The Mustajärvi orogenic gold occurrence, Central Lapland Greenstone Belt, northern Finland
1University of Oulu, Oulu Mining School, Geology
|Online Access:||PDF Full Text (PDF, 21.5 MB)|
|Persistent link:|| http://urn.fi/URN:NBN:fi:oulu-201905141779
Oulu : M. Müller,
|Publish Date:|| 2019-05-17
|Thesis type:||Master's thesis (tech)
The thesis gives the first description of the geological setting, alteration, mineralization style and structural control of the Mustajärvi gold occurrence in the Central Lapland Greenstone Belt, northern Finland. The applied methods mainly comprise drill core logging, bedrock mapping, thin section studies and microprobe analysis.
The Mustajärvi gold occurrence lies at the southern border of the Central Lapland Greenstone Belt, in proximity to the first-order transcrustal Venejoki thrust system. The occurrence is structurally controlled by the second-order Mustajärvi shear zone, which is located at the contact between Sodankylä Group siliciclastic metasediments and Savukoski Group mafic and ultramafic metavolcanic rocks. The outcropping gold-mineralized veins comprise a set of parallel quartz-tourmaline-pyrite veins that show typical pinch and swell features, with the vein widths ranging approx. from 0.15 to 1 m. At a depth of 90 m, a different mineralization style was recently discovered, comprising a 2-m-thick, quartz-poor, massive pyrite-mineralized zone grading 45.1 ppm Au. The geochemistry of both mineralization styles is typical for orogenic gold deposits, with strongly enriched elements comprising Au, B, Bi, C (CO₂), Te, and Se. Silver, As, Sb, and W are moderately elevated and positively correlate with gold. Atypical for orogenic gold deposits is the strong enrichment of Ni and Co. In unweathered rock, gold is hosted by Au- and Au-Bi-telluride micro-inclusions in pyrite, whereas strong weathering near the surface has caused a remobilization of gold, resulting in free gold, deposited mainly in the cracks of oxidized pyrite.
The thesis emphasizes the prospectivity for a more extensive gold mineralized system, especially at greater depths and encourages further exploration. Future research on the Mustajärvi occurrence could comprise mineralogical studies on the massive pyrite mineralization at depth, and age dating of the mineralization based on observed monazite within the mineralized veins.
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