Tungsten mineralization in hydrothermal quartz veins from the Nyakabingo,Gifurwe and Bugarama deposits in central Rwanda occurs as the iron-rich endmember ofthe wolframite solid solution series (ferberite) and in the particular form of reinite, whichrepresents a pseudomorph of ferberite after scheelite. Primary ferberite, reinite and latesecondary ferberite are characterized by their trace element chemistry and rare earthelement patterns. The replacement of scheelite by ferberite is also documented in the traceelement composition. Primary ferberite shows high Mg, Zn, Sc, V, Nb, In and Snconcentrations, but very low Ca, Pb, Sr and Ba contents. Reinite and late secondaryferberite display an uncommon trace element composition containing high concentrationsof Ca, Pb, Sr, Ba, As and Ga, but very low levels in Sn, Zr, Hf, In, Ti, Sc, Nb, Ta, Mg andZn. Late secondary ferberite replacing primary ferberite is characterized by additionalenrichments in Bi, Pb, As and Sb. The rare earth element patterns of reinite and secondaryferberite are also similar to hydrothermal scheelite. The formation of the tungsten depositsin central Rwanda is interpreted to be epigenetic in origin, and the hydrothermalmineralizing fluids are related to the intrusion of the G4-granites.

Organic acids, including humic acid, play a significant role in the weathering of minerals containing metals such as Pt and Pd. They are also among the reactants which are under consideration for new hydrometallurgical methods of liberating unconventional PGE ores (such as the oxidised ores of the Great Dyke, Zimbabwe, and at locations in the Bushveld Complex of South Africa where PGE-bearing layers have been exposed to weathering). In order to better understand the processes operating during weathering of PGE-bearing chromitite seams of the Bushveld Complex, chromite concentrate originating from a South African chromium mine was subjected to reaction with different concentrations of synthetic humic acid. The results confirm the greater mobility of palladium in the environment compared to platinum. Crushed chromite concentrate showed greater mobility of Cr, but not of Pd or Pt, compared to uncrushed concentrate. Increasing the concentration of humic acid increased the amount of Pd and Pt in solution. These experiments give insight into the processes that govern the weathering of chromitite in the Bushveld Complex. The main Pd– and Pt-bearing minerals are not enclosed within chromite but occur at grain boundaries. Thus, they can be liberated by disaggregation of chromite and infiltration of water along chromite grain boundaries. Once in solution, Pd is more mobile than Pt and is dispersed further. Organic acids play an important role during the weathering process as they are capable of enhancing the mobility of the PGE, particularly Pd.