Geology and mineralization in the Big Four Lake area, Shining Tree, Abitibi greenstone belt, Ontario

The purpose of the research work (in partnership with the Ontario Geological Survey (OGS) and International KRL Resources Corp. (KRL)) was to better define the geology and mineralization of the Big Four Lake area, close to Shining Tree in Ontario, part of the southwesthern Abitibi greenstone belt. Rocks of the area belong to the Deloro assemblage a suite of calc-alkaline mafic to felsic metavolcanic rocks, tholeiitic mafic to intermediate metavolcanic rocks, iron formation plus komatiites of the Kidd-Munro and Pacaud assemblages, all cut by later intrusions. The rocks are metamorphosed to the greenschist facies and for the most part are only slightly deformed.;In the field, the Red Dome rhyolite has a characteristic hematization whereas the South rhyolite is strongly sericitized and has conspicuous alkali feldspar phenocrysts. Based on the presence of columnar jointing, autoclastic fades, and its massive form, the Red Dome rhyolite is interpreted to be a dome. The South rhyolite was earlier interpreted to be pyroclastic because of the presence of pumice fragments in a breccia that surrounds massive facies. However, there is no evidence of explosive and hot emplacement, and there are red chert clasts within the breccia which is itself located close to a conglomerate containing fragments of the South rhyolite. Because these two facies surround massive rhyolite, the South rhyolite is also interpreted to be a dome.;Mineralization is characterized by pyrite, chalcopyrite, magnetite, hematite, malachite in quartz and carbonate veins that cut the Red Dome rhyolite. Exploration in the area focused on this mineralization because it was thought to be a VMS environment. The chlorite mineral chemistry is relatively iron-rich consistent with a metamorphic rather than hydrothermal origin and the rhyolites are not the Fill type typical of VMS.;It is concluded that the mineralization may have been hydrothermal, possibly from a small, or aborted, VMS system, or related to later regional felsic intrusions or even the Proterozoic intrusions, which may have provided heat to remobilize metals.;A 1:10,000 scale geological map was produced in order to characterize the distribution of Deloro assemblage rocks and the Proterozoic intrusions. Seven different units were defined: (1) Mafic to intermediate metavolcanic rocks, (2) South rhyolite, (3) Volcanoclastic rocks, (4) Red Dome rhyolite, (5) Sulfide---rich chemical metasedimentary rocks, (6) Matachewan dike swarm, and (7) Nipissing gabbro. Major and trace element geochemistry shows that unit 1 rocks are tholeiitic whereas units 2 and 4 are calc-alkaline. Accordingly the Deloro assemblage rocks (units 1 to 5) are interpreted to have formed in a back-arc setting. The mafic to intermediate metavolcanic rocks have affinities with N-type MORB and a volcanic-arc environment.