Geochemistry of the hydrothermal systems in the Jujuy Province, Argentina, and relationship with the regional geology.

PERALTA ARNOLD, Y.; CABASSI, J.; TASSI, F.; CAFFE, P.J.; VASELLI, O.

Viena

Congreso; European Geosciences Union General Assembly; 2016

European Geosciences Union

The western sector of the Jujuy province (22°-24° S), Argentina, basically consisting of thePuna region (from 3,500 to 4,700 m a.s.l.) that borders the Central Volcanic Zone (CVZ), ischaracterized by sub-meridional ridges that alternate with elongated basins and byextremely voluminous intermediate and silicic ignimbrite deposits, the latter being relatedto late miocenic and pliocenic calderas and central volcanic edifices. In this region, severalhydrothermal discharges with outlet temperatures up to 62°C occur. Among them, theCoranzulí and Pairique thermal emissions show a spatial relationship with miocenicvolcanic complexes, whereas other thermal manifestations (Queñual, Orosmayo, Pirquitas,Arizutar, Cono Panizo and Rachaite) are clearly controlled by the local structural setting.Most of these thermal waters have relatively high total dissolved solids (TDS up to 46,500mg/L), an alkaline-chloride composition and significant concentrations of B, NH4 and SiO2,i.e. they show the typical geochemical features of geothermal brine. Exceptions are theCoranzulí, Orosmayo and Rachaite springs, mainly fed by a shallow Na(Ca)-bicarbonateaquifer.The eastern sector of the province consists of the Eastern Cordillera, composed of aproterozoic basement constituted by the sedimentary sequences of the Puncoviscana Fm,and the Subandean Range, which shows wide east?vergence anticlines whose detachmentlevels are Silurian?Devonian shales. Both regions are separated by a major thrust that risesthe Proterozoic and Eopaleozoic sequences over the Subandean System.The thermal waters in the Eastern Cordillera, namely Termas de Reyes, are characterizedby alkaline-sulfate composition, temperature of ≈50°C and neutral pH.In contrast, in the Subandean Ranges, which is separated from the Eastern Cordillera by athrust rising Proterozoic and Eopaleozoic sequences over the Subandean System, the AguasCalientes springs are characterized by low temperature (from 21°C to 33°C) and a Cabicarbonatecomposition, with no significant evidences of deep fluid contribution.In spite of the clear differences in water chemistry of the thermal waters from the twodomains of the studied area, the chemical composition of the associated gas phases areinvariably dominated by CO2, with minor abundances of organic and atmospheric gases.The carbon isotopic signature of CO2 is consistent with that typically found inhydrothermal fluids, i.e. with significant contribution from a crustal source. Nevertheless,the R/Ra values indicate a significant fraction of mantle He (up to 17%). and the CO2/3Heratios are 2.9×1012, up to three orders of magnitude higher than the MORB value.