Sulfide mineralogy of the Suurikuusikko (Kittilä) gold deposit : a comparative study of the Au-rich Rimpi ore zone and the Au-poor Pikkurouravaara area

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http://urn.fi/URN:NBN:fi:hulib-202006012537
Title: Sulfide mineralogy of the Suurikuusikko (Kittilä) gold deposit : a comparative study of the Au-rich Rimpi ore zone and the Au-poor Pikkurouravaara area
Author: Hämäläinen, Roosa
Contributor: University of Helsinki, Faculty of Science
Publisher: Helsingin yliopisto
Date: 2020
Language: eng
URI: http://urn.fi/URN:NBN:fi:hulib-202006012537
http://hdl.handle.net/10138/315767
Thesis level: master's thesis
Discipline: Geologia
Abstract: 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.
Subject: Suurikuusikko
Kittilä
Lapland
Central Lapland Greenstone Belt
Orogenic gold
Arsenopyrite
Pyrite
Sulfides
Refractory gold


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