Ore mineralogy of the Kaapelinkulma gold deposit, Southern Finland

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http://urn.fi/URN:NBN:fi:hulib-201912134157
Title: Ore mineralogy of the Kaapelinkulma gold deposit, Southern Finland
Author: Karpoja, Anna
Contributor: University of Helsinki, Faculty of Science
Publisher: Helsingin yliopisto
Date: 2019
Language: eng
URI: http://urn.fi/URN:NBN:fi:hulib-201912134157
http://hdl.handle.net/10138/308249
Thesis level: master's thesis
Degree program: Geologian ja geofysiikan maisteriohjelma
Master's Programme in Geology and Geophysics
Magisterprogrammet i geologi och geofysik
Specialisation: Petrologia ja taloudellinen geologia
Petrology and Economic Geology
Berggrundsgeologi och ekonomisk geologi
Discipline: none
Abstract: Kaapelinkulma is an orogenic gold deposit located in the Paleoproterozoic Vammala Migmatite Belt (1.91 – 1.79 Ma) in Valkeakoski municipality in Southwestern Finland, and it is considered to have been formed in microcontinent collision during Svecofennian orogeny and has been classified as an orogenic gold deposit. Kaapelinkulma comprises a set of sub-parallel lodes in a tight array hosted within a sheared quartz-diorite unit inside a tonalitic intrusion. Gold occurrence is hosted by an en echelon type sheared quartz-dioritic dyke which forms a large xenolith inside synorogenic tonalite intrusion, surrounded by mica gneiss. It is estimated that Kaapelinkulma gold deposit contains at least, 168 Kt of ore containing 3.8 g/t Au. Textural setting, mineralogical association form and assemblage of gold, sulphides and telluride grains in Kaapelinkulma were studied with field-emission scanning electron microscopy, with electron probe microanalyzer and scanning electron microscopy. Ore minerals observed in Kaapelinkulma are: arsenopyrite, löllingite, pyrrhotite, pyrite and chalcopyrite. Other ore minerals identified are native bismuth, gold, scheelite, bismuth-tellurides and maldonite, which were all found in abundant amounts. Ore minerals occur as dissemination in intergranular spaces between silicate matrix, as polycrystal aggregates in quartz-veins and quartz clusters; and within shear zones. Gold in Kaapelinkulma is present as two generations: as single free native gold grains and as polycrystalline gold aggregates. Polycrystalline gold aggregates are grains formed from several mineral association and their combinations. Most common polycrystalline gold aggregates are formed from combination of: maldonite-native Au, Au-Bi alloys, Au-Ag grains and Au-hedleyite. Single free native gold grains are pure gold or gold-silver alloys. Free native gold grains can be found as intergranular, single grains in silicate matrix and adjacent or as a part of disseminated ore together with polycrystalline gold aggregates, bismuth and bismuth tellurides. Polycrystalline gold aggregates are found in disseminated ore, which are in close contact with quartz-veins and sulphide aggregates, or as inclusions in arsenopyrite-löllingite contact zones- or in other sulphides. Concentration of Au varies in native-gold grain from 76.83 to 97.87 wt% according to EPMA analyzes and from 50.03 to 100 wt% according to FE-SEM analyzes. Minor to moderate amounts of silver and copper were identified within the grains. Grain sizes of gold varies significantly from 7µm2 to 5mm2. Ore mineral paragenesis were observed to start when arsenopyrite and löllingite crystallized first, followed, partly simultaneously by pyrrhotite, pyrite and chalcopyrite. This was followed by crystallization of maldonite, first occurrence of native gold and bismuth, bismuth-tellurides, hedleyite and finally tellurides and main occurrence of gold. General ore forming process in Kaapelinkulma has been open space filling.


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