| Summary: | The Ersmarksberget gold mineralisation occurs in north-south striking, discontinuous quartz veins in the contact between a tonalitic intrusion (Juktan dome) and metagreywackes in the northern part of the Bothnian Basin, northern Sweden. The mineralisation is localised within sulphide and organic matter rich metasedimentary rocks. Metallurgical test work on the mineralisation produced low gold recoveries. Comprehensive ore mineralogy is essential for the gold recovery process. Forty polished thin sections were analysed using optical ore microscopy, scanning electron microscopy and the electron microprobe microanalyser. These analyses revealed that gold occurs as electrum with an Au:Ag ratio of approximately 5:1. The electrum occurs as free grains within quartz grain boundaries, intergrown with arsenopyrite crystals, filling in micro fractures and voids in the silicate matrix and arsenopyrite crystals, and clustered around the arsenopyrite-quartz grain boundaries. Arsenopyrite is the only sulphide associated with detectable electrum. The electrum has varying particle sizes ranging from less than 10 microns to more than 100 microns. A good fraction of the electrum is also submicroscopic. The gangue minerals include arsenopyrite, pyrite, chalcopyrite, galena, calcite, graphite and quartz. Low gold recoveries have been attributed to the chemical composition of the gold, reactive gangue minerals and preg-robbing minerals. Increasing the reaction time and/or reagents concentration, blinding of the gangue minerals, roasting of the ore or other oxidation methods will increase gold recoveries. Similarly an additional processing stage to eliminate or reduce some of the gangue minerals prior to cyanidation will improve gold recoveries. Validerat; 20110502 (anonymous)
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