Ti, the Faculty of The
The Geology of the Bajo El Durazno Porphyry Copper- Gold Prospect, Catamarca Province, Argentina
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Authors Allison, Antonia E.
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Link to Item http://hdl.handle.net/10150/249234 THE GEOLOGY OF THE BAJO EL DURAZNO PORPHYRY
COPPER-GOLD PROSPECT, r:ATAMARCA PROVINCE, ARGENTINA
by
A. E. ALLIEON
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DEPARTMENT OF GER=;`; :IENi_:ES
In F'ertiA.l Fulfillment of the Requirements For the Degree of
MASTER F Ss_IENCE
In the Graduate College
THE UNIVERSITY OF ARIZONA
1986 STATEMENT BY AUTHOR
This manuscript, prepared for publication in Economic Geology, has been submitted in partial fulfillment f requirements for an advanced degree at the University of Arizona and is deposited in the Antevs Reading Rool-ri to be made available to borrowers as are rnpiP=jf regular theses d dissertations.
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The Bajo El Durazno prospect is a small, gold -rich porphyry copper- type prospect located in Catamarca Province, northwest Argentina. It is one of a cluster of at least fourteen porphyry copper -type occurrences and numerous younger polymetallic epithermal veins, all of which are genetically related to the waning stages of magmatism that produced the Farallon Negro volcanic complex, an isolated Upper Miocene =hoshonit ii_ andesitic volcanic center.
Porphyry copper -type hydrothermal activity at the Bajo Ei Durazno prospect is associated with a small east -northeasterly elongated andesite porphyry stock.
The stock: was emplaced at 8.7 m.y. into comagmatic and petrologically similar andesitic volcanic breccias that form the highly dissected basal remnants of the main eruptive center of the volcanic complex. Intramineral, crudely radial ande-site porphyry dii : :es accompanied the development of concentric zones of hydrothermal alteration centered on the stock. These alteration Zones of potassium-silicate alteration in the stock and adjacent wallrocl : :s surrounded by an essentially coeval, weakly developed propylitic alteration zone. The propylitic alteration assemblage, which occurs as both pervasive replacement and as veinlets, consists mainly of chlorite, epidote, calcite, and magnetite, with lesser clays and zeolites. The potassium -silicate alteration zone is character iced by the replacement of primary minerals by secondary biotite, rtiagnetite, anhydrite, quartz, sericite, and calcite. Roughly coeval and coextensive with the earliest stages of potassium- silicate and propylitic alteration was a brief period of magnetite alteration conisisting mainly of well-banded magnetite + quart+ biotite veins. This volume also includes the development of irregular magnetite-rich masses in the stoci :: of probable late-magmatic origin. Major copper -gold mineralisation with rí inorsilver and molybdenum developed during later stages of potassium- silicate alteration after the magnetite alteration ev ent, although highest grade mineralization is commonly localized in areas of most intense magnetite alteration.
The bull:: of the mineralization occurs as veins within the stock and its (.4 ;lirocl::s near their mutual contact; these vPins cc nt Ain quartz, calcite, magnetite, pyrite, chaicopyrite, and lesser sericite, chlorite, orthoclase,=13sF, bic ititC, siderite, molybdenite, b rnite, sphelerite, galena, tFtrahedrite-tennentit _, and native gold.
Some i f the gold and silver occur in solid id solution in sulfide minerals, arad supergene enrichrilent of copper is not economically ¡' jlgnificant. Copper and gold grades are generally less than 0.4% and 1 ppm, respectively. The three early alteration assemblages were later overprinted by patchy arem of phyllie alteration consisting mainly of the assemblage sericite, quartz, pyrite, and anl-lydritelgypsum in an irregular northeasterly elongated halo. Phy llic alteration
is developed to its greatest extent in an irregular annular zone straddling the boundary between the potassium- silicate and propylitic alteration zones and i= generally coincident with the most highly fractured rocks in the prospect. Irregular patches of U+aeal:: to intense silicification are superimposed on all other alteration
types, and a number of distinctive, poorly mineralized, phyllically altered and
silicified fracture zones are distributed in a somewhat radial pAttPrn Around thP
stock. Low grade disseminated(?) gold mineralization is found over one square kilometer in phyllically and propylitically altered rocks surrounding the central
mineralized zone.
A fluid inclusion study has revealed the presence of two hypersaline liquid-rich fluid inclusion types having salinities of 73.0-87.0 and 50.0-79.5 weight
percent NaC1 + KCl equivalent, respectively, a single low salinity liquid-rich inclusion type (6.6-8.0 weight percent NaCl equivalent), and abundant vapor-rich
inclusions. Hematite, anhydrite, and a variety of unidentified opaque and
nonopaque minerals occur in many inclusions. Magnetite, pota=jiull'I-=,1l1c3tP, and phyllic-silicic alteration in s1lir1iiPc zc nP_ formed at temperatures between 71! °iw and CCL' + C and !,,PrP t;-,P product of the less ;aline f the two hypersaline fluids; this fluid episodically boiled. Copper-gold mineralization in potassium-silicate rocks probably peai :ed at about _7.q5Ì C. Fluid salinities .and temperatures gradually decreased with time, and during later stages of alteration they also da; rEa=ad with greater distance fr-om the hot center of the system, perhaps as a result of dilution. Although proof is lacking, the two high salinity fluids and the low salinity vapor may be magmatic in origin, and the low salinity fluid may represent a late-stage influx nf meteoric dater that encroached on the waning magmatic hydrothermal system. A depth of formation of 1.6 kilometers is estimated for the presently exposed portion fi the Bajo El Durazno prospect based i É the fluid inclusion data. Table! i Contents
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e Regional GeFi gf` E o a oe s s a a
RfIir'_l Structure And., T aasalalraIIOlQaCaaIIaataaatatl 5
Petrochemistry of Ïg'_i u= R:[t <= Tr! fthH+
Fal j I Ir_!n }eqr"i_;/k-iiran i_C iTÍiPEeXcaneanonnsenaseesaeoaanlanananaavsaacsnnnaQa¢es¢aa
Geology i_if the BA. jo 1 C lLi_tt a.`ni_i Prospecti1pect 33
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Volcanic Host r,lii-i:_eetaaaaQaae2la:IIas¢saelaaacsevIIaaso02traia:asaQSeelss 4
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Sediments and ri PdimL (i L R +J Rocks 42
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Alte-ration S i- l-1 e l Ì Ìe 45
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r-IirtkLilP aeasslaQaeaa77
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il ring fg P. i7-1L: it and t-iFP '_J= ana¢ascaa2ascaasaaceevaaneoeEaasaESevsaa
i_lPrT r 1 ii1 Ml1-r t ipr 1 Iris_+ Study of1 Vein1 in
- t l the1Centra-il i7Ñi ir rji_i-ii ii- 1 r?F1-iiii=! f VuasDaaev2sssvesaoQOaevnessaeavsec2aec 86
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r r_!r[pt3 .t'L.1l. aacaasPa1aFtuLLLi3 coma asaaaacaacaaa11acata
L t_ LI1 .41 .rS:_IdlEé53± =+F.1_i1i4E LIUt =,E_ii1=,=_I.(8 T!_° aaCao_Ias.id`.,i
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1 .J 0 f-!J= t = IJJ= ' ! 1'.1f-+! ! 1! T em r raÌ and Sra1 1=- i_ r i- L i t - C- ¡ d r-_} a . Fluids-...... 137
Significance o-1 Lf iPÌ fEpP A{ i L' Ti *jnd=
. in-i i i i t -yi }' }t`t} ilaasaa¢caavacaaanvavsazafaencvna as enno a¢enea ¢a a¢a¢savaea I :!
Origin á 1f Hydrothermal ;- i id= 140Í
Depth cif F ! t_;
{ _ ! l rqirHistoryof th P P aj± L i} ÿni Porphyry _ ÌÇp _ti-gfÊProspect 145
Ç i 1_r i n r i . 1 1 ; l j = t i a : f 11 f a f C 7 C : f f e 11 f ¢ ¢ .111127C2f f f 11 e f 6 0 7 11 6 : a f L f f 6 a 7 e 7: v a e f f e f 7 t 7 f 7 6 a CC 7 a a : e 6 11 f eC: e a C C a e t 7 f a C 7 11 1f
t: c 3 '. i7 Ii w l l' d g N -I 7 P [I t J 152
T -f 1 ' i_ 1 E i i= .J 7 1 7r C C D C t t a 7 7 f 7 1 7 7 DD f C C C t C 7 D f C 7 t 7 C C f C 7 6 t f 7 D tC 7 t 7 7 f C C C t f 7 7 f C 7 7: a 1 C t C: C C C7 C 7 D 7 C C C 7 t 7 C 7 D 7 C CC INTRODUCTION}N
The high desert country of the Farallon Negro region of Catamarca Province in northwestern Argentina (Figs. 1 and ) i= host to one of the greatest concentrations of base and precious metal mineralization identified in the entire country (Sister, 1 979). The mineralization is of ts&'' principal types, 'ritanganifer ou= polymetallic epithermal veins and disseminated porphyry copper-type, the latter associated with barren to mineralized breccia pipes in some localities. Both types of deposits are genetically associated with the waning stages of magmatism that produced the Farallon Negro volcanic complex, a major Upper Miocene shoshonitic andesitic volcanic center.
To date, all mining activity in the region has been limited to the small-scale near -surface exploitation of veins and has resulted in only a trivial historic production (Peirano, 1945), except for the two largest mines in the area. The larger of these two mines is at Capillitas (Fig. 2) where considerable copper and lesser lead, zinc, gold, and silver were produced in the early part of this century from veins associatedociated with a phyllically altered dacite porphyry stool:: and ignimbritic rhyolites (Peirano, 1 945; Gonzalez Bonorinco, 1950; Caches et al., 1971; F::.00ul::hars9::y and Mirre, 1976; Angelelli and Lima, 1972, 1 98o); the mine is now worl.::ed solely for- gem- quality rhooJcochrosite on an intermittent basis. The smaller Farallon Negro mine to the west (Fig. 2) has produced 350 tonnes/day of ore averaging 7.20 gm /tonne gold and 130 grt Tonne silver from a manganese-silver-gold vein since
1 978 (Sister, 1 979, Etults, 1 984).
The presence of numerous large areas of hydrothermal alteration in the
Farallon Negro region has been recognized for many decades, but the porphyry copper potential of these areas was not assessed until the period 1 96,3 -ï 968.
During this time, at least fourteen porphyry copper -type prospects were identified
(Fig. 2; Table 1), including the Pa io El Dutrazno prospect. Thee discoverieswere
l Fig. 1 - LocAtion mAp of the FArAllon NPgro region (Fig. 2) showing sOrne. crf the lithotectonic elements dcussPd in the tPt, including the PPru-nhile trench, the Easter Fracture Zone (E.F.7.), the onlAnd portion of the Ojos dP1 !=;a1Ado lineAmPnt (et-tAiPst-triAnding stippiPd pattern), And numbered physiographic provincPs (outlined by dAshPd linP..) of northwestern ArgPntinA, northern Chile!, and southwestern Bo livjA: 1- CordiliPrA de la Co.--FA (C01P.ngP); 2 - VAllP
Longitudinal (central Long1tuo1n1 VAllPy); - PrPcordillPrA ofChile; 4 - CordillPrA Principal (high AndPs); 5 - CordillPrA Frontal (Front RT-IngP); b - PrPcordillPr of
Argentina; 7 - Sierras PampeanAs (PAmpeAn - SiPrrA7.-, TrA,---.paropPanas
(Transpampean Range); 9 - Puna of ArgPntinA, AltiPlAno (high plAtPAu) Anci CordillPrA Occident] (Wet RAngP) rif Bo]iviai O - Cordillera Oriental (East Range) of ArgentinA And CnrdillPrA Central (nPntrAl Range) of Bolivia; 11 -iPrrAs Subandinas (Subandean Range) of Argentinand Cordillera Oriental (East Range) of PoliviA; 12 - L1.3nur. Charo-PampPAn:A (ChAco-FAmpPAn Plains). The curved shaded Plio-Quaternary sho=lhonitic volcanic belt of the Puna and Altiplano provinces is taken from DPruPTIP (1 978) the southern portion of the volcanic bPlt is based on limited sa.-.-mpling, and the Upper Miocene shoshonitic volcanic and inti u=.1vP rocks of the Farallon Negro region appear to form A southern continuAtion of this belt. In the immediate area of the Farallon Negro region (Fig. 2), the 1Pnsoid area shown by dotted lines is the simplified outline of A major regional structural flexure ; the solid black region within the structura] flexure represents the major outcrops of the Farallon Negro volcanic complex. X. = Major porphyry copper-type deposits of
Chile and Prospects of Argentina: C = Chuquicamata ; S = Salvador ; TV = Inca Viejo; P = El Pachon. Map compiled aft-Pr ZPntilli (1q74) t.A)ith slight additions And modifirAtinn,-,after Gonzalez Bonnrino (1q50), Garcia Bonatti Pt al. (1q77),
n 1 (1q11.7:),And!=:illito9]).(1Prub=, 70 °W 65°W
, 1I j i , ,\;, ., 1 1 I 1 \\\yA 1 1 1 , 9 f \\. f 1 1 f)l":\\\\ 1 it 1t \\ / ; f 1 _ 1txCt )-),-\\\,, t...... \ / , ; 10 i.,/Z ' . 1 tfl,\, 1 \ / 1 .... \ 1 I j ri 123 /9 i \ I / t Ì j \\\ i 1 1 J\`\ 11 1 Ii i 1 1 1 I \\ 1 1 /\\ t I / = ( tI f (a. Z I \\\\\< 1 1 1 I 1 I 1 1 1 1 .1 I i 1-1!-\\ t 12 Q j/ Z t a 1±-itqw\\ 1 f , I 1 f/xS I I / ( / f / / ¡\\ 1 9 San Miguel de I/ 1 a VTucuman ...... i- .I...... ! t .;.;..;.;.;.;...::-tt':: #Ï9 ; t:: Areao; FiFigure2 s ,,l'ç.;Ii:ç_\ ` ¡ / ¡ 1 ¡31(i/.A i ¡ 1 1 j j I/4(3f 1 .: i ¡ i 1 t 1 5¡61' 7 f f 1 1 1 1 i "'SanJuan i'Jtit 1 j ' '' f ; i ; ' . t t t Mendoza i'h%ttIt , Santiago+ t..% i%1,111. 70°W 65°W 0 200 400 km 1 . j Fig. 2 - General geology of thP FRrRllon NPgro rPgion showing locations of major miner.F.(1 occurrences.] photogeologic anomalies.] and hot springs. Fold axes are not shokaon beCaUSE of a lack of MR-p compiled and modifiPd Rffer Gonzalez
Bonorino (1950)7 Quartino (12F.2).] CiArciA (lciFicji] 1c17n, 12;71),
Servicio Nacional Minero (_eologic° (1972)5 Llambias (1;72)1 KoukhA.r,-----ky And Mirre
(1976)i Suchom Pl (1c1R0, Stu1t(19R4), Rvid this study. Stippled pRitr-rn for porphyry copper-type occurrences shows size and shR.pP hydrofhPrmAl altPrAtion halo as dPfinPH by thP oHtPr limit of ifiRior phyllicSa_rgillic alteration.
TriinPrR1 occurrences are as 1ollo1.4s: a) porphyry copper-type occurrences:
(1) E:ajo de Aqua Tapada; (2) Bajo El Durazno; (2) Bajo de Las- FarripifRs; (4) BAjo
Alufribrera; (5) Bajo di EspRntn; (F.) BRjo de Lo.1PjPnPs; (7) BRjo d Sn Lucas; (R)
VEkilerlfo;(g) BRio HPLsuntAs; (1 o: rpm:. AtAjo einrilidps Cprro Blanco to th'
_ . -2 th andcEirro B7...nro to (11*, ij n =1 =4--'n-"Li-*. (1.7j)
Mi Vida (Cerro Rico); (14) Filo ColorAdo; PpifhPrmAl vein-type occHrrPncPr...: (1F;)
7-1 (li;) C:;Rnfo 00-111l-190 arid LA .losPfR; (17) MRrho MuPrfn; (li;) S.n Jos P (Los
(Ici) AguA HP n-;oni,----:in; (20) Los ViE-co,---; (27) FRrAllon egro; (22) Alto HP LA.
B1PndR; Cerro A.fRio; (24) CRpillifRs. + + + + + + + + + 27' S cc EXPLANATION + + + + + + + + _ + + Campo de Arenal OD Z cc< Alluvium,Araucanense effluvium, sedimentary and colluvium rocks < v , V ^ L , ( A r A r mAy volcanicUpper Miocene complex: Farallón a] undifferentiated; Negro ( ,(( 7 J V A > ( , J r L j a r A A < r V . VV > AJ( r r A r A v + J r b] Alto de La Blenda monzonite stock > I ( ( , l A ( J (, J r J l < ( ). P V ( f > Calchaquense sedimentary rocks V<,i r v J r V r vA r J r r ' P r v ., 4 r, vr V 4.4 t, r UNCONFORMITY :'Y!r: A/h r V J i v r t J r A r + + i+ + + + + + + + + + + + Undifferentiated metamorphic rocks + + + + + + + { and granitoid batholiths .} c 10+ + + + ,I + ,1 ` + +f f + f f +13+ + + + + ++ + + + + + + + + + + + , + + + +++ + + + + + ++ + + +14 + + / Geologic contact -+ ++ + + + ++ + ++ + ` + I+ + + + + + + + + + + + + + + + + + + + + whereFault inferredstrike and dip, dashed + + + + + + + + + + ++ + + + + + + + + - + + + + + + + + 27 °30' - Hot spring 67 °W 0 SCALE 10 km 66 °15' y Epithermal veins: Mine o PorphyryPhotogeologic copper-type anomaly prospectProspect Ta-tble 1- Summary of the characteristics of ttAiPlvg, porphyry copper-type occurrencPc: in thP FRrgillon NPgro region ; no dAta gin= AvAilRhie for thP BRio de Los aiaTIES and Chauf.---.iyacu oceitrrPneg.... InformRtion compiled from Gonzalez Bonorino
(10,5m, riaret= (1 969I 9701 I q71), .---Prvicio Nacional Min Pro (7iPologico (1972)1
Koukhrsky F.And Mirre (i 976), !"-;iichomPl (I 93:3)!:"--;fulfs (1q84131 fhis study, nd unpublished data.; Kl Ar agi= dAtPs on biotitg. from McBride (1q72) Rnd McBride et a.l.
(1976). In the "Rs---,nrifPri intrusive phR=IPs" colirmn, "ear ly" rPfg-irs to rocks Prrip.1:=cPci prior to thP proqPnitor stock, .and these rocks pRrticuIRrly dikes and sill=--.1 m:=..y not be rg.1.=tPd to the genesisnfthP porphyry copper SYSterili i.e.,. they a-re
Pc-,sPntilly "I.A.JR1lrocks". HydrothPrmR1 A itPrAtion types rP as followc.: K = potassium-silicate; PerV. 0 = pervasive silicification; Diss. M = disseminated 1-11agnetit e. of the "quartz-magnetite" alteration subtype ; Q-1 vPins = qiiRri-z-magnPtitP veins of the "gliRrtz-DiRgnPtitP" RitPrRtion subtype ;J.-7; = phyllie; A
= Arc7fiic; AA = Advanegid Eì gillic; P = propylitic. :.-7;yrribols lisPd:::.:: = knnwn to bP prPs_--,Pnt; (A = kno;Ain to bP present but only tAieRkly developed ; x? = probRbly present
- = knotAin to b.= .--zt.-..sg.nt; ? = no informAtinn. AbbrPviAtionq used: F.N.v.c. = FArRllon
.-.ii=gro voicRnic complex.R1-nb. = CRmbrian; Ord. = Ordovician; Sil. = Silurian; incl.= inclHding. Fotssiltrn-----.ilie.--;Itg. giltPratinn (biotitizRtion) at P.A.ici drz. !:-;an Lira=IS inferred from Garei;Et's 0971) deseriptinn of"dioritP stock" in the ':enter of which
A H---iciti . porphyry stoci< w.,--4.=: intruded ; the "diorite " ariRS grAdRtional contacts tAiith surroHnding rocks, and the bintite content in it inrrPRsPs to 70 vnlwmP percent t. 0 w..p.r dr..,.I- hcz d.Rcif 17., por phyry ssi-ock, TABLE typePor Ph Occurrences y r y t:opper - (Age. -inFrogPnif m.y.) or Intrusive Rocks intrusive Phases Associated Walirocks Pi=rv. Diss. ChMHydrothermal0 Alteration M Veins - A AA P TapadaBajo de Agua. stockPDa.cite orphyry + 0,2.) andesite,stockEarly andi=s-ii-Pand andbasaltic andesite porphyry ande.site, (X) X centraldacitPporphyrypii----se;dacite porphyryitrimini=r-Alize=d porphyry pebble dikes; dikes-, bodiesirre.gular dikes; common brecciaRnds--mall Bajo El DiirAzno (R.7Andi=siti=stock + 0.4)or pebbledikes,FArly ba1ticmasses, dikes Rndi=sifi=rare and sill; andesite porphyry BA io dr= L. mpitA ..tockporphyryPhyodRciti= brecciasmallDacite weakly pipesporPhyry mineralized dikPs-; 2 rnonzonitdeF.r.i.v.c. La Blenda e(incl. Alto ock ) -7) (X) (X) B io L. A.lurn't--ii--Pr. stPorphyrynRciti= ock pebbledaciteEarly andPsit.= porphyrydikesporp commonyry dil.::es Ertoci:: dik and ; BajoBAio del di= Espanto !.--; sn LUCES Dacite PebbleDRcite- dikes porphyry common and quartz CAmb.Furuvo:. slates And (.7,1stockpor ph+ C1,7) r y hPAringp-,orp'nyryPipe barren di: i==.; breccia tourmaline- rPmnRnt=,quens-,egranite,porphyriticschists, sediments, Ord.-Sii.takha- of biotitP F.N.v.c. Porphyry Copper- Progenitor Intrusive Rocks Associated TABLE I (cont'd) PPrv. Disc. Q-MHydrothermal Alteration typeVallecito Occurrences stockTonalite(?)(Age in Tri.y.) Andesite dikes Intrusive Phases porphyryamphiboleOrd.-Sil. biotite- granite Wallrocks K Q M Veins S A AA P ? ElBajo Estanque de Las Junts stockporphyryAndesite Andesite porphyry "bodies" nrd.-Si].Ord.-Sil1porphyryCamb1 schistsgranite granite and PCI rphyry andpegmatiteporphyry F.N.v.c. and dikes, ? lAipst)BlancosouthCerro andBlancoAtajo Chico Cerro (incl. to porphyporphyryastocks2 rhyolitedante r y and rhyoliteSmall dante porphyry porphyry stocks and F.NaVoC.sediments,Calchaquensenrd.-Sil. granite, and M(Cerro i Vida Rico) (5.8r-nrriplexstock yPinndiorite 0.4) masses;andAndesite rhyolite large, porphyry, porphyry well rhyolite, andOrd-Sil.Camb. migmatites, schists, granite Filo Colorado stockporphyryDacite mineralized breccia pipe graniteentsCamb.and F.N.v.c and metasedi- nrd.-Sil. X X the result of a systematic regional exploration program of northwestArgentina known as Plan Cordillerano (Norte) or Plan NOA-1, which in the FarallonNegro region was carried out jointly by the United Nations Development Program, the
Direccion General de Fabricaciones Militares, and the Subsecretaria de Mineria
(Angelelli, 1970; Garcia, 1971; Serii ic c+it'+_ c+i nal ilinerc+ t +c+ 1_gi++ ci+, 1'_ái L; Ang+r+ ';+ and Lima, 1972, 1980; Sillitoe, ¡, 1 q81). Currently, all tort tf_ of the porphyry copper occurrences are in the early stages of exploration. The Mi Vidaprospect, with significant and somewhat unusual mineralization KnLl.:harsl::y Mirre, 1976), may not be developed for many years because of its difficult access.The Bajo La
Alumbrera prospect, located in much less difficult terrain, is currently being evaluated for development and has drill-proven reserves+f over 300 million tonnes of ore :averaging 0.49°á copper and 0.66grrlitonne gold ( Stults, 1qR4 ).(_1 !the better e-fpl red prospects, only the Mi Vida and Filo Colorado prospects (Fig. 2) possess hypogene mc+lybdenite mineralization and supergene copper mineralization that are potentially economically significant. In contrast, several of the porphyry copper-type prospects possess significant F ec is metal mineralization, commonly associated with elevated secondary magnetite concentrations ("quartz-magnetite" alteration). Because hypogene copper grades at most of these prospects average less than 0.4% copper, the future economic viability of most of them will primarily depend their low grade precious Is irEf?`tal potential.
"iy+ r ± herriEa, f activity has not `,'eceased in the Farallon Negro region.n. A number of geothermal hot springs, many of them highly saline, occur throughout the area (Fig. 2.), and several of these springs are presently depositing 'i'rEanganifFroL4s siliceous and calcareous sinters containing small quantities of gold and silver
+.Gc+nial?Z Bt_+nC+rin +, 1q50; Quartino, 1962; Sister, 1q6?., 1 q?q; Garcia, I qi ri1 1 q71_I,
1971).
r, PREVIOUS ;S W ORF::.
The geology, , struc turá, and mineral occurrences of the Farallon Negro region have been described in a number -f published and unpublished reports
{r al ranÉ , 1945 Gonzalez Bonor1TC, 1950; Quartino, 1962; Sister, 1963, l t79.
L1ambias, 1970, 1972; Figueroa, 1971; Caminos, 1972; Servicio tAacional Minero
Geologico, 1972; United Nations, 197:3; Angele 111 and Lima, 1972, 1980). Various aspects of the region have also been treated in more general studies devoted to porphyry ry coGper genetic models and surveys rif the metall geny rif western
- America (Stoll,y 64 , 1965; Hollister, 1974, 1978; _ illitoe, 1973b, 1976, 1977, 1979,,
1981; Sillitoe and Bonham, 1984.l. A large amount of new and important information on the geology, petrology, major and minor element geochemistry, geochronology, and magmatic and tectonic evolution of the Farallon Negro region and its mineral occurrences has also emerged from the Central Andean Metallagenetic Project l:C aelles et al., 1971; C1arl:: and "Lentilli, 1972; McBride, 1972; McNutt et al., 1975,
1979 McBrid e et al., 197rr Clark: et al., 1976,Dostal et al., 1977; Clark, 1977;
7entill i and Dost al, 1 977; Caelles, I q7q; Tilton et al., 1q81).
The Bajo El Durazno porphyry copper-gold prospect has been menti! ned or briefly described in many of the references cited above that were written after" its discovery in about 1968. The prospect is more fully described, however,oEev2r, ln a number
f largely unpublished reports dealing specifically with the porphyry copper prospects of the region (Garcia, 1959) and !Adth the Bajo El Durazno prospect in particular. Prior field iA)orl< at the Bajo El Durazno prospect includes several mapping studies of lithologies, hydrothermal alteration, mineralization, and structures conducted at different scales and in varying degrees of detail. The prospect has been explored by an extensive surface geochemical sampling program during which 653 rock chip samples collected ona50 meter grid pattern were analyzed for copper, gold, and molybdenum. Seventy-two f these samples were examined petrographically. The subsurface tA1a s explored by nine diamond drillhole_
in the period i, toL. 19F and byn an,7 induced p=+l--;-,,atlr}i; ( i re=1t1}Iit;'..1 survey1 .f of very limited extent; many drill core samples were also ',studied petrographically. Nearly ail of the previous work was conducted by personnel of the Ulreccion General de
Fabricaciones Militares in conjunction with ymAG (Yacimientos Mineros Agua, de
Elionisio), a state agency that controls Ì= the mineral rights to B3Jr El Durazno. The results of all prior work available to the author will tie integrated with the results of this study and presented in later sections.n=.
PURPOSE AND METHOD OF IN1°ErTIGATION
The present study is one of three projects of similar detail and scope carried out concurrently at the Bajo El Durazno, Bajo +fie Agua Tapada, and Bajo La
Alumbrera porphyry copper prospects. All three studies were supported by a United
Nations contract with J. M. Guilbert and the Laboratories of Economic lGeolog:,at the University of Arizona in conjunction with the Dirección General de
Fabricaciones Militares and the Institutoo de Investigaciones Mineras (Univ_r-sidad
Nac iona l de San Juan) of Argentina. Initial results of this worI :: and further exploration recommendations were presented in three separate unpublished reports to the United Nations (Suchcome1, 1983; Allison, 1 984; Stuits, ï' _,4). The three studies, now completed, represent the most detailed investigation of the geology of each of these three porphyry copper prospects undertaken to date.
This paper presents the result_ of three and one -half months offield work at Bajo El Durazno from May to Sieptembfer, 1983, and later laboratory studiPc. carried out at the University of Arizona in Tucson, Arizona. Mapping of lithe fogies, structures, vein assemblages, and alteration and mineralization types, intensities, and distribution was conducted at a scale of 1:2000 using an excellent topographic base map with 2.5 meter contour intervals prepared and provided by Fabricaciones Militares. In addition to field mapping, 26 fracture density measurements were taken, and the core from one drill hole tAias examined. A large number of surface roc k chip samples and a lesser number of drill core samples were collected,and several of these samples were analyzed for metals of economic interest or tor- major and minor elements; i le samples were thin sectioned and studied petrographically. The final phase of this project involved a fluid inclusion study of
selected doubly polished sections of quartz veins. Four of these sections were also
studied with the electron microprobe in an effort to establish the distribution of
selected elements, particularly gold, in several different vein types.
Before describing the Bajo El Durazno prospect in detail, the regional
geologic and tectonic setting of the Farallon Negro region will be briefly discussed.
The characteristics and petrography of the Farallon Negro volcanic complex will be
emphasized because of its genetic association with the development of vein- and
porphyry copper-type mineralization in the area
REGIONAL GEOLOGY
The Farallon Negro region is situated approximately 175 to 225 kilometers to
the east of the main Andean Cordillera (Cordillera Principal) and lies within the
Sierras Pampeanas physiographic province (Fig. 1). This large province is
characterized by elongate north- south -trending fault blocs.:: mountains bounded by
steep reverse faults and separated by intermontane basins partially filled with
continental sediments (Caminos, 1972; Caelles, 1979).
The oldest rocks in the area of Figure 2 are deep -water, mostly
fine -grained, clastic flysch-type marine sediments of probable uppermost
Proterozoic(?) to Middle Cambrian age (Caelles, I 979). These rocl.::s were
penetratively deformed and regionally metamorphosed in Late Ordovician -Early
Silurian times and now occur primarily as biotite -, muscovite-, quartz -, and
q garnet-bearing schists and p hyllitï_ ; with lesser amounts f s1Ates, horn f el_es, quartzites, amphibolites, gneis'=es, and migmatites tGonza1ez Bonorinie, 1950; j; Sister,
1963; Garcia, 1971; C a Iin =, 1972; Conzalez , 1975; Ko Ìlhar r and Mirre, 1 9 7 :;
Caelles, 1979). Metamorphism, which ranged from gr-eensE_hiat to amphibolite grade, was accompanied by the emplacement of minor pre- and 5yn-kinematic and majorr post-kinematic granitoid bathEeliths that mal::e up the bulk of the crystalline basement (Gonzalez Bonorino, 1950; Caelles, 147q). The sSrdovician-Silurian metamorphic-intrusive event has been dated by potassium-argon methods at 410 to
444 Ma in the immediate area of Figure 2 (McBride, 1972; McBride et al., 1976).
Unconformably overlyinging the crystalline basement are a group of Neogene continental sediments of variable thickness that have traditionallyinall; C+een subdivided into the younger Pliocene Araucanense and older Miocene(?) C alc haquei is+'_ se=_diments. These units are made up of arl::cesic and locally calcareous .,r- argillaceous sandstones and siltstones with minor conglomerates and rare limestones (Gonzalez Bonorino, 1950; 1; S1=ter, 196:3; Caminos, 1972; Kot1kher=k y and
Mirre, 1976; Caches,979). Much of the Calchaquense sequence consists J.i ?i beds and thin evaporite layers. Pyroclastic material and volcanic detritus bee_orrie increasingly more common towards the top i if the f_ralc ha`-'jUen1e sequence and are particularly abundant in the overlyinging Araucanensá _edinienta, with increasing
¡=r Ci:`.imlt; to i the Farallon Negro ii/Cili=eni[ i_ent_+t.
Throughout much if the area shown in Figure 2, volcanic rocks of the ! áNF=er-
Miocene Farallon Negro volcanic complex separate the Araucanense and
Cichaquense sediments along generally conformable contacts. These rocks crop out over an area of approximately 700 square kilometers and represent the highly dissected basal remains of one : f the largest Tertiary y i=ruptille centers in northwestern Argentina (Gonzalez E:onorino, 1950; Sister, 1963, 1979; Caches et al.,
1971; McBride, 1972; Caminos, 1 972; C:aelles, 1979). Most of the volcanism is believed to have been centered in a large graben where the thickest pile of vielcanic
10 rocks and the greatest number of sui_i=S!>..ilcclisii_ intrusions and mineralized oi=currences are now found. This volcanic center may rPi rP=Pnt the erosional1 remnant of a single large stratovolcano j rrie 6 110 meters high and thus comparable in size to some of the Quaternary stratovolcRn9 F?si the high Andes (Llarribias,
1970, 1 9 7 2, Sillitoe, 1 9-3 Thorpe _t al., 1q82;:illitcF and Bonham, l qi 4 ). SP :_r a discrete sate llitli volcanic r entPrs are al=i r'Pcrigniz Pdy as at Cerro Atajo,
Capillitas, i`=ii Vida, and between the Bajo de San Lucas and Bajo dP Las Juntas prospects (Fig. 2) (P ir no, 1945; Gonzalez E: n r ino, 1950; Quartino, 1962; Sister,
1q7c1; 1 r11,5; 1_ aelles et al., 1q71; Llarrihias, 1970, 1,1 9,2; Garcia, 1971;
McBride, 1972; i arincs, 1972; Sil l itoe, 1973b; C 3e11es, 1979; S i lt±e _ nd nhall1,
1984).
The bulk of the Farallon Negro volcanic complex is made up of anJe=itic
volcanic breccias with a fei.h, more siliceous or more rriafii_ breri_ias found locally
(Quartino, 1962; Sister, 1963). These breccias have not been differentiated in
previous reconnaissance mapping jtLtdies, but those brei=i_ia1 in the llirinitÿ` thP
Farallon Negro eriine, Bajo La Alumbrera, and Bajo El Durazno are all highly similar
and conform with desrri¡=tions of alloi= iasti:_ volcanic breccia= as defined by
Parsons (1969). Er =ci s of this t= are typical of the vent or cone complex -a ri= s
and are derived by underground brecciation i T previously consolidated flows,iws4
Cirei_cias, and intrusive rocks of the volcanic edifice; basement rocks may also be
afferted. The breccias are subsequently intruded into the vent walls=r- are
erupted as breccia flows (Parsons, 1969). Peripheral breccias of the Farallon Negro
volcanic rorriple::. desrritied in the literature (Peirano, 1945; Quartinn, l q,_°; Sister,
196:3; Llarritiias, 1 9?2) share many of the characteristics of the laharii_ brecrias
and epiclastic volcanic conglomerates of the alluvial facii?s classified by
Parsons (1969). Throughout much of the complex, the volcanic breccias are
interbedded with thin flows of andi?sitiC to basaltic composition and andesitiC toi rhy dacitic tuffs.
11 Within'iln thi main volcanic_ center, ? generalized =equanCe C 1' intrusion and hydrothermal alte:ratiEn and mineralization has been worked out (Llaïbla =, 1972;
Sister, 1979). First, 3 number cif basaltic- to 3nnesitii= sills and irregular aindesita stocks and endogenous domes were emplaced. This period A.LEt = followedwed ti;f the intrusion of numerous st(bvertiC31 to vertical basaltic to 3ni3esitii= dikes, principally along a northwest-trending belt, and to a lessi=r extent along northeast- to east-n, irthP3 st-rrendin'g belt. AL the interserti i ii,f these two belts and_itthe approximate centerif the graben the large m_inLi inite stock of Alto de
La Blenda was later passively emplaced. This more erE slon3lly resistant intr usive body now forms the highest point of elevation within the graben (:=950 siii=LFr3 above sea level; Sister, 1963) and may represent an old volcanic neck (Quartino, 1962).
Intrusive activity continued with the emplacement of numerous stocks, domes, and subvertic3l tn vertical dikes of andesite, 3ndi?1it+== porphyry, and dacite porphyry, followedwed by rhyodacite porphyry, and finally by rhyolite and rhyi_olito= porphyry.
Porphyry copper-type alteration and mineralization, with or without a=s _iatad
Cir'eo_Cia piposs (Table 1), were developed in association with =3t/3r3 i of the
_1dPs i t i c9 d3r i t i _' and rhyodacitic porphyry storks, RlhilG late-stage hydrothermal activity formed epithermal veins after the period i f rhyolite intrusion. In addition to the ro_: types describedabove, iqniriibritiC r fyo;ite s occur at Capillitas, and a complex syenodiorite stock hosts s porp`iy r-3' copper-type mineralization at Mi Vida
(Gonzalez Bonorino, 195f7; Caches et al., 1971; McBr 1972; Ki-3 r =ky and Mirre,
1976). Pebble dikes are common in some portions of the volcanic complex (Sister,
1963) and are often spatially related t! i porphyry copper-type, ='pith?r ma I vein-type, and geothermal hot spring-type h;'drotl-ier'irial activity, e.g. Agua da
Bionisio i965:i, Bajo del Espanto (Servicio Nacional Minero G _ C1 1 g i_! 1972),
Bajo de Agua Tapada tEuChcirrial, 1983), 3nd Bajo La Alumbrera (Stults,
An Upper Miocene age(6.nto l 0.6 m.y.) has been established for the Farallon
Negro volcanic complex on the basis Et KÍAr dates obtained for ten samples of the
12 complex by McBride (1972). Hctrnblende from a sample c +t "dacitr+, flow to +t he west of Bajo El Durazno ha= yielded an age of 10.6i,6 + 0.5 rri. y. for the basal portion of this complex. Four intrusions related to porphyry copper-type mineralization have been datF?'d at t_,,p to 8.8 m.y. tTabl+? I.l. At the Bajo El Durazno pr'trspect, a sample of phyllirally altered ande,=ite porphyry t,'elF,olF+ rock) t4as dated at 7.9 +0, _; Tri.y. and establishes that hydrothermal ac tiEjit y followed+wec tTíe emplacement of porphyry copper-related intrusions here (at 8.7 + i !.4 why.) within a period +d+t less than a rriillion years. Three other intrusions genetically unrelated to porphyry copper-type mineralization have yielded c rp arabl e dates of 7.7 + 0.3 r.y., 7.6 + 0,6 r.,., and
6.0 l + 0.2 liti. y.from the Alto ce La Blenda monzonite+nite = tc +c i:;, a rhyolite dike, and an andesite dike, re_pectik!ely. The close temporal relationships between intrusive and extrusive rc +cl::a in the Farallon Negro volcanic complex, along with field relationships and a number of petrologic similarities, have led many workers to conclude that these rocks are c rriagn-iatiF_ {.Llarrib+ia=, 19701 1,1 972; Gaellá= et al.,
1 971 ; `=illitoe, 1973b, 1 97 9, 19E:1; Gonzalez, 1975; Kv+ukhar _I-°, and Mirre, 1976;
Cael ies, 1979). Finally, cryptomelane in an ore sample from the Farallon Negro vein
mine has been dated at 2.6 + 0,8 m.y.. The cryptomelane is probably hÿ'pcEgene
(?=ister, 1979), but because its purity was unknown and because of strnng atritc+spheric contamination the age date is a minimum estimate (McBride, 1 9 7 2).
Despite these analytical uncertainties, the data suggest that there was a lapse in time between the terrriination of porphyry copper-type and the onset of epithermal vein-type hydrothermal activity in this region.
The Farallon Negro volcanic complex represents an isolated vol+_ani+_ renter at the eastern edge of a 200-250 kilometer wide north-south-trending petrographic age province within which Neogene (ca. 20-5 rri.y.) magmatic activit, was widespread. Prior to this ++Nerigene breakout" per-ind and since Middle Jurassic times, magmatic activity associated with the Andean orogeny in Chile and
Argentina was restricted to 2 l-4Q kilometer wide north-south-trending magmatic subprovinces that migrated eastward with time (Farrar" et al., 1970; i;C:'lart: and
Zentilli, 1972; Sillitoe, 1974,1976,1977,1981; "1c C'Nutt et a1., 1 975; Clark: et al.,
1976; Dostal et al., 1977; C:aelles, 1979; Tilton et al., 1981; Thorpe et al., 1'382).
The sudden widening of the belt of magmatism during the Neogene was followed in the Late Pliocene (ca. 3 Ma) and Quaternary by a sudden regression of magmatism to a positionalong the present Andean crestline and superimposed on the western edge of the Neogene magmatic subprovince. The broad eastward expansion of magmatism during the Neogene can be correlated With rapid rates of convergence of the Nazca and South American plates during the Miorene (ca. 20-9 Ma) at a time of major reorganization of plate boundaries in the Pacific and may reflect a downward er::tension of the zone of partial melting on and near a shallowly dipping subduction zone (Herron, 1 972; McNutt et al., I 9 7 5, 1 979). Following a brief period of slower rates of convergence of about 7 cm/year in the Late Miocene, convergence rates again increased to about 11 cm/year in the Pliocene (Zentilli, 1 974; i :aelles, 1979),
ri but reconstructions of the paleo- subduction zone at about latitude7S suggest that the angle of subduction has remained fairly constant at about 2R ID-30° for at least the past ten million ; ears (Cailles,1979). It is therefore not p orsible to correlate rates of convergence and angles of subduction with the westward regression of the belt of magmatism during the Late Pliocene and Quaternary.
These observations are in marked contrast to data obtained for Laramide intrusive rocl::s in the western United States (Coney and Reynolds, 1977).
Overlying all other rocl ::s in the Farallon Negro region along a slight unconformity are continental alluvial and crlluvial sediments, including thin tuffaceous beds, fanglomerates, and saline playa-lake sediments; areally extensive sand dunes and small hot spring deposits occur in some localities
(Gonzalez Bonorino, 1 950; Garcia, 1 969, 1 970, 1 971; Caminos, 197; C:aelles, 1 979).
Pleistocene glacial moraines are found at higher elevations to the eastnear Mi
r r-r Vida (not shown in Figure 2)) (Gonzalez Bonorino, 1950; Cariiin_is, i ,Koul::harsl::y
14 and Mirre, 197I;; (_ael ie s, i y7q).
REGIONAL STRUCTURE AND TECTONIC E-;
The major structural sty le of the Sierras Pampeanas ph}=siographic pror! i;!r7e is that of elongate north-south-trending tilted fault blocks bounded by regional high angle north- to n rth-northaast-trandin°a ral'ars+? faults (Quartinn, 1q62;
Stoll, 1°t4, Turner, , 1 íir! 1, Garcia, 1q71; erviE=io iac iGi ial 'iiierc iGec:lGgicc , l'! s.,,.
Caminos, 1972; C ae ilej, 1979). Wide amplitude folding along north- t north-
northeast-trending axes and lesser normal faulting accompanied the revárse
faulting but are subordinately developed. Repeated differential uplifts along the
major reverse faults in the re'giiln are responsible for the enormous relief f this
physiographic province where elevations range from a5 little as Inonmatar= toas
mLich as ,non meters above sea level (Caelles, 1979). Folding, faulting, and uplift,
vohic h began in the Lower Miocene(?), continue to the present as part s f the
widespread Andean oro'genf. in the Sierras Pampeanas physiographic province, this tectonic activity culminated in the F`iic-Pleistocene with accentuation of
preexisting strLictures.
Structural patterns Rioithir-i the Farallon Negro region deviate somewhat from
the gneral_ pattern described above. The volcanic _+_riple:. itself is localized withiniCnin'
a major r ragional structural flexure (cf. Figs. 1 and ,:) in which th+? predominant
structural grain ! f faults and fold axas trends east-northeastteaSt to n rthFa=t and
not north-south. This structural pattern is expressed in rocks at least as young a=J
the Late Pllot_PnF?, On Landsat photos the flexure appears as a di:scontiiC{(otij'',y
fault-bounded and lens-shaped structural depre=siïn elongated northeast-
southwest and surrounded on all side= by subparallei structures that rapidlylf
resume the north-south regional trend a short distance away from the flexure. T ha
origin of this flexure has not yet been studied in detail, but it lies along one of the
155 best defined and most important of 3 sFr 1P= of Lr-nst.Pr =.,-- 11nP3 mPnf 1 fhaT occur all along the west coast of South America, each approximately Éy p3rF'3nd icul3r to the
Peru-Chile trench. The rhar 3rtPr istiE_s of these lineament_ i E3vY been described elsewhere (Sillitoe, 1174; Zentilli, 1974; E,äraL3ngi and Isarks, lqfE,). A similar but smaller regional a l i 13r;ur 3 to the north-n+_trtj'rt;,,!3_t nfi Farallon NPgr" i in the Puna.t3 phy si_gr3r ir provinceas also been shownrwn to lie along a correspondingly smallerler transverse lineament f=Pg rs t ri andTurnr, 1q72). Th P i g in oft- P 11n F a t 3 l f that passes through the Farallon Negro volcanic complex, termed the Ojos¡o= de1
Salado lineament by Zentilli (1 974), has been attributed to several o_3ujes. i:l 3 tear fault in the subjacent subducting oceanic lithosphere tl_aelles, 1979); 2) the extension of the east-west-trending mantle hot line of the Easter fracture zone
(31_o called the Sala yGorilez Ridge) across plate boundaries and under the South
American continent (Bonatti et al., 1977); or 3) the presence of anomalous oceanic lithosphere created along the Easter fracture zone And subdurteoj under the western margin of South America (Eon3tti et al., 1977). The firsts f these explanations is strongly supported by available seismic evidence and is favored here, although the Easter fracture zone may h3=;`? provided3pí eTer entl3 l site along which tear faulting could occur.
The structural style of the Farallon Negro o regicon also deviates from that of the Sierras Pampeanas physiographic province in that the main magmatic renter of the Farallon Negro volcanic complex is situated within a graben bounded by steep
normal faults; this graben is located within the á::::acL centers T the regional4 structural flexure. Although some t the syn- and post-volcanic subsidence of this tectonic blo_ork may be related to domai uplift and grab*=n developmentover an upward stoping magma , body t:Koide and Battacharji, 1975)or to collapse consequent to the removal and upward transfer of magma as volcanism progressed (Garcia,
1971; Fyfe and McBirney, 1975), the graben le,a s apparently well developedprior too any volcanism in the area (L l3rrlbias, 1972). The main magmatic centerof the Farallon Negro volcanic complex therefore developed within R 1ocA11y tPn=i:nna i or extensional tectonic environment, in contrast to the surrounding compressive r-'giiËie. Magmatic activity in the Farallon Negr4, region developed during a period o. active tectonism, and structural patterns expressed in the vC,lc3nic complex form primarily in response to the regional structural regimP, on which second order effects related to intr usi! +n and volcanic r°rocr3= s?s (4r=r e superimposed (Garcia, 1971; Llambias, 1972). A number of east-west- to northwest-trending linear features that pass through the Farallon Negro volcanic complex have been recognized from satellite imagery (Figueroa, 1971; Servicio Nacional Minero
Geologico, 1972; K ui::harsl::y and Mirre, 1976; Bonatti et al., 1977; `=;ill itoe., 1981; this study). Although several of these lineaments are of subcontinental scale and probably cannot be traced continuously on the ground, others are reflected and paralleled by smaller mappable structures within the Farallon Negro t region itjelf, where fourur rfí3lor structural trends are recognized: 1 i west-nnrthwPst (N55c1-70()
W); i) northwest (t-4 2.ricE-5itOW.); =0 northeast (mainly N 25o -7:5o E); and 4)
ii ii east-northeast(N 55 -7 E). [he pronounced north- to north-northeast-trendinr structural grain of the central Andean orogen is poorly developed throughout mos of the Farallon Negro region, perhaps reflecting rotation or obliteration during ti- development of the regional structural flexure in the Flic- Fleistocene( ?). The foui major structural orientations are evident in the alinement of numerous mineral
occurrences and areas of hydrothermal alteration, in the geometries of intrusive,r vein, fault, and fracture patterns, and in the elongation of hydrothermal alterati halos surrounding porphyry copper- and vein -type occurrences. Of particular importance is the northwest- trending belt of seven identified porphyry copper prospects southward from Bajo de Agua Tapada to Bajo de Las Juntas (Fig. 2). Th largest dil::e swarm and the largest number of stocks and epitherrríal veins, most which are alined and elongated in a northwesterly direction, are also concentratE
17 within this belt. Clearly, this fracture zone contains sorite of the mostrepeatedly reactivated structures in the region, and some of the structures have considerable strike lengths, such as the Farallon Negro vein which has beentraced continuously for 15 kilometers (Sister, 1979). Another" important alinement of mineral occurrences and areas of hydrothermal alteration trends we=t -northwest
from the Mi Vida prospect through Bajo El Durazno to the Bajo deAguaTapada prospect. A third important structural feature of the Farallon Negro region is the
fracture zone and dil : :e swarm that trends east-northeast from the Alto i de La
Blenda stock through the Bajo El Durazno prospect. Finally, a small
northeast- trending lineament is visible on Landsat photos that connects the
identically alined phyllie alteration halos of the Bajo La Alumbrera and Bajo El
Durazno prospects, and another north-northwest-trending phatolinea cent several
tens of kilometers long passes through the Bajo El Durazno prospect. It should be
emphasized that although all four of the ma or structural orientations discussed
above are readily recognizable, the types of structures and the sense and
magnitude of any faulting along them are not yet well I. : :nrit.Am (Garcia, 196m.
Volcanic- and intrusive-related structures are poorly expressed throughout
the Farallon Negro volcanic complex. Radial dike, vein, fault, and fracture patterns
are best developed at the Bajo de Agua Tapada prospect(Suchomel, 1983) and on a
somewhat larger scale surrounding the Alto de La Blenda stock: (Quartino i, 1962;
Sister, 186==3; Liambias, 1972). Concentric structures are almost totally lacking,
except for what appear tri be several large, nested, discontinuous circular features visible on Landsat photos within the graben portion of the complex.
F=ETF=C;iC:'-iEMIBTi?''r' OF IGNEE}US ROCKS FROM THE
FAFALL{ N NEGRO VOLCANIC COMPLEX.
The major and minor element geochemistry of igneous rocks fromt he
18 Farallon Negro volcanic complex has been investigated in two previous studies
(Dostal et al., 1 977; Caelles, 1979). Based on a variety of criteria, Dostal et al.
197 7) concluded that these roci.::s belong to the shoshonite association, while
Caelles (1979) considered them to have high-potassium calc- alkaline affinities. A limited amount of information on Li and U concentration= (Zentilli and Dostal, 1977 and initial strontium ratios (McNutt et al., 1975) in igneous rocks, along with lead isotopic data for roce:: and ore leads (McNutt et al., 1979; Tilton et al., 1981), has also been obtained from the Farallon Negro region.
Major element analyses and CIPW norms for ten unaltered samples collected and analyzed by Caelles (1979) and new data for sixteen additional samples from the volcanic complex are listed in Table 2. The new data are for samples collected within the immediate environs of the Bajo de Agua Tapada, Bajo La Alumbrera, and
Bajo El Durazno porphyry copper prospects where fresh samples were difficult to obtain. Most of these samples are only weakly propylitized, but sample no. 3 was purposely collected from the most intensely biotitized rocks at Bajo El Durazno.
Although the analytical data for these samples undoubtedly reflect some chemical gains and losses due to alteration, the data are reasonably consistent with the analytical results for the ten samples of Caelles (1979). The raw analytical data for all 26 samples were normalized to 100% on a volatile -free basis in order to form an adjusted data base from which the CIPW normative mineralogies and total alkalies (Table 2) were calculated and on which the data in Table 3 and Figures 3 through 7 are based. It should be noted that since both ferric and ferrous iron were not reported separately in all of the analyses, some unavoidable discrepancies among the calculated norms for different samples have resulted. A suitable
Fe203 /FeO ratio could not be estimated with any degree of confidence that could be used to correct for either the effects of hydrothermal alteration or for analyses where only total iron (as FeO) was reported. Thus, in the lattercase, the standard practice of allotting 4/5 of the total iron to FeO and the remainder to
19 TAhilP 2 - Major in]PmPnt gPochPirfiliAtry Anri CTPW normsfor 26 samples of complex. Total alkalies, K.....alkia,,O, ignincius rocks frnM fhP Frxiliirirl Negro VrikAlliC -.-. and CIPW norms (..'ere calrulAtPd Using An Adj!tstPriHAta b-in in which the anAly---,ins
.:,.'Ere rincalculatind to 100% on avolAti1P-free basis. Samples 1, 2, 7, 8,121 13, and
14 from Bo de Aqua TApada and mplin---, 4, Fi¡ 155 Anri lf; from Bajo LaAluTribrera
were analyzed in Minndoza,Argentin:A, by the Dirincrion General deFabricaciones
MilitArins. 5:;AmplPs ?i,f--i, q, 10, And 11 from within or near thin BajoFl Durazno
prospect !.A.F.-i-2re analyzed in Tucsnn, Ar¡zonA,by -.7;1-::,,,lirii=. Labs, Tnca. :c;7-cfriPiPss 17
through 75 t./Pre rollinctind from a variety of location=(Table 4) And analyzed by
CaP11Ps (1 979) at Ouninn's Univinrsity, Kingston, Ontario.A dash (-) indicates: a) for
FPO: all iron reported As Fin.._r),; b) for LOI (loss onignition): not analyzed; and c) i...: .:t for CIF/ norms: a given normative mineral does not occur. 1 TABLE 2 5 l #1 7 10 ChemistrySample N o 3 (wts s` i i 2 C E t+ l a =c4 rtt,1 55.7 i-r. 13.60r_I7,9n 15.8 .f r1 1: =5,R7 # =3.15 52.1n1 rt,t i L+ . 16.1 5.2Lia 174,9P.57.40 5,15 a CLi 13.60 F.,29 15.2 4.41.8 58.015s0 4.01.8 Mn0Feb 10.n1 0 . 7.43_. ''' riL vt 0.08...:1i_4.2 s Ci- 0.17 - 1 7 0.21 - i1a140,n5 n,55t - i E,i t C ¡E .i.1 1 w CEn 1 .8v : i.. t :: 1.7 Mg() 4.58 f {{ r, 2.49 3.08 1..q . f Li 1 7 1n.:).5 i `iir A: aEt _- s 1 wf irf .L.t.4i.. 1ia!,- = i- 7ri . I 1 s ,Rn ist7.3.2.97 '8 fC 10.0n 2,,61 i ,, t 3.1lai_ 4e....oc a_*7 Pi. 2.17 t5 t , =L i . R R 3.13 .r r 1- 1.17 n.11;0.53 ] [ .1 9 r n.-I8 1.[in 0.0,55 :3q n.1747n,: t i t! . _ n,054 = 0.220 _ T + E 1 .F. ¿,,, i Ì q R -s :, =i r: s] - iLirt C 1 1 1.n1 - - 1 li la = 2.7 97.44 . ir, 95.3 q5.7 L 9 Total' +-t-.lt'i _ i t~ 5.4n.69 1.89 tt.i r 6, i 0.95,R 16.8 .: 0 1ni .i1 0.876.7 1.065 f 7.91 .68 .? 0.95 . i 1.:3 r? C 1.2r t-CIPW Norms 13.4 23.1 Ì j s'-t j + L+ s i i i = 17.2 1 ._ ' _ - 1.7 t 1 (-71.5 ii-,Ç't or `i i ' : t 24.0 16.7 -} 'i E 1 8 a4 2429.0 a'_+ =? a:+ 20.4 i ! R.3,wr ? iLiaL i E? :1 t..= .t+ - 274,0 +1,+ 11 , I 1r, 23.1. , . neEr#i_i 21.1 7-;.4 .i 18.9 r,L ai vt r, i18.1 , ! r. : iiat+ I 1 . f 3 1 tt. : r - - avi 1n.0 I .L . 1 ^ y 14.F. .l `i. . __ 'i 6.5 17.4 14.9 2.4 13 . ID 7 isi1 .__ ' R a - .- - .y E c 7 `` i 1 .5 1.1 r: is '1.z ,. t - _ i_at - 1 s6 -- 7rL.i. 0 ! ,1'1 C ± -:, t= I T_ì_1CiP 1 1 0. n 1 i In,rl il,6 99,5 1,r1 99.9 n,r_ '`. 0.3 == i 1_1i1,n 0.:; 1 1= 1. 1 r ,iar CD rv' LI.I ,r 1,~I Lf:l CCI r- r'Jl,i CD 04 11;1 ITI r'J r-CD rr:i r- ,- co o`, r-- L r J '. f 'J vr CO O w C' _. 1 e r''J I 1 1 I i;"'..I '.J.1r'JC C' U .I co co r.. I,:1 r- CD 1?1r',.", 0 ;IC) r- 1 1rl r-- -- r'J I.'J 04 Cr'
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incrPs--ring DtoRf:zt- for FRralinn tAiPrP r;:ilculAfPH using an
U;LLGlAta (sPfz, Plio-QuaternAry VolcAnic Rocks of the TABLE ChPmic7-17. Characteristics Ca lc-AlkAlinPAssociation South-central and Southern Andes ShOShOrlitiCAssociation A lk 111-13"--, sampleSmp1esFA.rallon no. ( Negro 22) CeP t wt % .A1 r) 4915 - 77 20 - 14 50.513.7 - 70- 13.7 Assorition43.719.5 - 67.5- 15.9 51,2:19.2 - -68.0 ot.A.) wtFe Enr% 7-111,:liPs ii.::1-crent .7: :3.3 - 9 low vpry 1ow .J... - -7 i . /4 .-7 C 1.8 - 12.4strong 5.4 - '74.1 wtkA) f % K f % -:.z.:. ;-::: , 0n »..4 t % t...1a._0 0.3 - 4.5 (1 , , ::,1 - 0.3 - 7 (1 0,55 -7%0 - 5.4 n = 1.1) !=h7trPez rOgraPhiC 1.3-0.4 1.8 - 0.4 2.4 - 0.2 - I% - - 1 irs op::::rpig p ::-:: 0cpxpig p x (nip CIF'W Nor MS (bi)(hb) (01) (bi)(bb) (01)op>:: (bi rare) foids 01 (npXoThopx) TAblP 4 - C:ompAri,---,on of rock nAmPs ohfAinPrifor the 26 igneous rock samples from the Farallon Negro volcanic romplP:::: using threediffPrPrif rod:: classificAfion schPrfiP'=. (s.---,P text). ::::AmplPs1 through Fi Rre vi-iirRnicrorks3and
,---ampiPc-- G through 1F. are subvolr-Anic intrusive rocks; similArdata are not available for samples 17 through ?R. A slAsh (I) indicAtPs that A particular rod:: composition falls on the boundary bPfkkiPPn two different rod:: nArriPs (cf. Figs. 3 And
7). :=:Ari-r,---,1P number, frir rock,-; from BAjo El DurAznocorrespondto f him fnUritAring
Sample numbPrs inFigure 11 And TAblP q::-: = BD-14n; f--; =BD-181-74; q =BD-78; i 0 =
BD-1521.; 1 i = BD-155. AbbrPviationsused: BAT =E:AjodeAqua TapAda; BD = BA jo El
Durazno;BLA = BsjoLA AlumhrPrA;FN= Farallon Negro minP; C: = C:ApillifAs mine; i'.-1\1 = Hi Vida; NW = norfht.A.IP'=.i. TABLE 4 SampleIUMbPr Sa Tr; ID 1 1 rnBATLOCatiOnS A Field ClAssification w Alkali (olivine) basalt High-K.Peccerillo IDA and Taylor (I 975) it 32 BLApiBDDP, A AndesiteAnde=.iteAndesitic flow flow volcanic brprctqbreccia AlkaliFold-bearingQuartzDacite (olivine) latite ]atte 1P1 ICO-bA Absarokite/shost-ionitPShoshoniteHigh-K f; BDBATt*--rid Of BD AndesiteAndP=.ifeAndPs--,itPAndesite: porphyry porphporphyry porphyry y r y AndesiteLatiteandesiteDacite High-KHigh-E::RTinakifp A.nriPii-PldarifP andesite andesite 131 7/ o BATBD DaciteAndeSiteA ndesite porphyry poi:.phyrporphyry DarDa.DaciteDarite riteite High-KHigh-f.:.High-F: dacite dacite ndesit e 171514 NWSirnBAT A of FN r-s.A DaciteD porphyry !Lt.: porphyryp porphyry AlkaliQuartzRhyolite (olivine) latite 1Puco-bAs7--klt LatiteAbs.=trokitP/High-l.:::BanakitellatiteHigh-I<. dacite basait 2270 NW cifofof BL.A.E:LEL/-. A W of BLA FN AndP=.itPnACitPAndesiteU:=0_ (olivinP) bas. LotAi-i
A.ftPrIrvineAnd ParAq:-.cr-(1 971), ..7;,./1-ribtils for FAralinn Negro dta,
S Tfie as -in Figure 10 6 4 2 0 45 50 55 wt % SiO2 60 65 70 75 _ _ Fig 5- )/Na._Ci vs. diagrm. Solin lineisbet_iit! (at mPthori, corrPlation coefficient = Orn:=0 frir FArRlion Pgro data e.xcuding sample. no, 22. FRrR11:--1 dRshPd linP is Rpproxivi-iRtP frPrid crf "typicl" continPnfAi rAlc-AlkRlinP rorks 7-IftPir .1AkPs and White (.1q72). :=-;yfribols Negro data
th in sRi-ciP Rs in Figu.r... 1 1 1 0 x I 1 - ...... x ft 4s A .-- - ...... "" ...... o 50 x ...... 1( ` r' s60 rt -. 70 wt % SIO - Fig, G -F.r1on N...,gro samples plotted on A =.tAndrii AFM diagram (A. =
+ :r F = Fp0 + 059Fp ; M = showing bound:R r y tholPiitir And cAlc-RikalinP fields after Irvine and Frgar (1971), :--.3ymbols same as-, in Figur..., 3,
Fig. 7 - C:IPW normativP of t\leciro samplesr: Itittida portion of the QAPF vcicanr rock clA....ifirafion diAgrAm of !=-rPri C:41t.r-fz; A = fPld..pAr; P = plAgiorlRsP; F = Number s and symboi S73.me as in Figure 3, Rhyolite Dacite Andesite Quartz latite 7 26X X 25 X24 21X Basalt 8 Foid - bearing .Latite 111MMIIII Alkali andesite 22 10 % P 35 %P latite 65%P 5A X17Al Alkali basalt 8 10 % F ti was followed during norm calculations, L+ui an Fi==,Or,F3t i ratio of 1:4 is not Fe L necessarily appropriate forsr- these rocks. Of the 26 samples analyzed, only sample no. 22 appears to be anomalous in that it is pot3ssiuftt-portr and tYt! re ii73itil_ than the other =aitipl%s. It to be more chemically ai::in to a t`r1 tlelltictr- alka-li (olivine) ba. = ait (Figs.throughgl-I ?g Tab le: 2 and 4) and for the most part i_ not included in the following ig disi=u=,=iLtn. The fiio=t significant chemical characteristic i tf the remaining 25 samples is their high content of potash. ch_ s honit i - rocks are, bdefinition, potassium-rich over a wide range of silica contents (Joplin, 1965, l?6P.! Dici::intson et Al., 1c1F[84 Jakes-,andWhite,1 972).Owing primarily to their high potash contant,.., the samples e [ ¡ , . alsoi"ia1erelatively high total alkali contents i, i.ti-.l wt / i-r.ti1 11a jt...jg Table 3}! and nearly one-third of them plot in the alkaline fieldofIrvin.... and Earagar (1q71) (Fig. 4). Although calc-alkaline and alkaline rocks may have similarly high total alkali contents, sodium usually prPdt]iïiiiîatt==,, over potassium 1á3di ¡g to i .a ¡::,_O/;,3t, ratio ttif less than InÑ at all '=;1n,.-, contents. Sh t ;¡'irtnitir rocks, i tr; the other hand, usually have I: rQ /tJa.,OJ ratios near to or greater than one at all silica contents, a i i.. feature generally agreed to t be the single most discriminatory geochemicali characteristic of this roci:: association (Table 3) (L-IrtF[lijl, 1q65, 1 qF_,R; Dickinson Pt al., 196= g Gillg 1970g Jakes and White, 1972; Williottg 1972; L _ f N v r e g 1973; Carmichael et al., 1974; Dostal et al., 1977; Geruelleg 1978; Hughes, 1982; Ea(er, 1982). All but five of the more mafie samples from the Farallon Negro area yield c /t=ç ; 0,7R and the average 0!tar0 ratio for all 26 samplesle= is 1.1 at an i i i average siliracontent of5q,8 weight percent i =. In Figure 5 it can be seen that I`: . 0/ta 0 values at any givers silica content are clearly higher for the Farallon t Negro samples than are those of "typical" 1=ontin=lntal c31t=-alkaline rocks. The A1,0, contents of the Farallon Negro t ri tr=:1 exhibit a wide variationtni in 3 Ti range tT 4.! v,elght percent Al__ }(Table 3), as is =haract?ristic of bot- high-potassium +_alc alkaline rocks and shoshonitic rocks 1.1aj::ta-r and Wi'iitijg 1q72). When plotted on a standard AFM diagram (Fig.6), the Farallon Negro dat a show little or no iron enrichment with respect to m3gi1e=it4fii andthP alkalies, and most of the samples plot in the calc-alkaline field of Irvine andBaragar (1971). Little or no iron enrichment is typicalaT both =ho1honit ;c and i==!li=-31k311 i= rocks, and distinguishes these two associations froïrt the 31ka I1F= and th! + lelltic magmaseries i 1 (Joplin, l95re, Irvine and Baragar,Ear a3r, 1'i 1;' al:e and 4tii t%=;r j'! ;. ln t 1.,1972; DeruPl 1P, 1q7P.). The rocks from the Farallon Negro rPgion are further distinguished from 31I::3i1' rocks in containing les=TiiJ.and Zr (Dostal et a1., I q77). In term= t f L the minor and trace elements (Dostal et 741., 1q77; +=aelles, 1 q7q:}, the Farallon Negro rocks ar e similar to most shoshoniticnitic roE_ka in being relatively enr li=hein strontium (20i -F,90 ppm), barium (200-950i -950 Rpï+i), rut,iJiu'rii (80-250 ppm), and the light rare earth elements (PEE), and in their relatively low K/Rb ratios (22L-33 ' (Jakes and White, 1971, 1972; Dostal et al., 1977; Baker, 1982; Et.4art, 1982; Hughes, 1982).. Dos tal et al. (1977) note that the REE concentrationsn= 3nd abundance pattern s of these rocks are comparable to those of sh! sh±+nitli rocks from Fiji (Gill, 1970) and New Guinea (Jakes and Gill, 1 q7n i, The modal niineralog ies of igneous rocks from the Farallon Negro volcanic complex are consistent with either the shoshonitic r cal'-alkaline rock asso' leti s, DPruP11P .q Chas, in fact, noted that these two rock associations are nearly petrographically indistinguishable in the ;Io ic3ni' r ck_ f Flio-..Qu3tern3ry age that he has studied from the central Andes. In terms of normative ïtlineralogi_°s, nineteen of the Farallon Negro samples are hy-norm3tive (Table 2), a feature apparently most characteristic of f =î toshGniti+_ rocks in rh+? central Andes :Deruelle, 1q78, Table 1). Five samples from the Farallon NegroE volcanic complex contain nepheline, olivine, or both in their norms (Table 2), and this occurrence, combined with the close field relationship between these undersaturated rocks and saturated rocks in the volcanic complex,are typical eatures of =h shonitic magmatic centers elsewhere, (Joplin, 1965, 1 `_r;8; vili, 1 ci7n; Er7ith,1q72; Willmott, 1972; Carmichael et al., 1974; Baker, 1982; E+A?art, 1982?. F?nck names for the twenty-six Farallon Negro were derived from three different classification systems and are listed in Table 4. The rough classification syst m used in the field tAias based purely +n f=gáscrpl_ m'sner a ogir features, and field names are not available for nae11es' (197q) ten samr` ies. The r'rii_I:s were then named on the basis of their CIPW normative mineralogies using the QAPF volcanic roci< classification n diagram of Strec-I::eisen (1 97q) (Fig. 7), and on the basis of their chemistry using the K,.,`! vs. SiO, variation diagram Ef Peccerillo i anb = Taylor (1 97E,) (Fig. 3). Only the latter system diStingi..iiShes between the th Elelltli-, calc-alkaline, and shCish initii_ r _icl:: associations, and it can be seen in Figure 3 that nearly two-thirds i f the samples plot in the high-potassium c-alE_-alkaline field, as does m C t of the "best-fit" line to these data. It is significant that the Farallon Negro area rocks are richer in potassium than the well-described sha shonitir rocks of Netf? Guinea Wakes and White, 1q72), and yet botht are considered ca lc-alkaline according to this system. As yet there is little consensus in the literature as to what the distinguishing criteria should be between high-potassium i_alC-alkaline rocks and si-iosh initic rocks, and the Farallon Negro samples might be considered transitional between these two3ss i± 13t inï1=. In view of the high potash contents, the {:[ The high potash contents of the Farallon Negroarea rocks are partif a regional chemical trend noted by Dostal et al. (1977)in Mesozoic to Recent volcanic ., rocks at this latitude. These workers documenteda systematic eastward ini=ree=_ in ::i contents (at any given Si content)inrockslocatedprogressively e_ L farther inboard from the subducting oceanic slab and at greaterdistances abovethe Benioff Lone. Similar (potash-depth) relations have been notedon many island arcs and continentalmargins at destructive plate boundaries (Dickinson and Hatrierton, 1967; Dickinson, 1968; Jakes and White, 1971, 1972; Lefevr"e,1973; Dupuy and Lefevre, 1974; Barberi et al., 1974; Deruelle, 1978), kivithnotable exceptions (Arculus and Johnson, 1978). In relatively mature continental oroge.ï il'- environments, the potash variation is characterized by the progression ri from Ca1i_-a1::a11nerocks near the trench through high-potassium '=a1c -a1ka11ne varieties trismall volumes of shoshonitic rocks located farthest from the trench (Jakes and White, 1971, 1972). The inboard setting of the Farallon Negro volcanic corYiple::;E n the landward side of the Andean orogen is therefore in agreement with the position of many subduction-related shc'shonitic rnagmatic centers elsewhere. This isolated volcanic Center probably forms the southern continuation of a series of shoshonitic volcanic Centers recognized in Peru, Bolivia, Chile, and northwestern Argentina within a narrow belt paralleling the Peru-Chile trench (Fig. 1 ) (Lefevr e, 197:::; Hormann et al., 1973; Dupuy and Le f evre4 1974; Dáraselle, 1978; Sillitoe, 1981). The tectonic setting for shc'shonitic magma t ísn" in the Farallon Ne'ar o area is broadly conformable with traditional concepts of shoshonitic magma generation duringa late stage of stabilization, consolidation, and welding in the development of a mature subduction-related oro genic belt (Joplin, 19651 1968; lakes and White, 1'472, Carmichael et al., 1974; Baker, 1982), but is most analogous to a setting in the southeastern tip ofPapuaNew Guinea where shoshonitic magmatism i-rnrf,ediately preceded a major episode of F' io-QLiaterna'i" 3' block-faultingltin'g and uplift (Smith, 1972). The Papua New Guinea e;:.ample differs in not having developed over an active subduction zone (Arculus and Johnson, 1 q 1R) GEOLOGY OF THE BAJO EL DURAZNO PROSPECT The Bajo El Durazno prospect, hereafter shortened to "Durazno",is situated near the eastern edge of the graben portion of the Farallon Negro volcanic complex.mpler :: The prospect occupies an elliptical topographic dP _prj ji n(a "bajo") elongated in a northeasterly direction and formed by differential weathering of vulnerable hydrolytic ally altered rori::j. Elevations r3 nge from aboutLFi[!i ito i 25 ¡5 meters ahilve sea level along the gently rolling hills of the flooriT the "bajo" toï 2655 meters along the peripheral rim of more ? r e=iatai lt pr CiF',f?itical iy alrer"ei] volcanic rocks. Mapping ai Durazno has encompassed three square : -1 i ieter s of the most jignifir3nt hydrothermal! itherliial a iter átliin including a peripheral margin T propylitiied rocks,I:.r-.+, Liut nar r rEt;ii zones i if prtyll ii_ .=:,ilt_ra E iiin extend beyond the mapped area in some localities. LITHO LOGIC 'Lit-AIT=; Vi..11i_Anir Host Ri id<1 The bulk of the host rocks at Durazno are andejitir vo j1=an ir [ir-erri3j with a fat..A, thin d isrrntinuj e j3 ti rA d r it e flows and sills. These rocks r3 % been collectively rn3F=pad as a single unitin Figure 8. The volcanic breccia is composed of fragments ! f andesite or andj`jlte porphyry y jnd local fragmentsof ¡r-e+E_rinjEJlidated viil=aiii_ breccia and F'rel_3mtlr laitl.?.lFalel_iiifi! basement rocks, all set in a fine-grained to microcrystalline groundmass of _indÑ=ite Ìi( andesit>=` porphyry. Fragments range in size from less than one centimeterup tii twofitpter-j in diameter, but are generally less than ten centimeters liing. Most fragments Pxhibit some degree of riiunding with jubartgul3r to subrounded fragments being mosti,r_ifiiTA?'in.There is 3 notable absence of jrrriarr:oujt puiitireiluj, tuffareonj,rir glassy material in both the fragments and the groundmass. , In detail, the l/iilr3nir breccia is highly variegated and consists of3 number of juhunitj that grade imperceptibly intil i_in3 another. This featurea_ made it difficult, if not impossible, to map individual subunitsor determine their bedding attitudes. All of the subunits appear to be i_iimpiiaitionallyidentical in terms of both their groundmass and contained fragments,and they differ primarily in the relative abundance of fragments andin the types, jiiej, volume percentages, and 34 Fig. E: - GPology ofhRAjo El Dil.rAzno coppPr-gold prospect showing ndesitic vokAffic brperiAsn d bA=Alfic anciPifP flows And s1s And lAi-pr porphyry copr--,Pr-rPlAfPd andPsifP porphyry infrItsions. Post-infritsivP sediments and sedimentary rocks- ha.',/e been omitted for ti iA sAl L n . J__, J r 7 Jrv r V L. r J > > ,_:.;, , _,,, ,,. N., 7 LV ^ < 4J .40.,....--,,,, .,',.,=,`,``;,`_-`,_- ; . V J,1 c ' " c. 1 7 . :" L < L J n L 1 7 VC _,_'---c' ì',f;,,;,';'',_ -,-:,,- , >V v 1 L. ., v v c V _' Miocene Farallon Negro Volcanic Complex b4 Intrusion breccias Volcanic breccias with minor flows and sills Li t holog is contact Andesite porphyry intrusions / Fault strike and dip showing F'f îej"joc r y = t s and ïrilc i opi".lenocr y = } = of quartz are ! Ij'ir -iÌtmrin. Hornblendeand jd "-1 t 1T F phenoi_r y_t_ never e:::reed 1i_i"'.,each by rill ir;lP ani; are usually le__ than five volume percent each, whereas plagioclase.: phencicr`-r'_t_ may range to 30 volume percent. The extremely fine-grained and occasionally l','l?"il_rF llti? groundfli3= = of the breccia and of the fragments themselves consists of interlocking anhedral grains of quartz, plagioclase (andesine?), and pote_1il!r! feldspar (=enldline'?), tt%ith one to five volume percent of disseminated anhedral to euhedral magnetite, up to about cine volume percent apatite, and trace amounts rif euhedral sphene, zircon, and anhedral hematite. Zircon and apatite are common as inclusions in plagioclase ph enricr á Jts. Accidental basement fragments in the breccia consist of i_rilAtei. angular to subangular fragments of ala_kite and massive opaque white quartz, up to two meters and one meter in diameter r e_pectl:'i-=ly.There 3r P Alsf subrlPdr Al white li_ri r 1nÑ megacv, r- supto three centimeters long, and r .banaslA r to r1 C r LidGc fragments of garnet-quartz-feldspar-mica gneiss up to 53 centimeters 3crrCs. Individual grains ff garnet or q uart z _ garn?t +f's =3 -till a aggregates , L i Ñd frrim these gneisses also occur in isolated localities. A few rare fragments of magnetite-altered ande_ iti= have also been found in the breccia. The dense, dark gray to black aphanitic basaltic ande_ite that make_ up the small volume r if flows and sills at Durazno differs from the main mass of volcanic breccia in being darker, more compact, and non-fragmental; it is generally r=i r+LL rf n F,rphyritic } butlocally small phenocrystsi jts 1 - .ifii_r+fr'ï'i_+n if=r'' =ts ! f quartz Li 11r- plagioclase occur. Much of the qr olnd(a jj_tthese roc:s con=ists1" an inter ïocl'::ing mesh of subhedra ï tabular plagiorlasecrystals. STria lï highly irregular and discontinuous basaltic andesite sills were found inonly oi"lF? ï +=??it; At Durazno roughly 350 meters south-southwest of the central stock. The sills appearto ba contini!C+us with several c,'lindriral feeder pipes(?) roughly one to two meters in diameter. Feeder pipes have also been recognized elsewhere in the Farallon Negro volcanic complex (Koul'::harsl'::y and Mirre, 1976). Basaltic andesite flows are found only in the northernmost portion n of the mapped area at Durazno. These flows are one to i twn waters thick, have vesicular flow tops, and are irregular and discontinuous along strike. The f ïo=nis have provided the only information on bedding attitudes within the mapped area (Fig. 8) and yield strikes and dips that vary dramatically over short distances. The distribution and attitudas of these flows seem to reflect a highly irregular paleo-depositional surface rather than the effects of later faulting or folding. Intrusive Roi_l'::s Intrusive rocks at Durazno consist entirely of medium gray andesite porphyry. This rock type occurs as a small elongate central =tori.::, several near-vertical dikes, a single small shallowly dipping sill less than a meter thick, and two small irregular masses near the stoci':: which are toi . small to showi_Fn Figure 8. The stock itself is irregular in plan view, partly owing ig t r' numerous,i4 _ , = meï !4 dike-like projections;Fns; its contacts vary over only a few meters from sharply linear and moderately to steeply dipping through wavy, irregular, and shallowly dipping to gradational, assimilative, and brecciated with fragments i f volcanic breccia.A large andesite porphyry domai structure with miarolitic cavities lined with quartz crystals and elongate roof pendants of volcanic breccia near its contacts alar, occurs just to the northwest of the mapped area. For ease of discussion, the six major dikes at Durazno have been named according to their position with respect to 37 the central stock, e.g. the South dike,the West dike, etc.. It i_ ?rflF'c+rtant to i note that the southern extensions of the North and t'clrthPsLdil::Ps, and almost all ofr the southeast extension of th*the Northwest dii::1= a= shown in FigureR are largely1y conjectural as they have been interpolated under sedimentary cover between available outcrops. Moreover, nearly all of the contacts of the unusually large East dike are obscured by sediments, and most of them have been mapped along cliff fact=`= or elt,ngaconspicuous break in slope in the sediments. The southwestern terminus _f this dike has a do rri_l appearance nearly identical in size and shape to + the dome already described, and the peak of this "dome" is the highest point of elevation within the mapped area. All of the intrusive rocks at Durazno are compositionally and petrographically identical to one another and to the andesitic groundmass and fragments of the volcanic breccia in all salient aspects. The intrusions exhibit a range of textural and mineralogical variations that differ in grain size and in the types, sizes, abundances, and relative pr i per ?=n= of different p =ni _r j ci minerals. Most of the different varieties of andesite porphyry found as intrusive rocks also occur as fragments or as g rolnd _ 3r C in the volcanic br?c_ - a, and only a few subtly distinct varieties ofider i Ñ porphyry occur PxrlusivPly as ° - r intrusive or volcanic ác fihases. The overall petrologic similarity between intrusive and volcanic rocks at Durazno argues strongly + or a =ofagra i= origin for these rocks. This interpretation is supported by the close temporal relationshipJ established by F::lkr age dating between the time of emplacement c {f an andesitr porphyry intrusion at Durazno at 8.7 + 0.4 m.y. and the 10.6i.6 + 0.5 rii.y. age obtained for volcanic rocks a few kilometers to the west (McBride, 1972; McBride et al., 1976). tt¡tineralogically, the andesite porphyry intrusions at Durazno consist of anhedral to euhedral phenocrysts of plagioclase up to 7 mm across, = nS-iFdr-a ito euhedral green pleochroic hornblende up to 10 rflm long, and anhedral brown pleochroic biotite usually less than about l mm acrossbut locally up to 3 mm in diameter. These minerals are set in a turbid, extremely fine-grainedgroundmass of interlocking anhedral grains of plagioclase, quartz, potassium feldspar(aenidiner`), and accessory magnetite, sphene, apatite, zircon, and hematite. Quartz never occurs as phenocr yst S. Plagioclase Flhen +c r f s± s are ubiquitousand occur in volumes of up to 30`!fi! they typically exhibit albite twinning and less commonly, Carlsbad or pericline twinning. These phe1=ECry=ts are usually larger, more equant in appearance, and more regularly spaced than the plagioclase r'hE_n Ecr;'stj in the volcanic breccia. They exhibit well-developed normalmal and oscillatory zoning with optically determined compositionsEn= ran'aing from ?ndejlnÑ (An,_,c )t i E labradorite (An) with an average of andesine (An.__). Many plagioclase phe%ocry_ts have turbid cores containing abundant minute inclusions withEr without ¡tAr'r-E_ttaly inclusion-free rims. Hornblende and biotite phenocryStS occur in quantitiesEf up to volume percent (usually :20 vol. %) and 10 vo lt ume pet-rent, respectively, And can Show considerable variation in their sizes and relative proportions over distances of only a few centimeters. Texturally, the andesite porphyry intrusions range from dense brownish-black aphanit iE_ rocks with only five to ten volume percent small plagioclase ph?it Ecr ySt j to coarser-grained porphyritic varieties in which plagioclase phenocrysts reach their maximum size of 7 mm in length. The textural variations are particularly well developed in the central StiErk and the large East dike, but they could not be mapped separately be+3ür e they grade imperceptiblyit1, into one another over distances of several CeLer S. It is possible that some orall ET these texturally distinct phases in the central stock are representative. of distinct but closely timed intrusive pulses. The aphanitic variety of andE==sit_ porphyry J is not common and occurstrs a 1 locally developed chili i3rgin= on some dlÌ:Pr and dli:elets, particularly with greater distance from the central stock,, but3l-rE along portions of the North and West dikPs in and near the stock. It alsooccurs as irregular masses in the center of the stock: a.id Ï_-Ai the =Ln_1{ = ijnl''thPa= Pfi n edge. Fiäld r P tat t {nsh1=`s aifi! {nqthe ie !r ar ious dikes., at Durazno illustrate some interesting features of the evolutionary¡uti tn3ry hi= t i {r- y of this porphyry system. First, many of the dikes, particularly the ='#x larger ones, display acrudely radial arrangement around the central stock:. Radi31 intrusive And Tr3ctur'e patterns surh t _ - 1 3.1+'- thirthis 3c are P usuallyinterpreted i-=! ir Ñ i*==_r 3 { i. i{4! 3t1;= of T! {i_F`fur CI`{3=3ii3nT . _ e i i t i a ' 1. . i- e -s (e.g. Titley and !-S3 iri{i , lf r j. LSecondly,-Ft=i 1iii j, almostii_`-=, 3Ï {1 i_`i3 1.3' 31 ! Durazno t y_3n be considered °intr3finer3i{ in the broadest sense of Kirkham (1q71). The Northwest dike, for example, is the ea r l ie== of thPl Ajr 3ikPs at Durazno n_ 3nd has bPPri , : { ?t3!::ly'3n{ pervasively14 = tl F3Ìiui1. . -r ilira altered;_r+i'ir cut by a variety of potassium-silicate and magnetite alteration =fein 3r r rlbl= g3r -F3v the s i I c ' All latPr dikes 3rá weakly pi npy11tized throughout their lengths and cut r`ri,'11ii=3lly altered rocks s 3t greater distances fr nm the stn{_k, buf 3r á themselves pi I, i li? 3 il f: altered to differing degrees nearer the stock:. .. Thá jlon1yttttF+ iT 3 ii C_ r i {.[T in ofi t differing intensities- of F'hyjlli{= alteration around these dikes, with the exception t {f the southernmost two dil:;e1Pt=, is taken to reflect the fact that dike emplacement did not in itself cause C'h:rllij= alteration or tinhancá its áfiTat=ttil;áná=á in adia{_ánt w3llroci:s. Nor ara the dikes less susceptible to phyllic alteration than the/ i 1_a nli breccia in terms of composition, permeability, fracturing characteristics, or other f3rti{r ;, bP{=atisá the entire range of F`hy'lli{= alteration a::tensi`t'3ïìej=, is found in them. Instead, the field relationships suggest that these dikás tear P PiiF' l-` .i_ i{ immediately after the main period i {t pro grading.ïig F'Ì t;r lliE= alteration at a tifflP when the hydrothermal fluids responsible for phyllie áiter3tl{1n had regressed to r A position along a narrow annular zone around the central stock. In purely relative terms, a comparison of the extensiveness of phyllie alteration developed in the five later major dikes suggests that the South and Northeast dikes are the earliest of these intrusions, followed by thP North dike, and finally by th e late-stage East and 40 West dikes. A third aspect of the Durazno porphyry system is thatthe v.le_ter n end of the stock originally tapered down and tA1as connected with the South dike.The northern end of tt-iis dike. was =ubseglsubsequently intensely then faulted left-laterally, and finally both the di1::e and the fault were cut and par sail;ti obliterated }by the late -stage- r t'e.tWest dike. TheseC ndand ot#-les field observations indicate that intrusion, faulting, and hydrothermal alteration and mineralization overlapped at several =tage= during th e evolution of thisporphyry 1rt¡I- . 1 L e¡ i in addition to the radial dil::F patterns and locally brecciated stock-wa ilrs=1F i:: contacts, several other lines of evidence r sgg== t that the emplacement o f the andesite porphyry intrusions at Durazno was s smFtAlhet f F 1rrE=fl_?l.I he central strick and almost all of the dió::e_., exhibit patchy areas where elongate hornblende and pla'giocia=e phenocr fat= are aligned along flow lines either in bands paralleling intrusive contacts, or in partial =piral= around lithic fragments or larger plagioclase phenocrysts indicatin'g rotationa1 movement. Subangular to well-rounded and strongly epidotized lithic fragments less than ten centimeters long are coiffon 1 found in all rii of the major dikes, and smaller, _ u ig Jla r fragments of magnetite + quartz veins or weakly propylitized_ flitled nss==it+= occasionally occur. Lithic fragments found in the central i st sE=f:: include isolated, a ¡igt tler"t+_, well-rounded fragments of intrusive andesite porphyry, volcanic breccia, leg-eLite +qL?artZ + biotite veins, pegmatitic granite from the basement, and in one place near the center of the stock, a small pod of massive chalcopyrite intergrown with pyrite, quartz, and euhedral gypsum crystals. The preferentially epidotized fragmentsthat areso,_orlrlion in thedikes donotoccurin thecentif =t _::.Irl addition to isolated lithic fragments of diverse origin, several small intrusion breccias occurur- in theth s_s?ntra' sstock. There, breccia= consist.i, of anllAr-tr, aubrounded fragments of all the different te::'tura ivarietiesf the intrusive andeslte porphyry supported by an andesite porphyry matrix that is te::::t?..lreil;, 41 identical to surrounding intrusiafe ro+_i::sin the immediate area. The breccia s thus grade outward into massive, n tn-t+r3C+_13t+=dporphyry simply by .=a sudden d3+=r33=á -in the number of fragments and do n +t havedistinct litho logic,g1+=i ;lt+=r3t1+ ln! or structural boundaries. These intrusion breccias occur insmall irregular patches,ies, locally along contacts, and also along 3 linear north-northwest-triidlnq zone that floors =., a narrow, steep -sided drainagei3ge ln the west part + t the stock (Fig.8). The linear breccia zone may represent an early zone of !Aie3R::ness, or perhaps afault active during the initial stages of emplacement of the stock. It may even correspond to the lineament of similar orientation noted on Landsat photos += that pa1se=, through the center of the prospect. An intrusionbreccia also occurs in tii+? west end of Lhethe West dike (Fig. 8) and includes small andesite fragments kiith veins and clots of fine-grained magnetite, in addition to fragments of intrusive andesite porphyry and volcanic breccia. Sediments and Sedimentary Rocks Well-lithified fluvial conglomerates and minor cross-bedded sandstones crop out along the present drainage system of the northern and eastern portions of the Durazno prospect. Similar conglomerates also form th+? base of a numbernf flat-lying alluvial terraces and grade upwards into unconsolidated colluvium. The colluviurn and alluvial conglomerates are both poorly to moderately bedded and sorted, with angular to sut+rounded and occasionally well-rounded indigenous and exotic pebbles, cobbles, and boulders supported by a coarse sandy matrix. =riïÌiP of the most recent colluvial rubble is clast-supported. In one place, white i oar si:?-graine+j e +ll? n sand overlies the r_ +l li I LIiu rii, presumably derived from +m r `iP extensive sand dunes located several kilometers t+7 the north i +t th+? prospect. The Durazno porphyryyry rnFper system was probably unroofed during the widespread and intense period of Pllc +-Plelstocene block-faultinglting that accentuatedlated the development of thP graben port-inn c+f the volcanic complex. A small basin was subsequently developed over the ProdPd northern and eastern portions of the Durazno prospect and toas partially filled withalluvium and locally dissected by fluvi?1 sand and gravel channels. The near-horizontal bedding ittitid3= tf the alluvium indicate that these sediments kagere depositedited ns earlier than perhaps the Late Pleistocene. The present drainag e system at Durazno fo' loih,r and has cut down through the older channels, in addition to creating new drainages, some of which dissect the alluvium and form the Plevated terraces now seen. T iese erosional patterns suggest at least ttA, periods j of renewed uplift in Recenttimes, anda similar theory of renewed uplift in the Late Quaternary has been proposed by Quartino (1353) for at least the graben portion f the Farallon Negro + volcanlc complex. Many ! +f the drainages at Durazno follow highly phyllii al c;r altered fracture systems, and some f these fractures may be unrecognized faults. Min r Pleistocene to Recent hot spring a _iit y is recorded in the _a i ï_ and chalcedony cement of conglomerates in two very small areas at Durazno. One of these conglomerates is unaltered and forms the base of an alluvial-colluvial terrace overlying the center of the West dike near where it cuts across the western contactofthe central stock. Here, abundant manganese oxides (Fig. 9) and iron oxides accompany the chalcedony cement as coatings on this 'rrilnera! and n pebbles. The other conglomerate is preserved as a thin veneer overlying the center- of the stock along the steep wall of a riverbed. Beneath the small area where these fluvial conglomerates are weakly silicified and cemented by calcite and ch?lcedony, a small seepage spring in the riverbed is presently depositing caliche and alum(?) and may represent the "last gasp" of the former hot spring. Similar late-=tage hot springs may once have been relatively common and widespread in at least the graben portion of the volcanic complex, because conglomerates at Bajo de Agua Tapada are also c erYlented by calcite, and locally, chalcedonic quartz (Suchomel, 1983). Sediments and sedimentary rocks have been omitted from'rri the ai=Lo'iiip3ny'iilg maps f the Durazno pr spe_t for the _a!::Ñtf clarity. Most linear TeaturW1 At 43 Fiq. 9 - Map of the Bajo El an-AZT-40 pro.pPct ...bowing thP distribution at the surface of: 1) visih1P coppPr ...ulfid.. and "oxide " minPr:z1lizAfion; 7) "high pyrite" zones in potassium-silicate, phyllically, and propylitically altered rocks; 3) dendritic a.nd spotty friang.anPsP oxide.; and 4) the outer limit of gypsum veining. The outer boundary of visible copper mineralizRtion is dashed where peripheral rock.. rP ob..curPd by sedimentary cover. Visible Cu mineralization Outer limit of gypsum TV veins Isolated occurrence of G r Lithologic contact visible Cu mineralization Fault (dashed where High pyrite zone inferred) ( >2 to IO vol. %) Manganese oxides 44 Durazno are not continuous in outcrop over long distances, and thus nearly all of the long linear intrusive contacts or alteration bf_sund3rl3j shown fn the accompanying maps represent the simplest interpretation fn rff geologic features hidden under sedimentary cover. HYDROTHERMAL ALTERATION The types and distribution of hydrothermaljermal 31tFration at the DuraznoÉ porphyry copper prospect are broadly similar to hydrothermal alteration patterns at many porphyry copper-type deposits around the world iLnt:w,lPll and (7iuilbert, 1970+ Rose, 1970; Gu i ibr_:rt and Lowell, 1974; Hi llli=tPi ,i q75). In generai terms, the hydrothermal alteration pattern at Durazno consists f # an early central potassium-silicate altered zone surrounded by a broad area of essentially coeval ' propylitic alteration. This pattern was subsequently overprinted by prograding and retrograding phyllie alteration which at present levels is developed t _f its greatest degree in an irregular annular Zone straddling the border between the Parlier potassium-silicate and propylitic alteration zones. Variations on the general porphyry copper "theme" at Durazno include anomalous preci îfu= metal mineralization, the development of early magnetite + quartz + biotite veins, patchy ! f andlocally intensei_ _ silicification, andand nt1 i ier f= of di-tinf_ti'eJ -ilicified pl-tyllically altered fracture z fnrs. Alteration Classification Scheme During field mapping at Durazno, the types, distribution, and intensities i ft hydrothermal alteration were quaiti ied according r o a classification scheme developed at the University of Arizona G, J. M, Guilbert (Guilbert and Park, i 9< á= ), Because the Bajo de Agua Tapada and Bajo La Alumbrera porphyry copper prospects were also mapped according to this system (Suchomel, 1983; Stu its, l q;,4) with constant comparisons of altered rocks from ali three prospects, the final alteration fapsofthese three porphyry systems arF internally "standardized", The alteration mapping system (employed here) consists of ttw!o pár aiijetei =. 45 The first parameter is designated by a letter" and represents a¡'arti:=ular set of intensive variables such as temperature, pressure, and crEFiifiF=A É c ±;mF'C=ition6± hydrothermal fluids. These variables, in effect, define a particular Alteration assemblage such as potassium-silicate. i.'[('), magnetite propylitic ('F"), phyllic argillic (IAA, and sili=i _ CO:). The second parameter is designated by a number frr+ff! one to ten that qElanLifi?_ thP e:::tPi'tt of hydrothermal alteration. The number indi=ate= the volume percentage Ì if minerais susceptible to a given alterationn effect that have actually been altered, e.g. primary quartz, a stable phase during many alteration reactionis usually not considered s"su=CPptiC-iP" -min+=ral, and the total volume f primary minerals is recalculated j tn{ i[i% without quartz beforeIr'e judging the total volume of minerals that h.3 vP Actually tPPn altPrPJ. TI-EP 1 to 10i designation is an abbreviation for 10%i'!t, tr 100% i% .a,1itÑred by volume. As an i*?xa'EïEple, a r'r_Eï j:: completely converted to a mixture of sericite, quartz, and pyrite is designated as "S-1 i", meaning that 100% It the susceptible minerals in this rock are phyllically altered. in using Li-Eis scheme it is important to r"M`ÏEi?''iEib*_r that the "susr=eptitle minerals" under consideration for a given alteration event may be the end products of a previous alteration event and not the original primary rock-forming minerals. A third parameter of the classification s,'stP'ifE has nnt been t us! F+1 Pd re C7?raurr_ it ._!_ =f found ndfo bet1_ïl, i rl_s r-rFniti = ;; Ñf for1 pr r_=` i_e l_pF'l1 rE. This parameter is an attempt t:= quantify the distributional characteristics of alteration, ranging from alteration minerals 1_i ImpletPly confined within a vPin through increasingly wider alteration selvages on veins tn 1_rliiEr'lPtPl:fi pervasive alteration. In this study, the distributional characteristics and intensities of a given altëration type in a particular volume of rock are combined using a weighted averaging procedure whose results indicate the total ?:ten t to which a givenii=n r JcI:. volume has been altered. For example, if an outcrop op with pervasive P;-Eylli_ alteration (S-2)r has superimposed on it a_nii'Epletely phyl li_all,¡ alter ed fracture system that makes up roughly :_; !% ! jf the volume of the outcrop, then thP total alteration thus consists of about 70% of S-i. and =:[l% of which converts n EE ' ! E s C - °7 to: ( 1 a :'JeL)4- (n.3 .. I1 }= 1!44[r" roundingoff,+designation f +T. Potassiurr! sil icate Alteration Potassium-silicate alTeration is 'AJeak1y developed in the center of the Durazno t porphl ry system 4 F ig. 10), and when viewed mega =i= +Fjicall jr, it appears to have affected the stork And thP volcanic breccia in a different fashion.in. ::l_lr'f3? e samples of the central stock seem almostcOiriF 1Gte lyunaltered. In contrast, the surrounding dark gray to blaci-.:, fine-grained volcanic breccia wa ilr"! c k= have clearly been pervasively biotitiied, primary plagioclase phenoE=r ;fst s in these rocks are clearly visible, but the brecciated textures are often obscured. Under the microscope it was found that these megascopic differences could be explained by the fact that potassium-silicate alteration is slightly more intensely developed in the volcanic breccia (K-03-4)).3-4)) ti-ian in the stock (K-(2-3)). FE -r-thPrm! re, the proportions of each secondary mineral are different in the two alteration n z! nF=. Biotite dominates the more intensa p sLas=ium-= 3lit=?te a ltar :Tii i¡n assemblage in the volcanic breccia as is typical'Í' this alteration type develoFi=_d in anJes itir (.4allroci::s elsewhere (Prise, 1q70; GuilbPr-t And Lowell, 1q74; Gustafson and Hunt, 19751. The secondary biotite is r+=adilj recognizable by its ±ine grained, shreddy, an6-ieJr.al habit, ;huai:: to absent plei 3chroisrrE, and pale browntopalP 'areenijÉ E-br wn color. In contrast, rliagn?tit=_ and calcite with lesser sericite are the most common alteration minerals in the stock. An additional difference between the twnwo al e aL; o assemblages is that =e_oar f quartz, ep i d ? _, 'al i^ i1e . hematite, and chlorite (pennine) are minor but characteristic minerals of the :-t3._4:alteration assemblage, while only the latter two of ï -es_ five minerals occur in trace allEol.lnt=, in the stork. Except for slight variations at the scale o# a thin section, the K-(2-3) and K-03-4) alteration assemblages are uniformly and . pervasively developed in the stockand its wallr{ ici :s, respectively. There is noE apparent Yegas=! pi_ or microscopic zoning patterntern f the extensiveness of 47 Fig. 10 - FotAssilirri-silicatpnd prnpylitic Ritgration. The inner boundary of Propylitic alteration is, thg, outer boundAry of Prit;vESiurri-Si lirate Bot1-1 thg, ArtliR1 observed AltPrAtion hAlo And its maxi-mum possible nriginal extent .re Both hAins arin PxtrApolAtPd under siAdtmentary cover as in MOSt of thg, NorthwPst dii alt Prf ion may be obscured by superimposed phyllie alteration! that isfrir modPrat&-_,ly to intgnsPly phyllicAlly AltPrPd whPrg, pAtr-hP=. of 1Pss intensely phyllically altPrgid !=:;--7:) rocks Ho not occur in which former potA=;siurn-silicAtg, altPrAtion could be rPrognizg,d. The nrirfhPAsf PionAtion of the pec;s1Hle: Potassium-silicate aiteration halo therefore rPflPefs the norfhPast PlongAtinn of thP phyllic AltPrAtinn hAlo And not nerPs.,-.Arily thP tritg, qPrii-riPf r j of the original potassium-silicate: alteration halo. POTASSIUM - SILICATE & PROPYLITIC ALTERATION K-(2-3) Known Lithologic contact + K - silicate K -(>3 -4) alteration Alteration boundary Possible maximum extent of K- ..%"silicate alteration (now obscured) P' Fault Propylitic alteration (P -2 to P-4) 4 g potassium-jlllrAte alteration in either rock type, and where not obscuredjcur'ed b}r superimposed phyllic alteration (i.e. )s-3), the K-03-4) alteration =E r!e seems to persist to the same degree out tothe, boundary withpr opylitic a it+.ratic =n. in Figure 10 the zone=, of potassium-silicate a1t*=raLi==n 3i=tHally observed are shown, along with the maximum possible extent of the original alteration halo whlrt might now be obscured by phyllie altereti =n. For ease of discussion, the characteristics of p =t3s1iuifi- t ii==at3 alteration will be described t+=lov.1 in terms t f rI_i+=i:: types because of the ! in i f +r ilt =_! rrP id tion between K-03-4i alteration in the volcanic breccia and K-(2-71) alteration in the stc+ck. However, it i_ importent + tnote that there Ar+.? few Siilel1Arpes within the stocktck that are too small to show on the accompanying maps that havebeenaltered in exactly the same fashion as the ii11l=_3nii= breccia, and the fc+1lo+.hfing disolssion of the biotitized vi ilraniE_ ``Ur e=_=_ia apr`1iP= to these 3rPaj3= lfit'=11.These moree bi tltizPd areas correspond j to = the aF°i-laniti=_ variety of intrusiveE jive ai ndesiTe porphyry described earliPr, but the textural distinctiveness of this rock is in part a pr iiifat y feature and not solely a result=f alteration. Plagioclase C'hfini Ic r f sLj in both the stock and the volcanic breccia have been altered primarily to a fine-grained intergrown mixture of aniledra i calcite and sericite along certain, presumably more 1elric compositional zones. Minor amounts of extremely fine-grained sïlreddy secondary biotite and disseminated anhedral chlorite and epidote also accompany the calcite and sericite as a replacement of plagioclase in theo1 c ani_ breccia, but of te=_ e lattPr ii, rals. only trace gl.iantitiesIf secondary biotite are locally found replacing plagioclase. in the stock. Hornblende phenocrysts are altered to i fine-graiiled mixturesI_1f disseminated anhedral iilagnetite with very minor calcite and secondary biotite in the stock. In the volcanic breccia, howevery hornblende is the fir= t mineral to be completely replaced by seco darJ biotite; t'e biotite is e:tr.1elJ fine-grained and shreddy and is often accompanied by minor amounts of disseminated an~!+.di 3l 49 chlorite, sericite, calcite, epidote,and either 'fiia'inet'te. or pyrite. Primary biotite phenocrysts are loi_all, altered to trace amountsof le.uE_oxenP in either rockii_k f jEpe! but generally the biotite phenocrysts in the stockand many =ma1Ée-r ones in the volcanic breccia are completely unaltered. Some primary biotiteFheno_r;sts in the e stoci:: are rimmed by secondary magnetite, however, and in the northeasternleastet"n F'! rtion of the stock many of them have been partially I_hloritlLed. Most lar'aer biotite=in the volcanic breccia have been part ly replaced by either secondary biotite withir- without disseminated p1 riti=Er by minor amounts of disse.minated chlorite and magnetite. The groundmass of the intrusive andesite porphyry containsintains =1'iiall amounts of fine-grained disseminated anhedr al sericite, calcite, and secondary magnetite with traces of netilatlte, only locally does it contain minor amount's of secondary biotite or traces of chlorite. In some places in the stock, secondaryj magnetite in any form is entirely absent without a corresponding increase in the abundance.i i sulfide. minerals that fiiight indicate thesulfidization of former magnetite. Disseminated pyrite, chalcopyrite, b! +rnite( ?), and na. ti't+e gold(?) also occur in the gr oi +ncii!a s = of f t he stock, but together r ar Fly e::'1_ePd i Ijjrr volume percent, The gr"oundmass of the potessiul!!-silii=ate altered tIËlll=Anic brÑ+_i_i:a contains a relatively constant amount of about15 to 20 [vCllufCta percent fine-qrained, intGrqranular, anhedral secondary biotite, ragardlass of distance from the stock. The secondary biotite is usually accompaniedifripe,nieJ b; small amounts of clear secondary quartz that occurs in clusters of interlCii=i::ing anhedrel grains in the grou ndr ars.These clusters occasionally form larger, iCg1r _Cr;- a1l ;t-ri-l=, smooth-sided irregular blebs up to about one or two millimeters long in ;h%hi_h the constituent grains display undulatory extinction. One to three volume percent pyriteas di_c lati C-= and ven i= is usually also present in the gr ou _If the volcanic breccia, and in places is as abundant as ten volume percent ("highhiah pyrite" zones, Fig. 9.}. Variable but small aifi=iunL= of fine-grained di=Saiilin.staij anhedral chlorite, sericite, calcite, epidote, hematite, and lesser i'::ac iliriita, the 5n latter 3s radiating plumose aggregates, also occur in the gr i1uÍ-Idii1ss along with local epidote L;ei41iets. Disseminated grains and clots of secondary magnetite erratically distributed in the gr f'3i Endffa ss t iT thin Lliot Ìt iiPd vf [ ii-A Ì 9c Lij Pi_i iAi Angi. Hp to 1 2 volume percent in some areas, but are =ni Ìrr- 3 y absent ii F hPr 1a Disseminated chalcopyrite ,n the gí 3=tEilld{ii+ss th ';iGi1-::ln i1_ bi Pf=r i3 r 3rP i;f exceeds cine volumeiLme p3r"t 3¡it and gel iP1' 3i iy dlminlshtm... with di=t ä ilrP from the stock. In the biotitized volcanic3Ìl11_ br"e1_c ia, some of ti-il_ jlr_1nd3r y biotite, regardless E iT occurrence, has been subsequently altered to either finP-gr"3iÌ11=13 sericite chlorite, i 1 r an i ii t e r" g r 1_I I Ìl mixture !_t t both `.i = h traces S 1f calcite; 4 Ì-i 3 few non-phyllically-i}I; li'1-3iit¡1 : -sii-l_r 3ia 3j i=3_T`e,the!"f secondary.1 1-chlorite1 1 r 3I L13 hasbeen ='=r i1=it--r{=j13l a ,3i1 some i f this ch or ite has definitely replaced secondary biotite. in general, the overall 3buÌl13=aÌ11_e.s i IT thP major 3 iter 3ti1 iÌ i minerals (by t{niLtiiF?) in the K-(2-20 3Ìid K-03-4) 3 itei Pd zones are I) K-(2-20: a7 biotite: 0-10%, uJua lly 1-2%; b) t i - ii ¡ia ` `-' R ' 1 .f i chlorite: , iLlil} (2%; J1r"11t ei - i i 1 n_ 9 i .t Ì F i t Ñ s i - ç ti},l' .i '1 iÌ" f i s i t} i,., a , tsi , Ls i- .so . - T 1 1 r . ¡ {cr Tepidote: _i,,,,, =i::3i3 Ifiite a_Ì Í:,, 11 magnetite: _11_i"lt,,i) i! F'; rite _i f_, a` _ha ci p frite <1%; t . hematite: <1%; i1 7 eL! :xPSP, n-1%; r ) K-03-4):3,Ìiiititl=. 3 rl . rf C n f r= -a 1f . . . _ a i f :!, 1_Ì 1_ - .- `-r = (1-1(1%; .:..L i ra, I.Ìchlorite: IÌ-flirite.;_ L, 3er"iil_e,2-10%;IJ f1-a 1a...it i-1 Ü rL., ! -iiar"t _ f) epidote: Í -2iLZ1 ? .,31'_sililite: ! i-i : q h) 'i3il _t t = i-;_. i'L ;1.Ì,`r- i1_e, ' . -11 1 sL:_ , j); chalcopyrite: (1%; riÑm3tite. (1-3%; I) áeui__I;':e{Ì+=. 0-1'.1*. In the above percentages, !_i`,'/, indicates that 3 glifeïi mineral is absent, while G% % iÌl13i;=ate= that mineral occurs in ubiquitous trace amounts. IÌl drill core from hole no 7 (Fig.i 1}, the only c rlr-e e:::aÌlliÌled, all gr"adAti3 En= between the }'::.-i.2-3i and K-03-4)3-4.} ?iter"eIj ? IÌle. described above are found in both r! id types, along iif ^more s=r i=itised or phy ilidai iy altered (sericite + quartz + pyrite) intercept...s. IÌl Addition i.n the LÌsur-i1 F'ervasiL,le potassium-silil_ate a'tPratiiiÌ,1 therer'P 31.o riots And ., e t= calcite intergrown with magnetite pyrite, and locally chalcopyrite inthe drilliicore. Below the oxide-sulfide boundary at approximately 9 meters depth, both se1_1 41dary unaltered white 1_Irth 1r.lä=a afin F°a ÌP purple anhydrite_ become easily Fig. 11 - SurfacP rock chip sm-npl.. locations .Rnd vertcs.i di;rmond drill holi. locations. Sarnp les shown were rol1Pcti.d for friAjOY Anri Tninor P1 PmPnt qPnchemistry, fluid inr-11v=1on study, 21Prtron TnicrnprnbPtLLdy, A.nd pPtrographyi but not all petrographic samples are shown fz;A.TriplP prefixe s "BD-" omittPd. (BD-) ciq and 159 are too closi.ly sp.red to show :;.arnpl... RD-1R'; from th P top of Ai larq,. ,..ndPsite porphyry ricirni ,---.tructi;r.. 100 - 200 meters r.ff the nrirthw..st corner of the Tni.p in the dirPriirirl shown; the Northt.A.IPcIt dis.q.ppP7-irs within 25 meters, of contacting this dom... For reson of scalPi drill hole s.; #7' nd #4 cA'nnot be shown sPpRr....t.ly because drill hoi.. #4 i= lorRtPd only RO cm to the eEt of drill hol.. #7. S.mp1e (BD-)1:::0 isdrill cone froTn drill hole #7. 10 Sample number and location Lithologic contact L 3Drill hole number and location eFault 52 visible, although neither was observed in 1us fAi_P A.TplPs. The AilhfriJi itP tflfa= i11ii seen in veins, but instead oc curr disseminated in thegrÌtndi-Ar -_a= . = , Ate3with secondary biotite.-ga= ÎpZra ll, visible secondary K-feldspar in drill core is for the most part fracture-controlled and occurs 3s veins or vein selvages u R to several e_tim_t=r= tiid e. It seems t_ olPrpi i -t An earlier episode of pCr t f3r ie biotite i anhydrite + magnetite alteration and is acc3 Fá nird by d sseiin=.L=d pyrite, some j ±j' which partly replaces the earlier magnetite. Some K-feldspar selvages are separated from the bi titii d areas by narrow = lva]es of more intense biotitization. The larger areas of K-feldspar 3 ÌtPr'3tinn ar"P not vi=t fr common, i`-lsile3f=rv3r"4 and usually secondary K-Tr jdjp3r is present in volumes iT :!!;L, and can only be detected in thin se{ tirin whPn sr3inPij with sodium i_obaiflnitfit3. It occurs as extremely fine-grained interstitial patr =s in the groUndY_s r an_- ar 1? iffu3e narrow leiii3g e= alonging vP inj, particularly along veins containing m3gni?tite, Trace quantities of secondary K-feldspar were seen replacing the more i=a icïi= cores of plagioclase p¡'i=nocr J st s only j' in rare =: :3'f{ip {ej4 and most F'i3 gi =ii=l3 jP p#-er-j trr y =t= are uncorroded and completely unaffected by jF+i_f_Ini3f y i r Chr+c l3=i=. T!-{e fart that the K-03-4) alteration assemblages in the volcanic breccia - nd in the small more Altered areas witi"in the strick 3r +__ id3nti± 3l in every dFt3 il, combined with the observed gradations between the K-t_r-;! and K-03-4) a lter Ati in assemblages in drill core, indicates that the mineralogical diffi==r'i=ni=3= [iPt::tsi=_i_n th3 two alteration assemblages are purely a functioniiç thP 3xt3iisil/irriîs= of potassium-silicate3te 3lt3r3tii n and are not a result of other factors, such As differences in primary rock composition, i, ± Ir the development of kaolinite and hematite through supergene processes-. Despite the presence of the secondary pr+t3sjiLte'{-be3riiig minerals orthoclase and biotite in the potassium-silicate alteration zone at Durazno, the relatively iw abundances of these minerals combined with microscopic textural relationships strongl j suggest that no significant%nt P+=it3sh mete =! it[3L'+s!I, .e. C -'"', ...I ..7.) addition of r'.,'..), l"ias occurred in this porphyry jlJij jtYiii.i hi potashash ri inteilt of a. L 4oeai':ly proFy lltliej volcanic breccia from Bajo de Aqua T = F3da (3.85% K ,O; sample n i. 2, Tabla i ) is similar to that of the most bj! ii itizrd vnli_ai-iir bri_i_i_ia from Durazno (2.7% K,0; sample n i. 3, Tabla 2). The ,Fc indir; biotite and K-feldsparat Durazno were pr ibab iY derived by redistribution in 1 if original r'._,0 at the expense cif iii3 i lc minerals and gr i 1und!iiass potash i P idrpar a 1 is bF_llaii[_d to i have 1 ii_our r adat other porphyry copper-type deposits, e.g. Bingham, Utah (Moore, 1978).Thu= the term "potassium-silicate" alteration, rather than "potassici' 31ti=?r3t'.on, i= employed here to refer to ± the central alteration zone at Durazno. Vein assemblages associated with the potaJ. iu1-='ll c= alteration event are responsible for all of the major copper-gold-(Ag-Mo) mineralization at Durazno. They are thus termed "main stage" veins. These veins occur throughout the potassiufii-silirati=_ alteration zone, but are largest and most abundant in the stock and its wallrocl::s Along and within 3 few tens of meters of the stock's western And southern contact. In some places at and near the surface where these ve ins arP abundant, spotty and, less often, dendritic manganese oxides occur a= coatingsin : i the veins and nearby wallv oj(Fig. ] }, se veins are generally iP 1 s than a __u five centimeters wide and follow a number of differentlyer ently i ir ii_1 Tti=d, flat to 1ti_ep iy dipping fractures. The veins are anasta mosing and commonly crosscutisscilt and offset each other, but they are not sufficiently closely spaced and randomly oriented toi form true st ii_kworks. Many of the main stage veins have center lines and occasional vugs lined with quartz crystals, Liut they are only rarely breccl3ted ± ir coarsely banded. By far the most common vein-forming mineral is clear quarti that occurs as interloi=king anhedral grains and less commonly as inwardly projecting euhedral rrystals; the quartz contains minor quantit?i=== of anhydrite, hematite, andi acicular rutile as minute solid inclusions. Smaller amounts of magnetite and calcite are almost as common as quartz in these veins, occurring together as center line fillings in many quartz-sulfide veins. Generally, the development of ïiiagni?titi= 54 appears to have decreased slightly with time. Calcite, i iÌ ;t EPrhP!' t-ind, iner _`= =t==d with time, culminating in an abundance of late-stage calcite_ + pyrite ;IPlns that rrosscut, offset, or follow earlier, reopened rfEln stage... ,1P1n=. Barren quartz veins are common and were formed tiirougrEi lut the period of main st gP vein dP!/FjoptiEent, The most common Jn ((ialn stage vein E as=emb ¡age consists of some combination F if quartz, magnetite, calcite, pyrite, and chalcopyrite. Siderite sometimes substitutes for calcite. Additional less commonim'CiEon veln-frr'ifiiTìg minerals, approximately in order of decreasing abundance, are sericite, chlorite (pennine), molybdenite, bornite, orthoclase, biotite, sphalerite, galena, tetrahedr1tF?-teilnaritite, and native gold (and silver?). All of the vein minerals occurEIr iiE a variety of different combinationsiiiations in any given vein and may or may not have alteration selvages of orthoclase,lt_lasef biotite, sericite, chlorite, quartz, or calcite, or fine-grained mixtures of quartz + orthoclase, r an intergr (Ain riili::tlire f some combination of quartz, sericite, and chlorite with r without calcite. Orthoclase veins and selvages and especially the rarer diffuse biotite veinlets and selvages were relatively early features and disappeared with time. A tentative paragenetic scheme for potassium-silirate alteration is presented in Figure 12. This diagram includes temporal relatinnships for alteration minerals occurring as pervasive phenocryst and groundmass replacement, as selvages on veins, and as vein minerals. The diagram also inrlude= paragenetic relationships for magnetite alteration idisrussGd later :I, a distinct alteration type believed to I be coeval with E potassium-silicate alteration but slightly earlier than most main stage veins. Figure 12 also shows later phyllie s.+nd supergene a ter ation as=erEb ,iages. in addition to the typical main stage :/eins described ab :/e, two less corm-non varieties of these veins also J ! rcur in potassium-silicate altered rocks. These veins are characterized by their brecciated textures but are similar to most main stage veins in other respects, such as in their laCi< of banding and in that quartz, where present, is clear and anhedral and usually contains minute solid inclusionst if 12 - Preliminary p3[3g@2@{)c scheme for potassium-silicate, ph/]]§rl32Ö SUp@rqGm@ ¿]{ºr¿{§O3. Aj{Pr3{jO2miG@r3j5 GSSOCj3{@Öw§{/ {/@m¿qn2{§{@ G]{@ration eÇ@S{ are iRC]Ud@Öi2 the @3{l/ S{JqPS of pO{355§Um -Silica{@ G]{@ia{§O2. Temporal §@)G0:1005h)PS ¿r@ 3Ró {)m @ S2G1@ )5 implied; the 31{GF3{§OR Even{ Ñ33 been @/P¿GÖ@d)g order toSÑO r@)G{)O95h)PSmOr@ C]@a§]y.5:iGº {@/{ for ÑiSGUSS§OS. POTASSIUM - SILICATE PHYLLIC SUPERGENE < > < > 1<--> BIOTITE ANHYDRITE 4Il 4.11.1.1 ORTHOCLASE CHLORITE 4IIMP SERICITE -? - QUARTZ MAGNETITE PYRITE CALCITE CHALCOPYRITE BORNITE MOLYBDENITE 4111111.1ww SPHALERITE GALENA ? TETRAHEDRITE - TENNANTITE ?i1? NATIVE Au (& Ag ? ) GYPSUM CHALCOCITE 111 COVELLITE NEODIGENITE i TIME 56 anhydrite, hematite, and acicular rutile. These two typ 'Pj a_ifi veins hPrP11i termed liblntis:}? b1 e! t=131t veins and "hydrothermaltti3l bref_3 13°i ;131ns. With respect to itrfe pot3=siuïfl-sili{_3tP alteration evi_nt, the abundance of Cii tit+= in theiibit'itite e_r iiveinsn ;rabundancer __ i intheiil f3úr1 breccia" veins suggest an early and 3 late origin for these vein i typPsg respectively, but confirmatory i_riis=-i=Littlng relationships with main stage veins are usually lacking. "Hydrothermal C{r"er{=i3°6 va?iTis are certainly later than the magnetite alteration event ber3Li=e they cut banded magnetite + quartz + biotite veins in places. "Biotite breccia" veins occur i! ii both the strict: and its inia iii s if_i.... A i ld dip anywhere fpr1 31-13 t r 90c. Th e; are g s_ n F+; 3¡l y one 1e t r1 three r G n t it!'1P t F Y" _, wide and d L f l 3 matrix appears black in hand sample owing to an abundance r lf both primary and secondary biotite and in some cases, magnetite. The major vein-forming minerals 3r"3 identicaltnthe roi={::-frirïtiing minerals in the stock itself, i.3. biotite, hornblende,iende, pl3glt"or f33Ñ, K-feldspar, quartz, and magnetite, and thus in some. respects these e veins ilg i jconsideredto be "intrusion breccia. These primary minerals, except quartz, are all partly altered to secondary ! oi ti ii lS_la_P, hii ititP, calcite, sericite, magnetite, and minor disseminatedf3LNd pf r"ite. The "biotite breccia" veins typically p o1 sG 1 1 flow lineations defined by elongate primary plagioclase äri hornblende crystals and zones of fine-grained secondary i {r'tht-!rl3=P i h3T 3c P 3l1 ned along { lÌ A lines paralleling t % e vein l s and -a r.i y wrapping around thP contained breccia fragments. These fragments consist of weakly r ' =: 3rr iuï-r 1 lr3 t 3 altered intrusive 3nd3r1T e porphyry, main stage veins,á nd banded magnetite -r quartz + biotite veins. Some ilblritlte breccia" veins gr 3di., into irregular bands of clear quartz along one edge identical to main stage veins, and locally this quartz contains disseminated auriferous1L'+_ 1_h3 lr py'r it3. The r P13tirinships in these veins illustrate the very close temporal and genetic relationships between "intru=irin1l, d3Lterii= potassium-=1l11^3te alteration,and vein-forming and mineralizing priiesse1. Furthermore, all of theseprocesses weree developed in rapid upward flow regimes that at times were suÎfii_li_nTlyvoient to lead to Lrerria}ion. Finally, the potassium-silicate alteration and copper-goldSd mineralization in the veins themselves obviously post-date ie th+? Tr+'i matirfn of some main atage veins and banded magnetite + quartz + biotite veins at deeper levels. f` E ' =Ti_sl_' "Hydrothermalmal c+recria" veins are mainly confinedtrftheI i_-en 'r"a !t i and i A _ H r a11, steeply dipping % !.These v ,na ArP widely =i . A c 3 d aid irregular in width (1-10 Ec'ErE wide) and continuity, but in places form some of the largest veins at Durazno, ranging up to l;:: i centimeters wide and 13 'rfE+=te.r_ long. The matrix1 f these veins contains highly var'iabif? proportionsf1` guArti, rál+=?tP, And sulfides, with minor disseminated magnetite, but calcite and sulfide minerals generally predominate. The sulfide minerals are mainly i=f iar=ál» crystalline pyrite with small and variable amounts of chalcopyrite. In F'lare= the iiiAtrlx is rr-iA.F_d with manganese oxides contains vugs lined with calcite crystals and less commonly, quartz or gypsum crystals. The fragments in these vein s areangular and ifEatri::::-aupported, and usually consist entirely of intrusive andejite porphyry, but locally small fragments of quartz ins ar e found. Both th e andesite porphyry t- -s e fragmentsandimmediateti_. e 1 1 f+? fr`i 1_i'. '= ar are weakly f_-'r"11_-r Itii+d (S-(1-2))fri 'll_i?f1¡ minor amounts of calcite, or both. Diffuse, moderately intense silicification 1 (Q-(4-F;)), pi a.i affects margins of these i ! in=in places forming d e selvages up to ten times the width of the vein itself. In some veins, all of the larger fragments are uniformly rimmed by ç-l;; mm widths of moderate silicification c:Q-(5-P.),! t.Alhi ie fragments less than one centimeter in1 diameter are r f'sÌEplPte?:!! S1 11=i1d 1.F?. rf--'¡-! _. j.ttn¡ ,iT the larger ff(-Eli"tJtlrÌÌiaIi, breccia" veins At 1.1E!rai ni i are distinctly radial in distribution, and a gouge-filled radial fault the edge (,_I f one r f them n+= a r- the center of the = t f_ k. In conclusion, it shouldL ? noted that F_r tiCn= Ï ff the stock and several of its dike-like projections cut across main stage veins and banded magnetite + quartz ± biotite veins developed in biotitized wallro+_ks during at leastone previous alteration and mineralization inÑl;Ñnt. The fragments 3 if 'iiiain stage ;iein= and banded magnetite fql_(3r"tz + biotite_ .vein= found in the "biotite breccia" veina?i-id within the stock itself also indicate that these two vein types wer_developedoprd adeeper levels during at least one previous. hydrothermal event. Finally, the texturallyall,r i_if variegated masses in the stock suggest that if is 3F tl(3 ll¡ a i k_ifÌiprir-ri?i. É li 3sP y timed, Coi'iipi+=ition3lly identical intruait+e pulses. The conclusion drawn from these and other observations!!3Tiont= i= that multiple episodes of intrusion, hydrothermal alteration, and mineralization in are represented in i the porphyry system at Durazno. It is suggested that, a high level t ,iT this system is presently exposed, ant? TrE3, thatwe are viewing a oor1iF'o=1t+_ of laTe-=t3ge intrusions that pierced through it; ieiri kn.!ri mineralized and potassium-silicate and magnetite altered hood zone(s), but were subsequently o nly weakly potassium-silicate altered themselves. Propyliti'= Alteration Pr opylitiC alteration oT the volcanic3nii= b'¡'eci_i3= at Durazno is uniformlyandi, tAteal::ly developed (P-(2-4)) throughout the peripheral pi frtioi n= of the mappedarea and dries not increase towardiward the central stock (Fig. 10). It does, ;, j'tt tt,tever, diminisht in intensity just outside the mapped area. The inner boundary of i=°r ir,'litit_ alteration correspondsSrre3F°ond3 tr4 the outer b_iunr3r; of potassium-silicate alteration, both of tk! (-[ i i= h are i= o n= i d e r" P d to have d t? S/ e l iG F? d =;' rl i= h r [i n i iu = ly A.= evideni_e anywhere for the superimposition ln =i i one upon the other. The precise boundary between these two alteration types is obscured by later phyllie alteration -i n many piaces. The uniform extensiveness of Fr-r , " i tir alterationter3tion at Durazno is in marked contrast to its variability at the Bajo de Agua Tapada and Bajo La Alumbrera prospects (Surho'tpel, 1983! EtL(lt=, 1984). This alte ration type = i3 better developed (P-(6-8)i at only one place at Durazno at the southwestern edge of tiffe mapped area where it occurs within 3 narrow linear zone some f_.3! i meter= it_tng and subparallel (N 4t,i i E) to the axis of elongation of the phyllie alteration halo. Propylitir: alteration consists of:-.-Lnvolume % epidnte9 5-15 volume`r`: çci calcite, and 5-20 volume % chlorite (pennine) all after htri-itlende, plagioclase,and[ biotite, along with 5-15 volume %,s magnetite and lesser 1ei1c xFne and hematite after hornblende and biotite. Variable but small amounts of sericite and 1::3? linite also occur locally as does scarce (usuallyally.l vol. %) disseminated pyr itP. Although the actual proportionFf each secondary mineral, especially e.F'idoLe and magnetite, may rh ange drastically over distances of only a Tgi!R1 cÑnt i(ilPtPr's, the }? tAl Amount of secondary alteration minerals remains fairly constant, a vt=r'eging between 2n'.L: and 40% of the original volume of susceptible minerals as indicated by the P-R.2-4! designation. The greater degree of propylitization in thP P-(6-R) zone dPst_i ibo?u above is primarily a result of increased amounts of chlorite, calcite, and kaolinitr. Propylitic vein assemblages are common but never abundant and consist ! f small ve-inlets i f1 some combination of epidote, calcite, quartz, magnetite, and less colfiyfi !nl jr pyrite, 3l1 generally lP=s than three mil iliï9PtPr = w1dP. A single iPCilit = veinlet (scolecite?) was noted just tC+ the south nT the southern +=xLf Pf(1ity n#' frit? mapped area.pido te, calcite, quartz, and zP tl iLPs may 3l4, occur as euhedral crystals lining in veins and veinlets in which they are major constituents Although many of the larger propylitic veins have filled open fracture=, smaller ones are often more diffuse and appear to t'r PF'resPnt local redistribution and replacement along micro fractures, rather than discrete fracture fillings. The same propylitic alteration assemblage described above for trie 1. tlt==init- host rocks is also uniformly and pervasively developed tn the same e:ftPnr (i.e. P-(2-4)) in nearly All ± f the dikes at Durazno throughouttu=ghout their lengths within the mapped area, although in places this assPrrib1ege is overprinted by later alteration events. As noted Parlier, most of these dikes are considered to have been emplaced during or following the waning stages of phyllie alteration at a distinctly later time than the major episode of F'rcpylitiz3t i n. Thus the propylitization f tf these dikes is probably deuteric in origin. Propylitic vein assemblages, usually vPinlets 4f t=pidotP and rarely calcite, are occasionally foundin the dikes. 60 Magnetite Alteration Magnetite alteration is a distinctive and somewhat unusual alteration type that takes several forms at Durazno (Fig. 13). It is a variAnt of_ tF "quartz-magnetite" alteration type foundi9lni..7taiseveral porphyry copper-type occurrences throughoutiugt-t iut trte Farallon in NegrE ir Pg1 in (TatilP 1 ) Rnd elsewhere 1979), but it is termed "magnetite" alteration at Durazno bPi+=atls}? it is not always associated with quartz here. Most commonly, magnetite alteration at Durazno occurs as banded magnit e+ quartz + bio i e veins and ! ' n1 rs less than one millimeter wide up to about. thr Pr-? centimeters wide with weakly sP1?fagPs in places. Similar banded veins are found in the center of the Bajo de Agua Tapada pro=Fectprospect. At Durazno,i thesei veins1 are found only, 1 1 in tthe potassium-silicate and pr"i i¡r ylitir alteration n irirj',e j with which they are t_ ie: `.ti=nsi vP, and essentially I_i IPtfa l, and =- e¡ do nit usually survive P /P1 wPRl; overprinted Fy1l1= at=ra =i! i (i.e. _S-4) because of the rapid alteration In i Ifi magnetite to hematite or .iii3qheïifite, and very rarely, pyrite. Textural evidence indicates that several aree=iIfbanded hematite +quartZ veins in strongly pi-tyllicallyf altered rocks were formerly banded magnetite + quartz veins, and these vein=, are included in Figure 13 as parti i the magnetite alteration event. Although (quartz-) magnetite alter at i E! i usually considered to be a subtype of potassium-silicate alteration (Sillitoe, 1979) and, for simplicity, is included with the potassium-silicate alteration a= semb1a ge in Figure 1 2, magnetite alteration at Durazno is a relatively early feature of both the potassium-silicate and propyliticiiiic a Ite.'r a ti=;i-t r'Lenta because the banded maql fet ite ¡- quartz + biotite veins are cut by main stage /ei1s in potassium-silicate alt erel rocks and rarely by epidote veinlets i n F`r%r'yf Iltized rocks. The most significant development of magnetite alteration occurs in the stock anin surrounding biotitized !nallriC:= within what are called `moderate " magnetite zones. Within these L Ine ;, banded magnetite + quartz + biotite veins compose about ten percent of therod::volume iIn average ! l.e. M-1), but locally are ,1 L Fig. 7 ! - A g ri _i Ì P = - f P i A t i ! r_ o c ..-ri s T i n g of b A n _i F d _ = g _ f + tr .! A r t-i_i ! i biotite v G i f É :=-. And i r P itS l P i 1 -i Ì 1 Ft t r !t =r i 3 i L F Ì Ï_r i_ ! i'Ii if= And+1 r i ié magnetite iq u A- t iP in' P t s F r F } l - f i__lÌ j ? l t P rG d r - _ k C_ sEtr P f IP lÍ small a r __s with tto i 5 And 20 vvolume percent d i= 1_ ifa t PH magnetite in F oF 3 l-i_ a l; and Z r _ L A =I 1 i lt m s ii r A t £ A l t r r G 3 rocks, r =. P _ f ' i _ 9 A r P not shown hr ï _ j they i r - considered c _ partof thi-'=P two alteration P`.jPt¡t_e Magnetite ± quartz ± biotite veins Lithologic contact V/A1Trace to weak (<1% by volume) Moderate (.?1 to 10% by volume) Alteration boundary X Isolated occurrence Fault 62 much more abundant ~Ç 0 vol. f ). Th e veinscrosscutrid f"Pgu=nL { y offsPt PA_h F ithir and are usually somewhat, 4. r^aiFl ..ji'iiif orientedleiteF Iut they do not form iii t i' Lie T sLo_ Cr ' s._ n places L h=veins.re Sheeted!Anri li [a - l! =_ : _r .?g=i iPr a t -oÏri sT sheeted 'leills! each oriented differently, cut each others 'v`eln- wider tri.=a1 about fl¡e centimeters up to maximum' i F of 25 centimeters A Luncommon ari_ 3tLsL fEj a r r -i -f r l t- f + r - l-. F_iiiiiF'i ssite i is coalescingsÊi' i3+ CFv i1.'_iLLiÍ-i r ' 'fCi3 Fier" '%lÏ-i.r TT hc= 13s 'r ,I si'i lF i~.; FPr" composites or not, often show evidence for repeated episodes Ì TT br eI_T_'sa il; Tn and have each time been recemented with banded magnetite +quartz + biotite material, tAif ille surroundinga l Ìr"oi_k= have not been br+_',F_ia Lpd. ER ch individuai vein T1r- velnlet is rhythmically and symmetrically banded i witn respect to the vein walls and exhibits "classic" epithermal open-space filling te: t1 Fr , including =P"}er lines and i Ì c t l a. ' f = lined with qü3rtL crystals. Most (usuallyci n vol. in i of the vein filling itself is made up of nuferC_- generations of inAf3rd lprojecting euC=dr3i quartz rri1 i-3 1_ that contain s_ar_e r Clld ir C l u r l ans ofnhl dr1 ! e. ,=!a. _ -Le, et-id rare 3'_lri i3r rf tliie. Very n3rrnf.4 bandsf tsf 3l { j' less ti teilTTIe fi,iiil'!"fiÑter wide, Tf 3nhedral t I Eui-iedral grains ET magnetite and feathery ' ='ä1P brown CT1E=it ¡le es.- radiating overgrowths on the terT'iiln3tieC ends of quartz crystals occur in no discernably l ='gIll=ir pattern with respect to T e3t=h =ithGr..A complete size. gr3d3Li in exi=ts from wider and F= Tn±lni Tu= bands ! Tfi r?dleL iil'3 biotite through very T13rr Tw and discontinuous- le b3i ldT of biotite completely encased in quartz ...long specific gr iFialth ZCllle_ tt ¡j3rrT_ïw growth zones ln quartz outlined by .eT faint brï itAijli=i I tint inferred to [Te due to minute inclusions i ii biotite. The biiiTite is ifi il?-piei ii_("iriiiF= 3nd is identified s islPl.t'i to the basis of color,li ire reli+_fi, and its identical 3p1='i==_.i r =.i l-?i_e to tF'iP hydrothermal biotite in potassium-silicate altered ¡i _:=! as it i= too fine-grained and 3'filCirphol!- for proper identiTir3tii=ï"i using optical Ti_i`_hniq!.le=. In s! iflfi= `%i=ins4 bli Tt1LG is absent. Massive fine-grainedle-gr=iilled '{i"E3 -_ini=T lte vein= up to i1 ,7Fi centimeters wide occur -in some places and ?re pert cif the =3'iifie 3lti==r3tliel¡ i=+,:-'el!tq these vi='sil= liFi_á jly rCint3in 3 small amount of lnter s tltl3i quartz, or have quartz F=r yst3 É-llili=+d 1fL!gsor 6:3 weakly silirified selvages. Much of the delicate banding in the typical banded magnetite + quartz + veins is an effect _i_A i jd hy AltPrna Ling Land=n f translucent gray to white quartz with clear quartz correspondingti_Ealternating inclusion-richn-riE-h and inclusion-free growth zones, respeoti1e1 fz The banded magnetite quartzbiotite veins Durazno differ from the tain s tage vein s intheir frequent brecriated textures, in their delicate and uniformr-m band ing, in the extremely tine-qr ained and translucent = 1u-=á! iT f of the quartz, and in containing numerous generations of inwardly projecting ÑL-PdrA [ quartz :ry1f r Ì=. TÌ _ f Also differ f om the main r ta g_ veins in f- AL they are not A;l_1 s l' n_A , but .}. C "i r-. - C r of , r t r , ° t-. -s , ° i Ei-taii Ena i;arear.E-tEate in TrEra r-c3r iJe_1 =E E °Ea Elriri:: t pe. LuE_I"I shrE ier° _ rc,_}atP, the veins maintain constant widths, and the symmetrical and regular banding in them is preserved. Outside of the "moderate" magnetite zones, the veins decrease markedly in abundance and width, particularly towards the center of the stock, And this i,a:f rease is even fi Era marked rE¡yE_ß outside the potassium-silicateata alta°t atl En zone. The magnetite + quartz veinlets that occur in propylitized roti::s And that a; P inferred to E ba part of ¡' Cha magnetite alteration a4.rF+j-ti{stia ily occur in erratic patchesEr as isolated veinleT j, but a few small areas [ EfEtÏ!od:jr_etETE! magnetite alteration ei P also found in pr opJ litiLeÚ rocks where, it might be added, ij! E evidence for the propylitizationiitlzatËon CE$ a former potassium-silicate alterationEn assemblage exists. All of the magnetite + qu3rîreinlets in pri p J lit i t ed rocks 1 j biotite, have a smaller number of wider bands (usually only :-D, to 5), and are usually less than about five millimeters wide. Some vein iet s are sheeted, but theyare never arcuate. The banded texture= and translucent, white to gray, fine-grained quartz of ti-tese veins helps to distinguish them from the magnetitequartz veinlets that are a part of propylitic alteration. A reletivEjly rare form of magnetite alterationoccurs locally in the rentrai arcuate tElEtoder ate,t magnetite zone in the stock: (Fig. 13). It consists of irregular. 64 discontinuous, and blebby masses up to about 0.5 meters long of t ; P-g 3in d magnetite with a small amount of quartz. TiiPsP only "iitod*=r31Ñly magnetic and thereforerP prNsutttably contain additional iIii iìl+?r 3! phases (biotite?) that rerft3in unidentified because this material 'jfa= not examined microscopically. The magnetiterfi3=S e=are probably late magmatic in origin and formed atátime when th;? porphyry host rock was still r-' 3r t i: molten and i n t a paC ~e of continuous brittle fracturing. They are cut by the bandPf, magnetite +quartz + biotite veins. The geo'fttetriesff the "moderate" magnetite zones, particularly the central arcuate. one, sugg_st that they were formed in irr3gL13Y 7P Y 1, 3 ; cylindrical s hP__ or irregular inverted bowl-shaped zones in the f_upnl.a portionrT j h3C3ri}r"31 stock and have now been trunCated and exposed by erosion. A small elliptical "moderate" magnetite zone in birft1t1z3d ilol_3nlf_ [fr3CCi3 t_f thf? north;.glP3fi _ff thP stock {:Fig. 170 may be the uppermost portionff another "magnetite-rich" cupola. developed over an 3s yet unexposed apophysis of the main intrusive body. The distinction of magnetite alteration as a separate alteration type at t r- r i Durazno iC p'ri'ft3r ;li- in responseC to ftheT f t3 discussionr byf _i fit =f3 i_1r ¡r itsit 3pF'3rF?n tAtidespr e3f; presencP, in one form or another, in gold-rich porphyry copper-type occurrences ;r,;i frld!itide. It could just as easily be argued, however, that the so-called "magnetite alteration"r if' at Durazno is simply one facet of both the p+_ft3ssiurit-silif=ate and propylitic alteration types as magnetite itself is weil known to be a characteristic 'iiiineral of both assemblages. In this study, the late magmatic, magnetite-rich masses in the stock, all massive magnetite \r einsand veinlets yielding open space filling textures, and the banded magnetite +quartz + biotite veins and /P inl P s in potassium-silicate and pri p,.itiÌ a 11; ' er Pd rr r'j oe included in the magnetite alteration event, while significant local c}_nc=ntr'at1nÉ?= of disseminated secondary magnetite in propylitized rocks (up to 1= vol. %:and more intensely potassium-silicate altered rocks (up to 20 Í v;_l. `f.i ar"e not il nf=l1_Ided because of their close textural inter"r"e13tionships '.}lith other secondary mineralsnf these respective alteration assi_iTiblages. Also not included in the magnetite alteration event are those magnetite-bearing veins that contain ïitlnPrR1= other than quartz and biotite and that lack:: the characteristic internal symmetry and open space. filling textures. Although the distinctions cited ab; e may bG ent l re y arbitrary, it is clear that the common, well-banded magnetite + quart= +birotitP veins constitute 3 unique textural-mineralt gìC vein type that is di=t'ni=t in its timing of formation and is not gradational with other vein types at Durazno. The lack:: of sulfide minerals in the late magmatic magnetite-rich masses in the stock:. and in all magnetite + quartz + biotite veins indicates that i rlat ively high oxygen fugacities and liiw sulfur fugacities prev3iled during the magnetite a lter ati in P:/ant. With ratei eneatE_1the fields of the relevant tCtinare 1 assemblages for primary minerals in the stock:: and secondary minerals associated kh:itf t magnetite andPotassium-silicatealteration (Fastrie, 1962, Fig, 7), it is clear that the ratio fr..:3 f,increased steadily from late magmatic conditions throughiuh ' 'L L magnetite alteration unti l sulfur fuga{ itles became high enough to stabilize pyrite and chalcopyrite during later stages of potassium-silicate alteration. Hematite seems to have been stable under magmatic conditionsins in the stock and during the entire perii id of hydrothermal alteration at Durazno. Therefore oxygen fugacities remained near the magnetite-hematite buffer 2FPi + 1.'L; r. = Fe rrrr (1 ) until the onset of phyllie alteration when magnetite was no longer s# able and pyrite-hematite equilibriurii Fjrrvailed. An absolute upper iiritit to the sulfur fugacity is set by the condensation reaction -L _ liq because native sulfur does not occur at Durazno. The equilibrium Ti{ + na =c + _1nr, = _ aTi-;i- + /25,-t 2 L L L sets a lower limit to the sulfur fugacities (Eastoe, 19E:2) that prevailed in All anhydrite -bearing rocks and veins. It is therefore suggested that as temperatures decreased from ïii3giYi3tiL conditions, oxygen tug3i=itie: remained near the iYi3gnetite-hPm3tite equilibrium boundary, while sulfur f+,.Egal=itlPj inE-}'"PAjÑd And conditions changed from aphene stable assemblages during intrusion and _r ; = t _ 1i T a t t i n to rutile-anhydrite-quartz =ta - ie. a r r Ñib = g e =_duringmagnetite 31terati in ii 1 ruti1e-anhydr itP-qisAr =i-p_°r'1}P-cFia1ci=i=';¡r"ite stable assemblages during potassium-silicate aeratiE in= Phy llir Alteration Phyllie alteration 3t Durazno is primarily developed within an irregular elliptical region elongated N 3i ìl i E _ rr+=tunlJing the centrai stock (Fig. 1 4) and superimposed on 311 rtr i?i/ioissl j` described a1ti=r3tini i ty T'3j{ h3 dir 3i=tii in! i; elnngatinn 1_ directly i in line with the projection i1Ì _is the direction _if elongation ! if the slightly larger ph;'1lir altera ti in zone at Bajo f L3 Alumbrera.1-i'itircr3. ThÑ peripheral margin of ?A1eak1y F'i-iY1li+=aliy altered riii=i::= at Durazno surrounding the zone of "major" phyllii= alteration actually represent_ propf iltil=31!y rltijrErd rilr-I::1 with very widely spaced narrow Tr .i_ti.lt e zones that 3r e i=:;teil=ii/e1;1 r°hyllii_ai1J 3 itPrPd4 but that L3nnnt be shown r PparA tl; At t r _ scale _ f Figure 1 =5 the S-00-2) designation fortheser Ér 1 is a i Ps u Z tL hP iPigL ejaveraging a Iter 3t t _i mapping procedure described in a previous section. S e vPrA ' f hPC Pra_r u r P zones F:%P d beyond tP mapped area, particularly along and to the south _f t N West a d East -PsP + 1Pc;_=._Prcrl _i A l; resistantr 3! t!= 3 :e cnntrn11PH the distribution =T iii(J t major drainages in the propylitized periphery of the "bajo".ill. Fot3ssiLÌ-argon dating (McBride, 1972; McBride et al.,975: ias er tabli =he7 that ph; lllc alteration at Durazno tolli_iwed the emplacement of the ie =entr 31 stock lAiiti'iin a period of i+=1= than 3 million years. The interi/al of intrusione potassium-silicate, ProF'y1iti!_a magnetite, and phy1 jii_ alteration is thPrPto';-P bracketed within the period from lm 8.7 + i_l.`r l4i.;'. to 7.9 + 0.3 ÌÌi. ;¡.. Because t_if re13t1< differing 31tfr'3t irils Ail ini'Pr"r"i_d 3 1t3r3tji:n boundaries Éd3=r13+: y 3 - iz in.t r 3C_ l 3tind 3rrrir = sPdi !i_ntr j 3?d may no f r-Pfl,nctrEt_ C ruci/I, p3i _3i is. The qiERr ti pyrite n veine of the phyllie =1t3r3í lon 3js+nrlibi3g3 minrg+_` imperceptibly with i =i'lriilar" a jj_'rClr'i!3ge= in the prit3i = =üm-=li it=.ätG 3iT3r:atir-:n zoi-1Fg and t;-!i? inner bii{ =3 ii±3r iin- for these 1iFins ari', drawn where ph}' ilii= A1rPr 3ti in envelopes similar Z ' --{. veins_. are._ minor or nonexistent in ÎET3,-_;,,I=r..r 1! S , =ilÏi_i'= !i'r ii r _°i_i'.ss PHYLLIC ALTERATION . Igo ( riIsiA/ 11"?., z1111:1! SR?, Jlih , i X 0 300m I I i 1 No phyllic alteration (S-0) T"NIMINNI >S-8 to S-10 Quartz ± pyrite veins (boundary >S-Oto S-2 L. I dotted where not defined bya >S-2 to S-4 lithologic contact, alteration boundary, or fault) >S-4 to S-6 X Isolated occurrence of quartz ± pyrite veins >S-6 to S-8 Lithologic contact Alteration boundary 68 Fault ., then '-=El iAFsd lnih;3ri to . position along a narrow aT!Ì4..IlAr ; i trjP around the stock. where phy 1 ili= alteration eventually dirfli n1=hi=d. Hi iv1+==aiPt", t;`sP T1P !d Ariz , , notlncon3l=tent with an inwardly advancing phyllie alteration, front, 4 or with an , ±r_iiirhti=i, ,, T- initial phase of widespread phyllie alteration CifF. 3n rd i=i!fAG1P. The iri=ire intense F=hyll;r alteration in irnrrPd-i3tP l;' =,r-r- Isnding thP stork may have beers in response to i higher thermal gradients, ;'y ijrngPt : ion 3 _Ti'1itlPj9 and Tr"3rtur"P den=itie3 there, but might just as easily h e due l a second pulse_ of phyllic alteration r iT limited distribution and duration. Perhaps the mo=t prominent feature of the Durazno porphyry copper prospect is the patchy and irregulareguiar rJijtr iCiutiC in of differing extents-, of pervasive phy llic alteration within the major phyllie alteration halo. T- sP i rr g ilr lt i= create a "jigsaw puzzle" ? ff ec t which -a _ been considerably simplified in Figure 1. M3ny of the boundaries between areas of differing p;-yilii= alteration degrees are gradational, but an equal nuËntier of them are linear and extremely sharp, and alteration development may change dramatically across these boundariesund3rle3 Civer distances of only a few centimeters up tCl one or two Ë!'1==tera. Only `r3ri='i;' do these sharp boundaries ; [i IrrE?3F=i_ind to faults or major Trac} re3; generally there is noi visible structural clntrCl The erratic development of phyllie alteration, although typical of smaller porphyry copper-type hydrothermal systems (Guilbert =nd -rie11, 1974), =ontr 3r tr with the uniformly pervasive nature the earlier r C ta=_ s;Hr- s11i_ a ? and prr ,il t' c 3lL t ' Ì - events and is also distinct frothe A' de zonesCif uniform pinyllic alteration that are common at the larger Bajo d_ Agua Tapada and Bajo La Alumbrera prospects i_. uchoËnel, 1983; _ tuit s, 1984)._Cne of the irre'aularltiel in the phyllir alteration halo are probably thP result dit; Pring primary perrneatii litie3 and porosities in the volcanic host rCiLr..l::1, in turn at partly a function ! I{ fragment content. The extent C If phyllie alteration A `I_E= correlates in a general way with the degrijr_+ of fracturing, and the most li'iten3rl`: fractured and phyllii=a lly altered rocks ; 3re roughlycoincident in an irregular annular zone around the stock. Aperfect correlation En between lnl=r_A sGd development of phyllic alteration and higherfracture_ density measurements,i.P. more fractured rocks,does not exist at Durazno, but such a correlation isprobably unreasonable to expert since the degree of mlE=r=ifracturingcannot be quantified by the fracture density measurement technique. An advantage of the erratic distribution of phyllic alterationis the e presence I f nl n-phyllical ly altered patchesii rock within more altered areas. These patches act as "windows" through which previous alteration assemblages can be identified and have made it possible, for in_t3i;rP, state with rPrt3lnt',+ that prop,¡lltle alteration does not augment toward the stock within the mapped area And have also aided in the closer definition thePx3=t positionEf the original pi Et3s=iurfi-si1iE=3te-prop;¡l?tie alteration boundary. However, ! thP latterboundary is not at all well constrained to 3 thP northeastclf the atrErl: which i= theonly areaE_Ef any s ìiP where phyllic alteration is broadly And unifnrffily pervasive Arid "islands" of non-pi'iyllic3lly altered rocks are lacking. r`hyllic alteration consists in the gradual r epl3cerfient of ail minerals, p::::cept quartz, by a fine-aE`"3ined mixture of sericite, quartz, pyrite, and very minor rutile and clay minerals. Pyrite is typicallyIn the order of one to three volume percent, usually as disseminations. In some ar e3= of moderate to P:tP _n l :.P phyllie alteration, including portions of the South dike, manganese c l:.:ide= occur as surficial coatings and impregnations (Fig. 9), particularly along larger fractures and some faults. Narrow (usually (3rfifwide)discontinuous vPin1=t= up to A maximum of sir` centimeters wide of quartz, quartz + pyrite (rarely handPd1, F',¡r ;tP-4 and locally calcite are erratically distributed throughout the phyllic alteration zones and though common, they are nowhere abundant. These llE=ilileta are occasionally sheeted, and locally they are bordered by weakly silicified (Q-(2-4)) selvages. 'J I_ i n quartz i= typically 3 translucent white to grayisho r bluish color. In solfie places, veins of either quartz or calcite are banded in shades of gray and white, and some calcite iieln= are tJuggy and linPd with ! alt itP Cr ;° ;TP is.At the surface, the quartz, quartz t pyrite, and pyrite vein.. produced during The phyllie alteration P1 Pnt merge imperceptibly with veinA._ s PiblagP_ r the c Pitra potassium-silicate alteration zonein+ through a de+_r P A=P in intensity ° and width f+t pFi, ll1+_a ily altered vPin envPlc+pP=. At Bajo La A iumt,.+rerA, a rPlAtl:lPly .n7P11 +_ÍFvPlnpPd "high py r 1tP++ hAl:t occurs around the outer edge of the phyllie a1L PY a r , _- z ! nPz T hP suggestion _f a similar, but poorly developed "high pyrite" halo also :=_ ur= at Dur?zîG (Fig. qj within which :: disseminated and lesser velnlPt pyrite +=++=+=í.ir= in í volumes+f up to In%."High pyrite zones are principally found in irregular patches in ph; ilis all} altered rocks along the outer edge of the major phyllic alteration i-fAir+; but also + ++_f í írli"íL'r opyllt izPd rocks that have been weakly and pervasively silicified.- weakly phyllically altered ( "high pyrite" zon+=_s referred to above are not to be confused with thP "high f pyritP++ zones locally found in potassium-silicate a iterP+j volcanic breccia which Ar"P Aljo shown in Figure 9. Gypsum veins are abundant at Durazno (Fig. 9) and range from thin films to veins 15 centimeters :i¡P. F;olP carry a few percent ofdisseminated pyrite,P d locally are accompanied by calcite, quartz, or k,yPaÌ:ly i.-5) =elvPg+='=. These veins arP relatively late features at Durazno and cut all other vein-,'pPPi but are themselves locally cut by calcite veins. `earl;all of the gypsum appearstr+ have formed as an integral part of phyllic alteration because gypsumvein= are proportionately more abundant in more intensely F'ff jlllCail¡' altered rocks. inmany areas of strong phyllie alteration i (`.'..r-i -1 0,1), gypsum apt=°eare to+ havP thoroughlyIgi fly permeated the rrtrï':: volume along every available.fracture. Where gypsum veins occur -in propylitized rocks,they are always bordereddt=red by wide, eì.:ien=ltt,e.lf phy'llirally Alter ád (S-(6-10)) selvage.s, A close spatial association between gypsum ?nd Fh; l lir alteration has been noted atother porphyry copper-type occurrences, such As Los Pelambres, Chile (Sillitoe., 1973a). Gypsum veins have also E b_en found along several faults, again associated with phyllic alter dtion, where they cement tectonically derived breccia fragments and may be intergrown with disseminated _ L r =dra , pyrite cubes up to one centimeter across. Although gypsum pseudomorphs after anhydritewere never seen, some gypsum 3n the center of the porphyrysystem may i"f3tt.Ie been ti trmPd irinrF` i ir less in n11tiç through sl..lper gef 1e hydration of formerttler anhydrite-bearing tileins because a fi3th! veins of intergrown gypsum, pyrite, rha lr tpy'rite, or magnetite occur in the central stock. In contrast to the occurrence of gypsum in other alteration zones, gypsum-bearing vein= within potassium-silicate altered rocks do tj"jot always Él3tle phyllic alteration sn :=_+ri,lelfiF==+s. Gypsum veins and veinlets of identical rharartertnthose nn the surface extend to rçtnsider3ble depth (ca. 100 l f mej ers) in several drill holes in and near the stock, with i +r withrut coexisting disseminated anhydrite. Fin3 i lJ, in31 Pw is! ia ted localities typically at or near the bott+t'Cti_ of drainages, gypsum without pyrite occurs as thin films along+ng t=pGn joint planel i n non-F'i i j? i':cá1i`¡' altered rock= anri i= undoubtedly of post-F'h_filir, scuper geÉte The gypsum now found in close spatial association n witf i phyllie alteration i may have been derived through hydration of anhydriteite in the potassiuii!-silirate alteration ztJne, with the components transported outward by supergene and redeposited in 1! rieÌ!! r"Ghighly fractured ph j'i lit=3lly alter-Pd r i _ s near T hP surface, or gypsum may have been transported outward a= an pRrt=+f F'i'!'flliralteration. Hcti..nfevpr, the widespread abundance of minute =olis3 anhydrite inclusions in rocks and veins well : ! It into the phyllie alteratii_;i! zone indicates that anhydrite ;,:1a= formerly tA,aidespr ead, and all hut the se pr'otect;=d inclusion=h.aVL been h; d " a t?d to gypsum l ear the surface. --y r 1 L P may havP been derived l initially from the potassium-silicate alt P =. Tiin ioni= trot !g r i.dl= = ou T ifn atlowPr temperatures and pressures during the phyllic AlLPrRt?±in i=E1i=+nT owing to its retrograde solubility (Blount and Dickson, n= l 97=:)q but there is =i,iíii. evidence = foI suggest that the solubility of anhydrite is j'r [igraiJe in more e 1a ll{ ie fluids (Holland1! iÌlar'id z y7 I ti . t-i=Ñ i i i and alin In,' , `, suchjl_i f ri--tIir i13ii. _ i-ii ¡ jr=ii- ij ¡ tCi ?i. for phyllie a iTir fin1 i EE::ir the Ciasisbasis i_T fluid inclusion e':lideni_e! Thus.1 ;Ti_ 1{11=i'{t-'i=1t liil,tfti-i' t Anhydrite {aras an integral component of the phyllie alteration assemblage, as is also the case at E1 '_=alvadi_r, 1_hili= % C:i{Est atsi=in Rnd Hunt, 1q75), .Find was not derived from potassium-silicate altered rocks. The large vLiluTiiesiT calcium arli=! sulfate now fixed as anhydrite/gypsum were presumably derived through liberation r. ' high levels i T calcium from im =' iAgi i[ iaSi. and hornblende thP ani]F+=iTi= ;TfJi_!:_ and i...,allrorks during the F=nt.alssiufii-silii_ate and phyllie alteration events, and through the decomposition of sulfur d?:idF9 the dominant s_1 .$1r =FP-iP _ at high temperatures and high oxygen activities, according to i the rPRi_t i± in 4 `_ ' p +4H. (-) = _; (--) ! 4 p 4 (Meyer and Hemley, 1967). AltPratirin :=,ilii_ie alteration at Durazno {.Fig. 15) takes two forms.m=. The first of as pPrvapervasive silicification to variable degrees in small irregular areas throughout. the prospect; it is apparently late stage in origin, as it is superimposed On a il previously described a types. Pervasive r ili=iTi_at -n ThP gradual replacement of all older minerals by fine-grained, clear to ilnJh iti=' Cr grayish quartz, beginning with the groundmass. For TÉtPr tPxtur Pj ArP commonlyij'tl f. preserved except where silicification has been more intense than about tt Q-__. The irregular distribution of this, alteration t; r= suggests no obvious structural control, although it is probably significant that portions ±T some dikes, such 31 the dike, are more silicified and F'F-iyllii=ally altPrijd th ian t! tPir wallrnl=ks. GP=ai Ì js_ Fig. 15 L i ` i_ l _alteration s__ i gr f_.= _ T pP r' =k 1 i iF- r - l i_ i _ zi } i n And r7 i i ra l i linearr y ,altered and r il lc - T ld r rc(re z r ( d r t t P d ? i r= ). i St off i f-_' wijigrTi_a a trktr"aglïit pr's_fi_E?0U:r i=:II1f=Li_:f I-n=1, a i:..,,r at-f_f i,iÏi.i.y =iiid ! 9 r i==-_ Li intense silicification r i i-Z r 1s intensely s i l-- 7 f1S.d R ¡ =RE can only be shown! An aC t -t C11 t -t -t r. - --i i ? r- _r- G, t I_ h f_ :++ A. I< I r- -1 i= 1 i l i_ r i i_i }-Ì7-1tf ii_ i_ _¡ i=i_! i F l' ! r, -i r- r- I_t i_ . . - t t A ¡,Tt_+ .T ii i? =ii.i! ir -it1 i= 1rif=f.a- ' - lint...)_ _lrf_>_+: ;Tr-.1f..tt- if rlt-f=,=,r r sediments. f lfii.1. = No pervasive silicification Silicified zones >Q -0 to Q -2 1 Lithologic contact >Q -2 to Q-4 Alteration boundary >Q -6 to Q-8 Fault 74 the weighted averaging alteration mapping E=he.+fEe, most of the silicified areas shownn t-igure 15 appear to be onlytrt,lF3i:: ;=' álrifie3., but ffin=tnt _. á= áÎ1t31n patches of more intense (Q-(6-10)) s i liá=ifir3til_!n that cannot hP shown at tiescale ofFigure 15.Themost inte c e_;1 ;r; }i rR _ion Durazno occurs within a small area along the southwestern = J3= of the 1 r _r1 withinpartiallybrPcri a ted intrusive andesite porphyry and volcanic breccia. L_1 l l_i . l _a! i_n!I-t á accompanied by a small = t i ck ! Ai É r : i { barren q u 3 rt z veins 31 Cn gwith b r d_ á magnetite 4- quartz + biotite >ieins and iiialarhite staining. This area is roughly coincident with some of the highest grade copper, gold, and molybdenum mineralization at Durazno as determined by surface geochemical sampling. The second form of sllirir alteration occurs as steeply dipping"i75) silicified and hi'3I'tl, phÿ'ilirailÿ altered (S-(8-10)) fracture zones. Tl il?sP typically linear zones form small bleached topographicIlr ! Èigh1 and rarely exceed two tffiá_ters in width or 100 ffiártPr= iil 13ngth. They are developed j ii!3somewhat radial pattern liïl all of the different alteration zones surrounding the central stock, but not within the stock: itself e::reF't for a small dike-like G'roiertion at its northwestern corner. They are particularly abundant along the main axis of elongation of the phyllie alteration n f iá los and several á f them occur in propylitized rocks. off the northeastern corner of the map. A few silicified zones occur near iár- .ali án=3 the z rClntaá_ts á f dik:es. Pervasive silicification within most silicified zones l;f=r "averages around Id Q-4, but locally increases to á Q-1 11. In general, the a: :tel:-lsit.P i1==L of silicification ln r e a = e = toward the centers of these zones R 1 d% F 1 t P abundance of iron oxides and locallyllf dl=sefttií`i_!ted or veiniet pyrite. Manganese oxides alsoi occur as surfirlal coatings on some silicified zones (Fig. 9). The central portionsiá ns iáf several of these fractures contain stol_kworks of translucent,vuggy, crystal-lined quartz veins and velnlets in Ì il Til_s uptriabout t ten centimeters = tn! ¡á..1á_! and d ti iis quartz contains minute anhydrite, hematite, and acicular rutile inclusions.in_, ===i i'Ètiie silicified zones are cut by small gypsumffi veins. Angular breccia fragments up to a meter Î= = also occur 1 the centers o - many of the silicified L- _s supported byA siliceous, leached, iron oxide-rich ;"{ f 3,Jr i it hi:+4 m a } matrix. _i he breccia! r a g{ttFn! 1 3= _ _al l; i nns i= t of ande= iti r volcanic b rerri a, but occasionally large fragmentsr if banded magnetite + quartz + veins or intrusive ?ndesite porphyry are found in silicified i rnp._with Ii l _anl= breccia t.:1.4i 1 rr=i< l A = -ng ia g ne = 1tF alteration. The combined ph,jllic -=ilic ;i_ alteratinn of thP frAgmPnt= }hPfftjPlves is highly variable and rangec_from none P:tPnC iv j, i.=. 1: + Q-(0-1W. Ì rasn i n ' a and malai=hite were noted in one silicified ; nne (at sample 1ni=ation BD-2?., Fig. I I s, and small masses and in G s of yellow c L ; pt ! r ; = t allin e jasperoidal quartz were found in a few isolated localities. The silicified zones at Durazno 'fftay be the pr fnr ly developed 3na ii lguPs Li {P mlnPr a1iieJ r Ad ia Ìi=S1F'i_r i 1iiJ vPln=. found at Bajo de Agua Tapada, although in terms tif volume, wholesale thtallroi_r:: replacement tf1` fin? grained silica dominates over fracture f iÉli ir quartz in the latter ve i-=. The silicified zones appear to have formed throughout much of the historyif evolution of the Durazno i porph]'r',' copper system. Some of them are at 1ceist es early as the magnetite alteration event because one silicified zone cut by3 tr ans =r c.P sPt nf sheeted magnetite + quartz leinlet= (at sample location BD-1 i7 , Fig.! 1 J. ntrÌPr silicified zones, and perhaps the majority of them, probably formed during the phjrllii Alteration P;/ant. This is suggested not nnly by the intense phyllic altPrAtinn n f these zones, =t also by the observation that several of themhPf grade outward along strike into tAtFai=:1,' non-_iiii=ifli_d Ì(j-l0-3,yje F'i-{yll';I_a¡ljr T a t PrPd (F.-(4-R)) ir a_tur= zones, some i} which crnt3 in quartz or gypsum veins.h? latter type of occurrence, in particular, suggests{- that the development of _ilii_ifii=i. zones may aiifipl, be an unusual manifestationif ph,'llif 3iteratiCfn. A =11ii=1fl=d ioni_ Along one contact iJT the (intr 3pi t,t}tllit_ } Northeast dlkF? has affected both !hi= dikf= and its tAtallrorks equally and contains breccia fragments Ef botht ri i=l: types; formation was undoubtedly coeval with at least the tA.'aning stages of phyllie alteration. The only silicified Lone (at =ample location BD-1 47, Fig. 1 i? that ?17 , contains fragmPiTt of intrusive rocks is_ _: =d within1 n few meters nf A small, partially brecciated and weakly silicified(Q-(2-4)) ande=ite porphyry dikelFt containing fragments of quartz veins. This occurrence and the rinse spatial association between several other silicified z ones A_:nd dikes = ugg=j t a genetic relationship between intrusion i aitd the deve j ip'}1}Prlt of silicified i? inPs. Furthermore, the hydrothermal breccias in the centers of so many of these silicified zones, combined}ed wit h fluid inclusion evidence for episodic ú{ i ir}= tri bP presented later, all =1gge=t that these zones are related t_ degassing phenomena of hydrothermal fluids in contact with underlying magma. The upward, often violent release of volatiles through these fracture zones may have led to the development of short-lived hot springs on the paleosurface. However, unlike the process that formed late-stAge pebble dike= at Fl nhilp ( riujtAfsrin and Hunt, 1q75), the rarity of subrr}tnde.d or rounded breccia fragments requires that the analogous situ3tion at Durazno operated over a shorter period of time, or a narrower vertical interval, or was less episodic in any given fracture zone. Pebble Dikes True pebble dikes are very poorly developed at Durazno and are distinctly different from the silicified zones. Although some of them are weakly =' lii=ifiPd ((Q-2), they are ph; ilically altered only where they occur in F'h; llically altered rocks. They range from one centimeter to one meter fh)1dP, up to 3bolit ten meters long, and are too small to i=hotli? on the accompanying 7iidps. The pebble dikes usually occur in closely f jpacad parallel or en echelon groups and are typically radia iin arrangement, particularly nearer the stock. Subrounded to well-rounded fragment= of volcanic breccia And, rarely, intrusive andesite porphyry usually les= than l= centimeters long, are the only rock types found in these dikes, in contrastto thP well developed pebble dikes at Ea jo de Agua Tapada and Bajo LA Alumbrera A i=onteiin a variet y of f rocl:: types, including abundant basement fragments (SuchomPl, l r °.,- S u t=, 1984). The weakly silicified but friable rod:: flour matrix of tsP -7 7 di:es at Durazno in places supports the peLblP fr AcI rent_ And elr PwhP;P fills interstices between fragments. Two cur ved radial f3ults- immediately south of the stork +_i1ntain pebble dike=, silicified zones, and tectonic bi' PI_!_iAt ion at different places along strike. Although geologic. rellTirnsnl's, . , . are in=ufitit=ienT. . . , ,to establish, , _, i _AT ,, t - e1 i there eT 1e timing of each ofthese = !lh = f r a_ L _, IS es ore I lithemselves developed relatively J P r A u j P At their northern ends, o F fault -d its b e c c i a fragments are cut by early r3gie_ite + quartz vPi nlPtr And t hP other iscut a..n obliterated by the stock. Although1ugh ±ne timing o! the pebble dikes with i F=p!_!_T toI the iiAinr hydrothermal alteration events At Durazno isîI_P-r-tain, such features arg, usually interpreted As l_+Tr_ s=egg. in origin rl elsewhere (e.g. Gustafson and Hunt, rl! _E, Hllli3t3r, lfrr: _,3tle, I=!_?.r-_,: MINERALIZATION AND GEOCHEMISTRY Anomalous r_nreft ati_n= of copper, r_ i y denur, 1=3d, zinc, gold, eil silver rti , occur in 3 aa!_sri!r t :/- rJI , i -t-,,ll lerenT r =,ettin'-, 3t u_rnl., In i errri=,- r rifL Iie- ifuture ,' exploitation3tion of this r'orF'hyr., copper-type prospect, however, only the Ì 1w-gr"eds_ pr, erirEl_! rrie73l rriiner 3 lií'-3i>_!n 3F°'e3r= T E[!!_ rf any- reel erI_tn[Irril= irr'3 ir =1nl=e. , i_: r n t r 3 li = !i != i-i e iri i!= 3 Anomaly . . TIi.- e -I ra'iit'ejt_ gr =- !e ne3 r -ur r E-t= r=, l_ !- i'j_-{,,,,g-,, /' __!u - ) rriner ? ,i= tili i á_ Jur=i=n: ! orrurr1 1 lnprii iLiii-,-,i ,li_3t .r i_a itFI ri rocks-J 'I ij the centerir í i the 'rf iri_t in!!, An irregular _ rne roughly 0.2= square i nrP T_ r r in Are-.ThP bulk oT 1r_ rri i n e r 3( i za t i_E li is made up r l f main s ta. g e veins, b _t 3small ¡' r í¡° _E r t io n consists lnlner grad= J ir C erii3rCd rlìera iiz a : ion and the uncommon, but lrll! high grade "biotite br,l=ri31Eveins and "hydrothermal breccia" veins-. Unlike rii_ n, other porphyry copper-type occurrences where major copper rriner?iizALi!li i_ foundin Ph r i l, =3 ,i y Altered rocks, copper at Durazno appears to have. been removed during phyllic alteration n 3s evidenced by 3 dramatic decrease in copper grades 3nd visible copper mineralization with increasing degree_ _ij'eriïFr+=ed aiterAti_i. Thus, the outer boundary of major copper rrtiiier eliL3ti l- at Durazno !_{!ii !_id=1 r r, roughly with tthP inner boundary of more. intense i-'rt,tllil= alteration (appr oxilË3átPl`,'E :`;-4). This boundary rather fortuitoLs if lies just ,ut_ - i ;P _f t P major ?P of vein-type mineralization.i Figures 16 through 1E; show thP ¡P=ult= of An Ñ::}Fits s11¡'TAi=1-_+ rock-chip giorhPiïti+=a { sampling program 1_Einl_?t I1=T=11 by F=+bt'ii=a.!_iE ir1e_ !"4i liCa.r"e=; Table 5 lists some of the pertinent data. regarding this P::piilr-_itiiln =zttr1¡P;r.ThPgPili 1 tPtit i!_al results define á central copper-molybdenumrrbdl=nt t'si; a r I i'!ftaly within !.?hil=r! the best gold valuesAre A, _i found; spatial zoning of these tr ree elements is not apparent. The distribution r f silver is un ni s it = n r r anA l, z d fnr duringr i = .: :Pi -ra _ tn survey. Within the central gPorriPmirá ianomaly, copper r1lntPnt r ángPá from 180 to 17n0 p f Cu with four more samr1 e s a. = = a , ' -=between 7:;27i0 And ç_ n pF iCu. The molybdenum content is very 11A and ranges,- from 1 = f=.::' Fpt Mo with two samples yielding 120 a nd 21F. ppm t M i 1 ,r Ps pP r t iv Fl;f . Gold values r A ng P fromn.11 t C!1.55 p F' ii t Au within the _P;tr 3 l copper anomaly Aith twoighPr grade samples assaying 2. 7 0 and 3.60 ppm Au, respectively. The 511rfPrrP gPilrhetiiirál sampling r-P=ult1 (Figs. 16, 171 and1 8) also define the presence of a lower grade Cu-Mo-Au core in the approximate_ center =sf the western portion of thP stock. Unlike molybdPnum and gold, the distribution of i;is iblp =ur"T ii_i_(l copper mineralization ;ri !__ltsid be mapped and is =ThE_11E1!1 in Figure ?.This map similarly shows a poorlylr ly min+=rPliiGdcore,but its position is more Ñ 1_F'ntr Pl ! f inrPtPd within i rtP stork. Ti tP cause of ; hi1 discrepancy appears to be surveying errorsin layingaIrt out the11 iF+1 i_ttfti{ j.)1 ti1p li:_-1:-' grid. _1 i.. :,!r-:'i= s ; Fti t7 nttgn 1F; must therefore bi'_ considered PfiÉtiP?tihPi pp(:!:':iritPfiG!Pti!JtítF` analytical r P=E_Ilt= should not l iP strictly correlated with geologic tP.Ptur"P j. although iithol_!giP ;;'távP been shown on these maps for reference. Tab1P R lists r hP gPorrlPÏÍtiral results=ltP Ti !rfnumber of samples i_ollP!_i Pd by the author (sample locations in Fig. 11). i 1nfi irtunatPl,, most of these á3iitpiPl wPrP mistakenly anái,'zPd for lead and zinc in /S!rgPiitina, instead of gold and silver As intended, and only a few samples brought ciAc;:: to thp UnitPd !=;T atP; for t !tF-tPr- Fi.-4.l 6 - Exploration geochPmic.J rPsults forcopper in surfacP-L rod.:: chip samples coliPcti.d by pPrsonnP1 of thP DirPrcion Generai de FabricarionPs MilitArPs (DGFM). SampiPs-,- were criliPctPd.on grid pRttPrn with 50 meter cPntPrs. Shaded regions show the distribution offlovi-1;-..lolts copper .-m-,--- defined by D(7iFil (s-PP Tb1P 5), aiong with significantly higher grArIP samplPs. Ther,-=,(A)d.:--tf;-1h,---kve not bppn smoothed or contoured. Where rsmpiP,--1 couldnot be collected bPc;--(usP Of sedimentary cover, the lines separating these samplepoints from adjacent norialous samples are da-shed; solid lines divide anomalousarid non-anomalous samplPs-- :PP tPxt for ,iiscu,--:.----%ion. 5 177 ppm Cu > 177 ppm Cu and <1000 ppm Cu ?1000 ppm Cu 80 } ; - k Y Fig. ' 7 - C: F l rfr 1 g e - _ r P i_ A. _ _ r u'_ for - gold li-.! r T= -e rock _ -Ì_ = e T A ie = collected . Dy ,---. =i1-lni--z1 of FN.,r..J irPf_f=1 fi i !=ef=¡-1Pra ! d-e Fabricacionesfrie= ti. D ri F Ì"! ! collected ongrid patternwithi it ç i! meter centers. Shaded regions _ris it,af the d i= i.r iLfutls fi-i of An? ilil7-t i_fu= g_i ii`= i3PT1nPd by D r-iFfti1 1. i if`.Ñ 5.f1 T a l c-4 with _ ' g ï l T i: t l l higher g ` e Samples.-_ ray,' data have not f j e T smoothed i p contoured. Where sn: i Fl F_ wer_ not collected bprRusp the PdgP rif thrzi sampling grid had been reached,f¡' where samples could Ìd nt=fT be f_i lif=f=tt=d because of 1=diiiiP! t A r !_ __lerS the lines1P_F_ iatlng these sample F 1' tr from ad13 _ji T eii_liiia ious =empie= are da=i-iÑi-i9 1Sf:lfJ lines divide aiiEfiiia f_fuA.nd non-A.nÉififA iour. sA ÍIiF' ie-r-ru See text for discuss-ion.ft 5a .5. 0.10 ppm Au > 0.10 ppm Au and < 0.50 ppm Au ? 0.50 ppm Au and <1.00 ppm Au a1.00 ppm Au 81 Fig. - Exp1or7-ition aimoch.=iffir-Al resultfor mr.lybdPnitm in surface. rock Cfl]: R Tri pfr collected by pc=r=.cinni=1 of this Direccion General de Fabricaciones Milit:ArPs (DriFIA). !:::;Arriplps were rollprtpd nn A grid F....Aft-Pm with FO meter c.=.ntc=rs. ShAdPd rPgion,--, show the distribufiion offlomioLL molybdenum defined by DriFM Tbl.= F.), Along with ,--lignificantly higher grade samples. T h e raw data have not been smoothed or controire d. lihprp could not biz collected of sedircent.ry cover, the lines separating these mpi E-2 points frOITI adjacent dashed;olid 1nes dvd e= A norr.A. loll -L.-. and non-A n om ird Pxt for discussion. <_ 12 ppm Mo > 12 ppm Mo and < 100 ppm Mo ? 100 ppm Mo 82 Lab le 5 - Data regardirlg the ,,,LtrfacP rock: cbi duLiíi1ri sRgipling program conducteri by the DirPccion CiPnPrRi di MilitarP=, at BRjo El Durazno. MPan, background, Rnd threshold vR1uP,-.1 for copper gold, Rnd molybdenum were c7-41citiRtPd by p.,---rsonnP1 nf the DirPccion riPnPrRldP FabricacionPs MilitRrPs using the graphical method of Lepeitier (1959) for Earripies in which these elements ere detected. C:elation cr.PfficiPnf.. for Cu-Au, Cu-Mo, and Au-Mc. el e E.' t pairs tA:ere calrulafed by the author for samples v.iith detectable copper, gold, and molybriPnum. CorrPiRtinn coPfficient,-, for these element Pairs, particularly Cu-Au, t.4ould probRbly exhibit an even grPRtPr positivP rorrP1Rtion if cAlrulations had been limited to samples confined to the central roppPr-gold-(molybdenum) anomaly where most of the detectable copper and molybdenum is foltnd. A close positive correlation bPft.A.IPPn copper and gold mineralization is typical of most gold-rich porphyry izopper-type (Sillitoe, 1979), eg, BRjo La AllimbrPra (5--;tults, TABLE EIPment AnalyzedSamples, No. of TotalOf SEirriples withNo. Detectable Cu, Au, or Mo: (ppm)Mean ValueBackground (ppm) ValueThrP=.hold (ppm) EiPmPrif Pair SampleNo. ofPairs Cr.1D-1y-relation Piffle:lent MoADi H 274651E153 251447641 142 11 0.19 18 50,05 177 12 Oil 0 Au-MoClt-MriCu-Au 202249439 0.4501590,54 r o tze0Ci - for SEeC_e_3 etmenL. aat -Chip samples colleci.d by the author at Exijo El Duraznn text for discussion). Sample number prefix "BD-" omitteri1 S.mpIes .n.iyzd by Skyline Labs!. Inc. in Tucson, Arizon. NA = rot Rnalyzind; - = not d'tectid iTt = magnetite.; qtz = quartz; \Jilts = veinlets; sIii. = silicified. TABLE F, Sample Type Sample No. Cu ( p p m ) t:/1s; (pp ï 7 ) Au ( Fpm ) Ag (pp ï r ) Pb ( p F° ï i) Zn Remarks samples:General check Cr:F,7 , LF,nn(F)2:::n0 1070 s NA NA _,r,40 i 74, 148 R57170 27n0 970 NA NA1n1Ci 2.2n1,F, staANA NANIA1. ni NA205n NAF,riqF, n 17711F.5 7 l Fi;nF,!48; i 10 - NA NANA , 2n80ln 1n8224 1 ":tt-r3+ ' ` core al.0o Magnetite 70 ! , altered rocks: Mi FdF?r'R t F?. 143 qgR 54!146 i NA t',110 n +.1! I,f_!7 r ,11 7 0.24.4i { ft 20rANA 1 3? t:;-71 ,145 ., _, . 450n1 nnn 50In 1 ,F, NA 1.0NA 7),n4n3n 274343 15qE , 1900 , 1 n NA NA .-, 1 5 t, i i, F is NA - if,:. ifsj E-_ i F., a , Z1. =tE=,} . -F 450 ,., t.91 -, !__ F °i 1 rc t'É! A s rj 80 A . i F : t 1NAi t:"R'i P; n E : t jÉ i. breccia" 11;0 FNA NA l '. 20,0R f 7 i .:l 1.0 NA NA "Hydrothermal ,:=14RI 2:37 F,Fij # n,:'s n,F, 70 Lcnn -: `F, breccia" treins: ==800 NA NA 940 11 zones:;l1ìE_ tled 1 6 A NA 0.3F. 3 f i NA T i1 4744 23 345-1 1 1 F 10 0.151f 1 = NAIi A NIANAn=L 2040 41qr.;q1 _ 156 ;_, 7 40 - NA NA 4n50 `avi44 vifsincludesalong cutting d l sheetedke-.;,,1 s il alic m i r ;o c i:: contact f zone ÷ -t r:= NAF.1 -c l 7i 7R 'ÿ= L - i'i EE'e ': NA 37'0110 i . E purposes could later be analyzed forthe precious me t a ls.r P gPn=i A 1 ì_riPì_k samples -in Tablerepresent main stage veins and di==e1"ffiliatFd rfiiner_+lizaLion4 and the results are in general agïee'fffent with the assay velue= obtained during the former geocrfemical sampling program. A sample taken from very near the so-called "low-grade" core (.i.e.! 7 ! pp!i4 Cu) ) a = defined by pr lgr =-i fi! yielded higher results (680 pFfs } than e : Fe r td, r_ lA J ev _r.In contrast L : observations at other gold-rich, quartz-magnetite-altered porphyry copper-type deposits (Sillitoe, 1979), assay values and 'ffle].asci ìpif= and microscopic studies of banded magnetite + quartz biotite veins at Q! r ezr1ìJ reveal th?t these t: F?iils are not well-mineralized themselves, but instead yield increasingly higher assay values for copper, gold, and silver - n proportion to the abundance of crosscutting mineralized main stagee sulfide veinlets in each sample. A geochemical peculiarity of the magnetite + quartz -t biotite vein= is their anli'tilelo11= content of zinc which is not typical of 'ifi=lst main _ta'ae veins. Although the frìrm in which }h i= z'-:f-io occurs is L!nknotAjn+ it is tentatively represented as an early pulse of spFialerite mineralization in the paragenetic scheme in Figur e 12. A single sample (BD-11:1) r{ a!"c i k t i tP breccia'F vein containing auriferous chalcopyrite yieldedrelatively high value for gold.rf the two "¡'}y dra=lthermâl breccia !! veins e:n3l`f Z?d, one sample (BD-81) yielded low values for copper, 9 glJlds and silver, while the other (BD-84) is considerably better mineralized with copper. Both samples are highly anomalous in zinc, and one (BD-84) is also anomalous in lead. The sulfide-rich matrix of these veins thereforeirY probably contains a small but variable amount jf aphalrr itei* galeftA, and s! li-osalts( .' ), in addition tcì the i::nl;kA,n s_!ì_ourren!_e i !t pyrite And lesser chalcopyrite. Drilling Results and Ore Reserves The subsurface of the Durazno prospect has been e::;pl!^lrGd by i iinP vertical diamond drill holes, most of which are located within or near the central copper-gold11d anomall' (drill hole locations shownlwi! in Figure i1 ). Tr ie results of thi= drilling program have been n i=! irfipile.d from i_'snpiibiÌ=hï_?; t'!=r i=if i7= And At e listed in Table i!. The s igniti+_a.nce t h:_ r iAsillt = 1s dl?'T iri_!"{t to =l=i=+?r't aln 1; ¡Ñt;,,i of the poor core recoveries. but anomalous quantities ci ppF r, molybdenum, _J lJ, and silverr occur in nearly all ot the drill CFr . Within the hyF_geÌe. sulfide zone, as_ aï values,- for these elements r erEiein r á13t';; _ jy r insTánt with d+?Pti-i. Primary i_nFF'*Ar grades are generally low, particularly in phyllically 3iti=r+='d rCi+=!':s (drill ho les #3 and #8). Based on the limited drilling information, a rough `_tlriiaZP of 13.5 million tonnes of ir it r!ts f i i_ il it ' 'tT s i f+ .J i i i=_ C ore r inE._ ' ,1,. , t0.78 : nnt,3.4 rii:i: nnAg,and . 0.004% si11t i1 t+=ntetiilPl;¡ propii=Pd fior F'_;=11bl}? ori_ r'P=: r"vF= At t¡-ii= ilr Aino F'r+_'tspcs_t. This estimate applies to the block of ground within the polygonal area defined by drill holes #4, #97 #7, #1, r-%- nd #5 tri a depth i of 100 ii= meters Ani_° do Ps not LAÌ'.P. into acrriunt any peripher&ier& miner alizatiiJn or the "low-grade" coreijrF_- f[+r which there is no dri ! linrinformation. Electron Microprobe{be t;tud',' of Veins in the Central Geo chemical A:io'iii3l,. A limited electron microprobe study of selected major and trace elements in four veins from+iii the central copper anomaly has been conducted primarily with the aim+;- drterriiinin'a the form and distribution of g[ilo. ThP = A?Cipl+== studied i.}1Pr"P prepared a= doubly polished T1 in sections a! J consist of two bsn7 die5n_ t i ÌP + ÿlsArtL + biotite veins (samples DD-99 and BD-161), a "biotite br+=+=i=ie" t/eln (sample. BD-150), and a "hydrothermal breccia" vein c( 1arifF'li_ BD-81). Detection limits for ir A ll elements i,.As e r F approximately i! i p p m. The scanning mode of the electron microprobe proved to be ineffective ?n the search for gold in all vein-forming minerals in all samples except for BD-16n. In this sample, detectable gold was found uniformly dispi_r"sed throughout i_ri.=ili_i iFif rite grains, probably in solid solution, and not as gold-rich domains_ would G? the cese with inclusions+r isolated grainsifi native gold or gold-bearing minerals. Point analyses of +=e. 30 micron diameter regions (the diameter of the electron beam) of pyrite, chalcopyrite, and heriietiT+_ grains !i+1er e more successful than thP Table_ 7 - SLITATilary of PxplorRtion drilling result s for nine vi==rtical di:4mond drill holes ..--it Exijo El DiirRznn rompiled from unpublished data. Drill hole locations, shown in Figure 11, Supergene enrichment data- are based on a-. combination of assay information .-3.nd minerAlogy. Where th.:-;e riAtA arP indicAtPd in pArg-mthPsP.-., :=1 highPr i-P:----tr-surfacP ss,7-1.,./ intPry7-11 my correspond to --7upergene Enrichment, but thPrP is little or no supporting rninPrA.logic Pv1d2nc..-4' thP highPr :-1ssAy ri.sult,--- may thPrPforP reflect hypogPriP TriinPrlizAtion. A dash (.-) indir;ItPs.-, that data are not 7-1-4:411:---iblP or .--:IrP inapplicable. TABLE 7 Supergene Enrichment HoleDni N o , ReCOVer y Core:Drill Collar E1 iz: (in) V . TotalD4-h ( m ) CI i M o Average Grade, (ppm) Au jAa Pb Zn Inter v ;711 Depth ( m ) ni AvPrage('t %)t Firade 21 76%-,..,,,i j , 2516 2:0,552q,2:0 72:00i 550 157 1 2,:ll1,73 1.60,8i; .7'8 154 NINone o n e - 57%44% :::=7.,,.....751F.2534 A ..) -; 1 00 34,40 2:200 1 ciF", f; 0,7:41.53 61:37.21.8 40 - 2:70 - 14-21R-1 R W,37)0.430,059 -7 - 2548--.::7525 : 112 3240n F. 0 n 100 0,7 0,:::01.2 42 . 0 - - (0-1;)(4-10) (0140) 8 - 750F,2521 174177.6 :,;(1-, 17001000100 l o o - o0.28 ,- 0 1 --_, scanning mode in detecting gold, and the analytical resultssits for a number of majorr and trace elements are given in Table 8. The material analyzed in this manner in the banded iii3gnetite + quartz + biotite veins consists s _fi tiny crosscutting ii3aiï! stage E;..i iride veinlets and d1s=,eminatlnn_ 3 inn'] mirr á f Rf Tl{r _s. The !iiPmAt Ët='! that was analyzed is an in situ oxidation ¡i F'r =idL=t after eit ier pyrite i r. chalcopyrite, and a itLi; cisgf t its optical lF'r CcF'er t ies conform iiiael f with those ! ±t Lietilatitc_', the dna iysjs obtainedained i'or this mineral indicate that it is impure. TLie point analyses conducted in this study yield expected a-,toiChi metr i± abundances for major elements, excluding oxygen, and will not be discussed further as it is the trace elements that are of most interest here. Ar sc=nir is ubiquitous, ranging from .'r560 to ! 700 ppm in chalcopyrite,ite, t.f}itri most vAluPs in F'; rite between ¡.'.'r_i{ i and 870 iF'F=rtl As. Arsenic contents are much more variable in hP'tri3titP (q70-1:::70 ppm As). Se?c=nii_iifi was detected in only half of the pyrite grains studied, but it is ubiquitous in the chalcopyrite grains analyzed, ranging from 30 to 250-tsiilSe..The remaining elements appear to be erratically distributed, in pi_crtinnscT t iP same mineral grain at points spaced only a few tens ? cf microns!nj 3p.=ár-t. s ¡ l,'_+r was detected more often than goi d in both rite and chalcopyrite, thi Ï P both gold and silver were detected in a larger percentage o7 chalcopyrite grainr than in pyrite grains. j o d and silver a'u=C range from L0-i y! r p ï Au _ n_ 7n-2F,0ppm Agi1 pyrite to 2! I-240 ppm ,iii_! and 10-1100F'pmAg1 chalcopyrite. Experimental work: nn synthetic pyrite and chalcopyrite±F'yrrite sl lc!!.h' s that these minerals can incorporatec= ate up to fr-. r i r- ` t } j " t 1 - C ? s Z s Lii I_i -ia n rs4_ii i {i Fi lAu,r F t ie i:! nl _ c i c-i solution3 Li= _c i- L.% rt 4-1 i . ! t_,Ti ! I high arsenic contents c_c?' thP DLir _tiri! i pyrites are typical of auriferous pyrites elsewhere (Boyle, 19791. Gold and silver data obtained for hematite in this 1tLid:, are itic=onc_lusi,leq but suggest some ?eari-iin'i and rem_1Ir,', these elementsclrini oxidation. Point analysesif magnetite were not performed in this study,,l=uit this mineral can theoretically accomodate =' quantities i fUPto i i.;_i_i_ipF'!"!1 t?.L i.Bi cy jP,c - i' '!, and occasionally as much aai. _2ci ppm i;.ioriÑ= .and Fleischer, 1q69). Table 3- Electron microprobe geors !Prr!'ira i results for =e1?rted major arid tre e iç P_ in p, itP_, r-Ai r_-iyr iTP_. And F rAt i t? r i-s in fnur veins from fFP = are r r r i r-ri_ `L Yr l=r r i r _i Anomaly `[ =i U_rr.i13 . r diameter regions in earh gr3iri, and detection limits are 3pi=ro".iTYiat_1jr 10 pGïri. Sample number prefix "BD-" omitted. NA = not Aiia1y iF`dg - = not dFi FrtP:j= 1'=P text for disru=r=rin`-!. E f; c, t _n 7 g s cr . t :: VNV t'.j n06 L 0 i L -- 001r g 1 { t Cr r E: L L - LzT 3',''o l / 11 r.J , tit f-7:EV 1 9 jr _ r ,1: t_ t t ¡t r , :Oi L .ci n r : VN gq 0 !F, -..4 . : r ti_ VNS - n O E fii L t7 t qr, 6 -1. . rr ' - VN 08 '1 . f r 0 t f i# 009 L I t r_. T ' ¡. t r_.i - r _t_ ¡r t' I I_, : T :.:1 t tj r. r:_ i - - 1 r V ', },i OLL 0.: 1q '' - 07.7. ¡ E - ; I = I L f:":0'17 = 7 =9' t- F: t 1 t_ #i' l ' 0;_ T t' 7i - fL r rl =7 I} L %r 3lL i_ L. ' rt0 l T? - ' ` . r, ', ' 2 t1 1-r t. r. L r: ' ! 1 t' ': i 1 ' tf.8 t tT;T'ft''.; E g OL I TYF f I , ,1Z .=t't E: tLE't=E: ¡ ! . a ,{T'. r . 1-ti a -r E L 'n. VN¿f t' - ! q g ~ 1 1 - 1 L g -. - L L ¡, 1 r L a a'- 1- OZ _, 1 L.. . t #< s_. t 1 tl 1='rt r a 9 J = . l L a L L _. _:,. } .' - 1 r i t tf 1_ rr1 ¡. ..± -r t tr. . 1:11 jÉT = E'1 . VN i - L EJ'!qC2,g1 - OL 1}EL7g 1 031r l 07. - 6,16'71E:Er f, 1. t 1' L.íi.!6 E' a' `' tI1 F RLg'At.g_ 7t z t ty.L=E. ori L .A ?i d Lr r»EIA _1 ttO,iti ;'R' 10 101 L VN O g i 7= L LL 0 t - - - E iE=ln'0 '0 t=L! g9'r:6-O'tg 0 6c'Lti 0 `Rt= 6L - - - - 1 L=L6 Lt' LO1i L t i 1 VN 09 - t 8L n L OZ - , tt0,3 L L - 7..g0' - r/PLL'r:g r,E6t'Ltf r, L 1 / _-ttttF1'a:: r1f L1 r L VN 01 - llc,`gLtr .,--: Ot I - n01ON 0 1 L - - ,,tfi'Csr - rtLtttL;r:gtl't'g ? ',_:g rrtr ! _ r,,_08'8tag9'otttl Ltt i, oCo P,?.J LL r1' r n! L rtT # ' L L Vt4 U- r?L 0.=!L ± _ L - - 0 gT - 061 - 1290'0 ' - F.8.'Iii 13 9 LL' = L1 V1, r/t tT r:F, L 70?1 f L Lí 111 .fVN R i Ci 6L 0n7 na', L - - -( f_1 - j'' LE: L 'tg L9!'i'"-?? T I L°J}LE7'{`f Li ' !-! F_ L rOLE - =fjf f 1 N i {i !?4iiSt?L, L 1 1P, I - {: l OS'r l :, iS) 1 Ott I.Jtt 1L {. L'J L rtJ atV;T+T , Ìa. r 8 't =1 Lt*LL`.' 9t ! a r 1611101 2 -_ Ct 6 - 1rVN l - - t=1 'n =,tt 't?g r=! 0'Ltt ES9'00g81`101- L 0L17OL - vrJVN Ot809L06L 001 - OgE rr 1 OL L - n91O1OL - - gLE'C:gLT1'tt.aL88'?. g g08'9tt6L_. Ll'Lt 6 9 t S'L09`001 l L'1 L1 L' i l6 b -- VN 018008 Ot01- 001ä 1 CJ l LOL08 - nO910'0 Ll'n L L ' C1 6gg'P96'?q`C:> LO t= ' C g ?t;C:'Lt> L8'9ttl E; tt '9 tt LE: ='.T.T' (% liA)Le401 TCE d LL ) 7il1 d) gE; ( lüdd)sv ( Rudd ) ai. (l!_idd ) a3 (ilidd) 6 V 8 318V' (Z= d.1 "V ( % l(y)) "0 CY. 4{'0 _i CY. -VO a3 aLdulteSkagi,ünr.1 paz,geIlv laral41;.1 TABLE R (cont. 1d) F.,--, f.: I.A.) :S --; .; ...) Cu Au Te AE Sb MineralAnalyzed SampleN u m b e r 81 51,867(tit.: f c.7.-,, ) 0,0c42 t ( w t P P m ) Se ( p p m ) ( p p ) p p NA ) ( p p m ) Ti.70 5Total( 7 0 t 4) Hematite 160 6o.94551:37Cl 0,08(3 I 40 I j NA NA f-=.1.FIR45 1 I. 5 7 161 00,05R 0 5 , 0 2 5 0,1.541 0 ?,0:";0 140i 00 7020 NA 5152.09452.479 .783 Consideringtheselow values, -!r eeMr Unlikelytht A.imuch, if any i f r hP goldat Durazno occurs = vritÌ-tin iitAqviPtitP. The small a ltii Eu Ïlt of data to Table Li and itsrather ¡=norquality Lft=!_-_.! I t = e of ïlt4.,J!?t.r!=r, high detection limits render un ='t'-ai-Ele>...! L for t_,,-TTti_tii_ i te=t=.. í in i a q A.l .at Ef P sense, there 1 P P m s `n_ _ biu. = _ positive or negative t_ì Er r e.lati! Ei! between j an ;'E2 f thP P1P3ltPnr 1 an ta i r'`z1=d.i hi=r"E==f írP, t¡ IP gold And=1?ii!_+rin the pyrite, chalcopyrite,Bndhematite grains probably do not ii _ici urBE Selertidi=1, tellurides, oras-, arsenic- or antimony-bearing rL i f S : altr Furthermore, t_ P lack of correlationYlatiÉln bett.ëjeEii gold and antimony r ulP1 ouf tj-tP y that gold Ed o]-cur = as aurostibite (Au=C7), a theoretically _l ftl n solid inclusion rra=r ipyrite with which itis isnstructtural (Eil yle, 197q). lt is I_ Ertf_?t fjr=d that qolri And s i Ì`:fPr occur as the native species, and possibly also ar electrum, at least in the four samples studied. Much of the native gold and silver in the central anomaly at Durazno undoubtedly occurs in complete solid solution in sulfide minerals, especially pyrite and chalcopyrite. The precious metals probably substitute for iron and copper in the crystal lattice, and selenium, tellurium, arsenic, and antimony probably substitute for sulfur (Boyle, I 97q). VisiLile native gi lld has [EGEjii noted in previous petrologic studies of Durazno drill ci r e (Unpub. data), but the proportions of q[ i] and silver occurring as "free" grains of the native metals, as solid solution in pnaSe= in sulfide l-nerals, and in silver-bearing s u ! To_ a Ls are not presently known. E1 t Abl i si-t iriq these proportions is of obvious us imp I i tai1!_e in tE==r"iii= f I fi the future economic feasibility Et extracting gold and silver from lotA, grade Durazno cires. At the Bajo La Alumbrera porphyry copper prospect, native gold is rarely intergrown with sulfides and occurs mainly in quartz veinlets and as disseminated submicroscopic particle=, often associated IrJitpervasive -i l i_i_atir n t' i n: a lPz, 197 5; Stults, 1984?; gold and silver also occur within pyrite and i_hali_opyrite grains (Fers. C orrtrrt., Stults, 1q84), prciba bly in solid solution. Gold occurs ma inly within p+,rrite and its oxidation products in the Farallon Negro epither"t;tal ileii! !.5i=-,ter, f 1963, i q ? q_; and thesepyrites may contain av 1Pni= i t a i ; i =i ni And L la fbi_1 9 Lt thisP at Durazno. Auriferouspyrite,741 k o occurs At Capillitas (ng:Ìe,,i and Lima, 1 980). The solid s o1 t in of precious mPt A is in sulfide .i i P a : s therefore appears to be a l"e.gionái characteristic of 'Min=i-AÉiLati=li Zn the Fai-ai-Ìi+n Negro area. Peripheral Low-grade Gold ld Ano'fftalIr Outside the central copper-gold anomaly, copper and molybdenum tË abLnda+i ii_e= generally fall below their threshold values, but anomalous gold (and silver?) mineralization F'ersists, albeit in lower amount'; than in the centrai zona. Erratic and anomalous gold values of up to0.78 pF=m Au occur in a broad area ! !t roughly one square kilometer ? ro!nd thin centrai anomaly A s shown_wn in Figurei o This broadly anomalous region is undoubtedly related to porphyry +_i+Feier--tyF=e hydrothermal phenomena and is not, for instance, part of a larger regional pattern, because the 3j__ e f , r3 l sampling grid is almost everywhere s_1fi=i?nt ; iarge t ! establish that ä no fa' s! s (>0.10 r p i ) gold d = ? _ not or Cur beyond t h e shadedr G g, onC ;('i liA,'il in Figure 17. The gold values within this anomalous region ¡ !Jo not ;ris ltil,i a consistent correlation ,fith any particular type or intensity of alteration or withi any other geologic feature such as faLiit=3 dikes, ; rr fracture density measurements. This lari:: !_+T correlation n tAas verified on several ! íccasis+ns by a comparison of assay infor'ffiation and geologylgy .at several of the ges +rhe;i'ical sampling locations in thefield;it was observed that anomalous gold occurs in both Propylitized and phyllically altered rocks and bears no relation to magnetite alteration. However, because not ail of the sample locations were =h=r=d in this manner, it is likely that some relationship_ have been missed. If distortion= of th>' sampling grid due to surveying errors are assumed t o be minimal, a =f1 a r i=on =lf Figure 17 with Figures 9 and 14 reveals that much, but by no mean= All, of the anomalous gold mineralization ïn i!_curs in ph.rili!_?ll'J altered rocks Along withweak correlation between gold mineraiization and some areas where either manganese` oxides or pyrite 3r-P more 3 3bund31-!t. The (rrlr'ï-P É3tlon between gold mineralization±n and pI f j lilc alteration does not appear to P..t _nd to';lii_r f=31ing gold abundances in more 3::t3n=iv31y phyllically á (t3r'3d r Cif kw, but ?lter'3t1on values at each sample location are not precisely î.É_lotAiÌi. E'3f3{;=f=' prC+py litl+_ and phylli+_ vein assemblages 3r-+= not abundant at Durazno, it is ext te mY É',: unlikely that much Ci lthe anomaloust= gold in the peripheral anomaly occurs in veinsnr iEl_ln Éi=t1. R3 thPr, the gold is probably in disseminated form 3a microscopicfpii_ir even submicroscopicplf p3rti_li=s of native metal, possibly along ï(!1f_i"Cifir"ái türi3s. By 3n31Qgl` with the distribution rj of gold in the central geof ham i+_al anomaly, some of the goldmay occur in solid solution in ii f pyrite, as the occurrence e o f pyrite is the only geologic featurelr"e tr f fïiiiion t;_iboth the phyllie and propylitle alteration assemblages and, ofcourse, the "high pyrite" i na=. Only one weight percent pyrite f=Cint3ining 20ppm Au is required to yield bulk rock assays of 0.20 l ppm Au, and all of these quantitiesare reasonable and well within the limits established by the present study. Since pyrite ismore abundant in F°hyllif3lly altered rocks than in propylitizedones, relatively more 3 gf ild may occur in pyrite than as "free" grains of native l'!li?t3ls to P'°.pl3in the %ii'ilAd positive correlation between anomalous gold min er31iz3ti on and phyllie 3 1t Fraf ion. The t><.ri?3I:, correlation between gnld mineralization fn 3ndmanganese +_i::11J3=i= not economically significant at Durazno. However, it iAia j noted earlier that hot springs and aFi t =ri3. veins i n the C 3 r 3llin Negro?g1ol are us_3 llJ ia igA n?s _- r ifh,suggesting some sort of chemical continuity in LFii? gi_nFaij of these l3 tPr hydrothermal systems and the earlier porphyry copper-typeones. Furthermore, iron and 'fii3tig3ni=_a oxides ara associated with high level disseminated native_ gold mineralization in the hi _d zone of 3 shoshonitic porphyry copper-type =yatf at Vund3, Fiji l.LatMr?nf*"=, I 97ED. Gold abundances in this ahi_+ahC+nitif systemseemtC! forr3 3La, with the degree of fracturing and with the original pCirC+=itÿ' and permeability of the pyr ot l3atif host rocks, rather than withany particular type or degree Cif alteration or with the distribution of pyrite. zones _r 3 z n o Ar i! not p Ar = 1_H l A l;I F1 All-"i A 11T d (Table_ 6),, although they i+_+ c 3ll y contain anomalous q u a nt1 T 1e s of i i_ i_i p F' P r 4lead, o';" silver.I r e copper, fflLily[idenull, gold, silver, and zinc 3huildal-i_P= Lh.A silicified zones, even 4 3F'F'are.S It i fE "anomalous",j3 iusi+, a r-e + probably not +'nha1 Fc+Ad relative 1 f i rock=. in the ir,edi3tearea, however, 3rl the only slgil 1=á} = g =_=h. (1. 3l feature 3F- ea. r j iï be Plgi ::3 t ed lead contents in i = o'i(IFriithese z i+n3 s: Other than the visible copper mineralization shown in Figure 94 'iii+Cg33ci=iF'1i=3li'}' peripheral Cu-Pb-Zn i 'ifin+='r"3l1z3 ti=i17 does not occur_;r 3t Durazno. Supergene cnrihrrent, Oxidation,3tion, 3n ; Lea chin; Supergene enrichment of copper is pi +[+r Ìy 3s'id 3rr"3ti+=3 1l,'Ft d=':;3 li ip3d At D!'[" jLnii ( i 3b le 7) and is ono real economic importance.3n+=+?. Wlthin =upFr"g3rir enriched zones, ch31c +=i! it3, +=t=iv3Ì lite, and n+=odig3Éilte replace chalcopyrite 31 id bornite along grain surfaces and internal fracture's and occasionally form thin coatings on pyrite f IJnI.tCE., 3t3, tl-iisthis stt_t+ fj ). The available; rl'i11ii i ?3Fdata 3 3 r"e in_4Tfii=i+ ± t=E1 +r .3h-_i i whether the present thin supergene enrichment blankets were originally formed at _! uniform depth, if they y rE! le+_t a former topographicigr 3F'I [lE_ 1ur 1 a+_e expression,f¡j,i_iF even whether the entire picture has J been L+_imF'li+_3te.d by faulting. It i= F'o=s ib1e, but highly unlikely, that a former =uF=e.rge.ne enrichment h13nke.t 3t Durazno h3 s c=PPt- completely er i_id+' d and 3 n3tA1 one has- developed il nRei_+rn t ti i'13=. C3e11es et 31. (1 q?i ), in discussing the poor development of =u¡1i=rgenÑ enrichment in the Fer311+_ili N3gr"i s region despite i 3,IitEr3CiZ:= climatic and g iri irr+_ii F'ho1i igic31 conditions, point t o1t that no supergene sulfide_ enrichment has occurred in contiguous northern Chili., since Oligocene times. Surficial oxidation and leaching are widespread at Durazno, and g±ieLhite, hematite, and lesser jarosite are common +n 3 = in situ oxidation tl F'rodu+=te after pyrite, especially in phyllically altered rocks. Magnetite is oxidized in situ to hematite or goethite, and much if thP magnetite and some f this hematite q4 (lÏfagher?tlt+j) are sufficiently magnetic to cling to steel. Transported iron ii::::-tdc`=,s usually gi tritF } are common along fra _ Tr P s throughout fP r _ r i_j A d transported t=ta t i : =} g+st iits} and ir s lt F often coat gypsum ielns.Iron oxides ar P bPst dPveloped in i-i"i+! sulfide-rich centers o! some sl Ilcl Ì iPd zones, "hydrothermal br'F+s_s_1a.!! veins, and faults, Aiiri .=ir"*? most obvious A= sur-TZs_ia i staining by brick-red goPthlte-hematite mixtures ss in !lhiqh pyrite" ii inej in pÌ-tylli+=ali}` 31tPr"P!_i rocks. DPspitP th3 tviijPspr-Pad r+rrtirr+=ilrP !3± iron oxides and r r=3siC¡a i [- ;wrr: -. aft Pr pyrite, most = rf the- pyrite aC Durazno is surprisingly fresh or only s 1 lg -t7y ;" d ii P -and if is not _ f Ali uncommon Tfind T fis- pyrite s=i n weathered Jut- i F ce s and in r rll! vi3irllb_ i a: L Ë_ = Lv t3Ì P} - al-= F yr1te is usually +±xid ii+_d (llsiai hitP r+r l_ss commonly i=rir yso+_+_ il3} but fresh chalcopyrite -Ar = i=-_r _PPn found on Faat ar Pd o= __f fACSs. Background GeochemistryiefiÍistr y "Background" base and prP+=i+_+u_ '((tPtA1 ilsli iPsfr+rtr:lP33:1y j°r- 1py i i! izsd dli':Ps at Durazno are listed in Table. ri (s= n-ipIP ;ri!_átinn= in Fig= ? 1 ), ThP rnppiár"}gr+id.Aild molybdenum values r bt 31 nF+ for these dikes 3 ; r similar fo laljP1 for sur r r!nd in w3llr i=k= a ltPr- ;tiQn `_'fTSI=ts. There no data a1..sllai..iit_ with which 1 e 1 1 e+ r_- t_ t i 11+ } 1 _ , r, rlr !J(i j 'r + ± i +l ! : r- ! } and zinc !] . T . } but = ( ( i + Ñ BD-781 1 i 1_ } and .1 Lri ;_i i may be slightly enriched in zinc, and i sái'(ip ¡s+ BD-78 may Miso bP snrichP in i 133d. Silver values for all the dikes arP p r - il T; p-_Ai!_rt gr_ÏJ! In AL-i1? 10 A.rPll_! P]background L' and precious (et3i values and sulfur contents } fresh t r R h j i 1 r r ^,li i? d volcanic b e _ cls from a variety of lo[r '_ tAilthin the triangular ares between the Bajo Agua Tapada, E:ajr+ Lal1luY(tGr=r-a} andt r 3 L n C prospects.o l s of the _ = '(i ri= locations .._ h r tA! a j Any evidence a ltPr3tlrl other than th)P31pr! p_'r'litiz3tion} nor any 3'l1di='1li_e of rlliiilCr"all: 3tii=1n! veining, or iron 1:.iPr staining. i.iinin'. It It can srbe =,PCii seen that t,tt at' least-1 in this' portionr-. of h31 Farallon Negro ii!s lranir- complex, tiäi=;::ground values for gold and silver are on the order r+f 0.02 p F' (( i Au and Ïp p ( ( i Ag. These 1/ A i Ll e = are t !a, ri to five t 1 m _ = greatert h a rl1 Table 9 - "Background" geochemical resulfs tOÌ s&ectd trace Piments in six major tAleakly propylitized ...7.-kndesite porphyry dikes at E:ajo El Durazno. Aio or three Separate analyses tAiere performed for different splits of sm pe. g A 1. - - -g '144- - byLot!.Lt it 172 1 r ura. rabricaLione.s ivrii,Lare.:.-, in Ar;ntins. Samp nu.-mbPr prPfix "RD-" omit not - not dpipci-pd, TABLE 9 Sample Cpr'M ) Cu (.. p p Mo Tri ) ( p p m ) Au A q Pb Zn Nos. F.F. 33 1 1102 (ppm) 1.4 (ppm) NA (ppm) NIA SouthRemarks dike -7, CI .-: -".",Fi 7R i. ..'/ / "7 0.08 NA NA 17032:0 NA 195:::01; NA East dike 1?.8 i...15,*: i 0.1E . NP, NA...... -J ., ri 60NA 122 NA Weit dike 153 28e.._. 1 0,04 NA NA1.3 70NA NA90 Northwest dike 154 NA NA1.8 70NA 7NA =I ....I North dike 1 F:5 13 'Ii 1 i Ch os NA NA1.1 30NA 175 NA Northeast dike TA.biP 10 - BA.ci :omplex. Samples: analyzed by the Direccion General dr-z. F.,--,bricAcionPs Milif:.irPs in ric:!:._.:Li.L.-=;5 p..rgentin.R. BD = background; - = not dPi-Pci-Pd. TABLE 1 0 1.. n ..--; 7=; a vi-ip le Cu M o AU i:. P p m ) A .q ( p P TA ) Pb ( f---. ..) ( p p rfi ) ( p P Tr! .) 1 F. i. p m ) ( -. P M .1 1 4 420 7 0 550 3 1 ,- 0 2 142 -0D , 270 :3/ i I n0 07 0 0i; oj L o 7504 1 0 503128 71 n0.020.02 , 0 7 1LO :21 .4 700610500 " "fz:, ...., 7 620450 the gold a nd silver contents of most tt a'tirrr.=f gtjft igneous roc-::s, P::f_+.pf for 1nmP tfasalta (T_lr Pi:la t n and WedeFfn;"!l, 1 ÿf;1 ; Thy. f1 1F'. T i i fl' Pf--E°g 1q7:---0. The quantities of precious j ifEeta i= could easily Cfi_ contained in primary i llcate filinPral=4 especially 'ifafic mineral_ such a_ hnrr-rCllenije, aHqitP, and biotite (iones And Fleischer, 19613 Til ling et El., 19? 3) and prri !a;='a a i_+_! in tr'ai equaÉ,lltlf==,ft sulfide ffilnPra 1=,. T! fP sulfur cclntPf ita of thP=f= rocks Ar P not 'ii!! lrri qr PAtPr than "typical" values, however (e.g. :300-400i-4[ i0 pp'i?! S9i Hr P-k1.3n And WPdPpri-!i;i c;1 s° Copper, fi!! i Ì r Lft...fei si t iliR and zinc f_onÌ Gnt = i t= thG `ti; i ti _A n';s_ br"f?t_f_1:,1 ,r-j a Ytj (A!1i h;¡"i f_; :;rJPi_ i f?d ran'f=, .TurP`::lan and Wedepohl, i`F,', Taylfr, : lyf,. but 1Pad i1 anomalouslyhigh., The reason for the lead enrichment is Lnkn ttAi;;, but it mA y bP _ffi As yPt unrecognized regional ge_fr;'lPffiical zoning pattern rrlatPd tn ¡iydr nthPr ifia i Activity. STRUCTURET RE ctl- r+ The rtr _itIr a i L!ri-1 Frr' at DJ.rI anf isri:rfi r-1 Li! a i ffliir F'l1=at-d lnFPrac}lcin between regionalandintrusive-centered stresses. The prospect itself is localized at the intersection Cf four rPglnlAl structural trends: 1 J a major n es t --_r ht.Aier t- trPnding:alinement of mineral occurrences and hydrotherffially altered Areas; 2: a nclrT;-l--SrrtrthwPst-trFnriing F'hntollnFA filent, 3) a small northeast-trending F'i-Ectolinf,--iarflPi-it that connecta Durazno with the Bajo f La Alumbrera prospect ; And 4) ar-E Paat-ncfrthPast-trending dike swarm and fracture ZGnP. Althoughthe of these trends- is indisputable, their precise role in ti-! localization c+{ the Durazno porphyry system and in later intrusive and hydrothermal Activity is open tn discussion. Intrusive-centered, i.e. radial and concentrii=, structures also Ea=ff=i=ur At Durazno and are most evident in the crudely radial arrangement of some andPllte porphyry dikes, pebble d1:e= 9 silicified zones, "hydrothermaltre_r-ia! = Alteration t_frfundAr iGJ, .+nd faults. Intrusive-centered patterns such as these _Er f= usually interpreted til reflect a shallow level of forceful emplacement where faafagenic stresses are able to overcome the regionaltrPs = r ifFor _]by tectonic forces and liti inStatic and hydrostatic loads (e.g. Koide and Bhattacharii, 1975; Titley and heldrlcl::, l978:. The intrusive-cenFered Patterns 3t Durazno hax./e been considerably modified by regional tectonics, h 1e}e ;IPi' , both in terms of structural control by pre-intrusion fracturing f the volcanic host rQrks and a= a result of continued regional tectonic ad1ii1t'i"ilP Ji s during t hP entire r;/olutloi i Of the porphyryi"y ropir-`ei system. The 'i{It+inPd rs=+giGn+l A n=1 1ntrLislllP-rPntPi Ñd =trui_ttlral effects are most evident in the dike Patterns 3t Úiira?no} which though crudely radi3l, have in detail been strictly r,JjITF" illFd in their Pmp iar+=iiiPnL by pre-existing fracture set= in the volcanic hrer==ias. The dr3''irlatir right angle turne along strike displayed by most dikes, Particularly the relatlt/ely early South dike; suggest that minor and continual shifts in regional tectonic stress regimes were important in determining which fracture set,-; acted As tens i! irla; or P:.LPn1i=+il?l o pPÌiing=_ rE`f: given time during the emplacement of the dikes. Structural Patterns at Durazno have been 'AiorÌ:ed out through ñ combinationn of traditional mapping methods and through the use of fracture density measurements. The latter technique involves the rrleasurement of the intensities and riPnt Ati! ns of fractures within one square meter areas and t/oa 1 performed at 26L different sitesJ 1 throughout the prospect (Fig..t . 11.!. Thet 1 sites1 I,.ere chosen...! .J = as1 to gain information on both ':/nlr=anir= and intrusive rrirks, and nn Pärh type i"l','drr+thPr friA! altE?'rAtion, including vein assemblages where r'r+'=sPnt. Fr fcti ir= t 1 1 1 1 I 1 -7_nJiti=,.,,/ taere 'rrleaured tt,irs += floe 1j j-'3:_Prr r j i1 r=,l T in tt3 . e 1re+t 7i1;e iecaueonly two of the three widely spared orthogonal joint sets there could be measured any given joint Piane that formed an outcrop face; both measurements were identical (O1,Ol9 cm '). The West dit. : :e example illustrates one of the dratAtarks to the fracture density measurement technique in that some fracture orientations cannot be measured if they are nearly Parallel to the outcrop fare where the measurements are being taken. A second dr3tjjb ck is that megascopically invisible fracture sets cannot be measured, and yet the intensity of 'microfr3cturing is stn important structural consideration. The results of the fracture density Fig. 19 - Çr aj_T4;Y'P density TilPA =il% immPii! ..if Ps =tfid ci sni o41i r+dt P1ui!=. =;ÑÑ f F1. .: ! i^ii1..i i.r-r- iE f I. <0.15cm1 0.23 Fracture density measurement site 1 and value in cm -1 0.15 to 0.20cm-1 Lithologic contact 0.20 to 0.25 cm-1 Fault or fracture density contour r'line (dashed where inferred) N, 0.25 to 0.30 cm-1 100 meesürements may Lie considered in terms of the two t j`pti=. of information to rhe:l 'leld.i 1 ) the intensity of fracturing,3=expressed in ?:`;3i=3 =3TÍt1ii+=tti =9 i.e. (sum of total lengths of all il i r 3ctui' e= in meters .' one square meter arPA. 1,''1001 with higher numbers representing more highly frA =tui"Pd rf rÍ :s4 And 2) th= =tti"üt_turA. 1 attitudes of all fracture sets at the measurement sites. - - =d The information_ .R the intensity of fracturing q ^EtiTie r what was readily observable in the TiPéd, na EEEi=lj , that C'r'_tpjf iitii!_d rocks Ar"P the ít'a=t fractured (rangt=: ! l,! lg-; i.27cm-I;mean: 0.15cm-1),p=+tA _sii!'ii"i-sil1(_a}r -=t1tPr'rrif i -, +i= = ' ' É ' r1- i1 rr+ i_ E `' : i I a1 mean: ° . rar ' =. 1e r=,_'f;E 't ' e1,1ii t'i"i'i r' Tr'3 ti r" i_.0.17-0.25 i_f-1'ii n. . cm and phyllic3lly altered rocks are the1 most1 1ntF=nrt=?'1: fractured .r"3i=`a n.1 -! _1!_a cm--1; mean: f .24cm-1).The contoured Tr t=tur r density measurement results are shown n in FigurP 1 q. A slight amount if geologic interpretation was involved in the contouring process, principally in that contour lines lh,i=¡"e modified along the west sidei i the Northi e9 andthe ]ate-stage, poorly fractured West 3i k P i s shown cutting across more highly fractured rocks.ii_k=. ft-EP contoured results in Figure show a -1 sAEell-fracti Eret i:;[f.2 lcrii .l central region elongated east-northeast parallel to the stock and the East and West dikes, with two outward projections the north E end southwest. This central regiCtn corresponds very generally, but not Ñ;::;.3ctl,`, to trie major zones of potassium-silicate e e!ld phyllic alteration. The 'ii"iojt highly fractured rocks rirrur in a discontinuous and irregular halo around ti'it? central stock and bordering the major central mineralized area. This halo generally correspondst_t intensely pnyllically altered rocks except for one measurement site locatedlMithin trie east end of thP stock. Because of =Gdiiilt=ntar y r tx/er, it i= not knott`¡l tAlf'ietht=r" more data would rGveal that this halo is actually continuous and annular in shape, but at least to the south, field observations make it f_!lllil:.Plyt s"iat more da tA. close the gap. Within the highly fractured and discontinuous halo, a central region of less intense i(n.2 Jcm-1)fracturing is apparent. This Jrriall oval region is situated near_ iecenter0, tiffe stock,ict.: :,allie its western Portion overlaps the t= enter -in 101 two-thirds of the poorly mineralized core =town in Figure. The structural attitudesif! 2ft aCLur'>= Aiid v*=i1-i sets Rt Chi-? 2R fracture density measurement siti=s are shown in 3 rrimpo=,itP rose diagram (Fig. 20a) and are then subdivided and represented in three additional rose diagrams according to whether the fractures were measured in pota==iuttt-=ilil=ate altered rocks (Fig. 7 2rb 1, phyliir3l; ái ta rd rnr s (Fig. 20c), _p r[p y li t ir3ll j 3t C Pd rocks(F'i gf 20d). Figures 20e and 20f also show composite ros_ d1 A g r a m= T nr silicified T i n sand faults, respectively, and include only thF? information originally draftF=iJ on the detailed mapping field sheets and not the inferred ir:::ten_iona and gen=r3liz3tiCns f : - shown in the simplified nia Ps in this pa P . C r uj _rai 3t tt li d r of zones and faults can be 1dequ3CPl'_r` shown in rose diágr Aril form bPi=Pusi_ both ther ii:: and lengths of all these features within the mapped area 3rP known, t ithi_.;' structural elements of interest such 3s joints, Tra_tlir Ps-g and tiil w o=P strikPs and dips were recorded at r3nsJCTr! localities L^.l-thin the prospect cannot be shown in rose diagram form because there isno ivia,:to ii==a+_ab ii=n the r"a1._iti :fi= isi'riip ;r'Tal îi=Ñii of each feature, i.e. eacri-! structural element can only fi bc_ ict.nieight:=i]cc the same as any other in the rose di3gr3m because their lengths were not r":Li=>_cr iJh_d. The i.J.33t= bese for these .a d d i C i i 3 n a i features is not la rg i=, enough t t i providea 'i iy sort of sC3ti=t'icr3l v3liditÿ' for the rosP cli3gr 3i91 i'ric_L!'i cd. All the structural data used to construct the rose diagrams (Fig. :-. l;i were first 3ssF`ssi=ir ä= to whether they could bP r F i3CF==d icrir srgir inái tectonics iitr LjtF-r=nt=r Fd stresses. The r3C h=J used to make th3s distinction providesa somewhat restrictive definition of radial and concentric structures ' nrdrrCce separ3te thP'rr{ from the large number of f g::notn?n regional structural F=ir iPl;CR t ;s ins, but hopefully this method provides a better and more objective G3si=, for ro'iripar i=on= cif the data in the rose diagrams. Intrusive-centered structures wPrP defined ii~ thP following manner. First, all fracture orientations 'rrie3 sured at each fracture density measurement site wPE P plotted in their proper locationsin3 i!il thP DLr!zn'= Fig. 20 - Rose diagrams of barren frRctures and veins fracture danity TriPA,..urPrriPnt sites (Fig. 20:=0, in poi-R=.siu-fri-silirti= altPrPri rocks only (Fig. 2o::' i in phyllir-lly AltPrPd rocks only (Fig. 70c), Rnd in propyliticRily RltPrPd roci<=-, only, inclm±ing dikes (Fig. 20d). Figures 20e and 20f show rose diagrams for zones 7-1nd fA.ults, respectively. Solid black shading represents possible regiona9 orientations 5 solid white repres-ents possible radii orienttionsl and stipp;ed pattern represents- possible concentric orienf7itionc.. Arc intPrv:--Rls R re ten degrees. SIRE' text for dis--,D1=-4sion. a b c d e f 103 g e l ng i map: Those T r- a c L u i e s that !; :! _ r" +_ p [+ i= n t i a l l f'r A I] l R 1i !-i n A t u r" P were p r" o j P +_ t t_ a] t,sl LI tte ren trr-s. rT! the prospect. _. t-+rl_+f It f r- f_i', r- t[Irr- rjt- r-i__ I Pt !+i i= nT r_r iNlL ;' the central stock, 38% intersected i t within a ]ti nr_l ,_ir=uz Rr A rPA rol _ sgh?; On meters in diameter. This!is +_ir-!_f 13r area +_ If rre=p_+nd= =tf,-I 1=t F: Rt_Tl;' tn thei Pat + rwer"13F' tll-_t;.A,ePn the poorly mineralized core _r+t wn in i Fi]t!r-F q and the á l:!a i area. of _t le_s intense i:0,20 cm? fracturing in the sTti1=k shown in Figure 19. This circular r e'?i+]n i= therefore inferred to be Lrl+= magmatic renter of Lh+ stock and the last portiontil kn t =++=r ;' =t3 ilize.i hP nC+i Lhlh!P= L 11=1LIF?+f the F`oor i`,' mineralized core !.Fig. 9) ï'fligrtt therefore 'ffi3r#:: the position i IT the 1RTr-_T RgP poorly+r i'` iiii'tl_r-R {iz=d West I]ikP where it issued from+fft the still fft3 11T Pn I_ r"t? i IT T hP 1= +E + iii tg ii itrtssit?P j; =t =ift. ThP IIi"fi3gïii3tiC center" defined abovei.hl3= subsequently used as a F't]inL of reference! and all structures w- oje projections intersected it are termed rA] i a ".Structures within ten degrees of being per"p+_'ndi+_ul3r to such 3 r3di31 trend 3re termed "concentric",IncFntril]'t4 r-e'a3r-d11=s= cif dip 3F-ig1r! or- +.J.tir-Fl_ti In, Obviously, ihi= 'ffil=titf d hRs its ..{P::nF'===r_j -in Lrit -t-LI ir_+ ne3rer- ii';t_+nLr..t_tl're iisL'il tthis t'j!I.ftt.ltitPLi+_ I_+ilt+_`r-!y the jar-Firer-, is. LIP, =L3Liti1=3. i,il::P., irtl_tII, _rta,.it will CIi-? fI_tt]nI to t IP r"3diiil' ']r- "concentric". This fact probably explains theair' ÌP F'Pr-cGntRgP (55%) of inferredJ intrusive-centered structures found in r it Cs i] f-r i- =3te 3ltjrP] rocks (Fig. 20b).!. r TheTrt definitions Iof"radial"3nI "concentric"rLr-.trTLr'f? t1i=,t_.= i,Fl = P[I {!P +tl not. 'ftil.PF¡- that these r trL=$Lr=s 3CL u 3 iJformedinresponse to intrusive-centered stresses, only that they might Ge interpreted that '.A?3,'. Thus, if .3 git%en ten degree arc interval in 3 particular rose diagram is made up almost entirely of r-3di31 Pntj concentric tr"Gnd=g thPn the structures involved are probably intrusive-+_GntPr-Pd 4rigin. Conversely,Inverr =Pl;!, if regional trends dominate 3 given arc interval, then 3i1,` inferred "radial" II+_t]n1=Pntri+^11 trends with this orientation 3rP F'r- =1b3b i ;' 3i1r1 regional in n3tLrPj and their inferred re13tiitn to the "magmatic l_+_ntPr " i1 lit+=r Pl,' l=+_incidÑnt31 and a function i+f lilr3tii +n. The data in Figure 2[i combined with field observations reveal =i complicatedi1=3Led 104 pattern of intrusive-centered structures modified and for thP most partnnijÌ_J Ì_ by five major structural _rient 3tinn'_: 1) ef?st-n_rthtAles t (N 70-_0f_tAr.J; L) i I i f f ! 3 i n ; r t h E ? s t (N - Ì1)3) north-south ' l - } E ) Ei; 4)l +r t 3 t N 1050 E); and ?.i east-nErtheast (N 55i_3 -i0f^_E E). There is al.st_i 3 minor Fa3st_tAlest trend (mainly N 801D-90(3 E z and 3 smaller proportion fTi ofi very minor structures that follow All pnssih ie orientations,ionj, p?rtif_u iar 1;° with iiiiE_r e=.f =1iig intensities of fracturing, as in phyllif_3lly altered rocks- (Fig. 2iÍc). With Lhf? P:::;E_3pLif_f#"i rfT the north-southTh-3outri tr"t_)'id4 the reiii3ining four major fr 3truE=LLir3 Ì orientations at Durazno correspond almost exactly to i thE= four major regional strirtcti_4r3i trends developed throughout the Farallon Negro region and described in an E earlier section. All five structural trends are represented to varying degrees by the fr3=L äY 3, vein, and fault t3t _=r rs of rocks of all alteration types i.Fig. 20i and by the orientation of individual small segments of intrusivE? contacts l.Fig. 8) and F'f iy siif= 31t}?r3tion boundaries i.ri'3. 14). There are, however, some uniglie characteristics of each of the five structural trends that are summarized below. The most important structural trend at Durazno is oriented east-iit fr"the3sL. it is the rrinst i='roïriinent direCtion of fracturing and faulting R.Figs. 203 3nd 2i,T.' 3l9d is rFTl_[t?s in the elongation of the _t rj the late-stage East and W PsL ri ` _I somej! 7ïi3 small i:3lFt 1 LÌ-if=the fiE= of Ï{ifr3 i) fri fi- fa tf. _i.,:..ir If_'i fr3f=t irif s-E'(Fig.4.f i. Ii .) and-a in the distribution of gypsum veins (Fig. 9). This orientation is of regional importance because it is the same as that of an early basaltic to t 3ndes itif= dike swarm that passes through and near Durazno and is radi3l to, but cut by, the Alto de L3 Blenda Trionzonite stock to ti-ie southwest ;L (Ll3rfitEi3s, 1972). 111trLisi;ir geometries at present 1Pvels of expo :sure F tÌ-ie;'Ff fr"P reflect only {yti;,f_f major stri.lCtLir3l orientations, an east-northeast trend and a radial dike pattern. Despite the Possible deep crustal control of the we=t-ri rtf iwf==t-,n_rth- nCrthwest-, and northeast-trending lineamentson the localization of trie C`..J±!r 3znr-i porphyry system, they 3re not well represented by intrusive patterns. In fact, the 105 only significant expression in =ifi the major r w+=st-northwP-t-t"?n iit;ÿ llnPRmi=nt at Durazno is that i_ is the s==f t d most prominent d-r ect 1 _y fracturing ! Fig. L!_i. The position i i,Ì the. n irth-northEAtPst -t'rf?nd ing 1ii1=3mF11tf iR = not báPÌ i a_i_Pi_t ainf?d ,_, {i- 1 t- r.,i -tir on the ground,n:i 4 but.Ì an earl; not _ri-ni=kTs;s.t-- =irrRiln (ca. '' N;_ÌI structuraltrui=tura1 control is Pvident in the ttht_k small d iÉ::elets the southern Pnd the prn1pi=ct and in the linear intrusion breccia within the stock (Fig. 6.'). The observed potassium-silicate a l t e aT io n halo seems to t e irregularly elongate inA , north-n i¡it,.tli?rsTPr k3HirPi=tlnii also (Fig. 10),1). Based on a variety of i_rn= =i=utting r"GlAtinn=hiFj=, thP Par"lia=t structural orientations seem to have been af t orthogonal =_T F it north-south- andpa=t._ t.=.it-?st-trending fractures. These fir R=_tur-P= Ar-P particularly dt=vi_ (npÑd in propylitized rocks (Fig. 20d), especially in the southeastern corner the prospect where they form prominent orthogonal joint sets. Early l'fiagni=_tite + quartz veins in propylitized rocks throughout the prnspPrt and associated with thP magnetite alteration event follow this orthogonal fracture _et, but are preferentially oriented north-south. Banded magnetite + quartz + biotite veins in potassium- silicate altered rocks follow a numberif orientation=, but exhibit a similar tendency to follow north-south-trending i r a=tur es and, to a lesser extent, wP jt - nk ±rthwest-t rending ones; ; ea sL-we=t-tre! fding structures seefii to i bi_ entirely ! absent in these r nk_ks. The early north-south- and east-west-trending orthogonal frai_Lur e set seemstt,ha,,Jf? been followediti<.lei] at about the same time as the onset of iintrull_kri by a northwest- and northeast-trending orthogonal fracture set. These two structural orientations are well developed in pOta=eili'iii-silit_ate and phyllically altered rocks, but not in propy litic ally altered ones (Fig. 20); an e::t_eF'tion is the northeasterly trending linear zone of more intensk? F=r Ir ylit ization in the southwestern c Irner of the prospect (Fig. 10).1). ir`k=ins associated with the potassium-silicate and phyllic alteration egents exhibit a number of or ientatii_ins, but the former show a slight tendency to follow northwest-trending } r a_.ur es, 106 while north-south and northwest orientations Are sciYrielA.ihaL 'rriC+; a prevalent for the latt?r The northwest- 3nd no_ th_ast trPnding nr tlo g! nA Ifr A=rlrP set seems?is irt_i have reached its greatest development during; and after the phyllic and silit-iC alteration events, zi.nd these _ ri nT 3T i_ns Ar j most PvidPnt in thP n rthF s er1y elongation of the phyllie alteration halo (Fig. 14), in t('ti p¡ edominance of nnrthtfliast- and, to a lesser extent, nC+rthea1t-trending F'i-lylli+= alteration boundaries and phylliCa 1ly altered fractures, in some syn- =tn+J post-}`hy"Ìii+= age dikes and faults, and in the nearly bimodal idi=trit;i!ti+_+n of silicified zCilnes (Figs. 15 and 20e). Li!::e the potassium-silicate alteration halo, the northeasterly elongation of the phyllie11iF alteration halo i and, incidentally, anomalous concentrations +_+f gC;ld and molybdenum (Figs. 17 and 18), are clearly independent of the geometry of the stock, but this orientation corresponds exactly to +tC"iG pht itoiine3 (riant that. connects Durazno with the Bajo La Alumbrera prospect to the southwest. The orthogonal n irtl fwP3tand l i +rtn3ajT LrPnlJing 1triSCtisr P3 3r a particularly weil developed along tÌ-ie major axis of elongation of i tha phyllic alteration halo within a zone roughly 400 meters wide. Many faults l.Fig. 8) and silicified zones (Fig. 1 5 ) and all F=eGt+le dikes are concentrated within Tl`tis zone, 3nd in 'rr13n, places there i_ 3 distinct p_r11di=i_: in the spacing of northwest-trending Ta ult j, dike segments, silicified i+-.+ n+s, and F.'l-tJ l lii= alteration boundaries,ie3, =,Ligge=? ing a common structural origin. Furthermore, because this zone passes th rCUgh the central stock, thee i=31 prthrCn31 tr 3_T r apatterns within_f ä11 iisjt -silicate and F'hyi li1=311 y altered rC+cl::s are undoubtedly also relatedlated tCi the same structural phenomenon, and these fractures are not 'iradia1" in potassium-silicate altered rocks as suggested in Figure 20b. The fact that northwest- and northeast-trending structures are very poorly developed in peripheral propylitized rocks (Fig. 20d) also suggests that these structures are nnt truly "regional" in origin. Tri±l_, despite the fact that airrtilar northwest- and northeast-trending fractures are found throughoutiL{gh+=+ut the Farallon Negro region tLlarYtbirts, 1972), their origin at Durazno _,eeri"i= to +[+i_ linked 107 in e n m i= way i,.n! i t h the 1_ 1_ n t r" a i zone i_i f e i o ;-!g 3 t j i s=i f th i== P h y i l 11_ 3 i t Pr 3 ti o n i jrl =i ,a. n d ti-te interpretation that the ii rt"e = .t-+r, PnT Pd fr 3ihrlr e= pi ;l i i=R ils alLPrPd F -+ 1_ e i-i - -' i-1 T T 1 i-1 + f., "regional"_ ' ii i_ °>- r C í-j - r e i iL' _ I }- i - i _+ rocksi1_ .3r i Ci n ì"(Fig.t. F' L 11 iil I/i 'lr.e 1is -rl_L i Z tf 11 r -tt elongation of the ph y i I i i_ alteration h A. i i+ is fi Ì-i;_ r F= Ii t of t ;hS o in tP r" r" = iR tP I_7 factors: i ? ti-te nCirthe3jter" iy 3i! Rq3t1=11-i of 3 i3r"g3r" intrusive body at depth, probably guided by the nnr"tÌ'ie3st-trending 'i n:F.imPnt, Rnd 2) A i9ilt }i-IF+.ásr3r iy trending set of fr 3-ir' e1 alongt theT 7Ri iii_3 . ïí"11=ng 1.1 rilL =I?11_'tfe+:.pL e__dl T 31J_ fa I Iit=;-1 11i1E_lt1_ i1=i7 ' i11ne9zones-, pebble dikes, 3nd phyllic alter3f ln' boundariPs af present i _iPi s of exposure. These fr 3=tur P=. pr i1b.3`ti j' pr"o,l idi-=i..s ;I iP pÎ lni-.1Fif1 channelsÌt_ie !1 fLlr the aF i Fj= =i i f meteoric i,43ter into the convecting `.3i3i i-É-tyi3r: iti-i='r iii3 1=1ystem 3t D`ir 3i¡i3 i1 The northwest- and ni IY'tri33=T-tr"3nd?{lg orthogonal }r"3rtur"e system may be related to normal faulting and graben fC1rmatiiln dLlring upward striping of 3 northeasterly elongated 1"f13g1ti3 body !.i':.Cilde and B3tt31=har"lig 1975:, probably followed by continued structural 3d1!!s-'Ltitent= as this intrusive body _o tied 3nd 1=Cintr"31=t1=d. É rrr_.3lisc of ti-ie periodicity among certain i!i ir'Lil'.A)3=t-tr'3nl;ii!g structures ni1t3d above, it seems--, likeÌy that many of the sharply linear ph;' 1i11= alteration b+_IClnd3'í"iF= at Durazno 3r'3, in fact, f3ilits that (-i3slÑ ;u::.t3p:i=e.d rocks of differing pl-lyiili= alteration degrees from different 1ti1lP i1 in the porphyry system, even lth:ilsgi-i these T3uits cannot now be recognized at the =ir f31_e1 FLUID INCLUSION STUDY INTRODUCTION SampleS Fluid 3nd solid inclusions were e::3ffiinel7 under the petrographic microscope in 1 1 8 thin sections of rrCi[ks And Veine from Durazno. 1 wl_n7,'-Tj-ir"eP F¡' the samples thle{e collected specifically for fluid inclusion study andwere l='t eF'3r"+=d3- doubly p ilshPd thin sections. The sampling process and the microscopic: Fl_ P.:`.?m1n3L1t_IE were considerably hindered by a la of tra '=_paL P-t 1 l a l ases and by the small sizeIf the inclusionsf #= Prif_I]Llnt=_+rPd1 Plag1 If_l3sP`hÑnrsl_r, y sf = in intrusive rocks and sPi_oindar"y '!!'ilnP3"als assni_ iaTFri with t["#F? i1ZagnPtiLi'-! pf =fG;`l1t1[, phyllic, and s 1 ,r i _ alteration ev_ntis and % lt " later hot springR tl- _ ;are milky or translucent I_ti.=.jing to an abundarlf_P of both And fluid inc 1#±=1i In=, These miniscule inclusions, most of them less than three microns 11 T+ a1 _ far too smalll to be studied ätthP highest magnification (400x)1;:;) Av ai jablP ì ljn the fluid inclusion stage, and they usually cannot P1n L P idPntifiPd as tn t; F P at : magnification. l n_-- t hp Hoi f ypolished thin sections, ]n l ; Pig ht that =_nta i n_d at le. st ten or more pi EtPntia-i! ¡'LI_.-_t[tiP fluid inclusions were finally chosen for heating and freezing tests. These thin sections include ttAti ra r F lPC L P_t P_=r P t 3 titiP rf magnetite alteration, four samples representative of p t%a= ;iu'Cf#-=llit=ate alteration and copper-gold mineralization,tn, ild two f 1a iilC' le= fromt'ifi a silicified zone representative of both silicic and phyllie alteration. The eight samples are briefly dPsrribPd in TatflP 11 And thPir locations are shown in Figure 1 1.Fivetf the samples were collected from '.nllthil-! or very T-ieaÏ the central copper-gold ai 1ofii31y! and the maximum vertical range represented by all Pi g! samples Ps i s only 5R.5 meters. The fluid inclusion to d3ta generated from these samples can therefore only bs P:::pPrtPd to yield 3 limited amountIf information on vertical or horizontal variationstf tht-? paleo-hydrothermaldrot¡'!ei-l'iial sr jteiri at Durazno.t. Meth f=.+d Fluid inclusions in the eight samples of vein quartz were studied at the University of Arizona using a gä =-fl tr,) heating and freezing stage designed and , modified after equipment originally developedtF'_+d at the l 1. S. Geological ='ur ;;t_+y.ThP fl stage is capabletf heating measurements to terriF'eratLlr"es above 7n ri-I_ and ` freezing ]eterfin=tl,ns to temperatures below - C:. Th e mPlting points n1 the pure substances chloroform(-63.50 rar brin tst rs_hlor1JP(-22.R°1, dis tillod if water (n.001:3 benzoic acid(122.4° cì, sodium nitrate( 306.8i_)! potassium Table ll - Description of samples used in fluid inclusion study (locRtions in Figure 11). .A.c-Isay dt for s:----ccrip1P--.: BD-1R, RD-2:3 9 BD-R11 BD99, And given in Table 6; electron microprobP rPsttlfs for sArriplPs En-R1 BD-99 RrA, given in Table E:, WallrOCkS TABLE 11 Vein Typg-, E:D-1Sample 6 No. VolcanicbrPcciA.TypeRock PropylificAlteration (P-(2-4))Type SilicicSampleAlteration + Represente phyllie Event() disseminatedVuggy, translucent pyrite quartz from centralwith Description BD-23 VolcAnie (3-5)withPropvlitic moderate overprint (P-(2-4)) phyllic Silicic + silicifiedchrysocolladisseminated\fuggy,storkiAiork translucent zone; fromof pyrite,silicified same central quArtz malachite,silicified zonestocki.A.inrk 1.0th zone arid as but 57 m f ArfhPr from stock along L-1 I-, - VolcAnicbreccia t P ot s. ssiu TA -silicate (K-03-4)) Ftnt.sPot A.-7um-silicatP QuartzandcontaininghasMain lined center stagewith i.A.rith minor line- coarselyquartz quartzdis--,seminated that vein iscrystalline crystals locally2.5 cm Openwidemagnetite; pyrite, RD-c477 Stock Pota'=1.=.1uTri-silicAtP (1<:-(2-3)) Potassium-silicAfP Mainasminor cement stage:fl calcite, cm of widequartz "hydr and +othertraces pyrite mal of + chrysocolla breccia"chalcopyritP vein ,b D - Stork phyllie (S-7) ovPrprint MagnPtifP brecciablebs;quartzFinelybanded(--F malachite) from banded+ cement biotite composite \feinAnd is vein banded 3 brecciafPd cm veinwith wide 20magnetite later cm; ripqrsPly magnetite k.Aidepyrite + CI :: Potassium-,;:ilicAtP Potassium-=.ilicAtP Mainfragmentsquartz stage material; And quartz cement fluid + pyrite inclusion are +idPntical magnPfifP data from BD-1 36 breccia. Ftrr;vifc F-4) (1-(-3) MAgnPtite wideBanded(Holevein 5 #7) mm magnetite At wide 32 m with dPpth + quartzcenter line;vein 3-4 brPcciAfPd; from drill core mm C:) wp; p, u=-.pd nitratame_,potassium dichromate (3g ;, And zinc r:Pt R1 (1-1-q. to calibrate the stage and yie.lded a calibration curve t¡'E3t was fo 700o1 .All temperature data pr_=e n T _d in this paper Ia ve been__rre_te d according -(1.n;F)Ti a7.-: given by the calibration t urvP, to the reletionship: Trcal nditatpJ Prior to any rEea tij Egi ir freezing measurements, each n Ì t4-EC Pigi"Ef poli=n'd sections was carefully examined under the petrographic microscope. and EkJ descr iLied accordingding ta_i the types, relative abundances, And di= Lr ibuti ti i of all mineral phases and Tluid inclusion t; Fer present. Chips containing the most usable fluid inclusions were hen broken from the thin =et_tini s. Ti 1P best fluid in=1us iiJs1= in Pac } E chip were e l e._LPd for study, mapped 3c _i i .i ng to distribution, and thenE i'i,Jiti 1 _¡ sketched and described according to i si; i= shape, origin (primary, secondary, etc.), 3i"id types and relative volume percentages of contained phases. Inclusions less than about five microns long were usually not selected for study because of difficulties in observing phase changes at the 40 ::. magnification employed ! iïi the fluid inclusion ste. The identification of hal- e and _ ,lf it e crystals, primar¡ end , eecond3r '1ine_ iusiins! and some estimates ri ¡ volume percentages are based rin thP criteria of Roedder ( 1q;1 , 19R4 Each hPating and freezing i_ sutjËrnt obtained in this study represents t h P average of Rf 1 Pas T two mPasurements taken during su__==r's_ runs, and n ±fFrt1t = tAt=r? determined to {.1L. Duplicate measurements had to fall within a 5.0-i C interval in order to be acceptable, and the reproducibility was usually better than :3.0i Freezing determinations were always made prior to ian;f heating of the sample chip so e a s to ia!, oid the possibility o.t distortion in or loss of data from ini=lusion= that migrEL stretch, leak:} or decrepitate on ileatin9. Nearly all of the freezing measurements attempted in this study were unsuccessful primarily because of observational difficulties. In most i=.ase=, ii=e could not be seeno form P1.1Pn ci upon freezing to i temperatures as low as -f- !_, nor could any melting be =,Fi==n when ie these frozen(?) inclusions wer _ slowly h heated to temperatures somewhat above _ Ì i 111 ;-. TeniperaturGs i'if first melting could not be determined for inclusions containing NaC1 or both NaC1 and KC1 daughter minerals. All heating measurements for a particular sample chip were obtained j in a series of increasing temperature iii=refni?nts in nt-der !_n s=insurl=r that Tf?11p`r"abIr"P data for phases that d is a -aa ra d at lower t j pe r a tui = =viere always ieasure d before the sample was subjected to higher temperatures. This approachiai_r- t;ta= necessary to avoid the effects i.i 1 stri=Li_hing, and also because some phases will not rßnL!t-_leatein cooling. Heating runs were generally not continued beyond É_I c-12,6 _. T if= abbreviations and terminology that will_ used in this Fa Fr are as follow_ (modified after F'nedder (1 q ;F i e1 '!R4 % iñ Tt (Pt) = temperature (and pressure)if trapping Th (Ph) = temperature (and pressure) of complete homogenization in a sing iF phase; this phase is usually stated as being either llaEi, h( L ), or vapor, Th!.Vi Th L-`,/ = temperature at which phase changes between liquid and vapor occur; if the vapor homogenizes in the liquid phase it is designated by Th; and if the liquid homogenizesizi=s in the vapor phase it is designated Ci''; Th L-V(V) Tin `f" = teti"F'erature of final irieltingidissolving of solid F'h3se X Compositional data for i"luid inclusions that do not contain ialite ±fr" =ylvite daughter minerals are obtained through freezing tests by .measuring Tm ice and 3fiplying the formula of Potter, Clynne, and Brown (1978). The compositioncif inclusions that contain halite, but not s y lvite daughter ifinerals, can Cie dete`f""inined through heating tests by measuring Tm N3i_1 and applying the fcirmula of Potter, T .1. . = rr - --. 1 . r. L . 13riiE_:, 3nd Gr it;,nr i 7asl!'ii1ngC n=f t.c_F'reen1- ?. lnboth, !-salinityt ris e:.-_: r L-e1=trUr_ r -r as weight percentL aCl eaui valent. Finally, the composition of inclusions that cont3in both halite and sylvite daughter minerals is deteririined through heating tPc-,tc. by measuring both Tm KC1 andTm Na-Cl for each inclusion and then using the H_O-NRC1-Krl ternary diagram (Fig. 21) according to the methoddescribed by RnPddPr (1q71, 1984). Both the approximate sAlinity of the fiuidPxprPssPd tm.ight percent NRC1 + KC1 equivalent, and the ApproximRtP weight pPrE:PEnfof both t..1R171 and Kr1 in the fluid ran be determined by this method. In this study,all compositional data derived from Figure 21 are roundind to this nParP,---1. 015 wt 7.and are considered prece within + 015wt The vAlidity of these results depends most importRntly on the assumption thRt the fluids rP RdPguRtPly represented by the system H2O-Nan1-KC1. This assumption mRy not be correct, but it is the best that can be done considering the paucity of relevant data for more complicated, rflulticomponent systems, and also since additional crimprEsitionR1 determinations suchc; 1PRchatP n1ysis (P.q. EastoP, 1978) or scanning electron microscopy(e.g. Metzger Pt al., 1q77; Wilson Pt al., lq8n) were not performed for solid and fluid phases in Durazno inclusions. No E:rushing tests were done on any Durazno inclusions. Visual examination of vapor bubbles suggests that if ariclifionAl fluid components do occur in these bubbles, they Rre present in very smil quantities because a double Trieni.;;E:us wRc- nPvPr seen. In the case of 170_, NR,--,h (1 97E three mole percent CO, is the approximate lower limit for visu.] detection. PETROGRAPHY Four petrographically distinct fluid inclusion types recognized att nitrZTIO havP been namPd Types Al B1 CI and D in Order of appArPnt HPED-PR.r.ing eì e salinity. The major characteristics of each type are summarized in FigureF., 21 through 24 And be discussed below. The gPriPrR1E-h.,-4rAc-fPristirs ct each -fluid inclusion type in the following discussion are derived fr OM the petrographic examination of all samples, but all percentages are derived solely from the 7`.715 inclusions that yielded temperature or compositional data, or both, on the fluid inclusion stage; these percentRge.s are considered representative of PRrh tyPP in 113 Fig. 21 - Portion of thi. ternary :Si diagraTri for the_ vapor-saturated Lystem Cl-NR1-1-kr1 showing isotherms in the Nal:1 fiPld. DgrA.m after ROerjuet 984.1Fig,8-255 pi 245). CompositionAi fields for Type A anci B fluid inclusions are indctd(sPP, fxt), ;Ilong t.Arifh five soltPd dta points for five nPcl Type B inclusions; that yiPlid-pdpuriou ri==',Ult=7. 13)1 I3eN r F, t rfv _ 1 0 , ! ii- FrequencyÍ *1L!nl '¡' i=1 temperaturehistogramsjf! r! +I forf _! all i Type.j C, and ilD 'InCÍI!jilii?= coded Arrf=rdiiiq to the AltPr =?'iCfn rypL of LhP sAm¡JiP= in which they II_r!t .f"ih = !i!3gnP} i1P A (tÑY.Ti[In,, i. = F'_t A=si!i!-_?l11_A;-+ .lfF+T"af i in and copper-cold T 9 t = ~, . . . . mineralization,, /L fCfn,--i-ri =_ phyllie .n1 i il:_,r .=ltr"tfCln in f iiCifi I_`..E1 . ; .:=1nP_,T . i-eAll +TIr. ;P+_Pq Type !-'i And C Î!!=i+_Ii y Ar"= probably secondary li! origin, If and al l the Type D arP pr"ObPbi;' primary in origin. Nu'fflbPr"= in the upper right of PA1=h Tisi rg( ai refer to the number of inclusions t har yielded Th L-V data greäfrr thanan Ì_I ! Ì ,_ T A t (.- . / , i . : Ail I j I-: n r `! _ i ! I n =r 1 fi f i I i Y . i_. J , i_a r C. `": ._. q M 5 ^` TmKCI o TmNaCI 46 C D %S+Q K `1 r ThL-V(L) 0 p T715 n n ThL-V(V) 6 200 300 n 400 Tt°C) 1r 500 600 . 700 Fig. 23s. - FringliPney vs 3 tPmpPrAtiirP histogrAm--;for all Type B inclusions coded Arrording to the AltPrRtirin type rif thesmp1inis in which they occur : M mAgnintitin .AltPrAtion! = Atinn ;zind copper-gold mininrAlizAtioni !=.; = phyllie And AltPrRtion in silicifiind zonins. Most of the inclusions are probably primary in origine The niymbg--rin the upper right of the Th L-V(L) histogram indicates the number of thAt yielded Th L-V(L) cir-- - then 692.6C, 30- 80-- 20 ThL-V(L) 71 70 60- pb. ..1 50,- TmNaCI 40 30- TmKCI 20 IM El K 0 S+Q 10 n a n r-E 100 200 300 400 500 600 700 T(°C) 116 Fig. - Frequency v. fPrripPrafurP histogr;rms for all Type B inclusions shotAiing fhP vArying stylPs of homogPnizAtion (see fPxf for disclissinn). The number the upper -right of the Th-V(L) hiqfogrrn the nurnhiPr of inclusinn=. yielded Th L-V(L) greater than 692.F,- C. All Th L-V(L) datA. greater than are be1iC.R2d e. erroneously iiigh ct.Ann:-.-- to mixed 1-74-lac---e (vapor + riquio; trapping. 30 - 20 ThL-V(L) 71 10 VI t17.7K 20 TmNaCI 10 n ThL-V(L) > 550°C ThL-V(L) > TmNaCI I ThL-V(L)< 550°C ThL-V(L) - TmNaCI ® ThL-V(L) < TmNaCI a No ThL-V(L) - TmNaCI data pair 200 300 400 500 600 700 T(°C) 117 Fig. 24 -Schem.tic illustrtions cf the four tiLnd inclusion types recognized Durazno, including the two different typP= of Type E inclusions snd the two different sizes and shapes of Type C inclusions. Abbreviations: V = vapor ; L = liquid; H = halitP; = yivite H A V D &L 118 gen1er31. Type A Type A fluid inclusions 3r+., highitPifipi=r3tur,=i, il_lpÑr- =RliniP inrlu=ioI-j= each of which at room temperature contain--; _ iArqP cube of ¡'".IA11Ti=y A 1A.r,RP cube of _., lv¡te; 3 small quantity of liquid, and313rqP, usu3lly nr'Ì1=priPi'ii_R1 ';{.por [iutsb1g, that _ r u F ;s 30 t i 50 percent the Ar=a (Fig. LT )a The refractive index of thee i! u j- i i!' G r j 3 1 i n+= fluid i i It iri i i i ll i_ li i i!1ris .iI!II_rtTri ' i ':.::_i t l! -Ì+_ iti_: E_to ithatT' i_i!the host quartz at roomii_im tempel 3tLt eg making the liquid-quartz lnt+_r-? á?_P v1rtLAl1 j' invisible until they arg, r= '3 ttPd. These inclusions 3rP rr 3 3rtPri st ; _3l y Pq.3nt in shape, ranging from ellipsoids Lii stout nPgRL1iig, cÌ t,',¡!1i A119 Riii.J they Arg, usually ig.J1 ti_iaIi Z Cmicrons inn,Whetheris i l a t=d or in'u,=_ t P s, - 11 Type A inclusions i n any sample possess `:%i=u311j! identical phase ratios-:, and no inclusions- in the proi=e1s of necking down 3 ñ i e r e seen. phases other than halite and sylvite areE+Jifilili+nin Type A inclusions. CfiÇ L e ;tli ist 3bLni3 3 nt is hematite ihij-lii_I I occursrr_rin . of{ i til=I inclusions 1tLi_i +iJ, +riltA r roughly one-third (32.1%) : if Type A inclusions rs int3in sffi3ll unidentified opaque minerals. Anhydrite is fairly rare, occurring in nn y 11.:::% thP inclusions studied, and all of these anhydrite-bearing Type A inclusions Is wi_+re found id ii-i-a. pa trii of quartz w. =r_ anhydrite solid inclusions were particularly abundant 1s3iplg, EU-2?,%. In 17.0% I% nf the inclusions,1=i ins, 3 single small, unidentified, birefringent, colorless salt was also found. These minerals appear similar to itiie various unidentified r 1 ir'lPss salts found in Type E inclusions and described bFlÌ ,Al,but they _rP ussR I i f , ÌI to temperatures of F,cJ"F-C. TypeB TyF'g, B inclusions contain ilypers=i iln3 fluids that homogenize i_iiff1p1e}rl`'}' in the liquid phase by s=+jt;'Irr vapor disappearance or halite d;=3F'F'eär ;ni_Ñ moderate t3 to high temperatures. At room temperature, ffiC+_t Type E: lnrlLsi+_+ns ront3in 3+=L1bF ofi h3lltg., 3 cube of sylt/ite, a variable 3triounit of liquid, and a v3F'i_r- C L uLLlP occupying I to volume pP( FÉt of PAc` inclusion (Fig. -2.4) They areEasily distinguished fi v i tType A inclusions J _ _ u1 PT ih 'r smAllPr halite and :; l1 t _ crystals and bPrRusP their Ar P _lPa 11ti_ i LlP r E`inm temperature. Type B inclusions have ä wide vari t, s nar = , including ,irregularly shapede1 ones ln the process of necking downs but over half 3of ff t!'_Ñ studied s En the fluid F; inclusionIIf i . stage1 _. _ occur asrPqIl.ntforms:1 fr as ...rï'='=at, ' i':f_i=rer-t?i=, 1 1 E-k=io°irf _r = 1 ïlf TypeJ vary from less than 5 microns up to !1 0 microns in their i ingPst dimension, but no inclusionsTri firl_+than about 35 microns long were because they had all nP rl.::F_d T jre lLinrsi! n=Lr'C_f contain a variety of d phases i! i addition ti _E halite and sylvite. As with Type A inclusions, iPma?itP is the abundant and n ¡r I S i= _ E _ E occurs_ li! ?r/ 9/,t f the1 Typer Binclusionsstudied. tui e '/, nht riti was}a found in34,2% i-f thTIp B inclusions. 'tp tot twounidentifiedopaque= I!grain= p~perinclusionE_ _ E_E andE upt to three unidentified nonopaque minerals per lnE= st =inn were seen in 27.R% and i =iL=%, r-P=PPrtitlE_?t=, of thP Type B inclusions stud EPdo The unidentified ni EnE ipaqLe minerals constitute less than five volume percent i Et each inclusion and are pr =EbA¡Ely true daughter iiiiliaral=/ ThP rarest Ìlfi these rriiïl=r".=.la, found in only one secondary Pnri two primary Type B in=, is a bright blue high relief mineral that occurred ásmall speck inEnE= inclusion and as small elongate grains in the other two inclusions. This mineral is insolublelut' to t erp e r atu- e r at least RC high as i:_ l_ " C:. The remaining unidentified n En EF-Lqul mineral's are all E_ Elorlas=,Er" oi_c asi>=E,;Allv 'a iii t.Aaiah or greenish, and their relief varias from high to approximatelyely that t Ef hP.lite These minera += are round to EE__t[i iE_ to Et=Ebt! iPr or rod-like in shape, and their birefringence ranges fr EJlrf high to 1 ver y loRA? None of them appeared tf 1[E6_ carbonates , or phylls laiiicatGe on the basis of relief, birefringence, or habit.Some =E? the colorless minerals are nucleated on small specks of hematite, and at least 1nP F=92í_=; _ mineral remained undissolved tf s temperatures C. One T``F=E_1' B î,E=l.iri Eli (sample BD-81) r- Entained three colorless soluble salts, in addition to halite and } r t % +E_ / thatdissolvedat e1peratfrP1 uT ¡ = r=}1vite, esrÑ;tfj The unidentified colorless minerals may be complex (hydrated?) ) r hlor'1d+=s, and , 1 possibly Ey a i1r+ sulfates,4 +'1j +! i.!Na,CAI FP, ;tt'9I n+3less11'.1.P E j'! E u,Al,?'1 Mn,1''j or ín +-' f + t - r n31 '.sti,! +- E' r r rt 1 F- ß r ' 1¡itii Gth_Er rtr' +JaTr+aiLa' f?hr += t pr-=.,PE-Ero_ t_ t=1ts?r i t i-t+ _ tiint 1 ..i f.. l 1 , 9 . ,1 3n salts has ++_n confirmed or inf =r r +=d (e.g. !"1t_r 3T 3 E. 1 ' /t , Fa_ ttEj-1q78; Wilson Pt al., 1980, KwLI. and Tan, l9Et1 ,R_EE_ddPr, 1984)./ Nearly one-fifth (1 i e. F.e%) i Ef the + Ty pP E; Ei_ EE.E_1E +ái1 1t ud1P+J lacked sylvite. These inclusions were always than 25 mic r=nr long =Td usually were some =¡ the smallest inclusions in any given Type B inclusion popui=it1oiM. At "Í"?r"1?1 these smaller" inclusions wer_ considered to be A di_t inr tly different type of inclusion. However, , bet a 4se they are virtually idPntl+=a1 the majority of Type B inclusions in terms of appearance, distribution among Type B inclusions, and proportion of 3dditir+n3l solid phases (hematite, anhydrite, etc.), they are also considered to be Type B inclusions. This interpretation is supported by the fact that thP homogenization behavior and all taïiip+=r'Stur"e data obtained for both types of inclusions are nearly indistinguishable. The similarities-. in Tm N3E_l temperatures-, in particular, indicate that inclusions lacking sy ÌtOlt3 ar P not much less aaiiT'!3 than , tii++?= +=_ntaln+n'=' ,' i!Ilt. rL 1_It isr+_n+=1L1 s!t E thati.ii3 theIi Type: p' inclusions1{= ÌEC tEiLthat Ealack k sylvite are met3st31=+1y supersaturated in i Type TypP [: inclusions contain 1+_+tt? salinity fluids that homogenize in the liquid phase by vapor ?i=aE='p'33r"?n+=e at moderate temperatures. These inclusions E_ont31n a large amount =+f liq!Eld! a vapor bubble that occupies roughly 1 to 15 1fnlu'fii+? percent of each in+=lE_E=iGE'! (Fig. 24.E, and rare hematite or 3nhydr'itP. TypaI_ inclusions typically occur in two distinctive shapesapas and sizes: o, Fis tPdiut-s itPd 15 to 25 micron, la_+13tad, round to elliptical inclusions l,,Jith jagged aEjgPa; and 2.} .a+a 121 le= than FI micron n =nIE=CEth-=1dE?d but lr-r"egLlAr" i ;'=i'IapE_Ej, An1 ¡:I ¡ar, elongated, j Er- r"oi1nded forms, either isolated tr" in trii eP di'ilÑr!1o!!Al r EmE_ of the tr E_ i_ =_ E Ì E 1 e r-{ . k iE clusters of Type C: inclusions ht' l nLl1 rhe r.ijÌ . { E EF ntht _ iers do not,1 and :-! although any of these clusters i3" b= the L ?1 f Eit nP=1ing down, no inclusions in the process of necking down were ob1Pf iiPd. Type Type D inclusionslils =onta i n low salinity fluids =id homogenize completely iÉ the vapor phase by liquid disappearance at moderate to rig n ti_p At_ P=. These inclusionsEi a E=si eraeirtli.all,r contain{LaIÌ ierr y1Lals i tf hematite =ir anhydrite but never rilitr`g _y',,,ite7t r or unidentified Ei=[Et_ iirLnJESE_IrlIli_rzr.r 5.1 'if9?EiEr.r E_.,.irT-t:."r r ' =ii 9:1irES_ _i, rlli_nr. _F P vapor-rich, varying from 50 to greater than 39I ! volumelme percent vapor rr" at room temperature (Fig. 241. Most Type D inclusions appear to EE_o¡-itain only a vapor phase because any liquid that may be pr e=enL is obscured by the high internai r PTIE=jE_i ii;it j, of the vapor-liquid menisci's. Occasionallyil;' 1=E{tiE= liquid Eran be seen where it hA been Ct En=entr"ated in a Cuspate pr-' ijE=ErLi=En rif the inclusion walls or at the apex of .a negative crystal. Typei inclusions _3 c S r in a; 17e variety { f shapes And sizes, including some = = process ! T necking diFl, bui they are rarelyiArgPr ft-IAn a 1 u. i 35 iirr o1C in their Jnger t dimension. When Primary, these inclusions Ì Pn- toÌ be irregularly angular or rounded in shape, wr{ile innumerable ¡e secondar -,, Type D inclusions oS_i_ur a = negative crystals. ' 1r---t^;- ` `IrE EiEJ-i Inclusions 1n=_ E1 Er E'.i {r r Er r" F``E=EJE i EE Other than halite and 1'f' itllte, hematite and anhydrite are ti ¡e m E`=t ='i i'ii1'fiiE=En solid phases seen in the inclusions studied.her e minerals usually - tnrtitltF less than one percent up to about five percent Ii-Fie t e_etal cavity t:iiElLl'ilii_, but some inclusionstn= contain larger" anr{ydrite crystals that TA e { iFl nearly twenty `:/E=eli''iile percent. Hierft{atlte and anhydrite ai!AJay1 remain u.sib111ol`:%E_d !:'The=n heated toi 1 6q2.F e%` [PPL for thr e e Type A inclusions in sample ED-== in which anhy7rit e And Ìi ÌI halite E1s=i El;;efJ eiTi"Il_'+ltanijEi!a i J between 613.617' C: and =l 5._fC:. Hr'iil=!t ite= and i77 I íií.. anhydrite crystals identical to those foundT n influidlïì_?_k i = ii nJ= lJ=11 occurRr" 3J= randomly dispersed solidËld Zn1=l;sji} lns -in i fhe quartz! f most sA mF1 1Ps, and locally they, _ {l? L j = l 1 F r 7 1i_+ -1 ! R = ar ~ l J 1 found alinedalonggrowth ii 1_ E J 0 '_. 1i ??f' ' occurs J= singleJ 1 t! 1 1? i pl'_tte, with or without a hexagonal bAsal ilEltli?-F-'e it 't/AF'ii== from orange. to red to iJl3i=k. depending on thickness..Extremely small, bright red a_ hPd r a- grains3 i n J t n at 1'r_-' rli_i^ir- in many fluid inclusionsJ 1 are1 J .=_1?1! i!_l Ìn_ilillr'111 to 1 i;P=!At 1iP9 regardless CiT whether" i=1r. not a?ar"ger plate of F hematite ._i Also iii_i_E lf = in r he seme inclusioni These small specks of hematite are often found in the centers ! 1f Anh}fdr irP, LIA?itG, and Pj lvite crystals and have apparently pr'[ltilidi_d nucleation E sites for these minerals; many halite and .'r_'-,' RvitF° crystals continued to ¡E1c?PaTi= on ¡MP-4P hematite grains after several heating and cooling run1 during :i_h the halite ir r 1l iTF was dissolved and recrystallized. Anhydrite is 1i il1Pwi"EPt1....P5 common th An hematite in 1 _1 Ñ rR'E inclusions.Tti_! i1y nd it toccurs inPi_'pPnlrntl of whether or not any hematite occurs in the same 1nclLRsion. Anhydrite wAs identified nn the basis rii' its moderately ï-Eigh relief, high birefringence, parallel extinction,in, 3nd tabular or 1 :` 1 Ì' f A 1 ` 11 i 1 é 1 t 3 = ! r'-li.i_ . lillc.' It i1is usually 1_ Îi i!rF-FJJ == tot 1pale R : but ii_i_aJi'_n??;' -ia, dir"t, orange tint, '!_111ib?y from incorporatedatPd hP'TPtit =!.'.R. Both hematite and anhydrite are randomly distr"ibuted among the fluid inclusions studied and =hi iw variable phaseL. se r'ati=..i1 among adjacent inclusions ithe same or lilifPrrPnT1 t,'F'PP.i hP=i_ two minerals are therefore tielieved to 1 be trapped solid inclusions, and not true datRghter" rfliner"ale R.E3stres 1973i. This interpretation i1 supported by thP fa==t that the number of fluid inclusions that contain anhydrite i e roughly proportional to t Ñ number of f 3olid anhydrite inclusions that occur in LI r nearby q1..RCi4 ti Most =f 1?id anhydrite inclusions a id fluid inclusions fa Ìwithin T_ _ same -_ i i e range, i. P. le s= than 5 microns to about :35 microns in length, bu L abundant solid anhydrite inclusions up to ' rmicronslong occur isample 1 _ Lr.It i r curious t h3 tA h i l = many of the smaller sciiid anhydrite inclusions have not been incorporated entir"ely (.Ai itl-lin fluid ini_lLsicinl, most hematite solid inÌ Le1i ne are n11A) found entirely withinn ii_ ir-' l fluid inclusions, j, rFr haps reflecting differences in surface properties. About nro.-fourth of the fl!!iE] inclusions _E]iC'E +]EiTA1EÌ tiny unidentified. opaque gr in=, sometimes with As many As two Ei Er three pPr inclusion. Ti-j=P highly irregular, nonmagnetic grains are i n+]i stingilisnab le from one another. They are beli e/P d to bP trA =ped solid incluions _Prau_E. they occur in a totally random fashion among :idj=+_Pnt inclusions, they Ar P insoluble to }PmNPr At{r rs as high :a_ {E ( F,t. i_, and they also occur as =,oliE] inclusionsin the nearby quartz. VariaC+I+? Amounts s ! E i accessory acicular f ut i ie also occur as solid [n-lu11 n= in the quartz ofitiostsamples. Ae; n { them project through -he walls of fluidi inclusions and probably caused the trapping of these lnE=lu=iC+ns. Entire rutile , 1 !eE^'+J 1es i]! ! not Et [Ei E_ur rÍ_EmplPtF+ _' within fluid inclusionsli1i! En= l- f t tA? G=i e r, Pin lid halite inclusions were not seen in any of the Durazno samples, nor is there any evidence, such a= cubic casts, r+ their former pr P= jÌ ]P. Furthermore, none of the fluid inclusions have s apes suggestive Ztrapping o n__ iLAit = inclusions,snj, and there are no EL!n!!a!!sE lly large halite crystals in Any of the fluid _= inclusionss tI E EiGttafi i not iasJ IIÑ nn =tin tr F`L.=01 except for two necked down Type B inclusions; T m ta:1 data for these twn inclusions =` j not inrluded in the arrompanying tabla= and figures. SÇ ' atia't RP lati1 Enj-iF' Ar1Ei Ì,.., , Generai visual examination En and idantiTiratiE=tTi of inclusions in the more than one hundred thin sections of all ro d< and alteration types within the i E# ItPr- boundary t: major phyllic al eration halowas hampered - y murky host ii nPr P l phasesÍ 1.f and t_by the smalllÏi E I sizeJ L 1 Ef inclusions.1 1 Thus1 it could not be P_ta+li=-e( with rert3int',' whether the abundances, relative proportions,ns, .and phase ratios of eaCi" of L he four fluid inclusion types changes away t rCf t P __n Pr of TP _;stei.Where inrlusi+ins could be identified, a slight decrease in overall abundance of fluid inclusions and hematite and anhydrite solid inclusions EjrEP js P e ll ! to occur ti u t lA. i s r E] from the stork. There is not, however, any discernible Ì E tiAl.lr d decrease I11L i-ie 124 proportion f # high salinity T fpeE:inclusions or vapor-rich T¡pe D ic ii! ;ii n ;, _r an increasee_a{3e in theproportion in of low salinity Type C. ini_lu1 i! °Ti_s 'Vapor-rich Type n inclus inns i1 Rr e by far- the most LE_!-iÏft Ïfton and iiJ abui id_int inclusion typeencountered in R11 saÏ Fles s particularly l n veins _ r alongy r ! Lr r in both È!c k_ and ! e i= , = n d 1rPi typically make up 50¡to over 90% of the ?drnÏiÍiab, e inclusions i i any sample. In mA " vÊ' =a the 'R test and t=t through-going fr aeiure= containPlanesof only secondary Type D inclusions; a similarj`tF origin for vapor-rich inclusioni was noted by Ri ÉFidder (1 q71 ) et the Bingham porphyry copper deposit, Utah.Type `F E inclusions, typically secondary and hi=Ïfieti=e- enh',rdrite-beering,andlPsser solid anhydrite and heÏftatite inclusions eF e el= _i e::tr i='Ïftely common inRll sample=, tthough generally somewhat less _ibundent than the vRp!ir-i lrh ini_141sinnsaSecondary TypeBand D inclusions,in=, ali_ing with irar-P. Ty pi= l_ inclusions andAtli tnd?nL enh`¡drite inclusions, were found in clear secondary quartz blebs in pot ==ilÏfi =i iii_eLi-?° ltereE` 1fE lcnir EI' e°_i-ir-,r and .ni to e i__i'=3`` ! s- extent in '.¡ oji__nt3. plagioclase grains. Thisii1 1a ifie fluid and solid inclusion population in wa1l=.i_i notedin banded magnetite + quartz veins and in adjacent quartzz and F lA ,oi iA_re g Ain==it the vein Ra llr É _j_ in "moderate." magnetite zones i n F r r p f lit ,? eor _c:: a Primaryani secondary Type D inclusions end secondary rnriyrIr'it =- end heme= itP-bç.se',ring T ypir_ E inclusions were noted in a "biotite breccia" vein (BD-160). In the eightsamples; studied more intensively, Type B ándD inclusions, along with lesser anhydrite and hematite solid inclusions, were the p( edrfinanL inclusion types found; _:_P p í±{1 are semples BD-23 (a silicified i =ne!and ED-99 ( .r banded magnetite t quartz f =;n} where ° Type A and D Tluid inclusions F'¡i=di=iïfiinati=d. Only +_ifle Type A inclusioni=i[¡ ;;1A 1 found in each of samples BD-16 {.e silicified zone) and BD-95 (a main stage vein), and they were not seen in any other samples. Type C inclusions are extremely rare , in all thin sections. Although they are moretn,tidel.' distributed than Type A inclusions, they were not found in two f ; i ¢ the eight samples (ED-8l and BD-17.:0)l) subjected to more intera=i? study; bnth ! i1' these samples were collectedTr ± ilif thP 125 atli_i.. Temporal RelationshipsiaLion=hip=1 Aifi ing Inclusions All {! 4.1"-.. fluid Is1i=4ti;=iinclusion i- t .=..1ra_ at3 L È"1 r ;..ii-r rcan _ii_i=1"r inC! i= r-i-`A r ti 3asni=i3tion witri cine 3nEither and it is often diffirult or impossible to establish the relative timing of tr3ppin=3 of each typa. Moreover, on the b3ji= of criteria for distinguishing between pritdry and secondary inclusions tF j adda r ; 1967, 19 7 1 ; 1979, 1q84), there appear f i Ì be both F' r" i i"i I a i" a` and = 1= i_ _i ri d a r"y fluid inclusions i_! 7 each t J: p e in each sample and oftenten !n,lithin the =3Ìm microscopic fiPld ;l ¡Pw. Difficulties in . . assigningr- r_ -1 - -1pr i.3r"- -ti' 3't or secondaryr fiYi' -1iiiT: _i - iili_isi=iÏ-i1inclusionsC 3r"i=' common li imosti1f!1 ' T : t fl.ridt 1 inl_llsin j t!diPC. 1h P1P 3 e compounded b y the repetitive episodesof frai_turing and annealing common in pclrphj'rtrJ i_Ci¡-'pPr environments by the fact that secondary inclusions can and frequently do migrate away from fractures with time; in t1t! ! cases,1- secondaryinclusions ilia jr be misinterpreted t! to be primary (Çi iPidd+?r ; 1q71, i 9L'4) At Durazno, the petrographic evidence is insufficient to establish whether different fluid inclusion typesi1" ?F'paire.1-it primary origin in the same sample occur in different generations of quartzIr" along different growth zones;.l i_ a rly ; this must be the c s e if the inclusions are indeed primary,! +r ti-ie interpretation of a primary origin for so'iiie or all of them is in error, because the three liquid-rich types could not have coexisted. Crosscutting relationshipsA IoÌg different generations _t secondary inclusions of different types 3( FU= .11A-11Y Also in=LfficiGnL to establish the r"alativ+? timing of each type, although some the ia Ñ1t}r _,turP= _n_f Rii F'_ i er .- of both Type L and V inclusions, a_d, as noted above, tilÑ very latest Ì r ai_Lur e:F., contain n only vapor-rich1 ypP D in°_iu1 1_}nc--. The overwhelming abundance of Type B and D inclusions in all samples, many nfwhich occur in random thr"aF dimensional arrays Thter'apirr 1ed .=.ititi?ilg ct r"',; i#=' ianar ; r sets=raL C isecondaryi?rinalr T.raType r,andi D inclusions,r- r suggeststt-iat1 iÌiai!_- of triatt!' areprimary 1 ' 1 7 ' T , 9 in origin. ,p1=_ and less i-iPfilatita- ani Anhydrite-bearing Type E inclusions of iJafinita pri'fflar"y origin were found LogFLhiCi"along _! single qri=3tA.¡Li izone in t a sample of a banded magnetite + quartz + biotitP vein (samplP BD-151); these incluc.ions were too small to sItjrzcf to hPAfing And frPP7ing tP,,,fs. Although th i..s-AmplP contained innurner.-able growth zones outlinPd by alfPrnafing inclusion-rich and inclusion-free zones, all of these inclusions were too small to identify ar---. to type, or even as to whether they efoere fluid or solid (magnetite? biofifP?) inclusionr. Thus Any fPrriporAl variafinns in the rharActPri=:fics of the hvdrofhPrmal fluids that produced these veins are u n TEMPERATURE MEi-V;UREMPTS AND COMPF):=;ITTON DPTERMINATION!----; Type A Data for both Tm KCI and Trri.1RC1 tAhz, r P obtained for 33 Type A inclusions 177 from samples BC-i 6, -23} and -99 (Fig. 21). Est i73ted salinitiPs of the fluidsis= inclusion=-, are remarkably high, ranging from 77san i-.r_thi;"ghL pPr _Pnt NaC1 +1,.:121 Trui '3i=nt.tt Thesersalinities3r_ higher t! than the highestsalinities 7`ÑL7 r F'cf irdPJ from fluid inclusions an rw¡-je.r Pc according g to i a r PE_P¡jL i_rimpi {R il! in 1 E j ji ici inclusion data (F'r F_dder, 19044 Table 44?). The field of iyr'F' A I ¡ ' _ i i r. T _ ' -ti t 1 - t.' .i. i -I r 7.i. i1 .)formsa linear trend r:1LI - _ 3_ i L ii=T j 'E=,= B sE i_ni! E!but L in iii' i. i iiis .a r _a l l y composed o } t lr i _ adjacent an d Ç a 7 - A lZ :r L3 r '- p[i l g } 1l d - CC ri P s - oni-ling to slightly different i_nliiprElitii_Ens of T j'rP i Ìilids in BD-2::: (lE fif)Pr i nigLi'r'Y KC 0 as i_i_'tiip3r i=d iFiitFi those in BD-99 3.i-iigFi=r K i;.). Tii3 lowest 1° r _ iiii i n t th _ Type A field corresponds to the single T jp e A inclusion "u ni " in BD-16, With decreasing 1? i n i. jalong T ' _ Type A trend, y a contents d = rr e ase fromi'iii 67. 0 to 46.5 weight percent, i1 contents ! PR E,P from7 i. 0 to 2R.0 AP ight percent, and Ki' i''l3 atomic ratios increase = from 0.27 to O.-7. intr!"PsLlnglj'S K.' f''i3 1n C'n-7,q (r1.L7-[I.ti +) 11 nearly identical to iti'i3T i f T j+pE_ B inclusions in the same =-.1 {'si "pO .a..'-i iI ',. a 33tur 3 not 'iiLiit3df by these ttat%r! ty'=`_,it inclusions ln C,D-23. Modal t3'tiipPr 3Lur P1 for Tm K_1 3ild Tm t',3i_1 differ between the two samples in which Type A inclusions are aL nda nt ,varying from Ti K_ 1ttr d_ 1=365° -!"r (_ ii ri ri fi r. t r. - t e - - . i and T ï t!'i ;_i ?. 't i"S E i i? .} = t,O + j : i ! iBD-2:3: ri T 't t6 { ._ !. 'tf i s E ! ! j = _ `, [ +! ni i_ ä n d E_i r l Tm ''i3r:l.'tii ±de) = L,45 + i in BD-99 (Fig. 22). Type A inclusionsin= hrE'ttir i'ii=niz3 ä t r ( temperatures overiO ail in_lu= i_n= that appeared LU be on the verge n i_E E_1 n E'ittE i'e-iiiin'-_,i at h,]i,Ft i were heated to Er-"j tai f,jÑsr'P3= r-_Efil+ ¡'É i':_i !_ .=E ii yet i]ii i not homogenize. True trapping temperatures were therefore probably in the range ?OOri -;niOrini because p3rt131 melting of the andesitic tiallr'i=lr;::3 should begin at tFiF_ - latter tP = L iiP . h_ density of the Type A fluids 3t the time of trapping cannot Arrur-3t3i'y' dPtiAf miinPd owing to a. lack of experimental data and uncertainties regarding temperatures and pressures of trapping. However, by analogy with the system Hr_-N3_l (lrus /3, 1975; H=.3=! 1976; R_ 3dd=r and E_ dn3r, 1980), and assuming trapping temperatures near 70n- 4 T P r T these 3Lthe ;rff, ! LC, time of trapping are estimated at 1 1-1 ._ gm.'E_=. V3 r 1R.t1ons in theTF=1fF'erat_4re -'nt,; compositionfi,iF=i 11fiiojÌ dat._ 1: bt_!iii=d for Ti'-=P A in_ lnrirns FrrbaLiy representative of those in the Tr F_ A;_i ot P%iat E.11E at the t it e _trapping. T he regular s ra p = = 7 constant Fha r j r a T i _ C4 and lack of evidence r i necking down suggest L hR L theseRr iRL i oFs 4 along withi 3h "t s and homogenization temperatures, 3 e not the result nPE_ki!g down or lP,..¡::ing! they also lnd i _R t F L R_.t Type A inclusions ká1=r = not generated through leaking or necking down of T,'F=a, B inclusions. ThP =tfï;_tatnt F'rlasa r"atirijrEtthis T.F,E_: 1 A inclusions and larf:,nfE?idPllra for thP sÍmLiltai_!E_fE1= trApping {ET these ?1Ed vapor-rich inclusions also indicate that mixed (vapor + liquid) phase trapping did not occur and---7rS i=not r es='i ins iblP for Er_!iP high i-ofi Egpn iiA Ì ii inLEi T=Er uri I=_. Moreover, L h P r e r e a t 1 i _ r r-r suggest 1 L a T Type? inclusions :e r e not L f a F-_ d from E LEi iiling f. El' condensing) fluids. Finally, the constant 1-'! a1P ratios,Es7 Rb jP1lcP of anomalously high Tm NaC1 temperatures, and la{_Ì:: of peLr i Egr RF°hiE_ evidence !i_Er- halite- precipitation n indif_3te= that their high salinities are not due to the random trapping of solid haliteiLe inE_ ! 11f fns. Type E__ Compositions - One r n dr e d and ninety-six TypeE:inclusion. yieldedi R f_ = for both T m _ an ri Tm N a_ . d st a (Fig. L i. form 3 line ar trend corresponding t! salinity range of 50.0 ir E= 79.5 weight percent Nal_ 1 f Kä_1 equivalent. With decreasing salinity á rng this trend, tai contents ] __rPR ._a from!m ti1_lto 29.0 weight F=Pf rent! S.rl contents increase from 1 = 0 to S 245 1r4Pi'r¡L percent,_ and K/Na 3trfliliE= ratios increase from 0.16 LrE 0.82. Densities of the Type B fluids at the time tYaFFing are estimated at 1.1-1.4 g!%=_ by analogy. with the _ysT Pf H.!1 -3f.1 (l. r1s{i{a l 1q75; HA R ,-1si 1976; Roedder and i1 iEl rj 1980). Temperature t a s- E r P n r s - Two hundred R n d t _ y -P igrt tF 266 T , p^ E: inclusions that contained sylvitP yielded Tm ::Cl. These tP'fiii!r`r =iLHrPs vary bPtwPPn 1[l[i° 1= and 1 a,ï a few l wf=r" and a highPr (sF'urirfus ') valtirs 1ÿ o also recorded ; Tm KC1(Triode.) = 135 + 7.) TernperAturP,. ohtAinPd for Tm N.Arl vary widely - c-` -3- -among Aii Type B inclurinnsi t.enth most values lying between3101--) and 5500 C; no inclusions irking sylvitP yiPldPd Tm NaC1 temperatures above 504- C. The Type E; Tm NaC1 data. yield two majnr modest 455 0 0 ÷5C And 495+ 5 and two minor mode t 375 4- 5L.. and 345 +_ _ 29A). Most (74.7%) of thP Type E; inciHsions for which Th L-V(L) could be obtained jii=sidPd Th Frr, with f PrripPr.af HrPsspanning a tAdde range o 0 0 . . C:) with Th L-V(L)(modP) = 7=95+ 5L. (Fig. Nearly one-quarter of the Type B Th L-V(L) temperatures are greater than592.50C. Homogenization Behavior - Type B fluid inclusions homogenize in the liquid phase in one of three ways: 1) by vapor disa.ppearance (Th(L) = Tb L-V(L) Tm NaC1); 2) by halite disappearance (Th(L) = Tm t....1ACI ) Tb L-V(L)); or :3) by the nPArly simultAnPous (+ 2,n° C) disAppPrAni-P of vapor ano halite (Th(L) = Th L-V(L) = Tm Nan1). Ail three stylPs of homogenization must be considered common A c- each s exhibited by some Type B inclusinns in PvPryampie, regardless of whether or not these inclusions contain sylvitP. Roughly one-quarter (24.2%) of the ?FA inclusinns thAt yiPldPd dAtA. for both Tm NAC1n d Th L-L' homngPnizPd by , vapor disappearance attempPrAtHres greatergreater than 692.6w TIT; NaC:1(mod.:---) =1' ± r". for _ _ y more than one-quarter (28.0,) of the 264 Type B inclusions homogenized by vapor disappearance at temperatures less than 692.50 0 and yielded1' (-1 ..mui - V(1.21.-r-. Hoot!.1-) - L., wirri-poorly1,y definPd mode for Tm Na.C1 445o + 5ED C:. one-halt (4F..4%.) of the Type E; inclusions homogenized by halte disappearance And yiPldPd two TITI NAG'. mod.. at 0 o 455 + n And 49nC: with A Th L-V(L) MOde at :3973- + L. An intPrPs-ting tendency was observed n that slightly over half (51 .E:%) of the inclusion, th:-.t contained sylvite homogenized by halite disappPArAncP, while slightly over half nf the inclusinns that did not contAin ,ylvite homogenized by vapor disappearance at temperatures 1Pss than hive Type D inclusions 130 homnsPnizPd by the nearly SiffiLlita.ne.OUS diSaPPEREtranCE Of vapornd h f; one of these fhAf contained =7-0-itP did sc bt;er ,P the remaining four that did not contRin syivite homogenized between , 0 2iF,4==1 Reasons for Va.riations in Temperature and Composition Dat.-3. - Much of the vAriation in the fPmpPrAfurP composifion,=.1 dAfa obfA'inPd for Type E 1c'n is believed to reflect vriations in the Type E: fluids at the time of trapping, It is 31,--.0 likely that the inclusions sfildiPH were frAppPd from sPveral different generations of Type E fluids. Syn- pct-trapping perturbations of Type E inclusions haVe also contributed to some of the =.-.r.LiffPr in the Stretching Anri leaking from natural causes in the field might be difficult to detect, rriAy be rPs-pon--zibiP for sorriP of the rAnrinmly highPr Tb L-V(L) fPmPPraflirP=7. HowPvPr vapor-rich inclusions, with halite or sylviiP d:=.1ighfPr .minPrPls were seen that might indicate that significant 1PAing of sAlinP, inclusionh.d occurred in nature. If is believed fh7-ff the effects of stretching in theboratory were minimized by fhe increTriental method of hP.=ifinq dPscribPd LPAking CL heating runs Was readily observable, especially on approaching Tm NEC] for inclusinns that homogenized by hAlifPdisappeara.ncel and these inclusions were not studied further1 Necking down of Type B inclus-inns hs HndoiibfPdly occurred in .Ii S1ñPiC, 3many inclusions in the process-, of necking down were ob..PrvPd5 clusters of inriHsinns with vAriAblP phase ratios in which necking down h.a.s been romplPtP arc not uncommon: Inc1usionth.t h-Arl obvinitsly necked down t.A.:ere mioidPri forstudy, butsome fPrripPri=iffirPnd rompnsifionAl v:---lriAfinns inthese might be due nPrking down fhRf. wentundetected. It is believed that the five inclusions fhAf liP nufsidPthe Type E: trendin FigHrP 21 owe their spurious compositions to having necked down, As with Type A inc.:IN-Lions, there is T. 0 pPfrographic evidence for the frRpping of solid 1-111t-P inclusions, no are thereny anomalously high Tm Nani f PmpPrafurPf. L:3 P>::CePt in ttA)0 ne=ckE.--d down 1ni_lu3lCisi=4 this process could tÊi3r efi=3ri_ not have contributed to salinity ei3r-13t1iifls r or to homogenization by ha i1tP dljaF'PÑaf R;ii_p suggested -n rt _ t t Ld1_r rí Ai- i iR d G ± : r,`r i % I!' tr -,. ' t , r- ! I.t i' 1 and 1 '. iii , i+, i , f. _I i 9 'f . 1.. i''. (vapor+ ' i! 1 i .iF' i r i trapping fi _ apparently affectedfei_te1; iiiai iy Type B inclusions, 3nri probably for of thP randomly r11gCier 05=0 C:) L-V(L) ti _iiiF`PrRtiJ.r *=s I_ibLRlrird for tifi='yii (Fig. 23.i. process 1_ evident in the tai 1ablP vapor to liquid (VIL) ratios in Type B inclusions r if the same generation, and by the fact that adjacent and apparently r r e_ I a l Type D inclusions show variable ratios. If these variable ratios were r 33Uit r if riPri:1ng down, then 3 riJr"r"PsF'r End ing variability in the phase ratiost1i irofT_;.`! i i i t i_r 'T"ii111+r "r in i¡t'C i_B i ,_i"r,_ 11r3 r i i! i_ ,`f Ì.ri L p::,7i_í_i, , lt, but l+?A_ T __i, not observed. Primary: Fe B inclusions in 1 _ - { "r samples 4rU-81 and BD-99) r = Ti i ria';fP preferentially tr aF+F+ed t,faF+s=irfi mixtures, and id nPar l y all yield very highi vapor disappearance temperatures(>550°_ 1.A few F1iii_r y 1sP Binclusionsin Pa_r AffiF1P_, h Pi Pt yield r{r 1da raC , ; lower vapor disappearance temperatures. Because Cr th types i } inclusions 1 n each sample y' e isimilar Tma _Z data, i t is - believed that only the lower temperature Th ( } data provide reliable estimates t r uP -V(L) temperatures.:1 nrP some_ these lower te fFr atur a L-V(L) data are actually lower than the rr !r r:=jFlo.rid1Ì"lg ! fif ' : dat_, it i_ li=af homogenization by halite disappearance 1 s more prevalent than T hP data reflect. Thus 1n these and i th_r samples--,, the real v Pl a = ii - - - F=7_between Th L - Y r L!and _ w Tm il NL_: 1 may C a masked b f erroneously high v a o r disappearancee !e r au f P _ a E!:f1dPnrP for Boiling - Evidence for mixed phase trapping and for thP simultaneous trapping Ci?' Tyr.--se E: liquids:, and Type Lj vapors indicates that Rt ìPa '- some of theT, Fe E: inclusion. weretrapped from boiling Cr condensing Although the necessary e : :: F' i= r 1 m i=' i 1 t a l data ,=_.i i' e lacking, t {-E eabundance _i i apparently ? é i coeval, "moderate j _t=r 3tri= J Type E: and D inclusions and the high salinities r Ef the Type B suggest t AT both types inclusions ==r_ trapped from boiling, and not r3 i ldPn=1ng fluids, by analogy with ± hiJ s- s,r Pifi _ H. 0-ia_ i !;rLrirA jan and Kennedy, 1962, Rf _ dd=r , 1984).i E ? many Fi rL ;; copper j'i¡rdr i ith+AE ii_ ! systems, boiling :f).r¡..`_- probably i=pisf idic owing to ipÌ"Cs1Lli 1= variations caused by alternating periods JE ri i mineral deposition +n at1iJ r Psi i ¡titisi¡i u ef r a_TLr i n9. Episodic [o1 ing might also havebeen_a s d by p=r i r _ -_ =a = fiLlrtLlaT?r+nE, or by the sporadic3d?c d i iLT?=in of Type B fluidsiid=, b f'¡i iw=+'I' =_!! init;' , tr ;_ = ; = !.i a,a+_n+ and + i_GY'rii39r-I i. t!~ri ;/i=r =i i_LF`i ?_i_ ii_-;- ii ii ifi` iii_.r 'L iF responsible for the = G rIIl(..j Pi iJ 1+A r.t Ll r P s of many v P i n s r E' u t h A n A jji 981 .i , and i1 w..1 1 ' -s i . -: p r. r r -1 . z = ; _i - - r - s r ; ' " i--s - r. F.i-S ! f i T ;/ i..i i i i_ i i_ Fj i occasionally ' !-! i1 ,_ T . i i i F 1 + i i iJ ! :1 E ¡i +ri 1 t i=:_s r ¡_ s? iby ;J i: iy drr+F ti _r 'iiia il; t're¡_r i.a Ted T P::;T L!r PE in E s i':ii=! [ Í'f SIGi i13+1 ._.i IirlT' Pd zones._ True Temperatures and Pressures of Trapping - the Type B wiAri_+ trapped At F'_T_. . conditions At or nPAr thP i_uE 1:tiA, then inclusions that ii ii by vapor 1 i_=-srr i temperatures 1 Tr. 550 that .}. - - s r' e ' r.- i rir_ r r r ' !, or LIIi iÌir'iii Li.! i ir 1?iii_iLii_ J?i..°r+rri,i_i+ of stir-.°iir.-.31 halite, should both ;¡ii?id Tt = Th L-V(L), and nn ti_mpPr?LilrP corrections ar+n l=P] F=-; . The _=rurrPr=P halite homogenization=r'frrin saline i, =luEion= needs ri [+a=a::::F'iained, tiri!h)F!!ar..This type of behavior has riPij (At Lr iri` I%Pd í'_ri several causes.I):?n PÌ=c effects producing d 'sH _ ii;_ isii during heating runs and a lag in , ° .j. i a i F t ¿ -r . ¡ fi_+ ! - ,1 T i'i _+ E rE I L!ii Ï`°L i 'i t F+i_ii 3 rí ri r i And i {';1 i r `"t y : 1 i i!=^-.ri + ':) i:: i Ï _ y í !9 E t'- ! A) ! + t ! d , r Pt a1., i q7_.n raland Rose,q=0)? necking down (Ahmad andrrEp. I i e0e R+_++=ddPr,s 1q84); 73i) trapping of solid halite inclusions l:Er wr+rid :=t Al., I ',?q; Ahmad and Rosei iro=n; ¢*.'.riAdd+=r, #i q14)9 4) solution of the inclusionEi! iF< wal I_ duringing t+=a! i!i'á runs t es llCii ig in increased inclusionii:II f lf Ilsi=_ End a change in the nature = of L ¡i-_` inclusion fluid b =_-u=` of additional dissolved silica (Chivas A_ d Wilkins 1977); r j trapping of inclusions at certain F'-T-::( c ri -d; t ?Cnr either }"G unsaturated sTat+iiiTÌ TiPid t_ir the sAtLlra±P+a J- i=litE° TiPid= generally At high pressures that are geologically unreasonable tP iEtrr _ et al., 1976; _ li{ e and KPir'er", 1979s Erw+_+0d et al., 1q79; RnPddiAr a nd Bodnar. 1980; R_e idr,.1984); or _ ) T r aF'F'ing cif inclusions fr ri'iil aLIF'ar" 1aT E lr.aT at,j in N=i!_ i!hl it i i =il.lt lJ =, "r3_ipi t 3t i_n of halite (Ea =tÌ =i 1982). None of the g_-lÌ g i_9 petrographic, temperature, p]r compositional data obtainedlncd in this study jupG pr"t A; ¡}' j if the Ee ab Et:, = r - . 7 = r theories, tii I1t the{ + lastL 4 E 1_LE ; E,r ! i i i_ i r_ .Ifi i-t !T 'Ñ p,,,,,B i1 pL: rii- SE i1t lí A r homogenize by I-i3liti_ disappearance t,,,iÑre. trapped from fluids that were boiling and supersaturated iÌ t iaC 1 ! thefi just as in the case _inclusions homogenizing by vapor di= 3pp33 r3tiCe at temperatures ti-13r! 550! (Th ,rt idP) = 455- C:), true trapping temperatures 3rjli;i}n by their Th L-V(L) temperatures '.Tt(TlCiije) - 39;O C J S pPr _t ui a - i or may have occurred periodically in i ás Fo- s e to toicooling and boiling. Mixed phase t r 3pGin'? may !-i vE_ contributed ] to E some i Ei Li ÍP rL.a Lte.r in 1i the Tr_-Vt L t data ¡ tori'nr l us ir =_homogenizing C j either v _,r l Ì ror halite di=3F'F'e.3r3n!_e and probably accounts for =_iiiie or all of the spar:5_ liitlt;p_," temperature inclusions that homogenize by vapor d1s-a.pF=erir'3t1ce Despite the effects of mixed phase trapping, a ll the L ( L ) data for B inclusions ._ -i; 3 normal frequency distribution pattern with 3 well-defined mode. This mi pl?p 3_p t__,ç . . t r r !_j i= 'i3EE]Fl' primarily 'ri'rrt3rilï tr iy'ini=li+in_1 r_ rr'r='.3nt3titl3r C ' tr of F'+t3_1=rEií-Tr_ r-i11:- 1i1iC3_3 _r 'Î-i ; alteration and main = t Age mineralization (g. 22) and corresponds to !Amodala pressure of trapping of about 135 bars according to available 3:=:F'3'i",iii3nt3l i13t3 - -g ( R P L d P r4 1984, n 8-25). Type C. Freezing determinations on Type_ iT _,Li- i = 3 Pq-_r r 3 i i _ not F.,- t i ==F tl! but two inclusions j iel ded salinity ]a t aT 5I56+ 0.07R _Ed + 0.028 weight percent ' 3_1 eauiva1ert, respectively. Type inclusions homogPnizP i_t iP liquid i_! phase. E tjr vapor d i = .3 F= I 3 .3 r 3 i i t= L t P : ' i= rF I! r P s i_ T= 200.1i3 i_i, 4_i 1 C: i:i -i i_ !andti ] exhibit ! ` _ modal vapor disappearancetG pai rr P n+ Ç_ + Ì _(Fig. LL)/1A. 'i_ ='__=inÌ the homogenization t ' i i p _ i_i t ii _ s r f these i_1 u _=i r probably reflect differences in the Type C.: fluids at the ti'iii3 of trapping. There is no ii=+Jiit;% iiii_1 ig petrographic evidence for stretching or íPakii!g these inclusions-, nor were any seen in ti`f, r'rS lrE-_+1sof necking n.( r t-t s of inclusions p. ,,,;-t- ?,i LÉ= 134 occasionally seen, but it is not knov.in t.,1/4.1h2fhPr vRriRb12 rRtios-, RrP thP rPsulf of necking down or mixed phase tra.pping. Since: there is no PvidPnr-P for the simultanPous trRpping of Type C and D inclusions, if is likely that these variabilities .re the result of necking If such is the case, Rnd Type C inclusions were not trapped from boiling (or ccinienng: tiLflds, fh-Pn fPmpPrRfure 0 0 0 ' - correction of -10C needs to be added the Tb L-qt.L.J oatA between n d O 0 I -1 C, nd correctinn of 10 needs to b :=1 r fprfippr.Ri- HA- Th L-WL) dRta (Potter, 1q77). The corrected modi temperature of trapping is therefore approximately 355c) C. The density of the inclusion fluids in the two Type C: inclusions that yielded salinity data was about 0.7 gm/cc at the time of trapping (unisov, 1975; Haas, 1976). Type D S]inities of Type D could not be determined bpicuse cf difficulties in observing the hPhAvior JT their ,,--.mAllquAntitiP=. of 1quìçi during freezing runs. Vapor-rich inclusions at porphyry copper deposits elsewhere gPnPrRlly have sllnitiPs in the range 0.4 to 7 weight percent N.C1 PguivalPnf, nci some vRpor-rich inclusions with even higher salinities may have fr:ppPd E--,omP high sAlinitv liquid (t.l.q.=.h, l976). At DurR7nol those Type D inclusion.; in t.4hich liquid could PA.sily be seen and which were therefore most suitable_ for freezing tests 4ere 697.1::» usually those that homogenized at temperatures greeter than NiRny of these inclusions may owe their high homogenization temperatures to hRving trapped vapnr + liquid mixtures, and thus the sAlinity data i-hRf might H P derived from them would als.--,o be erroneously high if the frRppPd liquid were highly sRlinPt The small quantities of liquid in Type D inclusions also mRde it very difficult to obtain homoge.nization temperatures for thPme tviRny of thPm . , _0 _o . homogenized in the te.mperaturP but o rri o m 7t o n 0 temperatures could usually not be P=itimAtPri more precisely than + 7FC: s.nd so LAP-re not recorded. Only five "modPrRP tPropPrRiu.rP" Type D inclusions- yieldc--_d more 135 Yeli3b e e=tilaLes _f homogenization temperatures, and the=_ data rangedfrom ri 0 ti _41 r l+ ÇCf to 4% i L.:).Many other Type D in_ ÌusiF -1 did not oii genii e i_i Ci n 'i e e t i n t i r i r e Fr L. The vapor b iE tf ri ä P ; these latter "high L P f fi p P r a t Lr e i S inclusions did not expand appreciably during iieaT1f7g t(i=r"jT_ir"i_ these inclusions probably homogenize at temperatures r=l1"dP"ably Ab[iI_ 700°11 E+=_r3t t _e t lf i irj_i=+r vat1 +n3 i difficulties, it i1 not knowni!ttn wi"ierher" these high temperature. inclusions predominate,+ffiinate, r tr even whether 3 continuuminLlum 3xi3L = between the "moderate and "high temperature" Type D ThF? hi'iri hF_+iii_+gPniz3tirin temperature= 0550° _i¡ many T.pe v inCiusioil1 may be due to the irar'piiig of vapor liquid mixtures.SOME'T;p e D inclusions -i P undoubtedly L rA r d s '-é-t r --'j ii f i two-phase ii apt irt liquid mixtures, beS_ai i1e groups of inclusions.luli ii isiir l1!r _i r i a L+ l i_ t- ratios are fairly common; these inclusions do not show signs of necking ig di ;r+.'i , 3 ithr+ugf i nerl::ed down inclusions !,h)er i_ observed el jPwher e . A= ingle Type D inclusion in a group of inclusions with variable WL r3Tïos Ñx!-iiCiltPd rrltlr?i b+=i i3'rii ír" whÑn it i irfrgenlzei:3tabout _, but this phenomenon is probably trir ri=sult Sij' a fortuitous combination i Ìf vapor and trapped llqi `i'Jn ii Most, lf not all, rT the í'fiQdPi At? _Pr PrzturPí (T ( ) =_' i_ -F,_ ? r_ ) T, Ff._D vapors are beliPtlPd tri have coexisted with boiling Type B fluids. Although the LPfiii-'+_r a i!!r N= At which boiling occurred ?r'-' not precisely kn! i'.'?n by compar lng r ,i Tl'tL-V t'fftt-'e i_ -atur"e_ Trti r" i-'re31 nL' rr iet,i3 i T,rPi; i-' Bu and 3 1 D in !_ir'''= r`.=, ri tn'=; !..rJ homogenization temperatures %i[ít= ineij forííÏfiiliJerate tPmpPYAtur P" iyC'i= D inclusions tall within the range 340f to Fin 0°C, a r3ngi= which its= lt ?lí='= within the broader range of T L-V(L) temperatures Ì LiL Ai nijJ for TypeE: í= Type D homogenization temperatures are therefore considered tl'ie best °__fifftA.tPe of t a minimum temperaturerangewithin which episodic boiling Type E: occurred and correspond to trapping pressuresLre _i a r r r ri _ F =l ; 100 iL7'5 L 3r c-- (Roedder, 1984, Fig. 8-25:. l -= - t %r DISCUSSIONI OF RESULTS Temporal and Spatial Variations., of `iys=1r: tht=r'iiiR i Fluids Primary Type B and D inclusions_=uL in most if not alll _ir samples f" 1 Durazno. Type B liquids, TYPE D vapors! j. or mixtures of rlr+T'¡'! RrP therefore considered to be the '!'i'imain +1I it f13l-ï +Tr il 'ii? i if!ilr i r_+=+i+t+frlr =_!Tr _ iifi-_ir 1i+=3 :e4 magnetite, and pI-Iyllir-=iiirir alterat i rlt being A irAi 1ab 1e for The t='roF'j.litir alteration event. Evidence presented Parlier for the simultaneous trapping of "moderate terp_ raT _ref i p ?B rri D alsn_ indicatesthat at least some quartz in th_ v Pi-s studied was FrP=l pit atPJ from boiling f 1uid=_.The Type B data therefore yield the following trapping temperatures and compositional o i r a iP= for each alterati: p =lPnt: Ìffa itit = aitPratl: n: 4!-çç Li E=,L.! l- 7qs5 weight percent C A1 + n_1 P tl % A 1 r , 2) r o t ar siuÌ-s i - i_atP P1L Fr-= i 1 =r and sa;r f'I f T i sta *s_ 'CE"iiPrliiPtl+i i: _{ l -54i l: 52.0-70.77 JCiÉ-.IL ''Na t:1i_+ 't,l_'l P.11/. 1Pi T'f 9 2900_520L phyllie and Pi1ir ir alteration: 40.O-57.C' fn1Pight rPr_Prt Nar i+ K_1 equivalent. Much of the overlap in the data is probably due to leaking, necking down,i ' :Pphase trapping, several generations cf Type. B fluids, P r_ difficulties in assigning a primary or secondary ÿ' +?rigil ¡ to +irl+= +i I si:=+rf=.,. DPspitPLfP=P ulirertalnties, it appears that Pali- i T'_= and temperatures gradually decreased from{'f1f trie period of magnetite alteration through potassium-silicate a iTer PLi! nt+ F°i'f111'=-_i1ïCl+= eiTe'r etirlrl. Each alteration assemblage, and perhaps also the peripheral low grade gold mineralization, 14a1 the product of episodicallyil} bi;i iilig saline fluids, in contrast to other porphyry copper-typeypF rfi r_It ¡'Pl'i[rrs I:,,Jrft_+rF+ p¡f jfl li+- _f it+'_r ;Tif Il-1 And GPr iF'hPr Rl gold mineralization were the product of relatively +ill_lLf= fluids ala 1h, 1'7476; Theodore Al., 1 qR2). Siri^fil3riti+== in ?'he data obtained for the two early magnetite + q.!Arri one hosted by the p_+tallil.lm-silif_P1f=_+ PiTPrcd stock ! BV-ricl.l And Ti i=+ other by propylitized volcanic Grerria 1P''1P'r a1 hundred meters from the 3Ta_il=i. (BD-136), , suggest that thP'r fifal and Pa liiiit 1gr adiP'nt = l}:fP'rPT1fiT!n1F 11 developed during this 137 early stag P of alteration. Sample BD-136 yielded the,highest trapping temperatures and 7.-,RlinitiPc: for all Type B inclusions, despite: its locRtion. The three mAin r--,tRgP veir(BD-E., ED-959 :q rid BD-130) I.A.ierP tal LOMPOSitiOn and temper u r E.-2 data for n-Type anuD - too limited to ..Psin tPrm,--, of spAtiRl/RriRtion--; in the porphyry systPm. The t.4-idPc.prPad RbundArirP of Type D inclusions, the widespread but scarce distribution of Type inclusions, and the very limited distribution of Type A inclusions do not appear to be :artifacts of sampling and probably reflect the originR1 distribution of these fluids withinthPouter boundRry ofthemAjor phyllir RltPration hRlo. h the interpretAtion is correct that all Type A and C: inclusions are secondary in the thin sections thenthese two fluids misthavP ltPrnatPd with .,--:hothPr Rnri t,,oith theType B and D TIJids, c:incP the latter fluids viere apparently both thP PRrliPst and thelatest fluids ineach sample. E.:;ignificance of the Type A and B Trends in the TPrnRry DiRgt-Rm The only consistent vRriRtions along the Type A and E trends (Fig. 21) are 1 38 decrPAsing NRC1 content, increasing 1::C1 contents5 RnH incrPac:ing K./Nwith decreasing salinity in both Type A and E inclusions. Also, the PxtrPrnP high Rnd iots' Pnd.; fl f the Type E trend do correlate t.ATith higher and lower trapping temperatures, respectively. The range of for both Type A E inclusions is cinsPly comparRb1P to the 1::./Na obtained for fluid inclusions at most porphyry roPPer deposits (Beane and Tifig--iy, 19R1),no sP\fPral rPRsorz. hRA/P hiPPn advRncPd to explain these changing ratios (Eastoe, 1978). Rock-fluid reaction is the most commonly cited explanation for changing in hydrothermal fluids, with higher l':/Na reflecting higher tempPraturPs of PquilihrRtion with K- And N:=1-t_DPAr in q minerals; the presencP of.11mit raisesthPsP rAtins even further (Orville, i 963 Fournier and TruesdP11, 197R; ERstoP, 197R; LRgachP Rnd WPisbrnH, 1977). 5:;incP the sRmplP (ED-12:F.) Yielding the t DurRzno yields the highest trapping temperatures for Type E inclusions and is also farthest from t'ne stock, while many of the lowest tPmpPrRturP Type E inclusions yield some of the highest K/Na vR1uPs5 it is clear that the DurRzno yield the nppositP of the predicted rPlationship. It is therefore not possible to est.biish that the high salinity Pnds of the Type A and E trends were generated under mAgmatic conditions purely on the bRsis of the crude measures of KINobtained by thermometric mPans,.. Trends similRr to those of the Type A and E inclusion=1Rt DurRzno (Fig. 71) have been found in other porphyry copper and molybdenum and base mPfRI skRrn deposits (Eastne, i 9761 1982; Erwood et al., 1979; Wilson et al., 1980; Kwak and Tan, 1981; Roeddez, 1984). Several workers have noted that these trends do not point towards the HO apex in the H2O-NaC1-KC1 tPcfriRrY diagram and so could not be generated by boiling or dilution Rlone (nlokP And KPs1Pr, 1979; Wilson Pt 1980; Beane and TitiPy, 19R1). Cloi Rnd not simply higher NRC:1 qnd Kr:1 contents. DivRiPnt cRtions such as calcium, iron, zinc, and magnesium greatly reduce the solubility of hRlitP Rnd sylvite Rnd Rlso 1nwPr vapor prPssurP.. And Tan, I q81; Poedde-x, 19E:4). If these additional solutes were known and cnitid be corrected for, either or both of the Type A a.nd B trends might be found to project towards the H 0 apex in the H,,u-N.B.L.1-r..L.1 tet nary diRgrRm, Rs demonstrated in other studies (Kwak Rnd Tan, 1981), and their parallellism might not be prP.:PrvPd. Thus it is pos..,ibiP thRt salinity vRriatinns of either the Type A or Type B inclusions, or both, may be a result of boiling or dilution by heated low .linty (Type C?) f1uds. There is, however, no evidence for boiling of Type A fluid..., Rnri their high trapping temperatures would seem to argue against dilution by low salinity, moderate temperature Type C: fluids. Given the compositional uncertainties, an interpretation of the Type A 7-1nd B trends is not presently possiblP. Origin of HydrothPrmal Fluids The origin ofthe four fluid inclusinn types foundRt Durazno is problematic. Evidence presented PArliPr suggests that the Type A Rmid inclusions were not trapped from boiling or condensing fluids, but that Type B fluids did boil episodically and generated some, ifnot all, of the"moderate temperRturP" Type D vapors-, Petrographic evidence suggeststhat theType A and C: fluids lternatPd with Parh other And with the Type B Snd D fluids, but the lack of apparent compositionalgradations between them and the high trapping temperatures of the Type A inclusions suggestthRtthese fluids were generatedn d transported to the site of trapping k.Ar¡thout This, Type R fluids could not have been generated through dilution of Type A. fluids by low sAlinity (Type C7') fluids and, as noted 140 7 . ' L , .}, , 1- . : T f t r 1 Y earlier, it is unlikely 1that ii compositional i i i i ti_=f=ri_ t ? + iy _ Ar fluids 1\i=ft=i P _i P : {. _ ' i f_ t 1 I i i n , ? S,.i 'j } j C= i i 1 L i_ F_ = If Ì i_7 Y _+ i_i Li i f' _ result i i 1 i ¡ si[i il variationsf f i Bfluids1 _ i_i 1mLiinA tii ij j i lf boiling, dilution,inç '_a{ fd changing P-T-.'. i_i_inHiT i_i í= at T! G s ÌtiF Of origin. The low salinity Type 6_ fluids are the only fluids detected in this study that were F fiA.l l rA F a t l_ of diluting irP TypP B ait[[uq- proof for such dilution is lacking since co'ifj =n=iti iiia 1F:.=+`siplPs intPr mPd i3}P bPTwPi_n the two were not seen; these 3:::311;p13= 3'iight be found with further study =, rir they simply might r -} ,j r}:- - ,- r= , - L . . - 11 - -r n i r.i1 Li {fluids ; ii ii i =ri iri =i+ i_k i I¡-=fÌ i : i =+ndid i ii not Ii i?i_iGiT T 1-a=1 . LisExtensive boiling of Type B fluids due to sudden high heat _¡lnwg with s ir without ..uddPn p r ij s 3 i¡' i= drops, could t i È i_ i i r" i? # i i_ _+ i i s' q+'` n+' ÌA rP f hP high K C l contents and JA I i n it i+-= J of the Type A fluids, because Na partitions more readily than K into the vapor phase (L3g3rhP 3nid Weisbrod, 1977). However, the lack: ii compositional gradations between the T5'pF? A, B, And r' fluidsiid1 And lack: Ì if evidence for boiling ! iT Type A and C iluid= argue 3g3inst the 3: .t3n3 i11e boiling of one fluid to create another, higher salinity T i Ii id. Er t = ie boiling ofi fir:a i n iL ; fluids such As ground waters to produce t hP highly saline fluids t y pi[al of porphyry copper environments is considered An unlii::_ l,, mechanism because;i= i_if the lAr-gi_+ am i[4iit= of t=::::ti==i-j;A' ¡"i=_At if required (Roedder, 1984), and temperatures greater Lh3n 3tiriut4no L.. are ditti+_u,t to maintain in shallow-seated p1 rp fl fcopper-typeydr oL_at isystems (N.A1 and Cunningham, 1974; Norton and Knight, 1977). Experimental data for the 3x=o iLti 1-; of residual, immiscible, aqueous fluide, from crystallizing m3g113 (Ki linr and Burnham, 1972; Burnham,fii,i 97q, l qRl ; Chiva =4 ,_. _.__. r = , 'G_T:' t ' t i r -- l_ ,-i á/_ t. _ i l/ i1 And 31/A I fLi i+== 1¡. ijiagr3.ir for icrI+' systemsy _ , i rr i If iir ir 7 i LL r'.epnÑdi 9_62 Rr=Jd=r,q_4) anli 4C , ii:Ñ _-J iPe _ 1PL , ci _ 7 q¡ i idi _AtP that ear I of the Type A, B, and _ifluids would have to have [ ?e¡ generated d =L dramatically different F`-T-. .+_+_+nuitinn= if they were all derived from the same. magmatic or IYIF=teili{lic source region. Differences iii trapping temperatures between 141 Li ÎP th1'-PP typPs ii ini= Ìus iniÎs are also s ign'fi3=aÌii. s Ìi i"or tlinA tP?_`,`g tiF1'-P is nn pPtr-ograG'hi= Ps?idP;ii_P for a. magmatic or 'ijirrtPiii-ic origin of Rlss¡ the inclusions f>_lund at Durazno. Inclusions of primary ffiagïf'sPtit= origin in phPrÎi_i_r;'_t 'ffilÎFr"Pi= in intrusive rocks P'{';= too small to identify, and there is no apparent outward irÎs=r PP=F in the proportionpor"ti3 [ 3Ci ¡ low salinity Type C inclusionsiu=ri! i( i= t aL might suggest a g ' r -s- r r !- l-i ti- - , r {.. 'f¡iGT Fi ír"i rir'[ +i i_Ir tlGe 'i 3rrLi i 3 ei il +i i íiIjitñ! i í i ir'í_idog,.ij. i!in_iÑ underlying eiR='í_ir ite sequences of the CPii_hP'¡üt_IÎ=P sedimentary ° unit where salinities tT up to 30 weight percent N a_ ' have b e=Î r eas[re_d from s pr1ngr passing through FPr P rocks i -= i rA-rng L _t it is difficult to imagine how n Î=' low salinity and two high salinity fluida; ,d C P derived from such A =i ir _F. Moreover, high rP l iÎit, inclusions RrP f_, Fi Ì al nf porphyry !}-r Pr systems (Nash, I q7 Beane and Tit ley, 1981; Poe_dde.r-g 1984.sg many of which are not located in areas where saline meteoric ic i,ir oceanic waters are available. It is t Î Ñ e proposed that t_ _ r invoking t e derivation f } all hydrothermal f l ---r in the porphyry copper environment from a single meteoric source '{-egi _iii or A 1i s'_i it_'ifP =-? 1fi _ f ii_ source rPginn dn not apF1; to the Durazno h y dr_ h_rfP1 system. In_ F e=dg it is -j : } z + : -t -. r . , , ¡- 3uti=1f trl.PF á i?Pr i iJrÎ ii ti-ií= ti_i 3i; Ii3_iffii ii_ !trT. e_i°s 1J ¡i_ig ! _31nr`=' i=rÎíJ j ili._ii fg . 1970 , Taylor, 1974 1979Gustafson and Hunt, , i-J N j_ r g1976; r a.s t r P, 19E.:2) might explain t! P Durazno t tid inclusion d a L a g with the h-g,; saline Type A. and E fluids derived from a magmatic source and the low salinity 1 jl pí= ! li_id derived fromi'ffi meteoric waters that in I lu::.pi_í fi-íF= 'i!iA'-iffiaf ií_ hydrothermal system:h%31ijEJ. The a g i t P t i i_3 3 i created through L h i_+ boiling _ f thej p e B fluids may (-i +--_7 v i_+ aided their i4li::::ilÎg with the lower density Type pP j_ fluids. The compositionalirÎP l d i=tii Îí_tiliPîrP=s of¡ the T s p_ A and B fluids s ugg e s L ç that t _ej were g j e v ? T ed under _ lg Î ,, c a n t l ; different P-'r conditions, either through P r_ =1 t i_ r an immiscible a q _11 1 phase f r 1 l crystallizing aÎJ e s I --P magma, + : :o - or without _1 _ :i= T i Îg high temperature 1PFor phase, _ r condensates fr nm highgÎ temperature ' Pp;L ,-- i _ d T s` o f fi this magmafag'ff! a t ri P n 1 P;' and M 3= i'i Pb b g1 q Ì R .l.The fluid i iÎ i= lr = i =í lÎ data are too limited 142 to dictingitish between the riAlativiA contribution of PAch of these prociAssP,--- !=';ovrie of the- "high tPmpPrafurP" Type D InClUSIOnE rciAy rPprP',.-.Pnt thP vapors PvolvPd directly from cooling magma, although certainly some of these high temperatures are result of mixed phase trapping from either boiling or conde_nsing fluids. The problem riA-mAins of how the Type A fluids were trapped at near magmatic tempPrAture_s in well crysfAllized quartz, yet a.ppariAntly alter nted with thP lower temperature, lower salinity, and volumetrically more s-ignificAnt Type B fluids wit h lit mixing. Depth of FormAtinn A proper stratigraphic reconstruction of the thickness of overburden at the time of PmplAciAmiAnt cif the DurAzno porphyry system is not possible because of lAck: of knotAiliAdgiA of the nAgirinal voicAnic stratigraphy. Quartino (1962) and SilUtoe (1 973b: estimate the thickness of overburden removed by erosion af greater than ti.00 or three kilometers in the graben portion of the volcanic crimplPx, but evidence from fluid inclur--,ion,-; depth of formation of the exposed portion of the porphyry systPm. Type Pi And D fluid inclusions provide the only P-7timAfP,.-, of pressures in open fractures throughout the period of hydrothermal alteration and minPralization. ThP PffPctE--, of oxfPrblirdPn Addition or rPmovAl through bActionic means, erosion, or continued addition of volcanic cover are assumed to be minor. Trapping pressures estimated from the Type B and D fluid inclusion data range from 100 t ri 225 bars (1(7,2.5 + 62.5 bars), and these pressures, in turn, correspond to 1-iPPth,, of emplacement of 370 to 830 meters under lithostAtic condition,,,, or 1000 to 2250 meters under hydrostatic rrinditirins. It seems reasonable to propose that pressures within fractures tAnArP initiAlly 5OiÑC)h2t greater than hydrostatic owing to the tortuousness of the poorly connected fracture systems as they first developed, particularly in the ',Jock:. With time the density of interconnected fractures steadily increased, And conditions must have become nearly hydrostatic. i Intermittent_ =i=1?li-fgi and reopening- of Ti ai_t_lrP= _!= ;"Ps:Rlt ifT ,1___s-ni'- adjustments, hydraulic fracturing including explosive 0r , a } i , F rP1G= _P, miÌPrA i deposition i ir solution, stress corrosion cracking,fgi and thermal stress fracturing probably iy a1l led to local and episodic flli!_L;_aTii_in= between i f;¡rdri +_TAti1= G'rPs= frP j =Ì"ld slightly higher pr='ssure1 As is typical of the dynamic evolution of porphyry . copper systemsl Ro_d der,q71, 1 98i Phillips1 9 Cunningham, 1 9 7 %Henley a nd McNabb, 1978; F e=-e andTitley, 1981). Along rith iÌ=rGajirg fracture densities, ,= vapor evolved from the boiling of Type B fluids and possibly A_- P1 ! l 0=d directly from the crystallizing magma began to i per fÌiCaTi_ the overlyinging r i ii_i'.=r in greater v _+1R_RmPs As evidenced J by LI !P abundance(da S ice i i i Type D inclusions.tso1 h+= combined effectTei_T of increasing fracture densities anb increasingr _1 = of overlying vapor grA Jua1ly reduced ; prp=sR_Rr =`= in individual frai=tfrr"e= from =nmFí.hThat more to somewhat le== than hydrostatic Rs th iA=:r' 1 t rC Yi f cooled R n d allowedf=ir th p1_ ln t i jl Et r+ +f -ç t- , , , s , , , boilingliRÌ i_+fi ypÑ LsB fluids.E lf1i+.1 1-f i+1f_ +_ti{ff t '_+f L!the f+'í+'}r-'LÌi of formation of rl-fethe presently exposed portion i if the Durazno ir'_+rpÌf yf' y s ystPfff is therefore pr l iti._fi.,.ily F i d 1hi 1 between the depth estimates given_.G . v = q or About Such .=s depth E is closely comparableLlle t 2 1 depths of formation ! iT 0.3-7:.0_:.0 estimated J for many otherfer r'! ir¡'h yr y copper occurrences. The depth estimate of 1.5 kilometers is appriixiiffaTe. First, it may be a minimum estimate if it Pr= _ T at - 1 regarding the boiling of Type fluidsAr_ incorrect ririf additional undetected j r1S +rji_i_iÌfbPn1 ib ie gases _iÍ_i_f Er iÌ Ì tl_ii= inclusion1 fluids tlPrPL: raising vapor pressures from ! hÌc.F P' %i_t_d here (Moore And N A _, ? ¡%!' , ,--rrs i i sr: :j [T} ,= . . I / -l' E ,jÏ i ( _ 1 s + 'i_}.i +iir ands Bodnar,s1 li !r1 _ ' f . It may also +t:M= a m il_limf ln Pst imatP if the integrated bulk dPÌÌsity of the fluid column overlying the Durazno Porphyry system was Ma = = than the 1.0 gm/cc density1 d to make Te pressure t i depth conversion,=i +Ì i, i.P. I on bars hydraulic pressure for each kilometer iRÌ d+_pts i. This situation could have occurred if A signifii+=Rî;t vniR!mPri ; lowdensity vapor filled ?' hÑ overlying pore spaces and frA+_tllre jf Iirtl_i_! ir'rGl1 A 1 suspended va Porir" bl Ibble1 in 144 overlying liquids 1q71; H P-, Py An_i McNabb, l c.17iri-_i _' =_ And¿o dnar , 1 9¿ I,r4 or 1f much ! iI } hf+ overlying fluid column }_oi- = ì=f Fri of low sA l inlf ¡r ' 'rrF, sntA:3 ti=r...r hPTPF' by F th i} {Jr`f I iZ¿' -iNr iiir-=:rr'f rF'i"ji L13= F91? I n;F ! ori! byi_} j I-i- Iil-¿- F` geothermalE' 3iiriiii3ly of rhP jrlrani_ Pditir= 'f3 eit the other h_nU, thP depth estimate may be a maximum e j T i T = 1t if the fluids contain d l c r o lP _' r Rl t_ addition t[FN.ii :1 3nd K121 that might lower vapor C ;Ps=irFr significantly fr['t _n_P estimated ( F o _ 7 d P i 4 1 J 7 1 9 1q84;t i r r r P and i a _ - ,q74; R s = d d = r and ==n î a r4 1980); ti if the integrated bulk: d_ns it / of the i_ri } ir g tli(id column !=ajgreater than 1.0 gril%fi=i= because[ifti-iP pr P+=i=nF_P t iT significant t gi1=1îf il1Ps of coolerleriig1-i (y =P i inP brine; t r3) l S pressures APr e intermediate _tAFPi h jdr _ C tt 1= R 71i1 L_ s t RT i- r1 FSrnPi_t i+ i i P} . i becauseGPi_ai_.P the i f.r"i .raif=rti'f=iA1P..r r r1 i Tri i_tPI n1. i ntnot TIPi _",1FjI .,i`f _Ì}i AiiL l' each otherandto the surface.(Haas,, . i andBodnar,i ¿ .orbecause tlîi= intervaln ii inPPtur" atPd ground above = L;'iP water t 3Cil= is ignored ! R FP+dd_r and Bodnar, s1 yPl lf. The depth estimate is jiiF.=an;ngirPs if tectonic, magmatic, or F- : f r l ¡-¿ r r , - l i i_F E i_+ ir ¡-.P r r i{E_ r i_ ii 1 ii ; Fi . r- j r, nt r-iti i i_ } i i h i{iPr31 F i-r-Jj!rP r-PFriiiori=i, ltboiling occurred within restricted tensional li_ipf`ning= created by tP!_t _i iii_ adjustments s (.G11}_i_idPr- and BrFdn3 ry FiEnLnGI1= 1--lI':Tr,F'Y OF THE E..10 EL D(1FAZinin F'Fi,1-..F'yYF'Y 1=t lF'F'EF'-r _1LD F°RraPiF'E¡?. The Bajo El Durazno Frrp-yr; copper-gold prospect i= one_ ofPP/Ñr 3, porphyry copper-type prospects that developed during tP waning _t 'iii3g'fii3tl3'(ii r it the Farallon in Nirgro volcanic complex, 3ifi3jrir Upper M?r }rF_nl=_ ai idPsiti'- volcanic +=entPr" w ifh jhr i1i-ir initir F'Ptr r`lrigir 3tTinlflF=3=. Hydrothermal l Piri=r =t tl+_3Ï i P11 mirier31iz3fir}n at Dura: ns=i are centered =1!'n143ïs=::indP'3lti. porphyry =fork emplaced at the 3ppr oxlïfi3tP lnt3r33rtiriiî ! iT LhrPP i inP3'iiiPnt= .nd 3nd33itlr dikG swarm. ThP ,--tori:: i1 i_irfstPrj c_i'ffi=giiiPY'si= and petrologically s-imil3r 145 andesitic iliiir3nir úri_ri=i3s and minor b3s3 ltir 3nd~11tP flows And Li !A# i i=1r_m the b3331 port inn:ifthe main eruptive center of the ;toii=3nii_ i=i Ifiii' lr::e lntr u=1i in! alteration! and mineralization were 311 accomplished `.r?ithii-i 3 period of Miss than million years between S.7 + 0.4 i[iey.o and 7.9 + 0.3 m.!. ag=ia This jr-'r e.1enL iy Fa,::,pri ji_d portion iii the Durazno porphyry 1i°1tPi{i pr--ibAtFly di_vPliÌFjPd At A dPpti{ Abolit 1.E, kilometers.i 3;::t{ir 31 variation_ within tn= =tori;: =L!ggÑ=t th3t intr-H=iitin may have r=ien Ai_ri=t'ff!pli1Ì-{Prl` in A sirr'iF+s of distinct! closely Limte+i], _tnd i=:iÏÌtp;i=i=loii311y identical intrusive pulses! =! P_ fwhirh tAiPri= forceful. Intrusion of L r +n stock nR j accompanied and followed by the emplacement i iT a jeT i if crudely r'3ria Í near-40i=rt?r31 3ndPlitt=_+ porphyryr-y diki_s. TiiTt"I_sit1i== 1_:intart1 of both ti-Ei== stork And the dikes reflect 3 pronounced regional structural i_i inT r" rti, both in terms l=it pre-intrusion fracturing i_i i the. volcanic host r:i r i:: 3 and i n termsii f continued tectonic 3iJ jL!3tm3nt3 throughout the ;_i tir; period of development T the porphyry copper Si/ Stern. alteration A t Durazno commenced w i F ii the f G i 4 (i 3 T i i=i i Icif An P3r1y central I zonP of w33;:: piit3l=iL!ffi-=iiir3tP aitPr3tinn centered on the 3L:irk 3nd 33á rP t t3 ir i r' r and surrounded by essentially i _ v31 tr., {ak Fi i Jl,t ~- alter/mt.-ion. Both al 3 3in types ar_ uniformly r ¡ s v r in surface e G r L P , P:-ltl! inÿ little 1 3 iiA[ A % r 1ati n with respect t 'F stock. C _t á - s i- r t l=3t 3!t'" 13 t l: n i n the! ltittstock 1is characterizedi+_tbyjiii''nt=Liti=_' ,t_3 i'_i'r F ,andf r_ rt i=rr i And i' i 1.' generally les=. well developed than that in surrounding {g w=liir s if=i::=, t4¡iG¡-e secondary , biotite! quartz! And Anhydrite characterize the potassium-silicate ._,iiti_r'atis_i1 assemblage.. Magnetite alteration, consisting primarily of banded r'ii3gni=_tltP + quartz + Lilotiti= veins and vi?inii=t=! developed during the P3rilG=t =t.=igi= ; : if potassium-silicate and i='riJF'f i itii= .=_.titer3tion and i3 r=te:::;tr'n3i'./i== with i both 31t3r3tiisn types. The P3riiP=t Tfi3nii-P=t3tii n of rEiAgn3titij 31tPr3tii in ori-Hr= in the stock 33 irregular m3gn3titF-rich mAss3= of probable. iati= magmatic origin. The later Gni Fiveins 3r"3bestdi_iIi_li_ij'Gd in the C'Çit3=fiL!m-=1iiratt= 31ii=r'=;L1_+n Loni= 146 where they commonly occur -in rinse F.-,patial sortticn Wit h ¡MP bit coppPr-gnld mineralizAtinn. MAin stage copper-gold (Ag, Mo) 1-ninera.lization developed As An intPgrAl part of potAssium-silirAfP AltPrAtion af slightly ]ter time than magnetite alteration. Highest grade minPrAlization occurs within the stork And it tAiallrricks withinAfew tens of TriPfPr'sof their mutual contacta A poorly rninPrAlizPd And fractured core is-, apparent in the stock and may represent the last Portion of the stock to crystallize. Jome of the mineralization in pof.-is----.ium-silicate altered rocks iin the form Of diss.-Pminafp-d chAlcop.:yrit.. (and bornifi=?, native gold?), but most of it occurs As main stage veine and their brPcciAfPd variant -7 "biotite breccia" veins and "hydrothermalbreccia' vPins, These veins contain ciHArtz, calcite, magnetite, pyrifP, chalcopyrite, and sPririfP, chlorifP, orthncia.,,P, biotite, siderite, molybdenite, bornifP, sphalPritP, gAlPnA, tPtrAhPdritP-fPnnAnfitP, and native gold. Much of the pyrite and chalcopyrite contains gold and silver in solid solution. Fragments of main ,tage vPins And magnetite + quartz i biotifP ht-PcciA" vPins, And in "hydrothermal brPrciA" veins, along with the local occurrence of main stage or 1-1-1AgnPfifP + c4:1Arf7 veins in volcanic breccia wallrorks that are cut by the sfock itEcif, Ail indicate that at least one period of pofassiHm-silicAtP And magnetite alteration pre-dated f him intrusion of some portions of the c.tock,The t.A.vil AlPAt fh - hf -f is A composifP of f intsionsrlifrion that pierced through their own mineralized And PArly-frirri-tPd and magnetite-altered hood zone(s), but were subsequently only i,A)PAkly potassilim-ilicAtP-altered fhPm,--P1./Ps. The occurrence of fragments of mAgnPtifP-altered rocks in dike-s And silicified zones at some distance from the --stock suggests that magnetite alteration is widP.--;r--TPAd At dPpth. FlirthPrmorP, the presence of rare frAgmPnfs--.,, of rriAgnPfifP-altered AnriPs-_-,ifP in the voicAnic hrPccia itself suggests that either mAgnPfitP AltPrAtionwA,-; widPc.prP.F..--,..d in the volcAnic complex, or that thesP brPrrias havP \fr-ry locAl origin a. no TrrEt y ["2 intimately 147 rPiRtpri to the Pvolufinn of the porphyry system. The. eRrly RifPrRtionPvents at Duraznn were followed by A period of phyllic alterations now found superimposedto differing degrees on ail earlier alteration s.ssemblages. The phyllic alteration assemblage consists primarily of F.-.-,e.ricites quartz, pyrites and anhydrite s the latter hydrated and remobilized near the surface to form the abundant gypsum vg.inc, that are a prominent feature of the prospect. At presently obsPrvRbie iPt,./Plss phyllie alteration is most extensively developed in an irregular annular zone surrounding the stork and straddling the former prnpylific-poiA=7siHm-c:ilieRfP alteration boundary. This more intense halo is broadly coincident with the most highly fractured rork:.--, at DurRzno. The entire phyllic alteration zone is elongated northeasterly and at an angle to the east-northeasterly elongation of the stock. The patchy distribution of phyllic alteration may be A function of variations in primary rod.:: pg.rmPahilifiPs and intensities of later fracturing, but it may Also reflect late-stage faulting that 1-17-vs juxtaposed rocks from different levels in the porphyry system. The entire irregular phyllic RltPrRtion halo is broadly coincident tAiith a p.P.riphg.ralow grade gold Anomaly surrounding the central mineralized zone. silicificationr -the11:2 hiny alteration4- event and is superimposed- on Ril prPvinH,-, alteration types, though it is proportionately more common in phyllically altered rocks. Most silicification is only wPaidy developed and is restricted to small isolated areas, but infPn..g. locally. "Poorly mineralized but extensively phyllically altered and silicified fracture zones were dPvPinpPd through the entire peì iod of hydrothermal alte.ration and arg. prirnRrily found in volcanic breccias along and generally paralleling the major axis of elongation of the phyllie alteration halo. The hydrothermally brecriatPd textures in the centers of many of these silicified :ones rnRy oc APsuit of V iolPnt volatile release of hydrothermal fluids in contact with underlying magma. The distribution of these fracture_ zones and the rare pPhblP dikes, along with the elongation of the phyllie alteration halos may reflect a northeasterly elongated intrusive body at 1 depth. F3t`1tlng Apparently occurred tr r r_rr ut the development rf f !h= porphyry system, a n d some dikes, silicified z n _ j and pe b b i e y rl is !tn former Tat Elr =! a 7l_1/4 of which were r'p af=titi.ate'd iaLPï . Evidencie from fluid inclusions indicates that magnetite and pCILas1"Ìu {{t =1-ils_L Alts='r'_itll_snq Along with ph y ills=-1! iii=i== 3ltPr- rL f? sfl in :1 ilÌi=lTi=d irsnPsg were j All the product of highly saline, episodicallyIly Ì_si 1!iftg hydrothermal fluids. primarily within the temperature range 310c3-490c10. __fF=F'pË -grlif7 f'i{inP'i"a lliAt {jfsi probably Ce a Ìe d at around 395w :.f { i J .Ì i { a f' have been precipitated from solutions - supersaturated i na .The peripheral low grade gold ii n_;3 liTT i i i mA y Al=o h3 _ been producedtr _T these same L f{ l l _ g saline fluids. Tt is suggested that these saline were llagmatil_ in origin, but were progressively diluted by ? i='ate'da low salinity meteoric waters during Bier stages of prfCA11 iu'i!{-=,1li1_.=aif_+ .=_,f¡{i_i i_- =, i '_alteration, dilution 1rÉ _t _ri'i_ atgreater distancesrJ it_ r_stock. After the development of the Durazno porphyry system, the prospect was eventually exposed Lf y erosion, probably a= A result ! fi P intense._,f n cl widespread period of hi locks_i:: Te :l,tll-ig and i tF'llf t during F i'iri-F le1= tl. s[ene times. A 1Ci{ = !l nearly circular depositional basin was developed :ver i P _Pntra ; northernq A, eastern portionstir lns E If the prospect, the shape of which !t!a'.° reflect .a column of 1l_== 1 ' }i ii r l-{ l l i f_ t i Fi. l i#_ ? er r1111n=l f' i=rLani'i a 11 f f rt-'_ e Fi r l_l=1:._ {a _a::.: i iFit~ fe±f{- to the t=a1fElt±r-fare'.r _ T- s w as later dissected by fluvial sand and gravel rhann els wf {rf=rP field relationships suggest Ri least two periodsfd1 rif renewed { Ip l'! t in Recent '._, rl i !"{ e i 1. i t e during the ?,a t e Pleistocene(?) or Holocene, two ver y sm All ¡¡ hot springsj _ active At Durazno over the center ! f the s Lo_and over i T r western contact. These springs deposited minor amounts of calcite, chalcedony, i{f_.En'-_!=i;iP=t_ . i it oxides, a n J iron E riF s in a fashion analogous thP hot = ig 1 that are =, j active in the Farallon i'1e'3r rE region today. A small seepage spring in a riverbed at Durazno 1s presently depositing small g_ A i t ir i e = caliche and alum(?) .a nd may represent the last marlife=tation Ofone of f these former hot springs. 149 l_i_) Ni l_ i1..F.:*; T0 NI _t TnF Bajo I El Durazno po! phyr y copper-gold pri_}=pe}_t 1s similar to iii}rt porphyry copper-type occurrences in terms- of spatial and te.iíliF'_}r _il patterns of intrusion, EfJ rnt- P r r a! aÌ ! P a L 1 _n5 And ii r Pv A t 1 i = ti onÌ} P presence 1T high salinity 3nd low salinity li='id-rit h inclusions and abundantJ 3pr_ r-i i =r inclusions L s i also iycal ofmany C r r J^ _i - p r-J rp n _ o i r r Y n r in 1 ( °i A.r - 5 1976g R Ì T d izj 1984).1 h e Durazno r r ! r _c Eis one of cluster of porphyry_F p e r prospects t ra } }_+vî E1il±jd i4'iti_iln A locally fPnçi_EnAl np' }==:::.LE=n= i }na i tectonic _Aitli_ig roughly f 5 l3 ! -lo i P t P t 1 inboard from a consuming r i A : P i A r g 1 n . Like i_s1 porphyry_ p occurrences (!_tll?lt i iPs 1q72, 1 ÚR 1 ; Guilbert, i JP. 1 ),! i iP development Ì iT ihEjsE_+ prospects is probably ultimately rP atPd fo iA r gP_-1 riP compresi" =_%Ptt il r And to f g_ Li i - of magmas through p A r1 1a melting Ì ÑAr A:.d Along A sub 1 A 1 Pi7 3L3bd1it_t?ng titslE_Esp3 iPr 1E_ slab. Variations in the general "porphyry f hPmPE} At,.} l_! r''=; zri _i 1ni= ii dP á genetic association with petrologicallylly _ilill Íar" jhCish=llllilE= volcanicjr=ailii= :And l intr'!l=je,P r-ni_k= and thP presence of significant precious metal mineralization, lÌ-ii=l!!diilg a peripheral lriw gradei gold1 1 anomaly.i_i Because1 I-j i_ 1 ofi'_ itsi JiA1lE_Eil Ei tilig, the Aii}Er ÌI E_ porphyry = t f ick at DDurazno is somewhat 1(i i 3 i 1 rr, more irregular, and'- =i ! i= id e r ..i b ly finer grained than many stocks associated with porphyry 1=1=1G'pPr"-typa ii Pr 3 l ii A _ i _n elsewhere. - Pr variations ir_ _d_ the presence of magnetite alteration, l i c a l ly intense s i? i_ i 1_a r ion 5 widespread r i p1 veining, a d a number of EÌi iilE_Éj, ;1, rii'ail;!t '{i j!1! 1 lii_3llsf' -1altered1 and silicified f fractureI zones. : Althoughlth iiugh saline "i"lt lid inclusionsÌ13 3 i llj evidence for boiling are L r-'F'i}=3l but not universal features of porphyry copper hydrothermal sy3t _m3, trithe salinities=ra lnitl J Pr JLiIYti3tP1 forfr some fluid i n 1= l t us i 3 i ns At Durazno are r" P'?fi 3 r" k 3 Ci ly high (upto I8 ' wt t % N 31..., 1 + K l_ 1 1f ` e' l.lit! % The Durazno F-porphyrylr-hJ r/+=tic.1J_t Eiii is;_,_t1-_C.li .tt ii1l_. i,1 inthatria itÉAii1_fii small11 Ïi hott springs apparently IAJPr-P activP in thP center of the =y3tFiii long after porphyry copper-type 1F10 d yhaehyroermacitcased. As with all porphyry copper systems (Guilbert, 1981)1 the AndPite porphyry intrusion,---. at Durazno are related to the I-type granite A ssociAtion of Chappell and White (1q74) And the magnetite sPriP.-. of IshihAra (1qR1). The gold-rirh nAtItrP of mineralization, though cleArly unrPlAtPd to thin or nonPxisfPnt --=iAlic crust (:}-iollistPri 1q75, 1q7R) cyr to An island Arc sPffing ov:---f oceanic basalt (Kesler, 1q73), mAy bP A r IA '.:-.ii 1 f of s-PvPrAl 1:Actor-L..: 1) the more MAI' ic composition of the host intrusions (f;o(ilbiArt And Lowell, I 974), in some respects comparAble to the diorite model of Hollister (1q75); 2) ./Prtical 7-on;;fion of gold (TiflPy, 1=47R) with high 1Pvel gold mineralization preserved from erosion because of its spffing within A. graben and because of the youthful age of the prospect, cf. VundA, Fiji (LAI.A.IrPnce, 1978); :3) uniquP physicnchemical conditions, especially high oxygen fuga.cities, at or near the site of emplacemPnf that fAvorPri the dPvPlopmPint rif 1...qwri-z-) mAgnPtifP AltPrAtinn (!=;illifnP, 1q7q); and 4) the pritAi.-;iilm-rich nature of shoshonific m.-3.gmatisrri in the Farallon Negro region (cf, i.::e=-.Pr Pt al., I ci77) cornparAhlP to gold-rich r-Thoshnnifir porphyry copper occurrences elsewhere such as Vunda, Fiji, GAlorP CrPP1<, British nolu-mbia., and Marian, Philippines (Sutherland Brown, 1q7R; 1-:Ps1.,---r et A-1., 1q7f----; lci77; Hollister, 1q7R; Lawrence, I 97R; !=;illitoP, 1q7q). The fact that precious TriPtaVE Are found in porphyry roper systems, epithPrmAl polymPtAllic veins-, and geothermal hot springs throughout the Farallon Negro region, combined .....iith the presence of detectable gold and s-ilve,- as "background" values in ordinAr y volcRnic rocks., 7411 indicate that A unique pr ECifil lc; mint. al "=.1c4nRfltrp" 1-(74s been maintAinPri through magmatic 2:nd hydrothermal processes more or lerz-is, continuously since the inception of volcanism in the area F.,ome ten million years ago. The lArgP number of minerAli,,zp.,-Ioccurrences and hydrothPrmally altered ..---krpAs, combined tArith the permeable and t.A.)Pli fractured nAtu.rP of largP volumes of potentially gold-bearing volcanic bri.cciAs, makP the Farallon Negro region ,3n exceptionally attractive exploration target for bull.: .minAblP, low grAdP prPciol ,---. metal deposits. 151 EDC:iEMENTS The author wishe.-s to express c-..inc-PrP gratitude to Mr. .1. M. Gifilbert for mRking th1 s uniquP Frniprf tñ ArgPntin:.---.. poEs1b11 ApprPciat inn is 741So inxtimnded tow.rds th tsff of fhp Tr,==.fifilfo dp Trr,./ps-f¡.-_-,arionpfs tt.linprs of fhp Uffiversid.-Ed r-TRrion.R1 dP !=;.n The frPP room and bo..=erd provided by ..iltviAD during thí. s-tAy in Farallon Negro rP gratefully :=icknow1PdgPd, fhP laboratory s.--,ervices and field support provided by thP DirPccion RPT1Pral de FabricacionPc. (.0i=4FM). ThP uthor i 10 gr.tifui for fhP TriAnytimu1.ting di=.cussions AJ1th Gonzalez of the !=iPrvicio MinPro r..cion:q1 in TurumsnTd with colleague's Andrew !:;tults A.nd !--;uchomPl. Finally, Dr. n..1. ER-,--i-oP is thankPd -for his valuable guidance and constructive criticism during the fluid inclusion stHdy. This project was supported by a.. United Nations E...;pecil !=;PrvirP Agreement (IndPx Nn. 2f;5R44 ojPct .=.yrnbol: Arg/R1-005) in conjunctinn kivith DM te Instituto de Investigaciones MinPrAs, 152 REFERENCES:E _; Ahmad, S. and Rose, A. 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Geol. !=;Hrvey iour.P.E.,23.1 V. 55 110. p. Potter, R. W. II, Clynne, M. A., and Brown, D. LI irj7F;, FrPPzing point depress-ion of aqueous sodium chloride solution=.: Econ. Geol.,i. 73, p. 284-285 Quartino, lqF83, Infprpt-pi-qcirri gpoc-4-jr.-== 6.1 Hisfrito vo1rniro de FArAllrin Nfragro (Provincia naf a ma c): An.is de Primer.' .lornadas Geolclqicas Argentinas (Buenos Aires:),v. 2, p. 2=i7-278 Roedder, E., 19R7, Fluid inclusion=. as ïÛP iPs of ore -roil& in BRrnPa.-,-, H. L., Pd., Geochemistry of HydrothPrmRinrP N.IPI.A.) York, Holt., RinPharf, Rnd Winston, P. FilFi-Fi74 RoPddinr, E 1971, Fluid inclusion studs n thP porphyry-typP :re dipcsitsf Bingham, Utah, Butte, MonfRnR, Rnd Color-Ado: Econ. Gect1v. GE, p. 98-130 RoPddPr, E.Pd., lq77, Foreword: Fluid .Inclusiorci r-Research ProcPPdingi----. cCOFFI, Ann Arbor, Mich., Univ. of Mii-hiqRn Pri.,=.=.1 v. I O, p. RoPddPr, E., 1c17c.i, FlHid inclusions as .amplPs of ore fluids in B.-E.--trnes, H. 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