The Canadian Mineralogist
Volume 34, pages 513-528 (1996)

HABIT AND COMPOSITION OF GOLD GRAINS IN QUARTZ VEINS FROM GREENSTONE BELTS: IMPLICATIONS FOR MECHANISMS OF PRECIPITATION OF GOLD

DOMINIQUE MICHEL
Ecole Nationale Supérieure de Géologie, BP 452, 54000 Nancy Cedex, France

GASTON GIULIANI
Institut Français de Recherche Scientifique pour le Développement en Coopération, TOA, UR13, 213, rue La Fayette, 75480 Paris Cedex 10 et Centre de Recherches Pétrographiques et Géochimiques, BP 20, 5450O Vandoeuvre-lès-Nancy Cedex, France

Abstract

The gold deposits of Fazenda Brasileiro (Bahia State) and Maria Lázara (Goiás State) in Brazil, and of Sigma at Val d'Or (Quebec) are mesothermal gold-quartz veins in greenstones terranes. The textural sites of gold grains, combined with variations in their Au:Ag ratio, show that most of the gold is secondary. Successive episodes of gold deposition in each textural site are linked to repeated deformation related to vein formation. The habit of gold grains shows the importance of surfaces of sulfides and Fe-rich minerals (tourmaline, biotite) for gold deposition. Gold reduction on earlier-formed minerals seems to be the main process of gold deposition in the three deposits studied.