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Isotopes in exploration

The Geological Survey of Finland has a growing research group in accessory mineral and metal isotope geochemistry of ore-forming systems. We are currently studying the radiogenic and stable isotope geochemistry of specific minerals cogenetic with ore minerals in select Finnish magmatic-hydrothermal copper and gold deposits.

Our instrumentation includes a Nu Plasma multi collector-ICP-MS and laser ablation system, which allows us to analyze the isotopic compositions of ore cogenetic minerals in situ, either on grain mounts or on thin sections (30-100 microns). Individual phases can also be identified using EMP, SEM or MLA and later on analyzed using our equipment. More common solution based analyses are also possible.

We are targeting ore cogenetic minerals such as U-rich accessory phases, sulfides, gold nuggets, apatite, scheelite and tourmaline for their isotopic compositions in order to determine the age of the mineralization, the provenance of the mineralizing fluids and the chemical parameters leading to the formation of the ore.

Primary U-rich accessory minerals such as zircon and metamorphic minerals such as monazite and titanite that crystallized at the same time as the ore formation processes. Based on in situ analyses (25-10 µm) of their U/Pb isotopic composition, we can date these phases and build constraints on the age of the mineralization relative to the country rocks. These analyses can be performed within minutes, on thin sections (30 µm), which reduces the sample preparation costs and provides petrographic constraints on the crystallization history.

Geological reservoirs have specific Pb, S and Sr isotopes that can be traced by the isotopic composition of cogenetic minerals such as sulfides (Pb, S), gold nuggets (Pb), scheelite and apatite (Sr).

Metal isotope geochemistry study of ore-forming systems is a new and growing field of research. Isotopic systems such as Cu, Fe, S, and B from individual phases will shed new light into the temperature and redox changes that are responsible for the precipitation of ore minerals from the mineralizing fluids leading to the formation of economic deposits.

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