In situ Lu-Hf dating of garnet and apatite as a means to understand fluid processes

Understanding fluid processes is crucial for understanding the magmatic and metamorphic evolution of rocks, as well as mineral transport and deposition. Targeted in situ geochronology of minerals which have interacted with fluids allow us to investigate the temporal evolution of these fluid systems. Combined with trace element analysis or stable isotope analysis it is possible to gain insight into the nature of source of mineralising fluids. These results contribute to modelling and understanding of mineral systems which can be used for targeting mineral deposits.

Early results of this work are presented based on situ Lu-Hf geochronology applied to garnet and apatite associated with a variety of mineral deposits from Finland. The mineral deposits are hosted in rocks are variable age (Archean to Proterozoic) and all were affected by the Svecofennian orogeny (1.92-1.78 Ga) causing deformation, metamorphism and/or remobilisation.

The Siilinjärvi P deposit hosted in a 2.61 Ga carbonatite (Karhu et al., 2001) presents apatite ages ranging from 2050 to 2260 Ma and calcite ages of 1800 to 1920 Ma indicating potentially several phases of fluid remobilisation which may be prior to or during the Svecofennian orogeny.

The Kiviniemi Sc deposit is hosted in a garnet bearing ferrodiorite (1857 ± 2 Ma, U-Pb zircon; Halkoaho et al 2020). In situ Lu-Hf analysis of garnet produces an age of 1824 ± 18 Ma and for apatite an age of 1835 ± 19 Ma.

Garnet and apatite geochronology has also been applied to Li bearing pegmatites in the Somera-Tammela pegmatite province in Southern Finland. Garnet gave an age of 1801 ± 53 Ma and apatite gives 1835 ± 26 Ma. A second sample produced a nearly identical apatite age of 1835 ± 15 Ma.

Two garnets were sampled from inferred Archean deposits, the Sotkamo silver Mine is hosted in the Tipasjärvi Greenstone Belt and the Hosko gold deposit hosted in the Ilomantsi Greenstone Belt. In Sotkamo garnet from a quartz vein hosting base metal mineralisation produced a Lu-Hf age of 1870 ± 27 Ma. Two garnet samples were dated from the Hosko gold deposit, in mineralised sediments, garnet associated with vein quartz produced an age of 1837 ± 4 Ma. A granitic vein cross-cutting the ore hosting sediments gave an age of 2620 ± 7 Ma, although this sample clearly recorded a resetting event with younger ages obtained from the rim.

Despite only have the age results so far, it is clear that the Svecofennian orogeny had a strong impact on mineral systems, reworking of deposits thought to be Archean.

 

Halkoaho, T., Ahven, M., Rämö, O.T., Hokka, J., Huhma, H., 2020, https://doi.org/10.1007/s00126-020-00952-2

Karhu, J.A., Mänttäri, I., Huhma, H., 2001. Radiometric ages and isotope systematic of some Finnish carbonatites. University Oulu, Res. Terrea, Ser. A. No. 19.8.