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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. Thedeposit was formed in an active continental rift basin during the Middle Jurassic, when subaerial volcanic activitywas contemporaneous with lacustrine sedimentation. The normal faults that delimited the horsts and grabens inthe deposit and the Sauzal fault, a detachment fault, acted as the main conduits for the upflow of hydrothermalfluids (Fig. 1). The deposit has a lithologic control and the highest metal anomalies are found in highly permeable andreactive autobrecciated mafic volcanoclastic rocks interbedded in the volcano-sedimentary Cañadón AsfaltoFormation (Fig. 1).The volcanic rocks are high-K basaltic andesite to dacite in composition and they have yielded LA-ICPMS U-Pbzircon ages of 173.9 ± 1.9 Ma and 170.8 ± 3.0 Ma. Lava flows surrounded by autobrecciated carapace weredominantly extruded in subaerial conditions whereas hyaloclastite and peperite facies suggest contemporaneoussubaqueous volcanism synchronous with sedimentation. Lacustrine sedimentation represented by calcareous mudstonewith pisolithic and laminated limestone, sandstone, chert and coal overlies the volcanic rocks. A Sr-rich evaporitelayer interbedded in the sedimentary succession indicates intermittent drying out of the lake and hypersalineconditions.The mineralization occurs in veins and hydrothermal breccias that lie on the structural hanging wall andfootwall of the main faults, in mineralized autobreccia and chaotic breccia at the contact between volcanic andsedimentary rocks, and disseminated in the organic- rich sedimentary rocks. The earliest mineral infill of veins andbreccias consists of calcite I and siderite with crustiform, cockade and platy textures, followed by two main episodesof 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 amountsof covellite and digenite replaced the early sulfides. Colloform bands of pyrite and marcasite show the highest Agcontents, although all sulfides and sulfosalts carry Ag. Sulfides are rarely observed in situ, on the walls of veins; theyare 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.Volcanic rocks next to veins and breccias are altered and replaced by albite, adularia, calcite, celadonite,quartz, chlorite, sulfides, chalcedony, several types of clay minerals (kaolinite and smectite with illite-smectitemixed layers and traces of tosudite), Fe-Mg-Mn carbonates and calcite II. In the sedimentary rocks, the clasts arereplaced by chalcedony, kaolinite and carbonates. Calcite and chalcedony also fill micro-veinlets.The sulfur isotopic signatures of sulfides hosted in veins and hydrothermal breccias (δ34S -0.9 to +8.3?) aremore restricted than the δ34S of sulfides hosted in the autobreccia, chaotic breccia and sedimentary rocks (δ34S -15.4 to +12.9). The sulfur isotopic signature of hydrothermal barite varies between +15.7 and +22.0?. The isotopiccomposition of celestite from the evaporite layer in the sedimentary rocks is 35.3?.Calcite I from veins has δ13C values between -4.4 and -0.3? and a δ18O of +19.7 to +22.1?. Calcite 2 also fromveins and breccias displays δ13C values between -3.5 and -1.7? and δ18O values of +11.2 to +15?. Chalcedony II infillof veins and breccias has δ18O between +16.2? and +18.4?. Late quartz infill of veins and breccias has a δ18O of+12.4?.Calcite I hosts aqueous fluid inclusions with a salinity close to 16.4 wt.% NaCl equiv. that homogenize between165.8 and 208.3°C. Barite hosts aqueous fluid inclusions that homogenize to liquid in the range of 111-169°C. Fluidinclusions in calcite II have lower salinity (0 to 4.2 wt.% NaCl equiv.) and homogenization temperatures (58 to 95°C)than calcite I.Early calcite I (siderite?) precipitated in veins and breccias from fluids with high salinity (~16.4% wt. NaCl eq.),temperatures (Th)