Mapping geology for the exploration of remote geothermal fields: The Tocomar Geothermal System case study (Puna plateau, Argentina)

BÁEZ, WALTER; GROPELLI, GIANLUCA; GIORDANO, GUIDO; AHUMADA, MARIA FLORENCIA; ALDEGA, LUCA; BECCHIO, RAUL; BIGI, SABINA; CARICCHI, CHIARA ; CHIODI, AGOSTINA; CORRADO, SVEVA; DE BENEDETTI, ARNALDO; FAVETTO, ALICIA; FILIPOVICH, RUBEN; FUSARI, ALESSANDRO; INVERNIZZI, CHIARA; MAFFUCCI, ROBERTA; NORINI, GIANLUCA; PINTON, ANNAMARIA; POMPOSIELLO, CRISTINA; TASSI, FRANCO; TAVIANI, SARA

Sicilia

Workshop; IAVCEI Commission on Volcano Geology - 3erd International Workshop - Etna-Aeolian islands; 2016

IAVCEI commission on Volcano Geology

The reconstruction of the stratigraphical and structural framework of geothermal areas is fundamental for understanding the relationships between cap rocks, reservoir and circulation of geothermal fluids and for planning the exploitation of geothermal fields. The Tocomar geothermal volcanic area (Puna plateau, Central Andes, NW Argentina) is located in an area characterised by numerous volcanic edifices active from Miocene to Holocene (e.g. from Aguas Calientes Caldera to Tuzgle volcano) and is crossed by the active NW?SE trans-Andean tectonic lineament known as the Calama?Olacapato?Toro (COT) fault system (Giordano et al., 2013; Norini et al., 2013).The stratigraphy of the Tocomar geothermal volcanic area allows recognising pre-Tocomar basin units and the volcano-sedimentary basin fill. The oldest basement unit outcropping in the Tocomar area is the Faja Eruptiva Fm. formed by Ordovician metagranites and minor mafic intrusive rocks that intrude the Pre-Paleozoic metasedimentary Puncoviscana Fm. Above the Palaeozoic basement rocks, the oldest rocks preserved are the Cretaceous?Oligocene syn-rift and post-rift sediments (Salta Group) that were deposited along narrow grabens in rapid subsidence (Salfity and Monaldi, 2006). They are made of poorly sorted conglomerates and breccias, and fluvial?lacustrine sandstones, with a typical red-purple colour (syn-rift Pirgua Subgroup). The youngest pre-Tocomar basin unit is represented by extensive dacitic crystal rich ignimbrite sheets sourced between 17 and 10 Ma from the Aguas Calientes Caldera (Petrinovic et al. 2010). The ignimbrites are extensively lithified with diffuse vapour phase crystallisation of the matrix.The Tocomar basin fill starts with a 12 m thick (minimum thickness) tabular and internally stratified medium to coarse sandstone, inter-bedded with thin mudstone levels, related to distal alluvial fans sedimentation (Red Sandstone unit). Coarse alluvial/fluvial sediments (Green Conglomerate unit) with a minimum thickness of 40 m filled the basin after a tectonic?-erosional phase. A coeval mafic phreatomagmatic activity is evidenced by pyroclastic surge deposits inter-bedded with the Green Conglomerate unit. Also, the presence of ~ 5m thick travertine at the top of the Green conglomerate unit points out the onset of the geothermal activity into the Tocomar basin. The Tocomar 1 pyroclastic sequence (fall/surge/flow deposits) related to a rhyoltic phreato-plinian eruption (0.55 Ma; Petrinovic and Colombo, 2006; Petrinovic et al., 2006) covers by a mild angular unconformity the Green Conlgomerate unit. This felsic volcanism was followed by other two rhyolitic phreatomagmatic eruptions associated with different vents that formed the pyroclastic sequences (dominantly surge deposits) of Tocomar 2 and Tocomar 3. Finally, thin alluvial, fluvial and aeolian recent deposits cover largely the Tocomar basin.