Nora Rubinstein

Processes Controlling Porosity and Permeability in Volcanic Reservoirs from the Austral and Neuquén Basins, Argentina

Volcanic rocks develop primary and secondary porosity and permeability, depending on both their lithology and the sequence of processes involved in their formation. Primary processes (welding, deuteric crystal dissolution, gas release, flow fragmentation, and crystal shattering) may lead to high porosity and permeability, the best example of which is a nonwelded ignimbrite with well-developed gas-pipe zones. Secondary processes (different types of alteration), however, tend to decrease primary porosity. However, certain secondary processes, such as dissolution and hydraulic fracturing, may contribute to enhance total porosity and permeability. These conclusions were developed through a systematic review of reservoir quality in volcanic rocks, integrating lithology and process interpretation with petrophysical data. The said information was taken from selected cores of volcanic rocks from the Serie Tobfera unit in the Austral Basin and the Precuyano unit in the Neuqun Basin, Argentina. A clear understanding of both primary and secondary processes may serve to predict the quality of volcanic reservoirs and could be used as a guide for oil and gas exploration and development in many parts of the world.

Subgreenschist facies metamorphism of metabasites from the Precordillera terrane of western Argentina; constraints on the later stages of accretion onto Gondwana

The metamorphic character of metabasites of the Precordillera terrane of western Argentina provides constraints which help to distinguish between contrasting models for the tectonic settings during the late stages of the accretion of this terrane onto the margin of Gondwana in late Lower Palaeozoic times. Metamorphic conditions are constrained to a low-temperature/low-pressure setting at ca. 250–350°C and 2–3 kbar, with geothermal gradients of between 30–35°C km −1 . The metamorphic character of the rocks and the derived P-T conditions demonstrate that the metamorphism did not develop in an ocean-floor setting as has been previously proposed. Neither is the metamorphic character compatible with being derived from enhanced heat flow in an extensional setting at the western edge of the Precordillera in the mid-Ordovician following docking against Gondwana. The most compatible model is one in which the metamorphism developed as a result of collision between the Chilenia and Precordillera terranes in early Devonian times. In addition, the metamorphism constrains the subduction depth to ca. 15 km had the leading western edge of the Precordillera been subducted westwards beneath Chilenia. However, the metamorphic character is perhaps better linked to subduction of the leading edge of Chilenia eastwards against the Precordillera.

Metallogeny of the Triassic-Jurassic Rift-Related Mineralizations in Argentina

The Triassic-Jurassic NNW-SSE extensional phase affected the western margin of Gondwana and marked the beginning of the fragmentation of this supercontinent. Extensional faulting began in the Late Permian in the northern areas, becoming younger to the south, i.e. Jurassic in Patagonia. During this stage extensive rhyolitic ignimbrite magmatism developed. The rift system, was filled with continental sediments that locally have intercalated bimodal, predominantly mafic, volcanic rocks. The region affected by the Triassic-Jurassic rifting in the Argentine territory hosts a number of deposits that have no direct link with magmatic activity and whose age has previously been attributed, in general, to the Mesozoic (Zappettini, 1999), or are herein ascribed to this event. These include the Se-rich polymetallic deposits, five elements deposits, Pb-Zn-Ag simple veins, epithermal Mn, fluorite and barite veins. The various types of deposits analyzed have a wide regional distribution reflecting the extent of the processes of rifting that started during the Triassic and reached the Cenozoic, generating thermal and fluid flow anomalies leading to hydrothermal mineralizing processes. Where there was a favorable source, fluids collected selected elements resulting in a mineralogical specialization and inhomogeneities in the distribution of the mineralization types along the belt affected by rifting. Characteristically, where the rifting affected Paleozoic or older sedimentary sequences, polymetallic and barite type deposits were generated. Where rifting affected acidic Triassic-Jurassic volcanic and volcaniclastic sequences the fluids originated fluorite mineralization. In the cases where there was an associated restricted basic magmatism or by leaching or volcanic sequences Mn ore bodies were formed. In all cases, this rift environment has been favorable for the emplacement of alkaline to subalkaline acid magmatic rocks. In these cases, besides the presence of fluorine and fluorine-rich fluids, evidenced by the presence of fluorite and / or topaz, there are associated REE mineralizations (Rodeo de los Molles, Sierras Pampeanas of San Luis and Rangel district, Puna of Salta), Mo-rich stockworks (Elsiren and German) and epithermal gold deposits (Pantanito, and, in general, the Au-Ag epithermal deposits of the Deseado Province). In this context the Pb-Ag polymetallic mineralization of the Navidad deposit formed in relation to a rift-related magmatism represented by volcanic and pyroclastic deposits associated with biochemical and epiclastic sediments deposited within the Canadon Asfalto hemigraben (e.g. Fernandez et al., 2008). From a global point of view, the fluorine, manganese and polymetallic deposits related to extensional environments may be associated with precious metal mineralization (Au-Ag) and Mo-rich porphyries, so that their presence can be used regionally as an exploration guide (e.g. Wallace, 2010).