Geology
Regional Geology
The oldest part of the Finnish bedrock was formed more than 3 billion years (Ga) ago. This period produced large volumes of plutonic rocks, that are presently seen as granite, gneisses, and migmatites. The greenstone belts of Eastern Finland were formed by volcanic activity and the deposition of volcanic and sedimentary rocks between 2.9-2.6 Ga.
The Tipasjärvi greenstone belt is the southernmost, isolated extension of the Archaean Kuhmo-Suomussalmi greenstone belt. The Tipasjärvi greenstone belt, roughly NNE-SSW oriented, 25 km long and a few km wide, is a T-shaped belt surrounded by gneisses. It has undergone multiple deformation phases and an amphibolite facies metamorphism, which has generally destroyed the primary volcanic or sedimentary features. The structural evolution comprises of two plastic deformational episodes, producing a tight synformal structure, that is later cut by a third brittle to a semi-brittle structural event. However, occasional primary features have been preserved in the north-western part of the belt enabling the determination of depositional top direction and reconstruction of a stratigraphic column.
Deposit Geology
The Silver Mine deposit’s geology is dominated by siliceous quartz porphyritic rocks, breccias, and layered tuffs and tuffites, indicating the eruption of volcanic rocks in the shallow water or subaerial environment. Based on the new age determinations, Silver Mine mineralization is interpreted to have been formed about 2.8 billion years ago. The Ag-Au-Zn-Pb-deposit is situated in the middle part of the volcanic succession. Mineralization is composed of numerous lenses along the six parallel mineralized layers. Pervasive lineation is the most important feature and controls the deposit.
The deposit is 400 m long and from 5 to 110 m wide, averaging 40 m. The deposit dips 65° to the southeast and plunges 60° to the south-southwest. Geophysical deep-penetrating “Sampo” surveys conducted by GTK in 2011 and 2013 indicate that the mineralized zone continues down to a depth of 1.5, or even 2, kilometers’.
The heterogeneous mineralized layers vary laterally in thickness and locally join up with the neighboring layers. The interspaces are not totally barren, but, rather, contain a weak dissemination of ore minerals. The ore can be divided into two main types, Ag-Au-type and Zn-Pb-Ag-type. However, these types are gradational and occur interchanged, thus cannot be separated in the mineable scale.
Mineralogy
The host rock for the Silver Mine mineralization is quartz-sericite schist. The ore mineralization appears both as the abundant sulfide-bearing quartz- and quartz-carbonate veins and sulfide dissemination in the host rock. The main minerals of the quartz-sericite schist are quartz, muscovite and biotite. The dominant carbonate mineral is ankerite.
Pyrite, pyrrhotite, sphalerite, and galena are the most common sulfide minerals. Sphalerite and galena are the main zinc- and lead-bearing minerals. The silver mineralogy is more diversified. The silver occurs as Ag-sulphides and Ag-sulfosalts, which are commonly associated with galena, either as inclusions or along the grain boundaries. The most common silver-bearing minerals are dyscrasite, freibergite, and pyrargyrite while allargentum, argentite, and native silver are less common. The gold occurs as electrum (gold-silver alloy) and in the lattice of silver minerals.