Primary and secondary ore textures in the West Ansil volcanic-hosted massive-sulphide deposit, Noranda mining camp, Rouyn-Noranda, Quebec

Abstract

The West Ansil deposit is the fi rst copper discovery in 25 years in the Noranda central camp. It has a combined indicated and inferred resource of about 1.2 Mt. Grades for the indicated resource are 3.4% Cu, 0.4% Zn, 1.4 g/t Au, and 9.2 g/t Ag. Locally, gold grades are up to 10 ppm within massive chalcopyrite ore, which is signifi cantly higher than other central camp deposits (e.g. Corbet). The bulk of the resource is located in three massive-sulphide lenses (upper, middle, and lower) that are hosted in the Rusty Ridge unit above the Lewis exhalite. The Lewis exhalite is a key marker that is also related to the nearby Ansil mine (1.58 Mt, 8.03% Cu, 0.09% Zn) about 1.8 km northeast of the West Ansil deposit. The overall distribution of massive-sulphide ore and stringer zones at the West Ansil deposit suggests either stacking of separate lenses or dissection of a single deposit by late-stage faulting and dyke formation. The mineralization in all three ore lenses consists of massive pyrrhotite+chalcopyrite±magnetite. Semimassive sphalerite is restricted to the upper and lower parts of the middle lens. Massive magnetite occurs in the upper and middle lenses, where it replaces massive pyrrhotite. A striking feature of the West Ansil deposit is the presence of abundant colloform and nodular pyrite (±marcasite) in the massive-sulphide deposits, especially at the top and bottom of the main massive-sulphide lens. The colloform pyrite is interpreted to be a product of supergene alteration of pyrrhotite. This alteration may be related to shearing at the contacts of the lenses that allowed penetration of groundwater into the massive-sulphide deposits.