U–Pb SHRIMP geochronology of Th-poor, hydrothermal monazite: An example from the Llallagua tin-porphyry deposit, Bolivia U. Kempe a, * , B. Lehmann b , D. Wolf a , N. Rodionov c , K. Bombach a , U. Schwengfelder a , A. Dietrich b a Institute of Mineralogy, TU Bergakademie Freiberg, Brennhausgasse 14, 09596 Freiberg, Germany b Institute of Mineralogy and Mineral Resources, TU Clausthal, Adolph-Roemer-Str. 2a, 38678 Clausthal-Zellerfeld, Germany c Centre of Isotopic Research, Karpinsky Russian Geological Research Institute (VSEGEI), Srednyi prospect 74, 199106 St. Petersburg, Russia Received 14 September 2007; accepted in revised form 27 May 2008; available online 14 June 2008 Abstract In situ U–Pb SHRIMP analysis of hydrothermal monazite virtually free of Th and poor in U (<0.2 ppm Th, 40–103 ppm U) from the world-class Llallagua tin porphyry deposit in Bolivia defines a mineralization age of 23.4 ± 2.2 Ma (MSWD 0.48) confirming earlier K–Ar sericite alteration age data. These ages are, however, in contrast with a weighted mean single crystal 207 Pb/ 206 Pb evaporation age of 39.3 ± 6.0 Ma, and a related Pb–Pb inverse isochron age of 42.4 ± 4.0 Ma (MSWD 0.66) on zircon from a post-porphyry dike, as well as with an earlier single crystal Sm–Nd apatite isochron age. Our data points to a significant time gap between emplacement of the ore-hosting porphyry intrusion (magmatism) and its hydrothermal overprint (tin mineralization), suggesting long-lived magmatic-hydrothermal activity in this part of the Andean back-arc crust. The decoupling of porphyry magmatism and hydrothermal activity may explain the unusual occurrence of relatively little fractionated felsic rocks together with extensive tin mineralization. Our study demonstrates the usefulness of the application of the U–Pb SHRIMP method to direct age determination of ore mineralization using Th-poor hydrothermal monazite even when dealing with geological young events. The common assump- tion of synchronous magmatism and hydrothermal ore formation in porphyry systems may not always be warranted. Ó 2008 Elsevier Ltd. All rights reserved. 1. INTRODUCTION During the last decades, the Th–U–Pb system of mona- zite has received increasing attention in geochronology due to (1) the occurrence of accessory monazite in a wide range of geological settings, (2) the elevated content of Th and— to a lesser extent—of U along with low concentrations of initial Pb commonly found in monazite from both mag- matic and metamorphic rocks, and (3) a high ‘‘closure tem- perature” (Dodson, 1973) for the Th–U–Pb system of monazite suggesting relatively high stability during low- temperature alteration (e.g., Scha ¨rer, 1984; Parrish, 1990; Wang et al., 1994; Harrison et al., 1995; Harrison et al., 2002). Estimates of the ‘‘closure temperature” vary from 530 °C up to 720 °C(Harrison et al., 2002). The elevated content of Th and U combined with a relative high concen- tration of radiogenic Pb in monazite from magmatic and metamorphic rocks has stimulated the application of chem- ical Th–U–Pb age determination with high spatial resolu- tion using the electron microprobe (e.g., Suzuki and Adachi, 1991; Montel et al., 1994; Cocherie and Albarede, 2001). These and other techniques of U–Th–Pb monazite geochronology were recently reviewed by Harrison et al. (2002). 0016-7037/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2008.05.059 * Corresponding author. E-mail address: kempe@mineral.tu-freiberg.de (U. Kempe). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 72 (2008) 4352–4366