U-Pb dating of skarn garnets from Bulgarian deposits

Calcic garnets from grossular-andradite (grandite) series have proven their ability to record the conditions and timing of their formation processes. Typically these minerals occur in skarn systems, together with other calc-silicates (diopside, epidote) and commonly host economic Cu, Zn-Pb-Ag, Au, Sn, W or Mo mineralization. Based on the U-content in the garnet structure, we used in-situ LA-ICP-MS U-Pb geochronology to determine the age record in more than 15 skarn deposits from different tectonic zones in Bulgaria. The data is partly complemented with ID-TIMS dating. The mineralogical, geochemical and petrological characteristics of the materials were described additionally. Both contact and infiltration skarns were studied.

The obtained data revealed that the garnet composition in terms of major elements does not affect the precision of age determination. Both andradite and grossular members yield age data with very high accuracy. The dating results, however, depend on the geochemical signature of the garnets and especially on the U-content and U/Pb ratio. Our data shows that skarn samples from the vicinities of magmatic bodies or along contacts of causative pegmatite veins usually have increased U-incorporation from several to more than 70 ppm, as suggested by their proximal position to the source. The contact skarn garnets formed by intrusion of silicate melts (or pegmatites) onto carbonate-rich hosts mostly produce precise ages, which are in good agreement with the geochronological zircon data about the magmatism in the studied regions (e.g. Central Pirin, Teshevo, Plana, Gutsal, Rila-West Rhodope, Sv. Nikola etc. plutons). The infiltration skarns, though, generally reveal ages with low accuracy and significant errors, mainly due to U-content below 1 ppm. The reason for the low U-concentration and U/Pb ratio is either connected with a primary U-deficit and its depletion in the garnet-precipitating fluids with time and space but might be also related to garnet retrograde hydrothermal alteration.

The time span of the Bulgarian skarn garnets is closely connected with the causative magmatic bodies. The studied skarns reveal Paleogene (~30-42 Ma - Central Pirin and Teshevo plutons and pegmatites from Rila-West Rhodope batholith; Djurkovo, Murzian and Zvezdel Pb-Zn deposits; ~ 58 Ma - skarns from Western Rila Mts., ~ 68 Ma – Babyak Mo-Ag-Au-W-Bi-Cu-Pb-Zn deposit), Cretaceous (~ 76 Ma- Gutsal pluton, 81 Ma - scheelite bearing skarns from the Plana pluton, 86 Ma – Iglika skarn deposit) and Paleozoic (~ 303 Ma – Martinovo Fe-skarn deposit) ages. Given the occurrence of Ca-garnet in contact rocks and hydrothermal ore deposits, our results highlight the potential of grandite as a powerful U-Pb geochronometer for dating magmatism and skarn-related mineralizations.

Acknowledgements. The study is partly supported by the DNTS 02/15 bilateral project between Bulgaria and the Russian Federation, financed by the Bulgarian National Science Fund.