U-PB DATING OF HYDROTHERMAL GARNET FROM SKARN DEPOSITS – IMPLICATIONS FOR PETROGENESIS AND ORE DEPOSITS

Skarns provide one of the clearest genetic links between magmatic events and ore deposits. U-Pb and/or Ar-Ar dating of associated intrusions has been used to bracket the age of skarn formation but there have been few attempts to directly date skarn minerals such as garnet, and attempts to date garnets in regional metamorphic terranes have led to uneven results. We report here the first direct dates for hydrothermal garnet from skarn deposits.

Clean, inclusion-free garnet fractions were separated from two different skarn deposits. Individual fractions separated on the basis of color gave U-contents of 3.0 to 32.3 ppm, with low to moderate proportions of initial common Pb. Initial common Pb compositions were determined on associated sulfide minerals.

The Shimyoka Fe skarn in Zambia occurs in an area of multiple intrusions. U-Pb zircon ages range from 560 Ma for the Hook granitoid massif, 551 Ma for syntectonic rhyolite in the Mwembeshi shear zone, 540±2 Ma for alkali-altered, post-tectonic granite, and 503±1.7 Ma for the latest magmatic pulse of gabbro. Garnets from four different skarn samples fall on or near concordia between 496 and 510 Ma, indicating that skarn is most likely related to the latest magmatic pulse and not to the earlier granitoids.

U-Pb single and multi-grain zircon analyses from the intrusion associated with the Porphyry Mtn. Cu skarn in Gaspe, Quebec scatter near concordia between 365-385 Ma, with evidence of both zircon inheritance and post-crystallization Pb-loss. A U-Pb ion probe single zircon analysis yielded an age of 371.2±4.7 Ma, in good agreement with a U-Pb titanite isochron age of 371.8±8.7 Ma. Nine U-Pb analyses of garnet fall on or near concordia between 340 and 380 Ma, with most clustering at ~360 Ma. A modified York-II regression through all but two of the garnet analyses, with the initial Pb composition pinned by an analysis of associated pyrrhotite, gives a 238U/206Pb isochron age of 362.7±3.5 Ma.

U-Pb garnet ages reported here are relatively imprecise; however garnets from both the Shimyoka and Porphyry Mtn. skarn systems give U-Pb ages that are in reasonable agreement with or slightly younger than the U-Pb ages of associated plutons. At Shimyoka this has allowed correlation of skarn mineralization with a specific intrusive event within a complex petrotectonic evolution.