Structural control on gold mineralization in the Satulinmäki and Riukka prospects, Häme Schist Belt, southern Finland

Abstract

The Satulinmaki and Riukka prospects located in the Hame Schist Belt in southern Finland are dominated by mafic and intermediate to felsic metavolcanic rocks of the Forssa Group formed in a continental arc setting. This magmatic belt formed some 1890 – 1880 Ma ago and has been deformed during the Svecofennian orogeny. The dominant penetrative foliation is represented by S n+1 , which is axial planar to cm- to dm-scale iscolinal F n+1 folds and thus parallel to the layering S n . Associated ductile shearing might reflect early thrusting. D n+2 post-dating peak metamorphism is characterized by small-scale to regional-scale refolding of F n+1 folds around ~SW-NE fold axes. This phase is transitional to development of SW-NE to WSW-ENE and NW-SE striking shear zones and faults formed due to dextral transpression during D n+3 at retrograde conditions crossing the brittle-ductile transition. Many mineralized quartz veins have formed during this event. Later faults and quartz veins and reactivation of pre-existing structures during D n+4 indicate rotation of the stress field to ~NESW oriented compression. A clear ~SW-NE trend of sulphide mineralization and elevated gold contents and the spatial association to D n+3 quartz veins, shear zones and faults suggest a strong structural control, typical of orogenic gold deposits, and that mineralization took place during D n+3 . The controlling structures, (i) WSW-ENE to SW-NE shear zones and faults and (ii) NW-SE oriented fault, are second and third order structures to major regional-scale shear zones. The fault zones and their intersection points impart a directional permeability so that the mineralising fluids were channelled along dilatant zones. Approx imately (W)NW-(E)SE trending faults being (re)activated as extensional faults or dilatant shear planes during D n+3 transpression with WNW-ESE to NW-SE oriented compression direction could have acted as conduits for fluids during upward flow from deeper crustal level. Future studies in this area are necessary to test if other prospects show similar timing and structural control, and future exploration for finding new targets should consider sites of (W)SW-(E)NE and NW-SE-striking second and third-order structures branching from major shear and fault zones.