Norman Russell

Gold deposition by sulfidation of ferrous Fe in the lacustrine sediments of the Pueblo Viejo district (Dominican Republic): The effect of Fe–C–S diagenesis on later hydrothermal mineralization in a Maar-Diatreme complex

Abstract The Pueblo Viejo district, located in the Cordillera Central of the Dominican Republic, contains large Au-Ag deposits associated with acid-sulfate alteration within spilites, conglomerates and carbonaceous sedimentary rocks that were deposited in a maar-diatreme complex. Much of the Au mineralization occurs in pyritic, carbonaceous siltstones of the Pueblo Viejo Maar-Diatreme Member of the Cretaceous Los Ranchos Formation. Pyrite is the only Fe-bearing phase in mineralized rock, whereas siderite is the dominant Fe-bearing phase in siltstones distal to mineralization. Disseminated pyrite occurs as framboids, cubes, pyritohedra, concretions and cement. Early framboids occur throughout the district. Au occurs as inclusions in later non-framboid disseminated pyrite (NFDP); an occurrence that is interpreted to be indicative of contemporaneous deposition. Pyrite framboids exhibit a wide range of δ34S cdt -values (−17.5 to +4.8‰) and are interpreted to have formed during biogenic reduction of pore-water sulfate. The NFDP yield restricted δ34S cdt -values ( x = −5.2‰ , s = ±2.4‰ , n = 43) similar to those obtained from later vein pyrite ( x = −6.4‰ , s = ±1.5‰ , n = 12). Alunite and barite have δ34S-values ranging from +18.8 to +21.6‰. The interpretation that the NFDP, vein pyrite, alunite and barite, and possibly even the framboidal pyrite share a common source of igneous sulfur is supported by the δ34S data. Siderite occurs as concretions and cement, contains abundant Mg (Fe0.75Mg0.19Mn0.03Ca0.02CO3) and has δ13C pdb - and δ18O smow -values ranging from −2.5 to +1.1%. and +14.6 to +19.5‰, respectively. These data are consistent with the interpretation that the siderite formed in lacustrine sediments and that the carbonate in the siderite is probably methanogenic, although contributions from oxidation of organic matter during biogenic sulfate reduction, thermal decarboxylation of organic matter, or magmatic vapor cannot be ruled out. Disseminated Au mineralization in the sedimentary rocks formed when a hydrothermal fluid encountered reactive Fe2+ in diagenetic siderite. The ensuing pyrite deposition consumed H2S and destabilized the Au (HS)−2 complex, leading to precipitation of Au. The capacity of the sedimentary rocks to consume H2S and precipitate Au was controlled by the amount of non-pyrite Fe present as siderite. The abundance of siderite was controlled by the extent of pyrite formation during diagenesis.

Age and Tectonic Setting of the Camaguey Volcanic-Intrusive Arc, Cuba: Late Cretaceous Extension and Uplift in the Western Greater Antilles

Ar/Ar ages determined on rocks and minerals from the Camagüey area in central Cuba provide age constraints on events that accompanied the northward migration of Cuba into the Caribbean region and its subsequent collision with the Bahamas Bank. Whole‐rock samples from the Camujiro and Piragua Formations, part of the Camagüey volcanic sequence, yielded Ar/Ar ages of 74–72 Ma, distinctly younger than the 100–80‐Ma ages indicated by fossils in interlayered sedimentary rocks. Syenite and granodiorite in the Camagüey batholith, which cut these volcanic rocks, yield generally similar Ar/Ar ages of 75–72 Ma for hornblende, biotite, and feldspar. These ages are interpreted to reflect relatively rapid uplift and cooling of most of the volcanic‐intrusive arc. Additional constraints on the timing of this uplift are provided by Ar/Ar ages of 71–75 Ma for rhyolite‐rhyodacite domes of the La Sierra Formation and 52 Ma for andesitic basalt of La Mulata Formation, which appear to have been emplaced onto erosion surfaces that resulted from this uplift. An average cooling rate of about 13°C/Ma, which prevailed during formation of the arc between about 96 and 80 m.yr., increased to about 40°C/Ma after 80 Ma and ended with formation of a paleosurface at about 75 Ma or slightly afterward. The rapid uplift and denudation arc necessary to form a Late Cretaceous paleosurface on the volcanic‐intrusive probably required an extensional tectonic environment, which could have been created during oblique convergence of the Greater Antilles arc with Yucatan as the arc migrated northward in Late Cretaceous time. Metamorphic rocks in the Escambray and Isle of Pines areas in western Cuba have ages similar to those indicated for uplift in the Camagüey area, suggesting that extension and related Late Cretaceous paleosurfaces were widespread in the western Greater Antilles during its northward migration into the Caribbean region.