Richard Kettler, et al., report that the Pueblo Viejo district is one of the largest producers of precious metals in the world, yielding more than 11,000 kg of Au annually. Gold mineralization at Pueblo Viejo is hosted in spilite, and coarse elastic and finely laminated, fine-grained carbonaceous sedimentary rocks of the Lower Cretaceous Los Ranchos Formation.
Mineralization was accompanied by sulfidation as evidenced by 1) the occurrence of siderite distal to mineralization and pyrite proximal to mineralization, 2) increased S over Fe ratios associated with Au mineralization, 3) the occurrence of native S in and adjacent to mineralization, and 4) the presence of sulfidized organic matter (organo-S compounds) in mineralized rocks.
Organic matter in the carbonaceous sedimentary rocks comprises vitrinite and pyrobitumen. Rock-Eval pyrolysis data indicate that this organic matter is overmature (HI is less than 55). The organic matter is S-rich (up to 14 wt% S) in rocks where all Fe has been sulfidized to pyrite. X-ray absorption near edge structure (XANES) analysis indicates that the C-bonded S is thiophenic. Early disseminated Au mineralization is postulated to have occurred in response to sulfidation of ferrous Fe to form pyrite.
The consequent decline in the activity of H2S in the mineralizing fluid would have destabilized Au bisulfide complexes and caused deposition of gold. The restriction of S-rich organic matter to rocks in which all Fe occurs as pyrite indicates that sulfidation of organic matter postdates sulfidation of ferrous Fe and therefore, deposition of much of the gold.
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