Potosi, a primary tin deposit in rondonia

The Potosi primary tin deposit is an exogreisen body contained within a near vertical breccia pipe, related to a nearby Rondonian tin-granite. Its spacial position appears to have been controlled by prominent fractures in the basement through which the granite and orebody were intruded. Different types of greisen have been recognized in the field which correspond to greater or lesser metasomatic alteration of the gneiss breccia fragments. Topazite forms the breccia matrix. It is believed that the Potosi orebody formed as result of volatile concentration (particularly f1uorine) beneath an impermeable carapace of the Pedra Branca granite and the subsequent escape of these volatiles along the basement fractures to surface, producing the breccia and tin/tungsten mineralization.