Genesis of the Loma Galena Pb-Ag deposit, Navidad district, Patagonia, Argentina, a sublacustrine hydrothermal system capped by an anoxic lake

BOUHIER, VERÓNICA; FRANCHINI, MARTA; TORNOS, FERNANDO; RAINOLDI, ANA LAURA; PATRIER PATRICIA; BEAUFORT DANIEL; BOYCE, ADRIAN; IMPICCINI, AGNES

ECONOMIC GEOLOGY AND THE BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS

SOC ECONOMIC GEOLOGISTS, INC

Año: 2023 vol. 118 p. 433 - 457

0361-0128

Loma Galena (978,852 t Pb - 206 Moz Ag) is one of eight epithermal deposits in the world-classNavidad Pb + Ag ± (Zn, Cu) district located in the Cañadón Asfalto continental foreland basin,northern Patagonia, Argentina. This basin formed during the Jurassic in an extensional tectonicregime during the breakup of Gondwana. Host rocks of Loma Galena comprise major faulted andtilted blocks of K-rich andesite to dacite lava flows (173.9-170.8 Ma; U/Pb ages for zircon)unconformably overlain by mudstone interbedded with stromatolitic and pisolitic limestones,sandstone, coal, and a Sr-rich evaporite layer deposited in a lacustrine environment. Themineralization occurs as disseminations in the organic-rich sedimentary rocks, in veins andhydrothermal breccias in the hanging- and footwalls of NW and NE normal faults, in volcanicautobreccias, and in a phreatic breccia at the contact between volcanic and sedimentary rocks.The earliest hydrothermal minerals identified at Loma Galena consist of colloform, crustiformand cockade calcite 1 (δ13Cfluid ?4.7 to +0.8?; δ18Ofluid +4.9 to +11.7?) and siderite precipitated inveins from fluids with temperatures of 212°-154°C. They are likely basinal brines having salinities of9.5 to 16.4 wt % NaCl equiv that have isotopically exchanged with metamorphic and igneousbasement rocks. The interaction of these fluids with the host volcanic rocks formed calcite, albite,adularia, and celadonite-glauconite group minerals followed by chlorite and siderite as fO2decreased. Fluids intermittently boiled, as evidenced by platy texture in calcite 1 and the presence ofadularia.Subsequent mineralizing stages contributed to the metal endowment of the Loma Galenadeposit. The abundance of organic-rich mudstone and the δ34S range (?15.4 to +12.9?) for sulfidessuggest that the bottom waters of the lake were anoxic and the loci of microbial evaporitic sulfate(δ34S +35?) reduction. Mixing of upflowing metal-rich basinal fluids carrying some S from depthwith this H2S-rich connate water efficiently precipitated Ag-bearing framboidal pyrite, colloformpyrite-marcasite, chalcopyrite, bornite, tennantite-tetrahedrite, sphalerite and galena as veins,4breccias and disseminations in host rocks and partially dissolved magmatic and hydrothermalfeldspar and calcite 1 in the altered volcanic rocks.Mineralization was followed by hydrothermal brecciation and successive precipitation ofchalcedony (δ18Ofluid +2.6 to +4.8?), barite (δ34S +15.7? to +22?; 183.8°-160.9ºC; 7.7% to 9.7 wt% NaCl equiv), calcite 2 (δ18Ofluid ?10.2 to ?3.7?; 95°-58ºC; 1.9% to 7.0 wt % NaCl equiv),strontianite, and quartz in brecciated veins and breccias, kaolinite (δ18Ofluid +2 to +6.2?), illitesmectite,smectite, and carbonates with minor chalcedony and barite in the volcanic rocks andcalcite, chalcedony and barite in the sedimentary rocks. A trend of decreasing salinity withdecreasing temperature and lowering δ18O of the fluids with time suggest dilution of the basinalfluids by mixing with Jurassic meteoric water (δ18O −9 to −5.2 ?).Loma Galena is a unique example of a polymetallic epithermal system formed in asublacustrine environment that promoted the efficient deposition and preservation of Ag-bearingsulfides, thereby contributing to the large size and relatively high grade of the deposit.