Using Re-Os to distinguish metamorphic-metallogenic events and their economic value: an example from northern Sweden

Details Using Re-Os to distinguish metamorphic-metallogenic events and their economic value: an example from northern Sweden Using Re-Os to distinguish metamorphic-metallogenic events and their economic value: an example from northern Sweden

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja.....7071s&link_type=abstract

EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #7071

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Molybdenite occurrences in northern Sweden are associated with a 1.9-1.75 Ga Proterozoic continental domain north of the active Skellefte mining district. The occurrences increase in regions where supracrustal rocks are underlain by large volumes of granitoid, with the occurrences primarily associated with aplites, pegmatites, and slightly foliated granites. Two exposed and drilled deposits occur at Allebouda and Munka. Previous investigations linked these molybdenum deposits to presumed granite cupolas, and a highly discordant U-Pb zircon age of 1767 ± 9 Ma at Allebouda provided supporting data for suggesting that major Mo deposits in Sweden were primarily associated with 1.80-1.77 Ga (Late Svecofennian) granites (&{uml;O}hlander and Billstr&{uml;o}m 1989). We report Re-Os ages for Allebouda-Munka molybdenites that support a metamorphic, rather than an intrusive-magmatic origin for many molybdenite occurrences in northern Sweden. Four molybdenite samples from Allebouda and three from Munka yield Re-Os ages that are consistent within sample, but vary from locality-to-locality within the deposits. The host rock for dated molybdenites varies from 10-0.1 m intervals of pristine aplite or pegmatite occurring foliation-parallel within supracrustal sections, with one sample taken from the margin of a 3 cm smoky quartz band within a supracrustal interval. The contacts between aplite-pegmatite-granite and supracrustal rocks are generally gradational with drastic mineralogic changes over the cm scale. The Re-Os ages indicate several distinct periods of molybdenite formation within the 1.87-1.76 Ga span. We suggest the molybdenites developed in association with pulses of silicic-granitic melt derived during protracted metamorphism of the hosting supracrustal rocks. We suggest the breakdown of biotite during moderate-high grade metamorphism provided Mo for these small molybdenite occurrences of erratic grade (e.g., Bingen and Stein 2003). In addition, the low Re concentrations for Allebouda-Munka molybdenites (0.5-5 ppm) coupled with the restricted size of the individual occurrences does not support a large scale magmatic system as the mineralizing source (Stein et al. 2001). Age dispersion, low Re concentrations in molybdenite, and small localized pockets of mineralization collectively do not provide intriguing possibilities for mineral exploration. Rather, the results of this study document the utility of the Re-Os chronometer in molybdenite for tracking more than 100 m.y. of metamorphic history in the Svecofennian of northern Sweden. (Work supported by Georange.) Bingen, B. and Stein, H. (in press) Granulite facies biotite dehydration melting determined by the Re-Os dating of molybdenite, Rogaland, S. Norway: Earth and Planetary Science Letters. &{uml;O}hlander, B. and Billstr&{uml;o}m, K. (1989) Regional implications of U-Pb zircon dating of two Proterozoic granites associated with molybdenite mineralized aplites in northern Sweden: GFF, v. 111, p. 229-238. Stein, H.J., Markey, R.J., Morgan, J.W., Hannah, J.L., and Scherstén, A. (2001) The remarkable Re-Os chronometer in molybdenite: how and why it works: Terra Nova, v. 13, p. 479-486.

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