Browsing by Subject "Orogenic gold"
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(2020)The Sukseton area is located in the northern part of Kittilä municipality, Central Lapland Greenstone Belt, approximately 15 km N from Suurikuusikko gold mine and 5 km NW from Iso-Kuotko orogenic gold deposit, between several large crustal scale thrust and shear zones. This area is a mix of different volcanic formations of Kittilä suite, felsic intrusion of Vuotso Complex and Paleoproterozoic intrusive rocks in the north. In addition to this, several porphyry dykes cut the Kittilä suite volcanic rocks around the area. Exploration work in this area started in the 1980’s when Outokumpu Oy found two minor gold and gold-copper mineralizations. In 2017, Agnico Eagle Finland Oy continued exploration in this area, intending to define the regional geology and the extent of the mineralizations. As a result of this exploration work, this study investigates more closely the regional geology, geochemistry, metamorphism and structural geology of the Sukseton area, as well as the geochronology of associated porphyry dykes. To understand and define the geology of the area, the following methods were used: geological bedrock and exploration trench mapping, interpretation of drill core loggings and several geophysical surveys, optical studies of polished thin sections and U–Pb dating a porphyry dyke sample. The metamorphic conditions of the area were studied through thorough petrological studies. In addition, an extensive geochemical and geotectonic classification of the rocks in the area was conducted. The Sukseton area composes mainly of different tholeiitic basalts and pyroclastic rocks with minor sulphide rich graphitic volcanic sediment and chert sections. Based on this study, these volcanic rocks originate from island arcs and mid-ocean ridges. With the help of geophysical surveys and field measurements, a couple of large fold structures were identified from the eastern part of the study area as well as a large shear zone in the middle of the area striking NE–SW. Porphyry dykes cut the volcanic rocks all around the area giving the minimum age of 1940±18 Ma for the volcanic rocks. Composition of porphyry dykes vary from rhyolites to basalt and they have similar geochemical characteristics with Nyssäkoski type felsic veins. The peak metamorphic conditions in the area represent high-P amphibolite facies metamorphism. Also, hydrothermal alteration is common in Sukseton and it can be metamorphic and magmatic in origin.
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(2020)The Suurikuusikko orogenic Au-deposit is located in the Central Lapland Greenstone Belt within the Kittilä Group volcano-sedimentary sequence. The deposit is hosted by almost vertically dipping Kiistala Shear Zone (KiSZ). In the Suurikuusikko Au-deposit, gold occurs mainly as refractory gold within the arsenopyrite and the pyrite. The Rimpi orebody is the northernmost orebody of the Suurikuusikko deposit. Pikkurouravaara is a unit located to the northeast of the KiSZ that also has arsenopyrite and pyrite with Au as refractory in the main zone; however, in Pikkurouravaara the Au-content is not significant. The goal of this work was to examine using mineralogical tools as distribution of Au in Rimpi and Pikkurouravaara. Studying the mineralogy of the Rimpi also provides information on whether the Rimpi orebody is similar to the other orebodies in the Suurikuusikko deposit. The methods used were optical- and reflective light microscopy, FE-SEM, EPMA, and LA-SC-ICP-MS. Petrographical observations show that the main minerals, texture, and alteration are the same in Rimpi and Pikkurouravaara. The main sulfides, arsenopyrite and pyrite, are also the same, however, morphology and habit are different. In Pikkurouravaara the sulfide grains are larger and more homogenous, while in Rimpi sulfides are more frequently zoned. Also, the trace element content of the sulfides between the study areas was significantly different. In Pikkurouravaara Se, Ni, and Mo occurrence is elevated in arsenopyrite as are Se, Cr, Ni, and Co in pyrites relative to Rimpi. Most likely, Pikkurouravaara and Rimpi are of different generations and enrichment of Au in Rimpi sulfides reflects the episodic formation of the sulfides. Some of the arsenopyrite and pyrite grains in Rimpi host visible gold between the sulfide grains and in fractures. These visible Au-bearing samples were compared to those Rimpi samples where Au occurs only as refractory. In samples with visible gold, the average Au grade in arsenopyrite and pyrite is relatively higher than in samples where Au is only refractory. Also, the S-content is higher in the former. The As-grades were equally variable in both Rimpi types. The average Au grade in arsenopyrites from Rimpi is 340 - 410 ppm, which is coherent with the previously reported average values of 398 ppm. The average gold content of Rimpi pyrites is 14 – 24 ppm, which is significantly lower than the previously reported average of 235 ppm. The major- and trace element data with mineral morphology suggests that the geological history of Pikkurouravaara has been less episodic than in Rimpi.
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