Timing of orogenic gold mineralization in southern Finland and its relationship to the Palaeoproterozoic Svecofennian tectonic evolution

Category Mineral processing
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Saalmann, Kerstin۱; Mنnttنri, Irmeli۱; Peltonen, Petri۱; Whitehouse, Martin۲
Holding Date 08 September 2008

Gold prospects in the Svecofennian domain of southern Finland represent various genetic types. Numerous small gold occurrences are hosted by quartz veins in shear zones. Because of a clear structural control on mineralization they are often classified as orogenic gold. The true age of mineralization is not well constrained, neither is their spatial, temporal and genetic relationship to specific tectonic events. Two gold prospects chosen for dating are located in the Häme belt (Somero-Tammela gold zone) and Vammala migmatite belt (Jokisivu deposit) belonging to the Southern Svecofennian Arc Complex and the Central Svecofennian Arc Complex, respectively. Samples of pre-, syn- and post-mineralization rocks have been taken to gain absolute ages of gold deposition by using a variety of techniques. The obtained ages in both areas show remarkable similarities. The dioritc host rock of the Jokisivu prospect has an age of 1882 ± 4/1881 ± 3 Ma, which is similar to the 1881 ± 3 Ma crystallization age of a felsic volcanic rock from the host rock succession in the Somero-Tammela gold zone providing the maximum age of gold deposition. Based on structural correlation with neighbouring areas the shear zone in Jokisivu hosting the auriferous quartz veins formed during 1.88-1.86 Ga regional-scale folding. The obtained 1.82-1.78 Ga ages (SIMS U-Pb zircon, titanite) from samples taken from the ore zone overlap with the 1.81-1.79 Ga age of a pegmatite dyke (SIMS U-Pb zircon, TIMS monazite) that post-dates the main mineralization.
The 1.82-1.80 Ga ages at Jokisivu are thus interpreted to reflect a second hydrothermal event associated with granitic magmatism that led to remobilization of the gold. The structural geology in the Somero-Tammela gold zone implies a close relationship of the mineralization to quartz veins and SW-NE trending shear zones formed during dextral transpression in retrograde metamorphic conditions. SIMS U-Pb zircon and titanite ages and Ar/Ar amphibole cooling ages of a gold-bearing quartz vein and shear zone spread between 1.84 and 1.74 Ga, but a 1.81-1.78 Ga old pegmatitic dyke intruding after or during the late stages of mineralization and 1.78-1.76 Ga Ar/Ar biotite cooling ages mark the minimum age for mineralization. Considering the structural evolution the age of structurally controlled mineralization can be estimated at 1.83-1.80 Ga. It overlaps with the age of gold remobilization in the Vammala migmatite belt further N. In both areas, the hydrothermal event is terminated by pegmatite dyke intrusions suggesting that hydrothermal fluid flow and heat supply was influenced by late Svecofennian granitic magmatism. The post-1.91 Ga Svecofennian tectonic evolution in southern Finland shows the potential for at least two orogenic gold mineralization episodes related to contractional episodes (1.88-1.86 and 1.83-1.80 Ga). The second event, however, seems to be characterized by gold remobilization rather than being a separate mineralizing event.