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Título: Genesis of the loma galena deposit, navidad district, patagonia Argentina
Autor(es): Bouhier, Verónica
Franchini, Marta
Tornos, Fernando
Rainoldi, Ana L.
Patrier, Patricia
Beaufort, Daniel
Fecha de publicación: 31-ago-2018
Es parte de: 15th Quadrennial International Associationon the Genesis of Ore Deposits Symposium
Resumen: Loma Galena (6410.8 t Ag, 997,130 t Pb) is one of the eight deposits of the world class Ag + Pb ± (Cu, Zn) Navidad district (19.670 t Ag and 1.32 Mt Pb) located in the west of the North Patagonian Massif of Argentina. The deposit was formed in an active continental rift basin during the Middle Jurassic, when subaerial volcanic activity was contemporaneous with lacustrine sedimentation. The normal faults that delimited the horsts and grabens in the deposit and the Sauzal fault, a detachment fault, acted as the main conduits for the upflow of hydrothermal fluids (Fig. 1). The deposit has a lithologic control and the highest metal anomalies are found in highly permeable and reactive autobrecciated mafic volcanoclastic rocks interbedded in the volcano-sedimentary Cañadón Asfalto Formation (Fig. 1). The volcanic rocks are high-K basaltic andesite to dacite in composition and they have yielded LA-ICPMS U-Pb zircon ages of 173.9 ± 1.9 Ma and 170.8 ± 3.0 Ma. Lava flows surrounded by autobrecciated carapace were dominantly extruded in subaerial conditions whereas hyaloclastite and peperite facies suggest contemporaneous subaqueous volcanism synchronous with sedimentation. Lacustrine sedimentation represented by calcareous mudstone with pisolithic and laminated limestone, sandstone, chert and coal overlies the volcanic rocks. A Sr-rich evaporite layer interbedded in the sedimentary succession indicates intermittent drying out of the lake and hypersaline conditions. The mineralization occurs in veins and hydrothermal breccias that lie on the structural hanging wall and footwall of the main faults, in mineralized autobreccia and chaotic breccia at the contact between volcanic and sedimentary rocks, and disseminated in the organic- rich sedimentary rocks. The earliest mineral infill of veins and breccias consists of calcite I and siderite with crustiform, cockade and platy textures, followed by two main episodes of mineralization. During the first mineralization stage precipitated framboidal pyrite and colloform pyrite, marcasite, galena and sphalerite. During the second stage chalcopyrite, bornite and tennantite-tetrahedrite with minor amounts of covellite and digenite replaced the early sulfides. Colloform bands of pyrite and marcasite show the highest Ag contents, although all sulfides and sulfosalts carry Ag. Sulfides are rarely observed in situ, on the walls of veins; they are usually found as transported fragments cemented by several pulses of chalcedony deposition with minor barite.Late infill consists of calcite II, barite II, quartz and strontianite. Hydrothermal breccias present voids with a geopetal infilling with detrital material of sand-silt size and variable composition (chalcedony, quartz, calcite, ankerite,barite, sulfides, rock fragments) with parallel lamination.
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