Abstract. Multiple reworking of ore deposits may disturb geochrono- logic systems and eventually render incorrect ages for ore deposi- tion 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-in- clusions include their genetic connection with various stages of ore formation and technical aspects, such as loss of daughter iso- topes by α-recoil and the initial isotopic composition of the daugh- ter 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-µ host. The philosophy behind the presented analytical approach is to “trade” radiogenic composi- tion 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 dem- onstrated 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. Keywords. Micro-inclusions, U-Pb dating, Ehrenfriederdorf, tin deposits 1 Introduction Precise and accurate dating of many kinds of ore depos- its is problematic because later hydrothermal and tectonic overprint (resulting in the repeated redistribution of ele- ments or the introduction of various metals at different time) may disturb geochronometers that are genetically and texturally related to early stages of deposit forma- tion. For instance, U deposits in the Aue-Niederschlema district (Erzgebirge, Saxony) yield ages of c. 280-270 Ma, 190 Ma, 120 Ma, 80 Ma, and 30 Ma (Förster 1996), the oldest age related with the introduction of U into the de- posit, the younger ages reflecting redistribution of U in conjunction with multiple tectonic reactivation of the Gera-Jachymov zone, along which numerous U deposits occur. Many of the ambiguities and complexities arising from the redistribution of the ore metals during multiple over- print and alteration can be avoided by (i) dating the eco- nomically interesting minerals, which is possible for min- erals of the columbite-tantalitetapiolite series (e.g. Romer and Wright 1992; Romer and Smeds 1996), molybdenite (e.g. Stein et al. 1998, 2001; Stein and Bingen 2002), and U-minerals (e.g. Ludwig and Simmons 1991; Förster 1996), (ii) by dating minerals that have formed cogenetically with the economically interesting minerals, which for skarn ores may involve titanite, vesuvianite, and garnet (e.g., Romer 1992; Romer and Soler 1995) and for hydrother- mal vein deposits may involve gangue minerals such as white mica or quartz (e.g. Franzke et al. 1996; Romer and Linnemann 2004), and (iii) by dating fluid inclusions in economically interesting minerals or associated gangue minerals (e.g. Glodny 1997; Pettke and Diamond 2000). In this paper, we demonstrate that U-Pb dating of micro- inclusions is technically possible and yields accurate re- sults as long as precautions are taken (i) to avoid micro- minerals along grain-boundaries and fractures and (ii) to sample in a way that compensates for recoil-related open-system behavior in the U-Pb system. 2 Advantages and disadvantages of the use of micro-inclusions in mineral dating Dating micro-inclusions entirely enclosed in other min- erals has the advantage that the host prevents interaction of the micro-inclusion with later fluids. The host-min- eral represents a container that makes the geochronologic system of the inclusion behave as a closed system even for inclusions (e.g., uraninite) that are known to react readily with fluids. The host mineral, however, should neither react with the inclusion nor incorporate those ele- ments that define the geochronologic system of the inclu- sion. Otherwise there may be exchange of the geochrono- logically relevant element between host and inclusion. The major problem in the use of micro-inclusions for dating originates from ambiguities in the genetic connection be- tween inclusion and host and the representativity of the inclusion. 3 Requirements on micro-inclusions to be suitable for dating There are basically three requirements that must be ful- filled for micro-inclusions to become potentially attrac- tive for dating attempts: (i) closed system, (ii) strong daughter-to-parent (D/P) fractionation, and (iii) high P contents. i. Closed system behavior requires that neither the P nor the D element is lost from the inclusion. Major pro- cesses for possible daughter loss include diffusion and recoil, both of which are more likely to become more important for smaller grain size. Loss by diffusion de- 7-25 Chapter 7-25 U-Pb dating of micro-inclusions: The age of the Ehrenfriedersdorf tin deposit (Erzgebirge, Germany) R.L. Romer, R. Thomas GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany