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Economic Geology and the Bulletin of the Society of Economic Geologists

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Institución detectada Período Navegá Descargá Solicitá
No detectada desde may. 2014 / hasta dic. 2023 GeoScienceWorld


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Estados Unidos

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Bill T. Fischer; Jean S. Cline

<jats:title>Abstract</jats:title> <jats:p>The 144 zone is a pseudobreccia-hosted, disseminated gold deposit that formed in the middle to late Cambrian Bonanza King dolostone along an unconformity with the underlying early to middle Cambrian Carrara limestone at Bare Mountain, southern Nevada. Underground mapping revealed spatial relationships between breccia types, host rocks, and alteration assemblages that are related to gold mineralization. Samples were collected along transects from low- to high-grade Au and were analyzed using petrography, applied reflectance spectroscopy, scanning electron microscopy, and electron probe microanalysis to characterize mineral assemblages and evaluate gold deportment.</jats:p> <jats:p>Two breccia types are identified. Breccia type 1 clasts consist of dolomite, dolomite with phengite, and quartz cemented in a quartz-rich matrix. Breccia type 2 has similar clasts of dolomite, dolomite with phengite, and quartz, but the matrix is phengite dominant. Neither breccia type has a preferred association with gold, which occurs with goethite that replaced pyrite in both breccias.</jats:p> <jats:p>Clast and matrix compositions and textures show that the two breccia types formed at the same time by selective dissolution and replacement of the lowermost Bonanza King dolomite. Fluid-rock reaction transformed massive dolomite into pseudobreccia. Quartz replacement of dolomite plus the precipitation of pyrite, Au, and phengite yielded the 144 zone pseudobreccia matrix. The geology that characterizes gold mineralization in the 144 zone can be applied to exploration throughout Bare Mountain. Other localities where the same stratigraphic contact is cut by silicic dikes of similar age provide drill targets in the mining district.</jats:p>

Pp. 1137-1150

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

Veronica Bouhier; Marta Franchini; Fernando Tornos; Ana L. Rainoldi; Patricia Patrier; Daniel Beaufort; Adrian J. Boyce; Warren Pratt; Agnes Impiccini

<jats:title>Abstract</jats:title> <jats:p>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 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 limestones, sandstone, coal, and an Sr-rich evaporite layer deposited in a lacustrine environment. The mineralization 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.</jats:p> <jats:p>The earliest hydrothermal minerals consist of veins of colloform, crustiform, and cockade calcite 1 (δ13Cfluid –4.7 to 0.8‰; δ18Ofluid 4.8–11.6‰) and siderite. The precipitating fluids were likely basement-exchanged basinal brines having salinities of 9.5–16.4 wt % NaCl equiv and temperatures of 154.7°–212°C. 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 bladed (platy) texture in calcite 1.</jats:p> <jats:p>Subsequent mineralizing stages contributed to the metal endowment of Loma Galena. The abundance of organic-rich mudstone and δ34S from –15.4 to 12.9‰ for sulfides suggests that the bottom waters of the lake were anoxic and the loci of microbial sulfate reduction (evaporites have δ34S 35‰). 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 addition 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.</jats:p> <jats:p>Mineralization was followed by hydrothermal brecciation and successive precipitation of chalcedony (δ18Ofluid 2.6–4.8‰), barite (δ34S 15.7–22‰; 160.9°–183.8°C; 7.7–9.7 wt % NaCl equiv), calcite 2 (δ18Ofluid –10.2 to –3.7‰, 58°–95°C; 1.9–7.0 wt % NaCl equiv), strontianite, and quartz in brecciated veins and breccias; kaolinite (δ18Ofluid 2–6.2‰), 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 suggests dilution of the basinal fluids by mixing with Jurassic meteoric water (δ18O −9 to −5.2‰).</jats:p> <jats:p>Loma Galena is a unique example of a polymetallic epithermal system formed in a sublacustrine anoxic 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.</jats:p>

Palabras clave: Economic Geology; Geochemistry and Petrology; Geology; Geophysics.

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