Trace element and ore mineralogy of the Osborne Lake mine, Manitoba

Abstract

The Osborne Lake mine of the Hudson Bay Mining and Smelting Company Ltd., is situated about 100 miles east of Flin-Flon, in the Snow-Chisel Lakes area, Manitoba. Pyrite, arsenopyrite, pyrrhotite, chalcopyrite and sphalerite are the dominant ore minerals, with some cobaltite and galena. Two periods of mineralization are recognized in these ores. All pyrrhotites are mixtures of both hexagonal Fe1-xs and monoclinic Fe1-xs. These mixtures contain from 8 to 87 Wt.% hexagonal Fe1-xs, and contain 46.7 to 47.4 At.% metals. The composition of coexisting hexagonal Fe1-xs and monoclinic Fe1-xs are 47.5 and 46.5 At.% metals respectively. X-ray fluorescence analyses indicate that the amount of Ni in bulk pyrrhotite samples varies from 481 to 51 p.p.m; the amount of Co varies from 922 to < 33 p.p.m; the amount of Mn varies from 1669 to < 37 p.p.m; and the amount of Se varies from 1395 to 698 p.p.m. The Co/Ni ratios in pyrrhotites range from < 0.0001 to 9.817. Sphalerite is relatively less abundant than pyrrhotite in these ores. Electron probe measurements indicate that iron in sphalerite varies from 15.61 to 3.76 mol% FeS, and the amount of manganese varies from 4.9 to 0.01 Wt.%. Cadmium was not detected. There is no correlation between the concentrations of iron and manganese in sphalerite. The partitioning of Co among coexisting pyrrhotite (Po), pyrite (Py), and arsenopyrite (AsP) in decreasing order of concentration is AsP>> Py > Po. Further pyrrhotite samples in which the Co/Ni ratios are less than unity are generally associated with arsenopyrite, suggesting that the low ratios may be due to the 'sponging' effect of arsenopyrite for Co. Hence it has been suggested that classification of ore bodies in the Flin Flon-Snow Lake mineralized belt into 'economic' and 'barren' types, based on Co/Ni ratios in pyrrhotite alone, may not be valid. An estimate of temperature and pressure, using composition of coexisting phases suggest that these ores at some period of their history have been heated to 350°-450°, and subjected to 500-1400 bars confining pressures.