The Geology of the Bajo El Durazno Porphyry Copper- Gold Prospect, Catamarca Province, Argentina Item Type text; Thesis-Reproduction (electronic) Authors Allison, Antonia E. Publisher The University of Arizona. Rights Copyright © is held by the author. Digital access to this material is made possible by the Antevs Library, Department of Geosciences, and the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author or the department. Download date 08/10/2021 22:01:36 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 A Prepublication Manuscript Submitted Ti, the Faculty of the 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. Brief quotations trorri this manuscript are allowablewithoutspecial permission, provided that accurate acknowledgementwledge'tiient of source is made. Requests -for permission for extended quotation from or reproductionif this manuscript in whole or in part may be granted by the head of the department, or the graduate student coordinator, when in their judgement the proposed use of the material is in the interests of scholarship. In all other instances, -r wever, permission must e obtained from the author. SIGNED: APPROVAL BY RESEARCH ADVISORY COMt%-°1ITTEE This manu =_ ias been approved for submission on the date shown below; CIL -s-fr 199g , )5V Gra e Student Dn rtator,or Date I Hea Department AB:=;Tr*'A!_T 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 r=` = q 1:' Ii !t f O dU i - t i _in 1 0 o n v s o c a a s n a o an s s r a v r v a o v r r e r r e o r o v a o a ar o a r o o a s e e n v o e n e a a v a G C D O ra r a o a r v a G s r o v a v n o n n r c o a c t r. 1 r r . 1 t;-i L- ;= r- k== ,r i r iu>-v li. F,L:r-posP aiilrl Method of Iniies1 ig._1i_'fiaavlaQOaCCII:aa:ECateaaaceeaDa20OnaEaC¢eaaeaascasaaDeaa 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 1aGeasQ¢aeesteeseaefeeaaoeaeeeaEeaaseeonicsaaaetaeoe¢escasassasaaaocoal 34 Volcanic Host r,lii-i:_eetaaaaQaae2la:IIas¢saelaaacsevIIaaso02traia:asaQSeelss 4 1 ... S( : r- . I¡ÿ{i ri`ri.j ~ . k '- 7 . 1 a G e C e a G n e e C C e a C C a Q CC e a G 1 C e C a e L C n a a C C C G e e C C e La a e a C C C e G C a C 1 G a C Sediments and ri PdimL (i L R +J Rocks 42 Hy 3 rct - C iAl QQDQQEQQDDCDEciao II DEQ45 Alte-ration S i- l-1 e l Ì Ìe 45 riT.='=ijiii-_i i ii- ^ ILNr a.L 1_ I nQaCaaCaoaal301223002eaIIQaCCESaCaaa2:taarII2L 1 Fi' ripy'liL ii- oaeasvaaasc:: :sa r+lagnlF1-i1-P;lfPr =! iifrieaavseesaaxaevaae:axsaesveveavsvcevveaveveaveveavaxaetaea2a r11 Phyllie-- l T P r ái i r n --. A.L_r a t i_nsossoanssanvavaassosaeeaeaasa aa:oae :eoeoa ea el: aoacna enc 2nna r-IirtkLilP aeasslaQaeaa77 iti-1 i -1 ^s -t {' }- -i 4iSet iii=li_ifi ii.i i1Ìii-iiii=ir¡rvaneesvvavcvCCSVavaataav:sevaeaaaasevevaavvavaveavv _ Pn± r ; 7_i rhP;'ra i Anomaly / 8 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 IPr i'iÑr =i l Li Ihi-6r AAF if i liJ ri(li.¡ ¡¡ i iii7-i l Ì¡ i_-r 1- 1 f P =I!_'.t _ILI L-! IL ,...! x e a c a s a P a a a: s a a e a a J.. ij- 1'1 ¡i.RL1r` ! _3_I.i ° r.ILr'.Sj aL.i pus.: ` IIL'L.IL! r .'_,/ -s r, !! 3! fÉlr_!G i(iI s-- i anaasaanaaCasaeeaaaeaea Ile: 1112eaasasCCSCacasaa = _1 J¡i ¢ a a n o sl a s s n a 0 e a e a o m a a a a a a a e a a a s a i r a C a x a a 0 s a a C 11 a a C a 4,= i:1Ì E, :awe s x a a E C a a a n a e t a o c 0 e as C 0 : a : t an : 8í i l .