- January 2003
Abstract and Figures
In French Guiana, three types of Proterozoic gold deposits can be recognized (gold-bearing tourmalinites, gold-bearing conglomerates, and mesothermal ore deposits) and used as metallogenic markers in the two-stage geodynamic evolution proposed by Vanderhaeghe et al. (1998) and updated at the Guiana Shield scale by Delor et al. (2000, 2001, 2003a, b this volume). During the first stage of crustal growth by calc-alkalic and TTG magmatic accretion, an early (pre-D1 deformation) stock of gold and disseminated sulphides associated with regional tourmalinization accumulated in the Paramaca volcaniclastic sequences in subaquatic conditions (example of the stratiform/stratabound Dorlin deposit). These gold-bearing tourmalinites were locally modified under upper–middle crust conditions during emplacement of discordant mesothermal quartz veins. During the second stage of crustal recycling and tectonic accretion, a wide variety of syntectonic ore deposits including gold-bearing conglomerate and gold mesothermal types were emplaced during all increments of D2 transcurrent deformation, which was responsible for the progressive deformation of the North Guiana Trough, from its creation through to its burial and final exhumation: – The gold-bearing conglomerate type of ore deposit, hosted by the Upper Detrital Unit in the North Guiana Trough, presents several facies markers of the progressive burial of the deposits, e.g. (i) gold-oxide “paleoplacer” of “Banket” type, (ii) debris-flow type with syndiagenetic and/or epigenetic deposits, (iii) epigenetic disseminated-sulphide facies, and (iv) epigenetic mesothermal quartz stockworks. Overall, it formed under low to medium-grade metamorphic conditions with, however, relicts of an earlier hydrothermal event as indicated by preserved hydrothermal pebbles in polygenic conglomerates. – The gold mesothermal type of ore deposit (dominant Au-Fe-Cu type associated with brittle-ductile faults and rare Au-As mesothermal deposits), hosted by the metavolcanic and metasedimentary rocks of the Paramaca Series and also by late-D2 small granite stocks, is always associated with major fault zones. Overall, it formed under low-grade metamorphic conditions. It is characterized by hydrothermal chlorite+ muscovite+albite+carbonates±biotite alteration. Muscovite +chlorite+carbonates predominate in hydrothermally altered metavolcanic and metasedimentary rocks, whereas albite+carbonates predominate in hydrothermally altered granites. Hydrothermal fluids have a variety of origins. Stable-isotope (C, H, O) data indicate a dominant participation of external fluids, near seawater, in the Dorlin subaquatic stratiform/stratabound deposit, whereas metamorphic fluids become dominant in ore deposits formed at deeper levels, such as gold-bearing conglomerates and mesothermal deposits. Sulphur isotopes, trace elements, and sulphide mineralogy show a consistent magmatic component, represented by magmatic fluids or by leaching of magmatic rocks. Key words: Gold deposits, Conglomerates, Tourmalinites, Mesothermal lodes, French Guiana. Full text link: http://geolfrance.brgm.fr/mineralisations-auriferes-tourmalinites-conglomerats-gites-mesothermaux-marqueurs-levolution
