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

2007 GSA Denver Annual Meeting (28–31 October 2007)
Paper No. 100-10

Presentation Time: 4:00 PM-4:15 PM

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, Trondheim, N-7491, Norway, rune.larsen@ntnu.no and STEIN, H.J., AIRIE Program, Department of Geosciences, Colorado State University, Fort Collins, CO 80523-1482 USA, and Geological Survey of Norway, Trondheim, N-7491, Norway 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 scheelite–molybdenite 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.
2007 GSA Denver Annual Meeting (28–31 October 2007) General Information for this Meeting
Presentation Handout (.doc format, 4937.0 kb)
Session No. 100 Metallogeny and Isotope Geochemistry—New Approaches, New Perceptions, New Paradigms Colorado Convention Center: 504 1:30 PM-5:30 PM, Monday, 29 October 2007 Geological Society of America Abstracts with Programs, Vol. 39, No. 6, p. 276
© Copyright 2007 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.