof the I STAGE I tI STAGE rlll STAGE products of hydrothermal alteration and the ore-ganguemineral assemblages,three types of K. FELDSPAR E BIOIITE - depositsin this metallogenicsubprovince associated sERrctlE - with the intrusion of the rhyodacite-daciteporphyry 9IOERITE - complex were identified: porphyry Mo-Cu-Au RHODOCHROSIIE |l- - deposits,acid-sulfate-rype deposits (the Cu-Au dis- ANKERIIS I ll tricts) and adularia-sericitetype deposits(the OUARTZ - I r Pb-Ag-Zn mineralizations). I I l. Offir Cu-Au PYRIIE I The La Mejicana,Los Bayos and - l- CIIALCOPYRIT€ districts seemto be spatially and temporally relatedto I F SPHATERIIE - - shallow hydrothermalsystems. This closerelationship vluRlz|lE - with a magmaticsource is made evident by a high 6ALENA - - I fugacity of sulfur, indicatedby the extensiveacid sul- covELUrE fate alterationand the assemblageenargite - covellite DIGEtl| I TE - pynte. The interaction of the sulfuric acid solutions NICKELINE - NICKEL. SKUIIERUDTTE (formed from SO2in contactwith water) and the host

RAMMELSBER6ITE I a rock producedthe advancedargillic alterationasso- SAFFLORIIE - - ciated with the ore. The depositsin the La Mejicana, FREIBERGIIE Los Bayos and Offir districts are, therefore,interpret- PYRAR6YRIIE I ed as the near-surface,low-temperature equivalents of PROUS'IIE - porphyry coppersystems. MIAREYFITE In contrast, the Pb-Ag-Zn (Cu-Ni-Co-Te-Sb) SIEPHANIIE I PEARCEIIE. PO!YBASITE - mineralizationsof the Caldera,Tigre and Cerro Negto AR6YROOIIE t districts, characterizedby the presenceof sericitic and BABITE I potassicalteration, show a diverse mineralogy that is ACANTHIIE I --l very different from that of the acid-sulfate-type SILVER -l -l deposits,consisting of rare base-metalsulfides and ALTAITE abundantsilver-bearing sulfosalts and tellurium- TELLURIUM :l Pb - A3 l.t bearing minerals in a gangueof siderite and ferroan MARCASITE il rhodochrosite.The structural characteristicsof the CALCIIE area,such as the fault density,are an importantcontrol on the extent of the mineralization. The structurally Ftc. 6. Parageneticsequence of minerals. enhancedpermeability allowed extensive circulation 678 TIIE CANADIAN MINERALOGIST of fluid in the host rocks. The K-feldspar- sericite ANGELELLT,V. (1984): YacimientosMetaltferos de la occurence indicates almost neutral pH conditions.In Repiblica Argentim. I y 1L Comisi6n de Investigaciones accordancewith Henley's ideas (in Heald et al. 1987) Cientificas,Buenos Aires, Argentina. about hydrothermallyactive systems,we believe that Bessr,H.G. (1953):Estudio Geol6gico-Econ6mico de la thesesilver-rich deposits with minor basemetals were mina "El Oro", departamentoChilecito, provincia de La probablyformed from relatively salinefluids (4.2 to Rioja. Anales IV. Dir. Nac. Geol. y Minerfa, Buenos 6.2 eq.Vo NaCl) with a low H2Scontent at a consider- Aires. able distancefrom the heatsource. which could lie at depth. An increasein the alkalinity as well as in the Brnnpmac, I. & DAvID,M. (1982):Native tellurium from oxygen fugacity of the argentiferousmineralizing Musariu, Brad Region, Metaliferi (Metalici) Mountains, solutionsis madeevident by the precipitationof abun- Romania.In :the State of the Art (G.C. dant native silver following acanthite. Amsfutzt, A. El Goresy, G. Frenzel, C. Kluth, G. Moh, The mineralogical assemblagesshow a three-stage A. Wauschkuhn& R.A. Zimmermann,eds.). Springer- Verlag,Berlin (283-295). temporal evolution of ttre compositionof the mineral- izing fluids. The first stageis characterizedby the BoorNspr,{onn,G. (1913):La formaci6nde plata met6hcay depositionof Cu, Zn and Pb sulfidesin a gangueof de los filones argentfferos en el Famatina. siderite.The sphaleriteis iron-rich. During the second (Comunicacionesmineras y mineral6gicas).Bol. Acad. stage,the depositionof gold and the main deposition Nac.de Cienciasde C6rdobal9.429440. of Ag-bearing- and Ni-Co-bearing mineralsin a gangueof ferroanrhodochrosite and quartztook place. (1922): El'Nevado de Famatina".Anales Minist, The sphaleriteformed at this stageshows enrichment deAgriculrura, Sec. Geol. y Minerfu,Buercs Aires 16(l). in Mn, Cd, Ge and As, and galena,in Bi, As, Hg and DB ALse, E. (1960):Descripci6n geol6gica de la hoja 16-d, Cd. StageIII is characterizedby the depositionof Chilecito,provincia de La Rioja. Dir. Nac. de Geol. y Te-bearingminerals in a gangueof calcite and quartz. Minerf,Buerns Aires, Bol.163. Only galenaof the third stageis enrichedin Te. Despitethe lack of l8O isotopicdata, we contendthat (1972):Sistema de Famatina. 1z Geologla Regional the mineralizing solutionsthat formed the Caldera, Argentina. Acad. Nac. de Ciencias de Cdrdoba l, Tigre and Cerro Negro depositshad an importantcom- 349-395. ponentof meteoricwater. From a lithotectonic point of view, the ore deposits GrrLBERr,J.M. & Penr, C.F., Jn. (1,986):The Geologyof on the easternside of the FamatinaRange seem to be OreDeposits. W.H. Freemanand Co., New York, N.Y. restricted to the back-arc basin of the Tertiary mag- Hr,alp, P.,Fomv, N.K. & Havsa, D.O. (1987):Comparative matic arcoand are directly relatedto the silicic to inter- anatomyof volcanic-hostedepithermal deposits: acid- mediatemagmatism of Plioceneage. These deposits sulfateand adularia-sericitetypes. Econ. Geol. 82, 1-26. formed in tensionalcracks that appearedfollowing the main stageof compression,late in the subduction LoSADA-CALDERON,A. & BLooM,M.S. (1990):Geology, phaseof the orogeniccycle. paragenesisand fluid inclusion studiesof high sulphida- tion epithermal/porphyrysystem, Nevados del Famatina AcrNowrplcevmNTs district, La Rioja province,Argentina. Pacific Rim 90 Congress,Australasian Institute of Mining and Metal- -464. The writers thank Dr. I.M. Cohen (CNEA, Iurgy (AUSM I M ), 457 Argentina) for his cooperationin the neutron-activa- (1975): tion analyses,Ing. T. Palacios,Dr. G. Vifla andMr. R. MARCos,O.R. Area de alteraci6nhidrotermal de los distritos mineros del sistemade Famatina,provincia de Gonzilez (CNEA, Argentina)for their assistancein La Rioja, Argentina.II Congresolbero-Americano de the electron-microprobeanalyses, Dr. Mangas Geol.Econ6mica II, 423445. Viiuelas for the fluid-inclusion study, Mrs. Elaine McGee (USGS,Reston) for her assistancein obtaining & Zerrrrwr, J.C.(1981): Geologiay Exploracidn scanningelectron micrographs,and Dr. Antonio del ProyectoNevados del Famatina. Ministerio de Arribas for use of the laboratoryfacilities of the Economla,Subsecretarla de Mineria, Servicio Minero Institute for Applied Geology, University of Nacional,Buenos Aires, Argentina. Salamanca,Spain. We also thank Robert F. Martin and two anonymousreviewers for providing useful McBruos,S.L. (1972):A Potassium-ArgonAge Investigation commentsthat improvedthe manuscript. of Igneousand MetamorphicRocks from Catamarcaand La Rioja provinces,Argentina. M.Sc. thesis,Queen's Univ., Kingston,Ontario. REFERENCFS MEror,z,V. (1981):Exploraci6n geol6gica del "Nevadode AIcHURCH, J. (1895): Expediciones Mineras. Memorias del Famattnt'. AnalesAcad. Nac. CienciasExactas F{sicas v Dpto. Nac. de Minerfa, Buenos Aires,20l-208. Nat.,Buenas Aires )O(XIII, 151-169. Ag-Te MINERALZATION, ARGENTINA 679

Raunoun, P. (1980): The Ore Minerals and their Tt'r.xrn, J.C.M. (1962):Estratigrafla del sectorcentml de la Intergrowths(2nd edition). Akademie-Verlag,Berlin. Sierra de Velazcoy la regi6n al oeste(La Rioja). Bol. Acad.Nac. Cienciasde C6rdoba43.5-54. Scuereuur, I., ANcelrllr, V. & Palacros,T. (1977): Mineralizaci6ndel distrito argendferoCerro Negro (mina - (1964): Descripci6n geol6gica de la Hoja 15c, "Peregrinay otms"), DepartamentoChilecito, La Rioja. Vinchina, provincia de La Rioja. Serv. Geol. Nac., Rev.Museo de I-a PlataN,103-1 18. BuenosAires I00.

Scorr, S.D.& Krssnr,S.A. (1973):Sphalerite composition in (1971):Descripci6n geol6gica de la Hoja 15d, the Zn-Fe-S systembelow 300oC.Econ. Geol.68, Famatina,provincia de La Rioja. Sery. Geol. Nac., 475-419. BuenosAires 126.

SrlpANrctc,P. & LTNARES,E. (1975):Catdlogo de edades radim6tricasdeterminadas para la RepriblicaArgentina. Asoc.Geol. Arg., Ser.B 3.

Tospt-u,G.H. (1978):Edad de la Formaci6nNegro Peinado, Siena de Famatina,l,a Rioja. Rev.Asoc. Geol. Arg. 33, ReceivedJanuary 22, 1993, revisedrnanuscript accepted 89. Janunry5, 1994.