Abstract
The Mustajärvi gold occurrence lies in the southern part of the Paleoproterozoic Central Lapland Greenstone Belt, in proximity to the first-order transcrustal Venejoki thrust fault system. The gold occurrence is structurally controlled by the second-order Mustajärvi shear zone, which is located at the contact between siliciclastic metasedimentary and mafic to ultramafic metavolcanic rocks. The main mineralization comprises a set of parallel veins and sulfidized rocks that are slightly oblique to the shear zone and are hosted by third-order structures likely representing Riedel R-type shears. The gold-mineralized rock at Mustajärvi is associated with pyrite that is present in 0.15- to 1-m-wide quartz-pyrite-tourmaline veins and in zones of massive pyrite in the host rocks with thicknesses ranging from 1.15 to 2 m. In unweathered rock, hypogene gold is hosted by Au- and Au-Bi-telluride micro-inclusions in pyrite, whereas strong weathering at near surface levels has caused a remobilization of gold, resulting in free gold deposited mainly in the cracks of oxidized pyrite. The geochemistry of both mineralization styles is typical of orogenic gold systems with strong enrichments comprising Au, B, Bi, CO2, Te, and Se; and less consistent anomalous amounts of Ag, As, Sb, and W. Unusual for orogenic gold deposits is the strong enrichment of Ni and Co, which leads to the classification of Mustajärvi as orogenic gold occurrence with atypical metal association.
Highlights
The Paleoproterozoic Central Lapland Greenstone Belt (CLGB, Fig. 1) has a total reported gold endowment of just over 10 Moz
The Venejoki thrust system has mostly been ignored as a gold exploration target, despite the fact that, regarding the timing and direction of deformation, it is sharing many features with the nearby Sirkka thrust zone which hosts a large proportion of known gold occurrences within the Central Lapland Greenstone Belt (Niiranen et al 2015)
Gold mineralization at Mustajärvi has a strong structural control, being hosted by third-order structures, possibly Riedel R-type shears, that splay off the second-order Mustajärvi shear zone which is likely to have formed due to the competency contrast between the highly albitized siliciclastic metasedimentary and the metavolcanic rocks in the mineralized area
Summary
The Paleoproterozoic Central Lapland Greenstone Belt (CLGB, Fig. 1) has a total reported gold endowment of just over 10 Moz. The Mustajärvi gold occurrence, lies in the southern part of the CLGB, in proximity to the first-order Venejoki thrust fault zone. The Venejoki thrust system has mostly been ignored as a gold exploration target, despite the fact that, regarding the timing and direction of deformation, it is sharing many features with the nearby Sirkka thrust zone which hosts a large proportion of known gold occurrences within the Central Lapland Greenstone Belt (Niiranen et al 2015). Seismic reflection data indicate the Venejoki fault zone can be traced to the base of the crust at a depth of approximately 42 km (Patison et al 2006; Niiranen et al 2014). Few gold occurrences are currently known in proximity to
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