Rock Chemistry and Alteration Processes in the Aluto-Langano Geothermal Field, Ethiopia

Abstract

In the Aluto-Langano geothermal field, two types of alteration trends have been recognized using different chemical diagrams: argillic and propylitic. The rock samples fiom the lateral outflow zone show a pronounced altration trend towards the phyllosilicate composition. This is in agreement with the comrnon occurrence of chlorite, smectite, illite and other clay minerals in these rocks. On the other hand, a few samples show a different trend towards the calcite and epidote composition where a relict propylitic alteration is prominent in these rocks. The rock samples fiom the upflow zone depict alteration towards the calcite and epidote composition which is consistent with the propylitic alteration characterizing these rock samples. The upflow zone is characterized by waters with upto 14 ppb of Au and up to 3.4 ppb of Ag. These metals can be deposited in the zones of lateral flow.