Grilles, séries, pixels. Le protocole Becher comme mécanisme proto-numérique chez Jörg Sasse, Thomas Ruff et Andreas Gursky

Sitôt inventée, la photographie fut exploitée comme outil d'observation et d'archivage des monuments, des paysages, des types humains ou encore des oeuvres d'art. Dans ce cadre, furent fréquemment adoptées des procédures de prise de vue strictes, paraissant à même d'assurer le caractère fiable et exploitable des représentations obtenues. À la période contemporaine, de façon récurrente, la photographie se trouve encore associée à des protocoles fixés au préalable ; ceux-ci sont établis à des fins variées et selon des modalités diversifiées. On peut parler de « protocole » dès lors que le photographe choisit consciemment d'observer lors de la prise de vue un certain nombre de règles (découlant ou non de la technique photographique) dont il fait explicitement un mode opératoire. Cet ouvrage tente d'explorer les causes et les enjeux de cette combinaison récurrente de la photographie avec une procédure préétablie.

The role of a transcrustal shear zone in orogenic gold mineralization at the Aijanahalli Mine, Dharwar Craton, South India

The Ajjanahalli gold mine is spatially associated with a Late Archean craton-scale shear zone in the eastern Chitradurga greenstone belt of the Dharwar craton, India. Gold mineralization is hosted by an similar to100-m-wide antiform in a banded iron formation. Original magnetite and siderite are replaced by a peak metamorphic alteration assemblage of chlorite, stilpnomelane, minnesotaite, sericite, ankerite, arsenopyrite, pyrite, pyrrhotite, and gold at ca. 300degrees to 350degreesC. Elements enriched in the banded iron formation include Ca, Mg, C, S, An, As, Bi. Cu, Sb, Zn, Pb, Se, Ag, and Te, whereas in the wall rocks As, Cu, Zn, Bi, Ag, and An are only slightly enriched. Strontium correlates with CaO, MgO, CO2, and As, which indicates cogenetic formation of arsenopyrite and Mg-Ca carbonates. The greater extent of alteration in the Fe-rich banded iron formation layers than in the wall rock reflects the greater reactivity of the banded iron formation layers. The ore fluids, as interpreted from their isotopic composition (delta(18)O = 6.5-8.5parts per thousand; initial Sr-87/Sr-86 = 0.7068-0.7078), formed by metamorphic devolatilization of deeper levels of the Chitradurga greenstone belt. Arsenopyrite, chalcopyrite, and pyrrhotite have delta(34)S values within a narrow range between 2.1 and 2.7 per mil, consistent with a sulfur source in Chitradurga greenstone belt lithologies. Based on spatial and temporal relationships between mineralization, local structure development, and sinistral strike-slip deformation in the shear zone at the eastern contact of the Chitradurga greenstone belt, we suggest that the Ajjanahalli gold mineralization formed by fluid infiltration into a low strain area within the first-order structure. The ore fluids were transported along this shear zone into relatively shallow crustal levels during lateral terrane accretion and a change from thrust to transcurrent tectonics. Based on this model of fluid flow, exploration should focus on similar low strain areas or potentially connected higher order splays of the first-order shear zone.