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The epithermal precious metal deposits at Pueblo Viejo in the Dominican Republic contain approximately 18 million ounces of gold and 95 million ounces of silver. Open-pit mining that began in 1975 has removed most of the oxide ore and exposed unweathered rocks at depths as much as 80 m below the original surface. Geologic mapping of these relatively fresh rocks has shown that mineralization is associated with a maardiatreme complex that was emplaced in volcanic rocks of the Lower Cretaceous Los Ranchos Formation.

Mineralization occurs mainly in sedimentary rocks that filled the maar crater. Diamond drilling to depths of 350 m did not reach the base of the sedimentary rocks, and the diatreme is interpreted to be present at greater depth. The initial maar-fill deposits consist of conglomerate, grit, and sandstone. The conglomerate contains clasts of plutonic rock and abundant quartz eyes, in addition to clasts of country rock. It is believed to be derived from an apron of pyroclastic rocks that were erupted from the diatreme and deposited on the rim of the maar. After an initial period of rapid sedimentation from the steep walls of the crater, quieter conditions prevailed in the maar, and thinly bedded carbonaceous sandstones and mudstones were deposited. These rocks contain abundant plant fossils as leaf and bark imprints, and silicified logs occur in shoreline deposits.

Hydrothermal alteration and sulfide mineralization began during formation of the diatreme and continued through the filling of the maar crater. Alteration extends well beyond the maar-diatreme complex, but precious metal mineralization is within it or immediately adjacent to it. The mineralization forms an incomplete ring around the perimeter of the intramaar sedimentary rocks, and the central maar area is barren. The configuration of the ore zones indicates that hydrothermal fluids ascended to the maar along the walls of the diatreme. In the maar-fill rocks, the form of mineralization was influenced by lithology. Where a permeable conglomerate horizon is present, mineralization took place by pyrite replacement of the conglomerate matrix. In areas where the ascending fluids encountered impermeable sandstone and mudstone layers, mineralization is in the form of sharp-walled veins and veinlets. The veins are layered and exhibit multiple episodes of mineralization due to repeated development of fluid overpressures beneath the impermeable beds.

The mineralized rocks were slightly eroded prior to their burial beneath Upper Cretaceous marine limestone. Recent erosion has also been slight, and the southern part of the maar sedimentary sequence remains covered by the limestone.

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