U–Pb SHRIMP geochronology of Th-poor, hydrothermal monazite: An example from the Llallagua tin-porphyry deposit, Bolivia
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...
Saved in:
Published in | Geochimica et cosmochimica acta Vol. 72; no. 17; pp. 4352 - 4366 |
---|---|
Main Authors | , , , , , , |
Format | Journal Article |
Language | English |
Published |
Elsevier Ltd
01.09.2008
|
Online Access | Get full text |
Cover
Loading…
Summary: | 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
207Pb/
206Pb 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 assumption of synchronous magmatism and hydrothermal ore formation in porphyry systems may not always be warranted. |
---|---|
ISSN: | 0016-7037 1872-9533 |
DOI: | 10.1016/j.gca.2008.05.059 |