MINERALOGY, MORPHOLOGY AND CHEMISTRY OF GOLD IN THE STONE LINE LATERITIC PROFILE OF THE POSSE DEPOSIT, CENTRAL BRAZIL

Primary Au mineralization at Posse is hosted within felsic meta-volcan ic rocks of the Proterozoic Mara Rosa volcano-sedimentary sequence. Th e weathered zone above on is characterized by a degraded laterite prof ile dominated by a saprolitic zone above which is a stone line rich in quartz and pisoliths. More than 80% of the Au in the primary ore is p resent as native Au intermixed with telluride minerals and the distrib ution of Ag within the Au grains is heterogeneous. During weathering t elluride minerals an readily oxidized leaving a Fe-oxide material inte rmixed with unaltered native Au. Both mechanical and chemical processe s have contributed to the redistribution of Au. The role of organic an d hydroxy complexes are thought to be important in the chemical remobi lization of Au near the surface. Reprecipitation of Au takes place in the saprorite resulting in Au grains with well-developed crystalline m orphology. The fineness of these grains is similar to that of primary Au but the distribution of Ag is typically homogeneous. Gold has also been reprecipitated, especially in the stone line, by adsorption onto iron-oxides, causing Au enrichment in the coarse fractions. The stone line and topsoil are dominated by relatively large primary and repreci pitated Au grains, due to the preferential mechanical removal of the f ine-grained Au by surficial erosion.