(Economic Geology, Vol. 85, 1990, pp. 1738-1758)
Phase equilibria and sulfur isotope thermometry show that Stage I alteration took place below 260oC at pH values between 2 and 3. Stage II pyrophyllite±diaspore formation occurred above 285oC, from a fluid with an initial pH of 1 to 1.5, while the silica caps formed below 220oC. Geologic constraints indicate that Stage I wallrock gold at Monte Negro was deposited by sulfidation of Fe-rich wallrock, 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 wallrock, which caused early deposition of "background" disseminated gold by sulfidation, and 3) later hydrothermal activity which formed impermeable silca 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.