Some observations on gold in the weathering profile at Garimpo Porquinho, an artisanal mine in the Tapajós Region, Brazilian Amazon

Abstract

At Garimpo Porquinho (Tapajós Province, Brazilian Amazon) gold-bearing quartz veins containing sulfides occur in a narrow zone affected by hydrothermal alteration. The artisanally mined veins are exposed in a saprolite zone extending down at least 9 m to the fresh rock and are covered by a 1 m thick residual soil. Lateral gold dispersion in the saprolite is not noticeable whereas in the soil gold dispersion has been observed as far as 2 m from the vein. Trace metals associated with gold include Ag, Bi, As and Sb. Copper and lead, present in sulfides, can also indicate the presence of gold. The study of gold particles in different levels of the profile shows that gold has been affected by weathering. In the upper levels of the profile, particles are smaller, more rounded and pitted by dissolution. Grains of primary gold containing up to 20% Ag are commonly exhibit silver-poor rims which may penetrate deep into the grain. The interface between the Ag-poor rims and the interior is very sharp. Such gold-rich rims likely formed by self-electrorefining of electrum grains during weathering.