Abstract
Geochemical investigations of till is a widely used method in metal exploration as the till commonly inherits the geochemical signature (including the metal contents) of the parent bedrock. In this investigation, over 2000 till samples were collected in the Sarvlaxviken area, southern Finland, where several polymetallic (Cu, Zn, Pb, As, Sn, W and In) veins recently have been discovered in Proterozoic crust along the border between Late Svecofennian granites and the Wiborg Batholith. The bedrock is commonly covered by compact and poorly sorted basal till, formed during the Late Weichselian glaciation event. Several glacial-transported boulders, with high contents of Cu, Zn, As, Sn, Mo and Bi and derived from the local bedrock, have also been discovered on top of the till and provide evidence for concealed mineralisation in the local bedrock under the till cover. The frequent distribution of till in the Sarvlaxviken area provides excellent conditions for the search of such hidden mineralisation by means of systematic till sampling, even if large farm field areas, composed of clay-rich sediments, and seawater-covered areas (Sarvlaxviken bay), had to be avoided in the sampling program. The till samples were collected during university courses and training programs led by the authors and were analysed in a cost-efficient and certified laboratory. Obtained geochemical data were statistically processed by using K-means clustering algorithms which can be used to treat large sets of geochemical data. The results provided anomalies that mainly occur in till with a thickness of <1 m and are considered to be derived from a local bedrock source. The discovered anomalies provide strong evidence for numerous undiscovered veins beneath the till cover.
Highlights
Till geochemical methods are considered an essential tool for mineral exploration in Finland (Sarala, 2015) and elsewhere in glaciated ter rains
Over 2000 till samples were collected in the Sarvlaxviken area, southern Finland, where several polymetallic (Cu, Zn, Pb, As, Sn, W and In) veins recently have been discovered in Proterozoic crust along the border between Late Sveco fennian granites and the Wiborg Batholith
The central and southeastern parts of the Fennoscandian Shield are dominated by juvenile Svecofennian crust that formed at c. 1.9 Ga after which it was subject to repeated phases of deformation and metamorphism (Andersson et al, 2006)
Summary
Till geochemical methods are considered an essential tool for mineral exploration in Finland (Sarala, 2015) and elsewhere in glaciated ter rains. Geochemical features of the local granite intrusions responsible for mineralisation were outlined by Nygård (2016) and Villar (2017) This contribution presents the results from a geochemical investigation of till in the Sarvlaxviken area, in which a wide spectrum of metal anomalies was revealed. The last phase reached locally anatectic conditions, resulting in migmatization, partial melting and formation of Late Svecofennian granites at 1.84 Ma. The metamorphosed Svecofennian crust was in turn intruded from 1.64 Ga to 1.50 Ga by anorogenic rapakivi granites, which constitute a charac teristic group of granites that formed in an extensional tectonic setting in the central-southeastern parts of the Fennoscandian Shield (Ramoand Haapala, 2005). Southern Finland (Fig. 1) provides an excellent example geological section detailing the evolution of Svecofennian meta-sedimentary for mations, mafic-felsic volcanic rocks and Early Svecofennian granitoids to the creation of anatectic Late Svecofennian granites and the devel opment of incipient continental rifts, including A-type rapakivi granites, lacustrine sandstone basins and tholeiitic dolerite dykes
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