Early gold-bearing quartz veins within the Riviere-Heva fault zone, Abitibi Subprovince, Quebec, Canada

The Malartic Lakeshore showing is a gold-bearing quartz vein system located within the major Riviere-Heva fault zone (RHFZ) of the southern Abitibi greenstone belt. This fault separates the 2702-2700 Ma felsic Heva Formation from the 2708 Ma mafic-ultramafic Dubuisson Formation. A swarm of thin diorite dykes with lamprophyric facies and gold-bearing quartz veins are present only on the Dubuisson side of the fault. The 30-70 cm thick gold quartz veins are boudinaged and folded. Veins are banded and associated with pyrite, chalcopyrite, galena, barite, and gold. The study area is characterized by a high degree of ductile deformation associated with the RHFZ and manifested by the southeast-trending "principal schistosity" (Sp). Stretching lineations plunge moderately to shallowly toward the southeast as a result of shortening followed by late directional shearing during a transpressive deformation. A sample from the Heva Formation yielded a zircon U-Pb age of 2698.2 + or - 0.8 Ma, and a diorite dyke produced an age of 2694.3 + or - 2.5 Ma. Quartz veins are crosscut by dykes, and both are affected by the Sp fabric, indicating an early emplacement with respect to the deformation. This situation contrasts with the orogenic gold veins found in association with major fault zones. A near-synvolcanic magmatic hydrothermal origin is proposed for this gold vein system. Because all subvertical units in the area are south facing, the presence of the older Dubuisson Formation over the younger Heva Formation is attributed to the RHFZ acting as a significant reverse or thrust fault.