U-Pb dating of micro-inclusions: The age of the Ehrenfriedersdorf tin deposit (Erzgebirge, Germany)

Multiple reworking of ore deposits may disturb geochronologic systems and eventually render incorrect ages for ore deposition or different stages of ore formation. Micro-inclusions, may beprotected from later disturbances by their host mineral and thus, yield reliable age information. Major problems in dating micro-inclusions include their genetic connection with various stages of ore formation and technical aspects, such as loss of daughter isotopes by α-recoil and the initial isotopic composition of the daugher element. The problem of daughter-loss from a high-μ micro-inclusion (μ=

238

U/

204

Pb) by α-recoil can be avoided by analyzing the inclusion together with its low-α host. The philosophy behind the presented analytical approach is to “trade” radiogenic composition of the daughter element (i.e., high

206

Pb/

204

Pb,

207

Pb/

204

Pb, and

208

Pb/

204

Pb) against closed-system behavior. This approach is demonstrated for <10 microns small uraninite inclusions in darkmica from Ehrenfriederdorf, Erzgebirge, Saxony. Our data demonstrate that micro-inclusions may yield precise and accurate age data for early ore forming processes.