Geology of the Skrukkelia molybdenite deposit, northwest margin of the Oslo Graben, Norway.

Abstract

The Skrukklia Mo-occurence is situated in Precambrian gneisses along the northwest margin of the igneous complex of the Oslo Region. Within the mineralized area a Permian granite suite (the Aurtjern granite) intrudes the gneisses 2 km from the western border of the main igneous complex. The Aurtjern granite is a multistage intrusion of biotite granite. The gneisses are further cut by Permian dykes and hydrothermal breccias, the most important of the latter being the Glasberget breccia with a core of mainly pegmatitic quartz, and the Dalen breccias with matrix mainly of rock flour. Weak propylitic alteration and a weak stockwork of pyrite-bearing veinlets, defining a pyrite halo enclosing the mineralized area and the intrusives, are the most extensive alteration types in the area. Agrillic alteration is observed in some drill-cores. Quartz-sericite-pyrite and K-silicate alteration and albitization are seen in subordinate amounts. A low-grade molybdenite mineralization is found over an area of more than 2 km, occuring as a stockwork of thin (< 2mm) veinlets in the gneisses and partly in granite porphyries. Quartz-molybdenite and quartz-pyrite-molybdenite are the main paragenic types. The sulphide mineralizations and the alteration patterns in the area can be interpreted both as a porphyry molybdenum-like occurence and as a roof zone mineralization of granitic intrusion.