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-volcanic rocks of the Proterozoic Mara Rosa volcano-sedimentary sequence. The weathered zone above ore is characterized by a degraded laterite profile 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 present as native Au intermixed with telluride minerals and the distribution of Ag within the Au grains is heterogeneous. During weathering telluride minerals are readily oxidized leaving a Fe-oxide material intermixed with unaltered native Au.

Both mechanical and chemical processes have contributed to the redistribution of Au. The role of organic and hydroxy complexes are thought to be important in the chemical remobilization of Au near the surface. Reprecipitation of Au takes place in the saprolite resulting in Au grains with well-developed crystalline morphology. 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 reprecipitated Au grains, due to the preferential mechanical removal of the fine-grained Au by surficial erosion.