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

VERÓNICA BOUHIER; MARTA FRANCHINI; FERNANDO TORNOS; PATRICE PATRIER; DANIEL BEAUFORT; ADRIAN BOYCE; AGNES IMPICCINI,

ECONOMIC GEOLOGY AND THE BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS

SOC ECONOMIC GEOLOGISTS, INC

Lugar: Littleton; 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-class Navidad 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 tectonic regime during the breakup of Gondwana. Host rocks of Loma Galena comprise major faulted and tilted 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. The mineralization occurs as disseminations in the organic-rich sedimentary rocks, in veins and hydrothermal breccias in the hanging- and footwalls of NW and NE normal faults, in volcanic autobreccias, and in a phreatic breccia at the contact between volcanic and sedimentary rocks. The earliest hydrothermal minerals identified at Loma Galena consist of colloform, crustiform and cockade calcite 1 (δ13Cfluid ?4.7 to +0.8?; δ18Ofluid +4.9 to +11.7?) and siderite precipitated in veins from fluids with temperatures of 212°-154°C. They are likely basinal brines having salinities of 9.5 to 16.4 wt % NaCl equiv that have isotopically exchanged with metamorphic and igneous basement 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 fO2 decreased. Fluids intermittently boiled, as evidenced by platy texture in calcite 1 and the presence of adularia. Subsequent mineralizing stages contributed to the metal endowment of the Loma Galena deposit. The abundance of organic-rich mudstone and the δ34S range (?15.4 to +12.9?) for sulfides suggest 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 depth with this H2S-rich connate water efficiently precipitated Ag-bearing framboidal pyrite, colloform pyrite-marcasite, chalcopyrite, bornite, tennantite-tetrahedrite, sphalerite and galena as veins, breccias and disseminations in host rocks and partially dissolved magmatic and hydrothermal feldspar and calcite 1 in the altered volcanic rocks. Mineralization was followed by hydrothermal brecciation and successive precipitation of chalcedony (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?), illite-smectite, smectite, and carbonates with minor chalcedony and barite in the volcanic rocks and calcite, chalcedony and barite in the sedimentary rocks. A trend of decreasing salinity with decreasing temperature and lowering δ18O of the fluids with time suggest dilution of the basinal fluids by mixing with Jurassic meteoric water (δ18O −9 to −5.2 ?). Loma Galena is a unique example of a polymetallic epithermal system formed in a sublacustrine environment that promoted the efficient deposition and preservation of Ag-bearing sulfides, thereby contributing to the large size and relatively high grade of the deposit.