Oiva gold-quartz dyke in the Lapland granulite belt, Laanila, northern Finland: Case study on high-voltage selective fragmentation for detailed mineralogical and analytical investigation

Abstract

A short-pulsed high-voltage selective fragmentation test was performed on samples from the Oiva gold-quartz dyke to liberate gold and related minerals using selFrag laboratory scale equipment. SelFrag was able to release individual grains of gold, electrum and other minerals of interest, preserving their original texture, shape and size, and provided an opportunity for detailed study of their morphology, surficial features, grain size and composition. The mineralogy of gold/electrum and associated ore minerals was studied by light and electron microscopy using various non-magnetic, heavy mineral concentrates (<200 μm and 2 mm-200 μm) of the fragmented samples. As a result, a number of gold/electrum grains were recovered from fragmented and preconcentrated samples. The composition of gold/electrum varied from about 27.15 to 96.06 wt% Au and from 3.5 to 72.85 wt% Ag, with an Ag/Au ratio of 0.04-2.68. Morphologically, gold/electrum grains were irregular, globular, branched, grooved, subrounded, flaky, and commonly <50 μm in size. Although chalcopyrite was relatively common, other sulphides were generally fine grained, disseminated and rare. Gold/electrum was goethite-quartz hosted and infrequently associated with silver telluride, bismuth telluride, native bismuth, Ag sulphide and Ag halide. Based on mineralogical evidence, the primary electrum might have been deposited with chalcopyrite and other base metal sulphides, and partially remobilized by either late-stage hydrothermal fluid or supergene processes for the depletion of its silver to form fine-grained native gold. Such secondary enrichment might also be supported by the presence of secondary acanthite, iodargyrite and covellite, which might only be possible to recover using selective fragmentation and consecutive preconcentration processes.