The large Pueblo Viejo oxide gold deposit was developed by weathering of a gold-quartz-pyrophyllite deposit that formed during sedimentation in a small basin in the upper part of the Los Ranchos Formation, one of the oldest volcanic units in the Greater Antilles. The basin is floored by conglomerates and agglomerates, which grade upward into standstones and carbonaceous sediments that include chert with thin pyrite layers. Plant fossils, which are common in the carbonaceous sediments, indicate that the basin was near a land mass.The mineralized zone is roughly funnel shaped and expands to its maximum horizontal dimensions in the carbonaceous sediments. The lower part of the funnel tube contains alunite and diaspore. Pervasive pyrophyllite alteration makes up most of the overlying tube and upper part of the zone, and locally extensive silicification makes up the top of the funnel. Supergene kaolinite is present in the upper part of the funnel.Metallic minerals, consisting mostly of pyrite, are found as disseminations, layers, and veins, with base and precious metal values being highest in the veins. Veins are most common in the carbonaceous sediments and diminish in abundance downward into the coarser clastic units. The veins contain abundant pyrite, sphalerite, and quartz; minor barite, enargite, and pyrophyllite; and traces of electrum, argentite, colusite, tetrahedrite-tennantite, geocronite, galena, and tellurides containing gold, silver, and copper. There is a strong correlation between Au and Zn in the upper parts of the mineralized zone and the Au:Ag ratio, which averages 1:7, decreases slightly with depth. Au is in electrum in the lower part of the mineralized zone and partly in tellurides in the upper part. Fluid inclusions indicate that fluids in the veins were very dilute, boiled at least locally, and reached temperatures of 130 degrees to 190 degrees C. Pyrophyllite stability requires minimum temperatures of about 260 degrees C for the hydrothermal system. Isotopic data indicate that most of the sulfur in the mineralization was derived from seawater, whereas the strontium and lead came from the Los Ranchos Formation, which probably also supplied the precious metals.The Pueblo Viejo system is similar to other gold-quartz-pyrophyllite deposits except that it contains no known mineable vein orebodies and, instead, contains numerous, narrow, closely spaced, irregular veins that can be mined by bulk methods. These veins are thought to have formed by hydraulic fracturing of the carbonaceous sediments caused by fluid pressures as the hot spring system developed. The size and shape of the mineralized system at Pueblo Viejo resemble active hot spring systems such as Wairakei, New Zealand, and its environment of formation probably resembled that of the Quaternary volcano chain which crosses Lake Managua in Nicaragua.