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Aetas II South American Symposium on Isotope Geology

Rb-Sr AGE OF THE LA JOSEFINA EPITHERMAL MINERALIZATION AND ITSRELATION WITH HOST VOLCANIC ROCKS. MACIZO DEL DESEADO, SANTA CRUZ /

PROVINCE, ARGENTINA /

R.R. Fernandez', H.J. Echeveste' , C.C.G. Tassinari 2 and LB. Schalamuk'

1 INREMI. Calle 47 N°522. 1900 La Plata, Argent ina. rfernand@museo.fcnym.unlp.edu.ar2 Instituto de Geociencias, USP. Rua do Lago, 562 -USP, (11348) Sao Paulo, Brasil.

Keywo rds: Age, Sr isotope, epithermal, Jurassic, Macizo del Deseado.

INTRODUCTION

The "Josefina" epithermal prospect is locatedin the central portion of the Deseado Massif (MD),which is a geological and geographical region thatcover the east and center of the Santa Cruz Provincein the southern Patagonia. It is characterized by alarge Middle to Upper Jurassic ignimbritic-rhyoliticplateau outcropping in a surface of about 30,000 knr'.Since the ·eighties, after the discovery of epithermalauriferous mineralizations in Jurassic volcanites,strong prospection activities began in this region ofthe Patagonia.

This paper present the isotopic data obtained atthe "Josefina" prospect from exploration activit iescarried out by the Instituto de Recursos Minerales inDeseado Massif. These data allow establishing atemporal connection between auriferous minera­lization and Jurassic volcanic activity.

Rb-Sr data of the Chon Aike Fm. have yieldedages of 161 ± 5 Ma (de Barrio, 1993) and 168 ± 2(Pankhurst et al., 1993) in the NW and NE of the MD,respectively. Alric et ai. (1996) obtained Ar/Ar datafor sanidines of the Chon Aike Frn., which yielded177 ± 0.7 Ma in the NE and 151.5±0....5 Ma in the NWof the MD. Arribas et al . (1996), in biotites fromignimbrites and acid lavas from the Chon Aike Fm. inthe area of the Josefina, obtained KIAr ages between148.8 and 153.2 ± 3.6. These same authors alsoobtained data on illite (142.3 to 152.4 ± 3.6 Ma) andadularia (138.5 ± 3.3 Ma) in veins of the Au-Ag CerroVanguardia district , and from adularia (124.8 to 142.6± 3.5 Ma) of the Ag-Au Manantial Espejo district .Therefore, the Jurassic volcanism at MD would bebetween 149 and 177 Ma. This significant range iscoherent with the extension of the ignimbritic plateau,although it may in part be due to the different

methodologies used. On the other hand, taking intoaccount a certa in "rejuvenation" because of adulariaalteration (Arribas et ai., 1996), the ages of themineral s accompanying the minera lization of Au andAg are in agreement with those of the hostedpiroclastic rocks.

GEOLOGY AND MINERALIZATION

Del Blanco et al. (1994), Echeveste et al.(1995), INREMI (1996), and Fernandez et al. ( 1996).have presented the. geological and metallogeniccharacteristics of the La Josefina prospect. The oldestunit, which 'outcrops in the NW of the area,corresp onds to the metamorphites of the La ModestaFm., which are covered by Jurassic volcanic rocksfrom the Chon Aike Fm. mainly (ignimbrites andrhyolites), and, to a lesser extent, by the Bajo PobrePm. (andesite-dacites). There are also exposures ofash fall tuffs (La Matilde Fm.) coeval with the former .Overlying the Chon Aike Fm., two discordant basalticIevelshave been recognized, with a thickness between5 and 20 m, which were assigned by Viera andMarquez (1975) to the Eocene. In the lower parts ofthe relief there are outcroppings of Quaternary basaltflows, which present a greater distributi on in the area.

The minera lization at the Josefina consists ofsystem of veins , veinlets (which occasionally formstockworks), breccias, and sinter. They are fillingstructures composed mainly by quartz, chalcedonyand opal (with scarce adularia), which carry nativegold, electrum, and basic metals (galena, sphalerite,and chalcopyrite) . There are also barite veins; thebarite is either an early mineral associated to coarse­grained quartz, or forms monomineralic up to 40 emthick veins. It is usually total or partially replaced byfine-gra ined silica (opal or chalcedony).

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Aetas II South American Symposium on Isotope Geo logy

30.0

15.0 20.0 25.0

Williamson (1968)f."i ' IMOj= 150-+: 4·..·"··..-t--:

I :

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, 81 RbrSr JI I

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.'

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78R~~~~~85 i I

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I0.820 t

Figure la: Errorchrona of the volcanic rocks from the ChonAike Fm. for the region "La Josefina",

Figure 1b: Errorchrona of altered rocks. The 87Sr/86Sr ratioof the bar ites is interpreted as coincident with the initialrat io of the mineralizing fluids

rocks is coherent with this picture, alt hough a bithigher than the va lues obtained by other authors inother areas of the Macizo del Deseado, which varybetween 0.7066 (Pankhurst et al., 1993) and 0.70588(de Barrio, 1993) .

RESULTS

The samples collected for the geoc hro­nologica l study of the Josefina prospect consisted insurrounded veins hydrothermally altered wall rocks,barite from veins associated to the auriferousmineralization, and unaltered rhyolitic domes andlavas, which were interpreted as the later volcanicevents of the Jurassic vo lcanism for this area byFernandez et al. (19 96) .

The isotopic Rb and Sr compositions weredeterm ined on whole-rock samples at the Instituto deGeociencias, Universidade de Sao Paulo , Brazil

ANALIZED MATERIALS

The hydrothermal alteration of the area appearswith different characteristics (Ro lando y Fernandez,1996; Tessone y Etcheverry, 1998). The mostimportant, because of the volume of rock involved, is

. the si licification as a "silica cap " associated tokaolinizatio n located in the northeast sector. The wallrocks of the veins and stockworks present a med iumsil icifi catio n with ser icite, illite, and occasionallyscarce dickite, form ing halos of 4 to 5 m widths. Insome sectors, the wall rock shows propilitizati on,with chlorite and ca lcite, and a part ial chloritizationof the biotit e with iron oxides segregation.

The results of the isotopic ana lysis are shownin Table 1. For the barite samples, only the rat io87Sr/86Sr was determined .

Samples align follow ing stra ight lines with arelative inc lination for the ages of 150 and 156 Ma(Figs. l .a and b). Figure 1.a was built with theisotopic values taken from samples of rhyolitic lavasof the Chon Aike Fm ., whereas Fig. l.b was built with

.values taken from samples of hydrothermally alteredhost -rocks, assuming the initial ratio to be the averageof that obtained for the barrettes accompanyingminera lizat ion (0.707 052). .

Th is ratio would represent that of the flu idsresponsible for both phenomena - hydrothermalalteration and mineralization A comparison betweenthe two graphics suggests that both phenomena - late The initial _ isotopic composit ion of thevolcanic activity and hydrothermal mineralization/ mineralizing flu ids, measured in 5 barite samplesalteration, 150±4 and 156±2 Ma respectively (2a) - associated to the mineralizat ion, varies betweenare to be cons idered as results of the same geo logical 0.70695 and 0.70719. These rati os ind icate a very lowevent, and are therefore contemporary. The initial variability and a coinc idence w ith the init ial rat io ofratio 87Sr/86Sr=0.70754 for the Chon Aike volcanic the unaltered and altered volcanic rocks .

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Aetas II South American Symposium on Isotope Geology

d b ' t . dil ti* Sda e : - r ISO0PICra lOS an error. ampes ana ze >y ISO OPIC I U IonSample N o . Material Rb Sr 87Rb/86Sr Error 87Srl86Sr Error

133 16/GE0 33 Rock altered 40.62 20.52 5.7355 0.0482 0.7196 2 0.00008133 17/GE034* Rock altered 164.86 12.06 39.9049 0.3656 1.79705 0.000 7513318/GE035* Rock altered 139.35 18.69 21.678 0.1 834 0.75525 0.00010133 19/GE0 36 Rock altered 135.4 8.8 44 .796 1.259 0.76 813 0.0001313320/GE037 Rock altered 316.5 54.I 16.985 0.47 9 0.73945 0.0000 71331 5/GE0 29* Lava 227.6 143.5 4.595 0.130 0.71776 0.0000813572/GE042 Lava 372.5 86.4 12.508 0.353 0.7322 1 0.0001113573/GE043 Lava 300.8 78.8 11.071 0.312 0.72901 0.0000713574/GE044 Lava 399.5 75.6 15.340 0.432 0.73853 0.0000813575/GE045* Lava 292.31 48.93 17.3537 0.1476 0.74465 0.0000 713576/GE046* Lava 337 .01 47.52 20.6155 0. 1710 0.75174 0.0000713709/Geo l Barite 0.7070 1 0.00008137 l"O/Ge02 Barite 0.70699 0.000091371 1/Ge03 Barite 0.70695 0.0000913712/Ge04 Barite ~ : 0.707 12 0.0000913713/Geo5 Barite 0.70719 0.00008

T bl I Rb S . t . t

The lasts without the barite samples have aninitial Sr rat io of 0,70 70.These results perm it interprettha t the flui d has suffered a strong interactio n with thehost-rocks and/or that the Sr, and possibly the gold,come from a homogeneous source, wh ich is probablythe hosted volcanic rock itself. ,;

CONCLUSIONS

The errorchrone Rb-Sr age of the latevolcan ic activity, which was determined in this paperfor the area of La Josefina, would be around 150 ± 4Ma, and it is coincident with the KJAr va lues obtainedby Arribas et al. (1996), which were between 149 and153 Ma. The thermal center, which represents thismagmatic activ ity, was responsible for themobilizat ion of the fluids caus ing the hydrothermalalteration, and bar ite, silica and preci ous metalsprecipitat ion, among others. The errorchrone Rb-Srage determined for the minera lizat ion is 156 ± 2 Ma.Both th is age and the previ ous one are coherentwithin the analytical error (2 sigma); therefore, thevolcan ic act ivity and mineralization shou ld beconsidered as geological processes genetically andtemporally linked occurred at the beg in of the UpperJurassic of Patagonia.

REFERENCES

Alr ic, V., Haller, M., Feraud, G., Bertrand, H. andZubia. M., 1996 . Cronologia 40Ar/39AI' de l volcanismoj urasico de la Patagonia Extrandina. XIII CongresoGeo logic o Argentino. V: 243-250.Arribas, A. Jr, Schalamuk., I.B., De Barrio R.E. andFernandez, R.R., 1996. Edades radimetricas demineralizaciones epitermales aurfferas del Mac izo delDeseado, provo de Santa Cruz. XXIX CongresoBrasileiro de Geologia. 7: 254-257.de Barrio, R.E., 1993 . El volcan ismo acido Jurasicoen el noroeste de Santa Cruz, Argentina. XIICongreso Ge ol6gico Argentino Aetas. IV: 189-198.Del Blanco M .A, Echavarria LE, Echeveste H.J .,Etcheverry R. 0 ., Tessone M.O. and Mondelo R.,1994. Estancia La Josefina, un nuevo prospectoaurffero en el Macizo de l Deseado. Provincia de SantaCruz, Argentina. Internationa l Mining Meeting,Aetas: 93-99 .Echeveste H.J., Echavarria L.E. and Tessone M.O.,1995. Prospecto aurifero "La Josefina ", un sistemahidrotermal tipa hot spring, Santa Cruz. Argent ina. VCongreso Nacional de Geologia Econ6mica. 223-233.Fernandez, R.R., Echcveste H.J., Echavarria L. andSchalamuk, LB., 1996. Contro l volcanico y tect6 nicode fa mineralizacion epitermal de l area La Josefina,

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Macizo del Deseado, Santa Cruz. Argentina. XIIICongreso Ge o16gic o Argentino y rn Congreso deHidrocarburos, Aetas. Ill: 41~54 .

INREMI, 1996. Informe geologico-minero final delProspecto Estancia La Josefina, Provincia de SantaCruz). FOMICRUZ "SE, Rio Gallegos . Inforrneinedito.Kawashita, K., 1972". 0 metodo Rb-Sr em rochassedimentares, Aplicacao para" as bacias do Parana eAmazonas. Tesis Doctoral Inedita,Pankhurst, RJ., Sruoga P. and Rapela, C.W ., 1993.Estudio geocronologic o Rb-Sr de los Complejos ChonAike y El Quemado a los 47°3 0' L.S . XII CongresoGeol6gico Argentino Aetas. IV : 171-178.Rolando P.A, and Fernandez R.R. , 1996 Alteracionhidrotermal del sector nor-oriental del prospectoepitermal La Josefina, Santa Cruz. III Reuni6n deMineralogia y Metalogenesis, Publicaci6n INRE:tvrrN°5 : 49-54.

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Steiger, R.H and Jaeger, E., 1977. Subcomission ongeochronology ruGS: Convention of the use of decayconstant ' in geo- and cosmochronology. Earth andPlanetary Science Letters, Vol. 36 (3): 359.367.Tessone, M.O. and Etcheverry, R.O., 1998.Alteraciones hidrotennales asociadas a alguno sprospectos epitennales del Macizo del Deseado,provincia de Santa Cruz, Argentina. IV Reunion deMineralogia y Metalogenia. Univ. Nac. del Sur: 267­274 .Viera, R.L. and Marquez, MJ., 1975. Infonnegeologico preliminar - Mosaico 4769 - III - D5 .Proyecto 19 NF - La Josefina. Servicio Mine roNacional (inedito).Williamson .H., 1968. Least-squares fitting of astraigh line . Canadian Journal of Physics, Vol. "46:1845- 1847.