RE-OS DATING OF OROGENIC W-MO DEPOSITS IN THE MID NORWEGIAN CALEDONIDES

LARSEN, R.B., Department of Geology and Mineral Resources Engineering, Norwegian University of Science and Technology (NTNU), Trondheim, N-7491, Norway and STEIN, H.J., AIRIE Program, Department of Geosciences, Colorado State University, Fort Collins, CO 80523-1482 USA, and CEED Centre of Excellence, University of Oslo, 0316 Oslo, P.O. Box 104, Norway, rune.larsen@ntnu.no

The western parts of the Mid-Norwegian Caledonides, i.e. the Helgeland and Röddingsfjell Nappe Complexes (HNC and RNC) are recognized as a regional W-Mo province comprising several sub-economic or economic scheelitemolybdenite deposits. The largest showings comprise the Målvika (in HNC) and the Bjellåtinden (in RNC) occurrences that are currently being explored for their economic potential. Average W grades are 0.8 and 1.8 wt% WO3 for the Målvika and Bjellåtinden deposits, respectively.

In this study, nine Re-Os molybdenite ages from four deposits in the Bjellåtinden area cluster at 430 ± 5 (scapolite skarn), 406 ± 1 (quartz veins), and 401 ± 3 Ma (quartz veins).

All deposits occur over eight kilometers in a shear zone that formed in supra-crustal meta-sedimentary rocks a few hundred meters above a Paleoproterozoic basement window, i.e. the typical structural setting for W-Mo deposits in the RNC.

Given these ages, scapolite skarn formation (430 Ma) coincides with high grade Scandian metamorphism whereas quartz vein formation (406-400 Ma) coincides with regional extensional shearing at lower metamorphic grades.

Curiously, granitic magmatism that is normally associated with W-Mo mineralization is not observed in the Bjellåtinden area. However, previous U-Pb dating in the HNC and RNC document batholith-scale magmatism at 435-430 Ma in HNC (Barnes et al., GSA annual meeting, 2006) and the 406-400 Ma ages coincide with the emplacement of granitic pegmatite swarms in the western part of the RNC.

The first W-Mo mineralizing event (430 Ma) formed scapolite (Me70-85) skarn associated with extremely CO2-rich fluids (Larsen, Mineralium Deposita, 26, 281-289, 1991) whereas the later events (406-400) formed more classical pyroxene-garnet-amphibole skarn associated with aqueous-carbonic saline fluids. Our study underlines the importance of long-lived shear zones as conduits for ore-forming fluids and documents that ore-forming events indeed may occur during high grade metamorphism.

Session No. 100

T65. Metallogeny and Isotope Geochemistry—New Approaches, New Perceptions, New Paradigms

Monday, 29 October 2007: 1:30 PM-5:30 PM

Geological Society of America Abstracts with Programs. Vol. 39, No. 6, p.276

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2007 GSA Denver Annual Meeting (28–31 October 2007)

RE-OS DATING OF OROGENIC W-MO DEPOSITS IN THE MID NORWEGIAN CALEDONIDES