R~.~l)\COs·n ll~.LL.'" SOClETA IT..\LlAS.\ III ~tl"~_II.."'I.(Xa .... ~_ 1'~_TROI.OI:I."'. l"il·f, \',4..N. 1'1" M7_Mi

MINERALOGY AND ENVIRONMENTOF FORMATIO OF THE Cu-Pb-Zn(Ag,Sb,As)

MINERALIZATIONS I THE 'ICCIOLETA DEPOSIT

MARCO INNOCENTILab. di microscopia elcllronica c (li microanalisi. Nuovo Pignone S.p.A., Via Matreucd 2, ~0127 Firen?e

PIER FRANCO LATTANZI, GIUSEPPE TANELLIDipanimelllO cli Scicn2l" ddla Terra dell'Universita, Via La Pi"" 4. 50121 Fi~nze

R1ASSU/Oo'TO. - Nel giacimenlo a pirile nussiva diNicciolela (Toscana), gli stadi finali dd processominerogcnico, lcgati alrattivita idrolermale conncssacon l'evento Appenninico, sono caratterinati clalladeposizione di solfuri di Zn·Cu·Fe·Pb, cui scguonodivcrsi mincrali di As e Sb (arsenopiritc, Slibina,lollingile e numcrosi solfosali, tra i quali tctraedriteargl"nlifcral.

Nell.t scquenza paragcnelia, il primo minttaledepostO appare esseTe la sf.tletile, che: reca trw:ttdi deform.lllionc: mecanica si.t dunile chc: fragile.I minerali s~jvi appaiono eucmialmente nonddonnali. La composizione chimio (microan.lisiSEM/EDS) ddl'arscnopirill:: " dei 5OIfO$.llli e assaivariabile, a lestimonianza di ricorrenti fiuttualionidei parametri chimico-fisici dell'ambiellle di depo­sizione.

Sulla basc ddla successione paragenetka, diinformnioni lermometriche: ricavale da dali i~

lopid e inclusioni lIuide, nondK: dei campi di stabi·lita dei vari minttali, la Icndenza ~raIe ddl'evo­lurione dell'.tmbiente dallo sl.tdio • Zn-Cu·Fe·Pb aqudlo AlSb pub essere approssim:all cia una dimi·nuzione di tempenlllura e al1ivit.l dello :wl£o, e daun aumenlO dellt' al1ivila di arscnico e .tntimonio.

ABSTRACT. - Zn·Cu·Fe-Pb sulfidcs, followed byAs-Sb minCT1lls (arsenopyrite, stibnite, lollingite andscveral sulfosalts) were form"d in the massive pyritcdeposit of Nicciolela (Tuscany, Italy) during thehydrothermal activity related 10 the Apenninic:"'-"('fIt.

Mincral deposit'on began in In environmentchllracterized by some tectonic disturo:ance, asrecorded by defonmrion features in sphaletite.Latt'r minerals appear 10 be essentially undefonned.

Chemical parameters of the: environlll('nt weresubjt'Ct to fluctuations, :as demonstrated by wideand random compositional variations of mosl slUdiedminerals. The general evolution trend from theZn·Cu·Fe-Pb stage to the As-Sb slage can benolll't:\'er be reasooably appro:.o:im:ated by .t dccre:asc:of temperature and ICtivity of s., and increase ofwivity of As Ind Sb.

Introduction

Niccioletll (Gros5eto, Tuscany) is currendythe major productive centre of the southernTuscany pyrite district. Mining activity takesplace today in milssive bodies (hereafterreferred to as Niccioleta A), completelyenclosed within a quartzitic·phyllitic complex(Filladi di Boccheggiano formation = F.B.;Paleozoic·Triassic?). Early mining worksexploited pyritic bodies (Niccioleta B) at thecontact between the F.B. and the overlyingdolomitic Calcilre Cavernoso formation(:::::: c.c.; Upper Triassic).

The geology, mineralogy and geochemistryof the Niccioletll pyrite deposits have beenextensively studied in the last years (sum.mary and references in LATTANZI & TANELLI,1984). CurreOl theories on ore genesisinvolve a Paleozoic·Triassic (?) volcano­sedimentary genesis of large massive pyriticbodies, which were metamorphosed andremobilized during the Apenninic event (MidTertiary to recent). In particular, to theApenninic eveOl is ascribed the emplacementof minor uneconomic Fe·Cu·Pb-Zn(As,Ag,Sb) mineral conceOlrations, in dear discordantattitude with respect to massive pyrite. Withthe aim of better understanding theenvironmeOl of ore deposition during theApenninic event, a study of these concen·trations was undertaken, mainly focused onthe mechanical behaviour of sulfide minerals,and on the chemiatl and textural charactersof Fe·Pb-As·Sb phases.

M. INNOCENTI, P. LATfANZI, C. TANELLI

NICCIOLCTlI. •

11(;) 03501 .p,Pl.ccp,'1n,.py,PI>-.. ,bnn

TABLE 1,\linera[ as!ocia/ion 01 lh~ sllldi~d samples

.1'. PY .ccp.'1n. Cd,

·P.I"/,<><:p.p...·.D>7

.1'.1"/.<><:1'.'1'" t.<I, ...a ....._Pl • ........

.1'. l"/.ccp.'1~. Cd, Ln._

.1'. py ,ccp.'1n, td. Pb-.. , bnn

.1'. Pl ,ccp.'1", ta.a ,'"

(brecriationl and plastic (formation of sliplines and deformation twins) deformation.A moderale annealing even I has panly erasedthe deformation features (GItEGORIO et al.,1980). Microploln analyses indicate FeScontents between .9 and 8.4 7r moles, andsmalJ amounts (less than .5 'le weight) ofCu, Cd and Mn (GItEGOlttO er al., 1980).

Small amounrs of tiny euhedral pyritecrystals, clearly recognizable from the coarsecrystals of early massive pyrite, appear tohave been depositc-d aher sphalerite.

Chalcopyrile is diffuse, although neverabundant. ] t is found, sometimes associatedwith the above described pyrite, alongfractures or at the border of sphalerite grains.It panly replaces both the Fe· and the Zn­sulfide; the symptoms of the «chalcopyritedisease,. in sphalerite (BARTON, 1978) areohen very evident. Mechanic deformationand/or annealing textures in chalcopyrireare relatively rare (fig. 1).

Galena is similarly diffuse, but always insmall amounts. Little galena may have bet:ncoprecipitared with the earliest sphaleritegeneration, but mostly it is later than

lIeelOlnll. "

". .127

"" .-••• .no..h ·tU

II0b .-". .05

~ Ul07

Abbrf!lmlf/u"s: IIPI =: >ItSf!oopyriu:; bnn =: bourroo­nile; brl =: benhierite·like: mineral (UTTANZI ..TANELl.\, 1980); up =: chaloopyrile:; gn =: gale:na;kb =: kobdlite:; Iz =: luzoniu.:; mar =: marcasite:;Pb-ss =: sulfosahs of the Pbs-Sb.S. join; py "" pyri·le:; sp = sphalcritc:; sib = slibnile: Id = Sb-richtcrms of rhc: tCIt:lhc:drite:·tennamite series; In = A~

rich lerms of the te:trahedritc:·tcnnamitc serie:s;x = Pb-Cu·Fe-Sb-As mineral (sc:c: leXI).

Samples for this study (table 1) were selectedamong about one hundred of hlmd specimenscollected underground during repeated visitslO Nicciolela A. A few additional samplesfrom the nowadays inaccessible Niccio­leta B, were kindly supplied by the miningstaff. Mineral assemblages and textures wereexamined using slandard reflected light mi·croscopy, integl'1lted, for sphalerite, bytransmitted light observations on doublypolished thin plates. To bring om the featuresof mechanic deformation. sphalerile wasetched with HI 57';, and chalcopyrite wilha NH~OH/H~O~ mixture. Microanalyseswere performed on a Cambridge Stereoscan250 scanning eleclron microscope, equippei.1with a Link 860 energy-dispersive spectra­meter, in Ihe electron microscopy andmicroanalyses laboratory of Nuovo l'ignoneS.p.A., Firenze. Accelerating polential was20 kv. The following elements were de­termined: Cu, Ag. Zn, Fe, Cd, Mn, Co, Ni,As, Sb, Se and S. Pure elemems were usedas standards, except for S (pyrite) and Pb(galena, after determination of the peakprofile on metallic Pb). The raw data wereelaborated on·line via computer applying theprogram of STATIIAM &< JONES (1978). Theprecision of the measuremems was estimatedby repeating three times the analyses ofselected points. Many analyzed minerals wereextremely small in size, and contaminationfrom neighboring phases could not always~ avoided. Only l'.nalyses wilh no evidenceof comamination, lmd with sums in ";?- wt.between 98 and 102 were accepted.

Mineralogy

Sphalerile is bv far the most abundantamong Cu.Pb-Zn minerals al Niccioleta, andoften the only one recognizable by eye. Indoubly polished thin section, the stratigraphyof sphalerite appears to consist of three mainunits, A, Band C, characterized by color,Fe coment and presence of inclusions ofother minerals (LATfANZI, 1982). Fluidinclusion homogenization lemperatures (nolcorrected for pressure) are betw~n 225" and300" C for units Band C (BELKIN et al.,1983). All sphalerite units show both brittle

Suml)ling and experimental

MINERALOGY AND ENVIRONMENT OF FORMATION ETC. 659

00 0.1 0.1 CII 0.1 '.1

Fig. I. _ Phoromkrogrlllph showing detail of chako.pyd1C: after etching wilh NH,OH/HA; some Ircuof abundanr Imsalic twinning, and no majorJdormllion fcllurcs, Ire shown.

~.l •••

1.1 0••

..3 1.0

J.t 1.0

UI<n.E MIO.

....••....

TABLE 2

Minimum, maximum and average contents(wt%) 0/ additional elements in arsenopyrilc

from Niccioleta

sphalerite, pyrite and chalcopyrite, and itpartly replaces all these sul6des.

Following these minerals, a number ofAs-Sb·Fe-Cu·Pb(Bi,Ag) minerals wereprecipitated, In very limited amounts,accompanied by quanz<alcite (gypsum)gangue. They occur along fractures or althe borders of the previously formed Cu-Fe­·Pb-Zn sul6des, often with evidence ofreplacement. These minerals can be groupedas follows:

1) phases of th(; Fe·As·S system: arseno­pyrite, in tiny neat rhomb-shaped crystals,sometimes rimmed by lollingite (fig. 2), andmarcasite. Textural relationships indicatethat arsenopyrite does nor belong to anyequilibrium sulfur·bulfering assemblage. TheAs/S ratio is therefore not applicable forgeothermomeuy (KRETSCHMAR & $cOTT,1976), and, indeed, it varies widely andrandomly even in the same polished section(fig. 3). Microanal}ses reveal the presencein arsenopyrite of several additional elementsbeside Fe, As and S (table 2). The com­positional fields of arsenopyrite of Niccioletain the Fe-As-S uiangular diagram arecompared in fig. 4 with composirional fieldsof arsenopyrite from other deposits. SomeNiccioleta analyses appear to be slightly As·and/or Fe-rich. This however might be ananalytical artifact, arising in p:trticular, forAs, from the use or elemental As as standard(KRETSCHMAR & Scon, 1976). LOlIingitereplacing arsenopyrite contains up to 2.8wt'/O S;

- = below detttlion limits, ~tillllllcd III about0.5 wrQD. Co and Ni arc alv.'ay$ bldo...., deltttionlimils.

Fig. 2. _ BackscatlcreJ electron image of lollingite(10) replacing arsc-nopyritC' torn) and pyrilC' (p)') inquam. gangue (qt).

M. INNOCENTI, P. LATTANZI, C. TANELLI

2) phases of Ihe Fe-Sb·S syslem: stibniteand a benhierile-like mineral (LATTANZI &

TANELLI, 1980). The latter appears in somecases 10 replace pyrite and stibnite; in lurn,supergenic transformalion of ir into stibnite

"--

,.

and pyrite (as documented for benhierite byRAMOOHR, 1980) is also observed;

3) phases of rhe Cu.(Fe).Pb-Sb.{Bi).Ssyslem:

.,___ ..... Sb/S.SO

•Sb 'f."

,

,

,

".'"'"

(,~

,,'"

,

1"00.',.

--"'/5

INOIVIl)U"L AN"LYSES

____ AJ+Sb/S+Se

LS

j"'..

'0 __

0.: L-,~.<e:::::;=::;:=;:::::::;:=;:::::::;:=:;::::::;=::=-"-~~~-~~~..,.---,,INOIVlOUAL ANALYSES

Fig. 3. - Variations of As/S and rh+Sb/Sc:+S nllios in arscnopyrite, plOllOO in order of increasingSb content. Ill; sample NO; b); sample: NIOa.

MINERALOGY AND ENVIRONMENT OF FORMATION ETC. (>6,

,-"- \.

".--

~___ NCI-NlOo.N_... 5ou....... l_-I,;,

__ ...... ..ERGlUND. EKST-"-'

___~ _l1IIOIllMOlD.CVJlII. ,.,

_____ "'__lKlEMM.lIil651

S(oS.1

Jll 31 :n XI

••

.A,

..Fig. 4. - COmpo~1I10mll hclJ~ ulfrom lil('nlll~.

31 • ~ ~ XI :n....1· 51>1 .,."

~t5enopyme 01 N,":.. luk,.I. "'mp.II .....1

"

B

f<o(·DMIlOCI r_1

•~

~

" » N N "\lOll h ...>lller ar.cnopyme analyses

a} bournonite, CuPbSbSJ (fig. 5). It containssmall amounts of Fe, As and Se, and itssulfur content is somewhat lower thanrequired for stoichiometry (table 3);

b) a mineral tentatively identified, on thebasis of the optical properties and ofsemiquantitative microanalysis, as kobel·lite, Pb~(Bi,Sb)~Sl1;

cl an unidentified phase of approximatecomposition Pb.•.(Cu,Fe)...(As,SbhSe (ta­ble 3). It occurs as a thin veinlet cuningacross a pyrite crystal and its compositiondoes not appear to be affected byanalytical interference with the ironsulfide. Its qualitative optical propertiesin reRected light are: color (in oil) olivegrey, fairly similar to terrahedrite; bi­reflectance and anisotropy weak, with novivid colors; internal reflections nOtobserved;

d) several phases of the pseudobinary PbS­-Sb:!:Ss join (fig. 6). Many of them areapparently strictly ternary Pb-Sb-S phases,as they contain no other element, at leastabove detections limits (estimated atabout .5 wt7t.); the sulfur content issomewhat lower than required for

om

....

.,

Fig. ,. - BackscsuC'red e1~lron imagC' of •scmscyitc (sm) _ bournonitC' (bnn) • chalcopyritC'(up) • arsenopyrit(' (op)') assemblag(' within fncllll'C$of pyrite (p)').

M2 M. INNOCENTI, P. LAlTANZI, G. TANELLI

TABLE 3Microanalyses of bournoni/e and of an uniden/ifii!d mineral from Nicciole/a

~,= UlfIDKIrPlrIBD MIlI'ERAL

SAKPLI Nel' SAKPLI NSc SNU'LI NI0b'

Ii:l_nt ~.._. I!:l_nt ~. .-.

Itl_nt ~. .t... ••.. 4l.5 1.04 .. 42.4 l.0' .. 3S.' 2.15

" 12 •• l.05 O. 12.S l.04 " 11.5 2.27.. ••• 0.01 .. .. '"' 1.51

~ ~ '"' 0.12 ~ ... 0.17

" 25.5 1.0' .. 22.1 a." .. 19.5 1.U.. .. ••• 0.03 ..• 11.5 3.00 , 11.1 3.00 , 23.3 '.00

"". ".2 "". 98.0 "". 98.'

.. = IVt'f"<lgC ot ~ analy$a; ~ = average of J anah-....-s;10 9 aroms of 5ulfur; - = below dcleaion limiu,

= rclcrroo 10 J atoms of sulfur;t-srimared ar abour 0.' Ilo't %.

- = referred

6~ .

• Sl VtT It. un

~,

I I

". ,1I I I

- I I

• • • • .. • • • .. Q Q • • • .. • • • " n -, I I I I I I I I I I I I I I I I I I I ,1Ib,$. 0lI .. .. " " " • • • .. • • .. • • " • • • "~..--

Fig. 6. - Individual analyses (arrows) of Pb-$b sulfosahs of Niccioleta reported 0010 lhe PbS-Sb,S"pscudobinary join according 10 lheir Pb/Sb alomic ratio. and compared wirh narural and synlhetk phasesoa:urring in the same compositional intervttl. Mineral X is described by J"NKOVIC et al. (1977); ,11other phases are reported with lhe nomenclature and compositions given by CXAIG et al. (197J).

slOichiometry . Two analy:zed pointsrevealed presence of As (.5 wt'/f) and ofSe (.7 WI~) respectively; olher analysesshow appreciable amounts of Zn and Fe,which however arise probably fromcontamination; notably, Fe CQnlents (upto 3.1 WI%) are markedly higher than

experimental findings for similar phasesby BoRTNIKQV el al. (1982). Theseanalyses were nOI reponed in fig. 6. Theidentification of all the encounteredphases is often difficult because theanalYlical error is relatively large, due to[he panial overlap of the PbM.. and SK..

srl8NITl

~"'TE

8EATHlelllTE

TH"A"EDII'TETENNANTlTE

~.

MINERALOGY AND ENVIRONMENT OF FORMATION ETC.

. ,--------

•·••·•

._---------------------------------

663

Frs· 7. - Inferred p:il11l~·t:nelic scquena: in the 5ludied umples. B line! C 5upenaipu in sphUerilerefer to types described by UTTA"''ZI (1982).

radiations, the compositional limits, aswell as stahility fields, of many Pb.-Sb-Sphases are not clearly established (BORT­NIKOV et al., 1982) and their opticalproperties are seldom distinctive. Takinginto account these uncenainties, we havetentatively identified the following sulfo­salts: sample NCI: semseyilc (PbllSb8S~.);

sample .N5c: semseyite and boulangerite(theotetically Pb~Sb~SlI); sample N lOa:robinsonite (Pbe-.sbIlS~1 according toCRAIG et al., 1973; see however BoRT­NIKOV et al., 1982), dadsonite and boulan­gerite. Moreover we notice analyseswhich may correspond to the 4: mineralX " of jANKOVIC et al. (1977) and to thesynthetic «phlise IV» (CRAIG ~t al.,1973) the presence of which in natutehas so far never been reported;

t') phases of the telrahedrite-tennantiteseries, with Sb-rich members largelyprevailing. Their general fotmula can bewritten as (CU,Ag),O"'1 (Fe,znb .... (Sb,As).. .... SI), with x = .18, Y = .46, andz = .32. Ag content averages 2.2 wt7o,and may be as high as 11.2 wt"Jc (INNQ-

CENTI Cl al., 1984). Tennantite is oftensupergenically ahered to luzonile.

Evolution of the ore-forming environ·menl

It is nOt always a simple task to establishthe paragenetic sequence ol the studiedminerals, because representative mutual con·taCls are relatively rare. A possible sequenceis reported in 6g. 7. Chalcopyrite and,therefore, all minerals following it has beenconsidered essentially posl-deformation, be­cause it lacks of abundant deformationtextures, as opposed to sphalerile. Themechanic and annealing behaviour of thetwo sul6des are rather similar, thus anyannealing even' which might have erased alldeformation features in chalcopyrite wouldprobably have crased those in sphalerite aswell (STANTON & GORMAN, 1968; GILL,1969; CLARK & KELLY, 1973; ATKINSON,1974; KELLY & CLARK, 197.:5).

For descriptive purposes, we can distinguishtwO main slag(..~ of mineralization. The first(Cu.Pb-Zn stage) was characterized by pre-

M. INNOCENT', P. LATTANZI, G. TANELLI

-"

iJ

-,

-~

Fig. 8. - Activity of su]fur.(cmperature plol of sulfidation equilibria at low presSUfc pertaining to thestudied aSSl::mblagcs. The low·temperature portion of most curves reprnent extrapolation from hightemperature equilibria, thus ilS position is speculative. Field I dC'fines the' conditions of sphaleriteformalion as deduced from Fe COntenl and fluid inclusion <ura; fidd 2 is the inferred possible range forthe environmC'nl during deposition of As-$b minerals. The sources of data for computing sulfidttionequilibria are rakC'J1 from BAlTON ar SKINNER (1979). Reaclions involving robinsonile and boulangerirC'arc shown for composirions PboSb..S., and Pb.Sb.s" rcspeclivdy (CUtG et aI., 1973).Abbreviations: apy = :trsenopyrirC'; bIg = boulangerire; brl = berthierite; 1!n = galena; gin =graronile;h·pa = hexagonal pyrrhorire; la = loellingile; m·po = monoclinic pyrrhotil"; Py = pyrile: rbn = robinS().nhe; Jtb = stibnire.

epitation of Zn-Cu-Fe-Pb sulfides, tM: s«ondby the formation of Cu(Ag)..Fe(Zn}Pb-Sb­-As(Bi) minerals (As-Sb stage).

Pressure throughout both stases was mostprobably confined below 500 bars (TANELLI,

1977; LATTANZI e TANELLI, 1984).The as,-T environment during sphalerite

deposition can be defined (6g. 8) on thebasis of the previously reported FeS contentsand fluid indusion homogenization tempe­ratures. As pointed out by GREGORIO et a1.(1980), the sphalerile as,.T field lies entirelywithin the stability field of pyrite, indicatingthat the large pyrite bodies acted as an ef­fective sulfur buffer during sphalerite de­position.

Brittle and plastic deformation texturesof sphalerite suggest that deformation of thismineral occurred al temperatures around300" C and prelosure not exceeding .500 bars(the boundaries between brittle and plasticbehaviour of sphalerite: CURK 5< KELLY,

1973; ATKINSON, 1974).Thermal conditions during precipitation

of the following minerals are somewhatspeculative. Fluid inclusions in calcite fillingsphalerite fractures may show homogenizationtemperatures as high as 3700 C (BELKIN etaI., 1983), but a generalized high thermalregime is in controst with the preservationof mechanic deformation textures in sphale·rite, which is ver}' sensitive to annealing

MINERALOGY AND ENVIRONMENT OF FORMATION ETC. 665

Fig. 9. - Possible evolution path (IrroW) withrespect to activities of sulfur and Irsenic of theore-forming' environment from pyrite stlbility toformltion of IOllingile. 'The 6eld boundaries forthe V1Irious phases Ire calculaled It 281- C (BA.TON,1%9)~ It lower temperalures, their Ibsolute positionswill chlnge, bUI it is assumed that the generaltopology does nOI vary.

the highest value of Q5: to be formed), whereasit does not allow the formation of gratonite(which is, among the Pb-As sulfosalts, theone stable at the lowest value of Q51 atconstant temperature).

The final appearance of lollingite re­placing arsenopyrite marks 11 drop of a'l acrossthe sulfidation boundary llrsenopyrite +As/lollingite. Pyrrhotite is exceedingly rarein the Cu-Pb·Zn·As-Sb mineralizations ofNicdoleta, and, in particular, it does notoccur in the studied samples. The topologicalrelations in the system Fe-As-S as shown in6g. 9 indicate that, if aAIl in the environmentis high enough, it is possible the evolutionfrom the stability 6eld of pyrite to that oflollingite in absence of pyrrhotite.

From 6gs. 8 and 9 it appears that thegeneral trend of the chemiclll environmentthroughout the As-Sb stage may be ap-

,_2 -I

log .....

," ----, /PYRITE

" >

r\~>•m~

" ~<>•m..

"PYRRHOTITE 1/

.."..

The simultaneous occurrence of all con·ditions, considering the sulfidation reactionslimiting the stability fields of the phases ofinterest, suggests that Q51 values during theAs-Sb stage were mostly confined betweenthe reaction curves arsenopyrite/pyrite+As,and pyrite/monoclinic pyrrhotite (6g. 8).The inferred a"t-T field is indeed compatiblewith the stability of pyrite, arsenopyrite,berthierite, and rohinsonite (which is, amongthe Pb-Sb-S phases encountered at Nicdoleta,the one requiring, at constant temperature,

(STANTON & GoRMAN, 1968). For galena,deposition temperatures around 200" Careindicaled by isotopic fractionation in pyrite.galena and sphalerite-galena pairs (CoRTECCIet al., 1980, 1983). Isotopic and fluidinclusion temperature estimates of 160"­180" C are reported for late vug crystalsof celestite (CoRTECCI et al., 1983). Con­sidering that As-Sb minerals appear 10 belater than phases of the Cu·Ph-Zn stage, wetentatively assume that precipitation of theAs·Sb association took place essentially in :the range 150"-250" C. 1

The wide compositional range, even atsmall scale, observed in arsenopyrite, Pb·sulfosalts and tetrahedrite (lNNOCENTt et al.,1984), can be interpreted as: either mosaicequilibrium (KoRzmNsKu, 1959), reflectingsmall scale fluctuations of activities of com­ponents, or disequilibrium precipitation.

I t is possible to estimate the range ofpossible values of 0', during As·Sb stage onthe basis of the following assumptions andobservations;

equilibrium conditions were at least ap­proached :n the deposit scale;arsenopyrite, berthierite-like mineral, Ph­sulfosalts and marcasite are more or lesscontemporaneous (6g. 7);

the sul6dation equilibria pertammg toberthierite are applicable to the ber.thierite-like mineral;

ignoring the small free energy changefor the transformation marcasite·pyrite(about 1 Kcal/mol: GRONVOLD & \'(TES­

TRUM, 1976), lhe ocOJrrence of marcasiteindicates that also in the As-Sb stagesulfur activity values were confinedmostly within the stability field of pyrite.

M. INNOCENTI, P. LATTANZ1, G. TANELLI

proximate<! by a progressive decrease of os:.This in turn may have been [he consequenceof many factors; among them, a progressivedecrease of IOta1sulfur in ~olution, as it wasseparated by sulfide precipitation. In thisrespect, we rec:;1I that cotunnite (PbCbl hasbeen found at Niceioleta A in vug associationwith euhedral pyritc, sphllleritc and galen:;(LAlTANZI '" TANElLl, 1978). Cotunnite didnOt appear to be an alteration mineral, butthe last cryslallization producl of the sameenvironment from which Ihe sulfides wereprecipitaled.

Conclusions

As described by LAlTANZI '" TANELLI(1984), the Apenn.inic event at iceioletaproduced recrystallization and partial hydro­thcrmal mobilization of the pre-existingsedimentary-exhalative pyritic bodies, thenskarn formation and finally formation of theCu-Pb-Zn and As-Sb associations.

Apparently, thesc two last stages ofmineralization look place under similarconditions both at Niceioleta A and at Nic­doleta B. Sphaleritc (± galenal was Ihe 6rst

mineral 10 form, in a low pressure environ­ment characlerized by temperalUres between225" and 300" C, lmd by some dcformativelIction, probably related to the posl-tortoniandistensive tectonics. Subsequent formationof pyrile, chalcopyrite and galena look placeat temperatures down to about 200" C, inlIbsence of significant Icctonic phenomena.The following stage was characterized byinuoduction of As and Sb in the environ­ment, at temperalUre possibly down to150" C. Chemical activity of componentsmay have fluctuated widely even at smallscale, bur the general evolution trend canbe approximated by a drop of os:, SO that,in spite of the buffering capacity of the largepyritic bodies, formation of lollingitc wasevenlUally possible. A possible cause of as:drop may have been the decrease of totalsulfur content in Ihe environment.

Acknowl~dl~mmts. - The Nuovo Pigoooc S.p.A.in Fir~ . • 00 espcci:lllly Dr. Ing. DIONISIO PIARDIand Dr. M.o\RY BECAGLI, are grarefully acknowledgedfor allowing access 10 analytical facilities utili~ed

in Ihis sludy. Financial supporl was provided byC.N.R_ (unlm di mincraiotlia c gcochimic:a deiscdimcmi, Filcnzc) and M.P.I. (gram 10.02.1721,Univcrsili: di Fircnzc).

REFERENCES

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