Geomorphological and hydrogeological features of the Poços de Caldas caldera analogue study sites

Abstract

The Osamu Utsumi mine and Morro do Ferro analogue study sites lie within the Pocos de Caldas plateau in Brazil, which is the eroded remnant of a caldera formed from an 80 million year old intrusion. Geomorphologically, both sites occupy watershed areas adjacent to small streams in the centre of the plateau. The climate of the area has a marked wet season from November to April and is dry the rest of the year. The streams are ephemeral in their upper reaches, tending to dry up in the dry season as they are fed by a declining base flow. In the wet season they exhibit flash floods fed by intense rainfall causing overland flow. The wet season also provides recharge to groundwater. The plateau is a stable feature and its surface has been eroding at an average rate of 12 m per million years over the last 50 million years. The hydrogeological conditions at the mine have been significantly changed by localised quarrying involving the removal of rock below the natural water table and destruction of interfluves. This has modified natural groundwater flow patterns and disturbed the movement of oxidising and reducing waters. Flow velocities have increased five-fold close to the quarry and flow paths have shortened. Morro do Ferro is a similar site accompanied by more intense weathering (laterite/clay) and by magnetite breccia dykes but the groundwater conditions remain in a natural state. Simplified numerical models of the two sites are used to estimate groundwater flow rates and the effects of mining activity. The hydraulic conductivity of the Osamu Utsumi mine model was calibrated to observed mine pumping rates and results indicate that Darcy flow rates of 2 mm day−1 or more may be expected. Residence and interaction times are probably quite short.