I did find some new info on Serra Pelada so I feel better now......

I did find some new info on Serra Pelada so I feel better now...

SUGGESTED GENETIC MODEL AND DISCUSSION

The Serra Leste Au-(Pd-Pt) mineralization is tentatively related to the hydrothermal alteration associated with the cooling of the diorite intrusion(s). The solubility of Au and PGE as Cl-complexes are of the same order of magnitude and drop rapidly with pH increase or ƒO2 decrease (Mountain and Wood 1988). The carbonaceous metasiltstone possibly acted as a chemical barrier, triggering focused deposition of precious metals through a rapid decrease of oxidation conditions of the hydrothermal fluid. This process was possibly coupled with carbonate dissolution of dolomitic marble that triggered open-space-filling processes, developed an external jasperoid envelope around the orebodies and caused a pH increase.

This depositional model is analogous to other sediment-hosted gold mineralizations (e.g. Berger and Bagby 1993) according to CVRD (1998) and Sillitoe (1998), although the metal content and mineralogical associations are quite distinct. The source(s) of precious metals are as yet unknown. One can speculate that Au, Pd and Pt could have been scavenged from maficultramafic rocks of the Rio Novo Sequence and/or from Luanga-type layered mafic-ultramafic intrusions.

The dioritic intrusion(s) and the carbonaceous metasiltstone constitute additional potential for Au and PGE sources, respectively. Except for the constraints derived from mineral equilibria, there is no direct data on the fluid. Thus, its source(s) and exact composition(s) are still to be established. However, the association with thorite and REE minerals, coupled with indirect evidence of a saline, oxidized and possibly acid fluid, suggest an intrusion-related model.

The presence of the hydrothermally altered dioritic intrusion(s) and the contactmetamorphic aureole support this hypothesis. The timing of mineralization is also unknown. The presence of shear zones within the diorite intrusion(s), together with the venulation and brecciation transgressive with respect to regional foliation indicate that magmatism and hydrothermal activity took place in the latest stages of evolution of the Cinzento strike-slip system.

http://www.ppegeo.igc.usp.br/index.php/rbg/article/download/10653/10151