The nature of gold in the Aitik Cu-Au deposit implications for mineral processing and mine planning
Sammanfattning: The Aitik mine is a large Cu-Au producer in Europe with an annual production of 28 million tons of ore and a planned expansion of production to 36 million tons of ore until year 2013. Large ongoing investments are done in the mine and a new processing plant is built to manage the capacity increase. The mine is situated 15 kilometers from the town Gällivare, in northern Norrbotten, Sweden. The ore is a large porphyry type deposit with low grades of disseminated copper (0,27 %) and gold (0,16 ppm). Porphyry copper ores usually have low recoveries when flotation is used as an extraction method and also the Aitik mine has shown historically low gold recoveries (< 50 %). The Aitik deposit is situated in 1.9 Ga Svecofennian volcanic and sedimentary rocks formed in a volcanic arc environment and later deformed and metamorphosed at amphibolite facies. The main ore zone consists of biotite and muscovite schist and gneisses. A thrust fault divides the main ore zone from the hanging wall that contains feldspar biotite amphibole gneiss. In the footwall the feldspar biotite amphibole gneiss also is found and an intrusion of quartz montzodiorite. In this study, horizontal and vertical zoning patterns were recognized in data based on the block model that consist of drill cores and drill ship samples since the mine started 1968. The vertical zoning shows a decrease in high (> 0,6 % Cu) copper grade areas and an increase in high (>0,3 ppm Au) areas towards the depth. An increase in the amphibole content is seen in the biotite gneiss towards depth. The gold mineralized areas are trending northeasterly and plunging northeast, and for copper north-northwest. The horizontal zoning can be seen in the middle of the ore zone where a high-grade area of disseminated sulfides in potassic altered biotite gneiss/schist exists. Further towards the hanging wall, a low grade, strongly sericite altered and pyrite-rich schist is common. Gold-rich areas are found close to the hanging wall in strongly sericite schist altered rock and in a part of a biotite schist that are strongly affected by K feldspar and epidote alteration and quartz and pyrite growth, but also in the amphibole rich parts close to the footwall. Copper dominated mineralization is mainly developed towards the hanging wall, as disseminated and massive veins of chalcopyrite. The rich dissemination in the centre of the ore body is suggested to be a part of the primary mineralization, and the locally higher concentrations in veins would originate from later remobilization. To get a better knowledge of how gold occurs at Aitik and how to increase the gold recovery during mining a detailed mineralogical investigation was done. This revealed that there is a change in gold associations towards depth. Commonly associated minerals with gold at 600 m and below are K-feldspar, biotite, plagioclase, quartz, chalcopyrite and pyrite. At higher levels in the mine gold is most commonly associated with chalcopyrite and pyrite. There is also a change in gold content in alloys towards depth where the gold decreases in electrum and amalgam but the amount of native gold grains increases. As the average gold and copper grades are relatively constant at the different levels in the mine this means that the gold associations that are not connected to sulfides (chalcopyrite) are going to tailings from the deeper part of the mine and the gold recovery as a consequence is lower today. Some other copper-gold deposits have implemented several different recovery methods to extract the gold from the ore, an example from Bajo de la Alumbrera (Chryssoulis 2001), Argentina with a similar ore type as Aitik have a combination of gravity and flotation in their recovery process. Based on the results on the mineralogy and the mineral associations it would be possible to combine additional recovery methods as gravimetric methods to the flotation that are used today. Lime used in the process might have a depressive affect on gold and should be further investigated. The Aitik deposit is interpreted to have a mixed origin, with an early porphyry copper style of mineralization overprinted by a later and minor iron-oxide Cu-Au (IIOCG) system (Wanhainen 2005). Gold bearing fluids at Aitik were compared with those at the minor epigenetic Cu-Au deposits Liikavaara East, Fridhem, Jårbojoki and Muorjevaara, that occur in the Nautanen deformation zone. According to earlier studies high salinity fluids are connected to the copper mineralization and in this study a connection between gold and CO2 is suggested based on the positive correlation between CO2 and gold that could be found in all deposits. However, the carbonic fluids varied in composition and includes pure CO2, CO2-CH4, CO2-H2O, or CO2 present as calcite in aqueous inclusions. The highest gold grades were associated with CO2 in silicates and CO2-CH4 in sulphides.
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