The Composition and Evolution of Fluids in Quartz Veins in Tonalites of the Kuittila Prospect, Hattu Schist Belt, Ilomantsi, Eastern Finland

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Title: The Composition and Evolution of Fluids in Quartz Veins in Tonalites of the Kuittila Prospect, Hattu Schist Belt, Ilomantsi, Eastern Finland
Author: Multanen, Jukka
Contributor: University of Helsinki, Faculty of Science
Publisher: Helsingin yliopisto
Date: 2019
Language: eng
Thesis level: master's thesis
Discipline: Geologia
Abstract: The Archean Hattu schist belt in Ilomantsi, eastern Finland, hosts several orogenic gold deposits and prospects, including the Pampalo gold mine. This study focuses on the quartz veins hosted by the Kuittila tonalite intrusion, which is a gold mineralized intrusion of the same age as the gold deposits in the surrounding metasediments. The aim of this study is to assess the relevance of magmatic-hydrothermal fluid contributions in the formation of orogenic gold deposits. Fluids of selected quartz veins located within Kuittila tonalite intrusion are studied to potentially get an answer to whether the Kuittila tonalite act simply as a trap for the gold precipitation, or if it can be considered as one of the potential sources for the gold. The analyzing methods comprise mineralogical and petrographic studies done with polarization and reflected microscope and electron microprobe (EPMA) along with fluid inclusion studies including microthermometry studies, Raman laser spectroscopy and laser-ablation-inductively coupled-mass spectrometry (LA-ICP-MS) analyses of selected fluid inclusions in different type of quartz veins. Seven different type of fluid inclusions were identified from selected quartz veins representing interpreted four different stages of quartz vein generation. Based on the petrography, the earliest fluids are aqueous-hydrocarbon bearing fluids, which are then followed by moderate salinity aqueous and aqueous-carbonic fluids, which are then accompanied by high salinity aqueous inclusions. Low salinity aqueous fluids are postdating the main mineralization stages. Fluids migrated through Kuittila tonalite contain features that are characteristic for fluids from both magmatic and metamorphic origin. The highly variable elemental compositions as well as the variable salinities and in particular molar Cl/Br ratios of different fluid types indicate that several pulses of fluids have migrated through the Kuittila tonalite with potentially variable origin of the fluids. The notably low molar Cl/Br ratios in all of the analysed fluid inclusion assemblages suggest that the contribution of metamorphic fluids have been significant even in the earliest fluids. The fact that in this study no gold were observed in the fluids with typical characteristics for magmatic fluids, and on contrary, those with gold signal shared more of the characteristics of metamorphic fluids would suggest that the magmatic fluids are not necessarily involved in the formation of the orogenic gold deposits in the Hattu schist belt and are not therefore mandatory in the formation of orogenic type gold deposits. It seems rather likely that the metamorphic fluids delivered potentially from country metavolcanites and metasediments are the sources for the gold and are precipitated in already formed fractures in the Kuittila zone.

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