The geology and alteration patterns of a disseminated, shear zone-hosted mesothermal gold deposit; the Francoeur 3 Deposit, Rouyn-Noranda, Quebec

The Francoeur 3 gold deposit, located approximately 15 km southwest of Rouyn-Noranda, Quebec, is one of several geologically similar Archean mesothermal gold deposits occurring along the Francoeur-Wasa shear zone, a second-order reverse structure possibly related to the Cadillac-Larder Lake break.The principal gold mineralization, which is completely hosted by the Francoeur-Wasa shear zone, is associated with disseminated pyrite and occurs in tabular zones of intense hydrothermal replacement. The mineralized zones consist of a buff- or gray-colored, very fine grained, carbonate-albite-pyrite assemblage which occurs in very different lithologies, including sheared andesite, red albitite dikes, and gabbro.Three types of hydrothermal alteration occur in the deposit. In chronological order, they are (1) a widespread pervasive preore carbonate-hematite-muscovite alteration associated with the intrusion of late kinematic albitite dikes, (2) an albite-pyrite alteration closely associated with the gold mineralization, and (3) a postore sulfate-bearing alteration. The distribution of alteration assemblages and replacement textures suggests evolution of fluid-rock interaction. The initial hydrothermal alteration involved the hydrolysis of metamorphic silicates and oxides to form carbonate, muscovite, and hematite with net additions in CO 2 , K, Rb, and TiO 2 . Subsequently, muscovite and hematite were replaced in the ore zones by albite and pyrite, respectively, resulting in K 2 O and Rb depletion and large enrichments in Na 2 O, S, W, and Au. These two alteration types are crosscut by late gypsum and anhydrite veinlets which are associated with late brittle extensional faults and possibly mark the last product of hydrothermal activity in the Francoeur-Wasa shear zone in this area.It is proposed that gold deposition occurred as a result of changes in physicochemical conditions of ore-forming fluids in response to progressive fluid-rock interaction. The combined effects of an increase in f (sub O 2 ) and a (sub (Na (super +) or K (super +) )) and reductions in both f (sub H 2 S) and pH controlled Au solubility, and very efficiently partitioned its deposition in the albite-pyrite alteration facies. This interpretation is consistent with preliminary sulfur isotope data which indicate that pyrite coexisting with gold is highly depleted in delta 34 S (-10.5 + or - 0.8ppm) compared with pyrite elsewhere in the shear zone, as a result of reduced S oxidation.The absence of an auriferous quartz-carbonate vein system and the occurrence of disseminated replacement mineralization at Francoeur suggest that this hydrothermal system remained relatively permeable through much of the hydrothermal process such that fluid flow operated under near-constant hydraulic gradient. Such a fluid regime is very different from that generally proposed for the formation of mesothermal gold quartz veins in which episodic fluid-pressure elevations above lithostatic pressure are thought to control fluid migration, wall-rock alteration, and gold deposition.