The Hyrkkölä U-Cu mineralization is located in south-western Finland, near
the Palmottu analog site, in crystalline, metamorphic bedrock. The age of
the mineralization is estimated to be between 1.8 and 1.7 Ga. The existence
of native copper and copper sulfides in open fractures in the near-surface
zone allows us to study the native copper corrosion process in conditions
analogous to a nuclear fuel waste repository.
From the study of mineral assemblages or paragenesis, it appears that the
formation of copper sulfide (djurleite, Cu1.934S) after native
copper (Cu°) under anoxic (reducing) conditions is enhanced by the
availability of dissolved hydrogen sulfide (HS) in the groundwater
circulating in open fractures in the near-surface zone. The minimum
concentration of HS in the groundwater is estimated to be of the order of
10-5 M (∼ 10-4 g/1) and the minimum pH value not
lower than about 7.8 as indicated by the presence of calcite crystals in the
same fracture.
The present study is the first one performed on occurrences of native copper
in reducing, neutral to slightly alkaline groundwaters. Thus, the data
obtained is of most relevance in improving models of anoxic corrosion of
copper canisters.