Geology and geochemistry of the Laird Lake property and associated gold mineralization, Red Lake greenstone belt, northwestern Ontario
Gelinas, Brigitte Rachel
Master of Science
Whole-rock and trace element geochemistry
Geology and mineralization
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The Laird Lake property encompasses the tectonic contact between the Balmer (2.99 to 2.96 Ga) and the Confederation (2.74 to 2.73 Ga) assemblages on the south-western end of the Red Lake greenstone belt, Northwestern Ontario. The purpose of this study was to determine the tectonic setting in which the assemblages formed, and to characterize the controls on and nature of the gold mineralization associated with the tectonic contact between the Balmer and Confederation assemblages. Detailed mapping of the area highlighted major differences between the two assemblages. The Balmer assemblage is typically composed of fine-grained, aphyric, locally pillowed mafic volcanic rocks, ultramafic intrusive and volcanic rocks with flow-breccia textures and local spinifex-bearing clasts, and banded-iron formations. In contrast, the Confederation assemblage consists of porphyritic (feldspar) or poikiloblastic (amphibole) mafic volcanic rocks intercalated with intermediate to felsic volcanic rocks that include crystal lapilli tuffs, crystal tuffs and tuffs. Syn-volcanic and syn- to post-D2 intrusions commonly cross-cut the volcanic packages. A regional foliation (~E-trending) is present throughout the volcanic rocks and increases in intensity at the tectonic contact between the two assemblages where a shear zone no thicker than 100 m is present within the Balmer assemblage. Whole-rock geochemical analyses were undertaken on 161 samples from the Laird Lake area. The Balmer assemblage is composed of tholeiitic mafic volcanic rocks with minor Al-undepleted komatiites, whereas the Confederation assemblage is composed of calc-alkalic mafic and intermediate to felsic volcanic rocks, which display FI, FII, and FIIIb rhyolite trends. Neodymium isotope analyses, in conjunction with trace element geochemistry, suggests that parts of the Balmer assemblage were weakly contaminated by an older intermediate basement. The data suggests both arc and back arc volcanism within the Confederation assemblage, with the arc rocks showing stronger a crustal component than the back arc rocks. U-Pb geochronology of volcanic and intrusive Confederation units yielded ages of 2741 ± 19 Ma (FI quartz-feldspar porphyritic crystal tuff) and 2737.68 ± 0.79 Ma (diorite). The geochemistry and age of the tuff correlates within error to the Heyson sequence of the Confederation, whereas the diorite is likely a syn-volcanic intrusion. The Balmer assemblage is interpreted to represent an oceanic plateau formed by plume magmatism on the margins of the North Caribou Terrane whereas the Confederation assemblage was likely built in an oceanic arc setting where both arc and back arc volcanism were occuring simultaneously. The presence of xenocrystic zircons within the 2741 Ma quartz-feldspar porphyritic crystal tuff suggest that melts within the main arc incorporated xenocrystic zircons during ascent through a thin Mesoarchean crustal fragment. Juxtaposition of the Confederation assemblage onto the Mesoarchean assemblages likely occurred between 2739-2733 Ma. Gold mineralization at the Laird Lake property is controlled by a D2 shear zone within the Balmer assemblage at the tectonic contact between the Balmer and Confederation assemblages. The mineralization is commonly found associated with a mineral banded texture, accompanied by disseminated arsenopyrite, pyrrhotite, pyrite ± chalcopyrite, similar to the features observed at the nearby Madsen gold mine. Oxygen isotope data from early- to syn-D2 gold-mineralized and barren quartz veins suggest both vein types originated from a similar source and overlap with δ18OH2O values of metamorphic, magmatic and meteoric waters, over crystallizing temperatures ranging from 300 to 500°C. Evidence for a second gold event (post-2702 ± 1 Ma) is characterized by the presence of gold-rich shears and lamprophyres cross-cutting the regional foliation, and the lack of overprinting amphibolite facies mineralogy. The Laird Lake property likely represents the continuation of the same mineralized structure found at both the Madsen and Starrat-Olsen mines and was later displaced as far as 10 km west by the dextral Laird Lake fault post-2704 Ma.