Abstract

This study was undertaken to determine the geologic factors that were important in making Pueblo Viejo, which contains over 600,000 kg of gold in relatively high-grade ore, one of the few acid sulfate deposits to be mined economically on a large scale. Orebodies at Pueblo Viejo, including Monte Negro, the focus of this study, are found around a maar-diatreme complex that formed in the upper part of the Lower Cretaceous Los Ranchos Formation. Ore is hosted by spilite, spilite-derived conglomerate, and intramaar carbonaceous sandstone and mudstone. Regional seawater alteration of these rocks was overprinted by two stages of advanced argillic alteration, both of which are associated with precious metals. Stage I produced deep alunite + quartz + pyrite and overlying kaolinite + quartz + pyrite assemblages and deposited gold in association with disseminated pyrite in wall rock. Stage II, which overprinted stage I, produced deep pyrophyllite + or - diaspore and an overlying (recrystallized) silica cap. Pyrite + or - sphalerite + or - enargite veins that probably formed in response to hydrofracturing of the silica cap contain gold grades of as much as 100 ppm. Vein density and assay data show that approximately 60 percent of the gold at Monte Negro is in disseminated stage I ore, with the remainder in stage II veins.Phase equilibria and sulfur isotope thermometry show that stage I alteration took place below 260 degrees C at pH values between 2 and 3. Stage II pyrophyllite + or - diaspore formation occurred above 285 degrees C, from a fluid with an initial pH of 1 to 1.5, while the silica caps formed below 220 degrees C. Geologic constraints indicate that stage I wall-rock gold at Monte Negro was deposited by sulfidation of Fe-rich wall rock, whereas stage II vein-hosted gold could have formed by cooling, boiling, or mixing with overlying ground water. Use of the programs SOLVEQ and CHILLER to evaluate the relative efficiencies of these ore-forming processes confirms that sulfidation was capable of depositing disseminated stage I gold, whereas stage II vein gold was probably deposited by boiling related to pressure release during hydrofracturing and subsequent mixing with ground water.These observations indicate that development of grade and tonnage at Pueblo Viejo was greatly facilitated by (1) the porous-permeable maar-diatreme complex, which hosted large, shallow hydrothermal cells and probably served as a conduit for rising magmatic gases, (2) the presence of iron-rich wall rock, which caused early deposition of "background" disseminated gold by sulfidation, and (3) later hydrothermal activity which formed impermeable silica caps that hydrofractured to create the high-grade vein overprint. Remobilization of stage I background gold into stage II veins might also have been an important factor in upgrading the deposits. Exploration for similar deposits should focus on areas of explosive volcanism in andesitic island arc terranes.

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