Till geochemistry of gold, arsenic and antimony in the Seinäjoki district, western Finland

Abstract

The data on Au, As, Sb, Ag, Co, Cu, Mn, Ni, Pb and W concentrations in the fine fraction of till (−0.06 mm) in the regional geochemical prospecting of Au is dealt with. The study was carried out in the Seinäjoki district in western central Finland. The bedrock of the area sampled is composed of variably migmatized volcanic-sedimentary schists and synorogenic granitoids of Middle Proterozoic age. Uneconomic Sb-As-, Pb-Zn-Ag-As-, W- and Sn-occurrences, with Au as a common constituent in the first two types of mineralization, are known to occur in the district. The overburden is composed mainly of sandy basal till deposited during the latest glacial movement from the north. Signs of older till, transported from northwest have also been encountered. The length of chemically detectable dispersal trains in the fraction of till analyzed seems to be not more than some hundred meters in the bottom till and a maximum of about 2 km in the surface till. Preglacially weathered bedrock is common under the till. The 300 km 2 study area was sampled on a 500 m by 500 m grid. Percussion drills with flow-through samplers were used to collect composite samples of the C-horizon at 1–2-m intervals. Normally, 200–300 g of till per sampling site were taken. Considering the regional geochemical variations typical of Finland, the Seinäjoki study area comprises remarkable anomalies for Au, As and Sb. The factor analysis and distribution patterns suggest that anomalous Au is mostly derived from different sources than anomalous Sb. Anomalous As is partly related to sources of Au and Sb, but also to sources of base metals which, on their part, strongly correlate with each other. The known areas of AuAs mineralization proved to be too small and too low-grade to be seen clearly in till with the sampling method employed. For Au the negative result may be due partly to poor reproducibility of the Au data. In contrast, the Sb pattern corresponds very well to known areas of Sb mineralization and indicates that the mineralization is more widespread than formerly known. Follow-up studies were made in five areas where the regional study revealed anomalous Au. Signs of mineralization were obtained in two targets. In one of them, mineralization, probably hosted in a mica gneiss, contained gold, arsenopyrite and iron sulphides. In another target, Au seemed to be related to a secondary enrichment, the primary source remaining uncertain. In the rest of the targets the source of the anomalies remain unresolved.