GEODYNAMIC SETTINGS OF ARCHEAN GREENSTONE BELTS AND THEIR INFLUENCE ON SYNVOLCANIC METALLOGENY: EXAMPLES FROM THE SUPERIOR PROVINCE

Archean greenstone belts of the Superior Province are host to many world class precious and base metal mineral deposits and are very important to the local economies in the Lake Superior region. The metallogeny of each greenstone belt is directly linked to their geodynamic setting and geological history. However, their geodynamic settings are, more often than not, poorly understood and the relative prospectivity of a belt for a particular type of metal deposit is ambiguous if minimal exploration or mining activity has taken place. This study was aimed at discovering subtle differences in the geodynamic settings of greenstone belts to explain variations in their metallogenic endowment. In the Wawa subprovince, of the Superior Province, many of the greenstone belts formed at ca. 2720 Ma and were previously interpreted to have formed in a rifted-arc to back-arc tectonic setting. While this interpreted tectonic setting is prospective for Cu-Zn-Pb volcanogenic massive sulfide (VMS) deposits, the ca. 2720 Ma greenstone belts show vastly different (discovered) endowments for this deposit type. Additionally, the prospectivity for magmatic Ni-Cu-PGE deposits also varies between the greenstone belts. Therefore, it is unlikely that that the belts formed in an identical tectonic and geodynamic setting.

Four ca. 2720 Ma greenstone belts in Ontario and Minnesota were characterized using mapping, and regional-scale trace element and isotopic (Nd and Pb) geochemistry. Greenstone belts that have past-producing VMS deposits have trace element geochemistry consistent with an extensional tectonic setting. Mafic rocks are mostly tholeiitic and felsic rocks are FIII-type rhyolites, which formed in high heat flow rift settings. Mantle Pb and Nd isotopic values indicate the absence of evolved crust. In contrast, greenstone belts containing magmatic Ni-Cu deposits and no VMS deposits are characterized by evolved Pb and Nd isotopic compositions suggesting interaction with older, evolved crust during petrogenesis. Felsic rocks are FI-type suggesting formation in the lower crust and a large proportion of mafic rocks have calc-alkalic affinities. Based on these key petrogenetic differences, it is proposed that the greenstone belts represent varying maturities of rifting within an arc geodynamic setting.