Mineralization and tectonic setting of the Norton Lake Region, Uchi subprovince

The Norton Lake region is located approximately 50 km northeast of Fort Hope within the northernmost unnamed assemblage of the Miminiska-Fort Hope greenstone belt of the Uchi Subprovince, northwestern Ontario. An investigation of the region was initiated to attempt to correlate this region of the Superior Province with tectonic assemblages located to the southwest within other greenstone belts of the Uchi Subprovince, and to understand the mineralization of the Norton Lake Cu-Ni-Co-PGE deposit. The geology of the area reveals a high degree of diversity within a relatively small area. The region is dominated by massive to pillowed mafic volcanic rocks with scattered pyroxenite and gabbro intrusions and sporadic sedimentary units. Trace element geochemistry has indicated that the volcanic rocks can be divided into three suites. Suite I consists of tholeiitic basalts to andesites characterized by weakly depleted to enriched LREE (La/Smcn= 0.8 – 1.2) and weakly fractionated HREE (Gd/Ybcn = 1.0 – 1.4) in conjunction with slightly negative to slightly positive niobium anomalies. Suite II’s tholeiitic basalts are characterized by LREE depleted basalts (La/Smcn= 0.5 0.7) with unfractionated HREE (Gd/Ybcn = 0.9 1.0) and flat to negative niobium anomalies. Suite III is subdivided into two subsets: IIIa, calc-alkaline dacites, and IIIb, calc-alkaline basalts. All Suite III samples are strongly enriched in LREE (La/Smcn= 4.8 and 4.7 for IIIa, 1.1 – 3.1 for IIIb) with pronounce negative niobium anomalies. HREE vary from highly fractionated in Suite IIIa (Gd/Ybcn = 2.3 and 1.8) to weakly fractionated and unfractionated in Suite IIIb (Gd/Ybcn = 0.8 to 1.3). The combination of trace element geochemistry and radiogenic isotope data indicates a complex tectonic environment. The sequence, from apparent oldest to youngest, of MORB (Suite II), subduction related volcanism (Suite III), ocean plateau (Suite I) and MORB (Suite II) is difficult to interpret. The obduction of an ocean plateau onto an immature island arc followed by the formation of a back-arc basin represents the most plausible explanation. The previous correlation of this region with the Northern Pickle assemblage of the Pickle Lake greenstone belt to the southwest, is supported by new trace element geochemistry and radiogenic isotope data. The Norton Lake Cu-Ni-Co-PGE deposit (delineated in 1981 as 944,000 tonnes at 0.72% Ni and 0.56% Cu) is hosted within an ultramafic intrusion located along the contact between an upper mafic volcanic unit and a lower sheared amphibolite unit. Study of the mineralogy of the deposit has revealed two stages of mineralization. The first, due to sulphide saturation of the ultramafic intrusion through assimilation of an iron formation, accounts for formation of the primary sulphide phases. The second, a hydrothermal remobilization and enrichment of Cu and PGEs, resulted in the formation of crosscutting chalcopyrite veins and various Pd tellurides. The majority of Pt within the deposit has been found to be contained within the sulphide phases either as microinclusions, below the detection limit of a microprobe, or in solid solution with the sulphides. The Norton Lake deposit shows a similarity to the Thierry Mine, located within the Northern Pickle assemblage, in both mineralization history and geological setting.