Genesis of the Loma Galena Pb-Ag Deposit, Navidad District, Patagonia, Argentina: A Unique Epithermal System Capped by an Anoxic Lake

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 Canadon 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 comprise major listric 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 lime-stones, sandstone, coal, and an Sr-rich evaporite layer deposited in a lacustrine environment. The mineraliza-tion occurs as disseminations in the organic-rich sedimentary rocks, in veins and hydrothermal breccia dikes in the hanging walls and footwalls of NW-and NE-striking normal faults, in volcanic autobreccias, and in a phreatic breccia at the contact of volcanic and sedimentary rocks. The earliest hydrothermal minerals consist of veins of colloform, crustiform, and cockade calcite 1 (delta 13Cfluid -4.7 to 0.8%o; delta 18Ofluid 4.8-11.6%o) and siderite. The precipitating fluids were likely basement-exchanged basinal brines having salinities of 9.5 to 16.4 wt % NaCl equiv and temperatures of 154.7 degrees to 212 degrees C. The inter-action 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 bladed (platy) texture in calcite 1. Subsequent mineralizing stages contributed to the metal endowment of Loma Galena. The abundance of organic-rich mudstone and delta 34S from -15.4 to 12.9%o for sulfides suggests that the bottom waters of the lake were anoxic and the loci of microbial sulfate reduction (evaporites have delta 34S 35%o). 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. The highest grade and tonnage of the ores are found in autobreccias at the junction of the uppermost lava flow and in the overlying mudstone, where the addi-tion of a strong microbial signature is recorded in sulfides. This event also led to partial dissolution of magmatic and hydrothermal feldspar and calcite 1 in the altered volcanic rocks. Mineralization was followed by hydrothermal brecciation and successive precipitation of chalcedony (delta 18Ofluid 2.6-4.8%o), barite (delta 34S 15.7-22%o; 160.9 degrees-183.8 degrees C; 7.7-9.7 wt % NaCl equiv), calcite 2 (delta 18Ofluid -10.2 to -3.7%o, 58 degrees-95 degrees C; 1.9-7.0 wt % NaCl equiv), strontianite, and quartz in brecciated veins and breccias; kaolinite (delta 18Ofluid 2-6.2%o), 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 decreas-ing temperature and lowering delta 18O of the fluids with time suggests dilution of the basinal fluids by mixing with Jurassic meteoric water (delta 18O -9 to -5.2%o). Loma Galena is a unique example of a polymetallic epithermal system formed in a sublacustrine anoxic envi-ronment that promoted the efficient deposition and preservation of Ag-bearing sulfides, thereby contributing to the large size and relatively high grade of the deposit.

Automated summary

Loma Galena is a polymetallic epithermal deposit located in the Canadon Asfalto continental foreland basin in northern Patagonia, Argentina. The mineralization occurs in organic-rich sedimentary rocks and is associated with faults, volcanic rocks, and sedimentary rocks. The earliest hydrothermal minerals are calcite and siderite veins. Subsequent mineralizing events led to the deposition of various ore minerals such as pyrite, chalcopyrite, sphalerite, and galena. The unique sublacustrine anoxic environment of Loma Galena contributed to the efficient deposition and preservation of the Ag-bearing sulfides.