Abstract

The Lupin deposit is located on the shores of Contwoyto Lake at lat 65 degrees 48' N, long 111 degrees 15'W and lies about 80 km south of the Arctic Circle. Since start-up in 1982 the mine has produced over 2.15 million ounces of gold from 6.66 million metric tons of ore grading 10.63 g/t. Total remaining reserves at the 1993 year end were 5.1 Mt at a grade of 9.11 g/t (1.65 Moz). This is sufficient for seven to eight years at the present annual production rate of about 200,000 oz. The center and west zones of the deposit are open to depth.The deposit is in an Archean metaturbidite sequence of the Contwoyto Formation, which has been mapped as part of the Yellowknife Supergroup of supracrustal metasedimentary and metavolcanic rocks making up part of the Slave geologic province. These rocks have been subjected to regional and contact metamorphism and to several phases of deformation. Six stages of Archean plutonism have been identified and all Archean rocks have been cut by Proterozoic diabase dike swarms.Gold mineralization in the Lupin deposit is commonly fine grained and associated with pyrrhotite and arsenopyrite in amphibolitic banded iron-formation. The gold-bearing portions of the host iron-formations are characterized by the presence of multiple quartz veins and the pervasive replacement of grunerite by hornblende. In sparsely mineralized portions of the host iron-formation, zoning of sulfides about quartz veins is clearly seen, with the arsenopyrite halo being more restricted around the veins than the pyrrhotite halo. In well-mineralized portions, quartz vein densities approach one vein per meter along strike. In these areas pyrrhotite distribution in the iron-formation is more pervasive and the rock takes on the appearance of a banded sulfide rock.D 1 and D 2 deformational events have resulted in steeply dipping fold limbs and steeply plunging fold noses at Lupin. The interaction of these two early events has formed a modified domal structure which characterizes the Lupin deposit. Fracturing along, and parallel to, F 2 folding in the Lupin deposit provided channelways within amphibolitic iron-formation during postpeak metamorphism. Fluids passing through these channelways introduced Au, Ag, and S plus other elements into the iron-formation. In a final phase of fluid movement, quartz veins filled the fractures.Geologic mapping, detailed sampling, and geochemical studies have clearly shown that both gold values and sulfide concentrations are commonly highest within 1 m of quartz vein-iron-formation contacts and decrease rapidly away from vein margins. In sparsely mineralized portions of the deposit, transitions from sulfide-rich iron-formation to sulfide-free iron-formation are common with increasing distances from quartz vein-iron-formation contacts. Gold-rich quartz veins with gold values up to several ounces per ton have been identified by detailed sampling.It is concluded that sulfide and gold concentrations at Lupin are related to a multiphase, pervasive quartz vein system present, to a greater or lesser extent, throughout the deposit. It is probable that gold-bearing metasomatic fluids circulating in the cooling sedimentary pile were channeled along structures resulting from earlier deformation.

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