Jurassic magmatism and Au-Ag mineralization in the Deseado Massif (Patagonia Argentina): Lead and sulfur isotopic studies

. Lead isotope analyses of volcanic rocks and hydrothermal minerals and sulfide isotope analyses of sulfurs from Au-Ag La Josefina prospect (Deseado Massif geological province, Argentina) are reported. Lead isotope ratios for sphalerite and pyrite have mean values of 18.48, 15.69 and 38.61 for 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb, respectively, and are similar to Pb isotope ratios for the Jurassic volcanic rocks of Bajo Pobre and Chon Aike Formations. δ 34 S ‰ values of sphalerite, pyrite and galena cluster between 1.3 and 3.8 ‰, suggesting that the sulfur at La Josefina was primarily of magmatic origin. These results are interpreted to indicate that the Jurassic volcanic rocks are closely related with low sulfidation Au-Ag occurrences.


Introduction
The Deseado Massif (DM) is located in the southern Argentinean Patagonia (Fig. 1). In this region, the most important unit is a volcanic complex of upper to middle Jurassic age with bimodal features marked by andesitic and rhyolitic magmas that conform the Bajo Pobre and Chon Aike Formations, respectively. These units crop out over more than half the area of a wide plateau. The geochemical analyses of the Jurassic volcanism show a calc-alkaline magmatism with a volcanic arc affinity.
These volcanic rocks are related to the early history of the Gondwana break-up and were deposited in the backarc Patagonian Andes during a long period (187 to 144 My) of extension . The geotectonic environment of Patagonia in the Jurassic times was characterized by very slow subduction in the Pacific margin of Gondwana (Ramos 1988). Pankhurst et al. (2000) support the idea that the volcanic rocks were probably generated by a variety of mechanism, among which the melting of pre-existing continental crust was a dominant process.
These volcanic rocks host the Au-Ag veins of Cerro Vanguardia, the principal ore deposit currently in production, and also host the Ag-ore shoot worked in the Marta Mine, as well as some other important prospects like Manantial Espejo, Huevos Verdes-Cerro Saavedra-El Pluma, El Dorado-Monserrat, Cerro Negro and La Josefina. Most of these prospects (Schalamuk et al. 1997) correspond to the low sulfidation epithermal type of deposits. The last prospect, currently under exploration, is located in the central part of the DM and the mineralization is represented by Au-Ag bearing vein systems (up to 300 g Au/t).
The objective of this paper is to present the lead and sulfur isotopic data from samples of hydrothermal sulfides and volcanic rocks obtained at the La Josefina prospect.
There are few isotopic studies of the DM, only two lead isotope data for the Jurassic volcanic rocks are indicated by Kay and Gorring (1999) and five lead isotope measurements on galena from several ore deposits of the DM were showed by Schalamuk et al. (1997) Isotope sulfur ratios carried out in samples from Au-Ag epitermal prospects (Cerro Vanguardia, Manantial Espejo, El Dorado-Monserrat and Cerro León) in the DM, were reported by Echavarría (1997), Schalamuk et al. (1998) and Jovic et al. (2004). The δ 34 S values range between -2.8 and 4.2 ‰, indicating a magmatic source for the sulfur.

La Josefina geology
The La Josefina prospect ( Fig. 1) includes lower Paleozoic low grade metamorphic muscovitic schists, metavolcanic and calk-silicate rocks with minor magnetitic levels and tourmalinites of La Modesta Formation (Moreira et al. 2005).
The Bajo Pobre and Chon Aike Formations represent the Jurassic volcanic rocks. The Bajo Pobre Formation, mainly andesitic-dacitic rocks, include porphyritic and aphanitic flows partially self-brecciated, porphyries, sills that intrude the low angle dip schist of the La Modesta Formation, and ring-dikes surrounding rhyolitic domes. The Chon Aike Formation cover approximately a 60 % of the area. The lower member outcrop on the west part, and consists of crystal-rich quartz-feldspar-biotite welded ignimbrite with some flatten pumice-rich levels that was dated at 153.2 ±3.6 My (K/Ar on biotite; Arribas et al.  Pará, Belém, Brazil 1996). On the eastern part dominate a welded pumicerich, crystal-poor welded ignimbrite whose lower levels carrying 5-20 cm crystal-rich-ignimbrite lithic fragments, whereas in the upper levels these lithics are smaller and sporadic. A rhyolitic dome-complex outcrops on 10 km 2 in the north-northwest of the area and some smaller rhyolitic domes extend to the northwest, both with slowly propylitic, pyritic and argillic patch of hydrothermal alteration. These domes intruded the Paleozoic metamorphic rocks (La Modesta Formation).
Paleogene and neogene basaltic flows cover the Jurassic volcanic rocks.

La Josefina Au-Ag epithermal ocurrences
The mineralization at the La Josefina prospect consists of system of low sulphidation epithermal veins and veinlets filled mainly by quartz and chalcedony with minor adularia, barite and platy calcite. The primary assemblages are composed by native gold, electrum, specularite and pyrite, arsenopyrite, galena with freibergite inclusions, sphalerite, chalcopyrite and bornite and a supergenic association consist in limonites, chalcocite-covelite, cerusiteanglesite and malachite.
These occurrences are distributed in a curved belt about N-S 12 km long and between 500 and 1200 m wide. The age of the mineralization is not well constrained; a Rb-Sr errorchron gave an age of 156 ± 2 My (Fernández et al. 1999). The veins are spatially associated with superficial epithermal occurrences, like hydrothermal eruption breccias, silica sinter terraces, geyserites, carbonate stromatolitic deposits, and steam heated blankets, mainly to the north of the prospect.

Samples
The samples collected for the lead isotopic study of the La Josefina prospect consisted in intermediate-acid Jurassic volcanic rock from Bajo Pobre and Chon Aike Formation (feldspar separates) and hydrothermal minerals from veins (pyrite and sphalerite). The isotopic Pb composition were determinate on 13 samples in the Pará-Iso Laboratory, Instituto de Geociencias, Universidade Federal do Pará, Belém, Brazil.
Seven sulfide paragenetic samples, including separates of pyrite (3), sphalerite (3), and galena (1), were analyzed for their sulfur isotope composition. Analyses were carried out at the Stable Isotopic Service from Salamanca University (Spain). The results are given as δ 34 S ‰ values relative to the CDT standard.

Results
The analyses of Pb in samples from La Josefina prospect are summarized in the Table 1 and plotted in the Zartman and Doe (1981) diagrams (Fig. 2) The lead isotope composition of these sulfides are in general slightly more radiogenic than the volcanic host rocks.
The analyses of δ 34 S ‰ are summarized in the Table 2. Sulfur samples in La Josefina show δ 34 S mineral values between 1.3 and 3.8 ‰. The δ 34 S high value for sphalerite (13.1 ‰) demonstrate an overall increase. This could be explained by changes of the hydrothermal fluid pH produced by the interaction with the volcanic rocks.

Conclusions
The δ 34 S values of pyrite, sphalerite and galena indicate that their sulfur was derived from a magmatic sulfur source with scarce supergene activity. Pb isotope ratios of pyrite and sphalerite, are most similar to those of the volcanic rocks and suggest that they are the likely source.
The correspondence of both sets of values with the orogenic growth curve of Zartman and Doe (1981) suggests further that varied sources contributed to the lead of the magma.
Lead isotopes obtained from galena samples by Schalamuk et al. (1997) although have slightly higher values, do not show significant differences from the samples studied here. The δ 34 S results in La Josefina are similar to those in several ore deposits in the DM.
Based on these features, there is a genetic link between the DM low sulfidation epithermal deposits and the bimodal magmatism of the Bajo Pobre and Chon Aike Formation.