The Lac Troilus gold-copper deposit, northwestern Quebec; a possible Archean porphyry system

Abstract

The Lac Troilus gold-copper deposit is located approximately 120 km north of Chibougamau, Quebec, within the northeastern part of the Evans-Frotet greenstone belt, an Archean, bimodal, supracrustal sequence of mafic to felsic volcanic flows and tuffs. Volcanic rocks in the vicinity of the deposit form an overturned sequence of porphyritic intermediate tuffs and flows and mafic lapilli tuffs. These rocks were intruded, post-tilting, by syn- to late tectonic felsite and feldspar porphyry dikes and sills. The bulk of the copper-gold mineralization is hosted within a zone of in situ breccia along the contact between the intermediate and mafic volcanic rocks. The Lac Troilus deposit has a drill-indicated geologic resource totaling approximately 60 million metric tons containing over 2.5 million oz gold, 3 million oz silver, and 60,000 metric tons copper.Mineralization consists of 2 to 3 percent sulfides with localized areas of up to 7 percent sulfides. The sulfides are principally chalcopyrite, pyrite, and pyrrhotite, with rare sphalerite. The sulfides form disseminations, tiny veinlets, and narrow semimassive seams and are controlled by both foliation and fractures. Gold generally occurs as electrum, although native gold and calaverite have been recognized. The electrum occurs as discrete grains, generally from 20 to 100 mu m in diameter, along sulfide grain boundaries and along fractures within the sulfides.The Lac Troilus deposit shows strong mineral and chemical zonation. The core area of wall-rock alteration, characterized by a zone of strong biotite alteration, is centered on a large felsic dike situated in the immediate structural footwall to the deposit. Moving outward, there is a decrease in the intensity of biotite alteration and an increase in plagioclase, epidote, and calcite alteration. Data currently available indicate that the zonation is asymmetric and is best developed upward into the hanging wall. Chemical zonation is indicated by strong potassium enrichment in the footwall which grades into a more sodium-enriched zone in the hanging wall. Calcium is depleted in all altered rocks adjacent to the copper-gold mineralization.The copper and iron sulfide minerals and gold show similar zoning patterns. The core area is characterized by a chalcopyrite-pyrrhotite-rich assemblage. Proceeding toward the structural hanging wall, there is a gradual decrease in chalcopyrite and pyrrhotite contents and a relative increase in pyrite. The potassic footwall rocks are primarily copper-bearing whereas the hanging-wall, sodium-enriched rocks tend to be gold rich and copper poor. The zone of brecciation, which has both sodium and potassium enrichment, is both gold and copper rich.Considering the size and nature of the wall-rock alteration zones, the metal zoning, grades, and characteristics of the host rocks, the Lac Troilus deposit is similar to, and may have formed in a manner analogous to, porphyry copper-gold deposits in the Phanerozoic, such as Island Copper, in British Columbia.